Acoustic wave device, filter, and multiplexer

Abstract

To provide an acoustic wave device capable of preventing a film from being peeled.SOLUTION: An acoustic wave device 100 comprises: a substrate 10; a piezoelectric layer 14 provided onto the substrate 10; a lower part electrode 12 and an upper part electrode 16 that are provided onto the substrate 10 while nipping at least one part of the piezoelectric layer 14; and an acoustic reflection film 30 that is provided to between the substrate 10 and the lower part electrode 12 in a resonance region 50 where the lower part electrode 12 and the upper part electrode 16 overlap with each other while nipping the piezoelectric layer 14; a high acoustic impedance film 32 and a low acoustic impedance film 34 having a pair of end part regions 34b nipping a center region 34a in a thickness direction of the substrate 10 are alternately laminated; the end part region 34b is an oxide silicon film having a less addition amount of fluorine and boron than that of the center region 34a; and the center region 34a is the oxide silicon film to which the fluorine and the boron are added.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to acoustic wave devices, filters, and multiplexers. [Background technology]

[0002] Filters and duplexers using piezoelectric thin film resonators are known as filters and duplexers for high-frequency circuits of wireless terminals such as mobile terminals. A piezoelectric thin film resonator includes a piezoelectric layer provided on a substrate, and a lower electrode and an upper electrode provided on the substrate with the piezoelectric layer sandwiched therebetween. The region where the lower electrode and the upper electrode overlap with the piezoelectric layer is the resonance region. A configuration is known in which an acoustic reflection film is provided between the substrate and the lower electrode in the resonance region, in which films with high and low acoustic impedance are alternately laminated. It is known to use a silicon oxide film or a silicon oxide film with fluorine, carbon, or boron added thereto as the low acoustic impedance film (for example, Patent Document 1).

[0003] Also, an acoustic wave device is known in which the temperature dependence of the frequency temperature coefficient and the like is reduced by using a silicon oxide film doped with fluorine or the like to replace O in the Si-O bond as a temperature compensation layer (for example, Patent Document 2). It is known that in order to suppress deterioration of electrical characteristics while reducing the temperature dependence of the frequency temperature coefficient and the like, the peak wave number of the stretching vibration of the Si-O bond and the fluorine concentration are set within a predetermined range (for example, Patent Document 3). It is also known that a silicon oxide film doped with fluorine is prone to hygroscopicity (for example, Non-Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2020 / 209152 [Patent Document 2] International Publication No. 2012 / 029354 [Patent Document 3] JP 2020-129724 A [Non-patent literature]

[0005] [Non-Patent Document 1] H. Miyajima, 23 others, "Challenge of low-k materials for 130, 90, 65 nm node interconnect technology and beyond", IEDM, 2004, pp329-332 Summary of the Invention [Problem to be solved by the invention]

[0006] In an acoustic wave device having an acoustic reflection film, a silicon oxide film containing other elements may be used as a low acoustic impedance film to suppress deterioration of electrical characteristics. However, in this case, peeling of the low acoustic impedance film may occur.

[0007] The present invention has been made in consideration of the above problems, and has an object to suppress film peeling. [Means for solving the problem]

[0008] The present invention relates to a piezoelectric element comprising: a substrate; a piezoelectric layer provided on the substrate; a pair of electrodes provided on the substrate sandwiching at least a portion of the piezoelectric layer; and a high acoustic impedance film and a low acoustic impedance film having an acoustic impedance lower than that of the high acoustic impedance film and a pair of end regions sandwiching a central region in a thickness direction of the substrate, the high acoustic impedance film and the low acoustic impedance film being alternately laminated, the pair of end regions containing less fluorine, boron, chlorine, tetrachlorine, or tetraethene than the central region. the acoustic wave device comprising: a silicon oxide film or a silicon nitride film having a small amount of at least one element selected from the group consisting of ruthenium, germanium, antimony, phosphorus, zinc, arsenic, tungsten, lead, praseodymium, ytterbium, lanthanum, calcium, tin, dysprosium, samarium, copper, europium, strontium, barium, sodium, bismuth, indium, silver, potassium, and rubidium, and an acoustic reflection film wherein the central region is a silicon oxide film having the at least one element added thereto.

[0009] In the above configuration, the central region can be configured to be a silicon oxide film to which at least one of fluorine and chlorine and one or more elements selected from boron, tellurium, germanium, antimony, phosphorus, zinc, arsenic, tungsten, lead, praseodymium, ytterbium, lanthanum, calcium, tin, dysprosium, samarium, copper, europium, strontium, barium, sodium, bismuth, indium, silver, potassium, and rubidium have been added.

[0010] In the above structure, the central region may be a silicon oxide film doped with fluorine and boron.

[0011] In the above-mentioned configuration, the pair of end regions may be a silicon oxide film or a silicon nitride film to which no other elements are intentionally added.

[0012] In the above configuration, the concentration of the at least one element in the central region may be 10 atomic % or less when the total of silicon, oxygen, and the at least one element is taken as 100 atomic %.

[0013] In the above configuration, the thickness of each of the pair of end regions may be smaller than the thickness of the central region.

[0014] In the above configuration, a total thickness of the pair of end regions may be 0.16 times or less the thickness of the piezoelectric layer in the resonance region.

[0015] In the above configuration, the total thickness of the pair of end regions may be 34% or less when the total thickness of the central region and the pair of end regions is taken as 100%.

[0016] In the above configuration, the ratio of the difference in thickness between one end region and the other end region of the pair of end regions to the average thickness of the one end region and the other end region can be ±10% or less.

[0017] In the above configuration, the high acoustic impedance film may be a metal film.

[0018] The present invention is a filter including the acoustic wave device described above.

[0019] The present invention is a multiplexer including the filter described above. Effect of the Invention

[0020] According to the present invention, film peeling can be suppressed. [Brief description of the drawings]

[0021] [Figure 1] FIG. 1(a) is a plan view of an acoustic wave device in accordance with a first embodiment, and FIG. 1(b) is a cross-sectional view taken along line AA of FIG. 1(a). [Diagram 2] 2A to 2C are cross-sectional views illustrating a method for manufacturing the acoustic wave device in accordance with the first embodiment. [Diagram 3] FIG. 3(a) is a plan view of an acoustic wave device according to a comparative example, and FIG. 3(b) is a cross-sectional view taken along line AA of FIG. 3(a). [Figure 4] FIG. 4 is a cross-sectional view of model A used in the simulation. [Diagram 5] FIG. 5 shows the simulation results of ΔY versus the thickness of the low acoustic impedance film in model A. [Figure 6] FIG. 6 is a cross-sectional view of model B used in the simulation. [Figure 7] FIG. 7 shows the simulation results of ΔY versus thickness of the end region of the low acoustic impedance film in model B. [Figure 8] 8(a) and 8(b) show simulation results of ΔY shown on different horizontal axes in FIG. [Figure 9] Figure 9(a) shows the simulation results of the admittance |Y| at the resonant frequency versus the thickness of the end region of the low acoustic impedance film in model B, and Figure 9(b) shows the simulation results of the admittance |Y| at the anti-resonant frequency versus the thickness of the end region. [Figure 10] 10(a) and 10(b) show the simulation results of admittance |Y| versus frequency in model B. [Figure 11] FIG. 11 shows the results of measuring the elastic modulus versus the concentration of the added element. [Figure 12] FIG. 12 shows the FTIR spectra of Samples A and B. [Figure 13] FIG. 13 shows the FTIR spectra of Samples A and C. [Figure 14] FIG. 14 is a diagram showing the FTIR spectra of Samples A and D to F. [Figure 15]FIG. 15 is a diagram showing the integrated intensity at the peak of the stretching vibration ω4 of the Si—O bond calculated from the FTIR spectra of Samples A to C. [Figure 16] FIG. 16 is a graph showing the integrated intensity at the peak of the stretching vibration ω4 of the Si—O bond versus the concentration of the added element. [Figure 17] FIG. 17 is a circuit diagram of a filter in accordance with the second embodiment. [Figure 18] FIG. 18 is a circuit diagram of a duplexer in accordance with the third embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Hereinafter, embodiments of the present invention will be described with reference to the drawings. EXAMPLES

[0023] Fig. 1(a) is a plan view of an acoustic wave device according to a first embodiment, and Fig. 1(b) is a cross-sectional view taken along line AA of Fig. 1(a). Fig. 1(a) mainly illustrates a substrate 10, a lower electrode 12, and an upper electrode 16. The X and Y directions are perpendicular to each other in the planar direction of the substrate 10, and the thickness direction of the substrate 10 is the Z direction. The acoustic wave device 100 according to the first embodiment is a piezoelectric thin film resonator including a lower electrode 12, a piezoelectric layer 14, and an upper electrode 16.

[0024] As shown in Figures 1(a) and 1(b), an acoustic reflection film 30 is provided on a substrate 10. The substrate 10 is, for example, a silicon substrate. The thickness of the substrate 10 is, for example, 100 µm to 1000 µm. A lower electrode 12 is provided on the acoustic reflection film 30. The lower electrode 12 includes, for example, a lower layer 12a and an upper layer 12b. The lower layer 12a is, for example, a chromium (Cr) film, and the upper layer 12b is, for example, a ruthenium (Ru) film. The thickness of the lower electrode 12 is, for example, 30 nm to 400 nm.

[0025] A piezoelectric layer 14 is provided on the acoustic reflection film 30 and the lower electrode 12. The piezoelectric layer 14 is, for example, an aluminum nitride film mainly composed of aluminum nitride having a principal axis in the (002) direction. The thickness of the piezoelectric layer 14 is, for example, 800 nm to 1500 nm. For a certain film to have a certain element as its main component, it is permitted that the certain film contains intentional or unintentional impurities other than the main component. When a certain element is the main component of a certain film, the concentration of the certain element is, for example, 50 atomic % or more, for example, 80 atomic % or more. When two elements are the main components, such as aluminum nitride, the sum of the aluminum concentration and the nitrogen concentration is, for example, 50 atomic % or more, for example, 80 atomic % or more, and the aluminum concentration and the nitrogen concentration are each, for example, 10 atomic % or more.

[0026] An upper electrode 16 is provided on the piezoelectric layer 14. The upper electrode 16 is provided on the piezoelectric layer 14 so as to have a region facing the lower electrode 12 with the piezoelectric layer 14 in between. A region where the lower electrode 12 and the upper electrode 16 (a pair of electrodes) overlap in a plan view, sandwiching at least a portion of the piezoelectric layer 14, is a resonance region 50. The resonance region 50 has, for example, an elliptical shape in a plan view, and is a region where, for example, an elastic wave in a thickness longitudinal vibration mode resonates. The upper electrode 16 includes, for example, a lower layer 16a and an upper layer 16b. The lower layer 16a is, for example, a ruthenium film, and the upper layer 16b is, for example, a chromium film. The thickness of the upper electrode 16 is, for example, 30 nm to 400 nm.

[0027] An additional film 24 is provided on the piezoelectric layer 14 and the upper electrode 16. The additional film 24 is provided to cover at least a part of the resonance region 50. The additional film 24 is, for example, a silicon oxide film, and may play a role in adjusting the frequency. The additional film 24 may also function as a protective film. The thickness of the piezoelectric layer 14 in the resonance region 50 is T1. The wavelength λ of an elastic wave, for example, of a thickness longitudinal vibration mode, resonated in the resonance region 50 is finely adjusted by the additional film 24, but is generally twice the thickness T1 of the piezoelectric layer 14.

[0028] The acoustic reflecting film 30 is formed by alternately stacking one or more high acoustic impedance films 32 and one or more low acoustic impedance films 34 having a lower acoustic impedance than the high acoustic impedance films 32 in the Z direction. The film thickness of each of the high acoustic impedance films 32 and the low acoustic impedance films 34 is, for example, about λ / 4. This allows the acoustic reflecting film 30 to reflect elastic waves. The number of layers of the high acoustic impedance films 32 and the low acoustic impedance films 34 can be set as desired.

[0029] The acoustic reflection film 30 overlaps with the resonance region 50 in a plan view and is larger than the resonance region 50. The acoustic reflection film 30 is made larger than the resonance region 50 in a plan view in order to suppress deterioration of characteristics by reflecting elastic waves that leak obliquely from the resonance region 50 by the acoustic reflection film 30. Note that the resonance region 50 is not limited to having an elliptical shape in a plan view and may have a rectangular shape or a polygonal shape having pentagons or more sides.

[0030] The high acoustic impedance film 32 is, for example, a tungsten film. The high acoustic impedance film 32 is not limited to a metal film, and may be, for example, an insulating film such as an aluminum nitride film. The low acoustic impedance film 34 has a central region 34a located in the center in the Z direction, and a pair of end regions 34b located on either side of the central region 34a. Since the high acoustic impedance film 32 and the low acoustic impedance film 34 are alternately stacked in the Z direction, the end region 34b of the low acoustic impedance film 34 is in contact with the high acoustic impedance film 32. The end region 34b of the low acoustic impedance film 34 is also in contact with the substrate 10, the lower electrode 12, and the piezoelectric layer 14. An embedded film 36 is provided on the side of the high acoustic impedance film 32. The embedded film 36 is, for example, a silicon oxide film to which other elements are not intentionally added. The embedded film 36 may be an insulating film such as a silicon oxide film or a silicon nitride film to which other elements are intentionally added.

[0031] The central region 34a is a silicon oxide film to which, for example, fluorine (F) or fluorine and boron (B) have been intentionally added. The edge region 34b is a silicon oxide film to which the amount of fluorine and boron added is less than that of the central region 34a and to which no other elements except silicon (Si) and oxygen (O) are intentionally added, except for fluorine and boron. The edge region 34b is a silicon oxide film to which no other elements except silicon and oxygen are intentionally added, except for silicon (SiO 2 ) film. A silicon oxide film to which other elements are not intentionally added is a film in which the concentration of other elements is less than 1.0 atomic % and the sum of the concentration of silicon and the concentration of oxygen is 99.0 atomic % or more. Conversely, a silicon oxide film to which other elements are intentionally added is a film in which the concentration of other elements is 1.0 atomic % or more. In the first embodiment, the end region 34b is a silicon oxide (SiO 2 ) membrane is shown as an example.

[0032] The thickness T3 of the end region 34b is smaller than the thickness T2 of the central region 34a, for example. The thickness T3 is, for example, 0.04λ or less. When the sum (T2+2×T3) of the thickness T2 of the central region 34a and the thickness T3 of the pair of end regions 34b is 100%, the thickness T3 is, for example, 17% or less.

[0033] The substrate 10 may be an insulating substrate or a semiconductor substrate other than a silicon 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.

[0034] The lower electrode 12 and the upper electrode 16 may be made of a single layer film or a laminate film of, for example, aluminum (Al), titanium (Ti), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), rhodium (Rh), or iridium (Ir) other than ruthenium and chromium.

[0035] The piezoelectric layer 14 may be made of, for example, zinc oxide (ZnO), gallium nitride (GaN), lead zirconate titanate (PZT), or lead titanate (PbTiO 3 ) may be used. Even when these are used, elastic waves are excited in the piezoelectric layer 14 mainly in a thickness longitudinal vibration mode. For example, single crystal lithium niobate or single crystal lithium tantalate may be used for the piezoelectric layer 14. In this case, elastic waves are excited in the piezoelectric layer 14 in which the displacement of the elastic wave vibrates in a direction substantially perpendicular to the Z direction (i.e., the strain direction relative to the thickness). This vibration is called thickness shear vibration.

[0036] Furthermore, the piezoelectric layer 14 may contain other elements, mainly made of aluminum nitride, to improve the resonance characteristics or the piezoelectricity. The piezoelectricity of the piezoelectric layer 14 is improved by using, as an additive element, for example, scandium (Sc), two elements of a group 2 element and a group 4 element, or two elements of a group 2 element and a group 5 element. This improves the effective electromechanical coupling coefficient. The group 2 element is, for example, calcium (Ca), magnesium (Mg), strontium (Sr), or zinc (Zn). The group 4 element is, for example, titanium (Ti), zirconium (Zr), or hafnium (Hf). The group 5 element is, for example, tantalum (Ta), niobium (Nb), or vanadium (V). Furthermore, the piezoelectric layer 14 may contain fluorine or boron, mainly made of aluminum nitride.

[0037] [Manufacturing method] 2(a) to 2(c) are cross-sectional views showing a method for manufacturing an acoustic wave device according to the first embodiment. As shown in FIG. 2(a), an acoustic reflection film 30 is formed on the upper surface of a substrate 10. The acoustic reflection film 30 is formed, for example, as follows. First, tetraethoxysilane (TEOS) and O, which are raw materials for silicon, are applied to the upper surface of the substrate 10. 2 Silicon oxide (SiO 2 Then, the end region 34b is formed by a TEOS and O 2 The gas contains F, the raw material for fluorine. 2 , C2 F 6 , C 3 F 8 Alternatively, a gas such as TEOS or O may be added to form a central region 34a made of a silicon oxide film doped with fluorine (SiOF). 2 The gas contains F, the raw material for fluorine. 2 , C 2 F 6 , C 3 F 8 Then, gases such as fluorine and boron are added to the silicon oxide (SiOFB) film, and triethyl borate (TEB) is added to form the central region 34a. 2 Silicon oxide (SiO 2 ) film. This forms a low acoustic impedance film 34 having a central region 34a and a pair of end regions 34b sandwiching the central region 34a. Next, a high acoustic impedance film 32 is formed using a sputtering method, a vacuum deposition method, or a CVD method, and then the high acoustic impedance film 32 is patterned into a desired shape using a photolithography method and an etching method. Next, a silicon oxide (SiO 2 After forming the buried film 36 made of a SiO 2 film, the buried film 36 is polished, for example, by a chemical mechanical polish (CMP) method until the upper surface of the high acoustic impedance film 32 is exposed. By repeating such a process, an acoustic reflection film 30 is formed in which the high acoustic impedance films 32 and the low acoustic impedance films 34 are alternately laminated.

[0038] As shown in FIG. 2(b), a lower layer 12a and an upper layer 12b are formed as the lower electrode 12 on the acoustic reflection film 30. The lower electrode 12 is formed by, for example, 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 method and an etching method. The lower electrode 12 may be formed by a lift-off method. Next, a piezoelectric layer 14 is formed on the acoustic reflection film 30 and the lower electrode 12. The piezoelectric layer 14 is formed by, for example, a sputtering method, a vacuum deposition method, or a CVD method.

[0039] As shown in FIG. 2(c), a lower layer 16a and an upper layer 16b are formed on the piezoelectric layer 14 as the upper electrode 16. The upper electrode 16 is formed by forming a film using, for example, a sputtering method, a vacuum deposition method, or a CVD method, and then patterning the film into a desired shape using a photolithography method and an etching method. The upper electrode 16 may be formed by a lift-off method. Next, an additional film 24 is formed on the piezoelectric layer 14 to cover the upper electrode 16. The additional film 24 is formed by forming a film using, for example, a sputtering method, a vacuum deposition method, or a CVD method, and then patterning the film into a desired shape using a photolithography method and an etching method. Next, the piezoelectric layer 14 is patterned into a desired shape using a photolithography method and an etching method. In this manner, the acoustic wave device according to the first embodiment is formed.

[0040] [Comparative Example] Fig. 3(a) is a plan view of an acoustic wave device according to a comparative example, and Fig. 3(b) is a cross-sectional view taken along line AA in Fig. 3(a). As shown in Fig. 3(a) and Fig. 3(b), in an acoustic wave device 500 according to the comparative example, the low acoustic impedance film 134 constituting the acoustic reflecting film 30 does not have a three-layer structure. The other configurations are the same as those in the first embodiment, and therefore will not be described.

[0041] [Simulation 1] 4 is a cross-sectional view of model A used in the simulation. As shown in FIG. 4, model A corresponds to an acoustic wave device according to a comparative example, and includes an acoustic reflection film 30 in which high acoustic impedance films 32 and low acoustic impedance films 134 are alternately laminated on a substrate 10. The high acoustic impedance films 32 and the low acoustic impedance films 134 are provided to extend over the entire substrate 10 in the surface direction of the XY plane. A lower electrode 12 is provided on the acoustic reflection film 30. A piezoelectric layer 14 is provided on the acoustic reflection film 30 and the lower electrode 12. An upper electrode 16 is provided on the piezoelectric layer 14 so as to have a resonance region 50 facing the lower electrode 12 with the piezoelectric layer 14 in between.

[0042] A simulation was performed for ΔY when the material of the low acoustic impedance film 134 was different for model A. ΔY was calculated by obtaining the difference between the absolute value of admittance |Y| at the resonance frequency fr and the absolute value of admittance |Y| at the anti-resonance frequency fa from the simulation results of the frequency characteristics of admittance |Y|. The simulation conditions are as follows. Note that the concentrations of fluorine and boron indicate the concentrations of fluorine and boron when the total of silicon, oxygen, fluorine and boron is 100 atomic % (the same applies below). Substrate 10: Silicon substrate High acoustic impedance film 32: 520 nm thick tungsten (W) film Low acoustic impedance film 134: Silicon oxide (SiO 2 ) film, silicon oxide film doped with 5.4 atomic percent fluorine (SiOF), or silicon oxide film doped with 3.9 atomic percent fluorine and 7.4 atomic percent boron (SiOFB) The lower layer 12a of the lower electrode 12: a chromium (Cr) film having a thickness of 55 nm Upper layer 12b of bottom electrode 12: 120 nm thick ruthenium (Ru) film Piezoelectric layer 14: 690 nm thick aluminum nitride (AlN) film Lower layer 16a of upper electrode 16: 150 nm thick ruthenium (Ru) film Upper layer 16b of upper electrode 16: 20 nm thick chromium (Cr) film Elastic wave wavelength λ: 1203nm~1689nm

[0043] 5 shows the simulation results of ΔY versus the thickness of the low acoustic impedance film in model A. As shown in FIG. 5, when a silicon oxide film with fluorine added (SiOF) is used as the low acoustic impedance film 134, the thickness of the low acoustic impedance film 134 is 100 nm, whereas the thickness of the low acoustic impedance film 134 is 100 nm. 2 When a silicon oxide film doped with fluorine and boron (SiOFB) was used for the low acoustic impedance film 134, ΔY was larger than when a silicon oxide film doped with fluorine (SiOF) was used.

[0044] Table 1 shows the SiO 2 1 is a table showing the physical properties of SiOF and SiOFB, and the physical properties of W used in the high acoustic impedance film 32. [Table 1] As shown in Table 1, SiOF is 2 5, the acoustic impedance of the low acoustic impedance film 134 is lower than that of the high acoustic impedance film 32, and SiOFB has a lower acoustic impedance than that of SiOF. Therefore, it can be said from the simulation result of Fig. 5 that the lower the acoustic impedance of the low acoustic impedance film 134, the more improved ΔY is. The reason why the lower the acoustic impedance of the low acoustic impedance film 134, the more improved ΔY is, is considered to be because the lower the acoustic impedance of the low acoustic impedance film 134, the greater the difference in acoustic impedance between it and the high acoustic impedance film 32, and therefore the reflectance of the elastic wave by the acoustic reflecting film 30 is improved.

[0045] In this way, by using a silicon oxide film doped with fluorine (SiOF) or a silicon oxide film doped with fluorine and boron (SiOFB) for the low acoustic impedance film 134, the resonance characteristics of the acoustic wave device can be improved. However, a silicon oxide film doped with elements other than silicon and oxygen has a tendency to easily absorb moisture. For this reason, when a SiOF film or a SiOFB film is used for the low acoustic impedance film 134, the low acoustic impedance film 134 may peel off. For example, the acoustic reflection film 30 is formed by forming the low acoustic impedance film 134, removing it from a film forming apparatus and putting it into another film forming apparatus to form the high acoustic impedance film 32. Therefore, the low acoustic impedance film 134 may be exposed to the atmosphere during movement between the apparatuses and absorb moisture. This may cause film peeling between the low acoustic impedance film 134 and the high acoustic impedance film 32, for example.

[0046] Therefore, in the first embodiment, a pair of end regions 34b, which are silicon oxide films with less fluorine and boron added than the central region 34a, are provided on either side of the central region 34a, which is a silicon oxide film with fluorine or fluorine and boron added. The central region 34a and the end regions 34b can be continuously formed in one film forming device, so that the central region 34a is prevented from being exposed to the atmosphere, and moisture absorption in the central region 34a is suppressed. Since the end regions 34b are silicon oxide films with less fluorine and boron added than the central region 34a, they are less likely to absorb moisture than the central region 34a, and therefore are less likely to absorb moisture even when exposed to the atmosphere. Therefore, moisture absorption of the low acoustic impedance film 34 is suppressed, and film peeling is suppressed.

[0047] [Simulation 2] 6 is a cross-sectional view of model B used in the simulation. As shown in FIG. 6, model B corresponds to the acoustic wave device according to Example 1, and includes an acoustic reflection film 30 in which high acoustic impedance films 32 and low acoustic impedance films 34 are alternately laminated on a substrate 10. The high acoustic impedance films 32 and the low acoustic impedance films 34 are provided to extend over the entire substrate 10 in the surface direction of the XY plane. A lower electrode 12 is provided on the acoustic reflection film 30. A piezoelectric layer 14 is provided on the acoustic reflection film 30 and the lower electrode 12. An upper electrode 16 is provided on the piezoelectric layer 14 so as to have a resonance region 50 facing the lower electrode 12 with the piezoelectric layer 14 in between.

[0048] For model B, a simulation was performed on ΔY when the thickness of each of the central region 34a and the end region 34b was varied while the total thickness (T2+2×T3) of the central region 34a and the pair of end regions 34b was kept constant. The simulation conditions were as follows. Substrate 10: Silicon substrate High acoustic impedance film 32: tungsten (W) film with a thickness of 520 nm Low acoustic impedance film 34: Thickness (T2+2×T3) is 330 nm Central region 34a of low acoustic impedance film 34: silicon oxide film (SiOFB) doped with 3.9 atomic % fluorine and 7.4 atomic % boron The end region 34b of the low acoustic impedance film 34: silicon oxide (SiO 2 )film The lower layer 12a of the lower electrode 12: a chromium (Cr) film having a thickness of 55 nm Upper layer 12b of bottom electrode 12: 120 nm thick ruthenium (Ru) film Piezoelectric layer 14: 690 nm thick aluminum nitride (AlN) film Lower layer 16a of upper electrode 16: 150 nm thick ruthenium (Ru) film Upper layer 16b of upper electrode 16: 20 nm thick chromium (Cr) film Elastic wave wavelength λ: 1311nm~1331nm

[0049] Fig. 7 shows the simulation results of ΔY versus thickness of the end region of the low acoustic impedance film in model B. In Fig. 7, when the thickness T3 of the end region 34b is 0 nm, the low acoustic impedance film 34 is formed only of the SiOFB film in the central region 34a. When the thickness T3 of the end region 34b is 165 nm, the low acoustic impedance film 34 is formed only of the SiOFB film in the end region 34b. 2 The dashed line in FIG. 7 indicates the case where the low acoustic impedance film 34 is made of SiO 2 This is ΔY when the film is formed only.

[0050] As shown in Fig. 7, when the thickness T3 of the end region 34b is 20 nm or less, ΔY is about the same as when the low acoustic impedance film 34 is formed only of a SiOFB film (when the thickness T3 is 0 nm). Up to a thickness T3 of the end region 34b of 70 nm, ΔY decreases as the thickness T3 increases. This is because an increase in thickness T3 is equivalent to a decrease in the thickness T2 of the central region 34a, which is a SiOFB film, and it is believed that ΔY decreases as the acoustic impedance of the low acoustic impedance film 34 as a whole increases. ΔY decreases up to a thickness T3 of 70 nm, but when the thickness T3 is 55 nm or less, when the low acoustic impedance film 34 is made of SiO 2 ΔY is larger than when the low acoustic impedance film 34 is formed only with a film. Therefore, by setting the thickness T3 of the end region 34b to 55 nm or less, it is possible to improve ΔY while suppressing peeling of the low acoustic impedance film 34 due to moisture absorption. From the viewpoint of improving ΔY, the thickness T3 of the end region 34b is preferably 50 nm or less, more preferably 40 nm or less, and even more preferably 30 nm or less.

[0051] When the thickness T3 of the end region 34b exceeds 70 nm, ΔY increases as the thickness T3 increases up to 110 nm, and when the thickness T3 exceeds 110 nm, ΔY decreases again as the thickness T3 increases up to 150 nm. Therefore, even when the thickness T3 is 80 nm or more and 140 nm or less, the low acoustic impedance film 34 is made of SiO 2 The reason why ΔY becomes larger again when the thickness T3 is 80 nm or more and 140 nm or less is unclear.

[0052] 8(a) and 8(b) show simulation results of ΔY shown on different horizontal axes in FIG. 7. The horizontal axis in FIG. 8(a) shows the thickness T3 of the end region 34b converted into λ. The horizontal axis in FIG. 8(b) shows the ratio of the thickness (2×T3) of the pair of end regions 34b to the thickness (T2+2×T3) of the low acoustic impedance film 34 taken as 100%. The dashed lines in FIG. 8(a) and FIG. 8(b) show the ratio of the thickness (T2+2×T3) of the low acoustic impedance film 34 to the thickness (T2+2×T3) of the low acoustic impedance film 34 taken as 100%. 2 This is ΔY when the film is formed only.

[0053] As shown in FIG. 8(a), when the thickness T3 of the end region 34b is 0.04λ or less, that is, when the total thickness (2×T3) of the pair of end regions 34b is 0.08λ or less, the low acoustic impedance film 34 is made of SiO 2 ΔY is larger than when the low acoustic impedance film 34 is made of only a SiO 2 film. Therefore, by making the total thickness (2×T3) of the pair of end regions 34b 0.08λ or less, it is possible to improve ΔY while suppressing film peeling due to moisture absorption of the low acoustic impedance film 34. Also, even when the thickness T3 is 0.06λ or more and 0.105λ or less, it is possible to improve ΔY by making the low acoustic impedance film 34 of SiO 2 film. 2 Compared to the case where the film is formed only, ΔY is larger.

[0054] As shown in FIG. 8B, when the ratio of the thickness (2×T3) of the pair of end regions 34b to the thickness (T2+2×T3) of the low acoustic impedance film 34 is 34% or less, the low acoustic impedance film 34 is made of SiO 2 ΔY is larger than when the low acoustic impedance film 34 is formed only with a SiO 2 film. Therefore, by setting the ratio of the thickness of the pair of end regions 34b to the thickness of the low acoustic impedance film 34 to 34% or less, it is possible to improve ΔY while suppressing film peeling due to moisture absorption of the low acoustic impedance film 34. Also, even when the ratio of the thickness of the pair of end regions 34b to the thickness of the low acoustic impedance film 34 is 45% or more and 85% or less, it is possible to improve ΔY by using the low acoustic impedance film 34 made of SiO 2 film. 2 Compared to the case where the film is formed only, ΔY is larger.

[0055] In FIG. 7, when the thickness T3 of the end region 34b is in the range of about 55 nm to 75 nm and in the range of about 140 nm to 165 nm, the low acoustic impedance film 34 is SiO 2 The reason why ΔY is smaller than when the film is formed only is as follows.

[0056] The simulation results of admittance |Y| at the resonant frequency and anti-resonant frequency under the simulation conditions of Fig. 7 are shown. Fig. 9(a) shows the simulation results of admittance |Y| at the resonant frequency versus the thickness of the end region of the low acoustic impedance film in model B, and Fig. 9(b) shows the simulation results of admittance |Y| at the anti-resonant frequency versus the thickness of the end region. The dashed lines in Fig. 9(a) and Fig. 9(b) indicate the results when the low acoustic impedance film 34 is made of SiO 2 is the admittance |Y| when it is formed only by the membrane.

[0057] 9(a), the admittance |Y| at the resonant frequency decreases almost monotonically as the thickness T3 of the end region 34b increases. This is because an increase in the thickness T3 of the end region 34b is equivalent to a decrease in the thickness T2 of the central region 34a, which is an SiOFB film, and therefore the acoustic impedance of the low acoustic impedance film 34 as a whole increases, causing the admittance |Y| to deteriorate.

[0058] 9B, the admittance |Y| at the anti-resonant frequency does not deteriorate monotonically even if the thickness T3 of the end region 34b increases. When the thickness T3 of the end region 34b is within the range of about 45 nm to 85 nm and about 135 nm to 165 nm, the admittance |Y| at the anti-resonant frequency is 2 The reason for this is as follows:

[0059] The simulation results of the frequency characteristics of admittance |Y| under the simulation conditions of Fig. 7 are shown below. Fig. 10(a) and Fig. 10(b) show the simulation results of admittance |Y| versus frequency in model B. Fig. 10(a) shows the simulation results of admittance |Y| versus frequency when the thickness T3 of end region 34b is 0 nm, 20 nm, 60 nm, and 80 nm. Fig. 10(b) shows the simulation results of admittance |Y| versus frequency when the thickness T3 of end region 34b is 100 nm, 110 nm, 120 nm, 150 nm, and 165 nm.

[0060] As shown in Fig. 10(a) and Fig. 10(b), when the thickness T3 of the end region 34b is 60 nm, 80 nm, and 150 nm, spurious signals occur near the anti-resonant frequency, and the admittance |Y| deteriorates. For this reason, it is considered that the admittance |Y| at the anti-resonant frequency deteriorates when the thickness T3 of the end region 34b is within the range of about 45 nm to 85 nm and about 135 nm to 165 nm in Fig. 9(b). It is unclear why spurious signals occur near the anti-resonant frequency and the admittance |Y| deteriorates when the thickness T3 of the end region 34b is 60 nm, 80 nm, and 150 nm.

[0061] Based on the above simulation results, in Example 1, the thickness T3 of the end region 34b is set to 55 nm or less. In other words, the thickness T3 of the end region 34b is set to 0.04λ or less. In yet another way, the thickness T3 of the end region 34b is set to 17% or less when the total thickness (T2+2×T3) of the thickness of the central region 34a and the thickness of the pair of end regions 34b is set to 100%.

[0062] [experiment] Silicon oxide (SiO 2 ) film, silicon oxide doped with fluorine (SiOF) film, silicon oxide doped with boron (SiOB) film, and silicon oxide doped with fluorine and boron (SiOFB) film were deposited, and the elastic modulus of each film was measured. SiO 2 The membrane is made of TEOS and O2 The SiOF film was formed by CVD using TEOS and O 2 Gas and C 2 F 6 The SiOB film was formed by CVD using TEOS and O 2 The SiOFB film was formed by CVD using TEOS and O 2 Gas and C 2 F 6 The films were formed by CVD using gas and TEB. The thickness of each film was 5000 nm. The elastic modulus was measured using the nanoindentation method. The concentration of the added elements was measured using the SIMS (Secondary Ion Mass Spectrometry) method.

[0063] Table 2 shows the measurement results of the concentration of the added element and the elastic modulus of each film formed. The concentration of the added element is the concentration when the total concentration of silicon, oxygen, and added element is taken as 100 atomic %. Figure 11 is a graph of the measurement results in Table 2. Figure 11 shows the measurement results of the elastic modulus versus the concentration of the added element. [Table 2] As shown in Figure 11 and Table 2, the elastic modulus decreased as the concentration of added fluorine and / or boron increased. Since acoustic impedance is proportional to the square root of the product of elastic modulus and density, the smaller the elastic modulus, the smaller the acoustic impedance. Below, we explain the reason why the elastic modulus decreased by adding fluorine and / or boron to silicon oxide.

[0064] The following samples A to F were prepared by forming each film on a silicon substrate by the above-mentioned CVD method. The concentrations of the additive elements in samples B to F were measured by the SIMS method. Sample A: 500 nm thick silicon oxide (SiO 2 )film Sample B: Silicon oxide (SiOF) film with a thickness of 500 nm and doped with 5.4 atomic % fluorine Sample C: Silicon oxide (SiOB) film with a thickness of 500 nm and doped with 3.8 atomic percent boron Sample D: A silicon oxide (SiOFB) film with a thickness of 500 nm and doped with 5.0 atomic % fluorine and 2.6 atomic % boron. Sample E: Silicon oxide (SiOFB) film with a thickness of 500 nm and doped with 4.5 atomic % fluorine and 4.1 atomic % boron. Sample F: A silicon oxide (SiOFB) film with a thickness of 500 nm and doped with 3.9 atomic % fluorine and 7.4 atomic % boron. The spectra of the stretching vibration of the Si-O bond of Samples A to F were measured by the Fourier transform infrared spectroscopy (FTIR method).

[0065] FIG. 12 is a diagram showing the FTIR spectra of samples A and B. In FIG. 12, Si-O(ω4) is a peak due to the stretching vibration ω4 of the Si-O bond. Si-F is a peak due to the Si-F bond. As shown in FIG. 12, sample B (SiOF film) has a higher FTIR spectrum than sample A (SiO 2 The wave number in Si-O(ω4) is higher than that in Si-O(ω4) (film). It is known that when the wave number of Si-O(ω4) is higher, the temperature coefficient of frequency (TCF) of the resonant frequency is improved and the Young's modulus (elastic modulus) is reduced, as described in Patent Document 2, for example. The reason why the TCF is improved and the Young's modulus (elastic modulus) is reduced when the wave number of Si-O(ω4) is higher is considered to be as follows.

[0066] When the wave number of Si-O(ω4) becomes high, SiO 4The Si-O-Si bond angle centered on the bridging oxygen of the tetrahedron becomes larger. When the bond angle becomes larger, the density decreases and the amount of volumetric shrinkage due to heating increases. For this reason, it is considered that when the wave number of Si-O(ω4) becomes higher, the TCF improves and Young's modulus (elastic modulus) decreases. The increase in the bond angle is considered to be due to the fact that oxygen, which is divalent and easily negatively charged, is replaced by fluorine, which is monovalent and easily negatively charged, causing the bond with silicon to break. For this reason, it is considered that even when a halogen element such as chlorine (Cl) is added instead of fluorine, the TCF improves and Young's modulus (elastic modulus) decreases due to the same mechanism. In addition, sample B (SiOF film) has a higher TCF than sample A (SiO 2 The integrated intensity of the stretching vibration ω4 peak of the Si-O bond is smaller than that of the SiO 4 This shows a decrease in crosslink density mediated by tetrahedral oxygen. This is believed to be due to the breaking of bonds with silicon as a result of the oxygen being replaced by fluorine. The decrease in crosslink density is also believed to be a factor in the decrease in elastic modulus. Thus, the SiOF film of sample B is similar to the SiO 2 Since the elastic modulus of the SiOF film is smaller than that of the SiO 2 The acoustic impedance is smaller than that of the membrane.

[0067] FIG. 13 shows the FTIR spectra of samples A and C. In FIG. 2 O 3 BO(BO phase) is a peak due to the stretching vibration of the bond between three-coordinated B and O. 2 The peaks in the BO-Si phase are due to the stretching vibration of the bond between 4-coordinated B and O. The peaks in the BO-Si phase are due to the BO-Si bond.

[0068] As shown in FIG. 13, sample C (SiOB film) is 2 membrane), the wave number is 1400 cm -1 Nearby BO(B 2 O 3 As shown in Fig. 11 and Table 2, the SiOB film has a SiO2 The elastic modulus is smaller than that of the film, but this is because the addition of boron results in a B 2 O 3 This is believed to be due to the formation of a phase. Sample C (SiOB film) was 2 The peak height of Si-O(ω4) is smaller than that of the Si-O(ω4) film. This is because the addition of boron increases the 2 O 3 The elastic modulus is larger than that of the SiO 2 This is thought to be due to the fact that the proportion of the SiO phase decreased, and the formation of three phases, silicon oxide, boron oxide, and a mixed crystal of silicon oxide and boron oxide, made it difficult for oxygen to bridge between the phases, resulting in a decrease in the bridge density. 2 Since the elastic modulus of the SiOB film is smaller than that of the SiO 2 The acoustic impedance is smaller than that of the membrane.

[0069] In the SiOB film, silicon is replaced by boron, and boron has a different valence from silicon, so it is believed that the elastic modulus will decrease due to a similar mechanism even if an element that replaces silicon and has a different valence is added instead of boron. It is also known that the peel energy of boron when bonded with oxygen is smaller than the peel energy of Si-O bonds from the literature "Estimation formula for Young's modulus, shear modulus, and Vickers hardness of aluminosilicate glass" Journal of the Japan Institute of Metals, Vol. 64, No. 3 (2000) pp. 177-183. Therefore, it is believed that the elastic modulus will decrease due to a similar mechanism if an element that has a smaller peel energy when bonded with oxygen than the peel energy of Si-O bonds is added instead of boron. Elements that can be used in place of boron include tellurium (Te), germanium (Ge), antimony (Sb), phosphorus (P), zinc (Zn), arsenic (As), tungsten (W), lead (Pb), praseodymium (Pr), ytterbium (Yb), lanthanum (La), calcium (Ca), tin (Sn), dysprosium (Dy), samarium (Sm), copper (Cu), europium (Eu), strontium (Sr), barium (Ba), sodium (Na), bismuth (Bi), indium (In), silver (Ag), potassium (K), and rubidium (Rb).

[0070] Fig. 14 shows the FTIR spectra of samples A and D to F. As shown in Fig. 14, samples D to F are SiOFB films doped with fluorine and boron, and therefore exhibit characteristics of both a SiOF film doped with only fluorine and a SiOB film doped with only boron.

[0071] FIG. 15 is a diagram showing the integrated intensity at the peak of the stretching vibration ω4 of the Si—O bond calculated from the FTIR spectra of Samples A to C. The integrated intensity was calculated using the peak waveform of the stretching vibration ω4 of the Si—O bond after separating the peak of the stretching vibration ω4 from other peaks. As shown in FIG. 15, Sample B (SiOF film) and Sample C (SiOB film) have a higher FTIR intensity than Sample A (SiO 2Assuming that the integrated intensity of the peak of the stretching vibration ω4 of the Si-O bond decreases as a linear function with respect to the amount of added fluorine or boron, the integrated intensity decreases by 1.3 when fluorine is added and by 3.2 when boron is added with an addition of 1.0 atomic %.

[0072] Table 3 shows the predicted change in the integrated intensity of the peak of the stretching vibration ω4 of the Si-O bond due to the addition of fluorine and the predicted change in the integrated intensity of the peak of the stretching vibration ω4 of the Si-O bond due to the addition of boron, which are predicted from the results of FIG. 15, for samples D to F. Also shown are the estimated value of the integrated intensity of the peak of the stretching vibration ω4 obtained from the predicted change, and the actual measured value of the integrated intensity of the peak of the stretching vibration ω4 calculated from the FTIR spectrum. FIG. 16 is a graph showing the results of Table 3. FIG. 16 is a diagram showing the integrated intensity of the peak of the stretching vibration ω4 of the Si-O bond versus the concentration of the added element. [Table 3]

[0073] As shown in Table 3 and FIG. 16, in samples D to F, the measured values ​​of the integrated intensity at the peak of the stretching vibration ω4 of the Si-O bond were smaller than the estimated values. This is believed to be because the addition of both fluorine and boron resulted in a complex phase structure and a decrease in crosslink density. From this result, it is believed that the addition of both fluorine and boron can reduce the crosslink density more than would be expected from the addition of either fluorine or boron alone, and can further reduce the elastic modulus. Therefore, even when the total addition concentration is the same, it is believed that the addition of both fluorine and boron is more preferable from the viewpoint of reducing the elastic modulus than the addition of either fluorine or boron alone.

[0074] As described above, according to the first embodiment, as shown in FIG. 1(b), the low acoustic impedance film 34 includes a central region 34a, which is a silicon oxide film doped with fluorine or fluorine and boron, and a pair of end regions 34b sandwiching the central region 34a. The end regions 34b are silicon oxide films doped with less fluorine and boron than the central region 34a. By making the central region 34a a silicon oxide film doped with fluorine or fluorine and boron, the resonance characteristics of the acoustic wave device are improved. By making the end regions 34b a silicon oxide film doped with less fluorine and boron than the central region 34a, the end regions 34b have a property of being less prone to absorbing moisture than the central region 34a, and therefore moisture absorption of the low acoustic impedance film 34 is suppressed. Therefore, according to the first embodiment, it is possible to suppress film peeling while improving the resonance characteristics.

[0075] In the first embodiment, the central region 34a is a silicon oxide film doped with fluorine or fluorine and boron. This can improve the resonance characteristics as shown in Fig. 5. The central region 34a is not limited to a silicon oxide film doped with fluorine or fluorine and boron, and may be other types of film. The central region 34a may be a silicon oxide film to which at least one of the following elements is added: fluorine (F), boron (B), chlorine (Cl), tellurium (Te), germanium (Ge), antimony (Sb), phosphorus (P), zinc (Zn), arsenic (As), tungsten (W), lead (Pb), praseodymium (Pr), ytterbium (Yb), lanthanum (La), calcium (Ca), tin (Sn), dysprosium (Dy), samarium (Sm), copper (Cu), europium (Eu), strontium (Sr), barium (Ba), sodium (Na), bismuth (Bi), indium (In), silver (Ag), potassium (K), and rubidium (Rb). Even in this case, the resonance characteristics can be improved. In addition, since a silicon oxide film to which other elements have been added has a tendency to easily absorb moisture, even when the above elements have been added, it is preferable to provide a pair of end regions 34b that sandwich the central region 34a and have a lower concentration of the added element than the central region 34a.

[0076] In the first embodiment, the central region 34a is a silicon oxide film to which fluorine and boron are added, and the pair of end regions 34b are silicon oxide films to which other elements are not intentionally added. By adding both fluorine and boron, as shown in Table 3 and FIG. 16, the elastic modulus can be further reduced beyond the amount of reduction in the elastic modulus predicted from the case in which only fluorine or only boron is added. Therefore, the acoustic impedance can be effectively reduced, and the resonance characteristics can be improved. In addition, in the silicon oxide film to which fluorine and boron are added, fluorine is substituted with oxygen, and boron is substituted with silicon. Therefore, compared with the silicon oxide film to which only fluorine or only boron is added, even if the total concentration of the added elements is increased, the change in the film quality is suppressed. If the film quality changes, for example, the film stress may change, causing film peeling, but the film peeling is suppressed by suppressing the change in the film quality. Therefore, the silicon oxide film to which fluorine and boron are added can have a higher concentration of the added elements than the silicon oxide film to which only fluorine or only boron is added, and therefore the elastic modulus can be reduced. From this point of view, the acoustic impedance can be reduced and the resonance characteristics can be improved. In addition, since the end region 34b is a silicon oxide film to which no other elements are added, moisture absorption by the low acoustic impedance film 34 can be effectively suppressed.

[0077] From the viewpoint of suppressing changes in film quality and suppressing film peeling, the concentration of the additive element in the central region 34a is preferably 10 atomic % or less, more preferably 9 atomic % or less, and even more preferably 8 atomic % or less. On the other hand, from the viewpoint of decreasing the elastic modulus to improve the resonance characteristics, the concentration of the additive element in the central region 34a is preferably 3 atomic % or more, more preferably 4 atomic % or more, and even more preferably 5 atomic % or more. The concentration of the additive element is the ratio of the additive element when the total of silicon, oxygen, and the additive element is 100 atomic %. From the viewpoint of increasing the concentration of the additive element to decrease the elastic modulus, the concentration of either boron or fluorine is preferably 0.5 to 2.0 times the concentration of the other, more preferably 0.6 to 1.8 times, and even more preferably 0.7 to 1.6 times.

[0078] As explained in Fig. 12, even when chlorine is added instead of fluorine, the elastic modulus decreases due to a mechanism similar to that in the case of adding fluorine. As explained in Fig. 13, even when tellurium, germanium, antimony, phosphorus, zinc, arsenic, tungsten, lead, praseodymium, ytterbium, lanthanum, calcium, tin, dysprosium, samarium, copper, europium, strontium, barium, sodium, bismuth, indium, silver, potassium, or rubidium is added instead of boron, the elastic modulus decreases due to a mechanism similar to that in the case of adding boron. Therefore, it is preferable that the central region 34a is a silicon oxide film to which at least one of fluorine and chlorine and at least one of boron, tellurium, germanium, antimony, phosphorus, zinc, arsenic, tungsten, lead, praseodymium, ytterbium, lanthanum, calcium, tin, dysprosium, samarium, copper, europium, strontium, barium, sodium, bismuth, indium, silver, potassium, and rubidium are added. As a result, as described above, the concentration of the added element in the central region 34a can be increased, and the elastic modulus can be reduced. Therefore, the acoustic impedance is reduced, and the resonance characteristics can be improved.

[0079] In addition, in the first embodiment, the thickness T3 of the end region 34b is smaller than the thickness T2 of the central region 34a, which makes it possible to lower the acoustic impedance of the entire low acoustic impedance film 34 and improve the resonance characteristics.

[0080] In the first embodiment, the total thickness (2×T3) of the pair of end regions 34b is 0.08λ or less. In other words, the total thickness (2×T3) of the pair of end regions 34b is 0.16 times or less the thickness T1 of the piezoelectric layer 14 in the resonance region 50. This allows the resonance characteristics to be improved as shown in FIG. 8(a). From the viewpoint of improving the resonance characteristics, the total thickness (2×T3) of the pair of end regions 34b is preferably 0.14 times (0.07λ) or less the thickness T1 of the piezoelectric layer 14, more preferably 0.12 times (0.06λ) or less, and even more preferably 0.10 times (0.05λ) or less. On the other hand, from the viewpoint of suppressing moisture absorption, the sum of the thicknesses of the pair of end regions 34b (2×T3) is preferably 0.008 times (0.004λ) or more of the thickness T1 of the piezoelectric layer 14, more preferably 0.012 times (0.006λ) or more, and even more preferably 0.016 times (0.008λ) or more.

[0081] In the first embodiment, when the total thickness (T2+2×T3) of the central region 34a and the pair of end regions 34b is 100%, the total thickness (2×T3) of the pair of end regions 34b is 34% or less. This allows the resonance characteristics to be improved as shown in FIG. 8(b). From the viewpoint of improving the resonance characteristics, the total thickness (2×T3) of the pair of end regions 34b is preferably 30% or less, more preferably 25% or less, and even more preferably 20% or less. On the other hand, from the viewpoint of suppressing moisture absorption, the total thickness (2×T3) of the pair of end regions 34b is preferably 2% or more, more preferably 4% or more, and even more preferably 6% or more. It is preferable that the thicknesses of the pair of end regions 34b are approximately equal to each other. It is preferable that the difference in thickness between one end region 34b and the other end region 34b of a pair of end regions 34b relative to the average thickness of the one end region 34b and the other end region 34b is ±10% or less, more preferably ±8% or less, and even more preferably ±5% or less.

[0082] From the viewpoint of suppressing moisture absorption by the low acoustic impedance film 34, it can be said that the thickness T3 of the end region 34b is preferably 2 nm or more, more preferably 4 nm or more, and even more preferably 5 nm or more.

[0083] In addition, in FIG. 7, FIG. 8(a), and FIG. 8(b), the central region 34a is a silicon oxide film (SiOFB) to which 3.9 atomic % of fluorine and 7.4 atomic % of boron are added, and the edge region 34b is a silicon oxide film (SiO 2 ) film is shown as an example. However, even if the concentration of fluorine and boron added in the central region 34a is different from that described above and fluorine and / or boron is added to the end region 34b, the magnitude of ΔY may change, but it is considered that the result will be similar with respect to the change in ΔY relative to the thickness T3 of the end region 34b. Also, even if at least one of the above-mentioned elements other than fluorine and boron is added, the magnitude of ΔY may change, but it is considered that the result will be similar with respect to the change in ΔY relative to the thickness T3 of the end region 34b.

[0084] The edge region 34b may be a silicon nitride film. Since silicon nitride film has a property of being difficult to absorb moisture, even if a silicon nitride film is used for the edge region 34b, moisture absorption of the low acoustic impedance film 34 can be suppressed. Even if a silicon nitride film is used for the edge region 34b, the edge region 34b can be formed continuously from the central region 34a by changing the gas type in the CVD method or the like. It is preferable that the silicon nitride film does not intentionally contain other elements.

[0085] In the first embodiment, the high acoustic impedance film 32 is a metal film. In this case, the acoustic impedance of the high acoustic impedance film 32 is likely to be high, so the difference in acoustic impedance between the high acoustic impedance film 32 and the low acoustic impedance film 34 is large, and the resonance characteristics are improved. On the other hand, when a silicon oxide film to which other elements are added is provided in contact with a metal film, the silicon oxide film to which other elements are added is likely to absorb moisture, so that film peeling is likely to occur between the silicon oxide film and the metal film. Therefore, when the high acoustic impedance film 32 is a metal film, it is preferable that the low acoustic impedance film 34 has a three-layer structure of a central region 34a and a pair of end regions 34b in order to suppress film peeling. In order to increase the acoustic impedance, the high acoustic impedance film 32 is preferably a tungsten (W) film or a molybdenum (Mo) film. The physical properties of W and Mo are shown in Table 4. [Table 4] EXAMPLES

[0086] FIG. 17 is a circuit diagram of a filter according to a second embodiment. As shown in FIG. 17, in a filter 200 according to the second embodiment, 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 P4 are connected in parallel between the input terminal Tin and the output terminal Tout. The acoustic wave device of the first embodiment may be used for at least one of the series resonators S1 to S4 and the parallel resonators P1 to P4. The numbers of the series resonators and the parallel resonators can be set as appropriate. Although a ladder-type filter is shown as an example of the filter, the filter may be a multimode filter.

[0087] In the second embodiment, the acoustic wave device according to the first embodiment is used as a filter, but the present invention is not limited to this. For example, the acoustic wave device may be used as an actuator such as an inkjet micropump, an RF-MEMS switch, or an optical mirror, or as an acceleration sensor, a gyro sensor, or an energy harvester sensor. EXAMPLES

[0088] FIG. 18 is a circuit diagram of a duplexer according to a third embodiment. As shown in FIG. 18, in the duplexer 300 according to the third embodiment, a transmission filter 60 is connected between a common terminal Ant and a transmission terminal Tx. A reception filter 62 is connected between the common terminal Ant and a reception terminal Rx. The transmission filter 60 passes a signal in a transmission band among high-frequency 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 62 passes a signal in a reception band among high-frequency 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 60 and the reception filter 62 may be the filter of the second embodiment. Although a duplexer has been shown as an example of a multiplexer, a triplexer or a quadplexer may also be used.

[0089] 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]

[0090] 10...substrate, 12...lower electrode, 12a...lower layer, 12b...upper layer, 14...piezoelectric layer, 16...upper electrode, 16a...lower layer, 16b...upper layer, 24...additional film, 30...acoustic reflecting film, 32...high acoustic impedance film, 34, 134...low acoustic impedance film, 34a...central region, 34b...end region, 36...embedded film, 50...resonance region, 60...transmitting filter, 62...receiving filter, 100, 500...acoustic wave device, 200...filter, 300...duplexer

Claims

1. A substrate; a piezoelectric layer provided on the substrate; a pair of electrodes provided on the substrate with at least a portion of the piezoelectric layer interposed therebetween; 13. An acoustic wave device comprising: an acoustic reflection film provided between the substrate and the pair of electrodes in a resonance region where the pair of electrodes overlap with the piezoelectric layer sandwiched therebetween, the acoustic reflection film being composed of alternately stacked high acoustic impedance films and low acoustic impedance films having an acoustic impedance lower than that of the high acoustic impedance film and having a pair of end regions sandwiching a central region in a thickness direction of the substrate, the pair of end regions being silicon oxide films or silicon nitride films having a smaller amount of at least one element selected from the group consisting of fluorine, boron, chlorine, tellurium, germanium, antimony, phosphorus, zinc, arsenic, tungsten, lead, praseodymium, ytterbium, lanthanum, calcium, tin, dysprosium, samarium, copper, europium, strontium, barium, sodium, bismuth, indium, silver, potassium, and rubidium added thereto than the central region, and the central region being a silicon oxide film added with the at least one element.

2. 2. The acoustic wave device of claim 1, wherein the central region is a silicon oxide film doped with at least one of fluorine and chlorine, and one or more of boron, tellurium, germanium, antimony, phosphorus, zinc, arsenic, tungsten, lead, praseodymium, ytterbium, lanthanum, calcium, tin, dysprosium, samarium, copper, europium, strontium, barium, sodium, bismuth, indium, silver, potassium, and rubidium.

3. The acoustic wave device according to claim 2 , wherein the central region is a silicon oxide film doped with fluorine and boron.

4. The acoustic wave device according to claim 3 , wherein the pair of end regions are made of a silicon oxide film or a silicon nitride film to which no other elements are intentionally added.

5. 5. The acoustic wave device according to claim 3, wherein the concentration of the at least one element in the central region is 10 atomic % or less, where the total of silicon, oxygen, and the at least one element is 100 atomic %.

6. The acoustic wave device of claim 5 , wherein a thickness of each of the pair of end regions is less than a thickness of the central region.

7. The acoustic wave device according to claim 6 , wherein a total thickness of the pair of end regions is equal to or less than 0.16 times a thickness of the piezoelectric layer in the resonance region.

8. The acoustic wave device according to claim 6 , wherein a total thickness of the pair of end regions is 34% or less when a total thickness of the central region and the pair of end regions is taken as 100%.

9. The acoustic wave device of claim 8 , wherein a difference in thickness between one end region and the other end region of the pair of end regions relative to an average thickness of the one end region and the other end region is ±10% or less.

10. The acoustic wave device of claim 9 , wherein the high acoustic impedance film is a metal film.

11. A filter comprising the acoustic wave device according to claim 1 or 2.

12. A multiplexer including a filter according to claim 11.

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