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Deposition of dense C:H films at elevated substrate temperature
Diamond and Related Materials, 2 (1993) 251-254
251
Deposition of dense C" H films at elevated substrate temperature A. von Keudell, W. M611er and R. Hytry Max-Planck-lnstitut ffir Plasmaphysik, EURATOM Association, W-8046 Garching (Germany)
Abstract C : H layers were prepared by using an electron cyclotron resonance (ECR) plasma from methane. The temperature of the substrate was varied up to 700 K at a gas pressure of 1.6 Pa. Despite low ion energies in the range of the order of the plasma potential, transparent C : H films were obtained at elevated temperature with a density of up to 2 g cm- 3. At a substrate temperature of 700 K during deposition the density of the films decreases with increasing density of atomic hydrogen in the plasma. A model is proposed to explain this change in density, which is supported by optical spectroscopy and measurements of the H : C ratio and the density of the films.
1. Introduction Hydrocarbon films are widely used as hard and IRtransparent coatings, but the deposition process of these layers is at present not well understood [1, 2]. Nevertheless, it is generally agreed that high ion energies (several hundred electronvolts or more) are necessary to obtain dense hydrocarbon films, as known from glow discharge experiments [3, 4]. However, recent results of Beulens et al. [5] indicate that equivalent film properties can be observed using very low ion energies (less than 1 eV) but very high fluxes with a cascaded arc plasma. This raises the question of whether effects other than ion bombardment, such as chemical reactions on the film surface, might be decisive for the film 'structure and composition. To study this question, we investigated the deposition of hydrocarbon films on heated substrates by using low ion energies in an electron cyclotron resonance (ECR) plasma. At high substrate temperatures the residence time of species on the surface is low, leading to an uncovered surface. This low coverage favours direct reactions of atomic hydrogen from the plasma with the growing layer and deposition via direct incorporation of the ions. The present paper investigates the structural change in the films due to these mechanisms.
2. Experimental details An ECR plasma from methane was used for the deposition of C : H layers on Si substrates. The plasma was generated in a rectangular standard waveguide (cross-section 7.2 cm × 3.4 cm) and was heated by microwaves in the TElo mode with a frequency of 2.45 GHz.
0925-9635/93/$6.00
A magnetic field (maximum about 40 mT) was applied in the direction of the waveguide. The transmitted and reflected powers were measured by directional couplers and the size of the plasma was estimated by observing the emission light. From these results the absorbed power per volume in the plasma was calculated. The substrate was placed on the inner wall of the waveguide. During deposition no additional biasing of the substrate was applied. Therefore the energy of the ions impinging on the growing layer reflects the floating potential in the range of the order of 10eV. The gas flow was measured by flow controllers and was about 14 sccm (standard cm 3 min- 1). The temperature of the substrate was varied from room temperature to 700 K by heating the vacuum vessel and measuring the temperature of the vessel with thermoelements. The change in film properties was investigated for films deposited at a pressure of 1.6 Pa, a microwave power input of 0.04 W cm- 3 and a substrate temperature of 700 K. The deposition rate was determined e x situ by measuring the thickness of the layers with a mechanical profilometer. The deposition rate was calculated from the thickness and the deposition time. The hydrogen and carbon contents of the films were determined by 2.6 MeV 4He+ elastic recoil detection (ERD) and 1.5 MeV proton-enhanced scattering (PES) respectively. From these data and the thickness of the films the density and the H : C ratio were calculated. The optical constants of the films were determined in the photon energy range 1.5-6 eV by analysis of reflectance data. A spectrometer (Perkin-Elmer Lambda 9) was used to measure the reflectance of the films near normal incidence relative to the measurement of an uncoated silicon substrate. With an optical model of this system the optical constants can be calculated from these data [6].
© 1993 - - Elsevier Sequoia. All rights reserved
252
A. yon Keudell et al. / Deposition of dense C.'H films
3. Results
i0 16
1.0 -
For the deposition of hydrocarbon films an adsorbed layer model has been proposed [7-9!. According to this model, the deposition rate is governed by the temperature-dependent coverage of the substrate with physisorbed neutral species from the plasma. Neutral species colliding with the substrate form an adsorbed layer by an attractive dipole-dipole interaction between the substrate and the adsorbed species. The reaction of atomic hydrogen from the plasma with the physisorbed species lowers the coverage, but the properties of the growing layer itself are not influenced. These neutral species cannot be chemisorbed immediately owing to their low translational energy, which is insufficient to overcome the activation barrier of a chemical bond. After a characteristic time these physisorbed species can be chemisorbed by the energy transfer from ions to the adsorbed species ("ion stitching"). The ions are accelerated in the plasma sheath and transfer their translational energy to the physisorbed molecules. This leads to the formation of new chemical bonds between the adsorbate and the growing layer. As the temperature increases, the residence time of the physisorbed species decreases exponentially. The rate of cross-linking by ions decreases with increasing coverage and thus the deposition rate is reduced with increasing temperature. At high substrate temperatures the coverage is close to zero and deposition by direct incorporation of ions becomes dominant. In this~ temperature region the surface is uncovered and the ~atomic hydrogen in the plasma reacts directly with the growing layer and influences the film structure. These deposition processes can be modelled by balance equations for the density of carbon atoms in an adsorbed layer and in the bulk. The fluxes of particles colliding with the surface are calculated simultaneously using a plasma model similar to that of Behringer [10]. The cross-sections for deposition and etching reactions were determined by fitting them to the measured temperature-dependent growth rate. The deposition via an adsorbate and the deposition via direct incorporation of ions can be identified as shown in Fig. I. It can be seen that at a substrate temperature above 450K the coverage is negligible and direct incorporation of ions becomes the main contribution to the deposition process. To study the interaction of atomic hydrogen with the growing layer in this temperature region, we varied the concentration of atomic hydrogen in the gas phase. This is achieved by variation of the microwave power input and the dwell time of the gas molecules in the plasma. A decrease in the absorbed energy per volume leads to a lower dissociation of methane and thus a decreasing density of atomic hydrogen in the plasma. The depen-
~o
8 i 0 1~_
_q~
---
DeposiUon via Adsorbate
....
Deposition via Ions
---
S,~a~__~Cov_~rage_
~
-
v
Model
-
.0.8
6 10as-
"0.6
4 10 is_
- 0.4
~
T
....'"
... 0
. . % ~..... .
~ ' " ' i ' 300 400
I
'--J
~ '
500
Temperature
~ t • • 600
0.0 700
(K)
Fig. 1. Comparison of measured deposition rates with model results (solid curve). The dominant deposition process changes at a temperature of about 425 K. The constant deposition rate above this temperature cannot be explained by the deposition via an adsorbed layer.
dence of the density of the hydrocarbon layers and the H : C ratio on the absorbed energy per volume in the plasma is shown in Fig. 2. The density of the hydrocarbon films increases from 0.8 to 2.2gcm -3 with a decrease in the absorbed energy per volume in the plasma. This leads to the conclusion that the density of the films depends strongly on the density of atomic hydrogen in the plasma due to chemical reactions on the surface. In contrast, the H : C ratio remains nearly constant at 0.2 and is independent of the absorbed energy, per volume in the plasma. Further information about the microstructure is obtained through the optical properties. The results are
3 o
Density
•
h/c-r~uo
2:1Itt
0.5 o14
- 0 . 3 ~l
-0.2 ~z I -0.I 0.5 0
0.01
I
I
0.02
0.03
0 0.04
A b s o r b e d E n e r g y ( J c m -3) Fig. 2. The deposition at 700 K is very sensitive to the surface reactions of the bulk ~vith the hydrogen in the plasma. A low hydrogen density leads to a dense hydrocarbon film. This result shows that dense hydrocarbon films can be produced by plasma-enhanced chemical vapour deposition without using high ion, energies.
A. yon Keudell et al. / Deposition of dense C : H films 2.5
l m d e n s i t y 2.2 g e m -a r.
2
© re
1.5 f i l m d e n s i t y 0.8 g c m
1 P h o t o n Energy (eV) Fig. 3. A high density of atomic hydrogen in the plasma leads to a graphite-like structure of the films with a low density. The index of refraction for these films decreases with Ephoton, similar to crystalline graphite. A low density of atomic hydrogen in the plasma leads to a diamond-like structure of the films with a high density. The index of refraction for these films is increasing, similar to crystalline diamond.
illustrated in Fig. 3. The slope of the index of refraction depends on the density of the films. For dense films the index of refraction is about 2 and an increase is observed with increasing photon energy. For films with a low density the index of refraction is about 1.5 and decreases with increasing photon energy.
4. Discussion
A prerequisite for an explanation of the observed structural changes in the films is a discussion of the possible surface reactions. As mentioned before, the surface is not covered with physisorbed species at high substrate temperatures, so the atomic hydrogen can react directly with the film surface and thus influence the film properties. This cannot be observed at room temperature, because the atomic hydrogen reacts only with the precursor for film growth in this temperature region and thus the growing layer is protected by the adsorbate. The structure of hydrogenated amorphous carbon films is mainly determined by the carbon bonds [11, 12]. For low temperature deposition a long-range local ordering may be impeded owing to a high concentration of hydrogen and a low mobility of the incorporated species. At elevated temperature, however, an increased local ordering may be expected owing to a decreasing hydrogen content and an increasing mobility. In this ordered structure the remaining hydrogen should mainly be found at the grain boundaries. Because of the low ion energies, this ordering is only little influenced by collision processes. To understand
253
this process of ordering, we want to discuss the different possibilities in the formation of such a network depending on the density of atomic hydrogen in the plasma. A high density of atomic hydrogen in the plasma results in a high probability of abstracting hydrogen atoms from the surface by formation of Ha molecules. This creates a large number of dangling bonds. These dangling bonds in turn can be saturated with atomic hydrogen. However, the dominant process will be the recombination of the dangling bonds themselves. This recombination changes the hybridization of the incorporated carbon atoms, leading to the formation of graphitic planes. Because of the stability of these graphitic planes, a further incorporation of carbon atoms is favoured at the edges of the planes. A destruction of these planes by the impinging ions is less probable owing to the low ion energy. A new graphitic plane is formed if an incoming carbon atom cannot find an adsorption place on an edge of a graphitic plane. Owing to the low translational energy of the impinging ions, no preferential orientation of these planes is expected. Therefore the network of graphitic planes with various orientations leads to films with lower density than pure graphite. This proposed structural model is supported by the observed slope of the index of refraction as shown in Fig. 3. This slope is similar to the decrease in the index of refraction in pure graphite. The value of the index of refraction is lower than for crystalline graphite 1,13]. This result corresponds to the low density of these graphite-like films, which is also lower compared with the density of pure graphite. From this viewpoint the optical measurements are consistent with the proposed structure and the measured density of the films. If the density of atomic hydrogen in the plasma is low, the surface is covered by a large amount of bonded hydrogen. Owing to the chemical protection afforded by this hydrogen, the hybridization of the carbon atoms incorporated on the surface is preserved. For the incorporation of ions colliding with the surface, it is only necessary to break C - H bonds on the surface. This will probably lead to a diamond-like structure, since the hybridization of the incorporated species cannot be changed by the recombination of dangling bonds. The density of this ordered structure is higher because of the three-dimensional shape of the resulting grains and their small average distance as compared with graphitic planes with various orientations. This formation of a more diamond-like structure is also supported by the slope of the index of refraction, which is increasing with increasing photon energy. The slope and the absolute value are similar to those for crystalline diamond 1,14]. This result is also in agreement with the relatively high densities of these films.
254
A. yon Keudell et al. / Deposition of dense C:H films
5. Summary
References
We have shown that dense hydrocarbon films can be prepared using a plasma process involving very low ion energy by a proper choice of process parameters such as substrate temperature and absorbed energy in the plasma. A surface reaction model has been proposed which explains the structural changes in the films due to the surface reactions with atomic hydrogen. It has been shown that in a deposition process at elevated substrate temperature and low ion energy a structural ordering can occur, leading to a diamond-like or graphite-like structure depending on the surface reactions with atomic hydrogen.
1 J. C. Angus, P. Koidl and S. Domitz, in J. Mort and F. Janson (eds.), Plasma Deposited Thin Films, CRC Press, Boca Raton, FL, 1986, p. 89. 2 Y. Catherine, Mater. Sei. Forum, 52-53 (1989) 175. 3 C. V. Deshpendey and R. F. Bunshah, J. Vac. Sei. Technol. A, 7 (1989) 2294. 4 A. Bubenzer, B. Dischler, G, Branat and P. Koidl, J. Appl. Phys., 54 (1983) 4590. 5 J. J. Beulens, A. J. M. Bunron and D. C. Schram, Surf. Coat. Technol., 47 (1991) 407. 6 N. Savvides, J. Appl. Phys., 59 (1986) 4133. 7 H. Deutsch, H. Kersten, S. Klagge and A. Rutscher, Contrib. Plasma Phys., 28 (1988) 149. 8 H. Kersten and G. M. W. Kroesen, J. Vac. Sei. Technol. A, 8 (1990) 38. 9 A. yon Keudell, W. M611er and R. Hytry, J. Appl. Phys. Lett., in press. 10 K. Behringer, Plasma Phys. Control. Fusion, 33 (1991) 997. 11 J. C. Angus and F. Jensen, d. Vac. Sei. Technol. A, 6 (1988) 1778. 12 J. Robertson, Surf Coat. Teehnol., 50 (1992) 185. 13 E. A. Taft and H. R. Philips, Phys. Rev., 138 (1965) A197. 14 H. R. Phillip and E. A. Taft, Phys. Rev., 127 (1962) 159.
Acknowledgments We thank G. Kerkloh for technical support and T. Langhoff and W. Fukarek for the ERD and PES measurements. Furthermore, we thank P. Reinke for helpful discussions.
251
Deposition of dense C" H films at elevated substrate temperature A. von Keudell, W. M611er and R. Hytry Max-Planck-lnstitut ffir Plasmaphysik, EURATOM Association, W-8046 Garching (Germany)
Abstract C : H layers were prepared by using an electron cyclotron resonance (ECR) plasma from methane. The temperature of the substrate was varied up to 700 K at a gas pressure of 1.6 Pa. Despite low ion energies in the range of the order of the plasma potential, transparent C : H films were obtained at elevated temperature with a density of up to 2 g cm- 3. At a substrate temperature of 700 K during deposition the density of the films decreases with increasing density of atomic hydrogen in the plasma. A model is proposed to explain this change in density, which is supported by optical spectroscopy and measurements of the H : C ratio and the density of the films.
1. Introduction Hydrocarbon films are widely used as hard and IRtransparent coatings, but the deposition process of these layers is at present not well understood [1, 2]. Nevertheless, it is generally agreed that high ion energies (several hundred electronvolts or more) are necessary to obtain dense hydrocarbon films, as known from glow discharge experiments [3, 4]. However, recent results of Beulens et al. [5] indicate that equivalent film properties can be observed using very low ion energies (less than 1 eV) but very high fluxes with a cascaded arc plasma. This raises the question of whether effects other than ion bombardment, such as chemical reactions on the film surface, might be decisive for the film 'structure and composition. To study this question, we investigated the deposition of hydrocarbon films on heated substrates by using low ion energies in an electron cyclotron resonance (ECR) plasma. At high substrate temperatures the residence time of species on the surface is low, leading to an uncovered surface. This low coverage favours direct reactions of atomic hydrogen from the plasma with the growing layer and deposition via direct incorporation of the ions. The present paper investigates the structural change in the films due to these mechanisms.
2. Experimental details An ECR plasma from methane was used for the deposition of C : H layers on Si substrates. The plasma was generated in a rectangular standard waveguide (cross-section 7.2 cm × 3.4 cm) and was heated by microwaves in the TElo mode with a frequency of 2.45 GHz.
0925-9635/93/$6.00
A magnetic field (maximum about 40 mT) was applied in the direction of the waveguide. The transmitted and reflected powers were measured by directional couplers and the size of the plasma was estimated by observing the emission light. From these results the absorbed power per volume in the plasma was calculated. The substrate was placed on the inner wall of the waveguide. During deposition no additional biasing of the substrate was applied. Therefore the energy of the ions impinging on the growing layer reflects the floating potential in the range of the order of 10eV. The gas flow was measured by flow controllers and was about 14 sccm (standard cm 3 min- 1). The temperature of the substrate was varied from room temperature to 700 K by heating the vacuum vessel and measuring the temperature of the vessel with thermoelements. The change in film properties was investigated for films deposited at a pressure of 1.6 Pa, a microwave power input of 0.04 W cm- 3 and a substrate temperature of 700 K. The deposition rate was determined e x situ by measuring the thickness of the layers with a mechanical profilometer. The deposition rate was calculated from the thickness and the deposition time. The hydrogen and carbon contents of the films were determined by 2.6 MeV 4He+ elastic recoil detection (ERD) and 1.5 MeV proton-enhanced scattering (PES) respectively. From these data and the thickness of the films the density and the H : C ratio were calculated. The optical constants of the films were determined in the photon energy range 1.5-6 eV by analysis of reflectance data. A spectrometer (Perkin-Elmer Lambda 9) was used to measure the reflectance of the films near normal incidence relative to the measurement of an uncoated silicon substrate. With an optical model of this system the optical constants can be calculated from these data [6].
© 1993 - - Elsevier Sequoia. All rights reserved
252
A. yon Keudell et al. / Deposition of dense C.'H films
3. Results
i0 16
1.0 -
For the deposition of hydrocarbon films an adsorbed layer model has been proposed [7-9!. According to this model, the deposition rate is governed by the temperature-dependent coverage of the substrate with physisorbed neutral species from the plasma. Neutral species colliding with the substrate form an adsorbed layer by an attractive dipole-dipole interaction between the substrate and the adsorbed species. The reaction of atomic hydrogen from the plasma with the physisorbed species lowers the coverage, but the properties of the growing layer itself are not influenced. These neutral species cannot be chemisorbed immediately owing to their low translational energy, which is insufficient to overcome the activation barrier of a chemical bond. After a characteristic time these physisorbed species can be chemisorbed by the energy transfer from ions to the adsorbed species ("ion stitching"). The ions are accelerated in the plasma sheath and transfer their translational energy to the physisorbed molecules. This leads to the formation of new chemical bonds between the adsorbate and the growing layer. As the temperature increases, the residence time of the physisorbed species decreases exponentially. The rate of cross-linking by ions decreases with increasing coverage and thus the deposition rate is reduced with increasing temperature. At high substrate temperatures the coverage is close to zero and deposition by direct incorporation of ions becomes dominant. In this~ temperature region the surface is uncovered and the ~atomic hydrogen in the plasma reacts directly with the growing layer and influences the film structure. These deposition processes can be modelled by balance equations for the density of carbon atoms in an adsorbed layer and in the bulk. The fluxes of particles colliding with the surface are calculated simultaneously using a plasma model similar to that of Behringer [10]. The cross-sections for deposition and etching reactions were determined by fitting them to the measured temperature-dependent growth rate. The deposition via an adsorbate and the deposition via direct incorporation of ions can be identified as shown in Fig. I. It can be seen that at a substrate temperature above 450K the coverage is negligible and direct incorporation of ions becomes the main contribution to the deposition process. To study the interaction of atomic hydrogen with the growing layer in this temperature region, we varied the concentration of atomic hydrogen in the gas phase. This is achieved by variation of the microwave power input and the dwell time of the gas molecules in the plasma. A decrease in the absorbed energy per volume leads to a lower dissociation of methane and thus a decreasing density of atomic hydrogen in the plasma. The depen-
~o
8 i 0 1~_
_q~
---
DeposiUon via Adsorbate
....
Deposition via Ions
---
S,~a~__~Cov_~rage_
~
-
v
Model
-
.0.8
6 10as-
"0.6
4 10 is_
- 0.4
~
T
....'"
... 0
. . % ~..... .
~ ' " ' i ' 300 400
I
'--J
~ '
500
Temperature
~ t • • 600
0.0 700
(K)
Fig. 1. Comparison of measured deposition rates with model results (solid curve). The dominant deposition process changes at a temperature of about 425 K. The constant deposition rate above this temperature cannot be explained by the deposition via an adsorbed layer.
dence of the density of the hydrocarbon layers and the H : C ratio on the absorbed energy per volume in the plasma is shown in Fig. 2. The density of the hydrocarbon films increases from 0.8 to 2.2gcm -3 with a decrease in the absorbed energy per volume in the plasma. This leads to the conclusion that the density of the films depends strongly on the density of atomic hydrogen in the plasma due to chemical reactions on the surface. In contrast, the H : C ratio remains nearly constant at 0.2 and is independent of the absorbed energy, per volume in the plasma. Further information about the microstructure is obtained through the optical properties. The results are
3 o
Density
•
h/c-r~uo
2:1Itt
0.5 o14
- 0 . 3 ~l
-0.2 ~z I -0.I 0.5 0
0.01
I
I
0.02
0.03
0 0.04
A b s o r b e d E n e r g y ( J c m -3) Fig. 2. The deposition at 700 K is very sensitive to the surface reactions of the bulk ~vith the hydrogen in the plasma. A low hydrogen density leads to a dense hydrocarbon film. This result shows that dense hydrocarbon films can be produced by plasma-enhanced chemical vapour deposition without using high ion, energies.
A. yon Keudell et al. / Deposition of dense C : H films 2.5
l m d e n s i t y 2.2 g e m -a r.
2
© re
1.5 f i l m d e n s i t y 0.8 g c m
1 P h o t o n Energy (eV) Fig. 3. A high density of atomic hydrogen in the plasma leads to a graphite-like structure of the films with a low density. The index of refraction for these films decreases with Ephoton, similar to crystalline graphite. A low density of atomic hydrogen in the plasma leads to a diamond-like structure of the films with a high density. The index of refraction for these films is increasing, similar to crystalline diamond.
illustrated in Fig. 3. The slope of the index of refraction depends on the density of the films. For dense films the index of refraction is about 2 and an increase is observed with increasing photon energy. For films with a low density the index of refraction is about 1.5 and decreases with increasing photon energy.
4. Discussion
A prerequisite for an explanation of the observed structural changes in the films is a discussion of the possible surface reactions. As mentioned before, the surface is not covered with physisorbed species at high substrate temperatures, so the atomic hydrogen can react directly with the film surface and thus influence the film properties. This cannot be observed at room temperature, because the atomic hydrogen reacts only with the precursor for film growth in this temperature region and thus the growing layer is protected by the adsorbate. The structure of hydrogenated amorphous carbon films is mainly determined by the carbon bonds [11, 12]. For low temperature deposition a long-range local ordering may be impeded owing to a high concentration of hydrogen and a low mobility of the incorporated species. At elevated temperature, however, an increased local ordering may be expected owing to a decreasing hydrogen content and an increasing mobility. In this ordered structure the remaining hydrogen should mainly be found at the grain boundaries. Because of the low ion energies, this ordering is only little influenced by collision processes. To understand
253
this process of ordering, we want to discuss the different possibilities in the formation of such a network depending on the density of atomic hydrogen in the plasma. A high density of atomic hydrogen in the plasma results in a high probability of abstracting hydrogen atoms from the surface by formation of Ha molecules. This creates a large number of dangling bonds. These dangling bonds in turn can be saturated with atomic hydrogen. However, the dominant process will be the recombination of the dangling bonds themselves. This recombination changes the hybridization of the incorporated carbon atoms, leading to the formation of graphitic planes. Because of the stability of these graphitic planes, a further incorporation of carbon atoms is favoured at the edges of the planes. A destruction of these planes by the impinging ions is less probable owing to the low ion energy. A new graphitic plane is formed if an incoming carbon atom cannot find an adsorption place on an edge of a graphitic plane. Owing to the low translational energy of the impinging ions, no preferential orientation of these planes is expected. Therefore the network of graphitic planes with various orientations leads to films with lower density than pure graphite. This proposed structural model is supported by the observed slope of the index of refraction as shown in Fig. 3. This slope is similar to the decrease in the index of refraction in pure graphite. The value of the index of refraction is lower than for crystalline graphite 1,13]. This result corresponds to the low density of these graphite-like films, which is also lower compared with the density of pure graphite. From this viewpoint the optical measurements are consistent with the proposed structure and the measured density of the films. If the density of atomic hydrogen in the plasma is low, the surface is covered by a large amount of bonded hydrogen. Owing to the chemical protection afforded by this hydrogen, the hybridization of the carbon atoms incorporated on the surface is preserved. For the incorporation of ions colliding with the surface, it is only necessary to break C - H bonds on the surface. This will probably lead to a diamond-like structure, since the hybridization of the incorporated species cannot be changed by the recombination of dangling bonds. The density of this ordered structure is higher because of the three-dimensional shape of the resulting grains and their small average distance as compared with graphitic planes with various orientations. This formation of a more diamond-like structure is also supported by the slope of the index of refraction, which is increasing with increasing photon energy. The slope and the absolute value are similar to those for crystalline diamond 1,14]. This result is also in agreement with the relatively high densities of these films.
254
A. yon Keudell et al. / Deposition of dense C:H films
5. Summary
References
We have shown that dense hydrocarbon films can be prepared using a plasma process involving very low ion energy by a proper choice of process parameters such as substrate temperature and absorbed energy in the plasma. A surface reaction model has been proposed which explains the structural changes in the films due to the surface reactions with atomic hydrogen. It has been shown that in a deposition process at elevated substrate temperature and low ion energy a structural ordering can occur, leading to a diamond-like or graphite-like structure depending on the surface reactions with atomic hydrogen.
1 J. C. Angus, P. Koidl and S. Domitz, in J. Mort and F. Janson (eds.), Plasma Deposited Thin Films, CRC Press, Boca Raton, FL, 1986, p. 89. 2 Y. Catherine, Mater. Sei. Forum, 52-53 (1989) 175. 3 C. V. Deshpendey and R. F. Bunshah, J. Vac. Sei. Technol. A, 7 (1989) 2294. 4 A. Bubenzer, B. Dischler, G, Branat and P. Koidl, J. Appl. Phys., 54 (1983) 4590. 5 J. J. Beulens, A. J. M. Bunron and D. C. Schram, Surf. Coat. Technol., 47 (1991) 407. 6 N. Savvides, J. Appl. Phys., 59 (1986) 4133. 7 H. Deutsch, H. Kersten, S. Klagge and A. Rutscher, Contrib. Plasma Phys., 28 (1988) 149. 8 H. Kersten and G. M. W. Kroesen, J. Vac. Sei. Technol. A, 8 (1990) 38. 9 A. yon Keudell, W. M611er and R. Hytry, J. Appl. Phys. Lett., in press. 10 K. Behringer, Plasma Phys. Control. Fusion, 33 (1991) 997. 11 J. C. Angus and F. Jensen, d. Vac. Sei. Technol. A, 6 (1988) 1778. 12 J. Robertson, Surf Coat. Teehnol., 50 (1992) 185. 13 E. A. Taft and H. R. Philips, Phys. Rev., 138 (1965) A197. 14 H. R. Phillip and E. A. Taft, Phys. Rev., 127 (1962) 159.
Acknowledgments We thank G. Kerkloh for technical support and T. Langhoff and W. Fukarek for the ERD and PES measurements. Furthermore, we thank P. Reinke for helpful discussions.