High-performance aluminum mirrors at far ultraviolet wavelengths require transparent dielectric materials as protective coatings to prevent oxidation. Reducing the thickness of this protective layer can result in additional performance gains by minimizing absorption losses, and provides a path toward high Al reflectance in the challenging wavelength range of 90 to 110 nm. We have pursued the development of new atomic layer deposition processes (ALD) for the metal fluoride materials of MgF2, AlF3 and LiF. Using anhydrous hydrogen fluoride as a reactant, these films can be deposited at the low temperatures required for large-area surface-finished optics and polymeric diffraction gratings. We also report on the development and application of an atomic layer etching (ALE) procedure to controllably etch native aluminum oxide. Our ALE process utilizes the same chemistry used in the ALD of AlF3 thin films, allowing for a combination of high-performance evaporated Al layers and ultrathin ALD encapsulation without requiring vacuum transfer. Progress in demonstrating the scalability of this approach, as well as the environmental stability of ALD/ALE Al mirrors are discussed in the context of possible future applications for NASA LUVOIR and HabEx mission concepts.
Numerous atomic and molecular transitions that provide important diagnostics for astrophysical research exist in the
Lyman-ultraviolet (LUV; 91.2 - 121.6 nm) and far-ultraviolet (FUV; 121.6 - 200 nm) bandpasses. Future astronomy and
planetary science missions require the development of mirror coatings with improved reflectance between 90 - 200 nm
which maintain optical performance in visible and IR wavelengths (320 - 2000 nm). Towards this end, we have developed
an atomic layer deposition (ALD) process for optical coatings to enhance the efficiency of future space observatories. We
measured the reflectance from 115-826 nm of sample optics, consisting of silicon wafers coated with lithium fluoride films
deposited via ALD. We also measured the reflectance of sample optics stored in various environments, and characterized
the effect of storage environment on visible and UV optical performance over week-long time scales. Minimal change in
optical performance was observed for wavelengths between 200 and 800 nm, regardless of storage environment.