APPLICATIONS OF a-Si FIELD EFFECT TRANSISTORS IN LIQUID CRYSTAL DISPLAYS AND IN INTEGRATED LOGIC CIRCUITS

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APPLICATIONS OF a-Si FIELD EFFECT

TRANSISTORS IN LIQUID CRYSTAL DISPLAYS AND IN INTEGRATED LOGIC CIRCUITS

P. Le Comber, A. Snell, K. Mackenzie, W. Spear

To cite this version:

P. Le Comber, A. Snell, K. Mackenzie, W. Spear. APPLICATIONS OF a-Si FIELD EFFECT TRAN-

SISTORS IN LIQUID CRYSTAL DISPLAYS AND IN INTEGRATED LOGIC CIRCUITS. Journal

de Physique Colloques, 1981, 42 (C4), pp.C4-423-C4-432. �10.1051/jphyscol:1981490�. �jpa-00220947�

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CoZloque C4, suppZ6ment au n0lO, Tome 42, octobre 1981 page c4-4.23

APPLICATIONS OF a-Si FIELD EFFECT TRANSISTORS IN LIQUID CRYSTAL DISPLAYS AND IN INTEGRATED LOGIC CIRCUITS

P.G. Le Comber, A.J. S n e l l , K.D. Mackenzie and W.E. Spear

Carnegie Laboratory o f Physics, t h e University, Dundee, DDl 4HN, Scotland, U.K.

A b s t r a c t . - This paper reviews t h e development and a p p l i c a t i o n s of a-Si i n s u l a t e d g a t e f i e l d e f f e c t t r a n s i s t o r s which have become a s u b j e c t o f much c u r r e n t i n t e r e s t . The p h y s i c a l p r i n c i p l e s underlying t h e o p e r a t i o n of t h e d e v i c e s , t h e i r f a b r i c a t i o n and t h e i r s t a t i c and dynamic c h a r a c t e r i s t i c s a r e d i s c u s s e d i n t h e f i r s t p a r t of t h e paper. Recent work on t h e a p p l i c a t i o n of t h e s e d e v i c e s i n a d d r e s s a b l e l i q u i d c r y s t a l d i s p l a y p a n e l s , i n b a s i c i n t e g r a t e d l o g i c c i r c u i t s and i n a d d r e s s a b l e image s e n s i n g a r r a y s i s t h e n reviewed i n some d e t a i l .

1. I n t r o d u c t i o n

I n r e c e n t y e a r s t h e r e has been a r a p i d l y growing i n t e r e s t i n t h e p r o p e r t i e s of amorphous (a-) S i prepared by t h e glow d i s c h a r g e p r o c e s s , p a r t i c u l a r l y w i t h a view t o i t s a p p l i c a t i o n s i n l a r g e - a r e a t h i n f i l m d e v i c e s . Many groups throughout t h e world a r e a c t i v e l y involved i n t h i s f i e l d and, a s can b e seen from t h e papers pres-

ented a t t h i s Conference, t h e range of p o s s i b l e a p p l i c a t i o n s of t h i s m a t e r i a l i s a l s o i n c r e a s i n g . Ever s i n c e t h e f i r s t r e p o r t (1,2) o f t h e doping of a-Si t h e r e has been c o n s i d e r a b l e i n t e r e s t i n i t s u s e a s a s o l a r c e l l m a t e r i a l ( 3 ) . This a p p l i c -

a t i o n probably p k e s e n t s t h e b i g g e s t c h a l l e n g e t o t h e m a t e r i a l p r e s e n t l y a v a i l a b l e b u t , because of i t s c o n s i d e r a b l e p o t e n t i a l , a t t r a c t s a g r e a t d e a l of investment i n both time and money. This a s p e c t i s d e a l t w i t h i n a number of papers a t t h i s conference and we s h a l l not t h e r e f o r e go i n t o f u r t h e r d e t a i l s h e r e .

One of t h e most a t t r a c t i v e f e a t u r e s of t h e glow d i s c h a r g e t e c h n i q u e i s i t s extreme v e r s a t i l i t y . For example, t h e b a r r i e r p r o f i l e of t h e a-Si j u n c t i o n can b e c o n t r o l l e d continuously over wide l i m i t s d u r i n g d e p o s i t i o n from t h e gas phase. By a s u i t a b l e c h o i c e of t h e doping p r o f i l e , a-Si d i o d e s have been prepared (4,5) t h a t p a s s c u r r e n t s up t o 40A cm-2 i n t h e forward d i r e c t i o n w i t h r e c t i f i c a t i o n r a t i o s up t o

lo5.

These r e s u l t s a r e encouraging and suggest t h a t a-Si d i o d e s may f i n d a p p l i c - a t i o n s i n c a s e s where some r e l a x a t i o n i n e l e c t r i c a l c h a r a c t e r i s t i c s from t h o s e of c r y s t a l l i n e j u n c t i o n s w i l l b e a c c e p t a b l e .

The v e r s a t i l i t y of t h e glow d i s c h a r g e p r o c e s s i s f u r t h e r i l l u s t r a t e d by t h e work of Shimizu e t a 1 (6,7) who demonstrated t h e u s e of a-Si a s t h e photo-receptor i n electrophotography. This a p p l i c a t i o n r e q u i r e s a m a t e r i a l w i t h a h i g h e l e c t r i c a l r e s i s t i v i t y and a h i g h p h o t o s e n s i t i v i t y . I n g e n e r a l , a-Si i s extremely photo-

conductive (8,9) and meets t h e second of t h e s e requirements. To o b t a i n a s u f f i c i e n t - l y h i g h r e s i s t a n c e f o r r e a s o n a b l e charge r e t e n t i o n a number of d i f f e r e n t approaches have been t r i e d , such a s blocking l a y e r s of doped a-Si (7,10,11) o r t h e a d d i t i o n of oxygen t o t h e gas plasma (12). The requirements f o r t h e p r o d u c t i o n of a v i d i c o n tube a r e s i m i l a r t o t h o s e of electrophotography. r h e f i r s t a-Si v i d i c o n was r e p o r t - ed by Imamura e t a 1 (13) who prepared t h e d e v i c e by s p u t t e r i n g i n an A r / H 2 gas mix- t u r e which produced a m a t e r i a l of s u f f i c i e n t l y l a r g e r e s i s t a n c e . The a l t e r n a t i v e approach of i n c o r p o r a t i n g a d i o d e - l i k e b l o c k i n g l a y e r (prepared by t h e glow d i s c h a r g e technique) was used by Shimizu e t a 1 ( 1 4 ) . I t i s claimed (13) t h a t t h e a-Si v i d i c o n h a s many advantages over conventional pickup t u b e s . F u r t h e r d e t a i l s on t h e p r e s e n t s t a g e of t h e development of a-Si f o r b o t h electrophotography and v i d i c o n tubes w i l l b e r e p o r t e d a t t h i s conference by D r . Shimizu.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981490

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Another advantage of t h e glow d i s c h a r g e technique i s t h a t i t can b e used t o d e p o s i t s e q u e n t i a l l y t h i n l a y e r s of d i f f e r e n t m a t e r i a l s , simply by changing t h e gas composition. This procedure has found i n c r e a s i n g use i n t h e production of t h i n f i l m f i e l d e f f e c t t r a n s i s t o r s ( t o be r e f e r r e d t o a s FETs i n t h e f o l l o w i n g ) , c o n s i s t - i n g of an amorphous s i l i c o n n i t r i d e g a t e d i e l e c t r i c and an a-Si a c t i v e l a y e r . The o r i g i n of t h i s device can be t r a c e d back t o 1972 when Spear and Le Comber (15) employed f i e l d e f f e c t s t r u c t u r e s t o i n v e s t i g a t e t h e d e n s i t y of s t a t e d i s t r i b u t i o n i n glow d i s c h a r g e a-Si. The subsequent use of s i l i c o n n i t r i d e a s t h e g a t e i n s u l a t o r (16) made i t p o s s i b l e t o extend t h e measurements of t h e d e n s i t y of s t a t e s over a wider range of energy. These experiments demonstrated t h a t a-Si h a s a remarkably low d e n s i t y of gap s t a t e s , and t h a t i t s c o n d u c t i v i t y can b e i n c r e a s e d by o r d e r s of magnitude w i t h t h e a p p l i c a t i o n of modest g a t e v o l t a g e s . The o b j e c t i v e of t h i s paper i s t o review t h e p o s s i b l e a p p l i c a t i o n s of t h e s e a-Si FETs. The p h y s i c a l p r i n c i p l e s underlying t h e o p e r a t i o n of t h e d e v i c e s , t h e i r f a b r i c a t i o n and t h e i r s t a t i c and dynamic c h a r a c t e r i s t i c s w i l l b e d i s c u s s e d i n some d e t a i l i n s e c t i o n 2.

The r e s u l t s o b t a i n e d by v a r i o u s groups w i l l a l s o b e compared. I n s e c t i o n 3 we s h a l l review t h e a p p l i c a t i o n of t h e s e d e v i c e s i n a d d r e s s a b l e l i q u i d c r y s t a l d i s p l a y panels, an a r e a t h a t looks extremely promising f o r t h e immediate f u t u r e . Within t h e l a s t year o r s o t h e r e has been an i n c r e a s i n g i n t e r e s t i n t h e a p p l i c a t i o n of a-Si PETS i n i n t e g r a t e d l o g i c c i r c u i t s and we s h a l l review t h e s e p o s s i b i l i t i e s i n s e c t i o n 4.

2. Device F a b r i c a t i o n and dc C h a r a c t e r i s t i c s of Elemental Device

The b a s i c d e s i g n of t h e e l e m e n t a l a-Si FET i s shown s c h e m a t i c a l l y i n t h e i n s e r t t o f i g . 1 . It i s a d i r e c t development of t h e arrangement o r i g i n a l l y used i n t h e f i e l d e f f e c t experiments (15,16,17) f o r t h e s t u d y of t h e l o c a l i z e d s t a t e d i s t r i b u t - i o n i n t h e m o b i l i t y gap of a-Si. A metal e l e c t r o d e , t y p i c a l l y 50 um wide, i s evap- o r a t e d onto a g l a s s s u b s t r a t e t o form t h e g a t e e l e c t r o d e . A t h i n i n s u l a t i n g l a y e r , t y p i c a l l y about 0.4 pm of amorphous s i l i c o n n i t r i d e (Si-N), i s t h e n d e p o s i t e d by t h e glow d i s c h a r g e t e c h n i q u e t o form t h e g a t e d i e l e c t r i c . An a-Si l a y e r , a l s o about 0.4 um t h i c k , i s next d e p o s i t e d onto t h e Si-N a g a i n by t h e glow d i s c h a r g e p r o c e s s . The f i n a l s t e p c o n s i s t s of d e p o s i t i n g t h e r e q u i r e d p a t t e r n of source and d r a i n c o n t a c t s on t o t h e a-Si s u r f a c e .

Fig.1. T r a n s f e r c h a r a c t e r i s t i c s of a-Si FET element. The d r a i n c u r r e n t ID i s p l o t t e d a g a i n s t t h e g a t e v o l t a g e VG f o r t h r e e d r a i n p o t e n t i a l s VD. (From ref.18)

1

¹

1 I

-2 0 2 I

Surface Field (105~!c/cml

6

Fig.2. Sheet conductance p l o t t e d a g a i n s t s u r f a c e f i e l d f o r a-Si FETs. Curve D:

Recent Dundee d a t a ; Curve P: Powell e t a 1 (19); Curve H-M: Hayama and Matsumura (20); Curve N-M; Neudeck and Malhotra (21).

I n a r e c e n t paper (18) w i t h D r . A . J . Hughes of RSRE, we showed t h a t convention- a l p h o t o l i t h o g r a p h i c techniques can b e a p p l i e d t o t h e f a b r i c a t i o n of a r r a y s of a-Si FETs f o r a p p l i c a t i o n i n l i q u i d c r y s t a l d i s p l a y p a n e l s ( s e e s e c t i o n 3). This i s an important p o i n t s i n c e i t i s e s s e n t i a l f o r any f u t u r e a p p l i c a t i o n s of t h e s e d e v i c e s t h a t l a r g e a r e a a r r a y s can b e produced reproduceably and cheaply.

The dc performance of a n elementary device w i t h a 40 um chan\el l e n g t h and a 500

um

channel width, produced by p h o t o l i t h o g r a p h i c t e c h n i q u e s , i s i l l u s t r a t e d by t h e t r a n s f e r c h a r a c t e r i s t i c s shown i n f i g . 1 . The source-drain curreaB ID i s p l o t t e d l o g a r i t h m i c a l l y a g a i n s t t h e g a t e v o l t a g e VG f o r d r a i n p o t e n t i a l s of 2V, LQV, and 20V.

It can b e seen t h a t w i t h +15V on t h e g a t e , d r a i n c u r r e n t s i n excess of 1 PA can b e achieved f o r d r a i n v o l t a g e s a s low a s 5V. I n t h e o f f - c o n d i t i o n , w i t h VG = 0 , t h e c u r r e n t through t h e d e v i c e drops below 1 0 - l l ~ . The remarkable r i s e i n ID _{i s} caused by an e l e c t r o n accumulation l a y e r formed a t t h e Si/Si-N i n t e r f a c e , which produces an e f f i c i e n t c u r r e n t p a t h between t h e s o u r c e and d r a i n e l e c t r o d e s .

A number of d i f f e r e n t groups have r e p o r t e d r e s u l t s f o r s i m i l a r a-Si FETs. To compare t h e d a t a , which has been o b t a i n e d f o r d i f f e r e n t d e v i c e geometries and u s i n g d i f f e r e n t g a t e d i e l e c t r i c s , we f o l l o w Powell e t a1 (19) and p l o t i n f i g . 2 t h e s h e e t conductance l o g a r i t h m i c a l l y a g a i n s t t h e f i e l d i n t h e semiconductor a t t h e i n s u l a t o r / a-Si i n t e r f a c e . The s u r f a c e f i e l d h a s been c a l c u l a t e d from E ~ V ~where e i / E ~ ~i s ~ t h e r e l a t i v e p e r m i t t i v i t y of t h e i n s u l a t o r , G S ~ t h e r e l a t i v e p e r m i t t i v i t y of t h e a-Si and d t h e t h i c k n e s s of t h e g a t e i n s u l a t o r . The curve denoted by D i s r e c e n t Dundee d a t a o b t a i n e d w i t h a 0.4 um t h i c k glow d i s c h a r g e Si-N i n s u l a t o r ; curve P i s from Powell e t a1 (19) a t t h e P h i l i p s Research L a b o r a t o r i e s who a l s o used a glow d i s c h a r g e SF-N f i l m of about 0 . 3 um t h i c k n e s s ; c u r v e H-M r e f e r s t o t h e d a t a of Hayama and Matsumura (20) u s i n g a 1 pm s i l i c o n d i o x i d e l a y e r , t h e r m a l l y grown on t h e s u r f a c e of h i g h l y doped s i n g l e - c r y s t a l s i l i c o n , and a 0.13 pm a-Si l a y e r d e p o s i t e d i n a dc glow d i s c h a r g e ; f i n a l l y c u r v e N-M r e f e r s t o t h e d a t a of Neudeck and Malhotra (21) f o r 0.3 pm Si02 and w i t h a t h e r m a l l y evaporated a-Si l a y e r about 0.06 pm t h i c k annealed f o r f o u r hours a t 400°C. I n p l o t t i n g t h e d a t a shown i n f i g . 2 we have taken t h e d i e l e c t r i c c o n s t a n t of Si-N a s 6 . 4 , of S i 0 2 a s 3.9 and t h a t of a-Si a t 11.8. For d e v i c e a p p l i c a t i o n s t h e t r a n s f e r c h a r a c t e r i s t i c s shown i n f i g s . 1 and 2 should have t h e f o l l o w i n g p r o p e r t i e s ( i ) low off-conductance, ( i i ) high on-conduct- ance, and ( i i i ) a r a p i d r i s e from t h e o f f - t o t h e on- s t a t e a t g a t e v o l t a g e s below 15V so t h a t devices a r e compatible w i t h modern i n t e g r a t e d c i r c u i t v o l t a g e l e v e l s . A l l t h e d a t a i n f i g . 2 s a t i s f y t h e f i r s t of t h e s e c r i t e r i a s o t h i s i s u n l i k e l y t o p r e s e n t any problems. The a b i l i t y t o o b t a i n a h i g h on-current i s c l e a r l y a much

l a r g e r c h a l l e n g e . From t h e d a t a p u b l i s h e d t o

8 d a t e , our own d e v i c e s a r e t h e b e s t i n t h i s

r e s p e c t g i v i n g over 1 pA a t VSD = 10V and VG = +15V even w i t h a 500 pm channel l e n g t h .

6 The t h i r d c r i t e r i o n i s a l s o r e l a t i v e l y

d i f f i c u l t t o achieve. Only curves P and D i n

ID f i g . 2 appear t o be s u i t a b l e i n t h i s r e s p e c t .

OJA

1

The o u t p u t c h a r a c t e r i s t i c s , p l o t t e d i n f i g . 3 , of t h e Dundee d e v i c e s show t h a t t h e on-current

L tends t o s a t u r a t i o n f o r g a t e v o l t a g e s between

10 and 15V compared w i t h , f o r example, t h e 50 t o l O O V r e q u i r e d by t h e d e v i c e s of Hayama and Matsumura. The devices of Powell e t a 1 (19) r e q u i r e even s m a l l e r VG ( < 5V) b u t , a s can b e

2 seen from f i g . 2 , t h e r a t e of r i s e of s h e e t con-

ductance a s a f u n c t i o n of VG i s almost e x a c t l y t h e same f o r t h e Dundee and t h e P h i l i p s d a t a , a t l e a s t a s f a r a s t h e l a t t e r extend. The main 0 d i f f e r e n c e l i e s i n t h e o f f s e t v o l t a g e

-

^{t h e}

0 10 20 30 LO P h i l i p s d e v i c e t u r n s on j u s t above z e r o where-

vD ( v )

a s t h e Dundee FETs s t a y o f f up t o about 5 v o l t s . Experiments i n our l a b o r a t o r y have Fig.3. Output c h a r a c t e r i s t i c s of shown t h a t i t i s p o s s i b l e t o vary t h e o f f s e t a-Si FET element. (From r e f . 1 8 ) . v o l t a g e by s u i t a b l e changes i n t h e a-Si prop- e r t i e s c l o s e t o t h e i n s u l a t o r / a - S i i n t e r f a c e .

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I t i s perhaps worth emphasising a t t h i s p o i n t t h a t t h e formation w i t h p o s i t i v e g a t e v o l t a g e of t h e h i g h l y conducting e l e c t r o n accumulation l a y e r , which determines t h e p r o p e r t i e s of t h e o n - s t a t e , i s governed by two fundamental m a t e r i a l p r o p e r t i e s : ( i ) a low d e n s i t y of l o c a l i z e d gap s t a t e s i n t h e bulk of t h e a-Si m a t e r i a l i t s e l f , and ( i i ) a low d e n s i t y of i n t e r f a c e s t a t e s a t t h e Si-N t o a-Si boundary. The poor response of t h e d e v i c e N-M i n f i g . 2 , f a b r i c a t e d from evaporated a-Si, may e a s i l y b e understood i n terms of ( i i ) s i n c e even annealed evaporated a-Si i s b e l i e v e d t o have a h i g h e r l o c a l i z e d gap s t a t e d e n s i t y than glow d i s c h a r g e m a t e r i a l ( 1 6 ) . The d i f f e r - ences i n t h e o t h e r samples may a l s o a r i s e a t l e a s t p a r t l y f o r t h i s r e a s o n , s i n c e l o c a l i z e d s t a t e d e n s i t i e s tend t o depend c r i t i c a l l y on t h e p r e c i s e c o n d i t i o n s i n t h e plasma d u r i n g t h e glow d i s c h a r g e process. It i s e q u a l l y l i k e l y however t h a t d i f f e r - e n t L a b o r a t o r i e s produce d e v i c e s w i t h d i f f e r e n t i n t e r f a c e s t a t e d e n s i t i e s . I n f a c t i t would b e s u r p r i s i n g i f t h e u s e of d i f f e r e n t i n s u l a t o r m a t e r i a l s d i d n o t produce d i f f e r e n t i n t e r f a c e s t a t e d e n s i t i e s . From t h e a d m i t t e d l y l i m i t e d d a t a i n f i g . 2 i t would appear a t t h i s s t a g e of t h e development t h a t glow d i s c h a r g e Si-N i s a s u p e r i o r g a t e m a t e r i a l t h a n thermally grown S i 0 2 b u t more work i s needed b e f o r e f i r m conclus- i o n s can b e drawn.

For l a r g e s c a l e a p p l i c a t i o n s t h e ac o p e r a t i o n of t h e FETs becomesof more r e l e v - ance t h a n t h e i r dc performance. We s h a l l d i s c u s s t h e ac c h a r a c t e r i s t i c s and a l s o t h e i r r e p r o d u c i b i l i t y and s t a b i l i t y i n t h e c o n t e x t of t h e experiments we have- c a r r i e d o u t on t h e use of a-Si FETs i n l i q u i d c r y s t a l d i s p l a y p a n e l s i n t h e follow- i n g s e c t i o n .

Recently we began an i n v e s t i g a t i o n of t h e e f f e c t of y - r a d i a t i o n from c o b a l t 60 on t h e dc c h a r a c t e r i s t i c s of t h e a-Si FETi. P r e l i m i n a r y r e s u l t s show t h a t doses up t o 11 Mrads, t h e maximum used t o d a t e , have r e l a t i v e l y l i t t l e e f f e c t . I n t h e devices i n v e s t i g a t e d so f a r t h e FETs r e t a i n e d t h e i r h i g h o f f r e s i s t a n c e and r e l a t - i v e l y h i g h on-conductance. More d e t a i l e d r e s u l t s w i l l b e p u b l i s h e d i n due c o u r s e . 3. A p p l i c a t i o n of a-Si FETs i n Addressable Liquid C r y s t a l E i s p l a y P a n e l s .

I n 1976, t h e Dundee group and c o l l e a g u e s a t RSRE, Malvern, j o i n t l y proposed t h e use of a-Si f i e l d e f f e c t d e v i c e s i n t h e a d d r e s s i n g of l i q u i d c r y s t a l m a t r i x d i s p l a y s a s an a l t e r n a t i v e t o t h e t h i n f i l m CdSe t r a n s i s t o r s which had been developed by Brody and co-workers (22). They reasoned t h a t an elemental c o v a l e n t l y bonded mater- i a l such a s a-Si should have d i s t i n c t advantages over t h e more complex 1 1 - V I com- pounds a s f a r a s e a s e of preparation,reproducibility and s t a b i l i t y a r e concerned.

Subsequently, Le Comber e t a 1 (23) p u b l i s h e d t h e d e s i g n and c h a r a c t e r i s t i c s of an a-Si i n s u l a t e d g a t e f i e l d e f f e c t t r a n s i s t o r s u i t a b l e f o r d r i v i n g l i q u i d c r y s t a l d i s - p l a y s , and experiments a t RSRE showed t h a t such a d e v i c e could switch l i q u i d c r y s t a l elements s a t i s f a c t o r i l y .

Liquid c r y s t a l d i s p l a y s a r e p o t e n t i a l l y v e r y a t t r a c t i v e f o r l a r g e - a r e a a p p l i c - a t i o n s a s they a r e low power d e v i c e s and have good v i s i b i l i t y i n h i g h ambient l i g h t c o n d i t i o n s . However, t h e d i f f i c u l t y of m u l t i p l e x i n g l a r g e a r r a y s of such d e v i c e s has so f a r l i m i t e d t h e i r use. I f a small n o n l i n e a r device, such a s an FET, i s used t o c o n t r o l each d i s p l a y element, much l a r g e r a r r a y s could i n p r i n c i p l e b e add- r e s s e d . Liquid c r y s t a l devices t y p i c a l l y r e q u i r e an a c d r i v e s i g n a l of 3-6V with- out dc component, a s t h e l a t t e r t e n d s t o degrade t h e m a t e r i a l . The d e v i c e responds t o t h e rms v a l u e of t h e a c s i g n a l , and h a s a t h r e s h o l d of 1-2V rms. Voltages below t h i s l e v e l w i l l n o t t u r n on t h e d i s p l a y .

We s h a l l b e g i n t h i s s e c t i o n by looking a t t h e f a b r i c a t i o n of t h e a-Si FET a r r a y and t h e n review i t s dynamic behaviour i n a l i q u i d c r y s t a l d i s p l a y (18).

3.1 F a b r i c a t i o n of Display Panel.

F i g u r e 4 shows a schematic diagram of a number of elements i n t h e d i s p l a y panel.

A t r a n s i s t o r i s i n c o r p o r a t e d i n a corner of each element of t h e a r r a y . The FETs a r e i n t e r c o n n e c t e d by means of X and Y buses, G1, G2... and S1, 52

...,

l i n k i n g g a t e and source c o n t a c t s , r e s p e c t i v e l y . The d r a i n c o n t a c t of each t r a n s i s t o r i s connected t o t h e IT0 s q u a r e s , D. From t h e s e c t i o n through t h e panel i n Fig.4b i t can b e s e e n t h a t t h e l i q u i d c r y s t a l m a t e r i a l i s sandwiched between t h e s u b s t r a t e c a r r y i n g t h e FETs and an IT0 coated g l a s s top p l a t e which i s normally r e t u r n e d t o ground. The

Fig.4a and b. Schematic lay-out of an Fig.5a and b . Design o f a-Si f i e l d e f f e c t a d d r e s s a b l e l i q u i d c r y s t a l panel. ( a ) t r a n s i s t o r element ( a ) s e c t i o n through p l a n view, (b) s i d e view. (From r e f . 1 8 ) d e v i c e , (b) FET i n p a r t of t h e m a t r i x

a r r a y . (From r e f . l 8 )

l i q u i d c r y s t a l element i s t h e r e f o r e i n s e r i e s w i t h t h e d r a i n c i r c u i t and behaves e l e c t r i c a l l y a s a c a p a c i t o r CLC w i t h some leakage r e s i s t a n c e RLC.

F i g u r e 5a shows a s e c t i o n through an i n d i v i d u a l a-Si d e v i c e and F i g . % i l l u s - t r a t e s t h e d e s i g n of t h e FET i n p a r t of t h e m a t r i x a r r a y . The f i r s t s t e p i n t h e f a b r i c a t i o n c o n s i s t s i n e t c h i n g t h e p a t t e r n of t r a n s p a r e n t conducting Indium T i n Oxide (ITO) t h a t forms t h e back c o n t a c t s of t h e l i q u i d c r y s t a l c e l l ( s e e a l s o Fig.4).

I n t h e p r e s e n t work we used an a r r a y of IT0 s q u a r e s of 1 mm s i d e - l e n g t h on t h e Corn- i n g g l a s s s u b s t r a t e . A chromium f i l m i s then evaporated and e t c h e d t o p r o v i d e t h e 60 u m wide g a t e e l e c t r o d e s G . The prepared s u b s t r a t e i s now c o a t e d w i t h 0.4 ym t h i c k l a y e r s of s i l i c o n n i t r i d e and s i l i c o n , b o t h d e p o s i t e d i n a n r f glow d i s c h a r g e . The unwanted s i l i c o n i s e t c h e d away, j u s t l e a v i n g t h e small i s l a n d s a t each FET s i t e

( F i g . 5 b ) . Contact h o l e s , denoted by A i n F i g . 5 , a r e etched through t h e n i t r i d e down t o t h e IT0 f o r connection t o t h e d r a i n c o n t a c t D. F i n a l l y , t h e aluminium t o p met- a l l i s a t i o n i s a p p l i e d t o form t h e s o u r c e and d r a i n c o n f i g u r a t i o n s , S and D. A t a l l s t a g e s t h e p r o c e s s i n g i s c a r r i e d o u t by s t a n d a r d p h o t o l i t h o g r a p h i c techniques and w i t h c o n v e n t i o n a l e t c h e s .

a-Si i s a h i g h l y s e n s i t i v e photoconductor ( 8 , 9 ) and i t i s important t o c o n s i d e r what e f f e c t s t r a y l i g h t might have on t h e c h a r a c t e r i s t i c s of t h e FET. Recent measurements have shown t h a t ambient l i g h t h a s l i t t l e i n f l u e n c e on t h e on-current b u t d e c r e a s e s t h e o f f - r e s i s t a n c e by an o r d e r of magnitude o r s o . I f t h i s proves u n d e s i r a b l e t h e n it should b e p o s s i b l e t o reduce t h i s e f f e c t by t h e i n c l u s i o n during f a b r i c a t i o n of a s u i t a b l e mask f o r t h e a-Si.

3 . 2 Dynamic Performance.

I n t h i s s e c t i o n we s h a l l c o n s i d e r t h e ac o p e r a t i o n of t h e elementary d e v i c e w i t h i t s l i q u i d c r y s t a l element. A more e x t e n s i v e d i s c u s s i o n i s g i v e n i n r e f . ( l 8 ) . To s i m u l a t e t h e o p e r a t i o n of t h e FETs i n an a d d r e s s a b l e a r r a y , t h e response of s i n g l e elements t o t y p i c a l p u l s e v o l t a g e s on t h e g a t e and source b u s e s was i n v e s t - i g a t e d . I n t h e s e experiments t h e d r a i n was connected t o ground through a 10 pF c a p a c i t o r t o s i m u l a t e t h e l i q u i d b r y s t a l c a p a c i t y CLC of t h e 1 mm IT0 elements ( s e e f i g . 4 ) . An important parameter, a s f a r a s m u l t i p l e x i n g i s concerned, i s t h e r i s e time of t h e p o t e n t i a l VLC a c r o s s t h e l i q u i d c r y s t a l element. I n Fig.6 t h e v o l t a g e

JOURNAL DE PHYSIQUE

F i g . 6 , Voltage l e v e l VLC a c r o s s t h e lOpF l i q u i d c r y s t a l element p l o t t e d a g a i n s t t h e width of t h e g a t e p u l s e n e c e s s a r y t o o b t a i n t h e v a l u e of VLC. (From r e f . 1 8 ) .

l e v e l VLC a c r o s s t h e lOpF element i s p l o t t e d a g a i n s t t h e width of t h e g a t e p u l s e C L , necess- a r y t o produce t h e v a l u e of VLC. I t can be seen t h a t a d r i v e p o t e n t i a l of 3V i s reached i n 30- 40 ps.

I n a n o t h e r s e r i e s of experiments d r i v i n g commercial l i q u i d c r y s t a l elements w i t h t h e a-Si FETs, i t was demonstrated t h a t c h a r g e r e t e n t i o n was l i m i t e d e n t i r e l y by t h e l i q u i d c r y s t a l prop-

e r t i e s t o about 40 ms. I n view of t h e above 30- 40 ps r e q u i r e d t o produce t h e d r i v e p o t e n t i a l f o r t h e l i q u i d c r y s t a l , i t can b e s e e n t h a t even w i t h

t h e p r e s e n t d e v i c e s i t i s p o s s i b l e i n p r i n c i p l e t o s c a n 1000 l i n e s of d i s p l a y i n t h i s time. I n p r a c t i c e , u s i n g a frame time of 25 m s , s a t i s f a c t - o r y o p e r a t i o n of a l i q u i d c r y s t a l element was achieved w i t h p u l s e s e q u i v a l e n t t o t h o s e used i n a 250 l i n e d i s p l a y .

S u c c e s s f u l 7 x 5 a r r a y s have been f a b r i c - a t e d i n our l a b o r a t o r y and we a r e p r e s e n t l y a s s - e s s i n g a 20 x 25 panel b e f o r e s c a l i n g up t o 100 x 100 and l a r g e r a r r a y s .

3.3 R e p r o d u c i b i l i t y and S t a b i l i t y of a-Si FETs.

An important f a c t o r i n t h i s development i s t h e r e p r o d u c i b i l i t y and u n i f o r m i t y of c h a r a c t e r i s t i c s of a r r a y s of elements. F i g u r e 7 shows t h e t r a n s f e r c h a r a c t e r - i s t i c s of a l i n e a r a r r a y of elements, spaced 2.5 nun a p a r t ( 1 8 ) . For c l a r i t y succ- e s s i v e c u r v e s have been d i s p l a c e d along t h e VG a x i s by 10V. The s i m i l a r i t y of

t h e s e curves demonstrates t h e remarkable

1 0 ' ~ 1 1 1 1 1 1 1 1 1 u n i f o r m i t y of performance t h a t can b e ach-

ieved i n a given d e p o s i t i o n . P r o b i n g t e s t s on a complete 7 x 5 a r r a y showed s i m i l a r e x c e l l e n t u n i f o r m i t y . A l l elements had --- Roff > 1011 S2 and a l l t h r e s h o l d v o l t a g e s V D = 2 V were w i t h i n ^i. 0.4V of t h e mean v a l u e .

Comparison between a r r a y s produced i n d i f f - e r e n t d e p o s i t i o n s i s a l s o v e r y encouraging.

So f a r , t h e i n f o r m a t i o n on long term s t a b i l i t y of t h e a-Si d e v i c e s i s

s t i l l

1 0 ' ~

v,

( V )

Fig.7. T r a n s f e r c h a r a c t e r i s t i c s of s i x FETs i n a l i n e a r a r r a y a s a t e s t of d e v i c e r e p r o d u c i b i l i t y . For c l a r i t y s u c c e s s i v e c h a r a c t e r i s t i c s have been d i s p l a c e d by 10V along t h e VG AXIS. (From r e f . 1 8 ) .

-

1 2 3 4 5 6

10 Volts

-

I t 1

somewhat l i m i t e d . I n a t e s t an element h a s now been run a t 100 Hz w i t h VS = k 10V and VG = 15V f o r 9 months, i . e . f o r over 2 x 10' s w i t c h i n g o p e r a t i o n s . There has been no s i g n of d e t e r i o r a t i o n o r change i n t r a n s f e r c h a r a c t e r i s t i c s of t h i s d e v i c e , which i s u n p a s s i v a t e d and unencapsulated.

4. A p p l i c a t i o n of a-Si FETs i n Logic C i r c u i t s and Image Sensors.

Recently, Matsumura, Hayama and coworkers a t t h e Tokyo I n s t i t u t e of Technology have shown t h a t an i n t e g r a t e d i n v e r t e r c i r c u i t (24) and an image s e n s o r (25) can be f a b r i c a t e d f r o n a-Si FETs. The devices used i n t h e i r work were made from a-Si dep- o s i t e d i n a dc glow d i s c h a r g e and w i t h low p r e s s u r e CVD S i 0 2 a s t h e g a t e i n s u l a t o r (24). Although t h e s e FETs were relatively leaky and had a slower t u r n on t h a n t h e i r previous d e v i c e s included i n f i g . 2 (curve H-M) they demonstrated t h e f e a s i b i l i t y of u s i n g t h e a-Si technology i n t h e s e c i r c u i t s . At t h e same time work i n our l a b o r a t - ory h a s been concerned w i t h s i m i l a r a p p l i c a t i o n s , i n i t i a l l y aimed a t p r o v i d i n g i n t - e g r a t e d a-Si d r i v e c i r c u i t s f o r t h e l i q u i d c r y s t a l d i s p l a y p a n e l s d i s c u s s e d i n s e c t i o n 3. I n t h e following we s h a l l review t h e work on t h e i n v e r t e r s and image s e n s o r s and r e p o r t b r i e f l y t h e a p p l i c a t i o n s of a-Si FETs i n N A N D and NOR g a t e s , i n a b i s t a b l e m u l t i v a b r a t o r and i n a s h i f t r e g i s t e r .

Three b a s i c c i r c u i t s f o r producing t h e i n v e r t e r l o g i c a r e shown i n f i g . 8 . Matsumura and Hayama used c i r c u i t ( c ) , which r e q u i r e s two FETs and an a d d i t i o n a l

supply VG. I n our own work we chose t h e simpler c i r c u i t ( a ) and made t h e load r e s i s t o r from an i n t e g r a t e d a-Si gap c e l l . This i s simpler and h a s t h e f u r t h e r advantage t h a t t h e a-Si r e s i s t o r r e q u i r e s l e s s space t h a n an FET l o a d . Fig.9 shows a s e c t i o n through t h e i n t e g r a t e d d e v i c e and a l s o i t s l a y o u t i n p l a n view. The i n p u t v o l t a g e i s a p p l i e d t o t h e g a t e of t h e FET, t h e source i s connected t o ground and t h e o u t p u t taken from t h e d r a i n connection. To o b t a i n t h e a p p r o p r i a t e v a l u e of

load r e s i s t a n c e a doped a-Si l a y e r , about 250A t h i c k and about 3 x

lo8

010

,

was d e p o s i t e d . This was e t c h e d away from t h e source-drain gap of t h e FET a f t e r t h e t o p A 1 m e t a l l i s a t i o n , l e a v i n g doped a-Si under t h e source and d r a i n , which had t h e added advantage of improving t h e e l e c t r i c a l c o n t a c t a t t h e s e e l e c t r o d e s .

The dashed curve i n f i g . 1 0 shows t h e t r a n s f e r c h a r a c t e r i s t i c s of such a d e v i c e having a l o a d of approximately 30M0 and measured w i t h a supply p o t e n t i a l of 15V.

The i n v e r t e r l o g i c i s c l e a r l y seen: t h e o u t p u t changes from about 14.5 t o 2V a s t h e i n p u t swings from about 5 t o 15V. The f u l l F i g . 8 . Three examples of i n v e r t e r l i h e i n f i g . 1 0 shows t h e c h a r a c t e r i s t i c s of

c i r c u i t s . t h r e e such d e v i c e s connected i n s e r i e s .

A s w e l l a s sharpening t h e c h a r a c t e r i s t i c s

a-Si Doped a-Si

t h i s demonstrates t h e important p o i n t t h a t t h e o u t p u t from one d e v i c e can b e success- f u l l y used a s t h e i n p u t f o r s u c c e s s i v e s t a g e s .

By simple e x t e n s i o n of t h e above b a s i c c i r c u i t s , we have produced l o g i c c i r c u i t s such a s N A N D and NOR g a t e s and a l s o t h e b i s t a b l e m u l t i v i b r a t o r t h e c i r c u i t of which i s shown i n f i g . 1 1 . The s w i t c h i n g times of t h e s e c i r c u i t s were t y p i c a l l y of t h e o r d e r of a ms, which would l i m i t t h e i r use t o f r e q u e n c i e s below 1kHz. However, we should emphasize t h a t t h e s e e x p l o r a t o r y a-Si d e v i c e s were i n no way optimised f o r speed of response. The main l i m i t a t i o n i n t h i s r e s p e c t i s t h e magnitude of t h e a-Si FET o n - r e s i s t a n c e

-

an a s p e c t which we w i l l r e t u r n t o b r i e f l y i n t h e n e x t s e c t i o n .

As s t a t e d p r e v i o u s l y , one o f t h e prim- - -

Fig.9. I n t e g r a t e d a-Si i n v e r t e r . a r y aims of o u r p r e s e n t work is t o i n v e s t - i g a t e t h e p o s s i b i l i t y of d r i v e c i r c u i t s f o r

JOURNAL DE PHYSIQUE

Fig.10. C h a r a c t e r i s t i c s of ( a ) s i n g l e Pig.11. C i r c u i t used f o r a-Si b i s t a b l e a-Si i n v e r t e r and (b) t h r e e i n v e r t e r s m u l t i v i b r a t o r and p u l s e sequence

connected i n s e r i e s . t y p i c a l l y observed.

t h e l i q u i d c r y s t a l p a n e l s . Of p a r t i c u l a r importance i n t h i s r e s p e c t i s t h e f a b r i c - a t i o n of a s h i f t r e g i s t e r t o s e q u e n t i a l l y a d d r e s s t h e g a t e buses G G2,

...

^{i n}

f i g . & . Recently we have made a f o u r s t a g e s h i f t r e g i s t e r u s i n g a-b?1 FETs i n which each s t a g e can s h i f t a 20 m s p u l s e by 10 m s . Although t h e s e c i r c u i t s a r e slow by c r y s t a l l i n e s t a n d a r d s we b e l i e v e optimised d e v i c e geometry w i l l c o n s i d e r a b l y improve m a t t e r s and t h a t t h e s e c i r c u i t s w i l l u l t i m a t e l y f i n d a u s e i n l e s s c r i t i c a l a p p l i c - a t i o n s .

F i n a l l y we review t h e work of Matsumura e t a 1 (25) and our own r e s u l t s on t h e a-Si image s e n s o r s . The c i r c u i t used by b o t h groups i s shown i n t h e t o p p a r t of f i g . 1 2 and i s t h a t o r i g i n a l l y suggested by Weimer e t a1 ( 2 6 ) . Light i n c i d e n t on t h e a-Si photoconductor charges up t h e c a p a c i t o r w h i l s t t h e FET i s o f f . The FET

i s t h e n p u l s e d on and t h e c a p a c i t o r d i s - charges r a p i d l y p r o v i d i n g an o u t p u t c u r r e n t t h e magnitude of which i s a f u n c t i o n of t h e l i g h t i n t e n s i t y a s shown i n f i g . 1 2 . Although t h e d e v i c e s of Matsumura e t a 1 (25) provide much lower c u r r e n t s t h a n t h o s e shown i n t h i s f i g u r e , t h e s e a u t h o r s s u g g e s t t h a t t h e i r p r e s e n t d e v i c e s can b e o p e r a t e d up t o f r e q u e n c i e s of about 2OkHz w i t h poss- i b l e a p p l i c a t i o n s i n l a r g e g a i n , l a r g e a r e a image s e n s o r s .

1o6-

10, (AMP53

Fig.12. C i r c u i t of a-Si image s e n s o r and measured o u t p ~ i t c u r r e n t a s a f u n c t i o n of l i g h t i n t e n s i t y f o r v a r i o u s p u l s e f r e q u e n c i e s .

1 mSec LIGHT WLSE IFREQ Yl

0-Sl PHOTO-CONDUCTOR

0-8

/o- o/

18-

^{Y=2Hz /O}

100 1000

LIGHT INTENSITY (LUX1

5 . L i m i t a t i o n s of t h e P r e s e n t a-Si FETs.

The work d e s c r i b e d above has c l e a r l y demonstrated t h e f e a s i b i l i t y of u s i n g a-Si FETs i n a number of p r a c t i c a l c i r c u i t s . I n p a r t i c u l a r , t h e d e v i c e s p r e s e n t l y a v a i l - a b l e a l r e a d y o f f e r a v i a b l e s o l u t i o n t o t h e problem of m u l t i p l e x i n g l i q u i d c r y s t a l d i s p l a y s ( s e c t i o n 3 ) . However, t h e f a c t o r t h a t appears most l i k e l y t o l i m i t t h e i r use i n o t h e r a p p l i c a t i o n s i s t h e i r r e l a t i v e l y slow s w i t c h i n g t i m e . This q u a n t i t y w i l l g e n e r a l l y b e determined by t h e time r e q u i r e d f o r t h e on-current t o charge c i r c u i t c a p a c i t a n c e s , s o t h a t an obvious improvement would b e achieved w i t h h i g h e r on-currents. A f i r s t s t e p i n t h i s d i r e c t i o n would b e a r e d u c t i o n i n t h e source- d r a i n s e p a r a t i o n , a t p r e s e n t 40 pm. As d i s c u s s e d i n s e c t i o n 2 , t h e p h y s i c a l f a c t o r s t h a t l i m i t t h e on-conductance a r e t h e i n t e r f a c e s t a t e d e n s i t y a t t h e a-Si t o

i n s u l a t o r boundary and t h e d e n s i t y of s t a t e s i n t h e a-Si. Even w i t h n e g l i g i b l e i n t e r f a c e s t a t e s t h e r a p i d l y r i s i n g t a i l s t a t e d i s t r i b a t i o n , which i s a fundamental p r o p e r t y of a-Si, w i l l l i m i t t h e amount of band bending such t h a t E ~ - E F s E ~ - E A Y

0.2 eV. Under t h e s e c o n d i t i o n s t h e s h e e t conductance i n a 0.1

um

t h i c k f i l m 1s e s t i m a t e d t o be about

~-l/n .

The b e s t d a t a (curve D) i n f i g . 2 approaches t h i s v a l u e a t t h e h i g h e s t g a t e v o l t a g e s b u t is approximately an o r d e r of magnitude lower a t VG = 15V. I t would t h e r e f o r e n o t seem unreasonable t o a n t i c i p a t e t h a t t h e h i g h l y v e r s a t i l e glow d i s c h a r g e p r o c e s s , which a t p r e s e n t p r o v i d e s t h e b e s t d e v i c e s , can b e used i n t h e f u t u r e t o produce m a t e r i a l t h a t w i l l go a t l e a s t some way t o overcome t h e p r e s e n t frequency l i m i t a t i o n s .

6. Conclusions.

The work reviewed above l e a d s t o t h e f o l l o w i n g conclusions:

( i ) glow d i s c h a r g e a-Si and s i l i c o n n i t r i d e f i l m s can b e p a t t e r n e d photolithographL i c a l l y by s t a n d a r d c r y s t a l l i n e semiconductor technology t o produce v i a b l e t h i n f i l m f i e l d e f f e c t t r a n s i s t o r s which o p e r a t e a t v o l t a g e s compatible w i t h t h o s e used i n modern i n t e g r a t e d c i r c u i t s .

( i i ) t h e r e p r o d u c i b i l i t y i n t h e c h a r a c t e r i s t i c s of d i f f e r e n t FET elements i s good and s o f a r s t a b l e o p e r a t i o n f o r over 2 x

lo9

s w i t c h i n g c y c l e s h a s been observed.

P a s s i v a t i o n o r e n c a p s u l a t i o n of t h e d e v i c e s does n o t appear t o b e necessary.

( i i i ) a-Si FETs have c o n s i d e r a b l e p o t e n t i a l a s s w i t c h i n g elements i n l i q u i d c r y s t a l d i s p l a y p a n e l s . P r e s e n t FETs could, i n p r i n c i p l e , b e used i n 1000 l i n e d i s p l a y s . ( i v ) t h e f e a s i b i l i t y of applying a-Si FETs i n l o g i c c i r c u i t s h a s been demonstrated.

I n v e r t e r s , NAND and NOR g a t e s , a b i s t a b l e m u l t i v i b r a t o r and a s h i f t r e g i s t e r have a l l been f a b r i c a t e d from a-Si FETs and i n t e g r a t e d a-Si r e s i s t o r s .

(v) by making use of t h e e x c e l l e n t photoconductivity of t h e m a t e r i a l , i n t e g r a t e d a-Si FET image s e n s o r s have been c o n s t r u c t e d .

Acknowledgments

The a u t h o r s would l i k e t o thank S t e w a r t Kinmond, Ally F a l c o n e r and Don M i t c h e l l f o r t h e i r t e c h n i c a l a s s i s t a n c e . Thanks a r e a l s o due t o I a n French f o r h i s h e l p w i t h t h e f a b r i c a t i o n and t e s t i n g of t h e a-Si s h i f t r e g i s t e r s . The s u p p o r t of t h e p r o j e c t by CVD and SRC i s g r a t e f u l l y acknowledged.

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