D.R. Boris¹, A.C. Kozen², S.G. Rosenberg², J.G. del Hoyo³, J.G. Richardson³, L.V. Rodriguez de Marcos^3,4, E.J. Wollack³, M.A. Quijada³, *S.G. Walton¹, V.D. Wheeler¹, E. Gray³, J.M. Woodward^2
1U.S. Naval Research Laboratory, Washington, DC; ²Postdoctoral Fellow, ASEE, Washington, DC; ³NASA Goddard Spaceflight Center, Greenbelt, MD; ⁴Catholic University of America
Astronomical instrumentation/telescopes operating in the Far Ultraviolet (FUV, 90-200nm) require the use of aluminum mirrors due to its high reflectivity over this wavelength range. Unfortunately, the native aluminum oxide layer formed in atmosphere is strongly absorbing in this wavelength range, requiring that the aluminum films be passivated with a dielectric/transparent layer that inhibits oxidation. Efficient optics in the FUV range are challenging due to the limited selection of protective transparent materials available for use on aluminum. A promising coating materials is AlF₃, which can protect the underlying aluminum and yields a theoretical reflectivity of » 50% down to 100 nm, if the coating is sufficiently thin. In this work, we explore the use of electron beam generated plasmas produced in an SF₆ background to simultaneously remove the native oxide layer, while depositing an AlF₃ capping layer to passivate the aluminum. In this work we analyze the effect of varying both ion energy and SF concentration on the FUV reflectance, thickness, composition, and surface morphology of the resulting AlF₃ protective layers. We also analyze the reflectivity of samples optimized at selected important FUV wavelengths for astronomical observations.