Composite substrate and method for producing the same

Abstract

Composite substrate (10) with: a piezoelectric substrate (12), one surface of which is a negatively polarized surface (12a) and the other surface of which is a positively polarized surface (12b); and a support substrate (14) connected to the positively polarized surface (12b) of the piezoelectric substrate (12) by direct bonding, wherein the support substrate (14) is a silicon substrate or a glass substrate.

Description

Technical area

[0001] The present invention relates to a composite substrate and a method for producing the same. State of the art

[0002] Acoustic wave devices contain interdigital transducer (IDT) electrodes formed on the surface of a piezoelectric substrate that receive a signal in a specific frequency band. In recent years, composite substrates obtained by bonding a thin piezoelectric substrate to a support substrate with a small thermal expansion coefficient have been used to improve the thermal characteristics of acoustic wave devices. One example of these composite substrates is a substrate that uses lithium tantalate or lithium niobate as the piezoelectric substrate and silicon or quartz as the support substrate (see PTL 1). Reference listPatent literature

[0003] PTL 1: JP 2006 319679 A. Relevant prior art can be found, for example, in JP H11 163668 A, which discloses a layered piezoelectric single-crystal substrate. Furthermore, JP 2001 036160 A discloses the fabrication of an electronic device, and JP 2008 205888 A discloses the fabrication of a piezoelectric vibration chip and a piezoelectric element. Summary of the inventionTechnical field

[0004] Generally, in a composite substrate, IDT electrodes are formed on a positively polarized surface of the piezoelectric substrate, and a negatively polarized surface of the piezoelectric substrate is bonded to a support substrate. To produce the composite substrate, the piezoelectric substrate and the support substrate are first bonded by a direct bonding method, and a surface of the piezoelectric substrate is then ground and polished to make the piezoelectric substrate thin. When bonding is performed by the direct bonding method, the bonding surface of the piezoelectric substrate and the bonding surface of the support substrate are irradiated with an argon beam.At this time, when the negatively polarized surface of the piezoelectric substrate is irradiated with the argon beam, the arithmetic average roughness Ra after irradiation is an area of ​​10 µm. 2 measured with an atomic force microscope (AFM), approximately 0.5 nm.

[0005] When joining by direct bonding, the bond strength is increased by reducing the arithmetic average roughness of the bonding surface, in addition to completely removing adhering substances such as an oxide layer or liquid on the surface and activating the surface. Therefore, it is desirable to reduce the arithmetic average roughness Ra of the bonding surface as much as possible.

[0006] The present invention was made to address this problem, and a primary object of the present invention is to increase the bonding strength in the composite substrate when the piezoelectric substrate and the support substrate are joined by the direct bonding method. Solution to the problem

[0007] The composite substrate of the present invention comprises: a piezoelectric substrate, one surface of which is a negatively polarized surface and the other surface of which is a positively polarized surface; and a support substrate connected to the positively polarized surface of the piezoelectric substrate by direct bonding, wherein the support substrate is a silicon substrate or a glass substrate.

[0008] In the composite substrate, the positively polarized surface of the piezoelectric substrate is directly bonded to a surface of the support substrate. Now, comparing the case where the positively polarized surface of the piezoelectric substrate is subjected to the direct bonding process with the case where the negatively polarized surface is subjected to the direct bonding process, the surface roughness of the former is better than that of the latter when the degree of removal from the surface of the piezoelectric substrates is identical. Accordingly, the bonding strength is higher in the former case than in the latter case when bonding is performed by the direct bonding method. Examples of the direct bonding method include ion beam irradiation such as with a rare gas (argon, etc.) and irradiation with a plasma or a neutral atom beam.

[0009] In the composite substrate according to the present invention, the negatively polarized surface of the piezoelectric substrate can be etched with a strong acid, and the etching rate at which the negatively polarized surface is etched with the strong acid can be higher than the etching rate at which the positively polarized surface is etched with the strong acid, and the etching rate at which the support substrate is etched with the strong acid can be higher than the etching rate at which the negatively polarized surface is etched with the strong acid. In the composite substrate, the positively polarized surface of the piezoelectric substrate is bonded to a surface of the support substrate, and therefore, the surface of the piezoelectric substrate is the negatively polarized surface that is etched with the strong acid at a higher rate.Accordingly, the time required to etch the surface of the piezoelectric substrate with the strong acid by a certain thickness, that is, the time during which the entire composite substrate is immersed in the strong acid during etching, is shorter than when the surface of the piezoelectric substrate were the positively polarized surface. Because the etching rate for the support substrate is higher than for the negatively polarized surface of the piezoelectric substrate, the support substrate is also etched while the entire composite substrate is immersed in the strong acid. However, because the immersion time in the strong acid is shorter than when the surface of the piezoelectric substrate were the positively polarized surface, as described above, progression in etching of the support substrate can be prevented to such an extent that the bonding strength is not impaired.Accordingly, in the composite substrate, the bonding strength between the support substrate and the piezoelectric substrate can be sufficiently ensured even after the piezoelectric substrate surface is etched with the strong acid.

[0010] In the composite substrate according to the present invention, the strong acid is preferably a hydrofluoric acid or a hydrofluoric acid. The use of a hydrofluoric acid or a hydrofluoric acid enables the surface of the piezoelectric substrate (negatively polarized surface) to be etched at a relatively high rate.

[0011] In the composite substrate according to the present invention, the support substrate preferably has a thermal expansion coefficient smaller than the thermal expansion coefficient of the piezoelectric substrate. Thus, when the composite substrate is used to fabricate an acoustic wave device, the dimensional variation of the piezoelectric substrate due to temperature variation can be reduced, and variations in frequency characteristics due to temperature variation of the acoustic wave device can be suppressed.

[0012] In the composite substrate according to the present invention, the piezoelectric substrate is preferably a lithium tantalate (LT) substrate or a lithium niobate (LN) substrate, and the support substrate is preferably a silicon substrate or a glass substrate. Because the LT substrate and the LN substrate have a large polarization vector, the etching rate at which the positively polarized surface is etched with the strong acid is likely to be greatly different from the etching rate at which the negatively polarized surface is etched with the strong acid, and applying the present invention is significant.In addition, applying the present invention is significant because the silicon substrate and the glass substrate are etched with the strong acid at a higher rate than in the case of the LT substrate and the LN substrate, and the degree of progress in etching must be suppressed by reducing the time during which each substrate is immersed in the strong acid.

[0013] A method for producing a composite substrate according to the present invention includes a bonding step of bonding a positively polarized surface of a piezoelectric substrate, one surface of which is a negatively polarized surface and the other surface of which is a positively polarized surface, to a surface of a support substrate by a direct bonding method to produce the composite substrate.

[0014] In the method for manufacturing a composite substrate, the positively polarized surface of the piezoelectric substrate is directly bonded to a surface of the support substrate. Comparing the case where the positively polarized surface of the piezoelectric substrate is subjected to the direct bonding process with the case where the negatively polarized surface is subjected to the direct bonding process, the surface roughness of the former is better than that of the latter. Accordingly, the bonding strength is higher in the former case than in the latter case when bonding is performed by the direct bonding method.

[0015] The method for manufacturing a composite substrate according to the present invention may further include a step of thinning the substrate by grinding and polishing the negatively polarized surface of the piezoelectric substrate of the composite substrate obtained in the bonding step, and an etching step of etching an affected layer generated in the negatively polarized surface with a strong acid in the substrate thinning step. In the piezoelectric substrate and the support substrate, an etching rate at which the negatively polarized surface is etched with the strong acid may be higher than an etching rate at which the positively polarized surface is etched with the strong acid, and an etching rate at which the support substrate is etched with the strong acid may be higher than the etching rate at which the negatively polarized surface is etched with the strong acid.In the substrate thinning step, an affected layer (a layer where the quality of the material has changed due to grinding and polishing) is created in the negatively polarized surface. In the etching step, the affected layer is etched and removed with strong acid. At this time, the time required to etch the affected layer created in the surface of the piezoelectric substrate with strong acid—that is, the time during which the entire composite substrate is immersed in the strong acid during etching—is shorter than when the surface of the piezoelectric substrate is the positively polarized surface. Because the etching rate for the support substrate is higher than that for the negatively polarized surface of the piezoelectric substrate, the support substrate is also etched while the entire composite substrate is immersed in the strong acid.However, because the immersion time in the strong acid is shorter than if the surface of the piezoelectric substrate were the positively polarized surface, as described above, the progression of etching of the support substrate can be prevented to such an extent that the bonding strength is not affected. Accordingly, in the composite substrate, the bonding strength between the support substrate and the piezoelectric substrate can be sufficiently ensured even after the surface of the piezoelectric substrate has been etched with the strong acid.

[0016] In the method for producing a composite substrate according to the present invention, hydrofluoric acid or hydrofluoric acid is preferably used as the strong acid. The use of hydrofluoric acid or hydrofluoric acid enables the surface of the piezoelectric substrate to be etched at a relatively high rate.

[0017] In the method for manufacturing a composite substrate according to the present invention, a substrate having a thermal expansion coefficient smaller than the thermal expansion coefficient of the piezoelectric substrate is preferably prepared as the support substrate. In this way, the dimensional variation of the piezoelectric substrate due to temperature variation can be reduced, and variation in frequency characteristics due to temperature variation of the acoustic wave device can be suppressed when the composite substrate is used to manufacture an acoustic wave device.

[0018] In the method for manufacturing a composite substrate according to the present invention, an LT substrate or an LN substrate is preferably prepared as the piezoelectric substrate, and a silicon substrate or a glass substrate is preferably prepared as the support substrate. Because the LT substrate and the LN substrate have a large polarization vector, the etching rate at which the positively polarized surface is etched with the strong acid is likely to be greatly different from the etching rate at which the negatively polarized surface is etched with the strong acid, and an application of the present invention is significant.In addition, an application of the present invention is significant because the silicon substrate and the glass substrate are etched with the strong acid at a higher rate compared with the LT substrate and the LN substrate, and need to be immersed in the strong acid for a shorter time, so that the degree of progress in the etching is suppressed. Short description of the drawings Fig. 1 is a perspective view of a composite substrate 10. Fig. Figure 2 is an explanatory drawing of a cutting angle of a wafer cut out of the piezoelectric single crystal. Fig. 3 is a cross-sectional view schematically showing production processes for the composite substrate 10. Fig. 4 is a perspective view schematically showing production processes for the composite substrate 10. Fig.5 is a perspective view of a one-port SAW resonator 30 fabricated using the composite substrate 10. Description of the embodiments

[0019] An embodiment of the present invention will now be described with reference to the drawings. Fig. 1 is a perspective view of a composite substrate 10 according to the embodiment.

[0020] In the composite substrate 10, a piezoelectric substrate 12 and a support substrate 14 are bonded by direct bonding. An exemplary direct bonding method is to first clean the bonding surfaces of the substrates 12 and 14, irradiate the bonding surfaces with an ion beam of a rare gas such as argon to activate the bonding surfaces, and then bond the substrates 12 and 14.

[0021] The piezoelectric substrate 12 is a substrate capable of propagating a surface acoustic wave (SAW). Examples of the material of the piezoelectric substrate 12 include lithium tantalate (LT), lithium niobate (LN), a lithium niobate-lithium tantalate solid solution single crystal, quartz, lithium borate, zinc oxide, aluminum nitride, langasite (LGS), and langatate (LGT). Among them, LT or LN is preferred. This is because LT and LN are suitable for high-frequency and broadband acoustic wave devices because LT and LN allow the SAW to propagate therethrough at a high rate and have a high electromechanical coupling coefficient. The thickness of the piezoelectric substrate 12 is not particularly limited and can be, for example, 0.2 to 50 µm. One surface of the piezoelectric substrate 12 is a negatively polarized surface 12a and the other surface of the piezoelectric substrate 12 is a positively polarized surface 12b.The etching rate at which the negatively polarized surface 12a is etched with a strong acid (e.g., hydrofluoric acid or hydrofluoric acid) is higher than that.

[0022] Etching rate at which the positively polarized surface 12b is etched with the strong acid. The positively polarized surface 12b of the piezoelectric substrate 12 is directly bonded to the support substrate 14, and the negatively polarized surface 12a is located on the outside. The negatively polarized surface 12a of the piezoelectric substrate 12 is etched with the strong acid.

[0023] Table 1 shows the rate at which different types of LT and LN are etched with hydrofluoric acid at 65°C. For these substances, the etch rate is higher for the negatively polarized surface than for the positively polarized surface. The cutting angle shown in Table 1 is calculated with reference to Fig. 2 described. Fig.Figure 2(a) shows a state before a piezoelectric single crystal is sliced ​​into wafers, where the X-axis represents the left-right direction, the Y-axis represents the vertical direction, and the Z-axis represents a direction perpendicular to the plane of the figure. In the piezoelectric single crystal, the direction of spontaneous polarization is a C-axis direction, that is, the Z-direction. For example, “LT36°Y” in Table 1 represents 36° rotated Y-cut LT, and means that when the Y-axis and Z-axis are rotated 36° in the same direction around the X-axis to obtain a new Y'-axis and a new Z'-axis, as shown in Fig. As shown in Figure 2(b), a wafer is cut so that the Y'-axis coincides with the normal to its top surface. The direction in which the acoustic wave propagates is the X-axis direction. [Table 1] Substrate type Etching rate with hydrofluoric acid at 65°C (µm / hr) positively polarized surface negatively polarized surface LN36°Y 0,08 1,3 LN45°Y 0,2 7,0 LN Z 0,5 16,3 LT36°Y 0,07 0,36 LT40°Y 0,12 1,15 LT42°Y 0,14 1,64

[0024] The support substrate 14 has a thermal expansion coefficient smaller than the thermal expansion coefficient of the piezoelectric substrate 12 and is bonded to the back surface of the piezoelectric substrate 12 by direct bonding. The support substrate 14, having a thermal expansion coefficient smaller than the thermal expansion coefficient of the piezoelectric substrate 12, can reduce the dimensional variation of the piezoelectric substrate 12 caused by temperature variation and prevent frequency characteristics from varying due to temperature when the composite substrate 10 is used as an acoustic wave device. The support substrate 14 is made of a material that is etched with the strong acid at a higher rate (e.g., 10 to 50 µm / hr) than the etching rate at which the negatively polarized surface 12a of the piezoelectric substrate 12 is etched with the strong acid.Examples of such a material for the support substrate 14 include silicon and glass (such as borosilicate glass and quartz glass). The thickness of the support substrate 14 is not particularly limited and may be, for example, 200 to 1200 µm.

[0025] A method of manufacturing the composite substrate 10 will now be described below with reference to Fig. 3 and Fig. 4 described. Fig. 3 and Fig. 4 are sectional views and a perspective view schematically showing production processes for the composite substrate 10.

[0026] The piezoelectric substrate 12, which is disc-shaped and has an orientation flat (OF), and the support substrate 14, which has the same shape as the piezoelectric substrate 12, are first prepared (see Fig. 3(a) and Fig.4(a)). The bonding surfaces of the substrates 12 and 14 are next cleaned to remove contaminants adhering to the bonding surfaces. Here, the bonding surface of the piezoelectric substrate 12 is the positively polarized surface 12b. The bonding surfaces of the substrates 12 and 14 are then irradiated with an ion beam of a rare gas, such as argon, to remove residual contaminants (such as an oxide layer and adhered substances) and to activate the bonding surfaces. The substrates 12 and 14 are then positioned so that the OFs of the substrates 12 and 14 coincide with each other at room temperature in a vacuum, and then the substrates 12 and 14 are bonded together (see Fig. 3(b) and Fig.4(b)). The negatively polarized surface 12a thereby becomes the surface of the piezoelectric substrate 12. The surface of the piezoelectric substrate 12 is next ground and polished to a predetermined thickness to make the piezoelectric substrate 12 thin (see Fig. 3(c) and Fig. 4(c)). An affected layer 12c is obtained in the surface of the piezoelectric substrate 12 after grinding and polishing. The affected layer 12c is a layer in which the quality of the material has been changed due to grinding and polishing. The entire bonded substrates are then immersed in the strong acid to remove the affected layer 12c of the piezoelectric substrate 12 by etching and to further thin the piezoelectric substrate 12, thus obtaining the composite substrate 10 (see Fig. 3(d) and Fig. 4(d)).

[0027] The thus obtained composite substrate 10 is then formed into an aggregate of a number of surface acoustic wave devices using a typical photolithographic technique and cut into individual surface acoustic wave devices by cutting. Fig. Figure 5 shows the state in which the composite substrate is formed into an array of single-port SAW resonators 30, which are surface acoustic wave devices. In each single-port SAW resonator 30, interdigital transducer (IDT) electrodes 32 and 34 and reflective electrodes 36 are formed on the surface of the piezoelectric substrate 12 using a photolithographic technique.

[0028] In the above-described composite substrate 10 according to the embodiment of the present invention, the positively polarized surface 12b of the piezoelectric substrate 12 is directly bonded to a surface of the support substrate 14. Now, comparing the case of irradiating the positively polarized surface 12b of the piezoelectric substrate with the ion beam with the case of irradiating the negatively polarized surface 12a with the ion beam, the surface roughness of the ion beam-irradiated surface is better in the former case than in the latter case. Accordingly, the bonding strength is higher in the former case than in the latter case when bonding is performed by the direct bonding method.

[0029] In addition, the surface of the piezoelectric substrate 12 is the negatively polarized surface 12a, which is etched with the strong acid at a high rate. Accordingly, the time required to etch the surface of the piezoelectric substrate by a certain thickness with the strong acid—that is, the time during which the entire composite substrate is immersed in the strong acid during etching—is shorter than if the surface of the piezoelectric substrate 12 were the positively polarized surface 12b. Because the etching rate for the support substrate is higher than for the negatively polarized surface 12a of the piezoelectric substrate 12, the support substrate is also etched while the entire composite substrate 10 is immersed in the strong acid.However, because the immersion time in the strong acid is shorter than if the surface of the piezoelectric substrate 12 were the positively polarized surface 12b as described above, the progression of etching of the support substrate 14 can be prevented to such an extent that the bonding strength is not affected. Accordingly, in the composite substrate 10, the bonding strength between the support substrate 14 and the piezoelectric substrate 12 can be sufficiently ensured even after the surface of the piezoelectric substrate 12 has been etched with the strong acid.

[0030] It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be embodied in various aspects within the scope of the present invention.

[0031] Although the piezoelectric substrate 12 was illustrated with the various types of LT and LN shown in Table 1 in the above embodiment, any piezoelectric substrate enables the same effect to be achieved as in the above embodiment, provided that the etching rate at which the negatively polarized surface of the piezoelectric substrate is etched with the strong acid is higher than the etching rate at which the positively polarized surface of the piezoelectric substrate is etched with the strong acid. In addition, any substrate enables the same effect to be achieved as in the above embodiment, provided that the etching rate at which the substrate is etched with the strong acid is higher than the etching rate at which the negatively polarized surface of the piezoelectric substrate is etched with the strong acid.

[0032] Although the piezoelectric substrate 12 and the support substrate 14 are joined by direct bonding using an ion beam in the above embodiment, a method of using a plasma beam or a neutral atom beam may be used instead of the method using an ion beam.

[0033] JP 2004 186938 A discloses that IDT electrodes are formed on the negatively polarized surface of a piezoelectric substrate in an acoustic wave device with a rear electrode on the surface of the piezoelectric substrate opposite the surface on which the IDT electrodes are formed, in order to prevent corrosion of the electrodes due to the effect of a local cell between the rear electrode and the IDT electrodes. However, this publication does not describe using a composite substrate for the acoustic wave device, bonding a piezoelectric substrate and a support substrate by a direct bonding method with an ion beam, or removing the affected layer on the piezoelectric substrate surface by etching using a strong acid.For this reason, it does not involve the problem of increasing the bonding strength of the composite substrate or the problem of separating the bonding boundaries of the composite substrate due to etching using a strong acid, and of course, a technique to solve these problems is not included. Therefore, the present invention cannot be easily devised by a person skilled in the art based on this publication. EXAMPLES[EXAMPLE 1]

[0034] A 42° Y-cut LT substrate (250 µm thick) was prepared as the piezoelectric substrate, in which the direction of acoustic wave propagation was the X-axis, and the Y-axis and Z-axis were rotated 42° around the X-axis. A Si (111) substrate (230 µm thick) was prepared as the support substrate. These two substrates were placed in a vacuum chamber at 2 × 10 -6(Pa), and the positively polarized surface of the piezoelectric substrate and one surface of the support substrate were irradiated with an argon beam for 60 seconds. After irradiation, the irradiated surfaces of the substrates were brought into contact with each other, and the substrates were pressed with 2000 kg and joined by direct bonding. An AFM measurement revealed that the arithmetic average roughness Ra of the positively polarized surface of the piezoelectric substrate irradiated with the argon beam under the same conditions as above was 0.3 nm (a 10 µm surface was measured). 2large area). The degree of removal from the positively polarized surface at this time was 1 nm. The bonded body was removed from the vacuum chamber, and the piezoelectric substrate was ground to 30 µm. The piezoelectric substrate was then polished to 25 µm with a tin surface plate while instilling a diamond slurry (particle diameter of 1 µm). The piezoelectric substrate was further polished to 20 µm with a urethane pad while instilling colloidal silica (particle diameter of 20 nm). As a result of the bonding strength of the bonded body after polishing, which was evaluated by a fracture-opening method, a surface energy of 1.8 J / cm was measured. 2The bonded body after polishing was then immersed in hydrofluoric acid at 65°C for one minute to remove the affected layer on the surface of the piezoelectric substrate by etching, thus obtaining the composite substrate. At this time, the etching degree was 20 nm, the surface roughness Ra after etching was 0.3 nm, and the separation length of the bonding boundaries after etching was 0.1 mm (the separation length extending from the outer periphery of the bonding surfaces to the substrate center, see Fig. 3 (d)). [COMPARISON EXAMPLE 1]

[0035] In Example 1, the negatively polarized surface of the piezoelectric substrate and a surface of the support substrate were directly bonded. An AFM measurement revealed that the arithmetic average roughness Ra of the negatively polarized surface of the piezoelectric substrate irradiated with the argon beam was 0.5 nm (measured area was 10 µm). 2 ). The degree of removal from the negatively polarized surface at this time was 1 nm. As a result of the bonding strength of the bonded body after polishing, a surface energy of 1.5 J / cm 2A composite substrate was obtained in the same manner as in Example 1, except that the bonded body was immersed in hydrofluoric acid for 10 minutes to remove the affected layer after grinding and polishing the positively polarized surface of the piezoelectric substrate. At this time, the etching degree was 23 nm, the surface roughness Ra after etching was 0.5 nm, and the separation length of the bond boundaries after etching was 0.5 mm.

[0036] In Example 1, the bonding strength between the piezoelectric substrate and the support substrate was increased compared to Comparative Example 1. In addition, the etching time for removing substantially the same amount of the affected layer was reduced to one-tenth of that in Comparative Example 1. Accordingly, in Example 1, the amount by which the support substrate was etched during etching was reduced, and the separation of the bond boundaries was reduced to one-fifth of that in Comparative Example 1. Industrial applicability

[0037] The present invention can be applied to an acoustic wave device such as a SAW filter. List of reference symbols 10 Composite substrate 12 piezoelectric substrate 12a negatively polarized surface 12b positively polarized surface 12c influenced layer 14 Support substrate 30 single-port SAW resonators 32, 34 IDT electrode 36 reflective electrode

Claims

[1] Composite substrate (10) with: a piezoelectric substrate (12), one surface of which is a negatively polarized surface (12a) and the other surface of which is a positively polarized surface (12b); and a support substrate (14) connected to the positively polarized surface (12b) of the piezoelectric substrate (12) by direct bonding, wherein the support substrate (14) is a silicon substrate or a glass substrate. [2] Composite substrate (10) according to claim 1, wherein the negatively polarized surface (12a) of the piezoelectric substrate (12) is etched with a strong acid, and wherein an etching rate at which the negatively polarized surface (12a) is etched with the strong acid is higher than an etching rate at which the positively polarized surface (12b) is etched with the strong acid, and an etching rate at which the support substrate (14) is etched with the strong acid is higher than an etching rate at which the negatively polarized surface (12a) is etched with the strong acid. [3] The composite substrate (10) of claim 2, wherein the strong acid is hydrofluoric acid or hydrofluoric acid. [4] The composite substrate (10) according to any one of claims 1 to 3, wherein the support substrate (14) has a thermal expansion coefficient smaller than a thermal expansion coefficient of the piezoelectric substrate (12). [5] The composite substrate (10) according to any one of claims 1 to 4, wherein the piezoelectric substrate (12) is a lithium tantalate substrate or a lithium niobate substrate. [6] A method for producing a composite substrate (10) comprising a bonding step of bonding a positively polarized surface of a piezoelectric substrate (12), one surface of which is a negatively polarized surface (12a) and the other surface of which is the positively polarized surface (12b), to a surface of a support substrate (14) by a direct bonding method to produce the composite substrate (10), wherein a silicon substrate or a glass substrate is used as the support substrate (14). [7] A method of manufacturing a composite substrate (10) according to claim 6, further comprising: a step of thinning the substrate by grinding and polishing the negatively polarized surface (12a) of the piezoelectric substrate (12) of the composite substrate (10) obtained in the bonding step; and an etching step of etching an affected layer produced in the negatively polarized surface (12a) in the step of thinning the substrate with a strong acid, wherein in the piezoelectric substrate (12) and the support substrate (14), an etching rate at which the negatively polarized surface (12a) is etched with the strong acid is higher than an etching rate at which the positively polarized surface (12b) is etched with the strong acid, and an etching rate at which the support substrate (14) is etched with the strong acid is higher than the etching rate at which the negatively polarized surface (12a) is etched with the strong acid. [8] A method for producing a composite substrate (10) according to claim 7, wherein hydrofluoric acid or hydrofluoric acid is used as the strong acid. [9] A method for manufacturing a composite substrate (10) according to any one of claims 6 to 8, wherein a substrate having a thermal expansion coefficient smaller than a thermal expansion coefficient of the piezoelectric substrate (12) is used as the support substrate (14). [10] A method for producing a composite substrate (10) according to any one of claims 6 to 9, wherein a lithium tantalate substrate or a lithium niobate substrate is prepared as the piezoelectric substrate.

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