Output Interdigital Transducers Research Articles

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.

Temperature Compensation of SAW Winding Tension Sensor Based on PSO-LSSVM Algorithm.

In this paper, a SAW winding tension sensor is designed and data fusion technology is used to improve its measurement accuracy. To design a high-measurement precision SAW winding tension sensor, the unbalanced split-electrode interdigital transducers (IDTs) were used to design the input IDTs and output IDTs, and the electrode-overlap envelope was adopted to design the input IDT. To improve the measurement accuracy of the sensor, the particle swarm optimization-least squares support vector machine (PSO-LSSVM) algorithm was used to compensate for the temperature error. After temperature compensation, the sensitivity temperature coefficient αs of the SAW winding tension sensor was decreased by an order of magnitude, thus significantly improving its measurement accuracy. Finally, the error with actually applied tension was calculated, the same in the LSSVM and PSO-LSSVM. By multiple comparisons of the same sample data set overall, as well as the local accuracy of the forecasted results, which is 5.95%, it is easy to confirm that the output error predicted by the PSO-LSSVM model is 0.50%, much smaller relative to the LSSVM's 1.42%. As a result, a new way for performing data analysis of the SAW winding tension sensor is provided.

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.

Nanostructured Silicon-Based Surface Acoustic Wave Sensor for CO2 Gas Detection

Surface acoustic wave (SAW) gas sensors with nanostructured material-based sensing layers are desirable in gas sensing owing to their high sensitivity, and their fast response and recovery time. In this work, a novel SAW gas sensor with a nanostructured silicon (Si)-based sensing layer with four pairs of aluminum-based input and output interdigital transducer (IDT) is developed and fabricated. A finite-element analysis (FEA) is used to determine the sensitivity while varying the height of the nanostructured Si. As a proof of concept, the SAW sensor is tested for carbon dioxide (CO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{2}}$ </tex-math></inline-formula> ) gas concentrations, ranging from 500 to 2000 ppm. The results demonstrate that the developed SAW sensor with a nanostructured Si-based sensing layer showed a maximum frequency shift of 4.62177 kHz with a response and a recovery time of 31 and 40.5 s, respectively, for a 2000-ppm CO2 gas concentration. The developed nanostructured Si as a sensing layer for the SAW gas sensor demonstrated higher sensitivity than previously reported devices and has promising potential to be used as a sensing layer for next-generation MEMS SAW gas sensors.

Printed Acoustic Sensor for Low Concentration Volatile Organic Compound Monitoring

Printed electronics employing flexible substrate offers prospective features for various applications such as, tactile sensing, energy harvesting, wearable electronics and acoustic wave sensors. In this work, an acoustic FPW (flexural plate wave) sensor is printed on thin and flexible PZT-PDMS (lead zirconate titanate-poly dimethyl siloxane) composite film with silver ink. The prototype FPW resonator has a resonant frequency of 22.65 MHz with an attenuation of -1.552 dBm. Gravimetric mass sensitivity of the sensor was measured by applying PDMS layers in between the input and output interdigital transducers (IDTs). The mass sensitivity was measured to be -7.8 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /g. The sensor is highly responsive to VOCs (volatile organic compounds) with PDMS as a sensing layer. Gas sensitivities with acetic acid and toluene concentrations were measured to be 0.66 and 160.63 kHz/ppm, respectively. The limit of detection for acetic acid and toluene were 10.9 and 0.03 ppm, respectively. Further, the sensor shows good repeatability and response time with both the VOCs. The thermal stability and high piezoelectric charge coefficient of PZT-PDMS composite compared to other piezoelectric composite and polymer substrates are advantageous for the flexible printed sensor. The reported FPW gas sensor shows potential for low concentration measurement of VOCs.

Finite element simulation for sensitivity measurement of a shear horizontal surface acoustic wave micro pressure sensor with a groove structure

Shear horizontal surface acoustic wave (SH-SAW) sensors have great application potential due to their advantages of easy integration, miniaturization and suitability in liquid environments. In this paper, the finite element method is used to analyse a new SH-SAW micro pressure sensor, in which there are many groove structures along the direction of wave propagation on the delay path. We use the transient response simulation method to calculate and analyse the output voltage signal at the output interdigital transducer and surface average stress at the delay path of this new SH-SAW sensor, and its pressure sensitivity is analysed by uniformly applying an appropriate surface pressure on the resonant beam formed after grooving. The simulation results show that the surface average stress can be enhanced in a certain range of groove depth during the vibration of the groove structure. When the groove depth and width are set to 0.7 μm and 0.5 μm, respectively, the sensitivity of the SH-SAW sensor with a groove structure is four times higher than that of the traditional SH-SAW sensor. The increase of pressure sensitivity is the result of the increase of average stress caused by the groove structure. The new groove structure SH-SAW sensor provides a new basis for research on high-sensitivity micro-pressure sensors and lays a foundation for subsequent device design and manufacture.

A Piezoelectric Flexural Plate Wave (FPW) Bio-MEMS Sensor with Improved Molecular Mass Detection for Point-of-Care Diagnostics

Abstract A piezoelectric FPW-sensor has been developed for a point of care device in this work. The Bio- MEMS FPW-sensor consists of an electrode configuration termed as an interdigital transducer (IDT) placed on a membrane. An input IDT excites and an output IDT detects the propagating acoustic waves through a PZT layer. Design optimizations and fabrication improvements of the FPW-sensor led to significantly reduced attenuation of the wave signal and the damping of the propagating waves between the IDTs. The working principle of mass loading is shown using different low-viscous liquids. A densitydependent sensitivity of -0.39 MHz/g/cm³ was evaluated. After the membrane was functionalized, the Bio-MEMS FPW-sensor was used to measure a specific chemokine in complex solution. By design improvements, the resolution was significantly increased from 0.7 Hz/nM to 14 Hz/nM.

Development of chipless and wireless underground temperature sensor system based on magnetic antennas and SAW sensor

A chipless and wireless underground sensor system was developed based on two magnetic coil antennas with magnetic cores and a surface acoustic wave (SAW) resonator sensor to monitor temperature variations around buried utilities from the ground. A ∼250 MHz alternating current from the magnetic antenna generates a SAW along the piezoelectric substrate, and the returned SAW energy owing to the reflection bars on the sensor is reconverted to magnetic flux by the sensor’s interdigital transducer (IDT) and subsequently transmitted to a reader via magnetic antenna. By observing changes in the center frequency of the SAW sensor with temperature, we were able to monitor the underground temperature variations in real time. The developed magnetic coil antenna has a large diameter Ni0.8Fe0.2 core wound by Cu coil, and a convex shape at the core end to converge the magnetic flux and enhance the readout distance. Two types of temperature sensors, a one-port SAW resonator and two-port SAW reflective delay line, were fabricated on a 128° YX LiNbO3 and their characteristics were compared in terms of soil temperature. SAW generation by magnetic antenna followed by SAW propagation along the piezoelectric substrate were each confirmed by the AC voltages derived at the output IDT on the SAW sensor, and the amplitude of the SAW was greatly dependent on the current applied to the coil. In soil testing, long readout distance between the reader and underground SAW sensor was observed. The temperature sensors provided stable performance in terms of underground temperature changes, soil type, and soil compactness at that readout distance. The resultant sensitivity and linearity for the SAW resonator temperature sensor was 0.3 MHz/ºC and 0.96, respectively. COMSOL and coupling of Mode (COM) modeling were also performed to find the optimal sensor system parameters and predict the results in advance.

Nano- and submicro-sized three-dimensional shape measuring system using a SAW-based capacitance sensor

SAW-based capacitance sensors were developed by measuring the changes in the voltage ratio between an output inter-digital transducer (IDT) and an input IDT, induced by the variation in the capacitance between hemi-spherical conductive tips with diameters on the order of micrometers. We derived the transfer and sensitivity functions, which describes the ratio of the variation in the output voltage to the variation in the input voltage with a 2-port network model of a SAW based on Mason’s equivalent electro-acoustic circuit and analytical model for the apex capacitance. We then conducted an experiment to verify and measure the transfer function and the 3-D shape. The system was found to have an average resolution of approximately 10 nanometers. We also used the constructed system to measure nano- and submicro-sized 3-D shapes, and verified that the measurement results were accurate.

Surface acoustic wave type electrode‐area‐weighted wavelet inverse‐transform processors with phase compensation

The main purpose of this research is to investigate a novel implementation method for a surface acoustic wave type (SAWT) electrode‐area‐weighted (EAW) wavelet inverse‐transform processor (WITP). The method of EAW is that the electrode areas of the input and output interdigital transducers (IDTs) are proportional to the envelope areas of the wavelet function (i.e. the two IDTs are identical). By this method, the SAWT EAW WITP is fabricated on X‐112°Y LiTaO3 substrate material. In the study, the diffraction problem and phase difference as two key problems are presented and the solution to two problems are implemented.

Development of an FPW Biosensor with Low Insertion Loss and High Fabrication Yield for Detection of Carcinoembryonic Antigen

In the last two decades, various flexural plate-wave (FPW)-based biosensors with low phase velocity, low operation frequency, high sensitivity, and short response time, have been developed. However, conventional FPW transducers have low fabrication yield because controlling the thickness of silicon/isolation/metal/piezoelectric multilayer floating thin-plate is difficult. Additionally, conventional FPW devices usually have high insertion loss because of wave energy dissipation to the silicon substrate or outside area of the output interdigital transducers (IDTs). These two disadvantages hinder the application of FPW devices. To reduce the high insertion loss of FPW devices, we designed two focus-type IDTs (fan-shaped and circular, respectively) that can effectively confine the launched wave energy, and adopted a focus-type silicon-grooved reflective grating structure (RGS) that can reduce the wave propagation loss. To accurately control the thickness of the silicon thin-plate and substantially improve the fabrication yield of FPW transducers, a 60 °C/27 °C two-step anisotropic wet etching process was developed. Compared with conventional FPW devices (with parallel-type IDTs and without RGS), the proposed FPW devices have lower insertion loss (36.04 dB) and higher fabrication yield (63.88%). Furthermore, by using cystamine-based self-assembled monolayer (SAM) nanotechnology, we used the improved FPW device to develop a novel FPW-based carcinoembryonic antigen (CEA) biosensor for detection of colorectal cancer, and this FPW-CEA biosensor has a low detection limit (5 ng/mL), short response time (<10 min), high sensitivity (60.16–70.06 cm2/g), and high sensing linearity (R-square = 0.859–0.980).

A mixing surface acoustic wave device for liquid sensing applications: Design, simulation, and analysis

This work presents the mixing wave generation of a novel surface acoustic wave (M-SAW) device for sensing in liquids. Two structures are investigated: One including two input and output interdigital transducer (IDT) layers and the other including two input and one output IDT layers. In both cases, a thin (1 μm) piezoelectric AlN layer is in between the two patterned IDT layers. These structures generate longitudinal and transverse acoustic waves with opposite phase which are separated by the film thickness. A 3-dimensional M-SAW device coupled to the finite element method is designed to study the mixing acoustic wave generation propagating through a delay line. The investigated configuration parameters include the number of finger pairs, the piezoelectric cut profile, the thickness of the piezoelectric substrate, and the operating frequency. The proposed structures are evaluated and compared with the conventional SAW structure with the single IDT layer patterned on the piezoelectric surface. The wave displacement along the propagation path is used to evaluate the amplitude field of the mixing longitudinal waves. The wave displacement along the AlN depth is used to investigate the effect of the bottom IDT layer on the transverse component generated by the top IDT layer. The corresponding frequency response, both in simulations and experiments, is an additive function, consisting of sinc(X) and uniform harmonics. The M-SAW devices are tested to assess their potential for liquid sensing, by dropping liquid medium in volumes between 0.05 and 0.13 μl on the propagation path. The interaction with the liquid medium provides information about the liquid, based on the phase attenuation change. The larger the droplet volume is, the longer the duration of the phase shift to reach stability is. The resolution that the output change of the sensor can measure is 0.03 μl.

A novel electrode‐area‐weighted method of implementing wavelet transform processor with surface acoustic wave device

SummaryThe objective of this research was to investigate the possibility of a novel electrode‐area‐weighted (EAW) method of implementing wavelet transform processor (WTP) with surface acoustic wave (SAW) device. The motivation for this work was prompted by a diffraction problem of the WTP using SAW device. In this paper, we propose a novel EAW method in order to solve the diffraction problem. When the electrode areas of the EAW wavelet interdigital transducer (IDT) (i.e. the input IDT) are designed according to the envelope areas of the wavelet function, the impulse‐response function of the EAW wavelet IDT is equal to the wavelet function, so that the novel EAW WTP using SAW device can be fabricated. In this study, we also present the diffraction problem, the substrate material, and the electrode number of the output IDT as three key problems, and the solutions to the three key problems are implemented. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Fabrication and characterization of highly sensitive ZnO/Si SAW device with Pd selective layer for F2 gas sensing

In this paper, ZnO nano thin film is prepared on n-type Si substrate with orientation using RF sputtering technique and a surface acoustic wave (SAW) based fluorine (F2) gas sensor integrated on ZnO with Palladium (Pd) on top as sensing layer is fabricated and characterized. The input and output interdigital transducer is fabricated of aluminum (Al) using nanolithography technique. The morphological characterization of ZnO thin film is done using scanning electron microscopy and atomic force microscopy. The XRD confirms the formation of ZnO nanostructured thin film. The voltage handling capability of the fabricated device is evaluated with negative and positive bias voltage. The study confirms that the SAW sensor based on ZnO/Si substrate and Pd sensing layer has good characteristic response of F2 gas with different concentration of gas from 0.5 to 20 ppm. The sensor shows good reproducibility and its sensitivity is approximately 2.4 kHz ppm?1.

Crosstalk suppression of CMOS compatible AlN based SAW devices on low resistive Si(100)

AbstractAluminum nitride (AlN) based high frequency surface acoustic wave (SAW) devices are produced by using a new CMOS compatible concept. Textured c‐axis oriented AlN films have been deposited on tungsten based embedded interdigital transducers (IDTs) by the reactive pulse DC‐sputter technique within the thermal budget of back end of line (BEOL) processes. Two SAW Rayleigh modes with different acoustic velocities were measured and identified by finite element method (FEM) simulation (Kaletta and Wenger, FEM Simulation of Rayleigh waves for CMOS compatible SAW devices based on AlN/SiO2/Si(100) (Elsevier Ultrasonics, available online 18 April 2013, ISSN 0041‐624X, 10.1016/j.ultras.2013.04.009) [1]). Electromagnetic crosstalk between in‐ and output IDTs via low resistive Silicon substrates is one of the most troublesome sources of integrated SAW devices. The impact of several device parameters like the acoustic aperture, the number of fingers and the delay line length on the electromagnetic crosstalk was studied and routes for optimizing SAW filter designs on low resistive Si(100) will be presented. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The interdigital transducer and its response in the case of the impact of the inhomogeneous surface acoustic wave

In the paper, the influence of the inhomogeneous surface acoustic wave and the tilt and shift of output interdigital transducer on the transmitted power coefficient and efficiency, is analyzed.

Development of a high sensitive pH sensor based on shear horizontal surface acoustic wave with ZnO nanoparticles

This study proposes a new pH sensor with a 230MHz center frequency based on surface acoustic wave (SAW). It consists of piezoelectric substrate, input and output interdigital transducer (IDT) and ZnO Nanoparticles (NPs) used for sensitive layer. The change in conductivity of ZnO NPs induced by pH solutions affects SAW velocity and attenuation. By measuring the frequency shift induced by change in SAW velocity, we can determine the pH value in a target solution. To improve sensitivity and stability of the sensor, we used ZnO Nanoparticles (NPs) and shear horizontal wave. ZnO NPs has some advantages over ZnO films, such as low price and simple fabrication procedure. The SH wave is suitable for layered structure and measurement in liquid. An optimal device structure was designed using coupling of mode (COM) analysis. Depending on the different pH values, a change in the center frequency was clearly observed. The measured frequency shift and linearity between pH 7 to pH 2 of the sensor with 1μm thick ZnO NPs were 144kHz and 0.957, respectively. In addition, the developed sensor exhibited good chemical resistance and pH stability.

Development of a high-sensitivity strain measurement system based on a SH SAW sensor

A strain measurement system based on a shear horizontal surface acoustic wave (SH SAW) was developed. The developed system is composed of a SAW microsensor, a printed circuit board (PCB), an adhesive and a strain gauge. When a compression force is applied to the PCB by the strain gauge, the PCB is bent so that external strain energy can be evenly delivered to the microsensor without any detachment of the sensor from the board. When a stretching force is applied to the PCB under the condition that one side of the PCB is fixed and the other side is modulated, the actual length of the SAW delay line between the two interdigital transducers (IDTs) is increased. The increase in the delay line length causes a change in the time for the propagating SAW to reach the output IDT. If strain energy is applied to the piezoelectric substrate, the substrate density is changed, which then changes the propagation velocity of the SAW. Coupling-of-modes modeling was conducted prior to fabrication to determine the optimal device parameters. Depending on the strain, the frequency difference was linearly modulated. The obtained sensitivity for stretching was 17.3 kHz/% for the SH wave mode and split electrode. And the obtained sensitivity for bending was 46.1 kHz/% for the SH wave mode and split electrode. The SH wave showed about 15% higher sensitivity than the Rayleigh wave, and the dog-bone PCB showed about 8% higher sensitivity than the rectangular PCB. The obtained sensitivity was about five times higher than that of existing SAW-based strain sensors.

Carbon Dioxide Gas Sensor Using SAW Device Based on ZnO Film

Carbon dioxide (CO2) gas sensor using surface acoustic wave (SAW) device based on Zinc oxide (ZnO) was developed and fabricated in this paper. The center frequency of SAW device is 203.528 MHz. The input and two output interdigital transducers (IDT) apodized by Morlet wavelet function can improve the side lobe rejection compared with uniform IDT. The ZnO film sensitive to CO2 gas was fabricated in measurement acoustic track of SAW device. Experiments results confirm that the CO2 gas sensor using SAW device based on ZnO film has good response characteristics to different concentrations CO2 gas. Furthermore, the CO2 gas sensor using SAW device based on ZnO film has good stability and linearity.

Frequency Response Optimization of Saw Filter using Canonical Genetic Algorithm

An optimization program is employed to design the inter-digital transducer (IDT) structure of the surface acoustic wave (SAW) transducer to aim at improvement in the transducer characteristics and to improve the frequency response of SAW filter. In this paper, Genetic algorithm is used to optimize the response of a SAW filter. Then, the comparison of the response of SAW filter is done by using GA and without GA. The goal of optimization is to find best value for each variable in order to achieve minima or maxima of an objective function within the given constraints. In present study, three design variables of SAW filter i.e. number of finger pairs in input IDT (Np1), number of finger pairs in `output IDT (Np2), center to center distance between two electrodes in a finger pair (d), are optimized. After getting the optimized parameters of SAW filter, the frequency response of bandpass SAW filter is obtained, which is better as compared to the frequency response of SAW filter getting from other methods in terms of bandwidth and ripples amplitude.