Eh. Wahl et al., SPATIALLY-RESOLVED MEASUREMENTS OF ABSOLUTE CH3 CONCENTRATION IN A HOT-FILAMENT REACTOR, DIAMOND AND RELATED MATERIALS, 6(2-4), 1997, pp. 476-480
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.