Diagnostics and modeling of moderate pressure microwave H2/CH4 plasmas obtained under pulsed mode

This paper deals with experimental investigation and modeling of pulsed H-2/CH4 plasmas used for diamond thin films deposition. Time-resolved measurements of the gas temperature determined from the Doppler broadening of the Balmer H-alpha line and of the discharge volume (V-pl) are reported. The measured temporal evolution of V-pl and input microwave power are used as input parameters for a thermochemical OD model that takes into account 28 species and 2 energy modes. This model enables us to estimate time-variation of the electron energy distribution function, gas temperature and chemical species densities. The time-variation of the gas temperature are non-monotonous and characterized by a sharp increase at the beginning of the pulse and a maximum temperature, (3000 K 250 mus after the pulse ignition). The in-pulse steady state temperature obtained from the model is in agreement with the measured one although a discrepancy is obtained on the shape of the early time-variation. The model is used to investigate the effect of duty cycle at a constant average power density on the dissociation degree in the discharge. Results shows a large enhancement of H-atom. and CH3 densities as the duty cycle is decreased.