Fabrication and tolerances of moth-eye structures for perfect antireflection in the mid-infrared wavelength region

摘要

Mid-infrared, 25 - 45 microns, is a very important wavelength region to investigate the physics of lower temperatureenvironments in the universe. There are few transparent materials in the range of mid-infrared exceptsilicon. However, the reflection on a silicon surface reaches 30 % because of its high refractive index (~3.4).To apply silicon to mid-infrared astronomical instruments, we need a way of antireflection and have adopteda moth-eye structure. This structure keeps durable under cryogenic environments, which is advantageous tomid-infrared instruments. We have fabricated three samples of the moth-eye structure on plane silicon surfacesby electron-beam photo-lithograph and reactive ion etching. The structures consist of many cones standing onsilicon surfaces. We have substantiated the transmittance of 96 % or higher in the wide range of 20 - 50 micronsand higher than 98 % at the maximum. The transmittance of moth-eye surfaces, however, is theoretically expectedas 100 %. We have examined the discrepancy between the transmittance of the theory and fabricationswith electromagnetic simulations. It has been revealed that shapes of the cones and gaps at the bottom of thecones seriously affect the transmittance. We have estimated a few tolerances for manufacturing the moth-eyestructures achieving sufficient transmittance of nearly 100 %.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.