Long term resilience of silver-based telescope mirrors with a single aluminum oxide protection layer deposited via thermal atomic layer deposition using H2O and pure ozone: a comparative study of progressive changes in surface structures during defect formation

Abstract

Using thermal atomic layer deposition (ALD), with trimethylaluminum (TMA) and water (H2O), it was found that coatings of aluminum oxide (AlOx) deposited on silver-based telescope mirrors operate as viable means to both protect the mirrors from corrosion and reap the benefits of the mirrors while only minimally impacting the mirror’s performance. However, as effective as AlOx coatings are in high temperature/high humidity (HTHH) testing, the mirrors tend to catastrophically fail after an extended period of time during the HTHH test. In order to further improve long-term resilience, the use of Pure Ozone (PO) as an oxygen precursor in replacing H2O is proposed and investigated. While the initial reflectance of the samples prepared with PO is slightly lower than those prepared with H2O, they exhibit better resilience during the HTHH test. In this paper, the study on progressive structural changes that occur in silver-based telescope mirrors protected by AlOx is described over several different stages during cyclic HTHH tests (i.e., temperature is cycled between two, high and low, temperatures). Two types of samples are prepared using either H2O or PO as an oxidation precursor and compared. The study reveals that the initial drop in spectral reflectance of the PO samples during the first HTHH cycle is associated with the surface reconfiguring itself in the presence of the extra energy which effectively tempers the surface to gain higher stability.