The evolution of the plasma potential in a HiPIMS discharge and its relationship to deposition rate

Abstract

An electron-emitting probe has been used to measure the temporal evolution of the plasma potential V p along a line from target (Ti) to substrate above the racetrack in a high-power impulse magnetron sputtering discharge pulsed at 100 Hz. The 20 ns time-resolution of the probe allowed us to observe the highly dynamic nature of V p as the discharge voltage waveform develops, with large negative V p values (−210 V) and strong potential gradients existing in the magnetic trap region in the first 6 to 8 µs. After 55 to 60 µs, V p is elevated towards ground potential (0 V) and the bulk electric field collapses. Outside the magnetic trap, i.e. on the open field lines, V p reveals much smaller axial and temporal variations, similar to those observed in conventional pulsed dc magnetrons.At standard conditions (Ar pressure of 0.54 Pa and 650 W average power), the results show that for over 50% of the 100 µs plasma 'on-time' the spatial structure of V p provides a large potential barrier for the sputtered post-ionized species so impeding their transport and deposition at the substrate. This barrier is reduced markedly (by 50%) through a small reduction in the magnetic field strength (33% at the target) so increasing the deposition rate by a factor of 6 at a typical position of the substrate (z = 100 mm). The structure of V p is marginally sensitive to changes in pressure (over the range 0.54 to 1.08 Pa), but more strongly dependent on the applied power (over the range 650 to 950 W).

Understanding deposition rate loss in high power impulse magnetron sputtering: I. Ionization-driven electric fields

“…The associated potential uphill extended a distance of 5 -6 cm from the sheath edge into the plasma, and had amplitudes in the range 6 -70 V. HiPIMS discharges should be possible to optimize with respect to such electric fields since they in these experiments varied with parameters that can be externally controlled as the magnetic field strength [6], the gas pressure [5,6], and the applied power [6]. It is also very likely that they can be influenced by varying the pulse shape and length: the z E fields are generally stronger closer to the target, and during the phases of breakdown and current rise [6].…”

An introduction to thin film processing using high-power impulse magnetron sputtering

“…Furthermore, Bugaev et al 66 and Bohlmark et al 67 showed that the magnetic confinement of the sputtered material affected the deposition rate. In a recent publication by Mishra et al, 68 it is reported that the deposition rate at the substrate position could be increased six times by weakening the magnetic field strength at the target surface by ;33%. Emmerlich et al 69 highlighted the nonlinear energy dependence of the sputtering yield, meaning that it does not make sense to compare HiPIMS and DCMS deposition rates for the same average power, if not taking this dependence into account.…”

Plasma potential mapping of high power impulse magnetron sputtering discharges

“…Not surprisingly, HiPIMS plasma has been extensively studied using Langmuir probes. [19][20][21][22][23] Various levels of time resolution have been demonstrated for selected positions of the probe, mostly focusing on the interesting region above the racetrack. However, measurements generally lack the survey character that may allow us to gain greater insights.…”