Enhancement of the deposition rate in reactive mid-frequency ac magnetron sputtering of hard and optically transparent ZrO2 films

Reactive mid-frequency ac magnetron sputtering with a pulsed reactive gas (oxygen) flow control was used for high-rate depositions of defect-free, hard and highly optically transparent stoichiometric ZrO2 films onto floating substrates. The depositions were performed using two strongly unbalanced magnetrons in a closed-field configuration. We used planar Zr targets of 100 mm diameter in argon‑oxygen gas mixtures at the argon pressure of 1 Pa. The repetition frequency was close to 85 kHz at the deposition-averaged target power density, which is the same for both magnetrons, from 5.6 Wcm− 2 to 15.7 Wcm− 2. The target-to-substrate distance was approximately 100 mm and the substrate temperatures were < 100 °C. For the deposition-averaged target power density of approximately 15.5 Wcm− 2, we achieved very high deposition rates (up to 100 nm/min), being up to 3 times higher than the corresponding maximum deposition rate achieved in the oxide mode, usually used for the ZrO2 films in industry. The high-quality, defect-free ZrO2 films with smooth surfaces (the root-mean-square roughness Rrms = 2.5-2.7 nm) were crystalline with a dominant monoclinic phase. They exhibited a hardness of 16 GPa, a refractive index of 2.20-2.21 and an extinction coefficient between 2 × 10− 4 and 6 × 10− 4 (both quantities at the wavelength of 550 nm).