Abstract

The art of designing optical coatings for micromirrors and other optical MEMS components is significantly different than for traditional optics. The thickness of a coating in a micromirror may be comparable to the thickness of the mirror structure itself, making it extremely susceptible to stress effects that will significantly warp the shape of the mirror and introduce optical aberrations. As a result, mechanical design considerations can supersede the importance of optical ones. A major source of mechanical stress in micromirror coatings is the thermal expansion differences between thin film layers. While the as-deposited stress can often be controlled during deposition, one has much less flexibility with thermal expansion and must create optomechanical design techniques to produce suitable mirrors. In this paper, we analyze typical micromirror structures for thermal expansion-induced aberrations and develop a semi-analytical design model that accurately predicts these aberrations as a function of the mirror structure, coating stress, and thermal expansion. It is further shown that these thermal expansion effects can be controlled with a high mechanical tolerance to thickness errors during manufacturing by using one or more additional coating compensation layers of appropriate thickness and thermal expansion coefficient.

© 2001 Optical Society of America