High stability of atmospheric pressure plasmas containing carbon tetrafluoride and sulfur hexafluoride

Abstract

The properties of electronegative discharges operating at atmospheric pressure have been investigated. The plasma source consisted of two parallel metal electrodes into which helium was fed with 0.5–8.0 Torr carbon tetrafluoride or sulfur hexafluoride. It was found that the ionization mechanism changed from the α- to the γ-mode at a critical RF power level. In pure helium, this resulted in an abrupt drop in the voltage with increasing current, whereas for the fluorine-containing plasmas, a smooth, continuous transition was observed along the I–V curve. The electron densities in the plasmas fed with 4.0 Torr CF4 and SF6 were calculated to be 7.0± 2.0 × 1011 cm−3, or about 30 times lower than that estimated for the pure helium case. Free electrons were consumed by electron attachment to the gas molecules, which boosted the density of negative ions to ∼1013 cm−3. Compared with pure He, the lower electron density in the fluorine-containing plasmas removed the ionization instabilities encountered in the γ -mode and made it possible to sustain these discharges over substantially larger areas.