Spokes in high power impulse magnetron sputtering plasmas

Abstract

High power impulse magnetron sputtering (HiPIMS) is a deposition technique where a metal magnetron target is sputtered in a high density plasma to synthesize thin layers with superior properties on a substrate material.These plasmas are characterised by short pulses in the range of 50 µs to 200 µs and very high peak powers in the range of several kW/cm 2 per target area.The understanding of these dynamic plasmas is of upmost interest for the further development of this coating technique.Fast camera measurements revealed the formation of localised ionisation zones in these plasmas that propagate with a velocity of the order km/s.In the case of a circular magnetron, the movement of these ionisation zones appears like the movement of a set of spokes, which led to this common expression spoke to illustrate the pattern formation in these high density plasmas.The analysis and understanding of the spoke phenomenon is still a matter of an open debate, because the analysis and the theoretical description 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.