Bulk acoustic wave filter and preparation method thereof

Abstract

The invention discloses a bulk acoustic wave filter, which is characterized in that the bulk acoustic wave filter comprises a plurality of series resonators and a plurality of parallel resonators, the resonant frequencies of the series resonators and the parallel resonators are different, and the series resonators or the parallel resonators have at least two different resonant frequencies; wherein the series resonator comprises a substrate, and a Bragg reflection layer, a first electrode layer, a piezoelectric layer, a second electrode layer and a passivation layer which are positioned on the substrate; the parallel resonator comprises a substrate, and a Bragg reflection layer, a first electrode layer, a piezoelectric layer, a first load layer, a second electrode layer and a passivation layer which are positioned on the substrate; the at least one series resonator or the at least one parallel resonator also includes a second load layer between the second electrode layer and the passivation layer. The bandwidth and out-of-band rejection performance of the filter can be improved.

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] The ladder filter is a device that uses a bulk acoustic wave resonator (BAW) to achieve signal filtering. It has the advantages of small size, high frequency, low loss, and good performance. It is widely used in wireless communications, radio frequency (RF) and microwave technologies, especially in RF front-end modules (RF Front-End Modules) in mobile communication devices.

[0003] Ladder filters use bulk acoustic wave resonators to construct a series of series and parallel resonator structures to achieve signal filtering within a specific frequency range. The series resonator is directly connected to the signal path and is responsible for setting the passband of the filter. Since the resonant frequency f0 of the series resonator is usually slightly higher than the resonant frequency f1 of the parallel resonator, the series resonator can allow the signal to pass smoothly within the operating frequency range of the filter while blocking lower frequency signals. Parallel resonators are usually connected to the ground and connected in parallel between series resonators. Their resonant frequency is slightly lower than that of the series resonator and is mainly used to suppress unwanted frequency components and prevent them from passing through the filter.

[0004] Traditional ladder filters usually have only two resonant frequencies: the resonant frequency f0 of the series resonator and the resonant frequency f1 of the parallel resonator. As the requirements for filter performance increase, in order to expand the bandwidth and improve the out-of-band suppression performance, a small resonant frequency is introduced into the series resonator or the parallel resonator. For example, a parallel resonator with a resonant frequency of f2 is added, and the frequency difference between the two parallel resonators with different resonant frequencies is very small.

[0005] In order to achieve these subtle frequency adjustments, it is usually necessary to fine-tune the frequency of the resonator by increasing the thickness of the first load layer. Since the frequency difference between f1 and f2 is very small, the thickness of the newly added first load layer must also be very thin. Although such a design can significantly improve the performance of the filter, it also places higher requirements on the manufacturing process. The extremely thin load layer can easily lead to process instability during the production process, increase the difficulty of manufacturing, and if not properly controlled, it may also have a negative impact on the final performance of the filter. Summary of the invention

[0006] In view of the above problems, an object of the present invention is to provide a bulk acoustic wave filter and a method for manufacturing the same.

[0007] According to a first aspect of the present invention, a bulk acoustic wave filter is provided, comprising a plurality of series resonators and a plurality of parallel resonators, wherein the resonant frequencies of the series resonators and the parallel resonators are different, and the series resonators or the parallel resonators have at least two different resonant frequencies; wherein the series resonator comprises a substrate, a Bragg reflection layer located on the substrate, a first electrode layer, a piezoelectric layer, a second electrode layer and a passivation layer; the parallel resonator comprises a substrate, a Bragg reflection layer located on the substrate, a first electrode layer, a piezoelectric layer, a first load layer, a second electrode layer and a passivation layer; and at least one series resonator or at least one parallel resonator further comprises a second load layer located between the second electrode layer and the passivation layer.

[0008] Optionally, the material of the first load layer and the material of the second electrode layer are both conductive materials, and the thickness of the first load layer controls the difference in resonant frequency between the series resonator and the parallel resonator.

[0009] Optionally, the second load layer material and the passivation layer material are both dielectric materials, and the thickness of the second load layer controls the resonant frequency difference between different series resonators and / or the resonant frequency difference between different parallel resonators.

[0010] Optionally, a difference in resonant frequency between the series resonators is smaller than a preset value; and / or a difference in resonant frequency between the parallel resonators is smaller than a preset value.

[0011] Optionally, the plurality of series resonators include a plurality of resonant frequencies, and a difference between the plurality of adjacent resonant frequencies is less than a preset value; and / or the plurality of parallel resonators include a plurality of resonant frequencies, and a difference between the plurality of adjacent resonant frequencies is less than a preset value.

[0012] Optionally, the preset value is 10 MHz.

[0013] Optionally, the material of the second electrode layer includes any one of gold, aluminum, titanium, tungsten, titanium tungsten, and molybdenum, and the material of the passivation layer includes any one of aluminum nitride, silicon nitride, and silicon oxide.

[0014] Optionally, the thickness of the second support layer is less than or equal to 500 angstroms.

[0015] According to another aspect of the present invention, a method for preparing a bulk acoustic wave filter is provided, comprising: sequentially forming a Bragg reflection layer, a first electrode layer, a piezoelectric layer, and a second electrode layer on a first region on a substrate to form a plurality of series resonators; sequentially forming a Bragg reflection layer, a first electrode layer, a piezoelectric layer, a first load layer, and a second electrode layer on a second region of the substrate to form a plurality of parallel resonators, wherein the resonant frequencies of the series resonators and the parallel resonators are different; forming a second load layer on the second electrode layer of at least one series resonator or at least one parallel resonator, so that the series resonator or the parallel resonator has at least two different resonant frequencies; and forming a passivation layer on the second load layer and the second electrode layer.

[0016] Beneficial Effects

[0017] According to the BAW filter and its preparation method of the embodiment of the present invention, the BAW filter includes a plurality of series resonators and a plurality of parallel resonators, the resonant frequencies of the series resonators and the parallel resonators are different, and the series resonators or the parallel resonators have at least two different resonant frequencies; wherein the series resonator includes a substrate, a Bragg reflection layer located on the substrate, a first electrode layer, a piezoelectric layer, a second electrode layer and a passivation layer; the parallel resonator includes a substrate, a Bragg reflection layer located on the substrate, a first electrode layer, a piezoelectric layer, a first load layer, a second electrode layer and a passivation layer; at least one series resonator or at least one parallel resonator also includes a second load layer located between the second electrode layer and the passivation layer. The present application adjusts the large resonant frequency difference between the series resonator and the parallel resonator through the first load layer, and adjusts the small resonant frequency difference between the multiple series resonators and / or the multiple parallel resonators through the second load layer, so as to improve the bandwidth and out-of-band suppression performance of the filter.

[0018] Furthermore, the second load layer is disposed between the second electrode layer and the passivation layer, and the thickness of the second load layer can be greatly increased to more accurately adjust the frequency difference, while improving the controllability and process tolerance in the manufacturing process without sacrificing the resonator performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and other objects, features and advantages of the present invention will become more apparent through the following description of the embodiments of the present invention with reference to the accompanying drawings, in which:

[0020] Figure 1 A circuit diagram of a bulk acoustic wave filter in the related art is shown;

[0021] Figure 2 A schematic diagram showing the structure of a bulk acoustic wave filter in the related art is shown;

[0022] Figure 3 A schematic diagram showing an output characteristic curve of a bulk acoustic wave filter in the related art;

[0023] Figure 4 A circuit diagram of a bulk acoustic wave filter provided by an embodiment of the present invention is shown;

[0024] Figure 5 A schematic structural diagram of a bulk acoustic wave filter provided by an embodiment of the present invention is shown;

[0025] Figure 6 Show Figure 4 Schematic diagram of output characteristic curve of BAW filter in FIG.

[0026] Figure 7 Another circuit diagram of a bulk acoustic wave filter provided by an embodiment of the present invention is shown;

[0027] Figure 8 Another schematic structural diagram of a bulk acoustic wave filter provided by an embodiment of the present invention is shown;

[0028] Fig. 9 Show Figure 7 Schematic diagram of output characteristic curve of BAW filter in FIG.

[0029] Fig.10 Another circuit diagram of a bulk acoustic wave filter provided by an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0030] Various embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. In each of the accompanying drawings, the same elements are represented by the same or similar reference numerals. For the sake of clarity, the various parts in the accompanying drawings are not drawn to scale.

[0031] The specific implementation of the present invention is further described in detail below in conjunction with the drawings and examples.

[0032] Figure 1 FIG. 4 is a circuit diagram of a bulk acoustic wave filter in the related art. Figure 2 FIG. 2 shows a schematic diagram of the structure of a bulk acoustic wave filter in the related art. Figure 1 As shown, the BAW filter includes a plurality of series resonators (1001-100n) and a plurality of parallel resonators (2001-200n), the plurality of series resonators have the same resonant frequency, which is f0; the plurality of parallel resonators have the same resonant frequency, which is f1, but there is a large resonant frequency difference between the resonant frequency f0 of the series resonator and the resonant frequency f1 of the parallel resonator, that is, the difference between the resonant frequency f0 and the resonant frequency f1 is greater than a predetermined value, for example, the difference between the two is greater than 150MHZ.

[0033] See also Figure 2The series resonator includes a substrate 100, a Bragg reflection layer 110 located on the substrate, a first electrode layer 120, a piezoelectric layer 130, a second electrode layer 140 and a passivation layer 150, and the parallel resonator includes a substrate 100, a Bragg reflection layer 110 located on the substrate, a first electrode layer 120, a piezoelectric layer 130, a first load layer 160, a second electrode layer 140 and a passivation layer 150.

[0034] In the related art, the resonance frequency difference between the series resonator and the parallel resonator is achieved by setting the first load layer 160 between the piezoelectric layer 130 and the second electrode layer 140. Since the resonance frequency difference between the series resonator and the parallel resonator is large, the thickness of the first load layer 160 is large and the process difficulty is low.

[0035] See also Figure 3 , multiple parallel resonators in the ladder filter have the same resonant frequency, each parallel resonator has a series resonator frequency and a parallel resonant frequency, wherein the series resonant frequency f1 is 4.007GHz and the parallel resonant frequency is 4.101GHz; similarly, multiple series resonators also have the same resonant frequency, each series resonant frequency has a series resonant frequency and a parallel resonant frequency, wherein its series resonant frequency is 4.101GHz and the parallel resonant frequency f0 is 4.190GHz. Therefore, the passband range of the BAW filter is 4.007GHZ-4.190GHz, which is the frequency range allowed to pass by the BAW filter.

[0036] However, as the requirements for filter performance increase, in order to expand the bandwidth and improve the out-of-band suppression performance, a small resonant frequency is introduced in the series resonator or parallel resonator. For example, a new series resonator with a resonant frequency of f2 is added (see Figure 4 ) or add a parallel resonator with a resonant frequency of f2 (see Figure 5 ), the resonant frequency f2 is very different from the resonant frequency of the original resonator.

[0037] In order to achieve these subtle frequency adjustments, it is usually necessary to fine-tune the frequency of the resonator by increasing the thickness of the first load layer. Since the frequency difference between f1 and f2 is very small, the thickness of the newly added first load layer must also be very thin. Although such a design can significantly improve the performance of the filter, it also places higher requirements on the manufacturing process. The extremely thin load layer can easily lead to process instability during the production process, increase the difficulty of manufacturing, and if not properly controlled, it may also have a negative impact on the final performance of the filter.

[0038] In order to solve the above problems, an embodiment of the present invention provides a bulk acoustic wave filter, including multiple series resonators and multiple parallel resonators, the resonant frequencies of the series resonators and the parallel resonators are different, and the series resonators or the parallel resonators have at least two different resonant frequencies; wherein the series resonator includes a substrate, a Bragg reflection layer located on the substrate, a first electrode layer, a piezoelectric layer, a second electrode layer and a passivation layer; the parallel resonator includes a substrate, a Bragg reflection layer located on the substrate, a first electrode layer, a piezoelectric layer, a first load layer, a second electrode layer and a passivation layer; at least one series resonator or at least one parallel resonator also includes a second load layer located between the second electrode layer and the passivation layer. The present application adjusts the large resonant frequency difference between the series resonator and the parallel resonator through the first load layer, and adjusts the small resonant frequency difference between multiple series resonators and / or multiple parallel resonators through the second load layer, so as to improve the bandwidth and out-of-band suppression performance of the filter.

[0039] Figure 4 A circuit diagram of a bulk acoustic wave filter provided by an embodiment of the present invention is shown. Figure 5 FIG. 2 shows a schematic diagram of the structure of a bulk acoustic wave filter provided by an embodiment of the present invention. Figure 6 As shown, the BAW filter includes a plurality of series resonators (1001-100n) and a plurality of parallel resonators (2001-200n), the resonant frequencies of the series resonators and the parallel resonators are different, wherein the series resonator includes at least 2 resonant frequencies, namely at least a resonant frequency f0 and a resonant frequency f2; the resonant frequencies of the parallel resonators are the same, both being f1.

[0040] See also Figure 5 The series resonator includes a substrate 100, a Bragg reflection layer 110 located on the substrate, a first electrode layer 120, a piezoelectric layer 130, a second electrode layer 140 and a passivation layer 150, and the parallel resonator includes a substrate 100, a Bragg reflection layer 110 located on the substrate, a first electrode layer 120, a piezoelectric layer 130, a first load layer 160, a second electrode layer 140 and a passivation layer 150.

[0041] The at least one series resonator further includes a second load layer 170 located between the second electrode layer 140 and the passivation layer 150 .

[0042] In this embodiment, the thickness of the first load layer 160 controls the difference in resonant frequency between the series resonator and the parallel resonator. The thickness of the second load layer 170 controls the difference in resonant frequency between different series resonators. The materials of the first load layer 160 and the second electrode layer 140 are both conductive materials (including metals or alloys), for example, including at least one of gold, aluminum, titanium, tungsten, titanium tungsten, and molybdenum, but not limited thereto. The material of the second load layer 170 and the material of the passivation layer 150 are both non-conductive dielectric materials, for example, including at least one of silicon nitride or aluminum nitride, but not limited thereto.

[0043] The difference in resonant frequency between the series resonators (1001-100n) is less than a preset value. The preset value is 10MHz. Since the material of the second electrode layer 140 is a conductive material, its material density is relatively large. If the second load layer 170 is still between the piezoelectric layer 130 and the second electrode layer 140, the thickness of the second load layer 170 is extremely thin due to the small frequency difference, the process is unstable, and the manufacturing difficulty is increased. If the second load layer 170 is arranged between the second electrode layer 140 and the passivation layer 150, since the material of the passivation layer is a dielectric material, its material density is much smaller than that of the conductive material. Under the same frequency offset conditions, the thickness of the second load layer 170 can be significantly increased.

[0044] For example, when the preset value is 10 MHz (i.e., f2-f0≤10 MHz), if the second load layer 170 is disposed between the piezoelectric layer 130 and the second electrode layer 140, the thickness of the second load layer 170 is less than or equal to 100 angstroms; if the second load layer 170 is disposed between the second electrode layer 140 and the passivation layer 150, the thickness of the second load layer 170 is less than or equal to 500 angstroms.

[0045] In this embodiment, the material of the second electrode layer includes any one of gold, aluminum, titanium, tungsten, titanium tungsten, and molybdenum, but is not limited thereto. The material of the passivation layer includes any one of aluminum nitride, silicon nitride, and silicon oxide, but is not limited thereto.

[0046] The second load layer is disposed between the second electrode layer and the passivation layer, and the thickness of the second load layer can be greatly increased to more accurately adjust the frequency difference, while improving the controllability and process tolerance in the manufacturing process without sacrificing the resonator performance.

[0047] See also Figure 6The parallel resonators have the same resonant frequency, where the series resonant frequency f0 is 4.007GHz and the parallel resonant frequency is 4.101GHz; the resonant frequencies of different series resonators have slight frequency differences, where the series resonant frequency is 4.101GHz, the parallel resonant frequency f1 is 4.190GHz, and the parallel resonant frequency f2 is 4.192GHz. The passband range of the BAW filter is 4.007GHZ-4.191GHz, which can expand the bandwidth of the BAW filter and improve the out-of-band suppression performance.

[0048] Figure 7 Another circuit diagram of a bulk acoustic wave filter provided by an embodiment of the present invention is shown. Figure 8 FIG. 2 shows another schematic diagram of the structure of a bulk acoustic wave filter provided by an embodiment of the present invention. Figure 7 As shown, the BAW filter includes a plurality of series resonators (1001-100n) and a plurality of parallel resonators (2001-200n), the series resonators and the parallel resonators have different resonant frequencies, wherein the resonant frequencies of the series resonators are the same, both being f0; and the parallel resonators include at least two resonant frequencies, namely at least a resonant frequency f1 and a resonant frequency f2.

[0049] See also Figure 8 The series resonator includes a substrate 100, a Bragg reflection layer 110 located on the substrate, a first electrode layer 120, a piezoelectric layer 130, a second electrode layer 140 and a passivation layer 150, and the parallel resonator includes a substrate 100, a Bragg reflection layer 110 located on the substrate, a first electrode layer 120, a piezoelectric layer 130, a first load layer 160, a second electrode layer 140 and a passivation layer 150.

[0050] The at least one parallel resonator further includes a second load layer 170 located between the second electrode layer 140 and the passivation layer 150 .

[0051] In this embodiment, the thickness of the first load layer 160 controls the difference in resonant frequency between the series resonator and the parallel resonator. The thickness of the second load layer 170 controls the difference in resonant frequency between different series resonators. The materials of the first load layer 160 and the second electrode layer 140 are both conductive materials (including metals or alloys), for example, including at least one of gold, aluminum, titanium, tungsten, titanium tungsten, and molybdenum, but not limited thereto. The materials of the second load layer 170 and the passivation layer 150 are both dielectric materials, for example, including at least one of silicon nitride or aluminum nitride, but not limited thereto.

[0052] The difference in resonance frequency between the parallel resonators (2001-200n) is less than a preset value. The preset value is 10 MHz. Since the material of the second electrode layer 140 is a conductive material, its material density is relatively large. If the second load layer 170 is still between the piezoelectric layer 130 and the second electrode layer 140, the thickness of the second load layer 170 is extremely thin due to the small frequency difference, the process is unstable, and the manufacturing difficulty is increased. If the second load layer 170 is arranged between the second electrode layer 140 and the passivation layer 150, since the material of the passivation layer is a dielectric material, its material density is much smaller than that of the conductive material, the thickness of the second load layer 170 can be significantly increased.

[0053] For example, when the preset value is 10 MHz (i.e., f2-f0≤10 MHz), if the second load layer 170 is disposed between the piezoelectric layer 130 and the second electrode layer 140, the thickness of the second load layer 170 is less than or equal to 100 angstroms; if the second load layer 170 is disposed between the second electrode layer 140 and the passivation layer 150, the thickness of the second load layer 170 is less than or equal to 500 angstroms.

[0054] In this embodiment, the material of the second electrode layer includes any one of gold, aluminum, titanium, tungsten, titanium tungsten, and molybdenum, but is not limited thereto. The material of the passivation layer includes any one of aluminum nitride, silicon nitride, and silicon oxide, but is not limited thereto.

[0055] The second load layer is disposed between the second electrode layer and the passivation layer, and the thickness of the second load layer can be greatly increased to more accurately adjust the frequency difference, while improving the controllability and process tolerance in the manufacturing process without sacrificing the resonator performance.

[0056] See also Fig. 9 , the resonant frequencies of different parallel resonators have slight frequency differences, among which the series resonant frequency f0 is 4.007GHz, the series resonant frequency f2 is 4.005GHz, and the parallel resonant frequency is 4.101GHz; the series resonators have the same resonant frequency, the series resonant frequency is 4.101GHz, and the parallel resonant frequency is 4.190GHz. The passband range of the BAW filter is 4.006GHZ-4.190GHz, which can expand the bandwidth of the BAW filter and improve the out-of-band suppression performance.

[0057] Fig.10 FIG. 2 shows another circuit diagram of a bulk acoustic wave filter provided by an embodiment of the present invention. Fig.10As shown, the bulk acoustic wave filter includes a plurality of series resonators (1001-100n) and a plurality of parallel resonators (2001-200n), and the resonance frequencies of the series resonators and the parallel resonators are different. Compared with the above embodiment, the plurality of series resonators (1001-100n) of this embodiment include a plurality of resonance frequencies f1-fn, and the difference between the plurality of adjacent resonance frequencies is less than a preset value; and / or, the plurality of parallel resonators (2001-200n) include a plurality of resonance frequencies f1'-fn', and the difference between the plurality of adjacent resonance frequencies is less than a preset value.

[0058] In this embodiment, each series resonator is provided with a second load layer 170 of different thicknesses, so that the plurality of series resonators have a plurality of resonant frequencies f1-fn. The resonant frequency of each parallel resonator is the same.

[0059] In an optional embodiment, each parallel resonator is provided with a second load layer 170 of different thicknesses, so that the plurality of parallel resonators have a plurality of resonant frequencies f1'-fn'. The resonant frequency of each series resonator is the same.

[0060] In an optional embodiment, each series resonator is provided with a second load layer 170 of different thickness, so that the multiple series resonators have multiple resonance frequencies f1-fn. Each parallel resonator is provided with a second load layer 170 of different thickness, so that the multiple parallel resonators have multiple resonance frequencies f1'-fn'.

[0061] The rest of this embodiment is the same as the above embodiment and will not be described again.

[0062] The bulk acoustic wave filter of the embodiment of the present invention includes a plurality of series resonators and a plurality of parallel resonators, the resonant frequencies of the series resonators and the parallel resonators are different, and the series resonator or the parallel resonator has at least two different resonant frequencies; wherein the series resonator includes a substrate, a Bragg reflection layer located on the substrate, a first electrode layer, a piezoelectric layer, a second electrode layer and a passivation layer; the parallel resonator includes a substrate, a Bragg reflection layer located on the substrate, a first electrode layer, a piezoelectric layer, a first load layer, a second electrode layer and a passivation layer; at least one series resonator or at least one parallel resonator also includes a second load layer located between the second electrode layer and the passivation layer. The present application adjusts the large resonant frequency difference between the series resonator and the parallel resonator through the first load layer, and adjusts the small resonant frequency difference between the multiple series resonators and / or the multiple parallel resonators through the second load layer, so as to improve the bandwidth and out-of-band suppression performance of the filter.

[0063] Furthermore, the second load layer is disposed between the second electrode layer and the passivation layer, and the thickness of the second load layer can be greatly increased to more accurately adjust the frequency difference, while improving the controllability and process tolerance in the manufacturing process without sacrificing the resonator performance.

[0064] The embodiment of the present invention also includes a method for preparing a bulk acoustic wave filter, which includes the following steps.

[0065] In step a, a Bragg reflection layer, a first electrode layer, a piezoelectric layer, and a second electrode layer are sequentially formed on a first region on a substrate to form a plurality of series resonators.

[0066] In step b, a Bragg reflection layer, a first electrode layer, a piezoelectric layer, a first load layer and a second electrode layer are sequentially formed on the second region of the substrate to form a plurality of parallel resonators, and the resonance frequencies of the series resonators and the parallel resonators are different.

[0067] In step c, a second load layer is formed on the second electrode layer of at least one series resonator or at least one parallel resonator, so that the series resonator or the parallel resonator has at least two different resonance frequencies.

[0068] In step d, a passivation layer is formed on the second electrode layer and the second support layer.

[0069] According to the embodiments of the present invention as described above, these embodiments do not describe all the details in detail, nor do they limit the invention to the specific embodiments described. Obviously, many modifications and changes can be made based on the above description. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can make good use of the present invention and the modified use based on the present invention. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A bulk acoustic wave filter, characterized in that: The device comprises a plurality of series resonators and a plurality of parallel resonators, wherein the resonant frequencies of the series resonators and the parallel resonators are different, and the series resonators or the parallel resonators have at least two different resonant frequencies; The series resonator includes a substrate, a Bragg reflection layer located on the substrate, a first electrode layer, a piezoelectric layer, a second electrode layer and a passivation layer; The parallel resonator comprises a substrate, a Bragg reflection layer located on the substrate, a first electrode layer, a piezoelectric layer, a first load layer, a second electrode layer and a passivation layer; The at least one series resonator or the at least one parallel resonator further includes a second load layer located between the second electrode layer and the passivation layer.

2. The bulk acoustic wave filter according to claim 1, characterized in that The material of the first load layer and the material of the second electrode layer are both conductive materials, and the thickness of the first load layer controls the difference in resonant frequency between the series resonator and the parallel resonator.

3. The bulk acoustic wave filter according to claim 1, wherein: The second load layer material and the passivation layer material are both dielectric materials, and the thickness of the second load layer controls the resonant frequency difference between different series resonators and / or the resonant frequency difference between different parallel resonators.

4. The bulk acoustic wave filter according to claim 3, characterized in that The difference in resonant frequency between the series resonators is smaller than a preset value; and / or the difference in resonant frequency between the parallel resonators is smaller than a preset value.

5. The bulk acoustic wave filter according to claim 4, characterized in that The plurality of series resonators include a plurality of resonant frequencies, and a difference between the plurality of adjacent resonant frequencies is less than a preset value; and / or the plurality of parallel resonators include a plurality of resonant frequencies, and a difference between the plurality of adjacent resonant frequencies is less than a preset value.

6. The bulk acoustic wave filter according to claim 4 or 5, characterized in that: The preset value is 10 MHz.

7. The bulk acoustic wave filter according to claim 1, wherein: The material of the second electrode layer includes any one of gold, aluminum, titanium, tungsten, titanium tungsten, and molybdenum. The material of the passivation layer includes any one of aluminum nitride, silicon nitride, and silicon oxide.

8. The bulk acoustic wave filter according to claim 1, wherein: The thickness of the second support layer is less than or equal to 500 angstroms.

9. A method for preparing a bulk acoustic wave filter, characterized in that: include: A Bragg reflection layer, a first electrode layer, a piezoelectric layer, and a second electrode layer are sequentially formed on a first region on a substrate to form a plurality of series resonators; A Bragg reflection layer, a first electrode layer, a piezoelectric layer, a first load layer and a second electrode layer are sequentially formed on the second region of the substrate to form a plurality of parallel resonators, wherein the resonance frequencies of the series resonators and the parallel resonators are different; forming a second load layer on the second electrode layer of at least one series resonator or at least one parallel resonator, so that the series resonator or the parallel resonator has at least two different resonant frequencies; A passivation layer is formed on the second support layer and the second electrode layer.

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