Abstract
More efficient and stable far ultraviolet (FUV) mirrors will enable future space observatories. Traditional FUV mirrors are based on MgF2-protected Al. AlF3 has been identified as a promising substitute for MgF2 to prevent Al oxidation. Hence, the reflectivity, stability, and morphology of AlF3-protected Al mirrors have been investigated as a function of deposition temperature of the AlF3 film. In this work, it is shown how AlF3 deposition temperature is an important parameter whose optimization ultimately yields valuable throughput enhancement and improved endurance to large storage periods. Al films were deposited at room temperature (RT) and AlF3 protective layers were deposited at temperatures ranging from RT to 350 °C. It was found that the optimum AlF3 deposition temperature was between 200 and 250 °C, yielding the largest FUV reflectance and a better stability of the mirrors, which had been stored in a desiccator environment. Increasing AlF3 deposition temperature resulted in an increase in film density, approaching bulk density at 250 °C. The morphology of Al and AlF3 films as a function of AlF3 deposition temperature was also investigated. The increase in the AlF3 deposition temperature resulted in a decrease of both Al and AlF3 surface roughness and in the growth of the grain width at the AlF3 outer surface. It also resulted in a trend for the prevalent (111) planes of Al nanocrystals to orient parallel to the coating surface.
Highlights
The far ultraviolet (FUV; here it refers to the 100–200-nm spectral range) encloses spectral lines of many fundamental atom and ion species of H, C, N, O, etc.; observation at such spectral lines is key for the communities of astronomy and astrophysics [1,2,3,4,5], solar physics [6], as well as atmosphere physics
AlF3 film thickness was set at 26 nm for maximum reflectance at 121.6-nm H Lyman α spectral line, an important line for astrophysics and solar physics
A first 5-nm thick AlF3 layer was deposited on Al at room temperature (RT) and the complementary 21-nm thick AlF3 film was deposited once the sample was heated at the desired temperature
Summary
Aluminum is the material with the largest reflectance down to the FUV [7]. Bare Al grows a thin film of native oxide in the air or even under high vacuum, and this very thin oxide film fully degrades Al reflectance in the FUV. The fast oxidation of Al is a difficulty to obtain durable FUV high-reflectance mirrors, namely that reflectance is maintained over time. It is necessary that space-instrument optics be stable due to the high cost or impossibility of replacement. Both for the use of Al mirrors in space and on earth, a transparent protective film is necessary to immediately overcoat the underlying Al film in order to avoid oxidation
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