Deformation measurement of MEMS components using optical interferometry

Current trends in miniaturization of microelectromechanical systems (MEMS) require the use of smaller and smaller components. Development of these microcomponents, e.g. microbeams in an accelerometer and membranes in a microphone, requires state-of-the-art test and measurement methodologies to inspect the deformation of the microcomponents for further understanding of their mechanical properties. In this paper we describe a system developed for testing deformation of a microbeam in an accelerometer under point-force load and a membrane in a microphone under applied voltage. The technique is based on optical interferometry. A collimated monochromatic beam is directed into an air wedge consisting of an optical reference plate and the microcomponent under test. The resulting interference fringe patterns from the air wedge are captured by a CCD camera mounted on a long working-distance microscope and subsequently stored in a computer. The fringe patterns that are related to deformations of the test object are analysed by a simple algorithm for recording both integral and fractional fringes. From the fringe pattern, deformation of the microbeam and membrane in the sub-micrometre range are obtained. The proposed method is potentially applicable to the in situ inspection of microcomponents in MEMS.