Design, optimisation and predicted performance of a micro-machined IR sensor that exploits the squeeze film damping effect to measure cantilever beam displacement

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 ΔT=0.21 mK, and an electronic noise-limited performance of NE ΔT=4 mK, being predicted for a 100×100 μm cantilever at 1 kHz measurement bandwidth.