Gaseous precursors of diamond-like carbon films: chemical composition of CH4/Ar plasmas

We have implemented a gas-phase chemistry numerical simulation of a typical radio frequency, low-pressure plasma. We have applied it to the study of CH4/Ar plasmas used for the deposition of diamond and diamond-like carbon films. Our results show that CH3 is the most abundant carbon-containing radical in pure methane discharges and in CH4/Ar mixtures at low argon concentrations, indicating that the gaseous precursor of the film is CH3. On the contrary, in discharges of methane highly diluted by argon, we find evidence of a transition in the gas-phase composition. Since the most abundant carbon-containing radical becomes the carbon dimer C2, while CH3 gets completely removed from the gas phase, we are led to the conclusion that an alternative growth mechanism should act in highly diluted CH4/Ar plasmas, where the role of the gaseous precursor of the film is played by C2radicals.