Diagnosing asymmetric bipolar HiPIMS discharges using laser Thomson scattering

The temporal evolution of the electron temperature T (e) and density n (e) has been measured at two positions on the centre-line of an asymmetrically pulsed bi-polar HiPIMS plasma using incoherent laser Thomson scattering (LTS). The magnetron was operated with a tungsten target in argon atmospheres. The results show that in the plasma afterglow when positive voltage pulses are applied (above a threshold of at least 200 V) significant heating of the electrons can occur in which T (e) can rise to values comparable to the those measured in HiPIMS on-time. The on-set of the rises in T (e) are significantly delayed relative to the start of the positive pulse, with the delay time decreasing with the magnitude of the positive voltage. The delay is only weakly dependent on the operating pressure. The presence of large positive pulses can also affect the local electron density with n (e) seen to decay significantly more quickly in the afterglow than for the corresponding unipolar pulsing case, in which no positive pulse is applied. The LTS measurements were complemented by a time-resolved study of the plasma optical emission (neutral argon and tungsten lines). With increasing positive potentials applied in the afterglow the Ar(I) line intensities grow consistent with increasing T (e). Interestingly, W(I) line intensities are detected in the afterglow with positive voltages >200 V despite the termination of all target sputtering, suggesting that tungsten is being re-sputtered from the vessel walls. With the aid of emissive probe measurements of the spatial and temporal evolution of plasma potential profile along the centre-line we discuss the phenomena of plasma electron heating and wall sputtering in the positive pulse. This is done in terms of the existence of a non-sustained reverse discharge, in which the vessel walls become an effective cathode.