Radical and film growth kinetics in methane radio-frequency glow discharges

The gas kinetics of a 30 mTorr radio-frequency methane glow discharge are studied as a function of methane depletion including conditions suitable for hard carbon thin-film deposition. Mass spectrometry is used to measure the partial pressures of the species C2H6, C2H4, C2H2, C3H8, C3H6, and C3H4. Net film growth was calculated using mass balance and corroborated by direct measurements of deposition rate. Using a combination of static and flowing discharge measurements, the net yields of C2H6, C2H4, and C2H2 are described using a simple analytic model. C2H6 is modeled as a production from CH3 recombination, and the production of C2H4 is modeled as reaction of CH with CH4 where the CH can be produced both by direct electron collisional dissociation of CH4 as well as reaction of CH2 with H. C2H2 production is modeled as arising principally from C2H4 depletion. The principal dissociation mechanism of these molecules appears to be electron collisional dissociation. The CH3 radical densities deduced from this analysis are in good agreement with threshold ionization radical measurements reported in the literature. In addition, the methane electron collisional dissociative branching is inferred to be approximately 68% CH3 and 32% CH2+CH. The results of this analysis when compared to the observed film yield imply that the CHn radicals do not make a significant direct contribution to amorphous carbon film growth. Rather, the film appears to result from the depletion products of discharge-produced gas molecules such as C2H6, C2H4, and C2H2. (C) 1996 American Institute of Physics.