Transverse mode suppression piston structure and surface wave filter with same

Abstract

The invention discloses a piston structure for transverse mode suppression of a surface acoustic wave filter, which is characterized in that the piston structure is configured at the tail end of an electrode finger strip of an interdigital electrode of an interdigital electrode transducer (IDT) of the surface acoustic wave filter and the corresponding part of a finger strip adjacent to the tail end of the electrode finger strip; and the upper surface of the piston structure is combined with the lower surface of the finger strip of the interdigital electrode, and the lower surface of the piston structure is combined with the upper surface of the piezoelectric substrate.

Description

Technical Field

[0001] The present invention relates to the field of filters, and particularly to a surface acoustic wave (SAW) filter having a lateral mode suppression piston structure. Background Art

[0002] Surface acoustic wave (SAW) is an elastic wave propagating along the surface of an object. A surface acoustic wave filter is a device that filters using the characteristics of surface acoustic waves. 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 are put forward for surface acoustic wave filters. Along with the development of surface acoustic wave filters towards higher frequencies, wider bandwidths, and smaller sizes, the influence of the lateral mode parasitized by the piezoelectric substrate on the passband flatness becomes increasingly non-negligible. Therefore, suppressing the lateral mode has become a major focus in the development and design of surface acoustic wave filters.

[0004] Currently, the commonly used lateral mode suppression methods include: interdigital electrode weighting method and Piston mode. Different from suppressing the lateral mode by the interdigital electrode weighting method, the piston technology can effectively ensure the Q value and size advantages of the device while effectively suppressing the lateral mode, and is the current research focus. The principle of the piston mode of surface acoustic wave is to increase mass loads at both ends of the finger bars of the interdigital electrode transducer (IDT) to create a low sound velocity region to modulate the main mode displacement distribution function to be consistent with the shape of the interdigital electrode, thereby retaining the main mode characteristics while suppressing the generation of the lateral mode. However, with the development of communication technology, the process implementation of the piston mode faces increasing difficulties. Summary of the Invention

[0005] One aspect of the object of the present invention is to provide a piston structure for lateral mode suppression of a surface acoustic wave filter, characterized in that the piston structure is configured at the ends of the electrode finger bars of the interdigital electrode of the interdigital electrode transducer (IDT) of the surface acoustic wave filter and at the corresponding parts of the finger bars adjacent to the ends of the electrode finger bars, and the piston structure is configured such that its upper surface is combined with the lower surface of the finger bars of the interdigital electrode, and its lower surface is combined with the upper surface of the piezoelectric substrate.

[0006] One aspect of the object of the present invention is to provide a piston structure for lateral mode suppression of a surface acoustic wave filter, and the piston structure is formed by a metal material having the same material as the finger bars of the interdigital electrode.

[0007] One aspect of the object of the present invention is to provide a piston structure for lateral mode suppression of a surface acoustic wave filter, and the piston structure is configured as a regular trapezoid, and its side walls form a specific angle with the upper surface of the piezoelectric substrate.

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

[0009] One aspect of the object of the present invention is to provide a piston structure for transverse mode suppression of a surface acoustic wave filter. The width of the upper surface of the piston structure is less than or equal to the width of the finger bars of the interdigital electrode, and its thickness is 20%-80% of the thickness of the finger bars of the interdigital electrode.

[0010] One aspect of the object of the present invention is to provide a piston structure for transverse mode suppression of a surface acoustic wave filter. The piezoelectric substrate is a piezoelectric-on-insulator substrate, and the piezoelectric substrate includes a piezoelectric layer, a temperature compensation layer, and a base layer. Among them, the piezoelectric layer is 42° lithium niobate with a thickness of 600 nanometers, the temperature compensation layer is silicon dioxide with a thickness of 500 nanometers, and the base layer is high-resistance silicon with a thickness of 1000 nanometers.

[0011] One aspect of the object of the present invention is to provide a piston structure for transverse mode suppression of a surface acoustic wave filter. The length L of the piston structure is configured according to the wavelength corresponding to the operating frequency of the surface acoustic wave filter.

[0012] One aspect of the object of the present invention is to provide a piston structure for transverse mode suppression of a surface acoustic wave filter. The piston structure configured as a positive trapezoid is formed on the piezoelectric substrate by a negative photoresist lift-off process.

[0013] One aspect of the object of the present invention is to provide a surface acoustic wave filter, including: a reflection grating, a bus bar, an interdigital electrode transducer (IDT), and the piston structure according to any one of the above. Among them, the bus bar includes a first bus bar and a second bus bar. Among them, the first electrode of the interdigital electrode transducer is connected to the input port of the surface acoustic wave filter through the first bus bar, and the second electrode of the interdigital electrode transducer is connected to the output port of the surface acoustic wave filter through the second bus bar. And among them, the reflection grating includes a first reflection grating and a second reflection grating, and the first reflection grating and the second reflection grating are arranged on both sides of the interdigital electrode transducer.

[0014] One aspect of the object of the present invention is to provide a surface acoustic wave filter, wherein the reflection grating is configured to have an interdigital structure, and the piston structure is arranged at the end of the finger bars of the electrode of the interdigital structure of the reflection grating. Description of the Drawings

[0015] From the following description in conjunction with the drawings, the above and other aspects, features, and advantages of the exemplary embodiments of the present disclosure will become clearer, wherein:

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

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

[0018] Figure 3 is a flowchart showing the manufacturing method of a piston structure according to an embodiment of the present invention; and

[0019] Figure 4 is a schematic diagram showing a surface acoustic wave filter with a piston structure according to an embodiment of the present invention. Detailed Description

[0020] Before proceeding with the following detailed description, it may be advantageous to set forth definitions of certain words and phrases used throughout this disclosure. The terms "include" and "comprise," and derivatives thereof, mean including but not limited to. 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 only one item in the list may be required. For example, "at least one of A, B, C" includes any 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 of ordinary skill in the art should understand that in many, if not most, instances, such definitions apply to the prior and future use of such defined words and phrases.

[0022] The following description of various embodiments of the principles of this disclosure in this patent application document with reference to the accompanying drawings is for illustrative purposes only and should not be construed in any way as limiting the scope of this disclosure. Those skilled in the art will understand that the principles of this disclosure may 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. Additionally, the processes depicted in the drawings do not necessarily require the specific order or sequential order shown to achieve the desired result. In certain embodiments, multitasking and parallel processing may be advantageous.

[0023] The text and the drawings are provided only as examples to assist 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, like reference numerals generally indicate like 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 the illustrated embodiments and examples may be changed without departing from the scope of this disclosure.

[0024] There are several ways to improve the piston structure of surface acoustic wave filters: 1) Effectively suppressing the transverse mode by adding mass loads at both ends of the interdigital electrodes. For example, a dielectric layer can be added at both ends of the electrodes to increase the mass. This dielectric layer can be on the lower surface or the upper surface of the electrodes to achieve the adjustment of the mass load. However, this method needs to overcome many process difficulties and has too high a cost. For example, this method needs to ensure a high bonding force between the dielectric layer and the substrate, which significantly increases the process complexity. In addition, the dimensional accuracy of the dielectric layer needs to be controlled to be equivalent to that of the metal of the interdigital electrode layer. Therefore, high-precision dielectric material coating equipment is required, resulting in a substantial increase in cost; 2) The transverse mode can be suppressed by configuring the duty cycle of the finger bars at both ends of the interdigital electrodes to be greater than that of the finger bars in the middle of the electrodes. However, this method is limited by the lithography limit size and has a limit in suppressing the transverse mode of medium and high-frequency devices; 3) The transverse mode can also be suppressed by increasing the thickness at both ends of the metal electrode finger bars and depositing an additional mass load metal layer at both ends of the finger bars. This method can overcome process limitations and is widely used for suppressing the transverse mode of medium and high-frequency devices. However, in the lithography - development process of thickening the piston layer, the presence of the interdigital electrode layer will affect the spin coating quality to a certain extent. This results in the piston size limit determined by the process limit being greater than the finger bar size limit of the interdigital electrodes, making this method difficult to implement; 4) A piston structure is formed by a multi-layer metal structure including multiple metals (such as Mo / Al) to achieve the modulation of surface acoustic waves. In this structure, the IDT electrode part is embedded in the piezoelectric layer, and the piston structure is formed by the multi-layer metal structure. However, due to the complex deposition and etching processes required for the multi-layer metal structure, the manufacturing cost and difficulty are increased. Since the piston structure mainly depends on the thickness and material density of the metal layer on the upper surface of the electrode, the piston structure formed by this method has limited effect on suppressing the transverse mode of high-frequency devices. Especially under the requirement of device miniaturization, it is difficult to meet the performance requirements of high-frequency surface acoustic wave filters.

[0025] In view of the above problems, the present invention proposes a novel piston structure, which significantly improves the transverse mode suppression effect and reduces the process complexity by optimizing the position, shape and material of the piston.

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

[0027] Reference Figure 1 , the piston structure is arranged at the ends of the electrode finger bars of the interdigital electrode and at the positions corresponding to the adjacent finger bars, that is, the piston structure is arranged on the straight line corresponding to the ends of the electrode finger bars of the interdigital electrode. For example, Figure 1On the straight line A-A' or B-B'. The upper surface of the piston structure is combined with the lower surface of the finger bar, and its lower surface is combined with the piezoelectric substrate. The finger bar and the 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 bar (see Figure 1 Part (a) in), equal to the width of the finger bar (see Figure 1 Part (b) in) or larger than the width of the finger bar (see Figure 1 Part (c) in). The length of the piston structure is set to L, where the length L is configured according to the transverse mode suppression requirement. For example, the length L of the piston is configured according to the wavelength (λ) corresponding to the operating frequency of the filter, and the value range of L is between 0.2λ and 0.8λ. According to an embodiment of the present invention, between the piston structures under the same finger bar, a dielectric layer can be configured to be filled. According to an embodiment of the present invention, between the piston structures under the same finger bar, the metal material for forming the finger bar can also be directly deposited. For example, the finger bars of the interdigital electrodes can be configured to directly deposit and cover the piston structure and the piezoelectric substrate between the piston structures. The finger bars of the interdigital electrodes are configured to be formed by a multi-layer metal structure, where the first layer of metal is configured to be a metal layer for increasing the bonding force between the electrode and the substrate (such as a titanium, nickel, cobalt layer, etc.).

[0028] According to an embodiment of the present invention, the piezoelectric substrate is configured to be a Piezoelectric on Insulator (POI) substrate, and the POI substrate includes a piezoelectric layer, a temperature compensation layer, and a base layer. The piezoelectric layer is 42°LT (lithium niobate, LiNbO 3 ) with a thickness of 600 nanometers, the temperature compensation layer is silicon dioxide with a thickness of 500 nanometers, and the base layer is high-resistance silicon with a thickness of 1000 nanometers.

[0029] The material of the piston structure can be the same as that of the finger bar, or other metal materials can be selected. According to an embodiment of the present invention, the piston structure can be formed by using the same metal material as the finger bar (such as Al). According to an embodiment of the present invention, the piston structure is configured to be directly located between the lower surface of the electrode finger bar and the piezoelectric substrate, rather than being embedded in the piezoelectric substrate. This design can more effectively modulate the main mode displacement distribution function, thereby enhancing the transverse mode suppression effect.

[0030] According to an embodiment of the present invention, the piston structure is configured to be a single-layer metal structure, rather than a multi-layer metal structure (such as a Mo / Al structure). The single-layer metal structure simplifies the process steps and reduces the manufacturing cost.

[0031] According to an embodiment of the present invention, the piston structure can be configured as a positive trapezoid, and its side wall forms a specific angle (such as 60°-80°) with the surface of the piezoelectric substrate to optimize the propagation characteristics of surface acoustic waves. Among them, the width of the upper surface of the positive 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 FIG. is a schematic diagram showing the admittance comparison between a resonator with a piston structure according to an embodiment of the present invention and a resonator without a piston structure at different frequency bands.

[0033] Reference Figure 2 , the blue curve represents the admittance curve of the resonator with the 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. It can be seen from the comparison results that the lateral mode interference of the resonator with the piston structure according to the embodiment of the present invention in the passband is significantly reduced, the admittance curve is smoother, and the introduction of the piston structure according to the embodiment of the present invention significantly improves the Q value of the resonator, greatly improving the in-band performance of the filter. According to Figure 2 , in the high-frequency part, the resonator with the piston structure according to an embodiment of the present invention has a significant lateral mode suppression effect and performance advantages.

[0034] Figure 3 FIG. is a flowchart showing a manufacturing method of a piston structure according to an embodiment of the present invention.

[0035] Reference 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, and the POI substrate includes a piezoelectric layer, a temperature compensation layer, and a base layer, where the piezoelectric layer is 42° LT (lithium niobate, LiNbO 3 ) with a thickness of 600 nanometers, the temperature compensation layer is silicon dioxide with a thickness of 500 nanometers, and the base layer is high-resistance silicon with a thickness of 1000 nanometers.

[0036] In step S302, a 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 photoresist, and a positive trapezoidal piston structure is fabricated on the piezoelectric substrate through a negative photoresist lift-off process. Preferably, the sidewall of the piston structure forms an angle of 60°-80° with the surface of the piezoelectric substrate. By forming an angle of 60°-80° between the sidewall of the piston structure and the surface of the piezoelectric substrate, it can be ensured that the interdigital electrodes deposited on the piston completely cover the contact surface, and there will be no voids when the interdigital electrodes are deposited on the piston, thereby avoiding the voids becoming crack source electrodes during the operation of the interdigital electrodes of the high-frequency surface acoustic wave filter and causing electrode failure, thus increasing the device reliability.

[0037] In step S303, an interdigital electrode structure is fabricated on the piezoelectric substrate and the piston structure by processing the second photoresist layer coated on the piezoelectric substrate and the piston structure. According to an embodiment of the present invention, for example, the second photoresist layer is a negative photoresist, and an interdigital electrode structure is fabricated on the piezoelectric substrate and the piston structure through a negative photoresist lift-off process. Preferably, the width of the upper surface of the piston structure is less than or equal to the finger width, and the thickness of the piston structure is 20%-80% of the finger thickness. According to an embodiment of the present invention, by configuring the width of the upper surface of the piston to be less than the finger width, the interdigital finger metal deposited on the upper surface of the piston can cover both sides of the piston structure, making the interdigital fingers more firmly bonded to the piston, thereby helping to improve the device reliability. In addition, considering that when the device frequency increases, there is a process limit for the finger width and it cannot be too small, by configuring the width of the upper surface of the piston structure to be less than the finger width, the process difficulty of the fingers will be greatly reduced.

[0038] By disposing the piston structure between the piezoelectric substrate and the interdigital electrodes, not only the manufacturing difficulty of the piston structure is reduced, but also the piston size limit is increased, the adjustment range of the piston structure for the transverse mode is expanded, and the transverse mode suppression effect of the resonator is improved while ensuring the minimum size of the resonator.

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

[0040] Reference Figure 4, the surface acoustic wave filter includes reflection gratings, bus bars, interdigital transducer (IDT), and piston structures. Among them, IDT 404-1 and 404-2 are connected to the input port 401 through the first bus bar; IDT 403 is connected to the output port through the second bus bar. Reflection gratings 405 and 406 are formed on both sides of IDT 403, IDT 404-1, and 404-2. According to an embodiment of the present invention, IDT 403, IDT 404-1, and 404-2 are configured with piston structures according to the embodiments of the present invention, that is, at the ends of the electrode fingers of the interdigital electrode and at the corresponding parts of the adjacent fingers to the finger, piston structures according to the embodiments of the present invention are formed. According to an embodiment of the present invention, reflection gratings 405 and 406 are configured as interdigital structures, and piston structures according to the embodiments of the present invention are also configured at the ends of the electrode fingers of the interdigital fingers.

[0041] Those skilled in the art should understand that although a surface acoustic wave filter with three interdigital transducers IDT is shown in Figure 4 , it is only for illustration, and appropriate modifications can be made to it without departing from the scope of the present invention. For example, the surface acoustic wave filter can be a Ladder structure filter, which can be configured to include only one series surface acoustic wave resonator and one parallel surface acoustic wave resonator, and the series resonator is connected to the input port and the output port through the first bus bar and the second bus bar, the third bus bar connects one end of the parallel resonator to the second bus bar, and the fourth bus bar connects the other end of the parallel resonator to the ground port.

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

[0043] The above embodiments of the present disclosure are only for easy description and help to comprehensively understand the present disclosure, rather than aiming to limit the scope of the present disclosure. Therefore, it should be understood that in addition to the above embodiments disclosed herein, all modifications and changes or forms of modifications and changes derived from the technical concept of the present disclosure fall within the scope of the present disclosure.

Claims

1. A piston structure for lateral mode suppression of a surface acoustic wave filter, characterized in that: The piston structure is arranged at the electrode finger end of the interdigital electrode of the interdigital electrode transducer IDT of the surface acoustic wave filter and at the corresponding position of the finger adjacent to the electrode finger end, and The piston structure is configured such that its upper surface is bonded to the lower surfaces of the fingers of the interdigital electrode, and its lower surface is bonded to the upper surface of the piezoelectric substrate.

2. The piston structure according to claim 1, characterized in that: The piston structure is formed of the same metal material as the fingers of the interdigitated electrodes.

3. The piston structure according to claim 1, characterized in that: The piston structure is configured as a regular trapezoid, and a side wall thereof forms a specific angle with an upper surface of the piezoelectric substrate.

4. The piston structure according to claim 3, characterized in that: The specific angle is 60°-80°.

5. The 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 interdigital electrode fingers, and the thickness thereof is 20% to 80% of the thickness of the interdigital electrode fingers.

6. The piston structure according to claim 1, characterized in that: The piezoelectric substrate is a piezoelectric substrate on insulator, and the piezoelectric substrate includes a piezoelectric layer, a temperature compensation layer and a base layer. Wherein, the piezoelectric layer is 600 nanometers thick 42° lithium niobate, the temperature compensation layer is 500 nanometers thick silicon dioxide, and the base layer is 1000 nanometers thick high-resistance silicon.

7. The 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 piston structure according to claim 3, characterized in that: The piston structure configured as a positive trapezoid is formed on the piezoelectric substrate by a negative resist lift-off process.

9. A surface acoustic wave filter, comprising: A reflector, a bus bar, an interdigital transducer (IDT), and a piston structure according to any one of claims 1 to 8, wherein: The bus bar includes a first bus bar and a second bus bar, wherein the first electrode of the IDT is connected to the input port of the surface acoustic wave filter through the first bus bar, and the second electrode of the IDT is connected to the output port of the surface acoustic wave filter through the second bus bar, and wherein, The reflection grating includes a first reflection grating and a second reflection grating, and the first reflection grating and the second reflection grating are arranged on both sides of the interdigital electrode transducer.

10. The surface acoustic wave filter according to claim 9, wherein: The reflection grating is configured to have an interdigital structure, and the piston structure is configured at the end of the electrode finger of the interdigital structure of the reflection grating.

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