Micromachined mid-infrared tunable Fabry-Perot filter

This contribution deals with the design, fabrication and test of a micromachined first order Fabry-Perot (FP) filter that is intended for use in advanced infrared gas analysis. To achieve optimal interference conditions within an FP filter, the roughness and the curvature of its reflectors must be minimized, while the parallelism between reflectors is maintained. The approach here minimizes mirror curvature by using 300 μm thick mirror carriers for the coplanar fixed and movable reflectors of the FP filter. Dielectric reflectance stacks consisting of deposited alternating layers of silicon dioxide (SiO₂) and polycrystalline silicon (Si) are used as the mirror reflectors and single and a multilayer antireflection coating is used at the silicon/air interface. The cavity spacing is electrostatically driven.