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Refractive optical elements are widely used in millimeter and sub-millimeterastronomical telescopes. High resistivity silicon is an excellent material fordielectric lenses given its low loss-tangent, high thermal conductivity andhigh index of refraction. The high index of refraction of silicon causes alarge Fresnel reflectance at the vacuum-silicon interface (up to 30%), whichcan be reduced with an anti-reflection (AR) coating. In this work we reporttechniques for efficiently AR coating silicon at sub-millimeter wavelengthsusing Deep Reactive Ion Etching (DRIE) and bonding the coated silicon toanother silicon optic. Silicon wafers of 100 mm diameter (1 mm thick) werecoated and bonded using the Silicon Direct Bonding technique at hightemperature (1100 C). No glue is used in this process. Optical tests using aFourier Transform Spectrometer (FTS) show sub-percent reflections for asingle-layer DRIE AR coating designed for use at 320 microns on a single wafer.Cryogenic (10 K) measurements of a bonded pair of AR-coated wafers also reachedsub-percent reflections. A prototype two-layer DRIE AR coating to reducereflections and increase bandwidth is presented and plans for extending thisapproach are discussed.

Deep Reactive Ion Etched Anti-Reflection Coatings for Sub-millimeter Silicon Optics