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PEALD AlF3 films were deposited using trimethylaluminum and SF6 plasma precursors in a modified Veeco Fiji G2 reactor. ALD growth windows (the range of process parameters resulting in ideal growth) were established using an in situ ellipsometer to monitor the fluoride growth rate directly on Al substrates and supplemented with post-deposition x-ray photoelectron spectroscopy to elucidate process-structure property relationships. Optimal AlF3 films had a growth rate of 0.75-0.8Å/cycle, F/Al ratio of ≈3, < 2 at% O, indicating that PEALD is a beneficial process technique towards achieving optical coatings on a variety of potential mirror materials. The influence of PEALD parameters on the FUV optical performance of Al mirrors overcoated with PEALD-AlF3 will be also discussed.
Efficient mirrors with high reflectivity over the ultra-violet, optical, and infra-red (UVOIR) spectral range are essential components in future space-based observatories. 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 optical performance to date, their reflectivity is still insufficient for future large telescope instrumentation in which several reflections are required.
Recently, a novel passivation procedure based on the exposure of bare Al to a fluorine containing electron beam generated plasma has been presented [1,2]. This research is framed in a collaboration between Goddard Space Flight Center (GSFC) and the U.S. Naval Research Laboratory (NRL), with plasma treatment carried out in NRL’s large area plasma processing system (LAPPS) using aluminum coated glass samples produced at GSFC coating facilities. The passivation of the bare 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 is performed at room temperature. In this work, details of the plasma process and in situ surface monitoring with spectroscopic ellipsometry are discussed. This novel procedure has demonstrated improved Al mirrors with state of the art far-ultraviolet (FUV) (λ = 90-200 nm) reflectivity (e.g. R=91% at 121.6 nm) paired with an excellent thickness control of the Al protective layer.
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