Intrinsic characteristics of thermally stable AlN Lamb wave resonators at high temperatures

The thermal compensation at high temperatures for aluminum nitride (AlN) Lamb wave resonators utilizing the lowest symmetric (S₀) mode is theoretically and experimentally demonstrated in this work. The turnover temperature can be designed at high temperatures by changing the normalized AlN film thickness (h_AlN/λ) and the normalized silicon oxide (SiO₂) layer thickness (h_SiO2/λ) in the AlN/SiO₂ composite layer. The AlN Lamb wave resonators were well temperature-compensated at 214°C and 430°C, respectively, by using different ratios of h_AlN/λ to h_SiO2/λ. Even though the intrinsic quality factor (Q) degrades and the intrinsic motional impedance (R_m) increases at high temperatures, a Lamb wave resonator shows a Q of 760 at its turnover temperature, 430°C. These results demonstrate that thermally stable AlN Lamb wave resonators have the great potential for harsh environment applications.