Temperature-stable orientations in LGS and LGN piezoelectric crystals for surface acoustic waves

The temperature characteristics of surface acoustic waves (SAW) propagating in LGS and LGN crystals are numerically analyzed. The optimal orientations that correspond to the zero value of the first-order temperature coefficient of delay (TCD) for SAW propagating in these crystals are considered. The second-order TCD for SAW is calculated for a wide range of operating temperatures. It is shown that the temperature dependences of the material constants of LGS and LGN crystals are strongly nonlinear. The characteristics of SAW propagating in a structure that consists of an isotropic layer overlying an LGS or LGN piezoelectric crystal are numerically calculated. It is shown that, in the presence of a thin aluminum layer of a certain thickness on the crystal surface, in some cases it is possible to extend the operating temperature range within which the TCD for SAW is equal to zero.     

Efficient intensity modulation due to surface acoustic waves on windows of the AlGaN/InGaN multiple quantum well laser

An alternative method for modulation of light generated by AlGaN/InGaN multiple quantum well laser with quartz antireflective (AR) coatings covering the resonator windows was modelled and studied theoretically. Disturbance of the piezoelectric coatings caused by surface acoustic waves (SAW) results in efficient intensity modulation of the laser beam. The model for the laser diode operating at DC demonstrates that the modulation factor can exceed 0.2 in sub-nanometer SAW displacement. For quarter-wave AR films, the carrier frequency of modulated beam has twice the frequency of SAW.     

Nonlinear frequency shift in surface acoustic wave resonator operating as a gas sensor

The shift in the resonance frequency of a two-port quartz surface acoustic wave (SAW) resonator operating as a gas sensor without a selective layer is studied versus the power of an SAW excited in the resonator. At working frequencies of the resonator (≈389 MHz) placed in the flow of moisture-containing nitrogen gas, an anomalously large positive shift of the resonance frequency is observed as the SAW power exceeds 1 mW. This shift is one order of magnitude larger than that due to the nonlinear amplitude-frequency effect, which is known for quartz SAW resonators. Possible physical mechanisms underlying this phenomenon are analyzed. Experimental data indicate that such a shift is associated with the influence of a powerful SAW on sorption processes taking place on the active surface of the resonator rather than being a direct consequence of heating of the SAW substrate by the powerful SAW.     

Vacuum-deposited wave-guiding layers on STW resonators based on LiTaO3 substrate as love wave sensors for chemical and biochemical sensing in liquids

A promising approach to apply the Love wave concept to commercially available low-loss surface acoustic wave (SAW) devices of the type Murata SAF 380 is presented. Thin wave-guiding layers of variable thickness are coated on the piezoelectric substrate of the devices. Two different layer materials were used: sputtered SiO₂ and a new polymer in this field, parylene C (poly-[2-chloro-p-xylylene]). Insertion loss, resonance frequency, frequency changes during protein precipitation and noise of the devices are discussed as a function of the thickness of the wave-guiding layer. It is demonstrated that the application of an optimized wave-guiding layer increases the sensitivity. When using SiO₂ as wave-guiding layer, an optimum layer thickness of 4 μm leads to a detection limit of 1.7 pg/mm². Therefore, the detection limit is improved by factor 7.7 as compared to uncoated SAW devices. Parylene-coated devices reach a detection limit of 2.9 pg/mm² at an optimum layer thickness of 0.5 μm. This corresponds to an improvement by factor 4.3. As the SAW devices used in this study are commercially available at low costs, applying appropriate wave-guiding layers permits an application as chemical or biochemical sensors with excellent sensitivities. Moreover, parylene-coated devices combine the sensitivity increase by excitation of Love waves with an excellent protective effect against corrosive attacks by the surrounding medium. Therefore, these sensors are most suitable for biosensing in conducting buffer solutions.     

MOCVD法生长SAWF用ZnO/Diamond/Si多层结构

使用等离子体辅助MOCVD系统在金刚石,硅衬底上成功地制备了氧化锌多层薄膜材料,通过两步生长法对薄膜质量进行了优化。XRD测试显示优化后的样品具有c轴的择优取向生长,PL谱测试表明样品经优化后不仅深能级发射峰消失,同时紫外发射峰增强。对优化后的样品的表面测试显示出较低的表面粗糙度。比较氧化锌多层薄膜结构的声表面波频散曲线,ZnO薄膜声表面滤波器受膜厚和衬底材料的影响较大。当ZnO薄膜较薄时,在它上面的传播速度将与衬底上的传播速度接近,与其他衬底上生长的薄膜相比,以金刚石这种快声速材料为衬底的ZnO多层薄膜结构,声表面波滤波器的中心频率将提高1倍左右。     

Thermal stabilization of SAW devices by metal films

The characteristics of surface acoustic waves (SAW) propagating in a structure that consists of a metal layer overlying an YX-cut quartz crystal are numerically analyzed for a wide range of operating temperatures. It is shown that the presence of a metal film of a certain thickness on the surface of an YX-cut quartz crystal considerably improves the thermal stability of the characteristics of SAW propagating in such a structure.     

应用于气体传感器的具有铝/金电极的单模式两端对声表面波谐振器

为改善气体传感器性能,通过器件优化设计获得了一种应用于气体传感器的具有低损耗、高品质因子(Q)的单模式两端对声表面波(SAW)谐振器。该谐振器由两个换能器、分置于换能器两边的短路栅反射器以及在换能器之间分布的用于敏感膜镀膜的约2.5mm金属薄层构成。谐振器采用铝/金双层电极以降低测试气体环境的腐蚀影响。利用经典耦合模(COM)理论对器件性能进行了仿真以提取优化的结构设计参数。基于仿真结果,实验研制了基于300MHz频率的新型铝/金电极SAW两端对谐振器,测试结果显示所研制器件具有较低损耗(〈7dB),较高Q值(-3000)以及单一谐振模式的特点,并且,以所研制的新型谐振器为频率控制单元的谐振器型振荡器表现出良好的频率稳定度(t15Hz/h),这对于改善气体传感器的检测下限及稳定性等性能指标具有重要意义。      

Characteristics of SAW UV sensors based on a ZnO/Si structure using third harmonic mode

This paper describes the characteristics of surface acoustic wave (SAW) ultraviolet (UV) sensors fabricated from a ZnO thin film using the third harmonic mode. A ZnO thin film was used as an active layer for UV detection, and a piezoelectric layer was sputtered using magnetron sputtering. The X-ray diffraction (XRD) and photoluminescence (PL) spectra showed that the ZnO sputtered onto Si(100) was highly (002)-oriented and had good optical properties. The two-port SAW resonator was based on an inter-digital transducer (IDT)/ZnO/Si structure and was fabricated and exposed under UV light at a wavelength of 380 nm. As a result, under a UV intensity of 3 mW/cm², the SAW UV sensor was greatly shifted by 400 kHz at the third harmonic mode compared to a frequency shift of 10 kHz in the fundamental mode.     

Development of two-port surface acoustic wave resonator with Al/Au electrodes for gas sensing

Simple and efficient surface acoustic wave(SAW)two-port resonators with low insertion loss and high Q-values on ST-X quartz substrate using a corrosion-proof A1/Au-stripe electrode structure are developed for gas sensing.It was composed of two shorted grating reflectors and adjacent intedigital transducers(IDT),and an active metal film in the cavity between the IDTs for the sensitive film coating.The devices are expected to provide good protection towards metal electrode for gas sensors application in chemically reactive environments.Excellent device performance as low insertion loss,high Q factor and single-mode are achieved by carefully selecting the metallic electrode thickness,cavity length and acoustic aperture.Prior to fabrication,the coupling of modes(COM)model was performed for device simulation to determine the optimal design parameters.The fabricated single-mode SAW resonator at operation frequency of 300 MHz range exhibits matched insertion loss of~6.5 dB and loaded Q factor in the 3000 range.Using the fabricated resonator as the feedback element,a dualresonator-oscillator with excellent frequency stability(0.1 ppm)was developed and evaluated experimentally,and it is significant for performance improvement of SAW gas sensor.     

Miniaturization of surface acoustic waves rotary motor

This paper presents the experimental study of a miniaturized surface acoustic waves (SAWs) rotary motor and the theoretical calculation. After the first success in SAW rotary motor operating at 9.85 MHz, a smaller rotary motor is designed. With the operating frequency of 30 MHz and the driving voltage of 120 V(p-p), the motor can rotate at a speed of 270 rpm.     

Multifrequency characterization of viscoelastic polymers and vapor sensing based on SAW oscillators

Simplified relations for the changes in SAW velocity and attenuation due to thin polymer coatings and vapor sorption are presented by making analytic approximations to the complex theoretical model developed earlier by Martin et al. [Anal. Chem. 66 (14) (1994) 2201–2219]. The approximate velocity relation is accurate within 4% for the film thicknesses up to 20% of the acoustic wavelength in the polymer film, and is useful for analyzing the mass loading, swelling and viscoelastic effects in SAW vapor sensors. The approximate attenuation relation is accurate within 20% for very thin films, (less than 2% of the acoustic wavelength in the film). Based on these relations, a new procedure for determination of polymer viscoelastic properties is described that exploits the frequency dependence of the velocity and attenuation perturbations, and employs multifrequency measurement on the same SAW platform. Expressions for individual contributions from the mass loading, film swelling and viscoelastic effects in SAW vapor sensors are derived, and their implications for the sensor design and operation are discussed. Also, a new SAW comb filter design is proposed that offers possibility for multimode SAW oscillator operation over a decade of frequency variation, and illustrates feasibility for experimental realization of wide bandwidth multifrequency SAW platforms.     

Transparent ZnO/glass surface acoustic wave based high performance ultraviolet light sensors

Surface acoustic wave(SAW) resonators are a type of ultraviolet(UV) light sensors with high sensitivity, and they have been extensively studied. Transparent SAW devices are very useful and can be developed into various sensors and microfluidics for sensing/monitoring and lab-on-chip applications. We report the fabrication of high sensitivity SAW UV sensors based on piezoelectric(PE) ZnO thin films deposited on glass substrates. The sensors were fabricated and their performances against the post-deposition annealing condition were investigated. It was found that the UV-light sensitivity is improved by more than one order of magnitude after annealing. The frequency response increases significantly and the response becomes much faster. The optimized devices also show a small temperature coefficient of frequency and excellent repeatability and stability, demonstrating its potential for UV-light sensing application.     

由于表面粗糙引起的激光声表面波色散的实验和理论研究

表面粗糙是材料制造过程中必有的副产物, 粗糙表面会引起其中传播的声表面波的速度发生变化. 在利用激光声表面波对材料性质进行评估时, 常用宽带的激光声表面波速度频散特性对材料性质进行反演. 为了研究表面粗糙度是否能作为反演的特征参数之一, 本文建立了激光在表面粗糙样品中激发声表面波、聚偏氟乙烯换能器宽带接收声表面波的实验装置来研究不同粗糙度表面对声表面波速度的影响; 理论上建立了激光在粗糙表面中激发声表面波的计算模型, 利用有限元法得到声表面波的时域特征, 并进一步得到声表面波的速度色散曲线, 理论结果和实验结果能很好地拟合. 这为利用激光声表面波对表面粗糙的评估提供理论和实验依据. 关键词： 表面粗糙 激光声表面波 速度色散 聚偏氟乙烯传感器 有限元法     

Active control of normal particle velocity at a boundary between two media

A spatially one-dimensional model of a plane active double layer between two homogeneous elastic half-spaces is studied analytically. The layer synthesizes a preset smooth trajectory of the controlled boundary between the media without any mechanical support. The outer layer of the coating is a piezoelectric, and the inner layer is a polymer that is transparent for low-frequency sound and opaque for high-frequency sound because of dissipation. An algorithm for controlling the piezoelectric elements of the layer on the basis of signals from surface particle-velocity sensors is proposed, and a method for measuring the particle velocity is developed. Conditions of stability and efficiency of the synthesis are formulated. It is shown that the active layer thickness can be much smaller than the wavelength corresponding to the minimal time scale of the boundary trajectory to be formed. The accuracy of the trajectory synthesis depends on the accuracy of measuring, computing, and actuating elements of the system but does not depend on the vibroacoustic characteristics of the half-spaces separated by the active layer or on the presence of smooth waves in these half-spaces. For the synthesis to be efficient, the operating frequency band and the dynamic range of sensors and actuators should be many times greater than the frequency band and the dynamic range of the trajectory to be formed.     

基于绝缘体上硅的一种改进的Mach-Zehnder声光调制器

传统的基于绝缘体上硅的Mach-Zehnder(MZ)声光调制器中,叉指换能器位于两臂的同一侧.为实现高的调制效率,声表面波的波峰和波谷分别调制MZ干涉仪的两臂,这要求控制MZ干涉仪两臂之间的距离为奇数倍声波半波长.但实际上由于传播过程中衬底材料的变化,声波波长会变大,这会导致两臂的间距难以准确设置.另一方面,声波在传播过程中经过MZ干涉仪的一臂后会发生衰减,降低了对另一臂的调制效果,影响了整体的调制效率.本文针对这些问题给出了一种解决方案,把叉指换能器放在MZ波导两臂之间,确保MZ干涉仪两臂到叉指电极中心距离相等.采用有限元法,首先对新提出的结构进行分析,然后通过声光互作用原理得到了材料的折射率变化;进而研究了波导类型、波导宽度、氧化锌厚度及叉指对数等因素对声光调制效率的影响,并对声光调制器的结构参数进行了优化以提高其性能.基于COMSOL Multiphysics的仿真结果表明,当条波导宽度为6μm,氧化锌只覆盖有叉指电极的部分且厚度为2.2μm,控制叉指电极数目为50对时,波导有效折射率变化在驱动电压为1 V时可以达到4.08×10~(-4),比传统结构提高了12%.     

表面声波传感器及其应用

本文简要介绍多种表面声波传感器的结构、特点、应用及其研究进展概况。表面声波传感器依其波型分类,主要包括瑞利波、水平切变声板波、乐甫波和兰姆波传感器;其应用领域根据作用特点大致可分为物理传感器和化学、生物传感器,前者较易实现,后者特异性强,需要针对具体情况作更细致研究。由于表面声波的传播速度和相位对周围环境的参量变化极为敏感,因此有关传感器具有很高的灵敏度和广泛的应用领域。     

Development of human IgE biosensor using Sezawa-mode SAW devices

This paper reports Sezawa-mode surface acoustic wave (SAW) devices with via-isolated cavity to construct the allergy biosensor. To fabricate Sezawa-mode SAW devices, the RF magnetron sputtering method for the growth of piezoelectric ZnO thin films are adopted and influences of the sputtering parameters are investigated. The optimal substrate temperature of 300 °C, RF power of 120 W and sputtering pressure of 2 Pa were used to deposit piezoelectric ZnO films with a smooth surface, uniform grain size and strongly c-axis-orientated crystallization. A back-etched SAW resonator is used in this study. The wet etching of (100)-oriented silicon wafers is used to form a back-side cavity which is critical to the formation of a hopper cavity for holding bio-analytes. The remaining membrane structure silicon thickness was 25 μm. In this report, the chrome (Cr, 12 nm)/gold (Au, 66 nm) layer was initially deposited onto the sensing area of SAW devices as the binding layer for biochemical sensor. The resonance frequency of the Sezawa-mode SAW device is 1.497 GHz. The maximum sensitivity of the Sezawa-mode is calculated to be 4.44 × 10⁶ cm²/g for human immunoglobulin-E (IgE) detection. The stability for human IgE detection is calculated to be 80% and the variation of the stability ±3% was obtained after several tests.     

Acoustic study of adsorbed liquid layers

Interference measurements of small variations in the velocity and attenuation of surface acoustic waves (SAWs) are used to investigate water layers up to 15 nm thick adsorbed on the surface of a lithium niobate crystal. The frequency dependence of the relative variation of the SAW velocity with the adsorption of water vapor is measured in the range from 40 to 400 MHz. Acoustic techniques are used to experimentally estimate the frequency dependence of the dielectric constant of adsorbed water and its dipole relaxation frequency along with the dependence of the adsorption layer thickness on the water vapor pressure in the surrounding medium. A simple expression is proposed for calculating the dispersion of the SAW velocity in a solid loaded with a thin liquid layer.     

Investigation of titanium nitride coating by broadband laser ultrasonic spectroscopy

We present a laser ultrasonic method to investigate a titanium nitride(TiN) coating specimen.The technique is based on the principle of surface acoustic wave (SAW) dispersion during acoustic propagation on a half-space with the presence of a thin layer.Due to the high efficiency of laser line-source excitation,we have been able to generate and detect a SAW with an excellent signal-to-noise ratio in a wide frequency band.An inverse fitting algorithm was employed to extract simultaneously the thickness and the elastic parameters of the TiN coating from the experimental SAW velocity dispersion curve.