CVD silicon carbide—a promising material for low-scatter mirrors

Abstract

Silicon carbide formed by chemical vapor deposition is a promising material to use for lightweight, low-scatter mirrors. It can be made optically dense with a minimum of growth defects and stress birefringence, and it can be polished to a low-scatter finish that is comparable to the best high-quality fused-silica optics. Roughness values of 0.3 Å rms have been measured on a Zygo heterodyne profiler and under 1 Å rms on a Talystep mechanical-contact profiler. The optical constants measured in the wavelength range from 1.6–25 μm show that there is a reststrahlen band in the 10–13μm region and that the reflectance exceeds 94% at 12 μm. Total integrated scattering (TTS) at 0.647 μm can be as low as 1 × 10−6, corresponding to an rms roughness of 5.2 Å. Bidirectional-reflectance distribution function (BRDF) values of less than 5 × 10−6 at a scattering angle of 10 from the specular direction and approximately 3 × 10−6 at 25–80° from the specular direction have been measured at 0.633 μm. The integrated BRDF values yield a roughness in good agreement with that measured by TIS. These BRDF values are approximately 11 orders of magnitude down from the intensity of the specular beam. Data measured on silicon carbide samples from different sources and polished by different companies will be shown.