Acoustic wave resonators and filters

Abstract

The present invention discloses an acoustic wave resonator and a filter. The acoustic wave resonator comprises: a substrate, a sound reflection structure is provided on the substrate, and a first electrode, a piezoelectric layer and a second electrode are sequentially stacked on the substrate, and the first electrode, the piezoelectric layer and the second electrode form a resonance effective area in the stacking direction; wherein the first electrode comprises at least two first partitions, and the thickness of at least two of the first partitions is different, and the connection between adjacent first partitions forms a first step, so that the surface of one side of the piezoelectric layer away from the substrate forms a second step; the second electrode comprises at least two second partitions, and the thickness of at least two of the second partitions is different, and the connection between adjacent second partitions forms a third step, and the second partition located on the second step forms a fourth step. The embodiment of the present invention improves the quality factor of the acoustic wave resonator and the filter, thereby improving its performance.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor technology, and in particular to an acoustic wave resonator and a filter. Background Art

[0002] With the development of technology, filters composed of acoustic wave resonators can meet the current requirements for high-performance RF filters and are a new RF filter solution. In the prior art, the acoustic wave resonator mainly includes a "sandwich" laminated structure composed of a top electrode, a piezoelectric layer and a bottom electrode, as well as a cavity under the bottom electrode. The principle can be briefly described as follows: using the piezoelectric properties of the material to convert electrical energy into acoustic energy, the sound waves reflect back and forth in the "sandwich" structure to form standing waves; when the frequency of the standing wave is the same as the frequency of the input signal, the electrical loss is minimal, and the input signal with a frequency of f can pass through the resonator. However, existing acoustic wave resonators have the problem of low quality factor (Q). Summary of the invention

[0003] The present invention provides an acoustic wave resonator and a filter to improve the quality factor and thus improve the performance.

[0004] According to one aspect of the present invention, there is provided an acoustic wave resonator, comprising:

[0005] A substrate, on which a sound reflection structure is provided, and on which a first electrode, a piezoelectric layer, and a second electrode are sequentially stacked, wherein the first electrode, the piezoelectric layer, and the second electrode form a resonance effective area in a stacking direction;

[0006] The first electrode includes at least two first partitions, and the thicknesses of the at least two first partitions are different, and the connection between adjacent first partitions forms a first step, so that the surface of the piezoelectric layer away from the substrate forms a second step;

[0007] The second electrode includes at least two second partitions, and the thicknesses of at least two second partitions are different. The connection between adjacent second partitions forms a third step, and the second partition located on the second step forms a fourth step.

[0008] Optionally, along the outward direction of the resonance effective area, the thicknesses of at least two of the first partitions increase sequentially.

[0009] Optionally, the thicker first partition partially overlaps with the resonance effective area or is arranged outside the resonance effective area.

[0010] Optionally, along the outward direction of the resonance effective area, the thicknesses of at least two of the first partitions are arranged alternately to form a phononic crystal structure.

[0011] Optionally, the phononic crystal structure partially overlaps with the resonance effective region or is arranged outside the resonance effective region.

[0012] Optionally, at the periphery of the phononic crystal structure, the thickness of the first partition is greater than or equal to the maximum thickness of the phononic crystal.

[0013] Optionally, the first electrode is provided with a groove, the groove is located at the outer periphery of the first step, and the groove and the piezoelectric layer are surrounded to form an air gap.

[0014] Optionally, the acoustic wave resonator further includes: a first isolation layer, wherein the first isolation layer is located between the first electrode and the substrate.

[0015] Optionally, a thickness of a portion of the first subarea in the first electrode is greater than a thickness of the first isolation layer.

[0016] Optionally, the first isolation layer is arranged on the periphery of the sound reflection structure.

[0017] Optionally, the first isolation layer includes at least one oxide layer and / or at least one amorphous silicon layer.

[0018] Optionally, the first isolation layer covers the sound reflection structure and extends to the periphery of the sound reflection structure;

[0019] The acoustic wave resonator further includes a second isolation layer located between the first isolation layer and the first electrode.

[0020] Optionally, the first isolation layer includes at least one oxide layer, and the second isolation layer includes at least one amorphous silicon layer.

[0021] Optionally, the second isolation layer covers the first isolation layer and extends outward to form a fifth step at an end of the first isolation layer, and the first electrode forms a sixth step on the fifth step.

[0022] According to another aspect of the present invention, there is provided a filter comprising the acoustic wave resonator according to any embodiment of the present invention.

[0023] In the embodiment of the present invention, the first electrode includes at least two first partitions, and the thickness of at least two first partitions is different, and the connection between adjacent first partitions forms a first step, so that the surface of the side of the piezoelectric layer away from the substrate forms a second step, and the second electrode includes at least two second partitions, and the thickness of at least two second partitions is different, the connection between adjacent second partitions forms a third step, and the second partition located on the second step forms a fourth step. That is, the acoustic wave resonator provided by the embodiment of the present invention includes a first step on the first electrode, a third step and a fourth step on the second electrode, that is, a stepped structure is provided on both the first electrode and the second electrode, so as to reduce the energy loss of the acoustic wave at the edge of the effective resonance area and improve the Q value of the resonator at the parallel resonance point. Further, in the embodiment of the present invention, due to the setting of the first step on the first electrode, even if the overall thickness of the piezoelectric layer is the same, a second step will be naturally formed on the piezoelectric layer; due to the setting of the second step on the piezoelectric layer, even if the thickness of the second electrode is the same, a fourth step will be formed on the second electrode. It can be seen that the thickness of the first electrode in the embodiment of the present invention has a height difference, thereby forming a first step, and also increasing the number of steps on the second electrode, which is further beneficial to improving the performance of the acoustic wave resonator.

[0024] 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

[0025] 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.

[0026] Figure 1 A schematic top view of an acoustic wave resonator provided by an embodiment of the present invention;

[0027] Figure 2 For along Figure 1 Schematic diagram of the cross-sectional structure of AA;

[0028] Figure 3 , Figure 4 A schematic diagram of a process flow of an acoustic wave resonator provided by an embodiment of the present invention;

[0029] Figure 5 A schematic top view of another acoustic wave resonator provided by an embodiment of the present invention;

[0030] Figure 6 For along Figure 5 Schematic diagram of the cross-sectional structure of the CC;

[0031] Figure 7 A schematic top view of another acoustic wave resonator provided by an embodiment of the present invention;

[0032] Figure 8 for Figure 7 Schematic diagram of the cross-section structure along DD;

[0033] Fig. 9 A schematic diagram of the structure of another acoustic wave resonator provided by an embodiment of the present invention;

[0034] Fig.10 A schematic structural diagram of another acoustic wave resonator provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0035] 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.

[0036] 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 variations thereof 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 that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0037] Figure 1 A schematic top view of an acoustic wave resonator provided by an embodiment of the present invention, Figure 2 For along Figure 1 Schematic diagram of the cross-sectional structure of AA. Figure 1 and Figure 2 , the acoustic wave resonator comprises:

[0038] A substrate 100, a sound reflection structure 110 is provided on the substrate 100, and a first electrode 200 (bottom electrode), a piezoelectric layer 300 and a second electrode 400 (top electrode) are sequentially stacked on the substrate 100, and the first electrode 200, the piezoelectric layer 300 and the second electrode 400 form a resonance effective area 500 in the stacking direction;

[0039] The first electrode 200 includes at least two first partitions 201, and the thicknesses of the at least two first partitions 201 are different. A first step 202 is formed at the connection between adjacent first partitions 201, so that a second step 302 is formed on the surface of the side of the piezoelectric layer 300 away from the substrate 100;

[0040] The second electrode 400 includes at least two second partitions 401 , and the thicknesses of the at least two second partitions 401 are different. The connection between adjacent second partitions 401 forms a third step 402 , and the second partition 401 located on the second step 302 forms a fourth step 403 .

[0041] Among them, on the first electrode 200, for the first partition 201 with a thinner thickness (for example, the first partition 201 located on the inner side), the first partition 201 with a thicker thickness (for example, the first partition 201 located on the outer side) is equivalent to thickening the first electrode 200. The first partition 201 located on the outer side includes an annular area and a connecting area, wherein the annular area is located between the connecting area and the resonance effective area 500.

[0042] This arrangement can reduce the resistance value of the first electrode 200, so that the electrical energy loss in the first electrode 200 is reduced, thereby improving the Q value of the resonator at the series resonance point. The resonance effective area 500 is also called the active area. The standing wave formed by the sound wave is mainly reflected in the resonance effective area 500. However, at the edge of the resonance effective area 500, there will be energy leakage. The acoustic wave resonator provided in an embodiment of the present invention includes a first step 202 on the first electrode 200, a third step 402 and a fourth step 403 on the second electrode 400, that is, a stepped structure is provided on both the first electrode 200 and the second electrode 400, thereby reducing the energy loss of the sound wave at the edge of the resonance effective area 500 and improving the Q value of the resonator at the parallel resonance point. Furthermore, in the embodiment of the present invention, due to the arrangement of the first step 202 on the first electrode 200, even if the overall thickness of the piezoelectric layer 300 is the same, a second step 302 will be naturally formed on the piezoelectric layer 300; due to the arrangement of the second step 302 on the piezoelectric layer 300, even if the thickness of the second electrode 400 is the same, a fourth step 403 will be formed on the second electrode 400. It can be seen that in the embodiment of the present invention, there is a height difference in the thickness of the first electrode 100, thereby forming the first step 202, and the number of steps on the second electrode 400 is increased, which is further conducive to the improvement of the performance of the acoustic wave resonator.

[0043] Continue to see Figure 1 and Figure 2 On the basis of the above embodiments, optionally, the thickness of at least two first partitions 201 increases successively along the direction B outward from the resonance effective region 500. In this way, the height of the steps increases successively along the direction B outward from the resonance effective region 500, so that the thickness of the first partition 201 corresponding to the resonance effective region 500 is the thinnest, which is conducive to improving the resonance frequency of the acoustic wave resonator.

[0044] Continue to see Figure 1 and Figure 2 On the basis of the above embodiments, optionally, the thicker first partition 201 partially overlaps with the resonance effective region 500 or is arranged outside the resonance effective region 500. This arrangement is conducive to forming a step at the edge of the resonance effective region 500, thereby reducing the energy loss of the sound wave at the edge of the resonance effective region 500 and improving the Q value of the resonator at the parallel resonance point.

[0045] Accordingly, Figure 3 , Figure 4 A schematic diagram of a process flow of an acoustic wave resonator provided by an embodiment of the present invention. Figure 3 and Figure 4 Based on the above embodiments, optionally, the process flow of the acoustic wave resonator includes the following steps:

[0046] S10 , providing a substrate 100 , and forming a sound reflection structure on the substrate 100 .

[0047] The material of the substrate 100 may be, for example, a Si substrate, and the reflective structure may be, for example, a cavity structure, which may be formed by etching. Optionally, the depth of the cavity structure ranges from 1 to 5 um.

[0048] S20 , forming a sacrificial layer 600 on the substrate 100 .

[0049] The sacrificial layer 600 may be, for example, a silicon oxide layer, such as a silicon dioxide oxide layer, which may be deposited by chemical vapor deposition. Optionally, the sacrificial layer 600 completely covers the acoustic reflection structure, and the film thickness of the sacrificial layer 600 is greater than the depth of the cavity structure and does not exceed 2 um.

[0050] S30 , removing the portion of the sacrificial layer 600 except for the portion in the cavity structure.

[0051] Optionally, a chemical mechanical polishing (CMP) process is used to polish the silicon oxide layer inside the cavity structure so that the height of the silicon oxide layer is consistent with that of the Si substrate outside the cavity.

[0052] S40, forming a first electrode thickening layer 210, and patterning it.

[0053] The first electrode thickening layer 210 is a part of the first electrode 200, and optionally, its material is the same as that of the first electrode 200. Exemplarily, the material of the first electrode 200 includes: tungsten (W), molybdenum (Mo) or platinum (Pt) and the like.

[0054] Optionally, the first electrode thickening layer 210 is formed by a deposition process and patterned by a photolithography process. The first electrode thickening layer 210 covers the annular area outside the active area, the wiring area and the electrode plate area, wherein the width of the annular area is 2-8um, and the thickness of the metal thickening layer ranges from 10-250nm.

[0055] S50 , forming a first electrode thin layer 220 to cover the first electrode thickened layer 210 , thereby forming a first step 202 .

[0056] S60, forming a piezoelectric layer 300, and further forming a second electrode A protruding layer 410.

[0057] Among them, the material of the piezoelectric layer 300 includes aluminum nitride (AlN) or zinc oxide (ZnO) and the like. The second electrode A raised layer 410 is a part of the second electrode 400, and optionally, its material is the same as that of the second electrode 400. Exemplarily, the material of the second electrode 400 includes: tungsten (W), molybdenum (Mo) or platinum (Pt) and the like. Optionally, the piezoelectric layer 300 with a thickness of 200 to 1500 nm is grown to cover the first electrode 200. Due to the presence of the first step 202 on the first electrode 200, the second step 302 is naturally formed on the piezoelectric layer 300. Further, the second electrode A raised layer 410 is deposited on the second step 302 and patterned, so that a top single-layer step can be formed on the second electrode A raised layer 410.

[0058] S70, forming a second electrode B protruding layer 420.

[0059] Optionally, a whole layer of second electrode B protrusion layer 420 is deposited and then patterned to retain second electrode B protrusion layer 420 only on second electrode A protrusion layer 410, so that there is a step between second electrode B protrusion layer 420 and second electrode A protrusion layer 410, thereby forming a top double-layer step.

[0060] S80 , etching a through hole on the piezoelectric layer 300 to expose the first electrode 200 .

[0061] S90, forming a second electrode thin layer 430, covering the second electrode A protruding layer 410 and the second electrode B protruding layer 420, to form three top steps.

[0062] SA0, etch release holes.

[0063] SB0, release cavity structure.

[0064] Optionally, hydrofluoric acid gas (VHF) and buffered oxide etchant (BOE) are used to release the cavity structure.

[0065] Through the above steps, the acoustic wave resonator provided by the embodiment of the present invention can be formed.

[0066] It should be noted that the embodiment of the present invention does not limit the number of steps of the first electrode 200 and the second electrode 400. Figure 2 to Figure 4 As shown, there is one step on the first electrode 200 and three steps on the second electrode 400. In other embodiments, there may be two steps on the first electrode 200 and three steps on the second electrode 400; or, there may be one step on the first electrode 200 and two steps on the second electrode 400; or, there may be three steps on the first electrode 200 and three steps on the second electrode 400, etc., which will not be described one by one.

[0067] In other embodiments, the steps on the first electrode 200 and the second electrode 400 may be arranged to increase in a direction other than a single direction. Figure 5 A schematic top view of another acoustic wave resonator provided by an embodiment of the present invention, Figure 6 For along Figure 5 Schematic diagram of the cross-section structure of CC. Figure 5 and Figure 6 In one embodiment of the present invention, optionally, along the direction B outward from the resonance effective region 500, the thicknesses of at least two first partitions 201 are alternately arranged to form a phononic crystal structure. Accordingly, a phononic crystal structure is also formed on the second electrode 400. The phononic crystal structure has a specific phase and intensity modulation capability for a sound wave of a specific frequency, thereby being able to suppress the propagation of sound waves within a certain frequency range and reduce the energy loss of sound waves at the edge of the resonance effective region 500.

[0068] Optionally, the phononic crystal structure may be one-dimensional, two-dimensional or three-dimensional, and its shape may be rectangular, polygonal or circular, etc.

[0069] Continue to see Figure 5 and Figure 6 On the basis of the above embodiments, optionally, the phononic crystal structure partially overlaps with the resonance effective region 500 or is arranged outside the resonance effective region 500. This arrangement is conducive to forming a step at the edge of the resonance effective region 500, thereby reducing the energy loss of the sound wave at the edge of the resonance effective region 500 and improving the Q value of the resonator at the parallel resonance point.

[0070] Continue to see Figure 5 and Figure 6 On the basis of the above embodiments, optionally, at the periphery of the phononic crystal structure, the thickness of the first partition 201 is greater than or equal to the maximum thickness of the phononic crystal. Wherein, the periphery of the phononic crystal structure is also located at the periphery of the resonance effective region 500, and the first electrode 200 of this part is thickened, which can reduce the resistance value of the first electrode 200, so that the electrical energy loss in the first electrode 200 is reduced, thereby improving the Q value of the resonator at the series resonance point.

[0071] Figure 7 A schematic top view of another acoustic wave resonator provided by an embodiment of the present invention, Figure 8 for Figure 7 Schematic diagram of the cross-section structure along DD. Figure 7On the basis of the above embodiments, optionally, the first electrode 200 is provided with a groove 209, the groove 209 is located at the periphery of the first step 202, and the groove 209 and the piezoelectric layer 300 are surrounded to form an air gap. The air gap, i.e., the cavity structure, can reflect sound waves, so the air gap and the step structure are combined to form a multi-level reflection structure, which further reduces energy leakage.

[0072] Exemplarily, when preparing the acoustic wave resonator, a sacrificial layer may be formed in the groove 209 , and finally the sacrificial layer is released to form the air gap.

[0073] On the basis of the above embodiments, optionally, the air gap may also be provided in the phononic crystal structure to form a multi-level reflection structure.

[0074] Fig. 9 A schematic diagram of the structure of another acoustic wave resonator provided by an embodiment of the present invention. Fig. 9 On the basis of the above embodiments, the acoustic wave resonator may further include: a first isolation layer 700, which is located between the first electrode 200 and the substrate 100. This arrangement can reduce the leakage of acoustic waves in the piezoelectric layer 300 toward the substrate 100 on the one hand; on the other hand, a sixth step 203 is further formed on the first electrode 200, so that the step structure has more levels, further improving the performance of the acoustic wave resonator.

[0075] Continue to see Fig. 9 On the basis of the above embodiments, optionally, the thickness of part of the first partition 201 in the first electrode 200 is greater than the thickness of the first isolation layer 700. This configuration makes the thickness of the first isolation layer 700 thinner, which is conducive to the thinning of the acoustic wave resonator.

[0076] Continue to see Fig. 9 On the basis of the above embodiments, optionally, the first isolation layer 700 is disposed at the periphery of the acoustic reflection structure 110. The embodiment of the present invention is arranged such that, within the resonance effective area 500, the first isolation layer 700 is not disposed at the lower part of the first electrode 200, which is conducive to expanding the material selection of the first isolation layer 700, thereby avoiding the influence of the first isolation layer 700 on the performance of the acoustic wave resonator. For example, the first isolation layer 700 includes at least one oxide layer and / or at least one amorphous silicon layer. In addition, the first isolation layer 700 located at the periphery of the acoustic reflection structure 110 can reflect the sound waves leaking from the edge, thereby improving the performance of the acoustic wave resonance.

[0077] Fig.10 A schematic diagram of the structure of another acoustic wave resonator provided by an embodiment of the present invention. Fig.10In another embodiment of the present invention, optionally, the first isolation layer 700 covers the acoustic reflection structure 110 and extends to the periphery of the acoustic reflection structure 110. The acoustic wave resonator further includes a second isolation layer 800, which is located between the first isolation layer 700 and the first electrode 200. This arrangement can reduce the leakage of acoustic waves in the piezoelectric layer 300 toward the substrate 100; and prevent the oxide released by the buffered oxide etchant (BOE) from corroding the AlN seed layer during the release of the cavity structure, causing frequency shift.

[0078] Continue to see Fig.10 Based on the above embodiments, optionally, the first isolation layer 700 includes at least one oxide layer, and the second isolation layer 800 includes at least one amorphous silicon layer. The advantage of this arrangement is that materials such as AlN seed layer and Mo grow better on the amorphous silicon layer than on the oxide layer.

[0079] Continue to see Fig.10 On the basis of the above embodiments, optionally, the second isolation layer 800 covers the first isolation layer 700 and extends outward to form a fifth step at the end of the first isolation layer 700, and the first electrode 200 forms a sixth step 203 on the fifth step.

[0080] An embodiment of the present invention further provides a filter, including an acoustic wave resonator as provided in any embodiment of the present invention, and its technical principles and effects are similar and will not be described in detail.

[0081] It should be understood that the various forms of processes shown above can be used to reorder, add or delete steps. For example, the steps described in the present invention can be executed in parallel, sequentially or in different orders, as long as the desired results of the technical solution of the present invention can be achieved, and this document does not limit this.

[0082] 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. An acoustic wave resonator, characterized in that: include: A substrate, on which a sound reflection structure is provided, and on which a first electrode, a piezoelectric layer, and a second electrode are sequentially stacked, wherein the first electrode, the piezoelectric layer, and the second electrode form a resonance effective area in a stacking direction; The first electrode includes at least two first partitions, and the thicknesses of the at least two first partitions are different, and the connection between adjacent first partitions forms a first step, so that the surface of the piezoelectric layer away from the substrate forms a second step; Along the outward direction of the resonance effective area, the thickness of at least two of the first partitions increases successively; the first partition with a thicker thickness partially overlaps with the resonance effective area or is arranged outside the resonance effective area; The second electrode includes at least two second partitions, and the thicknesses of at least two second partitions are different. The connection between adjacent second partitions forms a third step, and the second partition located on the second step forms a fourth step.

2. The acoustic wave resonator according to claim 1, characterized in that Along the outward direction of the resonance effective area, the thicknesses of at least two of the first partitions are arranged alternately to form a phononic crystal structure.

3. The acoustic wave resonator according to claim 2, characterized in that The phononic crystal structure partially overlaps with the resonance effective region or is arranged outside the resonance effective region.

4. The acoustic wave resonator according to claim 2, characterized in that At the periphery of the phononic crystal structure, the thickness of the first partition is greater than or equal to the maximum thickness of the phononic crystal.

5. The acoustic wave resonator according to claim 1, characterized in that The first electrode is provided with a groove, the groove is located at the outer periphery of the first step, and the groove and the piezoelectric layer are surrounded to form an air gap.

6. The acoustic wave resonator according to any one of claims 1 to 5, characterized in that Also includes: A first isolation layer is located between the first electrode and the substrate.

7. The acoustic wave resonator according to claim 6, characterized in that A thickness of a portion of the first subregion in the first electrode is greater than a thickness of the first isolation layer.

8. The acoustic wave resonator according to claim 6, characterized in that The first isolation layer is arranged on the periphery of the sound reflection structure.

9. The acoustic wave resonator according to claim 8, characterized in that The first isolation layer includes at least one oxide layer and / or at least one amorphous silicon layer.

10. The acoustic wave resonator according to claim 6, characterized in that The first isolation layer covers the sound reflection structure and extends to the periphery of the sound reflection structure; The acoustic wave resonator further includes a second isolation layer located between the first isolation layer and the first electrode.

11. The acoustic wave resonator according to claim 10, characterized in that The first isolation layer includes at least one oxide layer, and the second isolation layer includes at least one amorphous silicon layer.

12. The acoustic wave resonator according to claim 10, characterized in that The second isolation layer covers the first isolation layer and extends outward, forming a fifth step at an end of the first isolation layer, and the first electrode forms a sixth step on the fifth step.

13. A filter, characterized in that: Comprising the acoustic wave resonator as claimed in any one of claims 1 to 12.

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