Hard diamond-like carbon thin film deposited by multi pulse HiPIMS

抄録

1. Introduction

Diamond-like carbon (DLC) films have been widely used in industrial applications, especially cutting tools, sliding parts within automobile engines, and biomedical devices due to their unique properties. DLC films with a large amount of sp³ C bonds could have very high hardness and good wear characteristics. Because the ratio of sp³ C bonds to sp² C bonds strongly depends on the numbers of the incident carbon ions with an optimum kinetic energy, the discharge plasma with large number of carbon ions is required for the synthesis of dense and hard DLC film.

High-power impulse magnetron sputtering (HiPIMS) is one of ionized physical vapor deposition methods and is expected to synthesize hard DLC films. Up to now, a short-pulse HiPIMS was proposed to prepare hard DLC films [1], and then we proposed double pulse HiPIMS at a short pulse width and a relatively large target current [2]. In this study, DLC films are synthesized by a multi-pulse HiPIMS to achieve harder DLC films. The objective is to synthesize harder DLC films through the investigation on the relationship between the microstructure of DLC film and the setting parameters, such as the pulse number of HiPIMS, pulse interval, and pulse width.

2. Experiment

DLC thin films were fabricated via multi pulse HiPIMS at Ar gas pressures of 0.6 Pa and average power of 80 W. The multi pulse HiPIMS plasma was produced in a cylindrical vacuum chamber with 150 mm inner diameter and 220 mm height. As a typical example, the temporal waveform of the target current I_T(t) is shown in Fig. 1. The first HiPIMS plasma temporally evolved during a pulse-on time of the voltage source with a width of about 13 μs and the discharge current reached about -45 A corresponding to 1.7 A/cm². After the application of the first-pulse voltage for HiPIMS, multi-pulse voltages were applied to produce the multi pulse HiPIMS. The peak target current of the second and subsequent HiPIMS was about -40 A. The optimum energy of the energetic carbon ions for bombardment could be controlled by applying a negative pulse voltage of -100 V to the substrates. The film structure and the film density were analyzed using the Raman spectroscopy and X-ray reflectivity measurement.

3. Results and Discussion

Figure 2 shows the relationship between Raman parameters estimated from the measured Raman spectrum and N, where the number N is the number of short-pulse HiPIMS per one period. The measured Raman spectrum was assigned to the G (graphite) peak at 1540-1560 cm⁻¹ and D (disorder) peak at 1350-1370 cm⁻¹, respectively. According to the model proposed by Ferrari and Robertson [3], the I(D)/I(G) ratio is correlated with the size of sp² C network organized in aromatic rings, where I(D) and I(G) are the intensities of the D peak and G peak, respectively. In addition, the FWHM of the G peak provides insight into the structural disorder. As shown in Fig. 2, the FWHM of the G peak gradually increased from 192.1 cm^-1 to 203.7 cm^-1 with the increase in N. On the other hand, the I(D)/I(G) ratio was less sensitive to N for N ≧2, but the I(D)/I(G) ratio of the film synthesized at N ≧2 were smaller than that of the film synthesized at single pulse HiPIMS. These results indicate that the hardness of the films synthesized at N ≧2 is higher than that of the film synthesized by single HiPIMS due to the increase in the ratio of sp³ C bond to sp² C bond.

4. Conclusion

The structure of DLC film and the film density depended on the pulse number of the multi pulse HiPIMS.

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