Optics and Lasers in Engineering 45 (2007) 503–522 Design, optimisation and predicted performance of a micro-machined IR sensor that exploits the squeeze film damping effect to measure cantilever beam displacement P.D. Smith, C.R. Chatwin à , R.C.D. Young School of Science and Technology, University of Sussex, Falmer, Brighton, BN1 9QT, UK Received 6 June 2006; received in revised form 7 September 2006; accepted 4 October 2006 Available online 1 December 2006 Abstract We describe the theoretical modelling of an infrared (IR) sensor based on an oscillating bi-material cantilever in which the beam is quantified as a function of the squeeze-film damping ratio, by measurement of the forced damped resonance frequency or phase angle. The structure under consideration is composed of a silicon nitride cantilever beam, coated with an upper gold absorbing layer. A detailed description of the optimisation of the cantilever geometry is described, with the gap height being identified as the critical parameter. The influence of the length, width, absorber gap and thickness of the two layers on signal-to-noise ratio (snr) is also discussed and an optimum configuration identified for each parameter. Phase modulation measurement techniques are found to provide the highest measurement resolution, with a thermal mechanical noise-limited performance of NE DT ¼ 0.21 mK, and an electronic noise-limited performance of NE DT ¼ 4 mK, being predicted for a 100  100 mm cantilever at 1 kHz measurement bandwidth. r 2006 Elsevier Ltd. All rights reserved. Keywords: MEMS; Infrared; Squeeze-film damping; Micro-structure 1. Introduction Infrared (IR) sensors and thermal imaging technology have potential uses in a wide range of commercial, industrial, military and space research applications. These applications have conventionally relied upon two main classes of IR sensor, these being photon sensors and thermal sensors. [1] Photon sensors generally require cryogenic cooling. In addition, they may only be operated over very specific and relatively narrow range of wave- lengths. Because of the disadvantages associated with photon sensors, thermal sensors are generally favoured for the majority of low cost thermal imaging applications. Unfortunately, since in thermal sensors heating of the sensor element is an inherently slow process, this class of sensor is generally characterised by relatively low sensitiv- ity and slow response times. In order for thermal imaging applications to reach their full potential, a low cost, fast, un-cooled IR sensor, with high sensitivity, capable of operating across the full spectral range is required. One promising alternative to conventional thermal sensors is the use of Micro-Electro- Mechanical Systems (MEMS) technology. Sensors utilising this technology have been, almost exclusively, based upon micro-mechanical bi-material cantilever beams. With this type of MEMS IR sensor, incident IR radiation causes heating and hence bending of a bi-material cantilever beam. The incident IR radiation is then detected as a displacement of the bi-material cantilever beam. Many such devices have been reported, in which optical beam deflection techniques have been employed to measure the displacement of the beam. [2–6] Although these devices have the advantage of being capable of reaching the theoretical limit imposed by thermal mechanical noise, these techniques may prove difficult to incorporate in a compact rugged device. An alternative approach, more readily incorporated in a compact device, is direct capacitive sensing of cantilever ARTICLE IN PRESS 0143-8166/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2006.10.001 à Corresponding author. Tel.: +44 1273 678901. E-mail address: c.r.chatwin@sussex.ac.uk (C.R. Chatwin).