Non-uniform plasma distribution in dc magnetron sputtering: origin, shape and structuring of spokes

Abstract

Non-homogeneous plasma distribution in the form of organized patterns called spokes was first observed in high power impulse magnetron sputtering (HiPIMS). In the present work we investigate the spoke phenomenon in non-pulsed low-current dc magnetron sputtering (DCMS). Using a high-speed camera the spokes were systematically studied with respect to discharge current, pressure, target material and magnetic field strength. Increase in the discharge current and/or gas pressure resulted in the sequential formation of two, then three and more spokes. The observed patterns were reproducible for the same discharge conditions. Spokes at low currents and pressures formed an elongated arrowhead-like shape and were commonly arranged in symmetrical patterns. Similar spoke patterns were observed for different target materials. When using a magnetron with a weaker magnetic field, spokes had an indistinct and diffuse shape, whereas in stronger magnetic fields spokes exhibited an arrowhead-like shape.The properties of spokes are discussed in relation to the azimuthally dependent electronargon interactions. It is suggested that a single spoke is formed due to local gas breakdown and subsequent electron drift in the azimuthal direction. The spoke is self-sustained by electrons drifting in complex electric and magnetic fields that cause and govern azimuthally dependent processes: ionization, sputtering, and secondary electron emission. In this view plasma evolves from a single spoke into different patterns when discharge conditions are changed either by the discharge current, pressure or magnetic field strength. The azimuthal length of the spoke is associated with the electron-Ar collision frequency which increases with pressure and results in shortening of spoke until an additional spoke forms at a particular threshold pressure. It is proposed that the formation of additional spokes at higher pressures and discharge currents is, in part, related to the increased transport of charge towards the central part of the magnetron.

Plasma potential of a moving ionization zone in DC magnetron sputtering

“…As already suggested in Ref. 6, electrons predominantly interact with the background gas in DCMS, while in HiPIMS discharges, electrons interact with atoms from the target (sputtered and outgassed). The latter can be modulated in HiPIMS because the ionization occurs very close to the target (millimeters) and the refill time is determined by the sputtered atoms, which travel from the target to the location of the ionization.…”

“…A single ionization zone can be obtained when using a small magnetron at low working gas pressures and low discharge currents. 6,11 Our small magnetron was therefore operated at Ar pressure of 0.27 Pa and target potential of À270 V, which yielded 100 mA discharge current. To verify the presence and stability of a single ionization zone, the potential measurements were regularly compared with ICCD images.…”

“…6,7 In both regimes, ionization zones are observed for a wide range of discharge conditions. Ionization zones are stochastic, evolving patterns which occasionally become periodic, indicating selforganization of magnetron plasma.…”

“…Ionization zones are stochastic, evolving patterns which occasionally become periodic, indicating selforganization of magnetron plasma. [4][5][6] Conditions for stochastic versus regular patterns depend on many factors and have been explored in recent publications. 8,9 Generally, the pattern of ionization zones in DCMS tend to be periodic, which was one of the main reasons to utilize DCMS for the current study.…”

“…121, 063302-1 ionization zone forms at very low gas pressures while the number of zones increases when the pressure is increased. 6 Symmetric patterns are commonly observed when two or more zones are present. In HiPIMS discharges, the zones are triangular at low pressure, having an arrow-like shape, with the tip pointing in the E Â B direction, and more globular at higher pressure.…”