Spurious Mode Free 3.5GHz AlN Plate Mode Resonator with High FoM

This work reports a spurious mode free AlN plate mode resonator with high figure-of-merit (FoM) operating at 3.5 GHz. The reported resonator, which comprises of two sets of 0.27 μm-thick top molybdenum (Mo) interdigitated electrodes (IDE) fingers attached to a layer of 1 μm-thick AlN, has an IDE pitch (p) of 1μm and a finger width (w) of 0.5 μm defined by optical photolithography. Besides reducing the fabrication process complexity, having only IDE on the top surface of AlN in the fabrication process also increases the acoustic phase velocity (v
_p
) as a result of the exclusion of bottom Mo IDE which has a lower acoustic phase velocity than AlN, thus relaxing the lithographic and etching requirement of IDE pitch. Measurement results show that the resonant frequency (f) is around 3.5 GHz, with a spurious mode free frequency response over a 1 GHz frequency range from 3 GHz to 4 GHz. The effective electromechanical coupling coefficient (k
_eff
²
) is 3.15% and the Q factor (Q) is 1000, yielding an f·Q product of 3.5×10
¹²
and k
_eff
²
·Q of 31.5, respectively. Together with the series resistance of 3.11Ω and the parallel resistance of 874Ω, the proposed plate mode resonator is a promising candidate for mobile Band 42 applications.