CHARACTERIZATION OF HE/CH4 DC GLOW-DISCHARGE PLASMAS BY OPTICAL-EMISSION SPECTROSCOPY, MASS-SPECTROMETRY, AND ACTINOMETRY

The gas-phase kinetics responsible for the formation of some electronically excited radicals (CH) and atoms (H, He, Ar) in glow discharge plasmas of He-methane admixtures has been investigated under several conditions of gas composition and discharge current at a total initial pressure of 0.01 Torr. Actinometry has been used to characterize the microscopic plasma parameters and, in combination with mass spectrometry and optical emission spectroscopy, to establish the formation mechanism of excited species. A very good correlation between CH emission intensity and carbon deposition rate has been found under all conditions. The effective cracking kinetic constant for methane molecules depends on plasma conditions, its value ranging from almost-equal-to 1 to 5 X 10(10) cm3 s-1 as plasma current is increased, in good agreement with the expected value according to the actinometric results. A simplified kinetic model, accounting for all the observations reported in this work, is proposed.