Using an In-Situ Micromirror to Assist the Measurement of In-Plane Vibration of Microstructures

Heterodyne laser interferometry is an optical technique often used to measure displacement of surfaces along the wave vector direction of a measurement laser. For common microelectromechanical system (MEMS) testing setup, such laser wave vector is perpendicular to the substrate which the micromachined devices stand on. Therefore, this technique can only be used to characterize dynamics of the micro devices in the direction perpendicular to their substrate (out-of-plane motions) with the classic setup and it is not able to measure any motion that is parallel to the substrate (in-plane motions). In this study, in-situ micromirrors are fabricated onto a microstructure that is near the device to be measured by using a focused ion beam system. The micromirrors have a slant angle of approximate 45 degree to horizontal surface (or their substrate). By using the post-fabricated in-situ micromirror, the measurement laser of a heterodyne interferometer can be directed into horizontal plane which enables characterization of in-plane motions for micromechanical. To experimentally demonstrate the technique a micro cantilever fabricated using MetalMUMPs is used. The micro cantilever is excited by inplane electrostatic force. The results confirm the effectiveness of the method by the fact that the magnitude of the measured in-plane signal is increased by more than ten folds.