Temperature compensated AlN/SiO2 structures for Lamb wave resonators

Characteristics of lower symmetric and antisymmetric modes s₀ and a₀ (velocities, electromechanical coupling coefficients, TCF) were numerically investigated in Lamb wave resonators using AlN and AlN/SiO₂ membranes. Two configurations of AlN/SiO₂ structure were compared, with SiO₂ film located either at the bottom or on top of AlN. All characteristics were calculated as functions of SiO₂ film thickness with normalized AlN thickness varying between 0.1λ and 0.5λ. The influence of uniform Al electrode at the bottom of AlN on the Lamb wave characteristics has been studied. For symmetric mode s₀ it was found that SiO₂ film thickness, which is required for zero TCF, decreases from 60% to 25% of AlN thickness when the latter grows from 0.1λ to 0.5λ. The electromechanical coupling of s₀ mode in the temperature compensated structure depends on the presence of metal film at the bottom of AlN and drops with poor conductivity of this film. The behavior of the reflection coefficient per period of the grating reveals the crucial difference between the analyzed configurations.