Investigations of capacitively-coupled plasmas by electrostatic probe technique

This work reports on the electric characterization of capacitively-coupled RF plasmas by employing electrostatic (Langmuir) probes. Experiments with argon, oxygen and sulfur hexafluoride (SF₆) plasmas were carried out. The floating potential and cathode self-bias, as well as the shape of the Langmuir probe current-voltage characteristics were measured and interpreted. The current-voltage characteristic for SF₆ plasmas showed a strong variation in the floating potential, and a distortion in the region of electron retarding. An unexpected behavior of the curves was observed for the higher pressure discharge regime, at 100 mTorr. The cathode self-bias also was very different for SF₆ plasmas when compared to more electropositive oxygen and argon discharges. The results obtained pointed to the massive presence of negative ions in SF₆ plasmas.