4 The Formation Mechanism of Styrene Polymer Film by Glow Discharge Direct Method K. T S U N E T O and I. T A N I G U C H I
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Department of Electronics, Doshisha University, Kyoto, Japan
A wide variety of organic polymer thin films have been formed in a glow discharge. In 1960, Goodman described the formation of films in a glow discharge from hydrocarbon vapors (1). In 1963, Bradley et al. described films produced from many organic substances in a glow discharge (2). Polymerized thin films have been formed by several methods such as glow discharge, photolysis, electron bombardment and vacuum evaporation. The films formed in a glow discharge are pinhole free and possess a number of unique and desirable properties. The glow discharge methods are also simpler than the other methods from a processing point of view. Two different methods are used to form thin films in a glow discharge method, an a.c. or d.c. glow discharge is initiated between the electrodes, and the electrodes serve as the substrate for film formation. In the indirect method, the substrate is placed in the plasma formed by two electrodes or by an electrodeless discharge, and the polymerized film is formed on the substrate. The growth rate of film is greater with the direct method, and i t is easier to control the film thickness with the direct method. On the other hand, insulator materials can be used as substrates only in the indirect method. Also the bombardment of charged particles affects the film formation less in the indirect method than in the direct method. The general properties of polymer films formed in glow discharges have gained much attention, but rather few investigations of the formation mechanism have been made. It is desirable to understand the mechanism of film formation in order to improve film properties and stabilize film formation. Christy's theory of polymerization by electron bombardment contains an important idea which can be applied to the theory of direct glow discharge polymerization (3). Williams and Hayes emphasized the importance of monomer adsorption to the electrode in the direct method (4). Poll also described the process of film formation in the direct method (5). Yasuda et al. employed the indirect method, using an electrodeless discharge, and the results 0-8412-0510-8/79/47-108-065$05.00/0 © 1979 American C h e m i c a l Society
Shen and Bell; Plasma Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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66
P L A S M A POLYMERIZATION
showed that p o l y m e r i z a t i o n i n the vapor phase was important (6). M o r i t a e t . a l proposed a formation mechanism i n both the d i r e c t and i n d i r e c t methods using P o l l ' s model (7). The conclusions of previous i n v e s t i g a t i o n s of the polymeriza t i o n mechanism are: 1) Organic monomer i s adsorbed on the surface of an e l e c t r o d e , and p o l y m e r i z a t i o n proceeds by bombardment of p o s i t i v e i o n s . 2) Polymerization occurs i n the gas phase and polymerized p a r t i c l e s deposits on the substrate s u r f a c e . In general the two mechanisms described above c o e x i s t i n a glow discharge. The former mechanism predominates i n the d i r e c t method and the l a t t e r predominates i n the i n d i r e c t method. This paper i s p r i m a r i l y concerned with the d i r e c t method of glow discharge p o l y m e r i z a t i o n . The bombardment of p o s i t i v e ions plays an important r o l e i n f i l m formation by the d i r e c t method as mentioned above, but the bombarding i o n i t s e l f and the r e a c t i v e species i n the gas phase may c o n t r i b u t e to f i l m formation. The r e l a t i v e importance of these f a c t o r s depends on the discharge con d i t i o n s . A t h e o r e t i c a l equation f o r the growth r a t e of f i l m w i l l be presented from a phenomenological p o i n t of view and compared with experimental r e s u l t s . The theory takes i n t o account bombard ment by ions and transport of i o n s . Theory of f i l m growth r a t e This theory i s an extension of C h r i s t y ' s model, which takes e l e c t r o n bombardment i n the account. The bombarding e l e c t r o n s do not c o n t r i b u t e to f i l m formation. His experiments were done at a vapor pressure of about 10"^ Torr so r e a c t i o n s i n the vapor phase were i n s i g n i f i c a n t . However, the bombardment of p o s i t i v e ions at the cathode plays a l e a d i n g r o l e and the bombarding ions them s e l v e s p o s s i b l y c o n t r i b u t e to f i l m formation. I t i s p o s s i b l e that ions combine with other p a r t i c l e s or grow i n t o l a r g e i o n s . E l e c t r o n bombardment at the anode i s unimportant because the f i l m i s formed mainly on the cathode, as shown by d.c. glow discharge ex periments. The equation f o r f i l m growth r a t e must i n c l u d e the e f f e c t s of i o n bombardment and i o n t r a n s p o r t . Let Ρ be the number of mole cules per u n i t area which have been polymerized. The time v a r i a t i o n of these molecules i s given by
where Ν i s the number of adsorbed molecules on the e l e c t r o d e sur face per u n i t area, i i s the f l u x of i o n s , σ i s the c o l l i s i o n cross s e c t i o n f o r p o l y m e r i z a t i o n , 3 i s the r a t e of p o l y m e r i z a t i o n by transport of one i o n * The behavior of the adsorbed molecules on the e l e c t r o d e surface i s given by Ν dp' τ " dt
Shen and Bell; Plasma Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
(2)
4.
TSUNETO AND T A N i G u c H i
Styrene
Glow
67
Discharge
where F i s the number of molecules per u n i t area per u n i t time s t r i k i n g the e l e c t r o d e s u r f a c e , γ i s the s t i c k i n g c o e f f i c i e n t of molecules to the e l e c t r o d e surface, τ i s the mean l i f e time of adsorption, P i s the number of molecules per u n i t area polymer i z e d by i o n bombardment. In Eq. (2), 3 i s not included i n d P / d t s i n c e dN/dt i s not i n f l u e n c e d by the transport of i o n s . From Eq. (1) and Eq. (2) we get Ν and d P / d t ( x « t ) and by adding 3 i to d P / d t we get dP/dt. D e f i n i n g the growth r a t e of f i l m we hive R = V. dP/dt (3) 1
f
f
Downloaded by UNIV OF MELBOURNE on March 28, 2016 | http://pubs.acs.org Publication Date: September 24, 1979 | doi: 10.1021/bk-1979-0108.ch004
f
R
Y*F
(4)
-C 1+1/σ·τ·ΐ F
Ν =
(5)
YF*T
1+σ·τ· i where V i s the volume of one polymer molecule. The growth r a t e of f i l m and the number of d e n s i t y adsorbed monomer f o r v a r i o u s discharge c o n d i t i o n s are shown i n Table I, assuming that a,3 and γ are constant. Table I suggests the f o l l o w i n g observations: Table I.
\
T h e o r e t i c a l equations f o r growth r a t e .
στίρ
