Destruction of methane in low-pressure, electrodeless radio frequency plasma on quartz walls

Abstract

The destruction of methane in a low pressure, electrodeless radiofrequency discharge was studied by mass spectrometry. Plasmas were created in a quartz tube with the inner diameter of 3.6 cm. A coil with 6 turns was fixed onto the tube and connected to radio frequency (RF) generator via a matching network. Methane flows between 1.16 and 3.30 mbar.l/s and various RF powers up to 1200 W were used. Depending on gas flow and RF power, the discharge was either in E or H mode. The evolution of different hydrocarbon species versus discharge power was measured systematically by a differentially pumped mass spectrometer. No carbon deposit on the quartz walls was seen during the scans. The results showed that the destruction of methane depended on the flow rate as well as the discharge power and was accomplished already in the E mode. Well-pronounced maxima in the formation of both ethane and acetylene were observed at low gas flow rates. The observed products from radical recombination evolved with plasma conditions, and their contribution to the global carbon balance strongly decreased at high power-per-particle density. The functionality of secondary hydrocarbon formation with respect to the experimental parameters has been analyzed and a simple kinetic model is proposed in order to account for the observed trends.