Diagnostic Studies of a CH<INF>4</INF>/H<INF>2</INF> Microwave Plasma by Mass Spectrometry:  Ionic and Neutral Species

The major ionic and neutral product species in CH<INF>4</INF>/H<INF>2</INF> plasmas were studied as a function of the hydrogen composition in a 2.465-GHz microwave flow tube at 30-W power and 2.2-Torr total pressure. Mass spectrometry, coupled with Li⁺ ion attachment techniques, was used for identification of the products. Various neutral and ionic polymer molecules with carbon numbers of up to 12 were identified, and the relative concentration profiles of these species were determined as a function of H<INF>2</INF> composition in the feed gas. A possible reaction scheme to account for these observed profiles is suggested. From a comparison of the ionic species with the neutral species in this plasma, it is apparent that dissociative electron ionization is a likely process in the ion chemistry of the CH<INF>4</INF>/H<INF>2</INF> plasma. In addition, a general interpretation of the results was attempted for polymeric neutral product formation, particularly with regard to the role of hydrogen atom abstraction processes and radical processes.