Progressive structural changes in silver-based telescope mirrors with an aluminum oxide protection layer prepared via thermal atomic layer deposition using water and high purity ozone: a comparative study for long-term resilience

Silver-based astronomical telescope mirrors (Ag-mirrors) excels in their optical performance across the visible to infrared spectrum. However, without proper safeguarding, they degrade significantly in high temperature/humidity environmental durability tests. Our research explored low-temperature atomic layer deposition (ALD) to prepare a 60 nm aluminum oxide (AlOx) protection coating for Ag-mirrors. We compared two oxygen precursors: water (H2O) and high-purity ozone (PO). PO, with over 80% purity, enables efficient ALD processes at lower temperatures, addressing challenges posed by H2O. During environmental tests, PO samples outperformed H2O samples. PO samples exhibited a minimal reduction (12%) in optical reflectance in comparison to H2O samples that showed substantial reduction (30%). Additionally, PO samples displayed a mere 6 nm phase shift in ellipsometry compared to 74 nm of H2O samples, indicating better structural integrity. Structural analysis revealed that H2O samples experienced erratic changes, compromising integrity, while PO samples maintained their original structure.