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Influence of the deposition gas on the properties of plasma-deposited carbonized layers
Surface and Coatings Technology, 45 (1991) 353-358
353
Influence of the deposition gas on the properties of plasma-deposited carbonized layers D. B o u t a r d and W. M611er Max-Planck-Institut fiir Plasmaphysik, EURATOM Association, W-8046 Garching (F.R.G.)
Abstract Hard amorphous hydrocarbon films have been deposited by r.f. glow discharges in pure methane as well as methane hydrogen and methane helium mixtures. Helium or hydrogen addition at constant partial pressure of methane results in an increase of the growth rate in comparison with pure methane. Whereas helium is not incorporated into the film, an additional hydrogen uptake is found for hydrogen dilution. Both the film density and the index of refraction can be varied by the use of different gas mixtures. The results are explained qualitatively in terms of ballistic effects resulting from ion bombardment during film growth.
1. I n t r o d u c t i o n H y d r o g e n a t e d a m o r p h o u s c a r b o n films [1 4] s h o w specific p r o p e r t i e s w h i c h are of i n t e r e s t in a n u m b e r of a p p l i c a t i o n s , s u c h as t h e i r use as a n t i r e f l e c t i v e o p t i c a l coatings, p r o t e c t i v e l a y e r s a g a i n s t c h e m i c a l a t t a c k , e l e c t r i c a l l y i n s u l a t i n g films, w e a r - r e s i s t a n t c o a t i n g s a n d low Z i n n e r wall c o a t i n g s in fusion devices. T h e s t r u c t u r a l , c o m p o s i t i o n a l , m e c h a n i c a l a n d o p t i c a l p r o p e r t i e s of t h e s e films are k n o w n to v a r y w i t h i n r a t h e r wide r a n g e s . T h e r e f o r e , in o r d e r to o b t a i n specific a n d well-defined p r o p e r t i e s , it is d e s i r a b l e to c o r r e l a t e the film c h a r a c t e r i s t i c s w i t h the p a r a m e t e r s of the d e p o s i t i o n process. A c r u c i a l role for the q u a l i t y of the r e s u l t i n g films is k n o w n to be due to ion b o m b a r d m e n t d u r i n g film growth. T h e f o r m a t i o n of h i g h q u a l i t y " h a r d " films w i t h a h y d r o g e n c o n t e n t of less t h a n a b o u t 35 a t . % r e q u i r e s ion e n e r g i e s of s e v e r a l h u n d r e d e l e c t r o n v o l t s , w h e r e a s m o r e m o d e r a t e ion b o m b a r d m e n t r e s u l t s in p o l y m e r - l i k e l a y e r s [1]. I o n b o m b a r d m e n t effects r e s u l t f r o m the n u m b e r a n d e n e r g i e s of the i m p i n g i n g ions a n d c a n t h u s be influenced b o t h by a d j u s t m e n t of the p l a s m a p a r a m e t e r s (e.g. t h r o u g h the self-bias v o l t a g e [5] f o r m i n g across the s h e a t h ) a n d by the c o m p o s i t i o n of the p r o c e s s gas. I n the p r e s e n t i n v e s t i g a t i o n the influence of the gas c o m p o s i t i o n on the film p r o p e r t i e s h a s b e e n i n v e s t i g a t e d s y s t e m a t i c a l l y for m i x t u r e s of m e t h a n e , w h i c h is the s i m p l e s t s o u r c e for h y d r o c a r b o n deposition, w i t h h y d r o g e n a n d helium. Elsevier Sequoia/Printed in The Netherlands
354
2. E x p e r i m e n t a l details The films were deposited on the powered electrode in a largely asymmetric r.f. r eac t or at a frequency of 18.1MHz. The electrode, with an area of 25 cm 2, acquired a negative self-bias of 400 _+ 10 V at an r.f. power of 100 W, which was kept constant during all depositions. The discharge was confined around the substrate electrode by means of a grounded cylindrical mesh grid. The substrate t em pe r at ur e during deposition was between 370 and 400 K. The base pressure of the r e a c t or was about 5 × 10 .5 Pa. The total working pressure for the various gas mixtures was measured by means of a capacitance vacuumeter. The flow rates of methane and the added gases were varied by means of individual flow controllers. The methane partial pressures were calibrated against the gas flow by turning off the other gases in the absence of a plasma. Experiments were performed at constant partial pressure of methane and at constant total flow rate. In the latter case the total pressure increases with an increasing proportion of the added gas owing to its lower pumping speed with the turbomolecular pump. The layers were deposited on silicon single-crystal wafers with (111) surface orientation. The film growth rate was monitored in situ using optical interferometry with an He Ne laser. The geometrical thickness of the films was recorded ex situ using a surface profilometer. The areal densities of carbon and hydrogen were assessed by ion beam analysis using proton-enhanced cross-section scattering (PES) at 1.5MeV H ÷ and elastic recoil detection (ERD) with 2.6MeV He + respectively. From the thickness and areal densities the volume density of the films was calculated. The combination of thickness measurement and in situ laser interferometry also delivers the refractive index at 632.8 nm.
3. R e s u l t s In Fig. 1 the growth rates for different m e t h a n e - h y d r o g e n and methane helium mixtures are given as function of the total gas pressure during deposition. For a plasma ignited in pure methane the growth rate increases linearly with the gas pressure. When hydrogen or helium is added to a given partial pressure of methane, the growth rate increases with the total pressure by a factor of up to 2. The use of mixtures with either helium or hydrogen with identical total pressures and methane partial pressures leads to a nearly identical increase of the growth rate. Figure 2 indicates the film composition resulting from pure methane as well as m e t h a n e - h y d r o g e n and methane helium mixtures. For the case of hydrogen addition, both data obtained at constant partial pressure of methane and at constant total gas flow are shown. Whereas hydrogen addition increases the hydrogen content of the resulting film, helium addition is found to even slightly decrease it. The increase of the hydrogen content is independent of the isotopic species of the added hydrogen gas, in contrast to
355
/ 6
/
/
(,,)
s/
5 o
4
//= ~ P a
o o
/
3
o
_c
/ 2
/
o
C5
/
o
1 ~
-
5
PCH4 = 0,7 Pa
10
15
Total pressure
20
25
(Pa)
Fig. 1. Growth rate v s . total pressure of m e t h a n e - h y d r o g e n (open circles) and methane helium (open triangles) gas mixtures for different partial pressures of methane. Each solid line corresponds to a constant partial pressure of methane. The results are compared with the growth rate obtained with pure methane (full circles). (A growth rate of 1015 atoms cm 2 s-1 C corresponds to approximately 1 ~ s - l ) .
• ,[
pure OH 4
CH4+ H2
+x} PCH4 =0"7 Pa PCH4 = 5.4 Pa
CH 4 + H 2 CH4 + D2 CH 4 + He
A
0.8 .0 ._0 E
total flow = 60 sccm
0.7
~
_+/4-""
+
4=
+
06
°IiI (5 E
0.5
-r
0.4
'
x
.,x.......... -"-'x_~-
0.3 O
~ 0
5
10
15
20
25
Total pressure (Pa)
Fig. 2. Film c o m p o s i t i o n resulting from different gas compositions. T h e open symbols are obtained for dilution of m e t h a n e at c o n s t a n t total gas flow with m e t h a n e fractions ranging from 100% to 7%.
356
~
2.5
~6
2.0
1.5
li
C H 4 + H2 I
CH 4 + D2 CH 4 + He
e~ E (J
2.0
g
rO
/
a
1,5
/
/
/
/ 1
2
3
4
5
Total pressure
Fig. 3. Film density and index of refraction for different gas compositions at a constant total flow rate of 60 standard cm3 min 1. a n isotopic effect on the g r o w t h r a t e w h i c h h a s b e e n e s t a b l i s h e d in a p r e v i o u s s t u d y [6]. F i g u r e 3 d e m o n s t r a t e s t h a t the film density a n d the r e f r a c t i v e i n d e x c a n be v a r i e d by p r e s s u r e v a r i a t i o n a n d / o r by a d d i t i o n of h y d r o g e n or h e l i u m w i t h i n r a t h e r wide r a n g e s . B o t h the d e n s i t y a n d the r e f r a c t i v e i n d e x i n c r e a s e w i t h i n c r e a s i n g p r e s s u r e for p l a s m a s w i t h p u r e m e t h a n e . T h e d e n s i t y v a r i e s o n l y a little for h e l i u m a d d i t i o n a n d shows, in view of the e r r o r bars, o n l y a small isotopic effect for h y d r o g e n addition. T h e index of r e f r a c t i o n i n c r e a s e s significantly at i n c r e a s i n g h e l i u m c o n c e n t r a t i o n in the gas, w h e r e a s it d e c r e a s e s for p r o t i u m addition. An i n t e r m e d i a t e b e h a v i o u r is o b s e r v e d for deuterium.
4. D i s c u s s i o n U n d e r the p r e s e n t c o n d i t i o n s of p l a s m a o p e r a t i o n , i.e. at c o n s t a n t selfbias of the s u b s t r a t e a n d v a r y i n g gas p r e s s u r e a n d c o m p o s i t i o n , hydrog e n a t e d c a r b o n films c a n be d e p o s i t e d c o v e r i n g r a t h e r wide r a n g e s of c o m p o s i t i o n , d e n s i t y a n d r e f r a c t i v e index. We find h a r d ( " a - C : H " ) films w i t h a h y d r o g e n - t o - c a r b o n r a t i o a r o u n d 0.4 a n d a c o n t i n u o u s t r a n s i t i o n t o w a r d s m o r e p o l y m e r - l i k e films w i t h a r a t i o of up to 0.7. A t l o w e r p a r t i a l p r e s s u r e s of m e t h a n e a n d w i t h i n c r e a s e d a d d i t i o n of h y d r o g e n the films t e n d to b e c o m e m o r e p o l y m e r like. I n c o n t r a s t , D w o r s c h a k et al. [7] find a n i n c r e a s i n g h y d r o g e n c o n t e n t at i n c r e a s i n g p r e s s u r e w i t h p u r e a c e t y l e n e as s o u r c e gas.
357 With pure methane plasmas the deposition rate increases linearly with the gas pressure in agreement with former findings [8, 9] and is in quantitative agreement with the one reported by Zou et al. [9]. Contradictory results have been reported for the density of the films for different gases. Bubenzer et al. [8] describe the density as scaling with VB/P 1/2, where VB denotes the substrate bias and p the gas pressure, in the case of benzene. Accordingly, Dworschak et al. [7] observe a decreasing density at increasing pressure of acetylene. In contrast, Zou et al. [9] find the density to decrease with increasing bias for methane plasmas. The present work indicates an increasing density with increasing methane pressure. In all the above results the density ranges from 1.5 to 2.1g cm 3. Also, for the refractive index the present results fall into the range from 1.4 to 2.6 which is known for pure hydrocarbon plasmas under different conditions [2, 8, 10]. Even a merely qualitative interpretation of the effects of the gas pressure and its composition on the properties of the resulting films app¢ ~rs rather ambitious at the present state of knowledge. Plasma modelling as well as optical and mass spectrometric diagnostic studies [11-16] suggest t h a t CH3 radicals represent the main species from which the layer is deposited. In addition, hydrocarbon and hydrogen ions, which are accelerated in t h e plasma sheath in front of the substrate, impinge on the surface of the growing film. This ion bombardment is thought to influence the film properties through collisional effects which might lead to bond rearrangements, radiation damage and film densification [16, 17]. However, ion bombardment may also be necessary for the "stitching" of adsorbed radicals to the surface and for the removal of excess hydrogen from the growing film [6, 18]. A change of the total gas pressure may influence both the plasma conditions such as the ion or radical density and the energy of the ions, since the latter may undergo collisions with neutral atoms in the sheath, the number of which increases with increasing pressure. For the pure methane plasma a simple explanation for the increasing hydrogen content of the films at a pressure below 1 P a cannot readily be given. We presume that at low pressures more hydrogenic ions are formed relative to methyl radicals, which contribute by implantation to an increased hydrogen content. This in turn will reduce the density and the index of refraction. With hydrogen addition one has to assume that the hydrocarbon radical density will rather decrease since part of the r.f. power will be consumed for the ionization and dissociation of hydrogen. Nevertheless, the growth rate increases with increasing partial pressure of hydrogen. This finding is consistent with the picture that hydrogen ion bombardment promotes the attachment of adsorbed hydrocarbon radicals to the growing film. (This is also corroborated by recent isotope marker studies employing deuterium and deuterated methane [6].) Simultaneously, the hydrogen content increases, probably owing to an intensified implantation of hydrogen. In addition, the ion-induced release of hydrogen might become less effective for two reasons. (i) As the growth rate increases, any depth interval of the growing surface is
358 subject to ion i r r a d i a t i o n for s h o r t e r times. This, however, c a n n o t be responsible for the very steep i n c r e a s e of the h y d r o g e n c o n c e n t r a t i o n at a m e t h a n e p a r t i a l pressure of 0.7 Pa. (ii) I o n - i n d u c e d release of h y d r o g e n c a n be assumed to be m a i n l y due to the h e a v y h y d r o c a r b o n ions. Thus, at d e c r e a s i n g flux of h y d r o c a r b o n s as m e n t i o n e d above, more h y d r o g e n will r e m a i n in the film. Again, the density decreases with i n c r e a s i n g h y d r o g e n content. W i t h helium a d d i t i o n the c o m p o s i t i o n r e m a i n s c o n s t a n t in c o n t r a s t to h y d r o g e n addition. No decrease of the density is observed in a g r e e m e n t with the finding t h a t h e l i u m is n o t i n c o r p o r a t e d into h a r d h y d r o c a r b o n layers [19]. In c o n t r a s t , the c o m p a r i s o n with the results for pure m e t h a n e (Fig. 3) m i g h t i n d i c a t e a c o m p a c t i o n of the m a t e r i a l due to helium b o m b a r d m e n t . Accordingly, the r e f r a c t i v e index increases at i n c r e a s i n g h e l i u m c o n c e n t r a t i o n of the gas. The i o n - i n d u c e d c o m p a c t i o n a p p e a r s to be s t r o n g e s t for h e l i u m and w e a k e s t for p r o t i u m a d d i t i o n a n d m a y t h u s be a t t r i b u t e d to n u c l e a r collisions of the i n c i d e n t ions with the a t o m s of the g r o w i n g film. I n conclusion, we h a v e s h o w n t h a t the properties of plasma-deposited h y d r o c a r b o n layers c a n be a d j u s t e d in r a t h e r wide r a n g e s by h y d r o g e n or h e l i u m dilution of the process gas. Some of the results can be explained q u a l i t a t i v e l y in terms of ion i m p l a n t a t i o n a n d b o m b a r d m e n t effects. However, the i n t e r p r e t a t i o n r e m a i n s s p e c u l a t i v e as long as f u r t h e r i n f o r m a t i o n , e.g. on fluxes of ions a n d n e u t r a l s w h i c h impinge on the surface of the g r o w i n g film, is n o t available. C o r r e s p o n d i n g experiments are in p r e p a r a t i o n .
References 1 J. C. Angus, P. Koidl and S. Domitz, in J. Mort and F. Jansen (eds.), Plasma Deposited Thin Films, CRC Press,-Boca Raton, FL, 1985. 2 P. Couderc and Y. Catherine, Thin Solid Films, 146 (1987) 93. 3 H. C. Tsai and D B. Bogy, J. Vac. Sci. Technol. A, 5 (1985) 3287. 4 C. Weissmantel, E. Ackermann, K. Bewilogua, G. Hecht, H. Kupfer and B. Rau, J. Vac. Sci. Technol. A, 4 (1986) 2892. 5 J. W. Coburn and E. Kay, J. Appl. Phys., 43 (1972) 4965. 6 D. Boutard and W. M611er, J. Mater. Res., 5 (1990) 2451. 7 W. Dworschak, R. Kleber, A. Fuchs, B. Scheppat, G. Keller, K. Jung and H. Ehrhardt, Thin Solid Films, 89 (1990) 257. 8 A. Bubenzer, B. Dischler, G. Brandt and P. Koidl, J. Appl. Phys., 54 (1983) 4590. 9 J. W. Zou, K. Reichelt, K. Schmidt and B. Dischler, J. Appl. Phys., 65 (1989) 3914. 10 T. J. Moravec and J. C. Lee, J. Vac. Sci. Technol., 20 (1982) 338. 11 K. Tachibana, M. Nishida, H. Harima and Y. Urano, J. Phys. D: Appl. Phys., 17 (1984) 1727. 12 L. E. Kline, W. D. Partlow and W. E. Bies, J. Appl. Phys., 65 (1989) 70. 13 G. Smolinsky and M. J. Vasile, Int. J. Mass Spectrom. Ion Phys., 16 (1975) 137. 14 M. J. Vasile and G. Smolinsky, Int. J. Mass Spectrom. Ion Phys., 18 (1975) 179. 15 H. Toyoda, H. Kojima and H. Sugai, Appl. Phys. Lett., 84 (1979) 157. 16 G. L. Vandentop, M. Kawasaki, R. M. Nix, I. G. Brown, M. Salmeron and G. A. Somorjai, Phys. Rev. B, 41 (1990) 3200. 17 C. Weissmantel, J. Vac. Sci. Technol., 18 (1981) 179. 18 W. M611er and B. M. U. Scherzer, J. Appl. Phys., 64 (1988) 4860. 19 D. Boutard, I. Gudowska, B. M. U. Scherzer and W. M611er, J. Appl. Phys., 68 (1990) 2068.
353
Influence of the deposition gas on the properties of plasma-deposited carbonized layers D. B o u t a r d and W. M611er Max-Planck-Institut fiir Plasmaphysik, EURATOM Association, W-8046 Garching (F.R.G.)
Abstract Hard amorphous hydrocarbon films have been deposited by r.f. glow discharges in pure methane as well as methane hydrogen and methane helium mixtures. Helium or hydrogen addition at constant partial pressure of methane results in an increase of the growth rate in comparison with pure methane. Whereas helium is not incorporated into the film, an additional hydrogen uptake is found for hydrogen dilution. Both the film density and the index of refraction can be varied by the use of different gas mixtures. The results are explained qualitatively in terms of ballistic effects resulting from ion bombardment during film growth.
1. I n t r o d u c t i o n H y d r o g e n a t e d a m o r p h o u s c a r b o n films [1 4] s h o w specific p r o p e r t i e s w h i c h are of i n t e r e s t in a n u m b e r of a p p l i c a t i o n s , s u c h as t h e i r use as a n t i r e f l e c t i v e o p t i c a l coatings, p r o t e c t i v e l a y e r s a g a i n s t c h e m i c a l a t t a c k , e l e c t r i c a l l y i n s u l a t i n g films, w e a r - r e s i s t a n t c o a t i n g s a n d low Z i n n e r wall c o a t i n g s in fusion devices. T h e s t r u c t u r a l , c o m p o s i t i o n a l , m e c h a n i c a l a n d o p t i c a l p r o p e r t i e s of t h e s e films are k n o w n to v a r y w i t h i n r a t h e r wide r a n g e s . T h e r e f o r e , in o r d e r to o b t a i n specific a n d well-defined p r o p e r t i e s , it is d e s i r a b l e to c o r r e l a t e the film c h a r a c t e r i s t i c s w i t h the p a r a m e t e r s of the d e p o s i t i o n process. A c r u c i a l role for the q u a l i t y of the r e s u l t i n g films is k n o w n to be due to ion b o m b a r d m e n t d u r i n g film growth. T h e f o r m a t i o n of h i g h q u a l i t y " h a r d " films w i t h a h y d r o g e n c o n t e n t of less t h a n a b o u t 35 a t . % r e q u i r e s ion e n e r g i e s of s e v e r a l h u n d r e d e l e c t r o n v o l t s , w h e r e a s m o r e m o d e r a t e ion b o m b a r d m e n t r e s u l t s in p o l y m e r - l i k e l a y e r s [1]. I o n b o m b a r d m e n t effects r e s u l t f r o m the n u m b e r a n d e n e r g i e s of the i m p i n g i n g ions a n d c a n t h u s be influenced b o t h by a d j u s t m e n t of the p l a s m a p a r a m e t e r s (e.g. t h r o u g h the self-bias v o l t a g e [5] f o r m i n g across the s h e a t h ) a n d by the c o m p o s i t i o n of the p r o c e s s gas. I n the p r e s e n t i n v e s t i g a t i o n the influence of the gas c o m p o s i t i o n on the film p r o p e r t i e s h a s b e e n i n v e s t i g a t e d s y s t e m a t i c a l l y for m i x t u r e s of m e t h a n e , w h i c h is the s i m p l e s t s o u r c e for h y d r o c a r b o n deposition, w i t h h y d r o g e n a n d helium. Elsevier Sequoia/Printed in The Netherlands
354
2. E x p e r i m e n t a l details The films were deposited on the powered electrode in a largely asymmetric r.f. r eac t or at a frequency of 18.1MHz. The electrode, with an area of 25 cm 2, acquired a negative self-bias of 400 _+ 10 V at an r.f. power of 100 W, which was kept constant during all depositions. The discharge was confined around the substrate electrode by means of a grounded cylindrical mesh grid. The substrate t em pe r at ur e during deposition was between 370 and 400 K. The base pressure of the r e a c t or was about 5 × 10 .5 Pa. The total working pressure for the various gas mixtures was measured by means of a capacitance vacuumeter. The flow rates of methane and the added gases were varied by means of individual flow controllers. The methane partial pressures were calibrated against the gas flow by turning off the other gases in the absence of a plasma. Experiments were performed at constant partial pressure of methane and at constant total flow rate. In the latter case the total pressure increases with an increasing proportion of the added gas owing to its lower pumping speed with the turbomolecular pump. The layers were deposited on silicon single-crystal wafers with (111) surface orientation. The film growth rate was monitored in situ using optical interferometry with an He Ne laser. The geometrical thickness of the films was recorded ex situ using a surface profilometer. The areal densities of carbon and hydrogen were assessed by ion beam analysis using proton-enhanced cross-section scattering (PES) at 1.5MeV H ÷ and elastic recoil detection (ERD) with 2.6MeV He + respectively. From the thickness and areal densities the volume density of the films was calculated. The combination of thickness measurement and in situ laser interferometry also delivers the refractive index at 632.8 nm.
3. R e s u l t s In Fig. 1 the growth rates for different m e t h a n e - h y d r o g e n and methane helium mixtures are given as function of the total gas pressure during deposition. For a plasma ignited in pure methane the growth rate increases linearly with the gas pressure. When hydrogen or helium is added to a given partial pressure of methane, the growth rate increases with the total pressure by a factor of up to 2. The use of mixtures with either helium or hydrogen with identical total pressures and methane partial pressures leads to a nearly identical increase of the growth rate. Figure 2 indicates the film composition resulting from pure methane as well as m e t h a n e - h y d r o g e n and methane helium mixtures. For the case of hydrogen addition, both data obtained at constant partial pressure of methane and at constant total gas flow are shown. Whereas hydrogen addition increases the hydrogen content of the resulting film, helium addition is found to even slightly decrease it. The increase of the hydrogen content is independent of the isotopic species of the added hydrogen gas, in contrast to
355
/ 6
/
/
(,,)
s/
5 o
4
//= ~ P a
o o
/
3
o
_c
/ 2
/
o
C5
/
o
1 ~
-
5
PCH4 = 0,7 Pa
10
15
Total pressure
20
25
(Pa)
Fig. 1. Growth rate v s . total pressure of m e t h a n e - h y d r o g e n (open circles) and methane helium (open triangles) gas mixtures for different partial pressures of methane. Each solid line corresponds to a constant partial pressure of methane. The results are compared with the growth rate obtained with pure methane (full circles). (A growth rate of 1015 atoms cm 2 s-1 C corresponds to approximately 1 ~ s - l ) .
• ,[
pure OH 4
CH4+ H2
+x} PCH4 =0"7 Pa PCH4 = 5.4 Pa
CH 4 + H 2 CH4 + D2 CH 4 + He
A
0.8 .0 ._0 E
total flow = 60 sccm
0.7
~
_+/4-""
+
4=
+
06
°IiI (5 E
0.5
-r
0.4
'
x
.,x.......... -"-'x_~-
0.3 O
~ 0
5
10
15
20
25
Total pressure (Pa)
Fig. 2. Film c o m p o s i t i o n resulting from different gas compositions. T h e open symbols are obtained for dilution of m e t h a n e at c o n s t a n t total gas flow with m e t h a n e fractions ranging from 100% to 7%.
356
~
2.5
~6
2.0
1.5
li
C H 4 + H2 I
CH 4 + D2 CH 4 + He
e~ E (J
2.0
g
rO
/
a
1,5
/
/
/
/ 1
2
3
4
5
Total pressure
Fig. 3. Film density and index of refraction for different gas compositions at a constant total flow rate of 60 standard cm3 min 1. a n isotopic effect on the g r o w t h r a t e w h i c h h a s b e e n e s t a b l i s h e d in a p r e v i o u s s t u d y [6]. F i g u r e 3 d e m o n s t r a t e s t h a t the film density a n d the r e f r a c t i v e i n d e x c a n be v a r i e d by p r e s s u r e v a r i a t i o n a n d / o r by a d d i t i o n of h y d r o g e n or h e l i u m w i t h i n r a t h e r wide r a n g e s . B o t h the d e n s i t y a n d the r e f r a c t i v e i n d e x i n c r e a s e w i t h i n c r e a s i n g p r e s s u r e for p l a s m a s w i t h p u r e m e t h a n e . T h e d e n s i t y v a r i e s o n l y a little for h e l i u m a d d i t i o n a n d shows, in view of the e r r o r bars, o n l y a small isotopic effect for h y d r o g e n addition. T h e index of r e f r a c t i o n i n c r e a s e s significantly at i n c r e a s i n g h e l i u m c o n c e n t r a t i o n in the gas, w h e r e a s it d e c r e a s e s for p r o t i u m addition. An i n t e r m e d i a t e b e h a v i o u r is o b s e r v e d for deuterium.
4. D i s c u s s i o n U n d e r the p r e s e n t c o n d i t i o n s of p l a s m a o p e r a t i o n , i.e. at c o n s t a n t selfbias of the s u b s t r a t e a n d v a r y i n g gas p r e s s u r e a n d c o m p o s i t i o n , hydrog e n a t e d c a r b o n films c a n be d e p o s i t e d c o v e r i n g r a t h e r wide r a n g e s of c o m p o s i t i o n , d e n s i t y a n d r e f r a c t i v e index. We find h a r d ( " a - C : H " ) films w i t h a h y d r o g e n - t o - c a r b o n r a t i o a r o u n d 0.4 a n d a c o n t i n u o u s t r a n s i t i o n t o w a r d s m o r e p o l y m e r - l i k e films w i t h a r a t i o of up to 0.7. A t l o w e r p a r t i a l p r e s s u r e s of m e t h a n e a n d w i t h i n c r e a s e d a d d i t i o n of h y d r o g e n the films t e n d to b e c o m e m o r e p o l y m e r like. I n c o n t r a s t , D w o r s c h a k et al. [7] find a n i n c r e a s i n g h y d r o g e n c o n t e n t at i n c r e a s i n g p r e s s u r e w i t h p u r e a c e t y l e n e as s o u r c e gas.
357 With pure methane plasmas the deposition rate increases linearly with the gas pressure in agreement with former findings [8, 9] and is in quantitative agreement with the one reported by Zou et al. [9]. Contradictory results have been reported for the density of the films for different gases. Bubenzer et al. [8] describe the density as scaling with VB/P 1/2, where VB denotes the substrate bias and p the gas pressure, in the case of benzene. Accordingly, Dworschak et al. [7] observe a decreasing density at increasing pressure of acetylene. In contrast, Zou et al. [9] find the density to decrease with increasing bias for methane plasmas. The present work indicates an increasing density with increasing methane pressure. In all the above results the density ranges from 1.5 to 2.1g cm 3. Also, for the refractive index the present results fall into the range from 1.4 to 2.6 which is known for pure hydrocarbon plasmas under different conditions [2, 8, 10]. Even a merely qualitative interpretation of the effects of the gas pressure and its composition on the properties of the resulting films app¢ ~rs rather ambitious at the present state of knowledge. Plasma modelling as well as optical and mass spectrometric diagnostic studies [11-16] suggest t h a t CH3 radicals represent the main species from which the layer is deposited. In addition, hydrocarbon and hydrogen ions, which are accelerated in t h e plasma sheath in front of the substrate, impinge on the surface of the growing film. This ion bombardment is thought to influence the film properties through collisional effects which might lead to bond rearrangements, radiation damage and film densification [16, 17]. However, ion bombardment may also be necessary for the "stitching" of adsorbed radicals to the surface and for the removal of excess hydrogen from the growing film [6, 18]. A change of the total gas pressure may influence both the plasma conditions such as the ion or radical density and the energy of the ions, since the latter may undergo collisions with neutral atoms in the sheath, the number of which increases with increasing pressure. For the pure methane plasma a simple explanation for the increasing hydrogen content of the films at a pressure below 1 P a cannot readily be given. We presume that at low pressures more hydrogenic ions are formed relative to methyl radicals, which contribute by implantation to an increased hydrogen content. This in turn will reduce the density and the index of refraction. With hydrogen addition one has to assume that the hydrocarbon radical density will rather decrease since part of the r.f. power will be consumed for the ionization and dissociation of hydrogen. Nevertheless, the growth rate increases with increasing partial pressure of hydrogen. This finding is consistent with the picture that hydrogen ion bombardment promotes the attachment of adsorbed hydrocarbon radicals to the growing film. (This is also corroborated by recent isotope marker studies employing deuterium and deuterated methane [6].) Simultaneously, the hydrogen content increases, probably owing to an intensified implantation of hydrogen. In addition, the ion-induced release of hydrogen might become less effective for two reasons. (i) As the growth rate increases, any depth interval of the growing surface is
358 subject to ion i r r a d i a t i o n for s h o r t e r times. This, however, c a n n o t be responsible for the very steep i n c r e a s e of the h y d r o g e n c o n c e n t r a t i o n at a m e t h a n e p a r t i a l pressure of 0.7 Pa. (ii) I o n - i n d u c e d release of h y d r o g e n c a n be assumed to be m a i n l y due to the h e a v y h y d r o c a r b o n ions. Thus, at d e c r e a s i n g flux of h y d r o c a r b o n s as m e n t i o n e d above, more h y d r o g e n will r e m a i n in the film. Again, the density decreases with i n c r e a s i n g h y d r o g e n content. W i t h helium a d d i t i o n the c o m p o s i t i o n r e m a i n s c o n s t a n t in c o n t r a s t to h y d r o g e n addition. No decrease of the density is observed in a g r e e m e n t with the finding t h a t h e l i u m is n o t i n c o r p o r a t e d into h a r d h y d r o c a r b o n layers [19]. In c o n t r a s t , the c o m p a r i s o n with the results for pure m e t h a n e (Fig. 3) m i g h t i n d i c a t e a c o m p a c t i o n of the m a t e r i a l due to helium b o m b a r d m e n t . Accordingly, the r e f r a c t i v e index increases at i n c r e a s i n g h e l i u m c o n c e n t r a t i o n of the gas. The i o n - i n d u c e d c o m p a c t i o n a p p e a r s to be s t r o n g e s t for h e l i u m and w e a k e s t for p r o t i u m a d d i t i o n a n d m a y t h u s be a t t r i b u t e d to n u c l e a r collisions of the i n c i d e n t ions with the a t o m s of the g r o w i n g film. I n conclusion, we h a v e s h o w n t h a t the properties of plasma-deposited h y d r o c a r b o n layers c a n be a d j u s t e d in r a t h e r wide r a n g e s by h y d r o g e n or h e l i u m dilution of the process gas. Some of the results can be explained q u a l i t a t i v e l y in terms of ion i m p l a n t a t i o n a n d b o m b a r d m e n t effects. However, the i n t e r p r e t a t i o n r e m a i n s s p e c u l a t i v e as long as f u r t h e r i n f o r m a t i o n , e.g. on fluxes of ions a n d n e u t r a l s w h i c h impinge on the surface of the g r o w i n g film, is n o t available. C o r r e s p o n d i n g experiments are in p r e p a r a t i o n .
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