Composite substrate for surface acoustic wave device and method for manufacturing the same

Abstract

A piezoelectric composite substrate for a SAW device having small loss is provided. A composite substrate for a surface acoustic wave device according to one embodiment of the present invention has a piezoelectric single crystal thin film, a support substrate, and a first intermediate layer between the piezoelectric single crystal thin film and the support substrate. In the composite substrate, the first intermediate layer is in contact with the piezoelectric single crystal thin film, and a sound velocity of a transverse wave in the first intermediate layer is faster than a fast transverse wave in the piezoelectric single crystal thin film.

Description

[0001] Cross-reference to related applications

[0002] This non-provisional application claims priority to Japanese Patent Application No. 2020-086299, filed May 15, 2020, pursuant to 35 USC §119(a), the entire contents of which are incorporated herein by reference. Technical Field

[0003] This invention relates to a composite substrate for surface acoustic wave devices and a method for manufacturing the same, wherein a piezoelectric single crystal substrate is bonded to a support substrate. Background Technology

[0004] In recent years, data traffic has increased rapidly in the mobile communications market, represented by smartphones. To address this, it is necessary to increase the number of communication frequency bands, and it is also essential to miniaturize various devices (e.g., surface acoustic wave devices) and achieve high performance.

[0005] Piezoelectric materials (e.g., lithium tantalate (LT) and lithium niobate (LN)) are widely used in surface acoustic wave (SAW) devices, such as SAW filters. While these materials exhibit large electromechanical coupling coefficients and can extend the bandwidth of devices, a problem arises: their low temperature stability causes the applicable frequency to shift with temperature. This is because lithium tantalate or lithium niobate has a very high coefficient of thermal expansion.

[0006] To address this problem, a composite substrate has been proposed, which is obtained by bonding a material with a low coefficient of thermal expansion to lithium tantalate or lithium niobate and thinning one side of the piezoelectric material to a thickness of several μm to tens of μm. In this composite substrate, the thermal expansion of the piezoelectric material is suppressed by bonding a material with a low coefficient of thermal expansion (e.g., sapphire or silicon), thus improving temperature characteristics (Non-Patent Documents 1 and 2). Furthermore, Patent Document 1 discloses an acoustic wave device with a piezoelectric film. This acoustic wave device includes: a support substrate; a high-velocity acoustic film formed on the support substrate and having a volume sound velocity higher than that propagating through the piezoelectric film; a low-velocity acoustic film stacked on the high-velocity acoustic film and having a volume sound velocity slower than that propagating through the piezoelectric film; a piezoelectric film stacked on the low-velocity acoustic film; and an IDT electrode formed on one surface of the piezoelectric film.

[0007] Furthermore, Patent Document 2 discloses an acoustic wave device comprising: a support substrate; a dielectric layer stacked on the support substrate; a piezoelectric body stacked on the dielectric layer for propagating bulk waves; and an IDT electrode formed on one surface of the piezoelectric body. In this device, the dielectric layer includes a low-speed dielectric and a high-speed dielectric. In the low-speed dielectric, the propagation speed of the bulk wave, which is the main component of the acoustic wave, is slower than the sound speed of the acoustic wave propagating within the piezoelectric body; in the high-speed dielectric, the propagation speed of the bulk wave, which is the main component of the acoustic wave, is faster than the sound speed of the acoustic wave propagating within the piezoelectric body. When the sound speed of the main vibration mode when the dielectric layer is formed of a high-speed dielectric is VH and the sound speed of the main vibration mode when the dielectric layer is formed of a low-speed dielectric is VL, the dielectric layer is formed such that the sound speed of the main vibration mode in the acoustic wave device having the dielectric layer is VL < the sound speed of the main vibration mode < VH, and when the period of the IDT is λ, the thickness of the dielectric layer is 1λ or more.

[0008] Furthermore, Patent Document 3 discloses a composite substrate for a surface acoustic wave (SAW) device, comprising a piezoelectric single-crystal thin film and a support substrate. In this device, at the bonding interface between the piezoelectric single-crystal thin film and the support substrate, at least one of the piezoelectric single-crystal substrate and the support substrate has an uneven structure, and the ratio of the average length RSm of the unit cell in the cross-sectional curve of the uneven structure to the wavelength λ of the surface acoustic wave when used as a SAW device is 0.2 or more and 7.0 or less.

[0009] Existing technical references

[0010] Patent documents

[0011] Patent Document 1: Japanese Patent No. 5713025

[0012] Patent Document 2: Japanese Patent No. 5861789

[0013] Patent Document 3: Japanese Patent No. 6250856

[0014] Non-patent literature

[0015] Non-Patent Document 1: Temperature Compensation Technology for SAW-Duplexer Used in RF Front End of Smartphone, Dempa Shimbun High Technology, November 8, 2012

[0016] Non-patent literature 2: A study on Temperature-Compensated Hybrid Substrates for Surface Acoustic Wave Filters, 2010 IEEE International Ultrasonic Symposium Proceedings, pp. 637-640. Summary of the Invention

[0017] The problem to be solved by the present invention

[0018] However, when using the composite substrates of Patent Document 1 or Patent Document 2 to manufacture surface acoustic wave (SAW) filters, the following problem arises: so-called clutter or ripple noise is generated within the passband of the SAW filter or at higher frequencies. This noise is caused by the leakage of acoustic wave energy from the piezoelectric material into the low-velocity medium. This noise is generated due to reflections at the bonding interface between the piezoelectric crystal film and the supporting substrate, as well as the trapping of acoustic waves in the intermediate layer between the piezoelectric crystal film and the supporting substrate. This noise is undesirable because it degrades the frequency characteristics of the SAW filter and leads to increased losses.

[0019] In the composite substrate for surface acoustic wave devices described in Patent Document 3, both the piezoelectric single crystal thin film and the supporting substrate have an uneven structure, which is desirable because the uneven structure scatters unwanted waves, thereby suppressing the influence of reflected waves.

[0020] However, the inventors carefully investigated the possibility that the resistivity and thermoelectric properties of the single-crystal piezoelectric film might increase during the heat treatment processes described in Patent Document 3 for manufacturing a composite substrate for surface acoustic wave devices and for device manufacturing using the composite substrate. They discovered that the reason lies in the temperature variations during the process, which generate electric fields exceeding the coercive electric field in the uneven regions of the piezoelectric single crystal of the composite substrate. In extreme cases, this can lead to interference with the polarization of the single-crystal piezoelectric film.

[0021] In view of this, the object of the present invention is to provide a piezoelectric composite substrate with low loss for SAW devices.

[0022] Solution for solving the problem

[0023] A composite substrate for a surface acoustic wave device according to one embodiment of the present invention comprises: a piezoelectric single-crystal thin film, a supporting substrate, and a first intermediate layer between the piezoelectric single-crystal thin film and the supporting substrate. In the composite substrate, the first intermediate layer is in contact with the piezoelectric single-crystal thin film, and the sound velocity of the transverse waves in the first intermediate layer is faster than the sound velocity of the fast transverse waves in the piezoelectric single-crystal thin film. Preferably, the volume resistivity of the composite substrate is 1 × 10⁻⁶. 12 Below Ω·cm.

[0024] The present invention will be described using the case where the piezoelectric single crystal thin film is LiTaO3 (LT) as an example. An intermediate layer is disposed between the LT, which is the piezoelectric single crystal thin film, and the supporting substrate. If the velocity of the bulk wave (transverse wave) in the intermediate layer is slower than the velocity of the bulk wave (fast transverse wave) in the LT, the sound wave is easily trapped in the intermediate layer. Therefore, if the velocity of the transverse wave in the intermediate layer is made faster than the velocity of the slow transverse wave in the piezoelectric single crystal thin film in the composite substrate, the passband loss of the surface acoustic wave filter obtained using such a composite substrate 1 can be improved. Details will be described below.

[0025] In surface acoustic wave (SAW) filters obtained by forming periodic electrode structures on composite substrates, for example, in composite substrates where LT and Si are bonded by a 46° Y-axis rotation and the LT thickness is more than one wavelength, and the LT thickness excludes singularities in the dispersion curve, when the electrodes are electrically open, the velocity of the dominant mode of the SAW is 4060 m / s (the slowness as the reciprocal of the velocity of sound is 2.46 × 10⁻⁶ m / s). -3 The speed of sound is 3910 m / s when the electrode is electrically short-circuited (its slowness as the reciprocal of the speed of sound is 2.56 × 10⁻⁶ m / s). -3 s / m).

[0026] Therefore, surface acoustic waves (or leakage waves or SH waves) propagating along the surface of the LT from the electrode can couple with specific volume waves in the LT that can propagate inside the LT substrate. That is, as Figure 1 As shown in the calculated value of the slow surface of the LT cut by the Y-axis with a rotation of 46°, the main mode of the composite substrate structure (where, as explained above, the aforementioned LT cut by the Y-axis with a rotation of 46° and Si are bonded) can be coupled with a phase-matched bulk wave (slow transverse wave) that can propagate at a depth direction of about 22 degrees starting from the X-axis.

[0027] Figure 2 An example of a slow-speed surface is shown when an LT cut along the Y-axis at a rotation of 46° is used as a piezoelectric single-crystal thin film and Si3N4 is used as an intermediate layer. When Si3N4 is used as an intermediate layer, the transverse wave velocity of the intermediate layer can be made faster than the fast transverse wave velocity of the piezoelectric single-crystal thin film.

[0028] like Figure 2 As shown, when the speed of sound of the slow transverse wave in the intermediate layer is faster than that of the slow transverse wave in the piezoelectric single-crystal thin film, the slow transverse wave emitted from the X-axis of the LT cut at approximately 22° along a 46° Y-axis rotation is completely reflected by the intermediate layer even when it reaches the intermediate layer. Therefore, the bulk wave of the surface acoustic wave (or leakage wave or SH wave) propagating inward along the surface of the LT from the electrode is totally reflected by the intermediate layer and cannot remain in the intermediate layer.

[0029] As a result, no residual clutter is present in the intermediate layer near the main mode frequency. Therefore, performance degradation (e.g., ripple and loss in the filter's passband) can be prevented.

[0030] Furthermore, in one embodiment of the invention, the first intermediate layer is characterized by high sound speed and 10 -3 (g / m 2 The water vapor transmission rate is below 1000 m / day. This suppresses the diffusion of oxygen from the support substrate side of the intermediate layer to the piezoelectric single crystal film side, which in turn suppresses the increase of thermoelectricity in the piezoelectric single crystal film of the composite substrate and the generation of an electric field when the composite substrate is heat-treated. Here, the water vapor transmission rate is the value measured by the Mocon method at a temperature of 40°C and a relative humidity of 90%.

[0031] The material of the first intermediate layer can be any one of silicon oxynitride, silicon nitride, amorphous aluminum nitride, or aluminum oxide.

[0032] In this invention, the composite substrate may have at least a first intermediate layer and a second intermediate layer between the piezoelectric single-crystal thin film and the supporting substrate. Here, the first intermediate layer may be in contact with the piezoelectric single-crystal thin film. Preferably, the sound velocity of the transverse wave in the first intermediate layer is faster than the sound velocity of the fast transverse wave in the piezoelectric single-crystal thin film, and the sound velocity of the transverse wave in the second intermediate layer is slower than the sound velocity of the fast transverse wave in the piezoelectric single-crystal thin film.

[0033] In this invention, the second intermediate layer may contain oxygen. The second intermediate layer may contain any one of silicon dioxide, titanium dioxide, tantalum pentoxide, niobium pentoxide, and zirconium dioxide. Such an intermediate layer provides a composite substrate with high adhesive strength.

[0034] Here, the sound velocity of the transverse waves in the second intermediate layer is slower than that of the fast transverse waves in the piezoelectric single-crystal film, which allows for better suppression of clutter at frequencies higher than the dominant mode frequency. In other words, at frequencies higher than the dominant mode frequency, bulk waves radiate into the piezoelectric single-crystal film at angles even deeper than 22°. In this case, bulk waves will leak into the first intermediate layer. Since the sound velocity of the transverse waves in the second intermediate layer is slower than that of the fast transverse waves in the piezoelectric single-crystal film, the bulk waves can easily enter the second intermediate layer. However, the bulk waves that are reflected again in the supporting substrate are reflected again at the first intermediate layer and cannot easily return to the first intermediate layer. As a result, the bulk waves are confined in the second intermediate layer and have difficulty returning to the piezoelectric single-crystal film. Therefore, ripples at or above the passband of the filter can be prevented.

[0035] In this invention, any bonding interface between the piezoelectric single-crystal thin film and the supporting substrate (e.g., the bonding interface between the first intermediate layer and the layer adjacent to the first intermediate layer) can have an uneven structure. The ratio of the average length RSm of the cell in the cross-sectional curve of the uneven structure to the wavelength λ of the surface acoustic wave when used as a surface acoustic wave device can be between 0.2 and 5.0.

[0036] When the piezoelectric single-crystal thin film of this application has an uneven structure at the interface with the intermediate layer, the bulk wave in the approximately 22° direction is scattered by the uneven structure due to the main mode from the LT, and the component returning to the electrode can be significantly reduced. If the frequency of the main mode is fo and the radiation angle of the bulk wave from the electrode of the SAW device to the interior of the LT is θ, a reflected wave is generated at a frequency higher than fo, where this frequency is expressed as fo / cosθ, but the uneven structure will scatter the reflected wave.

[0037] In addition, the inventors studied the degree of the above-mentioned unevenness and found that when the ratio of the average length RSm of the unit in the cross-sectional curve of the uneven structure to the wavelength λ of the surface acoustic wave when used as a surface acoustic wave device is between 0.2 and 5.0, the composite substrate for surface acoustic wave devices of this application can maintain single polarization without losing the polarization of the piezoelectric single crystal, thereby completing the present invention.

[0038] In this invention, the piezoelectric material used to form the piezoelectric single crystal thin film can be lithium tantalate or lithium niobate.

[0039] In this invention, the supporting substrate can be any one of silicon wafers, sapphire wafers, alumina wafers, glass wafers, silicon carbide wafers, aluminum nitride wafers, silicon nitride wafers, and crystalline quartz.

[0040] The method for manufacturing a composite substrate according to an embodiment of the present invention is characterized by the steps of depositing an anti-diffusion layer on one side of a piezoelectric material substrate; further depositing an intermediate layer on the anti-diffusion layer; bonding a support substrate to the anti-diffusion layer; and thinning the other side of the piezoelectric material substrate. The method for manufacturing the composite substrate is characterized by heat-treating the composite substrate in a reducing or inert gas atmosphere containing nitrogen or hydrogen.

[0041] In this invention, it is preferred that the anti-diffusion layer is deposited by PVD or CVD.

[0042] In this invention, the bonding surface of the anti-diffusion layer and / or the bonding surface of the supporting substrate may be subjected to a surface activation treatment before being bonded to each other. The surface activation treatment may include any one of plasma activation, ion beam activation, and ozone water activation methods. Attached Figure Description

[0043] Figure 1The slow-speed surface of LT is shown as a Y-axis cut by rotating 46°.

[0044] Figure 2 The slowness representation of the LT and intermediate layer (Si3N4) cut by rotating 46° in the Y-axis in the YX plane is shown.

[0045] Figure 3 A cross-sectional view of the composite substrate structure is shown.

[0046] Figure 4 The process for manufacturing composite substrates is shown.

[0047] Figure 5 The waveform representing the characteristics (S11 frequency response) of the SAW filter formed on the composite piezoelectric substrate in Example 1 is shown.

[0048] Figure 6 A piezoelectric force microscopy (PFM) image of a cross-section of the composite substrate of Example 1 is shown.

[0049] Figure 7 The waveform representing the characteristics (S11 frequency response) of the SAW filter formed on the composite piezoelectric substrate in Comparative Example 1 before thermal cycling is shown.

[0050] Figure 8 A piezoelectric force microscopy (PFM) image of the cross-section of the composite substrate of Comparative Example 2 after thermal cycling is shown.

[0051] Figure 9 A piezoelectric force microscopy (PFM) image of the cross-section of the composite substrate of Comparative Example 3 after thermal cycling is shown. Detailed Implementation

[0052] exist Figure 3 The cross-sectional structure of the composite substrate 1 according to the present invention is shown in the figure. Figure 3 The composite substrate 1 shown has a piezoelectric single crystal thin film 2 on a supporting substrate 3. The piezoelectric single crystal thin film 2 has a high sound velocity. The piezoelectric single crystal thin film 2 is bonded to the supporting substrate 3 via an anti-diffusion layer 4 and an intermediate layer 5 to prevent oxygen diffusion.

[0053] The piezoelectric single-crystal thin film 2 is formed from lithium tantalate (LT) or lithium niobate (LN) as the piezoelectric material. The piezoelectric single-crystal thin film 2 preferably has single polarization. The supporting substrate 3 can be any one of silicon wafers, sapphire wafers, alumina wafers, glass wafers, silicon carbide wafers, aluminum nitride wafers, silicon nitride wafers, and crystalline quartz wafers.

[0054] The anti-diffusion layer 4 is sometimes referred to as the first intermediate layer in this invention. The anti-diffusion layer 4 is positioned in contact with the piezoelectric single-crystal thin film 2. The anti-diffusion layer 4 is formed such that the sound velocity of the transverse wave in the anti-diffusion layer 4 is faster than the sound velocity of the fast transverse wave in the piezoelectric single-crystal thin film 2. The anti-diffusion layer 4 has a density of 10... -3 (g / m 2 The water vapor transmission rate is below 1 / day. The anti-diffusion layer 4 can be formed from any one of silicon oxynitride, silicon nitride, amorphous aluminum nitride, or aluminum oxide.

[0055] An intermediate layer 5 is disposed between the anti-diffusion layer 4 and the support substrate 3. The intermediate layer 5 is sometimes referred to as the first intermediate layer, or, to distinguish it from the anti-diffusion layer 4, as the second intermediate layer. The intermediate layer 5 can be formed of an oxygen-containing material. More specifically, the intermediate layer can contain any of the following: silicon dioxide, titanium dioxide, tantalum pentoxide, niobium pentoxide, and zirconium dioxide. The intermediate layer 5 is formed such that the sound velocity of the transverse waves in the intermediate layer 5 is slower than the sound velocity of the fast transverse waves in the piezoelectric single-crystal thin film 2.

[0056] The uneven structure is formed at the bonding interface between the anti-diffusion layer 4 and the layer adjacent to the anti-diffusion layer 4 (in this embodiment, at the interface with the piezoelectric single crystal thin film 2 or at the interface with the intermediate layer 5). The uneven structure is formed such that the ratio of the average length RSm of the unit cell in the cross-sectional curve of the uneven structure to the wavelength λ of the surface acoustic wave when used as a surface acoustic wave device is 0.2 or more and 5.0 or less.

[0057] Figure 4 A flow chart of a method for manufacturing composite substrate 1 is shown. In this manufacturing method, a piezoelectric single-crystal wafer 2, which serves as a piezoelectric single-crystal thin film, is first prepared. Figure 4 S01), and by grinding or sandblasting one of its surfaces to form an uneven texture (S01), Figure 4 (S02 in the text). Additionally, an anti-diffusion layer 4 is formed on the uneven structure of the aforementioned piezoelectric single crystal thin film, and then an intermediate layer 5 is formed on this anti-diffusion layer 4. Figure 4 (S03 in the text). At this point, it is preferable to deposit the anti-diffusion layer 4 using PVD or CVD. Preferably, the intermediate layer 5 is also deposited using PVD or CVD. Then, the surface of the intermediate layer is mirror-finished by polishing. Figure 4 (S04 in the middle).

[0058] In parallel with the processing of the piezoelectric single crystal wafer described above, a support substrate 3 is fabricated. Figure 4(S11 in the text). The support substrate 3 can be any one of silicon wafers, sapphire wafers, alumina wafers, glass wafers, silicon carbide wafers, aluminum nitride wafers, silicon nitride wafers, and crystalline quartz wafers. The surface of the support substrate is mirror-finished by polishing. Instead of the piezoelectric single-crystal thin film 2, or in addition to the piezoelectric single-crystal thin film 2, the support substrate 3 can also be provided with a textured structure or an intermediate layer. In this case, the textured structure on the support substrate 3 can be formed by polishing with free abrasive particles, sandblasting, chemical etching, etc., and then an intermediate layer can be formed on the textured structure. Then, the intermediate layer can be mirror-finished by polishing to form an adhesive surface.

[0059] The polished surface of the intermediate layer 5 and the adhesive surface of the support substrate 3 are bonded together. Figure 4 (S21 in the text). Subsequently, the other side of the piezoelectric single-crystal wafer 2 (i.e., the side opposite to the side where the anti-diffusion layer 4 is formed) is ground and polished to thin it to the desired thickness. Thus, a composite substrate for surface acoustic wave devices can be manufactured. Figure 4 (S22). At this point, a surface activation treatment can be applied to the surfaces to be bonded beforehand. In this way, the bond strength can be increased. Plasma activation, ion beam activation, and ozone water activation can be used for surface activation treatment. In the plasma activation method, a plasma gas is introduced into a reaction vessel in which the wafer is placed, and a high-frequency plasma of about 100 W is formed under a reduced pressure of about 0.01 to 0.1 Pa, thereby exposing the wafer bonding surfaces to the plasma for about 5 to 50 seconds. As the plasma gas, oxygen, hydrogen, nitrogen, argon, or a mixture of these gases can be used.

[0060] After thinning, heat treatment is preferably performed in a reducing or inert gas atmosphere containing nitrogen or hydrogen to further improve the adhesive strength. For example, a hydrogen atmosphere can be used for a reducing atmosphere. For example, a nitrogen atmosphere can be used for an inert gas atmosphere.

[0061] Example

[0062] [Example 1]

[0063] A silicon nitride layer of approximately 800 nm was deposited on one side of a 150 mm diameter, Y-axis-cut LT wafer rotated 46° to form a diffusion barrier layer using PVD. Then, a silicon oxide film of approximately 3 μm thickness was formed on this diffusion barrier layer using CVD. Using this silicon oxide film as an intermediate layer, the film was polished and bonded to a p-type silicon wafer with a resistivity of 2000 Ω·cm. The LT wafer used has a diameter of approximately 1 × 10⁻⁶. 10 Volume resistivity in Ω·cm.

[0064] The surface of the LT wafer on which the silicon nitride layer is formed is processed into a rough and uneven structure by sandblasting. The rough and uneven structure has an average cell length RSm in a cross-sectional curve of 3 μm and Ra = 0.06 μm.

[0065] After bonding, a heat treatment was applied at 100°C for 48 hours in a nitrogen atmosphere. The LT layer was then thinned to a thickness of 10 μm by grinding and polishing. To further improve the bonding strength, a heat treatment was then performed at 250°C for 24 hours in a hydrogen atmosphere.

[0066] For the composite substrate manufactured as described above, the apparent volume resistivity of the composite substrate was measured using the method described in "JIS C 2139:2008 Solid electrical insulating materials—Methods for measuring volume resistivity and surface resistivity," and the volume resistivity was 5 × 10⁻⁶. 10 Ω·cm. The voltage applied when measuring volume resistivity is 500V.

[0067] Next, an Al film with a thickness of 0.14 μm was sputtered onto the composite substrate fabricated as described above, and a resist pattern with a linewidth of approximately 0.5 μm was formed by i-line exposure after resist coating. Then, the Al was etched by dry etching to form a ladder-type SAW filter. Additionally, a 50 nm silicon nitride layer was formed by sputtering onto the surface layer of the substrate on which the SAW filter was formed.

[0068] The characteristics of SAW filters are evaluated using RF probes and network analyzers, such as... Figure 5 As shown, a suitable waveform (frequency response of S11) is obtained. SAW filters with wavelengths of approximately 2.2 μm, 2.1 μm, and 1.9 μm are placed on the same wafer. Therefore, in this embodiment, if the wavelength λ is taken as the average length of the cell in the cross-sectional curve of the uneven structure of the LT bonding surface, then RSm / λ is 1.6, 1.5, and 1.4 for the SAW filters at each wavelength, respectively.

[0069] Next, the composite substrate with the SAW filter pattern fabricated as described above was passed through a reflow oven at 265°C six times, followed by 1000 thermal cycles from -40°C to 125°C. Afterward, the characteristics of the SAW filter were evaluated again using an RF probe and network analyzer, and the results were consistent with... Figure 5 same.

[0070] Next, piezoelectric force microscopy (PFM) images of the cross-section of the composite substrate, manufactured in the same manner as described above, were measured. The results are as follows: Figure 6 As shown, the LT portion is found to be uniformly polarized.

[0071] After passing the substrate through a reflow oven at 265°C six times and then cycling it 1000 times from -40°C to 125°C, a PFM image of the cross-section of the composite substrate with a SAW filter pattern was measured. The results are similar to... Figure 6 As shown, the LT portion is found to be uniformly polarized.

[0072] In Example 1 above, the Young's modulus and density of silicon nitride and SiO2, which served as intermediate layers, were measured by nanoindentation and X-ray reflectance (Xrr) methods, respectively. Table 1 shows the calculated sound velocities of the transverse waves of the silicon nitride film and SiO2 film obtained from the results of Example 1, as well as the aforementioned Young's modulus and density.

[0073] [Table 1]

[0074] silicon nitride <![CDATA[SiO2]]> <![CDATA[SiO 1.5 N 0.5 ]]> Young's modulus (GPa) 320 62 130 <![CDATA[Density (kg / m 3 )]]> 2800 2180 2260 The speed of sound of a transverse wave (m / s) 6700 3700 4865

[0075] The calculated sound velocity of the fast transverse wave in the x-axis direction of 46°Y-LiTaO3 is 4227 m / s.

[0076] [Example 2]

[0077] Except for the heat treatment performed in a nitrogen atmosphere instead of a hydrogen atmosphere to further improve the adhesion strength after LT layer thinning, the composite substrate was manufactured in the same manner as in Example 1. For this composite substrate, the apparent volume resistivity of the composite substrate was measured using the method described in "JIS C 2139:2008 Solid electrical insulating materials—Methods for measuring volume resistivity and surface resistivity," and the volume resistivity was 2 × 10⁻⁶. 11 Ω·cm. The voltage applied when measuring volume resistivity is 500V.

[0078] Next, in the same manner as in Example 1, a ladder-shaped SAW filter is formed on the composite substrate manufactured by the above method, and a 50 nm silicon nitride layer is formed by sputtering on the surface layer of the substrate on which the SAW filter is formed.

[0079] When evaluating the characteristics of a SAW filter using RF probes and a network analyzer, results are obtained similar to... Figure 5 The waveforms in the case are shown. SAW filters with wavelengths of approximately 2.2 μm, 2.1 μm, and 1.9 μm are placed on the same wafer. Therefore, in this embodiment, if the wavelength λ is taken as the average length of the cell in the cross-sectional curve of the uneven structure of the LT bonding surface, then RSm / λ is 1.6, 1.5, and 1.4 for the SAW filters at each wavelength.

[0080] Next, the composite substrate with the SAW filter pattern fabricated as described above was passed through a reflow oven at 265°C six times, followed by 1000 thermal cycles from -40°C to 125°C. Afterward, the characteristics of the SAW filter were evaluated again using an RF probe and network analyzer, and the results were consistent with... Figure 5 The situation is the same.

[0081] Furthermore, a PFM image of the cross-section of a composite substrate manufactured in the same manner as described above was measured. The results were similar to... Figure 6 As shown, the LT portion is found to be uniformly polarized.

[0082] After passing the substrate through a reflow oven at 265°C six times and then cycling it 1000 times from -40°C to 125°C, a PFM image of the cross-section of the composite substrate with a SAW filter pattern was measured. The results are similar to... Figure 6 As shown, the LT portion is found to be uniformly polarized.

[0083] [Example 3]

[0084] SiO2 of approximately 800 nm was deposited on one side of a 150 mm diameter, Y-axis-cut LT wafer with a diameter of 46° using CVD at a temperature of approximately 35 °C. 1.5 N 0.5 A diffusion-resistant layer was formed. Then, a silicon oxide film with a thickness of approximately 3 μm was formed on this diffusion-resistant layer using CVD. Using this silicon oxide film as an intermediate layer, the film was polished and bonded to a p-type silicon wafer with a resistivity of 2000 Ω·cm. The LT wafer used has a resistivity of approximately 5 × 10⁻⁶. 10 Volume resistivity in Ω·cm.

[0085] SiO is formed on the LT wafer through sandblasting. 1.5 N 0.5 The surface of the layer is processed into an uneven structure, which has an average unit length RSm in a cross-sectional curve of 3 μm and Ra = 0.06 μm.

[0086] After bonding, a heat treatment was applied at 100°C for 48 hours in a nitrogen atmosphere. The LT layer was then thinned to a thickness of 10 μm by grinding and polishing. To further improve the bonding strength, a heat treatment was then performed at 250°C for 24 hours in a hydrogen atmosphere.

[0087] For the composite substrate manufactured as described above, the apparent volume resistivity of the composite substrate was measured using the method described in "JIS C 2139:2008 Solid electrical insulating materials—Methods for measuring volume resistivity and surface resistivity," and the volume resistivity was 7 × 10⁻⁶. 10Ω·cm. The voltage applied when measuring volume resistivity is 500V.

[0088] Next, an Al film with a thickness of 0.14 μm was sputtered onto the composite substrate fabricated as described above, and a resist pattern with a linewidth of approximately 0.5 μm was formed by i-line exposure after resist coating. Then, the Al was etched by dry etching to form a ladder-type SAW filter. Additionally, a 50 nm silicon nitride layer was formed by sputtering onto the surface layer of the substrate on which the SAW filter was formed.

[0089] When evaluating the characteristics of a SAW filter using RF probes and a network analyzer, results are obtained similar to... Figure 5 The waveform of the situation.

[0090] Next, the composite substrate with the SAW filter pattern fabricated as described above was passed through a reflow oven at 265°C six times, followed by 1000 thermal cycles from -40°C to 125°C. Afterward, the characteristics of the SAW filter were evaluated again using an RF probe and network analyzer, and compared with... Figure 5 No change.

[0091] Furthermore, a PFM image of the cross-section of a composite substrate manufactured in the same manner as described above was measured. The results were similar to... Figure 6 As shown, the LT portion was found to be uniformly polarized. PFM images of the cross-section of the composite substrate with the SAW filter pattern were measured after the substrate was passed through a reflow oven at 265°C six times and then thermally cycled 1000 times from -40°C to 125°C. The results were similar to... Figure 6 As shown, the LT portion is found to be uniformly polarized.

[0092] [Example 4]

[0093] Except for the heat treatment performed in a nitrogen atmosphere instead of a hydrogen atmosphere to further improve the adhesion strength after LT layer thinning, the composite substrate was manufactured in the same manner as in Example 3. For this composite substrate, the apparent volume resistivity of the composite substrate was measured using the method described in "JIS C 2139:2008 Solid electrical insulating materials—Methods for measuring volume resistivity and surface resistivity," and the volume resistivity was 4 × 10⁻⁶. 11 Ω·cm. The voltage applied when measuring volume resistivity is 500V.

[0094] Next, in the same manner as in Example 3, a ladder-shaped SAW filter is formed on the composite substrate manufactured by the above method, and a 50 nm silicon nitride layer is formed by sputtering on the surface layer of the substrate on which the SAW filter is formed.

[0095] When evaluating the characteristics of a SAW filter using RF probes and a network analyzer, results are obtained similar to... Figure 5 The waveforms in the case are shown. SAW filters with wavelengths of approximately 2.2 μm, 2.1 μm, and 1.9 μm are placed on the same wafer. Therefore, in this embodiment, if the wavelength λ is taken as the average length of the cell in the cross-sectional curve of the uneven structure of the LT bonding surface, then RSm / λ is 1.6, 1.5, and 1.4 for the SAW filters at each wavelength.

[0096] Next, the composite substrate with the SAW filter pattern fabricated as described above was passed through a reflow oven at 265°C six times, followed by 1000 thermal cycles from -40°C to 125°C. Afterward, the characteristics of the SAW filter were evaluated again using an RF probe and network analyzer, and the results were consistent with... Figure 5 The situation is the same.

[0097] Furthermore, a PFM image of the cross-section of a composite substrate manufactured in the same manner as described above was measured. The results were similar to... Figure 6 As shown, the LT portion is found to be uniformly polarized.

[0098] After passing the substrate through a reflow oven at 265°C six times and then cycling it 1000 times from -40°C to 125°C, a PFM image of the cross-section of the composite substrate with a SAW filter pattern was measured. The results are similar to... Figure 6 As shown, the LT portion is found to be uniformly polarized.

[0099] [Examples 5 and 6]

[0100] A composite substrate was prepared in the same manner as in Example 1, except that an anti-diffusion layer of approximately 800 nm, as shown in Table 2, was deposited on one side of an LT wafer with a diameter of 150 mm and a Y-axis cut at a rotation of 46°, using PVD or PLD. For the composite substrate prepared as described above, the apparent volume resistivity of the composite substrate was measured using the method described in "JIS C 2139:2008 Solid Electrical Insulators—Methods for Measurement of Volume Resistivity and Surface Resistivity," and the volume resistivity was the value shown in Table 2. The voltage applied during the volume resistivity measurement was 500 V.

[0101] Next, an Al film with a thickness of 0.14 μm was sputtered onto the composite substrate fabricated as described above, and a resist pattern with a linewidth of approximately 0.5 μm was formed by i-line exposure after resist coating. Then, the Al was etched by dry etching to form a ladder-type SAW filter. Additionally, a 50 nm silicon nitride layer was formed by sputtering onto the surface layer of the substrate on which the SAW filter was formed.

[0102] When evaluating the characteristics of a SAW filter using RF probes and a network analyzer, results are obtained similar to... Figure 5 The waveforms in the case are shown. SAW filters with wavelengths of approximately 2.2 μm, 2.1 μm, and 1.9 μm are placed on the same wafer. Therefore, in this embodiment, if the wavelength λ is taken as the average length of the cell in the cross-sectional curve of the uneven structure of the LT bonding surface, then RSm / λ is 1.6, 1.5, and 1.4 for the SAW filters at each wavelength.

[0103] Next, the composite substrate with the SAW filter pattern fabricated as described above was passed through a reflow oven at 265°C six times, followed by 1000 thermal cycles from -40°C to 125°C. Afterward, the characteristics of the SAW filter were evaluated again using an RF probe and network analyzer, and the results were consistent with... Figure 5 The situation is the same.

[0104] Furthermore, a PFM image of the cross-section of a composite substrate manufactured in the same manner as described above was measured. The results were similar to... Figure 6 As shown, the LT portion is found to be uniformly polarized.

[0105] After passing the substrate through a reflow oven at 265°C six times and then cycling it 1000 times from -40°C to 125°C, a PFM image of the cross-section of the composite substrate with the SAW filter pattern was measured. The results are similar to... Figure 6 As shown, the LT portion is found to be uniformly polarized.

[0106] In Examples 5 and 6 above, the Young's modulus and density of the intermediate layer were measured by nanoindentation and X-ray reflectance (Xrr) methods, respectively. Table 2 shows the Young's modulus and density of Examples 5 and 6 above, as well as the calculated sound velocity of the transverse wave of the anti-diffusion layer obtained from the Young's modulus and density.

[0107] [Table 2]

[0108]

[0109] [Comparative Example 1]

[0110] Then, a silicon oxide film with a thickness of approximately 4 μm was formed on one side of an LT wafer with a diameter of 150 mm and a Y-axis cut at 46° rotation using CVD. This silicon oxide film was then used as an intermediate layer, polished, and bonded to a p-type silicon wafer with a resistivity of 2000 Ω·cm. The LT wafer used had a resistivity of approximately 5 × 10⁻⁶ Ω·cm. 10 Volume resistivity in Ω·cm.

[0111] The surface of the LT wafer on which the silicon oxide layer is formed is processed into a rough and uneven structure by sandblasting. The rough and uneven structure has an average unit length RSm in a cross-sectional curve of 3 μm and Ra = 0.06 μm.

[0112] After bonding, a heat treatment was applied at 100°C for 48 hours in a nitrogen atmosphere. The LT layer was then thinned to a thickness of 10 μm by grinding and polishing. To further improve the bonding strength, a heat treatment was then performed at 250°C for 24 hours in a nitrogen atmosphere.

[0113] For the composite substrate manufactured as described above, the apparent volume resistivity of the composite substrate was measured using the method described in "JIS C 2139:2008 Solid electrical insulating materials—Methods for measuring volume resistivity and surface resistivity," and the volume resistivity was 2×10⁻⁶. 12 Ω·cm. The voltage applied when measuring volume resistivity is 500V.

[0114] Next, an Al film with a thickness of 0.14 μm was sputtered onto the composite substrate fabricated as described above, and a resist pattern with a linewidth of approximately 0.5 μm was formed by i-line exposure after resist coating. Then, the Al was etched by dry etching to form a ladder-type SAW filter. Additionally, a 50 nm silicon nitride layer was formed by sputtering onto the surface layer of the substrate on which the SAW filter was formed.

[0115] The characteristics of the SAW filter were evaluated using an RF probe and a network analyzer, and the following results were obtained: Figure 7 The waveforms are shown. SAW filters with wavelengths of approximately 2.2 μm, 2.1 μm, and 1.9 μm were placed on the same wafer. Therefore, in this comparative example, if the wavelength λ is taken as the average length of the cell in the cross-sectional curve of the uneven structure of the LT bonding surface, then RSm / λ is 1.6, 1.5, and 1.4 for the SAW filters at each wavelength.

[0116] Next, the composite substrate with the SAW filter pattern fabricated as described above was passed through a reflow oven at 265°C six times, followed by 1000 thermal cycles from -40°C to 125°C. Afterward, the characteristics of the SAW filter were evaluated again using an RF probe and network analyzer, as well as its comparison with... Figure 7 Compared to the waveform in the previous example, the insertion loss increased by approximately 5 dB.

[0117] Furthermore, a PFM image of the cross-section of a composite substrate manufactured in the same manner as described above was measured. The results were similar to... Figure 6 As shown, the LT portion is found to be uniformly polarized.

[0118] After passing the substrate through a reflow oven at 265°C six times and then cycling it 1000 times from -40°C to 125°C, PFM images of the cross-section of the composite substrate with the SAW filter pattern were measured. The results are as follows: Figure 8 As shown, the polarization of the LT portion is disordered.

[0119] In Comparative Example 1 above, the Young's modulus and density of SiO2, which served as the intermediate layer, were measured by nanoindentation and X-ray reflectance (Xrr) methods, respectively. The results of Comparative Example 1 above and the calculated transverse wave velocity of the SiO2 film obtained from the Young's modulus and density are equal to the values ​​shown in Table 1.

[0120] [Comparative Example 2]

[0121] The composite substrate was prepared in the same manner as Comparative Example 1, except that the LT layer was thinned to a thickness of 10 μm by grinding and polishing, and the composite substrate was heat-treated at 250°C in an atmospheric atmosphere for 24 hours to further improve the adhesion strength.

[0122] For the composite substrate manufactured as described above, the apparent volume resistivity of the composite substrate was measured using the method described in "JIS C 2139:2008 Solid electrical insulating materials - Methods for measuring volume resistivity and surface resistivity", and the volume resistivity was 1×10⁻⁶. 14 Ω·cm. The voltage applied when measuring volume resistivity is 500V.

[0123] Next, an Al film with a thickness of 0.14 μm was sputtered onto the composite substrate fabricated as described above, and a resist pattern with a linewidth of approximately 0.5 μm was formed by i-line exposure after resist coating. Then, the Al was etched by dry etching to form a ladder-type SAW filter. Additionally, a 50 nm silicon nitride layer was formed by sputtering onto the surface layer of the substrate on which the SAW filter was formed.

[0124] When evaluating the characteristics of a SAW filter using RF probes and a network analyzer, the following results are obtained: Figure 7 The waveforms in the example are shown. SAW filters with wavelengths of approximately 2.2 μm, 2.1 μm, and 1.9 μm were placed on the same wafer. Therefore, in this comparative example, if the wavelength λ is taken as the average length of the cell in the cross-sectional curve of the uneven structure of the LT bonding surface, then RSm / λ is 1.6, 1.5, and 1.4 for the SAW filters at each wavelength.

[0125] Next, the composite substrate with the SAW filter pattern fabricated as described above was passed through a reflow oven at 265°C six times, followed by 1000 thermal cycles from -40°C to 125°C. Afterward, the characteristics of the SAW filter were evaluated again using an RF probe and network analyzer, as well as its comparison with... Figure 7 Compared to the waveform in the previous example, the insertion loss increased by approximately 6 dB.

[0126] Furthermore, a PFM image of the cross-section of a composite substrate manufactured in the same manner as described above was measured. The results were similar to... Figure 6 As shown, the LT portion is found to be uniformly polarized.

[0127] After passing the substrate through a reflow oven at 265°C six times and then cycling it 1000 times from -40°C to 125°C, a PFM image of the cross-section of the composite substrate with a SAW filter pattern was measured. The results are similar to... Figure 8 In the case shown, the polarization of the LT portion is disordered.

[0128] [Comparative Example 3]

[0129] Then, a silicon oxide film with a thickness of approximately 4 μm was formed on one side of an LT wafer with a diameter of 150 mm and a Y-axis cut at 46° rotation using CVD. This silicon oxide film was then used as an intermediate layer, polished, and bonded to a p-type silicon wafer with a resistivity of 2000 Ω·cm. The LT wafer used had a resistivity of approximately 1 × 10⁻⁶. 10 Volume resistivity in Ω·cm.

[0130] The surface of the LT wafer, on which the silicon oxide layer is formed, is processed into a rough and uneven structure by loose abrasive grains. The rough and uneven structure has an average unit length RSm in a cross-sectional curve of 12 μm and Ra = 0.3 μm.

[0131] After bonding, a heat treatment was applied at 100°C for 48 hours in a nitrogen atmosphere. The LT layer was then thinned to a thickness of 10 μm by grinding and polishing. To further improve the bonding strength, a heat treatment was then performed at 250°C for 24 hours in a nitrogen atmosphere.

[0132] For the composite substrate manufactured as described above, the apparent volume resistivity of the composite substrate was measured using the method described in "JIS C 2139:2008 Solid electrical insulating materials—Methods for measuring volume resistivity and surface resistivity," and the volume resistivity was 8 × 10⁻⁶. 12 Ω·cm. The voltage applied when measuring volume resistivity is 500V.

[0133] Next, an Al film with a thickness of 0.14 μm was sputtered onto the composite substrate fabricated as described above, and a resist pattern with a linewidth of approximately 0.5 μm was formed by i-line exposure after resist coating. Then, the Al was etched by dry etching to form a ladder-type SAW filter. Additionally, a 50 nm silicon nitride layer was formed by sputtering onto the surface layer of the substrate on which the SAW filter was formed.

[0134] When evaluating the characteristics of a SAW filter using RF probes and a network analyzer, the following results are obtained: Figure 7 The waveforms in the case are shown. SAW filters with wavelengths of approximately 2.2 μm, 2.1 μm, and 1.9 μm are placed on the same wafer. Therefore, in this comparative example, if the wavelength λ is taken as the average length of the cell in the cross-sectional curve of the uneven structure of the LT bonding surface, then RSm / λ for the SAW filters at each wavelength are 5.4, 5.7, and 6.3, respectively.

[0135] Next, the composite substrate with the SAW filter pattern fabricated as described above was passed through a reflow oven at 265°C six times, followed by 1000 thermal cycles from -40°C to 125°C. Afterward, the characteristics of the SAW filter were evaluated again using an RF probe and network analyzer, as well as its comparison with... Figure 7 Compared to the waveform in the previous example, the insertion loss increased by approximately 8 dB.

[0136] Furthermore, a PFM image of the cross-section of a composite substrate manufactured in the same manner as described above was measured. The results were similar to... Figure 6 As shown, the LT portion is found to be uniformly polarized.

[0137] After passing the substrate through a reflow oven at 265°C six times and then cycling it 1000 times from -40°C to 125°C, PFM images of the cross-section of the composite substrate with the SAW filter pattern were measured. The results are as follows: Figure 9 As shown, the polarization of the LT portion is disordered.

[0138] In Comparative Example 3 above, the Young's modulus and density of SiO2, which served as the intermediate layer, were measured by nanoindentation and X-ray reflectance (Xrr) methods, respectively. The results of Comparative Example 3 above, and the calculated transverse wave velocity of the SiO2 film obtained from the Young's modulus and density, are equal to the values ​​shown in Table 1.

[0139] [Comparative Example 4]

[0140] Except for the heat treatment to further improve the adhesion strength after LT layer thinning in an air atmosphere instead of a nitrogen atmosphere, the composite substrate was manufactured in the same manner as in Comparative Example 3.

[0141] For the composite substrate manufactured as described above, the apparent volume resistivity of the composite substrate was measured using the method described in "JIS C 2139:2008 Solid electrical insulating materials—Methods for measuring volume resistivity and surface resistivity," and the volume resistivity was 3 × 10⁻⁶. 14 Ω·cm. The voltage applied when measuring volume resistivity is 500V.

[0142] Next, an Al film with a thickness of 0.14 μm was sputtered onto the composite substrate fabricated as described above, and a resist pattern with a linewidth of approximately 0.5 μm was formed by i-line exposure after resist coating. Then, the Al was etched by dry etching to form a ladder-type SAW filter. Additionally, a 50 nm silicon nitride layer was formed by sputtering onto the surface layer of the substrate on which the SAW filter was formed.

[0143] When evaluating the characteristics of a SAW filter using RF probes and a network analyzer, results are obtained similar to... Figure 7 The waveforms in the case are shown. SAW filters with wavelengths of approximately 2.2 μm, 2.1 μm, and 1.9 μm are placed on the same wafer. Therefore, in this comparative example, if the wavelength λ is taken as the average length of the cell in the cross-sectional curve of the uneven structure of the LT bonding surface, then RSm / λ for the SAW filters at each wavelength are 5.4, 5.7, and 6.3, respectively.

[0144] Next, the composite substrate with the SAW filter pattern fabricated as described above was passed through a reflow oven at 265°C six times, followed by 1000 thermal cycles from -40°C to 125°C. Afterward, the characteristics of the SAW filter were evaluated again using an RF probe and network analyzer, as well as its comparison with... Figure 7 Compared to the waveform in the previous example, the insertion loss increased by approximately 9 dB.

[0145] Furthermore, a PFM image of the cross-section of a composite substrate manufactured in the same manner as described above was measured. The results were similar to... Figure 6 As shown, the LT portion is found to be uniformly polarized.

[0146] After passing the substrate through a reflow oven at 265°C six times and then cycling it 1000 times from -40°C to 125°C, a PFM image of the cross-section of the composite substrate with a SAW filter pattern was measured. The results are similar to... Figure 9 As shown, the LT portion is found to be uniformly polarized.

[0147] As can be seen from Examples 1-6 above, the sound velocity of the transverse wave in the anti-diffusion layer is preferably faster than the sound velocity of the fast transverse wave in the piezoelectric single-crystal thin film (LT). Preferably, the sound velocity of the transverse wave in the intermediate layer between the anti-diffusion layer and the support substrate is lower than the sound velocity of the fast transverse wave in the piezoelectric single-crystal thin film. Preferably, the ratio of the average length RSm of the unit cell in the cross-sectional curve of the uneven structure to the wavelength λ of the surface acoustic wave when used as a surface acoustic wave device is 0.2 or more and 5.0 or less.

Claims

1. A composite substrate for surface acoustic wave devices, comprising: A piezoelectric single-crystal thin film, a supporting substrate, and a first intermediate layer between the piezoelectric single-crystal thin film and the supporting substrate, wherein: The first intermediate layer is in contact with the piezoelectric single crystal thin film; The sound speed of the transverse wave in the first intermediate layer is faster than the sound speed of the fast transverse wave in the piezoelectric single crystal thin film. The water vapor permeability of the first intermediate layer is 10. -3 (g / m 2 ( / day) or less; The adhesive interface between the first intermediate layer and at least one layer adjacent to the first intermediate layer has an uneven structure, and The ratio of the average length RSm of the unit in the cross-sectional curve of the concave-convex structure to the wavelength λ of the surface acoustic wave when used as the surface acoustic wave device is greater than 0.2 and less than 5.

0.

2. The composite substrate according to claim 1, wherein, The first intermediate layer is any one of silicon oxynitride, silicon nitride, amorphous aluminum nitride, or aluminum oxide.

3. The composite substrate according to claim 1, wherein: A second intermediate layer is disposed between the first intermediate layer and the supporting substrate; and The sound velocity of the transverse wave in the second intermediate layer is slower than the sound velocity of the fast transverse wave in the piezoelectric single crystal thin film.

4. The composite substrate according to claim 3, wherein, The second intermediate layer comprises any one of silicon dioxide, titanium dioxide, tantalum pentoxide, niobium pentoxide, and zirconium dioxide.

5. The composite substrate according to claim 1, wherein, The volume resistivity of the composite substrate is 1×10⁻⁶. 12 Below Ω·cm.

6. The composite substrate according to claim 1, wherein, The piezoelectric single-crystal thin film is formed from lithium tantalate or lithium niobate.

7. The composite substrate according to claim 1, wherein, The supporting substrate is any one of silicon wafers, sapphire wafers, alumina wafers, glass wafers, silicon carbide wafers, aluminum nitride wafers, silicon nitride wafers, and crystalline quartz wafers.

8. The composite substrate according to claim 1, wherein, The piezoelectric single-crystal thin film has single polarization.

9. A method for manufacturing a composite substrate, comprising: The step of forming an uneven structure on one surface of a piezoelectric material substrate; The step of depositing an anti-diffusion layer on one side of the piezoelectric material substrate; The step of further depositing an intermediate layer on the anti-diffusion layer; The step of bonding the support substrate to the anti-diffusion layer; as well as The step of thinning the other side of the piezoelectric material substrate, The composite substrate is heat-treated in a reducing or inert gas atmosphere containing nitrogen or hydrogen. The bonding interface between the intermediate layer and at least one layer adjacent to the intermediate layer has an uneven structure, and The ratio of the average length RSm of the unit in the cross-sectional curve of the concave-convex structure to the wavelength λ of the surface acoustic wave when used as a surface acoustic wave device is greater than 0.2 and less than 5.

0.

10. The method for manufacturing a composite substrate according to claim 9, wherein, The anti-diffusion layer is any one of silicon oxynitride, silicon nitride, amorphous aluminum nitride, or aluminum oxide.

11. The method for manufacturing a composite substrate according to claim 9, wherein, In the step of depositing the anti-diffusion layer, the anti-diffusion layer is deposited by PVD or CVD.

12. The method for manufacturing a composite substrate according to claim 9, wherein, The intermediate layer comprises any one of silicon dioxide, titanium dioxide, tantalum pentoxide, niobium pentoxide, and zirconium dioxide.

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