Temperature Compensated AlN/SiO2 Structures for Lamb Wave Resonators

Characteristics of lower symmetric and anti-symmetric modes s(0) and a(0) (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 0.1 lambda and 0.5 lambda. The influence of uniform Al electrode at the bottom of AlN on the Lamb wave characteristics has been studied. For symmetric mode s(0) 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 0.1 lambda to 0.5 lambda. The electromechanical coupling of s(0) 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.