d15-Enhanced shear-extensional aluminum nitride resonators with kt > 4.4 for wide-band filters

摘要

This paper presents a novel aluminum nitride (AlN) resonator technology that fuses transverse (d31) and shear (d15) piezoelectric constants to realize a large electromechanical coupling factor (kt), while having a frequency defined by the lateral dimensions. An antisymmetric Lamb wave with co-existing in-plane extensional and thickness-shear elastic energy distribution is used to simultaneously exploit transverse and shear electromechanical coupling of AlN film. Specifically, the shear transduction (kt,15) benefits from the large d15 constant (due to the columnar single crystallinity of the film) as well as low in-plane (ε11) relative permittivity (due to the in-plane poly crystallinity) of the AlN film, and hence significantly improves the overall kt. A proof of concept device at 733MHz is demonstrated based on this technology, showing a Q of ∼1,400 with a kt of ∼4.4%. A ladder filter is also demonstrated, through electrical coupling of the d15-enhanced resonators, with a −3dB-bandwidth of 18MHz, at 745MHz.