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Synthesis and Properties of Conducting Films by Plasma Polymerization of Tetramethyltin R. K. SADHIR 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 Plasma polymerization of tetramethyltin has been investigated by using an inductively coupled tub ular reactor. Plasma polymerized thin films of tetramethyltin, produced i n the presence of oxy gen, were r e f l e c t i v e , having a conductivity i n the range 10 -10 (Ωcm)-1. A C/Sn ratio of 3344 + 325
Chemlok a d h e s i v e film interface Film-Al interface
>2552 + 300
II II
3740 + 225
2728 + 480 2090 + 650 1760 + 320
Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
42.
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Plasma Polymerization
of Tetramethyltin
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T e t r a m e t h y l T i n F i l m s D e p o s i t e d on and S t a i n l e s s S t e e l Brass F a i l u r e Mode l i g h t microscope inspection Film-brass
interface
Stainless F a i l u r e Mode l i g h t microscope inspection
Pull Strength psi 2904 +
235
tl
3168 + 360
It
3476 + 480
Chemlok a d h e s i v e rod i n t e r f a c e Film-brass i n t e r face
>2420
+
125
1320 + 390 968 + 455
Chemlok a d h e s i v e film interface Chemlok a d h e s i v e rod i n t e r f a c e Chemlok a d h e s i v e film interface tl
tt
Chemlok a d h e i s v e rod i n t e r f a c e
Steel Pull Strength psi >4004 ± 340 >2640 ± 235 >3388 ± 405 >4840 ± 465 >3388 ± 130 >3586 ± 340
Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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w a t e r f o r 2 h and w e r e p e r f o r m e d i n h o t w a t e r . The p u l l s t r e n g t h was r e d u c e d t o a b o u t o n e - t h i r d o f t h e v a l u e i n a i r . Kny e t a l . ( 1 8 ) have r e p o r t e d p u l l s t r e n g t h s o f glow-discharge-polymer i z e d m o n o b u t y l t r i v i n y l t i n f i l m s on aluminum and s t a i n l e s s steel. However, t h e y d i d n o t i n t r o d u c e O2 a t a l o w f l o w r a t e s i m u l t a n e o u s l y w i t h t h e monomer and a s a c o n s e q u e n c e d i d n o t e x p e r i e n c e i n t h e i r s y s t e m s t h e p l a s m a e t c h i n g i n t r o d u c e d b y an a b l a t i n g g a s s u c h a s 0£. A c c o r d i n g l y , t h e e x t e n t o f a t o m i c i n t e r f a c i a l m i x i n g was s m a l l and r e s u l t e d i n d i m i n i s h e d a d h e s i o n . I t i s noteworthy that the p u l l strengths a r e higher f o r the m e t a l l i c t i n - e n r i c h e d c o a t i n g s than f o r t h e c o a t i n g s c o n t a i n i n g higher c a r b o n - t o - t i n atomic r a t i o s . U n f o r t u n a t e l y , we were u n a b l e t o e x t r a c t f r o m t h e ESCA s t u d i e s i r r e f u t a b l e e v i d e n c e o f p r i m a r y bonds a t t h e f i l m - s u b s t r a t e i n t e r f a c e . On t h e b a s i s o f the h i g h e r p u l l s t r e n g t h s of f i l m s w i t h low c a r b o n - t o - t i n atomic r a t i o s and t h e l o w e r p u l l s t r e n g t h s o f t h e f i l m s w i t h a h i g h c a r b o n - t o - t i n r a t i o , i t i s e v i d e n t t h a t t i n somehow p l a y s a n i m p o r t a n t r o l e i n t h e i n t e r f a c i a l b o n d i n g mechanism. Indeed, t h e ESCA s t u d i e s o f Kny e t a l . d i d e s t a b l i s h t h e p r e s e n c e o f the bonding o f t i n t o t h e s u r f a c e o f aluminum v i a an oxygen b r i d g e bond. A t h i n m e t a l l i c f i l m (1000 A, c a r b o n - t o - t i n r a t i o l e s s t h a n 0.3) was d e p o s i t e d o n t o p o l y p r o p y l e n e and t h e p e r m e a b i l i t y determined. A w a t e r v a p o r p e r m e a b i l i t y o f 1.76 X 1 0 " c m (STP) cm cm"" S ^ c n T ^ H g " was o b t a i n e d , w h i c h i s o f t h e o r d e r o f v a l u e s u s u a l l y found f o r metal c o a t i n g s such as copper. (Table I I I ) . For r e f e r e n c e , t h e water vapor p e r m e a b i l i t y of t h e p o l y p r o p y l e n e f i l m i s (3.8 - 4.0) X 1 0 ~ c m ( S T P ) c m - c m ~ S" cm--lHgT- (19) The c o n t a c t a n g l e f o r w a t e r on t h e f i l m s d e p o s i t e d i n t h e m e t a l l i c r e g i o n a r e g i v e n i n T a b l e I V . The c o n t a c t a n g l e o f w a t e r on p o l y p r o p y l e n e i s 106° w h i c h was r e d u c e d t o 45° a f t e r t h e g l o w - d i s c h a r g e - p o l y m e r i z e d TMT f i l m was d e p o s i t e d . S i m i l a r values were o b t a i n e d on g l a s s . TMT f i l m s d e p o s i t e d b y g l o w d i s c h a r g e i n t h e absence o f oxygen e x h i b i t e d a c o n t a c t angle o f a p p r o x i m a t e l y 78°. T h e s e r e s u l t s s u g g e s t t h a t t h e TMT f i l m d e p o s i t e d i n the presence o f oxygen has t h e h i g h e s t s u r f a c e energy, which i s i n agreement w i t h t h e i n c r e a s e d m e t a l l i c b e h a v i o r o f s u c h films, i . e . high conductivities, luster etc. 0
1 4
2
3
1
9
3
1
2
1
L
Summary and C o n c l u s i o n s T h i n s h i n y m e t a l l i c t i n f i l m s c a n be p r o d u c e d b y p l a s m a p o l y m e r i z a t i o n o f TMT i n t h e p r e s e n c e o f o x y g e n . Such f i l m s h a v e a c o n d u c t i v i t y i n t h e r a n g e o f 1 0 - 1 0 Ω-ïcmT A carbon-to-tin ratio o f l e s s t h a n 2 on t h e s u r f a c e was e s s e n t i a l t o a c h i e v e a c o n ducting film. The t r a n s p a r e n t f i l m s were i n s u l a t i n g , and m i g h t c o n s i s t o f a n i r r e g u l a r n e t w o r k o f c a r b o n and t i n , b u t t h e r e was a t h r e s h o l d v a l u e o f t h e c a r b o n - t o - t i n r a t i o b e l o w w h i c h t h e s t r u c t u r e changed t o 3 - t i n w i t h p o s s i b l y some c a r b o n i n t h e interstices. 2
4
1
Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
42.
SADHIR AND JAMES
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Tetramethyltin
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Table III. Water Vapor Permeability of Barriers
p X 10 1 2 3 4 5
Polypropylene Polypropylene + TMT film (without oxygen) TMT film (without oxygen) Polypropylene + TMT film (in presence of oxygen) TMT film (in presence of oxygen)
cm^(STP cm cm (sec) cm Hg 40 .806 z
.00165 .161 .000176
Table IV. Contact Angle of the Metallic Films Deposited On Glass and Polypropylene Substrate
1 2 3 4
TMT deposited on glass (in absence of ablating gas) TMT deposited on glass (in presence of ablating gas) Polypropylene TMT deposited on polypropylene (in presnece of ablating gas)
Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
78° 48° 106° 46°
552
POLYMERS IN ELECTRONICS
The flow rate of oxygen played an important role in rate of deposition of the film and their conductivities. Too high a flow rate appreciably ablated the surface of the film producing pinholes and defects. At sufficiently low carbon-tin ratios, the films exhibited metallic behavior which was evidenced from the decrease in conductivity with increasing temperature. The electron diffraction lines were sharp and corresponded to 3-Sn. X-ray diffraction spectra of thicker metallic films showed major peaks at 30.7° and 31.2° corresponding to the (203) and (101) reflections of 3-Sn. Differential thermograms of powders scraped from the walls of the glass sleeves inserted in the reactor showed an endothermic peak at 232°C attributed to the melting of 3-Sn. A l l this evidence coupled with the SEM micro graphs clearly proves that the reflective films obtained in the manner described possessed the closely packed 3-Sn structure. Pull test results for a l l three substrates showed that the films enriched with tin have higher adhesive and cohesive strengths than those of higher carbon content. The metallic films acted as good water vapor permeability barriers in accord with what is expected for metal coatings. Literature Cited 1. Bradley, Α.; Hammes, J. P., J. Electrochem. Soc., 1963, 110, 15. 2. Tkachuk, Β. V.; Kobtsev, Yu. D.; Laurs, E. P.; Mikhalchenko, V. I.; Marusshi, N. Ya., Iv. Vyssh. Ucheben. Zaved Fiz., 1972, 15, 117. 3. Tkachuk, Β. V.; Marusshi, N. Ya; Laurs, E. P. Vysokomol. Soedin, Ser. A, 1973, 15, 2046. 4. Sadhir, R. K.; Saunders, Η. E. and James, W. J., submitted for Publication in the same ACS Symposium Series. 5. Kny, E.; Levenson, L. L . ; James, W. J.; and Auerbach, R. Α.; J. Phys. Chem., 1980, 84, 1635. 6. Hynecek, J., U.S. Patent 4,140,814, February 20, 1979. 7. Nomarski, G.; Weill, R. Α., Rev. Metall.(Paris), 1955, 55, 121. 8. Kny, E.; Levenson, L. L . ; James, W. J.; Auerbach, R. Α., Thin Solid Films, 1979, 64, 395. 9. Yasuda, H. K.; Stannett, V., J. Macromol. Sci.-Phys., 1969, B3(4), 589. 10. Yasuda, H. K., J . Appl. Polym. Sci., 1975, 19, 2529. 11. Havens, M. R.; Mayhan, K. G.; James, W. J., J. Appl. Polym. Sci., 1978, 22, 2793. 12. Yasuda, Η. K., ACS Symp. Ser., 1978, 108, 37. 13. Yasuda, Η. Κ., Contemporary Topics in Polymer Science, Ed. Shen, M. Plenum Publishing Corp., 1979, Vol. 3, 103. 14. Kny, E.; James, W. J.; Levenson, L. L . ; Auerbach, R. Α., Thin Solid Films, 1981, 85, 23. 15. Rice, D. W.; O'Kane, D. F., J. Electrochem. Soc., 1976, 123, 1308.
Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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16. Yasuda, H. K.; Marsh, H. C.; Brandt, E. S.; Reilley, C. Ν.; J. Polym. Sci.; Polym. Chem. Ed.; 1977, 15, 991. 17. Kim, G. S.; Baitinger, W. E.; Amy, J. W.; Winogard, N.; J . Electron. Spectr. Related Phen. 1974, 5, 351. 18. Kny, E.; Levenson, L. L . ; James, W. J.; and Auerbach, R. Α.; J. Vac. Sci. Technol.; 1979, 16(2), 359. 19. Lucari, J. J. and Brans, E. R. Mach. Des.; 1967, 39(12), 192. 20. Sadhir, R. Κ.; James, W. J.; and Auerbach, R. Α., Thin Solid Films, 1982, 97, 17. RECEIVED September 2,
1983
Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
