Electron density modulation in an asymmetric bipolar pulsed dc magnetron discharge

Abstract

This paper investigates the spatial and temporal variation in plasma electron density over a region between 5 and 10cm above the race-track region of a pulsed magnetron sputtering target. The pulse operation is performed using an asymmetric bipolar pulsed dc power supply, which provides a sequence of large negative “on-phase” voltage (−350V) and a small positive “reverse-phase” voltage (+10V) for 55% of the pulse duration (10μs). The electron density is measured using a floating microwave hairpin resonance probe. The results show electron expulsion from the target in the initial on phase, which propagates with a characteristic speed exceeding the ion thermal speed. In the steady state on phase, a consistent higher density is observed. A quantitative model has been developed to explain the resultant density drops in the initial on phase. While in the reverse phase, we observed an anomalous growth in density at a specific location from the target (d>7cm). The mechanism behind the increase in electron density has been attributed to the modulation in spatial plasma potential, which was measured earlier in the same apparatus using a floating emissive probe [J. W. Bradley et al., Plasma Sources Sci. Technol. 13, 189 (2004)].