Room Temperature Plasma-Etching and Surface Passivation of Far-Ultraviolet Al Mirrors Using Electron Beam Generated Plasmas
David Boris - U.S. Naval Research Laboratory

D.R. Boris1, A.C. Kozen2, S.G. Rosenberg2, J.G. del Hoyo3, J.G. Richardson3, L.V. Rodriguez de Marcos3,4, E.J. Wollack3, M.A. Quijada3, *S.G. Walton1, V.D. Wheeler1, E. Gray3, J.M. Woodward2
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 ยป 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.

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