Method for manufacturing a composite substrate and composite substrate

Abstract

It is easy to remove each substrate in the composite substrate, and the composite substrate peeling is inhibited in an unintended process. A manufacturing method of a composite substrate, in which a first bonding material is formed on a first surface of a first substrate, at least one recessed portion is formed inside a top view outer edge portion of the first substrate on the first surface, the first bonding material is formed along an inner wall of the recessed portion, and the space surrounded by the inner wall of the recessed portion is not filled, a second bonding material is formed on a second surface of a second substrate, and the first bonding material and the second bonding material are bonded in a region excluding the recessed portion.

Description

TECHNICAL FIELD

[0001] The technology disclosed in the present application relates to a manufacturing method of a composite substrate and a composite substrate. BACKGROUND

[0002] As a transistor of high output and high frequency, a high electron mobility transistor (HEMT) using a gallium nitride (GaN) film as an active layer is being put into practical use (for example, refer to Patent Literature 1).

[0003] In the above, in the case where a GaN transistor using a diamond as a substrate is adopted, in order to apply a transistor manufacturing process such as film formation, photolithography, or etching, a thin and curved polycrystalline diamond substrate is adhered with a support diamond substrate using an inorganic adhesive. Further, a composite substrate which is flat and has a moderate thickness is formed by the polycrystalline diamond substrate and the support diamond substrate.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

[0006] Patent Literature 1: Japanese Patent Application Laid-Open No. 2015-522213 SUMMARY

[0007] PROBLEMS TO BE SOLVED BY THE INVENTION

[0008] In the case where the above-described composite substrate is used, in order to remove the support diamond substrate from the polycrystalline diamond substrate, the composite substrate is immersed in a drug solution which can dissolve the inorganic adhesive.

[0009] However, it takes a relatively long time to dissolve the adhesive. In addition, if there is a similar process of using a drug solution in the transistor manufacturing process, the drug solution can be soaked in the inorganic adhesive, and the support diamond substrate can be peeled off from the diamond GaN substrate by mistake, and the transistor manufacturing process cannot be stably performed.

[0010] The technology disclosed in the present application is a technology completed in view of the above-described problems, and aims to provide a technology which easily removes each substrate in a composite substrate and suppresses peeling of the composite substrate in an unintended process.

[0011] MEANS FOR SOLVING THE PROBLEMS

[0012] According to the first aspect of the technology disclosed in the present specification, a first bonding material is formed on a first surface of a first substrate, at least one recess is formed on the first surface inside a top view outer edge portion of the first substrate, the first bonding material is formed along an inner wall of the recess and is not filled in a space surrounded by the inner wall of the recess, a second bonding material is formed on a second surface of a second substrate, the first bonding material and the second bonding material are bonded in a region excluding the recess in a state in which the space surrounded by the inner wall of the recess is maintained as a vacuum, and a composite substrate in a state in which the first bonding material and the second bonding material are bonded is immersed in a dissolving solution for dissolving the first bonding material and the second bonding material.

[0013] According to the second aspect of the technology disclosed in the present specification, a composite substrate for manufacturing a GaN transistor using a diamond as a substrate includes a first substrate having a first surface, a second substrate having a second surface, a first dissolvable bonding material formed on the first surface, and a second dissolvable bonding material formed on the second surface and bonded to the first bonding material, at least one recess is formed on the first surface inside a top view outer edge portion of the first substrate, the first bonding material is formed along an inner wall of the recess and is not filled in a space surrounded by the inner wall of the recess, the first bonding material and the second bonding material are bonded in a region excluding the recess, the space surrounded by the inner wall of the recess is a vacuum, and the first substrate and the second substrate are separated by dissolving the first bonding material and the second bonding material.

[0014] Effects of the Invention

[0015] According to the first aspect of the technology disclosed in the present specification, by immersing a dissolving solution into the space surrounded by the inner wall of the recess, the dissolving of the first bonding material and the second bonding material is advanced, and thus the time required for removing the first substrate can be shortened. In addition, by forming the recess inside the outer edge portion, the first bonding material and the second bonding material can be inhibited from dissolving and the composite substrate from peeling off in an unintended process.

[0016] According to the second aspect of the technology disclosed in the present specification, by immersing a dissolving solution into the space surrounded by the inner wall of the recess, the dissolving of the first bonding material and the second bonding material is advanced, and thus the time required for removing the first substrate can be shortened. In addition, by forming the recess inside the outer edge portion, the first bonding material and the second bonding material can be inhibited from dissolving and the composite substrate from peeling off in an unintended process.

[0017] Furthermore, objects, features, aspects and advantages of the technologies disclosed in this specification will become more apparent from the following detailed description and the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 A perspective view for illustrating an example of a configuration of a composite substrate involved in an embodiment.

[0019] Figure 2 A cross-sectional view for illustrating a cross section of A-A' in Figure 1

[0020] Figure 3 A diagram for illustrating an example of a semiconductor substrate and a support substrate configuring a composite substrate involved in an embodiment.

[0021] Figure 4 An enlarged plan view of a B region in Figure 3

[0022] A flowchart for illustrating a procedure of preparing a diamond-based GaN substrate involved in an embodiment. Figure 5

[0023] A flowchart for illustrating a procedure of preparing a support diamond substrate involved in an embodiment. Figure 6

[0024] A flowchart for illustrating a procedure of manufacturing a composite substrate involved in an embodiment. Figure 7

[0025] A flowchart for illustrating a procedure of forming a diamond-based GaN transistor involved in an embodiment. Figure 8

[0026] A cross-sectional view for illustrating an example of a state halfway through a procedure of preparing a diamond-based GaN substrate involved in an embodiment. Figure 9

[0027] A cross-sectional view for illustrating an example of a state halfway through a procedure of preparing a diamond-based GaN substrate involved in an embodiment. Figure 10

[0028] A cross-sectional view for illustrating an example of a state halfway through a procedure of preparing a diamond-based GaN substrate involved in an embodiment. Figure 11

[0029] A cross-sectional view for illustrating an example of a state halfway through a procedure of preparing a diamond-based GaN substrate involved in an embodiment. Figure 12

[0030] A cross-sectional view for illustrating an example of a state halfway through a procedure of preparing a diamond-based GaN substrate involved in an embodiment. Figure 13 A cross-sectional view for illustrating an example of a state halfway through a procedure of preparing a diamond-based GaN substrate involved in an embodiment.​

[0031] Figure 14 A cross-sectional view showing an example of a state halfway through a process of preparing a diamond substrate involved in the embodiment.

[0032] Figure 15 A cross-sectional view showing an example of a state halfway through a process of preparing a diamond substrate involved in the embodiment.

[0033] Figure 16 A plan view schematically showing an example of a resist mask pattern produced in the embodiment.

[0034] Figure 17 A cross-sectional view showing an example of a state halfway through a process of preparing a diamond substrate involved in the embodiment.

[0035] Figure 18 A cross-sectional view showing an example of a state halfway through a process of preparing a diamond substrate involved in the embodiment.

[0036] Figure 19 A cross-sectional view showing an example of a state halfway through a process of preparing a diamond substrate involved in the embodiment.

[0037] Figure 20 A cross-sectional view showing an example of a state halfway through a process of preparing a diamond substrate involved in the embodiment.

[0038] Figure 21 A cross-sectional view showing an example of a state halfway through a process of producing a composite substrate involved in the embodiment.

[0039] Figure 22 A cross-sectional view showing an example of a state halfway through a process of producing a composite substrate involved in the embodiment.

[0040] Figure 23 A cross-sectional view showing an example of a state halfway through a process of forming a diamond-based GaN transistor involved in the embodiment.

[0041] Figure 24 A cross-sectional view showing an example of a state halfway through a process of forming a diamond-based GaN transistor involved in the embodiment.

[0042] Figure 25 A cross-sectional view showing an example of a state halfway through a process of forming a diamond-based GaN transistor involved in the embodiment.

[0043] Figure 26 A cross-sectional view showing an example of a state halfway through a process of forming a diamond-based GaN transistor involved in the embodiment.

[0044] Figure 27A cross-sectional view for showing an example of a structure not having a space formed by a recess as a comparative example.

[0045] Figure 28 A cross-sectional view for showing an example of a configuration of a composite substrate involved in the embodiment.

[0046] Figure 29 A view for showing a state in which the composite substrate is immersed in an aqueous hydrofluoric acid solution.

[0047] Figure 30 A view for showing a state in which the composite substrate is immersed in an aqueous hydrofluoric acid solution.

[0048] Figure 31 A flowchart for showing a procedure of a process of manufacturing the composite substrate involved in the embodiment.

[0049] Figure 32 A cross-sectional view for showing an example of a state in the middle of the process of manufacturing the composite substrate involved in the embodiment.

[0050] Figure 33 A view for showing another example of a semiconductor substrate and a support substrate configuring the composite substrate involved in the embodiment. DETAILED DESCRIPTION

[0051] Hereinafter, the embodiment will be described with reference to the drawings. In the following embodiment, detailed features and the like are also shown for the purpose of technical explanation, but these are examples, and not all of them are necessarily essential features for the embodiment to be implemented.

[0052] Note that the drawings are schematically shown, and for the purpose of convenience of explanation, omission or simplification of configurations in the drawings is appropriately made. In addition, the mutual relationship of the size and position of the configurations and the like shown in different drawings is not necessarily correctly recorded, and can be appropriately changed. In addition, in the drawings such as plan views other than cross-sectional views, a section line is sometimes attached in order to make it easy to understand the contents of the embodiment.

[0053] In addition, in the explanation shown below, the same reference numerals are attached to the same configuration elements and illustrated for the same, and it is considered that the names and functions thereof are also the same. Therefore, in order to avoid repetition, detailed explanation thereof is sometimes omitted.

[0054] In addition, in the explanation described below, in the case where a certain configuration element is described as "provided with", "including", or "having" and the like, unless otherwise specified, it does not mean that the presence of other configuration elements is excluded.

[0055] In addition, in the following description, even when a term such as "first" or "second" is used, these terms are used for the sake of convenience in order to easily understand the content of the embodiments and are not limited to the order or the like that can be derived from these terms.

[0056] In addition, in the following description, expressions indicating relative or absolute positional relationships, such as "in a direction", "along a direction", "parallel", "orthogonal", "center", "concentric", or "coaxial", or the like, include cases where the positional relationship is strictly indicated and cases where the angle or the distance is displaced within a tolerance or a range in which the same degree of function is obtained, unless otherwise specified.

[0057] In addition, in the following description, expressions indicating an equal state, such as "same", "equal", "uniform", or "homogeneous", or the like, include cases where the state is strictly equal and cases where a difference is generated within a tolerance or a range in which the same degree of function is obtained, unless otherwise specified.

[0058] In addition, in the following description, even when a term such as "upper", "lower", "left", "right", "side", "bottom", "top", or "back" is used, these terms are used for the sake of convenience in order to easily understand the content of the embodiments and are not related to the direction at the time of actual implementation.

[0059] In addition, in the following description, in the case where it is described as "upper surface of...", "lower surface of...", or the like, in addition to the upper surface of the constituent element itself that is the object, a state in which another constituent element is formed on the upper surface of the constituent element that is the object is also included. That is, for example, in the case where it is described as "a constituent element B is provided on the upper surface of a constituent element A", it does not hinder the presence of another constituent element "C" between the constituent element A and the constituent element B.

[0060] In addition, in the following description, expressions indicating a shape, such as "quadrilateral" or "cylindrical", or the like, include cases where the shape is strictly indicated and cases where a recess, a protrusion, a chamfer, or the like is formed within a tolerance or a range in which the same degree of function is obtained, unless otherwise specified.

[0061] <First Embodiment>

[0062] Hereinafter, a manufacturing method of a composite substrate and a composite substrate related to the present embodiment will be described.

[0063] In the HEMT, which has been put into practical use as a transistor of high output and high frequency, by using GaN as an active layer, high withstand voltage can be achieved, and by adopting a HEMT structure, low resistance can be achieved. Therefore, the above-described HEMT can be used for large power.

[0064] However, due to the Joule heat generated in the transistor section when a large power is input, element destruction sometimes occurs.

[0065] Generally, a heat dissipation member called a heat sink is installed in a high-output transistor, but since the heat generation site is limited to a minute region even in the transistor, heat transport from the minute heat generation site to the heat sink becomes a problem.

[0066] As for the GaN film in which the transistor is formed, it is bonded to the heat sink while sandwiching, for example, a SiC (silicon carbide) substrate as a base material. In recent years, a structure in which diamond, which has a higher thermal conductivity than SiC, is used as the base material has been proposed.

[0067] If diamond is used as the base material, the heat generated in the minute region in the GaN film spreads in the lateral direction in the diamond layer. Then, the heat is transported to the wide region of the heat sink that bonds the diamond substrate and the GaN film, and as a result, the temperature reached by the heat generation site can be reduced.

[0068] In the structure in which diamond is used as the base material of the GaN film, a GaN thin film is crystallized and grown on the upper surface of a Si substrate or the upper surface of a SiC substrate, and further, a support substrate is bonded to the upper surface of the GaN thin film. Then, the Si substrate or the SiC substrate is removed.

[0069] Then, after a protective film such as a silicon nitride (SiN) film is formed on the lower surface of the GaN thin film that is exposed as a result, a polycrystalline diamond layer is formed by a chemical vapor deposition (CVD) method.

[0070] Then, by removing the support substrate, a structure in which the GaN thin film is formed on the upper surface of the polycrystalline diamond substrate can be obtained.

[0071] Further, if the transistor manufacturing process is applied to the GaN thin film, a GaN transistor using diamond as the base material can be obtained.

[0072] Hereinafter, the substrate in which the GaN thin film is formed on the upper surface of the polycrystalline diamond substrate is referred to as a diamond base GaN substrate.

[0073] In the case where the GaN transistor using diamond as the base material is adopted, in order to apply the transistor manufacturing process such as film formation, photolithography, or etching, the support diamond substrate is bonded to the thin and further curved polycrystalline diamond substrate using an inorganic adhesive. Then, the polycrystalline diamond substrate and the support diamond substrate form a composite substrate that is flat and has a moderate thickness.

[0074] If the polycrystalline diamond layer can be formed thickly by the CVD method, it is not necessary to additionally adhere the support diamond substrate as described above, and the transistor manufacturing process can be directly applied.

[0075] However, the film formation speed when forming the polycrystalline diamond layer by the CVD method is very slow, and thus it is difficult to form the polycrystalline diamond layer thickly by the CVD method.

[0076] Further, even if the polycrystalline diamond layer can be formed thickly by the CVD method, the substrate warping during the film formation process increases, and thus the transistor manufacturing process cannot be applied.

[0077] From these viewpoints, it is necessary to make the thickness of the polycrystalline diamond layer the minimum thickness, for example, about 100 μm, which functions as a heat transport material, and thus it is necessary to adhere the support diamond substrate to form a composite substrate.

[0078] In the case of the composite substrate manufactured as described above, in order to prevent warping during the heat process of the transistor manufacturing process, it is necessary to select materials having thermal expansion coefficients close to each other for the diamond substrate GaN substrate and the support diamond substrate which constitute the composite substrate.

[0079] From this viewpoint, as the support substrate, a diamond substrate manufactured separately is used. Further, as the method of adhering the support substrate, it is necessary to select a method in which the structure after the adhesion has heat resistance, and for example, adhesion using an inorganic adhesive is performed.

[0080] Then, it is necessary to remove the support diamond substrate from the composite substrate on which the transistor manufacturing process is completed. In this case, since the thickness of the diamond substrate GaN substrate is thin, for example, about 100 μm, in order to prevent breakage of the substrate, a mechanical method cannot be used for the removal of the support diamond substrate.

[0081] Thus, the composite substrate is immersed in a chemical solution, and further, the chemical solution is impregnated in the porous inorganic adhesive, and thus the support diamond substrate is removed from the diamond substrate GaN substrate.

[0082] In order to remove the support diamond substrate from the diamond substrate GaN substrate, the composite substrate is immersed in a chemical solution which can dissolve the inorganic adhesive.

[0083] However, a relatively long time is required to dissolve the adhesive. Further, if there is a process in which a similar chemical solution is used in the transistor manufacturing process, it is possible that the chemical solution is impregnated in the inorganic adhesive, and the support diamond substrate is accidentally peeled from the diamond substrate GaN substrate, and thus the transistor manufacturing process cannot be stably performed.

[0084] <Structure of Composite Substrate>

[0085] Next, the composite substrate according to the present embodiment will be described with reference to the drawings.

[0086] Figure 1 is a perspective view showing an example of a configuration of the composite substrate according to the present embodiment. As shown in the example of Figure 1 , the composite substrate 1 is provided with a diamond substrate 2 and a support diamond substrate 3.

[0087] Figure 2 is a cross-sectional view showing a cross section of A-A' in Figure 1 . As shown in the example of Figure 2 , a SiO2 bonding layer 22 and a SiO2 bonding layer 40A are formed at the bonding surface between the diamond substrate 2 and the support diamond substrate 3. Further, a space 43A is formed between these bonding layers.

[0088] Note that, as for the cross-sectional shape of the SiO2 bonding layer 40A and the cross-sectional shape of the space 43A, it is not limited to the example shown in Figure 2 . For example, as described later, the side surface of the recess of the upper surface of the support diamond substrate 3 is in a curved surface shape, and the SiO2 bonding layer formed at the side surface can also be in a curved surface shape.

[0089] Figure 3 is a view showing an example of a semiconductor substrate and a support substrate that configure the composite substrate according to the present embodiment. As shown in the example of Figure 3 , a processing recess 35 is formed at the bonding surface between the diamond substrate 2 and the support diamond substrate 3 of the support diamond substrate 3.

[0090] Figure 4 is an enlarged plan view of the B region in Figure 3 . As shown in the example of Figure 4 , the processing recess 35 is interrupted without reaching the outer edge portion 10 of the support diamond substrate 3, and the space inside the processing recess 35 is closed within the bonding surface between the support diamond substrate 3 and the diamond substrate 2.

[0091] <About the manufacturing method of the composite substrate>

[0092] Next, the manufacturing method of the composite substrate according to the present embodiment will be described.

[0093] The manufacturing method of the composite substrate according to the present embodiment is roughly divided into the following four processes, that is, a process of preparing a diamond substrate 2, a process of preparing a support diamond substrate 3, a process of manufacturing a composite substrate, and a process of forming a diamond substrate GaN transistor.

[0094] Figure 5 is a flowchart showing the process of preparing a diamond substrate 2 in the above.

[0095] Figure 6 A flowchart for showing the process of preparing the diamond substrate in the above.

[0096] Figure 7 A flowchart for showing the process of manufacturing the composite substrate in the above.

[0097] Figure 8 A flowchart for showing the process of forming the diamond substrate GaN transistor in the above.

[0098] The composite substrate related to the present embodiment is completed through the process of manufacturing the composite substrate shown in Figure 7 . Also, as for the process of forming the diamond substrate GaN transistor shown in Figure 8 , it is just a description of the latter half of the process using the composite substrate related to the present embodiment.

[0099] However, the composite substrate related to the present embodiment exerts an effect when the latter half of the process is implemented, and thus is described in detail in the present embodiment.

[0100] <Process of preparing the diamond substrate GaN substrate>

[0101] First, as for the process of preparing the diamond substrate GaN substrate, reference is made to Figure 5 , Figure 9 , Figure 10 , Figure 11 , Figure 1 , and Figure 13 .

[0102] Among them, Figure 9 , Figure 10 , Figure 11 , Figure 12 , and Figure 13 are cross-sectional views showing examples of the state halfway through the process of preparing the diamond substrate GaN substrate.

[0103] First, a Si substrate GaN substrate 15 as shown in the example in Figure 9 is manufactured.

[0104] The detailed process is as follows. First, for example, for a 2-inch Si substrate 11, a buffer layer 12 including an AlN (aluminum nitride) film or an AlGaN (aluminum gallium nitride) film, etc., a GaN film 13, and an AlGaN film 14 are successively epitaxially grown.

[0105] As for the buffer layer 12, in order to moderate the lattice mismatch with the Si substrate 11 in contact at the lower surface and the GaN film 13 in contact at the upper surface, and further, in order to moderate the stress contained in the GaN film 13, the composition ratio or film thickness, etc. thereof are adjusted.

[0106] The buffer layer 12 is sometimes a single layer, and sometimes a plurality of layers with different compositions are stacked. The thickness of the buffer layer 12 is, for example, 0.7 μm.

[0107] In addition, in the case of the AlGaN film 14, by epitaxially growing continuously on the upper surface of the GaN film 13, a high-concentration electron layer called two-dimensional electron gas is generated near the interface between the GaN film 13 and the AlGaN film 14, using the spontaneous polarization effect and piezoelectric polarization effect of AlGaN.

[0108] This electron layer is different from an electron layer formed by impurity addition, and the electrons are not easily affected by ion scattering, showing very high electron mobility.

[0109] Note that, as shown in FIG. 1, a cap layer including a GaN film is sometimes formed on the upper surface of the AlGaN film 14, or a layer in which an impurity such as Fe or C (carbon) is added is sometimes interposed in a part of the GaN film 13. In this way, the Si-based GaN substrate 15 is produced. Figure 9 Next, as shown in the example in FIG. 2, a support substrate adhesive layer 16 is formed on the upper surface of the AlGaN film 14 in the Si-based GaN substrate 15. Then, a support substrate 17 is further adhered to the upper surface of the support substrate adhesive layer 16.

[0110] Figure 10 Then, by removing the Si substrate 11 and the buffer layer 12, a GaN transfer substrate 18 is obtained.

[0111] In the case of the GaN transfer substrate 18, since it is subjected to a high-temperature process thereafter, the support substrate adhesive layer 16 and the support substrate 17 need to have high-temperature resistance.

[0112] In the present embodiment, as the support substrate adhesive layer 16, a laminated film of a SiN film and a SiO2 film is applied. Here, by disposing the SiN film on the side closer to the AlGaN film 14, an effect of suppressing the release of nitrogen (N) from the AlGaN film 14 or the GaN film 13 in a high-temperature process is obtained.

[0113] In addition, in the case of the SiO2 film in the laminated film as the support substrate adhesive layer 16, an effect of achieving strong adhesion with the support substrate 17 is obtained. In the case of this SiO2 film, by using a CVD method of TEOS (tetraethoxysilane), for example, a film is formed with a film thickness of 1.5 μm. Then, for example, annealing treatment at 700°C is performed, and further surface planarization treatment is performed. Then, the Si substrate as the support substrate 17 is adhered using a direct bonding method.

[0114]

[0115] ​​By using the direct bonding method, the adhesion strength that can withstand a high temperature process can be obtained between the SiO2 film and the Si substrate as the support substrate 17. In addition, no air is generated at the bonding interface.

[0116] As the support substrate adhesive layer 16, in addition to this, an inorganic adhesive having alumina or silica as a main component or the like can also be used.

[0117] Next, by removing the Si substrate 11 and the buffer layer 12, the GaN transfer substrate 18 is produced, and as for the removal of the Si substrate 11, for example, it can be performed by mechanical polishing.

[0118] On the other hand, the mechanical polishing speed of the buffer layer 12 composed of AlGaN or the like is very slow.

[0119] Therefore, at the time when the Si substrate 11 is removed and the buffer layer 12 is exposed, the mechanical polishing is temporarily stopped.

[0120] Then, the polishing conditions are changed to AlGaN polishing conditions, and the polishing is performed at a low speed, whereby the buffer layer 12 is removed. Then, the GaN film 13 is exposed.

[0121] In the present embodiment, mechanical polishing is used in the removal of the Si substrate 11 and the buffer layer 12, but gas phase etching can also be used by a reactive ion etching (RIE) method. In the case of both the mechanical polishing and the RIE method, only the processed surface is removed, and therefore it is suitable for the removal of the Si substrate 11 in the state where the support substrate 17 as the Si substrate is also provided on the opposite side as in the present embodiment.

[0122] As the material of the support substrate 17, if a material such as sapphire or the like is used instead of Si, a chemical liquid treatment using a chemical liquid such as fluoboric acid or the like can be applied in the removal of the Si substrate 11. If the chemical liquid treatment can be used, the ratio (i.e., the selection ratio) of the removal speed of the Si substrate 11 to the buffer layer 12 becomes very large, and therefore the removal can be reliably stopped at the time when the buffer layer 12 is exposed.

[0123] In the present embodiment, although a laminated film of a SiN film and a SiO2 film is used as the support substrate adhesive layer 16, the material is not limited to this, and other film types can be used, and an inorganic adhesive having alumina or silica or the like as a main component can be used.

[0124] Next, as shown in the example of Figure 11 In the lower surface of the GaN film 13 of the GaN transfer substrate 18, a protective layer 19 is formed. Then, by forming a polycrystalline diamond film 20 on the lower surface of the protective layer 19, a polycrystalline diamond film formation substrate 21 is produced.

[0125] The protective layer 19 is formed to protect the GaN film 13 by the protective layer 19 at the time of forming the polycrystal diamond film 20. In the present embodiment, the protective layer 19 is an amorphous Si thin film, which is formed by, for example, a plasma CVD method.

[0126] The film thickness of the protective layer 19 is a film thickness required for protecting the GaN film 13, and if the film thickness of the protective layer 19 is too thick, the protective layer 19 becomes an obstacle to heat conduction between the GaN film 13 and the polycrystal diamond film 20.

[0127] In the present embodiment, an amorphous Si having a film thickness of 20 nm is formed as the protective layer 19.

[0128] Note that the protective layer 19 is not limited to the case of being an amorphous Si, and can be another film type, and the protective layer 19 can not be formed as long as damage to the GaN film 13 is negligible.

[0129] In forming the polycrystal diamond film 20, for example, a microwave plasma CVD method using methane gas and hydrogen is applied. In forming the polycrystal diamond film by the microwave plasma CVD method, a polycrystal diamond film having high crystallinity can be obtained at a substrate temperature of 850°C.

[0130] If the temperature is higher or lower than this, the crystallinity of the polycrystal diamond film decreases and the graphite component increases.

[0131] In the present embodiment, the polycrystal diamond film 20 having a film thickness of 100 μm is formed in such a manner that the cooling conditions of the stage and the plasma power are adjusted so that the substrate temperature becomes 850°C.

[0132] As for the surface of the polycrystal diamond film 20, a concave-convex shape having a difference of 10 μm is formed in correspondence with the crystal grains. Therefore, the surface is planarized by mechanical polishing until the difference becomes 0.5 μm.

[0133] In the present embodiment, the polycrystal diamond film 20 is formed by a CVD method, but the method of forming the polycrystal diamond film is not limited thereto.

[0134] For example, a method of bonding another prepared polycrystal diamond substrate can be used. In the case of bonding the polycrystal diamond substrate, the surface of the GaN film 13 does not have to be protected, and therefore the protective layer 19 is not required. Therefore, it is advantageous from the viewpoint of heat transfer.

[0135] On the other hand, if an adhesive layer is required to be inserted instead of the protective layer 19 in order to increase the bonding force of the polycrystal diamond substrate, it becomes an obstacle to heat conduction, and therefore attention is required.

[0136] Next, as Figure 12As shown in the example, by removing the support substrate 17 and the support substrate adhesive layer 16, a diamond-based GaN substrate 2 is obtained. Figure 5 Step ST11 in the middle.

[0137] In this embodiment, the removal of the support substrate 17 and the support substrate adhesive layer 16 is carried out by dissolving and removing them by immersing them in a mixture of hydrofluoric acid and nitric acid.

[0138] In this case, the polycrystalline diamond film 20 is formed not only on the lower surface of the protective layer 19, which serves as the film-forming surface, but also on the side and upper surfaces of the GaN transfer substrate 18 (i.e., the upper surface of the support substrate 17). Therefore, after being dissolved and removed by hydrofluoric acid and nitric acid, the polycrystalline diamond film also remains on the side and upper surfaces of the GaN transfer substrate 18.

[0139] To prevent this, before performing the above-mentioned dissolution and removal, the following step can be added: that is, the step of removing excess polycrystalline diamond film by trimming (i.e., cutting off the end) the substrate end using a laser processing machine or the like.

[0140] The removal of the support substrate 17 and the support substrate adhesive layer 16 is not limited to removal by dissolution with a mixture of hydrofluoric acid and nitric acid. For example, it can be removed by mechanical polishing or by dry etching such as RIE.

[0141] However, when mechanical grinding is used to remove the material, even if the substrate end is trimmed with a laser as described above, sometimes a polycrystalline diamond film remains on the upper surface of the support substrate 17, which can become an obstacle to grinding. Therefore, care should be taken to ensure that the polycrystalline diamond film is removed.

[0142] Furthermore, in any case of mechanical grinding and dry etching, care must be taken not to damage the exposed AlGaN film 14 when the removal is completed.

[0143] In addition, Figure 12 The process of forming a SiN film with a thickness of about 100 nm on the upper surface of the AlGaN film 14 to protect the AlGaN film 14 is not shown.

[0144] Secondly, such as Figure 13 In the example shown, a SiO2 bonding layer 22 is formed on the lower surface of the polycrystalline diamond film 20 using a plasma CVD method with TEOS, for example, with a film thickness of 2 μm. Figure 5 Step ST12 in the middle.

[0145] Then, after heat treatment at 700°C in a nitrogen atmosphere, the surface of the SiO2 bonding layer 22 is planarized. Figure 5 Step ST13 in the middle.

[0146] In order to prevent oxidation of the diamond, the heat treatment needs to be performed in an inert furnace.

[0147] In addition, in the SiO2adhesion layer 22, since it is formed on the lower surface of the polycrystalline diamond film 20 having surface irregularities with a height difference of about 0.5 μm, the same surface irregularities are formed on the surface of the SiO2adhesion layer 22.

[0148] Therefore, in the planarization treatment after the heat treatment, for example, after performing the planarization treatment using mechanical polishing with a flat plate, the smoothing treatment using the chemical mechanical polishing (CMP) method can be combined.

[0149] Thus, the diamond-based GaN-based substrate 2 Figure 5 in which the SiO2adhesion layer 22 is formed (step ST14 in

[0150] Note that, after the smoothing treatment, the surface shape of the SiO2adhesion layer 22 was measured using an atomic force microscope (AFM), and the result was that the roughness was Ra = 0.3 nm.

[0151] <Process of preparing a support for a diamond substrate>

[0152] Next, the process of preparing a support for a diamond substrate will be described with reference to Figure 6 , Figure 14 , Figure 15 , Figure 16 , Figure 17 , Figure 18 , Figure 19 and Figure 20 .

[0153] Among them, Figure 14 , Figure 15 , Figure 17 , Figure 18 , Figure 19 and Figure 20 are cross-sectional views showing examples of the state during the process of preparing a support for a diamond substrate.

[0154] First, as shown in the example in Figure 14 , a diamond wafer is prepared (step ST21 in Figure 6 ).

[0155] Specifically, a polycrystalline diamond wafer 23 having a diameter of 2 inches and a plate thickness of 300 μm is prepared.

[0156] As for the polycrystal diamond wafer 23, mirror polishing is performed on both the upper and lower surfaces. In particular, as for the polished surface 24 which is the upper surface of the polycrystal diamond wafer 23, smoothing treatment is precisely performed by the CMP method.

[0157] Note that the surface shape of the polished surface 24 after the smoothing treatment was evaluated using an AFM, and as a result, the roughness was Ra = 0.5 nm.

[0158] Next, a protective SiN film 25 (step ST22 in Figure 6 is formed. Then, as shown in the example in Figure 15 , photolithography (step ST23 in Figure 6 ) is performed on this structure.

[0159] Specifically, on the polished surface 24 of the polycrystal diamond wafer 23, a SiN film (i.e., the protective SiN film 25) is formed in a film thickness of, for example, 400 nm using the plasma CVD method.

[0160] Then, a photoresist is formed on the upper surface of the protective SiN film 25, and a general photolithography process such as pattern exposure and development is performed. Then, a resist mask 26 is formed on the upper surface of the protective SiN film 25.

[0161] Figure 16 A plan view roughly showing an example of the resist mask pattern produced in the present embodiment.

[0162] As shown in the example in Figure 16 , the resist mask has a mask portion 28 and an opening portion 27. Also, the resist mask pattern has a stripe pattern orthogonal to an orientation flat 32 of the polycrystal diamond wafer 23, and a circumferential pattern along the outer edge portion 10 of the polycrystal diamond wafer 23.

[0163] In Figure 16 , the relative ratios and the number of stripes of each dimension are shown differently from actual ones for the sake of explanation, and in the present embodiment, the opening portion width 29 of the resist mask pattern is made 10 μm, the mask portion width 30 is made 50 μm, and the outer edge portion mask portion width 31 is made 200 μm.

[0164] Next, etching of the protective SiN film 25 is performed via the resist mask 26 (step ST24 in Figure 6 ).

[0165] In the present embodiment, the above-described etching is performed using the RIE method using a mixed gas of CF4and O2. Then, by removing the resist mask 26, a structure as shown in Figure 17The example shown in the figure has a SiN mask opening 33, and a pattern of a protective SiN film 25 forming the SiN mask portion 34 can be obtained on the upper surface of the polycrystalline diamond wafer 23.

[0166] In etching using the RIE method, not only the film thickness direction but also the pattern width variation caused by side etching needs to be considered. Under the conditions used in this embodiment, the width of the SiN mask opening 33 and the width of the SiN mask 34 are the same as the width of the resist mask pattern, which are 10 μm and 50 μm, respectively.

[0167] Furthermore, during the etching process using the RIE method, the upper surface of the polycrystalline diamond wafer 23 exposed at the bottom of the SiN mask opening 33 is also slightly damaged. However, this area is the part that will be etched in subsequent processes, so the damage here is not a problem.

[0168] It should be noted that, in the above, the etching of the protective SiN film 25 is performed using the RIE method, but it is not limited to this method. For example, wet etching can be used.

[0169] Secondly, such as Figure 18 As shown in the example, the upper surface of the polycrystalline diamond wafer 23 is etched using a protective SiN film 25 as a mask to obtain the supporting diamond substrate 3. Figure 6 (Step ST25 in the previous section). In this embodiment, the RIE method using O2 is applied.

[0170] Through the etching process described above, a processing recess 35 is formed at the lower part of the SiN mask opening 33. The processing recess 35 is located inside the outer edge 10 of the diamond substrate 3 when viewed from above. The processing depth 36 of the processing recess 35 is, for example, 10 μm.

[0171] In this embodiment, in order to maintain a large etching rate ratio between the protective SiN film 25 and the polycrystalline diamond wafer 23 (i.e., to maintain a large difference in etching rates), the process is carried out under conditions where the gas pressure ratio is high to suppress ion impact components on the etched surface.

[0172] Therefore, in the polycrystalline diamond wafer 23 located below the protective SiN film 25, the lateral etching is performed to about 5 μm, and the processing width 37 of the processing recess 35 is wider than the width of the SiN mask opening 33 in more than 90% of the region in the longitudinal direction of the recess 35, for example, becoming 20 μm.

[0173] As a result, the width of the area where etching is not performed, that is, the surface (referred to as a terrace portion 38) of the diamond polishing surface at the initial stage that is maintained, the terrace portion width 39, for example, becomes 40 μm. That is, in terms of the interval between the adjoining recessed portions 35, in 90% or more of the area in the longitudinal direction of the recessed portions 35, it is 0.1 μm or more and 100 μm or less. In addition, since the thickness of the outer edge portion 10 becomes 200 μm, the thickness of the outer edge portion 10 becomes twice or more the processing width 37 of the processing recessed portion. Figure 4 and Figure 16 Since the thickness of the outer edge portion 10 becomes 200 μm, the thickness of the outer edge portion 10 becomes twice or more the processing width 37 of the processing recessed portion.

[0174] Note that, in the present embodiment, the RIE method using O2is applied to etching of the polycrystal diamond wafer 23, but the etching method is not limited thereto.

[0175] Next, as shown in the example of Figure 19 , the structure shown in Figure 18 is immersed in an aqueous hydrofluoric acid solution to remove the protective SiN film 25 (step ST26 in Figure 6 ).

[0176] Here, in terms of the surface shape of the terrace portion 38, since it is the same as the polishing surface 24 of the polycrystal diamond wafer 23 obtained in step ST21 in Figure 6 , for example, the roughness Ra = 0.5 nm is maintained.

[0177] Next, as shown in the example of Figure 20 , on the upper surface of the support diamond substrate 3 on which the recessed portion processing has been performed, a SiO2adhesion layer 40 having a film thickness of 1 μm is formed by the CVD method using TEOS (step ST27 in Figure 6 ). Then, heat treatment is performed in a nitrogen atmosphere at 700°C.

[0178] Here, the surface shape of the terrace portion 41 after the heat treatment is measured, and as a result, for example, the roughness Ra = 0.5 nm is maintained.

[0179] It is also considered that depending on the film formation conditions or the heat treatment conditions of the SiO2described above, the surface roughness of the terrace portion 41 sometimes deteriorates. In this case, as shown in step ST28 in Figure 6 , the upper surface of the terrace portion 41 of the SiO2adhesion layer 40 can also be precisely polished by the CMP method.

[0180] In this way, the support diamond substrate 3 in the state where the SiO2adhesion layer 40 is formed is completed (step ST29 in Figure 6 ). Note that, in terms of the SiO2adhesion layer 40, it is formed along the inner wall of the processing recessed portion 35, and is not filled in the space 43 surrounded by the inner wall of the processed recessed portion 35.

[0181] <About the process of manufacturing the composite substrate>

[0182] Next, the process of manufacturing the composite substrate will be described with reference to Figure 7 , Figure 21 and Figure 22 .

[0183] Among them, Figure 21 and Figure 22 are cross-sectional views showing examples of the state during the process of manufacturing the composite substrate.

[0184] First, as shown in the example in Figure 21 , the SiO2 bonding layer 22 formed on the upper surface of the diamond substrate 2 and the SiO2 bonding layer 40 formed on the upper surface of the support diamond substrate 3 are treated with oxygen plasma 42 (step ST31 in Figure 7 ).

[0185] By this, the surface of the SiO2 bonding layer is activated, and further, adsorbs moisture in the atmosphere to become hydrophilic.

[0186] Note that, in the present embodiment, the oxygen plasma treatment is performed, but plasma treatment using an inactive gas such as argon or nitrogen can also be performed, and to more completely perform the hydrophilization treatment, the surface of the SiO2 bonding layer can be cleaned with pure water after the oxygen plasma treatment.

[0187] Next, as shown in the example in Figure 22 , the surfaces of the SiO2 bonding layers 40 and 22 that were hydrophilized in step ST31 are brought into contact with each other in the atmosphere to bond (step ST32 in Figure 7 ). At this time, the SiO2 bonding layer 40 and the SiO2 bonding layer 22 are bonded in the region excluding the processed recess 35.

[0188] If the SiO2 bonding layers are thus brought into contact with each other in the atmosphere, the diamond substrate 2 and the support diamond substrate 3 are fixed to each other by hydrogen bonds via the water molecules adsorbed to the surfaces.

[0189] At this time, since the contact area has an effect on the adhesive force, to increase the adhesive force, it is necessary to make the two surfaces that are brought into contact smooth. In the present embodiment, in terms of the roughness of the SiO2 bonding layers that are brought into contact, Ra = 0.3 nm and Ra = 0.5 nm, respectively, have sufficient smoothness for bonding.

[0190] In the present embodiment, both the diamond substrate 2 and the support diamond substrate 3 are transparent to visible light. Therefore, by visual observation, the contrast difference between the region appropriately bonded (i.e., the bonded region) and the region not bonded (the region in which the SiO2 bonding layers form a gap between each other, i.e., the unbonded region) can be confirmed.

[0191] According to the above-described visual observation, if the SiO2 bonding layers are brought into contact with each other in the atmosphere, the appearance in which the bonded region spontaneously expands from the portion where the two layers initially come into contact can be observed.

[0192] In terms of the contrast difference due to the presence or absence of bonding described above, sometimes it becomes difficult to distinguish due to the laminated structure including a plurality of transparent films. In addition, in terms of the contrast difference described above, even when the periodic groove structure forms the unbonded region, sometimes it becomes difficult to distinguish. However, in the present embodiment, a sufficiently distinguishable contrast difference can be observed.

[0193] Note that, in a case where there is a warp in either the diamond substrate 2 or the support diamond substrate 3, sometimes the bonded region does not spontaneously expand.

[0194] In this case, by applying pressure from the outside of the substrate using a roller or the like, the SiO2 bonding layers can also be brought into contact with each other to be bonded by force.

[0195] Generally, in a case where the substrates are bonded to each other in the atmosphere, the atmosphere is sandwiched at the bonding surface to become an isolated state, and sometimes it is closed and cannot escape. This becomes a bonding failure as an unbonded region (so-called gap).

[0196] However, in the present embodiment, since there is the space 43 periodically formed by machining the recess 35, it has the advantage that the atmosphere can escape to the space 43 and the atmosphere does not become isolated within the bonding surface.

[0197] The volume of the space 43 is sufficiently large with respect to the amount of atmosphere sandwiched, and therefore sometimes the rise in the internal atmospheric pressure also does not peel off the bonding.

[0198] Next, the two substrates to be bonded are heat-treated (step ST33 in FIG. 8) in a nitrogen atmosphere at 400°C. Figure 7

[0199] By this, the water molecules are detached from the hydrogen bonding portion of the bonding interface, and the hydrogen bonding of the bonding interface changes to siloxane bonding via an oxygen atom. Therefore, the adhesive force is greatly improved.

[0200] Note that, after the heat treatment, the atmosphere enclosed in the space 43 expands and the pressure increases, but does not peel off the bonding.

[0201] ​After the above processes, the composite substrate 1 related to this embodiment is completed. Figure 7 Step ST34 in the document will be explained. Figure 22 The composite substrate 1 shown is Figure 1 and Figure 2 The composite substrate 1 shown is the same.

[0202] To clarify, in the above, in Figure 7 Step ST32 and Figure 7 In step ST33, a substrate bonding method with hydrophilic treatment is described, but the substrate bonding method with hydrophilic treatment can also be applied to... Figure 10 The bonding method between the support substrate adhesive layer 16 and the support substrate 17.

[0203] However, since both the support substrate adhesive layer 16 and the support substrate 17 are flat substrates without recesses, poor bonding may occur due to atmospheric confinement, so care should be taken.

[0204] In addition, both the Si substrate 11 and the support substrate 17 are opaque to visible light, so near-infrared light is required when visually evaluating the bonding area.

[0205] For example, if a halogen lamp is used as the light source, and the transmitted light is captured by a CCD camera that excludes infrared cut-off filters, the presence or absence of poorly joined parts can be confirmed by the contrast difference.

[0206] <Process for forming diamond-based GaN transistors>

[0207] Secondly, regarding the process of forming diamond-based GaN transistors, refer to... Figure 8 , Figure 23 , Figure 24 , Figure 25 and Figure 26 Please provide an explanation.

[0208] in, Figure 23 , Figure 24 , Figure 25 and Figure 26 A cross-sectional view showing an example of a state during the process of forming a diamond-based GaN transistor.

[0209] First, such as Figure 23 As shown in the example, for composite substrate 1, a transistor process is applied ( Figure 8Step ST41 in the text. The contents of the transistor process are not described in detail here, but the transistor process includes, for example, ion implantation for device separation, metal thin film electrode formation, heat treatment for ohmic contact formation, surface protection SiN film formation, photolithography for pattern formation, wet etching or dry etching for pattern formation, etc.

[0210] Figure 23 The diagram schematically shows the state in which the AlGaN film 14 and GaN film 13, which serve as the upper surface of the composite substrate 1, form the electrode 44 constituting the transistor.

[0211] Secondly, such as Figure 24 As shown in the example, the supporting sapphire glass 45 is bonded to the composite substrate 1 on which the transistor is formed. Figure 8 Step ST42 in the middle.

[0212] The purpose of bonding the supporting sapphire glass 45 to the composite substrate 1 is to retain the substrate when the supporting diamond substrate 3 in the composite substrate 1 is removed.

[0213] In this embodiment, such as Figure 24 As shown in the example, a thermoplastic wax 46 is used to bond a support sapphire glass 45 having a diameter larger than that of the composite substrate 1 (e.g., 3 inches in diameter).

[0214] The reason for using sapphire glass here is to prevent it from being damaged during the subsequent immersion in hydrofluoric acid aqueous solution.

[0215] Secondly, such as Figure 25 As shown in the example, the composite substrate 1, bonded to the supporting sapphire glass 45, is immersed in a hydrofluoric acid aqueous solution 47, and the supporting diamond substrate 3 is removed. Figure 8 Step ST43 in the middle.

[0216] Both SiO2 bonding layer 22 and SiO2 bonding layer 40 are soluble in hydrofluoric acid aqueous solution 47, therefore, through this process, such as Figure 26 As shown in the example, these bonding layers are dissolved and removed.

[0217] like Figure 26 As shown in the example, the composite substrate 1 is separated into a diamond-based GaN substrate 2 in which a supporting sapphire glass 45 is mounted, and a supporting diamond substrate 3 in which the SiO2 bonding layer 40 is removed.

[0218] Then, the diamond-based GaN transistor (i.e., a structure in which the electrodes 44 and the like that constitute the transistor are formed on the upper surface of the diamond-based GaN substrate 2) is removed from the supporting sapphire glass 45 by heating the supporting sapphire glass 45 on a hot plate to soften the wax 46.

[0219] Then, the wax 46 attached to the diamond-based GaN transistor is completely removed using acetone. In this way, the diamond-based GaN transistor (i.e., the structure in which the electrodes 44 and the like that constitute the transistor are formed on the upper surface of the diamond-based GaN substrate 2) is completed (step ST44 in FIG. 4). Figure 8

[0220] <Concerning Space>

[0221] Next, the meaning of the space 43 formed using the processing recess 35 in the composite substrate 1 according to the present embodiment is described.

[0222] Figure 27 A cross-sectional view showing the structure of a composite substrate in which two substrates are bonded via a thin film, which is a comparative example that does not have a space formed by a recess.

[0223] Specifically, the Si substrate 49 located on the upper side and the Si substrate 50 located on the lower side are bonded with the SiO2 bonding layer 51 interposed therebetween. The film thickness 52 of the SiO2 bonding layer 51 is 2 μm.

[0224] Figure 27 A composite substrate in which the Si substrate 49 and the Si substrate 50 are immersed in an aqueous hydrofluoric acid solution having a concentration of 50% by weight for a certain period of time is shown. Therefore, in the composite substrate shown in FIG. 8, the SiO2 bonding layer 51 is etched from the end of the composite substrate by an etching distance 53. It is noted that the etching distance 53 is 100 μm per 1 hour. This is a value that is consistent with the etching rate of SiO2 that is generally known (1.5 μm / minute for an aqueous hydrofluoric acid solution having a concentration of 50% by weight). From this value, it is calculated that 280 hours are required for a process in which the etching distance is 25 mm in order to separate a 2-inch substrate, i.e., a substrate having a diameter of 50 mm. This length of time is not industrially efficient. Figure 27 On the other hand, in the composite substrate shown in FIG. 9, the space 43 formed by the processing recess 35 is filled with an aqueous hydrofluoric acid solution. Therefore, in the composite substrate shown in FIG. 9, the SiO2 bonding layer 51 is etched from the end of the composite substrate by an etching distance 54. It is noted that the etching distance 54 is 100 μm per 1 hour. This is a value that is consistent with the etching rate of SiO2 that is generally known (1.5 μm / minute for an aqueous hydrofluoric acid solution having a concentration of 50% by weight). From this value, it is calculated that 25 hours are required for a process in which the etching distance is 25 mm in order to separate a 2-inch substrate, i.e., a substrate having a diameter of 50 mm. This length of time is industrially efficient.

[0225] Figure 28 A cross-sectional view showing the structure of a composite substrate that contains the supporting diamond substrate 3 and the diamond-based GaN substrate 2 according to the present embodiment is shown. Figure 28 A composite substrate in which the Si substrate 49 and the Si substrate 50 are immersed in an aqueous hydrofluoric acid solution having a concentration of 50% by weight for a certain period of time is shown.

[0226] If the space 43 formed by the processing recess 35 is filled with an aqueous hydrofluoric acid solution, the etching rate of the etching distance 54 becomes the same as the etching rate of the SiO2 bonding layer 51. Therefore, the etching rate of the etching distance 54 is 100 μm per 1 hour. This is a value that is consistent with the etching rate of SiO2 that is generally known (1.5 μm / minute for an aqueous hydrofluoric acid solution having a concentration of 50% by weight). From this value, it is calculated that 25 hours are required for a process in which the etching distance is 25 mm in order to separate a 2-inch substrate, i.e., a substrate having a diameter of 50 mm. This length of time is industrially efficient. Figure 27 ​​The travel speed of the etching distance 53 of the comparative example shown in FIG. 6 is equal.

[0227] In the present embodiment, in order to remove the supporting diamond substrate 3 from the diamond-based GaN substrate 2, it is sufficient to etch the SiO2 bonding layer 40 and the SiO2 bonding layer 22 corresponding to the balcony width 39. In the present embodiment, the balcony width 39 is 40 μm, and therefore it is sufficient to etch half of the width, that is, 20 μm. By simple calculation, the etching is completed in 13 minutes, and the supporting diamond substrate 3 can be removed from the diamond-based GaN substrate 2.

[0228] Here, in order to fill the space 43 formed by the machining recess 35 with the aqueous hydrofluoric acid solution, the space 43 is required to function as a flow path extending up to the end of the supporting diamond substrate 3.

[0229] Note that, in the present embodiment, the width of the space 43 is made 20 μm with respect to the balcony width of 40 μm, but the respective dimensions are not limited thereto.

[0230] That is, if the balcony width is made narrow, the time required for removal of the supporting diamond substrate 3 is shortened, but if the balcony width is made excessively narrow, the balcony becomes easy to collapse and break.

[0231] On the other hand, if the balcony width is made wide, the bonding force between the diamond-based GaN substrate 2 and the supporting diamond substrate 3 increases, but even if the balcony width becomes wider than 100 μm, the bonding force is not greatly improved, and the time required for removal of the supporting diamond substrate 3 is merely prolonged. In view of the above, it is preferable that the balcony width be 0.1 μm or more and 100 μm or less.

[0232] In the space 43 formed by the machining recess 35, the aqueous hydrofluoric acid solution as a chemical solution is required to be immersed, and it is confirmed that if the width of the space 43 is narrow, the aqueous hydrofluoric acid solution does not sufficiently immerse. In the present embodiment, if the width of the space 43 is narrower than 1 μm, the aqueous hydrofluoric acid solution does not immerse.

[0233] On the other hand, if the width of the space 43 is excessively expanded, the diamond-based GaN substrate 2 and the supporting diamond substrate 3 constituting the composite substrate are respectively bent, and the following problem occurs: in the space 43, in the middle, the diamond-based GaN substrate 2 and the supporting diamond substrate 3 are pasted. In view of the above, it is preferable that the width of the space 43 be 1 μm or more and 1 mm or less.

[0234] As described above, if space 43 functions as a flow path and hydrofluoric acid aqueous solution is filled in space 43, the removal of the supporting diamond substrate 3 should become easy. However, in this embodiment, the immersion time required for the removal of the supporting diamond substrate 3 is sometimes significantly longer than expected.

[0235] It can be inferred that, for the relatively narrow space 43, which functions as a flow path, the wettability of the wall surface of the processed recess 35, and the alternation between the air first filled in the space 43 and the hydrofluoric acid aqueous solution subsequently filled in the space 43, become issues.

[0236] In this embodiment, in order to improve the wettability of the wall surface of the processed recess 35 in the above-mentioned problem, 0.1% by weight of a nonionic surfactant is added to the hydrofluoric acid aqueous solution.

[0237] Furthermore, to facilitate the filling of space 43 with hydrofluoric acid aqueous solution, an impregnation container is provided under reduced pressure. Under reduced pressure, the air filling space 43, which functions as a flow path, is released to the outside of space 43. Therefore, it is easy to fill space 43 with hydrofluoric acid aqueous solution.

[0238] It should be noted that in this embodiment, the addition of a surfactant and impregnation under reduced pressure can be performed, or neither can be performed. Furthermore, the types of surfactants added are not limited to those described above.

[0239] Furthermore, it was found that even with the addition of surfactants as described above, the immersion time required to remove the supporting diamond substrate 3 was sometimes longer than expected. This was particularly noticeable when the immersion container was not placed under reduced pressure.

[0240] Figure 29 This diagram illustrates the state of the composite substrate 56 immersed in an aqueous hydrofluoric acid solution 47. Figure 29 In order to aid understanding, the flow path is shown in the substrate surface, but since the flow path is actually sandwiched between substrates, it cannot be confirmed from the outside of the composite substrate 56.

[0241] exist Figure 29 In the process, the composite substrate 56 is impregnated with the flow path extending in a direction orthogonal to the liquid surface of the hydrofluoric acid aqueous solution 47.

[0242] on the other hand, Figure 30 This diagram illustrates the state of the composite substrate 57 immersed in an aqueous hydrofluoric acid solution 47. Even when Figure 30 In order to aid understanding, the flow path is shown in the substrate surface, but since the flow path is actually sandwiched between substrates, it cannot be confirmed from the outside of the composite substrate 57.

[0243] exist Figure 30 In the process, the composite substrate 57 is impregnated while the flow path extends parallel to the surface of the hydrofluoric acid aqueous solution 47.

[0244] In the inventor's experiments, it was learned that: Figure 30 As shown, when the composite substrate 57 is impregnated with the flow path extending parallel to the surface of the hydrofluoric acid aqueous solution 47, the removal of the supporting diamond substrate 3 is significantly delayed; that is, the impregnation time required to remove the supporting diamond substrate 3 is longer than expected.

[0245] The reason for the difference in the time required to remove the supporting diamond substrate 3 due to the different orientation of the composite substrate is that the intrusion of the liquid into the flow path comes from the density difference between the liquid and the air.

[0246] That is, compared to liquid medicine, air has a lower density, therefore... Figure 29 In the middle, air is drawn from the upper part of the flow path into the hydrofluoric acid aqueous solution 47, and conversely, the hydrofluoric acid aqueous solution 47 is drawn from the lower part of the flow path (i.e., Figure 29 The oriented planar portion of the composite substrate 56 is immersed in the composite substrate 56. As a result, the support diamond substrate 3 is removed from the bottom of the composite substrate 56.

[0247] In the inventor's experiments, if the impregnation is interrupted midway through the removal of the supporting diamond substrate 3 and the composite substrate 56 is observed with infrared transmitted light, it can be confirmed that the bonding separation occurs at the lower part of the composite substrate 56.

[0248] In contrast, Figure 30 Even though there is a density difference between the hydrofluoric acid aqueous solution 47 and the air, the lack of a height difference (depth difference from the liquid surface of the hydrofluoric acid aqueous solution 47) at both ends of the flow path makes it difficult for air to escape and for the hydrofluoric acid aqueous solution 47 to penetrate. Therefore, the time required to remove the supporting diamond substrate 3 is extended.

[0249] In the inventors’ experiments, if the impregnation was interrupted midway through the removal of the supporting diamond substrate 3 and the composite substrate 57 was observed with infrared transmitted light, it could be confirmed that the separation of the bonding was almost non-existent.

[0250] In view of the above phenomena, in order to remove the supporting diamond substrate 3 in a short time, such as Figure 29 As shown, it is important to impregnate the composite substrate with the flow path extending in a direction orthogonal to the surface of the hydrofluoric acid aqueous solution 47.

[0251] Alternatively, it is known that by configuring the flow path in a shape that intersects the flow paths in two directions (i.e., a lattice pattern) instead of extending it in one direction, the time required to remove the supporting diamond substrate 3 can be shortened even when the flow path is extended in any direction during immersion.

[0252] Figure 33 This figure shows another example of a semiconductor substrate and a support substrate constituting a composite substrate in relation to this embodiment.

[0253] like Figure 33 As shown in the example, a processing recess 35A and a processing recess 35B extending in a direction intersecting the processing recess 35A are formed at the bonding surface between the diamond substrate 3A and the diamond-based GaN substrate 2. The angle between the extending direction of the processing recess 35A and the extending direction of the processing recess 35B is, for example, 10° or more and 90° or less.

[0254] The two machined recesses form a shape (grid-like) that allows the flow paths in two directions to intersect. Thus, even when the flow paths extend in any direction, a height difference is generated at both ends of either flow path, allowing the hydrofluoric acid aqueous solution 47 to be immersed from the bottom of the flow path.

[0255] To explain, in addition to the shape that makes the flow paths intersect, for example, radial flow paths can also be formed, with multiple point-like terraces joining together, and the rest being joints that function as flow paths.

[0256] <About the Terrace Section>

[0257] In this embodiment, due to the use of Figure 16 The example shown uses a resist mask pattern to form the processing recess 35, thus forming a terrace portion around the entire periphery of the outer edge 10 supporting the diamond substrate 3. The space 43 is then enclosed by the terrace portion covering the entire periphery of the outer edge 10. The significance of the terrace portion covering the entire periphery will be explained below.

[0258] like Figure 8 As shown in step ST43, the removal of the supporting diamond substrate 3 is carried out using an aqueous hydrofluoric acid solution 47, but prior to this step, in Figure 8 Hydrofluoric acid aqueous solution is also used in step ST41 of the transistor process.

[0259] At this point, in the case of a structure that allows the supporting diamond substrate 3 to be removed in a short time, Figure 8 In step ST41, the supporting diamond substrate 3 was unintentionally removed, and it may no longer function as a supporting substrate.

[0260] In order to avoid such a situation, in the present embodiment, a deck portion is formed so as to extend over the entire circumference of the outer edge portion 10 in order to adjust the timing of the start of removal. Further, by this deck portion, the timing of immersion of the aqueous hydrofluoric acid solution into the space 43 is delayed.

[0261] In the present embodiment, the width in the radial direction of the deck portion formed in the outer edge portion 10 of the substrate is set to 200 μm. In this way, the immersion of the aqueous hydrofluoric acid solution into the space 43 can be delayed by about 120 minutes.

[0262] Note that, in the present embodiment, the width in the radial direction of the deck portion is 200 μm, but the width in the radial direction of the deck portion is not limited to this value and can be changed as needed.

[0263] In addition, in the present embodiment, the deck portion is formed along the (outermost) edge portion 10 of the substrate, but can be formed at a position that is moved inward by a certain width from the outermost edge portion of the substrate.

[0264] Generally, with respect to the end portion of the substrate, the pattern shape is sometimes disturbed due to the influence of the instability of the film thickness of the resist film used at the time of photolithography and the like. Therefore, in order to avoid this, a deck portion can be formed so as to extend over the entire circumference from the end portion of the substrate, for example, inward by about 1 mm.

[0265] <Second Embodiment>

[0266] A manufacturing method of a composite substrate and a composite substrate relating to the present embodiment will be described. Note that, in the following description, the same reference numerals are used to illustrate the same components as those described in the above-described embodiments, and detailed description thereof will be appropriately omitted.

[0267] <Regarding the Configuration of the Composite Substrate>

[0268] The composite substrate relating to the present embodiment is the same shape as the composite substrate described in the first embodiment, i.e., the composite substrate described in Figure 1 , Figure 2 , and Figure 22 , and the flow path is in a vacuum.

[0269] <Regarding the Manufacturing Method of the Composite Substrate>

[0270] Next, a manufacturing method of a composite substrate relating to the present embodiment will be described.

[0271] In the present embodiment, among the four processes, i.e., the process of preparing the diamond substrate, the process of preparing the support substrate, the process of manufacturing the composite substrate, and the process of forming the diamond substrate GaN transistor, only the process of manufacturing the composite substrate is different from the process shown in the first embodiment. Therefore, the difference will be described in detail.

[0272] <Process of Manufacturing the Composite Substrate>

[0273] The process of manufacturing the composite substrate will be described with reference to Figure 31 and Figure 32 .

[0274] Among them, Figure 31 is a flowchart showing the process of manufacturing the composite substrate. In addition, Figure 32 is a cross-sectional view showing an example of the state halfway through the process of manufacturing the composite substrate.

[0275] First, the diamond substrate GaN substrate 2 and the support substrate 3 are disposed so as to be separated from each other and so that the bonding surfaces thereof face each other in the bonding device chamber 58 maintained at a high vacuum (step ST51 in Figure 31 ).

[0276] Then, the bonding surfaces of the two substrates, i.e., the SiO2 bonding layer 22 and the SiO2 bonding layer 40, are subjected to activation treatment with the ion beam 60 emitted from the ion gun 59, respectively (step ST52 in Figure 31 ).

[0277] In the present embodiment, in the surface activation treatment, an argon ion beam is used. The purpose of the argon ion beam treatment here is to remove attachments or contaminants, etc. at the surface of the bonding surface, and to generate unbound ends (dangling bonds) at the surface of the bonding surface.

[0278] After the irradiation of the argon ion beam is completed, the two substrates are brought into contact while being maintained at a high vacuum in the bonding device chamber 58, and further subjected to pressurization. Thus, the two substrates are bonded (step ST53 in Figure 31 ).

[0279] The above-described surface activation treatment and bonding treatment are continuously performed under a high vacuum. Therefore, the unbound ends formed at the surface of the bonding surface are combined with the unbound ends in the bonding surface of the opposite side in contact in a state where they are not capped. Thus, a firm bond is formed between the two substrates.

[0280] According to this embodiment, the space 43 formed after joining the mating surfaces is made into a high vacuum of the same degree as that inside the joining device chamber 58. Therefore, when removing the supporting diamond substrate 3, the immersion solution can be rapidly immersed into the space 43. That is, depressurization within the space 43 can be achieved without placing the immersion solution itself under a depressurized environment. As a result, it is possible to remove the supporting diamond substrate 3 in a short time.

[0281] It should be noted that in this embodiment, the space 43 is made into a high vacuum, but it does not necessarily have to be a high vacuum. As long as it is negative pressure compared with atmospheric pressure, it is sufficient. Even if the pressure inside the space 43 is, for example, about 0.05 MPa, the effect can be fully achieved.

[0282] To clarify, from a bonding perspective, it is not preferable to perform the bonding method with hydrophilic treatment used in the first embodiment under reduced pressure in order to create a vacuum within space 43. This is because, although in Figure 21 The diagram shows that hydrogen bonds between water molecules at the interface surface are needed for fixation, but under reduced pressure, most of the water at the interface surface detaches, thus insufficient hydrogen bonding is achieved. Therefore, to create a vacuum within space 43, it is preferable to use... Figure 32 The method shown is used to perform the joining.

[0283] Furthermore, such as Figure 32 As shown in the example, even in the case of bonding in a vacuum, the risk of bonding peeling caused by the expansion of gas sealed in the gap (unbonded portion) or space 43 at the bonding interface can be suppressed in high-temperature or vacuum processes in transistor processes that are subsequent processes.

[0284] Thus, according to this embodiment, it is possible to fabricate a composite substrate while simultaneously reducing the time required to remove the supporting diamond substrate 3. Figure 31 Step ST54 in the middle.

[0285] <Regarding the effects of adopting the above-described implementation methods>

[0286] Next, examples of the effects produced by adopting the embodiments described above are shown. It should be noted that in the following description, the effects are described based on the specific configurations of the examples shown in the embodiments described above, and other specific configurations shown in the examples of this application specification may be substituted within the scope of producing the same effects.

[0287] Furthermore, this substitution can be performed across multiple implementations. That is, it can be a case where various components of the examples shown in different implementations are combined to produce the same effect.

[0288] According to the above-described embodiment, in the manufacturing method of the composite substrate, the first bonding material is formed on the first surface of the first substrate. Here, the first substrate corresponds to, for example, the support diamond substrate 3. Also, the first bonding material corresponds to, for example, the SiO2 bonding layer 40. Further, at least one recessed portion is formed on the first surface inside the outer edge portion 10 of the support diamond substrate 3 in plan view. Here, the recessed portion corresponds to, for example, the processed recessed portion 35. Further, the SiO2 bonding layer 40 is formed along the inner wall of the processed recessed portion 35. Also, the SiO2 bonding layer 40 is not filled in the space 43 surrounded by the inner wall of the processed recessed portion 35. Further, the second bonding material is formed on the second surface of the second substrate. Here, the second substrate corresponds to, for example, the diamond GaN substrate 2. Also, the second bonding material corresponds to, for example, the SiO2 bonding layer 22. Further, the SiO2 bonding layer 40 is bonded to the SiO2 bonding layer 22 in a region excluding the processed recessed portion 35.

[0289] According to such a configuration, by the dissolving solution being immersed in the space 43 surrounded by the inner wall of the processed recessed portion 35, the SiO2 bonding layer 40 and the SiO2 bonding layer 22 are dissolved in advance, and thus the time required for removal of the support diamond substrate 3 can be shortened. Also, by forming the processed recessed portion 35 inside the outer edge portion 10, the SiO2 bonding layer 40 and the SiO2 bonding layer 22 can be prevented from being dissolved and the composite substrate from being peeled off in an unintended process.

[0290] Note that even in the case where at least one of the other configurations exemplified in the present specification is appropriately added to the above-described configuration, that is, the case where the other configuration exemplified in the present specification is appropriately added to the above-described configuration without being mentioned as being appropriately added, the same effects can be obtained.

[0291] Also, the order of the respective processes can be changed without particular limitation.

[0292] Also, according to the above-described embodiment, the bonding of the SiO2 bonding layer 40 to the SiO2 bonding layer 22 is performed after the SiO2 bonding layer 40 and the SiO2 bonding layer 22 are activated using an oxygen plasma. According to such a configuration, the activated SiO2 bonding layer 40 and the activated SiO2 bonding layer 22 are bonded, and thus the composite substrate 1 can be formed.

[0293] Also, according to the above-described embodiment, the bonding of the SiO2 bonding layer 40 to the SiO2 bonding layer 22 is performed after the SiO2 bonding layer 40 and the SiO2 bonding layer 22 are activated using an ion beam in a vacuum environment. According to such a configuration, the space 43 formed in the composite substrate 1 after bonding is in a vacuum state, and thus the dissolving solution becomes easy to be immersed in the space 43.

[0294] In addition, according to the above-described embodiment, the composite substrate 1 in the state where the SiO2 bonding layer 40 is bonded to the SiO2 bonding layer 22 is immersed in a dissolving solution. According to such a configuration, the SiO2 bonding layer 22 and the SiO2 bonding layer 40 are dissolved by the dissolving solution, whereby the supporting diamond substrate 3 can be removed from the diamond-based GaN substrate 2.

[0295] In addition, according to the above-described embodiment, the dissolving solution is an aqueous hydrofluoric acid solution 47. According to such a configuration, the SiO2 bonding layer 22 and the SiO2 bonding layer 40 are dissolved by the aqueous hydrofluoric acid solution 47, whereby the supporting diamond substrate 3 can be removed from the diamond-based GaN substrate 2.

[0296] In addition, according to the above-described embodiment, the immersion of the composite substrate 1 in the dissolving solution is performed in a reduced-pressure environment. According to such a configuration, the air filled in the space 43 functioning as a flow path is easily released to the outside of the space 43, and thus the aqueous hydrofluoric acid solution 47 is easily filled in the space 43.

[0297] In addition, according to the above-described embodiment, the processing recess 35 is formed along a first direction in the first surface. Also, in the immersion of the composite substrate 1 in the dissolving solution, the composite substrate 1 is disposed so as to intersect the first direction with the liquid surface of the dissolving solution. According to such a configuration, the time required for the dissolving solution to reach the central portion of the supporting diamond substrate 3 from the dissolving solution immersed in the outer edge portion 10 of the supporting diamond substrate 3 is shortened, and thus the time required for the removal of the supporting diamond substrate 3 can be shortened.

[0298] According to the above-described embodiment, the composite substrate includes a supporting diamond substrate 3 having a first surface, a diamond-based GaN substrate 2 having a second surface, an SiO2 bonding layer 40 formed on the first surface, and an SiO2 bonding layer 22 formed on the second surface and bonded to the SiO2 bonding layer 40. Also, on the first surface, at least one processing recess 35 is formed inside the outer edge portion 10 of the supporting diamond substrate 3 in plan view. In addition, the SiO2 bonding layer 40 is formed along the inner wall of the processing recess 35, and a space 43 surrounded by the inner wall of the processing recess 35 is not filled. In addition, the SiO2 bonding layer 40 is bonded to the SiO2 bonding layer 22 in a region excluding the processing recess 35.

[0299] According to such a configuration, by the dissolving solution being immersed into the space 43 surrounded by the inner wall of the processed recess 35, the dissolving of the SiO2 bonding layer 40 and the SiO2 bonding layer 22 is advanced, and thus the time required for removal of the supporting diamond substrate 3 can be shortened. In addition, by forming the processed recess 35 inside the outer edge portion 10, the dissolving of the SiO2 bonding layer 40 and the SiO2 bonding layer 22 and the peeling of the composite substrate in an unintended process can be suppressed.

[0300] Note that even in the case where at least one of the other configurations exemplified in the present specification is appropriately added to the configuration described above, that is, the case where the other configurations exemplified in the present specification that are not mentioned as being appropriately added to the configuration described above are added, the same effects can be obtained.

[0301] In addition, according to the embodiment described above, the space 43 is a space extending in at least two directions along the first surface. Furthermore, the angle between the two directions in which the space 43 extends is 10° or more and 90° or less. According to such a configuration, the aqueous hydrofluoric acid solution 47 becomes easy to be immersed into the space 43 regardless of the direction in which the composite substrate is immersed in the aqueous hydrofluoric acid solution 47, and thus the time required for removal of the supporting diamond substrate 3 is not prolonged.

[0302] In addition, according to the embodiment described above, the width of the outer edge portion 10 is twice or more the interval between the processed recesses 35. According to such a configuration, the supporting diamond substrate 3 can be effectively prevented from being peeled off unintentionally in a process using a chemical solution other than the removal process of the supporting diamond substrate 3.

[0303] In addition, according to the embodiment described above, the width of the processed recess 35 is 1 μm or more and 1 mm or less. According to such a configuration, the dissolving solution can be effectively caused to be immersed into the space 43 inside the processed recess 35, and thus the time required for removal of the supporting diamond substrate 3 can be shortened.

[0304] In addition, according to the embodiment described above, a plurality of processed recesses 35 are provided. Furthermore, the interval between the adjacent processed recesses 35 is 0.1 μm or more and 100 μm or less. According to such a configuration, since the interval between the processed recesses 35 is 100 μm or less, the gap portion, that is, the portion of the diamond substrate GaN substrate 2 that is not supported, can be prevented from being deflected and changing the parallelism with the supporting diamond substrate 3. Thus, the parallelism between the bonded supporting diamond substrate 3 and the diamond substrate GaN substrate 2 can be maintained.

[0305] In addition, according to the above-described embodiments, the supporting diamond substrate 3 is a diamond substrate. According to such a configuration, the diamond-based GaN substrate 2 is applied to the transistor process without being bent by supporting the diamond-based GaN substrate 2 by the supporting diamond substrate 3.

[0306] <Modifications in the above-described embodiments>

[0307] In the above-described embodiments, the material, the material, the size, the shape, the relative arrangement relationship, or the conditions of implementation of each constituent element are sometimes described, but these are one example in all aspects and are not limited to the content described in the present specification.

[0308] Therefore, it is considered that an infinite number of modifications and equivalents that do not show examples are within the scope of the technology disclosed in the present specification. For example, cases in which at least one constituent element is modified, cases in which something is added or omitted, and cases in which at least one constituent element in at least one embodiment is extracted and combined with a constituent element in another embodiment are included.

[0309] In addition, as long as there is no contradiction, the constituent element described in the above-described embodiments as including "one" can include "more than one".

[0310] Further, each constituent element in the above-described embodiments is a conceptual unit, and cases in which one constituent element is composed of a plurality of structures, cases in which one constituent element corresponds to a part of a certain structure, and cases in which a plurality of constituent elements are included in one structure are included in the scope of the technology disclosed in the present specification.

[0311] In addition, for each constituent element in the above-described embodiments, a structure having another structure or shape is included as long as the same function is exerted.

[0312] In addition, the description in the present specification is referred to for all purposes associated with the present technology, and it is not considered that all are prior art.

[0313] In addition, in the above-described embodiments, in the case where the name of the material or the like is not specifically specified, as long as there is no contradiction, it is considered that, for example, an alloy or the like in which another additive is added to the material is included.

[0314] Explanation of Reference Signs

[0315] 1, 56, 57 composite substrate, 2 diamond substrate GaN substrate, 3, 3A support diamond substrate, 10 outer edge portion, 11, 49, 50 Si substrate, 12 buffer layer, 13 GaN film, 14 AlGaN film, 15 Si-based GaN substrate, 16 support substrate adhesive layer, 17 support substrate, 18 GaN transfer substrate, 19 protective layer, 20 polycrystalline diamond film, 21 polycrystalline diamond film forming substrate, 22, 40, 40A, 51 SiO2 bonding layer, 23 polycrystalline diamond wafer, 24 polishing surface, 25 protective SiN film, 26 resist mask, 27 opening portion, 28 mask portion, 29 opening portion width, 30 mask portion width, 31 outer edge portion mask portion width, 33 SiN mask opening portion, 34 SiN mask portion, 35 processing recessed portion, 36 processing depth, 37 processing width, 38, 41 balcony portion, 39 balcony portion width, 42 oxygen plasma, 43, 43A space, 44 electrode, 45 support sapphire glass, 46 wax, 47 aqueous hydrofluoric acid solution, 52 film thickness, 53, 54 etching distance, 58 bonding device chamber, 59 ion gun, 60 ion beam.

Claims

1. A method for manufacturing a composite substrate, wherein, A first bonding material is formed on the first surface of the first substrate. At least one recess is formed on the first surface, located inside the outer edge of the first substrate in a top view. The first bonding material is formed along the inner wall of the recess and is not filled in the space surrounded by the inner wall of the recess. A second bonding material is formed on the second surface of the second substrate. While maintaining a vacuum in the space surrounded by the inner wall of the recess, the first bonding material and the second bonding material are bonded in the region excluding the recess. Application of transistor processes, The composite substrate in which the first bonding material and the second bonding material are bonded can be separated by immersion in a solution used to dissolve the first bonding material and the second bonding material. The first substrate is a supporting diamond substrate. The second substrate is a diamond-based GaN substrate.

2. The method for manufacturing the composite substrate according to claim 1, wherein, The bonding of the first bonding material and the second bonding material is carried out after the first bonding material and the second bonding material are activated using oxygen plasma.

3. The method for manufacturing the composite substrate according to claim 1, wherein, Regarding the bonding of the first bonding material and the second bonding material, the bonding is carried out after activating the first bonding material and the second bonding material using an ion beam in a vacuum environment.

4. The method for manufacturing a composite substrate according to any one of claims 1 to 3, wherein, The solution is an aqueous solution of hydrofluoric acid.

5. The method for manufacturing a composite substrate according to any one of claims 1 to 3, wherein, The immersion of the composite substrate in the solution is carried out under reduced pressure.

6. The method for manufacturing a composite substrate according to any one of claims 1 to 3, wherein, The recess is formed along a first direction in the first surface. Regarding the immersion of the composite substrate in the solution, it is performed by arranging the composite substrate in a manner that intersects with the surface of the solution in the first direction.

7. A composite substrate for manufacturing GaN transistors using diamond as a substrate, comprising: First substrate having a first surface Second substrate having a second surface A soluble first bonding material formed on the first surface, and A soluble second bonding material formed on the second surface and bonded to the first bonding material. At least one recess is formed on the first surface, located inside the outer edge of the first substrate when viewed from above. The first bonding material is formed along the inner wall of the recess and is not filled in the space surrounded by the inner wall of the recess. The first bonding material is bonded to the second bonding material in the region excluding the recess. The space surrounded by the inner wall of the recess is a vacuum. The first substrate and the second substrate can be separated by dissolving the first bonding material and the second bonding material. The first substrate is a supporting diamond substrate. The second substrate is a diamond-based GaN substrate.

8. The composite substrate according to claim 7, wherein, The space is a space that extends along the first surface in at least two directions. The angle between the two directions of the spatial extension is greater than 10° and less than 90°.

9. The composite substrate according to claim 7 or 8, wherein, The width of the outer edge is more than twice the spacing between the recesses.

10. The composite substrate according to claim 7 or 8, wherein, The width of the recess is more than 1 μm and less than 1 mm.

11. The composite substrate according to claim 7 or 8, wherein, It has multiple recesses, The spacing between adjacent recesses is 0.1 μm or more and 100 μm or less.

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