Characteristics of MgxZn1-xO thin film bulk acoustic wave devices

Piezoelectric thin film zinc oxide (ZnO) and its ternary alloy magnesium zinc oxide (Mg/sub x/Zn/sub 1-x/O) have broad applications in transducers, resonators, and filters. In this work, we present a new bulk acoustic wave (BAW) structure consisting of Al/Mg/sub x/Zn/sub 1-x/O/n/sup +/-ZnO/r-sapphire, where Al and n/sup +/ type ZnO serve as the top and bottom electrode, respectively. The epitaxial Mg/sub x/Zn/sub 1-x/O films have the same epitaxial relationships with the substrate as ZnO on r-Al/sub 2/O/sub 3/, resulting in the c-axis of the Mg/sub x/Zn/sub 1-x/O being in the growth plane. This relationship promotes shear bulk wave propagation that affords sensing in liquid phase media without the dampening effects found in longitudinal wave mode BAW devices. The BAW velocity and electromechanical coupling coefficient of Mg/sub x/Zn/sub 1-x/O can be tailored by varying the Mg composition, which provides an alternative and complementary method to adjust the BAW characteristics by changing the piezoelectric film thickness. This provides flexibility to design the operating frequencies of thin film bulk acoustic wave devices. Frequency responses of devices with two acoustic wave modes propagating in the specified structure are analyzed using a transmission line model. Measured results show good agreement with simulation.     

Mg/sub x/Zn/sub 1-x/O: a new piezoelectric material

Piezoelectric ZnO thin films have been successfully used for multilayer surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices. Magnesium zinc oxide (Mg/sub x/Zn/sub 1-x/O) is a new piezoelectric material, which is formed by alloying ZnO and MgO. Mg/sub x/Zn/sub 1-x/O allows for flexibility in thin film SAW device design, as its piezoelectric properties can be tailored by controlling the Mg composition, as well as by using Mg/sub x/Zn/sub 1-x/O/ZnO multilayer structures. We report the metal-organic chemical vapor deposition (MOCVD) growth, structural characterization and SAW evaluation of piezoelectric Mg/sub x/Zn/sub 1-x/O (x<0.35) thin films grown on (011~2) r-plane sapphire substrates. The primary axis of symmetry, the c-axis, lies on the Mg/sub x/Zn/sub 1-x/O growth plane, resulting in the in-plane anisotropy of piezoelectric properties. SAW test devices for Rayleigh and Love wave modes, propagating parallel and perpendicular to the c-axis, were designed and fabricated. Their SAW properties, including velocity dispersion and piezoelectric coupling, were characterized. It has been found that the acoustic velocity increases, whereas the piezoelectric coupling decreases with increasing Mg composition in piezoelectric Mg/sub x/Zn/sub 1-x/O films.     

Influence of spinel substrate and over-layer for enhanced SAW and AO properties with KNbO3 thin film

Surface acoustic wave (SAW) propagation characteristics have been studied using modeling calculations for a potassium niobate (KNbO/sub 3/) thin film-layered structure with [001] and [110] orientation on a single crystal spinel (MgAl/sub 2/O/sub 4/) substrate, and a spinel buffer layer on silicon. Variation in the electromechanical coupling and acoustic attenuation has been compared. A significantly high value of coupling factor (k/sub max//sup 2/=23%) is obtained for the [001]KNbO/sub 3//spinel structure by introducing an optimum thickness of spinel over-layer for potential wide bandwidth SAW device applications. The dispersion characteristics with the [110] KNbO/sub 3/ orientation indicate an initial peak in the coupling coefficient value (k/sub max//sup 2/=8.8%) at a relatively low KNbO/sub 3/ film thickness that appears attractive for fabricating devices with thinner films. The KNbO/sub 3/ film with [001] orientation is found attractive for efficient acousto-optic (AO) device application with the formation of a symmetric waveguide structure (spinel(0.5 /spl mu/m)/KNbO/sub 3/(1.0 /spl mu/m)/spinel). A high value of k/sup 2/=23.5% with 50% diffraction efficiency has been obtained for the spinel(0.5 /spl mu/m)/KNbO/sub 3/(1.0 /spl mu/m)/spinel structure at 1 GHz SAW frequency and 633 nm optical wavelength at a very low input drive power of 15.4 mW.     

A piezoelectric-sound-resonance cavity for hydrogen gas detection

A new concept for hydrogen gas (H/sub 2/) detection has been developed. This concept is based on a piezoelectric-sound-resonance cavity (PSRC). Detection uses sound resonance and the acoustic property differences of gases as a sensing mechanism in which there is a significant difference in the sound velocity and acoustic impedance between H/sub 2/ and air. The PSRC sensor consists of two thin piezoelectric discs, separated by a small cavity. One disc excites a weak acoustic standing wave in the cavity, and the second senses the wave by monitoring the acoustic impedance characteristics of the cavity. Changes in H/sub 2/ concentration result in a shift of the sound resonance state. Investigations of a PSRC prototype have demonstrated a sensitivity limit of <10 ppm, a fast response time /spl sim/1 second, and a signal (phase and voltage) in proportion to the change in H/sub 2/ concentration (n) over the range of 10/sup -5/ < n < 0.2. These performance characteristics are far superior to those of other methods.     

一种由横波激发的特殊非线性声现象

目前对非线性超声的研究多集中在纵波激发的谐波性质以及对材料微观结构变化的实验检测上,横波激发的非线性声波性质少有研究。对横波激发的一维非线性声波方程入手,利用摄动法求解该方程,并改写为一阶偏微分方程,然后利用交错网格的有限差分形式进行数值求解。结果表明:采用横波激发,能产生线性横波和非线性纵波,且纵波的高次谐波内有两个信号,分别以纵波和横波两种速度传播。若采用较长的激发信号,纵波谐波能形成"拍"现象,成为一种奇特的声传播现象。     

Longitudinal leaky SAW resonators and filters on YZ-LiNbO/sub 3/

The high-phase velocity (above 6100 m/s in and aluminum (Al) grating on lithium niobate (LiNbO/sub 3/)) of the longitudinal leaky surface acoustic wave (SAW) (LLSAW) mode makes it attractive for application in high-frequency SAW ladder filters in the 2-5 GHz range. We investigate the dependence of one-port synchronous LLSAW resonator performance or YZ-LiNbO/sub 3/ on the metallization thickness and metallization ratio, both experimentally and theoretically. Our results indicate a strong dependence of the Q factor and resonance frequency on the aluminum thickness, with the optimal thickness that produces the highest Q values being about 8%. The optimal thickness increases with the metallization ratio. The observed behavior is interpreted with the help of simulations using a combined finite element method (FEM)/boundary element method (BEM) technique. As an application, bandpass filters have been fabricated in the 2.8 GHz frequency regime, based on LL-SAWs. The synchronous resonators constituting the ladder filters operate in the fundamental mode. The filters feature low insertion losses below 3 dB and wide relative passbands of 4.5-5%.     

Longitudinal critical angle singularities and their effect on V(Z)of the line-focus-beam acoustic microscope

In order to investigate a mechanism which causes a velocity difference between the longitudinal wave and leaky surface skimming compressional wave (LSSCW) observed in a line-focus-beam acoustic microscope, the analytic property of an acoustic reflection coefficient and its effect on a V(z) analysis were studied. A pole hidden in the unphysical Riemann sheet close to the longitudinal branch point is found to be responsible for the abrupt phase change at the longitudinal critical angle. This, together with an effect of a dominant Rayleigh wave pole, affects the V(z) measurement of the LSSCW. A method to estimate the longitudinal and shear wave velocities is discussed     

Calibration of Curie temperatures for LiTaO/sub 3/ single crystals by the LFB ultrasonic material characterization system

The line-focus-beam ultrasonic material characterization (LFB-UMC) system is applied to a standardized comparison and evaluation of the Curie temperatures, T/sub C/, exclusively used in evaluating the chemical compositions of commercial LiTaO/sub 3/ crystals by measuring the velocities of Rayleigh-type leaky surface acoustic waves (LSAWs), V/sub LSAW/. We measured V/sub LSAW/ and T/sub C/ (standardized) under the same T/sub C/ measurement conditions for 36/spl deg/Y X-LiTaO/sub 3/ single-crystal wafers produced by four manufacturers and related the results to the T/sub C/ (individual) measured by the individual manufacturers. The relationships between V/sub LSAW/ and T/sub C/ (individual) varied from one company to another, and a single straight line of the proportional relationship between V/sub LSAW/ and T/sub C/ (standardized) was obtained for all wafers regardless of the manufacturer. These experimental results clarify that the problem associated with T/sub C/ measurements lies in the measurement conditions and the absolute accuracy of the measurement instruments. Measurements of the center frequencies of SH-type surface acoustic wave (SAW) filter devices are compared with V/sub LSAW/ measurements. A method of calibrating T/sub C/ using this ultrasonic system is proposed to establish standardized specifications of SAW-device crystal wafers.     

基于相控阵激光超声的裂纹衍射增强试验研究

针对超声衍射时差法(TOFD)存在检测精度较差、区域检测可靠性不够和信号信噪比(signal-to-noise ratio, SNR)低等问题,提出了一种基于光纤皮秒激光器和高速旋转镜的相控阵激光超声裂纹检测方法。利用有限元方法模拟热弹机制,建立二维瞬态激光超声力-固耦合模型产生横波与纵波在缺陷处发生的衍射现象,分析了裂纹尖端不同奇异点、声波不同中心频率和相控阵激励源不同位置对声波衍射的影响,通过衍射信号的信噪比和位移幅值两个计算指标来分析变化规律,并进行了试验验证。结果表明:数值模拟与试验结果有较好的一致性,相控阵激光源较传统单束激光源对衍射信号幅值和信噪比有明显的增强作用,纵波衍射信号信噪比较理想;衍射信号幅值随裂纹尖端奇异点增加和声波中心频率减小而增大;信噪比随尖端奇异点增加而增大,随声波中心频率一定范围增加无明显变化,随激光源距离的增加呈现先增加后减小的趋势;缺陷定量分析时计算出的裂纹长度与实际裂纹的误差均不超过6.8%。     

Reflection of plate acoustic waves produced by a periodic array of mechanical load strips or grooves

The reflection of fundamental acoustic waves propagating in a thin piezoelectric plate by a periodic array of conducting strips of finite thickness or grooves has been theoretically and experimentally investigated. The analysis has shown that electrical shorting and mass loading affect the relationship of neighboring region impedances in a contrary manner. In some cases, these effects are comparable, and there exists a certain strip thickness for each piezoactive fundamental plate mode at which the reflection coefficient can become zero. A high efficiency of grooved reflector for plate acoustic waves was theoretically revealed. Experimental results for mass loading and grooved reflections, which have been obtained for an SH/sub 0/ wave propagating in the Y-X lithium niobate plate, are in a good agreement with the theory. They show a high efficiency of such reflectors and confirm the validity of using a model based on an equivalent circuit for the analysis of their operation. Investigations indicate that nearly 100% reflection of the SH/sub 0/ wave in the lithium niobate plate can be obtained with the use of a mass loading reflector containing 10 silver strips of thickness d/h=0.08 or a grooved reflector containing eight grooves of depth d/h=0.25. Here h is the plate thickness and d is the reflector thickness or depth.     

Magnetoelastic wave tunneling via a gap between ferroelectric crystals with relative longitudinal displacement

The tunneling of a plane monochromatic acoustic wave via a gap between two ferromagnetic crystals exhibiting relative longitudinal displacement has been studied. It is shown that, at a gap width comparable with the wavelength, the acoustic wave can exhibit complete transmission at the Damon-Eshbach mode frequency. If the gap width is much smaller than the wavelength, the complete transmission takes place at two resonance frequencies. Allowance for the longitudinal displacement of one crystal leads in all cases to violation of the resonance conditions, which results in a significant decrease in the transmission coefficient. The greater the velocity of crystal displacement, the stronger the decrease in the acoustic wave transmission though the gap between ferromagnetic crystals.     

Second harmonic generation of shear waves in crystals

Nonlinear self-interaction of shear waves in electro-elastic crystals is investigated based on the rotationally invariant state function. Theoretical analyses are conducted for cubic, hexagonal, and trigonal crystals. The calculations show that nonlinear self-interaction of shear waves has some characteristics distinctly different from that of longitudinal waves. First, the process of self-interaction to generate its own second harmonic wave is permitted only in some special wave propagation directions for a shear wave. Second, the geometrical nonlinearity originated from finite strain does not contribute to the second harmonic generation (SHG) of shear waves. Therefore, unlike the case of longitudinal wave, the second-order elastic constants do not involve in the nonlinear parameter of the second harmonic generation of shear waves. Third, unlike the nonlinearity parameter of the longitudinal waves, the nonlinear parameter of the shear wave exhibits strong anisotropy, which is directly related to the symmetry of the crystal. In the calculations, the electromechanical coupling nonlinearity is considered for the 6 mm and 3 m symmetry crystals. Complement to the SHG of longitudinal waves already in use, the SHG of shear waves provides more measurements for the determination of third-order elastic constants of solids. The method is applied to a Z-cut lithium niobate (LiNbO/sub 3/) crystal, and its third-order elastic constant c/sub 444/ is determined.     

High-Q whispering modes in empty spherical cavity resonators

This work presents the study of high-order modes in spherical cavity resonators. In general there are resonant mode families, degenerate in frequency, that "whisper" around the spherical surface. We call these whispering spherical (WS) mode sets. Each set includes the well-known whispering gallery (WG) mode, which propagates like a ray around the azimuth. Also, we identify a new mode, which we label the whispering longitudinal (WL) mode. This mode propagates as a wave front along the longitudinal direction. The rest of the degenerate set propagates like a combination of the WG and WL modes. We show that transverse electric WS modes have high geometric factors, greater than 2000, which increase linearly with frequency. This is an order of magnitude greater than that of a TM/sub 010/ cylindrical resonator. Also, Q-factors as high as 65,000 at 13.3 GHz were measured at room temperature.     

Thin film bulk acoustic wave filter

Thin film bulk acoustic wave (BAW) resonators (FBAR) are fabricated on a silicon nitride bridge using a ZnO piezolayer on a glass substrate and surface micromachining by standard thin film technology. These resonators exhibit a coupling constant k/sub t//sup 2/=7.8% at the first thickness extensional wave mode and are used as impedance elements in a ladder filter in the 1-GHz frequency band of mobile telecommunications. An electrical equivalent circuit is used to characterize the properties of the resonators and to show how the performance of the filter depends on the parameters of the resonators. 2.5% bandwidth, 2.8-dB insertion loss, and 35-dB selectivity are obtained in a filter with six resonators. The technology can be used to manufacture miniature microwave filters without any additional inductances.     

纵波在含有线性粘滞粘接层的粘接结构中的传播

研究了纵波垂直入射上下基体为同种介质且含有线性粘滞粘接层的三层板状粘接结构时声波的反射和透射特性,基于粘接界面的准静态模型(QSM)并结合各向同性线性粘滞体的本构方程,导出了含有粘滞系数和体积弹性模量的纵波反射和透射系数表达式。首先将纵波的准静态模型解和精确解进行对比,研究了准静态模型的适用条件。其次在界面处于不同接触形式(完好连接和粘滞型弱粘接)的情况下,分别讨论了考虑和不考虑界面相对质量对声反射和透射特性的影响。接着分析了粘滞粘接层参数的变化对纵波反射和透射特性的影响。最后在不大于0.1 MHz-mm的较小频厚积范围内简要阐述了如何鉴别界面形式。结果表明,准静态模型适用于较小频厚积的检测;是否考虑界面的相对质量对纵波的反射和透射特性影响不大;在特定的频厚积下,利用纵波的反射或透射系数方法结合准静态模型可有效辨别界面形式。研究成果可为实验时采用纵波垂直入射检测粘接结构提供一定的理论参考。     

Quantitative acoustic microscopy of anodized and coated aluminium at frequencies up to 1 GHz

Quantitative acoustic microscopy (QAM) has been used to measure surface wave velocities on polished, anodized and coated aluminium substrates, these materials being representative of those used for adhesive bonding in the aerospace industry. Good quality acoustic measurements were obtainable at frequencies between 225 and 980 MHz, despite the inhomogeneous nature of the oxide layer produced by phosphoric acid anodization (PAA). Good agreement was obtained between the surface acoustic wave dispersion measured on aluminium coated with 0.2 and 1.0 m PMMA, and that calculated by a simple isotropic layer model. The anodized aluminium was modelled as a transversely isotropic oxide layer on an aluminium substrate. At 0.2 m, the oxide layer was too thin for the comparison between measurement and calculation to be conclusive, but the calculations suggest that a change in porosity of 10% in a 0.6 m oxide layer, as obtained with an industry standard PAA treatment, should be readily detectable. The highly dispersive nature of some of the surface acoustic wave modes makes QAM extremely sensitive to small changes in the material parameters.     

Development of smart tongue devices for measurement of liquid properties

In this paper, we describe the design and characterization of shear horizontal surface acoustic wave devices for the analysis of liquid samples. Devices were fabricated on both 36/spl deg/ rotated Y-cut X-propagating LiTaO/sub 3/ and LiNbO/sub 3/ substrates. The design consists of a dual delay-line configuration where one delay line is metallized and shielded, while the other is left electrically active. Experiments to characterize the devices in terms of sensitivity, temperature dependence, and mass loading have been conducted and the results presented. Different liquid samples, i.e., water, orange juice, and milk, are 100% linearly separable using principal components analysis. In addition, it is possible to measure the fat content (/spl plusmn/0.1%) as well as the freshness of full (whole) milk.     

Unidirectional SAW transducer for gigahertz frequencies

A single-phase unidirectional transducer (SPUDT) structure using /spl lambda//4 and wider electrodes is introduced. The considerable difference between the reflectivity of short-circuited /spl lambda//4 electrodes and that of floating /spl lambda//2-wide electrodes on 128/spl deg/ lithium niobate (LiNbO/sub 3/) is exploited. The surface acoustic wave (SAW) device operating at 2.45 GHz has critical dimensions of about 0.4 /spl mu/m, accessible for standard optical lithography.