Atmospheric pressure methane-hydrogen dc micro-glow discharge for thin film deposition

Atmospheric pressure methane-hydrogen dc micro-plasmas were studied for thin film deposition. Numerical simulations were performed using a hybrid model. The model contained detailed reaction mechanisms for the gas-phase discharge and the surface reactions to predict the species densities in the discharge and the deposition characteristics and its growth rate. Twenty-three species and an eighty-one step reaction mechanism were considered for the gas-phase methane-hydrogen discharge. The surface chemistry consisted of eighteen species and eighty-four reaction steps. The simulations were carried out for a dc parallel plate electrode configuration with an inter-electrode gap of 200 mu m. An external circuit was also considered along with the discharge model and surface reactions. Basic plasma properties such as electron and species density, electric field, electron temperature and gas temperature were studied. Special attention was devoted to study the influence of discharge current and methane mass fraction on the plasma characteristics and the deposition characteristics and its rate. The H-3(+), CH5+ and C2H5+ concentrations were found to be the dominant hydrogen and hydrocarbon ions, respectively. It was found that in atmospheric pressure methane-hydrogen micro-plasmas, C2H6, C3H8, C2H4 and C2H2 are also present at high densities. CH2 and CH3 were found to be the main radicals, which are prominent growth species for diamond-like-carbon deposition. The simulations indicated significant gas heating in the entire regime of operation. Ion Joule heating was found to be dominant in the sheath whereas Franck-Condon heating and heavy particle reaction induced heating was dominant in the volume. A strong dependence of soot formation on gas temperature was observed. At higher discharge current and methane mass fraction the deposited film was predicted to have higher soot content. Mass spectrometry experiments were conducted to measure the methane conversion factor (cf) and the C2H2 density for different discharge currents. Both the methane cf and C2H2 density showed an increase with increasing discharge current. Predictions from simulations agreed favourably with the mass spectrometry measurements.