An ionisation region model of reactive high-power impulse magnetron sputtering of Ti in an Ar/N₂ atmosphere

Abstract

The ionisation region model (IRM) for high-power impulse magnetron sputtering discharges is extended to describe the reactive sputtering of titanium in an argon-nitrogen gas mixture. The aspects that are unique to the Ar/N2/Ti model are described and the additional input parameters required for the reactive case, along with the methods used to determine them, are explained in detail. The IRM is then applied to investigate four discharges with increasing nitrogen partial pressures and the results are used to illustrate how the shape of the discharge current influences the overall ionisation probability of the target metal. Differences in the discharge current evolution are explained by analysing how variations in the effective cost of ion–electron pair creation, as well as changes in the secondary electron emission (SEE) and sputtering yields, arise from the evolving composition of the cathode target (compound fraction), plasma volume, and discharge current. Additionally, working gas rarefaction and recycling, which limit and amplify the discharge current, respectively, are examined in detail. We find that, for the discharges operated primarily in argon, the initial current rise is determined by the SEE yield, while the rate at which the current grows in the following phase is limited by the target metal sputtering yield. In discharges operated in a predominantly nitrogen atmosphere, current growth is instead limited by the high effective cost of ionisation until the plasma and target metal densities are high enough to support significant self-sputter recycling.