Infrared laser diagnostics in methane chemical-vapor-deposition plasmas

Abstract

Tunable diode laser absorption spectroscopy has been used to measure the concentrations of reactive species in an rf (20 kHz) chemical-vapor-deposition reactor as a function of methane flow rate, pressure, current, and added inert gas. The IR measurements have been supplemented by mass spectrometry and optical emission spectroscopy. The reactive hydrocarbon species detected by IR were CH4, C2H2, C2H4, and C2H6, and the free radical CH3. No significant spatial variation in methyl radical concentration was measured between the parallel-plate electrodes under our conditions. The growth rate of amorphous carbon films on the lower (ground) electrode was found to vary linearly with the concentration of CH3 measured close to this electrode, and these results are compatible with a CH3 sticking coefficient of ≤0.02. One-dimensional modeling calculations using the facsimile program were carried out to simulate the variation of the concentration of the principle species with current (electron number density). Satisfactory agreement with the measurements of CH4, CH3, and C2H6 concentrations was obtained when ionization and ion molecule reactions, as well as reactions of neutral species, were included in the chemical model.