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

Abstract

This acoustic wave resonator comprises a support member provided with a support substrate, a piezoelectric layer provided on a main surface of the support member, and a plurality of functional electrodes provided respectively on opposing main surfaces of the piezoelectric layer, wherein: the piezoelectric layer includes a first piezoelectric body layer that has a polarization direction in a first direction, a second piezoelectric body layer that is provided between the first piezoelectric body layer and the main surface of the support member and that has a polarization direction in a second direction different from the first direction, and a low-density layer that is disposed between the first piezoelectric body layer and the second piezoelectric body layer and that has a density lower than the density of the first piezoelectric body layer and the density of the second piezoelectric body layer; the first piezoelectric body layer, the second piezoelectric body layer, and the low-density layer are single crystals; and the low-density layer includes, in a plan view, a first part and a second part having mutually different densities.

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 aspect 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 includes a first piezoelectric layer having a polarization direction in a first direction, a second piezoelectric layer provided between the first piezoelectric layer and the main surface of the support member and having a polarization direction in a second direction different from the first direction, and a low-density layer disposed between the first piezoelectric layer and the second piezoelectric layer and having a density lower than that of the first piezoelectric layer and the second piezoelectric layer, wherein the first piezoelectric layer, the second piezoelectric layer, and the low-density layer are single crystals, and the low-density layer includes a first portion and a second portion having densities different from each other in a planar view.

[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 the first embodiment. FIG. 2 is a cross-sectional view taken along line II-II' of FIG. 1. FIG. 3 is an explanatory diagram illustrating the relationship between the piezoelectric layer and the stress distribution of bulk waves. FIG. 4 is a diagram showing a cross-sectional TEM image of the piezoelectric layer according to the first example. FIG. 5 is a graph showing the relationship between the density ratio of the low-density layer according to the first example and the bonding strength. FIG. 6 is a table showing the relationship between the average density ratio and the thickness of the low-density layer of the elastic wave resonator according to the first example and the presence or absence of an unwanted wave reduction effect. FIG. 7 is a table showing the relationship between the in-plane density ratio of the low-density layer and the presence or absence of an unwanted wave reduction effect of the elastic wave resonator according to the first example. FIG. 8 is a table showing the density distribution in the planar direction of the low-density layer according to the first example and the first comparative example. FIG. 9 is a graph showing the relationship between the density ratio and the bonding strength of the low-density layer according to the second example. FIG. 10 is a table showing the relationship between the average density ratio and the thickness of the low-density layer of the elastic wave resonator according to the second example and the presence or absence of an unwanted wave reduction effect. FIG. 11 is a table showing the relationship between the in-plane density ratio of the low-density layer of the elastic wave resonator according to the second example and the presence or absence of the spurious wave reduction effect. FIG. 12 is a table showing the density distribution in the planar direction of the low-density layer according to the second example and the second comparative example. FIG. 13 is an explanatory diagram for explaining a method for manufacturing the elastic wave resonator according to the first example. FIG. 14 is a table showing the relationship between the surface roughness before bonding, the heating temperature after bonding, and the average density of the low-density layer of the piezoelectric layer according to the first example. FIG. 15 is a table showing the relationship between the surface roughness before bonding, the heating temperature after bonding, and the average density of the low-density layer of the piezoelectric layer according to the second example. FIG. 16 is a diagram showing the configuration of a communication device according to the second example. FIG. 17 is a cross-sectional view showing an elastic wave resonator according to a modified example.

[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 piezoelectric layer 21, a second piezoelectric layer 22 provided between the first piezoelectric layer 21 and the main surface of the support member 13, and a low-density layer 23 disposed between the first piezoelectric layer 21 and the second piezoelectric layer 22. That is, the piezoelectric layer 20 is configured by stacking the second piezoelectric layer 22, the low-density layer 23, and the first piezoelectric layer 21 in this order on the support member 13. The first piezoelectric layer 21, the low-density layer 23, and the second piezoelectric layer 22 are single crystals made of the same material. In this disclosure, being made of the same material refers to being made of a substance represented by the same empirical formula. The detailed configurations of the first piezoelectric layer 21, the low-density layer 23, and the second piezoelectric layer 22 will be described later with reference to FIG. 3 and subsequent figures.

[0020] In the present disclosure, the first piezoelectric layer 21, the low-density layer 23, and the second piezoelectric layer 22 constituting the piezoelectric layer 20 are "single crystal" meaning that all crystal orientations can be identified in each layer. In other words, if the first piezoelectric layer 21, the low-density layer 23, and the second piezoelectric 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 20 can be identified can be determined by measuring the cross section of each layer of the piezoelectric layer 20 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 with 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), molybdenum (Mo), or ruthenium (Ru), 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] 17 is a cross-sectional view showing an elastic wave resonator according to a modified example. As shown in FIG. 17, 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 ２ The 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 configuration of the first piezoelectric layer 21, the low-density layer 23, and the second piezoelectric layer 22 of the piezoelectric layer 20 will be described with reference to Fig. 3. Fig. 3 is an explanatory diagram for explaining the relationship between the piezoelectric layer and the stress distribution of the bulk wave.

[0028] 3, arrow P1 indicates the polarization direction of the first piezoelectric layer 21. Arrow P2 indicates the polarization direction of the second piezoelectric layer 22. As shown in FIG. 3, the first piezoelectric 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 of the first piezoelectric layer 21 (arrow P1) is a first direction D1. The second piezoelectric 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 of the second piezoelectric layer 22 (arrow P2) is a second direction D2 that is different from the first direction D1.

[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 piezoelectric layer 21, the low-density layer 23, and the second piezoelectric layer 22 intersect with the thickness direction (Z direction) of the piezoelectric layer 20.

[0030] Furthermore, the polarization directions of the first piezoelectric layer 21 and the second piezoelectric 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-direction and the Y-direction). The Z-direction component of the polarization direction (first direction D1) of the first piezoelectric layer 21 is opposite to the Z-direction component of the polarization direction (second direction D2) of the second piezoelectric layer 22. The horizontal direction component of the polarization direction (first direction D1) of the first piezoelectric layer 21 is opposite to the horizontal direction component of the polarization direction (second direction D2) of the second piezoelectric layer 22.

[0031] The low-density layer 23 is located between the first piezoelectric layer 21 and the second piezoelectric layer 22. The low-density layer 23 is made of the same material as at least one of the first piezoelectric layer 21 and the second piezoelectric layer 22, and is a single crystal having the same crystal structure and the same crystal orientation. The low-density layer 23 has a density lower than that of the first piezoelectric layer 21 and the second piezoelectric layer 22. The low-density layer 23 also has a density distribution in a planar view. In other words, the low-density layer 23 includes a first portion and a second portion having different densities in a planar view. For example, the first portion is a region having a relatively high density, and the second portion is a region having a relatively low density. However, both the first portion and the second portion have a lower density than the first piezoelectric layer 21 and the second piezoelectric layer 22.

[0032] 3, the polarization direction of the low-density layer 23 is not shown, but the polarization direction of the low-density layer 23 is the same as at least one of the polarization directions of the first piezoelectric layer 21 and the second piezoelectric layer 22. Alternatively, the low-density layer 23 may include a portion whose polarization direction is the first direction D1 and a portion whose polarization direction is the second direction D2.

[0033] 3 , the thickness t1 of the first piezoelectric layer 21 is equal to the thickness t2 of the second piezoelectric layer 22. The thickness t3 of the low-density layer 23 is thinner than the thickness t1 of the first piezoelectric layer 21 and the thickness t2 of the second piezoelectric layer 22. For example, the thickness t1 of the first piezoelectric layer 21 and the thickness t2 of the second piezoelectric layer 22 are each approximately 50 nm or more and 600 nm or less. The thickness t3 of the low-density layer 23 is approximately 0.2 nm or more and 30 nm or less. More preferably, the thickness t3 of the low-density layer 23 is approximately 0.2 nm or more and 20 nm or less.

[0034] 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%.

[0035] Furthermore, if the wavelength in the piezoelectric layer 20 of the high frequency signal transmitted between the upper electrode 31 and the lower electrode 32 is λ, the thickness t1 of the first piezoelectric layer 21 is λ / 2, and the thickness t2 of the second piezoelectric layer 22 is λ / 2. The thickness t3 of the low-density layer 23 is sufficiently thinner than λ / 2.

[0036] With the above-described configuration, the main bulk wave resonates at a half-wavelength in each of the first piezoelectric layer 21 and the second piezoelectric 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 piezoelectric layer 21 and the center in the Z direction of the second piezoelectric 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 low-density layer 23. As a result, even if the piezoelectric layer 20 has a configuration including the low-density layer 23, energy loss of the main bulk wave is suppressed.

[0037] Furthermore, for elastic waves, the larger the electromechanical coupling coefficient of the mode, the smaller the impact of energy loss in the low-density layer 23, and the smaller the electromechanical coupling coefficient of the mode, the larger the impact of energy loss in the low-density layer 23. Therefore, among the unnecessary waves different from the main bulk wave, the harmonics experience larger energy loss in the low-density layer 23 than the main bulk wave, and are suppressed.

[0038] On the other hand, the position in the Z direction where the stress is minimum for spurious waves different from the main bulk wave is shifted from the low-density layer 23. In other words, the stress generated by the spurious waves in the low-density layer 23 is greater than the stress generated by the main bulk wave. The low-density layer 23 has a density distribution in a planar view. Therefore, many spurious waves passing through different regions of the low-density layer 23 interfere with each other and are suppressed. Therefore, the elastic wave resonator 10 of this embodiment can reduce spurious waves while suppressing deterioration of the main bulk wave, compared to a configuration without the low-density layer 23. This allows the elastic wave resonator 10 to have improved resonator characteristics.

[0039] Here, examples of unwanted waves that are different from the main bulk waves include waves of a mode that originates from the upper surface of the upper electrode 31 or the lower surface of the lower electrode 32, and waves of a mode that originates from the edge of the membrane portion of the piezoelectric layer 20.

[0040] 4 to 7, the detailed configuration of the low-density layer 23 in the elastic wave resonator 10 according to the first embodiment will be described. In the elastic wave resonator 10 according to the first embodiment, the first piezoelectric layer 21, the second piezoelectric layer 22, and the low-density layer 23 are made of the same lithium niobate (LiNbO ３ ) single crystal.

[0041] Fig. 4 shows cross-sectional TEM photographs of the piezoelectric layer according to Example 1. The upper image in Fig. 4 is a TEM-DF (dark field) image of the piezoelectric layer 20, and the lower image in Fig. 4 is a TEM-BF (bright field) image showing an enlarged view of the interface portion between the first piezoelectric layer 21, the second piezoelectric layer 22, and the low-density layer 23 in the upper image in Fig. 4.

[0042] 4, in each of the TEM-DF image and the TEM-BF image, the low-density layer 23 is represented in a tone closer to black than the first piezoelectric layer 21 and the second piezoelectric layer 22. Here, in the cross-sectional TEM photograph, the higher the density, the whiter it appears, and the lower the density, the blacker it appears. This shows that the density of the low-density layer 23 is lower than the density of the first piezoelectric layer 21 and the density of the second piezoelectric layer 22.

[0043] Although not shown in the figure, the density distribution of the low-density layer 23 in a planar view (the first and second portions having different densities (see Figure 8)) can be confirmed by taking multiple cross-sectional TEM photographs shown in Figure 4 in different regions.

[0044] Figure 5 is a graph showing the relationship between the density ratio of the low-density layer and bonding strength in Example 1. The horizontal axis of graph 1 shown in Figure 5 represents the ratio of the average density of the low-density layer 23 to the density of the first piezoelectric layer 21 and the second piezoelectric layer 22. The average density of the low-density layer 23 represents the average density of the entire layer, including the first and second portions, which have different densities in a planar view. The vertical axis of graph 1 represents the bonding strength of the piezoelectric layer 20. The bonding strength was measured using a blade crack method, measuring the strength at which the first piezoelectric layer 21 and the second piezoelectric layer 22 crack and peel at the low-density layer 23, which is the boundary between them.

[0045] As shown in Fig. 5, the piezoelectric layer 20 of the first embodiment shows a tendency for the bonding strength to gradually decrease as the density ratio of the low-density layer 23 decreases. A significant decrease in bonding strength was observed when the ratio of the average density of the low-density layer 23 to the density of the first piezoelectric layer 21 and the second piezoelectric layer 22 increased from 36% to 34%. In addition, generally, a bonding strength of 0.5 J / m ２ It is known that the presence of the following layers results in insufficient resistance to mounting reflow, etc., and increases the defect rate. Taking this into consideration, in the first embodiment, it is preferable that the ratio of the average density of the low-density layer 23 to the densities of the first piezoelectric layer 21 and the second piezoelectric layer 22 is 36% or more.

[0046] Fig. 6 is a table showing the relationship between the ratio of the average density of the low-density layer, the thickness of the low-density layer, and whether or not the spurious wave reduction effect is achieved in the elastic wave resonator according to the first example. Table 1 shown in Fig. 6 shows the results of evaluating the resonance characteristics of evaluation samples fabricated with various different average densities and thicknesses of the low-density layer 23. In Table 1, "◯" indicates a sample in which the spurious wave reduction effect was achieved, and "X" indicates a sample in which the spurious wave reduction effect was not achieved. Note that a manufacturing method for low-density layers 23 with different average densities and thicknesses will be described later with reference to Figs. 13 to 15.

[0047] 6 , when the thickness of the low-density layer 23 is 0.1 nm and 0.15 nm, no spurious wave reduction effect is observed even when the average density of the low-density layer 23 is reduced to 85% of the density of the first piezoelectric layer 21 and the second piezoelectric layer 22. When the thickness of the low-density layer 23 is in the range of 0.2 nm to 5 nm, an spurious wave reduction effect is observed by setting the average density of the low-density layer 23 to 95% or less of the density of the first piezoelectric layer 21 and the second piezoelectric layer 22. Furthermore, when the average density ratio of the low-density layer 23 is 98%, no spurious wave reduction effect is observed in the region where the low-density layer 23 is 5 nm or less in thickness.

[0048] From the above results, in the first example, it is preferable that the thickness of the low-density layer 23 is 0.2 nm or more. In addition, it is preferable that the average density of the low-density layer 23 is 95% or less of the density of the first piezoelectric layer 21 and the second piezoelectric layer 22.

[0049] However, when an elastic wave resonator 10 was fabricated with the low-density layer 23 having a thickness of approximately 30 nm and the characteristics were measured, no degradation of the resonance characteristics was observed. This indicates that by setting the thickness of the low-density layer 23 in the range of 0.2 nm to 30 nm, it is possible to obtain the effect of reducing unwanted waves and suppress the degradation of the resonance characteristics of the main wave.

[0050] Fig. 7 is a table showing the relationship between the in-plane density ratio of the low-density layer and the presence or absence of the spurious wave reduction effect in the elastic wave resonator according to Example 1. Fig. 8 is a table showing the density distribution in the planar direction of the low-density layer according to Example 1 and Comparative Example 1.

[0051] 7 shows the relationship between the density ratio within the plane of the low-density layer 23 and the presence or absence of the spurious wave reduction effect when the average density of the low-density layer 23 relative to the densities of the first piezoelectric layer 21 and the second piezoelectric layer 22 is 95% and the thickness of the low-density layer 23 is 2 nm. The density ratio within the plane of the low-density layer 23 indicates the ratio between the density of the sparsest part and the density of the densest part in the density distribution within the plane of the low-density layer 23 ("lowest density in the second part of the low-density layer 23" / "highest density in the first part of the low-density layer 23").

[0052] As shown in Figure 7, when the in-plane density ratio of the low-density layer 23 is 97%, no unwanted wave reduction effect is observed. In contrast, when the in-plane density ratio of the low-density layer 23 is 94%, an unwanted wave reduction effect is observed. This is because the resonance state of unwanted waves adjacent in the planar direction differs depending on the in-plane density ratio of the low-density layer 23, and the mutual resonance state is inhibited. This suppresses the unwanted waves. Furthermore, the larger the in-plane density ratio of the low-density layer 23, the more the resonance state of adjacent unwanted waves is inhibited, and the greater the unwanted wave reduction effect. Therefore, it was shown that the unwanted wave reduction effect can be ensured by setting the in-plane density ratio of the low-density layer 23 to 94% or less.

[0053] Table 3 shown in Figure 8 shows the density distribution in the planar direction when the in-plane density ratio of the low-density layer 23 in Figure 7 is 94% (first embodiment) and when the in-plane density ratio is 97% (first comparative example).

[0054] The density distribution in the planar direction of the low-density layer 23 shown in Figure 8 was obtained by scraping the piezoelectric layer 20 so that the low-density layer 23 was exposed to the surface and then smoothing the surface. Subsequently, after irradiating the piezoelectric layer 20 with an Ar ion beam and cleaning the surface with a chemical solution in a vacuum, a uniform, ultrathin aluminum (Al) thin film was formed. The surface was measured in both the X and Y directions at a predetermined pitch using Auger electron spectroscopy (AES). In Figure 8, for example, measurements were taken over a 5-µm range at a 0.1 µm pitch in each of the X and Y directions. The relative density distribution in the planar direction was calculated from the ratio of aluminum (Al) to niobium (Nb). Figure 8 also shows values ​​normalized to the highest density point, which is 100%.

[0055] As shown in FIG. 8 , the low-density layer 23 of the first embodiment has a region having a relatively high density in the planar direction (hereinafter referred to as the first portion) and a region having a relatively low density (hereinafter referred to as the second portion) that are repeatedly arranged at a predetermined interval. The first and second portions are periodically and repeatedly arranged in both the X and Y directions. The relative density of the first portion is 100%, and the relative density of the second portion is 94%. That is, in the low-density layer 23 of the first embodiment, the ratio of the highest density in the first portion to the lowest density in the second portion is 94% or less. Furthermore, a region having a relative density of 95% to 99% is arranged between the first and second portions.

[0056] Similarly, the low-density layer 23 of the first comparative example has first and second portions repeatedly arranged at a predetermined interval in the planar direction. The relative density of the first portion is 100%, and the relative density of the second portion is 97%. Furthermore, a region having a relative density of 98% to 99% is arranged between the first and second portions.

[0057] As described above, in the first embodiment, the ratio of the average density of the low-density layer 23 to the densities of the first piezoelectric layer 21 and the second piezoelectric layer 22 is preferably 36% to 95%. The thickness of the low-density layer 23 is preferably 0.2 nm to 30 nm. The in-plane density ratio of the low-density layer 23 (the ratio between the highest density in the first portion and the lowest density in the second portion) is preferably 94% or less. This enables the acoustic wave resonator 10 according to the first embodiment to achieve an unwanted wave reduction effect while suppressing deterioration of the resonance characteristics of the main wave.

[0058] Second Example Next, a detailed configuration of the low-density layer 23 in an elastic wave resonator 10 according to a second example will be described with reference to Figs. 9 to 12. Unlike the first example described above, the elastic wave resonator 10 according to the second example has a first piezoelectric layer 21, a second piezoelectric layer 22, and a low-density layer 23 of the piezoelectric layer 20 made of the same lithium tantalate (LiTaO ３ ) single crystal.

[0059] Fig. 9 is a graph showing the relationship between the density ratio of the low-density layer and bonding strength according to Example 2. Fig. 10 is a table showing the relationship between the average density ratio and thickness of the low-density layer of the elastic wave resonator according to Example 2 and the presence or absence of an unwanted wave reduction effect. Fig. 11 is a table showing the relationship between the in-plane density ratio of the low-density layer of the elastic wave resonator according to Example 2 and the presence or absence of an unwanted wave reduction effect. Fig. 12 is a table showing the density distribution in the planar direction of the low-density layer according to Example 2 and Comparative Example 2.

[0060] 9 to 12, the elastic wave resonator 10 according to the second preferred embodiment exhibits the same results as the first preferred embodiment shown in FIGS. 5 to 8. That is, the piezoelectric layer 20 is made of lithium tantalate (LiTaO ３ In a configuration formed of a single crystal made of a piezoelectric material, the ratio of the average density of the low-density layer 23 to the densities of the first piezoelectric layer 21 and the second piezoelectric layer 22 is preferably 36% to 95%. The thickness of the low-density layer 23 is preferably 0.2 nm to 30 nm. The in-plane density ratio of the low-density layer 23 (the ratio of the highest density in the first portion to the lowest density in the second portion) is preferably 94% or less. As a result, the elastic wave resonator 10 according to the second example can achieve the effect of reducing unwanted waves while suppressing deterioration of the resonance characteristics of the main wave.

[0061] In the above-described embodiments and examples, the number of stacked piezoelectric layers and low-density layers, thicknesses, etc. are merely examples and can be changed as appropriate. For example, three or more piezoelectric layers may be stacked, with low-density layers provided between each piezoelectric layer.

[0062] 13 is an explanatory diagram illustrating a method for manufacturing an elastic wave resonator according to the first embodiment. As shown in FIG. 13 , the method for manufacturing elastic wave resonator 10 according to the present embodiment includes a first wafer bonding step (step S11), a first wafer thinning step (step S12), a lower electrode forming step (step S13), a sacrificial layer forming step (step S14), an intermediate layer forming step (step S15), a bonding step (step S16), a second wafer bonding step (step S17), a second wafer thinning step (step S18), an upper electrode forming step (step S19), a piezoelectric layer window opening step (step S20), and a space forming step (step S21).

[0063] In the first wafer bonding step (step S11), a first wafer 21S, which is the first piezoelectric layer 21 before processing, is bonded to the main surface of the transfer substrate WT.

[0064] In the first wafer thinning step (step S12), the first wafer 21S is ground to thin it, thereby forming the first piezoelectric layer 21. The second main surface 21b of the first piezoelectric layer 21 is smoothed by polishing. In this smoothing process, the surface of the piezoelectric layer is formed into a smooth surface having continuous regions with a surface roughness Rmax within a 1 μm × 1 μm area of ​​0.1 nm to 5 nm, and the size of adjacent irregularities within a 0.2 μm width is within the range of 0.1 nm to 5 nm.

[0065] In the lower electrode formation process (step S13), the lower electrode 32 is patterned by a method such as lift-off on the second main surface 21b of the first piezoelectric layer 21. At this time, a metal film such as a routing wiring (not shown) is also formed on the second main surface 21b of the first piezoelectric layer 21.

[0066] In the sacrificial layer formation step (step S14), a sacrificial layer 14S is formed on the second main surface 21b of the first piezoelectric layer 21 so as to cover a part of the lower electrode 32. The sacrificial layer 14S is made of, for example, zinc oxide.

[0067] In the intermediate layer formation process (step S15), an insulator is formed on the second main surface 21b of the first piezoelectric layer 21 to cover the lower electrode 32 and the sacrificial layer 14S, and the surface opposite the first piezoelectric layer 21 is then flattened and smoothed to form an intermediate layer 12 made of the insulator.

[0068] In the bonding step (step S16), the first piezoelectric layer 21, the lower electrode 32, etc. are bonded to the support member 13 via the intermediate layer 12. Thereafter, the transfer substrate WT is removed from the first piezoelectric layer 21.

[0069] In the second wafer bonding process (step S17), a second wafer 22S, which is the second piezoelectric layer 22 before processing, is bonded to the first main surface 21a of the first piezoelectric layer 21. The second wafer 22S is a piezoelectric material having a different polarization direction from that of the first piezoelectric layer 21. At this time, the surface of the second wafer 22S before bonding is subjected to a process similar to the smoothing process for the first piezoelectric layer 21. That is, the surface of the second piezoelectric layer 22 is formed into a smooth surface having continuous regions with smoothness such that the surface roughness Rmax within a 1 μm × 1 μm area is 0.1 nm to 5 nm and the size of adjacent irregularities within a 0.2 μm width is within the range of 0.1 nm to 5 nm.

[0070] Furthermore, a hydrophilic bonding method or a surface activation bonding method is used as a bonding method for the second wafer 22S, thereby directly bonding the first piezoelectric layer 21 and the second piezoelectric layer 22. When the first piezoelectric layer 21 and the second piezoelectric layer 22 in the above-described bonded surface state are bonded by the hydrophilic bonding method or the surface activation bonding method and then heated, a low-density layer 23 made of the same material and having the same crystal structure as the first piezoelectric layer 21 and the second piezoelectric layer 22 is formed at the bonding interface between the first piezoelectric layer 21 and the second piezoelectric layer 22 (a region near the interface where the polarization directions are different).

[0071] Fig. 14 is a table showing the relationship between the surface roughness before bonding, the heating temperature after bonding, and the average density of the low-density layer of the piezoelectric layer according to Example 1. Fig. 15 is a table showing the relationship between the surface roughness before bonding, the heating temperature after bonding, and the average density of the low-density layer of the piezoelectric layer according to Example 2.

[0072] 14 and Table 7 shown in Fig. 15, hatching is shown in the region where the average density of the low-density layer 23 is 0.34 or less, i.e., the region below the boundary value where the bonding strength between the first piezoelectric layer 21 and the second piezoelectric layer 22 drops significantly. Hatching is not shown in the region where the average density of the low-density layer 23 is 0.36 or more, i.e., the region above the boundary value where the bonding strength can be ensured.

[0073] 14 and 15, the surface roughness Ra before hydrophilic bonding was measured using an atomic force microscope (AFM). More specifically, the measurement was performed under the following conditions using an L-Trace II device manufactured by Hitachi High-Tech Science Corporation.

[0074] Main setting parameters Cantilever material: Si Scan area: 10 x 10 μm Scan frequency: 0.6 Hz Number of X and Y data: 1024 x 1024

[0075] 14 and 15 , in both the first and second examples, decreasing the surface roughness Ra before hydrophilic bonding increases the density of the low-density layer 23. In addition, increasing the heating temperature after hydrophilic bonding increases the density of the low-density layer 23.

[0076] More specifically, the low-density layer 23 has a density ratio near the boundary value when the surface roughness Ra of the piezoelectric layer (first piezoelectric layer 21 and second piezoelectric layer 22) before hydrophilic bonding is in the range of 0.9 nm to 1.1 nm and the heating temperature after bonding is in the range of 150° C. to 175° C. Furthermore, the low-density layer 23 has a density ratio near the boundary value when the surface roughness Ra of the piezoelectric layer before hydrophilic bonding is in the range of 1.4 nm to 1.5 nm and the heating temperature after bonding is in the range of 175° C. to 250° C.

[0077] More specifically, in the above-mentioned step S12, the first piezoelectric layer 21 is formed into a surface that repeatedly develops such that the surface roughness Ra is 0.075 nm or more and 1.5 nm or less, and the average spacing (≒ 1 / 2 period) between adjacent irregularities (Peak-Valley) is in the range of 10 nm or more and 100 nm or less.

[0078] In the second wafer bonding process (step S17), the surface of the second piezoelectric layer 22 is formed into a surface in which a surface roughness Ra within a 1 μm × 1 μm area is 0.075 nm to 1.5 nm, and the average spacing (≒ 1 / 2 period) between adjacent peak-valley irregularities is 10 nm to 100 nm, and the surface is repeatedly developed. The first piezoelectric layer 21 and the second piezoelectric layer 22 are directly bonded using hydrophilic bonding or surface activation bonding. Furthermore, by setting the heating temperature after bonding to 125°C to 350°C, a low-density layer 23 is formed at the bonding interface, which is made of the same material and has the same crystalline structure as the first piezoelectric layer 21 and the second piezoelectric layer 22, and has low density and unevenness in the planar direction.

[0079] Here, by manufacturing the low-density layer 23 with the above-mentioned surface roughness and heating temperature after bonding, it is possible to form the low-density layer 23 with a thickness of approximately 0.2 nm or more and 30 nm or less, an average density of approximately 30% or more and 98% or less of that of the first piezoelectric layer 21 and the second piezoelectric layer 22, and an in-plane density ratio (lowest density in the second part / highest density in the first part) of approximately 80% or more and 98% or less.

[0080] The thickness of the low-density layer 23 can be increased by increasing the surface roughness Ra of the first piezoelectric layer 21 and the second piezoelectric layer 22 before bonding and lowering the heating temperature after hydrophilic bonding. Furthermore, the average density of the low-density layer 23 can be reduced relative to the densities of the first piezoelectric layer 21 and the second piezoelectric layer 22 by increasing the surface roughness of the first piezoelectric layer 21 and the second piezoelectric layer 22 before bonding and lowering the heating temperature after hydrophilic bonding. To increase the in-plane density difference of the low-density layer 23, it is advisable to increase the surface roughness of the first piezoelectric layer 21 and the second piezoelectric layer 22 before bonding while increasing the Peak-Valley spacing of the surface before bonding, and further increase the heating temperature after hydrophilic bonding.

[0081] As described above, as shown in Figures 14 and 15, in steps S12 and S17, by appropriately adjusting the surface roughness Ra of the first piezoelectric layer 21 and the second piezoelectric layer 22 before hydrophilic bonding and the heating temperature after bonding, a low-density layer 23 having the desired density, thickness, and in-plane density ratio is formed.

[0082] 14 and 15 are merely examples and can be changed as appropriate. Conditions outside the ranges of surface roughness Ra and heating temperature shown in FIGS. 14 and 15 are also acceptable.

[0083] 13 , in the second wafer thinning step (step S18), the second wafer 22S is ground to thin it. This forms the second piezoelectric layer 22. The third main surface 22a of the second piezoelectric layer 22 is smoothed by polishing. In this embodiment, the second piezoelectric layer 22 is ground to have the same thickness as the first piezoelectric layer 21.

[0084] In the upper electrode formation process (step S19), the upper electrode 31 is formed on the third main surface 22a of the second piezoelectric layer 22. In this embodiment, the upper electrode 31 is formed by, for example, lift-off. At this time, a metal film such as wiring may be simultaneously formed on the third main surface 22a of the second piezoelectric layer 22. Furthermore, after the upper electrode 31 is formed, a layer made of an insulator such as a protective film may be formed.

[0085] In the piezoelectric layer window forming step (step S20), a window is formed in the piezoelectric layer 20. In this embodiment, a resist pattern is formed and dry etching is performed to form the open window 20W and a through-hole (not shown).

[0086] In the space forming step (step S21), etching gas or etching liquid is introduced into a through-hole (not shown) to remove the sacrificial layer 14S, thereby forming the recess 14.

[0087] After the above steps, the support member 13 is singulated. In the example of FIG. 13 , the region between the dashed dotted line E1 and the dashed dotted line E2, which does not have the functional electrodes (upper electrode 31 and lower electrode 32), is removed and singulated. This allows the elastic wave resonator 10 according to this embodiment to be manufactured. Note that the above-described steps are merely shown schematically and can be modified as appropriate.

[0088] 16 is a diagram showing the configuration of a communication device according to a second embodiment. The communication device 100 according to the second 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. 16, 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 the above-described acoustic wave resonator 10. 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 comprises: a first piezoelectric layer having a polarization direction in a first direction, a second piezoelectric layer provided between the first piezoelectric layer and the main surface of the support member and having a polarization direction in a second direction different from the first direction, and a low-density layer disposed between the first piezoelectric layer and the second piezoelectric layer and having a density lower than that of the first piezoelectric layer and the second piezoelectric layer, wherein the first piezoelectric layer, the second piezoelectric layer, and the low-density layer are single crystals, and the low-density layer includes a first portion and a second portion having densities different from each other in a planar view. (2) The elastic wave resonator according to (1), wherein the low-density layer is made of the same material as at least one of the first piezoelectric layer and the second piezoelectric layer and is a single crystal having the same crystal structure and the same crystal orientation. (3) The elastic wave resonator according to (1) or (2), wherein the density of the low-density layer is 36% or more of the density of the first piezoelectric layer and the density of the second piezoelectric layer. (4) The elastic wave resonator according to any one of (1) to (3), wherein the density of the low-density layer is 95% or less of the density of the first piezoelectric layer and the density of the second piezoelectric layer. (5) The elastic wave resonator according to any one of (1) to (4), wherein the thickness of the low-density layer is 0.2 nm or more and 30 nm or less. (6) The elastic wave resonator according to any one of (1) to (5), wherein in the low-density layer, the first portion is a region having a relatively high density and the second portion is a region having a relatively low density, and wherein a ratio of the highest density in the first portion to the lowest density in the second portion is 94% or less. (7) The elastic wave resonator according to any one of (1) to (6), wherein the polarization direction intersects with the thickness direction of the piezoelectric layer. (8) An acoustic wave filter including the acoustic wave resonator according to any one of (1) to (7). (9) A front-end module including the acoustic wave filter according to (8). (10) A communication device including the front-end module according to (9).

[0103] REFERENCE SIGNS LIST 10 Acoustic wave resonator 11 Support substrate 12 Intermediate layer 13 Support member 14 Recess 20 Piezoelectric layer 20a First main surface 20b Second main surface 21 First piezoelectric layer 22 Second piezoelectric layer 23 Low-density layer 31 Upper electrode 32 Lower electrode 100 Communication device 101 Front-end module 102 Antenna 113a, 113b Transmitting filter 117a, 117b Receiving filter 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 comprises: a first piezoelectric layer having a polarization direction in a first direction; a second piezoelectric layer provided between the first piezoelectric layer and the main surface of the support member, the second piezoelectric layer having a polarization direction in a second direction different from the first direction; and a low-density layer disposed between the first piezoelectric layer and the second piezoelectric layer, the low-density layer having a density lower than that of the first piezoelectric layer and the second piezoelectric layer, wherein the first piezoelectric layer, the second piezoelectric layer, and the low-density layer are single crystals, and the low-density layer includes a first portion and a second portion having densities different from each other in a planar view.

2. The elastic wave resonator according to claim 1, wherein the low-density layer is made of the same material as at least one of the first piezoelectric layer and the second piezoelectric layer, and is a single crystal having the same crystal structure and crystal orientation as the first piezoelectric layer and the second piezoelectric layer.

3. An elastic wave resonator according to claim 1 or 2, wherein the density of the low-density layer is 36% or more of the density of the first piezoelectric layer and the density of the second piezoelectric layer.

4. An elastic wave resonator according to any one of claims 1 to 3, wherein the density of the low-density layer is 95% or less of the density of the first piezoelectric layer and the density of the second piezoelectric layer.

5. An elastic wave resonator according to any one of claims 1 to 4, wherein the low-density layer has a thickness of 0.2 nm to 30 nm.

6. An elastic wave resonator according to any one of claims 1 to 5, wherein in the low-density layer, the first portion is a region having a relatively high density, the second portion is a region having a relatively low density, and the ratio of the highest density in the first portion to the lowest density in the second portion is 94% or less.

7. The elastic wave resonator according to any one of claims 1 to 6, wherein the polarization direction intersects with the thickness direction of the piezoelectric layer.

8. An elastic wave filter comprising an elastic wave resonator according to any one of claims 1 to 7.

9. A front-end module comprising the acoustic wave filter according to claim 8.

10. A communication device comprising the front-end module according to claim 9.

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