Deep Reactive Ion Etched Anti-Reflection Coatings for Sub-millimeter
  Silicon Optics
  Refractive optical elements are widely used in millimeter and sub-millimeter
astronomical telescopes. High resistivity silicon is an excellent material for
dielectric lenses given its low loss-tangent, high thermal conductivity and
high index of refraction. The high index of refraction of silicon causes a
large Fresnel reflectance at the vacuum-silicon interface (up to 30%), which
can be reduced with an anti-reflection (AR) coating. In this work we report
techniques for efficiently AR coating silicon at sub-millimeter wavelengths
using Deep Reactive Ion Etching (DRIE) and bonding the coated silicon to
another silicon optic. Silicon wafers of 100 mm diameter (1 mm thick) were
coated and bonded using the Silicon Direct Bonding technique at high
temperature (1100 C). No glue is used in this process. Optical tests using a
Fourier Transform Spectrometer (FTS) show sub-percent reflections for a
single-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 reached
sub-percent reflections. A prototype two-layer DRIE AR coating to reduce
reflections and increase bandwidth is presented and plans for extending this
approach are discussed.
