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.

Nucleation and Growth of Magnetron‐Sputtered Ag Nanoparticles as Witnessed by Time‐Resolved Small Angle X‐Ray Scattering

“…We can name this process a growth-transport instability. Such "coupled processes" instability can be actually an interesting analogue of the ionization-movement instabilities observed as fast "spokes" in closed-drift (E × B drift) devices such as magnetrons [31][32][33] or Hall thrusters [34,35] but at a much longer timescale. However, we should admit that details of these phenomena remain unclear at present and require further investigation.…”

Rotating Spokes, Ionization Instability, and Electron Vortices in Partially Magnetized E×B Plasmas

“…An intriguing feature of experiments on × devices is that spokes do not always rotate in the × direction. Retrograde motion (in the − × direction) has also been observed in the low current regime of HiPIMS or in direct current (DC) magnetrons 23,24 .…”

“…Rotating spokes have also been observed in Hall thrusters [17][18][19][20] and in pulsed planar magnetrons used in plasma processing (HiPIMS, for High Power Impulse Magnetron Sputtering) 1,21 . In some papers on Hall thrusters 17 or magnetrons 22 the CIV concept has been invoked to explain the presence of rotating spokes but there is no consensus on the physics of the rotating spokes in spite of important recent progress in the diagnostic and qualitative interpretation of these phenomena 23,24 . An intriguing feature of experiments on × devices is that spokes do not always rotate in the × direction.…”

Filamentation of capacitively coupled plasmas in large magnetic fields

“…The goal of this work is to offer new insights into a particular experimental configuration that could lead to the formation of filaments and to motivate continuous experimental studies. Although the motivation of this work is in the use of magnetized CCPs in the study of dusty plasmas, cross-magnetic field transport is also an important phenomenon in many applications of low temperature plasmas, such as magnetrons 24 and Hall effect thrusters. 25 In this work, we have found that the filaments are quasistationary structures that originate from statistical variations in the local plasma potential and charging of surfaces.…”