Study of an H<SUB>2</SUB>/CH<SUB>4</SUB> moderate pressure microwave plasma used for diamond deposition: modelling and IR tuneable diode laser diagnostic

Infra-red tuneable diode laser spectroscopy (IR TDLAS) has been used to detect and quantify the methyl radical and three stable carbon-containing species (CH₄, C₂H₂ and C₂H₆) in a moderate pressure microwave (f = 2.45 GHz) bell-jar reactor used for diamond films deposition. A wide range of experimental conditions was investigated, with typical pressure/power required to perform diamond deposition, i.e. pressure from 2500 to 12 000 Pa and power from 600 W to 2 kW, which means gas temperatures ranging from 2200 to 3200 K, when the power density increases from 9 to 30 W cm⁻³. Since TDLAS is a line of sight averaged technique, the analysis of the experimental data required the use of a one-dimensional non-equilibrium transport model that provides species density and gas temperature variations along the optical beam. This model describes the plasma in terms of 28 species/131 reactions reactive flow. The thermal non-equilibrium is described by distinguishing a first energy mode for the electron and a second one for the heavy species. Parametric studies as a function of power density and methane percentage in the gas mixture are presented. The good agreement obtained between measurement and one-dimensional radial calculations allows a validation of the thermo-chemical model, which can be used as a tool to enlighten the chemistry in the spatially non-uniform H₂/CH₄ microwave discharge used for diamond deposition. This is especially of interest for high power density discharge conditions that remain poorly understood.