Multilayered structures using thin plates of LiTaO₃ for acoustic wave resonators with high quality factor

Highlights

• Multilayered structures are investigated as materials for high quality resonators.

• The structures with LiTaO₃ plate bonded to Si or glass substrate were optimized.

• The optimal number and thicknesses of intermediate SiO₂ and AlN layers were found.

• Loss mechanisms were studied via visualization of acoustic waves.

• Structures combining high Q-factors with suppressed spurious modes were identified.

Abstract

Multilayered substrates, including thin plates of LiTaO₃ (LT) bonded to silicon (Si) or glass wafers either directly or via intermediate layers, were numerically investigated as potential materials for surface acoustic wave (SAW) resonators with high quality (Q)-factor required for the next generation of mobile communication systems. The propagation velocities and electromechanical coupling coefficients of shear horizontally (SH) polarized acoustic waves were estimated as functions of LT orientation and thicknesses of all layers in LT/Si, LT/SiO₂/Si, LT/glass, LT/AlN/glass and structures with several pairs of SiO₂/AlN layers between LT and the glass wafer. Optimal combinations of cut angle, LT and electrode thicknesses, as well as the number and thicknesses of intermediate layers, required for the construction of resonators with improved performance were observed for each analyzed structure. In the resonators employing LT/Si and LT/SiO₂/Si structures with 30°YX - 48°YX LT cuts, high electromechanical coupling k², reaching 11.6%, can be combined with high velocities up to 4000 m/s, zero TCF at the resonant frequency and Q-factors that are considerably higher than in the LSAW filters using regular LT substrates. To understand the loss mechanisms that limit resonator Q-factors in LT/glass, mechanical displacements that accompany wave propagation in multilayered structures were visualized. Investigation of the nature of acoustic modes and their transformations with number and thicknesses of the layers revealed that the low-velocity glass wafer can be used as a supporting substrate if an intermediate AlN layer or alternating pairs of low- and high-velocity layers, for example SiO₂/AlN, are introduced between LT and the glass wafer.

Introduction

Acoustic wave resonators with high quality (Q) factors are strongly required for the next generation of mobile communication systems (5G) characterized by a more efficient use of the frequency spectrum. The demand of narrow gaps between the specified frequency bands implies more stringent requirements for the performance of the surface acoustic wave (SAW) filters used in these systems. In SAW filters, Q-factors may be improved by proper selection of a substrate material and type of acoustic wave. Other requirements for the substrate materials used in low-loss high-frequency SAW devices, such as high electromechanical coupling and higher propagation velocities, reduce the list of suitable substrates to rotated YX cuts of LiNbO₃ (LN) and LiTaO₃ (LN), with LT being preferred for application in devices operating in a wide temperature range due to its lower temperature coefficient of frequency (TCF). In the temperature compensated SAW (TCSAW) filters TCF is usually improved by deposition of the silicon oxide (SiO₂) film over resonator structures [1], [2].

Shear-horizontally (SH) polarized leaky surface acoustic waves (LSAWs) propagating in θ°-rotated YX cuts of LT with θ = 42–48° combine electromechanical coupling coefficients k² = 8–9% with propagation velocities of approximately 4000 m/s but attenuate while propagating along the surface due to bulk acoustic wave (BAW) radiation. While energy leakage into the substrate can be suppressed by optimization of electrode thickness for selected LT orientations [3], propagation losses caused by this leakage are frequency-dependent. Therefore, the typical Q-factors that are achievable simultaneously at resonant (f_R) and anti-resonant (f_A) frequencies do not exceed 1500, which is insufficient for the next generation of SAW devices.

Resonators with high Q-factors and improved TCF can be constructed if the LT substrate is replaced by a thin plate (membrane) with the thickness that is smaller than the wavelength [4], [5]. For example, in 36°LT plates, a combination of high coupling k² = 19% and zero TCF can be realized if the plate thickness is less than 1 μm for devices operating at GHz frequencies [5]. While few experimental plate mode resonators using LT [4], and LN [6] thin plates have been reported and showed good performance, this type of device is unsuitable for mass production and commercial applications due to its use of thin and fragile membranes.

This problem can be solved by bonding of a thin LT or LN plate to a substrate [7], [8]. Leakage from a thin LT plate into the supporting substrate is impossible if the minimum BAW velocity in the substrate is higher than the SH wave velocity in LT. In this case, Q-factors are limited by other loss mechanisms, such as transverse SAW and BAW radiation, intrinsic losses in LT, and resistive losses in metal electrodes. In devices with low-velocity supporting substrates, leakage may be strong but can be reduced by the introduction of isolating high-velocity layers [9]. To meet the requirements of filter specifications defined in a wide temperature range, optimization of a device design is necessary including the proper LT plate orientation and thickness, the number and thicknesses of intermediate layers introduced between LT and supporting substrate, and metal electrode thickness.

This paper presents the simulation technique (Section 2) and results of a theoretical study of resonators using multilayered substrates with thin LT plates bonded to high-velocity silicon (Section 3) or low-velocity glass (Section 4) substrates either directly or via one or few intermediate layers. In addition to the optimization of the geometrical parameters of the analyzed multilayered structures, another goal of this research was to elucidate the loss mechanisms limiting the resonator Q-factors in the structures with different number, type and thicknesses of layers. The insight into the loss mechanisms was provided due to the visualization of the acoustic fields accompanying the propagating waves. All simulations were performed using a numerical technique that combines the Spectral Domain Analysis (SDA) of acoustic waves in a multilayered substrate with the Finite Element Method (FEM) analysis applied to the electrodes of a periodic grating or an interdigital transducer (IDT) used for the generation and reflection of acoustic waves in SAW resonators [10], [11].