Method for manufacturing composite substrate

Abstract

This method for manufacturing a composite substrate comprises: a step for obtaining a joined body by plasma-activating and joining a piezoelectric substrate and a silicon oxide layer formed on a main surface of a support substrate; a step for heating the obtained joined body to a predetermined first temperature and grinding the piezoelectric substrate to a predetermined thickness; a reheating step for reheating the joined body, in which the piezoelectric substrate has been ground, to a predetermined second temperature higher than the first temperature; and a regrinding step for further grinding the piezoelectric substrate in the joined body that has undergone the reheating step. The reheating step and the regrinding step are each repeated one or more times to form a composite substrate in which the support substrate and a piezoelectric layer comprising a piezoelectric material are joined via the silicon oxide layer.

Description

Composite substrate manufacturing method

[0001] The present invention relates to a method for manufacturing a composite substrate.

[0002] Conventionally, LN (LiNbO ３ : lithium niobate) and LT (LiTaO ３ Composite substrates are known that are formed by bonding a piezoelectric substrate made of a piezoelectric material such as lithium tantalate (LTTA) to a support substrate made of a material such as silicon, and are used in surface acoustic wave devices, etc. In recent years, in order to realize higher performance devices, a structure has been proposed for such composite substrates in which a silicon oxide layer is provided as an intermediate layer between the support substrate and the piezoelectric substrate.

[0003] Patent Document 1 discloses a method for manufacturing a composite wafer. This method for manufacturing a composite wafer includes the steps of implanting hydrogen atomic ions or hydrogen molecular ions from the surface to form an ion-implanted layer inside the oxide single crystal wafer, performing a surface activation treatment on at least one of the ion-implanted surface of the oxide single crystal wafer and the surface of a support wafer, and bonding the ion-implanted surface of the oxide single crystal wafer and the surface of the support wafer to obtain a bonded body. This method for manufacturing a composite wafer also includes the steps of heat-treating the bonded body at a temperature of 90°C or higher at which cracks do not occur, and irradiating the heat-treated bonded body with visible light to obtain an oxide single crystal thin film that is peeled along the ion-implanted layer and transferred onto the support wafer.

[0004] Patent Document 2 discloses that a bonded structure includes a support substrate, a piezoelectric material substrate made of a material selected from the group consisting of lithium niobate, lithium tantalate, and lithium niobate-lithium tantalate, and a bonding layer bonding the support substrate and the piezoelectric material substrate. The bonding layer is made of silicon oxide. When the bonding layer is divided into a bonding portion on the piezoelectric material substrate side and a bonding portion on the support substrate side, the nitrogen concentration in the bonding portion on the piezoelectric material substrate side is higher than the nitrogen concentration in the bonding portion on the support substrate side.

[0005] Japanese Patent Application Publication No. 2016-225537 International Publication No. 2019 / 130895

[0006] In conventional methods, the silicon oxide layer on the support substrate is formed by, for example, thermal oxidation. However, if the silicon substrate having the silicon oxide layer is bonded to the piezoelectric substrate by conventional techniques and then the piezoelectric substrate is processed or heated, the bonding strength may be insufficient, causing the piezoelectric substrate to peel off.

[0007] An object of the present invention is to provide a method for manufacturing a composite substrate in which peeling of the piezoelectric substrate is unlikely to occur even if the piezoelectric substrate is processed after bonding the support substrate and the piezoelectric substrate together.

[0008] a bonding step of bonding the main surfaces of the piezoelectric substrate and the silicon oxide layer to a predetermined thickness by plasma activation; a reheating step of reheating the bonded body from which the piezoelectric substrate has been ground to a predetermined second temperature higher than the first temperature; and a regrinding step of further grinding the piezoelectric substrate from the bonded body from which the reheating step has been performed. The reheating step and the regrinding step are each repeated one or more times to form a composite substrate in which the support substrate and the piezoelectric layer made of the piezoelectric material are bonded via the silicon oxide layer.

[0009] According to the present invention, it is possible to provide a method for manufacturing a composite substrate in which peeling of the piezoelectric substrate is unlikely to occur even if the piezoelectric substrate is processed after the support substrate and the piezoelectric substrate are bonded together.

[0010] 1 is a diagram showing the structure of a composite substrate according to one embodiment of the present invention; FIG. 2 is a flowchart showing a manufacturing process for a composite substrate according to one embodiment of the present invention; FIG. 3 is a diagram showing a process for forming a bonded body in a manufacturing process for a composite substrate according to one embodiment of the present invention; FIG. 4 is a table summarizing the results of checking whether or not the LN substrate has peeled off in Examples 1 to 3; FIG. 5 is a diagram showing an example of an observation image of a composite substrate when peeling of the LN substrate has occurred; FIG. 6 is a diagram showing an example of an observation image of a composite substrate when peeling of the LN substrate has not occurred; FIG. 7 is a table summarizing the results of measuring the increase in defect amount after high-temperature heating in Examples 1 to 3; and FIG. 8 is a diagram showing an example of defect distribution on the surface of an LN substrate before and after high-temperature heating.

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0012] <Configuration of Composite Substrate> Fig. 1 is a diagram showing the structure of a composite substrate 1 according to one embodiment of the present invention. The composite substrate 1 shown in Fig. 1 has a structure in which, from the top in the figure, a piezoelectric layer 11, a silicon oxide layer 12, a support substrate 13, and a silicon oxide layer 14 are stacked in this order, with the support substrate 13 and the piezoelectric layer 11 being bonded via the silicon oxide layer 12. As will be described later, the silicon oxide layer 14 does not necessarily have to be present.

[0013] The piezoelectric layer 11 is a layer made of a piezoelectric material. The piezoelectric material is appropriately selected depending on the application of the device in which the composite substrate 1 is used. The piezoelectric material is, for example, LiNbO ３ (LN) and LiTaO ３ (LT).

[0014] The silicon oxide layer 12 is a layer disposed between the piezoelectric layer 11 and the support substrate 13. In this embodiment, the silicon oxide layer 12 is made of SiO 2 , which is a dielectric material. ２ The silicon oxide layer 12 preferably has a thickness of, for example, 10 nm or more.

[0015] The support substrate 13 supports the entire composite substrate 1. The support substrate 13 and the piezoelectric layer 11 are bonded to each other with the silicon oxide layer 12 sandwiched therebetween. Any appropriate material can be used for the support substrate 13.

[0016] Silicon is preferred as the material for forming the support substrate 13. Alternatively, the support substrate 13 may be formed using a material containing silicon in its composition, such as sialon, cordierite, mullite, glass, quartz, or crystal. The thickness of the support substrate 13 is, for example, 0.2 to 1 mm, but any other appropriate thickness may be adopted.

[0017] The silicon may be single crystal silicon, polycrystalline silicon, or high-resistivity silicon, and the support substrate 13 may be SOI (Silicon on Insulator).

[0018] Typically, the sialon is a ceramic obtained by sintering a mixture of silicon nitride and alumina, and for example, Si ６－ｗ Al ｗ O ｗ N ８－ｗ Specifically, sialon has a composition in which alumina is mixed in silicon nitride, and w in the formula indicates the mixing ratio of alumina. w is preferably 0.5 or more and 4.0 or less.

[0019] Typically, the cordierite is 2MgO.2Al ２ O ３ 5SiO ２ The mullite is a ceramic having a composition of 3Al ２ O ３ 2SiO ２ ~2Al ２ O ３ SiO ２ It is a ceramic having a composition in the range of

[0020] The silicon oxide layer 14 is a layer disposed on one of the two main surfaces of the support substrate 13, the main surface to which the piezoelectric layer 11 is not bonded. In this embodiment, the silicon oxide layer 14 is made of SiO 2 , which is a dielectric material, similar to the silicon oxide layer 12 described above. ２ The thickness of the silicon oxide layer 14 is preferably equal to that of the silicon oxide layer 12, for example, 10 nm or more.

[0021] <Devices> The composite substrate 1 can be used as a material for various devices, such as optical modulators, surface acoustic wave (SAW) filters, and MEMS (Micro Electro Mechanical Systems).

[0022] <Method of Manufacturing Composite Substrate 1> Next, a method of manufacturing the composite substrate 1 will be described. Fig. 2 is a flowchart showing the manufacturing steps of the composite substrate 1 according to one embodiment of the present invention. Also, Figs. 3(A) to 3(E) are diagrams showing the process of forming a bonded body in the manufacturing steps of the composite substrate 1 according to one embodiment of the present invention, and show the state relative to the manufacturing steps shown in Fig. 2.

[0023] First, a support substrate 13 containing silicon is prepared, and silicon oxide layers 12 and 14 are formed on the main surfaces of the support substrate 13 (step S101: dielectric layer formation step). In this embodiment, the support substrate is thermally oxidized in this step. As a result, silicon oxide layers 12 and 14 are formed on the entire two main surfaces of the support substrate 13, respectively. Note that it is also possible to form the silicon oxide layer 12 only on one main surface, without forming the silicon oxide layer 14 on the other main surface.

[0024] Specifically, the silicon oxide layers 12 and 14 are formed by a thermal oxidation process using a heating device such as a heating furnace. That is, the support substrate 13 is placed in a heating furnace. Then, water vapor (H ２ O), or oxygen (O ２ ) gas is introduced and heated at a predetermined temperature for a predetermined time, whereby the surface of the support substrate 13 is thermally oxidized to form silicon oxide (SiO ２) film can be formed. By using a silicon oxide film formed by thermal oxidation as the silicon oxide layers 12, 14 in this way, it is possible to reduce the amount of foreign matter contaminating the silicon oxide layers 12, 14 compared to forming the silicon oxide layers 12, 14 by other film formation methods, such as chemical vapor deposition (CVD) or physical vapor deposition (PVD). In particular, when PVD is used, inert gases and moisture used in the film formation process tend to contaminate the silicon oxide layers 12, 14. However, thermal oxidation suppresses this contaminant contamination and forms a stable silicon oxide film that can be used as the silicon oxide layers 12, 14. Therefore, the composite substrate 1 is particularly useful when forming devices that are susceptible to impurities, such as waveguides.

[0025] In addition, when both the front and back surfaces of the support substrate 13 are thermally oxidized in the above-mentioned thermal oxidation treatment, SiO ２ In this way, by forming thin films made of the same material on both sides of the support substrate 13, it is possible to form SiO 2 layers with a certain thickness, for example, a thickness exceeding 10 μm as described above. ２ Even if a SiO 2 film is formed on only one surface of the support substrate 13, the occurrence of warping in the support substrate 13 can be suppressed, and deformation can be avoided. Therefore, when the piezoelectric substrate 11a and the silicon oxide layer 12 are bonded to each other in the bonding process described later, good bonding can be achieved. ２ When a film is formed, SiO ２ Depending on the thickness of the film and the support substrate 13, warping may occur in the support substrate 13 after film formation. As a result, it may be difficult to obtain good bonding quality in the bonding process, and it may be difficult to process the grinding process and re-grinding process that are performed thereafter, which will be described later. Therefore, it is preferable to form a SiO ２ When forming a film, the support substrate 13 and SiO ２ Taking into consideration the balance of the film, it may be necessary to take additional measures to suppress warping.

[0026] Furthermore, it is preferable to include a step of mirror-polishing the main surface of the support substrate 13 before step S101. Mirror-polishing flattens the main surface of the support substrate 13 on which the silicon oxide layer 12 is to be formed. This allows the silicon oxide layer 12 to be formed more uniformly, and also allows for more uniform bonding when the piezoelectric substrate 11a is bonded in a later step. The surface of the silicon oxide layer 12 formed in step S101 can also be polished and flattened. This improves the bonding strength when the piezoelectric substrate 11a is bonded in a later step.

[0027] Next, a piezoelectric substrate 11a made of a piezoelectric material is prepared, and the main surfaces of the piezoelectric substrate 11a are polished to a predetermined surface roughness (step S102: polishing step). In this embodiment, for example, an LN substrate is prepared as the piezoelectric substrate 11a, and its main surfaces are polished to a predetermined surface roughness. For example, the main surfaces of the piezoelectric substrate 11a are polished so that the arithmetic mean roughness Ra after polishing is 0.2 nm to 0.3 nm.

[0028] The order of steps S101 and S102 may be reversed.

[0029] Next, the main surface of the piezoelectric substrate 11a polished in step S102 and the main surface of the silicon oxide layer 12 formed on the main surface of the support substrate 13 in step S101 are arranged facing each other (FIG. 3A), and each is activated by plasma (step S103: activation step) (FIG. 3B). Here, plasma containing, for example, nitrogen or oxygen can be used. As a result, as shown in FIG. 3C, the LN (LiNbO ３ ) and SiO 2 constituting the silicon oxide layer 12 ２ and are activated to generate hydroxyl groups (OH groups) as hydrophilic functional groups. Therefore, this process can also be considered as a hydrophilization process in which the surfaces of the piezoelectric substrate 11a and the silicon oxide layer 12 are made hydrophilic by plasma.

[0030] The atmospheric pressure in the activation step is preferably 100 Pa or less, more preferably 80 Pa or less, and is preferably 30 Pa or more, more preferably 50 Pa or more.

[0031] In the activation step, in order to obtain a bonded body having high bonding strength and free from deterioration of the piezoelectric material, the temperature during plasma irradiation is preferably 150°C or less, and more preferably 100°C or less.

[0032] The energy during plasma irradiation is preferably 30 to 150 W. The product of the energy during plasma irradiation and the irradiation time is preferably 0.1 to 1.0 Wh. The plasma irradiation time is preferably 30 minutes or longer.

[0033] Next, the surfaces of the piezoelectric substrate 11a and the silicon oxide layer 12 after the activation process are bonded together (step S104: bonding process) ( FIG. 3(D) ). This bonding is performed, for example, by bringing the surfaces of the piezoelectric substrate 11a and the silicon oxide layer 12 into contact with each other and pressing them together with a predetermined pressure. As a result, the piezoelectric substrate 11a and the support substrate 13 are bonded together via the silicon oxide layer 12.

[0034] Then, the bonded body of the piezoelectric substrate 11a and the support substrate 13 obtained by the bonding process is heated (step S105: heating process) (FIG. 3E). Here, for example, the bonded body is placed in a heating device such as an oven and heated at a predetermined temperature for a predetermined time. In this heating process, at the interface (bonding surface) between the piezoelectric substrate 11a and the support substrate 13, [Li-OH]+[Si-OH]→[Li-O-Si]+H ２ O, [Nb-OH] + [Si-OH] → [Nb-O-Si] + H ２ The reaction of O causes the hydroxyl groups formed on the surfaces of the piezoelectric substrate 11a and the silicon oxide layer 12 to change into covalent bonds, generating water molecules. As a result, the piezoelectric substrate 11a and the support substrate 13 are firmly bonded together via the silicon oxide layer 12, forming a bonded body.

[0035] Next, the piezoelectric substrate 11a in the bonded body after the heating step is ground (step S106: grinding step). In this embodiment, the grinding step of step S106 is performed using a grindstone or the like so that the thickness of the ground piezoelectric substrate 11a is 2 μm to 100 μm, for example.

[0036] Then, the ground bonded body is reheated (step S107: reheating step). Here, similar to the heating step of step S105, for example, the bonded body that has been subjected to the grinding step of step S106 is placed in a heating device such as an oven and heated at a predetermined temperature for a predetermined time. The heating temperature at this time is preferably higher than the temperature in the heating step of step S105. This can further strengthen the bond between the piezoelectric substrate 11a and the support substrate 13 via the silicon oxide layer 12.

[0037] The heating step S105 and the reheating step S107 can also be considered as a step of annealing the bonded body of the piezoelectric substrate 11a and the support substrate 13 (annealing step).

[0038] Furthermore, the piezoelectric substrate 11a in the bonded body after the reheating step is ground again (step S108: re-grinding step). Here, similar to the grinding step of step S106, the piezoelectric substrate 11a is ground using a grindstone or the like so that the piezoelectric substrate 11a has a predetermined target thickness.

[0039] After the regrinding step of step S108, it is determined whether the piezoelectric substrate 11a has reached a predetermined final target thickness (step S109). Here, the final target thickness is set to, for example, 0.1 μm to 2 μm, and it is determined whether the piezoelectric substrate 11a has been ground to this final target thickness. If the result is that the piezoelectric substrate 11a has not yet been ground to the final target thickness, the process returns to the reheating step of step S107, and the bonded body in which the piezoelectric substrate 11a has been ground in the regrinding step of step S108 is reheated. On the other hand, if the piezoelectric substrate 11a has been ground to the final target thickness, the manufacturing process shown in the flowchart of FIG. 2 is terminated, and the finally obtained bonded body is referred to as the composite substrate 1. At this time, the piezoelectric substrate 11a ground to the final target thickness forms the piezoelectric layer 11 in the composite substrate 1.

[0040] Through the above steps, the reheating step of step S107 and the re-grinding step of step S108 are each repeated one or more times until the thickness of the piezoelectric substrate 11a reaches the predetermined final target thickness, thereby manufacturing the composite substrate 1.

[0041] The reheating step S107 and the regrinding step S108 may each be performed once or multiple times. The reheating step S107 and the regrinding step S108 may be performed any number of times until the piezoelectric substrate 11a finally reaches the final target thickness. This allows the composite substrate 1 having the piezoelectric layer 11 with any desired thickness to be manufactured.

[0042] Furthermore, when the reheating step S107 and the regrinding step S108 are each performed multiple times, it is preferable to perform the next reheating step at a temperature higher than the heating temperature in the previous reheating step. In this way, by repeating the reheating step and the regrinding step multiple times while gradually increasing the heating temperature in the reheating step, it is possible to reliably improve the bonding strength while preventing peeling of the piezoelectric substrate 11a during the regrinding step.

[0043] Examples for verifying the structure of the composite substrate according to the present invention will be specifically described below. Unless otherwise specified, the following procedures were carried out at room temperature.

[0044] Example 1 A bonded body was fabricated according to the manufacturing process described with reference to FIGS. 2 and 3. Specifically, a silicon substrate was prepared as the support substrate 13, and both the front and back surfaces were polished to a mirror finish. Then, a silicon oxide film (SiO ) was formed on both sides of the silicon substrate as the silicon oxide layer 12. ２ The silicon oxide film was formed by thermal oxidation to a thickness of 10 μm.

[0045] Next, a lithium niobate (LN) substrate was prepared as the piezoelectric substrate 11a, and the main surface of the LN substrate was polished to an arithmetic mean roughness Ra of 0.3 nm. Here, the arithmetic mean roughness was evaluated within a 10 μm field of view using an atomic force microscope (AFM).

[0046] The silicon substrate and the LN substrate were then plasma activated and bonded together to obtain a bonded assembly, which was then heated in an inert oven at 100°C for 10 hours in a non-oxidizing atmosphere.

[0047] Next, the surface of the LN substrate was ground with a grinder using a #1000 grindstone until the thickness of the LN substrate after grinding became any of 2 μm, 5 μm, 10 μm, 50 μm, and 100 μm.

[0048] Furthermore, each bonded body after grinding the LN substrate was reheated to enhance the bonding strength. The reheating temperature was set to 150°C, 200°C, 250°C, or 300°C, which was higher than the temperature in the heating step before grinding. The heating time was 10 hours, the same as in the heating step before grinding.

[0049] Then, the surface of the LN substrate of the reheated bonded body was re-ground with a grinder using a #1000 grinding stone to set the thickness of the LN substrate to the final target thickness of 0.1 μm, thereby producing the composite substrate 1 having the configuration shown in FIG.

[0050] Furthermore, the thickness of the LN substrate before reheating was varied among the five types (2 μm, 5 μm, 10 μm, 50 μm, 100 μm) and the reheating temperature was varied among the four types (150° C., 200° C., 250° C., 300° C.), and the above process was repeated while keeping the other conditions constant. In this way, composite substrates 1 were produced for all combinations of the thickness of the LN substrate before reheating and the reheating temperature, and each composite substrate 1 was visually inspected to see if peeling occurred in the LN substrate after regrinding.

[0051] Example 2 The piezoelectric substrate 11a before bonding was polished to a surface roughness different from that of Example 1. Specifically, an LN substrate was prepared as the piezoelectric substrate 11a, and the main surface of this LN substrate was polished so that the arithmetic mean roughness within a 10 μm field of view was Ra = 0.25 nm. Then, a bonded body was produced in which the piezoelectric substrate 11a and the support substrate 13 were bonded via the silicon oxide layer 12 using the same procedure as in Example 1. The resulting bonded body was subjected to the heating step, grinding step, reheating step, and regrinding step, respectively, while varying the thickness of the LN substrate in the grinding step and the reheating temperature in the reheating step, as in Example 1. In this way, composite substrates 1 were produced for all combinations of five types of LN substrate thickness (2 μm, 5 μm, 10 μm, 50 μm, 100 μm) and four types of reheating temperatures (150°C, 200°C, 250°C, 300°C), and each composite substrate 1 was visually inspected to see if peeling occurred in the LN substrate after regrinding.

[0052] Example 3 The piezoelectric substrate 11a before bonding was polished to a surface roughness different from that of Examples 1 and 2. Specifically, an LN substrate was prepared as the piezoelectric substrate 11a, and the main surface of this LN substrate was polished so that the arithmetic mean roughness within a 10 μm field of view was Ra = 0.2 nm. Then, a bonded body was produced in which the piezoelectric substrate 11a and the support substrate 13 were bonded via the silicon oxide layer 12 using the same procedures as in Examples 1 and 2. The resulting bonded body was subjected to the heating step, grinding step, reheating step, and regrinding step, respectively, while varying the thickness of the LN substrate in the grinding step and the reheating temperature in the reheating step, as in Examples 1 and 2. In this way, composite substrates 1 were produced for all combinations of five types of LN substrate thickness (2 μm, 5 μm, 10 μm, 50 μm, 100 μm) and four types of reheating temperatures (150°C, 200°C, 250°C, 300°C), and each composite substrate 1 was visually inspected to see if peeling occurred in the LN substrate after regrinding.

[0053] (Peeling of LN Substrate) Fig. 4 shows a table summarizing the results of checking whether or not peeling occurred in the LN substrates in Examples 1 to 3 described above. In Fig. 4, table (a) shows whether or not peeling occurred for each combination of LN substrate thickness and reheating temperature in Example 1. Table (b) shows whether or not peeling occurred for each combination of LN substrate thickness and reheating temperature in Example 2. Similarly, table (c) shows whether or not peeling occurred for each combination of LN substrate thickness and reheating temperature in Example 3. In these tables, cases where peeling was observed in the LN substrate after regrinding are indicated by "x", and cases where peeling was not observed are indicated by "o".

[0054] 4, it can be seen that the smaller the surface roughness of the piezoelectric substrate 11a before bonding, the less likely the LN substrate is to peel off during the regrinding process. It can also be seen that the thinner the LN substrate is before reheating (after the grinding process) and the lower the reheating temperature, the less likely the LN substrate is to peel off during the regrinding process.

[0055] 5 is a diagram showing an example of an observation image of the composite substrate 1 when peeling of the LN substrate occurs. The observation image shown in FIG. 5 shows that large peeling has occurred in the LN substrate (piezoelectric layer 11) arranged on the front surface side of the composite substrate 1.

[0056] 6 is a diagram showing an example of an observation image of the composite substrate 1 when no peeling of the LN substrate occurs. In the observation image shown in FIG. 6, no peeling is observed in the LN substrate (piezoelectric layer 11) arranged on the front surface side of the composite substrate 1.

[0057] (Increase in Defects Due to High-Temperature Heating) Furthermore, for each of the composite substrates 1 obtained in Examples 1 to 3 in which peeling of the LN substrate did not occur, the state of the surface of the LN substrate before and after high-temperature heating was observed using an optical appearance inspection device, and the amount of defects on the surface, i.e., the degree of progress of peeling of the LN substrate, was evaluated. At this time, defects measuring 10 μm or larger were counted, and the increase in the number of defects before and after high-temperature heating, i.e., the ratio of the increase in the number of defects before and after high-temperature heating to the number of defects before high-temperature heating, was calculated. Furthermore, the high-temperature heating was performed using the same inert oven as in the heating process and reheating process, under conditions of 500°C and 10 hours in a non-oxidizing atmosphere.

[0058] FIG. 7 shows a table summarizing the measurement results of the defect increase amount after high-temperature heating in Examples 1 to 3. In FIG. 7, table (a) shows the defect increase amount for each combination of LN substrate thickness and reheating temperature in Example 1. Table (b) shows the defect increase amount for each combination of LN substrate thickness and reheating temperature in Example 2. Similarly, table (c) shows the defect increase amount for each combination of LN substrate thickness and reheating temperature in Example 3. Note that these tables each show the defect increase amount, i.e., the ratio of the defect increase number before and after high-temperature heating to the defect number before high-temperature heating, expressed as a percentage. Furthermore, for combinations in which peeling of the LN substrate occurred during the regrinding process, "N / A" is entered instead of the defect increase amount, indicating that the combination was not subject to high-temperature heating.

[0059] 7, it can be seen that the smaller the surface roughness of the piezoelectric substrate 11a before bonding, the smaller the increase in defects after high-temperature heating. Furthermore, it can be seen that in order to keep the increase in defects to 20% or less, it is necessary to set the surface roughness of the piezoelectric substrate 11a before bonding to Ra = 0.25 nm or less, set the thickness of the LN substrate before reheating to 10 μm or less, and further set the reheating temperature to 250°C or more and 300°C or less.

[0060] 8 shows examples of defect distributions on the surface of an LN substrate before and after high-temperature heating. Fig. 8 shows the distribution of defects found by observing the surface of the LN substrate before and after high-temperature heating for each of the following cases: Example 1, where the thickness of the LN substrate before reheating is 10 μm and the reheating temperature is 150° C. (defect increase: 815%); Example 2, where the thickness of the LN substrate before reheating is 2 μm and the reheating temperature is 200° C. (defect increase: 256%); and Example 3, where the thickness of the LN substrate before reheating is 2 μm and the reheating temperature is 300° C. (defect increase: 4%). Fig. 8 shows that the number of defects increases after high-temperature heating in Examples 1 and 2, but that the number of defects remains almost unchanged after high-temperature heating in Example 3.

[0061] Among the conditions for keeping the defect increase to 20% or less, a regression analysis was conducted to determine the relationship between the surface roughness of the piezoelectric substrate 11a before bonding and the reheating temperature. It was found that the evaluation value A expressed by the following formula (1) should be within the range of 0 to 0.2. In formula (1), Ra represents the arithmetic mean roughness of the surface of the piezoelectric substrate 11a before bonding, and Th represents the reheating temperature. A=1.24+34.55×Ra+(−0.03)×Th+(Ra−0.24)×(Th−201.47)×(−0.36) ... (1)

[0062] From the above results, it can be seen that in order to strengthen the bond strength between the piezoelectric layer 11 and the silicon oxide layer 12 in the composite substrate 1 and to make it less likely for the piezoelectric layer 11 to peel off, it is desirable to set the surface roughness of the piezoelectric substrate 11a before bonding to an arithmetic mean roughness Ra of 0.25 nm or less and to repeat the reheating step and the re-grinding step one or more times. Furthermore, it can be seen that it is even more desirable to set the thickness of the LN substrate before reheating to 10 μm or less and to set the evaluation value A expressed by the above-mentioned formula (1) in the range of 0 to 0.2.

[0063] According to the embodiment of the present invention described above, the following advantageous effects are achieved.

[0064] (1) The method for manufacturing the composite substrate 1 includes the steps of: thermally oxidizing a support substrate 13 containing silicon to form a silicon oxide layer 12 on a main surface of the support substrate 13 (step S101: dielectric layer formation step); preparing a piezoelectric substrate 11a made of a piezoelectric material and having an arithmetic mean roughness of the main surface of 0.25 nm or less (step S102: polishing step); and plasma-activating and bonding the main surface of the piezoelectric substrate 11a and the main surface of the silicon oxide layer 12 to obtain a bonded body (step S103: activation step). The method includes the steps of: (a) heating the bonded body obtained to a predetermined first temperature (step S105); (b) grinding the piezoelectric substrate 11a of the bonded body after the heating step to a predetermined thickness (step S106); (c) reheating the bonded body from which the piezoelectric substrate 11a has been ground in the grinding step to a predetermined second temperature higher than the first temperature (step S107); and (d) regrinding the piezoelectric substrate 11a of the bonded body after the reheating step (step S108). The reheating step of step S107 and the regrinding step of step S108 are each repeated one or more times to form a composite substrate 1 in which the support substrate 13 and the piezoelectric layer 11 made of a piezoelectric material are bonded via the silicon oxide layer 12. By doing this, it is possible to improve the bonding strength between the support substrate 13 and the piezoelectric substrate 11a, and therefore it is possible to provide a manufacturing method for the composite substrate 1 in which the piezoelectric substrate 11a is less likely to peel off even if the piezoelectric substrate 11a is processed after the support substrate 13 and the piezoelectric substrate 11a are bonded together.

[0065] (2) When the arithmetic mean roughness of the main surface of the piezoelectric substrate 11a before bonding is Ra (nm) and the second temperature at which the bonded body is reheated in the reheating step is Th (°C), the evaluation value A based on Ra and Th is preferably within a range of 0 or more and 0.2 or less. Specifically, the evaluation value A calculated by the above-mentioned formula (1) is preferably within a range of 0 or more and 0.2 or less. In this way, in the composite substrate 1 in which the piezoelectric layer 11 is formed by grinding the piezoelectric substrate 11a to a predetermined final target thickness, the bonding strength between the piezoelectric layer 11 and the silicon oxide layer 12 can be made stronger, and peeling of the piezoelectric layer 11 can be made less likely to occur.

[0066] The present invention is not limited to the above-described embodiment, and can be implemented using any components without departing from the spirit of the present invention.

[0067] The above-described embodiments and modifications are merely examples, and the present invention is not limited to these details as long as the features of the invention are not impaired. Furthermore, although various embodiments and modifications have been described above, the present invention is not limited to these details. Other aspects that can be considered within the scope of the technical idea of ​​the present invention are also included within the scope of the present invention.

[0068] 1... composite substrate, 11... piezoelectric substrate, 11a... piezoelectric layer, 12... silicon oxide layer, 13... supporting substrate, 14... silicon oxide layer

Claims

1. A method for manufacturing a composite substrate, comprising: a step of thermally oxidizing a support substrate containing silicon to form a silicon oxide layer on a main surface of the support substrate; a step of preparing a piezoelectric substrate made of a piezoelectric material, the main surface having an arithmetic mean roughness of 0.25 nm or less; a step of plasma-activating and bonding the main surface of the piezoelectric substrate to a main surface of the silicon oxide layer to obtain a bonded body; a heating step of heating the bonded body to a predetermined first temperature; a grinding step of grinding the piezoelectric substrate in the bonded body after the heating step to a predetermined thickness; a reheating step of reheating the bonded body from which the piezoelectric substrate has been ground to a predetermined second temperature higher than the first temperature; and a regrinding step of further grinding the piezoelectric substrate in the bonded body after the reheating step, wherein the reheating step and the regrinding step are each repeated one or more times to form a composite substrate in which the support substrate and a piezoelectric layer made of the piezoelectric material are bonded via the silicon oxide layer.

2. A method for manufacturing a composite substrate according to claim 1, wherein, when the arithmetic mean roughness is Ra (nm) and the second temperature is Th (°C), an evaluation value A based on the Ra and the Th is in the range of 0 or more and 0.2 or less.

3. The method for manufacturing a composite substrate according to claim 2, wherein the evaluation value A is calculated by the following formula (1): A=1.24+34.55×Ra+(−0.03)×Th+(Ra−0.24)×(Th−201.47)×(−0.36) (1) 4. A method for manufacturing a composite substrate according to claim 1, wherein in the grinding step, the piezoelectric substrate is ground until the thickness of the piezoelectric substrate is 2 μm or more and 100 μm or less, and the reheating step and the re-grinding step are each repeated one or more times until the thickness of the piezoelectric substrate is 0.1 μm or more and 2 μm or less.

5. A method for manufacturing a composite substrate according to claim 1, wherein the second temperature is 250°C or higher and 300°C or lower.

6. A method for manufacturing a composite substrate according to claim 1, wherein the reheating step and the re-grinding step are each repeated multiple times while gradually increasing the second temperature.

7. A method for manufacturing a composite substrate according to claim 1, wherein the piezoelectric material is LN or LT.

Images