Temperature compensation of the AlN Lamb wave resonators utilizing the S1 mode

The temperature compensation techniques for the first-order symmetric (S₁) Lamb wave mode in the AlN Lamb wave resonators are firstly investigated in this paper. The S₁ mode simultaneously offers very high phase velocity (ν_p) and large coupling coefficient (k²) when h_AlN/λ is smaller than 0.4, but its thermal stability needs further improvement. The AlN/SiO₂ bilayer and SiO₂/AlN/SiO₂ sandwiched temperature compensation structures are investigated and compared in this study. The SiO₂/AlN/SiO₂ symmetric structure shows higher v_p and larger k² than the lowest-order quasi-symmetric (QS₁) mode traveling in the AlN/SiO₂ bilayer structure because the symmetric structure trapes more acoustic energy inside the AlN piezoelectric layer. Despite the trade-off between first-order temperature coefficient of frequency (TCF) and k², the SiO₂/AlN/SiO₂ structure can provide large k² and near-zero TCF at the same time with proper thickness selection of AlN and SiO₂. The temperature-compensated resonator utilizing the S₁ mode in the symmetrical SiO₂/AlN/SiO₂ sandwiched membrane can simultaneously offer excellent thermal compensation, and large k² at super-high frequency.