The ‘recycling trap’: a generalized explanation of discharge runaway in high-power impulse magnetron sputtering

Abstract

Contrary to paradigm, magnetron discharge runaway cannot always be related to self-sputtering. We report here that the high density discharge can be observed with all conducting targets, including low sputter yield materials such as carbon. Runaway to a high density discharge is therefore generally based on self-sputtering in conjunction with the recycling of gas atoms in the magnetic field-affected pre-sheath. A generalized runaway condition can be formulated, offering a pathway to a time-dependent model for high-power impulse magnetron sputtering that includes rarefaction and an explanation for the termination of runaway.

Drifting localization of ionization runaway: Unraveling the nature of anomalous transport in high power impulse magnetron sputtering

“…19 With the exception of the recent paper by Kozyrev et al, 33 the literature on HiPIMS generally does not specify anything about the localization of ionization processes other than that ionization processes are concentrated in the neartarget zone, i.e., the magnetic pre-sheath, which is determined by the arched magnetic field. Although Kozyrev et al 33 used quite different HiPIMS conditions in terms of current, voltage, target material, and so on, high speed photographs showed more-or-less periodic structures along the racetrack, which are similar to those shown here in Fig.…”

“…The enhanced supply of neutrals brings the opportunity to even further enhance the plasma density: This was previously discussed as runaway of self-sputtering and near-target gas trapping. 19 FIG . 10. (Color online) Thermalization of fast electrons by Coulomb interaction with the background electrons and ions.…”

“…Neutrals are provided by the background gas and the target, however only the flux from the surface can explain the magnitude of discharge currents observed in HiPIMS discharges. 19 The speed of the azimuthal motion of a dense plasma zone is affected by the inertia of ions accelerated to the target,…”

“…18 For low-yield materials, the high currents are related to a "gas recycling" mechanism, which is analogous to the self-sputtering process. 19 For both selfsputtering and gas-based sputtering, neutral atoms are released from the target surface, ionized in the near-target region, and then accelerated back to the target by the presheath and sheath electric field. Once returned to the target, the now energetic ions cause sputtering of surface atoms and the emission of secondary electrons.…”

Plasma potential mapping of high power impulse magnetron sputtering discharges

“…gas "recycling" (ionization-neutralization-ionization) can be an important process working in parallel to selfsputtering. 13 Magnetrons are characterized by a closed-drift of electrons: the large azimuthal Hall current, primarily caused by the E × B drift of electrons, exceeds the discharge current by a factor of 3-9 for direct current (dc) conditions. 14 This factor is smaller for HiPIMS conditions, 15 which can be associated with "anomalous" current transport across magnetic field lines based on collective processes (oscillations and instabilities).…”