Development of a Method to Evaluate the Surface of Single Crystal Substrates for Super-High-Frequency SAW Devices by UMS Technology

本研究利用线聚焦波束(LFB)和平面波超声材料表征系统的超声显微光谱(UMS)技术，对超高频声表面波(SHF)器件用LiNbO_3和LiTaO_3单晶的均匀性和表面改性/损伤问题进行了研究。本研究的目的是建立利用该技术进行定量分析和评价的方法。提高了LFB-UMC系统测量漏声表面波传播特性的相对和绝对精度。测定了LiNbO_3和LiTaO_3单晶的化学成分与声学物理常数的关系。这使得我们能够计算具有任意化学成分的晶体的LSAW和SAW的传播特性。直缝埋弧焊速度、居里温度、晶格常数、ET…之间的实验关系(校准线)用普通的直缝埋弧焊速度来评价晶体的各项性能，得到了更多的c。提出了一种通过评估市售LiTaO_3单晶的化学成分分布来改善晶体生长条件的方法，并成功地生长出了极其均匀的晶体。为了验证表面修饰/损伤的评估，我们制备了几种LiTaO_3基片，分别以质子交换层和磁区反转层为模型样品。用激光表面波焊速度估算的表面层厚度分辨率为纳米量级。扫描非线性介电显微镜的分辨率随着原子尺寸的增大而提高。测量了0.5 GHz、1 GHz和2 GHz频段LiTaO_3晶片上制作的声表面波滤波器的中心频率和声表面波速度，并通过测量声表面波速度预测了其化学成分的分布。结果表明，随着频率的升高，声表面波中心频率的变化和声表面波速度的计算值与实测值之间的差异变大。认为检测到了与衬底表层受损有关的问题。较少

In this study, ultrasonic microspectroscopy (UMS) technology, using the line-focus-beam (LFB) and plane-wave ultrasonic material characterization system, has been applied to the problems associated with the homogeneities and modified/damaged surface layers of LiNbO_3 and LiTaO_3 single crystals for super-high-frequency (SHF) surface acoustic wave (SAW) devices. The purpose of this study was to establish the quantitative analysis and evaluation method using this technology.1. The relative and absolute accuracies for the propagation characteristics of leaky SAW (LSAW) measured by the LFB-UMC system have been improved.2. Chemical composition dependences of the acoustical physical constants of LiNbO_3 and LiTaO_3 single crystals were determined. This enables us to calculate the propagation characteristics of LSAW and SAW for the crystals with an arbitrary chemical composition.3. Experimental relationships (calibration lines) among the LSAW velocity, Curie temperature, lattice constants, et … More c., were obtained to evaluate each property of the crystals by a common scale of LSAW velocity.4. A method for improving the crystal growth conditions through the evaluation of the chemical composition distributions of commercially available LiTaO_3 single crystals was proposed and an extremely homogeneous crystal was successfully grown.5. For the demonstration of evaluating modified/damaged surface layers, we fabricated several LiTaO_3 substrates with protonexchanged layers and domain-inverted layers as model specimens. The thickness resolution of the surface layers estimated by LSAW velocity was nanometer order.6. The resolution of scanning nonlinear dielectric microscopy was improved with an atomic size.7. Center frequencies and SAW velocities were measured for 0.5, 1, and 2 GHz-band SAW filters fabricated on LiTaO_3 wafers whose distributions in chemical composition were preexamined by measuring LSAW velocities. The results exhibited that variations of the center frequencies and differences between the calculated and measured SAW velocities became larger as the frequency became higher. It was considered that problems associated with the damaged surface layers of the substrates were detected. Less