Curvature compensation in micromirrors with high-reflectivity optical coatings

High-reflectivity coatings on micromirrors are critical to reduce reflection losses and absorptive heating. Unfortunately, coating stress induces an unwanted curvature in micromirrors. This effect is much more serious than in bulk optics because the thin coatings are similar in thickness to the structural material of the mirror. This paper describes a method to apply coatings that simultaneously achieve high reflectivity and optical flatness. The design theory is based on one-dimensional (1-D) static analysis and achieves curvature compensation with only a single additional coating layer. The technique is appropriate for any number of coating layers and includes both the elastic and plastic behaviors of the thin film layers. Plastic deformation is modeled using an empirically determined strain versus dielectric thickness curve. Experimental measurements of 200 mum x 200 mum x 3.5 mum polysilicon plates show mirror flatness better than lambda /10.