Physics, technology and applications of junction charge-coupled devices

applications of junction

charge-coupled devices

O -.1

Physics, technology and

applications of junction

charge-coupled devices

BIBLIOTHEEK TU Delft P 1697 4340

applications of junction

charge-coupled devices

Proefschrift

Ter verkrijging van de graad van doctor in

de technische wetenschappen aan de

Technische Hogeschool Delft, op gezag

van de rector magnificus,

prof. ir. B.P.Th. Veltman,

voor een commissie aangewezen door het

college van dekanen, te verdedigen op

donderdag 3 juni 1982 te 16.00 uur

door

Evert A n t o n i u s Wolsheimer

elektrotechnisch ingenieur > i

CONTENTS

1. INTRODUCTION 1

References 3

2. P R I N C I P L E S AND A P P L I C A T I O N S OF CHARGE-COUPLED DEVICES 5

2.1. Introduction 5 2.2. P r i n c i p l e s of the three types of buried-channel CCD's 5

2.3. Comparison of the three types of BCCD's l l 2.4. A p p l i c a t i o n s of charge-coupled devices 16

References 19

3. COMPUTER ANALYSIS OF JUNCTION CHARGE-COUPLED DEVICES 2 3

3.1. Introduction 23

3.2. One-dimensional a n a l y s i s of the JCCD 25 3.3. Two-dimensional c a l c u l a t i o n s on JCCD1s 36

3.4. A n a l y t i c a l formulation of d i f f u s e d and implanted 37

impurity p r o f i l e s

3.5. Numerical formulation 40

3.6. Results of the two-dimensional c a l c u l a t i o n s 42

3.6.1. Optimization of potential profiles in the 43 charge transport direction

3.6.2. Optimization of the lateral confinement of the 62 channeI

3.6.3. The use of V-groove etching in JCCD's 66

3.7. Scaling of j u n c t i o n charge-coupled devices 68

References 71 4. MEASUREMENT OF CCD CHARACTERISTICS 7 5 4.1. Introduction 75 4.2. F a b r i c a t i o n technology 7 6 4.2.1. Process #1 7 6 4. 2..2. Process #2 78 4. 2. 3. Process #3 8 1

4.3. Measurement of p o t e n t i a l p r o f i l e s 4.3.1. Measuring technique 4.3.2. Experimental data 4.4. Measurement of t r a n s f e r e f f i c i e n c y 4.4.1. Measuring technique 4.4.2. Experimental data

4.5. Measurement of dark current

4.5.1. Measuring technique 4.5.2. Experimental data

4.6. Conclusions References

5. ANALOG A P P L I C A T I O N S OF JUNCTION CHARGE-COUPLED DEVICES 115

5.1. Introduction 115 5.2. Charge d e t e c t i o n and charge i n j e c t i o n in JCCD's " 6

5.2.1. A linear charge-detection structure H 6

5.2.2. A linear charge-injection structure '21

5.2.2.1. I n t r o d u c t i o n 121 5.2.2.2. S i m u l a t i o n of the input s t r u c t u r e '23

5.2.2.3. Experiments '27

5.3. A programmable transversal JCCD f i l t e r '32

5.5.1. Introduction '32

5.3.2. Realization of the filter '34 5.3.3. Performance of the filter '36

5.3.4. Conclusions '41

References '42

Appendix A: Junction Charge-Coupled Logic '45

L i s t of symbols 165

Summary 168 Samenvatting 170 Acknowledgement 172 About the author 173

83 83 89 94 94 99 107 107 109 112 113

1. INTRODUCTION

In t h i s t h e s i s the p h y s i c s , t e c h n o l o g y and some a p p l i c a t i o n s o f j u n c t i o n c h a r g e - c o u p l e d d e v i c e s (JCCD's) a r e d e s c r i b e d . The c o n c e p t o f the JCCD was f i r s t d e s c r i b e d i n 1972 by Schuermeyer e t a l . [ 1 . 1 ] , a l t h o u g h no e x p e r i m e n t a l r e s u l t s were r e p o r t e d i n h i s p a p e r . Schuermeyer d e s c r i b e s how a CCD can be r e a l i z e d by r e p l a c i n g the MOS c a p a c i t o r , w h i c h i s used i n c o n v e n t i o n a l CCD's, by a r e v e r s e - b i a s e d pn j u n c t i o n ( r e s u l t i n g i n a j u n c t i o n c h a r g e - c o u p l e d d e v i c e ) , or by a r e v e r s e - b i a s e d S c h o t t k y b a r r i e r ( r e s u l t i n g i n a S c h o t t k y c h a r g e - c o u p l e d d e v i c e ) . B o t h the JCCD and the S c h o t t k y CCD a r e d e r i v e d from the b u r i e d - c h a n n e l CCD, w h i c h was i n v e n t e d i n 1972 by E s s e r [1.2] and Walden [1.3] a t about the same t i m e . The f i r s t p u b l i c a t i o n on CCD's was by B o y l e and Smith i n 1970 [ 1 . 4 ] , and d e s c r i b e s the w o r k i n g o f a s u r f a c e - c h a n n e l CCD. The f i r s t e x p e r i m e n t a l r e s u l t s on JCCD's were r e p o r t e d by K l e e f s t r a i n 1975 [ 1 . 5 ] , f o l l o w e d i n 1977 by a paper by K e l l n e r e t a l . [1.6] on the e x p e r i m e n t a l r e s u l t s o f a S c h o t t k y CCD r e a l i z e d i n GaAs t e c h n o l o g y . The JCCD d e s c r i b e d by K l e e f s t r a i n 1975 has some i m p o r t a n t a d v a n t a g e s over c o n v e n t i o n a l CCD's, such as the p o s s i b i l i t y f o r v e r t i c a l c h a r g e f l o w t h r o u g h the p - t y p e s i l i c o n g a t e s , the e x c e l l e n t a n t i - b l o o m i n g p r o p e r t i e s and the p o s s i b i l i t y t o i n t e g r a t e b i p o l a r t r a n s i s t o r s and j u n c t i o n FET's on the same c h i p w i t h o u t a d d i t i o n a l p r o c e s s i n g . An e x t e n s i v e d e s c r i p t i o n o f the p r o p e r t i e s o f the JCCD w i l l be g i v e n i n

t h i s t h e s i s .

The main p r o b l e m w i t h the f i r s t JCCD was the r a t h e r low t r a n s f e r e f f i c i e n c y , w h i c h was caused by l a r g e p a r a s i t i c w e l l s i n i t s p o t e n t i a l p r o f i l e . A t h e o r e t i c a l s o l u t i o n t o t h i s p r o b l e m was g i v e n by Herman e t a l . [ 1 . 7 ] . The JCCD's

which were f a b r i c a t e d by u s i n g the r e s u l t s o f t h e i r c a l c u l a t i o n s had a s i g n i f i c a n t l y b e t t e r t r a n s f e r e f f i c i e n c y t h a n the f i r s t JCCD's [ 1 - 8 ] * The p r o c e s s u s e d , however, was n o t v e r y r e p r o d u c i b l e and f u r t h e r r e s e a r c h was r e q u i r e d .

In t h i s t h e s i s the p r o p e r t i e s o f the JCCD a r e a n a l y z e d , and t h e o r e t i c a l and e x p e r i m e n t a l r e s u l t s a r e g i v e n . In

c h a p t e r 2 the b a s i c p r i n c i p l e s of the three types of

b u r i e d - c h a n n e l CCD's (the MOS BCCD, the S c h o t t k y CCD and the JCCD) a r e compared and some a p p l i c a t i o n s a r e m e n t i o n e d .

C h a p t e r 3 d e a l s with c a l c u l a t i o n s on the p o t e n t i a l p r o f i l e i n the JCCD. A model which o f f e r s a way t o c a l c u l a t e the p o t e n t i a l i n the JCCD i s d e s c r i b e d . S e v e r a l p o s s i b i l i t i e s f o r f a b r i c a t i n g JCCD's a r e d i s c u s s e d . I t i s shown t h a t the JCCD c e l l can be s c a l e d down c o n s i d e r a b l y w i t h o u t c a u s i n g d i s t o r t i o n s i n the p o t e n t i a l p r o f i l e .

The e x p e r i m e n t a l r e s u l t s , w h i c h a r e g i v e n i n c h a p t e r 4, d e a l w i t h JCCD's which a r e f a b r i c a t e d by making use o f the r e s u l t s o f the t h e o r e t i c a l model d e s c r i b e d i n c h a p t e r 3 . A c c e n t i n t h i s c h a p t e r i s l a i d on the measurement o f i m p o r t a n t JCCD c h a r a c t e r i s t i c s , s u c h as the p o t e n t i a l p r o f i l e , t r a n s f e r e f f i c i e n c y , c h a r g e - h a n d l i n g c a p a b i l i t y and dark c u r r e n t . Some i m p o r t a n t a p p l i c a t i o n s o f the JCCD a r e d e s c r i b e d i n c h a p t e r 5. The emphasis i s on a p p l i c a t i o n s which employ the u n i q u e f e a t u r e s o f the JCCD. C h a r g e - i n j e c t i o n and c h a r g e - d e t e c t i o n s t r u c t u r e s a r e d e s c r i b e d and e x p e r i m e n t a l r e s u l t s a r e g i v e n . The s t r u c t u r e s d e s c r i b e d a r e a p p l i e d i n a p a r a l l e l - i n , s e r i a l - o u t programmable t r a n s v e r s a l JCCD f i l t e r , which i s a l s o d e s c r i b e d i n t h i s c h a p t e r .

Appendix A c o n t a i n s a p r e p r i n t o f a paper by E.P May, C.L.M.v.d.Klauw, M . K l e e f s t r a and E.A.Wolsheimer on l o g i c c i r c u i t s u s i n g j u n c t i o n c h a r g e - c o u p l e d d e v i c e s ( J u n c t i o n C h a r g e - C o u p l e d L o g i c ) .

REFERENCES [1.1] F . L . S c h u e r m e y e r , R . A . B e l t , C.R.Young and J . M . B l a s i n g a m e , "New s t r u c t u r e s f o r c h a r g e - c o u p l e d d e v i c e s " , P r o c . I E E E , v o l . 60, p p . 1444-1445, Nov. 1972 [1.2] L . J . M . E s s e r , " P e r i s t a l t i c c h a r g e - c o u p l e d d e v i c e : a new t y p e o f c h a r g e - t r a n s f e r d e v i c e " , E l e c t r o n . L e t t . , v o l . 8, pp. 620-621, Dec. 1972

[1.3] R.H.Walden, R.H.Krambeck, R . J . S t r a i n , J.McKenna, N . L . S c h r y e r and G.E.Smith, "The b u r i e d - c h a n n e l c h a r g e - c o u p l e d d e v i c e " ,

B e l l S y s t . T e c h . J . , v o l . 51, pp. 1635-1640, S e p t . 1972

[1.4] W.S.Boyle and G.E.Smith, " C h a r g e - c o u p l e d s e m i c o n d u c t o r d e v i c e s " , B e l l S y s t . T e c h . J . , v o l . 49, pp. 587-593, A p r i l 1970 [1.5] M . K l e e f s t r a , " F i r s t e x p e r i m e n t a l b i p o l a r c h a r g e - c o u p l e d d e v i c e " , M i c r o e l e c t r o n . , v o l . 7, pp. 68-69, Dec. 1975

[1.6] W . K e l l n e r , H . B i e r h e n b k e and H.Kniepkamp, "A S c h o t t k y b a r r i e r CCD on GaAs",

i n P r o c . 23rd I n t . E l e c t r o n D e v i c e s M e e t i n g , W a s h i n g t o n , D.C., Dec. 1977

[1.7] G.C.Herman, C . D . H a r t g r i n g and M . K l e e f s t r a , " C a l c u l a t i o n o f p o t e n t i a l p r o f i l e s i n the j u n c t i o n

c h a r g e - c o u p l e d d e v i c e " , IEEE T r a n s * E l e c t r o n D e v i c e s , v o l . ED-25, pp. 845-847, J u l y 1978 [1.8] M . K l e e f s t r a and E.A.Wolsheimer, " J u n c t i o n c h a r g e - c o u p l e d d e v i c e s " , i n P r o c . 25th I n t . E l e c t r o n D e v i c e s M e e t i n g , W a s h i n g t o n , D.C., Dec. 1979

2. PRINCIPLES AND APPLICATIONS OF

CHARGE - COUPLED DEVICES

2,1 INTRODUCTION

The p u r p o s e o f t h i s c h a p t e r i s t o i n t r o d u c e t h e b a s i c p r i n c i p l e s o f b u r i e d - c h a n n e l c h a r g e - c o u p l e d d e v i c e s (BCCD's) and t o make a c o m p a r i s o n between t h e v a r i o u s t y p e s o f BCCD's. A d i s t i n c t i o n w i l l be made h e r e between MOS BCCD's, S c h o t t k y CCD's and J u n c t i o n CCD's. Some c h a r a c t e r i s t i c s , s u c h as i m p u r i t y p r o f i l e s , p o t e n t i a l p r o f i l e s , and i n p u t and o u t p u t c o n f i g u r a t i o n s o f t h e t h r e e t y p e s o f CCD's w i l l be compared. A p p l i c a t i o n s o f CCD's w i l l be i n d i c a t e d b r i e f l y .

2.2 PRINCIPLES OF THE THREE TYPES OF BURIED-CHANNEL CCD'S

The b a s i c e l e m e n t o f a l l t y p e s o f BCCD's i s a c a p a c i t o r which c a n c o n t a i n c h a r g e c a r r i e r s . I n s e m i c o n d u c t o r d e v i c e s two t y p e s o f c a p a c i t o r s c a n be d i s t i n g u i s h e d [ 2 . 1 ] , The f i r s t i s t h e m e t a l - o x i d e - s e m i c o n d u c t o r (MOS) c a p a c i t o r . The second t y p e i s formed by a r e v e r s e - b i a s e d d i o d e , where t h e i n s u l a t i o n i s formed by a d e p l e t e d s e m i c o n d u c t o r l a y e r . The c a p a c i t o r i n a b u r i e d - c h a n n e l MOS CCD i s formed by a c o m b i n a t i o n o f b o t h t y p e s o f c a p a c i t o r s [ 2 . 2 ] . The c a p a c i t o r i n both a S c h o t t k y CCD and a J u n c t i o n CCD i s formed by two r e v e r s e - b i a s e d d i o d e s . The former combines a m e t a l - s e m i c o n d u c t o r d i o d e and a pn d i o d e [ 2 . 3 ] , whereas t h e

l a t t e r c o n s i s t s o f two r e v e r s e - b i a s e d pn d i o d e s [ 2 . 4 ] . The t h r e e c o n f i g u r a t i o n s a r e shown i n F i g . 2.1. The c a p a c i t o r shown i n F i g . 2.1.a forms p a r t o f a b u r i e d - c h a n n e l MOS CCD. I f t h e n-type l a y e r i s l e f t o u t , a s u r f a c e - c h a n n e l MOS CCD

Fig. 2.1. Basic structures of the three types of buried-channel CCD's (not to scale): (a) MOS BCCD, (b) Schottky CCD, (a) Junction CCD.

w i l l r e s u l t [2.5, 2 . 6 ] . S c h o t t k y CCD's and J u n c t i o n CCD's can o n l y be r e a l i z e d as b u r i e d - c h a n n e l CCD's. I n a l l t h r e e d e v i c e s shown t h e c h a r g e c a r r i e r s a r e e l e c t r o n s , w h i c h a r e s i t u a t e d i n the n - t y p e l a y e r . In F i g . 2.2 t h e s p a c e - c h a r g e d e n s i t y , the e l e c t r i c f i e l d and t h e e l e c t r o s t a t i c p o t e n t i a l i n a b u r i e d - c h a n n e l MOS CCD a l o n g t h e l i n e y i n F i g . 2.1.a a r e d e p i c t e d . A c h a r g e p a c k e t i s p r e s e n t i n t h e c h a n n e l between yj and y% ; t h i s p a r t o f t h e n - t y p e l a y e r i s n o t d e p l e t e d . I n b u r i e d - c h a n n e l CCD's t h e h i g h e s t e l e c t r i c f i e l d o c c u r s i n t h e s e m i c o n d u c t o r i f no c h a r g e i s p r e s e n t [ 2 . 4 ] . From F i g . 2.2 i t c a n be c o n c l u d e d t h a t t h e maximum c h a r g e p a c k e t e q u a l s (2.1) ¥=y0 w h i l e t h e maximum e l e c t r i c f i e l d i s g i v e n by

e t a l 0, 0 NAT met

I f

' I

/

S .

I

/

Fig. 2.2. (a) Space-charge density, (b) electric field and (c) electrostatic potential in a MOS BCCD.

y=y-.

-A

From (2-1) and (2-2) i t c a n be c o n c l u d e d t h a t t h e maximum c h a r g e p a c k e t i s g i v e n by m S% m (2-3) A t an i m p u r i t y c o n c e n t r a t i o n o f 10 cm i n t h e n - t y p e 5 -1 l a y e r , t h e maximum e l e c t r i c f i e l d e q u a l s 4.10 V.cm , r e s u l t i n g i n a maximum c h a r g e p a c k e t o f about 12 -2 3.10 e l e c t r o n s . c m . In p r a c t i c a l CCD's t h i s w i l l be somewhat lower i n o r d e r t o p r e v e n t breakdown. In F i g s . 2.3 and 2.4 t h e s p a c e - c h a r g e d e n s i t y , t h e e l e c t r i c f i e l d and t h e e l e c t r o s t a t i c p o t e n t i a l a r e shown f o r a S c h o t t k y CCD and a J u n c t i o n CCD, r e s p e c t i v e l y . In a l l t h r e e d e v i c e s m e n t i o n e d

metal

I

(a) (b) (c)

Fig. 2.3. (a) Space-charge density, (b) electric field and (c) electrostatic potential in a Schottky CCD.

Fig. 2.4. (a) Space-charge density, (b) electric field and (c) electrostatic potential in a Junction CCD.

homogeneously doped n - t y p e and p - t y p e l a y e r s a r e assumed. The d e v i c e s have i n common t h e f a c t t h a t t h e maximum p o t e n t i a l ( t h e c h a n n e l p o t e n t i a l ) i s s i t u a t e d a t some d i s t a n c e from t h e s u r f a c e . I f t h e c h a r g e p a c k e t s a r e n o t

too l a r g e t h e y w i l l n o t come i n t o c o n t a c t w i t h t h e s u r f a c e o f t h e d e v i c e s . By c l o s e l y s p a c i n g a number o f one o f t h e t h r e e t y p e s o f c a p a c i t o r s a CCD w i l l r e s u l t . An example i s shown i n F i g . 2.5 f o r a t h r e e - p h a s e MOS BCCD. t=t m M m Y////////A 777777/

Fig. 2.5. Cross section of a MOS BCCD and transfer of charge packets.

The c h a n n e l ( t h e n - l a y e r ) i s d e p l e t e d by a p p l y i n g a h i g h p o t e n t i a l t o t h e d r a i n ( D ) , w h i l e t h e s o u r c e (S) can be used to i n j e c t c h a r g e i n t o t h e c h a n n e l . By a p p l y i n g t h e c o r r e c t v o l t a g e s t o t h e g a t e s , c h a r g e p a c k e t s c a n be s h i f t e d a l o n g t h e c h a n n e l . T h i s p r o c e s s i s shown i n F i g . 2.5 f o r a t h r e e - p h a s e CCD. The t h r e e - p h a s e c l o c k i n g scheme shown i n

t h i s f i g u r e i s t h e one t h a t was used w i t h t h e f i r s t CCD's. CCD's have been f a b r i c a t e d as two-, t h r e e - and f o u r - p h a s e d e v i c e s . Two-phase CCD's a r e a t t r a c t i v e as f a r as c l o c k i n g r e q u i r e m e n t s and c e l l s i z e a r e c o n c e r n e d , but a d d i t i o n a l t e c h n o l o g i c a l s t e p s a r e r e q u i r e d i n o r d e r t o o b t a i n u n i d i r e c t i o n a l c h a r g e t r a n s p o r t [ 2 . 6 ] . F o u r - p h a s e CCD's form a good t r a d e - o f f between t e c h n o l o g i c a l d i f f i c u l t i e s and c l o c k i n g r e q u i r e m e n t s , a l t h o u g h t h e c e l l s i z e i s l a r g e r when compared t o two- or t h r e e - p h a s e CCD's. S e v e r a l c l o c k i n g schemes e x i s t f o r f o u r - p h a s e CCD's, two o f w h i c h a r e shown i n F i g . 2.6.

Fig. S.6. Two examples of clocking schemes for a four-phase CCD : (a) four-phase push clock, (b) semi two-phase clock.

F i g u r e 2.6.a d e p i c t s a c l o c k i n g scheme, based on t h e same p r i n c i p l e as t h e one shown i n F i g . 2.5. Charge i s t r a n s f e r r e d from one g a t e t o t h e next and e v e r y a d j a c e n t g a t e s e r v e s as a s t o r a g e g a t e [ 2 . 6 ] . A more a t t r a c t i v e c l o c k i n g scheme i s shown i n F i g . 2.6.b. Two g a t e s a r e k e p t

a t a c o n s t a n t DC l e v e l , w h i l e the o t h e r two g a t e s a r e Used t o t r a n s f e r the c h a r g e p a c k e t s . The CCD i s i n f a c t used as a two-phase d e v i c e h e r e [ e . g . 2 . 7 ] . The c h a r g e - h a n d l i n g c a p a b i l i t y i s about h a l f t h a t o f the c l o c k i n g scheme shown i n F i g . 2.6.a, but t h e use o f the DC l e v e l s w i t h two o f the f o u r g a t e s i s v e r y a t t r a c t i v e , e s p e c i a l l y i n f i l t e r a p p l i c a t i o n s . The DC g a t e s can be used t o d e t e c t or i n j e c t c h a r g e w i t h o u t d i s t u r b i n g c l o c k i n t e r f e r e n c e .

T h r e e p r o c e s s e s which g o v e r n t h e t r a n s f e r o f c h a r g e c a r r i e r s from one w e l l t o a n o t h e r can be d i s t i n g u i s h e d

[2.5, 2 . 6 ] . I f t h e p o t e n t i a l w e l l s a r e assumed t o be r e c t a n g u l a r , t h e r e a r e no f r i n g i n g f i e l d s , and d i f f u s i o n w i l l be t h e main t r a n s f e r mechanism f o r s m a l l c h a r g e p a c k e t s . For l a r g e r c h a r g e p a c k e t s , s e l f - i n d u c e d f i e l d s w i l l d o m i n a t e t h e c h a r g e t r a n s f e r . For b u r i e d - c h a n n e l CCD's, however, t h e p o t e n t i a l w e l l s a r e not r e c t a n g u l a r and f r i n g i n g f i e l d s w i l l c a u s e t h e l a s t f r a c t i o n o f a c h a r g e p a c k e t t o be t r a n s f e r r e d to the next w e l l v e r y q u i c k l y . F r i n g i n g f i e l d s a r e t h e r e a s o n why b u r i e d - c h a n n e l CCD's s u c h as the JCCD can be o p e r a t e d a t much h i g h e r c l o c k f r e q u e n c i e s t h a n s u r f a c e - c h a n n e l CCD's. T h e r e i s no d i f f e r e n c e i n the c h a r g e - t r a n s p o r t mechanism o f the t h r e e d e v i c e s shown i n F i g . 2.1. The main d i f f e r e n c e s t h a t do e x i s t l i e i n t e c h n o l o g y , i n p u t and o u t p u t s t r u c t u r e s and l i g h t s e n s i t i v i t y .

2.3 COMPARISON OF THE THREE TYPES OF BCCD'S

The MOS BCCD, which i s the o l d e s t o f the t h r e e t y p e s o f b u r i e d - c h a n n e l CCD's, has been f a b r i c a t e d i n many d i f f e r e n t ways. MOS BCCD's e x i s t w i t h two-, t h r e e - and f o u r - p h a s e o r g a n i z a t i o n , m e t a l and p o l y s i l i c o n g a t e s , s i l i c o n - d i o x i d e

or combined s i l i c o n - d i o x i d e and s i l i c o n - n i t r i d e l a y e r s as i s o l a t i o n and homogeneous or p r o f i l e d n - t y p e l a y e r s [2.5, 2 . 6 ] . CCD's r e a l i z e d i n GaAs t e c h n o l o g y a r e a t t r a c t i v e f o r h i g h - s p e e d a p p l i c a t i o n s b e c a u s e o f the h i g h m o b i l i t y o f f r e e c h a r g e c a r r i e r s i n t h i s m a t e r i a l . A s t a b l e n a t i v e o x i d e , w h i c h can be used i n MIS s t r u c t u r e s , does not e x i s t i n GaAs

t e c h n o l o g y [ 2 . 8 ] . T h i s i s the r e a s o n why S c h o t t k y b a r r i e r g a t e s a r e used r a t h e r t h a n t h e c o n v e n t i o n a l MOS g a t e s . C l o c k f r e q u e n c i e s up t o 1 GHz have been r e a l i z e d , u s i n g s i n e waves i n s t e a d o f the c o n v e n t i o n a l c l o c k p u l s e s f o r

t r a n s p o r t i n g c h a r g e p a c k e t s [ 2 . 9 ] .

In MOS CCD's the g a t e s a r e made o v e r l a p p i n g , i n o r d e r to m i n i m i z e d i s t o r t i o n s i n the p o t e n t i a l p r o f i l e . An i m p o r t a n t c h a r a c t e r i s t i c o f JCCD's and S c h o t t k y CCD's i s the f a c t t h a t the g a t e s c a n n o t be made o v e r l a p p i n g , as can be seen from i n s p e c t i o n o f F i g . 2.1. T h i s r e q u i r e s some s p e c i a l measures i n o r d e r t o p r e v e n t d i s t o r t i o n i n the c h a n n e l p o t e n t i a l p r o f i l e i n the t r a n s p o r t d i r e c t i o n o f the JCCD, w h i c h can s e r i o u s l y hamper the c h a r g e t r a n s f e r p r o c e s s . An e x t e n s i v e d e s c r i p t i o n o f t h e s e measures i n the c a s e o f a JCCD can be found i n c h a p t e r 3. The f a c t t h a t the g a t e s c a n n o t be made o v e r l a p p i n g or even c l o s e l y s p a c e d i n a JCCD r e s u l t s i n a l a r g e r b a s i c c e l l . In S c h o t t k y CCD's the gap between the g a t e s can be made as s m a l l as 1.0 ym [ 2 . 8 ] , and the b a s i c c e l l i n t h i s t y p e o f CCD can be made n e a r l y as s m a l l as t h a t i n MOS CCD's.

An i m p o r t a n t c h a r a c t e r i s t i c o f a CCD i s t h e d a r k c u r r e n t . A d e p l e t e d n - t y p e l a y e r , as used i n a CCD, i s i n a n o n - e q u i l i b r i u m s i t u a t i o n . An empty w e l l w i l l be f i l l e d w i t h e l e c t r o n s i n a t i m e which depends on the g e n e r a t i o n r a t e o f c h a r g e c a r r i e r s . In MOS CCD's and JCCD's the d a r k c u r r e n t i s about 1-10 nA.cm and w i l l f i l l the c h a r g e w e l l s i n t h e s e d e v i c e s i n about 10 s [ 2 . 6 ] . In S c h o t t k y CCD's,

however, a s t o r a g e time o f 5 h o u r s has been r e p o r t e d [ 2 . 8 ] . The measurements have been p e r f o r m e d on a CCD r e a l i z e d i n G a j - ^ A l ^ A s , w h i c h has a bandgap o f 1.9 eV, and an i n t r i n s i c c a r r i e r c o n c e n t r a t i o n 10^ t i m e s s m a l l e r t h a n s i l i c o n , r e s u l t i n g i n a v e r y low d a r k c u r r e n t .

For o p t i c a l a p p l i c a t i o n s t h e quantum e f f i c i e n c y , t h e number o f e l e c t r o n s w h i c h a r e g e n e r a t e d per i n c i d e n t p h o t o n , s h o u l d be as h i g h as p o s s i b l e . As can be seen by i n s p e c t i o n o f F i g . 2.1 t h e m e t a l - g a t e MOS CCD and the S c h o t t k y CCD have an i m p o r t a n t d i s a d v a n t a g e when compared to JCCD's. M e t a l g a t e s w i l l r e f l e c t or a b s o r b p a r t s o f the e l e c t r o m a g n e t i c s p e c t r u m which c o u l d o t h e r w i s e g e n e r a t e e l e c t r o n - h o l e p a i r s i n t h e s e m i c o n d u c t o r . When t o o many c h a r g e c a r r i e r s a r e g e n e r a t e d i n one w e l l by i n c i d e n t l i g h t , t h e y may o v e r f l o w i n t o the a d j a c e n t c e l l s . T h i s e f f e c t i s c a l l e d " b l o o m i n g " and s p e c i a l measures have t o be t a k e n i n MOS CCD's t o p r e v e n t t h i s phenomenon. In S c h o t t k y CCD's and i n JCCD's e x c e l l e n t a n t i - b l o o m i n g p r o p e r t i e s e x i s t , as has been shown i n [2.8] and [ 2 . 1 0 ] .

For many a p p l i c a t i o n s CCD's need t o be e q u i p p e d w i t h c i r c u i t s t o i n j e c t and d e t e c t c h a r g e p a c k e t s . For a n a l o g a p p l i c a t i o n s , t h e i n j e c t i o n and d e t e c t i o n o f c h a r g e has t o be v e r y l i n e a r . C l a s s i c a l i n p u t methods, s u c h as the f i l l - a n d - s p i l l method and t h e d i o d e c u t - o f f i n p u t , have been d e s c r i b e d e x t e n s i v e l y i n l i t e r a t u r e [2.11, 2.12]. The d i f f e r e n c e s among t h e t h r e e t y p e s o f BCCD's a r e v e r y s m a l l as f a r as t h e s e c l a s s i c a l i n p u t s t r u c t u r e s a r e c o n c e r n e d . The JCCD, however, o f f e r s a unique method f o r i n j e c t i n g c h a r g e i n t o the channel.' When an n+ d i f f u s i o n i s p l a c e d i n a p - t y p e g a t e , a v e r t i c a l NPN t r a n s i s t o r i s formed, as shown i n F i g . 2.7.a. The base o f t h e t r a n s i s t o r i s formed by the g a t e , the n+ d i f f u s i o n i s t h e e m i t t e r , and the c h a n n e l o f

t h e CCD t h e c o l l e c t o r . An e q u i v a l e n t c i r c u i t i s shown i n F i g . 2.7.b. T h i s i n p u t s t r u c t u r e can be u t i l i z e d to

gate emitter base gate _emitter base n-layer p-substrate collector (a) (b)

Fig. 2.7. (a) Charge-injection structure in a JCCD and (b) equivalent circuit. r e a l i z e a v e r y f a s t and l i n e a r c h a r g e i n j e c t o r , as w i l l be shown i n c h a p t e r 5. One o f t h e i m p o r t a n t a d v a n t a g e s o f t h i s i n p u t s t r u c t u r e i s t h e f a c t t h a t i t c a n be p l a c e d anywhere i n t h e JCCD, t h u s c r e a t i n g the p o s s i b i l i t y t o i n j e c t c h a r g e anywhere i n t h e JCCD. T h i s i s h i g h l y v a l u e d f o r some a p p l i c a t i o n s , s u c h as f i l t e r s and m u l t i p l e x e r s . As f a r as c h a r g e - d e t e c t i o n c i r c u i t s a r e c o n c e r n e d , t h e r e i s h a r d l y any d i f f e r e n c e among t h e t h r e e t y p e s o f BCCD's i n n o n - d e s t r u c t i v e f l o a t i n g - g a t e d e t e c t o r s . The JCCD, however, o f f e r s a u n i q u e way t o d e t e c t c h a r g e d e s t r u c t i v e l y . In F i g . 2.8.a a JCCD c a p a c i t o r i s shown. Under c e r t a i n c o n d i t i o n s , w h i c h w i l l be e x p l a i n e d i n Appendix A, t h e e l e c t r o n s w i l l f l o w towards t h e p - t y p e g a t e , t h u s f o r m i n g the base c u r r e n t o f t h e v e r t i c a l PNP t r a n s i s t o r . The e m i t t e r o f t h i s t r a n s i s t o r i s formed by t h e p - t y p e g a t e , w h i l e t h e s u b s t r a t e a c t s as a c o l l e c t o r . The e q u i v a l e n t c i r c u i t i s shown i n F i g . 2.8.b. T h i s d e s t r u c t i v e c h a r g e - d e t e c t i o n c i r c u i t o f f e r s a v e r y e l e g a n t way t o d e t e c t c h a r g e p a c k e t s w i t h o u t r e q u i r i n g a d d i t i o n a l

gate p gate o gate o emitter it P

J

J \

J~

Fig. 2.8. (a) Charge-detection structure in a JCCD and (b) equivalent circuit. t e c h n o l o g i c a l s t e p s . An a d v a n t a g e o f t h i s o u t p u t c i r c u i t i s t h e b u i l t - i n a m p l i f i c a t i o n o f t h e c h a r g e p a c k e t s . W i t h t h e p r o c e s s u s e d , t h e c u r r e n t a m p l i f i c a t i o n i s about 100. A n o t h e r i m p o r t a n t c h a r a c t e r i s t i c o f t h e JCCD i s t h e f a c t t h a t b i p o l a r t r a n s i s t o r s and n - c h a n n e l j u n c t i o n FET's c a n be used t o r e a l i z e o n - c h i p c l o c k c i r c u i t r y and i n p u t and o u t p u t e l e c t r o n i c s . I t c a n be c o n c l u d e d t h a t t h e JCCD has some i m p o r t a n t a d v a n t a g e s o v e r S c h o t t k y CCD's and MOS CCD's, s u c h as a h i g h e r l i g h t s e n s i t i v i t y , b e t t e r a n t i - b l o o m i n g p r o p e r t i e s i n l i n e s e n s o r s and the f a c t t h a t t h e r e a r e more p o s s i b i l i t i e s f o r i n j e c t i o n and d e t e c t i o n o f c h a r g e p a c k e t s . The main d i s a d v a n t a g e o f t h e JCCD i s t h e f a c t t h a t t h e b a s i c c e l l c a n n o t be as s m a l l as t h a t o f t h e o t h e r two CCD t y p e s , when i d e n t i c a l l a y o u t r u l e s a r e u s e d .

2A APPLICATIONS OF CHARGE-COUPLED DEVICES

S i n c e t h e i r i n v e n t i o n i n 1969 CCD's have been used i n a l a r g e number o f a p p l i c a t i o n s . I n t h i s s e c t i o n a b r i e f o v e r v i e u w w i l l be g i v e n o f t h e s e a p p l i c a t i o n s . T h r e e main a r e a s c a n be d i s t i n g u i s h e d : - a n a l o g a p p l i c a t i o n s ( d e l a y l i n e s and f i l t e r s ) - d i g i t a l a p p l i c a t i o n s (memories and l o g i c f u n c t i o n s ) - o p t i c a l a p p l i c a t i o n s ( l i n e s e n s o r s and a r e a s e n s o r s ) The b a s i c b u i l d i n g b l o c k f o r a n a l o g a p p l i c a t i o n s o f CCD's i s t h e a n a l o g d e l a y l i n e . The maximum d e l a y t i m e i s d e t e r m i n e d by t h e d a r k c u r r e n t o f t h e d e v i c e , w h i l e t h e minimum d e l a y time i s l i m i t e d by t h e maximum c l o c k f r e q u e n c y . T h e r e a r e s e v e r a l a p p l i c a t i o n s o f t h e CCD d e l a y l i n e , s u c h as t v - g h o s t s u p p r e s s i o n and t i m e - e r r o r c o r r e c t i o n f o r r e c o r d i n g systems [ 2 . 6 ] . One o f t h e c h a r a c t e r i s t i c s o f a n a l o g d e l a y l i n e s u s i n g b u r i e d - c h a n n e l CCD's i s t h e h i g h range o f t h e time d e l a y , which c a n e x c e e d 1 0 ^ . A more advanced a n a l o g a p p l i c a t i o n i s t o use CCD's i n f i l t e r s

[2.5, 2.6, 2 . 7 ] . These c a n be d i v i d e d i n t o two g r o u p s , namely t r a n s v e r s a l and r e c u r s i v e f i l t e r s . The use o f programmable or f i x e d t r a n s v e r s a l f i l t e r s i s w i d e l y s p r e a d and much r e s e a r c h i s s t i l l g o i n g on i n t h i s a r e a . The s t r u c t u r e o f a CCD t r a n s v e r s a l f i l t e r i s shown i n F i g . 2.9. The i n p u t s i g n a l V¿ i s sampled and t h e c h a r g e p a c k e t

i s s h i f t e d a l o n g t h e CCD d e l a y l i n e . The s i z e o f t h e c h a r g e p a c k e t i s d e t e c t e d i n e v e r y c e l l and m u l t i p l i e d by a w e i g h t i n g f a c t o r . The o u t p u t s i g n a l s o f t h e

m u l t i p l i e r s a r e added i n t h e o u t p u t c i r c u i t o f t h e f i l t e r . The o u t p u t s i g n a l o f t h e f i l t e r a t time t=nT e q u a l s

sample Vin(nT) T j T T T T sample

II

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Fig. 2.9. Structure of a serial-in, parallel-out transversal CCD filter. k=N V ,(nT) out k=l h. . V . (nT-kT) k in (2.4) The i m p u l s e r e s p o n s e o f t h e f i l t e r i s d e t e r m i n e d by t h e w e i g h t i n g f a c t o r s . T h i s f i l t e r c a r r i e s out t h e c o n v o l u t i o n between t h e i n p u t s i g n a l and t h e i m p u l s e r e s p o n s e . The f i l t e r c a n be made programmable by making t h e w e i g h t i n g f a c t o r s v a r i a b l e . T h e r e a r e many a p p l i c a t i o n s f o r t h i s t y p e o f f i l t e r , s u c h as low-pass f i l t e r s , bandpass f i l t e r s , H i l b e r t t r a n s f o r m e r s and c h i r p - Z t r a n s f o r m e r s f o r s p e c t r a l a n a l y s i s [ 2 . 6 ] . F i l t e r s t r u c t u r e s u s i n g JCCD's have some a t t r a c t i v e f e a t u r e s , as w i l l be shown i n c h a p t e r 5.

D i g i t a l a p p l i c a t i o n s c a n be d i v i d e d i n t o two g r o u p s , memories and l o g i c c i r c u i t s . A CCD memory i s a dynamic s h i f t r e g i s t e r and t h e a c c e s s t i m e w i l l be l a r g e r t h a n t h a t f o r b i p o l a r or MOS random-access memories [ 2 . 6 ] . The r e f r e s h t i m e o f t h e memory w i l l be d e t e r m i n e d by t h e

d a r k - c u r r e n t g e n e r a t i o n . CCD memories, however, can be v e r y a t t r a c t i v e i n m a s s - s t o r a g e s y s t e m s w i t h a memory s i z e i n the megabit r a n g e . A v e r y a t t r a c t i v e p o s s i b i l i t y i s the c o m b i n a t i o n o f l a r g e memories and l o g i c c i r c u i t s on one c h i p .

L o g i c c i r c u i t s u s i n g CCD's have a l s o been the t o p i c o f r e s e a r c h [ 2 . 1 3 ] , i n c l u d i n g t h a t on m u l t i - v a l u e d l o g i c i n CCD's [2.14, 2 . 1 5 ] . JCCD's seem t o have some a d v a n t a g e s over o t h e r CCD's i n the f i e l d o f l o g i c c i r c u i t r y . A d e s c r i p t i o n o f l o g i c c i r c u i t s u s i n g JCCD's can be found i n Appendix A. The l o g i c c i r c u i t s make use o f the i n p u t and o u t p u t s t r u c t u r e s which were shown i n F i g . 2.7 and F i g . 2.8. The o p t i c a l a p p l i c a t i o n s o f CCD's can be d i v i d e d i n t o two main g r o u p s , namely l i n e s e n s o r s and a r e a s e n s o r s [ 2 . 6 ] . CCD l i n e s e n s o r s can be used i n o n e - d i m e n s i o n a l p o s i t i o n - d e t e c t i o n s y s t e m s . P a t t e r n - r e c o g n i t i o n s y s t e m s can be r e a l i z e d by m e c h a n i c a l l y s e n s i n g a second d i m e n s i o n . CCD a r e a s e n s o r s a r e used i n s o l i d - s t a t e i m a g e - s e n s i n g s y s t e m s . For p r o f e s s i o n a l a p p l i c a t i o n s v e r y l a r g e image s e n s o r s have been r e a l i z e d [ e . g . 2 . 1 6 ] . R e q u i r e m e n t s f o r r e a l i z i n g e c o n o m i c a l l y a t t r a c t i v e CCD s e n s o r s a r e a s m a l l c e l l s i z e , h i g h l i g h t s e n s i t i v i t y and good a n t i - b l o o m i n g p r o p e r t i e s . JCCD's can f u l f i l l o n l y the second r e q u i r e m e n t , f o r a s m a l l c e l l s i z e and good a n t i - b l o o m i n g p r o p e r t i e s i n an a r e a s e n s o r cannot be e a s i l y o b t a i n e d . The p e r f o r m a n c e of the JCCD as a l i n e s e n s o r i s d e s c r i b e d i n [ 2 . 1 0 ] .

REFERENCES [2.1] A.S.Grove, " P h y s i c s and t e c h n o l o g y o f s e m i c o n d u c t o r d e v i c e s " , W i l e y , New York, 1967 [2.2] L . J . M . E s s e r , " P e r i s t a l t i c c h a r g e - c o u p l e d d e v i c e : a new t y p e o f c h a r g e - t r a n s f e r d e v i c e " , E l e c t r o n . L e t t . , v o l . 8, pp. 620-621, Dec. 1972

[2.3] W . K e l l n e r , H . B i e r h e n b k e and H.Kniepkamp, "A S c h o t t k y b a r r i e r CCD on GaAs", i n P r o c . 23rd I n t . E l e c t r o n D e v i c e s M e e t i n g , W a s h i n g t o n , D.C., Dec. 1977 [2.4] M . K l e e f s t r a , "A s i m p l e a n a l y s i s o f CCD's d r i v e n by pn d i o d e s " , S o l i d - s t a t e E l e c t r o n . , v o l . 21, pp. 1005-1011, Aug. 1978

[2.5] C.H.Sequin and M.F.Tompsett, "Charge T r a n s f e r D e v i c e s " ,

i n Advances i n E l e c t r o n i c s and E l e c t r o n P h y s i c s , s u p p l . 8, Academic P r e s s , New Y o r k , 1975

[2.6] M.J.Howes and D.V.Morgan, " C h a r g e - C o u p l e d D e v i c e s and S y s t e m s " , W i l e y , C h i c h e s t e r , 1979 [2.7] H . W a l l i n g a , " C h a r g e - c o u p l e d d e v i c e s f o r a n a l o g s i g n a l p r o c e s s i n g " , Ph.D. T h e s i s , Twente U n i v e r s i t y o f T e c h n o l o g y , E n s c h e d e , 1980

[2.8] I.Deyhimy, R.C.Eden and J . S . H a r r i s , "GaAs and r e l a t e d h e t e r o j u n c t i o n c h a r g e - c o u p l e d d e v i c e s " , IEEE T r a n s . E l e c t r o n D e v i c e s , v o l . ED-27, pp. 1172-1180, June 1980 [2.9] I.Deyhimy, W . A . H i l l and R . J . A n d e r s o n , " C o n t i n u o u s l y c l o c k e d 1 GHz GaAs CCD", IEEE E l e c t r o n D e v i c e s L e t t . , v o l . EDL-2, pp. 70-72, Mar. 1981 [2.10] C . D . H a r t g r i n g and M . K l e e f s t r a , "Quantum e f f i c i e n c y and blooming s u p p r e s s i o n i n j u n c t i o n c h a r g e - c o u p l e d d e v i c e s " , IEEE J . S o l i d - S t a t e C i r c u i t s , v o l . SC-13, pp. 728-730, O c t . 1978 [2.11] H . W a l l i n g a , "A c o m p a r i s o n o f CCD a n a l o g u e i n p u t c i r c u i t c h a r a c t e r i s t i c s " , i n P r o c . CCD T e c h n o l o g y and A p p l i c a t i o n s C o n f . , E d i n b u r g h , S e p t . 1974

[2.12] C.H.Sequin and A.M.Mohsen, " L i n e a r i t y o f e l e c t r i c a l c h a r g e i n j e c t i o n i n t o c h a r g e - c o u p l e d d e v i c e s " ,

IEEE J . S o l i d - S t a t e C i r c , v o l . SC-10, p p . 81-92, A p r i l 1975

[2.13] T.A.Zimmerman, R . A . A l l e n and R.W.Jacobs, " D i g i t a l c h a r g e - c o u p l e d l o g i c (DCCL)",

IEEE J . S o l i d - S t a t e C i r c u i t s , v o l . SC-12, pp. 473-485, O c t . 1977

[2.14] H.G.Kerkhoff and H . D i j k s t r a , "The a p p l i c a t i o n o f CCD's i n m u l t i p l e - v a l u e d l o g i c " ,

C h a r g e - C o u p l e d D e v i c e s , E d i n b u r g h , S e p t - 1979

[2.15] H.G.Kerkhoff and M . L . T e r v o e r t , "The i m p l e m e n t a t i o n o f m u l t i p l e - v a l u e d f u n c t i o n s u s i n g c h a r g e - c o u p l e d d e v i c e s " ,

i n P r o c . 10th I n t e r n a t i o n a l Symposium on M u l t i p l e - V a l u e d L o g i c , E v a n s t o n , June 1980

[2.16] M.M.Blouke, J . E . H a l l and J . F . B r e i t z m a n n , "A 640 K i l o p i x e l CCD imager f o r s p a c e a p p l i c a t i o n s " ,

i n P r o c . 24th I n t . E l e c t r o n D e v i c e s M e e t i n g , W a s h i n g t o n , D.C., Dec. 1978

3. COMPUTER ANALYSIS OF JUNCTION

CHARGE-COUPLED DEVICES

r e s u l t i n g i n a h i g h t r a n s f e r i n e f f i c i e n c y . In a d d i t i o n , t h e c h a r g e - h a n d l i n g c a p a b i l i t y i s an i m p o r t a n t c h a r a c t e r i s t i c o f the JCCD, which i s d e t e r m i n e d by the i m p u r i t y p r o f i l e . O p t i m i z i n g both t h e t r a n s f e r e f f i c i e n c y and the c h a r g e - h a n d l i n g c a p a b i l i t y can l e a d t o c o n f l i c t i n g c o n c l u s i o n s about t h e i m p u r i t y p r o f i l e d e s i r e d . A n o t h e r i m p o r t a n t c h a r a c t e r i s t i c o f t h e JCCD i s t h e v a l u e o f t h e c h a n n e l p o t e n t i a l . T h i s v a l u e d e t e r m i n e s the h e i g h t o f the c l o c k p u l s e s r e q u i r e d t o t r a n s f e r c h a r g e t h r o u g h t h e JCCD, and thus s h o u l d n o t be t o o h i g h .

T h e r e a r e , however, more f a c t o r s which p u t c o n s t r a i n t s on t h e i m p u r i t y p r o f i l e . One o f t h e a d v a n t a g e s o f t h e JCCD over c o n v e n t i o n a l BCCD's i s the p o s s i b i l i t y f o r c h a r g e i n j e c t i o n t h r o u g h the g a t e s [3.3, 3. 4 ] , T h i s l i m i t s the p o s s i b l e v a r i a t i o n s i n the i m p u r i t y p r o f i l e s t o t h o s e c a s e s where NPN t r a n s i s t o r s can be r e a l i z e d . A n o t h e r c o n s t r a i n t on the p r o c e s s used f o r f a b r i c a t i n g JCCD's i s formed by t h e danger o f l e a k a g e between two a d j a c e n t g a t e s [ 3 . 3 ] . The p o t e n t i a l a l o n g the s i l i c o n s u r f a c e between two g a t e s has t o be h i g h enough i n o r d e r t o p r e v e n t h o l e c u r r e n t s from f l o w i n g between the g a t e s .

In t h i s c h a p t e r s e v e r a l p o s s i b i l i t i e s f o r f a b r i c a t i n g JCCD's w i l l be d e s c r i b e d , t a k i n g i n t o a c c o u n t t h e c o n s t r a i n t s m e n t i o n e d . The p o s s i b i l i t y f o r r e a l i z i n g NPN t r a n s i s t o r s and the r e q u i r e m e n t s f o r a h i g h c h a r g e - h a n d l i n g c a p a b i l i t y and a low c h a n n e l p o t e n t i a l can be t r e a t e d as o n e - d i m e n s i o n a l p r o b l e m s . C o n s e q u e n t l y , t h e s e r e q u i r e m e n t s w i l l be t r e a t e d f i r s t . The e q u a t i o n s which d e s c r i b e t h e p o t e n t i a l p r o f i l e i n s e m i c o n d u c t o r d e v i c e s c a n be s o l v e d a n a l y t i c a l l y o n l y i n v e r y s i m p l e c a s e s . In JCCD's even the o n e - d i m e n s i o n a l c a s e c a n n o t be s o l v e d a n a l y t i c a l l y w i t h o u t making v e r y c r u d e s i m p l i f i c a t i o n s , which makes t h e use o f n u m e r i c a l methods i n e v i t a b l e .

By u s i n g the c o n c l u s i o n s drawn from the o n e - d i m e n s i o n a l c a l c u l a t i o n s , the t w o - d i m e n s i o n a l p r o b l e m s o f t r a n s f e r e f f i c i e n c y and p o s s i b l e l e a k a g e between t h e g a t e s w i l l be c o n s i d e r e d . As has been shown by Herman e t a l . [ 3 . 5 ] , i t i s t h e o r e t i c a l l y p o s s i b l e t o a c h i e v e a p o t e n t i a l p r o f i l e w i t h v e r y s m a l l p a r a s i t i c w e l l s or b a r r i e r s i n a JCCD. A l t h o u g h JCCD's w i t h a t r a n s f e r i n e f f i c i e n c y as low as 2 . 1 0- 5 have been r e a l i z e d [3.6] by u s i n g t h e r e s u l t s o f t h e c a l c u l a t i o n s d e s c r i b e d i n [ 3 . 5 ] , t h e p r o c e s s used i s n o t r e p r o d u c i b l e . In t h i s c h a p t e r a method f o r f a b r i c a t i n g JCCD's which does n o t i n c l u d e any c r i t i c a l masking s t e p s i s a n a l y z e d . In a d d i t i o n to the a n a l y s i s o f the p o t e n t i a l p r o f i l e i n the c h a r g e t r a n s p o r t d i r e c t i o n , computer c a l c u l a t i o n s have been p e r f o r m e d on the l a t e r a l c o n f i n e m e n t o f the c h a n n e l . By c a l c u l a t i n g the p o t e n t i a l p r o f i l e i n a c o m p l e t e JCCD c e l l as a f u n c t i o n o f t h e g a t e v o l t a g e s , c o n c l u s i o n s can be drawn c o n c e r n i n g the i n f l u e n c e of r e s i d u a l d i s t o r t i o n s i n t h e p o t e n t i a l p r o f i l e on the t r a n s f e r e f f i c i e n c y . By i n s p e c t i n g the f r i n g i n g f i e l d s , an e s t i m a t i o n can be made o f t h e h i g h e s t c l o c k f r e q u e n c y a t which t h e d e v i c e s c a n be

o p e r a t e d . The s i z e o f the c h a r g e p a c k e t s p r e s e n t i n the c h a n n e l i n f l u e n c e s the s i z e o f the f r i n g i n g f i e l d s . The p o s s i b i l i t y f o r f a b r i c a t i n g JCCD's by means o f V - g r o o v e e t c h i n g w i l l be d e s c r i b e d . T h i s p r o c e s s i s based on s e p a r a t i n g the g a t e s by e t c h i n g V - g r o o v e s t h r o u g h the p - t y p e l a y e r [ 3 . 7 ] , A l t h o u g h the p o t e n t i a l p r o f i l e which can be r e a l i z e d w i t h V - g r o o v e e t c h i n g l o o k s p r o m i s i n g t h e o r e t i c a l l y , the t e c h n o l o g i c a l d i f f i c u l t i e s a r e l a r g e r t h a n i n the c a s e o f s e p a r a t e l y d i f f u s e d g a t e s , b e c a u s e the aluminum s t r i p s used f o r c o n t a c t i n g the g a t e s have t o c r o s s many V - g r o o v e s , w h i c h can s e r i o u s l y hamper the y i e l d .

By c h a n g i n g the t e c h n o l o g y , i t i s p o s s i b l e t o s c a l e down the JCCD. Some c o n s i d e r a t i o n s f o r s c a l i n g down JCCD's a r e g i v e n below.

3.2 ONE-DIMENSIONAL ANALYSIS OF THE JCCD

j The e l e c t r o s t a t i c p o t e n t i a l i n a JCCD i s d e s c r i b e d by t h e P o i s s o n e q u a t i o n „ p(xJyJz) r $(x,y,z)= (3.1) SSi T h i s can be w r i t t e n as S2 i>(y)--y f>(y) 'Si (3.2) i n the o n e - d i m e n s i o n a l c a s e . The s t r u c t u r e t o be a n a l y z e d i s shown i n F i g . 3.1 and c o n s i s t s o f a p - s u b s t r a t e , an

p-gate

n-layer

p-substrate

• depletion edge

- depletion edge

Fig. 3.1. One-dimensional model of a JCCD.

n - l a y e r and a p - t y p e g a t e . D u r i n g t h e a n a l y s i s i t i s assumed t h a t t h e n - l a y e r i s d e p l e t e d . D e p l e t i o n l a y e r s a l s o e x i s t i n t h e g a t e and i n t h e s u b s t r a t e . The w i d t h o f t h e s e d e p l e t i o n l a y e r s depends on t h e i m p u r i t y p r o f i l e s and c a n be c a l c u l a t e d a n a l y t i c a l l y o n l y i f v e r y c r u d e a p p r o x i m a t i o n s a r e made. The a p p l i c a b i l i t y i n t h a t c a s e w i l l be i n s u f f i c i e n t , so t h a t a n u m e r i c a l s o l u t i o n has t o be f o u n d . The n u m e r i c a l s o l u t i o n o f t h e P o i s s o n e q u a t i o n (3.2) i n a o n e - d i m e n s i o n a l c a s e i s a r e l a t i v e l y s i m p l e p r o b l e m . The i m p u r i t y c o n c e n t r a t i o n i s c a l c u l a t e d a t a number o f p o i n t s , and by i n t e g r a t i n g t w i c e , t h e p o t e n t i a l p r o f i l e w i l l r e s u l t . The l o c a t i o n o f t h e d e p l e t i o . n edges i n t h e p - l a y e r s c a n be found by u s i n g the boundary c o n d i t i o n s . The d e f i n i t i o n o f the v a r i a b l e s i s shown i n F i g . 3.1. The a s s u m p t i o n i s made t h a t t h e g a t e and t h e s u b s t r a t e a r e g r o u n d e d . The e l e c t r i c f i e l d and t h e p o t e n t i a l as a f u n c t i o n o f y a r e g i v e n by

*r w - * V + ^ [ p ^ W + p r%;] - % ] ( 3-3 )

The boundary c o n d i t i o n s f o r the e l e c t r i c f i e l d a t the d e p l e t i o n edges i n the g a t e s and i n the s u b s t r a t e can be w r i t t e n as

^depI.edge ~ ^ (3.5)

w h i l e the b o u n d a r y c o n d i t i o n f o r the e l e c t r o s t a t i c p o t e n t i a l a t the d e p l e t i o n edges i n the g a t e s and the s u b s t r a t e i s g i v e n by

*depl.edge = 0 (3-6>

The p o t e n t i a l p r o f i l e can now be c a l c u l a t e d by u s i n g the d e p l e t i o n - e d g e a p p r o x i m a t i o n (see a l s o page 3 6 ) . An a s s u m p t i o n i s made about the l o c a t i o n o f the d e p l e t i o n edge i n the g a t e . I f t h a t a s s u m p t i o n i s c o r r e c t , the v a l u e s o f b o t h the e l e c t r i c f i e l d and the e l e c t r o s t a t i c p o t e n t i a l a t the d e p l e t i o n edge i n the s u b s t r a t e , which a r e c a l c u l a t e d w i t h (3.3) and ( 3 . 4 ) , w i l l be z e r o .

I f t h e s e v a l u e s a r e not z e r o , the a s s u m p t i o n i s not c o r r e c t and has t o be changed i t e r a t i v e l y u n t i l the c o r r e c t

p o s i t i o n i s f o u n d . The c h a r g e - h a n d l i n g c a p a b i l i t y c a n be c a l c u l a t e d by i n t e g r a t i n g t h e i m p u r i t y c o n c e n t r a t i o n i n t h e n - l a y e r , as has been shown i n c h a p t e r 2.

As can be c o n c l u d e d from l i t e r a t u r e , t h e s i z e o f t h e f r i n g i n g f i e l d s i n a CCD i s i n f l u e n c e d by t h e d i s t a n c e between the S i - s u r f a c e and t h e c h a n n e l [ 3 . 8 ] . The l a r g e s t f r i n g i n g f i e l d s o c c u r i n CCD's where t h e c h a n n e l i s s i t u a t e d v e r y deep i n t h e b u l k o f t h e CCD. Thus i t c a n be c o n c l u d e d t h a t t h e i m p u r i t y c o n c e n t r a t i o n i n t h e s u b s t r a t e has t o be much lower t h a n t h e one i n t h e n - l a y e r . F u r t h e r , t h e

i m p u r i t y c o n c e n t r a t i o n i n t h e g a t e has t o be much h i g h e r than t h a t i n t h e n - l a y e r . A s u b s t r a t e i s c h o s e n w i t h an 1 4 - 3 i m p u r i t y c o n c e n t r a t i o n o f 2.10 cm . An n - e p i l a y e r i s grown on t h e s u b s t r a t e , w h i l e t h e g a t e s a r e formed by a d e p o s i t i o n o f b o r o n , f o l l o w e d by a d r i v e - i n s t e p . The p o s i t i o n o f t h e g a t e - e p i l a y e r j u n c t i o n c a n be v a r i e d by c h a n g i n g the d i f f u s i o n l e n g t h o f t h e b o r o n atoms. The i m p u r i t y p r o f i l e o f t h e b o r o n d i f f u s i o n c a n be e x p r e s s e d a p p r o x i m a t e l y as U s i n g an e p i l a y e r has t h e d i s a d v a n t a g e t h a t t h e r e w i l l be l a r g e v a r i a t i o n s i n t h e c h a n n e l p o t e n t i a l due t o v a r i a t i o n s i n t h e i m p u r i t y c o n c e n t r a t i o n i n t h e e p i l a y e r and i n i t s t h i c k n e s s . I n F i g . 3.2 t h e c a l c u l a t e d v a l u e o f t h e c h a n n e l p o t e n t i a l i s shown as a f u n c t i o n o f t h e donor c o n c e n t r a t i o n i n t h e e p i l a y e r . The t h i c k n e s s o f t h e l a y e r i n t h i s c a s e i s 7.0 u r n . In F i g . 3.3 the channel p o t e n t i a l i s d e p i c t e d as a f u n c t i o n o f t h e t h i c k n e s s o f t h e e p i l a y e r , w h i l e i t s 14 -3 i m p u r i t y c o n c e n t r a t i o n i s k e p t c o n s t a n t a t 7.10 cm . I t can be c o n c l u d e d t h a t t h e c h a n n e l p o t e n t i a l c a n show l a r g e (3.7)

*c h<V) 15 10 5 0 5 10 15 -Nd(10Kcm"3)

Fig. 3.2. Calculated value of the channel potential $ , as a function of the impurity concentration in the epilayer.

v a r i a t i o n s i f d e t e r m i n e d by the t h i c k n e s s and donor c o n c e n t r a t i o n o f the e p i l a y e r .

The c h a n n e l p o t e n t i a l and the c h a r g e - h a n d l i n g c a p a b i l i t y w i l l show l e s s v a r i a t i o n i f an e p i l a y e r w i t h a r e l a t i v e l y low i m p u r i t y c o n c e n t r a t i o n i s grown, f o l l o w e d by a p h o s p h o r u s i m p l a n t a t i o n . The number o f donor atoms c a n be c o n t r o l l e d v e r y w e l l i n t h i s way. A second a l t e r n a t i v e would be to l e a v e o u t the e p i l a y e r e n t i r e l y . T h i s would, however, make the f a b r i c a t i o n o f b i p o l a r t r a n s i s t o r s much more d i f f i c u l t , b e c a u s e no b u r i e d l a y e r can be used i n t h a t

depi(Hm)

Fig. 3. 3. Calculated value of the channel potential <j>e^ as a function

of the thickness of the epilayer.

c a s e . In a l l t h e c a s e s d e s c r i b e d i n t h i s c h a p t e r , an e p i l a y e r i s u s e d . For t h e p h o s p h o r u s i m p l a n t a t i o n t h e r e a r e t h r e e v a r i a b l e s : t h e number o f p h o s p h o r u s i o n s p e r u n i t a r e a , the i m p l a n t a t i o n e n e r g y and the d i f f u s i o n l e n g t h o f the atoms. The c o n c e n t r a t i o n o f p h o s p h o r u s atoms as a f u n c t i o n o f t h e d i s t a n c e y from t h e s u r f a c e c a n be w r i t t e n as

N (y) = p » h-n(<b£ > + 2D t )\ \ P P P f (3.8) exp 2(<AR •> + 2D t ) P P P (y - <R >)2 _ P 2(<AR >2 + 2D t ) P P P (y + <R >)2 P i n the c a s e o f an i m p l a n t a t i o n f o l l o w e d by an a n n e a l i n g s t e p . W h i l e b e a r i n g i n mind t h e r e q u i r e m e n t s on the p o t e n t i a l p r o f i l e , a c o m b i n a t i o n o f p a r a m e t e r s has t o be found f o r p r o c e s s i n g the JCCD's. In o r d e r to be a b l e to f a b r i c a t e b i p o l a r t r a n s i s t o r s , the p a r a m e t e r s have been combined i n a way t h a t l e a d s t o a j u n c t i o n d e p t h o f about 1.0 vm f o r the g a t e - n - l a y e r j u n c t i o n . In F i g . 3.4 t h e c h a r g e - h a n d l i n g c a p a b i l i t y i s shown as a f u n c t i o n o f t h e c h a n n e l p o t e n t i a l f o r s e v e r a l amounts o f i m p l a n t e d p h o s p h o r u s atoms and s e v e r a l d i f f u s i o n l e n g t h s of t h e i m p l a n t e d atoms. The i m p l a n t a t i o n e n e r g y i n t h i s c a s e i s 30 keV, w h i l e t h e i m p u r i t y c o n c e n t r a t i o n i n the e p i l a y e r 14 -3

i s 7.10 cm and the e p i l a y e r i s 7.0 urn t h i c k . The curves have been o b t a i n e d by v a r y i n g the d i f f u s i o n l e n g t h o f t h e p h o s p h o r u s atoms r e l a t i v e t o t h a t o f the b o r o n atoms. In a l l c a s e s the boron s u r f a c e c o n c e n t r a t i o n b e f o r e d i f f u s i o n

14 -2

i s 4.10 cm . S i m i l a r c a l c u l a t i o n s have been p e r f o r m e d w i t h a p h o s p h o r u s i m p l a n t a t i o n e n e r g y o f 180 keV. The r e s u l t s a r e shown i n F i g . 3.5. From F i g s . 3.4 and 3.5 two c o m b i n a t i o n s o f p a r a m e t e r s have been chosen and w i l l be used t h r o u g h o u t the t w o - d i m e n s i o n a l c a l c u l a t i o n s . The p a r a m e t e r s and the c a l c u l a t e d o n e - d i m e n s i o n a l f e a t u r e s o f t h e JCCD's i n t h e s e c a s e s a r e l i s t e d i n T a b l e 3.1. In F i g . 3.6 the i m p u r i t y p r o f i l e under a g a t e i s shown f o r the 30 keV i m p l a n t a t i o n . The j u n c t i o n d e p t h i s 1.0 urn i n t h i s c a s e .

Fig. 3.4. Charge-handling capability as a function of the channel

r——~— 7 % — 9

potential for several cases: (a) - 0.92 um, Q =2.10 cm~ ,

(b) 2 / 0&t& - 0.92 um, Qp - 4.10 cm , (a) 2/b~t^ - 0.92 um,

= 8.101S cm'2, (d) 2/DJT^ = 0.50 um, Qp - 2.1013 cm~2,

(e) - 0.50 um, Q - 4.1013 cm~2, (f) 2/D^ = 0.50 um,

13 -2

Fig. 3.5. Charge-handling capability as a function of the channel

potential for the cases given in Fig. 2.4, with an implantation energy of 180 keV.

Table 3.1

Results of the one-dimensional calculations 30 keV implantation 180 keV implantation channel potential 7.50 3.05 *ch,0 <V> charge-handling 6.5 1 01 1 1.0 1 01 2 c a p a b i l i t y (el.cm ) number of implanted 4.0 1 01 3 4.0 1 01 3 _2 phosphorus ions (cm ) phospnorus d i f f u s i o n 0.70 0.70 length (ym) boron d i f f u s i o n 0.50 0.50 length (urn) epilayer thickness (wm) 6.0 6.0 epilayer impurity concentration (cm ) 7.0 1 01 4 7.0 1 01 4 substrate impurity _3 concentration (cm ) 14 2.0 10 2.0 1 01 4

Fig. 3.6. Impurity profile under a JCCD gate, with the 30 keV phosphorus implantation described in Table 3.1.

3.3 TWO-DIMENSIONAL CALCULATIONS ON J C C D ' S Based on t h e r e s u l t s o f t h e o n e - d i m e n s i o n a l c a l c u l a t i o n s , a method w i l l be d e s c r i b e d t o o b t a i n a p o t e n t i a l p r o f i l e i n t h e JCCD which w i l l a l l o w e f f i c i e n t t r a n s f e r o f c h a r g e p a c k e t s t h r o u g h t h e c h a n n e l . T h i s i s , i n f a c t , a t h r e e - d i m e n s i o n a l p r o b l e m , which r e q u i r e s t h e s o l u t i o n o f t h e t h r e e - d i m e n s i o n a l P o i s s o n e q u a t i o n The s p a c e - c h a r g e d e n s i t y p i s a n o n - l i n e a r f u n c t i o n o f t h e e l e c t r o s t a t i c p o t e n t i a l <j> . T h i s f u n c t i o n i s g i v e n by t h e well-known B o l t z m a n n r e l a t i o n , which d e s c r i b e s t h e d e p l e t i o n i n a s e m i c o n d u c t o r . I t has been shown t h a t t h e use o f t h e d e p l e t i o n - e d g e a p p r o x i m a t i o n i n s t e a d o f t h e B o l t z m a n n r e l a t i o n can l e a d t o a c c u r a t e r e s u l t s [ 3 . 9 ] , and f u r t h e r m o r e t o a s i g n i f i c a n t r e d u c t i o n i n c o m p u t a t i o n a l e f f o r t . Some c o m p a r i s o n s between t h e r e s u l t s based on t h e d e p l e t i o n - e d g e a p p r o x i m a t i o n and t h o s e based on t h e B o l t z m a n n r e l a t i o n i n the c a s e o f a JCCD a r e g i v e n i n [ 3 . 7 ] . A f u r t h e r s i m p l i f i c a t i o n o f (3.9) can be made by assuming a t w o - d i m e n s i o n a l c o n f i g u r a t i o n . T h i s s i m p l i f i c a t i o n i s a l l o w e d i f d i s t a n c e s i n one d i r e c t i o n a r e much l a r g e r t h a n d i s t a n c e s i n t h e two o t h e r d i r e c t i o n s , which i s u s u a l l y t h e c a s e i n CCD's. W i t h t h e s e a s s u m p t i o n s , (3.9) can be w r i t t e n as 2 V §(x,y,z) = -p(x,y,z) (3.9) qN (x,y) (3.10)

f o r t h e d e p l e t e d p a r t o f t h e g a t e s and t h e s u b s t r a t e , and as Or + & ) $(x,y) = -* y _ (3.11) Si f o r t h e d e p l e t e d p a r t o f t h e n - l a y e r . I n t h e g a t e s and t h e s u b s t r a t e , t h e d e p l e t i o n - e d g e a p p r o x i m a t i o n i s implemented by assuming t h a t t h e a r e a s where t h e p o t e n t i a l i s V^ V h i g h e r t h a n <t>^ a r e d e p l e t e d . In the n - l a y e r the areas where t h e p o t e n t i a l i s VT V lower than <j> a r e assumed t o

be d e p l e t e d . The p o s i t i o n o f t h e d e p l e t i o n edge i s n o t f i x e d , b u t i s a f u n c t i o n o f t h e i m p u r i t y p r o f i l e , o f t h e g a t e v o l t a g e s a p p l i e d , and o f t h e s i z e o f t h e c h a r g e p a c k e t s . The a r e a s o u t s i d e t h e s e moving b o u n d a r i e s , t h e n e u t r a l a r e a s , a r e c o n d u c t i v e . The v a l u e o f t h e c o n d u c t i v i t y i s n o t i m p o r t a n t b e c a u s e t h e c u r r e n t s a r e z e r o i n t h i s s t a t i c c a s e . The n e u t r a l p a r t s o f t h e g a t e s , t h e n - l a y e r , and t h e s u b s t r a t e a r e e q u i p o t e n t i a l a r e a s . In o r d e r t o be a b l e t o s o l v e e q u a t i o n s (3.10) and ( 3 . 1 1 ) , a t w o - d i m e n s i o n a l d e s c r i p t i o n o f t h e i m p u r i t y p r o f i l e s i s n e c e s s a r y .

3.4 ANALYTICAL FORMULATION OF DIFFUSED AND IMPLANTED IMPURITY PROFILES

In t h e p r o c e s s used f o r f a b r i c a t i n g JCCD's, two d i f f e r e n t i m p u r i t y p r o f i l e s a r e u t i l i z e d . The f i r s t one, used f o r t h e g a t e s and f o r t h e i s o l a t i o n w a l l s , i s formed by a d e p o s i t i o n o f b o r o n , f o l l o w e d by a d r i v e - i n s t e p . T h i s i m p u r i t y p r o f i l e c a n be d e s c r i b e d by [3.10]

nfo(x,y) - N^(y).h(x) (3.12) where S, i s t h e b o r o n c o n c e n t r a t i o n as a f u n c t i o n o f y , as shown i n F i g . 3 . 7 . w mask _{0 x mask} y silicon

Fig. 3. 7. Model for the diffusions and implantations.

The f u n c t i o n h(x) i n c l u d e s t h e f i n i t e w i d t h o f t h e mask o p e n i n g . The f u n c t i o n ^ j / ^ i s assumed t o be g i v e n by t h e well-known G a u s s i a n d i s t r i b u t i o n T exp y 2(

W^

%

{1

The second t y p e o f i m p u r i t y p r o f i l e i s formed by a p h o s p h o r u s i m p l a n t a t i o n f o l l o w e d by a combined a n n e a l i n g and d r i v e - i n s t e p . I t c a n be d e s c r i b e d as

np(x,y) = N (y).g(x) (3.15) where N i s t h e p h o s p h o r u s c o n c e n t r a t i o n as a f u n c t i o n o f y , and g(x) i s a f u n c t i o n which i n c l u d e s t h e f i n i t e w i d t h o f t h e mask o p e n i n g . N (y) c a n be w r i t t e n as [3.11] N (y) = - —-j n P \2-n(<t£ > + 2D t )r \ P P P ) 1 (3.16) exp (y - <R >)' 1 P 2(<&R > + 2D t ) P P P exp (y + <R >)' _ P 2(<&R > + 2D t ) p p p The f u n c t i o n g(x) i s g i v e n by [3.7] x - W/2 g(x) = % erfa h(<Ax>2 + 2D t )\' \ P P

i