Corrosion barriers for silver-based telescope mirrors comparing PEALD and PVD of AlOx

Abstract

ABSTRACTAstronomical telescopes continue to demand high-endurance high-reflectivity silver mirrors that can withstand years of exposure in earth-based observatory environments. The University of California Observatories Astronomical Coatings Lab has undertaken development of protected silver coatings suitable for telescope mirrors that maintain high reflectivity at wavelengths from 340 nm through the mid-infrared spectrum. We present promising results of enhanced corrosion barriers using plasma-enhanced atomic layer deposition (PEALD) of aluminum oxide (AlOx) as a top barrier layer. Novel coating recipes developed with ion-assisted electron beam deposition (IAEBD) of materials including yttrium fluoride and oxides of yttrium, tantalum, and silicon are used to compare the endurance of physical vapor deposition-grown barriers with PEALD-grown barriers of similar thickness. Samples of these mirror coatings were covered with conformal layers of AlOx deposited by PEALD using trimethylaluminum as a metal precursor and plasma-activated oxygen as an oxidant gas. Samples of coating recipes with and without PEALD oxide undergo aggressive high temperature/high humidity (HTHH) environmental testing in which samples are exposed to an environment of 80% humidity at 80°C for ten days in a simple test set-up. HTHH testing show visible results suggesting that the PEALD oxide offers enhanced robust protection against chemical corrosion and moisture from an accelerated aging environment. Mirror samples are further characterized by reflectivity/absorption before and after deposition of oxide coatings. AlOx is suitable for many applications and has been the initial material choice for this study.