Optical EngineeringHolographic microscope for measuring displacements of vibrating microbeams using time-averaged, electro-optic holography
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An optical microscope, utilizing the principles of time- averaged hologram interferometry, is described for microelectromechanical systems (MEMS) applications. MEMS are devices fabricated via techniques such as microphotolithography to create miniature actuators and sensors. Many of these sensors are currently deployed in automotive applications which rely on, or depend on, the dynamic behavior of the sensor, e.g., airbag sensors, ride monitoring suspensions sensors, etc. Typical dimensions of current MEMS devices are measured in micrometers, a small fraction of the diameter of a human hair, and the current trend is to further decrease the size of MEMS devices to submicrometer dimensions. However, the smaller MEMS become, the more challenging it is to measure with accuracy the dynamic characteristics of these devices. An electro-optic holographic microscope (EOHM) for the purpose of studying the dynamic behavior of MEMS type devices is described. Additionally, by performing phase measurements within an EOHM image, object displacements are determined as illustrated by representative examples. With the EOHM, devices with surface sizes ranging from approximately 35x400 to 5x 18 µm are studied while undergoing resonant vibrations at frequencies as high as 2 MHz.