Elastic wave resonator, elastic wave filter, front end module, and communication device

Abstract

An elastic wave resonator according to the present invention includes: a support member including a support substrate; a piezoelectric layer provided on a main surface of the support member; and a plurality of functional electrodes respectively provided on opposite main surfaces of the piezoelectric layer. The piezoelectric layer includes: a first polarization layer having a polarization direction in a first direction; a second polarization layer provided between the first polarization layer and the main surface of the support member and having a polarization direction in a second direction opposite to the first direction; and a first polarization direction mixed layer disposed between the first polarization layer and the second polarization layer and including a first portion having a polarization direction in the first direction and a second portion having a polarization direction in the second direction. The first polarization layer, the second polarization layer, and the first polarization direction mixed layer are single crystals made of the same material.

Description

Acoustic wave resonator, acoustic wave filter, front-end module and communication device

[0001] The present invention relates to an acoustic wave resonator, an acoustic wave filter, a front-end module, and a communication device.

[0002] Patent Document 1 describes an acoustic wave resonator in which two piezoelectric layers are stacked between upper and lower electrodes, with the polarization directions of the two piezoelectric layers being opposite to each other.

[0003] US Patent Application Publication No. 2022 / 0321100

[0004] In the acoustic wave resonator disclosed in Patent Document 1, unwanted waves of various modes may be generated, which may deteriorate the resonator characteristics.

[0005] An object of the present invention is to provide an acoustic wave resonator, an acoustic wave filter, a front-end module, and a communication device that can improve the resonator characteristics.

[0006] An elastic wave resonator according to one embodiment includes a support member having a support substrate, a piezoelectric layer provided on a main surface of the support member, and a plurality of functional electrodes provided on opposing main surfaces of the piezoelectric layer, wherein the piezoelectric layer has a first polarization layer having a polarization direction in a first direction, a second polarization layer provided between the first polarization layer and the main surface of the support member and having a polarization direction in a second direction opposite to the first direction, and a first polarization direction mixed layer disposed between the first polarization layer and the second polarization layer and including a first portion having a polarization direction in the first direction and a second portion having a polarization direction in the second direction, and the first polarization layer, the second polarization layer, and the first polarization direction mixed layer are single crystals made of the same material.

[0007] An acoustic wave filter according to one aspect includes the acoustic wave resonator described above.

[0008] A front-end module according to one aspect includes the acoustic wave filter described above.

[0009] A communication device according to one aspect includes the above-described front-end module.

[0010] According to the acoustic wave resonator, the acoustic wave filter, the front-end module, and the communication device of the present invention, the resonator characteristics can be improved.

[0011] FIG. 1 is a plan view showing an elastic wave resonator according to a first embodiment. FIG. 2 is a cross-sectional view taken along line II-II' of FIG. 1. FIG. 3 is a schematic cross-sectional view illustrating the polarization direction of a piezoelectric layer. FIG. 4 is an explanatory diagram illustrating the polarization direction of a first polarization layer in a plane taken along line IV-IV' of FIG. 3. FIG. 5 is an explanatory diagram illustrating the polarization direction of a first polarization direction mixed layer in a plane taken along line V-V' of FIG. 3. FIG. 6 is an explanatory diagram illustrating the polarization direction of a first polarization direction mixed layer in a plane taken along line VI-VI' of FIG. 3. FIG. 7 is an explanatory diagram illustrating the polarization direction of a second polarization layer in a plane taken along line VII-VII' of FIG. 3. FIG. 8 is an explanatory diagram illustrating the relationship between a piezoelectric layer and the stress distribution of a bulk wave. FIG. 9 is a graph showing the relationship between the thickness of a polarization direction mixed layer and the energy loss rate of a bulk wave. FIG. 10 is an explanatory diagram illustrating a method for manufacturing an elastic wave resonator according to the first embodiment. Fig. 11 is a cross-sectional view showing an elastic wave resonator according to a second preferred embodiment of the present invention. Fig. 12 is a cross-sectional view showing an elastic wave resonator according to a third preferred embodiment of the present invention. Fig. 13 is a cross-sectional view showing an elastic wave resonator according to a fourth preferred embodiment of the present invention. Fig. 14 is a diagram showing the configuration of a communication device according to a fifth preferred embodiment of the present invention. Fig. 15 is a cross-sectional view showing an elastic wave resonator according to a modified preferred embodiment of the present invention.

[0012] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to these embodiments. Note that each embodiment described in the present disclosure is illustrative, and partial substitution or combination of configurations between different embodiments is possible. In modified examples and the second and subsequent embodiments, descriptions of matters common to the first embodiment will be omitted, and only differences will be described. In particular, similar effects resulting from similar configurations will not be mentioned in each embodiment.

[0013] First Embodiment Fig. 1 is a plan view showing an elastic wave resonator according to a first embodiment. Fig. 2 is a cross-sectional view taken along line II-II' of Fig. 1. An elastic wave resonator 10 according to the first embodiment is a resonator that utilizes bulk waves, i.e., a BAW (Bulk Acoustic Wave) element.

[0014] 1 and 2, the elastic wave resonator 10 includes a support member 13, a piezoelectric layer 20, an upper electrode 31, a lower electrode 32, and connection electrodes 41 and 42. As shown in Fig. 2, the lower electrode 32, the piezoelectric layer 20, the upper electrode 31, and the connection electrodes 41 and 42 are stacked in this order on the support member 13.

[0015] In the following description, the thickness direction of the piezoelectric layer 20 is referred to as the Z direction, the direction perpendicular to the Z direction is referred to as the X direction, and the direction perpendicular to the Z direction and the X direction is referred to as the Y direction. The X direction and the Y direction are each parallel to the surface (first main surface 20a) of the piezoelectric layer 20. In the following description, a plan view refers to the positional relationship when viewed from a direction perpendicular to the first main surface 20a of the piezoelectric layer 20 (Z direction).

[0016] The support member 13 is disposed opposite the second main surface 20b of the piezoelectric layer 20. The support member 13 includes a support substrate 11 and an intermediate layer 12. The support substrate 11 is made of silicon (Si), quartz crystal, or the like. The intermediate layer 12 is disposed between the support substrate 11 and the piezoelectric layer 20. The intermediate layer 12 is formed of an insulating material such as silicon oxide. Note that the support member 13 may be configured without the intermediate layer 12, with the piezoelectric layer 20 disposed on the support substrate 11. In other words, the piezoelectric layer 20 is bonded to the support substrate 11 directly or via the intermediate layer 12 (insulating layer).

[0017] A recess 14 (hollow portion) is formed on the surface of the support member 13 (intermediate layer 12) facing the second main surface 20b of the piezoelectric layer 20. The recess 14 is provided so as to overlap, in plan view, with the excitation region of the resonator formed by overlapping the piezoelectric layer 20, the upper electrode 31, and the lower electrode 32. This reduces energy loss of the bulk wave during excitation, resulting in good resonance characteristics.

[0018] The piezoelectric layer 20 is in the form of a flat plate having a first main surface 20a and a second main surface 20b opposite to the first main surface 20a. The piezoelectric layer 20 is made of lithium niobate (LiNbO ３ ) or lithium tantalate (LiTaO ３ The thickness of the piezoelectric layer 20 is not particularly limited, but is preferably 1 μm or less.

[0019] The piezoelectric layer 20 includes a first polarization layer 21, a second polarization layer 22 provided between the first polarization layer 21 and the main surface of the support member 13, and a first mixed polarization direction layer 23 disposed between the first polarization layer 21 and the second polarization layer 22. That is, the piezoelectric layer 20 is configured by stacking the second polarization layer 22, the first mixed polarization direction layer 23, and the first polarization layer 21 in this order on the support member 13. The first polarization layer 21, the first mixed polarization direction layer 23, and the second polarization layer 22 are single crystals made of the same material. Note that the detailed configurations of the first polarization layer 21, the first mixed polarization direction layer 23, and the second polarization layer 22 will be described later with reference to FIG. 3 and subsequent figures.

[0020] In the present disclosure, the term "single crystal" for the first polarization layer 21, the first polarization direction mixed layer 23, and the second polarization layer 22 constituting the piezoelectric layer 20 refers to the fact that all crystal orientations can be identified in each layer. In other words, if the first polarization layer 21, the first polarization direction mixed layer 23, and the second polarization layer 22 constituting the piezoelectric layer 20 are lithium niobate or lithium tantalate, these layers can be considered to be single crystals of lithium niobate or lithium tantalate if the a-axis and c-axis of the crystal can be identified. Whether the crystal orientation of the piezoelectric layer can be identified can be determined by measuring the cross section of each piezoelectric layer stacked along the Z direction using X-ray diffraction (XRD). The piezoelectric layer 20 is not limited to being single crystal, and may include partially amorphous regions and regions containing defects and grain boundaries due to manufacturing variations, etc.

[0021] The upper electrode 31 and the lower electrode 32 (plurality of functional electrodes) are provided on the opposing first and second principal surfaces 20a and 20b, respectively, of the piezoelectric layer 20. Specifically, the upper electrode 31 is provided on the first principal surface 20a of the piezoelectric layer 20. The lower electrode 32 is provided on the second principal surface 20b of the piezoelectric layer 20.

[0022] 1 and 2 , a portion of the upper electrode 31 and a portion of the lower electrode 32 overlap in the region overlapping with the recess 14. In other words, in the region overlapping with the recess 14, the piezoelectric layer 20 is disposed between the upper electrode 31 and the lower electrode 32 in the Z direction. This allows bulk waves to propagate between the upper electrode 31 and the lower electrode 32. In the following description, the region where the upper electrode 31 and the lower electrode 32 overlap in a planar view may be described as the excitation region of the resonator.

[0023] The upper electrode 31 extends in the X direction from the region overlapping with the recess 14, and is connected to a connection electrode 41 separate from the upper electrode 31. The lower electrode 32 extends from the region overlapping with the recess 14 to the opposite side to the upper electrode 31, and is connected to the connection electrode 42 through an opening OP formed in the piezoelectric layer 20.

[0024] The upper electrode 31 and the lower electrode 32 are formed of a metal such as aluminum (Al), platinum (Pt), copper (Cu), tungsten (W), or molybdenum (Mo), or an alloy containing at least one of these materials. The upper electrode 31 and the lower electrode 32 may be a laminated film. An adhesive layer such as Ti or NiCr may be provided between the upper electrode 31 and the lower electrode 32 and the support member 13 (intermediate layer 12).

[0025] 15 is a cross-sectional view showing an elastic wave resonator according to a modified example. As shown in FIG. 15, an acoustic multilayer film 15 may be provided in support member 13 instead of recess 14. Acoustic multilayer film 15 has a laminated structure of low acoustic impedance layers 15a, 15c, and 15e having a relatively low acoustic impedance and high acoustic impedance layers 15b and 15d having a relatively high acoustic impedance. Low acoustic impedance layers 15a, 15c, and 15e are made of, for example, SiO ２The high acoustic impedance layers 15b and 15d are, for example, metal layers such as W, Pt, and Mo, or dielectric layers such as hafnium oxide, tantalum oxide, tungsten oxide, and aluminum nitride. When the acoustic multilayer 15 is used, bulk waves can be confined within the piezoelectric layer 20 without using the recesses 14.

[0026] Although the upper electrode 31, the lower electrode 32, and the recess 14 are each rectangular in plan view, they are not limited to this and may be circular or have other shapes.

[0027] Next, the detailed configurations of the first polarization layer 21, first polarization direction mixed layer 23, and second polarization layer 22 of the piezoelectric layer 20 will be described with reference to Figures 3 to 7. Figure 3 is a schematic cross-sectional view for explaining the polarization directions of the piezoelectric layers. In Figure 3, arrow P1 indicates the polarization direction of the first polarization layer 21. Arrow P2 indicates the polarization direction of the second polarization layer 22. Arrows P3 and P4 indicate the polarization direction of the first polarization direction mixed layer 23.

[0028] As shown in FIG. 3 , the first polarization layer 21 is provided in contact with the upper electrode 31 and is located in the uppermost layer of the piezoelectric layer 20. The polarization direction (arrow P1) of the first polarization layer 21 is a first direction D1. The second polarization layer 22 is provided in contact with the lower electrode 32 and is located in the lowermost layer of the piezoelectric layer 20. The polarization direction (arrow P2) of the second polarization layer 22 is a second direction D2 opposite to the first direction D1. The first polarization direction mixed layer 23 is located in an inner layer between the first polarization layer 21 and the second polarization layer 22. The first polarization direction mixed layer 23 includes a first portion 23a whose polarization direction (arrow P3) is the first direction D1 and a second portion 23b whose polarization direction (arrow P4) is the second direction D2.

[0029] Here, the first direction D1 and the second direction D2 are directions that intersect with the X direction, the Y direction, and the Z direction, respectively. That is, the polarization directions of the first polarization layer 21, the first polarization direction mixed layer 23, and the second polarization layer 22 intersect with the thickness direction (Z direction) of the piezoelectric layer 20. The first direction D1 and the second direction D2 may be parallel to the Z direction. Also, in FIG. 3 , the boundary between the first portion 23 a and the second portion 23 b of the first polarization direction mixed layer 23 is shown as a zigzag dashed line. However, FIG. 3 is merely a schematic illustration, and the boundary between the first portion 23 a and the second portion 23 b may be formed with a random pattern including straight and curved portions.

[0030] More specifically, in the first polarization direction mixed layer 23, the polarization direction (first direction D1) of the first portion 23a is opposite to the polarization direction (second direction D2) of the second portion 23b. The polarization direction (first direction D1) of the first portion 23a of the first polarization direction mixed layer 23 is the same as the polarization direction (first direction D1) of the first polarization layer 21. The polarization direction (second direction D2) of the second portion 23b of the first polarization direction mixed layer 23 is the same as the polarization direction (second direction D2) of the second polarization layer 22.

[0031] The first portion 23a of the first polarization direction mixed layer 23 is in direct contact with the first polarization layer 21. The second portion 23b of the first polarization direction mixed layer 23 is in direct contact with the second polarization layer 22. When the first polarization direction mixed layer 23 is cut horizontally, the area ratio of the first portion 23a to the second portion 23b increases as the first polarization layer 21 is approached in the Z direction. In other words, the area ratio of the second portion 23b to the first portion 23a increases as the second polarization layer 22 is approached in the Z direction.

[0032] The polarization directions of the first polarization layer 21, the first polarization direction mixed layer 23, and the second polarization layer 22 will be described separately as a Z-direction component and a horizontal direction component (a direction parallel to a plane defined by the X and Y directions). The Z-direction component of the polarization direction (first direction D1) of the first polarization layer 21 is opposite to the Z-direction component of the polarization direction (second direction D2) of the second polarization layer 22. The horizontal direction component of the polarization direction (first direction D1) of the first polarization layer 21 is opposite to the horizontal direction component of the polarization direction (second direction D2) of the second polarization layer 22.

[0033] In the first polarization direction mixed layer 23, the Z-direction component of the polarization direction (first direction D1) of the first portion 23a is opposite to the Z-direction component of the polarization direction (second direction D2) of the second portion 23b, and the horizontal component of the polarization direction (first direction D1) of the first portion 23a is opposite to the horizontal component of the polarization direction (second direction D2) of the second portion 23b.

[0034] The Z-direction component of the polarization direction (first direction D1) of the first portion 23a of the first polarization direction mixed layer 23 is oriented in the same direction as the Z-direction component of the polarization direction (first direction D1) of the first polarization layer 21. The horizontal direction component of the polarization direction (first direction D1) of the first portion 23a of the first polarization direction mixed layer 23 is oriented in the same direction as the horizontal direction component of the polarization direction (first direction D1) of the first polarization layer 21.

[0035] The Z-direction component of the polarization direction (second direction D2) of the second portion 23b of the first polarization direction mixed layer 23 is oriented in the same direction as the Z-direction component of the polarization direction (second direction D2) of the second polarization layer 22. The horizontal direction component of the polarization direction (second direction D2) of the second portion 23b of the first polarization direction mixed layer 23 is oriented in the same direction as the horizontal direction component of the polarization direction (second direction D2) of the second polarization layer 22.

[0036] Fig. 4 is an explanatory diagram for explaining the polarization direction of the first polarization layer in a plane taken along line IV-IV' in Fig. 3. Fig. 5 is an explanatory diagram for explaining the polarization direction of the first polarization direction mixed layer in a plane taken along line V-V' in Fig. 3. Fig. 6 is an explanatory diagram for explaining the polarization direction of the first polarization direction mixed layer in a plane taken along line VI-VI' in Fig. 3. Fig. 7 is an explanatory diagram for explaining the polarization direction of the second polarization layer in a plane taken along line VII-VII' in Fig. 3.

[0037] In Figures 4 to 7, for each of the first polarization layer 21, the first polarization direction mixed layer 23, and the second polarization layer 22, the areas where the polarization direction is the first direction D1 are shown with a dot pattern, and the areas where the polarization direction is the second direction D2 are shown with diagonal lines.

[0038] 4 to 7 show the results of observing, using a scanning probe microscope (SPM), the polarization directions of the first polarization layer 21, the first polarization direction mixed layer 23, and the second polarization layer 22. Specifically, in PRM (Piezo Response Microscope) observation images of the horizontal surfaces of the first polarization layer 21, the first polarization direction mixed layer 23, and the second polarization layer 22, regions with different polarization directions appear as regions showing different colors.

[0039] As shown in Fig. 4, the polarization direction of the first polarization layer 21 is a first direction D1 in the entire region in plan view. Also, as shown in Fig. 7, the polarization direction of the second polarization layer 22 is a second direction D2 opposite to the first direction D1 in the entire region in plan view.

[0040] 5 and 6 , in plan view, the first polarization direction mixed layer 23 includes a first portion 23 a whose polarization direction is the first direction D1 and a second portion 23 b whose polarization direction is the second direction D2. The ratio of the area of ​​the first portion 23 a to the area of ​​the second portion 23 b varies depending on the position of the first polarization direction mixed layer 23 in the Z direction.

[0041] 5, in a region of the first polarization direction mixed layer 23 close to the first polarization layer 21 in the Z direction, the area of ​​the first portion 23a whose polarization direction is the first direction D1 is larger than the area of ​​the second portion 23b whose polarization direction is the second direction D2. In contrast, as shown in FIG. 6, in a region of the first polarization direction mixed layer 23 close to the second polarization layer 22 in the Z direction, the area of ​​the second portion 23b whose polarization direction is the second direction D2 is larger than the area of ​​the first portion 23a whose polarization direction is the first direction D1.

[0042] In this way, the first polarization direction mixed layer 23 is formed so that the area proportion of the first portion 23a increases the closer it is to the first polarization layer 21 in the Z direction, and the area proportion of the second portion 23b increases the closer it is to the second polarization layer 22.

[0043] Fig. 8 is an explanatory diagram for explaining the relationship between the piezoelectric layer and the stress distribution of the bulk wave. Fig. 9 is a graph showing the relationship between the thickness of the polarization direction mixed layer and the energy loss rate of the bulk wave.

[0044] As shown in Figure 8, the thickness t1 of the first polarization layer 21 is equal to the thickness t2 of the second polarization layer 22. The thickness t3 of the first polarization direction mixed layer 23 is thinner than the thickness t1 of the first polarization layer 21 and the thickness t2 of the second polarization layer 22. For example, the thickness t1 of the first polarization layer 21 and the thickness t2 of the second polarization layer 22 are each approximately 50 nm or more and 500 nm or less. For example, the thickness t3 of the first polarization direction mixed layer 23 is approximately 5 nm or more and 120 nm or less. If the thickness t3 of the first polarization direction mixed layer 23 is too thin, the spurious wave suppression effect may not be sufficiently exhibited. The thickness t3 of the first polarization direction mixed layer 23 is preferably set to a thickness that ensures that the lithium tantalate crystal and lithium niobate crystal exhibit sufficient piezoelectricity, and that ensures that the lithium tantalate crystal and lithium niobate crystal, which have a pseudo-ilmenite structure, contain three or more elementary lattices in the c-axis direction, i.e., 5 nm or more.

[0045] Note that "same thickness" is not limited to the case where the thickness is exactly the same, but also includes a difference in thickness ranging from -5% to +5%.

[0046] More specifically, the thickness t3 of the first polarization direction mixed layer 23 is thinner than 1 / 8 of the sum (t1 + t2) of the thicknesses of the first polarization layer 21 and the second polarization layer 22. Furthermore, the total thickness (t1 + t3 × 1 / 2) of the thickness t1 of the first polarization layer 21 and 1 / 2 of the thickness t3 of the first polarization direction mixed layer 23 in contact with the first polarization layer 21 is equivalent to the total thickness (t2 + t3 × 1 / 2) of the thickness t2 of the second polarization layer 22 and 1 / 2 of the thickness t3 of the first polarization direction mixed layer 23 in contact with the second polarization layer 22.

[0047] Furthermore, if the wavelength at which a high-frequency signal transmitted from among the elastic waves excited by the functional electrodes (upper electrode 31 and lower electrode 32) resonates in the piezoelectric layer 20 is λ, the thickness t1 of the first polarization layer 21 is λ / 2, and the thickness t2 of the second polarization layer 22 is λ / 2. The thickness t3 of the first polarization direction mixed layer 23 is thinner than λ / 8.

[0048] With the above-described configuration, the main bulk wave resonates at a half-wavelength in each of the first polarization layer 21 and the second polarization layer 22. The stress in the piezoelectric layer 20 caused by the main bulk wave is greatest at the center in the Z direction of the first polarization layer 21 and the center in the Z direction of the second polarization layer 22. The stress in the piezoelectric layer 20 caused by the main bulk wave is also greatest at the position where it overlaps with the first mixed polarization direction layer 23. As a result, even if the piezoelectric layer 20 has a configuration including the first mixed polarization direction layer 23, energy loss of the main bulk wave is suppressed.

[0049] On the other hand, the position in the Z direction where the stress is minimum for the spurious waves different from the main bulk waves is shifted from the first mixed polarization direction layer 23. In other words, the stress generated by the spurious waves in the first mixed polarization direction layer 23 is greater than the stress generated by the main bulk waves. This increases the energy loss of the spurious waves in the first mixed polarization direction layer 23, thereby suppressing the spurious waves. Therefore, the elastic wave resonator 10 of this embodiment can reduce the spurious waves while suppressing deterioration of the main bulk waves. This allows the elastic wave resonator 10 of this embodiment to improve its resonator characteristics.

[0050] Furthermore, for elastic waves, the larger the electromechanical coupling coefficient of the mode, the smaller the impact of energy loss in the first mixed polarization direction layer 23, and the smaller the electromechanical coupling coefficient of the mode, the larger the impact of energy loss in the first mixed polarization direction layer 23. Therefore, among the unnecessary waves different from the main bulk wave, the harmonics are suppressed because they experience larger energy loss in the first mixed polarization direction layer 23 than the main bulk wave.

[0051] Examples of unwanted waves that are different from the main bulk waves include waves in a mode influenced by the end of the upper electrode 31 or the lower electrode 32, and waves in a mode influenced by the edge of the membrane portion of the piezoelectric layer 20.

[0052] As described above, the first polarization layer 21, the first polarization direction mixed layer 23, and the second polarization layer 22 are integrally formed of the same piezoelectric material. In other words, there is no piezoelectric material or conductive material different from the first polarization layer 21, the first polarization direction mixed layer 23, and the second polarization layer 22 between the layers of the piezoelectric layer 20.

[0053] If a different material is provided between the piezoelectric layers 20, for example, the different elements may diffuse into the piezoelectric layers 20 and affect the piezoelectric properties of the piezoelectric layers 20, which may cause the resonance characteristics to change over time. Furthermore, the different material may become a layer that does not contribute to excitation, which may result in a deterioration of the resonance characteristics. Furthermore, if a conductive material is provided as the different material, parasitic capacitance may be generated, which may significantly deteriorate the resonator characteristics. Alternatively, if an amorphous piezoelectric material is provided as the different material, the layer may become a layer that does not contribute to excitation, which may result in a deterioration of the resonator characteristics.

[0054] In the elastic wave resonator 10 of this embodiment, the first polarization layer 21, the first polarization direction mixed layer 23, and the second polarization layer 22 are integrally formed from the same piezoelectric material, so that deterioration of the resonator characteristics can be suppressed compared to when different materials are provided between the layers.

[0055] The horizontal axis of the graph shown in Fig. 9 represents the thickness t3 of the first polarization direction mixed layer 23, and the vertical axis represents the energy loss rate of the main bulk wave. Fig. 9 shows the results of simulating the energy loss when the resonant wavelength of the main bulk wave is set to λ = 920 nm (4.5 GHz) and the thickness t3 is changed from 14.4 nm (= λ / 64) to 230 nm (= λ / 4).

[0056] 9 , when the thickness t3 of the first polarization direction mixed layer 23 is thinner than 115 nm (=λ / 8), the energy loss rate of the main bulk wave is suppressed to a range of approximately 0% to 2%. When the thickness t3 of the first polarization direction mixed layer 23 is 172.5 nm or less (=λ / 8×(3 / 2)), the energy loss rate of the main bulk wave is suppressed to a range of approximately 10% or less. When the thickness t3 of the first polarization direction mixed layer 23 is greater than 172.5 nm, the energy loss rate of the main bulk wave becomes greater than 10%.

[0057] This indicates that the energy loss of the main bulk wave can be effectively suppressed by making the thickness t3 of the first polarization direction mixed layer 23 thinner than λ / 8.

[0058] The above-described configuration of the elastic wave resonator 10 is merely an example and can be modified as appropriate. For example, the thickness t1 of the first polarization layer 21, the thickness t2 of the second polarization layer 22, the thickness t3 of the first mixed polarization direction layer 23, the resonant frequency, and other values ​​are merely examples and can be modified as appropriate depending on the characteristics required of the elastic wave resonator 10.

[0059] 10 is an explanatory diagram illustrating a method for manufacturing an elastic wave resonator according to the first embodiment. As shown in FIG. 10, in the method for manufacturing an elastic wave resonator, first, a transfer substrate 50 (e.g., a silicon substrate) is prepared, and a piezoelectric layer 20 is directly bonded to a main surface of the transfer substrate 50 (step ST1). The piezoelectric layer 20 is made of, for example, lithium niobate (LiNbO ３ In step ST1, the polarization direction (arrow P0) of the piezoelectric layer 20 is such that the +c plane faces the surface opposite to the bonding surface (first main surface 20a) with the transfer substrate 50.

[0060] Next, the second main surface 20b of the piezoelectric layer 20 is ground to be thinned (step ST2). At this time, a plurality of recesses are formed in the second main surface 20b of the piezoelectric layer 20.

[0061] Next, a metal layer 51 is formed on the second main surface 20b of the piezoelectric layer 20 (step ST3). In step ST3, the metal layer 51 is patterned so as not to be electrically connected to the transfer substrate 50. In steps ST3 and ST4, the piezoelectric layer 20 is shown enlarged in the thickness direction compared to step ST2 to make the drawings easier to see.

[0062] A reference potential Vref (for example, ground potential) is supplied to the transfer substrate 50, and while heating, a positive potential Vp is supplied to the metal layer 51. As a result, an electric field is applied to the piezoelectric layer 20.

[0063] By applying an electric field for a predetermined time, the polarization direction of the second main surface 20b of the piezoelectric layer 20 is reversed (step ST4). In step ST4, the electric field strength is controlled to be greater than the coercive electric field of the piezoelectric layer 20, thereby reversing the polarization direction from the second main surface 20b. Then, the polarization direction is reversed from the second main surface 20b to the center of the piezoelectric layer 20 in the thickness direction, forming a second polarization layer 22. Furthermore, a first polarization layer 21, whose polarization direction is maintained in the initial state, is formed from the center of the piezoelectric layer 20 in the thickness direction to the first main surface 20a.

[0064] The thickness of the second polarization layer 22 formed in step ST4 can be controlled by the electric field strength. For example, the second polarization layer 22 becomes thicker by increasing the positive potential Vp applied to the metal layer 51, and becomes thinner by decreasing the positive potential Vp.

[0065] Furthermore, the progress of polarization direction reversal of the piezoelectric layer 20 differs between the multiple recesses formed in the second main surface 20b in step ST2 and other portions (flat portions where no recesses are formed). For example, starting points for polarization direction reversal are formed in the recessed portions, making the polarization direction reversal more likely to progress in these portions than in regions where no recesses are formed. Then, by applying an appropriate electric field strength, a first polarization direction mixed layer 23 in which polarization directions are mixed is formed in the center of the thickness direction of the piezoelectric layer 20.

[0066] Through the above steps, the first polarization layer 21, the second polarization layer 22, and the first mixed polarization direction layer 23 disposed between the first polarization layer 21 and the second polarization layer 22 are formed. According to the manufacturing method of this embodiment, the polarization direction, thickness, etc. of the first polarization layer 21, the second polarization layer 22, and the first mixed polarization direction layer 23 can be controlled by applying an electric field to one piezoelectric layer 20. Therefore, it is not necessary to provide a different material, such as another piezoelectric material or a conductive material, between the piezoelectric layers 20. As a result, a piezoelectric layer 20 can be formed that has excellent reliability, moisture resistance, and good resonance characteristics.

[0067] 10 shows only the manufacturing process of the piezoelectric layer 20, but after step ST4, a lower electrode 32 and various wirings are formed on the second main surface 20b of the piezoelectric layer 20, and the piezoelectric layer 20 is bonded to the main surface of the support member 13 (see FIG. 2). As a result, the piezoelectric layer 20 is laminated on the support member 13 in the order of the second polarization layer 22, the first polarization direction mixed layer 23, and the first polarization layer 21 (see FIG. 1). Thereafter, the transfer substrate 50 is removed, and the upper electrode 31 and various wirings are formed. A recess 14 is formed in the support member 13, thereby completing the elastic wave resonator 10. Note that any method may be used for the processes from step ST4 onwards.

[0068] 11 is a cross-sectional view showing an elastic wave resonator according to a second embodiment. As shown in FIG. 11 , an elastic wave resonator 10A according to the second embodiment differs from the first embodiment in that a piezoelectric layer 20A further includes a third polarization layer 24 and a second polarization direction mixed layer 25.

[0069] The third polarization layer 24 is provided below the second polarization layer 22. That is, the third polarization layer 24 is provided between the second polarization layer 22 and the main surface of the support member 13 (see FIG. 2 ), and its polarization direction is the first direction D1. The second polarization direction mixed layer 25 is disposed between the second polarization layer 22 and the third polarization layer 24. Like the first polarization direction mixed layer 23, the second polarization direction mixed layer 25 includes a first portion whose polarization direction is the first direction D1 and a second portion whose polarization direction is the second direction D2.

[0070] The thickness t1 of the first polarization layer 21 is equal to the thickness t4 of the third polarization layer 24. The thickness t2 of the second polarization layer 22 is greater than the thickness t1 of the first polarization layer 21 and the thickness t4 of the third polarization layer 24. The bulk wave energy of the main resonance leaks to the functional electrode from the uppermost and lowermost polarization layers in contact with the functional electrode (here, the first polarization layer 21 and the third polarization layer 24 are referred to as the outer layers). Therefore, if the outer layer and the inner layer (the polarization layer located between the uppermost and lowermost polarization layers, here the second polarization layer 22) have the same thickness, unwanted waves are generated due to the energy leaked to the functional electrode. Therefore, taking into consideration that bulk wave energy leaks to the functional electrode in the outer layer, the thickness of the outer layer is made smaller than that of the inner layer so that the node of the main bulk wave stress becomes the functional electrode, thereby suppressing unwanted waves caused by bulk wave energy leaking to the functional electrode.

[0071] The total thickness (t1 + t3 × 1 / 2) of the thickness t1 of the first polarization layer 21 and half the thickness t3 of the first mixed polarization direction layer 23 in contact with the first polarization layer 21 is equal to the total thickness (t4 + t5 × 1 / 2) of the thickness t4 of the third polarization layer 24 and half the thickness t5 of the second mixed polarization direction layer 25 in contact with the third polarization layer 24. When the resonance of the main bulk wave is designed so that the sum of the thickness of the uppermost polarization layer (here, the first polarization layer 21) in contact with the functional electrode and a part of the mixed polarization direction layer (here, the first mixed polarization direction layer 23) adjacent to that polarization layer is half the wavelength, the midpoint of the mixed polarization direction layer is set to be the stress node of the main bulk wave. In other words, by making the sum of the thickness of the topmost polarization layer in contact with the functional electrode and half the thickness of the adjacent mixed polarization layer equal to the sum of the thickness of the bottommost polarization layer in contact with the other functional electrode and half the thickness of the adjacent mixed polarization layer, the energy loss of the main bulk wave due to the mixed polarization layer is minimized, and good resonance characteristics can be obtained.

[0072] With this configuration, elastic wave resonator 10A according to the second preferred embodiment can also suppress energy loss of the main bulk waves and reduce unwanted waves.

[0073] As described above, in the elastic wave resonator 10A of the second embodiment, the piezoelectric layer 20A has three or more polarization layers including the first polarization layer 21 and the second polarization layer 22, and two or more polarization direction mixed layers including the first polarization direction mixed layer 23 provided between the three or more polarization layers.

[0074] Of the multiple polarization layers, the total thickness of the uppermost polarization layer (e.g., the first polarization layer 21) in contact with the upper electrode 31 (one of the functional electrodes) and half the thickness of the polarization direction mixed layer (e.g., the first polarization direction mixed layer 23) in contact with the uppermost polarization layer is equal to the total thickness of the lowermost polarization layer (e.g., the third polarization layer 24) in contact with the lower electrode 32 (the other functional electrode) and half the thickness of the polarization direction mixed layer (e.g., the second polarization direction mixed layer 25) in contact with the lowermost polarization layer.

[0075] Third Embodiment Fig. 12 is a cross-sectional view showing an elastic wave resonator according to a third embodiment. In the first and second embodiments described above, the piezoelectric layer 20, 20A has two polarized layers and one mixed-polarization layer, or three polarized layers and two mixed-polarization layers, but is not limited to this. As shown in Fig. 12, an elastic wave resonator 10B according to the third embodiment differs from the first and second embodiments described above in that the piezoelectric layer 20B further includes a fourth polarized layer 26 and a third mixed-polarization layer 27.

[0076] The fourth polarization layer 26 is provided below the third polarization layer 24. That is, the fourth polarization layer 26 is provided between the third polarization layer 24 and the main surface of the support member 13 (see FIG. 2 ), and its polarization direction is the second direction D2. The third polarization direction mixed layer 27 is disposed between the third polarization layer 24 and the fourth polarization layer 26. Like the first polarization direction mixed layer 23 and the second polarization direction mixed layer 25, the third polarization direction mixed layer 27 includes a first portion whose polarization direction is the first direction D1 and a second portion whose polarization direction is the second direction D2.

[0077] The thickness t1 of the first polarized layer 21 is equal to the thickness t6 of the fourth polarized layer 26. The thickness t2 of the second polarized layer 22, which is an inner layer between the first polarized layer 21 and the fourth polarized layer 26, is equal to the thickness t4 of the third polarized layer 24. The thickness t2 of the second polarized layer 22 and the thickness t4 of the third polarized layer 24 are greater than the thickness t1 of the first polarized layer 21 and the thickness t6 of the fourth polarized layer 26.

[0078] The total thickness (t1 + t3 × 1 / 2) of the thickness t1 of the first polarization layer 21 and 1 / 2 of the thickness t3 of the first polarization direction mixed layer 23 in contact with the first polarization layer 21 is equal to the total thickness (t6 + t7 × 1 / 2) of the thickness t6 of the fourth polarization layer 26 and 1 / 2 of the thickness t7 of the third polarization direction mixed layer 27 in contact with the fourth polarization layer 26.

[0079] Furthermore, the total thickness (t2 + t3 × 1 / 2 + t5 × 1 / 2) of the thickness t2 of the second polarization layer 22, 1 / 2 of the thickness t3 of the first polarization direction mixed layer 23 in contact with the second polarization layer 22, and 1 / 2 of the thickness of the second polarization direction mixed layer 25 in contact with the second polarization layer 22 is equivalent to the total thickness (t4 + t5 × 1 / 2 + t7 × 1 / 2) of the thickness t4 of the third polarization layer 24, 1 / 2 of the thickness t5 of the second polarization direction mixed layer 25 in contact with the third polarization layer 24, and 1 / 2 of the thickness t7 of the third polarization direction mixed layer 27 in contact with the third polarization layer 24.

[0080] As described above, in the elastic wave resonator 10B of the third embodiment, the piezoelectric layer 20B has four or more polarization layers including the first polarization layer 21 and the second polarization layer 22, and three or more polarization direction mixed layers including the first polarization direction mixed layer 23, which are provided between the four or more polarization layers.

[0081] Of the multiple polarization layers, except for the topmost polarization layer (first polarization layer 21) in contact with the upper electrode 31 (one of the functional electrodes) and the bottommost polarization layer (e.g., fourth polarization layer 26) in contact with the lower electrode 32 (the other functional electrode), the sum of the thickness of each of the multiple inner polarization layers (e.g., second polarization layer 22, third polarization layer 24) located between the topmost polarization layer and the bottommost polarization layer and half the thickness of the polarization direction mixed layer in contact with each of the multiple inner polarization layers (first polarization direction mixed layer 23 and second polarization direction mixed layer 25 in contact with the second polarization layer 22, and second polarization direction mixed layer 25 and third polarization direction mixed layer 27 in contact with the third polarization layer 24) is equal to each other.

[0082] Fourth Embodiment Fig. 13 is a cross-sectional view showing an elastic wave resonator according to a fourth embodiment. In the third embodiment described above, the piezoelectric layer 20B includes four polarization layers and three mixed polarization layers, but this is not limiting. As shown in Fig. 13, an elastic wave resonator 10C according to the fourth embodiment differs from the first to third embodiments described above in that the piezoelectric layer 20C further includes a fifth polarization layer 28 and a fourth mixed polarization layer 29.

[0083] The fifth polarization layer 28 is provided below the fourth polarization layer 26. That is, the fifth polarization layer 28 is provided between the fourth polarization layer 26 and the main surface of the support member 13 (see FIG. 2 ), and its polarization direction is the first direction D1. The fourth polarization direction mixed layer 29 is disposed between the fourth polarization layer 26 and the fifth polarization layer 28. Like the first polarization direction mixed layer 23 to the third polarization direction mixed layer 27, the fourth polarization direction mixed layer 29 includes a first portion whose polarization direction is the first direction D1 and a second portion whose polarization direction is the second direction D2.

[0084] The thickness t1 of the first polarized layer 21 is equal to the thickness t8 of the fifth polarized layer 28. The thickness t2 of the second polarized layer 22, which is an inner layer between the first polarized layer 21 and the fifth polarized layer 28, is equal to the thickness t4 of the third polarized layer 24 and the thickness t6 of the fourth polarized layer 26. The thickness t2 of the second polarized layer 22, the thickness t4 of the third polarized layer 24, and the thickness t6 of the fourth polarized layer 26 are each thicker than the thickness t1 of the first polarized layer 21 and the thickness t8 of the fifth polarized layer 28.

[0085] The total thickness (t1 + t3 × 1 / 2) of the thickness t1 of the first polarization layer 21 and 1 / 2 of the thickness t3 of the first polarization direction mixed layer 23 in contact with the first polarization layer 21 is equal to the total thickness (t8 + t9 × 1 / 2) of the thickness t8 of the fifth polarization layer 28 and 1 / 2 of the thickness t9 of the fourth polarization direction mixed layer 29 in contact with the fifth polarization layer 28.

[0086] The total thickness (t2 + t3 × 1 / 2 + t5 × 1 / 2) of the thickness t2 of the second polarization layer 22, 1 / 2 of the thickness t3 of the first polarization direction mixed layer 23 in contact with the second polarization layer 22, and 1 / 2 of the thickness of the second polarization direction mixed layer 25 in contact with the second polarization layer 22 is 1 / 2 of the thickness t4 of the third polarization layer 24, 1 / 2 of the thickness t5 of the second polarization direction mixed layer 25 in contact with the third polarization layer 24, and 1 / 2 of the thickness t6 of the second polarization direction mixed layer 25 in contact with the third polarization layer 24. and the total thickness (t6 + t7 × 1 / 2 + t9 × 1 / 2) of the thickness t6 of the fourth polarization layer 26, 1 / 2 of the thickness t7 of the third polarization direction mixed layer 27 in contact with the fourth polarization layer 26, and 1 / 2 of the thickness t9 of the fourth polarization direction mixed layer 29 in contact with the fourth polarization layer 26.

[0087] In the above-described embodiments, the number of layers, thicknesses, and thickness relationships between layers of the multiple polarized layers and multiple polarization direction mixed layers are merely examples and can be changed as appropriate.

[0088] 14 is a diagram showing the configuration of a communication device according to a fifth embodiment. The communication device 100 according to the fifth embodiment is, for example, a mobile terminal such as a mobile phone, a smartphone, or a tablet terminal, or a personal computer equipped with a communication function. Alternatively, the communication device 100 may be a backhaul communication device that performs communication between base stations and communication between a base station and a core network.

[0089] As shown in FIG. 14, the communication device 100 includes a front-end module 101, an antenna 102, an RF-IC (Radio Frequency Integrated Circuit) 104, and a BB-IC (Baseband Integrated Circuit) 105.

[0090] The BB-IC 105 constitutes a baseband signal processing circuit and supplies a baseband signal to the RF-IC 104. The RF-IC 104 performs high-frequency signal processing of transmission and reception signals.

[0091] The front-end module 101 is connected between the antenna 102 and the RF-IC 104. In the communication device 100, a high-frequency transmission signal is output from the front-end module 101 to the antenna 102, and a high-frequency reception signal from the antenna 102 can be received by the front-end module 101.

[0092] Specifically, the front-end module 101 includes a switch SW, power amplifiers 111a and 111b, matching circuits 112a and 112b, transmit filters 113a and 113b, a multiplexer 114, matching circuits 115a and 115b, and low-noise amplifiers 116a and 116b.

[0093] When transmitting a high-frequency signal, the switch SW is switched to the side of the transmission filter 113a or the transmission filter 113b. The high-frequency transmission signal output from the RF-IC 104 is amplified by the power amplifier 111a or the power amplifier 111b, and input to the transmission filter 113a or the transmission filter 113b through the matching circuits 112a, 112b.

[0094] The band of the transmit filter 113a is, for example, n77 of the 5G NR (New Radio) standard. The frequency range of n77 of 5G NR is 3.3 GHz or higher and 4.2 GHz or lower. The band of the transmit filter 113b is, for example, n79 of the 5G NR standard. The frequency range of n79 of 5G NR is 4.4 GHz or higher and 5.0 GHz or lower.

[0095] The high frequency transmission signal that has passed through the transmission filter 113 a or 113 b is fed to the antenna 102 .

[0096] When receiving a high-frequency signal, the switch SW is switched to the side of the multiplexer 114. The multiplexer 114 includes receive filters 117a and 117b and a matching circuit 118. The receive filter 117a is in the n77 band of the 5G NR standard, for example. The receive filter 117b is in the n79 band of the 5G NR standard, for example.

[0097] A received signal from the antenna 102 passes through the receiving filter 117a or 117b via the matching circuit 118. The high-frequency received signal that has passed through the receiving filter 117a or 117b is transmitted to the RF-IC 104 via the matching circuits 115a, 115b and the low-noise amplifier 116a or 116b.

[0098] In the communication device 100 of this embodiment, at least one of the transmit filters 113a and 113b and the receive filters 117a and 117b of the multiplexer 114 is an acoustic wave filter including any one of the above-described acoustic wave resonators 10, 10A, 10B, and 10C. The front-end module 101 includes at least one of the transmit filters 113a and 113b and the receive filters 117a and 117b of the multiplexer 114. The communication device 100 also includes the front-end module 101.

[0099] 14 is merely a schematic diagram of the communication device 100 and can be modified as appropriate. The bands of the transmit filters 113a and 113b and the receive filters 117a and 117b are merely examples, and other communication standards can also be applied.

[0100] The above-described embodiment is intended to facilitate understanding of the present invention, and is not intended to limit the present invention. The present invention may be modified or improved without departing from the spirit and scope of the present invention, and equivalents thereof are also included in the present invention.

[0101] The present disclosure may also have the following configurations.

[0102] (1) An elastic wave resonator comprising: a support member including a support substrate, a piezoelectric layer provided on a main surface of the support member, and a plurality of functional electrodes provided on opposing main surfaces of the piezoelectric layer, wherein the piezoelectric layer has: a first polarization layer having a polarization direction in a first direction, a second polarization layer provided between the first polarization layer and the main surface of the support member, the second polarization layer having a polarization direction in a second direction opposite to the first direction, and a first polarization direction mixed layer disposed between the first polarization layer and the second polarization layer, the first polarization layer, the second polarization layer, and the first polarization direction mixed layer being a single crystal made of the same material. (2) The elastic wave resonator according to (1), wherein a thickness of the first polarization direction mixed layer is thinner than 1 / 8 of the sum of the thicknesses of the first polarization layer and the second polarization layer. (3) The elastic wave resonator according to (1) or (2), wherein a thickness of the first mixed-polarization layer is thinner than λ / 8, where λ is a wavelength at which a high-frequency signal transmitted from the elastic waves excited by the functional electrode resonates in the piezoelectric layer. (4) The elastic wave resonator according to any one of (1) to (3), wherein a total thickness of the first polarization layer and half the thickness of the first mixed-polarization layer in contact with the first polarization layer is equal to a total thickness of the second polarization layer and half the thickness of the first mixed-polarization layer in contact with the second polarization layer. (5) The elastic wave resonator according to any one of (1) to (4), wherein the piezoelectric layer has three or more polarization layers including the first polarization layer and the second polarization layer, and two or more mixed polarization layers including the first mixed polarization layer provided between the three or more polarization layers, and wherein a total thickness of the uppermost polarization layer in contact with one of the functional electrodes and half the thickness of the mixed polarization layer in contact with the uppermost polarization layer is equal to a total thickness of the lowermost polarization layer in contact with the other functional electrode and half the thickness of the mixed polarization layer in contact with the lowermost polarization layer.(6) The elastic wave resonator according to any one of (1) to (5), wherein the piezoelectric layer has four or more polarization layers including the first polarization layer and the second polarization layer, and three or more mixed polarization direction layers including the first mixed polarization direction layer provided between the four or more polarization layers, and wherein, except for the uppermost polarization layer in contact with one of the functional electrodes and the lowermost polarization layer in contact with the other functional electrode, the sum of each thickness of the inner polarization layers located between the uppermost polarization layer and the lowermost polarization layer and half the thickness of the mixed polarization direction layer in contact with each of the inner polarization layers is equal to each other. (7) The elastic wave resonator according to (1), wherein the piezoelectric layer includes: a third polarization layer provided between the second polarization layer and a main surface of the support member, the third polarization layer having a polarization direction in the first direction; and a second polarization direction mixed layer disposed between the second polarization layer and the third polarization layer, the second polarization direction mixed layer including a first portion having a polarization direction in the first direction and a second portion having a polarization direction in the second direction. (8) The elastic wave resonator according to (7), wherein a total thickness of the first polarization layer and half a thickness of the first polarization direction mixed layer in contact with the first polarization layer is equal to a total thickness of the third polarization layer and half a thickness of the second polarization direction mixed layer in contact with the third polarization layer. (9) The elastic wave resonator according to (7), wherein the piezoelectric layer includes: a fourth polarization layer provided between the third polarization layer and a main surface of the support member, the fourth polarization layer having a polarization direction in the second direction; and a third polarization direction mixed layer disposed between the third polarization layer and the fourth polarization layer, the third polarization direction mixed layer including a first portion having a polarization direction in the first direction and a second portion having a polarization direction in the second direction. (10) The elastic wave resonator according to (9), wherein a total thickness of the first polarization layer and half a thickness of the first polarization direction mixed layer in contact with the first polarization layer is equal to a total thickness of the fourth polarization layer and half a thickness of the third polarization direction mixed layer in contact with the fourth polarization layer.(11) The elastic wave resonator according to (9), wherein a total thickness of the second polarization layer, half the thickness of the first polarization direction mixed layer in contact with the second polarization layer, and half the thickness of the second polarization direction mixed layer in contact with the second polarization layer is equal to a total thickness of the third polarization layer, half the thickness of the second polarization direction mixed layer in contact with the third polarization layer, and half the thickness of the third polarization direction mixed layer in contact with the third polarization layer. (12) The elastic wave resonator according to (9), wherein the piezoelectric layer includes: a fifth polarization layer provided between the fourth polarization layer and a main surface of the support member, the fifth polarization layer having a polarization direction in the first direction; and a fourth polarization direction mixed layer disposed between the fourth polarization layer and the fifth polarization layer, the fourth polarization direction mixed layer including a first portion having a polarization direction in the first direction and a second portion having a polarization direction in the second direction. (13) The elastic wave resonator according to (12), wherein the total thickness of the first polarization layer and half the thickness of the first polarization direction mixed layer in contact with the first polarization layer is equal to the total thickness of the fifth polarization layer and half the thickness of the fourth polarization direction mixed layer in contact with the fifth polarization layer. (14) The elastic wave resonator according to (12), wherein a total thickness of the second polarization layer, half the thickness of the first polarization direction mixed layer in contact with the second polarization layer, and half the thickness of the second polarization direction mixed layer in contact with the second polarization layer is equal to a total thickness of the third polarization layer, half the thickness of the second polarization direction mixed layer in contact with the third polarization layer, and half the thickness of the third polarization direction mixed layer in contact with the third polarization layer, and a total thickness of the fourth polarization layer, half the thickness of the third polarization direction mixed layer in contact with the fourth polarization layer, and half the thickness of the fourth polarization direction mixed layer in contact with the fourth polarization layer. (15) The elastic wave resonator according to any one of (1) to (14), wherein the polarization direction intersects a thickness direction of the piezoelectric layer. (16) An acoustic wave filter including the acoustic wave resonator according to any one of (1) to (15). (17) A front-end module including the acoustic wave filter according to (16). (18) A communication device including the front-end module according to (17).

[0103] 10, 10A, 10B, 10C Acoustic wave resonator 11 Support substrate 12 Intermediate layer 13 Support member 14 Recess 20, 20A, 20B, 20C Piezoelectric layer 20a First main surface 20b Second main surface 21 First polarization layer 22 Second polarization layer 23 First polarization direction mixed layer 23a First portion 23b Second portion 24 Third polarization layer 25 Second polarization direction mixed layer 26 Fourth polarization layer 27 Third polarization direction mixed layer 28 Fifth polarization layer 29 Fourth polarization direction mixed layer 31 Upper electrode 32 Lower electrode 100 Communication device 101 Front-end module D1 First direction D2 Second direction

Claims

1. An elastic wave resonator comprising: a support member having a support substrate; a piezoelectric layer provided on a main surface of the support member; and a plurality of functional electrodes provided on opposing main surfaces of the piezoelectric layer, wherein the piezoelectric layer has: a first polarization layer whose polarization direction is a first direction; a second polarization layer provided between the first polarization layer and the main surface of the support member, the second polarization layer whose polarization direction is a second direction opposite to the first direction; and a first polarization direction mixed layer disposed between the first polarization layer and the second polarization layer, the first polarization layer including a first portion whose polarization direction is the first direction and a second portion whose polarization direction is the second direction, wherein the first polarization layer, the second polarization layer, and the first polarization direction mixed layer are single crystals made of the same material.

2. The elastic wave resonator according to claim 1, wherein the thickness of the first polarization direction mixed layer is less than 1 / 8 of the sum of the thicknesses of the first polarization layer and the second polarization layer.

3. An elastic wave resonator according to claim 1 or 2, wherein the thickness of the first polarization direction mixed layer is thinner than λ / 8, where λ is the wavelength at which a transmitted high-frequency signal, among the elastic waves excited by the functional electrode, resonates within the piezoelectric layer.

4. An elastic wave resonator according to any one of claims 1 to 3, wherein the total thickness of the first polarization layer and half the thickness of the first polarization direction mixed layer in contact with the first polarization layer is equal to the total thickness of the second polarization layer and half the thickness of the first polarization direction mixed layer in contact with the second polarization layer.

5. The elastic wave resonator according to any one of claims 1 to 4, wherein the piezoelectric layer has three or more polarization layers including the first polarization layer and the second polarization layer, and two or more mixed polarization direction layers including the first mixed polarization direction layer provided between the three or more polarization layers, and wherein the total thickness of the uppermost polarization layer in contact with one of the functional electrodes and half the thickness of the mixed polarization direction layer in contact with the uppermost polarization layer is equal to the total thickness of the lowermost polarization layer in contact with the other functional electrode and half the thickness of the mixed polarization direction layer in contact with the lowermost polarization layer.

6. The elastic wave resonator according to any one of claims 1 to 5, wherein the piezoelectric layer has four or more polarization layers including the first polarization layer and the second polarization layer, and three or more mixed polarization direction layers including the first mixed polarization direction layer provided between the four or more polarization layers, and wherein, except for the uppermost polarization layer in contact with one of the functional electrodes and the lowermost polarization layer in contact with the other functional electrode, the sum of the thickness of each of the plurality of inner polarization layers located between the uppermost polarization layer and the lowermost polarization layer and half the thickness of the mixed polarization direction layer in contact with each of the plurality of inner polarization layers is equal to each other.

7. The elastic wave resonator according to claim 1, wherein the piezoelectric layer comprises: a third polarization layer provided between the second polarization layer and the main surface of the support member, the third polarization layer having a polarization direction in the first direction; and a second polarization direction mixed layer disposed between the second polarization layer and the third polarization layer, the second polarization direction mixed layer including a first portion having a polarization direction in the first direction and a second portion having a polarization direction in the second direction.

8. The elastic wave resonator according to claim 7, wherein the total thickness of the first polarization layer and half the thickness of the first polarization direction mixed layer in contact with the first polarization layer is equal to the total thickness of the third polarization layer and half the thickness of the second polarization direction mixed layer in contact with the third polarization layer.

9. The elastic wave resonator according to claim 7, wherein the piezoelectric layer comprises: a fourth polarization layer provided between the third polarization layer and the main surface of the support member, the fourth polarization layer having a polarization direction in the second direction; and a third polarization direction mixed layer disposed between the third polarization layer and the fourth polarization layer, the third polarization direction mixed layer including a first portion having a polarization direction in the first direction and a second portion having a polarization direction in the second direction.

10. The elastic wave resonator according to claim 9, wherein the total thickness of the first polarization layer and half the thickness of the first polarization direction mixed layer in contact with the first polarization layer is equal to the total thickness of the fourth polarization layer and half the thickness of the third polarization direction mixed layer in contact with the fourth polarization layer.

11. The elastic wave resonator according to claim 9, wherein the total thickness of the second polarization layer, half the thickness of the first polarization direction mixed layer in contact with the second polarization layer, and half the thickness of the second polarization direction mixed layer in contact with the second polarization layer is equal to the total thickness of the third polarization layer, half the thickness of the second polarization direction mixed layer in contact with the third polarization layer, and half the thickness of the third polarization direction mixed layer in contact with the third polarization layer.

12. The elastic wave resonator according to claim 9, wherein the piezoelectric layer comprises: a fifth polarization layer provided between the fourth polarization layer and the main surface of the support member, the fifth polarization layer having a polarization direction in the first direction; and a fourth polarization direction mixed layer disposed between the fourth polarization layer and the fifth polarization layer, the fourth polarization direction mixed layer including a first portion having a polarization direction in the first direction and a second portion having a polarization direction in the second direction.

13. The elastic wave resonator according to claim 12, wherein the total thickness of the first polarization layer and half the thickness of the first polarization direction mixed layer in contact with the first polarization layer is equal to the total thickness of the fifth polarization layer and half the thickness of the fourth polarization direction mixed layer in contact with the fifth polarization layer.

14. The elastic wave resonator according to claim 12, wherein the total thickness of the second polarization layer, half the thickness of the first polarization direction mixed layer in contact with the second polarization layer, and half the thickness of the second polarization direction mixed layer in contact with the second polarization layer is equal to the total thickness of the third polarization layer, half the thickness of the second polarization direction mixed layer in contact with the third polarization layer, and half the thickness of the third polarization direction mixed layer in contact with the third polarization layer, and the total thickness of the fourth polarization layer, half the thickness of the third polarization direction mixed layer in contact with the fourth polarization layer, and half the thickness of the fourth polarization direction mixed layer in contact with the fourth polarization layer.

15. The elastic wave resonator according to any one of claims 1 to 14, wherein the polarization direction intersects with the thickness direction of the piezoelectric layer.

16. An acoustic wave filter comprising an acoustic wave resonator according to any one of claims 1 to 15.

17. A front-end module comprising the acoustic wave filter according to claim 16.

18. A communication device comprising the front-end module according to claim 17.

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