Acoustic wave devices, filters and multiplexers

Abstract

Acoustic wave device, filter, and multiplexer. An acoustic wave device includes: a supporting substrate; a piezoelectric layer disposed above the supporting substrate; comb electrodes disposed on the piezoelectric layer, each comb electrode including electrode fingers that excite acoustic waves; a temperature compensation film interposed between the supporting substrate and the piezoelectric layer and having a temperature coefficient of elastic constant opposite in sign to the temperature coefficient of elastic constant of the piezoelectric layer; a boundary layer interposed between the supporting substrate and the temperature compensation film, the acoustic velocity of bulk waves propagating through the boundary layer being higher than the acoustic velocity of bulk waves propagating through the temperature compensation film and lower than the acoustic velocity of bulk waves propagating through the supporting substrate; and an intermediate layer interposed between the supporting substrate and the boundary layer and having a Q factor smaller than the Q factor of the boundary layer.

Description

Technical Field

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

[0002] Surface acoustic wave resonators are known as acoustic wave resonators used in communication devices such as smartphones. It is known to bond a piezoelectric layer forming a surface acoustic wave resonator to a supporting substrate. It is known to adjust the thickness of the piezoelectric layer to be equal to or less than the wavelength of the surface acoustic wave (for example, as disclosed in Japanese Patent Application Publication No. 2017-034363). It is known to provide a temperature compensation film or a low-sound-velocity film having a lower sound velocity than the piezoelectric layer between the piezoelectric layer and the supporting substrate (for example, as disclosed in Japanese Patent Application Publication No. 2019-201345 and No. 2015-115870, U.S. Patent No. 10020796, and International Publication No. 2017 / 043427). It is known to provide a high-sound-velocity film (boundary layer) having a higher sound velocity than the piezoelectric layer between the low-sound-velocity film and the supporting substrate (for example, as disclosed in Japanese Patent Application Publication No. 2015-115870). Summary of the Invention

[0003] Spurious emission is reduced by providing a high acoustic velocity film (boundary layer) between the temperature compensation film and the support substrate or between the low acoustic velocity film and the support substrate. However, there is a need to further reduce spurious emission.

[0004] In a first aspect of the present disclosure, an acoustic wave device is provided, which includes: a supporting substrate; a piezoelectric layer, which is arranged above the supporting substrate; a pair of comb electrodes, which are arranged on the piezoelectric layer, each of the pair of comb electrodes including electrode fingers for exciting acoustic waves; a temperature compensation film, which is inserted between the supporting substrate and the piezoelectric layer and has a temperature coefficient of an elastic constant of the temperature compensation film and a temperature coefficient of an elastic constant of the piezoelectric layer with a sign opposite to that of the elastic constant of the piezoelectric layer; a boundary layer, which is inserted between the supporting substrate and the temperature compensation film, and has a sound velocity of a body wave propagating through the boundary layer that is higher than the sound velocity of a body wave propagating through the temperature compensation film and lower than the sound velocity of a body wave propagating through the supporting substrate; and an intermediate layer, which is inserted between the supporting substrate and the boundary layer and has a Q factor that is smaller than the Q factor of the boundary layer.

[0005] In a second aspect of the present disclosure, a filter including the above-mentioned acoustic wave device is provided.

[0006] In a third aspect of the present disclosure, a multiplexer including the above filter is provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1Ais a plan view of the acoustic wave resonator according to the first embodiment, and Figure 1B is a cross-sectional view of the acoustic wave resonator according to the first embodiment;

[0008] Figure 2A is a cross-sectional view of an acoustic wave resonator according to a first comparative example, and Figure 2B schematically illustrates transmission characteristics of an acoustic wave resonator according to a first comparative example;

[0009] Figure 3A is a cross-sectional view of an acoustic wave resonator according to a second comparative example, and Figure 3B schematically illustrates transmission characteristics of an acoustic wave resonator according to a second comparative example;

[0010] Figures 4A to 4C The magnitude |Y| of the admittance with respect to the frequency of the first embodiment and the second comparative example in Simulation 1 is illustrated;

[0011] Figure 5A and Figure 5B are Smith charts of impedance of the second comparative example and the first embodiment in simulation 1, respectively;

[0012] Figures 6A to 6E The magnitude of the admittance |Y| with respect to the frequency in simulation 2 is illustrated;

[0013] 7A to 7E They are Figures 6A to 6E A zoomed view around the middle spurious response 59;

[0014] Figure 8A and Figure 8B The main response ΔY and the spurious response maxΔY relative to the Q factor of the middle layer in simulation 2 are illustrated;

[0015] Figures 9A to 9E The magnitude of admittance |Y| with respect to frequency in simulation 3 is illustrated;

[0016] Figures 10A to 10E They are Figures 9A to 9E A zoomed view around the middle spurious response 59;

[0017] Figure 11A and Figure 11B The main response ΔY and the spurious response maxΔY with respect to the thickness T1 of the intermediate layer in simulation 3 are illustrated respectively;

[0018] Figure 12 The spurious response maxΔY with respect to the thickness and Q factor of the intermediate layer in simulation 4 is illustrated;

[0019] 13A to 13C The magnitude |Y| of the admittance of samples A and B and the second comparative example with respect to frequency in simulation 5 is respectively illustrated;

[0020] Figure 14A and Figure 14B The main response ΔY and the spurious response maxΔY in simulation 5 are illustrated respectively;

[0021] 15A to 15D A graph illustrating the response with respect to the thickness T2 of the boundary layer in Comparative Example 2 of Simulation 6;

[0022] Figure 16A and Figure 16B illustrative of the main response and spurious response with respect to the thickness T2 of the boundary layer in the first embodiment of simulation 6, respectively;

[0023] Figure 17 is a cross-sectional view of an acoustic wave resonator according to a first modification example of the first embodiment;

[0024] Figure 18A is a circuit diagram of a filter according to a second embodiment, and Figure 18B is a circuit diagram of a duplexer according to a first modification example of the second embodiment;

[0025] Figure 19A is a plan view of the acoustic wave resonator in the third embodiment, and Figure 19B is a cross-sectional view of an acoustic wave resonator in a third embodiment;

[0026] 20A to 20C is a schematic cross-sectional view of an intermediate layer in a third comparative example and a third embodiment;

[0027] Figures 21A to 21C is a schematic cross-sectional view of an intermediate layer in a third comparative example and a third embodiment;

[0028] 22A to 22D is a cross-sectional view illustrating a method of manufacturing an acoustic wave resonator in a third embodiment;

[0029] Figure 23 Schematic diagram showing the wafer bow (BOW) of each sample before and after bonding in the experiment;

[0030] Figure 24A and Figure 24B are cross-sectional views of acoustic wave resonators according to first and second modifications of the third embodiment; and

[0031] Figure 25A and Figure 25B are cross-sectional views of acoustic wave resonators according to third and fourth modifications of the third embodiment. DETAILED DESCRIPTION

[0032] Hereinafter, a description will be given of embodiments of the present disclosure with reference to the accompanying drawings.

[0033] First embodiment

[0034] In the first embodiment, an example in which the acoustic wave device has an acoustic wave resonator will be described. Figure 1A is a plan view of the acoustic wave resonator according to the first embodiment, and Figure 1B This is a cross-sectional view of an acoustic wave resonator according to the first embodiment. The direction in which the electrode fingers are arranged (the arrangement direction of the electrode fingers) is defined as the X direction, the direction in which the electrode fingers extend (the extension direction of the electrode fingers) is defined as the Y direction, and the direction in which the support substrate and the piezoelectric layer are stacked (the stacking direction of the support substrate and the piezoelectric layer) is defined as the Z direction. The X, Y, and Z directions do not necessarily correspond to the X-axis and Y-axis orientations of the crystal orientation of the piezoelectric layer. When the piezoelectric layer is a rotated Y-cut X-propagation substrate, the X direction corresponds to the X-axis orientation of the crystal orientation.

[0035] like Figure 1A and Figure 1B As shown, the piezoelectric layer 14 is disposed on the support substrate 10. The temperature compensation film 13 is interposed between the support substrate 10 and the piezoelectric layer 14. The boundary layer 12 is interposed between the temperature compensation film 13 and the support substrate 10. The intermediate layer 11 is interposed between the boundary layer 12 and the support substrate 10. The boundary surface between the support substrate 10 and the intermediate layer 11 is defined as boundary surface 30, the boundary surface between the intermediate layer 11 and the boundary layer 12 is defined as boundary surface 31, the boundary surface between the boundary layer 12 and the temperature compensation film 13 is defined as boundary surface 32, and the boundary surface between the temperature compensation film 13 and the piezoelectric layer 14 is defined as boundary surface 33. The thicknesses of the intermediate layer 11, boundary layer 12, temperature compensation film 13, and piezoelectric layer 14 are represented by T1, T2, T3, and T4, respectively. Thickness refers to the length of the substrate, layer, and film in the Z direction, which is the stacking direction of the support substrate 10 and the piezoelectric layer 14.

[0036] The acoustic wave resonator 26 is provided on the piezoelectric layer 14. The acoustic wave resonator 26 includes an interdigital transducer (IDT) 22 and a reflector 24. The reflector 24 is located on both sides of the IDT 22. The IDT 22 and the reflector 24 are formed by the metal film 16 on the piezoelectric layer 14.

[0037] The IDT 22 includes a pair of comb-shaped electrodes 20 facing each other. The comb-shaped electrodes 20 include electrode fingers 18 and bus bars 19 connecting the electrode fingers 18. The region where the electrode fingers 18 of the pair of comb-shaped electrodes 20 overlap is an overlap region 25. The length of the overlap region 25 is the opening length. The electrode fingers 18 of one comb-shaped electrode 20 are arranged alternately with the electrode fingers 18 of the other comb-shaped electrode 20 in at least a portion of the overlap region 25. In the overlap region 25, acoustic waves excited primarily by the electrode fingers 18 propagate primarily in the X direction. The pitch of the electrode fingers 18 of the pair of comb-shaped electrodes 20 is approximately equal to the wavelength λ of the acoustic waves. When the pitch of the electrode fingers 18 (the pitch between the centers of the electrode fingers 18) is represented by D, the pitch of the electrode fingers 18 of one comb-shaped electrode 20 is twice the pitch D. The reflectors 24 reflect the acoustic waves (surface acoustic waves) excited by the electrode fingers 18 of the IDT 22. Therefore, the acoustic waves are confined within the overlapping region 25 of the IDT 22 .

[0038] The piezoelectric layer 14 is, for example, a single-crystal lithium tantalate (LiTaO 3 ) layer or a single-crystal lithium niobate (LiNbO 3 ) layer, and is, for example, a rotated Y-cut X-propagation lithium tantalate layer or a rotated Y-cut X-propagation lithium niobate layer.

[0039] The support substrate 10 is, for example, a sapphire substrate, a silicon substrate, or a silicon carbide substrate. A sapphire substrate is a single-crystal or polycrystalline Al2O3 substrate. A silicon substrate is a single-crystal or polycrystalline silicon substrate. A silicon carbide substrate is a polycrystalline or single-crystal SiC substrate. The linear expansion coefficient of the support substrate 10 in the X direction is smaller than the linear expansion coefficient of the piezoelectric layer 14 in the X direction. This configuration can reduce the temperature dependence of the frequency of the acoustic wave resonator. When a hard material and / or a material with high thermal conductivity is selected as the support substrate 10, the acoustic velocity of the bulk wave propagating through the support substrate 10 is higher than the acoustic velocity of the bulk wave propagating through the boundary layer 12.

[0040] The temperature coefficient of the elastic constant of the temperature compensation film 13 is opposite in sign to the temperature coefficient of the elastic constant of the piezoelectric layer 14. For example, the piezoelectric layer 14 has a negative temperature coefficient of the elastic constant, while the temperature compensation film 13 has a positive temperature coefficient of the elastic constant. The temperature compensation film 13 is, for example, an additive-free silicon oxide (SiO2) film or a SiO2 film containing an additive element (such as, but not limited to, fluorine), and is, for example, an amorphous layer. This configuration can reduce the temperature coefficient of the frequency of the acoustic wave resonator. When the temperature compensation film 13 is a silicon oxide film, the speed of sound of the bulk wave propagating through the temperature compensation film 13 is lower than the speed of sound of the bulk wave propagating through the piezoelectric layer 14.

[0041] In order for the temperature compensation film 13 to have a temperature compensation function, the energy of the main response acoustic wave must be present to a certain extent within the temperature compensation film 13. Although the section where the energy of the surface acoustic wave is concentrated depends on the type of surface acoustic wave, the energy of the surface acoustic wave is generally concentrated in the section from the upper surface of the piezoelectric layer 14 to a depth of 2λ (λ is the wavelength of the acoustic wave), especially in the section from the upper surface of the piezoelectric layer 14 to a depth of λ. Therefore, the thickness T4 of the piezoelectric layer 14 is preferably 2λ or less, more preferably λ or less, and even more preferably 0.6λ or less.

[0042] The acoustic velocity of the bulk wave propagating through the boundary layer 12 is higher than the acoustic velocity of the bulk wave propagating through the temperature compensation film 13. Therefore, the acoustic wave serving as the main mode is confined in the piezoelectric layer 14 and the temperature compensation film 13. In addition, the acoustic velocity of the bulk wave propagating through the boundary layer 12 is lower than the acoustic velocity of the bulk wave propagating through the support substrate 10. The boundary layer 12 is, for example, polycrystalline or amorphous and is formed of an aluminum oxide film, a silicon nitride film, or an aluminum nitride film. The boundary layer 12 can be formed by stacking a plurality of layers made of different materials.

[0043] The Q factor of the intermediate layer 11 is lower than the Q factor of the boundary layer 12. The Q factor in a dielectric material is the inverse of the dielectric loss tangent tanδ. A dielectric material with a low Q factor has a large propagation loss. The sound velocity of the body wave propagating through the intermediate layer 11 can be higher or lower than the sound velocity of the body wave propagating through the boundary layer 12. For example, the intermediate layer 11 and the boundary layer 12 are layers without pores, and the material of the intermediate layer 11 is a material with a smaller Q factor than the material of the boundary layer 12. Alternatively, for example, the intermediate layer 11 is a porous layer having pores, and the boundary layer 12 is a non-porous layer. In this case, the material of the intermediate layer 11 can be the same as or different from the material of the boundary layer 12.

[0044] For example, a polycrystalline or amorphous inorganic insulating material can be used as the material of the intermediate layer 11. Examples of polycrystalline or amorphous inorganic insulating materials include, but are not limited to, silicon (Si), germanium (Ge), diamond, aluminum oxide (Al2O3), gallium nitride (GaN), lead zirconate titanate (PZT), zinc oxide (ZnO) or indium tin oxide (ITO), an oxide film, a nitride film or an oxynitride film (for example, an oxide film, a nitride film or an oxynitride film of silicon, aluminum, hafnium, tantalum, titanium or magnesium), carbon nanotubes or silicon oxide containing other elements (for example, SiOF, SiOC or SiOB). For example, a porous film can be used as the material of the intermediate layer 11. For example, porous silicon, porous polycrystalline silicon, porous aluminum oxide (nanoporous aluminum oxide) or porous polyimide can be used as the porous film. The porous film can be, for example, Figure 21B and Figure 21CThe film shown is formed by sputtering. Examples of the sputtered material include, but are not limited to, aluminum, aluminum oxide, aluminum nitride, silicon, silicon oxide, silicon nitride, aluminum-copper alloy, copper, gold, platinum, titanium, molybdenum, chromium, niobium, ITO, or PZT. For example, an organic insulating material such as a resin can be used as the material of the intermediate layer 11. Examples of the resin include, but are not limited to, epoxy resin, a mixture of epoxy resin and acrylic, acrylic, a mixture of an aluminum oxide deposited film and a resin, polyimide, and glass fiber.

[0045] The metal film 16 is a film primarily composed of, for example, aluminum (Al), copper (Cu), or molybdenum (Mo). An adhesive film, such as a titanium (Ti) film or a chromium (Cr) film, may be provided between the electrode fingers 18 and the piezoelectric layer 14. The adhesive film is thinner than the electrode fingers 18. An insulating film may be provided to cover the electrode fingers 18. The insulating film serves as a protective film or a temperature compensation film.

[0046] The wavelength λ of the acoustic wave is, for example, 1 μm to 6 μm. When two electrode fingers 18 are defined as a pair, the number of pairs of electrode fingers 18 is, for example, 20 to 300. The duty ratio of IDT 22 is the value obtained by dividing the width of electrode finger 18 by the pitch of electrode finger 18, and is, for example, 30% to 70%. The opening length of IDT 22 is, for example, 10λ to 50λ.

[0047] Table 1 lists the Young's modulus, Poisson's ratio, density, and acoustic velocity of the bulk wave of each material. The acoustic velocity V of the bulk wave can be calculated using the Young's modulus E, Poisson's ratio γ, and density ρ by the following equation (1).

[0048]

[0049] Table 1

[0050]

[0051] In Table 1, LT is single-crystal lithium tantalate, Al2O3 is polycrystalline aluminum oxide, SiO2 is amorphous silicon oxide, SA is sapphire (single-crystal aluminum oxide), LN is single-crystal lithium niobate, Si is polycrystalline silicon, AlN is polycrystalline aluminum nitride, SiN is polycrystalline silicon nitride, and SiC is polycrystalline silicon carbide.

[0052] As shown in Table 1, when a lithium tantalate substrate or a lithium niobate substrate is used as the piezoelectric layer 14 and a silicon oxide film is used as the temperature compensation film 13, the acoustic velocity of the bulk wave propagating through the temperature compensation film 13 is lower than the acoustic velocity of the bulk wave propagating through the piezoelectric layer 14. When an aluminum oxide film, an aluminum nitride film, or a silicon nitride film is used as the boundary layer 12, the acoustic velocity of the bulk wave propagating through the boundary layer 12 is higher than the acoustic velocity of the bulk wave propagating through the temperature compensation film 13. When a sapphire substrate or a silicon carbide substrate is used as the support substrate 10, the acoustic velocity of the bulk wave propagating through the support substrate 10 is higher than the acoustic velocity of the bulk wave propagating through the boundary layer 12. When the boundary layer 12 is formed of an aluminum oxide film, even if the support substrate 10 is a silicon substrate, the acoustic velocity of the bulk wave propagating through the support substrate 10 is higher than the acoustic velocity of the bulk wave propagating through the boundary layer 12.

[0053] First Comparative Example

[0054] Figure 2A is a cross-sectional view of an acoustic wave resonator according to a first comparative example, and Figure 2B The transmission characteristics of the acoustic wave resonator according to the first comparative example are schematically illustrated. Figure 2A As shown, in the first comparative example, neither the intermediate layer 11 nor the boundary layer 12 is provided. The acoustic velocity of the bulk wave propagating through the support substrate 10 is higher than the acoustic velocity of the bulk wave propagating through the temperature compensation film 13. Therefore, a slow acoustic wave 50 including an acoustic wave (e.g., a surface acoustic wave) serving as a main mode and a bulk wave is reflected by the boundary surface 34 between the temperature compensation film 13 and the support substrate 10. An acoustic wave 51 faster than the acoustic wave 50 passes through the boundary surface 34.

[0055] like Figure 2B As shown, a low-frequency region 54 corresponding to the acoustic wave 50 includes a main response 58 and a spurious response 59 due to a body wave. A frequency region 55 corresponding to the acoustic wave 51 is higher in frequency than the region 54. In the first comparative example, since the acoustic wave serving as the main mode is confined in the piezoelectric layer 14 and the temperature compensation film 13, the main response 58 increases. However, the body wave reflected by the boundary surface 34 causes a spurious response 59 (high-frequency spurious emission).

[0056] Second Comparative Example

[0057] Figure 3A is a cross-sectional view of an acoustic wave resonator according to a second comparative example, and Figure 3B The transmission characteristics of the acoustic wave resonator according to the second comparative example are schematically illustrated. Figure 3AAs shown, in the second comparative example, intermediate layer 11 is not provided. The acoustic velocity of the bulk wave propagating through boundary layer 12 is higher than the acoustic velocity of the bulk wave propagating through temperature compensation film 13, and the acoustic velocity of the bulk wave propagating through support substrate 10 is higher than the velocity of the bulk wave propagating through boundary layer 12. Therefore, slow acoustic wave 50, which includes an acoustic wave serving as a main mode, is reflected by boundary surface 32 between temperature compensation film 13 and boundary layer 12. Acoustic wave 52, which includes a bulk wave faster than acoustic wave 50, passes through boundary surface 32 and is reflected by boundary surface 35 between boundary layer 12 and support substrate 10. Acoustic wave 51, which is faster than acoustic wave 52, passes through boundary surfaces 32 and 35.

[0058] like Figure 3B As shown, low-frequency region 54 corresponding to acoustic wave 50 includes a main response 58. Frequency region 56 corresponding to acoustic wave 52 is higher in frequency than region 54 and includes a spurious response 59 due to body waves. Frequency region 55 corresponding to acoustic wave 51 is higher in frequency than frequency region 56. In the second comparative example, main response 58 is as large as in the first comparative example. Additionally, since acoustic wave 52, including body waves, passes through boundary layer 12, spurious response 59 is small compared to that in the first comparative example. Since acoustic wave 52, including body waves, does not leak to support substrate 10, losses are reduced. Although spurious response 59 caused by body waves reflected from boundary surface 35 can be reduced, spurious response 59 is not sufficiently small.

[0059] To reduce the spurious response 59 in the second comparative example, it is possible to thicken the boundary layer 12. In addition, to scatter the acoustic wave 52 at the boundary surface 35, it is possible to roughen the boundary surface 35. However, these methods increase the number of manufacturing steps and increase the difficulty of the manufacturing process.

[0060] In the first embodiment, the sharp reflection of acoustic waves 52 is reduced by providing an intermediate layer 11 having a low Q factor between boundary layer 12 and support substrate 10. As a result, spurious emission due to bulk waves is reduced. Since the acoustic velocity of bulk waves propagating through support substrate 10 is higher than that of bulk waves propagating through boundary layer 12, acoustic waves 52, including bulk waves, are prevented from passing through intermediate layer 11 and leaking to support substrate 10. Consequently, losses are reduced.

[0061] Simulation 1

[0062] The transmission characteristics of the first embodiment and the second comparative example were simulated. The simulation conditions are as follows.

[0063] Support substrate 10: sapphire substrate

[0064] Intermediate layer 11 : aluminum oxide layer, T1=1λ, Q=1 / 50×Q0, where Q0 represents the Q factor of the boundary layer 12 .

[0065] Boundary layer 12: aluminum oxide layer, T2 = 5λ

[0066] Temperature compensation film 13: silicon oxide film, T3 = 0.3λ

[0067] Piezoelectric layer 14: 42° rotated Y-cut X-propagated lithium tantalate substrate, T4 = 0.3λ

[0068] Metal film 16: Aluminum with a thickness of 0.1λ

[0069] Acoustic wavelength λ: 5μm

[0070] In the second comparative example, the intermediate layer 11 is not provided.

[0071] The sound velocity of the body wave propagating through each material used in the simulation is configured as follows.

[0072] Support substrate 10: 7068.2m / s

[0073] Intermediate layer 11: 4581.8m / s

[0074] Boundary layer 12: 4581.8 m / s

[0075] Temperature compensation film 13: 3683.5m / s

[0076] Piezoelectric layer 14: 3750.8m / s

[0077] Figures 4A to 4C The magnitude |Y| of the admittance with respect to the frequency in the first embodiment and the second comparative example in Simulation 1 is illustrated. Figure 4B and Figure 4C They are Figure 4A Enlarged view of range A and range B. Figure 4A and Figure 4B As shown, the amplitude of the main response 58 of the first embodiment is approximately equal to that of the second comparative example. Figure 4A and Figure 4C As shown, the amplitude of the spurious response 59 of the first embodiment is smaller than the amplitude of the spurious response of the second comparative example.

[0078] Figure 5A and Figure 5B are Smith charts of the impedance of the second comparative example and the first embodiment in Simulation 1. The Smith chart shows the impedance of the acoustic wave resonator in the frequency range of 500 MHz to 3000 MHz. Figure 5A and Figure 5B As shown, in the first embodiment, the difference in impedance due to high-frequency spurious emission is smaller than that in the second comparative example. As can be seen above, without changing the amplitude of the main response 58, the first embodiment can reduce the spurious response 59 compared to the second comparative example.

[0079] Simulation 2

[0080] In Simulation 2, the Q factor of the intermediate layer 11 was changed from 1×Q0 to 1 / 50×Q0. The case where the Q factor of the intermediate layer 11 was 1×Q0 corresponds to the second comparative example. Q0 represents the Q factor of the boundary layer 12. The other conditions are the same as those of the first embodiment of Simulation 1.

[0081] Figures 6A to 6E The magnitude |Y| of the admittance with respect to the frequency in Simulation 2 is illustrated. 7A to 7E They are Figures 6A to 6E Zoomed in view of the spurious response around 59.

[0082] Figure 8A and Figure 8B The main response ΔY and the spurious response maxΔY with respect to the Q factor of the middle layer in simulation 2 are respectively illustrated. The main response ΔY is Figures 6A to 6E The difference between the admittance |Y| at the resonant frequency and the admittance |Y| at the anti-resonant frequency around 750MHz. The spurious response maxΔY is 7A to 7E The largest ΔY among the ΔY responses in the range of 1000MHz to 2250MHz.

[0083] like Figures 6A to 6E and Figure 8A As shown, the amplitude of the main response 58 hardly changes even if the Q factor of the intermediate layer 11 changes. The main response ΔY when the Q factor of the intermediate layer 11 is 1×Q1 is approximately equal to the main response ΔY when the Q factor of the intermediate layer 11 is 1 / 50×Q2.

[0084] like 7A to 7E and Figure 8B As shown in FIG. 1 , as the Q factor of the intermediate layer 11 decreases, the spurious response 59 decreases. When the Q factor of the intermediate layer 11 becomes equal to or less than 0.2×Q0, the spurious response maxΔY decreases rapidly. The spurious response maxΔY when the Q factor of the intermediate layer 11 is 1 / 50×Q0 is approximately 1 / 7 of the spurious response maxΔY when the Q factor is 1×Q0.

[0085] As seen above, by adjusting the Q factor of the intermediate layer 11 to be smaller than the Q factor Q0 of the boundary layer 12 , the spurious response maxΔY can be reduced without changing the main response ΔY.

[0086] Simulation 3

[0087] In Simulation 3, the Q factor of the intermediate layer 11 is configured to be 1 / 10×Q0, and the thickness T1 of the intermediate layer 11 is changed from 0.2λ to 1λ. Other conditions are the same as those of the first embodiment in Simulation 1.

[0088] Figures 9A to 9E The magnitude |Y| of the admittance with respect to the frequency in Simulation 3 is illustrated. Figures 10A to 10E They are Figures 9A to 9E Zoomed in view around the middle spurious response 59. Figure 11A and Figure 11B The main response ΔY and the spurious response maxΔY with respect to the thickness T1 of the intermediate layer in Simulation 3 are illustrated respectively.

[0089] like Figures 9A to 9E and Figure 11A As shown, even when the thickness T1 of the intermediate layer 11 changes, the amplitude of the main response 58 hardly changes. Figures 10A to 10E and Figure 11B As shown, as the thickness T1 of the intermediate layer 11 increases, the spurious response 59 decreases. The spurious response maxΔY when the thickness T1 of the intermediate layer 11 is 1λ is approximately 1 / 2 of the spurious response maxΔY when the thickness T1 is 0.2λ. As can be seen above, the spurious response maxΔY can be reduced by increasing the thickness T1 of the intermediate layer 11.

[0090] Simulation 4

[0091] In Simulation 4, the spurious response maxΔY was simulated under the condition that the thickness T2 of the boundary layer 12 was configured to be 1.1λ and the Q factor and the thickness T1 of the intermediate layer 11 were changed. Figure 12 The spurious response maxΔY with respect to the thickness and Q factor of the intermediate layer in simulation 4 is illustrated. Figure 12 As shown, when the thickness T1 of the intermediate layer 11 is adjusted to be greater than or equal to 0.05λ and the Q / Q0 of the intermediate layer 11 is adjusted to be less than or equal to 0.2, the spurious response maxΔY is equal to or less than approximately -20 dB. When the Q / Q0 of the intermediate layer 11 is adjusted to be less than or equal to 0.1, the spurious response maxΔY is equal to or less than approximately -17.5 dB.

[0092] Simulation 5

[0093] In Simulation 5, the sound speed of the body wave propagating through the intermediate layer 11 is configured to be different from the sound speed of the body wave propagating through the boundary layer 12. The material and sound speed of the body wave of the intermediate layer 11 are configured as follows.

[0094] Sample A: Alumina, 4581.8 m / s

[0095] Sample B: sapphire, 7068.2 m / s

[0096] The Q factor of the intermediate layer 11 is configured to be 1 / 50×Q0′, where Q0′ represents the Q factor of the support substrate 10. The thickness T1 of the intermediate layer 11 is configured to be 1λ, and other conditions are the same as those of Simulation 1.

[0097] 13A to 13C The magnitude |Y| of the admittance with respect to frequency of the samples A and B and the second comparative example in simulation 5 is respectively illustrated. Figure 14A and Figure 14B The main response ΔY and the spurious response maxΔY in Simulation 5 are illustrated respectively.

[0098] like Figure 14A As shown in FIG. 1 , even when the acoustic velocity of the body wave propagating through the intermediate layer 11 changes, the main response ΔY is substantially the same as that of the second comparative example. 13A to 13C and Figure 14B As shown in Figure 1, when the acoustic velocity of the body wave propagating through the intermediate layer 11 is increased compared to that in Sample A, as in Sample B, the spurious response maxΔY increases, but is smaller than the spurious response maxΔY of the second comparative example. As can be seen above, even when the acoustic velocity of the body wave propagating through the intermediate layer 11 is made different from that of the body wave propagating through the boundary layer 12, high-frequency spurious emissions can be reduced without changing the amplitude of the main response. As is clear from the comparison between Samples A and B, as the acoustic velocity of the body wave propagating through the intermediate layer 11 decreases, the high-frequency spurious emissions are reduced more.

[0099] Simulation 6

[0100] In Simulation 6, the thickness T2 of the boundary layer 12 was varied. First, the thickness T2 of the boundary layer 12 was varied in the second comparative example in which the intermediate layer 11 was not provided. The conditions were the same as those of the second comparative example in Simulation 1, except that the thickness T2 was varied and the thickness T3 of the temperature compensation film 13 was set to 0.1λ.

[0101] 15A to 15D Response with respect to the thickness T2 of the boundary layer 12 in the second comparative example of Simulation 6 is illustrated. Figure 15A The main response is instantiated, and Figure 15B yes Figure 15A Enlarged view of the portion where the thickness T2 is 10λ or less. Figure 15C The spurious responses are illustrated, and Figure 15D yes Figure 15C Enlarged view of the portion where the thickness T2 is 10λ or less.

[0102] like Figure 15A and Figure 15B As shown, the main response ΔY does not change even when the thickness T2 of the boundary layer 12 changes from 0λ to 70λ. Specifically, when the thickness T2 becomes 1.1λ or less, the main response ΔY becomes slightly smaller, and when the thickness T2 becomes 1λ or less, the main response ΔY becomes even smaller.

[0103] like Figure 15C and Figure 15DAs shown in FIG, as the thickness T2 of the boundary layer 12 increases, the spurious response maxΔY decreases. Figure 15C As shown in , when the thickness T2 becomes less than 10λ, the spurious response maxΔY becomes larger. Figure 15D As shown in FIG. 2 , when the thickness T2 becomes less than 1.1, the spurious response maxΔY increases rapidly and becomes more than 20 dB.

[0104] Next, the thickness T2 of the boundary layer 12 is varied in the first embodiment. The conditions are the same as those of the first embodiment of Simulation 1 except that the thickness T2 is varied.

[0105] Figure 16A and Figure 16B The main response and the spurious response with respect to the thickness T2 of the boundary layer 12 in the first embodiment of the simulation 6 are respectively illustrated. Figure 16A As shown, the main response ΔY is almost independent of the thickness T2 of the boundary layer 12. When the thickness T2 becomes less than 1.1λ, the main response ΔY decreases slightly. Figure 16B As shown, the spurious response maxΔY is almost independent of the thickness T2 of the boundary layer 12. When the thickness T2 becomes 1.1λ or less, the spurious response maxΔY decreases slightly.

[0106] In Simulation 6, the thickness T3 of temperature compensation film 13 differs between the second comparative example and the first embodiment. Therefore, a simple comparison is inaccurate. However, in the first embodiment, unlike the second comparative example, even when the thickness T2 of boundary layer 12 is reduced, the main response ΔY does not decrease, and the spurious response maxΔY does not increase. As can be seen above, spurious responses can be reduced even when boundary layer 12 is thin. Therefore, the increase in the number of manufacturing steps due to an increase in the thickness of boundary layer 12 can be reduced.

[0107] exist Figure 3A In the second comparative example, the acoustic velocity of the body wave propagating through the boundary layer 12 is higher than the acoustic velocity of the body wave propagating through the temperature compensation film 13, and is lower than the acoustic velocity of the body wave propagating through the support substrate 10. Figure 3A As shown, the acoustic wave 52 including the bulk wave is reflected by the boundary surface 35 between the boundary layer 12 and the support substrate 10. Figure 3B As shown, a spurious response 59 is generated in a frequency range higher than the main response 58. Therefore, in the first embodiment, the intermediate layer 11 having a Q factor lower than that of the boundary layer 12 is provided between the support substrate 10 and the boundary layer 12. This structure reduces the spurious response 59 without deteriorating the main response 58, as shown in FIG. Figures 4A to 4C shown.

[0108] like Figure 12As shown, the thickness T1 of the intermediate layer 11 is adjusted to be equal to or greater than 0.1 times the average pitch D of the electrode fingers 18 (equal to or greater than 0.05λ), and the Q factor of the intermediate layer 11 is adjusted to be equal to or less than 0.2 times the Q factor Q0 of the boundary layer 12. This configuration results in a spurious response maxΔY equal to or less than approximately -20 dB. To reduce spurious responses, the thickness T1 of the intermediate layer 11 is preferably equal to or greater than 0.2 times the average pitch D of the electrode fingers 18 (equal to or greater than 0.1λ), and more preferably equal to or greater than 0.4 times the average pitch D of the electrode fingers 18 (equal to or greater than 0.2λ). The thickness T1 of the intermediate layer 11 is, for example, equal to or less than 10 times the average pitch D of the electrode fingers 18 (equal to or less than 5λ). To reduce spurious responses, the Q factor of the intermediate layer 11 is preferably equal to or less than 0.1 of the Q factor Q0 of the boundary layer 12, and more preferably equal to or less than 0.05. The Q factor of the intermediate layer 11 is greater than 0. The average pitch D of the electrode fingers 18 can be calculated by dividing the length of the IDT 22 in the X direction by the number of electrode fingers 18 in the acoustic wave resonator 26 .

[0109] like Figure 16A As shown, in order not to deteriorate the main response ΔY, the thickness T2 of the boundary layer 12 is preferably equal to or greater than 2.2 times (equal to or greater than 1.1λ) the average pitch D of the electrode fingers 18, and more preferably equal to or greater than 3.0 times (equal to or greater than 1.5λ) the average pitch D of the electrode fingers 18. Figure 15C and Figure 15D As shown, in the second comparative example, even when the thickness T2 of the boundary layer 12 is 1.1λ or more, as the thickness T1 increases, the spurious response maxΔY decreases. As the thickness of the boundary layer 12 increases, the number of manufacturing steps increases and the difficulty of the manufacturing process increases. On the other hand, as Figure 16B As shown, in the first embodiment, when the thickness T2 of the boundary layer 12 is 1.1λ or greater, the spurious response maxΔY remains almost unchanged. That is, in the first embodiment, even when the thickness T2 of the boundary layer 12 is not as thick as in the second comparative example, the spurious response can be reduced. Since the boundary layer 12 can be made thinner, the number of manufacturing steps and the level of difficulty in the manufacturing process can be reduced. Therefore, the thickness T2 of the boundary layer 12 is preferably equal to or less than 10 times the average pitch D of the electrode fingers 18 (equal to or less than 5λ), and more preferably equal to or less than 8 times the average pitch D of the electrode fingers 18 (equal to or less than 4λ).

[0110] like Figure 3BAs shown, in order for acoustic waves 52, including bulk waves, to pass through boundary layer 12, thickness T3 of temperature compensation film 13 is preferably equal to or less than 1.5 times the average pitch D of electrode fingers 18 (equal to or less than 0.75λ), and more preferably equal to or less than 1 times the average pitch D of electrode fingers 18 (equal to or less than 0.5λ). In order for temperature compensation film 13 to achieve a temperature compensation function, thickness T3 is preferably equal to or greater than 0.05 times the average pitch D of electrode fingers 18 (equal to or greater than 0.1λ), and more preferably equal to or greater than 0.1 times the average pitch D of electrode fingers 18 (equal to or greater than 0.2λ).

[0111] To ensure that the acoustic wave energy of the primary response resides within the temperature compensation film 13, the thickness T4 of the piezoelectric layer 14 is preferably equal to or less than twice the average pitch D of the electrode fingers 18 (equal to or less than 1λ), and more preferably equal to or less than one time the average pitch D of the electrode fingers 18 (equal to or less than 0.5λ). To ensure that the piezoelectric layer 14 functions properly, the thickness T4 of the piezoelectric layer 14 is preferably greater than or equal to 0.05 times the average pitch D of the electrode fingers 18 (greater than or equal to 0.1λ), and more preferably greater than or equal to 0.1 times the average pitch D of the electrode fingers 18 (equal to or greater than 0.2λ).

[0112] When most of the energy of the surface acoustic wave exists in the segment from the surface of the piezoelectric layer 14 to a depth of 2λ, in order to confine the main response acoustic wave to the piezoelectric layer 14 and the temperature compensation film 13 and reduce stray responses, the distance (T3+T4) between the first surface of the temperature compensation film 13 closer to the supporting substrate 10 and the second surface of the piezoelectric layer 14 closer to the comb electrode 20 is preferably equal to or less than 4 times the average pitch D of the electrode fingers 18 (equal to or less than 2λ), more preferably equal to or less than 3 times the average pitch D of the electrode fingers 18 (equal to or less than 1.5λ), and more preferably equal to or less than 2 times the average pitch D of the electrode fingers 18 (equal to or less than 1λ).

[0113] Since the acoustic waves 52 including the bulk waves are reflected by the intermediate layer 11 , the boundary layer 12 and the intermediate layer 11 are preferably in contact with each other, and the temperature compensation film 13 and the boundary layer 12 are preferably in contact with each other.

[0114] The acoustic velocity of the body wave propagating through the temperature compensation film 13 may be higher than the acoustic velocity of the body wave propagating through the piezoelectric layer 14. However, in order to make it more likely that the acoustic wave exists in the temperature compensation film 13, the acoustic velocity of the body wave propagating through the temperature compensation film 13 is preferably lower than the acoustic velocity of the body wave propagating through the piezoelectric layer 14. This configuration allows the temperature compensation film 13 to function more as a temperature compensation film. The acoustic velocity of the body wave propagating through the temperature compensation film 13 is preferably equal to or less than 0.99 times the acoustic velocity of the body wave propagating through the piezoelectric layer 14. When the acoustic velocity of the body wave propagating through the temperature compensation film 13 is too low, the possibility of the acoustic wave existing in the piezoelectric layer 14 is even smaller. Therefore, the acoustic velocity of the body wave propagating through the temperature compensation film 13 is preferably equal to or greater than 0.9 times the acoustic velocity of the body wave propagating through the piezoelectric layer 14.

[0115] The acoustic velocity of the body wave propagating through the boundary layer 12 is preferably equal to or greater than 1.1 times, and more preferably equal to or greater than 1.2 times, the acoustic velocity of the body wave propagating through the temperature compensation film 13. Furthermore, the acoustic velocity of the body wave propagating through the boundary layer 12 is preferably higher than the acoustic velocity of the body wave propagating through the piezoelectric layer 14. When the acoustic velocity of the body wave propagating through the boundary layer 12 is too high, the acoustic wave 52 including the body wave is reflected by the boundary surface 32 between the boundary layer 12 and the temperature compensation film 13. Therefore, the acoustic velocity of the body wave propagating through the boundary layer 12 is preferably equal to or less than 2.0 times, and more preferably equal to or less than 1.5 times, the acoustic velocity of the body wave propagating through the temperature compensation film 13.

[0116] The acoustic velocity of the body wave propagating through the support substrate 10 is preferably equal to or greater than 1.1 times, more preferably equal to or greater than 1.2 times, the acoustic velocity of the body wave propagating through the boundary layer 12. The acoustic velocity of the body wave propagating through the support substrate 10 is equal to or less than 2.0 times the acoustic velocity of the body wave propagating through the boundary layer 12.

[0117] By making the main components of boundary layer 12 and intermediate layer 11 substantially identical, the acoustic velocity of bulk waves propagating through boundary layer 12 and intermediate layer 11 can be made approximately equal. As in simulations 1 to 6 of the first embodiment, piezoelectric layer 14 is primarily composed of lithium tantalate or lithium niobate and is single-crystalline. Temperature compensation film 13 is primarily composed of silicon oxide and is polycrystalline or amorphous. Boundary layer 12 and intermediate layer 11 are primarily composed of aluminum oxide and are polycrystalline or amorphous. This configuration reduces spurious responses. When a layer is primarily composed of a certain material, this means that intentional or unintentional impurities in that layer are acceptable, and that the layer contains 50 atomic percent or more of the certain material, or 80 atomic percent or more of the certain material. For example, when boundary layer 12 is primarily composed of aluminum oxide, this means that boundary layer 12 contains, for example, at least 50 atomic percent of aluminum and oxygen, or at least 80 atomic percent of aluminum and oxygen, of the total composition.

[0118] First Modification of the First Embodiment

[0119] Figure 17 : is a cross-sectional view of an acoustic wave resonator according to a first modification of the first embodiment. Figure 17 As shown, the bonding layer 15 is inserted between the piezoelectric layer 14 and the temperature compensation film 13. The bonding layer 15 bonds the piezoelectric layer 14 to the temperature compensation film 13. When it is difficult to directly bond the piezoelectric layer 14 to the temperature compensation film 13, a bonding layer 15 may be provided. The bonding layer 15 is formed, for example, of an aluminum oxide film, a silicon film, an aluminum nitride film, a silicon nitride film or a silicon carbide film. In order not to impair the functions of the piezoelectric layer 14 and the temperature compensation film 13, the thickness T5 of the bonding layer 15 is preferably less than 20 nm, more preferably less than 10 nm. In order to maintain the function as the bonding layer 15, the thickness T5 is preferably greater than 1 nm, more preferably greater than 2 nm. In order to confine the acoustic wave of the main response in the piezoelectric layer 14, the sound velocity of the body wave propagating through the bonding layer 15 is preferably higher than the sound velocity of the body wave propagating through the temperature compensation film 13. The other structures are the same as those of the first embodiment, so their description is omitted.

[0120] In the first embodiment and its variations, when the acoustic waves primarily excited by the pair of comb-shaped electrodes 20 are shear horizontal (SH) waves, bulk waves are easily excited as unwanted waves. SH waves are excited when the piezoelectric layer 14 is a rotated Y-cut X-propagation lithium tantalate layer with an angle of 36° or greater and 48° or less. Therefore, in this case, it is preferable to provide the boundary layer 12. The acoustic waves primarily excited by the pair of comb-shaped electrodes 20 are not limited to SH waves and may be, for example, Lamb waves.

[0121] Second embodiment

[0122] Figure 18A : is a circuit diagram of a filter according to the second embodiment. Figure 18A As shown, one or more series resonators S1 to S3 are connected in series between the input terminal Tin and the output terminal Tout. One or more parallel resonators P1 and P2 are connected in parallel between the input terminal Tin and the output terminal Tout. At least one of the following resonators can be an acoustic wave resonator according to any of the first embodiment and its variations: the resonators are one or more series resonators S1 to S3 and one or more parallel resonators P1 and P2. The number of resonators in the ladder filter can be freely selected. The filter can be a multimode type filter.

[0123] First Modification of the Second Embodiment

[0124] Figure 18B FIG. 1 is a circuit diagram of a duplexer according to a first modification of the second embodiment. Figure 18BAs shown, the transmit filter 40 is connected between the common terminal Ant and the transmit terminal Tx. The receive filter 42 is connected between the common terminal Ant and the receive terminal Rx. The transmit filter 40 transmits signals within the transmit frequency band of the high-frequency signal input from the transmit terminal Tx as transmit signals to the common terminal Ant, while suppressing signals of other frequencies. The receive filter 42 transmits signals within the receive frequency band of the high-frequency signal input from the common terminal Ant as receive signals to the receive terminal Rx, while suppressing signals of other frequencies. At least one of the transmit filter 40 and the receive filter 42 may be the filter of the second embodiment.

[0125] A duplexer has been described as an example of a multiplexer, but the multiplexer may be a triplexer or a quadplexer.

[0126] Third embodiment

[0127] In the third embodiment, an example in which the acoustic wave device has an acoustic wave resonator will be described. Figure 19A is a plan view of an acoustic wave resonator according to a third embodiment, and Figure 19B This is a cross-sectional view of an acoustic wave resonator according to the third embodiment. The direction in which the electrode fingers are arranged (electrode finger arrangement direction) is defined as the X direction, the direction in which the electrode fingers extend (electrode finger extension direction) is defined as the Y direction, and the direction in which the support substrate and piezoelectric layer are stacked (the direction in which the support substrate and piezoelectric layer are stacked) is defined as the Z direction. The X, Y, and Z directions do not necessarily correspond to the X-axis and Y-axis orientations of the crystal orientation of the piezoelectric layer. When the piezoelectric layer is a rotated Y-cut X-propagation substrate, the X direction corresponds to the X-axis orientation of the crystal orientation.

[0128] like Figure 19A and Figure 19B As shown, a piezoelectric layer 114 is provided above a support substrate 110. An intermediate layer 112 is interposed between the support substrate 110 and the piezoelectric layer 114. Another intermediate layer 111 is interposed between the intermediate layer 112 and the support substrate 110. A bonding layer 113 is interposed between the intermediate layer 112 and the piezoelectric layer 114. The intermediate layers 111 and 112 and the bonding layer 113 form a multilayer film 115. The bottom surface of the multilayer film 115 is in contact with the support substrate 110, and the top surface of the multilayer film 115 is in contact with the piezoelectric layer 114. The thicknesses of the intermediate layers 111 and 112, the bonding layer 113, the piezoelectric layer 114, and the multilayer film 115 are denoted by T11, T12, T13, T14, and T15, respectively.

[0129] The acoustic wave resonator 126 is provided on the piezoelectric layer 114. The acoustic wave resonator 126 includes an IDT 122 and a reflector 124. The reflector 124 is located on both sides of the IDT 122 in the X direction. The IDT 122 and the reflector 124 are formed by the metal film 116 on the piezoelectric layer 114.

[0130] IDT 122 includes a pair of comb electrodes 120 facing each other. Comb electrodes 120 include electrode fingers 118 and bus bars 119 connecting electrode fingers 118. The region where the electrode fingers 118 of the pair of comb electrodes 120 overlap is an overlapping region 125. The length of overlapping region 125 in the Y direction is the opening length. The pair of comb electrodes 120 are arranged so that the electrode fingers 118 of one comb electrode 120 and the electrode fingers 118 of the other comb electrode 120 are substantially alternately arranged in at least a portion of overlapping region 125. Acoustic waves excited by the electrode fingers 118 in overlapping region 125 primarily propagate in the X direction. The pitch of the electrode fingers 118 of one comb electrode 120 in the pair of comb electrodes 120 corresponds to the wavelength λ of the acoustic wave. When the pitch of the electrode fingers 118 (the pitch between the centers of the electrode fingers 118) is represented by D, the pitch of the electrode fingers 118 of one comb electrode 120 is equal to twice the pitch D of the electrode fingers 118. The reflectors 124 reflect the acoustic waves (surface acoustic waves) excited by the electrode fingers 118 of the IDT 122. Therefore, the acoustic waves are confined to the overlapping region 125 of the IDT 122.

[0131] The piezoelectric layer 114 is, for example, a single-crystal lithium tantalate (LiTaO 3 ) layer or a single-crystal lithium niobate (LiNbO 3 ) layer, and is, for example, a rotated Y-cut X-propagation lithium tantalate layer or a rotated Y-cut X-propagation lithium niobate layer.

[0132] Support substrate 110 may be, for example, a sapphire substrate, a silicon substrate, a spinel substrate, a quartz substrate, a crystal substrate, an alumina substrate, or a silicon carbide substrate. A sapphire substrate is a single-crystal Al2O3 substrate, a silicon substrate is a single-crystal or polycrystalline silicon substrate, a spinel substrate is a polycrystalline MgAl2O4 substrate, a quartz substrate is an amorphous SiO2 substrate, a crystal substrate is a single-crystal SiO2 substrate, and a silicon carbide substrate is a polycrystalline or single-crystal SiC substrate. The linear expansion coefficient of support substrate 110 in the X direction is smaller than the linear expansion coefficient of piezoelectric layer 114 in the X direction. This configuration reduces the temperature dependence of the frequency of the acoustic wave resonator.

[0133] The intermediate layer 112 is formed, for example, of a temperature compensation film, and has a temperature coefficient of elastic constant that is opposite in sign to the temperature coefficient of elastic constant of the piezoelectric layer 114. For example, the piezoelectric layer 114 has a negative temperature coefficient of elastic constant, while the intermediate layer 112 has a positive temperature coefficient of elastic constant. The intermediate layer 112 is, for example, an insulating layer formed of an additive-free silicon oxide (SiO2) film or a SiO2 film containing an additive element such as, but not limited to, fluorine, and is, for example, an amorphous layer. This configuration reduces the temperature coefficient of the frequency of the acoustic wave resonator. When the intermediate layer 112 is formed of a silicon oxide film, the acoustic velocity of the bulk wave propagating through the intermediate layer 112 is lower than the acoustic velocity of the bulk wave propagating through the piezoelectric layer 114.

[0134] Intermediate layer 111 is a boundary layer or a high-acoustic-velocity layer, and the acoustic velocity of the bulk wave propagating through intermediate layer 111 is higher than the acoustic velocity of the bulk wave propagating through intermediate layer 112. Therefore, the acoustic wave is confined within piezoelectric layer 114 and intermediate layer 112. Furthermore, the acoustic velocity of the bulk wave propagating through intermediate layer 111 is lower than the acoustic velocity of the bulk wave propagating through support substrate 110. Intermediate layer 111 is, for example, polycrystalline or amorphous, and is an insulating layer formed of, for example, but not limited to, an aluminum oxide film, a silicon film, an aluminum nitride film, a silicon nitride film, or a silicon carbide film. Multiple layers made of different materials may be provided as intermediate layer 111.

[0135] The acoustic velocity of the bulk wave propagating through the bonding layer 113 is higher than that of the bulk wave propagating through the intermediate layer 112. The bonding layer 113 is, for example, polycrystalline or amorphous, and formed of, for example, an aluminum oxide film, a silicon film, an aluminum nitride film, a silicon nitride film, or a silicon carbide film.

[0136] The metal film 116 is a film primarily composed of, for example, aluminum (Al), copper (Cu), or molybdenum (Mo). An adhesive film, such as a titanium (Ti) film or a chromium (Cr) film, may be provided between the electrode fingers 118 and the piezoelectric layer 114. The adhesive film is thinner than the electrode fingers 118. An insulating film, such as a silicon oxide film or a silicon nitride film, may be provided to cover the electrode fingers 118. The insulating film serves as a protective film or a temperature compensation film.

[0137] The wavelength λ of the acoustic wave is, for example, 1 μm to 6 μm. When two electrode fingers 118 are defined as a pair, the number of pairs of electrode fingers 118 is, for example, 20 to 300. The duty cycle of IDT 122 is calculated by dividing the width of electrode finger 118 by the pitch of electrode finger 118, and is, for example, 30% to 70%. The opening length of IDT 122 is, for example, 10λ to 50λ.

[0138] IDT122 excites surface acoustic waves serving as the main mode in the piezoelectric layer 114. At this time, IDT 122 also excites unwanted waves such as body waves. The energy of the surface acoustic wave exists in a section from the upper surface of the piezoelectric layer 114 to a depth of approximately 2λ (λ is the wavelength of the acoustic wave), especially in a section from the upper surface of the piezoelectric layer 114 to a depth of λ. In contrast, unwanted waves such as body waves exist in a section from the upper surface of the piezoelectric layer 114 to a depth of more than 10λ. As the unwanted wave propagates downward, the energy of the acoustic wave leaks and the loss increases. On the other hand, when the body wave is reflected by the boundary surface on the way to the supporting substrate 110 and returns to the IDT122, this causes spurious emission.

[0139] Figures 20A to 21C Schematic cross-sectional views of the intermediate layer 111 in the third comparative example and the third embodiment. Figure 20A and Figure 21A corresponds to the third comparative example, and Figure 20B 、 Figure 20C 、 Figure 21B and Figure 21C Corresponding to the third embodiment. Figure 20A As shown in FIG. 1 , the intermediate layer 111 a in the third comparative example is non-porous, and almost no pores are formed in the intermediate layer 111 a. Figure 20B As shown, the middle layer 111b is porous, and pores 130 are formed in the middle layer 111b. Figure 20C As shown, the intermediate layer 111c is porous and has more pores 130 than the intermediate layer 111b. That is, the porosity of the intermediate layer 111c is greater than the porosity of the intermediate layer 111b. The porosity is the ratio of the total volume of the pores 130 to the total volume of each of the intermediate layers 111b and 111c.

[0140] like Figure 21A As shown, the intermediate layer 111d has a columnar structure having columnar grains 132. The surfaces between the grains 132 are grain boundaries 134. The grains 132 extend in the Z direction. That is, the width of the grain 132 in the Z direction is greater than the width of the grain 132 in the X direction. At least one grain 132 is provided from the bottom surface to the top surface of the intermediate layer 111d. The columnar structure can be formed using a sputtering method. Figure 21B As shown, the intermediate layer 111e has pores 130 formed in the grain boundaries 134. The pores 130 extend along the grain boundaries 134. Therefore, the width of the pores 130 in the Z direction is greater than the width of the pores 130 in the X direction. Figure 21CAs shown, pores 130 larger than those of the intermediate layer 111e are formed in the intermediate layer 111f. The porosity of the intermediate layer 111f is greater than that of the intermediate layer 111e. According to the Thornton Zone Model, when the intermediate layer 111 is formed by sputtering, the intermediate layer 111 having a porous columnar structure such as the intermediate layers 111e and 111f is obtained by adjusting the substrate temperature to a low level and the gas pressure to a high level. Under low gas pressure conditions, a non-porous intermediate layer 111d can be formed by sputtering. The porous intermediate layer 111e can be formed by adjusting the gas pressure to a high level, and the porous intermediate layer 111f having a greater porosity can be formed by adjusting the gas pressure to a higher level.

[0141] 22A to 22D is a cross-sectional view illustrating a method of manufacturing an acoustic wave resonator in the third embodiment. Figure 22A As shown, an intermediate layer 111 is formed on a support substrate 110. By adjusting the pressure of an inert gas (eg, argon) to be high when forming the intermediate layer 111 by sputtering, pores 130 are formed in the intermediate layer 111, as shown in FIG. Figure 21B and Figure 21C As shown. The intermediate layer 111 can be formed by vacuum evaporation or chemical vapor deposition (CVD). Figure 22B As shown, an intermediate layer 112 is formed on the intermediate layer 111, and a bonding layer 113 is formed on the intermediate layer 112. The intermediate layer 112 and the bonding layer 113 are formed by, for example, sputtering, vacuum evaporation, or CVD. The intermediate layers 111 and 112 and the bonding layer 113 form a multilayer film 115.

[0142] like Figure 22C As shown, the piezoelectric layer 114 is bonded to the bonding layer 113. The piezoelectric layer 114 is bonded using, for example, surface activation. Figure 22D As shown, the piezoelectric layer 114 is thinned by polishing or grinding its upper surface. For example, the piezoelectric layer 114 is thinned by chemical mechanical polishing (CMP). Through the above steps, a wafer is completed. Thereafter, a metal film 116 is formed on the piezoelectric layer 114 by vacuum evaporation and lift-off. This process forms an acoustic wave resonator 126 on the piezoelectric layer 114. The metal film 116 can be formed by sputtering and etching.

[0143] experiment

[0144] The amount of wafer warpage was measured under different film forming conditions of the intermediate layer 111. The wafer manufacturing conditions were as follows.

[0145] Support substrate 110: 500 μm thick sapphire substrate

[0146] Intermediate layer 111: Aluminum oxide layer with a thickness T11 of 7.2λ

[0147] Intermediate layer 112: silicon oxide layer with a thickness T12 of 0.2λ

[0148] Piezoelectric layer 114: 42° rotated Y-cut X-propagated lithium tantalate layer with a thickness T14 of 0.3λ

[0149] Acoustic wavelength λ: 1.5μm

[0150] Wafer size: 4 inches

[0151] Since the thickness T13 of the bonding layer 113 is about 10 nm, the stress caused by the bonding layer 113 can be substantially ignored.

[0152] Samples A to C were manufactured, each including an intermediate layer 111 formed under different film-forming conditions. The intermediate layer 111 was formed by sputtering using argon gas, and the argon gas pressure was adjusted to A<B<C. The density of the intermediate layer 111 formed under the same conditions as those for samples A to C was measured. The density of the intermediate layer 111 of each sample was as follows.

[0153] Sample A: 3.17 g / cm 3

[0154] Sample B: 3.15 g / cm 3

[0155] Sample C: 3.08 g / cm 3

[0156] As can be seen from the above, as the gas pressure when forming the intermediate layer 111 increases, the density of the intermediate layer 111 decreases. The intermediate layer 111 of sample A is considered to have a columnar structure and is similar to Figure 21A The intermediate layer 111d shown is non-porous and has almost no pores. The intermediate layer 111 of sample B is considered to have a columnar structure and is similar to Figure 21B The intermediate layer 111e of sample C is considered to have a columnar structure and is porous. Figure 21C The intermediate layer 111f of sample A is porous and has a higher porosity than the intermediate layer 111 of sample B. Assuming that the intermediate layer 111 of sample A is non-porous and the difference in density between samples A to C is determined only by the porosity, the porosity of samples A, B, and C is 0%, about 1%, and about 3%, respectively.

[0157] Measured before the piezoelectric layer 114 is bonded ( Figure 22B The amount of wafer bending before the bonding layer 113 is formed and the amount of wafer bending after the piezoelectric layer 114 is bonded and thinned ( Figure 22DThe amount of wafer bow is represented by BOW. BOW corresponds to the distance from the reference surface at the center of the wafer. When BOW is positive, the wafer is convexly curved, and the stress in the multilayer film 115 is compressive. When BOW is negative, the wafer is concavely curved, and the stress in the multilayer film 115 is tensile.

[0158] Figure 23 The wafer bending amount (BOW) of each sample before and after bonding in the experiment is shown. Since the wafer bending amount of sample A after bonding is too large, the piezoelectric layer 114 cannot be bonded. Figure 23 As shown, before bonding, the BOW of sample B is approximately 60% of the BOW of sample A. The BOW of sample C is equal to or less than 50% of the BOW of sample A. After bonding, the BOW of sample C is approximately 75% of the BOW of sample B. As can be seen above, it is believed that the porous intermediate layer 111 reduces the internal stress caused by the intermediate layer 111, thereby reducing the wafer bow.

[0159] In the third embodiment, the intermediate layer 111 (first intermediate layer) is inserted between the supporting substrate 110 and the piezoelectric layer 114 and is thicker than the piezoelectric layer 114. When the intermediate layer 111 is thick, the wafer bends due to the internal stress of the intermediate layer 111. Therefore, the intermediate layer 111 is made porous. This structure reduces the internal stress of the intermediate layer 111, thereby reducing the wafer bending. When the thickness T11 of the intermediate layer 111 is equal to or greater than 1.5 times, equal to or greater than 2 times, or equal to or greater than 3 times the thickness T14 of the piezoelectric layer 114, the wafer is more likely to bend due to the internal stress of the intermediate layer 111. Therefore, preferably, the intermediate layer 111 is made porous. In order to reduce the internal stress of the intermediate layer 111, the porosity of the intermediate layer 111 is preferably 0.1% or more, more preferably 0.5% or more, and further preferably 1.0% or more. When the porosity of the intermediate layer 111 is too large, the function of the intermediate layer 111 (for example, the function as a high acoustic velocity membrane) deteriorates. Therefore, the porosity of the intermediate layer 111 is preferably 20% or less, more preferably 10% or less, and further preferably 5% or less.

[0160] Although it is not necessary to provide the intermediate layer 112, when the intermediate layer 111 is arranged around the piezoelectric layer 114, the sound waves are scattered by the pores 130, so that the loss increases. Therefore, the intermediate layer 112 (second intermediate layer) is inserted between the intermediate layer 111 and the piezoelectric layer 114, and the porosity of the intermediate layer 112 is adjusted to be smaller than the porosity of the intermediate layer 111. This configuration makes it difficult for the sound waves to be scattered by the pores 130, thereby reducing the loss. When the intermediate layer 112 is non-porous, the porosity is 0%. In order to reduce the loss, the porosity of the intermediate layer 112 is preferably 0.5% or less, more preferably 0.1% or less, and preferably equal to or less than 1 / 2 of the porosity of the intermediate layer 111, more preferably equal to or less than 1 / 10 of the porosity of the intermediate layer 111. When the intermediate layer 112 is too thin, the loss will increase even if the intermediate layer 112 is provided. Therefore, the thickness T12 of the intermediate layer 112 is preferably equal to or greater than 1 / 10 of the thickness T14 of the intermediate layer 112, more preferably equal to or greater than 1 / 5 of the thickness T14 of the intermediate layer 112, and further preferably equal to or greater than 1 / 2 of the thickness T14 of the intermediate layer 112.

[0161] To reduce wafer bow, intermediate layer 111 is preferably thicker. Therefore, intermediate layer 111 is preferably thicker than intermediate layer 112, and thickness T11 of intermediate layer 111 is preferably equal to or greater than twice thickness T12 of intermediate layer 112, more preferably equal to or greater than five times thickness T12 of intermediate layer 112. Furthermore, thickness T11 of intermediate layer 111 is preferably equal to or greater than ¼ thickness T15 of multilayer film 115, more preferably equal to or greater than ½ thickness T15 of multilayer film 115, and further preferably equal to or greater than ¾ thickness T15 of multilayer film 115.

[0162] like Figure 21B and Figure 21C As shown, the intermediate layer 111 has a columnar structure, and pores 130 are provided in grain boundaries 134 between crystal grains 132. Therefore, the porous intermediate layer 111 can be formed by sputtering.

[0163] The temperature coefficient of the elastic constant of the intermediate layer 112 has a sign opposite to that of the temperature coefficient of the elastic constant of the piezoelectric layer 114. This configuration can reduce the temperature coefficient of the frequency of the acoustic wave device. The intermediate layer 112 is mainly composed of silicon oxide as such a material. For example, the sum of the O concentration and the Si concentration in the intermediate layer 112 is 50 atomic % or more, or 80 atomic % or more. The O concentration is, for example, 10 atomic % or more or 20 atomic % or more, and the Si concentration is, for example, 10 atomic % or more or 20 atomic % or more. The sound velocity of the body wave propagating through the intermediate layer 111 is higher than the sound velocity of the body wave propagating through the intermediate layer 112. Therefore, the acoustic wave is confined in the piezoelectric layer 114 and the intermediate layer 112. The sound velocity of the body wave propagating through the intermediate layer 111 is preferably equal to or greater than 1.1 times the sound velocity of the body wave propagating through the intermediate layer 112, and more preferably equal to or greater than 1.2 times. The speed of sound propagating through the intermediate layer 111 is preferably equal to or less than 2.0 times the speed of sound of the body wave propagating through the intermediate layer 112 , and more preferably equal to or less than 1.5 times.

[0164] Although the acoustic velocity of the bulk wave propagating through the intermediate layer 112 may be higher than the acoustic velocity of the bulk wave propagating through the piezoelectric layer 114, the acoustic velocity of the bulk wave propagating through the intermediate layer 112 is preferably lower than the acoustic velocity of the bulk wave propagating through the piezoelectric layer 114. The acoustic velocity of the bulk wave propagating through the intermediate layer 112 is preferably equal to or less than 0.99 times the acoustic velocity of the bulk wave propagating through the piezoelectric layer 114. When the acoustic velocity of the bulk wave propagating through the intermediate layer 112 is too low, the acoustic wave is less likely to exist in the piezoelectric layer 114. Therefore, the acoustic velocity of the bulk wave propagating through the intermediate layer 112 is preferably equal to or greater than 0.9 times the acoustic velocity of the bulk wave propagating through the piezoelectric layer 114.

[0165] When most of the energy of the surface acoustic wave exists in the section from the surface of the piezoelectric layer 114 to a depth of 2λ, in order to confine the energy of the acoustic wave serving as the main mode within the piezoelectric layer 114 and the intermediate layer 112 and to reduce spurious responses, the distance (T12+T13+T14) between the first surface of the intermediate layer 112 closer to the support substrate 110 and the second surface of the piezoelectric layer 114 closer to the comb electrodes 120 is preferably equal to or less than four times the average pitch D of the electrode fingers 118 (equal to or less than 2λ), and more preferably equal to or less than three times the average pitch D of the electrode fingers 118 (equal to or less than 1.5λ). The average pitch D of the electrode fingers 118 can be calculated by dividing the width of the IDT 122 in the X direction by the number of electrode fingers 118.

[0166] To ensure that acoustic wave energy is present in intermediate layer 112, piezoelectric layer 114 preferably has a thickness T14 equal to or less than twice the average pitch D of electrode fingers 118 (equal to or less than λ), and more preferably equal to or less than 1.2 times the average pitch D of electrode fingers 118 (equal to or less than 0.6λ). If piezoelectric layer 114 is too thin, acoustic waves are not excited. Therefore, piezoelectric layer 114 preferably has a thickness T14 equal to or greater than 0.2 times the average pitch D of electrode fingers 118 (equal to or greater than 0.1λ).

[0167] When the thickness T11 of the intermediate layer 111 is small, spurious emissions increase. Therefore, the thickness T11 of the intermediate layer 111 is preferably equal to or greater than 0.6 times the average pitch D of the electrode fingers 118 (equal to or greater than 0.3λ), more preferably equal to or greater than 1.4 times the average pitch D of the electrode fingers 118 (equal to or greater than 0.7λ), more preferably equal to or greater than 2 times the average pitch D of the electrode fingers 118 (equal to or greater than λ), and even more preferably equal to or greater than 4 times the average pitch D of the electrode fingers 118 (equal to or greater than 2λ).

[0168] The thickness T13 of the bonding layer 113 is preferably 20 nm or less, more preferably 10 nm or less, so as not to impair the functions of the piezoelectric layer 114 and the intermediate layer 112. To prevent the function of the bonding layer 113 from being impaired, the thickness T13 is preferably 1 nm or more, more preferably 2 nm or more. The bonding layer 113 may be omitted.

[0169] Piezoelectric layer 114 is primarily composed of lithium tantalate or lithium niobate and is single-crystalline, intermediate layer 112 is primarily composed of silicon oxide and is polycrystalline or amorphous, intermediate layer 111 and bonding layer 113 are primarily composed of aluminum oxide and are polycrystalline or amorphous, and support substrate 110 is a sapphire substrate or a silicon carbide substrate. When a film or layer is primarily composed of a certain material, this means that the intentional or unintentional inclusion of impurities in the film or layer is acceptable, and that the film or layer contains 50 atomic percent or more of the certain material, or 80 atomic percent or more of the certain material.

[0170] When the acoustic waves excited by the pair of comb electrodes 120 are primarily shear horizontal (SH) waves, bulk waves are more likely to be excited as unwanted waves. SH waves are excited when the piezoelectric layer 114 is a lithium tantalate layer with a small Y-cut angle of 36° or greater and 48°. The acoustic waves primarily excited by the pair of comb electrodes 120 are not limited to SH waves and may be, for example, Lamb waves.

[0171] First Modification of the Third Embodiment

[0172] Figure 24A : is a cross-sectional view of an acoustic wave resonator according to a first modification of the third embodiment. Figure 24AAs shown, in the first variant of the third embodiment, the bonding layer 113 is not provided. The intermediate layer 111 is thicker than the intermediate layer 112, and thicker than the piezoelectric layer 114. The intermediate layer 111 is porous, while the intermediate layer 112 is non-porous. For example, the intermediate layer 111 is mainly composed of aluminum oxide, and the intermediate layer 112 is mainly composed of silicon oxide. As seen above, the main component of the intermediate layer 111 may be different from the main component of the intermediate layer 112. As another example, the intermediate layers 111 and 112 may be mainly composed of silicon oxide. As seen above, the main components of the intermediate layers 111 and 112 may be the same. Other structures are similar to Figure 19A and Figure 19B The structure of the third embodiment shown is the same, so its description is omitted.

[0173] When the main component of the intermediate layer 112 is the same as the main component of the intermediate layer 111, for example, when both the intermediate layers 111 and 112 are mainly composed of silicon oxide or aluminum oxide, the density of the intermediate layer 112 is greater than the density of the intermediate layer 111. This configuration reduces the wafer bow caused by the internal stress of the intermediate layer 111. When the densities of the intermediate layers 111 and 112 are represented by ρ12 and ρ11, respectively, (ρ12-ρ11) / ρ11×100[%] is preferably 0.1% or more, more preferably 0.5% or more, and further preferably 1.0% or more. When the density of the intermediate layer 111 is too low, the function of the intermediate layer 111 (for example, the function as a high acoustic velocity membrane) deteriorates. Therefore, (ρ12-ρ11) / ρ11×100[%] is preferably 20% or less, more preferably 10% or less, and further preferably 5% or less.

[0174] Second Modification of the Third Embodiment

[0175] Figure 24B : is a cross-sectional view of an acoustic wave resonator according to a second modification of the third embodiment. Figure 24B As shown, in the second variation of the third embodiment, intermediate layer 111 is thinner than intermediate layer 112 and thicker than piezoelectric layer 114. Intermediate layer 111 is porous, while intermediate layer 112 is non-porous. The rest of the structure is the same as that of the first variation of the third embodiment, and a detailed description thereof is omitted.

[0176] Third Modification of the Third Embodiment

[0177] Figure 25A : is a cross-sectional view of an acoustic wave resonator according to a third modified example of the third embodiment. Figure 25AAs shown, in a third variation of the third embodiment, an intermediate layer 117 is interposed between intermediate layers 111 and 112. Intermediate layer 112 is non-porous. Intermediate layer 117 may be porous or non-porous. When intermediate layer 117 is porous, intermediate layer 111 may be porous or non-porous. When intermediate layers 111 and 117 are porous, the porosity of intermediate layer 117 may be the same as or different from the porosity of intermediate layer 111. For example, the porosity of intermediate layer 117 may be smaller than the porosity of intermediate layer 111. The main component of intermediate layer 117 may be the same as the main component of intermediate layer 111, the same as the main component of intermediate layer 112, or different from the main component of each of intermediate layers 111 and 112. Intermediate layer 117 may be thinner or thicker than intermediate layer 111.

[0178] As an example, intermediate layers 111 and 117 are primarily composed of aluminum oxide, and intermediate layer 112 is primarily composed of silicon oxide. Intermediate layers 111 and 117 are porous, intermediate layer 112 is non-porous, and the porosity of intermediate layer 117 is less than that of intermediate layer 111. As another example, intermediate layer 111 is primarily composed of aluminum oxide, and intermediate layers 117 and 112 are primarily composed of silicon oxide. Intermediate layer 111 is non-porous or porous, intermediate layer 117 is porous, and intermediate layer 112 is non-porous. The remaining structure is the same as that of the first modified example of the third embodiment, and its description is omitted here.

[0179] Fourth Modification of the Third Embodiment

[0180] Figure 25B : is a cross-sectional view of an acoustic wave resonator according to a fourth modified example of the third embodiment. Figure 25B As shown, in a fourth variation of the third embodiment, intermediate layers 117a and 117b are disposed between intermediate layers 111 and 112. Intermediate layer 112 is non-porous. Intermediate layers 117a and 117b may be porous or non-porous. When intermediate layer 117a or 117b is porous, intermediate layer 111 may be porous or non-porous. When at least two of intermediate layers 111, 117a, and 117b are porous, the porosity of at least two layers may be the same as or different from each other. The main component of intermediate layers 117a and 117b may be the same as the main component of intermediate layer 111, the same as the main component of intermediate layer 112, or different from the main component of each of intermediate layers 111 and 112. Intermediate layers 117a and 117b may be thinner or thicker than intermediate layer 111.

[0181] As an example, the intermediate layers 111, 117a, and 117b are primarily composed of aluminum oxide, and the intermediate layer 112 is primarily composed of silicon oxide. The intermediate layers 111 and 117a are porous, the intermediate layers 117b and 112 are non-porous, and the porosity of the intermediate layer 117a is smaller than the porosity of the intermediate layer 111. As another example, the intermediate layers 111 and 117a are primarily composed of aluminum oxide, and the intermediate layers 117b and 112 are primarily composed of silicon oxide. The intermediate layers 111, 117a, and 117b are porous, and the porosity of the intermediate layer 117a is smaller than the porosity of the intermediate layer 111. The intermediate layer 112 is non-porous. The other structures are the same as those of the first modified example of the third embodiment, and their description is omitted here.

[0182] When the porosity of the intermediate layer close to the piezoelectric layer 114 is large, the sound wave is scattered by the pores and the loss increases. Therefore, when three or more intermediate layers are provided as in the third and fourth modified examples of the third embodiment, the porosity of the intermediate layer closer to the piezoelectric layer 114 is preferably made smaller. For example, in the fourth modified example of the third embodiment Figure 25B , the porosity of the intermediate layer 111 > the porosity of the intermediate layer 117 a > the porosity of the intermediate layer 117 b > the porosity of the intermediate layer 112 .

[0183] Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.

Claims

1. An acoustic wave device, comprising: a supporting substrate; a piezoelectric layer, the piezoelectric layer being disposed above the supporting substrate; a pair of comb-shaped electrodes, the pair of comb-shaped electrodes being disposed on the piezoelectric layer, each of the pair of comb-shaped electrodes comprising electrode fingers for exciting acoustic waves; a temperature compensation film interposed between the support substrate and the piezoelectric layer and having a temperature coefficient of elastic constant opposite in sign to that of the piezoelectric layer; a boundary layer interposed between the support substrate and the temperature compensation film, wherein a sound velocity of a body wave propagating through the boundary layer is higher than a sound velocity of a body wave propagating through the temperature compensation film and lower than a sound velocity of a body wave propagating through the support substrate; as well as an intermediate layer interposed between the support substrate and the boundary layer and having a Q factor smaller than a Q factor of the boundary layer, The Q factor of the intermediate layer is equal to or less than 0.2 times the Q factor of the boundary layer.

2. The acoustic wave device according to claim 1, wherein The thickness of the intermediate layer is equal to or greater than 0.1 times an average pitch of the electrode fingers of the pair of comb-shaped electrodes.

3. The acoustic wave device according to claim 1 or 2, wherein The thickness of the boundary layer is equal to or greater than 2.2 times an average pitch of the electrode fingers of the pair of comb-shaped electrodes.

4. The acoustic wave device according to claim 1, wherein A distance between a first surface of the temperature compensation film closer to the support substrate and a second surface of the piezoelectric layer closer to the pair of comb-shaped electrodes is equal to or smaller than 4 times an average pitch of the electrode fingers of the pair of comb-shaped electrodes.

5. The acoustic wave device according to claim 1, wherein The boundary layer is in contact with the intermediate layer.

6. The acoustic wave device according to claim 1, wherein The thickness of the boundary layer is equal to or less than 10 times an average pitch of the electrode fingers of the pair of comb-shaped electrodes.

7. The acoustic wave device according to claim 1, wherein The main component of the boundary layer is the same as the main component of the intermediate layer.

8. The acoustic wave device according to claim 1, wherein The piezoelectric layer is mainly composed of lithium tantalate or lithium niobate and is single crystalline, the temperature compensation film is mainly composed of silicon oxide and is polycrystalline or amorphous, the boundary layer is mainly composed of aluminum oxide and is polycrystalline or amorphous, and the intermediate layer is mainly composed of aluminum oxide and is polycrystalline or amorphous.

9. A filter, comprising: An acoustic wave device according to any one of claims 1 to 8.

10. A multiplexer, comprising: The filter according to claim 9.

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