A thick shear mode bulk acoustic wave resonator without bottom electrode is excited laterally

Abstract

The application discloses a kind of without bottom electrode transverse excitation thickness shear type high overtone bulk acoustic wave resonator, including low acoustic loss substrate, piezoelectric film layer being arranged on substrate and the transverse electrode structure being arranged on the top of piezoelectric film layer. Among them, piezoelectric film layer and substrate do not set any metal electrode layer between them, and form continuous acoustic impedance matching interface.Working, through transverse electrode in piezoelectric layer generates plane direction electric field, excite thickness shear vibration mode, make acoustic wave propagate along thickness direction and form multiple reflection high overtone standing wave resonance in substrate.Compared with traditional structure, the application completely eliminates the acoustic impedance mismatch problem introduced by bottom electrode, significantly improves acoustic energy transmission efficiency and quality factor, obtains stable frequency spectrum comb response, simultaneously realizes the flexible regulation of modal volume by electrode design.The structure process is simple, compatible with standard semiconductor process, suitable for radio frequency filter, multiplexer and microwave acoustic system.

Description

Technical Field

[0001] This invention relates to the field of acoustic resonators, specifically to a high-overtone bulk acoustic resonator that does not require a bottom electrode for lateral excitation and thickness shearing. Background Technology

[0002] High overtone bulk acoustic resonators (HBARs) are widely used in fields such as radio frequency communication, low phase noise oscillators, and quantum acoustic systems due to their high quality factor (Q), high frequency operation capability, and multimode resonance characteristics.

[0003] Existing high-overtone bulk acoustic resonators generally employ a "metal / piezoelectric layer / metal" sandwich structure, where the bottom electrode is located between the piezoelectric layer and the substrate. This presents the following problems:

[0004] 1. Acoustic impedance mismatch problem: The introduction of the bottom electrode disrupts the continuous acoustic impedance matching between the piezoelectric layer and the substrate, resulting in enhanced sound wave reflection and reduced energy transmission efficiency;

[0005] 2. Unstable mode spacing (free spectral range FSR fluctuations): Due to multi-layer interface reflections, the spacing of higher-order overtone modes is uneven, which is not conducive to frequency comb and information storage applications.

[0006] 3. Severe lateral parasitic mode coupling: Complex structures (such as irregular electrodes) are required to suppress stray modes, increasing design complexity;

[0007] 4. Mode volume is not scalable: The resonant region is limited by the electrode size, making it difficult to achieve large mode volume and high energy storage;

[0008] 5. Complex Processes and High Costs: While some existing improved structures can replace the bottom electrode with a conductive substrate, such solutions typically rely on specific conductive substrates or superconducting electrode materials. This not only places high demands on the material system but also increases the complexity of the device fabrication process. Furthermore, these structures have poor compatibility with existing standard semiconductor processes, making large-scale integrated manufacturing difficult and resulting in high overall costs.

[0009] Therefore, there is an urgent need for a new structure that can achieve bottomless electrode, efficient acoustic impedance matching, scalable modal volume, and stray mode suppression while maintaining high Q and high frequency characteristics. Summary of the Invention

[0010] To address the aforementioned problems, this invention provides a transversely excited high-overtone thickness shear body acoustic resonator that does not require a bottom electrode. By exciting the thickness shear mode through a transverse electrode, it achieves efficient acoustic energy coupling and high-Q resonance, and significantly improves the acoustic impedance matching and mode control problems of traditional high-overtone volume acoustic resonators.

[0011] Technical solution:

[0012] A high-overtone bulk acoustic resonator without bottom electrode lateral excitation thickness shearing includes: a base, a piezoelectric thin film layer and a lateral electrode structure arranged sequentially from bottom to top, wherein the piezoelectric thin film layer is in direct contact with the base, and no metal electrode layer or conductive layer is disposed between the two.

[0013] The transverse electrode structure generates a transverse electric field along its plane in the piezoelectric thin film layer to excite thickness shear mode vibration, causing sound waves to propagate along the thickness direction of the substrate and form a high-overtone standing wave resonance with multiple reflections.

[0014] Furthermore, the piezoelectric thin film layer is made of lithium niobate; the substrate is made of high-resistivity silicon, sapphire, or silicon carbide.

[0015] Furthermore, the piezoelectric thin film layer is 128° Y-cut lithium niobate.

[0016] Furthermore, the thickness of the piezoelectric thin film layer is from 0.2 μm to 5 μm; the thickness of the substrate is from 30 μm to 600 μm.

[0017] Furthermore, the lateral electrode structure is a two-port electrode in the form of a coplanar waveguide, including a signal electrode and ground electrodes located on both sides of it.

[0018] Furthermore, the spacing between the signal electrode and the adjacent ground electrode is 20 μm to 600 μm.

[0019] Furthermore, the modal volume of the resonator can be controlled by adjusting the parameters of the transverse electrode structure.

[0020] The present invention also discloses a radio frequency device comprising the above-described high-overtone bulk acoustic resonator with lateral excitation thickness shearing without bottom electrode.

[0021] Furthermore, the radio frequency device is a radio frequency filter or a radio frequency multiplexer.

[0022] The present invention also discloses a method for operating a thickness shear-type high-overtone bulk acoustic resonator without a bottom electrode lateral excitation, comprising: applying a radio frequency signal to the lateral electrode structure to excite a thickness shear mode in a piezoelectric thin film layer; the vibration mode being coupled to a substrate to form a high-Q acoustic resonance.

[0023] Compared with traditional high-overtone solid acoustic resonators, the present invention has the following advantages:

[0024] This invention eliminates the need for a bottom electrode (including a metal electrode and a conductive substrate), fundamentally removing the abrupt change in acoustic impedance at the interface between the piezoelectric layer and the substrate, thus creating an acoustically continuous transmission path. This structural change significantly improves acoustic energy transmission efficiency (>99%), primarily because it achieves excellent acoustic impedance matching (impedance ratio of approximately 1.2) between the piezoelectric layer (e.g., LiNbO3) and the substrate (e.g., Si), significantly reducing interface reflection loss.

[0025] Thanks to efficient energy transfer and optimized acoustic confinement, this resonator exhibits outstanding overall performance: its quality factor Q is as high as 10. 3 -10 5 Meanwhile, the frequency-quality factor product f·Q can exceed 10. 13 This meets the requirements of high-frequency, high-Q applications. Furthermore, this structure provides unique design flexibility, allowing the resonant mode volume to be adjusted within a wide range (e.g., 0.008–0.064 mm) by varying the spacing of the top lateral electrodes. 3 This allows for regulation, breaking through the limitations of traditional electrode area on mode volume. In terms of frequency characteristics, it supports a frequency band of 0.1–1.8 GHz or even wider.

[0026] Finally, this solution eliminates the complex bottom electrode process, resulting in a simpler structure and good compatibility with standard CMOS manufacturing processes. It is well-suited for large-scale integration and mass production, and has significant industrial application value. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below.

[0028] Figure 1 The diagram shows a traditional HBAR structure, where (a) is a schematic diagram of the overall structure of a traditional longitudinal electric field-excited HBAR; (b) is a schematic diagram of the corresponding vibration modes excited by a traditional longitudinal electric field-excited HBAR; (c) is a schematic diagram of the overall structure of a longitudinal electric field-excited HBAR with a conductive substrate replacing the bottom metal electrode; and (d) is a schematic diagram of the vibration modes corresponding to a longitudinal electric field-excited HBAR with a conductive substrate replacing the bottom metal electrode.

[0029] Figure 2 The following is a schematic diagram of the X-HTBAR structure and the principle of the transverse electrode excitation in the embodiment of the present invention. (a) is a schematic diagram of the overall structure of the X-HTBAR with transverse excitation without bottom electrode; (b) is a schematic diagram of the principle of the transverse electrode generating an in-plane electric field and exciting the acoustic wave mode of the thickness shear body.

[0030] Figure 3 This is a schematic diagram of the device structure in an embodiment of the present invention;

[0031] Figure 4 This is a schematic diagram of the substrate structure in an embodiment of the present invention;

[0032] Figure 5 This is a spectral comb response diagram in an embodiment of the present invention;

[0033] Figure 6 This is a thickness shear modulus distribution diagram in an embodiment of the present invention;

[0034] Figure 7 This is a graph showing the quality factor of the device and the product of frequency and quality factor in an embodiment of the present invention.

[0035] Figure 8 This is a free spectral range diagram of the resonant modes of the device in an embodiment of the present invention.

[0036] Figure label:

[0037] 11-Piezoelectric material layer; 12-Bottom electrode; 13-Top electrode; 14-Insulating substrate; 15-Conductive substrate; 21-Lateral top electrode; 22-Piezoelectric thin film layer; 23-Substrate; 24-Thickness shear mode; 31-Electrode length; 32-Electrode width; 33-Electrode spacing; 51-Resonance mode number. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this invention clearer, the following detailed description, in conjunction with the accompanying drawings and embodiments, further illustrates the invention. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of the invention.

[0039] This embodiment discloses a high-overtone bulk acoustic resonator with lateral excitation and thickness shearing that does not require a bottom electrode. It can be applied to RF filtering, frequency reference, phonon information processing, and microwave acoustic systems. The resonator includes:

[0040] The system comprises a substrate, a piezoelectric thin film layer disposed on the substrate, and a transverse electrode structure disposed on top of the piezoelectric thin film layer. No metal electrode layer is disposed between the piezoelectric thin film layer and the substrate. The transverse electrode structure generates a transverse electric field along its planar direction within the piezoelectric thin film layer to excite thickness shear mode vibrations, causing sound waves to propagate along the thickness direction of the substrate and form high-overtone standing wave resonances therein. Specifically,

[0041] The substrate is a low-sound-loss substrate, preferably high-resistivity silicon (HR-Si), sapphire, or silicon carbide; its thickness is in the hundreds of micrometers range (e.g., 500 μm), preferably 30–600 μm; it serves as an acoustic Fabry-Perot cavity. The piezoelectric thin film layer material is lithium niobate (LiNbO3) or tantalum niobate, preferably 128° Y-cut lithium niobate; its thickness is 0.2–5 μm (preferably about 3 μm); it is integrated onto the substrate surface via bonding. The electrode structure consists of a metal electrode only on top of the piezoelectric layer; no bottom electrode is provided; the electrodes employ a laterally distributed dual-port structure (GSG structure), with an electrode spacing of 20–600 μm; the electrode material is chromium (Cr) / gold (Au) or other conductive materials. The lateral excitation mechanism is as follows: the electric field is mainly distributed along the plane of the piezoelectric layer; the thickness shear mode is excited; the sound wave enters the substrate vertically; and a high-overtone standing wave resonance with multiple reflections is formed in the substrate.

[0042] In this embodiment, the substrate is a 500 μm thick high-resistivity silicon substrate (resistivity > 10). 4 Ω·cm);

[0043] Piezoelectric layer: 3 μm thick, 128° Y-cut LiNbO3;

[0044] Electrode: 200 nm thick Au / 10 nm thick Cr; spacing 60 μm;

[0045] Test conditions: ambient air temperature;

[0046] Results: Frequency range 0.1–1.8 GHz; Q value 10 3 -10 5 ; FSR is approximately 5.8 MHz.

[0047] like Figure 1 The diagram shown is a schematic of a conventional high overtone bulk acoustic resonator (HBAR) in the prior art. Figure 1 (a) and Figure 1 As shown in (c), the structure includes a top electrode 13, a piezoelectric layer 11, and a bottom electrode 12, wherein the bottom electrode 12 is disposed between the piezoelectric layer 11 and the insulating substrate 14 or the conductive substrate 15. This structure introduces an acoustic impedance discontinuity interface between the piezoelectric layer and the substrate, leading to enhanced acoustic wave reflection, reduced energy transfer efficiency, and a tendency to cause multi-interface interference, resulting in mode spacing (FSR) fluctuations and stray mode coupling, primarily exciting longitudinal vibration modes, such as... Figure 1 (b) and Figure 1 As shown in (d). On the other hand, if a conductive substrate 15 is used instead of a metal bottom electrode, although the structure can be simplified to some extent, it will also limit the device's adaptability and compatibility with different substrate materials.

[0048] like Figure 2 (a) and Figure 2 Figure (b) shows the structure and working principle of the X-HTBAR (High Overtone Body Acoustic Resonator) without bottom electrode transverse excitation of the present invention. The structure includes a top transverse electrode 21, a piezoelectric thin film layer 22, and an insulating substrate 23. No bottom electrode is placed between the piezoelectric thin film layer 22 and the substrate 23, forming a continuous acoustic impedance interface. In this embodiment, the bottom electrode located between the piezoelectric layer and the substrate is removed, and the traditional longitudinal electric field excitation method is changed to transverse electrode excitation, retaining the insulating substrate as the resonant cavity. During operation, an alternating voltage is applied to the transverse electrode 21, generating a planar electric field within the piezoelectric layer, thereby exciting the thickness shear mode vibration 24. The acoustic wave couples vertically into the substrate and reflects off the upper and lower surfaces of the substrate to form an acoustic Fabry-Perot cavity, achieving high overtone standing wave resonance. Unlike the prior art, which mainly excites the longitudinal vibration mode through the longitudinal electric field between the upper and lower electrodes, this embodiment specifically excites the thickness shear mode through the in-plane transverse electric field between the coplanar transverse electrodes. In existing technologies, the significant difference in acoustic impedance between the metal bottom electrode, the piezoelectric layer, and the substrate leads to severe acoustic impedance mismatch at the interface. This results in numerous sound wave reflections, reducing the energy transfer efficiency into the substrate (high insertion loss) and causing multi-interface interference, leading to unstable peak resonant spacing (FSR). This application eliminates the metal bottom electrode, allowing direct connection between the piezoelectric layer (e.g., LiNbO3) and the substrate (e.g., Si). The acoustic impedances of both are close (impedance ratio approximately 1.2), forming a continuous and well-matched acoustic impedance interface. This enables efficient coupling of sound waves from the piezoelectric layer into the substrate, resulting in high energy transfer efficiency and a uniform and stable FSR, forming a regular "frequency comb."

[0049] like Figure 3 The diagram shown is a schematic representation of the overall structural parameters of the device of this invention. Key structural parameters such as electrode length 31, electrode width 32, and electrode spacing 33 are illustrated. By adjusting the electrode spacing 33, the lateral excitation region can be effectively controlled, thereby achieving scalable control of the acoustic field modal volume and energy distribution, which is an important structural feature distinguishing it from traditional HBARs.

[0050] like Figure 4 The diagram shows the substrate structure and sound propagation path of this invention. Sound waves propagate downwards from the piezoelectric layer 22 into the substrate 23, undergoing multiple reflections at the upper and lower interfaces of the substrate to form a standing wave structure along the thickness direction. The substrate thickness determines the longitudinal standing wave conditions and the free spectral range (FSR), thereby constructing a stable high-overtone resonant cavity.

[0051] like Figure 5The figure shows a schematic diagram of the frequency response and spectral comb structure of the device of the present invention. This figure illustrates the distribution of multiple resonance peaks over a wide frequency range, exhibiting a spectral comb structure with approximately equal spacing. The numbers labeled 1, 113, 114, and 227 in the figure represent resonance mode number 51. Compared to traditional structures, the present invention significantly reduces fluctuations in the free spectral range and improves spectral uniformity by eliminating reflection interference from the bottom electrode interface.

[0052] like Figure 6 The diagram shows the vibration distribution of the thickness shear mode in this invention. It illustrates the displacement distribution characteristics of the thickness shear mode in the piezoelectric layer and substrate. The vibration mainly involves shear displacement along the planar direction and propagates along the thickness direction, forming higher-order overtone modes in the substrate. This mode exhibits low radiation loss and high energy confinement capability, making it an important mechanism for achieving a high quality factor.

[0053] like Figure 7 The figure shows a schematic diagram of the quality factor and frequency-quality factor product (f×Q) performance of the device of the present invention. This figure illustrates the Q-value distribution of the device at different frequencies and its corresponding f×Q performance index. The results show that the structure of the present invention can achieve high Q (10⁻⁶) over a wide frequency range. 3 -10 5 ) and high f×Q (>10 12 This demonstrates its superior performance in radio frequency and microwave acoustic applications.

[0054] like Figure 8 The figure shows a schematic diagram of the free spectral range (FSR) distribution of the resonant modes in the device of this invention. This figure illustrates the variation of the frequency spacing (free spectral range) between different orders of resonant modes in a laterally excited high-overtone bulk acoustic resonator. The horizontal axis represents the resonant mode number (or frequency), and the vertical axis represents the frequency spacing (FSR) between adjacent modes. As can be seen from the figure, in the structure of this invention, the FSR between each order of resonant modes remains essentially constant, exhibiting good equidistant distribution characteristics and demonstrating stable acoustic Fabry-Perot cavity resonance behavior. In contrast, traditional bottom electrode structures, due to multi-interface reflections and acoustic impedance discontinuities, easily introduce additional phase delays, resulting in significant fluctuations in FSR with modal order.

[0055] It should be noted that this application fundamentally solves the series of performance bottlenecks caused by acoustic impedance mismatch in traditional structures by eliminating the bottom electrode and replacing it with a transverse electrode, thereby changing the excitation mode from the longitudinal mode to the thickness shear mode. The fundamental contradiction of the traditional "metal-piezoelectric-metal" sandwich structure is that the presence of the bottom electrode inevitably introduces acoustic impedance mismatch. Those skilled in the art have long optimized the structure while "retaining the bottom electrode," for example, by optimizing the electrode material and shape. This embodiment provides a fundamental improvement by eliminating the electrode, which directly causes the traditional longitudinal electric field excitation mode to fail. To this end, this embodiment creatively adopts a transverse electrode structure set on top of the piezoelectric layer to generate an in-plane electric field, thereby exciting the thickness shear vibration mode. This mode was chosen because its energy confinement characteristics are particularly suitable for bottom electrode-less architectures, enabling efficient acoustic energy coupling from the piezoelectric layer to the substrate. To further ensure that the sound waves can be transmitted to the resonant cavity to the maximum extent under this new excitation mechanism, this application specifically selected a substrate with an acoustic impedance close to that of the piezoelectric layer material to form a continuous acoustic channel. This collaborative design allows the invention to simplify the structure (de-complexify) while unexpectedly resolving many contradictions that are difficult to achieve simultaneously in traditional structures. This results in significant improvements in performance across multiple aspects, including high acoustic energy transmission efficiency, high Q value, stable spectral response, and flexible controllable modal volume, achieving unexpected technical effects. Specifically, the quality factor Q of the resonator in this invention can be greater than 10. 3 The product of frequency quality factors can be greater than 10. 12 Furthermore, its resonant mode volume can be flexibly adjusted within a wide range by regulating the spacing of the top transverse electrodes, for example, approximately 0.008–0.064 mm. 3 The continuous variation within the range allows for a balance between high-performance resonant characteristics and scalable device design requirements.

[0056] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-overtone bulk acoustic resonator with lateral excitation and thickness shearing without a bottom electrode, characterized in that, include: The base, piezoelectric thin film layer and lateral electrode structure are arranged sequentially from bottom to top, wherein the piezoelectric thin film layer is in direct contact with the base, and no metal electrode layer or conductive layer is disposed between the two; The transverse electrode structure generates a transverse electric field along its plane in the piezoelectric thin film layer to excite thickness shear mode vibration, causing sound waves to propagate along the thickness direction of the substrate and form a high-overtone standing wave resonance with multiple reflections.

2. The bulk acoustic resonator according to claim 1, characterized in that, The piezoelectric thin film layer is made of lithium niobate; the substrate is made of high-resistivity silicon, sapphire, or silicon carbide.

3. The bulk acoustic resonator according to claim 2, characterized in that, The piezoelectric thin film layer is 128° Y-cut lithium niobate.

4. The bulk acoustic resonator according to claim 1, characterized in that, The thickness of the piezoelectric thin film layer is 0.2 μm to 5 μm; the thickness of the substrate is 30 μm to 600 μm.

5. The bulk acoustic resonator according to claim 1, characterized in that, The lateral electrode structure is a two-port electrode in the form of a coplanar waveguide, including a signal electrode and ground electrodes located on both sides of it.

6. The bulk acoustic resonator according to claim 5, characterized in that, The distance between the signal electrode and the adjacent ground electrode is 20 μm to 600 μm.

7. The bulk acoustic resonator according to any one of claims 1-6, characterized in that, The modal volume of the resonator can be controlled by adjusting the parameters of the transverse electrode structure.

8. A radio frequency device, characterized in that, It includes at least one high-overtone body acoustic resonator without bottom electrode lateral excitation thickness shearing as described in any one of claims 1 to 7.

9. The radio frequency device according to claim 8, characterized in that, The radio frequency device is a radio frequency filter or a radio frequency multiplexer.

10. A method for operating the resonator according to claim 1, characterized in that, include: A radio frequency signal is applied to the transverse electrode structure to excite a thickness shear mode in the piezoelectric thin film layer; The vibration mode is coupled into the substrate to form a high-Q acoustic resonance.

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