S.G. Walton¹, J.P. Murphy², L. V. Rodriguez de Marcos³, J. del Hoyo⁴, M. A. Quijada⁴, V. D. Wheeler¹, and D. R. Boris^1
1U.S. Naval Research Laboratory, Washington, DC
² National Research Council Postdoctoral Associate Residing at the U.S. Naval Research Laboratory, Washington, DC
³Catholic University of America and NASA Goddard Space Flight Center (CRESST II), Greenbelt, MD
⁴NASA Goddard Space Flight Center, Greenbelt, MD
Efficient ultraviolet (UV) mirrors are essential components in space observatories for UV astronomy. Aluminum coated with metal-fluoride films are considered the baseline reflectors for NASA’s next flagship mission, the Habitable Worlds Observatory (HWO). Such mirrors exhibit the high reflectivity of aluminum in the UV, with the fluoride layer providing both protection against oxidation and transparency into the far ultraviolet (FUV). Despite the proven performance of materials such as magnesium fluoride (MgF2), further improvement is needed to meet the demands of large area reflectors (> 1m²) envisioned for next-generation space observatories. Indeed, the HWO reflectors will require uniform fluoride layers that are both thin (20-25 nm) and smooth (<1 nm RMS roughness) to meet the requisite reflectance uniformity (>> 99%) and improved reflectivity in the FUV portion of the spectrum than is possible with MgF₂.
Recently, a readily scalable, plasma-based passivation process was developed to produce a thin AlF3 layer on the surface of aluminum, which holds promise for meeting the stringent requirements for the HWO. 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. Interestingly, this process has the characteristics of classic aluminum anodization – either electrochemical or plasma – but with fluorine anions replacing oxygen anions as the reactant. 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. 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. This work partially supported by the Naval Research Laboratory base program.