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S. Miyagawa al. Frontier Carbon Technology New Diamondetand Vol. 16, No. 1 2006 MYU Tokyo 33 NDFCT 502 Electrically Conductive Diamond-Like Carbon Coatings Prepared by Plasma-Based Ion Implantation with Bipolar Pulses Soji Miyagawa*, Setsuo Nakao, Junho Choi, Masami Ikeyama and Yoshiko Miyagawa National Institute of Advanced Industrial Science and Technology (AIST), Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan (Received 14 October 2005; accepted 5 December 2005) Key words: DLC, PBII, conductivity, pulse An electrically conductive diamond-like carbon (DLC) coating technique has been developed by plasma-based ion implantation with bipolar pulses. Energetic electrons and ions, which were induced by positive and negative pulses, respectively, were bombarded alternately on a substrate in C7H8 plasma. As a result, the resistivity of DLC film decreased with increasing ion energy under electron bombardment, and reached 1 m1cm at –20 kV. The hardness of the film was 5.4 GPa, and the hydrogen atomic concentration was 10%. TEM observation showed that the electrically conductive DLC films are composed of clusters of graphitelike aggregates. The technique can be applied for conductive DLC coatings on a three-dimensional substrate with high adhesion strength. 1. Introduction DLC coatings are of technological interest for properties such as high hardness, low friction coefficients, and chemical inertness with corrosive resistance. DLC films are generally characterized by high electrical resistivity spanning a large range of values, from 102 to 1016 1cm, depending on the deposition technique and conditions.(1,2) The electrical resistivity of DLC films can be reduced by several orders of magnitude through incorporation of metals or nitrogen in the films.(3,4) Recently, superhard conductive carbon nanocrystallite films of 50 m1cm prepared by ECR sputtering have been reported.(5) Graphite has high electrical conductivity (53 +1cm on basal plane(5)) and possesses high chemical inertness against corrosion. However, as regards the mechanical properties of graphite films, the hardness and the adhesion strength to the substrate, such as stainless steel, are a serious hindrance for practical uses. Therefore, a carbon coating technique with high electrical conductivity, high hardness, and high adhesion strength to the substrate has been desired in many industries. * Corresponding author: e-mail: [email protected] 33 34 New Diamond and Frontier Carbon Technology, Vol. 16, No. 1 (2006) Various deposition methods for DLC coatings have been reported, for instance, ion beam deposition, CVD, cathodic arc, and plasma-based ion implantation/deposition (PBII/D). The PBII/D technique has the advantages of high adhesion strength coating because of an intermixing layer formed by energetic ion implantation, and of conformal coating on threedimensional substrates.(6) Recently, bipolar high-voltage pulses have been applied for the PBII technique,(7) in which pulsed glow discharge plasma is produced around the substrate by a positive pulse, and the ions are implanted/deposited on the substrate covered with hydrocarbon radicals using the subsequent negative high-voltage pulse. The benefits of this technique include heating of the substrate surface by electron bombardment during the deposition, in addition to the conformal coating on a large substrate and the simplicity of the design. In this paper, an electrically conductive DLC coating technique with the bipolar PBII/D is presented. 2. Experimentals DLC coating experiments were performed using PBII/D equipment at AIST-Chubu. The substrate was held at the center of the cylindrical vacuum chamber (640 mm in diameter, 700 mm in length), and a bipolar pulse voltage was applied to the substrate through a feedthrough, in which a thermocouple was embedded for monitoring the substrate temperature. The positive pulse worked both for the generation of pulsed glow discharge plasma around the substrate and for thermal annealing during DLC deposition by energetic electron bombardments. Target temperature was monitored with a thermocouple thermometer insulated from the grounded chamber. Three types of substrate were used in the experiments. Silica glass plates with a 0.2 mm thickness were used for measuring the electrical conductivity. Silicon wafers and stainless steel (SUS304) plates were used for film characterization and for practical interests, respectively. More details about the experimental setup and plasma characteristics have been described in ref. 7. The DLC coating process consists of Ar plasma sputter-cleaning of the substrate surface, a carbon ion implantation using CH4 plasma for increasing the adhesion strength of DLC to the substrate, and then a DLC deposition in C7H8 plasma. Positive and negative pulse voltages were varied within +2 to +5 kV and –5 to –20 kV, respectively, and their pulse frequencies were 3 to 5 kHz. The typical operating pressure was approximately 3×10–2 Pa. After the deposition, the DC electric resistivity was measured with four-point-probe equipment (Micro-Swiss) at room temperature, and the microhardness was measured using a microindentor (Hysitron). The corrosion behavior was assessed by recording potentiostatic anodic polarization curves in 0.5 M H2SO4 solution. The hydrogen concentration was determined by ERD using 2.8 MeV He ions with a 75° incident beam angle with respect to the surface normal. Raman D and G peak intensities and sp2/sp3 ratios were also measured using a Raman spectroscope (Renishaw, Ar:514.5 nm) and XPS (VG, Al:K_), respectively. The microstructure of DLC films was observed with a high resolution transmission electron microscope (HRTEM, JEOL, JEM2010) at an accelerating voltage of 200 kV. DLC film thickness was measured using a surface profilemeter. Target temperature can be increased up to 1000°C by increasing the positive pulse voltage and pulse frequency. In this experiment, most of the depositions were performed in the temperature range of 200°C to 450°C. 35 S. Miyagawa et al. 3. Results and Discussion Resistivity (1cm) Figure 1 shows a pulsed negative voltage dependence of the electrical resistivity of DLC films formed between –5 kV and –20 kV. Pulsed positive voltage (+4 kV), pulse frequency (3 kHz), deposition time (60 min), and all other parameters were kept constant. The result shows that the resistivity of DLC films decreases with increasing negative voltage, and reaches less than 1 mWcm at 20 kV. The target temperature increased with negative pulse voltage and it was approximately 400°C at 20 kV. Film thickness ranges from 0.6 to 1.1 mm depending on the negative voltage. Higher negative voltage induces an increase in ion current on the substrate and in plasma density, resulting in an increase in film thickness, while sputtering using energetic carbon ions and etching using atomic hydrogen at high temperature (chemical sputtering) induces a decrease in film thickness. As a result, the maximum deposition rate was approximately –10 kV. The resistivity can still be further decreased by increasing the pulsed positive voltage and/or pulse frequency, which led to an increase in the amount of electron bombardment on the substrates, and the value approached that of graphite above 800°C. The resistivity of DLC deposited on a stainless steel plate (resistivity 57 +1cm) under the same deposition conditions was also measured by the four-point-probe method, and the resistivity of the stainless steel plate coated with a DLC film of 1.0–0.5 +m thickness was 100–200 +1cm at –20 kV. In this case, an oxide layer of the stainless steel surface was removed by an Ar plasma cleaning process prior to the DLC deposition. No exfoliation of the film on the substrate was found even at a thickness more than 1 +m. A carbon composition gradient formed by CH4 ion implantation in the PBII/D process was effective in enhancing the adhesion strength of DLC films on the stainless steel substrate.(7,8) On corrosion behavior, the DLC coated films showed a marked decrease in anodic current density, which is more than two figures compared with those of the uncoated plate. Figure 2 shows the pulsed negative voltage dependence of the microhardness of DLC films. The hardness decreased to 5.4 GPa at –20 kV with increasing negative pulse voltage. Bias Voltage (kV) Fig. 1. Pulsed negative voltage dependence of electrical resistivity of DLC films deposited on glass substrate at positive voltage of 4 kV and pulse frequency of 3 kHz. 36 New Diamond and Frontier Carbon Technology, Vol. 16, No. 1 (2006) Intensity (counts) Hardness (GPa) The hardness is lower than that of conventional DLC films (10–20 GPa), but higher than that of glassy carbon (~3 GPa). The decrease in the hardness is due to the increase in graphite content, but the decrease seems to be restricted by a blocking of crack propagation by the randomly oriented graphite cluster, as shown later. Raman spectra of the DLC films deposited with negative pulse voltages of –5 kV and –20 kV are shown in Fig. 3. The spectra consist mainly of two peaks, a G peak around 1570 cm–1 and a D peak around 1350 cm–1. The ratio of the D to G peak intensities I(D)/I(G) ratio was calculated by fitting Gaussians to estimate the degree of ordering of sp2-hybridized carbon atoms. I(D)/I(G) ratio increased continuously with negative bias voltage from 2.4 at –5 kV to 3.4 at –20 kV, and G peak position also increased from 1575 cm–1 to 1594 cm–1, as shown in Fig. 4. It is well known that an increase in the amount of sp2-bonded atomic sites in amorphous carbon results in an increase in I(D)/I(G) ratio and a shift in G band position to a higher wave number.(2) Therefore, this provides a confirmation of the promotion of sp2- Bias Voltage (kV) Raman Shift (cm–1) I (D)/I (G) G Position (cm–1) Fig. 2 (left). Pulse negative voltage dependence of microhardness of electrically conductive DLC films deposited on Si substrate. The applied load was 100 mgf. Fig. 3 (right). Raman spectra of DLC films deposited at different pulsed negative voltages, –5 kV and –20 kV. Bias Voltage (kV) Fig. 4. Negative voltage dependences of I(D)/I(G) ratio and G position of Raman spectra of DLC films. 37 S. Miyagawa et al. bonding with increasing ion energy. XPS spectra were also measured for the film formed under the conditions of +4 kV and –20 kV, as shown in Fig. 5. The spectrum has been deconvoluted into three binding energies representing bonding states, and sp2/sp3 ratio can be estimated to be 0.8. Figure 6 shows the negative voltage dependence of the hydrogen concentration obtained from the ERD spectrum of DLC films. The hydrogen concentration decreases from 24% at –5 kV to 10% at –20 kV. Figure 7 shows an example of an HRTEM image of the DLC film obtained under the deposition conditions (+4 kV, –20 kV). The figure shows many fringe patterns, and the fringe spacing is approximately 0.34 nm, which corresponds to the lattice spacing of the graphite (002) plane. The direction of the lattice is oriented randomly and the film consists of densely distributed graphite clusters. 12000 8000 Hydrogen Fraction (at%) Intensity (counts) 10000 6000 4000 2000 0 280 282 284 286 288 Binding Energy (eV) 290 Bias Voltage (kV) Fig. 5 (left). C1s XPS spectra of DLC films (+4 kV, –20 kV). Fig. 6 (right). Hydrogen concentration of DLC films measured by ERD method. Fig. 7. HRTEM bright-field image of DLC films deposited at –20 kV. 38 New Diamond and Frontier Carbon Technology, Vol. 16, No. 1 (2006) The deposition takes place due to repeat sticking of neutral radicals and bombardment of energetic ions and electrons on the film surface. The energy deposition on the subplantation layer by energetic ions and electron bombardment leads to considerable local heating and hydrogen desorption. It is known that ion energy plays a critical role in the deposition, and the ion energy used in the deposition is much larger than the energy, around 100 eV per C ion, at which the highest fraction of sp 3 is formed. (2) Energetic ion bombardments result in an atomic redistribution of mobile hydrogen due to displacement of hydrogen from CH bonds, and the displaced hydrogen can then diffuse toward the surface under the electron bombardment and recombine with other hydrogen to form H2 molecules, which are desorbed from the film surface. This enhances the graphitization of growing DLC films. The surface topography of the DLC films prepared even at –20 kV was optically flat. This may be ascribed to the removal of dissociated carbon atoms by atomic hydrogen in plasma, because of predominant chemical sputtering (etching) in the temperature range.(9) The electrical conductivity of the DLC films at room temperature could be modeled as taking place by thermally activated conduction along linkages or chains of sp2 carbon atoms with a variable range and variable orientation hopping.(10,11) 4. Summary Electrically conductive DLC coatings on a three-dimensional substrate were performed using PBII/D with bipolar pulses. The resistivity of the film decreased with the pulsed negative voltage and reached 1 m1cm at –20 kV. It has been shown by Raman spectroscopy and by TEM observation that DLC films are composed of clusters of graphitelike aggregates. The electrically conductive DLC coated stainless steel can be used as an electrode in electrochemical studies, such as those involving water treatment and fuel batteries. References 1 2 3 4 5 6 7 8 9 10 11 A. Grill: Thin Solid Films 355/356 (1999) 189. J. Robertson: Mater. Sci. Eng. R37 (2002) 129. K. Baba and R. Hatada: Surf. Coat. Technol. 158/159 (2002) 373. H. Kinoshita, T. Hando and M. Yoshida: J. Appl. Phys. 89 (2001) 2737. S. Hirono, S. Umemura, M. Tomita and R. Kaneko: Appl. Phys. Lett. 80 (2002) 425. A. Anders: Handbook of Plasma Immersion Ion Implantation and Deposition (Wiley, New York, 2000). S. Miyagawa, S. Nakao, M. Ikeyama and Y. Miyagawa: Surf. Coat. Technol. 156 (2002) 322. K. C. Walter, M. Nastasi and C. Munson: Surf. Coat. Technol. 93 (1997) 287. G. Compagnini and L. Calcagno: Mater. Sci. Eng. R13 (1994) 193. R. U. Khan and S. R. P. Silva: Diamond Relat. Mater. 10 (2001) 224. C. Godet, N. M. J. Conway, J. E. Boutee, K. Bouamra, A. Grosman and C. Ortega: J. Appl. Phys. 91 (2002) 4154.