Dynamic characterization of MEMS cantilevers in liquid environment using a low-cost optical system

Taking advantage of an off-the-shelf, high-quality but inexpensive optical detector, namely a DVD optical head, a system has been developed for the dynamic characterization of the vertical (out-of-plane) displacement of MEMS structures. The novelty of the proposed setup, besides the proposed field of application, lies in the ability of the system to operate both in air and in a liquid environment. In order to give a representative experimental proof of the functionality of the system, a cantilever array was designed and fabricated via a commercially available multi-user polysilicon surface micromachining process (PolyMUMPs). The frequency response of a tiny silicon cantilever beam (150 µm length, 10 µm width and 1.5 µm thickness) was measured, both in air and in deionized water, and compared against theoretical models of the system. Despite the low-cost signal conditioning system used, it was possible to trace a very clear Bode plot, and to obtain good agreement between experimental data and theoretical predictions. The measurements indicate that this displacement sensor performs very well under many circumstances of interest which require continuous monitoring with large bandwidth. This sensor offers a good low-cost measurement solution for the single-point dynamic characterization of the MEMS devices, particularly when the frequencies involved are high and the working environment is unsuitable for other types of detectors.