Device for elastic waves

Abstract

Elastic wave device (1, 21, 31), comprising: a piezoelectric film (6), a low sound velocity film (5) arranged on the piezoelectric film and in which a sound velocity of a bulk wave propagating through the low sound velocity film is smaller than a sound velocity of a bulk wave propagating through the piezoelectric film, a high sound velocity film (4) stacked on a surface of the low sound velocity film on a side opposite to the piezoelectric film, and in which a sound velocity of a bulk wave propagating through the high sound velocity film is higher than a sound velocity of an elastic wave propagating through the piezoelectric film, a substrate (2) stacked directly or indirectly on a surface of the high-speed acoustic film on a side opposite to the low-speed acoustic film, and a bonding layer (7) located in the high sound velocity film (4) or in the low sound velocity film (5).

Description

TECHNICAL FIELD

[0001] The present invention relates to an elastic wave device having a structure in which a low acoustic velocity film and a high acoustic velocity film are stacked on a piezoelectric film. STATE OF THE ART

[0002] Elastic wave devices having a structure in which another material layer is sandwiched between a substrate and a piezoelectric film are known. Patent Document 1 mentioned below discloses an elastic wave device including a high acoustic velocity film, a low acoustic velocity film, and a piezoelectric film stacked in this order on a support substrate. This elastic wave device can enhance the quality factor.

[0003] In Patent Document 1, when manufacturing the elastic wave device, a support substrate is bonded to a multilayer body including a piezoelectric film, a low acoustic velocity film, and a high acoustic velocity film stacked on top of each other. This bonding can be performed by a method such as hydrophilization bonding, activation bonding, atomic diffusion bonding, or metal diffusion bonding.

[0004] A piezoelectric device according to Patent Document 2 comprises a first metal layer located on a bonding surface of a piezoelectric single-crystal substrate and a second metal layer located on a bonding surface of a support substrate, these metal layers being superimposed on each other to define a metal-bonded layer from which a semiconducting layer is formed by oxidation. Citation listPatent documents Patent document 1: WO 2012 / 086639 A1 Patent document 2: US 2014 / 0139075 A1 SUMMARY OF THE INVENTIONTechnical Problem

[0005] When the support substrate is bonded to other parts by the bonding method described in Patent Document 1, multiple layers are formed on the piezoelectric film side, which sometimes causes warping in the piezoelectric film due to mechanical stress in the formed layers. Therefore, ripples sometimes appear in the characteristics of the elastic wave device. Furthermore, an increase in the amount of warping sometimes causes cracking in the piezoelectric film during transportation.

[0006] It is an object of the present invention to provide an elastic wave device whose characteristics hardly deteriorate due to warping and in which breakage hardly occurs during transportation or the like. Solution to the problem

[0007] An elastic wave device according to the present invention includes a piezoelectric film, a low acoustic velocity film disposed on the piezoelectric film and in which a sound velocity of a bulk wave propagating through the low acoustic velocity film is smaller than a sound velocity of a bulk wave propagating through the piezoelectric film, a high acoustic velocity film stacked on a surface of the low acoustic velocity film on a side opposite to the piezoelectric film and in which a sound velocity of a bulk wave propagating through the high acoustic velocity film is higher than a sound velocity of an elastic wave propagating through the piezoelectric film, a substrate,which is stacked directly or indirectly on a surface of the high-speed film on a side opposite to the low-speed film, and a bonding layer located in the high-speed film or in the low-speed film.

[0008] An elastic wave device according to another specific embodiment of the present invention includes a piezoelectric film and a low acoustic velocity film disposed on the piezoelectric film, in which a sound velocity of a bulk wave propagating through the low acoustic velocity film is smaller than a sound velocity of a bulk wave propagating through the piezoelectric film, a high acoustic velocity substrate stacked directly or indirectly on a surface of the low acoustic velocity film on a side opposite to the piezoelectric film, and in which a sound velocity of a bulk wave propagating through the high acoustic velocity substrate is higher than a sound velocity of an elastic wave propagating through the piezoelectric film,and a bonding layer located in the low-speed acoustic film or in the high-speed acoustic substrate.

[0009] In another particular embodiment of the elastic wave device according to the present invention, the bonding layer contains a metal oxide or a metal nitride.

[0010] In another specific embodiment of the elastic wave device according to the present invention, the bonding layer includes a Ti layer, and the Ti layer has a thickness of 0.4 nm or more and 2.0 nm or less.

[0011] In another specific embodiment of the elastic wave device according to the present invention, the Ti layer has a thickness of 0.4 nm or more and 1.2 nm or less.

[0012] In another particular embodiment of the elastic wave device according to the present invention, the piezoelectric film is made of LiTaO 3 .

[0013] In another particular embodiment of the elastic wave device according to the present invention, the low acoustic velocity film is made of silicon oxide.

[0014] In another specific embodiment of the elastic wave device according to the present invention, the low acoustic velocity film is made of silicon oxide, the bonding layer is located in the low acoustic velocity film, the low acoustic velocity film includes a first low acoustic velocity layer located on a side of the bonding layer close to the piezoelectric film and a second low acoustic velocity layer located on a side of the bonding layer opposite to the piezoelectric film, and when it is assumed that an elastic wave used in the elastic wave device has a wavelength λ, the first low acoustic velocity layer has a thickness of 0.12λ or more.

[0015] In another particular embodiment of the elastic wave device according to the present invention, the first low acoustic velocity layer has a thickness of 0.22λ or more.

[0016] In another particular embodiment of the elastic wave device according to the present invention, the high acoustic velocity film is made of aluminum nitride or silicon nitride.

[0017] In another particular embodiment of the elastic wave device according to the present invention, the elastic wave device further includes an intermediate layer disposed between the high acoustic velocity film and the substrate. Advantageous effects of the invention

[0018] In the elastic wave device of the present invention, warpage hardly occurs during the formation of the bonding layer. Therefore, the electrical characteristics hardly deteriorate, and breakage hardly occurs during transportation or the like. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1(a) is a schematic cross-sectional elevation view of an elastic wave device according to a first embodiment of the present invention, and Fig. Figure 1(b) is a schematic plan view illustrating an electrode structure of the elastic wave device. Fig. 2 illustrates the resonance characteristics of elastic wave devices of the example according to the first embodiment of the present invention and the prior art example. Fig. 3 is a schematic cross-sectional elevation view of an elastic wave device according to a second embodiment of the present invention. Fig. 4 is a schematic cross-sectional elevation view of an elastic wave device according to a third embodiment of the present invention. Fig. 5 is a schematic cross-sectional elevation view of an elastic wave device not according to the present invention. Fig. 6 is a schematic cross-sectional elevation view of an elastic wave device according to a fifth embodiment of the present invention. Fig. 7 is a schematic cross-sectional elevation view of an elastic wave device not according to a sixth embodiment. Fig. Figure 8 illustrates the relationship between the thickness of a SiO 2 -Film and the quality factor. Fig. Figure 9 illustrates the relationship between the thickness of a Ti layer of a bonding layer and the quality factor. DESCRIPTION OF EMBODIMENTS

[0019] In the following, concrete embodiments of the present invention will be described with reference to the accompanying drawings, with the exception of Fig. 5 and Fig. 7, which are included for reference, so that the present invention may be better understood.

[0020] Fig. 1(a) is a schematic cross-sectional elevation view of an elastic wave device according to a first embodiment of the present invention.

[0021] An elastic wave device 1 includes a support substrate 2. A first silicon oxide film 3 is disposed on the support substrate 2. A high acoustic velocity film 4 is disposed on the first silicon oxide film 3. A second silicon oxide film serving as a low acoustic velocity film 5 is disposed on the high acoustic velocity film 4. As described later, the low acoustic velocity film 5 has a structure in which a low acoustic velocity layer 5a and a low acoustic velocity layer 5b are bonded to each other by a bonding layer 7. A piezoelectric film 6 is disposed on the low acoustic velocity film 5.

[0022] The low sound velocity film 5 means a film in which the sound velocity of a bulk wave propagating through the film is smaller than that of a bulk wave propagating through the piezoelectric film 6. The high sound velocity film 4 means a film in which the sound velocity of a bulk wave propagating through the film is higher than that of an elastic wave propagating through the piezoelectric film 6.

[0023] Although various elastic waves with different sound speeds and modes are excited by an IDT electrode, the elastic wave propagating through the piezoelectric film 6 means an elastic wave having a specific mode, which is used to obtain characteristics as filters and resonators.

[0024] An IDT electrode 8 is disposed on the piezoelectric film 6. The support substrate 2 may be made of any suitable material as long as the structure above the support substrate 2 can be maintained. Examples of the material include piezoelectrics such as sapphire, LiTaO 3 , LiNbO 3 and quartz; various ceramic materials such as alumina, magnesium oxide, silicon nitride, aluminum nitride, silicon carbide, zirconium oxide, cordierite, mullite, steatite, and forsterite; dielectrics such as glass; semiconductors such as silicon and gallium nitride; and resins. In this embodiment, the support substrate 2 is made of Si.

[0025] The first silicon oxide film 3 is not necessarily present, meaning the high acoustic velocity film 4 can be stacked directly on the support substrate 2. The high acoustic velocity film 4 can also be stacked indirectly over the support substrate 2, as in this embodiment, meaning it can be stacked over the support substrate 2 with the first silicon oxide film 3 in between.

[0026] The high acoustic velocity film 4 confines an elastic wave to a portion formed by the piezoelectric film 6 and the low acoustic velocity film 5, thus preventing the elastic wave from escaping to the structure below the high acoustic velocity film 4. In this embodiment, the high acoustic velocity film 4 is made of aluminum nitride. The high acoustic velocity film 4 may be made of materials such as aluminum nitride, aluminum oxide, silicon carbide, silicon nitride, silicon oxynitride, a DLC film, or diamond, as long as it confines the elastic wave. The high acoustic velocity film 4 may be made of a composite material such as a medium containing the above-mentioned material or a medium containing a mixture of the above-mentioned materials.

[0027] In order to confine the elastic wave to the portion formed by the piezoelectric film 6 and the low acoustic velocity film 5, the high acoustic velocity film 4 is desirably as thick as possible. The thickness of the high acoustic velocity film 4 is at least 0.5 times, and preferably at least 1.5 times, the wavelength λ of a surface acoustic wave.

[0028] The low acoustic velocity film 5 may be made of a suitable dielectric material in which the acoustic velocity of a bulk wave is smaller than that of a bulk wave propagating through the piezoelectric film 6. Examples of the material include silicon oxide, glass, silicon oxynitride, tantalum oxide, compounds prepared by adding fluorine, carbon, or boron to silicon oxide, and media predominantly containing the above-mentioned material.

[0029] As will be apparent from the production method described later, the bonding layer 7 is a part formed by metal diffusion bonding and is formed of Ti oxide in this embodiment.

[0030] A metal other than Ti may also be used. An example of such a metal is Al. Alternatively, the bonding layer 7 may be formed from a metal such as Ti or Al instead of metal oxides. To achieve electrical insulation, a metal oxide or a metal nitride is preferred. In particular, an oxide or nitride of Ti is suitably used to achieve high bonding strength.

[0031] In this embodiment, the piezoelectric film 6 consists of LiTaO 3 Alternatively, the piezoelectric film 6 may be made of a piezoelectric single crystal other than LiTaO 3 .

[0032] The IDT electrode 8 is made of Al in this embodiment. The IDT electrode 8 may be made of a suitable metal material such as Al, Cu, Pt, Au, Ag, Ti, Ni, Cr, Mo, W, or an alloy predominantly containing one or more of these metals. The IDT electrode 8 may have a structure in which a plurality of metal films made of these metals or alloys are stacked one on top of the other.

[0033] Although in Fig. 1(a) schematically illustrates an electrode structure as in Fig. 1(b), the IDT electrode 8 and the reflectors 9 and 10 arranged on both sides of the IDT electrode 8 in a propagation direction of the elastic wave are formed on the piezoelectric film 6. This forms a single-port type elastic wave resonator. However, the electrode structure including the IDT electrode in the present invention is not particularly limited. The electrode structure can be modified to form a ladder type filter, a longitudinally coupled resonator type filter, a grating type filter including a suitable resonator or resonators in combination, or a transversal type filter.

[0034] In the elastic wave device 1 according to this embodiment, the low acoustic velocity film 5 is disposed on the high acoustic velocity film 4, and the piezoelectric film 6 is disposed on the low acoustic velocity film 5. Therefore, the quality factor can be increased as in the elastic wave device described in Patent Document 1. Furthermore, in this embodiment, the bonding layer 7 formed by metal diffusion is disposed in the low acoustic velocity film 5, and thus warpage hardly occurs at the stage of a mother wafer in production. Therefore, warpage of the piezoelectric film 6 and the like also hardly occurs in the elastic wave device 1 finally obtained. This prevents deterioration of the characteristics.Furthermore, breakage of the piezoelectric film 6, the support substrate 2, and the like is unlikely to occur in a wafer transfer process during production and transportation of the products. This will be explained in more detail in the following description of a production method.

[0035] In the production of an elastic wave device 1, a first silicon oxide film 3 and a high acoustic velocity film 4 are stacked on a mother support substrate. Then, a second silicon oxide film is stacked on the high acoustic velocity film 4 to form a low acoustic velocity film 5, thereby obtaining a first multilayer body. Additionally, a second multilayer body is provided in which an IDT electrode is formed on one surface of a piezoelectric film and a silicon oxide film is formed on the other surface of the piezoelectric film.

[0036] A Ti layer is stacked on each of a surface of the silicon oxide film of the first multilayer body and a surface of the silicon oxide film of the second multilayer body. Then, the Ti layers of the first and second multilayer bodies are bonded together under heating by bringing them into contact with each other. In this case, Ti interdiffused in both Ti layers bonded together. As a result, a bonding layer 7 is formed by metal diffusion bonding. Further, oxygen is supplied from the silicon oxide films to the Ti layers. Therefore, the resulting bonding layer 7 is formed of Ti oxide. This provides sufficient electrical insulation and firmly bonds the first and second multilayer bodies together.

[0037] The obtained multilayer mother body is cut into individual elastic wave devices 1. Thus, an elastic wave device 1 can be obtained.

[0038] In this embodiment, since the bonding layer 7 is located in the low acoustic velocity film 5, warpage hardly occurs in the obtained multilayer mother body.

[0039] The present inventors have found that when the elastic wave device described in Patent Document 1 is obtained by performing metal diffusion bonding, warpage occurs in the piezoelectric film of a multilayer mother body. In the elastic wave device obtained by cutting a multilayer mother body in which warpage has occurred, ripples sometimes appear in electrical characteristics, such as resonance characteristics. Here, such warpage can be eliminated by performing heat-pressing after bonding. However, even if such a warpage elimination process is performed, the deteriorated electrical characteristics described above cannot be recovered. Therefore, it is considered that microcracks, for example, form in a piezoelectric thin film due to warpage.

[0040] As a result of further investigations of warpage, the present inventors have found that when the structures of the first and second multilayer bodies are selected so that the bonding layer 7 is formed in the low acoustic velocity film 5 as in this embodiment, the warpage can be effectively suppressed.

[0041] In Patent Document 1, a multilayer structure including a piezoelectric film, a low acoustic velocity film, and a high acoustic velocity film are bonded to a multilayer structure including a medium layer and a support substrate. Consequently, a large diaphragm stress acts on the piezoelectric film before bonding. Therefore, a relatively large warpage tends to occur in the piezoelectric film at the multilayer mother body stage.

[0042] In contrast, only a silicon oxide film is deposited on the piezoelectric film in the second multilayer body of this embodiment, and thus, no large diaphragm stress acts on the piezoelectric film. Since the mechanical stress acting on the piezoelectric film 6 is small even in a multilayer body obtained by bonding, warpage hardly occurs. This suppresses deterioration of electrical characteristics and also hardly causes cracking. This point will be described using a concrete experimental example.

[0043] A single-port elastic wave resonator was fabricated as the elastic wave device 1 described above. The number of pairs of electrode fingers of an IDT electrode was 100, the intersection width of electrode fingers was 20λ, and the wavelength determined by a center-to-center pitch of electrode fingers was 2.0 μm. The number of electrode fingers in reflectors 9 and 10 was 20. The IDT electrode 8 and reflectors 9 and 10 were formed of Al and had a thickness of 160 nm.

[0044] Fig. Figure 2 illustrates the resonance characteristics of the example according to the above embodiment with a solid line. For comparison, an elastic wave device was manufactured in the same manner as the example of the above embodiment, except that the bonding layer 7 was disposed in the first silicon oxide film 3. Fig. Figure 2 also illustrates the resonance characteristics of the elastic wave device of the prior art example with a broken line. As can be seen from Fig. As can be seen in Figure 2, a ripple appears between the resonance point and the anti-resonance point in the prior art example. In contrast, such a ripple does not appear between the resonance point and the anti-resonance point in the prior art example. Furthermore, the waveform at the resonance point in the prior art example is sharper than in the prior art example, and the peak-to-valley ratio of the impedance characteristics is also large.

[0045] As described above, it is believed that the resonance characteristics in the example are improved compared to the resonance characteristics in the prior art example because no microcracks are generated due to the warping described above.

[0046] Fig. 3 is a schematic cross-sectional elevation view of an elastic wave device according to a second embodiment of the present invention.

[0047] In an elastic wave device 21 of the second embodiment, a first silicon oxide film 3, a high acoustic velocity film 4, a low acoustic velocity film 5, a piezoelectric film 6, and an IDT electrode 8 are stacked on a support substrate 2. In the elastic wave device 21 of the second embodiment, a bonding layer 7 is provided in the high acoustic velocity film 4, that is, the high acoustic velocity film 4 includes high acoustic velocity layers 4a and 4b, and the bonding layer 7 is formed between the high acoustic velocity layer 4a and the high acoustic velocity layer 4b.

[0048] In this embodiment, a second multilayer body in which a low acoustic velocity film and a high acoustic velocity layer are arranged on a piezoelectric film can be provided in production. Therefore, warpage hardly occurs in the piezoelectric film. This suppresses deterioration of the electrical characteristics of the elastic wave device 21 as in the first embodiment, and also hardly causes breakage of the piezoelectric film at the wafer stage or in the elastic wave device 21 finally obtained.

[0049] Fig. 4 is a schematic cross-sectional elevation view of an elastic wave device according to a third embodiment of the present invention.

[0050] In an elastic wave device 31 of the third embodiment, a first silicon oxide film 3, a high acoustic velocity film 4, a second silicon oxide film 5B, a bonding layer 7, a third silicon oxide film 5A, a piezoelectric film 6, and an IDT electrode 8 are stacked on a support substrate 2 in this order. Here, the second silicon oxide film 5B and the third silicon oxide film 5A each serve as a low acoustic velocity film. In this embodiment, the bonding layer 7 is located at an interface between the second silicon oxide film 5B and the third silicon oxide film 5A, each serving as a low acoustic velocity film.

[0051] In this embodiment, a second multilayer body including a piezoelectric film can be provided during production. Therefore, warpage hardly occurs in the piezoelectric film. This suppresses deterioration of the electrical characteristics of the elastic wave device 31 and also hardly causes breakage of the piezoelectric film at the wafer stage or in the elastic wave device 31 that is finally obtained.

[0052] Fig. 5 is a schematic cross-sectional elevation view of an elastic wave device according to a fourth embodiment of the present invention, in which a first silicon oxide film 3, a high acoustic velocity film 4, a low acoustic velocity film 5, a piezoelectric film 6, and an IDT electrode 8 are stacked on a support substrate 2. A bonding layer 7 is located at an interface between the low acoustic velocity film 5 and the piezoelectric film 6.

[0053] In this case, a second multilayer body composed of a piezoelectric film can be provided during production. Therefore, warpage hardly occurs in the piezoelectric film. This suppresses deterioration of the electrical characteristics of the elastic wave device 41 and also hardly causes breakage of the piezoelectric film at the wafer stage or in the elastic wave device 41 that is finally obtained.

[0054] Fig. 6 is a schematic cross-sectional elevation view of an elastic wave device according to a fifth embodiment of the present invention.

[0055] In an elastic wave device 51, a low acoustic velocity film 55 is disposed on a high acoustic velocity substrate 52. A piezoelectric film 56 is disposed on the low acoustic velocity film 55. An IDT electrode 58 is formed on the piezoelectric film 56. Although not specifically illustrated, reflectors are disposed on both sides of the IDT electrode 58 in a propagation direction of the elastic wave, thereby forming a single-port type elastic wave resonator.

[0056] In this embodiment, the high acoustic velocity substrate 52 is used, and a high acoustic velocity film is not separately disposed. Since the low acoustic velocity film 55 and the high acoustic velocity substrate 52 are stacked below the piezoelectric film 56, the Q factor can also be increased in this embodiment. In this way, the high acoustic velocity substrate 52 can serve as both a high acoustic velocity film and a support substrate.

[0057] The high acoustic velocity substrate 52 is made of a suitable material in which the acoustic velocity of a bulk wave is higher than that of an elastic wave propagating through the piezoelectric film 56. In this embodiment, the high acoustic velocity substrate 52 is made of Si. The high acoustic velocity substrate 52 may be made of a suitable material that satisfies the above-described condition.

[0058] A bonding layer 7 is provided in the silicon oxide low acoustic velocity film 55, that is, the bonding layer 7 is disposed at an interface between a first low acoustic velocity layer 55a and a second low acoustic velocity layer 55b. Therefore, a second multilayer body obtained by superposing the IDT electrode 58 and the first low acoustic velocity layer 55a on the piezoelectric film 56 can be readily produced. Consequently, a large diaphragm stress hardly acts on the piezoelectric film 56 in the second multilayer body. Therefore, warpage hardly occurs in the piezoelectric film.

[0059] In production, a metal layer made of Ti or Al, for example, is formed on a surface of the second multilayer body where the low acoustic velocity layer is exposed. Then, a first multilayer body in which a low acoustic velocity layer is stacked on a high acoustic velocity mother substrate is provided. A metal layer made of Ti or the like is formed on the low acoustic velocity layer of the first multilayer body. Then, the first and second multilayer bodies are bonded to each other under heating by bringing the metal layers into contact with each other. Thus, the bonding layer 7 can be formed in the same manner as in the elastic wave device 1 of the first embodiment.

[0060] Subsequently, the obtained multilayer mother body can be cut into individual elastic wave devices 51.

[0061] In this embodiment, since the bonding layer 7 is arranged at the above-described position, warpage hardly occurs at the stage of a piezoelectric mother film during production. This suppresses deterioration of electrical characteristics and also hardly causes breakage and micro-cracking of the piezoelectric film 56 at the stage of a multilayer mother body and during product transportation.

[0062] Fig. 7 is a schematic cross-sectional elevation view of an elastic wave device according to a sixth embodiment not according to the present invention. In an elastic wave device 61, the bonding layer 7 is located at an interface between a piezoelectric film 56 and a low acoustic velocity film 55. The elastic wave device 61 is the same as the elastic wave device 51 except for this point.

[0063] In the elastic wave device 61, the bonding layer 7 is arranged at a position closer to the piezoelectric film 56. Therefore, warpage hardly occurs in the piezoelectric film 56 at the stage of a second multilayer body before bonding. This suppresses deterioration of electrical characteristics as in the elastic wave device 51 of the fifth embodiment. Furthermore, since warpage hardly occurs in the piezoelectric film 56 during the production process, breakage and microcracking hardly occur. Also, since warpage hardly occurs in the piezoelectric film 56 during product transportation, breakage and microcracking hardly occur.

[0064] In the structure including the high-acoustic-velocity substrate 52, as in the case of the elastic wave devices 51 and 61, another intermediate layer may be additionally stacked between the high-acoustic-velocity substrate 52 and the low-acoustic-velocity film 55, that is, the low-acoustic-velocity film 55 may be indirectly stacked over the high-acoustic-velocity substrate. In any case, in the structure including the high-acoustic-velocity substrate 52, the bonding layer 7 may be disposed in the low-acoustic-velocity film 55 or at an interface between the piezoelectric film 56 and the low-acoustic-velocity film 55.

[0065] Next, the relationship between the thickness of the low sound velocity film and the quality factor is described below.

[0066] Various elastic wave devices were prepared by changing the thickness of the first low acoustic velocity layer 55a of the elastic wave device 51 according to the method shown in Fig. 6. More specifically, a high acoustic velocity substrate 52 made of Si was used. An SiO 2 A 55 nm thick Ti film was used as the second low-speed layer 55b. A 0.5 nm thick Ti film was used as the bonding layer 7. A LiTaO 3 -film with a thickness of 600 nm was used as the piezoelectric film 56. The wavelength λ, which is determined by a center-to-center distance of electrode fingers of an ITD electrode, was 2 µm. The first low-acoustic-velocity layer 55a in contact with the piezoelectric film 56 was made of silicon oxide (SiO 2 ) and was designed to have different thicknesses.

[0067] Fig. Figure 8 illustrates the relationship between the thickness of a SiO 2 -film, which serves as the low sound velocity layer 55a, and the quality factor.

[0068] The quality factor increases as the thickness of the SiO 2 film, which serves as the low sound velocity layer 55a. When the thickness of the SiO 2 -film is 240 nm or more, i.e. 0.12λ or more, a high quality factor of more than 1000 is achieved. If the thickness of the SiO 2 film is 440 nm or more, i.e., 0.22λ or more, the quality factor varies little and is essentially constant. Therefore, the quality factor can be adjusted by adjusting the thickness of the SiO 2-film can be further increased to 0.22λ or more, and the variation of the quality factor can be suppressed. When the low acoustic velocity layer in contact with the piezoelectric film 56 is made of silicon oxide, the thickness of the SiO 2 -Film preferably 0.12λ or more and particularly preferably 0.22λ or more.

[0069] The thickness of the SiO 2 The film serving as the low sound velocity layer 55a is preferably 2λ or less. This reduces the membrane voltage.

[0070] Next, the relationship between the thickness of the Ti layer of the bonding layer and the quality factor is described.

[0071] Various elastic wave devices were prepared by changing the thickness of the Ti layer of the bonding layer 7 in the elastic wave device 31 according to the method shown in Fig. 4. More specifically, the high acoustic velocity film 4 was made of Si. The bonding layer 7 was formed of a Ti layer and a Ti oxide layer. The bonding layer 7 was formed such that the Ti oxide layer was located at a position close to the high acoustic velocity film 4 and the Ti layer was located at a position close to the piezoelectric film 6. The thickness of the Ti oxide layer was 50 nm. The low acoustic velocity film was made of SiO. 2 and had a thickness of 700 nm. The piezoelectric film 6 was made of LiTaO 3 and had a thickness of 600 nm. The wavelength λ of a surface acoustic wave serving as an elastic wave used in the elastic wave device 31 was 2 µm.

[0072] Fig.Figure 9 illustrates the relationship between the thickness of the Ti layer serving as the bonding layer and the quality factor.

[0073] The quality factor increases as the thickness of the Ti layer of the bonding layer decreases. In particular, when the thickness of the Ti layer is 2.0 nm or less, that is, 1 × 10 -3 λ or less, a high quality factor of more than 1000 is achieved. When the thickness of the Ti layer is 1.2 nm or less, that is, 0.6 × 10 -3 λ or less, the Q factor varies little and is essentially constant. Therefore, the Q factor can be improved by setting the thickness of the Ti layer of the bonding layer to 1.2 nm or less or 0.6 × 10 -3 λ or less, and the variation in the quality factor can be suppressed. The thickness of the Ti layer is preferably 2.0 nm or less, and more preferably 1.2 nm or less.

[0074] The thickness of the Ti layer is preferably 0.4 nm or more. This achieves a suitable bonding between the first multilayer body and the second multilayer body. LIST OF REFERENCE SYMBOLS 1 device for elastic waves 2 Support substrate 3 first silicon oxide film 4 Film with high speed of sound 4a, 4b Layer with high speed of sound 5 Film with low speed of sound 5a, 5b Layer with low sound speed 6 piezoelectric film 7 Bonding layer 8 IDT electrode 9, 10 Reflector 21, 31, 41, 51, 61 Device for elastic waves 52 Substrate with high speed of sound 55 Film with low speed of sound 55a, 55b Layer with low speed of sound 56 piezoelectric film 58 IDT electrode

Claims

[1] Elastic wave device (1, 21, 31), comprising: a piezoelectric film (6), a low sound velocity film (5) arranged on the piezoelectric film and in which a sound velocity of a bulk wave propagating through the low sound velocity film is smaller than a sound velocity of a bulk wave propagating through the piezoelectric film, a high sound velocity film (4) stacked on a surface of the low sound velocity film on a side opposite to the piezoelectric film, and in which a sound velocity of a bulk wave propagating through the high sound velocity film is higher than a sound velocity of an elastic wave propagating through the piezoelectric film, a substrate (2) stacked directly or indirectly on a surface of the high-speed film on a side opposite the low-speed film, and a bonding layer (7) located in the high sound velocity film (4) or in the low sound velocity film (5). [2] The elastic wave device (1, 21, 31) according to claim 1, wherein the high acoustic velocity film (4) is made of aluminum nitride or silicon nitride. [3] The elastic wave device (1, 21, 31) according to claim 1 or 2, further comprising an intermediate layer (3) disposed between the high acoustic velocity film (4) and the substrate (2). [4] Elastic wave device (51) comprising: a piezoelectric film (56) and a low sound velocity film (55) arranged on the piezoelectric film and in which a sound velocity of a bulk wave propagating through the low sound velocity film is smaller than a sound velocity of a bulk wave propagating through the piezoelectric film, a high acoustic velocity substrate (52) stacked directly or indirectly on a surface of the low acoustic velocity film on a side opposite to the piezoelectric film, and in which a sound velocity of a bulk wave propagating through the high acoustic velocity substrate is higher than a sound velocity of an elastic wave propagating through the piezoelectric film, and a bonding layer (7) located in the high sound velocity substrate (52) or in the low sound velocity film (55). [5] The elastic wave device (1, 21, 31, 51) according to any one of claims 1 to 4, wherein the bonding layer (7) contains a metal oxide layer or a metal nitride layer. [6] The elastic wave device (1, 21, 31, 51) according to any one of claims 1 to 5, wherein the bonding layer (7) includes a Ti layer, and the Ti layer has a thickness of 0.4 nm or more and 2.0 nm or less. [7] The elastic wave device (1, 21, 31, 51) according to claim 6, wherein the Ti layer has a thickness of 0.4 nm or more and 1.2 nm or less. [8] The elastic wave device (1, 21, 31, 51) according to any one of claims 1 to 7, wherein the piezoelectric film (6, 56) is made of LiTaO3. [9] An elastic wave device (1, 21, 31, 51) according to any one of claims 1 to 8, wherein the low acoustic velocity film (5, 55) is made of silicon oxide. [10] The elastic wave device (1, 21, 31, 51) according to claim 1 or 4, wherein the low acoustic velocity film (5, 55) is made of silicon oxide, and the bonding layer (7) is disposed in the low acoustic velocity film (5, 55), the low acoustic velocity film (5, 55) includes a first low acoustic velocity layer (5a, 55a) located on a side of the bonding layer close to the piezoelectric film, and a second low acoustic velocity layer (5b, 55b) located on a side of the bonding layer opposite to the piezoelectric film, and when an elastic wave used in the elastic wave device is assumed to have a wavelength λ, the first low acoustic velocity layer (5a, 55a) has a thickness of 0.12λ or more. [11] The elastic wave device (1, 21, 31, 51) according to claim 10, wherein the first low acoustic velocity layer (5a, 55a) has a thickness of 0.22λ or more.

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