Asymmetric plasma potential fluctuation in an inductive plasma source

Abstract

Summary form only given. Time averaged plasma potential in an inductive source has been measured by using an emissive probe. The inflection point method in the limit of zero emission is employed to determine the plasma potential. A T-shaped glass chamber whose dimensions are D=10 cm, L=35 cm is used. Ar plasma is generated by a planar four turn spiral coil antenna mounted on the outside the chamber end window. An aluminum flange, installed at the other end of chamber, serves as system ground. 2% thoriated tungsten wire (D=0.025 mm, L/spl sim/3 mm) is adopted as the emissive probe. All experiments have been performed in H-mode discharge at /spl sim/13 mTorr. In this source, capacitive coupling results in a fluctuating plasma potential. Taking derivative of the I-V characteristic curve of the emissive probe results in two peaks, corresponding to the RF fluctuation. The amplitude of the RF fluctuation, which is approximately given by the separation of the two peaks, has been reduced by introducing a gap between the antenna and the window and by Faraday shielding. The former increases the circuit impedance, the latter directly shields and controls the electrostatic coupling. Experimental data show the lower potential peak, representing more negative plasma potential, approximately constant as the RF fluctuation was reduced, while the higher potential peak, inferring more positive plasma potential, changed and approached the lower potential peak. The lower potential peak takes the role of the DC plasma potential and only the higher potential peak responds to the RF fluctuation.