High-rate reactive magnetron sputtering of zirconia films for laser optics applications

ZrO₂ exhibits low optical absorption in the near-UV range and is one of the highest laser-induced damage threshold (LIDT) materials; it is, therefore, very attractive for laser optics applications. This paper reports explorations of reactive sputtering technology for deposition of ZrO₂ films with low extinction coefficient k values in the UV spectrum region at low substrate temperature. A high deposition rate (64 % of the pure metal rate) process is obtained by employing active feedback reactive gas control which creates a stable and repeatable deposition processes in the transition region. Substrate heating at 200 °C was found to have no significant effect on the optical ZrO₂ film properties. The addition of nitrogen to a closed-loop controlled process was found to have mostly negative effects in terms of deposition rate and optical properties. Open-loop O₂ gas-regulated ZrO₂ film deposition is slow and requires elevated (200 °C) substrate temperature or post-deposition annealing to reduce absorption losses. Refractive indices of the films were distributed in the range n = 2.05-2.20 at 1,000 nm and extinction coefficients were in the range k = 0.6 × 10^-4 and 4.8 × 10^-3 at 350 nm. X-ray diffraction analysis showed crystalline ZrO₂ films consisted of monoclinic + tetragonal phases when produced in Ar/O₂ atmosphere and monoclinic + rhombohedral or a single rhombohedral phase when produced in Ar/O₂ + N₂. Optical and physical properties of the ZrO₂ layers produced in this study are suitable for high-power laser applications in the near-UV range.