Negative Piezoelectric-Based Electric-Field-Actuated Mode-Switchable Multilayer Ferroelectric FBARs for Selective Control of Harmonic Resonances Without Degrading Keff²

Mode-switchable ferroelectric thin-film bulk acoustic resonators (FBARs) are presented. Such resonators operate based on a dynamic nonuniform effective piezoelectricity in composite multilayer ferroelectrics with large electrostriction coefficients, like barium strontium titanate (BST). Harmonic resonance modes (nf
_o
) of a multilayer ferroelectric bulk acoustic wave (BAW) resonator can be selectively excited with an electromechanical coupling coefficient (K
_eff
²
) equal to the fundament mode, which is contrary to the trend K
_eff
²
∝ 1/n
²
exhibited by conventional piezoelectric BAW resonators. Such a device can selectively be set to resonate at its different resonance harmonics by generating a pattern of nonuniform piezoelectric coefficient proportional to the stress field of each mode with an application of a proper set of dc control voltages applied across the ferroelectric layers. Such a resonator allows for the design of a new class of band-switching filters. As an experimental validation, a mode-switchable FBAR and a bandswitching ladder-type filter based on a bilayer ferroelectric BST structure are designed and fabricated for the first time. The bilayer BST FBARs not only can be switched ON or OFF but also by choosing different bias configurations, two resonance modes at 2 and 3.6 GHz can be selectively excited having K
_eff
²
of 8% and 7%, respectively.