Bulk acoustic wave filter and preparation method thereof

Abstract

The invention discloses a bulk acoustic wave filter and a preparation method thereof. The bulk acoustic wave filter comprises an input terminal, an output terminal, at least one series resonator and at least one parallel resonator; the resonator comprises a substrate, a piezoelectric structure, a passivation layer and a protection layer which are arranged in a laminated mode. The thickness of the passivation layer meets a first preset thickness, so that the center frequency of the bulk acoustic wave filter reaches a first preset frequency; the thickness of the protection layer of the series resonator meets a second preset thickness, and the thickness of the protection layer of the parallel resonator meets a third preset thickness, so that the bandwidth of the bulk acoustic wave filter reaches the target bandwidth, and the second preset thickness and the third preset thickness have a thickness difference. According to the invention, the thickness of the passivation layer of the resonator is set to meet the first preset thickness, then the thickness of the protection layer of the series resonator is set to meet the second preset thickness, and the thickness of the protection layer of the parallel resonator is set to meet the third preset thickness, so that the bandwidth of the bulk acoustic wave filter can reach the target bandwidth.

Description

Technical Field

[0001] The present invention relates to the technical field of filters, and in particular to a bulk acoustic wave filter and a preparation method thereof. Background Art

[0002] During the processing of the BAW filter, in order to make the frequency and bandwidth of the BAW filter reach the target values, the surface of the BAW filter needs to be thinned.

[0003] The prior art generally deposits a frequency modulation layer on the surface of the resonator in the BAW filter, and then thins the frequency modulation layer on the resonator to make the minimum frequency or maximum frequency of the bandwidth of the BAW filter reach a target value.

[0004] The above method can only adjust the unilateral frequency of the bandwidth of the BAW filter. The bandwidth of the BAW filter still has a divergence phenomenon, which will affect the yield of the BAW filter. Summary of the invention

[0005] The present invention provides a bulk acoustic wave filter and a preparation method thereof, so as to solve the problem that the bandwidth of the existing bulk acoustic wave filter can only achieve unilateral frequency coincidence with the target bandwidth.

[0006] In a first aspect, an embodiment of the present invention provides a bulk acoustic wave filter, the bulk acoustic wave filter comprising an input terminal, an output terminal and at least two resonators, the at least two resonators comprising at least one series resonator and at least one parallel resonator, the series resonator being connected in series between the input terminal and the output terminal, and the parallel resonator being connected between one end of the series resonator and a ground terminal;

[0007] The resonator comprises a substrate, a piezoelectric structure, a passivation layer and a protective layer;

[0008] The piezoelectric structure is arranged on one side of the substrate, the passivation layer is arranged on a side of the piezoelectric structure away from the substrate, and the protective layer is arranged on a side of the passivation layer away from the piezoelectric structure;

[0009] The thickness of the passivation layer satisfies a first preset thickness, so that the center frequency of the BAW filter reaches a first preset frequency, wherein the first preset frequency is greater than a target center frequency, and a difference between the first preset frequency and the target center frequency satisfies a first preset range;

[0010] The thickness of the protective layer of the series resonator satisfies a second preset thickness, and the thickness of the protective layer of the parallel resonator satisfies a third preset thickness, so that the bandwidth of the bulk acoustic wave filter reaches the target bandwidth, wherein there is a thickness difference between the second preset thickness and the third preset thickness.

[0011] Optionally, the resonator further includes a cavity and a release window, the cavity penetrates at least a portion of the substrate, the release window penetrates the passivation layer and the piezoelectric structure, and is connected to the cavity; the protective layer includes a first protective layer, the first protective layer includes a first protective sub-portion and a second protective sub-portion;

[0012] The first protection sub-portion is arranged on a side of the passivation layer away from the piezoelectric structure, and the second protection sub-portion covers the inner wall of the release window and the inner wall of the cavity;

[0013] The thickness of the first protection section of the series resonator meets the second preset thickness, and the thickness of the first protection section of the parallel resonator meets the third preset thickness.

[0014] Optionally, the resonator further includes a cavity and a release window, the cavity penetrates at least a portion of the substrate, the release window penetrates the passivation layer and the piezoelectric structure, and is connected to the cavity; the protective layer includes a first protective layer and a second protective layer, the first protective layer includes a first protective sub-portion and a second protective sub-portion;

[0015] The first protection sub-portion is arranged on a side of the passivation layer away from the piezoelectric structure, and the second protection sub-portion covers the inner wall of the release window and the inner wall of the cavity; the second protection layer is arranged on a side of the first protection sub-portion away from the piezoelectric structure;

[0016] The sum of the thicknesses of the first protection section and the second protection layer of the series resonator meets the second preset thickness, and the sum of the thicknesses of the first protection section and the second protection layer of the parallel resonator meets the third preset thickness.

[0017] Optionally, the resonator further comprises a seed layer;

[0018] The seed layer is disposed between the substrate and the piezoelectric structure;

[0019] The seed layer includes an aluminum nitride film or a scandium aluminum nitride film.

[0020] In a second aspect, an embodiment of the present invention provides a method for preparing a bulk acoustic wave filter, which is used to prepare the bulk acoustic wave filter as described in the first aspect, and the preparation method comprises:

[0021] preparing a substrate;

[0022] preparing a piezoelectric structure on one side of the substrate;

[0023] A passivation layer having a first preset thickness is prepared on a side of the piezoelectric structure away from the substrate, so that the center frequency of the BAW filter reaches a first preset frequency, wherein the first preset frequency is greater than a target center frequency, and a difference between the first preset frequency and the target center frequency satisfies a first preset range;

[0024] Preparing a protective layer on a side of the passivation layer away from the piezoelectric structure;

[0025] The thickness of the protective layer of the series resonator and the thickness of the protective layer of the parallel resonator are thinned according to the size relationship between the bandwidth of the BAW filter and the target bandwidth, so that the bandwidth of the BAW filter reaches the target bandwidth, wherein the thickness of the protective layer of the series resonator satisfies the second preset thickness, and the thickness of the protective layer of the parallel resonator satisfies the third preset thickness, and there is a thickness difference between the second preset thickness and the third preset thickness.

[0026] Optionally, before thinning the thickness of the protective layer of the series resonator and the thickness of the protective layer of the parallel resonator according to the relationship between the bandwidth of the BAW filter and the target bandwidth, the method further includes:

[0027] The protective layer is thinned according to the relationship between the center frequency of the BAW filter and a second preset frequency so that the center frequency of the BAW filter reaches the second preset frequency, wherein the second preset frequency is less than the target center frequency, and the difference between the second preset frequency and the target center frequency satisfies a second preset range.

[0028] Optionally, the thickness of the protective layer of the series resonator and the thickness of the protective layer of the parallel resonator are reduced according to the size relationship between the bandwidth of the BAW filter and the target bandwidth, and the preparation method includes:

[0029] If the bandwidth of the BAW filter is greater than the target bandwidth, the thickness of the protective layer of each of the resonators is reduced so that the maximum frequency of the broadband of the BAW filter reaches the maximum frequency of the target bandwidth; the thickness of the protective layer of the parallel resonator is reduced so that the minimum frequency of the broadband of the BAW filter reaches the minimum frequency of the target bandwidth;

[0030] If the bandwidth of the BAW filter is smaller than the target bandwidth, the thickness of the protective layer of each of the resonators is thinned so that the minimum frequency of the broadband of the BAW filter reaches the minimum frequency of the target bandwidth; the thickness of the protective layer of the series resonator is thinned so that the maximum frequency of the broadband of the BAW filter reaches the maximum frequency of the target bandwidth.

[0031] Optionally, after preparing the substrate, the preparation method further comprises:

[0032] Prepare a first groove on one side of the substrate, wherein the first groove penetrates at least a portion of the substrate;

[0033] preparing a sacrificial layer in the first groove;

[0034] A piezoelectric structure is prepared on one side of the substrate, comprising:

[0035] Preparing the piezoelectric structure on one side surface of the sacrificial layer and one side surface of the substrate;

[0036] After preparing a passivation layer having a first preset thickness on a side of the piezoelectric structure away from the substrate, the preparation method further comprises:

[0037] preparing a release window in the passivation layer and the piezoelectric structure, wherein the release window penetrates the passivation layer and the piezoelectric structure;

[0038] The sacrificial layer is etched through the release window to prepare a cavity in the substrate, wherein the cavity is connected to the release window.

[0039] Optionally, a protective layer is prepared on a side of the passivation layer away from the piezoelectric structure, comprising:

[0040] Using an atomic vapor deposition method to prepare a first protective sub-portion of a first protective layer on a side of the passivation layer away from the piezoelectric structure, and to prepare a second protective sub-portion of the first protective layer on an inner wall of the release window and an inner wall of the cavity;

[0041] The method further comprises: thinning the thickness of the protective layer of the series resonator and the thickness of the protective layer of the parallel resonator according to the relationship between the bandwidth of the BAW filter and the target bandwidth, comprising:

[0042] The thickness of the first guard section of the series resonator and the thickness of the first guard section of the parallel resonator are reduced according to the size relationship between the bandwidth of the BAW filter and the target bandwidth.

[0043] Optionally, a protective layer is prepared on a side of the passivation layer away from the piezoelectric structure, comprising:

[0044] Using an atomic vapor deposition method to prepare a first protective sub-portion of a first protective layer on a side of the passivation layer away from the piezoelectric structure, and to prepare a second protective sub-portion of the first protective layer on an inner wall of the release window and an inner wall of the cavity;

[0045] Using chemical vapor deposition or physical vapor deposition to prepare the second protective layer on a side of the first protective section away from the piezoelectric structure;

[0046] The method further comprises: thinning the thickness of the protective layer of the series resonator and the thickness of the protective layer of the parallel resonator according to the relationship between the bandwidth of the BAW filter and the target bandwidth, comprising:

[0047] The thickness of the second protective layer of the series resonator and the thickness of the second protective layer of the parallel resonator are reduced according to the size relationship between the bandwidth of the BAW filter and the target bandwidth.

[0048] The technical solution of the embodiment of the present invention first sets the thickness of the passivation layer of the resonator to meet the first preset thickness, then sets the thickness of the protective layer of the series resonator to meet the second preset thickness, and the thickness of the protective layer of the parallel resonator to meet the third preset thickness, thereby realizing bilateral frequency adjustment of the bandwidth of the bulk acoustic wave filter, which can make the bandwidth of the bulk acoustic wave filter reach the target bandwidth, make the bandwidth of the bulk acoustic wave filter more convergent, and help to improve the yield of the bulk acoustic wave filter.

[0049] It should be understood that the contents described in this section are not intended to identify the key or important features of the embodiments of the present invention, nor are they intended to limit the scope of the present invention. Other features of the present invention will become easily understood through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0051] Figure 1 A schematic diagram of the structure of a bulk acoustic wave filter provided by an embodiment of the present invention;

[0052] Figure 2 A top view of a resonator provided by an embodiment of the present invention;

[0053] Figure 3 for Figure 2 A schematic cross-sectional structure diagram of the provided resonator along the section line AA';

[0054] Figure 4 for Figure 2 A schematic cross-sectional structure diagram of the provided resonator along the section line BB';

[0055] Figure 5A top view of another resonator provided by an embodiment of the present invention;

[0056] Figure 6 for Figure 5 A schematic diagram of a cross-sectional structure of the provided resonator along a section line CC';

[0057] Figure 7 for Figure 5 A schematic cross-sectional structure diagram of the provided resonator along the section line D-D';

[0058] Figure 8 A flow chart of a method for preparing a bulk acoustic wave filter provided by an embodiment of the present invention;

[0059] Fig. 9 A flow chart of another method for preparing a bulk acoustic wave filter provided by an embodiment of the present invention;

[0060] Fig.10 A flow chart of another method for preparing a bulk acoustic wave filter provided by an embodiment of the present invention;

[0061] Fig.11 A schematic diagram of a manufacturing process of a piezoelectric structure, a release window, an electrode block, a cavity and a passivation layer provided in an embodiment of the present invention;

[0062] Fig.12 A flow chart of another method for preparing a bulk acoustic wave filter provided by an embodiment of the present invention;

[0063] Fig.13 A flow chart of a method for preparing another bulk acoustic wave filter provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0064] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0065] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any of their variations are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices. The orientation or positional relationship indicated by the terms "upper", "lower", "left", "right", etc. is based on the orientation or positional relationship shown in the drawings, and is only used to illustrate the relative positional relationship between the components or components, and does not particularly limit the specific installation orientation of the components or components.

[0066] Figure 1 A schematic diagram of the structure of a bulk acoustic wave filter provided by an embodiment of the present invention, referring to Figure 1 The bulk acoustic wave filter in the embodiment of the present invention includes an input terminal in, an output terminal out and at least two resonators 30, the at least two resonators 30 include at least one series resonator 301 and at least one parallel resonator 302, the series resonator 301 is connected in series between the input terminal in and the output terminal out, and the parallel resonator 302 is connected between one end of the series resonator 301 and the ground terminal GND.

[0067] Figure 2 A top view of a resonator provided by an embodiment of the present invention, Figure 3 for Figure 2 A schematic diagram of the cross-sectional structure of the resonator provided along the section line AA', Figure 4 for Figure 2 The cross-sectional structure diagram of the resonator provided along the section line BB' is shown in FIG. Figure 2 , Figure 3 and Figure 4, the resonator 30 includes a substrate 31, a piezoelectric structure 32, a passivation layer 33 and a protective layer 34. The piezoelectric structure 32 is arranged on one side of the substrate 31, the passivation layer 33 is arranged on the side of the piezoelectric structure 32 away from the substrate 31, and the protective layer 34 is arranged on the side of the passivation layer 33 away from the piezoelectric structure 32. The thickness of the passivation layer 33 satisfies the first preset thickness so that the center frequency of the bulk acoustic wave filter reaches the first preset frequency, wherein the first preset frequency is greater than the target center frequency, and the difference between the first preset frequency and the target center frequency satisfies the first preset range. The thickness of the protective layer 34 of the series resonator 301 satisfies the second preset thickness, and the thickness of the protective layer 34 of the parallel resonator 302 satisfies the third preset thickness so that the bandwidth of the bulk acoustic wave filter reaches the target bandwidth, wherein there is a thickness difference between the second preset thickness and the third preset thickness.

[0068] It should be noted that the target center frequency refers to the center frequency of the target bandwidth, that is, the average value of the minimum frequency and the maximum frequency of the target bandwidth; the center frequency of the BAW filter refers to the center frequency of the bandwidth of the BAW filter, that is, the average value of the minimum frequency and the maximum frequency of the bandwidth of the BAW filter.

[0069] It is understandable that in the preparation Figure 1 When the BAW filter is shown, the substrate 31 of each resonator 30 is first prepared, and then the piezoelectric structure 32, the passivation layer 33 and the protective layer 34 are sequentially prepared on the substrate 31. After preparing the passivation layer 33, the center frequency of the bandwidth of the BAW filter can be made to reach a first preset frequency greater than the target center frequency by setting the thickness of the passivation layer 33 to meet the first preset thickness, wherein the difference between the first preset frequency and the target center frequency meets the first preset range, and the first preset range can be 100MHz-400MHz. Then, after preparing the protective layer 34, the bandwidth of the BAW filter can be made to reach the target bandwidth by setting the thickness of the protective layer 34 of each series resonator 301 to meet the second preset thickness and the thickness of the protective layer 34 of the parallel resonator 302 to meet the third preset thickness, that is, the minimum frequency of the bandwidth of the BAW filter reaches the minimum frequency of the target bandwidth, and the maximum frequency of the bandwidth of the BAW filter reaches the maximum frequency of the target bandwidth.

[0070] It should be noted that the embodiment of the present invention does not limit the specific values ​​of the first preset thickness, the second preset thickness and the third preset thickness, and those skilled in the art can set them according to actual conditions.

[0071] The embodiment of the present invention first sets the thickness of the passivation layer 33 of the resonator 30 to meet the first preset thickness, then sets the thickness of the protective layer 34 of the series resonator 301 to meet the second preset thickness, and the thickness of the protective layer 34 of the parallel resonator 302 to meet the third preset thickness, thereby realizing bilateral frequency adjustment of the bandwidth of the BAW filter, which can make the bandwidth of the BAW filter reach the target bandwidth, make the bandwidth of the BAW filter more convergent, and help to improve the yield of the BAW filter.

[0072] Based on the above embodiments, continue to refer to Figure 2 , Figure 3 and Figure 4 The resonator 30 in the embodiment of the present invention further includes a cavity 35 and a release window 36. The cavity 35 penetrates at least a portion of the substrate 31. The release window 36 penetrates the passivation layer 33 and the piezoelectric structure 32 and is connected to the cavity 35. The protective layer 34 includes a first protective layer 341. The first protective layer 341 includes a first protective sub-portion 341A and a second protective sub-portion 341B. The first protective sub-portion 341A is arranged on a side of the passivation layer 33 away from the piezoelectric structure 32, and the second protective sub-portion 341B covers the inner wall of the release window 36 and the inner wall of the cavity 35. The thickness of the first protective sub-portion 341A of the series resonator 301 meets the second preset thickness, and the thickness of the first protective sub-portion 341A of the parallel resonator 302 meets the third preset thickness.

[0073] Exemplarily, the protective layer in the embodiment of the present invention includes only one first protective layer 341. As a feasible implementation, the first protective layer 341 can be at least one of a silicon oxide film, an aluminum oxide film and a silicon nitride film generated by an atomic vapor deposition (ALD) method, and the silicon oxide film, the aluminum oxide film and the silicon nitride film have the characteristics of water isolation and resistance to corrosion by developer.

[0074] On the one hand, the first protective layer 341 is a water-proof film. By arranging the first protective sub-portion 341A of the first protective layer 341 to be located on the side of the passivation layer 33 away from the piezoelectric structure 32, and the second protective sub-portion 341B to cover the inner wall of the release window 36 and the inner wall of the cavity 35, the resonator 30 can be protected from the influence of water vapor and the reliability of the resonator 30 can be enhanced. On the other hand, the first protective layer 341 is a film resistant to corrosion by a developer, and the first protective sub-portion 341A of the first protective layer 341 of all resonators 30 in the bulk acoustic wave filter can be mask-thinned, and then the thickness of the first protective sub-portion 341A of each series resonator 301 can be achieved by mask-thinning to meet the second preset thickness and the thickness of the first protective sub-portion 341A of the parallel resonator 302 can be achieved to meet the third preset thickness, so that the bandwidth of the bulk acoustic wave filter reaches the target bandwidth, and the bandwidth of the bulk acoustic wave filter is more converged, which is conducive to improving the yield of the filter.

[0075] Figure 5 A top view of another resonator provided by an embodiment of the present invention, Figure 6 for Figure 5 A schematic diagram of the cross-sectional structure of the resonator provided along the section line CC', Figure 7 for Figure 5 The cross-sectional structure diagram of the resonator provided along the section line D-D' is shown in FIG. Figure 5 , Figure 6 and Figure 7 The resonator 30 further includes a cavity 35 and a release window 36. The cavity 35 penetrates at least a portion of the substrate 31. The release window 36 penetrates the passivation layer 33 and the piezoelectric structure 32 and is in communication with the cavity 35. The protective layer 34 includes a first protective layer 341 and a second protective layer 342. The first protective layer 341 includes a first protective sub-portion 341A and a second protective sub-portion 341B. The first protective sub-portion 341A is disposed on a side of the passivation layer 33 away from the piezoelectric structure 32, and the second protective sub-portion 341B covers the inner wall of the release window 36 and the inner wall of the cavity 35. The second protective layer 342 is disposed on a side of the first protective sub-portion 341A away from the piezoelectric structure 32. The sum of the thicknesses of the first protective sub-portion 341A and the second protective layer 342 of the series resonator 301 meets the second preset thickness, and the sum of the thicknesses of the first protective sub-portion 341A and the second protective layer 342 of the parallel resonator 302 meets the third preset thickness.

[0076] Exemplarily, the protective layer 34 in the embodiment of the present invention includes a first protective layer 341 and a second protective layer 342. As a feasible implementation, the first protective layer 341 can be at least one of a silicon oxide film, an aluminum oxide film and a silicon nitride film generated by an atomic vapor deposition (ALD) method. The second protective layer 342 can be a silicon oxide film generated by a chemical vapor deposition (CVD) or a physical vapor deposition (PVD) method. The silicon oxide film has the characteristic of resisting corrosion by a developer, and the second protective layer 342 of all resonators 30 in the bulk acoustic wave filter can be mask thinned, and then the sum of the thickness of the first protective section 341A and the second protective layer 342 of each series resonator 301 can be achieved by mask thinning to meet the second preset thickness and the sum of the thickness of the first protective section 341A and the second protective layer 342 of the parallel resonator 302 can meet the third preset thickness, so that the bandwidth of the bulk acoustic wave filter reaches the target bandwidth, the bandwidth of the bulk acoustic wave filter is more converged, and it is beneficial to improve the yield of the filter.

[0077] Compared with the solution of only setting a first protective layer 341, the present invention implements a protective layer 34 including a first protective layer 341 and a second protective layer 342. The presence of the second protective layer 342 can reduce the thickness of the first protective layer 341, thereby helping to reduce the time and cost of depositing the first protective layer 341 using the atomic vapor deposition (ALD) method.

[0078] Based on the above embodiments, Figure 3 , Figure 4 , Figure 6 and Figure 7 The resonator 30 further includes a seed layer 37. The seed layer 37 is disposed between the substrate 31 and the piezoelectric structure 32. The seed layer 37 includes an aluminum nitride film or a scandium aluminum nitride film.

[0079] Exemplarily, the seed layer 37 is disposed between the substrate 31 and the piezoelectric structure 32, which can protect the piezoelectric structure 32 during the preparation of the bulk acoustic wave filter, and can induce the growth of the bottom electrode layer 321 in the piezoelectric structure 32, thereby improving the growth of the piezoelectric layer 322 in the piezoelectric structure 32. The seed layer 37 can be an aluminum nitride film or a scandium aluminum nitride film formed by physical vapor deposition (PVD), or an aluminum nitride film or a scandium aluminum nitride film formed by metal organic chemical vapor deposition (MOCVD).

[0080] Optional, reference Figure 3 , Figure 4 , Figure 6 and Figure 7 The resonator in the embodiment of the present invention further includes an electrode block layer 38, which includes a first electrode block 381 and a second electrode block 382. The piezoelectric structure 32 includes a bottom electrode layer 321, a piezoelectric layer 322 and a top electrode layer 323, and the top electrode layer 323 includes a first top electrode subdivision 323A and a second top electrode subdivision 323B.

[0081] Specifically, the bottom electrode layer 321 is arranged on the side of the seed layer 37 away from the substrate 31. The piezoelectric layer 322 is arranged on the side of the bottom electrode layer 321 away from the substrate 31 and the side of the seed layer 37 away from the substrate 31. The first top electrode division 323A penetrates the piezoelectric layer 322 and is connected to the bottom electrode layer 321. The second top electrode division 323B is arranged on the side of the piezoelectric layer 322 away from the substrate 31. The first electrode block division 381 is arranged on the side of the first top electrode division 323A away from the bottom electrode layer 321. The second electrode block division 382 is arranged on the side of the second top electrode division 323B away from the piezoelectric layer 322. The passivation layer 33 is arranged on the side of the second top electrode division 323B away from the piezoelectric layer 322 and the side of the piezoelectric layer 322 away from the substrate 31. The release window 36 penetrates the passivation layer 33 and the piezoelectric layer 322 and is connected to the cavity 35. The protection layer 34 is disposed on a side of the passivation layer 33 away from the piezoelectric layer 322 , a surface of the release window in contact with the external air, and inner walls of the release window 36 and the cavity 35 .

[0082] The embodiment of the present invention further provides a method for preparing a bulk acoustic wave filter, which is used to prepare the bulk acoustic wave filter described in the above embodiment. Figure 8 A flowchart of a method for preparing a bulk acoustic wave filter provided by an embodiment of the present invention, referring to Figure 8 The method for preparing the bulk acoustic wave filter in the embodiment of the present invention includes:

[0083] S110, preparing a substrate.

[0084] S120, preparing a piezoelectric structure on one side of the substrate.

[0085] S130. Prepare a passivation layer having a first preset thickness on a side of the piezoelectric structure away from the substrate, so that the center frequency of the bulk acoustic wave filter reaches a first preset frequency, wherein the first preset frequency is greater than the target center frequency, and the difference between the first preset frequency and the target center frequency satisfies a first preset range.

[0086] S140, preparing a protective layer on a side of the passivation layer away from the piezoelectric structure.

[0087] S150. Thinning the thickness of the protective layer of the series resonator and the thickness of the protective layer of the parallel resonator according to the relationship between the bandwidth of the BAW filter and the target bandwidth, so that the bandwidth of the BAW filter reaches the target bandwidth, wherein the thickness of the protective layer of the series resonator satisfies the second preset thickness, and the thickness of the protective layer of the parallel resonator satisfies the third preset thickness, and there is a thickness difference between the second preset thickness and the third preset thickness.

[0088] Exemplarily, by adopting the above steps S120-S150, the following can be obtained: Figure 1 The BAW filter shown in Figure 1The series resonator 301 and the parallel resonator 302 in the BAW filter are shown in FIG. Figure 2-Figure 7 As shown, it includes a substrate 31, a piezoelectric structure 32, a passivation layer 33 and a protective layer 34. The piezoelectric structure 32 is arranged on one side of the substrate 31, the passivation layer 33 is arranged on the side of the piezoelectric structure 32 away from the substrate 31, and the protective layer 34 is arranged on the side of the passivation layer 33 away from the piezoelectric structure 32.

[0089] The embodiment of the present invention first sets the thickness of the passivation layer 33 of the resonator 30 to meet the first preset thickness, then sets the thickness of the protective layer 34 of the series resonator 301 to meet the second preset thickness, and the thickness of the protective layer 34 of the parallel resonator 302 to meet the third preset thickness, thereby realizing bilateral frequency adjustment of the bandwidth of the BAW filter, which can make the bandwidth of the BAW filter reach the target bandwidth, make the bandwidth of the BAW filter more convergent, and help to improve the yield of the BAW filter.

[0090] Fig. 9 A flowchart of another method for preparing a bulk acoustic wave filter provided by an embodiment of the present invention, Fig. 9 The embodiment shown enriches the process of the preparation method of the bulk acoustic wave filter, referring to Fig. 9 , the method for preparing the bulk acoustic wave filter in the embodiment of the present invention comprises:

[0091] S210, preparing a substrate.

[0092] S220 , preparing a piezoelectric structure on one side of the substrate.

[0093] S230. Prepare a passivation layer having a first preset thickness on a side of the piezoelectric structure away from the substrate so that the center frequency of the bulk acoustic wave filter reaches a first preset frequency, wherein the first preset frequency is greater than the target center frequency, and the difference between the first preset frequency and the target center frequency satisfies a first preset range.

[0094] S240, preparing a protective layer on a side of the passivation layer away from the piezoelectric structure.

[0095] S250. Thinning the protective layer according to the relationship between the center frequency of the BAW filter and the second preset frequency so that the center frequency of the BAW filter reaches the second preset frequency, wherein the second preset frequency is less than the target center frequency, and the difference between the second preset frequency and the target center frequency satisfies a second preset range.

[0096] Exemplarily, the second preset range may be 100 MHz-300 MHz.

[0097] S260. Thinning the thickness of the protective layer of the series resonator and the thickness of the protective layer of the parallel resonator according to the relationship between the bandwidth of the BAW filter and the target bandwidth so that the bandwidth of the BAW filter reaches the target bandwidth, wherein the thickness of the protective layer of the series resonator satisfies the second preset thickness, and the thickness of the protective layer of the parallel resonator satisfies the third preset thickness, and there is a thickness difference between the second preset thickness and the third preset thickness.

[0098] As a feasible implementation method, the thickness of the protective layer of the series resonator and the thickness of the protective layer of the parallel resonator are thinned according to the relationship between the bandwidth of the bulk acoustic wave filter and the target bandwidth. The preparation method includes:

[0099] If the bandwidth of the BAW filter is greater than the target bandwidth, the thickness of the protective layer of each resonator is reduced so that the maximum frequency of the broadband of the BAW filter reaches the maximum frequency of the target bandwidth. The thickness of the protective layer of the parallel resonator is reduced so that the minimum frequency of the broadband of the BAW filter reaches the minimum frequency of the target bandwidth.

[0100] If the bandwidth of the BAW filter is less than the target bandwidth, the thickness of the protective layer of each resonator is reduced so that the minimum frequency of the broadband of the BAW filter reaches the minimum frequency of the target bandwidth. The thickness of the protective layer of the series resonator is reduced so that the maximum frequency of the broadband of the BAW filter reaches the maximum frequency of the target bandwidth.

[0101] Fig.10 A flowchart of a method for preparing a bulk acoustic wave filter provided by an embodiment of the present invention, Fig.10 The embodiment shown not only enriches the process of the preparation method of the bulk acoustic wave filter, but also describes in detail how to prepare the piezoelectric structure on one side of the substrate and how to prepare the protective layer on the side of the passivation layer away from the piezoelectric structure. Fig.10 , the method for preparing the bulk acoustic wave filter in the embodiment of the present invention comprises:

[0102] S310, preparing a substrate.

[0103] S320, preparing a first groove on one side of the substrate, wherein the first groove penetrates at least a portion of the substrate.

[0104] For example, Fig.11 A schematic diagram of a manufacturing process of a piezoelectric structure, a release window, an electrode block, a cavity and a passivation layer provided in an embodiment of the present invention, with reference to Fig.11 As shown in FIG. 2 (a), a first groove 201 is prepared on one side of the substrate 31 , including: etching the first groove 201 penetrating at least a portion of the substrate 31 on one side of the substrate 31 .

[0105] S330 , preparing a sacrificial layer in the first groove.

[0106] Exemplary, reference Fig.11 As shown in FIG. 2( b ), a sacrificial layer 39 is prepared in the first groove 201 , including: depositing the sacrificial layer 39 in the first groove 201 of the substrate 31 , and grinding the sacrificial layer 39 by chemical mechanical planarization (CMP) so that the surface of the sacrificial layer 39 is in the same plane as the surface of the substrate 31 .

[0107] S340, preparing a piezoelectric structure on one side surface of the sacrificial layer and one side surface of the substrate.

[0108] Exemplary, reference Figure 3 , Figure 4 , Figure 6 and Figure 7 The resonator 30 of the BAW filter in the embodiment of the present invention further includes a seed layer 37. The piezoelectric structure 32 includes a bottom electrode layer 321, a piezoelectric layer 322 and a top electrode layer 323, and the top electrode layer 323 includes a first top electrode portion 323A and a second top electrode portion 323B.

[0109] Specifically, the seed layer 37 is disposed between the substrate 31 and the piezoelectric structure 32. The bottom electrode layer 321 is disposed on the side of the seed layer 37 away from the substrate 31. The piezoelectric layer 322 is disposed on the side of the bottom electrode layer 321 away from the substrate 31 and on the side of the seed layer 37 away from the substrate 31. The first top electrode portion 323A penetrates the piezoelectric layer 322 and is connected to the bottom electrode layer 321. The second top electrode portion 323B is disposed on the side of the piezoelectric layer 322 away from the substrate 31.

[0110] Therefore, reference Fig.11 In FIG. (c), before preparing the piezoelectric structure on one side surface of the sacrificial layer and one side surface of the substrate, the method for preparing the BAW filter in the embodiment of the present invention further includes: preparing a seed layer 37 on one side surface of the sacrificial layer 39 and one side surface of the substrate 31. Specifically, preparing the seed layer 37 on one side surface of the sacrificial layer 39 and one side surface of the substrate 31 includes: generating an aluminum nitride film or a scandium aluminum nitride film on one side surface of the sacrificial layer 39 and one side surface of the substrate 31 by physical vapor deposition (PVD) or metal organic chemical vapor deposition (MOCVD).

[0111] Continue to refer Fig.11 As shown in Figures (c), (d) and (e), a piezoelectric structure 32 is prepared on one surface of the sacrificial layer 39 and one surface of the substrate 31, including: preparing a piezoelectric structure on the side of the seed layer 37 away from the substrate 31 and on the side of the seed layer 31 away from the sacrificial layer 39.

[0112] Specifically, refer to Fig.11In (c), (d) and (e), the piezoelectric structure is prepared on the side of the seed layer 37 away from the substrate 31 and the side of the seed layer 31 away from the sacrificial layer 39, including: depositing a bottom electrode layer 321 on the side of the seed layer 37 away from the substrate 31 and the side of the seed layer 31 away from the sacrificial layer 39, and patterning the bottom electrode layer 321; depositing a piezoelectric layer 322 on the side of the bottom electrode layer 321 away from the substrate 31 and the side of the seed layer 37 away from the substrate 31, and patterning the piezoelectric layer 322 to etch a second groove 202; depositing a piezoelectric layer 322 on the side of the piezoelectric layer 322 away from the substrate A top electrode layer 32 is deposited on one side of the bottom 31 and on the inner wall of the second groove 202 and is patterned, wherein the top electrode layer 32 includes a first top electrode portion 323A and a second top electrode portion 323B, the first top electrode portion 323A penetrates the piezoelectric layer 322 and is connected to the bottom electrode layer 321 for leading out the bottom electrode layer 321, the second top electrode portion 323B is arranged on a side of the piezoelectric layer 322 away from the substrate 31, the second top electrode portion 323B, the piezoelectric layer 322 and the bottom electrode layer 321 at least partially overlap, and the overlapping area is the working area of ​​the resonator 30.

[0113] S350. Prepare a passivation layer having a first preset thickness on a side of the piezoelectric structure away from the substrate, so that the center frequency of the bulk acoustic wave filter reaches a first preset frequency, wherein the first preset frequency is greater than the target center frequency, and the difference between the first preset frequency and the target center frequency satisfies a first preset range.

[0114] Exemplary, reference Fig.11 In FIG. (f), a passivation layer 33 having a first preset thickness is prepared on the side of the piezoelectric structure 32 away from the substrate 11, including: depositing a passivation layer 109 on the side of the piezoelectric structure 32 away from the substrate 11, and patterning the passivation layer 109 to etch a third groove 203 and a fourth groove 204, wherein the third groove 203 exposes the first top electrode portion 323A, and the fourth groove 204 exposes a portion of the second top electrode portion 323B. It should be noted that the passivation layer 109 is insulating.

[0115] refer to Figure 3 , Figure 4 , Figure 6 and Figure 7 The resonator 30 of the BAW filter in the embodiment of the present invention further includes an electrode block layer 38, and the electrode block layer 38 includes a first electrode block 381 and a second electrode block 382. Specifically, the first electrode block section 381 is arranged on a side of the first top electrode section 323A away from the bottom electrode layer 321, and the second electrode block section 382 is arranged on a side of the second top electrode section 323B away from the piezoelectric layer 322.

[0116] Therefore, reference Fig.11In Figure (g), after preparing a passivation layer 33 that meets the first preset thickness on the side of the piezoelectric structure 32 away from the substrate 31, the preparation method of the bulk acoustic wave filter in the embodiment of the present invention also includes: depositing an electrode block layer 38 on the side of the passivation layer 33 away from the piezoelectric layer 322, on the inner wall of the third groove 203, and on the inner wall of the fourth groove 204, and patterning to form a first electrode block section 381 and a second electrode block section 382, ​​wherein the first electrode block section 381 is arranged on the side of the first top electrode section 323A away from the bottom electrode layer 321, and is connected to the bottom electrode 321 through the first top electrode section 323A, and the second electrode block 382 is arranged on the side of the second top electrode section 323B away from the piezoelectric layer 322, and is connected to the second top electrode section 323B.

[0117] S360, preparing a release window in the passivation layer and the piezoelectric structure, wherein the release window penetrates the passivation layer and the piezoelectric structure.

[0118] Exemplary, reference Fig.11 As shown in FIG. (h2), a release window 36 is prepared in the passivation layer 33 and the piezoelectric structure 32, including: etching the passivation layer 33 and the piezoelectric layer 322 in the piezoelectric structure 32 to form a release window 36 penetrating the passivation layer 33 and the piezoelectric layer 322. It should be noted that the release window 36 overlaps with the sacrificial layer 39 (cavity 35), but does not overlap with the bottom electrode layer 321 and the top electrode layer 323.

[0119] S370, etching the sacrificial layer through the release window to prepare a cavity in the substrate, wherein the cavity is connected to the release window.

[0120] Exemplary, reference Fig.11 In Figures (h1) and (h2), the sacrificial layer 39 is etched through the release window 36 to prepare a cavity 35 in the substrate 31, including: supplying VHF (hydrogen fluoride gas) to the sacrificial layer 39 through the release window 36 to release the sacrificial layer 39 and the cavity 35, and the cavity 35 is connected to the release window 36.

[0121] S380, preparing a protective layer on a side of the passivation layer away from the piezoelectric structure.

[0122] S390. Thinning the thickness of the protective layer of the series resonator and the thickness of the protective layer of the parallel resonator according to the relationship between the bandwidth of the BAW filter and the target bandwidth so that the bandwidth of the BAW filter reaches the target bandwidth, wherein the thickness of the protective layer of the series resonator satisfies the second preset thickness, the thickness of the protective layer of the parallel resonator satisfies the third preset thickness, and there is a thickness difference between the second preset thickness and the third preset thickness.

[0123] Fig.12 A flowchart of a method for preparing a bulk acoustic wave filter provided by an embodiment of the present invention, Fig.12 A detailed description is given on how to prepare a protective layer on the side of the passivation layer away from the piezoelectric structure, and how to reduce the thickness of the protective layer of the series resonator and the thickness of the protective layer of the parallel resonator according to the relationship between the bandwidth of the BAW filter and the target bandwidth. Fig.12 The method for preparing the bulk acoustic wave filter in the embodiment of the present invention includes:

[0124] S410, preparing a substrate.

[0125] S420 , preparing a first groove on one side of the substrate, wherein the first groove penetrates at least a portion of the substrate.

[0126] S430 , preparing a sacrificial layer in the first groove.

[0127] S440, preparing a piezoelectric structure on one side surface of the sacrificial layer and one side surface of the substrate.

[0128] S450. Prepare a passivation layer having a first preset thickness on a side of the piezoelectric structure away from the substrate, so that the center frequency of the bulk acoustic wave filter reaches a first preset frequency, wherein the first preset frequency is greater than the target center frequency, and the difference between the first preset frequency and the target center frequency satisfies a first preset range.

[0129] S460, preparing a release window in the passivation layer and the piezoelectric structure, wherein the release window penetrates the passivation layer and the piezoelectric structure.

[0130] S470, etching the sacrificial layer through the release window to prepare a cavity in the substrate, wherein the cavity is connected to the release window.

[0131] S480, using an atomic vapor deposition method to prepare a first protective portion of a first protective layer on a side of the passivation layer away from the piezoelectric structure, and to prepare a second protective portion of the first protective layer on an inner wall of the release window and an inner wall of the cavity.

[0132] S490. Thinning the thickness of the first protection section of the series resonator and the thickness of the first protection section of the parallel resonator according to the relationship between the bandwidth of the BAW filter and the target bandwidth, wherein the thickness of the first protection section of the series resonator satisfies the second preset thickness, and the thickness of the first protection section of the parallel resonator satisfies the third preset thickness.

[0133] Exemplarily, the resonator in the BAW filter obtained through steps S410-S490 is as follows: Figure 2 , Figure 3 and Figure 4 shown.

[0134] Fig.13 A flowchart of a method for preparing a bulk acoustic wave filter provided by an embodiment of the present invention, Fig.13A detailed description is given on how to prepare a protective layer on the side of the passivation layer away from the piezoelectric structure, and how to reduce the thickness of the protective layer of the series resonator and the thickness of the protective layer of the parallel resonator according to the relationship between the bandwidth of the BAW filter and the target bandwidth. Fig.13 The method for preparing the bulk acoustic wave filter in the embodiment of the present invention includes:

[0135] S510, preparing a substrate.

[0136] S520 , preparing a first groove on one side of the substrate, wherein the first groove penetrates at least a portion of the substrate.

[0137] S530 , preparing a sacrificial layer in the first groove.

[0138] S540, preparing a piezoelectric structure on one side surface of the sacrificial layer and one side surface of the substrate.

[0139] S550. Prepare a passivation layer having a first preset thickness on a side of the piezoelectric structure away from the substrate, so that the center frequency of the bulk acoustic wave filter reaches a first preset frequency, wherein the first preset frequency is greater than the target center frequency, and the difference between the first preset frequency and the target center frequency satisfies a first preset range.

[0140] S560, preparing a release window in the passivation layer and the piezoelectric structure, wherein the release window penetrates the passivation layer and the piezoelectric structure.

[0141] S570, etching the sacrificial layer through the release window to prepare a cavity in the substrate, wherein the cavity is connected to the release window.

[0142] S580, using an atomic vapor deposition method to prepare a first protective portion of a first protective layer on a side of the passivation layer away from the piezoelectric structure, and to prepare a second protective portion of the first protective layer on an inner wall of the release window and an inner wall of the cavity.

[0143] S590, preparing a second protective layer on a side of the first protective portion away from the piezoelectric structure by using a chemical vapor deposition method or a physical vapor deposition method.

[0144] S5100. Thin the thickness of the second protective layer of the series resonator and the thickness of the second protective layer of the parallel resonator according to the relationship between the bandwidth of the BAW filter and the target bandwidth, wherein the sum of the thickness of the first protective section and the second protective layer of the series resonator satisfies the second preset thickness, and the sum of the thickness of the first protective section and the second protective layer of the parallel resonator satisfies the third preset thickness.

[0145] Exemplarily, the resonator in the BAW filter obtained through steps S410-S490 is as follows: Figure 5 , Figure 6 and Figure 7 shown.

[0146] The above specific implementations do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A bulk acoustic wave filter, characterized in that: The BAW filter comprises an input terminal, an output terminal and at least two resonators, wherein the at least two resonators comprise at least one series resonator and at least one parallel resonator, wherein the series resonator is connected in series between the input terminal and the output terminal, and the parallel resonator is connected between one end of the series resonator and a ground terminal; The resonator comprises a substrate, a piezoelectric structure, a passivation layer and a protective layer; The piezoelectric structure is arranged on one side of the substrate, the passivation layer is arranged on a side of the piezoelectric structure away from the substrate, and the protective layer is arranged on a side of the passivation layer away from the piezoelectric structure; The thickness of the passivation layer satisfies a first preset thickness, so that the center frequency of the BAW filter reaches a first preset frequency, wherein the first preset frequency is greater than a target center frequency, and a difference between the first preset frequency and the target center frequency satisfies a first preset range; The thickness of the protective layer of the series resonator satisfies a second preset thickness, and the thickness of the protective layer of the parallel resonator satisfies a third preset thickness, so that the bandwidth of the bulk acoustic wave filter reaches the target bandwidth, wherein there is a thickness difference between the second preset thickness and the third preset thickness.

2. The bulk acoustic wave filter according to claim 1, characterized in that The resonator further comprises a cavity and a release window, wherein the cavity penetrates at least a portion of the substrate, and the release window penetrates the passivation layer and the piezoelectric structure and communicates with the cavity; the protective layer comprises a first protective layer, and the first protective layer comprises a first protective sub-portion and a second protective sub-portion; The first protection sub-portion is arranged on a side of the passivation layer away from the piezoelectric structure, and the second protection sub-portion covers the inner wall of the release window and the inner wall of the cavity; The thickness of the first protection section of the series resonator meets the second preset thickness, and the thickness of the first protection section of the parallel resonator meets the third preset thickness.

3. The bulk acoustic wave filter according to claim 1, wherein: The resonator further comprises a cavity and a release window, wherein the cavity penetrates at least a portion of the substrate, and the release window penetrates the passivation layer and the piezoelectric structure and communicates with the cavity; the protective layer comprises a first protective layer and a second protective layer, wherein the first protective layer comprises a first protective sub-portion and a second protective sub-portion; The first protection sub-portion is arranged on a side of the passivation layer away from the piezoelectric structure, and the second protection sub-portion covers the inner wall of the release window and the inner wall of the cavity; the second protection layer is arranged on a side of the first protection sub-portion away from the piezoelectric structure; The sum of the thicknesses of the first protection section and the second protection layer of the series resonator meets the second preset thickness, and the sum of the thicknesses of the first protection section and the second protection layer of the parallel resonator meets the third preset thickness.

4. The bulk acoustic wave filter according to claim 1, wherein: The resonator further comprises a seed layer; The seed layer is disposed between the substrate and the piezoelectric structure; The seed layer includes an aluminum nitride film or a scandium aluminum nitride film.

5. A method for preparing a bulk acoustic wave filter, for preparing the bulk acoustic wave filter according to any one of claims 1 to 4, characterized in that: The preparation method comprises: preparing a substrate; preparing a piezoelectric structure on one side of the substrate; A passivation layer having a first preset thickness is prepared on a side of the piezoelectric structure away from the substrate, so that the center frequency of the BAW filter reaches a first preset frequency, wherein the first preset frequency is greater than a target center frequency, and a difference between the first preset frequency and the target center frequency satisfies a first preset range; Preparing a protective layer on a side of the passivation layer away from the piezoelectric structure; The thickness of the protective layer of the series resonator and the thickness of the protective layer of the parallel resonator are thinned according to the size relationship between the bandwidth of the BAW filter and the target bandwidth, so that the bandwidth of the BAW filter reaches the target bandwidth, wherein the thickness of the protective layer of the series resonator satisfies the second preset thickness, and the thickness of the protective layer of the parallel resonator satisfies the third preset thickness, and there is a thickness difference between the second preset thickness and the third preset thickness.

6. The preparation method according to claim 5, characterized in that: Before thinning the thickness of the protective layer of the series resonator and the thickness of the protective layer of the parallel resonator according to the size relationship between the bandwidth of the BAW filter and the target bandwidth, the method further includes: The protective layer is thinned according to the relationship between the center frequency of the BAW filter and a second preset frequency so that the center frequency of the BAW filter reaches the second preset frequency, wherein the second preset frequency is less than the target center frequency, and the difference between the second preset frequency and the target center frequency satisfies a second preset range.

7. The preparation method according to claim 6, characterized in that: The thickness of the protective layer of the series resonator and the thickness of the protective layer of the parallel resonator are reduced according to the size relationship between the bandwidth of the bulk acoustic wave filter and the target bandwidth, and the preparation method includes: If the bandwidth of the BAW filter is greater than the target bandwidth, the thickness of the protective layer of each of the resonators is reduced so that the maximum frequency of the broadband of the BAW filter reaches the maximum frequency of the target bandwidth; the thickness of the protective layer of the parallel resonator is reduced so that the minimum frequency of the broadband of the BAW filter reaches the minimum frequency of the target bandwidth; If the bandwidth of the BAW filter is smaller than the target bandwidth, the thickness of the protective layer of each of the resonators is thinned so that the minimum frequency of the broadband of the BAW filter reaches the minimum frequency of the target bandwidth; the thickness of the protective layer of the series resonator is thinned so that the maximum frequency of the broadband of the BAW filter reaches the maximum frequency of the target bandwidth.

8. The preparation method according to claim 5, characterized in that: After preparing the substrate, the preparation method further comprises: Prepare a first groove on one side of the substrate, wherein the first groove penetrates at least a portion of the substrate; preparing a sacrificial layer in the first groove; A piezoelectric structure is prepared on one side of the substrate, comprising: Preparing the piezoelectric structure on one side surface of the sacrificial layer and one side surface of the substrate; After preparing a passivation layer having a first preset thickness on a side of the piezoelectric structure away from the substrate, the preparation method further comprises: preparing a release window in the passivation layer and the piezoelectric structure, wherein the release window penetrates the passivation layer and the piezoelectric structure; The sacrificial layer is etched through the release window to prepare a cavity in the substrate, wherein the cavity is in communication with the release window.

9. The preparation method according to claim 8, characterized in that: A protective layer is prepared on a side of the passivation layer away from the piezoelectric structure, comprising: Using an atomic vapor deposition method to prepare a first protective sub-portion of a first protective layer on a side of the passivation layer away from the piezoelectric structure, and to prepare a second protective sub-portion of the first protective layer on an inner wall of the release window and an inner wall of the cavity; The method further comprises: thinning the thickness of the protective layer of the series resonator and the thickness of the protective layer of the parallel resonator according to the relationship between the bandwidth of the BAW filter and the target bandwidth, comprising: The thickness of the first guard section of the series resonator and the thickness of the first guard section of the parallel resonator are reduced according to the size relationship between the bandwidth of the BAW filter and the target bandwidth.

10. The preparation method according to claim 8, characterized in that: A protective layer is prepared on a side of the passivation layer away from the piezoelectric structure, comprising: Using an atomic vapor deposition method to prepare a first protective sub-portion of a first protective layer on a side of the passivation layer away from the piezoelectric structure, and to prepare a second protective sub-portion of the first protective layer on an inner wall of the release window and an inner wall of the cavity; Using chemical vapor deposition or physical vapor deposition to prepare the second protective layer on a side of the first protective section away from the piezoelectric structure; The method further comprises: thinning the thickness of the protective layer of the series resonator and the thickness of the protective layer of the parallel resonator according to the relationship between the bandwidth of the BAW filter and the target bandwidth, comprising: The thickness of the second protective layer of the series resonator and the thickness of the second protective layer of the parallel resonator are reduced according to the size relationship between the bandwidth of the BAW filter and the target bandwidth.

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