d15-Enhanced shear-extensional aluminum nitride resonators with k2t > 4.4% for wide-band filters

This paper presents a novel aluminum nitride (AlN) resonator technology that fuses transverse (d
₃₁
) and shear (d
₁₅
) piezoelectric constants to realize a large electromechanical coupling factor (k
_t
²
), 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 (k
_t,15
²
) benefits from the large d
₁₅
constant (due to the columnar single crystallinity of the film) as well as low in-plane (ϵ
₁₁
) relative permittivity (due to the in-plane poly crystallinity) of the AlN film, and hence significantly improves the overall k
_t
²
. A proof of concept device at 733MHz is demonstrated based on this technology, showing a Q of ~1,400 with a k
_t
²
of ~4.4%. A ladder filter is also demonstrated, through electrical coupling of the d
₁₅
-enhanced resonators, with a -3dB-bandwidth of 18MHz, at 745MHz.