High rate reactive sputtering of Al2O3 coatings by HiPIMS

High rate reactive sputtering of insulating metal oxide coatings, e.g. alumina (Al2O3), by high power impulse magnetron sputtering (HiPIMS) is of strong interests, as the increased ionization of target species in a HiPIMS process can be used to improve the structure and properties of the coatings. The challenges for reactive sputtering of Al2O3 using HiPIMS include arc suppression on the target surface and low deposition rates due to target poisoning. In this study, AlxOy coatings were reactive sputtered by deep oscillation magnetron sputtering (DOMS), one version of HiPIMS, which generates large oscillatory micro-pulses within long modulation pulses. The process stability was evaluated by measuring micro arc counts on the target at different combinations of on and off times of the oscillatory micro-pulses. Virtually arc-free depositions for Al2O3 were observed by utilizing combinations of short oscillation micro-pulse on time (< 4 mu s) and longer oscillation micro-pulse off time (> 40 mu s) at a peak target current of 130 A. The mechanisms of generating arc-free discharge for insulating coating reactive sputtering using deep oscillatory micro-pulses are discussed. The hysteresis behavior with open-loop for reactive sputtering of AlxOy using DOMS was studied. The closed-loop control for reactive sputtering was achieved by controlling the oxygen partial pressure (P-O2) using the feedback signal from a remote penning plasma emission monitoring sensor, which generates an ionized sample plasma away from the deposition zone. The deposition rate, microstructure, and mechanical and optical properties of the AlxOy coatings deposited at different P-O2 in the transition region were investigated. Up to 56% of the metallic deposition rate has been achieved for obtaining stoichiometric Al2O3 coatings by controlling the P-O2 around 0.0143 Pa. The DOMS Al2O3 coatings show improved mechanical properties and optical transmission as compared to those obtained by pulsed dc magnetron sputtering reported in the literature.