Micro-sensors utilising the mode-localisation effect in electrostatically coupled MEMS resonators

Abstract

In response to a perturbation, the stiffness or mass of a MEMS resonator can change and previous research has utilised the resonant frequency shift to characterise the perturbation, be it strain or acceleration altering the resonator stiffness or the attachment of a biological element altering the resonator mass. More recently, research has focused on developing MEMS resonator sensors based on the mode-localisation effect, which is the name given to the effect where the mode shapes of an electrostatically coupled system are seen to 'localise' around one of the resonators when the stiffness or mass
of one changes.

By measuring the change in the mode shape of a coupled system, it is possible to achieve a greater sensitivity to a perturbation than by simply measuring the change in resonant frequency of a one degree-of-freedom system. Building on the previously reported work, the design, fabrication and characterisation of various designs and dimensions of coupled MEMS resonator devices has been performed, with the aim of experimentally determining the influence of the design and dimensions on the device sensitivity. A high-yield dicing-free silicon-on-insulator based process has been used to fabricate electrostatically-coupled MEMS resonator pairs with a thickness of 50 μm.

For a design consisting of two 410 μm long rectangular clamped-clamped beams, the sensitivity of the amplitude ratio at the in-phase mode shape has been increased up to 12 times by increasing the beam width from 10 μm to 20 μm. A second design, featuring a larger 310 x 60 μm rectangular block at the centre of the resonator, has shown a sensitivity up to 3.26 times greater than for the clamped-clamped beams and up to 9 times greater than the state-of-art, with reducing the anchor beam lengths down to 55 μm proving to be critical. Other devices fabricated with an alternative SOI-based process showed stiffness sensitivity up to 46 times greater than the state-of-the-art, but with the drawback that the fabrication process is of a much lower yield. Finally, through removal of up to 3.39 ng of material with a focused ion beam, mass sensing has been demonstrated with a coupled-resonator device, with a 5.4 times greater amplitude ratio response compared to the best value in the literature.

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Identifiers

ORCID for Harold Chong: orcid.org/0000-0002-7110-5761

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