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

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 mu s. After 55 to 60 mu s, 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 650W average power), the results show that for over 50% of the 100 mu s 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).