Tuning high power impulse magnetron sputtering discharge and substrate bias conditions to reduce the intrinsic stress of TiN thin films

Ion bombardment during film growth usually induces high compressive stress in compound thin film materials, resulting in rupture and failure of coated tools used in tribological applications. Hence, intrinsic stress generated during film growth can drastically limit the industrial appeal of deposition technologies such as high power impulse magnetron sputtering (HiPIMS). This work investigates how to reduce high stress levels by tuning the HiPIMS discharge conditions and selecting the appropriate substrate bias configuration. The strategy is based on optimizing the process discharge parameters, leading to HiPIMS discharges containing fewer multiply charged energetic metal ions, which is combined with pulsed substrate bias synchronized to the HiPIMS pulse to control the chemical nature of the incident ions, i.e., inert gas vs. metal ions. The study was performed during growth of TiN thin films, due to their relevance as a protective coating, and the intrinsic stress was measured in situ during film growth using the wafer curvature method and a multi-beam optical stress sensor. The results show that for standard HiPIMS discharges and biased substrates, the energetic metal ion bombardment results in very dense, compact microstructures, but highly stressed TiN films. On the other hand, when using the here proposed strategy mentioned above, the compressive stress was considerably reduced (by a factor 11) while retaining rather compact microstructures compared to direct current magnetron sputtered as well as non-biased HiPIMS samples.