Ion energy distribution and non-linear ion dynamics in BP-HiPIMS and ACBP-HiPIMS discharge

Investigating the ion dynamics in the emerging bipolar pulse high power impulse magnetron sputtering (BP-HiPIMS) discharge is necessary and important for broadening its industrial applications. Recently, an optimized plasma source operating the BP-HiPIMS with an auxiliary anode and a solenoidal coil is proposed to enhance the plasma flux and energy, named as ACBP-HiPIMS (‘A’-anode, ‘C’-coil). In the present work, the temporal evolutions of the ion velocity distribution functions (IVDF) in BP-HiPIMS and ACBP-HiPIMS discharges are measured using a retarding field energy analyser (RFEA). For the BP-HiPIMS discharge, operated at various positive pulse voltages U +, the temporal evolutions of IVDFs illustrate that there are two high-energy peaks, E 1 and E 2, which are both lower than the applied U +. The ratio of the mean ion energy E i,mean to the applied U + is around 0.55–0.6 at various U +. In ACBP-HiPIMS discharge, the IVDF evolution shows three distinguishable stages which has the similar evolution trend with the floating potential V f on the RFEA frontplate: (i) the stable stage with two high-energy peaks (E 2 and E 3 with energy respectively lower and higher than the applied U + amplitude) when the floating potential V f is close to the applied positive pulse voltage; (ii) the transition stage with low-energy populations when the V f drops by ∼20 V within ∼10 μs; and (iii) the oscillation stage with alternating E 2 and E 3 populations and ever-present E 1 population when the V f slightly decreases until to the end of positive pulse. The comparison of IVDFs in BP-HiPIMS and ACBP-HiPIMS suggests that both the mean ion energy and high-energy ion flux have been effectively improved in ACBP-HiPIMS discharge. The formation of floating potential drop is explored using the Langmuir probe which may be attributed to the establishment of anode double layer structure. The acceleration of ion at the double layer boundary is analysed using a theoretical model, in this way to clarify the oscillation in IVDF evolutions in ACBP-HiPIMS discharge.