Simulated effects of wet-etched induced surface roughness on IR transmission and reflection

We have constructed a finite element simulation where we investigate the effects of wet-etch-induced surface roughness on transmission and reflection of infrared light in the 8-12um band. A silicon wafer was wet-etched for 2 hours in a 10% KOH solution at 80 degrees C, scanned in an atomic force microscope, and the surface profile was recreated in COMSOL. Simulated plane waves of light and varying angles of incidence were then allowed to pass through this surface and the resulting effects on the reflection and transmission were investigated. Roughness was then amplified to investigate the effects of increased surface roughness. For the wavelengths investigated, an increase in transmission of 8% could be seen up to an RMS surface roughness of 800nm followed by a decrease, while the angles investigated showed an RMS dependent increase in transmission between 20 degrees and 40 degrees for RMS surface roughness' above 1000nm.