Surface reaction kinetics of CH₃ in CH₄ RF discharge studied by time-resolved threshold ionization mass spectrometry

The surface loss probability β of CH₃ radicals on a-C:H surface is determined by time-resolved threshold ionization mass spectrometry in pulsed RF discharges. In the post-discharge, the β value decreases from 0.011-0.015 to 0.001-0.002 in about 10ms. This time evolution suggests that the loss probability depends on incoming ion and radical fluxes on the surface. We present a simple model of the surface reaction kinetics in which the number of active chemisorption sites on the surface is determined by the balance between their production, due to ions and/or H, and their annihilation, due to CH₃ radicals. The model describes fairly well the observed decay of the CH₃ radical density and the time evolution of β determined experimentally in the afterglow. The maximum contribution of the CH₃ radicals to the deposition rate of a-C:H films was deduced to be about 60% of the total deposition rate determined experimentally.