S.G. Walton1, J. del Hoyo2, M.J. Johnson1, L.V. Rodriguez de Marcos3, M.E. Meyer4, J.P. Murphy1, M.A. Quijada2, M.G. Sales4, V.D. Wheeler1, D.R. Boris1
1U.S. Naval Research Laboratory, Washington, DC
2NASA Goddard Space Flight Center, Greenbelt, MD
3Catholic University of America and NASA Goddard Space Flight Center (CRESST II), Greenbelt, MD
4National Research Council Postdoctoral Associate Residing at the U.S. Naval Research Laboratory
Efficient ultraviolet (UV) mirrors are essential components in space observatories for UV astronomy. Aluminum mirrors with fluoride-based protective layers are commonly the baseline UV coating technology; these mirrors have been proven to be stable, reliable, and with a long flight heritage. However, despite their acceptable optical performance, it is still insufficient for future large telescopes in which several reflections are required.
Recently, a readily scalable, plasma-based passivation process was developed to produce a thin AlF3 layer on the surface of aluminum. The passivation process uses an electron beam generated plasma produced in a fluorine-containing background (SF6 or NF3), to simultaneously remove the native oxide layer while promoting the formation of an AlF3 layer with a tunable thickness. This process has the characteristics of classic aluminum anodization – either electrochemical or plasma – where oxygen is replaced by fluorine. The process takes advantage of the ability for electron beam driven plasmas produced in electronegative gas backgrounds to generate substantial densities of negative ions, which are utilized to grow the fluoride layer. While layer thickness scales with applied bias as expected, the growth rates are challenging to understand. In this presentation, we will discuss the process using operating parameter studies, plasma diagnostics, and materials characterization, with an eye on understanding the growth mechanisms and the potential for better process control.