Improving silver mirrors with aluminum oxynitride protection layers: variation in refractive index with controlled oxygen content by radiofrequency magnetron sputtering

Abstract

According to the demand for high-performance silver-based telescope mirrors, attempts are being made to develop surface coatings that protect the mirrors from corrosion. Aluminum nitride (AlN) is utilized for various optical coatings, and its high optical transparency and mechanical robustness make it potentially well suited as a protective coating of silver-based mirrors. However, according to our best knowledge, AlN with controlled oxygen content has never been used to protect silver mirrors. Various protective coatings based on AlN are prepared by RF magnetron sputtering. Specific amounts of oxygen are deliberately introduced to obtain aluminum oxynitride (AlON) protective layers with refractive indices within a certain range (i.e., high ∼2.1, medium ∼1.8, and low ∼1.6 at 400 nm). The designed protective layers are applied to two types of silver-based mirror test structures, and their performance was assessed in terms of optical reflectivity and structural integrity of the test structures that underwent environmental testing in a controlled atmosphere at 80°C with ∼80 % relative humidity. Comprehensive analysis on the samples before and after the environmental testing indicates that AlON-based 41- and 56-nm protective layers with medium refractive index outperform similar samples using AlON with higher or lower refractive index. We suggest that the benefits of the best AlON barrier with specific refractive index are likely associated with the optical, chemical, and structural characteristics produced with sputtering conditions that yield a nitrogen/oxygen ratio of approximately one.