High-rate reactive high-power impulse magnetron sputtering of hard and optically transparent HfO2 films

•Hard nanocrystalline HfO2 films were prepared.•High optical transparency of films in near-UV region was observed.•Very high deposition rates of densified HfO2 films were achieved.•Controlled reactive high-power impulse magnetron sputtering was used.•Relation for reactive HiPIMS deposition rate was proposed.

High-power impulse magnetron sputtering (HiPIMS) with a pulsed reactive gas (oxygen) flow control was used for high-rate reactive depositions of densified, highly optically transparent, stoichiometric HfO2 films onto floating substrates. The depositions were performed using a strongly unbalanced magnetron with a directly water-cooled planar Hf target of 100 mm diameter in argon–oxygen gas mixtures at the argon pressure of 2 Pa. The repetition frequency was 500 Hz at the deposition-averaged target power density from 29 W cm− 2 to 54 W cm− 2. The voltage pulse durations ranged from 50 μs to 200 μs. The target-to-substrate distance was 100 mm and the substrate temperatures were less than 165 °C. We showed that the HfO2 films can be prepared with very high deposition rates (up to 200 nm/min) at a deposition-averaged target power density of approximately 30 W cm− 2, which is relatively close to a target power density applicable in industrial HiPIMS systems. The films were nanocrystalline with a dominant monoclinic phase. They exhibited a hardness of 15–18 GPa, a refractive index of 2.07–2.12 and an extinction coefficient between ≤ 0.1 × 10− 3 and 0.6 × 10− 3 (both quantities at the wavelength of 550 nm). At 300 nm, the extinction coefficient was between 1.5 × 10− 3 and 7.0 × 10− 3. A simplified relation for the deposition rate of films prepared using a reactive HiPIMS was presented.