HYDROGEN-ATOM YIELD IN RF AND MICROWAVE HYDROGEN DISCHARGES

The hydrogen atom yield in pure-H-2 RF and microwave-sustained dischar ges is investigated both theoretically and experimentally. A particle balance model is developed that provides the concentrations of the H, H-2, H+, H-2+, and H-3+ species. It is also shown that an approximate solution of this model is adequate for calculating the concentration o f H atoms (required, for instance, in diamond film deposition) in the 0.1-10 torr range. Next, the validity of the actinometry technique app lied to the determination of the H-atom density in pure-H-2 discharges is examined Using this diagnostic, it is observed that the H-atom con centration decreases when the vessel wall temperature increases, owing to the increased efficiency of atomic hydrogen recombination on the w all. To overcome this effect, the discharge tube wall is cooled off wi th dimethylpolysiloxane, a low-loss dielectric liquid. It improves sig nificantly the H-atom concentration at 2450 MHz provided the pressure is typically below a few torr and the power density is not too high.