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].

Toward Ka Band Acoustics: Lithium Niobate Asymmetrical Mode Piezoelectric MEMS Resonators

“…It is also preferable that the mode has a large acoustic velocity so that scaling would not reduce the feature size to the extent that limits power handling. Recently, several modes (S0, SH0, and A1) have been demonstrated in LiNbO3 with high coupling factors [6][7][8][9][10]. Among these wave modes, A1 Lamb wave mode has the highest phase velocity, and has been demonstrated with very large kt 2 and Qs simultaneously in Z-cut LiNbO3 (kt 2 =28% and Q=500 at 5 GHz), and in Y-cut LiNbO3 (kt 2 =6.3% and Q=5341 at 1.7 GHz) [9] [10].…”

RF acoustic microsystems based on suspended lithium niobate thin films: advances and outlook

“…A well-established approach for quantifying acoustic damping is to compare resonators with the highest Q. The survey of the reported high k 2 resonators in thinfilm LiNbO 3 is shown in figure 9 [118], including SH0 [17,22,23,[27][28][29][30][31][32], S0 [13][14][15][16][17][18], and A1 mode devices [35, 36, 39-41, 48, 69, 119, 120], sorted by operating frequencies and film thickness. It is clear from figure 9(a) that devices at higher frequencies show higher damping, consistent with that measured in bulk LiNbO 3 [117].…”

“…Nevertheless, these values are among the highest reported in RF acoustic resonators. Examples of high Q resonators [15,22] are presented in figure 10, promising for frequency synthesis and timing applications.…”

“…This can be intuitively explained as the thin-film better confines both the acoustic and electric fields within a smaller volume, leading to stronger interaction through piezoelectricity. The commonly used modes include fundamental symmetric (S0) [13][14][15][16][17][18][19][20][21], fundamental shear horizontal (SH0) [22][23][24][25][26][27][28][29][30][31][32][33][34], 1st-order antisymmetric (A1) [35][36][37][38][39][40][41][42][43][44][45][46], thickness-shear (TS) [47-50], thicknessextensional (TE) [51][52][53][54][55], and higher-order Lamb modes [53,[56][57][58][59]. Studies on thin-film LiNbO 3 growth date back to the sputtering deposition in the 1960s [60].…”

Acoustic Loss in Thin-Film Lithium Niobate: An Experimental Study

“…1 (b)]. It includes SH0 [2], [5], [32]- [38], S0 [4]- [6], [39]- [41], and thickness-shear (comprising A1) mode devices [1], [3], [42]- [48] with high K 2 . Despite different designs and implementations, resonators at lower frequencies and using thicker LiNbO 3 tend to show higher Q.…”