Composite substrate, epitaxial wafer, method for producing composite substrate, method for producing epitaxial wafer, and method for producing device

Abstract

A composite substrate according to an embodiment of the present invention comprises a first single-crystal substrate layer and a second single-crystal substrate layer bonded to the first single-crystal substrate layer. The first single-crystal substrate layer, in the plane of a plan view from the thickness direction, has a first high-dislocation-density region having a dislocation density at least five times an average dislocation density of the first single-crystal substrate layer. The second single-crystal substrate layer, in the plane of a plan view from the thickness direction, has a second high-dislocation-density region having a dislocation density at least five times an average dislocation density of the second single-crystal substrate layer. In a plan view from the thickness direction, the first high-dislocation-density region and the second high-dislocation-density region do not overlap or overlie each other.

Description

Composite substrate, epitaxial wafer, method for manufacturing a composite substrate, method for manufacturing an epitaxial wafer, and method for manufacturing a device 【0001】 The present disclosure relates to a composite substrate, an epitaxial wafer, a method for manufacturing a composite substrate, a method for manufacturing an epitaxial wafer, and a method for manufacturing a device. 【0002】 Silicon carbide (SiC) has a breakdown electric field one order of magnitude larger and a bandgap three times larger than silicon (Si). In addition, silicon carbide (SiC) has properties such as a thermal conductivity about three times higher than that of silicon (Si). Therefore, silicon carbide (SiC) is expected to be applied to power devices, high-frequency devices, etc. In addition, devices using silicon carbide (SiC) can operate in a high temperature range of 150 °C or higher. In recent years, SiC epitaxial wafers have been used in semiconductor devices as described above. 【0003】 A semiconductor device using SiC is referred to as a SiC device. A SiC device is manufactured using a SiC epitaxial wafer. A SiC epitaxial wafer is obtained by laminating a SiC epitaxial layer on the surface of a SiC substrate. A SiC substrate is cut out from a SiC ingot (also referred to as a SiC boule). A SiC ingot is a SiC single crystal processed into a cylindrical shape. 【0004】 Consideration has been given to joining a plurality of SiC crystals. For example, Patent Document 1 discloses producing a seed crystal that serves as a nucleus for crystal growth of a SiC single crystal by joining a plurality of SiC crystals. 【0005】 For example, Patent Document 2 discloses a composite substrate in which a first silicon carbide layer and a second silicon carbide layer having a higher defect density than the first silicon carbide layer are joined. 【0006】 Chinese Patent No. 118186574, Japanese Patent Laid-Open No. 2023-9025 【0007】SiC substrates are generally flat before the SiC epitaxial layer is laminated onto them. Various processing steps are involved in fabricating semiconductor devices from SiC substrates. SiC substrates can warp during these processing steps. Processing steps that can cause warping of SiC substrates include, for example, epitaxial layer deposition, surface polishing, oxide film formation, and ion implantation. Warped substrates negatively affect the process of fabricating semiconductor devices. For example, warped substrates are difficult to focus on during photolithography. Also, warped substrates may come into contact with the walls of equipment or other parts during the transport process and be damaged. 【0008】 Because composite substrates are made by bonding different substrates together, they are more prone to warping than single substrates. For example, Patent Document 2 discloses a composite substrate in which the relationship between the defect density of the first silicon carbide layer and the second silicon carbide layer is defined. However, Patent Document 2 does not address the issue of warping, and merely defining the relationship between the defect densities may result in the composite substrate warping significantly when processed. 【0009】 This disclosure has been made in view of the above-mentioned problems and aims to provide a composite substrate, an epitaxial wafer, a method for manufacturing a composite substrate, a method for manufacturing an epitaxial wafer, and a method for manufacturing a device that are less prone to warping. 【0010】 The inventors, after diligent research, have found that it is possible to create a substrate that is less prone to warping, even if it is a composite substrate. 【0011】 This disclosure provides the following means to solve the above problems. 【0012】(1) The composite substrate according to the first embodiment comprises a first single-crystal substrate layer and a second single-crystal substrate layer bonded to the first single-crystal substrate layer. The first single-crystal substrate layer has a first high dislocation density region. The second single-crystal substrate layer has a second high dislocation density region. In a plan view from the thickness direction, the first high dislocation density region and the second high dislocation density region do not overlap. In the plane viewed from the thickness direction, the dislocation density of the first high dislocation density region is five times or more higher than the average dislocation density of the first single-crystal substrate layer. In the plane viewed from the thickness direction, the dislocation density of the second high dislocation density region is five times or more higher than the average dislocation density of the second single-crystal substrate layer. 【0013】 (2) In the composite substrate according to the above embodiment, the first high dislocation density region may be in the central region, and the second high dislocation density region may be in the outer peripheral region. The central region is a region within 50% of the diameter from the center, and the outer peripheral region is a region radially outward from the central region. 【0014】 (3) In the composite substrate according to the above embodiment, the second high dislocation density region may be in the central region, and the first high dislocation density region may be in the outer peripheral region. The central region is a region within 50% of the diameter from the center, and the outer peripheral region is a region radially outward from the central region. 【0015】 (4) In the composite substrate according to the above embodiment, when two regions flanking a first straight line passing through the center are defined as a first region and a second region, the first high dislocation density region may be located in the first region, and the second high dislocation density region may be located in the second region. 【0016】 (5) In the composite substrate according to the above embodiment, when two regions flanking a first straight line passing through the center are defined as the first region and the second region, and two regions flanking a second straight line perpendicular to the first straight line and passing through the center are defined as the third region and the fourth region, the first high dislocation density region may be located in the first region and the second region, and the second high dislocation density region may be located in the third region and the fourth region. 【0017】(6) In the composite substrate according to the above embodiment, the first single crystal substrate layer may have a first low dislocation density region in a plane viewed from the thickness direction in which the dislocation density is one-fifth or less of the average dislocation density of the first single crystal substrate layer, and the second single crystal substrate layer may have a second low dislocation density region in a plane viewed from the thickness direction in which the dislocation density is one-fifth or less of the average dislocation density of the second single crystal substrate layer. In a plane view from the thickness direction in which the first high dislocation density region may overlap with at least a part of the second low dislocation density region, and the second high dislocation density region may overlap with at least a part of the first low dislocation density region. 【0018】 (7) In the composite substrate according to the above embodiment, two regions flanking a first straight line passing through the center are designated as the first region and the second region, and two regions flanking a second straight line perpendicular to the first line and passing through the center are designated as the third region and the fourth region, and when the in-plane distribution of the total dislocations, including the dislocations of the first single crystal substrate layer and the dislocations of the second single crystal substrate layer, is evaluated, the difference between the dislocation density of the total dislocations in the first region and the dislocation density of the total dislocations in the second region may be 5% or less of the average dislocation density of the total dislocations, and the difference between the dislocation density of the total dislocations in the third region and the dislocation density of the total dislocations in the fourth region may also be 5% or less of the average dislocation density of the total dislocations. 【0019】 (8) In the composite substrate according to the above embodiment, the through-dislocation density of the first single-crystal substrate layer may be higher than the through-dislocation density of the second single-crystal substrate layer. 【0020】 (9) In the composite substrate according to the above embodiment, the through-dislocation density of the first single-crystal substrate layer may be 1.5 times or more and 110 times or less of the through-dislocation density of the second single-crystal substrate layer. 【0021】 (10) The composite substrate according to the above embodiment may have a bonding layer between the first single crystal substrate layer and the second single crystal substrate layer. 【0022】 (11) The composite substrate according to the above embodiment may have a SORI of 40 μm or less. 【0023】(12) An epitaxial wafer according to a second embodiment comprises a composite substrate according to the above embodiment and an epitaxial layer laminated on the second single crystal substrate layer of the composite substrate. 【0024】 (13) The epitaxial wafer according to the above embodiment may have a SORI of 50 μm or less. 【0025】 (14) A method for manufacturing a composite substrate according to a third embodiment comprises a first preparation step, a second preparation step, a bonding step, and a separation step. In the first preparation step, a first single crystal substrate layer is prepared, and a first high dislocation density region is identified in the first single crystal substrate layer where the dislocation density is five times or more higher than the average dislocation density. In the second preparation step, a second single crystal substrate is prepared, and a second high dislocation density region is identified in the second single crystal substrate where the dislocation density is five times or more higher than the average dislocation density. In the bonding step, the first single crystal substrate layer and the second single crystal substrate are bonded together such that the first high dislocation density region and the second high dislocation density region do not overlap in a plan view. In the separation step, a part of the second single crystal substrate is separated in the thickness direction to produce a second single crystal substrate layer. 【0026】 (15) A method for manufacturing an epitaxial wafer according to the fourth embodiment comprises the steps of preparing a composite substrate according to the above embodiment and forming an epitaxial layer on the second single crystal substrate layer of the composite substrate. 【0027】 (16) A method for manufacturing a device according to the fifth embodiment comprises the steps of preparing an epitaxial wafer according to the above embodiment and forming an element on the epitaxial layer to form a chip. 【0028】 The composite substrate and epitaxial wafer according to the above embodiment are less prone to warping even after processing. The method for manufacturing the composite substrate, the method for manufacturing the epitaxial wafer, and the method for manufacturing the device according to the above embodiment can suppress problems associated with wafer warping. 【0029】This is a plan view of the composite substrate according to the first embodiment. This is a cross-sectional view of the composite substrate according to the first embodiment. This is a schematic diagram showing a first example of the dislocation density distribution in the first single-crystal substrate layer and the second single-crystal substrate layer. This is a schematic diagram showing a second example of the dislocation density distribution in the first single-crystal substrate layer and the second single-crystal substrate layer. This is a schematic diagram showing a third example of the dislocation density distribution in the first single-crystal substrate layer and the second single-crystal substrate layer. This is a schematic diagram showing a fourth example of the dislocation density distribution in the first single-crystal substrate layer and the second single-crystal substrate layer. This is a diagram for explaining SORI. This is a diagram for explaining the manufacturing method of the composite substrate according to the first embodiment. This is a cross-sectional view of the epitaxial wafer according to the first embodiment. This is a cross-sectional view of the device according to the first embodiment. 【0030】 This embodiment will now be described in detail with reference to the drawings as appropriate. The drawings used in the following description may be enlarged for convenience to clearly illustrate the features of this embodiment, and the dimensional ratios of each component may differ from those of the actual components. The materials, dimensions, etc., exemplified in the following description are examples only, and this disclosure is not limited to them. It is possible to modify and implement these examples as appropriate without altering the essence of the invention. 【0031】 In this specification, individual orientations are indicated by [], collective orientations by <>, individual planes by (), and collective planes by {}. While crystallography dictates that negative exponents are represented by a "-" (bar) above the number, in this specification, the negative sign is placed before the number. 【0032】 First, let's define the directions. The thickness direction (crystal growth direction) of the composite substrate 1 is defined as the Z direction. The Z direction may be the <0001> direction, or it may be tilted by an offset angle relative to the <0001> direction. One direction of the plane perpendicular to the Z direction is defined as the X direction. The X direction is, for example, <11-20>. The Y direction is, for example, the <1-100> direction. 【0033】 "Composite Substrate" Figure 1 is a plan view of the composite substrate 1 according to this embodiment. The composite substrate 1 is a wafer that is approximately circular in plan view. 【0034】The diameter of the composite substrate 1 is, for example, 6 inches or more, preferably 8 inches or more, more preferably 10 inches or more, and even more preferably 12 inches or more. 【0035】 The diameter of the composite substrate 1 is, for example, 145 mm or more, preferably 149 mm or more. The diameter of the composite substrate 1 is, for example, 155 mm or less, preferably 151 mm or less. The diameter of the composite substrate 1 is, for example, 195 mm or more, preferably 199 mm or more. The diameter of the composite substrate 1 is, for example, 205 mm or less, preferably 201 mm or less. The diameter of the composite substrate 1 is, for example, 245 mm or more, preferably 249 mm or more. The diameter of the composite substrate 1 is, for example, 255 mm or less, preferably 251 mm or less. The diameter of the composite substrate 1 is, for example, 295 mm or more, preferably 299 mm or more. The diameter of the composite substrate 1 is, for example, 305 mm or less, preferably 301 mm or less. 【0036】 The composite substrate 1 may have a notch n for determining the direction of the crystal axis when viewed from the Z direction. The notch n is a groove cut out from the outer circumference of the composite substrate 1 toward the inside of the wafer. The notch n is located, for example, in the [1-100] direction from the center of the composite substrate 1. The composite substrate 1 may have an orientation flat instead of a notch n. The center is the center of the smallest circumscribed circle that is tangent to the outer circumference of the composite substrate 1. 【0037】 Figure 2 is a cross-sectional view of the composite substrate 1 according to this embodiment. The composite substrate 1 comprises a first single crystal substrate layer 11 and a second single crystal substrate layer 12. The first single crystal substrate layer 11 is in contact with the second single crystal substrate layer 12 and is bonded to the second single crystal substrate layer 12. The first single crystal substrate layer 11 and the second single crystal substrate layer 12 are bonded together during the manufacturing process and are originally separate substrates. 【0038】Both the first single-crystalline substrate layer 11 and the second single-crystalline substrate layer 12 contain, for example, SiC. Both the first single-crystalline substrate layer 11 and the second single-crystalline substrate layer 12 are made of, for example, SiC. The polytype of the first single-crystalline substrate layer 11 and the second single-crystalline substrate layer 12 is not particularly limited and may be any of 2H, 3C, 4H, and 6H. For example, the first single-crystalline substrate layer 11 and the second single-crystalline substrate layer 12 are 4H-SiC. 【0039】 Although it is ideal that the first single-crystalline substrate layer 11 and the second single-crystalline substrate layer 12 do not contain defects and dislocations inside, they contain defects and dislocations. The first single-crystalline substrate layer 11 and the second single-crystalline substrate layer 12 contain, for example, dislocations inside. Dislocations include through dislocations, basal plane dislocations, and micropipes. 【0040】 The dislocations in the first single-crystalline substrate layer 11 are measured, for example, on the second surface 11B. The dislocations in the second single-crystalline substrate layer 12 are measured, for example, on the first surface 12A. The first surface 11A is a surface facing the second surface 11B. The second surface 11B is a surface that joins the second single-crystalline substrate layer 12. The first surface 12A is a surface that joins the first single-crystalline substrate layer 11. The second surface 12B is a surface facing the first surface 12A. 【0041】 Dislocations can be observed, for example, using synchrotron radiation topography or X-ray topography. Dislocations can also be observed by etching the single-crystalline substrate using KOH or NaOH. 【0042】 Dislocations are caused by the displacement of atoms inside the crystal. Dislocations are caused by stress (strain) generated in a predetermined direction inside the crystal. The displacement inside the crystal that generates dislocations varies depending on the type of dislocations. For example, the displacement inside the crystal that generates basal plane dislocations is different from the displacement inside the crystal that generates through dislocations. 【0043】 In the first single-crystalline substrate layer 11 and the second single-crystalline substrate layer 12, dislocations do not uniformly exist in the plane and have a density distribution. Hereinafter, some specific examples will be given to explain the dislocation density distribution in the first single-crystalline substrate layer 11 and the second single-crystalline substrate layer 12. 【0044】FIG. 3 schematically shows a first example of the dislocation density distribution in the first single crystal substrate layer 11 and the second single crystal substrate layer 12. FIG. 3 is a plan view of each of the first single crystal substrate layer 11 and the second single crystal substrate layer 12 as viewed from the Z direction. The first single crystal substrate layer 11 is a plan view of the second surface 11B on the second single crystal substrate layer 12 side, and the second single crystal substrate layer 12 is a plan view of the first surface 12A on the first single crystal substrate layer 11 side. 【0045】 The first single crystal substrate layer 11 has, for example, a first high dislocation density region and a first low dislocation density region. The first high dislocation density region is a region where the dislocation density is 5 times or more higher than the average dislocation density of the first single crystal substrate layer 11. The first low dislocation density region is a region where the dislocation density is 1 / 5 or less of the average dislocation density of the first single crystal substrate layer 11. 【0046】 The average dislocation density of the first single crystal substrate layer 11 is obtained by dividing the number of dislocations on the entire surface of the first single crystal substrate layer 11 by the area of the first single crystal substrate layer 11. The substrate is divided into a predetermined size, and for each of the divided regions, it is determined whether it is a first high dislocation density region or a first low dislocation density region. The size of each of the divided regions is the same, for example, 10 mm × 10 mm. For example, each of the divisions is arranged without a gap as viewed from the Z direction, and the center of one of the plurality of divisions coincides with the center of the substrate. In each division, the dislocation density measured in a measurement region of 1.0 mm × 1.0 mm including the center is treated as the dislocation density in that division. The center of the division is the intersection of the diagonals of the division. 【0047】The second single-crystal substrate layer 12 has, for example, a second high dislocation density region and a second low dislocation density region. The second high dislocation density region is a region where the dislocation density is five times or more higher than the average dislocation density of the second single-crystal substrate layer 12. The second low dislocation density region is a region where the dislocation density is one-fifth or less of the average dislocation density of the second single-crystal substrate layer 12. The average dislocation density of the second single-crystal substrate layer 12 is obtained by dividing the number of dislocations across the entire surface of the second single-crystal substrate layer 12 by the area of ​​the second single-crystal substrate layer 12. Whether or not a region falls into the second high dislocation density region or the second low dislocation density region is determined for each section by dividing the substrate into predetermined sizes, similar to the first high dislocation density region and the first low dislocation density region. Each region is divided in the same way as the first single-crystal substrate layer 11. 【0048】 When viewed from the Z direction in a plan view, the positions of the first high dislocation density region and the second high dislocation density region are offset, and the first high dislocation density region and the second high dislocation density region do not overlap when viewed from the Z direction in a plan view. Furthermore, when viewed from the Z direction in a plan view, it is preferable that the positions of the first low dislocation density region and the second low dislocation density region are offset, and it is preferable that the first low dislocation density region and the second low dislocation density region do not overlap when viewed from the Z direction in a plan view. Furthermore, when viewed from the Z direction in a plan view, it is preferable that the first high dislocation density region and the second low dislocation density region overlap in at least a part, and it is preferable that the second high dislocation density region and the first low dislocation density region overlap in at least a part. 【0049】 In the first example shown in Figure 3, the first high dislocation density region is located in the central region 111. In the first example shown in Figure 3, the first low dislocation density region is located in the outer peripheral region 112. The central region 111 is the region within 50% of the diameter of the first single crystal substrate layer 11 from the center of the first single crystal substrate layer 11. The outer peripheral region 112 is the region radially outward from the central region 111. 【0050】 In the first example shown in Figure 3, the second high dislocation density region is located in the outer peripheral region 122. In the first example shown in Figure 3, the second low dislocation density region is located in the central region 121. The central region 121 is the region within 50% of the diameter of the second single crystal substrate layer 12 from the center of the second single crystal substrate layer 12. The outer peripheral region 122 is the region radially outward from the central region 121. 【0051】In the composite substrate 1 formed by joining the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12, the central region 111 faces the central region 121, and the outer peripheral region 112 faces the outer peripheral region 122. In the first example, the central region 111 containing the first high dislocation density region and the outer peripheral region 122 containing the second high dislocation density region do not overlap when viewed from the Z direction in a plan view. Also in the first example, when viewed from the Z direction in a plan view, the central region 111 containing the first high dislocation density region overlaps with the central region 121 containing the second low dislocation density region, and the outer peripheral region 122 containing the second high dislocation density region overlaps with the outer peripheral region 112 containing the first low dislocation density region. 【0052】 Figure 4 schematically shows a second example of the dislocation density distribution in the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12. Figure 4 is a plan view of the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 from the Z direction. The first single-crystal substrate layer 11 is a plan view of the second surface 11B on the side of the second single-crystal substrate layer 12, and the second single-crystal substrate layer 12 is a plan view of the first surface 12A on the side of the first single-crystal substrate layer 11. The second example differs from the first example in that the relationship between the high-density and low-density dislocation regions is reversed in both the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12. 【0053】 The first single-crystal substrate layer 11 has, for example, a first high dislocation density region and a first low dislocation density region. In the second example shown in Figure 4, the first high dislocation density region is located in the outer peripheral region 112. In the second example shown in Figure 4, the first low dislocation density region is located in the central region 111. 【0054】 The second single-crystal substrate layer 12 has, for example, a second high dislocation density region and a second low dislocation density region. In the second example shown in Figure 4, the second high dislocation density region is located in the central region 121. In the second example shown in Figure 4, the second low dislocation density region is located in the outer peripheral region 122. 【0055】In the composite substrate 1 formed by joining the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12, the central region 111 faces the central region 121, and the outer peripheral region 112 faces the outer peripheral region 122. In the second example, the outer peripheral region 112, which contains the first high dislocation density region, and the central region 121, which contains the second high dislocation density region, do not overlap when viewed from the Z direction in a plan view. Also in the second example, when viewed from the Z direction in a plan view, the outer peripheral region 112, which contains the first high dislocation density region, overlaps with the outer peripheral region 122, which contains the second low dislocation density region, and the central region 121, which contains the second high dislocation density region, overlaps with the central region 111, which contains the first low dislocation density region. 【0056】 Figure 5 schematically shows a third example of the dislocation density distribution in the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12. Figure 5 is a plan view of the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 from the Z direction. The first single-crystal substrate layer 11 is a plan view of the second surface 11B on the side of the second single-crystal substrate layer 12, and the second single-crystal substrate layer 12 is a plan view of the first surface 12A on the side of the first single-crystal substrate layer 11. 【0057】 The first single-crystal substrate layer 11 has, for example, a first high dislocation density region and a first low dislocation density region. In the third example shown in Figure 5, the first high dislocation density region is located in the first region 113. In the third example shown in Figure 5, the first low dislocation density region is located in the second region 114. In the first single-crystal substrate layer 11, the first region 113 is one of two regions that straddle the first straight line L1 passing through the center of the composite substrate 1, and the second region 114 is the other of two regions that straddle the first straight line L1 passing through the center of the composite substrate 1. 【0058】 The second single-crystal substrate layer 12 has, for example, a second high dislocation density region and a second low dislocation density region. In the third example shown in Figure 5, the second high dislocation density region is located in the second region 124. In the third example shown in Figure 5, the second low dislocation density region is located in the first region 123. In the second single-crystal substrate layer 12, the first region 123 is one of two regions that straddle the first straight line L1 passing through the center of the composite substrate 1, and the second region 124 is the other of two regions that straddle the first straight line L1 passing through the center of the composite substrate 1. 【0059】In the composite substrate 1 formed by joining the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12, the first region 113 faces the first region 123, and the second region 114 faces the second region 124. In the third example, the first region 113 containing the first high dislocation density region and the second region 124 containing the second high dislocation density region do not overlap when viewed from the Z direction in a plan view. Also in the third example, when viewed from the Z direction in a plan view, the first region 113 containing the first high dislocation density region overlaps with the first region 123 containing the second low dislocation density region, and the second region 124 containing the second high dislocation density region overlaps with the second region 114 containing the first low dislocation density region. 【0060】 In the example shown in Figure 5, the first straight line L1 divides the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 into two regions in the X direction. However, the first straight line L1 may also divide the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 into two regions in a direction inclined with respect to either the X direction or the Y direction, or in the Y direction. Furthermore, the positional relationship between the high dislocation density region and the low dislocation density region in the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 may be reversed. 【0061】 Figure 6 schematically shows a fourth example of the dislocation density distribution in the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12. Figure 6 is a plan view of the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 from the Z direction. The first single-crystal substrate layer 11 is a plan view of the second surface 11B on the side of the second single-crystal substrate layer 12, and the second single-crystal substrate layer 12 is a plan view of the first surface 12A on the side of the first single-crystal substrate layer 11. The fourth example differs from the first to third examples in that there are multiple high-dislocation-density regions in both the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12. 【0062】 In the fourth example shown in Figure 6, the first single-crystal substrate layer 11 can be divided into a first region 113 and a second region 114, which are flanked by a first straight line L1. In the fourth example shown in Figure 6, the first single-crystal substrate layer 11 can also be divided into a third region 115 and a fourth region 116, which are flanked by a second straight line L2. The second straight line L2 is perpendicular to the first straight line L1. The third region 115 is one of the two regions in the first single-crystal substrate layer 11 that flank the second straight line L2, and the fourth region 116 is the other region. 【0063】 In the fourth example shown in Figure 6, the first single-crystal substrate layer 11 has, for example, two first high dislocation density regions and a first low dislocation density region. In the fourth example, the first high dislocation density regions are located in the first region 113 and the second region 114, respectively. In the fourth example, the first low dislocation density region extends along the first straight line L1 and separates the two first high dislocation density regions. In the first single-crystal substrate layer 11 of the fourth example, the dislocation density distribution is preferably approximately symmetrical with respect to the first straight line L1 and preferably approximately symmetrical with respect to the second straight line L2. 【0064】 In the fourth example shown in Figure 6, the second single-crystal substrate layer 12 can be divided into a first region 123 and a second region 124 that straddle the first straight line L1. In the fourth example shown in Figure 6, the second single-crystal substrate layer 12 can also be divided into a third region 125 and a fourth region 126 that straddle the second straight line L2. The third region 125 is one of the two regions in the second single-crystal substrate layer 12 that straddle the second straight line L2, and the fourth region 126 is the other of the two regions that straddle the second straight line L2. 【0065】 In the fourth example shown in Figure 6, the second single-crystal substrate layer 12 has, for example, two second high dislocation density regions and a second low dislocation density region. In the fourth example shown in Figure 6, the second high dislocation density regions are located in the third region 125 and the fourth region 126, respectively. In the fourth example shown in Figure 6, the second low dislocation density region extends along the second straight line L2 and separates the two second high dislocation density regions. In the second single-crystal substrate layer 12 of the fourth example, the dislocation density distribution is preferably approximately symmetrical with respect to the first straight line L1 and preferably approximately symmetrical with respect to the second straight line L2. 【0066】 In the fourth example, the first high dislocation density region and the second high dislocation density region do not overlap when viewed from a planar perspective in the Z direction. In the fourth example, the two first high dislocation density regions overlap with, for example, at least a portion of the second low dislocation density region, and the two second high dislocation density regions overlap with, for example, at least a portion of the first low dislocation density region. In the fourth example, the first shortest line connecting the two first high dislocation density regions by the shortest distance and the second shortest line connecting the two second high dislocation density regions by the shortest distance intersect. Preferably, the first shortest line and the second shortest line are orthogonal. 【0067】 In the fourth example shown in Figure 6, the first straight line L1 divides the first single crystal substrate layer 11 into two regions in the X direction. However, the first straight line L1 may also divide the first single crystal substrate layer 11 into two regions in a direction inclined with respect to either the X or Y direction, or in the Y direction. Similarly, the second straight line L2 may also be a line in a direction inclined with respect to either the X or Y direction, or a line extending in the X direction. 【0068】 When evaluating the in-plane distribution of total dislocations, which is the sum of the dislocations in the first single-crystal substrate layer 11 and the dislocations in the second single-crystal substrate layer 12, the difference between the total dislocation density in the first region and the total dislocation density in the second region is preferably 5% or less of the average total dislocation density, and more preferably 3% or less. When the difference in dislocation density between the two regions separated by the first straight line L1 is small, the composite substrate 1 is less likely to warp even when a processing process is performed on it. 【0069】 The total dislocation density in the first region is obtained by dividing the sum of the dislocations in the first region 113 and the dislocations in the first region 123 by the area of ​​the first region 113. The total dislocation density in the second region is obtained by dividing the sum of the dislocations in the second region 114 and the dislocations in the second region 124 by the area of ​​the second region 114. The average dislocation density of the total dislocations is obtained by dividing the sum of the dislocations in the first single crystal substrate layer 11 and the dislocations in the second single crystal substrate layer 12 by the area of ​​the composite substrate 1. 【0070】 Furthermore, when evaluating the in-plane distribution of total dislocations, which is the sum of the dislocations in the first single-crystal substrate layer 11 and the dislocations in the second single-crystal substrate layer 12, the difference between the dislocation density of total dislocations in the third region and the dislocation density of total dislocations in the fourth region is preferably 5% or less of the average dislocation density of total dislocations, and more preferably 3% or less. When the difference in dislocation density between the two regions separated by the second straight line L2 is small, the composite substrate 1 is less likely to warp even when a processing process is performed on it. 【0071】The dislocation density of total dislocations in the third region is obtained by dividing the sum of dislocations in the third region 115 and the dislocations in the third region 125 by the area of ​​the third region 115. The dislocation density of total dislocations in the fourth region is obtained by dividing the sum of dislocations in the fourth region 116 and the dislocations in the fourth region 126 by the area of ​​the fourth region 116. 【0072】 In the composite substrate 1, it is preferable that the through-dislocation density of the first single-crystal substrate layer 11 is higher than that of the second single-crystal substrate layer 12. Through-dislocations include through-helical dislocations, through-edge dislocations, through-mixed dislocations, and micropipes. In an untreated single-crystal substrate, through-helical dislocations, through-edge dislocations, and through-mixed dislocations, excluding micropipes, cannot be confirmed by an optical microscope. If the through-dislocation density of the first single-crystal substrate layer 11 is higher than that of the second single-crystal substrate layer 12, the stress acting on the second single-crystal substrate layer 12 when an epitaxial layer is deposited on the second single-crystal substrate layer 12 can be relaxed. 【0073】 For example, when an epitaxial layer is deposited on a second single-crystal substrate layer 12, the second single-crystal substrate layer 12 is subjected to a stress that causes it to bend, with its center convex toward the epitaxial layer. The first single-crystal substrate layer 11 acts a stress on the second single-crystal substrate layer 12 that causes it to bend toward the opposite side due to the difference in threading dislocation density between the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12. This stress is relieved by the deposition of the epitaxial layer. This relationship is based on the relationship between the threading dislocation densities of the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12. Even if the relationship between the density of basal plane dislocations that strain in different directions and defects caused by factors other than strain is adjusted between the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12, it is difficult to adequately relieve the stress generated within the second single-crystal substrate layer 12 when the epitaxial layer is deposited. 【0074】 The threading dislocation density of the first single-crystal substrate layer 11 is preferably, for example, 1.5 times or more and 110 times or less than the threading dislocation density of the second single-crystal substrate layer 12. 【0075】Furthermore, among the through-dislocations on the second surface 11B of the first single-crystal substrate layer 11, those whose coordinates coincide with those on the first surface 12A of the second single-crystal substrate layer 12 are, for example, 0.5% or less. The coordinates are the XY coordinates with the center of the composite substrate 1 as the origin. 【0076】 Since the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 are bonded together, the coordinates of their respective threading dislocations hardly coincide at the bonding surface. 【0077】 The coordinates of the threading dislocations on the second surface 11B of the first single-crystal substrate layer 11 can be determined by measuring the second surface 11B after removing the second single-crystal substrate layer 12. For example, the location of the threading dislocations on the second surface 11B can be identified by classifying the etch pits formed by KOH etching of the second surface 11B. Alternatively, the location of the threading dislocations may be identified from the photoluminescence image of the second surface 11B. 【0078】 The coordinates of the threading dislocations on the first surface 12A of the second single crystal substrate layer 12 can be determined from the coordinates of the threading dislocations on the second surface 12B of the second single crystal substrate layer 12. Threading dislocations extend along the c-axis. Therefore, the coordinates of the threading dislocations on the first surface 12A can be determined using the coordinates of the threading dislocations on the second surface 12B of the second single crystal substrate layer 12, the film thickness of the second single crystal substrate layer 12, and the offset angle. For example, if the coordinates on the second surface 12B are (X1, Y1), the second single crystal substrate layer 12 has an offset angle θ in the X direction, and the film thickness of the second single crystal substrate layer 12 is T, then the coordinates on the first surface 12A of the second single crystal substrate layer 12 are (X1 - T / tanθ, Y1). 【0079】 Both the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 may be undoped SiC or dopant-doped SiC. The dopant is, for example, nitrogen. For example, the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 are n-type SiC single crystals. 【0080】The absolute value of the difference in dopant concentration between the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 is preferably 50% or less, more preferably 30% or less, and even more preferably 10% or less, of the average of the dopant concentrations of the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12. A smaller difference in dopant concentration between the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 makes the composite substrate 1 less prone to warping even after a processing step is performed on it. 【0081】 The dopant concentrations of the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 are the average values ​​of measurement results at multiple measurement points. The measurement points are, for example, the center of the substrate, multiple points spaced 10 mm apart in the <11-20> direction relative to the center, and multiple points spaced 10 mm apart in the <1-100> direction relative to the center. The dopant concentration at each measurement point can be measured, for example, using mercury CV or secondary ion mass spectrometry (SIMS). 【0082】 The total thickness of the composite substrate 1, including the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12, is preferably, for example, 198 μm or more. Having a predetermined thickness in the composite substrate 1 prevents the composite substrate 1 from warping significantly even when a processing process is performed on it. 【0083】 Furthermore, the total thickness of the composite substrate 1 is preferably, for example, 550 μm or less. A thicker composite substrate 1 has high resistance in the thickness direction. The composite substrate 1 is processed into a device after the epitaxial layer is formed. SiC devices often conduct electricity in the thickness direction of the composite substrate 1, and resistance in the thickness direction reduces the operating efficiency of the device. It is also possible to reduce the substrate resistance by slimming the first single crystal substrate layer 11 of the composite substrate 1, but hard SiC takes a long time to thin, which reduces the manufacturing efficiency of the device. 【0084】 The total thickness of the composite substrate 1 is preferably, for example, 250 μm or more and 800 μm or less, and more preferably 330 μm or more and 550 μm or less. 【0085】The thickness of the first single-crystal substrate layer 11 is, for example, 150 μm to 700 μm, and preferably 200 μm to 500 μm. The thickness of the first single-crystal substrate layer 11 can be calculated from the thickness of the original first single-crystal substrate and the amount of processing at each stage of the manufacturing process. When measuring from the bonded state, a method using light reflectance can be applied. The total thickness is measured, and the difference in light reflectance is measured when light is incident from the first single-crystal substrate layer 11 and when it is incident from the opposite second single-crystal substrate layer 12. By comparing this with a pre-prepared matrix table of carrier concentration and plate thickness and the relationship with reflectance, the thickness of the first single-crystal substrate layer 11 can be calculated non-destructively. If measurement involving destructive action is permitted, it is also possible to directly measure the thickness from cross-sectional observation in the thickness direction. 【0086】 The thickness of the first single-crystal substrate layer 11 is, for example, the average radial thickness measured along a straight line extending in the <11-20> direction. The thickness is measured, for example, at the center and at multiple measurement points arranged at 10 mm intervals from the center. The intervals between the multiple measurement points may be 15 mm, 20 mm, 25 mm, or 30 mm. 【0087】 The thickness of the second single-crystal substrate layer 12 is, for example, thinner than the thickness of the first single-crystal substrate layer 11. The thickness of the second single-crystal substrate layer 12 is, for example, 50 μm or more and 300 μm or less, preferably 100 μm or more and 250 μm or less. The thickness of the second single-crystal substrate layer 12 can be determined by measuring the original thickness of the second single-crystal substrate and understanding the amount of processing at each step, similar to the first single-crystal substrate layer 11. When measuring the thickness in the bonded state, it can also be measured using the same method as the first single-crystal substrate layer 11. The thickness of the second single-crystal substrate layer 12 is, for example, the average value of the measurement results at the same measurement points as the first single-crystal substrate layer 11. 【0088】The main surfaces of the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 may or may not have an offset angle with respect to the (0001) plane in the <11-20> direction. Preferably, the main surfaces of the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 have an offset angle in the <11-20> direction. Single crystals whose main surfaces have an offset angle are less likely to contain heterogeneous polymorphs internally, and the occurrence of heterogeneous polymorphs in the epitaxial layer growing on the main surface can be suppressed. The main surface of the first single-crystal substrate layer 11 is the first surface 11A or the second surface 11B, and the main surface of the second single-crystal substrate layer 12 is the first surface 12A or the second surface 12B. 【0089】 The offset angle is the angle between the plane perpendicular to the Z direction, which is the thickness direction of the composite substrate 1, and the (0004) plane. The offset angle in the <11-20> direction is, for example, greater than 0° and 10° or less, preferably 0.1° or more and 8° or less, more preferably 3.5° or more and 4.5° or less, and even more preferably 4°. 【0090】 Furthermore, the main surfaces of the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 may or may not have an offset angle with respect to the (0001) plane in the <1-100> direction. It is preferable that the main surfaces of the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 do not have an offset angle in the <1-100> direction. The offset angle of the main surfaces of the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 in the <1-100> direction may be, for example, 0.1° or more and less than 1.0°. 【0091】 Preferably, the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 have an offset angle in the <11-20> direction, for example, and do not have an offset angle in the <1-100> direction. 【0092】 The offset angle of the first single-crystal substrate layer 11 and the offset angle of the second single-crystal substrate layer 12 may be the same or different. Since the second single-crystal substrate layer 12 is bonded to the first single-crystal substrate layer 11, the offset angles may not be perfectly identical in many cases. 【0093】In the <11-20> direction, the difference between the offset angle of the first single crystal substrate layer 11 and the offset angle of the second single crystal substrate layer 12 is preferably less than 1.0°, and more preferably 0.50° or less. In the <11-20> direction, the difference between the offset angle of the first single crystal substrate layer 11 and the offset angle of the second single crystal substrate layer 12 may be 0.01° or more. When the difference in offset angles between the first single crystal substrate layer 11 and the second single crystal substrate layer 12 is small, a composite substrate 1 that is less prone to warping even after processing can be obtained. 【0094】 In the <1-100> direction, the difference between the offset angle of the first single crystal substrate layer 11 and the offset angle of the second single crystal substrate layer 12 is preferably 0.5° or less, and more preferably 0.30° or less. In the <1-100> direction, the difference between the offset angle of the first single crystal substrate layer 11 and the offset angle of the second single crystal substrate layer 12 may be 0.01° or more. When the difference in offset angles between the first single crystal substrate layer 11 and the second single crystal substrate layer 12 is small, a composite substrate 1 that is less prone to warping even after processing can be obtained. 【0095】 The composite substrate 1 may have a bonding layer between the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12. The bonding layer is located between the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12. The bonding layer contains a compound of Si and C. For example, the bonding layer is a layer in which 90 atoms or more of the constituent elements are Si and C, and Si and C are present in approximately a 1:1 ratio. The bonding layer may be crystalline, amorphous, or a layer in which both are present. The bonding layer may, in some cases, contain noble gas elements such as Ar and Ne, elements used for etching such as H, and dopant elements such as N, P, and B. The total amount of elements other than Si and C contained in the bonding layer is 10 atoms or less. 【0096】The boundary between the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 may contain voids. If there are too many voids, the bonding state may not be maintained when heated, so it is desirable to minimize the presence of voids. Preferably, the voids are less than 5% of the bonding area (approximately equivalent to the area of ​​the second surface 11B or the first surface 12A) in each interface region. The bonding layer is a layer formed by bonding the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 together. 【0097】 The bonding layer can sometimes be confirmed by, for example, magnifying the side (edge) of the substrate or by observing the cross-section with a transmission electron microscope (TEM). The thickness of the bonding layer may be, for example, between 0.5 nm and 10 nm. The thickness of the bonding layer can sometimes be measured directly by, for example, observing the cross-section with a TEM, or by scanning the Si-C bonding state using X-ray photoelectron spectroscopy (XPS). However, the resolution of the measuring device, such as a transmission electron microscope, must be considered. 【0098】 The surface roughness (Ra) of the first single crystal substrate layer 11 is preferably 10 nm or less, more preferably 0.5 nm or less, and even more preferably 0.3 nm or less. The surface roughness (Ra) of the first single crystal substrate layer 11 is the surface roughness (Ra) of the first surface 11A of the first single crystal substrate layer 11. 【0099】 The surface roughness (Ra) of the second single crystal substrate layer 12 is preferably 10 nm or less, more preferably 0.5 nm or less, and even more preferably 0.3 nm or less. The surface roughness (Ra) of the second single crystal substrate layer 12 is the surface roughness (Ra) of the second surface 12B of the second single crystal substrate layer 12. 【0100】 If the surface roughness of the first single-crystal substrate layer 11 and the second single-crystal substrate layer 12 is small, the probability of voids being introduced into the bonding interface when the two substrates are bonded together can be reduced. 【0101】The composite substrate 1 has, for example, a SORI of 40 μm or less. Preferably, the SORI of the composite substrate 1 is 35 μm or less, more preferably 30 μm or less, and even more preferably 20 μm or less. A composite substrate 1 with a small SORI is less prone to warping even when processed (for example, by depositing an epitaxial layer) on the composite substrate 1. 【0102】 SORI is one of the parameters that indicates the tendency of a substrate to warp. SORI is expressed as the sum of the normal distances from the lowest squares plane, which is calculated using the least squares method with all data on the substrate surface, when the back surface of the substrate is supported and measured without changing its original shape. Substrates with a large SORI are prone to warping during processing. Substrates with a small SORI are less prone to warping even after processing. 【0103】 Figure 7 is a schematic diagram illustrating the definition of SORI. As shown in Figure 7, when the least-squares plane S1 on the substrate surface is taken as the reference height (least-squares plane height), SORI represents the sum of the distance (a) between the height at the highest point HP on the substrate surface and the reference height, and the distance (b) between the height at the lowest point LP and the reference height. 【0104】 Next, a method for manufacturing the composite substrate 1 according to this embodiment will be described. Figure 8 is a diagram illustrating the method for manufacturing the composite substrate 1 according to this embodiment. The method for manufacturing the composite substrate 1 includes, for example, a first single crystal preparation step, a second single crystal preparation step, a bonding step, and a separation step. 【0105】 In the first single crystal preparation step, a first single crystal, which will become the first single crystal substrate layer 11 described above, is prepared. The first single crystal is a single crystal of SiC. The first single crystal may be prepared by sublimation, gas method, or solution method. The first single crystal substrate layer 11 is obtained by slicing the first single crystal to a desired thickness. The first single crystal is sliced ​​so that the total thickness of the composite substrate 1 is within a desired range. The slicing of the first single crystal can be done by known methods such as a wire saw. 【0106】In the second single crystal preparation step, a second single crystal, which will become the second single crystal substrate layer 12 described above, is prepared. The second single crystal is a single crystal of SiC. The second single crystal may be produced by sublimation, gas method, or solution method. The second single crystal has a lower threading dislocation density than the first single crystal, and it is preferable to produce it by sublimation using a seed crystal with few dislocation defects. For example, a second single crystal with a low threading dislocation density can be produced using a seed crystal produced by the RAF (Repeated a-face method), for example. Even when using a seed crystal with a high threading dislocation density, a second single crystal with a low threading dislocation density can be produced by annihilating nearby dislocations through crystal growth of sufficient length. 【0107】 A second single crystal substrate 13 is obtained by slicing the second single crystal to a desired thickness. The second single crystal can be sliced ​​using known methods such as a wire saw. 【0108】 When preparing the first single-crystal substrate layer 11 and the second single-crystal substrate 13, the dislocation density distribution of each is also confirmed. The dislocation density distribution can be determined, for example, by image analysis of an X-ray topographic image. Dislocation density is the density of dislocations including basal plane dislocations and through dislocations. By measuring the dislocation density distribution, the coordinates of the high-dislocation density region and the low-dislocation density region in the first single-crystal substrate layer 11 and the coordinates of the high-dislocation density region and the low-dislocation density region in the second single-crystal substrate 13 can be determined. 【0109】 Next, ions are implanted into the second single-crystal substrate 13. The ions to be implanted are, for example, hydrogen ions. By keeping the ion implantation energy constant, ion implantation regions (IPs) can be formed at the same height. By changing the ion implantation energy, the position where the ion implantation regions (IPs) are formed can be freely controlled. For example, the ion implantation regions (IPs) are formed at a position between 50 μm and 300 μm in the thickness direction from one surface of the second single-crystal substrate 13. 【0110】In the bonding process, the first single-crystal substrate layer 11 and the second single-crystal substrate 13 are bonded together. When bonding the first single-crystal substrate layer 11 and the second single-crystal substrate 13, the dislocation density distribution of each is checked before bonding. The first single-crystal substrate layer 11 and the second single-crystal substrate 13 are bonded together in such a way that stress does not concentrate in one direction after bonding. For example, at the bonding surface between the first single-crystal substrate layer 11 and the second single-crystal substrate 13, the high dislocation density region of the first single-crystal substrate layer 11 and the high dislocation density region of the second single-crystal substrate 13 are bonded together in such a way that they do not overlap. 【0111】 For example, in the example shown in Figure 3, by bonding a first single-crystal substrate layer 11, in which the dislocation density is high near the center and low near the outer edge, with a second single-crystal substrate 13, in which the dislocation density is low near the center and high near the outer edge, the difference in dislocation density distribution at the bonding surface is reduced. 【0112】 For example, in the example shown in Figure 4, by bonding a first single-crystal substrate layer 11, in which the dislocation density is high near the outer periphery and low near the center, with a second single-crystal substrate 13, in which the dislocation density is low near the outer periphery and high near the center, the difference in dislocation density distribution at the bonding surface is reduced. 【0113】 For example, in the example shown in Figure 5, by bonding a first single-crystal substrate layer 11, which has a high dislocation density in the -X direction and a low dislocation density in the +X direction, with a second single-crystal substrate 13, which has a low dislocation density in the -X direction and a high dislocation density in the +X direction, the difference in dislocation density distribution at the bonding surface is reduced. 【0114】 For example, in the example shown in Figure 6, by bonding a first single-crystal substrate layer 11, in which the dislocation density is high at both ends in the ±X direction, with a second single-crystal substrate 13, in which the dislocation density is high in the ±Y direction, the direction in which stress occurs is dispersed in the X and Y directions. 【0115】 The first single-crystal substrate layer 11 and the second single-crystal substrate 13 can be bonded together by activating their bonding surfaces and then stacking them and applying pressure. The bonding surfaces can be activated, for example, by irradiating them with Ar ions. By performing the bonding process, a bonded body is obtained in which the first single-crystal substrate layer 11 and the second single-crystal substrate 13 are joined together. 【0116】In the separation process, a portion of the second single-crystal substrate 13 is separated from the bonded body. The second single-crystal substrate 13 is separated in the thickness direction. For example, by heating the bonded body, the second single-crystal substrate 13 is separated along the ion implantation region IP. By separating the second single-crystal substrate 13 in the thickness direction, a thin film of the second single-crystal substrate layer 12 is obtained. 【0117】 By going through the process described above, a composite substrate 1 is obtained in which a second single crystal substrate layer 12 is bonded to a first single crystal substrate layer 11. 【0118】 In this embodiment, the composite substrate 1 is bonded so that the first high dislocation density region and the second high dislocation density region do not overlap, thereby suppressing localized increases in dislocation density distribution. When the difference in dislocation density distribution within the plane is small, it is possible to suppress the concentration of internal stress in one direction in the composite substrate 1. As a result, the composite substrate 1 in this embodiment has a small SORI and is less prone to warping even after processing. A composite substrate 1 that is less prone to warping is less likely to cause defects during device manufacturing and improves the yield of devices. 【0119】 Single-crystal substrates become more difficult to produce stably and more expensive as their thickness increases. Bonding multiple substrates together can reduce the cost of the substrate. Furthermore, producing thin, warp-resistant substrates inexpensively can lower the price of the device. 【0120】 "Epitaxial Wafer" Figure 9 is a cross-sectional view of an epitaxial wafer 2 according to this embodiment. The epitaxial wafer 2 comprises a composite substrate 1 and an epitaxial layer 21. The composite substrate 1 is as described above. 【0121】 The epitaxial wafer 2 has a SORI of, for example, 50 μm or less. More preferably, the SORI of the epitaxial wafer 2 is 42 μm or less, even more preferably 35 μm or less, and particularly preferably 20 μm or less. Epitaxial wafers 2 with a small SORI are less prone to warping even when processed (e.g., device formation) on the epitaxial wafer 2. 【0122】The epitaxial layer 21 is formed on the second single-crystal substrate layer 12 of the composite substrate 1. The epitaxial layer 21 is, for example, SiC. The epitaxial layer 21 may be undoped SiC or dopant-doped SiC. The epitaxial layer 21 is preferably, for example, nitrogen-doped n-type SiC. 【0123】 The thickness of the epitaxial layer 21 is, for example, 3 μm or more, preferably 5 μm or more, more preferably 10 μm or more, and even more preferably 15 μm or more. The thickness of the epitaxial layer 21 may also be, for example, 200 μm or less. 【0124】 The thickness of the epitaxial layer 21 is the average value of the thickness of the epitaxial layer 21 measured at different points in the radial direction of the epitaxial layer 21. The measurement points for the thickness of the epitaxial layer 21 are the same as the measurement points for the thickness of the composite substrate 1. The thickness of the epitaxial layer 21 can be determined, for example, by optical interferometry using FTIR (Fourier Transform Infrared Spectrophotometer). 【0125】 The epitaxial wafer 2 can be fabricated by depositing an epitaxial layer 21 onto the composite substrate 1 described above. The deposition of the epitaxial layer 21 can be carried out by known methods. 【0126】 Because the epitaxial wafer 2 according to this embodiment has the composite substrate 1 described above, it is less prone to warping even after processing. The warp-resistant epitaxial wafer 2 is less prone to defects during device manufacturing and improves the yield of devices. 【0127】 "Device" Figure 10 is a cross-sectional view of device 3 according to this embodiment. Device 3 is formed by creating a chip from an epitaxial wafer 2 by forming an element 34 on the epitaxial layer 21 of the epitaxial wafer 2. 【0128】 Device 3 includes, for example, a first single-crystal substrate layer 31, a second single-crystal substrate layer 32, an epitaxial layer 33, and an element 34. 【0129】The first single crystal substrate layer 31 is a chip made from the first single crystal substrate layer 11 described above. The first single crystal substrate layer 31 may also be formed by slimming the back surface of the first single crystal substrate layer 11 to create a thin film. 【0130】 The second single-crystal substrate layer 32 is a chipped version of the second single-crystal substrate layer 12 described above. The second single-crystal substrate layer 32 is equivalent to the second single-crystal substrate layer 12, except that it is a chipped version. 【0131】 The epitaxial layer 33 is a chip-formed version of the epitaxial layer 21 described above. The epitaxial layer 33 is equivalent to the epitaxial layer 21, except that it is chip-formed. 【0132】 The element 34 is formed in the epitaxial layer 33. The element is a combination of, for example, a transistor, capacitor, inductor, resistor, wiring, etc. In Figure 10, a transistor is shown as an example of the element 34. 【0133】 Device 3 can be fabricated, for example, by performing an element formation process, a slimming process, and a chip manufacturing process. 【0134】 In the element formation process, an element 34 is formed on the epitaxial layer 21. The element 34 can be formed by a known method. 【0135】 In the slimming process, at least a portion of the first single-crystal substrate layer 11 of the composite substrate 1 is removed. In the slimming process, the first single-crystal substrate layer 11 of the composite substrate 1 may be completely removed. The first single-crystal substrate layer 11 can be made of a material that is less expensive than the second single-crystal substrate layer 12. By using the composite substrate 1, the expensive and high-quality second single-crystal substrate layer 12 is not wasted, and the portion that is removed can be made of the inexpensive first single-crystal substrate layer 11. 【0136】 In the chipping process, the epitaxial wafer 2 is chipped for each element 34. By chipping, multiple devices 3 can be obtained from the epitaxial wafer 2. 【0137】 Since the device 3 according to this embodiment is manufactured using an epitaxial wafer 2 that is less prone to warping, defects are less likely to occur during manufacturing. 【0138】 While preferred embodiments of this disclosure have been described in detail above, this disclosure is not limited to any particular embodiment, and various modifications and changes are possible within the scope of the gist of this disclosure as described in the claims. 【0139】 For example, in the above embodiment, the case in which both the composite substrate 1 and the epitaxial layer 21 are made of SiC is illustrated, but the elements constituting them are not limited to SiC. For example, semiconductors such as GaN and GaInAs may be used for the composite substrate 1 and the epitaxial layer 21. Furthermore, the first single crystal substrate layer 11 and the second single crystal substrate layer 12 may be made of different elements. 【0140】 "Example 1" A first single-crystal substrate layer 11 was cut from a SiC single crystal with a diameter of 200 mm (8 inches). The thickness of the first single-crystal substrate layer 11 was 305 μm. The penetration dislocation density of the first single-crystal substrate layer 11 was 240 dislocations / cm². －２ The dopant concentration of the first single-crystal substrate layer 11 was 9 × 10⁻⁶. １８ atoms / cm ３ That was the case. 【0141】 Next, a second single-crystal substrate 13 was cut from a separately prepared SiC single crystal with a diameter of 200 mm (8 inches). An ion implantation region IP was formed inside the second single-crystal substrate 13 using an ion implantation method. Then, the first single-crystal substrate layer 11 and the second single-crystal substrate 13 were bonded together. When bonding the first single-crystal substrate layer 11 and the second single-crystal substrate 13, the dislocation density distribution of each bonding surface was measured to ensure that high-dislocation-density regions did not face each other. 【0142】 Next, a portion of the second single-crystal substrate 13 was separated to obtain the second single-crystal substrate layer 12. The thickness of the second single-crystal substrate layer 12 was 50 μm. The main surface of the second single-crystal substrate layer 12 was measured using X-ray topography to determine the threading dislocation density of the second single-crystal substrate layer 12. The threading dislocation density of the second single-crystal substrate layer 12 was 200 dislocations / cm². －２ The dopant concentration of the second single-crystal substrate layer 12 was 5 × 10⁻⁶. １７ atoms / cm ３ That was the case. 【0143】 The total thickness of the composite substrate 1 after bonding was 355 μm. The SORI of the surface of the second single-crystal substrate layer 12 was measured using the first single-crystal substrate layer 11 side of the composite substrate 1 as the support surface. The SORI of the composite substrate 1 was 20 μm. 【0144】 An epitaxial layer 21 was grown on the surface of the first single-crystal substrate layer 11 of the composite substrate 1. The epitaxial layer 21 is SiC containing nitrogen as a dopant. The dopant concentration of the epitaxial layer 21 is 5 × 10⁻¹⁶. １６ atoms / cm ３ The thickness of the epitaxial layer was set to 10 μm. The SORI of the surface of the epitaxial layer 21 was measured using the first single crystal substrate layer 11 side of the epitaxial wafer 2 as the support surface. The SORI of the epitaxial wafer 2 was 35 μm. 【0145】 "Comparative Example 1" Comparative Example 1 differs from Example 1 in that when bonding the first single crystal substrate layer 11 and the second single crystal substrate layer 12, the high dislocation density regions are bonded together so that they face each other. Other conditions are the same as in Example 1, and the same measurements were performed as in Example 1. 【0146】 The SORI of the composite substrate in Comparative Example 1 was 35 μm. The SORI of the epitaxial wafer in Comparative Example 1 was 55 μm. 【0147】 Comparing Example 1 with Comparative Example 1, the SORI of the composite substrate and epitaxial wafer of Example 1 was smaller than that of the composite substrate and epitaxial wafer of Comparative Example 1. In other words, the composite substrate and epitaxial wafer of Example 1 are less prone to warping during processing than the composite substrate and epitaxial wafer device of Comparative Example 1. 【0148】 Similar experiments were also conducted using a substrate with a diameter of 150 mm (6 inches), and the same results were obtained. 【0149】1. Composite substrate 2. Epitaxial wafer 3. Device 11, 31. First single-crystal substrate layer 12, 32. Second single-crystal substrate layer 13. Second single-crystal substrate 21, 33. Epitaxial layer 34. Element 111, 121. Central region 112, 122. Outer region 113, 123. First region 114, 124. Second region 115, 125. Third region 116, 126. Fourth region HP: Highest point IP: Ion implantation region LP: Lowest point n: Notch

Claims

1. A composite substrate comprising a first single-crystal substrate layer and a second single-crystal substrate layer bonded to the first single-crystal substrate layer, wherein the first single-crystal substrate layer has a first high dislocation density region, and the second single-crystal substrate layer has a second high dislocation density region, and the first high dislocation density region and the second high dislocation density region do not overlap in a plan view from the thickness direction; the dislocation density of the first high dislocation density region is five times or more higher than the average dislocation density of the first single-crystal substrate layer in a plane viewed from the thickness direction, and the dislocation density of the second high dislocation density region is five times or more higher than the average dislocation density of the second single-crystal substrate layer in a plane viewed from the thickness direction.

2. The composite substrate according to claim 1, wherein the first high dislocation density region is located in the central region, the second high dislocation density region is located in the outer peripheral region, the central region is a region within 50% of the diameter from the center, and the outer peripheral region is a region radially outward from the central region.

3. The composite substrate according to claim 1, wherein the second high dislocation density region is located in the central region, the first high dislocation density region is located in the outer peripheral region, the central region is a region within 50% of the diameter from the center, and the outer peripheral region is a region radially outward from the central region.

4. The composite substrate according to claim 1, wherein when two regions are defined as a first region and a second region, with respect to a first straight line passing through the center, the first high dislocation density region is located in the first region and the second high dislocation density region is located in the second region.

5. The composite substrate according to claim 1, wherein two regions flanking a first straight line passing through the center are defined as the first region and the second region, and two regions flanking a second straight line perpendicular to the first line and passing through the center are defined as the third region and the fourth region, the first high dislocation density region is located in each of the first region and the second region, and the second high dislocation density region is located in each of the third region and the fourth region.

6. The composite substrate according to claim 1, wherein the first single-crystal substrate layer has a first low dislocation density region in a plane viewed from the thickness direction, where the dislocation density is one-fifth or less of the average dislocation density of the first single-crystal substrate layer; the second single-crystal substrate layer has a second low dislocation density region in a plane viewed from the thickness direction, where the dislocation density is one-fifth or less of the average dislocation density of the second single-crystal substrate layer; and in a plane viewed from the thickness direction, the first high dislocation density region overlaps with at least a part of the second low dislocation density region, and the second high dislocation density region overlaps with at least a part of the first low dislocation density region.

7. The composite substrate according to claim 1, wherein two regions flanking a first straight line passing through the center are designated as the first region and the second region, and two regions flanking a second straight line perpendicular to the first line and passing through the center are designated as the third region and the fourth region, and when the in-plane distribution of the total dislocations, including the dislocations of the first single-crystal substrate layer and the dislocations of the second single-crystal substrate layer, is evaluated, the difference between the dislocation density of the total dislocations in the first region and the dislocation density of the total dislocations in the second region is 5% or less of the average dislocation density of the total dislocations, and the difference between the dislocation density of the total dislocations in the third region and the dislocation density of the total dislocations in the fourth region is 5% or less of the average dislocation density of the total dislocations.

8. The composite substrate according to claim 1, wherein the through-dislocation density of the first single-crystal substrate layer is higher than the through-dislocation density of the second single-crystal substrate layer.

9. The composite substrate according to claim 1, wherein the through-dislocation density of the first single-crystal substrate layer is 1.5 times or more and 110 times or less than the through-dislocation density of the second single-crystal substrate layer.

10. The composite substrate according to claim 1, wherein a bonding layer is provided between the first single-crystal substrate layer and the second single-crystal substrate layer.

11. The composite substrate according to claim 1, wherein the SORI is 40 μm or less.

12. An epitaxial wafer comprising a composite substrate according to claim 1, and an epitaxial layer laminated on the second single-crystal substrate layer of the composite substrate.

13. The epitaxial wafer according to claim 12, wherein the SORI is 50 μm or less.

14. A method for manufacturing a composite substrate, comprising: a first preparation step, a second preparation step, a bonding step, and a separation step, wherein in the first preparation step, a first single-crystal substrate layer is prepared and a first high-dislocation-density region is identified in the first single-crystal substrate layer where the dislocation density is five times or more higher than the average dislocation density; in the second preparation step, a second single-crystal substrate is prepared and a second high-dislocation-density region is identified in the second single-crystal substrate where the dislocation density is five times or more higher than the average dislocation density; in the bonding step, the first single-crystal substrate layer and the second single-crystal substrate are bonded together such that the first high-dislocation-density region and the second high-dislocation-density region do not overlap in a plan view; and in the separation step, a portion of the second single-crystal substrate is separated in the thickness direction to produce a second single-crystal substrate layer.

15. A method for manufacturing an epitaxial wafer, comprising the steps of: preparing a composite substrate according to claim 1; and forming an epitaxial layer on the second single-crystal substrate layer of the composite substrate.

16. A method for manufacturing a device, comprising the steps of: preparing an epitaxial wafer according to claim 12; and forming an element on the epitaxial layer to form a chip.

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