Method of tuning a resonator, method of forming a cavity of a resonator, and filter

Abstract

The present invention relates to a method of tuning a resonator, a method of forming a cavity of a resonator, and a filter, a method of forming a cavity of a bulk acoustic wave resonator, comprising the steps of: forming a sacrificial epitaxial structure mesa on a compound semiconductor substrate; forming an insulating layer on the sacrificial epitaxial structure mesa and the compound semiconductor substrate; polishing the insulating layer to form a polished surface using a chemical mechanical polishing process; forming a bulk acoustic wave resonator structure on the polished surface, wherein the bulk acoustic wave resonator structure is located on the sacrificial epitaxial structure mesa, wherein the bulk acoustic wave resonator structure comprises the steps of: forming a bottom electrode layer on the polished surface; forming a piezoelectric layer on the bottom electrode layer; and forming a top electrode layer on the piezoelectric layer; and etching the sacrificial epitaxial structure mesa to form a cavity, wherein the cavity is located under the bulk acoustic wave resonator structure.

Description

Technical Field

[0001] This invention relates to a bulk acoustic wave filter and a method for tuning a bulk acoustic wave resonator for the bulk acoustic wave filter, and more particularly to a method for having a bulk acoustic wave resonator capable of precisely tuning a bulk acoustic wave filter, a method for forming a cavity in the bulk acoustic wave resonator, and a bulk acoustic wave filter. Background Technology

[0002] Please see Figures 7A to 7D This is a cross-sectional schematic diagram of the fabrication process steps of a prior art method for forming a bulk acoustic filter. Figure 7A In this process, a groove 74 and a groove 74' are etched on an upper surface of a silicon substrate 75. A sacrificial layer 77 is formed on the silicon substrate 75, and then the sacrificial layer 77 is polished using a chemical mechanical planarization (CMP) process, so that the sacrificial layer 77 on the upper surface of the silicon substrate 75 is completely removed, forming a... Figure 7B The structure includes grooves 74 and 74', which are filled with sacrificial layer 77. Figure 7C In this process, a first bulk acoustic wave resonant structure 70 and a second bulk acoustic wave resonant structure 70' are formed on the upper surface of a silicon substrate 75. The first bulk acoustic wave resonant structure 70 and the second bulk acoustic wave resonant structure 70' each have a bottom electrode 71 and a piezoelectric layer 72 of the same thickness, and each of the first bulk acoustic wave resonant structure 70 and the second bulk acoustic wave resonant structure 70' has a top electrode 73 and a top electrode 73' of different thicknesses. The top electrode 73 and the top electrode 73' have a thickness difference 76. Figure 7D In the process, the sacrificial layer 77 is etched to fill the grooves 74 and 74', making the grooves 74 and 74' the two cavities of the first bulk acoustic resonant structure 70 and the second bulk acoustic resonant structure 70', respectively. Because the top electrode 73' is relatively thick, the resonant frequency of the second bulk acoustic resonant structure 70' is lower than that of the first bulk acoustic resonant structure 70, and there is a resonant frequency difference between the first bulk acoustic resonant structure 70 and the second bulk acoustic resonant structure 70'. This resonant frequency difference is related to the thickness difference 76.

[0003] However, using the thickness difference 76 between the top electrode 73 and the top electrode 73' to tune the resonant frequency difference between the first bulk acoustic resonant structure 70 and the second bulk acoustic resonant structure 70', and relying on the structural differences between the first bulk acoustic resonant structure 70 and the second bulk acoustic resonant structure 70' themselves to achieve the tuning of the resonant frequency difference, not only increases the complexity of manufacturing the first bulk acoustic resonant structure 70 and the second bulk acoustic resonant structure 70', but may also affect the characteristic performance of the first bulk acoustic resonant structure 70 and the second bulk acoustic resonant structure 70'.

[0004] In view of this, the inventors developed a simple design that avoids the above-mentioned disadvantages and has the advantage of low cost, taking into account considerations such as flexibility of use and economy, thus the present invention came into being. Summary of the Invention

[0005] To address the problems of existing technologies and achieve the desired effects, this invention provides a method for forming a cavity in a bulk acoustic wave resonator, comprising the following steps: Step A1: forming a sacrificial epitaxial mesa on a compound semiconductor substrate; Step A2: forming an insulating layer on the sacrificial epitaxial mesa and the compound semiconductor substrate; Step A3: polishing the insulating layer using a chemical mechanical planarization process to form a polished surface; Step A4: forming an integral acoustic wave resonator structure on the polished surface, wherein the bulk acoustic wave resonator structure is located on the sacrificial epitaxial mesa, wherein Step A4 includes the following steps: Step A41: forming a bottom electrode layer on the polished surface; Step A42: forming a piezoelectric layer on the bottom electrode layer; and Step A43: forming a top electrode layer on the piezoelectric layer; and Step A5: etching the sacrificial epitaxial mesa to form a cavity, wherein the cavity is located below the bulk acoustic wave resonator structure.

[0006] In the embodiment, in step A3, the insulating layer is ground so that the sacrificial epitaxial structure mesa is not exposed. The insulating layer between the bottom electrode layer and the sacrificial epitaxial structure mesa forms a frequency tuning structure. The frequency tuning structure has a thickness, and the bulk acoustic wave resonant structure has a resonant frequency. Thus, by adjusting the thickness of the frequency tuning structure, the resonant frequency of the bulk acoustic wave resonant structure can be tuned.

[0007] In an embodiment, the method further includes a step of forming a bottom etch stop layer on a compound semiconductor substrate, wherein a sacrificial epitaxial mesa is formed on the bottom etch stop layer; wherein the sacrificial epitaxial mesa includes a sacrificial epitaxial layer.

[0008] In the embodiments, (1) the compound semiconductor substrate is made of gallium arsenide; the sacrificial epitaxial layer is made of gallium arsenide; the bottom etch stop layer is made of indium gallium phosphide; or (2) the compound semiconductor substrate is made of indium phosphide; the sacrificial epitaxial layer is made of indium gallium arsenide; and the bottom etch stop layer is made of indium phosphide.

[0009] In the embodiment, the sacrificial epitaxial layer has a thickness between 50 nm and 5000 nm; and the bottom etch stop layer has a thickness between 20 nm and 500 nm.

[0010] Furthermore, the present invention also provides a method for tuning a bulk acoustic resonator for a bulk acoustic filter, comprising the following steps: Step B1: forming a plurality of sacrificial mesa on a substrate, wherein the plurality of sacrificial mesa includes at least one first sacrificial mesa and at least one second sacrificial mesa, wherein the height of one of the at least one first sacrificial mesa is greater than the height of one of the at least one second sacrificial mesa, and wherein there is a first height difference between the at least one first sacrificial mesa and the at least one second sacrificial mesa; Step B2: forming an insulating layer on the plurality of sacrificial mesa and the substrate; Step B3: polishing the insulating layer using a chemical mechanical planarization process to form a polished layer. Surface; Step B4: Forming a plurality of individual acoustic resonant structures on the polished surface, wherein the plurality of individual acoustic resonant structures includes at least one first volume acoustic resonant structure and at least one second volume acoustic resonant structure, the at least one first volume acoustic resonant structure and the at least one second volume acoustic resonant structure are respectively located on at least one first sacrificial structure mesa and at least one second sacrificial structure mesa, wherein step B4 includes the following steps: Step B41: Forming a bottom electrode layer on the polished surface; Step B42: Forming a piezoelectric layer on the bottom electrode layer; and Step B43: Forming a top electrode layer on the piezoelectric layer; and Step B5: Etching the plurality of sacrificial structure mesa to form A plurality of cavities, wherein the plurality of cavities are respectively located under a plurality of bulk acoustic resonant structures; wherein in step B3, the insulating layer is polished such that (1) at least one first sacrificial structure mesa is exposed and at least one second sacrificial structure mesa is not exposed, thereby the insulating layer located below the polished surface and below at least one second bulk acoustic resonant structure forms a second frequency tuning structure of at least one second bulk acoustic resonant structure, wherein the second frequency tuning structure has a thickness equal to a first height difference; or (2) at least one first sacrificial structure mesa and at least one second sacrificial structure mesa are not exposed, thereby located below the polished surface and below at least one first bulk acoustic resonant structure. The insulating layers beneath the resonant structure and beneath at least one second bulk acoustic wave resonant structure respectively form a first frequency tuning structure for at least one first bulk acoustic wave resonant structure and a second frequency tuning structure for at least one second bulk acoustic wave resonant structure. The first frequency tuning structure and the second frequency tuning structure have a first thickness difference equal to a first height difference. The at least one first bulk acoustic wave resonant structure and the at least one second bulk acoustic wave resonant structure have a first resonant frequency difference, which is related to the first height difference. Thus, by adjusting the first height difference, the first resonant frequency difference of the at least one first bulk acoustic wave resonant structure and the at least one second bulk acoustic wave resonant structure can be tuned.

[0011] In the embodiment, the substrate is a semiconductor substrate; wherein the material constituting the plurality of sacrificial mesa includes at least one selected from the group consisting of metals, alloys and epitaxial structures.

[0012] In the embodiment, the substrate is a compound semiconductor substrate, and step B1 includes the following steps: step B11: forming a sacrificial structure on the substrate, wherein the sacrificial structure includes a sacrificial epitaxial layer; step B12: etching the sacrificial structure to form a plurality of sacrificial structure mesa, such that the plurality of sacrificial structure mesa have the same height; and step B13: etching at least one first sacrificial structure mesa and at least one second sacrificial structure mesa or etching at least one second sacrificial structure mesa, such that at least one first sacrificial structure mesa and at least one second sacrificial structure mesa have a first height difference.

[0013] In the embodiment, the sacrificial structure further includes a first etch stop layer and a first fine tuning layer, wherein the sacrificial epitaxial layer is formed on the substrate, the first etch stop layer is formed on the sacrificial epitaxial layer, and the first fine tuning layer is formed on the first etch stop layer, wherein the first fine tuning layer has a thickness; wherein in step B13, the first fine tuning layer of at least one second sacrificial structure mesa is etched, such that at least one first sacrificial structure mesa and at least one second sacrificial structure mesa have a first height difference, thereby the first height difference is determined by the thickness of the first fine tuning layer.

[0014] In the embodiments, (1) the substrate is made of gallium arsenide; the sacrificial epitaxial layer is made of gallium arsenide; the first etch stop layer is made of aluminum arsenide or indium gallium phosphide; the first fine tuning layer is made of gallium arsenide; or (2) the substrate is made of indium phosphide; the sacrificial epitaxial layer is made of indium gallium arsenide; the first etch stop layer is made of indium phosphide; the first fine tuning layer is made of indium gallium arsenide.

[0015] In one embodiment, the thickness of the first fine tuning layer is between 1 nm and 300 nm; and the thickness of the first etch stop layer is between 1 nm and 50 nm.

[0016] In an embodiment, the method further includes a step of forming a bottom etch stop layer on the substrate, wherein a sacrificial structure is formed on the bottom etch stop layer; wherein the sacrificial epitaxial layer has a thickness between 50 nm and 5000 nm; wherein the bottom etch stop layer has a thickness between 20 nm and 500 nm; wherein (1) the substrate is made of gallium arsenide; the sacrificial epitaxial layer is made of gallium arsenide; the bottom etch stop layer is made of indium gallium phosphide; or (2) the substrate is made of indium phosphide; the sacrificial epitaxial layer is made of indium gallium arsenide; the bottom etch stop layer is made of indium phosphide.

[0017] Furthermore, the present invention also provides a method for tuning a bulk acoustic resonator for a bulk acoustic filter, comprising the following steps: Step C1: forming a plurality of sacrificial mesa on a substrate, wherein the plurality of sacrificial mesa have the same height, and wherein the plurality of sacrificial mesa includes at least one first sacrificial mesa and at least one second sacrificial mesa; Step C2: forming an insulating layer on the plurality of sacrificial mesa and the substrate; Step C3: polishing the insulating layer using a pre-chemical mechanical planarization process to form a pre-polished surface, thereby exposing the plurality of sacrificial mesa; Step C4: etching at least one first sacrificial mesa and at least one second sacrificial mesa, or etching at least one second sacrificial mesa, thereby exposing at least one first sacrificial mesa. The sacrificial structure platform and at least one second sacrificial structure platform have a first height difference, wherein the height of the at least one first sacrificial structure platform is greater than the height of the at least one second sacrificial structure platform; Step C5: Forming a plurality of individual acoustic resonant structures, wherein the plurality of individual acoustic resonant structures include at least one first-body acoustic resonant structure and at least one second-body acoustic resonant structure, wherein the at least one first-body acoustic resonant structure and at least one second-body acoustic resonant structure are respectively located on the at least one first sacrificial structure platform and on the at least one second sacrificial structure platform, wherein (a) Step C5 includes the following steps: Step C51: Forming a second polishing layer on the plurality of sacrificial structure platforms and the insulating layer, wherein the material constituting the second polishing layer is an insulator; Step C 52: A second polishing layer is polished using a chemical mechanical planarization process to form a polished surface, such that (1) at least one first sacrificial mesa is exposed and at least one second sacrificial mesa is not exposed, thereby forming a second frequency tuning structure of at least one second bulk acoustic resonant structure below the polished surface and below at least one second bulk acoustic resonant structure, wherein the second frequency tuning structure has a thickness equal to the first height difference; or (2) at least one first sacrificial mesa and at least one second sacrificial mesa are not exposed, thereby forming at least one second frequency tuning structure of at least one second frequency tuning structure below the polished surface and below at least one first bulk acoustic resonant structure and at least one second bulk acoustic resonant structure respectively. A first frequency tuning structure of a first bulk acoustic resonant structure and a second frequency tuning structure of at least one second bulk acoustic resonant structure, wherein the first frequency tuning structure and the second frequency tuning structure have a first thickness difference equal to a first height difference; step C53: forming a bottom electrode layer on the polished surface; step C54: forming a piezoelectric layer on the bottom electrode layer; and step C55: forming a top electrode layer on the piezoelectric layer; or (b) an extended plane coincides with the pre-polished surface, wherein step C5 includes the following steps: step C51': forming a second polishing layer on a plurality of sacrificial structure mesa and an insulating layer, wherein the material constituting the second polishing layer includes at least one selected from the group consisting of metals and alloys;Step C52': A second polishing layer is polished using a chemical mechanical planarization process to form a polished surface, such that a plurality of sacrificial structure mesa are not exposed; Step C53': The second polishing layer is patterned, wherein (1) in step C4, at least one second sacrificial structure mesa is etched; wherein the second polishing layer located above the extended plane, below the polished surface, and below at least one first bulk acoustic resonance structure forms a bottom electrode layer of at least one first bulk acoustic resonance structure; wherein the second polishing layer located above the extended plane, below the polished surface, and below at least one second bulk acoustic resonance structure forms a bottom electrode layer of at least one second bulk acoustic resonance structure; wherein the second polishing layer located below the extended plane and below the polished surface ... A second polishing layer beneath at least one second bulk acoustic wave resonant structure forms a second frequency tuning structure of at least one second bulk acoustic wave resonant structure, wherein the second frequency tuning structure has a thickness equal to a first height difference; or (2) in step C4, at least one first sacrificial structure mesa and at least one second sacrificial structure mesa are etched; wherein the second polishing layer located above the extended plane, below the polished surface, and beneath at least one first bulk acoustic wave resonant structure forms a bottom electrode layer of at least one first bulk acoustic wave resonant structure; wherein the second polishing layer located below the extended plane and beneath at least one first bulk acoustic wave resonant structure forms at least one first bulk acoustic wave resonant structure A first frequency tuning structure; wherein a second polishing layer located above the extended plane, below the polished surface, and below at least one second bulk acoustic wave resonant structure forms a bottom electrode layer of at least one second bulk acoustic wave resonant structure; wherein the second polishing layer located below the extended plane and below at least one second bulk acoustic wave resonant structure forms a second frequency tuning structure of at least one second bulk acoustic wave resonant structure; wherein the first frequency tuning structure and the second frequency tuning structure have a first thickness difference, the first thickness difference being equal to a first height difference; step C54': forming a piezoelectric layer above the polished surface; and step C55': forming a top electrode layer above the piezoelectric layer; or (c) step C5 includes the following steps: step Step C51”: A second polishing layer is formed on a plurality of sacrificial structure mezzanines and an insulating layer, wherein the material constituting the second polishing layer includes at least one of the following groups: metals, alloys and insulators; Step C52”: The second polishing layer is polished by a chemical mechanical planarization process to form a polished surface such that (1) at least one first sacrificial structure mezzanine is exposed and at least one second sacrificial structure mezzanine is not exposed, thereby forming a second frequency tuning structure of at least one second bulk acoustic resonant structure under the polished surface and under at least one second bulk acoustic resonant structure, wherein the second frequency tuning structure has a thickness equal to the first height difference;Or (2) at least one first sacrificial structure mesa and at least one second sacrificial structure mesa are not exposed, thereby forming a first frequency tuning structure of at least one first bulk acoustic resonant structure and a second frequency tuning structure of at least one second bulk acoustic resonant structure, respectively, under the polished surface and under at least one first bulk acoustic resonant structure and at least one second bulk acoustic resonant structure, wherein the first frequency tuning structure and the second frequency tuning structure have a first thickness difference, the first thickness difference being equal to the first height difference; Step C53”: Pattern the second polishing layer; Step C54”: Form a bottom electrode layer On the polished surface; step C55”: forming a piezoelectric layer on the bottom electrode layer; and step C56”: forming a top electrode layer on the piezoelectric layer; and step C6: etching a plurality of sacrificial structure mesa to form a plurality of cavities, wherein the plurality of cavities are respectively located below a plurality of bulk acoustic resonant structures; wherein at least one first bulk acoustic resonant structure and at least one second bulk acoustic resonant structure have a first resonant frequency difference, the first resonant frequency difference being correlated with a first height difference; thereby, by adjusting the first height difference, the first resonant frequency difference of at least one first bulk acoustic resonant structure and at least one second bulk acoustic resonant structure can be tuned.

[0018] In the embodiment, the substrate is a semiconductor substrate; wherein the material constituting the plurality of sacrificial mesa includes at least one selected from the group consisting of metals, alloys and epitaxial structures.

[0019] In the embodiment, the substrate is a compound semiconductor substrate; wherein step C1 includes the following steps: step C11: forming a sacrificial structure on the substrate, wherein the sacrificial structure includes a sacrificial epitaxial layer; and step C12: etching the sacrificial structure to form a plurality of sacrificial structure mesa, wherein the plurality of sacrificial structure mesa have the same height.

[0020] In the embodiments, (1) the sacrificial structure further includes a first etch stop layer and a first fine tuning layer, wherein the sacrificial epitaxial layer is formed on the substrate, the first etch stop layer is formed on the sacrificial epitaxial layer, and the first fine tuning layer is formed on the first etch stop layer, wherein the first fine tuning layer has a thickness; wherein in step C4, the first fine tuning layer of at least one second sacrificial structure mesa is etched, such that at least one first sacrificial structure mesa and at least one second sacrificial structure mesa have a first height difference, thereby the first height difference is determined by the thickness of the first fine tuning layer; or (2) the sacrificial structure further includes a first etch stop layer, a first fine tuning layer and a top etch stop layer. A stop layer, wherein a sacrificial epitaxial layer is formed on a substrate, a first etch stop layer is formed on the sacrificial epitaxial layer, a first fine tuning layer is formed on the first etch stop layer, and a top etch stop layer is formed on the first fine tuning layer, wherein the first fine tuning layer has a thickness; wherein step C4 includes the following steps: step C41: etching the top etch stop layer of at least one first sacrificial mesa and at least one second sacrificial mesa; and step C42: etching the first fine tuning layer of at least one second sacrificial mesa such that at least one first sacrificial mesa and at least one second sacrificial mesa have a first height difference, wherein the first height difference is determined by the thickness of the first fine tuning layer.

[0021] In the embodiments, (1) the substrate is made of gallium arsenide; the sacrificial epitaxial layer is made of gallium arsenide; the first etch stop layer is made of aluminum arsenide or indium gallium phosphide; the first fine tuning layer is made of gallium arsenide; and the top etch stop layer is made of indium gallium phosphide; or (2) the substrate is made of indium phosphide; the sacrificial epitaxial layer is made of indium gallium arsenide; the first etch stop layer is made of indium phosphide; the first fine tuning layer is made of indium gallium arsenide; and the top etch stop layer is made of indium phosphide.

[0022] In the embodiment, the thickness of the first fine tuning layer is between 1 nm and 300 nm; the first etch stop layer has a thickness between 1 nm and 50 nm; and the top etch stop layer has a thickness between 50 nm and 300 nm.

[0023] In an embodiment, the method further includes a step of forming a bottom etch stop layer on the substrate, wherein a sacrificial structure is formed on the bottom etch stop layer; wherein the sacrificial epitaxial layer has a thickness between 50 nm and 5000 nm; wherein the bottom etch stop layer has a thickness between 20 nm and 500 nm; wherein (1) the substrate is made of gallium arsenide; the sacrificial epitaxial layer is made of gallium arsenide; the bottom etch stop layer is made of indium gallium phosphide; or (2) the substrate is made of indium phosphide; the sacrificial epitaxial layer is made of indium gallium arsenide; the bottom etch stop layer is made of indium phosphide.

[0024] In an embodiment, in step C51', the material constituting the second polishing layer includes at least one selected from the group consisting of ruthenium, titanium, molybdenum, platinum, gold, aluminum, and tungsten.

[0025] Furthermore, the present invention also provides a bulk acoustic wave filter, comprising: an insulating layer, a plurality of bulk acoustic wave resonant structures, and one of the following structures A, B, and C; wherein the insulating layer is formed on a substrate, and wherein the insulating layer has a plurality of cavities; wherein the plurality of bulk acoustic wave resonant structures are respectively located on the plurality of cavities, wherein the plurality of bulk acoustic wave resonant structures include a first bulk acoustic wave resonant structure and a second bulk acoustic wave resonant structure, and the plurality of cavities include a first cavity and a second cavity, the first bulk acoustic wave resonant structure and the second bulk acoustic wave resonant structure respectively corresponding to the first cavity and the second cavity, wherein the first bulk acoustic wave resonant structure and the second bulk acoustic wave resonant structure have a first resonant frequency difference, wherein each A plurality of bulk acoustic resonant structures include: a bottom electrode layer, a piezoelectric layer, and a top electrode layer; wherein the bottom electrode layer is formed on an extending plane; the piezoelectric layer is formed on the bottom electrode layer; and the top electrode layer is formed on the piezoelectric layer; wherein structure A: the insulating layer has a polished upper surface, and the extending plane coincides with the upper surface of the insulating layer; wherein the second bulk acoustic resonant structure has a second frequency tuning structure, the second frequency tuning structure being formed below the extending plane and between the bottom electrode layer and the second cavity of the second bulk acoustic resonant structure, wherein the second frequency tuning structure has a thickness, the thickness being related to the first resonant frequency difference between the first bulk acoustic resonant structure and the second bulk acoustic resonant structure; structure B: the insulating layer has a polished upper surface, and the second frequency tuning structure is formed below the extending plane and between the bottom electrode layer and the second cavity of the second bulk acoustic resonant structure; The upper surface of the first bulk acoustic wave resonant structure and the second bulk acoustic wave resonant structure coincide with the upper surface of the insulating layer. The first and second bulk acoustic wave resonant structures each have a first frequency tuning structure and a second frequency tuning structure. The first frequency tuning structure is formed below the extended plane between the bottom electrode layer of the first bulk acoustic wave resonant structure and the first cavity. The second frequency tuning structure is formed below the extended plane between the bottom electrode layer of the second bulk acoustic wave resonant structure and the second cavity. The first and second frequency tuning structures have a first thickness difference, which is related to the first resonant frequency difference between the first and second bulk acoustic wave resonant structures. Structure C: A second polishing layer is formed on the insulating layer. The second grinding layer has a polished upper surface, and the extended plane coincides with the upper surface of the second grinding layer. The second grinding layer, located below the extended plane and between the bottom electrode layer of the first bulk acoustic resonant structure and the first cavity, forms a first frequency tuning structure of the first bulk acoustic resonant structure. The second grinding layer, located below the extended plane and between the bottom electrode layer of the second bulk acoustic resonant structure and the second cavity, forms a second frequency tuning structure of the second bulk acoustic resonant structure. The first frequency tuning structure and the second frequency tuning structure have a first thickness difference, which is related to the first resonant frequency difference between the first bulk acoustic resonant structure and the second bulk acoustic resonant structure.

[0026] In the embodiment, the substrate is a semiconductor substrate.

[0027] In the embodiments, the material constituting the first frequency tuning structure includes at least one selected from the group consisting of metals, alloys, and insulators; and the material constituting the second frequency tuning structure includes at least one selected from the group consisting of metals, alloys, and insulators.

[0028] In the embodiments, the bottom electrode layers of the first frequency tuning structure and the first bulk acoustic resonant structure are made of the same material; and the bottom electrode layers of the second frequency tuning structure and the second bulk acoustic resonant structure are made of the same material.

[0029] To further understand the present invention, preferred embodiments are described below in conjunction with accompanying drawings and reference numerals, and the specific composition and effects achieved by the present invention are explained in detail below. Attached Figure Description

[0030] Figures 1A to 1F This is a cross-sectional schematic diagram of the process steps of a specific embodiment of a method for forming a cavity in a bulk acoustic resonator according to the present invention.

[0031] Figure 1G , Figure 1H This is a cross-sectional schematic diagram of the process steps of another specific embodiment of the method for forming a cavity of a bulk acoustic resonator according to the present invention.

[0032] Figure 1I This is a cross-sectional schematic diagram of an epitaxial structure of a specific embodiment of a method for forming a cavity in a bulk acoustic resonator according to the present invention.

[0033] Figure 1J , Figure 1K This is a cross-sectional schematic diagram of the process steps of another specific embodiment of the method for forming a cavity of a bulk acoustic resonator according to the present invention.

[0034] Figure 1L This is a cross-sectional schematic diagram of another specific embodiment of the method for forming a cavity in a bulk acoustic resonator according to the present invention.

[0035] Figures 2A to 2F This is a cross-sectional schematic diagram of the manufacturing process steps of a specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0036] Figure 2G , Figure 2H This is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0037] Figure 2I , Figure 2JThis is a cross-sectional schematic diagram of two specific embodiments of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0038] Figures 2K to 2N This is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0039] Figures 3A to 3G This is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0040] Figure 3H , Figure 3I This is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0041] Figure 3J , Figure 3K This is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0042] Figure 3L This is a cross-sectional schematic diagram of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0043] Figures 4A to 4D This is a cross-sectional schematic diagram of the manufacturing process steps of a specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0044] Figure 4E , Figure 4F This is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0045] Figure 4G , Figure 4H This is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0046] Figure 4I This is a cross-sectional schematic diagram of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0047] Figures 4J to 4M This is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0048] Figures 5A-5CThis is a cross-sectional schematic diagram of the manufacturing process steps of a specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0049] Figure 5D This is a cross-sectional schematic diagram of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0050] Figures 5E to 5G This is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0051] Figures 5H to 5K This is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0052] Figure 5L , Figure 5M This is a cross-sectional schematic diagram of the manufacturing process steps of a specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0053] Figure 5N , Figure 5O This is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0054] Figure 5P This is a cross-sectional schematic diagram of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0055] Figures 6A to 6C This is a cross-sectional schematic diagram of the manufacturing process steps of a specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0056] Figures 6D to 6F This is a cross-sectional schematic diagram of three specific embodiments of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0057] Figure 6G This is a partially enlarged cross-sectional schematic diagram of a specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0058] Figure 6H This is a partially enlarged cross-sectional schematic diagram of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention.

[0059] Figures 7A to 7D This is a cross-sectional schematic diagram of the process steps of a prior art method for forming a bulk acoustic filter.

[0060] Explanation of icon numbers:

[0061] 1. Bulk acoustic resonator / First-stage bulk acoustic resonator;

[0062] 1' Second-body acoustic resonator;

[0063] 1” Third-body acoustic resonator;

[0064] 10 substrate;

[0065] 11. Insulation layer;

[0066] 12. Etching protective layer;

[0067] 13. Compound semiconductor substrates;

[0068] 20 Bottom etching stop layer;

[0069] 21. Sacrifice structure;

[0070] 22 First etch stop layer;

[0071] 23 First fine-tuning layer;

[0072] 24 Second etch stop layer;

[0073] 25. Second fine-tuning layer;

[0074] 26. Top etch stop layer;

[0075] 27. Sacrificial epitaxial layer;

[0076] 28. Sacrificial epitaxial structure;

[0077] 3-body acoustic resonant structure / First-body acoustic resonant structure;

[0078] 3' Second-body acoustic resonance structure;

[0079] 3” Third-body acoustic resonance structure;

[0080] 30 Bottom electrode layer;

[0081] 31. Piezoelectric layer;

[0082] 32. Top electrode layer;

[0083] 40. Cavity / First Cavity;

[0084] 40' Second cavity;

[0085] 40” Third cavity;

[0086] 41 Polished surface;

[0087] 42. Pre-polished surface;

[0088] 43. Extended plane;

[0089] 50 Frequency tuning structure / First frequency tuning structure;

[0090] 50' Second frequency tuning structure;

[0091] 50” Third frequency tuning structure;

[0092] 51. Second grinding layer;

[0093] 6. First sacrificial structural platform;

[0094] 6' Second sacrificial structure platform;

[0095] 6” Third sacrificial structure platform;

[0096] 60 Sacrificial epitaxial mesa;

[0097] 7. First-body acoustic resonator;

[0098] 7' Second-body acoustic resonator;

[0099] 70 First-body acoustic resonant structure;

[0100] 70' Second-body acoustic resonance structure;

[0101] 71 Bottom electrode;

[0102] 72 piezoelectric layers;

[0103] 73 Top electrode;

[0104] 73' Top electrode;

[0105] 74. Groove;

[0106] 74' Groove;

[0107] 75 Silicon substrate;

[0108] 76. Thickness difference;

[0109] 77. Sacrificial layer;

[0110] The thickness of the first etch stop layer in ET1;

[0111] The thickness of the first fine tuning layer in FT1;

[0112] The thickness of the second fine tuning layer in FT2;

[0113] HD1 First Height Difference;

[0114] HD2 second height difference;

[0115] T represents thickness;

[0116] The thickness of the second frequency tuning structure of T2;

[0117] The thickness of the third frequency tuning structure of T3;

[0118] TD1 First thickness difference;

[0119] TD2 Second thickness difference. Detailed Implementation

[0120] Please see Figures 1A to 1F This is a cross-sectional schematic diagram of the process steps of a specific embodiment of a method for forming a cavity in a bulk acoustic resonator according to the present invention. The method for forming a cavity in a bulk acoustic resonator according to the present invention includes the following steps: Step A1: (as shown in the diagram) Figure 1B As shown) a sacrificial epitaxial mesa 60 (28) is formed on a compound semiconductor substrate 13, including: (as shown) Figure 1A As shown, a sacrificial epitaxial structure 28 is formed on the compound semiconductor substrate 13 and (as shown) Figure 1B (As shown) Etching the sacrificial epitaxial structure 28 to form the sacrificial epitaxial structure mesa 60 (28); Step A2: (as shown) Figure 1C An insulating layer 11 is formed on the sacrificial epitaxial mesa 60 and the compound semiconductor substrate 13, wherein the material constituting the insulating layer 11 includes at least one selected from the group consisting of silicon nitride (SiN). x ), silicon dioxide (SiO2) and polymer; Step A3: (as shown) Figure 1D (As shown) The insulating layer 11 is polished using a chemical mechanical planarization process to form a polished surface 41; Step A4: (as shown) Figure 1E (As shown) An integral acoustic resonant structure 3 is formed on the polished surface 41, wherein the bulk acoustic resonant structure 3 is located above the sacrificial epitaxial mesa 60, wherein step A4 includes the following steps: step A41: forming a bottom electrode layer 30 on the polished surface 41; step A42: forming a piezoelectric layer 31 on the bottom electrode layer 30; and step A43: forming a top electrode layer 32 on the piezoelectric layer 31; and step A5: (as shown) Figure 1F(As shown) The sacrificial epitaxial mesa 60 is etched to form a cavity 40, which is located below the bulk acoustic wave resonator 3. In step A3, the insulating layer 11 is polished so that the sacrificial epitaxial mesa 60 is not exposed. The insulating layer 11 between the bottom electrode layer 30 and the sacrificial epitaxial mesa 60 forms a frequency tuning structure 50, which has a thickness T. The bulk acoustic wave resonator 3 has a resonant frequency F. By adjusting the thickness T of the frequency tuning structure 50, the resonant frequency F of the bulk acoustic wave resonator 3 can be tuned. The larger the thickness T of the frequency tuning structure 50, the smaller the resonant frequency F of the bulk acoustic wave resonator 3. Conversely, the smaller the thickness T of the frequency tuning structure 50, the larger the resonant frequency F of the bulk acoustic wave resonator 3. The method of forming a cavity of a bulk acoustic wave resonator according to the present invention is characterized by using a compound semiconductor substrate 13, using a sacrificial epitaxial structure 28 as a sacrificial layer, and polishing the insulating layer 11 using a chemical mechanical planarization process. Its advantage lies in its ability to precisely control the thickness T of the frequency tuning structure 50, which in turn helps to precisely tune the resonant frequency F of the bulk acoustic wave resonant structure 3. If the thickness T of the frequency tuning structure 50 is too thick, it will affect the resonant membrane state of the bulk acoustic wave resonant structure 3; therefore, the thickness T of the frequency tuning structure 50 needs to be less than 1000 nm. In some preferred embodiments, the thickness T of the frequency tuning structure 50 is equal to or less than 300 nm.

[0121] Please see Figure 1G , Figure 1H This is a cross-sectional schematic diagram of the process steps of another specific embodiment of the method for forming a cavity in a bulk acoustic resonator according to the present invention. Figure 1H The main process steps and formation of the embodiments shown Figure 1F The process steps of the illustrated embodiment are largely the same, except that in step A3, the insulating layer 11 is polished to expose the sacrificial epitaxial mesa 60 (e.g., Figure 1G (as shown); then a bulk acoustic resonance structure 3 is formed on the polished surface 41, and a sacrificial epitaxial mesa 60 is etched to form a cavity 40 (as shown). Figure 1H (As shown). Among them, the bulk acoustic resonance structure 3 does not possess the characteristics shown. Figure 1F The frequency tuning structure 50 shown is shown.

[0122] Please see Figure 1I This is a cross-sectional schematic diagram of an epitaxial structure, which is one of the specific embodiments of the method for forming a cavity in a bulk acoustic resonator according to the present invention. Figure 1I The main structure of the epitaxial structure of the embodiment is similar to Figure 1AThe epitaxial structures of the illustrated embodiments are generally the same, except that an etch protection layer 12 is formed on the lower surface of the compound semiconductor substrate 13. The function of the etch protection layer 12 is to protect the lower surface of the compound semiconductor substrate 13 from damage caused by etching during the process (especially wet etching etchants). The materials constituting the etch protection layer 12 include at least one selected from the group consisting of silicon nitride (SiNx), silicon oxide (SiO2), aluminum nitride (AlN), and photoresist. The preferred material constituting the etch protection layer 12 is silicon nitride (SiNx). Typically, after step A5, the etch protection layer 12 is removed to facilitate the substrate thinning process. In all other embodiments of the present invention, regardless of whether the substrate is a semiconductor substrate or a compound semiconductor substrate, the etch protection layer 12 can be formed to protect the lower surface of the semiconductor substrate or the compound semiconductor substrate.

[0123] Please see Figure 1J , Figure 1K This is a cross-sectional schematic diagram of the process steps of another specific embodiment of the method for forming a cavity of a bulk acoustic resonator according to the present invention. Figure 1J The epitaxial structure of the embodiment and Figure 1A The epitaxial structures of the illustrated embodiments are generally the same, except that they also include a bottom etch-stop layer 20, which is formed on the compound semiconductor substrate 13, and the sacrificial epitaxial structure 28 is formed on the bottom etch-stop layer 20. When the sacrificial epitaxial structure 28 is etched to form the sacrificial epitaxial structure mesa 60, the sacrificial epitaxial structures 28 around the sacrificial epitaxial structure mesa 60 are etched, and the etching terminates at the bottom etch-stop layer 20. The bottom etch-stop layer 20 is located below the sacrificial epitaxial structure mesa 60. Figure 1K The embodiments are by Figure 1J The embodiment of the bulk acoustic resonator is fabricated using an epitaxial structure. Figure 1K The main structure of the embodiments and Figure 1FThe main structure of the illustrated embodiment is largely the same, except that it also includes a bottom etch stop layer 20, which is formed on the compound semiconductor substrate 13. In step A2, an insulating layer 11 is formed on the sacrificial epitaxial mesa 60 and the bottom etch stop layer 20. Therefore, after the sacrificial epitaxial mesa 60 is etched in step A5, the cavity 40 is also located on the bottom etch stop layer 20. In some embodiments, the compound semiconductor substrate 13 is made of gallium arsenide (GaAs); the sacrificial epitaxial structure 28 is made of a sacrificial epitaxial layer, which is made of gallium arsenide (GaAs) and has a thickness between 50 nm and 5000 nm; the bottom etch stop layer 20 is made of indium gallium phosphide (InGaP) and has a thickness between 20 nm and 500 nm. In other embodiments, the compound semiconductor substrate 13 is made of indium phosphide (InP); the sacrificial epitaxial structure 28 is made of a sacrificial epitaxial layer made of indium gallium arsenide (InGaAs), wherein the sacrificial epitaxial layer has a thickness between 50 nm and 5000 nm; and the bottom etch stop layer 20 is made of indium phosphide (InP), wherein the bottom etch stop layer 20 has a thickness between 20 nm and 500 nm.

[0124] Please see Figure 1L This is a cross-sectional schematic diagram of another specific embodiment of the method for forming a cavity of a bulk acoustic resonator according to the present invention. Figure 1L The embodiments are also by Figure 1J The embodiment of the bulk acoustic resonator is fabricated using an epitaxial structure. Figure 1L The main structure of the embodiments and Figure 1K The main structures of the embodiments shown are largely the same, except that in step A3, the insulating layer 11 is polished to expose the sacrificial epitaxial mesa 60; then, a bulk acoustic resonant structure 3 is formed on the polished surface 41, and the sacrificial epitaxial mesa 60 is etched to form a cavity 40 (similar to...). Figure 1G , Figure 1H Therefore, the bulk acoustic resonance structure 3 does not possess the characteristics of... Figure 1K The frequency tuning structure 50 shown is shown.

[0125] In addition, please see Figures 2A to 2F This is a cross-sectional schematic diagram of the manufacturing process steps of a specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. Figure 2FAs shown, the structure of this embodiment includes at least one first bulk acoustic wave resonator 1 and at least one second bulk acoustic wave resonator 1' formed on a substrate 10. In this embodiment, the at least one first bulk acoustic wave resonator 1 may be a series resonator; and the at least one second bulk acoustic wave resonator 1' may be a shunt resonator. The at least one first bulk acoustic wave resonator 1 includes at least one first bulk acoustic wave resonant structure 3, a first frequency tuning structure 50, and at least one first cavity 40; the at least one second bulk acoustic wave resonator 1' includes at least one second bulk acoustic wave resonant structure 3', a second frequency tuning structure 50', and at least one second cavity 40'. A method for tuning a bulk acoustic wave resonator for a bulk acoustic wave filter according to the present invention includes the following steps: Step B1: (as shown) Figure 2B (As shown) A plurality of sacrificial mesa surfaces are formed on the substrate 10, wherein the plurality of sacrificial mesa surfaces include at least one first sacrificial mesa surface 6 and at least one second sacrificial mesa surface 6', wherein a height of the at least one first sacrificial mesa surface 6 is greater than a height of the at least one second sacrificial mesa surface 6', and wherein the at least one first sacrificial mesa surface 6 and the at least one second sacrificial mesa surface 6' have a first height difference HD1; in this embodiment, the substrate 10 may be a semiconductor substrate; the material constituting the plurality of sacrificial mesa surfaces includes at least one selected from the group consisting of: metals, alloys, and epitaxial structures; Step B2: (as shown) Figure 2C (As shown) An insulating layer 11 is formed on a plurality of sacrificial mesa and a substrate 10, wherein the material constituting the insulating layer 11 includes at least one selected from the group consisting of silicon nitride (SiNx), silicon oxide (SiO2), and polymer; Step B3: (as shown) Figure 2D (As shown) The insulating layer 11 is polished using a chemical mechanical planarization process to form a polished surface 41; Step B4: (as shown) Figure 2E As shown), a plurality of individual acoustic wave resonant structures are formed on the polished surface 41 (in all embodiments of the bulk acoustic wave filter of the present invention, the plurality of individual acoustic wave resonant structures are formed on an extended plane 43; and in this embodiment, the extended plane 43 coincides with the polished surface 41), wherein the plurality of individual acoustic wave resonant structures include at least one first bulk acoustic wave resonant structure 3 and at least one second bulk acoustic wave resonant structure 3', the at least one first bulk acoustic wave resonant structure 3 and at least one second bulk acoustic wave resonant structure 3' are respectively located above at least one first sacrificial structure mesa 6 and at least one second sacrificial structure mesa 6', wherein step B4 includes the following steps: step B41: forming a bottom electrode layer 30 on the polished surface 41; step B42: forming a piezoelectric layer 31 on the bottom electrode layer 30; and step B43: forming a top electrode layer 32 on the piezoelectric layer 31; and step B5: (as shown) Figure 2F(As shown) A plurality of sacrificial structural mesa surfaces are etched to form a plurality of cavities, wherein the plurality of cavities are located below a plurality of bulk acoustic resonant structures, and the plurality of cavities include at least one first cavity 40 and at least one second cavity 40', wherein the at least one first cavity 40 and at least one second cavity 40' are located below at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3', respectively. In step B3, the insulating layer 11 is polished so that at least one first sacrificial structural mesa surface 6 and at least one second sacrificial structural mesa surface 6' are not exposed, thereby the insulating layer 11 located below the polished surface 41 and below at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3', respectively, forms a first frequency tuning structure 50 of at least one first bulk acoustic resonant structure 3 and a second frequency tuning structure 50' of at least one second bulk acoustic resonant structure 3'. The first frequency tuning structure 50 and the second frequency tuning structure 50' have a first thickness difference TD1, which is equal to a first height difference HD1. The first frequency tuning structure 50 lowers the first resonant frequency F1 of at least one first bulk acoustic resonant structure 3, and the second frequency tuning structure 50' lowers the second resonant frequency F2 of at least one second bulk acoustic resonant structure 3'. However, since the second frequency tuning structure 50' is thicker than the first frequency tuning structure 50, the second resonant frequency F2 of the at least one second bulk acoustic resonant structure 3' is lowered than the first resonant frequency F1 of the at least one first bulk acoustic resonant structure 3. Therefore, at least one first bulk acoustic wave resonant structure 3 and at least one second bulk acoustic wave resonant structure 3' have a first resonant frequency difference FD1, which is related to the first thickness difference TD1 of the first frequency tuning structure 50 and the second frequency tuning structure 50'. That is, the first resonant frequency difference FD1 is related to the first height difference HD1 of the at least one first sacrificial structure mesa 6 and the at least one second sacrificial structure mesa 6'. Thus, by adjusting the first height difference HD1, the first resonant frequency difference FD1 of the at least one first bulk acoustic wave resonant structure 3 and the at least one second bulk acoustic wave resonant structure 3' can be tuned. Since the size of the substrate 10 is much larger than the size of the bulk acoustic wave resonator, when the insulating layer 11 is polished during the chemical mechanical planarization process, the amount of insulating layer 11 polished near the center of the substrate 10 is often different from the amount of insulating layer 11 polished away from the center of the substrate 10. However, for adjacent bulk acoustic wave resonators, especially multiple bulk acoustic wave resonators within the same bulk acoustic wave filter, the corresponding insulating layer 11 is polished by almost the same amount. The present invention is characterized in that the first thickness difference TD1 between the first frequency tuning structure 50 and the second frequency tuning structure 50' within the same bulk acoustic wave filter does not differ depending on whether the location is close to or far from the center of the substrate 10.In other words, the first resonant frequency difference FD1 between at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3' does not differ regardless of whether their positions are close to or far from the center of the substrate 10. The first resonant frequency difference FD1 between at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3' is only related to the first thickness difference TD1 between the first frequency tuning structure 50 and the second frequency tuning structure 50', that is, it is related to the first height difference HD1 between at least one first sacrificial structural mesa 6 and at least one second sacrificial structural mesa 6', and of course, it is also related to the type of material constituting the first frequency tuning structure 50 and the second frequency tuning structure 50'. By adjusting the first height difference HD1, or by selecting first frequency tuning structures 50 and second frequency tuning structures 50' made of different types of materials, the first resonant frequency difference FD1 between at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3' can be tuned. Furthermore, the first resonant frequency difference FD1 between at least one first bulk acoustic wave resonant structure 3 and at least one second bulk acoustic wave resonant structure 3' of the present invention does not differ depending on whether its location is close to or far from the center of the substrate 10, which is one of the features of the present invention and greatly helps in the subsequent trimming process. Since the first resonant frequency difference FD1 of at least one first bulk acoustic wave resonant structure 3 and at least one second bulk acoustic wave resonant structure 3' in each region on a whole wafer can be precisely controlled and does not change with its location, the time cost required for the trimming process can be significantly reduced. In some embodiments, the aforementioned substrate 10 may be a compound semiconductor substrate; the material constituting the plurality of sacrificial structure mesa is an epitaxial structure; and the aforementioned step B1 includes the following step: Step B11: (e.g.) Figure 2A (As shown) A sacrificial structure 21 is formed on the substrate 10; Step B12: The sacrificial structure 21 is etched to form a plurality of sacrificial structure mesa, wherein the plurality of sacrificial structure mesa includes at least one first sacrificial structure mesa 6 (21) and at least one second sacrificial structure mesa 6' (21), and the plurality of sacrificial structure mesa have the same height; and Step B13: (as shown) Figure 2B (As shown) Etch at least one first sacrificial structure mesa 6 and at least one second sacrificial structure mesa 6' or etch at least one second sacrificial structure mesa 6' such that at least one first sacrificial structure mesa 6 and at least one second sacrificial structure mesa 6' have a first height difference HD1.

[0126] Please see Figure 2G , Figure 2HThis is a cross-sectional schematic diagram of the fabrication steps of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. In this embodiment, the substrate 10 may be a semiconductor substrate; the material constituting the plurality of sacrificial mesa includes at least one selected from the group consisting of metals, alloys, and epitaxial structures. Figure 2H The main process steps and formation of the embodiments shown Figure 2F The process steps of the illustrated embodiment are largely the same, except that in step B3, the insulating layer 11 is polished such that at least one first sacrificial structure mesa 6 is exposed and at least one second sacrificial structure mesa 6' is not exposed (e.g., Figure 2G As shown), an insulating layer 11 located below the polished surface 41 (extended plane 43) and below at least one second bulk acoustic resonant structure 3' forms a second frequency tuning structure 50' of the at least one second bulk acoustic resonant structure 3'. Figure 2H As shown, the second frequency tuning structure 50' has a thickness T2, which is equal to the first height difference HD1. In this embodiment, there is no... Figure 2F The first frequency tuning structure 50 in the illustrated embodiment. Therefore, the first resonant frequency difference FD1 between at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3' is related to the thickness T2 of the second frequency tuning structure 50', that is, related to the first height difference HD1 between at least one first sacrificial structure platform 6 and at least one second sacrificial structure platform 6'. By adjusting the first height difference HD1, the first resonant frequency difference FD1 between at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3' can be tuned.

[0127] Please see Figure 2I This is a cross-sectional schematic diagram of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. Figure 2I The main structure of the embodiments and Figure 2F The illustrated embodiment is largely the same, except that it further includes a bottom etch stop layer 20, wherein the bottom etch stop layer 20 is formed on the substrate 10, the insulating layer 11 is formed on the bottom etch stop layer 20, and at least one first cavity 40 and at least one second cavity 40' are also located on the bottom etch stop layer 20. Figure 2I The main process steps and formation of the embodiments shown Figure 2FThe process steps of the illustrated embodiment are generally the same, except that, before step B11, a step of forming a bottom etch stop layer 20 on the substrate is included. Step B11 involves forming a sacrificial structure 21 on the bottom etch stop layer. In step B2, an insulating layer 11 is formed on a plurality of sacrificial structure mesa and the bottom etch stop layer 20. In this embodiment, the substrate 10 is a compound semiconductor substrate; the material constituting the plurality of sacrificial structure mesa (sacrificial structure 21) is an epitaxial structure. In some embodiments, the substrate 10 is made of gallium arsenide; the sacrificial structure 21 is made of a sacrificial epitaxial layer, which is made of gallium arsenide, wherein the sacrificial epitaxial layer has a thickness between 50 nm and 5000 nm; the bottom etch stop layer 20 is made of indium gallium phosphide, wherein the bottom etch stop layer 20 has a thickness between 20 nm and 500 nm. In other embodiments, the substrate 10 is made of indium phosphide; the sacrificial structure 21 is made of a sacrificial epitaxial layer, which is made of indium gallium arsenide, wherein the sacrificial epitaxial layer has a thickness between 50 nm and 5000 nm; and the bottom etch stop layer 20 is made of indium phosphide, wherein the bottom etch stop layer 20 has a thickness between 20 nm and 500 nm.

[0128] Please see Figure 2J This is a cross-sectional schematic diagram of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. The substrate 10 is a compound semiconductor substrate; the material constituting the plurality of sacrificial mesa is an epitaxial structure. Figure 2J The main structure of the embodiments and Figure 2I The embodiments shown are largely the same, except that in step B3, the insulating layer 11 is polished such that at least one first sacrificial mesa 6 is exposed and at least one second sacrificial mesa 6' is not exposed (similar to...). Figure 2G The insulating layer 11 located below the polished surface 41 (extended plane 43) and below at least one second bulk acoustic resonant structure 3' forms a second frequency tuning structure 50' of at least one second bulk acoustic resonant structure 3'.

[0129] Please see Figures 2K to 2N This is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. Figure 2K In the embodiment shown, substrate 10 is a compound semiconductor substrate; the material constituting sacrificial structure 21 is an epitaxial structure. Figure 2K The epitaxial structure of the embodiment and Figure 2AThe epitaxial structures of the illustrated embodiments are generally the same, except that the sacrificial structure 21 includes a sacrificial epitaxial layer 27, a first etch-stop layer 22, and a first fine-tuning layer 23. The sacrificial epitaxial layer 27 is formed on the substrate 10, the first etch-stop layer 22 is formed on the sacrificial epitaxial layer 27, and the first fine-tuning layer 23 is formed on the first etch-stop layer 22. Figure 2L As shown, the sacrificial structure 21 is etched into a plurality of sacrificial structure mesa, wherein the plurality of sacrificial structure mesa includes at least one first sacrificial structure mesa 6 and at least one second sacrificial structure mesa 6', and such that the plurality of sacrificial structure mesa have the same height (step B12). Figure 2M As shown, the first fine tuning layer 23 has a thickness FT1. The first fine tuning layer 23 is etched at least one second sacrificial mesa 6' such that the at least one first sacrificial mesa 6 and the at least one second sacrificial mesa 6' have a first height difference HD1 (step B13). Figure 2N Following this, schematic diagrams of steps B2, B3, and B4 were provided. Figure 2N At least one first sacrificial mesa 6 and at least one second sacrificial mesa 6' are etched away (step B5) to obtain the following: Figure 2F The embodiment shown. The first height difference HD1 is determined by the thickness FT1 of the first fine tuning layer 23, which helps to precisely adjust the first height difference HD1, that is, helps to precisely adjust the first resonant frequency difference FD1 between at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3'. In some embodiments, the substrate 10 is made of gallium arsenide (GaAs); the sacrificial epitaxial layer 27 is made of gallium arsenide (GaAs); the first etch stop layer 22 is made of aluminum arsenide (AlAs) or indium gallium phosphide (InGaP), wherein the first etch stop layer 22 has a thickness between 1 nm and 50 nm; the first fine tuning layer 23 is made of gallium arsenide (GaAs), wherein the thickness FT1 of the first fine tuning layer 23 is between 1 nm and 300 nm. In other embodiments, the substrate 10 is made of indium phosphide (InP); the sacrificial epitaxial layer 27 is made of indium gallium arsenide (InGaAs); the first etch stop layer 22 is made of indium phosphide (InP), wherein the first etch stop layer 22 has a thickness between 1 nm and 50 nm; and the first fine tuning layer 23 is made of indium gallium arsenide (InGaAs), wherein the thickness FT1 of the first fine tuning layer 23 is between 1 nm and 300 nm.

[0130] Please see Figures 3A to 3GThis is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. Using the method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention, at least one first bulk acoustic resonator 1 and at least one second bulk acoustic resonator 1' are formed (e.g., Figure 3G (As shown), including the following steps: Step C1: Forming a plurality of sacrificial mesa on a substrate 10, wherein the plurality of sacrificial mesa have the same height, wherein the plurality of sacrificial mesa includes at least one first sacrificial mesa 6 and at least one second sacrificial mesa 6'. In this embodiment, the substrate 10 may be a semiconductor substrate; the material constituting the plurality of sacrificial mesa includes at least one selected from the group consisting of: metals, alloys, and epitaxial structures; Step C2: (as shown) Figure 3A (As shown) An insulating layer 11 is formed on a plurality of sacrificial structural mesa and the substrate 10; Step C3: (as shown) Figure 3B (As shown) The insulating layer 11 is ground using a pre-chemical mechanical planarization process to form a pre-polished surface 42, exposing a plurality of sacrificial structure mesa surfaces; Step C4: ( Figure 3C (As shown) Etch at least one second sacrificial mesa 6' such that at least one first sacrificial mesa 6 and at least one second sacrificial mesa 6' have a first height difference HD1, wherein a height of at least one first sacrificial mesa 6 is greater than a height of at least one second sacrificial mesa 6'; Step C5: (as shown) Figures 3D to 3F(As shown) A plurality of individual acoustic resonant structures are formed, wherein the plurality of individual acoustic resonant structures include at least one first volume acoustic resonant structure 3 and at least one second volume acoustic resonant structure 3', wherein at least one first volume acoustic resonant structure 3 and at least one second volume acoustic resonant structure 3' are respectively located above at least one first sacrificial structure mesa 6 and at least one second sacrificial structure mesa 6', wherein step C5 includes the following steps: Step C51: forming a second polishing layer 51 on the plurality of sacrificial structure mesa and insulating layer 11, wherein the material constituting the second polishing layer 51 is an insulator, wherein the insulating material constituting the second polishing layer 51 includes at least one selected from the group consisting of silicon nitride (SiNx), silicon oxide (SiO2), aluminum nitride (AlN), and zinc oxide (ZnO); Step C52: polishing the second polishing layer 51 by a chemical mechanical planarization process to form a polished surface 41, such that at least one first sacrificial structure mesa 6 and at least one The second sacrificial structure platform 6' is not exposed, thereby forming a first frequency tuning structure 50 of at least one first bulk acoustic resonant structure 3 and a second frequency tuning structure 50' of at least one second bulk acoustic resonant structure 3', located below the polished surface 41 and below at least one first bulk acoustic resonant structure 3' and at least one second bulk acoustic resonant structure 3' respectively. The first frequency tuning structure 50 and the second frequency tuning structure 50' have a first thickness difference TD1, and the first thickness difference TD1 is equal to the first height difference HD1; Step C53: Form a bottom electrode layer 30 on the polished surface 41 (as mentioned above, a plurality of individual acoustic resonant structures are formed on an extended plane 43, in this embodiment the extended plane 43 coincides with the polished surface 41); Step C54: Form a piezoelectric layer 31 on the bottom electrode layer 30; and Step C55: Form a top electrode layer 32 on the piezoelectric layer 31; and Step C6: (as Figure 3G(As shown) A plurality of sacrificial structural mesa surfaces are etched to form a plurality of cavities, wherein the plurality of cavities are located below a plurality of bulk acoustic resonant structures, and the plurality of cavities include at least one first cavity 40 and at least one second cavity 40'. The at least one first bulk acoustic resonant structure 3 and the at least one second bulk acoustic resonant structure 3' have a first resonant frequency difference FD1, which is related to a first thickness difference TD1 between the first frequency tuning structure 50 and the second frequency tuning structure 50', i.e., related to a first height difference HD1; thus, by adjusting the first height difference HD1, the first resonant frequency difference FD1 between the at least one first bulk acoustic resonant structure 3 and the at least one second bulk acoustic resonant structure 3' can be tuned. In some embodiments, the aforementioned substrate 10 may be a compound semiconductor substrate; the material constituting the plurality of sacrificial structure mesa is an epitaxial structure; and the aforementioned step C1 includes the following steps: step C11: forming a sacrificial structure 21 on the substrate 10; and step C12: etching the sacrificial structure 27 to form a plurality of sacrificial structure mesa, wherein the plurality of sacrificial structure mesa have the same height.

[0131] Please see Figure 3H , Figure 3I This is a cross-sectional schematic diagram of the fabrication steps of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. In this embodiment, the substrate 10 may be a semiconductor substrate; the material constituting the plurality of sacrificial mesa includes at least one selected from the group consisting of metals, alloys, and epitaxial structures. Figure 3I The main process steps and formation of the embodiments shown Figure 3G The process steps of the illustrated embodiment are largely the same, except that in step C52, the second polishing layer 51 is polished until at least one first sacrificial structure mesa 6 is exposed and at least one second sacrificial structure mesa 6' is not exposed (e.g., Figure 3H As shown), a second polishing layer 51 located below the polished surface 41 (extended plane 43) and below at least one second bulk acoustic resonant structure 3' forms a second frequency tuning structure 50' (as shown). Figure 3I (As shown). The second frequency tuning structure 50' has a thickness T2, which is equal to the first height difference HD1. In this embodiment, there is no... Figure 3GThe first frequency tuning structure 50 in the illustrated embodiment. Therefore, the first resonant frequency difference FD1 between at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3' is related to the thickness T2 of the second frequency tuning structure 50', that is, related to the first height difference HD1 between at least one first sacrificial structure mesa 6 and at least one second sacrificial structure mesa 6'. By adjusting the first height difference HD1, the first resonant frequency difference FD1 between at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3' can be tuned. In this embodiment, the material constituting the second polishing layer 51 may include at least one selected from the group consisting of metals, alloys, and insulators.

[0132] Please see Figure 3J , Figure 3K This is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic wave filter according to the present invention. In this embodiment, the substrate 10 may be a semiconductor substrate; the material constituting the plurality of sacrificial mesa includes at least one selected from the group consisting of metals, alloys, and epitaxial structures. Figure 3K The main process steps and formation of the embodiments shown Figure 3G The process steps of the illustrated embodiment are generally the same, except that in step C5, a plurality of individual acoustic resonant structures are formed on an extended plane 43, wherein the extended plane 43 coincides with the pre-polished surface 42, and step C5 includes the following steps: Step C51': (as shown) Figure 3D (As shown) A second polishing layer 51 is formed on a plurality of sacrificial structure mesa and insulating layer 11, wherein the material constituting the second polishing layer 51 includes at least one selected from the group consisting of metals and alloys; in a preferred embodiment, the material constituting the second polishing layer 51 includes at least one selected from the group consisting of ruthenium, titanium, molybdenum, platinum, gold, aluminum, and tungsten; Step C52': (as shown) Figure 3E (As shown) A second polishing layer 51 is polished using a chemical mechanical planarization process to form a polished surface 41, such that the plurality of sacrificial structure mesa surfaces are not exposed; Step C53': (as shown) Figure 3J (As shown) Pattern the second polishing layer 51; step C54': form a piezoelectric layer 31 on the polished surface 41; and step C55': form a top electrode layer 32 on the piezoelectric layer 31. After step C6 etching to remove the plurality of sacrificial structure mesa, the following is formed: Figure 3KThe embodiment shown. In step C4, at least one second sacrificial structure mesa 6' is etched. A second polishing layer 51, located above the pre-polished surface 42 (extended plane 43), below the polished surface 41, and below at least one first bulk acoustic wave resonant structure 3, forms a bottom electrode layer 30 of at least one first bulk acoustic wave resonant structure 3; a second polishing layer 51, located above the pre-polished surface 42 (extended plane 43), below the polished surface 41, and below at least one second bulk acoustic wave resonant structure 3', forms a bottom electrode layer 30 of at least one second bulk acoustic wave resonant structure 3'; a second polishing layer 51, located below the pre-polished surface 42 (extended plane 43), and below at least one second bulk acoustic wave resonant structure 3', forms a second frequency tuning structure 50' of at least one second bulk acoustic wave resonant structure 3'. The second frequency tuning structure 50' has a thickness T2, which is equal to a first height difference HD1. By adjusting the first height difference HD1, the first resonant frequency difference FD1 of at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3' can be tuned.

[0133] Please see Figure 3L This is a cross-sectional schematic diagram of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. In this embodiment, the substrate 10 may be a semiconductor substrate; the material constituting the plurality of sacrificial mesa includes at least one selected from the group consisting of metals, alloys, and epitaxial structures. Figure 3L The main process steps and formation of the embodiments shown Figure 3G The process steps of the illustrated embodiment are largely the same, except that step C5 includes the following steps: Step C51”: (as shown) Figure 3D (As shown) A second polishing layer 51 is formed on a plurality of sacrificial structural mesa and an insulating layer 11, wherein the material constituting the second polishing layer 51 includes at least one selected from the group consisting of metals, alloys, and insulators; Step C52”: (as shown) Figure 3E (As shown) A second polishing layer 51 is polished using a chemical mechanical planarization process to form a polished surface 41, such that at least one first sacrificial mesa 6 and at least one second sacrificial mesa 6' are not exposed; Step C53”: (as shown) Figure 3J (As shown) Patterning of the second polishing layer 51; Step C54”: Forming a bottom electrode layer 30 on the polished surface 41 (extended plane 43); Step C55”: Forming a piezoelectric layer 31 on the bottom electrode layer 30; and Step C56”: Forming a top electrode layer 32 on the piezoelectric layer 31. The process is completed via step C6. Figure 3LIn the illustrated embodiment, a second polishing layer 51 located below the polished surface 41 and below at least one first bulk acoustic wave resonant structure 3 and at least one second bulk acoustic wave resonant structure 3' respectively forms a first frequency tuning structure 50 of at least one first bulk acoustic wave resonant structure 3 and a second frequency tuning structure 50' of at least one second bulk acoustic wave resonant structure 3'. The first frequency tuning structure 50 and the second frequency tuning structure 50' have a first thickness difference TD1, which is equal to a first height difference HD1. By adjusting the first height difference HD1, the first resonant frequency difference FD1 of at least one first bulk acoustic wave resonant structure 3 and at least one second bulk acoustic wave resonant structure 3' can be tuned.

[0134] exist Figure 3G as well as Figure 3I In the embodiment, in step C2 (such as...) Figure 3A As shown), a very thick insulating layer 11 is first formed, wherein the thickness of the insulating layer 11 must be greater than the height of the plurality of sacrificial structural platforms. In step C3 (as shown) Figure 3B As shown), a pre-chemical mechanical planarization process requires the thickness of the insulating layer 11 to be at least greater than or equal to the height of a plurality of sacrificial structure mesa. However, a drawback of the chemical mechanical planarization process is that when the required thickness is too thick, the uniformity of the polished surface becomes poor. In this embodiment, because the required thickness of the insulating layer 11 is very thick, the uniformity of the pre-polished surface 42 after grinding becomes poor. However, the second grinding layer 51 formed in step C51 is very thin (relative to the thickness of the insulating layer 11), only needing to be higher than the first height difference HD1. Therefore, the uniformity of the polished surface 41 formed after grinding the second grinding layer 51 in the chemical mechanical planarization process in step C52 does not become poor. Therefore, forming the bottom electrode layer 30 of at least one first bulk acoustic resonator 1 and at least one second bulk acoustic resonator 1' on the polished surface 41 will help improve the resonance characteristics of at least one first bulk acoustic resonator 1 and at least one second bulk acoustic resonator 1'. Similarly, Figure 3L The same applies to the embodiments described. And in... Figure 3K In one embodiment, a piezoelectric layer 31 is formed on the polished surface 41 for at least one first bulk acoustic resonator 1 and at least one second bulk acoustic resonator 1', which also helps to improve the resonance characteristics of the at least one first bulk acoustic resonator 1 and at least one second bulk acoustic resonator 1'.

[0135] The foregoing Figure 3G , Figure 3I , Figure 3K as well as Figure 3L The embodiments can also be derived from similar methods. Figure 2KThe structure is formed by epitaxial growth, wherein the substrate 10 is a compound semiconductor substrate, and the sacrificial structure 21 includes a sacrificial epitaxial layer 27, a first etch stop layer 22 and a first fine tuning layer 23. The sacrificial epitaxial layer 27 is formed on the substrate 10, the first etch stop layer 22 is formed on the sacrificial epitaxial layer 27, and the first fine tuning layer 23 is formed on the first etch stop layer 22. The first fine tuning layer 23 has a thickness FT1. In step C4, the first fine tuning layer 23 of at least one second sacrificial structure mesa 6' is etched, such that at least one first sacrificial structure mesa 6 and at least one second sacrificial structure mesa 6' have a first height difference HD1. The first height difference HD1 is determined by the thickness FT1 of the first fine tuning layer 23, which helps to accurately adjust the first height difference HD1, that is, helps to accurately adjust the first resonant frequency difference FD1 of at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3'.

[0136] Please see Figures 4A to 4D This is a cross-sectional schematic diagram of the manufacturing process steps of a specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. Formation Figure 4D The main process steps and formation of the embodiments shown Figure 3G The process steps in the illustrated embodiments are largely the same, except that step C4 is: (as shown) Figure 4A (As shown) At least one first sacrificial mesa 6 and at least one second sacrificial mesa 6' are etched such that the at least one first sacrificial mesa 6 and at least one second sacrificial mesa 6' have a first height difference, wherein a height of the at least one first sacrificial mesa 6 is greater than a height of the at least one second sacrificial mesa 6'. After step C51 (as shown) Figure 4B (as shown), step C52 (as shown) Figure 4C (as shown), steps C53 to C55 and step C6, to form as shown Figure 4D In the illustrated embodiment, the material constituting the second polishing layer 51 is an insulator. In this embodiment, the substrate 10 may be a semiconductor substrate; the materials constituting the plurality of sacrificial mesa include at least one selected from the group consisting of metals, alloys, and epitaxial structures.

[0137] Please see Figure 4E , Figure 4F This is a cross-sectional schematic diagram of the fabrication steps of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. In this embodiment, the substrate 10 may be a semiconductor substrate; the material constituting the plurality of sacrificial mesa includes at least one selected from the group consisting of metals, alloys, and epitaxial structures. Figure 4F The main process steps and formation of the embodiments shown Figure 4DThe process steps of the illustrated embodiment are generally the same, except that in step C52, (as shown) Figure 4E (As shown) The second polishing layer 51 is polished to at least such that the polished surface 41 (extended plane 43) coincides with the pre-polished surface 42 or is lower than the pre-polished surface 42, and at least one first sacrificial structure platform 6 and at least one second sacrificial structure platform 6' are not exposed. Figure 4F In some embodiments, the material constituting the second polishing layer 51 may include at least one of the following groups: metals, alloys, and insulators.

[0138] Please see Figure 4G , Figure 4H This is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic wave filter according to the present invention. In this embodiment, the substrate 10 may be a semiconductor substrate; the material constituting the plurality of sacrificial mesa includes at least one selected from the group consisting of metals, alloys, and epitaxial structures. Figure 4H The main process steps and formation of the embodiments shown Figure 3K The process steps in the illustrated embodiments are largely the same, except that step C4 is: (as shown) Figure 4A (As shown) At least one first sacrificial mesa 6 and at least one second sacrificial mesa 6' are etched such that the at least one first sacrificial mesa 6 and at least one second sacrificial mesa 6' have a first height difference, wherein the height of the at least one first sacrificial mesa 6 is greater than the height of the at least one second sacrificial mesa 6'; in step C5, a plurality of individual acoustic resonant structures are formed on an extension plane 43, wherein the extension plane 43 coincides with the pre-polished surface 42; after step C53': (as shown) Figure 4G (As shown) the second polishing layer 51 is graphically processed, and after steps C54', C55' and C6 (as shown) Figure 4HAs shown), the second polishing layer 51, located above the pre-polished surface 42 (extended plane 43), below the polished surface 41, and below at least one first bulk acoustic wave resonant structure 3, forms a bottom electrode layer 30 of at least one first bulk acoustic wave resonant structure 3; the second polishing layer 51, located below the pre-polished surface 42 (extended plane 43) and below at least one first bulk acoustic wave resonant structure 3, forms a first frequency tuning structure 50 of at least one first bulk acoustic wave resonant structure 3; the second polishing layer 51, located above the pre-polished surface 42 (extended plane 43), below the polished surface 41, and below at least one first bulk acoustic wave resonant structure 3, forms a bottom electrode layer 3 ... forms a bottom electrode layer 30 of at least one first bulk acoustic wave resonant structure 3. A second polishing layer 51, located below and beneath at least one second bulk acoustic wave resonant structure 3', forms a bottom electrode layer 30 of at least one second bulk acoustic wave resonant structure 3'; wherein the second polishing layer 51, located below the pre-polished surface 42 (extended plane 43) and beneath at least one second bulk acoustic wave resonant structure 3', forms a second frequency tuning structure 50' of at least one second bulk acoustic wave resonant structure 3'; wherein the first frequency tuning structure 50 and the second frequency tuning structure 50' have a first thickness difference TD1, the first thickness difference TD1 being equal to a first height difference HD1. By adjusting the first height difference HD1, the first resonant frequency difference FD1 of at least one first bulk acoustic wave resonant structure 3 and at least one second bulk acoustic wave resonant structure 3' can be tuned. The material constituting the second polishing layer 51 includes at least one selected from the group consisting of metals and alloys; in a preferred embodiment, the material constituting the second polishing layer 51 includes at least one selected from the group consisting of ruthenium, titanium, molybdenum, platinum, gold, aluminum, and tungsten.

[0139] Please see Figure 4I This is a cross-sectional schematic diagram of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. In this embodiment, the substrate 10 may be a semiconductor substrate; the material constituting the plurality of sacrificial mesa includes at least one selected from the group consisting of metals, alloys, and epitaxial structures. Figure 4I The main process steps and formation of the embodiments shown Figure 3L The process steps in the illustrated embodiments are largely the same, except that step C4 is: (as shown) Figure 4A (As shown) At least one first sacrificial mesa 6 and at least one second sacrificial mesa 6' are etched, such that the at least one first sacrificial mesa 6 and at least one second sacrificial mesa 6' have a first height difference, wherein the height of the at least one first sacrificial mesa 6 is greater than the height of the at least one second sacrificial mesa 6'; and wherein in step C52", the second polishing layer 51 is polished until the at least one first sacrificial mesa 6 and at least one second sacrificial mesa 6' are not exposed, and then in step C53", (as shown) Figure 4G (As shown) The second polishing layer 51 is patterned, and after steps C54” to C56” and step C6 (as shown) Figure 4IAs shown, the second polishing layer 51, located below the polished surface 41 (extended plane 43) and below at least one first bulk acoustic wave resonant structure 3 and at least one second bulk acoustic wave resonant structure 3' respectively, forms a first frequency tuning structure 50 of at least one first bulk acoustic wave resonant structure 3 and a second frequency tuning structure 50' of at least one second bulk acoustic wave resonant structure 3', wherein the first frequency tuning structure 50 and the second frequency tuning structure 50' have a first thickness difference TD1, which is equal to a first height difference HD1. By adjusting the first height difference HD1, the first resonant frequency difference FD1 of at least one first bulk acoustic wave resonant structure 3 and at least one second bulk acoustic wave resonant structure 3' can be tuned. In this embodiment, the material constituting the second polishing layer 51 may include at least one selected from the group consisting of metals, alloys, and insulators.

[0140] Please see Figures 4J to 4M This is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. Wherein... Figure 4J In the embodiment shown, substrate 10 is a compound semiconductor substrate; the material constituting sacrificial structure 21 is an epitaxial structure. Figure 4J The epitaxial structure of the embodiment and Figure 2L The epitaxial structures of the illustrated embodiments are generally the same, except that the sacrificial structure 21 includes a sacrificial epitaxial layer 27, a first etch-stop layer 22, a first fine-tuning layer 23, and a top etch-stop layer 26. Step C1 includes the following steps: Step C11: forming a sacrificial structure 21 on the substrate 10; and Step C12: etching the sacrificial structure 27 to form a plurality of sacrificial structure mesas, wherein the plurality of sacrificial structure mesas have the same height, and wherein the plurality of sacrificial structure mesas include at least one first sacrificial structure mesas 6 and at least one second sacrificial structure mesas 6'. The sacrificial epitaxial layer 27 is formed on the substrate 10, the first etch-stop layer 22 is formed on the sacrificial epitaxial layer 27, the first fine-tuning layer 23 is formed on the first etch-stop layer 22, and the top etch-stop layer 26 is formed on the first fine-tuning layer 23. After steps C2 and C3, a structure is formed as shown... Figure 4K The structure shown. Step C4 includes the following steps: Step C41: (as shown) Figure 4L (As shown) Etching at least one first sacrificial mesa 6 and at least one second sacrificial mesa 6' of the top etch termination layer 26; and step C42: (as shown) Figure 4M(As shown) At least one second sacrificial mesa 6' is etched with a first fine tuning layer 23, such that the at least one first sacrificial mesa 6 and the at least one second sacrificial mesa 6' have a first height difference HD1. The first fine tuning layer 23 has a thickness FT1, and thus the first height difference HD1 is determined by the thickness FT1 of the first fine tuning layer 23. This facilitates precise adjustment of the first height difference HD1, which in turn facilitates precise adjustment of the first thickness difference TD1 between the first frequency tuning structure 50 and the second frequency tuning structure 50', and also facilitates precise adjustment of the first resonant frequency difference FD1 between the at least one first bulk acoustic resonant structure 3 and the at least one second bulk acoustic resonant structure 3'. Figure 4M It can form such as Figure 4D , Figure 4F , Figure 4H or Figure 4I The illustrated embodiment. Figure 4M To form an epitaxial structure Figure 4D , Figure 4F , Figure 4H or Figure 4IIn the illustrated embodiment, in step C3, the insulating layer 11 is polished to expose the plurality of sacrificial mesa surfaces. During actual polishing, the plurality of sacrificial mesa surfaces located near the center of the substrate 10 and those located away from the center of the substrate 10 often cannot be exposed simultaneously. For example, when the plurality of sacrificial mesa surfaces located away from the center of the substrate 10 are exposed first, polishing must continue in order to expose the plurality of sacrificial mesa surfaces located near the center of the substrate 10. This results in the plurality of sacrificial mesa surfaces located away from the center of the substrate 10 being over-polished, and consequently, the thickness of the first fine tuning layer 23 of the plurality of sacrificial mesa surfaces located away from the center of the substrate 10 is polished to be thinner than the thickness of the first fine tuning layer 23 of the plurality of sacrificial mesa surfaces located near the center of the substrate 10. To prevent the first fine tuning layer 23 of the plurality of sacrificial mesa near the center of the substrate 10 from being ground to different thicknesses than the first fine tuning layer 23 of the plurality of sacrificial mesa far from the center of the substrate 10, a top etch stop layer 26 can be used to ensure that the thickness of the first fine tuning layer 23 of the plurality of sacrificial mesa near the center of the substrate 10 is equal to the thickness of the first fine tuning layer 23 of the plurality of sacrificial mesa far from the center of the substrate 10. In some embodiments, the substrate 10 is made of gallium arsenide; the sacrificial epitaxial layer 27 is made of gallium arsenide; the first etch stop layer 22 is made of aluminum arsenide or indium gallium phosphide, wherein the first etch stop layer 22 has a thickness between 1 nm and 50 nm; the first fine tuning layer 23 is made of gallium arsenide, wherein the thickness FT1 of the first fine tuning layer 23 is between 1 nm and 300 nm; and the top etch stop layer 26 is made of indium gallium phosphide, wherein the top etch stop layer 26 has a thickness between 50 nm and 300 nm. In other embodiments, the substrate 10 is made of indium phosphide; the sacrificial epitaxial layer 27 is made of indium gallium arsenide; the first etch stop layer 22 is made of indium phosphide, wherein the first etch stop layer 22 has a thickness between 1 nm and 50 nm; the first fine tuning layer 23 is made of indium gallium arsenide, wherein the thickness FT1 of the first fine tuning layer 23 is between 1 nm and 300 nm; and the top etch stop layer 26 is made of indium phosphide, wherein the top etch stop layer 26 has a thickness between 50 nm and 300 nm.

[0141] The foregoing embodiments of at least one first bulk acoustic resonator 1 and at least one second bulk acoustic resonator 1' formed by the method of the present invention for tuning a bulk acoustic resonator for a bulk acoustic filter (e.g.) Figure 2F , Figure 2H , Figure 2I , Figure 2J , Figure 3G , Figure 3I , Figure 3K , Figure 3L , Figure 4D , Figure 4F , Figure 4H as well as Figure 4I All embodiments of the above share a common feature: the bottom electrode layer 30 of any bulk acoustic resonant structure (3 or 3') is formed on an extending plane 43. The common structure of these embodiments includes: an insulating layer 11 formed on a substrate 10, wherein the insulating layer 11 has a plurality of cavities; a plurality of bulk acoustic resonant structures, each located on one of the cavities, wherein the plurality of bulk acoustic resonant structures includes at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3', and the plurality of cavities includes at least one first cavity 40 and at least one second cavity 40', and at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3'. 'Corresponding to at least one first cavity 40 and at least one second cavity 40 respectively', wherein at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3' have a first resonant frequency difference FD1, wherein each of the plurality of bulk acoustic resonant structures includes: a bottom electrode layer 30 formed on an extended plane 43; a piezoelectric layer 31 formed on the bottom electrode layer 30; and a top electrode layer 32 formed on the piezoelectric layer 31; and a tunable frequency structure; and the difference between these embodiments is that: (1) in Figure 2H , Figure 2J , Figure 3I as well as Figure 3K In the embodiments, the aforementioned tunable frequency structure includes structure A: the insulating layer 11 has a polished upper surface, and the extended plane 43 coincides with the upper surface of the insulating layer 11; wherein at least one second bulk acoustic resonant structure 3' has a second frequency tuning structure 50', the second frequency tuning structure 50' is formed below the extended plane 43 between the bottom electrode layer 30 of the at least one second bulk acoustic resonant structure 3' and the second cavity 40', wherein the at least one second frequency tuning structure 50' has a thickness T2, the thickness T2 being associated with the first resonant frequency difference FD1 between the at least one first bulk acoustic resonant structure 3 and the at least one second bulk acoustic resonant structure 3'; (2) in Figure 2F , Figure 2I , Figure 4F as well as Figure 4HIn the embodiment, the aforementioned tunable frequency structure includes structure B: the insulating layer 11 has a polished upper surface, and the extending plane 43 coincides with the upper surface of the insulating layer 11; wherein at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3' respectively have a first frequency tuning structure 50 and a second frequency tuning structure 50', wherein the first frequency tuning structure 50 is formed below the extending plane 43 between the bottom electrode layer 30 of the at least one first bulk acoustic resonant structure 3 and the first cavity 40, and the second frequency tuning structure 50' is formed below the extending plane 43 between the bottom electrode layer 30 of the at least one second bulk acoustic resonant structure 3' and the second cavity 40', wherein the first frequency tuning structure 50 and the second frequency tuning structure 50' have a first thickness difference TD1, and the first thickness difference TD1 is associated with the first resonant frequency difference FD1 of the at least one first bulk acoustic resonant structure 3 and the at least one second bulk acoustic resonant structure 3'; (3) in Figure 3G , Figure 3L , Figure 4D as well as Figure 4I In the embodiments described above, the aforementioned tunable frequency structure includes structure C: a second polishing layer 51 is formed on the insulating layer 11 and a plurality of cavities, wherein the second polishing layer 51 has a polished upper surface, and the extending plane 43 coincides with the upper surface of the second polishing layer 51; wherein the second polishing layer 51 below the extending plane 43, between the bottom electrode layer 30 of at least one first bulk acoustic wave resonant structure 3 and the first cavity 40, forms a first frequency tuning structure 50 of at least one first bulk acoustic wave resonant structure 3; wherein the second polishing layer 51 below the extending plane 43, between the bottom electrode layer 30 of at least one second bulk acoustic wave resonant structure 3' and the second cavity 40', forms a second frequency tuning structure 50' of at least one second bulk acoustic wave resonant structure 3'; wherein the first frequency tuning structure 50 and the second frequency tuning structure 50' have a first thickness difference TD1, and the first thickness difference TD1 is associated with the first resonant frequency difference FD1 of at least one first bulk acoustic wave resonant structure 3 and at least one second bulk acoustic wave resonant structure 3'. Figure 2F , Figure 2I , Figure 3G , Figure 3L , Figure 4D , Figure 4F , Figure 4H as well as Figure 4I In the embodiments described, the common feature is that: the bottom electrode layer 30 of at least one first bulk acoustic resonant structure 3 and the bottom electrode layer 30 of at least one second bulk acoustic resonant structure 3' are both formed on the extending plane 43; the first frequency tuning structure 50 and the second frequency tuning structure 50' are both formed below the extending plane 43. In this invention... Figure 2H , Figure 2J , Figure 3Ias well as Figure 3K In the embodiments, the common feature is that: the bottom electrode layer 30 of at least one first bulk acoustic resonant structure 3 and the bottom electrode layer 30 of at least one second bulk acoustic resonant structure 3' are both formed on the extension plane 43; the second frequency tuning structure 50' is formed below the extension plane 43.

[0142] Please see Figures 5A to 5C This is a cross-sectional schematic diagram of the fabrication steps of a specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. In this embodiment, the substrate 10 may be a semiconductor substrate; the material constituting the plurality of sacrificial mesa includes at least one selected from the group consisting of metals, alloys, and epitaxial structures. Figure 5C The main process steps and formation of the embodiments shown Figure 2F The process steps of the illustrated embodiment are largely the same, except that at least one first bulk acoustic resonator 1, at least one second bulk acoustic resonator 1', and at least one third bulk acoustic resonator 1'" are formed on the substrate 10; wherein in step B1, (as shown) Figure 5B(As shown) A plurality of sacrificial structural platforms include at least one first sacrificial structural platform 6, at least one second sacrificial structural platform 6', and at least one third sacrificial structural platform 6"; wherein the height of at least one first sacrificial structural platform 6 is greater than the height of at least one second sacrificial structural platform 6', wherein at least one first sacrificial structural platform 6 and at least one second sacrificial structural platform 6' have a first height difference HD1; wherein the height of at least one first sacrificial structural platform 6 is greater than the height of at least one third sacrificial structural platform 6", wherein at least one first sacrificial structural platform 6 and at least one third sacrificial structural platform 6" have a second height difference HD2; in step B4, the plurality of individual The acoustic resonance structure is formed on the polished surface 41 (extended plane 43), wherein the plurality of individual acoustic resonance structures include at least one first body acoustic resonance structure 3, at least one second body acoustic resonance structure 3', and at least one third body acoustic resonance structure 3'", wherein the at least one first body acoustic resonance structure 3, at least one second body acoustic resonance structure 3', and at least one third body acoustic resonance structure 3'" are respectively located above at least one first sacrificial structure mesa 6, at least one second sacrificial structure mesa 6', and at least one third sacrificial structure mesa 6'". In step B5, the plurality of sacrificial structure mesa are etched to form a plurality of cavities, wherein the plurality of cavities includes at least one first cavity 40. At least one second cavity 40' and at least one third cavity 40" are provided, wherein at least one first cavity 40, at least one second cavity 40' and at least one third cavity 40" are respectively located below at least one first bulk acoustic resonant structure 3, at least one second bulk acoustic resonant structure 3' and at least one third bulk acoustic resonant structure 3". In step B3, the insulating layer 11 is polished so that at least one first sacrificial structure mesa 6, at least one second sacrificial structure mesa 6' and at least one third sacrificial structure mesa 6" are not exposed, thereby being located below the polished surface 41 and respectively below at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3'. The insulating layer 11 beneath at least one third-body acoustic resonant structure 3” forms a first frequency tuning structure 50 of at least one first-body acoustic resonant structure 3, a second frequency tuning structure 50' of at least one second-body acoustic resonant structure 3', and a third frequency tuning structure 50' of at least one third-body acoustic resonant structure 3”. The first frequency tuning structure 50 and the second frequency tuning structure 50' have a first thickness difference TD1, which is equal to a first height difference HD1; and the first frequency tuning structure 50 and the third frequency tuning structure 50” have a second thickness difference TD2, which is equal to a second height difference HD2. By adjusting the first height difference HD1, the first resonant frequency difference FD1 of at least one first-body acoustic resonant structure 3 and at least one second-body acoustic resonant structure 3' can be tuned.By adjusting the second height difference HD2, a second resonant frequency difference FD2 between at least one first bulk acoustic resonant structure 3 and at least one third bulk acoustic resonant structure 3” can be tuned. In some embodiments, the aforementioned substrate 10 may be a compound semiconductor substrate; the material constituting the plurality of sacrificial mesa is an epitaxial structure; and the aforementioned step B1 includes the following steps: Step B11: (e.g.) Figure 5A (As shown) A sacrificial structure 21 is formed on the substrate 10; Step B12: Etch the sacrificial structure 21 to form a plurality of sacrificial structure mesa, wherein the plurality of sacrificial structure mesa includes at least one first sacrificial structure mesa 6 (21), at least one second sacrificial structure mesa 6' (21) and at least one third sacrificial structure mesa 6" (21), and such that the plurality of sacrificial structure mesa have the same height; and Step B13: (as shown) Figure 5B (As shown) Etch at least one first sacrificial mesa 6, at least one second sacrificial mesa 6', and at least one third sacrificial mesa 6" or etch at least one second sacrificial mesa 6' and at least one third sacrificial mesa 6" such that at least one first sacrificial mesa 6 and at least one second sacrificial mesa 6' have a first height difference HD1, and at least one first sacrificial mesa 6 and at least one third sacrificial mesa 6" have a second height difference HD2.

[0143] Please see Figure 5D This is a cross-sectional schematic diagram of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. In this embodiment, the substrate 10 may be a semiconductor substrate; the material constituting the plurality of sacrificial mesa includes at least one selected from the group consisting of metals, alloys, and epitaxial structures. Figure 5D The main process steps and formation of the embodiments shown Figure 5C The process steps of the illustrated embodiment are largely the same, except that in step B3, the insulating layer 11 is polished such that at least one first sacrificial structure mesa 6 is exposed, while at least one second sacrificial structure mesa 6' and at least one third sacrificial structure mesa 6" are not exposed. This allows the insulating layer 11, located below the polished surface 41 (extended plane 43) and below at least one second bulk acoustic resonant structure 3', to form a second frequency tuning structure 50' of the at least one second bulk acoustic resonant structure 3'; and the insulating layer 11, located below the polished surface 41 and below at least one third bulk acoustic resonant structure 3", to form a third frequency tuning structure 50" of the at least one third bulk acoustic resonant structure 3". The second frequency tuning structure 50' has a thickness T2, which is equal to the first height difference HD1; the third frequency tuning structure 50" has a thickness T3, which is equal to the second height difference HD2. In this embodiment, there is no... Figure 5CThe first frequency tuning structure 50 in the illustrated embodiment. Therefore, the first resonant frequency difference FD1 between at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3' is related to the thickness T2 of the second frequency tuning structure 50', that is, related to the first height difference HD1 between at least one first sacrificial structure platform 6 and at least one second sacrificial structure platform 6'; the second resonant frequency difference FD2 between at least one first bulk acoustic resonant structure 3 and at least one third bulk acoustic resonant structure 3' is related to the thickness T3 of the third frequency tuning structure 50', that is, related to the second height difference HD2 between at least one first sacrificial structure platform 6 and at least one third sacrificial structure platform 6'. By adjusting the first height difference HD1, the first resonant frequency difference FD1 between at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3' can be tuned; by adjusting the second height difference HD2, the second resonant frequency difference FD2 between at least one first bulk acoustic resonant structure 3 and at least one third bulk acoustic resonant structure 3' can be tuned.

[0144] Please see Figures 5E to 5G This is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. Figure 5E In the embodiment shown, substrate 10 is a compound semiconductor substrate; the material constituting sacrificial structure 21 is an epitaxial structure. Figures 5E to 5G The epitaxial structure of the embodiment and Figures 5A-5B The epitaxial structures of the illustrated embodiments are generally the same, except that the sacrificial structure 21 includes a sacrificial epitaxial layer 27, a second etch-stop layer 24, a second fine-tuning layer 25, a first etch-stop layer 22, and a first fine-tuning layer 23. The sacrificial epitaxial layer 27 is formed on the substrate 10, the second etch-stop layer 24 is formed on the sacrificial epitaxial layer 27, the second fine-tuning layer 25 is formed on the second etch-stop layer 24, the first etch-stop layer 22 is formed on the second fine-tuning layer 25, and the first fine-tuning layer 23 is formed on the first etch-stop layer 22. Figure 5F As shown, the sacrificial structure 21 is etched into a plurality of sacrificial structure mesa surfaces, wherein the plurality of sacrificial structure mesa surfaces include at least one first sacrificial structure mesa surface 6, at least one second sacrificial structure mesa surface 6', and at least one third sacrificial structure mesa surface 6'', and such that the plurality of sacrificial structure mesa surfaces have the same height. Figure 5GAs shown, the first fine tuning layer 23 has a thickness FT1, the first etch stop layer 22 has a thickness ET1, and the second fine tuning layer 25 has a thickness FT2. The first fine tuning layer 23 is etched onto at least one second sacrificial mesa 6', such that the at least one first sacrificial mesa 6 and the at least one second sacrificial mesa 6' have a first height difference HD1; the first fine tuning layer 23, the first etch stop layer 22, and the second fine tuning layer 25 are etched onto at least one third sacrificial mesa 6" such that the at least one first sacrificial mesa 6 and the at least one third sacrificial mesa 6" have a second height difference HD2. Figure 5G The structure can form such as Figure 5C The embodiment includes a first height difference HD1 determined by the thickness FT1 of the first fine tuning layer 23. This facilitates precise adjustment of the first height difference HD1, which in turn facilitates precise adjustment of the first thickness difference TD1 between the first frequency tuning structure 50 and the second frequency tuning structure 50', and also facilitates precise adjustment of the first resonant frequency difference FD1 between at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3'. The second height difference HD2 is determined by the thickness FT1 of the first fine tuning layer 23, the thickness ET1 of the first etch stop layer 22, and the thickness FT2 of the second fine tuning layer 25. This facilitates precise adjustment of the second height difference HD2, which in turn facilitates precise adjustment of the second resonant frequency difference FD2 between at least one first bulk acoustic resonant structure 3 and at least one third bulk acoustic resonant structure 3'. Figure 5G The structure can also form, for example, Figure 5DThe embodiment includes a first height difference HD1 determined by the thickness FT1 of the first fine tuning layer 23, which helps to precisely adjust the first height difference HD1, that is, helps to precisely adjust the thickness T2 of the second frequency tuning structure 50', that is, helps to precisely adjust the first resonant frequency difference FD1 of at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3'. The second height difference HD2 is determined by the thickness FT1 of the first fine tuning layer 23, the thickness ET1 of the first etch stop layer 22, and the thickness FT2 of the second fine tuning layer 25, which helps to precisely adjust the second height difference HD2, that is, helps to precisely adjust the thickness T3 of the third frequency tuning structure 50', that is, helps to precisely adjust the second resonant frequency difference FD2 of at least one first bulk acoustic resonant structure 3 and at least one third bulk acoustic resonant structure 3'. In some embodiments, the substrate 10 is made of gallium arsenide; the sacrificial epitaxial layer 27 is made of gallium arsenide; the first etch stop layer 22 is made of aluminum arsenide or indium gallium phosphide, wherein the first etch stop layer 22 has a thickness between 1 nm and 50 nm; the first fine tuning layer 23 is made of gallium arsenide, wherein the thickness FT1 of the first fine tuning layer 23 is between 1 nm and 300 nm; the second etch stop layer 24 is made of aluminum arsenide or indium gallium phosphide, wherein the second etch stop layer 24 has a thickness between 1 nm and 50 nm; and the second fine tuning layer 25 is made of gallium arsenide, wherein the thickness FT1 of the second fine tuning layer 25 is between 1 nm and 300 nm. In other embodiments, the substrate 10 is made of indium phosphide; the sacrificial epitaxial layer 27 is made of indium gallium arsenide; the first etch stop layer 22 is made of indium phosphide, wherein the first etch stop layer 22 has a thickness between 1 nm and 50 nm; the first fine tuning layer 23 is made of indium gallium arsenide, wherein the thickness FT1 of the first fine tuning layer 23 is between 1 nm and 300 nm; the second etch stop layer 24 is made of indium phosphide, wherein the second etch stop layer 24 has a thickness between 1 nm and 50 nm; and the second fine tuning layer 25 is made of indium gallium arsenide, wherein the thickness FT1 of the second fine tuning layer 25 is between 1 nm and 300 nm.

[0145] Please see Figures 5H to 5K This is a cross-sectional schematic diagram of the manufacturing process steps of another specific embodiment of the method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. In this embodiment, the substrate 10 is a compound semiconductor substrate; the material constituting the sacrificial structure 21 is an epitaxial structure. Figure 5K The illustrated embodiments are provided by Figure 5E The epitaxial structure shown is formed. Figure 5K The main process steps and formation of the embodiments shown Figure 3GThe process steps of the illustrated embodiment are generally the same, except that at least one first bulk acoustic resonator 1, at least one second bulk acoustic resonator 1', and at least one third bulk acoustic resonator 1" are formed on the substrate 10; wherein step C1 includes the following steps: Step C11: (as shown) Figure 5E (As shown) A sacrificial structure 21 is formed on the substrate 10, wherein the sacrificial structure 21 includes a sacrificial epitaxial layer 27, a second etch-stop layer 24, a second fine-tuning layer 25, a first etch-stop layer 22, and a first fine-tuning layer 23. The sacrificial epitaxial layer 27 is formed on the substrate 10, the second etch-stop layer 24 is formed on the sacrificial epitaxial layer 27, the second fine-tuning layer 25 is formed on the second etch-stop layer 24, the first etch-stop layer 22 is formed on the second fine-tuning layer 25, and the first fine-tuning layer 23 is formed on the first etch-stop layer 22; and step C12: (as shown) Figure 5F (As shown) Etching the sacrificial structure 27 to form a plurality of sacrificial structure mesa faces, wherein the plurality of sacrificial structure mesa faces have the same height, wherein the plurality of sacrificial structure mesa faces include at least one first sacrificial structure mesa face 6, at least one second sacrificial structure mesa face 6', and at least one third sacrificial structure mesa face 6''. Through steps C2 and C3, as shown, a structure is formed. Figure 5H The structure. Step C4: (as shown in...) Figure 5I (As shown) Etch at least one second sacrificial mesa 6' with a first fine tuning layer 23, such that the at least one first sacrificial mesa 6 and the at least one second sacrificial mesa 6' have a first height difference HD1; etch at least one third sacrificial mesa 6" with a first fine tuning layer 23, a first etch stop layer 22, and a second fine tuning layer 25, such that the at least one first sacrificial mesa 6 and the at least one third sacrificial mesa 6" have a second height difference HD2. The first fine tuning layer 23 has a thickness FT1, the first etch stop layer 22 has a thickness ET1, and the second fine tuning layer 25 has a thickness FT2. In step C5: forming a plurality of individual acoustic wave resonance structures, wherein the plurality of individual acoustic wave resonance structures includes at least one first-body acoustic wave resonance structure 3, at least one second-body acoustic wave resonance structure 3', and at least one third-body acoustic wave resonance structure 3''; wherein step C5 includes the following steps: step C51, step C52, step C53, step C54, and step C55. Through steps C51 and C52, a structure is formed as follows: Figure 5JThe structure includes a second polishing layer 51 polished to the point that at least one first sacrificial structure mesa 6, at least one second sacrificial structure mesa 6', and at least one third sacrificial structure mesa 6" are not exposed. This allows the second polishing layer 51, located below the polished surface 41 (extending plane 43) and below at least one first bulk acoustic resonance structure 3, at least one second bulk acoustic resonance structure 3', and at least one first bulk acoustic resonance structure 3" respectively, to form a first frequency tuning structure 50 of at least one first bulk acoustic resonance structure 3, a second frequency tuning structure 50' of at least one second bulk acoustic resonance structure 3', and a third frequency tuning structure 50" of at least one third bulk acoustic resonance structure 3". The first frequency tuning structure 50 and the second frequency tuning structure 50' have a first thickness difference TD1, and the first thickness difference TD1 is equal to a first height difference HD1. The first frequency tuning structure 50 and the third frequency tuning structure 50" have a second thickness difference TD2, and the second thickness difference TD2 is equal to the second height difference HD2. Then, through steps C53, C54, C55, and C6, they are formed as follows: Figure 5K The structure includes a plurality of cavities, including at least one first cavity 40, at least one second cavity 40', and at least one third cavity 40'. At least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3' have a first resonant frequency difference FD1. The first resonant frequency difference FD1 is related to a first thickness difference TD1 between the first frequency tuning structure 50 and the second frequency tuning structure 50', i.e., related to a first height difference HD1. Therefore, by adjusting the first height difference HD1, at least one first bulk acoustic resonant structure 3 and at least one second cavity 40' can be tuned. The second bulk acoustic resonant structure 3' has a first resonant frequency difference FD1. At least one first bulk acoustic resonant structure 3 and at least one third bulk acoustic resonant structure 3" have a second resonant frequency difference FD2. This second resonant frequency difference FD2 is related to the second thickness difference TD2 of the first frequency tuning structure 50 and the third frequency tuning structure 50", i.e., it is related to the second height difference HD2. Therefore, by adjusting the second height difference HD2, the second resonant frequency difference FD2 of at least one first bulk acoustic resonant structure 3 and at least one third bulk acoustic resonant structure 3" can be tuned. The material constituting the second polishing layer 51 is an insulator.

[0146] Please see Figure 5L , Figure 5M This is a cross-sectional schematic diagram of the fabrication process steps of a specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. In this embodiment, the substrate 10 is a compound semiconductor substrate; the material constituting the sacrificial structure 21 is an epitaxial structure. Figure 5M The main process steps and formation of the embodiments shown Figure 5KThe process steps of the illustrated embodiment are largely the same, except that in step C52, the second polishing layer 51 is polished until at least one first sacrificial structure mesa 6 is exposed, while at least one second sacrificial structure mesa 6' and at least one third sacrificial structure mesa 6" are not exposed (e.g., Figure 5L As shown), a second polishing layer 51 located below the polished surface 41 (extended plane 43) and below at least one second body acoustic resonance structure 3' forms a second frequency tuning structure 50' of at least one second body acoustic resonance structure 3'; a second polishing layer 51 located below the polished surface 41 and below at least one third body acoustic resonance structure 3'" forms a third frequency tuning structure 50' of at least one third body acoustic resonance structure 3'". Figure 5L As shown, the second frequency tuning structure 50' has a thickness T2, which is equal to the first height difference HD1; the third frequency tuning structure 50" has a thickness T3, which is equal to the second height difference HD2. In this embodiment, there is no... Figure 5K The first frequency tuning structure 50 in the illustrated embodiment. Therefore, the first resonant frequency difference FD1 between at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3' is related to the thickness T2 of the second frequency tuning structure 50', that is, related to the first height difference HD1 between at least one first sacrificial structure platform 6 and at least one second sacrificial structure platform 6'. By adjusting the first height difference HD1, the first resonant frequency difference FD1 between at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3' can be tuned; and the second resonant frequency difference FD2 between at least one first bulk acoustic resonant structure 3 and at least one third bulk acoustic resonant structure 3' is related to the thickness T3 of the third frequency tuning structure 50', that is, related to the second height difference HD2 between at least one first sacrificial structure platform 6 and at least one third sacrificial structure platform 6'. By adjusting the second height difference HD2, the second resonant frequency difference FD2 between at least one first bulk acoustic resonant structure 3 and at least one third bulk acoustic resonant structure 3' can be tuned. In this embodiment, the material constituting the second polishing layer 51 may include at least one of the following groups: metals, alloys, and insulators.

[0147] Please see Figure 5N , Figure 5O This is a cross-sectional schematic diagram of the fabrication steps of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. In this embodiment, the substrate 10 is a compound semiconductor substrate; the material constituting the sacrificial structure 21 is an epitaxial structure. Figure 5O The main process steps and formation of the embodiments shown Figure 5KThe process steps of the illustrated embodiment are generally the same, except that in step C5, a plurality of individual acoustic resonant structures are formed on an extended plane 43, wherein the extended plane 43 coincides with the pre-polished surface 42, and step C5 includes the following steps: Step C51': forming a second polishing layer 51 on a plurality of sacrificial structure mesa and insulating layer 11, wherein the material constituting the second polishing layer 51 includes at least one selected from the group consisting of metals and alloys; in a preferred embodiment, the material constituting the second polishing layer 51 includes at least one selected from the group consisting of ruthenium, titanium, molybdenum, platinum, gold, aluminum and tungsten; Step C52': (as Figure 5J (As shown) A second polishing layer 51 is polished using a chemical mechanical planarization process to form a polished surface 41, such that the plurality of sacrificial structure mesa surfaces are not exposed; Step C53': (as shown) Figure 5N (As shown) Patterning the second polishing layer 51; step C54': forming a piezoelectric layer 31 on the polished surface 41; and step C55': forming a top electrode layer 32 on the piezoelectric layer 31. After etching away the plurality of sacrificial structure mesa in step C6, the following is formed: Figure 5OThe embodiment shown. In step C4, at least one second sacrificial mesa 6' and at least one third sacrificial mesa 6' are etched; a second polishing layer 51 located above the pre-polished surface 42 (extended plane 43), below the polished surface 41, and below at least one first bulk acoustic wave resonant structure 3 forms a bottom electrode layer 30 of at least one first bulk acoustic wave resonant structure 3; a second polishing layer 51 located above the pre-polished surface 42 (extended plane 43), below the polished surface 41, and below at least one second bulk acoustic wave resonant structure 3' forms a bottom electrode layer 30 of at least one second bulk acoustic wave resonant structure 3'; a second polishing layer 51 located below the pre-polished surface 42 (extended plane 43) and below at least one second bulk acoustic wave resonant structure 3' forms at least one second bulk acoustic wave resonant structure 3'. A second frequency tuning structure 50', wherein the second frequency tuning structure 50' has a thickness T2, the thickness T2 of the second frequency tuning structure 50' being equal to a first height difference HD1; wherein a second polishing layer 51 located above the pre-polished surface 42 (extended plane 43), below the polished surface 41, and below at least one third body acoustic wave resonant structure 3" forms a bottom electrode layer 30 of at least one third body acoustic wave resonant structure 3"; wherein the second polishing layer 51 located below the pre-polished surface 42 (extended plane 43) and below at least one third body acoustic wave resonant structure 3" forms a third frequency tuning structure 50" of at least one third body acoustic wave resonant structure 3"; wherein the third frequency tuning structure 50" has a thickness T3, the thickness T3 of the third frequency tuning structure 50" being equal to a second height difference HD2. By adjusting the first height difference HD1, the first resonance frequency difference FD1 between at least one first body acoustic resonant structure 3 and at least one second body acoustic resonant structure 3' can be tuned; by adjusting the second height difference HD2, the second resonance frequency difference FD2 between at least one first body acoustic resonant structure 3 and at least one third body acoustic resonant structure 3' can be tuned.

[0148] Please see Figure 5P This is a cross-sectional schematic diagram of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. Formation Figure 5P The main process steps and formation of the embodiments shown Figure 5K The process steps of the illustrated embodiment are generally the same, except that step C5 includes the following steps: Step C51”: forming a second polishing layer 51 on a plurality of sacrificial mesa and insulating layer 11, wherein substrate 10 is a compound semiconductor substrate; the material constituting the plurality of sacrificial mesa is an epitaxial structure; wherein the material constituting the second polishing layer 51 includes at least one selected from the group consisting of: metals, alloys and insulators; Step C52”: (as Figure 5J(As shown) A second polishing layer 51 is polished using a chemical mechanical planarization process to form a polished surface 41, such that at least one first sacrificial mesa 6, at least one second sacrificial mesa 6', and at least one third sacrificial mesa 6" are not exposed; Step C53": (as shown) Figure 5N (As shown) Patterning of the second polishing layer 51; Step C54”: Forming a bottom electrode layer 30 on the polished surface 41 (extended plane 43); Step C55”: Forming a piezoelectric layer 31 on the bottom electrode layer 30; and Step C56”: Forming a top electrode layer 32 on the piezoelectric layer 31. The process is completed via step C6. Figure 5P In the illustrated embodiment, a second polishing layer 51 located below the polished surface 41 and respectively below at least one first bulk acoustic wave resonant structure 3, at least one second bulk acoustic wave resonant structure 3', and at least one third bulk acoustic wave resonant structure 3'' forms a first frequency tuning structure 50 of at least one first bulk acoustic wave resonant structure 3, a second frequency tuning structure 50' of at least one second bulk acoustic wave resonant structure 3'', and a third frequency tuning structure 50'' of at least one third bulk acoustic wave resonant structure 3''; wherein the first frequency tuning structure 50 and the second frequency tuning structure 50' have a first thickness difference TD1, which is equal to a first height difference HD1; wherein the first frequency tuning structure 50 and the third frequency tuning structure 50'' have a second thickness difference TD2, which is equal to a second height difference HD2. By adjusting the first height difference HD1, the first resonant frequency difference FD1 of at least one first bulk acoustic wave resonant structure 3 and at least one second bulk acoustic wave resonant structure 3' can be tuned. By adjusting the second height difference HD2, a second resonance frequency difference FD2 can be tuned between at least one first body acoustic wave resonance structure 3 and at least one third body acoustic wave resonance structure 3".

[0149] The foregoing Figure 5K , Figure 5M , Figure 5O as well as Figure 5P In other embodiments, it can also be derived from, for example... Figure 5A The structure is formed by (where the substrate 10 is a semiconductor substrate; the materials constituting the plurality of sacrificial structural mesa include at least one selected from the group consisting of metals, alloys and epitaxial structures).

[0150] Please see Figures 6A to 6C This is a cross-sectional schematic diagram of the fabrication steps of a specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. In this embodiment, the substrate 10 may be a semiconductor substrate; the material constituting the plurality of sacrificial mesa includes at least one selected from the group consisting of metals, alloys, and epitaxial structures. Figure 6A Structure and Figure 3BThe structures shown are largely the same, except that the plurality of sacrificial structural platforms include at least one first sacrificial structural platform 6', at least one second sacrificial structural platform 6', and at least one third sacrificial structural platform 6'. Figure 6C The main process steps and formation of the embodiments shown Figure 4D The process steps of the illustrated embodiment are largely the same, except that at least one first bulk acoustic resonator 1, at least one second bulk acoustic resonator 1', and at least one third bulk acoustic resonator 1" are formed; in step C1, the plurality of sacrificial structural mesa includes at least one first sacrificial structural mesa 6, at least one second sacrificial structural mesa 6', and at least one third sacrificial structural mesa 6"; and in step C4, at least one first sacrificial structural mesa 6, at least one second sacrificial structural mesa 6', and at least one third sacrificial structural mesa 6" are etched such that at least one first sacrificial structural mesa 6 and at least one second sacrificial structural mesa 6' have a first height difference HD1, and at least one first sacrificial structural mesa 6 and at least one third sacrificial structural mesa 6" have a second height difference HD2 (e.g., ...). Figure 6B(as shown); wherein in step C5, the plurality of individual acoustic resonance structures include at least one first body acoustic resonance structure 3, at least one second body acoustic resonance structure 3', and at least one third body acoustic resonance structure 3'”. At least one first body acoustic resonance structure 3, at least one second body acoustic resonance structure 3', and at least one third body acoustic resonance structure 3'” are respectively located on at least one first sacrificial structure platform 6, at least one second sacrificial structure platform 6', and at least one third sacrificial structure platform 6'”; in step C52, the second polishing layer 51 is polished so that at least one first sacrificial structure platform 6, at least one second sacrificial structure platform 6', and at least one third sacrificial structure platform 6'” are not exposed, thereby located below the polished surface 41 (extended plane 43) and respectively located on A second grinding layer 51 is formed beneath at least one first bulk acoustic resonant structure 3, at least one second bulk acoustic resonant structure 3', and at least one first bulk acoustic resonant structure 3'”, respectively forming a first frequency tuning structure 50 of at least one first bulk acoustic resonant structure 3, a second frequency tuning structure 50' of at least one second bulk acoustic resonant structure 3', and a third frequency tuning structure 50” of at least one third bulk acoustic resonant structure 3”; wherein the first frequency tuning structure 50 and the second frequency tuning structure 50' have a first thickness difference TD1, which is equal to a first height difference HD1; wherein the first frequency tuning structure 50 and the third frequency tuning structure 50” have a second thickness difference TD2, which is equal to a second height difference HD2. By adjusting the first height difference HD1, the first resonant frequency difference FD1 of at least one first bulk acoustic resonant structure 3 and at least one second bulk acoustic resonant structure 3' can be tuned. By adjusting the second height difference HD2, a second resonant frequency difference FD2 between at least one first body acoustic resonant structure 3 and at least one third body acoustic resonant structure 3” can be tuned. The material constituting the second polishing layer 51 is an insulator.

[0151] Please see Figure 6D This is a cross-sectional schematic diagram of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. In this embodiment, the substrate 10 may be a semiconductor substrate; the material constituting the plurality of sacrificial mesa includes at least one selected from the group consisting of metals, alloys, and epitaxial structures. Figure 6D The main process steps and formation of the embodiments shown Figure 6CThe process steps of the illustrated embodiment are generally the same, except that in step C52, the second polishing layer 51 is polished to at least such that the polished surface 41 (extended plane 43) coincides with the pre-polished surface 42 or is lower than the pre-polished surface 42, and wherein at least one first sacrificial structure mesa 6, at least one second sacrificial structure mesa 6', and at least one third sacrificial structure mesa 6" are not exposed. In this embodiment, the material constituting the second polishing layer 51 may include at least one selected from the group consisting of metals, alloys, and insulators.

[0152] Please see Figure 6E This is a cross-sectional schematic diagram of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. In this embodiment, the substrate 10 may be a semiconductor substrate; the material constituting the plurality of sacrificial mesa includes at least one selected from the group consisting of metals, alloys, and epitaxial structures. Figure 6E The main process steps and formation of the embodiments shown Figure 6C The process steps of the illustrated embodiment are generally the same, except that in step C5, a plurality of individual acoustic resonant structures are formed on an extended plane 43, wherein the extended plane 43 coincides with the pre-polished surface 42, wherein step C5 includes the following steps: step C51': forming a second polishing layer 51 on a plurality of sacrificial structure mesa and insulating layer 11, wherein the material constituting the second polishing layer 51 includes at least one selected from the group consisting of metals and alloys; in a preferred embodiment, the material constituting the second polishing layer 51 includes at least one selected from the group consisting of ruthenium, titanium, molybdenum, platinum, gold, aluminum and tungsten; step C52': polishing the second polishing layer 51 with a chemical mechanical planarization process to form a polished surface 41 such that the plurality of sacrificial structure mesa are not exposed; step C53': patterning the second polishing layer 51; step C54': forming a piezoelectric layer 31 on the polished surface 41; and step C55': forming a top electrode layer 32 on the piezoelectric layer 31. After etching away multiple sacrificial mesa in step C6, the following is formed: Figure 6EThe embodiment shown. A second polishing layer 51, located above the pre-polished surface 42 (extended plane 43), below the polished surface 41, and below at least one first bulk acoustic wave resonant structure 3, forms a bottom electrode layer 30 of at least one first bulk acoustic wave resonant structure 3; a second polishing layer 51, located below the pre-polished surface 42 (extended plane 43) and below at least one first bulk acoustic wave resonant structure 3, forms a first frequency tuning structure 50 of at least one first bulk acoustic wave resonant structure 3; a second polishing layer 51, located above the pre-polished surface 42 (extended plane 43), below the polished surface 41, and below at least one second bulk acoustic wave resonant structure 3', forms a bottom electrode layer 30 of at least one second bulk acoustic wave resonant structure 3'; a second polishing layer 51, located below the pre-polished surface 42 (extended plane 43) and below at least one second bulk acoustic wave resonant structure 3', forms at least one first frequency tuning structure 50 of at least one first bulk acoustic wave resonant structure 3'. A second frequency tuning structure 50' of a two-body acoustic resonant structure 3', wherein the first frequency tuning structure 50 and the second frequency tuning structure 50' have a first thickness difference TD1, the first thickness difference TD1 being equal to a first height difference HD1; wherein a second polishing layer 51 located above the pre-polished surface 42 (extended plane 43), below the polished surface 41, and below at least one third-body acoustic resonant structure 3', forms a bottom electrode layer 30 of at least one third-body acoustic resonant structure 3'; wherein the second polishing layer 51 located below the pre-polished surface 42 (extended plane 43) and below at least one third-body acoustic resonant structure 3', forms a third frequency tuning structure 50' of at least one third-body acoustic resonant structure 3', wherein the first frequency tuning structure 50 and the third frequency tuning structure 50' have a second thickness difference TD2, the second thickness difference TD2 being equal to a second height difference HD2. By adjusting the first height difference HD1, the first resonant frequency difference FD1 of at least one first-body acoustic resonant structure 3 and at least one second-body acoustic resonant structure 3' can be tuned. By adjusting the second height difference HD2, a second resonance frequency difference FD2 can be tuned between at least one first body acoustic wave resonance structure 3 and at least one third body acoustic wave resonance structure 3".

[0153] Please see Figure 6F This is a cross-sectional schematic diagram of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. Formation Figure 6F The main process steps and formation of the embodiments shown Figure 6CThe process steps of the illustrated embodiment are generally the same, except that step C5 includes the following steps: Step C51”: forming a second polishing layer 51 on a plurality of sacrificial mesa and insulating layer 11, wherein substrate 10 is a compound semiconductor substrate; the material constituting the plurality of sacrificial mesa is an epitaxial structure; wherein the material constituting the second polishing layer 51 includes at least one selected from the group consisting of metals, alloys and insulators; Step C52”: polishing the second polishing layer 51 by a chemical mechanical planarization process to form a polished surface 41, such that at least one first sacrificial mesa 6, at least one second sacrificial mesa 6' and at least one third sacrificial mesa 6” are not exposed; Step C53”: patterning the second polishing layer 51; Step C54”: forming a bottom electrode layer 30 on polished surface 41 (extended plane 43); Step C55”: forming a piezoelectric layer 31 on bottom electrode layer 30; and Step C56”: forming a top electrode layer 32 on piezoelectric layer 31. The process is completed by step C6. Figure 6F In the illustrated embodiment, a second polishing layer 51 located below the polished surface 41 and respectively below at least one first bulk acoustic wave resonant structure 3, at least one second bulk acoustic wave resonant structure 3', and at least one third bulk acoustic wave resonant structure 3'' forms a first frequency tuning structure 50 of at least one first bulk acoustic wave resonant structure 3, a second frequency tuning structure 50' of at least one second bulk acoustic wave resonant structure 3'', and a third frequency tuning structure 50'' of at least one third bulk acoustic wave resonant structure 3''; wherein the first frequency tuning structure 50 and the second frequency tuning structure 50' have a first thickness difference TD1, which is equal to a first height difference HD1; wherein the first frequency tuning structure 50 and the third frequency tuning structure 50'' have a second thickness difference TD2, which is equal to a second height difference HD2. By adjusting the first height difference HD1, the first resonant frequency difference FD1 of at least one first bulk acoustic wave resonant structure 3 and at least one second bulk acoustic wave resonant structure 3' can be tuned. By adjusting the second height difference HD2, a second resonance frequency difference FD2 can be tuned between at least one first body acoustic wave resonance structure 3 and at least one third body acoustic wave resonance structure 3".

[0154] The foregoing Figure 6C , Figure 6D , Figure 6E as well as Figure 6F In other embodiments, it can also be derived from, for example... Figure 5E The epitaxial structure is formed, wherein the substrate 10 is a compound semiconductor substrate; the material constituting the sacrificial structure 21 is an epitaxial structure.

[0155] The foregoing Figure 5E The sacrificial structure 21 may also include an etch stop layer 26 formed on the first fine tuning layer 23 (not shown in the figure), thereby forming Figure 6C , Figure 6D , Figure 6E as well as Figure 6F Examples of embodiments. The function of this top etch stop layer 26 is similar to... Figure 4J The top etch stop layer 26 has the same function. In order to avoid the first fine tuning layer 23 of the plurality of sacrificial structural mesa located near the center of the substrate 10 being ground to different thicknesses than the first fine tuning layer 23 of the plurality of sacrificial structural mesa located away from the center of the substrate 10, the top etch stop layer 26 can be used to ensure that the thickness of the first fine tuning layer 23 of the plurality of sacrificial structural mesa located near the center of the substrate 10 is maintained equal to the thickness of the first fine tuning layer 23 of the plurality of sacrificial structural mesa located away from the center of the substrate 10.

[0156] In the present invention Figure 5C , Figure 5K , Figure 5P , Figure 6C , Figure 6D , Figure 6E as well as Figure 6F In the embodiments of the present invention, the common feature is that: the bottom electrode layer 30 of at least one first bulk acoustic resonant structure 3, the bottom electrode layer 30 of at least one second bulk acoustic resonant structure 3', and the bottom electrode layer 30 of at least one first bulk acoustic resonant structure 3" are all formed on the extending plane 43; the first frequency tuning structure 50, the second frequency tuning structure 50', and the first frequency tuning structure 50" are all formed below the extending plane 43. Figure 5D , Figure 5M as well as Figure 5O In the embodiments, the common feature is that: the bottom electrode layer 30 of at least one first bulk acoustic resonant structure 3, the bottom electrode layer 30 of at least one second bulk acoustic resonant structure 3', and the bottom electrode layer 30 of at least one first bulk acoustic resonant structure 3" are all formed on the extension plane 43; the second frequency tuning structure 50' and the first frequency tuning structure 50" are both formed below the extension plane 43.

[0157] The foregoing Figure 3G , Figure 3I , Figure 3K , Figure 3L , Figure 4D , Figure 4F , Figure 4H , Figure 4I , Figure 5C , Figure 5D , Figure 5K , Figure 5M , Figure 5O , Figure 5P , Figure 6C , Figure 6D , Figure 6E as well as Figure 6F All of the embodiments can be as follows Figure 2I or Figure 2J The embodiment further includes a bottom etch stop layer 20, wherein the bottom etch stop layer 20 is formed on the substrate 10, the insulating layer 11 is formed on the bottom etch stop layer 20, and at least one first cavity 40 and at least one second cavity 40' are also located on the bottom etch stop layer 20. The substrate 10 is a compound semiconductor substrate; the material constituting the plurality of sacrificial mesa (sacrificial structure 21) is an epitaxial structure.

[0158] Please see Figure 6G This is a partially enlarged cross-sectional schematic diagram of a specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. Figure 6G The structure of this invention Figure 1F , Figure 1K , Figure 2F , Figure 2I as well as Figure 5C A partially enlarged cross-sectional schematic diagram of the embodiment is shown. The bottom metal layer 30 of the bulk acoustic wave resonant structure 3 can gradually thin out at its edges in a relatively gentle manner, and the piezoelectric layer 31 of the bulk acoustic wave resonant structure 3 also gradually thins out at the edges of the bottom metal layer 30 in a relatively gentle manner. This allows the crystallization of the piezoelectric layer 31 near the edges of the bottom metal layer 30 to maintain a good state, preventing crystallization cracks or breakage. Therefore, Figure 6G In this structure, the bottom metal layer 30 gradually thins out at the edges in a relatively gentle manner, which is a preferred embodiment. In other embodiments of the present invention, the edges of the bottom metal layer 30 of the bulk acoustic resonant structure 3 (or bulk acoustic resonant structure 3', or bulk acoustic resonant structure 3") also have a similar shape. Figure 6G The structure features a bottom metal layer 30 that gradually thins in a relatively gentle manner. Please refer to [link / reference]. Figure 6H This is a partially enlarged cross-sectional schematic diagram of another specific embodiment of a method for tuning a bulk acoustic resonator for a bulk acoustic filter according to the present invention. Figure 6H The structure of this invention Figure 3L as well as Figure 5P A partially enlarged cross-sectional schematic diagram of an embodiment. In this diagram, besides the bottom metal layer 30 of the bulk acoustic resonance structure 3 gradually thinning at its edges in a relatively gentle manner, the second polishing layer 51 also gradually thins at its edges in a relatively gentle manner. In the present invention... Figure 4I as well as Figure 6F In one embodiment, the second polishing layer 51 can also gradually thin out at the edges in a relatively gentle manner.

[0159] In embodiments of the present invention, if the thickness of the frequency tuning structure 50 (or frequency tuning structure 50', or frequency tuning structure 50”) is too thick, it will affect the resonant membrane state of the bulk acoustic wave resonant structure 3 (or bulk acoustic wave resonant structure 3', or bulk acoustic wave resonant structure 3”). Therefore, the thickness of the frequency tuning structure 50 (or frequency tuning structure 50', or frequency tuning structure 50”) needs to be less than 1000 nm. In some preferred embodiments, the thickness of the frequency tuning structure 50 (or frequency tuning structure 50', or frequency tuning structure 50”) is equal to or less than 300 nm.

[0160] The above description describes specific embodiments of the present invention and the technical means used. Many changes and modifications can be derived from the disclosure or teachings herein, which can still be regarded as equivalent changes to the concept of the present invention. The effects produced do not exceed the essential spirit covered by the specification and drawings, and should all be regarded as within the technical scope of the present invention.

[0161] In summary, based on the content disclosed above, this invention can indeed achieve its intended purpose and provide a valuable invention with significant industrial applicability. Therefore, an invention patent application is hereby filed in accordance with the law.

Claims

1. A method for forming a cavity in a bulk acoustic resonator, comprising the following steps: Step A1: Form a sacrificial epitaxial mesa on a compound semiconductor substrate; Step A2: Form an insulating layer on the sacrificial epitaxial mesa and the compound semiconductor substrate, wherein the thickness of the insulating layer is greater than the height of the sacrificial epitaxial mesa; Step A3: Grind the insulating layer using a chemical mechanical planarization process to form a polished surface; Step A4: Forming a bulk acoustic resonant structure on the polished surface, wherein the bulk acoustic resonant structure is located on the sacrificial epitaxial mesa, wherein step A4 includes the following steps: Step A41: Form a bottom electrode layer on the polished surface; Step A42: Form a piezoelectric layer on the bottom electrode layer; and Step A43: Form a top electrode layer on the piezoelectric layer; and Step A5: Etch the sacrificial epitaxial mesa to form a cavity, wherein the cavity is located beneath the bulk acoustic resonant structure.

2. The method for forming a cavity of a bulk acoustic resonator according to claim 1, wherein in step A3, the insulating layer is ground such that the sacrificial epitaxial mesa is not exposed, wherein the insulating layer between the bottom electrode layer and the sacrificial epitaxial mesa forms a frequency tuning structure, wherein the frequency tuning structure has a thickness and the bulk acoustic resonator has a resonant frequency, thereby the resonant frequency of the bulk acoustic resonator can be tuned by adjusting the thickness of the frequency tuning structure.

3. The method for forming a cavity in a bulk acoustic resonator according to claim 1, further comprising the step of forming a bottom etch stop layer on the compound semiconductor substrate, wherein the sacrificial epitaxial mesa is formed on the bottom etch stop layer; wherein the sacrificial epitaxial mesa comprises a sacrificial epitaxial layer.

4. The method for forming a cavity of a bulk acoustic resonator according to claim 3, wherein (1) the compound semiconductor substrate is made of gallium arsenide; the sacrificial epitaxial layer is made of gallium arsenide; and the bottom etch stop layer is made of indium gallium phosphide; or (2) the compound semiconductor substrate is made of indium phosphide; the sacrificial epitaxial layer is made of indium gallium arsenide; and the bottom etch stop layer is made of indium phosphide.

5. The method for forming a cavity in a bulk acoustic resonator according to claim 3, wherein the sacrificial epitaxial layer has a thickness between 50 nm and 5000 nm; wherein the bottom etch stop layer has a thickness between 20 nm and 500 nm.

6. The method of forming a cavity in a bulk acoustic resonator according to claim 1, wherein the material constituting the insulating layer comprises at least one selected from the group consisting of silicon nitride, silicon oxide, and polymers.

7. The method for forming a cavity in a bulk acoustic resonator according to claim 1, wherein step A1 comprises the following steps: A sacrificial epitaxial structure is formed on the compound semiconductor substrate; as well as The sacrificial epitaxial structure is etched to form the sacrificial epitaxial structure mesa.

8. The method for forming a cavity in a bulk acoustic resonator according to claim 7, further comprising the step of forming an etch protection layer on the lower surface of the compound semiconductor substrate before etching the sacrificial epitaxial structure to form the sacrificial epitaxial structure mesa.

9. The method of forming a cavity in a bulk acoustic resonator according to claim 8, wherein the material constituting the etched protective layer comprises at least one selected from the group consisting of silicon nitride, silicon oxide, aluminum nitride, and photoresist.

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