On selective ion acceleration in bipolar HiPIMS: A case study of (Al,Cr)2O3 film growth

Abstract

Selective ion acceleration using a synchronized substrate bias is a common way to tailor the microstructure and intrinsic stress of films grown by high-power impulse magnetron sputtering (HiPIMS), owing to the high degree of sputtered metal ionization and the inherent time separation between different ionic species in the ion fluxes at the substrate position. Here we show that it is possible to achieve selective acceleration of ionic species with different ion masses by employing a synchronized positive reversed pulse (Urev) on the sputtering target itself, after the end of the main HiPIMS pulse, i.e., bipolar HiPIMS (BP-HiPIMS), if the substrate is grounded. The evidence is provided by growing (Al,Cr)2O3 films using BP-HiPIMS where the time delay (Δτacc) between the HiPIMS-pulse and the positive reversed pulse as well as the length of the positive reversed pulse (τacc) are varied. In this way, both film stresses and film crystal structures are altered. The obvious drawback of BP-HiPIMS, that the ion-accelerating potential cannot be applied during the HiPIMS-pulse itself, has been minimized by using short HiPIMS pulses of 20 μs during which the peak of the substrate ion current density (Js) occurs well after the end of the HiPIMS-pulse indicating that the main portion of the ion fluxes can be accelerated by Urev. An important observation is that the temporal evolution of Js did not change as the different reversed pulse parameters (Urev, Δτacc, and τacc) were altered. This is evidence, that in these experiments, the dominating ion-acceleration occurs in the plasma sheath at the substrate, i.e., similar to the case when synchronized substrate bias is utilized.