Spokes in high power impulse magnetron sputtering plasmas

High-power impulse magnetron sputtering is a deposition technique where a metal magnetron target is sputtered in a high-density plasma to synthesise thin layers with superior properties on a substrate material. These plasmas are characterised by short pulses in the range of 50 mu s to 200 mu s and very high peak powers in the range of several kW cm(-2) per target area. Understanding these dynamic plasmas is of upmost importance for the further development of this coating technique. Fast camera measurements have revealed the formation of localised ionisation zones in these plasmas, which propagate with a velocity of the order km s(-1). In the case of a circular magnetron, these ionisation zones appear to move like a set of spokes, which has led to the expression spoke being commonly used to illustrate the pattern formation in these high-density plasmas. Analysing, understanding and theoretically describing the spoke phenomenon is still a matter of open debate, which is hampered by the inherent complexity of these plasmas. In this paper, we review the experimental observations of the spoke phenomenon and highlight several approaches for their theoretical explanation.