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Evaluation of the Sheath Factor in Electronegative Gas Plasmas

The influence of the change in the sheath structure on the Langmuir probe characteristics in an electronegative gas plasma was evaluated quantitatively using the reduction rate of the ion saturation current in a SF6/Ar double plasma. Here, the density ratio of negative ions to positive ions, a, and ion mass were estimated from the phase velocity of the ion acoustic wave and the mass analysis with a quadruple mass spectroscopy (QMS), respectively. It was found that the sheath factor, which represents the change in the sheath structure, increases when a > 0.6, and that it agrees qualitatively with the expansion rate of sheath surface area calculated theoretically.