Theoretical study on SAW characteristics of layered structuresincluding a diamond layer

Diamond has the highest surface acoustic wave (SAW) velocity among all materials and thus can provide much advantage for fabrication of high frequency SAW devices when it is combined with a piezoelectric thin film. Basic SAW properties of layered structures consisting of a piezoelectric material layer, a diamond layer and a substrate were examined by theoretical calculation. Rayleigh mode SAW's with large SAW velocities up to 12,000 m/s and large electro-mechanical coupling coefficients from 1 to 11% were found to propagate in ZnO/diamond/Si, LiNbO₃/diamond/Si and LiTaO₃/diamond/Si structures. It was also found that a SiO₂/ZnO/diamond/Si structure can realize a zero temperature coefficient of frequency with a high phase velocity of 8,000-9,000 m/s and a large electro-mechanical coupling coefficient of up to 4%     

High temperature stable GHz-range low-loss wide band transducersand filter using SiO2/LiNbO3, LiTaO3

GHz-range low-loss transducers and filters are required for communication systems, especially mobile telephone communication systems. Many types of low insertion-loss transducers and filters utilizing the high electromechanical coupling coefficient (K²) materials such as LiNbO₃ and LiTaO₃ have been developed. Unfortunately, these materials have large temperature coefficients of the frequency (TCF). In this paper, SAW substrates with high coupling coefficients and low propagation attenuations and small temperature coefficient of frequency in the GHz-range are theoretically and experimentally investigated. The experimental results show very low propagation loss of 0.02 dB/λ ₀ and larger K² than those of the substrates of LiNbO₃ and LiTaO₃ at the TCF of below -5 ppm/°C at 1~2 GHz-range. The low-loss filter results using internal reflection types of IDT show the insertion loss of about 2.9 dB at 1 GHz and 4.9 dB at 2 GHz under the TCF's of 0 and +20 ppm/°C. These materials are applicable for devices at GHz-range because SiO₂ thickness is very thin such as below 1 μm and the center frequency shift of the filter versus SiO₂ thickness is very small     

Characterization of surface acoustic wave propagation in multi-layered structures using extended FEM/SDA software

This paper describes the characterization of SAW propagation in layered substrate and overlayered structures. The software based on the finite element method and spectral domain analysis was newly developed and applied to the characterization of SAW propagation under an infinitely-long Al interdigital transducer on a rotated Y-cut LiTaO₃/sapphire substrate. Because of the finite LiTaO₃ thickness, a series of spurious resonances appears. It is shown that the excitation strength of the spurious resonances changes with frequency as well as the rotation angle, which reflects the frequency and rotation angle dependence of the energy leakage. Next, the analysis was carried out for SAWs propagating in a SiO₂ layer/Al IDT/42°YX-LiTaO₃ structure. It is shown that the influence of the SiO₂ layer is significantly dependent on the location where the SiO₂ layer is deposited. In particular, it is shown that when the SiO₂ layer is deposited only on top of the electrodes, the SAW reflectivity increases compared with when the SiO₂ layer is deposited between and on top of electrodes.     

High-accuracy standard specimens for the line-focus-beam ultrasonic material characterization system

We prepared standard specimens for the line-focus-beam ultrasonic material characterization system to obtain absolute values of the propagation characteristics (phase velocity and attenuation) of leaky surface acoustic waves (LSAWs). The characterization system is very useful for evaluating and analyzing specimen surfaces. The calibration accuracy of these acoustic parameters depends on the accuracy of acoustical physical constants (elastic constants, piezoelectric constants, dielectric constants, and density) determined for standard specimens. In this paper, we developed substrates of non piezoelectric single crystals (viz., gadolinium gallium garnet [GGG], Si, and Ge) and an isotropic solid (synthetic silica [SiO₂] glass) as standard specimens. These specimens can cover the phase velocity range of 2600 to 5100 m/s for Rayleigh-type LSAWs. To determine the elastic constants with high accuracy, we measured velocities by the complex-mode measurement method and corrected diffraction effects. Measurements of bulk acoustic properties (bulk wave velocity and density) were conducted around 23°C, and bulk wave velocities were obtained with an accuracy of within ±0.004%. We clearly detected differences in acoustic properties by comparing the obtained results with the previously published values; the differences were considered to be due to differences of the specimens used. We also detected differences in acoustic properties among four SiO₂ substrates produced by different manufacturers     

A method of determining acoustical physical constants for piezoelectric materials by line-focus-beam acoustic microscopy

We developed a new method of determining acoustical physical constants (elastic constant, piezoelectric constant, dielectric constant, and density) of piezoelectric materials with high accuracy. This method acquires velocities of leaky surface acoustic waves (LSAWs) excited on the water-loaded specimen surface, measured by line-focus-beam (LFB) acoustic microscopy, and bulk velocities of longitudinal and shear waves, measured with planewave transducers replacing the LFB device in the same system, together with the dielectric constants and density measured independently, for a small number of specimens. For LiNbO₃ and LiTaO₃ crystals, we demonstrated that we could accurately determine the constants by choosing proper propagation directions of LSAWs and bulk waves for three principal X-, Y-, and Z-cut specimens and one rotated Y-cut specimen [(104) plate for LiNbO₃ and (012) plate for LiTaO₃]. The accuracy is nearly the same as that for the constants determined only from the bulk wave velocities     

High-Q AlN/SiO2 symmetric composite thin film bulk acoustic wave resonators

High-Q, bulk acoustic wave composite resonators based on a symmetric layer sequence of SiO₂-AlN-SiO₂ sandwiched between electrodes have been developed. Acoustic isolation was achieved by means of deep silicon etching to obtain membrane type thin film bulk acoustic wave resonators (TFBARs). Three different device versions were investigated. The SiO₂ film thicknesses were varied (0 nm, 70 nm, 310 nm, and 770 nm) while the piezoelectric AlN film had a constant thickness of 1.2 μm. The sputter-deposited AlN film grown on the amorphous, sputter-deposited SiO₂ layer exhibited a d_33,f of 4.0 pm/V. Experimental results of quality factors (Q) and coupling coefficients (k_t²) are in agreement with finite element calculations. A Q of 2000 is observed for the first harmonic of the 310 nm oxide devices. The most intense resonance of the 770 nm oxide device is the third harmonic reaching Q factors of 1450. The temperature drift reveals the impact of the SiO₂ layers, which is more pronounced on the first harmonic, reducing the TCF to 4 ppm/K for the 3rd harmonic of the 310 nm oxide devices.     

乐甫波的液体粘度传感研究

利用部分波理论和边界条件精确推导,对“粘性液体/非压电薄膜/压电基底”3层乐甫波结构建模,分析得出随叉指周期变小,液体粘度灵敏度提高,传播衰减增大;随“薄膜厚度与波长之比”增大,液体的粘度灵敏度随之先减小再增大,液体密度灵敏度几乎为零。优化设计了以36°钽酸锂和SiO₂为基底和薄膜的乐甫波器件,制作并测试实物,实验结果与数值仿真基本一致,表明了理论模型的正确性。     

Multistrip couplers for surface acoustic wave sensor application

We present a new surface acoustic wave gas sensor utilizing a multistrip coupler on LiNbO₃ and LiTaO₃ substrates and using copper phthalocyanine as a sensor layer for NO₂. The sensor signal originates from surface conductivity changes induced by the adsorbed NO₂. This variation in conductivity leads to a changing coupling efficiency of the multistrip coupler and thus to variations in the insertion loss of the device. Rayleigh and shear wave devices with operating frequencies of 170 and 243 MHz have been tested. A sensitivity of better than 1 ppb NO₂ in air was achieved     

Change in piezoelectric boundary acoustic wave characteristics with overlay and metal grating materials

This paper describes how the characteristics of shear-horizontal type piezoelectric boundary acoustic waves (PBAWs) change with combination of different overlay and metal grating materials. It is shown that PBAWs are supported in various structures provided that highly piezoelectric material(s) are employed as structural member(s). For verification, numerical simulation of different material combinations is done. The results are in good agreement with the qualitative prediction. That is, large electromechanical coupling factor K² is obtainable when materials having small mass densities shear modulus c₄₄ and shear velocity VBS; and materials having extremely large shear modulus c₄₄ are chosen, respectively, for overlay and metallic grating. When YX-LiNbO₃ is assumed as a substrate, for example, the best choice seems to be SiO₂ and Au for overlay and metallic grating, respectively. Although metals with extremely large ρ and c₄₄ such as W and Ta offer large K², they may not be acceptable for practical PBAW applications because of their large electric resistivity.     

低温SiO_(2)气凝胶基复合相变材料的制备与性能分析EI北大核心CSCD

以SiO_(2)气凝胶为支撑材料,通过物理吸附法制备定形SiO_(2)气凝胶基复合相变材料(PCCs),再利用密封盒进行二次封装。探究SiO_(2)气凝胶与相变材料的最佳配比,并对复合相变材料的微观结构、化学成分、孔结构、相变特性、热可靠性、定形能力和隔热性能进行表征。结果表明:含有质量分数为80%相变材料的SiO_(2)气凝胶复合相变材料(LS-80)具有最佳吸附比,并且在相变过程中显示了良好的定形能力,其熔点和熔融潜热分别为-15.6℃和170.2 J/g;同时SiO_(2)气凝胶的成功吸附使得LS-80的比表面积、孔径和孔容大小下降至59 m^(2)/g,13 nm和0.2 cm^(3)/g;20次冷热循环后,封装后相变材料的相变潜热减少了13.4%,而SL-80只减少了2.8%,表现出良好的热可靠性能;SiO_(2)气凝胶的添加使得复合相变材料导热系数降低,隔热能力增强。该结果为SiO_(2)气凝胶复合相变材料在冷链物流领域的应用提供了实验依据。     

基于FTIR和RBF网络的二元脂肪酸/SiO2相变储湿复合材料热湿性能优选预测

采用溶胶-凝胶法制备二元脂肪酸（BFA）/SiO₂相变储湿复合材料,既具有相变调温性能,又具有储湿调湿性能,还能满足无机材料的相容性。研究溶液pH值、超声波功率、去离子水用量、无水酒精用量及BFA用量对BFA/SiO₂相变储湿复合材料结构的影响规律,通过FTIR分析BFA/SiO₂相变储湿复合材料的嵌合机制。通过RBF网络,研究BFA/SiO₂相变储湿复合材料的结构参数与热湿性能优选预测模型。结果表明：BFA/SiO₂相变储湿复合材料中BFA与SiO₂仅仅是物理嵌合。最优热湿性能的BFA/SiO₂相变储湿复合材料制备工艺参数：pH值为3.64、超声波功率为120 W、去离子水用量为1.45 mol、无水乙醇用量为0.78 mol和BFA用量为0.079 mol。预测结果：30%相对湿度（RH）~90% RH湿容量为0.1676 g/g、相变焓为41.89 J/g。测试结果：30% RH~90% RH湿容量为0.1653 g/g、相变焓为41.22 J/g。     

超声波辅助制备癸酸-棕榈酸@改性SiO2调温调湿复合材料超声波辅助制备癸酸-棕榈酸@改性SiO2调温调湿复合材料

以硅烷偶联剂改性SiO₂为壁材,癸酸（DA）-棕榈酸（PA）为芯材,利用超声波辅助溶胶-凝胶法制备DA-PA@改性SiO₂调温调湿复合材料,分析了硅烷偶联剂用量、超声波功率、超声波时间和超声波温度对DA-PA@改性SiO₂调温调湿复合材料粒径的影响,以及相关性能。结果表明,利用超声波辅助溶胶-凝胶法制备DA-PA@改性SiO₂调温调湿复合材料,可以显著降低粒径尺寸和减小粒径分布。当硅烷偶联剂用量为4.0 g、超声波功率为120 W、超声波时间为100 min和超声波温度为60℃时,DA-PA@改性SiO₂调温调湿复合材料的粒径较小且粒径分布较窄,即d₉₀=87.36 nm、d₅₀=63.34 nm、d₁₀=44.02 nm和d₉₀-d₁₀=43.34 nm,在相对湿度40.0%~65.0%范围内的平衡含湿量为0.1864~0.2379 g/g,相变温度为20.23~23.59℃,相变潜热为40.91~46.72 J/g,稳定性能良好。      

Strain effect in single-crystal silicon-based multilayer surface acoustic wave structures

A method for calculating the characteristics of surface acoustic wave (SAW) propagation in a deformable piezoelectric multilayer medium is presented. The effect of longitudinal and lateral mechanical strain on the SAW phase velocity in a (ZnO or AIN)/SiO₂/Si thin film structure for {001}, {111} and {110} silicon crystal planes within the temperature range 293–673 K is studied. The effects of thickness and internal mechanical stresses in the ZnO or A1N piezoelectric film and SiO₂ dielectric film on the sensitivity of the SAW phase velocity to strains in the structure are investigated. The Si{110}-based SAW structure with the SAW wavevector oriented in the 10 direction is shown to possess maximum operating frequency sensitivity to both longitudinal and lateral strain. The parameters of SAW structure stable to mechanical loads are determined. ZnO and SiO₂ layer deposition on silicon is shown to increase the SAW phase velocity sensitivity to longitudinal strain and to decrease its sensitivity to lateral strain in the structure.     

Fabrication of SiO2 Microcantilever Using Isotropic Etching With ICP

This paper reports a new design and microfabrication process for high sensing guard-armed silicon dioxide (SiO₂ ) microcantilever sensor, which can be widely used in chemical, environmental and biomedical applications. One sensor platform consists of two SiO₂ cantilever beams as the sensing and reference elements, two connecting wings, and three guard arms. The guard arms prevent damage to the cantilever beam from collision. To efficiently release the SiO₂ cantilevers from the silicon substrate, an isotropic etch method using inductively coupled plasma (ICP) was developed. The isotropic etching with ICP system provides an advantage in good profile control and high etching rate than wet isotropic etching or conventional RIE (reactive ion etching); however, it has not been gained many attentions. In this work, the effects of chamber pressure, plasma source power, substrate power, SF₆ flow rate relating with Si etching rate, undercutting rate, and isotropic ratio were investigated and discussed. The optimum isotropic etching process achieved a 9.1 mum/min etch rate, 70% isotropic ratio, and 92% etching uniformity. The SiO₂ cantilever sensor was fabricated and the cantilever beam was successfully released using a patterned photoresist layer as an etching mask. This plasma isotropic etching release processing can be also applied to release other SiO₂ or metal suspended MEMS structures, such as bridges and membranes, with a fast etch rate and reasonable isotropic ratio.     

基于MoS2/SiO2范德华异质结的VO2薄膜转移打印研究

近年来, 柔性电子器件由于在物联网、生物电子等领域的潜在应用引起了研究者的广泛关注。功能氧化物材料在柔性聚合物中的集成已被证明是实现高性能柔性电子器件的有效方式。由于功能氧化物薄膜通常需要高温制备, 直接在柔性聚合物基底上合成高质量的氧化物薄膜仍然是一个巨大的挑战。本研究提出了一种基于MoS₂/SiO₂范德华异质结转移打印大面积VO₂薄膜的方法, 即利用MoS₂和SiO₂薄膜亲疏水性能的不同, 可以仅使用去离子水解离MoS₂/SiO₂范德华异质结界面, 成功将Si/SiO₂/MoS₂/SiO₂/VO₂ 多层膜结构上的VO₂薄膜转印到Si、SiO₂/Si以及柔性基底上。X射线衍射(XRD)结果显示, 转印前后VO₂薄膜的晶体结构没有差异, 变温Raman光谱和变温红外反射光谱证明了转印前后VO₂薄膜良好的金属-绝缘体转变性能。本研究提供了一种有效的功能氧化物薄膜转印方法, 在不引入牺牲层和腐蚀性溶剂的条件下, 实现了VO₂薄膜在任意基底上的低温集成, 为柔性可穿戴电子器件的研制提供了一种新思路。     

核壳材料Co3O4@SiO2催化环己基过氧化氢分解

环己基过氧化氢(CHHP)分解是环己烷无催化氧化工艺制备环己醇和环己酮的重要反应步骤.本研究以Co3O4纳米颗粒为内核,十六烷基三甲基溴化铵(CTAB)为模板剂,正硅酸四乙酯(TEOS)为硅源,采用模板法制备了核壳结构材料Co3O4@SiO2.考察了SiO2壳层制备条件:乙醇和水的比例、CTAB的浓度和TEOS的用量对...     

Residual stress development in Pb(Zr,Ti)O3/ZrO2/SiO2 stacks for piezoelectric microactuators

The residual stress of multilayers in piezoelectric microelectromechanical systems structures influences their electromechanical properties and performance. This paper describes the development of residual stress in 1.6 μm Pb(Zr_0.52,Ti_0.48)O₃ (PZT)/0.3 μm ZrO₂/0.5 μm SiO₂ stacks for microactuator applications. The residual stresses were characterized by wafer curvature or load-deflection measurements. PZT and zirconia films were deposited on 4-in. (100) silicon wafers with 0.5 μm thick thermally grown SiO₂ by sol–gel processes. After the final film deposition, the obtained residual stress of PZT, ZrO₂, and SiO₂ were 100–150, 230–270, and − 147 MPa, respectively. The average stress in the stack was  80 MPa. These residual stresses are explained in terms of the thermal expansion mismatch between the layers and the substrate. Load-deflection measurements were conducted to evaluate localized residual stresses using released circular diaphragms. The load-deflection results were consistent with the average stress value from the wafer curvature measurements. It was found that more reasonable estimates of the stack stresses could be obtained when mid-point vertical deflection data below 6 μm were used, for diaphragms 0.8–1.375 mm in diameter.     

Full-wave analysis of piezoelectric boundary waves propagating along metallic grating sandwiched between two semi-infinite layers

This paper describes full-wave analysis of piezoelectric boundary acoustic waves (PBAWs) propagating along a metallic grating sandwiched between 2 semi-infinite layers. In the analysis, the finite element method (FEM) is used for the grating region while the spectral domain analysis (SDA) is applied for an isotropic overlay region as well as a piezoelectric substrate region. The combination of the FEM and SDA makes the numerical analysis very fast and precise. As an example, the analysis was made on the PBAWs propagating in an SiO₂ overlay/ Cu grating/rotated Y-cut LiNbO₃ structure. It is shown that both the shear-horizontal (SH) type and Rayleigh-type PBAWs are supported in the structure, and that their velocities are very close to each other. Thus spurious responses due to the Rayleigh-type PBAW should completely be suppressed for device implementation. Discussions are made in detail on the influence of Cu grating thickness, substrate rotation angle, and metallization ratio on excitation and propagation characteristics of the SH- and Rayleigh-type PBAWs.     

电极与介电层褶皱接触对压电式柔性电子皮肤性能的影响

提出了一种基于压电效应制备柔性电子皮肤的简单方法。为了研究纳米改性对柔性电子皮肤各层性能的影响，首先以纳米SiO₂粒子作为改性体，以聚二甲基硅氧烷(PDMS)作为基体，制备出SiO₂/PDMS复合柔性衬底，解决了在PDMS上磁控溅射沉积电极材料产生裂纹的现象，成功获得能够稳定工作的柔性电极。然后用钛酸钡/碳纳米管/聚二甲基硅氧烷(BaTiO₃/CNTs/PDMS)复合材料作为功能层，制备出一种五层结构的高灵敏性柔性电子皮肤，并找到一种通过改变基板粗糙度的简单方法构建电极与介电层的褶皱接触，进而提升柔性电极的电导率与柔性电子皮肤的压电响应信号。      

癸酸-棕榈酸/SiO_2相变储湿复合材料的制备与表征

以SiO2为载体,癸酸-棕榈酸为相变材料,采用溶胶-凝胶法制备了癸酸-棕榈酸/SiO2相变储湿复合材料,利用等温吸放湿法、步冷曲线法、FTIR、SEM、激光粒度分析仪(LPSA)、BET、TGA和DSC等对其进行性能测试和表征。结果表明:癸酸-棕榈酸/SiO2相变储湿复合材料的载体材料SiO2具有"笼"结构,其大量微孔形成三维空间网格结构既可以通过癸酸-棕榈酸与SiO2的物理嵌合方式包裹与束缚相变材料,又可以通过SiO2的亲水性能吸附水分子,具有良好的热湿性能。癸酸-棕榈酸/SiO2相变储湿复合材料表面光滑圆润无明显凹陷,颗粒粒径较小且均匀性较好(粒径为82.14nm、孔体积为0.010 7mL/g、孔比表面积为25.16m2/g、孔平均直径为26.63nm),属于纳米级有机相变芯材/无机基体复合材料;相变温度为19.88~23.13℃、相变潜热为38.55~42.56J/g,癸酸-棕榈酸的质量分数约为31.59%,满足人体舒适度的温度范围,适合在建筑领域中广泛应用。