SPATIALLY-RESOLVED MEASUREMENTS OF ABSOLUTE CH3 CONCENTRATION IN A HOT-FILAMENT REACTOR

Methyl radicals are generated in a hot-filament diamond synthesis reac tor using a resistively heated tungsten filament (20 mm long) in a slo wly flowing mixture of 0.5% CH4 in H-2. The UV absorbance of CH3 is me asured during deposition using a line-of-sight optical technique calle d cavity ring-down spectroscopy (CRDS). Measurements are carried out a t 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 concentra tion throughout the reactor to the substrate temperature. At some oper ating 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 agreeme nt with two-dimensional models of the deposition environment, and is a ttributed to the effect: of Soret diffusion on the balance of the prim ary methyl production/destruction reaction. (C) 1997 Elsevier Science S.A.