Cylindrical magnetron discharges. II. The formation of dc bias in rf-driven discharge sources

Abstract

When a steady-state capacitive rf discharge is sustained between two electrodes whose surfaces in contact with the plasma have different areas, a negative dc self-bias usually develops on the smaller electrode. For ratios of electrode areas greater than about three, the self-bias is typically 80%–90% of the zero-to-peak potential of the applied rf voltage. However, in cylindrical-post magnetron discharges which are driven by rf power, the self-bias is often only 10%–20% of the zero-to-peak applied voltage. Since these discharges are of interest to both sputtering and plasma-assisted etching, we have investigated the formation of the dc self-bias as a function of applied magnetic field strength. We studied discharges using different diameter post electrodes at rf frequencies of 1.8 and 13.56 MHz. The self-bias voltages were indeed found to be considerably less than those observed in nonmagnetron discharges. Electrostatic probes were used to measure the interelectrode potential distribution. From these results we conclude that the reduced dc self-bias in rf driven cylindrical magnetron discharges can be explained in terms of the effect of the axial magnetic field on electron transport to the cylindrical-post electrode during that portion of the rf cycle when the post electrode is serving as an anode.