Micromachining porous silicon Fabry–Pérot filters operating in the long wavelength infrared range

Porous silicon is a promising material to form low-cost all-silicon optical filters by leveraging the ability to tune the porosity to achieve multilayer thin films with alternating refractive indices. In this work, porous silicon based Fabry-Perot filters operating in the longwave (8-12 µm) infrared range were demonstrated. Surface micromachining processes were optimised by developing techniques to achieve stress relief, uniform porosity and flat surfaces, resulting in suspended filters with aperture size up to 600×600 µm2. The filters were evaluated by modelling and measuring the optical and mechanical properties. The findings provide a pathway to fabricate optical filters for multispectral thermal imaging applications.