Resonance enhanced multiphoton ionization probing of H atoms and CH sub(3) radicals in a hot filament chemical vapour deposition reactor

Resonance enhanced multiphoton ionization spectroscopy has been used to provide spatially resolved in situ measurements of H atom and CH sub(3) 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 (CH sub(4) and C sub(2)H sub(2)), the hydrocarbon/H sub(2) process gas mixing ratio, the total pressure and flow rate, and the temperatures of both the filament and substrate. H atoms number of densities are observed to be a maximum at the hot filament surface, to be independent of the H sub(2) pressure, p(H sub(2)), in the range 5-55 Torr, and to drop monotonically with increasing radial distance from filament, d. In contrast, the CH sub(3) radical number density arising both in dilute CH sub(4)/H sub(2) and C sub(2)H sub(2)/H sub(2) gas mixtures is found to scale with the hydrocarbon input as 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 CH sub(3) radical formation is dominated by gas phase reactions both when using CH sub(4) and C sub(2)H sub(2) as the carbon sources gas. The formation mechanism of CH sub(3) radicals in a hot filament reactor operating with C sub(2)H sub(2)/H sub(2) gas mixtures is considered further.