This Letter reports on an advanced out-of-plane bending mode for aluminum-nitride (AlN)-actuated cantilevers. Devices of different thickness were fabricated and characterized by optical and electrical measurements in air and liquid media having viscosities up to 615 cP and compared to the classical out-of-plane bending and torsional modes. Finite element method eigenmode analyses were performed showing excellent agreement with the measured mode shapes and resonance frequencies. Quality factors (Q-factor) and the electrical behavior were evaluated as a function of the cantilever thickness. A very high Q-factor of about 197 was achieved in deionized water at a low resonance frequency of 336 kHz, being up to now, the highest quality factor reported for cantilever sensors in liquid media. Compared to the quality factor of the common fundamental out-of-plane bending mode, a 5 times higher Q-factor was achieved. Furthermore, the strain related conductance peak of the roof tile-shaped mode is superior. Compared to any out-of-plane bending mode, this combination of most beneficial properties is unique and make this mode superior for a large variety of resonator-based sensing applications.

It is the objective of this paper to report on the performance of piezoelectric MEMS resonators for viscosity and density measurements at elevated temperatures. A custom-built temperature controlled measurement setup is designed for fluid temperatures up to 100 °C. Piezoelectric single-side clamped resonators are fabricated, excited in 2nd order of the roof tile-shaped mode (13-mode) and exposed to several liquids (i.e. D5, N10, N35, PAO8, olive oil, ester oil and N100). At the next step, these results are analysed applying a straightforward evaluation model, thus demonstrating that with piezoelectric MEMS resonators the density (i.e. from kg m ⁻³ to kg m ⁻³ ) and viscosity (i.e. from mPa s to mPa s) values of liquids can be precisely determined in a wide range. Compared to standard measurement techniques, the results show for the first parameter a mean deviation of about 1.04% at 100 °C for all the liquids investigated. For the second parameter, the standard evaluation model implies a systematic deviation in viscosity with respect to the calibration being N35 in this study. This inherent lack of strength has a significant influence on the accuracy, especially at 100 °C due to fluids having a viscosity reduced by a factor of 30 for N100 compared to room temperature. This leads to relative deviations of about 23% at 100 °C and indicates the limits of the evaluation model.

This letter reports on higher orders of an advanced out-of-plane bending mode in aluminium-nitride (AlN)-actuated cantilever plates achieving the highest quality factors (Q-factor) of cantilever-based MEMS (micro electromechanical system) resonators in liquids up to now. Devices based on a 20 μm thick silicon cantilever were fabricated and characterised by optical and electrical measurements in air and in different liquids. Furthermore, finite element method eigenmode analyses were performed, showing an excellent agreement with the measured mode shape and the electrical characteristics. The highest Q-factor was achieved in deionised water with Q = 366, operated at the 10th order mode at a resonance frequency less than 4 MHz. This is the highest value ever measured in liquid media with a cantilever-based MEMS resonator up to now and exceeds the Q-factors of state of the art resonators in liquids in the given resonance frequency range by a factor of about 4. Furthermore, the strain related conductance peak of the multi roof tile-shaped modes is superior, showing great potential for further electrode design optimisation. Compared to common out-of-plane bending modes, this combination of most beneficial properties is unique, making this type of vibration mode the first choice for a large variety of resonator-based liquid-phase sensing applications.