Design and characterization of a micromachined Fabry–Perot vibration sensor for high-temperature applications

We have designed and characterized a MEMS-based Fabry–Perot device (MFPD) to measure vibration at high temperatures. The MFPD consists of a micromachined cavity formed between a substrate and a top thin film structure in the form of a cantilever beam. When affixed to a vibrating surface, the amplitude and frequency of vibration are determined by illuminating the MFPD top mirror with a monochromatic light source and analyzing the back-reflected light to determine the deflection of the beam with respect to the substrate. Given the device geometry, a mechanical transfer function is calculated to permit the substrate motion to be determined from the relative motion of the beam with respect to the substrate. Because the thin film cantilever beam and the substrate are approximately parallel, this two-mirror cavity arrangement does not require alignment or sophisticated stabilization techniques. The uncooled high-temperature operational capability of the MFPD provides a viable low-cost alternative to sensors that require environmentally controlled packages to operate at high temperature. The small size of the MFPD (85–175 µm) and the choice of materials in which it can be manufactured (silicon nitride and silicon carbide) make it ideal for high-temperature applications. Relative displacements in the sub-nanometer range have been measured and close agreement was found between the measured sensor frequency response and the theoretical predictions based on analytical models.