On Electron Heating and Ion Recycling in High Power Impulse Magnetron Sputtering Discharge

It was generally assumed that the magnetron sputtering discharge is maintained by the sheath acceleration of secondary electrons, emitted from the cathode target upon ion impact, as described by the well-known Thornton equation, which originally was formulated to give the minimum required voltage to sustain the discharge [1] . However, it has been demonstrated recently that Ohmic heating of electrons outside the cathode sheath is roughly of the same order as heating due to acceleration across the sheath in dc magnetron sputtering (dcMS) discharges [2] . When operated as high power impulse magnetron sputtering (HiPIMS) discharge Ohmic heating of the plasma electrons is found to dominate over sheath acceleration by typically an order of magnitude [3] . In HiPIMS discharge a high density plasma is created by applying high power pulses at low frequency and low duty cycle to a magnetron sputtering device. Here we discuss the large discharge currents, the discharge current composition at the target surface, and the electron energy distribution [4] in HiPIMS discharges. We discuss the role of self-sputter (SS-) recycling and working gas recycling within the discharge. We find that above a critical current density J crit ≈ 0.2 A/cm 2 , a combination of self-sputter recycling and working gas-recycling is the general case [5] . For high self-sputter yields, the discharges become dominated by SS-recycling, and contains only a few energetic secondary electrons. For low self-sputter yields, the discharges operated above J crit are dominated by working gas recycling, and secondary electrons play a more important role. We explore a discharge with Al target which develops almost pure self-sputter recycling, a discharge with Ti target that exhibits a mix of self-sputter recycling and working gas-recycling [6] , and discharges with graphite target and a reactive Ar/O 2 gas mixture with poisoned target where working gas-recycling dominates [7] .