INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING J. Micromech. Microeng. 16 (2006) 2618–2631 doi:10.1088/0960-1317/16/12/015 Design and characterization of a micromachined Fabry–Perot vibration sensor for high-temperature applications P M Nieva 1 , N E McGruer 2 and G G Adams 2 1 University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada 2 Northeastern University, 360 Huntington Avenue, Boston, MA 02115, USA E-mail: pnieva@uwaterloo.ca Received 14 July 2006, in final form 2 October 2006 Published 7 November 2006 Online at stacks.iop.org/JMM/16/2618 Abstract 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. (Some figures in this article are in colour only in the electronic version) 1. Introduction MEMS sensors for harsh environments are recognized as essential for reducing weight and volume, in strategic market sectors such as automotive, aerospace, communications, oil-well/logging equipment, turbomachinery, and nuclear power [1, 2]. Typical temperatures for the automotive and aerospace systems range from 200 ◦ C to 600 ◦ C. Higher temperatures up to and above 900 ◦ C can be found in extremely harsh environments, such as turbine engines, nuclear power generators, etc. Silicon (Si) is well suited for the development of a wide range of MEMS sensing elements. However, conventional Si-based MEMS sensors containing pn-junctions suffer from severe performance degradation and failure above 200 ◦ C due to excessive leakage currents [2]. Presently, when the environment temperature is too high, the electronics must reside in cooler areas, either remotely located or actively cooled. The additional weight, in the form of longer wires, more connectors, and/or bulky and expensive cooling systems, adds undesired size and weight to the system. It also increases complexity and potential for failure. MEMS sensors that can be placed closer to the ultimate point of use will reduce weight, decrease interconnection complexity and improve machine reliability [1, 2]. 0960-1317/06/122618+14$30.00 © 2006 IOP Publishing Ltd Printed in the UK 2618