Deposition of Diamond-Like Carbon Films by Excimer Lasers Using Frozen Source Gases
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Fig. 1 The experimental arrangemnt of laser ablation used for depositing DLC films by using frozen gases.
The laser beam was introduced into the chamber through a quartz window. We used an ArF laser and a KrF laser (Lambda Physik model EMG 102). The repetition frequency of the laser was set at 10 Hz. To control the laser power density, we adjusted the spot size of laser beams on the target by a quartz lens with a focal length of 35 cm. Since the frozen gas layers were lost quickly by irradiation, it was necessary to move the laser beam constantly across the target to avoid exposing copper surfaces. The substrate used in the present experiment was quartz plate. To control substrate temperature, we placed it on a heated holder at a distance of 4 cm from the target after surface cleaning. Normally, quartz surfaces were cleaned ultrasonically in methanol and rinsed by pure water. When deposited films did not adhere strongly enough and tended to peel off from the substrate, we removed residual hydrocarbon contamination by exposing quartz surfaces to the UV light generated by a deuterium lamp for 15 min at 200oC. 1'
RESULTS First, we show the dependence of film thickness on substrate temperature. The result is shown in Fig. 2. The films were produced in a deposition period of 15 min. The power density of the ArF laser and the KrF laser was normally 9 X l0' W/cm2 and 5 X 10" W/cm2, respectively. The growth rate was greater for the frozen acetylene target than for the frozen acetone target. In either case, the deposition rate did not change significantly with the substrate temperature. A broad laser Raman peak was observed near 1540 cm-' from the films produced by the present method, as shown in Fig. 3. The spectra were taken for the films deposited from acetylene at 200'C with the ArF laser and at 300'C with the KrF laser, while from frozen acetone the film was deposited at 200'C with the ArF laser. The signal-to-noise ratio was high for the spectrum observed in the film deposited from frozen acetone, because it was taken by a different instrument. The observed spectra are characteristic of DLC films.' -.. In addition to the Raman spectra, we measured the electrical resistivity of the films, which was typically 3 X 10 Q - CM.Also, we confirmed chemical inertness of the films in a HF/HNO 3 (1:1) solution. Furthermore, the optical absorption coefficient in the IR and visible regions closely resembled what we observed previously for DLC films produced by
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