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2 September 2015 Low-stress silicon cladding made by pulsed-ion-assisted evaporation
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Abstract
A low-stress, polishable, silicon (Si) cladding process for lightweight mirrors is presented. The cladding process is based on the thermal evaporation of silicon in the presence of low-energy argon ions. The process utilizes an ion bombardment technique whereby the coating stress of a silicon film is manipulated periodically from compressive to tensile in order to achieve a low net stress for the complete layer. A Si cladding with little intrinsic stress is desirable to minimize bending that would otherwise distort the figure of very lightweight mirrors. The process has yielded silicon claddings up to 100-microns thick, with less than 85 MPa of compressive stress. This polishable Si cladding was specifically designed for silicon carbide mirror substrates, however, it is also suitable for graphite composite, beryllium, and aluminum mirror substrates, which may be difficult or impossible to polish to sufficient mirror quality without a specialized coating.
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David A. Sheikh "Low-stress silicon cladding made by pulsed-ion-assisted evaporation", Proc. SPIE 9574, Material Technologies and Applications to Optics, Structures, Components, and Sub-Systems II, 95740C (2 September 2015); https://doi.org/10.1117/12.2191445
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