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What Degrades the Performance Of Silver Thin Film Mirrors Use In Telescopes? (Instrumentation / Astronomy)

High-performance astronomical telescopes benefit from durable broadband mirrors based on silver (Ag). Advantages of Ag include higher reflectivity and lower emissivity in the thermal infrared spectrum when compared to aluminum mirrors currently in prevalent use. There have been some successful implementations of Ag mirrors; notably those used on the Gemini telescopes. However, few ground-based telescopes choose to utilize Ag mirrors in actual observatory environments, which reveals the elusiveness of Ag mirrors. The deep blue and UV spectra are very important for many astronomical research programs; however, even the Ag mirrors used in the Gemini telescope suffer from compromised deep blue and UV reflectivity due to the presence of an optically absorbing NiCrN adhesion film placed on a Ag film. In addition, bare Ag tarnishes easily in the presence of oxygen and especially sulfur in Earth’s atmosphere, and it experiences rapid corrosion via salt formation with halides. Therefore, reflective surfaces of Ag films must be immediately covered by an optically transparent protection coating to avoid tarnish and maintain original reflectivity for years.

Both the reflective Ag film and the subsequent protection coating are routinely deposited at room temperature by various physical vapor deposition (PVD) techniques including sputtering, evaporation, and cathodic arc deposition. However, extensive literature indicates that PVD techniques produce protection coatings with superior durability against corrosion when carried out above room temperature. Apart from PVD, atomic layer deposition (ALD) offers protection coatings with performance often superior to those deposited by PVD techniques. For instance, in our previous work, the highly uniform, conformal, and virtually pinhole-free nature of Al2O3 protection coatings deposited by ALD demonstrated superior durability over comparable Al2O3 deposited by electron-beam evaporation. In our other previous work, it was found that ALD based Al2O3 protection coatings on Ag exhibited higher durability with higher ALD processing temperature. However, before further work can be done to maximize durability of Ag mirrors covered with protection coatings by ALD done at an optimal elevated processing temperature, the effects of such elevated processing temperatures on properties of Ag films themselves and of the entire Ag mirror structure must be thoroughly studied.

Fig. 1 Illustration of the three types of Ag mirror samples showing details of layered structures with nominal thicknesses. (a) UCO Ag 2018, (b) Bare-Ag, and (c) Al2O3/Ag. © Kobayashi et al.

Thus, in their recent study, Kobayashi and colleagues designed and fabricated high-performance Ag mirrors with a new benchmark. The resulting Ag mirrors were annealed (i.e., post-fabrication annealing) at various temperatures to investigate the viability of introducing thermal processes during and/or after fabrication in improving overall optical performance and durability of protected silver mirrors. In their experiments, Ag mirror samples were deposited by electron-beam evaporation and subsequently annealed at various temperatures in the range from 60 °C to 300 °C, and then the mirror samples underwent an environmental stress test at 80 °C and 80% humidity for 10 days.

They found that, while all the mirror samples annealed below 200 °C showed negligible corrosion after undergoing the stress testing, those annealed below 160 °C presented spectral reflectivity comparable to or higher than that of as-deposited reference samples. In contrast, the mirror samples annealed above 200 °C exhibited significant degradation after the stress testing. The comprehensive analysis indicated that delamination and voids caused by the growth of Ag grains during the annealing are the primary mechanisms of the degradation.

“Formation of void defects and delamination between the Ag thin film and adjacent barrier film, caused by Ag grain growth during annealing at temperatures higher than 160 °C, are suggested as the major failure mechanisms. Our experiment suggests that the common practice, for Ag-based protected mirrors, of not using post-fabrication processing at elevated temperature should be re-evaluated for the sake of improving performance and durability.”

— concluded authors of the study

Featured image: SEM images of as-deposited and 500 °C-annealed Bare-Ag and Al2O3/Ag samples. Scale bar represents 500 nm. © Kobayashi et al.

Reference: David M. Fryauf, Andrew C. Phillips, Nobuhiko P. Kobayashi, “Critical processing temperature for high performance protected silver thin film mirrors”, ArXiv, pp. 1-18, 2021. https://arxiv.org/abs/2104.08233

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