Love Wave Devices Research Articles

FEM Modeling Strategies: Application to Mechanical and Dielectric Sensitivities of Love Wave Devices in Liquid Medium.

This paper presents an extended work on the Finite Element Method (FEM) simulation of Love Wave (LW) sensors in a liquid medium. Two models are proposed to simulate the multiphysical response of the sensor. Both are extensively described in terms of principle, composition and behavior, making their applications easily reproducible by the sensor community. The first model is a Representative Volume Element (RVE) simulating the transducer and the second focuses on the sensor's longitudinal (OXZ) cut which simulates the multiphysical responses of the device. Sensitivity of the LW device to variations in the rheological and dielectric properties of liquids is estimated and then compared to a large set of measurements issued from LW sensors presenting different technological characteristics. This integral approach allows for a deeper insight into the multiphysical behavior of the LW sensor. This article also explores the advantages and drawbacks of each model. Both are in good accordance with the measurements and could be used for various applications, for which a non-exhaustive list is proposed in the conclusion.

Development of Love Wave-Based Ice Sensor Incorporating a PDMS Micro-Tank

A Love wave-based sensing chip incorporating a polydimethylsiloxane (PDMS) micro-tank was proposed for ice sensing. The waveguide effect in the SU-8/ST-90°X quartz structure confines the acoustic energy in the SU-8 thin film, which is beneficial to improving the mass loading sensitivity and reducing longitudinal coupling attenuation in liquid. Self-compensation in temperature effect can be achieved by using the reverse polarity of the temperature coefficient (TCF) of SU-8 and ST-90°X. The micro-tank made of PMDS material was bonded on the acoustic wave propagation path to protect the interdigital electrodes and provide a sensitive area. Coupling of models (COM) and finite-element method (FEM) were employed to extract the optimal design parameters allowing low insertion loss of Love wave device, and the influence of PDMS toward Love wave propagation was also analyzed theoretically to determine the PDMS design. Using the standard photolithographic technique and molding method, a Love wave-based ice sensing chip was prepared to operate at 200 MHz and characterized by being connected to the amplitude detection circuit, and the corresponding icing sensing characteristics were studied. High sensitivity (11.894 mV/mm) and fast response (170 ms) were successfully achieved.

Sensitivity of Love surface waves to mass loading

The sensitivity of the phase velocity vp of Love surface waves to mass loading is a very important characteristic of Love wave devices. In this paper, we present a novel approach to evaluate the sensitivity of Love surface waves to loading with an infinitesimal layer of mass of a surface density σkg/m2. To this end, the we developed analytical formulas for the mass coefficient of sensitivity Sσvp=(1/vp)dvp/dσ [m2/kg] and phase velocity gradients −dvp(f)/df and −dvp(h1)/dh1, where f and h1 stand, respectively, for frequency of the Love wave and thickness of the guiding surface layer. We also established analytical formulas that relate the mass sensitivity Sσvp with 1) the relative slope (gradient) −(1/vp)dvp/dh1 of the phase velocity dispersion curve vp(h1), and 2) the relative slope (gradient) −(1/vp)dvp/df of the phase velocity dispersion curve vp(f). These analytical formulas have been developed using full wave theory. We have discovered that the maxima of the mass sensitivity Sσvpf, Sσvph1 and maxima of the relative gradients (-(1/vp)dvp)⁄df, −(1/vp)dvp/dh1, occur virtually at the same values of fandh1. Comparing with the Perturbation Method and Finite Element Method (FEM), the analytical formulas established in this paper display some advantages, such as very low execution time of the mass sensitivity , and perhaps more importantly a possibility for a direct parametric optimization of the Love wave waveguide as a function of its material parameters, thickness of the guiding surface layer and wave frequency .

Finite element simulation of Love wave sensor for the detection of volatile organic gases

The three-dimensional (3D) finite element (FE) simulation and analysis of Love wave sensors based on polyisobutylene (PIB) layers/SiO2/ST-90°X quartz structure are presented in this paper, as well as the investigation of coupled resonance effect on the acoustic properties of the devices. The mass sensitivity of the basic Love wave device with SiO2 guiding layers is solved analytically. And the highest mass sensitivity of 128 m2/kg is obtained as h s/λ = 0.175. The sensitivity of the Love wave sensors for sensing volatile organic compounds (VOCs) is greatly improved due to the presence of coupled resonance induced by the PIB nanorods on the device surface. The frequency shifts of the sensor corresponding to CH2Cl2, CHCl3, CCl4, C2Cl4, CH3Cl and C2HCl3 with the concentration of 100 ppm are 1.431 kHz, 5.507 kHz, 13.437 kHz, 85.948 kHz, 0.127 kHz and 17.879 kHz, respectively. The viscoelasticity influence of the sensitive material on the characteristics of SAW sensors is also studied. By taking account of the viscoelasticity of the PIB layers, the sensitivities of the SAW sensors with the PIB film and PIB nanorods decay in different degree. The gas sensing property of the Love wave sensor with PIB nanorods is superior to that of the PIB films. Meanwhile, the Love wave sensors with PIB sensitive layers show good selectivity to C2Cl4, making it an ideal selection for gas sensing applications.

Drop-casted Graphene Oxide Love wave sensor for detection of humidity and VOCs

The rising demand for sensitive analytical techniques have led to a great deal of research interest in the recent years, which has also generated much public awareness of health risks caused by environmental humidity and air pollution. This study reports advancements in highly-sensitive detection methods using Love wave devices fabricated from Graphene Oxide (GO). Under this, solutions of GO were prepared and used to achieve improved efficiency of the oxidation process and enhance the sensitivity of Relative Humidity (RH) and Volatile Organic Compounds (VOCs) detections. This work demonstrates that the detection limit of RH could be set very low due to the certain sensitivity levels. Similarly, GO prepared by us exhibited low detection limit for VOCs, proving the multi-functionality of GO rather than alternative sensing materials. The experiments conducted at room temperature and realized fast response and recovery times. This Love wave sensor provides high accuracy under full scale exposure of target analytes. Eventually, these ultrasensitive GO based devices can pave the way for a wide range of high-sensitivity detection applications.

Magnetic anisotropy controlled FeCoSiB thin films for surface acoustic wave magnetic field sensors

Surface acoustic wave magnetic field sensors based on guided Love waves using the ΔE effect of a magnetostrictive thin film have been shown to be promising candidates for the measurement of weak fields at low frequencies as required for biomagnetic applications or as current sensors benefitting from the large dynamic range and bandwidth. The deposition of soft magnetic films with high magnetostriction is, however, more challenging on piezoelectric substrates such as quartz than on silicon. Thermally induced anisotropic expansion during the deposition process or during post-deposition magnetic field annealing leads to uniaxial stresses acting on the films, which makes the precise control of magnetic anisotropy difficult. Accordingly, this work analyzes the influence of the deposition process and heat treatment on the performance of Love wave devices. ST-cut quartz based delay line surface acoustic wave sensors with a SiO2 guiding layer are employed, and a 200 nm layer of amorphous magnetostrictive (Fe90Co10)78Si12B10 is used as the sensitive element. Magneto-optical imaging is performed for magnetic domain characterization, and the sensor performance is characterized in terms of bias field dependent phase sensitivity and frequency dependent phase noise. By performing a low temperature deposition in an external magnetic field, considerable improvement in limits of detection at biomagnetic relevant frequencies down to 70 pT/Hz at 10 Hz and 25 pT/Hz at 100 Hz is achieved.

A Weak Form Nonlinear Model for Thermal Sensitivity of Love Wave Mode on Layered Structures.

A precise theoretical model for the thermal sensitivity of Love wave mode is significant in the structure design, temperature compensation, and the prediction of thermal behavior. This article proposes a weak form nonlinear model to calculate the thermal sensitivity of Love waves on arbitrary layered structures. The third-order material constants, as well as the thermal stress and strain tensors between the substrate, electrodes, and wave-guiding layer, are considered in the model. The 9 ×9 effective elastic and the 3 ×9 effective piezoelectric matrixes are imported into the nonlinear constitutive equations and boundary conditions using weak form expressions. A temperature-compensated Love wave mode resonator on a layered ZnO/interdigital transducer (IDT)/quartz structure is obtained. The theoretical model is verified through the comparison of experimental and analytical results. The model is beneficial for the design of Love wave devices and sensors.

Love-wave devices with continuous and discrete nanocrystalline diamond coating for biosensing applications

The integration of diamond layers brings biocompatibility and enhanced stability of biomolecules to Love wave devices for promising biosensor applications. Love-Wave Surface Acoustic Wave (LW-SAW) devices consisting of an ST-cut quartz substrate with a silicon oxide layer coated by a thin diamond layer have been fabricated. The effect of nucleation and growth of diamond on the properties of LW-SAW devices was studied. The Love-wave phase velocity was shown to be affected by the diamond coating and dependent on its thickness. Experimental results are in a good agreement with simulations carried out with COMSOL Multiphysics software. Deposition of isolated diamond grains results in a reduction in velocity in the Love waves, which is attributed to a mass loading effect. Conversely, coalesced diamond layers increase the stiffness of the surface, which results in faster propagation of the Love waves. Improved entrapment of the Love waves in the guiding layer was observed in the case of isolated diamond grains, which should ensure higher sensitivity for this type of coating. Diamond coated LW-SAW devices can be used for biosensing applications with appropriated functionalization of the diamond surface.

Mesoporous Titania-Coated Biosensor and FEM Model for Highly Sensitive Detection of Low Molecular Weight Targets

This paper presents the interest of a highly sensitive biosensor coated with a TiO2 mesoporous film as a sensitive layer. The main novelty is related to the modeling of the device and simulation by using the finite element method with a COMSOL multiphysics software as a good way to take into account the physical properties of porous 3-D layers. The strategy of using such Love wave devices, with 3-D porous layers, offering further easy functionalization, aims not only to increase the amount of targets caught on the sensor surface but also to enhance the detection mechanism by a higher perturbation of the Love wave acoustic energy, which could be trapped inside the 3-D sensitive layer. First, as a proof of concept, experimental devices with a 3-D titania mesoporous layer were realized, and they have shown a good agreement with simulated results. Furthermore, experimental tests with several Newtonian liquids are investigated, in a range of viscosities from 1 to 7 cP, which are typical of those that concern our biochemical applications. The sensitivity with a 300 nm thick porous sensing layer was ten times that of the bare device, with interesting dynamical issues to be further studied, giving rise to the great potentialities of such architectures for biological detection of low weight biochemical targets.

Multiple-layer guided surface acoustic wave (SAW)-based pH sensing in longitudinal FiSS-tumoroid cultures

A constitutively increased intracellular pH that is higher than the extracellular pH is emerging as a hallmark of cancer and determining pH could play a significant role in the measurement of drug responsiveness of tumor cells. However, a non-invasive, touch-free and real-time pH sensing as a research tool is lacking and remains a major unmet need. The purpose of the current study is to investigate a microfluidic surface acoustic wave (SAW) sensor platform capable of monitoring pH in cell and tumoroid cultures. A novel multi-layer guided SAW sensor integrated into a microfluidic channel was investigated theoretically and experimentally in detail for pH bio-sensing. Sensitivity and capability of the layer guided Love wave device was modeled using the finite element simulation. The model was verified experimentally, and a study monitoring pH of cell growth media is presented. This novel pH sensor is based on a 13.91 MHz center frequency SAW device coated with ZnO (500 nm) and IrO2 (30 nm) layers to increase the sensitivity. A change in mechanical and electrical properties of the conductive IrO2 layer was observed resulting from electrical corrosion induced by pH solutions affecting the charge distribution, SAW phase velocity and attenuation. By measuring the frequency shift induced by the change in SAW phase velocity between the test group and control group, the pH value of cell culture media from H460 cancer cell culture plates from day 0 to day 5 can easily be determined. To improve the sensitivity and stability of the sensor, a finite element method was used to optimize the layer thicknesses. Taken together, the results of experiments show the potential application of this device to be integrated with microfluidic channels and used in determining pH changes in longitudinal tumor cell cultures.

Study on Fabrication of ZnO Waveguide Layer for Love Wave Humidity Sensor Based on Magnetron Sputtering.

The ZnO waveguide layer for the Love wave humidity sensor was fabricated by radio frequency (RF) magnetron sputtering technique using ZnO as the target material. To investigate the effect of RF magnetron sputtering temperature on the ZnO waveguide layer and Love wave device, a series of Love wave devices with ZnO waveguide layer were fabricated at different sputtering temperatures. The crystal orientation and microstructure of ZnO waveguide was characterized and analyzed, and the response characteristics of the Love wave device were analyzed by network analyzer. Furthermore, a humidity measurement system is designed, and the performance of the Love wave humidity sensor was measured and analyzed. The research results illustrate that the performance of the ZnO waveguide layer is improved when the sputtering temperature changes from 25 °C to 150 °C. However, when the sputtering temperature increases from 150 °C to 200 °C, the performance of the ZnO waveguide layer is degraded. Compared with the other sputtering temperatures, the ZnO waveguide layer fabricated at 150 °C has the best c-axis orientation and the largest average grain size (53.36 nm). The Love wave device has the lowest insertion loss at 150 °C. In addition, when the temperature of the measurement chamber is 25 °C and the relative humidity is in the range of 10% to 80%, the fabricated Love wave humidity sensor with ZnO waveguide layer has good reproducibility and long-term stability. Moreover, the Love wave humidity sensor has high sensitivity of 6.43 kHz/RH and the largest hysteresis error of the sensor is 6%.

Enhanced Sensitivity of a Love Wave-Based Methane Gas Sensor Incorporating a Cryptophane-A Thin Film.

A Love wave-based sensing chip incorporating a supramolecular cryptophane A (CrypA) thin film was proposed for methane gas sensing in this work. The waveguide effect in the structure of SiO2/36° YX LiTaO3 will confine the acoustic wave energy in SiO2 thin-film, which contributes well to improvement of the mass loading sensitivity. The CrypA synthesized from vanillyl alcohol by a double trimerisation method was dropped onto the wave propagation path of the sensing device, and the adsorption to methane gas molecules by supramolecular interactions in CrypA modulates the acoustic wave propagation, and the corresponding frequency shifts were connected as the sensing signal. A theoretical analysis was performed to extract the coupling of modes for sensing devices simulation. Also, the temperature self-compensation of the Love wave devices was also achieved by using reverse polarity of the temperature coefficient in each media in the waveguide structure. The developed CrypA coated Love wave sensing device was connected into the differential oscillation loop, and the corresponding gas sensitive characterization was investigated. High sensitivity, fast response, and excellent temperature stability were successfully achieved.

Finite element simulation of a highly sensitive SH-SAW delay line sensor with SiO $$_{2}$$ 2 micro-ridges

The paper presents 3D finite element simulation of an SH-SAW delay line sensor to investigate coupled resonance with SiO $$_{2}$$ micro-ridges designed along the wave propagation direction on a 36 $$^{\circ }$$ -YX LiTaO $$_{3}$$ substrate. Time response simulation is used to calculate the displacements and voltages at the output IDT. The device generates a shear wave with horizontal displacement component ten times greater than vertical displacement component. Mass loading is simulated by applying an equivalent surface mass density of double-stranded DNA uniformly on the SiO $$_{2}$$ ridges. At critical ridge height, both substrate and ridge vibrate in unison leading to a sharp change in the phase shift from negative to positive causing coupled resonance. In the presence of coupled resonance, the device offers high mass sensitivity of 1295 m $$^{2}$$ /kg which is about 20 times greater in comparison to the mass sensitivity of a conventional SiO $$_{2}$$ based Love wave device. The increase in mass sensitivity at coupled resonance is attributed to the increase in the area-averaged stress at the interface of micro-ridge and substrate, going above 8.5 MPa. As ridge height is varied, a transition occurs in the mode of vibration leading to a 5 dB swing in the insertion loss of the device near the coupled resonant height.

Highly sensitive Love wave acoustic biosensor for uric acid

In the present work, zinc oxide (ZnO) thin film based Love wave acoustic device has been exploited for realization of a biosensor. These Love wave devices have been fabricated on 36°YX lithium tantalate with ZnO thin film deposited using rf sputtering technique as the guiding layer. Polydimethylsiloxane (PDMS) microchannels have been prepared and integrated on the propagation path on the fabricated device. Detection of uric acid has been demonstrated using the developed biosensing device by measuring the shift in center frequency on interaction with uric acid. The developed uric acid sensor paves way towards the realization of handheld biosensor for future wireless sensing technology.

Characteristics of surface acoustic waves in (11 $$\bar 2$$ 2 ¯ 0)ZnO film/R-sapphire substrate structures

(11\(\bar 2\) 0)ZnO film/R-sapphire substrate structure is promising for high frequency acoustic wave devices. The propagation characteristics of SAWs, including the Rayleigh waves along [0001] direction and Love waves along [1ī00] direction, are investigated by using 3 dimensional finite element method (3D-FEM). The phase velocity (v p), electromechanical coupling coefficient (k 2), temperature coefficient of frequency (TCF) and reflection coefficient (r) of Rayleigh wave and Love wave devices are theoretically analyzed. Furthermore, the influences of ZnO films with different crystal orientation on SAW properties are also investigated. The results show that the 1st Rayleigh wave has an exceedingly large k 2 of 4.95% in (90°, 90°, 0°) (11\(\bar 2\) 0)ZnO film/R-sapphire substrate associated with a phase velocity of 5300 m/s; and the 0th Love wave in (0°, 90°, 0°) (11\(\bar 2\) 0)ZnO film/R-sapphire substrate has a maximum k 2 of 3.86% associated with a phase velocity of 3400 m/s. And (11\(\bar 2\) 0)ZnO film/R-sapphire substrate structures can be used to design temperature-compensated and wide-band SAW devices. All of the results indicate that the performances of SAW devices can be optimized by suitably selecting ZnO films with different thickness and crystal orientations deposited on R-sapphire substrates.

Simulation of a Love wave device with ZnO nanorods for high mass sensitivity

The paper presents 3D finite element simulation and analysis of Love wave resonator with different guiding layer materials and investigation of the coupled resonance effect with ZnO nanorods on the device surface. Analytical estimation of phase velocity and mass sensitivity of Love wave device with SiO2, ZnO, gold, SU-8, and parylene-C as guiding layer materials is performed for comparative analysis. Simulations are carried out to study the variation in electromechanical coupling coefficient, displacement profile and frequency response of the Love wave resonator. SU-8 offers high mass sensitivity of 1044 m2/kg while gold layer provides maximum K2 of 8.6%. In comparison to SiO2 and ZnO, polymers exhibit sharp rise and fall in K2 within a narrow range of normalized layer thickness (0.03–0.1). ZnO nanorods of varying height and surface nanorod density are designed over the Love wave resonator with SiO2 as the waveguiding layer. In the presence of coupled resonance, the nanorods and substrate vibrate in unison causing an increase in average stress and mass sensitivity but leads to decrease in the electromechanical coupling coefficient of the device. Surface nanorod packing density of 25 μm−2 offers high mass sensitivity of 1304 m2/kg that is 20 times greater in comparison to the mass sensitivity of a plain Love wave device.

Propagations of Rayleigh and Love waves in ZnO films/glass substrates analyzed by three-dimensional finite element method**Project supported by the National Natural Science Foundation of China (Grant No. 11304160), the Natural Science Foundation of Jiangsu Provincial Higher Education Institutions, China (Grant No. 13KJB140008), and the Foundation of Nanjing University of Posts and Telecommunications, China (Grant No. NY213018).

Propagation characteristics of surface acoustic waves (SAWs) in ZnO films/glass substrates are theoretically investigated by the three-dimensional (3D) finite element method. At first, for ZnO films/glass substrates, the simulation results confirm that the Rayleigh waves along the [0001] direction and Love waves along the direction are successfully excited in the multilayered structures. Next, the crystal orientations of the ZnO films are rotated, and the influences of ZnO films with different crystal orientations on SAW characterizations, including the phase velocity, electromechanical coupling coefficient, and temperature coefficient of frequency, are investigated. The results show that at appropriate , Rayleigh wave has a maximum k2 of 2.4% in (90°, 56.5°, 0°) ZnO film/glass substrate structure; Love wave has a maximum k2 of 3.81% in (56°, 90°, 0°) ZnO film/glass substrate structure. Meantime, for Rayleigh wave and Love wave devices, zero temperature coefficient of frequency (TCF) can be achieved at appropriate ratio of film thickness to SAW wavelength. These results show that SAW devices with higher k2 or lower TCF can be fabricated by flexibly selecting the crystal orientations of ZnO films on glass substrates.

Electro‐mechanical properties of inkjet‐printed graphene oxide nanosheets

This study deals with the deposition of graphene oxide (G‐O) thin films by inkjet printing method on Love wave devices, forming up G‐O nanosheets in between the inter‐digitated transducers (IDTs) of the device. Experimental results were compared with 3D finite element calculations. Enhanced properties were observed by amalgamating ultra‐thin G‐O films with SiO2 and lead to optimized guided surface acoustic wave propagation. The structural model of the G‐O films provided useful information regarding the Young and the Shear modulus of the device, which are found near 470 and 196 GPa, respectively. The resulting Maxwelian viscoelastic components of G‐O sheets were compared theoretically and experimentally to each other based on the providing losses on the Love wave devices through the scattering parameters (S‐parameters). Similarly, the theoretical and experimental resonance frequency data showed a very good agreement. This behavior is attributed to the different mechanisms, thus combining the viscoelastic and gravimetric effects of the G‐O on the Love wave devices. Finally, The G‐O film was characterized using colocalized AFM/Raman imaging in order to assess its roughness and chemical homogeneity.

Inkjet-Printed Graphene Oxide Thin Layers on Love Wave Devices for Humidity and Vapor Detection

We report inkjet printing as an alternative deposition method for low-cost gas or humidity sensors based on graphene oxide (GO)-coated Love wave devices. Our inkjet printing method paves the way toward massive, large-area industrial production of multi-layered GO chemical sensing films for volatile organic compounds and relative humidity (RH) detection applications. The adsorption of vapor compounds on GO led to the sensitivities of 30 Hz/ppm, 24 Hz/ppm, and 2.4 kHz/1% of ethanol (C2H6O), toluene (C7H8), and RH, respectively. Electrical, gaseous, and RH characterization analyses showed that this GO-based inkjet printing method is one of the most promising features for inexpensive and high-speed patterning devices for high demanding gas trace or RH sensing applications.

Nanostructured PMMA-coated Love wave device as a platform for protein adsorption studies

The development of nanostructured surfaces can enhance molecular interaction with sensors by increasing the available surface area. The associated potential for increased sensitivity towards analytes is of significance to the analytical and biophysical sector. In this work we explore the effect of nanostructures created on the surface of an acoustic wave device based on a Love wave geometry. Nanopillars with a diameter of 35nm and a height range of 60–280nm were created on top of a PMMA waveguide film by oxygen plasma etching. The effect of the pillar height on the acoustic response was tested by measuring the adsorption of BSA; increasing the pillar height up to 155nm led to an increase in the acoustic phase response associated with wave velocity. At higher surface roughness, wave-scattering effects dominated: at nanopillar heights larger than 200nm these effects were so severe that the sensors could not be used for protein sensing. In addition to phase, amplitude measurements were used to derive information on the structure of the adsorbed protein film and its dependency on the nanopillar height. This work demonstrates the importance of controlling the roughness of the waveguiding layer of a Love wave device, since the nanostructure-dependent response has considerable effects on designing sensitive biochip platforms for clinical and diagnostic applications.