Thermally invariant dielectric coatings for micromirrors

Thermal expansion-induced curvature becomes a major effect in micromirrors as the mirror diameter exceeds 100 μm. Such mirrors are used for optical switching, scanning, and many other applications. By using multilayer coatings instead of a single metal reflector, one can use the mechanical properties of the multilayer to create mirrors with zero curvature across temperature. We demonstrate the fabrication of such thermally invariant mirrors using dielectric coatings. A semianalytic model based on free-plate elastic theory is developed that uses empirical parameters in place of the true thermal expansion coefficients of the coating materials. Micromirrors are demonstrated that maintain their design curvature to within λ/60 for λ = 633 nm across an operating range from 21 °C to 58 °C.