Experimental investigation of the chemistry in a capacitively coupled hydrocarbon/oxygen radio frequency discharge

The densities and temperatures of methane, oxygen, the methyl radical, carbon monoxide and carbon dioxide have been measured using absorption spectroscopy in a capacitively coupled radio frequency (rf) (13.56 MHz) discharge with varied mixtures of methane and molecular oxygen as feed gases under flowing conditions (flow rate ≤10 sccm, total pressure 100 Pa, input power ≤80 W). Whereas neither translational nor rovibrational temperatures rise much above room temperature, up to 90% of the feed gas species are dissociated, and the methyl radical attains densities of the order of 1017-1018 m-3. The observed depletion of the feed gases and the production of methyl as a function of the applied rf power are described consistently by means of analytical expressions derived from simple rate equations of the volume chemistry. Some experimental findings, however, cannot be explained by volume reactions only, indicating that surface processes probably play an important role in the overall plasma chemistry.