Connection between target poisoning and current waveforms in reactive high-power impulse magnetron sputtering of chromium

Abstract

Global models of high-power impulse magnetron sputtering (HiPIMS) plasmas in the literature predict a unique connection between target current waveform and oxidation state of the target (metallic versus poisoned): in the metallic mode, the current waveform reaches a plateau due to metal atom recycling, in the poisoned mode a triangular current waveform is predicted driven by plasma gas recycling. This hypothesis of such a unique connection is tested by measuring the surface chemical composition of chromium magnetron targets directly during reactive high-power impulse magnetron sputtering (r-HiPIMS) by spatially resolved x-ray photoelectron spectroscopy (XPS). The sputtering setup was connected to the ultra-high vacuum XPS spectrometer so that the targets could be transferred between the two chambers without breaking the vacuum. The O2/Ar feed gas ratio, the input power and the pulse frequency of the HiPIMS plasmas were varied. The racetrack oxidation state was measured for different plasma parameters and correlated to the target current waveform shape. It was found that a shift of the target operation from the poisoned mode at low powers to the metallic mode at high powers when operating the discharge at 20 Hz pulse frequency occurs. The transition between these modes was directly correlated with analysis of the Cr2p core level peak on the complete target area. A unique correlation between the metallic and poisoned state of the target and the plateau and triangular current waveform was identified for very low powers and very high powers. In the intermediate power range, such a unique connection is absent. It is argued that the presence of already a small fraction of metal on the target may induce a plateau current waveform despite a significant oxidation of the target. This implies a finite contribution of metal sputtering during the pulse that dominates the recycling and leads to a plateau current waveform. Consequently, the shape of current waveforms cannot easily be connected to target poisoning, but a more detailed modeling of the recycling mechanisms is required.

Temporal studies of titanium ionised density fraction in reactive HiPIMS with nitrogen admixture

“…In HiPIMS, due to the significantly higher ionisation, some form of back-attraction of the produced ions to the target surface should be considered [37,78], and so these data are not applicable. A similar effect of increased target coverage on the discharge parameters has previously been reported [79]. This agrees with our current findings where current waveform changes to a triangular shape are accompanied by suppressed sputtering and thus compound coverage of the target.…”

Ion-induced secondary electron emission of oxidized nickel and copper studied in beam experiments

“…However, for example, in typical magnetron discharges, the values of j O / j Ar + are much smaller and, therefore, a total oxide coverage does not occur. For example, Layes et al [53] applied j O / j Ar + ∼2 and measured Θ = 0.8 for titanium oxide, being consistent with the here used model.…”

Plasma-surface interactions

“…Optical emission spectroscopy characterisation of the sputtering process and plasma properties is presented by Ries et al [17] for a reactive argon-nitrogen (Ar/N 2 (10/1)) DC magnetron discharge for sputtering of a chromium target. Layes et al [18] experimentally study the connection between target poisoning and current waveforms in reactive high-power impulse magnetron sputtering of chromium. They find a unique correlation between the metallic and poisoned state of the target and the plateau and triangular shape of the current waveform at very low and very high powers, while it is absent at intermediate power.…”