Virtual phase CCD technology - IEEE Xplore

VIRTUAL PHASE CCD TECHNOLOGY

.J. Hynecek

T e x a Isn s t r u m e n t Isn c o r p o r a t e d P . O . Box225936 Dallas,Texas75265 t h es u r f a c e

ABSTRACT

Inthisarticle a new c o n c e p to f a s i n g l e phase C C D , c a l l e d" v i r t u a lp h a s e " ,i sd e s c r i b e d . V i r t u a lp h a s et e c h n o l o g ye m p l o y so n l y a single l e v e gl a t ee l e c t r o d es t r u c t u r e T . h i st e c h n o l o g y r e p r e s e n t s a new approach t o f a b r i c a t i o n o f l a r g ea r e a C C D d e v i c e sw h i c hh a v eb o t hh i g hp e r f o r m a n c ea n dh i g hy i e l d . An example o f t h e a p p l i c a t i o n o f v i r t u a l phasetechnology t o t h ef a b r i c a t i o no f a 490 x , n ds e v e r a il m p o r 328 C C D imager i sp r e s e n t e d a t a n td e s i g n and p e r f o r m a n c ep a r a m e t e r sa r ed i s cussed.

D E S C R I P T I O N OF OPERATION OF THE

VP C C D

I nt h ef o l l o w i n gs e c t i o n st h ed i s c u s s i o n will b el i m i t e dt oN - c h a n n e ld e v i c e s ,w i t ht h e u n d e r s t a n d i n gt h a tt h eP - c h a n n e le q u i v a l e n tc a n be o b t a i n e ds i m p l yb yr e v e r s i n gt h es i g n so fp o Pt e n t i a l s and t h et y p eo d f o p a n t sB . oth channea l n dN - c h a n n e vl i r t u a p l h a s ed e v i c e sh a v e been b u i l t and o p e r a t e ds u c c e s s f u l l y . V i r t u a lp h a s et e c h n o l o g yi sa ne x t e n s i o no f t w op h a s eb u r i e dc h a n n e lt e c h n o l o g yi nw h i c ho n e phase i sb u i l ti n t ot h es i l i c o ns u r f a c e . The V P d e v i c e sa r e b a s i co p e r a t i n gp r i n c i p l e so f d e s c r i b e db e l o w . I nF i g u r e 1 a t y p i c a lp o t e n t i a lp r o f i l ef o r a b u r i e dc h a n n e l CCD i s shown. The g a t ep o t e n and maximum t i a l i s i n d i c a t e d by V . t h es u r f a c e 9' p o t e n t i a l sa r ei n d i c a t e d by thesymbols @ s and $m r e s p e c t i v e l y . One i m p o r t a n tf e a t u r eo ft h eb u r i e dc h a n n e l s t r u c t u r ei st h a te l e c t r o n sa r es t o r e d away f r o m Xm and a r et h e r e t h es u r f a c ei nt h ev i c i n i t yo f f o r ef r e ef r o mt h eu n d e s i r a b l ei n f l u e n c eo fs u r f a c es c a t t e r i n g and t r a p sA . n o t h e irm p o r t a n t f e a t u r eo ft h eb u r i e dc h a n n e ls t r u c t u r e ,n o t e d a l s o by o t h e r s( l ) ,i sc a l l e dt h ep o t e n t i a lp i n n i n g phenomena. As t h eg a t ep o t e n t i a l V is 9 loweredtoward more n e g a t i v eb i a s ,t h es u r f a c e potential i sa l s ol o w e r e du n t i l i t reaches

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c a no c c u p yo n l y a v e r yt h i nr e g i o no fs p a c en e a r t h es u r f a c eo ft h es i l i c o n ,t h ep o t e n t i a lp r o f i l e w i t h i nt h es i l i c o n will n o t be a f f e c t e d by t h e amount o f h o l e s needed t o compensateany additional f i e l di n d u c e db yt h eg a t e . As a r e s u l t ,t h ev a l u e some p o s i t i v e minimum, o f @ will a l s ob ep i n n e dt o

@, min, r e g a r d l e s s o f f u r t h e r l o w e r i n g o f t h e g a t e remains pos im min t i v e ,s i g n a lc h a r g e( e l e c t r o n s )c a nb ee f f e c t i v e l y s t o r e do rt r a n s f e r r e du n d e rt h ea c c u m u l a t e dl a y e r o fh o l e s ,i n d e p e n d e n to ff u r t h e r changes o ft h e v o l t a g eo nt h eo v e r l y i n gg a t e .I nt h i sc a s et h e h o l e sa c c u m u l a t e da tt h es u r f a c ea c t as a " v i r t u a l " g a t ef o rr h eu n d e r l y i n gb u r i e dc h a n n e lr e g i o n . T h i sb a s i cp r i n c i p l ei st h ec o r n e r s t o n eo ft h e virtuap l h a s et e c h n o l o g y . The use o f t h i s p r i n c i p l ef o rt h ec o n s t r u c t i o no fc h a r g et r a n s f e r dev i c e s will now b ee x p l a i n e d . p o t e n t i aS l . i n ctehpei n n e d

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One f u n d a m e n t a le l e m e n t ,p i x e l ,o rb i to f a virC C D i sd i v i d e di n t of o u rs e c t i o n s as t u a lp h a s e will i n d i c a t e di nF i g u r e2 . The f i r s t t w os e c t i o n s be c a l l e dt h ec l o c k e dp h a s e ,w i t ht h eb a r r i e rb e i n g S e c t i o n 1 , and t h ew e l l ,S e c t i o n 2 . The secondtwo s e c t i o n sa r ec a l l e dt h ev i r t u a p l h a s e .T h i sp h a s e 3 , and a i s a l s o composed o f a b a r r i e r , S e c t i o n w e l l ,S e c t i o n 4. All f o u rs e c t i o n sa r e assumed t o have a u n i f o r mb u r i e dc h a n n e ld o p i n g( t h i si sn o t shown i nF i g u r e2 ) . The p o t e n t i a w l elu l n d e rt h e c l o c k e dp h a s ei sc r e a t e db yp l a c i n g a s h a l l o w Nt y p ei m p l a n tw i t h i nS e c t i o n 2,and i t i si n d i c a t e d o f p l u ss i g n s . The v i r t u a l t h e r eb yt h e .s e r i e s a deep N - t y p ed o p i n gi nS e c t i o n phase i s formedby 3 , w i t ha ne v e nh e a v i e rd o s ei nS e c t i o n 4 t of o r m a p o t e n t i aw l e l tl h e r e .S i n c et h i sN - t y p ed o p i n g i sr e l a t i v e l yl a r g e , i t would be necessary t o b i a s t h eg a t et o a l a r g en e g a t i v ep o t e n t i a li no r d e rt o o b t a i nt h ed e s i r e ds u r f a c ep o t e n t i a p l inning. Howi f a s h a l l o wP - t y p e e v e r ,t h i si sn o tn e c e s s a r y i m p l a n to f a s u f f i c i e n t dose i s p l a c e d c l o s e t o t h e 3 and 4 . T h i s s u r f a c eo ft h es i l i c o ni nS e c t i o n s i si n d i c a t e db y a s e r i e so fm i n u ss i g n si nF i g u r e2 . I f t h e dose o ft h i sd o p i n gi sl a r g ee n o u g h ,t h e s u r f a c ep o t e n t i a l will s t a yp i n n e dt oz e r of o rp o s i t i v e as w e l l as f o rn e g a t i v ep o t e n t i a l so nt h eg a t e . I n t h i s c a s et h eg a t ee l e c t r o d eo v e rt h ev i r t u a l phase i ss u p e r f l u o u s andcan a c t u a l l y be e l i m i n a t e d , s i n c e i t does n o th a v ea n ye f f e c to nt h ep o t e n t i a l p r o f i l e i n t h e s i 1 i c o n . We c a n ,t h e r e f o r e ,h a v e twotypes o f v i r t u a l p h a s ed e v i c e s , one w i t h a con-

611

T h e r ea r et h r e em a j o rc o m p o n e n t so ft h et o t a ld a r k c u r r e n ti n a CCD. These a r et h es u r f a c e compon e n t , due t o t h e S i O p - S i i n t e r f a c es t a t e s ;d e p l e G-R centers t i o nr e g i o nc o m p o n e n t ,w h e r et h eb u l k s u p p l yt h ec u r r e n t ;a n dt h ed i f f u s i o nc o m p o n e n t , due t o t h e m i n o r i t y c a r r i e r d i f f u s i o n f r o m t h e n e u t r a lu n d e p l e t e db u l ko ft h es i l i c o n . O f these t h r e e ,t h es u r f a c ec o m p o n e n ti sd o m i n a n ti nm o s t C C D s t r u c t u r e s .F o rt h ev i r t u a l b u r i e dc h a n n e l phase CCD, h o w e v e r ,t h es u r f a c e component i s s i g i t i si no t h e rt e c h n o l o n i f i c a n t l ys m a l l e rt h a n gies. The r e a s o nf o rt h er e d u c t i o no ft h es u r f a c e component o f t h e d a r k c u r r e n t i n v i r t u a l phasedev i c e si st h a ti nt h ev i r t u a l p h a s er e g i o n ,t h e SiOp-Si i n t e r f a c es t a t e sa r ef i l l e d by h o l e sF . or s t a n d a r db u r i e dc h a n n e ld e v i c e we h a v e( 4 ) :

t i n u o u sg a t ee l e c t r o d eo v e rt h ec l o c k e dp h a s ea r e a and t h e v i r t u a l p h a s ea r e a( a si n d i c a t e di nF i g u r e 2)andone w i t h a p a t t e r n e dg a t ee l e c t r o d ew h e r e o n l yt h ec l o c k e dp h a s eh a s an o v e r l y i n gg a t e s t r u c t u r e , as will b ed e s c r i b e dl a t e r . The c h a r g et r a n s f e rp r o c e s si si l l u s t r a t e di n maximum p o t e n t i a l sw e r e F i g u r e 3 , w h e r eo n l yt h e p l o t t e d f o r a s e c t i o no ft h e C C D channel i n t h e d i r e c t i o no fc h a r g et r a n s f e r . From F i g u r e 3 i t i sa p p a r e n tt h a tt h eo p e r a C C D i se q u i v a l e n tt ot h e t i o no ft h ev i r t u a lp h a s e CCD w i t ho n ep h a s e o p e r a t i o no ft h et w op h a s e b i a s e da t some DC p o t e n t i a l and t h eo t h e rc l o c k e d belowandabove t h i sp o t e n t i a l , as d e s c r i b e de l s e where(2) . The a d v a n t a g eo ft h ev ir t u a lp h a s e s t r u c t u r ei st h a tt h e DC b i a s e de l e c t r o d ei sn o t b u i l to v e rt h eg a t ed i e l e c t r i cb u ti sa c t u a l l y b u i l ti n t ot h es u r f a c eo ft h es i l i c o n and i s b i a s e da t h es u b s t r a t ep o t e n t i a l .T h i sf e a t u r e has s i g n i f i c a n ta d v a n t a g e sn o to n l yf o re a s eo f f a b r i c a t i o n ,b u ta l s oi np e r f o r m a n c e , as will be d i s c u s s e di nt h ef o l l o w i n gs e c t i o n s .

B u tf o rt h ev i r t u a lp h a s er e g i o n i t canbe easily d e r i v e d ,f o l l o w i n gt h e same a n a l y s i sg i v e ni n( 4 ) , that

Us E -So

APPLICATION OF THE VP CONCEPT AREA IMAGERS TO LARGE

(ni/2)

.

( 2 ni/NA)

where NA i st h ed o p i n gc o n c e n t r a t i o na tt h es u r -

A l a r g ea r e a C C D imager was s e l e c t e d as a t e s t v e h i c l ef o dr e m o n s t r a t i o no tf h ev i r t u apl h a s e CCD concept.Thisimagerhas 490 x 328 elementsand i sd e s i g n e dt oo p e r a t ei n a TV c o m p a t i b l e ,f r a m e s t o r e mode. The p a r a l l e ls e c t i o ni sd i v i d e di n t o a s e p a r a t e image i n t e g r a t i o na r e a andimage stor245 x 328 e l e m e n t s , as i l l u age area,eachhaving s t r a t e di nF i g u r e 4. Dn t h eb o t t o mo ft h es t o r a g e a r e a i s a s e r i a lr e g i s t e rw h i c ha l l o w s one v i d e o l i n e a t a t i m et ob ec l o c k e dt ot h eo u t p u ta m p l i fier. The o u t p u ta m p l i f i e ri s a s t a n d a r df l o a t i n g g a t es t r u c t u r ew i t hg a t er e s e tc a p a b i l i t y( 3 ) . The t o p o l o g yo f a s i n g l e c e l l i s a l s o shown i n Figure 4. R e g i o n so n ea n dt h r e ea r eb a r r i e r s , w h i l er e g i o n st w oa n df o u ra r ew e l l s . A crosss e c t i o no ft h ed e v i c ei nv a r i o u ss t a g e so ff a b r i c a t i o ni s shown i nF i g u r e 5 .

A t y p i c a l image o b t a i n e df r o mt h i sd e v i c ei s shown i nF i g u r e 6 . The m o d u l g t i o nt r a n s f e rf u n c t i o n was measured a t h. = 6000A. i t i s 58% a t t h e N y q u i s t limit. The c h a r g et r a n s f e re f f i c i e n c y measuredby o p t i c a ls i g n a li n j e c t i o ni s CTE = 0.99997. The maximum u s a b l ew e l lp o p u l a t i o ni s NW = 400,000e-.The r e s u l t i n gd e v i c ei s an N-

f a c eo ft h ed i r t u a lp h a s er e g i o n . N,

n

The f a c t o r h i /

makes t h es u r f a c ec o n t r i b u t i o nt ot h ed a r kc u r -

r e n it nt h i sr e g i o nn e g l i g i b l e I. na d d i t i o n , it i sa l s op o s s i b l et ob i a st h ec l o c k e d phase d u r i n g t h ei n t e g r a t ep e r i o da t a n e g a t i v eg a t ep o t e n t i a l a n dt h e r e b yf o r c eh o l e st ot h es u r f a c eo v e rt h e e n t i r e imagerarea. The darkcurrentmeasured u n d e rt h e s ec o n d i t i o n sf o rt h e 490 x 328 v i r t u a l 0 . 4 na/cm2,which i sa p p r o x i m a t e l y phaseimageris an o r d e ro fm a g n i t u d el o w e rt h a nf o rt w op h a s e d e v i c e so ft h e same g e o n e t r y and f a b r i c a t e d on t h e same m a t e r i a l .

A f e wp h o t o g r a p h so ft h ed a r kc u r r e n ts i g n i t u r e a t room t e m p e r a t u r ef o r 490 x 328 v i r t u a l phase C C D a tv a r i o u si n t e g r a t ec o n d i t i o n sa r e 7 a and 7 b.Spectralresponse shown i nF i q u r e d a t ai s shown i nF i g u r e 8. CONCLUSIONS

490 x 328 The p e r f o r m a n c ed a t ao b t a i n e df r o m p i x e li m a g e rd e m o n s t r a t et h a tt h ev i r t u a lp h a s e t e c h n o l o g yi s a v i a b l e a l t e r n a t i v e f o r f a b r i c a t i o no fl a r g ea r e ai m a g e r s .

c h a n n e lv i r t u a lp h a s e CCD w i t h a p a t t e r n e ds i n g l e p o l y s i l i c o ng a t e .T h i st y p eo fd e v i c ei se a s i l y m a n u f a c t u r e da n dy i e l d se x c e l l e n tr e s u l t si nt h e f r o n ts i d ei l l u m i n a t e d mode o fo p e r a t i o n . The unc o v e r e dr e g i o n so ft h ec h a n n e (l v i r t u a p l h a s e )d o n o tb l o c kt h el i g h t ,h e n c et h ed e v i c eh a sh i g h quantum e f f i c i e n c y ,p a r t i c u l a r l yi nt h eb l u er e g i o no ft h es p e c t r u m compared t o o t h e r p o l y s i l i c o n g a t es t r u c t u r e CCDs.

Theabsence o f i n t e r l e v e ls h o r t s ,l o wd a r k current,simplicityoffabrication,high quantum e f f i c i e n c yi nt h ef r o n ts i d ei l l u m i n a t i o n mode, as w e l l as s i m p l i c i t y o f c l o c k i n g , make t h i s t e c h n o l o g yp a r t i c u l a r l ya t t r a c t i v ef o rh i g h volumeand o lw c o s t p r o d u c t i o n . ACKNOWLEDGEMENTS The a u t h o r w o u l d l i k e t o t h a n k D r . H H. . Hosack f o r h e l p f u l comments o nt h em a n u s c r i p t , Dr. H. Dean M e r c e rf o rt h ed e v i c el a y o u t and mask des i g n , M r . H. Ray S h i e l d sf o rh e l pw i t hd e v i c e c h a r a c t e r i z a t i o n and t e s t i n g , and t h e management of t h eC e n t r a lR e s e a r c hL a b o r a t o r i e sf o rp r o v i d i n g

The l a s t andmostimportantperformanceparameter o f t h e V P CCD t o b ed i s c u s s e di st h ed a r k current. The a b s o l u t ev a l u eo ft h ed a r kc u r r e n t as w e l l a st h es p a t i a ld i s t r i b u t i o no ft h el o c a l d a r kc u r r e n ts o u r c e s( b l e m i s h e s )i sc r u c i a lf o r CCD asanimager. t h es u c c e s s f u lo p e r a t i o no ft h e

26.2 612

.

s u p p o r tf o rt h ed e v e l o p m e n to fv i r t u a lp h a s et e c h no1 ogy

.

REFERENCES T e t s u o Yamada, HaruoOkano,andNobuoSuzuki, "The E v a l u a t i o no fB u r i e d - C h a n n e lL a y e ri n BCCDIs1', IEEE T r a n s a c t i o n s on E . D . , May 1978, Vol. ED-25, NO. 5 , p. 544. C a r l o H. SequinandMichael F . Tompsett,Charge T r a n s f e rD e v i c e s , AcademicPress,Inc., N.Y.,

1975, P . 39.

3.

Figure J . E. H a l l , J . F. B r e i t z m a n n , M . M . Blouke,and J . T. C a r l o , "A M u l t i p l eO u t p u t CCD Imager f o r I m a g i n gP r o c e s s i n gA p p l i c a t i o n s " ,P r o c e e d i n s of t h eIEDM, Washington, D . C . , December 197

+

A. S . Grove,PhysicsandTechnologyofSemiconductorDevices,JohnWileyandSons,Inc., 1967, pp. 136, 145.

Maximum p o t e n t i a lp r o f i l ef o rt w od i f i s plotted f e r e n tg a t eb i a sc o n d i t i o n s i nt h ed i r e c t i o no fc h a r g et r a n s f e r .

N.Y. IMAGE IMTEGRATE A R f A

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4.

T o p o l o g ya n dt h es i n g l ep i x e d l imensions o f t h e 490x328 f r a m es t o r ei m a g e r

T

AFTERN+DIODEDIFFUSION

Figure 1 .

P o t e n t i a lp r o f i l ei n a t y p i c aN l -type CCD d e v i c e as f u n c t i o n b u r i e dc h a n n e l o ft h ed i s t a n c ei n t ot h es i l i c o n

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Figure 2.

D i v i s i o no f a s i n g l e p i x e l o f t h e v i r t u a l p h a s e CCD i n t o t h e c l o c k e d p h a s e It i s a n dt h ev i r t u apl h a s er e g i o n s . imassumed t h a tu n i f o r mb u r i e dc h a n n e l p l a n ti sp r o v i d e d .

AFTER, VIRTUALPHASEIMPLANTS

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5.

C r o s ss e c t i o n o f t h ed e v i c e stages o f f a b r i c a t i o n

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26.2 613

Figure 7b. F i g u r e 6.

Image o ft h ep a r to ft h e IEEE stand a r df a c s i m i l et e s tc h a r to b t a i n e d by t h e v i r t u a l phaseimageroperated inthefull frame mode a t 5MHz

1000

D a r kc u r r e n ts i g n a t u r e o f t h e 490 x 528 v i r t u a l phase C C D imager a f t e r 7 seconds o f i n t e g r a t i o n a t 25OC

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490 x F i g u r e7 a .D a r kc u r r e n ts i g n a t u r eo ft h e 328 v i r t u a l phase C C D imager a f t e r 300msec. o fi n t e g r a t i o na t 25OC.

26.2 614

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10

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Spectralresponse o f t h e 490 x 328 v i r C C D imager w i t ht h ep a t t e r n t u a lp h a s e e dp o l y s i l i c o ng a t es t r u c t u r e .