Elastic wave device and elastic wave filter

Abstract

An elastic wave device (1) comprises a support substrate (320); a piezoelectric substrate bonded to the support substrate (320); and an IDT electrode (10) disposed on the piezoelectric substrate. The piezoelectric substrate includes a piezoelectric layer (310) made of lithium niobate. When the film thickness of the IDT electrode (10) is T and the electrode finger pitch of the IDT electrode (10) is P, T / P is less than 0.1.

Description

Acoustic wave device and acoustic wave filter

[0001] The present invention relates to an acoustic wave device and an acoustic wave filter.

[0002] Patent Document 1 discloses a surface acoustic wave filter having an oxide crystal substrate, a piezoelectric crystal substrate bonded to the oxide crystal substrate, and an IDT (InterDigital Transducer) electrode formed on the piezoelectric crystal substrate. It claims that by optimizing the thickness of the piezoelectric crystal substrate and the electrode finger duty, it is possible to provide surface acoustic waves that utilize odd-order harmonics.

[0003] Japanese Patent Application Laid-Open No. 2021-118366

[0004] As the capacity of mobile communications increases, there is a demand for acoustic wave devices and acoustic wave filters with higher frequencies and wider bandwidths.

[0005] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an acoustic wave device and an acoustic wave filter that can operate at higher frequencies and with a wider bandwidth.

[0006] In order to achieve the above object, an elastic wave device according to one aspect of the present invention includes a support substrate, a piezoelectric substrate bonded to the support substrate, and an IDT electrode disposed on the piezoelectric substrate, wherein the piezoelectric substrate includes a piezoelectric layer made of lithium niobate, and where T is the film thickness of the IDT electrode and P is the electrode finger pitch of the IDT electrode, T / P is less than 0.1.

[0007] An acoustic wave filter according to one aspect of the present invention includes the above-described acoustic wave device, and has a passband that includes the frequencies of odd-order harmonics of the acoustic wave device.

[0008] According to the present invention, it is possible to provide an acoustic wave device and an acoustic wave filter that have a higher frequency and a wider bandwidth.

[0009] FIG. 1 is a plan view and a cross-sectional view of an elastic wave device according to an embodiment. FIG. 2A is a cross-sectional view of an elastic wave device according to a first modification of an embodiment. FIG. 2B is a graph showing the resonance characteristics of an elastic wave device according to the first modification of an embodiment. FIG. 3A is a cross-sectional view of an elastic wave device according to a second modification of an embodiment. FIG. 3B is a graph showing the resonance characteristics of elastic wave devices according to the first and second modifications of an embodiment. FIG. 4A is a cross-sectional view of an elastic wave device according to a third modification of an embodiment. FIG. 4B is a graph showing the relationship between the electrode finger duty and the impedance ratio of an elastic wave device. FIG. 4C is a graph showing the relationship between the normalized film thickness and the impedance ratio of an elastic wave device.

[0010] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the embodiments described below are all comprehensive or specific examples. The numerical values, shapes, materials, components, arrangements and connection forms of the components shown in the following embodiments are merely examples and are not intended to limit the present invention.

[0011] It should be noted that the drawings are schematic diagrams in which emphasis, omission, or adjustment of proportions has been appropriately made to illustrate the present invention, and are not necessarily strictly illustrated, and may differ from the actual shapes, positional relationships, and proportions. In the drawings, the same reference numerals are used to denote substantially the same components, and redundant explanations may be omitted or simplified.

[0012] In the circuit configuration of the present disclosure, "connected between A and B" means connected to both A and B between A and B.

[0013] Furthermore, terms indicating the relationship between elements, such as "parallel" and "perpendicular," terms indicating the shape of elements, such as "rectangle," and numerical ranges do not only represent strict meanings, but also include substantially equivalent ranges, for example, including an error of a few percent.

[0014] In addition, in the present disclosure, the passband of an acoustic wave device or an acoustic wave filter is defined as the frequency band between two frequencies that are 3 dB higher than the minimum insertion loss value within the passband.

[0015] Furthermore, in the resonance characteristics of the elastic wave device of the present disclosure, the resonance frequency and anti-resonance frequency are derived, for example, by contacting an RF probe with two input / output electrodes of the elastic wave device when the elastic wave device is not connected to other circuit elements, and measuring the reflection characteristics (impedance characteristics) using a network analyzer or the like.

[0016] In addition, in this disclosure, the term "major component of a material" refers to a component that accounts for more than 50% by weight of the material. The major component may be present in any one of single crystal, polycrystalline, and amorphous states, or in a mixture of these states.

[0017] 1 is a plan view and a cross-sectional view of an elastic wave device 1 according to an embodiment. As shown in the figure, the elastic wave device 1 includes a support substrate 320, a piezoelectric layer 310, an IDT electrode 10, a reflective electrode 20, and a dielectric film 40. Note that the elastic wave device 1 shown in FIG. 1 is intended to illustrate a typical structure of an elastic wave resonator that constitutes the elastic wave device 1, and the number and length of electrode fingers that constitute the IDT electrode 10 are not limited thereto.

[0018] As shown in FIG. 1A, an IDT electrode 10 and a reflective electrode 20 are disposed on one main surface of a piezoelectric layer 310. The piezoelectric layer 310 is made of lithium niobate (LiNbO ３ ) In other words, for example, lithium niobate or a material containing lithium niobate as a main component can be used for the piezoelectric layer 310. In this embodiment, the piezoelectric layer 310 constitutes a piezoelectric substrate.

[0019] The thickness of the piezoelectric layer 310 is, for example, 0.05 μm or more and 0.5 μm or less. The second Euler angle of the piezoelectric layer 310 is, for example, 20° or more and 40° or less (30°±10°). In other words, the piezoelectric layer 310 is a (30±10)° Y-cut X-propagation LiNbO ３ The piezoelectric crystal is a lithium niobate crystal cut along a plane whose normal is an axis rotated (30±10) degrees from the Y axis around the X axis, and in which surface acoustic waves propagate in the X axis direction.

[0020] When the piezoelectric layer 310 contains lithium niobate, the electromechanical coupling coefficient k ２ Since the thickness and second Euler angle of the piezoelectric layer 310 are within the above-described ranges, it is possible to efficiently excite the third harmonic.

[0021] The support substrate 320 is disposed on the other principal surface of the piezoelectric layer 310 and is a substrate that supports the piezoelectric layer 310, the IDT electrode 10, and the reflective electrode 20. The support substrate 320 functions as a high acoustic velocity layer in which the acoustic velocity of the bulk waves propagating through it is faster than that of the bulk waves propagating through the piezoelectric layer 310. This can prevent the elastic waves propagating through the piezoelectric layer 310 from leaking to the support substrate 320 side. Note that the support substrate 320 does not have to be a high acoustic velocity layer.

[0022] Support substrate 320 is made of, for example, a non-oxide material, such as silicon (Si), silicon carbide (SiC), or a material containing these materials as its main component. Silicon and silicon carbide have high thermal conductivity, so using these materials for support substrate 320 can improve the heat dissipation of acoustic wave device 1.

[0023] The dielectric film 40 is formed so as to cover the IDT electrode 10 and the reflective electrode 20. The dielectric film 40 is a layer intended to protect the IDT electrode 10 and the reflective electrode 20 from the external environment, adjust the frequency-temperature characteristics, and increase the moisture resistance. ２ The dielectric film 40 does not necessarily have to be included in the acoustic wave device 1 .

[0024] Next, the configurations of the IDT electrode 10 and the reflective electrode 20 will be described.

[0025] As shown in FIG. 1A, the IDT electrode 10 has electrode fingers 11a and 11b and busbar electrodes 12a and 12b.

[0026] The plurality of electrode fingers 11a are arranged parallel to one another. The plurality of electrode fingers 11b are arranged parallel to one another. The plurality of electrode fingers 11a and the plurality of electrode fingers 11b are arranged parallel to one another so as to be interdigitated with one another.

[0027] The busbar electrode 12a is arranged to connect one ends of the electrode fingers 11a to each other. The busbar electrode 12a extends in a direction (x-axis direction) intersecting the extension direction of the electrode fingers 11a (y-axis direction in FIG. 1 ). The busbar electrode 12b is arranged to connect one ends of the electrode fingers 11b to each other. The busbar electrode 12b extends in a direction (x-axis direction) intersecting the extension direction of the electrode fingers 11b (y-axis direction in FIG. 1 ). The busbar electrode 12a and the busbar electrode 12b are arranged opposite each other, with the electrode fingers 11a and the electrode fingers 11b sandwiched between them. The other ends of the electrode fingers 11a face the busbar electrode 12b, and the other ends of the electrode fingers 11b face the busbar electrode 12a.

[0028] The reflecting electrodes 20 are arranged on both sides of the IDT electrode 10 so as to be adjacent to the IDT electrode 10 in a direction perpendicular to the extension direction of the electrode fingers 11 a and 11 b (x-axis direction). The reflecting electrodes 20 confine a predetermined acoustic wave signal excited by the IDT electrode 10 within the IDT electrode 10. Note that the acoustic wave device 1 does not necessarily have to include the reflecting electrodes 20.

[0029] As shown in FIG. 1B , the IDT electrode 10 and the reflective electrode 20 have, for example, a laminated structure of multiple metal layers. In this embodiment, the IDT electrode 10 and the reflective electrode 20 each have, in order from the piezoelectric layer 310 side, a first metal layer, a second metal layer, and a third metal layer. The first metal layer is made of, for example, titanium (Ti), the second metal layer is made of, for example, aluminum (Al), and the third metal layer is made of, for example, titanium (Ti). That is, the first metal layer can be made of, for example, titanium or a material containing titanium as its main component, the second metal layer can be made of, for example, aluminum or a material containing aluminum as its main component, and the third metal layer can be made of, for example, titanium or a material containing titanium as its main component.

[0030] Here, the electrode parameters of the IDT electrode 10 will be described.

[0031] The electrode finger pitch P of the IDT electrode 10 is determined by the repetition period of the electrode fingers 11 a and 11 b. In other words, the electrode finger pitch P is defined as (L + S), where L is the line width of the electrode fingers 11 a and 11 b and S is the space width between adjacent electrode fingers 11 a and 11 b.

[0032] The electrode finger duty D of the IDT electrode 10 is the line width occupancy rate of the electrode fingers 11 a and 11 b, and is the ratio of the line width L to the sum of the line width L and the space width S, and is defined as L / (L+S). The film thickness of the IDT electrode 10 is T.

[0033] In the IDT electrode 10, when the interval between adjacent electrode fingers is not constant, the electrode finger pitch P of the IDT electrode 10 is set to be equal to or smaller than the average electrode finger pitch P of the IDT electrode 10. ＡＶＥ The average electrode finger pitch P of the IDT electrode 10 is defined as ＡＶＥ is defined as Di / (Ni-1), where Ni is the total number of electrode fingers 11a, 11b included in the IDT electrode 10, and Di is the center-to-center distance between the electrode finger located at one end of the IDT electrode 10 and the electrode finger located at the other end in the acoustic wave propagation direction.

[0034] In addition, when the electrode finger duty D of the IDT electrode 10 is not constant, the electrode finger duty D of the IDT electrode 10 is equal to the average electrode finger duty D of the IDT electrode 10. ＡＶＥ The average electrode finger duty D of the IDT electrode 10 is defined as ＡＶＥ The total number of electrode fingers 11a and 11b included in the IDT electrode 10 is Ni, and the total line width obtained by adding the line width L of (Ni-1) electrode fingers is L. ＡＬＬ The total space width obtained by adding up the (Ni-1) space widths S included in the IDT electrode 10 is S ＡＬＬ In this case, L ＡＬＬ / (L ＡＬＬ +S ＡＬＬ ) is defined as

[0035] The electrode finger pitch P and electrode finger duty D of the IDT electrode 10 can be measured by using a scanning electron microscope (SEM), a scanning transmission electron microscope (STEM), or a transmission electron microscope (TEM) to view the main surface of the piezoelectric substrate on which the IDT electrode 10 is formed in a plan view and / or to view a cut surface perpendicular to the extension direction of the electrode fingers 11 a and 11 b in a cross section, thereby measuring the line width L and space width S.

[0036] In elastic wave device 1 according to this preferred embodiment, the normalized thickness (T / P) defined by the thickness T of IDT electrode 10 and the electrode finger pitch P is less than 0.1.

[0037] Because the wavelength of the third harmonic is one-third of the wavelength of the fundamental wave, if the film thickness T of the IDT electrode 10 is large, the third harmonic is absorbed by the IDT electrode 10, resulting in a decrease in the electric flux density of the third harmonic in the piezoelectric layer 310. In contrast, by setting the normalized film thickness (T / P) to less than 0.1, the third harmonic is more easily propagated through the piezoelectric layer 310, and it is possible to increase the electric flux density and excitation intensity of the third harmonic in the piezoelectric layer 310.

[0038] In other words, since the piezoelectric layer 310 is made of lithium niobate, the elastic wave device 1 and the elastic wave filter using it can be made to have a wide bandwidth, and since the normalized film thickness (T / P) is less than 0.1, odd-order (third-order) harmonics can be efficiently utilized, so the elastic wave device 1 and the elastic wave filter using it can be made to have a high frequency.

[0039] Furthermore, in elastic wave device 1, piezoelectric layer 310 is bonded to support substrate 320 without a low acoustic velocity layer, and therefore the absence of a low acoustic velocity layer limits excitation of the fundamental wave, which has a longer wavelength than the third harmonic, thereby reducing the electric flux density of the fundamental wave. Meanwhile, because the third harmonic has difficulty reaching below piezoelectric layer 310, the electric flux density of the third harmonic does not decrease even without a low acoustic velocity layer. This allows elastic wave device 1 to reduce the excitation intensity of the fundamental wave while maintaining the excitation intensity of the third harmonic.

[0040] In addition, in elastic wave device 1 according to this preferred embodiment, electrode finger duty D of IDT electrode 10 is 0.65 or more and 0.9 or less. When electrode finger duty D is 0.65 or more, third harmonic waves, which have shorter wavelengths than the fundamental wave, are more likely to couple between adjacent electrode fingers, thereby increasing the excitation intensity of the third harmonic waves. Furthermore, when electrode finger duty D is 0.9 or less, the spacing between the electrode fingers of IDT electrode 10 can be accurately created during the manufacturing process of elastic wave device 1.

[0041] In elastic wave device 1 according to this embodiment, an adhesion layer having a lower Young's modulus than support substrate 320 and piezoelectric layer 310 may be disposed between support substrate 320 and piezoelectric layer 310. The adhesion layer may be made of, for example, silicon oxide (SiOx: e.g., silicon dioxide) or a material containing silicon oxide as a main component. The thickness of the adhesion layer is 50 nm or less.

[0042] When the support substrate 320 functions as a high acoustic velocity layer, the support substrate 320 is hard, and therefore when it is bonded to the piezoelectric layer 310, a strong stress is applied to the piezoelectric layer 310 due to thermal history or the like, which may cause the piezoelectric layer 310 to peel off from the support substrate 320. In contrast, by providing the adhesion layer, it is possible to prevent the piezoelectric layer 310 from peeling off from the support substrate 320.

[0043] 2. Configuration of Elastic Wave Device 1A According to Modification 1 Next, the configuration of elastic wave device 1A according to Modification 1 of the embodiment will be described.

[0044] 2A is a cross-sectional view of an elastic wave device 1A according to a first modification of the embodiment. As shown in the figure, the elastic wave device 1A includes a support substrate 320, a piezoelectric layer 310, a low acoustic velocity layer 330, a high acoustic velocity layer 340, an IDT electrode 10, a reflecting electrode 20, and a dielectric film 40. The elastic wave device 1A according to this modification differs from the elastic wave device 1 according to the embodiment in that it includes the low acoustic velocity layer 330 and the high acoustic velocity layer 340. Therefore, the following description of the elastic wave device 1A according to this modification will focus on the different components and omit a description of the same components as those of the elastic wave device 1 according to the embodiment.

[0045] The piezoelectric layer 310 has the IDT electrode 10 and the reflective electrode 20 disposed on one main surface thereof. The piezoelectric layer 310 may be made of, for example, lithium niobate or a material containing lithium niobate as a main component. The second Euler angle of the piezoelectric layer 310 is, for example, 20° to 40° (30°±10°).

[0046] The high acoustic velocity layer 340 is disposed between the piezoelectric layer 310 and the support substrate 320, and is a layer through which the acoustic velocity of the bulk waves propagating through the layer 340 is faster than that of the bulk waves propagating through the piezoelectric layer 310. The high acoustic velocity layer 340 may be made of, for example, silicon nitride (SiN) or a material containing silicon nitride as a main component.

[0047] The high acoustic velocity layer 340 may be made of ceramics such as aluminum nitride (AlN), silicon nitride (SiN), zirconia, cordierite, mullite, steatite, forsterite, spinel, and sialon; dielectrics such as aluminum oxide, silicon oxynitride, DLC (diamond-like carbon), and diamond; or materials containing the above materials as main components. The spinel includes aluminum compounds containing oxygen and one or more elements selected from Mg, Fe, Zn, Mn, and the like. Examples of the spinel include MgAl ２ O ４ , FeAl ２ O ４ , ZnAl ２ O ４ , MnAl ２ O ４ Examples include:

[0048] The low acoustic velocity layer 330 is disposed between the piezoelectric layer 310 and the high acoustic velocity layer 340, and is a layer in which the acoustic velocity of the bulk waves propagating through the low acoustic velocity layer 330 is slower than that of the bulk waves propagating through the piezoelectric layer 310. The low acoustic velocity layer 330 may be made of, for example, silicon oxide (e.g., silicon dioxide) or a material containing silicon oxide as a main component.

[0049] In addition, the low acoustic velocity layer 330 may be made of a dielectric material such as glass, silicon oxide, silicon oxynitride, lithium oxide, tantalum oxide, or a compound of silicon oxide with fluorine, carbon, or boron added, or a material containing the above materials as its main component.

[0050] The piezoelectric layer 310 , the low acoustic velocity layer 330 and the high acoustic velocity layer 340 constitute the piezoelectric substrate 30 .

[0051] The support substrate 320 is bonded to the piezoelectric substrate 30 and supports the piezoelectric substrate 30. The support substrate 320 is, for example, a silicon substrate with a crystal orientation of

[111] and a third Euler angle of 73°.

[0052] That is, the acoustic wave device 1A has a layered structure in which the support substrate 320, the high acoustic velocity layer 340, the low acoustic velocity layer 330, the piezoelectric layer 310, and the IDT electrode 10 are arranged in this order from the negative z-axis direction.

[0053] The above-described laminated structure of the piezoelectric substrate 30 makes it possible to significantly increase the Q value at the resonant frequency and the antiresonant frequency compared to a conventional structure using a single-layer piezoelectric substrate. In other words, an acoustic wave resonator with a high Q value can be configured, and an acoustic wave filter with low insertion loss can be configured using the acoustic wave resonator.

[0054] 2B is a graph showing the resonance characteristics of elastic wave device 1A in accordance with Modification 1 of the embodiment. Table 1 shows the parameters of elastic wave device 1A.

[0055]

[0056] As shown in Table 1, the normalized film thickness (T / P) is 0.066, which satisfies (T / P) < 0.1. As shown in FIG. 2B , elastic wave device 1A has a fundamental resonant frequency fr1, a third-harmonic resonant frequency fr3, and a fifth-harmonic resonant frequency fr5, at which impedance is minimized, and a fundamental antiresonant frequency fa1, a third-harmonic antiresonant frequency fa3, and a fifth-harmonic antiresonant frequency fa5, at which impedance is maximized. The third-harmonic resonant frequency fr3 is 3280 MHz, and the antiresonant frequency fa3 is 3410 MHz. The third-harmonic resonant frequency ratio band (the frequency difference between the antiresonant frequency and the resonant frequency divided by the center frequency of the antiresonant frequency and the resonant frequency) is 3.9%. Furthermore, the third-harmonic impedance ratio (the ratio of the impedance at antiresonant frequency fa3 to the impedance at resonant frequency fr3) is 80 dB or greater.

[0057] In elastic wave device 1A according to this modification, IDT electrode 10 has a normalized thickness (T / P) of 0.066 and an electrode finger duty ratio D of 0.8, which increases the electric flux density and excitation intensity of the third harmonic in piezoelectric layer 310, resulting in an elastic wave resonator having a large impedance ratio and a large resonance ratio band at third harmonics of 3 GHz or higher. This makes it possible to provide elastic wave device 1A with higher frequencies and wider bands, and an elastic wave filter using elastic wave device 1A.

[0058] Note that, in elastic wave device 1A according to this modification, instead of piezoelectric substrate 30, an energy trapping layer and piezoelectric layer 310 may be stacked in this order from support substrate 320 side.

[0059] The energy trapping layer is composed of one or more layers, and the velocity of the bulk acoustic waves propagating through at least one of the layers is greater than the velocity of the acoustic waves propagating near the piezoelectric layer. For example, the energy trapping layer may have a laminated structure of a low acoustic velocity layer and a high acoustic velocity layer. The low acoustic velocity layer is a film in which the acoustic velocity of the bulk waves in the low acoustic velocity layer is slower than the acoustic velocity of the acoustic waves propagating through the piezoelectric layer. The high acoustic velocity layer is a film in which the acoustic velocity of the bulk waves in the high acoustic velocity layer is faster than the acoustic velocity of the acoustic waves propagating through the piezoelectric layer. The support substrate may also be the high acoustic velocity layer.

[0060] The energy trapping layer may also be an acoustic impedance layer having a configuration in which low acoustic impedance layers with a relatively low acoustic impedance and high acoustic impedance layers with a relatively high acoustic impedance are alternately stacked.

[0061] [3. Configuration of Elastic Wave Device 1B According to Modification 2] Next, the configuration of elastic wave device 1B according to Modification 2 of the embodiment will be described.

[0062] 3A is a cross-sectional view of an elastic wave device 1B according to a second modification of the embodiment. As shown in the figure, the elastic wave device 1B includes a support substrate 320, a piezoelectric layer 310, a low acoustic velocity layer 330, a high acoustic velocity layer 340, a metal layer 350, an oxide layer 360, an IDT electrode 10, a reflecting electrode 20, and a dielectric film 40. The elastic wave device 1B according to this modification differs from the elastic wave device 1A according to the first modification in that it includes the metal layer 350 and the oxide layer 360. Therefore, the following description of the elastic wave device 1B according to this modification will focus on the different components and omit a description of the same components as those of the elastic wave device 1A according to the first modification.

[0063] Each of the IDT electrode 10 and the reflective electrode 20 has a first metal layer and a second metal layer, in this order from the piezoelectric layer 310 side. The first metal layer is made of, for example, titanium (Ti), and the second metal layer is made of, for example, aluminum (Al) and copper (Cu). That is, the first metal layer can be made of, for example, titanium or a material containing titanium as its main component, and the second metal layer can be made of, for example, aluminum and copper, or a material containing aluminum and copper as its main components.

[0064] The piezoelectric layer 310 has the IDT electrode 10 and the reflective electrode 20 disposed on one main surface thereof. The piezoelectric layer 310 may be made of, for example, lithium niobate or a material containing lithium niobate as a main component. The second Euler angle of the piezoelectric layer 310 is, for example, 20° to 40° (30°±10°).

[0065] The high acoustic velocity layer 340 is disposed between the piezoelectric layer 310 and the support substrate 320, and is a layer through which the acoustic velocity of the bulk waves propagating through the layer 340 is faster than that of the bulk waves propagating through the piezoelectric layer 310. The high acoustic velocity layer 340 may be made of, for example, silicon nitride or a material containing silicon nitride as a main component.

[0066] The low acoustic velocity layer 330 is disposed between the piezoelectric layer 310 and the high acoustic velocity layer 340, and is a layer in which the acoustic velocity of the bulk waves propagating through the low acoustic velocity layer 330 is slower than that of the bulk waves propagating through the piezoelectric layer 310. The low acoustic velocity layer 330 may be made of, for example, silicon oxide (e.g., silicon dioxide) or a material containing silicon oxide as a main component.

[0067] The metal layer 350 is disposed between the low acoustic velocity layer 330 and the high acoustic velocity layer 340. The metal layer 350 is made of, for example, aluminum (Al). That is, the metal layer 350 can be made of, for example, aluminum or a material containing aluminum as a main component.

[0068] Oxide layer 360 is disposed between metal layer 350 and high acoustic velocity layer 340. Oxide layer 360 may be made of, for example, silicon oxide (e.g., silicon dioxide) or a material containing silicon oxide as a main component. Note that oxide layer 360 does not necessarily have to be included in acoustic wave device 1B.

[0069] The piezoelectric layer 310, the low acoustic velocity layer 330, the high acoustic velocity layer 340, the metal layer 350, and the oxide layer 360 constitute the piezoelectric substrate 30B.

[0070] The support substrate 320 is bonded to the piezoelectric substrate 30 B and supports the piezoelectric substrate 30 B. The support substrate 320 is, for example, a silicon substrate with a crystal orientation of

[111] and a third Euler angle of 73°.

[0071] In other words, the elastic wave device 1B has a layered structure in which, from the negative z-axis side, a support substrate 320, a high acoustic velocity layer 340, an oxide layer 360, a metal layer 350, a low acoustic velocity layer 330, a piezoelectric layer 310, and an IDT electrode 10 are arranged in this order.

[0072] The laminated structure of the piezoelectric substrate 30B described above makes it possible to significantly increase the Q value at the resonant frequency and the antiresonant frequency compared to a conventional structure using a single layer of piezoelectric substrate. In other words, an acoustic wave resonator with a high Q value can be configured, and an acoustic wave filter with low insertion loss can be configured using the acoustic wave resonator.

[0073] 3B is a graph showing the resonance characteristics of elastic wave device 1A according to Modification 1 and elastic wave device 1B according to Modification 2. Table 2 shows the parameters of elastic wave device 1B.

[0074]

[0075] As shown in Table 2, the normalized film thickness (T / P) is 0.046, which satisfies (T / P) < 0.1. As shown in FIG. 3B , elastic wave device 1B has a fundamental resonant frequency fr1, a third harmonic resonant frequency fr3, and a fifth harmonic resonant frequency fr5, at which impedance is at a minimum, and an anti-resonant frequency fa1, a third harmonic anti-resonant frequency fa3, and a fifth harmonic anti-resonant frequency fa5, at which impedance is at a maximum. Elastic wave device 1B's resonant frequency fr3 and anti-resonant frequency fa3 are 4 GHz or higher, and the impedance ratio of the third harmonic is 75 dB or higher. Note that in FIG. 3B , the film thicknesses of the layers constituting elastic wave device 1A are different from those listed in Table 1 to match the fundamental, third harmonic, and fifth harmonic frequencies of elastic wave device 1A and elastic wave device 1B.

[0076] In elastic wave device 1B according to this modification, IDT electrode 10 has a normalized thickness (T / P) of 0.046 and an electrode finger duty ratio D of 0.8, which increases the electric flux density and excitation intensity of the third harmonic in piezoelectric layer 310, resulting in an elastic wave resonator with a large impedance ratio and a large resonance ratio band at third harmonics of 4 GHz or higher. This makes it possible to provide elastic wave device 1B with higher frequencies and a wider band.

[0077] Furthermore, in elastic wave device 1B according to this modification, the impedance ratio of the third harmonic is greater than the impedance ratio of the fundamental wave. In elastic wave device 1B, metal layer 350 limits excitation of the fundamental wave, thereby reducing the electric flux density of the fundamental wave. Meanwhile, because the third harmonic has difficulty reaching below low acoustic velocity layer 330, the presence of metal layer 350 does not affect the propagation of the third harmonic, and the electric flux density of the third harmonic does not decrease. As a result, in elastic wave device 1B, the excitation intensity of the fundamental wave is reduced while maintaining the excitation intensity of the third harmonic. Therefore, an elastic wave filter using elastic wave device 1B can achieve low loss in the passband and high attenuation in the attenuation band.

[0078] [4. Configuration of Elastic Wave Device 1C According to Modification 3] Next, the configuration of an elastic wave device 1C according to Modification 3 of the embodiment will be described.

[0079] 4A is a cross-sectional view of an elastic wave device 1C according to a third variation of the embodiment. As shown in the figure, the elastic wave device 1C includes a support substrate 320, a piezoelectric layer 310, a high acoustic velocity layer 340, an oxide layer 360, a non-oxide layer 370, an IDT electrode 10, a reflecting electrode 20, and a dielectric film 40. The elastic wave device 1C according to this variation differs from the elastic wave device 1 according to the embodiment in that the high acoustic velocity layer 340, the oxide layer 360, and the non-oxide layer 370 are provided. Therefore, the following description of the elastic wave device 1C according to this variation will omit a description of the same components as those of the elastic wave device 1 according to the embodiment and will focus on the different components.

[0080] The piezoelectric layer 310 has the IDT electrode 10 and the reflective electrode 20 disposed on one main surface thereof. The piezoelectric layer 310 is made of lithium niobate. That is, the piezoelectric layer 310 can be made of, for example, lithium niobate or a material containing lithium niobate as a main component. The second Euler angle of the piezoelectric layer 310 is, for example, 20° to 40° (30°±10°).

[0081] When piezoelectric layer 310 contains lithium niobate, the electromechanical coupling coefficient k ２Since the second Euler angle range of the piezoelectric layer 310 can be increased, a wide-band, low-loss acoustic wave filter can be formed using the acoustic wave device 1C. Furthermore, since the piezoelectric layer 310 has the second Euler angle range, it is possible to efficiently excite the third harmonic.

[0082] The high acoustic velocity layer 340 is disposed between the piezoelectric layer 310 and the support substrate 320, and is a layer through which the acoustic velocity of the bulk waves propagating through the layer 340 is faster than that of the bulk waves propagating through the piezoelectric layer 310. In this modification, the high acoustic velocity layer 340 is bonded to the piezoelectric layer 310 without the low acoustic velocity layer 330. The high acoustic velocity layer 340 may be made of, for example, silicon nitride or a material containing silicon nitride as a main component.

[0083] Oxide layer 360 is disposed between high acoustic velocity layer 340 and support substrate 320. Oxide layer 360 may be made of, for example, silicon oxide (e.g., silicon dioxide) or a material containing silicon oxide as a main component. Note that oxide layer 360 does not necessarily have to be included in acoustic wave device 1C.

[0084] Non-oxide layer 370 is disposed between oxide layer 360 and support substrate 320. Non-oxide layer 370 may be made of, for example, silicon nitride or a material containing silicon nitride as a main component. Note that non-oxide layer 370 does not necessarily have to be included in elastic wave device 1C.

[0085] The piezoelectric layer 310, the high acoustic velocity layer 340, the oxide layer 360, and the non-oxide layer 370 constitute the piezoelectric substrate 30C.

[0086] The support substrate 320 is bonded to the piezoelectric substrate 30 C and supports the piezoelectric substrate 30 C. The support substrate 320 is, for example, a silicon substrate with a crystal orientation of

[111] and a third Euler angle of 73°.

[0087] That is, the elastic wave device 1C has a layered structure in which the support substrate 320, the non-oxide layer 370, the oxide layer 360, the high acoustic velocity layer 340, the piezoelectric layer 310, and the IDT electrode 10 are arranged in this order from the negative z-axis direction.

[0088] The laminated structure of the piezoelectric substrate 30C described above makes it possible to significantly increase the Q value at the resonant frequency and the antiresonant frequency compared to a conventional structure using a single layer of piezoelectric substrate. In other words, an acoustic wave resonator with a high Q value can be configured, and an acoustic wave filter with low insertion loss can be configured using the acoustic wave resonator.

[0089] Table 3 shows the parameters of the elastic wave device 1C.

[0090]

[0091] From Table 3, the normalized film thickness (T / P) is 0.056. Because the wavelength of the third harmonic is one-third of the wavelength of the fundamental wave, if the film thickness T of the IDT electrode 10 is large, the third harmonic is absorbed by the IDT electrode 10, resulting in a decrease in the electric flux density of the third harmonic in the piezoelectric layer 310. In contrast, by setting the normalized film thickness (T / P) to 0.056, it is possible to increase the electric flux density and excitation strength of the third harmonic in the piezoelectric layer 310.

[0092] FIG. 4B is a graph showing the relationship between the electrode finger duty D and the impedance ratio of the third harmonic in an elastic wave device. FIG. 4B also shows the relationship between the electrode finger duty D and the impedance ratio of the third harmonic for elastic wave devices having the parameters shown in Table 3 except for the electrode finger duty D. As shown in the figure, the impedance ratio of the third harmonic increases as the electrode finger duty D increases. This shows that in elastic wave device 1C, the impedance ratio of the third harmonic is 40 dB or greater when the electrode finger duty D is 0.65 or greater. Furthermore, it shows that in elastic wave device 1C, the impedance ratio of the third harmonic is 60 dB or greater when the electrode finger duty D is 0.70 or greater.

[0093] FIG. 4C is a graph showing the relationship between the normalized thickness (T / P) and the impedance ratio of the third harmonic for an elastic wave device. FIG. 4C also shows the relationship between the normalized thickness (T / P) and the impedance ratio of the third harmonic for elastic wave devices having the parameters shown in Table 3, excluding the thickness T of IDT electrode 10. As shown in the figure, the impedance ratio of the third harmonic increases as the normalized thickness (T / P) decreases. This indicates that in elastic wave device 1C, the impedance ratio of the third harmonic is 40 dB or greater when the normalized thickness (T / P) is less than 0.1. Furthermore, it indicates that in elastic wave device 1C, the impedance ratio of the third harmonic is 60 dB or greater when the normalized thickness (T / P) is less than 0.07.

[0094] In other words, since the piezoelectric layer 310 is made of lithium niobate and has a normalized thickness (T / P) of less than 0.1, it is possible to provide an elastic wave device 1C with a higher frequency and a wider bandwidth, and an elastic wave filter using the elastic wave device 1C.

[0095] In addition, in acoustic wave device 1C, piezoelectric layer 310 is preferably made of lithium niobate and has a normalized thickness (T / P) of less than 0.07, which makes it possible to provide an acoustic wave device with higher frequencies and wider bandwidths, and an acoustic wave filter using the acoustic wave device.

[0096] Furthermore, in elastic wave device 1C, piezoelectric layer 310 is joined to high acoustic velocity layer 340 without a low acoustic velocity layer. Therefore, the absence of a low acoustic velocity layer limits excitation of the fundamental wave, which has a longer wavelength than the third harmonic, thereby reducing the electric flux density of the fundamental wave. Meanwhile, because the third harmonic has difficulty reaching below piezoelectric layer 310, the electric flux density of the third harmonic does not decrease even without a low acoustic velocity layer. This allows elastic wave device 1C to reduce the excitation intensity of the fundamental wave while maintaining the excitation intensity of the third harmonic.

[0097] In elastic wave device 1C according to this modification, electrode finger duty D of IDT electrode 10 is 0.65 or more and 0.90 or less. When electrode finger duty D is 0.65 or more, third harmonic waves, which have shorter wavelengths than the fundamental wave, are more likely to couple between adjacent electrode fingers, thereby increasing the excitation intensity of the third harmonic waves. When electrode finger duty D is 0.9 or less, the spacing between the electrode fingers of IDT electrode 10 can be accurately created during the manufacturing process of elastic wave device 1C.

[0098] In acoustic wave device 1C, electrode finger duty D is preferably 0.70 or more and 0.90 or less. This makes it possible to provide an acoustic wave device with a higher frequency and a wider bandwidth, and an acoustic wave filter using the acoustic wave device.

[0099] In elastic wave device 1C according to this modification, an adhesion layer having a lower Young's modulus than high acoustic velocity layer 340 and piezoelectric layer 310 may be disposed between high acoustic velocity layer 340 and piezoelectric layer 310. The adhesion layer may be made of, for example, silicon oxide (e.g., silicon dioxide) or a material containing silicon oxide as a main component. The thickness of the adhesion layer is 50 nm or less.

[0100] Because the high acoustic velocity layer 340 is hard, when it is bonded to the piezoelectric layer 310, a strong stress is applied to the piezoelectric layer 310, which may cause the piezoelectric layer 310 to peel off from the high acoustic velocity layer 340. In contrast, by providing the above-described adhesive layer, it is possible to prevent the piezoelectric layer 310 from peeling off from the high acoustic velocity layer 340.

[0101] [5. Effects, etc.] As described above, elastic wave device 1 according to the embodiment and elastic wave devices 1A to 1C according to Modifications 1 to 3 include support substrate 320, a piezoelectric substrate bonded to support substrate 320, and IDT electrode 10 disposed on the piezoelectric substrate. The piezoelectric substrate includes piezoelectric layer 310 made of lithium niobate. When the film thickness of IDT electrode 10 is T and the electrode finger pitch of IDT electrode 10 is P, T / P is less than 0.1.

[0102] In this configuration, piezoelectric layer 310 is made of lithium niobate, which enables the acoustic wave device and the acoustic wave filter using the same to have a broadband, and normalized thickness (T / P) of less than 0.1 enables the efficient use of odd-order harmonics, thereby enabling the acoustic wave device and the acoustic wave filter using the same to have a high frequency. As a result, acoustic wave devices 1 and 1A-1C with a high frequency and a broadband can be provided.

[0103] Furthermore, for example, in acoustic wave devices 1 and 1A to 1C, electrode finger duty D of IDT electrode 10 is not less than 0.65 and not more than 0.90.

[0104] According to this, the electrode finger duty D of 0.65 or more can increase the excitation intensity of the third harmonic. Furthermore, the electrode finger duty D of 0.9 or less can accurately create the spacing between the electrode fingers of IDT electrode 10 in the manufacturing process of the acoustic wave device.

[0105] Furthermore, for example, in acoustic wave devices 1 and 1A to 1C, support substrate 320 is made of a non-oxide material.

[0106] Furthermore, in the acoustic wave device 1 , for example, the acoustic velocity of the bulk waves propagating through the support substrate 320 is faster than that of the bulk waves propagating through the piezoelectric layer 310 .

[0107] This can prevent the elastic waves propagating through the piezoelectric layer 310 from leaking to the support substrate 320 side.

[0108] Furthermore, for example, in the acoustic wave devices 1 and 1A to 1C, the support substrate 320 includes silicon or silicon carbide.

[0109] This improves the heat dissipation properties of elastic wave devices 1 and 1A to 1C.

[0110] Furthermore, for example, in elastic wave devices 1 and 1A to 1C, the thickness of piezoelectric layer 310 is not less than 0.05 μm and not more than 0.5 μm.

[0111] This makes it possible to efficiently excite the third harmonic.

[0112] Furthermore, for example, in elastic wave devices 1 and 1A to 1C, the second Euler angle of piezoelectric layer 310 is not less than 20° and not more than 40°.

[0113] This makes it possible to efficiently excite the third harmonic.

[0114] For example, in the elastic wave device 1A according to variant example 1, the piezoelectric substrate 30 further includes a high acoustic velocity layer 340 disposed between the piezoelectric layer 310 and the support substrate 320, the high acoustic velocity layer 340 having a bulk wave propagating therethrough at a higher acoustic velocity than the bulk wave propagating through the piezoelectric layer 310, and a low acoustic velocity layer 330 disposed between the piezoelectric layer 310 and the high acoustic velocity layer 340, the low acoustic velocity layer 330 having a bulk wave propagating therethrough at a lower acoustic velocity than the bulk wave propagating through the piezoelectric layer 310.

[0115] This allows for a higher Q value at the resonant frequency and antiresonant frequency compared to a conventional structure using a single-layer piezoelectric substrate. In other words, an elastic wave resonator with a high Q value can be configured, and elastic wave filters with low insertion loss can be configured using elastic wave device 1A.

[0116] Furthermore, for example, in the acoustic wave device 1A, the high acoustic velocity layer 340 includes silicon nitride, and the low acoustic velocity layer 330 includes silicon oxide.

[0117] Furthermore, for example, in an elastic wave device 1B in accordance with Modification 2, a piezoelectric substrate 30B further includes a metal layer 350 disposed between the low acoustic velocity layer 330 and the high acoustic velocity layer 340 .

[0118] As a result, metal layer 350 limits the excitation of the fundamental wave, thereby reducing the electric flux density of the fundamental wave. Meanwhile, because the third harmonic wave has difficulty reaching below low acoustic velocity layer 330, the presence of metal layer 350 does not affect the propagation of the third harmonic wave, and the electric flux density of the third harmonic wave does not decrease. This allows elastic wave device 1B to reduce the excitation intensity of the fundamental wave while maintaining the excitation intensity of the third harmonic. Therefore, an elastic wave filter using elastic wave device 1B can achieve low loss in the passband and high attenuation in the attenuation band.

[0119] Furthermore, in the acoustic wave device 1B, the metal layer 350 is made of aluminum.

[0120] For example, in the elastic wave device 1C according to variant example 3, the piezoelectric substrate 30C further includes a high acoustic velocity layer 340 disposed between the piezoelectric layer 310 and the support substrate 320, which has a bulk wave propagating at a higher acoustic velocity than the bulk wave propagating through the piezoelectric layer 310.

[0121] In this configuration, piezoelectric layer 310 is joined to high acoustic velocity layer 340 without a low acoustic velocity layer, and the absence of a low acoustic velocity layer limits excitation of the fundamental wave, which has a longer wavelength than the third harmonic, thereby reducing the electric flux density of the fundamental wave. Meanwhile, because the third harmonic has difficulty reaching below piezoelectric layer 310, the electric flux density of the third harmonic does not decrease even without a low acoustic velocity layer. This allows elastic wave device 1C to reduce the excitation intensity of the fundamental wave while maintaining the excitation intensity of the third harmonic.

[0122] Furthermore, for example, in the acoustic wave device 1C, the high acoustic velocity layer 340 includes silicon nitride.

[0123] Furthermore, for example, in elastic wave device 1C, piezoelectric substrate 30C further includes an adhesion layer disposed between piezoelectric layer 310 and high acoustic velocity layer 340 and having a lower Young's modulus than piezoelectric layer 310 and high acoustic velocity layer 340 .

[0124] This can prevent the piezoelectric layer 310 from peeling off from the high acoustic velocity layer 340 .

[0125] Furthermore, for example, in the elastic wave device 1C, the adhesion layer contains silicon oxide and has a thickness of 50 nm or less.

[0126] In this case, since the adhesive layer is thin, it is possible to prevent the piezoelectric layer 310 from peeling off from the high acoustic velocity layer 340 without affecting the propagation characteristics of the fundamental wave and the third harmonic wave.

[0127] An acoustic wave filter according to the embodiment includes acoustic wave device 1 and any one of 1A to 1C, and includes in its passband the frequencies of odd-order harmonics of acoustic wave device 1 and any one of 1A to 1C.

[0128] This makes it possible to provide an acoustic wave filter that is high frequency, wide bandwidth, low loss, and high attenuation.

[0129] Other Embodiments While the acoustic wave device and acoustic wave filter according to the present invention have been described above with reference to exemplary embodiments and modifications, the present invention is not limited to the exemplary embodiments and modifications. The present invention also includes other embodiments realized by combining any of the components in the exemplary embodiments and modifications, as well as modifications obtained by applying various modifications to the exemplary embodiments and modifications that would occur to those skilled in the art without departing from the spirit of the present invention.

[0130] For example, in the above-described embodiment and modified examples, elastic wave devices 1 and 1A to 1C are disclosed that utilize the third harmonic among odd-order harmonics. However, the elastic wave device and elastic wave filter of the present invention are also applicable to elastic wave devices and elastic wave filters that utilize not only the third harmonic but also odd-order harmonics that are located on the higher frequency side than the third harmonic.

[0131] The following describes features of the acoustic wave devices and acoustic wave filters described based on the above-described embodiment and modifications.

[0132] <1> An acoustic wave device comprising: a support substrate; a piezoelectric substrate bonded to the support substrate; and an IDT electrode disposed on the piezoelectric substrate, wherein the piezoelectric substrate includes a piezoelectric layer made of lithium niobate, and wherein T / P is less than 0.1, where T is a film thickness of the IDT electrode and P is an electrode finger pitch of the IDT electrode.

[0133] <2> The acoustic wave device according to <1>, wherein the electrode finger duty of the IDT electrode is equal to or greater than 0.65 and equal to or less than 0.90.

[0134] <3> The acoustic wave device according to <1> or <2>, wherein the support substrate is made of a non-oxide material.

[0135] <4> The acoustic wave device according to any one of <1> to <3>, wherein the supporting substrate has a bulk wave propagating therethrough at a sound velocity faster than that of the bulk wave propagating through the piezoelectric layer.

[0136] <5> The acoustic wave device according to any one of <1> to <4>, wherein the support substrate includes silicon or silicon carbide.

[0137] <6> The acoustic wave device according to any one of <1> to <5>, wherein the piezoelectric layer has a thickness of 0.05 μm or more and 0.5 μm or less.

[0138] <7> The acoustic wave device according to any one of <1> to <6>, wherein the second Euler angle of the piezoelectric layer is equal to or greater than 20° and equal to or less than 40°.

[0139] <8> The elastic wave device according to any one of <1> to <7>, wherein the piezoelectric substrate further includes: a high acoustic velocity layer disposed between the piezoelectric layer and the support substrate, the high acoustic velocity layer providing a bulk wave having a higher acoustic velocity than the bulk wave propagating through the piezoelectric layer; and a low acoustic velocity layer disposed between the piezoelectric layer and the high acoustic velocity layer, the low acoustic velocity layer providing a bulk wave having a lower acoustic velocity than the bulk wave propagating through the piezoelectric layer.

[0140] <9> The acoustic wave device according to <8>, wherein the high acoustic velocity layer includes silicon nitride, and the low acoustic velocity layer includes silicon oxide.

[0141] <10> The acoustic wave device according to <8> or <9>, wherein the piezoelectric substrate further includes a metal layer disposed between the low acoustic velocity layer and the high acoustic velocity layer.

[0142] <11> The acoustic wave device according to <10>, wherein the metal layer is made of aluminum.

[0143] <12> The elastic wave device according to any one of <1> to <7>, wherein the piezoelectric substrate further includes a high acoustic velocity layer disposed between the piezoelectric layer and the support substrate, the high acoustic velocity layer allowing bulk waves to propagate through the high acoustic velocity layer at a higher acoustic velocity than bulk waves propagating through the piezoelectric layer.

[0144] <13> The acoustic wave device according to <12>, wherein the high acoustic velocity layer contains silicon nitride.

[0145] <14> The acoustic wave device according to <12> or <13>, wherein the piezoelectric substrate further includes an adhesion layer disposed between the piezoelectric layer and the high acoustic velocity layer and having a Young's modulus lower than those of the piezoelectric layer and the high acoustic velocity layer.

[0146] <15> The acoustic wave device according to <14>, wherein the adhesion layer contains silicon oxide, and the adhesion layer has a thickness of 50 nm or less.

[0147] <16> An acoustic wave filter comprising the acoustic wave device according to any one of <1> to <15>, wherein the passband includes frequencies of odd-order harmonics of the acoustic wave device.

[0148] INDUSTRIAL APPLICABILITY The present invention can be widely used as an acoustic wave device and an acoustic wave filter disposed in a front end portion of communication devices such as mobile phones.

[0149] REFERENCE SIGNS LIST 1, 1A, 1B, 1C Acoustic wave device 10 IDT electrode 11a, 11b Electrode fingers 12a, 12b Bus bar electrode 20 Reflecting electrode 30, 30B, 30C Piezoelectric substrate 40 Dielectric film 310 Piezoelectric layer 320 Support substrate 330 Low acoustic velocity layer 340 High acoustic velocity layer 350 Metal layer 360 Oxide layer 370 Non-oxide layer

Claims

1. An acoustic wave device comprising: a support substrate; a piezoelectric substrate bonded to the support substrate; and an IDT (InterDigital Transducer) electrode disposed on the piezoelectric substrate, wherein the piezoelectric substrate includes a piezoelectric layer made of lithium niobate, and where T is the film thickness of the IDT electrode and P is the electrode finger pitch of the IDT electrode, T / P is less than 0.

1.

2. The acoustic wave device according to claim 1, wherein the electrode finger duty of the IDT electrode is not less than 0.65 and not more than 0.

90.

3. The acoustic wave device according to claim 1, wherein the support substrate is made of a non-oxide material.

4. The acoustic wave device according to claim 1, wherein the supporting substrate allows bulk waves to propagate at a higher acoustic velocity than bulk waves propagating through the piezoelectric layer.

5. The acoustic wave device according to any one of claims 1 to 4, wherein the support substrate includes silicon or silicon carbide.

6. The acoustic wave device according to any one of claims 1 to 5, wherein the piezoelectric layer has a thickness of 0.05 μm or more and 0.5 μm or less.

7. The acoustic wave device according to any one of claims 1 to 6, wherein the second Euler angle of the piezoelectric layer is equal to or greater than 20° and equal to or less than 40°.

8. The elastic wave device according to any one of claims 1 to 7, wherein the piezoelectric substrate further includes: a high acoustic velocity layer disposed between the piezoelectric layer and the support substrate, the high acoustic velocity layer providing bulk waves propagating at a higher acoustic velocity than bulk waves propagating through the piezoelectric layer; and a low acoustic velocity layer disposed between the piezoelectric layer and the high acoustic velocity layer, the low acoustic velocity layer providing bulk waves propagating at a lower acoustic velocity than bulk waves propagating through the piezoelectric layer.

9. The acoustic wave device according to claim 8, wherein the high acoustic velocity layer includes silicon nitride, and the low acoustic velocity layer includes silicon oxide.

10. The acoustic wave device according to claim 8 or 9, wherein the piezoelectric substrate further includes a metal layer disposed between the low acoustic velocity layer and the high acoustic velocity layer.

11. The acoustic wave device according to claim 10, wherein the metal layer is made of aluminum.

12. The elastic wave device according to any one of claims 1 to 7, wherein the piezoelectric substrate further includes a high acoustic velocity layer disposed between the piezoelectric layer and the support substrate, the high acoustic velocity layer allowing bulk waves to propagate through the high acoustic velocity layer at a higher acoustic velocity than bulk waves propagating through the piezoelectric layer.

13. The acoustic wave device according to claim 12, wherein the high acoustic velocity layer includes silicon nitride.

14. The elastic wave device according to claim 12 or 13, wherein the piezoelectric substrate further includes an adhesive layer disposed between the piezoelectric layer and the high acoustic velocity layer, the adhesive layer having a Young's modulus lower than those of the piezoelectric layer and the high acoustic velocity layer.

15. The acoustic wave device according to claim 14, wherein the adhesion layer contains silicon oxide, and the thickness of the adhesion layer is 50 nm or less.

16. An acoustic wave filter comprising the acoustic wave device according to any one of claims 1 to 15, wherein the passband includes the frequencies of odd-order harmonics of the acoustic wave device.

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