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 μ × 200 μ × 3.5 μ polysilicon plates show mirror flatness better than λ/10.
Bibliographical noteFunding Information:
Manuscript received September 15, 2000; revised April 6, 2001. This work was supported by DARPA under Contract F33615-00-1-1625 and Seagate Technologies. Subject Editor D.-I. Cho. The authors are with the Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455 USA (e-mail: firstname.lastname@example.org). Publisher Item Identifier S 1057-7157(01)05265-9.
- Mirror flatness
- Optical coatings
- Plastic deformation