5-GHz Antisymmetric Mode Acoustic Delay Lines in Lithium Niobate Thin Film

Abstract

We present the first group of acoustic delay lines (ADLs) at 5 GHz, using the first-order antisymmetric (A1) mode in Z-cut lithium niobate thin films. The demonstrated ADLs significantly surpass the operation frequency of the previous works with similar feature sizes, because of its simultaneously fast phase velocity, large coupling coefficient, and low-loss. In this work, the propagation characteristics of the A1 mode in lithium niobate is analytically modeled and validated with finite element analysis. The design space of A1 ADLs is then investigated, including both the fundamental design parameters and those introduced from the practical implementation. The implemented ADLs at 5 GHz show a minimum insertion loss of 7.9 dB, an average IL of 9.1 dB, and a fractional bandwidth around 4%, with delays ranging between 15 ns to 109 ns and the center frequencies between 4.5 GHz and 5.25 GHz. The propagation characteristics of A1 mode acoustic waves have also been extracted for the first time. The A1 ADL platform can potentially enable wide-band high-frequency passive signal processing functions for future 5G applications in the sub-6 GHz spectrum bands.

Low-Loss 5-GHz First-Order Antisymmetric Mode Acoustic Delay Lines in Thin-Film Lithium Niobate

“…The ADL consists of 50-nm aluminum electrodes on top of a suspended 540-nm 128 • Y-cut LiNbO 3 thin film. The thickness of the LiNbO 3 is selected to enable a center frequency around 5 GHz, while the thickness of aluminum is chosen to avoid notable cutoff of aluminum that confines acoustic waves and prevents propagation [32]. The orientation selection will be shown in Section II-B.…”

“…is selected as 1.8 μm to place the passband above the cutoff Table I. frequency of A1 [32]. Each cell includes a pair of transduction electrodes (L E = /4), connected to the signal and ground, respectively, and a floating electrode (L F = /2) [33].…”

“…2(a) and (b), respectively. To identify the optimal wafer cut and in-plane orientations, the coupling coefficient (K 2 ) is first evaluated under the quasistatic approximation as [32], [35]…”

“…Nevertheless, it remains difficult to further scale the operating frequencies above 4 GHz. To address the issues, the A1 mode in LiNbO 3 with high phase velocities has been exploited to demonstrate ADL at 5 GHz while only requiring a large feature size of 500 nm [32]. However, the A1 ADL prototypes still suffer from 6-dB bidirectional transducer loss and severe in-band This work is licensed under a Creative Commons Attribution 4.0 License.…”

“…In this article, we have included a detailed analysis of unidirectional transducer design, piezoelectric substrate orientation selection, and key design parameters. Compared to the state-of-the-art ADLs with similar feature sizes, this article presents a substantial enhancement in both the IL (4.9 dB less) [32] and operating frequency (3 times higher) [26], [27], [29], [30]. It is organized as follows.…”