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Deposition of highly adhesive amorphous carbon films with the use of preliminary plasma-immersion ion implantation
Surface and Coatings Technology 156 (2002) 311–316
Deposition of highly adhesive amorphous carbon films with the use of preliminary plasma-immersion ion implantation S.P. Bugaev, K.V. Oskomov*, N.S. Sochugov Institute of High-Current Electronics SD RAS, 4 Akademichesky Ave., Tomsk 634055, Russia
Abstract The usability of the following two types of plasma generators for deposition of highly adhesive a-C:H films on the large area substrates has been studied: (1) a source of plasma generated by means of a non-self-sustained arc discharge in low-pressure gas; and (2) an ion-plasma source on the basis of a Hall current accelerator with closed electron drift. The distinctive features of both sources are: (a) the possibility of the generation of extended flows (up to 2 m) of relatively dense plasma (;1010 cmy3); and (b) control of the plasma ionization degree, allowing realization of both preliminary plasma-immersion ion implantation (PIII) of a substrate and subsequent plasma-immersion ion-assisted deposition (PIID) of a-C:H film. The results of experimental investigations into the characteristics of the sources in different operational regimes are presented. Taking into account the probe measurements of plasma parameters, both generators have been optimized to operate in the PIII and PIID regimes. Characteristics of the pulsed negative bias applied to the substrate in both regimes have also been determined. It was shown that both sources allowed deposition of a diamond-like film on large-area substrates with a growth rate of 100–300 nm hy1 . A hard (20–30 GPa) a-C:H coating containing approximately 60% of carbon atoms with sp3 hybridization and having satisfactory adhesion to the substrate can be obtained if short (;60 ms) high-voltage (;6 kV) bias pulses are applied to the substrate. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Plasma immersion ion implantation (PIII); Plasma immersion ion-assisted deposition (PIID); a-C:H; Adhesion
1. Introduction Since the first works by Konrad et al. w1x, plasmaimmersion ion implantation (PIII) and plasma-immersion ion-assisted deposition (PIID) have been widely used as material surface modification techniques. This approach, in particular, is convenient for the deposition of highly adhesive and hard a-C:H coatings on different substrates, as collaboratively demonstrated by LANL and General Motors w2x. To create large volumes of hydrocarbon plasma, different types of glow discharge are mainly used: DC w3x, pulsed w4x, capacitive RF w5x and inductive RF w6x. The first three types are characterized by moderate power (up to 1 kW), low operating pressure (0.01–0.1 Pa) and, hence, low plasma density (108 –109 cmy3) and low growth rate of the DLC (0.1 mm hy1). In order to increase the productivity of the a-C:H film PIID process, it is necessary to use *Corresponding author. Tel.: q7-382-2-258-651; fax: q7-382-2259-410. E-mail address: [email protected] (K.V. Oskomov).
denser hydrocarbon plasma. The inductive RF glow discharge plasma is characterized by a comparatively high ion concentration (1010 –1011 cmy3) w6x. However, this is applicable to the PIID of a-C:H films only if a hydrocarbon gas is diluted by the inert one w7x, which again results in growth rates of the coatings of approximately 0.1 mm hy1. A more detailed comparative analysis of different methods of plasma generation for the PIID of diamond-like films is presented in w8x. In this paper, we studied the possibility of an alternative approach of PIII of large-area substrates followed by PIID of a-C:H films. This is based on the use of two types of extended (up to 2 m long) remote plasma sources: (1) a source of plasma generated by means of a non-self-sustained arc discharge; and (2) an ionplasma source on the basis of a Hall current accelerator with closed electron drift. These sources are notable for reliability, simplicity of construction and operation. In spite of the rather large length, these devices can be adjusted to generate relatively dense (ni;109 –1011 cmy3) and uniform w(Dni yni)s"10%x plasma, the
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 0 9 9 - 3
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Fig. 1. Experimental set-up for the PIII of substrates and PIID of the a-C:H films: 1, vacuum chamber; 2, diffusion pump; 3, non-self-sustained arc-discharge plasma source; 4, closed-drift plasma-ion source; 5, substrate; 6, high-voltage substrate-bias pulse power supply; and 7,8, DC power supplies.
parameters of which can be varied over a wide range w9,10x. Hence, continuous back-and-forth movement of the plasma source along the substrate during deposition or implantation processes provides realization of largescale PIII and PIID. This approach is more convenient for processing of flat large-area substrates than those ones previously listed. The two types of device mentioned have been used for plasma cleaning of large glass substrates before metal and oxide film deposition w9– 11x. The purpose of this paper is to show the principle of the possibility of using this approach and the plasma sources for preliminary PIII of large-area substrates followed by PIID of amorphous hydrogenated carbon films.
walls at a distance of approximately 30 cm relative to the water-cooled sample holder (5). Silicon wafers, titanium and stainless steel plates were used as substrates. A generator of high-voltage negative substrate bias pulses (6) with a width of 40–60 ms, amplitude of up to 15 kV and frequency of up to 1 kHz was used in the experiments. To ignite the discharge in the vacuum chamber, DC power supplies (7, 8) were used. A plasma generator on the basis of a non-selfsustained arc discharge (Fig. 2) combines the features of thermionic and hollow cathode sources w12,13x. The combined cathode of this plasma generator consists of a 280-mm-long and 120-mm-diameter hollow metal cylinder (1) encircling a heated tungsten filament (2). The case of the plasma generator (4) and the chamber walls that are at earth potential are used as the anode. Applying DC voltage of the order of 60–70 V to the cathode, the non-self-sustained discharge can be ignited in the chamber. Current of the discharge can reach 50 A and is controlled by the operating gas pressure and the filament current. The magnetic field increasing the plasma ionization degree and stabilizing the discharge is generated by means of electromagnets (3). A more detailed description of the plasma generator can be found in w14x. Another plasma generator used in the experiments (Fig. 3) presents an extended (30 cm long) closed-drift ion source consisting of an anode (1) placed inside a case (cathode) (2) that is, as a rule, connected to the vacuum chamber walls and has earth potential. In the cathode gap (4), a magnetic field of approximately 1300 G is created by means of the permanent magnets (3). This source can operate in the ion-beam generation regime, plasma generation regime or transition regime, depending on the anode voltage and pressure in the chamber w15x.
2. Experimental To demonstrate the applicability of the approach to preliminary PIII of substrates and PIID of amorphous hydrogenated carbon films, laboratory-scale equipment was used in the first stage of the investigations. The experimental set-up consisted of a vacuum chamber (1) with a volume of approximately 0.2 m3 that was evacuated by a diffusion pump (2) (Fig. 1). The plasma sources (3,4) were symmetrically placed at the chamber
Fig. 2. Non-self-sustained arc-discharge plasma source: 1, hollow cathode; 2, hot filament; 3, electromagnets; and 4, case of the source.
S.P. Bugaev et al. / Surface and Coatings Technology 156 (2002) 311–316
Fig. 3. Closed-drift plasma ion source: 1, anode; 2, case (cathode); 3, permanent magnets; and 4, channel.
The plasma parameters were measured by means of a plane Langmuir probe with a guard ring and the area of the collecting surface equal to 1 cm2. In addition to the plasma characteristics, the properties of a-C:H films as a function of the deposition conditions were studied. The main tool for analysis of the coating quality was Raman spectroscopy (a DFS-52 spectrophotometer, LOMO, Russia) and the main criterion was the ratio of the integral intensities of the D-peak (;1400 cmy1) and G-peak (;1600 cmy1) in the film spectra w16x. The adhesion and hardness of the coatings were qualitatively determined by the scratch test. In this case, a rubber with abrasive SiO2 particles at a 1-kg load applied perpendicular to the sample surface was moved alternatively along it. 3. Results and discussion 3.1. Main characteristics of the plasma sources The probe measurement results of the non-self-sustained low-pressure arc discharge source plasma and the plasma generated by the ion-plasma source with closed electron drift for different operation regimes are presented in Table 1. The plasma concentration of the nonself-sustained arc discharge is observed to linearly depend on the discharge current Id. It should be noted that, in all cases, plasma filled the chamber almost uniformly, and thus, in the substrate region, no spatial non-uniformity of the plasma parameters within the limits of measurement accuracy ("20%) was observed. The possibility of successful application of thermionic sources for the PIII of different materials and PIID of different coatings, including diamond-like carbon, was demonstrated by Matossian and co-workers w4,12,18x.
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High discharge current (up to 100 A) and high plasma density (up to 1011 cmy3), as well as easy controllability, are the advantages of this type of plasma generator. However, the presence of the tungsten filament reduces the technological potential of this plasma generator due to the limited operation resource, especially in reactive environments (oxygen, air, hydrocarbon, etc.). Hence, the next step was to study the possibility of application of the closed-drift ion-plasma source free from the above-mentioned shortcomings for PIII of substrates and subsequent PIID of a-C:H films. In the case of the closed-drift ion-plasma source, three different operational regimes can be described (Table 1). The first regime is realized at pressures lower than 0.3 Pa. The discharge current of 30 mA corresponds to a discharge voltage of 1000 V. In these conditions, plasma with a density of 109 cmy3 and an ion beam propagating in straight lines from the anode through the gap in the cathode simultaneously exist (ion regime). When the pressure is higher than 0.3 Pa, the conditions for volume glow-discharge ignition having a 0.6-A current and 600–700-V voltage are created in our experiment. In this operation regime, the plasma concentration is 2=1010 cmy3. Its distribution profile becomes more uniform, although the directed ion beam still exists (ion plasma or transition regime). Finally, with subsequent increase in the operating pressure up to 0.5 Pa, the discharge current and plasma density can be increased to 3 A and ;4=1010 cmy3 (plasma regime), respectively. The discharge voltage drops to 300 V and the plasma concentration profile becomes uniform within the limits of measurement errors. In the ion-plasma and plasma regimes, the linear plasma density profiles are nearly uniform. At present, the closed-drift ion sources are fabricated at the Institute of High Current Electronics; the length is up to 2 m and the linear uniformity of the ion beam and plasma concentration are no less than 10–20%. The values obtained for the plasma concentration were used for calculation of the ion current density at the emission boundary of the plasma by the Bohm formula, and the thickness of the cathode layer formed Table 1 Plasma parameters Plasma source
P (Pa)
Id (A)
Te (eV)
ni (cmy3)
ji (mAycm2)
Not-self-sustained arc-discharge plasma source
0.5 0.2 0.2
10 5 1
3.7 3.7 3.7
3.4=1010 1.7=1010 3.4=109
0.92 0.46 0.09
Closed-drift ion-plasma source
0.5 0.3 0.2
4=1010 2=1010 109
1 0.52 0.03
3 0.6 0.03
4 4 4
P, pressure in the chamber; Id, discharge current; Te , electron temperature; ni, ion concentration; and ji, ion current density.
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near the substrate when the negative polarity bias voltage is applied. As observed from Table 1, the characteristics of the plasma generated by the closed-drift ion source in the ion-plasma and plasma regimes of operation coincide within the limits of measurement errors, with discharge current of Ids5 and 10 A, respectively, for the non-self-sustained arc discharge plasma. It is shown below that these regimes are of greatest interest for PIID of the a-C:H films and preliminary PIII of the substrates. Operating pressure values in the cases considered are also close to each other. Hence, we can conclude that the physical rules for the PIII and PIID processes for the non-self-sustained arc discharge regimes considered and the corresponding operation regimes of the closeddrift ion source will be identical. 3.2. Preliminary PIII of substrates and PIID of a-C:H films Experiments on a-C:H film deposition from the nonself-sustained arc discharge plasma in methane without substrate bias have shown that the growth rate and the characteristics of the films obtained depend on the discharge current. For example, the rate of film growth drops by from 3 mm hy1 to 500 nm hy1 as the discharge current decreased from 10 to 5 A and the operational pressure decreased from 0.5 to 0.2 Pa. The plasma concentration, according to the probe measurements, decreases by only two-fold, from 3.4=1010 to 1.7=1010 cmy3 (Table 1). In our case, regime of the operation with lower discharge current (Ids5 A) is optimum for PIII realization, as the a-C:H film growth rate is low enough to completely etch the film at the high-voltage bias applied to the substrate. According to the Child–Langmuir law, in this regime the width of the double layer for 12-kV bias voltage pulses is equal to 6 cm, which approximately coincides with the free pass length at a 0.2-Pa pressure. The best PIII results have been obtained at pulse amplitude, width and repetition rate for the substrate bias voltage of 10–12 kV, 45 ms and 660 Hz, respectively. As previously mentioned, a methane plasma with the necessary characteristics in this case can be generated both by means of the nonself-sustained arc discharge (Ps0.2 Pa, Ids5 A, Uds 50–60 V) and the closed-drift ion source (Ps0.3 Pa, Ids0.6 A, Uds600 V). The results of scratch tests on the a-C:H films obtained have shown that, in order to achieve adhesion to the silicon, stainless steel and titanium substrates, it is necessary to carry out preliminary PIII for 30 min. Using the value calculated for the current density in methane in the PIII regime, a dose of 8.6=1016 cmy2 was obtained. This agrees with the literature data: in w2x, the authors obtained adhesion of the a-C:H film to aluminum after preliminary PIII of carbon into the
Table 2 a-C:H film PIID conditions 噛
Gas
Usub (kV)
v (nm sy1)
EC (eV atomy1)
IDyIG
1 2 3 4 5 6 7 8
CH4 CH4 CH4 CH4 CH4 yAr (2:1) CH4 CH4 yAr (1:4) CH4 yAr (1:4)
0 1.2 2 4 2.5 6 0.5 1
0.78 0.55 0.48 0.18 0.06 0.09 0.07 0.06
0 40 70 110 150 180 240 420
– – – 1.1 0.8 0.8 0.8 0.9
Usub, amplitude of substrate bias pulses; v, a-C:H film growth rate; EC, energy per carbon atom deposited; and ID yIG , Raman spectral Dand G-peak integral intensity ratio.
substrate from RF discharge plasma in acetylene at a dose of 1017 cmy2. Non-self-sustained low-pressure discharge can also be used for PIII of a-C:H films. In contrast to the PIII regime, the optimum regime for PIID is the non-selfsustained arc discharge with high current (Ps0.5 Pa, Ids10 A, Uds50–60 V) and corresponding plasma regime of the closed-drift ion source operation (Ps0.5 Pa, Ids3 A, Uds300 V). In these cases, maximum plasma concentration in the substrate region (Table 1) and, hence, maximum growth rates of the a-C:H film, are achieved. The main parameter influencing the structure and properties of amorphous carbon and hydrocarbon films is the energy EC per one deposited carbon atom w17x, which depends on the balance of the ion and radical flow to the growing coating surface. The radical flow density was determined from the a-C:H film growth rate and density in the absence of substrate bias. The ion current density was evaluated using the Bohm formula and on the assumption of the previously determined plasma parameters. Table 2 presents the experimental conditions for aC:H film deposition, the value calculated for EC, and the ratio (ID yIG) of integral intensities of the D- and Gpeaks. The ion flow to the substrate was practically unchanged (the ion current density was ji f0.9–1 mA cmy2), since the plasma parameters, as well as length and frequency of the bias voltage pulses (tsubs60 ms, f subs660 Hz), remained approximately constant. The radical flow to the surface was changed due to the use of methane and argon mixtures with different CH4 content. The bias voltage pulse amplitude was varied within the limits Usubs0–6 kV. Fig. 4 presents Raman spectra of the a-C:H films. It is evident that with an increase in EC higher than 100 eV, spectral changes characterize the transition from polymer-like to diamond-like films. When EC is in the range from 150 to 240 eV, the diamond-like carbon content in the coating reaches a maximum (in accor-
S.P. Bugaev et al. / Surface and Coatings Technology 156 (2002) 311–316
Fig. 4. Raman spectra for the a-C:H films obtained by the PIID method in accordance with the experimental conditions presented in Table 2.
dance with the minimum of the ratio of the D- and Gpeak integral intensities). According to Qian et al. w19x, the value of ID yIGs0.8 is characteristic of a-C:H films containing 50–60% of diamond-like carbon. With subsequent increase in EC, the film graphitization process begins. Hence, the optimum energy per carbon atom is in the range of 150–240 eV. The film growth rate is 200–300 nm hy1, which is 1.5–2-fold higher than in the case of glow-discharge RF plasma generated in large (;1 m3) volumes w2x. As well as the energy per deposited carbon atom, suitable energy of the ions bombarding the growing aC:H film, i.e. the bias pulse amplitude, is also important. As follows from Raman spectra (Fig. 4), the coatings obtained at ECG180 eV are characterized by the fact that the G-peak is shifted to lower wavenumbers in comparison with ECF180 eV. According to Wei et al. w20x, this can be related to relaxation of the intrinsic stresses in the film under the action of more intensive ion bombardment. Moreover, this effect is more pronounced at the maximum amplitude of the bias voltage pulses (Usubs6 kV, Fig. 4, spectrum 噛6). At the same time, the ratio of ID yIG obtained from Raman spectra is slightly changed, indicating that the proportion of diamond-like phase in the films is still higher than 50%. An improvement in coating adhesion can also occur due to ion mixing at the film–substrate interface and between the film layers. 4. Conclusion The possibility of preliminary PIII and PIID of diamond-like a-C:H films from non-self-sustained arc discharge plasma, as well as from the plasma generated by the closed-drift ion source, has been experimentally shown. The plasma parameters were studied by means of a Langmuir probe. The ion-plasma operation regime
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of the closed-drift ion source (Ps0.3 Pa, Ids0.6 A, Uds600 V), as well as the non-self-sustained arc discharge with moderate discharge current (Ps0.2 Pa, Ids5 A, Uds50–60 V), was shown to be optimum for PIII. At the same time, the plasma operation regime of the closed-drift ion source (Ps0.5 Pa, Ids3 A, Uds 300 V), as well as the non-self-sustained arc discharge with maximum discharge current (Ps0.5 Pa, Ids10 A, Uds50–60 V), is optimum for a-C:H film PIID. Owing to the absence of a hot filament and better scalability, the closed-drift ion source is more attractive from the technological point of view than the non-self-sustained arc-discharge plasma source. The pulsed bias parameters at the substrate optimum for the preliminary PIII (Usubs12 kV, ts45 ms, f s 660 Hz) and PIID of a-C:H films (Usubs6 kV, ts60 ms, f s660 Hz) have been determined. Diamond-like a-C:H films with maximum content of sp3 carbon were obtained in the energy interval ECs150–240 eV per deposited carbon atom. The coatings obtained at ECG180 eV are characterized by better adhesion to the substrate that can be related to relaxation of intrinsic stresses in the film under the action of more intensive ion bombardment. This effect is more notable at the maximum amplitude of the bias voltage pulses (Usubs 6 kV). Improvement in coating adhesion can also take place due to ion mixing at the film–substrate interface, as well as between the film layers. References w1x J.R. Conrad, J.L. Radtke, R.A. Dodd, F.J. Worzala, N.C. Tran, J. Appl. Phys. 62 (1987) 4591. w2x B.P. Wood, I. Henins, W.A. Reass, et al., Nuclear Instruments and Methods in Physics B96 (1995) 429. w3x J. Chen, J.R. Conrad, R.A. Dodd, J. Mater. Eng. Perf. 2 (1993) 839. w4x J.N. Matossian, R. Wei, Surf. Coat. Technol. 85 (1996) 92. w5x K.C. Walter, M. Nastasi, C. Munson, Surf. Coat. Technol. 93 (1997) 287. w6x M. Tuszewski, I. Henins, M. Nastasi, et al., IEEE Trans. Plasma Sci. 26 (1998) 1653. w7x X.M. He, J.-F. Bardeau, K.C. Walter, M. Nastasi, J. Vac. Sci. Technol. A17 (1999) 2525. w8x A. Anders (Ed.), Handbook of Plasma Immersion Ion Implantation and Deposition, John Wiley & Sons, New York, 2000. w9x L.G. Vintizenko, N.N. Koval’, P.M. Shchanin, V.S. Tolkachev, Instrum. Exp. Techniques 43 (3) (2000) 375. w10x N. Vershinin, B. Straumal, K. Filonov, Thin Solid Films 351 (1999) 190. w11x S.P. Bugaev, N.S. Sochugov, Surf. Coat. Technol. 131 (2000) 472. w12x J.N. Matossian, D.M. Goebel, US Patent 5 218 179, Hughes Aircraft Company, Los Angeles, CA, 1991. w13x J.N. Matossian, D.M. Goebel, US Patent 5 296 272, Hughes Aircraft Company, Los Angeles, CA, 1993. w14x P.M. Shchanin, N.N. Koval’, D.P. Borisov, Izv. VUZ Fiz. 37 (3) (1994) 115, In Russian.. w15x V.V. Zhurin, H.R. Kaufman, R.S. Robinson, Plasma Sources Sci. Technol. 8 (1999) R1.
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w16x R. Vuppuladhadium, H.E. Jackson, R.L.C. Wu, J. Appl. Phys. 77 (1995) 2714. w17x S. Aisenberg, J. Vac. Sci. Technol. A 8 (1990) 2150. w18x J.N. Matossian, D.M. Goebel, Surf. Coat. Technol. 85 (1996) 86.
w19x F. Qian, R.K. Singh, S.K. Dutta, P.P. Pronko, Appl. Phys. Lett. 67 (1995) 3120. w20x Q. Wei, R.J. Narayan, A.K. Sharma, et al., J. Vac. Sci. Technol. A 17 (1999) 3406.
Deposition of highly adhesive amorphous carbon films with the use of preliminary plasma-immersion ion implantation S.P. Bugaev, K.V. Oskomov*, N.S. Sochugov Institute of High-Current Electronics SD RAS, 4 Akademichesky Ave., Tomsk 634055, Russia
Abstract The usability of the following two types of plasma generators for deposition of highly adhesive a-C:H films on the large area substrates has been studied: (1) a source of plasma generated by means of a non-self-sustained arc discharge in low-pressure gas; and (2) an ion-plasma source on the basis of a Hall current accelerator with closed electron drift. The distinctive features of both sources are: (a) the possibility of the generation of extended flows (up to 2 m) of relatively dense plasma (;1010 cmy3); and (b) control of the plasma ionization degree, allowing realization of both preliminary plasma-immersion ion implantation (PIII) of a substrate and subsequent plasma-immersion ion-assisted deposition (PIID) of a-C:H film. The results of experimental investigations into the characteristics of the sources in different operational regimes are presented. Taking into account the probe measurements of plasma parameters, both generators have been optimized to operate in the PIII and PIID regimes. Characteristics of the pulsed negative bias applied to the substrate in both regimes have also been determined. It was shown that both sources allowed deposition of a diamond-like film on large-area substrates with a growth rate of 100–300 nm hy1 . A hard (20–30 GPa) a-C:H coating containing approximately 60% of carbon atoms with sp3 hybridization and having satisfactory adhesion to the substrate can be obtained if short (;60 ms) high-voltage (;6 kV) bias pulses are applied to the substrate. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Plasma immersion ion implantation (PIII); Plasma immersion ion-assisted deposition (PIID); a-C:H; Adhesion
1. Introduction Since the first works by Konrad et al. w1x, plasmaimmersion ion implantation (PIII) and plasma-immersion ion-assisted deposition (PIID) have been widely used as material surface modification techniques. This approach, in particular, is convenient for the deposition of highly adhesive and hard a-C:H coatings on different substrates, as collaboratively demonstrated by LANL and General Motors w2x. To create large volumes of hydrocarbon plasma, different types of glow discharge are mainly used: DC w3x, pulsed w4x, capacitive RF w5x and inductive RF w6x. The first three types are characterized by moderate power (up to 1 kW), low operating pressure (0.01–0.1 Pa) and, hence, low plasma density (108 –109 cmy3) and low growth rate of the DLC (0.1 mm hy1). In order to increase the productivity of the a-C:H film PIID process, it is necessary to use *Corresponding author. Tel.: q7-382-2-258-651; fax: q7-382-2259-410. E-mail address: [email protected] (K.V. Oskomov).
denser hydrocarbon plasma. The inductive RF glow discharge plasma is characterized by a comparatively high ion concentration (1010 –1011 cmy3) w6x. However, this is applicable to the PIID of a-C:H films only if a hydrocarbon gas is diluted by the inert one w7x, which again results in growth rates of the coatings of approximately 0.1 mm hy1. A more detailed comparative analysis of different methods of plasma generation for the PIID of diamond-like films is presented in w8x. In this paper, we studied the possibility of an alternative approach of PIII of large-area substrates followed by PIID of a-C:H films. This is based on the use of two types of extended (up to 2 m long) remote plasma sources: (1) a source of plasma generated by means of a non-self-sustained arc discharge; and (2) an ionplasma source on the basis of a Hall current accelerator with closed electron drift. These sources are notable for reliability, simplicity of construction and operation. In spite of the rather large length, these devices can be adjusted to generate relatively dense (ni;109 –1011 cmy3) and uniform w(Dni yni)s"10%x plasma, the
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 0 9 9 - 3
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S.P. Bugaev et al. / Surface and Coatings Technology 156 (2002) 311–316
Fig. 1. Experimental set-up for the PIII of substrates and PIID of the a-C:H films: 1, vacuum chamber; 2, diffusion pump; 3, non-self-sustained arc-discharge plasma source; 4, closed-drift plasma-ion source; 5, substrate; 6, high-voltage substrate-bias pulse power supply; and 7,8, DC power supplies.
parameters of which can be varied over a wide range w9,10x. Hence, continuous back-and-forth movement of the plasma source along the substrate during deposition or implantation processes provides realization of largescale PIII and PIID. This approach is more convenient for processing of flat large-area substrates than those ones previously listed. The two types of device mentioned have been used for plasma cleaning of large glass substrates before metal and oxide film deposition w9– 11x. The purpose of this paper is to show the principle of the possibility of using this approach and the plasma sources for preliminary PIII of large-area substrates followed by PIID of amorphous hydrogenated carbon films.
walls at a distance of approximately 30 cm relative to the water-cooled sample holder (5). Silicon wafers, titanium and stainless steel plates were used as substrates. A generator of high-voltage negative substrate bias pulses (6) with a width of 40–60 ms, amplitude of up to 15 kV and frequency of up to 1 kHz was used in the experiments. To ignite the discharge in the vacuum chamber, DC power supplies (7, 8) were used. A plasma generator on the basis of a non-selfsustained arc discharge (Fig. 2) combines the features of thermionic and hollow cathode sources w12,13x. The combined cathode of this plasma generator consists of a 280-mm-long and 120-mm-diameter hollow metal cylinder (1) encircling a heated tungsten filament (2). The case of the plasma generator (4) and the chamber walls that are at earth potential are used as the anode. Applying DC voltage of the order of 60–70 V to the cathode, the non-self-sustained discharge can be ignited in the chamber. Current of the discharge can reach 50 A and is controlled by the operating gas pressure and the filament current. The magnetic field increasing the plasma ionization degree and stabilizing the discharge is generated by means of electromagnets (3). A more detailed description of the plasma generator can be found in w14x. Another plasma generator used in the experiments (Fig. 3) presents an extended (30 cm long) closed-drift ion source consisting of an anode (1) placed inside a case (cathode) (2) that is, as a rule, connected to the vacuum chamber walls and has earth potential. In the cathode gap (4), a magnetic field of approximately 1300 G is created by means of the permanent magnets (3). This source can operate in the ion-beam generation regime, plasma generation regime or transition regime, depending on the anode voltage and pressure in the chamber w15x.
2. Experimental To demonstrate the applicability of the approach to preliminary PIII of substrates and PIID of amorphous hydrogenated carbon films, laboratory-scale equipment was used in the first stage of the investigations. The experimental set-up consisted of a vacuum chamber (1) with a volume of approximately 0.2 m3 that was evacuated by a diffusion pump (2) (Fig. 1). The plasma sources (3,4) were symmetrically placed at the chamber
Fig. 2. Non-self-sustained arc-discharge plasma source: 1, hollow cathode; 2, hot filament; 3, electromagnets; and 4, case of the source.
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Fig. 3. Closed-drift plasma ion source: 1, anode; 2, case (cathode); 3, permanent magnets; and 4, channel.
The plasma parameters were measured by means of a plane Langmuir probe with a guard ring and the area of the collecting surface equal to 1 cm2. In addition to the plasma characteristics, the properties of a-C:H films as a function of the deposition conditions were studied. The main tool for analysis of the coating quality was Raman spectroscopy (a DFS-52 spectrophotometer, LOMO, Russia) and the main criterion was the ratio of the integral intensities of the D-peak (;1400 cmy1) and G-peak (;1600 cmy1) in the film spectra w16x. The adhesion and hardness of the coatings were qualitatively determined by the scratch test. In this case, a rubber with abrasive SiO2 particles at a 1-kg load applied perpendicular to the sample surface was moved alternatively along it. 3. Results and discussion 3.1. Main characteristics of the plasma sources The probe measurement results of the non-self-sustained low-pressure arc discharge source plasma and the plasma generated by the ion-plasma source with closed electron drift for different operation regimes are presented in Table 1. The plasma concentration of the nonself-sustained arc discharge is observed to linearly depend on the discharge current Id. It should be noted that, in all cases, plasma filled the chamber almost uniformly, and thus, in the substrate region, no spatial non-uniformity of the plasma parameters within the limits of measurement accuracy ("20%) was observed. The possibility of successful application of thermionic sources for the PIII of different materials and PIID of different coatings, including diamond-like carbon, was demonstrated by Matossian and co-workers w4,12,18x.
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High discharge current (up to 100 A) and high plasma density (up to 1011 cmy3), as well as easy controllability, are the advantages of this type of plasma generator. However, the presence of the tungsten filament reduces the technological potential of this plasma generator due to the limited operation resource, especially in reactive environments (oxygen, air, hydrocarbon, etc.). Hence, the next step was to study the possibility of application of the closed-drift ion-plasma source free from the above-mentioned shortcomings for PIII of substrates and subsequent PIID of a-C:H films. In the case of the closed-drift ion-plasma source, three different operational regimes can be described (Table 1). The first regime is realized at pressures lower than 0.3 Pa. The discharge current of 30 mA corresponds to a discharge voltage of 1000 V. In these conditions, plasma with a density of 109 cmy3 and an ion beam propagating in straight lines from the anode through the gap in the cathode simultaneously exist (ion regime). When the pressure is higher than 0.3 Pa, the conditions for volume glow-discharge ignition having a 0.6-A current and 600–700-V voltage are created in our experiment. In this operation regime, the plasma concentration is 2=1010 cmy3. Its distribution profile becomes more uniform, although the directed ion beam still exists (ion plasma or transition regime). Finally, with subsequent increase in the operating pressure up to 0.5 Pa, the discharge current and plasma density can be increased to 3 A and ;4=1010 cmy3 (plasma regime), respectively. The discharge voltage drops to 300 V and the plasma concentration profile becomes uniform within the limits of measurement errors. In the ion-plasma and plasma regimes, the linear plasma density profiles are nearly uniform. At present, the closed-drift ion sources are fabricated at the Institute of High Current Electronics; the length is up to 2 m and the linear uniformity of the ion beam and plasma concentration are no less than 10–20%. The values obtained for the plasma concentration were used for calculation of the ion current density at the emission boundary of the plasma by the Bohm formula, and the thickness of the cathode layer formed Table 1 Plasma parameters Plasma source
P (Pa)
Id (A)
Te (eV)
ni (cmy3)
ji (mAycm2)
Not-self-sustained arc-discharge plasma source
0.5 0.2 0.2
10 5 1
3.7 3.7 3.7
3.4=1010 1.7=1010 3.4=109
0.92 0.46 0.09
Closed-drift ion-plasma source
0.5 0.3 0.2
4=1010 2=1010 109
1 0.52 0.03
3 0.6 0.03
4 4 4
P, pressure in the chamber; Id, discharge current; Te , electron temperature; ni, ion concentration; and ji, ion current density.
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near the substrate when the negative polarity bias voltage is applied. As observed from Table 1, the characteristics of the plasma generated by the closed-drift ion source in the ion-plasma and plasma regimes of operation coincide within the limits of measurement errors, with discharge current of Ids5 and 10 A, respectively, for the non-self-sustained arc discharge plasma. It is shown below that these regimes are of greatest interest for PIID of the a-C:H films and preliminary PIII of the substrates. Operating pressure values in the cases considered are also close to each other. Hence, we can conclude that the physical rules for the PIII and PIID processes for the non-self-sustained arc discharge regimes considered and the corresponding operation regimes of the closeddrift ion source will be identical. 3.2. Preliminary PIII of substrates and PIID of a-C:H films Experiments on a-C:H film deposition from the nonself-sustained arc discharge plasma in methane without substrate bias have shown that the growth rate and the characteristics of the films obtained depend on the discharge current. For example, the rate of film growth drops by from 3 mm hy1 to 500 nm hy1 as the discharge current decreased from 10 to 5 A and the operational pressure decreased from 0.5 to 0.2 Pa. The plasma concentration, according to the probe measurements, decreases by only two-fold, from 3.4=1010 to 1.7=1010 cmy3 (Table 1). In our case, regime of the operation with lower discharge current (Ids5 A) is optimum for PIII realization, as the a-C:H film growth rate is low enough to completely etch the film at the high-voltage bias applied to the substrate. According to the Child–Langmuir law, in this regime the width of the double layer for 12-kV bias voltage pulses is equal to 6 cm, which approximately coincides with the free pass length at a 0.2-Pa pressure. The best PIII results have been obtained at pulse amplitude, width and repetition rate for the substrate bias voltage of 10–12 kV, 45 ms and 660 Hz, respectively. As previously mentioned, a methane plasma with the necessary characteristics in this case can be generated both by means of the nonself-sustained arc discharge (Ps0.2 Pa, Ids5 A, Uds 50–60 V) and the closed-drift ion source (Ps0.3 Pa, Ids0.6 A, Uds600 V). The results of scratch tests on the a-C:H films obtained have shown that, in order to achieve adhesion to the silicon, stainless steel and titanium substrates, it is necessary to carry out preliminary PIII for 30 min. Using the value calculated for the current density in methane in the PIII regime, a dose of 8.6=1016 cmy2 was obtained. This agrees with the literature data: in w2x, the authors obtained adhesion of the a-C:H film to aluminum after preliminary PIII of carbon into the
Table 2 a-C:H film PIID conditions 噛
Gas
Usub (kV)
v (nm sy1)
EC (eV atomy1)
IDyIG
1 2 3 4 5 6 7 8
CH4 CH4 CH4 CH4 CH4 yAr (2:1) CH4 CH4 yAr (1:4) CH4 yAr (1:4)
0 1.2 2 4 2.5 6 0.5 1
0.78 0.55 0.48 0.18 0.06 0.09 0.07 0.06
0 40 70 110 150 180 240 420
– – – 1.1 0.8 0.8 0.8 0.9
Usub, amplitude of substrate bias pulses; v, a-C:H film growth rate; EC, energy per carbon atom deposited; and ID yIG , Raman spectral Dand G-peak integral intensity ratio.
substrate from RF discharge plasma in acetylene at a dose of 1017 cmy2. Non-self-sustained low-pressure discharge can also be used for PIII of a-C:H films. In contrast to the PIII regime, the optimum regime for PIID is the non-selfsustained arc discharge with high current (Ps0.5 Pa, Ids10 A, Uds50–60 V) and corresponding plasma regime of the closed-drift ion source operation (Ps0.5 Pa, Ids3 A, Uds300 V). In these cases, maximum plasma concentration in the substrate region (Table 1) and, hence, maximum growth rates of the a-C:H film, are achieved. The main parameter influencing the structure and properties of amorphous carbon and hydrocarbon films is the energy EC per one deposited carbon atom w17x, which depends on the balance of the ion and radical flow to the growing coating surface. The radical flow density was determined from the a-C:H film growth rate and density in the absence of substrate bias. The ion current density was evaluated using the Bohm formula and on the assumption of the previously determined plasma parameters. Table 2 presents the experimental conditions for aC:H film deposition, the value calculated for EC, and the ratio (ID yIG) of integral intensities of the D- and Gpeaks. The ion flow to the substrate was practically unchanged (the ion current density was ji f0.9–1 mA cmy2), since the plasma parameters, as well as length and frequency of the bias voltage pulses (tsubs60 ms, f subs660 Hz), remained approximately constant. The radical flow to the surface was changed due to the use of methane and argon mixtures with different CH4 content. The bias voltage pulse amplitude was varied within the limits Usubs0–6 kV. Fig. 4 presents Raman spectra of the a-C:H films. It is evident that with an increase in EC higher than 100 eV, spectral changes characterize the transition from polymer-like to diamond-like films. When EC is in the range from 150 to 240 eV, the diamond-like carbon content in the coating reaches a maximum (in accor-
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Fig. 4. Raman spectra for the a-C:H films obtained by the PIID method in accordance with the experimental conditions presented in Table 2.
dance with the minimum of the ratio of the D- and Gpeak integral intensities). According to Qian et al. w19x, the value of ID yIGs0.8 is characteristic of a-C:H films containing 50–60% of diamond-like carbon. With subsequent increase in EC, the film graphitization process begins. Hence, the optimum energy per carbon atom is in the range of 150–240 eV. The film growth rate is 200–300 nm hy1, which is 1.5–2-fold higher than in the case of glow-discharge RF plasma generated in large (;1 m3) volumes w2x. As well as the energy per deposited carbon atom, suitable energy of the ions bombarding the growing aC:H film, i.e. the bias pulse amplitude, is also important. As follows from Raman spectra (Fig. 4), the coatings obtained at ECG180 eV are characterized by the fact that the G-peak is shifted to lower wavenumbers in comparison with ECF180 eV. According to Wei et al. w20x, this can be related to relaxation of the intrinsic stresses in the film under the action of more intensive ion bombardment. Moreover, this effect is more pronounced at the maximum amplitude of the bias voltage pulses (Usubs6 kV, Fig. 4, spectrum 噛6). At the same time, the ratio of ID yIG obtained from Raman spectra is slightly changed, indicating that the proportion of diamond-like phase in the films is still higher than 50%. An improvement in coating adhesion can also occur due to ion mixing at the film–substrate interface and between the film layers. 4. Conclusion The possibility of preliminary PIII and PIID of diamond-like a-C:H films from non-self-sustained arc discharge plasma, as well as from the plasma generated by the closed-drift ion source, has been experimentally shown. The plasma parameters were studied by means of a Langmuir probe. The ion-plasma operation regime
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of the closed-drift ion source (Ps0.3 Pa, Ids0.6 A, Uds600 V), as well as the non-self-sustained arc discharge with moderate discharge current (Ps0.2 Pa, Ids5 A, Uds50–60 V), was shown to be optimum for PIII. At the same time, the plasma operation regime of the closed-drift ion source (Ps0.5 Pa, Ids3 A, Uds 300 V), as well as the non-self-sustained arc discharge with maximum discharge current (Ps0.5 Pa, Ids10 A, Uds50–60 V), is optimum for a-C:H film PIID. Owing to the absence of a hot filament and better scalability, the closed-drift ion source is more attractive from the technological point of view than the non-self-sustained arc-discharge plasma source. The pulsed bias parameters at the substrate optimum for the preliminary PIII (Usubs12 kV, ts45 ms, f s 660 Hz) and PIID of a-C:H films (Usubs6 kV, ts60 ms, f s660 Hz) have been determined. Diamond-like a-C:H films with maximum content of sp3 carbon were obtained in the energy interval ECs150–240 eV per deposited carbon atom. The coatings obtained at ECG180 eV are characterized by better adhesion to the substrate that can be related to relaxation of intrinsic stresses in the film under the action of more intensive ion bombardment. This effect is more notable at the maximum amplitude of the bias voltage pulses (Usubs 6 kV). Improvement in coating adhesion can also take place due to ion mixing at the film–substrate interface, as well as between the film layers. References w1x J.R. Conrad, J.L. Radtke, R.A. Dodd, F.J. Worzala, N.C. Tran, J. Appl. Phys. 62 (1987) 4591. w2x B.P. Wood, I. Henins, W.A. Reass, et al., Nuclear Instruments and Methods in Physics B96 (1995) 429. w3x J. Chen, J.R. Conrad, R.A. Dodd, J. Mater. Eng. Perf. 2 (1993) 839. w4x J.N. Matossian, R. Wei, Surf. Coat. Technol. 85 (1996) 92. w5x K.C. Walter, M. Nastasi, C. Munson, Surf. Coat. Technol. 93 (1997) 287. w6x M. Tuszewski, I. Henins, M. Nastasi, et al., IEEE Trans. Plasma Sci. 26 (1998) 1653. w7x X.M. He, J.-F. Bardeau, K.C. Walter, M. Nastasi, J. Vac. Sci. Technol. A17 (1999) 2525. w8x A. Anders (Ed.), Handbook of Plasma Immersion Ion Implantation and Deposition, John Wiley & Sons, New York, 2000. w9x L.G. Vintizenko, N.N. Koval’, P.M. Shchanin, V.S. Tolkachev, Instrum. Exp. Techniques 43 (3) (2000) 375. w10x N. Vershinin, B. Straumal, K. Filonov, Thin Solid Films 351 (1999) 190. w11x S.P. Bugaev, N.S. Sochugov, Surf. Coat. Technol. 131 (2000) 472. w12x J.N. Matossian, D.M. Goebel, US Patent 5 218 179, Hughes Aircraft Company, Los Angeles, CA, 1991. w13x J.N. Matossian, D.M. Goebel, US Patent 5 296 272, Hughes Aircraft Company, Los Angeles, CA, 1993. w14x P.M. Shchanin, N.N. Koval’, D.P. Borisov, Izv. VUZ Fiz. 37 (3) (1994) 115, In Russian.. w15x V.V. Zhurin, H.R. Kaufman, R.S. Robinson, Plasma Sources Sci. Technol. 8 (1999) R1.
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w16x R. Vuppuladhadium, H.E. Jackson, R.L.C. Wu, J. Appl. Phys. 77 (1995) 2714. w17x S. Aisenberg, J. Vac. Sci. Technol. A 8 (1990) 2150. w18x J.N. Matossian, D.M. Goebel, Surf. Coat. Technol. 85 (1996) 86.
w19x F. Qian, R.K. Singh, S.K. Dutta, P.P. Pronko, Appl. Phys. Lett. 67 (1995) 3120. w20x Q. Wei, R.J. Narayan, A.K. Sharma, et al., J. Vac. Sci. Technol. A 17 (1999) 3406.