Analysis of the chemistry in CH4/O2 plasmas by means of absorption spectroscopy and a simple numerical model

The chemistry in CH4/O2 rf discharges has been investigated by comparing molecular concentrations measured by tunable diode laser absorption spectroscopy with a hypothetical plasma composition calculated by a simple chemical rate equation system. This comparison allowed us to investigate the reaction paths of molecules, which undergo long reaction chains, and to gather information on the high-energy tail of the electron energy distribution function (EEDF). The procedure has been applied to a capacitively and two inductively coupled plasma sources with strongly varied external parameters (p=10-100 Pa, Q=2-60 SCCM) and for a wide range of electron densities (ne=1014-1017 m-3). In mixtures of CH4 and O2 the main production and depletion channels of two stable products, CO and CO2, have been obtained, and their relative importance under modified operational conditions has been studied. Furthermore, the analysis of the molecular densities of CH4, CH3, C2H2, and C2H6 dependent on the input power in an inductively coupled discharge with pure CH4 as source gas allowed us to determine a range of EEDFs, which are compatible with the measured concentrations