Low-Loss and Wideband Acoustic Delay Lines

Abstract

This paper demonstrates low-loss acoustic delay lines based on shear-horizontal waves in thin-film LiNbO3 for the first time. Due to its high electromechanical coupling, the shear-horizontal mode is suited for producing devices with large bandwidths. Here we show that shear-horizontal waves in LiNbO3 thin films are also excellent for implementing low-loss acoustic delay lines based on unidirectional transducers. The high acoustic reflections and large transducer uni-directionality induced by the mechanical loading of the electrodes on a LiNbO3 thin film provide a great trade-off between delay line insertion loss and bandwidth. The directionality for two different types of uni-directional transducers has been characterized. Delay lines with variations in the key design parameters have been designed, fabricated and measured. One of our fabricated devices has shown a group delay of 75 ns with an IL below 2 dB over a 3 dB bandwidth of 16 MHz centered at 160 MHz (FBW=10%). The measured insertion loss for other devices with longer delays and different numbers of transducer cells are analyzed, and the loss contributing factors and their possible mitigation are discussed.

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

“…Below fc_open, the section with the input transducer is equivalent to an A1 mode resonator [57]- [60]. The acoustic impedance difference caused by different electrical boundary conditions acts as reflective boundaries [7], [41]. The resonant modes below fc_open are marked with (i)-(iii) in the frequency response [ Fig.…”

“…Another effect of more severe mass loading is the larger ripples in IL and group delay. These are caused by more significant internal reflections at the edge of the electrodes [34], [41]. The edge reflections are of two origins, namely the electrically induced Γe and the mechanically induced Γm [41].…”

“…While Γe does not change with electrode thickness, Γm is larger for thicker metals. To study the influence quantitatively, the reflection generated at the interface between the parts with and without electrodes, namely the step-up reflection coefficient, Γsu, is studied using the simulation procedure in [41], [61]. A slab of LiNbO3 partially covered with metal is modeled in 2D with PMLs on the lateral ends for absorbing the reflected waves.…”

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

“…Second, the slow phase velocities (v p < 5 km/s) of the incumbent SAWs require narrow electrodes for high-frequency transduction, which causes severe electrical loading and limits the operating frequency to below 2.5 GHz [25]. Recently, leveraging the larger k 2 and reflection per wavelength of plate wave modes in thin-film LiNbO 3 , ADLs have been demonstrated using fundamental shear-horizontal (SH0) [26]- [28] and fundamental symmetric (S0) modes [29]- [31], achieving notably lower insertion loss (IL) and wider FBW between 0.1 and 2 GHz. Nevertheless, it remains difficult to further scale the operating frequencies above 4 GHz.…”

“…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.…”