Resonance enhanced multiphoton ionization probing of H atoms and CH3 radicals in a hot filament chemical vapour deposition reactor

Abstract

Resonance enhanced multiphoton ionization spectroscopy has been used to provide spatially resolved in situ measurements of H atom and CH3 radical relative number densities and the local gas temperature in a hot filament reactor used for diamond chemical vapour deposition (CVD). Parameters varied include the hydrocarbon (CH4 and C2H2), the hydrocarbon/H2 process gas mixing ratio, the total pressure and flow rate, and the temperatures of both the filament and substrate. H atom number densities are observed to be a maximum at the hot filament surface, to be independent of the H2 pressure, p(H2), in the range 5–55 Torr, and to drop monotonically with increasing radial distance from the filament, d. In contrast, the CH3 radical number density arising both in dilute CH4/H2 and C2H2/H2 gas mixtures is found to scale with the hydrocarbon input gas pressure, and to be rather constant for d<4 mm and to decline thereafter. These direct measurements serve to reinforce the consensus view that H atom production during diamond CVD in a hot filament reactor arises as a result of dissociative adsorption on the hot filament surface, whereas CH3 radical formation is dominated by gas phase reactions both when using CH4 and C2H2 as the carbon source gas. The formation mechanism of CH3 radicals in a hot filament reactor operating with C2H2/H2 gas mixtures is considered further.