Stroboscopic phase-shifting interferometry for dynamic characterization of optical MEMS

Macro-scale optical components with surface flatness better than 25 nm over large areas (more than 1x1 mm) are widely available. However, the flatness of optical MEMS devices (for example micro-mirrors and -diffraction gratings) is often considerably worse. In addition to static deformation caused by film stresses and stress gradients, dynamic mechanical effects, such as air drag and excitation of higher-order resonant modes, cause surface deformations that are difficult to predict using theoretical or finite-element models. These deformations can cause significant degradation to optical performance. Dynamic measurements of nanometer-scale displacements across the entire surface of a micro-minor are difficult or impossible to perform with conventional MEMS metrology techniques such as SEM, AFM, and optical microscopy. Stroboscopic interferometry,(1,2) however, can be used to measure time-slice images that show three-dimensional motion of fast-moving MEMS devices, with vertical resolution better than I nm. In this paper, we report the application of this technique to dynamic characterization of fold-up surface-micromachined structures and show how the method can be used to provide new insights into the optical and mechanical behavior of scanning micro-mirror devices.