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A GaAs cermet gate charge-coupled device LeNoble, Maurice 1989

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A GaAs C E R M E T G A T E C H A R G E - C O U P L E D D E V I C E By MAURICE  LeNOBLE  B . A . S c , University of British Columbia,  1984  A THESIS S U B M I T T E D IN P A R T I A L F U L F I L L M E N T O F T H E REQUIREMENTS FOR T H E D E G R E E OF D O C T O R OF PHILOSOPHY  in T H E F A C U L T Y O F G R A D U A T E STUDIES Department of Electrical Engineering  We accept this thesis as conforming to the required standard  THE UNIVERSITY  OF BRITISH April  1989  © Maurice LeNoble,  1989  COLUMBIA  In  presenting  degree freely  at  this  the  available  copying  of  department publication  of  in  partial  fulfilment  of  the  University  of  British  Columbia,  I  agree  for  this or  thesis  reference  thesis by  this  for  his thesis  and  study.  scholarly  or for  her  of  financial  £leC • £rt(jr.  T h e U n i v e r s i t y o f British 1956 M a i n M a l l Vancouver, Canada V 6 T 1Y3 Date  DE-6(3/81)  /  CLju  Columbia  /m  further  purposes  gain  shall  that  agree  may  representatives.  permission.  Department  I  requirements  It not  be is  that  the  Library  permission  granted  by  understood be  for  allowed  an  advanced  shall for  the that  without  head  make  extensive of  copying my  it  my or  written  Abstract  T h e design, i m p l e m e n t a t i o n a n d evaluation of a 64-pixel, 4-phase G a A s cermet gate charge-coupled device ( C M C C D ) are described.  It is demonstrated that the sig-  nal charge confinement a n d the signal charge capacity of the C M C C D are m a x i m i z e d when t h i n , h i g h l y doped active layers are used for implementing the device. T h e cerm e t / G a A s j u n c t i o n w i t h i n a n interelectrode gap of the C M C C D forms a barrier similar to a m e t a l / G a A s Schottky barrier, as revealed by a n investigation of the dc currentvoltage characteristic of a c e r m e t / G a A s Schottky barrier diode. A transmission line model is described for the c e r m e t / G a A s j u n c t i o n w i t h i n a n interelectrode gap of the C M C C D a n d is used to demonstrate the relationship of the surface potential variation along the gap as a f u n c t i o n of the clock frequency a n d the m a t e r i a l parameters.  It  is shown that the surface potential v a r i a t i o n is monotonic for a l l frequencies, w h i c h is desirable for m i n i m i z i n g the f o r m a t i o n of energy troughs w i t h i n the active layer. E n ergy troughs trap a n d release charge f r o m passing charge packets, causing unwanted signal dispersion. A two-dimensional computer model is used to determine a theoretical m a x i m u m frequency of operation of the C M C C D . It is shown that a short transport electrode length for a fixed transport electrode p i t c h is preferable as it results i n the m a x i m u m h i g h frequency performance of the C M C C D for the lowest clock power. A computer s i m u l a t i o n of a single electrode transfer of a charge packet is demonstrated using the two-dimensional computer model. T h e computer s i m u l a t i o n indicates that efficient charge transfer takes place, suggesting that the C M C C D w i l l have good performance. A G a A s C M C C D w i t h an on-chip G a A s M E S F E T source follower amplifier has been p r o d u c e d using a six mask level fabrication procedure. T h e C M C C D a n d the output source follower amplifier are demonstrated to operate at 100 M H z . Charge transfer efficiencies of 1.00 a n d 0.998 for 100 M H z operation are obtained for the C M C C D using the impulse response m e t h o d a n d the insertion loss method.  ii  Table of Contents  Abstract  ii  List of Symbols  v  L i s t of Tables  x  List of F i g u r e s  xi  Acknowledgements  xiv  1  Introduction  1  2  Theory  6  2.1  P r i n c i p l e of O p e r a t i o n  6  2.2  One-dimensional P o t e n t i a l D i s t r i b u t i o n s  12  2.3  A c t i v e Layer Specification  17  3  4  The C e r m e t / G a A s Junction  20  3.1  B a r r i e r Properties  20  3.2  Surface Potentials  25  3.3  Verification  32  Two-dimensional G a A s C M C C D Model  38  4.1  Geometric Representation  38  4.2  Device Equations  38  4.3  F i n i t e Difference G r i d a n d the C o m p u t a t i o n a l K e r n e l  40  4.4  F i n i t e Difference Equations for u(x,y)  41  4.5  B o u n d a r y C o n d i t i o n s for u(x, y)  44  4.6  F i n i t e Difference E q u a t i o n for J(x, y)  45  4.7  D i s c r e t i z a t i o n of the C o n t i n u i t y E q u a t i o n  46  4.8  B o u n d a r y Conditions for n(x, y)  48 in  4.9  N u m e r i c a l Solution of the Difference Equations  4.10 C o m p u t e r Simulations  . . . .  49 • 52  5  Device Fabrication  59  6  Testing and Evaluation  66  7  Comments  73  7.1  Summary  73  7.2  Considerations for Future W o r k  .  Bibliography  76 78  Appendices A  B N L E x p e r i m e n t 787  83  B  P o l a r T r a n s f o r m a t i o n of V(y)  89  C  Newton's M e t h o d  91  D  Detailed Device Fabrication Procedure  92  E  Test C i r c u i t for V H F O p e r a t i o n  97  iv  List of S y m b o l s  a(y,u)  real part of (L  A,...,G  constants  Ads  logarithmic amplitude  AQ  amplitude constant  A*  modified Richardson's constant  b(y,u>)  imaginary part of (L  c(u>)  real part of  Ci  integration constant  —  g  y)j(oj)  g  -  y) (w) 7  L j(u>) g  C  m a x i m u m potential contour  COM  distributed cermet f i l m capacitance  CD  distributed depletion layer capacitance  C'o/p  parasitic output capacitance  CCM  l u m p e d cermet film capacitance  CD  l u m p e d depletion layer capacitance  d(u)  imaginary part of L 7(cu)  df  differential contour vector  E  normalized electric field  EQ  critical electric field  Ei  intrinsic energy  EQ  empirical constant  f  clock frequency  M  5  c  fl  + l  finite  difference equation  fmax  m a x i m u m clock frequency  g, h  functions comprising the finite difference equation  H(y,u)  normalized surface potential amplitude v  i,j  x,y g r i d point indices  i ux,jaux  x,y  iint  active layer d e p t h g r i d point index  imax  wafer thickness g r i d point index  I(y)  dc tangential current  IQ  saturation current  j ax  pixel length g r i d point index  J  electron current density  Jo  saturation electron current density  k  time-step index  a  m  k  a u x i l i a r y g r i d point indices  stop time index  max  k\, &2  integration constants  /  iteration index  L  interelectrode gap length  g  L  transport electrode p i t c h  p  m  integer index  rii  intrinsic carrier density  Np k ea  number of pixel transfers between the peak of the observed impulse response a n d the peak of the ideal impulse response  Nc  effective density of states i n the conduction b a n d  Nr>  u n i f o r m donor density  NT  number of single electrode transfers  fi  normalized electron density  n  normalized c e r m e t / G a A s j u n c t i o n electron density  no  normalized m e t a l / G a A s j u n c t i o n electron density  ND  normalized u n i f o r m donor density  gap  vi  p  number of pixels  q  electron charge  Qintj  interface charge density  Q  signal charge density  Q ,max  m a x i m u m signal charge density  Qph.i+Ph.2  phase 1 a n d phase 2 signal charge density  Qph.2+Pk.3  phase 2 and phase 3 signal charge density  r  geometric series constant  RCM  distributed cermet f i l m resistance  RL  load resistance  Rs  series resistance  RCM  l u m p e d cermet f i l m resistance  S  derivative of the tangential current I(y)  t  time  s  s  tdei  delay time  t  clock transition p e r i o d  T  temperature  u  normalized potential  tr  Ug p  normalized c e r m e t / G a A s j u n c t i o n potential  Uo  normalized m e t a l / G a A s j u n c t i o n potential  UT  t h e r m a l voltage  v  normalized electron quasi-Fermi potential  V(y)  dc surface potential  V(y,u)  ac surface potential  V  applied gate voltage  a  g  potential difference across the gap  V  gap  Vi ,Ii n  n  diode t e r m i n a l voltage a n d current  vii  Vp  pinch-off voltage  Vo  incident voltage  w  cermet gate w i d t h  w  space charge region depth  n  w  relaxation parameter  r  x,y  spatial coordinates  X{,yj  spatial position at the  Xi t  active layer depth  n  i,j  t h  g r i d point  coordinate of the m a x i m u m potential x  wafer thickness  x,y  x, y unit vectors  max  x  ,  rr-component or y-component  y  Umax  p i x e l length  VmLtV-mR  transport electrode endpoints  Y  distributed shunt admittance  z  complex variable  Z  distributed series impedance  8  diode ideality factor  Sjg p  relative position along the gap  Sui  normalized potential difference between the F e r m i  a  max  level a n d the intrinsic energy level at St  time increment  Sx  g r i d spacing i n the x - d i r e c t i o n  Sx  u n i f o r m active layer s u b d o m a i n g r i d spacing  a  Sx{,Sy  g r i d spacings at the  fitpmax  charge confinement  ^Ci+i  correction factor viii  i,j  t h  g r i d point  dielectric constant of G a A s charge transfer efficiency propagation constant electron drift m o b i l i t y low-field electron drift m o b i l i t y electron drift velocity r a d i a n frequency electrostatic potential m a x i m u m potential m i n i m u m value of the m a x i m u m potential c e r m e t / G a A s Schottky barrier height m e t a l / G a A s Schottky barrier height m e t a l / G a A s j u n c t i o n potential electron quasi-Fermi potential length constant electron transit time surface potential phase shift electron drift velocity function fractional constant b i n a r y variable finite difference variable ordinary iterated solution for Cij./t+i relaxed solution for Ci,j,k+i  ix  List of Tables I  A s u m m a r y of the c e r m e t / G a A s Schottky barrier parameters  II  T h e d i s t r i b u t e d circuit parameters of the c e r m e t / G a A s contact  III  T h e lengths used i n the two-dimensional computer m o d e l  40  IV  A s u m m a r y of the equations used i n the C M C C D m o d e l  50  V  T h e measured threshold voltages of the G a A s C M C C D  68  VI  T h e signal levels applied to the G a A s C M C C D for operation at 100 M H z 69  VII  T h e charge transfer efficiencies of the G a A s C M C C D for 100 M H z operation  x  26 . . .  31  72  List of F i g u r e s 1.1  T h e cross-sectional views of a single p i x e l of a G a A s C G C C D a n d a GaAs C M C C D  3  A cross-sectional view of the 4-phase G a A s C M C C D w i t h representative signal levels applied to the device nodes  7  T h e theoretical generation of a charge packet at the i n p u t section of a G a A s C M C C D using the diode cutoff method  9  T h e theoretical transfer of a charge packet t h r o u g h one p i x e l of a 4-phase GaAs C M C C D  11  2.4  T h e output sequence of a G a A s C M C C D  13  2.5  T h e abrupt charge a p p r o x i m a t i o n used i n the one-dimensional analysis of the potential tp(x) underneath the center of a C M C C D transport electrode  14  T h e potential d i s t r i b u t i o n under a C M C C D transport electrode for a fixed surface potential a n d a variable signal charge density  16  T h e potential d i s t r i b u t i o n under a C M C C D transport electrode for an e m p t y well c o n d i t i o n a n d a variable surface potential  16  T h e two electrode model for the charge storage mode w i t h i n a G a A s CMCCD  18  T h e c e r m e t / G a A s Schottky barrier diode used to investigate the barrier properties of the c e r m e t / G a A s j u n c t i o n  21  T h e dc current-voltage characteristic of the c e r m e t / G a A s Schottky barrier diode  21  T h e distributed resistive gate Schottky barrier diode model of the cerm e t / G a A s Schottky barrier diode  22  A differential length of the distributed resistive gate Schottky barrier diode m o d e l  22  T h e c e r m e t / G a A s transmission line model of the u n i f o r m c e r m e t / G a A s contact w i t h i n an interelectrode gap of a G a A s C M C C D  27  3.6  A differential length of the c e r m e t / G a A s transmission line m o d e l . . . .  27  3.7  T h e variation of the normalized surface potential along the gap of a G a A s C M C C D as a f u n c t i o n of frequency  30  T h e v a r i a t i o n of the normalized h i g h frequency surface potential along the gap of a G a A s C M C C D as a f u n c t i o n of the ratio CD/CCM  33  2.1 2.2 2.3  2.6 2.7 2.8 3.1 3.2 3.3 3.4 3.5  3.8  xi  3.9  T h e test structure a n d the test circuit used to demonstrate the validity of the transmission line model for the c e r m e t / G a A s contact w i t h i n a gap of a G a A s C M C C D  33  3.10 T h e a m p l i t u d e response a n d the phase response of the c e r m e t / G a A s test circuit  35  3.11 T h e l u m p e d equivalent circuit of the c e r m e t / G a A s transmission line test circuit  36  3.12 T h e theoretical a m p l i t u d e response of the c e r m e t / G a A s test circuit . . .  37  3.13 T h e theoretical phase response of the c e r m e t / G a A s test circuit  37  4.1  T h e unit cell for modeling the G a A s C M C C D  39  4.2  T h e m a i n finite difference g r i d for the G a A s C M C C D model  39  4.3  T h e nine-point c o m p u t a t i o n a l kernel  42  4.4  T h e nonlinear electron velocity-field characteristic of G a A s  48  4.5  T h e flow d i a g r a m for the two-dimensional computer simulations  53  4.6  T h e m a x i m u m frequency of operation of a 4-phase G a A s C M C C D as a function of the clock voltage a m p l i t u d e a n d the transport electrode length 55  4.7  T h e simulated single electrode transfer of a charge packet  56  4.8  T h e theoretical charge transfer efficiency as a function of time for the G a A s C M C C D obtained f r o m the simulated single electrode transfer of a charge packet  57  5.1  A m i c r o p h o t o g r a p h of the fabricated G a A s C M C C D  60  5.2  A m i c r o p h o t o g r a p h of the i n p u t section of the G a A s C M C C D  61  5.3  A m i c r o p h o t o g r a p h of the output section of the G a A s C M C C D  61  5.4  A transmission electron microphotograph of the C r : S i O ( 45 w t . % C r ) film  64  A m i c r o p h o t o g r a p h of the plasma etch profile of a 5 m i c r o n square v i a etched t h r o u g h a 1.8 m i c r o n thick p o l y i m i d e f i l m  64  6.1  T h e w i r e - b o n d i n g configuration for packaging the G a A s C M C C D  . . . .  67  6.2  T h e insertion loss of the on-chip G a A s M E S F E T source follower amplifier measured f r o m 300 k H z to 200 M H z  67  T h e qualitative demonstration of the performance of the G a A s C M C C D for 100 M H z operation  70  T h e impulse response of the G a A s C M C C D for 100 M H z operation . . .  70  5.5  6.3 6.4  xii  6.5  T h e insertion loss of the G a A s C M C C D for 100 M H z operation  6.6  T h e theoretical insertion loss of the G a A s C M C C D for 100 M H z operation 71  A.l  A cross-sectional view of the B N L Experiment 787 rare kaon decay spectrometer  85  T h e ideal range stack p h o t o m u l t i p l i e r tube output waveform for a p i o n to m u o n to electron decay sequence  86  A.2 A.3  71  A system block diagram of the d a t a acquisition system e m p l o y i n g a GaAs C M C C D  86  A.4  A G a A s C M C C D operating i n the frequency compression mode  87  E.l  T h e schematic d i a g r a m of the test circuit used to operate the C M C C D i n the V H F b a n d  97  xiii  Acknowledgements  I w o u l d like to express m y thanks to m y thesis supervisor, Professor Lawrence Y o u n g , for his assistance a n d guidance throughout this work. I w o u l d also like to thank P r o fessor D a v i d L . Pulfrey, Professor R i c h a r d R . Johnson a n d M r . J o h n V . Cresswell for co-supervising this project. M y deepest thanks go out to M r . J o h n V . Cresswell, w h o is a Senior Research Engineer at T R I U M F , for i n t r o d u c i n g me to the b r o a d subject of G a A s C C D s . M r . Cresswell a n d I have spent many long hours exploring the nuances of these devices. He has m y heartfelt thanks for his invaluable assistance throughout this project. A special note of thanks to D r . R . Sahai of R o c k w e l l International C o r p o r a t i o n for the discussions that we h a d on the various points of interest that we share on the subject of G a A s C C D s .  T h e knowledge gained f r o m these discussions proved to be  quite beneficial. There are a number of people that I a m indebted to for their help dealing w i t h the marry technical aspects concerning this project: R a y m o n d B u l a , M i l e s Constable, H i r o s h i K a t o , Y v o n n e Langley, Peter L e N o b l e , M i c h a e l LeRoss, Tony Leugner, N a o m i Shibaoka, Peter Townsley a n d D a v i d Webster. I would also like to thank m y colleagues i n the S o l i d State G r o u p w i t h i n the Department of E l e c t r i c a l Engineering at U B C for the assistance they have given me. F i n a l l y , to m y beautiful wife, Eveline, for having the patience to endure—I express my love a n d appreciation.  xiv  Chapter 1  Introduction  T h e original proposal of a G a A s charge-coupled device was made i n 1972 by Schuermeyer et a l [1],  T h e transport electrodes of the proposed C C D structure consisted  of m e t a l / s e m i c o n d u c t o r Schottky barriers instead of the M O S structures c o m m o n l y used w i t h silicon C C D s . T h e Schottky barrier C C D has the advantage that it can be fabricated on G a A s using well developed G a A s M E S F E T fabrication techniques. T h i s is i m p o r t a n t , as w i l l be described later, for the monolithic integration of G a A s C C D s w i t h a u x i l i a r y G a A s M E S F E T support circuits. A 3-phase G a A s capacitive gate C C D ( C G C C D ) employing m e t a l / G a A s Schott k y barriers separated by narrow ( approximately 1 m i c r o n long ) dielectric filled i n terelectrode gaps was demonstrated i n 1977 by K e l l n e r et al [2], a n d shortly thereafter i n 1978 by D e y h i m y et al [3].  O n e , two a n d four-phase G a A s C G C C D s have  been p r o d u c e d a n d operated since the i n i t i a l demonstrations of the 3-phase G a A s C G C C D [4,5,6].  G a A s C C D s have wider operating b a n d w i d t h s t h a n silicon C C D s  as a result of the approximately five times greater electron m o b i l i t y w i t h i n G a A s at low to moderate electric fields. A l i m i t a t i o n for the high frequency operation of a silicon C C D is a t t r i b u t e d to the lower b a n d w i d t h s a n d greater power requirements of the support circuits integrated w i t h the device [1,7]. T h e highest clock frequency that a silicon C C D has been operated at, that this author is aware of, is 180 M H z w h i c h was reported by Esser a n d Sangster [8]. In comparison, a G a A s C M C C D was operated at 4.2 G H z as demonstrated by Sovero et al [9]. T h e best reported charge transfer efficiencies ( C T E s ) attained by G a A s C C D s approach 0.9999 for C C D clock frequencies l y i n g between 1.0 M H z a n d 1.0 G H z [10,9]. T h i s level of performance has made the G a A s C C D desirable for signal processing applications that extend into the U H F b a n d 1  ( 0.30 G H z - 1 . 1 2 G H z ) [11,12,13,14,15,16,17]. A l i m i t a t i o n that is encountered w i t h using G a A s C G C C D s i n U H F signal processing applications arises f r o m the difficulty of m o n o l i t h i c a l l y integrating G a A s C G C C D s w i t h G a A s M E S F E T s . T h i s difficulty is a consequence of the different active layer requirements of the two devices. A G a A s C G C C D is t y p i c a l l y fabricated on a 1-2 m i c r o n deep n-type e p i t a x i a l layer grown on a n~-buffer on a semi-insulating G a A s substrate. T h e donor density for the n-type epitaxial layer is chosen to lie w i t h i n the range f r o m 10  15  cm  - 3  to 1 0  16  cm  - 3  to m a i n t a i n a reasonable pinch-off voltage suitable for the high  frequency operation of the C G C C D . T h e active layer parameters described above for the G a A s C G C C D are not o p t i m a l for a t y p i c a l G a A s M E S F E T w h i c h requires a thinner, more h i g h l y doped active layer. It is i m p r a c t i c a l to grow segregated regions of doped G a A s for C G C C D s a n d M E S F E T s using epitaxy, as the present e p i t a x i a l growth methods are not well suited to this task [18]. I o n - i m p l a n t a t i o n w o u l d y i e l d selectively doped regions of G a A s , b u t possesses some difficulty p r o v i d i n g the deep active layers required for the fabrication of a G a A s C G C C D [19]. T h e performance of a G a A s C G C C D i n a signal processing a p p l i c a t i o n is further l i m i t e d by the charge transfer loss that arises f r o m the f o r m a t i o n of energy troughs w i t h i n the active layer bounded on the G a A s surface by the interelectrode gaps [7,20]. A n energy t r o u g h w i t h i n a gap of a G a A s C G C C D has a m i n i m u m electron energy less t h a n that of the regions under the transport electrodes adjacent to the gap. T h i s energy t r o u g h w i l l f o r m w i t h i n a gap of a G a A s C G C C D as a result of a non-monotonic surface p o t e n t i a l d i s t r i b u t i o n along the gap [7].  D u r i n g the charge transfer process,  the energy t r o u g h captures a quantity of charge f r o m a charge packet as it passes t h r o u g h the region of m i n i m u m energy. T h e captured electrons are transferred to the C G C C D o u t p u t at a later time or lost t h r o u g h recombination resulting i n increased signal dispersion. D e y h i m y et al [7] used a two-dimensional electrical analog for the G a A s C G C C D to show that the energy trough w i t h i n a gap of a G a A s C G C C D was 2  Dielectric  Active  layer  N *i B  10  fj,m  Y*-<1.0  \  cm  ^1.0  J  fxm  CGCCD  Cermet  Active  -»-|  layer  iV^Ra 10  cm  \*->1.0  t  <1.0  I  nm  fj.m  CMCCD  Figure 1.1: The cross-sectional views of a single pixel of a GaAs CGCCD and a GaAs C M C C D illustrating the basic physical differences between the two devices.  considerably reduced when the gap length was decreased. It was demonstrated that a gap length of less than 1 micron would result in the formation of a minimal energy trough in a GaAs CGCCD having a 2 micron deep active layer uniformly doped with a donor density of 1 • 10  16  cm . -3  The GaAs C M C C D demonstrated in 1982 by Higgins et al overcomes the practical limitations of using a GaAs CGCCD in a signal processing application [18]. The crosssectional views of a single pixel of a GaAs CMCCD and a GaAs CGCCD are shown in Figure 1.1. The C M C C D is fabricated on a GaAs M E S F E T compatible active layer enabling the C M C C D to be monolithically integrated with MESFET support circuitry. The transport electrodes of a C M C C D are thinner than the transport electrodes of a C G C C D and are separated by a wider interelectrode gap. A thinner transport electrode provides an increased tangential electric field component under the electrode, resulting in improved charge transfer within the CMCCD [18,21]. The wider interelectrode separation of the C M C C D provides a considerable reduction in the dimensional tolerance  3  required to fabricate the device. T h e G a A s surface comprising the interelectrode gaps of a G a A s C M C C D is encapsulated w i t h a cermet f i l m . T h e c e r m e t / G a A s j u n c t i o n was demonstrated i n 1974 b y W r o n s k i et al [22] to f o r m a Schottky barrier w i t h a series gate impedance comprised of a parallel resistance a n d capacitance. D u r i n g the operation of the C M C C D , a current w i l l flow f r o m a transport electrode t h r o u g h the cermet film to a n adjacent transport electrode w h e n a voltage difference exists between the two electrodes.  T h e current  t h r o u g h the cermet film establishes a potential d i s t r i b u t i o n along the c e r m e t / G a A s j u n c t i o n that varies monotonically i n the direction of flow. W a l d e n et al [23] determ i n e d , u s i n g two-dimensional computer m o d e l i n g , that a monotonic surface potential variation across each of the interelectrode gaps of a C C D w i l l prevent the f o r m a t i o n of energy troughs w i t h i n the device. T h i s result suggests that a C M C C D w i l l have reduced charge transfer loss d u r i n g operation, and consequently w i l l exhibit i m p r o v e d performance. T h e performance of the G a A s C M C C D has been demonstrated i n two signal processing applications: a h i g h speed G a A s detector a r r a y / C M C C D multiplexer [24] a n d a G a A s V H F / U H F agile bandpass filter [25]. These applications have been developed by a group at the R o c k w e l l International Microelectronics Research a n d Development Center ( T h o u s a n d O a k s , C a l i f o r n i a ) and are the only demonstrated applications of G a A s C C D s that this author is aware of. T h e h i g h speed G a A s detector a r r a y / C M C C D multiplexer has been developed for an acousto-optic s p e c t r u m analyzer. In this a p p l i c a t i o n , an array of t h i r t y - t w o G a A s photodiodes are m u l t i p l e x e d by a 64-pixel, 4-phase G a A s C M C C D using a side-feed arrangement. T h e photodiodes a n d the C M C C D are interconnected by a gating circuit provided by G a A s M E S F E T s integrated on-chip. T h e currents generated b y each of the photodiodes are integrated onto h o l d capacitors p r o d u c i n g t h i r t y - t w o discrete charge packets. T h e charge packets are injected into the C M C C D channel i n a parallel manner 4  using the M E S F E T gating circuit a n d are subsequently transferred to the  CMCCD  output for further processing. R e a l - t i m e signal processing w i t h this device has been demonstrated using a C M C C D clock frequency of 1.0 G H z . T h e G a A s V H F / U H F agile bandpass filter has been developed for frequency selective filtering applications. T h i s device employs G a A s C M C C D s arranged i n a pipeorgan structure to provide weighted sampling, programmable delay a n d s u m m i n g of analog signals.  Supervisory functions are provided b y G a A s M E S F E T circuits i n -  tegrated m o n o l i t h i c a l l y w i t h the C M C C D s .  T h e C M C C D agile bandpass filter has  demonstrated i n excess of 60 d B of d y n a m i c range for lowpass, bandpass a n d highpass filter operations u s i n g a 1.0 G H z i n p u t sampling rate [26].  T h e two G a A s  CMCCD  applications described above have demonstrated the ability to m o n o l i t h i c a l l y integrate the C M C C D w i t h other circuits to provide sophisticated signal processing functions. A G a A s C M C C D signal processing system is presently being developed at T R I U M F ( T r i - U n i v e r s i t y M e s o n Facility, Vancouver ) to satisfy the i n s t r u m e n t a t i o n requirements for a nuclear physics e x p e r i m e n t — B N L E x p e r i m e n t 787.  A 64-pixel, 4-  phase G a A s C M C C D comprises an essential part of a w i d e b a n d d a t a acquisition system capable of recording 250 M H z b a n d l i m i t e d analog signals.  T h e nuclear physics  experiment a n d the G a A s C M C C D w i d e b a n d d a t a acquisition system are described i n Appendix A . T h e purpose of this work is to provide a theoretical a n d p r a c t i c a l development of a 64-pixel, 4-phase G a A s C M C C D . T h e design, implementation a n d evaluation of this device are described i n the following chapters.  5  Chapter 2 Theory  2.1  P r i n c i p l e of O p e r a t i o n  A G a A s C M C C D functions as a programmable delay line. T h e i n p u t signal is applied to the i n p u t o h m i c contact ( I / O ) a n d is sampled by the i n p u t section at fixed time intervals p r o d u c i n g a sequence of discrete charge packets.  T h e charge packets are  sequentially injected into the C M C C D transport region where they are transferred to the output o h m i c contact ( 0 / P ) under the control of the quadrature clocks. A t the output section of the C M C C D , the charge packets are converted to an analog signal corresponding to the original i n p u t signal delayed by an amount of time  Uel  =  (2.1)  J Jc  where t^i is the delay time assuming ideal operation, p is the number of pixels comprising the C M C C D a n d f  c  is the C M C C D clock frequency. A cross-sectional view of  a 4-phase G a A s C M C C D w i t h representative signal levels applied to the device nodes is shown i n F i g u r e 2.1. T h e signals applied to the nodes of the C M C C D are engaged i n a sequential manner d u r i n g the i n i t i a l start-up of the device.  T h e b o t t o m surface of the semi-  insulating G a A s substrate, the input ohmic contact a n d the output o h m i c contact are biased first, to the reference potential of 0 volts. N e x t , the control gates G i , G and G  3  2  are biased negatively w i t h respect to the ohmic contacts by amounts that are  less t h a n or equal to the pinch-off voltage of the C M C C D active layer ( t y p i c a l l y -2.0 volts ), depleting the volume of semiconductor under these gates. T h e quadrature clocks w i t h voltage levels of 0 volts a n d -5 volts are subsequently applied to the  CMCCD,  transferring the remaining electrons w i t h i n the channel to the output of the device 6  •*HH*~  Time  displacement  R/G  R/S  B/D Ph. 2 G  Ph. 1  Ph.  Ph.  3  4 Ph.  4  e  I/P B/O TJJU  rm  —  TJJJInnr  CMCCD Active Layer  Output  Input  B/S  Figure 2.1: A cross-sectional view of the 4-phase GaAs C M C C D with representative signal levels applied to the device nodes. 7  where they are removed. T h e i n p u t signal is applied to the i n p u t o h m i c contact once the fully depleted c o n d i t i o n is achieved w i t h i n the channel. Charge packet generation a n d injection at the input section of the G a A s C M C C D is accomplished using the diode cutoff method developed by Sequin a n d M o h s e n [27]. T h e i n p u t signal is ac coupled to the input ohmic contact a n d offset by a positive dc bias, resulting i n an applied input signal ranging positively f r o m 0 volts. T h e phase 3 a n d phase 4 clock signals are ac coupled to the two i n p u t control gates G i a n d G , 2  ensuring synchronization between the generation of the charge packet at the i n p u t section a n d the subsequent injection of the charge packet into the transport region. A n apparent negative time delay is added to the phase 3 clock signal a p p l i e d to the i n p u t control gate G i to m i n i m i z e forward charge injection d u r i n g the charge packet generation sequence. T h e negative time delay is achieved by delaying the phase 3 clock signal a p p l i e d to G i by a positive amount equal to the clock period less a s m a l l t i m e , displacement ( typically 0.5 nanoseconds for a 10 nanosecond clock p e r i o d ). Figure 2.2 illustrates the theoretical sequence of events that occur to create a discrete charge packet under the control gate G  2  using the diode cutoff m e t h o d . Initially,  the active layer extending f r o m the v i c i n i t y of the r i g h t - h a n d edge of the input ohmic contact to the right-hand edge of G  2  is depleted of electrons.  Electrons flow into the  potential well formed under G i f r o m the i n p u t ohmic contact d u r i n g the positive transition of the delayed phase 3 clock applied to G j .  A steady-state c o n d i t i o n for the  electron density d i s t r i b u t i o n w i t h i n the potential well under Gi is achieved d u r i n g the intervening time p r i o r to the positive transition of the phase 4 clock applied to G . Elec2  trons f r o m the i n p u t o h m i c contact and f r o m the potential well formed under G i flow into the potential well formed under G clock applied to G . 2  2  d u r i n g the positive transition of the phase 4  A steady-state electron density d i s t r i b u t i o n is achieved w i t h i n  the composite potential well formed under b o t h of the i n p u t control gates d u r i n g the remainder of the positive half cycle of the delayed phase 3 clock. D u r i n g the negative 8  I/p  1 1:  2  I/P  3  4  1  G,  iiiiiiiiiiiiiiiiiiiiiiurr  Depleted  Figure 2.2: The theoretical generation of a charge packet at the input section of a GaAs C M C C D using the diode cutoff method [27]. 9  t r a n s i t i o n of the delayed phase 3 clock applied to G i , the electrons residing w i t h i n the potential well formed under this gate are swept out t h r o u g h the i n p u t o h m i c contact leaving a discrete charge packet w i t h i n a potential well residing under the control gate G . T h e m a g n i t u d e of the localized charge packet w i t h i n this potential well is a function 2  of the active layer d e p t h , the active layer donor density d i s t r i b u t i o n a n d the voltage difference between the i n p u t ohmic contact a n d the control gate G  2  at the time of the  negative t r a n s i t i o n of the delayed phase 3 clock applied to the control gate G i . A f u l l well of charge is produced under the control gate G  2  when this voltage difference is at  its m i n i m u m value ( approximately 0 volts ). T h e charge packet residing i n the potential well formed under G  2  is subsequently  injected into the transport region where it is transferred to the o u t p u t o h m i c contact. T h e injection of the charge packet into the transport region occurs on the positive t r a n s i t i o n of the phase 1 clock. F i g u r e 2.3 illustrates the theoretical transfer of a charge packet t h r o u g h one pixel of a 4-phase G a A s C M C C D . A s i n d i c a t e d i n F i g u r e 2.3, the charge packet occupies a potential well that spans two transport electrodes d u r i n g the transfer process. T h i s is a consequence of the 4-phase clocking scheme that is used. T h e directionality of charge m o t i o n is achieved by the tangential electric fields that arise w i t h i n the C M C C D channel as a result of the differences between the clock voltage levels applied to the transport electrodes. T h e clocking scheme chosen for operating the C M C C D is i m p o r t a n t as it determines the complexity of b o t h the C M C C D and the clock circuits. T h e 4-phase structure was chosen for the C M C C D as it has the advantage of not requiring ' b u i l t - i n ' directionality of charge m o t i o n , resulting i n reduced fabrication requirements to produce the device. T h e reduced fabrication requirements for p r o d u c i n g the 4-phase C M C C D are made at the expense of increased clock circuit complexity. T h e quadrature clocks that are needed to operate the 4-phase C M C C D require a modest level of circuit design sophistication to achieve the wide bandwidths necessary to operate the device i n 10  1  Ph. 2  / i  Ph. 3  2 i  Ph. 4 Ph. 1  I  Ph. 1  Ph. 2  Transfer  I  Ph. 3  Ph. 4  Ph. 1  EKXXX>J  rVx-x^yi  direction  F i g u r e 2.3: T h e theoretical transfer of a charge packet t h r o u g h one p i x e l of a 4-phase GaAs C M C C D . 11  the U H F b a n d . T h e key issue is the difficulty o b t a i n i n g stable w i d e b a n d 90° phase shifts between successive clock phases. T h e current G a A s M E S F E T integrated circuit technology is capable of p r o v i d i n g a solution to this difficulty [12,13,15,16]. A charge packet transferred to the potential well residing under the final phase 4 transport electrode of the G a A s C M C C D is transmitted to the o u t p u t o h m i c contact on the negative excursion of the phase 4 clock.  T h e output o h m i c contact of the  C M C C D is precharged to 0 volts during the positive half cycle of the phase 3 clock using the external reset G a A s M E S F E T . T h e reset M E S F E T is disabled d u r i n g the negative half cycle of the phase 3 clock allowing the output o h m i c contact to float at its precharged value. T h e electrons passing t h r o u g h the potential well f o r m e d under G 3 exit the C M C C D t h r o u g h the floating output ohmic contact, charging the parasitic capacitance Co/p a n d d r i v i n g the output ohmic contact voltage negatively w i t h respect to its precharged value. A f u l l well of charge a r r i v i n g at the o u t p u t o h m i c contact w i l l drive the o u t p u t o h m i c contact voltage to its most negative level. T h e signal produced at the o u t p u t o h m i c contact is buffered f r o m the external output signal processing 0  c i r c u i t r y using a M E S F E T source follower amplifier integrated m o n o l i t h i c a l l y w i t h the C M C C D . There is usually some distortion observed i n the signal obtained f r o m the output of the source follower amplifier w h i c h is a consequence of the passive feedthrough of the quadrature clocks to the output ohmic contact. T h i s is reflected as a level change at each occurrence of a clock transition and is illustrated i n the output sequence shown i n F i g u r e 2.4. 2.2  One-dimensional Potential Distributions  T h e one-dimensional solution of Poisson's equation for the potential d i s t r i b u t i o n u n derneath the center of a C M C C D transport electrode, perpendicular to the surface, was determined using the abrupt charge a p p r o x i m a t i o n shown i n F i g u r e 2.5.  T h e charge  d i s t r i b u t i o n i l l u s t r a t e d i n Figure 2.5 is similar to the one used by Hansell [20] w i t h the 12  Q to  0/P  s  Ph. 4 Floating  R/G  Precharge  B/0 — Empty well Feedthrough Signal  — Full  displacement  due to  R/G  R/S  well Q /C s  0//p  B/D  CMCCD Output:  Ph. 4 ivwwza  o/p  O/P  fwvwza  —  B/0  B/S  Figure 2.4: The output sequence of a GaAs CMCCD. The signal obtained from the output of the source follower amplifier includes the effects of passive feedthrough from the clocks to the output ohmic contact.  13  1 — dL Charge  Abrupt Active  O.O  Dercsxty  layer  to  ,  SI  DC  Profile  ,,  substrate  int  Distance  xrxto  GaAs:  cc  Figure 2.5: The abrupt charge approximation used in the one-dimensional analysis of the potential ip(x) underneath the center of a C M C C D transport electrode.  exception that the depth of the space charge region is variable. Poisson's equation for the illustrated charge distribution is d ip(x) _  qN  2  D  (2.2)  0 < x < w„  dx*  for the space charge region within the active layer and d xp{x) 2  dx  2  0  (2.3)  w < x < x n  r  for the quasi-neutral region within the active layer and for the semi-insulating substrate. The signal charge \Q \ = qN (x -w ) s  D  int  (2.4)  n  resides within the channel defined by the quasi-neutral region w  n  < x < x, . n<  Here  xp(x) is the potential, No is the uniform active layer donor density, q is the charge of 14  an electron, e is the dielectric constant of G a A s , X{ is the active layer depth, x  max  ni  is  the wafer thickness a n d w is the depth of the space charge region under the transport n  electrode. T h e potential ip(x) is related to the intrinsic energy Ej(x) w i t h i n the G a A s by the relationship —qip(x) — Ei(x) —  Ei(x ). max  T h e b o u n d a r y conditions are described below. T h e surface potential is equal to the potential difference V between the F e r m i level at the surface a n d the F e r m i level g  at the b o t t o m of the substrate less the potential difference between the m e t a l / G a A s Schottky barrier height at the surface </>BM,O a n d the m e t a l / G a A s Schottky barrier height at the b o t t o m of the substrate V>(0) = %l> =  V  0  g  (f>BM,x x ma  -  (<f>  - <t>BM,x )  BMfi  max  ;  (2.5)  the potential a n d the electric field across the interface at x = w are continuous n  i>(w _)  V(^n )  =  n  ,  +  dx  (2.6) (2.7)  dx  and the reference energy level is  Ei(x ). max  T h e solutions for equation 2.2 a n d equation 2.3 using the b o u n d a r y conditions 2.5,.. .,2.7 are qN x  (qN w x  2  D  D  ^( ) x  =  «  n  qN w 7)—" D  max  +  \  2  n  ^  \  x  + *Po /  0< x< w  n  . (2.8)  x  max  and // \ (QNpwl ip(x) = - I — \  \ h ip I  qN w  x  D  1  0  2 n  h tyo w <x< n  / X  x  max  .  (2.9)  Z6  max  T h e potential variation h o l d i n g the surface potential ipo constant at zero volts a n d varying the magnitude of the signal charge \Q \ w i t h i n the C M C C D is shown i n F i g u r e 2.6 S  and s i m i l a r l y the potential variation for the empty well condition \Q \ = 0 w i t h i n the S  C M C C D a n d v a r y i n g the surface potential is shown i n F i g u r e 2.7. T h e peak poten15  f — d. P'otential  Distributions  Distance  into  GaAs:  oc  Figure 2.6: The potential as a function of position underneath the center of a C M C C D transport electrode. The surface potential ^ held constant at zero volts and the signal charge density is varied. 0  f — d Potentxal  1 S  Distributions  Distance  irtto  GaAs:  ac  Figure 2.7: The potential as a function of position underneath the center of a C M C C D transport electrode. The signal charge density is held at the empty well condition \Q \ = 0 and the surface potential i p is varied. S  0  16  t i a l w i t h i n the C M C C D active layer occurs near the depletion region b o u n d a r y at the location  x  m  T h e corresponding m a x i m u m potential "4>  ZQi\f)X  \  max  2.3  X  %max'  max  at this location is  y  max  ^X  max  •^max J  A c t i v e L a y e r Specification  T h e design of a G a A s C M C C D requires choosing an active layer depth  and an  active layer donor density No that are compatible w i t h G a A s M E S F E T s . T h e pinchoff voltage of a t y p i c a l n-type depletion mode G a A s M E S F E T usually lies between -3 volts a n d -1 volt constraining the pinch-off voltage of the C M C C D active layer to lie between these two values.  T h e active layer parameters X{  a n d No for the  ni  G a A s C G C C D were determined by D e y h i m y et a l [7] a n d by H a n s e l l [20] to provide a predetermined m a x i m u m potential  i p  m  a  x  T h e active layer parameters of the G a A s  .  C M C C D were determined using a new m e t h o d w h i c h simultaneously maximizes the charge confinement a n d the signal charge capacity. Consider the two adjacent transport electrodes of a C M C C D i l l u s t r a t e d schematically i n F i g u r e 2.8. U n d e r the left-hand electrode resides a f u l l well of charge as defined by equation 2.4 w i t h w  n  equal to a s m a l l fraction </? « 0 of the active layer d e p t h = qN (l  \Qs,max\  D  ~ <p)x  int  « qN X D  int  Xi  nt  (2.12)  .  Furthermore, assume that the left-hand electrode is biased to the most positive clock voltage level such that  ipo,ieft  = 0 volts. T h e resultant m a x i m u m potential under this  electrode using equation 2.11 is  ^.-Wl-P^ ^)^**!) 2  \  ^Zmax  x  17  max  I  .  (2.13)  int max, left  max, right  Figure 2 . 8 : The two electrode model for the charge storage mode within a GaAs CMCCD. A full well of charge resides in the potential well formed under the left-hand electrode. The two electrodes are biased such that 4> x,ieft > *J>max,ri h.t • ma  g  Similarly, an empty well resides under the right-hand electrode which is biased to the most negative clock voltage level such that  rpo,ri ht g  =  V'min <  volts. The maximum  0  potential that results under the right-hand electrode using equation 2 . 1 1 is I Vmax.right  '• _ „/, i ^min ~ V>min + „ „ , ^y U^max 2  €t  JY  ' , (1 I1 + N  ~ . \Nint' l min ./. int ~ I— •L'm.a.x' •''max X  x  l ,  .  /( X i n t + I 1 ~ 2 max \ 1  x  _|_  x  ]  nt  mi  \ qN x] D  I  maxJ  X  *s  %nt  2f (2.14)  The configuration described above corresponds to the case where a charge packet is confined to a potential well residing under the left-hand electrode as a result of a blocking voltage applied to the right-hand electrode. It is necessary that il>max,ri ht g  for charge confinement.  potential difference between  The charge confinement  rp x,ieft ma  and  8tp  max  t/Wx.ie/t  is defined as the  The following equation for  ip ,rightmax  >  6tp x ma  is obtained from equations 2 . 1 3 and 2 . 1 4  Hmax  =  lj>max,Uft  ~ ^max,right  where it has been assumed that  «  x  max  (|Vw|  ~  \V \) ~ p  (|Vw|  ~  ^  )  (2-15)  and  ^> X{ t n  2 \V \ =  <  P  18  | ^ , n |  (2-16)  is the m a g n i t u d e of the pinch-off voltage for a uniformly doped n-type active layer [28]. T h e following relationship for the m a x i m u m signal charge density \Q , \ oc 1 ^ 1  (2.17)  s max  is obtained f r o m equation 2.12 a n d equation 2.16. T h e values of X{ a n d Nrj for the G a A s C M C C D are determined using the design nt  equations 2.15, 2.16 a n d 2.17 w i t h the assumption that the pinch-off voltage V is a conp  stant value. T o simultaneously m a x i m i z e the charge confinement 6ip charge capacity  Q ,max s  max  a n d the signal  it is necessary to use a t h i n , h i g h l y doped active layer. E q u a -  tions 2.15 a n d 2.17 indicate that 8tp the active layer depth #;  n<  max  and Q  Stmax  approach m a x i m u m values when  approaches a m i n i m u m value. U n d e r the assumption of a  constant pinch-off voltage, equation 2.16 indicates that the active layer donor density No approaches a m a x i m u m value when the active layer depth approaches a m i n i m u m value. T h e above qualitative analysis supports one of the p r i n c i p a l advantages of a G a A s C M C C D , the signal charge capacity a n d the charge packet confinement are o p t i m u m for devices fabricated on active layers that are suitable for M E S F E T s . T h e G a A s C M C C D s that were fabricated as part of this research u t i l i z e d epi-wafers possessing a u n i f o r m active layer donor density of 4.5 • 1 0  16  cm  - 3  a n d an active layer depth of  0.25 microns, corresponding to a pinch-off voltage of approximately -2.0 volts.  19  Chapter 3 The C e r m e t / G a A s Junction  3.1  Barrier Properties  A distributed resistive gate Schottky barrier ( S B ) diode model is described i n this sect i o n . T h i s m o d e l was used to determine the barrier properties of the c e r m e t / G a A s junction f r o m the measured dc current-voltage characteristic of a fabricated c e r m e t / G a A s SB-diode. T h e experimental current-voltage measurements were conducted w i t h the planar c e r m e t / G a A s S B - d i o d e illustrated i n F i g u r e 3.1. T h e diode consisted of a C r : S i O ( n o m inal 45 w t . % C r ) cermet gate attached at one end to a gold contact, a A u - G e / N i / A u o h m i c contact separated f r o m the cermet gate by a 5.0 m i c r o n gap a n d a n active layer possessing a donor density of 4.5 • 1 0  16  cm  - 3  to a depth of 0.25 microns. T h e dc current  t h r o u g h the diode as a function of the applied dc voltage difference between the gold cermet gate contact a n d the ohmic contact was measured using a H e w l e t t - P a c k a r d H P 4145A Semiconductor Parameter A n a l y z e r connected to a W e n t w o r t h probe station. T h e measurements were conducted i n the dark to m i n i m i z e photocurrent  generation  w i t h i n the diode. T h e diode current I{ measured for discrete i n p u t voltages V{ l y i n g n  n  between -5 volts a n d +5 volts is shown i n F i g u r e 3.2. It is apparent u p o n inspection of Figure 3.2 that the c e r m e t / G a A s S B - d i o d e exhibits rectification properties similar to that of a m e t a l / G a A s S B - d i o d e possessing a large series gate resistance. T h e dc operation of the c e r m e t / G a A s S B - d i o d e illustrated i n Figure 3.1 is modeled using the distributed resistive gate S B - d i o d e m o d e l shown i n F i g u r e 3.3. T h e resistor Rs is the series resistance ( ohms ) between the gold cermet gate contact a n d the active region of the diode plus the series resistance of the bulk G a A s , a n d RCM is the distributed resistance ( o h m s / u n i t length ) of the cermet 20  film.  u  1  Ohmic  g  Contact  1  ^^^^^^Cermet  Gold Contact  o >^  vy,  o  ^  ^^^^^^^^^  •«  0.01  cm  ^  —  5.0  fim  1  Active Isolated  16  N =4.5 D  10  Layer Isolated  —"3  cm  x =0.25 int  fim  Figure 3.1: The cermet/GaAs Schottky barrier diode used to investigate the barrier properties of the cermet/GaAs junction. I— V  16 14  Characteristic  1 =3.6 rtA 0  12  (3 =1.17  ^10  <j> =0.64 volts Bc  a  M eaaured, data °  6 Co Q5  o  3  Theory  4 2 0 -2 -6  a  a  a  -4  a  a  a  a  u  -2 0 2 Bias Voltage (V)  4  6  Figure 3.2: The dc current-voltage characteristic of the cermet/GaAs Schottky barrier diode. The solid line is obtained after fitting equation 3.20 to the data. 21  Ohmic  contact  F i g u r e 3.3: T h e d i s t r i b u t e d resistive gate Schottky barrier diode m o d e l of the cerm e t / G a A s Schottky barrier diode.  — i( y)  RCM y d  wwwvw+  dV(y)  -  v(y)  'SB  dl(y)  dy  F i g u r e 3.4: A differential length of the distributed resistive gate Schottky barrier diode model. 22  T h e current n o r m a l to the c e r m e t / G a A s j u n c t i o n is modeled using the distributed S B - d i o d e , DSBA differential length of the distributed resistive gate S B - d i o d e m o d e l is shown i n F i g u r e 3.4. T h e differential voltage drop along this length a n d the differential current n o r m a l to the c e r m e t / G a A s j u n c t i o n are dV(y)  =  dl(y)  =  R Ml(y)dy  (3.1)  C  wJ  (3.2)  dy  \WT)  6XP  0  where y is the position variable, w is the cermet gate w i d t h , 0 is the ideality factor, U~T is the t h e r m a l voltage a n d Jo is the saturation current density. T h e saturation current density is given by [29] Jo = A * T where  A*  e x  2  P  ( ^ )  is the modified Richardson's constant,  barrier height a n d T is the temperature.  (3.3)  CJ>BC  is the c e r m e t / G a A s  Schottky  Differentiating equation 3.2 w i t h respect to  the variable y gives cPI  wJ„  w  =  I  V  WT {im)^  \  dV  •  exp  '  (3 4)  S u b s t i t u t i n g equation 3.1 a n d equation 3.2 into equation 3.4 yields ^ - a l ^ - - b l dy dy  =  0  (3.5)  2  where  a =  RCM/0UT  and  are constants.  b — WRCMJO/0UT  T h e nonlinear second-order differential equation 3.5 describes the spatial variation of the tangential current along the c e r m e t / G a A s j u n c t i o n . T h i s equation can be solved analytically i n the following manner [30]. Substitute (3.6)  S = f dy  and fl_dS dy  2  _ dSdl dy  di  23  dy  _ dS_ g  di  ^  ^  into equation 3.5 to give the following first-order linear differential equation + b)I = 0  S ^ - ( a S al  .  (3.8)  T h e solution to equation 3.8 is obtained using separation of variables  - -l (s b  S  n  a  \  +  b  - ) = -f a  a  J  +  2  (3.9)  Cl  where C\ is a constant of integration determined as follows. A s the tangential current 7. approaches zero the derivative of the tangential current w i t h respect t o the variable y also approaches zero. Hence, the b o u n d a r y c o n d i t i o n for equation 3.9 is = 0) = 0  S(I  (3.10)  w h i c h yields c = - - I n (:  a  (3.11)  \ a,  S u b s t i t u t i n g equation 3.11 into equation 3.9 gives _ 6, / , aS\ al S - - In I 1 + — ) = — a \ b 2  2  (3.12)  •  J  E q u a t i o n 3.2, equation 3.6 a n d the relationship b/a — wJ yields 0  S =  exp  '  V  (3.13)  S u b s t i t u t i n g equation 3.13 into equation 3.12 gives exp  (v(y)\  _YM_  =  1  \PU )  PU  T  T  %)  q2/  '  (3.14)  2b  Rearranging equation 3.14 results i n a n expression for the current I(y) as a f u n c t i o n of the voltage V(y) i(y)  = io  exp  (m)_m_  1  (3.15)  where l2wpU J T  Io = ±  — = ± a  24  0  •CM  (3.16)  is the s a t u r a t i o n current. T h e sign of the saturation current is chosen to be the same as the sign of the voltage V(y). T h e ideality factor, the saturation current a n d the Schottky barrier height of the c e r m e t / G a A s Schottky barrier were determined f r o m the measured current-voltage d a t a of the c e r m e t / G a A s S B - d i o d e shown i n Figure 3.1. T h e d i s t r i b u t e d resistive gate S B - d i o d e m o d e l illustrated i n F i g u r e 3.3 yields V  = R I  in  S  + V(0)  I N  (3.17)  for the dc t e r m i n a l parameters of the c e r m e t / G a A s S B - d i o d e . T h e diode voltage V(0) is obtained f r o m equation 3.15 w i t h 1(0) = I  I N  U n d e r f o r w a r d bias conditions where V(0)/3U~T  > 3, equation 3.18 yields  V(0)^23U \n(j^j  .  T  (3.19)  S u b s t i t u t i n g equation 3.19 into equation 3.17 gives V  m  = RJ  in  + 23U \n(j^j T  .  (3.20)  F i t t i n g equation 3.20 to the measured current-voltage d a t a of the c e r m e t / G a A s S B diode for Vi > 0.2 volts yields the solid line illustrated i n F i g u r e 3.2 a n d the barrier n  parameters listed i n Table I. 3.2  Surface Potentials  It was demonstrated i n the previous section that the c e r m e t / G a A s j u n c t i o n forms a Schottky barrier. T h i s characteristic is used to control the surface p o t e n t i a l w i t h i n the interelectrode gaps of the G a A s C M C C D . It w i l l be demonstrated i n this section using a c e r m e t / G a A s transmission line m o d e l that the surface p o t e n t i a l varies monotonically along the gap of a G a A s C M C C D . 25  Parameter T  Value 300 K 0.0259 volts 100 firn  U  T  w  7.8 • 1 0 ~  A*  A///m -K  8  2  Rs RcM  287.9 kQ 55 kfl/ftm  Io  1.17 3.60 n A 0.64 volts  <t>BC  2  Table I: A s u m m a r y of the c e r m e t / G a A s Schottky barrier parameters.  T h e proposed c e r m e t / G a A s transmission line model of a u n i f o r m c e r m e t / G a A s contact w i t h i n a n interelectrode gap of a G a A s C M C C D is illustrated i n F i g u r e 3.5 w i t h a differential length of the c e r m e t / G a A s transmission line shown i n F i g u r e 3.6. T h e d i s t r i b u t e d series impedance Z ( o h m s / u n i t length ) is modeled using the parallel network [22] comprised of the distributed cermet f i l m resistance RCM ( o h m s / u n i t length ) a n d the distributed cermet f i l m capacitance CCM ( farads-unit length )  2  RCM  3.21  JWRCMCCM  1 +  where u is the r a d i a n frequency. T h e distributed shunt admittance Y ( siemens/unit length ) is modeled using the distributed depletion layer capacitance of the active layer  CD ( f a r a d s / u n i t length ) Y=juC  D  .  (3.22)  T h e position along the gap is denoted by the variable y w i t h y = 0 defined at the righth a n d edge of the left-hand transport electrode a n d y = L defined at the left-hand edge G  of the r i g h t - h a n d transport electrode. T h e spatial v a r i a t i o n of the surface potential along the gap is described by the voltage wave equation for a u n i f o r m transmission line [31]. F o r h a r m o n i c a l l y v a r y i n g 26  Cermet  Left  Right  electrode  GaAs  active  electrode  layer  y  y=0  Figure 3.5: The cermet/GaAs transmission line model of the uniform cermet/GaAs contact within an interelectrode gap of a GaAs CMCCD.  Zdy  y 'CM  U +  VWWVVV\A R,CM  J Ydy  dV(y,o})  V(y.io)  dy  Figure 3.6: A differential length of the cermet/GaAs transmission line model. 27  voltages this equation is d V(y,u) 2  where the factor e  3Wt  (3.24)  + k e~^  y  x  1?  (3.23)  has been suppressed. T h e solution to equation 3.23 is V(y, u>) = k e^  where &  .  = YZV{y,u)  y  2  k are integration constants a n d 7 ( 0 ; ) is the propagation constant 2  /  JUJRCMCD 1 +  JUJRCMCCM UJRCMCD  \  ^ 1 + (URCMCCM)  .  2  j  exp  — arctan  1  (  2  (3.25)  \WRCMCCM  T h e b o u n d a r y conditions imposed o n the solution 3.24 are V{y = 0)  =  V  (3.26)  V(y = L )  =  0  (3.27)  g  0  where V > 0 is the incident voltage a m p l i t u d e . T h e integration constants ki a n d k 0  2  are obtained by s u b s t i t u t i n g the b o u n d a r y conditions 3.26 a n d 3.27 into equation 3.24 to give k  =  k  =  x  2  —  — l  V  T  , ,  (  3  .  2  .  8  )  (3.29)  S u b s t i t u t i n g equation 3.28 a n d equation 3.29 into equation 3.24 yields the solution for the surface p o t e n t i a l along the gap as a f u n c t i o n of position a n d frequency  % » ) =  5%fM „ F  .  (3.30)  smh[L 7(cj)J 3  T h e surface potential V(y,u)  is conveniently expressed i n polar f o r m as V(y,u)  = V H(y,u;)L6(y,u>) 0  28  (3.31)  where H(y,u)  is the normalized magnitude of the surface potential i  ' c o s h [a(y,u)] - cos [b(y,u)}\ c o s h [c(o;)] — cos [<i(u>)] 2  2  2  2  (3.32)  2  and Q(y,u>) is the phase shift of the surface potential 0 ( y , c j ) = a r c t a n (coth[a(y,u;)] tan[&(y,u>)]) — arctan (coth[c(u;)] tan[d(u;)]) T h e f u n c t i o n s a(y,u),  a(y,u)  +  (WRCMCCM)  -  +  - y) sin  g  (U>RCMCCM)  2  1 / - arctan ( —  +  (URCMCCM)  +  g  cos  n  2  L  g  1  —  ,(3.35)  \WRCMCCM  - arctan .  \LORCMCCM  1 / sin - arctan I — 2  (OJRCMCCM)'  {3.34)  KOJitCM^CM  - arctan I ——  2  URCMCD  =  1  (  1  2  L  yi  c o s  I  =  i  y)  2  (L  (3.33)  are  (g  URCMCD  =  ^1  d(u)  a n d d(u>)  L  yjl  c(u)  c{u)  URCMCD  = yjl  b(y,u)  b(y,u),  .  1  .  (3.36)  / -  (3.37)  —  \OJRCM^CM  T h e derivation of the polar f o r m of V(y,o>) is described i n A p p e n d i x B . T h e f u n c t i o n H(y,u>)  described by equation 3.32 decreases monotonically for the  choice of b o u n d a r y conditions 3.26 a n d 3.27 used above.  Interchanging the b o u n d -  ary conditions w i l l result i n the surface potential increasing monotonically, as the cerm e t / G a A s transmission line model illustrated i n Figure 3.5 is symmetric. It is sufficient to show that equation 3.32 satisfies the condition 9H(y,u>) dy  < 0  (3.38)  w=constant  to demonstrate that the surface potential varies monotonically on the interval 0 < y < L  g  for a l l frequencies u > 0. Differentiating equation 3.32 w i t h respect to the variable  y gives dH(y,u) dy  _ cosh [a(y, u)} sinh [a(y, u>)] ^ff  1  + cos [b(y, u)} sin [6(y, u)]  (cosh [a(y, u)} - cos [6(y, u)]) * (cosh [c(u)} - cos [d(u)]) ' 2  2  2  2  (3.39) 29  Surface  O.O  O.S Distance  1.0  Potentials  along  1.5  the  Z.O Gap: y  2.5  (/umj  3.0  Figure 3.7: The variation of the normalized surface potential along the gap of a GaAs C M C C D as a function of frequency.  The functions a(y,u;), b(y,u)), c(u) and d(uj) are positive functions of y and a>, and the derivatives da(y,uj)/dy  and db(y,u>)/dy are negative functions of a;. Consequently, the  derivative of the normalized surface potential H(y,u>) with respect to the variable y is negative, satisfying the condition 3.38. The variation of the normalized surface potential along a gap of a GaAs CMCCD as a function of frequency is illustrated in Figure 3.7.  Equation 3.32 and the parameter  values listed in Table II were used to produce the curves. The parameter values were obtained using a Hewlett-Packard HP-4275A Multi-Frequency L C R Meter and an Alessi probe station to perform low-frequency ( 10 kHz ) impedance measurements on a fabricated cermet/GaAs test structure. It is apparent from Figure 3.7 that the surface potential decreases monotonically along the gap for all positive frequencies. There are two special cases which are of interest: the case when the frequency approaches zero and the case when the frequency approaches infinity. 30  Parameter RCM  Value 55 k f i / ^ m  CCM  pF-fim  0.8  c L  Scale Factor X /127 127/L, 5  127L,  15 f F / f x m 3.0 fim  D  3  —  Table II: T h e distributed circuit parameters of the c e r m e t / G a A s contact.  T h e dc surface potential variation along a gap of a G a A s C M C C D is linear.  The  propagation constant j(u>) is zero when UJ = 0 resulting i n a n indeterminate f o r m for the normalized surface potential H(y,u> — 0). L ' H o s p i t a l ' s rule is used to resolve the indeterminate f o r m  H(y,  U  = 0)  =  lim  S i n h  smh[L 7(u;)] (L -y)cosh.[(L -y)j(u)]  -Y("HO H  fl  m  g  g  L cosh[L 7(a;)]  7M-0  3  5  (3.40)  E q u a t i o n 3.40 is i n t u i t i v e l y correct.  A t low frequencies the electric current through  the c e r m e t / G a A s contact w o u l d be d o m i n a n t l y through the distributed cermet f i l m resistance RCM a n d w o u l d result i n a linear surface potential v a r i a t i o n . T h i s i n t u i t i v e argument was probably the basis for deriving the name resistive to the original cermet gate C C D described i n reference [18].  gate  CCD applied  T h e adjective  resistive  used i n the cited reference is considered to be a misnomer [32] as it implies that the surface potential v a r i a t i o n along a gap of a G a A s C M C C D is established v i a resistive c o n d u c t i o n only, a n d does not take into consideration the effect of capacitive coupling w i t h i n the cermet f i l m at higher clock frequencies. T h e adjective cermet has been used instead to avoid the implications of the term  resistive.  T h e h i g h frequency normalized surface potential variation along a gap of a G a A s 31  C M C C D is H(y,u  sinh  -» co) =  [y/^( > ~ y)] L  (3.41)  where the h i g h frequency value for the propagation constant 7(0;) is determined using equation 3.25 (3.42) E q u a t i o n 3.41 is independent of the distributed cermet f i l m resistance RCM-  This  suggests that a wide range of cermet f i l m resistivities can be used i n the design of a G a A s C M C C D . A requirement that must be satisfied b y the cermet film resistivity is that it must be large enough to comply w i t h the power constraints of the C M C C D quadrature clock drivers. T h e capacitive coupling between the cermet film a n d the u n d e r l y i n g G a A s is responsible for establishing the h i g h frequency surface potential variation along a gap of a G a A s C M C C D . F i g u r e 3.8 illustrates the effect of different ratios of CD/CCM the normalized h i g h frequency surface potential H(y,u  O  N  —> 00). It is apparent f r o m the  curves illustrated i n Figure 3.8 that it is desirable to m i n i m i z e the ratio CDJCCM order to m a i n t a i n a nearly linear surface potential v a r i a t i o n for a l l frequencies.  M  The  reason for this is that a v i r t u a l equipotential zone extends along the surface into the gap near the r i g h t - h a n d transport electrode at y = L  G  for large ratios of  CD/CCM-  T h e extent of this zone increases w i t h this ratio. T h i s is undesirable as the tangential electric field w i t h i n the active layer would be reduced underneath the equipotential zone along the surface, creating a source of potential loss of performance i n a G a A s C M C C D operating at h i g h frequencies.  M i n i m i z i n g the ratio CD/CCM  w o u l d reduce  this negative effect.  3.3  Verification  Frequency response measurements performed o n a 2-port c e r m e t / G a A s test structure were used to test the validity of the c e r m e t / G a A s transmission line m o d e l . T h e test 32  Surface  Potentials  O . O O . O  0  .  5  1.0  Distance  1.5  along  the  2  .  Ga-p:  0  2  .  5 3  .  0  (fjunx)  y  F i g u r e 3.8: T h e v a r i a t i o n of the n o r m a l i z e d h i g h frequency surface potential along the gap of a G a A s C M C C D as a f u n c t i o n of the ratio C D / C C M [)  50 ohmT}  HP-85047A Port  Port 64  Ohmic  1  fingers-  j-e- 3.0  Ohmic  contact Port  fim  contact  2  Cermet Active  A  Port 50  region  ohm  B  )HP-85047A  F i g u r e 3.9: T h e test structure a n d the test circuit used to demonstrate the validity of the transmission line m o d e l for the c e r m e t / G a A s contact w i t h i n a gap of a G a A s CMCCD. 33  structure was fabricated on a n-type active layer ( ND = 4 . 5 - 1 0  16  cm  - 3  , x,  ni  = 0.25 m i -  crons ) a n d consisted of two interleaved arrays of sixty-four, 3 m i c r o n long T i - P t - A u Schottky barriers encapsulated w i t h a n o m i n a l 5000 A thick cermet f i l m deposition of C r : S i O ( n o m i n a l 45 w t . % C r ).  T h e separation between adjacent m e t a l  fingers  was 3 microns. T w o A u / G e - N i - A u o h m i c contacts were p r o v i d e d at each end of the 100 m i c r o n wide active region. T h e test structure was packaged i n a leadless chip carrier w h i c h p e r m i t t e d external connections to be made to the two ports.  The ohmic  contacts were connected to the reference potential a n d the two electrode arrays were connected to the measurement apparatus using 50 o h m r i g i d copper coaxial cables. T h e test structure a n d the test circuit are shown schematically i n F i g u r e 3.9. T h e frequency response measurements consisted of measuring the a m p l i t u d e response a n d the phase response of the c e r m e t / G a A s test structure. T h e measurement apparatus consisted of a H e w l e t t - P a c k a r d H P - 8 7 5 3 B network analyzer a n d an H P 85047A s-parameter test set.  T h e 50 o h m port A a n d port B terminals of the s-  parameter test set were connected to the two ports of the test structure. T h e a m p l i t u d e response a n d the phase response were measured using a 0 d B m , 300 k H z - 5 0 0 M H z swept rf signal a n d are shown i n F i g u r e 3.10.  T h e reference levels are i n d i c a t e d i n each of  the two plots b y the arrows. T h e marker triangle labeled w i t h the number ' 1 ' coincides w i t h the m a x i m u m observed phase shift of 53.1° at 12.8 M H z . T h e l u m p e d equivalent circuit shown i n F i g u r e 3.11 was used to m o d e l the theoretical frequency response of the c e r m e t / G a A s transmission line test circuit. transmission line length L  G  The  = 3 microns is m u c h less t h a n the effective wavelength of  the c e r m e t / G a A s transmission line below 500 M H z operation, hence the one hundred twenty-seven parallel connected c e r m e t / G a A s transmission lines are modeled approxi m a t e l y u s i n g the l u m p e d elements RCM, CCM a n d CD- T h e l u m p e d element values are obtained f r o m the distributed element values listed i n Table II after m u l t i p l y i n g by the scale factors listed i n the t h i r d c o l u m n of this table. T h e theoretical a m p l i t u d e 34  CHI  S21  START  CH 2 S21  START  l o g I'lflG  3 dB/  REFBclB  . 3 0 0 0 0 0 MHz  phase  7.5  STOP  °/  REF 0  . 3 0 0 0 0 0 MHz  0  STOP  i: - 1 7 . 258 dB  500.000  1:  500.000  0 0 0 MHz  53.108  0  0 0 0 MHz  F i g u r e 3.10: T h e a m p l i t u d e response ( 3 d B / d i v i s i o n ) a n d the phase response ( 7.5°/division ) of the c e r m e t / G a A s test circuit measured f r o m 300 k H z to 500 M H z .  35  'CU  o +  4AA/VWW\r R cu  V(L.,a>)$-*i  V(0,G>)  o-  o-  F i g u r e 3.11: T h e l u m p e d equivalent circuit of the c e r m e t / G a A s transmission line test circuit.  response of the test circuit is Ada = - 2 0 log  V(L ,u) g  = - 1 0 log  + (CVRLRCMCCM)  Rj  (RL  + RCM)  + (UR RCM[C  2  L  D  \  2  + C }) CM  2  )  (3.43) and the theoretical phase response of the test circuit is $ = a r c t a n (WRCMCCM)  — arctan ( '  U  \  ^  L  ^  M  ^ ^  ^CM) j  RL + RCM  (3.44)  J  T h e theoretical a m p l i t u d e a n d phase responses of the c e r m e t / G a A s test circuit are i l l u s t r a t e d i n Figures 3.12 a n d 3.13, respectively.  T h e theoretical a n d measured re-  sponses are i n reasonable agreement, s u p p o r t i n g the c e r m e t / G a A s transmission line m o d e l described i n the previous section.  T h e deviation between the theoretical a n d  measured responses is a result of the parasitic components associated w i t h the interconnect w i r i n g between the c e r m e t / G a A s test circuit a n d the network analyzer, which are neglected i n the above analysis. 36  Figure 3.12: The theoretical amplitude response of the cermet/GaAs test circuit for frequencies lying between dc and 500 M H z . Phase  70  Response  Measured  data  a  O O  100  200 Preqxtertcy  300 400 (MHz)  500  Figure 3.13: The theoretical phase response of the cermet/GaAs test circuit for frequencies lying between dc and 500 M H z . 37  Chapter 4 Two-dimensional G a A s C M C C D  4.1  Model  Geometric Representation  T h e u n i t cell for the 4-phase G a A s C M C C D model shown i n F i g u r e 4.1 consists of a twodimensional slice t h r o u g h a single p i x e l . T h e slice is assumed to lie on a plane coincident w i t h the central axis of the C M C C D so that the potential a n d the charge density are considered invariant along the axis n o r m a l to this plane.  C a r t e s i a n coordinate axes  are defined as i n d i c a t e d , w i t h the origin located at the intersection between the upper b o u n d a r y segment a n d the left-hand b o u n d a r y segment. T h e u n i t cell occupies a d o m a i n comprised of two subdomains: the active layer s u b d o m a i n (0 < x < x , , 0 < y < n t  (xint  x =  5: x < x ,0 max  Xi t n  <  y x) ma  a n d the semi-insulating substrate s u b d o m a i n  A l o n g the intersection of the two subdomains at  y < y x)ma  is an i n t e r n a l b o u n d a r y segment.  T h e upper b o u n d a r y segment at x =  consists of the u n i o n of two b o u n d a r y segment sets.  0  T h e first set includes the four  transport electrode b o u n d a r y segments a n d the second set consists of the five cerm e t / G a A s j u n c t i o n b o u n d a r y segments. T h e lengths used i n the computer model are listed i n Table III. 4.2  Device Equations  T h e equations used to describe the v a r i a t i o n of b o t h the p o t e n t i a l a n d the electron density i n nondegenerate n-type G a A s are [33]: V u ( a : , y) = a[n(x, y) J(x,y)  — qmU fJ.(x,y)[Vn(x,y) T  (4.1)  N]  2  D  +  n(x,y)E(x,y)]  (4.2)  and dn(x,y) dt  = — V - J(x,y) qrii  38  (4.3)  Ph. VlL  1  Ph. Z  VlR  KOmH  VZL  t  Ph. 3  VzR  ^  ^  VsL  VsR  Ph. 4 V 4L  x^^<x  d  V4R  K^y^x  (0,0) Active  ( o,y ) max  layer  i>( XvVj)  (x ,0)  ( invVmiuc) X  int  Semi—insulating  substrate  (mtix''y  ( a*>°)  I  X m  max)  F i g u r e 4.1: T h e unit cell for modeling the G a A s C M C C D .  Ph. 3a uMMMnn  1 Jm  Ph. 2 3XL  Ph. 3  3IR  3st  IUMMM  » M M M  Ph. 4 3$R  J*R j  3AL M  M  M  ^  ^^^^^^^^^"^^^^^^^^^"^^^^^^^^^^^^^^^^^^^^ ^  i  max 3 max  F i g u r e 4.2: T h e m a i n finite difference for the G a A s C M C C D m o d e l .  39  Parameter  Value 0.25 nm 100 fxm 23.8 / m i  int  x  max  int  x  x  Umax VmR  — VmL  3.0 n  m  VlL Vmax y(m+l)L  f° m = 1,..., 4 1.4 (JLVO. r  V4R  1.4 jim.  — VmR  3.0 fj,m for m = 1 , 2 , 3  Table III: T h e lengths used i n the two-dimensional computer m o d e l .  where n{ is the intrinsic carrier density of G a A s , No — No/rii density, n(x, y) = n(x, y)/rti  is the n o r m a l i z e d donor  is the normalized electron density, u(x, y) = ip(x, y)/Ur is  the n o r m a l i z e d potential, E(x, y) = — Wu(x, y) is the n o r m a l i z e d electric field, J(x, y) is the electron current density and a = qni/eUx  is a constant. T h e m i n o r i t y carriers a n d  the electron generation/recombination processes w i t h i n the active layer are neglected i n the m o d e l .  4.3  Finite Difference Grid and the Computational Kernel  T h e model equations 4 . 1 , . . .,4.3 are discretized o n a finite difference g r i d superimposed onto the u n i t cell.  T h e potential is computed at each m a i n g r i d point l y i n g w i t h i n  the u n i t cell, while the electron density is computed at each m a i n g r i d point l y i n g w i t h i n the active layer subdomain.  A n a u x i l i a r y g r i d is interleaved w i t h the m a i n  g r i d l y i n g w i t h i n the active layer s u b d o m a i n , o n w h i c h , the intermediate calculation of the electron current density is made. T h e a u x i l i a r y g r i d points are located at the m i d p o i n t s of the m a i n g r i d intervals. T h e m a i n g r i d layout w i t h i n the u n i t cell is shown i n F i g u r e 4.2. T h e m a i n g r i d is composed of 21 by 120 grid points w i t h 1 < 1 ^  j  5:  jmax  i <  i  max  =  21 a n d  = 120. T h e g r i d spacing along the z'-axis is u n i f o r m w i t h i n the active  layer interval 1 <  i <  ii  nt  = 11 a n d is n o n u n i f o r m w i t h i n the semi-insulating substrate 40  interval i{  nt  < i < i  max  . A u n i f o r m g r i d spacing is maintained along the j - a x i s of the  unit cell. T h e auxiliary g r i d w i t h i n the active layer s u b d o m a i n is shifted f r o m the m a i n g r i d as described above w i t h (i ,j ) aux  = (i + \,j + |).  aux  T h e g r i d spacings 6x,- = x  — x , a n d Sy = y  t + 1  — yj w i t h i n the unit cell are  J + 1  determined f r o m the geometry of the unit cell a n d the number of g r i d points l y i n g w i t h i n the region of interest.  T h e constant g r i d spacing along the z-axis w i t h i n the  active layer s u b d o m a i n is Sxi = 8x = - — — — = 0.025 microns for 1 < i < i , a  —1  nt  .  (4.4)  W i t h i n the semi-insulating substrate s u b d o m a i n the grid spacing along the i-axis is n o n u n i f o r m a n d is defined using a finite geometric series 6xi - < f a a r , _ , i n t  for i  < i <  int  i  m a  x  1  -  (4-5)  where r is a constant determined f r o m the s u m m a t i o n Imax —1  -Pint  ^moi  /  ,  yimax—  ^  —  6xj  iint  )fig  -  .  (4.6)  1 — r T h e value of the constant r is 2.364316 for 6x  a  = 0.025 microns a n d x  — Xi  m a x  nt  —  100 microns. T h e u n i f o r m g r i d spacing along the jf-axis w i t h i n the unit cell is fiy —  _ Q_2 microns for 1 < i < j  y  max  Jmax  m a x  —1  .  (4.7)  1 -  T h e potential or the electron density at each g r i d point ( i , j ) is evaluated using the discrete nine-point c o m p u t a t i o n a l kernel shown i n Figure 4.3. T h e potential a n d the electron density are associated w i t h the five m a i n g r i d points a n d the components of the electron current density are coupled w i t h the four a u x i l i a r y g r i d points.  4.4  Finite Difference Equations for  u(x,y)  T h e generalized two-variable Taylor series for the dimensionless potential u(x, y) near the point (xi,yj) u(xi±6x,yj  is ±8y)  =  u(x,y)\ ,  + (^±8x^  {xi y])  41  ± 8 y ^  u(x,y)\ ^ (xi  o.56y —  —W  °- y  " - 0 * -  5S  °- y  — * " 0 * ~  56  0.56 x  °- y 56  —  t  Figure 4.3: The nine-point computational kernel used in the calculation of the potential or the electron density within the unit cell. The five disks represent the main grid points and the four circles denote the interleaved auxiliary grid points.  1 / \  +  d  (  d \  d  2  d \  m  ^.{ ai d-y) ±6l  where +Sx = 6x{, —Sx = 8xi_i  "(*•*•*> + •••  ±6y  <-> 4  8  and m is an integer variable. Applying equation 4.8 to  each of the four main grid points surrounding the central point of the computational kernel and neglecting third and higher order terms of the resulting Taylor series yields the following set of four equations:  Ui-lJ  =  u  Ui,j-1  =  U  i,j+l  =  U  =  U  d  ',j  Sx ! 2  £y  d i,i  6 y  dy  U  +  6  y  f  '  j  2  +  d  c u  U i  d y  iJ + bxi  d Ox  i  '  j  itj  42  +  '  d  1  (4.10)  2  * y  U  Sx  d  2  dx  2  u  (4.9)  2  Ui  2  +  d  2  d*y "  by U  2  a  2  "  '  (4.11)  2  Uij  (4.12)  where the s h o r t h a n d n o t a t i o n  =  Uij  is used.  u(x,y)\( ,yj) Xi  A d d i n g equation 4.9 to  equation 4.12 gives d  1  2  2  dx  '  2  8xi_i  + 8xi  / Sx{_i + 6 x A  8xi_i  l  1  I  j  J  8xi_i8xi  1 '  8x{  l 3  , + 1  '  (4.13)  J  a n d similarly, a d d i n g equation 4.10 t o equation 4.11 yields d  1  2  dy ' 2  - 2uij  8y  Ui j  +u  .  )  i } j + 1  (4-14)  2  T h e s u m of equation 4.13 a n d equation 4.14 produces the finite difference equation for the L a p l a c i a n of the potential u j t  _  2  2 8x^(8x^1  1,3  +  ^  +  1 +  _l_  + 8xi) ~ l  Sx (Sx . t  2  l 3  + Sx ) '  2  i  8y '~  1,3  Ui+1  1  2  _ I  u  2 8y  ySx^Sxi  1  2  u t  hi  •  i  i  E q u a t i o n 4.15 can be simplified when the point  -  (4  15)  resides w i t h i n the active layer sub-  d o m a i n . R e c a l l that the g r i d spacing along the i-axis w i t h i n the active layer s u b d o m a i n is a constant, as defined by equation 4.4. Hence, equation 4.15 reduces to 1  V72 V  U  "  =  PxZ ^ U  1 +  2 ( 6 x + 8 y) 2  Py- ^ U  ~  8x6y 2  a  1  2  a  ^  2  +  S^y^  1 1  +  Px?* * 1  '  ( 4 > 1 6 )  T h e discretization of the Poisson equation 4.1 w i t h i n the active layer s u b d o m a i n is obtained u s i n g equation 4.16  2(8 x 2  + 8 y) 2  a  ¥X-Py—^  +  1  1  p-y '^  PxZ ^  U  +  „ U  (4  '  17)  a n d w i t h i n the semi-insulating substrate s u b d o m a i n using equation 4.15 2  1  0Xi-i(0Xi_i  + oxi)  (^8x{-i8xi  8 y) 2  8y z  1 , 3  8y 2  1 , 3 + 1  8xi(8xi-i  + 8xi)  where i t is assumed that no charge is present w i t h i n this region. 43  ^  ^  4.5  Boundary Conditions for u(x, y)  T h e b o u n d a r y conditions for the potential tt,-j are a n extension of the one-dimensional b o u n d a r y conditions described i n C h a p t e r 2. T h e normalized reference potential along the lower b o u n d a r y segment at i = i  is assigned a value of zero  max  (4.19)  , = 0  Ui  T h e unit cell is considered to be part of a repetitive structure resulting i n a periodic b o u n d a r y c o n d i t i o n for the potential that is described by the following two equations  (4.20)  and (4.21) T h e potential along the internal b o u n d a r y segment at i = ii t is continuous a n d satisfies n  Gauss's law d  d OX  '"'  QintJ  OX '">'  ,J  (4.22)  0  J  where the interface charge density Qi t,j  is assumed to be zero. F r o m equations 4.1,  4.9, 4.14 a n d n o t i n g that 8xi _i  = 8x o b t a i n  n  int  — 8xi  int  2 - D) ~ 7 J — " w - W  8x  a  N  2  dx ' '^ U  int  a  8x 2  1  2(8 x 2  c  + 8 y) 2  a  (4.23)  Similarly, equation 4.1 w i t h the right-hand side set to zero, equation 4.12 a n d equat i o n 4.14 y i e l d d OX  8x  2(8 x  2  8y 2  1  + 8 y)  2  a  2  a  8*x 8 y  '  2  Uiint  a  J  +  2  ^y " " U  n  J + 1  +  8x 2  c  (4.24) 44  S u b s t i t u t i n g equation 4.23 a n d equation 4.24 into equation 4.22 gives a_ / 2 \ iint,j n  1  \  fj  — ^ D )  —  1  fi2^T int-i<i Ui  'p^  +  2{8 x + 8 y) 6x8y 2  U i i n  " j  1  2  a  2  2  a  1  ~'J  Ui  +  S  2  y  u  iin*J+*  +  S  1 *x  U i i n t a  +  l J  '  '  1  T h e p o t e n t i a l along a transport electrode b o u n d a r y segment along the upper b o u n d a r y segment at i = 1 is defined as U  °  =  v\  ~ (tBMfi  ~ <t>BM,x )} mal  (4.26)  •  T h e p o t e n t i a l along a n interelectrode gap b o u n d a r y segment along the upper b o u n d a r y segment at i = 1 is a p p r o x i m a t e d as a linear f u n c t i o n of the voltages a p p l i e d to the two adjacent transport electrodes, which is consistent w i t h the c e r m e t / G a A s j u n c t i o n theory presented i n C h a p t e r 3. T h e potential along a gap b o u n d a r y segment is u  where V \ jt g%  e  1 S  gap  TT WaJeft + SJgapVgap ~ (4>BC ~ 4>BM,x )]  ( -27) 4  max  UT  the potential of the transport electrode to the i m m e d i a t e left of the gap,  Vg p is the p o t e n t i a l difference across the gap relative to this electrode a n d 0 < 8j a  gap  < 1  is the relative position along the gap. 4.6  Finite Difference Equation for  J(x,y)  T h e S c h a r f e t t e r - G u m m e l ( S G ) m e t h o d [34,35] is used for discretizing the electron current density equation 4.2. T o illustrate this m e t h o d , the positive x-component of the current density  J  i  +  i  j w i l l be determined.  T h e x-component of the electron current density at a point ( x , y) using equat i o n 4.2 is J ( , y) = qniU fJ. (x, x  x  x  T  y)  —n(x, ox  y) + n(x, y)E (x, x  y)  (4.28)  where the superscript x denotes the x-component of the variable. R e a r r a n g i n g equat i o n 4.28 results i n a first order differential equation for the electron density —n(x,y)-an(x,y)-b-0  45  (4.29)  where a =  —E (x,y)  and b =  x  J (x y)/qn UTlJ' (x,y). x  y  T h e Scharfetter-Gummel  X  i  m e t h o d assumes that the electron current density a n d the electric field w i t h i n the semiconductor vary more slowly t h a n the electron density a n d consequently can be app r o x i m a t e d as local constants. T h i s a p p r o x i m a t i o n enables equation 4.29 t o be solved a n a l y t i c a l l y u s i n g an integrating factor exp [—a(x — a;,-)] t o give n(x,y) = - (exp [a(x - a;,-)] - 1) + n(xi,y)exp a  [a(x - x -)]  (4.30)  t  where n(x -,y) is the i n i t i a l c o n d i t i o n . A s s i g n i n g n(x,-,y) = n j , fixing x = x,- + 8x and s  t  setting n(xi + 8x ,y) a  J  i +  y  = n  i + 1  a  j yields  =  O.bqriiUTfi^ij  +  (n,- j+n,- -)E i ] + 1  [E L I+  J  f +  coth(Q.oE ijSx )(n j i+  TJ  a  i+1  - n^) (4.31)  i i  where the x-components of the electron current density, the electric field a n d the elect r o n drift m o b i l i t y are c o m p u t e d at the a u x i l i a r y g r i d point (z +  S i m i l a r finite  difference equations are obtained for the three remaining components of the electron current density s u r r o u n d i n g the point  + Jij_i  J  4.7  i ) j +  i  ( n ^ + n^E^A  =  ,  (4.32)  0.5gnitf ^j-i[£,j_i^ r  +  (n.-j-x + n . j ) ^ - - ! ]  =  O.bqniUT^xiEi^cothiQ.bEi^SyXni^  +  (n  id+1  ,  +n ,)E ,} it  (4.33) - n^)  .  itJ+  (4.34)  Discretization of the Continuity Equation  T h e electron continuity equation 4.3 is descretized using the C r a n k - N i c o l s o n equat i o n [36]  8t  2  —n(x ,y ,t ) i  46  j  k+1  +  —n{xi,yj,t ) k  =  (  '  V  J  i  *  M  1  +  V  '  ''  Ji  3  k)  (  4  '  3  5  )  where 8t is the t i m e increment a n d the subscript A; is the discrete time-step. T h e spatial derivatives o n the r i g h t - h a n d side of equation 4.35 are discretized using central differences of the f o r m V.J,-  J, i=  ' * ' ~ t-U +  J  J  +  c  ^ h -  8x  ^ - h  J  { A M  )  by  a  S u b s t i t u t i n g equations 4.31,.. .,4.34 into equation 4.36 yields  ^ - -' V J  [  =  ^  )^ n  +  {  si—)""-*  6x  a  )  +  =  /J,U~TE  v coth(Q.5E8x ). a  V-  •  1  h  2  - ',j n  8y  l" - ^— -) ^ + {  +  where v  —  2  jxZ  n  is the nonlinear electron  '  (4 37)  drift velocity of G a A s  and v  =  A finite difference equation for the electron density at the point  for the k + 1 time-step is obtained u p o n s u b s t i t u t i n g equation 4.37 into equation 4.35 st  to give 0  =  ^ — »  t  h h k + 1 +  {^  -  T t )  —8y  +  + +  jn _  T  V• J  i J i f c  J  Y  + jnij  ik  + [  ^  -  n  j  n  ,  ^  ^ J n  i + 1 J M  i  .  (4.38)  T h e electron drift velocity of G a A s is a nonlinear f u n c t i o n of the electric  field  strength. T o incorporate this into the m o d e l , the e m p i r i c a l relationship between the 47  Velocity 25  1  i  O  5 Electric  Field,  vs  1  IO Field  15 (x10 V/cm)  20  s  Figure 4.4: The nonlinear electron velocity-field characteristic of GaAs obtained from the empirical equation 4.39, developed by Chang and Fetterman [37].  electron velocity and the electric field strength developed by Chang and Fetterman is used [37]  /J. U E 0  T  v = y/l + (\U E\ - E yE u(\U E\ T  0  c  2  T  (4.39)  - EQ)  where U(\U~TE\ —E ) is a unit step function equal to zero for |£/;r-E7| < Eo- The remaining 0  constants are: / i = 7500 cm /V-sec, E = 2800 V/cm and E 0  2  0  c  = 1100 V/cm. The  nonlinear electron velocity-field characteristic obtained from this equation is shown in Figure 4.4. 4.8  Boundary Conditions for  n(x,y)  The electron density along the upper boundary segment at i = 1 is assumed to be equal to the equilibrium electron density at the surface neglecting Schottky barrier height lowering. For the metal/GaAs junction the electron density is 48  n  °  =  -n7  eXP  l"-^"j  ( 4  '  4 0 )  and for the c e r m e t / G a A s j u n c t i o n the electron density is n  a a p  =  N — e x p (^ - <t>Bc\ — j  ,  (4.41)  c  A  A  U  where Nc is the effective density of states w i t h i n the conduction b a n d of G a A s . T h e periodic b o u n d a r y c o n d i t i o n for the electron density is described b y the following two equations «i,o = n  i J m a x  (4.42)  =  (4.43)  and ra.jm«+i for 1 < i < i{ .  T h e electron density along the internal b o u n d a r y segment at i = ii  nt  nt  is assumed to be equal to the e q u i l i b r i u m electron density along this b o u n d a r y = where Sui  e x  P  ,j - ° ^  m o  J  (4.44)  is the normalized potential difference between the F e r m i level a n d the  max  intrinsic energy level along the b o t t o m of the substrate at i = i 4.9  max  .  N u m e r i c a l S o l u t i o n of the Difference E q u a t i o n s  N e w t o n i t e r a t i o n w i t h successive relaxation [20,38,39,40] is used for solving the C M C C D model equations summarized i n Table I V . T h e finite difference equations summarized i n this table are expressed as the s u m of two functions fi+i  (C-i,j,fc+n Cij-i,fc+i> (i,j,k+i->Ci,j+i,k+i-> Ci+i,j,fc+i) +  = 0  = 0  (4.45)  where I = 0 , 1 , 2 , 3 , . . . is the iteration counter, ( represents either the potential u or the electron density h a n d the function h is a constant d u r i n g the k + 1  st  time-step.  T h e functions g a n d h are g = A(<_  hhk+1  +  £Cl_i,* i + CO+i + D$j i,k i +  +  49  +  +  ECUJMI (  4  - ) 46  Potential  E l e c t r o n Density  Region of A p p l i c a t i o n  4.17  4.38  A c t i v e layer s u b d o m a i n  4.18  —  Semi-insulating substrate s u b d o m a i n  4.19  —  Lower b o u n d a r y segment  4.20  4.42  L e f t - h a n d b o u n d a r y segment  4.21  4.43  R i g h t - h a n d b o u n d a r y segment  4.25  4.44  Internal b o u n d a r y segment  4.26  4.40  Electrode b o u n d a r y segments  4.27  4.41  G a p boundary segments  Table I V : A s u m m a r y of the equations used i n the C M C C D m o d e l .  and h = Fn , where A,...,G tion f  l k+1  (4.47)  + G  itj k  are constants obtained f r o m the finite difference equations. T h e func-  defined by equation 4.45 is a function of either the potential u  electron density n  l k+1  l k+1  or the  i m p l y i n g that the finite difference equations are decoupled. T h i s  enables the potential a n d the electron density distributions t o be computed independently at each discrete time-step.  T h e solution for the potential d i s t r i b u t i o n w i t h i n  the unit cell is iterated first, followed b y the iterated solution for the electron density d i s t r i b u t i o n w i t h i n the active layer s u b d o m a i n . T h e potential d i s t r i b u t i o n at the k + 1 time-step w i t h i n the unit cell is iterated st  first using the electron density d i s t r i b u t i o n computed at the k t r o n density d i s t r i b u t i o n at the k-\-\  th  time-step, as the elec-  time-step is unavailable. T h e rate of convergence  st  of the iterates is accelerated i f the electron density d i s t r i b u t i o n at the k  th  replaced by a n estimated d i s t r i b u t i o n for the k+l  time-step is  time-step. T h i s estimate is obtained  at  using the B o l t z m a n n equation a n d is n  where  i,j,k+i  Vij k — <j>n(xi,yj,t )/UT t  k  ~ e '^ + ~ -u i  k  1  Vi  i  k  =  n j  )  j  i  f  c  e "  u  * '  J  ' >  f  e  e  u  ' ' . > . * + i  (4.48)  is the n o r m a l i z e d quasi-Fermi potential for the electrons 50  w i t h i n the active layer subdomain. S u b s t i t u t i n g equation 4.48 into equation 4.47 yields the revised equation for the function h h = Fn  exp[{(u  l  i<jtk  iJM1  - u.-j,*)] + G  (4.49)  where £ = O i f £ = n o r £ = l i f £ = u . A Cfc+i  single N e w t o n iteration step consists of adding a correction factor S(  k+l  =  Cfc+i to each of the u n k n o w n variables i n equation 4.45 using the following  —  relation described i n A p p e n d i x C  T h e following expansion is obtained for equation 4.50  n  _  . xfi  fi  i  * W  , ci  d/fe+i  d  fk+l  , ci  dfl+i  r  ,  dfL+i  r  (A K I ^  dfk+1  S u b s t i t u t i n g equation 4.45 into equation 4.51 and recalling the definitions for the functions g a n d h given by equations 4.46 and 4.49 yields the following equation for a single N e w t o n iteration of the variable £  0  =  AC£i  Jifc+1  + BCijIi,*+i + (f + Fn  + ^cgi+i + £ c £ i +  G  J > f  i+  ^.-^(i -  ililfc  £ exp  eC-  lfc+  J > f l  - u^)]) £Ji i +  - «.-.;.*)]  i)-«p  .  (4.52)  A single N e w t o n iteration step through the m a t r i x of unknowns proceeds i n the usual reading order. T h e rate of convergence of the sequence of iterates is accelerated using successive relaxation. T h e ordinary iterated solution for C ' j j t + i i - obtained f r o m s  equation 4.52  C'S+i  =  +  - ( C + F*ij*t  ^  P  [t(u'  itjM1  - Uij, )]) k  _ 1  {ACl l +  JMl  +  S  d,  f c + 1  (4.53)  G) 51  where the most recently iterated values for the variables are used. solution C S r  k + 1  The  accelerated  u s i n g successive relaxation is (4.54)  T h e o r d i n a r y iterated solution is obtained f r o m equation 4.54 if w 4.10  r  = 1.  Computer Simulations  A flow d i a g r a m for a two-dimensional computer simulation is illustrated i n F i g u r e 4.5. A s i m u l a t i o n begins f r o m an i n i t i a l guess for the potential a n d the electron density distributions w i t h i n the unit cell for the i n i t i a l bias conditions at k — 0.  T h e bias  voltages are adjusted, the time-step counter k is incremented b y 1 a n d the s i m u l a t i o n proceeds. T h e potential u at each g r i d point w i t h i n the u n i t cell is iterated u n t i l the m a x i m u m absolute residual for the potential w i t h i n the active layer s u b d o m a i n is less t h a n 0.0005. T h e electron density at each g r i d point w i t h i n the active layer s u b d o m a i n is subsequently iterated u n t i l the m a x i m u m relative error for the electron density is less t h a n 0.001.  T h e s i m u l a t i o n continues u n t i l a stop time k  max  is reached.  This  simulation procedure was used to produce the two-dimensional p o t e n t i a l a n d charge density distributions for investigating the m a x i m u m frequency of operation a n d the charge transfer performance of a 4-phase G a A s C M C C D . T h e theoretical m a x i m u m frequency of operation of a G a A s C M C C D was determ i n e d using the single electron transit time model developed by D e y h i m y et al [7] a n d P r o k o p ' e v [41]. T h e transit time r required for an electron to travel w i t h i n the fully depleted active layer between the centers of two adjacent transport electrodes is given by the line integral (4.55)  52  Initialize  u^  n  Qi  w  and  V7  y ( )  k=k+1  Adjust  Biases  I Compute  Uyjg+f  I Compute  I  Save u  ,  idk+1  n  y f c + /  n  iJk+1  and W  U f c + /  Z3Z k>k  max  \ T  Stop  F i g u r e 4.5: T h e flow d i a g r a m for the two-dimensional computer simulations. 53  where C  m  is the curve coinciding w i t h the m a x i m u m potential contour between the elec-  trode centers, u (x (y),  y) a n d u (x (y),  x  y) are the x-component a n d the y-component  y  m  m  of the electron velocity vector along this curve, df = xdx + ydy is the differential contour vector a n d x ( y ) is the depth of the m a x i m u m potential as a f u n c t i o n of the m  position y between the electrode centers. E q u a t i o n 2.10 is used to determine a n approximate value for the depth of the m a x i m u m potential. If w = x , n  ND = 4.5 • 1 0  16  cm  - 3  ,x  max  n t  = 0.25 microns,  ^> x,- a n d |^ | < 10 volts then the second a n d t h i r d terms n<  0  on the r i g h t - h a n d side of equation 2.10 are negligible which gives  x {y)  &w  m  n  = x  (4.56)  int  for the d e p t h of the m a x i m u m potential between the electrode centers.  Substituting  equation 4.56 into equation 4.55 yields  T !=S  where L  p  I"'  Jo  -A—,  (4-57)  v {x y) y  inU  is the distance between the two adjacent transport electrode centers. T h e  electron transit time is computed w i t h the electron velocity v (xi ,y)  described b y  y  nt  equation 4.39 w i t h the y-component of the n o r m a l i z e d electric field E (xi ,y) y  ni  deter-  m i n e d f r o m a static two-dimensional potential d i s t r i b u t i o n . T h e theoretical m a x i m u m frequency of operation f  max  of a 4-phase G a A s C M C C D  is /m«« = ^ 4r  •  (4.58)  T h e m a x i m u m frequency of operation of a 4-phase G a A s C M C C D as a f u n c t i o n of the clock voltage a m p l i t u d e and.the interelectrode gap length for a constant transport electrode p i t c h L is illustrated i n Figure 4.6. A s indicated i n this figure, the m a x i m u m p  frequency of operation of the C M C C D increases w i t h the clock voltage amplitude a n d w i t h the interelectrode gap length. T h i s relationship is i n t u i t i v e l y correct as the electron 54  •Mobx-imiMrrt  Opera-tiriff Frequency  1 2 3 4 Clock Voltage Am.jplitixdie (-volts)  5  F i g u r e 4.6: T h e m a x i m u m frequency of operation of a G a A s C M C C D as a f u n c t i o n of the clock voltage a m p l i t u d e a n d the transport electrode length.  transit time is d o m i n a t e d by the time required for the electron to travel t h r o u g h the lowfield region u n d e r n e a t h the transport electrode. T h e electron transit time underneath the transport electrode is reduced by increasing the fringing field penetration f r o m the adjacent transport electrode. T h i s is accomplished either by increasing the clock voltage a m p l i t u d e or by r e d u c i n g the transport electrode length. It w o u l d appear f r o m the above description, that the transport electrodes of a G a A s C M C C D s h o u l d have a m i n i m u m length i n order to achieve the m a x i m u m operating b a n d w i d t h possible for the lowest clock power requirements. T h e charge transfer performance of a G a A s C M C C D was investigated i n a manner s i m i l a r to that used by S o d i n i et al [42]. A simulated single electrode transfer of a half f u l l well charge packet ( Q — 0.5 • I O s  - 1 0  c o u l / c m ) was performed a n d is illustrated  i n F i g u r e 4.7. T h e charge packet i n i t i a l l y resides under the phase one a n d phase two transport electrodes a n d is transferred to the region under the phase two a n d phase 55  <=0 pS  t=SS pS  Mm  t=ee ps  F i g u r e 4.7: T h e simulated single electrode transfer of a charge packet. T h e transport electrode l e n g t h is 3.0 microns. 56  Charge  0.4 O  20  Transfer  40  Efficiency  60  80  Time (-picosecond,s)  too  F i g u r e 4.8: T h e theoretical charge transfer efficiency as a f u n c t i o n of time for the G a A s C M C C D o b t a i n e d f r o m the simulated single electrode transfer of a charge packet.  three transport electrodes. T h e quadrature clock voltage f u n c t i o n consisted of a 2 volt a m p l i t u d e t r a p e z o i d a l pulse w i t h 100 picosecond edge transitions. A time increment of 8t = 0.1 picoseconds was used i n the s i m u l a t i o n . T h e theoretical charge transfer efficiency as a f u n c t i o n of time for the G a A s C M C C D was o b t a i n e d f r o m the s i m u l a t i o n results. T h e charge transfer efficiency i]{t) is defined as the ratio of the charge transferred to the transfer well to the charge i n i t i a l l y residing i n the storage well. F o r the s i m u l a t i o n of the 4-phase G a A s C M C C D Qph.2+Ph.3(t) Qph.i+Ph.2(t  =  (4.59) o)  F i g u r e 4.8 illustrates the theoretical charge transfer efficiency of the G a A s  CMCCD  o b t a i n e d f r o m the computer s i m u l a t i o n described above. T h i s figure indicates that the packet of charge is essentially fully transferred at the completion of the clock t r a n s i t i o n period t  tr  = 100 picoseconds.  T h i s result implies that the simulated transfer of the 57  charge packet was not transit time l i m i t e d a n d that the 4-phase G a A s C M C C D should exhibit good charge transfer at clock frequencies approaching f  c  = l/4i  < r  = 2.5 G H z .  Sovero et al [9] measured a charge transfer efficiency of 0.99 per transfer for a G a A s C M C C D operating at a clock frequency of 2.5 G H z , supporting the above theoretical result.  58  Chapter 5  Device Fabrication  F i g u r e 5.1 shows a microphotograph of a 64-pixel, 4-phase G a A s C M C C D . T h e i n p u t section is located on the left-hand end of the device a n d is shown i n detail i n F i g u r e 5.2. T h e control gates G i a n d G  2  are n o m i n a l l y 5 microns i n length a n d are separated by  2 m i c r o n gaps f r o m the i n p u t o h m i c contact, f r o m the first transport electrode a n d f r o m each other, respectively. There are 256 transport electrodes comprising the sixtyfour pixels w i t h i n the transport section of the C M C C D . T h e transport electrodes are 3 microns i n length a n d are separated by 3 m i c r o n gaps.  T h e phase one transport  electrodes a n d the phase three transport electrodes are interconnected along the lower side of the device while the phase two transport electrodes a n d the phase four transport electrodes are interconnected along the upper side of the device. T h e entire transport section is encapsulated w i t h a cermet film. T h e output section is located at the righth a n d end of the device a n d is shown i n detail i n F i g u r e 5.3. T h e o u t p u t o h m i c contact, the control gate G3 a n d the output source follower amplifier comprise this section. T h e control gate G3 is 5 microns i n length a n d is separated by 2 m i c r o n gaps f r o m the final transport electrode and f r o m the output o h m i c contact. T h e C M C C D channel is n o m i n a l l y 100 microns wide. T h e G a A s wafer that was used for p r o d u c i n g the C M C C D was a n u n d o p e d (100) oriented semi-insulating substrate onto which an n-type e p i t a x i a l layer was grown. T h e substrate was grown using the l i q u i d encapsulated C z o c h r a l s k i technique [43] a n d h a d a sheet resistivity exceeding 10  7  o h m - c m . Metal-organic chemical vapour phase  deposition [44] was used to grow the n-type active layer onto the substrate. T h i s layer consisted of a n o m i n a l 1-2 m i c r o n thick n~-buffer layer onto w h i c h the 0.25 m i c r o n n-type active layer was grown.  T h e active layer was u n i f o r m l y doped, w i t h No 59  =  Figure 5.1: A microphotograph of the fabricated G a A s C M C C D . T h e b o n d i n g pads are 100 m i c r o n squares.  4.5 • 1 0  16  cm" . 3  T h e fabrication of the G a A s C M C C D required six mask levels that employed a 2.0 m i c r o n m i n i m u m design rule. T h e mask levels provided the patterns for fabricating the ohmic contacts, the isolated active regions, the m e t a l / G a A s Schottky barriers, the c e r m e t / G a A s Schottky barriers, the interconnect vias a n d the second level metallization. Conventional contact lithography was used to produce the device. A detailed list of the fabrication steps is described i n A p p e n d i x D . T h e o h m i c contacts [45] of the C M C C D a n d the M E S F E T s were fabricated i n i tially. A 1.2 m i c r o n thick positive photoresist f i l m was patterned onto the wafer surface. A n o m i n a l 1200 A A u - G e ( 12 w t . % G e ), 200 A N i a n d 1400 A A u o h m i c contact metallization was sequentially deposited onto the wafer surface using t h e r m a l evaporation a n d electron-beam evaporation i n a h i g h v a c u u m chamber. T h e unwanted metal was removed f r o m the wafer surface using the photoresist liftoff m e t h o d [46]. 60  The  F i g u r e 5.2: A microphotograph of the input section of the G a A s C M C C D .  61  ohmic contacts were completed by alloying the ohmic contact m e t a l l i z a t i o n w i t h the underlying G a A s . To achieve a planar device structure, m u l t i p l e energy p r o t o n isolation implants were used to isolate the active device regions. E a r l y investigators of the G a A s C G C C D used a mesa etch to achieve the required isolation [47,48]. A l t h o u g h this technique is simple to implement a n d provides good isolation, it has the drawback that the subsequent lithography is hampered by the different elevations between the mesa plateaus a n d the surrounding valleys. A planar G a A s C G C C D was realized using Schottky barrier channel stops [49] to isolate the active device regions. A channel stop must completely surround the active device region to be effective, which is a disadvantage as it becomes difficult to r u n first level metallizations directly between isolated regions. P r o t o n bombardment was used to isolate the active device regions of a G a A s C G C C D [50]. T h i s m e t h o d has the desirable feature that it does not alter the G a A s surface profile a n d thus does not have the associated problems of the above isolation techniques. T h e active regions for the C M C C D and the M E S F E T s were electrically isolated using a sequence of three p r o t o n implants at different beam energies. A n o m i n a l 7 m i cron thick patterned photoresist f i l m was used as a barrier to protect the active device regions during the implants. T h e exposed G a A s was sequentially b o m b a r d e d using protons at i o n energies of 180 k e V , 90 k e V a n d 30 k e V . Fluences of 1 0 5 • 10  13  cm  - 2  13  cm  - 2  and  were used for the first two implants a n d the final i m p l a n t , respectively.  N o post-implantation anneal was performed. G o o d electrical isolation was achieved, w i t h the measured resistivity of the deactivated G a A s exceeding 10 o h m - c m . 5  T h e m e t a l / G a A s Schottky barriers comprising the transport electrodes a n d the M E S F E T gates were patterned using the photoresist liftoff m e t h o d .  A nominal  500 A T i , 100 A P t a n d 2150 A A u multilayer film was sequentially electron b e a m evaporated onto the wafer surface t h r o u g h a 1.2 m i c r o n thick photoresist mask. 62  The  photoresist a n d the unwanted metallization were removed i n an ultrasonic N - m e t h y l 2-pyrrolidone solvent b a t h . T h e c e r m e t / G a A s Schottky barriers w i t h i n the interelectrode gaps of the C M C C D were patterned using the photoresist liftoff process.  A n o m i n a l 5000 A thick f i l m of  C r - S i O ( n o m i n a l 45 w t . % C r ) was rf diode sputtered f r o m a 6 inch composite target onto the wafer surface t h r o u g h a 2.1 m i c r o n thick photoresist mask. T h e target was separated f r o m the substrate table by 1.5 inches a n d was sputtered at 13.56 M H z i n a 10 m T o r r argon environment [51]. T h e input power to the target was approximately 250 watts rms, achieving a dc target bias of -800 volts relative to the substrate table. It was observed that a target bias of less t h a n -1000 volts was detrimental to the photoresist f i l m . A n extended 24 hour chamber preconditioning p e r i o d was required prior to the 30 minute deposition to achieve a u n i f o r m C r : S i O f i l m . T h e photoresist and the unwanted cermet f i l m were removed i n an ultrasonic N-methyl-2-pyrrolidone solvent b a t h subsequent to the deposition. F i g u r e 5.4 shows a transmission electron microphotograph of the structure of the C r : S i O  film.  T h e dark areas correspond to  the regions of highest atomic density and are believed to be the result of c h r o m i u m compounds [52].  Energy dispersive x-ray analysis ( E D X ) was used to ascertain the  chemical composition of the film and it was f o u n d to be 41.7 weight percent c h r o m i u m , which is i n agreement w i t h the manufacturer's target specification of 45 weight percent chromium. A 1.8 m i c r o n thick interlayer dielectric film of p o l y i m i d e was used to protect the active G a A s surface a n d to separate the two m e t a l l i z a t i o n levels f r o m each other.  A  layer of d i l u t e d D u Pont P Y R A L I N PI-2550 p o l y i m i d e was applied to the G a A s wafer surface using a spin-on technique [53]. T h e p o l y i m i d e was d i l u t e d to a lower viscosity using D u Pont T-9039 thinner at a 1:1 d i l u t i o n ratio. T h e p o l y i m i d e was i m i d i z e d i n a controlled forced air convection oven using a low temperature 250 °C heating cycle for nearly 3 hours [54]. T h i s heating cycle was below the eutectic temperature of the o h m i c 63  Figure 5.4: A transmission electron microphotograph of the C r : S i O ( 45 wt. % C r ) f i l m at 150,000 times magnification. T h e number ' 1 ' i n the label corresponds to a height of 2.0 m m at this magnification.  Figure 5.5: A microphotograph of the plasma etch profile of a 5 m i c r o n square v i a etched through a 1.8 m i c r o n thick polyimide film. 64  contacts [45] a n d the annealing temperature of the p r o t o n isolation implants [55,56] and consequently d i d not alter the electrical characteristics of these fabricated structures. T h e interconnect vias between the first level m e t a l l i z a t i o n a n d the second level metallization were chemically etched t h r o u g h the i m i d i z e d p o l y i m i d e f i l m using a three step p l a s m a etch process employing p l a s m a enhanced chemical vapour etching [57,58]. A n o m i n a l 600 A thick t i t a n i u m f i l m was deposited onto the i m i d i z e d p o l y i m i d e surface using electron beam evaporation. T h e surface of the t i t a n i u m f i l m was subsequently covered w i t h a 1.2 m i c r o n thick patterned photoresist mask. T h e exposed regions of the t i t a n i u m film were etched t h r o u g h the photoresist mask using a C F / 0 4  2  plasma,  transferring the photoresist pattern to the t i t a n i u m film. T h e exposed regions of the p o l y i m i d e film were etched t h r o u g h the t i t a n i u m mask using an O2 p l a s m a , transferring the original photoresist pattern to the polyimide film. T h e photoresist film was also removed f r o m the t i t a n i u m surface during this etch. T h e t i t a n i u m mask was removed i n a final C F 4 / O 2 plasma etch. F i g u r e 5.5 shows the resultant vertical etch profile of a 5 m i c r o n square interconnect v i a etched t h r o u g h the p o l y i m i d e film using the above p l a s m a etch process. T h e second level m e t a l l i z a t i o n was patterned using the photoresist liftoff m e t h o d . A n o m i n a l 500 A T i a n d 4000 A A u multilayer m e t a l l i z a t i o n was sequentially electron beam evaporated onto the wafer surface t h r o u g h a 2.1 m i c r o n thick photoresist mask a n d the subsequent liftoff was performed i n an ultrasonic acetone b a t h , completing the fabrication of the G a A s C M C C D . T h e intermediate t i t a n i u m layer provided the required adhesion between the polyimide film a n d the second level m e t a l l i z a t i o n gold layer.  65  Chapter 6 T e s t i n g and E v a l u a t i o n  A series of dc threshold voltage measurements were performed on the G a A s C M C C D p r i o r to packaging the device to determine if the control gates a n d the transport electrode arrays were functional. A Tektronix T E K - 5 7 6 curve tracer attached to an Alessi probe station was used to make these measurements. T h e i n p u t o h m i c contact a n d the output ohmic contact of the C M C C D were used as the d r a i n a n d the source, respectively. A 5 volt d r a i n to source bias was applied to the device. E a c h of the three control gates and each of the four transport electrode arrays were biased, i n t u r n , negatively w i t h respect to the source node u n t i l no further change was observed i n the d r a i n to source current. T h e observed gate to source voltage corresponding to this c o n d i t i o n was recorded as the threshold voltage. Table V contains a list of the measured threshold voltages of the G a A s C M C C D . T h e C M C C D was mounted i n a 32 p i n ceramic flat package. A discrete D E X C E L 2502 G a A s M E S F E T die was also mounted i n the package for use as a reset switch at the output o h m i c contact of the C M C C D . T h e C M C C D , the on-chip G a A s M E S F E T source follower amplifier a n d the discrete G a A s M E S F E T die were wire-bonded i n the package using the configuration shown i n F i g u r e 6.1.  T h i s packaging configuration  resulted i n a m i n i m u m parasitic capacitance Co/p at the output ohmic contact of the C M C C D . T h i s is desirable for obtaining m a x i m u m output signal amplitudes f r o m the C M C C D , as the signal charge a r r i v i n g at the output o h m i c contact is converted to a voltage w i t h a n a m p l i t u d e inversely p r o p o r t i o n a l to  Co/p-  T h e on-chip G a A s M E S F E T source follower amplifier buffered the output ohmic contact of the C M C C D f r o m the external output electronics. It consisted of two depletion mode 2 m i c r o n by 30 m i c r o n M E S F E T s configured i n a totem pole arrangement 66  Figure 6.1: T h e wire-bonding configuration used to interconnect the C M C C D , the on-chip G a A s M E S F E T source follower amplifier a n d the discrete G a A s M E S F E T die.  Figure 6.2: T h e insertion loss of the on-chip G a A s M E S F E T source follower amplifier measured f r o m 300 k H z to 200 M H z .  67  Gate Gi G  Threshold Voltage (Volts) -2.4 -2.45 -2.4  2  G Ph. 1 3  -1.8  Ph. 2  -1.8  Ph. 3 Ph. 4  -1.85 -1.7  Table V : T h e measured threshold voltages of the G a A s C M C C D .  which provided a low capacitance, h i g h impedance load to the output o h m i c contact of the C M C C D . A threshold voltage of approximately -2.2 volts a n d a saturation current of approximately 5.0 milliamperes was measured for these transistors using a Tektronix T E K - 5 7 9 curve tracer. T h e insertion loss of the source follower amplifier terminated i n 50 o h m source a n d load impedances was measured f r o m 300 k H z to 200 M H z using a H e w l e t t - P a c k a r d H P - 8 7 5 3 A network analyzer a n d an H P - 8 5 0 4 6 A s-parameter test set. T h e measured insertion loss is shown i n F i g u r e 6.2. T h e G a A s C M C C D was operated i n the V H F b a n d at 100 M H z a n d was evaluated for operation at this frequency using the impulse response m e t h o d [59] a n d the insertion loss method [60]. T h e C M C C D was operated using the signal levels listed i n Table V I , w h i c h were provided by a test circuit comprised of emitter-coupled logic ICs a n d discrete GaAs M E S F E T s .  A schematic diagram of the C M C C D test circuit is provided i n  A p p e n d i x E . Charge injection into the C M C C D was obtained using the diode cutoff method described i n C h a p t e r 2. T h e test circuit h a d a b a n d w i d t h of approximately 150 M H z a n d was the l i m i t i n g factor for testing the C M C C D at higher clock frequencies. A T e k t r o n i x P G - 5 0 2 250 M H z pulse generator a n d a T E K - 7 9 0 4 oscilloscope frame mounted w i t h a 7A24 d u a l trace amplifier a n d a 7 B 9 2 A d u a l timebase unit were used for the impulse response measurement a n d a H e w l e t t - P a c k a r d H P - 8 7 5 3 A network analyzer w i t h an H P - 8 5 0 4 6 A s-parameter test set was used for the insertion loss measurement. 68  Figure 6.3: T h e qualitative demonstration of the performance of the G a A s C M C C D for 100 M H z operation.  Figure 6.4: T h e impulse response of the G a A s C M C C D for 100 M H z operation. 69  Figure 6.5: T h e insertion loss of the G a A s C M C C D for 100 M H z operation.  Frequency , ,—  10\  O -nnn nn n u  u  Response ,  rj = 0.998  t  3 ^-10  &-20 Measured N  -30  data.  a  Theory  s  §-40 -50 O  10 Input  20 Signal  30 Frequency  40 (MHz)  50  Figure 6.6: T h e theoretical insertion loss of the G a A s C M C C D for 100 M H z operation. 70  Gate I/P  Signal level(s) ( volts ) 0 to  +0.5  Gx  -0.8 to -5.8  G Ph. 1  +0.3 to -4.7 0 to -5.0  Ph. 2  0 to -5.0  2  Ph. 3 Ph. 4 R/G G B/D 3  B/S R/S  0 0 0 -2.7 +5.0 -5.2 0  to to to to to to to  -5.0 -5.0 -5.0 -2.7 +5.0 -5.2 0  Table V I : T h e signal levels applied to the G a A s C M C C D for operation at 100 M H z .  A qualitative demonstration of the performance of the G a A s C M C C D for 100 M H z operation is shown i n F i g u r e 6.3. T h e oscillograph contained i n this figure displays the C M C C D input signal along the upper signal trace a n d the processed C M C C D output signal along the lower signal trace. T h e input signal was obtained by passing a trapezoidal pulse through a passive lowpass filter having a cutoff frequency of 20.5 M H z . T h e damped oscillations were a result of the filter response to the 1 nanosecond transitions of the input pulse. T h e processed C M C C D output signal was obtained by filtering the buffered C M C C D output signal using a lowpass filter similar to the one used at the i n p u t . Figure 6.3 demonstrates the good signal fidelity of the C M C C D for 100 M H z operation. Figure 6.4 shows an oscillograph of the impulse response of the C M C C D  for  100 M H z operation. T h e C M C C D input signal is along the upper signal trace a n d the buffered C M C C D output signal is along the lower signal trace. T h e i n p u t signal consisted of a 5 nanosecond wide, 2.4-volt amplitude, 1 nanosecond t r a n s i t i o n trapezoidal impulse.  T h e buffered C M C C D output waveform contains the m o d u l a t i o n envelope  of the impulse a n d the passive feedthrough of the quadrature clocks. T h e m o d u l a t i o n 71  Charge Transfer Efficiency  Method Impulse Response Insertion Loss  1.00 0.998  Table V I I : T h e charge transfer efficiencies of the G a A s C M C C D for 100 M H z operation.  envelope of the impulse consists of a single pulse transient delayed b y 640 nanoseconds w i t h respect to the i n p u t signal. T h e charge transfer efficiency of the C M C C D was determined f r o m the C M C C D impulse response using the calculation [59] N  where N  T  (6.1)  = N (l-ri)  peak  T  is the number of single electrode transfers t h r o u g h the C M C C D a n d  N  peak  is the number of pixel transfers between the peak of the observed C M C C D impulse response a n d the peak of the ideal C M C C D impulse response. T h e computed charge transfer efficiency is listed i n Table V I I . T h e insertion loss of the G a A s C M C C D for 100 M H z operation is shown i n F i g ure 6.5. A -10 d B m swept frequency sinusoidal signal spanning the range f r o m 300 k H z to the N y q u i s t frequency of 50 M H z was applied to the i n p u t o h m i c contact of the C M C C D a n d the insertion loss of the device was measured. F i g u r e 6.5 indicates that the C M C C D has a nearly u n i f o r m insertion loss over the entire 50 M H z i n p u t signal b a n d w i d t h , w h i c h is indicative of good performance. T h e charge transfer efficiency of the C M C C D was determined by fitting the equation [60]  A  D  B  = 20 log A e x p 0  -N (l T  - ?y)  to the measured data. Here AJ,B is the insertion loss, A u> is the i n p u t signal frequency a n d f  c  cos  0  (f  (6.2)  is a constant a m p l i t u d e t e r m ,  is the C M C C D clock frequency. T h e calculated  charge transfer efficiency using the insertion loss m e t h o d is listed i n Table V I I a n d the curve fit to the measured d a t a is shown i n F i g u r e 6.6.  72  Chapter 7 Comments  7.1  Summary  C o n t r i b u t i o n s were made towards developing the G a A s C M C C D for h i g h frequency sign a l processing applications. T h e design, implementation a n d evaluation of the C M C C D were considered a n d are summarized i n this section. T h e design equations for determining the active layer requirements of the G a A s C M C C D were described i n C h a p t e r 2. T h e design protocol that was outlined assumes that the active layer was u n i f o r m l y doped a n d was constrained to have a pinch-off voltage that was t y p i c a l of an n-type depletion mode M E S F E T . It was demonstrated that the f u l l well charge confinement a n d the full well capacity of the C M C C D were simultaneously m a x i m i z e d if the device was fabricated on a t h i n , highly doped active layer.  T h i s result suggested that the o p t i m u m C M C C D active layer was similar to  the active layer of a low to m e d i u m power n-type depletion mode G a A s M E S F E T , which was advantageous when the two devices were integrated monolithically. It was indicated i n C h a p t e r 1 that the G a A s C G C C D could not be m o n o l i t h i c a l l y integrated w i t h G a A s M E S F E T s i n a simple manner as the C G C C D was t y p i c a l l y fabricated on thick, lightly doped active layers w h i c h were not directly compatible w i t h M E S F E T s . It was demonstrated i n C h a p t e r 3 that a c e r m e t / G a A s Schottky barrier diode exhibits rectification properties similar to that of a m e t a l / G a A s Schottky barrier diode w i t h a large series resistance.  T h e Schottky barrier height a n d the ideality factor of  the c e r m e t / G a A s j u n c t i o n were determined using a distributed resistive gate Schottky barrier diode m o d e l of the fabricated planar c e r m e t / G a A s Schottky barrier diode. A Schottky barrier height of 0.64 e V a n d an ideality factor of 1.17 were determined for the c e r m e t / G a A s j u n c t i o n . 73  A transmission line model described i n C h a p t e r 3 for the c e r m e t / G a A s j u n c t i o n w i t h i n an interelectrode gap of the C M C C D was used to demonstrate that the surface potential d i s t r i b u t i o n along the gap was monotonic for a l l frequencies.  A differential  length of the transmission line model consisted of a differential series impedance and a differential shunt admittance. T h e series impedance modeled the cermet f i l m a n d was comprised of a parallel resistance a n d capacitance. T h e shunt admittance modeled the depletion layer capacitance of the u n d e r l y i n g G a A s . It was determined f r o m an analysis of the transmission line model that the h i g h frequency surface potential v a r i a t i o n along a gap of the C M C C D was independent of the d i s t r i b u t e d cermet f i l m resistance.  This  was an important result as it indicated that the operation of the C M C C D was not critically dependent u p o n the distributed cermet f i l m resistance, provided that it was large enough to satisfy the power constraints of the quadrature clocks. It was further demonstrated i n C h a p t e r 3 using the transmission line model that the distributed cermet f i l m capacitance was preferably greater t h a n the distributed depletion layer capacitance i n order to m a i n t a i n a nearly linear surface potential variation along the gap of a C M C C D for all frequencies.  T h i s was desirable for achieving an o p t i m a l  u n i f o r m tangential electric field d i s t r i b u t i o n w i t h i n the C M C C D active layer to assist charge transfer. A two-dimensional computer model for investigating the operation of the G a A s C M C C D was described i n C h a p t e r 4.  A unit cell representing a single pixel of a 4-  phase G a A s C M C C D consisted of a d o m a i n comprised of the active layer s u b d o m a i n a n d the semi-insulating substrate subdomain. T h e transport electrodes were defined as equipotential boundaries a n d the interelectrode gaps were defined as linear potential boundaries.  A finite difference grid was superimposed onto the unit cell, on which  the semiconductor equations were solved. A N e w t o n iteration scheme w i t h successive relaxation was used to solve the finite difference equations for the potential a n d the electron density. C o m p u t e r simulations for the static potential distributions w i t h i n the  74  C M C C D were used to determine the theoretical m a x i m u m frequency of operation of the device as a f u n c t i o n of the interelectrode gap length a n d the peak clock voltage amplitude for a constant transport electrode p i t c h .  It was demonstrated that the  C M C C D transport electrodes should have a m i n i m u m length to achieve the m a x i m u m frequency of operation for the lowest possible power requirements.  A s i m u l a t i o n of  the d y n a m i c single electrode transfer of a half f u l l well of charge i n 100 picoseconds was demonstrated. T h i s s i m u l a t i o n indicated that the charge packet was essentially fully transferred by the end of the transfer interval, suggesting that the C M C C D w i l l demonstrate good performance at frequencies approaching 2.5 G H z . A six mask level fabrication process for p r o d u c i n g the G a A s C M C C D was described i n C h a p t e r 5.  Conventional contact lithography was used to fabricate the  device. T h e six mask levels provided the patterns for the o h m i c contacts, the p r o t o n isolation implants, the m e t a l / G a A s Schottky barriers, the c e r m e t / G a A s Schottky barriers, the interconnect vias a n d the second level m e t a l l i z a t i o n . T h e A u - G e / N i / A u ohmic contacts, the T i / P t / A u m e t a l / G a A s Schottky barriers a n d the C r : S i O c e r m e t / G a A s Schottky barriers were patterned directly on the n-type active layer. Three p r o t o n i m plants at different beam energies were used to isolate the C M C C D active region a n d the M E S F E T source follower amplifier active region, m a i n t a i n i n g a planar device structure. A p o l y i m i d e interlayer dielectric film was used to separate the first level m e t a l l i z a t i o n f r o m the second level m e t a l l i z a t i o n . Connections between the two m e t a l l i z a t i o n levels were made t h r o u g h interconnect vias that were p l a s m a etched t h r o u g h the polyimide film. T h e operation of the G a A s C M C C D was described i n C h a p t e r 6. T h e dc threshold voltage measurements were used to select the C M C C D f r o m the fabricated devices. T h e C M C C D , the on-chip M E S F E T source follower amplifier a n d a discrete G a A s M E S F E T die were wire-bonded i n a 32 p i n ceramic package. T h e M E S F E T was used as a reset switch on the output ohmic contact of the C M C C D . A test circuit was used to 75  provide the signals to the packaged components. T h e diode cutoff m e t h o d described i n C h a p t e r 2 was used to inject charge into the C M C C D . T h e C M C C D was operated using a clock frequency of 100 M H z a n d was evaluated at this operating frequency using the impulse response m e t h o d a n d the insertion loss m e t h o d . T h e C M C C D demonstrated good performance at 100 M H z clock frequency w i t h charge transfer efficiencies of 1.00 and 0.998 calculated respectively using the above two evaluation techniques. 7.2  Considerations for F u t u r e W o r k  T h i s work was focused on the design, implementation and evaluation of a 64-pixel, 4-phase G a A s C M C C D . T h e issues which could be addressed i n further developing this device are described i n this section. T h e 64-pixel, 4-phase G a A s C M C C D that was developed i n this work was not fully o p t i m i z e d . T h e fabricated C M C C D h a d an active layer w i t h a u n i f o r m donor density of 4.5 • 1 0  16  cm  - 3  to a depth of 0.25 microns. These active layer parameters were  satisfactory for demonstrating the operation of the C M C C D w i t h a G a A s M E S F E T , but would not necessarily yield the best possible device performance. T h e 3 m i c r o n C M C C D transport electrode length was chosen for convenience, i n order that the fabrication requirements to produce the device would be reduced. A revision to the above C M C C D structure would consist of using a C M C C D active layer w i t h a u n i f o r m donor density of approximately 2.0 • 1 0  17  cm  - 3  to a depth of approximately 0.1 microns which  is more consistent w i t h the active layer requirements of a n o m i n a l -2.0 volt n-type depletion mode G a A s M E S F E T . Furthermore, the analysis described i n C h a p t e r 2 indicates that the revised active layer parameters are preferred, as the signal charge confinement a n d the signal charge capacity of the C M C C D w o u l d be i m p r o v e d . T h e revised C M C C D transport electrode length would be 1.0 microns or less i n order that the high frequency performance a n d the associated power requirements of the C M C C D would be i m p r o v e d as described i n C h a p t e r 4. 76  T h e monolithic integration of the peripheral support electronics w i t h the G a A s C M C C D is essential for o b t a i n i n g m a x i m u m performance f r o m the device. In particular the output reset switch a n d the output signal processing c i r c u i t r y should be directly integrated w i t h the C M C C D . T h i s level of integration would increase the operating b a n d w i d t h , increase the d y n a m i c range and increase the signal to noise ratio of the C M C C D . T h i s would be a consequence of the reduction i n the parasitic component values attached to the output node of the C M C C D . A 2-phase G a A s C M C C D structure could also be considered for further investigation as it would m a x i m i z e the u t i l i z a t i o n of the active device area by achieving a greater p i x e l density a n d it would significantly reduce the clock driver circuit requirements. Hansell developed a castellated 2-phase G a A s C G C C D that exhibited a charge transfer efficiency of 0.93 [20]. It was determined by Hansell that the reduced charge transfer efficiency of the 2-phase C G C C D was largely due to the presence of energy troughs w i t h i n the active layer volume bounded on the surface by the interelectrode gaps. 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Gallium Arsenide House, D e d h a m , M A , 1984.  Processing  Techniques,  pages 44-45.  Artech  A . Piotrowska, A . G u i v a r c ' h , a n d G.Pelous. O h m i c contacts to I I I - V comp o u n d semiconductors: a review of fabrication techniques. Solid-State Electronics, 26(3):179-197, 1983. R . E . W i l l i a m s . Gallium Arsenide House, D e d h a m , M A , 1984.  Processing  Techniques,  pages 125-127. A r t e c h  I. Deyhimy, J . S . H a r r i s , J r , R . C . E d e n , R . J . A n d e r s o n , a n d D . D . E d w a l l . A n u l t r a high speed G a A s C C D . Int. Electron Devices Meeting, 619-621, 1979. I. Deyhimy, R . J . A n d e r s o n , a n d S. Lane. G a A s C C D . In IEEE Electronics Aerospace Systems Conference, pages 151-154, I E E E , New Y o r k , 1980.  and  M . D . C l a r k , C . L . A n d e r s o n , R . A . Jullens, a n d G . S . K a m a t h . P l a n a r sealed-channel g a l l i u m arsenide Schottky-barrier charge-coupled devices. IEEE Trans. Electron Devices, E D - 2 7 ( 6 ) : 1 1 8 3 - 1 1 8 8 , J u n . 1980. Y . Z . L i u , R . J . A n d e r s o n , I. Deyhimy, a n d L . R . Tomasetta. Proton-bombardment isolated G a A l A s / G a A s charge-coupled devices. Electron. Lett., 16(9):327-329, A p r . 1980. H . M a t i n o a n d T . U s h i r o d a . Effect of substrate bias o n properties of rf-sputtered C r - S i O films. IBM J. Res. Develop., 576-579, N o v . 1977. M . Mager. U B C Metallurgy, private communication. D u Pont Company. P Y R A L I N polyimide coatings: preliminary information b u l letin spin coating techniques. M a y 1985. B u l l e t i n : P C - 2 . D u Pont Company. Bulletin: P C - 1 .  P Y R A L I N polyimide coatings for electronics.  A p r . 1982.  A . G . Foyt, W . T . L i n d l e y , C M . Wolfe, a n d J . P . Donnelly. Isolation of j u n c t i o n devices i n G a A s using proton bombardment. Solid-State Electronics, 12:209-214, 1969. D . C . D ' A v a n z o . P r o t o n isolation for G a A s integrated circuits. IEEE tron Devices,  Trans.  Elec-  E D - 2 9 ( 7 ) : 1 0 5 1 - 1 0 5 8 , J u l . 1982.  J . W . C o b u r n . Plasma-assisted etching. Plasma  2 ( l ) : l - 4 0 , 1982. 81  Chemistry  and Plasma  Processing,  [58] F . D . E g i t t o , F . E m m i , R . S . H o r w a t h , a n d V . V u k a n o v i c . P l a s m a etching of organic materials. 1. p o l y i m i d e i n 0 - C F . J. Vac. Sci. Technol. B, 3(3):893-904, M a y 1985. 2  4  [59] C H . Sequin a n d M . F . Tompsett. Press, New Y o r k , 1975.  Charge  Transfer  Devices,  pages 73-76. A c a d e m i c  [60] G . F . Vanstone, J . B . G . Roberts, a n d A . E . L o n g . T h e measurement of the charge residual for C C D transfer using impulse a n d frequency responses. Solid-State Electronics, 17:889-895, 1974. [61] J . V . Cresswell, I. C a r v a h l o , M . L e N o b l e , O . Berolo, a n d R . K u l e . A 500 M H z C C D serial analog memory. IEEE Trans. Nucl. Sci., N S - 3 3 ( 1 ) : 9 0 - 9 1 , Feb. 1986. [62] D . A . B r y m a n . T R I U M F , private communication. [63] B N L , P r i n c e t o n , T R I U M F C o l l a b o r a t i o n . Current struction K  +  Status TT+VV.  of Detector  for  AGS Experiment  Goals,  R&D, Design,  #787—A  Study  and  of the  ConDecay  Technical Report, T R I U M F , O c t . 1985.  [64] J . V . Cresswell, S. A h m a d , E . W . Blackmore, D . A . B r y m a n , N . K a h n , Y . K u n o , a n d T . N u m a o . A c y l i n d r i c a l drift chamber for the measurement of K —> irvv decay. IEEE  Trans.  Nucl.  Sci, 3 5 ( l ) : 4 6 0 - 4 6 3 , Feb. 1988.  [65] J . V . Cresswell. T R I U M F , private communication.  82  Appendix A  B N L Experiment  787  T h e development of the G a A s C M C C D was inspired by the need for a w i d e b a n d d a t a acquisition system for E x p e r i m e n t 787, w h i c h is currently being prepared for implement a t i o n at the Brookhaven N a t i o n a l L a b o r a t o r y [61]. T h i s nuclear physics experiment is being conducted collaboratively w i t h scientists a n d engineers f r o m B r o o k h a v e n , Princeton U n i v e r s i t y a n d T R I U M F . T h e experimental goals of E x p e r i m e n t 787, the relevant technical aspects of the detector apparatus for this experiment a n d the a p p l i c a t i o n of a C M C C D i n the d a t a acquisition system for i n s t r u m e n t i n g the detector are described i n this appendix. T h e development of a comprehensive S t a n d a r d M o d e l for describing the interactions that occur amongst the elementary subatomic particles is of current interest to nuclear physicists. T h e existence of three neutrino generations has been established w i t h i n the current framework of this model a n d must be experimentally verified.  A  test for the number of neutrino generations w i l l be attempted i n E x p e r i m e n t 787 by directly observing a n d measuring the rate of decay of a kaon to a p i o n a n d n e u t r i n o / a n t i neutrino p a i r . T h i s p a r t i c u l a r decay sequence is extremely rare a n d is anticipated to occur once i n approximately every ten b i l l i o n kaon decays [62].  Should the observed  decay rate lie i n the v i c i n i t y of the expected rate, then a positive test for the existence of three neutrino generations w i l l have been made. It has been suggested that new generations of neutrinos, or perhaps, new elementary particles may exist if the observed rate of decay of a kaon to a p i o n and n e u t r i n o / a n t i - n e u t r i n o p a i r is greater t h a n about five times the anticipated rate [63]. A sophisticated rare kaon decay spectrometer is currently being developed for E x p e r i m e n t 787 to provide the required detection capability for observing the decay 83  of a kaon to a p i o n a n d n e u t r i n o / a n t i - n e u t r i n o pair.  A cross-sectional view of this  apparatus displaying its relevant features is shown i n F i g u r e A . l . T h e detector is c y l i n d r i c a l i n shape w i t h overall dimensions of approximately 6 metres i n length b y 5 metres i n diameter.  T h e target is located along the central axis of the detector  core a n d is surrounded by a c y l i n d r i c a l drift chamber [64] that is enclosed w i t h i n a scintillation counter range stack. A burst of highly energetic kaons f r o m the B N L accelerator arrives along the central axis of the detector penetrating the target.  T h e m a j o r i t y of incident kaons  are stopped w i t h i n the target a n d decay into other particles. T h e newly formed decay particles traverse the detector i n a manner that is dependent u p o n their energy, mom e n t u m a n d lifetime. T h e c y l i n d r i c a l drift chamber is used to m o n i t o r the energies and the trajectories of the particles as they are emitted f r o m the target.  T h e pions that  result f r o m a decaying kaon pass through the c y l i n d r i c a l drift chamber a n d are u l t i mately stopped w i t h i n the scintillation counter range stack where they decay into other particles. A p r i m a r y f u n c t i o n of the scintillation counter range stack is to provide the positive identification of the pions that emerge f r o m the drift chamber. T h i s is achieved by tracking the decay of the p i o n to a m u o n a n d the subsequent decay of the m u o n to an electron using energy versus time measurements. These interactions are detected as electrical signals at the output of the p h o t o m u l t i p l i e r tubes that are attached to the scintillation counter range stack.  T h e ideal output waveform obtained f r o m a range  stack p h o t o m u l t i p l i e r tube for the p i o n to m u o n to electron decay sequence is shown in Figure A . 2 . T h e waveform shown i n F i g u r e A . 2 is a simplification of the complex series of interactions that occur between the energetic particles a n d the nuclear instruments. In principle, the observed pulses tend to pile up onto each other due to the previous history w i t h i n the spectrometer. Consequently, the waveform illustrated i n F i g u r e A . 2 for the practical case w i l l consist of many superimposed pulses having peak separations that 84  /  VETO  PHOTOTUBES  1  BEAM  • * - B E A M MWPC  •TARGET SC1NT;  RANGE STACK PHOTOTUBES  TARGET RM.SUPPORT"  i  Figure A . l : A cross-sectional view of the B N L E x p e r i m e n t 787 rare kaon decay spectrometer. 85  Decay  Energy  us  Time  50  40  30  >»•  20  -  •<s> CO  o  ^10  O  o  10  20 Time  30 (ns)  40  50  60  Figure A.2: The ideal output waveform for a pion to muon to electron decay sequence obtained from a photomultiplier tube attached to the end of the scintillation counter range stack. The leading peak corresponds to the energy deposited by a pion to muon decay and the trailing peak corresponds to the energy deposited by the subsequent muon to electron decay. The vertical bars represent discrete pulse amplitudes obtained for a 2 nanosecond sampling rate [62].  Range  Stack  PMT  500  A/D  Data  Converter  MHz  Routing  Off-line Processing  Off-line Processing  Timebase  Off-line Trigger  Processing  Figure A.3: A system block diagram of the GaAs C M C C D based data acquisition system for the analog to digital conversion of a signal from a range stack photomultiplier tube [65]. 86  Figure A . 4 : T h e 64-pixel, 4-phase G a A s C M C C D p r o v i d i n g frequency compression. T h e i n p u t signal along the upper trace ( 20 n s / c m ) acquired at 483 M H z consists of two superimposed 30 ns pulses. T h e output signal along the lower trace ( 1 / / s / c m ) shows the processed i n p u t signal after frequency compression.  vary f r o m zero to many tens of nanoseconds.  A d a t a acquisition system employing a  64-pixel, 4-phase G a A s C M C C D is currently being developed at T R I U M F for recording these waveforms. A block diagram of the G a A s C M C C D data acquisition system [65] is shown i n Figure A . 3 . In this application the G a A s C M C C D provides frequency compression of a 250 M H z b a n d - l i m i t e d analog input signal. T h e i n p u t signal applied to the C M C C D is obtained f r o m a photomultiplier tube attached to the scintillation counter range stack. A n externally generated acquisition trigger pulse enables the application of a 500 M H z high frequency clock to the C M C C D . T h e acquisition cycle occurs for 128 nanoseconds filling the C M C C D w i t h sixty-four discrete samples of the i n p u t signal.  The  two nanosecond resolution of the input signal is considered sufficient for discriminating the two energy peaks that are observed d u r i n g the p i o n to m u o n to electron decay se87  quence [62]. Subsequent to the completion of the d a t a acquisition cycle, a 7.81 M H z low frequency clock pulse burst is applied to the C M C C D compressing the acquired signal by a factor of sixty-four. T h e compressed signal is transmitted to an analog to digital converter a n d the b i n a r y d a t a resulting f r o m the analog to d i g i t a l transformation is routed to a d a t a bus for sparse d a t a processing a n d d i s t r i b u t i o n to off-line computer resources. F i g u r e A . 4 shows an oscillograph i l l u s t r a t i n g the p r e l i m i n a r y results obtained for a 64-pixel, 4-phase G a A s C M C C D operating i n the frequency compression mode.  88  Appendix B P o l a r T r a n s f o r m a t i o n of V ( y )  T h e complex h a r m o n i c a l l y varying surface potential is given as  %^)= 'Vff"^ • sMl|  smh[L 7(u;)] s  where the factor e  3ujt  has been suppressed. L e t =  ( 9-yh(u) L  — C(UJ) + jd(u>)  L ^(u) g  w i t h 7 ( 0 ; ) defined as 7(<*>) =  WRCMCD yjl +  and  a(y,L>)+]b(y,u)  J  (  exp - arctan I — .2  (URCMCCM)  1  —— .  XURCM^CM'  •  T h e functions a(y,u>), b(y,u>), c(ui) a n d d(to) are WRCMCD  a(y,u)  {L - y) cos g  ^Jl +  {WRCMCCM)  2  I  URCMCD , \fl +  (If, - y) sin  (URCMCCM)  2  WRCMCD  c(u)  Lg  (URCMCCM)  , y^l +  2  1 COS  2  1 / - arctan ( —  arctan .  1  2  (WRCMCCM)  1  ——  KURCM&CM  1  g  \ yjl +  1  KUJKCM^CM  / L sin - arctan I ——  URCMCD  d(u)  1 / - arctan I —  —  \URCM&CM  Substitute equation B . 2 and equation B . 3 into equation B . l yields \r( \ v{y,u) =  sinh(a + j6) ^-77—;—-Vo sinh(c + jd) gjd  ^  cg  jd  e [cos(6) + j sin(o)] — e~ [cos(o) — j sin(o)] a  a  e [cos(d) + j sin(cf)] — e c  _ c  V  [cos(d) — j sin(rf)]  sinh(a) cos(6) + j cosh(a) sin(6)  Vo  sinh(c) cos(cf) + j cosh(c) sin(d) 89  0  Transforming equation B . 9 into polar f o r m yields  f c °t ( °J s i n S) (^ffiffj)  J s i n h ( a ) cos (fe) + cosh (a) sin (6)Z arctan 2  2  2  2  )/sinh (c) cos (rf) + cosh (c) sin (rf)Z arctan 2  2  2  2  U s i n g the trigonometric identities sinh (2)  =  cosh (» -  sin (^)  =  l-cos (z)  2  2  2  1  and  (B.H)  (B.12)  2  gives V(y,u) = V H(y,u)ie(y,Lo)  (B.13)  0  where H(y,u>) is the normalized magnitude of the surface potential  TT(„ .-A H  y y >  u  I  ) -  12  \  ^  COsh2  cosh  r  /  ~ ° C  \i  [C(UJ)\  s 2  2  ^ ' ^ ^1* y  U  r  \i  — cos^ |a(w)J  m ) U  (B.14J  /  a n d Q(y,u>) is the phase shift of the surface potential 0(y,u>) = arctan (coth[a(y,a;)] tan[6(y,u>)]) — arctan (coth[c(o;)] tan[<i(a;)])  90  . (B.15)  Appendix C Newton's Method Newton's method [39] is a numerical technique for finding the roots of a general function /(C)  = 0  a  n  a  is described i n this appendix.  Consider a point f u n c t i o n /(C)  that is i n the v i c i n i t y of a root of the function /(C)-  The  can be expanded i n a T a y l o r series about the point Q to give  / ( O = / ( O k . + (c A root of the equation /(C)  c,)/'(C)k. +  • • • + —xc - o r / m!  ( m )  (c.i)  (0k. + • • • .  = 0 can be obtained approximately by replacing /(C)  with  the first two terms of the expansion given i n equation C . I  /(C)lc, + (C-CO/'(C)lc. = o •  (C2)  Rearranging equation C . 2 a n d performing the function evaluations at Q gives < = G - $ §  •  (C3)  T h e value of £ computed i n this manner is an improved estimate for the o r i g i n a l root (i of the function /(C),  a n d can replace Q i n equation C . 3 to provide an even better  estimate for the root. T h i s can be w r i t t e n i n the generalized f o r m W+i  =  £(£1  W  f r o m w h i c h the correction factor for the I  th  (CA)  iterate is  $C' = C' -C' • +1  (C5)  F r o m equations C.4 a n d C . 5 one obtains  /(C) + *C7'(C) - o  (c.6)  which forms the basis of the N e w t o n iteration technique for numerically solving the set of finite difference equations listed i n Table I V . 91  Appendix D D e t a i l e d Device F a b r i c a t i o n P r o c e d u r e 1. O h m i c contact f o r m a t i o n 1.1— 3 m i n . immersion i n an ultrasonic acetone b a t h . 1.2— 3 m i n . sequential immersion i n each of hot trichloroethylene, hot acetone, a n d hot 2-propanol baths. 1.3— 1 m i n . N wafer dry. 2  1.4— Spin-coat the wafer @ 4000 R P M for 30 sec. w i t h A Z 4110 photoresist. 1.5— Softbake the photoresist coated wafer i n a forced air oven @ 90 ± 2 °C for 30 m i n . 1.6— Expose the photoresist coated wafer to 405 n m , 4.5 m W c m light for 14 sec t h r o u g h the 'ohmic contact' mask.  - 2  ultraviolet  1.7— Spray develop the exposed photoresist using A Z 4 0 0 K developer d i l u t e d to a volume ratio of 1:3 w i t h D I H 0 . 2  1.8— Immerse the wafer i n a 20 ml:200 m l , N H O H : D I H 0 oxide etch b a t h for 30 sec. 4  2  1.9— 1 m i n . D I H 0 rinse a n d subsequent 1 m i n . N wafer dry. 2  2  1.10— Sequentially evaporate the following metal films under high v a c u u m 1.10.1— 1150 A A u - G e ( 12 w t . % G e ), 1.10.2— 200 A N i , 1.10.3— 1400 A A u . 1.11— 3 m i n . immersion i n an ultrasonic acetone b a t h to lift-off the unwanted metal. 1.12— 3 m i n . immersion i n an ultrasonic acetone b a t h . 1.13— 3 m i n . sequential immersion i n each of hot trichloroethylene, hot acetone, a n d hot 2-propanol baths. 1.14—  1 m i n . N wafer dry. 2  1.15— A l l o y o h m i c contacts @ 468 °C for 1.5 m i n . i n a zone controlled quartz tube furnace w i t h 0.8 1/min N flowing t h r o u g h the tube. 2  2. P r o t o n isolation implants 92  2 . 1 — 3 m i n . immersion i n an ultrasonic acetone b a t h . 2.2— 3 m i n . sequential immersion i n each of hot trichloroethylene, hot acetone, a n d hot 2-propanol baths. 2 . 3 — 1 m i n . N wafer dry. 2  2.4— Spin-coat the wafer @ 4000 R P M for 30 sec. w i t h A Z 4620 photoresist. 2 . 5 — Softbake the photoresist coated wafer i n a forced air oven @ 90 ± 2 °C for 30 m i n . 2.6— Expose the photoresist coated wafer to 405 n m , 4.5 m W c m light for 98 sec t h r o u g h the ' p r o t o n i m p l a n t ' mask  - 2  ultraviolet  2.7— Spray develop the exposed photoresist using A Z 4 0 0 K developer d i l u t e d to a volume ratio of 1:3 w i t h D I H 0 . 2  2.8— Postbake the photoresist coated wafer i n a forced air oven @ 120 ± 2 °C for 30 m i n . 2.9— P e r f o r m the multiple-energy p r o t o n implants 2 . 9 . 1 — E i = 180 k e V , D i = 1 0  cm" ,  13  2  2.9.2— E  2  = 90 k e V , D = 1 0  2.9.3— E  3  = 30 k e V , D = 5 • 1 0  2  13  cm" , 2  3  13  cm" . 2  2.10— 15 m i n . immersion i n a hot N-methyl-2-pyrrolidone b a t h to remove the photoresist. 3. M e t a l / G a A s Schottky barrier formation 3 . 1 — 3 m i n . immersion i n an ultrasonic acetone b a t h . 3.2— 3 m i n . sequential immersion i n each of hot trichloroethylene, hot acetone, a n d hot 2-propanol baths. 3.3— 1 m i n . N wafer dry. 2  3.4— Spin-coat the wafer @ 4000 R P M for 30 sec. w i t h A Z 4110 photoresist. 3 . 5 — Softbake the photoresist coated wafer i n a forced air oven @ 90 ± 2 °C for 30 m i n . 3.6— Expose the photoresist coated wafer to 405 n m , 4.5 m W c m ultraviolet light for 14 sec t h r o u g h the ' m e t a l / G a A s Schottky barrier' mask. - 2  3.7— Spray develop the exposed photoresist using A Z 4 0 0 K developer d i l u t e d to a volume ratio of 1:3 w i t h D I H 0 . 2  93  3.8— Immerse the wafer i n a 20 ml:200 m l , N H O H : D I H 0 oxide etch b a t h for 30 sec. 4  2  3.9— • 1 m i n . D I H 2 O rinse a n d subsequent 1 m i n . N wafer dry. 2  3.10— Sequentially evaporate the following metal films under h i g h v a c u u m 3.10.1— 500 A T i , 3.10.2— 100 A P t , 3.10.3— 2150 A A u . 3 . 1 1 — 15 m i n . immersion i n a hot N-methyl-2-pyrrolidone ultrasonic b a t h to liftoff the unwanted metal. 3.12— 5 m i n . immersion i n an ultrasonic acetone b a t h . 4. C e r m e t / G a A s Schottky barrier formation 4 . 1 — 3 m i n . immersion i n an ultrasonic acetone b a t h . 4 . 2 — 3 m i n . sequential immersion i n each of hot trichloroethylene, hot acetone, a n d hot 2-propanol baths. 4 . 3 — 1 m i n . N wafer dry. 2  4.4— Spin-coat the wafer @ 4000 R P M for 30 sec. w i t h A Z 4210 photoresist. 4 . 5 — Softbake the photoresist coated wafer i n a forced air oven @ 90 ± 2 °C for 30 m i n . 4 . 6 — Expose the photoresist coated wafer to 405 n m , 4.5 m W c m ultraviolet light for 28 sec t h r o u g h the ' c e r m e t / G a A s Schottky barrier' mask. - 2  4 . 7 — Spray develop the exposed photoresist using A Z 4 0 0 K developer d i l u t e d to a volume ratio of 1:3 w i t h D I H 0 . 2  4.8— Postbake the photoresist coated wafer i n a forced air oven @ 120 ± 2 °C for 30 m i n . 4 . 9 — Immerse the wafer i n a 20 ml:200 m l , N H O H : D I H 0 oxide etch b a t h for 30 sec. 4  2  4.10— 1 m i n . D I H 0 rinse a n d subsequent 1 m i n . N wafer dry. 2  2  4 . 1 1 — rf diode sputter C r - S i O ( 45 w t . % C r . ) onto the surface —frequency: 13.56 M H z , — b a c k g r o u n d chamber pressure: < 2 • 1 0 mTorr, —gas: A r , — d e p o s i t i o n chamber pressure: 10 m T o r r , - 6  94  — r f forward power: w 250 W a t t s , rf reflected power: < 13 W a t t s resulting i n a target bias of -800 volts, — P r e c o n d i t i o n time: 24 hrs., — D e p o s i t i o n time: 30 m i n . 4.12— 5 m i n . immersion i n a hot N-methyl-2-pyrrolidone ultrasonic b a t h to lift-off the unwanted cermet f i l m . 4.13— 5 m i n . immersion i n an ultrasonic acetone b a t h . 5. Interconnect  via formation  5 . 1 — 3 m i n . immersion i n an ultrasonic acetone b a t h . 5 . 2 — 3 m i n . sequential immersion i n each of hot trichloroethylene, hot acetone, a n d hot 2-propanol baths. 5 . 3 — Immerse the wafer i n a 20 ml:200 m l , N H O H : D I H 0 oxide etch b a t h for 30 sec. 4  2  5.4— 1 m i n . D I H 2 O rinse a n d subsequent 1 m i n . N wafer dry. 2  5 . 5 — Spin-coat the wafer @ 4000 R P M for 1 m i n . w i t h D u Pont P Y R A L I N P I 2550 p o l y i m i d e d i l u t e d to a volume ratio of 1:1 w i t h D u Pont T-9039 t h i n ner. 5.6— Imidize p o l y i m i d e i n a forced air oven @ 250 ± 2 °C for 3 hrs. 5.7— Evaporate 600 A of T i onto the p o l y i m i d e surface under h i g h v a c u u m . 5.8— Spin-coat the wafer @ 4000 R P M for 30 sec. w i t h A Z 4110 photoresist. 5.9— Softbake the photoresist coated wafer i n a forced air oven @ 90 ± 2 °C for 30 m i n . 5.10— E x p o s e the photoresist coated wafer to 405 n m , 4.5 m W c m light for 14 sec through the 'interconnect vias' mask.  ultraviolet  - 2  5.11— Spray develop the exposed photoresist using A Z 4 0 0 K developer d i l u t e d to a volume ratio of 1:3 w i t h D I H 0 . 2  5.12— P l a s m a etch the interconnect vias — b a c k g r o u n d chamber pressure: < 25 m T o r r , — r f power: 150 W a t t s , 5.12.1— PcF* = 256 m T o r r , P m T o r r for 90 sec.  0 2  =  23 m T o r r a n d P  5.12.2— P F = 0 m T o r r , P o = 250 m T o r r a n d for 10 m i n . C  4  2  95  Pchamber  c k m t e r  =  =  279  250 m T o r r  5.12.3— P F = 256 m T o r r , P o m T o r r for 90 sec. C  4  2  =  23 m T o r r a n d  P  =  c h a m b e r  279  6. Second level interconnect metal formation 6 . 1 — 3 m i n . immersion i n an ultrasonic acetone b a t h . 6.2— 3 m i n . sequential immersion i n each of hot trichloroethylene, hot acetone, a n d hot 2-propanol baths. 6.3— 1 m i n . N wafer dry. 2  6.4— Spin-coat the wafer @ 4000 R P M for 30 sec. w i t h A Z 4210 photoresist. 6.5— Softbake the photoresist coated wafer i n a forced air oven @ 90 ± 2 °C for 30 m i n . 6.6— Expose the photoresist coated wafer to 405 n m , 4.5 m W c m light for 28 sec t h r o u g h the 'second level m e t a l l i z a t i o n ' mask.  - 2  ultraviolet  6.7— Spray develop the exposed photoresist using A Z 4 0 0 K developer d i l u t e d to a volume ratio of 1:3 w i t h D I H 0 . 2  6.8— Sequentially evaporate the following metal films under h i g h v a c u u m 6.8.1— 500 A T i , 6.8.2— 4000 A A u . 6.9— 3 m i n . immersion i n an ultrasonic acetone b a t h to lift-off the unwanted metal. 6.10— 3 m i n . immersion i n an ultrasonic acetone b a t h . 6.11— 3 m i n . sequential immersion i n each of hot trichloroethylene, hot acetone, a n d hot 2-propanol baths. 6.12—  1 m i n . N wafer dry. 2  96  Appendix E Test C i r c u i t for V H F O p e r a t i o n  T h e G a A s C M C C D was operated i n the V H F b a n d at 100 M H z using the test circuit illustrated i n Figure E . l .  Figure E . l : T h e schematic d i a g r a m of the test circuit used to operate the C M C C D i n the V H F b a n d .  97  

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