Fabrication of Conductive Polyimide-Gate Transistors - American

33 Fabrication of Conductive Polyimide-Gate Transistors

Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch033

DAVID R. DAY Department of Electrical Engineering and Computer Science, and Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 One route to achieve a useful conducting polymer is to combine the conduction properties of a graphite -like material with the processability of common polymers. Such a material can be realized by first processing a common polymer and then pyrolyzing it in its final form. An organic material which exhibits both excellent stability and high conductivity is heat-treated or pyrolyzed polyimide. Pyrolysis is normally carried out at temperatures of 650°C or above, and conductivity typically increases from 10 to 10 (ohm-cm). Conductive polyimide may be useful in integrated circuits as a gate material because of its conductive properties and its ability to withstand high temperatures. Conductive polyimide gates for field effect transistors (FETs) were fabricated and have been found to be similar to conventional gate transistors. The processing steps and transistor characteristics are discussed. -18

2

1-

Polyimide irreversibly forms a conductive material when heated above 650°C in an inert ambient (1-5). During the heating process, the conductivity increases from that of a good insulator, 10~ (ohm-cm) , to 100 (ohm-cm)" (2). This final conductivity is comparable to that of doped polyacetylene. Unlike polyacetylene, however, conductive polyimide (CPI) is stable, i.e., it retains conductivity when exposed to air and moisture, and it can withstand temperatures greater than 1200°C in an inert ambient. Elemental analysis of the CPI indicates that significant amounts of oxygen and nitrogen remain in the carbonaceous matrix following pyrolysis and that it is not simply graphite (6). Several conduction mechanisms have been proposed including charging-energy-limited tunneling and variable-range hopping (2-4). In addition to its conductive properties, CPI is 18

1

0097-6156/84/0242-0423506.00/0 © 1984 American Chemical Society Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

424

POLYMERS IN ELECTRONICS


e a s i l y p r o c e s s e d and l i t h o g r a p h i c a l l y p a t t e r n e d by e i t h e r w e t chemical or plasma e t c h i n g before heat-treatment* These p r o p e r t i e s make C P I u s e f u l as a g a t e f o r f i e l d - e f f e c t transistors i n integrated circuits. A l t h o u g h CPI i s not s u f f i c i e n t l y c o n d u c t i v e t o be c o n s i d e r e d f o r h i g h - c u r r e n t c o n d u c t i o n l i n e s i n a c i r c u i t , i t i s a n e x c e l l e n t c a n d i d a t e as a gate m a t e r i a l . Since the main f u n c t i o n of a gate i s to d e l i v e r a v o l t a g e but pass o n l y low amounts of c u r r e n t , e x t r e m e l y h i g h c o n d u c t i v i t i e s s u c h as t h o s e c h a r a c t e r i s t i c o f common m e t a l s a r e n o t r e q u i r e d , a l t h o u g h t h e c o n d u c t i v i t y o f t h e g a t e m a t e r i a l does a f f e c t t h e speed o f t h e d e v i c e . The a b i l i t y t o w i t h s t a n d h i g h temperatures i s another important f a c t o r i n the choice of a gate m a t e r i a l as w i l l be d e s c r i b e d b e l o w . Background C o n v e n t i o n a l M e t a l - O x i d e - S e m i c o n d u c t o r (MOS) t r a n s i s t o r s t y p i c a l l y i n c l u d e p o l y s i l i c o n as a g a t e m a t e r i a l . Polysilicon is o f t e n u s e d i n s t e a d o f a l u m i n u m o r some o t h e r m e t a l t o t a k e advantage of the s o - c a l l e d s e l f - a l i g n e d - g a t e process. In t h i s p r o c e s s t h e g a t e i s u s e d as a mask d u r i n g t h e s o u r c e - d r a i n i m p l a n t so t h a t t h e edge o f t h e s o u r c e a n d d r a i n a l i g n t o t h e edge o f t h e g a t e ( F i g u r e 1). T h i s m i n i m i z e s the o v e r l a p capacitance between the gate and e i t h e r source o r d r a i n . During the required s u b s e q u e n t a n n e a l o f t h e s o u r c e - d r a i n i m p l a n t (1000 t o 1 2 0 0 ° C ) , a p a s s i v a t i o n o x i d e c a n be g r o w n o v e r t h e p o l y s i l i c o n ( F i g u r e 1). C P I c a n a l s o be u s e d i n a s e I f - a l i g n m e n t g a t e p r o c e s s . Although CPI i s not s e l f - p a s s i v a t i n g , non-conducting p o l y i m i d e c a n be d e p o s i t e d o v e r t h e C P I f o l l o w i n g t h e s o u r c e - d r a i n a n n e a l t o a c t as a n i n s u l a t i o n l a y e r (7)· Thus C P I c a n be i n c o r p o r a t e d i n a p r o c e s s sequence t o p r o v i d e t h e same b e n e f i t s as p o l y s i l i c o n technology. The C P I t e c h n o l o g y , h o w e v e r , i s more e a s i l y c a r r i e d o u t and demands f e w e r p r o c e s s s t e p s . A c o n v e n t i o n a l p o l y s i l i c o n g a t e p r o c e s s p r o c e e d s as f o l l o w s : 1. 2. 3. 4. 5. 6. 7. 8. 9.

v a p o r d e p o s i t p o l y s i l i c o n on w a f e r d e p o s i t p h o t o r e s i s t and p a t t e r n etch p o l y s i l i c o n remove p h o t o r e s i s t i m p l a n t w a f e r t o dope s o u r c e , d r a i n and p o l y s i l i c o n h i g h t e m p e r a t u r e a n n e a l i n oxygen to f o r m s o u r c e , d r a i n , conductive p o l y s i l i c o n gate d e p o s i t and p a t t e r n p h o t o r e s i s t etch contact cuts i n oxide remove p h o t o r e s i s t

and

T h i s i s n o t an e n t i r e p r o c e s s b u t i t s u f f i c e s f o r c o m p a r i s o n t o the CPI process. In the CPI process described below, a p h o t o s e n s i t i v e p o l y i m i d e ( c u r r e n t l y a v a i l a b l e f r o m EM C h e m i c a l Co.) i s p r o p o s e d t o s i m p l i f y p r o c e s s i n g . A n a n a l o g o u s C P I p r o c e s s

Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

33.

DAY

Conductive

Polyimide-Gate

Transistors:

IMPLANT


M

425

Fabrication

IMPLANT

I I I I I . i I · I. .1 I T 1 I :·;l POLYSILICON

S.O

CPI

2

1-^—1

Si

n+ POLYSILICON

Si0

2

CPI

PI

3b.

F i g u r e 1. Comparison of c o n v e n t i o n a l process to proposed CPI-gate process.

polysilicon-gate


426


s t a r t i n g a t t h e same p o i n t as above f o r t h e p o l y s i l i c o n w o u l d be as f o l l o w s : 1· 2. 3.


4. 5.

d e p o s i t and p a t t e r n p o l y i m i d e ( t o become g a t e m a t e r i a l ) i m p l a n t t o dope s o u r c e a n d d r a i n ( p o l y i m i d e i s i m p l a n t e d but a t t h i s stage remains n o n c o n d u c t i v e ) high temperature anneal i n n i t r o g e n to form source, d r a i n , and c o n d u c t i v e p o l y i m i d e g a t e d e p o s i t and p a t t e r n p o l y i m i d e ( t h i s w i l l s e r v e as i n s u l a t i o n ) etch contact cuts

N o t e t h a t t h e p r o p o s e d C P I p r o c e s s segment c o n t a i n s f o u r f e w e r steps. A schematic of both s t r u c t u r e s a f t e r t h i s p a r t i a l p r o c e s s i n g i s i n d i c a t e d i n F i g u r e 1. The p u r p o s e o f t h i s r e s e a r c h was t o d e t e r m i n e t h e f e a s i b i l i t y of i n c o r p o r a t i n g CPI i n t o f i e l d - e f f e c t t r a n s i s t o r s . Due t o t i m e l i m i t a t i o n s and a v a i l a b i l i t y o f v a r i o u s e q u i p m e n t , s o u r c e a n d d r a i n s w e r e f o r m e d by t h e r m a l d i f f u s i o n r a t h e r t h a n by i o n i m p l a n t a t i o n as p r o p o s e d i n t h e p r e c e d e d d i s c u s s i o n . As a r e s u l t o f t h i s s l i g h t change i n p r o c e s s s e q u e n c e , m e t a l - g a t e FETs c o u l d be f a b r i c a t e d a s a c o n t r o l f o r t h e C P I d e v i c e s . I n a d d i t i o n , a n o n - p h o t o s e n s i t i v e P I was u s e d t h u s r e q u i r i n g e x t r a p h o t o r e s i s t d e p o s i t i o n and p a t t e r n i n g s t e p s . Experimental P-channel f i e l d - e f f e c t t r a n s i s t o r s were f a b r i c a t e d b o t h w i t h CPI gates and w i t h aluminum gates as c o n t r o l s . A f t e r c o n v e n t i o n a l p r o c e s s i n g up t o and i n c l u d i n g g r o w t h o f t h e g a t e o x i d e , t e s t w a f e r s w e r e c o a t e d w i t h p o l y a m i c a c i d (DuPont P I - 2 5 4 5 ) w h i l e t h e c o n t r o l s w e r e c o a t e d w i t h a l u m i n u m . The p o l y a m i c a c i d was i m i d i z e d w i t h a f i n a l bake a t 390°C. A p h o t o r e s i s t mask was d e p o s i t e d and p a t t e r n e d on b o t h t h e t e s t and c o n t r o l w a f e r s . The p o l y i m i d e was p l a s m a e t c h e d i n o x y g e n a n d t h e a l u m i n u m was e t c h e d i n a p h o s p h o r i c - a c e t i c - n i t r i c s o l u t i o n (PAN ETCH). After p h o t o r e s i s t removal, t h e p o l y i m i d e t e s t w a f e r s were heat t r e a t e d a t 700°C f o r 15 m i n u t e s . B o t h C P I and a l u m i n u m g a t e w a f e r s w e r e t h e n c o a t e d w i t h p o l y a m i c a c i d a g a i n a n d i m i d i z e d a t 390°C. Contact c u t s were plasma etched through t h e p o l y i m i d e ( u s i n g a p h o t o r e s i s t mask) and b u f f e r e d HF was u s e d t o e t c h t h e s i l i c o n d i o x i d e u n d e r n e a t h . A f t e r a l u m i n u m was d e p o s i t e d and p a t t e r n e d , b o t h t e s t and c o n t r o l w a f e r s were s i n t e r e d i n f o r m i n g gas a t 400°C f o r 10 m i n u t e s . A C P I s t r i p was f a b r i c a t e d b e s i d e e a c h FET w i t h f o u r a l u m i n u m c o n t a c t s t o a l l o w C P I '4 p o i n t ' c o n d u c t i v i t y measurements. A summary o f t h e w a f e r p r o c e s s i s g i v e n b e l o w : 1. 2.

CLEAN WAFERS ( o r g a n i c , o x i d e r e m o v a l , i n o r g a n i c ) GROW F I E L D OXIDE(1100°C, 0 , H 0 , 0 ) 2

2

2


33.


3.

4. 5. 6. 7. 8. 9· 10. 11. 12. 13. 14. 15. 16. 17.

DAY

Conductive

Polyimide-Gate

Transistors: Fabrication

427

A ) DEPOSIT PHOTORESIST, PREBAKE B) EXPOSE AND DEVELOP, POSTBAKE Y PHOTOLITHOGRAPHY C) ETCH SILICON DIOXIDE D) STRIP PHOTORESIST E ) CLEAN WAFER DOPANT DEPOSITION ( b o r o n ) BORON GLASS REMOVAL ( H F ) DOPANT D R I V E - I N ( 1 2 0 0 ° C ) AND OXIDIZE SOURCE-DRAIN PHOTOLITHOGRAPHY - ETCH GATE REGION GROW GATE OXIDE ( 1 1 0 0 ° C ) , 0 ) CONTROL WAFERS POLYIMIDE DEPOSITION 9. A L DEPOSITION PHOTOLITHOGRAPHY - POLYIMIDE 1 0 . PHOTOLITH- A L HIGH TEMPERATURE POLYIMIDE BAKE ( 7 0 0 ° C ) 1 1 . CLEAN WAFER POLYIMIDE d e p o s i t i o n PHOTOLITHOGRAPHY - POLYIMIDE SOURCE-DRAIN CONTACT ETCH ALUMINUM DEPOSITION PHOTOLITHOGRAPHY ANNEAL ( 4 0 0 ° C ) 2

RESULTS AND DISCUSSION The p a t t e r n e d and s u b s e q u e n t l y p y r o l y z e d p o l y i m i d e f i l m s on s i l i c o n wafers adhered w e l l to the s u b s t r a t e w i t h o u t c r a c k i n g or p e e l i n g ( F i g u r e 2). The m e a s u r e d c o n d u c t i v i t y o f t h e C P I was 10 (ohm-cm) . T h i s v a l u e i s a b o u t t e n t i m e s l o w e r t h a n t h a t r e p o r t e d i n r e f e r e n c e (2) and i s l i k e l y due t o t h e r e l a t i v e l y s h o r t t e s t w a f e r h e a t i n g p e r i o d o f 30 m i n u t e s . I t i s s u s p e c t e d t h a t i f h i g h e r temperatures or extended t i m e s d u r i n g heat t r e a t m e n t a r e u s e d , c o n d u c t i v i t y s h o u l d i n c r e a s e by a f a c t o r o f 10. A s c h e m a t i c and p h o t o g r a p h o f t h e f a b r i c a t e d t r a n s i s t o r a r e shown i n F i g u r e 3. N o t e t h a t t h e c o n t r o l d e v i c e s a r e i d e n t i c a l t o the t e s t devices except t h a t the CPI i s r e p l a c e d w i t h aluminum. As a t e s t f o r i n t e g r i t y o f t h e s e c o n d p o l y i m i d e i n s u l a t i o n l a y e r , t h e s o u r c e e l e c t r o d e was made t o c r o s s d i r e c t l y o v e r t h e C P I gate. I n b o t h t h e t e s t and c o n t r o l d e v i c e s , no s h o r t i n g o f s o u r c e t o g a t e e l e c t r o d e s was o b s e r v e d , a n i n d i c a t i o n o f good i n s u l a t i o n b e t w e e n t h e a l u m i n u m and t h e u n d e r l y i n g C P I . A l t h o u g h most c o n t a c t s f r o m a l u m i n u m t o C P I and a l u m i n u m t o s i l i c o n w e r e good, a few were t o t a l l y open. This o b s e r v a t i o n suggests that some r e s i d u e may have r e m a i n e d i n t h e b o t t o m s o f t h e v i a s a f t e r the p l a s m a c o n t a c t e t c h (9). The 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 o f t h e C P I - and a l u m i n u m - g a t e t r a n s i s t o r s a r e i l l u s t r a t e d i n F i g u r e 4. The d r a i n c u r r e n t - g a t e v o l t a g e c h a r a c t e r i s t i c s are s i m i l a r for both devices. The C P I d e v i c e , h o w e v e r , has a s l i g h t l y h i g h e r t h r e s h o l d v o l t a g e and never c o m p l e t e l y t u r n s o f f . T h i s may r e s u l t f r o m c o n t a m i n a t i o n of the gate oxide from i m p u r i t i e s i n the p o l y i m i d e or from the p y r o l y s i s chamber i n w h i c h t h e h e a t t r e a t m e n t was p e r f o r m e d .



428


Figure 2 .

Patterned and pyrolyzed CPI on SiO

?

substrate.


DAY

Conductive Polyimide-Gate

Transistors: Fabrication


33.

F i g u r e 3.

A) S c h e m a t i c o f C P I - g a t e F E T . B) O p t i c a l m i c r o g r a p h o f C P I - g a t e FET ( d a r k material is CPI).


429



-10

-8

-6

-4-2

0

2

Voate (volts)

Figure 4. Comparison of drain current for both CPI- and aluminum-gate FETs.


33. DAY

431 Conductive Polyimide-Gate Transistors: Fabrication

The difference between the CPI- and aluminum-gate devices is minimal and suggests that CPI is a good candidate as a gate material. Although the absolute conductivity of CPI is less than the more conventional polysilicon, CPI processing is much simpler and cheaper than polysilicon technology. To become competitive in device 'speed' with polysilicon (and the current heavily researched suicides), CPI would have to have equivalent conduction levels. If the conductivity of CPI could be increased by 2 or 3 orders of magnitude through alteration of polyimide chemistry, incorporation of more conductive graphite particles, or possibly by doping, CPI gate technology could become extremely useful. Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch033

Acknowle dgment s The author thanks E.W. Maby and S.D. Senturia for their helpful comments and discussion and DuPont de Nemours and Company for providing the polyamic acid solutions. Literature Cited 1. Bruck, S.D. J. of Polymer Sci. (part C) 1967, 17, 169 2. Brom, H.B.; Tomkiewitz, Y.; Aviram, Α.; Broers, A. Solid State Comm. 1980, 35, 135 3. Gittleman, J.I.; Sichel, E.K. J. Electronic Mat. 1981,10, 327 4. Sichel, E.K.; Emma, T. Solid State Comm. 1982, 41, 747 5. Lin, J.W.P.; Epstein, A.J.; Dudek, L.P.; Rommelmann, H.; Organic Coatings and Plastics Chemistry (ACS) 1980,43, 446 6. Conley, R.T.; Guadiana, R.A. "Thermal Stability of Polymers"; M. Dekker Inc.: New York, 1970, chap. 10 7. See for example Samuelson, G.; Organic Coatings and Plastics Chemistry (ACS) 1980, 43, 446 8. Day, D.R.; Senturia, S.D. J. of Electronic Mat. 1982, 11, 441 RECEIVED September 2,

1983


Fabrication of Conductive Polyimide-Gate Transistors - American

Recommend Documents