Interdigital Transducers Research Articles

Circular SAW Resonators: Influence of Sensitive Element Dimensions on Strength Characteristics and First Experimental Samples.

In preceding research endeavors, the frequency characteristics of a ring resonator on surface acoustic waves made of various materials were studied. Investigations encompassed fixation techniques within the housing, the impact of external variables on these components, and the most efficient configuration of the interdigital transducer within the ring resonator to curtail bandwidth. This current study is dedicated to investigating the correlation between sensitivity and the highest measurable acceleration concerning the dimensions of these sensitive elements. Furthermore, it involves assessing the attributes of produced experimental samples to verify the simulation results. The results obtained represent the possibility of creating a micromechanical accelerometer that can be used in the automotive industry as a g-sensor shock, as well as in industries where the numerical value of high overloads is required.

Low temperature coefficient of frequency AlN Lamb wave resonator using groove structure between interdigital transducers

The lithographically tunable and small size features of Lamb wave resonators (LWRs) take a bright future for their use in high frequency narrow band filters. Resonant frequency drift due to temperature variation has a large impact on the narrower passband and a temperature coefficient of frequency (TCF) closer to 0 can broaden the stable temperature range of the resonator. This paper proposes a method of etching grooves in the area of the piezoelectric layer not covered by the Interdigital transducer (IDT) electrodes to improve the temperature stability of the Lamb resonator. Through the finite element method and theoretical analysis, the phase velocity, group velocity, and TCF dispersion curve of different modes of the resonator with groove structure were determined. The reason for the change of TCF under different normalized thicknesses of AlN was explained through the conversion of the LWR from a contour mode resonator (CMR) to a Cross-Sectional Lamé Mode Resonator. Simulation and test have shown that etching grooves have the effect of reducing TCF by adjusting the electromechanical coupling coefficient. The test shows that 205 nm-depth grooves can lower the TCF of the S0 mode IDT-Open LWR from −13.3 to −5.1 ppm/°C and S1 mode from −36.8 to −9.0 ppm/°C, respectively. The TCF of S0 and S1 mode LWRs decreased by 61.7% and 75.5%, respectively, after 205 nm groove etching. The groove etching method greatly raises the temperature stability of the LWR, enabling Lamb wave filters to operate stably over a wider temperature range.

Prediction of directivity characteristics of piezoelectric macro-fiber composite interdigital transducers: a semi-analytical approach

The employment of guided waves for structural health monitoring applications has attracted substantial attention in recent years. Piezoelectric transducers are frequently employed in these systems for elastic wave generation and acquisition. Their dynamic properties, i.e., direction-dependent frequency characteristics, are an important feature for designing a reliable monitoring strategy. Although analysis of the transducer directivity pattern can be performed using finite element models, these usually tend to be computationally very expensive, especially, for parametric and optimization studies. In this paper, a semi-analytical methodology is proposed that can predict the far-field responses of complex transducers of arbitrary shape and structure mounted on elastic plates. The approach reported here couples analytical far-field predictions with a local FE model evaluating the traction field generated by the transducer. Thus, the FE model captures the near-field responses, while the analytical part predicts the far-field responses. The implementation of this semi-analytical method requires development of analytical and numerical frameworks. The former includes computation of dispersion curves of the inspected plate, the stress and displacement profiles of wave modes and the excitability curves, while the latter includes estimation of the traction field between the transducer and the plate using the FE model. Owing to the popularity of piezoelectric macro-fiber composite (MFC) interdigital transducers, in this paper, we apply the proposed method to simulate the directivity patterns of the MFC IDTs of different configurations. To validate the reliability of the proposed method the simulated patterns are compared with experimental results obtained using Laser Doppler Vibrometer (LDV).

Acoustofluidics-enhanced biosensing with simultaneously high sensitivity and speed

Simultaneously achieving high sensitivity and detection speed with traditional solid-state biosensors is usually limited since the target molecules must passively diffuse to the sensor surface before they can be detected. Microfluidic techniques have been applied to shorten the diffusion time by continuously moving molecules through the biosensing regions. However, the binding efficiencies of the biomolecules are still limited by the inherent laminar flow inside microscale channels. In this study, focused traveling surface acoustic waves were directed into an acoustic microfluidic chip, which could continuously enrich the target molecules into a constriction zone for immediate detection of the immune reactions, thus significantly improving the detection sensitivity and speed. To demonstrate the enhancement of biosensing, we first developed an acoustic microfluidic chip integrated with a focused interdigital transducer; this transducer had the ability to capture more than 91% of passed microbeads. Subsequently, polystyrene microbeads were pre-captured with human IgG molecules at different concentrations and loaded for detection on the chip. As representative results, ~0.63, 2.62, 11.78, and 19.75 seconds were needed to accumulate significant numbers of microbeads pre-captured with human IgG molecules at concentrations of 100, 10, 1, and 0.1 ng/mL (~0.7 pM), respectively; this process was faster than the other methods at the hour level and more sensitive than the other methods at the nanomolar level. Our results indicated that the proposed method could significantly improve both the sensitivity and speed, revealing the importance of selective enrichment strategies for rapid biosensing of rare molecules.

Acoustic wave-based single photon shifter for solid-state sources

Controlling the frequency of nonclassical light is essential for the implementation of quantum computation, communication, and the integration of various quantum systems. However, there is a practical absence of easy-to-integrate frequency-shift devices for solid-state single-photon sources. Here, we propose an integrated single-photon frequency shifter that utilizes acousto-optic modulation. The device is composed of two interdigital transducers (IDTs) for generating acoustic waves on a lithium niobate on insulator (LNOI) platform, along with a silicon waveguide that is periodically positioned at the nodes of the acoustic wave to enhance the interaction length. We achieved a low half-wavelength voltage length product Vπ×L of 0.18 V cm. With a driving frequency of 129.7 MHz and a driving voltage of 10 V, a frequency shift of up to ± 405 GHz is realized with near-unity conversion efficiency. Our findings illustrate the feasibility of deterministic on-chip quantum spectral control, which is pivotal for constructing hybrid quantum networks.

О возможности создания эффективных устройств на поверхностных акустических волнах на частотах выше 6 ГГц

In the paper the interaction of surface acoustic waves (SAWs) with reflectors and interdigital transducers (IDTs) with high aspect ratio electrodes (HAR-electrodes) are investigated. The conditions for the SAW resonant reflection and propagation and the effective IDT conversion of the SAW power into electrical power at the load have been obtained. The real possibility of the effective SAW devices creation (filters, delay lines, resonators, RFID tags) at frequencies above 6 GHz (up to 12-15 GHz) using HAR-electrodes was demonstrated.

Spectrum-Clean Dispersion-Engineered YX-LN/SiO₂/Si Wideband SH-SAW Resonators With Crossed Interdigital Transducers

Spectrum-Clean Dispersion-Engineered YX-LN/SiO₂/Si Wideband SH-SAW Resonators With Crossed Interdigital Transducers

An Improved Method to Compute the Mutual Capacitance between Interdigital Transducers in Radio Frequency Surface Acoustic Wave Filters

This paper proposes an improved method to calculate the mutual capacitance between interdigital transducer (IDT) electrodes to enhance the accuracy of the traditional coupling-of-modes (COM) model, which is commonly used to simulate surface acoustic wave (SAW) filters and duplexers. In this method, the boundary element method (BEM) is adopted to obtain the capacitance per unit length in a layered medium, while the partial capacitance (PC) method is used to derive the effective relative permittivity of the multi-layered IDT. Numerical results from commercially available software are provided for comparison with the results calculated using the proposed method. The consistent results verify the validity and accuracy of this method, which also demonstrates significantly faster calculation speed compared to commercially available software. Precise electrical response prediction of a dual-mode SAW (DMS) filter can be achieved by applying this method to the COM model, and this ultra-fast calculation method can also be included in filter design optimization.

Quasi-Bessel surface acoustic wave for dynamic acoustic manipulation

Acoustic manipulation using surface acoustic wave has aroused widespread interest in life sciences, biomedical, and bioanalytical chemistry. Acoustic manipulation for different applications requires different acoustic fields. Bessel beams are non-diffractive and re-constructable, bringing possibility and versatility of acoustic manipulation integrated on microfluidic chips. To date, there are a few studies on constructing Bessel surface acoustic waves. Moreover, there is still a lack of dynamic acoustic manipulation using Bessel surface acoustic waves propagating along a surface of piezoelectric substrate with simple and high-precision devices. Here, we design a device with two omnidirectional equifrequency interdigital transducers to form a quasi-Bessel surface acoustic wave by means of coherent interference. The proposed device avoids influences of anisotropy on its operating frequency, making its quasi-Bessel beam accurately and stably conform to the predetermined design acoustic field. This acoustic field could control micrometer to submicrometer particles and dynamically move particles along lateral direction and axial direction of the propagation of quasi-Bessel beam. A phenomenon similar to negative force appeared when the two-micron spherical particles were manipulated. The quasi-Bessel beam formed by our device can provide a versatile movement for on-chip acoustic manipulation.

Digital twin of surface acoustic wave transceivers for a computational design of an optimal wave guiding layer thickness

AbstractDetection of biomarkers is exploited in lab-on-a-chip devices by means of Love type Surface Acoustic Waves (SAW). Finger type arrangement of electrodes, used for InterDigital-Transducers (IDT), perform well to create and detect SAW by using electro-mechanical coupling. Efficiency of such a transceiver depends on design parameters such as chosen material orientation, thickness, placement of electrodes. An optimized design reduces production costs, hence, we need a digital twin of the device with multiphysics simulations that compute deformation and electric field. In this study, we develop a framework with the open-source package called FEniCS for modal and transient analyses of IDTs by using the Finite Element Method (FEM). Specifically, we discuss all possible sensor design parameters and propose a computational design guideline that determines the “best” thickness parameter by maximizing mass sensitivity, thus, efficiency for a Love surface acoustic wave sensor.

Grooving and Absorption on Substrates to Reduce the Bulk Acoustic Wave for Surface Acoustic Wave Micro-Force Sensors.

Improving measurement accuracy is the core issue with surface acoustic wave (SAW) micro-force sensors. An electrode transducer can stimulate not only the SAW but also the bulk acoustic wave (BAW). A portion of the BAW can be picked up by the receiving transducer, leading to an unwanted or spurious signal. This can harm the device's frequency response characteristics, thereby potentially reducing the precision of the micro-force sensor's measurements. This paper examines the influence of anisotropy on wave propagation, and it also performs a phase-matching analysis between interdigital transducers (IDTs) and bulk waves. Two solutions are shown to reduce the influence of BAW for SAW micro sensors, which are arranged with acoustic absorbers at the ends of the substrate and in grooving in the piezoelectric substrate. Three different types of sensors were manufactured, and the test results showed that the sidelobes of the SAW micro-force sensor could be effectively inhibited (3.32 dB), thereby enhancing the sensitivity and performance of sensor detection. The SAW micro-force sensor manufactured using the new process was tested and the following results were obtained: the center frequency was 59.83 MHz, the fractional bandwidth was 1.33%, the range was 0-1000 mN, the linearity was 1.02%, the hysteresis was 0.59%, the repeatability was 1.11%, and the accuracy was 1.34%.

Rational Design of a Surface Acoustic Wave Device for Wearable Body Temperature Monitoring.

Continuous monitoring of vital signs based on advanced sensing technologies has attracted extensive attention due to the ravages of COVID-19. A maintenance-free and low-cost passive wireless sensing system based on surface acoustic wave (SAW) device can be used to continuously monitor temperature. However, the current SAW-based passive sensing system is mostly designed at a low frequency around 433 MHz, which leads to the relatively large size of SAW devices and antenna, hindering their application in wearable devices. In this paper, SAW devices with a resonant frequency distributed in the 870 MHz to 960 MHz range are rationally designed and fabricated. Based on the finite-element method (FEM) and coupling-of-modes (COM) model, the device parameters, including interdigital transducer (IDT) pairs, aperture size, and reflector pairs, are systematically optimized, and the theoretical and experimental results show high consistency. Finally, SAW temperature sensors with a quality factor greater than 2200 are obtained for real-time temperature monitoring ranging from 20 to 50 °C. Benefitting from the higher operating frequency, the size of the sensing system can be reduced for human body temperature monitoring, showing its potential to be used as a wearable monitoring device in the future.

Ternary Holey Carbon Nanohorn/Potassium Chloride/Polyvinylpyrrolidone Nanohybrid as Sensing Film for Resistive Humidity Sensor

The study presents findings on the relative humidity (R.H.) sensing capabilities of a resistive sensor. This sensor utilizes sensing layers composed of a ternary nanohybrid, consisting of holey carbon nanohorn (CNHox), potassium chloride (KCl), and polyvinylpyrrolidone (PVP), with mass ratios of 7/1/2, 6.5/1.5/2, and 6/2/2 (w/w/w). The sensing structure comprises a silicon substrate, a SiO2 layer, and interdigitated transducer (IDT) electrodes. The sensing film is deposited on the sensing structure via the drop-casting method. The sensing layers’ morphology and composition are investigated through Scanning Electron Microscopy (SEM) and RAMAN spectroscopy. The resistance of thin-film sensors based on ternary hybrids increased with exposure to a range of relative humidity (R.H.) levels, from 0% to 100%. The newly designed devices demonstrated a comparable response at room temperature to that of commercial capacitive R.H. sensors, boasting excellent linearity, swift response times, and heightened sensitivity. Notably, the studied sensors outperform others employing CNHox-based sensing layers in terms of sensitivity, as observed through manufacturing and testing processes. It elucidates the sensing mechanisms of each constituent within the ternary hybrid nanocomposites, delving into their chemical and physical properties, electronic characteristics, and affinity for water molecules. Various alternative sensing mechanisms are considered and discussed, including the reduction in holes within CNHox upon interaction with water molecules, proton conduction, and PVP swelling.

Anti-plane waves in a liquid-loaded piezo-flexo-electric layered model with interface energy

The basic design of a Love wave (LW) bio-sensor contains the loading of a viscoelastic liquid on top of a layered structure with distinct viscoelastic properties. Changes in the characteristics of the propagating acoustic wave caused by biochemical interactions at the sensing area can be detected at the output Inter digital transducer (IDT). The propagation of the anti-plane (AP) wave is discussed in this work in a layered structure of piezo-flexo-electric (PFE) layer bonded with a PFE half-space having a soft reinforced layer at the interface. The free surface of the PFE layer is loaded with viscous liquid. The viscosity of the loaded liquid introduces losses and results in the damping of the wave. For the formulation of the problem with interface energy, the Gurtin–Murdoch approach is used. Using suitable conditions, a dispersion relation for propagating waves is derived in complex form. On separating the dispersion relation in real and non-real parts, the expressions relating the phase and damp velocities with wave number are derived. The obtained theoretical results are portrayed for the numerical data of PFE materials and distinct data of the interface layer. The obtained results are validated with pre-existing literature.

Surface Acoustic Wave Resonator Chip Setup for the Elimination of Interfering Conductivity Responses.

A surface acoustic wave (SAW) resonator chip setup is presented that eliminates interfering signal responses caused by changes in the electrical environment of the surrounding media. When using a two-port resonator, applying electrically shielding layers between the interdigital transducers (IDTs) can be challenging due to the limited dimensions. Therefore, a layered setup consisting of an insulating polymer layer and a conductive gold layer was preferred. The SAW resonators were provided with polycarbonate housings, resulting in SAW resonator chips. This setup enables easy application of a wide range of coatings to the active part of the resonator surface, while ensuring subsequent electrical and fluidic integration of the resonator chips into a microfluidic array for measurements. The signal responses of uncoated SAW resonators and those with polymer coatings with and without a gold layer were tested with aqueous potassium chloride (KCl) solutions up to 3 mol/L, corresponding to conductivities up to 308 mS/cm. The use of a polymer coating at the thickness of the first Love mode resonance and a conductive gold layer completely reduced the electrical impact on the SAW resonator signal response, making small signals resulting from changes in viscosity and density of the KCl solutions visible.

Optimization and operation of interdigital transducer to improve signal-to-noise ratio

This study presents a novel optimization method for Lamb wave mode selective transducers, surpassing traditional interdigital transducers (TIDT) by employing non-uniform electrode widths to suppress harmonic components. Utilizing a frequency domain finite element model (FDFEM), the coupling mechanism between the double-sided interdigital transducer (DS-IDT) and the waveguide is analyzed, forming the optimization basis. By discussing key parameters affecting wavelength selection characteristics, an optimization scheme is formulated. Employing a multi-objective genetic algorithm, the strategy focuses on minimizing the ratio of stray to target mode as the cost function, resulting in significant signal-to-noise ratio (SNR) improvements. Compared to TIDTs, SNR gains of at least 4.7 dB at 70 kHz and 8.8 dB at 400 kHz are achieved. Laboratory experiments validate the superior mode selection performance of the optimized interdigital transducer (OIDT) over ordinary piezoelectric wafer (OPW) and TIDT, demonstrating a significant reduction of 48.82 % in stray S0 mode amplitude at 70 kHz and 94.77 % in stray A2 mode amplitude at 400 kHz. This method ensures high SNR for target mode signals, simplifying feature extraction and enhancing structural health monitoring capabilities.

CARBON NANOHORNS – BASED NANOCOMPOSITES AS SENSING LAYERS FOR ROOM TEMPERATURE RESISTIVE OXYGEN SENSING: PRELIMINARY RESULTS

This paper presents the oxygen sensing response of a resistive sensor employing sensing layers based on a binary matrix nanocomposite such as carbon nanohorns/polyvinylpyrrolidone and oxidized carbon nanohorns/polyvinylpyrrolidone, both at 9/1 w/w/ mass ratio. The sensing structure comprises a silicon substrate, a SiO2 layer, and interdigitated transducers (IDT) electrodes, on which the sensing layer is deposited via the drop-casting method. The thin film's morphology and composition are examined through scanning electron microscopy (SEM) and RAMAN spectroscopy. The oxygen sensing capability of each carbon nanohorns composite-based sensing layer was analyzed by applying a current between the two electrodes and measuring the voltage difference when varying the O2 from 0% to 100% in dry nitrogen. Experiments reveal that in the case of pristine carbon nanohorns / PVP matrix nanocomposite, the resistance increases. In contrast, in the case of oxidized carbon nanohorns, the resistance of the sensitive layer decreases with increasing oxygen concentration. The results are explained by considering the differences from structural and electrical points of view between the two types of nanocarbonic materials. Moreover, previous resistive RH sensing measurements in humid air and humid nitrogen using oxidized carbon nanohorns as sensing elements proved helpful in better understanding and discriminating between the chemisorption/physisorption of oxygen molecules at carboxyl functional sites and graphitic carbon sites.

A Complete LTspice Simulation Model for SAW Devices

A complete circuit model of Surface Acoustic Wave (SAW) devices with straight interdigital transducers is presented. This equivalent circuit can be implemented in LTspice and contains variable design characteristics that can be easily changed to see their effect on the electrical response of the final SAW device design. Later a two port Surface Acoustic Wave device with 13-finger-IDT electrodes is fabricated and its electrical characterization is examined using a vector network analyzer. The test results are then compared with the equivalent circuits’ simulation output results for the verification. The results show an acceptable agreement between the experimental and simulation results for a wide frequency range. This paper offers an easy method to create an equivalent LTSpice model to determine the electrical response of a SAW device before fabrication. The model can also be used to simulate the behaviour of the circuits containing SAW devices using LTSPICE tool.

Optically Controlled Nano-Transducers Based on Cleaved Superlattices for Monitoring Gigahertz Surface Acoustic Vibrations.

Surface acoustic waves (SAWs) convey energy at subwavelength depths along surfaces. Using interdigital transducers (IDTs) and opto-acousto-optic transducers (OAOTs), researchers have harnessed coherent SAWs with nanosecond periods and micrometer localization depth for various applications. These applications include the sensing of small amount of materials deposited on surfaces, assessing surface roughness and defects, signal processing, light manipulation, charge carrier and exciton transportation, and the study of fundamental interactions with thermal phonons, photons, magnons, and more. However, the utilization of cutting-edge OAOTs produced through surface nanopatterning techniques has set the upper limit for coherent SAW frequencies below 100 GHz, constrained by factors such as the quality and pitch of the surface nanopattern, not to mention the electronic bandwidth limitations of the IDTs. In this context, unconventional optically controlled nanotransducers based on cleaved superlattices (SLs) are here presented as an alternative solution. To demonstrate their viability, we conducted proof-of-concept experiments using ultrafast lasers in a pump-probe configuration on SLs made of alternating AlxGa1-xAs and AlyGa1-yAs layers with approximately 70 nm periodicity and cleaved along their growth direction to produce a periodic nanostructured surface. The acoustic vibrations, generated and detected by laser beams incident on the cleaved surface, span a range from 40 to 70 GHz, corresponding to the generalized surface Rayleigh mode and bulk modes within the dispersion relation. This exploration shows that, in addition to SAWs, cleaved SLs offer the potential to observe surface-skimming longitudinal and transverse acoustic waves at GHz frequencies. This proof-of-concept demonstration below 100 GHz in nanoacoustics using such an unconventional platform might be useful for realizing sub-THz to THz coherent surface acoustic vibrations in the future, as SLs can be epitaxially grown with atomic-scale layer width and quality.

Detection of a Submillimeter Notch-Type Defect at Multiple Orientations by a Lamb Wave A0 Mode at 550 kHz for Long-Range Structural Health Monitoring Applications.

The early detection of small cracks in large metal structures is a crucial requirement for the implementation of a structural health monitoring (SHM) system with a low transducers density. This work tackles the challenging problem of the early detection of submillimeter notch-type defects with a semielliptical shape and a groove at a constant width of 100 µm and 3 mm depth in a 4.1 mm thick aluminum plate. This defect is investigated with an ultrasonic guided wave (UGW) A0 mode at 550 kHz to investigate the long range in thick metal plates. The mode selection is obtained by interdigital transducers (IDTs) designed to operate with a 5 mm central wavelength. The novel contribution is the validation of the detection by pulse-echo and pitch and catch with UGW transducers to cover a distance up to 70 cm to reduce the transducers density. The analysis of scattering from this submillimeter defect at different orientations is carried out using simulations with a Finite Element Model (FEM). The detection of the defect is obtained by comparing the scattered signals from the defect with baseline signals of the pristine laminate. Finally, the paper shows that the simulated results are in good agreement with the experimental ones, demonstrating the possible implementation in an SHM system based on the efficient propagation of an antisymmetric mode by IDTs.