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Tantalum pentoxide, a non conventional gate insulator for MOS devices Eguizabal-Rivas, Antonio L. 1984

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TANTALUM PENTOXIDE, A NON CONVENTIONAL GATE INSULATOR FOR MOS DEVICES by ANTONIO L. EGUIZABAL-RIVAS Inge n i e r o C i v i l - E l e c t r i c o , U n i v e r s i d a d Tecnica d e l Estado (Santiago, C h i l e , 1970) A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE THE FACULTY OF GRADUATE STUDIES Department of E l e c t r i c a l E n gineering We accept t h i s T h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH November 1984 © Antonio L. E g u i z a b a l COLUMBIA Rivas, 1984 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and study. I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the head o f my department o r by h i s o r her r e p r e s e n t a t i v e s . I t i s understood t h a t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be allowed without my w r i t t e n p e r m i s s i o n . Department of E l e c t r i c a l E n g i n e e r i n g The U n i v e r s i t y of B r i t i s h Columbia 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 Date November 6th, 19 84 DE-6 (2/79) i i ABSTRACT Non c o n v e n t i o n a l gate i n s u l a t o r s f o r MOS d e v i c e s are g e n e r a l l y d i e l e c t r i c s that depart c o n s i d e r a b l y from the c l a s s i c S i 0 2 used e x t e n s i v e l y i n t h i s technology. The work presented here r e f l e c t s the r e s e a r c h and development of an e x i s t i n g compound, T a 2 0 5 , and i t s a p p l i c a t i o n as a gate i n s u l a t o r f o r both MOS c a p a c i t o r s and t r a n s i s t o r s . The oxide i s grown both t h e r m a l l y and a n o d i c a l l y from pure s p u t t e r e d tantalum metal over s i l i c o n wafers. S u c c e s f u l d i e l e c t r i c s s u i t a b l e f o r gate i n s u l a t o r s were obtained using both methods. High r e l a t i v e p e r m i t t i v i t y (^26-28) being c h a r a c t e r i s t i c of tantalum pentoxide, o f f e r s c o n s i d e r a b l e advantage over c l a s s i c s i l i c o n d i o x i d e gate i n s u l a t o r s , however higher leakage c u r r e n t s (100 to 1000 times g r e a t e r ) were encountered i n MOS C a p a c i t o r samples at room temperature. A method f o r p r o c e s s i n g the tantalum metal was developed using the l i f t o f f technique, and i t was s u c c e s s f u l l y a p p l i e d to both MOS c a p a c i t o r s and f i e l d e f f e c t t r a n s i s t o r s . Furthermore, d e v i c e s were f a b r i c a t e d i n the form of MOS T r a n s i s t o r s , which e x h i b i t e d good Id vs. Vds c h a r a c t e r i s t i c s , with Vgs as a parameter. Gate leakage c u r r e n t s were low, as a double d i e l e c t r i c T a 2 0 5 over S i 0 2 s t r u c t u r e was used as gate i n s u l a t o r . A small s i g n a l model of t h i s c l a s s of d e v i c e s i s presented, that takes i n t o account the non zero gate leakage c u r r e n t . Another s u c c e s s f u l technique, i n t e r f a c i a l o x i d a t i o n of T a 2 0 5 over S i , was used i n f a b r i c a t i n g MOS C a p a c i t o r s that y i e l d e d a l s o low leakage c u r r e n t s and high s p e c i f i c c a p a c i t a n c e s . The purpose of t h i s T h e s i s i s to r e p o r t the development at the U n i v e r s i t y of B r i t i s h Columbia of the double gate i n s u l a t o r MOSFET technology based on the Tantalum Pentoxide-S i l i c o n D i oxide ( T a 2 0 5 / S i 0 2 ) heteromorphic s t r u c t u r e . T A B L E O F C O N T E N T S A B S T R A C T i i T A B L E O F C O N T E N T S i v L I S T O F T A B L E S v i i L I S T O F F I G U R E S x A C K O W L E D G E M E N T S x i i C H A P T E R 1 I N T R O D U C T I O N 1 C H A P T E R 2 A N O V E R V I E W OF N O N C O N V E N T I O N A L I N S U L A T O R S 6 C H A P T E R 3 T H E T H E O R Y O F T H E T a 2 0 5 - S i 0 2 D O U B L E D I E L E C T R I C S T R U C T U R E 2 9 3 . 1 S T E A D Y S T A T E A N A L Y S I S , 3 6 3 . 2 T R A N S I E N T A N A L Y S I S 3 8 3 . 3 E F F E C T O F T H E D O U B L E D I E L E C T R I C G A T E I N S U L A T O R O N T H E M O S F E T P E R F O R M A N C E 41 3 . 4 E F F E C T OF A H I G H P E R M I T T I V I T Y G A T E I N S U L A T O R ON T H E M O S F E T P E R F O R M A N C E 4 6 3 . 5 E N E R G Y B A N D S O F T H E T A N T A L U M P E N T O X I D E I N S U L A T O R 5 0 3 . 6 D O U B L E D I E L E C T R I C M O S F E T S T R U C T U R E 5 2 C H A P T E R 4 F A B R I C A T I O N A N D P R O C E S S I N G O F MOS C A P A C I T O R D E V I C E S 5 4 4 . 1 MOS C A P A C I T O R S W I T H T H E R M A L T a 2 0 5 A S I N S U L A T O R . 5 4 4 . 1 . 1 T H I C K N E S S A N D S H E E T R E S I S T I V I T Y M E A S U R E M E N T S 5 5 4 . 1 . 2 S C R I B I N G A N D M A R K I N G 5 5 V 4 . 1 . 3 P E R O X I D E A C I D C L E A N I N G 5 7 4 . 1 . 4 R F S P U T T E R I N G 5 7 4 . 1 . 5 T H E R M A L O X I D A T I O N 5 7 4 . 1 . 6 A L U M I N I U M D E P O S I T I O N 5 9 4 . 1 . 7 P H O T O L I T H O G R A P H Y 5 9 4 . 1 . 8 C O N T A C T M E T A L L I Z A T I O N 6 0 4 . 2 MOS C A P A C I T O R S W I T H D O U B L E D I E L E C T R I C S T R U C T U R E 6 0 4 . 3 P R O C E S S A N D F A B R I C A T I O N C O M M E N T S 61 4 . 4 T H E L I F T O F F T E C H N I Q U E ON T A N T A L U M F I L M S 6 4 4 . 5 MOS C A P A C I T O R S W I T H A N O D I C T a 2 0 5 A S I N S U L A T O R . . 6 5 4 . 5 . 1 A N O D I Z A T I O N I N C I T R I C A C I D E L E C T R O L Y T E S O L U T I O N 6 9 4 . 5 . 2 A N O D I Z A T I O N I N H 3 P O „ E L E C T R O L Y T E S O L U T I O N . 7 2 4 . 6 I N T E R F A C I A L O X I D A T I O N MOS C A P A C I T O R S 7 3 C H A P T E R 5 R E S U L T S A N D M E A S U R E M E N T S ON MOS C A P A C I T O R S 7 6 5 . 1 E L L I P S O M E T R Y 7 6 5 . 2 C - V M E A S U R E M E N T S 7 7 5 . 3 I - V M E A S U R E M E N T S 81 5 . 4 H I L L O C K F O R M A T I O N I N V E S T I G A T I O N 8 5 5 . 5 D I S C U S S I O N O F R E S U L T S 9 3 5 . 5 . 1 E L L I P S O M E T R Y 9 3 5 . 5 . 2 C - V C U R V E S 9 9 5 . 5 . 3 I - V C U R V E S 1 1 4 5 . 6 I N T E R F A C I A L O X I D A T I O N MOS C A P A C I T O R S 121 C H A P T E R 6 F A B R I C A T I O N A N D P R O C E S S I N G O F M T A O S F I E L D E F F E C T T R A N S I S T O R S 1 2 3 v i 6 . 1 S H E E T R E S I S T I V I T Y D E T E R M I N A T I O N 1 2 5 6 . 2 S I L I C O N T H E R M A L O X I D A T I O N 1 2 6 6 . 3 T H I C K O X I D E P H O T O L I T H O G R A P H Y 1 2 7 6 . 4 B O R O N P R E D E P O S I T I O N 1 2 8 6 . 5 B O R O N D R I V E - I N 1 2 8 6 . 6 G A T E P H O T O L I T H O G R A P H Y 1 2 9 6 . 7 T H I N O X I D E P R O C E S S S I M U L A T I O N U S I N G S U P R E M 1 2 9 6 . 8 P R E P A R A T I O N O F D E V I C E W A F E R S F O R A N O D I Z A T I O N . . . 1 3 2 6 . 9 P E R O X I D E - A C I D C L E A N I N G O F A L L W A F E R S 1 3 3 6 . 1 0 A L U M I N I U M E V A P O R A T I O N 1 3 3 6 . 1 1 P H O T O L I T H O G R A P H Y F O R L I F T O F F 1 3 3 6 . 1 2 M I C R O S C O P E E X A M I N A T I O N A F T E R L I F T O F F 1 3 4 6 . 1 3 D E T E R M I N A T I O N O F T H E S W E L L I N G F A C T O R S 1 3 4 6 . 1 4 P R E L I M I N A R Y R F S P U T T E R I N G O F T A N T A L U M 1 3 6 6 . 1 5 P R E L I M I N A R Y T H E R M A L O X I D A T I O N 1 3 7 6 . 1 6 P R E L I M I N A R Y A N O D I C O X I D A T I O N 1 3 8 6 . 1 7 R F S P U T T E R I N G O F T A N T A L U M 1 3 9 6 . 1 8 T H E R M A L O X I D A T I O N O F T A N T A L U M 1 3 9 6 . 1 9 A N O D I C O X I D A T I O N O F T A N T A L U M 1 4 0 6 . 2 0 P R E L I M I N A R Y M I C R O S C O P E E X A M I N A T I O N 1 4 3 6 . 2 1 L I F T O F F P A T T E R N I N G 1 4 3 6 . 2 2 M I C R O S C O P E E X A M I N A T I O N A F T E R L I F T O F F 1 4 4 6 . 2 3 P E R O X I D E - A C I D C L E A N I N G 1 4 5 6 . 2 4 S O U R C E A N D D R A I N T H I N G A T E O X I D E R E M O V A L 1 4 5 6 . 2 5 M I C R O S C O P E P H O T O G R A P H Y 1 4 5 6 . 2 6 A L U M I N I U M D E P O S I T I O N F O R C O N T A C T S 1 4 6 6 . 2 7 D R A I N A N D S O U R C E C O N T A C T P H O T O L I T H O G R A P H Y 1 4 7 6 . 2 8 E T C H I N G ON B A C K O F W A F E R A N D A u D E P O S I T I O N 1 4 7 6 . 2 9 F I N A L M I C R O S C O P E E X A M I N A T I O N 1 4 8 C H A P T E R 7 R E S U L T S A N D M E A S U R E M E N T S O N M T A O S F I E L D E F F E C T T R A N S I S T O R S 1 5 4 7 . 1 T E S T I N G A N D M E A S U R E M E N T P R O C E D U R E 154 7 . 2 D I S C U S S I O N O F R E S U L T S 1 6 3 7 . 2 . 1 C - V C U R V E S O N D O U B L E D I E L E C T R I C I N S U L A T O R . 1 6 3 7 . 2 . 2 I - V C U R V E S O N T H E D O U B L E D I E L E C T R I C G A T E I N S U L A T O R 1 6 9 7 . 2 . 3 G A T E T H R E S H O L D V O L T A G E 1 6 9 7 . 2 . 4 T H E O U T P U T C U R V E S 1 7 0 7 . 2 . 5 T H E T R A N S F E R C U R V E S 1 7 3 7 . 2 . 6 P U L S E R E S P O N S E O F T H E DD M O S F E T s 1 7 4 7 . 2 . 7 S P I C E S I M U L A T I O N O F M O S F E T C H A R A C T E R I S T I C S 1 7 5 7 . 3 D O U B L E D I E L E C T R I C M O S F E T E Q U I V A L E N T C I R C U I T 181 C H A P T E R 8 S U M M A R Y A N D C O N C L U S I O N S 1 8 3 R E F E R E N C E S 1 8 7 A P P E N D I X I _ „ C - V A N D I - V C U R V E S O F MOS C A P A C I T O R S & e e — A d d & a d u m . •ti'ru^e-F—s-epa-F-a-t-e—e-O'V-e'i? A P P E N D I X I I C O M P U T E R S O U R C E P R O G R A M S 1 9 5 A P P E N D I X I I I L A B O R A T O R Y P R O C E S S I N G D E T A I L S 2 1 2 A P P E N D I X I V S P I C E A N D S U P R E M S I M U L A T I O N R E S U L T S 2 2 2 v i i i LIST OF TABLES 3.1 Double D i e l e c t r i c MOSFET Parameters 45 3.2 S i n g l e D i e l e c t r i c MOSFET Parameters 47 3.3 MOS C a p a c i t o r I n s u l a t o r F i g u r e Of Me r i t ..49 4.1 S i n g l e D i e l e c t r i c Thermal MOS C a p a c i t o r s 56 4.2 Thermal O x i d a t i o n Of Tantalum On S i l i c o n 58 4.3 Double D i e l e c t r i c MOS C a p a c i t o r s 61 4.4 S i n g l e D i e l e c t r i c Anodic MOS C a p a c i t o r s 73 4.5 I n t e r f a c i a l O x i d a t i o n Of Tantalum On S i l i c o n 75 5.1 S p u t t e r i n g Of Ta On Glass Samples 88 5.2 Oxide Thickness Determination By E l l i p s o m e t r y 99 5.3 Resume Of Thermal Oxide MOS-SD C a p a c i t o r s C-V Curves 105 5.4 Resume Of Thermal Oxide MOS-DD C a p a c i t o r s C-V Curves 106 5.5 Resume Of Anodic Oxide MOS-SD C a p a c i t o r s C-V Curves 107 5.6 C a l c u l a t e d R e l a t i v e D i e l e c t r i c Constant Of Thermal T a 2 0 5 1 08 5.7 C a l c u l a t e d R e l a t i v e D i e l e c t r i c Constant Of Anodic T a 2 0 5 1 09 5.8 F l a t b a n d Voltage, Capacitance And F i x e d Charge Of S i n g l e D i e l e c t r i c T a 2 0 5 MOS C a p a c i t o r s : 110 5.9 F l a t b a n d Voltage, Capacitance And F i x e d Charge Of Double D i e l e c t r i c MOS C a p a c i t o r s 111 5.10 F l a t b a n d Voltage, Capacitance And F i x e d Charge Of Anodic T a 2 0 5 MOS C a p a c i t o r s 112 5.11 Resume Of Schottky I-V Curves And C a l c u l a t e d ix O p t i c a l Value Of The R e l a t i v e D i e l e c t r i c Constant For Thermal T a 2 0 5 116 5.12 Resume Of Schottky I-V Curves And C a l c u l a t e d O p t i c a l Value Of The R e l a t i v e D i e l e c t r i c Constant For Anodic T a 2 0 5 117 5.13 Photoconduction In Tantalum Oxide MOS C a p a c i t o r s .120 5.14 I n t e r f a c i a l O x i d a t i o n MOS C a p a c i t o r s , C-V R e s u l t s 122 5.15 I n t e r f a c i a l O x i d a t i o n MOS C a p a c i t o r s , I-V R e s u l t s 122 6.1 Device Substrate Marking And Measured R e s i s t i v i t y .126 6.2 SUPREM S i m u l a t i o n R e s u l t s 130 6.3 Dry Thermal O x i d a t i o n Of S i 0 2 131 7.1 Summary Of Double D i e l e c t r i c MOSFET C h a r a c t e r i s t i c s 1 74 X LIST OF FIGURES 3.1 The General Double D i e l e c t r i c S t r u c t u r e 31 3.2 E q u i v a l e n t P e r m i t t i v i t y Of A Double I n s u l a t o r .....44 3.3 Energy Band Diagram Of The A l - T a 2 0 5 - n S i S t r u c t u r e .50 3.4 Double D i e l e c t r i c MOSFET S t r u c t u r e 53 4.1 A n o d i z a t i o n C e l l And Equipment 68 4.2 A n o d i z a t i o n C e l l Voltage Under Constant Current ...70 4.3 A n o d i z a t i o n C e l l Current Under Constant V o l t a g e ...71 5.1 C-V Measuring System For MOS C a p a c i t o r s 79 5.2 I-V Measuring System For MOS C a p a c i t o r s 83 5.3 Dark F i e l d Photograph (560X), 500 A Ta RFS On Glass(sample G500) 90 5.4 Dark F i e l d Photograph (560X), 500 A Ta MES On S i l i c o n (sample BNR500) 90 5.5 Dark F i e l d Photograph (140X), 200 A Ta RFS On Thin S i 0 2 On S i l i c o n (sample. 4T6) 91 5.6 Dark F i e l d Photograph (140X), 50 A Ta RFS On Thin S i 0 2 On S i l i c o n (sample 2T6) 91 5.7 Dark F i e l d Photograph (140X), 500 A Ta RFS On Thin S i 0 2 On S i l i c o n (sample 3T7) 92 5.8 Dark F i e l d Photograph (140X), 1000 A Ta RFS On Thin S i 0 2 On S i l i c o n (sample 4T7) 92 5.9 E l l i p s o m e t r i c Data Vs. Time, Sample BNR500 94 5.10 E l l i p s o m e t r i c Data Vs. Time, Sample BNR1000 95 5.11 T r a n s i e n t E l l i p s o m e t r y , Sample BNR500 97 5.12 T r a n s i e n t E l l i p s o m e t r y , Sample BNR1000 98 6.1 MTAOS Anodic O x i d a t i o n Under Constant Current 141 6.2 MTAOS Anodic O x i d a t i o n Under Constant V o l t a g e 142 x i 6.3 MTAOS T r a n s i s t o r ; D r a i n , Source And Gate D e t a i l s ..149 6.4 MTAOS T r a n s i s t o r , Contact Window Area D e t a i l 149 6.5 O v e r a l l View Showing MOSFET And R-S F l i p F l o p 150 6.6 MTAOS T r a n s i s t o r Contact M e t a l l i z a t i o n D e t a i l s ....150 6.7 R-S F l i p F l o p Contact M e t a l l i z a t i o n Windows 151 6.8 MOS C a p a c i t o r Area, Contact M e t a l l i z a t i o n 151 6.9 O v e r a l l View Showing MOSFET And D i f f u s e d R e s i s t o r .152 6.10 Contact Pads And Alignment Markers 152 6.11 NOR Gate And R-S F l i p F l o p O v e r a l l View 153 6.12 I n t e r c o n n e c t i o n And Contact Pad D e t a i l 153 7.1 System For P l o t t i n g MOSFET S t a t i c Curves 156 7.2 S t a t i c Output Curve, Sample MTAOS3 Thermal 157 7.3 S t a t i c Output Curve, Sample MTAOS4 Anodic 158 7.4 S t a t i c T r a n s f e r Curve, Sample MTAOS3 Thermal 159 7.5 S t a t i c T r a n s f e r Curve, Sample MTAOS4 Anodic 160 7.6 System For Measuring The MOSFET Pulse Response ....162 7.7 C-V Curve On Double D i e l e c t r i c Test Wafer (sample MOSCTest 200A Thermal 165 7.8 C-V Curve On Double D i e l e c t r i c Test Wafer (sample MOSCTest 200A Anodic 166 7.9 C-V Curve Of MOSFET Gate, Thermal Sample MTAOS3 ...167 7.10 C-V Curve Of MOSFET Gate, Anodic Sample MTAOS5 ...168 7.11 I-V Curve On MOSFET Gate, Thermal Sample MTAOS3 ..171 7.12 I-V Curve On MOSFET Gate, Anodic Sample MTAOS4 ...172 7.13 Double D i e l e c t r i c MOSFET Output Curves, Sample MTAOS 3 176 7.14 Double D i e l e c t r i c MOSFET Sat u r a t e d T e s t , Sample MTAOS3 176 7.15 Double D i e l e c t r i c . MOSFET Pulse Test (Turn On), Sample MTAOS3 177 7.16 Double D i e l e c t r i c MOSFET Pulse Test (Turn O f f ) , Sample MTAOS3 177 7.17 Double D i e l e c t r i c MOSFET Output Curves, Sample MTAOS4 178 7.18 Double D i e l e c t r i c MOSFET Sat u r a t e d T e s t , Sample MTA0S4 178 7.19 Double D i e l e c t r i c MOSFET Pulse Test (Turn On), Sample MTAOS4 179 7.20 Double D i e l e c t r i c MOSFET Pulse Test (Turn O f f ) , Sample MTAOS4 179 7.21 Leaky Gate In MOSFET, Anodic T a 2 0 5 , Sample MTAOS4 180 7.22 Leaky Gate In MOSFET, Thermal T a 2 0 5 , Sample MTAOS3 180 7.23 Double D i e l e c t r i c MOSFET E q u i v a l e n t C i r c u i t 182 x i i i ACKNOWLEDGEMENTS I wish to thank my S u p e r v i s o r , Dr. Lawrence Young f o r h i s guidance and encouragement on the work presented in t h i s T h e s i s . My thanks to Dr. David P u l f r e y for h i s i n t e r e s t and e n l i g h t n i n g c o n v e r s a t i o n s on the s u b j e c t . A l s o I am indebted to Dr. Peter Janega ( S o l i d S tate Laboratory at UBC) f o r h i s most valued a s s i s t a n c e i n many p r o c e s s i n g d e t a i l s , and i n p r e p a r i n g the i n t e r f a c i a l o x i d a t i o n samples; to Dr. J a m i l Ahmed (now with M i c r o t e l P a c i f i c Research) f o r suggesting the p o s s i b l e use of the L i f t o f f Technique on tantalum metal; to Dr. David Smith (now with B a l l a r d Research) f o r h i s i n i t i a l a s s i s t a n c e and he l p on the s u b j e c t , and f o r a l l o w i n g me the use of some of h i s computer i n t e r f a c i n g software. My e x p r e s s i o n of g r a t i t u d e to Ms. C a r l a Miner of B e l l Northern Research i n Ottawa, f o r supplying the Magnetron Enhanced S p u t t e r i n g samples f o r MOS c a p a c i t o r s . My thanks to Mr. James Henderson of the Science D i v i s i o n , Main L i b r a r y , f o r h i s a s s i s t a n c e i n the Computer A s s i s t e d ' L i b r a r y Search f o r o b t a i n i n g the r e f e r e n c e m a t e r i a l . The a s s i s t a n c e of Ms. Angela Runnals, Ms. V i c t o r i a Lyons-Lamb and Mr. Jon N i g h t i n g a l e of the Computer Centre i s a l s o acknowledged, as they made p o s s i b l e to compose t h i s T h e s i s using the FMT e d i t o r under the MTS o p e r a t i n g system. I a l s o wish to acknowledge the i n f i n i t e p a t i e n c e that my wife Brenda and our c h i l d r e n have demonstrated d u r i n g a l l these years of hard work and study. Without the encouragement of other members of my f a m i l y , i n p a r t i c u l a r my mother and bro t h e r , t h i s work would have not been x i v p o s s i b l e . To a l l them, I e x p r e s s my deep i n d e b t e d n e s s . I d e d i c a t e t h i s work t o a man t h a t fought many b a t t l e s i n l i f e , but h e r o i c a l l y l o s t the l a s t one: my f a t h e r . 1 CHAPTER 1 INTRODUCTION The term non-conventional d i e l e c t r i c s i s used to d e s c r i b e those which depart c o n s i d e r a b l y from the c l a s s i c S i 0 2 used i n MOS technology as gate i n s u l a t o r s , d e v i c e i s o l a t i o n , d i f f u s i o n masking, implant r e s i s t and g e n e r a l s u r f a c e p a s s i v a t i o n . Notwithstanding the n a t u r a l advantages of the s i l i c o n d i o x i d e , numerous authors have f o l l o w e d r e s e a r c h i n a l t e r n a t e d i e l e c t r i c s s u i t a b l e f o r use i n MOS d e v i c e s . In the present work, one of such non-conventional i n s u l a t o r s i s used, namely Tantalum Pentoxide ( T a 2 0 5 ) , e i t h e r o b t a i n e d by thermal or anodic o x i d a t i o n of tantalum metal to form MOS c a p a c i t o r s and f i e l d e f f e c t d e v i c e s on s i l i c o n s u b s t r a t e s . Large S c a l e , Very Large Scale I n t e g r a t e d and f u t u r e U l t r a Large S c a l e I n t e g r a t e d C i r c u i t s w i l l r e q u i r e t e c h n o l o g i e s and d e v i c e s capable of performing when s c a l e d down to 1 micron f e a t u r e s and beyond, i n order to achieve the l a r g e number of d e v i c e s per d i c e (>300,000) i f t h i s microtechnology i s to be s u c c e s s f u l . At t h i s present time ( e a r l y 1984), major problem areas seem to have not been r e s o l v e d y e t , namely the s o l u t i o n to e l e c t r o m i g r a t i o n induced f a i l u r e s , i n which the ions that form the i n t e r c o n n e c t i n g metal l i n e s ( u s u a l l y Aluminium) are t r a n s p o r t e d by the extremely high c u r r e n t d e n s i t i e s , i . e . , " e l e c t r o n wind", of t y p i c a l l y 10,000 A/cm 2 [Black, 1969]; and the s u b t h r e s h o l d conduction i n MOS t r a n s i s t o r s , i n which 2 the device does not turn o f f , i . e . , a s m a l l , but non-zero d r a i n c u r r e n t s t i l l flows d e s p i t e of the e f f o r t s of the a p p l i e d gate v o l t a g e to do so [Troutman, 1974]. The l a t t e r i s p a r t i c u l a r l y c r i t i c a l to the o p e r a t i o n of c i r c u i t s where low leakage (the device i s i n the c u t o f f or OFF c o n d i t i o n ) c u r r e n t s are r e q u i r e d . The subthreshold c u r r e n t s do not s c a l e p r o p e r l y and t h i s p r e s e n t s a problem f o r very small d e v i c e s [Dennard et a l . , 1974]. As a consequence, the r e d u c t i o n of the s u b t h r e s h o l d e f f e c t i s of great importance i n a t t a i n i n g s u c c e s s f u l d e v i c e s f o r VLSI and ULSI. The high p e r m i t t i v i t y d i e l e c t r i c s p r e s e n t l y used in gate i n s u l a t o r s f o r Metal Oxide Semiconductor F i e l d E f f e c t T r a n s i s t o r s (MOSFET's) have d e f i n i t i v e advantages. A c l o s e examination of the equations [Sze, 1969] that r e l a t e the d r a i n c u r r e n t with i t s geometry, t h r e s h o l d v o l t a g e and d r a i n v o l t a g e , r e v e a l s that the former i s i n d i r e c t r e l a t i o n with the i n s u l a t o r r e l a t i v e d i e l e c t r i c c o n s t a n t . A s i m i l a r e f f e c t , and perhaps the most important, i s on the device transconductance gm: i t i s a l s o d i r e c t l y p r o p o r t i o n a l to the gate i n s u l a t o r d i e l e c t r i c c o n s t a n t . Another important parameter, the t h r e s h o l d v o l t a g e , i s reduced by about the same f a c t o r , which i s c r u c i a l i n the design of MOS t r a n s i s t o r s f o r analog ( l i n e a r ) a p p l i c a t i o n s . The channel conductance, s u f f e r s a more complicated change, as i t depends on both i n s u l a t o r c a p a c i t a n c e (which i n c r e a s e s with the d i e l e c t r i c constant) and on the t h r e s h o l d v o l t a g e (which d e c r e a s e s ) . I t can be s a i d then, that a general improvement 3 on the MOSFET device performance can be accomplished by the use of high p e r m i t t i v i t y gate i n s u l a t o r s . Tantalum pentoxide ( T a 2 0 5 ) has a r e l a t i v e d i e l e c t r i c constant of 27, about seven times l a r g e r than s i l i c o n d i o x i d e ( S i 0 2 ) . T h e r e f o r e , in theory, a s e v e n f o l d i n c r e a s e i n performance should be expected. Because of i t s negative i n t e r f a c e charge d e n s i t y , T a 2 0 5 can h e l p in f u r t h e r reducing the gate t h r e s h o l d v o l t a g e [Seki et a l . , 1984] Another promising a p p l i c a t i o n i s i n a v o i d i n g the gate i n s u l a t o r breakdown by e i t h e r the i n t e r n a l f i x e d charge generated f i e l d , or the one caused by the a p p l i c a t i o n of a gate v o l t a g e to the c l a s s i c gate oxide. As the device i s s c a l e d down i n i t s three dimensions, the gate i n s u l a t o r t h i c k n e s s i s reduced by the same f a c t o r K, hence l i m i t i n g the maximum a p p l i e d gate v o l t a g e by oxide breakdown. The a p p l i e d gate v o l t a g e has then a lower bound, the turn-on v o l t a g e , and a higher bound, the i n s u l a t o r breakdown v o l t a g e . T h i s s i t u a t i o n can be avoided, i f a double d i e l e c t r i c s t r u c t u r e i s used i n s t e a d of the s i n g l e l a y e r , s i l i c o n d i o x i d e gate i n s u l a t o r [Angle and T a l l e y , 1978], If a t h i c k l a y e r of high d i e l e c t r i c constant m a t e r i a l ( i . e . , tantalum pentoxide) i s d e p o s i t e d over a t h i n l a y e r of low d i e l e c t r i c constant m a t e r i a l ( i . e . , s i l i c o n d i o x i d e ) , the l a t t e r i s p r o t e c t e d from breakdown f i e l d s , as most of the vol t a g e drop a c r o s s the compound d i e l e c t r i c w i l l appear i n the higher p e r m i t t i v i t y m a t e r i a l , the t h i c k e r tantalum pentoxide. 4 In the double i n s u l a t o r s t r u c t u r e , the p r o b a b i l i t y of p i n h o l e s c o i n c i d i n g i n the same l o c a t i o n i s v a n i s h i n g l y s m a l l , thus producing a much b e t t e r q u a l i t y i n s u l a t o r . Amorphous s i l i c o n d i o x i d e i s s t r u c t u r a l l y porous and has high p e r m e a b i l i t y to water vapour and m i g r a t i o n of a l k a l i i o n s . By a p p l y i n g a second outer d i e l e c t r i c onto the inner one, these e f f e c t s can be reduced. Tantalum pentoxide has a much denser s t r u c t u r e , and p o s s i b l y r e t a r d s the ion m i g r a t i o n . The double d i e l e c t r i c i n s u l a t i n g s t r u c t u r e i s used i n the present work as the gate i n s u l a t o r f o r both MOSFET and MOS c a p a c i t o r d e v i c e s . The s u c c e s s f u l d e v i c e s proved to be f u n c t i o n a l t r a n s i s t o r s , with moderate transconductance and f a s t s w i t c h i n g p u l s e response. The MOS c a p a c i t o r s have good C-V curves and low leakage c u r r e n t s . I t i s q u i t e p o s s i b l e that the s i n g l e d i e l e c t r i c high p e r m i t t i v i t y i n s u l a t o r and the double d i e l e c t r i c s t r u c t u r e have d i r e c t a p p l i c a t i o n i n f u t u r e VLSI and ULSI t e c h n o l o g i e s . Already, the high d i e l e c t r i c constant tantalum pentoxide has found use i n a VLSI 256/512 k b i t dynamic Random Access Memory (RAM), which aims towards the 1 Mbit memory I n t e g r a t e d C i r c u i t [Ohta et a l . , 1980]. i The purpose of t h i s T h e s i s i s to present the measurements and r e s u l t s o b tained from experimental MOS C a p a c i t o r s f a b r i c a t e d with anodic and thermal tantalum pentoxide; and from the experimental MOSFET's u t i l i z i n g a double d i e l e c t r i c ( T a 2 0 5 ) gate i n s u l a t o r s t r u c t u r e , i n which the tantalum oxide was ob t a i n e d by thermal and anodic 5 processes. T h i s T h e s i s i s d i v i d e d i n t o e i g h t chapters and four appendices. The second chapter c o n t a i n s an overview of the pre v i o u s work done by s e v e r a l authors in t h i s f i e l d . In the t h i r d chapter, the theory of the double d i e l e c t r i c i n s u l a t o r i s presented, as l a t e r on a d e v i c e w i l l be developed around t h i s technology. The f o u r t h and s i x t h chapters c o n t a i n a l l the i n f o r m a t i o n r e l a t e d to the p r o c e s s i n g of the MOS c a p a c i t o r and double d i e l e c t r i c d e v i c e . In the f i f t h c hapter, the measurements performed on the MOS c a p a c i t o r s and r e s u l t s are presented. The seventh chapter c o n t a i n s the measurements made on the double d i e l e c t r i c MOSFET's (MTAOS devi c e s ) and the r e s u l t s o b t a i n e d . In chapter e i g h t , the summary and c o n c l u s i o n s of t h i s work are presented. 6 CHAPTER 2 AN OVERVIEW OF NON CONVENTIONAL INSULATORS A l a r g e number of authors have repo r t e d many d i f f e r e n t p r o p e r t i e s of non c o n v e n t i o n a l d i e l e c t r i c s . In t h i s chapter, I have attempted to present a summary of p u b l i s h e d works on the s u b j e c t . These cover t h i n and t h i c k f i l m s of s e v e r a l compounds used as d i e l e c t r i c s i n a non c o n v e n t i o n a l way. Despite e a r l y d i f f i c u l t i e s i n o b t a i n i n g good counter e l e c t r o d e c o n t a c t s , s u c c e s s f u l tantalum pentoxide t h i n f i l m s were f i r s t o b t ained by anodic o x i d a t i o n of s p u t t e r e d tantalum metal [Berry and Sloan, 1959], to be used i n low value c a p a c i t o r s f o r p r i n t e d c i r c u i t s . T y p i c a l c a p a c i t a n c e d e n s i t i e s of 10 nF/mm2 and c a p a c i t a n c e s of 30 nF with leakage c u r r e n t s of 10-100 nA were obtained f o r samples made on ceramic s u b s t r a t e s . Thin f i l m s of a n o d i c a l l y o x i d i z e d aluminium were a l s o produced f o r the same a p p l i c a t i o n and a c e r t a i n degree of s i m p l i c i t y over the tantalum v e r s i o n was claimed, s i n c e A l metal was evaporated i n s t e a d s p u t t e r e d and the same was used as c o n t a c t and for anodic d i e l e c t r i c formation [Huber and Haas, 1960]. A d e t a i l e d study of the anodic oxides of the so c a l l e d "valve metals", i . e . , the e l e c t r o l y t i c a l l y formed oxides of Ta, Nb, A l , Zr, Hf, W, B i , Sb and o t h e r s , was given c o n c i s e l y i n book form by Young in 1961, with great a t t e n t i o n devoted to tantalum and the p r o p e r t i e s of i t s t h i n f i l m o x i d e s . 7 A n o d i c a l l y grown t h i n f i l m s of s i l i c o n oxide (SiO), aluminium oxide ( A l 2 0 3 ) , tantalum pentoxide ( T a 2 0 5 ) , z i r c o n i u m oxide ( Z r 0 2 ) and t i t a n i u m oxide ( T i 0 2 ) were f a b r i c a t e d , u sing evaporation techniques f o r e v a l u a t i n g t h e i r conduction and negative r e s i s t a n c e at low f r e q u e n c i e s (60 Hz); although the a p p l i c a t i o n was not intended f o r MOS d e v i c e s , but r a t h e r i n s w i t c h i n g and r e c t i f y i n g [Hickmott, 1962]. A study done on a M e t a l - I n s u l a t o r - M e t a l (MIM) s t r u c t u r e of the form Ta-Ta 20 5-Au r e v e a l e d that the e l e c t r o n i c t r a n s p o r t mechanism obeys an Ohmic c h a r a c t e r i s t i c at low f i e l d s and that at high f i e l d s i t f o l l o w s a P o o l e - F r e n k e l type emission at room temperatures. At high f i e l d s and low temperatures ( i . e . 77K), the e l e c t r o n i c conduction i s governed by a Fowler-Nordheim type emission. Measurements gave an a c t i v a t i o n energy of 0.1 eV. A d i s c o n t i n u i t y i n the oxide p r o p e r t i e s was noted around 50 nm [Mead, 1962]. A r a t h e r unusual c u r r e n t - v o l t a g e c h a r a c t e r i s t i c s was observed f o r niobium, t i t a n i u m and tantalum oxides, when prepared as t h i n f i l m s and t h e i r oxides obtained by the thermal p r o c e s s . Regions of negative r e s i s t a n c e are found to be c u r r e n t c o n t r o l l e d , as opposed to the vo l t a g e c o n t r o l l e d n egative r e s i s t a n c e of the Tunnel (Esaki) diode [Chopra, 1963]. Thin f i l m c i r c u i t s using r e s i s t o r s made of tantalum n i t r i d e (TaN) and c a p a c i t o r s made of anodic tantalum oxide were s u c c e s s f u l l y a p p l i e d i n c r e a t i n g the f i r s t h y b r i d c i r c u i t s , i . e . , i n c o r p o r a t i n g d i s c r e t e b i p o l a r t r a n s i s t o r s 8 to a common ceramic s u b s t r a t e , i n making both analog and d i g i t a l c i r c u i t s [Berry, 1963], L a t e r a method f o r producing high value r e s i s t o r s was developed, by s p u t t e r i n g tantalum i n a p a r t i a l atmosphere of argon and oxygen [Pendergast, 1963]. Furthermore, a pro d u c t i o n s t y l e open ended vacuum system f o r d e p o s i t i o n of tantalum f i l m s f o r manufacturing r e s i s t o r s was repor t e d to make 4 m i l l i o n square inches of metal f i l m per y e a r - s h i f t [Balde, Charschan and Dineen, 1 964] . Reactive s p u t t e r i n g of tantalum metal with subatmospheric p a r t i a l p r e ssure gas pres s u r e s was used to d e p o s i t f i l m s of tantalum n i t r i d e (TaN), tantalum pentoxide ( T a 2 0 5 ) , tantalum c a r b i d e (TaC and Ta 2C) t i t a n i u m n i t r i d e (TiN) and oxides ( T i 0 2 ) , Niobium N i t r i d e s (NbN and Nb 2N); f o r the f a b r i c a t i o n of two dimensional h y b r i d t h i n f i l m c i r c u i t s . I t was noted that the i n t r o d u c t i o n of Oxygen as a r e a c t i v e component, had a d r a s t i c e f f e c t i n the e l e c t r i c a l p r o p e r t i e s of the s p u t t e r e d f i l m s . By c o n t r o l l i n g i t s p a r t i a l p r e s s u r e , r e p r o d u c i b l e v a l u e s of sheet r e s i s t i v i t y were o b t a i n e d . I t was found that most n i t r i d e s and c a r b i d e s have superconducting p r o p e r t i e s and that niobium n i t r i d e , NbN, has one of the hig h e s t t r a n s i t i o n temperatures [Gerstenberg, 1964]. The d i s c o v e r y of beta tantalum [Read and Altmann, 1965] sparked more i n t e r e s t i n the r e s e a r c h of t h i n f i l m s made of t h i s m a t e r i a l , as i t o f f e r e d improved e l e c t r i c a l p r o p e r t i e s over i t s p r e d e c e s s o r s . I t has higher r e s i s t i v i t y , a sma l l e r temperature c o e f f i c i e n t of r e s i s t a n c e and i t s 9 superconducting t r a n s i t i o n temperature i s much l e s s . Beta tantalum i s formed in a s p u t t e r i n g system when an atmosphere of argon i s introduced at a p a r t i a l pressure of 10 to 30 mTorr and the t o t a l pressure ( i . e . , vacuum q u a l i t y ) of the other gases i s 10 uTorr. i t was found that a conver s i o n to normal tantalum (body centered c u b i c s t r u c t u r e ) takes p l a c e , when beta tantalum ( t e t r a g o n a l ) i s heated in vacuum to about 750 C. Evidence of p h o t o e l e c t r i c p r o p e r t i e s has been found f o r f i l m s made of Nb 20 5 over metal s u b s t r a t e s , as knowledge of the conduction mechanism through the oxide f i l m can be obta i n e d by photoresponse s t u d i e s [Hickmott, 1966]. E l e c t r o n i c photoconduction i s a l s o found in t h i n f i l m s of T a 2 0 5 , with sharp i n c r e a s e s of conduction c u r r e n t when i r r a d i a t e d with u l t r a v i o l e t l i g h t , i n which the i n c r e a s e i n c u r r e n t was found to be approximately p r o p o r t i o n a l to the i n t e n s i t y of the l i g h t [Young, 1961]. A t h i n f i l m p h o t o c e l l , u s i n g t h e r m a l l y o x i d i z e d niobium metal, has been f a b r i c a t e d , and i t was found that a l i n e a r r e l a t i o n e x i s t s between the photocurrent and l i g h t i n t e n s i t y , as experimental measurements i n d i c a t e d . However, the t r a n s i e n t response i s very slow (50-100 sec) and the co n v e r s i o n e f f i c i e n c y i s two orde r s of magnitude below the c o n v e n t i o n a l j u n c t i o n type s i l i c o n c e l l s . For Ta and T i anodic oxides a l i n e a r r e l a t i o n e x i s t s between photocurrent and photovoltage, although such a c h a r a c t e r i s t i c i s not d e s i r a b l e i n a p r a c t i c a l p h o t o c e l l [Chopra and Bobb, 1963]. Chemical vapour d e p o s i t i o n techniques (CVD) have been 10 a p p l i e d t o the f a b r i c a t i o n of MOS c a p a c i t o r s , i n which f i l m s of T i 0 2 , T a 2 0 5 and N b 2 0 5 were o b t a i n e d by p y r o l y s i s of the a p p r o p i a t e organo-metal compound. Lead o x i d e (PbO) f i l m s were a l s o d e p o s i t e d , u s i n g the same CVD t e c h n i q u e s , f o r use i n V i d i c o n s . A wide range of s u b s t r a t e s , from s i l i c o n t o p l a t i n u m was used [Wang, Z a i n i n g e r and D u f f y , 1970]. Bismuth t i t a n a t e f i l m s (Bi„Ti 30 1 2) w i t h a v e r y h i g h r e l a t i v e p e r m i t t i v i t y (=160) have been r e p o r t e d f o r use i n M e t a l - I n s u l a t o r - M e t a l c a p a c i t o r s , i n s p i t e of the s t o i c h i o m e t r y c o n t r o l problems i n the s p u t t e r i n g of t h i s compound. High d i s s i p a t i o n f a c t o r s which depend s t r o n g l y on f r e q u e n c y and heavy c o n d u c t i o n c u r r e n t s were found [Szedon and T a k e i , 1971 ] . Tantalum p e n t o x i d e ( T a 2 0 5 ) t h i n f i l m s have been c o n s i d e r e d f o r a p p l i c a t i o n as d i e l e c t r i c s f o r Microwave I n t e g r a t e d C i r c u i t s (MIC's), due t o i t s h i g h ' r e l a t i v e d i e l e c t r i c c o n s t a n t and m o d e r a t e l y h i g h Q (=100) at microwave f r e q u e n c i e s . The so c a l l e d Tantalum System d i e l e c t r i c s f o r MIC's are based on m etal s p u t t e r i n g and l a t e r a n o d i c o x i d a t i o n on an a l u m i n a s u b s t r a t e t e c h n o l o g i e s . The g u i d e wavelength Xg can be reduced, and t h e degree of s i z e r e d u c t i o n as w e l l as i n t e g r a t i o n can be i n c r e a s e d , i f the r e l a t i v e d i e l e c t r i c c o n s t a n t of the s u b s t r a t e i s made l a r g e r . T h i s i s d e t e r m i n e d by the e f f e c t i v e d i e l e c t r i c c o n s t a n t e e f f , which i s a d i r e c t f u n c t i o n of the r e l a t i v e d i e l e c t r i c c o n s t a n t er of the s u b s t r a t e . Hence, by u s i n g s u b s t r a t e s w i t h l a r g e er ( i . e . T a 2 0 5 ) , a l a r g e r degree of i n t e g r a t i o n and s i z e r e d u c t i o n can be a c h i e v e d [ C a u l t o n , 11 1971]. A r a t h e r unorthodox approach to a non-conventional gate i n s u l a t o r was a p p l i e d to a MOSFET v o l t a g e d i v i d e r , i n which the gate e l e c t r o d e i s made to s l i d e over s i l i c o n e o i l ( l a t e r g l y c e r i n e ) , with the gate oxide being underneath. T h i s formed a c a p a c i t i v e v o l t a g e d i v i d e r , with the MOSFET d r a i n c u r r e n t being a f u n c t i o n of the movable gate e l e c t r o d e p o s i t i o n [Okamoto and Ugai, 1971]. Thermal o x i d a t i o n of s p u t t e r e d tantalum metal over s i l i c o n was made f e a s i b l e i n the form of a Si-Ta and S i - T a -A l s t r u c t u r e s , p l a c e d under heat treatment i n d i f f e r e n t atmospheres of N 2/0 2 and N 2/H 20 mixtures. The k i n e t i c s of oxygen i n c o r p o r a t i o n show an approximate square root dependence with time. I t was re p o r t e d that the Ta-Si i n t e r f a c e e x h i b i t e d n e g l i g i b l e i n t e r d i f f u s i o n when annealed. Because of the r e l a t i v e l y low temperature process used (525 C), i t i s s a i d t h at the tantalum t h i n f i l m s are t e c h n o l o g i c a l l y compatible with the S i l i c o n System. T h i s i s due to the s t a b i l i t y shown by s i l i c o n , the S i - S i 0 2 i n t e r f a c e and aluminium metal at t h i s temperature [Croset and V e l a s c o , 1971 ]. A t h i n f i l m c a p a c i t o r of s i l i c o n d i o x i d e over tantalum pentoxide has been s u c c e s s f u l l y f a b r i c a t e d , i n which S i 0 2 was s p u t t e r e d over anodized T a 2 0 5 over a ceramic s u b s t r a t e . The c a p a c i t a n c e d e n s i t y i s dominated by the s i l i c o n d i o x i d e f i l m , because of i t s s m a l l e r d i e l e c t r i c constant as compared with anodic tantalum oxide. T h i s gave a high degree of r e p r o d u c i b i l i t y , as the d e s i r e d c a p a c i t a n c e d e n s i t y of the 12 S i 0 2 / T a 2 0 5 t h i n f i l m c a p a c i t o r i s c o n t r o l l e d by the t h i c k n e s s of the s i l i c o n d i o x i d e f i l m . The temperature c o e f f i c i e n t of c a p a c i t a n c e (TCC) can be a d j u s t e d t o compensate the n e g a t i v e temperature c o e f f i c i e n t of r e s i s t a n c e (TCR) of t a n t a l u m n i t r i d e r e s i s t o r s (used i n RC networks based on the Tantalum System), by p r o p e r s e l e c t i o n of the s i l i c o n d i o x i d e t h i c k n e s s [ S a t o , Sato and Okamoto, 1973]. Thermal o x i d a t i o n of t a n t a l u m f i l m s d e p o s i t e d by e l e c t r o n beam e v a p o r a t i o n on s i l i c o n , have been p r e p a r e d i n o r d e r t o study the Si-Ta-{Ta o x i d e } s t r u c t u r e . The c r i t e r i a of a maxima i n the o x i d e ' s apparent r e f r a c t i v e index was used i n d e c i d i n g the complete o x i d a t i o n of the t a n t a l u m m e t a l . The e l a p s e d time f o r t h i s maximum v a l u e i s the o x i d a t i o n time of the f i l m . I t was c o n c l u d e d t h a t some i n t e r a c t i o n w i t h the S i s u b s t r a t e o c u r r e d d u r i n g the o x i d a t i o n of the Ta f i l m , namely the c o - o x i d a t i o n of a s m a l l , but non-zero amount of s i l i c o n . T h i s i n t e r a c t i o n a t the S i - T a i n t e r f a c e has a c t u a l l y moved i n t o the S i c r y s t a l , i n a s i m i l a r f a s h i o n as the S i - S i 0 2 i n t e r f a c e when t h e r m a l o x i d a t i o n t a k e s p l a c e . I t was c l a i m e d t h a t the p r o p e r t i e s of the o x i d e f i l m and the S i - { T a oxide} i n t e r f a c e can be c o n t r o l l e d by the S i / T a r a t i o i n the o x i d e f i l m [Revesz, A l l i s o n , K i r k e n d a l l and R e y n o l d s , 1974]. An i n t e r e s t i n g i n s u l a t o r s t r u c t u r e i s made u s i n g e v a p o r a t e d o x i d e f i l m s of t u n g s t e n (W0 3) and molybdenum (Mo0 3) o x i d e s , sandwiched between o r t h o g o n a l l y e v a p o r a t e d A l e l e c t r o d e s . I t was e s t a b l i s h e d t h a t a S c h o t t k y type b a r r i e r 1 3 e x i s t s between the c o n t a c t s and oxide by v e r i f y i n g the l i n e a r i t y of the 1/C2 versus a p p l i e d v o l t a g e p l o t and i t was concluded that a compound b a r r i e r , i . e . , an i n s u l a t i n g l a y e r adjacent to a space charge l a y e r e x i s t s between the oxide f i l m and the e l e c t r o d e [Padmanabhan and Sathianandan, 1975]. Another use of tantalum pentoxide i s i n the f i e l d of a n t i r e f l e c t i o n c o a t i n g s f o r s i l i c o n s o l a r c e l l s , s i n c e i t s r e f r a c t i v e index (2.23) i s very c l o s e to the optimum value of 2.3 r e q u i r e d i n t h i s a p p l i c a t i o n . The thermal o x i d a t i o n of tantalum i s a low temperature process that r e s u l t s i n a n o n c r y s t a l l i n e oxide with a S i l i c o n - { T a oxide] s t r u c t u r e of high p e r f e c t i o n . T h i s i s of great importance i f a high quantum y i e l d i s d e s i r e d at short wavelengths, as with a good i n t e r f a c e , the c a r r i e r s generated i n i t s surroundings reach the n-p j u n c t i o n without recombination. Niobium pentoxide has a l s o s i m i l a r p r o p e r t i e s with a r e f r a c t i v e index of 2.37, c l o s e to the optimum [Revesz, A l l i s o n and Reynolds, 1976]. MOS c a p a c i t a n c e measurements using the C-V p l o t techniques are h e l p f u l i n determining the c h a r a c t e r i s t i c s of the semiconductor- oxide i n t e r f a c e , as the d e n s i t y of i n t e r f a c e s t a t e s can be obtained i n v a r i o u s ways, in p a r t i c u l a r by the Terman method [Sze, 1969]. Thermally o x i d i z e d samples of s p u t t e r e d tantalum have been prepared f o r t h i s purpose, and observing the f i n a l oxide t h i c k n e s s , i t gave twice that of the o r i g i n a l f i l m . I t was noted that a n n e a l i n g i n forming gas (N 2/H 2 m i x t u r e ) , reduced the d e n s i t y of i n t e r f a c e s t a t e s by a f a c t o r of 4, while the 1 4 oxide charge remained f i x e d and negative [Revesz and A l l i s o n , 1976]. E l l i p s o m e t r i c o b s e r v a t i o n s of the o p t i c a l p r o p e r t i e s of thermally o x i d i z e d T a 2 0 5 on s i l i c o n have shown that a g r a d i e n t i n the o p t i c a l r e f r a c t i v e index e x i s t s at the s u b s t r a t e - o x i d e i n t e r f a c e and that the index a c t u a l l y i n c r e a s e s s l i g h t l y from i t s S i - T a 2 0 5 boundary towards the outer end of the oxide. The e x i s t e n c e of t h i s r e f r a c t i v e index g r a d i e n t i s somewhat c o n t r o v e r s i a l , as i n i t s a c t u a l measurement, a t h i n (=10 nm) l a y e r of oxide was etched at a time, and i t s index measured by e l l i p s o m e t r y using a m u l t i l a y e r model. Measurements performed on a GaAs s u b s t r a t e show that a g r a d i e n t with opposite slope e x i s t s , as compared with the s i l i c o n s u b s t r a t e case. Hence, the g r a d i e n t i n the r e f r a c t i v e index (and i t s slope) of the Ta oxide depends l a r g e l y on the s u b s t r a t e were they are grown [Revesz, Reynolds and A l l i s o n , 1976]. Chemical Vapour D e p o s i t i o n (CVD) i s a technique by which an o r g a n o - m e t a l l i c compound i s d e p o s i t e d by p y r o l y s i s with the a s s i s t a n c e of a c a r r i e r gas, u s u a l l y a mixture which c o n t a i n s oxygen among o t h e r s . Tantalum pentoxide f i l m s have been d e p o s i t e d using t h i s method, with the advantage of being a low temperature (300-500 C) process, that r e s u l t s i n oxides with amorphous s t r u c t u r e s , s i m i l a r to the t h e r m a l l y grown tantalum o x i d e s . The disadvantages of t h i s approach are mainly i n the vast complexity of the d e p o s i t i o n equipment and the a v a i l a b i l i t y of a s u i t a b l e tantalum organic compound. F i l m s d e p o s i t e d by CVD show smooth 15 s u r f a c e s and good a d h e s i o n , w i t h an index of r e f r a c t i o n c l o s e t o 2.3 and an o p t i c a l bandgap of 4.4 eV. C a p a c i t a n c e measurements gave a r e l a t i v e d i e l e c t r i c c o n s t a n t of 22-25 and C-V p l o t s i n d i c a t e d t h a t t h e r e was no d i f f e r e n c e among t h r e e d i f f e r e n t c r y s t a l o r i e n t a t i o n s of p - s i l i c o n s u b s t r a t e s , a l l showing no h y s t e r e s i s . Some d i s t o r t i o n of the C-V c u r v e s was a t t r i b u t e d t o f a s t s u r f a c e s t a t e s . The c o n d u c t i o n mechanism a t DC was e s t a b l i s h e d t o be b u l k l i m i t e d , w i t h a P o o l e - F r e n k e l e m i s s i o n a t low c u r r e n t d e n s i t i e s and space charge l i m i t e d a t h i g h e r d e n s i t i e s . A S c h o t t k y p l o t ( l o g C u r r e n t v s . Square Root V o l t a g e ) r e v e a l e d a two-slope c u r v e , and from the l i n e a r p o r t i o n ( f i r s t s l o p e ) a d i e l e c t r i c c o n s t a n t of 5.3 was o b t a i n e d , which f i t s the o p t i c a l c o n s t a n t v a l u e , but not the one o b t a i n e d by c a p a c i t a n c e measurements. I n t e r e s t i n g l y , the CVD S i 0 2 f i l m s a l s o show c u r r e n t s a t u r a t i o n , but a t much lower c u r r e n t s ( s i x o r d e r s of magnitude) than T a 2 0 5 f i l m s [ K a p l a n , B a l o g and Frohman-Bentchkowsky, 1976]. F u r t h e r r e s e a r c h work on the S i - T a 2 0 5 i n t e r f a c e has r e v e a l e d t h a t a s i g n i f i c a n t i n t e r a c t i o n o c c u r s w e l l below the temperature r e q u i r e d t o form t a n t a l u m s i l i c i d e ( T a S i 2 ) or s i l i c o n d i o x i d e ( S i 0 2 ) . T h i s phenomenon o c c u r s i n the d e p o s i t i o n of the t a n t a l u m m e t a l f i l m , t o which s i l i c o n i n c o r p o r a t e s i n the p r o c e s s . I t i s c l a i m e d t h a t t h i s i n t e r a c t i o n i s the cause f o r the r e f r a c t i v e index g r a d i e n t of the Ta o x i d e f i l m . G r a v i m e t r i c a n a l y s i s show t h a t t h e r m a l l y o x i d i z e d T a 2 0 5 f i l m s c o n t a i n a s i g n i f i c a n t amount of s i l i c o n and the r e s u l t s g i v e a r a t i o of 0.85 Si-Atom/Ta-1 6 atom in the oxide f i l m . Secondary Ion Mass Spectroscopy (SIMS) a n a l y s i s seem to strengthen t h i s hypothesis at l e a s t in a q u a l i t a t i v e sense. The i n t e r f a c e i n t e r a c t i o n i s e x p l a i n e d by assuming that oxygen f a c i l i t a t e s the i n c o r p o r a t i o n of s i l i c o n by g r a i n boundary d i f f u s i o n due to the formation of strong S i - 0 bonds [Revesz and K i r k e n d a l l , 1976]. The R u t h e r f o r d B a c k s c a t t e r i n g (RBS) a n a l y s i s technique has been a p p l i e d to study the Ta{oxide}-Si boundary and the c h a r a c t e r i s t i c s of the oxide l a y e r i t s e l f . In the RBS a n a l y s i s , l i g h t and high energy ions ( t y p i c a l are helium p o s i t i v e , ions with an energy of 1-5 MeV) are used to bombard the s u r f a c e to be s t u d i e d , and by c o l l e c t i n g data from the e l a s t i c s c a t t e r e d ( i . e . bounced back) energy spectrum, knowledge of the s u r f a c e and near s u r f a c e composition can be o b t a i n e d . A depth c o n c e n t r a t i o n p r o f i l e from the energy s p e c t r a i s then obtained from the He ions r a t e of i n e l a s t i c energy l o s s . R e s u l t s from t h i s technique on Ta f i l m s show s i g n i f i c a n t impurity e f f e c t s due to g e t t e r i n g d u r i n g the e l e c t r o n beam d e p o s i t i o n from a high p u r i t y source. In p a r t i a l thermal tantalum o x i d e s , they e x h i b i t s u b s t a n t i a l oxygen i n c o r p o r a t i o n , and i n c o n t r a s t with anodic T a 2 0 5 , there i s no sharp boundary between the Ta metal and the tantalum pentoxide l a y e r . I t was found that the r e f r a c t i v e index v a r i e s with the oxide t h i c k n e s s , d e c r e a s i n g s l i g h t l y with t h i n n e r oxides, as r e p o r t e d p r e v i o u s l y by Revesz et a l (1974 and 1976). The formation of Si-O-Ta bonds has been p o i n t e d to be r e s p o n s i b l e f o r t h i s e f f e c t , and the 1 7 i n c o r p o r a t i o n of s i l i c o n to the T a 2 0 5 i s noted f o r t h i s i n t e r a c t i o n [Hirvonen, Revesz and K i r k e n d a l l , 1976]. Anodic t i t a n i u m - t a n t a l u m f i l m s have been prepared f o r use in c a p a c i t o r TM (tantalum-tantalum-oxide/metal) s t r u c t u r e s , by the standard s p u t t e r i n g and a n o d i z a t i o n method. The f i l m s had v a r y i n g atom f r a c t i o n s of tantalum to t i t a n i u m and the r e s u l t s gave an i n c r e a s e d r e l i a b i l i t y (lower f a i l u r e r a te) under reverse b i a s , as compared with the tantalum oxide c a p a c i t o r s . Furthermore, i t has been proposed that a l l o y i n g agents i n t o tantalum have the e f f e c t of i n c r e a s i n g the symmetry of the DC conductance c h a r a c t e r i s t i c at room temperature [Peters and Schwartz, 1977].' A conduction mechanism study of another p o t e n t i a l i n s u l a t o r f i l m , vanadium pentoxide ( V 2 0 5 ) i n d i c a t e d that f o r both forward and reverse b i a s ( i . e . , the gate i s p o s i t i v e under forward b i a s ) the I-V c h a r a c t e r i s t i c e x h i b i t e d a combination of d i f f e r e n t conduction and emission e f f e c t s and that no s i n g l e one c o u l d account f o r the c a r r i e r t r a n s p o r t i n the MOS s t r u c t u r e . For example, under reverse b i a s c o n d i t i o n s , the experimental I-V measurements showed that at low v o l t a g e s (0.4 V) an Ohmic r e g i o n e x i s t s ; a Schottky type r e l a t i o n ( l o g I i s p r o p o r t i o n a l to the square root of V) between 0.4-0.7 V; and a Square Law area f o r v o l t a g e s above 1.25 V and then f o r higher v o l t a g e s , a t r a n s i t i o n region i n which the c u r r e n t passes from a Schottky type r e l a t i o n , to a l i n e a r one and then to a Square Law one. A s i m i l a r s i t u a t i o n e x i s t s under forward b i a s , with d i f f e r e n c e s i n the region 1 8 extensions and s l o p e s of the I-V c h a r a c t e r i s t i c . Capacitance measurements, C-V p l o t s , a l s o i n d i c a t e complex r e l a t i o n s h i p s with v o l t a g e and frequency. It was found that above a c e r t a i n c r i t i c a l frequency, the c a p a c i t a n c e became frequency independent, under both forward and reverse b i a s c o n d i t i o n s . An i n v e r s e square law c h a r a c t e r i s t i c dominates the C-V r e l a t i o n s h i p , with d i f f e r e n t s l o p e s under forward or reverse b i a s , i . e . , with forward b i a s the c a p a c i t a n c e decreases with v o l t a g e , whereas with reverse b i a s , i t i n c r e a s e s with v o l t a g e . A c e r t a i n degree of d e v i a t i o n from t h i s r e l a t i o n s h i p was found at v o l t a g e s c l o s e to zero. A p l o t of the i n v e r s e square of the Capacitance vs. Voltage gave a curve which has a slope, such that when i n t e r s e c t e d at the o r d i n a t e (voltage) a x i s , g i v e s a value Vb (the b a r r i e r h e i g h t ) , independent of frequency [Mackus, S u l i , Torok and Hevesi, 1977]. Thin f i l m c a p a c i t o r s , f a b r i c a t e d by s p u t t e r i n g tantalum metal over g l a s s s u b s t r a t e s and then producing tantalum oxide by the a n o d i z a t i o n method, which gave a f i n i s h e d c a p a c i t o r area of 0.1 mm2, were used i n the experimental d e t e r m i n a t i o n of the AC p r o p e r t i e s of these. The e f f e c t of a n n e a l i n g the c a p a c i t o r s i n n i t r o g e n at temperatures between 250-350 C was s t u d i e d , and the authors e s t a b l i s h e d that an improved c a p a c i t a n c e s t a b i l i t y , lower l o s s e s and b e t t e r temperature c o e f f i c i e n t of c a p a c i t a n c e (TCC) was obtained a f t e r the n i t r o g e n a n n e a l i n g . The metal d e p o s i t i o n process, Magnetron Enhanced S p u t t e r i n g (MES), proved to give b e t t e r q u a l i t y f i l m s , of lower DC leakage, than those prepared by 19 the c o n v e n t i o n a l DC Diode s p u t t e r equipment [Rottersman, B i l l and Gerstenberg, 1978]. C a p a c i t o r s made of a n o d i c a l l y o x i d i z e d tantalum n i t r i d e (TaN), using t h i n f i l m technology have a l s o s u c c e s s f u l l y been f a b r i c a t e d on gl a z e d ceramic s u b s t r a t e s . Reactive s p u t t e r i n g of TaN i s fo l l o w e d by anodic o x i d a t i o n , with c o u n t e r e l e c t r o d e s d e p o s i t e d by e v a p o r a t i o n . These c a p a c i t o r s showed s u p e r i o r performance as compared with the previous (Ta-oxide) metal c a p a c i t o r s . Lower TCC and d i s s i p a t i o n f a c t o r s were ob t a i n e d , with h i g h endurance at to heat treatment and improved r e l i a b i l t y [Ooken, Ohwada, Okamoto and Kamei, 1978]. Tantalum pentoxide a l s o e x h i b i t s a c o u s t o - o p t i c p r o p e r t i e s , and Bragg c e l l s using a s u r f a c e a c o u s t i c wave (SAW) have been f a b r i c a t e d f o r use i n i n t e g r a t e d o p t i c a l RF spectrum a n a l y z e r s . T h i s oxide i s r e p r e s e n t a t i v e of a group of m a t e r i a l s (namely s i l i c o n n i t r i d e , z i n c oxide and g l a s s ) which have low o p t i c a l l o s s , r e f r a c t i v e i n d i c e s g r e a t e r than S i 0 2 and can be d e p o s i t e d by c o n v e n t i o n a l techniques. For example, T a 2 0 5 had an o p t i c a l r e f r a c t i v e index of 2.15, an o p t i c a l l o s s of 1.5-2.0 dB/cm, a s u r f a c e a c o u s t i c wave v e l o c i t y of 3500-3700 m/s, and an a c o u s t i c l o s s of 7.0-7.5 dB/cm, when used i n a t h i n f i l m waveguide made on s i l i c o n s u b s t r a t e s [ H i c k e r n e l l , Davis and Ri c h a r d , 1978]. Double d i e l e c t r i c s t r u c t u r e s , which i n c o r p o r a t e both T a 2 0 5 and S i 0 2 have been analyzed f o r p o t e n t i a l use in n o n v o l a t i l e memory d e v i c e s . Anodic and thermal T a 2 0 5 i s formed over g l a s s or s i l i c o n s u b s t r a t e s , and then aluminium 20 e l e c t r o d e s are d e p o s i t e d f o r determining the c h a r a c t e r i s t i c s of the i n s u l a t o r f i l m . Theory p r e d i c t s that by a p p l y i n g a v o l t a g e between the gate e l e c t r o d e and s u b s t r a t e , a charge can be s t o r e d at the i n t e r f a c e between both i n s u l a t o r s . I f t h i s s t r u c t u r e i s used as the gate i n s u l a t o r i n a MOS d e v i c e , the presence of t h i s charge a f f e c t s the t h r e s h o l d v o l t a g e of the t r a n s i s t o r . Hence, a memory c e l l can be c o n s t r u c t e d , as the presence or absence of the i n t e r f a c e charge can d e f i n e the l o g i c a l s t a t e of the d e v i c e . The conduction mechanism through the T a 2 0 5 / S i 0 2 double d i e l e c t r i c i s , under high f i e l d c o n d i t i o n s ( i . e . , the " w r i t e " c y c l e ) , P o o l e - F r e n k e l . At moderate f i e l d s , i t i s the temperature dependent ohmic component that predominates. Only MOS c a p a c i t o r s of M e t a l - T a { o x i d e } - S i ( o x i d e } - S i l i c o n (MTOS) s t r u c t u r e were used i n the experimental work [Angle and T a l l e y , 1978]. Thin f i l m t r a n s i s t o r s , f a b r i c a t e d with anodized T a 2 0 5 as gate i n s u l a t o r have been developed, which r e p l a c e d the usual A l 2 0 3 and e l i m i n a t e d the frequent c l e a n i n g of masks d u r i n g manufacturing. The gate i n s u l a t o r was developed from the a n o d i z a t i o n of tantalum o x y n i t r i d e , which was used as the gate e l e c t r o d e . T h i s combination produced matched temperature c o e f f i c i e n t s between gate i n s u l a t o r and e l e c t r o d e . The r e s u l t a n t anodic tantalum pentoxide had a high r e l a t i v e d i e l e c t r i c constant of 22, which produced d e v i c e s with high transconductance, thus e l i m i n a t i n g the n e c e s s i t y f o r double gate t r a n s i s t o r s [ K a l f a s s and Lueder, 1979]. 21 Experimental work of tantalum pentoxide f i l m s grown on GaAs s u b s t r a t e s showed that good i n s u l a t i n g f i l m s were obtained f o r both thermal and anodic o x i d e s . The o p t i c a l p r o p e r t i e s of these f i l m s i n d i c a t e that a pronounced i n t e r a c t i o n takes p l a c e d u r i n g the thermal growth of T a 2 0 5 , s i m i l a r to the i n t e r a c t i o n on s i l i c o n s u b s t r a t e s . Better i n s u l a t i n g p r o p e r t i e s were obtained with anodic p r o c e s s i n g of Ta, as a reduced f i l m - s u b s t r a t e i n t e r a c t i o n was claimed, due to l e s s i n c o r p o r a t i o n of Ga and/or As i n t o the tantalum pentoxide f i l m . However, i t was not p o s s i b l e to o b t a i n MOS c a p a c i t a n c e - v o l t a g e curves [ N i s h i and Revesz, 1979]. A m i c r o e l e c t r o d e a r r a y , using T a 2 0 5 on sapphire, was developed f o r p r o d u c t i o n of l o c a l i z e d e l e c t r i c a l e x c i t a t i o n of the f i b r e bundles i n the a u d i t o r y nerve. The a r r a y serves as the i n t e r f a c e between the nervous system and the implanted e l e c t r o n i c s of an a u d i t o r y c o c h l e a r p r o s t h e s i s . The e l e c t r o d e s are made of tantalum metal, i n s u l a t e d from the conducting medium and from each other by a n o d i c a l l y formed tantalum pentoxide, with an o v e r a l l p a s s i v a t i o n of s i l i c o n n i t r i d e . T h i s p r o v i d e d a s t r u c t u r e with s i g n i f i c a n t advance i n the mechanical and e l e c t r i c a l s t a b i l i t y r e q u i r e d to s u r v i v e the continous e l e c t r i c a l s t i m u l a t i o n i n the nervous system f l u i d s without metal d i s s o l u t i o n due to the i n e r t n e s s of tantalum and i t s oxides [May, Shamma and White, 1979]. C a p a c i t o r s made with t h i n f i l m anodic tantalum oxide were used f o r studying the e f e c t s of humidity on the f a i l u r e r a t e of these. Two d i s t i n c t f a i l u r e modes were e s t a b l i s h e d : 22 p o i n t breakdown caused by randomly d i s t r i b u t e d weak spots r e s u l t i n g from m a t e r i a l p r o c e s s i n g d e f e c t s , and edge f a i l u r e s which are l o c a l i z e d at the exposed edge of the c o u n t e r e l e c t r o d e when only s u b j e c t e d to reverse b i a s [Adolt and Melroy, 1980]. A Dynamic Random Access Memory (DRAM) using the Quadruply S e l f A l i g n e d (QSA) MOSFET and a stacked high storage c a p a c i t o r was developed i n the c o n s t r u c t i o n of a 256 kb i t memory using 1.5 um d o u b l e - p o l y s i l i c o n - S i process.The word l i n e i s p o l y S i and the b i t l i n e i s A l . The storage c a p a c i t o r uses anodic T a 2 0 5 as a hig h p e r m i t t i v i t y i n s u l a t o r , which i s c r u c i a l i n r e a l i z i n g the c a p a c i t a n c e on the r e q u i r e d small area of 7.4 ixm2. The leakage c u r r e n t through the i n s u l a t o r was small enough that i t d i d not a f f e c t the o p e r a t i o n of memory c e l l , nor i t s h o l d time. Experimental 512 k b i t s and 1 Mbit i n t e g r a t e d c i r c u i t s have a l s o been designed using the same technology [Ohta, Yamada, Sa i t o h , S h i r a k i , Nakamura, Shimizu and T a r u i , 1980]. The conduction phenomena i n tantalum pentoxide f i l m s shows that they obey the Schottky theory at low f i e l d s , and at higher f i e l d s , the P o o l e - F r e n k e l model p r e v a i l s . However there i s experimental evidence that an anomalous Poole-F r e n k e l conduction takes p l a c e at high f i e l d s i n anodic T a 2 0 5 . Heat treatment of samples using anodic tantalum f i l m s , r e v e a l s that an i n c r e a s e i n c o n d u c t i v i t y takes p l a c e a f t e r 10 minutes i n oxygen or vacuum, and i f t h i s time i s exceeded i n a second heat treatment i n oxygen, the c o n d u c t i v i t y i s decreased a g a i n . A s h i f t from the anomalous 2 3 P o o l e - F r e n k e l i n t o the normal one i s then observed. These changes were a t t r i b u t e d to v a r i a t i o n s i n the donor d e n s i t y to t r a p d e n s i t y r a t i o and i n the donor l e v e l energy [Matsumuto, Susuki and Yabumoto, 1980]. A new kind of t h i n f i l m t r a n s i s t o r (TFT) that uses a r a t h e r novel gate i n s u l a t o r , a t h i n polymer f i l m of p o l y t e t r a f l u o r o e t h y l e n e ( i . e . , t e f l o n ) was f a b r i c a t e d to study the MIS s t r u c t u r e using organic i n s u l a t o r s . The i n s u l a t o r was a p p l i e d by e l e c t r o n gun e v aporation on a Te semiconductor s u b s t r a t e . However, the d e v ice showed i n s t a b i l i t y a f t e r a few weeks of been f a b r i c a t e d , caused by slow d r i f t i n the i n s u l a t o r - semiconductor i n t e r f a c e . T h i s i s an improvement over the p r e v i o u s TFT f a b r i c a t e d with the same polymer over a CdSe f i l m , which was q u i t e i n s t a b l e and d e t e r i o r a t e d the d e v i ce in a few days [De Vos and Hindryckx, 1980]. The o p t i c a l and e l e c t r i c a l p r o p e r t i e s of Ta thermal oxides have been determined by a n a l y z i n g the f i l m s on s i l i c o n s u b s t r a t e s by e l l i p s o m e t r i c , C-V and I-V methods. The o x i d a t i o n time f o r the tantalum f i l m s i n dry oxygen was o b t ained by examination of the e l l i p s o m e t r i c parameters P s i and D e l t a as a f u n c t i o n of time. A f t e r a c e r t a i n amount, they had a very slow r a t e of change, i n d i c a t i n g that a complete c o n v e r s i o n from metal to oxide had taken p l a c e . Any f u r t h e r change in these parameters c o u l d be a t t r i b u t e d to a n n e a l i n g e f f e c t s and slow changes in s t o i c h i o m e t r y . In c o n t r a s t with a p r e v i o u s work by Revesz et a l . , a tapered r e f r a c t i v e index t r a n s i t i o n l a y e r , represented by f i v e 24 l a y e r s of 1.5 nm t h i c k was found, i n s t e a d of the graded r e f r a c t i v e index through the oxide t h i c k n e s s . The C-V curves i n d i c a t e d the presence of a negative oxide charge at f l a t b a n d , h y s t e r e s i s and a r e l a t i v e d i e l e c t r i c constant of 26. The I-V curves r e v e a l e d a two slope Schottky p l o t , which i s t y p i c a l of tantalum pentoxide f i l m s [Smith and Young, 1981]. M e t a l - I n s u l a t o r - M e t a l (MIM) d e v i c e s have been developed f o r use as n o n - l i n e a r d e v i c e s i n m u l t i p l e x i n g L i q u i d C r y s t a l D i s p l a y s (LCDs). A MIM d e v i c e , using T a 2 0 5 as the i n s u l a t o r , was optimized f o r t h i s a p p l i c a t i o n , by p a r t i a l l y a n o d i z i n g a s p u t t e r e d metal f i l m on a g l a s s s u b s t r a t e . A computer program was used to determine the parameters i n the Poole-F r e n k e l conduction model to optimize the performance of the MIM switch. .Furthermore, n i t r o g e n doping enhanced the c o n d u c t i v i t y parameter f o r b e t t e r m u l t i p l e x i n g [ B a r a f f , Long, MacLaurin, Miner and S t r e a t e r , 1981]. Waveguide f i l m s prepared with tantalum pentoxide over S i 0 2 are important, s i n c e t h i s c l a s s of t h i n f i l m s has a wide range of r e f r a c t i v e index with low propagation l o s s ( l e s s than 1 dB/cm at 633 nm wavelength). I r r a d i a t i o n using a C0 2 l a s e r produces a decrease in the r e f r a c t i v e index, up to 2%. T h i s decrease i s c o n s i d e r e d to be r e l a t e d to the T a 2 0 5 c r y s t a l l i z a t i o n process from i t s amorphous s t a t e , when exposed to intense l a s e r r a d i a t i o n . T h i s i n turn a f f e c t s the p e r m i t t i v i t y of the tantalum oxide [ T e r u i and Kobayashi, 1981]. Bismuth oxide i s another i n s u l a t o r m a t e r i a l that has 25 been used i n the f a b r i c a t i o n of M e t a l - I n s u l a t o r -Semiconductor (MIS) c a p a c i t o r s . The f i l m was d e p o s i t e d by r e a c t i v e ion s p u t t e r i n g and l a t e r annealed i n a i r . The B i 2 0 3 c a p a c i t o r y i e l d e d a r e l a t i v e d i e l e c t r i c constant of 25, and a tan5 ( l o s s f a c t o r ) of 0.002 at 35 kHz, making i t u s e f u l f o r p r o d u c t i o n of low frequency c a p a c i t o r s . The C-V curves show a s h i f t i n the negative d i r e c t i o n with r e l a t i v e l y small h y s t e r e s i s [Raju and Talwai, 1981]. When tantalum f i l m s are d e p o s i t e d over s i l i c o n s u b s t r a t e s by s p u t t e r i n g , a s t r e s s f i e l d i s induced, which produces a c u r v a t u r e of the s u b s t r a t e . A MIS device was used as a p h o t o v o l t a i c c e l l i n order to study the s t r e s s e f f e c t in i t s performance. I t was found that the induced s t r e s s produced l a r g e changes i n the m i n o r i t y c a r r i e r d i f f u s i o n l e n g t h and recombination time, as w e l l as the recombination time and capture c r o s s s e c t i o n . Moreover, the short c i r c u i t c u r r e n t , open c i r c u i t v o l t a g e and f i l l f a c t o r were reduced [ L a l e v i c and Murty, 1981]. Indium phosphide, InP, was a n o d i c a l l y o x i d i z e d to form an gate i n s u l a t o r i n a MOSFET accumulation mode d e v i c e . The oxide had low enough i n t e r f a c e s t a t e d e n s i t y to be used as gate i n s u l a t o r . No C-V or I-V data was r e p o r t e d , and the oxide r e q u i r e d a nnealing i n order to reduce the C-V h y s t e r e s i s loop. The device had a gate l e n g t h of 500 um and a gate width of 2 mm. The measured estimated e l e c t r o n m o b i l i t y , A t e f f , was 1500 cm 2/Vs [Yamamoto and Uemura, 1981], GaAs technology f o r Microwave I n t e g r a t e d C i r c u i t s (MICs) makes use of high q u a l i t y tantalum oxide c a p a c i t o r s 26 i i n i n t e r s t a g e c o u p l i n g of microwave a m p l i f i e r s . The c a p a c i t o r has a M e t a l - I n s u l a t o r - M e t a l (MIM) s t r u c t u r e and they were i n t e g r a l l y s p u t t e r e d ( r e a c t i v e l y ) on the semi-i n s u l a t i n g GaAs s u b s t r a t e . A s t a i r c a s e shape i s then formed by l i f t - o f f and p a t t e r n i n g . T y p i c a l c a p a c i t a n c e s i n the range of 50-200 pF were obtained, with an oxide t h i c k n e s s of 150 nm, a r e l a t i v e d i e l e c t r i c constant of 20-25 and a l o s s f a c t o r (tan5) of 0.03 at 1 MHz. The leakage c u r r e n t had an e x p o n e n t i a l r e l a t i o n with a p p l i e d v o l t a g e [ E l t a , Chu, Mahoney, C e r r e t a n i and Courtney, 1982], A n o d i z a t i o n of aluminium has a l s o produced d i e l e c t r i c s f o r MIC c a p a c i t o r s , i n the form of A l 2 0 3 , with a t y p i c a l value of 30 pF, and a r e l a t i v e d i e l e c t r i c constant of 8. The f i g u r e of merit erE, the product of the r e l a t i v e p e r m i t t i v i t y and the d i e l e c t r i c s t r e n g t h , i n d i c a t e s the q u a l i t y i n terms of bandwidth and area f o r a given m a t e r i a l [Binet,1982 ] . Fur t h e r work i n the T a 2 0 5 MIC c a p a c i t o r s i n d i c a t e s that a two stage m o n o l i t h i c IF a m p l i f i e r , o p e r a t i n g in the 1.2 to 2.8 GHz range and using these as c o u p l i n g elements, i s q u i t e f e a s i b l e . The r e a c t i v e MIM process produced a c a p a c i t a n c e d e n s i t y of 1500 pF/mm2, and a very good q u a l i t y d i e l e c t r i c was ob t a i n e d with an i n s e r t i o n l o s s of 1 dB at 16 GHz f o r a 29 pF c a p a c i t o r [Chu, Mahoney, E l t a Courtney, F i n n , P i a c e n t i n i and Donnelley, 1983]. Ion S e n s i t i v e F i e l d E f f e c t T r a n s i s t o r s ( i S F E T ' s ) , f a b r i c a t e d with SOS ( s i l i c o n on sapphire) technology and tantalum pentoxide as gate i n s u l a t o r produced the hi g h e s t pH 27 s e n s i t i v i t y among three other d i e l e c t r i c s . ISFETs do not have a metal gate e l e c t r o d e , and i t i s the e l e c t r o l y t e i n which they are immersed which a c t s as the gate e l e c t r o d e , making them very u s e f u l as pH d e t e c t o r s . The T a 2 0 5 gate ISFET e x h i b i t s a l i n e a r r e l a t i o n between output v o l t a g e and the s o l u t i o n pH, with a 90% response time of a few seconds. It was concluded that the T a 2 0 5 f i l m was the most s u i t a b l e as sensing m a t e r i a l f o r hydrogen a c t i v i t y measurements [Akiyama, U j i h i r a , Okabe Sugano and N i k i , 1982]. Langmuir-Blodgett f i l m s are s i n g l e molecular l a y e r s , formed by the removal of a a m p h i p h i l i c molecule from an aqueous s o l u t i o n with a p o l a r s u b s t r a t e ( i . e . , aluminium o x i d e ) . The a m p h i p h i l i c molecule i s a simple f a t t y a c i d , which has a h y d r o p h i l i c t e r m i n a t i o n immersed in the aqueous s o l u t i o n and a hydrophobic end i n the a i r . The r e l a t i v e p e r m i t t i v i t y of such a molecule can be s e l e c t e d by choosing a s u i t a b l e CH 2 chain l e n g t h and by s u b s t i t u t i n g a d i v a l e n t ion bond f o r f o r the h y d r o p h i l i c t e r m i n a t i o n . A t h i n f i l m FET using amorphous s i l i c o n and L-B f i l m techniques as gate i n s u l a t o r was developed. The L-B l a y e r not only i s p i n h o l e f r e e but a l s o has a high d i e l e c t r i c breakdown s t r e n g t h and e x h i b i t s many of the c h a r a c t e r i s t i c s of an i d e a l gate i n s u l a t o r . [ P i t t , 1983; L l o y d , Petty and Roberts, 1983] F u r t h e r a n a l y s i s of thermal T a 2 0 5 f i l m s on s i l i c o n s u b s t r a t e s show that the amount of s i l i c o n i n c o r p o r a t e d to the oxide depends on the r e s i d u a l pressure d u r i n g the tantalum metal e v a p o r a t i o n . As the vacuum q u a l i t y i s reduced, l e s s amount of s i l i c o n i s i n c o r p o r a t e d to the 28 oxide. A l s o , i t was found that the the r e l a t i v e d i e l e c t r i c c onstant and the leakage c u r r e n t decrease as the amount of S i i n c r e a s e s in the oxide f i l m . [Hasegawa, Ogawa, Wada and Nakano, 1 9 8 3 ] . An i n t e r e s t i n g technique f o r growing a heteromorphic d i e l e c t r i c l a y e r , T a 2 0 5 over S i 0 2 , was developed f o r producing high q u a l i t y storage c a p a c i t o r s f o r a V L S I dynamic RAM. The technique i s based i n the o x i d a t i o n of a s i l i c o n s u b s t r a t e covered with a t h i n T a 2 0 5 l a y e r . The i n t e r f a c i a l o x i d a t i o n of S i takes place by wet o x i d a t i o n , and a high q u a l i t y double d i e l e c t r i c i s then produced, thus a v o i d i n g separate o x i d a t i o n s of Ta and S i . A memory c e l l was f a b r i c a t e d using t h i s technique [Kato, I t o , Taguchi, Nakamura and Ishikawa, 1 9 8 3 ] , A t h i n f i l m t r a n s i s t o r (TFT) with tantalum pentoxide as gate i n s u l a t o r and d e p o s i t e d by Magnetron Enhanced S p u t t e r i n g has r e c e n t l y being developed. The device has a p-channel s t r u c t u r e , with a fused quartz s u b s t r a t e and A l doped source and d r a i n r e g i o n s . Reactive Ion E t c h i n g (RIE) was used to p a t t e r n the T a 2 0 5 . The device e x h i b i t s a t h r e s h o l d v o l t a g e of - 2 . 5 V and a transconductance of 70MS. The f a b r i c a t i o n of the device allowed a maximum temperature of 6 2 5 C , which i s compatible with g l a s s s u b s t r a t e s and tantalum pentoxide [ S e k i , Unagami and Tsujiyama, 1 9 8 4 ] , 29 CHAPTER 3 THE THEORY OF THE T a 2 0 5 - S i 0 2 DOUBLE DIELECTRIC STRUCTURE Con s i d e r a b l e amount of work was done i n double d i e l e c t r i c i n s u l a t o r s , which were based on the Meta l -N i t r i d e - O x i d e - S i l i c o n (MNOS) system -used i n memory d e v i c e s -such as n o n - v o l a t i l e dynamic and s t a t i c c e l l s i n E l e c t r i c a l l y Programmable Read Only Memories (EPROMs). The MNOS Memory T r a n s i s t o r was s t u d i e d by Wallmark and Scott (1969), Ross (1969) and Frohman-Bentchkowsky (1970). The t h e o r e t i c a l aspects of t h i s device have great importance, as the MNOS and MTAOS behaviour i s q u i t e s i m i l a r , although not i d e n t i c a l . They a l s o have i n common a t h i n l a y e r of S i 0 2 , immediately above the s u b s t r a t e . The e l e c t r i c a l performances are s i m i l a r , and Angle (1976) e s t a b l i s h e d that memory de v i c e s c o u l d a l s o be f a b r i c a t e d using MTAOS technology, which had su p e r i o r c h a r a c t e r i s t i c s over the MNOS de v i c e . The double heteromorphic Ta{ox}/Si{ox} d i e l e c t r i c s t r u c t u r e has been developed as a r e s u l t of i n v e s t i g a t i o n s on l a r g e p e r m i t t i v i t y i n s u l a t i n g t h i n f i l m s f o r a p p l i c a t i o n s i n MOS C a p a c i t o r s [Angle, 1976], memory de v i c e s [Angle and T a l l e y , 1978] and more r e c e n t l y storage MOS c a p a c i t o r s f o r dynamic Random Access Memories of VLSI p r o p o r t i o n s [Ohta et al. , 1 9 8 2 ] . G e n e r a l l y speaking, the Metal-Double Insulator-Semiconductor (MDIS) s t r u c t u r e s have s i m i l a r p r o p e r t i e s , namely a 30 d i s s i m i l a r f i e l d d i s t r i b u t i o n i n each i n s u l a t o r , d i f f e r e n t e l e c t r o n i c conduction (emission) mechanisms and under c e r t a i n c o n d i t i o n s , a charge r e t e n t i o n or memory e f f e c t . S e v e r a l areas of common i n t e r e s t to m i c r o e l e c t r o n i c s and the resear c h i n double i n s u l a t i n g s t r u c t u r e s are i d e n t i f i a b l e : 1) The LSI and VLSI d e n s i t i e s r e q u i r e i n c r e a s e ^ d e v i c e s e a l i n g . 2) F u r t h e r s c a l i n g i n MOS de v i c e s r e q u i r e s t h i n n e r oxides and i n s u l a t o r s . 3) Device s c a l i n g decreases the maximum gate v o l t a g e due to i n s u l a t o r breakdown. 4) I n s u l a t o r s of l a r g e d i e l e c t r i c constant produce the r e q u i r e d c a p a c i t a n c e i n p r o p o r t i o n a l l y l e s s area, but in most cases an in c r e a s e i n leakage c u r r e n t i s observed. 5) Breakdown of a t h i n s c a l e d down oxide can be avoided i f a l a r g e r p e r m i t t i v i t y i n s u l a t o r i s pl a c e d between i t s e l f and the gate e l e c t r o d e . 6) The l a r g e d i e l e c t r i c constant of Tantalum Pentoxide can be combined with the e x c e l l e n t i n s u l a t i n g p r o p e r t i e s of S i l i c o n D i o x i d e , thus producing a double d i e l e c t r i c with a breakdown v o l t a g e governed by the high p e r m i t t i v i t y oxide t h i c k n e s s , while m a i n t a i n i n g a low l e v e l of leakage c u r r e n t . 7) In the double i n s u l a t o r s t r u c t u r e , the p r o b a b i l i t y of p i n h o l e s c o i n c i d i n g i n the same l o c a t i o n i s n e g l i g i b l e , thus producing a b e t t e r q u a l i t y i n s u l a t o r . 31 8) Amorphous S i l i c o n D ioxide i s s t r u c t u r a l l y porous and i t has high p e r m e a b i l i t y to water vapour and mig r a t i o n of a l k a l i i o n s . The a p p l i c a t i o n of a second outer d i e l e c t r i c reduces these e f f e c t s . Tantalum Pentoxide has a much denser s t r u c t u r e , p o s s i b l y r e t a r d i n g the ion mi g r a t i o n through i t . 9) Double d i e l e c t r i c s t u c t u r e s e x h i b i t memory e f f e c t s , hence they can be used as memory c e l l s i f p r o p e r l y designed. In t h i s work, our main e f f o r t i s conc e n t r a t e d i n the development of a T a 2 0 5 / S i 0 2 i n s u l a t o r f o r MOSFET a p p l i c a t i o n s as opposed to a memory c e l l f o r e i t h e r RAMs or DRAMs. The MOSFET device that i n c o r p o r a t e s the tantalum p e n t o x i d e - s i 1 icon d i o x i d e gate i n s u l a t o r i s termed a Met a l -Tantalum{0xide}- Si1 icon{Oxide}-Si1 icon d e v i c e , or MTAOS s t r u c t u r e . The inner i n s u l a t o r i s immediately above the su b s t r a t e , and the outer i n s u l a t o r i s below the gate contact e l e c t r o d e . F i g u r e 3.1 gi v e s a c r o s s s e c t i o n of such a double d i e l e c t r i c , with the main f e a t u r e s and dimensions shown. The c r i t i c a l areas are the i n t e r f a c e s between the T a 2 0 5 and S i 0 2 i n s u l a t o r s and to a l e s s e r degree, the i n t e r f a c e between the top (gate) e l e c t r o d e and the T a 2 0 5 . The i n t e r f a c e between the S i 0 2 and the s i l i c o n s u b s t r a t e i s w e l l known [Grove et a l . , 1964; Grove et a l . , 1965; Gray, 1969] and i t i s not analyzed here. Of great importance i s the p h y s i c a l model f o r c a r r i e r t r a n s p o r t under steady s t a t e and t r a n s i e n t c o n d i t i o n s . The Gate Contact Substrate The Al-Ta 20 5-Si02-nSi Structure gure 3.1 The General Double Dielectric Structure 3 3 d i f f e r e n c e s i n the composition of each d i e l e c t r i c makes the exact a n a l y s i s q u i t e complicated, p a r t i c u l a r l y f o r the t r a n s i e n t case. When a v o l t a g e i s a p p l i e d to the double i n s u l a t o r s t r u c t u r e , conduction c u r r e n t s flow, however, the mechanism that governs these are q u i t e d i f f e r e n t f o r the tantalum pentoxide and s i l i c o n d i o x i d e . The dominant c u r r e n t t r a n s p o r t mechanism f o r S i 0 2 has been e s t a b l i s h e d to be e l e c t r o d e l i m i t e d due to Fowler-Nordheim emission [ L e n z l i n g e r and Snow, 1969]. T h i s type of conduction mechanism i s caused by t u n n e l i n g through the b a r r i e r i n t o the i n s u l a t o r conduction band. In the case of T a 2 0 5 , the c a r r i e r t r a n s p o r t i s bulk l i m i t e d by P o o l e - F r e n k e l type emission [Mead, 1962; Angle, 1976; Angle and T a l l e y , 1978], although i t i s g r e a t l y i n f l u e n c e d by the e x i s t e n c e of t r a p p i n g c e n t r e s . For example, i f the t r a p p i n g d e n s i t y i s low, normal Poo l e - F r e n k e l conduction takes p l a c e , but i f i t i s high (as compared with the donor d e n s i t y ) , compensated or anomalous P o o l e - F r e n k e l emission dominates. Anodic tantalum oxide seems to f o l l o w these c o n s i d e r a t i o n s , however the thermal tantalum oxide e x h i b i t s a more complex behaviour, depending on the s i g n of the a p p l i e d b i a s . Angle (1976) e x p l a i n e d t h i s complex behaviour by o b s e r v i n g that under p o s i t i v e gate b i a s , normal P-F conduction takes p l a c e , but under negative b i a s space charge l i m i t e d conduction determines the c u r r e n t flow, a t t r i b u t e d to s u r f a c e s t a t e s formed at the A l ( g a t e } - T a 2 0 5 i n t e r f a c e . The P o o l e - F r e n k e l e f f e c t r e p r e s e n t s the i n c r e a s e i n e l e c t r i c a l c o n d u c t i v i t y by lowering the Coulombic p o t e n t i a l b a r r i e r , when i t i n t e r a c t s 34 with an e l e c t r i c f i e l d . Space charge conduction r e s u l t s from c a r r i e r s i n j e c t e d i n t o the i n s u l a t o r where there i s no compensation charge pre s e n t . C o n s i d e r i n g the displacement and conduction c u r r e n t d e n s i t y c o n t r i b u t i o n s by v i r t u e of the Ramo-Shockley theorem, the t e r m i n a l c u r r e n t s f o r the T a 2 0 5 and S i 0 2 i n s u l a t o r s can be expressed as: C - t - d V 1 - 1 4 -W I T d T + - r f o j t ( z ) d z ) O . l a ) e dV n r s W s ^ 3 7 f + - i - J ° j s ( z ) d z ) (3.1b) Where At and As are the c r o s s s e c t i o n areas; et and es are the T a 2 0 5 and S i 0 2 d i e l e c t r i c c o n s t a n t s ; t and s are the i n s u l a t o r t h i c k n e s s e s ; Vt and Vs are the r e s u l t i n g v o l t a g e s i n each i n s u l a t o r and the v a r i a b l e r r e p r e s e n t s time. The term j t ( z ) i s the conduction c u r r e n t d e n s i t y i n the tantalum pentoxide, given by the P o o l e - F r e n k e l e f f e c t , and j s ( z ) i s the conduction c u r r e n t d e n s i t y in the s i l i c o n d i o x i d e and determined by Fowler-Nordheim t u n n e l i n g . The dependency on the v a r i a b l e z i n d i c a t e s that a c u r r e n t d e n s i t y d i s t r i b u t i o n e x i s t s along that a x i s . In the P o o l e - F r e n k e l emission, the c u r r e n t d e n s i t y u s u a l l y can be expressed as: V c P f E t e x p ( " i J - t V ^ V ^ i ^ ( 3 - 2 ) 3 5 Where Et i s the e l e c t r i c f i e l d i n the tantalum pentoxide and the q u a n t i t y $ represents the b a r r i e r h e i g h t . For the Fowler-Nordheim emission, the c u r r e n t d e n s i t y i s expressed as: J = E 2 exp(-E /E ) /o o \ s f n s ^ o s \ 3.3) Where Es i s the f i e l d i n the s i l i c o n d i o x i d e . The c h a r a c t e r i s t i c constants Cfn and Eo depend on the e f f e c t i v e mass and b a r r i e r h e i g h t . N o t i c e that i n both cases, the c u r r e n t d e n s i t i e s are strong f u n c t i o n s of the a p p l i e d f i e l d . Furthermore, upon the a p p l i c a t i o n of a gate v o l t a g e Va to the double i n s u l a t o r s t r u c t u r e , an e l e c t r i c f i e l d i s c r e a t e d in each i n d i v i d u a l i n s u l a t o r . Gauss' Law p r e s c r i b e s that the c o n t i n u i t y of the displacement D at the i n t e r f a c e between i n s u l a t o r s s h a l l be maintained: O. '"X e.E^ - e E = (3 4) t t s s 0 Qi i s the charge per u n i t area at the i n t e r f a c e . The sum of the v o l t a g e drops across each d i e l e c t r i c has to be equal to the t o t a l a p p l i e d v o l t a g e . Then, c o n s i d e r i n g the metal semiconductor work f u n c t i o n 0ms, we have: V = - t E ^ -sE + l b + <b (3 5) a t s y s Tms v o . j y 36 The s i l i c o n s u r face p o t e n t i a l i s represented by \ps and the c o n v e n t i o n a l d e f i n i t i o n of p o t e n t i a l d i f f e r e n c e (drop) i s used. F i n a l l y , the e l e c t r i c charge has to be conserved at a l l times: (3.6) In g e n e r a l , an i n t e r f a c e charge Qi w i l l appear between both d i e l e c t r i c s , at the i n t e r f a c e . The dependence of t h i s i n t e r f a c e charge upon time and geometry of the i n s u l a t i n g s t r u c t u r e , can be obtained by simultaneous s o l u t i o n of Equations 3.4, 3.5, 3.6, and c o n s i d e r i n g the P-F T a 2 0 5 emission and the F-N mechanism in the S i 0 2 ; not an easy task! {3.1} STEADY STATE ANALYSIS: The d i f f e r e n t conduction mechanisms r e s p o n s i b l e f o r the c u r r e n t t r a n s p o r t i n the S i 0 2 and T a 2 0 5 are a l s o the cause f o r c r e a t i n g an accumulation of c a r r i e r s at the i n t e r f a c e between d i e l e c t r i c s . Upon the a p p l i c a t i o n of an e x t e r n a l f i e l d , caused by the gate v o l t a g e , e l e c t r o n i c conduction f o l l o w i n g the Fowler-Nordheim emission takes p l a c e i n the s i l i c o n d i o x i d e and conduction f o l l o w i n g P o o l e - F r e n k e l takes p l a c e i n the tantalum pentoxide. These being q u i t e d i f f e r e n t , c r e a t e a c u r r e n t d i s c o n t i n u i t y , which leads to a charge accumulation at the i n t e r f a c e . T h i s charge 37 a c c u m u l a t i o n , i n t u r n , a d j u s t s the e l e c t r i c f i e l d d i s t r i b u t i o n , u n t i l c u r r e n t c o n t i n u i t y i s e s t a b l i s h e d . The s o l u t i o n o b t a i n e d by s t e a d y s t a t e a n a l y s i s g i v e s the e l e c t r i c f i e l d i n each d i e l e c t r i c Et and Es, as w e l l as the charge a t the i n t e r f a c e Q i . Under s t e a d y s t a t e c o n d i t i o n s , both c u r r e n t s J s and J t a r e e q u a l , and t h e r e f o r e , the charge a t the i n t e r f a c e i s n e g l i g i b l e . By s u b s t i t u t i n g these c o n d i t i o n s i n E q u a t i o n s 3.4 and 3.5 above, we o b t a i n the f i e l d Es i n the S i 0 2 : (V -<J> )/s E = 2 — § — m s 3 ? ) s . e 1 + _ t s_ s e t 1 + (3.8) T h i s l a s t e q u a t i o n r e p r e s e n t s the r e d u c t i o n i n the S i 0 2 e l e c t r i c f i e l d due t o the a d d i t i o n of a second d i e l e c t r i c of t h i c k n e s s t and r e l a t i v e p e r m i t t i v i t y e t . U s i n g v a l u e s of es=3.8, et=27, s=200 A, and t=l000 A, a r e d u c t i o n f a c t o r of 0.587 i s o b t a i n e d . By s i m i l a r r e a s o n i n g we o b t a i n the f i e l d Et i n the T a 2 0 5 : E, = t F~~ ( 3 . 9 ) 1 + -2 On the o t h e r hand, i f the i n t e r f a c e charge Qi i s non-zero, 38 the i n t e r n a l e l e c t r i c f i e l d s due to t h i s charge, can be c a l c u l a t e d i n each i n d i v i d u a l d i e l e c t r i c : For the T a 2 0 5 i n s u l a t o r : e s 1 o s Ys vms' E = (3.10) ° t e. + —=— e t s s For the S i 0 2 i n s u l a t o r : / £ t Q./e + — £ - ( I J J +<j> ) I o t r s ms E s= + (3.11) e + - J - e. s t t {3.2} TRANSIENT ANALYSIS: Upon the a p p l i c a t i o n of a sudden v o l t a g e at the gate, both i n s u l a t o r s w i l l have c e r t a i n f i e l d s a p p l i e d and conduction c u r r e n t s w i l l flow as p r e s c r i b e d b e f o r e . The Law of C o n s e r v a t i o n of Charge r e q u i r e s t h a t : d Q ( V ,T.) X Ct = J 4 _ ( V .T) - J ( V , T ) V v a ' T ' " u s ^ V a ' T ; (3.12) d T The r a t e of change i n the i n t e r f a c e charge w i l l be d i r e c t l y p r o p o r t i o n a l to the d i f f e r e n c e i n c u r r e n t d e n s i t y f o r both d i e l e c t r i c s , at a given a p p l i e d gate v o l t a g e and time. The s o l u t i o n to t h i s p a r t i a l d i f f e r e n t i a l equation of two independent v a r i a b l e s can be obtained i f more i n f o r m a t i o n of the d e t a i l e d conduction mechanisms i s a v a i l a b l e . For example 39 Ross and Wallmark, ( 1 9 6 9 ) s t u d i e d the t r a n s i e n t behaviour of a MNOS s t r u c t u r e , and explained, the conduction mechanism at the i n t e r f a c e between d i e l e c t r i c s , as a f u n c t i o n of t r a p p i n g c e n t r e s of the donor type which communicate with the s i l i c o n , when a f i e l d i s a p p l i e d to the s t r u c t u r e . In t h e i r mathematical d e r i v a t i o n , they assumed a monoenergetic t r a p l e v e l , and the charge t r a n s f e r was assumed to be through d i r e c t t u n n e l i n g between t r a p s t a t e s i n the composite i n s u l a t o r . These authors obtained a r e l a t i o n f o r the t r a n s f e r r e d charge, that depended l o g a r i t h m i c a l l y on the a p p l i e d gate pulse d u r a t i o n and e x p o n e n t i a l l y on the amplitude of the a p p l i e d gate p u l s e . C o n s i d e r i n g the f a c t that the i n t e r f a c e charge i s a f u n c t i o n of a p p l i e d v o l t a g e and time, i f Equation 3.4 i s s u b s t i t u t e d i n Equation 3.5, the general time v a r y i n g s o l u t i o n i s found f o r both i n s u l a t o r e l e c t r i c f i e l d s : (V -U, -« ) Q i / £ o E t= a s ms + ( 3 < 1 3 ) fc + e t + — £ s s ( Va-V*ms> ° ± / e ° E 8=F 5 - ( 3 . 1 4 ) t — — + s -§- e. + e t t s Where the r e l a t i o n Qi=Qi(Va,t) i s v a l i d f o r t>to, the i n s t a n t of gate v o l t a g e a p p l i c a t i o n . Then the corresponding v o l t a g e s a c r o s s each d i e l e c t r i c are given by: 40 (V -Til -<J> ) (3.15) { 1 + s } {e /s + e t / t > t e s (V -ip -<J> ) a r s ms V = s + (3.16) t { 1 + s Very l i t t l e has been p u b l i s h e d on a s o l u t i o n f o r the term Qi=Qi(Va,t) under t r a n s i e n t c o n d i t i o n s . Frohman-Bentchkowsky (1970) analyzed the MNOS case by o b t a i n i n g a computer s o l u t i o n f o r Equation 3.12, but he gi v e s no d e t a i l s . Ross and Wallmark (1969) obtained a s o l u t i o n c o n s i d e r i n g a MNOS s t r u c t u r e and analyzed the f i l l e d t r a p s t a t e s using t u n n e l i n g t r a n s i t i o n p r o b a b i l i t i e s through a r e c t a n g u l a r b a r r i e r . F o l l o w i n g these l a s t authors, we have: Q i(x)=qXN i(x o,0){0.5 7 7 + l n ( t / t o ) - E i ( - t / t o ) } (3.17) The f u n c t i o n E i ( - t / t o ) converges r a p i d l y to zero f o r t>to, and X i s q u i t e c l o s e to 1 Angstrom. In the case of the composite d i e l e c t r i c T a 2 0 5 - S i 0 2 , more knowledge has to be a c q u i r e d of the int i m a t e conduction mechanisms at the i n t e r f a c e , i n order to eval u a t e Equation 3.12 i n a b e t t e r way. Once again, the i n t e r f a c e p r o p e r t i e s are d e f i n i n g the o v e r a l l behaviour of a MDIS system. 41 {3.3} EFFECT OF THE DOUBLE DIELECTRIC GATE INSULATOR ON THE MOSFET PERFORMANCE: In the double i n s u l a t o r s t r u c t u r e , an e q u i v a l e n t i n s u l a t o r t h i c k n e s s d i can be d e f i n e d : e s d± = s + — — t (3.18) T h i s can be v e r i f i e d by c o n s i d e r i n g the c a p a c i t o r s Ct and Cs in s e r i e s , and c a l c u l a t i n g the t o t a l c a p a c i t a n c e C T : C C, s t C T = (3.19) C + C. s t A l s o , an e q u i v a l e n t c a p a c i t a n c e C i per u n i t area that r e f l e c t s the combination of both d i e l e c t r i c s can be w r i t t e n as: s o C. = (3.20) d . a. T h e r e f o r e , the t o t a l c a p a c i t a n c e per u n i t area i n the double gate i n s u l a t o r i s l e s s than each s i n g l e i n s u l a t o r c a p a c i t a n c e . T h i s i s q u i t e obvious, as the i n s u l a t o r s can be represented by two c a p a c i t o r s i n s e r i e s , each with a value of c a p a c i t a n c e per u n i t area c o r r e s p o n d i n g to each d i e l e c t r i c . Of i n t e r e s t are the e q u i v a l e n t c a p a c i t a n c e to S i 0 2 42 ca p a c i t a n c e r a t i o : C. C 1 + t e s (3.21 ) s s e t T h i s r a t i o r e p r e s e n t s the r e d u c t i o n i n ca p a c i t a n c e as compared with the s i l i c o n d i o x i d e c a p a c i t a n c e . For t y p i c a l v alues of s=200 A, t=1000 A, es=3.8 and et=27, the r a t i o i s 0.587. T h e r e f o r e , as the e l e c t r i c f i e l d a cross the S i 0 2 i s reduced, the e q u i v a l e n t c a p a c i t a n c e i s a l s o reduced by the same f a c t o r . Conversely, an e q u i v a l e n t p e r m i t t i v i t y can be d e f i n e d , i f the c a p a c i t o r i s c o n s i d e r e d to have a t h i c k n e s s of t+s u n i t s . From the e q u i v a l e n t c a p a c i t a n c e e x p r e s s i o n , Equation 3.20: C. = t + s ' i e (3.22) s + — — t e t Then, the e q u i v a l e n t p e r m i t t i v i t y can be d e f i n e d as: ej, = t+s (3.23) s / e g + t / e t A graph of i t s f u n c t i o n i s shown in F i g u r e 3.2, with s e v e r a l curves given f o r d i f f e r e n t i n s u l a t o r compounds. No t i c e that f o r a l a r g e r a t i o of i n s u l a t o r t h i c k n e s s e s , the e q u i v a l e n t p e r m i t t i v i t y approaches that of the outer (and l a r g e r ) 43 i n s u l a t o r d i e l e c t r i c c o n s t a n t . The f o l l o w i n g Equations d e f i n e the op e r a t i o n of a MOSFET, using the s i m p l i f i e d model [Sze, 1969]: x d = "TT ^ c i { ( V V v d - * v a } <3-24> N o t i c e that Id, the d r a i n c u r r e n t , i s p r o p o r t i o n a l to C i , the i n s u l a t o r c a p a c i t a n c e . The t h r e s h o l d v o l t a g e i s given by: V T = 2*b + V f b + { 2 £ S ^ A , D ( 2 V } % / C i „ O R l IcT N o t i c e that the t h r e s h o l d v o l t a g e V ^ i s i n v e r s e l y r e l a t e d to C i , which i s i s given by: C. = — (3.26) x d. The device transconductance i s : 1-uC.Vj. (3.27) L i d Which i s d i r e c t l y p r o p o r t i o n a l to C i . In the case of the channel conductance, the s i t u a t i o n i s s l i g h t l y more comp l i c a t e d : 4 4 CP cn CM cn C M >-r i -> tTM-r i a. L U r u -2 *=» «=* cr. > n «3 -a .00 Zr0 2 er=30 Ta 20 5 er=27 er=8 Al 20 3 e r r « I S i 3 N 4 r=6 Al 20 3 S-00 10.00 I S . 0 0 £0.00 ZS.00 RflTID DT INCULPTDR T H I C K N E S S E S 50 . 0 0 Figure 3.2 Equivalent Permittivity of a Double Insulator Structure 45 g d = - E - y C i ( V g " V (3.28) The channel conductance remains approximately the same, si n c e C i has i n c r e a s e d , but V T h a s decreased. Based on the above reasoning, Table 3.1 g i v e s the r e s u l t s o btained u s i n g the S i 0 2 i n s u l a t o r c a p a c i t a n c e as r e f e r e n c e . TABLE 3.1 DOUBLE DIELECTRIC MOSFET PARAMETERS Tantalum Pentoxide on S i l i c o n Dioxide t/s=5.0 t/s=2.0 t/s=1.0 I d i / I d s 0.587 0.780 0.877 V T i / V T s = 1 .704 =1.282 =1.140 Ci/Cs 0.587 0.780 0.877 gmi/gms 0.587 0.780 0.877 gdi/gds = 1 = 1 = 1 The s u b s c r i p t s x s ' and x i ' r e f e r to the S i 0 2 and e q u i v a l e n t i n s u l a t o r parameters, t and s are the t h i c k n e s s of the tantalum pentoxide and s i l i c o n d i o x i d e , r e s p e c t i v e l y . 46 N o t i c e that the channel conductance gd remains approximately constant as the r a t i o t / s i s v a r i e d . The transconductance, i n s u l a t o r c a p a c i t a n c e and d r a i n c u r r e n t i n c r e a s e with d i m i n i s h i n g t / s . The gate t h r e s h o l d v o l t a g e decreases as t/s i s reduced. {3.4} EFFECT OF A HIGH PERMITTIVITY GATE INSULATOR ON THE MOSFET PERFORMANCE: In the case of a s i n g l e d i e l e c t r i c l a y e r of gate oxide, which has high r e l a t i v e p e r m i t t i v i t y , the same pre v i o u s reasoning i s v a l i d f o r the MOSFET device r e l a t i o n s , with the excep t i o n that the r e l a t i v e d i e l e c t r i c constant of the T a 2 0 5 i s 27, and that of the S i 0 2 i s 3.8. Using these v a l u e s the r a t i o of et to es i s then 7.11. The c a l c u l a t e d q u a n t i t i e s , f o r tantalum pentoxide, s i l i c o n n i t r i d e (er=7.6) and aluminium oxide (er=8) using the S i 0 2 i n s u l a t o r as comparison, are shown i n Table 3.2. 47 TABLE 3.2 SINGLE DIELECTRIC MOSFET PARAMETERS Tantalum Pentoxide, S i l i c o n N i t r i d e and Aluminium Oxide T a 2 0 5 / S i 0 2 et/es=7.11 Si 3 N a / S i 0 2 en/es=2.0 A l 2 0 3 / S i 0 2 ea/es=2.11 I d i / I d s 7.11 2.0 2.11 =0.141 =0.500 =0.475 Ci/Cs 7.11 2.0 2.11 gmi/gms 7.11 2.0 2.11 gdi/gds = 1 = 1 = 1 48 I t q u i t e c l e a r that the high d i e l e c t r i c constant i n s u l a t o r as gate oxide w i l l , with a l l other parameters remaining equal, i n c r e a s e the transconductance, i n s u l a t o r c a p a c i t a n c e and d r a i n c u r r e n t . N o t i c e that the t h r e s h o l d v o l t a g e i s reduced by the same f a c t o r . The channel conductance remains approximately the same. The a b i l i t y of the MOS C a p a c i t o r to withstand an a p p l i e d v o l t a g e and s t o r e a given charge i s measured by the f i g u r e of merit c 0 e r E , were E i s the maximum (breakdown) e l e c t r i c f i e l d . T h i s i s the c a p a c i t o r charge storage f a c t o r [ G l a s e r , Subak-Sharpe, 1977; B i n e t , 1982], i t i s based on the f l a t p l a t e c a p a c i t o r equation and the f a c t that the best c a p a c i t o r bandwidth i s obtained when the r a t i o e r / t i s l a r g e s t . The bandwidth of a c a p a c i t o r (used f o r c o u p l i n g c i r c u i t s i n MIC's) i s that i n which the impedance shunting e f f e c t s due to p a r a s i t i c c a p a c i t a n c e s i s small compared with the system impedance. However, the t h i c k n e s s t i s l i m i t e d by t=E/Va, were Va i s the maximum a p p l i e d v o l t a g e . F o l l o w i n g Binet (1982), Table 3.3 g i v e s the f i g u r e of merit f o r s e v e r a l widely used i n s u l a t o r s , i n which the maximum (breakdown) f i e l d i s a p p l i e d . The tantalum pentoxide i n s u l a t o r has the h i g h e s t f i g u r e of m e r i t , followed by aluminium oxide and s i l i c o n n i t r i d e . The use of e i t h e r s i n g l e i n s u l a t o r or double i n s u l a t o r s t r u c t u r e i n a MOS t r a n s i s t o r w i l l be a f u n c t i o n of the design o b j e c t i v e s . In theory, the s i n g l e , high p e r m i t t i v i t y T a 2 0 5 gate i n s u l a t o r can o f f e r c o n s i d e r a b l e advantages over 49 the compound s t r u c t u r e , but as we s h a l l see i n the f o l l o w i n g c h a p t e r s , the e l e c t r o n i c conduction ( i . e . leakage) c u r r e n t s , impose a s e r i o u s l i m i t a t i o n to the tantalum pentoxide monoinsulator. The double l a y e r i n s u l a t o r , with S i 0 2 immediately over the S i s u b s t r a t e , can reduce c o n s i d e r a b l y the e l e c t r o n i c conduction, i f the t h i c k n e s s of t h i s i n s u l a t o r i s such ( t y p i c a l l y >10 nm) that t u n n e l i n g through the b a r r i e r i s avoided. TABLE 3.3 MOS CAPACITOR INSULATOR FIGURE OF MERIT S i l i c o n Dioxide, S i l i c o n N i t r i d e , Aluminium Oxide and Tantalum Pentoxide MATERIAL er F i e l d , E [MV/cm] erE [MV/cm] e 0 e r E [pfV/mm 2] S i 0 2 3.8 4 15.2 1 3458 Si 3N a 7.6 1 0 76 67290 A 1 2 0 3 8 1 0 80 70832 T a 2 0 5 27 4 108 95623 50 {3.5} ENERGY BANDS OF THE TANTALUM PENTOXIDE INSULATOR: An e f f o r t was made to compile enough data from t h i s i n s u l a t o r as to produce an energy band diagram. L i t t l e data i s a v a i l a b l e i n the l i t e r a t u r e r e g a r d i n g the e s s e n t i a l parameters such as e l e c t r o n a f f i n i t y and m e t a l - i n s u l a t o r b a r r i e r p o t e n t i a l s . I n t e r n a l photoemission i s a powerful t o o l that i s used to o b t a i n the e l e c t r o n a f f i n i t y of i n s u l a t o r s [Goodman, 1968]. However, in the case of T a 2 0 5 no measurements have been made [Goodman, 1984], which confirms the u n a v a i l a b i l i t y of p u b l i s h e d f i g u r e s . Angle (1976) pr o v i d e s a sketch of a band diagram i n h i s Ph.D. T h e s i s , but he p r o v i d e s no d e t a i l s or r e f e r e n c e s on h i s source f o r the tantalum pentoxide band s t r u c t u r e data. The value of h i s m e t a l - i n s u l a t o r b a r r i e r height c o i n c i d e s (0.72 eV) with the one given by Young (1961) of 0.71 eV. A l s o the value of the bandgap energy (4.2 eV) i s c o i n c i d e n t with the ones obtained by previous authors [ Z a i n i n g e r et a l . , 1969; Kaplan et a l . , 1973; Revesz et a l . , 1976], Based on the data p r o v i d e d by Angle, an energy band diagram i s presented i n F i g u r e 3.3, f o r an A l - T a 2 0 5 - n S i s t r u c t u r e i n e q u i l i b r i u m (no e x t e r n a l gate p o t e n t i a l a p p l i e d ) . 51 Vacuum Level qXj=3.38eV — A l H—Ta205—I-—n Si S^ m- M e t a l - I n s u l a t o r B a r r i e r P o t e n t i a l J2-S : S i l i c o n - I n s u l a t o r B a r r i e r P o t e n t i a l Xj : I n s u l a t o r E l e c t r o n A f f i n i t y [j"g; I n s u l a t o r Bandgap Energy A ; Voltage across I n s u l a t o r with zero gate v o l t a g e Figure 3.3 Energy Band Diagram of the Al-Ta 20 5-nSi Structure 5 2 { 3 . 6 } DOUBLE DIELECTRIC MOSFET STRUCTURE: The c r o s s s e c t i o n of t h i s d e v i c e i s shown i n F i g u r e 3 . 4 . I t s d i m e n s i o n s a re t y p i c a l and r e f l e c t the ones used i n t h i s work. The t r a n s i s t o r has the d r a i n and sour c e p-type j u n c t i o n s on a n - S i s u b s t r a t e , w i t h windows c u t i n the f i e l d o x i d e t o p r o v i d e adequate c o n t a c t s . The gate a r e a c o n t a i n s the double i n s u l a t o r s t r u c t u r e , bounded by the s u b s t r a t e and the gate metal c o n t a c t . An i n v e r s i o n p-type c h a n n e l i s formed under the g a t e , when a n e g a t i v e v o l t a g e i s a p p l i e d t o the g a t e , thus p r o v i d i n g enhancement mode o p e r a t i o n . For prope r o p e r a t i o n , the d r a i n - s o u r c e b i a s has t o be n e g a t i v e . The s u b s t r a t e or b u l k , has a c o n t a c t p r o v i d e d by a bottom g o l d m e t a l l i z a t i o n . * 53 l IIIIIIIIIIUIIIIIIIIIIIIIII t-SiOo B  n Si T y p i c a l Dimensions: Gate Length: 10 ym Gate Oxide T h i c k n e s s : 6 Substrate Au F i e l d Oxide: 600 nm J u n c t i o n Depth: 1 ym S i 0 2 : T a 2 ° 5 200 A : 1000 A Proposed Symbol G o- -o B Figure 3.4 Double D i e l e c t r i c MOSFET Structure. 54 CHAPTER 4 FABRICATION AND PROCESSING OF MOS CAPACITOR DEVICES B e f o r e the f i n a l MTAOS s t r u c t u r e c o u l d be de v e l o p e d , i t was n e c e s s a r y t o study i n g r e a t d e t a i l t he f a b r i c a t i o n and p r o c e s s i n g of t a n t a l u m p e n t o x i d e s i n g l e d i e l e c t r i c MOS c a p a c i t o r s . Then a s t e p f u r t h e r c o u l d be made i n t o the T a 2 0 5 - S i 0 2 double d i e l e c t r i c MOS C a p a c i t o r f a b r i c a t i o n . When these were e s t a b l i s h e d as s u c c e s s f u l p r o c e s s i n g t e c h n o l o g i e s , the double d i e l e c t r i c MTAOS d e v i c e c o u l d o n l y then be f a b r i c a t e d . {4.1} MOS CAPACITORS WITH THERMAL Ta 2O s AS INSULATOR: C o n s i d e r a b l e amount of time was devoted t o the p r o c e s s i n g of MOS C a p a c i t o r s w i t h a s i n g l e i n s u l a t i n g f i l m of T a 2 0 5 , and s e v e r a l t e c h n i q u e s f o r p a t t e r n i n g the Ta metal were used. The f o l l o w i n g g e n e r a l p r o c e s s i n g s t e p s were f o l l o w e d : T h i c k n e s s and f o u r p o i n t r e s i s t i v i t y measurements, S c r i b i n g and marking. P e r o x i d e - A c i d c l e a n i n g u s i n g the RCA p r o c e s s . RF S p u t t e r i n g of t a n t a l u m m e t a l . Thermal O x i d a t i o n i n dry oxygen. Aluminium e v a p o r a t i o n f o r gate e l e c t r o d e s . P a t t e r n i n g of aluminium metal by p h o t o l i t h o g r a p h y , Back c o n t a c t by aluminium e v a p o r a t i o n . 55 {4.1.1} THICKNESS AND SHEET RESISTIVITY MEASUREMENTS: Thickness measurements were performed with a Mitutoyo DGS-E Gauge d i a l c a l i p e r . Four Point r e s i s t i v i t y measurements were done using a Hewlett-Packard 6186C Current Source, a Fluke 8000A or 8050A D i g i t a l Voltmeter and a K u l i c k e and S o f f a Model 3007, No.130, four p o i n t probe with an i n t e r p r o b e spacing of 0.025 inches. {4.1.2} SCRIBING AND MARKING: S c r i b i n g and marking was done with a standard diamond pen, on the wafer's back, immediately above the f l a t . A s p e c i a l code and date was i n s c r i b e d , unique to each i n d i v i d u a l wafer, so that f u t u r e i d e n t i f i c a t i o n c o u l d be made easy. A l a r g e amount of samples was prepared, as shown in Table 4.1. T A B L E 4 . 1 S I N G L E D I E L E C T R I C T H E R M A L MOS C A P A C I T O R S S A M P L E N A M E T a T H I C K N E S S [ A ] S U B S T R A T E N L 5 0 0 n T y p e N 2 1 0 0 0 n T y p e B N R 5 0 0 5 0 0 P T y p e B N R 1 0 0 0 1 0 0 0 P T y p e S a m p l e A 5 0 0 P T y p e S a m p l e B 1 0 0 0 P T y p e 5 0 0 A L i f t 5 0 0 P T y p e 1 O O O A L i f t 1 0 0 0 P T y p e M O S C 1 5 0 0 n T y p e M O S C 2 1 0 0 0 n T y p e M O S C 3 5 0 0 n T y p e M O S C 4 1 0 0 0 n T y p e M O S C 5 5 0 0 n T y p e M O S C 6 5 0 0 n T y p e M O S C 7 1 0 0 0 n T y p e M O S C 8 1 0 0 0 n T y p e 57 {4.1.3} PEROXIDE ACID CLEANING: Cl e a n i n g of the wafers followed the RCA SSEE-100 a c i d -peroxide process [Kern and Puotinen, 1970], as d e t a i l e d i n the Appendix I I I . R i n s i n g i n d e i o n i z e d water f o l l o w e d each step, which produced a smooth, shiny and very c l e a n s u r f a c e . {4.1.4} RF SPUTTERING: Radio Frequency S p u t t e r i n g of Ta metal on the a l r e a d y c l e a n S i s u b s t r a t e s , was accomplished using a Perkin-Elmer 3140 Randex s i n g l e t a r g e t system, mounted on a NRC 703 Automatic Valve C o n t r o l High Vacuum System. The s p u t t e r i n g was performed i n Argon, with a p a r t i a l p r e ssure of 26 mTorr. The vacuum system evacuated the chamber to 10" 6 T o r r , as monitored by the b a s e p l a t e I o n i z a t i o n Gauge (CHA I n d u s t r i e s IG-101P Ion Tube and C o n s o l i d a t e d Vacuum Corp. G1C-110A I o n i z a t i o n Vacuum Gage). Under these c o n d i t i o n s , the s p u t t e r i n g r a t e was 15 nm/min, which was p r e v i o u s l y determined by D. Smith using the Sloan Angstrometer method [Smith and Young, 1981]. {4.1.5} THERMAL OXIDATION: Thermal o x i d a t i o n of the Ta f i l m s was performed i n a Thermco Products Corp. Mini Brute r e s i s t a n c e heated quartz tube (5 cm) furnace, with a Ana Lock 201 C o n t r o l l e r . The temperature in the c e n t r e was set to 500 C, with +5 C d i f f e r e n c e at the edges, i n order to compensate f o r heat l o s s , thus o b t a i n i n g a " f l a t " temperature p r o f i l e . Dry oxygen was manually r e g u l a t e d by a Brooks R-2-15-A tube 58 flowmeter, and set to a flow of 1.0 l/min (a Brooks tube reading of 9.4 cm). The o x i d a t i o n time, a c r i t i c a l parameter, was v a r i e d c o n s i d e r a b l y as d e s c r i b e d below, i n order to determine i t s optimum value in f u n c t i o n of the oxide q u a l i t y . TABLE 4.2 THERMAL OXIDATION OF TANTALUM ON SILICON SAMPLE NAME Ta THICKNESS TEMPERATURE TIME N1 500 A 500 C 60 min N2 1 000 A 500 C 120 min BNR500 500 A 500 C 93 min BNR1000 1 000 A 500 C 187 min SampleA 500 A 500 C 3.5 hrs SampleB 1000 A 500 C 3.5 hrs 500ALift 500 A 500 C 6.5 hrs 1OOOALift 1 000 A 500 C 10 hrs MOSC1 500 A 400 C 5 hrs MOSC2 1000 A 400 C 7 hrs MOSC3 500 A 600 C 5 hrs MOSC4 1000 A 600 C 7 hrs MOSC5 500 A Broken under p r o c e s s i n g MOSC6 500 A 600 C 5 hrs MOSC7 1000 A 400 C 1 week MOSC8 1000 A 600 C 7 hrs 59 {4.1.6} ALUMINIUM DEPOSITION: I n i t i a l l y , some samples had an aluminium gate d e p o s i t e d using a Veeco model VE-400 E l e c t r o n Beam equipment and a s t e n c i l metal mask. The b e l l j a r was evacuated to 10" 5 T o r r before d e p o s i t i o n took p l a c e . T h i s method was r e p l a c e d by the one d e s c r i b e d below, s i n c e the p o s s i b i l i t y e x i s t e d of i n t r o d u c i n g an unknown f a c t o r , namely the high energy r a d i a t i o n from the E-Beam that c o u l d implant A l i n the oxide, or damage the S i s u b s t r a t e at the i n t e r f a c e . In e i t h e r case, the r e s u l t s would be a f f e c t e d [Miner, 1981]. Aluminium metal was t h e r m a l l y evaporated (as opposed to E l e c t r o n Beam, which i s a high energy process) using a CHA I n d u s t r i e s Model SE-600-RP Evaporator with a Auto Tech II c o n t r o l l e r . High purity. 1% s i l i c o n aluminium wire (Cominco ALA 1793, 0.045" d i a . ) was cut i n t o hoops of 2 cm l e n g t h s , and hung on the tungsten f i l a m e n t s . The chamber pressure was 10" 6 T o r r , before any e v a p o r a t i o n took p l a c e . The evaporated t h i c k n e s s ranged between 600 and 1000 nm, as given by a I n f i c o n Model 321 Quartz C r y s t a l F i l m Thickness Monitor (a d e n s i t y of 2.73 was used f o r aluminium). {4.1.7} PHOTOLITHOGRAPHY: Pho t o l i t h o g r a p h y was used to p a t t e r n the aluminium e l e c t r o d e s , a dot and r i n g mask was used with a negative p h o t o r e s i s t process as d e s c r i b e d i n d e t a i l i n Appendix I I I . The dots have an area of 0.7854 mm2 (1 mm diameter), and the r i n g s p r o v i d e e l e c t r i c a l i s o l a t i o n . 60 {4.1.8} BACK CONTACT METALLIZATION: The back contact was made by e v a p o r a t i n g aluminium metal, using the same method d e s c r i b e d above f o r the gate e l e c t r o d e s . Again thermal e v a p o r a t i o n was f a v o r e d . Gold i s the metal u s u a l l y used i n t h i s a p p l i c a t i o n , however the l a r g e amount of wafers processed i n d i c a t e d that the cost would be p r o h i b i t i v e , thus aluminium proved to be an economical a l t e r n a t i v e with known p r o c e s s i n g techniques. {4.2} MOS CAPACITORS WITH DOUBLE DIELECTRIC STRUCTURE: The double d i e l e c t r i c T a 2 0 5 - S i 0 2 was used i n a number of samples to evaluate i t s c h a r a c t e r i s t i c s . T h i s method of f a b r i c a t i o n r e q u i r e s f i r s t the formation of a t h i n s i l i c o n oxide l a y e r . Then a l a y e r of tantalum metal i s a p p l i e d , f o l l o w e d by a dry thermal o x i d a t i o n . A f t e r c l e a n i n g and marking, the p r o c e s s i n g steps are as f o l l o w s : a) Dry thermal o x i d a t i o n i n oxygen. b) Tantalum RF s p u t t e r i n g . c) Aluminium e v a p o r a t i o n . d) P a t t e r n i n g using p h o t o l i t h o g r a p h y . e) Back contact m e t a l l i z a t i o n . The thermal o x i d a t i o n was accomplished i n the same r e s i s t a n c e furnace used f o r the tantalum o x i d a t i o n , as d e s c r i b e d above. E x p e r i m e n t a l l y , i t was determined that the o x i d a t i o n r a t e was very small [ T a r r , 1980], about 1 A/min., f o r an oxygen flow of 1 l/min. The a d d i t i o n a l steps used i n 61 t h i s procedure are the same as b e f o r e . The samples prepared using t h i s technique a re: TABLE 4.3 DOUBLE DIELECTRIC MOS CAPACITORS SAMPLE NAME S i 0 2 THICKNESS Ta THICKNESS 20S20T 20 A 20 A 20S50T 20 A 50 A 20S100T 20 A 100 A 20S200T 20 A 200 A 50S100T 50 A 100 A 50S200T 50 A 200 A 50S500T 50 A 500 A 50S1000T 50 A 1000 A The tantalum metal when f u l l y c onverted i n t o i t s oxide, w i l l show an i n c r e a s e in t h i c k n e s s , by a f a c t o r of approximately 2, (the " s w e l l i n g " f a c t o r ) as d e t a i l e d i n Chapter 6. Hence, the v a l u e s on the l a s t column w i l l produce a corresponding T a 2 0 5 f i l m t h i c k n e s s twice as l a r g e . {4.3} PROCESS AND FABRICATION COMMENTS: T h i s method of f a b r i c a t i o n produced the r e q u i r e d MOS C a p a c i t o r s f o r the i n i t i a l e v a l u a t i o n of the thermal T a 2 0 5 f i l m on s i l i c o n s u b s t r a t e s . The s t r u c t u r e then has a A l -T a 2 0 5 - S i c o n f i g u r a t i o n . The c o n v e r s i o n of the tantalum metal f i l m i n t o oxide i s a rath e r lengthy process, and although p r e v i o u s authors gave 62 some i n d i c a t i o n of the time r e q u i r e d f o r complete thermal o x i d a t i o n of the metal f i l m [Revesz et al.,1974; Smith and Young, 1981], i t was f e l t that some experimentation should take p l a c e to determine the optimum o x i d a t i o n time i n terms of more c r i t i c a l parameters as conduction (leakage) c u r r e n t for a given v o l t a g e , and Capacitance-Voltage c h a r a c t e r i s t i c s . Hence, a l a r g e amount of samples was produced, with d i f f e r e n t o x i d a t i o n times, which v a r i e d from a few minutes ( f o r very t h i n Ta f i l m s ) to s e v e r a l hours ( f o r t h i c k e r Ta f i l m s ) , and in one case f o r a few days. Furthermore, i n some cases, the temperature was i n c r e a s e d from the usual 500 C to 600 C, and i n some cases reduced to 400 C, i n order to determine the e f f e c t of the temperature v a r i a t i o n s on the C-V and I-V c h a r a c t e r i s t i c s . The f i l m f i n a l c o l o r was p u r p l e - b l u e f o r the 50 nm samples, and g o l d - y e l l o w f o r the 100 nm samples. T h i s c o l o r d i f f e r e n c e was q u i t e h e l p f u l i n i d e n t i f y i n g each sample, i f the case r e q u i r e d . In both cases, the c o l o r s are intense and q u i t e s t r i k i n g . The S i s u b s t r a t e was i n most cases n type, however p type was a l s o used, i n order to e s t a b l i s h p o s s i b l e d i f f e r e n c e s i n the q u a l i t y of the Al-Ta o x i d e - S i c a p a c i t o r s . No attempt was made to use d i f f e r e n t s u b s t r a t e o r i e n t a t i o n s , as t h i s would have v a s t l y i n c r e a s e d the amount of v a r i a b l e s in t h i s work, and hence the t o t a l time r e q u i r e d to complete the p r o j e c t . In two cases (samples BNR500 and BNR1000), i t was p o s s i b l e to o b t a i n Ta f i l m s d e p o s i t e d by Magnetron Enhanced S p u t t e r i n g (MES), i n order to e v a l u a t e any p o s s i b l e 63 d i f f e r e n c e s i n the performance of the f i n i s h e d MOS c a p a c i t o r s . S i l i c o n w a f e r s were sent t o the S o l i d S t a t e L a b o r a t o r i e s of B e l l N o r t h e r n Research i n Ottawa, O n t a r i o ; and p r o c e s s e d by MES. A d i f f e r e n t c l e a n i n g method was used, as d e t a i l e d i n Appendix I I I . B e s i d e s t h i s i n i t i a l d i f f e r e n c e , the p r o c e s s f o l l o w e d f o r MOS C a p a c i t o r s f a b r i c a t i o n , was e x a c t l y the same as i n d i c a t e d b e f o r e . S e v e r a l a t t e m p t s were made t o p r o c e s s the Ta and T a 2 0 5 f i l m s i n a way t h a t would y i e l d the MOS C a p a c i t o r s w i t h the o x i d e f i l m o n l y under the A l gat e e l e c t r o d e , and not over the e n t i r e s u b s t r a t e . E t c h i n g of the Ta or Ta{oxide} f i l m was mandatory, i f the p r e s e n t p r o c e s s i n g t e c h n o l o g y was t o be f u r t h e r extended t o the f a b r i c a t i o n of MOSFETs. The au t h o r had many f r u s t r a t i n g e x p e r i e n c i e s i n t r y i n g t o p a t t e r n the Ta m e t a l or Ta o x i d e f i l m on S i s u b s t r a t e s . The c h e m i c a l s o l u t i o n s used went from the c l a s s i c a l 10% HF and B u f f e r e d HF, t o more s t r o n g e r m i x t u r e s of c o n c e n t r a t e d hot HF, and even dangerous m i x t u r e s l i k e " p i r a h n a e t c h " (a hot s o l u t i o n of H 2 S O 4 and H 2 0 2 ) and "marabunta e t c h " (a hot s o l u t i o n of H N O 3 and NH aF), names d e r i v e d from a s p e c i e s of Amazonian a n t s , which devour e v e r y t h i n g upon t h e i r passage. The r e s u l t s were v e r y u n s a t i s f y i n g , and i n most c a s e s severe damage r e s u l t e d , e i t h e r t o the p h o t o r e s i s t or t o the s u b s t r a t e , i n p a r t i c u l a r when the "Amazonian" s o l u t i o n s were used. I t was amazing t o o b s e r v e both Ta and T a 2 0 5 f i l m s r e s t untouched by such s t r o n g c h e m i c a l s . 64 {4.4} THE LIFTOFF TECHNIQUE ON TANTALUM FILMS: A b e t t e r p r o c e s s i n g method was i n need, and the use of the L i f t o f f technique was developed f o r use with tantalum metal f i l m s . It i s based on removing c h e m i c a l l y ( i . e . , e t c h i n g ) a t h i n l a y e r of aluminium metal, d e p o s i t e d f i r s t over the S i s u b s t r a t e , then p a t t e r n e d with a negative image of the r e q u i r e d f i n a l shape. Then the tantalum f i l m i s de p o s i t e d and by e t c h i n g the u n d e r l y i n g aluminium, the f i n a l p o s i t i v e p a t t e r n i s obtained. The d e p o s i t i o n methods vary c o n s i d e r a b l y , but i n t h i s case, the Aluminium metal was evaporated and the tantalum metal was RF s p u t t e r e d , using the same methods d e s c r i b e d above. The f o l l o w i n g steps were used, a f t e r the RCA SSEE100 c l e a n i n g step mentioned bef o r e : 1. Aluminium evaporation f o r l i f t o f f . 2. P a t t e r n i n g of A l metal by p h o t o l i t h o g r a p h y , with negative image. 3. RF S p u t t e r i n g of tantalum metal. 4. L i f t o f f of unwanted tantalum by aluminium e t c h i n g . Then the p r o c e s s i n g was continued as u s u a l , i . e . , the A l metal evaporation f o r gate e l e c t r o d e s , e x a c t l y as d e s c r i b e d above. The f i n a l e t c h i n g r e q u i r e d c o n s i d e r a b l e time, from 60 to 90 minutes, depending on the o r i g i n a l A l t h i c k n e s s , which v a r i e d from 600 to 1000 nm. I n i t i a l l y , no r e a c t i o n was n o t i c e d , but a f t e r 10-20 minutes, gas bubbles appeared on the wafer's s u r f a c e , i n d i c a t i n g t h a t the A l metal was at t a c k e d by the etchant s o l u t i o n . Towards the end of t h i s 65 process, i t was n o t i c e d that the tantalum metal s u r f a c e changed from a smooth appearance to a coarse and w r i n k l e d t e x t u r e . The sample(s) then were c a r e f u l l y removed, r i n s e d in d e - i o n i z e d water and g e n t l y rubbed with a s o f t foam swab. T h i s i s done to remove the unwanted w r i n k l e d Ta f i l m . -I n t e r e s t i n g l y , i f the swab was made of c o t t o n ( i . e . , a common Q t i p s o l d i n d r u g s t o r e s ) , t h i s one would s e v e r e l y s c r a t c h the tantalum s u r f a c e , as seen under a standard o p t i c a l microscope, or even naked eye. A foam swab (Sof-Swab, Clean Room Products, Bay Shore, New York) proved to leave a c l e a n , s c r a t c h f r e e s u r f a c e . I t was necessary to leave the sample(s) in the e t c h i n g s o l u t i o n f o r another 30-45 minutes, in order to remove the r e s i d u a l Al/Ta from the wafer. The end r e s u l t was a c l e a n , w e l l d e f i n e d p a t t e r n of tantalum metal, with no damage to the s i l i c o n s u b s t r a t e . A f t e r t h i s , then the process c o u l d continue as d e s c r i b e d b e f o r e . {4.5} MOS CAPACITORS WITH ANODIC T a 2 0 5 AS INSULATOR: Another p o s s i b l e way of producing tantalum pentoxide f i l m s i s by anodic o x i d a t i o n of a t h i n l a y e r of Ta metal. S e v e r a l authors have reported t h i s method [Berry, 1959; Young, 1961; Dell'Oca et a l . , 1971] as a very r e l i a b l e one that produces f i l m s of s u p e r i o r q u a l i t y and, of course, much f a s t e r than the dry thermal o x i d a t i o n of Ta f i l m s . However, the p r e v i o u s work only d e a l t with i n s u l a t i n g s u b s t r a t e s ( f o r example g l a s s or alumina) or conductive ones (a p l a t e of A l or Ta) which f u r t h e r s i m p l i f i e d the problem of making 6 6 e l e c t r i c a l c o n t a c t to i t . In t h i s case, the s u b s t r a t e was s i l i c o n , a semiconductor, and a p r a c t i c a l s o l u t i o n had to be found f o r making a good e l e c t r i c a l c ontact to the Ta f i l m on S i . A p o s s i b l e s o l u t i o n i s to allow the c u r r e n t to flow through the c r o s s s e c t i o n of the s i l i c o n s u b s t r a t e , i f the proper type i s chosen. By t a k i n g i n t o account the c u r r e n t d i r e c t i o n i n the s u b s t r a t e , and n o t i n g that the tantalum e l e c t r o d e i s the anode, the e l e c t r o n i c c u r r e n t flow d i c t a t e s that a p - S i type s u b s t r a t e i s the only p o s s i b l e c h o i c e , as t h i s w i l l make the Ta-pSi diode forward b i a s e d . If a n-Si s u b s t r a t e i s used as the case i s , the Ta-nSi diode i s then reversed b i a s e d and the a n o d i z a t i o n c u r r e n t w i l l be bl o c k e d . Hence, a good d i r e c t contact must e x i s t to the s i l i c o n s u r f a c e , i f the s u b s t r a t e i s p type. T h i s was accomplished by e t c h i n g a small s t r i p c l o s e to the f l a t , i n the wafer's f a c e . Under these c o n d i t i o n s , i t i s then f e a s i b l e to a n o d i c a l l y o x i d i z e a t h i n l a y e r of Ta metal p r e v i o u s l y d e p o s i t e d on the s i l i c o n s u b s t r a t e . However, care s h o l d be taken NOT to grow an anodic oxide of s i l i c o n , which would produce a l a y e r of S i 0 2 under the a l r e a d y formed anodic T a 2 0 5 . T h i s c o n d i t i o n can be de t e c t e d by monitoring the v o l t a g e across the a n o d i z a t i o n c e l l , as a f u n c t i o n of time, when a constant c u r r e n t flows through i t . The r a t e of v o l t a g e i n c r e a s e dV/dt i s constant f o r a constant c u r r e n t d e n s i t y through the e l e c t r o l y t e [ D e l l ' Oca et a l . , 1971], The metal w i l l be f u l l y converted i n t o i t s oxide, when the v o l t a g e a c r o s s the c e l l reaches V l i m i t of the constant c u r r e n t source. At t h i s p o i n t the 6 7 process i s i n t e r r u p t e d . A l s o , any gas escaping from the anode ( u s u a l l y oxygen), i n d i c a t e s that a f u r t h e r change i s t a k i n g p l a c e on the s i l i c o n s u r f a c e : the growth of a S i 0 2 l a y e r . I t i s a l s o known, that by a p p l y i n g a constant v o l t a g e to the c e l l , a f t e r the constant c u r r e n t process, i t i s p o s s i b l e to o b t a i n a b e t t e r q u a l i t y oxide f i l m [Young, 1961]. T h i s i s based on experimental evidence that a c e r t a i n " h e a l i n g " e f f e c t takes p l a c e on weak spots or areas i n the oxide f i l m i t s e l f . The a n o d i z a t i o n c e l l and a s s o c i a t e d equipment are shown in F i g u r e 4.1, f o r both constant c u r r e n t and constant v o l t a g e p r o c e s s e s . Based on these p r i n c i p l e s , the f o l l o w i n g steps were used in producing anodic tantalum on s i l i c o n s u b s t r a t e s , a f t e r the RCA SSEE100 c l e a n i n g p r o c e s s : 1. RF S p u t t e r i n g of tantalum metal. 2. Back c o n t a c t by aluminium e v a p o r a t i o n . 3. Anodic o x i d a t i o n at constant c u r r e n t , v o l t a g e monitored as a f u n c t i o n of time. 4. A p p l i c a t i o n of constant v o l t a g e to the c e l l , c u r r e n t monitored as a f u n c t i o n of time. 5. R i n s i n g i n d e - i o n i z e d water to remove a l l t r a c e s of e l e c t r o l y t e s o l u t i o n . The p r o c e s s i n g then con t i n u e s as before, with the A l e v a p o r a t i o n f o r gate e l e c t r o d e s , being the next step. Table 4.4 g i v e s f u r t h e r d e t a i l s of the wafers and e l e c t r o l y t e s used. 68 Constant Current Phase Figure 4. 1 Anodization C e l l and Equ ipment 69 {4.5.1} ANODIZATION IN CITRIC ACID ELECTROLYTE SOLUTION: A good c o n t a c t i s r e q u i r e d to the anode, and the back A l contact performs t h i s f u n c t i o n q u i t e w e l l . A c u r r e n t d e n s i t y of 1 mA/cm2 was used f o r the Constant Current process, a value that has been proven to form good q u a l i t y f i l m s [Young, 1961]. The s u r f a c e area of a standard 2 inch S i wafer i s very c l o s e to 20 cm 2, and a c u r r e n t source set to 20 mA was used (Hewlett-Packard 6186C) with a maximum vo l t a g e s e t t i n g of 100 v o l t s . The v o l t a g e a c r o s s the c e l l was recorded as a f u n c t i o n of time, with a ch a r t recorder (Moseley Autograph 7100BM S t r i p Chart Recorder). The anodic o x i d a t i o n of tantalum f i l m s i s a r e l a t i v e l y f a s t process, in 3-5 minutes a f i l m of 100 nm i s f u l l y converted to i t s oxide. The o x i d a t i o n time i s determined from the r a t e of change of the c e l l v o l t a g e , as e x p l a i n e d b e f o r e . T h i s i s i n sharp c o n t r a s t with the time under thermal o x i d a t i o n , a few hours. I t i s q u i t e amazing to observe the sudden change in c o l o r of the Ta s u r f a c e , denoting the formation of an oxide f i l m , i n the f i r s t few seconds. The f i n a l c o l o r was l i g h t blue f o r a 50 nm Ta sample, and g o l d f o r a 100 nm sample, i n agreement with the thermal oxide f i l m s , f o r the same tantalum metal t h i c k n e s s . At t h i s p o i n t the Constant Current process i s i n t e r r u p t e d , to be f o l l o w e d by the a p p l i c a t i o n of a Constant V o l t a g e f o r a c e r t a i n time. The a p p l i e d v o l t a g e corresponds to the o p e r a t i n g v o l t a g e of the MOS C a p a c i t o r , i n t h i s case not more than 20 v o l t s . Based on data given by Del l ' O c a et a l . (1971), 1-3 h r s . , i s the usual time f o r t h i s p r o c e s s . T h e r e f o r e , a constant v o l t a g e of 20 V was a p p l i e d 7 0 Sample M05C10 .1M Citric Acid J=1mA/cm2 Vcell • (V) 60-t (min) 0 1 2 3 A Figure 4.2 Anodization Cell Voltage under Constant Current 71 Sample MOSC10 .1 M Citric Acid V= 15 Volts Icell* (mA) 6-I 54 AH 34 H T 5 ~T"~ 10 15 —r— 20 t (min) 25 Figure 4.3 Anodization C e l l Current under Constant Voltage 72 to the c e l l f o r 1 hr., and the leakage c u r r e n t monitored with, an e l e c t r o m e t e r ( K e i t h l e y 602) d r i v i n g a c h a r t r e c o r d e r . The r e s u l t s of these measurements are shown in F i g u r e s 4.2 and 4.3. Note the sharp change of v o l t a g e under constant c u r r e n t and the d i m i n i s h i n g leakage c u r r e n t under constant v o l t a g e . {4.5.2} ANODIZATION IN H 3PO„ ELECTROLYTE SOLUTION: The c l a s s i c e l e c t r o l y t e f o r anodic o x i d a t i o n of tantalum has been an aqueous s o l u t i o n of c i t r i c a c i d [Berry, 1959 and 1963; Young, 1961; McLean, 1966], however i t was decided that another e l e c t r o l y t e should be a l s o used in order to compare the f i l m q u a l i t y and the r e s u l t i n g MOS c a p a c i t o r performance. The r e s u l t s given by Randall et a l . (1965), i n d i c a t e that an aqueous s o l u t i o n of phosphoric a c i d produces a b e t t e r q u a l i t y f i l m , as i t reduces the i o n i c c o n d u c t i v i t y i n MOS d e v i c e s that use tantalum oxides. S o l u t i o n s of 0.1 M were prepared in both cases, as t h i s c o n c e n t r a t i o n had produced good r e s u l t s f o r p r e v i o u s authors [Young, 1961]. Both e l e c t r o l y t e s o l u t i o n s were used, and d i f f e r e n c e s were apparent once the anodic o x i d a t i o n took p l a c e and the V ( t ) c h a r t s (constant c u r r e n t ) compared. The anodic samples were then r i n s e d i n d e - i o n i z e d water, to remove a l l t r a c e s of the e l e c t r o l y t e s o l u t i o n s , and then b o i l e d i n i s o p r o p y l a l c o h o l to remove any water in the wafers. Appendix III g i v e s these i n f u r t h e r d e t a i l . 73 TABLE 4 . 4 SINGLE DIELECTRIC ANODIC MOS CAPACITORS SAMPLE NAME Ta THICKNESS [A] ELECTROLYTE M0SC9 500 C i t r i c A c i d MOSC10 500 C i t r i c A c i d MOSC11 1 000 Phosphoric A c i d MOSC12 500 Phosphoric A c i d MOSC13 500 C i t r i c A c i d MOSC14 500 Phosphoric A c i d MOSC15 1 000 C i t r i c A c i d MOSC16 500 Phosphoric A c i d MOSC17 500 C i t r i c A c i d MOSC18 1 000 C i t r i c A c i d { 4 . 6 } INTERFACIAL OXIDATION MOS CAPACITORS: A new technique, I n t e r f a c i a l O x i d a t i o n , was used in f a b r i c a t i n g MOS c a p a c i t o r s , as t h i s a llows the o x i d a t i o n of s i l i c o n through a f i l m of t h e r m a l l y grown tantalum pentoxide. T h i s process departed from the one repo r t e d by pr e v i o u s authors [Kato et a l . , 1983], in which a f i l m of tantalum pentoxide was d e p o s i t e d on s i l i c o n s u b s t r a t e s , and then f o l l o w e d by a wet o x i d a t i o n at 800C. The p r i n c i p l e behind t h i s procedure i s that the o x i d i z i n g gases w i l l t r a v e l through the outer T a 2 0 5 f i l m and grow a f i l m at the i n t e r f a c e of S i 0 2 . However, no i n d i c a t i o n was given i n t h e i r paper on the tantalum pentoxide f i l m and i t s o r i g i n s . The i n s u l a t i n g s t r u c t u r e i s then a double d i e l e c t r i c one, with an outer i n s u l a t o r of T a 2 0 5 and an inner i n s u l a t o r of S i 0 2 . 74 The procedure followed by t h i s w r i t e r and Dr. P. Janega, was s l i g h t l y d i f f e r e n t and i t has the advantage of growing thermal oxides f o r both Ta metal and S i . The p r o c e s s i n g steps are as f o l l o w s : 1. Thickness and four p o i n t r e s i s t i v i t y measurements. 2. S c r i b i n g and marking. 3. P e r o x i d e - A c i d c l e a n i n g using the RCA p r o c e s s . 4. RF S p u t t e r i n g of tantalum metal. 5. Thermal o x i d a t i o n of tantalum i n dry oxygen. 6. Wet o x i d a t i o n of s i l i c o n . 7. Aluminium e v a p o r a t i o n f o r gate e l e c t r o d e s . 8. P a t t e r n i n g of A l metal by p h o t o l i t h o g r a p h y . 9. Back con t a c t by g o l d e v a p o r a t i o n . The f i r s t f i v e steps have a l r e a d y been d i s c u s s e d p r e v i o u s l y , and they are d e t a i l e d i n Appendix I I I . The wafers had 500 A of Ta metal d e p o s i t e d by RFS and were o x i d i z e d t h e r m a l l y , f o l l o w i n g the same method as d e s c r i b e d before and given i n Appendix I I I , the only d i f f e r e n c e being an oxygen flow of 1.5 1/min, i n s t e a d of the usual 1 1/min. The c o l o r was deep p u r p l e - b l u e , i n d i c a t i n g that the Ta metal was p r o p e r l y o x i d i z e d . The wafers were then i n t r o d u c e d i n t o the wet o x i d a t i o n furnace at 800C, f o r v a r y i n g p e r i o d s of time and c y c l e s , i n order to determine the e f f e c t of o x i d a t i o n time on the q u a l i t y of the double d i e l e c t r i c s t r u c t u r e . Table 4.5 summarizes the r e s u l t s . 75 TABLE 4.5 INTERFACIAL OXIDATION OF TANTALUM ON SILICON SAMPLE Ta THICKNESS WET OXIDATION CYCLE MTJ 1 500 A 3-54-3 min. MTJ 2 500 A 3-114-3 min. MTJ 3 500 A 3-54-3 min. MTJ 4 500 A 3-24-3 min. MTJ 5 300 A None The wet o x i d a t i o n gas flows are given i n Appendix I I I , the f i r s t d i g i t i n d i c a t e s the time of oxygen flow, the second of oxygen + hydrogen and the l a s t of oxygen only. Examination of the samples showed that the Ta{oxide} f i l m d i d not d e t e r i o r i a t e d u r i n g the wet o x i d a t i o n . Aluminium was de p o s i t e d by use of the E-Beam technique and l a t e r p atterned by p h o t o l i t h o g r a p h y using p o s i t i v e p h o t o r e s i s t . Gold was de p o s i t e d on the wafers back using a l s o the E-Beam equipment. The sample marked MTJ1, with e l e c t r o d e s in p l a c e , was annealed i n n i t r o g e n at 500C f o r 3 min. The remaining ones had no an n e a l i n g treatment. 76 CHAPTER 5 RESULTS AND MEASUREMENTS ON MOS CAPACITORS Once the MOS c a p a c i t o r s and de v i c e s were f a b r i c a t e d , a proper set of systematic mesurement procedures was implemented. In the case of MOS c a p a c i t o r s these c o n s i s t e d o f : 1) E l l i p s o m e t r i c Determination of Oxide T h i c k n e s s . 2) Capacitance-Voltage (C-V) Curves. 3) Curr e n t - V o l t a g e (I-V) Curves. {5.1} ELLIPSOMETRY: The oxide t h i c k n e s s d e t e r m i n a t i o n was done using a Rudolf Model 43603-200E E l l i p s o m e t e r c o n t r o l l e d by a PDP8/E D i g i t a l Equipment C o r p o r a t i o n minicomputer; with a RL-01 d i s c d r i v e , magnetic tape u n i t , A/D and D/A c o n v e r t e r s , running OS/8 under r e a l time and using p r e v i o u s l y r e p o r t e d techniques [Hopper et a l . , 1975; C o r n i s h et a l . , 1973] and software developed by D. Smith (1980). A Spectra P h y s i c s Model 133 helium-neon l a s e r , p r ovided the l i g h t source, a beam of red c o l o r at 632.8 nm. The e l l i p s o m e t e r angle of in c i d e n c e was 70° and measurements made i n zones I and I I I . The e l l i p s o m e t r i c parameters * and. A were then converted to t h i c k n e s s using the tr a n s p a r e n t s i n g l e l a y e r model equations, given the r e f r a c t i v e index of the f i l m . S e v e r a l measurements are u s u a l l y taken, and these are then averaged. 77 The software r e s i d i n g i n the computer c o n t r o l s the r o t a t i o n of a p a i r of small stepper motors, which i n turn d r i v e , through reducing gear boxes the Analyzer and P o l a r i z e r o p t i c s of the E l l i p s o m e t e r . The angular p o s i t i o n of these are a c c u r a t e l y given by two r e s o l v e r s ( s h a f t encoders), coupled v i a reducing gear boxes to the Analyzer and P o l a r i z e r motor d r i v e n u n i t s . The c o n t r o l system i s then a p o s i t i o n feedback type and the software r e s i d e n t i n the computer attempts to f i n d a balance ( n u l l ) of the t r a n s m i t t e d l i g h t as sensed by a photodetector p l a c e d at the output of the Analyzer o p t i c s . An i n i t i a l n u l l or balance i s i n i t i a l l y performed manually, i n order to gi v e a good s t a r t i n g p o i n t to the computer c o n t r o l software, thus minimizing the p o s i b i l i t y of an e r r o r and a l s o reducing the measurement time. {5.2} C-V MEASUREMENTS: The Capacitance-Voltage (C-V) measurements were obtained a l s o by a computer c o n t r o l l e d method [Boyd, 1981]. The measuring system i s shown i n F i g u r e 5.1. A Boonton Model 71A Capacitance Meter, connected to an o p t i c a l l y and e l e c t r i c a l l y s h i e l d e d t e s t j i g box pro v i d e d the ca p a c i t a n c e i n f o r m a t i o n to the computer, v i a the A/D i n t e r f a c i n g u n i t . The b i a s i n g v o l t a g e i s pro v i d e d by a D/A c o n v e r t e r and i t i s o f f s e t by an opposing v o l t a g e source, which can be manually a d j u s t e d to a d e s i r e d v a l u e . T h i s a l l o w s the user to obt a i n a C-V p l o t f o r negative gate v o l t a g e s , without r e s o r t i n g to s p e c i a l and expensive D/A c o n v e r t e r s . The b i a s i n g v o l t a g e i s 7 8 monitored by a Dana Model 5900 D i g i t a l Voltmeter, which i n turn serves as a feedback A/D c o n v e r t e r . Then by appropiate commands from the computer, the gate b i a s v o l t a g e can be incremented and the c a p a c i t a n c e measured. A r e s i d e n t software module, CV.PG was used i n o b t a i n i n g the C-V curves. The source program allows f o r p l o t t i n g the experimental data, a t h e o r e t i c a l curve, c a l c u l a t i n g the f l a t b a n d c a p a c i t a n c e and p r o v i d e s c a l c u l a t e d values of oxide t h i c k n e s s , s u r f a c e s t a t e d e n s i t y , e t c . [Boyd, 1981]. The curves were p l o t t e d by a Houston Instrument Complot XY p l o t t e r , d r i v e n by the PDP8/E minicomputer. T h i s arrangement p r o v i d e d a f a s t and e f f i c i e n t way to o b t a i n the vast amount of C-V curves from a l a r g e amount of samples, which each c o n t a i n e d s e v e r a l MOS c a p a c i t o r s . From the C-V curves, a great amount of i n f o r m a t i o n can be o b t a i n e d . The i n s u l a t o r (oxide) t h i c k n e s s can be c a l c u l a t e d from the accumulation l a y e r c a p a c i t a n c e , i f the c a p a c i t o r area and i n s u l a t o r p e r m i t t i v i t y i s known. The i r r e g u l a r i t i e s i n the curve i n d i c a t e u s u a l l y the presence of s u r f a c e s t a t e s , of the " f a s t " kind, as they f o l l o w the sweep component a p p l i e d to the Capacitance Meter. The h y s t e r e s i s , i f any, and i t s o r i e n t a t i o n can r e v e a l the presence of the mobile charge i n the i n s u l a t o r . By comparing with an i d e a l curve around the o r i g i n , the f l a t b a n d c a p a c i t a n c e and v o l t a g e can be obtained. The f l a t b a n d v o l t a g e i s given by: V f b = V s " Q f / C o x (5.1) 79 PDP8/E BUS PDP8/E BUS •-Capacitance PDP8/E BUS — Voltage Bias BOONTON 71A o C-Meter and Ox ~ 0 9 Tungsten Wire to vac pump Shielded Test Box rass Block Figure 5.1 C-V Measuring System for MOS Capacitors. 80 Where the metal to semiconductor work f u n c t i o n i s given by 0ms and Qf i s the net charge at f l a t b a n d per u n i t area [Sze, 1969]. Then, the f l a t b a n d c a p a c i t a n c e can be c a l c u l a t e d : £ o x C f b ~~ e (5.2) t + o x /kT/e N _ ox q s A,D The s l o p e of the C-V curve can be used to c a l c u l a t e the " f a s t " s u r f a c e s t a t e d e n s i t y : C OX N s s ( f a s t ) = — c T c T — ( A V a " 2 < M (5.3) The q u a n t i t y AVa re p r e s e n t s the a c t u a l slope of the curve and q<p =Eg/2 + kTln {ni/Na} . F The "slow" surface s t a t e s , caused by p o l a r i z a t i o n of the i n s u l a t o r and mobile i o n i c charges, produces h y s t e r e s i s in the C-V curve. A good estimate of these i s given by [North, 1980]: C N i n \ = — — — AV, (5 4) ss(slow) _ , h \o..t> 3qcj)F Where the h y s t e r e s i s width around f l a t b a n d i s represented by AVh. The C-V data can be r e p l o t t e d i n the form of the i n v e r s e of the c a p a c i t a n c e squared v s . the gate b i a s v o l t a g e (1/C 2 vs V). L i n e a r e x t r a p o l a t i o n of the r e p l o t t e d data to the 81 v o l t a g e a x i s g i v e s the d i f f u s i o n or b u i l t - i n p o t e n t i a l [Sze, 1969]. T h i s method was used in papers by Padmanahban (1975) in r e l a t ion with W O 3 and Mo0 3 f i l m s , and by Makus (19*77) in t h e i r work on V 2 0 5 . {5.3} I-V MEASUREMENTS: The I-V data was obtained by measuring the DC c u r r e n t flow through the MOS c a p a c i t o r s , under both p o s i t i v e (Al e l e c t r o d e ) and negative gate voltage.. A computer source program was w r i t t e n by t h i s author, that would s e q u e n t i a l l y increment the gate b i a s v o l t a g e and then read the c u r r e n t f l o w i n g i n the c i r c u i t . Then the data i s s t o r e d t e m p o r a r i l y and the user has the o p t i o n of p l o t t i n g a l i n e a r (I vs. V) p l o t or a so c a l l e d Schottky ( l o g I vs. i/V) . The d e t a i l s of the source program w r i t t e n i n FORTRAN IV which runs i n the PDP/8E minicomputer, are given i n Appendix I I . The I-V measuring system i s shown in F i g u r e 5.2. A K e i t h l e y Model 602 E l e c t r o m e t e r i s used to measure the leakage c u r r e n t , which in turn i s connected to a K e i t h l e y Model 399 I s o l a t i n g ( b u f f e r ) A m p l i f i e r . Then i t feeds a Tyco Model 404 DVM-A/D co n v e r t e r , which i s i n t e r f a c e d with the computer. The gate v o l t a g e i s p r o v i d e d by a D/A c o n v e r t e r , d r i v e n by the r e s i d e n t software i n the minicomputer, and i t i s monitored by a Dana Model 5900, which serves as both i n d i c a t i n g DVM and A/D c o n v e r t e r . There i s a p r o t e c t i v e c u r r e n t l i m i t i n g r e s i s t o r p l a c e d d i r e c t l y at the output of the D/A c o n v e r t e r , u s u a l l y set to 100 ohms, which has a n e g l i g i b l e e f f e c t on the measurements. T h i s arrangement allows use of the 82 computer and i n t e r f a c i n g c i r c u i t s to a c q u i r e I-V data i n a f a s t and e f f i c i e n t way. The software was w r i t t e n so that the user chooses the instrument s c a l e s , maximum gate v o l t a g e and number of increments, which are entered v i a keyboard commands i n t o the computer. There i s a c h o i c e of p l o t t i n g the data, with and without axes ( f o r s e v e r a l curves i n the same shee t ) , and of a L i n e a r or Schottky graph. A l l s c a l i n g i s done a u t o m a t i c a l l y and the axes l a b e l l e d a c c o r d i n g the type of p l o t and range of recorded v a l u e s . A l l measurements were made in an e l e c t r i c a l l y and o p t i c a l l y s h i e l d e d box, which c o n t a i n e d the t e s t j i g . Care was taken to a v o i d ground loops and n o i s e , as the measured leakage c u r r e n t s were in the nA range. E x t e n s i v e use of s h i e l d e d low l o s s c a b l e s was made and the t e s t j i g was cleaned with i s o p r o p y l a l c o h o l , before each batch of mesurements were taken. T h i s ensured that any s u r f a c e leakage of the exposed conductors (due to dust, contamination or water vapour) was minimal. The i n t e n t i o n of p r o v i d i n g the I-V data i n L i n e a r or Schottky form i s i n the f i r s t case to g i v e an o v e r a l l idea of the I-V c h a r a c t e r i s t i c and i n the second to o b t a i n more information, of the i n s u l a t o r , by examining the slope of l o g I vs. i/V. From t h i s one, i t i s p o s s i b l e to c a l c u l a t e the o p t i c a l (high frequency) value of the i n s u l a t o r p e r m i t t i v i t y . However, the c l a s s i c q u e s t i o n of whether the conduction mechanism i s Schottky or P o o l e - F r e n k e l has to be answered f i r s t , otherwise the c a l c u l a t e d er(°°) w i l l be i n e r r o r by a f a c t o r of 2. T h i s can be seen from the e x p r e s s i o n 8 3 Shielded Test Box PDP67E BUS PDP8/E BUS PDP8/E BUS Isolating Amp, Keithley 399 Figure 5.2 I-V Measuring System for MOS Capacitors 84 of the P-F c o n d u c t i o n : J = quN E expC-qA + 3 f /E) /kT ( 5 g ) P P When t h i s e x p r e s s i o n i s p l o t t e d i n the Schottky form, and the p l o t i s a s t r a i g h t l i n e , i t s slope g i v e s the Poole-F r e n k e l lowering constant /3pf : 0 = { 2 f 2 (5.6) o r The p e r m i t t i v i t y can then be c a l c u l a t e d from: 2 TT B f e r ( o o ) = E - g — ( 5 < 7 ) e a o -On the other hand, the Schottky conduction c u r r e n t d e n s i t y i s given by: J s = A*T exp ( - q c f ) s + J23 p f/E)/kT (5.8) Where A* i s the Richardson c o n s t a n t . The s l o p e , i f the Schottky p l o t i s a s t r a i g h t l i n e , i s then h a l f of the Poole-F r e n k e l mechanism. U n f o r t u n a t e l y , there i s no easy way of determining whether the conduction i s P-F or S, from the examination of a s t r a i g h t l i n e p l o t i n the form of l o g I vs. i/V. More i n f o r m a t i o n from the conduction process i s r e q u i r e d , and i t has been suggested that from i n t e r n a l photoemission measurements [ D e l l ' O c a , et a l . , 1971], the F r e n k e l vs. Schottky dilemma can be s o l v e d . The 85 photoemission t h r e s h o l d energy i s a f u n c t i o n of the a p p l i e d v o l t a g e a c r o s s the i n s u l a t o r , at the i n t e r f a c e between the metal and i n s u l a t o r , hence d e f i n i n g Schottky type emission [Goodman, 1968]. In t h e i r now c l a s s i c paper, Angle and T a l l e y (1978) i n d i c a t e d that the kind of conduction mechanism can be determined by examining the forward (gate p o s i t i v e ) and reverse (gate negative) b i a s Schottky p l o t s . If the s l o p e s are the same and have n e a r l y the same i n t e r c e p t , under both reverse and forward b i a s , Poole-F r e n k e l conduction i s t a k i n g p l a c e ; i . e . , the conduction i s b u l k - l i m i t e d and not e l e c t r o d e l i m i t e d . I f Schottky emission i s r e s p o n s i b l e , then the d i f f e r e n c e s i n work f u n c t i o n s between the A l and the S i s u b s t r a t e s w i l l l e a d to forward and reverse c u r r e n t s of s e v e r a l orders of magnitude d i f f e r e n t . As i t can be seen l a t e r , not a l l the T a 2 0 5 samples have measured s t r a i g h t l i n e Schottky p l o t s , which suggests that the r e a l conduction mechanism i s more e l a b o r a t e than the a l r e a d y proposed ones. Furthermore, forward and reverse b i a s p l o t s have l a r g e c u r r e n t d i f f e r e n c e s of s e v e r a l orders of magnitude. {5.4} HILLOCK FORMATION INVESTIGATION: Through our experimental work, i t was found that when Ta metal was RF Sputtered on c l e a n S i s u b s t r a t e s , h i l l o c k s or "measles" appeared on the s u r f a c e . These small protuberances are formed probably as a consequence of thermal s t r e s s e s on the Ta metal d u r i n g and a f t e r d e p o s i t i o n 86 [Miner, 1981]. Close examination with the microscope under dark f i e l d ( F i g u r e s 5.3 to 5.8) r e v e a l s a " s t a r sky" or "milky way" p a t t e r n , which i n d i c a t e s a r e l a t i v e l y high area d e n s i t y of h i l l o c k s . However, i t was found that t h i s i s q u i t e t y p i c a l of RF Sputtered tantalum [Galeener et a l . , 1980, Westwood et a l . , 1975], and an attempt was made to e s t a b l i s h a p o s s i b l e l i n k between contamination l e v e l s , h i l l o c k s and p o s s i b l y p i n h o l e formation. It i s i n t e r e s t i n g to n o t i c e that i n the paper by Galeener et a l . , niobium pentoxide e x h i b i t s a s i m i l a r s u r f a c e c o n d i t i o n , but to a l e s s e r degree. These authors a l s o r e p o r t e d that i n s i l i c o n n i t r i d e , h i l l o c k s were not seen. They i n d i c a t e d that these are q u i t e s m a l l , i n the case of Ta, 100 A i s a t y p i c a l l a t e r a l dimension. H i l l o c k growth a l s o appears as an unwanted source of d e f e c t s i n evaporated A l over S i s u b s t r a t e s , and they have l a r g e dimensions, t y p i c a l l y a height of 10 times the d e p o s i t e d t h i c k n e s s . T h i s c r e a t e s a very s e r i o u s problem i f the p a s s i v a t i o n or p h o t o r e s i s t f a i l s to cover them [Santoro and T o l l i v e r , 1971]. In more recent work [Jakson and L i , 1982], the h i l l o c k and v o i d growth on t h i n f i l m s (1 um or l e s s ) d e p o s i t e d on r e l a t i v e l y massive s u b s t r a t e s i s governed by thermal compressive s t r e s s e s and the growth k i n e t i c s are c o n t r o l l e d by a bulk process, as opposed to g r a i n boundary d i f f u s i o n . Under o b s e r v a t i o n , h i l l o c k s w i l l grow and s h r i n k with temperature changes, however a c e r t a i n degree of h y s t e r e s i s e x i s t s , as repeated c y c l i n g promotes growth. In order to q u a n t i f y the p o s s i b l e e f f e c t of contamination 87 (mainly dust p a r t i c l e s ) , c l e a n and " d i r t y " g l a s s samples (Corning 7059) were prepared and introduced to the RF S p u t t e r i n g equipment. Tantalum d e p o s i t i o n was done i n an argon gas atmosphere at a r a t e of 100 A/min., under a forward RF power of 120 W. R e s i d u a l gases i n d i c a t e d a pressure of 1-2X10~ 6 T o r r , and the argon p a r t i a l p r essure was 25-30 mTorr. Microscope examination under dark f i e l d f o l l o w e d . Only a r e l a t i v e measure i s given, as no a c t u a l count of the number of h i l l o c k s per u n i t area was done. R e s u l t s are given i n Table 5.1. T A B L E 5 . 1 R F S P U T T E R I N G O F T a O N G L A S S S A M P L E S S A M P L E N A M E C O N D I T I O N T H I C K N E S S H I L L O C K D E N S I T Y G1 0 0 C l e a n 1 0 0 A N o r m a l G 5 0 0 C l e a n 5 0 0 A N o r m a l G 5 0 0 D . D i r t y 5 0 0 A H i g h I G 5 0 0 C l e a n 5 0 0 A N o r m a l 3 1 0 1 8 1 C C l e a n 5 0 0 A N o r m a l 3 1 0 1 8 1D D i r t y 5 0 0 A N o r m a l 0 4 0 2 8 1 C C l e a n 5 0 0 A ( * ) L o w 0 4 0 2 8 1 D D i r t y 5 0 0 A ( * ) L o w 1 1 0 2 8 1 C l C l e a n 5 0 0 A ( # ) V e r y L o w 1 1 0 2 8 1 C 2 C l e a n 5 0 0 A ( # ) V e r y L o w 1 8 0 2 8 1 C 1 C l e a n 5 0 0 A ( $ ) L o w 1 8 0 2 8 1 C 2 C l e a n 5 0 0 A ( $ ) L o w 1 8 0 2 8 1 C 1 C l e a n 1 0 0 0 A (fc) H i g h 1 8 0 2 8 1 C 2 C l e a n 1 0 0 0 A ( & ) H i g h N o t e s : ( * ) 0 . 2 2 nm f i l t e r i n s t a l l e d i n A r g o n l i n e . ( # ) R F S E q u i p m e n t t h o r o u g h l y c l e a n e d . ( $ ) V a c u u m 1 0 " 7 T o r r . ( & ) S a m p l e s p u t t e r e d a g a i n . 8 9 I t i s not c o n c l u s i v e whether the c l e a n l i n e s s of the g l a s s sample i s r e l a t e d to the d e n s i t y of h i l l o c k s , however there i s some c o r r e l a t i o n beteween the contamination of. the RFS equipment and the amount of h i l l o c k s . I t appears that the e f f e c t of c l e a n i n g , decontaminating and i n s t a l l i n g a submicron f i l t e r i n the Ar gas l i n e produced some p o s i t i v e e f f e c t s , as the h i l l o c k growth d i m i n i s h e d . There seems to be a l s o some c o r r e l a t i o n with the s t a b i l i t y of the plasma du r i n g s p u t t e r i n g . E x p e r i m e n t a l l y , we observed that when the plasma behaved in an e r r a t i c way, with secondary a r c s moving randomly, the h i l l o c k growth was a f f e c t e d . No c o n c l u s i v e evidence c o u l d be found, as the number of samples i s too low, the randomness of the secondary a r c s cannot be a c c u r a t e l y d e s c r i b e d , and not i n a l l cases was there a n o t i c e a b l e e f f e c t . Microscope examination under dark f i e l d c o n d i t i o n s show no c o r r e l a t i o n with o b s e r v a t i o n s done in a t r a n s m i s s i o n microscope (Union O p t i c a l , m e t a l l u r g i c a l , Model MeC-3Bi) in attempting to f i n d p i n h o l e s . The g l a s s samples examined with the t r a n s m i s s i o n u n i t gave very l i t t l e p i n h o l e count (10-20) over the e n t i r e 2" wafer. F i g u r e s 5.3 to 5.8 show the " s t a r sky" p a t t e r n f o r s e v e r a l Ta-RFS g l a s s and S i l i c o n samples, and Ta-MES on s i l i c o n . I t appears that the MES samples (BNR s u p p l i e d ) have l e s s h i l l o c k d e n s i t y , although the m a g n i f i c a t i o n i s h i g h e r . Upon examination under the t r a n s m i s s i o n u n i t , the p i n h o l e d e n s i t y i s l e s s than the RFS samples, which again i n d i c a t e s perhaps a b e t t e r q u a l i t y Ta f i l m f o r the MES samples. Figure 5.4 Dark F i e l d Photograph (560X), 500 A Ta Magnetron Enhanced Sputtered on S i l i c o n (sample BNR500). F i g u r e 5.5 Dark F i e l d Photograph (140X), 200 A Ta RF Sputtered on t h i n S i 0 2 on S i l i c o n (sample 4T6). F i g u r e 5.6 Dark F i e l d Photograph (140X), 50 A Ta RF Sputtered on t h i n S i 0 2 on S i l i c o n (sample 2T6). F i g u r e 5.7 Dark F i e l d Photograph (140X), 500 A Ta RF Sputtered on t h i n S i 0 2 on S i l i c o n (sample 3T7). • F i g u r e 5.8 Dark F i e l d Photograph (140X), 1000 A Ta RF Sputtered on t h i n S i 0 2 on S i l i c o n (sample 4T7). 9 3 {5.5} DISCUSSION OF RESULTS: {5.5.1} ELLIPSOMETRY: Data obtained from the samples BNR500 and BNR1000, i n which the e l l i p s o m e t r i c parameters * and A were p e r i o d i c a l l y measured dur i n g the dry thermal o x i d a t i o n , i s shown i n graph form i n F i g u r e 5.9 and 5.10. Sample BNR500 (500 A Ta t h i c k n e s s ) e x h i b i t s a r a t h e r sharp peak of 48 degrees f o r * at 15 min, as opposed to a broad peak of 50 degrees but at 60 min o x i d a t i o n time f o r f o r sample BNR1000 (1000 A Ta t h i c k n e s s ) . The parameter A decreases m o n o t o n i c a l l y with i n c r e a s i n g time. The apparent d i s c o n t i n u i t y of the A curves i s due to i t s modulo 2n p r o p e r t y , i . e . , they- repeat themselves a f t e r 360 degrees. It i s expected that both parameters assume a constant value a f t e r long time. I n d i c a t i o n s of t h i s are apparent from the BNR500 sample curves, and a t r e n d i s given in the curves f o r the sample BNR1000. T h i s i n d i c a t e s t h a t , a f t e r a p e r i o d of time, slow changes take p l a c e i n the e l l i p s o m e t r i c p r o p e r t i e s of the oxide. F o l l o w i n g Smith and Young (1981), t h i s i s i n t e r p r e t e d as the metal being e n t i r e l y converted i n t o oxide, with slow changes a t t r i b u t e d to changes in s t o i c h i o m e t r y and s t r u c t u r a l a n n e a l i n g . I t i s i n t e r e s t i n g to observe that the "P and A curves vs. time o b t a i n e d i n t h i s work are s i m i l a r to the ones p u b l i s h e d by these authors. They report a sharp peak of 46 degrees at 20 min. f o r the * parameter, f o r a 400 A t h i c k tantalum f i l m that i s t h e r m a l l y o x i d i z e d . A s i m i l a r s i t u a t i o n e x i s t s f o r A, i n which the 9 4 360 n r- 80 Time(min) Figure 5.9 Ellipse-metric Data vs. Time, Sample BNR500 95 Figure 5.10 Ellipsometric Data vs. Time, Sample BNR1000 9 6 s t a r t i n g value i s c l o s e to 130 degrees, and the curve f o l l o w s a s i m i l a r shape. The s h i f t i n the BNR1000 curve, as compared with the BNR500 sample, represents the a d d i t i o n a l time r e q u i r e d to f u l l y o x i d i z e a t h i c k e r (twice) f i l m of tantalum metal. We can then s a f e l y say that a 500 A Ta f i l m w i l l be f u l l y o x i d i z e d i n the above sense a f t e r 75 min., and that a longer time, estimated to be 120 min. i s r e q u i r e d to o x i d i z e a 1000 A Ta f i l m sample. F i n a l l y , a p l o t of A vs. * was made, as the f i l m i s grown i n both BNR500 and BNR1000 samples. F i g u r e s 5.11 and 5.12 are computer p l o t s of a c t u a l data. These graphs can be termed " t r a n s i e n t " , as the f i l m grows, the e l l i p s o m e t r i c parameters approach t h e i r f i n a l value a f t e r a c e r t a i n time, i . e . , the "steady s t a t e or permanent" c o n d i t i o n . I t i s i n t e r e s t i n g to n o t i c e that the f i n a l values r e s t on a locus that corresponds to the general shape of the A-* curves f o r a grown oxide, and that i n i t i a l l y l a r g e changes of A take p l a c e with small changes of T h i s c o u l d be a property p e c u l i a r to the growth of thermal tantalum pentoxide that should be f u r t h e r i n v e s t i g a t e d . Thickness d e t e r m i n a t i o n of tantalum oxide i n s i l i c o n s u b s t r a t e s was made on s e v e r a l samples. In these, a r e f r a c t i v e index of 2.22 f o r the T a 2 0 5 [Young, 1961; Smith and Young, 1981] with a s u b s t r a t e index of 3.86-J0.025 was used to perform the t h i c k n e s s computation using the program ANALYS [Boyd, 1981], runing i n the PDP8/E minicomputer. T h i s program i s based on the s i n g l e l a y e r , o p t i c a l l y t r ansparent 9 7 a? <0 •=* v c-CM" 45 60 * 20 *25 35 * 40 50 *65 75 •»• 30 LU Q as LU a c *T=0 min os-* 1 0 0.00 15 T 1~ T T 1 IS. 00 30.00 ^S.00 CO.00 7S.00 30.00 PSI / DECKCeC Figure 5.11 Transient Ellipsometry, Sample BNR500 98 CT * 6 0 6 5 * * VO 7 5 rvi" to CO QL L U • en - I IT) * x = 0 m i n B . BE * 10 2 5 2 0 3 5 3 0 • ^ 4 0 4 5 ^ 5 5 * 5 0 T T " T -1S.H0 30.BB 1S.BB CB.BB P S I / DEGREES —I 7S. BB —1 30 . BB Figure 5.12 Transient Ellipsometry, Sample BNR1000 99 model and no allowances are made f o r the t r a n s i t i o n l a y e r at the i n t e r f a c e between the Ta{oxide} and s u b s t r a t e [Revesz et a l . , 1974 and 1976 ( f o r the graded r e f r a c t i v e index model); Smith and Young, 1981 ( f o r the tapered model)]. The r e s u l t s are given i n Table 5.2. TABLE 5.2 OXIDE THICKNESS DETERMINATION BY ELLIPSOMETRY SAMPLE Ta THICKNESS PROCESS OXIDE THICKNESS N2 400 A Thermal 1173.3 A N3 400 A Thermal 1173.2 A MOSC17 500 A Anodic 890.5 A MOSC 1 8 1 000 A Anodic 1530.1 A MTEST.T 500 A Thermal 792.4 A MTEST.A 500 A Anodic 893.3 A {5.5.2} C-V CURVES These curves were obtained by the method alr e a d y d e s c r i b e d , using the program CV [Boyd, 1981]. The shape of these v a r i e d , and i t depended on s e v e r a l parameters such as the oxide t h i c k n e s s , whether the oxide i s anodic or thermal, s u b s t r a t e , p r o c e s s i n g , and a n n e a l i n g . In the case of thermal grown oxides, the temperature of o x i d a t i o n i s a l s o a f a c t o r . With the anodic o x i d e s , the e l e c t r o l y t e s o l u t i o n a l s o p l a y s a r o l e . Surface s t a t e s appear i n some curves, and they tend to d i s t o r t i t s shape, p a r t i c u l a r l y around the i n v e r s i o n r e g i o n . I t was n o t i c e d that i n the A l - T a 2 0 5 - S i samples, the accumulation region was d i f f i c u l t to o b t a i n , independently 100 of the s u b s t r a t e type. T h i s was not the case i n the double d i e l e c t r i c samples, i . e . , those with A l - T a 2 0 5 - S i 0 2 - S i s t r u c t u r e . However, i n some cases, d i f f i c u l t y was encountered i n o b t a i n i n g a d e f i n e d i n v e r s i o n ; i . e . , the curve i s q u i t e "noisy", i n d i c a t i n g the presence of slow s u r f a c e s t a t e s ( s i n c e they f o l l o w the r e l a t i v e l y slow gate v o l t a g e ramp). The f a c t that accumulation i s d i f f i c u l t to o b t a i n i n the s i n g l e d i e l e c t r i c samples, i n d i c a t e s that r e l a t i v e l y l a r g e conduction (leakage) c u r r e n t s are f l o w i n g . T h i s c o r r e l a t e s with the o b t a i n a b l e accumulation region i n the double d i e l e c t r i c MOS c a p a c i t o r s , in which the S i 0 2 l a y e r prevents e x c e s s i v e e l e c t r o n i c c onduction. Furthermore, examination of the I-V curves, as shown l a t e r , c o r r o b o r a t e s t h i s a n a l y s i s . Another i n t e r e s t i n g c h a r a c t e r i s t i c of the s i n g l e d i e l e c t r i c c a p a c i t o r s , i s that a r e d u c t i o n of ca p a c i t a n c e manifests i t s e l f when approaching the accumulation region from a weak i n v e r s i o n . T h i s phenomena i s not w e l l understood, and i t seems to be r e l a t e d to a decrease i n apparent c a p a c i t a n c e due to an i n c r e a s e i n leakage c u r r e n t s at the onset of accumulation. S i m i l a r r e s u l t s were observed by other r e s e a r c h e r s [ N i s h i o k a , 1984], In some cases, l a r g e h y s t e r e s i s i n the C-V curve was observed, probably due to the mobile ( i o n i c ) oxide charge i n the d i e l e c t r i c . However, the lack of c o n s i s t e n c y i n o b t a i n i n g curves with h y s t e r e s i s i n d i c a t e s that t h i s e f f e c t i s process dependent and not a d e f i n e d p r o p e r t y of the i n s u l a t o r . For example, two anodic grown T a 2 O s samples (MOSC 9 and 10) show l a r g e h y s t e r e s i s , but upon f a b r i c a t i n g more 101 samples with the same c i t r i c a c i d p r o c e s s , very l i t t l e , o r no h y s t e r e s i s i s observed (samples MOSC 13, 15, 17 and 18). One double d i e l e c t r i c MOS c a p a c i t o r sample (4T7, 1000 A Ta th i c k n e s s over 50 A of S i 0 2 ) e x h i b i t s a l a r g e h y s t e r e s i s of 5 V, approaching the very l a r g e h y s t e r e s i s curves obtained by Angle and T a l l e y (1978) f o r t h e i r memory c a p a c i t o r s . Another p e c u l a r i t y p e r t i n e n t to the s i n g l e T a 2 0 5 MOS c a p a c i t o r s which were processed t h e r m a l l y at 600C (an inc r e a s e of 100C above the "normal" temperature of 500C), was the t o t a l absence of MOS c a p a c i t o r behaviour and very l a r g e conduction c u r r e n t s . T h i s i s p o s s i b l y due to a change in the c r y s t a l s t r u c t u r e , from amorphous ( i . e . , short range order) to c r y s t a l l i n e ( l a r g e range o r d e r ) . Then r e -c r y s t a l l i z a t i o n takes p l a c e and the i n s u l a t o r behaves more l i k e a conductor than an i n s u l a t o r . Previous work i n d i c a t e s that T a 2 0 5 r e c r y s t a l l i z e s at 650-700C f o r r e a c t i v e l y s p u t t e r e d f i l m s annealed i n n i t r o g e n [Kimura et a l . , 1983]. No f i e l d e f f e c t was observed f o r sample MOSC 3 and 4 processed at 600C. However, one sample (MOSC6, 500 A Ta) produced a reasonable C-V curve, except that a l a r g e decrease i n ca p a c i t a n c e was recorded at accumulation and that leakage c u r r e n t s were a l s o h i g h . The C-V r e s u l t s are presented i n a summary form i n Tables 5.3 5.4 and 5.5, f o r the MOS S i n g l e D i e l e c t r i c (MOS-SD) thermal the MOS Double D i e l e c t r i c (MOS-DD) and MOS S i n g l e D i e l e c t r i c C a p a c i t o r s . By using the value of c a p a c i t a n c e i n deep accumulation, which i s the oxide c a p a c i t a n c e Cox, i t i s p o s s i b l e to 102 c a l c u l a t e the r e l a t i v e d i e l e c t r i c constant from: C t ox ox e = (5.9) A £ 0 The area A i n most cases i s 0.7854x10" 6 m2, that of a 1 mm diameter dot. The t h i c k n e s s tox, was obtained from e l l i p s o m e t e r measurements or from c a l c u l a t i o n s using the s w e l l i n g f a c t o r . The c a l c u l a t e d r e l a t i v e d i e l e c t r i c constant from the C-V curves i s given i n Tables 5.6 (thermal) and 5.7 ( a n o d i c ) . The value of the f l a t b a n d v o l t a g e Vfb ( i . e . , that which makes the s u r f a c e p o t e n t i a l \//s=0, thus producing f l a t b a n d s ) , i s o b t a i n e d from the C-V p l o t by f i r s t c a l c u l a t i n g the f l a t b a n d c a p a c i t a n c e Cfb with the equation that i n v o l v e s the Debye l e n g t h [Sze, 1969; p.435]: R - 1 cfb ZIZZZZ (5.io) 1/C +(/kT/e N , / a ) ' ox v ' s A D ' 1 1 The value of the s u b s t r a t e doping Na or Nd i s obtained from the four p o i n t r e s i s t i v i t y measurements and I r v i n ' s c h a r t s . T h i s value of Cfb i s then entered i n t o the C-V curve and a c o r r e s p o n d i n g Vfb i s obtained. With the l a t t e r i t i s p o s s i b l e to c a l c u l a t e the f i x e d charge i n the oxide Qfc [Sze, 1969; p.468]: C Q = —22E_(d> - v „ ) n type (5.11) rc q rtis ID C Q f c = q ( V f b + W p type (5.12) 1 03 T h i s i s a s t r a i g h t f o w a r d c a l c u l a t i o n i f the v a l u e of 0ms, the m e t a l - s e m i c o n d u c t o r work f u n c t i o n d i f f e r e n c e i s known. However, i n s u f f i c i e n t d a t a or none i s a v a i l a b l e on the work f u n c t i o n p r o p e r t i e s of m e t a l s on T a 2 0 5 - S i s t r u c t u r e s . I t i s p o s s i b l e t o compute the v a l u e of 0ms from the e x p r e s s i o n o b t a i n e d by a n a l y s i s of the MIS band diagram [ G l a s e r , Suback-Sharpe, 1977; Sze, 1969]: ^ms = *m " ( x + E g / 2 q " V n type (5.13a) *ms = * m " ( x + E g / 2 q + V P type (5.13b) Where 0m and x are the m e t a l work f u n c t i o n and the s i l i c o n e l e c t r o n a f f i n i t y r e s p e c t i v e l y . In the band diagram f o r the A l - T a 2 0 5 - S i s t r u c t u r e [ A n g l e , 1976] the f o l l o w i n g v a l u e s a r e used [ S z e , 1969]: q0m=4.1 eV qx=4.45 eV The bandgap energy Eg of the s i l i c o n s emiconductor i s taken here t o be 1.12 eV. The l a s t q u a n t i t y \pF i s a f u n c t i o n of s u b s t r a t e i m p u r i t y c o n c e n t r a t i o n : kT ^ F = — — I n (.n-/tog) n type (5.14a) kT i>j? = — — l n ( N A / n ± ) p type (5.14b) 104 Were \//F i s the d i f f e r e n c e between the Fermi l e v e l of the semiconductor s u b s t r a t e and i t s i n t r i n s i c v a l u e . Using an average doping c o n c e n t r a t i o n of 4.5xl0 1* cm""3 and an i n t r i n s i c c o n c e n t r a t i o n of 1 . 8x10 10 cm" 3, the c a l c u l a t e d value of \pF i s -0.263 V f o r n type and +0.263 V f o r p type s u b s t r a t e s . With these q u a n t i t i e s , the values of 0ms f o r n and p m a t e r i a l can be c a l c u l a t e d : 0ms=-O.647 V n-Si s u b s t r a t e 0ms=-1.173 V p - S i s u b s t r a t e F i n a l l y , the f l a t b a n d v o l t a g e , c a p a c i t a n c e and f i x e d oxide charge i s given i n Tables 5.8 (thermal) and 5.9 ( a n o d i c ) . 1 T A B L E 5 . 3 R E S U M E O F T H E R M A L O X I D E M O S - S D C A P A C I T O R S C - V C U R V E S S A M P L E / T H I C K N E S S T E M P / T I M E A C C U M U L A T I O N C A P . C O M M E N T S N 2 / 4 0 0 A 5 0 0 C / 8 0 m i n 5 7 5 0 0 p F / c m 2 A c c . D i f f . N 3 / 4 0 0 A 5 0 0 C / 3 2 0 m i n 8 5 0 0 p F / c m 2 A c c . D i f f . B N R 5 0 0 / 5 0 0 A 5 0 0 C / 9 3 m i n 7 6 2 5 0 p F / c m 2 L o w H y s t . B N R 1 0 0 0 / 1 0 0 0 A 5 0 0 C / 1 8 7 m i n 9 4 0 6 3 p F / c m 2 A c c . D i f f . S a m p l e A / 5 0 0 A 5 0 0 C / 2 1 O m i n 1 6 0 0 0 0 p F / c m 2 A c c . D i f f . S a m p l e B / 1 0 0 0 A 5 0 0 C / 2 1 O m i n 8 4 0 0 0 p F / c m 2 A c c . D i f f . 1 0 0 0 A M O S / 1 0 0 0 A 5 0 0 C / 3 6 0 m i n 1 2 0 0 0 0 p F / c m 2 P e a k A c c . 5 0 0 A L i f t / 5 0 0 A 5 0 0 C / 3 9 0 m i n 1 7 5 0 0 0 p F / c m 2 A c c . D i f f . M O S C 1 / 5 0 0 A 4 0 0 C / 3 0 0 m i n n / a n / a N o MOS C a p . M O S C 2 / 1 0 0 0 A 4 0 0 C / 4 2 0 m i n n / a n / a N o MOS C a p . M O S C 3 / 5 0 0 A 6 0 0 C / 3 0 0 m i n n / a n / a N o MOS C a p . M O S C 4 / 1 0 0 0 A 6 0 0 C / 4 2 0 m i n n / a n / a N o MOS C a p . M O S C 6 / 5 0 0 A 6 0 0 C / 3 0 0 m i n 1 3 0 0 0 0 p F / c m 2 P e a k A c c . M O S C 7 / 1 0 0 0 A 4 0 0 C / 1 w k 1 5 0 0 0 0 p F / c m 2 P e a k A c c . M O S C 8 / 1 0 0 0 A 6 0 0 C / 4 2 0 m i n 9 8 7 5 0 p F / c m 2 P e a k A c c . N o t e s : 1 . A c c . D i f f . : A c c u m u l a t i o n D i f f i c u l t . 2 . H y s t . : H y s t e r i s i s . 1 0 6 T A B L E 5 . 4 R E S U M E O F T H E R M A L O X I D E M O S - D D C A P A C I T O R S C-S A M P L E / T H I C K N E S S T E M P / T I M E A C C U M U L A T I O N C A P . 1 T 6 / 2 0 S 2 0 T A 5 0 0 C / 3 m i n 3 3 8 7 5 p F / c m 2 5 0 0 C / 7 . 5 m i n 2 7 0 3 2 p F / c m 2 2 T 6 / 2 0 S 5 0 T A 3 T 6 / 2 0 S 1 0 0 T A 4 T 6 / 2 0 S 2 0 0 T A 1 T 7 / 5 0 S 1 0 0 T A 2 T 7 / 5 0 S 2 0 0 T A 3 T 7 / 5 0 S 5 0 0 T A 5 0 0 C / 1 5 m i n 5 0 0 C / 3 0 m i n 5 0 0 C / 1 5 m i n 5 0 0 C / 3 0 m i n 5 0 0 C / 7 5 m i n 2 8 4 3 8 p F / c m 2 2 8 7 5 0 p F / c m 2 1 3 3 7 5 p F / c m 2 2 4 0 0 0 p F / c m 2 2 0 0 0 0 p F / c m 2 4 T 7 / 5 0 S 1 0 0 0 T A 5 0 0 C / 1 5 0 m i n 2 0 5 0 0 p F / c m : •V C U R V E S C O M M E N T S I n v . S S S m o o t h L o w H y s t . I n v . S S I n v . S S P e a k A c c . L o w H y s t . L a r g e H y s t N o t e s : 1 . I n v . S S : I n v e r s i o n w i t h S u r f a c e S t a t e s . TABLE 5.5 RESUME OF ANODIC OXIDE MOS-SD CAPACITORS C-V CURVES SAMPLE/THICKNESS PROCESS ACCUMULATION CAP. COMMENTS MOSC9/500 A C i t r i c Ac i d 25000 pF/cm 2 Large Hyst MOSC10/500 A C i t r i c Ac i d 1 5000 pF/cm 2 Large Hyst MOSC11/1000 A Phosp . Ac i d 76875 pF/cm 2 Peak Acc. MOSC12/500 A Phosp .Acid 87500 pF/cm 2 Peak Acc. MOSC13/500 A C i t r i c Ac i d 431 25 pF/cm 2 A c c . D i f f . MOSC14/500 A Phosp .Acid 70000 pF/cm 2 A c c . D i f f . MOSC15/1000 A C i t r i c Ac i d 51875 pF/cm 2 Smooth MOSC16/500 A Phosp . Ac i d 88750 pF/cm 2 Peak Acc. MOSC17/500 A C i t r i c Ac i d 77000 pF/cm 2 Low Hyst. MOSC18/1000 A C i t r i c Ac i d 82750 pF/cm 2 Peak Acc. T A B L E 5 . 6 C A L C U L A T E D R E L A T I V E D I E L E C T R I C C O N S T A N T O F T H E R M A L T a 2 S A M P L E T a T H I C K N E S S T E M P / T I M E e r N 2 4 0 0 A 5 0 0 C / 8 0 m i n 5 . 5 7 N 3 4 0 0 A 5 0 0 C / 3 2 0 m i n 0 . 8 2 B N R 5 0 0 5 0 0 A 5 0 0 C / 9 3 m i n 9 . 2 4 B N R 1 0 0 0 1 0 0 0 A 5 0 0 C / 1 8 7 m i n 2 2 . 7 8 S a m p l e A 5 0 0 A 5 0 0 C / 2 1 O m i n 1 9 . 3 8 S a m p l e B 1 0 0 0 A 5 0 0 C / 2 1 O m i n 2 0 . 3 4 1 0 0 0 A M O S 1 0 0 0 A 5 0 0 C / 3 6 0 m i n 2 9 . 0 6 5 0 0 A L i f t 5 0 0 A 5 0 0 C / 3 9 0 m i n 21 . 2 0 M O S C 6 5 0 0 A 6 0 0 C / 3 0 0 m i n 1 5 . 7 5 M O S C 7 1 0 0 0 A 4 0 0 C / 1 w e e k 3 6 . 3 2 M O S C 8 1 0 0 0 A 6 0 0 C / 4 2 0 m i n 2 3 . 9 1 T A B L E 5 . 7 C A L C U L A T E D R E L A T I V E D I E L E C T R I C C O N S T A N T O F A N O D I C T a S A M P L E T a T H I C K N E S S P R O C E S S e r M O S C 9 5 0 0 A C i t r i c A c i d 3 . 0 3 M O S C 1 0 5 0 0 A C i t r i c A c i d 1 . 8 2 M O S C 1 1 1 0 0 0 A P h o s p . A c i d 18 . 6 2 M O S C 1 2 5 0 0 A P h o s p . A c i d 1 0 . 6 0 M O S C 1 3 5 0 0 A C i t r i c A c i d 5 . 2 2 M O S C 1 4 5 0 0 A P h o s p . A c i d 8 . 4 8 M O S C 1 5 1 0 0 0 A C i t r i c A c i d 1 2 . 5 6 M O S C 1 6 5 0 0 A P h o s p . A c i d 1 0 . 7 5 M O S C 1 7 5 0 0 A C i t r i c A c i d 9 . 3 3 M O S C 1 8 1 0 0 0 A C i t r i c A c i d 2 0 . 0 4 T A B L E 5 . 8 F L A T B A N D V O L T A G E , C A P A C I T A N C E A N D F I X E D C H A R G E O F S I N G L E D I E L E C T R I C T a 2 0 5 MOS C A P A C I T O R S S A M P L E T Y P E C f b [ p f / c m 2 ] V f b [ V ] Q f c [ e / c m 2 ] N 2 n 2 8 5 9 5 + 1 . 8 5 - 8 . 9 6 X 1 0 1 1 N 3 n 7 3 9 5 -1 . 6 7 + 5 . 4 3 x 1 0 1 ° B N R 5 0 0 P 3 2 2 9 5 + 0 . 5 6 - 2 . 9 2 X 1 01 1 B N R 5 0 0 P 3 5 1 1 1 + 1 . 3 5 + 8 . 2 2 X 1 01 0 S a m p l e A P 41 4 9 4 - 6 . 5 7 - 7 . 7 3 X 1 01 2 S a m p l e B P 3 3 6 0 8 - 6 . 4 9 - 4 . 0 2 X 1 01 2 1 0 0 0 A M O S n 3 8 1 9 2 - 1 0 . 2 4 + 7 . 1 8 x 1 0 1 2 5 0 0 A L i f t P 4 2 4 3 7 - 8 . 0 3 - 1 . 0 1 X 1 0 1 3 M O S C 6 n 3 9 1 51 - 2 . 0 0 + 1 . 1 0 x 1 0 1 2 M O S C 7 n 4 0 7 8 9 - 1 . 2 9 + 6 . 0 2 X 1 0 1 1 M O S C 8 n 3 5 7 4 4 - 5 . 0 0 + 2 . 6 8 x 1 0 1 2 111 T A B L E 5 . 9 F L A T B A N D V O L T A G E , C A P A C I T A N C E A N D F I X E D C H A R G E O F D O U B L E D I E L E C T R I C MOS C A P A C I T O R S S A M P L E T Y P E C f b [ p f / c m 2 ] V f b [ V ] Q f c [ e / c m 2 ] 1 T 6 n 2 1 2 3 1 - 2 . 9 5 + 4 . 8 7 x 1 0 1 1 2 T 6 n 1 8 3 2 4 - 4 . 3 7 + 6 . 2 8 X 1 0 1 1 3 T 6 n 1 8 8 6 3 - 9 . 1 7 + 1 . 5 1 x 1 0 1 2 4 T 6 n 1 8 9 3 1 - 7 . 6 7 + 1 . 2 5 X 1 0 1 2 1T7 n 1 3 3 7 5 - 3 . 3 3 + 2 . 2 4 X 1 0 1 1 2 T 7 n 1 6 7 4 1 - 8 . 2 6 + 1 . 1 3 x 1 0 1 2 3 T 7 n 1 6 0 5 3 - 0 . 2 4 - 5 . 7 1 X 1 0 1 0 4 T 7 . n 1 4 9 4 1 - 5 . 6 3 + 6 . 3 4 X 1 0 1 1 T A B L E 5 . 1 0 F L A T B A N D V O L T A G E , C A P A C I T A N C E A N D F I X E D C H A R G E O F A N O D I C T a 2 0 5 M O S C A P A C I T O R S S A M P L E T Y P E C f b [ p f / c m 2 ] V f b [ V ] Q f c [ e / c m 2 ] M O S C 9 n 1 7 2 8 6 - 5 . 3 5 + 7 . 3 4 X 1 0 1 1 M O S C 9 n 1 7 2 8 6 + 2 . 2 8 - 4 . 5 6 x 1 0 1 1 M O S C 1 0 n 1 1 8 3 2 - 1 . 1 3 ' - 4 . 5 2 X 1 0 1 0 M O S C 1 0 n 1 1 8 3 2 + 2 . 1 6 - 2 . 6 3 X 1 0 1 1 M O S C 1 1 n 3 2 4 0 6 - 2 . 2 4 + 7 . 6 4 X 1 0 1 1 M O S C 1 2 n 3 4 1 5 5 - 2 . 0 5 + 7 . 6 6 X 1 0 1 1 M O S C 1 3 n 2 4 3 6 7 + 0 . 5 1 - 3 . 1 1x10 1 1 M O S C 1 4 n 3 1 1 1 8 - 0 . 3 9 - 1 . 1 2 x 1 0 1 1 M O S C 1 5 n 2 6 9 3 5 + 1 . 6 1 - 7 . 3 0 X 1 0 1 1 M O S C 1 6 n 3 4 3 4 3 - 3 . 6 6 + 2 . 1 7 x 1 0 1 2 M O S C 1 7 n 3 2 4 2 8 - 2 . 2 1 + 7 . 5 1 X 1 0 1 1 M O S C 1 8 n 3 3 4 0 6 - 4 . 0 2 + 1 . 7 4 X 1 0 1 2 1 1 3 I t can be c o n c l u d e d t h e n , t h a t from the above measurements and c a l c u l a t i o n s t h a t o p e r a t i o n a l MOS c a p a c i t o r s can be f a b r i c a t e d u s i n g t h e r m a l and a n o d i c p r o c e s s e s f o r o b t a i n i n g t a n t a l u m p e n t o x i d e . F u r t h e r m o r e , double d i e l e c t r i c MOS c a p a c i t o r s a l s o show good o p e r a t i o n a l c h a r a c t e r i s t i c s . In p a r t i c u l a r , from the r e s u l t s p r e s e n t e d i n T a b l e s 5.6 t o 5.10, i t can be s a i d t h a t : 1. The c a l c u l a t e d v a l u e of the r e l a t i v e d i e l e c t r i c c o n s t a n t v a r i e s a c c o r d i n g t o the t h i c k n e s s of the f i n a l T a 2 0 5 f i l m and i n the case of the t h e r m a l o x i d e , v a r i e s w i t h the o x i d a t i o n t i m e . With t h i n o x i d e f i l m s and s h o r t o x i d a t i o n t i m e s , the v a l u e of er i s l e s s . T h i s e f f e c t i s a l s o n o t i c e d by p r e v i o u s a u t h o r s [ N i s h i o k a et a l . , 1984], and i t was a t t r i b u t e d t o the f o r m a t i o n of a v e r y t h i n l a y e r of S i 0 2 a t the i n t e r f a c e . Revesz et a l . (1974) r e p o r t e d t h a t s i l i c o n i n c o r p o r a t e s t o the t a n t a l u m o x i d e a t the i n t e r f a c e , w i t h c o - o x i d a t i o n i t e r a c t i o n d u r i n g the growth of t h e r m a l T a 2 0 5 , and as a consequence, the r e f r a c t i v e index of the T a 2 0 5 f i l m d e c r e a s e d w i t h the o x i d e t h i c k n e s s . The t h e r m a l o x i d e samples g i v e a h i g h e r d i e l e c t r i c c o n s t a n t than the a n o d i c o x i d e samples, p o s s i b l y due t o an i n c r e a s e i n p o r o s i t y or inhomogeneity i n the f i l m . These a r e l e s s than the v a l u e r e p o r t e d by Young (1961) of 27, f o r a M e t a l - I n s u l a t o r - M e t a l (MIM) c a p a c i t o r ; by Smith and Young (1981) of 26 f o r a p-MOS c a p a c i t o r w i t h t h e r m a l Ta o x i d e ; but h i g h e r than the ones o b t a i n e d by Revesz and A l l i s o n (1976) of 11.4 f o r t h e r m a l t a n t a l u m o x i d e 1 1 4 on s i l i c o n s u b s t r a t e s . 2. The f l a t b a n d v o l t a g e s are mostly n e g a t i v e , and they vary i n magnitude with the type of process and s u b s t r a t e . The thermal oxide sample e x h i b i t e d the h i g h e s t f l a t b a n d v o l t a g e s , and the anodic oxide samples the lowest. The double d i e l e c t r i c MOS c a p a c i t o r s have f l a t b a n d v o l t a g e s between these. 3. The f i x e d charge i n the oxide, Qfc, v a r i e s in magnitude and s i g n , depending on the s u b s t r a t e and the nature of the oxide. The thermal T a 2 0 5 on n type samples have mostly a p o s i t i v e charge and the p type samples e x h i b i t mostly a negative charge, with i t s magnitude in the range of 2.9x1U 1 1 to 1 X 1 0 1 3 charges/cm 2. In the double d i e l e c t r i c samples, Qfc i s mostly p o s i t i v e , with a range of 2.2 X10 1 1 to 1.5 X10 1 2 charges/cm 2. The anodic T a 2 0 5 e x h i b i t s mostly a p o s i t i v e charge i f the oxide was formed i n phosphoric a c i d , and mostly a negative charge i f formed in c i t r i c a c i d , with the same s u b s t r a t e type. I t s magnitude i s somewhat smal l e r as compared with the thermal oxides, with a range of 4.5 X10 1 1 to 2.2 X10 1 2 charges/cm 2. Thermal tantalum oxide as r e p o r t e d by p r e v i o u s authors [Revesz and A l l i s o n , 1976; Smith and Young, 1981], has a negative charge i n the oxide when made on p type s u b s t r a t e s . I t s magnitude was i n the range 6 x 1 0 1 1 - 5 x 1 0 1 2 . {5.5.3} I-V CURVES: These curves were obtained by the method a l r e a d y 1 1 5 d e s c r i b e d . From the S c h o t t k y graphs, i t i s p o s s i b l e t o o b t a i n the v a l u e of the o p t i c a l r e l a t i v e d i e l e c t r i c c o n s t a n t , which i s e q u a l t o the square of the index of r e f r a c t i o n . By u s i n g the S c h o t t k y e m i s s i o n model, and by o b t a i n i n g the s l o p e 31nJ/3v/E i n the e x p e r i m e n t a l c u r v e s , the er(«) can be c a l c u l a t e d . T h i s assumes t h a t the S c h o t t k y p l o t i s a s t r a i g h t l i n e . In most c a s e s however, a best f i t t o a s t r a i g h t l i n e can be o b t a i n e d . In o t h e r c a s e s , t h e r e a r e s e v e r a l s l o p e s d e n o t i n g a complex c o n d u c t i o n phenomena, which d e p a r t s c o n s i d e r a b l y from e i t h e r the P o o l e - F r e n k e l or S c h o t t k y models. The f o l l o w i n g e x p r e s s i o n was used t o c a l c u l a t e the S c h o t t k y s l o p e : E l " E 2 , „h , „h 2 e = { — ^ } ( l n E ^ - l n E 2 2 ) - S L - { 5 # 1 5 ) l n J , - l n J 0 ( k T ) 2 1 2. 0 A p p r o p r i a t e c o n v e r s i o n s were done, as our S c h o t t k y p l o t s a r e g i v e n i n l o g 1 0 I v s . /V form. A summary of r e s u l t s i s g i v e n i n T a b l e 5.11. As a l r e a d y mentioned, most of the S c h o t t k y p l o t s a r e not s t r a i g h t l i n e s , i n d i c a t i n g t h a t the e m i s s i o n mechanism cannot be r e p r e s e n t e d by a P o o l e - F r e n k e l or S c h o t t k y model. Each p o r t i o n of c o n s t a n t s l o p e can be i n t e r p r e t e d as a v a l u e of c o n s t a n t o p t i c a l r e l a t i v e p e r m i t t i v i t y . The number of s l o p e changes i n the S c h o t t k y p l o t s p r o v i d e an i n d i c a t i o n of how the e m i s s i o n d e p a r t s from t h e s e c l a s s i c a l models. These ar e a l s o g i v e n i n T a b l e 5.11. From p r e v i o u s a u t h o r s [ K a p l a n et a l . , 1976; Smith and Young, 1981], the t y p i c a l S c h o t t k y p l o t of T a 2 0 5 on S i l i c o n s u b s t r a t e s i s a two s l o p e c u r v e , 1 1 6 with a somewhat sharp knee. In i t , the high f i e l d region has a slope that i s c l o s e to the o p t i c a l value of the r e l a t i v e d i e l e c t r i c constant. However, i n t h i s work we have found that some samples e x h i b i t two and three s l o p e s . Mead (1962) prov i d e s i n f o r m a t i o n of h i s MIM samples and they have three d i f f e r e n t s l o p e s . In h i s work, he notes that there i s an ohmic r e g i o n at low f i e l d s and a exp(i/V) region at higher f i e l d s , when the conduction c h a r a c t e r i s t i c i s p l o t t e d i n Schottky form. TABLE 5.11 RESUME OF SCHOTTKY I-V CURVES AND CALCULATED OPTICAL VALUE OF THE RELATIVE DIELECTRIC CONSTANT FOR THERMAL T a 2 0 5 SAMPLE/TYPE PROCESS er (o>) COMMENTS N2/n Thermal/500C 3.032 Curve w/null N3/n Thermal/500C 1 .925 Two Slopes Nl/n Thermal/500C 5.455 Two Slopes BNR500/p Thermal/500C 8.523 Curve w/null BNR1000/p Thermal/500C 21.056 Two Slopes SampleA/p Thermal/500C 2.424 Two Slopes SampleB/p Thermal/500C 3.867 Two Slopes 1OOOAMOS/n Thermal/500C 1.895/1.213 Three Slopes 5 0 0 A L i f t / p Thermal/500C 1.364/1.435 Three Slopes lOOOALift/p Thermal/500C 1.213 Curve M0SC6/n Thermal/600C 50.928 Two Slopes MOSC7/n Thermal/400C 174.643 Two Slopes MOSC8/n Thermal/600C 3.564 Two Slopes 1 1 7 T A B L E 5 . 1 2 R E S U M E O F S C H O T T K Y I - V C U R V E S A N D C A L C U L A T E D O P T I C A L V A L U E O F T H E R E L A T I V E D I E L E C T R I C C O N S T A N T F O R A N O D I C T a 2 0 5 S A M P L E / T Y P E P R O C E S S e r (co) C O M M E N T S M O S C 9 / n C i t r i c A c i d 3 . 491 T w o S l o p e s M O S C 1 0 / n C i t r i c A c i d 1 8 . 3 3 4 S t r . L i n e f : M O S C 1 3 / n C i t r i c A c i d 2 5 . 0 4 7 C u r v e M O S C 1 4 / n P h o s p . A c i d 1 7 . 2 4 0 T h r e e S l o p e M O S C 1 5 / n C i t r i c A c i d 4 . 3 7 8 T h r e e S l o p e M O S C 1 6 / n P h o s p . A c i d 1 2 . 7 3 2 C u r v e M O S C 1 7 / n C i t r i c A c i d 6 . 7 3 4 C u r v e M O S C l 8 / n C i t r i c A c i d 1 7 . 0 5 5 C u r v e 1 18 In some samples, the c a l c u l a t e d values of er(°°) from the Schottky p l o t s are q u i t e unreasonable. T h i s r e i n f o r c e s the f a c t that n e i t h e r a Po o l e - F r e n k e l or Schottky emission mechanisms can account f o r the conduction c u r r e n t s . Mead (1962) proposed three regions with t h e i r r e s p e c t i v e conduction mechanisms: at low a p p l i e d v o l t a g e s and high ( i . e . , room) temperatures, an ohmic c h a r a c t e r i s t i c p r e v a i l s ; at high f i e l d s and low temperatures, f i e l d i o n i z a t i o n of trapped e l e c t r o n s i n the conduction band i s r e s p o n s i b l e f o r the c u r r e n t flow, y i e l d i n g a Fowler-Nordheim type emission; f i n a l l y at high - f i e l d s and high temperatures, the c u r r e n t flow i s f i e l d enhanced by thermal e x c i t a t i o n of trapped e l e c t r o n s i n t o the conduction band, producing a Poole-F r e n k e l type emission. Kaplan, Balog and Frohman-Bentchkowsky (1976) c o n s i d e r a bulk l i m i t e d P-F emission at low f i e l d s , and a t r a n s i t i o n towards a space charge l i m i t e d c u r r e n t at higher f i e l d s . They provide an e m p i r i c a l r e l a t i o n of the form J a V 2 ' a . Angle and T a l l e y (1978) argue that the conduction mechanisms are q u i t e d i f f e r e n t i n the anodic and thermal o x i d e s . From t h e i r experimental data, they concluded that under forward b i a s , P-F type conduction takes p l a c e f o r thermal oxides under forward b i a s and under reverse b i a s , a space charge l i m i t e d conduction mechanism dominates, which i s a t t r i b u t e d to the s u r f a c e s t a t e s at the Al-Ta{oxide} i n t e r f a c e formed dur i n g p r o c e s s i n g . A c l e a r evidence of photoconduction was found i n the MOS c a p a c i t o r s . I t was n o t i c e d that when the samples were i l l u m i n a t e d , l a r g e v a r i a t i o n s i n * conduction c u r r e n t took 1 19 pl a c e , as dete c t e d by the el e c t r o m e t e r used i n measuring the leakage c u r r e n t . Some samples e x h i b i t e d more s e n s i t i v i t y to the i n c i d e n t l i g h t than o t h e r s . The l i g h t source was a Westinghouse Heat Ray ( i n f r a r e d ) lamp, 250 W, 115 V, pl a c e d e x a c t l y 25 cm above the c e n t r e of the sample. Some samples e x h i b i t e d more s e n s i t i v i t y to the a p p l i e d l i g h t that o t h e r s , and from the c a l c u l a t e d r a t i o s of photocurrent i n c r e a s e , as given i n Table 5.11, i t suggests that t h i s phenomena i s not process dependent, but r e l a t e d to the t r a p s i n the oxide's conduction band, to which photo generated e l e c t r o n s use as stepping stones i n the i n s u l a t o r bandgap. T A B L E 5 . 1 3 P H O T O C O N D U C T I O N I N T A N T A L U M O X I D E MOS C A P A C I T O R S S A M P L E P R O C E S S P H O T O C U R R E N T N 2 T h e r m a l / 5 0 0 C 3 . 5 1 T 6 T h e r m a l / 5 0 0 C 3 . 7 2 T 6 T h e r m a l / 5 0 0 C 1 5 . 8 3 T 6 T h e r m a l / 5 0 0 C 2 . 3 4 T 6 T h e r m a l / 5 0 0 C 1 . 0 1 T 7 T h e r m a l / 5 0 0 C 1 3 3 . 3 2 T 7 T h e r m a l / 5 0 0 C 2 . 0 3 T 7 T h e r m a l / 5 0 0 C 3 . 2 4 T 7 T h e r m a l / 5 0 0 C 1 . 3 N1 T h e r m a l / 5 0 0 C 8 . 0 BNR . 5 0 0 T h e r m a l / 5 0 0 C 6 . 3 S a m p l e A T h e r m a l / 5 0 0 C 1 . 0 S a m p l e B T h e r m a l / 5 0 0 C 5 . 6 1 0 0 0 A M O S T h e r m a l / 5 0 0 C 3 . 5 5 0 0 A L i f t T h e r m a l / 5 0 0 C 1 . 2 1 O O O A L i f t T h e r m a l / 5 0 0 C 2 . 0 M O S C 6 T h e r m a l / 6 0 0 C 1 . 5 M O S C 7 T h e r m a l / 4 0 0 C 3 2 . 0 M O S C 8 T h e r m a l / 6 0 0 C 3 . 2 M O S C 1 4 A n o d i c / P h o s p . 5 . 5 M O S C 1 5 A n o d i c / C i t r i c 2 0 . 5 M O S C 1 6 A n o d i c / P h o s p . 1 . 3 M O S C 1 7 A n o d i c / C i t r i c 1 1 . 0 M O S C 1 8 A n o d i c / C i t r i c 1 . 8 121 {5.6} INTERFACIAL OXIDATION MOS CAPACITORS: Both C-V and I-V curves were ob t a i n e d f o r these samples. I t was noted that i n general the C-V curves are of e x c e l l e n t q u a l i t y , with w e l l d e f i n e d accumulation and i n v e r s i o n r e g i o n s . In the f i r s t , the sample had a smooth t r a n s i t i o n towards accumulation, with none of the d i f f i c u l t i e s t h a t appeared i n the s i n g l e d i e l e c t r i c T a 2 0 5 MOS c a p a c i t o r s . The acumulation region i s a l s o without v i s i b l e bumps, that i n d i c a t e the presence of s u r f a c e s t a t e s . T h i s i n d i c a t e s that the q u a l i t y of the double d i e l e c t r i c s t r u c t u r e i s very good and that the method of growing the S i 0 2 l a y e r under the T a 2 0 5 i s q u i t e s u c c e s s f u l . The r e s u l t s from the I-V curves a l s o i n d i c a t e that t h i s process i s q u i t e s u c c e s s f u l , as the e l e c t r o n i c conduction (leakage) c u r r e n t i s q u i t e s m a l l , of comparable or s m a l l e r magnitude than the previous double d i e l e c t r i c p r o c e s s . Tables 5.12 and 5.13 summarize the r e s u l t s obtained from both C-V and I-V data. 1 2 2 T A B L E 5 . 1 4 I N T E R F A C I A L O X I D A T I O N MOS C A P A C I T O R S , C - V R E S U L T S S A M P L E WET O X . T I M E I N S U L A T O R A C C U M U L A T I O N C A P . M T J 1 3 - 5 4 - 3 m i n T a 2 0 5 / S i 0 2 5 1 8 7 5 p F / c m 2 M T J 1 3 - 5 4 - 3 m i n S i 0 2 4 6 5 0 0 p F / c m 2 M T J 2 3 - 1 1 4 - 3 m i n T a 2 0 5 / S i 0 2 7 6 7 5 0 p F / c m 2 M T J 2 3 - 1 1 4 - 3 m i n S i 0 2 5 6 5 6 3 p F / c m 2 M T J 3 3 - 5 4 - 3 m i n T a 2 O s / S i 0 2 8 8 7 5 0 p F / c m 2 M T J 3 3 - 5 4 - 3 m i n S i 0 2 7 5 0 0 0 p F / c m 2 M T J 4 3 - 2 4 - 3 m i n T a 2 0 5 / S i 0 2 1 0 6 2 5 0 p F / c m 2 M T J 4 3 - 2 4 - 3 m i n S i 0 2 1 1 7 5 0 0 p F / c m 2 M T J 5 N / A T a 2 0 5 1 7 5 0 0 0 p F / c m 2 T A B L E 5 . 1 5 I N T E R F A C I A L O X I D A T I O N MOS C A P A C I T O R S , I - V R E S U L T S S A M P L E I N S U L A T O R L E A K A G E C U R R E N T S C H O T T K Y S L O P E S M T J 1 D o u b l e 0 . 5 2 n A a t 10 V T w o M T J 2 D o u b l e 0 . 3 0 n A a t 10 V T w o M T J 3 D o u b l e 0 . 5 2 n A a t 10 V T w o M T J 4 D o u b l e 13 n A a t 10 V T w o M T J 5 T a 2 0 5 1 4 . 5 MA a t 10 V T h r e e 1 23 CHAPTER 6 FABRICATION AND PROCESSING OF MTAOS FIELD EFFECT TRANSISTORS The MTAOS a c t i v e d e v i c e s were f a b r i c a t e d using the al r e a d y developed techniques f o r the MOS c a p a c i t o r s , i n p a r t i c u l a r the l i f t o f f process and anodic o x i d a t i o n of tantalum on s i l i c o n s u b s t r a t e s . The p r o c e s s i n g i s based on a mod i f i e d v e r s i o n of the standard p-MOS technology used i n the S o l i d State Laboratory, E l e c t r i c a l E n g i n e e r i n g Department, at t h i s U n i v e r s i t y . The o b j e c t i v e was to f a b r i c a t e a T a 2 0 5 / S i 0 2 double i n s u l a t o r p-Channel Enhancement type MOSFET, and to demonstrate i t s f e a s i b i l i t y as a new a c t i v e device of the MOS f a m i l y . Two main avenues were fo l l o w e d : the thermal and anodic o x i d a t i o n methods f o r p r e p a r i n g tantalum pentoxide. T h i s r e s u l t e d i n two groups of n-type S i s u b s t r a t e s , each d e d i c a t e d to the i n d i v i d u a l p r e p a r a t i o n method. In order to independently v e r i f y the q u a l i t y of the f i n a l double d i e l e c t r i c s t r u c t u r e , s e v e r a l s u b s t r a t e s were a l s o s i m u l t a n e o u s l y processed f o r dot and r i n g MOS c a p a c i t o r s . F u r t h e r C-V and I-V measurements of these w i l l a c c u r a t e l y r e v e a l the i n s u l a t o r performance. The f o l l o w i n g general p r o c e s s i n g steps were f o l l o w e d : 1) Thickness and four p o i n t r e s i s t i v i t y measurements. 2) S c r i b i n g and marking. 3) P e r o x i d e - a c i d c l e a n i n g using the RCA pr o c e s s . 4) Thermal o x i d a t i o n of s u b s t r a t e s , u s i n g the "wet" 1 24 hydrogen- oxygen method. F i e l d oxide t h i c k n e s s t a r g e t : 600 nm. 5) P h o t o l i t h o g r a p h y of source and d r a i n windows on f i e l d o x ide. 6) Source and d r a i n boron d i f f u s i o n s : p r e d e p o s i t i o n and dr i v e - i n . 7) Gate p h o t o l i t h o g r a p h y , window in f i e l d oxide. 8) Thermal o x i d a t i o n , using the "dry" oxygen method, for the t h i n S i 0 2 gate i n s u l a t o r . 9) P e r o x i d e - a c i d c l e a n i n g , e x c l u d i n g the HF step. 10) Aluminium thermal e v a p o r a t i o n in p r e p a r a t i o n f o r l i f t o f f . 11) P h o t o l i t h o g r a p h y of aluminium fo r l i f t o f f . 12) RF S p u t t e r i n g of tantalum metal. 13) Thermal o x i d a t i o n of f i r s t group i n dry oxygen. 14) Anodic o x i d a t i o n of second group in c i t r i c a c i d . 15) L i f t o f f p a t t e r n i n g of tantalum pentoxide. 16) I n s p e c t i o n of a l l wafers under microscope. 17) P e r o x i d e - A c i d c l e a n i n g , e x c l u d i n g the HF step. 18) D r a i n and source p h o t o l i t h o g r a p h y to remove t h i n gate oxide. 19) I n s p e c t i o n and photography of a l l wafers under microscope. 20) P e r o x i d e - A c i d c l e a n i n g , e x l u d i n g the HF step. 21) Aluminium ev a p o r a t i o n by e l e c t r o n beam technique fo r source, d r a i n and gate c o n t a c t s . 22) P h o t o l i t h o g r a p h y of aluminium source, d r a i n and gate c o n t a c t s . 125 23) Thick oxide e t c h i n g on wafer's back, p r e p a r a t i o n f o r s u b s t r a t e (back) c o n t a c t . 24) Gold evaporation by E l e c t r o n Beam technique f o r back c o n t a c t . 25) F i n a l microscope i n s p e c t i o n of a l l wafers. Photography. The f i r s t three steps were a l r e a d y d e s c r i b e d i n d e t a i l under Chapter 4, and they are not repeated here. A l l wafers were s c r i b e d with a code and date, so that they c o u l d be e a s i l y and uniquely i d e n t i f i e d . Six MTAOS device wafers were processed, together with three general and one S i 0 2 c o n t r o l wafers, a t o t a l ' of ten s u b s t r a t e s . {6.1} SHEET RESISTIVITY DETERMINATION: The four p o i n t r e s i s t i v i t y measurements are standard procedure to v e r i f y the sheet r e s i s t i v i t y and o b t a i n the impurity c o n c e n t r a t i o n v i a I r v i n curves f o r s i l i c o n . A l l wafers were cleaned using the RCA procedure with a l l i t s s t e p s . The s u b s t r a t e s used were n-type s i l i c o n , of the f o l l o w i n g c h a r a c t e r i s t i c s : Manufacturer: Monsanto Lot No. 4002, S e r i a l ' No. Di-45640, Date 3/5/76 Dopant: Phosphorous, N type. R e s i s t i v i t y : 8-10 ohm-cm. Thi c k n e s s : 11-12 m i l s ; Diameter: 1.98-2.02 inches. 126 Two groups of wafers are used i n t h i s stage: the ones that w i l l have the tantalum pentoxide t h e r m a l l y grown and those in which the oxide w i l l be grown a n o d i c a l l y . These are given in Table 6.1, with the sheet r e s i s t i v i t y r e s u l t s . TABLE 6.1 DEVICE SUBSTRATE MARKING AND MEASURED RESISTIVITY MARKING TYPE RESISTIVITY [ohm-cm] MTAOS1 Thermal 9.337 MTAOS2 Thermal 10.171 MTAOS3 Thermal 9.528 MTAOS4 Anodic 9.438 MTAOS5 Anodic 9.856 MTAOS6 Anodic 9.221 MOS I C o n t r o l 9.976 MOS II C o n t r o l 9.259 MOS I I I C o n t r o l 9.044 Si02 I S i 0 2 9.472 SI02 II S i 0 2 10.637 {6.2} SILICON THERMAL OXIDATION: The c l e a n e d wafers were then i n t r o d u c e d to a p r e v i o u s l y c o n d i t i o n e d o x i d a t i o n furnace ( F a i r c h i l d with Wheelco/Barber Coleman temperature c o n t r o l l e r s ) f o r t h i c k oxide ( t a r g e t v a l u e : 600 nm). The temperature was set to 1100°C±5 with the a i d of a pre v i o u s thermocouple temperature p r o f i l e and i n t e r n a l r e g u l a t i n g system. The gas flows were set as 1 27 d e s c r i b e d in Appendix I I I . A f t e r 2 h r s . 45 min., the samples were removed, c o o l e d f o r a few minutes and t h e i r t h i c k n e s s checked a g a i n s t the C o l o r Chart f o r s i l i c o n oxides and by e l l i p s o m e t r i c measurements, using the equipment d e s c r i b e d before i n Chapter 4. The r e s u l t s are the f o l l o w i n g : Sample * A Thickness [nm] Si02I 15.72 122.72 591.1 Si02II 19.27 108.06 602.8 The c o l o r , of the grown f i l m as observed by naked eye was pink. Which, from the Color Chart corresponds to 600 nm. {6.3} THICK OXIDE PHOTOLITHOGRAPHY: The mask set used throughout t h i s process was a l r e a d y a v a i l a b l e , and i t c o n t a i n s a sample of s e v e r a l a c t i v e and p a s s i v e d e v i c e s , hence the name "Smorgasbord" attached to i t . The d e v i c e s a r e : a) A r e s i s t o r , p-n diode and MOS c a p a c i t o r . b) A MOS T r a n s i s t o r . c) A L o g i c I n v e r t e r , 2 Input NOR gate and RS F l i p - F l o p . P h o t o l i t h o g r a p h y was used to e t c h the source and d r a i n windows through the f i e l d o x i d e . Negative p h o t o r e s i s t a p p l i c a t i o n , exposure, development, e t c h i n g , and s t r i p p i n g d e t a i l s are given i n Appendix I I I . The wafers were then i n s p e c t e d under a microscope f o r r e s o l u t i o n and under/over e t c h i n g . They gave e x c e l l e n t r e s u l t s with good l i n e r e s o l u t i o n and proper e t c h i n g . 128 {6.4} BORON PREDEPOSITION: The d i f f u s i o n process has two ste p s : the p r e d e p o s i t i o n and d r i v e - i n of the i m p u r i t i e s . Since the s u b s t r a t e s are n-type, we are i n t e r e s t e d i n c r e a t i n g two p+ regions f o r the d r a i n and source. Boron i s used as acceptor impurity, i n the form of BBr 3, which i s passed through the p r e d e p o s i t i o n furnace ( F a i c h i l d , with Wheelco/Barber Coleman temperature c o n t r o l l e r s ) together with other gases ( f o r d e t a i l s see Appendix I I I ) . The furnace i s p r e c o n d i t i o n e d ("predoped") fo r one hour before the s l i c e s are int r o d u c e d , and the temperature c a r e f u l l y set to 1090±5°C. A h a l f - s l i c e , t e s t wafer was used to check the p r e d e p o s i t i o n step, before the dev i c e wafers were i n t r o d u c e d . A f t e r c y c l i n g and c o o l i n g i t s r e s i s t i v i t y was measured, using the four p o i n t probe method. At t h i s p o i n t , a c e r t a i n degree of d i f f i c u l t y was encountered, as the t a r g e t value of 2.0 ohm-cm c o u l d not be achieved. I n i t i l l y , the four p o i n t probe t e s t gave values that were too high (8-11 fl-cm), a second i t e r a t i o n gave a value of 6.8 fi-cm, and f i n a l l y a t h i r d one gave a sheet r e s i s t i v i t y of 2.6-2.8 fi-cm. The measured r e s i s t i v i t y was 1.95-1.99 ohm-cm, very c l o s e to the t a r g e t v a l u e . T o t a l time was 23 min. {6.5} BORON DRIVE-IN: Before the d r i v e - i n of i m p u r i t i e s , the "boron g l a s s " , formed d u r i n g the pr e v i o u s p r e d e p o s i t i o n step, was removed. I t was noted that by p l a c i n g the wafers i n d e - i o n i z e d water ("wetting") before e t c h i n g i n HF, the r e s u l t s improved 129 c o n s i d e r a b l y . I t appears that the adhered s u r f a c e water slows the i n i t i a l r e a c t i o n , so that there i s a slow i n i t i a l e t c h i n g r a t e . The d r i v e - i n furnace (Thermco P a c e s e t t e r II) was f i r s t set to the proper temperature (1090±1°C), allowed to s e t t l e and then p r o f i l e d with a Pt-Rd thermocouple. The l a t t e r was i n agreement with the s e t t i n g . A f t e r c o n d i t i o n i n g , the s l i c e s were allowed i n t o the furnace and c y c l e d as d e t a i l e d i n Appendix I I I . T o t a l time was 2 h r s . The samples showed t y p i c a l green t r a c k s when examined with naked eye. At t h i s p o i n t no smears or evidence of contamination c o u l d be seen. {6.6} GATE PHOTOLITHOGRAPHY: The gate window now can be cut using p h o t o l i t h o g r a p h y . T h i s r e q u i r e d a mask alignment step, performed i n a Kasper Instruments Model 17A Mask A l i g n e r , with an u l t r a v i o l e t l i g h t source and a d j u s t a b l e exposure time. Negative p h o t o r e s i s t was used, and developed a u t o m a t i c a l l y i n a K u l i c k e and S o f f a Model 693 P h o t o r e s i s t Spray Developer, etched and s t r i p p e d as d e t a i l e d i n Appendix I I I . {6.7} THIN OXIDE PROCESS SIMULATION USING SUPREM: A f t e r t h i s step, the gate o x i d a t i o n can be performed. No data was a v a i l a b l e f o r a c c u r a t e l y growing a t h i n 200 A t h i c k S i 0 2 l a y e r over the s i l i c o n s u b s t r a t e , using the furnace equipment a v a i l a b l e i n the S o l i d S tate Lab. A process s i m u l a t i o n program, SUPREM, runing under MTS, was used to simulate the growth of the s i l i c o n d i o x i d e at high 130 temperatures i n an o x i d i z i n g atmosphere. S e v e r a l combinations of temperatures and o x i d a t i o n time i n a dry oxygen atmosphere were used. The r e s u l t s are as f o l l o w s ( d e t a i l s i n Appendix I V ) : TABLE 6.2 SUPREM SIMULATION RESULTS Time [min] Temperature [°C] Oxide Thickness [A] 3 1000 140 5 1000 200 3 1090 224 3 1090 224 10 1090 349 38.5 1090 754 60 1090 998 80 1090 1198 85 1090 1244 90 1090 1290 For the t h i n gate oxide, the process s e l e c t e d i s then 5 min. i n dry oxygen, furnace temperature at 1000 C. Before the d e v i c e wafers were i n t r o d u c e d i n the furnace, the c y c l e was checked using t e s t samples p r e v i o u s l y prepared and cle a n e d . The Color Chart a v a i l a b l e does not pr o v i d e data below 500 A f o r s i l i c o n , so that only e l l i p s o m e t r i c measurements c o u l d be c a r r i e d out. The above SUPREM r e s u l t s were used as a s t a r t i n g p o i n t i n determining the r i g h t c y c l e c o n d i t i o n s f o r a t a r g e t S i 0 2 131 t h i c k n e s s of 200 A. Test wafers were then c l e a n e d and prepared f o r a sequence of o x i d a t i o n s and e l l i p s o m e t r i c measurements, u n t i l the proper t a r g e t value was obt a i n e d . When the s e l e c t e d SUPREM value of 5 min. at 1000 C was used, the measured t h i c k n e s s was 110-120 A, which i s too low for t h i s a p p l i c a t i o n . I t i s important to note that the a c t u a l o x i d a t i o n c y c l e used c o n s i s t s of f i v e s t e p s : a) 5 min. 0 2, Purge. b) Samples Introduced. c) 3 min. 0 2, P a s s i v a t i o n . d) X min. 0 2+HCl, Slow O x i d a t i o n . e) 20 min. N 2, Annealing. In step d ) , the a c t u a l time was v a r i e d f o r each i n d i v i d u a l t e s t wafer (given by the l e t t e r X), and then i t s t h i c k n e s s measured. The r e s u l t s are given i n Table 6.3. TABLE 6.3 DRY THERMAL OXIDATION OF S i 0 2 WAFER 0 2+HCl Time [min] Thickness [nm] TEST 200A 2 13.74 TEST 200A 5 17.47 TEST 200A 7 20.72 TEST 200A (etched) 7 17.12 TEST 200A (etched) 10 23.95 TEST 200A (etched) 8 21.04 1 32 Based on these r e s u l t s , the l a s t entry on Table 6.3 i s q u i t e a c c e p t a b l e . The m o d i f i e d c y c l e i s then: a) 5 min. 0 2, Purge. b) Samples Introduced. c) 3 min. 0 2, P a s s i v a t i o n . d) 8 min. 0 2+HCl, Slow O x i d a t i o n . e) 20 min. N 2, An n e a l i n g . The device and c o n t r o l wafers were then i n t r o d u c e d to a p r e v i o u s l y c o n d i t i o n e d and p r o f i l e d furnace ( F a i r c h i l d with Wheelco/Barber Coleman C o n t r o l l e r s ) , at a temperature of 1000 C. {6.8} PREPARATION OF DEVICE WAFERS FOR ANODIZATION: At t h i s p o i n t the d e v i c e wafers were separated i n t o the anodic and thermal groups, as the next p r o c e s s i n g steps are q u i t e d i f f e r e n t . Since the anodic device wafers have an o v e r a l l t h i n oxide, i t i s necessary to remove a small area, so to have a d i r e c t c o n t a c t to the s i l i c o n s u b s t r a t e f o r anodic o x i d a t i o n ( t h i s i s the same process developed d u r i n g the anodic MOS c a p a c i t o r p r o c e s s i n g i n Chapter 4 ). Ph o t o l i t h o g r a p h y with n e g a t i v e p h o t o r e s i s t was used, and a small s t r a i g h t p i e c e of a diamond cut wafer used to cover the f l a t edge in the d e v i c e wafer. T h i s s l i g h t l y reduces the y i e l d , but there i s no a l t e r n a t i v e . 1 33 {6.9} PEROXIDE-ACID CLEANING OF ALL WAFERS: A l l c o n t r o l and device samples were clea n e d using a mo d i f i e d v e r s i o n of the RCA process, which excludes the HF etch, otherwise the t h i n oxide w i l l be removed. {6.10} ALUMINIUM EVAPORATION: Aluminium thermal evaporation followed, as the f i r s t s tep i n p r e p a r a t i o n f o r L i f t o f f . T h i s was done i n a CHA Evaporator, p r e v i o u s l y loaded with high p u r i t y A l wire, as d e s c r i b e d i n Chapter 4. The monitored f i n a l t h i c k n e s s was c l o s e to 500 nm. E x p e r i m e n t a l l y , t h i s w r i t e r found that when the l i f t o f f A l was too t h i c k (around 1000 nm) the e t c h i n g time was q u i t e long and underetching took p l a c e , as compared with a t h i n n e r l a y e r (around 500 nm) of d e p o s i t e d A l . Smaller values can be used, and l e s s e t c h i n g time w i l l be r e q u i r e d , however the r e s o l u t i o n can be impaired due to o v e r - e t c h i n g . {6.11} PHOTOLITHOGRAPHY FOR LIFTOFF: The L i f t o f f p a t t e r n was d e l i n e a t e d using p h o t o l i t h o g r a p h y with negative p h o t o r e s i s t . At t h i s p o i n t , a c a r e f u l examination of the a v a i l a b l e masks was done. The set d i d not have an exact opposite (negative m i r r o r image) p a t t e r n f o r our s p e c i f i c a p p l i c a t i o n . Instead of d e s i g n i n g a new one, with the c o n s i d e r a b l e delay i n v o l v e d , i t was d i s c o v e r e d that by combining a Gate Contact mask (negative v e r s i o n ) with a Gate Window mask, the exact area of evaporated aluminium c o u l d be removed. T h i s r e q u i r e d a 1 34 c r i t i c a l double alignment and exposure, which c o n s i d e r i n g the q u a l i t y of our Mask A l i g n e r , was not an easy s t e p . A "guinea p i g " wafer was used to v e r i f y the r e s u l t s and q u a l i t y of our procedure. A c l o s e examination under the microscope confirmed our e x p e c t a t i o n s and r e v e a l e d that t h i s technique was a good one. Only the device wafers were processed with t h i s method. {6.12} MICROSCOPE EXAMINATION AFTER LIFTOFF: I n s p e c t i o n under the microscope r e v e a l e d that a l l d e v i c e wafers had e x c e l l e n t alignment, good r e s o l u t i o n and that the double exposure technique worked very w e l l , with good d e f i n i t i o n around the gate area. Some "ragging" was v i s i b l e under higher m a g n i f i c a t i o n , which w i l l not make the l i f t o f f edge very s t r a i g h t . T h i s suggests perhaps some degree of underetching of the A l metal. Some flaws were evident on the edges (tweezer h a n d l i n g ) , which are q u i t e normal. Except f o r these, the device wafers e x h i b i t e d very good q u a l i t y . {6.13} DETERMINATION OF THE SWELLING FACTOR S: Before the tantalum metal c o u l d be d e p o s i t e d by s p u t t e r i n g , i t was necessary to determine the t h i c k n e s s r e q u i r e d f o r a t a r g e t t h i c k n e s s of tantalum pentoxide. In the o x i d a t i o n process, a l l the metal w i l l be converted ( i d e a l l y ) i n t o oxide, independently of whether the method i s thermal or anodic. The f i n a l oxide w i l l be t h i c k e r t hat the o r i g i n a l metal f i l m , and i t can be s a i d that " s w e l l i n g " 1 35 takes p l a c e . Hence a s w e l l i n g f a c t o r S can be d e f i n e d , and c a l c u l a t e d c o n s i d e r i n g the molecular weights, d e n s i t i e s , areas and t h i c k n e s s of the i n i t i a l metal and f i n a l oxide f i1ms: m T a 2 0 5 6 T a 2 0 5 (A t ) = £_2 L2.— (6.D ^ a 6 T a ( A T a fcTa ] C l e a r l y , the areas are equal, and by using m{0}=16 gr and m{Ta}=l80.88 gr, the molecular weight m{Ta 20 5}=441.76 gr i s o b t a i n e d . From the CRC (Chemical Rubber Company, 57th E d i t i o n , 1977) P h y s i c s and Chemistry Handbook, the d e n s i t y value of tantalum and from Young (1961) the tantalum pentoxide d e n s i t y a r e : 6{Ta}=16.6 gr/cm 3 5{Ta 20 5}=7.95 gr/cm 3 Then, the s w e l l i n g f a c t o r S i s d e f i n e d as the r a t i o of the tantalum pentoxide t h i c k n e s s to the tantalum t h i c k n e s s : S = t T a 2 ° 5 2 t ^ Ta (6.2) The f a c t o r 2 i n the denominator a r i s e s from the f a c t that two atoms of tantalum are r e q u i r e d to form T a 2 0 5 , i f the s t o i c h i o m e t r y i s r i g h t . By using the above molecular weight f i g u r e s , the S f a c t o r i s then: mTa205 6Ta. -s = =2.55 ( 6 > 3 ) -Ta 5 T a 2 0 5 136 It i s important to compare t h i s value with e x p e r i m e n t a l l y measured ones. Revesz et a l . (1976), i n h i s work on thermal tantalum oxides, gave a f i n a l oxide t h i c k n e s s of 60 nm f o r an i n i t i a l metal f i l m t h i c k n e s s of 28 nm; the S f a c t o r i s then 2.14. T h i s compares q u i t e favourably with the t h e o r e t i c a l value of 2.55. I t q u i t e reasonable then to expect d o u b l i n g the o r i g i n a l metal t h i c k n e s s v a l u e , when f u l l y c onverted to oxide. {6.14} PRELIMINARY RF SPUTTERING OF TANTALUM: In t h i s work, and from p r e v i o u s MOS C a p a c i t o r experience, two standard values of tantalum metal t h i c k n e s s are used: 500 and 1000 A, which when f u l l y o x i d i z e d w i l l y i e l d a gate oxide t h i c k n e s s of 1000 and 2000 A f o r the tantalum pentoxide. In t h i s case only 500 A of Ta f i l m w i l l be used. P r e v i o u s l y c l e a n t e s t wafers were int r o d u c e d i n t o the RF S p u t t e r i n g equipment. A good recommended p r a c t i c e , f o l l o wed through our experimental work, was to p r e l i m i n a r y s p u t t e r at low power f o r 10-15 min., i n order to e l i m i n a t e any p o s s i b l i t y of contamination, due to chamber/target i m p u r i t i e s . We a l s o recommend that c l e a n g l a s s samples should be p l a c e d s i d e by s i d e to the wafer ( p r e f e r a b l y two). T h i s accomplishes two f u n c t i o n s : holds the s u b s t r a t e i n p l a c e , under the c e n t r e of the t a r g e t , thus e l i m i n a t i n g any p o s s i b l i t y of movement due to v i b r a t i o n ( p a r t i c u l a r l y when the roughing pump i s i n a c t i o n ) and by a i r being removed from under the wafer; secondly the g l a s s r e c e i v e s an equal 1 37 amount of d e p o s i t e d metal, hence these can be used f o r microscope (dark f i e l d w i l l show the h i l l o c k or measle count, and a t r a n s m i s s i o n one w i l l r e v e a l the p i n h o l e d e n s i t y ) examination. The s p u t t e r i n g r a t e was determined before by D. Smith (using the Sloan Angstrometer and stepwise etch method). At a forward RF power of 160 W ( r e f l e c t e d power <5 W), the r a t e i s 294 A/min. The f i l m t h i c k n e s s of the Ta metal can be determined using e l l i p s o m e t r y , by t a k i n g measurements at two or more d i f f e r e n t e l l i p s o m e t e r a n g l e s . Westwood (1975), g i v e s s e v e r a l values for t r i o d e s p u t t e r e d f i l m s , a l l complex with a f a i r l y l a r g e imaginary component. Furthermore, these f i l m s seem to be q u i t e r e f l e c t i v e , and not t r a n s p a r e n t as the e l l i p s o m e t r i c technique r e q u i r e s . Examination e i t h e r by naked eye or microscope shows a " m i r r o r " l i k e s u r f a c e , which r e f l e c t s most of the i n c i d e n t l i g h t . {6.15} PRELIMINARY THERMAL OXIDATION: A s i n g l e t e s t wafer was t h e r m a l l y o x i d i z e d in a dry Oxygen atmosphere at 500 C, i n p r e v i o u s l y c o n d i t i o n e d and s t a b i l i z e d furnace ( M i n i b r u t e , Thermco Products Corp., with Analock 201 C o n t r o l l e r s ) . The gas flow was set to 1 l/min, and at the furnace ends, the temperature was set to +5C above the c e n t r e . The o x i d a t i o n time was based on p r e v i o u s MOS c a p a c i t o r work, and a time of 90 min was decided to be optimum for f u l l c o n v e r s i o n of metal i n t o oxide. The wafer was then removed and allowed to c o o l . I t s c o l o r was b r i g h t i n t e n s e blue, c l e a n with no t r a c e s of contamination. 1 38 E l l i p s o m e t r i c d e t e r m i n a t i o n of the f i l m t h i c k n e s s gave a value of 80 nm. {6.16} PRELIMINARY ANODIC OXIDATION: Anodic o x i d a t i o n was performed on another t e s t wafer, with A l d e p o s i t e d by e v a p o r a t i o n i n the back, to assure a good c o n t a c t . The e l e c t r o l y t e was a 0.1 M s o l u t i o n of c i t r i c a c i d i n d e - i o n i z e d water. The a n o d i z a t i o n c e l l was the same as d e s c r i b e d i n Chapter 4, the same equipment set-up was used and a c u r r e n t d e n s i t y of of 1 mA/cm2 was passed through the c e l l . Because of the 0 r i n g that holds the wafer a g a i n s t i t s back brass c o n t a c t , the Ta metal at the perimeter i s not o x i d i z e d , and t h i s a c t s as a conductor around the wafer, thus a s s u r i n g that the c u r r e n t w i l l be u n i f o r m l y d i s t r i b u t e d on the exposed area. The wafer was c a r e f u l l y r i n s e d i n d . i . water and dryed i n b o i l i n g i s o p r o p y l a l c o h o l . I t s c o l o r was medium dark blue, and some t r a c e s of d e t e r i o r a t i o n were e v i d e n t . A l s o , some bubbling was n o t i c e d during the l a t t e r p art of a n o d i z a t i o n , which i n d i c a t e s breakdown by " s p a r k i n g " [Young, 1961]. A n a l y s i s of our procedure r e v e a l e d that the Constant Current power supply was set to a v o l t a g e l i m i t of 100 V, which i s too high and as the oxide was grown, some areas (weak spots and probably p i n h o l e s ) were s u b j e c t to e l e c t r i c f i e l d s beyond breakdown. T h i s experience was i n v a l u a b l e f o r s u c c e s s f u l l y p r o c e s s i n g the anodic d e v i c e wafers. The f i l m t h i c k n e s s , as determined e l l i p s o m e t r i c a l l y , gave a value of 90 nm. 1 3 9 { 6 . 1 7 } R F S P U T T E R I N G O F T A N T A L U M : A f t e r t h i s p r e l i m i n a r y e x p e r i m e n t , t h e r e m a i n i n g d e v i c e w a f e r s c o u l d b e p r o c e s s e d . R F s p u t t e r i n g w a s a c c o m p l i s h e d o n e a c h w a f e r i n d i v i d u a l l y , a s o u r e x p e r i e n c e i n d i c a t e d t h a t t w o o r m o r e w a f e r s c o u l d n o t b e u n i f o r m l y c o v e r e d w i t h T a , d u e t o a n o t i c e a b l e t h i c k n e s s g r a d i e n t a w a y f r o m t h e t a r g e t c e n t r e . E a c h w a f e r w a s i n t r o d u c e d a c c o m p a n i e d b y t w o p r e v i o u s l y c l e a n e d ( R C A p r o c e s s ) a n d l a b e l e d g l a s s s a m p l e s ( C o r n i n g 2 9 4 7 ) . T h e s p u t t e r i n g i s d o n e i n a r g o n , w i t h a p a r t i a l p r e s s u r e o f 2 6 m T . R e s i d u a l g a s e s h a d a m e a s u r e d p r e s s u r e b e t t e r t h a n 1 0 ~ 6 T o r r . U n d e r t h e s e c o n d i t i o n s t h e s p u t t e r i n g r a t e i s 2 9 4 A / m i n a t 1 6 0 W f o r w a r d a n d <5 W r e f l e c t e d p o w e r . A l l s a m p l e s h a d a n e s t i m a t e d d e p o s i t e d t h i c k n e s s o f 5 0 0 A . N a k e d e y e e x a m i n a t i o n i n d i c a t e s t h a t t h e s u r f a c e i s m i r r o r - l i k e a n d f r e e f r o m c o n t a m i n a t i o n a n d i m p u r i t i e s . { 6 . 1 8 } T H E R M A L O X I D A T I O N O F T A N T A L U M : A l l t h e r m a l d e v i c e s a m p l e s w e r e i n t r o d u c e d t o a p r e v i o u s l y c o n d i t i o n e d f u r n a c e ( a l r e a d y d e s c r i b e d b e f o r e ) a t 5 0 0 C , a n d w i t h a d r y o x y g e n f l o w r a t e o f 1 1 / m i n . T h e t o t a l o x i d a t i o n t i m e w a s 9 0 m i n , a t w h i c h p o i n t t h e w a f e r s w e r e r e m o v e d a n d l e f t t o c o o l b e f o r e f u r t h e r h a n d l i n g . T h e o x i d i z e d T a s u r f a c e h a d a d e e p b l u e - p u r p l e c o l o r i n t h e a r e a s w e r e t h e A l w a s e t c h e d . T h e f i l m s e e m s t o b e t r a n s p a r e n t , a s t h e A l c a n b e s e e n q u i t e w e l l , a l t h o u g h t h e b l u i s h f i l m i s e v e r y w h e r e . 1 40 {6.19} ANODIC OXIDATION OF TANTALUM: Before the anodic o x i d a t i o n c o u l d proceed, A l metal was t h e r m a l l y evaporated onto the back of each anodic sample. The CHA equipment was used, and a t h i c k n e s s of 7500 A was d e p o s i t e d as i n d i c a t e d by the Thickness Monitor. The a n o d i z a t i o n process c o n s i s t e d of two main steps: a) Constant Current Mode: A n o d i z a t i o n . b) Constant Voltage Mode: H e a l i n g of weak spots. The wafers were anodized in a 0.1 M C i t r i c A c i d s o l u t i o n at a constant c u r r e n t d e n s i t y of 1 mA/cm2. The v o l t a g e l i m i t on the Constant Current Power Supply was set to 50 V, as t h i s avoids the problems encountered i n S e c t i o n {6.16}. As u s u a l , a c h a r t recorder monitored the v o l t a g e V ( t ) a c r o s s the a n o d i z a t i o n c e l l . At t h i s p o i n t , the c o l o r of the wafer changed from m e t a l l i c tantalum, to pale yellow and f i n a l l y to i n t e n s e blue. I t i s q u i t e i n t e r e s t i n g to observe t h i s c o l o r change i n a matter of 2-3 min. The graph of V ( t ) under constant c u r r e n t mode i s shown in F i g u r e 6.1, when i t reaches V l i m i t , the Ta metal i s f u l l y converted i n t o oxide. The b a s i c assumption under the constant v o l t a g e mode, i s that most of the weak spots and p i n h o l e s w i l l be "healed", in a not very w e l l understood process [Dell'Oca et a l . , 1971], but has been e x p e r i m e n t a l l y v e r i f i e d to reduce the e l e c t r o n i c conduction (leakage c u r r e n t ) through the f i l m . A constant v o l t a g e of 15 V was a p p l i e d to the c e l l , f o r 60 min., and the c u r r e n t flow I ( t ) monitored with a c h a r t 141 Sample MTA0S6 .1M Citric Acid J=1mA/cm2 Figure 6.1 MTAOS Anodic Oxidation under Constant Current. 142 Sample MTAOS6 .1M Citric Acid V=10Volts I cell i (uA) 600 -500 H 400 H 300 H 200 1 100 H 0 Figure 6.2 MTAOS Anodic Oxidation under Constant Voltage. 1 4 3 r e c o r d e r , a s s h o w n i n F i g u r e 6 . 2 . A d e c r e a s e o f c u r r e n t w i t h t i m e i s c l e a r l y n o t i c e d . A f t e r a n o d i z a t i o n t o o k p l a c e , a l l w a f e r s w e r e t h o r o u g h l y r i n s e d i n d e - i o n i z e d w a t e r , b o i l e d i n i s o p r o p y l a l c o h o l a n d d r y e d i n a N 2 j e t . { 6 . 2 0 } P R E L I M I N A R Y M I C R O S C O P E E X A M I N A T I O N : A p r e l i m i n a r y e x a m i n a t i o n o f a l l w a f e r s u n d e r t h e m i c r o s c o p e r e v e a l e d t h a t w e l l d e l i n e a t e d f e a t u r e s a r e v i s i b l e , w i t h g o o d r e s o l u t i o n a n d c o n t a m i n a t i o n f r e e s u r f a c e s . T h e t y p i c a l " s t a r s k y " o r " m i l k y w a y " o n t h e T a { o x i d e } a r e a s , w a s o b t a i n e d u n d e r d a r k f i e l d r e v e a l i n g t h e f o r m a t i o n o f h i l l o c k s o r " m e a s l e s " . S o m e " r a g g i n g " w a s e v i d e n t i n t h e g a t e a r e a , p r o b a b l y d u e t o A l u n d e r e t c h i n g o r u n d e r d e v e l o p i n g o f t h e p h o t o r e s i s t . I t w a s n o t i c e d t h a t a s i g n i f i c a n t c o l o r d i f f e r e n c e e x i s t s b e t w e e n t h e a n o d i c a n d t h e r m a l o x i d e w a f e r s . T h e f i r s t o n e s s h o w a b l u e / l i g h t - b l u e c o l o r a n d t h e l a t t e r , a d e e p b l u e - p u r p l e c o l o r . { 6 . 2 1 } L I F T O F F P A T T E R N I N G : A d e v i c e w a f e r w a s i n t r o d u c e d i n t o a h o t ( 7 0 C ) e t c h i n g s o l u t i o n o f p h o s p h o r i c a c i d a n d d e - i o n i z e d w a t e r ( 1 : 1 r a t i o ) , i n o r d e r t o c h e c k t h e q u a l i t y o f t h e p r o c e s s . A f t e r 5 m i n . , a c l o u d o f g a s b u b b l e s f o r m e d a b o v e t h e w a f e r . T h e t e m p e r a t u r e w a s c l o s e l y m o n i t o r e d w i t h a t h e r m o m e t e r p l a c e d i n t h e b e a k e r c o n t a i n i n g t h e s o l u t i o n a n d w a f e r . A f t e r o n e h o u r , t h e A l s h o w s s i g n s o f d e t e r i o r a t i o n a n d i t h a s a c t u a l l y w r i n k l e d . T h e w a f e r w a s a l l o w e d a n o t h e r 3 0 m i n . i n 1 44 the s o l u t i o n , a f t e r which was removed, r i n s e d i n d . i . water. A Sof-Swab (Clean Room Products, Bay Shore, New York) was used to remove, with great care, the w r i n k l e d A l with Ta{oxide} on top, procedure mentioned in Chapter 4. If care i s taken, the surface can be q u i c k l y cleaned, by l i b e r a l use of d . i . water r i n s i n g and the a c t i o n of the Sof-Swab. Any stubborn spots can be removed by p l a c i n g the wafer back in the e t c h i n g s o l u t i o n f o r another 15 min., perhaps f o r 3-4 times or u n t i l r e q u i r e d . A l l device (anodic and thermal) wafers were processed s u c c e s s f u l l y i n a s i m i l a r way. {6.22} MICROSCOPE EXAMINATION AFTER LIFTOFF: Under b r i g h t f i e l d , a l l device wafers show a c l e a n s u r f a c e , with no t r a c e s of l i f t o f f Al/Ta{oxide}, f e a t u r e s such as l i n e s , d r a i n s , gates and sources are very w e l l d e l i n e a t e d , i n d i c a t i n g good r e s o l u t i o n . Dark f i e l d examination shows the t y p i c a l " s t a r sky" p a t t e r n of h i l l o c k s , i n the areas i n which the tantalum metal was o x i d i z e d . For comparison, a c l e a n , bare sample of s i l i c o n wafer (of same c h a r a c t e r i s t i c s as the processed ones) d i d not r e v e a l the " s t a r sky" or "milky way" p a t t e r n , when examined under dark f i e l d . I t i s then q u i t e c o n c l u s i v e , that the d e p o s i t i o n of tantalum metal on s i l i c o n o r i g i n a t e s such a p a t t e r n . A more e x t e n s i v e d i s c u s s i o n on t h i s s u b j e c t was done i n Chapter 5. 1 45 {6.23} PEROXIDE-ACID CLEANING: A m o d i f i e d RCA c l e a n i n g process was used, i n which the h y d r o f l u o r i c a c i d (HF) step was not used, as t h i s would have at t a c k e d the very t h i n (200 A) gate S i oxide. {6.24} SOURCE AND DRAIN THIN GATE OXIDE REMOVAL: Since the t h i n gate oxide was grown everywhere on the wafer s u r f a c e , i t i s necessary to remove i t from the source and d r a i n , to provide a r e l i a b l e c o n t a c t . T h i s was accomplished by p h o t o l i t h o g r a p h y using a negative p h o t o r e s i s t process. E t c h i n g was performed i n a b u f f e r e d HF s o l u t i o n at a rate of 850 A/min, for a t o t a l time of 30 sees. {6.25} MICROSCOPE PHOTOGRAPHY: Examination under the microscope r e v e a l e d that a l l wafers were in e x c e l l e n t c o n d i t i o n . I t was d e c i d e d then to o b t a i n a set of c o l o r negative p i c t u r e s of the devices f a b r i c a t e d . A Wild Model MPS20 Negative F i l m camera was used in c o n j u n c t i o n with a Model M20 Microscope. The combination proved e x c e l l e n t f o r microphotography work. F i g u r e s 6.3 to 6.5 show the d i f f e r e n t d e v i c e s captured by the camera. No t i c e the blue-green c o l o r of the tantalum pentoxide, the pink-orange background of the t h i c k f i e l d oxide and pale yellow of the source and d r a i n d i f f u s i o n s . The c o l o r s are q u i t e s t r i k i n g c o n s i d e r i n g that no c o n t a c t m e t a l l i z a t i o n (Al) has been a p p l i e d y e t . 1 46 {6.26} ALUMINIUM DEPOSITION FOR CONTACTS: The f i n a l c o n t a c t m e t a l l i z a t i o n was done by d e p o s i t i n g high p u r i t y aluminium metal using the E l e c t r o n Beam method. Previous work [Solomon, 1974; Janega, 1983] i n d i c a t e d that c o n s i d e r a b l e amount of Na can be d e p o s i t e d using the thermal evaporation method, i n which A l metal i s melted under high vacuum i n Tungsten f i l a m e n t s . Apparently, the h i g h l y mobile sodium i s present i n the W f i l a m e n t s . Experimental c o n f i r m a t i o n was evident by measuring the t h r e s h o l d v o l t a g e V of c o n v e n t i o n a l MOSFET's f a b r i c a t e d i d e n t i c a l l y , except for the f i n a l c o n t a c t m e t a l l i z a t i o n [Janega, 1983]. The measured V was higher i n the d e v i c e s with t h e r m a l l y evaporated A l than those with E-Beam d e p o s i t e d metal; which i n d i c a t e s the presence of a charge i n the gate oxide ,due to mobile i o n s . The E l e c t r o n Beam equipment (Veeco Model VE400) was thoroughly cleaned, i t s hearth sandblasted, cleaned i n hot acetone and i s o p r o p y l a l c o h o l . The wafer c a r r i e r s were in s p e c t e d and cleaned i n a hot 5% s o l u t i o n of HC1 and HN0 3, si n c e p r e v i o u s users had d e p o s i t e d t i t a n i u m and n i c k e l . The e n t i r e u n i t was reassembled and t e s t e d f o r p o s s i b l e l e a k s . P r e v i o u s l y c l e a n samples (using the m o d i f i e d RCA process as in 6.22) were p l a c e d i n the E-Beam equipment c a r o u s e l . A f t e r a good vacuum was obtained (<4X10 6 T o r r ) , 1000 nm of metal was d e p o s i t e d as i n d i c a t e d by the Thickness Monitor. Upon a s a t i s f a c t o r y i n s p e c t i o n , then the remaining wafers were processed i n the same way,. C o n t r o l wafers were a l s o i n t r o d u c e d to the E-Beam equipment, to monitor the q u a l i t y 1 47 of the f i n a l double i n s u l a t o r s t r u c t u r e . {6.27} DRAIN AND SOURCE CONTACT PHOTOLITHOGRAPHY: T h i s step r e q u i r e d mask alignment, using the con t a c t mask together with the Mask A l i g n e r d e s c r i b e d b e f o r e . As one of the f i n a l p r o c e s s i n g steps, great a t t e n t i o n was e x e r c i z e d in o b t a i n i n g a c l o s e to p e r f e c t alignment. The top m e t a l l i z a t i o n (contact) l a y e r was p a t t e r n e d by ph o t o l i t h o g r a p h y using p o s i t i v e p h o t o r e s i s t , as per d e t a i l s given i n Appendix I I I . Care was taken to c l o s e l y monitor the e t c h i n g process, as n e i t h e r under or o v e r e t c h i n g i s wanted. The c o n t r o l wafers (dot and r i n g ) etched at a f a s t e r r a t e , than those with the device p a t t e r n . T h i s i s probably due to the l a r g e r exposed s u r f a c e f o r the dot and r i n g p a t t e r n , as compared with the device ones. A l s o , gas bubbles (H 2) form very q u i c k l y at the exposed A l s u r f a c e . T h i s has a more pronounced e f f e c t on the very f i n e device f e a t u r e s , slowing the r a t e of e t c h i n g . {6.28} ETCHING ON BACK OF WAFER AND Au DEPOSITION: Before the Au back co n t a c t c o u l d be evaporated onto the wafer's back, i t s back has to be etched very c a r e f u l l y to remove the t h i c k oxide grown i n the i n i t i a l s t e p s . A small Nalgene beaker, with a diameter s l i g h t l y s maller than the 2" wafer was used. The e t c h i n g s o l u t i o n was 48% HF, and the wafer was handled very c a r e f u l l y , p l a c e d on top of the beaker f o r 60 sees. The fumes from the strong HF s o l u t i o n are s u f f i c i e n t to remove by e t c h i n g the t h i c k oxide on the 148 wafer's back. Thorough r i n s i n g i n d . i . water was f o l l o w e d by b o i l i n g in i s o p r o p y l a l c o h o l to remove any t r a c e s of water. Gold was d e p o s i t e d on the back of each wafer using the thermal evaporation o p t i o n a v a i l a b l e i n the Veeco VE400 E-Beam equipment. A f t e r a good vacuum (<10~ 5 T o r r ) was obtained, 320 nm of Au was d e p o s i t e d as i n d i c a t e d by the Thickness Monitor. It was decided that annealing i n Nitrogen was not to be done, because from our p r e v i o u s work i n d i c a t i o n s were that the gate leakage c u r r e n t would i n c r e a s e by s e v e r a l orders of magnitude. {6.29} FINAL MICROSCOPE EXAMINATION: A l l d e v i c e and c o n t r o l wafers were c a r e f u l l y examined under the microscope. T h i s showed that e x c e l l e n t r e s u l t s were obtained and that a l l wafers e x h i b i t e d p r o p e r l y etched c o n t a c t m e t a l l i z a t i o n , good alignment and r e s o l u t i o n . Photography of most samples was accomplished u s i n g the same microscope and negative f i l m camera arrangement d e s c r i b e d i n 6.24. The p i c t u r e s obtained are shown in F i g u r e s 6.6 to 6.12. The aluminium m e t a l l i z a t i o n i s c l e a r l y v i s i b l e as l i g h t grey, under which the source, d r a i n and gate c o n t a c t windows areas can be seen. The source and d r a i n d i f f u s i o n s are shown in l i g h t p u r p l e , and the background c o l o r i s the f i e l d ( t h i c k ) oxide. The p i c t u r e s a l s o show s e v e r a l areas of the mask, namely the RS F l i p - F l o p , 2 Input NOR gate and MOS T r a n s i s t o r . 149 F i g u r e 6.4 MTAOS T r a n s i s t o r , Contact Window area d e t a i l . 150 F i g u r e 6.5 O v e r a l l view showing MOSFET and R-S F l i p F l o p . Figure 6.6 MTAOS T r a n s i s t o r Contact M e t a l l i z a t i o n d e t a i l s . F i g u r e 6 . 8 MOS C a p a c i t o r A r e a , c o n t a c t m e t a l l i z a t i o n . 152 Figure 6.10 Contact pads and alignment markers. 1 53 F i g u r e 6.12 I n t e r c o n n e c t i o n and c o n t a c t pad d e t a i l . 1 54 CHAPTER 7 RESULTS AND MEASUREMENTS ON MTAOS FIELD EFFECT TRANSISTORS The d e v i c e s f a b r i c a t e d as d e s c r i b e d i n the preceding Chapters, were t e s t e d using c o n v e n t i o n a l methods, i n order to determine t h e i r o v e r a l l performance. Of great i n t e r e s t and importance i n MOS technology are the f o l l o w i n g device parameters: 1) The C-V and I-V curves of the gate i n s u l a t o r . 2) The Gate Threshold v o l t a g e . 3) The Drain C u r r e n t - V o l t a g e (output) curves with Gate Volta g e as a parameter. 4) The Drain Current vs. Gate Voltage ( t r a n s f e r ) c u r v e s . 5) The device transconductance and channel conductance. 6) The pulse response, r i s e and f a l l times of the Drain output v o l t a g e waveform. {7.1} TESTING AND MEASUREMENT PROCEDURE: The C-V and I-V curves were obtained using the method and equipment d e s c r i b e d i n Chapter 5. The only d i f f e r e n c e was the use of the Wafer Probing Microscope (Micromanipulator Model 1800 AO Prober with AO Instruments Model 570 microscope). T h i s made i t p o s s i b l e to make a good con t a c t to the A l metal pads i n the d i c e , and to s e l e c t the proper mask quadrant. The l a t t e r i s a consequence of the 1 55 mask o r g a n i z a t i o n , as they a r e made i n such a way t o reduce t h e i r number, a t the expense of y i e l d . Four p a t t e r n s a r e p r i n t e d i n the same mask, hence o n l y one d i c e i s f u n c t i o n a l and the r e m a i n i n g t h r e e a r e scra m b l e d . The gate t h r e s h o l d v o l t a g e was o b t a i n e d by i n s p e c t i o n of the I d v s . Vds c u r v e , when the gate i s con n e c t e d t o the d r a i n . The p r o j e c t i o n of the c u r v e t o the Vds a x i s g i v e s the V T. T h i s i s a s t a n d a r d i n d u s t r y t e s t on MOS d e v i c e s . The d r a i n c u r r e n t f o l l o w s then the r e l a t i o n Id=0Vds 2. The V can a l s o be o b t a i n e d by i n s p e c t i n g the t r a n s f e r c u r v e s as e x p l a i n e d below. The output c u r v e s were o b t a i n e d w i t h a T e k t r o n i x 577 T r a n s i s t o r Curve T r a c e r and they p r o v i d e the q u a s i s t a t i c c h a r a c t e r i s t i c s w i t h the gate v o l t a g e v a r y i n g as a s t a i r c a s e f u n c t i o n . P i c t u r e s were o b t a i n e d of the v a r i o u s d e v i c e s , and they a r e shown i n F i g u r e s 7.13 t o 7.22. An attempt was made t o o b t a i n the s t a t i c o u t p u t c u r v e s . T h i s was done u s i n g the c i r c u i t of F i g u r e 7.1. The gate v o l t a g e was a c c u r a t e l y f i x e d w i t h the a t t e n u a t o r ( f o r p r e c i s e gate v o l t a g e c o n t r o l ) and then the d r a i n v o l t a g e was s l o w l y swept m a n u a l l y . The r e s u l t i s a s e t of p a r a m e t r i c c u r v e s , which a re q u i t e c l o s e t o the ones o b t a i n e d w i t h the c u r v e t r a c e r , as shown i n F i g u r e s 7.2 and 7.3. The t r a n s f e r c u r v e s were o b t a i n e d by s t a t i c measurements, u s i n g the t e s t equipment a r r a n g e d as shown i n F i g u r e 7.1. By ke e p i n g the d r a i n v o l t a g e f i x e d as a parameter, the gate v o l t a g e was swept manually and the d r a i n c u r r e n t measured. The r e s u l t a n t c u r v e s a r e shown i n F i g u r e s 1 56 6 HP70A4A XY Plotter 9 1/2 Anatek — 25D Vsupply * J [mA Decade Voltage Divider (GRI^AAH) Id! $ Rl 500\rv (GR1A32K) Figure 7.1 System for Plotting MOSFET Static Curves. -vds(v) 158 igure 7.3 Static Output Curve, Sample MTAOS4 Anodic. 159 F i g u r e 7 . 4 S t a t i c T r a n s f e r C u r v e , S a m p l e M T A O S 3 T h e r m a l . 160 cn > i f - o Figure 7.5 S t a t i c Transfer Curve, Sample MTAOS4 Anodic. 161 7.4 and 7.5. The gate t h r e s h o l d v o l t a g e can be obtained by i n s p e c t i n g these curves at the p o i n t where the d r a i n c u r r e n t f a l l s to a n e g l i g i b l e v a l u e , i . e . the c u t o f f p o i n t . Above t h i s t h r e s h o l d , a p p r e c i a b l e d r a i n c u r r e n t flows, and below i t , the c u r r e n t i s very small or near zero. The t r a n s i e n t or pulse response was obtained using the c i r c u i t shown in F i g u r e 7.6. A Microdot Model F210B Fu n c t i o n Generator was the pulse source and a T e k t r o n i x 5440 O s c i l l o s c o p e with camera attachment (Tektronix C-5C) was used to re c o r d the output waveform. Measurements were made at 10 and 100 kHz, 50% duty c y c l e square wave, r e p e t i t i o n f r e q u e n c i e s . Care was taken to minimize the l e a d lengths at both gate and d r a i n c o n n e c t i o n s , as any s t r a y inductance and/or ca p a c i t a n c e can a f f a c t the r e s u l t s . T e s t s were performed with Vdd=-6 V, a r e s i s t i v e (carbon) l o a d r e s i s t o r of 1 kfl, and a gate pulse of -5 V peak. By c o r r e c t l y t r i g g e r i n g the o s c i l l o s c o p e , i t i s p o s s i b l e to examine the r i s i n g and f a l l i n g edge of the d r a i n output v o l t a g e p u l s e , as the t r i g g e r p o l a r i t y can be s e l e c t e d . . Delayed time measurements were h e l p f u l i n ob s e r v i n g with g r e a t e r d e t a i l the pulse edges and shape at the output. 162 Anatek 25D Vsupply CbypassT" 22uF TT tant. 7^ DUT -6 to -8V 2? PULSE Generator Microdot F210B Rl 1kn. 10X Probe (TekP6062B) Oscilloscope: Tektronix 5440 with 5A48 Dual Trace Amp. 5B42 Delayed Time Base Figure 7.6 System for Measuring the MOSFET Pulse Response. 163 {7.2} DISCUSSION OF RESULTS {7.2.1} C-V CURVES ON DOUBLE DIELECTRIC INSULATOR: As mentioned i n the pre v i o u s chapter, a d d i t i o n a l samples were prepared and processed, so that the q u a l i t y of the double d i e l e c t r i c i n s u l a t o r could be ev a l u a t e d independently from the device wafers. The curves are shown in F i g u r e s 7.8 and 7.9. From them, i t can be seen that a good q u a l i t y double i n s u l a t o r i s obtained, with w e l l d e f i n e d accumulation and i n v e r s i o n r e g i o n s , l i t t l e h y s t e r e s i s , and g e n e r a l l y a smooth curve that i n d i c a t e s that s u r f a c e s t a t e s have small or no i n f l u e n c e on the i n s u l a t o r performance. T h i s i s e s s e n t i a l f o r the proper and s u c c e s s f u l o p e r a t i o n of the double d i e l e c t r i c MOSFET, and the high q u a l i t y C-V curves obtained are i n d i c a t i v e that the technology i s not only f e a s i b l e , but a l s o s u c c e s s f u l . S i m i l a r r e s u l t s are obtained f o r both anodic and thermal T a 2 0 5 samples, except that s i g n i f i c a n t l y l e s s accumulation c a p a c i t a n c e i s obtained f o r the anodic process, as compared with the thermal tantalum oxide. T h i s i s probably due to a inhomogeneous or porous f i l m , as d i s c u s s e d i n Chapter 5. As a consequence, the d i e l e c t r i c constant of the f i l m i s reduced, r e s u l t i n g i n l e s s oxide c a p a c i t a n c e which i s i n t e r p r e t e d i n the C-V p l o t to correspond to the ca p a c i t a n c e i n accumulation. An attempt was made to o b t a i n the C-V gate c h a r a c t e r i s t i c in the MOSFET d e v i c e s . T h i s would be the d e f i n i t i v e i n d i c a t i o n of the double d i e l e c t r i c i n s u l a t o r q u a l i t y . The C-V p l o t t i n g equipment a l r e a d y d e s c r i b e d , was connected to the wafer probing microscope, with s p e c i a l 1 64 a t t e n t i o n given to s t r a y c a p a c i t a n c e s and le a d l e n g t h s . The Capacitance Meter was c a r e f u l l y n u l l e d and a d j u s t e d . A s u c c e s s f u l C-V p l o t was obtained between the gate and su b s t r a t e f o r the thermal T a 2 0 5 d e v i c e s , as shown i n F i g u r e 7.9. In the case of the anodic Ta{oxide}, a curve with l a r g e h y s t e r e s i s was obtained (Figure 7.10) , i n d i c a t i n g that f o r a p a r t i c u l a r wafer, the q u a l i t y of the double i n s u l a t o r approached those of a memory d e v i c e s , as d e s c r i b e d i n the paper by Angle and T a l l e y (1978). I t i s i n t e r e s t i n g to n o t i c e that the C-V curve f o r the thermal Ta oxide sample i s s i m i l a r of that of a low frequency MOS c a p a c i t o r C-V p l o t [Grove, 1967; Penney and Lau, 1979]. T h i s i s due to the f a c t that i n the i n v e r s i o n r e g i o n , m i n o r i t y c a r r i e r s have to be generated t h e r m a l l y , u s u a l l y from e l e c t r o n - h o l e p a i r s . However these recombine at a c e r t a i n r a t e . The C-V p l o t i s obtained by a small AC s i g n a l superimposed on a l i n e a r sweep, which i s used to measure the change of c a p a c i t a n c e with v o l t a g e . I f the sweep r a t e i s constant, we have two p o s s i b i l i t i e s : the recombination r a t e can be such that not enough m i n o r i t y c a r r i e r s are generated; or a s u f f i c i e n t number m i n o r i t y c a r r i e r s are generated to form an i n v e r s i o n l a y e r that f o l l o w s the sweep v o l t a g e . In the f i r s t case, a low frequency C-V p l o t i s obtained, and no i n v e r s i o n takes p l a c e f o r l a r g e p o s i t i v e gate v o l t a g e . In the second case, a high frequency curve with a d e f i n e d i n v e r s i o n r e gion i s obtained. In the case of a MOSFET, i n which the gate and channel are bounded by the source and d r a i n r e g i o n s , these p r o v i d e an e l e c t r i c a l connection f o r the i n v e r s i o n l a y e r , 165 G A T E V D L T f l G E Figure 7.7 C-V Curve on Double Dielectric Test Wafer (sample MOSCTest 2 0 0 A thermal). 166 Figure 7.8 C-V Curve on Double D i e l e c t r i c Test Wafer (sample MOSCTest 2 0 0 A anodic). 167 F i g u r e 7 . 9 C - V C u r v e o f M O S F E T G a t e , T h e r m a l S a m p l e M T A O S 3 . 168 C A T C V D L T P C C F i g u r e 7 . 1 0 G - V C u r v e o f M O S F E T G a t e , A n o d i c S a m p l e M T A O S 5 . 169 a c t i n g as a sink or source of m i n o r i t y c a r r i e r s to the e x t e r n a l c i r c u i t . Thus, f o r l a r g e gate v o l t a g e , no i n v e r s i o n i s produced and the c a p a c i t a n c e i s then that of accumulation, i n t e r p r e t e d as the gate i n s u l a t o r c a p a c i t a n c e . {7.2.2} I-V CURVES ON THE DOUBLE DIELECTRIC GATE INSULATOR: The I-V measurements gave an idea of the conduction (leakage) c u r r e n t of the gate under normal o p e r a t i o n . The curves were obtained i n a q u a s i s t a t i c way, i n order to a v o i d any c a p a c i t i v e e f f e c t s . The r e s u l t s are shown i n F i g u r e s 7.11 f o r a t y p i c a l thermal oxide sample and i n F i g u r e 7.12, for an anodic process wafer. D i f f e r e n c e s between the leakage c u r r e n t s are l a r g e , i n d i c a t i n g that the anodic oxide has more leakage than i t s thermal c o u n t e r p a r t . A l s o , the p o s i t i o n of the t h r e s h o l d of gate conduction depends on the a p p l i e d source to d r a i n v o l t a g e , Vds. T h i s was not the case of the thermal Ta{oxide} samples, i n which a s m a l l , but c o n s i s t e n t leakage c u r r e n t flows. {7.2.3} GATE THRESHOLD VOLTAGE: A standard procedure to determine the t h r e s h o l d v o l t a g e V T, i s to perform a p l o t of the d r a i n c u r r e n t Id vs. d r a i n to source v o l t a g e Vds, with the gate connected to the d r a i n c i r c u i t . T h i s ensures that the t r a n s i s t o r i s f u l l y s a t u r a t e d , thus the square law c h a r a c t e r i s t i c of the Ids vs. Vds curve. From the curve obtained, the t h r e s h o l d v o l t a g e i s that which above i t , there i s a p p r e c i a b l e flow of d r a i n c u r r e n t , and below i t , there i s very l i t t l e . T h i s i s 1 70 obtained by i n s p e c t i o n of the T r a n s f e r or Saturated curves, the l a t t e r are shown i n . F i g u r e s 7.14 and 7.18. {7.2.4} THE OUTPUT CURVES: The d r a i n c u r r e n t vs. D r a i n to source v o l t a g e , with the Gate v o l t a g e as a parameter (output c h a r a c t e r i s t i c s ) was obtained both with the T r a n s i s t o r Curve Tr a c e r (Tektronix 577) and i n a q u a s i s t a t i c way. T h i s i s because the curve t r a c e r generates a s t a i r c a s e p u l s e i n the gate c i r c u i t , and some concern e x i s t e d whether improper responses c o u l d be c r e a t e d . Both methods gave s i m i l a r r e s u l t s , however i t was found that more d e v i c e s were damaged using the curve t r a c e r , than with the manual q u a s i s t a t i c method. T h i s was i n s p i t e of a l l p r e c a u t i o n s taken to ensure than no s t a t i c d i s c harge was damaging the gate i n s u l a t o r by oxide breakdown. E x c e l l e n t q u a l i t y curves were obtained ( F i g u r e s 7.13 and 7.17) i n d i c a t i n g that the d e v i c e s behave e l e c t r i c a l l y l i k e MOSFET's and that they present gain i f connected in a c i r c u i t . In some wafers, the y i e l d was low and some t r a n s i s t o r s had e x c e s s i v e gate leakage, probably due to p r o c e s s i n g f a u l t s . An example i s given i n F i g u r e s 7.21 and 7.22. A l s o , the anodic Ta{oxide} wafers were very s e n s i t i v e to a p p l i e d v o l t a g e s and i t was q u i t e easy to damage them. The order of connecting the probes in the Wafer Probing equipment i s a l s o important: the l e a s t damage was with the source probe f i r s t , gate second and then the d r a i n . Voltage s p i k e s (pulses of short d u r a t i o n ) a l s o c o n t r i b u t e to a lower y i e l d : f o r example, the Wafer Probing Microscope i s equipped 1 7 1 igure 7.11 I-V Curve on MOSFET Gate, Thermal Sample MTAOS3. 172 0) > > t o i I-CO > LO CN II I > o h LO O >> co cn< —r-to T" LO I CO r~ CM Figure 7.12 I-V Curve on MOSFET Gate, Anodic Sample MTAOS4. 173 with a f l u o r e s c e n t lamp and b a l l a s t (!), which generates a b e a u t i f u l spike when the lamp i s switched on or o f f . A f t e r a few d i s a s t e r s , we simply d i s c o n n e c t e d the lamp and grounded i t s l e a d s . An examination of F i g u r e s 7.3, 7.13 and 7.17 i n d i c a t e s that for low v a l u e s of Vds, the c o r r e s p o n d i n g d r a i n c u r r e n t does not have a l i n e a r v a r i a t i o n . Moreover, the gate v o l t a g e has l i t t l e e f f e c t on t h i s phenomenon, which shows that the channel conductance does not f o l l o w the usual r e l a t i o n s h i p with Vds and Ids, i . e . the slope of the curves c l o s e to the o r i g i n are q u i t e d i s t i n c t and equal to the channel conductance. The reasons behind t h i s unusual behaviour are not known, but i t i s p o s s i b l e that the changes in channel conductance are r e l a t e d to e l e c t r o n t r a p p i n g i n the s i l i c o n . {7.2.5} THE TRANSFER CURVES: These were obtained by the q u a s i s t a t i c method, manually a d j u s t i n g the gate v o l t a g e , and measuring the d r a i n c u r r e n t , with the d r a i n v o l t a g e as a parameter. The slope of these curves g i v e s the transconductance of the d e v i c e . These i n d i c a t e that the d e v i c e s e x h i b i t e d moderate values of gm, as summarized in Table 7.1. Again, curves were obtained f o r both anodic and thermal Ta{oxide} samples. The f i r s t ones presented a smaller transconductance as compared with the l a t t e r . T h i s i s probably due to a reduced i n s u l a t o r c a p a c i t a n c e , as d e s c r i b e d b e f o r e . The curves are shown in F i g u r e s 7.4 and 7.5. 1 74 {7.2.6} PULSE RESPONSE OF THE DD MOSFET's: Another important parameter i s the s w i t c h i n g time of the d e v i c e , c h a r a c t e r i z e d as the ton and t o f f times, corresponding to the time to turn ON and t u r n OFF the t r a n s i s t o r . A pulse generator and f a s t o s c i l l o s c o p e were used, as shown in F i g u r e 7.6. Short leads around the d e v i c e , and good grounds assured a minimum of s t r a y c a p a c i t a n c e and inductance. S e v e r a l measurements were made, one at 10 kHz and another at 100 kHz square waves. Delayed sweep was used to expand the t r a c e around the s w i t c h i n g p o i n t s , i n order to a c c u r a t e l y measure ton and t o f f . Some d i f f e r e n c e s e x i s t between the anodic and thermal Ta{oxide} samples. G e n e r a l l y , the anodic v e r s i o n i s slower than the thermal oxide samples, when used i n the same c i r c u i t . The p i c t u r e s of the a c t u a l scope t r a c e s are shown in F i g u r e s 7.15, 7.16, 7 . 1 9 and 7.20. Table 7.1 summarizes the r e s u l t s obtained i n the MOSFET measurements. TABLE 7.1 SUMMARY OF DOUBLE DIELECTRIC MOSFET CHARACTERISTICS THERMAL Sample MTAOS3 Cox 185000 pf/cm 2 Gate Leakage 1 nA at Vgs=-5 V Gate Threshold - 2.0 V gm at Vgs=-3 V 300 juS gm at Vgs=-6 V 1750 MS ton 400 ns t o f f 220 ns ANODIC Sample MTAOS4 84000 pf/cm 2 4 MA at Vgs=-5 V - 2.5 V 475 MS 1125 MS 500 ns 250 ns 175 {7.2.7} SPICE SIMULATION OF MOSFET CHARACTERISTICS: In order to v e r i f y the performance of the double d i e l e c t r i c d e v i c e s , the s i m u l a t i o n program SPICE was used in c o n j u n c t i o n with the d e v i c e parameters and e x t e r n a l c i r c u i t components. T h i s was-done using the same c i r c u i t values as used in the a c t u a l pulse t e s t ; p l u s the c i r c u i t s t r a y c a p a c i t a n c e s . The MOSFET geometry parameters were obtained from the microscope i n s p e c t i o n of the d e v i c e s (channel l e n g t h and width, d r a i n and source areas and p e r i m e t e r s ) . The t h r e s h o l d v o l t a g e and transconductance parameter were obtained from experimental v a l u e s . The DC T r a n s f e r Curves i n d i c a t e that the device has a t h r e s h o l d c l o s e to -2.5 V and the T r a n s i e n t A n a l y s i s shows a f a s t turn on i n 10 ns and a turn o f f i n 70 ns. The simulated device i s then much f a s t e r than the a c t u a l one, as the i n t e r n a l j u n c t i o n and gate o v e r l a p c a p a c i t a n c e s were not taken i n t o account dur i n g the s i m u l a t i o n . A l s o , the small s t r a y inductances of connecting wires and c a b l e s were not i n c l u d e d in the simulated model. The combined e f f e c t of these r e s u l t i n slower p u l s e response fo r the measured d e v i c e . R e s u l t s of the SPICE s i m u l a t i o n are given i n Appendix IV. 176 Figure 7.13 Double D i e l e c t r i c MOSFET Output Curves, Sample MTAOS 3 (Vgs step 0.5 V, Hor. 1 V/d i v . , V e r t . 0.1 mA/div.) Figure 7.14 Double D i e l e c t r i c MOSFET Saturated T e s t , Sample MTAOS 3 (Vgs=Vds, Hor. 1 V/div., V e r t . 0.2 mA/div.) 1 77 Figure 7.15 Double D i e l e c t r i c MOSFET Pulse Test (Turn On), Sample MTAOS 3 (top t r a c e i s Vgs input, lower i s Vds output) Figure 7.16 Double D i e l e c t r i c MOSFET Pulse Test (Turn O f f ) , Sample MTAOS 3 (top t r a c e i s Vgs input, lower i s Vds output) 178 Figure 7.17 Double D i e l e c t r i c MOSFET Output Curves, Sample MTAOS4 (Vgs step 0.5 V, Hor. 1 V/d i v . , Vert 0.1 mA/div.) Figure 7 . 1 8 Double D i e l e c t r i c MOSFET Saturated T e s t , Sample MTAOS4 (Vgs=Vds, Hor. 1 V / d i v . , V e r t . 0.1 mA/div.) 1 7 9 F i g u r e 7 . 1 9 D o u b l e D i e l e c t r i c M O S F E T P u l s e T e s t ( T u r n O n ) , S a m p l e M T A O S 4 ( t o p t r a c e i s V g s i n p u t , l o w e r i s V d s o u t p u t ) F i g u r e 7 . 2 0 D o u b l e D i e l e c t r i c M O S F E T P u l s e T e s t ( T u r n O f f ) , S a m p l e M T A O S 4 ( t o p t r a c e i s V g s i n p u t , l o w e r i s V d s o u t p u t ) 180 F i g u r e 7.21 Leaky Gate i n MOSFET, Anodic T a 2 0 5 , Sample MTAOS4 (Vgs step 0.5 V, Hor. 1 V/div., V e r t . 0.2 mA/div.) Figu r e 7.22 Leaky Gate i n MOSFET, Thermal T a 2 0 5 , Sample MTAOS3 (Vgs step 0.5 V, Hor. 1 V/div., V e r t . 0.2 mA/div.) 181 {7.3} DOUBLE DIELECTRIC MOSFET EQUIVALENT CIRCUIT: Based on the experimental c h a r a c t e r i s t i c s of the MTAOS t r a n s i s t o r , a Small S i g n a l model can be made. The e q u i v a l e n t c i r c u i t i s shown i n F i g u r e 7.23, and i t i s based on the s i n g l e d i e l e c t r i c MOSFET model [Millman and H a l k i a s , 1972], with the a d d i t i o n of a f i n i t e input conductance (Ggs) due to the i n c r e a s e d gate leakage and an the input c a p a c i t a n c e due to the double d i e l e c t r i c gate i n s u l a t o r with i n c r e a s e d p e r m i t t i v i t y (Cgs). The c a p a c i t a n c e Cgd appears as a r e s u l t of the o v e r l a p between the d r a i n and gate areas. I t s e f f e c t i s to modify the input and output admittances Y i n and Yout due to M i l l e r e f f e c t . 182 MTAOS FET Small Signal Equivalent Circuit A f t e r M i l l e r T r a n s f o r m a t i o n : Y = g + jo){C + ( l - A ) C ,} gs Jgs J gs gd Y d s = 3ds + J ^ d - A ) / A } C C T d Where A i s the c i r c u i t g a i n Figure 7.23 Double Dielectric MOSFET Equivalent Circuit. 183 CHAPTER 8 SUMMARY AND CONCLUSIONS In t h i s work, we have presented the theory, design and development of a MOSFET with a double d i e l e c t r i c gate i n s u l a t o r , u t i l i z i n g the T a 2 0 5 / S i 0 2 i n s u l a t o r s t r u c t u r e . The i n i t i a l MOS c a p a c i t o r work, paved the road f o r the s u c c e s s f u l development of MTAOS F i e l d E f f e c t D e v i c e s . We have demonstrated that t h i s technology i s f e a s i b l e , r e p r o d u c i b l e , and r e l i a b l e . I t can be extended to I n t e g r a t e d C i r c u i t s of more complex nature, without e x c e s s i v e h a r d s h i p s , thus opening new avenues of r e s e a r c h and development f o r an e n t i r e l y new member of the MOS f a m i l y . No attempts were made to reduce the s i z e of our f i n a l d e v i c e s , which f o r today's standards are q u i t e l a r g e (channel l e n g t h =*lO/im). T h i s w i l l be p a r t of f u t u r e work on t h i s f a m i l y of d e v i c e s , aimed towards the VLSI and ULSI t e c h n o l o g i e s . The MOS c a p a c i t o r performance was w e l l w i t h i n the range of r e p o r t e d work by p r e v i o u s a u t h o r s , except perhaps the conduction c h a r a c t e r i s t i c s f o r our d e v i c e s . The double d i e l e c t r i c d e v i c e s e x h i b i t e d very small leakage (conduction) c u r r e n t s , thus rendering them u s e f u l i n gate i n s u l a t o r s f o r MOSFET's. Photoconduction phenomena was q u i t e evident f o r these d e v i c e s , and i t i s another area of p o s s i b l e r e s e a r c h , s t i l l not q u i t e explored nor understood. High c a p a c i t a n c e d e n s i t i e s (50000 pf/cm 2) were o b t a i n e d i n the s i n g l e ( T a 2 0 5 ) capac i t o r s . 184 The Double D i e l e c t r i c MOSFET d e v i c e s presented good output and t r a n s f e r c h a r a c t e r i s t i c s , moderate transconductance and f a s t s w i t c h i n g times. Gate leakage c u r r e n t was very low f o r the thermal tantalum pentoxide d e v i c e s , and somewhat higher f o r the anodic v e r s i o n . The t h r e s h o l d v o l t a g e was 2-2.5 V. The d e v i c e s were s t a b l e and the y i e l d obtained was compatible with l a b o r a t o r y p r o d u c t i o n l e v e l s . From t h i s work, the f o l l o w i n g can be concluded: 1. The tantalum pentoxide i n s u l a t o r i s a f e a s i b l e d i e l e c t r i c f o r use i n MOS technology. 2. Both anodic and thermal tantalum oxides can be grown as t h i n f i l m s on s i l i c o n s u b s t r a t e s . They are compatible with standard MOS p r o c e s s i n g and f a b r i c a t i o n . 3. The p r o p e r t i e s of an i n s u l a t i n g f i l m can be d e r i v e d from MOS c a p a c i t o r C-V c u r v e s . The I-V p l o t s provide a d d i t i o n a l i n f o r m a t i o n on the conduction mechanisms. 4. S i n g l e and Double d i e l e c t r i c MOS c a p a c i t o r s using the A l - T a 2 0 5 - S i and A l - T a 2 0 5 - S i 0 2 - S i s t r u c t u r e s are p o s s i b l e and p r a c t i c a l . 5. Double D i e l e c t r i c MOS F i e l d E f f e c t T r a n s i s t o r s , using tantalum pentoxide over s i l i c o n d i o x i d e are a developed, f e a s i b l e and s u c c e s s f u l technology at the U n i v e r s i t y of B r i t i s h Columbia. 6. The i n t e r f a c i a l o x i d a t i o n of s i l i c o n below tantalum pentoxide proved to be a s u c c e s s f u l technique i n the p r o c e s s i n g of double d i e l e c t r i c MOS c a p a c i t o r s . 185 7. Both MOS c a p a c i t o r s and t r a n s i s t o r s can be improved by b e t t e r p r o c e s s i n g and f a b r i c a t i o n methods, in p a r t i c u l a r the p a t t e r n i n g of tantalum and tantalum o x i d e s . 8. That t h i s author has v e r i f i e d once again that Thomas A. Edison was a b s o l u t e l y c o r r e c t i n s t a t i n g t h at " e v e r y t h i n g takes 10% i n s p i r a t i o n and 90% p e r s p i r a t i o n " . If f u t u r e work i s attempted, the f o l l o w i n g areas of development are recommended to be r e f i n e d : 1. A b e t t e r and more s c i e n t i f i c study should be made on the formation of h i l l o c k s and p i n h o l e s on the d e p o s i t e d Ta metal on s i l i c o n s u b s t r a t e s . In p a r t i c u l a r f o r RF s p u t t e r i n g , as t h i s i s q u i t e a popular technique. 2. The problem of contamination of the tantalum pentoxide f i l m should addressed. A l k a l i ions are known to cause havoc i n s i l i c o n d i o x i d e , the q u e s t i o n i s then: Do the a l k a l i ions ( i . e . sodium) or other ions a f f e c t the q u a l i t y of the T a 2 0 5 i n s u l a t o r ? 3. A compatible p a t t e r n i n g of Ta metal and i t s oxides with s i l i c o n has to be s t u d i e d and developed. A trend e x i s t s towards Reactive Ion E t c h i n g (RIE), using plasma e t c h i n g equipment, which o f f e r s i n t e r e s t i n g s o l u t i o n s . [Seki et a l . , 1983]. 4. Besides the standard RFS d e p o s i t i o n method, others such 186 as E l e c t r o n Beam and Reactive RF s p u t t e r i n g should be developed. In p a r t i c u l a r , one that d e p o s i t s d i r e c t l y the tantalum pentoxide over the s u b s t r a t e , with minimum damage, should be c l o s e to i d e a l . L i t t l e or no data e x i s t s on the E l e c t r o n A f f i n i t y , Work Function d i f f e r e n c e s and B a r r i e r Height of tantalum pentoxide on s i l i c o n . Even worse i s the s i t u a t i o n in the double d i e l e c t r i c s t r u c t u r e . Energy Band diagram data should be compiled, i f a b e t t e r understanding of t h i s i n s u l a t o r i s r e q u i r e d . Memory devi c e s should be i n v e s t i g a t e d . The double d i e l e c t r i c MOSFET, with very t h i n s i l i c o n d i o x i d e , should e x h i b i t enough h y s t e r e s i s so that a memory c e l l can be b u i l t u s ing a s i n g l e t r a n s i s t o r . I d e a l f o r Dynamic Random Acess Memories (DRAM's), and c o n s i d e r i n g the amount of i n t e r e s t on the 1 Mbit RAM, i t should be worthwhile. The i n t e r f a c i a l o x i d a t i o n of s i l i c o n below tantalum pentoxide should be f u r t h e r developed. T h i s i s an i n t e r e s t i n g technique that o f f e r s many s o l u t i o n s i n the p r o c e s s i n g of the Double D i e l e c t r i c s t r u c t u r e . 187 REFERENCES A d o l t , A.R., and Melroy, D.O., 1980, "Humidity E f f e c t s on Reverse Bi a s T e s t i n g of Ta F i l m C a p a c i t o r s " , Proceedings of the 18th Annual Conference on R e l i a b i l i t y P h y s i c s , pp. 39-43. 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Z a i n i n g e r , K.H., and Wang, C.C., 1969, "Thin F i l m D i e l e c t r i c M a t e r i a l s f o r M i c r o e l e c t r o n i c s " , Proceedings of the IEEE, v o l . 57, no. 9, pp. 1564-1570. 1 9 5 A P P E N D I X I I C O M P U T E R S O U R C E P R O G R A M S { A 2 . 1 } F O R T R A N I V P R O G R A M F O R O B T A I N I N G I - V C U R V E S : T h e f o l l o w i n g i s a l i s t i n g o f t h e s o u r c e c o d e w r i t t e n i n F O R T R A N I V , w h i c h i s u s e d t o o b t a i n t h e I - V d a t a i n t h e f o r m o f C u r r e n t - V o l t a g e p l o t s , i n e i t h e r L i n e a r o r S c h o t t k y g r a p h s . T h e m a i n p r o g r a m a n d i t s a s s e m b l e r r o u t i n e s r u n u n d e r t h e O S / 8 o p e r a t i n g s y s t e m i n t h e P D P 8 / E . S o m e o f t h e r o u t i n e s w e r e p r o v i d e d b y D . S m i t h , t h e y a r e w r i t t e n i n t h e O S / 8 a s s e m b l e r c o d e , o c t a l a d d r e s s i n g a n d R A L F m n e m o n i c s . T h i s s o u r c e c o d e a n d i t s c a l l a b l e s u b r o u t i n e s r u n i n g u n d e r t h e o p e r a t i n g s y s t e m w i t h t h e r e q u i r e d h a r d w a r e , p r o d u c e s a g r a p h o f t h e I - V d a t a : C F I L E L E A K Y C M O S I V I N M S T E P S ( L E S S T H A N 1 5 0 ) R E A L V O L T , C I N C , T I M E , S V O L T , T V O L T D I M E N S I O N V O L T ( 1 6 0 ) , C I N C ( 1 6 0 ) , T I M E ( 1 6 0 ) D I M E N S I O N S V O L T ( 1 6 0 ) , T V O L T ( 1 6 0 ) , S P A N N ( 1 6 0 ) , S T R O M ( 1 6 0 ) C A L L P L O T S ( . 0 0 5 , 0 ) I F S T = 2 0 4 8 W R I T E ( 4 , 1 ) 1 F O R M A T ( I X , ' T H I S I S A N I V M E A S U R E M E N T P R O G R A M ' ) 81 W R I T E ( 4 , 6 ) 6 F O R M A T ( 1 H O , ' N U M B E R O F I N C R E M E N T S : ' , $ ) R E A D ( 4 , 7 ) M 7 F O R M A T ( F 1 0 . 0 ) W R I T E ( 4 , 5 ) 5 F O R M A T ( I X , ' M A X I M U M V O L T A G E [ V O L T S ] : ' , $ ) R E A D ( 4 , 9 ) V M A X 9 F O R M A T ( F 1 0 . 5 ) W R I T E ( 4 , 1 1 ) 11 F O R M A T ( 1 X , ' K E I T H L E Y S C A L E : ' , $ ) R E A D ( 4 , 1 2 ) S K E I T H 12 F O R M A T ( E 7 . 0 ) W R I T E ( 4 , 8 ) 8 F O R M A T ( 1 X , ' T Y C O S C A L E : ' , $ ) R E A D ( 4 , 2 ) T Y C O S C 1 9 6 2 F O R M A T ( F 6 . 0 ) W R I T E ( 4 , 4 ) T Y C O S C 4 F O R M A T ( 1 X T Y C O S C A L E : ' , F 6 . 0 , ' V ) . W R I T E ( 4 , 3 ) 3 F O R M A T ( 3 X , ' I ' , 3 X , ' T I M E ' , 5 X , ' V O L T ' , 5 X , ' S V O L T ' , 8 X , ' T V O L C I N I T I A L I Z E A R R A Y T O Z E R O DO 6 6 1 = 1 , M V O L T ( I ) = 0 6 6 C O N T I N U E O L V O L T = 0 C A L L R E S E T DO 6 0 1 = 1 , M C I N C ( I ) = V M A X / M V O L T ( I ) = O L V O L T + C I N C ( I ) O L V O L T = V O L T ( l ) C A L L D A C 1 6 ( V O L T ( I ) ) C A L L T I M E X ( T I M E ( I ) , S V O L T ( I ) , I F S T ) C A L L T Y C O ( T V O L T ( I ) ) T V O L T ( I ) = ( T V O L T ( l ) * T Y C O S C * S K E I T H ) / 1 0 0 0 0 W R I T E ( 4 , 1 0 ) I , T I M E ( I ) , V O L T ( I ) , S V O L T ( I ) , T V O L T ( I ) 10 F O R M A T ( 1 X , I 3 , F 9 . 4 , F 1 0 . 6 , F 1 0 . 6 , E 1 5 . 5 ) 6 0 C O N T I N U E C S E T D A C O U T P U T T O Z E R O W H E N F I N I S H E D V O L T ( M + 1 ) = 0 C A L L D A C 1 6 ( V O L T ( M + 1 ) ) D A T A Y E S , N O / ' Y ' , ' N ' / C ' A S K I F G R A P H I S R E Q U I R E D 2 9 C O N T I N U E W R I T E ( 4 , 3 0 ) 3 0 F O R M A T ( 1 X , ' P L O T G R A P H ? Y / N : ' , $ ) R E A D ( 4 , 2 8 ) A N S W 2 8 F O R M A T ( A 1 ) I F ( A N S W . E Q . N O ) GO T O 4 2 C A S K WHAT K I N D O F P L O T W R I T E ( 4 , 2 6 ) 2 6 F O R M A T ( 1 X , ' L I N E A R P L O T ? Y / N : ' , $ ) R E A D ( 4 , 2 4 ) S W A N 2 4 F O R M A T ( A 1) I F ( S W A N . E Q . Y E S ) GO T O 4 0 W R I T E ( 4 , 2 2 ) 2 2 F O R M A T ( 1 X , ' S C H O T T K Y P L O T ? Y / N : ' , $ ) R E A D ( 4 , 2 0 ) R E P L Y 2 0 F O R M A T ( A l ) I F ( R E P L Y . E Q . Y E S ) GO T O 3 4 4 0 C O N T I N U E C L I N E A R P L O T 3 2 W R I T E ( 4 , 5 6 ) 5 6 F O R M A T ( 1 X , ' P L O T A X E S ? Y / N : ' , $ ) R E A D ( 4 , 4 8 ) R E S P 4 8 F O R M A T ( A I ) I F ( R E S P . E Q . N O ) GO T O 5 2 C A L L P S C A L E ( S V O L T , 6 , M , 1) C A L L P S C A L E ( T V O L T , 8 , M , 1 ) X 0 = S V O L T ( M + 1 ) X I N C = S V O L T ( M + 2 ) Y 0 = T V O L T ( M + 1 ) 1 9 7 Y I N C = T V 0 L T ( M + 2 ) C A L L A X I S ( 0 , 0 , ' V O L T A G E [ V O L T S ] ' , - 1 5 , 6 , 0 , X O , X I N C ) C A L L A X I S ( 0 , 0 , ' C U R R E N T [ A M P S ] ' , 1 4 , 8 , 9 0 , Y 0 , Y I N C ) C A L L S Y M B O L ( 1 . 5 , 8 . 5 , . 1 7 5 , ' L I N E A R I V P L O T ' , 0 , 1 4 ) 5 2 C O N T I N U E C A L L L I N E ( S V O L T , T V O L T , M , 1 , 0 , 0 ) C A L L X Y P L O T ( 0 , 0 , 3 ) GO T O 4 2 3 4 C O N T I N U E C S C H O T T K Y P L O T C C A L C U L A T E S Q R T V O L T A G E A N D L O G C U R R E N T DO 7 0 1 = 1 , M S P A N N ( I ) = S Q R T ( A B S ( S V O L T ( I ) ) ) S T R O M ( I ) = - A L O G 1 0 ( A B S ( T V O L T ( I ) ) ) 7 0 C O N T I N U E W R I T E ( 4 , 7 9 ) 7 9 F O R M A T ( 1 X , ' P L O T A X E S ? Y / N : ' , $ ) R E A D ( 4 , 5 8 ) R E P O N 5 8 F O R M A T ( A 1 ) I F ( R E P O N . E Q . N O ) GO T O 6 2 C A L L P S C A L E ( S P A N N , 6 , M , 1 ) C A L L P S C A L E ( S T R O M , 8 , M , - 1) X S 0 = S P A N N ( M + 1 ) X S I N C = S P A N N ( M + 2 ) Y S 0 = S T R O M ( M + 1 ) Y S I N C = S T R O M ( M + 2 ) C A L L A X I S ( 0 , 0 , ' S Q R T V O L T A G E [ S Q R T ( V O L T S ) ] ' , - 2 6 , 6 , 0 , X S C A L L A X I S ( 0 , 0 , ' - L O G 10 C U R R E N T [ L O G 1 0 ( A M P S ) ] ' , 2 8 , 8 , 9 0 , C A L L S Y M B O L ( 1 . 5 0 , 8 . 5 , . 1 7 5 , ' S C H O T T K Y I V P L O T ' , 0 , 1 6 ) 6 2 C O N T I N U E C A L L L I N E ( S P A N N , S T R O M , M , 1 , 0 , 0 ) C A L L X Y P L O T ( 0 , 0 , 3 ) 4 2 C O N T I N U E W R I T E ( 4 , 7 1 ) 71 F O R M A T ( 1 X , ' C O N T I N U E G R A P H ? Y / N : ' , $ ) R E A D ( 4 , 8 0 ) W O R D 8 0 ' F O R M A T ( A 1 ) I F ( W O R D . E Q . N O ) GO T O 81 GO T O 2 9 E N D { A 2 . 2 } F O R T R A N C A L L A B L E S U B R O U T I N E S : T h e s e a r e s u b r o u t i n e s t h a t i n t e r f a c e t o t h e R e a l T i m e C l o c k , o p e r a t e t h e 16 b i t D i g i t a l t o A n a l o g C o n v e r t e r , a n d a c q u i r e d a t a f r o m t h e D a n a a n d T y c o D i g i t a l V o l t m e t e r s : / ******************************* / * S U B R O U T I N E S W ( I , J ) * / * S U B R O U T I N E R E L A Y ( I ) * / * S U B R O U T I N E D A C 1 6 ( V ) * y * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * / / / T H E S E F O R T R A N C A L L A B L E S U B R O U T I N E S W I L L R E A D / T H E C O N S O L E S W I T C H R E G I S T E R OR O P E N A R E L A Y . / C A L L I N G S E Q U E N C E : / C A L L S W ( I , J ) / I = S W I T C H R E G I S T E R N U M B E R ( 0 - 1 1 ) / J = 1 I F S W I T C H I S S E T , 0 O T H E R W I S E / C A L L R E L A Y ( I ) / I = R E L A Y N U M B E R ( 0 - 3 ) / C A L L D A C 1 6 ( V ) / V = V O L T A G E WORD ( 0 - 6 5 5 3 5 ) / / T H E A R G U M E N T S O F S W ( I , J ) A N D R E L A Y ( I ) C A N B E R E A L / O R I N T E G E R . / / / S E T U P , R E L A Y , S E C T SW E X T E R N SW8 E X T E R N R E L A Y 8 E X T E R N W 1 E X T E R N D A C 8 E X T E R N A 8 B A S E 0 J S A S E T U P / G E T A R G U M E N T A N D R T N P T R ' S T R A P 4 SW8 / C A L L 8 M O D E R O U T I N E S T A R T D S E T X # I R / S E T I N D E X T O T H I S R O U T I N E F L D A % 0 , 2 / G E T P T R T O A N S W E R F S T A 3 / A N D S T O R E I T S E T X W 1 S T A R T F X T A 0 F S T A % 3 / G I V E A N S T O C A L L E R F L D A 3 0 / R T N T O C A L L E R J A C / 0 ; 0 / R E T U R N G O E S H E R E S T A R T D S E T X # I R / S E T T O R O U T I N E ' S I N D E X F L D A % 0 , 1 / G E T P T R T O A R G F S T A 3 / A N D S T O R E I T S E T X W 1 S T A R T F F L D A % 3 / U S E R A R G T O F A C A T X 0 / P A S S T O 8 M O D E J A S E T U P / E N T R Y R E L A Y / O P E N A R E L A Y J S A S E T U P / G E T A R G F R O M C A L L E R 199 / / DAC16, / / #IR, #64K, #25, TRAP4 RELAY8 /CALL 8 MODE ROUTINE /ARG IS IN FPP XR3 FLDA 30 /RTN TO CALLER JAC ENTRY DAC 1 6 JSA SETUP /GET ARG AND RTN PTR'S FLDA% 3 /GET THE VOLTAGE WORD FMUL #64K /AND SCALE TO THE DAC RANGE FDIV #2 5 /DIVIDE BY THE GAIN OF /THE DAC (ASSUMED TO BE 25) ALN 0 /MAKE IT AN INTEGER FSTA A8 /PASS TO 8-MODE TRAP4 DAC8 /CALL 8-MODE ROUTINE FLDA 30 /GET RETURN ADDRESS JAC /RETURN TO CALLER OOOO /THIS ROUTINE'S INDEX 0001 /XR1 0002 /XR2 F 6553. 5 /FULL SCALE FOR DAC F 25.0 /GAIN OF THE DAC / / / / / / / / / DEC 8 SECT8 DECOD SUBROUTINE ENTRY NAME ENTRY DEC8 ENTRY SW8 ENTRY RELAY8 LIST OF DATA ENTRY POINTS ENTRY V8A ENTRY RANG 8 ENTRY SGN ENTRY W3 ENTRY W2 ENTRY W1 DECODE VOLTAGE WORDS TO DECIMAL START WITH VOLTAGE WORD 3 n u TAD W3 /GET VOLTAGE WORD 3 AND K17 /MASK BITS 8-11 DCA V8A /AND STORE TAD W3 /GET THE WORD AGAIN RTR /ROTATE RIGHT 4 TIMES RTR AND K17 /MASK BITS 8-1 1 DCA V8B /AND STORE 2 0 0 / / / / / / P O S , / / / R A N G , / RO, / R1 , T A D R A L R T L R T L A N D D C A W3 K1 7 V 8 C / G E T T H E WORD A G A I N / R O T A T E L E F T / 5 T I M E S / M A S K B I T S 8 - 1 1 / A N D S T O R E U N P A C K V O L T A G E WORD 2 T A D A N D D C A T A D R T R R T R A N D D C A W 2 K1 7 V 8 D W 2 K1 7 V 8 E / G E T V O L T A G E WORD 2 / M A S K B I T S 8 - 1 1 / A N D S T O R E / G E T T H E WORD A G A I N / R O T A T E R I G H T 4 T I M E S / M A S K B I T S 8 - 1 1 / A N D S T O R E U N P A C K V O L A T G E WORD 1 T A D W 1 / G E T V O L T A G E WORD 1 R T L / R O T A T E L E F T 2 T I M E S A N D K1 / M A S K B I T 11 D C A V 8 F / A N D S T O R E D E T E R M I N E P O L A R I T Y T A D A N D S N A J M P T A D D C A J M P C L A C L L D C A W 1 K 4 0 0 P O S K1 S G N R A N G S G N / G E T V O L T A G E WORD 1 / M A S K A C 0 3 / I S I T Z E R O ? / Y E S . S I G N I S P O S I T I V E / N O . S E T S I G N C O D E = 1 / G O T O R A N G E T E S T / S E T S I G N C O D E = 0 D E T E R M I N E V O L T A G E R A N G E S T A R T B Y T E S T I N G F O R . I V R A N G E T A D W 2 / G E T V O L T A G E WORD 2 A N D K 6 0 0 0 / M A S K A C 0 0 A N D A C 0 1 S Z A / A C = 0 ? J M P RO / N O . T R Y N E X T R A N G E D C A R A N G 8 / Y E S . S E T R A N G E C O D E = 0 J M P O U T / R E T U R N T R Y I V R A N G E C L A C L L T A D W 2 / G E T V O L T A G E WORD 2 A N D K 5 4 0 0 / M A S K A C 0 0 , A C 0 2 , A N D A C 0 3 S Z A / A C = 0 ? J M P R1 / N O . T R Y N E X T R A N G E T A D K1 / Y E S . S E T R A N G E C O D E = D C A R A N G 8 J M P O U T / R E T U R N T R Y 1 OV R A N G E C L A C L L T A D W 2 / G E T V O L T A G E WORD 2 A N D K 5 0 0 0 / M A S K A C 0 0 A N D A C 0 2 S Z A / A C = 0 ? 2 0 1 J M P R 2 / N O . T R Y N E X T R A N G E T A D K 2 / Y E S . S E T R A N G E C O D E = 2 D C A R A N G 8 J M P O U T / R E T U R N / T R Y 1 0 0 V R A N G E R 2 , C L A C L L T A D W 2 / G E T V O L T A G E WORD 2 A N D K 4 4 0 0 / M A S K A C 0 0 A N D A C 0 3 S Z A / A C = 0 ? J M P R 3 / N O . T R Y N E X T R A N G E T A D K 3 / Y E S . S E T R A N G E C O D E = 3 D C A R A N G 8 J M P O U T / R E T U R N / ' M U S T 1 B E 1 0 0 0 V R A N G E R 3 , C L A C L L T A D K 4 / S E T R A N G E C O D E = 4 D C A R A N G 8 O U T , C D F C I F 0 / R E T U R N T O C A L L I N G J M P % D E C 8 / R O U T I N E / / / / R E A D T H E S W I T C H R E G I S T E R A N D R E T U R N W I T H / X R 3 = 1 I F B I T I S S E T S W 8 , 0 L A S / R E A D T H E R E G I S T E R WORD D C A R A N G 8 / A N D S T O R E I T T A D W 1 / G E T B I T P O I N T E R C M A C L L C M L / S E T U P F O R M A S K I N G D C A W 1 R A R / R O T A T E L I N K U N T I L I S Z W 1 / X R 3 = 0 J M P . - 2 A N D R A N G 8 / M A S K S W I T C H WORD S Z A C L A / I F B I T I S S E T , S E T X R 3 = 1 I S Z W 1 C I F C D F 0 / R T N T O C A L L E R J M P % SW8 / / / / E N E R G I Z E T H E S P E C I F I E D R E L A Y R E L A Y 8 , 0 C L A C L L T A D W 1 A N D K 3 / M A S K I T T O M A K E S U R E 6 3 5 4 / E N E R G I Z E T H E R E L A Y C L A C L L C I F C D F 0 / R T N T O C A L L E R J M P % R E L A Y 8 / K1 , 1 K 2 , 2 K 3 , 3 K 4 , 4 K 1 7 , T 7 2 0 2 K 4 0 0 , 4 0 0 K 4 4 0 0 , 4 4 0 0 K 5 0 0 0 , 5 0 0 0 K 5 4 0 0 , 5 4 0 0 R 6 0 0 0 , 6 0 0 0 V 8 A , 0 / L I S T O F D E C O D E D V 8 B , 0 / V O L T A G E WORDS V 8 C , 0 V 8 D , 0 V 8 E , 0 V 8 F , 0 R A N G 8 , 0 / R A N G E C O D E WORD S G N , 0 / S I G N C O D E WORD W 3 , 0 / C O D E D V O L T A G E WORDS W 2 , 0 / F R O M C A L L I N G R O U T I N E W l , 0 / / / / / / / / P H O T 8 , P A , / / / / / S E C T 8 P H 0 8 L I S T O F S U B R O U T I N E N A M E S E N T R Y P H O T 8 E N T R Y G A N 8 E N T R Y C L O K 8 E N T R Y R C L K 8 E N T R Y S T E P L I S T O F D A T A E N T R Y P O I N T S E N T R Y T 8 L E N T R Y V 2 E N T R Y V 3 E N T R Y E R R 8 E N T R Y F A S T 8 E N T R Y G 8 L R E A D P H O T O D E T E C T O R A N D S T O R E D A T A WORD I N E R R 8 0 C L A C L L T A D C H A N 6 3 2 3 6 3 2 1 J M P P A 6 3 2 4 C M A D C A E R R 8 C D F C I F 0 J M P % P H O T 8 / G E T C H A N N E L S E L E C T / O U P U T T O D E V I C E 3 2 / C O N V E R S I O N D O N E ? / N O . T R Y A G A I N / L O A D T H E D A T A WORD / C O M P L E M E N T I T / A N D S T O R E I T / R E T U R N TO C A L L I N G / R O U T I N E G E T T H E G A I N W O R D , A D D C H A N N E L N U M B E R A N D S T O R E I N C H A N 2 0 3 / / / G A N 8 / / / / C L O K 8 / / / F L G , / / / / T E S T , / / / G A I N WORD I S I N T H E R A N G E 0 - 7 A N A L O G I N P U T O N . C H A N N E L #17 0 C L A C L L T A D G 8 L / G E T T H E G A I N WORD ( 0 - 7 ) R A L / S H I F T L E F T T O B I T S 6 - 8 R T L T A D K 7 / A D D T H E A N A L O G C H A N # D C A C H A N / A N D S T O R E I T T A D C H A N / S E L E C T T H E NEW G A I N 6 3 2 3 C D F C I F 0 / R E T U R N T O C A L L I N G J M P % G A N 8 / R O U T I N E R E A D C L O C K A N D D A N A 0 I N I T I A L I Z E S Y S T E M D I R E C T C O M M A N D C L A C L L T A D F A S T 8 / G E T S U P E R F A S T C O D E T A D K 2 0 0 0 / A D D A C 0 1 ( 1 ) T A D C H A N / A D D T H E C H A N N E L S E L E C T 6 3 2 3 / O U T P U T T O D E V I C E 3 2 T E S T F O R S Y S T E M R D Y - - G R O U N D T R U E , A C 0 6 C L A C L L 6 3 1 1 / L O A D F R O M D E V I C E 31 A N D K 4 0 / M A S K A C 0 6 S Z A / I S I T Z E R O ? J M P F L G / N O . T E S T A G A I N G E T T I M E , T H E N S T A R T C O N V E R S I O N - - S E T S Y S D I R T O 0 , W A I T , T H E N B R I N G H I G H A G A I N . R E A D C L O C K W H I L E W A I T I N G T A D F A S T 8 / G E T S U P E R F A S T C O D E T A D C H A N / A D D C H A N N E L S E L E C T C O D E 6 3 7 1 / C L O C K R E A D Y ? J M P T E S T / N O . T R Y A G A I N 6 3 2 3 / Y E S . O U T P U T D A T A WORD 6 3 6 1 / L O A D LOW O R D E R C L O C K WORD D C A T 8 L / A N D S T O R E I T 6 3 6 2 / L O A D H I G H O R D E R C L O C K WORD D C A T 8 H / A N D S T O R E I T N O P / W A I T B E F O R E B R I N G I N G S Y S T E M N O P / D I R E C T C O M M A N D H I G H A G A I N N O P N O P T A D F A S T 8 / L O A D S U P E R F A S T WORD T A D C H A N / A D D C H A N N E L S E L E C T WORD T A D K 2 0 0 0 / A D D A C 0 1 ( 1 ) 6 3 2 3 / O U T P U T T O D E V I C E 3 2 W H I L E W A I T I N G F O R E N D O F S I G N B I T F R O M T H E 1 2 - B I T C O N V E R S I O N , T I M E WORDS S T R I P O F F 2 0 4 / / / D F L G , / / / / / / / R C L K 8 , / / / / / / S T E P , B A C K , C L A C L L T A D T 8 L / G E T LOW O R D E R T I M E WORD R A R / R O T A T E 1 B I T TO T H E R I G H T D C A T 8 L / A N D S T O R E A C R A L / G E T B I T 11 D C A S T 8 L / A N D S T O R E I T T A D T 8 H / R E P E A T W I T H H I G H O R D E R WORD R A R D C A T 8 H R A L D C A S T 8 H C H E C K F O R E N D O F C O N V E R S I O N D A T A R E A D Y — G R O U N D T R U E , A C 0 4 C L A C L L 6 3 1 1 / L O A D F R O M D E V I C E 31 A N D K 2 0 0 / M A S K A C 0 4 S Z A / I S I T Z E R O ? J M P D F L G , / N O . T R Y A G A I N R E A D I N B C D V O L T A G E WORD A N D S T O R E I N V 1 , V 2 , V 3 6 3 1 4 / L O A D V O L T A G E WORD 3 D C A V 3 / A N D S T O R E I T 6 3 1 2 / L O A D V O L T A G E WORD 2 D C A V 2 / A N D S T O R E I T 6 3 1 1 / L O A D V O L T A G E WORD 1 D C A VI / A N D S T O R E I T C D F C I F 0 / R E T U R N T O C A L L I N G J M P % C L O K 8 / R O U T I N E R E S E T T H E 1 0 0 n M I C R O S E C O N D C L O C K 6 3 7 4 / R E S E T P U L S E C D F C I F 0 J M P % R C L K 8 S T E P T H E S P E C I F I E D S T E P P I N G MOTOR A N D W A I T F O R 1 T H E S P E C I F I E D T I M E B E F O R E : n R E T U R N I N G u C L A C L L T A D M2 / G E T M O T O R S E L E C T C O D E 6 3 3 2 / S E L E C T T H E MOTOR 6 3 3 4 / S T E P T H E MOTOR C L A C L L T A D S L O / G E T T H E W A I T T I M E WORD C I A / G E T I T S N E G A T I V E I A C / I N C R E M E N T A C C U M U L A T O R S Z A / A C I S Z E R O ? J M P B A C K / N O . J U M P T O B A C K C D F C I F 0 / Y E S . R E T U R N T O C A L L I N G 2 0 5 J M P % S T E P / R O U T I N E / / / / T A B L E O F C O N S T A N T S A N D D A T A WORDS K 4 0 , 4 0 K 2 0 0 , 2 0 0 K 2 0 0 0 , 2 0 0 0 C H A N , 7 / C H A N N E L S E L E C T C O D E K 7 , 7 E R R 8 , 0 / P H O T O D E T E C T O R E R R O R T 8 L , 0 / L O W O R D E R C L O C K WORD S T 8 L , 0 T 8 H , 0 / H I G H O R D E R C L O C K . W O R D S T 8 H , 0 V1 , 0 / D A N A V O L T A G E WORDS V 2 , 0 V 3 , 0 F A S T 8 , 0 / S U P E R F A S T C O D E WORD G 8 L , 0 / G A I N S E L E C T WORD S L O , 0 / D E L A Y WORD F O R S T E P M 2 , 0 / M O T O R S E L E C T WORD / / / / / / / / R E A D 8 S E C T 8 R E A 8 S U B R O U T I N E E N T R Y N A M E E N T R Y R E A D 8 E N T R Y D A C 8 L I S T O F D A T A E N T R Y P O I N T S E N T R Y A 8 E N T R Y D 8 S U B R O U T I N E T O E N C O D E R S 0 C L A C L L T A D I S C 8 6 3 3 2 N O P N O P N O P N O P N O P N O P N O P C L A C L L 6 3 0 2 D C A 6 3 0 4 D C A T A D A N G H A N G L A N G L R E A D A N D D E C O D E S H A F T / G E T E N C O D E R S E L E C T WORD / A N D O U T P U T I T / W A I T F O R M U X T O / S E T T L E / G E T H I G H O R D E R A N G L E / A N D S T O R E I T / G E T LOW O R D E R A N G L E / A N D S T O R E I T / G E T LOW O R D E R WORD 2 0 6 A N D K1 7 / M A S K B I T S 8 - 1 1 D C A E 8 / A N D S T O R E D E C O D E D B C D T A D A N G L / G E T LOW O R D E R WORD A G A I N R T R R T R / R O T A T E R I G H T 2 T I M E S A N D K1 7 / M A S K B I T S 8 - 1 1 D C A D 8 / A N D S T O R E D E C O D E D B C D T A D A N G L / G E T LOW O R D E R A G A I N R T L R T L / R O T A T E L E F T 5 T I M E S R A L A N D K1 7 / M A S K B I T S 8 - 1 1 D C A C 8 / A N D S T O R E D E C O D E D B C D T A D A N G H / G E T H I G H O R D E R WORD A N D K1 7 / M A S K B I T S 8 - 1 1 D C A B 8 / A N D S T O R E D E C O D E D B C D T A D A N G H / G E T H I G H O R D E R A G A I N R T R R T R / R O T A T E R I G H T 2 T I M E S A N D K1 7 / M A S K B I T S 8 - 1 1 D C A A 8 / A N D S T O R E D E C O D E B C D C D F C I F 0 / R E T U R N T O C A L L I N G J M P % R E A D 8 / R O U T I N E / / / / S U B R O U T I N E T O O U T P U T A V O L T A G E WORD T O T H E / 1 6 - B I T D / A C O N V E R T E R D A C 8 , 0 C L A C L L T A D B 8 / G E T M O S T S I G WORD A N D K1 7 / M A S K LOW 4 B I T S R A R R T R R T R / R O T A T E I N T O H I G H 4 B I T S D C A A N G H / A N D S A V E I T T A D C 8 / G E T L E A S T S I G WORD R T R R T R / R O T A T E R I G H T 4 B I T S A N D K 3 7 7 / M A S K 8 LOW O R D E R T A D A N G H / G E T H I G H B I T S C M A / C O M P L E M E N T 61 7 3 / A N D O U T P U T T O T H E D A C T A D C 8 / G E T L E A S T S I G WORD A G A I N A N D K1 7 / M A S K LOW O R D E R 4 B I T S R A R R T R R T R / R O T A T E I N T O H I G H 4 B I T S C M A / C O M P L E M E N T 6 4 2 3 / A N D O U T P U T T O T H E D A C C L A C L L C D F C I F 0 / R E T U R N T O C A L L I N G J M P % D A C 8 / R O U T I N E / / / 2 0 7 K1 7 , 1 7 K 3 7 7 , 3 7 7 A N G H , 0 / H I G H O R D E R A N G L E WORD A N G L , 0 / L O W O R D E R A N G L E WORD A 8 , 0 / L I S T O F D E C O D E D A N G L E B 8 , 0 / W O R D S I N B C D C 8 , 0 D 8 , 0 E 8 , 0 I S C 8 , 0 / E N C O D E R S E L E C T C O D E / F I L E : T Y C O / / * * / * S U B R O U T I N E T Y C O ( V O L T ) / * * * * / * R E A D T H E V O L T A G E F R O M T H E T Y C O * / * V O L T M E T E R A N D R E T U R N T H E V O L T A G E * / * T O T H E C A L L I N G R O U T I N E . / * * / * N O T E : T H E V O L T A G E I S N O T S C A L E D * / * T O T H E V O L T A G E R A N G E O F * / * T H E V O L T M E T E R . S C A L I N G / * M U S T B E D O N E B Y T H E C A L L I N G / * R O U T I N E . A L S O , T H E V O L T A G E / * WORD I S A L W A Y S A P O S I T I V E / * Q U A N T I T Y . / * * / / / C A L L I N G S E Q U E N C E : / / C A L L T Y C O ( V O L T ) / / W H E R E : V O L T = V O L T A G E R E A D F R O M T H E / T Y C O V O L T M E T E R . / / / S E C T 8 D V M 8 8 E N T R Y T Y C O / / / / R E A D T H E V O L T A G E F R O M T H E T Y C O D V M , / D E C O D E T H E V O L T A G E , A N D R E T U R N T O T H E / R A L F C A L L I N G R O U T I N E . / D V M 8 , 0 / P D P 8 M O D E R O U T I N E E N T R Y F L A G , C L A C L L 6 3 2 4 / G E T T H E F L A G WORD 2 0 8 A N D S Z A J M P 6 3 0 1 D C A 6 3 1 1 A N D D C A T A D A N D D C A T A D R T R R T R A N D D C A / T A D A N D D C A / T A D R T R R T R A N D D C A / T A D R A L R T L R T L A N D D C A / C D F J M P % / / / K1 7 , 1 7 K 3 7 , 3 7 K 2 0 0 0 , 2 0 0 0 / / D V M L , 0000 D V M H , 0000 V H 1 , 0 0 0 0 V H 2 , 0000 V L 1 , 0 0 0 0 V L 2 , 0000 V L 3 , 0000 / / / C I F K2000 F L A G D V M L K 3 7 D V M H D V M H K1 7 V H 2 D V M H R1 7 V H 1 D V M L K1 7 V L 3 D V M L K1 7 V L 2 D V M L K1 7 V L 1 0 D V M 8 / M A S K T H E F L A G B I T / I F D V M R E A D Y , C O N T I N U E / E L S E , C H E C K F L A G A G A I N / G E T T H E LOW O R D E R V O L T A G E / W O R D / A N D S A V E I T / G E T T H E H I G H O R D E R V O L T A G E / W O R D / M A S K T H E LOW O R D E R B I T S / A N D S A V E T H E WORD / G E T T H E WORD / M A S K T H E P R O P E R B I T S / A N D S A V E T H E M / G E T T H E WORD A G A I N / R O T A T E R I G H T 4 T I M E S / M A S K T H E F I R S T B I T / A N D S A V E I T / G E T T H E LOW O R D E R WORD A G A I N / M A S K T H E F I R S T B C D D I G I T / A N D S A V E I T / G E T T H E F U L L WORD A G A I N / R O T A T E 4 T I M E S T O T H E R I G H T / M A S K N E X T B C D D I G I T / A N D S A V E I T / G E T T H E F U L L WORD A G A I N / R O T A T E 5 T I M E S T O T H E L E F T / M A S K F I N A L B C D D I G I T / A N D S A V E I T / R E T U R N TO R A L F / R O U T I N E / L O W O R D E R V O L T A G E WORD / H I G H O R D E R V O L T A G E WORD / D E C O D E D V O L T A G E WORDS 2 0 9 / # B A S E , F 0 . 0 # X R r F 0 . 0 # V O L T , F 0 . 0 # T E M P , F 0 . 0 # T E N 1 , F 1 0 . 0 / B A S E 0 T Y C O , S T A R T D F L D A 3 0 F S T A # G O B A K F L D A 0 S E T X #XR S E T B # B A S E B A S E # B A S E L D X 1 , 1 F S T A # B A S E F L D A % # B A S E , 1 F S T A # V O L T / / S T A R T F T R A P 4 D V M 8 S E T X V H 1 X T A 0 F M U L # T E N 1 F S T A # T E M P X T A 1 F A D D # T E M P F M U L # T E N 1 . F S T A # T E M P X T A 2 F A D D # T E M P F M U L # T E N 1 F S T A # T E M P X T A 3 F A D D # T E M P F M U L # T E N 1 F S T A # T E M P X T A 4 F A D D # T E M P F S T A % # V O L T # G O B A K , J A / B A S E P A G E R E G / I N D E X R E G ' S 0 - 2 / P O I N T E R T O A R G / T E M P O R A R Y S T O R A G E / U S E C A L L E R S B A S E P A G E F O R NOW / S T A R T O F R A L F R O U T I N E / G E T R E T U R N A D D R E S S / A N D S A V E I T / G E T P O I N T E R T O A R G / S E T I N D E X R E G T O T H I S P A G E / S A M E F O R B A S E R E G / P U T 1 I N T O I N D E X R E G 1 / S A V E P O I N T E R / A N D S A V E I T / S T A R T F L O A T I N G P O I N T / C A L L T H E P D P 8 M O D E R O U T I N E / S E T I N D E X R E G T O P D P 8 M O D E A R E A / G E T M O S T S I G D I G I T / M U L T I P L Y B Y 10 / A N D S A V E T H E R E S U L T / G E T N E X T D I G I T / A D D T O T E M P / M U L T I P L Y B Y 10 / A N D S A V E T H E R E S U L T / G E T N E X T D I G I T / A D D T O T E M P / M U L T I P L Y B Y 10 / A N D S A V E T H E R E S U L T / G E T N E X T D I G I T / A D D T O T E M P / M U L T I P L Y B Y 10 / A N D S A V E T H E R E S U L T / G E T L E A S T S I G D I G I T / A D D T O T E M P / P A S S T O T H E C A L L I N G R O U T I N E / R E T U R N T O T H E C A L L I N G / R O U T I N E { A 2 . 3 } F O R T R A N I V P R O G R A M F O R O B T A I N I N G E Q U I V A L E N T P E R M I T T I V I T Y C U R V E S : T h i s s o u r c e c o d e c a l c u l a t e s t h e e q u i v a l e n t p e r m i t t i v i t y o f a d o u b l e i n s u l a t o r s t u c t u r e . G i v e n t h e r e l a t i v e d i e l e c t r i c c o n s t a n t o f t h e i n n e r a n d o u t e r d i e l e c t r i c s a n d 2 1 0 t h e i r r a t i o of t h i c k n e s s e s , i t produces a p l o t of the e q u i v a l e n t p e r m i t t i v i t y as a f u n c t i o n of the i n s u l a t o r t h i c k n e s s r a t i o . The number of increments has to be s u p p l i e d . C F I L E P E R M I Q C T H I S P R O G R A M C A L C U L A T E S A N D P L O T S T H E E Q U I V A L E N T P E R M C O F A D O U B L E I N S U L A T O R S T R U C T U R E , W I T H T / S A S A P A R A M E R E A L A , B , X , Y D I M E N S I O N X ( 2 0 0 ) , Y ( 2 0 0 ) , T I N C ( 2 0 0 ) C A L L P L O T S ( 0 . 0 0 5 , 0 ) W R I T E ( 4 , 1 ) 1 F O R M A T ( 1 X , ' N U M B E R O F I N C R E M E N T S : ' , $ ) R E A D ( 4 , 2 ) M 2 F O R M A T ( F 1 0 . 0 ) W R I T E ( 4 , 3 ) 3 F O R M A T ( 1 X , ' M A X . T H I C K N E S S R A T I O O F I N S U L A T O R I I T O I : R E A D ( 4 , 4 ) R 4 F O R M A T ( F 1 0 . 0 ) 14 C O N T I N U E W R I T E ( 4 , 5 ) 5 F O R M A T ( 1 X , ' P E R M I T T I V I T Y O F I N S U L A T O R I : ' , $ ) R E A D ( 4 , 6 ) B 6 F O R M A T ( F 1 0 . 0 ) W R I T E ( 4 , 7 ) 7 F O R M A T ( 1 X , ' P E R M I T T I V I T Y O F I N S U L A T O R I I : ' , $ ) R E A D ( 4 , 8 ) A 8 F O R M A T ( F 1 0 . 0 ) C I N I T I A L I Z E A R R A Y T O Z E R O DO 9 I = 1 , M X ( l ) = 0 Y ( l ) = 0 T I N C ( I ) = 0 9 C O N T I N U E O L D T = 0 DO 10 1 = 1 , M T I N C ( I ) = R / M X ( I ) = O L D T + T I N C ( l ) O L D T = X ( l ) Y ( I ) = B * ( ( 1 + X ( I ) ) / ( 1 + X ( I ) * ( B / A ) ) ) 10 C O N T I N U E C A S K I F G R A P H I S R E Q U I R E D D A T A Y E S , N O / ' Y ' , ' N ' / W R I T E ( 4 , 1 1 ) 11 F O R M A T I [ 1 X , ' P L O T A X E S ? Y / N : ' , $ ) R E A D ( 4 , 1 2 ) R E S P 12 F O R M A T ( A 1 ) 21 1 I F ( R E S P . E Q . N O ) GO T O 13 C A L L P S C A L E ( X , 6 , M , 1 ) C A L L P S C A L E ( Y , 8 , M , 1 ) X 0 = X ( M + 1 ) X I N C = X ( M + 2 ) Y 0 = Y ( M + 1 ) Y I N C = Y ( M + 2 ) C A L L A X I S ( 0 , 0 , ' R A T I O O F I N S U L A T O R T H I C K N E S S E S ' , - 3 0 , 6 , C A L L A X I S ( 0 , 0 , ' E Q U I V A L E N T P E R M I T T I V I T Y ' , 2 3 , 8 , 9 0 , Y 0 , Y 13 C O N T I N U E C A L L L I N E ( X , Y , M , 1 , 0 , 0 ) C A L L X Y P L O T ( 0 , 0 , 3 ) GO T O 14 E N D 212 APPENDIX III LABORATORY PROCESSING DETAILS Some f u r t h e r d e t a i l s of the s o l i d s t a t e l a b o r a t o r y processes are now given i n g r e a t e r d e t a i l . These have been fo l l o w e d through in the f a b r i c a t i o n of MOS c a p a c i t o r s and d e v i c e s . In some cases, they are m o d i f i e d to s u i t compatible requirements of the f i l m s and s u b s t r a t e s . The reagent c o n c e n t r a t i o n s of the v a r i o u s chemicals are as f o l l o w s : H 2 0 2 , 30%; NH„OH, 28-30%; HF, 48%; HC1, 37-38%; U2SOlt , 95-96%. {A3.1} THE RCA SSEE100 CLEANING PROCEDURE FOR SILICON SUBSTRATES: T h i s i s a standard c l e a n i n g process f o l l o w e d i n the S o l i d S tate Laboratory, Departement of E l e c t r i c a l E n g i n e e r i n g at UBC [Kern and Puotinen, 1970]. The peroxide-a c i d c l e a n i n g process i s as f o l l o w s : 1. Prepare a s o l u t i o n i n a 5:1:1 r a t i o of hot (60 C) de-i o n i z e d water : hydrogen peroxide ( H 2 0 2 ) : ammonium hydroxide (NH„OH). T y p i c a l amounts are 300 ml:60 ml:60 ml, that f i t s very w e l l i n a 500 ml beaker. 2. Immerse the w a f e r s / s l i c e s i n above s o l u t i o n f o r 10 min. 3. Rinse the w a f e r s / s l i c e s i n d e - i o n i z e d water f o r 2 min. in f i r s t water cascade, then f o r 8 min. i n the second water cascade. 213 4. Prepare a 10% HF s o l u t i o n i n Nalgene beaker. T y p i c a l amounts are 450 ml H 20 and 50 ml HF. 5. Immerse the w a f e r s / s l i c e s i n s o l u t i o n 4) f o r 30 sees. 6. Rinse the w a f e r s / s l i c e s i n d . i . water per 3) above. 7. Prepare a s o l u t i o n in a 5:1:1 r a t i o of hot (60 C) d . i . water : h y d r o c h l o r i c a c i d (HC1) : hydrogen peroxide ( H 2 0 2 ) . T y p i c a l amounts are 300 ml:60 ml:60 ml. 8. Immerse the w a f e r s / s l i c e s i n s o l u t i o n 7) f o r 10 min. 9. Rinse the w a f e r s / s l i c e s i n d . i . water per 3) above. 10. Immerse w a f e r s / s l i c e s ( s t i r r i n g ) i n beaker with 400 ml hot (60 C) i s o p r o p y l a l c o h o l . {A 3.2} PHOTOLITHOGRAPHY: Negative (Waycoat HR200) and p o s i t i v e (Waycoat HPR204) p h o t o r e s i s t s are used i n d i f f e r e n t p r o c e s s i n g s t e p s . Exposure to UV l i g h t i s normally part of the mask alignment process. Development i s u s u a l l y done a u t o m a t i c a l l y f o r negative p h o t o r e s i s t and manually f o r the p o s i t i v e ones. {A3.2.1} Negative P h o t o r e s i s t : a) Place w a f e r / s l i c e s i n d r y i n g oven f o r 30 min. at 200 C. Let samples c o o l i n c l e a n t r a y before a p p l y i n g p h o t o r e s i s t . b) Spin w a f e r s / s l i c e s i n spi n n e r , once, at 5000 rpm f o r 15 sees. without p h o t o r e s i s t . c) C a r e f u l l y apply p h o t o r e s i s t with eyedropper, repeat 1b) above. 214 d) Prebake a l l wafers i n oven f o r 15 min., at 60 C. e) Place w a f e r / s l i c e s one at a time i n mask a l i g n e r , expose f o r 8 sees. f) Develop in automatic developer. g) Postbake i n oven f o r 15 min. at 135 C. Let samples c o o l . {A3.2.2} P o s i t i v e P h o t o r e s i s t : a) Place wafers i n d r y i n g oven f o r 2 h r s . at 300 C. Let samples c o o l i n c l e a n t r a y before a p p l y i n g p h o t o r e s i s t . b) Spin w a f e r s / s l i c e s i n spi n n e r , once, at 5000 rpm for 20 sec. without p h o t o r e s i s t . c) C a r e f u l l y apply p h o t o r e s i s t with eyedropper, repeat 2b) above. d) Prebake a l l wafers i n oven f o r 30 min. at 105 C. e) Place w a f e r / s l i c e s one at a time i n mask a l i g n e r , expose f o r 12 sees. f) Develop i n 1:3 s o l u t i o n of Waycoat P o s i t i v e LSI Developer and d e - i o n i z e d water f o r 60 sees. g) Postbake i n oven f o r 30 min. at 125 C. Let samples c o o l . {A3.2.3} E t c h i n g : {A3.2.3.1} S i l i c o n D i o x i d e : a) Dip a l l w a f e r / s l i c e s i n d. i . water before HF e t c h i n g , t h i s w i l l assure that no gas bubbles are formed. b) Pour 450 ml of b u f f e r e d HF s o l u t i o n i n Nalgene beaker. 2 1 5 c ) S l o w l y i m m e r s e a l l w a f e r / s a m p l e s i n s o l u t i o n . E t c h i n g r a t e i s a p p r o x i m a t e l y 8 5 0 A / m i n . d ) R i n s e a l l s a m p l e s f o r 2 + 8 m i n . i n d e - i o n i z e d w a t e r . e ) I n s p e c t u n d e r m i c r o s c o p e , e t c h e d a r e a s s h o u l d b e d u l l g r a y a n d a l s o i n w a t e r , t h e y a r e h y d r o p h o b i c . f ) E t c h a g a i n i f r e q u i r e d . R i n s e t h o r o u g h l y . g ) P l a c e a l l s a m p l e s i n b o i l i n g i s o p r o p y l a l c o h o l . D r y o v e r b e a k e r i n a l c o h o l v a p o u r s . I f r e q u i r e d u s e N 2 j e t . { A 3 . 2 . 3 . 2 } A l u m i n i u m : a ) P r e p a r e a s o l u t i o n o f 1:1 o f p h o s p h o r i c a c i d ( H 3 P O „ ) i n d e - i o n i z e d w a t e r , c a r e f u l l y s t i r r . H e a t t o 6 0 C a n d m a i n t a i n t e m p e r a t u r e , a n i m m e r s i o n t h e r m o m e t e r i s r e c o m m e n d e d . b ) C a r e f u l l y p l a c e a t e s t w a f e r / s l i c e i n s o l u t i o n . S l o w m o v e m e n t s a r e r e q u i r e d a s s o l u t i o n i s q u i t e v i s c o u s . E t c h i n g r a t e v a r i e s a n d s o m e e x p e r i m e t a t i o n i s a d v i s e d . T h e r m a l l y e v a p o r a t e d A l e t c h e s v e r y s l o w l y ( 5 0 0 nm i n 1 0 - 1 5 m i n . ) , E - B e a m d e p o s i t e d A l i s f a s t ( 5 0 0 nm 2 - 3 m i n . , o r l e s s ) . G a s b u b b l e s f o r m o n t h e s u r f a c e w h e n e t c h i n g t a k e s p l a c e . I f t e s t s a m p l e i s s u c c e s f u l , r e m a i n i n g o n e s c a n b e t r e a t e d . c ) R i n s e a l l s a m p l e s f o r 2+8 m i n . i n d e - i o n i z e d w a t e r . d ) I n s p e c t u n d e r m i c r o s c o p e , c h e c k f o r u n d e r / o v e r e t c h i n g , r e s o l u t i o n , w e a k s p o t s e t c . e ) E t c h a g a i n i f r e q u i r e d . D o n o t o v e r e t c h . f ) P l a c e a l l s a m p l e s i n b o i l i n g i s o p r o p y l a l c o h o l . D r y o v e r b e a k e r i n a l c o h o l v a p o u r s . I f r e q u i r e d u s e N 2 j e t . 216 {A3.2.4} S t r i p p i n g the P h o t o r e s i s t : {A3.2.4.1} Negative: Two p o s s i b i l i t i e s e x i s t , one that i s compatible with A l metal and that i s incompatible, i . e . i t w i l l remove i t by etching.• These are: {A3.2.4.1.1} M i c r o s t r i p process: A l compatible. a) Place wafer/samples in hot (60-70C) M i c r o s t r i p f o r 5 min. b) Immerse wafer/samples i n hot xylene I (60-70C) f o r 5 min. c) Place samples i n hot xylene II (60-70C) f o r 5 min. d) Immerse wafers i n hot i s o p r o p y l a l c o h o l (60-70C) f o r 5 min. e) Dry blow in n i t r o g e n j e t . {A3.2.4.1.2} Chromic A c i d process: incompatible with A l . a) Pour c a r e f u l l y 400 ml of s u l f u r i c a c i d (H 2SO«) in beaker, add 3 s p o o n f u l l s of chromic t r i o x i d e ( C r 0 3 ) and heat up to 60C, s t i r r thoroughly. T h i s mixture i s deadly and i t should be handled with great care and r e s p e c t . b) Prepare beaker with f r e s h d e - i o n i z e d water. c) Slowly immerse the wafer/samples i n the s o l u t i o n f o r 2 min. A l t e r n a t e a t o t a l of three times between the s o l u t i o n and d. i . water. d) Give a f i n a l r i n s e i n d e - i o n i z e d water f o r 2+8 min. e) Place a l l samples in b o i l i n g i s o p r o p y l a l c o h o l . Dry over beaker i n a l c o h o l vapours. If r e q u i r e d use N 2 j e t . 217 {A3.2.4.2} P o s i t i v e : a) Place wafer/samples i n acetone f o r 60 min. b) Rinse a l l samples i n f r e s h acetone. c) Dry i n Nitrogen j e t . {A3.3} SILICON OXIDATION-FIELD OXIDE: The t h i c k f i e l d oxide ( t y p i c a l l y 600 nm) i s grown from the s u b s t r a t e by "wet" o x i d a t i o n i n a furnace at 1100±5C. The gas flows are a d j u s t e d as f o l l o w s : 1. Oxygen: 1.0 l/min. 2. Hydrogen: 1.6 l/min. 3. N i t r o g e n : 1.0 l/min. 4. Hydrogen C h l o r i d e (HC1): 50 cc/min. C y c l e : 5-5-120-5-30 min.: 1 . 5 min. 0 2 2. 5 min. 0 2+HCl 3. 120 min. H 2+0 2+HCl 4. 5 min. 0 2 5. 30 min. N 2 {A3.4} SILICON OXIDATION-GATE OXIDE: The t h i n gate oxide ( t y p i c a l l y 20 nm) i s grown from the s u b s t r a t e by "dry" o x i d a t i o n i n a furnace at 1000±5C. T h i s process was a r e s u l t of s i m u l a t i o n using SUPREM and experimental v e r i f i c a t i o n : 1. Oxygen: 1.0 l/min. 218 2. N i t r o g e n : 1.6 l/min. 3. Hydrogen C h l o r i d e (HC1): 50 cc/min. C y c l e : 5-3-8-20-min.: 1. 5 min. 0 2 2. 3 min. 0 2 3. 8 min. 0 2+HCl 4. 20 min. N 2 '5 {A3.5} TANTALUM THERMAL OXIDATION: The tantalum metal i s o x i d i z e d by "dry" o x i d a t i o n i n a furnace at 500±1C. The gas flow i s a d j u s t e d as f o l l o w s : 1. Oxygen: 1.0 l/min. C y c l e : 45 to 90 min. u n t i l f u l l y done, f o r 50-100 nm Ta metal. 1. Thermally o x i d i z e f o r 45-90 min. i n 0 2 {A3.6} SOURCE-DRAIN DIFFUSIONS: The d i f f u s i o n process i s d i v i d e d i n t o p r e d e p o s i t i o n and dr i v e - i n . {A3.6.1} P r e d e p o s i t i o n : a) Prepare two h a l f wafers, n-type m a t e r i a l , cleaned per RCA process. b) Set the gas flows as f o l l o w s : 1. N i t r o g e n , coarse: 2000 cc/min. 2. Oxygen, f i n e : 15 cc/min. 219 3. N i t r o g e n , f i n e ( s o u r c e ) : 60 cc/min. c) P r e c o n d i t i o n furnace and predope boat, without wafer/samples. d) C y c l e : 3-18-2 min.: 1. 3 min. N 2 coarse + 0 2 f i n e . 2. 18 min. N 2 coarse + 0 2 f i n e + N 2 f i n e (BBr 3 s o u r c e ) . 3. 2 min. N 2 + 0 2 f i n e . {A3.6.2} D r i v e - i n : a) Set the gas flows as f o l l o w s : 1. Oxygen: 1.5 l/min. 2. Hydrogen: 2.4 l/min. 3. Hydrogen C h l o r i d e (HC1): 60 cc/min. b) C y c l e : 5-80-30-5 min.: 1. 5 min. 0 2 o n l y . 2. 80 min. 0 2 + HC1. 3. 30 min. H 2 + 0 2 + HC1. 4. 5 min. 0 2 o n l y . {A3.6.3} BORON GLASS ETCHING: The g l a s s y s u r f a c e formed d u r i n g the p r e d e p o s i t i o n stage has to be removed by e t c h i n g : a) Prepare 400 ml of b u f f e r e d HF s o l u t i o n . b) Dip a l l wafer/samples in d e - i o n i z e d water. c) Immerse samples i n e t c h i n g s o l u t i o n s l o w l y . Leave f o r 90 sees. d) Rinse i n d e - i o n i z e d water f o r 2+8 min. e) B o i l i n i s o p r o p y l a l c o h o l to remove a l l t r a c e s of 220 water. Dry in N 2 j e t i f r e q u i r e d . {A3.7} POST ANODIC OXIDATION CLEANING: A f t e r the anodic o x i d a t i o n takes p l a c e , the wafer/samples are cleaned with the f o l l o w i n g p r o c e s s : 1. Rinse wafer/sample i n anodic c e l l holder f o r 10 min in d e - i o n i z e d water. 2. C a r e f u l l y remove sample and place in s i n g l e wafer h o l d e r . 3. Place sample i n b o i l i n g t r i c h l o r o e t h y l e n e . 4. Immerse sample i n b o i l i n g i s o p r o p y l a l c o h o l . Dry i n vapours on top of beaker. 5. Dry i n n i t r o g e n j e t i f requi'red. {A3.8} THE BNR CLEANING PROCESS: As some of the samples had Tantalum d e p o s i t e d using the MES technique at B e l l Northern Research i n Ottawa, the f o l l o w i n g steps were used i n c l e a n i n g the wafers [Miner, 1981]: 1. U l t r a s o n i c a g i t a t i o n i n t r i c h l o r o e t h y l e n e f o r 5 min. 2. U l t r a s o n i c a g i t a t i o n i n acetone f o r 5 min. 3. Immersion i n Alconox and d e - i o n i z e d water s o l u t i o n with u l t r a s o n i c a g i t a t i o n . 4. Long r i n s e i n flowing d e - i o n i z e d water f o r 3 hours. 5. Dip i n HF. 6. Spray r i n s e i n i s o p r o p y l a l c o h o l and blow dry with f i l t e r e d n i t r o g e n . 221 {A3.9} INTERFACIAL OXIDATION OF SILICON: The process f o l l o w e d a dry o x i d a t i o n of tantalum at 500C, d e p o s i t e d by RFS, and then a wet o x i d a t i o n of S i l i c o n at 800C. T h i s c r e a t e s a double oxide s t r u c t u r e , with T a 2 0 5 on top of S i 0 2 • The gas flows were set as f o l l o w s : 1. Oxygen: 1.5 1/min. 2. Hydrogen: 2.5 1/min. The c y c l e used was 3-X-3 min., with X v a r y i n g between 24 and 1 14 minutes: a) 3 min.: 0 2 b) X min: 0 2 + H 2 c) 3 min: 0 2 The furnace used i s the P a c e s e t t e r I I , r e s i s t a n c e heated, 2'' quartz tube. 222 APPENDIX IV SPICE AND SUPREM SIMULATION RESULTS The s i m u l a t i o n of the double d i e l e c t r i c MOSFET DC and t r a n s i e n t c h a r a c t e r i s t i c s was accomplished u s i n g SPICE. The o x i d a t i o n was s i m u l a t e d u s i n g SUPREM. i ******.*06- 19-84 ******** SPICE 2G.1 (150CT80) ******** 20: 08 : 3 1 **** * ODOUBLE DIELECTRIC MOSFET SPICE SIMULATION 0**** INPUT LISTING TEMPERATURE = 27.000 DEG C 0*********************************************************************** * T h i s i s the T r a n s i e n t Response of a MTAOSFET, designed *and f a b r i c a t e d by A. E g u i z a b a l (1983). * VIN 4 O PULSECO -11 50NS 2NS 2NS 200NS 1000NS) RS 4 3 600 RIN 3 O 500 CIN 3 O 10PF VDD 1 0 DC -6 CBYP 1 0 22UF RL I 2 IK COUT 2 O 15PF ROUT 2 O 10MEG M1 O 3 2 O M0D1 L=10U W=680U AS=70000P AD=70000P PD=1600U PS=1S00U .MODEL M0D1 PMOS VT0=-2.5 KP=6.82E-3 .DC VIN 0 -11 -0.5 .PLOT DC V(2) .PLOT TRAN V(2) .TRAN 10NS 500NS ONS . END 1 ****************06- 19-84 ************************ SPICE 2G.1 (150CT80) ************************20:08:31 **************** ODOUBLE DIELECTRIC MOSFET SPICE SIMULATION 0**** MOSFET MODEL PARAMETERS TEMPERATURE = 27.000 DEG C 0* ************************************************************************************************************************ 223 M0D1 OTYPE PMOS OLEVEL 1.OOO OVTO -2.500 OKP 6.82E-03 ^****************06~19-84 ************************ SPICE 2G.1 (150CT80) ************************20:08:31 **************** ODOUBLE DIELECTRIC MOSFET SPICE SIMULATION 0**** DC TRANSFER CURVES TEMPERATURE = 27.000 DEG C X VIN V(2) X -6.000E+00 -4.000E+00 -2.000E+00 0.0 0 0 -5 999E+00 * -5 OOOE-01 -5 999E+00 * -1 OOOE+OO -5 999E+00 * -1 500E+00 -5 999E+00 * -2 OOOE+OO -5 999E+00 * -2 500E+00 -5 999E+00 * -3 OOOE+OO -5 999E+00 * -3 500E+00 -5 999E+00 * -4 OOOE+OO -5 999E+00 * -4 500E+00 -5 999E+00 * -5 OOOE+OO -5 999E+00 * -5 500E+00 -5 999E+00 * -6 OOOE+OO -6 583E-02 * -6 500E+00 -2 927E-02 * -7 OOOE+OO -1 918E-02 * -7 500E+00 - 1 431E-02 * -8 OOOE+OO - 1 142E-02 * -8 500E+00 -9 505E-03 * -9 OOOE+OO -8 142E-03 * -9 500E+00 -7 121E-03 * - 1 000E+01 . -6 328E-03 * - 1 050E+01 -5 694E-03 * - 1 100E+01 -5 176E-03 * Y 1 ****************06- 19-84 ************************ SPICE 2G .'1 (150CT80) ************************20:08:31 **************** ODOUBLE DIELECTRIC MOSFET SPICE SIMULATION 0**** INITIAL TRANSIENT SOLUTION TEMPERATURE = 27.000 DEG C NODE VOLTAGE NODE VOLTAGE NODE VOLTAGE NODE VOLTAGE 224 ( 1) -6.0000 ( 2) -5.9994 ( 3 ) 0.0 ( 4 ) 0.0 VOLTAGE SOURCE CURRENTS NAME CURRENT VIN 0.0 VDD 5.999E-07 TOTAL POWER DISSIPATION 3.60E-06 WATTS 1****************06- 19-84 ************************ SPICE' 2G . 1 (150CT80) ************************20 * 08 : 3 1 *************** * ODOUBLE DIELECTRIC MOSFET SPICE SIMULATION 0**** OPERATING POINT INFORMATION TEMPERATURE = 27.000 DEG C MOSFETS 0 OMODEL ID VGS VDS VBS M1 M0D1 0.0 5.999 5.999 5 . 999 ^************** **06- 19-84 ************************ SPICE 2G 1 (150CT8O) ************************20•08:31*************** * ODOUBLE DIELECTRIC MOSFET SPICE SIMULATION 0**** TRANSIENT ANALYSIS TEMPERATURE = 27.000 DEG C TIME V(2) -6.OOOE+00 -4.OOOE+OO -2.OOOE+00 0.0 0 0 -5 999E+00 * 1 000E -08 -5 999E+00 * 2 000E -08 -5 999E+00 * 3 OOOE -08 -5 999E+00 * 4 000E -08 -5 999E+00 * 5 OOOE -08 -5 999E+00 * 6 OOOE -08 -8 229E-03 7 OOOE -08 -1 188E-02 8 OOOE -08 -9 499E-03 9 OOOE -08 - 1 404E-03 1 OOOE -07 -9 442E-03 1 100E -07 -1 439E-03 1 200E -07 -9 394E-03 1 300E -07 - 1 475E-03 1 400E -07 -9 348E-03 1 500E -07 -1 511E-03 1 600E -07 -9 303E-03 1 700E -07 -1 546E-03 1 800E -07 -9 258E-03 1 900E -07 - 1 581E-03 2 OOOE -07 -9 213E-03 2 100E -07 - 1 G16E-03 2 200E -07 -9 169E-03 2 300E -07 - 1 650E-03 2 400E -07 -9 126E-03 2 500E -07 1 015E-02 2 600E -07 - 1 788E+00 2 700E -07 -3 835E+00 2 800E -07 -4 890E+00 2 900E -07 -5 425E+00 3 OOOE -07 -5 710E+00 3 100E -07 -5 855E+00 3 200E -07 -5 927E+00 * 3 300E -07 -5 963E+00 * 3 400E -07 -5 981E+00 * 3 500E -07 -5 990E+00 * 3 600E -07 -5 995E+00 * 3 700E -07 -5 997E+00 * 3 800E -07 -5 998E+00 * 3 900E -07 -5 999E+00 * 4 OOOE -07 -5 999E+00 * 4 100E -07 -5 999E+00 * 4 200E -07 -5 999E+00 * 4 300E -07 -5 999E+00 * 4 400E -07 -5 999E+00 4 500E -07 -5 999E+00 * 4 600E -07 -5 999E+00 * 4 700 E -07 -5 999E+00 * 4 800E -07 -5 999E+00 * 4 900E -07 -5 999E+00 * 5 OOOE -07 -5 999E+00 * Y 0 JOB CONCLUDED 0 TOTAL JOB TIME 0.0 1 226 *** STANFORD UNIVERSITY PROCESS ENGINEERING MODELS PROGRAM *** *** VERSION 0-05 *** 1. . . .TITLE GATE OXIDE 2 . . ..GRID DYSI=0.001, DPTH=0 2 , YMAX=1.0 3 . . ..SUBST ORNT=100, E,LEM = P, C0NC=4.OE14 4 . . ..PLOT TOTL=Y 5 . . ..PRINT HEAD=Y 6 . . ..STEP TYPE=OXID, TIME=3, TEMP=1090, MODL=DRYO 7 . . . . END 1 GATE OXIDE STEP tt 1 OXIDATION IN DRY OXYGEN TOTAL STEP TIME = 3.0 MINUTES INITIAL TEMPERATURE = 1090.00 DEGREES C. OXIDE THICKNESS = 2.2444E-02 MICRONS LINEAR OXIDE .GROWTH RATE = 2.492192E-03 MICRONS/MINUTE PARABOLIC OXIDE GROWTH RATE = 3.647341E-04 MICRONS!2/MINUTE OXIDE GROWTH PRESSURE = 1.00000 ATMOSPHERES I OXIDE I I DIFFUSION I I COEFFICIENT I SILICON DIFFUSION COEFFICIENT SEGREGATION COEFFICIENT I SURFACE I I TRANSPORT I COEFFICIENT I PHOSPHORUS I 5.23156E-06 I 1.22179E-03 I 10.000 SURFACE CONCENTRATION = -4.375506E+14 ATOMS/CM!3 JUNCTION DEPTH I SHEET RESISTANCE j I 125136. OHMS/SOUARE 3.95133E-02 I NET ACTIVE CONCENTRATION OXIDE CHARGE = 1 SILICON CHARGE = 3 TOTAL CHARGE = 3 INITIAL CHARGE = 3 011304E+08 987205E+10 9973 18E+ 10 999999E+10 IS IS IS O. 253 99 . 7 99.9 % OF TOTAL % OF TOTAL % OF INITIAL CHEMICAL CONCENTRATION OF PHOSPHORUS OXIDE CHARGE = 1.011304E+08 IS SILICON CHARGE = 3.987205E+10 IS TOTAL CHARGE = 3.997318E+10 IS 0. 253 99. 7 99.9 % OF TOTAL % OF TOTAL % OF INITIAL 227 INITIAL CHARGE = 3.999999E+10 GATE OXIDE I STEP I DEPTH I (UM) I 14 -0.02 *-15 TIME = 3.0 MINUTES. CONCENTRATION (LOG ATOMS/CC) 16 17 18 19 20 21 0.0 1 .00 2 .00 3 .00 228 i i i i i I I I I I i i i i I i 4.00 1 SUPREM END . J> 1 *** STANFORD UNIVERSITY PROCESS ENGINEERING MODELS PROGRAM *** *** VERSION 0-05 *** 1....TITLE GATE OXIDE 2....GRID DYSI=O.001, DPTH=0.2, YMAX=1.0 3....SUBST 0RNT=10O, ELEM=P, C0NC=4.0E14 4 . . . .PLOT TOTL = Y 5....PRINT HEAD=Y 6....STEP TYPE=OXID, TIME = 80.0, TEMP=1090, M0DL = DRYO 7 . . . .END 1 GATE OXIDE 'STEP tt 1 OXIDATION IN DRY OXYGEN TOTAL STEP TIME = 80.0 MINUTES INITIAL TEMPERATURE = 1090.00 DEGREES C. OXIDE THICKNESS = 0.1198 MICRONS LINEAR OXIDE GROWTH RATE = 2.492192E-03 MICRONS/MINUTE PARABOLIC OXIDE GROWTH RATE = 3.647341E-04 MICRONS!2/MINUTE OXIDE GROWTH PRESSURE = 1.00000 ATMOSPHERES I OXIDE I SILICON I I SURFACE I I DIFFUSION I DIFFUSION I SEGREGATION I TRANSPORT I COEFFICIENT I COEFFICIENT I COEFFICIENT I COEFFICIENT I PHOSPHORUS I 5.23156E-06 I 1.22179E-03 I 10.000 I 3.95133E-02 I SURFACE CONCENTRATION = -4.474924E+14 ATOMS/CM!3 JUNCTION DEPTH I SHEET RESISTANCE j I 126788. OHMS/SQUARE NET ACTIVE CONCENTRATION OXIDE CHARGE = 4.517386E+08 SILICON CHARGE = 3.935OO2E+10 TOTAL CHARGE = 3.980176E+10 INITIAL CHARGE = 3.999999E+10 IS 1.13 % OF TOTAL IS 98.9 % OF TOTAL IS 99.5 % OF INITIAL 229 CHEMICAL CONCENTRATION OF PHOSPHORUS OXIDE CHARGE = 4.517386E+08 IS 1.13 % OF TOTAL SILICON CHARGE = 3.935002E+10 IS 98.9 % OF TOTAL TOTAL CHARGE = 3.980176E+10 IS 99.5 % OF INITIAL INITIAL CHARGE = 3.999999E+10 1 GATE OXIDE I STEP = 1 TIME = 80.0 MINUTES. I DEPTH I CONCENTRATION (LOG ATOMS/CC) (UM) I 14 15 16 17 18 19 -0.12 * * I I I I I * I I I I I * I I I I I * I I I I I * I I I I I * I I I I I * I I I I I * I I I I I * I I I I I 0.0 * * 1 * 1 I I I I 1 * 1 I I I I 1 * 1 I I' I I 1 * 1 I I I I 1 * 1 I I I I 1 * 1 I I I I 1 * 1 I I I I 1 * 1 I I I I 1 .00 2 .00 3.00 21 230 4.00 1 SUPREM END.b 1 *** STANFORD UNIVERSITY PROCESS ENGINEERING MODELS PROGRAM *** *** VERSION 0-05 *** 1. . . .TITLE GATE OXIDE 2 . . ..GRID DYSI=0.001, DPTH=0 2 , YMAX=1.0 3 . . .SUBST 0RNT=100, ELEM=P, C0NC=4.0E14 4 . . ..PLOT TOTL=Y 5 . . ..PRINT HEAD=Y 6 . . .STEP TYPE=OXID, TIME=3, TEMP=1000, MODL=DRYO 7 . . . . END 1 GATE OXIDE STEP # 1 OXIDATION IN DRY OXYGEN TOTAL STEP TIME = .3.0 MINUTES INITIAL TEMPERATURE = 1000.00 DEGREES C. OXIDE THICKNESS = 1.4006E-02 MICRONS LINEAR OXIDE GROWTH RATE = 7.479377E-03 MICRONS/MINUTE PARABOLIC OXIDE GROWTH RATE = 1.739819E-04 MICRONS!2/MINUTE OXIDE GROWTH PRESSURE = 1.00000 ATMOSPHERES I OXIDE I I DIFFUSION I I COEFFICIENT I SILICON DIFFUSION COEFFICIENT SEGREGATION COEFFICIENT I SURFACE I I TRANSPORT I COEFFICIENT I PHOSPHORUS I 6.36610E-07 I 1.35026E-04 I 10.000 SURFACE CONCENTRATION = -4.947440E+14 ATOMS/CM!3 JUNCTION DEPTH I SHEET RESISTANCE 1 I 124966. OHMS/SOUARE 1.19299E-02 I 231 NET ACTIVE CONCENTRATION OXIDE CHARGE = 6. SILICON CHARGE = 3. TOTAL CHARGE = 3, INITIAL CHARGE = 3. 576280E+07 992655E+10 999231E+10 999999E+10 IS IS IS 0. 1G4 99 .8 100. % OF TOTAL % OF TOTAL % OF INITIAL CHEMICAL CONCENTRATION OF PHOSPHORUS OXIDE CHARGE = 6 SILICON CHARGE = 3 TOTAL CHARGE = 3 INITIAL CHARGE = 3 GATE OXIDE 576280E+07 992655E+10 999231E+10 999999E+10 IS IS IS 0. 164 99.8 100. I I DEPTH I (UM) I 14 -0.01 *-STEP 1 15 % OF TOTAL % OF TOTAL % OF INITIAL TIME = 3.0 MINUTES. CONCENTRATION (LOG ATOMS/CC) 16 17 18 19 0.0 1 .00 2.00 21 232 3 .00 4.00 1 SUPREM END . o 1 *** STANFORD UNIVERSITY PROCESS ENGINEERING MODELS PROGRAM *** *** VERSION 0-05 *** 1. . ..TITLE GATE OXIDE 2 . . ..GRID DYSI=0.001, DPTH=0 2, YMAX=1.0 3 . . ..SUBST ORNT=100, ELEM=P, C0NC=4.OE14 4 . . .PLOT TOTL=Y 5 . . ..PRINT HEAD=Y 6 . . ..STEP TYPE=OXID, TIME=5, TEMP=1000, MODL=DRYO 7 . . . . END 1 GATE OXIDE STEP tt 1 OXIDATION IN DRY OXYGEN TOTAL STEP TIME = 5.0 MINUTES INITIAL TEMPERATURE = 1000.00 DEGREES C. OXIDE THICKNESS = 2.0074E-02 MICRONS LINEAR OXIDE GROWTH RATE = 7.479377E-03 MICRONS/MINUTE PARABOLIC OXIDE GROWTH RATE = 1.739819E-04 MICRONS!2/MINUTE OXIDE GROWTH PRESSURE - 1.00000 ATMOSPHERES I OXIDE I SILICON I I SURFACE I I DIFFUSION I DIFFUSION I SEGREGATION I TRANSPORT I COEFFICIENT I COEFFICIENT I COEFFICIENT I COEFFICIENT I PHOSPHORUS I 6.36610E-07 I 1.35026E-04 I 10.000 I 1.19299E-02 I 233 SURFACE CONCENTRATION JUNCTION DEPTH -5.045019E+14 ATOMS/CM!3 SHEET RESISTANCE 125066. OHMS/SQUARE NET ACTIVE CONCENTRATION OXIDE CHARGE = 9. SILICON CHARGE = 3. TOTAL CHARGE = 3. INITIAL CHARGE = 3. 653334E+07 989433E+10 999086E+10 999999E+10 IS IS IS 0.24 1 99.8 100/ % OF TOTAL % OF TOTAL % OF INITIAL CHEMICAL CONCENTRATION OF PHOSPHORUS OXIDE CHARGE = 9 SILICON CHARGE = 3 TOTAL CHARGE = 3 INITIAL CHARGE = 3 GATE OXIDE I STEP I DEPTH I (UM) I 14 -0.02 * 653334E+07 989433E+10 999086E+10 999999E+ 10 IS IS IS 0.241 99 . 8 100. 1 15 % OF TOTAL % OF TOTAL % OF INITIAL TIME = 5.0 MINUTES. CONCENTRATION (LOG ATOMS/CC) 16 17 18 19 0.0 1 .00 21 c "a XI O o o o o o A P P E N D I X I C - V A N D I - V C U R V E S O F MOS C A P A C I T O R S A D D E N D U M T O T H E M . A . S c . T H E S I S T A N T A L U M P E N T O X I D E , A N O N C O N V E N T I O N A L G A T E I N S U L A T O R F O R MOS D E V I C E S b y A N T O N I O L . E G U I Z A B A L R I V A S T H E U N I V E R S I T Y O F B R I T I S H J a n u a r y 1 9 8 4 © A n t o n i o L . E g u i z a b a l C O L U M B I A R i v a s S A M P L E N 2 , S I N G L E D I E L E C T R I C n T y p e , T h e r m a l 5 0 0 C . CM • u CO cm LU Q_ LL. CL LO S . tr LU • CC tr n z => a. en LU EL £_> Z tr i— n u t r a . t r Sample N2 n Type S i l i c o n -I IS. BB -1 z s SCHOTTKY I V PLOT pj-j Sample N2 n Type S i l i c o n Gate negative . 1 1 1 1 — 1 1 0.H 0 B.8B 1-G0 Z.10 S.ZH 1.B0 ^-S0 SORT VDLTflGE CSORTCVDLTS3D S A M P L E N 3 , S I N G L E D I E L E C T R I C n T y p e , T h e r m a l 5 0 0 C . G A T E V D L T f l G E SORT V D L T f l G E C S O R T C V D L T S D 3 S A M P L E 1 T 6 , D O U B L E D I E L E C T R I C n T y p e , T h e r m a l 5 0 0 C . -1" SCHOTTKY I V PLOT CO Sample 1T6 n Type S i l i c o n Gate negative cp C M co-at in' to" C M I 1 T T T T B . BB B.SB l.GB Z.tB S.ZB SORT VOLTAGE CSORTCVDLTS3D S A M P L E 2 T 6 , D O U B L E D I E L E C T R I C n T y p e , T h e r m a l 5 0 0 C . S C H O T T K Y I V P L O T Sample 2T6 n Type S i l i c o n Gate negative T T T .00 1.60 Z.T0 3.Z0 t - 0 0 SORT VDLTflGE CSORTCVDLTS7D Md.</ S A M P L E 3 T 6 , D O U B L E D I E L E C T R I C n T y p e , T h e r m a l 5 0 0 C . SCHDTTKT I V PLDT Sample 3T6 n Type S i l i c o n Gate negative SORT VDUTflGE CSDRTCVDLTS33 S A M P L E 4 T 6 , D O U B L E D I E L E C T R I C n T y p e , T h e r m a l 5 0 0 C . Sample 4T6 n Type S i l i c o n -i 1 1 r- 1 1Z.BB -Z.BB 6.BB 10.BB Z6. G A T E V D L T f l G E J'9' S C H O T T K Y I V P L O T CD Sample 4T6 n Type S i l i c o n Gate negative to CM" «* t N en" to' CM 0 .BB — i - 1 r - r 1 B.SB l.GB Z.1B S.ZB *-BB SORT VOLTAGE CSDRTCVDLTS3 3 t. SB S A M P L E 1 T 7 , D O U B L E D I E L E C T R I C n T y p e , T h e r m a l 5 0 0 C . GATE VDLTflGE ,22 JSC S C H O T T K Y I V P L O T Sample 1T7 n Type S i l i c o n Gate negative CM" in-to ' T 0.00 T T T T 0.80 1.G0 Z.*t0 S.ZB 1. SDRT VDLTflGE CSORTCVDLTS33 00 Si S A M P L E 2 T 7 , D O U B L E D I E L E C T R I C n T y p e , T h e r m a l 5 0 0 C . Sample 2T7 GATE VDLTflGE 2b°l S C H O T T K Y I V P L D T Sample 2T7 n Type S i l i c o n Gate negative to CM" CM m" to in-to' CM T T B.8B 1.6B S.ZB "».BB SDRT VOLTAGE CSORTCVDLTS3 3 BB S A M P L E 3 T 7 , D O U B L E D I E L E C T R I C n T y p e , T h e r m a l 5 0 0 C . GATE VDLTAGE 2'8 S C H O T T K Y I V P L O T Sample 3T7 n Type S i l i c o n Gate negative I 1 1 1 1 .80 l . C B Z.*tB 3.ZB t . B B SQRT VDLTflGE CSORTCVDLTS33 -1 1. 8 S A M P L E 4 T 7 , D O U B L E D I E L E C T R I C n T y p e , T h e r m a l 5 0 0 C . v3'0 , #6f C Q K T V O L T A G E C S O R T C V D L T C 3 3 2& S A M P L E NI, S I N G L E D I E L E C T R I C n T y p e , T h e r m a l 5 0 0 C . i S O R T V O L T A G E CSQRTf.VDL.TC3 3 S A M P L E S 7 , S I N G L E D I E L E C T R I C n T y p e , T h e r m a l 5 0 0 C . 3-5' 9&% SORT VDLTPGE CSDKTCVDLTC33 S A M P L E B N R 5 0 0 , S I N G L E D I E L E C T R I C p T y p e , T h e r m a l 5 0 0 C . GATE VDLTAGE S O R T V D L T f l G E C S O R T C V D L T S 3 3 S A M P L E B N R 1 0 0 0 , S I N G L E D I E L E C T R I C p T y p e , T h e r m a l 5 0 0 C . GATE VDLTflGE S C H Q T T K T I V P L D T CP Sample BNR1000 p Type S i l i c o n Gate p o s i t i v e s: ex o _J CJ T ~T T 0 . 0 0 0 . t 0 0 . 6 0 1.Z0 1.C0 Z . 0 0 Z.^Z S O R T V D L T f l C E C S O R T C V D L T S 3 3 S A M P L E S a m p l e A , S I N G L E D I E L E C T R I C p T y p e , T h e r m a l 5 0 0 C . .4-3" GflTE VDLTflGE 21< S C H O T T K Y I V P L D T Sample SAMP LEA p Type S i l i c o n Gate p o s i t i v e to CM cr to xn-to" —r 1 00 t . Sf T T 0 . 0 0 0 . 6 0 4.G0 Z . t 0 S . Z 0 S Q R T V D L T f l G E C S Q R T C V D L T S 3 3 S A M P L E S a m p l e B , S I N G L E D I E L E C T R I C p T y p e , T h e r m a l 5 0 0 C . Sample SAMPLEB GATE VOLTAGE #7 S Q R T V O L T f l C C C C Q R T C V D L T S 3 3 S A M P L E 1 0 0 0 A M O S , S I N G L E D I E L E C T R I C n T y p e , T h e r m a l 5 0 0 C . ,4-9' Sample 1000AMDS n Type Silicon GATE VDLTflGE SQRT VDLTflGE [SQRTCVDLTS33 S A M P L E 5 0 0 A L i f t , S I N G L E D I E L E C T R I C p T y p e , T h e r m a l 5 0 0 C . GATE VDLTPCE S C H D T T K T I V P L D T CD Sample 5 0 0 A L i f t p Type S i l i c o n Gate p o s i t i v e I te to' cy T~ T T T 0-00 H.S0 1.00 1-S0 2.00 Z.S0 SQRT VDLTflGE CSORTCVDLTS33 -1 3.00 S A M P L E l O O O A L i f t , S I N G L E D I E L E C T R I C p T y p e , T h e r m a l 5 0 0 C . SCHOTTKY I V PLDT CD Sample l O O A L i f t n Type S i l i c o n Gate negative to C\J-CO-CD tO to' (M 0 .00 T T T 0.80 1.G0 Z.^0 3-ZB SORT VOLTAGE CSQRTCVDLTS3 2 t . 80 S A M P L E M O S C 6 , S I N G L E D I E L E C T R I C n T y p e , T h e r m a l 6 0 0 C . S C H O T T K Y I V P L O T Sample MOSC6 n Type S i l i c o n Gate negative I 1 1 1 1 .80 1.60 Z.tB 3.Z0 *-00 SQRT VDLTflGE CSQRTCVDLTS33 1 <K 6 S A M P L E M 0 S C 7 , S I N G L E D I E L E C T R I C n T y p e , T h e r m a l 4 0 0 C . Sample MOSC7 n Type S i l i c o n 3Z.00 -ZZ.BI -I r 1 -1Z.00 -Z.00 8.00 G f l T E w V D L T A G E 18. 00 ZS S C H D T T K T I V P L D T Sample MOSC7 n Type S i l i c o n Gate negative S A M P L E M O S C 8 , S I N G L E D I E L E C T R I C n T y p e , T h e r m a l 6 0 0 C . S C H D T T K Y I V P L D T CM" to 0_ s: cr •—' ^ Q _J t— 2 Ul OL a: CJ • e» LS Q _l 1 C— -Sample MOSC8 n Type S i l i c o n Gate nega t ive 0.00 T T T 0.80 1.G0 Z . t B 3.Z0 *K00 SORT VDLTflEE CSDRTCVDLTSD1 *. 80 65' S A M P L E M 0 S C 9 , S I N G L E D I E L E C T R I C n T y p e , A n o d i c , C i t r i c A c i d . GATE VDLTflGE 3o( S C H O T T K Y I V P L D T tn n s: tr C5 CD" Sample MOSC9 n Type S i l i c o n Gate negat ive LU LY CJ C M 2 <=5 to CD 0.00 T 0.80 1.90 Z.^0 3.20 SORT V D L T f l G E C S Q R T C V D L T S33 —1— 1 <K 00 "K SI S A M P L E MOSC10, S I N G L E D I E L E C T R I C n T y p e , A n o d i c , C i t r i c A c i d . &9 3*3 \5o SCHOTTKY IV PLDT Sample MOSC10 n Type S i l i c o r Gate negative CM" _ c o n CO" S A M P L E M0SC11, S I N G L E D I E L E C T R I C n T y p e , A n o d i c , P h o s p h o r i c A c i d . CD' K < ^ <y «- -s: (-} LO ~ CV UJ D_ U_ «=» in~ \ < ^ cr LU £V cr "*" i — r i 2 3 -(V [ | I cn~ CL LU z CM -t-i cr cr Sample M0SC11 n Type S i l i c o n 1 r — 1 1 1~ 1 -35.00 -ZS.00 -15.00 -S.00 5.00 15.00 ZS . 0 0 GATE VDLTflGE S A M P L E M 0 S C 1 2 , S I N G L E D I E L E C T R I C n T y p e , A n o d i c , P h o s p h o r i c A c i d . 3 & r c <=» * C M i-5 1 CJ ac LU C L LU LI-LY cr 2 CY LU a. LU CJ Z tr CJ cr a . cr CJ LP CM CD-CM: Sample M0SC12 n Type S i l i c o n , 1 1 - 3 S . 0 0 -ZS . .00 - I S . 0 0 - S . 0 0 S. 00 IS.0 0 Z S . 0 0 GATE VDLTf lGE S A M P L E M 0 S C 1 3 , S I N G L E D I E L E C T R I C n T y p e , A n o d i c , C i t r i c A c i d . CD' X. (M «- -s : Qi UJ CL LL. D_ in~ \ e» cr 111 VL CC i — r> Z (V 1 1 1 cn ~ D. LU t_J Z cr >— i - i £J CC D_ CC Sample M0SC13 n Type S i l i c o n 1 1 1 1 -35.00 -ZS.00 - I S . 0 0 -5.00 5.00 GATE VDLTflGE -1 1 1SV00 25. S C H D T T K Y I V P L O T Sample M0SC13 n Type S i l i c o n Gate negative -j 1 1 1 1 0.80 1.60 Z.ta 3.20 t.00 SQRT V D L T f l G E C S Q R T C V O L T S 3 3 -I S A M P L E M 0 S C 1 4 , S I N G - L E D I E L E C T R I C n T y p e , A n o d i c , P h o s p h o r i c A c i d . :Sl3 ^ 7 SCHOTTKY IV PLDT Sample M0SC14 n Type S i l i c o n Gate negative 1 1 1 1 l.GB Z.^B 3.ZB t.BB RT VDLTflGE CSORTCVDLTS11 S A M P L E M 0 S C 1 5 , S I N G L E D I E L E C T R I C n T y p e , A n o d i c , C i t r i c A c i d . SCHDTTKT IV PLDT Sample M0SC15 n Type S i l i c o n Gate negative CM cn' in-to' T 0.00 T 0.80 1.E0 Z.50 3.Z0 5.00 SQRT VDLTflGE CSDRTCVDLTS31 *8\ 3<i S A M P L E M 0 S C 1 6 , S I N G L E D I E L E C T R I C n T y p e , A n o d i c , P h o s p h o r i c A c i d . GATE VOLTAGE S C H Q T T K T I V P L D T CD Sample M0SC16 n Type S i l i c o n Gate negative to cn • to CM 0.00 T T T T 0.80 1.60 Z . t 0 3.Z0 5.00 SQRT VDLTflGE CSQRTCVDLTS1D I <r . 80 ,8*7 I S A M P L E M 0 S C 1 7 , S I N G L E D I E L E C T R I C n T y p e , A n o d i c , C i t r i c A c i d . G A T E V D L T f l G E $2) S C H D T T K T I V P L O T Sample MOSC17 n Type S i l i c o n Gate negative CM" T 0 .00 0.80 1.G0 Z-tB 3.Z0 SORT VDLTflGE CSQRTCVDLTSD3 —I 1 5.00 5.81 1^/ S A M P L E M 0 S C 1 8 , S I N G L E D I E L E C T R I C n T y p e , A n o d i c , C i t r i c A c i d . Sample M0SC18 con G A T E V D L T f l G E S C H O T T K Y I V P L D T CO Sample MOSC18 n Type S i l i c o n Gate n e g a t i v e CO est' CM cn' to tn' to' CM 0.00 T 0.80 1.60 Z.50 3-20 5.00 SQRT VDLTflGE CSORTCVDLTS33 5. 80 S A M P L E M T J 1 , D O U B L E D I E L E C T R I C n T y p e , I n t e r f a c i a l O x i d a t i o n . 9$ 3 S C H O T T K Y I V P L O T Sample MTJ1 n Type S i l i c o n Gate negative S Q R T V D L T f l G E C S O R T C V D L T S J 1 S A M P L E M T J 2 , D O U B L E D I E L E C T R I C n T y p e , I n t e r f a c i a l O x i d a t i o n . &1 GATE VDLTflGE SCHDTTKT IV PLOT Sample MTJ2 n Type S i l i c o n Gate negative oo 0.00 —I 0 . SB 1. £0 Z . 50 3. Z0 I 5.00 5. SB SQRT V D L T f l G E C S Q R T £ V D L T S 3 3 S A M P L E M T J 3 , D O U B L E D I E L E C T R I C n T y p e , I n t e r f a c i a l O x i d a t i o n . G A T E V O L T A G E 31 S C H D T T K T I V P L O T Sample MTJ3 n Type S i l i c o n Gate negative to' - r B . SB 1. GB Z. t B 3. ZB I " "t. 0 0 —1 5. SB S Q R T V D L T f l G E C S O R T C V D L T S P D S a m p l e M T J 4 , D O U B L E D I E L E C T R I C n T y p e , I n t e r f a c i a l O x i d a t i o n . .^1 Sample MTJ4 n Type S i l i c o n 00 n a. 00 i t S C H O T T K Y I V P L O T Sample MTJ4 n Type S i l i c o n Gate negative T T T -r 0.S0 1.G0 Z.50 3 . Z 0 SORT VDLTflGE C S Q R T C V O L T S 3 3 —1 1 5.00 5. S a m p l e M T J 5 , D O U B L E D I E L E C T R I C n T y p e , I n t e r f a c i a l O x i d a t i o n . G A T E V D L T f l G E S Q R T V D L T f l G E C S D R T C V D L T S 3 D 

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