Thermal field structure for crystal diameter expansion

Abstract

The invention discloses a thermal field structure for crystal diameter expansion, which comprises a crucible, a graphite cover, a seed crystal, an outer ring and a diameter expansion piece, the seed crystal is installed on the inner wall of the graphite cover, the outer ring is installed between the graphite cover and the crucible, and the crucible, the outer ring and the graphite cover jointly define a growth chamber; the diameter expanding piece comprises a diameter expanding ring, the diameter expanding ring extends upwards to form an annular fixing part, the diameter expanding piece is fixed between the seed crystal and the outer ring, the inner wall of the annular fixing part is tightly attached to the back face of the seed crystal, the top wall of the annular fixing part is tightly attached to the inner wall of the graphite cover, and the outer walls of the annular fixing part and the diameter expanding ring are tightly attached to the inner wall of the outer ring. The inner side of the expanding ring is provided with a cavity structural member, the inner diameter of the cavity structural member is equal to the diameter of the seed crystal, and the wall thickness of the inner wall of the cavity structural member is 2-3mm. According to the invention, high-quality diameter expansion can be realized, so that silicon carbide crystals with larger sizes can be obtained.

Description

Technical Field

[0001] The invention relates to the field of semiconductor technology, and in particular to a thermal field structure for crystal diameter expansion. Background Art

[0002] The relevant technology points out that at present, all domestic 6-inch silicon carbide substrate production lines have been put into operation, and the 8-inch Mos-grade P-grade substrate sheets of major domestic substrate manufacturers have been shipped in batches. In addition, in terms of 6-inch substrates, as the international market's requirements for the technical indicators of 6-inch SiC substrate products become more and more stringent. In terms of technology, first-tier substrate manufacturers need to fundamentally improve the substrate yield, and how to achieve the transition from 6-inch to large-size silicon carbide substrates, or the production of larger diameter crystal technology reserves, all require accelerated research and development.

[0003] In order to improve market competitiveness and reduce the cost of silicon carbide single crystals, it is necessary to prepare large-size seed crystals under the premise of stabilizing the crystal growth yield and processing yield. Therefore, how to achieve high-quality crystal diameter expansion is to fundamentally solve the reserve of large-size single crystal preparation technology, which is crucial to the development of enterprises. Summary of the invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention proposes a thermal field structure for crystal diameter expansion, which can achieve high-quality diameter expansion to obtain a larger-sized silicon carbide crystal.

[0005] To achieve the above-mentioned purpose, the thermal field structure for crystal expansion of the present invention comprises a crucible, a graphite cover, a seed crystal, an outer ring and an expansion member, wherein the seed crystal is mounted on the inner wall of the graphite cover, the outer ring is mounted between the graphite cover and the crucible, and the crucible, the outer ring and the graphite cover jointly define a growth chamber; the expansion member comprises an expansion ring, the expansion ring extends an annular fixing portion upward, the expansion member is fixed between the seed crystal and the outer ring, the inner wall of the annular fixing portion is in close contact with the back of the seed crystal, and the top wall thereof is in close contact with the inner wall of the graphite cover, the annular fixing portion and the outer wall of the expansion ring are in close contact with the inner wall of the outer ring, the inner side of the expansion ring has a cavity structure member, the inner diameter of the cavity structure member is equal to the diameter of the seed crystal, and the inner wall thickness of the cavity structure member is 2-3 mm.

[0006] Preferably, the seed crystal of the present invention includes a first seed crystal and a second seed crystal, the first seed crystal is connected to the upper surface of the second seed crystal, the first seed crystal and the second seed crystal are an integral structure, the back of the first seed crystal is provided with a concave arc chamfer structure, the concave smooth chamfer structure and the back of the second seed crystal together define an S-shaped structure 1, the inner wall of the annular fixing portion is provided with an S-shaped structure 2 matched with the S-shaped structure 1, and the diameter expansion member and the seed crystal are tightly fitted together through the interference fit of the S-shaped structure 1 and the S-shaped structure 2. In this embodiment, the diameter expansion member and the seed crystal are interference fit, and there is no gap between the two, which limits the generation of polycrystals at the edge of the seed crystal in the initial stage, thereby further limiting the situation where polycrystals erode the single crystal area during the diameter expansion process due to the generation of polycrystals, and improving the quality of the crystal.

[0007] Preferably, the arc chamfered structure of the first seed crystal and the second S-shaped structure of the present invention are both polished to prevent scratches on the seed crystal.

[0008] Preferably, the cavity structure of the present invention includes a straight line segment and a horizontal line segment, the top of the straight line segment is connected to the bottom wall of the annular fixing portion, the bottom of the straight line segment is connected to the side wall of the expansion ring through the horizontal line segment, the height of the straight line segment is equal to the height of the expansion ring, the straight line segment is the inner wall of the cavity structure, the wall thickness of the straight line segment is 2.5 mm, and the straight line segment, the horizontal line segment, the annular fixing portion and the expansion ring jointly define a cavity structure.

[0009] Preferably, the cavity structure of the present invention includes a straight line segment and an oblique line segment, the top of the straight line segment is connected to the bottom wall of the annular fixing portion, the bottom of the straight line segment is connected to the side wall of the expansion ring through the oblique line segment, the angle between the oblique line segment and the side wall of the expansion ring is α, 120°≤α≤160°, the height of the straight line segment is 550μm, the straight line segment is the inner wall of the cavity structure, the wall thickness of the straight line segment is 3mm, and the straight line segment, the oblique line segment, the annular fixing portion and the expansion ring jointly define a cavity structure.

[0010] Furthermore, the present invention also includes a porous filter plate with an opening at the bottom, the porous filter plate is placed at the bottom of the crucible, and its side walls are tightly attached to the inner wall of the crucible, and the porous filter plate is filled with a powder source.

[0011] Preferably, an annular fixing piece extends radially outward from the upper portion of the annular fixing portion of the present invention, and the annular fixing piece fills the gap between the outer ring and the graphite cover to further seal the growth chamber.

[0012] Preferably, the material of the expansion ring of the present invention is Ta, W and their carbides.

[0013] Compared with the prior art, the present invention has the following advantages:

[0014] 1) The expansion member of the present invention tightly wraps the seed crystal, exposing only the area of ​​the extension surface facing the powder source, and through the interference fit between the expansion ring and the seed crystal, the generation of polycrystals at the edge of the seed crystal in the initial stage is limited, thereby further limiting the erosion of polycrystals into the single crystal area during the expansion process due to the generation of polycrystals.

[0015] 2) The overall axial structure of the thermal field of the present invention remains unchanged, and the overall axial temperature gradient of the thermal field remains unchanged. Through the cavity structure of the expansion ring, a large radial temperature gradient is created at the outer edge of the extension surface, so as to utilize radial expansion to achieve crystal expansion growth.

[0016] 3) The inner diameter of the expansion ring of the present invention is equal to the diameter of the seed crystal. During the growth of the crystal, its outer edge grows closely to the expansion ring, which limits the generation of polycrystalline at the edge of the crystal. In addition, since the inner side of the expansion ring of the present invention is provided with a cavity structure, and the inner wall of the cavity structure does not exceed 3mm, the wall thickness is relatively thin, so that the stress of the crystal growth process can be released, and while maintaining the expansion, the crystal stress is effectively prevented from being too large. In other words, the present invention uses the design of a metal thin wall (metal foil structure) so that the metal thin wall can be squeezed toward the cavity during the growth process of the crystal, thereby obtaining more growth chambers in the lateral direction, thereby realizing the free expansion of the crystal to obtain a larger crystal.

[0017] 4) While the diameter of the present invention is expanding, as the crystal diameter increases and under the action of high temperature, the metal thin wall (inner wall of the cavity structure) breaks, at which time the cavity structure is connected to the growth cavity as a whole, and the crystal edge component polycrystals can gather here, thereby achieving the purpose of polycrystal collection.

[0018] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a schematic diagram of a thermal field structure for crystal expansion according to an embodiment of the present invention;

[0020] Figure 2 A schematic diagram of a partial structure of a thermal field structure for crystal expansion according to an embodiment of the present invention;

[0021] Figure 3 is a schematic diagram of a late stage of crystal growth of a thermal field structure for crystal expansion according to an embodiment of the present invention;

[0022] Figure 4 is a schematic diagram of a thermal field structure for crystal expansion according to an embodiment of the present invention;

[0023] Figure 5A schematic diagram of a partial structure of a thermal field structure for crystal expansion according to an embodiment of the present invention;

[0024] Figure 6 is a schematic diagram of a late stage of crystal growth of a thermal field structure for crystal expansion according to an embodiment of the present invention;

[0025] Reference numerals:

[0026] 100: thermal field structure for crystal expansion;

[0027] 110: Quartz cover;

[0028] 120: Insulation felt;

[0029] 130: electromagnetic induction coil;

[0030] 140: graphite cover;

[0031] 150: outer ring;

[0032] 160: crucible; 161: porous filter plate; 162: powder source;

[0033] 170: seed crystal; 1701: first seed crystal; 17011: arc chamfer structure; 17012: S-type structure 1; 1702: second seed crystal; 171: graphite paper;

[0034] 180: diameter expansion member; 1801: annular fixing portion; 18011: S-shaped structure II; 18012: annular fixing member; 1802: diameter expansion ring; 1803: cavity structure member; 18031: straight line segment; 18032: cavity structure; 18033: horizontal line segment; 18034: oblique line segment. DETAILED DESCRIPTION

[0035] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0036] The disclosure below provides many different embodiments or examples to realize different structures of the present invention. In order to simplify the disclosure of the present invention, the parts and settings of specific examples are described below. Of course, they are only examples, and the purpose is not to limit the present invention. In addition, the present invention can repeat reference numbers and / or letters in different examples. This repetition is for the purpose of simplicity and clarity, and does not itself indicate the relationship between the various embodiments and / or settings discussed. In addition, the examples of various specific processes and materials provided by the present invention, but those of ordinary skill in the art can be aware of the applicability of other processes and / or the use of other materials.

[0037] Embodiment 1

[0038] Reference below Figures 1 to 6 A thermal field structure 100 for crystal expansion according to an embodiment of the present invention is described, comprising a quartz cover 110, a thermal insulation felt 120, an electromagnetic induction coil 130, a crucible 160, a graphite cover 140, a seed crystal 170, an outer ring 150, a diameter expander 180 and a porous filter plate 161, wherein the seed crystal 170 is mounted on the inner wall of the graphite cover 140, and the outer ring 150 is mounted between the graphite cover 140 and the crucible 160, wherein the crucible 160, the outer ring 150 and the graphite cover 140 jointly define a growth chamber; the diameter expander 180 comprises a diameter expander ring 1802, and the diameter expander ring 1802 extends upwardly out of an annular fixing portion 161. 801, the inner diameter of the annular fixing part 1801 is smaller than the inner diameter of the diameter expansion ring 1802. In this embodiment, the diameter expansion ring 1802 and the annular fixing part 1801 are an integral structure. The diameter expansion member 180 is fixed between the seed crystal 170 and the outer ring 150. The inner wall of the annular fixing part 1801 is close to the back of the seed crystal 170, and its top wall is close to the inner wall of the graphite cover 140. The outer walls of the annular fixing part 1801 and the diameter expansion ring 1802 are close to the inner wall of the outer ring 150. The inner side of the diameter expansion ring 1802 has a cavity structure 18032. The inner diameter of the cavity structure member 1803 is equal to the diameter of the seed crystal 170. The inner wall thickness of the diameter expansion ring 1802 is 2-3mm. The material of the diameter expansion member 180 in this embodiment is a refractory metal, such as Ta, W and its carbide. The diameter expansion member 180 in this embodiment is made of Ta.

[0039] refer to Figure 1 As shown, specifically, the crucible 160 of this embodiment is made of graphite. The seed crystal 170 is bonded to the center of the inner wall of the graphite cover 140 through the graphite paper 171, and the graphite cover 140 and the outer ring 150, and the outer ring 150 and the crucible 160 are assembled together through threaded connection, and then the whole is placed in the quartz cover 110, and the insulation felt 120 is filled between the quartz cover 110 and the graphite cover 140, the outer ring 150 and the crucible 160, and the electromagnetic induction coil 130 surrounds the outside of the quartz cover 110 to heat the inside of the crucible 160.

[0040] refer to Figure 2 As shown, in some embodiments of the present invention, the seed crystal 170 includes a first seed crystal 1701 and a second seed crystal 1702, the first seed crystal 1701 is connected to the upper part of the second seed crystal 1702, the first seed crystal 1701 and the second seed crystal 1702 are an integral structure, the back of the first seed crystal 1701 is provided with a concave arc chamfer structure 17011, the concave arc chamfer structure 17011 and the back of the second seed crystal 1702 together define an S-shaped structure 17012, the inner wall of the annular fixing portion 1801 is provided with an S-shaped structure 2 18011 matched with the S-shaped structure 1 17012, and the expansion member 180 and the seed crystal 170 are tightly fitted together through the interference fit of the S-shaped structure 1 17012 and the S-shaped structure 2 18011. In this embodiment, the diameters of the first seed crystal 1701 and the second seed crystal 1702 are both 205 mm, the thickness of the first seed crystal 1701 is 250 μm, and the thickness of the second seed crystal 1702 is 550 μm.

[0041] Specifically, the S-shaped structure 17012 on the back of the seed crystal 170 is interference-fitted with the S-shaped structure 18011 of the annular fixing portion 1801 of the diameter expander 180, so that the contact surface between the seed crystal 170 and the diameter expander 180 is tightly fitted together without a gap. At the same time, the outer wall of the annular fixing portion 1801 of the diameter expander 180 and the outer wall of the diameter expander 150 are tightly fitted together. In other words, there is no gap between the outer wall of the diameter expander 180 and the inner wall of the outer ring 150 to ensure the sealing of the growth chamber. Since a certain gap will still remain between the graphite cover 140 and the outer ring 150 after they are assembled, in order to further ensure the sealing of the growth chamber, the upper part of the annular fixing portion 1801 radially extends outward to form an annular fixing member 18012, which fills the gap between the outer ring 150 and the graphite cover 140, thereby achieving further sealing of the growth chamber.

[0042] In some embodiments of the present invention, in order to avoid damage to the seed crystal 170, the arc chamfer structure 17011 and the S-shaped structure 18011 of this embodiment are polished to prevent the surface of the arc chamfer structure 17011 or the S-shaped structure 18011 from scratching the seed crystal 170, thereby effectively reducing the production cost of the enterprise.

[0043] In some embodiments of the present invention, the cavity structure 1803 includes a straight line segment 18031 and a horizontal line segment 18033, the top of the straight line segment 18031 is connected to the bottom wall of the annular fixing portion 1801, and the bottom of the straight line segment 18031 is connected to the side wall of the expansion ring 1802 through the horizontal line segment 18033, the height of the straight line segment 18031 is equal to the height of the expansion ring 1802, the straight line segment 18031 is the inner wall of the cavity structure 1803, the wall thickness of the straight line segment 18031 is 2.5 mm, and the wall thickness of the horizontal line segment 18033 is 2.5 mm. Optionally, the wall thickness of the straight line segment 18031 and the horizontal line segment 18033 can also be 2 mm, 2.3 mm, 2.4 mm, 2.8 mm or 3 mm. The straight line segment 18031, the horizontal line segment 18033, the annular fixing portion 1801 and the expansion ring 1802 jointly define the cavity structure 18032. The diameter expander 180 of this embodiment is integrally formed by an annular fixing portion 1801, a diameter expander ring 1802 and a cavity structure 1803. It can be understood that in this embodiment, the height of the straight line segment 18031 is equal to the height of the diameter expander ring 1802, and the wall thickness of the horizontal line segment 18033 is relatively thin, so the height of the cavity structure 18032 is substantially the same as the height of the diameter expander ring 1802, providing a space for the crystal to expand.

[0044] Furthermore, in some embodiments of the present invention, in order to improve the growth quality of the crystal, the present invention further includes a porous filter plate 161, the lower part of the porous filter plate 161 has an opening, the porous filter plate 161 is placed at the bottom of the crucible 160, and the left and right side walls of the porous filter plate 161 are respectively tightly fitted with the inner wall of the crucible 160, and the powder source 162 is installed inside the porous filter plate 161. During the crystal growth process, the porous filter plate 161 can filter out impurities in the silicon carbide gas, preventing these impurities from participating in the crystal growth, thereby avoiding defects in the crystal, and thus improving the growth quality of the crystal. On the other hand, the porous filter plate 161 is conducive to the uniform growth of the crystal, reduces the deviation during the crystal growth process, and further improves the growth quality of the crystal.

[0045] In this embodiment, the S-shaped structure 17012 on the back of the seed crystal 170 is interference fit with the S-shaped structure 18011 of the annular fixing portion 1801 of the expansion member 180. It can be understood that the seed crystal 170 is tightly wrapped by the annular fixing portion 1801 of the expansion member 180, and only the extended surface of the bottom of the seed crystal 170 facing the powder source 162 area is exposed. Since there is no gap contact between the back of the seed crystal 170 and the annular fixing portion 1801, the generation of polycrystals at the edge of the seed crystal 170 in the initial stage is effectively limited, and the erosion of polycrystals to the single crystal area during the expansion process due to the generation of polycrystals is further limited, thereby improving the growth quality of the crystal.

[0046] refer to Figure 3As shown, since the diameter expansion member 180 of this embodiment is made of Ta, and the inner wall of the cavity structure member 1803 is 2.5 mm, it can be understood that the inner wall of the cavity structure member 1803 is only a thin metal wall. As the crystal grows, its diameter is constantly increasing. Under the increase of the crystal diameter and the high temperature, the inner wall of the cavity structure member 1803 is broken under the extrusion of the crystal. At this time, the cavity structure 18032 is integrated with the growth chamber, so that the crystal can continue to expand and grow inside the cavity structure 18032, and the stress is released, so that the crystal can be freely expanded while preventing the crystal stress from being too large. At the same time, the crystal edge component polycrystals can be gathered inside the cavity structure 18032, achieving the purpose of polycrystal collection and facilitating the processing of later processing. In addition, the sealed growth chamber of this embodiment further limits the growth of polycrystals at the edge of the crystal, effectively improving the growth quality of the crystal.

[0047] The expansion structure of the present embodiment can only realize the gradual expansion of the crystal from the seed crystal 170 surface toward the growth surface. In other words, the diameter of the obtained crystal gradually increases from top to bottom. Therefore, the crystal expansion of the present embodiment is unequal diameter expansion. The stress caused by the unequal diameter and sharp edge of the crystal during subsequent processing is relatively large, which can easily cause damage to the crystal and is not conducive to reducing the production cost of the enterprise.

[0048] Embodiment 2

[0049] This embodiment is substantially the same as the embodiment, and the same components are numbered the same. The difference is that: Figure 4 and Figure 5 As shown, the material of the diameter expansion member 180 of this embodiment is made of W. The cavity structure member 1803 includes a straight line segment 18031 and an oblique line segment 18034. The top of the straight line segment 18031 is connected to the bottom wall of the annular fixing portion 1801. The bottom of the straight line segment 18031 is connected to the side wall of the diameter expansion ring 1802 through the oblique line segment 18034. The angle between the oblique line segment 18034 and the outer wall of the diameter expansion ring 1802 is α, 120°≤α≤160°. In this embodiment, the angle between the oblique line segment 18034 and the side wall of the diameter expansion ring 1802 is α=120°. Of course, it can also be adjusted according to actual conditions. The angle α between the oblique line segment 18034 and the outer wall of the expansion ring 1802 is set to 125°, 130°, 240°, etc. The height of the straight line segment 18031 is 550μm. The straight line segment 18031 is the inner wall of the cavity structure 1803. The wall thickness of the straight line segment 18031 is 3mm. The straight line segment 18031, the oblique line segment 18034, the annular fixing portion 1801 and the expansion ring 1802 jointly define the cavity structure 18032. The cavity structure 1803 is located at the lower part of the seed crystal 170.

[0050] The height of the cavity structure 18032 in this embodiment is shorter than that in the first embodiment. Figure 6As shown, the inner diameter of the expansion ring 1802 of this embodiment is 240mm. During the crystal growth process, the crystal grows along the inner wall of the expansion ring 1802. Since the height of the cavity structure 18032 is short, in the early stage of crystal growth, the stress at the edge of the crystal is not enough to break the inner wall (straight line segment 18031) of the expansion ring 1802. Therefore, the crystal will grow downward along the straight line segment 18031 of the cavity structure 1803, and expand outward along the oblique line segment 18034 until it is expanded to contact the outer wall of the expansion ring 1802, and then continue to grow downward close to the side wall of the expansion ring 1802 to complete the expansion growth of the crystal, so that the crystal diameter is gradually expanded from the seed crystal 170205mm to no more than 240mm. In this embodiment, the effective expansion (maximum 240mm) part continues to grow according to the equal diameter size, so as to obtain more effective crystals. When processing the crystal in the later stage, it is only necessary to cut the crystal of the equal diameter part from the entire crystal as an effective crystal. Compared with Example 1, most of the crystals in this example are of equal diameter and expanded diameter, so fewer crystals are wasted and more large-size crystals are obtained. In subsequent processing, only the crystals of the equal-diameter part are processed, which avoids the problem of sharp edges, facilitates crystal processing, and obtains more large-size substrate sheets.

[0051] Other components of the thermal field structure 100 for crystal expansion according to the embodiment of the present invention, such as the crucible 160, the quartz cover 110, the electromagnetic induction coil 130, etc., and operations are known to those skilled in the art and will not be described in detail here.

[0052] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0053] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0054] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, an electrical connection, or a communication; it can be a direct connection, or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0055] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may mean that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Moreover, a first feature being "above", "above" or "above" a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being "below", "below" or "below" a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

[0056] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0057] Although the embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the claims and their equivalents.

Claims

1. A thermal field structure for crystal expansion, characterized in that: It includes a crucible, a graphite cover, a seed crystal, an outer ring and an expansion member, the seed crystal is installed on the inner wall of the graphite cover, the outer ring is installed between the graphite cover and the crucible, and the crucible, the outer ring and the graphite cover jointly define a growth chamber; the expansion member includes an expansion ring, the expansion ring extends an annular fixing portion upward, the expansion member is fixed between the seed crystal and the outer ring, the inner wall of the annular fixing portion is in close contact with the back of the seed crystal, and the top wall thereof is in close contact with the inner wall of the graphite cover, the annular fixing portion and the outer wall of the expansion ring are in close contact with the inner wall of the outer ring, the inner side of the expansion ring is provided with a cavity structure, the inner diameter of the cavity structure is equal to the diameter of the seed crystal, and the inner wall thickness of the cavity structure is 2-3mm.

2. A thermal field structure for crystal expansion according to claim 1, characterized in that: The seed crystal includes a first seed crystal and a second seed crystal connected to the lower part of the first seed crystal, the first seed crystal and the second seed crystal are an integral structure, the back of the first seed crystal is provided with a concave arc chamfer structure, the concave arc chamfer structure and the back of the second seed crystal together define an S-shaped structure one, the inner wall of the annular fixing portion is provided with an S-shaped structure two that matches the S-shaped structure one, and the expansion piece and the seed crystal are tightly fitted together through the interference fit of the S-shaped structure one and the S-shaped structure two.

3. The thermal field structure for crystal expansion according to claim 1, characterized in that: The arc chamfered structure and the second S-shaped structure are both polished.

4. A thermal field structure for crystal expansion according to any one of claims 1 to 3, characterized in that: The cavity structure member includes a straight line segment and a horizontal line segment. The top of the straight line segment is connected to the bottom wall of the annular fixing portion, and the bottom of the straight line segment is connected to the side wall of the expansion ring through the horizontal line segment. The height of the straight line segment is equal to the height of the expansion ring. The straight line segment is the inner wall of the cavity structure member, and the wall thickness of the straight line segment is 2.5 mm. The straight line segment, the horizontal line segment, the annular fixing portion and the expansion ring jointly define a cavity structure.

5. A thermal field structure for crystal expansion according to any one of claims 1 to 3, characterized in that: The cavity structure member includes a straight line segment and an oblique line segment, the top of the straight line segment is connected to the bottom wall of the annular fixing portion, the bottom of the straight line segment is connected to the side wall of the diameter expansion ring through the oblique line segment, the angle between the oblique line segment and the side wall of the diameter expansion ring is α, 120°≤α≤160°, the height of the straight line segment is 550μm, the straight line segment is the inner wall of the cavity structure member, the wall thickness of the straight line segment is 3mm, and the straight line segment, the oblique line segment, the annular fixing portion and the diameter expansion ring jointly define a cavity structure.

6. A thermal field structure for crystal expansion according to claim 5, characterized in that: It also includes a porous filter plate with an opening at the bottom, the porous filter plate is placed at the bottom of the crucible, and its side walls are closely attached to the inner wall of the crucible, and a powder source is installed inside the porous filter plate.

7. The thermal field structure for crystal expansion according to claim 1, characterized in that: An annular fixing piece is radially extended outward from the upper part of the annular fixing portion, and the annular fixing piece is filled in the gap between the outer ring and the graphite cover.

8. The thermal field structure for crystal expansion according to claim 1, characterized in that: The material of the diameter expansion ring is Ta, W and their carbides.

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