Measuring Motions of MEMS
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Measuring Motions of MEMS
Dennis M. Freeman Testing microelectromechanical systems (MEMS) includes testing a variety of electrical and mechanical signals. While powerful tools are available to test the electrical behavior of microfabricated systems, relatively few tools are available to measure micromechanical behavior. Motions of MEMS are commonly measured using two electrodes: one attached to a moving part and one attached to a stationary reference point. Relative motions of the electrodes change the capacitance between them, and this change is detected electrically. Although the reference electrode can be external, both the sense and reference electrodes used in this “capacitive probe” method are more commonly integrated into the MEMS design. Interpreting changes in capacitance as motion is complicated by the fact that threedimensional (3D) motion can generate ambiguous changes in capacitance. For example, the capacitance of a “comb” sensor (Figures 1 and 2) changes if the teeth move along their long axis (in-plane) or out-of-plane. Thus, multiple electrode pairs are generally needed to determine 3D motions. Furthermore, since the changes in capacitance are small, large-area electrodes are typically required. Thus, integrating electrodes to sense every possible mode of motion of a MEMS of even moderate complexity may not be practical or even possible. Tools for in situ measurement of motions of micromechanical structures are therefore critical to the effective design and fabrication of MEMS. One convenient method is laser Doppler velocimetry,1,2 in which a laser beam is focused to a spot size that is small compared to the target of interest, and the reflected light is mixed with a reference beam to generate interference. Commercial systems (e.g., Polytec, Waldbronn, Germany) are sensitive to motions as small as a few nanometers. While very effective for measuring out-ofplane motions, the mirrorlike surfaces of many MEMS complicate measurements in other directions because little light is scat-
MRS BULLETIN/APRIL 2001
tered back along the incident path unless the path is perpendicular to the surface. Optical imaging provides the ability to measure 2D motions of any visible part without the need to integrate additional components into the MEMS. If the general form of the motion is known (e.g., if it is sinusoidal), then the magnitude of the motion can be determined by comparing a
blurry image of a moving part with a second image of the part taken when it is not moving. This technique has been applied to determine the resonant frequency and quality of tuning of MEMS.3 Quality of tuning is a quantitative measurement that indicates whether a structure is easily excited by the resonant (peak) frequency as compared with other frequencies. Arbitrary periodic motions can be measured from sequences of stop-action images taken with stroboscopic illumination (Figure 1). By analyzing such sequences using algorithms originally developed for robot vision,4,5 motions as small as a few nanometers can be resolved.6–8 This appl
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