Over 10% of ${k}_{\text{eff}}^{{2}}$ Demonstrated by 2-GHz Spurious Mode-Free Sc0.12Al0.88N Laterally Coupled Alternating Thickness Mode Resonators

In this letter, a group of laterally coupled alternating thickness (LCAT) mode resonators based on 12% scandium-doped aluminum nitride (Sc
_0.12
Al
_0.88
N) are reported. The LCAT mode resonators are realized in-house by sandwiching a layer of 1-μm-thick Sc
_0.12
Al
_0.88
N between the top two sets and the bottom two sets of interdigitated electrode (IDE) fingers, all made of molybdenum (Mo). The experimental results show that an effective electromechanical coupling coefficient (k
_eff
²
) of more than 10% and loaded quality factors (QL) as high as 870 are obtained, while the motional resistance of the resonators is kept as low as 0.68 Ω. The figure-of-merit (FoM) which is defined as k
_eff
²
QL and phase velocity (vp) is subsequently derived to be 87 and 7840 m/s, respectively. By changing the pitch of the IDE fingers, the resonators can operate from 1.96 GHz to 2.11 GHz without significant perturbation to their performance, corresponding to a lithographic frequency tuning range of 7.5%. In addition, clean spectra without any spurious modes are obtained within a 1-GHz frequency range for all fabricated devices of various pitches, making a significant step forward in the commercialization path of the reported resonators in the applications of the highband selection filters integrated on a single chip for mobile communication.