Radio-frequency glow discharges in methane gas: modelling of the gas-phase physics and chemistry

A methane discharge fluid model is developed, and subsequently combined with a simple gas-phase chemical kinetics model. The aim is to provide better understanding of the charged species dynamics, and their interaction with the gas-phase kinetics in a CH4 plasma. Swarm data are used as input in the fluid model, which predicts the ion and electron densities, electric fields and ionization rates as a function of space and time in the radio-frequency period. Results show that, due to detachment, the negative ion density in CH4 is of order 10-2 that of electrons; a capacitive discharge behaviour is observed analogous to that of an electropositive gas. The effects of electrode spacing (2-6 cm), gas pressure (80 mTorr to 1 Torr) and radio-frequency current (2.2-3.4 mA cm-2 0.06-0.15 W cm-2) are studied and compared successfully with experimental data. The time-averaged, spatially-resolved electron density and energy, the set of cross sections for CH2 and CH3 dissociation, together with an assumption about the form of the electron energy distribution function, are subsequently used as input in a simplified one-dimensional gas-phase kinetic model. The model predicts the CH2 and CH3 spatial profiles, which compare well with experimental data.