Low-Stress Silicon Cladding for Silicon-Carbide Mirrors

Silicon carbide (SiC) mirrors are expected to replace beryllium mirrors in future missile interceptors. However, many SiC manufacturing processes create materials that are difficult or impossible to grind and polish to mirror quality. A relatively thick layer of silicon coating may be applied to SiC substrates in order to reduce the processing time required to figure the mirror. In addition, high-quality silicon coatings may be polished to achieve excellent specularity. In Phase I, Surface Optics Corporation (SOC) demonstrated a low-stress silicon cladding for SiC mirrors using a novel ion-assisted evaporation (IAD) technique. A cladding with little intrinsic stress is essential to minimize bending that would otherwise distort the figure of lightweight mirrors. Excessive coating stress may also cause cracking or delamination from the substrate. In Phase II, SOC will optimize the process parameters to minimize coating stress and maximize the deposition rate. SOC will determine by experiment, the initial SiC substrate micro-roughness, which balances the trade between coating adhesion and desirable polishing characteristics.