MEASUREMENT OF ABSOLUTE CH3 CONCENTRATION IN A HOT-FILAMENT REACTOR USING CAVITY RING-DOWN SPECTROSCOPY

Methyl radicals were generated in a hot-filament diamond synthesis rea ctor using a resistively heated tungsten filament (length, 20 mm) in a slowly flowing mixture of 0.5% CH4 in H-2. The UV absorbance of CH3 w as measured during deposition using a new line-of-sight optical techni que called cavity ring-down spectroscopy (CRDS). Measurements were car ried out at 213.9 mm, a wavelength at which the CH3 absorption cross-s ection has been shown by others to be independent of the temperature o ver a large range. We observed that the CH3 absolute concentration var ied strongly as a function of the position between the substrate and t he filament, and its value was strongly influenced by the substrate te mperature.