Micromachined porous silicon Fabry-Pérot long wavelength infrared filters

Porous silicon Fabry-Pérot (PS-FP) filters operating in the long wavelength infrared (LWIR) range are demonstrated and studied. Porous silicon (PS) offers many advantages for such filters, including improved optical and mechanical properties compared to other LWIR filter materials, and the ability to modulate porosity to form multilayer structures that are compatible with conventional semiconductor technologies. This work discusses the optical and the mechanical characterizations of what are the first MEMS-based suspended PS-FP filters operating in the LWIR range. Methods developed to model and characterize PS-FP filters are discussed, with characterization a challenge for such porous structures considering their low refractive index and low Young's modulus. The transmittance measurements show that introducing notches at the four corners of the top mirror minimizes the bowing of the top mirror significantly. The stress measurements of attached films show that the lattice mismatch at the PS/silicon interface is likely the key factor controlling the stress in multilayer PS thin films. The micromachining techniques developed to form a resonance cavity within PS-FP filters can also be tuned to reduce the stress mismatch and the absorption loss in the LWIR range. This study shows exciting promise for the use of micromachined PS-based filters for LWIR operation.