<title>Optical measurement methods to study dynamic behavior in MEMS</title>

The maturing designs of moving microelectromechanical systems (MEMS) make it more-and-more important to have precise measurements and visual means to characterize dynamic microstructures. The Berkeley Sensor&Actuator Center (BSAC) has a forefront project aimed at developing these capabilities and at providing high-speed Internet (Supernet) access for remote use of its facilities. Already in operation are three optical-characterization tools: a stroboscopic-interferometer system, a computer-microvision system, and a laser-Doppler vibrometer. This paper describes precision and limitations of these systems and discusses their further development. In addition, we describe the results of experimental studies on the different MEMS devices, and give an overview about high-speed visualization of rapidly moving MEMS structures. Advanced testing methods for the dynamics of microdevices are necessary to develop reliable, marketable microelectromechanical systems (MEMS) (1). The main purpose for MEMS testing is to provide feedback to the design-and- simulation process in an engineering development effort. This feedback should include device behavior, system parameters, and material properties. An essential part of a more effective microdevice development is high-speed visualization of the dynamics of MEMS structures. The growing number of MEMS related projects at Berkeley have lead to the essential need for advanced testing and characterization facilities for microdevices. We are establishing an optical MEMS-metrology laboratory for the increasingly diverse types of micromechanical devices developed at Berkeley or by the industrial partners of BSAC. At this time we have assembled three setups to measure and characterize the motions of MEMS microstructures: • One setup is a Stroboscopic Microscopic Interferometer System that has been developed at BSAC. The system can measure both in-plane and out-of-plane motions in a single experiment. In-plane motions are measured with subpixel resolutions better than 5nm. Out-of-plane deflections are recorded with nm accuracy for structures either at rest or vibrating at frequencies up to 1MHz. Stroboscopy and digital-image processing are employed to determine in-plane motions with sub-pixel resolution. Stroboscopic interferometry is used to determine out-of-plane deflections at defined phases for a periodically moving structure. We have developed this system to study the excitation of complex mechanical modes in micromachined devices. • A second system was developed at MIT and is called a "Computer-Micro-Vision System". It employs stroboscopy and image-processing techniques to track the motions of moving rigid-body structures. Through a focus variation, this system can measure out-of-plane as well as in-plane motions. Frequencies up to 100kHz have been investigated in our laboratory. • A third system is a commercial Laser-Doppler Vibrometer (Polytec PI), which can be used to measure transient out- of-plane motion on one spot. With a microscope adaptor, the measuring laser beam is coupled into a microscope and is focused to a spot smaller than 1µm in diameter. This system measures the Doppler shift of the reflected laser beam and, therefore, the velocity of a moving microstructure. The laser-Doppler instrument can measure out-of- plane motions having frequencies as high as 1.5MHz. Different MEMS devices, for example: micromirrors, read/write heads for hard disks, acceleration sensors, and electroacoustic high-frequency elements have been studied using the three different experimental facilities. In addition, we use the white-light interferometer (Wyko NT3300) from Veeco Instruments for accurate static-profile characterizations of complex structures.