The statistics of spoke configurations in high-power impulse magnetron sputtering discharges

Abstract

Ionisation zones, or spokes to which the discharge self-organises during a high-power impulse magnetron sputtering (HiPIMS) pulse, are a recently investigated phenomenon; however, adequately describing or representing these highly dynamical features is a challenge. As the spokes rotate above the target their properties can change over time, with splitting or merging frequently occurring. Here we investigate the evolution of quasi-stable spoke configurations (modes) during the HiPIMS pulse by simultaneously employing six flush-mounted strip probes evenly distributed over the target and through observation by a fast camera. This arrangement was used to track the changes in the spoke configuration. The effect of the discharge current at two different pressures on the spoke configuration was statistically examined. A large amount of data was evaluated to claim that at the pressure of 0.2 Pa, there exists an equal probability for spokes to merge and to split. By contrast, spoke configurations at a pressure of 4 Pa exhibited a strong driving force towards higher spoke mode numbers, which was reflected in a significantly higher occurrence of spoke splitting over spoke merging. A simple phenomenological model describing spoke merging and splitting is presented.