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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 long flight heritage. However, despite their acceptable optical performance, the single-bounce reflectance values are still too low for use in optical systems in which several reflections are required. Recently, a novel passivation procedure based on the self-fluorination of bare Al has been presented [1, 2]. This research is framed in a collaboration between the Goddard Space Flight Center (GSFC) and the Naval Research Laboratory (NRL), and the experiments are carried out in the Large Area Plasma Processing System (LAPPS) at NRL using bare aluminum samples coated at GSFC coating facilities. The passivation of the oxidized Al is accomplished by using an electron-beam generated plasma produced in a fluorine-containing background to simultaneously remove the native oxide layer while promoting the formation of an AlF3 passivation layer with tunable thickness. Importantly, this new treatment uses benign precursors (SF6) and does not require high substrate temperatures. This novel procedure has demonstrated improved Al mirrors with enhanced FUV reflectivity. Examples of mirrors tuned at several key FUV wavelengths are provided. The LAPPS has been recently upgraded to include a new spectroscopic ellipsometer for real-time, in situ measurements of film thickness and optical constants of the fluoride layer during the plasma treatment. Since this new capability requires precise knowledge of the complex refractive index (n,k) of AlF3, we present optical constants in the 90-2500 nm range obtained from Al mirrors previously prepared using the LAPPS process. The derived optical properties from the AlF3 passivation layer show similar optical properties in the FUV when compared with PVD- and ALD- hot-deposited AlF3.
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