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PECVD of hydrogenated diamond-like carbon films from CH4–Ar mixtures: growth chemistry and material characteristics
Diamond and Related Materials 13 (2004) 1361–1365
PECVD of hydrogenated diamond-like carbon films from CH4 –Ar mixtures: growth chemistry and material characteristics G. Cicalaa,*, P. Brunob, A.M. Losaccoc a
IMIP-CNR wyo Department of Chemistry, University of Bari, Bari, Italy b Department of Chemistry, University of Bari, Bari, Italy c Centro Laser, S.P. Casamassima km3, 70010, Valenzano, Bari, Italy
Abstract Hydrogenated diamond-like carbon film growth from CH4 –Ar plasmas has been investigated under the effect of negative DC self-bias voltage. The deposition rate, the optical emission spectroscopy data and the material properties have been examined for understanding the growth mechanism. It is suggested that the sputter-etching process in competition with the growth process is responsible for the decrease of deposition rate at high bias voltage (i.e. high radio frequency power and low pressure). The clear and important roles of Ar ions and H atoms in sputter-etching process are well evidenced by the joined analysis of deposition rate, emitting species intensities and modified material properties. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: CH4 –Ar plasmas; Hydrogenated diamond-like carbon films; Optical emission; Material properties
1. Introduction Hydrogenated amorphous carbon films (a-C:H) produced via plasma enhanced chemical vapour deposition (PECVD) with diamond-like properties have historically and successfully been used in several fields like scratchresistant and wear protective coatings. The various applications are largely dictated by their chemical inertness, biocompatibility, hardness, optical transparency and thermal stability w1x. Although the extreme simplicity of the CH4 monomer gas, the plasma processes responsible for the film formation are very complex and a complete understanding of the growth mechanism is still unclear and lacking. So far, most studies w2,3x on this topic have been done for a-C:H films produced from pure CH4 gas and grown on the anode. A comparative investigation on a-Si:H and a-C:H films obtained by SiH4 and CH4 plasmas, respectively, has been reported by Perrin et al. w2x who believe that the films are mainly formed by SiH3 and CH3 growth precursors. The great difference in the surface reaction kinetics is due to the different molecular structure of the pyramidal SiH3 on a-Si:H and the planar CH3 on a-C:H although *Corresponding author. Tel.: q39-80-544-2102; fax: q39-80-5442024. E-mail address: [email protected] (G. Cicala).
the same formula of sylil and methyl radicals. Further, the weak physisorption and the low surface reactivity of CH3 is supposed to be strengthened and enhanced under H atom flux andyor ion bombardment. Recently, von Keudell et al. w3x provided a direct identification of the synergism between CH3 radical and hydrogen atoms during the growth by using in situ real-time ellipsometry. The sticking coefficient of methyl is found to enhance up to two orders of magnitude (from ;10y4 to ;10y2) in presence of H atoms. Hard a-C:H films (i.e. diamond-like carbon films), however, are produced on the capacitively coupled radio frequency (r.f.) electrode of a parallel plate reactor, which operates at bias voltage higher than that of the anode. For the fact that the role of ions cannot be neglected during the growth as generally assumed for deposition on the grounded electrode, nowadays many efforts are addressed to the formulation of a mechanism which takes into account also the ion activity w1,3,4x. In 1996, Mantzaris et al. w4x have proposed the ion-stitching mechanism to explain the incorporation of the adsorbed hydrocarbon radical induced by bombardment of low-energy ions. High-energy ions make the sputtering process significant. Other research groups w1,3x have claimed the activity of ions via physical subplantation process.
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.10.049
G. Cicala et al. / Diamond and Related Materials 13 (2004) 1361–1365
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Table 1 Experimental parameters for deposition of a-C:H films grown on the glass and crystalline silicon placed onto the cathode Set
r.f. power (W)
Total pressure (mTorr)
DC self-bias voltage, Vb (V)
I II
50–350 250
300 50–500
(y60, y270)W (y170, y400)P
In this paper, we report data on the deposition and properties of a-C:H films produced from CH4 –Ar mixture at the fixed CH4 percentage (% CH4) of 21.4. This value is the lowest one in which the growth occurs and the deposition rate is constant as detailed in our recent paper w5x. The control of the deposition and properties of a-C:H films has been carried out by changing the growth conditions such as total pressure and r.f. power. These two process key-parameters change substantially the negative DC self-bias voltage w6x, developed on the cathode and consequently the ion bombardment of the growing film surface. The optical emission spectroscopy (OES) for in situ diagnostics of CH4 –Ar plasmas has been utilized to get insight into the discharge processes and monitor the plasma influence on the a-C:H deposition. Thus, particular attention has been devoted to investigate the growth kinetics, the evolution of active species responsible for the growth and sputter-etching processes and the influence of ion bombardment on the material properties. 2. Experimental The depositions of a-C:H coatings were performed in a reactive ion etching Plasma Tech DP800 reactor operating at 13.56 MHz. In this apparatus both the anode and the cathode were water-cooled at approximately 25 8C and the r.f. power was coupled to the bottom electrode holding the silicon and glass substrates. The negative DC self-bias voltage (Vb), created on the cathode connected via a matching box with a blocking capacitor to an r.f. generator, allows the biasing of conductive, semiconductive and insulating substrates placed on it. Two sets of a-C:H films were prepared and studied by changing r.f. power (W) and total pressure (P) as listed in Table 1 and keeping constant the following experimental parameters: CH4 flow rate (FCH4), 15 sccm; Ar flow rate (FAr), 55 sccm; total flow rate (Ftot), 70 sccm, CH4 percentage (%CH4s (FCH4)y(FCH4qFAr)=100), 21.4; and deposition temperature, 25 8C. The thickness and the surface roughness of the films were measured using a surface profiler KLATENCOR䉸 P-10. The hydrogen content (cH) and the optical energy gap (E04, (the energy at which the absorption coefficient is equal to 104 ycm) and ET (the Tauc energy as derived by the equation ahvsB(hvy
ET)2) were evaluated by Fourier transform infrared (FTIR NICOLET IMPACT 400 D spectrophotometer) and ultraviolet–visible (Cary 3 UVyVis spectrophotometer Varian) absorption spectra, as reported in detail elsewhere w5x. The root mean square (rms) surface roughness (ss) was also evaluated on a squared 20=20 mm2 area of the sample surface image obtained by atomic force microscopy (AFM, Thermomicroscopes Autoprobe CP) on two different regions. Water contact angles (WCA) were measured by a Rame’-Hart contactangle goniometer (mod. A-100) at 25 8C in static and dynamic modes using the sessile drop technique (a double distilled water droplet of 2 ml). The dynamic advancing and receding angles (uA, uR) were recorded as the probe water was added to and withdrawn from the drop, respectively. The average values of WCA were evaluated by performing three independent measurements on the coating surface. The plasma phase was monitored in situ by OES (spectroscope AvaSpec-2048 Model) that detects the emission of excited species in the spectral range (200–900 nm). The main emission lines observed in CH4 –Ar plasmas are Ar* at 750.39 nm, Arq* at 387.53, 387.21, 386.85 nm, atomic hydrogen lines of the Balmer series (Ha at 656.28 nm, Hb at 486.13 nm, Hg at 434.05 nm), H2* at 602.80, 622.48 nm and CH* (system A 2DyX 2P at 431.42 nm and 432.4) w7,8x. 3. Results and discussion The left- and right-hand ordinates of Fig. 1 collect the emission intensity of Ar normalized to its pressure (Ar*yPAr)W, (Ar*yPAr )P and the (Vb)W, (Vb)P data of r.f. power and pressure sets, respectively. The Ar*yPAr trends represent the fractional density of electron with energy )13.5 eV and exhibit trends akin to Vb. (Vb)W,
Fig. 1. DC self-bias voltages, (Vb)W, (Vb)P, and emission intensities (Ar*yPAr)W, (Ar*yPAr)P as a function of r.f. power and pressure.
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Fig. 2. Emission intensities (a) (CH*)W, (Arq*)W, (Ha)W; (b) (CH*)P, (Arq*)P, (Ha)P as a function of DC self-bias voltage.
(Vb)P vary as expected from the equation VbA(Wy P)1y2 of Ref. w6x. Fig. 2a and b show the evolution of emitting species of r.f. power and pressure runs, respectively. The Arq* increases strongly with increasing the Vb, (at high r.f. power Fig. 2a and low pressure Fig. 2b). The CH* increases linearly in Fig. 2a and remains almost constant in Fig. 2b. The Ha exhibits opposite trends as a function of Vb depending on the variation of r.f. power (Fig. 2a) or pressure (Fig. 2b). The deposition rates of r.f. power and pressure set, indicated as (rD)W and (rD)P, respectively, are displayed
Fig. 3. Deposition rates (rD)W, (rD)P as a function of DC self-bias voltage.
Fig. 4. The solid and empty symbols refer to: (a) hydrogen content, (cH)W, (cH)P; (b) optical energy gap, (E04)W, (E04)P, (ET)W, (ET)P; (c) advancing and receding WCA (uA)W, (uR)W, (uA)P, (uR)P and (d) rms surface roughness (ss)W, (ss)P variation of r.f. power and pressure sets, respectively, as a function of DC self-bias voltage.
in Fig. 3 as a function of Vb. The (rD)P decreases ˚ at linearly, whereas the (rD)W increases up to 3.06 Ays y180 V and then it decreases abruptly as the Vb increases. Let us now examine Fig. 4a–d, the dependence of material properties, such as the hydrogen content (cH), the optical gap (E04, ET), the advancing (uA) and receding (uR) WCA and the rms surface roughness (ss), on the bias voltage. The solid and empty symbols refer to Vb variation of r.f. power and pressure sets, respectively. The cH, E04, ET, uA, uR decrease, whereas the ss increases with increasing the Vb. It is worth noting that in Fig. 4a–d at intermediate values of Vb the (cH)W, (cH)P; (E04)W, (E04)P; (ET)W, (ET)P; (uA)W, (uA)P; (uR)W, (uR)P; and (ss)W, (ss)P data are more
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highly scattered due to the overlap of Vb values obtained by a different combination of r.f. power and pressure. However, the global trend is still preserved as the single ones (r.f. power and pressure). In particular, in Fig. 4c the limit between hydrophobic and hydrophilic character is taken at u of 658 w9x. The results illustrated in Figs. 1–4 highlight that the increasing energy of ions bombarding a growing film on the cathode induces: (a) marked variation of the deposition rate and the emitting species intensities and (b) modification of the material properties. High bias voltage promotes the increase of Arq density (Figs. 2a and b) and notice that the highest Arq intensity is achieved at the highest Vb values (Fig. 2b). The behaviour of CH* and Ha is very different with increasing the Vb obtained from the r.f. power and pressure sets; in particular, the Ha trends are opposite. The rD dependence on the r.f. power indicates the presence of two distinct regimes: (1) low and (2) high bias voltage regime. In the low regime the increase of CHx (xs0–3) radicals and H atoms (qualitatively related to CH* and Ha) are responsible for the growth, whereas in the high regime the strong increase of Ar ions and H atoms much more than CHx radicals (Fig. 2a) enhances the sputter-etching process with respect to the growth, causing the observed decrease in the deposition rate (Fig. 3). The (rD)P also decreases as Vb increases but goes down slightly and ˚ the lowest value (1.17 Ays) reached at y400 V is still ˚ higher than (rD)W one (0.28 Ays) at y270 V. The explanation for this could be due to the different density of CHx, H and Arq (qualitatively related to CH*, Ha and Arq*) species affecting the growth, the etching and the sputtering rates which determine the net deposition rate. In particular, the OES results could allow us to determine which of these last two processes occur predominantly; further investigations, however, need to clarify this last point. Even though, we are aware that emission intensities have to be taken with caution as the actinometry validity has been shown to fail in pure CH4 w10x and has not yet been established in CH4 –Ar mixtures, the mere comparison between Fig. 2a–b and Fig. 3 allows us to observe that the higher the Arq*, Ha, the higher the sputter-etching process. Finally, the decrease of either (rD)P or (rD)W with increasing the negative Vb, is an evidence that there is a competition between the film formation and the film sputtering andy or etching accordingly the OES trends of CH*, Ha and Arq*. Such a competition has already been observed by us w5x by changing the CH4 percentage in the range (0– 100%). The modifications of the film composition, optical energy gap, surface free energy and roughness are found to depend on the energy of ion bombardment, too. The decrease of cH in the film at increasing Vb could be due to the higher efficiency of hydrogen removal by the impact of high-energy ions. The optical gap values
reflect mainly the cH variation. An attempt to quantify the changes in wetting properties of the a-C:H coatings has been done. In this respect, the assessment of surface energy based on the film wettability is important to characterize the coating stability and, therefore, the processes taking place at the water-film interface. The wetting properties show a correlation with the surface roughness and the hydrogen incorporated in the films which vary as a function of bias voltage (Fig. 4a–d). In particular, at low Vb (high pressure and low r.f. power) the WCA increase by indicating that the reduced ion bombardment increases the hydrogen in the bulk sample as well as at its surface and decreases the surface roughness. The roughness decrease induces an increase of WCA, according to the Wenzel equation w11x because the measured contact angle is less than 908 cos ursr cos us
(1)
where r is the roughness factor (defined as the ratio between the increased area of the rough surface and the geometrical surface area) and ur and us are the contact angles for a rough and smooth surface, respectively. In conclusion, higher values of WCA ()658), i.e. lower values of surface free energy, are indicative of good protective properties and weak interaction between the water and the coating. 4. Conclusions In this paper, we have sought to clarify the growth kinetics of a-C:H film through the investigation of the dependence of rD, active reactive species and film properties under the effect of ion bombardment. The behaviour of the Ha, Arq* emissions has been taken as an indicator of the etching and sputtering processes occurring simultaneously to the growth process. Inspection of Figs. 2 and 3 shows a qualitative satisfactory agreement between the decrease of deposition rate and the increase of the sputtering-etching rate based on the interplay of Ha and Arq* species. Despite the considerations that the quantitative determination of H and Arq density is hard from OES measurements, we want to stress that this joined analysis allows us to establish that the higher the H*, Arq* intensities, the lower the deposition rate of carbon films. The present results, however, provide unambiguous evidence that the DC self-bias voltage affects the excited species intensities, which strongly modify the deposition rate and the material properties (composition, transparency, surface free energy and roughness). Specifically, the wetting properties of a-C:H, influenced by the outermost surface layers, have been evaluated to monitor the surface reactivity of the film and the quality of their protective action. The wettability variations have been related to
G. Cicala et al. / Diamond and Related Materials 13 (2004) 1361–1365
the different degree of surface passivation with hydrogen and the surface roughness. Acknowledgments The authors are grateful to Dr G.S. Senesi (IMIPCNR Sezione di Bari) for AFM measurements. References w1x J. Robertson, Mater. Sci. Eng. R 37 (2002) 129. w2x J. Perrin, M. Shiratani, P. Kae-Nune, H. Videlot, J. Jolly, J. Guillon, J. Vac. Sci. Technol. A 16 (1998) 278.
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w3x A. von Keudell, M. Meier, C. Hopf, Diamond Relat. Mater. 11 (2002) 369. w4x N.V. Mantzaris, E. Gogolides, A.G. Boudouvis, A. Rhallabi, G. Turban, J. Appl. Phys. 79 (1996) 3718. w5x G. Cicala, P. Bruno, M.A. Losacco, G. Mattei, Surf. Coat. Sci., in press. w6x Y. Catherine, P. Couderc, Thin Solid Films 144 (1986) 265. w7x A.R. Striganov, N.S. Sventiskii, Tables of Spectral Lines of Neutral and Ionized Atoms, IFI Plenum, New York, 1968. w8x R.W.B. Pearse, A.G. Gaydon, The Identification of Molecular Spectra, fourth ed., Chapman and Hall, London, 1976. w9x E.A. Vogler, Adv. Coll. Interf. Sci. 74 (1998) 69. w10x A. Pastol, Y. Catherine, J. Phys. D: Appl. Phys. 23 (1990) 799. w11x R.N. Wenzel, J. Phys. Colloid Chem. 53 (1949) 1466.
PECVD of hydrogenated diamond-like carbon films from CH4 –Ar mixtures: growth chemistry and material characteristics G. Cicalaa,*, P. Brunob, A.M. Losaccoc a
IMIP-CNR wyo Department of Chemistry, University of Bari, Bari, Italy b Department of Chemistry, University of Bari, Bari, Italy c Centro Laser, S.P. Casamassima km3, 70010, Valenzano, Bari, Italy
Abstract Hydrogenated diamond-like carbon film growth from CH4 –Ar plasmas has been investigated under the effect of negative DC self-bias voltage. The deposition rate, the optical emission spectroscopy data and the material properties have been examined for understanding the growth mechanism. It is suggested that the sputter-etching process in competition with the growth process is responsible for the decrease of deposition rate at high bias voltage (i.e. high radio frequency power and low pressure). The clear and important roles of Ar ions and H atoms in sputter-etching process are well evidenced by the joined analysis of deposition rate, emitting species intensities and modified material properties. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: CH4 –Ar plasmas; Hydrogenated diamond-like carbon films; Optical emission; Material properties
1. Introduction Hydrogenated amorphous carbon films (a-C:H) produced via plasma enhanced chemical vapour deposition (PECVD) with diamond-like properties have historically and successfully been used in several fields like scratchresistant and wear protective coatings. The various applications are largely dictated by their chemical inertness, biocompatibility, hardness, optical transparency and thermal stability w1x. Although the extreme simplicity of the CH4 monomer gas, the plasma processes responsible for the film formation are very complex and a complete understanding of the growth mechanism is still unclear and lacking. So far, most studies w2,3x on this topic have been done for a-C:H films produced from pure CH4 gas and grown on the anode. A comparative investigation on a-Si:H and a-C:H films obtained by SiH4 and CH4 plasmas, respectively, has been reported by Perrin et al. w2x who believe that the films are mainly formed by SiH3 and CH3 growth precursors. The great difference in the surface reaction kinetics is due to the different molecular structure of the pyramidal SiH3 on a-Si:H and the planar CH3 on a-C:H although *Corresponding author. Tel.: q39-80-544-2102; fax: q39-80-5442024. E-mail address: [email protected] (G. Cicala).
the same formula of sylil and methyl radicals. Further, the weak physisorption and the low surface reactivity of CH3 is supposed to be strengthened and enhanced under H atom flux andyor ion bombardment. Recently, von Keudell et al. w3x provided a direct identification of the synergism between CH3 radical and hydrogen atoms during the growth by using in situ real-time ellipsometry. The sticking coefficient of methyl is found to enhance up to two orders of magnitude (from ;10y4 to ;10y2) in presence of H atoms. Hard a-C:H films (i.e. diamond-like carbon films), however, are produced on the capacitively coupled radio frequency (r.f.) electrode of a parallel plate reactor, which operates at bias voltage higher than that of the anode. For the fact that the role of ions cannot be neglected during the growth as generally assumed for deposition on the grounded electrode, nowadays many efforts are addressed to the formulation of a mechanism which takes into account also the ion activity w1,3,4x. In 1996, Mantzaris et al. w4x have proposed the ion-stitching mechanism to explain the incorporation of the adsorbed hydrocarbon radical induced by bombardment of low-energy ions. High-energy ions make the sputtering process significant. Other research groups w1,3x have claimed the activity of ions via physical subplantation process.
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.10.049
G. Cicala et al. / Diamond and Related Materials 13 (2004) 1361–1365
1362
Table 1 Experimental parameters for deposition of a-C:H films grown on the glass and crystalline silicon placed onto the cathode Set
r.f. power (W)
Total pressure (mTorr)
DC self-bias voltage, Vb (V)
I II
50–350 250
300 50–500
(y60, y270)W (y170, y400)P
In this paper, we report data on the deposition and properties of a-C:H films produced from CH4 –Ar mixture at the fixed CH4 percentage (% CH4) of 21.4. This value is the lowest one in which the growth occurs and the deposition rate is constant as detailed in our recent paper w5x. The control of the deposition and properties of a-C:H films has been carried out by changing the growth conditions such as total pressure and r.f. power. These two process key-parameters change substantially the negative DC self-bias voltage w6x, developed on the cathode and consequently the ion bombardment of the growing film surface. The optical emission spectroscopy (OES) for in situ diagnostics of CH4 –Ar plasmas has been utilized to get insight into the discharge processes and monitor the plasma influence on the a-C:H deposition. Thus, particular attention has been devoted to investigate the growth kinetics, the evolution of active species responsible for the growth and sputter-etching processes and the influence of ion bombardment on the material properties. 2. Experimental The depositions of a-C:H coatings were performed in a reactive ion etching Plasma Tech DP800 reactor operating at 13.56 MHz. In this apparatus both the anode and the cathode were water-cooled at approximately 25 8C and the r.f. power was coupled to the bottom electrode holding the silicon and glass substrates. The negative DC self-bias voltage (Vb), created on the cathode connected via a matching box with a blocking capacitor to an r.f. generator, allows the biasing of conductive, semiconductive and insulating substrates placed on it. Two sets of a-C:H films were prepared and studied by changing r.f. power (W) and total pressure (P) as listed in Table 1 and keeping constant the following experimental parameters: CH4 flow rate (FCH4), 15 sccm; Ar flow rate (FAr), 55 sccm; total flow rate (Ftot), 70 sccm, CH4 percentage (%CH4s (FCH4)y(FCH4qFAr)=100), 21.4; and deposition temperature, 25 8C. The thickness and the surface roughness of the films were measured using a surface profiler KLATENCOR䉸 P-10. The hydrogen content (cH) and the optical energy gap (E04, (the energy at which the absorption coefficient is equal to 104 ycm) and ET (the Tauc energy as derived by the equation ahvsB(hvy
ET)2) were evaluated by Fourier transform infrared (FTIR NICOLET IMPACT 400 D spectrophotometer) and ultraviolet–visible (Cary 3 UVyVis spectrophotometer Varian) absorption spectra, as reported in detail elsewhere w5x. The root mean square (rms) surface roughness (ss) was also evaluated on a squared 20=20 mm2 area of the sample surface image obtained by atomic force microscopy (AFM, Thermomicroscopes Autoprobe CP) on two different regions. Water contact angles (WCA) were measured by a Rame’-Hart contactangle goniometer (mod. A-100) at 25 8C in static and dynamic modes using the sessile drop technique (a double distilled water droplet of 2 ml). The dynamic advancing and receding angles (uA, uR) were recorded as the probe water was added to and withdrawn from the drop, respectively. The average values of WCA were evaluated by performing three independent measurements on the coating surface. The plasma phase was monitored in situ by OES (spectroscope AvaSpec-2048 Model) that detects the emission of excited species in the spectral range (200–900 nm). The main emission lines observed in CH4 –Ar plasmas are Ar* at 750.39 nm, Arq* at 387.53, 387.21, 386.85 nm, atomic hydrogen lines of the Balmer series (Ha at 656.28 nm, Hb at 486.13 nm, Hg at 434.05 nm), H2* at 602.80, 622.48 nm and CH* (system A 2DyX 2P at 431.42 nm and 432.4) w7,8x. 3. Results and discussion The left- and right-hand ordinates of Fig. 1 collect the emission intensity of Ar normalized to its pressure (Ar*yPAr)W, (Ar*yPAr )P and the (Vb)W, (Vb)P data of r.f. power and pressure sets, respectively. The Ar*yPAr trends represent the fractional density of electron with energy )13.5 eV and exhibit trends akin to Vb. (Vb)W,
Fig. 1. DC self-bias voltages, (Vb)W, (Vb)P, and emission intensities (Ar*yPAr)W, (Ar*yPAr)P as a function of r.f. power and pressure.
G. Cicala et al. / Diamond and Related Materials 13 (2004) 1361–1365
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Fig. 2. Emission intensities (a) (CH*)W, (Arq*)W, (Ha)W; (b) (CH*)P, (Arq*)P, (Ha)P as a function of DC self-bias voltage.
(Vb)P vary as expected from the equation VbA(Wy P)1y2 of Ref. w6x. Fig. 2a and b show the evolution of emitting species of r.f. power and pressure runs, respectively. The Arq* increases strongly with increasing the Vb, (at high r.f. power Fig. 2a and low pressure Fig. 2b). The CH* increases linearly in Fig. 2a and remains almost constant in Fig. 2b. The Ha exhibits opposite trends as a function of Vb depending on the variation of r.f. power (Fig. 2a) or pressure (Fig. 2b). The deposition rates of r.f. power and pressure set, indicated as (rD)W and (rD)P, respectively, are displayed
Fig. 3. Deposition rates (rD)W, (rD)P as a function of DC self-bias voltage.
Fig. 4. The solid and empty symbols refer to: (a) hydrogen content, (cH)W, (cH)P; (b) optical energy gap, (E04)W, (E04)P, (ET)W, (ET)P; (c) advancing and receding WCA (uA)W, (uR)W, (uA)P, (uR)P and (d) rms surface roughness (ss)W, (ss)P variation of r.f. power and pressure sets, respectively, as a function of DC self-bias voltage.
in Fig. 3 as a function of Vb. The (rD)P decreases ˚ at linearly, whereas the (rD)W increases up to 3.06 Ays y180 V and then it decreases abruptly as the Vb increases. Let us now examine Fig. 4a–d, the dependence of material properties, such as the hydrogen content (cH), the optical gap (E04, ET), the advancing (uA) and receding (uR) WCA and the rms surface roughness (ss), on the bias voltage. The solid and empty symbols refer to Vb variation of r.f. power and pressure sets, respectively. The cH, E04, ET, uA, uR decrease, whereas the ss increases with increasing the Vb. It is worth noting that in Fig. 4a–d at intermediate values of Vb the (cH)W, (cH)P; (E04)W, (E04)P; (ET)W, (ET)P; (uA)W, (uA)P; (uR)W, (uR)P; and (ss)W, (ss)P data are more
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highly scattered due to the overlap of Vb values obtained by a different combination of r.f. power and pressure. However, the global trend is still preserved as the single ones (r.f. power and pressure). In particular, in Fig. 4c the limit between hydrophobic and hydrophilic character is taken at u of 658 w9x. The results illustrated in Figs. 1–4 highlight that the increasing energy of ions bombarding a growing film on the cathode induces: (a) marked variation of the deposition rate and the emitting species intensities and (b) modification of the material properties. High bias voltage promotes the increase of Arq density (Figs. 2a and b) and notice that the highest Arq intensity is achieved at the highest Vb values (Fig. 2b). The behaviour of CH* and Ha is very different with increasing the Vb obtained from the r.f. power and pressure sets; in particular, the Ha trends are opposite. The rD dependence on the r.f. power indicates the presence of two distinct regimes: (1) low and (2) high bias voltage regime. In the low regime the increase of CHx (xs0–3) radicals and H atoms (qualitatively related to CH* and Ha) are responsible for the growth, whereas in the high regime the strong increase of Ar ions and H atoms much more than CHx radicals (Fig. 2a) enhances the sputter-etching process with respect to the growth, causing the observed decrease in the deposition rate (Fig. 3). The (rD)P also decreases as Vb increases but goes down slightly and ˚ the lowest value (1.17 Ays) reached at y400 V is still ˚ higher than (rD)W one (0.28 Ays) at y270 V. The explanation for this could be due to the different density of CHx, H and Arq (qualitatively related to CH*, Ha and Arq*) species affecting the growth, the etching and the sputtering rates which determine the net deposition rate. In particular, the OES results could allow us to determine which of these last two processes occur predominantly; further investigations, however, need to clarify this last point. Even though, we are aware that emission intensities have to be taken with caution as the actinometry validity has been shown to fail in pure CH4 w10x and has not yet been established in CH4 –Ar mixtures, the mere comparison between Fig. 2a–b and Fig. 3 allows us to observe that the higher the Arq*, Ha, the higher the sputter-etching process. Finally, the decrease of either (rD)P or (rD)W with increasing the negative Vb, is an evidence that there is a competition between the film formation and the film sputtering andy or etching accordingly the OES trends of CH*, Ha and Arq*. Such a competition has already been observed by us w5x by changing the CH4 percentage in the range (0– 100%). The modifications of the film composition, optical energy gap, surface free energy and roughness are found to depend on the energy of ion bombardment, too. The decrease of cH in the film at increasing Vb could be due to the higher efficiency of hydrogen removal by the impact of high-energy ions. The optical gap values
reflect mainly the cH variation. An attempt to quantify the changes in wetting properties of the a-C:H coatings has been done. In this respect, the assessment of surface energy based on the film wettability is important to characterize the coating stability and, therefore, the processes taking place at the water-film interface. The wetting properties show a correlation with the surface roughness and the hydrogen incorporated in the films which vary as a function of bias voltage (Fig. 4a–d). In particular, at low Vb (high pressure and low r.f. power) the WCA increase by indicating that the reduced ion bombardment increases the hydrogen in the bulk sample as well as at its surface and decreases the surface roughness. The roughness decrease induces an increase of WCA, according to the Wenzel equation w11x because the measured contact angle is less than 908 cos ursr cos us
(1)
where r is the roughness factor (defined as the ratio between the increased area of the rough surface and the geometrical surface area) and ur and us are the contact angles for a rough and smooth surface, respectively. In conclusion, higher values of WCA ()658), i.e. lower values of surface free energy, are indicative of good protective properties and weak interaction between the water and the coating. 4. Conclusions In this paper, we have sought to clarify the growth kinetics of a-C:H film through the investigation of the dependence of rD, active reactive species and film properties under the effect of ion bombardment. The behaviour of the Ha, Arq* emissions has been taken as an indicator of the etching and sputtering processes occurring simultaneously to the growth process. Inspection of Figs. 2 and 3 shows a qualitative satisfactory agreement between the decrease of deposition rate and the increase of the sputtering-etching rate based on the interplay of Ha and Arq* species. Despite the considerations that the quantitative determination of H and Arq density is hard from OES measurements, we want to stress that this joined analysis allows us to establish that the higher the H*, Arq* intensities, the lower the deposition rate of carbon films. The present results, however, provide unambiguous evidence that the DC self-bias voltage affects the excited species intensities, which strongly modify the deposition rate and the material properties (composition, transparency, surface free energy and roughness). Specifically, the wetting properties of a-C:H, influenced by the outermost surface layers, have been evaluated to monitor the surface reactivity of the film and the quality of their protective action. The wettability variations have been related to
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the different degree of surface passivation with hydrogen and the surface roughness. Acknowledgments The authors are grateful to Dr G.S. Senesi (IMIPCNR Sezione di Bari) for AFM measurements. References w1x J. Robertson, Mater. Sci. Eng. R 37 (2002) 129. w2x J. Perrin, M. Shiratani, P. Kae-Nune, H. Videlot, J. Jolly, J. Guillon, J. Vac. Sci. Technol. A 16 (1998) 278.
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w3x A. von Keudell, M. Meier, C. Hopf, Diamond Relat. Mater. 11 (2002) 369. w4x N.V. Mantzaris, E. Gogolides, A.G. Boudouvis, A. Rhallabi, G. Turban, J. Appl. Phys. 79 (1996) 3718. w5x G. Cicala, P. Bruno, M.A. Losacco, G. Mattei, Surf. Coat. Sci., in press. w6x Y. Catherine, P. Couderc, Thin Solid Films 144 (1986) 265. w7x A.R. Striganov, N.S. Sventiskii, Tables of Spectral Lines of Neutral and Ionized Atoms, IFI Plenum, New York, 1968. w8x R.W.B. Pearse, A.G. Gaydon, The Identification of Molecular Spectra, fourth ed., Chapman and Hall, London, 1976. w9x E.A. Vogler, Adv. Coll. Interf. Sci. 74 (1998) 69. w10x A. Pastol, Y. Catherine, J. Phys. D: Appl. Phys. 23 (1990) 799. w11x R.N. Wenzel, J. Phys. Colloid Chem. 53 (1949) 1466.