Spatially resolved measurements of absolute CH3 concentration in a hot-filament reactor

Methyl radicals are generated in a hot-filament diamond synthesis reactor using a resistively heated tungsten filament (20 mm long) in a slowly flowing mixture of 0.5% CH4 in H2. The UV absorbance of CH3 is measured during deposition using a line-of-sight optical technique called cavity ring-down spectroscopy (CRDS). Measurements are carried out at 213.9 nm, a wavelength at which the CH3 absorption cross section has been shown, by others, to be independent of temperature over a large range. We observe a strong sensitivity of the methyl radical concentration throughout the reactor to the substrate temperature. At some operating conditions, we also observe the methyl radical concentration to peak at a location several millimeters from the filament surface. This behavior of CH3 with distance from filament is in qualitative agreement with two-dimensional models of the deposition environment, and is attributed to the effect of Soret diffusion on the balance of the primary methyl production/destruction reaction.