Elastic wave devices

Abstract

To provide an elastic wave device that can suppress damage to the piezoelectric layer. To provide an elastic wave device that can improve the heat dissipation of the piezoelectric layer. [Solution] The elastic wave device 1 is an elastic wave device that propagates elastic waves and comprises a Si substrate 11 including holes 16, a SiC film 12 formed on the surface 11a side of the Si substrate 11, a piezoelectric layer 14 made of a piezoelectric material formed on the surface 12a side of the SiC film 12, and an IDT (Inter-Digital Transducer) electrode 15 formed on the surface 14a side of the piezoelectric layer 14. The back surface 12b of the SiC film 12 is exposed at the bottom of the holes 16 in the Si substrate 11.

Description

Technical Field

[0001] The present invention relates to an elastic wave device. More specifically, the present invention relates to an elastic wave device that propagates elastic waves.

Background Art

[0002] With the full-scale spread of communication devices such as smartphones, the need to improve the communication capacity and communication speed of mobile wireless communication systems has increased explosively. A filter device is mounted on the communication device. The filter device constitutes a filter circuit that removes radio wave signals having frequencies outside the frequency band required for communication and disturbance noise from the radio wave signal input to the communication device, and extracts only the radio wave signal in the desired frequency band. As this filter device, there are SAW (Surface Acoustic Wave) devices, BAW (Bulk Acoustic Wave) devices, and LC resonators. As the filter device of communication devices heretofore, particularly SAW devices, BAW devices, etc. have been adopted. For example, SAW devices are adopted in communication frequency bands of less than 0.2 to 3.0 GHz, and BAW devices are adopted in communication frequency bands exceeding 1.5 to 3.5 GHz. By using SAW devices, BAW devices, etc. as the filter device of communication devices, steep filter characteristics (high Q value) can be obtained in a frequency band with a large number of communication bands.

[0003] The SAW device is mainly composed of a piezoelectric substrate and an IDT (Inter-Digital Transducer) electrode (also called a comb electrode or a grating electrode) formed thereon. As the piezoelectric material constituting the piezoelectric substrate, LT (LiTaO3, lithium tantalate), LN (LiNbO3, lithium niobate), or quartz is often used, and there are also many other materials having piezoelectric characteristics. In particular, LT and LN are suitable as the piezoelectric substrate of the elastic wave device for filters because they have a large coupling coefficient as piezoelectric materials.

[0004] Generally, in a SAW device, an electrical signal is input to the input electrodes of the IDT electrodes, and strain is generated in the piezoelectric substrate by the voltage applied from the input electrodes, and surface acoustic waves propagate in the piezoelectric substrate. The surface acoustic waves propagating in the piezoelectric substrate are converted into an electrical signal at the output electrodes of the IDT electrodes and output. The frequency of the generated acoustic wave is defined by the distance between the electrodes in the IDT electrodes and the conditions of the electrodes (such as material density, thickness, or width). By combining a plurality of SAW devices as resonators, a filter circuit that allows only electrical signals in a specific frequency band to pass through can be obtained.

[0005] The frequency band operable in conventionally manufacturable SAW devices is said to be up to around 3.5 GHz at most. For this reason, the development of SAW devices operable in the SUB-6 frequency band (approximately 3.3 GHz to 6 GHz) used in 5G (the fifth-generation mobile communication system) and the frequency band around 5 GHz frequently used in wireless LAN (Local Area Network) and Wifi is underway.

[0006] In order to increase the operating frequency of a SAW device to a frequency of, for example, 3.5 GHz or higher, a SAW device having a structure in which a piezoelectric substrate made of LT or LN or the like and a substrate made of Si (silicon), which is a material with a high propagation speed of acoustic waves, are bonded has been proposed. In this SAW device, when the piezoelectric substrate and the substrate made of Si or the like function as a propagation layer of acoustic waves, the energy of the acoustic wave concentrates in the piezoelectric substrate where the propagation speed of the acoustic wave is relatively slow. As a result, the propagation speed of the acoustic wave in the SAW device can be improved, and the operating frequency of the SAW device can be improved, compared with the case where a single piezoelectric substrate is used as the propagation layer of acoustic waves without bonding a substrate made of Si or the like.

[0007] Further, Patent Document 1 below discloses a SAW device including a piezoelectric substrate, a support substrate having a thermal expansion coefficient smaller than that of the piezoelectric substrate, and a SiC (silicon carbide) thin film layer bonded to the piezoelectric substrate.

[0008] However, these structures had limitations in improving the operating frequency of SAW devices. Therefore, plate wave devices have been proposed as a method to further improve the operating frequency of elastic wave devices. A plate wave device is a type of elastic wave device that includes a thin film propagation layer with a thickness sufficiently thin relative to the wavelength, and in which the upper and lower surfaces of the plate-shaped propagation layer are in contact with air. In a plate wave device, elastic waves propagating within the propagation layer undergo total internal reflection at both the upper and lower surfaces of the propagation layer, concentrating the energy of the elastic waves within the propagation layer and exciting plate waves, including several types of Lamb waves and SH waves. As a result, by utilizing plate waves with a high propagation speed, the operating frequency of the elastic wave device can be improved.

[0009] Conventional plate wave devices are disclosed in Patent Document 2 below. Patent Document 2 discloses an elastic wave device comprising a silicon substrate, a silicon oxide film, a piezoelectric layer made of LT or LN, and a plurality of electrodes. The silicon substrate includes at least a portion of a cavity facing a portion of the piezoelectric layer. Because the silicon substrate includes a cavity, the upper and lower surfaces of the piezoelectric layer are in contact with air, and the piezoelectric layer becomes a plate-shaped propagation layer that propagates elastic waves. The silicon oxide film is provided between the silicon substrate and the piezoelectric layer. Plate wave devices have the characteristic of being able to increase the propagation speed of elastic waves and improve the operating frequency of the device. In addition, plate wave devices can increase the electromechanical coupling coefficient of elastic waves, and when used as a filter device, they can pass electrical signals with a wide frequency band. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2019-161634 [Patent Document 2] Patent No. 7103528 [Overview of the project] [Problems that the invention aims to solve]

[0011] Conventional elastic wave devices, such as those described in Patent Document 2, required a thin piezoelectric layer to generate plate waves. This resulted in insufficient strength in the piezoelectric layer, making it prone to damage. This problem was particularly pronounced when elastic wave devices were incorporated into portable communication devices susceptible to impact, such as smartphones and wearable devices.

[0012] Furthermore, because the thermal conductivity of LT or LN constituting the piezoelectric layer is very low, conventional elastic wave devices have the problem of poor heat dissipation of the piezoelectric layer. This problem causes heat to accumulate in the piezoelectric layer, causing it to become hot. As a result, the electrodes of the piezoelectric layer change, making it difficult to propagate elastic waves, and increasing the likelihood of wiring failure due to short circuits. In addition, repeated thermal contraction of the piezoelectric layer, which has a different coefficient of thermal expansion than the electrode layer, can cause metal fatigue of the IDT electrode, potentially leading to the IDT electrode delaminating from the piezoelectric layer.

[0013] The present invention aims to solve the above problems, and one of its objectives is to provide an elastic wave device that can suppress damage to the piezoelectric layer.

[0014] Another object of the present invention is to provide an elastic wave device that can improve the heat dissipation performance of the piezoelectric layer. [Means for solving the problem]

[0015] An elastic wave device according to one aspect of the present invention is an elastic wave device for propagating elastic waves, comprising a Si substrate containing holes, a SiC film formed on the surface side of the Si substrate, a piezoelectric layer made of a piezoelectric material formed on the surface side of the SiC film, and an IDT electrode formed on the surface side of the piezoelectric layer, wherein the back surface of the SiC film is exposed at the bottom of the holes in the Si substrate.

[0016] Preferably, in the above-described elastic wave device, the piezoelectric layer is made of lithium tantalate or lithium niobate, and the piezoelectric layer has a thickness of 18 nm to 5.0 μm.

[0017] In the above-described elastic wave device, the SiC film preferably has a thickness of 5 nm to 4.0 μm.

[0018] Preferably, the above-described elastic wave device further comprises a bonding layer formed between the piezoelectric layer and the SiC film. [Effects of the Invention]

[0019] According to the present invention, it is possible to provide an elastic wave device that can suppress damage to the piezoelectric layer. Furthermore, according to the present invention, it is possible to provide an elastic wave device that can improve the heat dissipation performance of the piezoelectric layer. [Brief explanation of the drawing]

[0020] [Figure 1] This is a cross-sectional view showing the configuration of an elastic wave device 1 in one embodiment of the present invention. [Figure 2] This is the first figure showing a method for manufacturing an elastic wave device 1 according to one embodiment of the present invention. [Figure 3] This is a second figure showing a method for manufacturing an elastic wave device 1 according to one embodiment of the present invention. [Figure 4] This is a third figure showing a method for manufacturing an elastic wave device 1 according to one embodiment of the present invention. [Figure 5] This is the fourth figure showing a method for manufacturing an elastic wave device 1 according to one embodiment of the present invention. [Figure 6] This is the fifth figure showing a method for manufacturing an elastic wave device 1 according to one embodiment of the present invention. [Figure 7] This is the sixth figure showing a method for manufacturing an elastic wave device 1 according to one embodiment of the present invention. [Figure 8] This figure shows the displacement directions of the S-mode and A-mode of Lamb waves, and the propagation modes of SH waves. [Figure 9] This figure shows the setting conditions and simulation results of the SH0 mode elastic wave in one embodiment of the present invention. [Figure 10]This graph shows the simulation results of the relationship between the propagation speed of the SH0 mode elastic wave and the admittance Y in one embodiment of the present invention. [Figure 11] This figure shows the setting conditions and simulation results of the S0 mode elastic wave in one embodiment of the present invention. [Figure 12] This graph shows the simulation results of the relationship between the propagation velocity of an S0 mode elastic wave and admittance Y in one embodiment of the present invention. [Modes for carrying out the invention]

[0021] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0022] Figure 1 is a cross-sectional view showing the configuration of an elastic wave device 1 in one embodiment of the present invention.

[0023] Referring to Figure 1, the elastic wave device 1 (an example of an elastic wave device) is an elastic wave device that propagates elastic waves. The elastic wave device 1 has a structure in which a thin SiC layer 12 is added to a piezoelectric layer 14 made of LT or LN as a propagation layer for propagating elastic waves. The elastic wave device 1 comprises a Si substrate 11 (an example of a Si substrate), a SiC layer 12 (an example of a SiC layer), a bonding layer 13 (an example of a bonding layer), a piezoelectric layer 14 (an example of a piezoelectric layer), and an IDT electrode 15 (an example of an IDT electrode).

[0024] The Si substrate 11 includes a front surface 11a and a back surface 11b. The front surface 11a is the surface facing the SiC layer 12, and the back surface 11b is the surface opposite to the SiC layer 12. The Si substrate 11 preferably has a thickness of 100 μm or more and 3000 μm or less, and more preferably 300 μm or more. By having a thickness of 100 μm or more (more preferably 300 μm or more), the mechanical strength of the SiC layer 12 can be reinforced.

[0025] The Si substrate 11 further contains a hole 16. When viewed from above, the Si substrate 11 has a closed curve shape surrounding the hole 16. The hole 16 penetrates the Si substrate 11 in the thickness direction. The bottom of the hole 16 exposes the back surface 12b of the SiC layer 12.

[0026] The SiC layer 12 is formed on the surface 11a side of the Si substrate 11. The SiC layer 12 includes a surface 12a and a back surface 12b. The back surface 12b of the SiC layer 12 is in contact with the surface 11a of the Si substrate 11. The SiC layer 12 preferably has a thickness of 5 nm or more and 4.0 μm or less. A thickness of 5 nm or more allows for effective support of the piezoelectric layer 14. A thickness of 4.0 μm or less prevents the elastic wave propagation layer (piezoelectric layer 14 and SiC layer 12) from becoming too thick, allowing the elastic waves generated within the propagation layer to be plate waves.

[0027] The piezoelectric layer 14 is formed on the surface 12a side of the SiC layer 12. The piezoelectric layer 14 includes a surface 14a and a back surface 14b. The piezoelectric layer 14 is made of a piezoelectric material, preferably LT or LN. It is preferable that the SiC layer 12 and the piezoelectric layer 14 are joined together. The piezoelectric layer 14 preferably has a thickness of 0.02λ or more and 0.6λ or less, where λ is the wavelength of the propagating elastic wave. A thickness of 0.02λ for the piezoelectric layer 14 corresponds to the lower limit of the thickness at which the plate wave is effectively excited. In a typical case (for example, when λ = 4.0 μm), a thickness of 0.02λ or more and 0.6λ or less corresponds to a thickness of 80 nm or more and 2.4 μm or less. In typical cases in the frequency bands of LTE and SUB-6 used in commonly used mobile communication equipment, the wavelength λ is about 0.9 μm to 8 μm. When the wavelength λ is between 0.9 μm and 8.3 μm, a thickness of 0.02 λ to 0.6 λ corresponds to a thickness of 18 nm to 5.0 μm. When the thickness of the piezoelectric layer 14 is the lower limit of 0.02 λ and the wavelength λ is 0.9 μm, the thickness of the piezoelectric layer 14 is 18 nm. A piezoelectric layer 14 with a thickness of 18 nm has low mechanical strength and is extremely fragile on its own. However, the piezoelectric layer 14 can be reinforced by the SiC layer 12 (especially by bonding), preventing damage to the piezoelectric layer 14. Furthermore, when the thickness of the piezoelectric layer 14 is the upper limit of 0.6 λ and the wavelength λ is 8.3 μm, the thickness of the piezoelectric layer 14 is 5.0 μm. By making the piezoelectric layer 14 less than 5.0 μm thick, the elastic wave propagation layer (piezoelectric layer 14 and SiC layer 12) does not become too thick, and the elastic waves generated within the propagation layer can be made into plate waves.

[0028] The SiC layer 12 and the piezoelectric layer 14 may be formed by bonding them together or by a manufacturing method such as film deposition. When the SiC layer 12 and the piezoelectric layer 14 are bonded together, a bonding layer 13 exists between the SiC layer 12 and the piezoelectric layer 14. The bonding layer 13 is in contact with the surface 12a of the SiC layer 12 and the back surface 14b of the piezoelectric layer 14. The bonding layer 13 has a thickness of, for example, 4 nm to 13 nm. When formed by a manufacturing method such as film deposition, the bonding layer 13 does not exist, and the SiC layer 12 and the piezoelectric layer 14b are in contact with each other.

[0029] The IDT electrode 15 is formed on the surface 14a side of the piezoelectric layer 14. The spacing between electrodes in the IDT electrode 15, as well as the material and thickness of the electrodes, are set based on the frequency characteristics used in the elastic wave device 1. The IDT electrode 15 is made of Al (aluminum) or the like.

[0030] When an AC signal is applied to the IDT electrode 15, vibrations of a wavelength corresponding to the spacing between the electrodes in the IDT electrode 15, as well as the material and thickness of the electrodes, are excited. These vibrations become plate waves and propagate through the propagation layers (SiC layer 12 and piezoelectric layer 14).

[0031] Since holes 16 are formed in the Si substrate 11, the elastic wave device 1 becomes a plate wave device, and the SiC layer 12 and the piezoelectric layer 14 constitute a plate-shaped propagation layer of the plate wave device. The upper surface 14a of the piezoelectric layer 14 and the lower surface 12b of the SiC layer 12 are both in contact with air. In order to generate plate waves, the combined thickness of the SiC layer 12 and the piezoelectric layer 14 is preferably set to a thickness of about 10 to 30% of the wavelength of the elastic wave. Since the propagation speed of low-mode elastic waves in the SiC layer 12 is faster than the propagation speed in the piezoelectric layer 14, low-mode elastic waves mainly propagate in the piezoelectric layer 14, and the energy of the propagating elastic waves is concentrated in the piezoelectric layer 14.

[0032] The SiC layer 12 may have insulating properties (resistivity greater than 100 Ωcm). Generally, in an elastic wave device, the greater the electromechanical coupling coefficient of the elastic wave propagation layer, the greater the difference between the resonance frequency and the anti-resonance frequency of the elastic wave, and a wider bandwidth can be obtained when a filter is configured using the elastic wave device. Here, the electromechanical coupling coefficient calculated from the resonance frequency and the anti-resonance frequency is the effective electromechanical coupling coefficient K eff 2 is used. When the elastic wave propagation layer includes the SiC layer, the effective electromechanical coupling coefficient K eff 2 of the elastic wave propagation layer regarding the S0 mode and SH0 mode elastic waves decreases compared to the case where the elastic wave propagation layer does not include the SiC layer. However, when the SiC layer 12 has insulating properties (resistivity greater than 100 Ωcm), the decrease in the effective electromechanical coupling coefficient K eff 2 of the propagation layer regarding the S0 mode and SH0 mode elastic waves can be suppressed to some extent.

[0033] The SiC layer 12 may have conductive properties (resistivity less than 100 Ωcm). When the SiC layer 12 has conductive properties (resistivity less than 100 Ωcm), the back surface 14b side of the piezoelectric layer 14 is equalized in potential, and in the case of the SH1 mode, the excitation of the elastic wave is promoted. Also, by appropriately adjusting the thickness of the piezoelectric layer 14 and the cut angle of the piezoelectric substrate 141 (the cut angle based on the orientation flat having a predetermined crystal orientation when processing an ingot formed by the Cz method into a wafer shape), etc., the effective electromechanical coupling coefficient K eff 2 regarding the SH1 mode elastic wave can be made larger than the effective electromechanical coupling coefficient K eff 2 when the elastic wave propagation layer does not include the SiC layer.

[0034] Next, a method for manufacturing the elastic wave device 1 in an embodiment of the present invention will be described using FIGS. 2 to 7.

[0035] Refer to Figure 2 to prepare the Si substrate 11. The surface 11a of the Si substrate 11 faces upward in Figure 2, and the back surface 11b of the Si substrate 11 faces downward in Figure 2. The plane orientation of the surface 11a of the Si substrate 11 is arbitrary, but it is preferably a (100) plane or a (111) plane. To adjust the conductivity, the Si substrate 11 may contain impurities such as Al (aluminum) or P (phosphorus). The Si substrate 11 may be amorphous. The plane orientation of the surface 11a of the Si substrate 11 is, for example, a (111) plane. The Si substrate 11 may have an n-type conductivity or it may be semi-insulating. The plane orientation of the surface 11a of the Si substrate 11 may be a (110) plane or a (010) plane. The Si substrate 11 has, for example, a diameter of 6 inches.

[0036] Next, a single-crystal SiC layer 12 is formed on the surface 11a of the Si substrate 11. This yields a structure 51 containing the Si substrate 11 and the SiC film 12. The SiC layer 12 may be formed by homoepitaxial growth of SiC on a SiC underlayer obtained by carbonizing the surface 11a of the Si substrate 11 using methods such as MBE (Molecular Beam Epitaxy), CVD (Chemical Vapor Deposition), or LPE (Liquid Phase Epitaxy). The SiC layer 12 may also be formed solely by carbonizing the surface 11a of the Si substrate 11. Furthermore, the SiC layer 12 may be formed by heteroepitaxial growth on the surface 11a of the Si substrate 11 (or with a buffer layer in between). Impurities may be intentionally doped into the SiC layer 12 to adjust its conductivity.

[0037] When a SiC layer 12 is formed on the surface 11a of a Si substrate 11 by carbonization, homoepitaxial growth, or heteroepitaxial growth, the SiC layer 12 has a 3C-type crystal structure. When the SiC layer 12 has a 3C-type crystal structure, the surface orientation of the surface 12a of the SiC layer 12 is preferably (111), (100), or (110). The off-angle of the surface 12a of the SiC layer 12 is preferably 0° or more and 10° or less.

[0038] After forming the SiC layer 12, the surface 12a of the SiC layer 12 may be polished using the CMP (Chemical Mechanical Polishing) method to thin the SiC layer 12 to the required thickness. The SiC layer 12 preferably has a thickness of 5 nm to 4.0 μm, and more preferably 1 μm or less. However, it is difficult to fabricate bulk SiC substrates with the above thickness range. Therefore, it is desirable to form a SiC layer 12 of any thickness using this method.

[0039] Referring to Figure 3, a piezoelectric substrate 141 made of piezoelectric material is prepared. The piezoelectric substrate 141 will become the piezoelectric layer 14 in a later process. The piezoelectric substrate 141 may be a single crystal, twin, or ceramic.

[0040] Next, the structure 51 obtained in the process shown in Figure 2 is positioned so that the surface 12a of the SiC layer 12 faces upward in Figure 3. Also, the back surface 141b of the piezoelectric substrate 141 is positioned so that it faces downward in Figure 3 (the back surface 141b of the piezoelectric substrate 141 corresponds to the back surface 14b of the piezoelectric layer 14). In this state, the surface 12a of the SiC layer 12 and the back surface 141b of the piezoelectric substrate 141 are joined together. When joining, it is preferable that the SiC layer 12 satisfies at least one of the following two conditions (a) and (b): (a) the full width at half maximum of the X-ray rocking curve of the surface orientation of the surface 12a of the SiC layer 12 is greater than 0 and 2000 arcsec or less, and (b) the full width at half maximum of the orientation difference distribution of the surface 12a of the SiC layer 12 as measured by electron backscatter diffraction is greater than 0 and 2000 arcsec or less. This makes the surface 12a of the SiC layer 12 suitable for bonding. Furthermore, in order to ensure reliable contact with the back surface 141b of the piezoelectric substrate 141, the arithmetic mean roughness Ra of the surface 12a of the SiC layer 12 is preferably greater than 0 and 1 nm or less, and more preferably 0.7 nm or less.

[0041] Next, the surface 12a of the SiC layer 12 and the back surface 141b of the piezoelectric substrate 141 are joined using a room-temperature activated bonding method or the like. Room-temperature activated bonding is a technique that removes the adsorbed and oxide layers present on the surface 12a of the SiC layer 12 and the back surface 141b of the piezoelectric substrate 141 by energy shock such as ion bombardment, and joins them at room temperature by contact alone, directly utilizing the surface energy of the surface 12a of the SiC layer 12 and the back surface 141b of the piezoelectric substrate 141. When using room-temperature activated bonding, 1 × 10⁻⁶ -5 Pa or less, preferably 1 × 10⁻⁶ -6 In a reduced pressure below Pa and at room temperature (for example, a temperature between 10°C and 30°C), energy particles are irradiated onto the surface 12a of the SiC layer 12 and the back surface 141b of the piezoelectric substrate 141, as indicated by arrow AR1. This removes adsorbed substances such as gas, water, organic matter, or oxygen from the surface 12a of the SiC layer 12 and the back surface 141b of the piezoelectric substrate 141. The energy particles consist of neutral atoms such as ions, Ar (argon), Kr (krypton), or Ne (neon), or cluster ions. It is preferable that the energy particles consist of Ar.

[0042] When energy particles are irradiated onto the surface 12a of the SiC layer 12 and the back surface 141b of the piezoelectric substrate 141, amorphous layers 131 and 132 appear on the surface 12a of the SiC layer 12 and the back surface 141b of the piezoelectric substrate 141, respectively, which are layers in which the SiC constituting the SiC layer 12 and the material constituting the piezoelectric substrate 141 have become amorphous due to the collision of energy particles. Each of the amorphous layers 131 and 132 has a thickness, for example, greater than 0 and less than or equal to 5 nm. Next, amorphous layer 131 and amorphous layer 132 are brought into contact with each other, as shown by arrow AR2.

[0043] Referring to Figure 4, as a result of the contact between amorphous layer 131 and amorphous layer 132, the surface 12a of the SiC layer 12 and the back surface 141b of the piezoelectric substrate 141 are joined. Between the surface 12a of the SiC layer 12 and the back surface 141b of the piezoelectric substrate 141, a bonding layer 13 consisting of the two types of amorphous layers 131 and 132 appears. The piezoelectric substrate 141 becomes the piezoelectric layer 14. Note that amorphous layers 131 and amorphous layer 132 may also be formed by forming a Si thin film on the surface 12a of the SiC layer 12 and the back surface 141b of the piezoelectric substrate 141, respectively, using sputtering or CVD, and then irradiating the Si thin film with energy particles.

[0044] Furthermore, any method can be used to bond the surface 12a of the SiC layer 12 to the back surface 141b of the piezoelectric substrate 141 (the back surface 14b of the piezoelectric layer 14). In addition to room-temperature activated bonding, metal diffusion bonding and hydrophilic bonding methods can also be used. By using such bonding methods, the Si substrate 11 and the SiC layer 12 can be strongly bonded.

[0045] The metal diffusion bonding method involves interposing a metal between the surface 12a of the SiC layer 12 and the back surface 141b of the piezoelectric substrate 141. By applying heat and pressure to both the SiC layer 12 and the piezoelectric substrate 141, a bond is formed by the diffusion of atoms at the bonding surface. When the metal diffusion bonding method is used, a bonding layer 13 containing the interposed metal (e.g., Al, Cu (copper), Ti (titanium), or Cr (chromium)) is formed between the surface 12a of the SiC layer 12 and the back surface 141b of the piezoelectric substrate 141. Simultaneously, the effect of equipotentializing the back surface 141b of the piezoelectric substrate 141 is also expected.

[0046] The hydrophilic bonding method involves forming an oxide film layer, such as a hydrophilic SiO2 (silicon oxide) layer, on the surface 12a of the SiC layer 12 and the back surface 141b of the piezoelectric substrate 141, respectively, using heat treatment in an oxygen atmosphere, oxidation with oxidizing chemicals, or CVD. The SiO2 layers on the surface 12a of the SiC layer 12 and the back surface 141b of the piezoelectric substrate 141 are brought into contact with each other, and a bond is formed by applying heat and pressure. When the hydrophilic bonding method is used, a bonding layer 13 containing oxygen atoms such as SiO2 is formed between the surface 12a of the SiC layer 12 and the back surface 141b of the piezoelectric substrate 141.

[0047] Referring to Figure 5, after forming the piezoelectric layer 14, the surface 14a of the piezoelectric layer 14 may be polished using a CMP method or the like. This removes a portion of the piezoelectric layer 14 (the portion shown by the dotted line in Figure 5), and the piezoelectric layer 14 may be thinned to the required thickness. The position of the surface 14a of the piezoelectric layer 14 will recede.

[0048] Referring to Figure 6, next, an IDT electrode 15 is formed on the surface 14a of the piezoelectric layer 14, for example, using a vapor deposition method.

[0049] Next, holes 16 (Figure 1) are formed in the Si substrate 11. The holes 16 can be formed at any time after the formation of the SiC layer 12, and may be formed before the formation of the IDT electrode 15 or before the formation of the piezoelectric layer 14. The holes 16 are formed, for example, by the following method.

[0050] A mask layer 91 made of SiC or the like is formed on the outer edge of the back surface 11b of the Si substrate 11. The central part RG1 of the back surface 11b of the Si substrate 11 is removed using the mask layer 91 as a mask. The removal of the central part RG1 is performed by any method, for example, by mechanical polishing such as sandblasting.

[0051] As a result of removing the central RG1, a groove 92 is formed on the back surface 11b of the Si substrate 11, with the material constituting the Si substrate 11 as the bottom surface. The groove 92 has a depth that does not penetrate the Si substrate 11. Due to the presence of the groove 92, the thickness of the central part of the Si substrate 11 becomes thinner than the thickness of the outer edge of the Si substrate 11.

[0052] Referring to Figure 7, the bottom surface RG2 of the groove 92, which is part of the Si substrate 11, is removed by wet etching. As a result of removing the bottom surface RG2, the groove 92 becomes a hole 16 that penetrates the Si substrate 11 in the thickness direction. The back surface 12b of the SiC layer 12 is exposed at the bottom surface of the hole 16. By employing wet etching as the method for removing the bottom surface RG2, damage to the SiC layer 12 during the removal of the bottom surface RG2 can be suppressed. Alternatively, the hole 16 may be formed in the Si substrate 11 using only wet etching without forming the groove 92 by any other method. After the formation of the hole 16, the mask layer 91 is removed. When wet etching is employed as the method for removing the bottom surface RG2, the Si substrate 11 is etched isotropically. As a result, the width of the hole 16 (length in the horizontal direction in Figure 7) decreases as it approaches the back surface 12b of the SiC layer 12.

[0053] For wet etching of the bottom surface RG2, a mixed acid containing oxidizing acids such as hydrofluoric acid and nitric acid, or an aqueous solution of KOH (potassium hydroxide) can be used as the chemical agent. To suppress etching of the SiC layer 12 and to improve the quality of the SiC layer 12, it is preferable to use a mixed acid consisting of hydrofluoric acid and nitric acid as the chemical agent for wet etching of the Si substrate 11.

[0054] When wet etching the bottom surface RG2, it is preferable to move the SiC layer 12 and the Si substrate 11 relative to the chemical solution used for wet etching. In particular, in order to avoid the SiC layer 12 being damaged by the pressure received from the chemical solution while the SiC layer 12 and the Si substrate 11 are being moved, it is preferable to move the SiC layer 12 and the Si substrate 11 in a direction in a plane parallel to the surface 12a of the SiC layer 12. Spin etching is the most preferred method of wet etching.

[0055] The mask layer 91 is made of a material that is insoluble in at least one of the following: a chemical solution containing an acid and hydrofluoric acid that have an oxidizing effect on Si, and an alkaline aqueous solution composed only of components that do not have an oxidizing effect on Si. The mask layer 91 may be made of, for example, SiC, SiN, SiO2, or photoresist. The holes 16 may be formed by any method. In addition to the above method, the holes 16 may be formed by chemical etching methods such as dry etching or wet etching, mechanical processing methods such as sandblasting, machining centers, or lasers, or by any combination of these processing methods.

[0056] Through the above process, the elastic wave device 1 shown in Figure 1 is obtained.

[0057] Figure 8 shows the displacement directions of the S-mode and A-mode of Lamb waves, and the propagation modes of SH waves. In Figures 8(a) to 8(d), the direction of the arrows indicates the displacement direction of the elastic wave, while in Figures 8(e) and (f), the paper thickness direction indicates the displacement direction of the elastic wave.

[0058] Referring to Figure 8, the plate waves propagating through the elastic wave device 1 are classified into Lamb waves (primarily components in the elastic wave propagation direction and the piezoelectric thickness direction) and SH waves (primarily SH components) depending on the displacement component. Lamb waves are further classified into symmetric modes (S modes) and antisymmetric modes (A modes). When folded at a line half the thickness of the propagation layer, those with overlapping displacements are called symmetric modes, and those with opposite displacements are called antisymmetric modes. The subscript number indicates the number of nodes in the thickness direction. Here, an A1 mode Lamb wave is a first-order antisymmetric mode Lamb wave.

[0059] [Effects of the embodiment]

[0060] Even in a thin film state, the SiC layer possesses high mechanical strength (e.g., tensile strength exceeding 2 GPa) and high heat dissipation (e.g., thermal conductivity of approximately 200 W / mK). In the above embodiment, since the piezoelectric layer made of piezoelectric material is formed on the surface side of the SiC layer, the mechanical strength of the piezoelectric layer can be reinforced by the SiC layer, thereby suppressing damage to the piezoelectric layer. Furthermore, heat from the piezoelectric layer can be efficiently transferred to the SiC layer, improving the heat dissipation of the piezoelectric layer.

[0061] In addition, the SiC layer has high hardness and exhibits little change in hardness due to temperature changes. Therefore, it is possible to suppress fluctuations in properties due to temperature changes. Furthermore, the propagation speed of low-mode elastic waves such as S0 mode and SH0 mode within the SiC layer is faster than the propagation speed of elastic waves within the piezoelectric layer made of LT or LN, so it is possible to increase the propagation speed of low-mode elastic waves such as S0 mode and SH0 mode within the propagation layer in an elastic wave device.

[0062] [Examples]

[0063] Figure 9 shows the setting conditions and simulation results of the SH0 mode elastic wave in one embodiment of the present invention. Figure 10 is a graph showing the simulation results of the relationship between the propagation speed and admittance Y of the SH0 mode elastic wave in one embodiment of the present invention.

[0064] Referring to Figures 9 and 10, the inventors of the present invention virtually created the elastic wave devices of Examples 1 and 2 of the present invention and the elastic wave device of Comparative Example 1, and performed simulations of the SH0 mode elastic waves generated by the elastic wave devices.

[0065] As examples 1 and 2 of the present invention, elastic wave devices having the structure shown in Figure 1 were fabricated, and various parameters in the elastic wave device were set to different values ​​from each other. These parameters included the thickness h of the IDT electrode made of Al. AL , the spacing MR of the IDT electrodes made of Al AL , thickness h of the piezoelectric layer made of LN LN , and the thickness h of the SiC layer SiC This includes the thickness h of the Al IDT electrode in Example 1 of the present invention. AL The spacing between the IDT electrodes made of Al is MR, with a value of 0.048λ (where λ is the wavelength of the elastic wave). AL The thickness of the piezoelectric layer made of LN is 0.44. LN 0.20λ, thickness h of the SiC layer SiC The thickness h of the Al IDT electrode in Example 2 of the present invention was set to 0.05λ. AL The spacing MR of the IDT electrodes made of Al is 0.05λ. AL The thickness of the piezoelectric layer made of LN is 0.46. LN 0.30λ, thickness h of the SiC layer SiC It was set to 0.20λ.

[0066] As Comparative Example 1, an elastic wave device with a structure obtained by removing the SiC layer from the structure shown in Figure 1 was fabricated, and various parameters were set. Specifically, the thickness h of the Al IDT electrode in Comparative Example 1 was set. AL The spacing MR of the IDT electrodes made of Al is 0.04λ. AL The thickness of the piezoelectric layer made of LN is 0.50 h LN I set it to 0.05.

[0067] The simulation results for SH0 mode elastic waves generated in an elastic wave device show the propagation velocity V at the resonance point. fr , propagation velocity V at the anti-resonance point fa, the ratio bandwidth FBW between the anti-resonant frequency and the resonant frequency, and the effective electromechanical coupling coefficient K eff 2 The effective electromechanical coupling coefficient K was obtained. eff 2 V is the propagation velocity at the resonance point. fr and the propagation velocity V at the anti-resonance point fa Based on this, it was calculated using the following formula (1).

[0068]

number

[0069] ...(1)

[0070] According to the simulation results, the propagation velocity V at the resonance point of Example 1 of the present invention is fr and the propagation velocity V at the anti-resonance point fa These values ​​are 4094.7 m / s and 4763.3 m / s, respectively, and the propagation velocity V at the resonance point of Comparative Example 1 fr and the propagation velocity V at the anti-resonance point fa This was higher than (3028.6 m / s and 3899.6 m / s, respectively). Similarly, the propagation velocity V at the resonance point of Example 2 of the present invention was also higher. fr and the propagation velocity V at the anti-resonance point fa These values ​​are 4276.9 m / s and 4870.6 m / s, respectively, and the propagation velocity V at the resonance point of Comparative Example 1 fr and the propagation velocity V at the anti-resonance point fa This was higher than before. This indicates that the inclusion of a SiC layer in the elastic wave device increases the propagation speed of SH0 mode elastic waves by about 1.4 times, improving its adaptability to high frequencies.

[0071] On the other hand, the effective electromechanical coupling coefficient K of each of the present invention examples 1 and 2 eff 2 These are 30.26% and 26.74%, respectively. Effective electromechanical coupling coefficient K for each of the Invention Examples 1 and 2 eff 2 The effective electromechanical coupling coefficient K of Comparative Example 1 is shown. eff 2This is lower than (44.65%), indicating a narrower bandwidth. However, the effective electromechanical coupling coefficient K of each of the Invention Examples 1 and 2 eff 2 This remains above 20%. Therefore, it is presumed that a sufficiently large bandwidth can be secured when using an elastic wave device as an elastic wave filter, and no practical problems will arise.

[0072] Figure 11 shows the setting conditions and simulation results of the S0 mode elastic wave in one embodiment of the present invention. Figure 12 is a graph showing the simulation results of the relationship between the propagation speed and admittance Y of the S0 mode elastic wave in one embodiment of the present invention.

[0073] Referring to Figures 11 and 12, the inventors of the present invention virtually created the elastic wave devices of Examples 3 and 4 of the present invention and the elastic wave device of Comparative Example 1, and performed simulations of the SH0 mode elastic waves generated by the elastic wave devices.

[0074] As examples 3 and 4 of the present invention, elastic wave devices having the structure shown in Figure 1 were fabricated, and various parameters in the elastic wave devices were set to different values ​​from each other. Specifically, the thickness h of the Al IDT electrode in example 3 of the present invention AL The spacing between the IDT electrodes made of Al is MR, where λ is the wavelength of the elastic wave. AL The thickness of the piezoelectric layer made of LN is 0.30 h LN 0.10λ, thickness h of the SiC layer SiC The thickness h of the Al IDT electrode in Example 4 of the present invention was set to 0.05λ. AL The spacing MR of the IDT electrodes made of Al is 0.04λ. AL The thickness of the piezoelectric layer made of LN is 0.48. LN 0.30λ, thickness h of the SiC layer SiC It was set to 0.12λ.

[0075] Simulation results of S0 mode elastic waves generated in an elastic wave device show the propagation velocity V at the resonance point. fr , propagation velocity V at the anti-resonance point fa, the relative bandwidth FBW between the anti-resonant frequency and the resonant frequency, and the effective electromechanical coupling coefficient K eff 2 This was obtained.

[0076] According to the simulation results, the propagation velocity V at the resonance point of Example 3 of the present invention is fr and the propagation velocity V at the anti-resonance point fa These values ​​are 7199.3 m / s and 7773.4 m / s, respectively, and the propagation velocity V at the resonance point of Comparative Example 1 fr and the propagation velocity V at the anti-resonance point fa This was higher than (5112.8 m / s and 5937.2 m / s, respectively). Similarly, the propagation velocity V at the resonance point of Example 4 of the present invention was also higher. fr and the propagation velocity V at the anti-resonance point fa These values ​​are 6315.4 m / s and 7050.9 m / s, respectively, and the propagation velocity V at the resonance point of Comparative Example 1 fr and the propagation velocity V at the anti-resonance point fa This was higher than before. This indicates that the inclusion of a SiC layer in the elastic wave device increases the propagation speed of S0 mode elastic waves by about 1.4 times, improving its adaptability to high frequencies.

[0077] On the other hand, the effective electromechanical coupling coefficient K of each of the present invention examples 3 and 4 eff 2 These are 17.00% and 23.264%, respectively. Effective electromechanical coupling coefficient K for each of the present invention examples 3 and 4. eff 2 The effective electromechanical coupling coefficient K of Comparative Example 1 is shown. eff 2 This is lower than (30.26%), indicating a narrower bandwidth. However, the effective electromechanical coupling coefficient K of each of the present invention examples 3 and 4 eff 2 This remains above 17%. Therefore, it is presumed that a large bandwidth can still be secured when using elastic wave devices as elastic wave filters, and that there will be no practical problems.

[0078] [others]

[0079] The elastic wave device 1 in the above-described embodiment may be used as a parallel resonator or a series resonator that constitutes an elastic wave filter.

[0080] The embodiments and examples described above should be considered in all respects as illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than by the foregoing description, and all modifications within the meaning and scope equivalent to the claims are intended to be included. [Explanation of Symbols]

[0081] 1. Elastic wave devices (an example of an elastic wave device) 11. Si (Silicon) Substrate (An example of a Si substrate) 11a Si substrate surface 11b Main surface of Si substrate 12 SiC (Silicon Carbide) Layer (An example of a SiC layer) Surface of the 12a SiC layer 12b Back surface of the SiC layer 13. Bonding layer (an example of a bonding layer) 14. Piezoelectric layer (an example of a piezoelectric layer) 14a Surface of the piezoelectric layer 14b Back surface of the piezoelectric layer 15. IDT (Inter-Digital Transducer) electrode (an example of an IDT electrode) 16 Holes in Si substrate 51 Structures 91 Mask Layer 92 grooves in Si substrate 131,132 Amorphous Layers 141 Piezoelectric substrate 141b Back surface of piezoelectric substrate Center of the back surface of the RG1 Si substrate Bottom surface of grooves in RG2 Si substrate

Claims

1. An elastic wave device that propagates elastic waves, A Si substrate containing holes, The SiC film formed on the surface side of the Si substrate, A piezoelectric layer made of a piezoelectric material is formed on the surface side of the SiC film, The piezoelectric layer comprises an IDT (Inter-Digital Transducer) electrode formed on the surface side, An elastic wave device in which the back surface of the SiC film is exposed at the bottom of the hole in the Si substrate.

2. The piezoelectric layer is made of lithium tantalate or lithium niobate. The elastic wave device according to claim 1, wherein the piezoelectric layer has a thickness of 18 nm or more and 5.0 μm or less.

3. The acoustic wave device according to claim 1, wherein the SiC film has a thickness of 5 nm or more and 4.0 μm or less.

4. The elastic wave device according to claim 1, further comprising a bonding layer formed between the piezoelectric layer and the SiC film.

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