Observation of multiple charge states and high ion energies in high-power impulse magnetron sputtering (HiPIMS) and burst HiPIMS using a LaB₆ target

The charge-state-resolved ion energies of high-power impulse magnetron sputtering (HiPIMS) discharges were measured, using a LaB₆ target, as a function of charging voltage, pulse length, pulse frequency and 'on/off' time ratio within applied HiPIMS bursts. The highest charge states can reach '+2' and '+3' for boron and lanthanum ions, respectively. At high discharge powers, the B/La ion ratio can exceed the respective atom ratio in the target producing B-rich plasma with up to 98% boron ions. In the case of two-segmented bursts with high 'on/off' time ratios, La³⁺ is the dominating lanthanum ion species and the ion energy distribution of B⁺ shows a pronounced high-energy tail extending up to 750eV. The measured plasma compositions, ion charge states and ion energies are discussed within the established framework of HiPIMS discharges and the recent postulation that potential humps are associated with drifting ionization zones. The recorded high B/La ion ratios are a result of complex effects related to particle fluxes in the HiPIMS plasma of compound targets, as explained with the help of an expanded schematic representation of self-sputtering and gas atom recycling. The high energies of the B⁺ ions are based on a combination of the self-sputtering of boron, backscattering of incident boron ions on lanthanum atoms in the target and acceleration by the potential hump. Further evidence for potential humps is provided by the observed charge-state dependence of ion energies and features between the thermal peak and high-energy tail of the ion energy distribution functions.