Interdigital Transducers Research Articles

Temperature and strain SAW/BAW sensors on metallic substrates with RFID capability

Surface acoustic wave (SAW) strain sensors require an adhesive—typically a glue—for the mounting on the part under measurement. This comes with strain-transfer, reproducibility and aging issues. In this paper, we propose a novel glue-less solution where the SAW sensor is directly fabricated onto the surface of interest, here a metallic substrate. Here, we study the layered structure ZnO/Ti numerically and experimentally, with ZnO as the piezoelectric layer and titanium as the substrate. In this structure, both bulk acoustic waves and SAW can propagate, and we used both of them to monitor temperature variations up to 300 °C and strain levels up to 1800 μϵ. Moreover, reflective delay line (R-DL) designs were used, to provide the future users with radio frequency identification functionalities. In order to overcome the limitations due to the relatively low electromechanical coupling coefficient of ZnO/Ti, a specific R-DL layout with connected inter-digital transducers was used. The obtained experimental results confirm that the proposed glue-less R-DL structure is a promising solution for the independent evaluation of temperature and strain, with identification.

Velocity and Direction Adjustment of Actuated Droplets Using the Standing Wave Ratio of Surface Acoustic Waves (SAW)

Droplet actuation using surface acoustic wave (SAW) technology has recently been widely employed in “lab-on-a-chip” applications. In this article, SAWs generated by interdigital transducers (IDTs) were used to actuate microdroplets, where their velocity and direction could be adjusted by changing only the excitation phase shift, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ</i> , of the voltage applied to the two IDTs. Specifically, the equation of the vibration of the mixed traveling surface acoustic waves (TSAWs) and standing surface acoustic waves (SSAWs) formed on the substrate surface operating in the exciter–exciter mode has been reported by the authors. An analytical expression for the acoustic standing wave ratio has been derived and given as a function of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ</i> and the spatial phase difference. It can be seen from the expression that the directions of the TSAWs will be opposite to each other, in the range of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ</i> given by (0, π) and (π, 2π), and the component of the TSAWs can be adjusted by changing <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ</i> in each direction. Following the theoretical analysis discussed here, two IDTs have been excited to generate a mixture of TSAWs and SSAWs. A series of experiments was carried out to control the velocity and direction of the actuated droplets, by changing only <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ</i> . In addition, an experiment performed to compare the techniques shows that the upper limit of the velocity of the actuated droplets can be significantly increased using the exciter–exciter mode, showing that it has the potential to be an alternative method to the traditional exciter–absorber mode.

A Numerical Study of Collective Cell Migration in a Microchannel Driven by Surface Acoustic Wave (SAW) Device

Collective cell migration is involved in a variety of biological contexts, including tissue morphogenesis, wound healing, and cancer invasion. Many studies have revealed that chemical, mechanical, and electrical stimulation all affect cell migration. Although an acoustic stimulus has been shown to influence cell migration in the past, the underlying mechanism is still unknown. A computational model that accounts for acoustic–structure interaction was constructed in this study to simulate the formation of a surface acoustic wave (SAW) field and the application of the acoustic pressure field on collective cell migration. A group of cells within a microchannel device and two ports of interdigitated transducers (IDTs) with different wavelengths were modeled. The stresses within cells were investigated as it was influenced by substrate displacement and pressure acoustic in the cell media generated by the SAW device. As a result, we observed the local stress within cells near the solid-fluid interfaces. For propagating SAW, the shorter wavelength of IDTs (600 μm) attributed to high stress at the cell’s top and bottom as compared to the SAW device with the longer wavelength (1000 μm). The standing SAW occurred underneath collective cells. The results of standing SAW on cell stress at the bottom confirm that the SAW device can be useful to regulate the abnormalities cellular activities associated with cell migration.

Acoustical control of charge movement in an organic semiconductor

We show experimentally the directional acoustic charge transport of optically generated charge carriers in an organic semiconductor at room temperature by surface acoustic waves (SAWs) generated on a lithium niobate substrate utilizing an easy and straightforward structure consisting of two identical interdigital transducers (IDTs) and a corresponding collector diode facing each IDT. An orthogonal arrangement of the two IDTs has been used to analyse the impact of SAW drag force on the charge carriers by interacting two SAWs. In addition, we show the frequency dependency of the acoustic dragging force on charge carriers by orthogonally interacting SAW beams of different frequencies. This relation provides the idea of using one or more SAWs to achieve an acoustic manoeuvring of charge carrier transport. The diagonal propagation of the carriers through orthogonal SAW beams of the same frequencies was also verified experimentally, thus giving rise to the possibility of an acoustic charge transfer device that may be employed as a fundamental gate for information processing based on the charge transport with the help of SAW.

A Comprehensive Review of Surface Acoustic Wave-Enabled Acoustic Droplet Ejection Technology and Its Applications.

This review focuses on the development of surface acoustic wave-enabled acoustic drop ejection (SAW-ADE) technology, which utilizes surface acoustic waves to eject droplets from liquids without touching the sample. The technology offers advantages such as high throughput, high precision, non-contact, and integration with automated systems while saving samples and reagents. The article first provides an overview of the SAW-ADE technology, including its basic theory, simulation verification, and comparison with other types of acoustic drop ejection technology. The influencing factors of SAW-ADE technology are classified into four categories: fluid properties, device configuration, presence of channels or chambers, and driving signals. The influencing factors discussed in detail from various aspects, such as the volume, viscosity, and surface tension of the liquid; the type of substrate material, interdigital transducers, and the driving waveform; sessile droplets and fluid in channels/chambers; and the power, frequency, and modulation of the input signal. The ejection performance of droplets is influenced by various factors, and their optimization can be achieved by taking into account all of the above factors and designing appropriate configurations. Additionally, the article briefly introduces the application scenarios of SAW-ADE technology in bioprinters and chemical analyses and provides prospects for future development. The article contributes to the field of microfluidics and lab-on-a-chip technology and may help researchers to design and optimize SAW-ADE systems for specific applications.

Theory of Synthesis of Asymmetrical Delay Line with the Surface Acoustic Wave

The aim of this publication is to design a procedure for the synthesis of an IDT (interdigital transducer) with diluted electrodes. The paper deals with the surface acoustic waves (SAW) and the theory of synthesis of the asymmetrical delay line with the interdigital transducer with diluted electrodes. The authors developed a theory, design, and implementation of the proposed design. They also measured signals. The authors analysed acoustoelectronic components with SAW: PLF 13, PLR 40, delay line with PAV 44 PLO. The presented applications have a potential practical use.

Review of Surface Acoustic Wave Sensors for the Detectionand Identification of Toxic Environmental Gases/Vapours

Detection and identification of toxic environmental gases have assumed paramount importance precisely in the defense, industrial and civilian security sector. Numerous methods have been developed for the sensing of toxic gases in the environment ever since surface acoustic wave (SAW) technology came into existence. Such SAW sensors called electronic nose (E-Nose) sensor use the frequency response of a delay line/resonator. SAW device is focused and given importance. The selective coating between input and output interdigital transducers (IDTs) in the SAW device is responsible for corresponding changes in operating frequency of the device for a specific gas/vapour absorbed from the environment. A suitable combination of well-designed SAW delay lines with selective coatings not only help to improve sensor sensitivity and selectivity but also leads to the minimization of false frequency alarms in the E-Nose sensor. This article presents a comprehensive review of design, development, simulation and modelling of a SAW sensor for potential sensing of toxic environmental gases.

Rapid particle concentration for immunofluorescence enhancement by a surface acoustic wave microchip

At the initial stages of pathogenesis, biomarkers are generated in an insignificant concentration. Hence, the rapid screening for these low-abundance indicators is immensely critical for early disease diagnosis. Enhancement of the immunofluorescence signal emitted from little concentrated biomarkers can aid their detection among other molecules in complex body fluids. Here, we developed an advanced bead-based immunosensing technique combining surface acoustic wave (SAW) for a rapid and sensitive on-chip detection of the diabetic retinopathy biomarker lipocalin 1. As a result, the implementation of the SAW microchip-enabled particle mixing, separation, and concentration in the droplet delivered a 3.6-fold enhanced immunofluorescence signal within 5 s. The integration of fluorescent signal enhancement by SAW, along with the high sensitivity of the immunoassay, featured a limit of detection of 40 pg/mL in 3 min with a sample volume as low as 4 µL. A simple addition of cover glass on the SAW microchip allowed the immuno-sensing process to be completed without any direct sample contact with the interdigital transducer, minimizing the risk of cross-contamination and damage to the chip due to frequent washing. This study demonstrated the potential of the developed SAW device and insights into its utility as an early diagnostic tool.

Microfluidics-assisted synthesis of hydrogel microparticles with acoustic-magnetic control

Using one microfluidic device integrating acoustic and magnetic control, we propose a novel preparation method of magnetic fluorescent polyacrylamide hydrogel microspheres and calcium alginate hydrogel microparticles in this paper. The effect of the two interdigital transducers in different position on the droplets generation is studied. By tuning the input parameters of the two focused interdigital transducers (FIDTs), the excited focused surface acoustic wave (FSAW) can induce the generation of magnetic droplets with different sizes flexibly. Then the prepared magnetic droplets are cross-linked after crossing the continuous phase into the laminar flow induced by magnetic force. This method can avoid the direct reaction between the dispersed phase and the continuous phase in the channel, and enable the synthesis of magnetic fluorescent polyacrylamide hydrogel microspheres and calcium alginate hydrogel microparticles more easy and simple.

Contribution of Ribbon-Structured SiO2 Films to AlN-Based and AlN/Diamond-Based Lamb Wave Resonators.

New designs based on S0 Lamb modes in AlN thin layer resonating structures coupled with the implementation of structural elements in SiO2, are theoretically analyzed by the Finite Element Method (FEM). This study compares the typical characteristics of different interdigital transducer (IDTs) configurations, involving either a continuous SiO2 cap layer, or structured SiO2 elements, showing their performance in the usual terms of electromechanical coupling coefficient (K2), phase velocity, and temperature coefficient of frequency (TCF), by varying structural parameters and boundary conditions. This paper shows how to reach temperature-compensated, high-performance resonator structures based on ribbon-structured SiO2 capping. The addition of a thin diamond layer can also improve the velocity and electromechanical coupling coefficient, while keeping zero TCF and increasing the solidity of the membranes. Beyond the increase in performance allowed by such resonator configurations, their inherent structure shows additional benefits in terms of passivation, which makes them particularly relevant for sensing applications in stern environments.

Through-Holes Design for Ideal LiNbO3 A1 Resonators.

This paper proposes a method to realize ideal lithium niobate (LiNbO3) A1 resonators. By introducing subwavelength through-holes between the interdigital transducer (IDT) electrodes on the LiNbO3 surface, all unfavorable spurious modes of the resonators can be suppressed completely. It is convenient and valid for various IDT electrode parameters and different LiNbO3 thicknesses. Also, this method does not require additional device fabrication steps. At the same time, these through-holes can greatly reduce the suspended area of the LiNbO3 thin film, thus significantly improving the design flexibility, compactness, mechanical stability, temperature stability, and power tolerance of the resonators (and subsequent filters). It is expected to become an important means to promote the practical application of LiNbO3 A1 filters and even all Lamb waves filters.

Preparation of MoS2 Nanospheres using a Hydrothermal Method and Their Application as Ammonia Gas Sensors Based on Delay Line Surface Acoustic Wave Devices.

An ammonia sensor based on a delay-line surface acoustic wave (SAW) device is developed in this study by coating the delay line area of the device with a nano-structured molybdenum disulfide (MoS2) sensitive material. A SAW device of 122 MHz was designed and fabricated with a pair of interdigital transducers (IDTs) defined on a 128° y-cut LiNbO3 substrate using photolithography technologies, and the aluminum IDT electrodes were deposited by a DC magnetron sputtering system. By adjusting the pH values of precursor solutions, molybdenum disulfide (MoS2) nanospheres were prepared with various structures using a hydrothermal method. Finally, an NH3 gas sensor with high sensitivity of 4878 Hz/ppm, operating at room temperature, was successfully obtained. The excellent sensitivity performance may be due to the efficient adsorption of NH3 gas molecules on the surfaces of the nanoflower-like MoS2, which has a larger specific surface area and provides more active sites, and results in a larger change in the resonant frequency of the device due to the mass loading effect.

Multiphysics modeling of printed surface acoustic wave thermometers

Surface acoustic wave (SAW) transducers are a class of sensors and actuators that operate on the fundamental principle of piezoelectricity. Aerosol jet printing and other additive manufacturing techniques have resulted in the low-cost fabrication of low-power and small-footprint SAW devices that are suitable for sensing in high-temperature and radioactive environments. In this work, we developed a series of temperature-dependent finite element models for a SAW transducer consisting of printed silver interdigitated transducers (IDTs) deposited onto piezoelectric lithium niobate. Modeling accuracy was evaluated experimentally from room temperature to 200 °C using an aerosol-jet-printed SAW thermometer. A time-domain study enabled visualization of the wave propagation and successfully guided the denoising of the scattering parameter measurement. Additionally, frequency-domain models using traditional modal analysis or the unique port boundary condition feature in COMSOL Multiphysics accurately predicted the temperature-driven natural frequency drift in the SAW thermometer. The finite element models developed in this study serve to facilitate the computer-aided design of future SAW transducers for applications in harsh environments.

Influence of electrode apodization weighted structure on the performance of SAW temperature sensors

In order to improve the comprehensive performance of the surface acoustic wave (SAW) temperature sensors and reduce clutter interference, the optimized Chebyshev window function, Blackman window function and Hamming window function are used to carry out the apodization weighting design for interdigital transducer (IDT) and reflective grating respectively. The influence of electrode apodization weighted structure on the performance of SAW temperature sensors is systematically studied through experiments. The results show that the apodization weighted design of IDT and reflective gate can effectively improve the resonance peak intensity and reduce the side lobe interference; the IDT and the reflection grating are designed with apodization weighting at the same time, compared with the single reflection grating weighting, the volatility of the side lobe of the Chebyshev window function and the Hamming window function increases, the volatility of the side lobe of the Blackman window function decreases; after electrode apodization optimization, the resonance frequency, Q value of the SAW sensor and the sensitivity to temperature increases; using a hamming window optimized electrode structure, the peak value of S11 is −36.73 dB and the Q value is 15 030. In order to ensure the signal processing ability and comprehensive performance of the sensor, an optimized Hamming window should be used for the electrode design of the SAW sensors.

An efficient modeling approach for wavelet transform processors using surface acoustic wave devices

Wavelet transform processors (WTPs) using surface acoustic wave devices provide an effective solution to utilize wavelet transform technology in practical applications. This work proposes a novel model of WTPs for predicting their performance characteristics, which might be utilized for single-electrode type and double-electrode type interdigital transducers (IDTs), as there is currently no universal, quick, and accurate simulation technology for WTPs (IDTs). The admittance matrix, transfer function, and insertion loss for both IDTs and WTPs in conjunction with a two-port electrical network are all evaluated by the model. Furthermore, some WTPs samples with a piezoelectric substrate of ST-X quartz and IDTs of Al thin film were fabricated to validate the accuracy of the simulation, with a center frequency and a scale of 60 MHz and 0.215, respectively. A comparison of the simulated and measured results found that the relative error of the frequency is only 0.67%, the maximum relative error of the bandwidth is 5.79% and the relative error of the insertion loss is only 1.24%. The experimental results show that the model is extremely accurate in effective frequency band, and its application will accelerate the design and production of WTPs, promoting the use of WTPs in signal processing systems.

Acoustofluidics: A Versatile Tool for Micro/Nano Separation at the Cellular, Subcellular, and Biomolecular Levels

AbstractSeparation of micro/nanoparticles, such as cellular, subcellular and biomolecular, has attracted increasing attention because of their remarkable potential applications in various fields, including chemistry, physics, medicine, etc. Among different micro/nanoparticle separation methods, acoustofluidics, which combines acoustics and microfluidics, has drawn the interest of researchers due to its biocompatibility, high efficiency and free labeling. In this review, the basic constitutions, mechanisms, and materials of acoustofluidics are described. Subsequently, sorts of delicately designed acoustofluidic devices, including diverse bulk acoustic wave (BAW) microfluidics and surface acoustic wave (SAW) microfluidics, are discussed, covering principles, advantages, limitations and applications in separation. Besides the introduction of advances of micro/nanoparticle separation in the BAW microfluidics, the SAW microfluidics are elaborated in detail with a focus on various configurations of interdigital transducers (IDTs), comprising straight IDT, slanted‐finger IDT, chirped IDT and focused IDT. Microfluidic systems of the acoustofluidics involve the forms of straight channels, serpentine channels, and droplets. Additionally, besides simply structured acoustofluidics, acoustofluidics integrated with other structures are also mentioned. Finally, the prospects and limitations of acoustofluidics in micro/nanoparticle separation are also discussed. The acoustofluidics reviewed here is envisioned as a versatile tool for micro/nanoparticle separation at the cellular, sub‐cellular, and biomolecular levels.

Optimal Overall Dimensions of a Surface Acoustic Waves Ring Resonator

Introduction . In previous works, the authors considered the frequency characteristics of sensitive elements made of various materials in the form of a ring resonator on surface acoustic waves (SAW), along with their fixing methods in the housing, the influence of external factors, and an optimal topology of the interdigital transducer of the ring resonator. Further, the need arose to study the dependence of the sensitivity of the sensitive element and the maximum acceleration load on its dimensions, as well as to analyze the characteristics of the manufactured experimental samples in comparison with the simulated values. Aim . To determine optimal dimensions of the sensitive element of a ring resonator and to confirm the adequacy of the constructed models by comparing the characteristics of experimental samples with those obtained by computer simulation. Materials and methods . The theoretical part of the research was carried out using the finite element method. Mathematical processing was implemented in AutoCAD and COMSOL Multiphysics. Results . Three overall dimensions of the sensitive element of a ring resonator were proposed: 1500, 3000 and 4500 µm. The characteristics of sensitive elements made of lithium niobate with the above dimensions were studied. Thus, the resonance frequency for 1500, 3000 and 4500 µm samples comprised 207.99, 104.10 and 68.99 MHz, respectively. The maximum acceleration experienced by a cantilever with a radius of 1500, 3000 and 4500 µm was found to be 191 132, 84 958 and 37 514g, respectively. Dependence graphs of the maximum acceleration and sensitivity on the ratio of the radius of the console to its height are presented. The adequacy of the constructed model was confirmed, i. e., the resonance frequency for 1500, 3000 and 4500 µm experimental samples comprised 218.17 MHz (4.67 % discrepancy with computer simulation), 109.23 MHz (4.69 %) and 72.88 MHz (5.34 %), respectively. Conclusion . The sensitivity and maximum acceleration load of the sensitive element of a SAW ring resonator directly depends on the ratio of the cantilever radius to its height, with higher sensitivity values correlating to lower values of maximum acceleration load. For each material, these dependencies are unique. The interdigital transducer bus size has little effect on the frequency response. The previously presented simulations were confirmed by experimental samples with a difference in resonance frequencies of less than 5.5 %.

Intercellular Calcium Waves and Permeability Change Induced by Vertically Deployed Surface Acoustic Waves in a Human Cerebral Microvascular Endothelial Cell Line (hCMEC/D3) Monolayer.

The ultrasound-mediated blood-brain barrier (BBB) opening with microbubbles has been widely employed, while recent studies also indicate the possibility that ultrasound alone can open the BBB through a direct mechanical effect. However, the exact mechanisms through which ultrasound interacts with the BBB and whether it can directly trigger intracellular signaling and a permeability change in the BBB endothelium remain unclear. Vertically deployed surface acoustic waves (VD-SAWs) were applied on a human cerebral microvascular endothelial cell line (hCMEC/D3) monolayer using a 33-MHz interdigital transducer that exerts shear stress-predominant stimulation. The intracellular calcium response was measured by fluorescence imaging, and the permeability of the hCMEC/D3 monolayer was assessed by transendothelial electrical resistance (TEER). At a certain intensity threshold, VD-SAWs induced an intracellular calcium surge that propagated to adjacent cells as intercellular calcium waves. VD-SAWs induced a TEER decrease in a pulse repetition frequency-dependent manner, thereby suggesting possible involvement of the mechanosensitive ion channels. The unique VD-SAW system enables more physiological mechanical stimulation of the endothelium monolayer. Moreover, it can be easily combined with other measurement devices, providing a useful platform for further mechanistic studies on ultrasound-mediated BBB opening.

UV Sensor Based on Surface Acoustic Waves in ZnO/Fused Silica.

Zinc oxide (ZnO) thin films have been grown by radio frequency sputtering technique on fused silica substrates. Optical and morphological characteristics of as-grown ZnO samples were measured by various techniques; an X-ray diffraction spectrum showed that the films exhibited hexagonal wurtzite structure and were c-axis-oriented normal to the substrate surface. Scanning electron microscopy images showed the dense columnar structure of the ZnO layers, and light absorption measurements allowed us to estimate the penetration depth of the optical radiation in the 200 to 480 nm wavelength range and the ZnO band-gap. ZnO layers were used as a basic material for surface acoustic wave (SAW) delay lines consisting of two Al interdigitated transducers (IDTs) photolithographically implemented on the surface of the piezoelectric layer. The Rayleigh wave propagation characteristics were tested in darkness and under incident UV light illumination from the top surface of the ZnO layer and from the fused silica/ZnO interface. The sensor response, i.e., the wave velocity shift due to the acoustoelectric interaction between the photogenerated charge carriers and the electric potential associated with the acoustic wave, was measured for different UV power densities. The reversibility and repeatability of the sensor responses were assessed. The time response of the UV sensor showed a rise time and a recovery time of about 10 and 13 s, respectively, and a sensitivity of about 318 and 341 ppm/(mW/cm2) for top and bottom illumination, respectively. The ZnO/fused silica-based SAW UV sensors can be interrogated across the fused silica substrate thanks to its optical transparency in the UV range. The backlighting interrogation can find applications in harsh environments, as it prevents the sensing photoconductive layer from aggressive environmental effects or from any damage caused by cleaning the surface from dust which could deteriorate the sensor's performance. Moreover, since the SAW sensors, by their operating principle, are suitable for wireless reading via radio signals, the ZnO/fused-silica-based sensors have the potential to be the first choice for UV sensing in harsh environments.

Super-High-Frequency Low-Loss Sezawa Mode SAW Devices in a GaN/SiC Platform.

This paper presents a comprehensive study of the performance of Sezawa surface acoustic wave (SAW) devices in SweGaN QuanFINE® ultrathin GaN/SiC platform, reaching frequencies above 14 GHz for the first time. Sezawa mode frequency scaling is achieved due to the elimination of the thick buffer layer typically present in epitaxial GaN technology. Finite element analysis (FEA) is first performed to find the range of frequencies over which the Sezawa mode is supported in the grown structure. Transmission lines and resonance cavities driven with Interdigital Transducers (IDTs) are designed, fabricated, and characterized. Modified Mason circuit models are developed for each class of devices to extract critical performance metrics. We observe a strong correlation between measured and simulated dispersion of the phase velocity (vp) and piezoelectric coupling coefficient (k2). Maximum k2 of 0.61% and frequency-quality factor product (f.Qm) of 6×1012 s-1 are achieved for Sezawa resonators at 11 GHz, with a minimum propagation loss of 0.26 dB/λ for the two-port devices. Sezawa modes are observed at frequencies spanning up to 14.3 GHz, achieving a record high in GaN microelectromechanical systems (MEMS) to the best of the authors' knowledge.