MEMS and MOEMS resonant frequencies analysis by Digital Holography Microscopy (DHM)

Digital Holographic Microscopes (DHMs) have unique features especially relevant for static and dynamical MEMS characterization. They provide both 3D topography with interferometric resolution and intensity image in a single acquisition at camera rate, without any lateral or vertical scanning. In this presentation, DHM is operated in conjunction with a laser pulsed stroboscopic module providing synchronization of camera, laser pulses, and MEMS excitation signal up to 25 MHz. Three methods for DHM analysis of resonant frequencies are presented with concrete examples. The first method, gives the more general, precise and complete information. Sine wave excitation with increasing (or decreasing) frequencies is used. For each frequency, laser pulses are used to “freeze” the movement of the MEMS. Using the stroboscopic synchronization, each period of the excitation signal is sampled at high resolution, and the topography of the MEMS can be measured at each of those samples points. As implemented, the change of frequency is a continuous sweep: quantitative values in term of displacement amplitude and Bode diagrams can be measured for nonlinear resonances as well. The second method uses sine wave excitation with increasing (or decreasing) frequencies. For each frequency, the optical signal is integrated over an entire number of periods of the MEMS. At resonance, constructive and destructive interference build up on the intensity images. It enables fast frequency scan over large ranges. But it provides neither quantitative values of displacement amplitude, nor Bode diagrams. The third method is to measure the system response to an impulse or chirp excitation signal for instance, and to make a Fourier analysis of this response to determine resonant frequencies. This method is less sensitive as it spreads the excitation energy in many frequencies.