Modelling of the power dissipation and rovibrational heating and cooling in SiH4-H2 RF glow discharges

Abstract

The electrical power dissipation and the vibrational (v) and rotational-translational (RT) heating and cooling mechanisms in SiH4-H2 radio-frequency (RF) glow discharges are analysed. Detailed balance of the various contributions leads to determination of the steady state v and RT temperatures TV and TR depending on the discharge geometry, total and partial pressures, wall temperature and power density. In low-power SiH4-dominated discharges (0.1-0.2 Torr range), v excitation is mostly due to low-energy electron-SiH4 collisions, but in high-power discharges, dominated by H2 (0.1-1 Torr), v excitation proceeds via collisional relaxation of hot H atoms and exothermic chemical reactions. v cooling is mostly controlled by v-RT transfer and thermal diffusion to the walls and by electron impact dissociation of v excited molecules. In any case the contribution of spontaneous decomposition highly v excited SiH4 molecules (pyrolysis) remains negligible. The model is tested on various experimental situations and the computed values of TV and TR are in fair agreement with available measurements by IR emission spectroscopy and CARS. It is also shown that the spatial temperature profiles may induce thermophoresis on particulates generated within the discharge.