  This work reports an acoustic solidly mounted resonator (SMR) at 18.64 GHz,
among the highest operating frequencies reported. The device is built in
scandium aluminum nitride (ScAlN) on top of silicon dioxide (SiO2) and tantalum
pentoxide (Ta2O5) Bragg reflectors on silicon (Si) wafer. The stack is analyzed
with X-ray reflectivity (XRR) and high-resolution X-ray diffraction (HRXRD).
The resonator shows a coupling coefficient (k2) of 2.0%, high series quality
factor (Qs) of 156, shunt quality factor (Qp) of 142, and maximum Bode quality
factor (Qmax) of 210. The third-order harmonics at 59.64 GHz is also observed
with k2 around 0.6% and Q around 40. Upon further development, the reported
acoustic resonator platform can enable various front-end signal-processing
functions, e.g., filters and oscillators, at future frequency range 3 (FR3)
bands.
