FinLN: 3D Fin Lithium Niobate Acoustic Resonators

Abstract

We demonstrate a three-dimensional fin lithium niobate (FinLN) acoustic resonator fabricated from thick lithium niobate on insulator using deep ion etching and sidewall electrode patterning. The FinLN geometry enables strong three-dimensional acoustic confinement and enhanced electromechanical coupling while maintaining a compact device footprint. The fabricated devices operate near 300 MHz and exhibit an effective electromechanical coupling coefficient of 6.2% with a mechanical quality factor of 430, in good agreement with finite-element simulations. Compared to planar surface acoustic wave resonators fabricated on the same wafer, the FinLN configuration achieves an 1.8x enhancement in effective coupling. This work establishes FinLN as a promising platform for compact, high-performance RF MEMS and piezo-optomechanical systems.

Figures (5)

• Figure 1: Schematic illustration of the fin lithium niobate (FinLN) acoustic resonator. Two unit cells are shown, each containing one pair of interdigital transducer (IDT) electrodes patterned across the fin. The fin width, fin height, and acoustic wavelength are indicated.
• Figure 2: Finite-element simulation results of the unit cell of the FinLN acoustic resonator. (a) Electromechanical coupling coefficient $k_t^2$ and resonance frequency $f_s$ as a function of fin width with the acoustic wavelength fixed at 12 $\mu$m. (b) $k_t^2$ and $f_s$ as a function of acoustic wavelength with the fin width fixed at 2 $\mu$m. (c) Electrical admittance magnitude with the acoustic wavelength fixed at 12 $\mu$m and the fin width fixed at 2 $\mu$m. (d) Resonance mode shape of unit cell at 290 MHz using the same parameters as in (c).
• Figure 3: Fabrication process flow of the FinLN devices: (1) starting from a thick Z-cut LNOI wafer with a 5 $\mu$m LN device layer, (2) Ar ion etching to define the fin structure, and (3) sidewall electrode formation by e-beam evaporation using glancing-angle deposition (GLAD), depositing 15 nm Ti and 200 nm Al.
• Figure 4: Images of the fabricated fin lithium niobate (FinLN) acoustic resonator. (a) Optical microscope image showing a top-view overview of the device. (b) Scanning electron microscope (SEM) image of the top view. (c) False-colored zoom-in SEM image of the device side view, where light blue indicates the ground electrode and light purple indicates the signal electrode.
• Figure 5: Electrical characterization of FinLN acoustic resonators with different acoustic wavelengths. Measured admittance magnitude $Y_{11}$ (solid lines) and MBVD model fits (dashed lines) for devices with acoustic wavelengths of $\lambda = 20\,\mu\mathrm{m}$, $16\,\mu\mathrm{m}$, and $12\,\mu\mathrm{m}$, shown from left to right. The effective electromechanical coupling coefficient $k_{\mathrm{eff}}^{2}$ extracted from the fitting is indicated in each panel. An inset in the $\lambda = 12\,\mu\mathrm{m}$ panel shows the measured Bode quality factor $Q$ as a function of frequency.