GAS-PHASE KINETICS DURING MICROWAVE PLASMA-ASSISTED DIAMOND DEPOSITION - IS THE HYDROCARBON PRODUCT DISTRIBUTION DICTATED BY NEUTRAL-NEUTRAL INTERACTIONS

The mole fractions of H, H-2, CH3, CH4, and C2H2 that were generated in a microwave plasma under conditions typical for diamond deposition are reported. When the methane percentage in the feed gas was < 1%, the measured concentrations were surprisingly similar to those reported for a hot-filament system operated under nearly identical conditions. However, as the methane percentage was increased beyond 1%, a noticeable deviation from the hot-filament results became apparent. In the hot-filament case, surface-induced dissociation of hydrogen on the filament surface was significantly inhibited at this high methane level and lead to a pronounced reduction of the H-atom concentration. But, in the microwave system where dissociation is a homogeneous process, no such reduction was observed. Other observations were: (i) The acetylene concentration scaled as the square of the methyl concentration; (ii) The CH4 + H <--> CH3 + H-2 reaction was found to be at the same level of nonequilibrium as in the hot-filament case. These results, plus arguments based on general principles of plasma chemistry, are taken as indications that the hydrocarbon chemistry in the microwave system is as strongly dictated by neutral-neutral reactions as in a thermal system. The primary effect of the plasma is perhaps its role in dissociating hydrogen molecules to H atoms without having to raise the gas temperature significantly.