Transverse mode suppression piston structure and surface acoustic wave filter with such structure

Abstract

This utility model discloses a transverse mode suppression piston structure and a surface acoustic wave filter having the structure, wherein the piston structure is disposed at the end of the interdigital electrode strip of the interdigital electrode transducer (IDT) of the surface acoustic wave filter and at the corresponding part of the interdigital electrode strip adjacent to the end of the interdigital electrode strip, and the piston structure is configured such that its upper surface is combined with the lower surface of the interdigital electrode strip, and its lower surface is combined with the upper surface of the piezoelectric substrate.

Description

Technical Field

[0001] This utility model relates to the field of filters, and in particular to a surface acoustic wave (SAW) filter with a transverse mode suppression piston structure. Background Technology

[0002] Surface acoustic waves (SAWs) are elastic waves that propagate along the surface of an object. A SAW filter is a device that uses the characteristics of SAWs for filtering. Due to its advantages of low insertion loss, good rectangularity, small size, and low cost, it is widely used in mobile communication systems.

[0003] With the development of 5G technology, higher requirements have been placed on surface acoustic wave (SAW) filters. As SAW filters develop towards higher frequencies, wider bandwidths, and smaller sizes, the influence of transverse modes parasitic on piezoelectric substrates on passband flatness has become increasingly significant. Therefore, suppressing transverse modes has become a major focus in the development and design of SAW filters.

[0004] Currently, commonly used transverse mode suppression methods include the interdigitated electrode weighting method and the Piston mode. Unlike the interdigitated electrode weighting method, the Piston mode effectively suppresses transverse modes while maintaining the device's Q-value and size advantages, making it a current research focus. The principle of the Piston mode in surface acoustic wave (SAW) is to add mass loads to both ends of the interdigitated electrode transducer (IDT) to create a low-velocity region to modulate the dominant mode displacement distribution function, ensuring it aligns with the shape of the interdigitated electrodes. This suppresses transverse mode generation while preserving the dominant mode characteristics. However, with the development of communication technology, the fabrication of the Piston mode faces increasing challenges. Utility Model Content

[0005] The purpose of this invention is, on the one hand, to provide a transverse mode suppression piston structure for a surface acoustic wave (SAW) filter, characterized in that the piston structure is disposed at the end of the interdigital electrode strip of the interdigital electrode transducer (IDT) of the SAW filter and at the corresponding position of the strip adjacent to the end of the interdigital electrode strip, and the piston structure is configured such that its upper surface is combined with the lower surface of the interdigital electrode strip, and its lower surface is combined with the upper surface of the piezoelectric substrate.

[0006] One objective of this invention is to provide a transverse mode suppression piston structure for a surface acoustic wave filter, the piston structure being formed of the same metal material as the finger material of the interdigital electrode.

[0007] One objective of this invention is to provide a transverse mode suppression piston structure for a surface acoustic wave filter, wherein the piston structure is configured as a positive trapezoid, and its sidewalls form a specific angle with the upper surface of the piezoelectric substrate.

[0008] One objective of this invention is to provide a transverse mode suppression piston structure for a surface acoustic wave filter, wherein the specific angle is 60°-80°.

[0009] One objective of this invention is to provide a transverse mode suppression piston structure for a surface acoustic wave filter, wherein the width of the upper surface of the piston structure is less than or equal to the width of the interdigital electrode strip, and its thickness is 20% to 80% of the thickness of the interdigital electrode strip.

[0010] One objective of this invention is to provide a transverse mode suppression piston structure for a surface acoustic wave filter, wherein the piezoelectric substrate is a piezoelectric substrate on an insulator, and the piezoelectric substrate includes a piezoelectric layer, a temperature compensation layer and a base layer, wherein the piezoelectric layer is a 600 nm thick 42° lithium niobate, the temperature compensation layer is a 500 nm thick silicon dioxide, and the base layer is a 1000 nm thick high-resistivity silicon.

[0011] One objective of this invention is to provide a transverse mode suppression piston structure for a surface acoustic wave (SAW) filter, wherein the length L of the piston structure is configured according to the wavelength corresponding to the operating frequency of the SAW filter.

[0012] One objective of this invention is to provide a transverse mode suppression piston structure for a surface acoustic wave filter, wherein the piston structure, configured as a positive trapezoid, is formed on a piezoelectric substrate by a negative adhesive peeling process.

[0013] One aspect of the present invention is to provide a surface acoustic wave (SAW) filter, comprising: a reflective grating, a busbar, an interdigital electrode transducer (IDT), and a piston structure according to any one of the foregoing, wherein the busbar comprises a first busbar and a second busbar, wherein a first electrode of the interdigital electrode transducer is connected to the input port of the SAW filter via the first busbar, and a second electrode of the interdigital electrode transducer is connected to the output port of the SAW filter via the second busbar, and wherein the reflective grating comprises a first reflective grating and a second reflective grating, the first reflective grating and the second reflective grating being disposed on both sides of the interdigital electrode transducer.

[0014] One objective of this invention is to provide a surface acoustic wave filter, wherein the reflector grating is configured to have an interdigitated structure, and the piston structure is disposed at the end of the electrode fingers of the interdigitated structure of the reflector grating. Attached Figure Description

[0015] The above and other aspects, features and advantages of exemplary embodiments of the present disclosure will become clearer from the following description taken in conjunction with the accompanying drawings, in which:

[0016] Figure 1This is a schematic diagram showing the transverse mode suppression piston structure of a surface acoustic wave filter according to an embodiment of the present invention;

[0017] Figure 2 This is a schematic diagram showing a comparison of the admittance of a resonator with a transverse mode suppression piston structure according to an embodiment of the present invention and a resonator without a piston structure at different frequency bands;

[0018] Figure 3 This is a flowchart illustrating a method for manufacturing a transverse mold suppression piston structure according to an embodiment of the present invention; and

[0019] Figure 4 This is a schematic diagram showing a surface acoustic wave filter having a transverse mode suppression piston structure according to an embodiment of the present invention. Detailed Implementation

[0020] Before proceeding with the detailed description below, it may be advantageous to define certain words and phrases used throughout this disclosure. The terms “comprising” and “including” and their derivatives mean, but are not limited to, “including”. The phrase “at least one”, when used with a list of items, means that different combinations of one or more of the listed items may be used, and that only one item in the list may be required. For example, “at least one of A, B, and C” includes any one of the following combinations: A, B, C, A and B, A and C, B and C, A and B and C.

[0021] Definitions of other specific words and phrases are provided throughout this disclosure. Those skilled in the art will understand that, in many, if not most, cases, such definitions apply to the prior and future use of the words and phrases thus defined.

[0022] The various embodiments of the principles of this disclosure described below in conjunction with the accompanying drawings are for illustrative purposes only and should not be construed as limiting the scope of this disclosure in any way. Those skilled in the art will understand that the principles of this disclosure can be implemented in any suitably arranged system or device. In some cases, the actions described in this disclosure may be performed in a different order and still achieve the desired result. Furthermore, the processes depicted in the drawings do not necessarily require a specific order or sequential sequence to achieve the desired result. In certain embodiments, multitasking and parallel processing may be advantageous.

[0023] The text and accompanying drawings are provided by way of example only to aid in understanding this disclosure. They should not be construed as limiting the scope of the claims appended to this disclosure in any way. Throughout the drawings, the same reference numerals generally indicate the same elements. Although certain embodiments and examples have been provided, it will be apparent to those skilled in the art, based on the content of this disclosure, that changes may be made to the illustrated embodiments and examples without departing from the scope of this disclosure.

[0024] Several approaches exist for improving the piston structure of surface acoustic wave (SAW) filters: 1) Effectively suppressing transverse modes by adding mass loads at both ends of the interdigital electrodes. For example, a dielectric layer can be added to both ends of the electrodes to increase mass. This dielectric layer can be placed on the lower or upper surface of the electrodes to adjust the mass load. However, this method requires overcoming numerous technological challenges and is too costly. For instance, this method requires ensuring high adhesion between the dielectric layer and the substrate, significantly increasing the process complexity. Furthermore, the dimensional accuracy of the dielectric layer needs to be controlled to be comparable to that of the interdigital electrode metal layer, requiring high-precision dielectric material deposition equipment, leading to a substantial increase in cost; 2) Transverse mode suppression can be achieved by configuring the duty cycle of the fingers at both ends of the interdigital electrodes to be greater than that of the fingers in the middle of the electrodes. However, this method is limited by the dimensional constraints of photolithography, limiting its effectiveness in suppressing transverse modes in mid-to-high frequency devices; 3) The thickness at both ends of the metal electrode fingers can also be increased by depositing a mass-loaded layer at both ends of the fingers. 4) Using a multilayer metal structure (e.g., Mo / Al) to form a piston structure for surface acoustic wave modulation (SAW) modulation. This method overcomes process limitations and is widely used for SAW in mid-to-high frequency devices. However, in the photolithography-development process of thickening the piston layer, the presence of the interdigitated electrode layer affects the homogenization quality to some extent. This results in the piston size limit, determined by process limitations, being greater than the interdigitated electrode finger size limit, making this method difficult to implement. 5) Forming a piston structure using a multilayer metal structure (e.g., Mo / Al) to incorporate multiple metals, where the IDT electrode is partially embedded in the piezoelectric layer, and the piston structure is formed using a multilayer metal structure. However, the multilayer metal structure requires complex deposition and etching processes, increasing manufacturing costs and difficulty. Since the piston structure mainly depends on the thickness and material density of the metal layer on the electrode surface, the piston structure formed by this method has limited SAW effect on high-frequency devices, especially under the miniaturization requirements of devices, making it difficult to meet the performance requirements of high-frequency SAW filters.

[0025] To address the aforementioned problems, this invention proposes a novel piston structure that significantly improves the lateral mold suppression effect by optimizing the piston's position, shape, and material, while simultaneously reducing process complexity.

[0026] Figure 1 This is a schematic diagram showing the transverse mode suppression piston structure of a surface acoustic wave filter according to an embodiment of the present invention.

[0027] refer to Figure 1 The lateral mode suppression piston structure is disposed at the end of the electrode finger strip of the interdigital electrode and at the corresponding position of the adjacent finger strip. That is, the lateral mode suppression piston structure is disposed on the straight line corresponding to the end of the electrode finger strip of the interdigital electrode. For example... Figure 1 On the straight line A-A' or B-B'. The upper surface of the piston structure is bonded to the lower surface of the finger strip, and its lower surface is bonded to the piezoelectric substrate. The finger strip and piston structure are in the width direction perpendicular to the aperture direction. In the width direction, the width of the piston structure can be smaller than the width of the finger strip (see...). Figure 1 Part (a) of the text, equal to the width of the bar (see [reference]). Figure 1 Part (b) of the finger strip or greater than the width of the finger strip (see part (b)) Figure 1 Part (c) of the diagram. The length of the piston structure is set to L, wherein the length L is configured according to the transverse mode suppression requirements, for example, according to the wavelength corresponding to the operating frequency of the filter. The piston length L is configured such that the value of L is within the range of 0.2. ~0.8 Between. According to embodiments of the present invention, a dielectric layer can be configured to fill the piston structures below the same finger strip. According to embodiments of the present invention, a metallic material for forming the finger strip can also be directly deposited between the piston structures below the same finger strip. For example, the finger strip of the interdigital electrode can be configured to be directly deposited covering the piston structure and the piezoelectric substrate between the piston structures. The finger strip of the interdigital electrode is configured to be formed by a multilayer metal structure, wherein the first metal layer is configured as a metal layer (e.g., titanium, nickel, cobalt, etc.) to increase the adhesion between the electrode and the substrate.

[0028] According to an embodiment of the present invention, the piezoelectric substrate is configured as a piezoelectric on insulator (POI) substrate, wherein the POI substrate includes a piezoelectric layer, a temperature compensation layer and a base layer, wherein the piezoelectric layer is 600 nm thick 42°LT (lithium niobate, LiNbO3), the temperature compensation layer is 500 nm thick silicon dioxide, and the base layer is 1000 nm thick high-resistivity silicon.

[0029] The piston structure can be made of the same material as the finger strip, or other metallic materials can be selected. According to an embodiment of the present invention, the piston structure can be formed using the same metallic material as the finger strip (e.g., Al). According to an embodiment of the present invention, the piston structure is configured to be located directly between the lower surface of the electrode finger strip and the piezoelectric substrate, rather than embedded in the piezoelectric substrate. This design can more effectively modulate the master mode displacement distribution function, thereby enhancing the lateral mode suppression effect.

[0030] According to embodiments of this invention, the piston structure is configured as a single-layer metal structure, rather than a multi-layer metal structure (e.g., a Mo / Al structure). The single-layer metal structure simplifies the manufacturing process and reduces production costs.

[0031] According to an embodiment of the present invention, the piston structure can be configured as a trapezoid, with its sidewalls forming a specific angle (e.g., 60°-80°) with the surface of the piezoelectric substrate to optimize the propagation characteristics of surface acoustic waves. The width of the upper surface of the trapezoidal piston structure is less than or equal to the width of the electrode fingers, and its thickness is 20% to 80% of the finger thickness.

[0032] Figure 2 This is a schematic diagram showing a comparison of the admittance of a resonator with a transverse mode suppression piston structure according to an embodiment of the present invention and a resonator without a transverse mode suppression piston structure at different frequency bands.

[0033] refer to Figure 2 The blue curve represents the admittance curve of the resonator with the transverse mode suppression piston structure according to an embodiment of the present invention, and the red curve represents the admittance curve of the resonator without the piston structure. The comparison results show that the resonator with the transverse mode suppression piston structure according to an embodiment of the present invention significantly reduces transverse mode interference in the passband, and the admittance curve is smoother. The introduction of the transverse mode suppression piston structure according to an embodiment of the present invention significantly improves the Q value of the resonator, greatly improving the in-band performance of the filter. Figure 2 In the high-frequency range, the resonator with the transverse mode suppression piston structure according to the embodiments of the present invention has significant transverse mode suppression effect and performance advantages.

[0034] Figure 3 This is a flowchart illustrating a method for manufacturing a transverse mold suppressing piston structure according to an embodiment of the present invention.

[0035] refer to Figure 3 In step S301, a piezoelectric substrate is formed. According to an embodiment of the present invention, the piezoelectric substrate is configured as a piezoelectric on insulator (POI) substrate. The POI substrate includes a piezoelectric layer, a temperature compensation layer, and a base layer. The piezoelectric layer is 600 nm thick 42°LT (lithium niobate, LiNbO3), the temperature compensation layer is 500 nm thick silicon dioxide, and the base layer is 1000 nm thick high-resistivity silicon.

[0036] In step S302, a lateral mode suppression piston structure is fabricated on the piezoelectric substrate by processing the first photoresist layer coated on the piezoelectric substrate. According to an embodiment of the present invention, for example, the first photoresist layer is a negative resist, and a positive trapezoidal piston structure is fabricated on the piezoelectric substrate using a negative resist peeling process. Preferably, the sidewalls of the piston structure form an angle of 60°-80° with the surface of the piezoelectric substrate. By forming an angle of 60°-80° between the sidewalls of the piston structure and the surface of the piezoelectric substrate, it can be ensured that the interdigitated electrodes deposited on the piston completely cover the contact surface, preventing voids from appearing when the interdigitated electrodes are deposited on the piston. This avoids voids becoming crack sources and causing electrode failure when the interdigitated electrodes of the high-frequency surface acoustic wave filter are in operation, thus increasing device reliability.

[0037] In step S303, an interdigitated electrode structure is fabricated on the piezoelectric substrate and piston structure by processing the second photoresist layer coated on the piezoelectric substrate and the lateral mode suppression piston structure. According to an embodiment of the present invention, for example, the second photoresist layer is a negative resist, and the interdigitated electrode structure is fabricated on the piezoelectric substrate and piston structure using a negative resist peeling process. Preferably, the width of the upper surface of the piston structure is less than or equal to the width of the finger strip, and the thickness of the piston structure is 20% to 80% of the thickness of the finger strip. According to an embodiment of the present invention, by configuring the width of the upper surface of the piston to be less than the width of the finger strip, the interdigitated metal deposited on the upper surface of the piston can cover both sides of the piston structure, making the interdigitated metal bonded to the piston more firmly, thereby helping to improve device reliability. Furthermore, considering that the width of the interdigitated metal has a process limit that cannot be too small when the device frequency increases, configuring the width of the upper surface of the piston structure to be less than the width of the finger strip will significantly reduce the processing difficulty of the finger strip.

[0038] By placing the piston structure between the piezoelectric substrate and the interdigitated electrodes, the fabrication difficulty of the piston structure is reduced, the piston size limit is increased, and the adjustment range of the piston structure for transverse modes is expanded. This improves the transverse mode suppression effect of the resonator while ensuring that the resonator size is minimized.

[0039] Figure 4 This is a schematic diagram showing a surface acoustic wave filter having a transverse mode suppression piston structure according to an embodiment of the present invention.

[0040] refer to Figure 4The surface acoustic wave (SAW) filter includes a reflector grating, a busbar, an interdigital electrode transducer (IDT), and a transverse mode suppression piston structure. IDTs 404-1 and 404-2 are connected to the input port 401 via a first busbar; IDT 403 is connected to the output port via a second busbar. Reflector gratings 405 and 406 are formed on both sides of IDTs 403, 404-1, and 404-2. According to an embodiment of the present invention, IDTs 403, 404-1, and 404-2 are configured with a piston structure according to an embodiment of the present invention, that is, the piston structure is formed at the end of the electrode fingers of the interdigital electrode and at the corresponding positions of adjacent fingers. According to an embodiment of the present invention, reflector gratings 405 and 406 are configured as interdigital structures, and a piston structure according to an embodiment of the present invention is also configured at the end of the electrode fingers of the interdigital electrodes.

[0041] Those skilled in the art should understand that, although Figure 4 The diagram shows a surface acoustic wave (SAW) filter with three interdigitated electrode transducers (IDTs), but it is merely an example and can be appropriately modified without departing from the scope of this invention. For instance, the SAW filter can be a ladder-structure filter, which can be configured to include only one series SAW resonator and one parallel SAW resonator. The series resonator is connected to the input and output ports via a first and a second busbar, a third busbar connects one end of the parallel resonator to the second busbar, and a fourth busbar connects the other end of the parallel resonator to a ground port.

[0042] Those skilled in the art will understand that the various illustrative logic blocks, modules, circuits, and steps described in this disclosure can be implemented in hardware, software, or a combination of both. Whether such a set of functions is implemented in hardware or software depends on the specific application and the design constraints imposed on the overall system. Those skilled in the art may implement the described set of functions in different ways for each specific application, but such design decisions should not be construed as departing from the scope of this disclosure.

[0043] The above embodiments of this disclosure are merely for ease of description and to aid in a comprehensive understanding of this disclosure, and are not intended to limit the scope of this disclosure. Therefore, it should be understood that all modifications and alterations, or forms of modifications and alterations, derived from the technical concept of this disclosure, other than those disclosed herein, fall within the scope of this disclosure.

Claims

1. A transverse mold suppression piston structure, characterized in that, The piston structure is configured at the end of the interdigital electrode strip of the interdigital electrode transducer (IDT) of the surface acoustic wave filter and at the corresponding position of the adjacent finger strip. The piston structure is configured such that its upper surface is coupled to the lower surface of the interdigital electrode's fingers, and its lower surface is coupled to the upper surface of the piezoelectric substrate.

2. The transverse mold suppression piston structure according to claim 1, characterized in that, The piston structure is formed using the same metallic material as the finger material of the interdigital electrode.

3. The transverse mold suppression piston structure according to claim 1, characterized in that, The piston structure is configured as a trapezoid, with its sidewalls forming a specific angle with the upper surface of the piezoelectric substrate.

4. The transverse mode suppression piston structure of claim 3, wherein, The specific angle is 60°-80°.

5. The transverse mold suppression piston structure according to claim 1, characterized in that, The width of the upper surface of the piston structure is less than or equal to the width of the interdigitated electrode strip, and its thickness is 20% to 80% of the thickness of the interdigitated electrode strip.

6. The transverse mold suppression piston structure according to claim 1, characterized in that, The piezoelectric substrate is a piezoelectric substrate on an insulator, and the piezoelectric substrate includes a piezoelectric layer, a temperature compensation layer, and a substrate layer. The piezoelectric layer is a 600 nm thick 42° lithium niobate, the temperature compensation layer is a 500 nm thick silicon dioxide, and the substrate layer is a 1000 nm thick high-resistivity silicon.

7. The transverse mold suppression piston structure according to claim 1, characterized in that, The length L of the piston structure is configured according to the wavelength corresponding to the operating frequency of the surface acoustic wave filter.

8. The transverse mode suppression piston structure of claim 3, wherein, The piston structure, configured as a positive trapezoid, is formed on a piezoelectric substrate using a negative adhesive peeling process.

9. A surface acoustic wave filter comprising: The reflective grating, busbar, interdigitated electrode transducer (IDT), and the transverse mode suppression piston structure according to any one of claims 1-8, characterized in that, The busbar includes a first busbar and a second busbar. The first electrode of the interdigital electrode transducer is connected to the input port of the surface acoustic wave filter via the first busbar, and the second electrode of the interdigital electrode transducer is connected to the output port of the surface acoustic wave filter via the second busbar. The reflective grating includes a first reflective grating and a second reflective grating, which are disposed on both sides of the interdigital electrode transducer.

10. The surface acoustic wave filter according to claim 9, characterized by, The reflective grating is configured to have an interdigitated structure, and the piston structure is disposed at the end of the electrode fingers of the interdigitated structure of the reflective grating.

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