Study of an ion-ion plasma by biasing a large planar probe with square waveforms

摘要

An electrical study of an ion-ion plasma by an intrusive planar probe biased with a square waveform is presented here. The experiments are performed in the downstream chamber of a Helicon source in a "pure" ion-ion plasma region in SF6. For frequencies in the kHz range applied to the planar probe, the positive and negative ions saturations currents are alternately measured following the applied waveform. Whereas for low frequencies (in the Hz range), we observe a decrease of the negative ion saturation current collected on the probe during the positive bias period, while the positive ion saturation current is constant versus time. We suggest that the phenomenon is due to a dielectric layer deposited on the reactor walls. When negative ions are collected on the probe, the plasma compensate the lost of negative charge by positive charge going to the wall. Due to the dielectric layer on the reactor walls this positive ion current is limited. However, for the positive ions collected on the probe, the current is instantly compensated by mobile electrons flowing along the field lines.