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

在本研究中，利用线聚焦光束（LFB）和平面波超声材料表征系统，将超声显微光谱（UMS）技术应用于超高频（SHF）表面声波（SAW）器件中LiNbO_3和LiTaO_3单晶的均匀性和修饰/损伤表面层相关问题。本研究的目的是建立该技术的定量分析和评价方法。提高了LFB-UMC系统测量漏声SAW （LSAW）传播特性的相对精度和绝对精度。测定了LiNbO_3和LiTaO_3单晶声物理常数的化学组成依赖性。这使我们能够计算任意化学成分晶体的LSAW和SAW的传播特性。得到了LSAW速度、居里温度、晶格常数等之间的实验关系（校准线），用LSAW速度的通用尺度来评价晶体的各项性质。提出了一种通过评价市售LiTaO_3单晶的化学成分分布来改善晶体生长条件的方法，并成功地生长出了一种极均匀的晶体。为了验证对改性/损伤表面层的评价，我们制作了几种具有质子交换层和畴反转层的LiTaO_3衬底作为模型样品。LSAW速度估计的表层厚度分辨率为纳米级。扫描非线性介质显微镜的分辨率随着原子尺寸的增大而提高。测量了在LiTaO_3晶片上制备的0.5、1和2 ghz波段声表面波滤波器的中心频率和声表面波速度，通过测量LSAW速度预先检测了LiTaO_3晶片在化学成分中的分布。结果表明，随着频率的增加，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也越来越大。人们认为，检测到与衬底表层受损有关的问题。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