Resonant MEMS Piezoelectric Balances and Strain-Gauges




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Microelectromechanical (MEMS) resonators can be used as high sensitivity frequency output nano-balances to detect small masses of biomolecules at low concentrations for biosensing applications. For this reason, the micro-resonator is engaged in an oscillation circuit as a frequency source where adsorption of masses to the surface of the MEMS resonator causes a shift in its mechanical resonance frequency. Despite their high mass sensitivity, mechanical and electrical performance of micro-structures are deteriorated at liquid media due to viscosity and electrical conductivity of biological solutions. Proper strategies are required to improve performance of such sensors at liquid phase to enable their operation as real-time high-sensitivity mass sensors. In this work, an investigation has been conducted on Thin film Piezoelectric-on-Substrate (TPoS) resonators as the main platform aiming at development of resonant sensors with improved performance at liquid phase. For such purpose, two distinctive classes of resonant nano-balances have been introduced using TPoS structures. In a Micro-Resonator-on-Membrane (MRoM) a thin membrane has been used which provides an electrical isolation while minimizing surface interface of the resonator with liquid. High liquid phase quality factors (Qs) up to 410 have been achieved using such structures without electrical interference of liquid on the resonator. MRoMs have been used for detection of bio-analytes in liquid phase. MRoMs fabricated on 15 µm thick substrate, operating in the 1st width extensional (WE) mode, have demonstrated high mass sensitivities up to -18.9 Hz.cm2/ng. A higher mass sensitivity of -88.9 Hz.cm2/ng has also been achieved for operation in the 3rd length extensional (LE) mode for MRoMs made out of a 10 µm thick substrate. In an alternate approach, rectangular resonators were coupled with two half-ring arms on their sides which allows operation of the structure in an elliptical resonance mode. Similar to disk resonators due to sliding of moving parts of the resonator in parallel to liquid, operation of such resonators in an elliptical resonance mode provides high liquid phase Qs. Quality factors up to 200 have been exhibited for elliptical mode resonators. Moreover, thanks to stronger transduction of rectangular plates, an elliptical resonator has shown a much lower motional resistance (Rm) of ~5.5 kΩ in comparison to disk resonators.

In addition to use of TPoS resonators as high sensitivity mass sensors, their potential applications as resonant strain-gauges have also been studied. A laterally vibrating TPoS resonator can be used as or as a part of a micro-cantilever. In this work, TPoS micro-resonators have been embedded as strain-gauges in force/displacement sensors. Three different designs have been proposed in use of such resonant strain-gauges in high sensitivity profilometers. Such designs include a micro-cantilever vibrating in a length extensional mode, embedding micro-resonators along cantilevers and coupling a micro-resonator with cantilever extensions. The highest displacement sensitivity of 1.49 kHz/µm has been demonstrated for a TPoS resonator operating in the 1st length extensional mode which was coupled with triangular cantilever extensions on it sides.



Microelectromechanical systems, Resonators, Quality factor meters, Piezoelectric devices, Biosensors, Strain gages, Electric displacement


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