Diagnostic studies of species concentrations in a capacitively coupledRF plasma containing CH4-H2-Ar

Abstract

Three plasma diagnostic methods, tunable infrared diode laserabsorption spectroscopy, optical emission spectroscopy and microwaveinterferometry have been used to monitor concentrations of transient andstable molecules, CH3, CH4, C2H2, C2H6, and of electrons incapacitively coupled CH4-H2-Ar radiofrequency (RF) plasmas(fRF = 13.56 MHz, p = 100 Pa, ϕtotal = 66 sccm) for various dischargepower values (P = 10-100 W) and gas mixtures. The degree of dissociation ofthe methane precursor varied between 3% and 60%. The methyl radicalconcentration was found to be in the order of 1012 molecules cm-3and the electron concentration in the order of 1010 cm-3. Themethyl radical concentration and the concentrations of the stable C-2hydrocarbons, C2H2 and C2H6, produced in the plasma, increasedwith discharge power. The fragmentation rates of the methane precursor andconversion rates to the measured C-2 hydrocarbons were estimated in dependenceon discharge power. Radial distributions of the electron and methyl radicalconcentrations, and of the gas temperature were measured for the first timesimultaneously in the plasma region between the discharge electrodes. Themeasurements allow us to draw qualitative conclusions on the main chemicalprocesses and the plasma chemical reaction paths.