An acoustic surface wave resonator with a core-shell cylindrical structure phononic crystal reflector

Abstract

The application discloses a surface acoustic wave resonator with a core-shell cylindrical structure phononic crystal reflector, comprising a first substrate, a second substrate, a third substrate, a piezoelectric layer arranged on the first substrate, an input electrode arranged on the piezoelectric layer, an output electrode and a phononic crystal reflector; the input electrode and the output electrode jointly form an interdigital transducer and are arranged at the center of the upper surface of the piezoelectric layer; the phononic crystal reflector is composed of two phononic crystal arrays, and the two phononic crystal arrays are symmetrically arranged on the two sides of the interdigital transducer. On the basis of the multilayer substrate structure resonator, the traditional metal reflector is replaced by the phononic crystal, so that the reflection efficiency of the surface acoustic wave resonator is improved, and the SAW resonator with high quality factor is designed.

Description

Technical Field

[0001] This invention belongs to the field of acoustic resonators and radio frequency filters, and relates to a surface acoustic wave resonator using a core-shell cylindrical phononic crystal reflector grating. Background Technology

[0002] Surface acoustic wave (SAW) devices possess characteristics such as high quality factor and miniaturization, and are widely used in wireless communication, sensing, and filtering. However, SAW resonators suffer from a relatively low quality factor (Q). For filters, a high quality factor allows for a steeper passband edge, which is beneficial for improving frequency selectivity. Therefore, with the increasingly dense allocation of wireless communication frequency bands, improving the quality factor of SAW resonators is of great significance for their application.

[0003] The quality factor characterizes the energy retention capability of a resonator, and improving it hinges on reducing energy loss during resonance. During mechanical vibration, the resonator radiates sound waves, some of which cannot be effectively converted into electrical energy by the interdigital transducer (IDT), resulting in energy leakage and becoming a significant factor affecting the quality factor. Existing surface acoustic wave (SAW) resonators typically incorporate metal electrode reflectors on both sides of the IDT to reflect some of the leaked sound waves and reduce losses. However, the suppression effect of this reflective structure is mainly concentrated within a limited frequency range, making it difficult to effectively suppress broadband sound energy leakage, thus limiting its effectiveness in improving the quality factor.

[0004] Phononic crystals are artificially constructed periodic structural materials that, through structural parameter design, can form band gaps within a specific frequency range, thereby suppressing the propagation of mechanical waves at the corresponding frequencies. Based on the band gap characteristics of phononic crystals, constructing a phononic crystal structure that covers the resonator's operating frequency and has a relatively wide band gap can effectively block the outward propagation of surface acoustic waves, significantly reduce energy dissipation, and thus effectively improve the resonator's quality factor. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a surface acoustic wave resonator employing a core-shell cylindrical phononic crystal reflector grating. The technical solution is as follows:

[0006] It includes a first substrate, a second substrate, a third substrate, a piezoelectric layer disposed on the first substrate, an input electrode disposed on the piezoelectric layer, an output electrode, and a phononic crystal reflector;

[0007] The input electrode and the output electrode together constitute an interdigital transducer and are located at the center of the upper surface of the piezoelectric layer; the phononic crystal reflector is composed of two phononic crystal arrays, and the two phononic crystal arrays are symmetrically arranged on both sides of the interdigital transducer.

[0008] Preferably, the phonon crystal array comprises a plurality of phonon crystal units arranged in a matrix, wherein the phonon crystal units are nested cylindrical structures.

[0009] Preferably, the phononic crystal array is a phononic crystal unit with 1 row and 4 columns, and the spacing between the phononic crystal units is 1 μm.

[0010] Preferably, the overall cylindrical radius of the phononic crystal unit is 300 nm and the height is 400 nm; the radius of the inner nested cylinder is 90 nm-180 nm.

[0011] Preferably, the inner nested cylinder of the phonon crystal unit is made of Al2O3, AlN, or AlScN, and the outer layer is made of W.

[0012] Preferably, both the input electrode and the output electrode are made of aluminum and have a thickness of 200 nm.

[0013] Preferably, the first substrate is made of silicon dioxide and has a thickness of 1 μm.

[0014] Preferably, the material of the second substrate is polycrystalline silicon, and the thickness is 5 μm.

[0015] Preferably, the third substrate is made of silicon and has a thickness of 5 μm.

[0016] Preferably, the piezoelectric layer is made of lithium tantalate and has a thickness of 6 μm.

[0017] Compared with the prior art, the present invention has at least the following beneficial effects:

[0018] The present invention aims to improve the acoustic wave reflection efficiency by introducing a phononic crystal structure into a multilayer IHP SAW resonator instead of a traditional metal reflective grating structure, thereby obtaining a SAW resonator with high admittance ratio and high quality factor by leveraging the bandgap characteristics of the phononic crystal.

[0019] By employing high acoustic impedance material W as the main body of the phonon crystal and combining it with low-loss materials such as Al2O3, AlN, and AlScN, a hybrid material phonon crystal structure is formed. Through the rational design of the phonon crystal's shape and size, a strict match is achieved between the surface acoustic wave frequency and the phonon crystal's bandgap. This allows the SAW resonator to achieve high reflection efficiency while reducing energy loss, thereby improving the quality factor.

[0020] The surface acoustic wave resonator with a phononic crystal reflector provided by this invention forms a phononic crystal array by setting cylindrical phononic crystal units arranged in an array on the surface of a piezoelectric layer, thereby forming a phononic crystal reflector to replace the traditional electrode reflector. It can reflect the sound waves leaking at the interdigitated electrodes, reduce sound energy loss, and improve the quality factor of the resonator. At the same time, compared with the traditional electrode reflector design, the phononic crystal reflector has a wider acoustic bandgap, which can suppress the propagation of sound waves in the entire bandgap frequency range, thus making the suppression of sound energy loss more effective. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the surface acoustic wave resonator using a core-shell cylindrical phononic crystal reflector grating according to an embodiment of the present invention;

[0022] Figure 2 This is a top view of the phononic crystal array of a surface acoustic wave resonator employing a core-shell cylindrical phononic crystal reflector grating according to an embodiment of the present invention.

[0023] Figure 3 This is a three-dimensional structural diagram of the phononic crystal unit of the surface acoustic wave resonator employing a core-shell cylindrical phononic crystal reflector grating according to an embodiment of the present invention.

[0024] Figure 4 The image shows a top view and Brillouin zone of the phononic crystal unit of the surface acoustic wave resonator employing a core-shell cylindrical phononic crystal reflector grating according to an embodiment of the present invention.

[0025] Figure 5 The bandgap generated by the phononic crystal unit of the surface acoustic wave resonator using a core-shell cylindrical phononic crystal reflector grating in an embodiment of the present invention;

[0026] Figure 6 shows a comparison of the admittance of the surface acoustic wave resonator with a core-shell cylindrical phononic crystal reflector grating according to an embodiment of the present invention and the surface acoustic wave resonator with a traditional reflector grating structure. Specifically, Figure 6(a) shows a comparison of the admittance of the surface acoustic wave resonator with a core-shell cylindrical phononic crystal reflector grating structure made of W and Al2O3 mixed materials and the surface acoustic wave resonator with a traditional reflector grating structure; Figure 6(b) shows a comparison of the admittance of the surface acoustic wave resonator with a core-shell cylindrical phononic crystal reflector grating structure made of W and AlN mixed materials and the surface acoustic wave resonator with a traditional reflector grating structure; and Figure 6(c) shows a comparison of the admittance of the surface acoustic wave resonator with a core-shell cylindrical phononic crystal reflector grating structure made of W and AlScN mixed materials and the surface acoustic wave resonator with a traditional reflector grating structure.

[0027] Figure 7 This is a top view of the phononic crystal array of the surface acoustic wave resonator employing a phononic crystal reflector grating with a core-shell cylindrical inner radius gradient structure, according to an embodiment of the present invention.

[0028] Figure 8 shows a comparison of the admittance of the surface acoustic wave resonator with the phononic crystal reflector grating structure with a core-shell cylindrical inner layer radius gradient structure according to an embodiment of the present invention, and the surface acoustic wave resonator with the traditional reflector grating structure. Specifically, Figure 8(a) shows a comparison of the admittance of the surface acoustic wave resonator with the phononic crystal reflector grating structure with a core-shell cylindrical inner layer radius gradient structure using W and Al2O3 mixed materials, and the surface acoustic wave resonator with the traditional reflector grating structure using W and Al2O3 mixed materials; Figure 8(b) shows a comparison of the admittance of the surface acoustic wave resonator with the phononic crystal reflector grating structure with a core-shell cylindrical inner layer radius gradient structure using W and Al2O3 mixed materials, and the surface acoustic wave resonator with the traditional reflector grating structure using W and Al2O3 mixed materials; and Figure 8(c) shows a comparison of the admittance of the surface acoustic wave resonator with the phononic crystal reflector grating structure with a core-shell cylindrical inner layer radius gradient structure using W and Al2O3 mixed materials, and the surface acoustic wave resonator with the traditional reflector grating structure using W and Al2O3 mixed materials. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0030] Conversely, this invention encompasses any substitutions, modifications, equivalent methods, and solutions made within the spirit and scope of the invention as defined in the claims. Furthermore, to provide a better understanding of the invention, certain specific details are described in detail below. However, those skilled in the art will fully understand the invention even without these detailed descriptions.

[0031] See Figure 1 This is a schematic diagram of a surface acoustic wave resonator using a core-shell cylindrical phononic crystal reflector grating according to an embodiment of the present invention. This embodiment provides a surface acoustic wave resonator with a phononic crystal reflector, whose operating frequency is 1918MHz-1986MHz. The specific structure is as follows: Figure 1 As shown, it includes: a first substrate 1, a second substrate 2, a third substrate 3, a piezoelectric layer 4 disposed on the first substrate 1, an input electrode 5 and an output electrode 6 disposed on the upper surface of the piezoelectric layer 4, and a phononic crystal reflector 7.

[0032] The input electrode 5 and the output electrode 6 are located at the center of the upper surface of the piezoelectric layer 4 and are arranged in a cross pattern to form an interdigital transducer, which is used to realize the input and output of electrical signals in the electromechanical conversion process of the piezoelectric layer 4. The input electrode 5 can convert electrical energy into sound waves to form resonance based on the inverse piezoelectric effect, and the output electrode 6 can convert the generated sound wave signal into an electrical signal for output based on the direct piezoelectric effect.

[0033] like Figure 2As shown, the phononic crystal reflector 7 consists of two phononic crystal arrays, each array consisting of 4×1 nested cylindrical phononic crystal units. These units are positioned on both sides of the interdigital transducer formed by the input electrode 5 and the output electrode 6 to reflect the acoustic energy dissipated from the interdigital electrodes.

[0034] Example 1:

[0035] In this embodiment, the device sequentially includes a first substrate 1, a second substrate 2, a third substrate 3, a piezoelectric layer 4, an input electrode 5, an output electrode 6, and a phonon crystal reflector 7. The first substrate 1 is made of silicon dioxide, a material with good thermal and chemical stability, and has a thickness of 1 μm, providing a flat and stable support base for the device. The second substrate 2 is made of polycrystalline silicon and has a thickness of 5 μm, and the third substrate 3 is made of silicon and also has a thickness of 5 μm, to enhance the overall mechanical strength and process compatibility of the structure. The piezoelectric layer 4 is disposed on the substrate structure and is made of lithium tantalate (LiTAN). The piezoelectric layer 4, with a thickness of 6 μm, is used to realize the energy conversion between electrical signals and surface acoustic waves. Input electrode 5 and output electrode 6 are respectively disposed on the surface of the piezoelectric layer 4, and both are made of aluminum with a thickness of 0.2 μm to form an interdigital transducer structure for the excitation and reception of surface acoustic waves. The phononic crystal reflector 7 consists of two phononic crystal arrays, each containing 4×1 phononic crystal units in a nested cylindrical structure. Each phononic crystal unit has a height of 0.4 μm, an outer cylinder radius of 0.3 μm, and an inner nested core cylinder radius of 0.09 μm. The specific structure is as follows... Figure 3 As shown, the outer cylindrical shell 8 is made of high acoustic impedance tungsten, while the inner nested core cylinder 9 can be made of low dielectric loss materials such as alumina, aluminum nitride, or aluminum scandium nitride. The spacing between adjacent phonon crystal units is 1 μm.

[0036] Phononic crystals constructed using high acoustic impedance materials generate acoustic band gaps within a specific frequency range. Sound wave propagation within the band gap is suppressed, and the frequency range and location of the band gap can be controlled by changing the geometry and size of the phononic crystal unit.

[0037] Meanwhile, to reduce the intrinsic dielectric loss introduced by a single-metal phonon crystal structure, this embodiment selects materials with low dielectric loss characteristics, such as alumina, aluminum nitride, and scandium nitride, and combines them with high acoustic impedance metal materials to construct a hybrid material phonon crystal structure. By introducing low dielectric loss materials into the phonon crystal unit, the energy attenuation of the phonon crystal within the operating frequency range can be effectively reduced, thereby improving the device's quality factor and overall acoustic performance. In the hybrid material phonon crystal structure, due to the large radial acoustic impedance difference between the inner and outer layer materials, a significant acoustic impedance abrupt change is formed at the interface, which is beneficial for opening the phonon bandgap over a wider frequency range and further deepening the bandgap depth. This structure can significantly enhance the phonon crystal's ability to reflect transversely leaked sound waves and effectively suppress the propagation of sound energy in undesirable directions. Therefore, by rationally selecting the acoustic parameters and geometric dimensions of the inner and outer layer materials, the hybrid material phonon crystal structure described in this embodiment optimizes the phonon bandgap characteristics while reducing dielectric loss, making it suitable for acoustic devices with high requirements for sound wave confinement and low-loss performance.

[0038] In this embodiment, the analysis of the band structure characteristics of the core-shell cylindrical phonon crystal depends not only on the intuitive geometric arrangement, but more profoundly on solving the eigenvalues ​​of the wave equation under periodic boundary conditions. The propagation of sound waves in the composite medium follows the displacement-field wave equation:

[0039] ;

[0040] in, C(r) and C(r) are the spatially dependent density and elastic tensors, respectively. Due to the introduction of the core-shell structure, the material parameters in R... in and R out A step change occurs at the point, which leads to strong scattering of Bloch waves at the lattice boundary.

[0041] like Figure 4 As shown, wave vector k The sweeping within the irreducible Brillouin zone reveals the formation mechanism of the acoustic bandgap. The first bandgap is mainly generated by Bragg scattering, where the traveling wave is converted into a standing wave when the wavelength of the sound wave and the lattice constant a (1 μm in this embodiment) satisfy the constructive interference condition, and the kinetic energy cannot be transferred across the cell. The second bandgap exhibits obvious local resonance characteristics. This is because the high-density tungsten shell and the low-modulus inner core form a resonant unit similar to a "mass-spring," generating a negative equivalent mass effect at a specific frequency (such as around 1900 MHz), thus producing an extremely deep attenuation domain. This dual-mechanism coupling design allows the reflective grating to achieve a reflectivity of more than 20 cycles compared to a traditional metal grating within a very small physical size (only 4 columns of cells), significantly reducing the device size.

[0042] Hasaya k Through such Figure 4 By performing sweep calculations within the irreducible Brillouin zone, the band structure of the phononic crystal and its corresponding phonon bandgap range can be obtained. This can be achieved by analyzing the wave vector... k Variation analysis along a predetermined high-symmetry path within the first Brillouin zone can comprehensively characterize the propagation characteristics of sound waves in phononic crystals. Figure 5 A schematic diagram of the band structure of the core-shell cylindrical phononic crystal reflector grating used in this invention is shown. As shown in the figure, the horizontal axis represents the wave vector parameter in the first Brillouin zone. k The band structure varies sequentially along a highly symmetric path Γ–X–M–Γ, with the vertical axis representing the corresponding acoustic frequency. Multiple band curves in the figure correspond to the dispersion relations of different acoustic modes in the phononic crystal, reflecting the propagation behavior of sound waves in this periodic structure. In the band structure, the gray shaded areas represent the acoustic bandgap regions formed by the phononic crystal within the corresponding frequency range, including the first and second bandgap. Sound waves within the bandgap frequency range cannot exist in the phononic crystal structure as propagation modes; their propagation is significantly suppressed or completely prohibited. Therefore, the phononic crystal structure can effectively reflect incident surface acoustic waves and confine acoustic energy within a predetermined region, thereby reducing transverse acoustic energy leakage and improving the energy confinement capability and operational stability of acoustic devices.

[0043] As shown in Figure 6, by comparing the simulation results of the phononic crystal reflector grating structure surface acoustic wave resonator in this embodiment with the traditional metal reflector grating structure surface acoustic wave resonator, (a) compares the phononic crystal reflector grating with aluminum oxide core cylinder 9 and tungsten outer cylindrical shell 8 with the ordinary metal reflector grating; (b) compares the phononic crystal reflector grating with aluminum nitride core cylinder 9 and tungsten outer cylindrical shell 8 with the ordinary metal reflector grating; and (c) compares the phononic crystal reflector grating with aluminum scandium nitride core cylinder 9 and tungsten outer cylindrical shell 8 with the ordinary metal reflector grating. It can be seen that the admittance ratio of the phononic crystal reflector grating structure surface acoustic wave resonator in this embodiment is larger, and the admittance curve is relatively smooth. The simulation results prove the feasibility of the scheme in this embodiment.

[0044] Example 2:

[0045] In this embodiment, the substrate material and overall structure of the resonator are consistent with those of Embodiment 1. Therefore, the basic structure will not be repeated here; only the specific parameters and improvements will be described. The phononic crystal reflector 7 consists of two phononic crystal arrays disposed on one side of the resonator. Each phononic crystal array contains 4×1 phononic crystal units in a nested cylindrical structure. Figure 7 As shown, Figure 7This is a top-view schematic diagram of a single-sided phonon crystal array for a resonator. Each phonon crystal unit has a height of 0.4 μm and the radius of the outer cylinder is 0.3 μm. Unlike Embodiment 1, in this embodiment, the radius of the nested core cylinders within each phonon crystal unit cell in the phonon crystal reflector array gradually increases along the direction away from the interdigital transducer (IDT) of the resonator, i.e., it gradually increases from the side closer to the IDT towards the outside of the resonator. Specifically, the radii of the nested core cylinders are 0.09 μm, 0.12 μm, 0.15 μm, and 0.18 μm, respectively. The outer cylindrical shell 8 is made of high acoustic impedance tungsten, and the inner nested core cylinder 9 can be made of low dielectric loss materials such as alumina, aluminum nitride, or scandium nitride. The spacing between adjacent phonon crystal units is 1 μm.

[0046] In this embodiment, the inner core structure of the phonon crystal unit is designed as a gradually changing structure that moves away from the interdigital transducer (IDT). Specifically, the inner core size is smaller on the side closer to the IDT and gradually increases on the side farther away. This allows the equivalent mass density, local resonant frequency, and acoustic impedance parameters of the phonon crystal unit to change continuously with spatial position. Consequently, the entire phonon crystal array transforms from a traditional uniform periodic structure into a reflection region where acoustic parameters are gradually distributed along the propagation direction. This gradual design effectively avoids strong abrupt changes in acoustic parameters between adjacent phonon crystal units, thereby reducing acoustic mode mismatch caused by acoustic impedance discontinuities at the interface and suppressing coupling between stray modes. Simultaneously, this gradual structure breaks the strict periodicity condition of the phonon crystal to some extent, making it difficult for transverse stray modes to form stable long-range propagation paths, thus further weakening the propagation capability of transversely leaked sound waves. Therefore, the gradient phonon crystal reflection structure described in this embodiment is beneficial to improving the energy confinement capability and quality factor of acoustic devices, and can effectively improve the electrical response characteristics of devices, making the admittance curve smoother, thereby improving the overall performance and operational stability of devices.

[0047] While retaining the phononic crystal structure and its technical effects described in Embodiment 1, this embodiment further optimizes the structural parameters of the phononic crystal reflector. Specifically, the inner core radius of the phononic crystal unit near the interdigital transducer in the phononic crystal array is set to be relatively small, and the inner core radius is gradually increased along the direction away from the interdigital transducer, thereby creating a structure in which the acoustic parameters of the entire phononic crystal reflector are continuously and gradually distributed along the propagation direction. Through the above-mentioned gradual design, the acoustic parameters such as the equivalent mass density, local resonant frequency, and acoustic impedance of the phononic crystal unit are continuously varied in space, thereby effectively reducing the abrupt change in acoustic impedance between the phononic crystal reflector and adjacent acoustic structures, reducing reflection loss and acoustic mode mismatch at the interface. At the same time, this gradual phononic crystal structure can correspond to different local bandgap characteristics at different spatial locations, enabling multiple bandgaps to be superimposed within the frequency range, thereby widening the overall coverage of the acoustic bandgap. Therefore, the phononic crystal reflector described in this embodiment can significantly enhance the suppression capability of leakage acoustic waves over a wide frequency range, further improve the acoustic energy confinement effect of the surface acoustic wave resonator, which is conducive to improving the quality factor of the device and improving its admittance response characteristics, thereby improving the overall performance and operational stability of the surface acoustic wave resonator.

[0048] As shown in Figure 8, by comparing the simulation results of the surface acoustic wave resonator with the gradient phononic crystal reflector grating structure in this embodiment with the surface acoustic wave resonator with the traditional metal reflector grating structure, (a) compares the phononic crystal reflector grating with the core cylinder 9 having a gradient radius and being made of aluminum oxide and the outer cylindrical shell 8 being made of tungsten with the ordinary metal reflector grating; (b) compares the phononic crystal reflector grating with the core cylinder 9 having a gradient radius and being made of aluminum nitride and the outer cylindrical shell 8 being made of tungsten with the ordinary metal reflector grating; (c) compares the phononic crystal reflector grating with the core cylinder 9 having a gradient radius and being made of aluminum scandium nitride and the outer cylindrical shell 8 being made of tungsten with the ordinary metal reflector grating. It can be seen that the surface acoustic wave resonator with the phononic crystal reflector grating structure in this embodiment has a larger admittance ratio and a relatively smooth admittance curve. This simulation result proves the feasibility of the scheme in this embodiment.

[0049] The multilayer heterogeneous integrated substrate (IHP SAW structure) employed in this invention aims to achieve ultra-high phase velocity and excellent energy confinement. The first substrate 1 is a 1μm silicon dioxide layer, whose main function is to act as an acoustic impedance buffer layer. Its low elastic constant adjusts the lateral scattering characteristics of sound waves, ensuring that the master mode electromechanical coupling coefficient is not affected by the introduction of phonon crystals. The combination of the second substrate 2 (polycrystalline silicon) and the third substrate 3 (monocrystalline silicon) provides robust mechanical support and utilizes the high acoustic velocity characteristics of silicon to guide the Rayleigh surface acoustic wave (SAW) mode, reducing volume wave loss transmitted into the deeper substrate.

[0050] The piezoelectric layer 4 uses 6μm lithium tantalate, a thickness optimized through simulation to ensure optimal impedance matching in the 5G RF band around 1.9GHz. A core-shell phononic crystal is directly etched or grown on the surface of the piezoelectric layer, forming a good interface coupling with the lithium tantalate piezoelectric layer. By precisely controlling the density and elastic parameters of the dielectric layers such as silicon dioxide, polysilicon, and silicon, the admittance curve can be further smoothed and ripple eliminated.

[0051] The multilayer heterogeneous substrate structure used in this invention not only provides mechanical support but also constitutes a complex acoustic waveguide system. The first substrate (silicon dioxide) serves as a low-velocity acoustic medium layer, forming a significant acoustic impedance difference with the upper piezoelectric layer (lithium tantalate). This configuration simulates the "acoustic potential well" effect, confining most of the acoustic energy near the surface of the piezoelectric layer and preventing it from being converted into bulk acoustic waves and dissipated into the deeper substrate.

[0052] The introduction of the second substrate (polycrystalline silicon) and the third substrate (monocrystalline silicon) further restricts the residual longitudinal component through their unique anisotropic elastic characteristics. Polycrystalline silicon exhibits acoustic properties between silicon dioxide and silicon, and its mechanical strength and thermal stability are superior to silicon dioxide. This avoids excessive scattering caused by the abrupt change in acoustic impedance from silicon dioxide to silicon, and prevents stress, warping, or cracking problems caused by an excessively thick single silicon dioxide layer. Simulation data shows that when the silicon dioxide layer thickness is precisely controlled at 1 μm, the phase velocity of the master mode Rayleigh SAW is improved by approximately 15% compared to a single substrate, providing a larger design margin for 5G high-frequency filtering. The phononic crystal reflector grating is located above this waveguide layer, and the silicon dioxide layer at its bottom actually acts as the "acoustic ground plane" of the phononic crystal unit, enhancing the cylindrical unit's ability to capture transverse shear waves, thus increasing the total energy storage density E. stored Significant improvement was achieved.

[0053] In Example 2, the radius of the inner nested core cylinder is gradually distributed along the direction away from the interdigital transducer (IDT) (e.g., 90nm –>120nm –>150nm –>180nm). This non-uniform periodic design is one of the key innovations of this invention, which breaks the strict translational symmetry, allowing the equivalent mass density and local resonant frequency of the phonon crystal unit to change continuously with spatial position.

[0054] The technical advantages of this gradient design are as follows: Smooth impedance transition: It reduces the abrupt change in acoustic impedance between the IDT region and the reflector region, thereby significantly suppressing acoustic mode mismatch at the interface and eliminating in-band spurious responses common in traditional structures. Bandwidth broadening effect: Since cylindrical units of different radii correspond to different local bandgap frequencies, multiple bandgaps overlap and superimpose in the frequency domain, thereby broadening the overall acoustic bandgap coverage and enhancing the ability to suppress leakage acoustic waves over a wide frequency range. Energy field confinement: Compared with traditional gates that produce significant lateral leakage, the core-shell gradient structure of this invention can tightly confine Rayleigh surface acoustic wave energy to the center of the resonant cavity, significantly improving the admittance ratio.

[0055] To address the in-band ripple problem commonly found in high-quality factor resonators, the gradient radius design proposed in Embodiment 2 incorporates deep signal processing logic. Traditional constant-size reflective gratings generate steep phase transitions at the reflection boundaries, inducing coupling of transverse standing wave modes. The gradient radius (90nm to 180nm) of this invention physically constitutes an acoustic impedance transformer.

[0056] The small-radius elements near the center of the IDT have a smaller scattering cross-section, allowing the main mode to smoothly enter the reflection region; while the gradually increasing radius at the periphery provides increasingly stronger reflection intensity. This spatial energy distribution modulation makes the envelope of the sound wave during reflection tend towards a Gaussian distribution rather than a rectangular distribution, fundamentally suppressing sidelobes in the Fourier transform sense, i.e., smoothing the admittance curve Y. Furthermore, since different radii correspond to different bandgap centers, this "broadening effect" ensures that even with ±5% errors in the manufacturing process, the resonant point still falls steadily within the coverage of the combined bandgap, greatly improving the product's industrial robustness.

[0057] Preparation method example:

[0058] An embodiment of the fabrication method of the core-shell structured phononic crystal resonator of the present invention mainly includes the following steps:

[0059] 1. Substrate treatment: A high-resistivity silicon substrate is provided, and a polycrystalline silicon layer and a silicon dioxide layer are grown sequentially by chemical vapor deposition (CVD), followed by surface planarization.

[0060] 2. Piezoelectric thin film integration: Lithium tantalate wafers are integrated onto a composite substrate using bonding and thinning processes to form a piezoelectric layer structure.

[0061] 3. Electrode patterning: Photoresist is spin-coated onto the piezoelectric layer, and interdigitated transducers and busbars made of aluminum are formed through exposure, development and evaporation processes.

[0062] 4. Core-shell cylinder construction: Using electron beam lithography (EBL) and deep reactive ion etching (DRIE) techniques, phonon crystal unit holes are etched in a predetermined area of ​​the piezoelectric layer. Subsequently, a low-loss dielectric material core is precisely filled into the holes using atomic layer deposition (ALD) technology. Finally, a high acoustic impedance tungsten shell is covered by sputtering to form a closed core-shell nested structure.

[0063] 5. Subsequent processing: Remove excess masks, package the device, and finally obtain a high-Q surface acoustic wave filter.

[0064] The working principle of this invention is as follows:

[0065] Surface acoustic wave (SAW) resonators operate based on SAW propagating along a solid surface, utilizing piezoelectric materials to achieve the interconversion between electrical energy and mechanical energy. These SAW resonators typically employ interdigital transducers composed of cross-arranged electrodes to handle energy input and output. When an external electric field is applied to the input electrodes, the piezoelectric material deforms under the inverse piezoelectric effect, thereby exciting SAW waves on the solid surface, which then propagate along the surface. The frequency of the SAW wave can be controlled by adjusting the spacing between the interdigital electrodes, and its resonant frequency can be defined as:

[0066] ;

[0067] Where: f is the resonant frequency, v is the speed of surface acoustic wave propagation, and λ is the spacing between the interdigitated electrodes;

[0068] According to common knowledge in the field, the quality factor (Q) of a resonator can be defined by the following formula:

[0069] Q = 2π × ;

[0070] Where: Q is the quality factor, E stored E represents the energy stored in the resonator. dissipated This represents the energy lost in each electromechanical conversion cycle;

[0071] This shows that reducing energy loss can effectively improve the quality factor of a device.

[0072] A phononic crystal is formed by arranging two materials with significantly different acoustic impedances in a periodic manner. Its periodic structure generates an acoustic bandgap within a predetermined frequency range, thereby suppressing the propagation of sound waves within that range. The frequency range and location of the acoustic bandgap can be adjusted by changing the geometric dimensions of the phononic crystal unit. The dispersion relation of sound waves within the phononic crystal is as follows:

[0073] ω = v·k;

[0074] Where ω is the angular frequency, v is the wave speed in the medium, and k represents the wave vector;

[0075] Due to the high acoustic velocity of tungsten metal, it exhibits a significant acoustic impedance difference with lithium tantalate. Therefore, this invention constructs a columnar phononic crystal structure with tungsten as the main component, and further innovates by nesting low-dielectric-loss materials such as alumina, aluminum nitride, and aluminum scandium nitride inside the tungsten pillars. This maintains the high reflectivity of the tungsten phononic crystal pillars for surface acoustic waves while reducing the dielectric loss of the pure tungsten metal phononic crystal structure for surface acoustic waves. This results in E... dissipated This reduction improved the device's quality factor.

[0076] The surface acoustic wave resonator described in this invention is based on the synergistic effect of piezoelectricity and phononic crystal bandgap theory. When a high-frequency electrical signal is applied to the interdigital transducer, the piezoelectric layer undergoes elastic deformation under the inverse piezoelectric effect, and its particle vibration energy E stored Propagation occurs along the surface. Traditional metal reflectors, limited by the Bragg reflection bandwidth, cannot effectively suppress higher-order overtones and stray modes with lateral overflow, resulting in energy dissipation E. dissipated This increases, which in turn limits the improvement of the quality factor Q.

[0077] This invention introduces a core-shell cylindrical phononic crystal structure, which uses two materials with a large difference in acoustic impedance (e.g., a high-impedance outer shell of tungsten W and a low-impedance core medium of Al₂O₃) arranged periodically in space. This structure not only follows the Bragg scattering mechanism, using wave interference caused by periodicity to eliminate propagation modes at specific frequencies, but also introduces a local resonance mechanism. Through the resonance of a single core-shell unit, it strongly couples with the incident elastic wave, forming a broad and deep acoustic bandgap. Within the bandgap frequency range, the dispersion relation ω = v·k of the elastic wave is distorted, and the group velocity tends to zero, thereby forcibly confining the acoustic energy within the resonant cavity and greatly reducing end leakage.

[0078] The structure described in this invention, while achieving the core objective of a high Q value, also ensures the thermal stability of the RF device under high-power operating conditions. Due to the adoption of a gradient thermal management structure design from low thermal conductivity to high thermal conductivity using silicon dioxide, polycrystalline silicon, and silicon, the thermal self-heating effect of the device under continuous wave (CW) excitation is effectively mitigated while maintaining acoustic constraints. When the core-shell structure of the phononic crystal unit expands due to heat, its internal stress can be locally compensated through the heterogeneous interface, avoiding the deformation failure that is prone to occur in traditional large-area metal reflective gratings.

[0079] Through this multi-dimensional technological innovation, the surface acoustic wave resonator designed in this invention maintains a high admittance ratio while its temperature coefficient of frequency (TCF) is also significantly optimized, making it suitable for high-precision frequency selection tasks in extreme environments.

[0080] It should be noted that in the two embodiments provided above, if the operating frequency of the surface acoustic wave resonator is changed, it is necessary to adjust the size of the phononic crystal array of the phononic crystal reflector 7, the size of the phononic crystal unit, and other parameters so that the phononic crystal bandgap covers the resonant frequency.

[0081] Specifically, such as Figure 2 or Figure 7 The diagram shows a schematic of a core-shell cylindrical phononic crystal. Each rectangular region in the diagram corresponds to a unit cell of the phononic crystal. The unit cell has several characterizing parameters, which significantly influence the acoustic properties of the phononic crystal. Specifically, the lattice constant of the cylindrical phononic crystal characterizes the regular spacing between the centers of adjacent phononic crystal pillars in the crystal lattice structure; this lattice constant is one of the key parameters for establishing the phonon bandgap characteristics. Furthermore, the diameter of the phononic crystal pillars defines the width of a single phononic crystal pillar in the phononic crystal array and affects the fill fraction of the unit cell. The fill fraction can be defined as the ratio of the cross-sectional area of ​​the phononic crystal pillar to the total cross-sectional area of ​​the unit cell. The fill fraction is an important factor affecting the phonon bandgap intensity and the phononic crystal's ability to reflect sound waves of specific frequencies. Therefore, by controlling the above geometric parameters, the acoustic behavior of the phononic crystal can be regulated to meet different acoustic management needs, thus facilitating its application in the design and implementation of acoustic devices.

[0082] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A surface acoustic wave resonator employing a core-shell cylindrical phononic crystal reflector grating, characterized in that, It includes a first substrate, a second substrate, a third substrate, a piezoelectric layer disposed on the first substrate, an input electrode disposed on the piezoelectric layer, an output electrode, and a phononic crystal reflector; The input electrode and the output electrode together constitute an interdigital transducer, which is located at the center of the upper surface of the piezoelectric layer; the phononic crystal reflector is composed of two phononic crystal arrays, which are symmetrically arranged on both sides of the interdigital transducer. The phonon crystal array comprises a plurality of phonon crystal units arranged in a matrix, wherein the phonon crystal units are nested cylindrical structures; The phononic crystal array consists of 1 row and 4 columns of phononic crystal units, with a spacing of 1 μm between the phononic crystal units; The overall cylindrical radius of the phononic crystal unit is 300 nm, and the height is 400 nm; the inner nested cylinders have radii of 90 nm to 180 nm. The inner nested cylinder of the phononic crystal unit is made of Al2O3, AlN, or AlScN, and the outer layer is made of W. In the phononic crystal reflective grating array, the radius of the core cylinder nested in the inner layer of each phononic crystal unit cell is gradually distributed along the direction away from the interdigital transducer of the resonator, that is, it gradually increases from the side closer to the interdigital transducer to the outside of the resonator.

2. The surface acoustic wave resonator employing a core-shell cylindrical phononic crystal reflector grating according to claim 1, characterized in that, Both the input and output electrodes are made of aluminum and have a thickness of 200 nm.

3. The surface acoustic wave resonator employing a core-shell cylindrical phononic crystal reflector grating according to claim 1, characterized in that, The first substrate is made of silicon dioxide and has a thickness of 1 μm.

4. The surface acoustic wave resonator employing a core-shell cylindrical phononic crystal reflector grating according to claim 1, characterized in that, The second substrate is made of polycrystalline silicon and has a thickness of 5 μm.

5. The surface acoustic wave resonator employing a core-shell cylindrical phononic crystal reflector grating according to claim 1, characterized in that, The third substrate is made of silicon and has a thickness of 5 μm.

6. The surface acoustic wave resonator employing a core-shell cylindrical phononic crystal reflector grating according to claim 1, characterized in that, The piezoelectric layer is made of lithium tantalate and has a thickness of 6 μm.

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