Surface Acoustic Wave Structures Research Articles

Light-activated SAW sensor structures with photoconductive polymer films for DMMP detection

We report the light-activated surface acoustic wave (SAW) investigations of DMMP (dimethyl methylphosphonate) sensing based on photoconductive polymer films such as: (rr)P3HT (regio-regular Poly-3-hexylthiophene) and SilPEG1.4. These SAW sensor structures were composed of ∼205 MHz quartz dual delay lines with a novel switchable digital generator set-up and a thin (∼50–100 nm) polymer films created by means of a simple spray coating method. The white light, low power laser LED (blue, green, red) and seven LED’s sources were applied for light activation of the SAW structures. The relative sensitivity increase (frequency shifts in light vs. the non activated state) at 2 ppm of DMMP is ∼275 for SilPEG1.4 with a blue LED (∼460 nm), to even ∼340 for (rr)P3HT with a yellow LED (∼585 nm) activation. This last structure has also acceptable stability and repeatability with the limit of detection ∼133 ppb, the response/recovery times ∼5–100 s / ∼240 s, and sensitivity coefficient ∼1.013 kHz/ppm in ppb ranges. The observed substantially enhanced light-activated SAW sensitivity should be interpreted in category of the acoustoelectric interactions, because the acoustoelectric parameter, estimated by resistive transducer results, has the greatest value for a yellow and blue LED light illumination.

Design and analysis of high-sensitivity multilayered structure surface acoustic wave sensor for dichloromethane gas detection

Micro- and nano-sensors are key devices used in various fields from medical to environmental science. With the progress of micro- and nano-fabrication technology and the use of newly designed materials, sensor design has been significantly improved, leading to the development of high-performance gas sensors. A novel multilayer structure gas sensor based on surface acoustic wave (SAW) is proposed for the detection of volatile harmful gas dichloromethane. The gas sensing characteristics of single-layer and multilayer SAW sensors were studied by using finite element method. The influence of geometric parameters on the sensor performance was analyzed and the optimal structure was determined. The propagation characteristics of SAW were obtained by transient simulation of the whole structure. The sensitivity of the sensor is evaluated by placing the sensor in different concentrations of dichloromethane gas. The results show that the sensitivity of the PIB/AlN/Diamond/Si SAW gas sensor is as high as 66.9 ppm/Hz under the optimal geometry structure, which is nearly 30 times higher than that of other single-layer and multilayer SAW sensors. This work provides a good solution for the design of high-sensitivity SAW sensors.

The Behavior of Gold Metallized AlN/Si- and AlN/Glass-Based SAW Structures as Temperature Sensors.

Thin AlN piezoelectric layers have been deposited on high resistivity Si and glass substrates by reactive RF magnetron sputtering, in order to manufacture one-port gigahertz operating surface acoustic wave (SAW)-type resonators to be used as temperature sensors. The growth morphology surface topography, crystallographic structure, and crystalline quality of the AlN layers have been analyzed. Advanced nanolithographic techniques have been used to manufacture structures having interdigitated transducers with fingers and finger interdigit spacing width in the range of 250-170 nm. High resonance frequency ensures the increase of the sensitivity, but also of its normalized value, the temperature coefficient of frequency (TCF). The resonance frequency shift versus temperature has been measured in the -267°C-+150°C temperature range, using a cryostat setup adapted for on wafer microwave measurements up to 50 GHz. The sensitivity and the TCF were determined in the 25 °C-150 °C temperature range.

Electrical Performances of a Surface Acoustic Wave Device With Inter-Digital Transducers Electrodes in Local Resonances

Abstract In this paper, we investigate numerically the coupling of the Rayleigh mode with the micro-wall resonance modes in inter-digital transducers (IDTs) electrodes of surface acoustic wave (SAW) devices. We perform a finite element analysis (FEA) of the SAW features using an implemented model using comsol Multiphysics® software. The SAW structure comprises identical transmitter and receiver IDTs electrodes, with different electrode heights (he). The proposed FEA study is based on the extraction of reflection (S11) and transmission (S21) coefficients of the SAW device. The IDTs are considered to be a micro-wall phononic crystal acting as local resonators at frequencies inside the SAW passband. The locally resonance gap is strongly dependent on the he value, and S11 and S21 parameters are affected by the SAW energy absorption in the IDTs system. We have chosen two he values (0.5 and 3 µm) to study low and high aspect ratios of micro-walls, corresponding respectively to Bragg-type and resonance-type bandgaps appearing near the SAW central frequency. At the SAW resonance frequency, the return (S11) and the insertion (S21) losses are reduced. S21 is reduced by 12.73 and 18.49 dB for he = 0.5 and 3 µm, respectively, accompanied by an increase in the quality factor, and S11 parameter is reduced by 1.357 and 4.98 dB for he = 0.5 and 3 µm, respectively.

Surface Acoustic Wave Characteristics with a Layered Structure of IDT/θ° YX-LiTaO3/SiO2/AlN/diamond

A new layered surface acoustic wave (SAW) structure was proposed by Murata Co., Ltd recently. It was reported that such structure could achieve an incredible high performance including a higher quality factor Q and electromechanical coupling coefficient K2. For deeply understanding propagating characteristics and optimizing the performance of the SAWs in such structure, a layered structure of IDT/θ° YX-LiTaO3/SiO2/AlN/Diamond with different structural parameters were theoretically investigated by FEM method. The calculated admittance shows that four eigenmodes simultaneously exist in such layered structure including the main mode SH SAW. And compared with the Traditional SAW structure, the main mode could achieve a higher value of K2 with sacrificing its velocity a little. Different metal layers (Au, Al, and Cu) were examined as the electrode material. With Au employed, the K2 of the main mode is a little larger resulting from better suppression of the spurious modes. The optimum thickness of electrode and piezoelectric layer are 0.2 and 0.02λ, respectively. In this case, the K2 for the SH SAW achieves its maximum value of 12.20% with a large phase velocity of 3608 m/s. Furthermore, the pure SH SAW can be obtained with the Y-cut Euler angle θ from 0° to 60°, where its K2 has a wide range of 8.70 to 13.69%. Consequently, the work provides a theoretical guide for designing SAW devices of different bandwidth and operation frequency with such structure.

Microfluidic Acoustic Metamaterial SAW Based Sensor

Introduction. Microacoustic sensors based on surface acoustic wave (SAW) devices allow the sensor integration into a wafer based microfluidic analytical platforms such as lab-on-a-chip. Currently exist various approaches of application of SAW devices for liquid properties analysis. But this sensors probe only a thin interfacial liquid layer. The motivation to develop the new SAW-based sensor is to overcome this limitation. The new sensor introduced here uses acoustic measurements, including surface acoustic waves (SAW) and acoustic methamaterial sensor approaches. The new sensor can become the starting point of a new class of microsensor. It measures volumetric properties of liquid analytes in a cavity, not interfacial properties to some artificial sensor surface as the majority of classical chemical and biochemical sensors.Objective. The purpose of the work is to find solutions to overcome SAW-based liquid sensors limitations and the developing of a new sensor that uses acoustic measurements and includes a SAW device and acoustic metamaterial.Materials and methods. A theoretical analysis of sensor structure was carried out on the basis of numerical simulation using COMSOL Multiphysics software. Lithium niobate (LiNbO3) 127.86° Y-cut with wave propagation in the X direction was chosen as a substrate material. Microfluidic structure was designed as a set of rectangular shape channels. A method for measuring volumetric properties of liquids, based on SAW based fluid sensor concept, comprising the steps of: (a) providing sensor structure with the key elements: a SAW resonator, a high-Q set of liquid-filled cavities and intermediate layer with artificial elastic properties between them; (b) measuring of resonance frequency shift, associated with the resonance in liquid-filled cavity, in the response of weakly coupled resonators of SAW resonator loaded by periodic microfluidic structure; (c) determination of volumetric properties of the fluid on the basis of a certain relationship between the speed of sound in liquid, the resonant frequency of the set of liquid-filled cavities, and the geometry design of the cavity.Results. The new sensor approach is introduced. The eigenmodes of the sensor structure with a liquid analyte are carried out. The characteristic of sensor structure is determined. The key elements of introduced microfluidic sensor are a SAW structure, an acoustic metamaterial with a periodic set of microfluidic channels. The SAW device acts as electromechanical transducer. It excites surface waves propagating in the X direction lengthwise the periodic structure and detects the acoustic load generated by the microfluidic structure resonator. The origin of the sensor signal is a small frequency change caused by small variations of acoustic properties of the analyte within the set of microfluidic channels.Conclusion. The principle of the new microacoustic sensor, which can become the basis for creating a new class of microfluidic sensors, is shown.

Non-leaky longitudinal acoustic modes in ScxAl1-xN/sapphire structure for high-temperature sensor applications

Multilayered structures based on wide bandgap nitride piezoelectric thin films are very attractive for high-temperature surface acoustic wave (SAW) sensor applications. In this respect, scandium aluminum nitride (ScAlN) films are of particular interest as they combine enhanced piezoelectric properties and slower acoustic wave velocities when the Sc content steadily increases up to 40%. This property offers the possibility to combine slow ScAlN films on fast substrates like sapphire, to generate higher-order SAW modes which often show a better electromechanical coupling coefficient k2 compared to zero-order modes. In this letter, we show that low-attenuated longitudinal SAW can be generated in the ScxAl1-xN/sapphire structure, for the x parameter varying in a large range. This theoretical result is then confirmed by the experimental investigation of SAW resonators based on highly textured (002) Sc0.09Al0.91N films sputtered on c-cut sapphire substrates. It is finally shown that the use of electrodes based on metals with high density can lead to SAW structures offering a unique combination between a large bandgap over 5 eV, a k2 value beyond 1%, and a high SAW velocity near 10 000 m/s.

A Fast Optimization Algorithm of FEM/BEM Simulation for Periodic Surface Acoustic Wave Structures

The accurate analysis of periodic surface acoustic wave (SAW) structures by combined finite element method and boundary element method (FEM/BEM) is important for SAW design, especially in the extraction of couple-of-mode (COM) parameters. However, the time cost is very large. With the aim to accelerate the calculation of SAW FEM/BEM analysis, some optimization algorithms for the FEM and BEM calculation have been reported, while the optimization for the solution to the final FEM/BEM equations which is also with a large amount of calculation is hardly reported. In this paper, it was observed that the coefficient matrix of the final FEM/BEM equations for the periodic SAW structures was similar to a Toeplitz matrix. A fast algorithm based on the Trench recursive algorithm for the Toeplitz matrix inversion was proposed to speed up the solution of the final FEM/BEM equations. The result showed that both the time and memory cost of FEM/BEM was reduced furtherly.

SAW propagation characteristics of TeO3/3C-SiC/LiNbO3 layered structure

Surface acoustic wave (SAW) devices based on Lithium Niobate (LiNbO3) single crystal are advantageous because of its high SAW phase velocity, electromechanical coupling coefficient and cost effectiveness. In the present work a new multi-layered TeO3/3C-SiC/128° Y-X LiNbO3 SAW device has been proposed. SAW propagation properties such as phase velocity, coupling coefficient and temperature coefficient of delay (TCD) of the TeO3/SiC/128° Y-X LiNbO3 multi layered structure is examined using theoretical calculations. It is found that the integration of 0.09λ thick 3C-SiC over layer on 128° Y-X LiNbO3 increases its electromechanical coupling coefficient from 5.3% to 9.77% and SAW velocity from 3800 ms−1 to 4394 ms−1. The SiC/128° Y-X LiNbO3 bilayer SAW structure exhibits a high positive TCD value. A temperature stable layered SAW device could be obtained with introduction of 0.007λ TeO3 over layer on SiC/128° Y-X LiNbO3 bilayer structure without sacrificing the efficiency of the device. The proposed TeO3/3C-SiC/128° Y-X LiNbO3 multi-layered SAW structure is found to be cost effective, efficient, temperature stable and suitable for high frequency application in harsh environment.

GaN Membrane Supported SAW Pressure Sensors With Embedded Temperature Sensing Capability

This paper presents the fabrication and characterization of a GHz operating surface acoustic wave (SAW)-based pressure sensor on a 1.2- $\mu \text{m}$ -thin GaN membrane. Two types of interdigitated transducers are manufactured using electron beam nanolithography to obtain finger and interdigit spacing widths, one with 170 nm and the other 200-nm-half pitch. Micromachining techniques are used to obtain the 1.2- $\mu \text{m}$ -thin membrane. The resonance frequency shift of the SAW, the pressure sensitivity, $s_{p}$ , as well as the pressure coefficient of frequency (PCF), were experimentally determined and analyzed, both for the Rayleigh as well as for the symmetrical Lamb propagation mode, in the 1 to 7 Bar pressure range. Record values for $s_{p}$ (up to 6 MHz/Bar) and PCF (up to 537 ppm/Bar) have been obtained, especially for the symmetrical Lamb propagation mode also due to the very high frequency operation (5–11.5 GHz). The effect of different orientations of the SAW device (in the $\textrm {[1}\bar {\textrm {1}} \textrm {00]}$ and $\textrm {[11}\bar {\textrm {2}} \textrm {0]}$ directions) on the frequency response and sensitivity is also analyzed. The possibility to determine simultaneously the pressure and the temperature with the same SAW structure operating as a dual sensor has been demonstrated.

Influence of Au-Based Metallization on the Phase Velocity of GaN on Si Surface Acoustic Wave Resonators

This letter presents a comparison between the phase velocity of Al and Ti/Au metallized GaN/Si-based surface acoustic wave structures. The resonance frequency (and consequently the phase velocity) is higher for Ti/Au-based metallization, especially when the acoustic wavelength has comparable values with the GaN layer thickness. Simulation results are in excellent agreement with the experiment. For example, the phase velocity on a 1- $\mu \text{m}$ -thin GaN layer with 200-nm finger/interdigit spacing width of the interdigitated transducer (IDT) is 3707 m/s (simulated value: 3840 m/s) for 100-nm-thin Al metallization and 4360 m/s (simulated value: 4240 m/s) for Ti/Au 5-/95-nm-thin metallization. This behavior was explained considering the effects of the significantly higher value of the acoustic impedance of Au compared with Al. The simulated mode shapes have confirmed this explanation. The advantage of the Ti/Au metallization lies in the possibility to obtain a higher resonance frequency for the same topology of the IDT.

Development and experimental verification of a finite element method for accurate analysis of a surface acoustic wave device

Accurate analysis of surface acoustic wave (SAW) devices is highly important due to their use in ever-growing applications in electronics, telecommunication and chemical sensing. In this study, a novel approach for analyzing the SAW devices was developed based on a series of two-dimensional finite element method (FEM) simulations, which has been experimentally verified. It was found that the frequency response of the two SAW device structures, each having slightly different bandwidth and center lobe characteristics, can be successfully obtained utilizing the current density of the electrodes via FEM simulations. The two SAW structures were based on XY Lithium Niobate (LiNbO3) substrates and had two and four electrode finger pairs in both of their interdigital transducers, respectively. Later, SAW devices were fabricated in accordance with the simulated models and their measured frequency responses were found to correlate well with the obtained simulations results. The results indicated that better match between calculated and measured frequency response can be obtained when one of the input electrode finger pairs was set at zero volts and all the current density components were taken into account when calculating the frequency response of the simulated SAW device structures.

Sezawa Propagation Mode in GaN on Si Surface Acoustic Wave Type Temperature Sensor Structures Operating at GHz Frequencies

This letter investigates the applicability of Sezawa propagation mode for various GaN/Si surface acoustic wave (SAW) temperature sensor structures. First, different acoustic propagation modes in GaN on Si single SAW resonators, operating at GHz frequencies, were evaluated. The variation of the phase velocity versus the normalized thickness of the GaN layer is analyzed experimentally for different propagation modes, on the SAW structures manufactured using e-beam lithography. The different acoustic modes were also identified using finite-element method calculations. The effective coupling coefficients for the Sezawa mode are determined and show larger values than those obtained for the Rayleigh mode. The sensitivities obtained for the temperature sensor structures are 1.8 times higher for Sezawa than for the fundamental Rayleigh mode.

GaN/SiC based surface acoustic wave structures for hydrogen sensors with enhanced sensitivity

In this article, we present an application of GaN/SiC heterostructures for Surface Acoustic Wave (SAW) hydrogen sensors with enhanced sensitivity. We demonstrate that the mass-loading sensitivity of such sensors can be increased by using shorter acoustic wavelengths and by confining the acoustic wave energy in an epitaxial waveguide. The effect of different Rayleigh-like modes on the mass-loading sensitivity is also discussed. The SAW wavelength-dependent phase and group velocity and electromechanical coupling constants were determined from electrical measurements. The outcome was used to design a microwave SAW delay-line oscillator. Its sensing characteristics are presented. The oscillator frequency shift was larger than 60kHz after exposure to 1000ppm H2/N2. The sensor reaction time was about 12s for the same sensing conditions at room temperature.

Numerical and Experimental Analysis of the Response of a SAW Structure with WO3 Layers on Action of Carbon Monoxide

Abstract The paper presents the results of an analysis of gaseous sensors based on a surface acoustic wave (SAW) by means of the equivalent model theory. The applied theory analyzes the response of the SAW sensor in the steady state affected by carbon monoxide (CO) in air. A thin layer of WO3 has been used as a sensor layer. The acoustical replacing impedance of the sensor layer was used, which takes into account the profile of the concentration of gas molecules in the layer. Thanks to implementing the Ingebrigtsen equation, the authors determined analytical expressions for the relative changes of the velocity of the surface acoustic wave in the steady state. The results of the analysis have shown that there is an optimum thickness of the layer of CO sensor at which the acoustoelectric effect (manifested here as a change in the acoustic wave velocity) is at its highest. The theoretical results were verified and confirmed experimentally

On-chip temperature-compensated Love mode surface acoustic wave device for gravimetric sensing

Love mode surface acoustic wave (SAW) sensors have been recognized as one of the most sensitive devices for gravimetric sensors in liquid environments such as bio sensors. Device operation is based upon measuring changes in the transmitted (S21) frequency and phase of the first-order Love wave resonance associated with the device upon on attachment of mass. However, temperature variations also cause a change in the first order S21 parameters. In this work, shallow grooved reflectors and a “dotted” single phase unidirectional interdigitated transducer (D-SPUDT) have been added to the basic SAW structure, which promote unidirectional Love wave propagation from the device's input interdigitated transducers. Not only does this enhance the first-order S21 signal but also it allows propagation of a third-order Love wave. The attenuation coefficient of the third-order wave is sufficiently great that, whilst there is a clear reflected S11 signal, the third-order wave does not propagate into the gravimetric sensing area of the device. As a result, whilst the third-order S11 signal is affected by temperature changes, it is unaffected by mass attachment in the sensing area. It is shown that this signal can be used to remove temperature effects from the first-order S21 signal in real time. This allows gravimetric sensing to take place in an environment without the need for any other temperature measurement or temperature control; this is a particular requirement of gravimetric biosensors.

GaN/Si based single SAW resonator temperature sensor operating in the GHz frequency range

The paper presents the manufacturing and characterization of GaN based SAW (surface acoustic wave) type temperature sensors. In contrast with most SAW sensor structures, manufactured on classical piezoelectric materials where delay lines or two port resonators have been used, in this paper, the resonance frequency shift vs. temperature for a single resonator structure was used for temperature measurements. It is demonstrated that the single resonator SAW structures ensure better performances in terms of sensitivity and losses compared with two port resonator structures. The sensor structure was manufactured using deep submicron e-beam nano-lithographic process on GaN/Si (finger width and interdigit spacing of 200nm) and resonance frequencies higher than 5GHz have been obtained at room temperature. The high resonance frequency ensures an increase of the sensitivity of the sensor structure. The sensor was characterized by “on wafer” resonance frequency shift and sensitivity measurements in the 23–150°C temperature range using S11 reflection parameter. The resonance frequency shift vs. temperature and the sensitivity for the sensor structures assembled on a special ceramic carrier, were measured, in a cryostat, in the −268–+150°C temperature range. Sensitivities higher than 300kHz/°C, (corresponding to values higher than 50ppm/°C) have been obtained.

Local temperature determination in power loaded surface acoustic wave structures using Raman spectroscopy

The temperature at the surface of surface acoustic wave (SAW) devices is a critical parameter not only for their design but also for the understanding of failure mechanisms like acoustomigration. We report about a contactless measurement method using Raman spectroscopy for the temperature determination on interdigital transducers (IDTs) of SAW test structures. A power load of up to 3.5 W/mm2 at an operating frequency of 130 MHz was used to generate travelling Rayleigh waves on a lithium niobate 128° YX substrate. Here, the local temperature on the surface between the finger electrodes of the IDT increases by 60 K. In addition to Raman spectroscopy, the reversible shift of the resonance frequency of the SAW test structure was used to determine and calibrate the local temperature of external loaded samples. These experiments show good agreement with temperatures obtained by Raman spectroscopy based evaluations.

Simulation of Characteristics of ZnO/Diamond/Si Surface Acoustic Wave

Based on the recursive stiffness matrix method, the effective surface permittitivity model of multilayered Surface Acoustic Wave structure was established. By this model the phase velocity dispersive of ZnO/Si layered structure was calculated. The calculation results is in agreement with the experimental results ,which verifies effectiveness and accuracy of the model. Furthermore, the model was also carried out for the determination of the phase velocity and electromechanical coupling coefficients of ZnO/Diamond/Si structure. The best combination of high velocity and high coupling coefficient of the structure was obtained, which provided a good reference for the design of the high performance and high capability Surface Acoustic Wave device.

Simulation of characteristics of ZnO/diamond/Si structure surface acoustic wave

Based on the recursive stiffness matrix method, the effective surface permittitivity model of multilayered surface acoustic wave structure is established. By this model, the frequency dispersive characteristic of phase velocity for the ZnO/Si layered structure is calculated. The calculation results are in agreement with the experimental results, which verifies the effectiveness and accuracy of the model. Furthermore, the model is also established for the determination of the phase velocity and electromechanical coupling coefficients of ZnO/Diamond/Si structure. The best combination of high velocity and high coupling coefficient of the structure is obtained, which provides a good reference for the design of a high-performance and high-capability surface acoustic wave device.