Surface acoustic wave resonator and manufacturing method therefor

Abstract

The present application relates to a surface acoustic wave resonator and a manufacturing method therefor. The surface acoustic wave resonator comprises a substrate, a piezoelectric composite layer, and an electrode layer. The piezoelectric composite layer is located between the substrate and the electrode layer. The piezoelectric composite layer comprises a first piezoelectric layer and a second piezoelectric layer. The second piezoelectric layer is located between the first piezoelectric layer and the substrate. Under excitation of an electric field applied by the electrode layer, the piezoelectric composite layer generates a first-mode wave, and a second-mode wave is concomitantly generated during the propagation of the first-mode wave within the piezoelectric composite layer. The first piezoelectric layer is configured to confine the energy of the first-mode wave to propagate within the first piezoelectric layer. The second piezoelectric layer is configured to guide the leakage of the second-mode wave towards the substrate. The surface acoustic wave resonator achieves effective suppression of spurious modes during operation of the resonator, thereby helping to improve the overall operation performance of the resonator.

Description

A surface acoustic wave resonator and its fabrication method

[0001] This application claims priority to Chinese patent application No. 202411966179.6, filed with the China National Intellectual Property Administration on December 27, 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of semiconductor device technology, and in particular to a surface acoustic wave resonator and its fabrication method. Background Technology

[0003] Surface acoustic wave (SAW) resonators are widely used in various acoustic devices, such as RF filters, duplexers, delay lines, discriminators, and modulators, due to their resonant characteristics. These devices play a crucial role in wireless communication, particularly in the input / output RF filters of receivers and transmitters, enabling efficient wireless communication across multiple frequency bands.

[0004] Piezoelectric surface acoustic wave (SAW) resonators are based on a piezoelectric layer and interdigital transducers formed on its surface. Based on the piezoelectric effect, under the excitation of an electric field applied to the interdigital transducers, the dominant mode SAW is generated in the piezoelectric layer and propagates along the propagation region formed by the interdigital electrodes in the interdigital transducers. Simultaneously with the generation of the dominant mode SAW, multiple stray mode waves are also generated in the piezoelectric layer. These stray modes are unwanted acoustic resonances that occur concurrently with the dominant mode, and they typically have different propagation characteristics (e.g., velocity, direction, and frequency). These stray mode waves affect the resonator's output performance; for example, they not only affect the filter's passband performance but also its out-of-band and high-frequency performance, reducing power endurance and linearity (including harmonic distortion and intermodulation distortion).

[0005] Therefore, how to provide a surface acoustic wave resonator and its fabrication method that can effectively suppress stray mode waves and improve the resonator's performance has become an important technical problem that needs to be solved by those skilled in the art.

[0006] It should be noted that the above introduction to the technical background is only for the purpose of providing a clear and complete explanation of the technical solutions of this application and facilitating understanding by those skilled in the art. It should not be assumed that these technical solutions are known to those skilled in the art simply because they have been described in the background section of this application. Summary of the Invention

[0007] Based on this, the purpose of this application is to provide a surface acoustic wave resonator and its manufacturing method, which solves the problem of suppressing stray mode waves during the operation of surface acoustic wave resonators in the prior art.

[0008] To achieve the above and other related objectives, in a first aspect, a surface acoustic wave (SAW) resonator is provided, the SAW resonator comprising: a substrate, a piezoelectric composite layer, and an electrode layer;

[0009] The piezoelectric composite layer is located between the substrate and the electrode layer; the piezoelectric composite layer includes a first piezoelectric layer and a second piezoelectric layer, with the second piezoelectric layer located between the first piezoelectric layer and the substrate;

[0010] The piezoelectric composite layer is excited to generate a first mode wave under the electric field applied by the electrode layer. During the propagation of the first mode wave in the piezoelectric composite layer, a second mode wave is generated. The first piezoelectric layer is configured to confine the energy of the first mode wave to the first piezoelectric layer for propagation, and the second piezoelectric layer is configured to guide the second mode wave to leak toward the substrate.

[0011] In one embodiment, the first mode wave includes horizontal shear sound waves, and the second mode wave includes bulk sound waves.

[0012] In one embodiment, the acoustic impedance of the second mode wave propagating in the first piezoelectric layer is less than the acoustic impedance of the second mode wave propagating in the second piezoelectric layer.

[0013] In one embodiment, the material of the first piezoelectric layer includes YX spin-cut lithium tantalate, and the material of the second piezoelectric layer includes YX spin-cut lithium niobate; the thickness of the first piezoelectric layer ranges from 1 μm to 30 μm; and the thickness of the second piezoelectric layer ranges from 5 μm to 20 μm.

[0014] In one embodiment, the thickness of the first piezoelectric layer ranges from 2 μm to 10 μm, the material of the first piezoelectric layer is 42°YX spin-cut lithium tantalate, and the material of the second piezoelectric layer is 70°YX spin-cut lithium niobate.

[0015] In one embodiment, the surface acoustic wave resonator further includes an intermediate layer located between the first piezoelectric layer and the second piezoelectric layer, and / or, the intermediate layer located between the substrate and the second piezoelectric layer.

[0016] In one embodiment, the intermediate layer is located between the first piezoelectric layer and the second piezoelectric layer, and the intermediate layer is used to match the acoustic impedance and phase matching between the first piezoelectric layer and the second piezoelectric layer.

[0017] In one embodiment, the intermediate layer is located between the substrate and the second piezoelectric layer, and the thickness of the intermediate layer is at the atomic level.

[0018] In one embodiment, the material of the intermediate layer includes at least one of acoustic insulating material and electrical insulating material; the material of the substrate includes at least one of silicon, low-temperature co-fired ceramic, high-temperature co-fired ceramic, silicon carbide, alumina, glass, quartz and metal.

[0019] In one embodiment, the electrical insulating material includes at least one of semi-insulating polycrystalline silicon, aluminum nitride, aluminum boron nitride, and silicon dioxide.

[0020] In one embodiment, the piezoelectric composite layer is directly bonded to the substrate.

[0021] In one embodiment, the electrode layer is made of at least one of aluminum, copper, platinum, gold, silver, titanium, nickel, chromium, molybdenum and tungsten, or an alloy of any of the aforementioned metals.

[0022] In one embodiment, the thickness of the electrode layer ranges from 0.1 μm to 0.5 μm.

[0023] Secondly, a method for fabricating a surface acoustic wave resonator is provided, including the following steps:

[0024] Provide a substrate;

[0025] A piezoelectric composite layer is formed on the substrate, and an electrode layer is formed on the piezoelectric composite layer, wherein the piezoelectric composite layer includes a first piezoelectric layer and a second piezoelectric layer, and the second piezoelectric layer is located between the first piezoelectric layer and the substrate;

[0026] The piezoelectric composite layer is excited to generate a first mode wave under the electric field applied by the electrode layer. During the propagation of the first mode wave in the piezoelectric composite layer, a second mode wave is generated. The first mode wave and the second mode wave are different at least in their propagation mode. The first piezoelectric layer is configured to confine the energy of the first mode wave to the first piezoelectric layer for propagation, and the second piezoelectric layer is configured to guide the second mode wave to leak toward the substrate.

[0027] In one embodiment, the method of forming a piezoelectric layer on the substrate includes a direct bonding connection method.

[0028] The aforementioned surface acoustic wave (SAW) resonator, by replacing the traditional single-layer piezoelectric layer with a piezoelectric composite layer comprising a first piezoelectric layer and a second piezoelectric layer, achieves this. The first piezoelectric layer is configured to confine the energy of the first mode wave generated within the piezoelectric composite layer, while the second piezoelectric layer guides the leakage of the second mode wave, generated during the propagation of the first mode wave, towards the substrate. This prevents the second mode wave from propagating as a stray mode within the first piezoelectric layer and affecting the propagation characteristics of the first mode wave, effectively suppressing stray modes during resonator operation and improving the overall performance of the resonator. Furthermore, by optimizing the materials and thicknesses of the first and second piezoelectric layers, it is possible to achieve complete suppression of stray modes in certain frequency ranges while effectively confining the energy of the first mode wave and suppressing the PSC effect, thereby enhancing the resonator's performance and competitiveness and expanding its application scenarios.

[0029] The above-mentioned method for manufacturing surface acoustic wave resonators is simple and easy to implement, and is suitable for mass production. Attached Figure Description

[0030] Figure 1 is a schematic diagram of the surface acoustic wave resonator provided in an embodiment of this application;

[0031] Figure 2 is a schematic diagram of the displacement component amplitude of a first mode wave propagating in a first piezoelectric layer according to an embodiment of this application;

[0032] Figure 3 is a graph showing the relationship between the phase velocity and electromechanical coupling coefficient of a first mode wave propagating in a first piezoelectric layer as a function of the thickness of the first piezoelectric layer, according to an embodiment of this application.

[0033] Figure 4 is an admittance curve of a resonator with a first piezoelectric layer as the piezoelectric layer provided in an embodiment of this application;

[0034] Figure 5 is a graph showing the relationship between the admittance of a resonator provided in an embodiment of this application and the thickness of the first piezoelectric layer.

[0035] Figure 6 is a graph showing the variation of the admittance curve of the resonator provided in the embodiment of this application with the tangential angle of the second piezoelectric layer;

[0036] Figure 7 is a comparison of the conductance curves of the resonators provided in the embodiments of this application and the comparative examples;

[0037] Figure 8 is a comparison of the susceptance curves of the resonators provided in the embodiments of this application and the comparative examples.

[0038] Explanation of reference numerals in the attached figures: 1-Surface acoustic wave resonator; 10-Substrate; 20-Piezoelectric composite layer; 21-First piezoelectric layer; 22-Second piezoelectric layer; 30-Electrode layer. Detailed Implementation

[0039] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the specific embodiments of this application are described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of this application. However, this application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0040] In the description of this application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0041] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0042] In this application, unless otherwise expressly specified and limited, the terms "installation," "connection," "joining," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise expressly limited. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0043] In this application, unless otherwise expressly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "on top of," and "over" the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0044] It should be noted that when an element is referred to as being "fixed to" or "set on" another element, it can be directly on the other element or there may be an intervening element. When an element is considered to be "connected to" another element, it can be directly connected to the other element or there may be an intervening element. The terms "vertical," "horizontal," "upper," "lower," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only possible implementation.

[0045] This application provides a surface acoustic wave resonator 1. Please refer to Figure 1, which shows a schematic diagram of the structure of the surface acoustic wave resonator 1. The surface acoustic wave resonator 1 includes a substrate 10, a piezoelectric composite layer 20, and an electrode layer 30.

[0046] Specifically, the piezoelectric composite layer 20 is located between the substrate 10 and the electrode layer 30. The piezoelectric composite layer 20 includes a first piezoelectric layer 21 and a second piezoelectric layer 22. The second piezoelectric layer 22 is located between the first piezoelectric layer 21 and the substrate 10. The piezoelectric composite layer 20 generates a first mode wave under the excitation of the electric field applied by the electrode layer 30. During the propagation of the first mode wave within the piezoelectric composite layer, a second mode wave is generated. The first piezoelectric layer 21 is configured to confine the energy of the first mode wave within the first piezoelectric layer 21 for propagation, and the second piezoelectric layer 22 is configured to guide the second mode wave to leak towards the substrate 10.

[0047] In some embodiments, the first mode wave includes a horizontal shear wave, and the second mode wave includes a bulk acoustic wave. That is, the resonator structure provided in this application suppresses stray modes caused by the propagation of bulk acoustic waves through the second piezoelectric layer 22, thereby ensuring good propagation characteristics of the horizontal shear wave and guaranteeing the operating performance of the resonator with the horizontal shear wave as the dominant mode. Of course, in other embodiments, the first mode wave and the second mode wave can also be other modes of acoustic waves.

[0048] In some embodiments, the acoustic impedance Z1 of the second mode wave propagating in the first piezoelectric layer 21 is less than the acoustic impedance Z2 of the second mode wave propagating in the second piezoelectric layer 22. When Z1 < Z2, the energy of the second mode wave encounters resistance when penetrating from the first piezoelectric layer 21 to the second piezoelectric layer 22, causing the energy to leak from the first piezoelectric layer 21 to the second piezoelectric layer 22 and gradually attenuate, preventing it from propagating in the first piezoelectric layer 21 and affecting the propagation characteristics of the first mode wave confined by the first piezoelectric layer 21. However, if Z1 > Z2, during the leakage of the second mode wave from the first piezoelectric layer 21 to the substrate 10, strong reflection occurs at the contact interface between the first piezoelectric layer 21 and the second piezoelectric layer 22, and even at the contact interface between the second piezoelectric layer 22 and the substrate 10. This can actually enhance the signal of stray modes, which is detrimental to the propagation characteristics of the first mode wave.

[0049] In this embodiment of the application, by configuring the first piezoelectric layer 21 to confine the energy of the first mode wave within the first piezoelectric layer 21 for propagation, the first mode wave can be directly bounded, thus forming a directly bounded SAW resonator structure. Since the first mode wave, which is the primary operating mode, propagates within a specific region (i.e., the first piezoelectric layer 21), energy loss during propagation can be reduced, improving the transmission efficiency of the first mode wave and consequently enhancing the resonator's performance. To avoid the adverse effects of the radiation of the second mode wave on the propagation of the first mode wave, based on acoustic considerations, the selection and optimization of parameters such as the crystal material type, cutting direction, and thickness of the first piezoelectric layer 21 and the second piezoelectric layer 22 are used. On the one hand, the propagation direction of the second mode wave in the piezoelectric composite layer 20 is changed, allowing the second piezoelectric layer 22 to selectively guide the second mode wave towards the substrate 10. On the other hand, by controlling the acoustic impedance characteristics of the second mode wave propagating in the first piezoelectric layer 21 and the second piezoelectric layer 22, reflection of the second mode wave at the interface between different structural layers is further avoided, thus preventing the aggravation of stray modes and effectively suppressing stray modes. This also satisfies the effects of device miniaturization and high output characteristics.

[0050] In some embodiments, the material of the first piezoelectric layer 21 includes YX spin-cut lithium tantalate. The thickness of the first piezoelectric layer 21 ranges from 1 μm to 30 μm (inclusive), and can be 6 μm, 10 μm, 15 μm, 20 μm or 25 μm.

[0051] Please refer to Figure 2, which shows a schematic diagram of the displacement component amplitude when the first mode wave propagates in the first piezoelectric layer. Since the surface acoustic wave (e.g., the first mode wave) causes a small displacement of the medium (e.g., the first piezoelectric layer 21) when it propagates, this small displacement can be decomposed into components along three orthogonal directions: x, y, and z. u, v, and w shown in Figure 2 are components of the total displacement caused by the first mode wave propagating in the first piezoelectric layer 21. These three components have equal displacement in the x, y, and z three-dimensional coordinate system. However, due to the anisotropy of the crystal in the first piezoelectric layer 21, the above three components differ in the coordinate system along the crystal direction. As can be seen from Figure 2, when the distance from the upper surface of the first piezoelectric layer 21 downward into the first piezoelectric layer 21 exceeds 5 μm, the three components u, v, and w are all 0. That is, at any position in the first piezoelectric layer 21 where the distance from the upper surface of the first piezoelectric layer 21 exceeds 5 μm, the first mode wave has no sound field (i.e., no intensity distribution of the first mode wave), and the displacement amplitude is almost zero. This means that using the first piezoelectric layer 21 with a thickness greater than 5 μm can ensure that the energy of the first mode wave is confined within the first piezoelectric layer 21, and the thickness of the first mode wave leaking into the substrate 10 is negligible.

[0052] Furthermore, for direct bounded surface acoustic wave (SAW) resonators, parasitic surface conduction (PSC) occurs during operation, potentially leading to unwanted current leakage to the substrate 10. For example, PSC can result from the direct bonding process at the interface of highly anisotropic crystal materials (e.g., the contact interface between the silicon substrate and the piezoelectric layer) when bonding the piezoelectric layer to the substrate 10. This bonding process may cause crystal structure degradation (which could be due to thermal stress, mechanical stress, or other physicochemical effects during bonding), generating free charges in the interface region. These free charges may originate from chemical changes during bonding or material defects (such as vacancies or impurities). These free charges form a conductive layer on the surface of the silicon substrate 10, leading to the PSC effect. The PSC effect reduces the effective resistivity of the substrate 10, increasing high-frequency signal loss and thus affecting the resonator's performance (especially RF performance). In short, the presence of the PSC effect leads to power consumption and performance degradation in the resonator.

[0053] Please refer to Figure 3, which shows the relationship between the phase velocity and electromechanical coupling coefficient k2 of the first mode wave propagating in the first piezoelectric layer as a function of the thickness of the first piezoelectric layer. As can be seen from Figure 3, the phase velocity and electromechanical coupling coefficient of the first mode wave tend to stabilize when the thickness of the first piezoelectric layer 21 is greater than 2 μm (especially 3 μm). Due to the presence of the PSC effect, some acoustic wave energy does not propagate through the piezoelectric layer but propagates through the substrate or other materials, which reduces the electromechanical coupling of the piezoelectric material, thereby reducing the electromechanical coupling coefficient. However, if the PSC effect is effectively suppressed, the acoustic wave will mainly propagate through the piezoelectric layer, which will increase the phase velocity and electromechanical coupling coefficient. Therefore, when the thickness of the piezoelectric layer increases to a certain value (e.g., 2 μm and 3 μm as shown in Figure 3), the phase velocity and electromechanical coupling coefficient no longer change with increasing thickness. This indicates that the PSC effect is effectively suppressed because a piezoelectric layer of a certain thickness can effectively prevent the propagation of acoustic wave energy through non-piezoelectric paths. In summary, the relatively thick first piezoelectric layer 21 can effectively limit the energy of the first mode wave, while suppressing the PSC effect to prevent the first mode wave from leaking through the second piezoelectric layer 22 or even the substrate 10, thus ensuring the working performance of the resonator.

[0054] Although the displacement amplitude of the first mode wave is essentially zero in the portion of the first piezoelectric layer 21 located away from the electrode layer 30 when the first piezoelectric layer 21 is relatively thick (e.g., greater than 2 μm, 3 μm, 5 μm, etc.), other modes of waves can propagate at any location in the aforementioned portion, resulting in strong stray modes within the entire first piezoelectric layer 21. For example, a second mode wave generated alongside the first mode wave propagates simultaneously within the first piezoelectric layer 21, constituting a stray mode. Please refer to Figure 4, which shows the admittance curve of a resonator using the first piezoelectric layer as the piezoelectric layer. As can be seen from the real and imaginary admittance curves in Figure 4, strong broadband spurious signals appear in both the out-of-band (OOB) and high-frequency (HF) ranges. The spurious signals are manifested as significant peaks on the curves. For example, strong spurious peaks are observed in the range below the resonant frequency (1965MHz in Figure 4), between the resonant frequency and the anti-resonant frequency (2040MHz in Figure 4), and above the anti-resonant frequency, especially the bulk acoustic spurs appearing in the area indicated by the dashed box.

[0055] Furthermore, to achieve good acoustic wave propagation performance, the first piezoelectric layer 21 typically possesses excellent waveguide characteristics. Therefore, under normal circumstances, the propagation loss of the second-mode wave within the first piezoelectric layer 21 is low, making it difficult to suppress its propagation solely by adjusting the first piezoelectric layer 21 itself. In addition, due to differences in the acoustic properties of the materials, acoustic impedance mismatches between different materials can easily lead to high acoustic wave reflection. For example, a reflection boundary exists at the contact interface between the first piezoelectric layer 21 and the substrate 10, which further exacerbates the stray mode effects of the second-mode wave. For instance, referring to Figure 5, which shows the resonator's admittance as a function of the thickness of the first piezoelectric layer 21, the admittance curve remains essentially unchanged when the thickness of the first piezoelectric layer 21 varies within the range of 0.6λ to 1.2λ (λ being the wavelength of the resonator). This means that adjusting the thickness of the first piezoelectric layer 21 has a negligible effect on suppressing stray modes. Therefore, it is impossible to suppress stray modes by adjusting the thickness of the first piezoelectric layer 21. That is, although a larger thickness of the first piezoelectric layer 21 can effectively limit the energy of the first mode wave, it cannot effectively suppress the adverse effects of stray mode waves on the resonator output characteristics. Therefore, the thickness range of the first piezoelectric layer 21 provided in this embodiment is a numerical range summarized to achieve good limitation of the energy of the first mode wave and suppression of the PSC effect. The thickness of the first piezoelectric layer 21 can be adjusted within the above range based on the required surface acoustic wave propagation characteristics and resonator parameters (e.g., phase velocity, electromechanical coupling coefficient k2, and quality factor Q).

[0056] In some embodiments, the material of the second piezoelectric layer 22 includes YX spin-cut lithium niobate. The thickness of the second piezoelectric layer 22 ranges from 5 μm to 20 μm (inclusive), and can be 10 μm, 15 μm, or 18 μm.

[0057] As described above, a relatively thick first piezoelectric layer 21 can effectively limit the energy of the first mode wave and simultaneously suppress the PSC effect, thus ensuring the resonator's performance to a certain extent. However, a relatively thick piezoelectric layer is not conducive to meeting the miniaturization requirements of the resonator and weakens the original advantages of the direct bounded resonator. Furthermore, it still faces the problem of significant spurious mode influence. Therefore, a relatively thin first piezoelectric layer 21 can be used to basically satisfy the requirements of effective first mode wave limitation and PSC effect suppression, while adjusting the piezoelectric layer structure of the resonator. However, a relatively thin first piezoelectric layer 21 faces the problem of higher spurious mode influence. Therefore, in this embodiment, a second piezoelectric layer 22 is added, and the material and structure of the second piezoelectric layer 22 are optimized to suppress the propagation of the second mode wave in the first piezoelectric layer 21, thereby further improving the resonator's performance. The second piezoelectric layer 22 provided in this embodiment is particularly suitable for suppressing spurious modes in resonators with a thinned YX spin-cut lithium tantalate piezoelectric layer.

[0058] In some embodiments, the thickness of the first piezoelectric layer 21 ranges from 2 μm to 10 μm (inclusive), and can be 3 μm, 5 μm, or 8 μm. The material of the first piezoelectric layer 21 is 42°YX spin-cut lithium tantalate, and the material of the second piezoelectric layer 22 is 70°YX spin-cut lithium niobate.

[0059] Please refer to Figure 6, which shows the admittance curve of the resonator according to an embodiment of this application as a function of the tangential angle of the second piezoelectric layer. As can be seen from Figure 6, when the tangential angle of the second piezoelectric layer 22 is in the range of 60° to 80°, the spurious modes are suppressed compared to those with other tangential angles. This is manifested in a reduction in the height and number of spurious spikes. The suppression effect is particularly optimal at a tangential angle of 70°, with no spurious spikes appearing in the range of 1800MHz to 2100MHz. This indicates that while the first mode wave is well confined to propagate in the first piezoelectric layer 21, the second mode wave in the resonator can be effectively suppressed by optimizing the material and tangential angle of the second piezoelectric layer 22, thereby reducing the spurious effects caused by the second mode wave propagating alongside the first mode wave in the first piezoelectric layer 21.

[0060] Please refer to Figures 7 and 8. Figure 7 shows a comparison of the conductance curves of the resonators provided in the embodiments of this application and the comparative examples, and Figure 8 shows the susceptance curves of the resonators provided in the embodiments of this application and the comparative examples. The dashed lines in the two figures correspond to the curves of the embodiments of this application, and the solid lines correspond to the curves of the comparative examples. The only difference between the embodiments of this application and the comparative examples is that in the embodiments of this application, the piezoelectric composite layer 20 is composed of a first piezoelectric layer 21 and a second piezoelectric layer 22, while in the comparative examples, the first piezoelectric layer 21 is used as the piezoelectric layer. The material, thickness, and overall structure of the first piezoelectric layer 21 in the embodiments of this application and the first piezoelectric layer 21 in the comparative examples are the same (including the substrate 10 and the electrode layer 30). As can be seen from Figures 7 and 8, the number and height of spurious peaks in the conductance and susceptance curves of the resonator provided in this embodiment are significantly reduced, and there are no spurious peaks between the resonant frequency and the anti-resonant frequency. This indicates that the piezoelectric composite layer 20 structure in this embodiment has a prominent effect on suppressing spurious modes, and the stopband performance of the resonator is not reduced. This means that the resonator structure provided in this embodiment can be applied to a variety of filters.

[0061] In some embodiments, the piezoelectric composite layer 20 is directly bonded to the substrate 10. Direct bonding is a technique that connects two surfaces at the molecular interface without the need for adhesives or intermediate layers, relying on van der Waals forces or chemical reactions to form structural bonds between materials. That is, there are no other structural layers between the piezoelectric composite layer 20 and the substrate 10; the piezoelectric composite layer 20 is in direct contact and connection with the substrate 10. This allows for matching of the thermal expansion coefficients between the piezoelectric composite layer 20 and the substrate 10, thereby reducing internal stress and strain caused by temperature changes. Residual stress and strain between the piezoelectric composite layer 20 and the substrate 10 do not require additional compensation, which can reduce the impact of temperature changes on the resonator frequency, thereby improving the temperature coefficient of frequency (TCF) of the resonator. This ensures that the resonator maintains a stable frequency at different temperatures without frequency temperature drift. In contrast, if traditional bonding techniques requiring adhesive layers are used, the presence of the adhesive layer may introduce additional interfacial stress and strain. These stresses and strains need to be compensated for in the overall structural design of the resonator, increasing the complexity of the device structure. Meanwhile, using a material with good thermal conductivity to make the substrate 10 helps to quickly disperse the heat generated when the resonator is working, prevent the resonator from overheating, and thus improve the stability and durability of the resonator under high power conditions.

[0062] In some embodiments, the surface acoustic wave resonator further includes an intermediate layer (not shown) located between the first piezoelectric layer 21 and the second piezoelectric layer 22, and / or, the intermediate layer is located between the substrate 10 and the second piezoelectric layer 22.

[0063] In some embodiments, the intermediate layer is located between the first piezoelectric layer 21 and the second piezoelectric layer 22. The thickness and material selection of the intermediate layer are used to match the acoustic impedance and phase matching between the first piezoelectric layer 21 and the second piezoelectric layer 22, which is beneficial for the effective excitation of the first mode wave. Simultaneously, when the first piezoelectric layer 21 and the second piezoelectric layer 22 are connected by bonding, the intermediate layer can also enhance the connection strength between them, improving the mechanical stability of the resonator.

[0064] In some embodiments, the intermediate layer is located between the substrate 10 and the second piezoelectric layer 22, and the thickness of the intermediate layer is at the atomic level (the specific thickness depends on the material of the intermediate layer). In this case, the piezoelectric composite layer 20 and the substrate 10 no longer strictly conform to the definition of "direct bonding". The intermediate layer is introduced between the two interfaces to be bonded as a seed layer of the piezoelectric composite layer 20. This can avoid the problem of poor crystal structure matching when the two are in direct contact, which would lead to residual stress and strain at the bonding interface (and even delamination and stacking). This improves the bonding quality and increases the bonding strength, especially when both the substrate 10 and the second piezoelectric layer 22 have high anisotropy. At the same time, since the thickness of the intermediate layer is at the atomic level, it will not affect the acoustic propagation effect of the first mode wave in the first piezoelectric layer 21 (or the effect is negligible).

[0065] In some embodiments, the material of the intermediate layer includes at least one of an acoustic insulating material and an electrical insulating material. The electrical insulating material can reduce charge scattering at the bonding contacts and improve bonding quality; the electrical insulating material includes at least one of semi-insulating polycrystalline silicon, aluminum nitride, aluminum boron nitride, and silicon dioxide.

[0066] In some embodiments, the substrate 10 is made of at least one of silicon (Si, for example,

[0111] Si), low-temperature co-fired ceramic (LTCC), high-temperature co-fired ceramic (HTCC), silicon carbide (SiC), alumina (Al2O3), glass, quartz, and metal. The substrate 10 primarily serves as a support structure in the resonator. The choice of substrate 10 material significantly impacts the resonator's power handling capability and TCF (thermal conductivity factor). Therefore, it is necessary to select a suitable material for the substrate 10 based on the actual application scenario of the resonator. The coefficient of thermal expansion of the substrate material directly affects the resonator's TCF, and a higher TCF reduces the resonator's usable effective bandwidth. Simultaneously, the thermal conductivity and thermal stability of the substrate material directly affect the resonator's power handling capability. For example, SiC material has excellent thermal conductivity, resulting in relatively superior power handling capability for resonators based on SiC substrates. Therefore, the material of the substrate 10 is rationally selected based on the actual application scenario of the resonator.

[0067] In some embodiments, the electrode layer is made of at least one of aluminum, copper, platinum, gold, silver, titanium, nickel, chromium, molybdenum, and tungsten, or an alloy primarily composed of any of the aforementioned metals. Optionally, the thickness of the electrode layer ranges from 0.1 μm to 0.5 μm (inclusive), and can be 0.2 μm or 0.4 μm.

[0068] The surface acoustic wave (SAW) resonator of this application replaces the traditional single-layer piezoelectric layer with a piezoelectric composite layer, comprising a first piezoelectric layer and a second piezoelectric layer. The first piezoelectric layer is configured to confine the energy of a first-mode wave generated in the piezoelectric composite layer to propagate within the first piezoelectric layer. The second piezoelectric layer is configured to guide the leakage of a second-mode wave generated during the propagation of the first-mode wave towards the substrate. This prevents the second-mode wave from propagating as a stray mode within the first piezoelectric layer and affecting the propagation characteristics of the first-mode wave, thus effectively suppressing stray modes during resonator operation and improving the overall performance of the resonator. Furthermore, by optimizing the materials and thicknesses of the first and second piezoelectric layers, it is possible to achieve complete suppression of stray modes in certain frequency ranges while effectively confining the energy of the first-mode wave and suppressing the PSC effect, thereby enhancing the resonator's performance and competitiveness and expanding its application scenarios.

[0069] This application also provides a method for fabricating a surface acoustic wave (SAW) resonator, which can be used to fabricate the SAW resonator described above or other suitable resonator structures. Referring to Figure 1, the fabrication method includes the following steps:

[0070] Provide a substrate 10;

[0071] A piezoelectric composite layer 20 is formed on the substrate 10, and an electrode layer 30 is formed on the piezoelectric composite layer 20. The piezoelectric composite layer 20 includes a first piezoelectric layer 21 and a second piezoelectric layer 22, and the second piezoelectric layer 22 is located between the first piezoelectric layer 21 and the substrate 10.

[0072] The piezoelectric composite layer 20 generates a first mode wave under the excitation of the electric field applied by the electrode layer 30. During the propagation of the first mode wave in the piezoelectric composite layer 20, a second mode wave is generated. The first piezoelectric layer 21 is configured to confine the energy of the first mode wave within the first piezoelectric layer 21 for propagation. The second piezoelectric layer 22 is configured to guide the second mode wave to leak toward the substrate 10.

[0073] In some embodiments, the method of forming the piezoelectric composite layer 20 on the substrate 10 includes a direct bonding connection method.

[0074] The fabrication method of the surface acoustic wave resonator in this embodiment is simple and easy to implement, and is suitable for mass production.

[0075] In summary, the surface acoustic wave (SAW) resonator of this application, by adjusting the traditional single-layer piezoelectric layer to a piezoelectric composite layer, comprising a first piezoelectric layer and a second piezoelectric layer, wherein the first piezoelectric layer is configured to confine the energy of the first mode wave generated in the piezoelectric composite layer to propagate within the first piezoelectric layer, and the second piezoelectric layer is configured to guide the leakage of the second mode wave generated during the propagation of the first mode wave toward the substrate, thereby preventing the second mode wave from propagating as a stray mode within the first piezoelectric layer and affecting the propagation characteristics of the first mode wave, effectively suppressing stray modes during the operation of the resonator and contributing to improving the overall performance of the resonator. Furthermore, by optimizing the materials and thicknesses of the first and second piezoelectric layers, it is possible to achieve complete suppression of stray modes in certain frequency ranges while effectively confining the energy of the first mode wave and suppressing the PSC effect, thereby improving the resonator's performance and competitiveness and expanding its application scenarios. The fabrication method of the SAW resonator of this application is simple and easy to implement, suitable for mass production. Therefore, this application effectively overcomes the various shortcomings of the prior art and has high industrial applicability.

[0076] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0077] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A surface acoustic wave resonator, characterized in that, include: Substrate, piezoelectric composite layer and electrode layer; The piezoelectric composite layer is located between the substrate and the electrode layer; the piezoelectric composite layer includes a first piezoelectric layer and a second piezoelectric layer, with the second piezoelectric layer located between the first piezoelectric layer and the substrate; The piezoelectric composite layer is excited to generate a first mode wave under the electric field applied by the electrode layer. During the propagation of the first mode wave in the piezoelectric composite layer, a second mode wave is generated. The first piezoelectric layer is configured to confine the energy of the first mode wave to the first piezoelectric layer for propagation, and the second piezoelectric layer is configured to guide the second mode wave to leak toward the substrate.

2. The surface acoustic wave resonator according to claim 1, characterized in that: The first mode wave includes horizontal shear sound waves, and the second mode wave includes volume sound waves.

3. The surface acoustic wave resonator according to claim 2, characterized in that: The acoustic impedance of the second mode wave propagating in the first piezoelectric layer is less than the acoustic impedance of the second mode wave propagating in the second piezoelectric layer.

4. The surface acoustic wave resonator according to claim 2, characterized in that: The material of the first piezoelectric layer includes YX spin-cut lithium tantalate, and the material of the second piezoelectric layer includes YX spin-cut lithium niobate; the thickness of the first piezoelectric layer ranges from 1 μm to 30 μm; and the thickness of the second piezoelectric layer ranges from 5 μm to 20 μm.

5. The surface acoustic wave resonator according to claim 4, characterized in that: The thickness of the first piezoelectric layer ranges from 2 μm to 10 μm, the material of the first piezoelectric layer is 42°YX spin-cut lithium tantalate, and the material of the second piezoelectric layer is 70°YX spin-cut lithium niobate.

6. The surface acoustic wave resonator according to claim 1, characterized in that: The surface acoustic wave resonator further includes an intermediate layer located between the first piezoelectric layer and the second piezoelectric layer, and / or, the intermediate layer located between the substrate and the second piezoelectric layer.

7. The surface acoustic wave resonator according to claim 6, characterized in that: The intermediate layer is located between the first piezoelectric layer and the second piezoelectric layer, and the intermediate layer is used to match the acoustic impedance and phase matching between the first piezoelectric layer and the second piezoelectric layer.

8. The surface acoustic wave resonator according to claim 6, characterized in that: The intermediate layer is located between the substrate and the second piezoelectric layer, and the thickness of the intermediate layer is at the atomic level.

9. The surface acoustic wave resonator according to claim 6, characterized in that: The material of the intermediate layer includes at least one of acoustic insulating materials and electrical insulating materials; the material of the substrate includes at least one of silicon, low-temperature co-fired ceramics, high-temperature co-fired ceramics, silicon carbide, alumina, glass, quartz and metal.

10. The surface acoustic wave resonator according to claim 9, characterized in that: The electrical insulating material includes at least one of semi-insulating polycrystalline silicon, aluminum nitride, aluminum boron nitride, and silicon dioxide.

11. The surface acoustic wave resonator according to any one of claims 1 to 10, characterized in that: The piezoelectric composite layer is directly bonded to the substrate.

12. The surface acoustic wave resonator according to claim 1, characterized in that: The electrode layer is made of at least one of aluminum, copper, platinum, gold, silver, titanium, nickel, chromium, molybdenum and tungsten, or an alloy of any of the aforementioned metals.

13. The surface acoustic wave resonator according to claim 12, characterized in that: The thickness of the electrode layer ranges from 0.1 μm to 0.5 μm.

14. A method for fabricating a surface acoustic wave resonator, characterized in that, Includes the following steps: Provide a substrate; A piezoelectric composite layer is formed on the substrate, and an electrode layer is formed on the piezoelectric composite layer, wherein the piezoelectric composite layer includes a first piezoelectric layer and a second piezoelectric layer, and the second piezoelectric layer is located between the first piezoelectric layer and the substrate; The piezoelectric composite layer is excited to generate a first mode wave under the electric field applied by the electrode layer. During the propagation of the first mode wave in the piezoelectric composite layer, a second mode wave is generated. The first piezoelectric layer is configured to confine the energy of the first mode wave to the first piezoelectric layer for propagation, and the second piezoelectric layer is configured to guide the second mode wave to leak toward the substrate.

15. The method for fabricating a surface acoustic wave resonator according to claim 14, characterized in that, Methods for forming a piezoelectric layer on the substrate include direct bonding.

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