Theoretical study of thermally stable SiO2/AlN/SiO2 Lamb wave resonators at high temperatures

Abstract

Aluminum nitride (AlN) and silicon dioxide (SiO2) bilayer structure has been widely utilized in temperature-compensated micromechanical resonators as SiO2 has unique positive temperature coefficients of elasticity. However, the thermal expansion mismatch would cause large bending deformation and stress distribution in the resonant plate. In this study, a symmetrical SiO2/AlN/SiO2 sandwiched structure is proposed to reduce the temperature-induced deformation in the asymmetrical AlN/SiO2 bilayer plate. The thermal compensation at high temperatures for the Lamb wave resonators utilizing the lowest-order symmetric (S0) mode in the SiO2/AlN/SiO2 sandwiched structure is theoretically investigated herein. While operation temperature rises from room temperature to 600 °C, the temperature-induced bending deformation in the symmetrical SiO2/AlN/SiO2 composite plate is much less than that in the AlN/SiO2 composite plate conventionally used for temperature compensation. Furthermore, the different material properties of the AlN and SiO2 layers make the displacements of the S0 mode not purely symmetric with respect to the neutral axis, whereas the symmetrical SiO2/AlN/SiO2 sandwiched membrane still can enable a pure S0 mode which shows higher phase velocity and larger electromechanical coupling coefficient than the lowest-order quasi-symmetric (QS0) mode traveling in the AlN/SiO2 bilayer membrane. With proper thickness selection of AlN and SiO2, the S0 mode in the symmetrical SiO2/AlN/SiO2 sandwiched membrane can simultaneously offer excellent thermal compensation, high phase velocity, large electromechanical coupling coefficient, and small thermally induced deformation at high temperatures.