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

Abstract

The thermal compensation at high temperatures for aluminum nitride (AlN) Lamb wave resonators utilizing the lowest symmetric (S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ) 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">AlN</sub> /λ) and the normalized silicon oxide (SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) layer thickness (h <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SiO2</sub> /λ) in the AlN/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> composite layer. The AlN Lamb wave resonators were well temperature-compensated at 214°C and 430°C, respectively, by using different ratios of h <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">AlN</sub> /λ to h <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SiO2</sub> /λ. Even though the intrinsic quality factor (Q) degrades and the intrinsic motional impedance (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> ) 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.