Simulation of Atmospheric Pressure Methane-Hydrogen Microdischarge for Diamond Like Carbon (DLC) Film Deposition

Summary form only given. Plasma assisted chemical vapor (PACVD) reactors are frequently used to deposit amorphous carbon layers on materials. These layers also called diamond-like-carbon layers, can be deposited on a variety of substrates by PACVD using different kinds of plasmas, which are generally maintained at low pressure. Operating the plasma at low pressure has several drawbacks which include expensive vacuum systems and high maintenance cost. Atmospheric pressure glow discharges are attractive for a wide range of material processing application largely due to their operation flexibility afforded by the removal of the vacuum system. Atmospheric pressure micro glow discharge of the cold type has been generated in our laboratory. Experimental studies are being performed to use the micro glow discharge for deposition. A hybrid model has been developed to simulate direct current methane-hydrogen plasma operating at atmospheric pressure. The model developed consists of the momentum and energy conservation for the multi-component gas mixture, and continuity equations for each component of the mixture (electrons, ions radicals and neutrals). The model considers a drift-diffusion approximation for the species fluxes. The species considered include 6 neutral species, 8 ionic species, 5 radicals and the electron. The electric field is obtained from the simultaneous solution of the Poisson´s equation. The electron induced reaction rates, electron mobility and diffusion coefficients were obtained by solving a zero-dimensional Boltzmann equation. In the model two or more vibrational excitations for every neutral was included, since a considerable traction of the electron energy is lost in the vibrational excitation reactions. Two dimensional simulations were carried out for a pin-plate electrode configuration together with an external circuit. The results obtained indicated H₃ ⁺ and CH₅ ⁺ to be the dominant ionic species. Volum- etric production of C₂H₅ ⁺ and CH₅ ⁺ were observed and H₃ ⁺ ion density indicated a larger concentration at the sheaths. The radical density indicated abundance of CH₃ and CH₅ radicals, which are the key radicals for DLC depositions. The simulations further indicated the discharge to be a normal glow discharge having a normal current density of 22 A cm^-2. The simulations will help to optimize the plasma parameters (power, gas flow and gas mixture) for good quality deposition.