Theoretical analysis of SAW propagation characteristics under the strained medium and applications for high temperature stable high coupling SAW substrates

The important properties required for SAW substrates are a large electromechanical coupling coefficient (k²), small temperature coefficient of frequency (TCF), low propagation loss, etc. At present, there is no single crystal which satisfies all the above mentioned requirements. Recently the efforts to develop new SAW substrate with large k² and small TCF have investigated. New KNbO₈ single crystal substrates with extremely large electromechanical coupling coefficient (k²=53%) and zero TCF around room temperature are developed. LiNbO₃ has good properties as a SAW substrate with a large size. Unfortunately LiNbO₃ possesses the defect of large values of TCF. In this paper, SAW bonded composite substrates with a large k², small TCF, low propagation loss and no dispersion using conventional bonders are investigated theoretically and experimentally. The propagation characteristics of SAW in the strained piezoelectric crystal using the higher-order elasticity theory have been analyzed. The theoretical results show zero TCF on LiNbO₃/SiO₂ substrates. At room temperature a relatively thin LiNbO₃ is firmly bonded to a glass material with a small thermal expansion coefficient by using ultraviolet rays to stiffen the bonder. The experimental results of LiNbO₃/glass substrate showed a TCF of -19 ppm/°C. The propagation properties were almost the same as those of the single crystal.