Low-Stress Silicon Cladding for Surface Finishing Large UVOIR Mirrors

In this Phase I research, ZeCoat Corporation demonstrated a low-stress silicon cladding process for surface finishing large UVOIR mirrors. A polishable cladding is desired for SiC optics so they may be figured in less time, and so they may be polished to levels suitable for UVOIR astronomy. ZeCoat has filed a provisional US patent application for the technology. The proposed process is directly scalable to SiC mirrors several meters in diameter. The process is based on a novel, low temperature, ion-assisted, evaporation technique (IAD), whereby the coating stress of a silicon film may be manipulated from compressive to tensile, in order to produce a near-zero net stress for the complete layer. A Si cladding with little intrinsic stress is essential to allow thick coatings to be manufactured without cracking. A low stress coating also minimizes substrate bending that would otherwise distort the figure of very lightweight mirrors.

Normal incidence 4-meter class UVOIR telescopes have been cited as a high priority by multiple government review panels including; the National Research Council's (NRC) study of NASA's Space Technology Roadmap and Priorities, The Office of the Chief Technologist, The Cosmic Origins Program and NWNH Decadal.

The ability to apply silicon cladding over a very large area offers the possibility of coating hundreds of smaller SiC optics at a time, thereby driving the cost down. In exceptionally high volumes it is conceivable that the price to mass produce a Si-SiC optic could become competitive with traditional Zerodur and ULE optics. Thousands of small laboratory mirrors, high-energy laser mirrors and microlithographic mirrors, are sold throughout the world in places like "Edmond Scientific". The ability to economically figure and polish high-performance SiC substrates, would open up many new markets.

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