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

TL;DR

This paper presents a novel deep reactive ion etching (DRIE) technique for creating anti-reflection coatings on silicon optics at sub-millimeter wavelengths, achieving sub-percent reflectance and promising for astronomical applications.

Contribution

It introduces a new DRIE-based AR coating process for silicon optics, including bonding techniques and a two-layer prototype to enhance bandwidth and reduce reflections.

Findings

Sub-percent reflection achieved at 320 microns wavelength.

Successful cryogenic testing at 10 K showing low reflectance.

Prototype two-layer coating extends bandwidth and reduces reflections.

Abstract

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.