Mode Surface Acoustic Wave Research Articles

Pure SH-SAW propagation, transduction and measurements on KNbO/sub 3/

Potassium niobate (KNbO3) supports the electromechanically active pure shear horizontal surface acoustic wave (SH-SAW) mode along Z-axis cylinder orientations, Euler angles (phi, 90 degrees, 0 degrees), in which two uncoupled wave solutions exist: a purely mechanical sagittal Rayleigh SAW and a piezoelectrically stiffened pure SH-SAW. Within this family of cuts, a maximum electromechanical coupling coefficient for the pure SH-SAW, K2 = 53%, is observed along (0 degrees, 90 degrees, 0 degrees). This pure SH-SAW orientation also has the maximum value of electromechanical coupling observed along rotated Y-cut X propagation directions, Euler angles (0 degrees, theta, 0 degrees). The use of the pure SH-SAW mode is attractive for liquid-sensing applications because the SH-SAW is modestly attenuated by the adjacent liquid, unlike the generalized SAW (GSAW), which has particle displacement normal to the surface. This work investigates propagation and excitation properties of the SH-SAW and the shear horizontal bulk acoustic wave (SH-BAW) on single crystal KNbO3, Euler angles (0 degrees, 90 degrees, 0 degrees). Interdigital transducer (IDT) arrays are analyzed using boundary element method (BEM) techniques, addressing IDT properties such as: power partitioning between the SH-SAW and SH-BAW, SH-BAW radiation as a function of wave vector direction and radiation angle, and overall IDT impedance. The percentage of SH-SAW power to total input power is above 98% for IDTs containing 1.5 to 5.5 wavelengths of active electrodes with surrounding metalized regions. For nonmetalized regions outside the IDT, the ratio drops to between 1 and 2%, showing the importance of an energy trapping structure for efficient SH-SAW excitation and propagation along this orientation. Simulated and experimental IDT admittance results are compared, verifying the validity of the analysis performed. The reported measurements on the frequency variation with temperature indicate that the orientation considered is temperature compensated at about 8 degrees C. The surface of the SH-SAW devices fabricated have been loaded with deionized water and showed additional 1.6 dB transmission loss with respect to the unloaded surface, verifying the suitability of the pure SH-SAW mode on KNbO3 for liquid sensor applications.

High-frequency surface acoustic wave devices based on LiNbO3∕diamond multilayered structure

This research is focused on the development of a new technology for the fabrication of high-frequency surface acoustic wave (SAW) filters based on diamond and lithium niobate (LiNbO3) materials. LiNbO3 films are in situ deposited at 490 °C on polycrystalline diamond substrates by radio-frequency magnetron sputtering; a multistep growth process is proposed to deposit oriented LiNbO3 films with a smooth surface. Conventional interdigital transducers are fabricated on top of the piezoelectric LiNbO3 layer by standard optical lithography. We report experimental results for a SAW bandpass filter operating at a frequency above 2.30 GHz. We have observed that Rayleigh SAW modes are excited within this structure, with extremely high phase velocities (up to 12000m∕s). This latter result illustrates the greatest advantage of using diamond as an acoustic substrate. It allows the fabrication of SAW devices operating in the gigahertz frequency range using standard optical lithography.

Microacoustic investigations of different structural forms of silicon

Silicon in its different structural forms is the most widely element in all modern microtechnological fields and in near future nanotechnological applications. Despite the great deal of interest in electronic, magnetic, and optical properties of all these types, only very little work is reported on their elastic properties. In this context, we determine the acoustic parameters: longitudinal, transverse and Rayleigh velocities as well as their corresponding acoustic impedances. Then, using angular spectrum model, we calculate their reflectance function and the acoustic materials signatures of these types of semiconductors. It is found that all V(z) signatures show an oscillatory behavior due to constructive and destructive interferences between different propagating surface acoustic wave modes. The values of wave velocities are found to change according to atomic arrangements as well as to defect density in different Si types. Similar variations are also noticed for their impedances as well as their elastic moduli.

Functional Passive Sensor System Using Reflective SAW Delay Line

ABSTRACTA new passive and remote SAW sensor system having ID tag function is presented. Surface acoustic wave (SAW) devices have been widely used as a chemical and physical sensor for measuring the environment. SAW devices have been also used as identification tags (ID tags). The majority of the work reported on SAW sensors to date has used Rayleigh mode SAW and are concerning a delay line. The other hand, it has been shown that the sensors utilizing the shear horizontal mode SAW (SH-SAW) can be designed to sense the liquid properties. The SH-SAW has the unique characteristic of complete reflection at a 90 degree edge normal to the direction of the propagation. This characteristic makes it possible to construct a reflective delay line using the edge reflection. The reflective delay line can be used for as passive and remote sensing.In this paper, a passive and remote SAW sensor with a function of identification tags has been designed by using the reflective SH-SAW delay line. The reflective delay line was constructed on the 36 YX LiTaO3 substrate having a 90 degree edge normal to the direction of the SAW propagation. Passive and remote SAW sensor consists of several reflective delay lines having different propagation length. The passive sensor and ID tags were made by the multi-channel delay line. The responses of the sensor and ID tags using the edge reflection of SH-SAW are obtained in time domain. Several experiments were performed to verify the performances of the multi-channel edge reflection sensor. The basic characteristics of the reflective SAW sensor were obtained by measuring the amplitude and phase of reflected wave. The obtained results showed that the reflective SH-SAW sensor was very effective for sensing the physical and chemical properties.Wireless systems are also suggested for the effective operation of the passive SAW sensor. A novel and simple electronic circuit system for accurately measuring the phase characteristics of responses form multi-channel is proposed. In the application, the reflective delay line has been mounted on cantilever to monitor the strain experimentally. The variation of the SH-SAW velocity on the delay line caused by a strain was measured as the passive SAW sensor response. The velocity varied in proportion to the strain applied to the propagation surface. The results showed that the passive sensor was very effective to measure the strain in a wireless mode and it was found that the system is very suitable to health and safety monitoring of building or airplane wing.Finally it will be shown some applications of the wireless sensor with ID tags on the measurement of the conductivity and viscosity of same liquids.

Comparison of Conductometric Gas Sensitivity of Surface Acoustic Wave Modes in Layered Structures

Properties of propagation modes for a layered ZnO/XZ LiNbO3 surface acoustic wave sensor have been investigated both theoretically and experimentally. By calculating the effective permittivity function, it was shown that after the ZnO deposition two distinct propagation modes have been generated: a Rayleigh and a shear horizontal. A layer of InOx was deposited to provide a conductometric sensing mechanism to NO2. It was found that the sensor is at least one order of magnitude more sensitive when operating in Rayleigh mode than in the shear horizontal mode. This sensitivity was found to be in good agreement with calculations for low concentrations of NO2, however discrepancies were observed for higher concentrations.

Sezawa mode SAW pressure sensors based on ZnO/Si structure

Surface acoustic wave (SAW) devices have been shown to be suitable for many sensor applications. One of these applications is pressure sensor. In this study we investigate the performance of SAW pressure sensors formed with ZnO/Si(001) structure. The pressure sensitivities of Rayleigh mode as well as the Sezawa mode are studied as a function of normalized thickness (kh = 2pihZnO/lambda). The experimental results show an opposite strain effect in the ZnO layer and Si substrate. A theoretical approach based on the perturbation method has been developed for the evaluation of pressure sensitivity in the Sezawa mode. Experimental and theoretical results obtained for the ZnO/Si SAW sensor prepared with kh = 1.18 are in good agreement. For kh < or = 1.2, the ZnO contribution to the sensor sensitivity can be neglected in the Sezawa mode in which ZnO acts mainly as an electromechanical conversion layer.

Surface acoustic wave ultraviolet photodetectors using epitaxial ZnO multilayers grown on r-plane sapphire

A surface acoustic wave (SAW) ultraviolet (UV) photodetector is made of a zinc oxide (ZnO) based epitaxial multilayer structure on an r-plane sapphire (r-Al2O3) substrate. Piezoelectric and semiconducting ZnO layers are used for SAW excitation and photodetection, respectively. A thin Mg0.2Zn0.8O layer grown between the two ZnO layers isolates the semiconducting layer from the piezoelectric one. In contrast to previously reported SAW UV detectors on GaN and LiNbO3, the Sezawa SAW mode in the ZnO∕r-Al2O3 system is used for its high acoustic velocity and large maximum effective piezoelectric coupling constant. The interaction of the SAW with the photogenerated carriers in the semiconducting ZnO layer results in a phase shift and an insertion loss change, as functions of light wavelength and power. The ZnO SAW UV detector can be used as a passive zero-power remote wireless UV sensor.

Human Immunoglobulin Adsorption Investigated by Means of Quartz Crystal Microbalance Dissipation, Atomic Force Microscopy, Surface Acoustic Wave, and Surface Plasmon Resonance Techniques

Time-resolved adsorption behavior of a human immunoglobin G (hIgG) protein on a hydrophobized gold surface is investigated using multitechniques: quartz crystal microbalance/dissipation (QCM-D) technique; combined surface plasmon resonance (SPR) and Love mode surface acoustic wave (SAW) technique; combined QCM-D and atomic force microscopy (AFM) technique. The adsorbed hIgG forms interfacial structures varying in organization from a submonolayer to a multilayer. An "end-on" IgG orientation in the monolayer film, associated with the surface coverage results, does not corroborate with the effective protein thickness determined from SPR/SAW measurements. This inconsistence is interpreted by a deformation effect induced by conformation change. This conformation change is confirmed by QCM-D measurement. Combined SPR/SAW measurements suggest that the adsorbed protein barely contains water after extended contact with the hydrophobic surface. This limited interfacial hydration also contributed to a continuous conformation change in the adsorbed protein layer. The viscoelastic variation associated with interfacial conformation changes induces about 1.5 times overestimation of the mass uptake in the QCM-D measurements. The merit of combined multitechnique measurements is demonstrated.

Li diffusion in epitaxial (11 $$\bar 2$$ 0) ZnO thin films

Zinc oxide (ZnO) possesses many interesting properties, such as a wide energy bandgap, large photoconductivity, and high excitonic binding energy. Chemical-vapor-deposition-grown ZnO films generally show n-type conductivity. A compensation doping process is needed to achieve piezoelectric ZnO, which is needed for surface acoustic wave (SAW), bulk acoustic wave, and micro-electromechanical system devices. In this work, a gas-phase diffusion process is developed to achieve piezoelectric (11\(\bar 2\)0) ZnO films. Comparative x-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements confirmed that high crystal quality and good surface morphology were preserved after diffusion. Photoluminescence (PL) measurements show a broad band emission with a peak wavelength at ∼580 nm, which is associated with Li doping. The SAW, including both Rayleigh-wave and Love-wave modes, is achieved along different directions in piezoelectric (11\(\bar 2\)0) ZnO films grown on an r-plane sapphire substrate.

SAW COM-parameter extraction in AlN/diamond layered structures.

Highly c-axis oriented aluminum nitride (AlN) thin piezoelectric films have been grown on polycrystalline diamond substrates by pulsed direct current (DC) magnetron reactive sputter-deposition. The films were deposited at a substrate temperature below 50 degrees C (room temperature) and had a typical full width half maximum (FWHM) value of the rocking curve of the AlN-002-peak of 2.1 degrees. A variety of one-port surface acoustic wave (SAW) resonators have been designed and fabricated on top of the AlN films. The measurements indicate that various SAW modes are excited. The SAW phase velocities of up to 11.800 m/s have been measured. These results are in agreement with calculated dispersion curves of the AlN/diamond structure. Finally, the coupling of modes parameters have been extracted from S11 measurements using curve fitting for the first SAW mode, which indicate an effective coupling K2 of 0.91% and a Q factor of about 600 at a frequency of 1050 MHz.

Anisotropic effects in surface acoustic wave propagation from a point source in a crystal

Strong anisotropic effects in the propagation of surface acoustic waves (SAWs) from a point-like source are studied experimentally and theoretically. Nanosecond SAW pulses are generated by focused laser pulses and detected with a cw probe laser beam at a large distance from the source compared to the SAW wavelength, which allows us to resolve fine intricate features in SAW wavefronts. In our theoretical model, we represent the laser excitation by a localized impulsive force acting on the sample surface and calculate the far-field surface response of an elastically anisotropic solid to such a force. The model simulates the measured SAW waveforms very well and accounts for all experimentally observed features. Using the data obtained for the (111) and (001) surfaces of GaAs, we describe a variety of effects encountered in the SAW propagation from a point source in crystals. The most interesting phenomenon is the existence of cuspidal structures in SAW wavefronts resulting in multiple SAW arrivals for certain ranges of the observation angle. Cuspidal edges correspond to the “phonon focusing” directions yielding sharp peaks in the SAW amplitude. A finite SAW wavelength results in “internal diffraction” whereby the SAW wavefront spreads beyond the group velocity cusps. Degeneration of a SAW into a transverse bulk wave is another strong effect influencing the anisotropy of the SAW amplitude and making whole sections of the SAW wavefront including some phonon focusing directions unobservable in the experiment. The propagation of a leaky SAW mode (pseudo-SAW) is affected by a specific additional effect i.e. anisotropic attenuation. We also demonstrate that many of the discussed features are reproduced in “powder patterns”, a simple technique developed by us earlier for visualization of SAW amplitude anisotropy.

LGX pure shear horizontal SAW for liquid sensor applications

This paper reports predicted and measured properties of the pure shear horizontal (SH) mode for the LGX family of crystals, which includes langasite (LGS), langanite (LGN), and langatate (LGT). These crystals are of the trigonal class 32 group, as quartz, and they exhibit the SH symmetry type uncoupling for the Euler angles (0/spl deg/, /spl theta/, 90/spl deg/). This surface acoustic mode, also known as surface transverse wave (STW), is especially attractive for liquid sensing due to the moderate damping observed in liquid or viscous environments. Numerical and experimental propagation data presented for the SH mode on LGX (0/spl deg/, /spl theta/, 90/spl deg/) includes phase velocity (v/sub p/), electromechanical coupling coefficient (K/sup 2/), temperature coefficient of delay (TCD), fractional change in frequency with respect to temperature (/spl Delta/f/fo), penetration depth, metal strip reflectivity, and excitation of spurious plate modes as a function of /spl theta/. High electromechanical coupling and zero temperature coefficient of delay (TCD) along LGX Euler angles (0/spl deg/, /spl theta/, 90/spl deg/), /spl theta/ between 10/spl deg/ and 25/spl deg/, with penetration depths comparable to surface acoustic wave (SAW) devices are disclosed. In particular, along LGT (0/spl deg/, 13.5/spl deg/, 90/spl deg/), the experimental results reported with resonators and delay line structures verify the high electromechanical coupling (0.8%) for a SH SAW mode, about 10 times stronger than the 36/spl deg/ Y rotated quartz SH orientation, and the existence of zero TCD around 140/spl deg/C. The phase velocity of 2660 m/s is within 0.2% of the calculated value, which is about 55% below the phase velocity of 36/spl deg/ Y quartz, thus leading to smaller STW devices. The penetration depth of 6.5 wavelengths is eight times more shallow than 36/spl deg/ Y quartz, thus providing significant SH mode energy trapping close to the surface. With such positive predicted and measured coupling and propagation characteristics, these orientations are appropriate for the fabrication of high coupling, zero TCD, smaller, and highly sensitive STW devices for filtering, frequency control, and liquid sensor applications.

Surface acoustic wave propagation characteristics of aluminum nitride thin films grown on polycrystalline diamond

Diamond has the highest surface acoustic wave (SAW) velocity among all materials and thus provides substantial advantages for the fabrication of high-frequency SAW devices when combined with a piezoelectric thin film. The properties of layered film structures consisting of a piezoelectric material layer, aluminum nitride (AlN), and a polycrystalline diamond layer grown on a silicon substrate have been examined. Highly textured AlN thin films have been sputter-deposited onto polycrystalline diamond substrates at room temperature. X-ray diffraction analysis of the multilayer structure as well as atomic force microscope images of the AlN surface indicate that the deposited AlN films were c-axis oriented with a full width at half maximum of the rocking curve of the AlN-002-peak of 2.1°. The thickness of the AlN layer was of 4.3 μm, whereas the diamond layer was 50 μm thick and resting on a 3-in Si wafer. Standard one-port SAW resonators with aluminum (Al) metallization have been subsequently fabricated and evaluated. Experimental results indicate that the fundamental as well as higher Rayleigh SAW modes are excited. Thus, the phase velocities of the first, second, third, and fourth mode are found to be 6.850, 10.000, 11.800, and 14.450 m/s respectively. A very good out-of-band rejection of −50 dB was obtained.

Nonlinear coupling of surface and quasitransverse bulk modes in cubic crystals.

In crystals the speed of the surface acoustic wave mode may approach that of the lowest-speed quasitransverse bulk mode in some directions of propagation. Under these circumstances, it is possible for energy to be transferred from the surface mode to the bulk mode by nonlinear coupling. In the present paper we investigate the possibilities for mode coupling in the (001), (110), and (111) planes of cubic crystals. A condition is given for determining the range of propagation directions with significant coupling, and numerical results are provided for eight different crystals with a range of anisotropy ratios. It is shown that even for significant excitation amplitudes the coupling is negligible for most propagation directions in the aforementioned surface cuts.

Shear-Horizontal-Type Surface Acoustic Waves on Quartz with Ta2O5 Thin Film

The propagation characteristics of two modes of surface acoustic waves (SAWs) on ST quartz with a tantalum pentoxide (Ta2O5) thin film are investigated theoretically and experimentally. One is the pure shear-horizontal (SH)-type SAW and the other is the Love-wave-type SAW converted from the leaky SAW (LSAW). For these SAWs, stronger energy trapping effects on the surface can be expected by loading the substrate with the Ta2O5 thin film, because this film has a relatively high density. In fact, the film thickness of the Ta2O5 film required to obtain the minimum insertion loss and the maximum K2 for the SH-type SAW on ST-90°X quartz is around 0.01 λ, which is approximately one-fourth of that reported for silicon dioxide film. Furthermore, the attenuation of the LSAW on ST-33°X quartz was reduced by loading with the Ta2O5 film because the LSAW becomes the Love-wave-type SAW without attenuation.

Novel Love mode surface acoustic wave based immunosensors

Love mode surface acoustic wave (SAW) immunosensors were developed. The Love mode guiding layers in these sensors are ZnO and SiO 2 thin films. It is shown that mass sensitivity of the devices with ZnO layer are larger than that of with SiO 2 guiding layers. A system comprising dual delay line, based on ZnO/90° rotated ST-cut quartz crystal devices, was set up for conducting the immunosensing experiments. In these experiments adsorption of rat immunoglobulin G onto the active area of the sensor was monitored. Upon exposure to solutions containing IgG, the operational frequency of the system changed due to the interaction of IgG and a gold surface on the active area of the sensor. Mass sensitivities were measured and frequency responses to various IgG concentrations are presented.

Liquid sensor probe using reflecting SH-SAW delay line

A new liquid sensor probe has been designed by using SH-SAW device. Shear horizontal mode surface acoustic wave (SH-SAW) has a unique characteristic of complete reflection at the free edges of the substrate. The SH-SAW are excited on a 36° YX LiTaO 3 and the right angle edge of the substrate has been used to reflect the SAW. Multi-channel reflecting SAW delay lines were constructed on the substrate. Both the phase and amplitude of the SH-SAW reflected from the edge were measured as a sensor response. Several experiments were performed to verify the edge reflection type SAW sensor in liquid phase. A sensing system configuration is also suggested for the effective operation of the sensor probe. An electronic circuits system for accurately measuring the phase characteristics of the reflected wave and for detecting the output from the multi-channel SAW sensor probe was shown.

Theoretical mass, liquid, and polymer sensitivity of acoustic wave sensors with viscoelastic guiding layers

The theoretical sensitivity of Love wave and layer-guided shear horizontal acoustic plate mode (SH-APM) sensors for viscoelastic guiding layers and general loading by viscoelastic materials is developed. A dispersion equation previously derived for a system of three rigidly coupled elastic mass layers is modified so that the second and third layers can be viscoelastic. The inclusion of viscoelasticity into the second, wave guiding layer, introduces a damping term, in addition to a phase velocity shift, into the response of the acoustic wave system. Both the waveguiding layer and the third, perturbing layer, are modeled using a Maxwell model of viscoelasticity. The model therefore includes the limits of loading of both nonguided shear horizontal surface acoustic wave and acoustic plate mode (APM) sensors, in addition to Love wave and layer-guided SH-APM sensors, by rigidly coupled elastic mass and by Newtonian liquids. The three-layer model is extended to include a viscoelastic fourth layer of arbitrary thickness and so enable mass deposition onto an immersed Love wave or layer-guided SH-APM sensor to be described. A relationship between the change in the complex velocity and the slope of the complex dispersion curve is derived and the similarity to the mass and liquid sensor response of quartz crystal microbalances is discussed. Numerical calculations are presented for the case of a Love wave device in vacuum with a viscoelastic waveguiding layer. It is shown that, while a particular polymer relaxation time may be chosen such that the effect of viscoelasticity on the real part of the phase speed is relatively small, it may nonetheless induce a large insertion loss. The potential or the use of insertion loss as a sensor parameter is discussed.

Guided propagation of surface acoustic waves in AlN and GaN films grown on4H–SiC(0001)substrates

The propagation modes of surface acoustic waves ~SAW! in AlN and GaN layers are identified for the case of the acoustic wave being confined in the epitaxial layers by the fast-velocity SiC substrate. Higher-order Rayleigh modes emerge when the SAW wavelength is reduced to be below the layer thickness. We also report on the observation of an extremely-high-velocity SAW mode, which exists when the SAW wavelength is about an order of magnitude larger than the layer thickness. These guided modes enable us to utilize SAW velocities which can be even larger than the Rayleigh velocity in the substrate while maintaining the large electromechanical coupling coefficient of the overlayer.

Surface and pseudo surface acoustic waves in langatate: predictions and measurements.

Langatate (LGT, La3Ga(5.5)Ta(0.5)O14) is a recent addition to materials of the trigonal crystal class 32, which is the same crystal class as quartz, langasite, langanite, and gallium phosphate. Langatate has several attractive acoustical properties, in particular: a measured bulk acoustic wave (BAW) resonator quality factor frequency product (Qf) of 16 million, comparable to that of AT cut quartz; high-piezoelectric coupling orientations, up to 0.5% for surface acoustic waves (SAWs), about five times larger than that of ST-X quartz; low power flow angle orientations in the vicinity of high coupling orientations; phase velocities about 20% smaller than those of ST-X quartz, facilitating the production of smaller, lower frequency devices; the existence of pseudo SAW modes for higher frequency applications. In this paper SAW contour plots of the phase velocity (vp), the electromechanical coupling coefficient (K2), the temperature coefficient of delay (TCD), and the power flow angle (PFA), are given showing the orientations in space in which high coupling is obtained, with the corresponding TCD, PFA, and vp characteristics for these orientations. This work reports experimental results on the SAW temperature fractional frequency variation (delta f/fo) and the TCD for several LGT orientations on the plane with Euler angles: (0 degrees, 132 degrees, psi). The temperature behavior has been measured directly on SAW wafers from 10 to 200 degrees C, and the results are compared with numerical predictions using our recently measured temperature coefficients for LGT material constants. This research also has uncovered temperature compensated orientations, which we have experimentally verified with parabolic behavior, turnover temperatures in the 130 to 160 degrees C range, and delta f/fo within 1000 ppm variation from 10 to 260 degrees C, appropriate for higher temperature device applications. Regarding the pseudo surface acoustic waves (PSAWs), results of calculations are presented for both the PSAW and the high velocity PSAW (HVPSAW) for some selected, rotated cuts. This study shows that propagation losses for the PSAWs of about 0.01 dB/wavelength, and phase velocities approximately 20% higher than that of the SAW, exist along specific orientations for the PSAW, thus showing the potential for somewhat higher frequency SAW device applications on this material, if required.