Elastic wave device, filter, and method for manufacturing elastic wave device

Abstract

To provide an elastic wave device capable of improving a Q value while improving an electromechanical coupling coefficient k2.SOLUTION: An elastic wave device 100 includes: a substrate 10; a lower electrode 12 provided on the substrate 10; an upper electrode 20 provided on the lower electrode 12; a first piezoelectric film 16 that forms a resonance region 50 while at least part of the first piezoelectric film is sandwiched between the lower electrode 12 and the upper electrode 20, and that is aluminum nitride doped with an impurity element that increases piezoelectricity; and a second piezoelectric film 18 that is located in a central region 54 that is a region falling inside the resonance region 50, that is provided between the lower electrode 12 and the first piezoelectric film 16, and that is aluminum nitride in which a doping amount of an impurity element that increases piezoelectricity is less than a doping amount in the first piezoelectric film 16.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an acoustic wave device, a filter, and a method for manufacturing an acoustic wave device. [Background technology]

[0002] As filters and duplexers for high-frequency circuits of wireless terminals such as mobile terminals, filters and duplexers using piezoelectric thin film resonators are known. The piezoelectric thin film resonator includes a piezoelectric film provided on a substrate, and a lower electrode and an upper electrode provided on either side of the piezoelectric film. The region where the lower electrode and the upper electrode face each other with the piezoelectric film in between is a resonance region where an elastic wave is excited. When an aluminum nitride film is used for the piezoelectric film, it is known to add scandium, a group 2 element or a group 12 element and a group 4 element, or a group 2 element or a group 12 element and a group 5 element to the aluminum nitride in order to enhance the piezoelectricity (for example, Patent Documents 1-3). It is also known to insert an insertion film into the piezoelectric film in the peripheral region of the resonance region, not in the central region of the resonance region, in order to improve the Q value (for example, Patent Documents 1-3). It is also known to provide a piezoelectric film in the central region of the region where the lower electrode and the upper electrode overlap, and provide a non-piezoelectric film in the peripheral region, thereby improving the Q value (for example, Patent Documents 4 and 5). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-207910 A [Patent Document 2] JP 2017-158161 A [Patent Document 3] JP 2019-186691 A [Patent Document 4] U.S. Patent No. 8,330,325 [Patent Document 5] U.S. Patent No. 8,796,904 Summary of the Invention [Problem to be solved by the invention]

[0004] In order to improve the Q value, if an insertion film or a non-piezoelectric film is provided in the peripheral region where the lower electrode and upper electrode overlap as described in Patent Documents 1-5, the piezoelectricity in the peripheral region decreases, and the electromechanical coupling coefficient k2 decreases.

[0005] The present invention has been made in consideration of the above-mentioned problems, and has an object to improve the Q value while improving the electromechanical coupling coefficient k2. [Means for solving the problem]

[0006] The present invention is an acoustic wave device comprising a substrate, a lower electrode provided on the substrate, an upper electrode provided on the lower electrode, a first piezoelectric film which is at least partially sandwiched between the lower electrode and the upper electrode to form a resonance region and which is made of aluminum nitride doped with an impurity element that enhances piezoelectricity, and a second piezoelectric film which is located in a central region that is within the resonance region and is provided between the lower electrode and the first piezoelectric film, and which is made of aluminum nitride doped with a smaller amount of the impurity element than the first piezoelectric film.

[0007] In the above configuration, the upper surface of the first piezoelectric film in the peripheral region that surrounds the central region along the outer periphery of the resonance region can be configured to have a rough surface shape with greater surface roughness than the upper surface of the first piezoelectric film in the central region.

[0008] In the above structure, the impurity element can be a combination of a Group 2 element or a Group 12 element and a Group 4 element, a combination of a Group 2 element or a Group 12 element and a Group 5 element, or a Group 3 element.

[0009] In the above configuration, the second piezoelectric film may be an aluminum nitride film that is not doped with the impurity element.

[0010] In the above configuration, the second piezoelectric film may have a thickness of 15 nm or more.

[0011] In the above configuration, the first piezoelectric film may be an aluminum nitride film doped with magnesium and hafnium as the impurity elements.

[0012] In the above configuration, the total thickness of the first piezoelectric film and the second piezoelectric film in the central region can be the same as the thickness of the first piezoelectric film in a peripheral region that surrounds the central region along the outer periphery of the resonance region.

[0013] In the above configuration, the second piezoelectric film may be embedded in the lower electrode.

[0014] In the above configuration, the side surface of the second piezoelectric film can be configured to have repeated projections and recesses having a depth of 0.5H or more and 1.5H or less, where H is the total thickness of the lower electrode, the second piezoelectric film, the first piezoelectric film, and the upper electrode in the central region.

[0015] In the above configuration, the distance between the outer periphery of the resonance region and the second piezoelectric film can be configured to be 2.4H or more and 3.0H or less, where H is the total thickness of the lower electrode, the second piezoelectric film, the first piezoelectric film, and the upper electrode in the central region.

[0016] The present invention is an acoustic wave device comprising a substrate, a lower electrode provided on the substrate, an upper electrode provided on the lower electrode, and a piezoelectric film comprising a first portion which is at least partially sandwiched between the lower electrode and the upper electrode to form a resonance region and which is made of aluminum nitride doped with an impurity element that enhances piezoelectricity, and a second portion which is located in a central region which is within the resonance region and is provided between the lower electrode and the first portion, and which is made of aluminum nitride doped with a smaller amount of the impurity element than the first portion.

[0017] The present invention is a filter including the above-described acoustic wave device.

[0018] The present invention is a method for manufacturing an acoustic wave device, comprising the steps of: forming a lower electrode on a substrate; forming an upper electrode on the substrate; forming a first piezoelectric film, which is aluminum nitride doped with an impurity element that enhances piezoelectricity, so that at least a portion of the first piezoelectric film is sandwiched between the lower electrode and the upper electrode to form a resonance region before forming the upper electrode; and forming a second piezoelectric film, which is aluminum nitride doped with a smaller amount of the impurity element than the first piezoelectric film, so that the second piezoelectric film is located in a central region that is within the resonance region and is provided between the lower electrode and the first piezoelectric film, before forming the first piezoelectric film. Effect of the Invention

[0019] According to the present invention, it is possible to improve the Q value while improving the electromechanical coupling coefficient k2. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1(a) is a plan view of an acoustic wave device in accordance with a first embodiment, FIG. 1(b) is a plan view of a second piezoelectric film in the first embodiment, and FIG. 1(c) is a cross-sectional view taken along line AA of FIG. 1(a). [Diagram 2] 2(a) to 2(d) are cross-sectional views illustrating a method for manufacturing the acoustic wave device in accordance with the first embodiment. [Diagram 3] 3(a) to 3(c) are cross-sectional views of the sample used in Experiment 1. [Figure 4] 4(a) to 4(e) are schematic diagrams of images of the sample used in Experiment 1 observed by an atomic force microscope. [Diagram 5] FIG. 5(a) is a perspective view showing the wurtzite crystal structure of aluminum nitride, and FIG. 5(b) is an image of abnormal crystals occurring in the first piezoelectric film. [Figure 6] 6(a) and 6(b) are cross-sectional views of acoustic wave devices according to comparative examples 1 and 2. FIG. [Figure 7] FIG. 7 is a cross-sectional view of an acoustic wave device in accordance with Comparative Example 3. As shown in FIG. [Figure 8] 8(a) and 8(b) are cross-sectional views of the first piezoelectric film in the first embodiment. [Figure 9] FIG. 9 shows the simulation results of the Q value of the anti-resonant frequency versus the insertion amount of the insertion film in Comparative Example 3. [Figure 10] FIG. 10A is a cross-sectional view of an acoustic wave device in accordance with the second embodiment, and FIG. 10B is a cross-sectional view of an acoustic wave device in accordance with a modified example of the second embodiment. [Figure 11] FIG. 11(a) is a plan view of a second piezoelectric film in Example 3, and FIG. 11(b) is an enlarged plan view of a portion of FIG. 11(a). [Figure 12] 12(a) to 12(c) are plan views of a second piezoelectric film in a modified example of the third embodiment. [Figure 13] FIG. 13A is a cross-sectional view of an acoustic wave device in accordance with the fourth embodiment, and FIG. 13B is a cross-sectional view of an acoustic wave device in accordance with a modified example of the fourth embodiment. [Figure 14] FIG. 14 is a circuit diagram of a filter in accordance with the fifth embodiment. [Figure 15] FIG. 15 is a circuit diagram of a duplexer in accordance with the sixth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Hereinafter, with reference to the drawings, an embodiment of the present invention will be described taking an acoustic wave device as an example of an electronic component. EXAMPLES

[0022] Fig. 1(a) is a plan view of an acoustic wave device 100 according to a first embodiment, Fig. 1(b) is a plan view of a second piezoelectric film 18 in the first embodiment, and Fig. 1(c) is a cross-sectional view taken along line AA of Fig. 1(a). Fig. 1(a) mainly illustrates a lower electrode 12, a piezoelectric film 14, and an upper electrode 20. As shown in Figs. 1(a) to 1(c), the acoustic wave device 100 according to the first embodiment is a piezoelectric thin film resonator including a lower electrode 12, a piezoelectric film 14, and an upper electrode 20.

[0023] The lower electrode 12 is provided on a substrate 10. A gap 30 is formed between the flat upper surface of the substrate 10 and the lower electrode 12. The gap 30 can have various shapes, for example, a rectangular shape or a dome shape in cross section. The substrate 10 is, for example, a silicon (Si) substrate, and has a thickness of 100 μm to 1000 μm. The lower electrode 12 has a two-layer structure, for example, a lower layer that is a chromium (Cr) film and an upper layer that is a ruthenium (Ru) film, and has a thickness of 30 nm to 400 nm.

[0024] A piezoelectric film 14 is provided on the lower electrode 12. The piezoelectric film 14 includes a first piezoelectric film 16 (first portion) and a second piezoelectric film 18 (second portion). The first piezoelectric film 16 and the second piezoelectric film 18 are aluminum nitride films mainly composed of aluminum nitride (AlN). The main component may be a total of 50 atomic % or more of aluminum atoms and nitrogen atoms, or 80 atomic % or more. The second piezoelectric film 18 is provided on the lower electrode 12 so as to overlap the gap 30 in a plan view. The second piezoelectric film 18 is in contact with the upper surface of the lower electrode 12. The thickness of the second piezoelectric film 18 is 15 nm or more. The first piezoelectric film 16 is provided on the lower electrode 12 and the substrate 10 so as to cover the second piezoelectric film 18. The first piezoelectric film 16 is in contact with the lower electrode 12 around the second piezoelectric film 18. The thickness of the first piezoelectric film 16 is 800 nm to 1500 nm.

[0025] The first piezoelectric film 16 is an aluminum nitride film doped with an impurity element that enhances the piezoelectricity. For example, the impurity element is a combination of a group 2 element or a group 12 element and a group 4 element, or a combination of a group 2 element or a group 12 element and a group 5 element, or a group 3 element. This improves the piezoelectricity of the first piezoelectric film 16, and can improve the effective electromechanical coupling coefficient k2. The group 2 element is, for example, magnesium (Mg), calcium (Ca), or strontium (Sr). The group 12 element is, for example, zinc (Zn). The group 4 element is, for example, titanium (Ti), zirconium (Zr), or hafnium (Hf). The group 5 element is, for example, vanadium (V), niobium (Nb), or tantalum (Ta). The group 3 element is, for example, scandium (Sc). In the first embodiment, a case in which magnesium and hafnium are doped as impurity elements that enhance the piezoelectricity of the first piezoelectric film 16 is shown as an example. The total doping amount of magnesium and hafnium is more than 5 atomic % and not more than 20 atomic %. The definition of the doping amount will be described later.

[0026] The second piezoelectric film 18 is an aluminum nitride film doped with a smaller amount of impurity element that enhances piezoelectricity than the first piezoelectric film 16. In the first embodiment, the second piezoelectric film 18 is an aluminum nitride film that is not doped with an impurity element that enhances piezoelectricity. Not doped with an impurity element that enhances piezoelectricity means that the film is not intentionally doped with an impurity element that enhances piezoelectricity, for example, the amount of the impurity element that enhances piezoelectricity is 0.5 atomic % or less.

[0027] An upper electrode 20 is provided on the piezoelectric film 14. The upper electrode 20 is provided on the piezoelectric film 14 so as to have an area facing the lower electrode 12 with the piezoelectric film 14 in between. The area where the lower electrode 12 and the upper electrode 20 face each other with the piezoelectric film 14 in between is a resonance area 50. The resonance area 50 is an area where an elastic wave in a thickness longitudinal vibration mode resonates. The resonance area 50 has, for example, an elliptical shape in a plan view. In a plan view, the size of the gap 30 is the same as or larger than the resonance area 50. The resonance area 50 may have a polygonal shape such as a rectangle or a pentagon in a plan view. The upper electrode 20 has a two-layer structure, for example, a lower layer that is a Ru film and an upper layer that is a Cr film, and has a thickness of 30 nm to 400 nm.

[0028] The total thickness of the lower electrode 12, the piezoelectric film 14, and the upper electrode 20 in the central region 54 is defined as H. The wavelength λ of the elastic wave in the thickness extensional vibration mode excited in the resonance region 50 is approximately twice the total thickness H of the lower electrode 12, the piezoelectric film 14, and the upper electrode 20.

[0029] A hole 35 is provided in the lower electrode 12. The hole 35 communicates with the void 30 via an introduction path 33 below the lower electrode 12. The hole 35 and the introduction path 33 are for introducing an etching liquid into the sacrificial layer used to form the void 30 when etching the sacrificial layer.

[0030] The second piezoelectric film 18 is not provided in the outer peripheral region 52 in the resonance region 50, but is provided between the lower electrode 12 and the first piezoelectric film 16 in the central region 54. The central region 54 is a region in the resonance region 50 that includes the center of the resonance region 50. The central region 54 does not have to be the geometric center. The outer peripheral region 52 is a region in the resonance region 50 that includes the outer periphery of the resonance region 50 and surrounds the central region 54 along the outer periphery. The outer peripheral region 52 is, for example, strip-shaped or ring-shaped. The second piezoelectric film 18 has an elliptical shape that is generally the same as the resonance region 50 in a plan view. Therefore, the side surface of the second piezoelectric film 18 generally follows the outer periphery of the resonance region 50. The second piezoelectric film 18 is in contact with the lower electrode 12 in the central region 54 and is provided between the lower electrode 12 and the first piezoelectric film 16.

[0031] The first piezoelectric film 16 is provided from the central region 54 to the outer peripheral region 52 so as to cover the second piezoelectric film 18. The first piezoelectric film 16 is in contact with the second piezoelectric film 18, and is in contact with the lower electrode 12 in the outer peripheral region 52. The first piezoelectric film 16 is in contact with the upper electrode 20 in the resonance region 50.

[0032] As the substrate 10, in addition to a Si substrate, for example, an insulating substrate or a semiconductor substrate such as a sapphire substrate, a spinel substrate, an alumina substrate, a quartz substrate, a glass substrate, a ceramic substrate, or a gallium arsenide substrate can be used. As the lower electrode 12 and the upper electrode 20, in addition to Ru and Cr, for example, a single layer film of aluminum (Al), titanium (Ti), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), rhodium (Rh), iridium (Ir), or the like, or a laminated film thereof can be used.

[0033] [Manufacturing method] 2(a) to 2(d) are cross-sectional views showing a method for manufacturing the acoustic wave device 100 according to the first embodiment. As shown in FIG. 2(a), a sacrificial layer 38 for forming the void 30 is formed on the flat upper surface of the substrate 10. The thickness of the sacrificial layer 38 is, for example, 10 nm to 100 nm. The sacrificial layer 38 is selected from materials that are easily dissolved in an etching solution or etching gas, such as magnesium oxide, zinc oxide, germanium, or silicon oxide. The sacrificial layer 38 is formed by forming a film using a sputtering method, a vacuum deposition method, or a CVD (Chemical Vapor Deposition) method, and then patterning the film into a desired shape using a photolithography method and an etching method. The sacrificial layer 38 may be formed by a lift-off method. The shape of the sacrificial layer 38 corresponds to the planar shape of the void 30. Next, a lower electrode 12 is formed on the sacrificial layer 38 and the substrate 10. The lower electrode 12 is formed by forming a film using a sputtering method, a vacuum deposition method, or a CVD method, and then patterning the film into a desired shape using a photolithography technique and an etching technique. The lower electrode 12 may also be formed by a lift-off method.

[0034] 2(b), the second piezoelectric film 18 is formed on the lower electrode 12 so as to overlap the sacrificial layer 38 in a plan view. The second piezoelectric film 18 is formed by forming a film using a sputtering method, a CVD method, or an ALD (Atomic Layer Deposition) method, and then patterning the film into a desired shape using a photolithography technique and an etching technique.

[0035] As shown in FIG. 2(c), the first piezoelectric film 16 is formed on the substrate 10 and the lower electrode 12 so as to cover the second piezoelectric film 18. The first piezoelectric film 16 is formed by sputtering, CVD, or ALD. The first piezoelectric film 16 and the second piezoelectric film 18 form the piezoelectric film 14. The upper electrode 20 is formed on the first piezoelectric film 16. The upper electrode 20 is formed by forming the film by sputtering, vacuum deposition, or CVD, and then patterning the film into a desired shape by photolithography and etching. The upper electrode 20 may be formed by a lift-off method. If necessary, the first piezoelectric film 16 is patterned into a desired shape by photolithography and etching.

[0036] As shown in FIG. 2(d), an etching solution for the sacrificial layer 38 is introduced into the sacrificial layer 38 below the lower electrode 12 through the hole 35 and the introduction path 33 (see FIG. 1(a)). This removes the sacrificial layer 38. The medium for etching the sacrificial layer 38 is preferably a medium that barely etches the materials constituting the resonator other than the sacrificial layer 38. By removing the sacrificial layer 38, a gap 30 is formed between the lower electrode 12 and the substrate 10. In this manner, the acoustic wave device 100 in accordance with the first embodiment is formed.

[0037] [Experiment 1] 3(a) to 3(c) are cross-sectional views of samples A to E for which experiment 1 was performed. As shown in FIG. 3(a) to 3(c), the experiment was performed on three types of structures. As shown in FIG. 3(a), the first structure was a structure in which a sacrificial layer 38, a lower electrode 12, and a first piezoelectric film 16 were formed in this order on a substrate 10, and sample A of this structure was fabricated. As shown in FIG. 3(b), the second structure was a structure in which a sacrificial layer 38, a lower electrode 12, a second piezoelectric film 18, and a first piezoelectric film 16 were formed in this order on a substrate 10, and samples B, C, and D of this structure were fabricated. As shown in FIG. 3(c), the third structure was a structure in which a sacrificial layer 38, a lower electrode 12, and a second piezoelectric film 18 were formed in this order on a substrate 10, and sample E of this structure were fabricated. The sacrificial layer 38 was formed by vacuum deposition. The lower electrode 12 was formed by sputtering. The first piezoelectric film 16 and the second piezoelectric film 18 were formed by sputtering.

[0038] The thickness and material of each film in Samples A to E are as follows. Common conditions for samples A to E Substrate 10: Si substrate Sacrificial layer 38: Magnesium oxide film with a thickness of 50 nm Lower electrode 12: a two-layer structure consisting of a lower layer made of a 50 nm thick Cr film and an upper layer made of a 70 nm thick Ru film. Conditions of Sample A First piezoelectric film 16: AlN film with a thickness of 400 nm and doped with Mg and Hf (the total doping amount of Mg and Hf is 11 atomic %) Conditions for Sample B Second piezoelectric film 18: an AlN film having a thickness of 15 nm and not doped with impurity elements that enhance the piezoelectricity First piezoelectric film 16: an AlN film having a thickness of 385 nm and doped with Mg and Hf (the total doping amount of Mg and Hf is 11 atomic %) Conditions for Sample C Second piezoelectric film 18: an AlN film having a thickness of 30 nm and not doped with impurity elements that enhance the piezoelectricity First piezoelectric film 16: an AlN film having a thickness of 370 nm and doped with Mg and Hf (the total doping amount of Mg and Hf is 11 atomic %) Conditions for Sample D Second piezoelectric film 18: an AlN film having a thickness of 50 nm and not doped with impurity elements that enhance the piezoelectricity First piezoelectric film 16: an AlN film having a thickness of 350 nm and doped with Mg and Hf (the total doping amount of Mg and Hf is 11 atomic %) Conditions of Sample E Second piezoelectric film 18: an AlN film having a thickness of 400 nm and not doped with impurity elements that enhance the piezoelectricity

[0039] The top surfaces of the first piezoelectric film 16 of Samples A to D and the top surface of the second piezoelectric film 18 of Sample E were observed using an atomic force microscope (AFM), and the arithmetic mean roughness Ra was measured. Figures 4(a) to 4(e) are schematic diagrams of the atomic force microscope observation images of Samples A to E used in Experiment 1.

[0040] In the case where the first piezoelectric film 16 is formed directly on the lower electrode 12, as in sample A in Fig. 3(a), it was confirmed that a large number of large abnormal crystals 72 were generated in the first piezoelectric film 16, as shown in Fig. 4(a). The arithmetic mean roughness Ra of the upper surface of the first piezoelectric film 16 was 5.47 nm. When the half-width of the X-ray diffraction rocking curve of the first piezoelectric film 16 was measured, the half-width was about 4.4 nm.

[0041] In the case where the first piezoelectric film 16 is formed on the second piezoelectric film 18 as in samples B, C, and D in FIG. 3(b), it was confirmed that the abnormal crystals 72 of the first piezoelectric film 16 are reduced compared to sample A, as shown in FIG. 4(b) to FIG. 4(d). In addition, since the abnormal crystals 72 are reduced in the order of sample B, sample C, and sample D, it was also confirmed that the abnormal crystals 72 are reduced as the second piezoelectric film 18 becomes thicker. The arithmetic mean roughness Ra of the upper surface of the first piezoelectric film 16 in sample B was 3.40 nm. The arithmetic mean roughness Ra of the upper surface of the first piezoelectric film 16 in sample C was 1.74 nm. The arithmetic mean roughness Ra of the upper surface of the first piezoelectric film 16 in sample D was 1.83 nm. In addition, when the half-width of the X-ray diffraction rocking curve of the first piezoelectric film 16 of sample D was measured, the half-width was about 4.4 nm.

[0042] In the case where the first piezoelectric film 16 is not provided, as in sample E in Fig. 3(c), it was confirmed that almost no abnormal crystals 72 were generated in the second piezoelectric film 18, as shown in Fig. 4(e). The arithmetic mean roughness Ra of the upper surface of the second piezoelectric film 18 in sample E was 2.50 nm. When the half-width of the X-ray diffraction rocking curve of the second piezoelectric film 18 was measured, the half-width was about 3.0 nm.

[0043] Aluminum nitride (AlN) generally has a crystal structure of a wurtzite structure that is energetically stable. FIG. 5(a) is a perspective view showing the crystal structure of the wurtzite structure of AlN. Aluminum (Al) atoms are indicated by reference numerals 74 and nitrogen (N) atoms are indicated by reference numerals 75, and are hatched for clarity of the drawing. The wurtzite structure has a c-axis orientation in which the plane parallel to the substrate 10 is the c-plane. Since the second piezoelectric film 18 is an AlN film that is not doped with impurity elements that enhance the piezoelectricity, the wurtzite structure grows in the c-axis direction. For this reason, abnormal crystal growth is unlikely to occur in the second piezoelectric film 18. Therefore, it is considered that in sample E, as shown in FIG. 4(e), almost no abnormal crystals 72 were observed in the second piezoelectric film 18.

[0044] On the other hand, when AlN is doped with an impurity element such as Mg or Hf that enhances the piezoelectricity, the impurity atoms such as Mg and Hf are replaced with Al atoms 74 in the wurtzite structure. That is, the impurity atoms enter the aluminum sites. In this application, the atomic concentration of the impurity element when the total amount of the aluminum element atoms and the doped impurity element atoms is 100 atomic % is defined as the doping amount of the impurity element. For example, when the doping amount of the impurity element is 10 atomic %, one of the Al atoms 74 in FIG. 5(a) is replaced with the impurity atom.

[0045] Impurity elements such as Mg and Hf are basically doped almost uniformly into the AlN film. However, when the doping amount of the impurity element is large or due to the influence of the underlayer, etc., there may be localized locations where most of the Al atoms 74 are replaced by impurity atoms. When most of the Al atoms 74 are replaced by impurity atoms, the crystal structure may not be a wurtzite structure and the axis may be tilted. It is believed that abnormal crystal growth occurs when the wurtzite structure of AlN doped with the impurity element grows again along this tilted axis.

[0046] FIG. 5(b) is an image diagram of abnormal crystals 72 occurring in the first piezoelectric film 16. In FIG. 5(b), impurity atoms are indicated by the reference numeral 76. As shown in FIG. 5(b), the AlN film doped with an impurity element that enhances the piezoelectricity basically grows with a wurtzite crystal structure, but when the doping amount of the impurity element is large, a portion 77 where most of the Al atoms are replaced by the impurity atoms may occur locally. At the portion 77, the axis of the crystal structure may be tilted from the c-axis, and the wurtzite structure of the impurity-doped AlN thereafter grows with the axis tilted, which is thought to cause the abnormal crystals 72 to occur.

[0047] Therefore, in sample A, as shown in FIG. 4(a), many abnormal crystals 72 were generated in the first piezoelectric film 16, and the unevenness of the upper surface became large. On the other hand, in samples B, C, and D, the first piezoelectric film 16 is provided on the second piezoelectric film 18. The second piezoelectric film 18 is an AlN film that is not doped with impurity elements that enhance the piezoelectricity, so the generation of abnormal crystals 72 is suppressed. By forming the first piezoelectric film 16 using such a second piezoelectric film 18 as a base film, it is considered that the abnormal crystals 72 of the first piezoelectric film 16 are reduced due to the influence of the second piezoelectric film 18, as shown in FIG. 4(b) to FIG. 4(d). Since the influence of the second piezoelectric film 18 on the formation of the first piezoelectric film 16 is reduced when the second piezoelectric film 18 is thin, it is considered that the abnormal crystals 72 of the first piezoelectric film 16 are reduced when the second piezoelectric film 18 is thick.

[0048] [Comparative Example] 6(a) and 6(b) are cross-sectional views of acoustic wave devices 900 and 910 according to Comparative Examples 1 and 2. As shown in Fig. 6(a), in the acoustic wave device 900 according to Comparative Example 1, the first piezoelectric film 16 is provided directly on the lower electrode 12, and the second piezoelectric film 18 is not provided between the lower electrode 12 and the first piezoelectric film 16. The other configurations are the same as those in Example 1, and therefore will not be described.

[0049] 6(b), in an acoustic wave device 910 in accordance with Comparative Example 2, the second piezoelectric film 18 is provided over the entire resonance region 50. The other configurations are the same as those in Example 1, and therefore will not be described.

[0050] [Experiment 2] Acoustic wave devices 900 and 910 according to Comparative Examples 1 and 2 were fabricated, and the Q value and electromechanical coupling coefficient k2 of each were measured. The thickness and material of each layer of the fabricated acoustic wave devices 900 and 910 are as follows. Substrate 10: Si substrate Lower electrode 12: a two-layer structure consisting of a lower layer made of a 50 nm thick Cr film and an upper layer made of a 70 nm thick Ru film. Second piezoelectric film 18: an AlN film having a thickness of 50 nm and not doped with impurity elements that enhance the piezoelectricity First piezoelectric film 16: an AlN film having a thickness of 350 nm and doped with Mg and Hf (the total doping amount of Mg and Hf is 11 atomic %) Upper electrode 20: a two-layer structure consisting of a lower layer made of a Ru film with a thickness of 70 nm and an upper layer made of a Cr film with a thickness of 50 nm

[0051] When the Q value and the electromechanical coupling coefficient k2 of the acoustic wave device 900 in accordance with Comparative Example 1 were measured, the maximum Q value was 940, and the electromechanical coupling coefficient k2 was 8.0%. When the Q value and the electromechanical coupling coefficient k2 of the acoustic wave device 910 in accordance with Comparative Example 2 were measured, the maximum Q value was 940, and the electromechanical coupling coefficient k2 was 7.5%.

[0052] These experimental results show that even when the second piezoelectric film 18 is provided between the lower electrode 12 and the first piezoelectric film 16, the Q value and electromechanical coupling coefficient k2 are equivalent to those when the second piezoelectric film 18 is not provided.

[0053] 7 is a cross-sectional view of an acoustic wave device 920 according to Comparative Example 3. As shown in FIG. 7, in the acoustic wave device 920 according to Comparative Example 3, the second piezoelectric film 18 is not provided between the lower electrode 12 and the first piezoelectric film 16, and instead, an insertion film 60 is provided in the first piezoelectric film 16. The insertion film 60 is provided in the outer periphery region 52 in the resonance region 50, but not in the central region 54. Therefore, the insertion film 60 is provided along the outer periphery of the resonance region 50 so as to surround the central region 54. The insertion film 60 is provided continuously not only in the outer periphery region 52 but also in the region surrounding the resonance region 50 outside the outer periphery region 52.

[0054] The insertion film 60 is formed of a material having a smaller Young's modulus and / or acoustic impedance than the first piezoelectric film 16. For example, the insertion film 60 is a silicon oxide film, but may be a single layer film of Al, Au, Cu, Ti, Pt, Ta, Cr, or the like, or a laminated film of these. By providing the insertion film 60 in the outer peripheral region 52, the sound velocity of the elastic wave in the outer peripheral region 52 becomes different from the sound velocity of the elastic wave in the central region 54, so that a confinement effect of the elastic wave occurs and the elastic wave is less likely to leak out of the resonance region 50. Therefore, the Q value can be improved.

[0055] However, in Comparative Example 3, the piezoelectricity is reduced in the outer peripheral region 52 into which the insertion film 60 is inserted because the insertion film 60 is not a piezoelectric film. That is, even if an aluminum nitride film doped with an impurity element that enhances the piezoelectricity is used as the first piezoelectric film 16 in order to improve the electromechanical coupling coefficient k2, the effect of improving the electromechanical coupling coefficient k2 is reduced.

[0056] In contrast, according to the first embodiment, as shown in Figs. 1(a) to 1(c), the first piezoelectric film 16 is provided in the resonance region 50, and the second piezoelectric film 18 is located in the central region 54, which is a region that is contained inside the resonance region 50, and is provided between the lower electrode 12 and the first piezoelectric film 16. The second piezoelectric film 18 is an aluminum nitride film in which the doping amount of impurity elements that increase the piezoelectricity is smaller than that of the first piezoelectric film 16. Therefore, as shown in Fig. 4(e), the second piezoelectric film 18 is less likely to generate abnormal crystals 72. Since the first piezoelectric film 16 is provided on the second piezoelectric film 18 in the central region 54, the generation of abnormal crystals 72 is reduced due to the influence of the second piezoelectric film 18, which is the base film, as shown in Figs. 4(b) to 4(d). On the other hand, in the peripheral region 52, the first piezoelectric film 16 is provided on the lower electrode 12, so many large abnormal crystals 72 are generated as shown in Fig. 4(a). Therefore, the crystal state of the first piezoelectric film 16 differs between the central region 54 and the peripheral region 52. Therefore, the acoustic velocity of the elastic wave differs between the central region 54 and the peripheral region 52, and an elastic wave confinement effect occurs, making it difficult for the elastic wave to leak out of the resonance region 50. This improves the Q value. Furthermore, the abnormal crystals 72 generated in the first piezoelectric film 16 in the central region 54 are reduced, thereby suppressing the frequency variation. Furthermore, as in the above-mentioned experiment 2, the electromechanical coupling coefficient k2 is equal regardless of the presence or absence of the second piezoelectric film 18. Therefore, even when the second piezoelectric film 18 is provided in the central region 54, the electromechanical coupling coefficient k2 is equal to that when the second piezoelectric film 18 is not provided. Therefore, by using an aluminum nitride film doped with an impurity element that enhances the piezoelectricity as the first piezoelectric film 16, the electromechanical coupling coefficient k2 can be improved. Therefore, according to the first embodiment, the electromechanical coupling coefficient k2 can be improved while improving the Q value.

[0057] In Example 1, the first piezoelectric film 16 and the second piezoelectric film 18 are observed as separate films as shown in Fig. 1(c), but the present invention is not limited to this case and may be observed as a single film without observing the boundary. In this case, the piezoelectric film 14 can be said to be composed of a first portion (corresponding to the first piezoelectric film 16) doped with a large amount of impurity element that enhances piezoelectricity and a second portion (corresponding to the second piezoelectric film 18) doped with a small amount of impurity element that enhances piezoelectricity.

[0058] 8(a) and 8(b) are cross-sectional views of the first piezoelectric film 16 in the first embodiment. FIG. 8(a) shows a cross-section of the first piezoelectric film 16 in the central region 54, and FIG. 8(b) shows a cross-section of the first piezoelectric film 16 in the peripheral region 52. As shown in FIG. 8(a) and FIG. 8(b), the upper surface of the first piezoelectric film 16 has a larger surface roughness in the peripheral region 52 than in the central region 54. This is believed to be due to the following reasons. As described above, the first piezoelectric film 16 is more likely to have large abnormal crystals 72 in the peripheral region 52 than in the central region 54. The more large abnormal crystals 72 are generated, the larger the unevenness of the upper surface becomes. For this reason, it is believed that the upper surface of the first piezoelectric film 16 has a larger surface roughness in the peripheral region 52 than in the central region 54.

[0059] Thus, in Example 1, the upper surface of the first piezoelectric film 16 in the outer peripheral region 52 has a rough surface shape with a larger surface roughness than the upper surface of the first piezoelectric film 16 in the central region 54. This indicates that the abnormal crystals 72 occurring in the first piezoelectric film 16 are more numerous in the outer peripheral region 52 than in the central region 54. Therefore, since the surface roughness of the upper surface of the first piezoelectric film 16 is larger in the outer peripheral region 52 than in the central region 54, it can be said that the crystal state of the first piezoelectric film 16 is different between the central region 54 and the outer peripheral region 52. Therefore, the sound speed of the elastic wave is different between the central region 54 and the outer peripheral region 52, and the leakage of the elastic wave from the resonance region 50 to the outside is suppressed. In the above-mentioned Experiment 2, the elastic wave device 900 according to Comparative Example 1 does not have the second piezoelectric film 18, and the first piezoelectric film 16 is provided on the lower electrode 12. The arithmetic mean roughness Ra of the upper surface of the first piezoelectric film 16 at this time is assumed to be about 5.5 nm from the experimental result of Sample A in FIG. 4(a). In the acoustic wave device 910 according to the comparative example 2, the first piezoelectric film 16 is provided on the second piezoelectric film 18 having a thickness of 50 nm. The arithmetic mean roughness Ra of the upper surface of the first piezoelectric film 16 is assumed to be about 1.8 nm from the experimental result of the sample D in FIG. 4(d). Considering that the acoustic wave device 900 according to the comparative example 1 and the acoustic wave device 910 according to the comparative example 2 have the same electromechanical coupling coefficient k2 in the experiment 2, the arithmetic mean roughness Ra of the upper surface of the first piezoelectric film 16 in the peripheral region 52 is preferably 1.5 to 4.5 times, more preferably 2.0 to 4.0 times, and even more preferably 2.5 to 3.5 times, of the arithmetic mean roughness Ra of the upper surface of the first piezoelectric film 16 in the central region 54. The roughness of the upper surface of the first piezoelectric film 16 is not limited to the observation by AFM. For example, if the roughness of the upper surface can be compared by visual observation, visual observation may be used.

[0060] In the first embodiment, the second piezoelectric film 18 is an aluminum nitride film that is not doped with an impurity element that enhances the piezoelectricity. This effectively reduces the abnormal crystals 72 that occur in the first piezoelectric film 16 in the central region 54. This allows the crystal state of the first piezoelectric film 16 to be significantly different between the central region 54 and the peripheral region 52. The second piezoelectric film 18 may be doped with an impurity element that enhances the piezoelectricity, but from the viewpoint of reducing the abnormal crystals 72 that occur in the first piezoelectric film 16, it is preferable that the doping amount is small. The doping amount of the impurity element is preferably 5 atomic % or less, more preferably 3 atomic % or less, and even more preferably 1 atomic % or less.

[0061] In Example 1, the thickness of the second piezoelectric film 18 is 15 nm or more. This makes it possible to effectively reduce the abnormal crystals 72 generated in the first piezoelectric film 16, as in the above-mentioned Experiment 1. Therefore, the crystal state of the first piezoelectric film 16 can be greatly different between the central region 54 and the peripheral region 52, and the leakage of the elastic wave from the resonance region 50 can be effectively suppressed. From the viewpoint of reducing the abnormal crystals 72 generated in the first piezoelectric film 16, the thickness of the second piezoelectric film 18 is preferably 30 nm or more, more preferably 40 nm or more, and even more preferably 50 nm or more. On the other hand, if the first piezoelectric film 16 becomes thinner as the second piezoelectric film 18 becomes thicker, the effect of improving the electromechanical coupling coefficient k2 is weakened. For this reason, the thickness of the second piezoelectric film 18 is preferably 20% or less of the thickness of the first piezoelectric film 16, more preferably 15% or less, and even more preferably 10% or less.

[0062] In the first embodiment, the impurity element doped in the first piezoelectric film 16 to enhance the piezoelectricity is a combination of magnesium, a group 2 element, and hafnium, a group 4 element. This allows the electromechanical coupling coefficient k2 to be increased. The impurity element doped in the first piezoelectric film 16 to enhance the piezoelectricity may be other than magnesium and hafnium. The impurity element may be a combination of a group 2 element (e.g., magnesium, calcium, strontium) or a group 12 element (e.g., zinc) and a group 4 element (e.g., titanium, zirconium, hafnium), a combination of a group 2 element or a group 12 element and a group 5 element (e.g., vanadium, niobium, tantalum), or a group 3 element (e.g., scandium). The electromechanical coupling coefficient k2 is increased when these impurity elements are doped. These impurity elements are also substituted for the aluminum site of aluminum nitride. The names of the groups of elements are in accordance with the IUPAC (International Union of Pure and Applied Chemistry) notation.

[0063] In the first embodiment, the total doping amount of magnesium and hafnium doped in the first piezoelectric film 16 is more than 5 atomic % and less than 20 atomic %. By making the total doping amount more than 5 atomic %, the electromechanical coupling coefficient k2 can be effectively increased. From the viewpoint of increasing the electromechanical coupling coefficient k2, the total doping amount is preferably 8 atomic % or more, more preferably 10 atomic % or more, and even more preferably 12 atomic % or more. On the other hand, if the total doping amount is too high, even if the second piezoelectric film 18 is provided, the effect of reducing the abnormal crystals 72 generated in the first piezoelectric film 16 is weakened. By making the total doping amount less than 20 atomic %, the abnormal crystals 72 generated in the first piezoelectric film 16 in the central region 54 can be effectively reduced, and the crystal state of the first piezoelectric film 16 can be greatly different between the central region 54 and the peripheral region 52. From the viewpoint of reducing the occurrence of abnormal crystals 72 in the first piezoelectric film 16, the total doping amount is preferably 18 atomic % or less, more preferably 16 atomic % or less, and further preferably 14 atomic % or less.

[0064] [simulation] A simulation was performed on the acoustic wave device 920 in accordance with Comparative Example 3 shown in Fig. 7. The simulation was performed for the Q value of the anti-resonant frequency versus the insertion amount of the insertion membrane 60 into the resonance region 50 (i.e., the width of the outer circumferential region 52). The simulation conditions were as follows. Substrate 10: Si substrate Lower electrode 12: a two-layer structure consisting of a lower layer made of a 55 nm thick Cr film and an upper layer made of a 130 nm thick Ru film. First piezoelectric film 16: AlN film with a thickness of 700 nm Insertion film 60: SiO2 film with a thickness of 75 nm Upper electrode 20: a lower layer made of a Ru film having a thickness of 130 nm and an upper layer made of a Cr film having a thickness of 55 nm

[0065] Fig. 9 shows the simulation results of the Q value of the anti-resonant frequency versus the insertion amount of the insertion film 60 in Comparative Example 3. In Fig. 9, the horizontal axis represents the insertion amount of the insertion film 60 into the resonance region 50 (the width of the outer peripheral region 52), and the vertical axis represents the Q value of the anti-resonant frequency. As shown in Fig. 9, the Q value is maximized when the insertion amount is approximately 0.7λ and 1.3λ.

[0066] Therefore, in the first embodiment, it is considered that the Q value of the anti-resonant frequency is increased by setting the width of the outer peripheral region 52 where the second piezoelectric film 18 is not provided to about 0.7λ or about 1.3λ. Here, from the viewpoint of ease of manufacturing the second piezoelectric film 18, it is preferable that the width of the outer peripheral region 52 is relatively large. Therefore, in the first embodiment, the width of the outer peripheral region 52 where the second piezoelectric film 18 is not provided is set to 1.2λ or more and 1.5λ or less. In other words, the distance between the outer periphery of the resonance region 50 and the second piezoelectric film 18 is set to 2.4H or more and 3.0H or less, where H is the total thickness of the lower electrode 12, the second piezoelectric film 18, the first piezoelectric film 16, and the upper electrode 20 in the central region 54. This makes it possible to obtain a large Q value while ensuring ease of manufacturing the second piezoelectric film 18. From the viewpoint of obtaining a large Q value, the distance between the outer periphery of the resonance region 50 and the second piezoelectric film 18 is preferably 2.5H or more and 2.8H or less, more preferably 2.6H or more and 2.7H or less. EXAMPLES

[0067] Fig. 10(a) is a cross-sectional view of an acoustic wave device 200 according to a second embodiment. As shown in Fig. 10(a), in the acoustic wave device 200 according to the second embodiment, the upper surface of the first piezoelectric film 16 is flattened by, for example, a chemical mechanical polishing (CMP) method. Therefore, the thickness of the piezoelectric film 14 is the same in the central region 54 and the peripheral region 52. The other configurations are the same as those in the first embodiment, and therefore will not be described.

[0068] According to the second embodiment, the total thickness of the first piezoelectric film 16 and the second piezoelectric film 18 in the central region 54 is the same as the thickness of the first piezoelectric film 16 in the peripheral region 52. This makes the resonant frequency of the elastic wave excited in the resonance region 50 a constant value. Note that "the thicknesses are the same" does not necessarily mean that they are completely the same, but also means that differences due to manufacturing errors are allowed.

[0069] FIG. 10(b) is a cross-sectional view of an acoustic wave device 210 according to a modification of the second embodiment. As shown in FIG. 10(b), in the acoustic wave device 210 according to the modification of the second embodiment, the second piezoelectric film 18 is embedded in the lower electrode 12, and the upper surface of the second piezoelectric film 18 and the upper surface of the lower electrode 12 are flush with each other. As in the second embodiment, the upper surface of the first piezoelectric film 16 is flattened by, for example, a CMP method. The other configurations are the same as those in the first embodiment, and therefore the description is omitted. In this way, the second piezoelectric film 18 may be embedded in the lower electrode 12. As a result, the thickness of the first piezoelectric film 16 becomes approximately the same in the central region 54 and the peripheral region 52, and therefore the effect of confining the acoustic wave is increased. EXAMPLES

[0070] FIG. 11(a) is a plan view of the second piezoelectric film 18 in the third embodiment, and FIG. 11(b) is an enlarged plan view of a part of FIG. 11(a). As shown in FIG. 11(a) and FIG. 11(b), in the third embodiment, the side surface of the second piezoelectric film 18 is repeatedly formed with unevenness 26 consisting of recesses 22 and protrusions 24. The recesses 22 and the protrusions 24 are, for example, substantially rectangular in plan view. The depth D of the recesses 22 is greater than λ / 4 and smaller than 3λ / 4. The width W1 of the recesses 22 and the width W2 of the protrusions 24 are, for example, greater than λ / 4 and less than 2λ. The width W1 of the recesses 22 and the width W2 of the protrusions 24 may be the same size. The same size means that the difference between the widths W1 and W2 is 5% or less. The depth D of the recesses 22 is the depth of the deepest part when the bottom surface of the recesses 22 is curved, etc. The width W1 of the recess 22 is defined as the width of the portion closest to the center of the resonance region 50 when the two side surfaces of the recess 22 are inclined toward each other. Similarly, the width W2 of the protrusion 24 is defined as the width of the portion closest to the center of the resonance region 50 when the two side surfaces of the protrusion 24 are inclined toward each other. The center of the resonance region 50 is, for example, the center of gravity of the resonance region 50. The other configurations of the acoustic wave device according to Example 3 are the same as those of Example 1, and therefore description thereof will be omitted.

[0071] In the third embodiment, when the total thickness of the lower electrode 12, the second piezoelectric film 18, the first piezoelectric film 16, and the upper electrode 20 in the central region 54 is H, the concaves and convexes 26 having a depth of 0.5H to 1.5H are repeatedly formed on the side surface of the second piezoelectric film 18. This makes it possible to cancel and weaken the standing waves reflected at the interface between the central region 54 and the peripheral region 52, thereby suppressing spurious. From the viewpoint of weakening the standing waves, the depth D of the concaves 22 is preferably 0.75H to 1.25H, more preferably 0.8H to 1.2H, and even more preferably 0.9H to 1.1H.

[0072] In the third embodiment, the width W1 of the recess 22 and the width W2 of the protrusion 24 are 0.5H or more and 4.0H or less. When the width W1 of the recess 22 is narrow and the width W2 of the protrusion 24 is wide, or when the width W1 of the recess 22 is wide and the width W2 of the protrusion 24 is narrow, the ratio of the standing wave reflected by the recess 22 and the standing wave reflected by the protrusion 24 becomes significantly different. Therefore, the effect of weakening the standing wave becomes small. By setting the width W1 of the recess 22 and the width W2 of the protrusion 24 to 0.5H or more and 4.0H or less, the ratio of the standing wave reflected by the recess 22 and the standing wave reflected by the protrusion 24 can be suppressed from being significantly different, and the standing wave can be effectively weakened. From the viewpoint of weakening the standing wave, the width W1 of the recess 22 and the width W2 of the protrusion 24 are preferably 1.0H or more and 3.0H or less, and more preferably 1.5H or more and 2.5H or less.

[0073] In the third embodiment, the irregularities 26 are repeatedly provided on the entire side surface of the second piezoelectric film 18. This makes it possible to effectively weaken standing waves. From the viewpoint of weakening standing waves, the irregularities 26 are preferably provided on 50% or more of the side surface of the second piezoelectric film 18, more preferably on 70% or more, and even more preferably on 90% or more.

[0074] 12(a) to 12(c) are plan views of the second piezoelectric film 18 in a modified example of the third embodiment. As shown in FIG. 12(a), the unevenness 26 may be repeatedly provided on each of the side surfaces of the second piezoelectric film 18 that face the center of the elliptical resonance region 50 in the long axis direction, and the unevenness 26 may not be provided on the side surfaces that face the center of the elliptical resonance region 50 in the short axis direction. As shown in FIG. 12(b), the unevenness 26 may be repeatedly provided on each of the side surfaces of the second piezoelectric film 18 that face the center of the elliptical resonance region 50 in the short axis direction, and the unevenness 26 may not be provided on the side surfaces that face the center of the elliptical resonance region 50 in the long axis direction. As shown in FIG. 12(c), the unevenness 26 may be repeatedly provided on each of the side surfaces of the second piezoelectric film 18 that face the center of the elliptical resonance region 50 in the long axis direction and the short axis direction, and there may be an area between them where the unevenness 26 is not provided. As described above, the side surface of the second piezoelectric film 18 may have a flat surface with a width wider than the recesses 22 between closely adjacent protrusions 24 or the protrusions 24 between the recesses 22. The unevenness 26 provided on each of the side surfaces facing each other in the major axis direction and / or the minor axis direction is preferably five or more recesses 22 and five or more protrusions 24, and more preferably ten or more. EXAMPLES

[0075] Fig. 13(a) is a cross-sectional view of an acoustic wave device 300 in accordance with the fourth embodiment. As shown in Fig. 13(a), in the acoustic wave device 300 in accordance with the fourth embodiment, a recess is formed in the upper surface of the substrate 10, thereby forming a gap 30 between the substrate 10 and the lower electrode 12. The gap 30 may be formed so as to penetrate the substrate 10. The other configurations are the same as those in the first embodiment, and therefore description thereof will be omitted.

[0076] FIG. 13(b) is a cross-sectional view of an acoustic wave device 310 according to a modification of the fourth embodiment. As shown in FIG. 13(b), in the acoustic wave device 310 according to the modification of the fourth embodiment, an acoustic reflection film 40 is formed under the lower electrode 12 in the resonance region 50. The acoustic reflection film 40 is formed by alternately providing a film 42 having a low acoustic impedance and a film 44 having a high acoustic impedance. The thicknesses of the films 42 and 44 are, for example, λ / 4. The number of layers of the films 42 and 44 can be set arbitrarily. The acoustic reflection film 40 may be formed by laminating at least two types of layers having different acoustic characteristics with a gap therebetween. The substrate 10 may be one of the at least two types of layers having different acoustic characteristics of the acoustic reflection film 40. For example, the acoustic reflection film 40 may be configured such that one layer of a film having a different acoustic impedance is provided in the substrate 10. The other configurations are the same as those of the first embodiment, and therefore will not be described.

[0077] As in Example 1, Example 2 and its modifications, Example 3 and its modifications, and Example 4, the piezoelectric thin film resonator may be an FBAR (Film Bulk Acoustic Resonator) in which a gap 30 is formed between the substrate 10 and the lower electrode 12 in the resonance region 50. As in the modification of Example 4, the piezoelectric thin film resonator may be an SMR (Solidly Mounted Resonator) including an acoustic reflection film 40 that reflects an elastic wave propagating through the piezoelectric film 14 under the lower electrode 12 in the resonance region 50. EXAMPLES

[0078] FIG. 14 is a circuit diagram of a filter 400 according to a fifth embodiment. As shown in FIG. 14, in the filter 400, one or more series resonators S1 to S4 are connected in series between an input terminal Tin and an output terminal Tout. One or more parallel resonators P1 to P3 are connected in parallel between the input terminal Tin and the output terminal Tout. The parallel resonators P1 to P3 are connected between a path between the input terminal Tin and the output terminal Tout and a ground terminal. The acoustic wave devices according to the first embodiment, the second embodiment and its modified examples, the third embodiment and its modified examples, and the fourth embodiment and its modified examples can be used for at least one of the series resonators S1 to S4 and the parallel resonators P1 to P3. The number of resonators in the ladder filter can be set appropriately. EXAMPLES

[0079] FIG. 15 is a circuit diagram of a duplexer 500 according to a sixth embodiment. As shown in FIG. 15, in the duplexer 500, a transmission filter 80 is connected between a common terminal Ant and a transmission terminal Tx. A reception filter 82 is connected between the common terminal Ant and a reception terminal Rx. The transmission filter 80 passes a signal in a transmission band among signals inputted from the transmission terminal Tx to the common terminal Ant as a transmission signal, and suppresses signals of other frequencies. The reception filter 82 passes a signal in a reception band among signals inputted from the common terminal Ant to the reception terminal Rx as a reception signal, and suppresses signals of other frequencies. At least one of the transmission filter 80 and the reception filter 82 can be the filter according to the fifth embodiment. Although a duplexer has been described as an example of a multiplexer, a triplexer or a quadplexer may be used.

[0080] The acoustic wave device is not limited to the above, but may be a MEMS or a sensor.

[0081] Although the embodiment of the present invention has been described in detail above, the present invention is not limited to such specific embodiment, and various modifications and variations are possible within the scope of the gist of the present invention described in the claims. [Explanation of symbols]

[0082] 10...substrate, 12...lower electrode, 14...piezoelectric film, 16...first piezoelectric film, 18...second piezoelectric film, 20...upper electrode, 22...recess, 24...protrusion, 26...concave, 30...void, 33...introduction path, 35...hole, 38...sacrificial layer, 40...acoustic reflecting film, 42...film with low acoustic impedance, 44...film with high acoustic impedance, 50...resonance region, 52...periphery region, 54...central region, 60...insertion film, 72...abnormal crystal, 74...aluminum atom, 75...nitrogen atom, 76...impurity atom, 77...location, 80...transmitting filter, 82...receiving filter, 100, 200, 210, 300, 310, 900, 910, 920...acoustic wave device, 400...filter, 500...duplexer

Claims

1. A substrate; a lower electrode provided on the substrate; an upper electrode provided on the lower electrode; a first piezoelectric film made of aluminum nitride doped with an impurity element that enhances piezoelectricity, the first piezoelectric film being at least partially sandwiched between the lower electrode and the upper electrode to form a resonance region; An elastic wave device comprising: a second piezoelectric film located in a central region that is within the resonance region, between the lower electrode and the first piezoelectric film, and made of aluminum nitride doped with a smaller amount of the impurity element than the first piezoelectric film.

2. The acoustic wave device according to claim 1 , wherein an upper surface of the first piezoelectric film in a peripheral region surrounding the central region along the outer periphery of the resonance region has a rough surface shape having a surface roughness greater than that of the upper surface of the first piezoelectric film in the central region.

3. 3. The acoustic wave device according to claim 1, wherein the impurity element is a combination of a Group 2 element or a Group 12 element and a Group 4 element, a combination of a Group 2 element or a Group 12 element and a Group 5 element, or a Group 3 element.

4. The acoustic wave device according to claim 1 , wherein the second piezoelectric film is an aluminum nitride film that is not doped with the impurity element.

5. The acoustic wave device according to claim 4 , wherein the second piezoelectric film has a thickness of 15 nm or more.

6. The acoustic wave device according to claim 1 , wherein the first piezoelectric film is an aluminum nitride film doped with magnesium and hafnium as the impurity elements.

7. The elastic wave device of claim 1 or 2, wherein the total thickness of the first piezoelectric film and the second piezoelectric film in the central region is the same as the thickness of the first piezoelectric film in a peripheral region surrounding the central region along the outer periphery of the resonance region.

8. The acoustic wave device according to claim 1 , wherein the second piezoelectric film is embedded in the lower electrode.

9. 3. The elastic wave device of claim 1, wherein the side surface of the second piezoelectric film has repeated irregularities having a depth of 0.5H or more and 1.5H or less, where H is the total thickness of the lower electrode, the second piezoelectric film, the first piezoelectric film, and the upper electrode in the central region.

10. 3. The elastic wave device of claim 1 or 2, wherein the distance between the outer periphery of the resonance region and the second piezoelectric film is 2.4H or more and 3.0H or less, where H is the total thickness of the lower electrode, the second piezoelectric film, the first piezoelectric film, and the upper electrode in the central region.

11. Substrate and a lower electrode provided on the substrate; an upper electrode provided on the lower electrode; an elastic wave device comprising: a piezoelectric film comprising: a first portion which is at least partially sandwiched between the lower electrode and the upper electrode to form a resonance region and which is made of aluminum nitride doped with an impurity element that enhances piezoelectricity; and a second portion which is located in a central region which is within the resonance region, and is provided between the lower electrode and the first portion, and which is made of aluminum nitride doped with a smaller amount of the impurity element than the first portion.

12. A filter comprising the acoustic wave device according to claim 1 or 11.

13. forming a lower electrode on a substrate; forming an upper electrode on the substrate; forming a first piezoelectric film made of aluminum nitride doped with an impurity element that enhances piezoelectricity, such that at least a portion of the first piezoelectric film is sandwiched between the lower electrode and the upper electrode to form a resonance region, before forming the upper electrode; A method for manufacturing an acoustic wave device, comprising: before forming the first piezoelectric film, forming a second piezoelectric film made of aluminum nitride doped with a smaller amount of the impurity element than the first piezoelectric film, so that the second piezoelectric film is located in a central region that is within the resonance region and is provided between the lower electrode and the first piezoelectric film.

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