The chemistry of the radiofrequency ethane discharge

Abstract

The ionic and neutral species produced in an RF discharge in ethane have been examined by mass spectrometry. The most energetic processes occur in the sheath between the bulk plasma and the RF electrode. Primary reactions are observed in this region, while secondary and less energetically induced reactions are observed near the reactor walls. The major ions found near the RF electrode contain less than four carbon atoms and consist of CH 3 +, C 2H 2 +, C 2H 3 +, C 2H 5 +, C 3H 2 +, C 3H 3 +, and C 3H 5 +. The ions found near the wall contain as many as six carbon atoms but consist mainly of C 2H 4 +, C 2H 5 +, C 3H 3 +, C 3H 5 +, C 3H 7 +, and C 4H 5 + through C 4H 9 +. When sufficient energy is available to the electrons the major reaction channel for ethane is, C 2H 6 → C 2H 5 → C 2H 5 +. When less energetic electrons are available, channel C 2H 6 → C 2H 4 → C 2H 4 + dominates. The ion chemistry observed in a given region of the discharge is consistent with the energetics of that region and known ion—molecule processes. The neutral composition is also dependent upon position in the discharge. The ratio of hydrogen produced to ethane consumed is a function of reaction conditions (increasing with increasing plasma density) but is always greatest in the vicinity of the RF electrode.