Effect of image degradation on nm-scale MEMS FFT optical displacement measurements

This paper examines the optical measurement of nm-scale MEMS in-plane motion using microphotographs, and the effect of degraded images on these results. Two common forms of image degradation are: blurring caused by the dynamic (AC) motion of the actuator and focus effects due to changes in focal plane or media changes. This paper address whether consistent nm-scale results can still be obtained when the image is thus changed. The motion of MEMS thermal actuators was measured by taking a series of undegraded microphotographs at varying DC voltages that produce displacements of ~2.5 microns. A FFT based algorithm was used to analyze the phase of periodic structures in the image and measure displacements with precision on the order of 20-100 nm. Pristine undegraded measurements were compared with both artificially blurred images as well as manually defocused images. It was found that the FFT method was robust to large amounts of blurring and defocusing.