Investigation on effects of thermo-mechanical coupling on residual stress in lithium niobate in surface acoustic wave device

Abstract

Surface acoustic wave (SAW) devices are subject to thermomechanical coupling during operation, and when the temperature rises sharply to 180 °C, the surface of the lithium niobate crystal substrate generates nonuniform thermal stresses, thus leaving large residual stresses after complete cooling, which seriously influences its service performance and life. In this study, the thermal residual stresses of lithium niobate wafers were tested by nanoindentation tests, and the nanoindentation test process was simulated. The inverse calculation of the parameters of the lithium niobate constitutive model was carried out by comparing the load-displacement curves obtained from the finite element calculations with the experimental results. A multiscale model of the SAW device was established, its operation process was simulated, and then the generation law of residual stress was further discussed. The nanoindentation test results show that the thermal residual stress is positively correlated with the heating time under the effect of the same temperature; however, at the heating temperature up to about 180 °C, the lithium niobate crystal enters the high-temperature hyperelastic interval, and the yield stress of the crystal increases significantly at this stage. The constitutive model curves obtained by finite element calculations are in good agreement with the experimental results, and the thermal residual stress value of lithium niobate can reach 2/3 of its yield stress outside the high-temperature hyperelastic interval by comparison tests. The calculation results of the multi-scale model of the SAW device indicate that excessive thermal stress is the main cause of damage during the thermo-mechanical coupling of the SAW device. The polarization direction of lithium niobate crystals may cause the development of damage cracks, increasing the heat dissipation can effectively reduce the thermal stress to which it is subjected, and better extend the service life of the SAW device.