24 August 2015 Current progress in the characterization of atomic layer deposited AlF3 for future astronomical ultraviolet mirror coatings
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Reflective aluminum (Al) mirrors for astronomical telescopes are traditionally protected by a transmissive overcoat. The optical, mechanical and chemical properties of this overcoat material strongly affect the spectral reflective properties and durability of the mirror system. We are developing atomic layer deposited metal fluorides and assessing their applicability for future astronomical space missions in the ultraviolet and visible wavelengths. We are currently performing depositions on silicon wafers to serve as a basis for the metal-fluoride on Al depositions. In this paper we present reflectance, surface roughness, environmental storage and polarization sensitivity results of thin layers of AlF3 on silicon. Atomic layer deposited coatings of AlF3 grown at 100 and 200 °C yield good optical characteristics deduced from reflectance measurements from 90 – 800 nm and spectroscopic ellipsometry measurements from 200 – 800 nm, which are consistent with calculations from optical constants derived by our group and from the literature. Atomic force microscopy (AFM) measurements demonstrate a 15% increase in surface roughness for a ~25 nm film with respect to a silicon reference. Temporary storage in a gN2 box minimally affects the UV reflectance of ~30 nm of AlF3 on Si. Overall, these coatings have proven to be versatile and optically stable in the early phases of development.
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Christopher Samuel Moore, Christopher Samuel Moore, John Hennessy, John Hennessy, Eliot Kersgaard, Eliot Kersgaard, April D. Jewell, April D. Jewell, Shouleh Nikzad, Shouleh Nikzad, Kevin France, Kevin France, } "Current progress in the characterization of atomic layer deposited AlF3 for future astronomical ultraviolet mirror coatings", Proc. SPIE 9601, UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XIX, 96010X (24 August 2015); doi: 10.1117/12.2186723; https://doi.org/10.1117/12.2186723

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