Layered Surface Acoustic Wave Device Research Articles

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.

Enhancement of effective electromechanical coupling factor by mass loading in layered surface acoustic wave device structures

This paper describes a drastic enhancement of the effective coupling factor by mass loading in layered surface acoustic wave (SAW) device structures such as the ScAlN film/Si substrate structure. This phenomenon occurs when the piezoelectric layer exhibits a high acoustic wave velocity. The mass loading decreases the SAW velocity and causes SAW energy confinement close to the top surface where an interdigital transducer is placed. It is shown that this phenomenon is obvious even when an amorphous SiO2 film is deposited on the top surface for temperature compensation. This enhancement was also found in various combinations of electrode, piezoelectric layer, and/or substrate materials. The existence of this phenomenon was verified experimentally using the ScAlN film/Si substrate structure.

Investigation into Mass Loading Sensitivity of Sezawa Wave Mode-Based Surface Acoustic Wave Sensors

In this work mass loading sensitivity of a Sezawa wave mode based surface acoustic wave (SAW) device is investigated through finite element method (FEM) simulation and the prospects of these devices to function as highly sensitive SAW sensors is reported. A ZnO/Si layered SAW resonator is considered for the simulation study. Initially the occurrence of Sezawa wave mode and displacement amplitude of the Rayleigh and Sezawa wave mode is studied for lower ZnO film thickness. Further, a thin film made of an arbitrary material is coated over the ZnO surface and the resonance frequency shift caused by mass loading of the film is estimated. It was observed that Sezawa wave mode shows significant sensitivity to change in mass loading and has higher sensitivity (eight times higher) than Rayleigh wave mode for the same device configuration. Further, the mass loading sensitivity was observed to be greater for a low ZnO film thickness to wavelength ratio. Accordingly, highly sensitive SAW sensors can be developed by coating a sensing medium over a layered SAW device and operating at Sezawa mode resonance frequency. The sensitivity can be increased by tuning the ZnO film thickness to wavelength ratio.

Surface acoustic wave devices on AlN/3C–SiC/Si multilayer structures

Surface acoustic wave (SAW) propagation characteristics in a multilayer structure including a piezoelectric aluminum nitride (AlN) thin film and an epitaxial cubic silicon carbide (3C–SiC) layer on a silicon (Si) substrate are investigated by theoretical calculation in this work. Alternating current (ac) reactive magnetron sputtering was used to deposit highly c-axis-oriented AlN thin films, showing the full width at half maximum (FWHM) of the rocking curve of 1.36° on epitaxial 3C–SiC layers on Si substrates. In addition, conventional two-port SAW devices were fabricated on the AlN/3C–SiC/Si multilayer structure and SAW propagation properties in the multilayer structure were experimentally investigated. The surface wave in the AlN/3C–SiC/Si multilayer structure exhibits a phase velocity of 5528 m s−1 and an electromechanical coupling coefficient of 0.42%. The results demonstrate the potential of AlN thin films grown on epitaxial 3C–SiC layers to create layered SAW devices with higher phase velocities and larger electromechanical coupling coefficients than SAW devices on an AlN/Si multilayer structure. Moreover, the FWHM values of rocking curves of the AlN thin film and 3C–SiC layer remained constant after annealing for 500 h at 540 °C in air atmosphere. Accordingly, the layered SAW devices based on AlN thin films and 3C–SiC layers are applicable to timing and sensing applications in harsh environments.

A Surface Acoustic Wave Ethanol Sensor with Zinc Oxide Nanorods

Surface acoustic wave (SAW) sensors are a class of piezoelectric MEMS sensors which can achieve high sensitivity and excellent robustness. A surface acoustic wave ethanol sensor using ZnO nanorods has been developed and tested. Vertically oriented ZnO nanorods were produced on a ZnO/128∘ rotated Y-cut LiNbO3 layered SAW device using a solution growth method with zinc nitrate, hexamethylenetriamine, and polyethyleneimine. The nanorods have average diameter of 45 nm and height of 1 μm. The SAW device has a wavelength of 60 um and a center frequency of 66 MHz at room temperature. In testing at an operating temperature of 270 with an ethanol concentration of 2300 ppm, the sensor exhibited a 24 KHz frequency shift. This represents a significant improvement in comparison to an otherwise identical sensor using a ZnO thin film without nanorods, which had a frequency shift of 9 KHz.

UV detection based on a ZnO∕LiNbO3 layered surface acoustic wave oscillator circuit

This study elucidates the combination of an oscillator circuit with a high-frequency amplifier, a matching network, and a layered surface acoustic wave device for detecting ultraviolet (UV) light. The oscillator circuit shows an excellent performance with the resonance frequency of 109.34MHz and phase noise of −107.137dB at 100kHz. The frequency shift that is caused by the interaction between the acoustic wave and carriers upon excitation by UV light in the ZnO thin film is discussed. The frequency variation of the oscillator steadily increases with the intensity of the UV light. An extreme frequency shift of 63.75kHz was observed as the UV light intensity reached 1250μW∕cm2.

Utilization of phononic-crystal reflective gratings in a layered surface acoustic wave device

In this paper, a design that combines two-port surface acoustic wave (SAW) devices and phononic crystals (PCs) acting as reflected gratings is demonstrated. Finite-difference time-domain method is used to analyze SAWs encountering the PC and optimize the design. A layered ZnO/Si SAW device and a square lattice PC composing of cylindrical holes on silicon were fabricated. With the PC of 15-layer cylinders, experimental insertion loss shows a 7 dB improvement at 212 MHz at central frequency. In addition, the size of gratings is reduced significantly as compared to the traditional gratings with hundreds of metal strips.

Layered SAW gas sensor based on CSA synthesized polyaniline nanofiber on AlN on 64° YX LiNbO 3 for H 2 sensing

A 64° YX LiNbO 3 surface acoustic wave (SAW) transducer was fabricated and then aluminium nitride (AlN) layer was deposited on the active area of the device as acoustic wave guiding layer. Structural studies revealed that the deposited AlN thin films have strong preferential c-axis orientation. Morphological analysis showed that the AlN layer is compact with grain dimensions of less than 80 nm. Polyaniline nanofibers were polymerized from the reactions in which an oxidant was rapidly mixed with the aniline monomer in an acidic environment. The polymer was synthesized in camphor sulfonic acid (CSA) to obtain 50 nm average diameter polyaniline nanofibers. These nanofibers were deposited on the layered SAW device and were tested towards hydrogen (H 2) gas while operating at room temperature. The device demonstrated a large and reproducible response to different concentrations of the H 2 gas making it an ideal candidate for H 2 sensing at room temperature.

Very high frequency SAW devices based on nanocrystalline diamond and aluminum nitride layered structure achieved using e-beam lithography

Very high frequency surface acoustic wave (SAW) devices based on the AlN/diamond layered structure are fabricated by direct writing using e-beam lithography on the nucleation side of nanocrystalline diamond (NCD) films deposited by microwave plasma assisted chemical vapor deposition process. The NCD nucleation side is characterized from the point of view of microstructure, morphology and surface topography. Surface roughness as low as 6 nm is reached, which enhances the deposition of AlN film on this flat surface. The interdigital transducers IDTs made in aluminum with lateral resolution down to 600 nm are successfully patterned on the AlN/NCD layered structure with an adapted technological process. Experimental results show that the Rayleigh wave and the higher mode are generated. A high frequency around 4 GHz (mode 1) is obtained for the considered layered structure SAW device, exhibiting a phase velocity of 9200 m/s taking into account the wavelength of 2.4 μm. This value agrees well with calculated values determined from dispersion curves of phase velocity.

The Preparation of Piezoelectric ZnO Films by RF Magnetron Sputtering for Layered Surface Acoustic Wave Device Applications

Crystalline structures, intrinsic stress, and surface roughness of piezoelectric ZnO films deposited on sapphire substrates by rf magnetron sputtering and sequential post-annealing treatments were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. The rf power used and O2/(Ar+O2) gas ratio greatly affected the crystallinity and surface roughness of the ZnO film. A c-axis preferred orientation with superior surface roughness was achieved at a specific rf power and O2/(Ar+O2) gas flow ratio. In addition, the original intrinsic stress was also markedly released (stress=0.325 ×1010 dyn/cm2) and a uniform surface (Ra=2.42 nm) was produced by a post-annealing treatment at a temperature of 400 °C under oxygen ambient. An acoustic wave at the center frequency of 240 MHz (hZnO/λ=0.1) was obtained from the ZnO/sapphire-layered surface acoustic wave (SAW) device using this optimized ZnO film.

Layered WO3/ZnO/36° LiTaO3 SAW gas sensor sensitive towards ethanol vapour and humidity

Ethanol sensing in dry and humid air by layered surface acoustic wave (SAW) devices is presented. The transducing platform is based on 36° YX LiTaO3 layered SAW device, utilising a 1.2μm zinc oxide (ZnO) intermediate layer and 150nm tungsten trioxide (WO3) sensing layer. Sheet conductivity calculations show that maximum sensitivity is achieved for ZnO layer thickness between 1 and 1.5μm. Sensor performance was analyzed in terms of response magnitude as a function of operational temperature and different relative humidity (RH). Frequency shifts of 119, 90 and 86kHz towards 500ppm of ethanol in synthetic air were observed for 0, 25 and 50% RH, respectively. All RH levels were measured at 20°C. At an operating temperature of 300°C, the largest response towards 500ppm of ethanol was observed. Response magnitude was found to decrease with increasing RH and decreasing operating temperature. Furthermore, the effect of elevated temperatures on the sensors surface morphology is characterised by AFM and SEM techniques. It is suggested that the morphological modifications, due to elevated temperatures play an important role in the sensing behaviour of the WO3 films.

A novel weighted method for layered SAW filters using slanted finger interdigital transducers

In this paper, we propose a novel weighted method to improve the inclined pass-band problem that is intrinsic to surface acoustic wave (SAW) filters with slanted finger interdigital transducers (SFITs). By the dispersive characteristics of SAW in layered piezoelectric substrates, the present method could make the inclined pass-band flatter or the bandwidth wider. We derived a coupling-of-modes model for the calculation of the frequency responses of layered SAW devices with SFITs and calculated the dispersive characteristics of an AlN/silicon layered piezoelectric substrate, such as the phase velocity and the electromechanical coupling coefficient. Then we utilized the dispersive electromechanical coupling coefficient to flatten the inclined pass-band and the dispersive phase velocity to widen the bandwidth. Two designed examples of wide-band layered SAW filters using SFITs on AlN/silicon substrates are presented to prove the validity of the proposed method: one is designed to flatten the inclination of the pass-band; the other, to widen the bandwidth. Results show that the dispersions of layered piezoelectric media are favourable for improving the performance of SFIT-based SAW filters.

Layered SAW hydrogen sensor with modified tungsten trioxide selective layer

Layered surface acoustic wave (SAW) devices are investigated for sensing hydrogen (H 2) concentrations less than 1% in air. Platinum (Pt) and gold (Au) catalyst activated tungsten trioxide (WO 3) selective layers are investigated. The SAW sensors consist of two thin film metal interdigital transducers (IDTs) on a 36° Y-cut, X-propagating LiTaO 3 substrate. A ZnO guiding layer is used to confine the acoustic energy at the active surface of the device for increased sensitivity. In this paper, the fabrication of an Au–WO 3 and a Pt–WO 3 based layered SAW device are described. The sensor response have been analysed in terms of frequency shift as a function of different hydrogen concentrations and operating temperatures. The responses of the catalyst activated WO 3 sensors show much higher sensitivity when compared against a layered SAW sensor employing only a bare WO 3 selective layer. Frequency shifts of 705 and 118 kHz towards 1% H 2 in air were observed for the Au–WO 3 and Pt–WO 3 sensors, respectively. Characterization by scanning electron microscope (SEM) of the Au catalyst activated tungsten trioxide sensor is also presented.

An experimental study on the ZnO/sapphire layered surface acoustic wave device

In this study, ZnO thin films with different thicknesses were deposited on C-plane sapphire substrates by radio-frequency magnetron sputtering. Properties of interdigital transducer/ZnO/sapphire layered surface acoustic wave device including surface wave velocity, electromechanical coupling coefficient, and propagatiion loss were measured and discussed. Combining the effective permittivity approach and the coupling-of-modes (COM) model, the dispersion characteristics and frequency responses of the ZnO/sapphire layered structure were simulated. By substituting the measured parameters into the COM model, the simulated frequency response has shown good agreement with the experimental results.

A Layered Surface Acoustic Wave ZnO/LiTaO<SUB>3</SUB> Structure with a WO<SUB>3</SUB> Selective Layer for Hydrogen Sensing

A layered surface acoustic wave (SAW) gas sensor is investigated for hydrogen concentrations of 0.5% and 1% in air at different operational temperatures. The device structure is based on a layered SAW device fabricated on a 36° X-cut, Y-propagating LiTaO3 substrate. A ZnO guiding layer is employed to increase sensor sensitivity, and a thin film of WO3 provides the selectivity toward hydrogen gas. Such a structure has the advantage of confining the acoustic wave energy at the surface of the device, which increases the sensitivity of the device. In this paper, the fabrication of the ZnO/36° YXLiTaO3 sensor is described, and the sensor's response features are analyzed in terms of response time, recovery time, and response magnitude as a function of operational temperature. Furthermore, the hydrogen interaction with the WO3 selective layer is briefly described.

Analysis of Surface Acoustic Waves in Layered Piezoelectric Media and its Applications to the Design of Saw Devices

ABSTRACTIn this paper, we utilized a Stroh based formulation for solving problems of surface waves in layered piezoelectric media, and then, applied it to analyze surface acoustic wave (SAW) devices. The determination of the optimal cut of a piezoelectric crystal and the choice of the best propagation of SAW devices were given. The dispersion induced by a thin metal layer on SAW propagation in a SAW device was analyzed and discussed. Finally, we applied the formulation to calculate the effective permittivity and phase velocity dispersion of a LiNbO3/Diamond layered SAW device. Both of the null frequency bandwidth and the insertion loss of the dispersive SAW device were obtained.

BULK AND SURFACE ACOUSTIC WAVES IN ANISOTROPIC SOLIDS

In this paper methods for analyzing acoustic propagation characteristics for bulk and surface acoustic waves in anisotropic piezoelectric multilayers are described. The methods's conceptual usefulness is demonstrated by examples showing how problems of guided wave propagation in complicated layered surface acoustic wave device geometries are simplified. The formulation reduces the acoustoelectric equations to a first order ordinary matrix differential equation in the variables that must be continuous across interfaces. The solution to these equations is a transmission matrix that maps the variables from one layer face to the other. Interface boundary conditions for a planar multilayer are automatically satisfied by multiplying the individual transmission matrices in the appropriate order thus reducing the problem to imposing boundary conditions appropriate to the remaining free surface. The dimensionality of the problem being independent of the number of layers is a significant advantage. A classification scheme for reducing problem dimensionality, based on an understanding of crystal symmetry properties, further simplifies surface acoustic wave problems.

Bulk and Surface Acoustic Waves in Anisotropic Solids

In this paper methods for analyzing acoustic propagation characteristics for bulk and surface acoustic waves in anisotropic piezoelectric multilayers are described. The methods's conceptual usefulness is demonstrated by examples showing how problems of guided wave propagation in complicated layered surface acoustic wave device geometries are simplified. The formulation reduces the acoustoelectric equations to a first order ordinary matrix differential equation in the variables that must be continuous across interfaces. The solution to these equations is a transmission matrix that maps the variables from one layer face to the other. Interface boundary conditions for a planar multilayer are automatically satisfied by multiplying the individual transmission matrices in the appropriate order thus reducing the problem to imposing boundary conditions appropriate to the remaining free surface. The dimensionality of the problem being independent of the number of layers is a significant advantage. A classification scheme for reducing problem dimensionality, based on an understanding of crystal symmetry properties, further simplifies surface acoustic wave problems.