Experimental measurements and numerical simulations of the gas composition in a hot-filament-assisted diamond chemical-vapor-deposition reactor

Molecular-beam mass spectroscopy was used to measure the gas composition near a growing diamond surface in a hot-filament-assisted chemical-vapor-deposition reactor. The dependencies of the gas composition on changes in (1) the carbon mole fraction in the reactor feed XC, (2) the identity of the inlet carbon source (CH4 versus C2H2), and (3) the surface temperature TS, were studied. For XC≤0.02, the gas composition appeared to be nearly independent of the identity of the inlet hydrocarbon source and depended only on the C/H ratio in the feed gas. At higher values of XC, catalytic poisoning of the hot filament resulted in different product distributions in these two systems. Increasing the surface temperature affected changes in the hydrocarbon composition; the dependencies of the CH3 and C2H2 mole fractions on TS can each be characterized as having an activation energy of 3±1 kcal/mol. Surprisingly, the H-atom mole fraction was independent of TS. These results suggest that reported temperature sensitivities of film growth properties are primarily due to changes in the kinetics of surface processes rather than changes in the gas composition near the surface. A numerical model of the process is presented. In the study of the compositional change as a function of XC, the code gives good prediction for the methane case but grossly underestimates the methane and methyl concentrations for the acetylene case. The H-atom mole fraction is predicted to increase by ×7 if the H destruction probability on the diamond surface is expected to have an activation energy of 7.3 kcal/mol. Good agreement with experimental data can be obtained, however, if H loss by lateral transport to the walls is taken into account.