Incorporating the gas flow in a numerical model of rf discharges in methane

A two-dimensional modified fluid model for a capacitively coupled rf discharge in methane, used for the deposition of diamond-like carbon layers, is presented. The gas velocity calculated with a computational fluid dynamics code is used as an input for the modified fluid model. Convection is taken into account as an additional transport mechanism as well as diffusion and migration. The calculations show that the gas flow results in a shift of the maximum of the densities of the plasma species toward the grounded electrode. It is shown that this shift has a large effect on the fluxes of the ions and radicals toward the substrate where the deposition occurs. As a result, the deposition rate will increase if the gas velocity has a component directed to the surface and it will decrease in the opposite case. However, the uniformity of the layer can become worse when the convection velocity is too high for the reactor geometry studied here.