Graphite Crucible

Abstract

The goal is to improve the quality of the grown crystals by eliminating as much as possible the possibility of introducing heteromorphisms and defects into the SiC single crystals being manufactured. [Solution] A graphite crucible for growing a SiC single crystal on the first main surface of a seed crystal substrate by sublimation recrystallization, wherein the graphite crucible is provided with a base that protrudes inward from the housing of the graphite crucible, the base having a mounting surface for attaching the second main surface of the seed crystal substrate and a side surface in contact with the periphery of the mounting surface, and the base has heat dissipation characteristics such that the amount of heat dissipated upward through the peripheral portion in contact with the side surface is smaller than the amount of heat dissipated upward through the central portion surrounded by the peripheral portion.

Description

Technical Field

[0001] The present invention relates to a graphite crucible.

Background Art

[0002] A method for manufacturing a SiC single crystal ingot by sublimation recrystallization is widely known. For example, in Patent Document 1, raw material powder obtained by heating and synthesizing a carbon raw material and a silicon raw material is placed in a graphite crucible, the graphite crucible is heated to sublime silicon carbide, and a SiC single crystal is grown on a SiC seed crystal substrate to manufacture a SiC single crystal ingot.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Heterogeneous polytypes and defects may be introduced into SiC single crystal ingots manufactured by sublimation recrystallization using a graphite crucible. Since SiC single crystal ingots are used as substrates for electronic devices and the like, a high-quality single crystal ingot that excludes heterogeneous polytypes and defects as much as possible is required by the market. Therefore, an object of the present invention is to eliminate the possibility of introducing heterogeneous polytypes and defects as much as possible and improve the quality of the grown crystal.

Means for Solving the Problems

[0007] During the crystal growth process, crystals growing from multiple starting points may collide with each other. Figure 13B shows a grown crystal that has grown further from the state shown in Figure 13A. The portion that has grown further from Figure 13A is not shown with hatched lines. The crystal grows from the edge 11e toward the center along the arrow 11d. The surface 11c shown in Figure 13B is an example of a surface where crystals growing from multiple starting points have collided. A mismatch in the crystal lattice occurs at surface 11c. Surface 11c then becomes the starting point for the mismatch in the crystal lattice that causes heteromorphisms and defects.

[0008] In contrast, when the shape of the growing crystal is convex downwards, the most protruding part of the growing crystal is limited to a single point. As will be explained in detail later, the point labeled "11p" in Figure 3 is the most protruding part of the growing crystal that is convex downwards. Point 11p, the most protruding part, becomes the starting point of growth, and the crystal grows in various directions from this starting point. Unlike when the growing crystal has a concave shape downwards, since there is only one starting point for growth, collisions between growing crystals are suppressed, and as a result, heteromorphisms and defects are less likely to occur.

[0009] As described above, the possibility of introducing heteromorphisms or defects varies depending on the shape of the grown crystal. To reduce the possibility of heteromorphisms or defects being introduced, it is desirable to obtain a grown crystal that exhibits a downward-convex shape.

[0010] As will be explained in more detail later, the shape of the isothermal surface inside the crucible when growing crystals resembles the shape of the grown crystal. In other words, if the shape of the isothermal surface inside the crucible is convex downwards when growing crystals, a grown crystal exhibiting a convex downwards shape can be obtained. Therefore, we decided to design a graphite crucible in which the isothermal surface is convex downwards. The inventors focused on the heat dissipation characteristics of the base, which is a part of the graphite crucible, and completed the following graphite crucible.

[0011] The present invention relates to a graphite crucible for growing a SiC single crystal on the first main surface of a seed crystal substrate by sublimation recrystallization, The graphite crucible is provided with a base that protrudes inward from the housing of the graphite crucible, The base has a mounting surface for attaching the second main surface of the seed crystal substrate and a side surface that is in contact with the periphery of the mounting surface. The base has heat dissipation characteristics such that the amount of heat dissipated upward through the surrounding portion in contact with the side surface is less than the amount of heat dissipated upward through the central portion surrounded by the surrounding portion.

[0012] In this specification, the principal surface of the seed crystal substrate refers to the surface having a larger area than other surfaces (the side surfaces of the seed crystal substrate). On the seed crystal substrate, the surface exposed to the internal space of the graphite crucible and on which the SiC single crystal is grown is called the first principal surface, and the surface attached to the base is called the second principal surface. The mounting surface of the base is the surface on the base to which the seed crystal substrate is attached. The mounting surface is the flat surface on the bottom of the base. The side surface of the base is the surface located radially outward of the base. As will be described later, the base may be shown divided into multiple parts, in which case the side surface of the base exists across the multiple divided parts.

[0013] This specification explains the terms heat removal, heat removal amount, and heat removal characteristics used herein. Heat removal refers to the phenomenon in which heat is transported from the main surface of the seed crystal substrate through the base to the outside of the graphite crucible. Heat removal amount refers to the amount of heat transported from the main surface of the seed crystal substrate through the base to the outside of the graphite crucible, that is, the amount of heat removed from inside the graphite crucible to the outside. Heat removal characteristics refer to the properties related to the magnitude of the heat removal amount. This specification describes the heat removal characteristics of a base on which a seed crystal substrate can be attached. The presence of heat removal characteristics in the base means that the temperature of the main surface of the seed crystal substrate and the growth crystal formed on that main surface can be maintained at a lower temperature than the temperature of the sublimation gas inside the graphite crucible, which contributes to the recrystallization of the sublimation gas.

[0014] Therefore, the temperature distribution of the base in contact with the seed crystal substrate within the graphite crucible is important. An isothermal surface serves as an indicator of the temperature distribution of the base within the graphite crucible. As mentioned above, the amount of heat removed from the base differs between the peripheral and central parts. This local difference in heat removal from the base affects the temperature distribution of the base, and consequently, the temperature distribution within the crucible, including the seed crystal substrate fixed to the base. The temperature distribution within the crucible is represented as an isothermal surface within the crucible. In the case of the graphite crucible described above, the amount of heat removed is relatively small in the peripheral part and relatively large in the central part. As a result, the surface of the seed crystal substrate becomes colder in the central part and hotter towards the periphery of the seed crystal substrate, forming a downward-convex isothermal surface.

[0015] One method for locally adjusting the heat dissipation characteristics of the base is to create a gap in a part of the base. The gap needs to be designed to facilitate the formation of downward-convex growth crystals. For example, the base may have a shape in which the surrounding part has a gap extending from the side and the central part does not have a gap. A base with such a shape may be designed. Additional features of the base will be described below, but these descriptions are intended to indicate that a base with additional features may be designed.

[0016] The gap closer to the side surface may be larger than the gap closer to the central portion. Thereby, the heat extraction amount at the position closer to the side surface becomes smaller than that at the position closer to the central portion, making it easier to form a downwardly convex grown crystal.

[0017] The gap may include a portion where the gap becomes larger as it approaches the side surface. Thereby, the heat extraction amount becomes smaller as it approaches the side surface, making it easier to form a downwardly convex grown crystal.

[0018] The gap can be formed in various ways. One method is to configure the pedestal from two members and provide a gap between the two members. For example, the pedestal may include a mounting portion for attaching the seed crystal substrate and a base portion for supporting the mounting portion, and the mounting portion and the base portion may be configured to be separable.

[0019] The gap may be provided between the mounting portion and the base portion.

[0020] The mounting portion may be in a flat plate shape. In this specification, when there is a plate-shaped object, if the local plate thickness difference of the object is less than 20 μm, the object is regarded as "flat plate-shaped", and the plate thickness difference of less than 20 μm is considered a manufacturing error.

[0021] The maximum dimension of the gap in the normal direction of the bottom surface may be 500 μm or less.

Advantages of the Invention

[0022] Thereby, it is possible to eliminate as much as possible the possibility of introducing different polytypes and defects in the manufactured SiC single crystal, and improve the quality of the grown crystal.

Brief Description of the Drawings

[0023] [Figure 1] It is a cross-sectional view of the first embodiment of the graphite crucible. [Figure 2A] It is an enlarged cross-sectional view of the vicinity of the pedestal of the graphite crucible in FIG. 1. [Figure 2B] This is a diagram for explaining the details of the pedestal in FIG. 1. [Figure 3] This is a diagram used to explain the method for measuring a growing crystal. [Figure 4] This is a diagram showing a first modification example of the pedestal. [Figure 5] This is a diagram showing a second modification example of the pedestal. [Figure 6] This is a diagram showing a third modification example of the pedestal. [Figure 7] This is a diagram showing a fourth modification example of the pedestal. [Figure 8] This is a diagram showing a fifth modification example of the pedestal. [Figure 9] This is a diagram showing a sixth modification example of the pedestal. [Figure 10] This is a diagram showing a seventh modification example of the pedestal. [Figure 11] This is a diagram showing an eighth modification example of the pedestal. [Figure 12] This is an enlarged cross-sectional view near the pedestal of the second embodiment of the graphite crucible. [Figure 13A] This is a cross-sectional view showing a growing crystal with a concave shape downward. [Figure 13B] This is a diagram showing a growing crystal that has grown further from the state in FIG. 13A.

Embodiments for Carrying Out the Invention

[0024] Hereinafter, the description will be made with reference to the drawings as appropriate. Note that each drawing disclosed in this specification is schematically illustrated. That is, the dimensional ratios on the drawings do not necessarily match the actual dimensional ratios, and the dimensional ratios also do not necessarily match between the drawings.

[0025] In this specification, the XYZ coordinate system will be described with appropriate reference. In this specification, when expressing directions, positive and negative directions are distinguished, and are indicated with a sign, such as "+X direction" and "-X direction". When directions are expressed without distinguishing between positive and negative directions, they are simply described as "X direction". That is, in this specification, when simply described as "X direction", both "+X direction" and "-X direction" are included. The same applies to the Y direction and Z direction. In this embodiment, the X direction and Y direction are horizontal directions, and the -Z direction is the direction of gravity.

[0026] <First Embodiment> [Overview of Graphite Crucibles] A graphite crucible of the first embodiment will be described. The graphite crucible 100 is used to grow a SiC single crystal by sublimation recrystallization. Figure 1 is a cross-sectional view of the graphite crucible 100 of this embodiment. As shown in Figure 1, the graphite crucible 100 has a container 1 having an opening in the upward direction (+Z direction), a lid 3 that closes the opening of the container 1 and forms an internal space partitioned from the outside, and a base 4 for mounting the seed crystal substrate 10. In Figure 1, the raw material 2 and the seed crystal substrate 10 are located inside the graphite crucible 100, but the raw material 2 and the seed crystal substrate 10 are not components of the graphite crucible 100. Figure 1 shows the growth crystal 11 that may be formed on the seed crystal substrate 10 by a dashed line. In this specification, the expression "may be formed on the seed crystal substrate 10" does not mean "may be formed on the +Z side of the seed crystal substrate 10," as shown in Figure 1, but rather "may be formed in contact with the main surface of the seed crystal substrate 10." The curve 5 drawn with a dashed line on the -Z side of the grown crystal 11 represents an isothermal surface. In a cross-sectional view like Figure 1, the isothermal surface appears as an isotherm. Details of the isothermal surface will be described later.

[0027] Container 1 is a hollow cylindrical shape, and the raw material 2 is contained within it. The raw material 2 is single-crystal or polycrystalline SiC powder. The raw material 2 may also be a mixture of silicon powder and carbon powder. Container 1 in this embodiment consists of a bottom on the -Z side, a top on the +Z side, and a side located between the bottom and top. The side extends upward from the periphery of the bottom to the periphery of the top. The bottom is circular. The top is circular and has a through hole into which a lid 3 can be fitted.

[0028] The outer circumference 3e of the lid 3 and the inner surface 1i of the through hole provided at the top of the container 1 are both threaded and fitted together. The lid 3 is attached to the container 1 by rotating it and screwing it into the through hole provided in the container 1. When the lid 3 is attached to the container 1, a closed space is formed inside the graphite crucible 100, separated from the outside of the graphite crucible 100.

[0029] A base 4 is fixed to the lid 3, and the base 4 protrudes from the inner wall of the lid 3. The lid 3 and the base 4 may be integrally formed. The shapes of the container 1 and lid 3 shown in this embodiment are examples, and other shapes are also possible. For example, the entire top of the container 1 may be composed of the lid 3, and the lid 3 may be connected to the side of the container 1. Alternatively, a part of the side of the container 1 may be composed of the lid 3, and the part of the side that constitutes the lid 3 may be connected to the side of the container 1. The bottom and top of the container 1, and the lid 3, do not have to be circular in shape; for example, they may be square or hexagonal. The container 1 has a columnar shape with a constant size in the Z direction, but it may have a shape where the size is not constant in the Z direction. The lid 3 does not have to be a shape that fits onto the container 1.

[0030] [Overview of the base] The outline of the base 4 will be described with reference to Figures 1 and 2A. Figure 2A is an enlarged cross-sectional view of the base 4 fixed to the lid 3 and the area around the base 4. In this embodiment, the base 4 is divided into a +Z side component and a -Z side component. The +Z side component is the base 4a. The base 4a is closer to the lid 3 and supports the mounting portion 4b. The -Z side component is the mounting portion 4b for attaching the seed crystal substrate 10. The mounting portion 4b is further from the lid 3 than the base 4a. The base 4a and the mounting portion 4b are configured to be separable from each other. Note that Figure 2A shows the seed crystal substrate 10 attached to the mounting portion 4b, but does not show the growth crystal 11 and isothermal surface 5 formed on the seed crystal substrate 10.

[0031] Both the base 4a and the mounting portion 4b are rotating bodies centered on an axis along the Z-axis. As shown in Figure 2A, the base 4 can be distinguished into a central portion Ce including the central axis and a ring-shaped peripheral portion Ci located radially outside the central portion Ce. When using the graphite crucible 100, the base portion 4a of the central portion Ce of the base 4 is positioned so as to be in contact with the mounting portion 4b. On the other hand, in the peripheral portion Ci of the base 4, the base portion 4a does not contact the mounting portion 4b, and there is a gap G1 between the base portion 4a and the mounting portion 4b. The gap G1 is configured to extend from the peripheral side surface of the base 4 toward the radial center of the container 1. In the peripheral portion Ci, the gap G1 becomes larger as it approaches the side surface of the base 4. When using the graphite crucible 100, the shape and dimensions of the gap G1 do not change and always maintain the same shape and size. However, this change in shape and dimensions excludes the effects of thermal expansion and contraction.

[0032] The gap G1 in the base 4 creates differences in localized heat dissipation characteristics within the base 4, allowing the isothermal surface 5 to be adjusted. The reason for this is explained below. When the graphite crucible 100 is heated in the heating furnace, the internal space of the graphite crucible 100 rises in temperature. At this time, the base 4, which protrudes into the internal space, transfers the heat that the seed crystal substrate 10 in contact with the base 4 receives from the internal space to the lid 3 due to its heat dissipation characteristics. The heat transferred to the lid 3 is then released upwards from the graphite crucible. In other words, the base 4 has a heat dissipation function that removes heat from inside the graphite crucible 100 upwards from the graphite crucible 100.

[0033] The base 4 has a heat dissipation path extending along the Z direction. However, the ease with which heat escapes through the heat dissipation path is not uniform within the base 4. The ease with which heat escapes through the heat dissipation path differs between the peripheral part Ci, where there is a gap G1 in the middle of the heat dissipation path, and the central part Ce, where there is no gap G1 (or the gap G1 is so small that it can be said to be virtually nonexistent). The amount of heat dissipated through the heat dissipation path passing through the central part Ce of the base 4 is large, while the amount of heat dissipated through the peripheral part Ci, where the gap G1 exists, is small. The reason why the amount of heat dissipated differs depending on the presence or absence of the gap G1 and the size of the gap G1 is that heat is not easily transferred through the gap G1. Within the peripheral part Ci, the gap G1 becomes larger as you get closer to the side, so the amount of heat dissipated in the peripheral part Ci decreases as you get closer to the side of the base 4.

[0034] In Figure 2A, the arrows labeled Q1 to Q4 indicate the magnitude and direction of heat removal. A large amount of heat removal, Q1, is obtained in the central Ce portion where the heat removal is high. Then, as you move from the central Ce portion closer to the side of the base 4, the amount of heat removal gradually decreases to Q2, Q3, and Q4. A large amount of heat removal means that the cooling effect of the seed crystal substrate 10 in the graphite crucible 100 is high, and the temperature decrease of the seed crystal substrate 10 also means that the isothermal surface 5 is partially directed further inward (i.e., in the -Z direction) of the graphite crucible 100. Therefore, as shown in Figure 1, in the central Ce portion where the heat removal effect is high and the temperature decreases easily, the isothermal surface 5 passes further inward (i.e., towards the -Z side) of the graphite crucible 100, while near the side of the peripheral Ci portion where the heat removal effect is low and the temperature does not decrease easily, the isothermal surface 5 passes closer to the seed crystal substrate 10. As a result, the isothermal surface 5 takes on a convex shape downwards, as shown in Figure 1.

[0035] One of the factors that determines the amount of crystal growth is temperature. The amount of crystal growth changes with temperature. The lower the temperature of the surface of the seed crystal substrate 10, the more easily the sublimation gas is cooled and recrystallized, resulting in a larger amount of crystal growth per unit time. When the supply of sublimation gas to the crystal growth area is sufficient and uniform, the crystal grows as each part of the growth surface competes for the sublimation gas that reaches it, so that the crystal growth surface becomes isothermal. As a result, the shape of the grown crystal 11 resembles the shape of the isothermal surface 5. For example, if the isothermal surface 5 is concave downwards, the grown crystal 11 will also be concave downwards, and as shown in Figure 1, if the isothermal surface 5 is convex downwards, the grown crystal 11 will also be convex downwards.

[0036] As described above, when the shape of the growing crystal 11 is concave downwards, multiple growth starting points are generated. From each of these starting points, crystals grow in various directions. Consequently, during the growth process, crystals growing in diverse directions from multiple starting points may collide with each other. As a result, heteromorphisms and defects are more likely to occur. On the other hand, when the shape of the growing crystal is convex downwards, the growing crystal protrudes most in the central region. As described above, when the shape of the growing crystal is convex downwards, the growth starting point is the single central point that protrudes the most, and crystals grow in various directions, i.e., radially, from this central point. Consequently, during the growth process, collisions between crystals from multiple locations and multiple directions are reduced. As a result, heteromorphisms and defects are less likely to occur.

[0037] As described above, in the central part Ce, the base 4a is in contact with the mounting part 4b, while in the surrounding part Ci, the base 4a is not in contact with the mounting part 4b. By using a base 4 with a gap G1 between the base 4a and the mounting part 4b, the amount of heat removed in the central part can be increased, making the grown crystal convex downwards and suppressing the occurrence of heteromorphisms and defects. Furthermore, since the size of the gap G1 correlates with the amount of heat removed, the amount of heat removed in the central part, the amount of heat removed in the surrounding part, and the difference in the amount of heat removed between the central part and the surrounding part can be controlled by adjusting the local size of the gap G1. This makes it possible to adjust the shape of the isothermal surface and, consequently, the shape of the grown crystal.

[0038] [Details of the base] The details of the base 4 will be explained with reference to Figure 2B. Figure 2B is a cross-sectional view illustrating the details of the base 4 shown in Figure 1. In Figure 2B, the base portion 4a and the mounting portion 4b that constitute the base 4 are shown separated for clarity of explanation. The base portion 4a has a mating surface 4as for fitting together with the mounting portion 4b. The side surface of the base 4 is composed of the side surface 4af of the base portion 4a and the side surface 4bf of the mounting portion 4b.

[0039] In the mating surface 4as, point 4ap, which lies on the axis of rotation of the base 4a, is the point that protrudes the most (is located furthest towards the -Z side) of the base 4a. The protruding point 4ap is located lower (towards the -Z side) than the edge 4ae of the base 4a. In this embodiment, the mating surface 4as is a part of a spherical surface having a constant curvature, but is not limited to this. The mating surface 4as may be a conical surface with the protruding point 4ap as its vertex, or a surface that includes a stepped portion.

[0040] The mounting portion 4b of this embodiment will now be described. The mounting portion 4b has a mating surface 4bs that is aligned to contact the base portion 4a, and a mounting surface 4bm on the opposite side of the mating surface 4bs for attaching the seed crystal substrate 10. The seed crystal substrate 10 may be attached to the mounting surface 4bm with an adhesive, or it may be attached by sandwiching the seed crystal substrate 10 between the mounting surface 4bm and a substrate support member (not shown), thereby pressing the seed crystal substrate 10 against the mounting surface 4bm. In this embodiment, both the mating surface 4bs and the mounting surface 4bm are planar in shape along the XY plane. The mounting portion 4b of this embodiment is flat as a whole. Point 4bp is the center point of the mating surface 4bs and is located on the axis of rotation of the mounting portion 4b. Regarding "planar shape," considering that there are manufacturing tolerances in actual products, in this specification, if the difference in local irregularities on the surface is less than 20 μm, the surface is considered to be "planar."

[0041] The mounting portion 4b is moved in the direction of arrow A4 so that its center point 4bp contacts point 4ap on the base portion 4a, thereby fixing the mounting portion 4b to the base portion 4a. As a result, the two mating surfaces (4as, 4bs) come into contact at points 4ap and 4bp. A small gap G1 is left between the contact points of points 4ap and 4bp, but near the contact points of points 4ap and 4bp, the heat dissipation characteristics may be similar to those at the contact points due to the influence of adhesives, etc., as described later. The maximum value of the gap G1 is the height difference Δ4a in the Z direction between the edge 4ae and point 4ap. In this embodiment, when the mating surface 4bs of the mounting portion 4b is flat, the height difference Δ4a is represented by the gap in the Z direction between the edge 4ae and the edge 4be.

[0042] The maximum value of the gap G1 (in this embodiment, the height difference Δ4a in the Z direction between the edge 4ae and point 4ap) is preferably 20 μm or more and 500 μm or less. If the maximum dimension of the gap G1 is too large, the heat dissipation effect will be lost, but by setting it to 500 μm or less, the heat dissipation effect can be appropriately exerted. If the maximum dimension of the gap G1 is too small, there will be no local difference in the amount of heat dissipated, but by setting it to 20 μm or more, a sufficient local difference in the amount of heat dissipated can be obtained. The maximum value of the gap G1 (height difference Δ4a) is more preferably 30 μm or more and 300 μm or less, and even more preferably 50 μm or more and 200 μm or more.

[0043] The height h1 corresponding to the sum of the maximum thickness of the base 4a and the thickness of the lid 3 is preferably 5 mm to 100 mm, more preferably 10 mm to 80 mm, and more preferably 20 mm to 50 mm. The thickness h2 of the mounting portion 4b is preferably 1.5 mm to 40 mm, more preferably 5 mm to 35 mm, and more preferably 10 mm to 30 mm. The diameter 4ar of the base 4a and the diameter 4br of the mounting portion 4b are both greater than or equal to the diameter of the seed crystal substrate, for example, preferably 100 mm to 250 mm. The diameter 4ar of the base 4a and the diameter 4br of the mounting portion 4b may be the same value or different values. If they are different values, it is more preferable that the diameter 4ar of the base 4a is larger than the diameter 4b of the mounting portion 4b.

[0044] Adhesive can be used to fix the divided base parts (base 4a and mounting part 4b) together. This specification includes descriptions of base parts such as base 4a and mounting part 4b being in "contact" with each other. It should be noted that in this specification, "contact" between base parts includes not only the state in which the base parts are in direct contact with each other, but also the state in which the base parts are connected to each other via adhesive.

[0045] As such an adhesive, it is preferable to use one that does not contain metal elements (except silicon). It is preferable to use an organic adhesive. When the graphite crucible 100 is heated, the adhesive decomposes and partially carbonizes, and the adhesive properties inherent in the adhesive components themselves are lost. However, as the adhesive carbonizes, the mounting part 4b is fixed to the base part 4a, so the state in which the mounting part 4b and the base part 4a are bonded can be maintained. In addition, gas may be generated from the adhesive during the decomposition process, but the gap G1 in the base 4 allows the gas generated from the adhesive to be discharged. The same adhesive used to fix the seed crystal substrate 10 to the base 4 may be used as the above adhesive.

[0046] Because the carbides in the adhesive exhibit high thermal conductivity, the adhesive can be used to adjust the heat dissipation characteristics of the gap G1. For example, the application density of the adhesive applied between the two mating surfaces (4as, 4bs) can be varied depending on the position of the mating surfaces (4as, 4bs). For instance, by making the application density of the adhesive in the central part Ce of the base 4 greater than the application density of the adhesive in the surrounding part Ci of the base 4, the amount of heat dissipated through the heat dissipation path passing through the central part Ce can be increased, while the amount of heat dissipated in the surrounding part Ci can be decreased. Methods for applying adhesive with varying application densities include (1) dispersing the adhesive in a dotted pattern within the mating surfaces (4as, 4bs) and varying the size or spacing of each dot, or (2) applying the adhesive in a ring-like pattern (drawing multiple rings with a common center and different diameters) within the mating surfaces (4as, 4bs) and varying at least one of the thickness and spacing of each ring. When using an adhesive to adjust the heat dissipation characteristics of gap G1, the two mating surfaces (4as, 4bs) may be parallel.

[0047] The sublimation gas from raw material 2 may penetrate the gap G1 sandwiched between the two mating surfaces (4as, 4bs), causing polycrystals to grow inside the gap G1. Since it is difficult to control the distribution of polycrystals growing inside the gap G1, the growth of polycrystals in the gap G1 causes variations in the heat dissipation characteristics, leading to a deviation from the desired heat dissipation characteristics. Therefore, a flexible graphite sheet may be attached to the side of the base 4 so as to cover the gap G1. The flexible graphite sheet blocks the entrance to the gap G1, preventing the sublimation gas from penetrating into the gap G1. This suppresses the generation of polycrystals inside the gap G1. An adhesive can be used to attach the flexible graphite sheet. The same adhesive used to fix the seed crystal substrate 10 to the base 4 can be used as such.

[0048] Flexible graphite sheets, also known as expanded graphite sheets, are manufactured by expanding acid-treated natural graphite and compressing it into a sheet. Flexible graphite sheets are mainly composed of carbon atoms. Specifically, flexible graphite sheets have a structure in which multiple layers (graphene) of benzene ring structures spread in a two-dimensional manner are stacked in many parts. Therefore, the surface of the flexible graphite sheet is a chemically stable surface in which the benzene ring structure, that is, the carbon atoms, form a regular honeycomb structure. As a result, even if a sublimation gas with sufficient supersaturation comes into contact with the surface of the flexible graphite sheet, nucleation by the sublimation gas is unlikely to occur, and the rate of polycrystalline growth on the surface of the flexible graphite sheet is reduced. The thickness of the flexible graphite sheet is preferably, for example, 0.3 mm to 2 mm, and more preferably 0.5 mm to 1.5 mm.

[0049] The flexible graphite sheet may be placed to fill the inside of the gap G1. Since heat is not easily transferred to the flexible graphite sheet, especially in the thickness direction, placing the flexible graphite sheet inside the gap G1 is expected to have the effect of preventing sublimation gas from entering the inside of the gap G1, as well as further reducing the amount of heat removed from the Ci around the base 4.

[0050] [Gap size and shape of growing crystals] Experiments were conducted to investigate the relationship between the presence or absence of gap G1, the size of gap G1, and the degree of downward protrusion of the grown crystal. The experiment involved growing crystals on a seed crystal substrate 10 using graphite crucibles equipped with four different bases, and examining the degree of downward protrusion of the grown crystals. The results of crystal growth were confirmed over a maximum of 120 hours to clearly show the protrusion ratio of the grown crystals for each sample, and the results are shown in Table 1 below.

[0051] For sample numbers S1 to S3, the graphite crucible 100 used had the same base 4 as in the first embodiment described above. That is, all graphite crucibles included a base 4 with a gap G1, and the base 4 had a downwardly convex base 4a and a flat mounting portion 4b, with the base 4a and mounting portion 4b in contact with each other at the center (4ap, 4bp). However, the size of the gap G1 differed depending on the sample. The height difference Δ4a of the base 4a of the base 4 (in other words, the maximum value of the gap G1) differed for sample numbers S1 to S3.

[0052] Sample number S4 is a graphite crucible for comparative example. This graphite crucible does not have a gap G1 and has a base with uniform heat dissipation. In this base, the height difference Δ4a of the base 4a can be expressed as 0. Other conditions are the same as in the first embodiment.

[0053] The experimental conditions for sample numbers S1 to S4 are the same except for the height difference Δ4a of the base 4, the shape and dimensions of the graphite crucible 100 other than the base 4, and the conditions regarding the raw materials and seed crystal substrate. Referring to Figure 3, the protrusion ratio L1 / L2 (%) of the produced crystal will be explained. L1 is the Z-direction length (mm) of the grown crystal at the center of the grown crystal. L2 is the Z-direction length (mm) of the grown crystal at the edge of the seed crystal substrate 10. The value obtained by dividing L1 by L2 and multiplying by 100 is shown as the protrusion ratio (%) of the produced crystal.

[0054] [Table 1]

[0055] Table 1 shows that in the case of base 4, which does not have a gap G1 and has a uniform heat dissipation rate, as in sample number S4, the grown crystal was slightly concave. In contrast, in samples S1 to S3, which have a height difference Δ4a (i.e., the size of the gap G1 in the Z direction), the grown crystal was convex downwards. Furthermore, as the size increased to 80 μm, 130 μm, and 200 μm, more protruding grown crystals were obtained. However, it is not the case that the more protruding the grown crystal 11, the better; the goal should be to obtain a grown crystal 11 with a desired shape. To this end, the relationship between the heat dissipation characteristics of base 4 (size of gap G1 in Table 1) and the shape of the grown crystal can be obtained experimentally, the results can be stored in the form of a table or function, and base 4 can be designed by applying the stored results of a grown crystal with the desired degree of protrusion.

[0056] [First variation of the base] Figure 4 shows the first modified form of the base 4. Parts common to the above-described embodiment are omitted from the description. Except for the matters described below, it can be implemented in the same way as the above-described embodiment. The same applies to the second and subsequent modifications. The base 4a includes the main body portion 4a1 and the tip portion 4a2. The tip portion 4a2 is frustoconical in shape. The base 4a and the mounting portion 4b do not contact at a point, but contact over a surface. Because the area of ​​the contact region between the base 4a and the mounting portion 4b is increased, a larger area can be secured for the central portion which has a large amount of heat dissipation. In addition, in Figure 4, the generatrix 4q representing the conical surface of the tip portion 4a2 is shown as a straight line with a constant angle of inclination, but the generatrix 4q may be stepped, or as in the above-described embodiment, the generatrix 4q may be a curve that bulges or curves from the straight line of inclination. The generatrix 4q can take on various forms, and this is also true for the second and subsequent modifications.

[0057] [Second variation of the base] Figure 5 shows a second modified example of the base 4. Similar to the first modified example, the base 4 of the base 4 includes a columnar main body portion 4a1 and a frustoconical tip portion 4a2. However, the diameter on the +Z side of the frustoconical tip portion 4a2 is smaller than the diameter of the columnar main body portion 4a1.

[0058] [Third variation of the base] Figure 6 shows a third modified example of the base 4. The mating surface 4as of the base 4a is a flat surface along the XY plane, and the mating surface 4bs of the mounting portion 4b is a surface that is convex upward. The base 4a and the mounting portion 4b may be in contact at a point or in contact over a surface.

[0059] [Fourth variation of the pedestal] Figure 7 shows a fourth modified example of the base 4. In the above embodiments and the first and third modified examples, only one of the mating surfaces (4as, 4bs) of the base 4a or the mounting portion 4b had a convex surface. However, in the fourth modified example shown in Figure 7, both mating surfaces (4as, 4bs) have convex surfaces. The base 4a and the mounting portion 4b may be in contact at a point or at a surface.

[0060] [Fifth variation of the pedestal] Figure 8 shows a fifth modified example of the base 4. The mating surface 4as of the base 4a is convex, while the mating surface 4bs of the mounting portion 4b is concave. The mating surfaces 4as and 4bs are in contact in the central portion and have a gap G1 in the peripheral portion. The gap G1 between the mating surfaces 4as and 4bs is formed because the curvature of the mating surface 4as is greater than the curvature of the mating surface 4bs. This combination of convex and concave surfaces is excellent in terms of installation accuracy when fixing the mounting portion 4b to the base 4a and operability for the fixing work. The base 4a and the mounting portion 4b may be in contact at a point or in contact over a surface.

[0061] [Sixth variation of the pedestal] Figure 9 shows a sixth modified example of the base 4. The mating surface 4as of the base 4a is concave, while the mating surface 4bs of the mounting portion 4b is convex. The mating surfaces 4as and 4bs are in contact in the central portion and have a gap G1 in the peripheral portion. The gap G1 between the mating surfaces 4as and 4bs is formed because the curvature of the mating surface 4bs is greater than the curvature of the mating surface 4as. This combination of convex and concave surfaces is excellent in terms of installation accuracy when fixing the mounting portion 4b to the base 4a and operability for the fixing work. The base 4a and the mounting portion 4b may be in contact at a point or in contact over a surface.

[0062] [Seventh variation of the pedestal] Figure 10 shows a seventh modified example of the base 4. The central portion of the mating surface 4as of the base 4a contacts the central portion of the mating surface 4bs of the mounting portion 4b, while the peripheral portion of the mating surface 4as of the base 4a does not contact the peripheral portion of the mating surface 4bs of the base 3b. The peripheral portion has a first peripheral portion close to the side surface 4af of the base 4a and a second peripheral portion located between the first peripheral portion and the central portion. The gap G1 between the base 4a and the mounting portion 4b in the first peripheral portion is smaller than the gap G1 between the base 4a and the mounting portion 4b in the second peripheral portion. As in this modified example, the base 4 may have a form in which the gap G1 decreases from the center outward in the radial direction. By adopting a base 4 in which the size of the gap G1 can be freely adjusted in the radial direction, the isotherms can be adjusted to the desired shape, and consequently, the shape of the grown crystal can be adjusted.

[0063] [Eighth variation of the pedestal] Figure 11 shows the eighth modified form of the base 4. In this embodiment, the base 4 is a base 4 in which the base portion 4a and the mounting portion 4b are integrated. The body of the base 4 has a mounting surface 4bm for attaching the seed crystal substrate 10. The body of the base 4 has a radial recess. This recess is a gap G1 that creates a difference in the amount of heat dissipation. The recess can be formed by wire electrical discharge machining or the like. In this embodiment, the gap G1 is formed along the XY plane, but the gap G1 may be inclined with respect to the XY plane. In this modified form, the entire side surface 4f excluding the recessed area becomes the side surface of the base 4.

[0064] [Details of the components that make up the graphite crucible, excluding the base] The container 1, lid 3, and base 4 that make up the graphite crucible 100 are all mainly composed of graphite. In this specification, "main component" means that the carbon content of the graphite is 90 wt% or more of the total mass. The carbon content of the container 1, lid 3, and base 4 is more preferably 99 wt% or more, and even more preferably 99.9 wt% or more. The carbon content of the container 1, lid 3, and base 4 may also be different. Furthermore, the graphite used in the graphite crucible may be graphite obtained by the CIP (Cold Isostatic Press) molding method, graphite obtained by the mold molding method, or graphite obtained by the extrusion molding method. The graphite molding method may be different for the container 1, lid 3, and base 4.

[0065] The thickness of the sides of container 1 is preferably, for example, 5 mm or more and 30 mm or less. The thickness of the bottom of container 1 is preferably, for example, 3 mm or more and 20 mm or less.

[0066] Gas guide members may be placed inside the graphite crucible 100 to guide the sublimation gas generated from the raw material 2 to the crystal growth surface. In this embodiment, multiple gas guide members are arranged downward from the outer circumference of the seed crystal substrate. The gas guide members have a taper. Multiple gas guide members may be arranged so as to be in contact with each of the edges of the seed crystal substrate 10. In this case, the multiple gas guide members support the seed crystal substrate 10 so as to press against the mounting portion 4b of the base 4, thereby fixing the seed crystal substrate 10 to the mounting portion 4b. Due to variations in the processing accuracy of the graphite material, the pressing force applied by the multiple gas guide members may not always be uniform. However, since the gap G1 placed in the base 4 is expected to buffer the variations in the pressing force of the gas guide members, a relatively uniform pressing force can be obtained.

[0067] [Seed crystal substrate] The seed crystal substrate 10 is made of a SiC single crystal. Sublimation gas in contact with the seed crystal substrate 10 is cooled upon contact with the substrate 10, and a crystal is grown on the seed crystal substrate 10 while aligning the crystal orientation of the seed crystal substrate 10. The base 4 and the seed crystal substrate 10 may be chemically bonded with an adhesive or mechanically bonded with screws. It is preferable that the surface on which the crystal of the seed crystal substrate 10 is grown be positioned as horizontally as possible (parallel to the XY plane).

[0068] <Second Embodiment> Figure 12 is an enlarged cross-sectional view of the base area of ​​the graphite crucible of the second embodiment. The second embodiment will be described focusing on the parts that differ from the above-described embodiment and its modifications. In the second embodiment, parts that are common to the above-described embodiment and its modifications will be omitted from the description. Except for the matters described below, it can be carried out in the same manner as the above-described embodiment.

[0069] In the first embodiment, all bases 4 exhibited localized differences in heat dissipation within the base 4 due to the presence and size of the gap G1. In contrast, the base 4 of the second embodiment does not have a gap G1. Instead, the base 4 of the second embodiment is composed of an inner base 4c with relatively high thermal conductivity and an outer base 4d with relatively low thermal conductivity. This makes it possible to form a base 4 with heat dissipation characteristics in which the amount of heat dissipated in the peripheral portion in contact with the side surface of the base 4 is smaller than the amount of heat dissipated in the central portion surrounded by the peripheral portion.

[0070] The difference in thermal conductivity between the two bases (4c, 4d) is caused by differences in graphite parameters such as their composition (carbon content) and density. Differences in graphite parameters can be obtained by using different graphite molding methods or different types of raw materials to produce the graphite. For example, two types of graphite materials with different thermal conductivity can be prepared, and the graphite material with high thermal conductivity can be used for the inner base 4c, while the graphite material with low thermal conductivity can be used for the outer base 4d, thereby creating a difference in heat removal between the inner and outer bases 4 in the radial direction. Alternatively, graphite materials with anisotropic thermal conductivity can be used, and the inner base 4c can be positioned so that the direction of high thermal conductivity aligns with the Z direction, while the outer base 4d can be positioned so that the direction of low thermal conductivity aligns with the Z direction, thereby creating a difference in heat removal between the inner and outer bases 4 in the radial direction.

[0071] In this embodiment, the outer base 4d is ring-shaped, with the inner base 4c fitting into a through hole in its center, but it is not limited to this configuration. For example, the ring-shaped outer base 4d may be cut radially, and the outer base 4d may be composed of multiple ring components. The -Z end faces of the inner base 4c and the outer base 4d together constitute a mounting surface 4cdm for attaching the seed crystal substrate 10. In this embodiment, the entire outer surface 4f of the outer base 4d becomes the side surface of the base. The surfaces 4m located below the outer base 4d and the inner base 4c become the mounting surfaces for attaching the seed crystal substrate.

[0072] The first embodiment and its modifications, as well as the second embodiment, have been described above. The present invention is not limited in any way to the embodiments and their modifications described above, and various improvements and modifications are possible without departing from the spirit of the invention. It is also possible to implement multiple embodiments and modifications in combination. For example, the members with different thermal conductivity from the second embodiment may be applied to the pedestal having a gap from the first embodiment. [Explanation of Symbols]

[0073] 1: Container 1i: Inner surface of the through hole (of the container) 2: Raw materials 3: Lid 3e: Outer circumference (of the lid) 4: Pedestal 4a: Base (of a pedestal) 4a1: Main body (base) 4a2: (The) tip of the base 4a3: Main body (base) 4a4: Peripheral part (of the base) 4ab: Bottom surface (where the base touches the lid) 4ae: (The edge of the base) 4af: (The side of the base) 4ap: (base) projection 4as: mating surface (of the base) 4b: Mounting part (of the base) 4be: (The edge of the mounting part) 4bf: Side view (of the mounting part) 4bm: Mounting surface (for mounting the seed crystal substrate) 4bp: Center point (of the mounting part) 4bs: (The mating surface of the mounting part) 4c: Inner base 4cdm: Mounting surface (for attaching the seed crystal substrate) 4d:Outer pedestal 4f: Side (of the base) 4m: Mounting section 4q: Bus bar 5: Isothermal surface 10: Seed crystal substrate 11: Growth Crystal 100: Graphite Crucible

Claims

1. A graphite crucible for growing a SiC single crystal on the first main surface of a seed crystal substrate by sublimation recrystallization, The graphite crucible is provided with a base that protrudes inward from the housing of the graphite crucible, The base has a mounting surface for attaching the second main surface of the seed crystal substrate, and a side surface that is in contact with the periphery of the mounting surface. The graphite crucible is characterized in that the base has a heat dissipation characteristic such that the amount of heat dissipated upward through the surrounding portion in contact with the side surface is less than the amount of heat dissipated upward through the central portion surrounded by the surrounding portion.

2. The graphite crucible according to claim 1, characterized in that the base has a shape in which the surrounding portion has a gap extending from the side and the central portion does not have a gap.

3. The graphite crucible according to claim 2, characterized in that the gap located near the side is larger than the gap located near the central portion.

4. The graphite crucible according to claim 3, characterized in that the gap includes a portion in which the gap becomes larger as it approaches the side surface.

5. The graphite crucible according to any one of claims 1 to 4, wherein the base comprises a mounting portion for attaching the seed crystal substrate and a base portion for supporting the mounting portion, and the mounting portion and the base portion are configured to be separable.

6. The graphite crucible according to claim 5, characterized in that the gap is provided between the mounting portion and the base portion.

7. The graphite crucible according to claim 5, characterized in that the mounting portion is flat.

8. A graphite crucible according to any one of claims 1 to 4, characterized in that the maximum dimension of the gap in the direction normal to the bottom surface of the base is 500 μm or less.

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