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Self-aligned gallium arsenide MESFETs for microwave integrated circuits Sutherland, David B. 1988

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SELF-ALIGNED  GALLIUM ARSENIDE INTEGRATED  MESFETs  FOR MICROWAVE  CIRCUITS  by DAVID B . A . S c ,  A  THESIS THE  B.  SUTHERLAND  Queen's  SUBMITTED  IN  REQUIREMENTS MASTER  University,  PARTIAL  1986  FULFILMENT  FOR THE DEGREE  OF APPLIED  OF  SCIENCE  in THE  FACULTY  Department  We  accept to  THE  OF GRADUATE  of  E l e c t r i c a l  t h i s the  thesis  required  UNIVERSITY  David  B.  Engineering  conforming  standard  OF BRITISH  July ©  as  STUDIES  COLUMBIA  1988  Sutherland,  1988  OF  In presenting  this thesis in partial fulfilment  of the  requirements for an advanced  degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department  or  by  his  or  her  representatives.  It  is  understood  that  copying  or  publication of this thesis for financial gain shall not be allowed without my written permission.  Department The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3  DE-6G/81)  ABSTRACT  A refractory gallium  a r s e n i d e MESFETs h a s  s a m p l e and  hold c i r c u i t .  the p a r a s i t i c factor t o be  self-aligned  end  gate  been developed  The  p r o c e s s has  i n t h e i r microwave performance. compatible  18GHz on  with  FETs  microwave probes Ottawa O n t a r i o . s a m p l e and was  The producing  o f TiW  and at  anneal  the  The  an u n d e r c u t  gate  metal  was  c o n t a c t s t o GaAs.  end  I t was  Centre  in  used  found  and  the gate  an e v a p o r a t e d  metal  a  to  to  test  sampling  rate  plasma  etch  airbridges  Metal-Insulator-Metal  yielded  t h e d i e l e c t r i c were  a  single  rapid  thermal suitable  MESFETs  with by  technique  the was  MESFETs  procedures evaporation  capacitors using s i l i c o n  developed.  alloy  fabricated A  ion  suitable  r e s i s t a n c e of s e l f - a l i g n e d  using  (MIM)  provide  maintained  Also,  for  self-aligned  that a  implant process.  overlayer.  up  MHz.  r e s i s t a n c e when compared t o t h o s e  t o reduce  fabricating  Research  implant  characteristics  more c o n v e n t i o n a l s e l e c t i v e developed  the  structure for  developed  chip  with  included  f o l l o w i n g the s e l f - a l i g n e d  reduced  on  amplifiers  tested  equipment t o 2 5 0  ' T'  designed  measured  were a l s o  maximum  process  limiting  g a i n was  Communications  The  a  reduce  which allowed  buffer  for to  A method o f s p u t t e r i n g a t h e r m a l l y s t a b l e  TiW/GaAs S c h o t t k y  using  on  a  A mask s e t was  Usable  microwave probes  the t e s t  refractory  Schottky  5GHz  fabrication  implantation.  applied  b e e n shown t o  Cascade Inc. probes  hold operation.  l i m i t e d by  and  process  r e s i s t a n c e o f MESFETs w h i c h c a n be  m i c r o w a v e m e a s u r e m e n t s t o be made. to  fabrication  nitride  for and as  The  effect  of  gate  resistance  performance of the s e l f - a l i g n e d modeling  with  the  EEsof  MESFETs  Inc.  Touchstone.  The m o d e l i n g showed  capable  giving  of  greater  devices  implant  operation  o f t h e s a m p l e and h o l d  version  o f SPICE t h a t  model.  The s i m u l a t i o n s  be u s e d  f o r gigahertz  that  with  the  was  microwave  high  selective  on  investigated software  frequency  gain  circuit  showed  was  Sussman  iii  are  than  are  geometry.  The  simulated Fort  t h a t t h e s a m p l e and  sampling.  by  package,  s e l f - a l i g n e d MESFETs  t h e same d e s i g n  included the  microwave  using  GaAs hold  a  MESFET could  TABLE OF CONTENTS  Page  ABSTRACT  i i  L I S T OF FIGURES  v i i  ACKNOWLEDGMENT  x  1.  1  2.  INTRODUCTION 1.1  GaAs MESFET T e c h n o l o g i e s  1  1.2  Microwave Performance  6  1.3  T h e Sample a n d H o l d  8  1.4  Sample a n d H o l d  Performance  8  1.5  Sample a n d H o l d  Structures  9  SAMPLE AND HOLD DESIGN AND LAYOUT 2.1  The Sampling  2.2  The Hold C a p a c i t o r  14  2.3  Buffer Amplifiers  16  2.4 3.  Gate  11 12  Layout  17  FABRICATION 3.1  Special  19 Structures  19  3.1.1  Fine Line Resist  Profiles  3.1.2  Self-aligned  3.1.3  Post-anneal  3.1.4  Airbridges  26  3.1.5  MIM C a p a c i t o r s  29  'T' G a t e S t r u c t u r e Gate T h i c k e n i n g  iv  19 21 24  Page  3.2  3.3  The R e f r a c t o r y Metal/GaAs  Contact  31  3.2.1  Effect  o f a Furnace Anneal  33  3.2.2  Effect  o f Rapid Thermal Anneal  34  3.2.3  Effect  o f Change o f TiW C o m p o s i t i o n  35  The Microwave  Integrated  Circuit  Fabrication 36  Process  4.  41  MEASUREMENT AND TESTING 4.1  4.2  S t a t i c Measurements  41  4.1.1  T h e TiW/GaAs S c h o t t k y J u n c t i o n  41  4.1.2  Sheet R e s i s t a n c e o f Implanted Layers  43  4.1.3  Ohmic C o n t a c t R e s i s t a n c e  45  4.1.4  Source S e r i e s R e s i s t a n c e  45  4.1.5  S h e e t R e s i s t a n c e o f TiW F i l m s  48  4.1.6  Isolation  49  4.1.7  MESFET C h a r a c t e r i s t i c  4.1.8  Threshold Voltage  51  4.1.9  Low F r e q u e n c y T r a n s c o n d u c t a n c e  53  Microwave  Curves  55  Measurements  4.2.1  S-Parameters  o f MESFETs  4.2.2  S-Parameters  of the Buffer  55 Amplifier 58  Stage 4.3  Testing  50  o f t h e Sample a n d H o l d C i r c u i t s  v  60  Page 5.  5.1  5.2  6.  63  ANALYSIS AND MODELING M o d e l i n g o f MESFETs  63  5.1.1  Scattering  65  5.1.2  Maximum G a i n o f MESFETs  parameters  S i m u l a t i o n o f Sample a n d H o l d O p e r a t i o n  66 69  71  CONCLUSION  REFERENCES  72  APPENDIX A - F a b r i c a t i o n P r o c e d u r e  78  APPENDIX B - I n p u t L i s t i n g s f o r M o d e l i n g APPENDIX C - M i c r o w a v e G a i n D e f i n i t i o n s  vi  84 91  List  Figure  of  Figures  Description  Page  1.1.  Recessed gate f a b r i c a t i o n process  3  1.2.  Refractory process  4  1.3.  S e l e c t i v e implant  1.4.  A high frequency equivalent GaAs MESFET in the configuration  s e l f - a l i g n e d gate  fabrication  5  f a b r i c a t i o n process circuit for a common-source  6  1.5.  Basic  1.6.  MESFET s a m p l i n g s w i t c h  10  2.1.  Sample and  11  2.2.  Triple  2.3.  Interdigitated capactitor layout  2.4.  Cross s e c t i o n of (MIM)  s t r u c t u r e o f s a m p l e and  hold  schematic diagram  g a t e s a m p l i n g FET  Metal  13  cross-section  15  Insulator  Metal 16  capacitor  2.5.  Sample and  2.6.  Mask s e t o v e r a l l l a y o u t  3.1.  SEM s h o w i n g r e s i s t p r o f i l e w i t h 2/-/m g a p s Production of 'T' gate s e l f - a l i g n e d implant  3.2.  8  hold  hold  17  with buffer amplifiers  18 of  1  lines 20  structure  3.3.  SEM showing 'T' gate structure self-aligned ion implantation  3.4.  Procedure o v e r l a y e r on  3.5.  TiW  3.6.  Airbridge technique  3.7.  Airbridge evaporation  for depositing a s e l f - a l i g n e d gates  for for metal  fabrication and l i f t o f f vii  23 25 26  gate with A l overlayer f a b r i c a t i o n by  22  a  conventional by  single  27 28  3.8.  SEM o f a i r b r i d g e  29  3.9.  SEM o f MIM  30  3.10.  Doping p r o f i l e tail obtained from the mercury contact probe on a sample a n n e a l e d a t 8 0 0 ° C f o r 25 m i n u t e s w i t h TiW encapsulant  34  Doping p r o f i l e o b t a i n e d from t h e mercury c o n t a c t p r o b e on a s a m p l e g i v e n a n RTA a t 8 5 0 ° C f o r 60s w i t h TiW e n c a p s u l a n t  35  Doping p r o f i l e o b t a i n e d from t h e mercury c o n t a c t p r o b e on a s a m p l e s p u t t e r e d with W f o i l on t h e TiW t a r g e t t h e n given an RTA a t 9 5 0 ° C f o r 5s  36  3.13.  The m i c r o w a v e i n t e g r a t e d c i r c u i t e m p l o y i n g s e l f - a l i g n e d MESFETs  37  3.14.  A s e l f - a l i g n e d microwave FET  40  3.15.  A completed  40  4.1.  (kT/q) l n ( I ) v s . V for a TiW/GaAs S c h o t t k y j u n c t i o n  4.2.  I vs. V f o r TiW (solid) and Ti/Pd/Au (dashed) S c h o t t k y j u n c t i o n s t o GaAs  43  4.3.  Pad t o pad r e s i s t a n c e v s . pad for ohmic contacts to n transmission line  separation implanted  44  4.4.  Current d i s t r i b u t i o n s measurement  resistance  4.5.  Rend = d V / d l vs. 1/I for selective i m p l a n t a n d s e l f - a l i g n e d MESFETs  47  4.6.  M e a s u r e d r e s i s t a n c e o f TiW a n d o f A l o v e r l a y e r v s . number o f s q u a r e s  49  4.7.  I-V c h a r a c t e r i s t i c s for a d e p l e t i o n mode MESFET with l e n g t h a n d 200^m g a t e w i d t h  self-aligned 0.8 ^m gate  I-V c h a r a c t e r i s t i c s for a enhancement mode MESFET w i t h l e n g t h a n d 200^m g a t e w i d t h  self-aligned 0.8 gate  3.11.  3.12.  capacitor  sample and h o l d a m p l i f i e r self-aligned  +  4.8.  4.9.  d s  process  g  f o r end  46  d  TiW  g s  viii  with  50  -/l vs. V f o r a) a d e p l e t i o n MESFET b) a n enhancement MESFET d  42  and  51 52  4.10.  Low f r e q u e n c y t r a n s c o n d u c t a n c e ( g ) vs. gate bias (V ) for a 100 /JUI w i d e s e l f - a l i g n e d MESFET  54  Mean w i t h s t a n d a r d d e v i a t i o n l i m i t s o f |S2i| in dB for self-aligned FETs (solid curves) and selective implant FETs (dashed c u r v e s )  56  S m i t h c h a r t p l o t s o f mean with standard d e v i a t i o n l i m i t s o f a) Su and b) S22 from 2 t o 18GHz for self-aligned FETs ( s o l i d c u r v e s ) and s e l e c t i v e i m p l a n t F E T s (dashed curves)  57  |S | i n dB f o r b u f f e r amplifier circuit implemented w i t h s e l f - a l i g n e d FETs  .58  m  g s  4.11.  4.12.  4.13. 4.14.  4.15. 4.16.  5.1. 5.2. 5.3.  5.4.  5.5.  5.6.  C.l.  21  Smith c h a r t p l o t s o f a) Su f r o m 0.1 t o 5GHz for buffer circuit Sample and hold c o n f i g u r a t i o n used  and b) S amplifier  2 2  59  layout and for testing  probing 60  Oscilloscope traces of input and output s i g n a l s f r o m a s a m p l e and h o l d w i t h g u a r d g a t e s b i a s e d a t -0.8V  62  Lumped e l e m e n t  64  model o f MESFET  | S i | v s . f r e q u e n c y from model measured d a t a ( p o i n t s ) 2  S and S from (solid lines) (cross-hatch) X 1  2 2  2 to and  (lines)  18GHz from measured  and 65  model data 66  Maximum gain of self-aligned MESFETs from model (lines) and measured data f r o m s e l f - a l i g n e d and s e l e c t i v e implant devices (points)  67  S i m u l a t e d i n p u t and o u t p u t w a v e f o r m s a sample and hold under the c o n d i t i o n s o f s e c t i o n 4.2  68  S i m u l a t e d i n p u t and o u t p u t w a v e f o r m s a s a m p l e and h o l d f o r 2.7GHz s a m p l i n g a 360MHz s i g n a l Power g a i n b l o c k d i a g r a m  ix  for test for of 70 .....91  Acknowledgement  The work on t h e s a m p l e a n d h o l d the Defence  D r . L. Young,  f o r s u g g e s t i n g many o f t h e i d e a s o f t h i s Mr. H i r o s h i  on  was s u p p o r t e d by  R e s e a r c h E s t a b l i s h m e n t , Ottawa.  I t h a n k my s u p e r v i s o r , and  circuit  fabricating  Kato  i s t o be t h a n k e d  for his  guidance  work.  forhis  collaboration  devices.  I a l s o thank  David Hui  f o r writing  t h a t were u s e d t o p r e p a r e some o f t h e f i g u r e s  x  software in  this  routines thesis.  1.  The  INTRODUCTION  o b j e c t o f t h e work d e s c r i b e d  examine t h e a p p l i c a t i o n implantation technology  of  a  gallium  i n this  arsenide  t o microwave d e v i c e s  t o a GaAs s a m p l e a n d h o l d  circuit.  u s i n g a r e f r a c t o r y metal gate  c h a r a c t e r i s t i c s were m e a s u r e d a n d compared from d e v i c e s implant  fabricated  process.  by  the  GaAs MESFET  The  m a j o r i t y o f work on  mode MESFETs f o r d i g i t a l  FET  GaAs  (SDFL),  those  obtained  conventional the  ultimate  the technology.  self-aligned  fabrication  logic  circuits.  implemented  only  The f i r s t  (BFL) a n d  GaAs  digital  Schottky  depletion  mode  T h e s e a p p r o a c h e s were s u c c e s s f u l i n a c h i e v i n g h i g h  Diode  devices.  speeds  [ 1 , 2 ] however, h i g h power d i s s i p a t i o n p r e v e n t e d  GaAs  selective  t o t h e d e v e l o p m e n t o f enhancement  a p p r o a c h e s , B u f f e r e d FET L o g i c  Logic  microwave  Technologies  t e c h n o l o g i e s has been r e l a t e d  logic  with  fabricated  and  M o d e l i n g was u s e d t o d e t e r m i n e  p e r f o r m a n c e t h a t c a n be a c h i e v e d  1.1  more  to  to  particular  were  Static  was  self-aligned  and i n  Devices  process.  thesis  using  their  use  approach o f D i r e c t Coupled  FET  i n VLSI a p p l i c a t i o n s . The Logic  l o w power GaAs l o g i c  (DCFL)  using  Enhancement-Depletion  FET  Logic  r e q u i r e s t h e development o f a c o n s i s t e n t f a b r i c a t i o n f o r enhancement mode d e v i c e s require very 0.1  -  tight  [3].  Enhancement  0 . 2 V ) i n order  t o allow  f o r adequate 1  technique  mode  control of the threshold voltage voltage  (EDFL)  devices  (typically swing  and  noise  [4],  margins  unmodulated channel  Also  i t  is  necessary  to  make  o f a n enhancement mode d e v i c e s h o r t b e c a u s e  it  i s d e p l e t e d by s u r f a c e s t a t e p i n n i n g o f t h e Fermi  is  therefore highly resistive The  most  important  unmodulated channel self-aligned process  regions n used.  type  methods  implant  o r a w a f e r w i t h an n i s  opened  over  p h o t o r e s i s t p a t t e r n t h a t i s used MESFETs.  used  to  The channel  is  In the i s used  type  to define so  that  is  must  be  to  dope  the  gate active  layer  regions  gates  b a s i c types  of self-aligned  GaAs MESFETs  have been demonstrated.  Implantation  f o r N^-layer Technology,"  uses a temporary implant. a second  The gate gate  gate  structure  the  with  the  Schottky  process  the  or  SAINT  [6].  "Self-Aligned process  the  the Schottky  other process uses a r e f r a c t o r y metal the h i g h temperature The r e f r a c t o r y  post-implant self-aligned  involves undercutting a lateral  One,  block  'dummy  1  [7]  self-aligned anneal,  contact.  g a t e w h i c h must  then The  withstand  anneal. gate  gate  gap between t h e s e l f - a l i g n e d itself.  in  technologies for  i s removed f o r t h e p o s t - i m p l a n t  i s d e p o s i t e d t o form  gate  characterized  implant  to  the  of  o r d e r t o g i v e r e p r o d u c i b l e t h r e s h o l d v o l t a g e s o f MESFETs Two  is  using  MESFETs  well  the  recessed  i s deposited. A d i f f i c u l t y with the recessed gate  that the gate recess etch  the  and  epitaxial  channel  etched  gate  d e s i r e d t h r e s h o l d v o l t a g e a r e o b t a i n e d when t h e metal  and  reduce  r e s i s t a n c e are the recessed  1.1), a single  A window  level  [5].  implantation technologies.  (figure  the  process  (figure  1.2)  [8] s o a s t o l e a v e a s m a l l source/drain  and  the gate  T h i s i s done t o d e c r e a s e  and  t o i n c r e a s e t h e r e v e r s e breakdown v o l t a g e o f  n  gate  +  implant  capacitance  the  Schottky  contacts  while  channel.  The reduced  expected  using 1.3).  a  the length  the device  selective  For a  be  and the gate  reduced  beyond  the  unmodulated  MESFET  transconductance  implant  fabricated  using  s e t by  that  technology a  selective  the source/drain  i s d e f i n e d b y t h e mask l a y o u t  the limits  c a n be  over  fabrication  i m p l a n t p r o c e s s , t h e l a t e r a l gap between implant  of  l e n g t h o f the unmodulated channel  t o increase  achieved (figure  minimizing  and  lithography  and  mask  - pattern f o r device wells  K* region  - implant  3.1. GaAs substrate  - anneal  Au6e  4,  p.n.  - pattern f o r ohmic contacts - evaporate and l i f t o f f AuGe - alloy  n1  IS  1  p.fl.  - pattern f o r Schottky gate - etch channel - evaporate and l i f t o f f gate  source  Schottky gate drain  —'-isr-'— Figure  1.1.  Recessed  completed MESFET  gate  fabrication  process  +  cannot  alignment.  P.fl.  n  4  P.R.  K" region  - pattern f o r device wells - implant channel n~  S.I. BtAs substrate  -  anneal  - sputter TiW gate metal - pattern and l i f t o f f A l dummy gates  P.R.  fi U  N* region  - undercut dummy gates - pattern and implant source/drain n'  T l - N * region  MlBt  TU pte  i  1 _ J 1II  w  11  I  - pattern and l i f t o f f AuGe ohmic contacts - alloy - completed MESFET  Figure  1.2.  Refractory  self-aligned  gate  fabrication  process  5  P.R. H" region  — p a t t e r n and implant device wells n"  S.I. GaAs substrate  - anneal  P.R. N* region  pattern and implant source/drain n  tr region  AuGe  i  1 1  - evaporate and l i f t o f f AuGe 1  Schottky gate  1  i  11  1  - pattern for ohmic contacts  - alloy  i - pattern and l i f t o f f Schottky  (  Figure  1.3.  Selective  implant  - completed MESFET  f a b r i c a t i o n  process  gate  6 1.2.  Microwave Performance  An e q u i v a l e n t high-frequency  circuit  [9]  for  m o d e l i n g o f t h e GaAs MESFET i n t h e common-source  configuration  i s shown  the  model  intrinsic  t h a t was u s e d b y L i e c h t i  includes parasitic  i n figure 1 . 4 .  as  well  as  The f i g u r e  the extrinsic  shows model  both which  elements.  1 INTRINSIC M O D E L  -dg  r  GATE g O — A W  R  d  DRAIN  AVV  0  'ds  SOURCE  SOURCE  Figure  1.4.  A high  frequency  equivalent  circuit  for a  GaAs  MESFET i n t h e common-source c o n f i g u r a t i o n .  Using  the  intrinsic  model  assuming t h e feedback c a p a c i t a n c e neglected, C  g s  the c u t o f f frequency  i s equal  tog V B  shown C  d g  i n figure  i s very  / T where  small  1.4 and  and can  the current  be  through  i s given by  2  n C  ge  2  rr T  [1.1]  Here  T  =  the channel, L  L/  i s the c a r r i e r  V e  i s the e l e c t r o n  i s the modulated channel  oscillation  /  transit  saturation d r i f t  length.  The  through  velocity  maximum  where u n i t y power g a i n i s  r o a x  time  and  frequency  achieved  is  of  given  by: de  Equation non-unilateral  1.2  can  extrinsic  [1.2]  be  generalized  GaAs MESFET model  [9]  for  the  as  1^2  de R  It  c a n be  source s e r i e s  +  g  using  gate  Also,  2nf  R . ( de  T  R  g  C  [1.3]  )  dg  are  critical The  than  and  the  determining  the  refractory  MESFETs  self-aligned  in  L  with  low  self-aligned source  series  g a t e s c a n be made  can  those  smaller  fabricated  implant or recessed gate techniques since the from  devices  the  gate s e r i e s  the r e f r a c t o r y gate r e s i s t a n c e significant.  mask  have  R e f r a c t o r y gate metal  gate m e t a l i z a t i o n s used effective  B  lithography  self-aligned  circuits.  +  yield  length i s undercut  result,  e  o f a MESFET.  can  [10].  optical  selective  R  resistance R  technique  resistance  +  >•  seen t h a t the l e n g t h o f the gate  frequency response gate  R  gate  resistance R  g a t e m e t a l may  in  g  may  be  be d e s i r a b l e  As  a  microwave  c a n be more r e s i s t i v e  i n o t h e r p r o c e s s e s and  actual  length.  application  by  than  the  as a r e s u l t ,  the  greater. i f the e f f e c t  on t h e m i c r o w a v e p e r f o r m a n c e o f t h e  Plating of  the  device  is  8 1.3  T h e Sample a n d H o l d  The consists 1.5.  basic  structure  o f a sampling  of  switch  )  a  and  sample a  hold  and  hold  circuit  capacitor  (figure  sampling switch hold node  hold . capacitor'  Input  Figure  The is  1.5.  s t r u c t u r e o f sample and h o l d  most common a p p l i c a t i o n o f s a m p l e a n d h o l d  i n analogue t o d i g i t a l  used  conversion.  A  sample  a t t h e f r o n t end o f an analogue t o d i g i t a l  to relieve ADCs.  some o f t h e a p e r t u r e  The s h o r t  provides  acquisition  a constant  take  place.  1.4  Sample a n d H o l d  The its  Basic  Output  time problems time  signal level  of  period  the  hold  is  converter  (ADC)  associated  with  sample  f o r A/D  and  conversion  hold to  Performance  main measures o f a sample and h o l d ' s  a c q u i s i t i o n time,  and  circuits  input  performance are  impedance, droop r a t e ,  and  output  impedance. The specification  acquisition  time  is  o f a sample and h o l d .  the  key  performance  I t i s t h e time required t o  s t o r e on the  the  hold  s a m p l e and  hold  maximum s a m p l i n g The  hold  on  i f the  to  impedance input  charge  The  It  can  output  directly  become  to  a  connected to the  hold  input  signal  i s presented and  is  input  to  be  For  the  sample  and  to  supply  the  then  very  impedance path.  required  capacitor,  the  the  frequency  limited.  r a t e of decay  of  the  important  i t  approaches  the  i f the  s a m p l e and  hold  i f  hold  node  load.  . If  the  hold  output, then the  capacitor  l o a d may  affect  is the  rate.  Sample and  The sample  Hold  Structures  m o s t common  and  holds  disadvantage of the the  hold.  impedance i s i m p o r t a n t  must d r i v e c u r r e n t  1.5  s a m p l e and  limits  at  rate.  The  droop  time  h o l d may  i s the  signal present  acquisition  the  hold  s a m p l e and  droop r a t e  potential.  of  signal  the  of the  s a m p l e and  impedance l i n e  response of the  sampling  The  importance of the  a high  current  input.  level  frequency of the  depends upon t h e example,  c a p a c i t o r the  diode bridge  are  of  diode  must be of  very the  in characteristics  sampling  bridges  diode bridge  sampled r e p r e s e n t a t i o n Variations  types  and  well balanced to give input across  signal a  cause i n t e g r a t e d diode bridges  results.  Also,  complementary s t r o b e  f e e d t h r o u g h from b e i n g  at  wafer to  pulses observed  used A  FETs.  arrangement i s the  therefore  prevent pulse  switches  are on  major  fact  that  an  accurate  the  output.  surface  give  in  could  inconsistent required the  to  output.  GaAs d i o d e b r i d g e s a m p l e  and  holds  P o u l t o n e t a l [11] and b y Wong and The  MESFET  implemented  sampling  have  been  Fawcett  [12].  switch  i n GaAs f o r h i g h s p e e d  (figure  described  1.6)  sampling.  The  a single  fabrication MESFET a s  s a m p l i n g p u l s e and nonuniformities.  the  capacitance  sampling  (C  capacitance. t h e h o l d node  g s  )  As to  gate  One is  i t i s n o t as s e n s i t i v e disadvantage  that  its  a result, a  the sampling  certain  extent.  of  gate  using to  e t . a l . [13]  MESFET and  [14].  Figure  1.6.  MESFET s a m p l i n g  switch  to a  source node  pulse i s fed through GaAs  of  i t requires  a c t s as a v o l t a g e d i v i d e r w i t h t h e h o l d  s w i t c h e s a r e d e s c r i b e d by A k e r s al  that  often  advantages  a MESFET o v e r t h e d i o d e b r i d g e a r r a n g e m e n t a r e t h a t only  is  by  to  sampling  Swierkowski  et  2.  The Barta  and  sample Rode  sampling gate amplifier that  SAMPLE AND  and  [15]•  The  (figure  of Rutherford  testing  hold  followed  stage  and  circuit  circuit by  a  in hold  probes allow  on  to  require a regular contact  was  used  GaAs and  a  high  using  by  MESFET buffer  modified  allow  performance  made b u t  a  capacitor layout  order  The  that  consisted of  The  probes.  LAYOUT  followed  hold  2.1).  [16]  o f sample  HOLD DESIGN AND  from  frequency  Cascade  Inc.  c h i p microwave measurements t o pad  be  arrangement.  Buffer  amplifier  Sampling gate  rt Pula« i n v  bito  o Hold Capacitor *T  in  Figure  2.1.  Sample and  hold 11  <  >  schematic  diagram.  +0  RF  out  12 The  sample  implementation integrated  2.1  in  circuit  and  hold  circuit  both  selective  was  and  designed  self-aligned  t e c h n o l o g i e s u s i n g e i g h t mask  1)  Registration  for  subsequent l e v e l  2)  n~  FET  channels.  3)  TiW  4)  n  5)  AuGe/Ni  ohmic c o n t a c t s .  6)  Dielectric  MIM  7)  Schottky  d i o d e s , MIM  8)  Au  a i r b r i d g e body, b o n d i n g  The  implant  self-aligned implant  +  Barta  and  Rode  tops, airbridge  addressed  of the sampling  applied  to the centre  •guard  gates.  "the  thickening.  of  pulse  g a t e by t h e u s e  of a  triple  2.2.  pulse to the  h o l d was 'sampling*  pulse  d i s p l a c e d by t h e triple  t h e s a m p l e and  pad  problem  smaller gate  They  found  that  i n p u t and  t o the  hold  when  rather  feedthrough  could  momentary f o r w a r d c o n d u c t i o n o f t h e g u a r d  and  footings.  the  pulse  than  the  was outer  R u t h e r f o r d showed u s i n g S P I C E s i m u l a t i o n s  the reduction i n  single  bottoms.  Gate  node o f t h e s a m p l e and  carriers  alignment.  g a t e s , MIM  g a t e s a m p l i n g MESFET a s shown i n f i g u r e  1  levels.  insulator.  f e e d t h r o u g h w i t h a MESFET s a m p l i n g  feedthrough  implant  FET s o u r c e s / d r a i n s .  plating  Sampling  for  pulse.  g a t e s , a l l o f 1+m  hold  layouts.  For  be gates  attributed which  comparative  length,  were  that to  remove  purposes,  included  in  13  Guard gates  Sampling gate  Ohmic contact  nnn N Channel  Source S.I. 6a As substrate  F i g u r e 2.2.  Another  T r i p l e gate sampling  effect  FET c r o s s - s e c t i o n .  e x a m i n e d b y R u t h e r f o r d was t h a t  p r o p a g a t i o n on t h e g a t e .  TiW g a t e  metal  (=*3n/square ) a n d b e i n g o v e r t h e c h a n n e l IfF/Mm  2  capacitance per unit  transmission applied  i t  has  resistive  approximately  area depending  on g a t e b i a s .  The  l i n e nature o f t h e gate causes  a phase s h i f t  o f an  signal  along i t s width.  The e f f e c t  f o r s m a l l width gates such as those used circuits,  i s quite  of signal  b u t becomes i m p o r t a n t  i s not  i ndigital  f o r analogue  important integrated  a n d power MESFETs.  S P I C E s i m u l a t i o n s show t h a t a p u l s e i n p u t w o u l d t a k e a b o u t t o propagate To  100/jm a l o n g a g a t e o f l e n g t h 0.5/jm.  reduce t h i s phase s h i f t  effect,  the  d e s i g n e d b y R u t h e r f o r d was d r i v e n b y a c o n t a c t connected t-junction  at  2Ops  the  center  of  the gate's  s t r u c t u r e o f t h e sampling  gate  w i t h t h e Cascade I n c . microwave probes source c o n t a c t pad o f t h e sampling  sampling pad  60 jjm was  which  width.  not  to  be  was The  compatible  because i t r e q u i r e d  gate  gate  split.  the This  restriction the gate  on  c o n t a c t pad  of the  sampling  l a y o u t made i t  MESFET f r o m one  necessary end  in  to  drive  the  present  design.  2.2  The  Hold  The the  signal  constant  Capacitor  i n p u t time frequency  value  gate  arrangements.  half  due  the  g i v e n by the  o f 25ps f o r b o t h The  to parasitic  to the  fixed  hold  60Mm w i d e FET o f 100 Q  order  sampling  be  sample  capacitance  FET  was  node c a p a c i t a n c e were r e d u c e d  .  typically  an  single is  has  hold  a  channel  node. input  sampling  approximately and  half  node  resistance  capacitor  s e t t o 0.13pF i n o r d e r t o  significantly  In a d d i t i o n , p u l s e  sampling  of the hold  and  time  capacitance.  o f 0.25pF.  the d e p l e t i o n l a y e r  of the  chosen t o g i v e  the t r i p l e  limits  input  depletion layer capacitance  The  then  hold  The  the product  h o l d node c a p a c i t a n c e  and  and  sampled.  o f t h e h o l d c a p a c i t a n c e was  constant  A  of the  can  r e s i s t a n c e and  time  due  that  i s approximately  MESFET c h a n n e l The  constant  I f the value  give  the  a  a  total  nonlinearities  c a p a c i t a n c e might cause s i g n a l and  such  of the hold c a p a c i t o r  from t h i s v a l u e ,  feedthrough  for  in  signal  in  distortion.  droop r a t e  would  increase. Two the  types  s a m p l e and  interdigitated 2.3) .  o f h o l d node c a p a c i t o r s were  hold layouts.  The  f i n g e r s of Schottky  first metal  incorporated  c o n s i s t e d o f 2 pm with  2^m  gaps  in wide  (figure  15  GND  Figure The (figure  2.3.  Interdigitated capactitor  s e c o n d was a m e t a l  2.4).  An MIM  than an i n t e r d i g i t a t e d difficult shorting as  capacitor requires  Pinholes  of i t s terminals.  alloying capacitors  to  form  i s quite  on t h e o t h e r  value  more  T h e TiW m e t a l i z a t i o n c o u l d b e  used  contacts  low as r e p o r t e d  i n this  is  cause  could  be  so by  employed  quite  design.  as  rough  in  after  the  yield  of  these  Barta  and  Rode  [15].  h a n d r e m a i n s smooth e v e n a f t e r  t h e r e f o r e was u s e d  but  area  could  AuGe t e n d s t o become ohmic  structure  substrate  i n the d i e l e c t r i c  t h e MIM b o t t o m p l a t e o r A u G e / N i design.  (MIM)  less  c a p a c i t o r of equal  to fabricate.  Rutherford's  TiW  i n s u l a t o r metal  layout.  annealing  and  16  Figure  2.4.  Cross  section  of  Metal  Insulator  Metal  (MIM)  capacitor.  2.3  Buffer  Amplifiers  A s a m p l e and h o l d switch  and a h o l d  signal  source i s high,  amplifier current  a high  capacitor.  i n front  of  i t may the  c a n be as s i m p l e  be  impedance  sampling  at the  output.  where sample and h o l d ' s  quantizers  which have l a r g e i n p u t  in  hold with figure  2.5.  to  switch  capacitor.  input  This  is  add  to  sampling  a  the  and o u t p u t b u f f e r s  is  shown  t o the  capacitor  [17].  sees  in  A  the  i f the  current  case  the  buffer  provide  a r e added i n f r o n t o f capacitance  of  Similarly,  be n e e d e d s o t h a t t h e h o l d  conversion  and  necessary  i s required to drive significant  a b u f f e r may  as a  However, i f t h e i m p e d a n c e  needed t o charge t h e h o l d  sample and h o l d output,  circuit  A/D flash  sample  schematically  17  input  output sampling  buffer  buffer  switch hold  high  Figure  2.5.  output  about  logic  output  Sample a n d h o l d w i t h  3dB g a i n a n d t o i n s u r e t h a t t h e h o l d by t h e l o a d .  i n v e r t e r with  Hornbuckle, Van T u y l  and  Provided the  has  and E s t r e i c h [18-21]. the  node  been  droop  buffered  examined  gate  input  but  impedance  was  b u f f e r would o n l y  serve  has  sufficient  current  to increase c i r c u i t  not  as i s .  a p p e a r s a s a 0.25pF c a p a c i t o r t h r o u g h  the input s i g n a l path  by  A similar amplifier  sampling  b e c a u s e t h e sample and h o l d has a h i g h sample and h o l d  i n the design  T h e a m p l i f i e r was a  feedback  c o u l d have been added b e f o r e  The  buffer amplifiers.  b u f f e r a m p l i f i e r was i m p l e m e n t e d  r a t e was u n a f f e c t e d FET  impedence  capacitor  input  to give  low  hold  impedence  An  node  lOOfi. drive,  complexity.  2.4 L a y o u t  The buffer  p a d arrangements f o r sample and h o l d s ,  a m p l i f i e r and  compatible  with  discrete  MESFETs  were  t h e Cascade I n c . microwave  an  designed  probes.  isolated to  be  18  Figure  E l e m e n t s 1-8 for  implementation  9-11  were  2.6.  Mask  on t h e c h i p  set overall  layout  i n a self-aligned  selectively  implanted.  shown gate 12-14  e l e m e n t s i n c l u d i n g V a n D e r Pauw c r o s s , line,  and r i n g - d o t  included  so t h a t  metalization  could  structures. the be  A  resistivity determined.  TiW of  fat  layout  i n f i g u r e 2.6 were technology were FET,  stepped the  whereas  diagnostic transmission resistor  self-aligned  was gate  3.  3.1  FABRICATION  Special Structures  Before performing a f u l l  fabrication  run,  experiments  were conducted t o develop procedures f o r making s t r u c t u r e s t h a t were r e q u i r e d f o r the sample and h o l d s .  3.1.1  Fine Line Resist P r o f i l e s  The most common technique f o r on GaAs i s l i f t - o f f .  The  patterning  lift-off  process  metalizations  involves  opening  windows i n a l a y e r of p o s i t i v e p h o t o t o r e s i s t by exposure t o  UV  l i g h t f o l l o w e d by development.  the  wafer  by  evaporation.  The  p h o t o r e s i s t s t r i p p e r such  as  d e p o s i t e d on top o f  the  behind  where  metal  only  The metal i s d e p o s i t e d wafer  is  acetone.  photoresist the  then The  is  windows  on  soaked metal  lifted were  in that  off  opened  a was  leaving to  the  substrate. Difficulties  can  arise  in  lift-off  p r o f i l e of the patterened p h o t o r e s i s t i s  if  rounded  the  sidewall  or  tapered.  Evaporated metal w i l l then form a continuous f i l m which may not lift  o f f o r may l e a v e metal  pattern.  It  'wings  1  on the edges of the d e f i n e d  i s t h e r e f o r e most d e s i r a b l e t o c r e a t e an  or r e v e r s e t a p e r e d s i d e w a l l r e s i s t p r o f i l e . using  techniques  d i e l e c t r i c assisted  that  involve  lift-off  two  [22],  t e c h n i q u e was employed i n t h i s work. 19  levels however  T h i s can of a  undercut be  resist  done or  chlorobenzene  20  An overhang s t r u c t u r e can be c r e a t e d i n a o f r e s i s t by soaking the r e s i s t  i n a developer  as chlorobenzene p r i o r t o development. the r e s i s t  The  single inhibitor  surface  i s m o d i f i e d by the soak so t h a t i t  so as t o l e a v e an overhang.  3.1  resist  1 pm  chlorobenzene t e c h n i q u e .  profile  produced  such  layer  develops  than the u n d e r l y i n g r e s i s t i s an SEM o f a  layer  of  slower Figure  using  I t shows good v e r t i c a l s i d e w a l l s  the and  undercut.  Figure 3.1. gaps.  SEM showing r e s i s t p r o f i l e o f l^m l i n e s w i t h  2/JIP.  21  3.1.2  Self-aligned  A  " I " gate s t r u c t u r e  self-aligned sidewalls  gate process.  may  source/drain is  'T* G a t e S t r u c t u r e  serve  i s not necessary f o r the refractory A  both  refractory as  the  t o leave a l a t e r a l  refractory capacitance  mask  gate.  gap b e t w e e n  structure the  T h e e f f e c t . o f t h e gap  n  +  is  , r e d u c e MESFET s h o r t c h a n n e l  with  itself  heavy  3.2)  regions  and  to  n  [23].  (figure  decrease  effects,  t h e r e v e r s e b r e a k d o w n v o l t a g e [24] s o t h a t  vertical  f o r the  i m p l a n t and as t h e S c h o t t k y g a t e  advantageous however i f a T g a t e  used  gate  +  It is the gate  and i n c r e a s e  larger values of V  d s  may b e s w i t c h e d when compared w i t h MESFETs f a b r i c a t e d w i t h no T gate  structure.  orientation The similar CF  4  The l a t e r a l  sensitivity  also  helps  t o produce t h e T gate  by S a d l e r and Eastman  p l a s m a e t c h o f t h e TiW was u s e d  rather than a r e a c t i v e  to  reduce  gate  [25,26].  t e c h n i q u e used  t o t h a t used  gap  ion etch.  to  structure  [10] e x c e p t  produce  the  that  was a  undercut  22  Sputter TiU  -RlP.R.  P.R.  Pattern  For dummy gates  EuBporate RI GsRs substrate  LlFtoFF GsRs substrate  Etch TiU in CF, pi asms 4  Figure  3.2.  Production of  'T' g a t e  structure for  self-aligned  implant.  The determining suitable  most  critical  of  this  the d u r a t i o n of the plasma e t c h  undercut.  composition  aspect  and  The  situation  was  process  was  required  to  complicated  thickness of the r e f r a c t o r y  metal  was  when  in give the  changed.  For t h i s r e a s o n , the wafer was o p t i c a l l y  checked  d u r i n g the e t c h i n g p r o c e s s t o  an  time.  An SEM o f  a  described process i s  Figure 3.3.  'T'  structure  shown i n f i g u r e  SEM showing  ion implantation.  gate  determine  ' T ' gate  periodically  appropriate  produced  using  etch the  3.3.  structure  for  self-aligned  24 3.1.3.  Post-anneal  Gate  Thickening  F o r t h e b e s t microwave performance o f a desirable that the r e s i s t i v i t y discussed  i n s e c t i o n 1.2.  conductive gate  gate  structure  self-aligned  annealing Sadler 'T'  result,  i n place  during  and  t o p s were  alloying  left  A l dummy g a t e s  small  after  of  of  t h e 'T'  annealing  occured  the  e f f e c t s when A l , during  were u s e d t o s e r v e  severe  during  C r o r Pd/Au dummy g a t e s  i n place  highly  Unfortunately,  metal  the  were  used.  N i , Au,  o r Pt  annealing. as  the  n  as  As  a  implant  +  t h e y were removed f o r t h e a n n e a l .  Geissberger self-aligned  gates  et  a l . [28] d e p o s i t e d  after  SiON t h e n  patterning  Rutherford  [16]  required  and  implant.  +  c y c l e when e i t h e r  mask, t h e n  be  w o u l d be t o l e a v e t h e 'dummy' g a t e  [27] r e p o r t e d s i m i l a r  gate  o f t h e gate metal  i ti s  T h e s i m p l e s t way t o o b t a i n a  source/drain n  interdiffusion  MESFET,  and  gates  on  annealing by p l a n a r i z i n g  suggested  a l l MESFET  metal  evaporating an  an  a  top of  layer  overlayer  electroplating  of  o f Au.  technique  t o be c o n t a c t e d by a removable  that metal  i n t e r c o n n e c t web a n d a l l a r e a s where p l a t i n g was n o t d e s i r e d t o be  masked o f f . I n t h i s work, a n e v a p o r a t i o n method u s i n g two l e v e l s  p h o t o r e s i s t was d e v e l o p e d the  self-aligned  hardbaking overlayer width The  gates.  a lower  level  f o r obtaining a The t e c h n i q u e of resist  from c o n t a c t i n g t h e n  and alignment  procedure  +  metal  overlayer  of on  i n v o l v e d p l a n a r i z i n g and  in  order  regions  and  to to  prevent  the  allow  line  t o l e r a n c e s on t h e o v e r l a y e r t o be  i s o u t l i n e d i n f i g u r e 3.4.  relaxed.  25  m  P.R.  Spin  -TiU  on  p h o t o r e s i s t  Hardbake  CeR> s u b s t r a t e  P.R. P l a n a r i r e  r e s i s t  i n  CeRs kubfcirete  - R i P.R.  \  etch  o f  p h o t o r e s i s t  P.R. Spin  TiU  P.R.  0. 2  on  Pattern  upper  l e v e l  p h o t o r e s i s t  u s i n g  gate  mask  CBRS m b i trBie  Evaporate D i s s o l v e  Figure  3.4.  self-aligned  Procedure  for  shown  because  in  poor  lower  l e v e l  figure  3.5.  alignment  The y i e l d  o f  o v e r l a y e r r e s i s t  metal  The of  g a t e w i t h an  structure the  overlayer  on  Al  to  was the  Al  overlayer  not  symmetric  TiW  o f g a t e s w i t h t h e o v e r l a y e r was  a d h e s i o n o f t h e A l t o t h e TiW.  planarization  a  A l  gates.  precise  possible.  l i f t o f f  depositing  A n SEM o f a TiW s e l f - a l i g n e d is  and  e t c h was n o t s u f f i c i e n t  t h e t o p o f t h e TiW g a t e .  It is  possible  was  not  l o w due t o that  t o c l e a r the r e s i s t  the from  26  Figure  3.1.4.  3.5.  TiW g a t e w i t h  Airbridges  Airbridges are  an  integral  microwave i n t e g r a t e d c i r c u i t f o r making c o n n e c t i o n s for  A l overlayer.  spiral  inductors,  interconnects. parasitic  metalizations The utilizes  and as a second l e v e l a r e used  associated with  on a d i e l e c t r i c  most  common  of  (MMIC) p r o c e s s e s .  t o the top electrode  Airbridges  capacitance  part  monolithic  They  o f MIM  are  used  capacitors,  of metalization f o r  because them t h a n  there  i s  there  less  i s for  layer.  technique  two l e v e l s o f r e s i s t ,  most  f o r producing  an evaporation,  airbridges  an etch,  and  an  27  electroplating  step  [29].  This procedure i s outlined  i n figure  3.6.  EwBporated T l  Pet tern photoresist P.R.  P.R.  F i r s t leuel P.R.  EuBporate T i ( lBBBS)  GaRs Substrate  A  S  V-  Socond l  M  l  For a i r b r i d g e Footings  Second E l e c t r o p l a t e d flu  P.R.  leuel P R .  P a t t e r n tecond I sue! photoresist  for  B i r b r l d g e bodies P.R.  F i r s t leoet P . R .  P.R E l e c t r o p l a t e exposed t i t o n l u t with gold  Befit Substrate  V-  Dissolve upper photoresist  Etch  level  In Acetone  titftniu*  I n HF  D i t t o ! w e lower level  Goflt  A  Figure  3.6.  Substrate  photoresist  i n Deetone  y^A i r b r i d g e f a b r i c a t i o n by a c o n v e n t i o n a l  technique.  A t e c h n i q u e was d e v e l o p e d t o two  levels of resist 3.7).  (figure  lower l e v e l was a p p l i e d . the  lower  to give  and  a  The p r o c e d u r e resist  single involved  produce  evaporation  f r o m b e i n g d i s s o l v e d when t h e  of resist  good m e t a l  so t h a t  sidewall  and  using liftoff  a hardbake t o prevent the  T h e c h l o r o b e n z e n e t r e a t m e n t was  level  airbridges  second  not  level  applied  i t s edge p r o f i l e was  to  rounded  coverage.  P a t t e r n photoresist For Birbri.dcjfc Footings  H&rdbeke r e s i s t  P a t t e r n second teuel photoresist For e i r b r i d g e bodies  EuBporBte Ru  Figure  3.7.  liftoff.  Airbridge  f a b r i c a t i o n by  single  evaporation  and  Gold technique [30].  g i v i n g them a s h e e t  Thicker metal  strength not  a i r b r i d g e s c o u l d be  f o r long  normally  interconnects circuit. method  An  a  span a i r b r i d g e s . concern,  3.1.5  MIM  to  The  employed  in  0.050/square  improve  sheet  the  mechanical  resistivity  for  this  short  sample  and  the  is span hold  liftoff  3.8.  3.8.  SEM  of a i r b r i d g e .  Capacitors  Metal-insulator-metal extensively  of about  airbridge fabricated using  i s shown i n f i g u r e  Figure  l/urn t h i c k w i t h  particularly  those  o f an  to  resistivity  i s sometimes u s e d  s u c h as SEM  made up  i n MMIC d e s i g n s  (MIM) f o r RF  capacitors tuning  and  are  bypassing.  used The  MIM  consists  two  metal p l a t e s .  thinner be  a thin  than the  l a y e r of d i e l e c t r i c  Since  the  airbridge  with thick d i e l e c t r i c airbridge The  [31],  can  be  in this of  nitride  layer.  was  dielectric  e v a p o r a t e d as  lower e l e c t r o d e . that  was  an  fabricated  as  Figure  3.9  top  generally  metal  avoid  use  top  electrode  footing  and  TiW  SEM  of  an  o f MIM  capacitor  must An of  enhanced  of  800&  was  much  more a r e a .  The  shows an  SEM  shorting.  work u s e d a p l a s m a  described.  3.9.  plate  element r e q u i r e s  approximately  airbridge  Figure  is  between  f a b r i c a t e d without the  chemical vapor deposition f o r the  the  i n order to  however s u c h an  MIMs f a b r i c a t e d  sandwiched  dielectric  bottom metal p l a t e ,  c o n n e c t e d w i t h an  MIM an  of  silicon  of  the  u s e d as  MIM  MIM the  capacitor  31  3.2  The  R e f r a c t o r y Metal/GaAs  The process  most c r i t i c a l  i s that  aspect  elemental  of  good  such  for  reproducible  Tungsten  the  desirable  of 8 00°C  and  properties  h i g h temperature  s t r e s s when a n n e a l e d  [33].  alloy  to give better et  a l . [35]  GaAs/Ti  of T i W Q3  ?  found  that  w a s r e p o r t e d by  i f t h e a l l o y was  r e a c t i o n degraded  such  on  gates  Also,  without  s p u t t e r e d under p a r t i c u l a r In t h i s  work, a n  alloy  A procedure c o n t a c t probe annealing requiring mercury requiring  was  involving devised  parameters a complete contact  on  electrodes  is to  be  used  Mukherjee  i n titanium,  the  a  barrier.  self-aligned  t u n g s t e n has  been used  for  problems  used  when  as t h e r e f r a c t o r y  metal.  the  for  was [42].  Electronics  run  i t  stress  reduce  TiW/GaAs  fabrication  probe  tungsten.  [34]  has  test  the  thermal  nitrides  t h e MSI to  GaAs  and  conditions to was  to  of the Schottky  adhesion  o f TiW  due  on  Yokoyama e t a l .  silicides  pure  easily  electrical  stability  for  An  sputtering.  low  too r i c h  materials  as r e f r a c t o r y metal  been p u b l i s h e d [36-41]. self-aligned  the q u a l i t y  other  of  peel  a d h e s i o n t o GaAs t h a n p u r e  C o n s i d e r a b l e work process  0  after  or greater.  by  chemical  f o u n d t o c r a c k and  metal  i s t h e most  deposition  [ 3 2 ] , h o w e v e r i t was  An  implant  characteristics  as t u n g s t e n  characterized has  self-aligned  Schottky  annealing at temperatures  r e f r a c t o r y metal  resistivity  the  of the high-temperature-stable r e f r a c t o r y  g a t e w h i c h must e x h i b i t post-implant  Contact  to  effects  mercury  of  various  interface be  C-V  evaporated  Inc.  conducted. profiling  on  without  samples  The  without [43].  Instead,  two  mercury  contact with  the  contacts to the t o back  electrodes  implanted  are  temporarily  s u r f a c e o f a specimen.  s u r f a c e a c t a s two  Schottky  The  diodes  into  mercury  placed  back  [44]. The  series  capacitance  of the  two  Schottky  measured between t h e mercury c o n t a c t s w i t h m o d e l 4275A LCR greater than very  drawn  meter.  t h a t of the  c l o s e t o the  reverse  bias  conventional The implanted obtained  Since  C-V  other,  capacitance  was  applied  curves  the  and  area the  of the to  the  doping  a  t o be  with  w i t h TiW  present  1)  Etch wafer surface  2)  Implant without  3)  Light  4)  Sputter with  5)  Anneal with  6)  Remove TiW  7)  Obtain  chemical  2*  to  allow  on t h e  profiles  of  surface to  be  steps.  3^m.  patterning  (blanket  implant  3xl0  1 2  cm~ ).  etch. TiW.  TiW i n CF  doping  as 4  A  made.  doping  a m i n i m a l number o f p r o c e s s i n g  was  junction.  contact  profiles  much  capacitance  contact  small  was  Packard  c o n t a c t was  measured  f o l l o w i n g t e s t procedure allowed wafers annealed  Hewlett  o f one  small  junctions  encapsulant  using test  plasma.  p r o f i l e with mercury  probe.  parameters.  2  33 3.2.1  Effect  of a Furnace  Tests  using  encapsulated furnace  quarter  furnace  observed  on  annealing, furnace.  furnace.  pulled  from  procedure, greatly  the  annealed  surface  oxidized.  A  on  on  TiW  in  the  removal  residual film  a f t e r the  was  oxidized  TiW  the  the  TiW  added t o t h e  b o a t was  Another wait heated  of  surface  nitrogen  was part  of  TiW  the  pushed t o the  allowed  after  the  flow  during  in  the  substrates  heated part the  furnace.  surface  during  boat Using  annealing  of was  this was  reduced.  on  the  reducing  of a high  a CF  4  plasma e t c h  s u b s t r a t e was atmosphere.  TiW,  o b s e r v e d e v e n on The  could  be  f i l m was  obtained  s a m p l e s w h i c h were a p p a r e n t l y profiles  of the  a  residual  p r e s u m e d t o be  showed o n l y 3.10.  a tail  of  from  some  c l e a r e d of the activated  surface  samples annealed  t e m p e r a t u r e GaAs/TiW r e a c t i o n r a t h e r t h a n  Doping p r o f i l e s  figure  TiW  made  f o r 15 m i n u t e s w i t h  o x i d a t i o n of the  After film  was  gases flowed  the  been  surface  oxidation  a load lock before  the  The  were  plasma.  4  h y d r o g e n gas The  had  minutes.  GaAs s u b s t r a t e  prevent  probe  that  a p p e a r e d t o be  r e m o v e d i n a CF To  in  the  mercury  wafers  a t 8 00°C f o r 25  from the  was  the  Anneal  evidence oxidation.  areas  surface  dopant  in  as  of  the  film.  The  seen  in  34  Figure  3.10.  c o n t a c t probe TiW  Doping  profile  tail  obtained  on a s a m p l e a n n e a l e d  from  the  mercury-  a t 8 00°C f o r 25 m i n u t e s  with  encapsulant.  3.2.2  Effect  o f Rapid Thermal  Similar  results  subjected to a rapid seconds.  Anneal  were  observed  thermal  By v a r y i n g t h e RTA  anneal time  probe  c o u l d be  t h e RTA  a t 8 5 0 ° C f o r 60s  under Gaussian positions 1.6xl0  1 2  increased.  The  on two  wafer 2  950  temperature,  °C  for  i t was  observed u s i n g the  in  figure  t o doping p r o f i l e s  quarters  ± 0 . 4 x l 0 c m ~ o r 40110%. 1 2  at  wafers 5  found  mercury  b e s t r e s u l t s were o b t a i n e d u s i n g  a s shown  curves f i t t e d  quarter  (RTA)  and  t h a t t h e amount o f a c t i v a t e d d o p a n t  for  gave  an  3.11.  The  obtained  activated  area at  dose  6 of  35  l x io  .1  .2  .3  DEPTH  Figure  3.11.  Doping p r o f i l e  (MICRONS)  obtained  p r o b e o n a s a m p l e g i v e n a n RTA  at  from  the mercury  850 ° C  contact  f o r 60s w i t h  TiW  encapsulant.  3.2.3  Effect  o f Change o f TiW C o m p o s i t i o n  Further  improvement  increasing the tungsten The  tungsten  attaching using 50%  content  film.  content o f the sputter  a piece o f tungsten  the surface  proportionately  a c t i v a t i o n was o b t a i n e d b y  o f the sputtered  c o n d u c t i v e epoxy. of  i n doping  reduce  foil  The f o i l area  of  to which  film  deposited  was  the target  test  procedure  was  was  s a m p l e s s p u t t e r e d w i t h t h e TiW t a r g e t a n d W f o i l  on  i n t h e RTA.  samples without  The r a p i d thermal  encapsulation  anneals  but with  a  p r o x i m i t y t o p r o v i d e some a r s e n i c o v e r p r e s s u r e .  by  target  approximately expected  t h e amount o f t i t a n i u m i n t h e  The mercury c o n t a c t  annealed  increased  the sputtering covered  TiW.  deposited  conducted i n place  were GaAs  to  on then  conducted wafer  in  I t h a s been  found t h a t  significant  c a p l e s s RTA  out-diffusion  i s performed with  The  activated  RTA  a t 950°C  dose measured  no  arsenic  f r o m two w a f e r  f o r 5 s e c o n d s was 2 . 8 x l 0  .2  3.12.  Doping p r o f i l e  p r o b e on a s a m p l e  The Microwave  obtained  Integrated  f o r the registration  self-aligned p r o c e d u r e was  Circuit  measured.  Details  cm" or  an  70±5%.  2  contact  on t h e TiW t a r g e t  then  Fabrication Process  etched about  supplier's  i n figure  etch. 3.13.  g a t e s i s n o t shown not  1 2  given  from t h e mercury  T h e a l i g n m e n t mask was t h e n u s e d t o  steps are outlined  quarters  f o r 5s.  remove a n y damage c a u s e d b y t h e  quarters  a  (Blcren)  The GaAs s u b s t a t e s were i n i t i a l l y  [46].  when  overpressure [45].  ±0.2xl0  sputtered with W f o i l  g i v e n a n RTA a t 9 5 0 ° C  3.3  1 2  occurs  .3  tteplh  Figure  of arsenic  conducted  3  polishing pattern  The s u b s e q u e n t  process  the  figure  until  after  of the process are given  3.13 the  wafer  processing  The m e t a l o v e r l a y e r on  in  to  because devices  i n a p p e n d i x A.  the the were  37  5 i I i con n i i r i de  5.1.  a)  GaRs s u b s t r a t e  S u b s t r a t e c l e a n e d , e t c h e d , and about  silicon  300A  nitride  deposited.  1 n  region  5 ( 1 icon n i t r i d e GaFIs s u b s t r a t e  5.1.  b)  ^  P h o t o r e s i s t patterned f o r device w e l l s then implanted n " ,  TiU  n  region 5.1.  EaRs s u b s t r a t e  c)  Annealed i n furnace w i t h s i l i c o n n i t r i d e c a p .  and  about  of TiW s p u t t e r e d on s u r f a c e .  3 000A  3 [  Rl-  P.R.  P.R.  Cap removed  P.R.  O n  reg i 5.1.  d)  Gafls s u b s t r a t e  P a t t e r n e d f o r dummy gates and MIM bottoms.  About 5000A  Al  evaporated on s u r f a c e .  Figure  3.13.  employing  The  microwave  self-aligned  MESFETs.  integrated  circuit  process  TiU  RI  I  n  i  i  ^  wwwMmy///////^ n region 5.1.  GaRs sub&trate  5.1.  GaRs s u b s t r a t e  Al lifted off.  B n  f)  region  Undercut etched i n CF„ plasma.  B  -  PR  ^  n  regions '  ^  r e g i on  g)  P a t t e r n e d and implanted n + .  RuGe  b)  Removed A l , annealed i n RTA, p a t t e r n e d and evaporated  ohmic c o n t a c t s .  F i g u r e 3.13.  continued  AuGe  i)  L i f t e d off  AuGe,  alloyed  deposited s i l i c o n n i t r i d e  to  form  ohmic  contacts,  and  dielectric.  PR.  n n  j)  regions  region  R e s i s t d e f i n e d f o r H I M and s i l i c o n n i t r i d e  etched.  Ti/Pd/Ru  i  _1_ n n  X)  reaions  regio'  T i / P d / A u S c h o t t k y metal e v a p o r a t e d , and l i f t e d  MESFET  MIM  off.  Schottky  diode  RuEe  1)  Au a i r b r i d g e s e v a p o r a t e d , sample and h o l d s completed.  Figure  3.13.  continued  40  Scanning e l e c t r o n micrographs of a microwave FET sample  and  hold  circuit  s e l f - a l i g n e d technique are  fabricated shown  in  and  using  the  described  figures  3.14  and  respectively.  100/jm  F i g u r e 3.14.  A s e l f - a l i g n e d microwave FET.  2 00/L/m  F i g u r e 3.15.  a  A completed sample and h o l d a m p l i f i e r .  3.15  4.  4.1  MEASUREMENT AND TESTING  S t a t i c Measurements  S t a t i c measurements were made on MESFETs and d i a g n o s t i c s t r u c t u r e s u s i n g a Hewlett parameter a n a l y z e r .  model  4145A  semiconductor  The measurements were used t o e v a l u a t e the  b e n e f i t s of using the  self-aligned  s e l e c t i v e implant p r o c e s s . i n t h e modeling o f d e v i c e s  4.1.1  Packard  process  rather  than  the  A l s o , measured parameters were used (chapter 5 ) .  The TiW/GaAs Schottky J u n c t i o n  Measurements on t h e TiW/GaAs Schottky j u n c t i o n were made t o determine whether s u i t a b l e c h a r a c t e r i s t i c s e x i s t e d a f t e r the r a p i d thermal  anneal o f t h e s e l f - a l i g n e d  d e s i r a b l e t h a t t h e Schottky gate barrier  height  and  low  of  reverse  a  n  +  implant.  MESFET  leakage  Enhancement mode MESFETs which operate w i t h  have current  t h e gate  It is a  large [47]. forward  b i a s e d r e q u i r e t h e b a r r i e r t o be s u f f i c i e n t l y h i g h t o a l l o w adequate v o l t a g e swing [ 4 8 ] . The r e v e r s e important  leakage  an  current i s  i n microwave d e v i c e s f o r low n o i s e a p p l i c a t i o n s [49].  The c u r r e n t - v o l t a g e r e l a t i o n s h i p o f a diode  neglecting  series  r e s i s t a n c e i s g i v e n by  I  for  kT V >>  [4.1]  q  P l o t s o f t h e l o g a r i t h m o f forward diode c u r r e n t I versus 41  42 applied voltage V currents  f o r TiW  f r o m 30nA t o  of such curves  gave an i d e a l i t y  leakage  current of I  -0.5V.  The s t a t e d v a l u e s f o r data  gates  factor  = 6 ± 2nA a t  r  f r o m 10  linear for  linear  portion  o f n = 1.20±0.05 a  a r e t h e mean a n d  collected  were  The s l o p e o f t h e  IOO/LJA.  reverse  deviation  Schottky  reverse  bias  estimated  MESFETs  on  and a of  standard  two  quarter  wafers. k T  ln(I)  I I T  1  1/' s l o p s  —  -800 I -.5000 Figure  4.1.  The by  •  1 . 20  2500/div ln(I/amps)  vs. V  for a  the  o f Schottky  'knee'  in  the  contacts diode  r e l a t i o n s h i p when p l o t t e d o n a n a p p r o p r i a t e The  knee v o l t a g e  higher not  than  2.000  ( V)  self-aligned  junction.  b a r r i e r heights  observing  «= n  -  (kT/q)  TiW/GaAs S c h o t t k y  /  obtained  was o b t a i n e d  undergo thermal  f o r TiW/GaAs with  processing  were  current-voltage  current scale  contacts  was  t h e Ti/Pd/Au contacts (figure  4.2).  compared  [50].  slightly  which d i d  (cnA) 10.00  1 1 1  // //  1.000 /div  ll II  II  If  0000 -.5000  4.2.  Figure  Schottky  4.1.2  I v s . V f o r TiW  R e s i s t a n c e o f Implanted  Although  implanted source,  (solid)  thermal  interface,  i t must  drain  and  of n  +  other  be  reduce  Ti/Pd/Au  to  f o rcarrying  v o l t a g e probes  activate  doped  n  .  +  structures.  [51].  the  other  U s i n g t h e V a n d e r Pauw  o n two q u a r t e r w a f e r s  the  of the  The  d o p e d r e g i o n s was m e a s u r e d u s i n g b o t h  current while  TiW/GaAs  t h e sheet r e s i s t i v i t y  sheet Van d e r  The V a n d e r  i s a f o u r t e r m i n a l s t r u c t u r e i n w h i c h two  locations  (dashed)  Layers  sufficient  regions  Pauw c r o s s a n d t r a n s m i s s i o n l i n e  used  and  a n n e a l i n g must n o t damage t h e  donors and thus  resistivity  cross  2.000  ( V)  j u n c t i o n s t o GaAs.  Sheet  Schottky  .2500/div  VF  Pauw  terminals  two  are  technique  f a b r i c a t e d by t h e  used at  are as 10  self-aligned  process  r e s i s t a n c e s o f 190±4 0 o / s q u a r e  gave s h e e t  doped n  +  (2.0xl0  doped n  2  (3.0x10  resistances  and 1700 ± 3 0 0 n / s g u a r e  ions/cm )  1 3  ions/cm  o f 270  ).  fi/square  for  Sadler  [27]  regions  f o r regions  obtained  a n d 1600 fi / s q u a r e  on  n  sheet and  +  n~  regions respectively. The s h e e t obtained two of  resistance of the n  f r o m t r a n s m i s s i o n l i n e -measurements  quarter wafers. a series  of  The t r a n s m i s s i o n l i n e  ohmic  2,4,6,8, a n d lO^m pad  doped  +  contact  on t h e n  resistance versus  slope of the f i t t e d  +  implanted  spacing  +  structure  with  mask  layer.  sheet  was  also  a t 10 l o c a t i o n s on consisted  spacings  A plot  i s shown i n f i g u r e  the n  line,  pads  layer  o f pad  4.3.  of to  From t h e  r e s i s t a n c e was  found  to  b e R B h = 200 ± 60 n / s q u a r e .  K  o H  p  0  1 0.02  1  1 OJW  1  1 0.06  1  1 0.08  1  1 0.1  '  ' 0.12  1  ~ 0.14 r  Number of Souorva  Figure  4.3.  contacts  to n  Pad t o p a d r e s i s t a n c e v s . p a d s e p a r a t i o n f o r ohmic +  implanted  transmission  line.  45  4.1.3  Ohmic C o n t a c t  Resistance  The  'y'  intercept of  represents  the  r e s i s t a n c e o f two  product  of contact  contact  resistance of  Source S e r i e s  One  xeason  self-aligned  fitted  0.4 ± 0 . 2 by  pad  C5mm.  process  was  The  using the  'end'  Contact  in series.  The  a  normalized  resistances  Price  make  current through the  forward  source  to  by  [53].  4.3  of  [52].  f o r f a b r i c a t i n g MESFETs u s i n g t h e  gate  MESFET w h i l e  figure  Resistance  The  the  in  width gave  M u r a k a m i and  resistance small. passing  line  ohmic c o n t a c t s  r e s i s t a n c e and  0.3 fimm h a v e b e e n r e p o r t e d  4.1.4  the  the  source  s e r i e s r e s i s t a n c e can Schottky  drain contact  as  refractory  be  series measured  gate contact  a  voltage  probe  r e s i s t a n c e o f t h e MESFET i s t h e n d e f i n e d  R  d e  «  e n d  d  of  as  [4.2]  I  9 where V I  g  i s the  d s  being  passed  distributed the  end  drain-source  from gate t o source.  along  the  r e s i s t a n c e as  r e s i s t a n c e of the  length of the defined  conducting R  Here  a  is  a  voltage  constant  • n d  .  =  that  that results Since the  channel  from  current  gate current  (figure  4.4),  i n c l u d e s a c o n t r i b u t i o n from channel R  e  is  + a R  is  then the  R . c h  [4.3]  ch  determined  by  the  current  d i s t r i b u t i o n i n t h e channel. this  way  is  resistance.  then  Several  series resistance  an  The end r e s i s t a n c e as measured i n  overestimate  techniques  of  for  the  source  determining  the  of  the channel doping p r o f i l e i n order t o e l i m i n a t e t h e e f f e c t  of  the channel r e s i s t a n c e on t h e measured end r e s i s t a n c e .  Lee  et  al.  channel  resistance  shown  can  be  that  the  contribution  determined  directly  by  the  source  nature  [56] have  [53-55] r e q u i r e knowledge o f  series  of  the  measuring  d i f f e r e n t i a l end r e s i s t a n c e f o r v a r i o u s v a l u e s o f d r a i n I  d  the  current  applied.  Sou rce  Gat.  Dr8 i n  R..  Rcn  Figure  4.4.  Current  distribution  for  end  resistance  measurement. The measured d r a i n - s o u r c e drain currents Yds  where  =  voltage  are applied i s given  when  both  g  d  a r e the source  and  and  by  ( I d - r l g ) R e + I d Rch + l ( n k T / q I d ) + Rd I d  and R  gate  drain  series  [4.4]  resistances  47 respectively  [56].  R  The  *nd  source  From e q u a t i o n 4.2 we f i n d  =  R  e  +  ^  series  k  T  /qi )  [4.5]  d  resistance  may  then  f i t t i n g a l i n e o f s l o p e nkT/q t o a p l o t o f R resistance  measurements  self-aligned value of the  and two s e l e c t i v e source  t e c h n i q u e was R self-aligned  were  e  series  made  at  implant resistance  10  be vs.  e n d  l/l . d  locations  quarter  R  g  on  by End two  wafers.  The  using  this  obtained  = 3 8 ± 7Q f o r s e l e c t i v e and  found  =  5 ± lfi  for  devices.  F i g u r e 4.5. Rend = d V  ds  s e l f - a l i g n e d MESFETs.  /dig  vs. 1/I for selective d  implant  and  48 4.1.5  Sheet Resistance  The  o f TiW  speed o f s i g n a l propagation  increases  as t h e r e s i s t i v i t y  [57-60].  The s h e e t  measured u s i n g chip  layout.  applying along  Films  of  on t h e g a t e o f a  the  gate  metal  resistance of the sputtered  MESFET  is  film  was  included  on  the  The s t r u c t u r e c o n s i s t e d o f s i x c o n t a c t s ,  two  for  the stepped r e s i s t o r  current  the length  pattern  t o t h e TiW r e s i s t o r of the resistor.  was a l s o u s e d t o m e a s u r e s h e e t  TiW  reduced  and f o u r as v o l t a g e  probes  The s t e p p e d r e s i s t o r  resistance after  the  pattern  overlayer  o f A l was e v a p o r a t e d on t o p o f t h e TiW. The  measured r e s i s t a n c e R = V  VI  M e a s u r e d '  in  f i g u r e 4.6 a g a i n s t  probes used. at  low  the  The measured s h e e t  applicable  frequencies,  the  effective  o f t h e m e t a l i z a t i o n s becomes s m a l l e r  as a  At  effect.  The  high  skin  p and p e r m e a b i l i t y 6  =  f  depth  M  J  where / i s t h e s i g n a l f r e q u e n c y at  Profilometer  40GHz  are  t h i c k and t h e A l o v e r l a y e r  films  the skin effect  at frequencies The  resistances  slopes of  R  s h  =  a  and  the  depths  lOOOOA  t h a t t h e TiW f i l m  than  with  [4.6]  [ 6 1 ] . The s k i n  was  material  of  by  400OA ± 200  o n l y becomes i m p o r t a n t  greater  of  of  result  1^2  4100A  scans revealed  6  LJ i s g i v e n  1  P  ( " /  result,  c  are  skin  TiW  plotted  resistivities  resistivity  and  is  t h e number o f s q u a r e s b e t w e e n t h e v o l t a g e  frequencies.  thickness  ,. . applied  Al  respectively. was  A  of  3700 ± 2 0 0 &  thick. for  the  As  a  metal  40GHz.  lines  0.87 ± 0.06  in  figure  ^/square  4.6 for  gave the  sheet TiW  m e t a l i z a t i o n and R overlayer. R = 3.7 the  Sadler  fi/square  TiW f i l m  =  0.11±0.01 [27]  fi/square  reported  a  of  work may the  be  sputter  a  f o r TiW w i t h  sheet  on a 2000A" TiW f i l m .  i n this  tungsten content resistivity  B h  resistance  Al of  The l o w e r r e s i s t i v i t y o f result  deposited  o f T i i s eight times that  the  of  the  film.  enhanced The  bulk  of W [62].  o  0  20  40  Numbar of Squares  Figure  4.6.  M e a s u r e d r e s i s t a n c e o f TiW  and  of  TiW  with  Al  o v e r l a y e r v s . number o f s q u a r e s .  4.1.6  Isolation  Transmission  line  s t r u c t u r e s 48 0^m  long  and s e p a r a t e d  30/jm o f s e m i - i n s u l a t i n g GaAs were u s e d t o m e a s u r e t h e provided  b y t h e s e m i - i n s u l a t i n g GaAs s u b s t r a t e .  A  by  isolation  current  of  50  200 ± 50nA was measured when a  potential  across the transmission l i n e contacts.  of  10V was  applied  This i s t y p i c a l of the  i s o l a t i o n a c h i e v e d i n many GaAs f a b r i c a t i o n p r o c e s s e s [ 6 3 ] .  4.1.7  MESFET C h a r a c t e r i s t i c  Curves  Drain current c h a r a c t e r i s t i c s f o r a mode MESFET f a b r i c a t e d figure  typical  depletion  by t h e s e l f - a l i g n e d p r o c e s s a r e shown i n  4.7. Id  (mA)  Vgs (V)  50.00  0.0  -0.5  5.000 /div  -1.0  -1.5  -2.0  0000 0000  F i g u r e 4.7.  -2.5  3000/div  I-V c h a r a c t e r i s t i c s f o r a  ( V)  3.000  self-aligned  depletion  mode MESFET w i t h 0.8/jm gate l e n g t h and 2 00^/m gate width.  The was 3.0x10  channel implant dose used f o r d e p l e t i o n cm  .  Enhancement mode  MESFETs  mode MESFETs  were  fabricated  using  a lighter  current  channel  implant dose of  2.0x10  c h a r a c t e r i s t i c s f o r s u c h a MESFET a r e  cm'  .  Drain  in  figure  2  shown  4.8. Id  (mA)  Vg«  (V)  0.6  .0000  Figure  4.8.  Vds  I-V  0.8^m  4.1.8  Voltage  Plots versus the 0.5  mA  fitting  < I  of the  < 5mA.  a line  to  Sample p l o t s a r e depletion  gate length  square  root  and  200Mm g a t e  of  the  applied gate-source voltage d  The  threshold  s u c h p l o t s and shown  mode d e v i c e s .  w a f e r s gave V . =  3.000  ( V)  c h a r a c t e r i s t i c s f o r a s e l f - a l i g n e d enhancement  mode MESFET w i t h  Threshold  .3000/div  -1.7  in  g s  voltage  drain  figure  4.9  V was h  f o r the  for 10  -/T  current  &  were n e a r l i n e a r  extrapolating  Measurements o f  ±0.2V  V  width.  for  determined  by  to  0.  Vl^  =  enhancement  MESFETs on  d e p l e t i o n mode  2  and  quarter  devices.  Figure  4.9.  -/T^ v s . V g s f o r a ) a d e p l e t i o n MESFET  and  b ) a n e n h a n c e m e n t MESFET.  53  4.1.9  Low Frequency  Transconductance  The transconductance o f a MESFET, g (R h  t h e channel r e s i s t a n c e resistances  ( R and R s  =  =  — — -  dV  m  and source  , can be r e l a t e d t o and d r a i n  dCl/(R»+Rd+Rcr,)) Vds dVga  Vds  f-dReh  (R»+Rd+Rch)  [4.7]  [ dVge  Z  The s m a l l source and d r a i n s e r i e s r e s i s t a n c e s w i t h s e l f - a l i g n e d MESFETs s h o u l d t h e r e f o r e r e s u l t d e v i c e t r a n s c o n d u c t a n c e when compared MESFETs.  m  with  associated  i n improved  selective  implant  The measured transconductance o f 10 MESFETs from two  s e l f - a l i g n e d and two g  series  ) by  d  dl  g  )  C  B  - 140 ± 10  selective  mS/mm and  A sample curve o f g given i n figure  m  4.10.  g  m  =  implant 100  ± 10  quarter mS/mm  wafers  gave  respectively.  v s . gate b i a s f o r a s e l f - a l i g n e d MESFET i s  Figure (  v g s  4.10.  Low f r e q u e n c y t r a n s c o n d u c t a n c e  ) f o r a 100pm w i d e s e l f - a l i g n e d  M  E  S  F  E  T  .  (g ) v s . g a t e ffi  bias  55  4.2  Microwave Measurements  Microwave network  measurements  analyzer  and  microwave p r o b i n g Centre  a  Cascade  apparatus  i n Ottawa O n t a r i o .  at  bonding  calibration it  pads  using  the  HP 8510  54  automated  Communications probes  characteristic  [64].  an  I n c . model  Because  Research  are  coplanar  impedance  the  o f t h e measurement s y s t e m r i g h t  was n o t n e c e s s a r y  wires  made  The m i c r o w a v e  w a v e g u i d e s t h a t b r i n g a 50Q device  were  probes  The  probing  a t t h e probe  arrangement  and  bonded  M i c r o w a v e m e a s u r e m e n t s were made on  t h e A l o v e r l a y e r was e v a p o r a t e d  4.2.1  S - P a r a m e t e r s o f MESFETs  wafers  scattering  fabricated  wafers  measured u s i n g standard  by  the  the  Cascade  p a r a m e t e r s were computed  microwave  self-aligned  jigs. devices  on t h e TiW g a t e s .  process  selective  and  implant  Inc. probing  f o r both  and e i g h t s e l e c t i v e  two  quarter  process  stage.  shorted gates.  measured forward  types  implant  f r o m t h e c o m p u t a t i o n s due t o a p p a r e n t as  requiring  of  devices.  MESFETs were  fabrication  were  Estimated  d e v i a t i o n s f r o m t h e mean m a g n i t u d e a n d p h a s e o f  self-aligned  bond  p a r a m e t e r s o f 80 MESFETs f r o m two q u a r t e r  by t h e s e l f - a l i g n e d  fabricated  to  the  before  The  tips,  allowed  o n - w a f e r measurement o f d e v i c e s a n d c i r c u i t s w i t h o u t mounted,  the  allowed  t o de-embed d e v i c e m e a s u r e m e n t s f r o m  and f i x t u r e e f f e c t s .  c h i p s t o be d i c e d ,  to  these Five  excluded  failures  such  F i g u r e 4.11 shows t h e mean m a g n i t u d e o f  the  transmission coefficient  S  2 1  with  deviation  limits  i n dB f r o m  curves)  d s  The s e l f - a l i g n e d  showed g r e a t e r g a i n i n t h e  selective V  2 t o 18GHz.  implant devices  = 4V.  Smith  (dashed  chart plots  self-aligned selective  were  implant devices.  sensitivity the gate  devices  than  and S  x l  2 2  V  d i d the =  g s  (solid  OV  and  a r e shown i n f i g u r e  The measured S - p a r a m e t e r s f o r t h e more  uniform  those  of  the  result  of  the  r e s i s t a n c e t o misalignment  of  This i s  o f the source s e r i e s  for selective  system  curves) with  of S  4.12 a) a n d b) r e s p e c t i v e l y .  50n  devices  than  likely  a  implant devices.  e.ooo 1 1  1  3 j  k 3.000 >  >  s N  \  N -2.000 2.000  Figure for  4.11.  \  s\  10.00  (dashed c u r v e s ) .  FETs  (solid  curves)  X  s*.1  1  18.00  FHEQ-GHZ  Mean w i t h s t a n d a r d d e v i a t i o n  self-aligned  j 1  limits  and s e l e c t i v e  o f |S  2  |i n  implant  dB  FETs  Figure  4.12.  limits  o f a) S  FETs  (solid  Smith c h a r t p l o t s a n d b) S  curves)  2 2  o f mean w i t h  from  and s e l e c t i v e  2 to  standard deviation  18GHz  implant  FETs  for  self-aligned  (dashed  curves).  58  4.2.2  S-Parameters  The b u f f e r circuit  Amplifier  a m p l i f i e r was n e e d e d  t o present a s u f f i c i e n t l y  node t o p r e v e n t output  of the Buffer  signal  signal  t o a 50Q  implemented w i t h layout.  high  droop,  and  line.  An  S-parameters  i n t h e sample  and  hold  impedance  the  hold  to  present  isolated  s e l f - a l i g n e d F E T s was  The measured  Figure  4.13 shows t h a t  for five  50O s y s t e m up t o 2.6GHz. is  nearly  matched  a n open  The i n p u t  circuit  and  t o 50O a s s e e n i n f i g u r e  output  buffered amplifier  on  the  provided  4.13 gain  of the is  chip  amplifiers  figures  impedance  the  the  buffer  in  the amplifier  to  buffer  included  w e r e a v e r a g e d a n d t h e r e s u l t s a r e shown 4.14.  Stage  very  and in  a  amplifier closely  4.14.  6.000  JO 1.000  2.550  Figure  4.13.  '21  in  dB  for  implemented w i t h s e l f - a l i g n e d FETs.  buffer  FREQ-GHZ  amplifier  5.000  circuit  59  Figure to  4.14.  S m i t h c h a r t p l o t s o f a)  5GHz f o r b u f f e r a m p l i f i e r  circuit.  S^  and  b)  S  2 2  from  0.1  4.3.  Testing  Using applied was  and H o l d  t o t h e RF  signal  oscilloscope  Figure  4.15.  Circuits  t h r e e microwave probes, a input  was  sinusoidal  o f t h e s a m p l e and h o l d ,  f e d i n t o the sampling gate,  input  used  o f t h e Sample  observed  at  a  signal pulse  and t h e s a m p l e d v e r s i o n the  output  on  a  was train  of  the  sampling  ( f i g . 4.15).  Sample  for testing.  and h o l d  layout  and p r o b i n g  configuration  O s c i l l o s c o p e t r a c e s o f a 10MHz s i n u s o i d a l a t t h e s a m p l e and sampling figure  hold input,  g a t e , and  4.16  a).  the  The  an  80MHz p u l s e t r a i n  sampled sample  signal  are  hold  output  signal  and  signal  of the  level  i n t h e 12ns  amplitude  sampling  of  the  sampling  signal.  t o the sampling  i n p u t and  s i g n a l s when t h e  output  3 6MHz and The  0.8ns. for  r a t e was  maximum t e s t e d  sampling The  As  feedthrough  o f the sampling  rate  frequency  pulse  t h e RF  output  of the probe  As  with  by  the  time  that  generator  was  fall  nearly  [16]  sinusoidal  showed  gate r a t h e r than a not  observed The  lower  used  guard  induced  i t was  and gate  the  two  gates of the in  the  I t i s suspected  of the  h o l d was  stage configuration,  when  a c t e d a s an RF  the e f f e c t  o f t h e s a m p l e and  single  a microwave probe  t o measure t h e d e v i c e s . 'bias' probe  a  increased  p u l s e t o t h e o u t p u t o f a sample  T h i s e f f e c t was  a result,  the  250MHz.  momentary c u r r e n t w h i c h w o u l d o t h e r w i s e be gate contact.  of  limited and  became  h o l d were n o t c o n t a c t e d b y  arrangement used t h e low  was  minimum r i s e  t y p e s o f s a m p l e a n d h o l d were m e a s u r e d . s a m p l e and  2%  F i g u r e 4.16b) shows t h e  by R u t h e r f o r d  implemented w i t h a t r i p l e  s a m p l i n g MESFET.  of the  l e s s than  lOOmV  a  i n c r e a s e d t o 250MHz.  frequencies g r e a t e r than conducted  in  was  droop  i n p u t f r e q u e n c y was  a result,the pulse t r a i n  sampling  the  shown  feedthrough  o b t a i n e d w i t h t h e T e k t r o n i x 115  Simulations  hold  gate.  the sampling  available pulse generator. c o u l d be  The  p u l s e t o t h e s a m p l e and h o l d o u t p u t was  IV p u l s e a p p l i e d  to  The  p e r i o d was  applied  fed into  output  sampled r e p r e s e n t a t i o n o f t h e i n p u t s i n u s o i d . held  signal  that  choke t o i n the  guard  suppressed. not p o s s i b l e  any  guard  gates  on  Because t o use  a  62 fourth  microwave  supplying  probe  the biases V  d d  for and V.  the s  guard  to the  gate  contact  while  circuit.  strobe i n > input > output >  input >  output >  Figure  4.16.  O s c i l l o s c o p e t r a c e s o f i n p u t and  from a sample and h o l d . with  s e l f - a l i g n e d FETs.  The s a m p l e and  hold  output was  signals  implemented  5.  ANALYSIS AND  M o d e l i n g o f t h e microwave  performance  done u s i n g t h e E E s o f I n c . s o f t w a r e m o d e l i n g was  primarily  gate resistance  same a s t h a t  on t h e s e l f - a l i g n e d  package,  Touchstone.  shown i n f i g u r e  and h o l d  of  was The the  and  to  c a n be e x p e c t e d f o r g a t e s g i v e n  1.4.  A  Touchstone  version  i n c l u d e d t h e Sussman F o r t GaAs MESFET m o d e l  5.1.  MESFETs  device performance  The MESFET m o d e l u s e d b y  s i m u l a t e "the s a m p l e  of  intended t o determine the e f f e c t  e s t i m a t e t h e improvement t h a t metal overlayer.  MODELING  of  [65,66]  is  SPICE was  a  the that  used t o  devices.  M o d e l i n g o f MESFETs  The  potential  along the width of a s e l f - a l i g n e d  not g e n e r a l l y u n i f o r m because nature. effect  A  of the gate's  lumped e l e m e n t m o d e l was  of a distributed  lumped e l e m e n t m o d e l  used  gate r e s i s t a n c e  (figure  each h a v i n g the p r o p e r t i e s  5.1)  MESFET.  63  transmission to  and  consisted  o f a 1/8  slice  gate  of  approximate capacitance.  of  eight the  is line the The  MESFETs,  self-aligned  64  output port  F i g u r e 5.1.  The s e t t o 1/8 4.1.9.  The  Lumped e l e m e n t m o d e l o f MESFET.  transconductance of the  low  source  o f e a c h o f t h e MESFET e l e m e n t s  frequency  and  drain  series  were g i v e n v a l u e s e i g h t t i m e s measured from gate  series  determined (section  resistance from  applied  The  C-V  gate-source  measurements  t o the Schottky  that of C  for  g  g s  .  measured f o r t h e listing  4.1.4. of  contact.  (namely C R  d s  and  R^  d g  ,C  in R  end  The  the  value  The d c  ,  3.2.3)  C ) d s  ,  the  width  was  TiW  area with  film  given that zero  other capacitors and  d  of  was  g s  R  resistance  gate  capacitance C  section and  s  r e s i s t a n c e of the  (section  t h e S - p a r a m e t e r s o b t a i n e d by  input  1/8  the  to the capacitance per u n i t  m o d e l o f t h e MESFET 1/10  g r e a t e r than  t h e measured s h e e t  4.1.5).  from  R  measured  resistances,  the devices i n s e c t i o n  value corresponding found  value  was  was bias  in  were g i v e n  a  the  values  were c h o s e n t o g i v e a g o o d f i t o f the  model  s e l f - a l i g n e d MESFETs i n  with the values of the  to  those  section  components  lumped e l e m e n t m o d e l f o r T o u c h s t o n e i s i n a p p e n d i x  that  were  4.2.1.  The  used B.  in  the  65 5.1.1  Scattering  The  parameters  lumped e l e m e n t m o d e l was u s e d t o o b t a i n  f o r MESFETs w i t h g a t e s e r i e s  resistance R  g a t e s a n d f o r MESFETs  TiW  overlayer.  with  S-parameters  gates  obtained  g  c o r r e s p o n d i n g t o TiW  with  from  an  modeling  m e a s u r e m e n t s o f s e l f - a l i g n e d MESFETs a r e shown i n and  5.3.  The e f f e c t  o f the gate overlayer  2.000  Figure  5.2.  measured d a t a  |S | 21  vs.  (points).  frequency  from  Al  (or  and  Au) from  figures  5.2  i s seen p r i m a r i l y i n  FREQ-GHZ  10.00  S-parameters  model  18.00  (lines)  and  66  .2  Figure  5.3.  lines)  and  5.1.2  Maximum  when  port  is  always  cause  the  output part  negative  a  S  of  from  2  of  the  be  r e s i s t i v e r  c  or  is  gain  of  unstable.  In  has  with  a  from  two  conjugate  the  matched  )  18GHz  provided.  impedance  L  to  because  device  T  -r  model  (solid  MESFETs  attainable to  2  2  (cross-hatch).  simultaneous  impedances  (  1  available  conjugately  c o e f f i c i e n t  2  data  Gain  device  loop  when  and  maximum  output not  X 1  measured  The achieved  S  .5  an  The  match  maximum a v a i l a b l e  gain  this  terminations  magnitude  of  is and  required  a  device input  impedance  y i e l d s  port  terminations  case,  the  input  with  a  are  provided.  source greater  or  negative  load  than  may or real The  r e f l e c t i o n  unity.  A b r i e f summary o f power g a i n s S-parameters i s g i v e n maximum a v a i l a b l e  i n Appendix  and t h e i r r e l a t i o n s h i p t o  C.  gain of self-aligned  Figure  5.4  devices  as  f r o m m e a s u r e d S - p a r a m e t e r s a n d f r o m t h e lumped The  maximum s t a b l e  available  frequency  implant  gain  than  1.  the  Data  maximum the  The model  gave  from  model.  from  included.  overlayer  measured  determined  element of  the  the  greater selective  devices.  2.000  Figure  i s also  a gate  was  place  L  devices  device with  in  \T \ >  C  implant  the self-aligned  high  i s plotted \T \ > l o r  g a i n where  measured s e l e c t i v e of  gain  shows  5.4.  20.00  FREQ-GHZ  Maximum g a i n o f s e l f - a l i g n e d  (lines)  and  measured  implant  devices  data  (points).  from  MESFETs  self-aligned  40.00  from  and  model  selective  By m o d e l i n g of  the velocity  a GaAs MESFET, M a l o n e y a n d  relationship  between  the  length L .  The model gave  parameters  used  o f c a r r i e r s through Frey  cutoff /  =  T  27GHz.  gave  a  frequency  model  A cutoff  theoretical  and  for L  25GHz  i n t h e Touchstone  MESFET g a v e / = -J^Q—=  [67]  t h e channel  of  =  the  0.8 pm.  the  frequency  gate The  self-aligned of /  T  = 30GHz  9°  was o b t a i n e d b y Chao e t a l . [68] f r o m Morgan elements they  a n d Howes  o f a MESFET  can  oscillation  be /  [69] h a v e shown t h a t  ( R , Rd, R , B  a  x  and C  g  neglected, m  a MESFET w i t h L = 0.3pm.  then  the  d s  i f the  ) are  so  maximum  parasitic  small  frequency  that of  i s g i v e n by [ 6 9 ] :  33 /  From  figure  MESFET m o d e l e d L  (GHz)  5.4,  with  an  f  =  >  maM  Al  L  4 0GHz  f o r the  overlayer  at 0.8pm, e q u a t i o n 5.1 i n d i c a t e s  [5.1]  (pm)  that  on  the  self-aligned gate.  the effect of parasitics  on t h e microwave performance  o f t h e MESFET i s s m a l l .  et  =  al.  [29]  obtained  MESFETs w i t h L = 0.5pm.  /  m < x x  2 0GHz  Since  with  selective  North implant  69 5.2.  S i m u l a t i o n of Sample and Hold  The v a l u e s  f o r Rg,  Rd,  Rs,  Operation  C  g s  ,  Ca , g  Cds  and  used  for  modeling w i t h Touchstone were used i n the SPICE s i m u l a t i o n s the sample and h o l d .  A lumped element  S i m u l a t i o n o f the sample and h o l d o f s e c t i o n 4.2  model  under  the  was  also  test  gave an output waveform ( f i g u r e 5.5)  t h a t observed i n f i g u r e  used.  conditions similar  to  4.16.  10  0  of  20  30  40  SO  Tlm« ( n a )  F i g u r e 5.5.  Simulated  i n p u t and output waveforms f o r a  and h o l d under the t e s t c o n d i t i o n s o f s e c t i o n  Simulation at higher  sampling  4.2.  frequency  a l l the  T = lOOps i f a gate o v e r l a y e r was  MESFETs  in  the  circuit.  An  5.6)  (figure  showed t h a t the a c q u i s i t i o n time o f the sample and be as s m a l l as  sample  hold  could  evaporated  acquisition  time  on of  70  T = 500ps was bridge using  obtained  scheme.  by Wong and  Fawcett  Swierkowski et a l .  a GaAs MESFET s a m p l e and  [14]  [3-2] u s i n g achieved  T  a  diode  =  2 0Ops  hold.  -0.2  0  1  3  2  s  4  Tim* ( n » )  Figure and  5.6.  hold  Simulated  output  f o r 2.7GHz s a m p l i n g o f a 3 60MHz  Complete i n p u t included  i n p u t and  i n appendix  listings B.  f o r the  waveforms f o r a  sample  signal.  SPICE  simulations  are  6.  A  gallium  circuit  arsenide  fabrication  g a t e s was  microwave  integrated  p r o c e s s e m p l o y i n g MESFETs w i t h  self-aligned  developed.  The  with lower source s e r i e s t h a n MESFETs f a b r i c a t e d fabrication  of  CONCLUSIONS  monolithic  self-aligned resistance  by  implant y i e l d e d  and  selective  larger  Measured  microwave  scattering  parameters  MESFETs were more u n i f o r m a c r o s s  s u r f a c e and  between  selective  i m p l a n t MESFETs.  available  gain of  that  of  the  selective  gate metalization resistance  of  o v e r l a y e r was A  and  of  the  were  reduced.  self-aligned  A  the  of  wafer  measured  that  the  of  from  maximum  superior  resistance  method  the  GaAs  MESFETs w o u l d be  i m p l a n t MESFETs i f t h e  was  the  those  M o d e l i n g showed  self-aligned  to  of  the  reducing  the  gates  by  evaporating  circuit  was  fabricated  a  metal  demonstrated.  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Gonzalez, "Microwave Transistor Amplifiers, Analysis and Design," Prentice-Hall Inc., Englewood N.J., pp.  92-94,  (1984).  APPENDIX A - F a b r i c a t i o n P r o c e d u r e  The d e t a i l s listed  of the  developed  fabrication  process  are  as f o l l o w s :  1)  Scribe  3 i n c h wafer.  2)  Degrease i n  boiling  acetone  followed  by  rinse  in  b o i l i n g methanol and blow d r y . 3)  GaAs e t c h  i n 8 : 1 : 1 / H S 0 : H 0 : H 0 f o r 2m45s 2  3pm o f GaAs f r o m w a f e r 4)  Rinse  4  2  2  2  -  removes  surface.  i n D e - I o n i z e d (D.I.) H 0 2  cascade  f o r 4min  and  blow d r y . 5)  Cleave wafer i n t o  6)  Oxide etch  7)  Plasma d e p o s i t i)  NH  i n 10% NH OH f o r 15s a n d b l o w d r y . 4  nitride. Parameters: Temp:  3  Pressure:  ii)  silicon  plasma p r e c l e a n .  3  Gas: NH  NH  quarters.  500mTorr  Power:  F l o w : 42.5sccm/min  3  Silicon  nitride  G a s e s : He, S i H , 4  Pressure:  Time:  deposition. NH  1500mTorr  Power:  He F l o w : 500sccm/min SiH NH 8)  Spin  3  Flow:  4  Flow:  Time:  lmin  300°C 100W/500cm  2  4min  42.5sccm/min  on S h i p l e y  S o f t bake  2  540sccm/min  1400-30 p h o t o r e s i s t  30s. 9)  l00W/500cm  Parameters:  Temp:  3  300°C  a t 95°C f o r 25min. 78  (PR) a t 4700rpm f o r  79 10)  Expose  under  registration  mask #1 a t 25mW/cm  11)  P o s t e x p o s u r e b a k e a t 95°C  12)  Develop  13)  R i n s e i n D.I. H 0  14)  Remove n i t r i d e  f o r 45s.  2  f o r 20min.  i n MF312 d e v e l o p e r f o r 5 0 s . cascade  2  f o r l m i n and blow d r y .  from r e g i s t r a t i o n  marks i n  Buffered  HF  f o r 30s. 15)  R i n s e i n D.I. H 0  cascade f o r lOmin  16)  Etch registration  marks i n 5:2:240/NH OH:H O :H  2  and blow d r y . 4  2  2  0  2  for  50s. 17)  R i n s e i n D.I. H 0  18)  Remove  cascade  2  PR  methanol 19) ..24)  in  boiling  f o r 4min a n d b l o w d r y . acetone  followed  by  boiling  and blow d r y .  Pattern  f o r i m p l a n t w i t h n " mask #2 same a s  steps  8-13. 25)  Channel  implant 29  Parameters:  Species: Energy:  12  .  Si  Dose: 2.0 - 3.0x10  90 - 125 KeV  26)  Remove p h o t o r e s i s t  27)  R i n s e i n D.I. H 0  28)  Thicken s i l i c o n  29)  F u r n a c e a n n e a l c h a n n e l i m p l a n t a t 850°C  30)  Remove s i l i c o n  cascade  2  i)  With  i n 80°C  microstrip. f o r 4min. a n d b l o w d r y .  n i t r i d e a s i n 7)  (time=2min). f o r 25min.  nitride.  B u f f e r e d HF f o r 7min a n d  lOmin  D.I.  cascade  rinse. ii)  CF  4  plasma.  Parameters: Gas:  CF  Temp:  4  Pressure:  300mTorr  Power:  CF.  140sccm/min  Time:  Flow:  100°C 100W/500cm 2min  2  80  A  31)  Light etch  i n l:l:240/NH OH:H O :H O f o r 3sec.  32)  D.I. r i n s e  f o r 4min i n c a s c a d e  33)  Oxide e t c h  i n 10% NH OH f o r 15s a n d b l o w d r y .  'T' g a t e  ion  4  structure with  suitable using  S p u t t e r TiW r e f r a c t o r y g a t e Parameters:  2  bath.  undercut steps  f o r self-aligned  34)..43) :  metal.  B i a s s p u t t e r mode Gas:  Argon  Pressure:  Patterning  2  4  i m p l a n t a t i o n was o b t a i n e d  34)  2  Power: 2 00W/180cm 2 0mTorr  o f l^m l i n e s was a c h i e v e d  T i m e : 2 0min  using steps  35)..40)  35)  Spin  36)  S o f t b a k e a t 7 0 ° C f o r 25min a n d a l l o w t o c o o l .  37)  Soak i n c h l o r o b e n z e n e f o r 7min a n d b l o w d r y .  38)  E x p o s e u n d e r TiW dummy g a t e  mask #3 a t 25mW/cm  39)  D e v e l o p i n MF-312 d e v e l o p e r  f o r 60s.  40)  Rinse  41)  Evaporate  42)  Liftoff  o n S h i p l e y 1400-30 p h o t o r e s i s t a t 4700rpm f o r 3 5 s .  f o r 45s.  and blow d r y .  5000A A l .  metal  i nboiling  Parameters:  acetone  followed  by  boiling  and blow d r y .  Undercut e t c h i n CF  4  plasma.  Gas: CF  CF Flow: 4  Pattern f o r n  +  Temp:  4  Pressure:  44) ..49)  2  i n DI w a t e r c a s c a d e f o r l m i n .  methanol r i n s e 43)  2  300mTorr  Power:  140sccm/min  implant  with  Time:  100°C 100W/500cm  2  15min  mask #4 a s s t e p s 8 - 1 3 .  81 50)  Source/drain implant. Parameters:  Species: Energy:  51)  Remove p h o t o r e s i s t  52)  Rinse  53)  Remove A l dummy g a t e s  54)  Rinse  55)  Anneal  i n D.I.  i n D.I.  in  Si  Dose: 1.0 - 2 . 0 x l 0  i n 80°C  H 0 cascade 2  microstrip. f o r 4min. a n d b l o w d r y .  i n 50% H P 0 3  H 0 cascade 2  a t 5 5 ° C f o r 2min.  f o r 4min. a n d b l o w d r y . f o r 5s  o v e r l a y e r o n t h e TiW g a t e s was e v a p o r a t e d T h e s e s t e p s were a c t u a l l y  i nthe fabrication.  t o A l a s was u s e d  (capless  i n proximity).  c o m p l e t i o n o f t h e d e v i c e s b u t would point  4  s o u r c e / d r a i n i n RTA a t 950 ° C  steps 56)..67).  1 3  130 - 150 KeV  a n n e a l w i t h GaAs w a f e r  A metal  2 9  be  as  described  performed  better  done  after  at  this  E v a p o r a t i n g Pd/Au w o u l d b e p r e f e r r e d  here.  56)  S p i n o n S h i p l e y 1400-30 p h o t o r e s i s t  57)  S o f t b a k e a t 9 5 ° C f o r 25min.  58)  Hardbake  i ) ramp o v e n f r o m  a t 4700rpm f o r 3 0 s .  140 ° C  to  150 °C  over  20min. ii) 59)  l e t c o o l t o 120°C o v e r  Planarization etch i n 0 Parameters:  Gas: 0  2  0 60) ..65)  Evaporate  Temp: 3 00mTorr  F l o w : 2 00sccm/min  P a t t e r n second TiW  66)  2  plasma.  2  Pressure:  level  20min.  of resist  Power: 100W/500cm Time:  60min  for liftoff  dummy g a t e mask #8 a s i n s t e p s 5000A" A l .  100°C  35-40.  using  2  82 67)  Liftoff  metal  i n boiling  methanol r i n s e 68) ..73)  Pattern  acetone  followed  by  boiling  and blow d r y .  f o rl i f t o f f  w i t h o h m i c mask #5 a s i n s t e p s  35-40. 74)  E v a p o r a t e 2 000A AuGe t h e n 300& N i .  75)  Liftoff  metal  i n boiling  acetone  followed  by  boiling  methanol r i n s e and blow d r y . 76)  Alloy  77)  Test  ohmic c o n t a c t s FETs.  Steps 7 8 ) . . I l l ) dielectric  78)  i n f u r n a c e a t 4 2 5 ° C f o r 2min.  were  f o r formation  of  the  silicon  nitride  l a y e r a n d t h e MIM t o p p l a t e .  Plasma d e p o s i t Parameters:  silicon  nitride.  G a s e s : He, S i H Pressure:  4  , NH  Temp: 3 0 0 ° C  3  1500mTorr  Power:  He F l o w : 500sccm/min SiH NH  3  4  F l o w : 42.5sccm/min  S p i n on W a y c o a t HRN 200 n e g a t i v e  80)  S o f t b a k e a t 6 0 ° C f o r 20min.  81)  E x p o s e a t 25mW/cm  82)  Develop i n xylene  83)  Rinse  84)  Etch n i t r i d e  85)  Rinse  86)  Remove p h o t o r e s i s t  i n isopropyl  photoresist.  f o r 10s. f o r 90s. alcohol  i n HF t o l e a v e  and blow d r y . islands  i n D.I. water cascade bath  boiling  T i m e : 8min  F l o w : 540sccm/min  79)  2  100W/500cm  in  methanol r i n s e  boiling  a t MIM  sites.  f o r lOmin. acetone  and blow d r y .  followed  by  2  83  S t e p s 87-95 a r e n o t r e q u i r e d c o n t a c t s and  87)..92)  selective  Pattern steps  93)  Oxide  94)  Evaporate  95)  Liftoff  for  Schottky  diode  implant gates.  for liftoff  using Schottky p l a t e  t l as i n  35-40.  e t c h i n 10%  methanol  f o r MIMs b u t  NH 0H. 4  3000A o f T i / P d / A u  metal  in boiling  rinse  Schottky metal.  acetone  followed  by  boiling  and b l o w d r y .  A i r b r i d g e s were f a b r i c a t e d  i n steps  96)..111).  96)  S p i n on S h i p l e y  1400-30 p h o t o r e s i s t  97)  S o f t b a k e a t 9 5 ° C f o r 25min.  98)  Expose under  99)  Develop  S c h o t t k y mask #7  a t 4700rpm f o r 30s.  a t 25mW/cm  2  f o r 45s.  i n MF312 d e v e l o p e r f o r 50s.  100)  R i n s e i n DI w a t e r  101)  Hardbake  cascade  f o r l m i n . and b l o w d r y .  i ) ramp o v e n f r o m  140  °C  to  150  °C  over  20min. ii) 102) ..107)  l e t c o o l t o 12 0°C o v e r 2 0min.  P a t t e r n second Au  p l a t e mask #8  108)  E v a p o r a t e Au  109)  Liftoff methanol  level  of r e s i s t  for  liftoff  using  a s i n s t e p s 35-4 0.  airbridges.  metal rinse  in boiling  acetone  f o l l o w e d by  boiling  and b l o w d r y .  110)  Remove l o w e r l e v e l  resist  111)  R i n s e i n D.I. water  112)  T e s t s a m p l e and h o l d  i n hot  cascade  microstrip.  f o r 4 min.  amplifiers.  and b l o w d r y .  APPENDIX B - I n p u t L i s t i n g s  B.l  !  Touchstone  f o r Modeling  Input  Lumped e l e m e n t  model o f s e l f - a l i g n e d  MESFET  DIM FREQ GHZ RES OH IND NH CAP PF LNG M I L TIME PS COND /OH ANG DEG VAR G =» 0.0025 T =• 6  A/V) ( T  Cl  =  0.035  Gl  « 1E-8  pF) A/V)  R I •= 20  Ohms)  C2 =  0.004  C3 -  0.004  C4 -  0.004  R4 -  4000  RS = 24 RG = 5  ps)  <c  d g  pF) pF)  (c  d 6  pF) Ohms)  (R.  Ohms)  ( o r RG = 0.63 f o r g a t e w i t h A l o r Au o v e r l a y e r )  84  RD = 24  (Ohms)  L=0  (nH)  CKT SRL 4 2 R R D L=0 A  SRL 3 1 R R G L=0 A  FET2 3 4 0 G G T T F F C G S C 1 GGS G1 R I R 1  CDG C2  CDC C3  CDG C2  CDC C3  CDG C2  CDC C3  FET2 9 10 0 G G T T F F C G S C 1 GGS G1 R I R 1 CDG C2  CDC C3  A  A  A  A  A  A  A  A  + C D S C 4 RDS R4 R S R S A  A  A  SRL 6 2 R R D L=0 A  SRL 5 3 R R G L=0 A  FET2 5 6 0 G G T T F F C G S C 1 GGS G1 R I R 1 A  A  A  A  A  A  A  A  + C D S C 4 RDS R4 R S R S A  A  A  SRL 8 2 R R D L=0 A  SRL 7 5 R R G L=0 A  FET2 7 8 0 G G T T F F C G S C 1 GGS G1 R I R 1 A  A  A  A  A  A  A  A  + C D S C 4 RDS R4 R S R S A  A  A  SRL 10 2 R R D L=0 A  SRL 9 7 R R G L=0 A  A  A  A  A  A  A  A  A  + C D S C 4 RDS R4 R S R S A  A  A  SRL 12 2 R R D L=0 A  SRL 11 9 R R G L=0 A  FET2 11 12 0 G G T T F F C G S C 1 GGS G1 R I R 1 CDG C2 CDC C3 A  + C D S C 4 RDS R4 A  A  A  A  A  A  A  A  A  RS RS A  SRL 14 2 R R D L=0 A  SRL 13 11 R R G L=0 A  FET2 13 14 0 G G T T F F C G S C 1 GGS G1 R I R 1 CDG C2 CDC C3 A  + C D S C 4 RDS R4 A  A  A  RS RS  SRL 16 2 R R D L=0 A  A  A  A  A  A  A  A  SRL 15 13 R R G  L=0  A  FET2 15 16 0 G G  T T F F C G S C 1 GGS G1 R I R 1 C D G C 2  A  + C D S C 4 RDS R4 A  A  A  A  A  A  A  CDC C3 A  RS RS  A  A  SRL 18 2 R R D L=0 A  SRL 17 15 R R G  L=0  A  FET2 17 18 0 G G A  + C D S C 4 RDS R4 A  A  DEF2P 1 2  T T F F C G S C 1 GGS G1 R I R 1 C D G C 2 A  A  A  RS RS A  TRA  OUT TRA DB[S21] GR1 TRA DB[S12] GR2 TRA DB[GMAX] J  TRA SB1 TRA  Sll  TRA S22 !  TRA SB2  FREQ SWEEP  2 40 2  RANGE  2 40 2  GRID  GR1  -2 8 1  GR2  -40 0 4  GR3  0 20 2  GR3  A  A  A  CDC C3 A  87 B.2  Spice  Input  Sample a n d h o l d - t e s t c o n d i t i o n s *Cycle Controls .OPTIONS ITL4=1000 ITL5=0 LIMPTS=2000 NOPAGE .WIDTH OUT=80  * •Active  Elements  * •Sampling  Switch  BSWI 10 20 3 RSAG12 60  * * F i x e d H o l d Node  Capacitance  CHOLD 3 0 13OFF  * •Amplifier BIN  4 3 0 RSAG12 67  BFB 4 5 0 RSAG12 23 BLPU 2 4 4 RSAG12  45  BPU 2 4 6 RSAG12 90 Dl  6 7 TD4 66  D2 7 8 TD4 66 D3 8 5 TD4 66 BPD 5 1 1  RSAG12 90  •Standard A c t i v e Element  Models  •model i s f o r 1 um s l i c e  o f MESFET o r DIODE  * .MODEL RSAG12 GASFET(VT0=-1.7, VBI=1.23, RG=1, + BETA=9E-5,  LAMBDA=0.055, CGS0=1.0FF,  + IS=2.0E-15, RD=600, RS=600,  ALPHA=2.3,  CGD=0.1FF,  CDS=0.05  TAU=3.0PS)  * .MODEL TD4 D(IS=1.24E-14, RS=1300, + VJ=0.72, EG=1.42,  N=1.2, TT=2PS,  CJ0=8.0E  BV=8, I B V = l E - 3 )  * •Independent VDD  2 0 DC  VSS 1 0 DC VIN  Sources  5.25 -2.25  10 0 S I N ( - 0 . 6 5 0.32  36MEGHZ)  VCTRL 20 0 P U L S E ( - 3 . 0 -1.9 700PS 700PS 700PS 700PS 3700PS)  •Transient Analysis •.TRAN .TRAN  Parameters  T S T E P TSTOP <TSTART  TMAX  UIO  100PS 50NS  * •Output  Parameters  .PRINT DC V ( 1 0 ) V ( 3 ) V ( 5 ) .PRINT TRAN V ( 1 0 ) V ( 2 0 ) V ( 3 ) V ( 5 ) .END  89  Sample a n d h o l d  *Cycle  - ultimate  performance  Controls  .OPTIONS ITL4=1000 ITL5=0 L I M P T S = 2 0 0 0 .WIDTH  NOPAGE  OUT=80  * •Active  Elements  •Sampling  Switch  BSWI 10 20 3 RSAG12 60  * • F i x e d H o l d Node  Capacitance  CHOLD 3 0 13OFF  * •Amplifier BIN  4 3 0 RSAG12 67  BFB  4 5 0 RSAG12 23  BLPU 2 4 4 RSAG12 45 BPU DI  2 4 6 RSAG12 90 6 7 TD4 66  D2 7 8 TD4 66 D3 8 5 TD4 66 BPD 5 1 1  RSAG12 90  * •Standard •model  *  A c t i v e Element  i s f o r 1 um s l i c e  Models o f MESFET o r DIODE  90 .MODEL RSAG12 GASFET(VTO=-l.7, VBI=1.23, RG=0.1, ALPHA=2.3, + BETA=9E-5, LAMBDA=0.055, CGS0=1.0FF, CGD=0.1FF, + IS=2.0E-15, RD=600, RS=600,  CDS=0.05FF,  TAU=3.0PS)  * .MODEL TD4 D(IS=1.24E-14, RS=1300, N=1.2, TT=2PS, CJO=8.0E-15, + VJ=0.72, EG=1.42,  BV=8, I B V = l E - 3 )  * •Independent VDD  2 0 DC  Sources  5.25  VSS 1 0 DC -2.25 VIN  10 0 S I N ( - 0 . 6 5 0.32  360MEGHZ)  VCTRL 20 0 P U L S E ( - 3 . 0 -2.9 70PS 20PS 20PS 20PS 370PS) * •Transient Analysis .TRAN  10PS  Parameters  5NS  * •Output  Parameters  .PRINT DC V ( 1 0 ) V ( 3 ) V ( 5 ) .PRINT TRAN V ( 1 0 ) V ( 2 0 ) V ( 3 ) V ( 5 ) .END  APPENDIX C - M i c r o w a v e G a i n  Definitions  out  Input matching netwo *<  Output matching network  Mtsm  r  Figure  The scattering  ou t  gains  C.l.  ( o r power  parameters  gain  block  ratios)  CI  diagram.  are related  to  the  o f t h e MESFET b y [ 7 0 ] :  Transducer power gain in 50-ohm system  Transducer power gain for arbitrary T and T c  Power  50  L  Power gain with input conjugate matched  G = \S \ T  2  [C.l]  2I  0-l ci )|s | 0-ir | ) r  l  J  l  2 1  l(i - s „ r ) ( i  2  12  |s | (i-|r | ) J  t  |i-s 2rj(i-|5;,| )  l^.l (>-|r j')  c  c  J  l  |5 ,[ 2  2  1-|S |  2  22  (forr = 0) c  91  [C.3]  J  =  |»-S„r | (l-|5„| )  L  n  2  J  2l  i-|5 | (for 1^ = 0)  :  c  n  21  2  Available power gain with output conjugate matched  L  |s |  J  21  [C.2]  -s T )-s s r r \  c  G=  t  [C.4]  92  |5 ,t (<-ircl )(i-|rj ) 2  :  Unilateral transducer power gain  2  2  11 — S r | ! 1 — S ^ r j  ™  Maximum unilateral transducer power gain  _  n  _  O TUm»»  c  :  2  L  |S;i| (l — | S | ) ( l — | S | )  J  2  3  [C-6]  2  M  2J  [C.7]  Maximum available power gain  5  Maximum stable power gain  {k-jiS-\)  _ |S il |s |  .. _  2  '  |2  where k i s t h e s t a b i l i t y  factor  1 -1S,,| -|S | 2  2  : :  -!  as given by:  + 1A|\ ,  2 |S S | 12  "  >  forstability  !  21  [C.9]  here |A| = | S S n  and  r  and  can be r e l a t e d  c  and r  2 2  t  - S, S 2  2 1  |  [ C I O ]  a r e t h e s o u r c e and l o a d  reflection  coefficients  t o impedances by:  r  c  =  Z  ~  c  z  C  r  -  Z  °  z +z  l  [C.ll]  o  ~ °  ^-zTTT  z  tc.i2]  

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