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Acoustic Transducers and Their Applications

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A special issue of Micromachines (ISSN 2072-666X). This special issue belongs to the section "A：Physics".

Deadline for manuscript submissions: closed (10 April 2024) | Viewed by 12757

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Special Issue Editors

Dr. Songsong Zhang

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Guest Editor

School of Microelectronics, Shanghai University, Shanghai 200444, China
Interests: piezoelectric material; MEMS device process; piezoelectric transducer design; acoustic sensing model; acoustic device algorithm and system-level application

Dr. Qiaozhen Zhang

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Guest Editor

College of Information, Mechanical and Electrical Engineering, Shanghai Normal University, Shanghai 200234, China
Interests: Intelligent sensing technology and system; RF micro-acoustic devices for mobile communication; modeling and simulation of piezoelectric thin-film micro-acoustic devices

Dr. Xing Haw Marvin Tan

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Guest Editor

Institute of High Performance Computing, A*STAR, Singapore 138632, Singapore
Interests: piezoelectric transducer design and simulation; piezoelectric material property extraction; bulk acoustic wave (BAW) devices; acoustic meta-surfaces (including meta-lens); beam steering

Prof. Dr. Kenya Hashimoto

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Guest Editor

School of Integrated Circuit Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610056, China
Interests: surface and bulk acoustic wave devices
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Acoustics is one of the key forms of mechanical energy that is exerted everywhere in our world. It helps with sensing, actuating, and communicating, and even helps in medical or industrial matters. With a broad operating spectrum, it serves as the most pivotal tool in various implementations, ranging from airborne applications to studies through different media, especially for needs beyond the physical limitations of other mechanisms (e.g., optical or electromagnetic waves). Alongside the progression of advanced transducer technology, more acoustic device alternatives have been optimized for conventional applications. In addition, consistent research efforts in the field of frontier transducers enable rapid paradigm shifts in many emerging acoustic applications, which coherently accelerate the development of other peripheral technologies, such as thin-film materials used in acoustic transducers, circuits for acoustic devices, modules, algorithms, system integration, and even the integration of acoustic sensors into smart systems. In order to promote research efforts and advocate for continuous innovation in this field, in this Special Issue the current state of the art in the field of ‘acoustic transducers and their applications’ will be presented, covering a wide range of related topics, including, but not limited to, the following:

New piezoelectric materials: ceramics, thin films, single crystals, polymers, composites, 2D materials, etc.;
Acoustic transducers: acoustic sensors and actuators, circuits for acoustic devices, modules, algorithms, and integrated systems;
Piezoelectric devices/acoustic transducers: fabrication, testing, characterization, design, modeling, simulation, manufacturing, 3D printing, packaging, and system integration;
Acoustic transducer applications: nondestructive testing, acoustic arrays for holograms and beam steering, acoustic ranging, acoustic lenses, meta-materials and meta-surfaces, energy harvesting, medical imaging, wearable sensors, biomedical applications, virtual reality/augmented reality, and other emerging applications for the metaverse.

Dr. Songsong Zhang
Dr. Qiaozhen Zhang
Dr. Xing Haw Marvin Tan
Prof. Dr. Kenya Hashimoto
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Micromachines is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

Acoustic transducers and their design, simulation, testing, and characterization
Piezoelectric MEMS device
Thin-film piezoelectric materials and material properties
MEMS integration process
Acoustic modules, algorithms, circuits, and systems
Acoustic lenses and meta-surfaces
Acoustic transducer arrays and their applications
Implementation and usage of acoustic devices for various applications.

Published Papers (13 papers)

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Research

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16 pages, 6766 KiB

Open AccessArticle

Electromagnetic Acoustic Detection of Pipe Defects Hidden above T-Type Support Structures with Circumferential Shear Horizontal Guided Wave

by Xingjun Zhang, Jinjie Zhou, Yang Hu, Yao Liu and Xingquan Shen

Micromachines 2024, 15(4), 550; https://doi.org/10.3390/mi15040550 - 20 Apr 2024

Viewed by 309

Abstract

When pipe defects are generated above the T-type support structure location, it is difficult to distinguish the reflection signals caused by the weld bead at the support structure from the reflection echoes of pipe defects. Therefore, in order to effectively detect pipe defects, a waveform subtraction method with a circumferential shear horizontal (CSH) guided wave is proposed, which is generated by an electromagnetic acoustic transducer (EMAT). First, a CSH₀ guided wave mode with a center frequency of 500 kHz is selected to establish a three-dimensional model with and without pipe defects above the support structure. Following this, the influence of different widths of support structures on the echo signal is compared. Moreover, simulation and experimental results are used to compare the influence of different welding qualities on the detection results. Finally, the waveform subtraction method is used to process the simulation and experimental signals, and the influence of pipe defects with different lengths and depths is discussed. The results show that the non-through crack defect of 5 mm × 1 mm (length × depth) can be detected. The results show that this method can effectively detect the cracks by eliminating the influence of the weld echo, which provides a new concept for the detection of the defect above the support structure. Full article

(This article belongs to the Special Issue Acoustic Transducers and Their Applications)

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16 pages, 7169 KiB

Open AccessArticle

Design and Characterization of Surface Acoustic Wave-Based Wireless and Passive Temperature Sensing System

by Zhixin Zhou, Hui Wang and Liang Lou

Micromachines 2024, 15(4), 544; https://doi.org/10.3390/mi15040544 - 18 Apr 2024

Viewed by 262

Abstract

The surface acoustic wave (SAW) temperature sensor has received significant attention due to its wirelessly powered, battery-free, and chipless capabilities. This paper proposes a wireless sensing system comprising a one-port SAW resonator, helix antenna, and transceiver circuit. The SAW resonator used in this system is based on aluminum nitride (AlN) thin film, which exhibits high velocity and excellent piezoelectric properties. Simulations and experiments were conducted to investigate the performance of the designed SAW resonator. A helix antenna was also designed using finite element simulation to facilitate signal transmission between the SAW temperature sensor and the transceiver. An impedance-matching network was introduced between the helix antenna and the SAW resonator to optimize signal transmission. When the wireless SAW temperature sensor was placed within a certain distance of the mother antenna, the reflection peak of the SAW resonator was observed in the spectrum of the return signal. The frequency of the echo signal increased almost linearly as the temperature increased during the temperature tests. The fitted temperature coefficient of frequency (TCF) was −31.34 ppm/°C, indicating that the wireless temperature sensing system has high-temperature sensitivity. Full article

(This article belongs to the Special Issue Acoustic Transducers and Their Applications)

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12 pages, 9703 KiB

Open AccessArticle

A Novel Nondestructive Testing Probe Using AlN-Based Piezoelectric Micromachined Ultrasonic Transducers (PMUTs)

by Jiawei Yin, Zhixin Zhou and Liang Lou

Micromachines 2024, 15(3), 306; https://doi.org/10.3390/mi15030306 - 23 Feb 2024

Viewed by 722

Abstract

Ultrasonic nondestructive testing (NDT) usually utilizes conventional bulk piezoelectric transducers as transceivers. However, the complicated preparation and assembly process of bulk piezoelectric ceramics limits the development of NDT probes toward miniaturization and high frequency. In this paper, a 4.4 mm × 4.4 mm aluminum nitride (AlN) piezoelectric micromachined ultrasonic transducer (PMUT) array is designed, fabricated, characterized, and packaged for ultrasonic pulse–echo NDT of solids for the first time. The PMUT array is prepared based on the cavity silicon-on-insulator (CSOI) process and packaged using polyurethane (PU) material with acoustic properties similar to water. The fabricated PMUT array resonates at 2.183 MHz in air and at around 1.25 MHz after PU encapsulation. The bandwidth of the packaged PMUT receiver (244 kHz) is wider than that of a bulk piezoelectric transducer (179 kHz), which is good for axis resolution improvement. In this work, a hybrid ultrasonic NDT probe is designed using two packaged PMUT receivers and one 1.25 MHz bulk transmitter. The bulk transmitter radiates an ultrasonic wave into the sample, and the defect echo is received by two PMUT receivers. The 2D position of the defect could be figured out by time-of-flight (TOF) difference, and a 30 mm × 65 mm detection area is acquired. This work demonstrates the feasibility of applying AlN PMUTs to ultrasonic NDT of solids and paves the way toward a miniaturized NDT probe using AlN PMUT technology. Full article

(This article belongs to the Special Issue Acoustic Transducers and Their Applications)

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12 pages, 6110 KiB

Open AccessArticle

Stepped-Tube Backside Cavity Piezoelectric Ultrasound Transducer Based on Sc_0.2AI_0.8N Thin Films

by Xiaobao Li, Haochen Lyu, Ahmad Safari and Songsong Zhang

Micromachines 2024, 15(1), 72; https://doi.org/10.3390/mi15010072 - 29 Dec 2023

Viewed by 819

Abstract

This paper presents a novel piezoelectric micromachined ultrasonic transducer (PMUT) with theoretical simulation, fabrication, and testing. Conventional methods using a PCB or an external horn to adjust the PMUT acoustic field angle are limited by the need for transducer size. To address this limitation, the stepped-tube (expanded tube) backside cavity PMUT has been proposed. The stepped-tube PMUT and the tube PMUT devices have the same membrane structure, and the acoustic impedance matching of the PMUT is optimized by modifying the boundary conditions of the back cavity structure. The acoustic comparison experiments show that the average output sound pressure of the stepped-tube backside cavity PMUT has increased by 17%, the half-power-beam-width (θ_-3db) has been reduced from 55° to 30° with a reduction of 45%, and the side lobe level signal is reduced from 147 mV to 66 mV. In addition, this work is fabricated on an eight-inch wafer. The process is compatible with standard complementary metal oxide semiconductor (CMOS), conditions are stable, and the cost is controllable, plus it facilitates the batch process. These conclusions suggest that the stepped-tube backside cavity PMUT will bring new, effective, and reliable solutions to ranging applications. Full article

(This article belongs to the Special Issue Acoustic Transducers and Their Applications)

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27 pages, 3788 KiB

Open AccessArticle

Modeling a Fluid-Coupled Single Piezoelectric Micromachined Ultrasonic Transducer Using the Finite Difference Method

by Valentin Goepfert, Audren Boulmé, Franck Levassort, Tony Merrien, Rémi Rouffaud and Dominique Certon

Micromachines 2023, 14(11), 2089; https://doi.org/10.3390/mi14112089 - 12 Nov 2023

Viewed by 788

Abstract

A complete model was developed to simulate the behavior of a circular clamped axisymmetric fluid-coupled Piezoelectric Micromachined Ultrasonic Transducer (PMUT). Combining Finite Difference and Boundary Element Matrix (FD-BEM), this model is based on the discretization of the partial differential equation used to translate the mechanical behavior of a PMUT. In the model, both the axial and the transverse displacements are preserved in the equation of motion and used to properly define the neutral line position. To introduce fluid coupling, a Green’s function dedicated to axisymmetric circular radiating sources is employed. The resolution of the behavioral equations is used to establish the equivalent electroacoustic circuit of a PMUT that preserves the average particular velocity, the mechanical power, and the acoustic power. Particular consideration is given to verifying the validity of certain assumptions that are usually made across various steps of previously reported analytical models. In this framework, the advantages of the membrane discretization performed in the FD-BEM model are highlighted through accurate simulations of the first vibration mode and especially the cutoff frequency that many other models do not predict. This high cutoff frequency corresponds to cases where the spatial average velocity of the plate is null and is of great importance for PMUT design because it defines the upper limit above which the device is considered to be mechanically blocked. These modeling results are compared with electrical and dynamic membrane displacement measurements of AlN-based (500 nm thick) PMUTs in air and fluid. The first resonance frequency confrontation showed a maximum relative error of 1.13% between the FD model and Finite Element Method (FEM). Moreover, the model perfectly predicts displacement amplitudes when PMUT vibrates in a fluid, with less than 5% relative error. Displacement amplitudes of 16 nm and 20 nm were measured for PMUT with 340 µm and 275 µm diameters, respectively. This complete PMUT model using the FD-BEM approach is shown to be very efficient in terms of computation time and accuracy. Full article

(This article belongs to the Special Issue Acoustic Transducers and Their Applications)

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19 pages, 10163 KiB

Open AccessArticle

Acoustic Characterization of Transmitted and Received Acoustic Properties of Air-Coupled Ultrasonic Transducers Based on Matching Layer of Organosilicon Hollow Glass Microsphere

by Xinhu Xu, Liang Zhang, Hulin Guo, Xiaojie Wang and Lingcai Kong

Micromachines 2023, 14(11), 2021; https://doi.org/10.3390/mi14112021 - 30 Oct 2023

Cited by 1 | Viewed by 885

Abstract

An air-coupled transducer was developed in this study, utilizing hollow glass microsphere-organosilicon composites as an acoustically matching layer, which demonstrated outstanding acoustic performance. Firstly, a comparison and analysis of the properties and advantages of different substrates was carried out to determine the potential application value of organosilicon substrates. Immediately after, the effect of hollow glass microspheres with different particle sizes and mass fractions on the acoustic properties of the matching layer was analyzed. It also evaluated the mechanical properties of the matching layer before and after optimization. The findings indicate that the optimized composite material attained a characteristic acoustic impedance of 1.04 MRayl and an acoustic attenuation of 0.43 dB/mm, displaying exceptional acoustic performance. After encapsulating the ultrasonic transducer using a 3D-printed shell, we analyzed and compared its emission and reception characteristics to the commercial transducer and found that its emission acoustic pressure amplitude and reception voltage amplitude were 34% and 26% higher, respectively. Finally, the transducer was installed onto a homemade ultrasonic flow meter for practical application verification, resulting in an accuracy rate of 97.4%. Full article

(This article belongs to the Special Issue Acoustic Transducers and Their Applications)

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15 pages, 2523 KiB

Open AccessArticle

Quasi-3D Model for Lateral Resonances on Homogeneous BAW Resonators

by Carlos Udaondo, Carlos Collado and Jordi Mateu

Micromachines 2023, 14(11), 1980; https://doi.org/10.3390/mi14111980 - 25 Oct 2023

Cited by 1 | Viewed by 763

Abstract

Lateral modes are responsible for the in-band spurious resonances that appear on BAW resonators, degrading the in-band filter response. In this work, a fast computational method based on the transmission line matrix (TLM) method is employed to model the lateral resonances of BAW resonators. Using the precomputed dispersion curves of Lamb waves and an equivalent characteristic impedance for the TE₁ mode, a network of transmission lines is used to calculate the magnitude of field distributions on the electrodes. These characteristics are specific to the stack layer configuration. The model’s implementation is based on nodal Y matrices, from which particle displacement profiles are coupled to the electric domain via piezoelectric constitutive relations. Consequently, the input impedance of the resonator is obtained. The model exhibits strong agreement with FEM simulations of FBARs and SMRs, and with measurements of several SMRs. The proposed model can provide accurate predictions of resonator input impedance, which is around 200 times faster than conventional FEM. Full article

(This article belongs to the Special Issue Acoustic Transducers and Their Applications)

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12 pages, 4964 KiB

Open AccessArticle

Multiphysics Modeling and Analysis of Sc-Doped AlN Thin Film Based Piezoelectric Micromachined Ultrasonic Transducer by Finite Element Method

by Xiaonan Liu, Qiaozhen Zhang, Mingzhu Chen, Yaqi Liu, Jianqiu Zhu, Jiye Yang, Feifei Wang, Yanxue Tang and Xiangyong Zhao

Micromachines 2023, 14(10), 1942; https://doi.org/10.3390/mi14101942 - 18 Oct 2023

Cited by 1 | Viewed by 1046

Abstract

This paper presents a Piezoelectric micromechanical ultrasonic transducer (PMUT) based on a Pt/ScAlN/Mo/SiO₂/Si/SiO₂/Si multilayer structure with a circular suspension film of scandium doped aluminum nitride (ScAlN). Multiphysics modeling using the finite element method and analysis of the effect of [...] Read more.

k_{e f f}^{2}

) and the station sensitivity of the PMUT cell are performed. The calculation results show that the resonant frequency of the ScAlN-based PMUT can be above 20 MHz and its

k_{e f f}^{2}

monotonically rise with the increasing doping concentrations in ScAlN. In comparison to the pure AlN thin film-based PMUT, the static receiving sensitivity of the PMUT based on ScAlN thin film with 35% Sc doping concentration is up to 1.61 mV/kPa. Meanwhile, the static transmitting sensitivity of the PMUT is improved by 152.95 pm/V. Furthermore, the relative pulse-echo sensitivity level of the 2 × 2 PMUT array based on the Sc doping concentration of 35% AlN film is improved by 16 dB compared with that of the cell with the same Sc concentration. The investigation results demonstrate that the performance of PMUT on the proposed structure can be tunable and enhanced by a reasonable choice of the Sc doping concentration in ScAlN films and structure optimization, which provides important guidelines for the design of PMUT for practical applications. Full article

(This article belongs to the Special Issue Acoustic Transducers and Their Applications)

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12 pages, 9121 KiB

Open AccessArticle

Partially Etched Piezoelectric Film Filled with SiO₂ Structure Applied to A1 Mode Resonators for Transverse Modes Suppression

by Zhenyi Yu, Yu Guo, Sulei Fu, Baichuan Li, Peisen Liu, Shuai Zhang and Zongqin Sun

Micromachines 2023, 14(9), 1745; https://doi.org/10.3390/mi14091745 - 07 Sep 2023

Viewed by 889

Abstract

With the arrival of the Fifth Generation (5G) communication era, there has been an urgent demand for acoustic filters with a high frequency and ultrawide bandwidth used in radio-frequency (RF) front-ends filtering and signal processing. First-order antisymmetric (A1) lamb mode resonators based on LiNbO₃ film have attracted wide attention due to their scalable, high operating frequency and large electromechanical coupling coefficients (K²), making them promising candidates for sub-6 GHz wideband filters. However, A1 mode resonators suffer from the occurrence of transverse modes, which should be addressed to make these devices suitable for applications. In this work, theoretical analysis is performed by finite element method (FEM), and the admittance characteristics of an A1 mode resonator and displacement of transverse modes near the resonant frequency (f_r) are investigated. We propose a novel Dielectric-Embedded Piston Mode (DEPM) structure, achieved by partially etching a piezoelectric film filled with SiO₂, which can almost suppress the transverse modes between the resonant frequency (f_r) and anti-resonant frequency (f_a) when applied on ZY-cut LiNbO₃-based A1 mode resonators. This indicates that compared with Broadband Piston Mode (BPM), Filled-broadband Piston Mode (FPM) and standard structures, the DEPM structure is superior. Furthermore, the design parameters of the resonator are optimized by adjusting the width, depth and filled materials in the etched window of the DEPM structure to obtain a better suppression of transverse modes. The optimized A1 mode resonator using a DEPM structure exhibits a transverse-free response with a high f_r of 3.22 GHz and a large K² of ~30%, which promotes the application of A1 mode devices for use in 5G RF front-ends. Full article

(This article belongs to the Special Issue Acoustic Transducers and Their Applications)

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15 pages, 8629 KiB

Open AccessArticle

Reem-Shape Phononic Crystal for Q Anchor Enhancement of Thin-Film-Piezoelectric-on-Si MEMS Resonator

by Mohammed Awad, Temesgen Bailie Workie, Jing-Fu Bao and Ken-ya Hashimoto

Micromachines 2023, 14(8), 1540; https://doi.org/10.3390/mi14081540 - 31 Jul 2023

Cited by 3 | Viewed by 942

Abstract

This paper proposes a reem-shaped phononic crystal for the performance enhancement of TPoS resonators. The proposed phononic crystal offers an ultra-wide acoustic band gap that prevents energy leakage through the supporting substrate upon its placement at the anchoring boundary, resulting in significant improvements in the resonator quality factor. Simulated results show reem-shape phononic crystals generate a band gap up to 175 MHz with a BG of 90% and enhance the anchor quality factor from 180,000 to 6,000,000 and the unloaded quality factor from 133,000 to 160,000, representing 33.3-fold and 1.2-fold improvements, respectively. Full article

(This article belongs to the Special Issue Acoustic Transducers and Their Applications)

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16 pages, 6358 KiB

Open AccessArticle

A Compact Piezo-Inertia Actuator Utilizing the Double-Rocker Flexure Hinge Mechanism

by Pingping Sun, Chenglong Lei, Chuannan Ge, Yunjun Guo and Xingxing Zhu

Micromachines 2023, 14(6), 1117; https://doi.org/10.3390/mi14061117 - 26 May 2023

Viewed by 1100

Abstract

With a simple structure and control method, the piezo-inertia actuator is a preferred embodiment in the field of microprecision industry. However, most of the previously reported actuators are unable to achieve a high speed, high resolution, and low deviation between positive and reverse velocities at the same time. To achieve a high speed, high resolution, and low deviation, in this paper we present a compact piezo-inertia actuator with a double rocker-type flexure hinge mechanism. The structure and operating principle are discussed in detail. To study the load capacity, voltage characteristics, and frequency characteristics of the actuator, we made a prototype and conducted a series of experiment. The results indicate good linearity in both positive and negative output displacements. The maximum positive and negative velocities are about 10.63 mm/s and 10.12 mm/s, respectively, and the corresponding speed deviation is 4.9%. The positive and negative positioning resolutions are 42.5 nm and 52.5 nm, respectively. In addition, the maximum output force is 220 g. These results show that the designed actuator has a minor speed deviation and good output characteristics. Full article

(This article belongs to the Special Issue Acoustic Transducers and Their Applications)

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16 pages, 4990 KiB

Open AccessArticle

A Real Time Method Based on Deep Learning for Reconstructing Holographic Acoustic Fields from Phased Transducer Arrays

by Shuai Wang, Xuewei Wang, Fucheng You, Yang Li and Han Xiao

Micromachines 2023, 14(6), 1108; https://doi.org/10.3390/mi14061108 - 24 May 2023

Viewed by 1133

Abstract

Phased transducer arrays (PTA) can control ultrasonic waves to produce a holographic acoustic field. However, obtaining the phase of the corresponding PTA from a given holographic acoustic field is an inverse propagation problem, which is a mathematically unsolvable nonlinear system. Most of the existing methods use iterative methods, which are complex and time-consuming. To better solve this problem, this paper proposed a novel method based on deep learning to reconstruct the holographic sound field from PTA. For the imbalance and randomness of the focal point distribution in the holographic acoustic field, we constructed a novel neural network structure incorporating attention mechanisms to focus on useful focal point information in the holographic sound field. The results showed that the transducer phase distribution obtained from the neural network fully supports the PTA to generate the corresponding holographic sound field, and the simulated holographic sound field can be reconstructed with high efficiency and quality. The method proposed in this paper has the advantage of real-time performance that is difficult to achieve by traditional iterative methods and has the advantage of higher accuracy compared with the novel AcousNet methods. Full article

(This article belongs to the Special Issue Acoustic Transducers and Their Applications)

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Review

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19 pages, 5149 KiB

Open AccessFeature PaperEditor’s ChoiceReview

Surface Acoustic Wave Humidity Sensor: A Review

by Maria Muzamil Memon, Qiong Liu, Ali Manthar, Tao Wang and Wanli Zhang

Micromachines 2023, 14(5), 945; https://doi.org/10.3390/mi14050945 - 27 Apr 2023

Cited by 3 | Viewed by 1919

Abstract

The Growing demands for humidity detection in commercial and industrial applications led to the rapid development of humidity sensors based on different techniques. Surface acoustic wave (SAW) technology is one of these methods that has been found to provide a powerful platform for humidity sensing owing to its intrinsic features, including small size, high sensitivity, and simple operational mechanism. Similar to other techniques, the principle of humidity sensing in SAW devices is also realized by an overlaid sensitive film, which serves as the core element whose interaction with water molecules is responsible for overall performance. Therefore, most researchers are focused on exploring different sensing materials to achieve optimum performance characteristics. This article reviews sensing materials used to develop SAW humidity sensors and their responses based on theoretical aspects and experimental outcomes. Herein the influence of overlaid sensing film on the performance parameters of the SAW device, such as quality factor, signal amplitude, insertion loss, etc., is also highlighted. Lastly, a recommendation to minimize the significant change in device characteristics is presented, which we believe will be a good step for the future development of SAW humidity sensors. Full article

(This article belongs to the Special Issue Acoustic Transducers and Their Applications)

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