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

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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