Group iii nitride substrate and method for manufacturing same

Abstract

The present invention provides a group III nitride substrate with reduced susceptibility to scratches. A group III nitride substrate according to an embodiment of the present invention is substantially disk-shaped, and has a first main surface and a second main surface facing each other, and side surfaces that connect the first main surface and the second main surface. The side surfaces include a first flat surface, and the surface roughness Ra of the first flat surface is 0.07 μm or less.

Description

Group III element nitride substrate and manufacturing method thereof

[0001] The present invention relates to a Group III element nitride substrate and a method for manufacturing the same.

[0002] 2. Description of the Related Art Group III element nitride substrates are used as substrates for various devices such as light emitting diodes, semiconductor lasers, and power ICs.

[0003] The Group III element nitride substrate can be obtained by epitaxially growing a thick Group III element nitride crystal on a substrate such as a sapphire substrate as a base substrate, as described in Patent Document 1. Typically, the grown crystal is subjected to processing such as grinding and polishing to obtain a Group III element nitride substrate having a desired shape.

[0004] Japanese Patent Application Laid-Open No. 2005-136167

[0005] However, scratches are observed on the obtained Group III element nitride substrate, which may result in, for example, a decrease in yield.

[0006] In view of the above, a main object of the present invention is to provide a Group III element nitride substrate in which the occurrence of scratches is suppressed.

[0007] 1. A Group III element nitride substrate according to an embodiment of the present invention is substantially disc-shaped and has a first main surface and a second main surface facing each other, and a side surface connecting the first main surface and the second main surface, the side surface including a first flat surface, and the surface roughness Ra of the first flat surface being 0.07 μm or less. 2. In the Group III element nitride substrate described in 1 above, the first flat surface may be an orientation flat. 3. In the Group III element nitride substrate described in 1 or 2 above, the first flat surface may be an m-plane or an a-plane of a Group III element nitride crystal. 4. In the Group III element nitride substrate described in any one of 1 to 3 above, the first flat surface may be an m-plane of a Group III element nitride crystal. 5. In the Group III element nitride substrate described in any one of 1 to 4 above, the surface roughness Ra of the upper first flat surface may be 0.05 μm or less. 6. In the Group III element nitride substrate according to any one of items 1 to 5 above, the surface roughness Ra of the first flat surface may be 0.02 μm or more. 7. In the Group III element nitride substrate according to any one of items 1 to 6 above, the side surface may include a second flat surface. 8. In the Group III element nitride substrate according to item 7 above, the surface roughness Ra of the second flat surface may be 0.07 μm or less. 9. In the Group III element nitride substrate according to item 7 or 8 above, the difference between the surface roughness Ra of the first flat surface and the surface roughness Ra of the second flat surface may be 0.02 μm or less.

[0008] 10. A method for manufacturing a Group III element nitride substrate according to an embodiment of the present invention is the method for manufacturing a Group III element nitride substrate according to any one of 1 to 9 above, and includes preparing a Group III element nitride substrate for processing, processing a side surface of the Group III element nitride substrate for processing, processing a main surface of the Group III element nitride substrate for processing, and cleaning the Group III element nitride substrate after the processing.

[0009] According to an embodiment of the present invention, it is possible to provide a group III element nitride substrate in which the occurrence of scratches is suppressed.

[0010] FIG. 3 is a schematic cross-sectional view showing the general configuration of a group III element nitride substrate according to one embodiment of the present invention. FIG. 4 is a plan view of the group III element nitride substrate shown in FIG. 1. FIG. 5 is a view showing a manufacturing process of a group III element nitride substrate according to one embodiment. FIG. 6 is a view continuing from FIG. 3A. FIG. 7 is a view continuing from FIG. 3B. FIG. 8 is a schematic cross-sectional view showing an outline of an example of polishing the side surface of a group III element nitride substrate for processing. FIG. 9 is a schematic cross-sectional view showing the general configuration of a device substrate according to one embodiment of the present invention. FIG. 10 is a schematic cross-sectional view showing the general configuration of a bonded substrate according to one embodiment of the present invention.

[0011] Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to these embodiments. In order to clarify the description, the drawings may schematically show the width, thickness, shape, etc. of each part compared to the embodiments, but these are merely examples and do not limit the interpretation of the present invention. Furthermore, in the drawings, the same or equivalent elements are given the same reference numerals, and duplicate explanations may be omitted.

[0012] A. Group III Element Nitride Substrate Fig. 1 is a schematic cross-sectional view showing the general configuration of a Group III element nitride substrate according to one embodiment of the present invention, and Fig. 2 is a plan view of the Group III element nitride substrate shown in Fig. 1. The Group III element nitride substrate 10 is substantially disk-shaped (wafer-shaped) and has a first main surface 11 and a second main surface 12 that face each other and are connected via a side surface 13. A flat surface (first flat surface) is formed on at least a portion of the side surface 13.

[0013] As shown in FIG. 2 , the outer periphery of the group III element nitride substrate 10 has an arc-shaped portion 14 and a linear portion 15 corresponding to the flat surface of the side surface 13. The arc-shaped portion 14 is continuous with the linear portion 15. The linear portion 15 is typically an orientation flat. Two linear portions 15 are formed on the outer periphery of the group III element nitride substrate 10 shown in FIG. 2 . Specifically, the side surface 13 of the group III element nitride substrate 10 includes two flat surfaces (a first flat surface and a second flat surface). One of the two linear portions 15 is formed longer than the other. For example, the longer linear portion (flat surface) 15 can function as a main orientation flat, and the shorter linear portion (flat surface) can function as a sub-orientation flat. The two linear portions 15 are arranged perpendicular to each other.

[0014] The side surface 13 preferably includes a first flat surface having a surface roughness Ra of 0.07 μm or less. By forming such a first flat surface on the side surface 13, a Group III element nitride substrate 10 in which the occurrence of scratches is suppressed can be obtained. For example, the occurrence of scratches on the main surfaces 11 and 12 can be suppressed. The surface roughness Ra of the first flat surface is more preferably 0.05 μm or less. By forming such a first flat surface on the side surface, a Group III element nitride substrate 10 in which the occurrence of scratches is extremely effectively suppressed can be obtained.

[0015] As shown in the figure, the side surface 13 may include a second flat surface in addition to the first flat surface. The surface roughness Ra of the second flat surface is preferably 0.07 μm or less, and more preferably 0.05 μm or less.

[0016] The surface roughness Ra of the first flat surface is preferably 0.02 μm or more. For example, if the surface roughness Ra of the first flat surface is made too low, the surface roughness Ra of other portions of the side surface 13 (e.g., the second flat surface) may instead become larger. When the side surface 13 includes a second flat surface, the difference between the surface roughness Ra of the first flat surface and the surface roughness Ra of the second flat surface is preferably 0.02 μm or less, more preferably 0.01 μm or less. The surface roughness Ra of the second flat surface is, for example, 0.02 μm or more.

[0017] The size of the group III element nitride substrate 10 can be appropriately set depending on the purpose. The diameter of the group III element nitride substrate 10 is, for example, 25 mm or more and 350 mm or less, and may be 50 mm or more, 75 mm or more, or 100 mm or more. A group III element nitride substrate with a large size (e.g., a diameter of 75 mm or more) can improve the productivity of large-sized elements, for example. The larger the size, the more likely scratches tend to occur. However, according to the embodiment of the present invention, the occurrence of scratches can be effectively suppressed even when the size is large.

[0018] In plan view, the length of the linear portions 15 can be set appropriately depending on, for example, the size of the Group III element nitride substrate 10. When the diameter of the Group III element nitride substrate 10 is 50 mm to 150 mm, the length of the linear portions 15 is, for example, 5 mm to 20 mm.

[0019] The thickness of the group III element nitride substrate 10 is, for example, 200 μm to 1000 μm, preferably 250 μm to 900 μm, more preferably 300 μm to 800 μm, and even more preferably 350 μm to 750 μm.

[0020] The group III element nitride substrate is composed of group III element nitride crystals. Examples of group III elements that make up the group III element nitride include aluminum (Al), gallium (Ga), and indium (In). These may be used alone or in combination of two or more. Specific examples of group III element nitrides include aluminum nitride (Al ｘ N), gallium nitride (Ga ｙ N), indium nitride (In ｚ N), aluminum gallium nitride (Al ｘ Ga ｙ N), gallium indium nitride (Ga ｙ In ｚ N), aluminum indium nitride (Al ｘ In ｚ N), aluminum gallium indium nitride (Al ｘ Ga ｙ In ｚIn each chemical formula in parentheses, typically, x+y+z=1.

[0021] The Group III element nitride may contain a dopant. Examples of the dopant include p-type dopants such as beryllium (Be), magnesium (Mg), strontium (Sr), cadmium (Cd), iron (Fe), manganese (Mn), and zinc (Zn), and n-type dopants such as silicon (Si), germanium (Ge), tin (Sn), and oxygen (O). These may be used alone or in combination.

[0022] In the above-mentioned Group III element nitride crystal, the <0001> direction is typically the c-axis direction, the <1-100> direction is typically the m-axis direction, and the <11-20> direction is typically the a-axis direction. Furthermore, the crystal plane perpendicular to the c-axis is the c-plane, the crystal plane perpendicular to the m-axis is the m-plane, and the crystal plane perpendicular to the a-axis is the a-plane.

[0023] In one embodiment, the thickness direction of the Group III element nitride substrate 10 is substantially the c-axis direction. Specifically, the thickness direction of the Group III element nitride substrate 10 is parallel or approximately parallel to the c-axis. The first main surface 11 is substantially a Group III element polar plane on the (0001) plane side, and the second main surface 12 is substantially a nitrogen polar plane on the (000-1) plane side. Specifically, the first main surface 11 may be parallel to the (0001) plane or may be inclined with respect to the (0001) plane. The inclination angle of the first main surface 11 with respect to the (0001) plane is, for example, 10° or less, 5° or less, 2° or less, or 1° or less. The second main surface 12 may be parallel to the (000-1) plane or may be inclined with respect to the (000-1) plane. The inclination angle of the second main surface 12 with respect to the (000-1) plane is, for example, 10° or less, or may be 5° or less, 2° or less, or 1° or less. Here, the thickness direction is the direction perpendicular to the main surface, that is, the normal direction to the main surface.

[0024] When the thickness direction of the group III element nitride substrate 10 is substantially the c-axis direction, for example, the linear portion (flat surface) 15 may be substantially at least one of the m-plane and the a-plane. For example, the first flat surface may be substantially the m-plane. When two linear portions (flat surfaces) 15 are formed on the group III element nitride substrate 10 as shown in the figure, one may be substantially the m-plane and the other may be substantially the a-plane. The a-plane may have a larger surface roughness Ra than the m-plane, which may be a plane that is easier to cleave (cleavage plane). For example, if the surface roughness Ra of the m-plane is made too low, the surface roughness Ra of the a-plane may instead be larger.

[0025] B. Manufacturing Method A manufacturing method of a group III-nitride substrate according to one embodiment of the present invention includes preparing a group III-nitride substrate for processing, processing a side surface of the group III-nitride substrate for processing, processing a main surface of the group III-nitride substrate for processing, and cleaning the group III-nitride substrate after processing.

[0026] B-1. Preparation of Group III Element Nitride Substrate for Processing The Group III element nitride substrate for processing is typically prepared by fabricating a free-standing substrate. The free-standing substrate is fabricated, for example, by a method including: preparing a seed crystal substrate having a base substrate and a seed crystal film, growing a Group III element nitride crystal on the seed crystal film of the seed crystal substrate, and removing the base substrate from the Group III element nitride crystal.

[0027] 3A to 3C are diagrams showing steps for fabricating a group III element nitride substrate (a group III element nitride substrate for processing) according to one embodiment. Fig. 3A shows a state in which a seed crystal film 22 is formed on an upper surface 21a of a base substrate 21 having an upper surface 21a and a lower surface 21b facing each other, thereby completing a seed crystal substrate 20.

[0028] The base substrate may be, for example, a substrate having a shape and size that allows a Group III nitride substrate having the desired shape and size to be fabricated. Typically, the base substrate is disk-shaped with a diameter of 50 mm to 350 mm. The thickness of the base substrate is, for example, 300 μm to 2000 μm.

[0029] Any suitable substrate can be used as the base substrate. The base substrate is typically made of a single crystal. Examples of materials that can be used for the base substrate include sapphire, crystal-oriented alumina, silicon, gallium oxide, aluminum gallium nitride, gallium arsenide, and silicon carbide (SiC).

[0030] The thickness of the seed crystal film is, for example, 0.2 μm or more and 10 μm or less, and preferably 1 μm or more and 5 μm or less. Any appropriate material can be used as the material for the seed crystal film. A group III element nitride is typically used as the material for the seed crystal film. The details of the group III element nitride are as described above.

[0031] The seed crystal film can be formed by any appropriate method. A typical method for forming the seed crystal film is a vapor phase growth method. Specific examples of the vapor phase growth method include metal organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), pulsed excited deposition (PXD), molecular beam epitaxy (MBE), and sublimation. Among these, the MOCVD method is preferably used.

[0032] The formation of the seed crystal film by the MOCVD method includes, for example, a first formation step and a second formation step, in this order. Specifically, in the first formation step, a first layer (low-temperature grown buffer layer) (not shown) is formed on a base substrate at a temperature T1 (e.g., 450°C to 550°C), and in the second formation step, a second layer (not shown) is formed at a temperature T2 (e.g., 1000°C to 1200°C) higher than temperature T1. The thickness of the first layer is, for example, 20 nm to 50 nm. The thickness of the second layer is, for example, 1 μm to 4 μm.

[0033] Next, a group III element nitride crystal is grown on the seed crystal film 22 of the seed crystal substrate 20 to form a group III element nitride crystal layer 16, thereby obtaining a layered substrate 30 as shown in FIG. 3B . The degree of growth of the group III element nitride crystal (the thickness of the group III element nitride crystal layer 16) can be adjusted depending on the desired thickness of the group III element nitride substrate. Any appropriate direction can be selected as the growth direction of the group III element nitride crystal depending on the application, purpose, etc. Specific examples include the normal directions to the c-plane, a-plane, and m-plane, and the normal directions to planes inclined relative to the c-plane, a-plane, and m-plane.

[0034] Group III element nitride crystals can be grown by any appropriate method. The method for growing Group III element nitride crystals is not particularly limited, as long as it is a method that can achieve a crystal orientation that roughly follows the crystal orientation of the seed crystal film. Specific examples of methods for growing Group III element nitride crystals include vapor phase growth methods such as metal organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), pulsed excited deposition (PXD), molecular beam epitaxy (MBE), and sublimation; liquid phase growth methods such as flux deposition, ammonothermal deposition, hydrothermal deposition, and sol-gel deposition; and solid phase growth methods that utilize grain growth of powder. These methods can be used alone or in combination of two or more.

[0035] Preferably, a flux method (e.g., a Na flux method) is employed as a method for growing Group III element nitride crystals. Details of such growth methods are described, for example, in Japanese Patent No. 5244628, and growth may be performed by adjusting various conditions of the described growth method as appropriate. Specifically, Group III element nitride crystals can be grown by adjusting various conditions using a crystal manufacturing apparatus that includes a pressure-resistant vessel capable of supplying pressurized nitrogen gas, a turntable that can rotate within the pressure-resistant vessel, and an outer vessel placed on the turntable.

[0036] Growth of Group III nitride crystals by the flux method is typically carried out using a crucible as a growth container. Specifically, the base substrate or seed crystal substrate is placed at a predetermined position within the crucible, and raw materials are then filled in. The crucible containing the base substrate or seed crystal substrate is typically placed with a lid on in a nitrogen-containing atmosphere at a predetermined pressure and temperature for growth.

[0037] The raw material is, for example, a melt composition containing a flux, a Group III element, and, if necessary, a dopant. The flux preferably contains at least one of an alkali metal and an alkaline earth metal, and more preferably metallic sodium. Typically, the flux and a metal source material are mixed together. As the metal source material, an elemental metal, an alloy, a metal compound, etc. can be used, but from the viewpoint of ease of handling, an elemental metal is preferably used.

[0038] The crucible (including the lid) can be made of any suitable material that can be used in the flux method. Examples of the crucible material include alumina, yttria, and YAG (yttrium aluminum garnet). The crucible material may be a single crystal or a polycrystal (ceramic). The ceramic may have a high relative density, such as by HIP treatment, to give it translucency.

[0039] As described above, the growth can be carried out in a nitrogen-containing atmosphere. The growth atmosphere can contain other gases in addition to nitrogen. The other gases are preferably inert gases such as argon, helium, and neon.

[0040] The pressure of the atmosphere during growth may be set to any appropriate pressure. For example, the pressure of the atmosphere during growth may be 1 MPa or more, 2 MPa or more, or 3 MPa or more, from the viewpoint of preventing evaporation of the flux. On the other hand, the pressure of the atmosphere during growth may be 50 MPa or less, or 10 MPa or less, from the viewpoint of preventing the growth apparatus from becoming large-scale.

[0041] The temperature of the atmosphere during growth can be set to any appropriate temperature, for example, 700°C to 1000°C, or may be 800°C to 900°C.

[0042] The growth is preferably carried out while rotating the crucible (base substrate). For example, the covered crucible is placed in the outer container of the crystal production apparatus and placed on a turntable, and the turntable is rotated (e.g., on its axis) in this state to rotate the crucible. The rotation speed is, for example, 5 rpm to 40 rpm. The rotation direction can be set to any appropriate direction. In a preferred embodiment, during growth, clockwise and counterclockwise rotations are repeated at a predetermined cycle. The time (retention time) for maintaining rotation at a predetermined rotation speed in one rotation direction is, for example, 10 seconds to 1000 seconds.

[0043] After growth of the group III element nitride crystal, as shown in FIG. 3C , the base substrate 21 is removed from the group III element nitride crystal (group III element nitride crystal layer 16) to obtain a free-standing substrate 32. Typically, as shown in the figure, the free-standing substrate 32 may include the group III element nitride crystal layer 16 and a seed crystal film 22. For example, the free-standing substrate 32 is obtained by separating the group III element nitride crystal layer 16 from the base substrate 21. The group III element nitride crystal may be separated from the base substrate by any appropriate method. Examples of methods for separating the group III element nitride crystal include a method of spontaneously separating the group III element nitride crystal from the base substrate by utilizing the difference in thermal contraction between the group III element nitride crystal and the base substrate during a cooling step after growth of the group III element nitride crystal, a separation method using chemical etching, and a laser lift-off method using laser light irradiation. Alternatively, the free-standing substrate may be obtained by, for example, grinding and removing the base substrate 21, or by cutting the base substrate using a cutting machine such as a wire saw.

[0044] B-2. Processing of Group III Element Nitride Substrate for Processing The Group III element nitride substrate for processing (e.g., a free-standing substrate) can be subjected to any appropriate processing. Specific examples of processing include grinding and polishing the side surfaces of the Group III element nitride substrate for processing. Typically, the side surfaces are ground to obtain a Group III element nitride substrate having the desired shape and size. The abrasive grain size of the grinding stone (e.g., a diamond grinding stone) is, for example, 20 μm to 60 μm. Typically, the side surfaces are polished to obtain the predetermined surface roughness Ra. Polishing of the side surfaces can be performed by any appropriate method.

[0045] The polishing of the side surface of the Group III element nitride substrate for processing is carried out using, for example, a polishing tape. A polishing tape is typically a laminate of a base material and a polishing layer containing abrasive grains as an abrasive. FIG. 4 is a schematic cross-sectional view showing an example of polishing the side surface of a Group III element nitride substrate for processing. In the example shown in FIG. 4, a long polishing tape T is held by a roll (not shown), and the side surface of the Group III element nitride substrate for processing 18 is pressed against a pressing portion P, and the polishing tape T is fed in the direction of the arrow shown in FIG. 4. This polishes the side surface of the Group III element nitride substrate for processing 18. The pressing portion P is composed of, for example, a rubber pad.

[0046] The polishing conditions can be controlled so as to obtain the predetermined surface roughness Ra. In the example shown in Fig. 4, for example, a Group III element nitride substrate having the predetermined surface roughness Ra can be obtained by controlling the type (grit) of the polishing tape T, the pressure with which the substrate 18 is pressed against the pressing part P, the polishing time, the feed rate of the polishing tape T, and the like. Typically, arc-shaped portions and linear portions are formed in advance on the outer periphery of the Group III element nitride substrate to be processed by grinding, and then polishing is performed. In this case, the polishing conditions may be slightly changed from the viewpoint of obtaining the same degree of surface roughness Ra for the arc-shaped portions and the linear portions. For example, the pressure with which the Group III element nitride substrate 18 to be processed is pressed against the pressing part P may be slightly changed for the arc-shaped portions and the linear portions.

[0047] Another specific example of processing is grinding and polishing (e.g., lapping, chemical mechanical polishing (CMP)) of the main surface of the group III nitride substrate for processing. For example, the main surface is ground and polished to thin and flatten it to the desired thickness. For example, the seed crystal film 22 can be removed by processing the main surface, leaving only the group III nitride crystal layer 16 (only a single crystal growth layer).

[0048] Other specific examples of processing include chamfering the outer peripheral edge, removing a process-affected layer, and removing residual stress that may result from a process-affected layer.

[0049] B-3. ​​Cleaning After processing, the Group III element nitride substrate is typically subjected to a cleaning treatment. Cleaning can be performed using any appropriate cleaning solution. Examples of cleaning solutions include organic solvents such as acetone and ethanol, water, alkaline cleaning solutions, and acidic cleaning solutions. These can be used alone or in combination of two or more.

[0050] Cleaning using the cleaning solution can be carried out by any appropriate method, such as a method of immersing the Group III element nitride substrate in the cleaning solution and, if necessary, subjecting the substrate to various treatments such as ultrasonic treatment, or a method of rinsing the Group III element nitride substrate with the cleaning solution.

[0051] The number of washings is not particularly limited, and the washing liquid and washing method used for each washing may be selected appropriately.

[0052] According to an embodiment of the present invention, the occurrence of scratches can be suppressed. Specifically, by satisfying the predetermined surface roughness Ra on the side surface of a Group III element nitride substrate, the occurrence of scratches can be suppressed. Through repeated trial and error, we have found that the occurrence of scratches can be suppressed by controlling the condition of the side surface. When the surface roughness Ra of the side surface exceeds a predetermined value, foreign matter is likely to be retained on the side surface. For example, it is believed that foreign matter retained on the side surface during substrate cleaning may reattach to the main surface and cause scratches. Examples of foreign matter that can cause scratches include fine particles that can be generated by processing such as grinding and polishing (typically, processing the side surface). These fine particles may be derived from the Group III element nitride substrate. Another example of foreign matter is a fixing material (e.g., resin bond, wax) used to fix the substrate to a processing machine during processing. However, the fixing material can be removed by cleaning (e.g., cleaning with an organic solvent). In contrast, it is difficult to completely remove fine particles that can be generated by the above processing, even with powerful cleaning such as ultrasonic cleaning or cleaning with a brush. By controlling the surface roughness Ra of the side surface, it is possible to reduce foreign matter that can be retained on the side surface and to effectively suppress the occurrence of scratches. Note that the surface roughness Ra of the side surface can be evaluated on a flat surface from the viewpoint of ease of measurement.

[0053] C. Applications In one embodiment, a functional layer may be formed on the group III element nitride substrate. FIG. 5 is a schematic cross-sectional view showing an outline of the configuration of a device substrate according to one embodiment of the present invention. The device substrate 40 has a group III element nitride substrate 10 and a functional layer 42 formed on a first main surface (e.g., a group III element polar surface) 11 of the group III element nitride substrate 10. The functional layer 42 is typically formed by epitaxially growing a crystal.

[0054] The functional layer can function as, for example, a light-emitting layer, a rectifying element layer, a switching element layer, or a power semiconductor layer. In one embodiment, a Group III element nitride crystal is used as the material constituting the functional layer. Examples of Group III elements constituting the Group III element nitride include Ga (gallium), Al (aluminum), and In (indium). These elements can be used alone or in combination of two or more.

[0055] In addition, in a state where the functional layer 42 is formed on the group III element nitride substrate 10 (in a state where it has been made into the element substrate 40), the second main surface (e.g., the nitrogen-polar surface) 12 of the group III element nitride substrate 10 may be subjected to processing such as grinding or polishing.

[0056] In another embodiment, the group III element nitride substrate can be bonded to a support substrate. Fig. 6 is a schematic cross-sectional view showing an outline of the configuration of a bonded substrate according to one embodiment of the present invention. The bonded substrate 60 has a group III element nitride substrate 10 and a support substrate 62 arranged on the second main surface (e.g., nitrogen-polar surface) 12 side of the group III element nitride substrate 10. Although not shown, any appropriate epitaxial film can be formed on the group III element nitride substrate 10 of the bonded substrate 60.

[0057] The thickness of the support substrate 62 is, for example, 100 μm to 1000 μm. Any appropriate substrate can be used as the support substrate. Specifically, the support substrate may be made of a single crystal or a polycrystalline material.

[0058] Although not shown, the laminated substrate may further include any appropriate layer, the type, function, number, combination, arrangement, etc. of such layers may be appropriately determined depending on the purpose.

[0059] In one embodiment, the bonded substrate can be obtained by directly bonding the Group III element nitride substrate and the support substrate. For example, the bonding surface of the support substrate and / or the bonding surface of the Group III element nitride substrate can be surface-activated, and then the Group III element nitride substrate and the support substrate can be bonded to each other to obtain a bonded substrate.

[0060] The bonded substrate according to the embodiment of the present invention may have, for example, a bonding layer (not shown) disposed between the Group III element nitride substrate and the support substrate. In this case, the bonded substrate can be obtained by, for example, forming a bonding layer on the support substrate and / or the Group III element nitride substrate, and then bonding the Group III element nitride substrate and the support substrate via the bonding layer. During bonding, the bonding surfaces may be surface-activated.

[0061] From the viewpoint of obtaining excellent bonding strength, the bonding layer preferably contains at least one material selected from the group consisting of tantalum oxide, alumina, aluminum nitride, silicon carbide, sialon, and silicon oxide.

[0062] Typically, the sialon is a ceramic obtained by sintering a mixture of silicon nitride and alumina, and for example, Si ６－ａ Al ａ O ａ N ８－ａ Specifically, sialon has a composition in which alumina is mixed in silicon nitride, and a in the formula indicates the mixing ratio of alumina. a is preferably 0.5 or more and 4.0 or less. The silicon oxide is typically Si （１－ｂ） O ｂ (wherein 0.008≦b≦0.408).

[0063] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. The surface roughness Ra of a substrate (wafer) is a value measured by the following measurement method. <Surface roughness Ra> Surface roughness Ra was measured on a flat surface with a measurement width (length) of 4 mm using a small surface roughness measuring instrument (manufactured by Mitutoyo Corporation, "SURFTEST SJ-410") in accordance with ISO 4287:1997. For example, when the processing conditions of the side surface of a substrate are substantially the same, the surface roughness Ra on the flat surface and the surface roughness Ra on the curved surface can be estimated to be approximately the same.

[0064] Experimental Example 1-1 (Preparation of c-plane gallium nitride substrate) A roughly disk-shaped c-plane gallium nitride substrate (wafer) was prepared, having a diameter of 2 inches, a thickness of 400 μm, an orientation flat (m-plane) length of 16 mm, and a sub-orientation flat (a-plane) length of 8 mm, as shown in Figure 2. Specifically, the procedure is as follows.

[0065] 1. Preparation of Seed Crystal Substrate A seed crystal substrate was prepared by forming a gallium nitride film having a thickness of 3 μm on a sapphire substrate having a diameter of 58 mm by MOCVD.

[0066] 2. Growth of Gallium Nitride Crystals Gallium nitride crystals were grown using a crystal manufacturing apparatus equipped with a pressure-resistant container capable of supplying pressurized nitrogen gas, a turntable rotatable within the pressure-resistant container, and an outer container placed on the turntable. The obtained seed crystal substrate was placed in an alumina crucible in a nitrogen atmosphere glove box. Next, metallic gallium and metallic sodium were filled into the crucible so that the atomic ratio Ga / (Ga + Na) (mol%) was 15 mol%, and the crucible was covered with an alumina plate. In this state, the crucible was placed in a stainless steel inner container, which was then placed in a stainless steel outer container capable of containing the inner container, and the outer container was closed with a lid equipped with a nitrogen inlet pipe. In this state, the outer container was placed on a turntable installed in the heating section of the crystal manufacturing apparatus, and the pressure-resistant container of the crystal manufacturing apparatus was covered and sealed. Next, the pressure-resistant container was evacuated using a vacuum pump. Next, the heating section was operated to heat the heating space to 870°C, while nitrogen gas was introduced into the pressure vessel until the pressure reached 4.3 MPa, and the outer vessel was rotated around its central axis at a speed of 20 rpm in a constant clockwise and counterclockwise direction. This state was maintained for 100 hours, and a 400 μm thick gallium nitride crystal was grown.

[0067] After that, the pressure was reduced to atmospheric pressure by naturally cooling it to room temperature, and then the lid of the pressure vessel was opened and the crucible was removed from inside. The solidified metallic sodium in the crucible was removed, and the seed crystal substrate on which the gallium nitride crystal had grown was recovered.

[0068] Then, at room temperature, an ultraviolet laser was irradiated from the sapphire substrate side of the seed crystal substrate on which the gallium nitride crystal had been grown to decompose the gallium nitride film on the seed crystal substrate, and the grown gallium nitride crystal was separated from the sapphire substrate. The peripheral portion of the separated gallium nitride crystal was ground using a diamond grinding wheel (grit size: 45 μm) and processed into the shape shown in Figure 2, thereby preparing a c-plane gallium nitride substrate. The surface roughness Ra of the side surface (at the orientation flat and sub-orientation flat) of the prepared c-plane gallium nitride substrate was approximately 1 μm.

[0069] (Polishing of Substrate Side Surfaces) The side surfaces of the prepared c-plane gallium nitride substrates were polished using an edge polisher ("SFF-200TX" manufactured by Mipox Corporation) as shown in FIG. 4. Specifically, the polishing tape was fed out while the side surface of the substrate was pressed against a rubber pad via a long polishing tape held by a roll (not shown). Polishing was carried out under the following conditions. <Polishing conditions> Polishing tape: Diamond film manufactured by Mipox Corporation, grit size #50 to #2000 Pressure of substrate pressed against rubber pad: 6N to 15N Polishing time: 120 to 1500 seconds

[0070] (Grinding and polishing of the substrate principal surface) Next, the c-plane gallium nitride substrate was fixed to a ceramic processing platen, and the surface was ground using a grinder with a grinding stone of grit size #1000 (processing amount: approximately 100 μm to 300 μm), and then mirror-finished using a lapping device (processing amount: approximately 20 μm). In the lapping process, a tin surface plate was used from the perspective of maintaining quality. In addition, in the lapping process, abrasive grains with particle diameters (Dp) of 3 μm, Dp 0.5 μm, and Dp 0.2 μm were used in this order. The back surface of the c-plane gallium nitride substrate was also ground in the same way as the front surface, and then mirror-finished.

[0071] (Cleaning) Next, the c-plane gallium nitride substrate was set on a U-shaped sample carrier and subjected to acetone cleaning. Specifically, ultrasonic treatment using acetone (treatment time: 3 minutes) was repeated twice. Then, pure water cleaning was repeated twice. Specifically, the sample carrier on which the substrate was set was rocked in pure water for 10 minutes, and then the pure water was replaced and rocked for 10 minutes. The acetone cleaning and pure water cleaning were then repeated once more. Finally, pure water cleaning was performed by continuously pouring pure water into a beaker containing the sample carrier on which the substrate was set for 10 minutes. In this way, a gallium nitride substrate for evaluation (a-plane surface roughness Ra: 0.07 μm, m-plane surface roughness Ra: 0.01 μm) was obtained.

[0072] [Experimental Examples 1-2 to 1-13] Gallium nitride substrates for evaluation were obtained in the same manner as in Experimental Example 1-1, except that the polishing conditions for polishing the substrate side surfaces were changed within the ranges described above. The surface roughness Ra of the a-plane and the surface roughness Ra of the m-plane of the gallium nitride substrates for evaluation obtained in each Experimental Example are as shown in Table 1.

[0073] [Experimental Examples 2-1 to 2-10] Gallium nitride substrates for evaluation were obtained in the same manner as in Experimental Examples 1-1 to 1-10, except that a c-plane gallium nitride substrate having a diameter of 4 inches, a thickness of 400 μm, an orientation flat (m-plane) length of 32.5 mm, and a sub-orientation flat (a-plane) length of 15 mm was prepared, and the polishing time of the substrate side surface was changed accordingly (due to the increase in wafer size). The surface roughness Ra of the a-plane and the surface roughness Ra of the m-plane of the gallium nitride substrates for evaluation obtained in each experimental example are as shown in Table 2.

[0074] [Experimental Examples 3-1 to 3-10] Gallium nitride substrates for evaluation were obtained in the same manner as in Experimental Examples 1-1 to 1-10, except that a c-plane gallium nitride substrate with a diameter of 6 inches, a thickness of 400 μm, an orientation flat (m-plane) length of 47.5 mm, and a sub-orientation flat (a-plane) length of 20 mm was prepared, and the polishing time of the substrate side surface was changed accordingly (due to the increase in wafer size). The surface roughness Ra of the a-plane and the surface roughness Ra of the m-plane of the gallium nitride substrates for evaluation obtained in each experimental example are as shown in Table 3.

[0075] <Evaluation> The number of scratches present on the surface of the gallium polar surface of the obtained gallium nitride substrate for evaluation (after cleaning) was counted by analyzing images taken with a microscope (Keyence Corporation, VHX6000). The evaluation results are summarized in Tables 1 to 3.

[0076]

[0077]

[0078]

[0079] Group III nitride substrates according to embodiments of the present invention can be used, for example, as substrates for various semiconductor devices.

[0080] REFERENCE SIGNS LIST 10 Group III element nitride substrate 11 First main surface 12 Second main surface 13 Side surface 14 Arc-shaped portion 15 Linear portion (flat surface) 16 Group III element nitride crystal layer 18 Group III element nitride substrate for processing 20 Seed crystal substrate 21 Base substrate 21a Upper surface 21b Lower surface 22 Seed crystal film 30 Laminated substrate 32 Freestanding substrate 40 Element substrate 42 Functional layer 60 Bonded substrate 62 Support substrate

Claims

1. A Group III element nitride substrate having a generally disk-like shape, a first main surface and a second main surface opposing each other, and a side surface connecting the first main surface and the second main surface, wherein the side surface includes a first flat surface, and the surface roughness Ra of the first flat surface is 0.07 μm or less.

2. The Group III nitride substrate of claim 1, wherein said first flat surface is an orientation flat.

3. The Group III nitride substrate according to claim 1, wherein the first flat surface is an m-plane or an a-plane of a Group III nitride crystal.

4. The Group III nitride substrate according to claim 1, wherein the first flat surface is an m-plane of a Group III nitride crystal.

5. The Group III element nitride substrate according to claim 1, wherein the surface roughness Ra of said first flat surface is 0.05 μm or less.

6. The Group III element nitride substrate according to claim 1, wherein the surface roughness Ra of said first flat surface is 0.02 μm or more.

7. The Group III nitride substrate of claim 1, wherein the side surface includes a second planar surface.

8. The Group III element nitride substrate according to claim 7, wherein the second flat surface has a surface roughness Ra of 0.07 μm or less.

9. The Group III element nitride substrate according to claim 7, wherein the difference between the surface roughness Ra of said first flat surface and the surface roughness Ra of said second flat surface is 0.02 μm or less.

10. A method for producing a Group III element nitride substrate according to any one of claims 1 to 9, comprising: preparing a Group III element nitride substrate for processing; processing a side surface of the Group III element nitride substrate for processing; processing a main surface of the Group III element nitride substrate for processing; and cleaning the Group III element nitride substrate after the processing.