Characteristics of sub-atmospheric pressure glow discharge plasmas for preparation of a-C:H films

A new method of synthesis of a-C:H films using a pulsed glow discharge plasma with a mixed gas of helium and nitrogen at sub-atmospheric pressure of 2 kPa was proposed. Characteristics of the pulsed glow discharge plasma produced with a pair of parallel plate electrodes by applying high-repetition bipolar voltage pulses were investigated using optical emission spectroscopy. The ratio of intensities of the 0-0 transitions of the second positive system of nitrogen molecule (337.1 nm) and the first negative system of ionized nitrogen molecule (391.4 nm) was measured to investigate the electric field strength in the discharge plasma. The spatial structure of the plasma emission was similar with that of a typical glow discharge. It was also found that an intensive electric field appeared near the cathode electrode during negative ripple pulse voltages induced by an inductor added between the electrodes and the power supply. A uniform a-C:H film on a silicon substrate with hardness of 7 GPa was successfully prepared with the pulsed glow discharge plasma with the ripple pulse voltage application, where acetylene was used for the precursor gas. The deposition rate is 0.2 μm/min which is approximately 10 times higher than that of the conventional plasma CVD method under the low gas pressure. It is discussed that a rough estimate of incident ion energy onto the negatively biased substrate using a collisional plasma sheath model indicates the possibility of increase in the incident ion energy that is needed for enhancement of fraction of sp3 carbon bond due to decrease in the neutral gas pressure from 100 kPa to 2 kPa.