Surface Acoustic Wave Wave Research Articles

Design and implementation of shear-horizontal mode surface acoustic wave formaldehyde gas sensor

Surface acoustic wave (SAW) sensors have garnered significant attention due to high performance in gas detection. Love wave as the special mode of SAW has greater attractiveness for molecules detection in gas or liquid. To investigate the detection performance of Love wave SAW (Love-SAW) devices towards toxic gases, the influence of interdigitated electrodes on the propagating performance of Rayleigh and shear horizontal (SH) waves along ST-X and ST-90°X quartz substrates was analyzed by finite element method (FEM). The effect of the guiding layer based on different substrates of SAW devices has been discussed by changing the thickness of the zinc oxide (ZnO) layer. The results indicate that the SH wave excited on ST-90°X quartz substrate couples into the Love wave mode through the ZnO guiding layer, and Rayleigh waves in ST-X quartz do not exhibit mode coupling. The Love-SAW sensor covering the ZnO guiding layer (0.5 µm) has shown high mass sensitivity. The vibration distribution of SH wave excited by different thicknesses of electrodes is different, indicating that the mass-loading is one of the reasons for the transformation of SH wave into Love wave. Furthermore, the 50 ppm, 95 ppm and 110 formaldehyde-load leads the short-circuit resonance frequency of the polyetherimide (PEI) covered Love-SAW devices to drop by 100.1 kHz, 200.2 kHz and 300.3 kHz, respectively. The maximum detection sensitivity reached about 2.73 kHz/ppm. This work can provide valuable insights for designing and investigating the SAW formaldehyde gas sensors.

Measuring Velocity, Attenuation, and Reflection in Surface Acoustic Wave Cavities Through Acoustic Fabry-Pérot Spectra.

Surface acoustic wave (SAW) cavities have been widely applied as electronic bandpass filters, sensors, microfluidic tweezers, and, in recent years, as devices for coupling with quantum systems. Here we propose a novel method of analyzing acoustic Fabry-Pérot spectra, by analogy with optical cavities, to determine the free surface velocity and attenuation of SAW waves, as well as the reflection of interdigital transducers (IDTs), all of which are crucial design parameters. In our experiment, two-port SAW resonators, consisting of two IDTs laterally separated by a free surface cavity length, are used to generate SAWs on 128° Y-X lithium niobate that are trapped between the two IDTs which also act as Bragg reflectors. Resonant cavity peaks can be observed through the electrical S11 (reflection) spectrum measured on one IDT. The free spectral range and linewidths of cavity peaks are then measured to obtain the free surface SAW velocity, SAW propagation attenuation coefficient, and IDT reflection phase and amplitude. Our method of analyzing Fabry-Pérot spectra provides an intuitive approach for determining key characteristics of SAW waves and cavities.

FEM Simulation of a High-Performance 128°Y-X LiNbO3/SiO2/Si Functional Substrate for Surface Acoustic Wave Gyroscopes.

To obtain a high-performance surface acoustic wave (SAW) gyroscope substrate, the propagation characteristics and gyroscopic effect of Rayleigh waves in a 128°Y–X LiNbO3/SiO2/Si (LNOI) functional substrate were investigated with a three-dimensional finite element method. The influence of LNOI structural parameters on Rayleigh wave characteristics, including the phase velocity (vp), electromechanical coupling coefficient (K2) and temperature coefficient of frequency (TCF), were analyzed. The results demonstrate that the SiO2 layer compensates for the negative TCF of 128°Y–X LiNbO3 and enhances the K2 of the LNOI substrate. The Rayleigh wave velocity change of the LNOI substrate after rotations in different directions was studied. The gyroscope gain factor (η) represents the strength of the gyroscopic effect in the differential traveling wave SAW gyroscope and is defined. The ηy and ηz of the LNOI substrate with different structural parameters were investigated. Finally, an LNOI substrate with an hLN/λ of 0.2 and an hSiO2/λ of 0.05 was obtained by balancing the characteristic parameters, with a K2 of 3.96%, TCF of −18.75 ppm/°C and ηy of 0.26. The LNOI substrate has a better gyroscopic effect and temperature stability than the 128°Y–X LiNbO3 crystal. The LNOI substrate meets device miniaturization and integration needs.

Engineering the beat phenomenon of quasi-Rayleigh waves for regions with minimal Surface Acoustic Wave (SAW) amplitude

Surface Acoustic Waves (SAW) are prevalent in a variety of scientific fields. Typically, the ability to manipulate these surface waves has a significant impact on the efficacy of any SAW-based device. In this paper, we present a simple method for designing localized regions that have minimal SAW amplitude. These localized regions have valley-like drops in SAW amplitude and are achieved by using the spatial beat phenomenon of quasi-Rayleigh waves. Controlling the beat phenomenon allows us to manipulate where and how these localized regions appear in the waveguide. In particular, the localized regions can be designed to have variable widths and can be positioned consecutively due to minimal wave scattering. The design methodology and physics are described in detail, and several supplementary simulations are provided to demonstrate the efficacy of the proposed method. Lastly, we briefly discuss how the proposed approach could potentially be exploited to implement a localized high-pass filter and enables SAW wave propagation that avoids surface obstacles.

High quality and low loss surface acoustic wave SAW resonator based on chromium-doped AlN on sapphire

This paper aims to investigate the performances of surface acoustic wave (SAW) resonators based on chromium-doped aluminum nitride (AlCrN) piezoelectric thin film on c-cut sapphire substrate. The electromechanical properties of AlCrN (mass density, elastic, dielectric and piezoelectric constants) are obtained from density functional theory calculations for different Cr-doping concentrations. Piezoelectricity is enhanced for Al1 − xCrxN with different x concentrations. Using finite element analysis, the electroacoustic parameters such as phase velocity, electromechanical coupling factor K2 of surface acoustic modes are determined for different AlCrN thickness-to-wavelength ratios and different Cr-doping concentrations. Higher order SAW modes arise at Cr concentration x = 25%. The electrical admittance and scattering parameter S21 for the fundamental SAW modes are determined. High K2, low loss and high-quality factor are found after incorporating Cr dopant. The comparison of the obtained results with the available numerical and experimental data on AlN doped with Scandium Sc has shown a considerable improvement of SAW devices characteristics, confirming that AlCrN can be adopted as a new piezoelectric material for high performances SAW devices.

Excitation of Surface Acoustic Waves and Lamb Waves at Superhigh Frequencies in a Diamond-Based Piezoelectric Layered Structure

The authors have carried out an experimental and theoretical study of the propagation of surface acoustic waves (SAW)—Rayleigh, Sezawa and SH-modes, as well as Lamb waves in Me-IDT/AlN/(100) diamond (Me = Pt, Al) piezoelectric layered structures with a SAW resonator configuration. The types of modes are identified and the dispersion curves of the phase velocities and electromechanical coupling coefficients for SAW and Lamb waves are obtained. The excitation of Lamb waves up to a frequency of 7.3 GHz was observed experimentally. The resonance curves for Lamb waves have a higher quality factor Q compared to surface acoustic waves propagating on the same substrates. At a frequency of about 7 GHz, the loaded Q-factor reaches 3400, while the quality parameter Q × f = 2.4 × 1013 Hz. An unusual effect of a significant increase in the Q-factor of Lamb waves with the frequency increasing from 760 at 1.5 GHz to 3400 at 7 GHz is noted.

Mode Analysis of Pt/LGS Surface Acoustic Wave Devices.

Platinum (Pt) gratings on langasite (LGS) substrates are a widely used structures in high temperature surface acoustic wave (SAW) devices. Multiple modes can be excited in PtLGS SAW devices owing to the heavy weight of the Pt electrode and leaky waves in the LGS substrate. In this work, we report on a detailed mode analysis of PtLGS SAW devices, where three commonly used LGS cuts are considered. A three-dimensional (3D) finite element method (FEM) numerical model was developed, and the simulation and experiment results were compared. The experiment and simulation results showed that there are two modes excited in the PtLGS SAW devices with Euler angle (0, 138.5, 27) and (0, 138.5, 117), which are Rayleigh-type SAW and SH-type leaky wave, respectively. Only the Rayleigh-type mode was observed in the PtLGS SAW devices with Euler angle (0, 138.5, 72). It was found that the acoustic velocities are dependent on the wavelength, which is attributed to the change of wave penetration depth in interdigital transducers (IDTs) and the velocity dispersion can be modulated by the thickness of the Pt electrode. We also demonstrated that addition of an Al2O3 passivation layer has no effect on the wave modes, but can increase the resonant frequencies. This paper provides a better understanding of the acoustic modes of PtLGS SAW devices, as well as useful guidance for device design. It is believed that the Rayleigh-type SAW and SH-type leaky waves are potentially useful for dual-mode sensing applications in harsh environments, to achieve multi-parameter monitoring or temperature-compensation on a single chip.

Comparative Study of Binding Constants from Love Wave Surface Acoustic Wave and Surface Plasmon Resonance Biosensors Using Kinetic Analysis

Biosensors are used in a variety of fields for early diagnosis of diseases, measurement of toxic contaminants, quick detection of pathogens, and separation of specific proteins or DNA. In this study, we fabricated and evaluated the capability of a high sensitivity Love wave surface acoustic wave (SAW) biosensor. The experimental setup was composed of the fabricated 155-MHz Love wave SAW biosensor, a signal measurement system, a liquid flow system, and a temperature-control system. Subsequently, we measured the lower limit of detection (LOD) of the 155-MHz Love wave SAW biosensor, and calculated the association and dissociation constants between protein G and anti-mouse IgG using kinetic analysis. We compared these results with those obtained using a commercial surface plasmon resonance (SPR) biosensor. We found that the LOD of the SAW biosensor for anti-mouse IgG and mouse IgG was 0.5 and 1 microg/ml, respectively, and the resultant equilibrium association and dissociation constants were similar to the corresponding values obtaining using the commercial SPR biosensor. Thus, we conclude that the fabricated 155-MHz Love wave SAW biosensor exhibited the high sensitivity of the commercial SPR biosensor and was able to analyze the binding properties of the ligand and receptor by kinetic analysis similarly to the commercial SPR biosensor.

X-ray imaging of the surface acoustic wave propagation in La3Ga5SiO14 crystal

Direct imaging of a 10 μm surface acoustic wave (SAW) propagation in a La3Ga5SiO14 crystal was obtained on the European Synchrotron Radiation Facility (ESRF) synchrotron radiation source in the sagittal diffraction geometry using the Talbot effect. Imaging of the SAW wave field with the period 10 μm was observed at a distance corresponding to the half of the Talbot distance. It is shown that the presence of the growth banding in the crystal does not influence the propagation of acoustic wave-fields in the crystal nor does it cause a distortion of the SAW wave front.

Integrated microfluidics system using surface acoustic wave and electrowetting on dielectrics technology.

This paper presents integrated microfluidic lab-on-a-chip technology combining surface acoustic wave (SAW) and electro-wetting on dielectric (EWOD). This combination has been designed to provide enhanced microfluidic functionality and the integrated devices have been fabricated using a single mask lithographic process. The integrated technology uses EWOD to guide and precisely position microdroplets which can then be actuated by SAW devices for particle concentration, acoustic streaming, mixing and ejection, as well as for sensing using a shear-horizontal wave SAW device. A SAW induced force has also been employed to enhance the EWOD droplet splitting function.

Love wave SAW biosensors for detection of antigen-antibody binding and comparison with SPR biosensor

In this study, 77 and 155 MHz Love wave surface acoustic wave (SAW) biosensors were fabricated to detect antigen-antibody reactions in liquid environments. The single-interdigital transducer (IDT) on the quartz crystal substrate was found to be not suitable for Love wave SAW biosensor because of its signal distortion due to triple transit echo (TTE) between the IDT fingers. As a waveguide layer for the Love wave SAW biosensor, SU-8 photoresist was coated on a quartz crystal substrate with double-IDT patterns. Anti-mouse IgG was immobilized on the sensing area with a protein G layer bound to a gold surface. The minimum detectable concentrations of mouse IgG for 77 and 155 MHz Love wave SAW biosensors and a surface plasmon resonance (SPR) biosensor were found to be 2.5, 1, and 1 μg/mL, respectively. The fabricated Love wave SAW and SPR biosensors showed similar performances in detectable concentrations of mouse IgG and in coefficient of determination.

The numerical analysis of general SAW and leaky wave devices using approximate Green's function representations

The Green's function or boundary element method (BEM) is the best available technique for rigorous surface acoustic wave (SAW) device analysis. However, its computational cost usually means that it cannot be applied directly to devices with complex, nonperiodic electrode structures. In this paper, approximate forms for the Green's function are employed. They are based on rigorous representations, they can represent the Green's function to any required degree of accuracy, and they can be applied to any type of substrate and acoustic wave. The use of this type of approximation for practical device analysis is considered, and computational procedures are presented that can exploit the special approximate Green's function structure. It is shown that highly efficient computational algorithms can be constructed, in which the computational effort increases linearly with the number of electrodes in the device. These methods can be applied to any type of device structure, and they do not require any empirically derived parameters. The practical application of the methods is illustrated by examples of longitudinally coupled resonator filter (LCRF) designs implemented using leaky wave cuts of lithium tantalate. Agreement between theory and experiment is excellent, even for devices of this complexity.

Study on non-contact linear motors driven by surface acoustic waves

Experimental measurements and theoretical analyses of non-contact linear motors driven by surface acoustic waves (SAWs) through liquid layers are presented, in which the slider is suspended at the surface of the liquid layer contained in a glass cell located at the surface of the stator. The experimental results show that the slider velocity is essentially proportional to the exciting voltage of the SAW (as the voltage in excess of a threshold) and decreases approximately exponentially with the thickness of the liquid layer increase (as the layer in excess of some thickness). To explain the experimental phenomena, according to the acoustic mode transition from Rayleigh wave to leaky SAW wave and the generation of acoustic streaming at the solid–liquid boundary, a simplified theoretical model is introduced to simulate the slider motion. These results demonstrate that the theoretical calculations essentially explain the experimental measurements. This kind of non-contact motors are hopeful to be used as manipulators in biomedical engineering and/or micromechanical systems, etc.

Spin dynamics in (110) GaAs quantum wells under surface acoustic waves

Long spin transport lengths $(g60\text{ }\ensuremath{\mu}\text{m})$ independent of temperature up to approximately 80 K are demonstrated in (110) GaAs quantum wells using surface acoustic waves (SAWs). Study of the dynamics of spins aligned along the [110] direction shows that, in addition to the intrinsic absence of the D'yakonov-Perel' spin-relaxation mechanism [Sov. Phys. Semicond.20, 110 (1986)], the Bir-Aronov-Pikus mechanism [Sov. Phys. JETP 42, 705 (1976)] is also suppressed due to the type-II carrier confinement imposed by the SAW piezoelectric potential. Experimental evidence is provided for suppression of the spin relaxation via motional narrowing effects induced by the mesoscopic carrier confinement in narrow stripes along the SAW wave front, thus demonstrating the tuning of the spin-relaxation rates with the acoustic power.

Love파를 이용한 저점성 유체 점도 측정용 표면 탄성파 센서 개발

Love wave is one of the shear horizontal waves and it can propagate between two layers in liquid without energy loss. The SAW (surface acoustic wave) sensor using Love wave is very useful for real time measurement of the viscosity of liquid with high sensitivity. In this study, the 77 MHz and 155 MHz Love wave SAW sensors were fabricated and use to measure the viscosity of low viscous liquid. To generate the surface acoustic wave, the inter-digital transducers were fabricated on the quartz crystal wafer. In order to obtain the optimal thickness of the coating film (novolac photoresist) generating the Love wave on the surface of SAW device, theoretical calculation was performed. The performances of fabricated Love wave SAW sensors were tested. As test liquid, pure water and glycerol solutions having different concentrations were used. Since the determination coefficients of the regression equations for measuring the viscosity of liquid are greater than 0.98, the developed Love wave SAW sensors in this study will be very useful for precise measurement of viscosity of liquid.

The theoretical analyse of the surface acoustic waves propagated on the surface of the bulk materials with subsurface defect

A theoretical model based on finite element method is used to analyse the properties of surface acoustic wave (SAW) propagated on the surface of the bulk materials with subsurface defect. The simulation results reveal the modes conversion between SAW and Lamb wave at the border of the defect and explain the diffusion properties of SAW propagated on the bulk material with subsurface defect. The modes conversion and wave reflection at the border of the defect are theoretically explained why there are a lot of riffles after the SAW passed through Al sample with circle subsurface defect, which detected by laser ultrasonic method. The relationships between the wavelength of wave and the physical state of the defect in the bulk material are also fully described in theoretical simulation.

Damascene technique applied to surface acoustic wave devices

A new process technology for the wafer-level fabrication of surface acoustic wave (SAW) structures in piezoelectric LiNbO3 is reported. The complete Cu-based metallization system is embedded in the surface by applying the damascene technology. This technique includes ion based dry etching procedures, metallization by sputter deposition (physical vapor deposition) and electrochemical deposition, and finally planarization by chemical-mechanical polishing. The process steps were monitored by means of different evaluation techniques. SAW wave field measurements provided evidence for a good surface quality after planarization.

Examination of surface acoustic wave reflections by observing acoustoelectric current generation under pulse modulation

The authors investigate surface acoustic wave (SAW) reflections using the frequency dependence of the acoustoelectric current through a one-dimensional channel. By pulse modulating the transducer power with a short pulse width (0.2μs) and matching the pulse repetition period to the delay time of a specific reflection path, the authors detect individual reflected SAW wave packets. Fourier transform results show that reflections from the sample edge behind the transducer and Bragg reflections from the second transducer have the strongest effects. The reflection power coefficients for these are estimated to be ≈0.29 and ≈0.99, respectively.

Surface acoustic wave properties of freestanding diamond films

"Ideal" diamond has the highest acoustic velocity of any material known, and is of great interest as a substrate material for high frequency surface acoustic wave (SAW) device structures. However, little is known of the acoustic wave propagation properties of polycrystalline diamond grown by chemical vapour deposition (CVD) techniques, the commercially accessible form of this material. We report on propagation of laser-generated SAW on three forms of freestanding CVD diamond samples, "white" polycrystalline, "black" polycrystalline, and "highly oriented" diamond. Despite differing sample nature, SAW waves propagating along the smooth (nucleation) side of the diamond showed similar velocities in the range 10600-11900 ms(-1). These results are discussed in terms of the potential of each form of CVD diamond for SAW device fabrication.

Acoustoelectric currents in manganite thin films

Recently an anomalous acoustoelectrical (AE) effect produced by surface acoustic waves (SAW) in thin La 0.67Ca 0.33MnO 3 films deposited on a piezoelectric LiNbO 3 substrate was discovered [Phys. Rev. Lett. 87 (2001) 146602]. The SAW, launched along the distinct crystallographic axis (+ z) of the structure, induces a dc current I AE in the film, which in the ordinary AE effect would be odd with respect to the SAW wave vector q . However, in the investigated case I AE contains, along with the ordinary odd component, an anomalous one, which is even in q . The anomalous effect dominates near the metal–insulator transition, while the ordinary effect prevails at high and low temperatures. The anomalous effect appears to depend on strong modulation of the film conductivity resulting from the elastic deformations carried by the SAW. We have investigated the influence of an in-plane magnetic field H on the magnitude of I AE, and found an increase of about 50% when H was parallel to q . The increase was limited to the anomalous component of I AE. We speculate that this may be caused by magnetic field induced changes in the conductivity dependence on pressure. The ordinary AE current was left essentially unchanged. Apparently the boost given to this component by the increase of conductivity in a magnetic field is compensated by the corresponding SAW attenuation decrease. We have also conducted I AE measurements with the SAW split into two components of equal amplitude, propagating in the film in opposite directions. As a result, we have obtained cancellation of the ordinary I AE component and augmentation of the anomalous one, i.e. we have confirmed our understanding of the AE effects.