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Fabrication process and characteristics of a silicon strip detector 1985

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FABRICATION PROCESS AND CHARACTERISTICS OF A SILICON STRIP DETECTOR by DAVID J . MILLS B.A.Sc, U n i v e r s i t y of B r i t i s h Columbia, 1983 THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Applied MASTER OF vSCIENCE in THE FACULTY OF GRADUATE STUDIES Department of E n g i n e e r i n g P h y s i c s 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 COLUMBIA A p r i l 1985 © David John M i l l s 1985 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of i ^ ^ g j u ^r\TilAr^ 1/ (y/ The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 ^ t e Apr]/ - X f DE-6 (3/81) ABSTRACT The c u r r e n t and p o s s i b l e uses of semiconductor s o l i d s t a t e d e t e c t o r s in nuclear p h y s i c s are b r i e f l y d i s c u s s e d . The theory of s o l i d s t a t e d e t e c t o r s i s d i s c u s s e d with emphasis on the s i l i c o n PIN diode d e t e c t o r . A f a b r i c a t i o n process f o r s i l i c o n s u r f a c e b a r r i e r p o s i t i o n s e n s i t i v e s o l i d s t a t e d e t e c t o r s has been developed at UBC based on the work of J.B.A. England. A f a b r i c a t i o n process r e c i p e i s i n c l u d e d . A prototype s u r f a c e b a r r i e r d e t e c t o r system has been b u i l t and t e s t e d at UBC and TRIUMF using t h i s p r o c e s s . The device has 1 mm p o s i t i o n r e s o l u t i o n i n one d i r e c t i o n , an a c t i v e area of 40 mm in diameter and a mass t h i c k n e s s of 55 mg/cm2. The measured e f f i c i e n c y f o r 50 MeV pions i s 70% and expected rate c a p a b i l i t y i s i n excess i f 1 MHz per s t r i p . The de t e c t o r e f f i c i e n c y i s l i m i t e d by a marginal s i g n a l - t o - n o i s e r a t i o . TABLE OF CONTENTS CHAPTER I: INTRODUCTION . . 1 What i s a s o l i d s t a t e d e t e c t o r ? 1 General p r o p e r t i e s of SS d e t e c t o r s 3 M o t i v a t i o n for d e t e c t o r development at TRIUMF 4 CHAPTER I I : PRINCIPLES OF OPERATION 8 Ba s i c band theory of semiconductors and charge gener a t i o n 8 Bulk type SS d e t e c t o r s , r e s i s t i v i t y of semiconductors 11 Diode d e t e c t o r s , e l e c t r i c a l c h a r a c t e r i s t i c s 14 Charge c o l l e c t i o n time 20 Energy r e s o l u t i o n , n o i s e , leakage c u r r e n t ..23 Leakage c u r r e n t , r a d i a t i o n damage 26 D e p l e t i o n depth, energy r e s o l u t i o n 27 Transmission d e t e c t o r s 28 P o s i t i o n r e s o l u t i o n ...29 CHAPTER I I I : FABRICATION OF DEVICES 31 Ion Implanted SS d e t e c t o r s 31 Surface b a r r i e r SS d e t e c t o r s ....34 P r e l i m i n a r y e l e c t r i c a l c h a r a c t e r i s t i c s of s u r f a c e b a r r i e r d e t e c t o r s 39 CHAPTER IV: SOLID STATE DETECTOR SYSTEM TEST 43 P r e a m p l i f i e r e l e c t r o n i c s 43 i v Alpha source t e s t s , d e t e c t o r asymmetry 46 Beta source t e s t s 50 Observed noise of system 53 Beam t e s t design 53 Beam t e s t , p o s i t i o n r e s o l u t i o n 55 Beam t e s t , noise and e f f i c i e n c y 60 Beam t e s t , b i a s v o l t a g e and e f f i c i e n c y 67 Timing of SS d e t e c t o r s i g n a l s 70 CHAPTER V: SUMMARY AND CONCLUSION 7 2 Noise, improvements 72 Future c o n s i d e r a t i o n s 74 BIBLIOGRAPHY 7 5 APPENDICES: Appendix A: M o d i f i e d England process f o r surf a c e b a r r i e r d e t e c t o r s ....76 L I S T OF TABLES T a b l e 2.1 C h a r g e c a r r i e r m o b i l i t i e s o f s e m i c o n d u c t o r s v i L I S T OF FIGURES F i g u r e 1.1 G e n e r i c s o l i d s t a t e d e t e c t o r 2 F i g u r e 1.2 TRIUMF M13 p i o n b e a m l i n e 5 F i g u r e 1.3 P h o t o g r a p h s o f p r o t o t y p e d e t e c t o r 7 F i g u r e 2.1 C h a r g e g e n e r a t i o n i n SS d e t e c t o r 10 F i g u r e 2.2 C h a r g e c a r r i e r c o n c e n t r a t i o n i n s e m i c o n d u c t o r s 13 F i g u r e 2.3 P I N d i o d e .15 F i g u r e 2.4 Band d i a g r a m s o f pn j u n c t i o n 16 F i g u r e 2.5 E l e c t r i c a l c h a r a c t e r i s t i c s o f a r e v e r s e b i a s e d pn j u n c t i o n 18 F i g u r e 3.1 I m p l a n t a t i o n f a b r i c a t i o n t e c h n i q u e 32 F i g u r e 3.2 C r o s s s e c t i o n o f t h e s u r f a c e b a r r i e r d e t e c t o r 35 F i g u r e 3.3 C r o s s s e c t i o n o f t h e d e t e c t o r mount 38 F i g u r e 3.4 L e a k a g e c u r r e n t o f p r o t o t y p e 41 F i g u r e 4.1 P h o t o g r a p h o f d e t e c t o r p r e a m p l i f i e r s ....44 F i g u r e 4.2 C i r c u i t d i a g r a m o f d e t e c t o r p r e a m p l i f i e r s a n d b i a s v o l t a g e 45 F i g u r e 4.3 R e s p o n s e o f d e t e c t o r t o a l p h a p a r t i c l e s 47 F i g u r e 4.4 Asymmetry o f r e s p o n s e o f d e t e c t o r t o a l p h a p a r t i c l e s 48 F i g u r e 4.5 R e s p o n s e o f d e t e c t o r t o b e t a p a r t i c l e s ..51 F i g u r e 4.6 N o i s e i n d e t e c t o r / p r e a m p l i f i e r s 52 F i g u r e 4.7 D a t a a c q u i s i t i o n e l e c t r o n i c s f o r beam t e s t 54 v i i F i g u r e 4.8 Beam t e s t apparatus 56 Fi g u r e 4.9 S c a t t e r p l o t of d e t e c t o r p o s i t i o n versus wire chamber p o s i t i o n 57 Fi g u r e 4.10 S c a t t e r p l o t of d e t e c t o r p o s i t i o n versus wire chamber p o s i t i o n , noise only 58 Fi g u r e 4.11 P o s i t i o n r e s o l u t i o n of d e t e c t o r 59 Fi g u r e 4.12 Beam p r o f i l e as seen with SS d e t e c t o r ...61 F i g u r e 4.13 M u l t i p l i c i t y of d e t e c t o r events 63 F i g u r e 4.14 E f f i c i e n c y and noise of SS det e c t o r with and without noise r e j e c t i o n 64 F i g u r e 4.T5 D i s t r i b u t i o n of muon l a b angle f o r pion decay 65 Fi g u r e 4.16 Histograms of p a r t i c l e angles in the beam t e s t 66 Fi g u r e 4.17a G r a p h i c a l summary of e f f i c i e n c y and noise of the SS d e t e c t o r 68 F i g u r e 4.17b G r a p h i c a l summary of e f f i c i e n c y and noise continued 69 F i g u r e 4.18 Time spectrums of s i g n a l s from SS de t e c t o r 71 ACKNOWLEDGEMENTS I would l i k e to express my thanks to.Dr. L. Young f o r the use of the s o l i d s t a t e l a b o r a t o r y i n the E l e c t r i c a l E n g i n e e r i n g department at UBC. I would a l s o l i k e to thank Dr. P. Janega and Dr. J.B.A. England f o r t h e i r t e c h n i c a l advice i n s o l i d s t a t e f a b r i c a t i o n t e c h n i q u e s . I am indebted to J . C r e s w e l l , D. Maas, G. S h e f f e r and P. A. Amaudruz f o r t e c h n i c a l a d v i c e on e l e c t r o n i c s and nuclear experimental techniques. Many thanks to P. Amaudruz and the PISCAT group f o r time and h e l p i n the beam experiments. Thanks are due to my s u p e r v i s o r Dr. K a r l Erdman and to Dr. R.R. Johnson and Dr. D. G i l l f or encouragement and guidance d u r i n g t h i s p r o j e c t . CHAPTER I INTRODUCTION The use of s o l i d s t a t e d e t e c t o r s in nuclear p h y s i c s i s i n c r e a s i n g . The development of f a b r i c a t i o n technology has both reduced t h e i r cost and improved t h e i r q u a l i t y . What i s a s o l i d s t a t e d e t e c t o r ? The SS ( s o l i d s t a t e ) d e t e c t o r i s a s l a b of semiconducting c r y s t a l with an i n t e r n a l e l e c t r i c f i e l d generated by an e x t e r n a l v o l t a g e a p p l i e d across i t . A charged p a r t i c l e or X-ray i s det e c t e d when i t s passage through the c r y s t a l generates charge c a r r i e r s which are swept out of the c r y s t a l by the i n t e r n a l e l e c t r i c f i e l d and are dete c t e d e l e c t r o n i c a l l y as a c u r r e n t pulse (Figure 1.1). SS d e t e c t o r s can be c l a s s i f i e d as e i t h e r bulk or diode types. In bulk d e t e c t o r s the i n t r i n s i c c o n d u c t i v i t y of the semiconductor i s very low due to hig h c r y s t a l p u r i t y or charge compensation techniques. One only needs to a t t a c h ohmic c o n t a c t s to the normally i n s u l a t i n g c r y s t a l to produce a d e t e c t o r . The diode d e t e c t o r has l e s s s t r i n g e n t r e s t r i c t i o n s on the bulk p r o p e r t i e s of the semiconducting m a t e r i a l and uses the d e p l e t i o n zone c r e a t e d i n a re v e r s e b i a s e d pn j u n c t i o n as i t s a c t i v e r e g i o n . T h i s a c t i v e region can extend p a r t i a l l y or 2 FIGURE 1. 1 BIAS VOLTAGE S c h e m a t i c of a s e m i c o n d u c t o r c h a r g e d p a r t i c l e d e t e c t o r . 3 c o m p l e t e l y t h r o u g h t h e c r y s t a l s l a b . G e n e r a l p r o p e r t i e s o f SS d e t e c t o r s S o l i d s t a t e d e t e c t o r s c a n be d e s i g n e d f o r t i m i n g o f - p a r t i c l e s , e n e r g y r e s o l u t i o n , p o s t i t i o n r e s o l u t i o n o r some c o m b i n a t i o n o f t h e t h r e e . The i n t r i n s i c p r o p e r t i e s o f t h e s e d e t e c t o r s make them v e r s a t i l e d e v i c e s w i t h some u n i q u e p r o p e r t i e s . SS d e t e c t o r s d e s i g n e d f o r s m a l l c a p a c i t a n c e a n d h i g h i n t e r n a l e l e c t r i c f i e l d s h ave p u l s e r i s e t i m e s i n t h e o r d e r o f n a n o s e c o n d s w i t h p u l s e w i d t h s o f t e n s o f n a n o s e c o n d s . A c c u r a t e t i m i n g o f e v e n t s a nd h i g h r a t e a p p l i c a t i o n s a r e t h u s p o s s i b l e . P o s i t i o n r e s o l u t i o n i n SS d e t e c t o r s i s a c h i e v e d by p a t t e r n i n g t h e c h a r g e c o l l e c t i n g e l e c t r o d e s . W i t h s t a n d a r d s e m i c o n d u c t o r p h o t o l i t h o g r a p h i c t e c h n i q u e s , S i d e t e c t o r s have been b u i l t w i t h 5 um r e s o l u t i o n f o r minimum i o n i z i n g p a r t i c l e s (REF 6 ) . SS d e t e c t o r s c a n be u s e d t o d e t e r m i n e t h e e n e r g y o f s t o p p e d p a r t i c l e s o r t h e e n e r g y l o s s o f t r a n s m i t t e d p a r t i c l e s . The c h a r g e p u l s e p r o d u c e d by t h e p a r t i c l e i s d i r e c t l y p r o p o r t i o n a l t o t h e e n e r g y l o s t i n t h e a c t i v e r e g i o n o f t h e d e t e c t o r , a n d may be m e a s u r e d down t o a few p e r c e n t a c c u r a c y (REF 4 ) . H i s t o r i c a l l y , t h i s h a s been t h e most common u s e f o r SS d e t e c t o r s . A s i n g l e d e t e c t o r o r s t a c k o f t o t a l l y d e p l e t e d d e t e c t o r s i s commonly u s e d t o s t o p l o w e n e r g y p a r t i c l e s o r X - r a y s , a n d m e a s u r e t h e i r e n e r g y . The energy r e s o l u t i o n or minimum energy d e t e c t a b l e i n a s o l i d s t a t e d e t e c t o r i s l i m i t e d p r i m a r i l y by the random f l u c t u a t i o n of c u r r e n t flowing i n the absence of p a r t i c l e s . T h i s leakage c u r r e n t depends on the i n t e g r i t y of the o r i g i n a l semiconductor m a t e r i a l and on the f a b r i c a t i o n techniques used. R a d i a t i o n damage can a l s o c o n t r i b u t e to leakage c u r r e n t , and the l i f e of an SS d e t e c t o r i s l i m i t e d by the i n c r e a s e of the leakage c u r r e n t to some c r i t i c a l value. The most s i g n i f i c a n t development i n these d e t e c t o r s has been the r e d u c t i o n of leakage c u r r e n t to l e v e l s as low as 40 nA/cm2 (REF 6). These d e t e c t o r s are t y p i c a l l y l e s s than 5 cm i n diameter and g r e a t e r than 100 m t h i c k (=25 mg/cm 2). T h i s r e l a t i v e l y small area and l a r g e mass t h i c k n e s s i s l e s s of a drawback when one c o n s i d e r s that other p o s i t i o n s e n s i t i v e or timing d e t e c t o r s r e q u i r e bulky l i g h t p ipes and p h o t o m u l t i p l i e r s or gas containment systems. At present, p o s i t i o n s e n s i t i v e s o l i d s t a t e d e t e c t o r s need l a r g e a r r a y s of p r e a m p l i f i e r s , but in fu t u r e even t h i s may be avoided with a m p l i f i e r s or CCD readouts being b u i l t on the same semiconductor wafer as the d e t e c t o r . At present SS d e t e c t o r s are used e x t e n s i v e l y i n low energy p h y s i c s due to t h e i r good behavior in high vacuum. M o t i v a t i o n f o r d e t e c t o r development at TRIUMF The high r a t e c a p a b i l i t y of p o s i t i o n s e n s i t i v e SS d e t e c t o r s was the primary m o t i v a t i o n f o r b u i l d i n g them at TRIUMF. The TRIUMF M13 pion beamline can have p i + f l u x e s i n excess of 2X 1 0 6 / S . A high r a t e d e t e c t o r can be used i n t h i s 5 FIGURE 1.2 M13 p i o n beam l i n e a t TRIUMF. B1 and B2 a r e d i p o l e b e n d i n g m a g n e t s ; F l a n d F2 a r e t h e f o c u s l o c a t i o n s o f t h e momentum s e l e c t i o n s l i t s . channel as a beam p r o f i l e monitor f o r d i a g n o s t i c s or as a t o o l in s c a t t e r i n g experiments. C u r r e n t l y the momentum r e s o l u t i o n of the pion beam i s s e l e c t e d by a p a i r of s l i t s l o c a t e d at focuses downstream of a bending magnet (F1, F2 in F i g u r e 1.2). A SS d e t e c t o r l o c a t e d at one of these focuses can be used to tag the p o s i t i o n and t h e r e f o r e the momentum of each p a r t i c l e as i t t r a v e r s e s the channel. The s l i t s can then be opened to inc r e a s e the pion f l u x without degrading the momentum r e s o l u t i o n . In a s i m i l a r manner, a s o l i d s t a t e d e t e c t o r at the s c a t t e r i n g t a r g e t can be used as a p o s i t i o n monitor to improve the energy r e s o l u t i o n of the spectrometer. A p o s i t i o n s e n s i t i v e , s i l i c o n , s o l i d s t a t e d e t e c t o r has been b u i l t and t e s t e d by the PISCAT (pion s c a t t e r i n g ) experimental group at TRIUMF. The de v i c e was made using s i l i c o n s u r f a c e b a r r i e r techniques. I t has a r e s o l u t i o n of one m i l l i m e t e r i n one d i r e c t i o n and an a c t i v e area of 4 cm in diameter and i s 230 jum t h i c k (55 mg/cm2 ), ( F i g u r e 1.3). S u r f a c e b a r r i e r , p o s i t i o n s e n s i t i v e , s o l i d s t a t e d e t e c t o r manufactured at UBC. The p r e - a m p l i f i e r readout e l e c t r o n i c s are v i s i b l e . CHAPTER II PRINCIPLES OF OPERATION Basic band theory of semiconductors and charge generation A semiconductor i s a c r y s t a l with a gap in the a v a i l a b l e energy s t a t e s for e l e c t r o n s . The s t a t e s above the gap are normally empty and are c o l l e c t i v e l y known as the conduction band. S i m i l a r l y , the valence band i s the c o l l e c t i o n of normally f u l l s t a t e s below the band gap. In any c r y s t a l at zero temperature e l e c t r o n s are l y i n g in the lowest energy s t a t e s p o s s i b l e without v i o l a t i n g the P a u l i e x c l u s i o n p r i n c i p l e . At zero temperature the Fermi energy i s d e f i n e d as the energy above which no e l e c t r o n s are seen. At non-zero temperatures, the Fermi energy, E p ' d e f i n e s the thermal d i s t r i b u t i o n of e l e c t r o n s or holes (REF 8 ) : .-(E c-E F/kT) n = N e c - ( E p - E v / K T ) p = N e ^ v Equations 2.1 where n and p are the r e s p e c t i v e d e n s i t i e s of e l e c t r o n s and h o l e s , E c i s the e l e c t r o n energy in the conduction band, E v i s the h o l e energy in the valence band, N c and N v are d e n s i t i e s of s t a t e s f o r the conduction and valence bands r e s p e c t i v e l y and kT i s the temperature in energy u n i t s . Holes are e l e c t r o n v a cancies in the almost f u l l valence band and behave l i k e p o s i t i v e charged p a r t i c l e s with a mass c l o s e to the mass of the e l e c t r o n ( e f f e c t i v e mass). Both e l e c t r o n s and holes have an e f f e c t i v e mass in the c r y s t a l which depends on the s t a t e occupied by the p a r t i c l e and on i t s d i r e c t i o n of motion in the c r y s t a l l a t t i c e . E f f e c t i v e masses of charge c a r r i e r s i n semiconductors u s u a l l y are between 10% and 100% of the e l e c t r o n mass and holes can be " h e a v i e r " or " l i g h t e r " than e l e c t r o n s . I n t r i n s i c semiconductors are c h a r a c t e r i z e d by the Fermi energy l y i n g c l o s e to the c e n t r e of the band gap. I n s u l a t o r s are the same but with a l a r g e r band gap. In a s o l i d s t a t e d e t e c t o r the a c t i v e volume of semiconducting c r y s t a l normally a c t s l i k e an i n s u l a t o r . The non-conducting s t a t e has an empty conduction band (no e l e c t r o n charge c a r r i e r s ) and a f u l l valence band (no hole charge c a r r i e r s ) . When a charged p a r t i c l e passes through the c r y s t a l i t s c a t t e r s e l e c t r o n s i n t o the a v a i l a b l e energy s t a t e s in the conduction band, l e a v i n g behind empty s i t e s (holes) i n the valence band. T h i s e x c i t e d c o n f i g u r a t i o n q u i c k l y decays by phonon e x c i t a t i o n , r a d i a t i v e decay or s c a t t e r i n g from valence e l e c t r o n s (generating more e l e c t r o n - h o l e p a i r s ) u n t i l a -1 2 q u a s i - s t a b l e s t a t e i s reached (10 s ) . T h i s longer l i v e d s t a t e i s c h a r a c t e r i z e d by an equal number of e l e c t r o n s and h oles in the conduction and valence bands. These charge c a r r i e r s are f r e e to d r i f t i n the i n t e r n a l e l e c t r i c f i e l d of the d e t e c t o r and form the d e t e c t e d c u r r e n t pulse ( F i g u r e 2.1). 10 FIGURE 2 . 1 I L_. 1 _ . I" CONDUCTION I BAND | BAND GAP I L_. L. I !<£> ( Q I , i>- I r - I 1°" A VALENCE BAND EQUILIBRIUM AT TIME OF IONIZATION 1 ps AFTER IONIZATION > - C D cr. UJ LU Schematic of charge gener a t i o n process i n the a c t i v e r e g i o n of a s o l i d s t a t e d e t e c t o r . The s i z e of t h i s c u r r e n t p u l s e , Q, i s p r o p o r t i o n a l to the energy l o s s , AE, of the de t e c t e d p a r t i c l e i n the a c t i v e r egion and i s given by Q = qKAE/E • gap Equation 2.2 where Q i s the charge generated i n the d e t e c t o r ( e i t h e r h oles or e l e c t r o n s ) , E i s the band gap of the semiconductor (1.12 gap 3 r eV f o r S i ) and q i s the e l e c t r o n i c charge. K i s a f a c t o r l e s s than one that accounts f o r energy l o s s e s other than i o n i z a t i o n , f o r example phonon e x c i t a t i o n . K i s about 1/3 i n Si as 3.2 eV i s the average energy r e q u i r e d to produce an e l e c t r o n hole p a i r (REF 7). Gas i o n i z a t i o n chambers r e q u i r e about 30 eV per i o n i z a t i o n and o f t e n use avalanche charge m u l t i p l i c a t i o n near c o l l e c t i o n w i r e s . Avalanche charge m u l t i p l i c a t i o n a l s o occurs i n semiconductors with a high i n t e r n a l e l e c t r i c f i e l d . T h i s e f f e c t i s not e a s i l y c o n t r o l l e d , however, and o f t e n causes t o t a l breakdown, so i t i s not used in semiconductor charged p a r t i c l e d e t e c t o r s . Bulk type SS d e t e c t o r s , r e s i s t i v i t y of semiconductors Bulk SS d e t e c t o r s are made from high r e s i s t i v i t y semiconductor. An e x t e r n a l v o l t a g e , V, a p p l i e d to c o n t a c t e l e c t r o d e s generates an i n t e r n a l e l e c t r i c f i e l d i n the d e t e c t o r s l a b . For a s l a b of t h i c k n e s s d, the e l e c t r i c f i e l d s t r e n g t h i s un i f o r m l y V/d. One l i m i t a t i o n of bulk d e t e c t o r s i s the f i n i t e r e s i s t i v i t y of the c r y s t a l due to th e r m a l l y e x c i t e d charge c a r r i e r s . The leakage c u r r e n t of bulk d e t e c t o r s can be c a l c u l a t e d from semiconductor p r o p e r t i e s . In thermal e q u i l i b r i u m the c a r r i e r c o n c e n t r a t i o n i s given by Equations 2.1 or np = n^ 2 - e -E AT gap Equation 2.3 where n and p are the d e n s i t y of e l e c t r o n s and holes r e s p e c t i v e l y , n. i s the i n t r i n s i c c a r r i e r c o n c e n t r a t i o n , E ^ J i ' gap i s the semiconductor band gap, kT i s the ab s o l u t e temperature i n energy u n i t s . A p r o p o r t i o n a l i t y constant dependent on d e n s i t y of s t a t e s i s omitted. The i n t r i n s i c c a r r i e r c o n c e n t r a t i o n i s shown i n (Figure 2.2) f o r Ge, S i , GaAs. The e l e c t r o n leakage c u r r e n t per u n i t area, J n , in an i d e a l bulk d e t e c t o r (n=p=n^) i s (REF 8): J n = qn^M Equation 2.4 where q i s the e l e c t r i c charge, n^=n i s the e l e c t r o n c a r r i e r c o n c e n t r a t i o n , E i s the i n t e r n a l e l e c t r i c f i e l d and u i s the e l e c t r o n m o b i l i t y . For example, at room temperature with a 1kV/cm e l e c t r i c f i e l d , the best leakage c u r r e n t s f o r Ge, S i , GaAs bulk d e t e c t o r s are 14 A/cm 2, 3 mA/cm2 and 2 uh/cm2 r e s p e c t i v e l y . Of these three only GaAs has an acceptable behavior at room temperature. I n t r i n s i c charge c a r r i e r c o n c e n t r a t i o n of Ge, S i , and GaAs as a f u n c t i o n of i n v e r s e temperature ( a f t e r Sze Ref. 8 ) . In a non-ideal bulk d e t e c t o r , leakage c u r r e n t can be produced by the i n j e c t i o n of charges i n t o the c r y s t a l by the conta c t e l e c t r o d e s i f s p e c i a l care i s not taken to make the c o n t a c t s ohmic. Unwanted donor or acceptor i m p u r i t i e s can c o n t r i b u t e charge c a r r i e r s as can mid-band genera t i o n and recombination s i t e s caused by i m p u r i t i e s or c r y s t a l f a u l t s . F u r t h e r d i s c u s s i o n of leakage c u r r e n t i s in c l u d e d in the s e c t i o n on diode d e t e c t o r s . Recent improvements in the q u a l i t y of commercially a v a i l a b l e semiconductors are making bulk d e t e c t o r s more f e a s a b l e . Bulk d e t e c t o r s made from l a r g e band gap semiconductors such as GaAs or GdS may be common i n the f u t u r e . Diode d e t e c t o r s ' e l e c t r i c a l c h a r a c t e r i s t i c s In diode type SS d e t e c t o r s , i t i s the d e p l e t i o n zone of a reverse b i a s e d pn j u n c t i o n that p r o v i d e s the s e m i - i n s u l a t i n g a c t i v e volume f o r charged p a r t i c l e d e t e c t i o n . The e l e c t r i c a l p r o p e r t i e s of a l l j u n c t i o n SS d e t e c t o r s are approximated by the PIN diode (Figure 2.3). The p + n - n + diode w i l l be co n s i d e r e d here although the p +p~n* i s s i m i l a r . Let us co n s i d e r the i n t e r f a c e of the h i g h l y doped p + r e g i o n and the l i g h t l y doped n~ r e g i o n . T h i s pn j u n c t i o n can be made by d i f f u s i n g or implanting a c l a s s III dopant element such as boron i n t o an n" s u b s t r a t e or by the c o n s t r u c t i o n of a su r f a c e b a r r i e r . In a p type doped semiconductor, there are fewer e l e c t r o n s per l a t t i c e s i t e than i n i n t r i n s i c m a t e r i a l FIGURE 2.3 Xo X Schematic of PIN diode and d e f i n i t i o n of c o o r d i n a t e x. 16 F I G U R E 2.4a I O N I Z E D A C C _ E P T O R _ S _ I T E S 6 § 5 e 9 g g ) e e ê 5>5> ® Q ~ + f + + t + + + M O B I L E H O L E S qVo D E P L E T I O N ! Z O N E J M O B I L E E L E C T R O N S @ © S> ® I O N I Z E D © € D O N O R S I T E S Ef F I G U R E 2.4b Band diagrams for pn junction with (a) no external a p p l i e d voltage, (b) an a p p l i e d voltage of V a. E c and Ev are the energies of the bottom of the conduction band and the top of the valence band r e s p e c t i v e l y ; Ef i s the Fermi energy and q i s the e l e c t r o n i c charge. and the Fermi l e v e l i s consequently s h i f t e d downward i n energy toward the bottom of the band gap. S i m i l a r l y the Fermi l e v e l i s s h i f t e d upward toward the top of the band gap i n an n type semiconductor. In the absence of any e x t e r n a l f i e l d , there i s a r e d i s t r i b u t i o n of charges such that the Fermi l e v e l i s constant throughout the c r y s t a l . T h i s leaves a region d e p l e t e d of f r e e charge c a r r i e r s near the j u n c t i o n . The energy l e v e l s of the valence and conduction bands are a f u n c t i o n of the l o c a l charge d i s t r i b u t i o n and thus change acr o s s the pn j u n c t i o n ( F i g u r e 2.4). In e q u i l i b r i u m the d e p l e t i o n zone b a r r i e r i s p r e c i s e l y l a r g e enough to prevent d i f f u s i o n of holes i n t o the n region or e l e c t r o n s i n t o the p region i n the l a r g e c o n c e n t r a t i o n g r a d i e n t s . When an e x t e r n a l v o l t a g e V i s a p p l i e d a c r o s s the j u n c t i o n with the same sign as V 0 ( r e v e r s e b i a s ) , f r e e charges move away from the j u n c t i o n and leave a l a r g e r d e p l e t i o n zone; the b a r r i e r height i n c r e a s e s p r o p o r t i o n a t e l y (Figure 2.4). In a t o t a l l y d e p l e t e d PIN diode, the d e p l e t i o n zone extends acr o s s the c r y s t a l to the n + r e g i o n . In the approximation that the n" or i n t r i n s i c region of the diode has a much lower dopant d e n s i t y than e i t h e r the p + or n + regions and assuming that the d e p l e t i o n zone i s t o t a l l y d e p l e t e d of charge c a r r i e r s , one can c a l c u l a t e the d e p l e t i o n width and i n t e r n a l e l e c t r i c f i e l d shape as a f u n c t i o n of a p p l i e d v o l t a g e V (REF 7, Fig u r e 2.5). FIGURE 2.5 CHARGE DENSITY ELECTRIC FIELD STRENGTH ELECTRIC POTENTIAL X X FULLY DEPLETED PARTIALLY DEPLETED Q u a l i t a t i v e charge d e n s i t y , e l e c t r i c f i e l d and p o t e n t i a l as a f u n c t i o n of p o s i t i o n i n a reverse b i a s e d PIN diode. Charge d e n s i t y i s p r o p o r t i o n a l to the c o n c e n t r a t i o n of uncompensated i o n i z e d dopant i o n s . For an SS d e t e c t o r that i s not f u l l y d e p l e t e d , the d e p l e t i o n width, W, i s given by (REF 7) W2 = 2 ( V 0 + V a ) e s / q p n Equation 2.5 where V 0 i s the b u i l t in d e p l e t i o n p o t e n t i a l with no a p p l i e d v o l t a g e , V i s the a p p l i e d v o l t a g e , e i s the d i e l e c t r i c a s constant of the . semiconductor, p n i s the d e n s i t y of dopant ions i n the n~ region and q i s the e l e c t r o n i c charge. The e l e c t r i c f i e l d i s c a l c u l a t e d by i n t e g r a t i n g the charge d e n s i t y i n the d e p l e t i o n zone and i s thus l i n e a r (REF 8): E(x) = (W-x)p n/e s 0 < x < W = 0 elsewhere Equations 2.5 where x i s d e f i n e d in F i g u r e 2.3. For a t o t a l l y d e p l e t e d diode the d e p l e t i o n width i s f i x e d at the width, x 0 , of the i n t r i n s i c r e g i o n . The e l e c t r i c f i e l d i s s t i l l l i n e a r with the same slope as i n a p a r t i a l l y d e p l e t e d diode, but now has maximum and minimum values (E , E . ) max min dependent on the a p p l i e d v o l t a g e (Figure 2.5). AE = E max -E min = q p f l x 0 / * s E(x) = (x 0-x)AE/x 0+E 0<x<xo = 0 elsewhere V a + V 0 = x 0 ( E m i n + A E / 2 ) Equations 2.7 The e l e c t r i c p o t e n t i a l at any p o i n t i n the c r y s t a l i s found by i n t e g r a t i n g the e l e c t r i c f i e l d . The p o t e n t i a l d i f f e r e n c e across the whole c r y s t a l i s the a p p l i e d v o l t a g e , V , plus the a b u i l t - i n p o t e n t i a l , V 0 (Figure 2.5). T o t a l l y d e p l e t e d PIN diodes are convenient d e t e c t o r s because the d e p l e t i o n width ( a c t i v e region) and c a p a c i t a n c e are c o n s t a n t s . The hig h frequency c a p a c i t a n c e , C, of the PIN diode with d e p l e t i o n width W i s the same as a p a r a l l e l p l a t e c a p a c i t o r of the same width (REF 8) C = Aes/W Equation 2.8 where A i s the area of the diode. Charge c o l l e c t i o n time The s i g n a l response time i n a SS d e t e c t o r i s dependent on the d r i f t time of charge c a r r i e r s and on the RC time constant of the d e t e c t o r / p r e a m p l i f i e r system. The d r i f t v e l o c i t y , v, of c h a r g e c a r r i e r s i n a s e m i c o n d u c t o r i s p r o p o r t i o n a l t o t h e e l e c t r i c f i e l d , E, i n t h e c r y s t a l (REF 8 ) . v = M E E q u a t i o n 2.9 (REF 8) where u i s t h e c a r r i e r m o b i l i t y . The m o b i l i t y f o r e l e c t r o n s i s h i g h e r t h a n t h a t f o r h o l e s a nd v a r i e s w i t h t e m p e r a t u r e , d o p i n g c o n c e n t r a t i o n and s e m i c o n d u c t o r t y p e . C a r r i e r m o b i l i t i e s f o r l i g h t l y d o ped s e m i c o n d u c t o r s a t 300 K a r e shown i n T a b l e 2.1 f o r Ge, S i and GaAs. T a b l e 2.1 C a r r i e r m o b i l i t i e s a t 300 k ( c m 2 / V s ) Ge S i GaAs H o l e s 1 900 450 400 E l e c t r o n s 3900 1 500 8500 I t s h o u l d a l s o be n o t e d t h a t t h e d r i f t v e l o c i t i e s i n t h e s e t h r e e s e m i c o n d u c t o r s s t a r t t o s a t u r a t e a b o v e f i e l d s o f 3000 V/cm a t a v a l u e n e a r 1 0 7 cm/s (REF 8 ) . F o r a f u l l y d e p l e t e d SS d e t e c t o r o f t h i c k n e s s d, t h e l o n g e s t d r i f t t i m e i s g i v e n b y : t = / d d x / V ( x ) = / d d x / M E ( x ) ( d / M A E ) I n ( E / E • ) max' min E q u a t i o n 2.10a where E m _ . E m . , AE a r e d e f i n e d i n E q u a t i o n s 2.7. F o r a ma x m l n p a r t i a l l y d e p l e t e d d e t e c t o r E . i s z e r o b u t , a s t h e e l e c t r i c min ' f i e l d i s l i n e a r , 90% of t h e c h a r g e i s c o l l e c t e d i n t 9 Q = <.9W/ME m a x)ln<10) E q u a t i o n 2.10b where W i s t h e d e p l e t i o n w i d t h . I n a b u l k d e t e c t o r t h e c o l l e c t i o n t i m e i s t = d / M E E q u a t i o n 2.10c where E i s V / d , t h e u n i f o r m e l e c t r i c f i e l d . F o r a h y p o t h e t i c a l 200 m i c r o n S i d e t e c t o r s i m i l a r t o t h e one b u i l t a t TRIUMF, t h e c o l l e c t i o n t i m e w o u l d be t = t 0 l n ( E /E/ • ,) 0 max' ( m i n ) E q u a t i o n 2.1Od where t 0 = 4 ns f o r e l e c t r o n s a n d t 0 = 13 ns f o r t h e s l o w e r h o l e s . The c a p a c i t a n c e f o r s u c h a d e t e c t o r i s a b o u t 50 p F / c m 2 . T h i s d e t e c t o r made w i t h s m a l l a r e a s t r i p s , f a s t e l e c t r o n i c s a n d o p e r a t e d w i t h a l a r g e b i a s v o l t a g e w i l l h a v e p u l s e . r i s e t i m e s on t h e o r d e r o f n a n o - s e c o n d s a nd p u l s e s e p a r a t i o n down to 30 ns , depending on e l e c t r o n i c s response time. The e f f i c i e n c y f o r charge c o l l e c t i o n i n SS d e t e c t o r s i s very high as the c a r r i e r l i f e t i m e i s long i n the d e p l e t i o n zone. C a r r i e r l i f e t i m e s are on the order of m i l l i s e c o n d s i n high r e s i s t i v i t y s i l i c o n at room temperature. GaAs has s h o r t e r c a r r i e r l i f e t i m e s (-10 M S ) due to allowed r a d i a t i v e e l e c t r o n hole recombination, but l i f e t i m e s in the d e p l e t i o n zone are s t i l l long compared to c o l l e c t i o n time. Energy r e s o l u t i o n , n o i s e , leakage c u r r e n t It has a l r e a d y been s t a t e d that a charge c o l l e c t e d by a SS d e t e c t o r i s p r o p o r t i o n a l to the energy d e p o s i t e d i n the a c t i v e region of the d e v i c e . The energy r e s o l u t i o n , or the minimum energy d e t e c t a b l e , i s l i m i t e d by the noise i n the system. Noise i n SS d e t e c t o r s i s due to the v a r i a t i o n i n charge generated by a mono-energetic p a r t i c l e and the background noise i n t r i n s i c to the d e t e c t o r / a m p l i f i e r system. The d i s t r i b u t i o n i n the amount of charge generated by a stopped mono-energetic p a r t i c l e has a s t a t i s t i c a l v a r i a n c e a s s o c i a t e d with i t dependent on the average energy, £, r e q u i r e d to produce an e l e c t r o n - h o l e p a i r . The width of the i d e a l pulse i n terms of energy i s (REF 7): W = 2.96(HE) 1 / 2 Equation 2.11 where E i s the energy of the i n c i d e n t p a r t i c l e . Because of the small s i z e of £ (3.2 eV f o r S i ) t h i s u n c e r t a i n t y due to s t a t i s t i c s i s n e g l i g i b l e f o r SS d e t e c t o r s . The s t a t i s t i c a l e r r o r i n measuring a 6 MeV alpha p a r t i c l e with a S i d e t e c t o r i s only 11 KeV (REF 7). Gas p r o p o r t i o n a l chambers or s c i n t i l l a t o r s have a l a r g e r e f f e c t i v e energy per d e t e c t e d e l e c t r o n (30 eV to 500 eV), and consequently have l a r g e r s t a t i s t i c a l e r r o r s i n energy r e s o l u t i o n . The i n t r i n s i c noise in a reverse biased j u n c t i o n i s due mostly to shot noise.and thermal n o i s e . Shot n o i s e , due to f i n i t e c a r r i e r charge, i s d i r e c t l y p r o p o r t i o n a l to leakage c u r r e n t , I s . The mean square shot n o i s e c u r r e n t i s (REF 8) <i 2> = 2qBI n ^ s N Equation 2.12 where B i s the frequency bandwidth of i n t e r e s t . Thermal n o i s e i s prop'ortional to temperature and to the conductance, G, of the device (REF 8): <i 2> = 4kTBG n Equation 2.13 Both these noise mechanisms have a uniform or white frequency spectrum and are t h e r e f o r e l i m i t e d by the bandwidth of i n t e r e s t . A t h i r d source of noise, f l i c k e r n o i s e , i s not important i n the high frequency band of i n t e r e s t f o r SS d e t e c t o r s due to i t s 1/f frequency spectrum. For a given leakage c u r r e n t I , the noise expected i s (REF 8) <i 2> = 2qBI ( i + e ^ V / K T ) n ^ s Equat ion 2.14 where q i s the e l e c t r o n i c charge, B i s the bandwidth of i n t e r e s t , V i s the b i a s v o l t a g e and kT i s the temperature energy. The second (thermal) term has been s i m p l i f i e d using the Shockly diode equation to c a l c u l a t e conductance. E m p i r i c a l l y , one can d e t e c t a c u r r e n t pulse approximately one t e n t h of the magnitude of the leakage c u r r e n t . The leakage c u r r e n t i s p a r t i a l l y d e s c r i b e d by the t h e o r e t i c a l d i f f u s i o n c u r r e n t ( f i r s t term, Equation 2.15) and g e n e r a t i o n c u r r e n t (second term, equation 2.15) i n a pn step j u n c t i o n diode: JR = V A = q / V r p ' ( n i V N D ) + q n i W / T e Equat ion 2.15 where D i s the d i f f u s i o n constant f o r h o l e s , r and r are P P e the r e s p e c t i v e hole and e l e c t r o n l i f e t i m e s , n^ i s the i n t r i n s i c c a r r i e r c o n c e n t r a t i o n , N Q i s the donor c o n c e n t r a t i o n in the n r e g i o n , and W i s the d e p l e t i o n width. R e c a l l that the i n t r i n s i c c a r r i e r c o n c e n t r a t i o n i n c r e a s e s with temperature (Equation 2.3) while l i f e t i m e decreases; the leakage c u r r e n t thus has a strong temperature dependence. The l i f e t i m e for c a r r i e r s i s a l s o reduced by i m p u r i t i e s (generation and recombination s i t e s ) i n the c r y s t a l and by l a t t i c e f a u l t s . F u r t h e r c o n t r i b u t i o n s to leakage c u r r e n t come from i n j e c t i o n of c a r r i e r s from the back contact in a t o t a l l y depleted d e t e c t o r as w e l l as r e s i s t i v e leakage at the edges of the d e v i c e . In surface b a r r i e r d e t e c t o r s , v o l t a g e dependent Schottky emission c o n t r i b u t e s to the leakage c u r r e n t and su r f a c e s t a t e s o f t e n dominate generation c u r r e n t in the d e p l e t i o n zone. Leakage c u r r e n t i n room temperature SS d e t e c t o r s i s the primary e n g i n e e r i n g problem due to the l a r g e s u r f a c e area and d e p l e t i o n depth r e q u i r e d f o r u s e f u l d e v i c e s . Leakage c u r r e n t , r a d i a t i o n damage R a d i a t i o n c o n t r i b u t e s to leakage c u r r e n t and noise by causing l a t t i c e f a u l t s and by isotope p r o d u c t i o n . In the absence of slow neutrons, i t i s the former mechanism which dominates r a d i a t i o n damage. L a t t i c e damage causes charge t r a p s which slow d e t e c t i o n response times, reduce charge e f f i c i e n c y , and i n c r e a s e leakage c u r r e n t . Heavy ions d e p o s i t more energy in a d e t e c t o r and do p r o p o r t i o n a t e l y more damage. C r y s t a l l a t t i c e damage can sometimes be annealed out by low temperature ( l e s s than 400°C) heat treatment i n implanted or d i f f u s e d d e v i c e s . Ion implanted d e t e c t o r s have been shown to have a l i f e t i m e of 1 0 1 1 - 1 0 1 4 p a r t i c l e s / c m 2 i n a muon f l u x (REF 9). Surface b a r r i e r d e t e c t o r s tend to have s h o r t e r l i f e e x p e c t a n c i e s . In a slow neutron f l u x 3 0 S i (3% abundant) can absorb a neutron and s u b s e q u e n t a l l y /3 decay to phosphorus. Phosphorus i s a donor dopant and w i l l make the c r y s t a l more n type. In a d d i t i o n , decay of r a d i o - i s o t o p e s i n the d e t e c t o r a c t i v e r e g i o n w i l l add noise s i g n a l s . D e p l e t i o n depth, energy r e s o l u t i o n To measure the energy of stopped p a r t i c l e s i n an SS d e t e c t o r , i t i s necessary that the p a r t i c l e stop i n the a c t i v e d e p l e t i o n zone. A stack of t o t a l l y d e p l e t e d SS d e t e c t o r s can have more than 99% of t h e i r volume d e p l e t e d and are i d e a l f o r measuring the energy of medium to low energy p a r t i c l e s or X-rays. To measure low energy p a r t i c l e s , a s i n g l e s o l i d s t a t e d e t e c t o r with an a p p r o p r i a t e l y l a r g e d e p l e t i o n depth i s used. The low energy l i m i t of r e s o l u t i o n i s reached when the range of the p a r t i c l e of i n t e r e s t approaches the t h i c k n e s s of the dead zone at the s u r f a c e of the d e t e c t o r . T h i s dead zone i s c h a r a c t e r i z e d by the t h i c k n e s s of the metal c o n t a c t s on the s u r f a c e and the d i s t a n c e to the edge of the d e p l e t i o n zone. The dead zone can be reduced to hundreds of Angstroms in s u r f a c e b a r r i e r d e t e c t o r s . The ab s o l u t e t h i c k n e s s of the dead zone can be estimated by measuring the energy of a de t e c t e d p a r t i c l e as a f u n c t i o n of the angle of i n c i d e n c e f o r a c o l l i m a t e d monoenergetic source. Transmission d e t e c t o r s SS d e t e c t o r s are o f t e n used to detect p a r t i c l e s by t h e i r energy l o s s in t r a n s m i s s i o n . Assuming that the energy l o s s i s small compared to the t o t a l energy of the p a r t i c l e s , the d e t e c t e d pulse i s p r o p o r t i o n a l to the d e p l e t i o n depth. As i t i s normally d e s i r e a b l e to reduce s c a t t e r i n g , f u l l y d e p l e t e d t h i n d e t e c t o r s are used. When measuring the energy l o s s of a t r a n s m i t t e d p a r t i c l e , one i s a l s o measuring the s u b s t a n t i a l energy s t r a g g l e (^20% of energy l o s s ) . Even with such a l a r g e b u i l t - i n u n c e r t a i n t y , energy r e s o l u t i o n i s used to d i s t i n g u i s h p a r t i c l e s with d i f f e r e n t r a t e s of energy l o s s i n matter. The primary problem in a t r a n s m i s s i o n d e t e c t o r i s to d e t e c t the small s i g n a l s a s s o c i a t e d with t h i n d e t e c t o r s and p a r t i c l e s with low energy l o s s . The s i g n a l - t o - n o i s e r a t i o of t o t a l l y d e p l e t e d d e t e c t o r s can o f t e n be improved by i n c r e a s i n g the b i a s v o l t a g e across the d e v i c e . According to Equations 2.7 and 2.10, t h i s speeds up the charge c o l l e c t i o n , i n c r e a s i n g the height of the c u r r e n t p u l s e s . However, in a t o t a l l y d e p l e t e d d e t e c t o r with a l a r g e a p p l i e d b i a s v o l t a g e , i n j e c t i o n c u r r e n t at the back contact can become a problem as can breakdown e f f e c t s . The breakdown v o l t a g e s of implanted or d i f f u s e d PIN diodes are l a r g e r than s u r f a c e b a r r i e r devices and a l l types behave b e t t e r with very high r e s i s t i v i t y i n t r i n s i c r e g i o n s . Breakdown occurs i f the i n t e r n a l e l e c t r i c f i e l d exceeds a c r i t i c a l value (somewhat higher than 10 5 V/cm) or i f the d e p l e t i o n zone moves through the back contact (or f r o n t c o n t a c t in s u r f a c e b a r r i e r d e v i c e s ) . These processes are c a l l e d avalanche breakdown and punchthrough r e s p e c t i v e l y . Minimum i o n i z i n g p a r t i c l e s can be de t e c t e d i n room temperature SS d e t e c t o r s as t h i n as 100 microns i f care i s taken to reduce no i s e . T h i s corresponds to a s i g n a l of around 10" e l e c t r o n s . It should be emphasized that the s i g n a l to noise behavior of any SS d e t e c t o r i s i n t i m a t e l y r e l a t e d to the readout e l e c t r o n i c s . It i s the behavior of the d e t e c t o r / e l e c t r o n i c s system .that should be optimized f o r any given a p p l i c a t i o n . P o s i t i o n r e s o l u t i o n P o s i t i o n s e n s i t i v e d e t e c t o r s are made by p a t t e r n i n g the charge c o l l e c t i n g e l e c t r o d e s on the surface of the d e t e c t o r . P h o t o l i t h o g r a p h i c techniques developed for the e l e c t r o n i c s i n d u s t r y can reproduce f e a t u r e s with one micron dimensions. Less e x a c t i n g techniques have been used by Kemmer et a l to produce SS s t r i p d e t e c t o r s with 5 micron r e s o l u t i o n i n one d i r e c t i o n ( R E F - 6 ) . T h i s high r e s o l u t i o n was accomplished using 20 Mm s t r i p spacing, an a m p l i f i e r on every t h i r d s t r i p and charge i n t e r p o l a t i o n between readout a m p l i f i e r s . The t h e o r e t i c a l l i m i t on p o s i t i o n r e s o l u t i o n in SS d e t e c t o r s may be the width of i o n i z a t i o n t r a c k s i n SS d e t e c t o r s or the d i f f u s i o n l e n g t h , L, of c a r r i e r s before they are c o l l e c t e d . (REF 8) L = ( D t ) l / 2 Equation 2.16a where D i s the d i f f u s i o n constant f o r charge c a r r i e r s and t i s the time of i n t e r e s t . The d i f f u s i o n constant i s r e l a t e d to c a r r i e r m o b i l i t y u and temperature (REF 8): D = MkT/q Equation 2.16b E l e c t r o n s i n s i l i c o n w i l l d i f f u s e an average of 6 /um i n 10 ns at room temperature. D e t e c t o r s made with CCD readouts or i n t e g r a t e d p r e a m p l i f i e r s c o u l d make l a r g e area high r e s o l u t i o n two dimensional d e t e c t o r s more fe a s a b l e by reducing the d e n s i t y of e l e c t r o n i c s r e q u i r e d f o r a high r e s o l u t i o n small area d e t e c t o r . In a l l types of d e t e c t o r s i t i s important to p r o t e c t ( p a s s i v a t e ) the sur f a c e of the semiconductor from i m p u r i t i e s in the environment (REF 8). In a p o s i t i o n s e n s i t i v e SS d e t e c t o r the su r f a c e should be p a s s i v a t e d between e l e c t r o d e s t r i p s . Two examples of s u r f a c e p a s s i v a t i o n techniques are used i n the two processes d e s c r i b e d i n Chapter I I I . CHAPTER III FABRICATION OF DEVICES With a recognized need for SS d e t e c t o r s at TRIUMF, i t was decided that p o s i t i o n s e n s i t i v e s i l i c o n d e v i c e s should be f a b r i c a t e d at UBC. P o s i t i o n s e n s i t i v e d e v i c e s are a v a i l a b l e commercially but they are s t i l l very expensive. The f a b r i c a t i o n work was done p r i m a r i l y in the E l e c t r i c a l E n g i n e e r i n g S o l i d State L a b o r a t o r y . The main t h r u s t of the work was to reproduce the ion i m p l a n t a t i o n techniques used by Kemmer et a l (REF 6) and the s u r f a c e b a r r i e r techniques used by England (REF 3,5,12). England's s u r f a c e b a r r i e r technique proved to be simpler and more s u c c e s s f u l , and w i l l t h e r e f o r e be emphasized in t h i s d i s c u s s i o n . Ion implanted SS d e t e c t o r s The implanted d e t e c t o r developed by Kemmer (REF 6) i n 1980 i s a standard PIN diode as d e s c r i b e d i n Chapter I I , but made in a p a t t e r n of s t r i p s so as to be p o s i t i o n s e n s i t i v e . A f a b r i c a t i o n flow c h a r t i s shown in F i g u r e 3.1. T h i s type of d e t e c t o r i s a t t r a c t i v e because of i t s very low leakage c u r r e n t (Kemmer c l a i m s 40 nA/cm 2), even when bia s e d w e l l above the t o t a l d e p l e t i o n v o l t a g e . A l s o d e s i r e a b l e i s the q u a l i t y of the s u r f a c e p a s s i v a t i o n that makes the d e t e c t o r almost immune to environmental contamination a f t e r f a b r i c a t i o n . I t was not p o s s i b l e to make a low leakage SS d e t e c t o r u s i n g t h i s FIGURE 3.1 Clean 5000K n type wafer Grow 2000A oxide (lOhrs 1040C dry oxygen) Pa t t e r n oxide with p h o t o l i t h o g r a p h y Implant Boron (15ReV 5x1u'%m 3 Arsenic (30KeV 5x1 O'Vcm'3) Anneal implant damage (30min 600C i n n i t r o g e n ) Deposit aluminum by evaporation (1000A) Pat t e r n aluminum by ph o t o l i t h o g r a p h y Kemmer's implanted d e t e c t o r f a b r i c a t i o n process. Note that the s i l i c o n s u r f a c e i s p r o t e c t e d from the environment by e i t h e r aluminum or s i l i c o n - o x i d e ( R e f . 6 ) . technique i n the development time a v a i l a b l e . The best d e t e c t o r b u i l t using a modified process had a leakage of 10 juA/cm2 and was not uniform from s t r i p to s t r i p . The most common problem in any s o l i d s t a t e f a b r i c a t i o n i s c l e a n l i n e s s . The l a r g e number of steps in t h i s process make contamination of the wafer more l i k e l y i f a h i g h l y c o n t r o l l e d c l e a n environment i s not a v a i l a b l e f o r the f a b r i c a t i o n p r o c e s s . The E.E. l a b o r a t o r y at UBC lacked t h i s q u a l i t y of c l e a n l i n e s s in both dry and wet p r o c e s s i n g steps. In an environment l i k e the E.E. l a b o r a t o r y that i s not completely c o n t r o l l e d . Wet steps l i k e p h o t o l i t h o g r a p h y can introduce i m p u r i t i e s to the s u r f a c e of the d e t e c t o r wafer and high temperature steps l i k e .oxidation and a n n e a l i n g can d r i v e i m p u r i t i e s i n t o the bulk regions of the d e t e c t o r . The i n t e g r i t y of the c r y s t a l l a t t i c e i s a l s o dependent on i t s temperature h i s t o r y . High temperature treatment can lower the r e s i s t i v i t y of the semiconductor even in a clean envi ronment. Since ion i m p l a n t a t i o n doping a l s o damages the s u r f a c e of the c r y s t a l t h i s damage must be annealed out at high temperatures. The temperature as a f u n c t i o n of time used in the anneal process i s very important. During the anneal process dopant ions d i f f u s e and are i n c o r p o r a t e d i n t o the c r y s t a l l a t t i c e . At the same time, the c r y s t a l f a u l t s heal themselves or move towards the s u r f a c e . The amount of a n n e a l i n g necessary i s dependent upon the energy and dose of the implanted dopant atoms and upon the d e s i r e d dopant " a c t i v a t i o n " (the percentage of dopant i n c o r p o r a t e d i n t o the c r y s t a l l a t t i c e , the r e s t remaining i n t e r s t i t i a l ) . Kemmer recommends annealing at 600°C f o r 30 minutes, but the best r e s u l t s at UBC were achieved with an 800°C anneal f o r 16 hours (REF 14). Others recommend high temperature s h o r t anneal processes (REF 13). The anneal process c e r t a i n l y a f f e c t s the leakage c u r r e n t of implanted d e t e c t o r s , and f u r t h e r experimentation i s r e q u i r e d to optimize t h i s p r o c e s s at UBC. An o p t i o n that may be c o n s i d e r e d in the f u t u r e i s sur f a c e a n n e a l i n g with high power arc lamps. The Kemmer process w i l l probably have to be mo d i f i e d before c o n s i s t e n t r e s u l t s can be . achieved i n a l a b o r a t o r y without e x t e n s i v e environmental c o n t r o l . For example, lower temperature s p u t t e r i n g or plasma techniques c o u l d be used to de p o s i t the mask/passivation oxide. A s u r f a c e e t c h before o x i d a t i o n may prove to be b e t t e r than the RCA c l e a n i n g technique used at UBC. G e n e r a l l y speaking, i t i s not always p o s s i b l e to t r a n s p l a n t a f a b r i c a t i o n process from one l a b o r a t o r y to another without c a r r y i n g out a time-consuming redevelopment procedure. Surface b a r r i e r SS d e t e c t o r s A s u r f a c e b a r r i e r f a b r i c a t i o n process was developed more than ten years ago at the U n i v e r s i t y of Birmingham by J.B.A. England (REF 10). His d e t e c t o r i s an n" wafer with a Si / S i - o x i d e / G e tunnel diode on both s u r f a c e s ( F i g u r e 3.2). FIGURE 3.2 11OOA A l 650A Ge 20A-50A SiO, 2 30/un S i (5000.rt.cm n type) W/1777777777777777777Z Cross s e c t i o n of the s u r f a c e b a r r i e r s t r i p d e t e c t o r b u i l t at UBC (modified England technique, Ref. 3,5,10,12). Front and back are i d e n t i c a l except f o r p a t t e r n s i n aluminum. Surface e l e c t r o d e s of A l or Au are used to reduce the sheet r e s i s t a n c e of the device and to provide a p a t t e r n f o r p o s i t i o n s e n s i t i v e d e t e c t o r s . The f a b r i c a t i o n process i s short, r e q u i r i n g only one day in the l a b o r a t o r y . T h i s reduces the p r o b a b i l i t y of contamination or other mistakes during manufacture. The d e t e c t o r c r y s t a l i s never heated above 100°C in the England process and thus i t s bulk p r o p e r t i e s are not degraded. Surface b a r r i e r SS d e t e c t o r s do have s h o r t e r l i v e s in r a d i a t i o n and, i n theory, a higher leakage c u r r e n t than a s i m i l a r implanted d e v i c e . The low c o s t and simple process f o r f a b r i c a t i o n of s u r f a c e b a r r i e r d e v i c e s may make these problems unimportant. The r e c i p e d e t a i l e d i n Appendix A i s the best of s e v e r a l attempts at d u p l i c a t i n g England's process, and i s only s l i g h t l y m o d i f i e d from h i s recommendations (REF 12). Two low leakage d e t e c t o r wafers were f a b r i c a t e d u s i n g t h i s process with aluminum m e t a l i z a t i o n . S e v e r a l p a r t s of the process proved to be q u i t e important. As p r e v i o u s l y mentioned, c l e a n l i n e s s i s of primary importance. I t was found that wafers that were not etched produced diodes with l a r g e leakage c u r r e n t s , even i f they were c a r e f u l l y c l e a n e d . England a l s o c l a i m s that d e i o n i z e d f i l t e r e d water i s not c l e a n enough f o r the process and recommends only d i s t i l l e d , f i l t e r e d , d e i o n i z e d water. S i m i l a r l y , u t e n s i l s and chemicals must be very c l e a n and pure. Only a f t e r the edges are p r o t e c t e d by epoxy and the s u r f a c e i s p r o t e c t e d by germanium i s the d e t e c t o r r e l a t i v e l y safe from contamination of the s u r f a c e . The germanium prov i d e s e x c e l l e n t s u r f a c e p a s s i v a t i o n due to i t s g e t t e r i n g e f f e c t s ( a b i l i t y to t r a p i m p u r i t i e s ) ; t h i s i s one of the a t t r a c t i v e p r o p e r t i e s of the England s u r f a c e b a r r i e r d e t e c t o r . E t c h i n g of the s i l i c o n s u r f a c e p r o v i d e s an i d e a l o p p o r t u n i t y to choose the t h i c k n e s s of the f i n a l d e v i c e . However, i t was found that even with continuous a g i t a t i o n of the etch bath and c a r e f u l quick quenching of the etch, the S i s u r f a c e was l e f t m ottled or r i p p l e d . T h i s perhaps c o u l d be prevented by d i l u t i n g the etchant s l i g h t l y . With 10 um removed from each s u r f a c e (wafer reduced from 250 to 230 Mm), the uneven etch d i d not appear to e f f e c t the behavior of the d e t e c t o r . The type of epoxy used for edge p a s s i v a t i o n of these s u r f a c e b a r r i e r d e t e c t o r s i s a l s o very important. The epoxy must cover the area not p r o t e c t e d by Ge. Transene epoxy 50 and the CIBA epoxy (mentioned i n Appendix A) are designed to have a high r e s i s t i v i t y to prevent ohmic leakage. They are of low v i s c o s i t y to reduce the chance of an a i r bubble touching the s i l i c o n s u r f a c e (another source of leakage c u r r e n t (REF 3 ) ) . These are a l s o "amine type" epoxies, which prevent a c o n d u c t i v e s u r f a c e i n v e r s i o n l a y e r from forming underneath them i n the s i l i c o n . England recommends p a s s i v a t i n g the edges with epoxy before e v a p o r a t i n g Ge. I t was found that t h i s was not necessary as long as the evaporated f i l m s were kept away FIGURE 3.3 EVAPORATED A l CONTACT SILICON CIRCUIT EPOXY BOARD Cross s e c t i o n of the d e t e c t o r c i r c u i t board mount and edge p a s s i v a t i o n . from the wafer edges (Figure 3.2). The process developed at UBC uses a second A l eva p o r a t i o n to make contact to the mounting c i r c u i t board (Figure 3.3). I t i s p o s s i b l e , as recommended by England, to do both the c o n t a c t evaporation and the s t r i p e v a p o r a t i o n through a s i n g l e shadow mask. Care must be taken, however, to a v o i d e l e c t r i c a l l y s h o r t i n g s t r i p s together i f the shadow mask cannot touch the wafer with the epoxy ramp i n the way. U l t r a s o n i c wirebonding (used e x t e n s i v e l y in the semiconductor i n d u s t r y ) i s another technique a v a i l a b l e to make e l e c t r i c a l contact to the wafer. Wirebonding tends to be time consuming, f r a g i l e and may damage the t h i n f i l m s t r u c t u r e of a s u r f a c e b a r r i e r d e t e c t o r , and so i s not recommended. P r e l i m i n a r y e l e c t r i c a l c h a r a c t e r i s t i c s of s u r f a c e b a r r i e r d e t e c t o r s Before mounting the completed d e t e c t o r wafers, the leakage c u r r e n t and the r e s i s t a n c e between s t r i p s was measured to assess the q u a l i t y of the d e v i c e s . The r e s i s t a n c e between s t r i p s on the d e t e c t o r must be much grea t e r than the input impedance of the p r e a m p l i f i e r s to prevent c r o s s t a l k . C a p a c i t i v e c o u p l i n g can a l s o l e a d to c r o s s t a l k but i s not so e a s i l y measured or c o n t r o l l e d . The sheet r e s i s t a n c e of the germanium l a y e r i s high enoug'h that the r e s i s t a n c e between s t r i p s was measured at between 40 Kfl and 100 Kfl. T h i s i s more than adequate s t r i p i s o l a t i o n . To measure leakage c u r r e n t , a m u l t i p o i n t probe was used to apply a b i a s v o l t a g e to three adjacent s t r i p s . The c u r r e n t flowing in the middle s t r i p c o u l d then be measured in a s i t u a t i o n s i m i l a r to the o p e r a t i n g c o n f i g u r a t i o n . The d e t e c t o r s produced with the m o d i f i e d England r e c i p e had a t y p i c a l c u r r e n t v o l t a g e (I/V) behavior p i c t u r e d i n F i g u r e 3.5. The 1 M A leakage per s t r i p at -30 V was seen to be ac c e p t a b l e as the wafer was expected to be t o t a l l y d e p l e t e d at t h i s v o l t a g e . The I/V behavior was q u i t e c o n s i s t e n t a c r o s s the wafer with o n l y one or two s t r i p s being more than 50% above the average. I t was l a t e r found that these high leakage s t r i p s were more n o i s y than the r e s t i n the d e t e c t o r that was t e s t e d . One of the s u r p r i s i n g r e s u l t s of the I/V measurements i n the l a b was the asymmetry of the d e t e c t o r . In theory, the symmetric manufacturing techniques should produce a d e t e c t o r that can be b i a s e d with a v o l t a g e of e i t h e r p o l a r i t y . When a negative v o l t a g e i s a p p l i e d to the pat t e r n e d or f r o n t s i d e of the d e t e c t o r , the f r o n t s urface b a r r i e r j u n c t i o n i s reverse b i a s e d and ,the d e p l e t i o n zone^ grows from the f r o n t toward the back. The e l e c t r i c f i e l d in t h i s negative b i a s e d d e t e c t o r causes p o s i t i v e c u r r e n t p u l s e s to be seen on the d e t e c t o r s t r i p s when p a r t i c l e s are d e t e c t e d . S i m i l a r l y , a p o s i t i v e b i a s e d d e t e c t o r w i l l have the back j u n c t i o n reverse b i a s e d while the f r o n t i s forward b i a s e d ; the d e t e c t o r c u r r e n t p u l s e s w i l l now be n e g a t i v e . Symmetry i n the leakage c u r r e n t was expected but not observed. A n e g a t i v e b i a s proved to give a lower leakage c u r r e n t . T h i s asymmetry can only be caused by FIGURE 3.4 D E T E C T O R S T R I P 3 6 BIAS VOLTAGE (V) Leakage c u r r e n t of one s t r i p (1 mir,X30mm) i n the SS d e t e c t o r detScto? V ° l t a g e r e v e r s e biases the p a t t e r n e d s i d e of the the d i f f e r e n t geometry of the aluminum l a y e r s ( f r o n t i s p a t t e r n e d , back i s n o t ) , as a l l other q u a l i t i e s of the s u r f a c e s are i d e n t i c a l . A f t e r the wafers were mounted in epoxy, the asymmetry was reduced t o a f a c t o r of 3, i n d i c a t i n g that edge e f f e c t s are p a r t i a l l y r e s p o n s i b l e . Even more p u z z l i n g i s that an asymmetry was a l s o seen in charge c o l l e c t i o n i n the f i n i s h e d d e t e c t o r . T h i s charge asymmetry w i l l be f u r t h e r d i s c u s s e d in Chapter IV. Current v o l t a g e c h a r a c t e r i s t i c s show that t h i s type of s u r f a c e b a r r i e r d e t e c t o r i s not symmetric as expected. A l l s o l i d s t a t e d e t e c t o r s act to some extent l i k e a p h o t o d e t e c t o r . The England s u r f a c e b a r r i e r d e t e c t o r i s mostly s h i e l d e d from photons by the aluminum and germanium l a y e r s . There i s , however, an i n c r e a s e of about 3 ixh/cxn2 leakage c u r r e n t seen when the device i s operated i n d a y l i g h t . T h i s i n c r e a s e i s roughly constant at a l l non-zero b i a s v o l t a g e s . P r e l i m i n a r y t e s t s showed that the d e t e c t o r wafers manufactured with the m o d i f i e d England process were a c c e p t a b l e and ready f o r charged p a r t i c l e t e s t s . CHAPTER IV SOLID STATE DETECTOR SYSTEM TEST A d e t e c t o r a m p l i f i e r system was assembled- using the s u r f a c e b a r r i e r d e t e c t o r s manufactured at UBC. Pion beam t e s t s were performed on the system to measure the e f f i c i e n c y f o r d e t e c t i n g a t r a n s m i t t e d charged p a r t i c l e and the s i g n a l - t o - n o i s e r a t i o of the system. The 1 mm r e s o l u t i o n was confirmed. A l s o measured was the charge g e n e r a t i o n / c o l l e c t i o n e f f i c i e n c y f o r alpha and beta p a r t i c l e s . P r e a m p l i f i e r e l e c t r o n i c s The p r e a m p l i f i e r s used f o r the prototype system are c u r r e n t s e n s i t i v e t h i c k f i l m h y b r i d s a v a i l a b l e from Laben 1 i n I t a l y (part number MSD2). These a m p l i f i e r s were designed f o r s i l i c o n s t r i p d e t e c t o r s and are high speed (2 ns r i s e t i m e ) with a low input impedance. P a r t i c u l a r l y u s e f u l f o r s t r i p d e t e c t o r s i s t h e i r high d e n s i t y (four channels per square inch) and low noise behavior (15 nA RMS e q u i v a l e n t i n p u t ) . The gain was measured at (4±.5) mV/juA in the prototype c i r c u i t . For small s i g n a l s these a m p l i f i e r s work e q u a l l y w e l l f o r p o s i t i v e or negative p u l s e s . In summary, the a m p l i f i e r s are l i n e a r and of lower noise and higher speed than the s i l i c o n d e t e c t o r . They are good for o b s e r v i n g i t s behavior, but higher g a i n would have been u s e f u l . A photograph of the 1 D i v i s i o n e Laben, 20133 Milano, t e l e x 312451 FIGURE 4. 1 Current s e n s i t i v e p r e a m p l i f i e r a r r a y . In s p i t e of the high d e n s i t y Laben h y b r i d s , two l a y e r s of c i r c u i t boards are used to read out the 20 s t r i p s from each s i d e of the d e t e c t o r . 45 F I G U R E 4 . 2 10 K J T to Cu I—I EH W a o EH U H EH W Q B I A S " W W ' V O L T A G E .01 MF in EH D a. EH D o L A B E N M S D 2 A M P L I F I E R S P r e a m p l i f i e r a n d b i a s v o l t a g e c i r c u i t . d e t e c t o r / p r e a m p l i f i e r system i s i n c l u d e d i n Figure 4.1, and a c i r c u i t diagram i s shown i n Fi g u r e 4.2. Alpha source t e s t s , d e t e c t o r asymmetry Charge c o l l e c t i o n and gener a t i o n c h a r a c t e r i c t i c s of the SS d e t e c t o r were measured with alpha p a r t i c l e s i g n a l s . A s t r i k i n g asymmetry was observed i n charge c o l l e c t i o n e f f i c i e n c y and time. F i g u r e 4.3 shows the p r e a m p l i f i e r output pulses seen for dete c t e d alpha p a r t i c l e s from a 2""Cm source on the j u n c t i o n si d e of the d e t e c t o r with s e v e r a l negative b i a s v o l t a g e s . The c h a r a c t e r i s t i c s h o r t e n i n g of pulse width arid i n c rease i n pulse height can be seen as the b i a s v o l t a g e i s i n c r e a s e d . The charge c o l l e c t e d from the d e t e c t o r i s 230 fmC estimated from the area of the v o l t a g e pulse and the gain of the p r e a m p l i f i e r . The alpha energy i s 5.8 MeV; when reduced to 4.8 MeV by l o s s e s i n 1 cm of a i r , t h i s g i v e s a value of k=.34 in Equation 2.2 (3.3 eV per e l e c t r o n hole p a i r ) . T h i s charge ge n e r a t i o n e f f i c i e n c y i s the same as the 1/3 expected. U n c e r t a i n t i e s i n these c a l c u l a t i o n s are on the order of 20%. The c o l l e c t i o n times were seen to be as f a s t as expected from Equations 2.10; the p u l s e s p i c t u r e d i n F i g u r e 4.3 have r i s e times on the order of 7 ns. The charge c o l l e c t i o n behavior of the d e t e c t o r i n t h i s c o n f i g u r a t i o n i s as expected from theory. .Crosstalk between adjacent s t r i p s and adjacent a m p l i f i e r s was observed to be l e s s than 20%. 47 FIGURE 4.3 c) BIAS=-32V d) BIAS=-39V Curium 244 alpha p u l s e s f o r d i f f e r e n t b i a s v o l t a g e s (measured a f t e r the p r e a m p l i f i e r s ) . N o t i c e that charge c o l l e c t e d (area of pulse) i s roughly constant while the pu l s e height and r i s e time vary. ALPHA SOURCE ON FRONT SIDE ALPHA SOURCE ON BACK SIDE Response of de t e c t o r to 5.8 MeV alpha p a r t i c l e s for d i f f e r e n t b i a s p o l a r i t i e s and s u r f a c e s of entrance f o r the p a r t i c l e s . Sketches of the i n t e r n a l e l e c t r i c f i e l d and the i n i t i a l i o n i z a t i o n d i s t r i b u t i o n are i n c l u d e d . The l e f t hand s i d e of the sketches represents the fr o n t of the det e c t o r (patterned side) where the p r e a m p l i f i e r s are connected. When t h e b i a s v o l t a g e i s r e v e r s e d o r t h e a l p h a s o u r c e i s p l a c e d on t h e o p p o s i t e s i d e o f t h e d e t e c t o r f r o m t h e j u n c t i o n , t h e b e h a v i o r c h a n g e s i n a m y s t e r i o u s way. F i g u r e 4.4 shows t h e p u l s e s o b s e r v e d f o r t h e f o u r p o s s i b l e c o m b i n a t i o n s o f b i a s v o l t a g e p o l a r i t y a nd s o u r c e p o s i t i o n . A l s o shown a r e t h e q u a l i t a t i v e e l e c t r i c f i e l d a n d i o n i z a t i o n d i s t r i b u t i o n s . T h e s e o b s e r v a t i o n s c a n be u s e d t o e s t i m a t e t h e d e p l e t i o n d e p t h of t h e d e t e c t o r , t h e c h a r g e c o l l e c t i o n e f f i c i e n c y a n d t h e d r i f t t i m e o f t h e c h a r g e c a r r i e r s . The c h a r g e d e p o s i t e d i n t h e d e p l e t i o n z o n e (-area o f p u l s e ) i s s e e n t o be t h e same f o r a p a r t i c l e s e n t e r i n g e i t h e r t h e f r o n t o r t h e b a c k o f t h e d e t e c t o r ( f o r a g i v e n b i a s v o l t a g e ) . As t h e r a n g e o f t h e a i s a t most 28 Mm'in s i l i c o n , t h i s i m p l i e s t h a t t h e d e t e c t o r i s t o t a l l y d e p l e t e d a t a b i a s v o l t a g e o f ±32 V. The same r e s u l t s were n o t s e e n a t a b i a s v o l t a g e o f ±25 V, i n d i c a t i n g t h a t t h e d e t e c t o r i s o n l y s l i g h t l y o v e r - d e p l e t e d a t 32 V. The d i f f e r e n c e i n p u l s e a r e a s s e e n f o r d i f f e r e n t p o l a r i t i e s o f b i a s v o l t a g e i n d i c a t e s t h a t t h e c h a r g e c o l l e c t i o n e f f i c i e n c y f o r h o l e s ( n e g a t i v e b i a s v o l t a g e on t h e f r o n t s t r i p s i d e ) i s t h r e e t i m e s b e t t e r t h a n t h a t f o r e l e c t r o n s . T h i s e f f e c t seems t o i n d i c a t e t h e p r e s e n c e o f t r a p p i n g s i t e s f o r e l e c t r o n s e i t h e r i n t h e c r y s t a l b u l k o r i n t h e s u r f a c e . The e x t r e m e w i d e n i n g o f t h e d e t e c t o r p u l s e s e e n when t h e a p a r t i c l e s e n t e r on t h e n o n - j u n c t i o n s i d e o f t h e d e t e c t o r i s another unexpected e f f e c t . The c o l l e c t i o n time p r e d i c t e d by Equations 2.10 are at most the order of 20 ns. In a d d i t i o n , the c o l l e c t i o n times are p r e d i c t e d to be l a r g e r f o r holes than e l e c t r o n s , which i s not seen i n the data. T h i s pulse time length e n i n g e f f e c t i s c o r r e l a t e d with the a b s o l u t e s t r e n g t h of the e l e c t r i c f i e l d where the a energy i s d e p o s i t e d . The d i s c r e p a n c i e s from Equations 2.10 can p o s s i b l y be e x p l a i n e d by space charge e f f e c t s on the l o c a l e l e c t r i c f i e l d due to the l a r g e charge d e n s i t i e s ^ g e n e r a t e d by a p a r t i c l e s . Beta source t e s t s 0 p a r t i c l e s from a 1 0 6 R u source were used to observe the d e t e c t o r response to small s i g n a l t r a n s m i t t e d p a r t i c l e s . 1 0 6 R u decays to 1 0 6 R h which i n turn |3 decays with a peak energy of 3.5 MeV. The peak energy l o s t by the 0 p a r t i c l e i n 230 urn of s i l i c o n i s about 250 KeV when p a r t i c l e s from the low end of the energy spectrum are almost stopped by the d e t e c t o r . Photographs of the d e t e c t o r response to 0 p a r t i c l e s are shown in F i g u r e 4.5. The p u l s e s are about 30 ns long and show no evidence of the length e n i n g observed f o r a p a r t i c l e p u l s e s . The 1/3 charge r e d u c t i o n e f f e c t i s s t i l l seen, however, when the b i a s v o l t a g e i s switched from negative to p o s i t i v e . As a r e s u l t of the charge c o l l e c t i o n asymmetry of the prototype, the e l e c t r o n i c s f o r f u r t h e r t e s t s were designed to r e c e i v e p o s i t i v e p u l s e s from a negative b i a s e d d e t e c t o r . FIGURE 4.5 20ns per d i v i s i o n 5mV per d i v i s i o n D etector p u l s e s seen ( a f t e r p r e a m p l i f i e r s ) f o r beta p a r t i c l e s from a Ruthenium 106 source. 52 FIGURE 4.6 5mV/div 2 0 n s / d i v N o i s e s e e n a t o u t p u t o f p r e a m p l i f i e r s . Observed noise of system The noise seen on an o s c i l l i s c o p e i n the output of the p r e a m p l i f i e r was about 1 mV peak to'peak (.2 M A RMS e q u i v a l e n t input to p r e a m p l i f i e r ) ( F i g u r e 4.6). What i s i n t e r e s t i n g i s that t h i s noise d i d not change with b i a s v o l t a g e on the d e t e c t o r or with exposure of the d e t e c t o r to d a y l i g h t . The noise appeared to be mostly caused by the p r e a m p l i f i e r s ( i n s p i t e of the s p e c i f i c a t i o n s ) , as i t was only s l i g h t l y reduced when the de t e c t o r was removed. T h i s low noise behavior c o u l d only be achieved when a b a t t e r y was used f o r the d e t e c t o r b i a s v o l t a g e , as power s u p p l i e s proved to be a s i g n i f i c a n t source of n o i s e . Beam t e s t design The prototype d e t e c t o r was t e s t e d in the M13 pion beamline at TRIUMF. The e f f i c i e n c y f o r d e t e c t i n g p a r t i c l e s was measured f o r d i f f e r e n t p a r t i c l e types and energies at d i f f e r e n t s e t t i n g s of d e t e c t o r b i a s v o l t a g e . Estimates were a l s o made of the noise l e v e l of the system. It was not p o s s i b l e to measure the high ra t e performance of the d e t e c t o r as the beam f l u x was too low during the experimental time. Only one wafer was t e s t e d e x t e n s i v e l y . The d e t e c t o r system was l i n k e d to the CAMAC data a c q u i s i t i o n system as shown i n F i g u r e 4.7. The s i g n a l s from the p r e a m p l i f i e r s on the 40 d e t e c t o r s t r i p s were d i s c r i m i n a t e d i n d i v i d u a l l y by three LeCroy 2735A u n i t s . The l o g i c a l ECL SOLID STATE DETECTOR ANALOGUE SIGNALS 40 CHANNELS CD a w MALU GATES, TDC START FROM SCINTILLATOR COINCIDENCE Block diagram of de t e c t o r data for the pion beam t e s t . a q u i s i t i o n e l e c t r o n i c s o u t p u t p u l s e s f r o m t h e 2735A's were r e a d i n t h e c o u n t i n g room by CAMAC m a j o r i t y l o g i c u n i t s . The 2735A a m p l i f i e r d i s c r i m i n a t o r s do n o t g i v e an a n a l o g u e s i g n a l o u t p u t so t h e s i z e o f t h e s i g n a l p u l s e s c o u l d n o t be r e c o r d e d d i r e c t l y . I n a d d i t i o n t o t h e s t r i p d e t e c t o r , two d e l a y - l i n e m u l t i - w i r e p r o p o r t i o n a l c h a m b e r s were u s e d f o r r a y t r a c i n g a n d two s c i n t i l l a t o r s were u s e d f o r a t r i g g e r ( F i g u r e 4 . 8 ) . The d e l a y l i n e c h a m b e r s had a r e s o l u t i o n o f 1 mm i n t h e v e r t i c a l o r Y d i r e c t i o n and 2 mm i n t h e X d i r e c t i o n . The s t r i p d e t e c t o r was o r i e n t e d t o be p o s i t i o n s e n s i t i v e i n t h e Y d i r e c t i o n . The M13 b e a m l i n e was s e t a t two momentum v a l u e s d u r i n g t h e t e s t . W i t h t h e c h a n n e l s e t a t 128 MeV/c ( s l i t s open t o g i v e 10% Ap/p momentum window) t h e beam i s r o u g h l y 9 5% n*, 3% M + a n d 2% e + . H e a v i e r p a r t i c l e s a r e s t o p p e d i n t h e f i r s t s c i n t i l l a t o r . The p i o n e n e r g y a t t h i s momentum i s 50 MeV. W i t h t h e c h a n n e l s e t a t 96 MeV/c (30 MeV p i o n s ) t h e f r a c t i o n o f e l e c t r o n s and muons g o e s up t o a b o u t 8% e a c h . By u s i n g t i m e o f f l i g h t i n f o r m a t i o n , i t i s p o s s i b l e t o d i s t i n g u i s h t h e t h r e e p a r t i c l e s i n e a c h e v e n t o b s e r v e d . A r a n g e o f p a r t i c l e s f r o m minimum i o n i z i n g up t o more t h a n t w i c e minimum i o n i z i n g was a v a i l a b l e f o r t h e t e s t . A p p r o x i m a t e l y 30,000 d e t e c t o r e v e n t s were r e c o r d e d f o r e a c h d e t e c t o r c o n f i g u r a t i o n . Beam t e s t , p o s i t i o n r e s o l u t i o n M e a s u r ement of t h e SS d e t e c t o r p o s i t i o n r e s o l u t i o n was p e r f o r m e d by r a y t r a c i n g w i t h t h e w i r e c h a m b e r s . T h i s was done FIGURE 4.8 S C I N T I L L A T O R 2 W I R E C H A M B E R 2Q (1 3cmx13cm) W I R E C H A M B E R (13cmx13cm) 1, S O L I D S T A T E D E T E C T O R S C I N T I L L A T O R 1 w EH z w o FIGURE 4 . 9 e E z o in O cu K O E-CJ H ca a to 40 0 39 5 39 0 38 5 3E 0 37. 5 37 0 36 9 36 0 35 35 34 34 33 33 32 32 31 31 30 30 24 13 23 20 20 19 I—I 21 C - 9. 5 : 9 0 I B 5 : 6 0 : 7 5 1 7 o : 6 s : 6 0 - . 5. 9 : 9 o : 4 9 : 4. 0 i 3. 9 1 3 o : 2 5 : 2 0 : 1 l 9 : o - I E I :iBX . . : l»K: . . . . :IBS: :I»S: :t*S :ICS. : «•«. : :Xf : :X»S : : IH, : :X««,: ::>•••: : IMi: : ft. : . : tS*.: . . . : :X««i :ICS.: : I S S I S » , : . . : l»Si: : iSS. . : . >Gi: . IS. : . . 1 I . 1 - I OO -0 50 0.00 0 .90 t o o 1 50 a. 00 3 .90 WIRE CHAMBER TRACEBACK (cm) S c a t t e r p l o t of SS d e t e c t o r p o s i t i o n ( s i n g l e or adjacent double h i t s ) versus wire chamber traceback p o s i t i o n . FIGURE 4.10 E z o co O a. a o E-< u w E-> o co CO «0 3<f 39. 3B 3B 37 37 36 36. 35 35 34 34 3 3 3 3 3 ; 3 ? 31 31 30 30 2 " re 2B 17. 27 St I > I I > X < : . : X XI . X i . X . S S . • X X S i X >SX». X . S X i S ' S i I I X X S S X • ISS. i i S X i i X i X i X X i S i i I n I S . I S I C . I M M . i.XS. WIRE CHAMBER TRACEBACK (cm) S c a t t e r p l o t of d e t e c t o r p o s i t i o n versus wire chamber traceback with only " n o i s e " i n c l u d e d . Note the excess n o i s e on s t r i p s 17 and 23. FIGURE 4.11 ' ' ' ' ' ' ' ' ' I ' ' • ' ' ' ' ' ' I ' I I I i i I i i K I I I 360. — 240. — j 120. — I WIRE CHAMBER TRACEBACK (mm) Wire chamber traceback p o s i t i o n at d e t e c t o r f o r h i t s of s t r i p 20. Note that the r e s o l u t i o n of the WC traceback o n l y 1mm at best. more to t e s t the t o t a l experimental apparatus and to measure noise than to show that 1 mm s t r i p s g i v e 1 mm r e s o l u t i o n . A s c a t t e r p l o t of wire chamber Y traceback versus d e t e c t o r p o s i t i o n i s shown i n F i g u r e 4.9. Only p a r t i c l e s that were t r a c e d back through the SS d e t e c t o r were co n s i d e r e d f o r a n a l y s i s . A s l i c e of t h i s p l o t , the c o r r e l a t i o n of s t r i p 20 and traceback p o s i t i o n , i s shown i n F i g u r e 4.11. The 1.5 mm width of the peak i n F i g u r e 4.11 i s e x a c t l y what one expects from the c o r r e l a t i o n of two 1 mm r e s o l u t i o n instruments. The events that do not l i e on the s c a t t e r p l o t d i a g o n a l (Figure 4.10) were con s i d e r e d as n o i s e . Beam t e s t , noise and e f f i c i e n c y The prototype system was found to have a low s i g n a l - t o - n o i s e r a t i o . Minimum i o n i z i n g p a r t i c l e s l o s e about 90 KeV in the d e t e c t o r . T h i s i s a s i g n a l of only .16 ULA based on a 30 ns pulse width and 3.2 eV per e l e c t r o n hole p a i r . The p r e a m p l i f i e r / d e t e c t o r RMS nois e has been measured at approximately .2 uA (Figure 4.5). P r e l i m i n a r y r e j e c t i o n of n o i s e i n the s t r i p d e t e c t o r was accomplished by r e j e c t i n g a l l d e t e c t o r events with more than one s t r i p h i t unless i t was a p a i r of adjacent s t r i p s that f i r e d . A t y p i c a l beam p r o f i l e as measured by the SS d e t e c t o r i s shown in F i g u r e 4.12. Half i n t e g e r bins represent events where adjacent s t r i p s f i r e d . Real double events were r u l e d out as the beam f l u x was too low (1 KHz); only at f l u x e s over 1 MHz i s p i l e u p a n o n - n e g l i g i b l e problem. T h i s m u l t i p l e h i t FIGURE 4.12 1 80Q\ 1 2 0 0 . 6 0 Q . H Beam p r o f i l e as seen with the s o l i d s t a t e d e t e c t o r . Half i n t e g e r bins represent adjacent double s t r i p h i t s . noise r e j e c t i o n system w i l l i d e n t i f y a percentage of noise s i g n a l s equal to the p r o b a b i l i t y of a r e a l p a r t i c l e being d e t e c t e d . Only when no p a r t i c l e passes through the d e t e c t o r or when a r e a l p a r t i c l e pulse i s not d i s c r i m i n a t e d can a noise s i g n a l be i n t e r p r e t e d as a r e a l event. The l a r g e s t e f f e c t of the n o i s y system was to reduce the peak e f f i c i e n c y of the d e t e c t o r at low d i s c r i m i n a t o r t h r e s h o l d s . The number of s t r i p s f i r e d per p a r t i c l e i s shown in F i g u r e 4.13; events in bin zero represent p a r t i c l e s that were t r a c e d back through the d e t e c t o r but were not d i s c r i m i n a t e d . F i g u r e 4.14 shows the d e t e c t o r e f f i c i e n c y and noise versus d i s c r i m i n a t o r t h r e s h o l d with no noise r e j e c t i o n and with the r e j e c t i o n method d e s c r i b e d above. E f f i c i e n c y i s d e f i n e d as the number of p a r t i c l e s detected by the SS d e t e c t o r d i v i d e d by the number t r a c e d back through i t . The n o i s e i s d e f i n e d as d e t e c t o r events that do not c o i n c i d e with the wire chamber traceback ( F i g u r e 4.10). S t r i p s 17 and 23 show an ex c e s s i v e amount of n o i s e . The r e s i d u a l noise seen a f t e r m u l t i p l e h i t noise r e j e c t i o n i s mostly due to pion decay to muons in the t e s t apparatus. The it* l i f e t i m e , r, i s 2.6 X 10~ 8 s. The f r a c t i o n , f, of pions that decay i n a short d i s t a n c e x i s f =*X//3C7T Equation 4.1 where | 3 , 7 , are the r e l a t i v i s t i c parameters a s s o c i a t e d with the pion in the l a b frame. For 50 MeV pions, 4.2% w i l l decay i n the 30 cms between the s t r i p d e t e c t o r and the f i n a l wire FIGURE 4.13 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 2 4 0 0 0 . — 2 4 0 0 0 . — 1 6 0 0 0 . 1 6 0 0 0 . — 8 0 0 0 . — L 8 0 0 0 . — 0. ~Tjn 1 1 1 1 1 1 1 5. 0 1 0. 0 MULTIPLICITY THRESHOLD=0.75V - TI 1 1 1 1 1 1 1 T T T " 5. 0 10. MULTIPLICITY THRESHOLD=1.0V ' ' ' ' ' • ' ' ' ' ' • ' • ' ' ' i i 2 4 0 0 0 . — I 6 0 Q O . — 8 0 0 0 . 2 4 0 0 0 . — - 1 6 0 0 0 . — 8 0 0 0 . — I I I | I I I l I I I I l | - 5 . 0 10. D 0 . I i ' ' • } 1 1 I 1 i i i i . TI I I l I I I I I I I I I I 5 . 0 1 0 . 0 MULTIPLICITY THRESHOLD^1. 5V MULTIPLICITY THRESHOLD=2.0V M u l t i p l i c i t y (number of s t r i p s f i r e d per event) f o r d i f f e r e n t d i s c r i m i n a t o r t h r e s h o l d s . Only s i n g l e s and some doubles are kept as good events. FIGURE 4.14 128NeV/c PARTICLES ( 9 3 % PIONS) 1 1 i i i 0.0 0.5 1. 0 1. 5 2.0 2.5 DISCRIMINATOR THRESHOLD ( V ) ' Dete c t o r e f f i c i e n c y and noise f o r a -32V b i a s v o l t a g e . Net data i n c l u d e s only s i n g l e or adjacent double SS d e t e c t o r h i t s . FIGURE 4.15 D i s t r i b u t i o n of l a b angle of muons from decay of 50MeV pi o n s . FIGURE 4.16 3600 . — I 2 O 0 . — ANGLE (mRADIANS) (ALL EVENTS) 1600. — 800. — I i I i i i i i i i i i I I I n 1111111 n 111111111 IT=T 0 40 8b 120 ANGLE (mRADIANS) (DETECTOR HITS) n 1111111 I I ] 1111 160 2D0 6 0 . — 0 4b Bb 120 160 200 ANGLE (mRADIANS) (NOISE HITS) Angle of p a r t i c l e ray through apparatus (beam c e n t r e l i n e d e f i n e d as z e r o ) . N o t i c e c o r r e l a t i o n between nois e s i g n a l s and a n g l e ; t h i s c o r r e l a t i o n i s caused by pion to muon decay i n the aparatus. chamber, the d i s t r i b u t i o n of the r e s u l t i n g muon angle i s shown in F i g u r e 4.15 (REF 11). The product muons are d e t e c t e d in the wire chambers but due to the non-zero angle to the o r i g i n a l pion path, an erroneous traceback i s c a l c u l a t e d i n most cases. A 3-4% traceback " n o i s e " i s expected in the p i o n t e s t data due to ir t o (i decay in the t e s t apparatus. F i g u r e 4.16 shows the c o r r e l a t i o n between angle of traceback path and n o i s e . T h i s c o r r e l a t i o n supports the TI to u n o i s e h y p o t h e s i s . Beam t e s t , b i a s v o l t a g e and e f f i c i e n c y The e f f i c i e n c y of the d e t e c t o r was measured at three d i f f e r e n t b i a s v o l t a g e s . A summary of the r e s u l t s i s represented g r a p h i c a l l y i n F i g u r e s 4.17. U n f o r t u n a t e l y , only negative b i a s e s c o u l d be used due to the f i x e d input p o l a r i t y of the 2735A d i s c r i m i n a t o r s . As was expected, the e f f i c i e n c y f o r d e t e c t i o n improves with i n c r e a s e d p a r t i c l e energy l o s s . The e f f i c i e n c y a l s o i n c r e a s e s with i n c r e a s e d b i a s v o l t a g e . In the range t e s t e d , the d e t e c t o r i s of l i t t l e use f o r d e t e c t i n g minimum i o n i z i n g e l e c t r o n s ( l e s s than 50% e f f i c i e n t , l a r g e amount of noise) and only of marginal use f o r the 50 MeV pions i t was designed f o r (70% e f f i c i e n t , low net n o i s e ) . A f u r t h e r i n c r e a s e in b i a s v o l t a g e would not be l i k e l y to improve the performance of the d e t e c t o r as t h i s would i n c r e a s e leakage c u r r e n t . Excess noise was d e t e c t e d on s t r i p s 17 and 23, and both of these s t r i p s were seen to have leakage c u r r e n t s s e v e r a l times the average ( = 10-20 /xA/cm2). FIGURE 4.17a 100 -- LU 9 0 " CO o 8 0 " 70 - O 60 - LU M 50 - (_) U_ 40 - Ll_ W 3 0 - I— W 20 - « 10 - 0 4 96MeV/c PIONS: DELTR E 190KeV I I I L 25V B i n s 32V B i n s 39V B i n s • EFFICIENCY D NOISE 1 1 1 T— 0. 0 0 . 5 1.0 1. 5 2 .0 2 .5 DISCRIMINATOR THRESHOLD ( V) 100 UJ so H co o 8 ( H 70 - O 60 H LU I—I 50 H o £ 40 Ll_ LU LU 30 - 20 - 10 - 0 0. 0 128MeV/c PIONS: DELTA E 150KeV 25V B i n s 32V B i n s 39V B i n s • EFFICIENCY a NOISE — r ~ 1.0 —r~ 1.5 0 . 5  0 1.5 2.0 2 .5 DISCRIMINATOR THRESHOLD (V) Summary of SS de t e c t o r e f f i c i e n c i e s and noise f o r v a r i o u s d e p o s i t e d e n e r g i e s , b i a s v o l t a g e and d i s c r i m i n a t o r t h r e s h o l d s , Only s i n g l e or adjacent double h i t s are i n c l u d e d . See a lso f igure 4.17b. 69 FIGURE 4 . 17b 100 L U 9° C O 70 (_) 60 - LU 50 40 30 - 20 -2 * * 10 - 128MeV/c MUONS: i i DELTA E -1 1 OKeV 25V BIBS 32V BIBS 39V BIBS • EFFICIENCY • NOISE 2. 5 1 1 1 r~ 0.0 0. 5 1.0 1.5 2.0 DISCRIMINATOR THRESHOLD ( V) 100 UJ 90 H CO 70 - >-* C J 60 H L U H 50 H C J 40 - 30 - 20 - 10 0 L U 128MeV/c ELECTRONS: 1 i i DELTA E 86KeV 2SV BIBS 32V BIBS 39V BIBS • EFFICIENCY • NOISE • 1 1 1 1— 0.0 0 .5 1. 0 1.5 2.0 DISCRIMINATOR THRESHOLD ( V) 2. 5 SS d e t e c t o r e f f i c i e n c i e s and noi s e . Timing of SS de t e c t o r s i g n a l s Timing information f o r some s t r i p s was recorded. D i s c r i m i n a t e d s i g n a l s from the s t r i p d e t e c t o r were made a v a i l a b l e by using an ECL/NIM/ECL converter (Figure 4.7). These s i g n a l s were recorded i n a TDC (time to d i g i t a l c o n v e r t e r ) s t a r t e d by the two s c i n t i l l a t o r s . A time spectrum for one s t r i p i s shown in F i g u r e 4.18. Even with the p r i m i t i v e time over t h r e s h o l d d i s c r i m i n a t i o n , the width of the time peak (time j i t t e r ) was only 10 ns. The width of the data a c q u i s i t i o n gate to the m a j o r i t y l o g i c u n i t c o u l d then be set at 40 ns, thus reducing random n o i s e . The time spectrum a l s o showed evidence of RF pickup by the d e t e c t o r . . At low d i s c r i m i n a t o r s e t t i n g s , noise peaks sepa r a t e d by 43 ns ( c h a r a c t e r i s t i c of the c y c l o t r o n ) c o u l d be seen i n the time spectrum. FIGURE 4.18 ' 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 t 240. H 160. H 80. r r v v v v T r r r r r r r V V V V T ) •i i v v v •[' v \ i' v v v v v y v v v v v v v v v y v i 320 400 80 160 240 TIME (250ps/bin) DISCRIMINATOR THRESHOLD*0.75V ' • ' • 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 180. H 1 20. —\ 60. H I I I 1 I l"l T I I I I I n I I I I I i^V*rf 1 T I ITI ITI I I I I T I T T T I I I I T T I 160 240 320 400 TIME (250ps/bin) DISCRIMINATOR THRESHOLDS.25V Time spectrum of s i g n a l s from one s t r i p . At low t h r e s h o l d noise i s c l e a r l y v i s i b l e with a component having a 43ns p e r i o d . CHAPTER V SUMMARY AND CONCLUSION A s u r f a c e b a r r i e r s o l i d s t a t e d e t e c t o r system was developed at TRIUMF s p e c i f i c a l l y f o r p o s i t i o n r e s o l u t i o n of 50 MeV pions at high f l u x e s . The e f f i c i e n c y of the prototype d e t e c t o r system was measured at 70% f o r these p a r t i c l e s at low r a t e s . The ra t e c a p a b i l i t y of the device i s expected to be i n excess of 1 MHz per s t r i p as the c h a r a c t e r i s t i c s i g n a l l e n g t h was 30 ns. The 1 mm r e s o l u t i o n and 40 mm a c t i v e diameter of the device are adequate f o r beam p r o f i l e monitor, a c t i v e momentum s l i t , and t a r g e t p o s i t i o n monitor a p p l i c a t i o n s i n the Ml 3 beamline. The device would be more d e s i r e a b l e i f p o s t i o n r e s o l u t i o n c o u l d be achieved i n both d i r e c t i o n s . The m u l t i p l e s c a t t e r i n g and energy s t r a g g l e a s s o c i a t e d with the 55 mg/cm2 d e t e c t o r i s acceptable f o r the a p p l i c a t i o n s proposed although t h i n n e r d e t e c t o r s would a l s o be d e s i r e a b l e . Noise, improvements The low s i g n a l to noise r a t i o and r e s u l t i n g low e f f i c i e n c y of the prototype d e t e c t o r i s due to one or more of three sources. There i s the i n t r i n s i c n o i s e from the d e t e c t o r c r y s t a l , RF ele c t r o - m a g n e t i c pickup, and e l e c t r o n i c s n o i s e . E x c e s s i v e leakage c u r r e n t noise was observed on two s t r i p s of the prototype d e t e c t o r . To remedy t h i s problem, more work must be done i n the s o l i d s t a t e l a b o r a t o r y . A l l that may be necessary i s the manufacture of s e v e r a l more d e t e c t o r wafers to smooth out problems with c l e a n l i n e s s . As with a l l f a b r i c a t i o n processes, the q u a l i t y of the product w i l l vary from run to run and the best d e v i c e s must be chosen f o r use as d e t e c t o r s . The s t r i p s of the SS d e t e c t o r act as an a e r i a l f o r ambient RF power. The pickup observed for the unshielded d e t e c t o r was small as the e l e c t r o n i c s were c a r e f u l l y s h i e l d e d and grounded. To f u r t h e r reduce t h i s problem, the d e t e c t o r / p r e a m p l i f i e r system should operate in a Faraday cage with f o i l windows, or i n s i d e the beam pipe. E l e c t r o n i c s noise appeared to be the biggest problem in the prototype system. On most of the d e t e c t o r s t r i p s , a p p l i c a t i o n of bias v o l t a g e d i d not increase the n o i s e seen on the output of the p r e a m p l i f i e r s . T h i s noise was seen to be the same s i z e as pulse expected from minimum i o n i z i n g p a r t i c l e s . The LeCroy a m p l i f i e r d i s c r i m i n a t o r s were operated very c l o s e to the minimum t h r e s h o l d p o s s i b l e (at which p o i n t o s c i l l a t i o n o c c u r s ) , and they may a l s o have c o n t r i b u t e d a p o r t i o n of the noise s i g n a l s . Higher gain low n o i s e combination p r e a m p l i f i e r and d i s c r i m i n a t o r h y b r i d s are being considered f o r the next g e n e r a t i o n of e l e c t r o n i c s . These de v i c e s , a v a i l a b l e from Newmarket Microsystems in England, a l s o have an analogue output u s e f u l f o r measuring the energy l o s s of p a r t i c l e s i n the d e t e c t o r . Future c o n s i d e r a t i o n s In the immediate fu t u r e at TRIUMF, development w i l l continue on d e v i c e s s i m i l a r to the non-integrated prototype d e t e c t o r . Two dimensional r e s o l u t i o n d e v i c e s are p o s s i b l e , but problems stemming from the asymmetry observed in the prototype device may have to be overcome. Thinner d e t e c t o r s can a l s o be made i f the s i g n a l to noise r a t i o i s i n c r e a s e d . The next step i n the development of p o s i t i o n s e n s i t i v e s o l i d s t a t e d e t e c t o r s w i l l be i n t e g r a t i o n of the f i r s t stage of a m p l i f i c a t i o n on the same wafer as the d e t e c t o r . T h i s w i l l probably be accomplished with GaAs technology. The maximum s i z e of s o l i d s t a t e d e t e c t o r s w i l l soon expand to the 8 inch diameter wafers now a v a i l a b l e . 75 REFERENCES 1 E . E . H a l l e r , H.W,Kramer, W.A.Higinbothom, Mat.Res.Soc. V o l 16, "Nuclear R a d i a t i o n Detector M a t e r i a l s " , H o l l a n d Press (a) J.H.Howes, J.Watling, P207 (b) A.Musa, J.P.Ponpon, M.Hage-Ali, P225 2 E.M.Lawson, Nuc.Inst.Meth. 180(1981)651 3 J.B.A.England, "Techniques in Nuclear S t r u t u r e P h y s i c s " , Part 1, Macmillan Press 4 A.A.Konova, Nuc.Inst.Meth. 160(1979)115 5 J.B.A.England et a l , Nuc.Inst.Meth. 196(1982)149 6 J.Kemmer et a l , Nuc.Inst.Meth. 205(1983)99 7 W.J.Price, "Nuclear R a d i a t i o n D e t e c t i o n " , second e d i t i o n , McGraw H i l l 8 S.M.Sze, "Physics of Semiconductor Devices", John Wiley and Sons 9 P.Borgeaud et a l , "The E f f e c t of R a d i a t i o n on the Energy R e s o l u t i o n of Ion Implanted S i l i c o n D e t e c t o r s " , Commissariat a l ' E n e r g i e Atomique Centre d'Etudes N u c l e a i r e s de Sac l a y , France 10 J.B.A.England, V.W.Hammer, Nuc ..I nst .Meth. 96(1971)81 11 R. Ta c i k , Phd T h e s i s , UBC 1984 12 J.B.A.England, P r i v a t e communication, Dec 1984 13 J.Yah-Min Lee, IEEE T r a n s a c t i o n s on E l e c t r o n Devices, Vol.ED-28,No.4, A p r i l 1981 14 P.Janega, P r i v a t e communication, May 1983 APPENDIX A MODIFIED ENGLAND PROCESS FOR SURFACE BARRIER DETECTORS S t a r t with high r e s i s t i v i t y n" s i l i c o n wafers: p o l i s h e d both s i d e s , - 5000 ficm, c a r r i e r l i f e t i m e ^3 ms. Clean the s l i c e i n s o l v e n t (2-Propanol, Acetone) and then in d i s t i l l e d d e i o n i z e d water with u l t r a s o u n d . ( A l l wet chemicals and u t e n s i l s must be very c l e a n , a l l water should be d i s t i l l e d d e i o n i z e d ) . B o i l the s l i c e g e n t l y in con c e n t r a t e d n i t r i c a c i d f o r 5 minutes. D i l u t e and decant with d i s t i l l e d d e i o n i z e d water. B o i l i n water f o r 5 minutes, d i l u t e and decant with c o l d water. Etch the wafer, i n room temperature CP4A (40% HF: co n c e n t r a t e d a c e t i c a c i d : c o n c e n t r a t e d n i t r i c a c i d / 3 : 3 : 5 ) . The wafer must be c o n s t a n t l y a g i t a t e d and tends to form a " r i p p l e d " s u r f a c e anyway. CP4A etches at about 20 jim/min (per s i d e ) . At l e a s t 10 Mm should be removed. Quench the wafer i n H 20. D i l u t e and decant with water (50% d i l u t i o n , 5 t i m e s ) . O x i d i z e the wafer i n 1% potassium dicromate s o l u t i o n . P lace i t i n the c o l d s o l u t i o n and heat to 80 C, wait 10 minutes, d i l u t e and decant. At t h i s p o i n t , i f contamination can be observed as s t r e a k s under an i n f r a - r e d lamp, one may want to s t a r t over. Dry the s l i c e i n N 2 and p l a c e i n evaporator. o Evaporate 560-750 A of Germanium (p doped semiconductor grade Ge was used with a s t a r t i n g pressure of 10~ 6 Torr) onto both s i d e s of the s i l i c o n , the diode i s now formed and p a s s i v a t e d by the Ge. o o 9. Evaporate 210 A Au or 1100 A A l onto both s u r f a c e s . S t r i p p a t t e r n s are made on one s u r f a c e with a wire shadow mask in c o n t a c t with the wafer ( s t a r t at 10~ 6 Torr p r e s s u r e ) . 10. Glue the ground plane to the c i r c u i t board mounting s u r f a c e with conducting epoxy (Epotek H44). Make an epo.xy "ramp" with Transene epoxy 50 (or CIBA AY105:HY956, 6:1 by weight) and evaporate c o n t a c t s to the c i r c u i t board through a shadow mask (Fi g u r e 3.3). The t r a c e s on the c i r c u i t board should be g o l d . Often the evaporated contact t r a c e s must be probed with a p e n c i l p o i n t to make a good e l e c t r i c a l contact i f aluminum i s used f o r the metal e v a p o r a t i o n s . The aluminum s t r i p s form an i n s u l a t i n g oxide l a y e r very q u i c k l y in a i r .

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