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Vibrational modes of higher order in micromachined resonators exhibit low damping in liquid environments, which facilitates accurate sensing even in highly viscous liquids. A steady increment in mode order, however, results in sound dissipation effects at a critical mode number ncrit, which drastically increases damping in the system. Basic understanding in the emerging of sound dissipation in micromachined resonators is therefore of utmost importance, when an application of higher mode orders is targeted. For that reason, we experimentally investigated in this paper the appearance of sound dissipation in higher order non-conventional vibrational modes in MEMS plate resonators in liquids. The results are compared to those of an analytical model and of finite element method analyses. Micromechanical piezoelectric resonators were fabricated and characterized in sample fluids with a dynamic viscosity μfluid ranging from 1 to 5 mPa s and density values ρfluid ranging from 0.774 up to 0.835 kg l−1. Quality factors up to 333 are obtained for the eighth mode order in model solution with a dynamic viscosity of 1 mPa s. By monitoring the resonance and damping characteristics as a function of mode order, sound dissipation effects occur, observed by the detection of increased damping, starting at mode number n = 8, which is in good agreement to the predictions of an analytical model and to finite element method simulations. At the critical mode number ncrit, a reduction in quality factor up to 50% is measured. The results show a direct correlation of ncrit and the density of the fluid, which agrees to theory. The lowest value of 8 for ncrit is obtained in a sample liquid with the lowest density value of 0.774 kg l−1, followed by ncrit = 9 in a sample liquid with ρfluid = 0.782 kg l−1 and ncrit = 10 in a sample liquid with ρfluid = 0.835 kg l−1. These findings are of particular interest for sensing applications in low dense liquids, as sound dissipation effects emerge even at lower mode numbers. [ABSTRACT FROM AUTHOR]

In this contribution, we discuss the implementation of a novel microelectromechanical-systems (MEMS)-based energy harvester (EH) concept within the technology platform available at the ISAS Institute (TU Vienna, Austria). The device, already presented by the authors, exploits the piezoelectric effect to convert environmental vibrations energy into electricity, and presents multiple resonant modes in the frequency range of interest (i.e. below 10 kHz). The experimental characterisation of a sputter deposited aluminium nitride piezoelectric thin-film layer is reported, leading to the extraction of material properties parameters. Such values are then incorporated in the finite element method model of the EH, implemented in Ansys Workbench™, in order to get reasonable estimates of the converted power levels achievable by the proposed device solution. Multiphysics simulations indicate that extracted power values in the range of several µW can be addressed by the EH-MEMS concept when subjected to mechanical vibrations up to 10 kHz, operating in closed-loop conditions (i.e. piezoelectric generator connected to a 100 kΩ resistive load). This represents an encouraging result, opening up the floor to exploitations of the proposed EH-MEMS device in the field of wireless sensor networks and zero-power sensing nodes. [ABSTRACT FROM AUTHOR]

The reliable characterization of piezoelectric thin film properties is important for a tailored integration into micro-electromechanical systems (MEMS). In this work a method for the determination of d 33 and d 31 of piezoelectric thin films is presented by comparing Laser Doppler Vibrometry (LDV) displacement curves to finite element simulations (FEM) and applied to AlN and Sc x Al 1-x N (x = 27%). A rectangular 2-port electrode design for 2D FEM modelling enables the evaluation of d 33 and d 31 by a full implementation into a MatLab based fit function. This allows also to account for the substrate-support adhesive bond strength in the FEM evaluation process. Doing so, insufficient bond strength can be compensated. [ABSTRACT FROM AUTHOR]

In this contribution, we discuss a novel design concept of a high-performance wideband MEMS vibration energy harvester (EH), named four-leaf clover (FLC EH-MEMS) after its circular shape featuring four petal-like mass-spring systems. The goal is to enable multiple resonant modes in the typical range of vibrations scattered in the environment (i.e., up to 4-5 kHz). This boosts the FLC conversion capability from mechanical into electrical energy exploiting the piezoelectric effect, thus overcoming the common limitation of cantilever-like EHs that exhibit good performance only in a very narrow band of vibration (i.e., fundamental resonant mode). The FLC concept is first discussed framing it into the current state of the art, highlighting its strengths. Then, after a brief theoretical introduction on mechanical resonators, the FLC EH-MEMS device is described in details. Finite Element Method (FEM) analyses are conducted in the ANSYS Workbench™ framework. A suitable 3D model is built up to perform modal simulations, aimed to identify mechanical resonant modes, as well as harmonic analyses, devoted to study the mechanical and electrical behaviour of the FLC EH-MEMS (coupled field analysis). The work reports on experimental activities, as well. Physical samples of the FLC EH-MEMS device are fabricated within a technology platform that combines surface and bulk micromachining. Thereafter, specimens are tested both with a laser doppler vibrometer measurement setup as well as with a dedicated shaker-based setup, and the results are compared with simulations for validation purposes. In conclusion, the FLC EH-MEMS exhibits a large number of resonant modes scattered in the tested range of vibrations (up to 15 kHz) already starting from frequencies as low as 200 Hz, and expected levels of converted power better than 10 µW. [ABSTRACT FROM AUTHOR]

Aluminum nitride thin films are commonly used as active layer in micro-/nanomachined devices due to their piezoelectric properties. In order to predict the performance of advanced device architectures, careful modelling and simulation using techniques such as finite element analysis are of the utmost importance. An accurate knowledge of the corresponding thin film material properties is therefore required. This work focuses on the mechanical properties residual stress and Young's modulus over a wide thickness range from 100 to 1200 nm. The load-deflection technique is used to measure the bending curve of a circumferentially clamped, circular aluminum nitride diaphragm under a uniformly distributed pressure load. The bending curves are analyzed using an advanced analytical approach rather than commonly used models for load-deflection methods, thus resulting in a higher accuracy. It is found that the Young's modulus is nearly independent of film thickness, whereas the tensile residual stress exhibits a maximum at a thickness of about 600 nm. A thorough discussion of possible error sources is presented and approaches to minimize their impact are discussed. [ABSTRACT FROM AUTHOR]

A micromachined hot-film anemometer on a high-pressure stable substrate is demonstrated to be suitable for "on-board" measurements of various injection quantities, which are needed in modern direct injection systems for optimum performance of diesel, as well as gasoline, engines. To estimate the velocity profile in the injection nozzle during the injection pulse, two-dimensional finite element method calculations are performed. To consider the substrate effect on the performance of the thermal injection rate sensor, an IR imaging system is used to determine the heat loss via the robust glass ceramic substrate up to thin-film temperatures of 450 K in air. Using both the numerical and the experimental results, the temperature sensitivity and velocity sensitivity of hot-film anemometers, operated in the constant current mode in a fuel atmosphere, can be modeled analytically. Finally, an overview about the results of the latest injection rate measurements at fuel pressures up to 60 MPa and drive pulse lengths between 0.2-5 ms is given. Comparing these signals of the thermal thin-film sensor to injection volume measurements, performed with an injection amount indicator at a high-pressure hydraulic test bench, the mass flow measurements can be calibrated. One empiric parameter is necessary, which is the exponent of a power law dependence on the maximum sensor amplitude during injection. [ABSTRACT FROM PUBLISHER]

In this study, the impact of residual tensile stress on a membrane-type piezoelectric MEMS (Micro Electromechanical Systems) microphone is investigated both theoretically by FEM (finite element method) simulations and experimentally by stress measurements, by manufacturing devices and by recording the acoustic performance. Based on these findings, a design is proposed composing of segmented electrodes which are arranged close to the silicon frame as well as on the inner part of the membrane. The MEMS microphone features an aluminum nitride layer which allows a piezoelectric read-out, and multiple metal bottom and top electrodes distributed radially and azimuthally. FEM simulations were done over a large range of different layer thicknesses, membrane radii, and tensile stress levels, which indicate the existence of a sweet-spot when targeting a maximum output signal. The data showed, that thicker membranes perform better under tensile stress than thinner membranes. Furthermore, a relation between output signal and residual stress level was established, which showed that the FEM predictions agree well with the measurement results of piezoelectric microphones. [Display omitted] • Thicker MEMS membranes preferred over thinner counterparts when tensely pre-stressed. • Finite element analyses revealed sweet spot for maximizing piezoelectric output. • Design propositions for stressed closed membranes without stress relaxing measures. • Microphone measurements and finite element methods results agree well. [ABSTRACT FROM AUTHOR]

Piezoelectric scandium aluminium nitride (Sc x Al 1− x N) thin films offer a large potential for the application in micro electromechanical systems, as advantageous properties of pure AlN thin films are maintained, but combined with an increased piezoelectric actuation and sensing potential. Sc x Al 1− x N thin films with x = 27% have been prepared by DC reactive magnetron sputtering to find optimized deposition parameters to maximize the piezoelectric constants d 33 and d 31 . For the accurate and simultaneous measurement of these constants Laser Doppler Vibrometry has been applied and compared to finite element (FEM) simulations. The electrode design has been optimized to rotational symmetric structures enabling a 180° phase shifted excitation, so that a straight-forward comparison of experimental displacement curves with those obtained from FEM is feasible. [ABSTRACT FROM AUTHOR]