Atomic Layer Deposited Protective Coating of Aluminum Oxide on Silver-based Telescope Mirror A Comparison Between a Pure Ozone and H2O Precursor

Abstract

Although silver-based telescope mirrors excel over other materials such as gold and aluminum in the visible-infrared spectral range, they require robust protective coatings to overcome their inherent low durability. Our research shows that a single-layer of aluminum oxide (AlOx) deposited through thermal atomic layer deposition (ALD) using trimethylaluminum (TMA) and water (H2O) at low temperatures (~60{\deg}C) serves as an acceptable protective coating without adversely impacting the optical performance of the mirrors. While silver-based mirrors protected with a single-layer of AlOx perform decently in the field, in environmental tests under high-humidity at high-temperature conditions that accelerate underlying failure mechanisms, they degrade quickly, suggesting that there is room for improvement. This paper describes a study that compares the performance and endurance of two sets of silver-based mirrors protected by a single-layer of AlOx prepared by thermal ALD with two types of oxygen precursors: H2O and pure ozone (PO). The study shows that while the two types of samples, regardless of their oxygen precursors, initially have comparable spectral reflectance, the reflectance of the samples with AlOx protective coatings prepared with PO remain nearly constant 1.6 times longer than those with AlOx protective coatings prepared with H2O in the environmental test, suggesting promising characteristics of AlOx protective coatings prepared with PO.