Piezoelectric microelectromechanical system (piezo-MEMS)-based mass sensors including the piezoelectric microcantilevers, surface acoustic waves (SAW), quartz crystal microbalance (QCM), piezoelectric micromachined ultrasonic transducer (PMUT), and film bulk acoustic wave resonators (FBAR) are highlighted as suitable candidates for highly sensitive gas detection application. This paper presents the piezo-MEMS gas sensors' characteristics such as their miniaturized structure, the capability of integration with readout circuit, and fabrication feasibility using multiuser technologies. The development of the piezoelectric MEMS gas sensors is investigated for the application of low-level concentration gas molecules detection. In this work, the various types of gas sensors based on piezoelectricity are investigated extensively including their operating principle, besides their material parameters as well as the critical design parameters, the device structures, and their sensing materials including the polymers, carbon, metal-organic framework, and graphene. 

/pdf/a-review-of-piezoelectric-mems-sensors-and-actuators-for-gas-1jojc69k.pdf

A review of piezoelectric MEMS sensors and actuators for gas detection application

Realization of a DNA biosensor using inverted Lamb wave MEMS resonator based on ZnO/SiO2/Si/ZnO membrane.

This paper reports on a design journey for a laterally-excited bulk acoustic wave resonator (XBAR) with an adjustable piezoelectric coupling coefficient (K 2). The vibration principle and resonant characteristics of XBAR are investigated by theoretical and simulation analysis, and the way to adjust K 2 by introducing two grooves in the area between interdigital electrodes (IDEs) is proved to be effective by using the finite-element method (FEM) simulation and the Modified Butterworth–Van Dyke model with series capacitor C r (MBVD-C r ) first, and then the impact of groove depth (H g) and groove width (W g) on K 2 of XBAR is theoretically analyzed. Specifically, K 2 can be effectively adjusted by setting different values of H g and W g. After optimization, the maximum regulation range of K 2 is from 2.02% to 45.05%. The fluted XBAR has remarkable potential in building an XBAR bandpass filter for specific frequency bands, which greatly optimizes the XBAR filter design.

Laterally-excited bulk acoustic wave resonator with an adjustable piezoelectric coupling coefficient

Temperature monitoring in extreme environments presents new challenges for MEMS sensors. Since aluminum nitride (AlN)/scandium aluminum nitride (ScAlN)-based surface acoustic wave (SAW) devices have a high Q-value, good temperature drift characteristics, and the ability to be compatible with CMOS, they have become some of the preferred devices for wireless passive temperature measurement. This paper presents the development of AlN/ScAlN SAW-based temperature sensors. Three methods were used to characterize the temperature characteristics of a thin-film SAW resonator, including direct measurement by GSG probe station, and indirect measurement by oscillation circuit and antenna. The temperature characteristics of the three methods in the range of 30–100 °C were studied. The experimental results show that the sensitivities obtained with the three schemes were −28.9 ppm/K, −33.6 ppm/K, and −29.3 ppm/K. The temperature sensor using the direct measurement method had the best linearity, with a value of 0.0019%, and highest accuracy at ±0.70 °C. Although there were differences in performance, the characteristics of the three SAW temperature sensors make them suitable for sensing in various complex environments.

Development of Temperature Sensor Based on AlN/ScAlN SAW Resonators

This work demonstrates a laterally excited shear bulk acoustic wave resonator (XBAR) based on a 300-nm-thick ZY-cut Lithium Niobate thin film, which takes advantages of high frequency, large electromechanical coupling coefficient (kt2) and high quality factor (Q). A bi-electrode structure was proposed to weaken the effects of mechanical and electrical loading introduced by the metal electrodes, while enhancing the coupling between the electric field and stress. In addition, the electrode configurations including interdigitated electrodes (IDEs) number, metal coverage and effective electrode length were studied to eliminate spurious modes and improve resonators' performance. The fabricated devices can achieve the resonant frequency of 5.89 GHz using the first-asymmetric (A1) mode Lamb wave in suspended LiNbO3 thin film. Combined with the high operating frequency and high kt2 characteristics of the A1 mode Lamb wave, the measured results show that the designed bi-electrode structure XBAR achieves a kt2 value of 14 % and a high Q value of 300. 

Study of electrode structures for 5.89 GHz A1 mode Lamb wave resonators to achieve fs×kt2×Q > 247×109 Hz

Laterally excited bulk acoustic resonators (XBARs) using interdigital electrodes are currently limited in potential 5G applications due to the accompanied spurious modes and small capacitance. This Letter presents an XBAR with scattering vias in electrodes, named SV-BAR. The non-parallel boundaries thoroughly disrupt the coherent formation of standing waves in the lateral direction, thereby suppressing the higher-order spurious modes. The duty factor of SV-BAR is optimized as 0.5 using COMSOL Multiphysics finite element analysis for the 50 Ω impedance matching. The SV-BAR is fabricated using a 390 nm Z-cut LiNbO3 on an insulator substrate and exhibits an impressive ratio of Zp/Zs near 64.5 dB, a Kt2 of 21.80% at 4.762 GHz, as well as a large value f · Bode_Qmax · Kt2 (3.43 × 1011). The measured temperature coefficient of frequency is −69.8 ppm/°C, which can be compensated using SiO2 thin films. The SV-BAR provides an effective solution for high-performance radio frequency filters for 5G communication. 

A laterally excited bulk acoustic resonator with scattering vias in electrodes

Surface acoustic wave (SAW) resonators based on AlN/ScAlN composite thin films with dual reflection structure demonstrate substantial improvement in acoustic performance. In this work, the factors affecting the final electrical performance of SAW are analyzed from the aspects of piezoelectric thin film, device structure design and fabrication process. AlN/ScAlN composite films can effectively solve the problem of abnormal grains of ScAlN, improve the crystal orientation, and reduce the intrinsic loss and etching defects. The double acoustic reflection structure of the grating and groove reflector can not only reflect the acoustic wave more thoroughly, but also helps to release the film stress. Both structures are beneficial to obtain a higher Q-value. The new stack and design results in large Qp and figure of merit among SAW devices working at 446.47 MHz on silicon up to 8241 and 18.1, respectively.

Boosting Performance of SAW Resonator via AlN/ScAlN Composite Films and Dual Reflectors.

A resonator with an appropriate electromechanical coupling coefficient (Kt2) is crucial for filter applications in radio communication. In this paper, we present an effective method to tune the Kt2 of resonators by introducing different materials into a lithium niobate (LiNbO3) piezoelectric matrix. The effective piezoelectric coefficients e33eff and e15eff of composite materials with four different introduced materials were calculated. The results show that the e15eff of SiO2/LiNbO3 composite piezoelectric material was mostly sensitive to an increase in the width of introduced SiO2 material. Simultaneously, the simulation of a laterally excited bulk wave resonator (XBAR) with SiO2/LiNbO3 composite material was also carried out to verify the change in the Kt2 originating from the variation in e15eff. The achievable n79 filter using the SiO2/LiNbO3 composite material demonstrates the promising prospects of tuning Kt2 by introducing different materials into a LiNbO3 piezoelectric matrix.

/pdf/tunable-electromechanical-coupling-coefficient-of-a-m1ujxc7w.pdf

Tunable Electromechanical Coupling Coefficient of a Laterally Excited Bulk Wave Resonator with Composite Piezoelectric Film

The development of MEMS acoustic resonators meets the increasing demand for in situ detection with a higher performance and smaller size. In this paper, a lithium niobate film-based S1 mode Lamb wave resonator (HF-LWR) for high-sensitivity gravimetric biosensing is proposed. The fabricated resonators, based on a 400-nm X-cut lithium niobate film, showed a resonance frequency over 8 GHz. Moreover, a PMMA layer was used as the mass-sensing layer, to study the performance of the biosensors based on HF-LWRs. Through optimizing the thickness of the lithium niobate film and the electrode configuration, the mass sensitivity of the biosensor could reach up to 74,000 Hz/(ng/cm2), and the maximum value of figure of merit (FOM) was 5.52 × 107, which shows great potential for pushing the performance boundaries of gravimetric-sensitive acoustic biosensors.

/pdf/a-high-sensitivity-gravimetric-biosensor-based-on-s1-mode-22mx0zv0.pdf

A High-Sensitivity Gravimetric Biosensor Based on S1 Mode Lamb Wave Resonator

A first-order ( S1 ) Lamb wave resonator (LWR) with a trapezoid grooves configuration on the top surface of the aluminum nitride membrane between interdigital electrodes is proposed in this paper. Both the simulation and experiment results demonstrate that the keff2 can be enhanced with the depth of the trapezoid grooves increasing. Meanwhile, the spurious modes can be suppressed by the trapezoid grooves. A 75% improvement on the keff2 has been achieved in a 2.45 GHz S1 mode LWR with trapezoid grooves with spurious modes free. The keff2 reaches up to 5.47% without quality factor degradation.

Effective electromechanical coupling coefficient ( keff2 ) enhancement of a Lamb wave resonator with trapezoid grooves configuration

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