Velocity and displacement measurements of microelectromechanical systems using laser vibrometry

An instrumentation suite for 3D-motion measurement of displacement and velocity of microelectromechanical systems (MEMS) is described. The instrument suite includes an optical microscope with an integrated laser vibrometer and computer-controlled instrumentation for measurement of real-time device dynamics. A long focal-length optical microscope (NA = 0.28) provides mum-scale spatial resolution for positioning the laser beam (lambda = 633nm) on the MEMS structure for nm-resolution interferometry and for imaging through a vacuum viewport. Integrated micromachined mirrors reflect the laser beam into the plane of the MEMS to measure the in-plane motion of the microstructure. The measurement of velocity or displacement of mum-scale features normal to the plane of a reflective MEMS structure is done directly on the MEMS surface. Decoders provide simultaneous measurements of displacement with resolution up to 4nm in a 50kHz bandwidth and velocity with a resolution to 0.5mum/s in a 1.5 MHz bandwidth. The instrument suite includes electronics and software for real-time recording and display of system variables including velocity and displacement. Programmable experimental variables include applied voltage, laser position, optical focus, chamber pressure and temperature. The system can measure device parameters including natural frequency and quality factor.;The instrument suite is used to characterize single-crystal Silicon MEMS as a demonstration of its capabilities. Devices studied include lateral actuators and a large displacement XY stage. Characterization of the electromechanical responses of a torsional resonator and a lateral tunneling accelerometer are also presented. The tunneling current response and the electromechanical device performance is measured for the accelerometer. The motion of the torsional resonator can be described by the Mathieu equation. Maps of the stability regions are measured to characterize the response of the device. Resonant frequency, quality factor, linear spring constant, cubic spring constant, and mass are measured experimentally or determined by models. The measurement errors are discussed. Resonant mode maps, phase plots (dx vs. x), pressure dependence data, and other measurements are included to further demonstrate the performance of the instrumentation suite.