Measurement of the plasma potential adjacent to the substrate in a midfrequency bipolar pulsed magnetron

Abstract

Using an emissive probe, the temporal evolution of plasma potential Vp in front of an electrically isolated substrate in an asymmetric pulsed dc magnetron has been determined. The discharge pulsing frequency was 100 kHz, with a 50% duty cycle. Through a scheme of externally biasing the emissive probe, it was found that the time response of the probe could be improved greatly, and a resolution of 20 ns was achieved. This good response revealed that Vp is highly modulated by the transient cathode potential, following it closely and varying from a value just above the ground potential in the pulse “on” phase, up to a value of +277 V during the positive overshoot in the “reverse” pulse phase. During the whole pulse cycle, Vp was found to remain above the most positive surface in the discharge. The results confirm our previous prediction for Vp, based on energy-resolved mass spectrometry [Bradley et al., Plasma Sources Sci. Technol. 11, 165 (2002)], which indicated that ions must be created at high positive plasma potentials. However, measurements here show that the substrate floating potential Vf is also strongly modulated and the difference Vp−Vf, which determines the ion bombarding energy, always remains below 40 V during steady phases of the discharge throughout the pulse cycle.