43 Plasma Polymerized Organometallic Thin Films: Preparation and Properties R. K. SADHIR and H. E. SAUNDERS
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Research and Development Center, Westinghouse Electric Corporation, Pittsburgh, PA 15235 W. J. JAMES Department of Chemistry and Graduate Center for Materials Research, University of Missouri—Rolla, Rolla, MO 65401
We review the recent work on plasma polymerization of organometallic monomers, and discuss the preparation and properties of organotin and organogermanium mon omers. Various saturated and unsaturated monomers (such as t e t r a e t h y l t i n , t e t r a v i n y l t i n , hexamethylditin and tetramethylgermanium) have been polymerized i n a glow discharge, and the mechanism of polymeri zation has been elucidated. Films were also pre pared by glow discharge polymerization from the above mentioned monomers i n the presence of oxygen. Films of varying conductivity were prepared by these two methods, the presence of oxygen giving r i s e to semi conducting films. The C/Sn and C/Ge r a t i o s , as de termined by ESCA, vary with the reactor operating parameters and with the location of the substrate within the reactor. Thin films prepared from tetramethylgermanium also show semiconducting and insula ting behavior, depending upon their location within the plasma reactor. V a r i o u s methods h a v e been u s e d f o r d i s p e r s i n g m e t a l s i n c o n v e n t i o n a l p o l y m e r i c systems, such as c o e v a p o r a t i v e t e c h n i q u e s , ( 1 ) s o l u t i o n g r o w t h t e c h n i q u e s ( 2 ) and i o n i m p l a n t a t i o n . ( 3 ) However, plasma p o l y m e r i z a t i o n has r e c e i v e d r e l a t i v e l y l i t t l e a t t e n t i o n in this field. The s y n t h e s i s o f o r g a n o m e t a l l i c f i l m s p r o d u c e d by p l a s m a p o l y m e r i z a t i o n t e c h n q i u e s i s a n a t t r a c t i v e p r o s p e c t , s i n c e i t c a n be e n v i s a g e d t h a t c a r e f u l c h o i c e o f an o r g a n o m e t a l l i c monomer and c l o s e c o n t r o l o f t h e o v e r a l l c o m p o s i t i o n o f t h e p r o d u c t by the c o n t r o l o f the p r o c e s s i n g p a r a m e t e r s would g r e a t l y e x t e n d t h e s c o p e o f t h e s e f i l m s i n e l e c t r i c a l , m a g n e t i c and optical applications. I n t h i s p a p e r we r e v i e w t h e r e c e n t w o r k on p l a s m a p o l y m e r i z a t i o n o f o r g a n o m e t a l l i c monomers and d i s c u s s t h e room t e m p e r a t u r e 0097-6156/84/0242-0555506.00/0 © 1984 American Chemical Society In Polymers in Electronics; Davidson, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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p r e p a r a t i o n , p r o p e r t i e s , and c o m p o s i t i o n o f g l o w d i s c h a r g e p o l y m e r i z e d t e t r a e t h y l t i n , h e x a m e t h y l d i t i n , t e t r a v i n y l t i n and t e t r a m e t h y l g e r m a n i u m . A p o s s i b l e mechanism by w h i c h g l o w d i s c h a r g e p o l y m e r i z a t i o n o f t h e s e monomers o c c u r s i s d i s c u s s e d .
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L i t e r a t u r e Review T h e r e h a s b e e n a c o n s i d e r a b l e amount o f w o r k done on t h i n f i l m d e p o s i t i o n o f p u r e o r g a n i c v a p o r s by g l o w d i s c h a r g e p o l y m e r i z a t i o n i n the p a s t . ( 4 ) H o l l a h a n and R o s i e r ( 5 ) h a v e d i s c u s s e d t h e p r e p a r a t i o n o f i n o r g a n i c t h i n f i l m s by t h e g l o w d i s c h a r g e p r o c e s s , and t h e i r u s e s i n t h e e l e c t r o n i c s i n d u s t r y . I n most c a s e s , t h e s u b s t r a t e i s h e a t e d t o 200°C o r above t o f o r m t h e s e inorganic films. The i n i t i a l w o r k on p l a s m a p o l y m e r i z a t i o n o f o r g a n o s i l i c o n compounds was done by T k a c h u k e t a l . ( 6 - 8 ) who p r e p a r e d s i l o x a n e and οrganosiloxane f i l m s u n d e r d i f f e r e n t p r o c e s s i n g c o n d i t i o n s , and s t u d i e d r a t e s o f d e p o s i t i o n . The f i l m s were c h a r a c t e r i z e d by IR and ESR. Some o f t h e more r e c e n t p u b l i c a t i o n s show t h a t t h e p l a s m a p o l y m e r i z a t i o n o f o r g a n o s i l i c o n monomers y i e l d s f i l m s e x h i b i t i n g e x c e l l e n t p r o p e r t i e s , s u c h as h i g h t h e r m a l s t a b i l i t y ^ ^ and h i g h d i e l e c t r i c c o n s t a n t . ( 1 0 , 1 1 ) S u c h f i l m s h a v e f o u n d u s e a s d i e l e c t r i c s f o r m i c r o e l e c t r o n i c s , and i n o p t i c a l ( 1 2 , 13) and b i o m e d i c a l a p p l i c a t i o n s . ( 1 4 ) The t e c h n o l o g y o f p l a s m a f o r m a t i o n o f m e t a l - c o n t a i n i n g p o l y m e r s i n t h e f o r m o f t h i n f i l m s d a t e s f r o m 1963, when B r a d l e y and Hammes(15) p r e p a r e d s p e c i m e n s f r o m some f o r t y d i f f e r e n t m a t e r i a l s , and s t u d i e d t h e i r e l e c t r i c a l c o n d u c t i v i t i e s . I n c l u d e d i n t h e s t u d y w e r e o r g a n i c compounds o f i r o n , t i n , t i t a n i u m , m e r c u r y , s e l e n i u m , and a r s e n i c . The p r e s e n c e o f a m e t a l o r t r a n s i t i o n element i n the polymer d i d not l e a d to s p e c i a l e l e c t r i c a l p r o p e r t i e s compared t o t h e p u r e l y o r g a n i c p o l y m e r s s t u d i e d . T k a c h u k and c o w o r k e r s ( 1 6 , 1 7 ) h a v e r e p o r t e d t h a t o r g a n o t i n p o l y m e r f i l m s p r e p a r e d i n a g l o w d i s c h a r g e c a n be u s e d as i n s u l a t i n g l a y e r s on m i c r o e l e c t r o n i c d e v i c e s , a s p r o t e c t i v e c o a t i n g s , and a s i n t e r m e d i a t e a d h e s i v e l a y e r s . S u b s e q u e n t thermal treatment of such c o a t i n g s can produce t h i n f i l m s of d i f f e r e n t p r o p e r t i e s . For example, p y r o l y s i s of o r g a n o t i n polymer f i l m s can produce t i n o x i d e c o a t i n g s . Kny e t a l . ( 1 8 ) h a v e r e p o r t e d t h e p r o d u c t i o n and p r o p e r t i e s o f s e m i c o n d u c t i n g f i l m s from t e t r a m e t h y l t i n h a v i n g c o n d u c t i v i t i e s between 2 Χ 10" and 10~2ft-l rl. Thermal treatment of such f i l m s i n c r e a s e d the con d u c t i v i t y t o a maximum v a l u e o f 10 ^"* cm~ . T h e s e w o r k e r s h a v e a l s o d e t e r m i n e d t h e c o m p o s i t i o n o f s u c h f i l m s by x - r a y p h o t o e l e c t r o n s p e c t r o s c o p y (XPS) and A u g e r e l e c t r o n s p e c t r o s c o p y (AES) . (19) Kny e t a l . ( 2 0 ) h a v e a l s o s t u d i e d t h e polymer/substrate i n t e r f a c e c o m p o s i t i o n and a d h e s i o n o f g l o w d i s c h a r g e - f o r m e d o r g a n o t i n p o l y m e r s . The d e p o s i t i o n o f a t r a n s p a r e n t conductive l a y e r o f t i n ( I I ) o x i d e on a s u b s t r a t e by g l o w d i s c h a r g e p o l y m e r i z a t i o n o f a m i x t u r e o f t e t r a m e t h y l t i n and c a r b o n d i o x i d e has b e e n 0
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In Polymers in Electronics; Davidson, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
Downloaded by MONASH UNIV on November 19, 2015 | http://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch043
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SADHIR ET AL.
Plasma Polymerized Organometallic
Thin Films
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t h e s u b j e c t o f a U.S. p a t e n t . ( 2 1 ) M o r o s o f f and P a t e l ( 2 2 ) i n v e s t i g a t e d the p r e p a r a t i o n of plasma polymers c o n t a i n i n g the t r a n s i t i o n m e t a l s , i r o n and c o b a l t , and e l u c i d a t e d t h e i r c h e m i c a l properties. The t r a n s i t i o n m e t a l s were i n t r o d u c e d i n t o t h e p l a s m a as t h e v o l a t i l e o r g a n o m e t a l l i c compounds, p e n t a c a r b o n y l i r o n and c y c l o p e n t a d i e n y l - d i c a r b o n y l c o b a l t , and t h e r e s u l t i n g f i l m s were c h a r a c t e r i z e d by F T I R , ATR, ESCA and c y c l i c v o l t a m m e t r y . A n d e r s o n and S p e a r ( 2 3 ) p r e p a r e d amorphous germanium c a r b i d e by d e c o m p o s i t i o n o f a m i x t u r e o f g e r m a n i a and e t h y l e n e i n a r . f . glow discharge at e l e v a t e d temperatures. R e c e n t l y , plasma p o l y m e r i z e d v i n y l f e r r o c e n e f i l m s have been d e p o s i t e d on g r a p h i t e and p l a t i n u m e l e c t r o d e s , ( 2 4 , 2 5 ) and t h e c h a r a c t e r i s t i c s of these e l e c t r o d e s have been s t u d i e d . There h a v e b e e n a few p u b l i c a t i o n s ( 2 6 - 2 9 ) d e s c r i b i n g a method i n v o l v i n g s i m u l t a n e o u s p l a s m a e t c h i n g and p o l y m e r i z a t i o n i n t h e same system, r e s u l t i n g i n the s y n t h e s i s of m e t a l - c o n t a i n i n g f l u o r o p o l y m e r s . I n t h i s method, t h e m e t a l t o be i n c o r p o r a t e d i n t h e polymer f i l m i s used as a cathode i n a c a p a c i t i v e l y c o u p l e d r e a c t o r , and a f l u o r i n a t e d h y d r o c a r b o n i s u s e d as a p o l y m e r i z i n g / e t c h a n t gas. By u s i n g t h i s t e c h n i q u e , v a r i o u s p o l y m e r i c f i l m s c o n t a i n i n g germanium, molybdenum, c o p p e r , t i n and chromium h a v e been o b t a i n e d . XPS and MS were e m p l o y e d t o d e t e r m i n e t h e compos i t i o n and s t r u c t u r e o f t h e f i l m s . S h u t t l e w o r t h ( 3 0 ) has u s e d two d i s t i n c t r o u t e s t o o b t a i n a m e t a l i n t h e gas p h a s e and s u b s e q u e n t l y i n c o r p o r a t e i t i n t o the growing plasma polymer, namely, s p u t t e r i n g f r o m a t i n s u r f a c e by use o f an i n e r t gas p l a s m a and by t h e use o f a v o l a t i l e m e t a l compound ( C r ( C O ) ^ ) . The a n a l y s i s o f t h e p o l y m e r f i l m s was c a r r i e d o u t by u s i n g ESCA and i t was shown t h a t t h e s e t e c h n i q u e s p r o d u c e a m e t a l compound d i s p e r s e d i n t h e p l a s m a p o l y m e r w i t h a w i d e r a n g e o f m e t a l - p o l y m e r compositions. L i e p i n s e t a l . ( 3 1 ) h a v e r e p o r t e d t h e use o f g l o w d i s c h a r g e on o r g a n o m e t a l l i c monomers s u c h as t a n t a l u m (V) ethoxide, dimethyl mercury, d i e t h y l mercury, t e t r a m e t h y l l e a d , t e t r a e t h y l l e a d and t r i m e t y l b i s m u t h t o d e p o s i t t h i n , m e t a l c o n t a i n i n g c o a t i n g s on m i c r o s p h e r e s u s e d as t a r g e t s f o r f u s i o n reactors. Experimental Glow d i s c h a r g e o r g a n o m e t a l l i c f i l m s w e r e p r e p a r e d i n an i n d u c t i v e l y c o u p l e d p l a s m a r e a c t o r ( 2 0 ) ( F i g u r e 1 ) . The r e a c t o r c o n s i s t e d o f a c y l i n d r i c a l p y r e x g l a s s t u b e (100 cm X 7 cm). The w h o l e s y s t e m was e v a c u a t e d t o t h e m i l l i t o r r r a n g e by means o f a r o t a r y pump. The monomer g a s e s o r v a p o r s w e r e i n t r o d u c e d by a l e a k v a l v e a t one end o f t h e r e a c t o r a t t h e d e s i r e d f l o w r a t e , and t h e p r e s s u r e was r e c o r d e d by means o f a t h e r m o c o u p l e gauge. The g e n e r a t o r f o r a c t i v a t i n g t h e p l a s m a was o p e r a t e d a t 3.9 MHz frequency. P r e c l e a n e d g l a s s s l i d e s (2.5 X 2.5 X 0.045 cm) and a l u m i n u m p l a t e s (1 X 1 X 0.0625 cm) were u s e d a s s u b s t r a t e s f o r f i l m
In Polymers in Electronics; Davidson, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
Downloaded by MONASH UNIV on November 19, 2015 | http://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch043
POLYMERS IN ELECTRONICS
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