Lamb wave micromechanical resonators formed in thin plates of lithium niobate

Abstract

We study and compare high coupling symmetric and shear mode Lamb wave resonators realized in thin plates of X-cut lithium niobate. Fundamental mode bar resonators with a plate width of 20 µm, a plate thickness of 1.5 µm, apertures of 50, 90 and 130 µm and acoustic wave propagation rotated 30◦ (symmetric) and 170◦ (shear) to the +y-axis were realized on a single die for direct comparison. As expected, the symmetric Lamb wave resonators exhibited a higher sound velocity of ~6400 m/s when compared to the shear velocity of ~3900 m/s. The shear mode resonators, however, were found to have a significantly higher effective piezoelectric coupling coefficient of 16.3%, compared to a maximum of 9.1% for the symmetric Lamb wave resonators. In addition, the shear mode resonators were found to be less sensitive to the device aperture and to have fewer spurious responses. Based on these results, the shear mode resonators were selected for scaling to higher operating frequencies. A shear mode lithium niobate Lamb wave resonator operating at 350 MHz has been demonstrated with an effective piezoelectric coupling of 16%, a quality factor in air of 2200 and a device figure-of-merit of 420, among the highest reported for Lamb wave resonators [1-3].