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Plasma polymerization of CF4 + H2 mixtures on the surface of polyethylene and polyvinylidene flouride substrates
Surface and Coatings Technology, 45 (1991) 369-378
369
P l a s m a p o l y m e r i z a t i o n of CF 4 + H 2 m i x t u r e s on the surface of p o l y e t h y l e n e a n d p o l y v i n y l i d e n e fluoride s u b s t r a t e s P. M o n t a z e r R a h m a t i , F. Arefi and J. A m o u r o u x Laboratoire de Gdnie des Procddds Plasma, Universitd Pierre et Marie Curie, ENSCP, 11 rue P. et M. Curie, 75231 Paris (France)
Abstract A non-equilibrium low pressure plasma was established in a bell-jar type reactor with a corona discharge configuration of electrodes (hollow blade electrode-grounded cylinder) at a low pressure (150 Pa), with a 70 kHz excitation source, for the plasma polymerization of CF 4 + H 2 mixtures on polyethylene (PE) as well as on the fluorine-containing substrate material polyvinylidene fluoride (PVDF). The effect of the feed composition, treatment time and the nature of the substrate both on the chemical structure of the deposited film and on the growth mechanism were analysed. In particular, it was found that the structure of the film was highly dependent on the hydrogen concentration in the feed and on the treatment time. Furthermore, it became clear that the polymerization rate on a PE substrate decreased with increasing treatment time and that the fluorine constituent in a PVDF substrate participated in the etching processes. Therefore, as compared with a PE substrate, a higher concentration of hydrogen in the CF4 + H 2 mixtures is needed to form a polymer on a PVDF substrate.
1. I n t r o d u c t i o n A great deal of work has been carried out in recent years in the field of surface treatment of different materials by thin-film deposition of plasmap o l y m e r i z e d f l u o r i n a t e d m o n o m e r s i n o r d e r t o p r o d u c e s u r f a c e s w i t h imp r o v e d p r o p e r t i e s s u c h a s a d h e s i o n o f t h e film, c o n d u c t i v i t y , w e t t i n g a b i l i t y and chemical selectivity. 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 f l u o r o c a r b o n s is a l w a y s a c c o m p a n i e d b y etching processes. It has been reported that the ratio of concentrations [ F ] / [ C F x ] i n f l u o r o c a r b o n d i s c h a r g e s is c h a r a c t e r i s t i c o f t h e i r a b i l i t y t o e t c h o r c a u s e p o l y m e r i z a t i o n [1]. K a y a n d D i l k s [2] a n d Y a s u d a [3] n o t e d t h a t i n d i s c h a r g e s i n p e r f l u o r o c a r b o n s s u c h a s CF4 a n d C2F6, t h e d o m i n a n t p r o c e s s is e t c h i n g r a t h e r t h a n p o l y m e r i z a t i o n , b u t w h e n a r e d u c i n g a g e n t (e.g. H2, C2F4 etc.) is p r e s e n t i n the discharge then polymerization predominates. D'Agostino and coworkers studied the effect of the addition of hydrogen, a s c a v e n g e r o f f l u o r i n e a t o m s , t o Cn F(2n+2) d i s c h a r g e s [4, 5]. I n a l l c a s e s t h e fluorine atom concentration fell to a minimum at a hydrogen percentage v a r y i n g b e t w e e n 10% a n d 18% a n d t h e C F a n d CF2 r a d i c a l c o n c e n t r a t i o n s increased with the percentage of hydrogen. Elsevier Sequoia/Printed in The Netherlands
370 Emission spectroscopy using the actinometric technique developed by Coburn and Chen [6] and D'Agostino and coworkers [4, 5] has made possible the determination of the relative concent rat i on trends among the detected excited species (CF2, F, CH, H) as a function of the different parameters relating to the discharge. In our study, we used argon as actinometer in optical emission spectroscopy measurements, X-ray photoelectron spectroscopy (XPS) to determine the surface composition, contact angle measurements for evaluating the surface tension and gravimetric measurement for the polymerization rate.
2. E x p e r i m e n t a l details The experimental apparatus used for this work was a bell-jar type reactor with a corona discharge configuration of electrodes (hollow blade type electrode-grounded cylinder). This particular configuration of electrodes was chosen in order to simulate the industrial processes used for the surface t r e a t m e n t of polymers. The experimental details have been described elsewhere [7]. The discharge was established by means of an industrial 800 W excitation source with a frequency of 70 kHz. The experiments were carried out under the following conditions: pressure, 150 Pa; power, 170 W; frequency, 70 kHz; Q = 100 200 sccm.
3. R e s u l t s Emission spectroscopy was used to characterize the excited species in the discharge and the spectra obtained for the C F 4 ~ - H 2 mixtures showed the following features: two different continua centred approximately at 300 and 580 nm which were attributed to CF2 ÷ ions and CF3 radicals [7] and lines at 685.6, 703.7 and 739.8 nm corresponding to atomic fluorine. Furthermore, Ha and Hfl lines were detected at 656.2 and 486 nm respectively. Because of the energy relationships in the discharge no CF2 radicals were detected; this is the predominant species present in an r.f. glow discharge in fluorocarbons [7]. The relative concentrations of the various active species were determined by means of actinometry using argon as the actinometer; the relative concentrations of atomic fluorine and excited CF2 + ions are shown in Fig. 1 as a function of the additive percentage of hydrogen in the feed. The fluorine atom co n ce nt r a t i on passed t hr ough a minimum at 2% hydrogen owing to the formation of the unreactive stable compound HF, and then increased again. The intensity of the argon line (the actinometer) reached a maximum at 2% hydrogen (Fig. 2) which suggests t hat the mean energy of the electrons was at a maximum at this point. Therefore the decomposition of CF4, for hydrogen percentages up to 2%, must have been taking place by direct
371 1.2 i 1,0 ~ 0,8 0.6
CF2+
0.4 02.
0
i
J
|
20
40
60
%H2
80
Fig. 1. Relative concentration of fluorine atoms (685.6 nm) and CF2+ ions (310 nm) obtained by actinometry vs. % H2 in the C F 4 + H2 + 2% Ar mixture (p = 150 Pa; P = 170 W; Q = 200 cm~rain-l; d = 10 ram). .~ 1,2 ... ,0 0,8 o,6
"-4.... 0,4 1 0.2 0o0
""m •
0
.
•
20
40
60
80
%H2
100
Fig. 2. Relative intensity of argon (750.4 nm) obtained by actinometry vs. % H2 in the CF4 + H2 + 2% Ar mixture (p = 150 Pa; P = 170 W; Q = 200 cm3 min-1; d = 10 mm). e l e c t r o n i c i m p a c t . F r o m t h i s p o i n t on, t h e i n t e n s i t y of t h e a c t i n o m e t e r d e c r e a s e d , w h i c h s u g g e s t s t h a t t h e m e a n e n e r g y of t h e e l e c t r o n s d e c r e a s e d . It h a s b e e n s h o w n e l s e w h e r e t h a t in N 2 + H2 m i x t u r e s t h e d i s t r i b u t i o n f u n c t i o n of t h e e n e r g y of t h e e l e c t r o n s ( D F E E ) i n c r e a s e s w i t h t h e h y d r o g e n p e r c e n t a g e in t h e feed b e c a u s e n i t r o g e n p u m p s t h e v i b r a t i o n a l l e v e l s [8]. In t h e s a m e way, for C F 4 + H 2 m i x t u r e s h y d r o g e n seems to p u m p e n e r g y , p r o b a b l y v i b r a t i o n a l l y [9]. T h e r e f o r e , b e a r i n g in m i n d t h e a b o v e , i . e . t h e d e c r e a s e in t h e D F E E w i t h t h e h y d r o g e n p e r c e n t a g e , t h e i n c r e a s e in t h e p o p u l a t i o n of f l u o r i n e a t o m s a n d CF2 ÷ i o n s in t h e f u n d a m e n t a l s t a t e ( F i g . 1') c a n n o t be a t t r i b u t e d to d i r e c t e l e c t r o n - m o l e c u l e i m p a c t s . So a n o t h e r mecha= n i s m m u s t be p r o p o s e d to t a k e a c c o u n t of t h e i n c r e a s e in a t o m i c f l u o r i n e a n d CF2 +. T h e f o l l o w i n g m e c h a n i s m is t h e r e f o r e p r o p o s e d , in w h i c h v i b r a t i o n a l l y e x c i t e d h y d r o g e n p l a y s a n i m p o r t a n t role. H2(v) + CF4 -~ CF3 + H F + H CF3 + H --* CF2 + H F + H2(v) --* CF2 + H F + H
(1) (2)
372
CF2 + H --* CF + HF + H2(v) --* CF + HF + H
(3)
H F + H--*H2+ F 2HF + H2(v) --* 2H2 + 2F
(4)
Equation (4) represents an endothermic reaction series which has been studied in detail by Polanyi [10]. In fact, the above mechanisms become more important as the hydrogen content increases in the feed, leading to a decrease in the DFEE. Indeed, for very small percentages of hydrogen in the feed, the mean energy of the electrons remains too high (around 6 eV for this particular energetic discharge) to excite the hydrogen molecules to different vibrational levels [11]. As soon as the hydrogen percentage increases, the energy of the electrons decreases and most of the hydrogen is probably vibrationally excited, which can give rise to the proposed mechanism. The polymerization rate, which was recorded by gravimetric measurements, was greater with 2% hydrogen in the C F 4 discharge (Fig. 3). This was due to two phenomena. First of all, the concentration of fluorine atoms (known to be an etching agent) was at a minimum at this point. Secondly, the high mean energy of the electrons at this point (Fig. 2) caused the formation of reactive sites on the polymer surface by bombardment with energetic electrons and ions; these sites reacted with the CF2 species in the discharge, resulting from the decomposition of CF 4 by direct electronic impact, giving rise to the formation of a polymer. XPS experiments performed on the deposited fluorocarbon films confirmed the emission spectroscopic results, i.e. the fluorine content (F:C) was greatest for 2% hydrogen, which corresponds to the point where the relative concentration of excited fluorine atoms (known to be an etching agent) was at a minimum and emphasizes the role of the CF2 ÷ species in the polymerization mechanism (Fig. 4). At this point the area of the peaks shown in Fig. 4 corresponding to the functional groups CF3, CF2 and CF is at a maximum and that of the C--CFx peak is at a minimum. The concentration of fluorine-containing groups (CF, CF2 and CF3) then decreased and that of 3O ¢= ,=
2
20"
f
10'
0 0
|
|
|
|
20
40
60
80
F i g . 3. P o l y m e r i z a t i o n r a t e o f C F 4 + H 2 + 2 % A r m i x t u r e ( p = 150 P a ; Q = 200 c m a m i n 1; p = 170 W ; t = 2.7 m i n ) .
% H2
vs.
100
% H 2 in the feed on a PE substrate
373
-
'!
/I I~~
Fls/Cls=0,89 Ols/Cls=0,1
8.3
i
i
286,5
291
-1
•
295,5 B E ~¢v) CF4+I07Ji 2 Fls/Cls=0,52 Ols/Cls=0,1
&; 3.(.
286,5
i
295,5 B.E(ev)
I,~C_CF
19.2
ii
291 x
cF4÷5o~
Fls/Cls--0,28 Ols/Cls=O'16
~,s
~i
.........
=--
295,5
-
BE
(ev)
Fig. 4. V a r i a t i o n of C l s photo-peak of PE film t r e a t e d w i t h different p e r c e n t a g e s of H 2 in CF 4 + H 2 discharges (p = 150 Pa; P = 170 W; Q = 200 cm 3 min-1; t = 2.7 rain).
374 C - - C F x g r o u p s i n c r e a s e d w i t h a n i n c r e a s e in the h y d r o g e n f r a c t i o n in the m i x t u r e (Fig. 4). This w a s p r o b a b l y b e c a u s e the i n c r e a s e in the l a t t e r type g a v e rise to an i n c r e a s e in the q u a n t i t y of fluorine a t o m s ( b e y o n d 2% h y d r o g e n ) as well as h y d r o g e n species (Fig. 1) in the discharge. T h e r e f o r e , the r a t e at w h i c h the p o l y m e r w a s e t c h e d by h y d r o g e n a n d fluorine a b l a t i o n i n c r e a s e d w h e n c o m p a r e d w i t h the c o m p e t i t i v e p o l y m e r i z a t i o n p h e n o m e n a . F u r t h e r m o r e , this m a k e s c l e a r the i m p o r t a n t role of t h e n a t u r e of the s u b s t r a t e in the g r o w t h m e c h a n i s m s of the f l u o r o c a r b o n film. I n d e e d it was f o u n d t h a t w i t h a m i x t u r e of C F 4 a n d 2% h y d r o g e n a h y d r o f l u o r i n a t e d p o l y m e r was f o r m e d on a P E s u b s t r a t e as soon as the d i s c h a r g e w a s started, w h e r e a s w i t h l o n g e r t r e a t m e n t t i m e s w h e n the fluorine c o n t e n t of the d e p o s i t e d p o l y m e r was i n c r e a s e d (F:C = 1:1), the p o l y m e r i z a t i o n r a t e decreased; t h a t is, e t c h i n g b e c a m e c o m p e t i t i v e w i t h p o l y m e r deposition. F i g u r e 5 shows the p l a t e a u o b t a i n e d for t h e p o l y m e r i z a t i o n r a t e ( a f t e r 1.5 min) w a s due to a n e q u i l i b r i u m e s t a b l i s h e d b e t w e e n the p o l y m e r i z a t i o n a n d e t c h i n g processes. XPS r e s u l t s o b t a i n e d w i t h t h e s e s a m p l e s s h o w e d t h a t the F:C r a t i o of the deposit r e m a i n e d a l m o s t c o n s t a n t while the s u r f a c e of the p e a k a t 286.6 eV a t t r i b u t e d to C - - C F x g r o u p s i n c r e a s e d w i t h time. A t the s a m e time, a d e c r e a s e of the p e a k at 285 eV c o r r e s p o n d i n g to
I C
I
a n d / o r C H 2 - - C H z f u n c t i o n a l g r o u p s h a s b e e n p o i n t e d o u t (Fig. 6). T h e r e f o r e , a h y d r o g e n loss by the p o l y m e r l e a d i n g to a m o r e c r o s s l i n k e d s t r u c t u r e c a n be s u g g e s t e d for w h e n the t r e a t m e n t t i m e was increased. T h e P V D F s u b s t r a t e m a t e r i a l was itself h y d r o f l u o r i n a t e d f r o m t h e start, w i t h a n F:C r a t i o e q u a l to unity, a n d t h e r e f o r e w i t h this p a r t i c u l a r feed c o m p o s i t i o n ( C F 4 -~- 2% H2) , for s h o r t t r e a t m e n t times (less t h a n 75 s), t h e r e was no p o l y m e r d e p o s i t i o n b u t only e t c h i n g of the s u b s t r a t e . T h i s is consist e n t w i t h the p h e n o m e n a o b s e r v e d w i t h the P E s u b s t r a t e u s i n g l o n g e r t r e a t m e n t times ( m o r e t h a n 30 s) (Fig. 7). As soon as the fluorine c o n t e n t of 140 "~120-
~
.~
100 -
4020 •
t
0
, 1
2
3
,
|
4
5
t(min)
Fig. 5. Polymerization rate of CF4 + 2% H2 + 2% Ar mixture strate (p = 150 Pa; Q = 200 cm3 min 1; p = 170 W).
vs.
treatment time on a PE sub-
375 102 CPS 11,7 9,37,4.
t=0.227 mln
CF
C-C 5,4. CH2-C
F1s/CIs=0.95
3,~*.
01s/Cls=O.05
'~\--4~.~. I,'" ~ 102
,
,
---r B E (ev)
ii,'~ 9,3
]!!'~~ C-CFx
7,5,
t=0,54 rain
5,1" i
Fls/Cls=l 3,4
Ols/Cls=0.05
1,41 io z
ll,t 9,5
7,7
t=1.36 rain
5,8
Fls/Cls=l
3,9 2,1
Ols/Cls=0.04
I
L
(ev) -r BE(ev
102
IO,~ 8,3_ 6,2-
t=2.72 rain
",I_
Els/Cls=0.9
2,0
Ols/Cls=0.1
I
286.5
I
291.5
296.5
E L
(eV)
Fig. 6. Variation of C ls photo-peak of PE film treated for different treatment times in CF4 + 2% H2 discharges (p = 150 Pa; P = 170 W; Q = 200 cm~ min-1). t h e d e p o s i t e d p o l y m e r d e c r e a s e d , t h e n t h i s m a t e r i a l b e g a n to b u i l d u p o n t h e s u r f a c e o f t h e P V D F . W i t h l o n g e r t r e a t m e n t t i m e s ( m o r e t h a n 75 s), u n d e r our experimental conditions, the chemical composition of the deposited f l u o r o c a r b o n film b e c a m e t h e s a m e o n b o t h o f t h e s u b s t r a t e s .
376 6O Polymerization
i4o 20
"~
0 -20 -40 -60
Etching
-80 0.0
0,5
1.0
1.5
|
|
2,0
2,5
t(min)
3,0
F i g . 7. P o l y m e r i z a t i o n r a t e v s . t r e a t m e n t t i m e i n a C F 4 + 2 % H 2 + 2 % A r m i x t u r e s u b s t r a t e ( p = 150 P a ; Q = 200 c m a r a i n a; p = 170 W).
on a PVDF
25, ._~ E
20'
--~ 1 5 '
10,
,
0 0
2
|
4
6
8
10
%H2
12
F i g . 8. P o l y m e r i z a t i o n r a t e o f C F 4 -}-H 2 + 2 % A r m i x t u r e v s . % H 2 i n t h e f e e d o n a P V D F s u b s t r a t e ( p = 150 P a ; Q = 200 c m 3 m i n 1; p = 170 W; t = 2.7 m i n ) .
Therefore, since fluorine contained in the substrate contributes to the growth mechanisms, the optimum hydrogen percentage in the feed (corresponding to the maximum polymer growth rate) will not be the same with a PVDF substrate as with a PE substrate. The polymerization rate on PE is greatest with a hydrogen fraction equal to 2% (Fig. 3), whereas with the fluorine-containing substrate PVDF, the optimum hydrogen proportion is in the order of 4% (Fig. 8). Contact angle measurements carried out on the deposited films by an image-processing system [12] with two liquids (formamide and water) provided the possibility of calculating the surface tension of the deposited films by the Owens and Kaelble method. These results (Fig. 9) confirm the XPS results showing the surface tension was lowest with a feed containing 2% hydrogen, i.e. when fluorine content (F:C ratio) of the film was at a maximum.
4. C o n c l u s i o n
In this work the plasma polymerization of C F 4 + H2 mixtures was studied by means of a corona discharge configuration of electrodes at low pressure
377 50 Total S.T
2~]
DispersiveS.T
10.
Polar S.T
0
20
40
60
so
loo
%H2
Fig. 9. Variation of the surface tension (polar and dispersive components) of the deposited polymer surface on a PE substrate with water and formamide couple v s . % H2 in C F 4 + H 2 + 2 % Ar discharge (p = 150 Pa; Q = 200 cm3 min-1; P = 170 W; f = 70 kHz; t = 2.7 min).
(150 Pa) on two s u b s t r a t e s , P E a n d PVDF. T h e difference in the r e s u l t s confirmed the i m p o r t a n c e of the use of d i a g n o s t i c m e t h o d s s u c h as o p t i c a l e m i s s i o n s p e c t r o s c o p y for the c o n t r o l of the c h e m i c a l a n d e n e r g e t i c properties of the p l a s m a . T h e s e p r o p e r t i e s , a r i s i n g f r o m the c h e m i c a l n a t u r e of the excited species, c a n b e c o m e quite different d e p e n d i n g on the e x c i t a t i o n f r e q u e n c y [7] a n d e l e c t r o d e c o n f i g u r a t i o n [7], as well as the w o r k i n g p r e s s u r e w h i c h will be discussed in a n o t h e r p a p e r in the n e a r future. F r o m the o p t i c a l e m i s s i o n s p e c t r o s c o p y r e s u l t s a r e a c t i o n m e c h a n i s m was p r o p o s e d to e x p l a i n w h a t h a p p e n e d in t h e h o m o g e n e o u s p l a s m a p h a s e w h e n h y d r o g e n was a d d e d to the C F 4 discharge; f u r t h e r m o r e , in the h e t e r o g e neous plasma-substrate system simultaneous polymerization and etching p h e n o m e n a w e r e observed, t h e i r r e l a t i v e r a t e s d e p e n d i n g on the a t o m i c fluorine c o n t e n t of the h e t e r o g e n e o u s p l a s m a - s u b s t r a t e b o u n d a r y layer. T h i s a c c o u n t s for t h e difference o b s e r v e d in the p o l y m e r i z a t i o n r a t e of C F 4 + H2 m i x t u r e s on different s u b s t r a t e s . It h a s t h u s b e e n c l e a r l y s h o w n t h a t the d e p o s i t i o n m e c h a n i s m s of C F 4 + H 2 m i x t u r e s d e p e n d on t h e t r e a t m e n t t i m e a n d the n a t u r e of the s u b s t r a t e on w h i c h t h e y are deposited. I n the case of f l u o r i n e - c o n t a i n i n g s u b s t r a t e PVDF, t h e fluorine c o n s t i t u e n t in the s u b s t r a t e c o n t r i b u t e s to the g r o w t h m e c h a n i s m s . T h e r e f o r e the o p t i m u m h y d r o g e n p e r c e n t a g e in the feed ( c o r r e s p o n d i n g to the m a x i m u m p o l y m e r g r o w t h r a t e ) will n o t be the s a m e for the two s u b s t r a t e s .
A ck n o w l ed g m e nt T h e a u t h o r s w o u l d like to t h a n k Electricit6 de F r a n c e for its financial support.
378
References 1 R. d'Agostino, D. de Benedictis and F. Cramarossa, Plasma Chem. Plasma Process., 4(1) (1984) 1. 2 E. Kay and A. Dilks, Thin Solid Films, 78 (1981) 309. 3 H. Yasuda, in J. L. Vessen and W. Kern (eds.), Thin Film Process, Academic Press, New York, 1978, p. 361. 4 R. d'Agostino, F. Cramarossa, V. Colaprico and R. d'Ettole, J. Appl. Phys., 54(3) (1983) 1284. 5 R. d'Agostino, F. Cramarossa and F. Fracassi, Thin Solid Films, 143 (1986) 163. 6 J. W. Coburn and M. Chen, J. Appl. Phys., 51 (1980) 3134. 7 P. Montazer Rahmati, F. Arefi, J. Amouroux and A. Ricard, Proc. 9th ISPC, (IUPAC), Pugnochiuso, Italy, 1989, p. 1195. 8 S. de Benedictis, A. Gicquel and F. Cramarossa, Proc. 8th ISPC, Tokyo, Japan, 1987, p. 631. 9 M. Capitelli and M. Dilonardo, J. Chem. Phys., 20 (1977) 417. 10 J. C. Polanyi, Acc. Chem. Res., 5 (1972) 161. 11 Von Engel, Electric Plasmas: their Nature and Uses, Taylor and Francis Ltd., London and New York, 1983. 12 P. Montazer Rahmati, F. Arefi and J. Amouroux, French Patent 8807726.
369
P l a s m a p o l y m e r i z a t i o n of CF 4 + H 2 m i x t u r e s on the surface of p o l y e t h y l e n e a n d p o l y v i n y l i d e n e fluoride s u b s t r a t e s P. M o n t a z e r R a h m a t i , F. Arefi and J. A m o u r o u x Laboratoire de Gdnie des Procddds Plasma, Universitd Pierre et Marie Curie, ENSCP, 11 rue P. et M. Curie, 75231 Paris (France)
Abstract A non-equilibrium low pressure plasma was established in a bell-jar type reactor with a corona discharge configuration of electrodes (hollow blade electrode-grounded cylinder) at a low pressure (150 Pa), with a 70 kHz excitation source, for the plasma polymerization of CF 4 + H 2 mixtures on polyethylene (PE) as well as on the fluorine-containing substrate material polyvinylidene fluoride (PVDF). The effect of the feed composition, treatment time and the nature of the substrate both on the chemical structure of the deposited film and on the growth mechanism were analysed. In particular, it was found that the structure of the film was highly dependent on the hydrogen concentration in the feed and on the treatment time. Furthermore, it became clear that the polymerization rate on a PE substrate decreased with increasing treatment time and that the fluorine constituent in a PVDF substrate participated in the etching processes. Therefore, as compared with a PE substrate, a higher concentration of hydrogen in the CF4 + H 2 mixtures is needed to form a polymer on a PVDF substrate.
1. I n t r o d u c t i o n A great deal of work has been carried out in recent years in the field of surface treatment of different materials by thin-film deposition of plasmap o l y m e r i z e d f l u o r i n a t e d m o n o m e r s i n o r d e r t o p r o d u c e s u r f a c e s w i t h imp r o v e d p r o p e r t i e s s u c h a s a d h e s i o n o f t h e film, c o n d u c t i v i t y , w e t t i n g a b i l i t y and chemical selectivity. 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 f l u o r o c a r b o n s is a l w a y s a c c o m p a n i e d b y etching processes. It has been reported that the ratio of concentrations [ F ] / [ C F x ] i n f l u o r o c a r b o n d i s c h a r g e s is c h a r a c t e r i s t i c o f t h e i r a b i l i t y t o e t c h o r c a u s e p o l y m e r i z a t i o n [1]. K a y a n d D i l k s [2] a n d Y a s u d a [3] n o t e d t h a t i n d i s c h a r g e s i n p e r f l u o r o c a r b o n s s u c h a s CF4 a n d C2F6, t h e d o m i n a n t p r o c e s s is e t c h i n g r a t h e r t h a n p o l y m e r i z a t i o n , b u t w h e n a r e d u c i n g a g e n t (e.g. H2, C2F4 etc.) is p r e s e n t i n the discharge then polymerization predominates. D'Agostino and coworkers studied the effect of the addition of hydrogen, a s c a v e n g e r o f f l u o r i n e a t o m s , t o Cn F(2n+2) d i s c h a r g e s [4, 5]. I n a l l c a s e s t h e fluorine atom concentration fell to a minimum at a hydrogen percentage v a r y i n g b e t w e e n 10% a n d 18% a n d t h e C F a n d CF2 r a d i c a l c o n c e n t r a t i o n s increased with the percentage of hydrogen. Elsevier Sequoia/Printed in The Netherlands
370 Emission spectroscopy using the actinometric technique developed by Coburn and Chen [6] and D'Agostino and coworkers [4, 5] has made possible the determination of the relative concent rat i on trends among the detected excited species (CF2, F, CH, H) as a function of the different parameters relating to the discharge. In our study, we used argon as actinometer in optical emission spectroscopy measurements, X-ray photoelectron spectroscopy (XPS) to determine the surface composition, contact angle measurements for evaluating the surface tension and gravimetric measurement for the polymerization rate.
2. E x p e r i m e n t a l details The experimental apparatus used for this work was a bell-jar type reactor with a corona discharge configuration of electrodes (hollow blade type electrode-grounded cylinder). This particular configuration of electrodes was chosen in order to simulate the industrial processes used for the surface t r e a t m e n t of polymers. The experimental details have been described elsewhere [7]. The discharge was established by means of an industrial 800 W excitation source with a frequency of 70 kHz. The experiments were carried out under the following conditions: pressure, 150 Pa; power, 170 W; frequency, 70 kHz; Q = 100 200 sccm.
3. R e s u l t s Emission spectroscopy was used to characterize the excited species in the discharge and the spectra obtained for the C F 4 ~ - H 2 mixtures showed the following features: two different continua centred approximately at 300 and 580 nm which were attributed to CF2 ÷ ions and CF3 radicals [7] and lines at 685.6, 703.7 and 739.8 nm corresponding to atomic fluorine. Furthermore, Ha and Hfl lines were detected at 656.2 and 486 nm respectively. Because of the energy relationships in the discharge no CF2 radicals were detected; this is the predominant species present in an r.f. glow discharge in fluorocarbons [7]. The relative concentrations of the various active species were determined by means of actinometry using argon as the actinometer; the relative concentrations of atomic fluorine and excited CF2 + ions are shown in Fig. 1 as a function of the additive percentage of hydrogen in the feed. The fluorine atom co n ce nt r a t i on passed t hr ough a minimum at 2% hydrogen owing to the formation of the unreactive stable compound HF, and then increased again. The intensity of the argon line (the actinometer) reached a maximum at 2% hydrogen (Fig. 2) which suggests t hat the mean energy of the electrons was at a maximum at this point. Therefore the decomposition of CF4, for hydrogen percentages up to 2%, must have been taking place by direct
371 1.2 i 1,0 ~ 0,8 0.6
CF2+
0.4 02.
0
i
J
|
20
40
60
%H2
80
Fig. 1. Relative concentration of fluorine atoms (685.6 nm) and CF2+ ions (310 nm) obtained by actinometry vs. % H2 in the C F 4 + H2 + 2% Ar mixture (p = 150 Pa; P = 170 W; Q = 200 cm~rain-l; d = 10 ram). .~ 1,2 ... ,0 0,8 o,6
"-4.... 0,4 1 0.2 0o0
""m •
0
.
•
20
40
60
80
%H2
100
Fig. 2. Relative intensity of argon (750.4 nm) obtained by actinometry vs. % H2 in the CF4 + H2 + 2% Ar mixture (p = 150 Pa; P = 170 W; Q = 200 cm3 min-1; d = 10 mm). e l e c t r o n i c i m p a c t . F r o m t h i s p o i n t on, t h e i n t e n s i t y of t h e a c t i n o m e t e r d e c r e a s e d , w h i c h s u g g e s t s t h a t t h e m e a n e n e r g y of t h e e l e c t r o n s d e c r e a s e d . It h a s b e e n s h o w n e l s e w h e r e t h a t in N 2 + H2 m i x t u r e s t h e d i s t r i b u t i o n f u n c t i o n of t h e e n e r g y of t h e e l e c t r o n s ( D F E E ) i n c r e a s e s w i t h t h e h y d r o g e n p e r c e n t a g e in t h e feed b e c a u s e n i t r o g e n p u m p s t h e v i b r a t i o n a l l e v e l s [8]. In t h e s a m e way, for C F 4 + H 2 m i x t u r e s h y d r o g e n seems to p u m p e n e r g y , p r o b a b l y v i b r a t i o n a l l y [9]. T h e r e f o r e , b e a r i n g in m i n d t h e a b o v e , i . e . t h e d e c r e a s e in t h e D F E E w i t h t h e h y d r o g e n p e r c e n t a g e , t h e i n c r e a s e in t h e p o p u l a t i o n of f l u o r i n e a t o m s a n d CF2 ÷ i o n s in t h e f u n d a m e n t a l s t a t e ( F i g . 1') c a n n o t be a t t r i b u t e d to d i r e c t e l e c t r o n - m o l e c u l e i m p a c t s . So a n o t h e r mecha= n i s m m u s t be p r o p o s e d to t a k e a c c o u n t of t h e i n c r e a s e in a t o m i c f l u o r i n e a n d CF2 +. T h e f o l l o w i n g m e c h a n i s m is t h e r e f o r e p r o p o s e d , in w h i c h v i b r a t i o n a l l y e x c i t e d h y d r o g e n p l a y s a n i m p o r t a n t role. H2(v) + CF4 -~ CF3 + H F + H CF3 + H --* CF2 + H F + H2(v) --* CF2 + H F + H
(1) (2)
372
CF2 + H --* CF + HF + H2(v) --* CF + HF + H
(3)
H F + H--*H2+ F 2HF + H2(v) --* 2H2 + 2F
(4)
Equation (4) represents an endothermic reaction series which has been studied in detail by Polanyi [10]. In fact, the above mechanisms become more important as the hydrogen content increases in the feed, leading to a decrease in the DFEE. Indeed, for very small percentages of hydrogen in the feed, the mean energy of the electrons remains too high (around 6 eV for this particular energetic discharge) to excite the hydrogen molecules to different vibrational levels [11]. As soon as the hydrogen percentage increases, the energy of the electrons decreases and most of the hydrogen is probably vibrationally excited, which can give rise to the proposed mechanism. The polymerization rate, which was recorded by gravimetric measurements, was greater with 2% hydrogen in the C F 4 discharge (Fig. 3). This was due to two phenomena. First of all, the concentration of fluorine atoms (known to be an etching agent) was at a minimum at this point. Secondly, the high mean energy of the electrons at this point (Fig. 2) caused the formation of reactive sites on the polymer surface by bombardment with energetic electrons and ions; these sites reacted with the CF2 species in the discharge, resulting from the decomposition of CF 4 by direct electronic impact, giving rise to the formation of a polymer. XPS experiments performed on the deposited fluorocarbon films confirmed the emission spectroscopic results, i.e. the fluorine content (F:C) was greatest for 2% hydrogen, which corresponds to the point where the relative concentration of excited fluorine atoms (known to be an etching agent) was at a minimum and emphasizes the role of the CF2 ÷ species in the polymerization mechanism (Fig. 4). At this point the area of the peaks shown in Fig. 4 corresponding to the functional groups CF3, CF2 and CF is at a maximum and that of the C--CFx peak is at a minimum. The concentration of fluorine-containing groups (CF, CF2 and CF3) then decreased and that of 3O ¢= ,=
2
20"
f
10'
0 0
|
|
|
|
20
40
60
80
F i g . 3. P o l y m e r i z a t i o n r a t e o f C F 4 + H 2 + 2 % A r m i x t u r e ( p = 150 P a ; Q = 200 c m a m i n 1; p = 170 W ; t = 2.7 m i n ) .
% H2
vs.
100
% H 2 in the feed on a PE substrate
373
-
'!
/I I~~
Fls/Cls=0,89 Ols/Cls=0,1
8.3
i
i
286,5
291
-1
•
295,5 B E ~¢v) CF4+I07Ji 2 Fls/Cls=0,52 Ols/Cls=0,1
&; 3.(.
286,5
i
295,5 B.E(ev)
I,~C_CF
19.2
ii
291 x
cF4÷5o~
Fls/Cls--0,28 Ols/Cls=O'16
~,s
~i
.........
=--
295,5
-
BE
(ev)
Fig. 4. V a r i a t i o n of C l s photo-peak of PE film t r e a t e d w i t h different p e r c e n t a g e s of H 2 in CF 4 + H 2 discharges (p = 150 Pa; P = 170 W; Q = 200 cm 3 min-1; t = 2.7 rain).
374 C - - C F x g r o u p s i n c r e a s e d w i t h a n i n c r e a s e in the h y d r o g e n f r a c t i o n in the m i x t u r e (Fig. 4). This w a s p r o b a b l y b e c a u s e the i n c r e a s e in the l a t t e r type g a v e rise to an i n c r e a s e in the q u a n t i t y of fluorine a t o m s ( b e y o n d 2% h y d r o g e n ) as well as h y d r o g e n species (Fig. 1) in the discharge. T h e r e f o r e , the r a t e at w h i c h the p o l y m e r w a s e t c h e d by h y d r o g e n a n d fluorine a b l a t i o n i n c r e a s e d w h e n c o m p a r e d w i t h the c o m p e t i t i v e p o l y m e r i z a t i o n p h e n o m e n a . F u r t h e r m o r e , this m a k e s c l e a r the i m p o r t a n t role of t h e n a t u r e of the s u b s t r a t e in the g r o w t h m e c h a n i s m s of the f l u o r o c a r b o n film. I n d e e d it was f o u n d t h a t w i t h a m i x t u r e of C F 4 a n d 2% h y d r o g e n a h y d r o f l u o r i n a t e d p o l y m e r was f o r m e d on a P E s u b s t r a t e as soon as the d i s c h a r g e w a s started, w h e r e a s w i t h l o n g e r t r e a t m e n t t i m e s w h e n the fluorine c o n t e n t of the d e p o s i t e d p o l y m e r was i n c r e a s e d (F:C = 1:1), the p o l y m e r i z a t i o n r a t e decreased; t h a t is, e t c h i n g b e c a m e c o m p e t i t i v e w i t h p o l y m e r deposition. F i g u r e 5 shows the p l a t e a u o b t a i n e d for t h e p o l y m e r i z a t i o n r a t e ( a f t e r 1.5 min) w a s due to a n e q u i l i b r i u m e s t a b l i s h e d b e t w e e n the p o l y m e r i z a t i o n a n d e t c h i n g processes. XPS r e s u l t s o b t a i n e d w i t h t h e s e s a m p l e s s h o w e d t h a t the F:C r a t i o of the deposit r e m a i n e d a l m o s t c o n s t a n t while the s u r f a c e of the p e a k a t 286.6 eV a t t r i b u t e d to C - - C F x g r o u p s i n c r e a s e d w i t h time. A t the s a m e time, a d e c r e a s e of the p e a k at 285 eV c o r r e s p o n d i n g to
I C
I
a n d / o r C H 2 - - C H z f u n c t i o n a l g r o u p s h a s b e e n p o i n t e d o u t (Fig. 6). T h e r e f o r e , a h y d r o g e n loss by the p o l y m e r l e a d i n g to a m o r e c r o s s l i n k e d s t r u c t u r e c a n be s u g g e s t e d for w h e n the t r e a t m e n t t i m e was increased. T h e P V D F s u b s t r a t e m a t e r i a l was itself h y d r o f l u o r i n a t e d f r o m t h e start, w i t h a n F:C r a t i o e q u a l to unity, a n d t h e r e f o r e w i t h this p a r t i c u l a r feed c o m p o s i t i o n ( C F 4 -~- 2% H2) , for s h o r t t r e a t m e n t times (less t h a n 75 s), t h e r e was no p o l y m e r d e p o s i t i o n b u t only e t c h i n g of the s u b s t r a t e . T h i s is consist e n t w i t h the p h e n o m e n a o b s e r v e d w i t h the P E s u b s t r a t e u s i n g l o n g e r t r e a t m e n t times ( m o r e t h a n 30 s) (Fig. 7). As soon as the fluorine c o n t e n t of 140 "~120-
~
.~
100 -
4020 •
t
0
, 1
2
3
,
|
4
5
t(min)
Fig. 5. Polymerization rate of CF4 + 2% H2 + 2% Ar mixture strate (p = 150 Pa; Q = 200 cm3 min 1; p = 170 W).
vs.
treatment time on a PE sub-
375 102 CPS 11,7 9,37,4.
t=0.227 mln
CF
C-C 5,4. CH2-C
F1s/CIs=0.95
3,~*.
01s/Cls=O.05
'~\--4~.~. I,'" ~ 102
,
,
---r B E (ev)
ii,'~ 9,3
]!!'~~ C-CFx
7,5,
t=0,54 rain
5,1" i
Fls/Cls=l 3,4
Ols/Cls=0.05
1,41 io z
ll,t 9,5
7,7
t=1.36 rain
5,8
Fls/Cls=l
3,9 2,1
Ols/Cls=0.04
I
L
(ev) -r BE(ev
102
IO,~ 8,3_ 6,2-
t=2.72 rain
",I_
Els/Cls=0.9
2,0
Ols/Cls=0.1
I
286.5
I
291.5
296.5
E L
(eV)
Fig. 6. Variation of C ls photo-peak of PE film treated for different treatment times in CF4 + 2% H2 discharges (p = 150 Pa; P = 170 W; Q = 200 cm~ min-1). t h e d e p o s i t e d p o l y m e r d e c r e a s e d , t h e n t h i s m a t e r i a l b e g a n to b u i l d u p o n t h e s u r f a c e o f t h e P V D F . W i t h l o n g e r t r e a t m e n t t i m e s ( m o r e t h a n 75 s), u n d e r our experimental conditions, the chemical composition of the deposited f l u o r o c a r b o n film b e c a m e t h e s a m e o n b o t h o f t h e s u b s t r a t e s .
376 6O Polymerization
i4o 20
"~
0 -20 -40 -60
Etching
-80 0.0
0,5
1.0
1.5
|
|
2,0
2,5
t(min)
3,0
F i g . 7. P o l y m e r i z a t i o n r a t e v s . t r e a t m e n t t i m e i n a C F 4 + 2 % H 2 + 2 % A r m i x t u r e s u b s t r a t e ( p = 150 P a ; Q = 200 c m a r a i n a; p = 170 W).
on a PVDF
25, ._~ E
20'
--~ 1 5 '
10,
,
0 0
2
|
4
6
8
10
%H2
12
F i g . 8. P o l y m e r i z a t i o n r a t e o f C F 4 -}-H 2 + 2 % A r m i x t u r e v s . % H 2 i n t h e f e e d o n a P V D F s u b s t r a t e ( p = 150 P a ; Q = 200 c m 3 m i n 1; p = 170 W; t = 2.7 m i n ) .
Therefore, since fluorine contained in the substrate contributes to the growth mechanisms, the optimum hydrogen percentage in the feed (corresponding to the maximum polymer growth rate) will not be the same with a PVDF substrate as with a PE substrate. The polymerization rate on PE is greatest with a hydrogen fraction equal to 2% (Fig. 3), whereas with the fluorine-containing substrate PVDF, the optimum hydrogen proportion is in the order of 4% (Fig. 8). Contact angle measurements carried out on the deposited films by an image-processing system [12] with two liquids (formamide and water) provided the possibility of calculating the surface tension of the deposited films by the Owens and Kaelble method. These results (Fig. 9) confirm the XPS results showing the surface tension was lowest with a feed containing 2% hydrogen, i.e. when fluorine content (F:C ratio) of the film was at a maximum.
4. C o n c l u s i o n
In this work the plasma polymerization of C F 4 + H2 mixtures was studied by means of a corona discharge configuration of electrodes at low pressure
377 50 Total S.T
2~]
DispersiveS.T
10.
Polar S.T
0
20
40
60
so
loo
%H2
Fig. 9. Variation of the surface tension (polar and dispersive components) of the deposited polymer surface on a PE substrate with water and formamide couple v s . % H2 in C F 4 + H 2 + 2 % Ar discharge (p = 150 Pa; Q = 200 cm3 min-1; P = 170 W; f = 70 kHz; t = 2.7 min).
(150 Pa) on two s u b s t r a t e s , P E a n d PVDF. T h e difference in the r e s u l t s confirmed the i m p o r t a n c e of the use of d i a g n o s t i c m e t h o d s s u c h as o p t i c a l e m i s s i o n s p e c t r o s c o p y for the c o n t r o l of the c h e m i c a l a n d e n e r g e t i c properties of the p l a s m a . T h e s e p r o p e r t i e s , a r i s i n g f r o m the c h e m i c a l n a t u r e of the excited species, c a n b e c o m e quite different d e p e n d i n g on the e x c i t a t i o n f r e q u e n c y [7] a n d e l e c t r o d e c o n f i g u r a t i o n [7], as well as the w o r k i n g p r e s s u r e w h i c h will be discussed in a n o t h e r p a p e r in the n e a r future. F r o m the o p t i c a l e m i s s i o n s p e c t r o s c o p y r e s u l t s a r e a c t i o n m e c h a n i s m was p r o p o s e d to e x p l a i n w h a t h a p p e n e d in t h e h o m o g e n e o u s p l a s m a p h a s e w h e n h y d r o g e n was a d d e d to the C F 4 discharge; f u r t h e r m o r e , in the h e t e r o g e neous plasma-substrate system simultaneous polymerization and etching p h e n o m e n a w e r e observed, t h e i r r e l a t i v e r a t e s d e p e n d i n g on the a t o m i c fluorine c o n t e n t of the h e t e r o g e n e o u s p l a s m a - s u b s t r a t e b o u n d a r y layer. T h i s a c c o u n t s for t h e difference o b s e r v e d in the p o l y m e r i z a t i o n r a t e of C F 4 + H2 m i x t u r e s on different s u b s t r a t e s . It h a s t h u s b e e n c l e a r l y s h o w n t h a t the d e p o s i t i o n m e c h a n i s m s of C F 4 + H 2 m i x t u r e s d e p e n d on t h e t r e a t m e n t t i m e a n d the n a t u r e of the s u b s t r a t e on w h i c h t h e y are deposited. I n the case of f l u o r i n e - c o n t a i n i n g s u b s t r a t e PVDF, t h e fluorine c o n s t i t u e n t in the s u b s t r a t e c o n t r i b u t e s to the g r o w t h m e c h a n i s m s . T h e r e f o r e the o p t i m u m h y d r o g e n p e r c e n t a g e in the feed ( c o r r e s p o n d i n g to the m a x i m u m p o l y m e r g r o w t h r a t e ) will n o t be the s a m e for the two s u b s t r a t e s .
A ck n o w l ed g m e nt T h e a u t h o r s w o u l d like to t h a n k Electricit6 de F r a n c e for its financial support.
378
References 1 R. d'Agostino, D. de Benedictis and F. Cramarossa, Plasma Chem. Plasma Process., 4(1) (1984) 1. 2 E. Kay and A. Dilks, Thin Solid Films, 78 (1981) 309. 3 H. Yasuda, in J. L. Vessen and W. Kern (eds.), Thin Film Process, Academic Press, New York, 1978, p. 361. 4 R. d'Agostino, F. Cramarossa, V. Colaprico and R. d'Ettole, J. Appl. Phys., 54(3) (1983) 1284. 5 R. d'Agostino, F. Cramarossa and F. Fracassi, Thin Solid Films, 143 (1986) 163. 6 J. W. Coburn and M. Chen, J. Appl. Phys., 51 (1980) 3134. 7 P. Montazer Rahmati, F. Arefi, J. Amouroux and A. Ricard, Proc. 9th ISPC, (IUPAC), Pugnochiuso, Italy, 1989, p. 1195. 8 S. de Benedictis, A. Gicquel and F. Cramarossa, Proc. 8th ISPC, Tokyo, Japan, 1987, p. 631. 9 M. Capitelli and M. Dilonardo, J. Chem. Phys., 20 (1977) 417. 10 J. C. Polanyi, Acc. Chem. Res., 5 (1972) 161. 11 Von Engel, Electric Plasmas: their Nature and Uses, Taylor and Francis Ltd., London and New York, 1983. 12 P. Montazer Rahmati, F. Arefi and J. Amouroux, French Patent 8807726.