Elastic wave device

Abstract

This invention provides an elastic wave device that is less prone to generating high-order modes. The elastic wave device (1) has a piezoelectric film (3) stacked on an energy sealing layer. An IDT electrode (4) and a first dielectric film (7) are stacked on the first main surface (3a) of the piezoelectric film (3), and a second IDT electrode (5) and a second dielectric film (8) are stacked on the second main surface (3b). When the product of the density and thickness of the second dielectric film (8) on the second main surface (3b) side is larger than that of the first dielectric film (7) on the first main surface (3a) side, the sum of the product of the density and thickness of the electrode fingers of the first IDT electrode (4) covered by the first dielectric film (7) is set to be smaller than the sum of the product of the density and thickness of the electrode fingers of the second IDT electrode (5).

Description

Technical Field

[0001] This invention relates to an elastic wave device having IDT electrodes disposed on both sides of a piezoelectric film. Background Technology

[0002] Conventional elastic wave devices with IDT electrodes disposed on both sides of a piezoelectric film are known. For example, in the elastic wave device described in Patent Document 1 below, a waveguide enclosure structure based on a high-speed wave propagation layer, a low-speed wave propagation layer, and a piezoelectric film are sequentially stacked on a support substrate. Furthermore, a low-speed wave propagation layer and a waveguide enclosure structure are stacked on the piezoelectric film. The high-speed acoustic component comprises a high-speed acoustic material in which the speed of sound is higher than the speed of sound propagating in the piezoelectric film. In addition, the first IDT electrode and the second IDT electrode are arranged to face each other with the piezoelectric film in between. However, the low-speed wave propagation layer and the waveguide enclosure structure stacked on the piezoelectric film can also be omitted.

[0003] Prior art literature

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Publication No. 2018-506930 Summary of the Invention

[0006] The problem the invention aims to solve

[0007] In conventional elastic wave devices as described in Patent Document 1, stray noise caused by higher-order modes is sometimes generated.

[0008] The purpose of this invention is to provide an elastic wave device that is less prone to generating high-order modes.

[0009] Technical solutions for solving the problem

[0010] The elastic wave device of the first invention of this application comprises: a piezoelectric film having a first main surface and a second main surface facing each other; and an IDT electrode disposed in contact with the piezoelectric film, the IDT electrode having: a first IDT electrode disposed on the first main surface of the piezoelectric film; and a second IDT electrode disposed on the second main surface of the piezoelectric film. The elastic wave device further comprises: an energy-sealing layer disposed on the second main surface side of the piezoelectric film for sealing the energy of the elastic wave propagating on the piezoelectric film. A piezoelectric film; and a first dielectric film and a second dielectric film respectively covering the first IDT electrode and the second IDT electrode on the first main surface and the second main surface of the piezoelectric film, or, the first dielectric film and the second dielectric film are not provided on one of the first main surface and the second main surface, but the first dielectric film or the second dielectric film of the first IDT electrode or the second IDT electrode is covered on the other of the first main surface and the second main surface, regarding the first... A first dielectric film and a second dielectric film, (a) having a volume wave velocity lower than the velocity of an elastic wave propagating in the piezoelectric film, or (b) having a volume wave velocity higher than the velocity of an elastic wave propagating in the piezoelectric film, or (c) the first dielectric film having a volume wave velocity lower than the velocity of an elastic wave propagating in the piezoelectric film, and the second dielectric film having a volume wave velocity higher than the velocity of an elastic wave propagating in the piezoelectric film, wherein the product of density and film thickness in the first dielectric film and the second dielectric film is smaller. The sum of the product of the density of the electrode fingers and the film thickness of the IDT electrode, whether it is covered by one of the dielectric films or not covered by either of the first or second dielectric films, is less than the sum of the product of the density of the electrode fingers and the film thickness of the IDT electrode, which is covered by the dielectric film with the larger product of the density and film thickness of the first or second dielectric film, or disposed on the main surface opposite to the IDT electrode not covered by either of the first or second dielectric films.

[0011] The second invention of this application is an elastic wave device comprising: a piezoelectric film having a first main surface and a second main surface facing each other; and an IDT electrode configured to be in contact with the piezoelectric film, the IDT electrode comprising: a first IDT electrode disposed on the first main surface of the piezoelectric film; and a second IDT electrode disposed on the second main surface of the piezoelectric film. The elastic wave device further comprises: an energy-sealing layer disposed on the second main surface side of the piezoelectric film for sealing the energy of the elastic wave propagating on the piezoelectric film; and a first dielectric film and a second dielectric film respectively covering the first IDT electrode and the second IDT electrode on the first main surface and the second main surface of the piezoelectric film, or, on one of the first main surface and the second main surface, the first dielectric film and the second dielectric film are not disposed, but on the other of the first main surface and the second main surface. The first dielectric film or the second dielectric film covering the first IDT electrode or the second IDT electrode, wherein the first dielectric film has a volume wave velocity higher than the sound velocity of the elastic wave propagating in the piezoelectric film, and the second dielectric film has a volume wave velocity lower than the sound velocity of the elastic wave propagating in the piezoelectric film, wherein the sum of the density and thickness product of the electrode fingers of the IDT electrode covered by the dielectric film with a smaller density-to-thickness product of the first dielectric film and the second dielectric film, or not covered by either the first dielectric film or the second dielectric film, is greater than the sum of the density and thickness product of the electrode fingers of the IDT electrode covered by the dielectric film with a larger density-to-thickness product of the first dielectric film and the second dielectric film, or disposed on the main surface opposite to the IDT electrode not covered by either the first dielectric film or the second dielectric film.

[0012] The third invention of this application is an elastic wave device comprising: a piezoelectric film having a first main surface and a second main surface facing each other; and an IDT electrode disposed in contact with the piezoelectric film, the IDT electrode having: a first IDT electrode disposed on the first main surface of the piezoelectric film; and a second IDT electrode disposed on the second main surface of the piezoelectric film. The elastic wave device further comprises: a support substrate having a cavity located on the second main surface side of the piezoelectric film; and a first dielectric film and a second dielectric film respectively disposed on the first main surface and the second main surface of the piezoelectric film, covering the first IDT electrode and the second IDT electrode, respectively; or, the first dielectric film and the second dielectric film are not disposed on one of the first main surface and the second main surface, but the first dielectric film or the second dielectric film of the first IDT electrode or the second IDT electrode is disposed on the other of the first main surface and the second main surface. Regarding the first dielectric film and the second dielectric film... 2. A dielectric film, (a) having a volume wave velocity lower than the velocity of an elastic wave propagating in the piezoelectric film, or (b) having a volume wave velocity higher than the velocity of an elastic wave propagating in the piezoelectric film, or (c) the first dielectric film having a volume wave velocity lower than the velocity of an elastic wave propagating in the piezoelectric film, and the second dielectric film having a volume wave velocity higher than the velocity of an elastic wave propagating in the piezoelectric film, wherein the product of the density and film thickness in the first dielectric film and the second dielectric film is small. The sum of the product of the density of the electrode fingers and the film thickness of the IDT electrode, whether it is covered by the dielectric film or not covered by either the first dielectric film or the second dielectric film, is less than the sum of the product of the density of the electrode fingers and the film thickness of the IDT electrode covered by the dielectric film with the larger density-film-thickness product of the first dielectric film and the second dielectric film, or disposed on the main surface opposite to the IDT electrode not covered by either the first dielectric film or the second dielectric film.

[0013] The fourth invention of this application is an elastic wave device comprising: a piezoelectric film having a first main surface and a second main surface facing each other; and an IDT electrode disposed in contact with the piezoelectric film, the IDT electrode comprising: a first IDT electrode disposed on the first main surface of the piezoelectric film; and a second IDT electrode disposed on the second main surface of the piezoelectric film. The elastic wave device further comprises: a support substrate having a cavity located on the second main surface side of the piezoelectric film; and a first dielectric film and a second dielectric film respectively disposed on the first main surface and the second main surface of the piezoelectric film, covering the first IDT electrode and the second IDT electrode, or, on one of the first main surface and the second main surface, the first dielectric film and the second dielectric film are not disposed, but the first IDT electrode is disposed on the other of the first main surface and the second main surface. The first dielectric film or the second dielectric film of the second IDT electrode, wherein the first dielectric film has a volume wave velocity higher than the sound velocity of the elastic wave propagating in the piezoelectric film, and the second dielectric film has a volume wave velocity lower than the sound velocity of the elastic wave propagating in the piezoelectric film, wherein the sum of the density and thickness product of the electrode fingers of the IDT electrode, which is covered by a dielectric film with a smaller density-to-thickness product in the first and second dielectric films, or which is not covered by either the first or second dielectric film, is greater than the sum of the density and thickness product of the electrode fingers of the IDT electrode, which is covered by a dielectric film with a larger density-to-thickness product in the first and second dielectric films, or disposed on the main surface opposite to the IDT electrode not covered by either the first or second dielectric film.

[0014] Hereinafter, the first to fourth inventions mentioned above will be collectively referred to as the present invention.

[0015] Invention Effects

[0016] According to the present invention, it is possible to provide an elastic wave device that is less prone to generating high-order modes. Attached Figure Description

[0017] Figure 1 (a) and Figure 1 (b) is a front sectional view of the elastic wave device according to the first embodiment of the present invention and a schematic top view showing the electrode structure.

[0018] Figure 2 This is a graph showing the relationship between the normalized film thickness and the coupling coefficient of the higher-order mode of the second IDT electrode in the first embodiment.

[0019] Figure 3This is a graph showing the relationship between the distance from the starting point and the shear stress along the film thickness direction in the piezoelectric film of Comparative Example 1's elastic wave device.

[0020] Figure 4 This is a graph showing the relationship between the distance from the starting point and the shear stress of the piezoelectric film along the film thickness direction in the elastic wave device of Embodiment 1.

[0021] Figure 5 This is a graph showing the impedance characteristics of the elastic wave device of Example 1 and Comparative Example 1.

[0022] Figure 6 This is a front sectional view used to illustrate a modified example of the first embodiment.

[0023] Figure 7 This is a front sectional view of the elastic wave device according to the second embodiment of the present invention.

[0024] Figure 8 This is a graph showing the relationship between the sound velocity of the first dielectric film and the second dielectric film, the normalized film thickness of the second IDT electrode, and the coupling coefficient of the higher-order modes.

[0025] Figure 9 This is a simplified front sectional view used to illustrate the function of the hypersonic film as an energy-sealing layer in a structure in which a hypersonic film and a piezoelectric film are stacked on a support substrate, where the thickness of the hypersonic film is thick.

[0026] Figure 10 This is a simplified front sectional view used to illustrate that in a structure in which a hypersonic film and a piezoelectric film are stacked on a support substrate, the hypersonic film, when thin, does not become an energy-sealing layer.

[0027] Figure 11 This is a front sectional view of the elastic wave device according to the third embodiment of the present invention.

[0028] Figure 12 This is a graph showing the relationship between the normalized film thickness of the second IDT electrode and the coupling coefficient of the SH1 mode, which is a higher-order mode, in the case where the sound velocity of the third dielectric film is low and the sound velocity of the third dielectric film is high in the third embodiment.

[0029] Figure 13 This is a front sectional view of the elastic wave device according to the fourth embodiment of the present invention.

[0030] Figure 14 This is a diagram showing the relationship between the sound velocity of the first dielectric film and the second dielectric film, the normalized film thickness of the second IDT electrode, and the coupling coefficient of the SH1 mode as a higher-order mode in the structure in which a hypersonic film that does not serve as an energy sealing layer is provided on the second main surface of the piezoelectric film in the fourth embodiment.

[0031] Figure 15 This is a front sectional view of the elastic wave device according to the fifth embodiment of the present invention.

[0032] Figure 16 This is a front sectional view of the elastic wave device according to the sixth embodiment of the present invention. Detailed Implementation

[0033] Hereinafter, specific embodiments of the present invention will be described with reference to the accompanying drawings, thereby clarifying the present invention.

[0034] Furthermore, it should be noted in advance that the embodiments described in this specification are illustrative, and partial substitutions or combinations of structures can be made between different embodiments.

[0035] Figure 1 (a) and Figure 1 (b) is a front sectional view of the elastic wave device of the first embodiment and a schematic top view showing the electrode structure.

[0036] The elastic wave device 1 has a piezoelectric film 3. The piezoelectric film 3 is not particularly limited, but in this embodiment, it comprises a lithium tantalate film that cuts at 42° Y and propagates in the X direction. The piezoelectric film 3 has a first main surface 3a and a second main surface 3b facing each other. A first IDT electrode 4 is disposed on the first main surface 3a of the piezoelectric film 3. In practice, as... Figure 1 As shown in (b), reflectors 6a and 6b are provided on both sides of the elastic wave propagation direction of the first IDT electrode 4.

[0037] A second IDT electrode 5 is also disposed on the second main surface 3b of the piezoelectric film 3. Reflectors are also disposed on both sides of the second IDT electrode 5 in the direction of elastic wave propagation. Thus, an elastic wave resonator is formed. The elastic wave device 1 is an elastic wave resonator.

[0038] The electrode fingers of the first IDT electrode 4 and the electrode fingers of the second IDT electrode 5 are positioned opposite each other through the piezoelectric film 3. The electrode fingers of the first IDT electrode and the electrode fingers of the second IDT electrode can be in phase or out of phase.

[0039] A first dielectric film 7 is provided on the first main surface 3a of the piezoelectric film 3, thereby covering the first IDT electrode 4. A second dielectric film 8 is also provided on the second main surface 3b, thereby covering the second IDT electrode 5. Furthermore, the piezoelectric film 3 is laminated on the support substrate 2 with the second dielectric film 8 in between. That is, the piezoelectric film 3 is indirectly laminated on the support substrate 2 from the second main surface 3b side.

[0040] In the elastic wave device 1, the first dielectric film 7 and the second dielectric film 8 have a volume wave velocity lower than the sound velocity of the elastic wave propagating in the piezoelectric film 3. Furthermore, the support substrate 2 contains a high-velocity acoustic material, which has a volume wave velocity higher than the sound velocity of the elastic wave propagating in the piezoelectric film 3. That is, the support substrate 2 is a high-velocity acoustic component containing a high-velocity acoustic material, functioning as an energy-sealing layer for confining the energy of the elastic wave within the piezoelectric film 3.

[0041] The characteristic of this embodiment is that the sum of the product of the density and thickness of the electrode fingers of the IDT electrode coated by the dielectric film with the smaller product of density and thickness in the first dielectric film 7 and the second dielectric film 8 is less than the sum of the product of the density and thickness of the electrode fingers of the IDT electrode coated by the dielectric film with the larger product of density and thickness in the first dielectric film 7 and the second dielectric film 8. In this embodiment, the product of density and thickness of the first dielectric film 7 is less than the product of density and thickness of the second dielectric film 8. Therefore, the sum of the product of density and thickness of the electrode fingers of the first IDT electrode 4 is less than the sum of the product of density and thickness of the electrode fingers of the second IDT electrode 5. As a result, higher-order modes are suppressed. This will be clarified by describing Example 1 and Comparative Example 1 below.

[0042] As an example 1, an elastic wave device with the following structure was prepared.

[0043] The finger spacing of the first IDT electrode 4 and the second IDT electrode 5 is set to 1 μm, and the duty cycle of both is set to 0.5. The wavelength determined by this finger spacing is denoted as λ. λ = 2 μm.

[0044] First dielectric film 7: SiO2 film, with a thickness of 0.03λ.

[0045] First IDT electrode 4: Al film, with a film thickness of 0.08λ.

[0046] Piezoelectric film 3: A LiTaO3 film propagating along the X-axis at a 42° Y-angle, with a film thickness of 0.4λ.

[0047] Second dielectric film 8: SiO2 film, with a thickness of 0.2λ.

[0048] Second IDT electrode 5: It is set as a Pt film, and various changes were made to the standardized film thickness of the second IDT electrode 5.

[0049] Support substrate 2: Sapphire

[0050] In addition, the thickness of the first dielectric film 7 and the second dielectric film 8 refers to the thickness from the first main surface 3a and the second main surface 3b of the piezoelectric film 3 to the outer surface of the first dielectric film 7 and the second dielectric film 8.

[0051] Figure 2 This is a graph showing the relationship between the normalized film thickness and the coupling coefficient of the higher-order mode (SH1 mode) of the second IDT electrode 5 in the above-described elastic wave device 1. Furthermore, hereinafter, the term "normalized film thickness" refers to the film thickness normalized using wavelength λ. Figure 2 In Comparative Example 1, the case where the normalized film thickness of the second IDT electrode 5 is 0.01λ represents the situation where the mass load generated by the first IDT electrode 4 and the second IDT electrode 5 are equal. Here, the mass load is equivalent to the sum of the product of the density of the electrode fingers of the first IDT electrode 4 and the second IDT electrode 5 and their film thickness.

[0052] Moreover, in Figure 2 In this embodiment, the standardized film thickness of the second IDT electrode 5 is larger than that of Comparative Example 1. In Example 1, the film thickness of the second IDT electrode 5 was set to 0.025λ. Figure 2 It is clear that in Example 1, the normalized film thickness of the second IDT electrode 5 is 0.025λ, and the coupling coefficient of the higher-order mode is almost 0%. In contrast, in Comparative Example 1, the coupling coefficient of the higher-order mode is as large as 0.18%. Therefore, it can be seen that, according to Example 1, the higher-order mode can be suppressed.

[0053] In addition, according to Figure 2 It is clear that the normalized film thickness of the second IDT electrode 5 becomes greater than 0.01λ. Preferably, the normalized film thickness of the second IDT electrode 5 is 0.015λ or more, at which point the coupling coefficient of the higher-order mode becomes 0.08% or less. More preferably, the normalized film thickness of the second IDT electrode 5 is 0.02λ or more, at which point the coupling coefficient of the higher-order mode becomes 0.01% or less. Further preferably, if it is 0.025λ as in Example 1, then it is known that almost no higher-order modes are generated.

[0054] Figure 5 The solid line shows the impedance characteristics of Example 1, and the dashed line shows the impedance characteristics of Comparative Example 1. Figure 5 Within the circle indicated by arrow A, in Comparative Example 1, a large response caused by higher-order modes appeared near 2.4 GHz. In contrast, it can be seen that, according to Example 1, the response of these higher-order modes was effectively suppressed.

[0055] As described above, the reason why the SH1 mode, which is a higher-order mode, could be suppressed in Example 1 compared to Comparative Example 1 can be attributed to the following reasons. Although the first dielectric film 7 and the second dielectric film 8 are made of the same material, their thicknesses differ at the portions in contact with the piezoelectric film 3. On the other hand, the electrode films in contact with the piezoelectric film 3 are the first IDT electrode 4 and the second IDT electrode 5, and in Comparative Example 1, their mass loads are the same. Therefore, the stresses on the first principal surface 3a and the second principal surface 3b of the piezoelectric film 3 become unequal. Therefore, as... Figure 3 As shown, in Comparative Example 1, an asymmetric stress distribution was generated in the thickness direction of the piezoelectric film 3. In this case, the shear stress in the thickness direction of the piezoelectric film 3 becomes asymmetric relative to the center of the piezoelectric film 3. That is, the symmetry of the stress distribution in the thickness direction is broken. Therefore, since the coupling coefficient is determined by the product of the electric field and the stress, it is impossible to reduce the coupling coefficient.

[0056] In contrast, such as Figure 4 As shown, in Example 1, the distribution of shear stress in the thickness direction of the piezoelectric film 3 is designed to be symmetrical. That is, the symmetry is excellent. As a result, the coupling coefficient can be reduced.

[0057] As described above, by making the mass loads generated by the first dielectric film 7 and the second dielectric film 8 have the same magnitude based on the mass load relationship generated by the first IDT electrode 4 and the second IDT electrode 5, higher-order modes can be effectively suppressed.

[0058] In addition, although a first dielectric film 7 and a second dielectric film 8 are provided in this embodiment, the first dielectric film may not be provided, for example.

[0059] Figure 6 The diagram shows a modified example of the elastic wave device according to the first embodiment. In the elastic wave device 1A, the first dielectric film 7 is not provided. The elastic wave device 1A is the same as the elastic wave device 1 in other aspects. In this invention, the first dielectric film may not be laminated on the first main surface 3a of the piezoelectric film 3. In this case, since there is no mass load on the dielectric film for the first IDT electrode 4, the side with the larger mass load generated by the dielectric film becomes the second IDT electrode 5. Therefore, it is acceptable as long as the sum of the product of the density of the electrode fingers of the first IDT electrode 4 not covered by the first dielectric film and the film thickness is less than the sum of the product of the density of the electrode fingers of the second IDT electrode 5 and the film thickness.

[0060] Conversely, the second dielectric film 8 can also be omitted. In this case, it is sufficient as long as the sum of the product of the density of the electrode fingers of the second IDT electrode 5 and the film thickness is less than the sum of the product of the density of the electrode fingers of the first IDT electrode 4 and the film thickness.

[0061] Figure 7 This is a front sectional view of the elastic wave device according to the second embodiment of the present invention. The elastic wave device 21 is structurally identical to the elastic wave device 1 of the first embodiment. Therefore, its description is omitted by referring to the same reference numerals.

[0062] In the elastic wave device 21 of the second embodiment, the sum of the product of the density and thickness of the electrode fingers of the first IDT electrode 4, which is covered by the first dielectric film 7 (where the product of the density and thickness of the first dielectric film 7 and the second dielectric film 8 is small), is set to be greater than the sum of the product of the density and thickness of the electrode fingers of the second IDT electrode 5, thereby suppressing the generation of higher-order modes. The following specific construction example will be used to illustrate this.

[0063] As an example 2, the following elastic wave device was prepared.

[0064] The electrode finger spacing of the first IDT electrode 4 and the second IDT electrode 5 is 1 μm, the duty cycle is 0.5, and λ is 2 μm.

[0065] First dielectric film 7: film thickness is 0.04λ

[0066] First IDT electrode 4: Cu film, with a film thickness of 0.06λ.

[0067] Piezoelectric film 3: A LiTaO3 film with a thickness of 0.45λ, cut at 50° Y-axis and propagating along the X-axis.

[0068] Second dielectric film 8: film thickness is 0.3λ

[0069] The second IDT electrode 5 is a Cu film, and its thickness has been varied.

[0070] Support substrate 2: Silicon

[0071] In Example 2, the Young's modulus of the first dielectric film 7 or the second dielectric film 8 was set to be the same as or four times that of SiO2. If the Young's modulus of the first dielectric film 7 and the second dielectric film 8 increases, the volume wave velocity increases. Furthermore, the effect of the mass load on the first IDT electrode 4 and the second IDT electrode 5 was determined in cases where the volume wave velocity is low relative to the velocity of the elastic wave propagating in the piezoelectric film 3, and in cases where the volume wave velocity is high. Additionally, in the specific examples of Examples 3 and 4 described later, the Young's modulus was also changed to adjust the velocity of sound.

[0072] Furthermore, the mass load generated by the first dielectric film 7 and the second dielectric film 8 is related as follows: the first dielectric film 7 < the second dielectric film 8. That is, the product of the density and thickness of the first dielectric film 7 is set to be less than the product of the density and thickness of the second dielectric film 8.

[0073] Figure 8 This is a graph showing the relationship between the sound velocity of the first dielectric film 7 and the second dielectric film 8 in Embodiment 2 above, and the relationship between the normalized film thickness of the second IDT electrode 5 and the coupling coefficient of the higher-order modes. Additionally, in Figure 8 The sound velocity relationship shown next to the graph illustrates the relationship between the sound velocity of the first dielectric film 7 and the sound velocity of the second dielectric film 8. For example, "low speed / low speed" indicates that when the first dielectric film 7 is a low-speed dielectric film and its Young's modulus is the same as SiO2, the second dielectric film 8 is also a low-speed dielectric film, meaning its Young's modulus is the same as SiO2. Here, "low speed" means that the propagating volume wave sound velocity is lower than the elastic wave sound velocity, and "high speed" means that the propagating volume wave sound velocity is higher than the elastic wave sound velocity. High speed is the case where the Young's modulus is 4 times that of SiO2.

[0074] according to Figure 8 It is clear that, except in the case where the second dielectric film 8 is a low-speed dielectric film and the first dielectric film 7 is a high-speed dielectric film, as long as the relationship between the mass load generated by the first dielectric film 7 and the mass load generated by the second dielectric film 8 is set to the same relationship as the relationship between the mass load generated by the first IDT electrode 4 and the second IDT electrode 5, higher-order modes can be effectively suppressed. Therefore, regarding the first dielectric film and the second dielectric film, it is sufficient if (a) they have a volume wave velocity lower than the sound velocity of the elastic wave propagating in the piezoelectric film, or (b) they have a volume wave velocity higher than the sound velocity of the elastic wave propagating in the piezoelectric film, or (c) the first dielectric film has a volume wave velocity lower than the sound velocity of the elastic wave propagating in the piezoelectric film, and the second dielectric film has a volume wave velocity higher than the sound velocity of the elastic wave propagating in the piezoelectric film.

[0075] However, when the second dielectric film 8 is a high-velocity film, the effect of reducing the coupling coefficient of higher-order modes is relatively small. Therefore, it is preferable that the second dielectric film 8 is a low-velocity film.

[0076] Furthermore, it is most preferable that both the first dielectric film 7 and the second dielectric film 8 are low-velocity films. In this case, it is known that when the normalized film thickness of the second IDT electrode 5 is greater than 0.06λ, the coupling coefficient of the higher-order modes can be made very small, less than 0.37%.

[0077] This is because, in the case where the second dielectric film 8 is a hypersonic film, it functions as an energy-sealing structure by being bonded to a silicon substrate that serves as a hypersonic component.

[0078] like Figure 9As shown, the second dielectric film 8A has a film thickness tA. When the film thickness tA is thick, the piezoelectric film 3 will be in contact with the energy-sealing structure. At the boundary between the piezoelectric film 3 and the second dielectric film 8A, which is a hypersonic film, the displacement of the elastic wave acts as an evanescent wave (see reference). Figure 9 On the other hand, at the boundary surface opposite to the piezoelectric film 3, i.e., the first principal surface 3a, the displacement of the piezoelectric film 3 acts as a propagating wave. Therefore, the symmetry of the shear stress in the piezoelectric film 3 is compromised. Consequently, the coupling coefficient of higher-order modes cannot be reduced.

[0079] In contrast, such as Figure 10 As shown, if the second dielectric film 8B, which is a thin hypersonic film with a thickness tB, is configured to be in contact with the second main surface 3b of the piezoelectric film 3, it will not form an energy-sealing structure. That is, as Figure 10 As indicated by the middle arrow, higher-order mode leakage occurs, thereby improving the symmetry of the elastic wave distribution along the thickness direction of the piezoelectric film 3, which is preferable. Furthermore, it is preferable that the thickness of the second dielectric film 8B, which serves as a hypersonic film, is 1λ or less. The thickness of the hypersonic film, which can form an energy-sealing structure, is determined by the relationship between the speed of the elastic wave propagating in the piezoelectric body and the speed of the transverse body wave of the hypersonic film. This has been disclosed, for example, in Japanese Patent No. 5835480, the contents of which are incorporated herein by reference.

[0080] Figure 11 This is a front sectional view of the elastic wave device according to the third embodiment of the present invention. The elastic wave device 31 is structurally identical to the elastic wave device 1 of the first embodiment, except that it has a third dielectric film 32. Therefore, the same reference numerals are used to denote the same parts, and their descriptions are omitted.

[0081] The third dielectric film 32 is stacked between the support substrate 2 and the second dielectric film 8. In this way, in the present invention, a third dielectric film 32 may also be further stacked on the second main surface 3b side of the piezoelectric film 3, in addition to the second dielectric film 8. In this case, on the side where the second IDT electrode 5 is provided, i.e., the second main surface 3b side of the piezoelectric film 3, the mass load generated by the dielectric film can be considered as the sum of the mass load generated by the second dielectric film 8 and the mass load generated by the third dielectric film 32. This will be clarified by describing the elastic wave device of Embodiment 3 below.

[0082] The electrode finger spacing of the first IDT electrode 4 and the second IDT electrode 5 is 1 μm, the duty cycle is 0.5, and λ is 2 μm.

[0083] First dielectric film 7: SiN film, with a thickness of 0.1λ.

[0084] First IDT electrode 4: Al film, with a film thickness of 0.08λ.

[0085] Piezoelectric film 3: A LiTaO3 film propagating along the X-axis at a 42° Y-angle, with a film thickness of 0.4λ.

[0086] Second dielectric film 8: SiO2 film, with a thickness of 0.15λ.

[0087] Third dielectric film 32: film thickness is 0.2λ

[0088] The second IDT electrode 5 is a Pt film, and the film thickness was varied. The film thickness was 0.01λ when it achieved the same mass load as the first IDT electrode 4.

[0089] Support substrate 2: SiC substrate

[0090] Figure 12 This is a graph showing the relationship between the normalized film thickness and the coupling coefficient of the higher-order mode of the second IDT electrode when the third dielectric film 32 is a low-velocity film and when the third dielectric film 32 is a high-velocity film.

[0091] In this embodiment, the first dielectric film 7 is a hypersonic film, and the second dielectric film 8 is a hyposonic film. Therefore, the relationship between the mass load generated by the first IDT electrode 4 and the mass load generated by the second IDT electrode 5 can be set to be the opposite of the relationship between the mass loads of the first dielectric film 7 and the second dielectric film 8. Furthermore, when the third dielectric film 32 is a hypersonic film, as mentioned above, it can be considered as part of the energy-sealing structure. Therefore, it is sufficient to consider the second dielectric film 8 as the mass load film for the piezoelectric film 3. Therefore, since the mass load generated by the first dielectric film 7 is relatively large, it is preferable that the mass load generated by the second IDT electrode 5 is relatively large as well.

[0092] Therefore, as Figure 12 As shown, when the third dielectric film 32 is a hypersonic film, it is sufficient to make the normalized film thickness of the second IDT electrode 5 0.01λ thick.

[0093] On the other hand, when the third dielectric film 32 is a low-velocity film, the elastic wave Q generated by the third dielectric film 32 becomes non-negligible. That is, it does not function as an energy-sealing structure. Therefore, the mass load generated by the dielectric film on the second main surface 3b of the piezoelectric film 3 becomes the sum of the mass load generated by the second dielectric film 8 and the mass load generated by the third dielectric film 32. In this case, it is sufficient to make the mass load generated by the first IDT electrode 4 greater than the mass load generated by the second IDT electrode 5.

[0094] Therefore, according to Figure 12 It is clear that when the third dielectric film 32 is a low-velocity film, the thickness of the standardized film of the second IDT electrode 5 is only 0.01λ thinner than the thickness of the first IDT electrode 4 when the mass load is the same.

[0095] Figure 13 This is a front cross-sectional view of the elastic wave device according to the fourth embodiment of the present invention. The elastic wave device 41 has a third dielectric film 42. Furthermore, it is structurally identical to the elastic wave device 31. The result shown in the description of the fourth embodiment is the case where the third dielectric film 42 does not function as an energy sealing layer. The following elastic wave device has been prepared.

[0096] The electrode finger spacing of the first IDT electrode 4 and the second IDT electrode 5 is 1 μm, the duty cycle is 0.5, and λ is 2 μm.

[0097] First dielectric film 7: film thickness is 0.05λ

[0098] The first IDT electrode 4 consists of an Al film and a Ti film, with the Al film having a thickness of 0.08λ and the Ti film having a thickness of 0.01λ. Furthermore, the Ti film is located on the first principal surface 3a side of the piezoelectric film 3.

[0099] Piezoelectric film 3: A LiNbO3 film with a thickness of 0.3λ, cut at 30° Y-axis and propagating along the X-axis.

[0100] Second dielectric film 8: film thickness is 0.15λ

[0101] The third dielectric film 42: SiO2 film, with a thickness of 0.2λ.

[0102] The second IDT electrode 5 is a Mo film, and the film thickness was varied. The film thickness was set to 0.025λ to achieve the same mass load as that generated by the first IDT electrode 4.

[0103] Support substrate 2: diamond

[0104] Figure 14 This is a graph showing the relationship between the sound velocity of the first dielectric film 7 and the second dielectric film 8, the normalized film thickness of the second IDT electrode 5, and the coupling coefficient of the SH1 mode as a higher-order mode. Additionally, Figure 14 The sound velocity relationship is the sound velocity of the first dielectric film 7 / the sound velocity of the second dielectric film 8. For example, low speed / low speed indicates that the first dielectric film 7 is a low-sound-velocity film and the second dielectric film 8 is also a low-sound-velocity film. The third dielectric film 42 is a low-sound-velocity film containing SiO2 with a thickness of 0.2λ. Here, similarly to Example 2, the Young's modulus of the first dielectric film 7 and the second dielectric film 8 were changed, and the sound velocity relationship of the first dielectric film 7 / the sound velocity relationship of the second dielectric film 8 was changed.

[0105] The third dielectric film 42 is a low-velocity film. The first dielectric film 7 and the second dielectric film 8 are sufficiently thin. Therefore, the second dielectric film 8 itself does not become an energy-sealing layer. Therefore, the mass load film on the second main surface 3b side of the piezoelectric film 3 is the sum of the mass loads generated by the second dielectric film 8 and the third dielectric film 42.

[0106] according to Figure 14 It is clear that, similar to the case in Example 2, even if the second dielectric film 8 is a high-velocity film, as long as the ratio of the velocity of sound of the first dielectric film 7 to the velocity of sound of the second dielectric film 8 is low / low, low / high, or high / high, the normalized film thickness of the second IDT electrode 5 can be set to be greater than the mass load generated by the first IDT electrode 4. This reduces the coupling coefficient of higher-order modes.

[0107] Furthermore, when the velocity of sound between the first dielectric film 7 and the second dielectric film 8 is high speed / low speed, it is sufficient to specifically reduce the mass load generated by the second IDT electrode 5. That is, it is sufficient to make the normalized film thickness of the second IDT electrode 5 thinner than 0.025λ.

[0108] Furthermore, in this case, it is understood that in order to more effectively suppress higher-order modes, the second dielectric film 8 in contact with the piezoelectric film 3 is preferably a low-velocity film. Moreover, if the second dielectric film 8 in contact with the piezoelectric film 3 is a high-velocity film, as described above, it is preferable not to constitute an energy-sealing layer.

[0109] Figure 15 This is a front cross-sectional view of the elastic wave device according to the fifth embodiment of the present invention. In the elastic wave device 61, the support substrate 62 has a recess 62b open on its upper surface 62a. A laminate of the piezoelectric film 3, the first dielectric film 7, and the second dielectric film 8 is laminated to cover the recess 62b. Thus, a cavity 62c is provided. In this way, the elastic wave device of the present invention may also have a structure in which the piezoelectric film 3 is provided above the cavity 62c. In the elastic wave device 61, similarly to the first to fourth embodiments, the magnitude of the mass load generated by the first IDT electrode 4 and the second IDT electrode 5 can be selected based on the sound velocity and mass load magnitude of the first dielectric film 7 and the second dielectric film 8. Thus, higher-order modes can be suppressed.

[0110] Figure 16This is a front cross-sectional view of the elastic wave device according to the sixth embodiment of the present invention. In the elastic wave device 71, an acoustic reflection layer 72 is provided as an energy sealing layer between the support substrate 2 and the second dielectric film 8. Thus, in the present invention, the acoustic reflection layer 72 can also be used as an energy sealing layer. The acoustic reflection layer 72 has high acoustic impedance layers 72a, 72c, 72e with relatively high acoustic impedance, and low acoustic impedance layers 72b, 72d, 72f with relatively low acoustic impedance. Regarding the materials constituting such high acoustic impedance layers 72a, 72c, 72e and low acoustic impedance layers 72b, 72d, 72f, suitable materials that satisfy the above-described acoustic impedance relationships can be used.

[0111] Furthermore, in this invention, as described above, the piezoelectric film is composed of LiTaO3 or LiNbO3, but the material of the piezoelectric film is not limited to these. That is, in addition to lithium tantalate or lithium niobate, aluminum nitride, zinc oxide, etc., can also be used. Additionally, the thickness of the piezoelectric film is preferably, for example, less than three times the smaller of the electrode finger spacing of the first IDT electrode and the second IDT electrode.

[0112] Furthermore, regarding the material of the support substrate, in addition to sapphire, silicon, and diamond, a variety of insulators or semiconductors can be used. Moreover, in the case of a stacked energy-sealing structure, the support substrate can also be made of a low-velocity material.

[0113] Explanation of reference numerals in the attached figures

[0114] 1. 1A…elastic wave device;

[0115] 2...support base plate;

[0116] 3…piezoelectric film;

[0117] 3a, 3b... First and second main faces;

[0118] 4, 5... IDT electrodes 1 and 2;

[0119] 6a, 6b... reflectors;

[0120] 7…First dielectric film;

[0121] 8, 8A, 8B… Second dielectric film;

[0122] 21, 31, 41… elastic wave devices;

[0123] 32, 42, 52… Third dielectric film;

[0124] 61…elastic wave device;

[0125] 62...support base plate;

[0126] 62a…upper surface;

[0127] 62b…concave;

[0128] 62c…cavity;

[0129] 71…elastic wave device;

[0130] 72…acoustic reflector layer;

[0131] 72a, 72c, 72e… high acoustic impedance layers;

[0132] 72b, 72d, 72f... Low acoustic impedance layers.

Claims

1. An elastic wave device, comprising: A piezoelectric film having a first principal surface and a second principal surface opposite to each other; and The IDT electrode is configured to be in contact with the piezoelectric film. The IDT electrode has: A first IDT electrode is disposed on the first main surface of the piezoelectric film; and The second IDT electrode is disposed on the second main surface of the piezoelectric film. The elastic wave device also includes: An energy-sealing layer, disposed on the second main surface side of the piezoelectric film, is used to confine the energy of elastic waves propagating on the piezoelectric film within the piezoelectric film; and The first and second main surfaces of the piezoelectric film are respectively provided with a first dielectric film and a second dielectric film covering the first IDT electrode and the second IDT electrode, respectively; or, the first dielectric film and the second dielectric film are not provided on one of the first and second main surfaces, but the first dielectric film or the second dielectric film of the first IDT electrode or the second IDT electrode is covered on the other of the first and second main surfaces. Regarding the first dielectric film and the second dielectric film, (a) they have a volume wave velocity lower than the velocity of the elastic wave propagating in the piezoelectric film, or (b) they have a volume wave velocity higher than the velocity of the elastic wave propagating in the piezoelectric film, or (c) the first dielectric film has a volume wave velocity lower than the velocity of the elastic wave propagating in the piezoelectric film, and the second dielectric film has a volume wave velocity higher than the velocity of the elastic wave propagating in the piezoelectric film. The sum of the product of the density and film thickness of the electrode fingers of the IDT electrode, whether covered by the dielectric film with the smaller product of density and film thickness of the first dielectric film and the second dielectric film, or not covered by either the first dielectric film or the second dielectric film, is less than the sum of the product of the density and film thickness of the electrode fingers of the IDT electrode, whether covered by the dielectric film with the larger product of density and film thickness of the first dielectric film and the second dielectric film, or disposed on the main surface opposite to the IDT electrode not covered by either the first dielectric film or the second dielectric film.

2. An elastic wave device, comprising: A piezoelectric film having a first principal surface and a second principal surface opposite to each other; and The IDT electrode is configured to be in contact with the piezoelectric film. The IDT electrode has: A first IDT electrode is disposed on the first main surface of the piezoelectric film; and The second IDT electrode is disposed on the second main surface of the piezoelectric film. The elastic wave device also includes: An energy-sealing layer, disposed on the second main surface side of the piezoelectric film, is used to confine the energy of elastic waves propagating on the piezoelectric film within the piezoelectric film; and The first and second main surfaces of the piezoelectric film are respectively provided with a first dielectric film and a second dielectric film covering the first IDT electrode and the second IDT electrode, respectively; or, the first dielectric film and the second dielectric film are not provided on one of the first and second main surfaces, but the first dielectric film or the second dielectric film of the first IDT electrode or the second IDT electrode is covered on the other of the first and second main surfaces. The first dielectric film has a volume wave velocity higher than the sound velocity of the elastic wave propagating in the piezoelectric film, and the second dielectric film has a volume wave velocity lower than the sound velocity of the elastic wave propagating in the piezoelectric film. The sum of the product of the density and film thickness of the electrode fingers of the IDT electrode, whether covered by a dielectric film with a smaller density-to-film-thickness product in the first and second dielectric films or not covered by either the first and second dielectric films, is greater than the sum of the product of the density and film thickness of the electrode fingers of the IDT electrode, whether covered by a dielectric film with a larger density-to-film-thickness product in the first and second dielectric films or disposed on the main surface opposite to the IDT electrode not covered by either the first and second dielectric films.

3. The elastic wave device according to claim 1 or 2, wherein, The energy-sealing layer is a high-velocity component containing a high-velocity material in which the propagating volume wave has a sound speed higher than that of the elastic wave propagating in the piezoelectric film.

4. The elastic wave device according to claim 1 or 2, wherein, The energy sealing layer is an acoustic reflection layer with a low acoustic impedance layer and a high acoustic impedance layer.

5. An elastic wave device, comprising: A piezoelectric film having a first principal surface and a second principal surface opposite to each other; and The IDT electrode is configured to be in contact with the piezoelectric film. The IDT electrode has: A first IDT electrode is disposed on the first main surface of the piezoelectric film; and The second IDT electrode is disposed on the second main surface of the piezoelectric film. The elastic wave device also includes: A support substrate having a cavity is located on the second main surface side of the piezoelectric film; and The first and second main surfaces of the piezoelectric film are respectively provided with a first dielectric film and a second dielectric film covering the first IDT electrode and the second IDT electrode, respectively; or, the first dielectric film and the second dielectric film are not provided on one of the first and second main surfaces, but the first dielectric film or the second dielectric film of the first IDT electrode or the second IDT electrode is covered on the other of the first and second main surfaces. Regarding the first dielectric film and the second dielectric film, (a) they have a volume wave velocity lower than the velocity of the elastic wave propagating in the piezoelectric film, or (b) they have a volume wave velocity higher than the velocity of the elastic wave propagating in the piezoelectric film, or (c) the first dielectric film has a volume wave velocity lower than the velocity of the elastic wave propagating in the piezoelectric film, and the second dielectric film has a volume wave velocity higher than the velocity of the elastic wave propagating in the piezoelectric film. The sum of the product of the density and film thickness of the electrode fingers of the IDT electrode, whether covered by a dielectric film with a smaller density-to-film-thickness product in the first and second dielectric films or not covered by either the first and second dielectric films, is less than the sum of the product of the density and film thickness of the electrode fingers of the IDT electrode, whether covered by a dielectric film with a larger density-to-film-thickness product in the first and second dielectric films or disposed on the main surface opposite to the IDT electrode not covered by either the first and second dielectric films.

6. An elastic wave device, comprising: A piezoelectric film having a first principal surface and a second principal surface opposite to each other; and The IDT electrode is configured to be in contact with the piezoelectric film. The IDT electrode has: A first IDT electrode is disposed on the first main surface of the piezoelectric film; and The second IDT electrode is disposed on the second main surface of the piezoelectric film. The elastic wave device also includes: A support substrate having a cavity is located on the second main surface side of the piezoelectric film; and The first and second main surfaces of the piezoelectric film are respectively provided with a first dielectric film and a second dielectric film covering the first IDT electrode and the second IDT electrode, respectively; or, the first dielectric film and the second dielectric film are not provided on one of the first and second main surfaces, but the first dielectric film or the second dielectric film of the first IDT electrode or the second IDT electrode is covered on the other of the first and second main surfaces. The first dielectric film has a volume wave velocity higher than the sound velocity of the elastic wave propagating in the piezoelectric film, and the second dielectric film has a volume wave velocity lower than the sound velocity of the elastic wave propagating in the piezoelectric film. The sum of the product of the density and film thickness of the electrode fingers of the IDT electrode, whether covered by a dielectric film with a smaller density-to-film-thickness product in the first and second dielectric films or not covered by either the first and second dielectric films, is greater than the sum of the product of the density and film thickness of the electrode fingers of the IDT electrode, whether covered by a dielectric film with a larger density-to-film-thickness product in the first and second dielectric films or disposed on the main surface opposite to the IDT electrode not covered by either the first and second dielectric films.

7. The elastic wave device according to any one of claims 1 to 6, wherein, The thickness of the piezoelectric film is less than three times the smaller of the electrode finger spacing of the first IDT electrode and the second IDT electrode.

8. The elastic wave device according to any one of claims 1 to 7, wherein, The piezoelectric film contains lithium niobate or lithium tantalate.

9. The elastic wave device according to any one of claims 1 to 8, wherein, The electrode fingers of the first IDT electrode and the electrode fingers of the second IDT electrode are in phase.

10. The elastic wave device according to any one of claims 1 to 8, wherein, The electrode fingers of the first IDT electrode and the electrode fingers of the second IDT electrode are in opposite phase.

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