Surface Acoustic Wave Humidity Sensors Based on(1120) ZnO Piezoelectric Films Sputtered on R-Sapphire Substrates

ZnO films on R-sapphire substrates are prepared and characterized by x-ray diffraction and scanning electron microscopy, which indicate that the thin films are well crystallized with （1120） texture. Love wave and Rayleigh wave are used for fabrications of humidity sensors, which are excited in [1100] and [0001] directions of the （1120） ZnO piezoelectric films, respectively. The experimental results show that both kinds of sensors have good humidity response and repeatability, and the performances of the Love wave sensors are better than those of the Rayleigh wave sensors at room temperature. Moreover, the theoretical calculations of the mass sensitivity of the sensors are a/so carried out and the calculated results are in good agreement with the experimental measurements.

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.

Influence of roughness on the detection of mechanical characteristics of low-k film by the surface acoustic waves

The surface acoustic wave （SAW） technique is a precise and nondestructive method to detect the mechanical charac- teristics of the thin low dielectric constant （low-k） film by matching the theoretical dispersion curve with the experimental dispersion curve. In this paper, the influence of sample roughness on the precision of SAW mechanical detection is inves- tigated in detail. Random roughness values at the surface of low-k film and at the interface between this low-k film and the substrate are obtained by the Monte Carlo method. The dispersive characteristic of SAW on the layered structure with rough surface and rough interface is modeled by numerical simulation of finite element method. The Young＇s moduli of the Black DiamondTM samples with different roughness values are determined by SAWs in the experiment. The results show that the influence of sample roughness is very small when the root-mean-square （RMS） of roughness is smaller than 50 nm and correlation length is smaller than 20 μm. This study indicates that the SAW technique is reliable and precise in the nondestructive mechanical detection for low-k films.

Surface Acoustic Wave Device with Reduced Insertion Loss by Electrospinning P(VDF–TrFE)/ZnO Nanocomposites

Surface acoustic wave(SAW) devices have been utilized for the sensing of chemical and biological phenomena in microscale for the past few decades. In this study, SAW device was fabricated by electrospinning poly(vinylidenefluoride-co-trifluoroethylene)(P(VDF-TrFE)) incorporated with zinc oxide(ZnO) nanoparticles over the delay line area of the SAW device. The morphology, composition, and crystallinity of P(VDF-TrFE)/ZnO nanocomposites were investigated. After measurement of SAW frequency response, it was found that the insertion loss of the SAW devices incorporated with ZnO nanoparticles was much less than that of the neat polymer-deposited device. The fabricated device was expected to be used in acoustic biosensors to detect and quantify the cell proliferation in cell culture systems.

Characteristics of surface acoustic waves in (1120) ZnO film/R-sapphire substrate structures

（1120）ZnO film/R-sapphire substrate structure is promising for high frequency acoustic wave devices. The propagation characteristics of SAWs, including the Rayleigh waves along [0001] direction and Love waves along [1100] direction, are investigated by using 3 dimensional finite element method （3D-FEM）. The phase velocity （vp）, electromechanical coupling coefficient （k2）, temperature coefficient of frequency （TCF） and reflection coefficient （r） of Rayleigh wave and Love wave devices are theoretically analyzed. Furthermore, the influences of ZnO films with different crystal orientation on SAW properties are also investigated. The results show that the 1st Rayleigh wave has an exceedingly large/d of 4.95% in （90°, 90°, 0°） （1120）ZnO film/R-sapphire substrate associated with a phase velocity of 5300 m/s; and the 0th Love wave in （0°, 90°, 0°） （1120）ZnO film/R-sapphire substrate has a maximum k2 of 3.86% associated with a phase velocity of 3400 m/s. And （1120）ZnO film/R-sapphire substrate structures can be used to design temperature-compensated and wide-band SAW devices. All of the results indicate that the performances of SAW devices can be optimized by suitably selecting ZnO films with different thickness and crystal orientations deposited on R-sapphire substrates.

Propagations of Rayleigh and Love waves in ZnO films/glass substrates analyzed by three-dimensional finite element method

Propagation characteristics of surface acoustic waves（SAWs） in ZnO films/glass substrates are theoretically investigated by the three-dimensional（3D） finite element method. At first, for（11ˉ20） ZnO films/glass substrates, the simulation results confirm that the Rayleigh waves along the [0001] direction and Love waves along the [1ˉ100] direction are successfully excited in the multilayered structures. Next, the crystal orientations of the ZnO films are rotated, and the influences of ZnO films with different crystal orientations on SAW characterizations, including the phase velocity, electromechanical coupling coefficient, and temperature coefficient of frequency, are investigated. The results show that at appropriate h/λ, Rayleigh wave has a maximum k^2 of 2.4% in（90°, 56.5°, 0°） ZnO film/glass substrate structure; Love wave has a maximum k^2 of 3.81% in（56°, 90°, 0°） ZnO film/glass substrate structure. Meantime, for Rayleigh wave and Love wave devices, zero temperature coefficient of frequency（TCF） can be achieved at appropriate ratio of film thickness to SAW wavelength. These results show that SAW devices with higher k^2 or lower TCF can be fabricated by flexibly selecting the crystal orientations of ZnO films on glass substrates.

Improved electromigration reliability of surface acoustic wave devices using Ti/Al-Mo/Ti/Al-Mo electrodes

In order to obtain both high electromigration （EM） reliability and free-dimensional control in high-frequency surface acoustic wave （SAW） devices, 4-layered Ti/Al-Mo/Ti/Al-Mo electrode films were investigated on 128° Y-X LiNbO3 substrates by sputtering deposition. The resuits indicated that the 4-layered films had an improved EM reliability compared to conventional Al-0.5wt.%Cu films. Their lifetime is approximately three times longer than that of the Al-0.5wt.%Cu films tested at a current density of 5 x 107 A/cm^2 and a temperature of 200℃. Moreover, the 4-layered films were easily etched in reactive ion etching and fine-dimensional control was realized during the pattern replication for high-frequency SAW devices. For the 4-layered films, an optimum Mo quantity and sputtering parameters were very significant for high EM reliability.

Effect of working pressure and temperature on ZnO film deposited on free-standing diamond substrates

The structure characteristic and electric performance of ZnO film deposited on nucleation side of free-standing diamond substrates under different heating temperatures (Th) of substrate and working pressures (p) were studied. The structure of the ZnO films tested by X-ray diffraction shows that ZnO film of high c-axis orientation is deposited on the nucleation side of free-standing diamond substrate which is extremely smooth when Th=250 ℃ and p=0.4 Pa. After annealing at 480 ℃ in N2 atmosphere, the SEM and the AFM analyses demonstrate that the c-axis orientation of ZnO film is obviously enhanced. The resistivity of ZnO films also increases up to 8×105 ■·cm which is observed by I?V test.

AlN-based surface acoustic wave resonators on platinum bottom electrodes for high-temperature sensing applications

Surface acoustic wave（SAW） resonators with Pt/AlN/Si and Pt/AlN/Pt/Si configurations were fabricated by lift-off photolithography techniques. High-temperature performances of both configurations were investigated for temperature sensor applications. AlN films grown on Ptcoated Si substrates exhibit a lower（002） preferred orientation than those grown on Si substrates. The center frequencies of Pt/AlN/Si and Pt/AlN/Pt/Si configurations at room temperature are 424.1 and 456.4 MHz, respectively.The SAW was limited by Pt bottom electrodes to propagate in AlN layer. The temperature coefficient of frequency（TCF） values of Pt/AlN/Si and Pt/AlN/Pt/Si configurations are-51.6 × 10^-6 and-69.2 × 10^-6℃^-1, respectively.Compared with that of Pt/AlN/Si configuration, the TCF value of Pt/AlN/Pt/Si configuration decreases by 34.1 %.AlN resonator with the Pt floating bottom electrodes provides a large, quasi-constant temperature sensitivity which is suitable for temperature sensor applications.

Breath monitoring,sleep disorder detection,and tracking using thin-film acoustic waves and open-source electronics

Apnoea,a major sleep disorder,affects many adults and causes several issues,such as fatigue,high blood pressure,liver conditions,increased risk of type II diabetes,and heart problems.Therefore,advanced monitoring and diagnosing tools of apnoea disorders are needed to facilitate better treatment,with advantages such as accuracy,comfort of use,cost effectiveness,and embedded computation capabilities to recognise,store,process,and transmit time series data.In this work we present an adaptation of our apnoea-Pi open-source surface acoustic wave(SAW)platform(Apnoea-Pi)to monitor and recognise apnoea in patients.The platform is based on a thin-film SAW device using bimorph ZnO and Al structures,including those fabricated as Al foils or plates,to achieve breath tracking based on humidity and temperature changes.We applied open-source electronics and provided embedded computing characteristics for signal processing,data recognition,storage,and transmission of breath signals.We show that the thin-film SAW device out-performed standard and off-the-shelf capacitive electronic sensors in terms of their response and accuracy for human breath-tracking purposes.This in combination with embedded electronics makes a suitable platform for human breath monitoring and sleep disorder recognition.

High Q,high frequency,high overtone bulk acoustic resonator with ZnO films

Bulk acoustic wave resonators with piezoelectric films have been widely explored for the small size and high quality factor （Q） at GHz. This paper describes a high overtone bulk acoustic resonator （HBAR） based on AI/ZnO/AI sandwich layers and c-axis sapphire substrate. ZnO film with high quality c-axis orientation has been obtained using DC magnetron sputtering. The fabricated HBAR presents high Q at the multiple resonances from a 0.5-4.0 GHz wide band with a total size （including the contact pads） of 0.6 mm×0.3 mm×0.4 mm, The device exhibits the best acoustic coupling at around 2.4 GHz, which agrees with the simulation results based on the one-dimensional Mason equivalent circuit model. The HBAR also demonstrates Q values of 30 000, 25 000, and 6500 at 1.49, 2.43, and 3.40 GHz, respectively. It is indicated that the HBAR has potential applications for the low phase noise high frequency oscillator or microwave signal source.

基于ZnO压电薄膜的柔性声表面波器件

提出并制备了基于聚酰亚胺柔性衬底的声表面波(SAW)器件。在柔性聚酰亚胺衬底上低温反应磁控溅射沉积了ZnO压电薄膜;采用X射线衍射仪,扫描电子显微镜,原子力显微镜等设备对ZnO薄膜进行了检测,结果表明:ZnO薄膜晶粒呈柱状生长并且(002)择优取向,膜粗糙度小于9nm,适合制作压电器件。在柔性衬底上制备了基于ZnO压电薄膜的SAW器件,该器件表现出良好的谐振性能。采用矢量网络分析仪检测器件的传输曲线,实验结果与仿真结果具有很好的一致性。随着波长减小,谐振频率和相速度增加,当ZnO厚度为4μm,波长为8μm时,器件的谐振频率为268MHz,相速度为2 144m/s,机电耦合系数为1.1%;当ZnO厚度增加时,此叠层结构的声表面器件的叠层声速也增加。

ZnO薄膜的研究进展

光泵浦紫外激光的获得使ZnO成为热门的新型光电材料 ,文章从ZnO的基本特性、光电特性、生长方法、用途和应用前景等方面综述了ZnO的最新研究与进展 ,并对ZnO今后的发展作了展望。

声表面波ZnO薄膜紫外探测器的响应机制研究

通过射频磁控溅射法在以128°Y-X LiNbO3为压电衬底的声表面波(SAW)小波器件上沉积了一层ZnO薄膜作为紫外光敏感膜,利用网络分析仪对所制备探测器的紫外光响应特性进行了测试。实验结果表明,在波长365nm、光强210μW/cm2的紫外光照射下,探测器的频移量最大可达到37kHz,且具有良好的可重复性。探测器的紫外光响应过程和暗场恢复过程均包含了一个快过程和一个慢过程,前者决定于ZnO薄膜表面氧气分子的吸附与解吸附过程,而后者则决定于外界氧气分子与ZnO内部本征缺陷间的慢交换过程。最后,结合声电效应和半导体光电导效应分析给出了探测器紫外光响应过程和暗场恢复过程的理论公式。该文对基于ZnO薄膜的高灵敏度SAW紫外探测器响应机制的揭示,为其瞬态特性的改善和实用化提供了思路。

溶胶-凝胶法制备声表面波器件用Li∶ZnO薄膜

用溶胶凝胶法在玻璃载玻片上制备Li∶ZnO薄膜 ,涂膜采用旋涂法。薄膜通过二次涂膜而形成。用X射线衍射、原子力显微镜、扫描电镜和电阻仪表征和测量了薄膜的特性和性能。实验结果表明 :通过改变每层涂膜热处理的温度 ,可使Li∶ZnO薄膜具有更好的c轴择优取向性、平整均匀、颗粒细小和高的薄膜电阻率 (≈ 10 7Ω·cm) ,这些性能可满足声表面波器件的使用要求。薄膜形成时每层采用不同成膜热处理温度可使薄膜有较好性能 。

基于ZnO/Si结构的声表面波器件设计研究

微机电系统(MEMS)工艺已被广泛用于制造各种硅基薄膜器件。声表面波(SAW)器件是性能优良的MEMS器件。该文利用多物理耦合场软件COMSOL Multiphysics仿真了氧化锌/硅(ZnO/Si)结构SAW谐振器,并得到其S11参数。对应于仿真,该文制造了该种结构的SAW器件。实验所用的ZnO通过射频磁控溅射制备,所制备的ZnO具有良好的(002)取向。实验测得的ZnO/Si结构SAW器件的中心频率为111.6MHz,与仿真结构接近。

新型波浪加权式高性能声表面波滤波器设计

射频滤波器是移动通信中信号传输频率选择、噪声滤除干扰的不可或缺的关键器件。压电薄膜多层衬底结构的声表面波滤波器因具有高矩形度、低损耗等优点在近年来被广泛应用,但其谐振器固有的横向模式会极大影响滤波器性能。为此提出了一种新型的波浪加权式叉指设计的声表面波谐振器,该谐振器通过使用正弦函数作为交叠长度加权包络曲线,能够在不恶化谐振器性能的前提下有效抑制谐振器中的横向模式。将波浪加权式叉指设计的谐振器按照梯形拓扑结构级联,设计并制备了一款尺寸为1.1 mm×0.9 mm×0.55 mm的高性能滤波器。该滤波器中心频率为2.593 GHz,相对带宽为8.8%,插入损耗为0.52 dB。实物测试结果与仿真基本一致。

ZnO/IDT/金刚石结构SAW滤波器的设计和制作

传统材料制备的声表面波(SAW)滤波器的中心频率f大多位于较低的频段(f<500 MHz),但是无线通信频段逐渐向着高频化的方向发展,因此在SAW器件领域中引入高声速材料如金刚石以提高中心频率成为一种必然趋势。基于金刚石材料、采用ZnO/IDT/金刚石新型结构,设计了交错叉指结构和镜像阻抗连接结构的叉指换能器(IDT)参数。通过优化的工艺制备流程,获得了金刚石SAW滤波器样品。最后采用RF探针台对样品进行了性能测试和数据分析。测试结果表明制备的SAW滤波器样品的测试结果与理论值比较符合,交错叉指结构滤波器获得了很低的插损,镜像阻抗连接结构滤波器有效地提高了带外抑制度。样品IDT指宽2μm,SAW滤波器中心频率可达0.775 GHz。

ZnO薄膜生长及声表面波性能研究

采用传统射频磁控溅射技术,通过引入Si O2缓冲层以及调节工作气压的方法,在Si衬底上制备具有高度(1120)择优取向的Zn O薄膜.采用X射线衍射技术和原子力显微镜分析Zn O薄膜的晶体特性和择优取向.研究发现,引入Si O2缓冲层能显著减小Zn O/Si O2/Si三层结构声表面波器件的温度延迟系数(temperature coefficient of delay,TCD),当Si O2缓冲层厚度为200 nm时,Zn O薄膜同时具有(0002)和(1120)择优取向,且TCD值仅为2×10-6℃-1左右,说明器件温度稳定性佳.当工作气压降低时,Zn O(1120)择优取向增强,相应的声表面波器件的机电耦合系数(K2)也增大.在大机电耦合系数和高温度稳定性的Zn O/Si O2/Si三层结构的基础上,有望制作出高性能的Love波声表面波生物传感器.

ZnO薄膜声表面波型SF6气体传感器的研究

提出了一种基于ZnO薄膜的声表面波型SF6气体传感器.将利用微乳液法制备的ZnO颗粒制作在声表面波器件上,形成了一种对SF6气体具有物理吸附和脱吸附作用的ZnO薄膜.应用小波函数加权的输入换能器和具有抑制体声波作用的多条耦合器到声表面波器件中,传感器的频率响应特性得到了提高.声表面波器件的双声路结构消除了因外界测量条件改变引起的测量误差,进一步提高了传感器的可靠性和准确性.实验结果表明,基于ZnO薄膜的声表面波型SF6气体传感器具有较好的重复性和较强的连续测量特性,在测量范围内对各种浓度的SF6气体具有好的响应特性,传感器在φ(SF6)为0.5×10-6到20×10-6范围内的线性灵敏度大约为7.3 kHz/10-6,但对CO2、N2具有一定的交叉灵敏度.