https://www.bnjohnson.webs.com/biosensing_review.pdf

Biosensing using dynamic-mode cantilever sensors: a review.

A systematic design procedure to tailor the modal response of micro-resonators based on flexible plates with piezoelectric films is demonstrated. Sensors/actuators were designed by optimizing the surface electrode shapes in the plane dimensions. A numerical finite element procedure, which considers the effective surface electrode covering the piezoelectric film as a binary function on each element, has been implemented. Two design goals are considered: (i) optimized response (actuation or sensing) in a given mode; (ii) implementation of a modal transducer by filtering specific modes. For a given mode in a plate with arbitrary boundary conditions, our calculations allowed us to predict the top electrode layout reaching higher displacement in resonance than any other electrode design for the same structure. Microcantilevers and microbridges were fabricated and their modal response characterized by laser Doppler vibrometry. In comparison to a conventional square-shaped electrode, our experiments show that the implemented designs can increase the response in any desired resonant mode and simultaneously attenuate the contributions from other unwanted modes, by simply shaping the surface electrodes. Enhancement ratios as high as 42 dB, relative to a full-size electrode case, are demonstrated. The limitations imposed by the fabrication are also discussed.

Modal optimization and filtering in piezoelectric microplate resonators

Medium damping influences on the resonant frequency and quality factor of piezoelectric circular microdiaphragm sensors (PCMSs) are investigated theoretically and experimentally in this paper. The acoustic radiation and viscosity damping as the two main sources of energy dissipation in a medium virtually added the mass of the diaphragm and therefore decrease the frequency and Q-factor of the diaphragm. The magnitude of medium damping inversely depends on the radius-to-thickness ratio. An increase in this ratio is the trend in the fabrication of thin microdiaphragms by MEMS fabrication processes, which implies the higher influence of medium damping on the dynamic behavior of microdiaphragms. The fabricated PCMSs were tested in vacuum, air, and ethanol. The Q-factor and the resonant frequency of the device increase by almost seven times, 4.7% from air to 0.05 atmpressure, respectively. The Q-value drops from 111.195 in air to 23.908 in ethanol. Throughout this work, theoretical and experimental values were compared and a fairly good correlation was observed.

https://iopscience.iop.org/article/10.1088/0960-1317/21/4/045002/pdf

Medium damping influences on the resonant frequency and quality factor of piezoelectric circular microdiaphragm sensors

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.

Characterization of a roof tile-shaped out-of-plane vibrational mode in aluminum-nitride-actuated self-sensing micro-resonators for liquid monitoring purposes

In this study, residual stress influences on the vibrational behavior of piezoelectric circular microdiaphragm-based biosensors are investigated theoretically and experimentally. The piezoelectric microdiaphragm was first fabricated by combining sol–gel PZT thin film and MEMS technology. The stress measurements by the wafer curvature method and the micro-Raman technique demonstrate that high tensile stresses are generated in the upper films (Pt/PZT/Pt), while the silicon oxide layer experiences a compressive stress. After backside etching of the microdiaphragm, the suspended membrane method was used to measure the average stress and equivalent Young's modulus of the diaphragm, which were 96 MPa and 106.4 GPa, respectively. The dynamic behavior of the fabricated sensor under this stress was then investigated. A comprehensive mechanics model based on vibration modes is presented and the natural frequencies of the diaphragm are obtained. A nondimensional tension parameter is defined, and effects of this parameter on the resonant frequency of the diaphragm are presented. It was concluded that both flexural rigidity and tension contribute to the resonant frequency of the diaphragm sensor. Finally, the resonant frequencies of the fabricated sensor were measured by impedance analysis and laser vibrometry techniques. These frequencies were compared with their theoretical counterparts and a good agreement was observed.

https://iopscience.iop.org/article/10.1088/0960-1317/20/1/015007/pdf

Investigation of residual stress and its effects on the vibrational characteristics of piezoelectric-based multilayered microdiaphragms

This paper gives a precise insight into the frequency response of self-actuating cantilevers, extended to high-order modes An original approach is presented, based on a mixed experimental scheme regarding both impedance and laser Doppler interferometry measurements, performed with an impedance analyser and a vibrometer, respectively, and supported by some computational analysis as well The experimental results obtained by the two different techniques agree reasonably well, as for the location of the resonances in the spectrum is concerned However, certain particular modes, torsionally shaped and detected by the vibrometer, show no impedance change due to symmetry reasons A computational model has been successfully employed and utilized to shed some light on this phenomenon The model demonstrates that the induced reaction charge redistribution cancels out only for those particular torsional modes featuring an anti-symmetrical nature; as a consequence, no impedance change is to be expected in the resonances This explanation perfectly fits the impedance measurements carried out, including the missing torsional modesCorrections were made to this article on 10 April 2007 The greyscale figures were replaced with colour versions

Laser vibrometry and impedance characterization of piezoelectric microcantilevers

The electromechanical response of piezoelectrically-actuated AlN micromachined bridge resonators has been characterized using laser interferometry and electrical admittance measurements. We compare the response of microbridges with different dimensions and buckling (induced by the initial residual stress of the layers). The resonance frequencies are in good agreement with numerical simulations of the electromechanical behavior of the structures. We 
show that it is possible to perform a rough tuning of the resonance frequencies by allowing a determined amount of built-in stress in the microbridge during its fabrication. Once the resonator is made, a DC bias added to the AC excitation signal allows to fine-tune the frequency. Our microbridges yield a tuning factor of around 88 Hz/V for a 500 μm-long microbridge.

/pdf/tunable-mechanical-resonator-with-aluminium-nitride-6c85i63bqv.pdf

Tunable mechanical resonator with aluminium nitride piezoelectric actuation

Aluminum nitride is lately being considered as a promising candidate for its use as the actuator in piezoelectrically-actuated MEMS due to its good piezoelectric and mechanical properties, high chemical stability and full compatibility with conventional silicon technologies. In this work we present the mechanical response of doubly-clamped microbridges with piezoelectric actuation by a sputtered AlN film. A complete technology for the fabrication of the microbridges on silicon substrates using surface micromachining has been developed. The mechanical response of the microbridges under electrical excitation has been measured. Finite element method (FEM) computations have been carried out in order to analyze the static and dynamic response of devices with several configurations and to determine their optimum design. These simulations include the influence of the properties of the materials, the initial residual stress and the different geometries on the device operation. Although the qualitative behavior of the microbridges is well predicted, a significant discrepancy is observed between the measured displacement of the beam and the simulated response. The measured values of the out-of-plane displacement of the bridge are near ten times greater than those obtained in the simulations. Some of the possible causes of these discrepancies are widely discussed.

P. Sanz

Papers

Laser vibrometry and impedance characterization of piezoelectric microcantilevers

Tunable mechanical resonator with aluminium nitride piezoelectric actuation

Simulation, fabrication, and testing of aluminium nitride piezoelectric microbridges