Effect of annealing on properties and performance of HfO₂/SiO₂ optical coatings for UV-applications

The field of ultraviolet (UV)-laser applications is currently experiencing rapid growth in the semiconductor processing, laser micromachining and biomedical markets. Key enablers for these technologies are optical coatings used to manipulate and guide laser beams in a targeted manner. As laser power, laser fluence and pulse frequencies increase, the demands on the physical properties of the coating materials become more stringent. Ion beam sputtering is a technique that allows producing optical coatings with the low losses required in these applications. In this study, we investigate the influence of ion beam sputtering (IBS) parameters on the optical properties of HfO₂ and SiO₂ single layers as well as the impact of annealing duration at 475 °C for anti-reflective (AR) and highly reflective (HR) optical coatings at 355 nm. For HfO₂ sputtered from a metal target the O₂ flow during the coating process is a key parameter to reduce absorption. SiO₂ single layers exhibit improved transmission in the UV-range as the ion beam energy for the sputtering process is reduced. Furthermore, a complex behavior for film stress, absorption, surface roughness and coating structure was unraveled as a function of annealing duration for AR- and HR-coatings at 355 nm. The reflectance of the HR-mirror after optimized annealing exceeded 99.94% at 355 nm and a high laser induced damage threshold (LIDT) of 6.9 J/cm² was measured after 2 hours of annealing. For the AR-coating a LIDT-value of 15.7 J/cm² was observed after 12 hours of annealing.