Tuning the characteristics of Al2O3 thin films using different pulse configurations: Mid-frequency, high-power impulse magnetron sputtering, and their combination

The growth of alumina (Al2O3) films by reactive magnetron sputtering in oxidizing atmosphere is complicated due to the difficulty to keep the process stable within the transition zone and the formation of insulating layers on the target, which may lead to arcing events. To minimize those issues, an active feedback reactive sputtering control was explored to deposit Al2O3 thin films by two sputtering sources: high-power impulse magnetron sputtering (HiPIMS) and mid-frequency (MF) at three different powers (2000, 2500 and 3500 W) and two deposition temperatures (200 and 450 °C). A combination of MF + HiPIMS at 2500 W was also studied. X-ray diffraction (XRD) revealed the formation of γ-Al2O3 polycrystalline films, except those deposited at 200 °C, which were amorphous. In fact, more energetic depositions (higher power, use of HiPIMS instead MF) lead to films with larger grain density and refractive index, up to values close to α-Al2O3. HiPIMS deposition induced films with higher values of compressive stress than MF, which is caused by the higher energy and intensity of the ions impinging on substrate and its lower deposition rate. The combination of MF + HiPIMS led to higher deposition rates, lower compressive stress, and lower crystallinity compared with HiPIMS alone.