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Deposition of a-C:H films with an ECWR-reactor at 27 MHz: plasma diagnostics and film properties
Surface and Coatings Technology 142᎐144 Ž2001. 342᎐347
Deposition of a-C:H films with an ECWR-reactor at 27 MHz: plasma diagnostics and film properties P. Awakowicz a,U , R. Schwefel a , P. Scheubert a , G. Benstetter b a
Chair for Physic of Electrotechnology, Uni¨ ersity of Technology, Munich, Germany b Uni¨ ersity of Applied Sciencesr FH Deggendorf, Deggendorf, Germany
Abstract For the first time, an electron᎐cyclotron-wave resonance ŽECWR. source was used to deposit thin amorphous hydrocarbon Ža-C:H. films. The deposition experiments have been supported by intensive plasma diagnostics with Langmuir probe ŽLP. measurements and energy mass spectrometry ŽEMS.. The LP-investigations yielded a set of external parameters for homogeneously grown hard films at deposition rates of approximately 1.5 mrh. By calibrating the EMS-system for particle number densities of stabile hydrocarbons and by using an appropriate fit-formula to evaluate absolute ion flux densities, the growth rates were in good agreement with predictions of the ‘thermally activated re-etching’-model ŽTR-model.. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Deposition; ECWR-reactor; Plasma diagnostics; Film properties
1. Introduction Since the last 30 years, hard carbon and hydrocarbon films have been deposited from a variety of different ion and plasma sources w1᎐3x. These films represent a broad spectrum of high-tech materials with extraordinary mechanical, electrical optical and chemical properties w4x. By applying appropriate process parameters, the film qualities are adapted to various technological tasks where many commercial applications have been reviewed w5,6x. Different aspects of deposition, film characterization and film properties were tried to model and to compare with measurements w7,8x, but especially the surface processes partly remain vague. In this paper, a new source for producing a-C:H films was investigated, namely an electron cyclotron wave resonance ŽECWR. reactor operated at 27.12 MHz w9x. Like in ECR sources, the electron cyclotron resonance
is used to generate a dense plasma at low-pressure values. Since the ECWR source is operated in the radio frequency Žr.f.. regime, the advantages of a lower magnetic field strength, the occurring self-bias and the perfect automatic matching support the investigations. In order to find a set of external parameters to deposit hard carbon films on silicon substrates, an automated Langmuir probe ŽLP. system ŽAPS3. was used for plasma diagnostics. Additionally, an energy᎐mass spectrometer ŽEMS. system ŽEQP 300. was calibrated to evaluate the neutral particle number densities of hydrocarbons occurring in the deposition plasmas. By using a formula for the determination of absolute ion flux densities w10x, the combination of LP and EMS measurements provided total carbon fluxes to the substrates, which were in good agreement with the deposition model of ‘thermally activated re-etching’ w11x.
2. Experimental U
Corresponding author. E-mail address: [email protected] ŽP. Awakowicz..
The ECWR plasma source and the underlying principle is described in several publications w9,12x. The r.f.-
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 3 1 3 - 5
P. Awakowicz et al. r Surface and Coatings Technology 142᎐144 (2001) 342᎐347
power is delivered by a single turn coil mounted outside a quartz cylinder Ždiameter s 20 cm, and height s 40 cm.. The horizontal d.c.-magnetic field is generated by a coil with distributed windings around the cylinder. In order to prevent capacitive coupling, a grounded Faraday shield is used. The bottom of the quartz cylinder is inserted into a stainless steel housing with a load lock system and several flanges for diagnostic purposes. The substrate holder is located just below the quartz cylinder. Its radial position is at r s 5 cm. For external biasing, the holder is connected to a d.c.-power supply Ž0᎐360 V.. The reactor was equipped with an in-house built Langmuir probe ŽLP. system, which is shown in Fig. 1. In general, this diagnostic is based on a small cylindrical tungsten wire Ždiameter s 50 m, and length s 5᎐10 mm. kept in the plasma w13x. A voltage ramp Ž"80 V, max.. was applied between the probe tip and reactor’s grounded electrode or wall while the current to the probe was measured. Place-dependent measurements in radial direction were enabled by a high-speed step motor and membrane bellows, which served as ultra high vacuum feed-through. Spatial profiles with a step distance of 1 cm and a total length of 40 cm were obtained in 1 s. While remaining in the zero-position the probe tip is drawn back into a quartz capillary tube. Because of this and the fast shift, coating of the probe tip was prevented. Since calculations and measurements partly revealed strong second and third harmonics of the r.f. ground wave, a passive compensation method has been chosen. The r.f. compensation is achieved by the capacitive coupling of a floating potential electrode to the plasma. This electrode is connected to the shield of a coaxial wave-guide, a band pass filter system and a high ohmic impedance converter. The probe current itself is measured by an amplifier and digitalized by a 16-bit ArD-converter. An electrically decoupled PC serves for controlling and signal processing. A fast automatic evaluation of electron density, mean electron energy, floating and plasma potential supports the improvement of the deposition parameters. Probe theory and different evaluation methods is described elsewhere w14,15x. Neutral particle number densities and ion flux densities are measured by a commercially available energy mass spectrometer ŽEQP 300, Hiden Analytical., which is described in w10,15x. Both diagnostic systems and the substrate are symmetrically placed to the middle axis in the plasma at r s 5 cm. Cutted silicon wavers Ž350 m. were used as substrates for film deposition. The temperature-dependent film growth was investigated by means of a calibrated microprocessor controlled current heating of the substrates. Film thickness is kept constant at approximately 1 m. Hardness is measured by a nanoindenter.
343
Fig. 1. Automated Langmuir probe system APS3.
The penetration depth of the indenter was chosen to be 100 nm.
3. Diagnostics of ECWR-plasmas 3.1. Langmuir probe In Fig. 2, the electron density and mean electron energy at an optimized parameter set for film deposition is shown. The r.f.-power of 100 W has been chosen for substrate temperatures below 350 K. The admixture of argon and methane Ž30:20 sccm. was used in order to enhance the deposition rate. All following deposition experiments were performed with a total pressure of 1 Pa, which has been chosen as a compromise between high electron density and a good spatial homogeneity. The homogeneity in electron density at 1 Pa has been found to be better than 3% over an area of 18 cm in diameter w9x. 3.2. Mass spectrum and calibration for neutral particles Fig. 3 shows a mass spectrum of neutral particles in
Fig. 2. Optimized electron density and mean electron energy as a function of pressure Žr.f. powers 100 W, methane:argon s 30:20 sccm.. The uncertainty is approximately 20%.
344
P. Awakowicz et al. r Surface and Coatings Technology 142᎐144 (2001) 342᎐347
Fig. 3. Neutral mass spectrum.
the deposition plasma at 100-W r.f.-power, 1-Pa total pressure and argon:methane s 30:20 sccm flowrates. This spectrum mainly contained C 1 H x and C 2 H y hydrocarbons. From cracking patterns of the stabile hydrocarbons CH 4 , C 2 H 2 , C 2 H 4 and C 2 H 6 produced in the ionizer of the EMS, it is known that the high concentration of the methyl radical ŽCH 3 . has not been produced in the plasma zone w10,15x. The same holds for the H-atom concentration, which may be evaluated only with ionization mass spectrometry. The calibration for neutral particle number densities was based on the aforementioned cracking pattern and an iterative solution scheme w10,15x. 3.3. Mass spectrum and calibration for charged particles As in case of neutrals, only C 1 H x and C 2 H y ions have been detected ŽFig. 4.. In contrast to the neutral particle’s measurements, H 3 ions showed the highest concentrations in the hydrogen group. For calibrating the system response of the EMS for ions, a formula was
Fig. 4. Ion mass spectrum.
Fig. 5. Energy distribution of argon Ž40 amu. ions Žr.f. powers 100 W, methane:argon s 30:20 sccm, and pressure s 1 Pa..
used which yielded the sensitivity in dependence on the channeltron effectivity, an energy-dependent transmission at constant ion mass and a mass-dependent transmission at constant energy w10x. 3.4. Ion energy distribution Fig. 5 shows the ion energy distribution for mass 40 amu Žargon.. As can be seen in the figure, the capacitive coupling due to the non-zero area of the r.f.-coil is not completely suppressed by the grounded Faraday shield. Therefore, the measured mean value of ion energy ² E : is located at approximately 32 eV without additional d.c.-bias. The half width at half maximum of the energy distribution is detected to be approximately 4 eV. 3.5. Comparison of LP and EMS measurements It is well-known that charged particles in plasmas gain most of their translational energy in the sheath shielding the plasma bulk from the confining walls. Since this energy increase is caused by the strong potential drop in the sheath, which equals the difference of plasma and floating potential, both diagnostic systems may be compared in this respect. Fig. 6 gives a comparison between the plasma potential Vp l observed with the LP and two distinguished points of the ion energy distribution at various total pressures where ² E : means the mean value of the distribution and Emax the maximum energy measured in the various plasmas. At low pressures, the sheath is believed to be collisionless and the mean value of the ion energy should be equal to the difference of plasma and floating potential. At higher pressures, the ions undergo collisions when they pass the sheath. Therefore, their mean energy should decrease with respect to the potential drop in the sheath.
P. Awakowicz et al. r Surface and Coatings Technology 142᎐144 (2001) 342᎐347
Fig. 6. Comparison of Langmuir probe and ion energy distribution measurements.
4. Deposition of a-C:H films The deposition conditions have been 100-W r.f.power, 1-Pa total pressure, variable d.c.-bias and a gas mixture of argon:methane of 30:20 sccm. Fig. 7 shows the hardness measured with a nanoindenter and the deposition rate at various pressure values. Both curves revealed the same dependence on pressure. Where the increase in deposition rate at increasing pressures was caused by an enhancement of the flux densities of both, the neutral and ionized hydrocarbons, the increase in hardness is not completely understood but also observed in expanding cascaded arcs w16x. It is thought to be due to an enhancement of the flux density of ions bombarding the surface and thereby, increasing the density of dangling bonds. Due to this increase, the surface of the growing films and therefore gave rise to a better network in the material w17x. Fig. 8 gives the increase in hardness and sp 3 content Žcalculated after w18x. in the films at increasing bias voltage. At low external d.c.-bias, the hardness of the films should be caused by the self-bias of the capacitive coupling. The relative low energy of approximately 32 eV seems to be high enough for reasonable hard films
345
Fig. 8. Hardness and sp 3-content of a-C:H films as a function of d.c.-bias.
Ž10 GPa.. If the external bias is increased to values of more than 200 V, the increase in hardness saturates. Since there was a voltage drop across the growing film, the d.c.-bias may not be identified with the ion energy at the substrate surface, but remarkable lower. Nevertheless, an enhancement of the ion energy above a special value will no longer increase the hardness of the films.
5. Flux measurement and modeling 5.1. Flux densities of neutral and ionized species Based on the calibrated system response N Ž k . for a neutral species k, the relative number densities of the detected species n k have been calculated by: nk Z Ž k . N Ž 16. s s ck n16 Z Ž 16. N Ž k . where ZŽ k . is the measured count rate for species k and the index k denotes the species mass in amu. The sum of all detected hydrogen, hydrocarbon and argon species gives the total pressure: pt o t s kTg Ý Ž c k n16 . k
where Tg is the spectroscopically measured gas temperature and k is Botzmann’s constant. By inserting the mean thermal velocity of neutral particles ² ¨ :, the flux densities are given: jk ,ges s
Fig. 7. Hardness and deposition rates of a-C:H films as a function of pressure.
1 ²¨ : ⭈ nk s nk 4
(
kTg . 2 mn
Fig. 9 shows the flux densities of the mainly detected neutral species as a function of increasing pressure. Based on the pressure-dependent electron densities
346
P. Awakowicz et al. r Surface and Coatings Technology 142᎐144 (2001) 342᎐347
Fig. 9. Flux densities of neutrals occurring in the deposition plasma. The uncertainty is approximately 35%.
and electron temperatures from LP measurement, the fluxes of the methyl radical and the hydrogen atom are calculated with an electron-induced collision model w19x. For calculations of the rate coefficients, a Maxwellian electron energy distribution function was assumed. In a similar way, the ion flux densities have also been calculated where the system response was chosen after w10x. Keeping in mind that the EMS instrument primary measures ion flux densities and not ion number densities in SIMS-mode, quasi neutrality and Bohm’s velocity at the sheath edge were used to evaluate the ion fluxes, given in Fig. 10. In this figure, the strong increase of the CHq 5 species with increasing pressure is remarkable. In the same way, the sum over all hydrocarbon ions yielded an increase too. By inserting these fluxes into the model of ‘thermally activated re-etching’ ŽTR-model w11x. with a substrate temperature of approximately 300 K, the resulting carbon flux to the growing film was calculated. This model considers adsorbed methyl radicals partly covering the dangling bonds on the surface. Hydrocarbon ions are bombarding the growing film and remain in the layer without being sputtered or backscattered. The role of hydrogen ions lies in a hydrogen abstraction, which
Fig. 10. Flux densities of ions occurring in the deposition plasma Ž jq c denotes the total flux of carbon containing ions.. The uncertainty is approximately 35%.
Fig. 11. Comparison of growth rate and carbon fluxes due to the TR-model Ždensity s 2 grcm3 ..
enhances the dangling bond density on the surface. The neutral hydrogen flux to the surface is responsible for re-etching of carbon and an increase in hydrogen concentration of the growing film. In Fig. 11, this calculated growth rate was compared to the measured one, where the density of the deposited films measured by Rutherford backscattering is approximately 2 grcm3.
6. Conclusions It was the purpose of the present paper, to deposit hydrogenated carbon films with a new reactor concept. Furthermore, plasma diagnostics and film analytics are used to optimize the process conditions and to correlate film properties with plasma parameters. By evaluating neutral and ion flux densities to the substrate and inserting these in the model of ‘thermally activated re-etching’, good agreement is found to the measured deposition rates. References w1x S. Aisenberg, R. Chabot, J. Appl. Phys. 42 Ž1971. 2953. w2x L. Holland, S.M. Ojha, Thin Solid Films 58 Ž1979. 107. w3x Y. Catherine, Diamond and diamond-like films and coatings, in: R.E. Clausing, L.L. Horton, J.C. Angus, P. Koidl ŽEds.., NATO ASI Series B, vol. 266, Plenum, New York, 1991, p. 193. w4x P. Koidl, Ch. Wild, B. Dischler, J. Wagner, M. Ramsteiner, Mat. Sci. Forum 52r53 Ž1989. 41. w5x H. Tsai, D.B. Bogy, J. Vac. Sci. Technol. A 5 Ž1987. 3287. w6x A.H. Lettington, C. Smith, Diam. Rel. Mater. 1 Ž1992. 805. w7x Y. Lifshitz, S.R. Kasi, J.W. Rabalais, W. Eckstein, Phys. Rev. B 41 Ž15. Ž1990. 10468. w8x J. Robertson, Surf. Coat. Technol. 50 Ž1992. 185. w9x W. Kasper, Thesis, Technical University of Munich, Germany, 1993. w10x P. Pecher, Thesis, University of Bayreuth, Germany, 1997. w11x W. Moller, W. Fukarek, K. Lange, A.V. Keudell, W. Jacob, ¨ Jpn. J. Appl. Phys. 34 Ž1995. 2163. w12x W. Kasper, P. Awakowicz, Proceedings of the Twelfth International Conference on Plasma chemistry ŽISPC 95., August 21᎐25, ISPC, Minneapolis, MN, USA, 1995, 2239.
P. Awakowicz et al. r Surface and Coatings Technology 142᎐144 (2001) 342᎐347 w13x I. Langmuir, Gen. Electr. Rev. 27 Ž1924. 449. w14x J.D. Swift, M.J.R. Schwar, Electrical Probes for Plasma Diagnostics, IIliffe Books Ltd, London, 1970. w15x P. Awakowicz, Mat. Sci. Forum 287r288 Ž1998. 3. w16x J.J. Beulens, A.J.M. Buuron, D.C. Schram, Surf. Coat. Technol. 47 Ž1991..
347
w17x W. Jacob, Thin Solid Films 326 Ž1998.. w18x C. Martino De, F. Demichelis, A. Tagliaferro, Diam. Relat. Mater. 4 Ž1995.. w19x K. Behringer, Plasma Phys. Control Fusion 33 Ž9. Ž1991. 997.
Deposition of a-C:H films with an ECWR-reactor at 27 MHz: plasma diagnostics and film properties P. Awakowicz a,U , R. Schwefel a , P. Scheubert a , G. Benstetter b a
Chair for Physic of Electrotechnology, Uni¨ ersity of Technology, Munich, Germany b Uni¨ ersity of Applied Sciencesr FH Deggendorf, Deggendorf, Germany
Abstract For the first time, an electron᎐cyclotron-wave resonance ŽECWR. source was used to deposit thin amorphous hydrocarbon Ža-C:H. films. The deposition experiments have been supported by intensive plasma diagnostics with Langmuir probe ŽLP. measurements and energy mass spectrometry ŽEMS.. The LP-investigations yielded a set of external parameters for homogeneously grown hard films at deposition rates of approximately 1.5 mrh. By calibrating the EMS-system for particle number densities of stabile hydrocarbons and by using an appropriate fit-formula to evaluate absolute ion flux densities, the growth rates were in good agreement with predictions of the ‘thermally activated re-etching’-model ŽTR-model.. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Deposition; ECWR-reactor; Plasma diagnostics; Film properties
1. Introduction Since the last 30 years, hard carbon and hydrocarbon films have been deposited from a variety of different ion and plasma sources w1᎐3x. These films represent a broad spectrum of high-tech materials with extraordinary mechanical, electrical optical and chemical properties w4x. By applying appropriate process parameters, the film qualities are adapted to various technological tasks where many commercial applications have been reviewed w5,6x. Different aspects of deposition, film characterization and film properties were tried to model and to compare with measurements w7,8x, but especially the surface processes partly remain vague. In this paper, a new source for producing a-C:H films was investigated, namely an electron cyclotron wave resonance ŽECWR. reactor operated at 27.12 MHz w9x. Like in ECR sources, the electron cyclotron resonance
is used to generate a dense plasma at low-pressure values. Since the ECWR source is operated in the radio frequency Žr.f.. regime, the advantages of a lower magnetic field strength, the occurring self-bias and the perfect automatic matching support the investigations. In order to find a set of external parameters to deposit hard carbon films on silicon substrates, an automated Langmuir probe ŽLP. system ŽAPS3. was used for plasma diagnostics. Additionally, an energy᎐mass spectrometer ŽEMS. system ŽEQP 300. was calibrated to evaluate the neutral particle number densities of hydrocarbons occurring in the deposition plasmas. By using a formula for the determination of absolute ion flux densities w10x, the combination of LP and EMS measurements provided total carbon fluxes to the substrates, which were in good agreement with the deposition model of ‘thermally activated re-etching’ w11x.
2. Experimental U
Corresponding author. E-mail address: [email protected] ŽP. Awakowicz..
The ECWR plasma source and the underlying principle is described in several publications w9,12x. The r.f.-
0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 3 1 3 - 5
P. Awakowicz et al. r Surface and Coatings Technology 142᎐144 (2001) 342᎐347
power is delivered by a single turn coil mounted outside a quartz cylinder Ždiameter s 20 cm, and height s 40 cm.. The horizontal d.c.-magnetic field is generated by a coil with distributed windings around the cylinder. In order to prevent capacitive coupling, a grounded Faraday shield is used. The bottom of the quartz cylinder is inserted into a stainless steel housing with a load lock system and several flanges for diagnostic purposes. The substrate holder is located just below the quartz cylinder. Its radial position is at r s 5 cm. For external biasing, the holder is connected to a d.c.-power supply Ž0᎐360 V.. The reactor was equipped with an in-house built Langmuir probe ŽLP. system, which is shown in Fig. 1. In general, this diagnostic is based on a small cylindrical tungsten wire Ždiameter s 50 m, and length s 5᎐10 mm. kept in the plasma w13x. A voltage ramp Ž"80 V, max.. was applied between the probe tip and reactor’s grounded electrode or wall while the current to the probe was measured. Place-dependent measurements in radial direction were enabled by a high-speed step motor and membrane bellows, which served as ultra high vacuum feed-through. Spatial profiles with a step distance of 1 cm and a total length of 40 cm were obtained in 1 s. While remaining in the zero-position the probe tip is drawn back into a quartz capillary tube. Because of this and the fast shift, coating of the probe tip was prevented. Since calculations and measurements partly revealed strong second and third harmonics of the r.f. ground wave, a passive compensation method has been chosen. The r.f. compensation is achieved by the capacitive coupling of a floating potential electrode to the plasma. This electrode is connected to the shield of a coaxial wave-guide, a band pass filter system and a high ohmic impedance converter. The probe current itself is measured by an amplifier and digitalized by a 16-bit ArD-converter. An electrically decoupled PC serves for controlling and signal processing. A fast automatic evaluation of electron density, mean electron energy, floating and plasma potential supports the improvement of the deposition parameters. Probe theory and different evaluation methods is described elsewhere w14,15x. Neutral particle number densities and ion flux densities are measured by a commercially available energy mass spectrometer ŽEQP 300, Hiden Analytical., which is described in w10,15x. Both diagnostic systems and the substrate are symmetrically placed to the middle axis in the plasma at r s 5 cm. Cutted silicon wavers Ž350 m. were used as substrates for film deposition. The temperature-dependent film growth was investigated by means of a calibrated microprocessor controlled current heating of the substrates. Film thickness is kept constant at approximately 1 m. Hardness is measured by a nanoindenter.
343
Fig. 1. Automated Langmuir probe system APS3.
The penetration depth of the indenter was chosen to be 100 nm.
3. Diagnostics of ECWR-plasmas 3.1. Langmuir probe In Fig. 2, the electron density and mean electron energy at an optimized parameter set for film deposition is shown. The r.f.-power of 100 W has been chosen for substrate temperatures below 350 K. The admixture of argon and methane Ž30:20 sccm. was used in order to enhance the deposition rate. All following deposition experiments were performed with a total pressure of 1 Pa, which has been chosen as a compromise between high electron density and a good spatial homogeneity. The homogeneity in electron density at 1 Pa has been found to be better than 3% over an area of 18 cm in diameter w9x. 3.2. Mass spectrum and calibration for neutral particles Fig. 3 shows a mass spectrum of neutral particles in
Fig. 2. Optimized electron density and mean electron energy as a function of pressure Žr.f. powers 100 W, methane:argon s 30:20 sccm.. The uncertainty is approximately 20%.
344
P. Awakowicz et al. r Surface and Coatings Technology 142᎐144 (2001) 342᎐347
Fig. 3. Neutral mass spectrum.
the deposition plasma at 100-W r.f.-power, 1-Pa total pressure and argon:methane s 30:20 sccm flowrates. This spectrum mainly contained C 1 H x and C 2 H y hydrocarbons. From cracking patterns of the stabile hydrocarbons CH 4 , C 2 H 2 , C 2 H 4 and C 2 H 6 produced in the ionizer of the EMS, it is known that the high concentration of the methyl radical ŽCH 3 . has not been produced in the plasma zone w10,15x. The same holds for the H-atom concentration, which may be evaluated only with ionization mass spectrometry. The calibration for neutral particle number densities was based on the aforementioned cracking pattern and an iterative solution scheme w10,15x. 3.3. Mass spectrum and calibration for charged particles As in case of neutrals, only C 1 H x and C 2 H y ions have been detected ŽFig. 4.. In contrast to the neutral particle’s measurements, H 3 ions showed the highest concentrations in the hydrogen group. For calibrating the system response of the EMS for ions, a formula was
Fig. 4. Ion mass spectrum.
Fig. 5. Energy distribution of argon Ž40 amu. ions Žr.f. powers 100 W, methane:argon s 30:20 sccm, and pressure s 1 Pa..
used which yielded the sensitivity in dependence on the channeltron effectivity, an energy-dependent transmission at constant ion mass and a mass-dependent transmission at constant energy w10x. 3.4. Ion energy distribution Fig. 5 shows the ion energy distribution for mass 40 amu Žargon.. As can be seen in the figure, the capacitive coupling due to the non-zero area of the r.f.-coil is not completely suppressed by the grounded Faraday shield. Therefore, the measured mean value of ion energy ² E : is located at approximately 32 eV without additional d.c.-bias. The half width at half maximum of the energy distribution is detected to be approximately 4 eV. 3.5. Comparison of LP and EMS measurements It is well-known that charged particles in plasmas gain most of their translational energy in the sheath shielding the plasma bulk from the confining walls. Since this energy increase is caused by the strong potential drop in the sheath, which equals the difference of plasma and floating potential, both diagnostic systems may be compared in this respect. Fig. 6 gives a comparison between the plasma potential Vp l observed with the LP and two distinguished points of the ion energy distribution at various total pressures where ² E : means the mean value of the distribution and Emax the maximum energy measured in the various plasmas. At low pressures, the sheath is believed to be collisionless and the mean value of the ion energy should be equal to the difference of plasma and floating potential. At higher pressures, the ions undergo collisions when they pass the sheath. Therefore, their mean energy should decrease with respect to the potential drop in the sheath.
P. Awakowicz et al. r Surface and Coatings Technology 142᎐144 (2001) 342᎐347
Fig. 6. Comparison of Langmuir probe and ion energy distribution measurements.
4. Deposition of a-C:H films The deposition conditions have been 100-W r.f.power, 1-Pa total pressure, variable d.c.-bias and a gas mixture of argon:methane of 30:20 sccm. Fig. 7 shows the hardness measured with a nanoindenter and the deposition rate at various pressure values. Both curves revealed the same dependence on pressure. Where the increase in deposition rate at increasing pressures was caused by an enhancement of the flux densities of both, the neutral and ionized hydrocarbons, the increase in hardness is not completely understood but also observed in expanding cascaded arcs w16x. It is thought to be due to an enhancement of the flux density of ions bombarding the surface and thereby, increasing the density of dangling bonds. Due to this increase, the surface of the growing films and therefore gave rise to a better network in the material w17x. Fig. 8 gives the increase in hardness and sp 3 content Žcalculated after w18x. in the films at increasing bias voltage. At low external d.c.-bias, the hardness of the films should be caused by the self-bias of the capacitive coupling. The relative low energy of approximately 32 eV seems to be high enough for reasonable hard films
345
Fig. 8. Hardness and sp 3-content of a-C:H films as a function of d.c.-bias.
Ž10 GPa.. If the external bias is increased to values of more than 200 V, the increase in hardness saturates. Since there was a voltage drop across the growing film, the d.c.-bias may not be identified with the ion energy at the substrate surface, but remarkable lower. Nevertheless, an enhancement of the ion energy above a special value will no longer increase the hardness of the films.
5. Flux measurement and modeling 5.1. Flux densities of neutral and ionized species Based on the calibrated system response N Ž k . for a neutral species k, the relative number densities of the detected species n k have been calculated by: nk Z Ž k . N Ž 16. s s ck n16 Z Ž 16. N Ž k . where ZŽ k . is the measured count rate for species k and the index k denotes the species mass in amu. The sum of all detected hydrogen, hydrocarbon and argon species gives the total pressure: pt o t s kTg Ý Ž c k n16 . k
where Tg is the spectroscopically measured gas temperature and k is Botzmann’s constant. By inserting the mean thermal velocity of neutral particles ² ¨ :, the flux densities are given: jk ,ges s
Fig. 7. Hardness and deposition rates of a-C:H films as a function of pressure.
1 ²¨ : ⭈ nk s nk 4
(
kTg . 2 mn
Fig. 9 shows the flux densities of the mainly detected neutral species as a function of increasing pressure. Based on the pressure-dependent electron densities
346
P. Awakowicz et al. r Surface and Coatings Technology 142᎐144 (2001) 342᎐347
Fig. 9. Flux densities of neutrals occurring in the deposition plasma. The uncertainty is approximately 35%.
and electron temperatures from LP measurement, the fluxes of the methyl radical and the hydrogen atom are calculated with an electron-induced collision model w19x. For calculations of the rate coefficients, a Maxwellian electron energy distribution function was assumed. In a similar way, the ion flux densities have also been calculated where the system response was chosen after w10x. Keeping in mind that the EMS instrument primary measures ion flux densities and not ion number densities in SIMS-mode, quasi neutrality and Bohm’s velocity at the sheath edge were used to evaluate the ion fluxes, given in Fig. 10. In this figure, the strong increase of the CHq 5 species with increasing pressure is remarkable. In the same way, the sum over all hydrocarbon ions yielded an increase too. By inserting these fluxes into the model of ‘thermally activated re-etching’ ŽTR-model w11x. with a substrate temperature of approximately 300 K, the resulting carbon flux to the growing film was calculated. This model considers adsorbed methyl radicals partly covering the dangling bonds on the surface. Hydrocarbon ions are bombarding the growing film and remain in the layer without being sputtered or backscattered. The role of hydrogen ions lies in a hydrogen abstraction, which
Fig. 10. Flux densities of ions occurring in the deposition plasma Ž jq c denotes the total flux of carbon containing ions.. The uncertainty is approximately 35%.
Fig. 11. Comparison of growth rate and carbon fluxes due to the TR-model Ždensity s 2 grcm3 ..
enhances the dangling bond density on the surface. The neutral hydrogen flux to the surface is responsible for re-etching of carbon and an increase in hydrogen concentration of the growing film. In Fig. 11, this calculated growth rate was compared to the measured one, where the density of the deposited films measured by Rutherford backscattering is approximately 2 grcm3.
6. Conclusions It was the purpose of the present paper, to deposit hydrogenated carbon films with a new reactor concept. Furthermore, plasma diagnostics and film analytics are used to optimize the process conditions and to correlate film properties with plasma parameters. By evaluating neutral and ion flux densities to the substrate and inserting these in the model of ‘thermally activated re-etching’, good agreement is found to the measured deposition rates. References w1x S. Aisenberg, R. Chabot, J. Appl. Phys. 42 Ž1971. 2953. w2x L. Holland, S.M. Ojha, Thin Solid Films 58 Ž1979. 107. w3x Y. Catherine, Diamond and diamond-like films and coatings, in: R.E. Clausing, L.L. Horton, J.C. Angus, P. Koidl ŽEds.., NATO ASI Series B, vol. 266, Plenum, New York, 1991, p. 193. w4x P. Koidl, Ch. Wild, B. Dischler, J. Wagner, M. Ramsteiner, Mat. Sci. Forum 52r53 Ž1989. 41. w5x H. Tsai, D.B. Bogy, J. Vac. Sci. Technol. A 5 Ž1987. 3287. w6x A.H. Lettington, C. Smith, Diam. Rel. Mater. 1 Ž1992. 805. w7x Y. Lifshitz, S.R. Kasi, J.W. Rabalais, W. Eckstein, Phys. Rev. B 41 Ž15. Ž1990. 10468. w8x J. Robertson, Surf. Coat. Technol. 50 Ž1992. 185. w9x W. Kasper, Thesis, Technical University of Munich, Germany, 1993. w10x P. Pecher, Thesis, University of Bayreuth, Germany, 1997. w11x W. Moller, W. Fukarek, K. Lange, A.V. Keudell, W. Jacob, ¨ Jpn. J. Appl. Phys. 34 Ž1995. 2163. w12x W. Kasper, P. Awakowicz, Proceedings of the Twelfth International Conference on Plasma chemistry ŽISPC 95., August 21᎐25, ISPC, Minneapolis, MN, USA, 1995, 2239.
P. Awakowicz et al. r Surface and Coatings Technology 142᎐144 (2001) 342᎐347 w13x I. Langmuir, Gen. Electr. Rev. 27 Ž1924. 449. w14x J.D. Swift, M.J.R. Schwar, Electrical Probes for Plasma Diagnostics, IIliffe Books Ltd, London, 1970. w15x P. Awakowicz, Mat. Sci. Forum 287r288 Ž1998. 3. w16x J.J. Beulens, A.J.M. Buuron, D.C. Schram, Surf. Coat. Technol. 47 Ž1991..
347
w17x W. Jacob, Thin Solid Films 326 Ž1998.. w18x C. Martino De, F. Demichelis, A. Tagliaferro, Diam. Relat. Mater. 4 Ž1995.. w19x K. Behringer, Plasma Phys. Control Fusion 33 Ž9. Ž1991. 997.