A method for producing a silicon carbide single crystal

Abstract

The application provides a preparation method of a silicon carbide single crystal and belongs to the technical field of semiconductor material preparation. The application places silicon carbide raw material for crystal growth in a graphite crucible for loading, then assembles a first graphite support ring, places a seed crystal with a coating on the first graphite support ring, and makes the silicon surface of the seed crystal with the coating face upward; then assembles a second graphite support ring on the first graphite support ring, places a porous graphite plate on the second support ring, and makes a space reserved between the porous graphite plate and the seed crystal with the coating; lays a layer of silicon carbide powder on the surface of the porous graphite plate, then covers a graphite cover, heats, and performs a crystal growth process to obtain a silicon carbide single crystal. The application sets a carbon coating on the back of the seed crystal, can effectively prevent the volatilization of silicon vapor on the back of the seed crystal, and further prevents the corrosion on the back of the seed crystal; meanwhile, through the optimization design of the seed crystal support structure, the corrosion on the back of the seed crystal can be further prevented, and high-quality silicon carbide single crystals can be finally obtained.

Description

Technical Field

[0001] This invention relates to the field of semiconductor material preparation technology, and in particular to a method for preparing silicon carbide single crystals. Background Technology

[0002] Semiconductor chip structures include substrates, epitaxy, and device structures. The substrate typically provides support, the epitaxy consists of specific thin films required for the device, and the device structure is a topological structure with a specific circuit pattern, fabricated using processes such as photolithography and etching. Third-generation semiconductor materials mainly include silicon carbide (SiC) and gallium nitride (GaN). Compared to first- and second-generation semiconductor materials, they have higher band gaps, breakdown voltages, electrical conductivity, and thermal conductivity, and will replace the first two generations of semiconductor materials in high-temperature, high-voltage, high-power, and high-frequency applications. Among these, due to the lack of large-size single crystals of gallium nitride, silicon carbide is more widely used. For example, the main forms of third-generation semiconductor materials are silicon carbide-based silicon carbide epitaxial devices and silicon carbide-based gallium nitride epitaxial devices. Furthermore, the thermal conductivity of silicon carbide is approximately three times that of gallium nitride, resulting in stronger thermal conductivity, longer device lifespan, higher reliability, and a smaller required heat dissipation system.

[0003] After years of research, the technology for growing SiC single crystals using the physical vapor transport (PVT) method has become increasingly mature. Currently, SiC single crystal growth typically uses a graphite crucible. Specifically, the SiC raw material is placed in the lower part of the growth chamber, and a seed crystal is bonded and fixed to a graphite cap at the top of the growth chamber. By controlling the temperature and pressure conditions of the growth chamber, the SiC raw material sublimates from the lower part of the chamber and then rises to the seed crystal for accumulation and growth, ultimately obtaining a SiC single crystal. However, the coefficient of thermal expansion of graphite at 2000–2200℃ is 6 × 10⁻⁶. -6 K -1 Meanwhile, the axial and radial coefficients of thermal expansion of 4H-SiC single crystal at the same temperature are 5.2 × 10⁻⁶. -6 K -1 and 4.6×10 -6 K -1 The difference in the coefficient of thermal expansion will lead to greater thermal stress, which in turn will cause mechanical stress, resulting in excessive stress in the final substrate product, too many dislocation defects, and even microtube proliferation.

[0004] To overcome the above problems, existing methods involve adding graphite paper buffers between the seed crystal and the graphite cap. This involves bonding one side of the seed crystal to the graphite paper and the other side of the graphite paper to the graphite cap to reduce stress. However, bonding the seed crystal to the graphite paper and the graphite paper to the graphite cap is difficult and time-consuming, and can easily lead to uneven heat dissipation on the back of the seed crystal, resulting in corrosion at higher temperatures. Furthermore, graphite paper itself has low strength and is not dense enough, making it susceptible to corrosion from silicon vapor. Once localized corrosion pits appear, the corrosion rate in that area accelerates, leading to polycrystalline growth. Polycrystalline growth is faster than monocrystalline growth, which can compress the monocrystalline crystal, causing stress, cracking, or polymorphic defects. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing silicon carbide single crystals. The method provided by this invention can avoid the problem of uneven heat dissipation caused by the bonding and fixing of seed crystals, graphite paper and graphite caps in existing methods, and can effectively prevent corrosion on the back of the seed crystal, thereby obtaining high-quality silicon carbide single crystals.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for preparing silicon carbide single crystals, comprising the following steps:

[0008] A coated seed crystal is provided, the coated seed crystal comprising a seed crystal and a carbide coating disposed on the silicon surface of the seed crystal;

[0009] A crystal growth crucible is provided, which, from bottom to top, comprises a loading graphite crucible, a first graphite support ring, a second support ring, a porous graphite plate, and a graphite cap.

[0010] Silicon carbide raw material for crystal growth is placed in the loading graphite crucible. A first graphite support ring is assembled on the loading graphite crucible containing the silicon carbide raw material for crystal growth. The coated seed crystal is placed on the first graphite support ring with the silicon surface of the coated seed crystal facing upward. Then, a second graphite support ring is assembled on the first graphite support ring. The porous graphite plate is placed on the second support ring with a space reserved between the porous graphite plate and the coated seed crystal. A silicon carbide powder layer is laid on the surface of the porous graphite plate. Then, the graphite cover is placed on top, and the plate is heated to perform crystal growth treatment to obtain silicon carbide single crystal.

[0011] Preferably, the thickness of the carbonized coating is 150–200 μm.

[0012] Preferably, the method for preparing the coated seed crystal includes the following steps:

[0013] Phenolic resin is mixed with propylene glycol methyl ether acetate to obtain a phenolic resin solution;

[0014] Graphite emulsion, ethanol and phenolic resin are mixed to obtain a diluted graphite emulsion solution;

[0015] The phenolic resin solution is coated onto the silicon surface of the seed crystal, and after drying, a first phenolic resin adhesive layer is formed on the silicon surface.

[0016] The graphite emulsion diluent is coated on the surface of the first phenolic resin adhesive layer, and after drying, a graphite emulsion coating is formed on the surface of the first phenolic resin adhesive layer.

[0017] The phenolic resin solution is coated on the surface of the graphite emulsion coating, and after drying, a second phenolic resin adhesive layer is formed on the surface of the graphite emulsion coating.

[0018] Graphite powder is coated on the surface of the second phenolic resin adhesive layer to form a graphite powder coating, which is then sintered and cured to obtain a seed crystal with a coating.

[0019] Preferably, the graphite in the graphite emulsion has a D50 of <400nm; and the thickness of the graphite emulsion coating is 60-80μm.

[0020] Preferably, the graphite emulsion diluent is coated by spraying, the spraying distance is 30-40cm, and the compressed air pressure is 0.2-0.4MPa.

[0021] Preferably, the D50 of the graphite powder is 1-5 μm; the thickness of the graphite powder coating is 70-130 μm.

[0022] Preferably, the graphite powder is coated by spraying, the spraying distance is 30-50cm, and the pressure of the compressed air is 0.2-0.4MPa.

[0023] Preferably, the phenolic resin content in the phenolic resin solution used to prepare the first phenolic resin adhesive layer and the second phenolic resin adhesive layer is independently 15-35 wt%; the thickness of the first phenolic resin adhesive layer and the second phenolic resin adhesive layer is independently 2-4 μm.

[0024] Preferably, the porous graphite plate has a thickness of 5–10 mm, a porosity of 40–60%, and a pore size of 8–30 μm.

[0025] Preferably, the thickness of the silicon carbide powder layer is 20-30 mm; the particle size of the silicon carbide powder used to lay the silicon carbide powder layer is 40-60 mesh.

[0026] This invention provides a method for preparing silicon carbide single crystals. In this invention, a silicon carbide coating is applied to the back of the seed crystal (i.e., the silicon surface of the seed crystal). Simultaneously, through optimized design of the seed crystal support structure, such as using a porous graphite plate with a silicon carbide powder layer on its surface, the uneven heat dissipation problem caused by the bonding and fixing of the seed crystal, graphite paper, and graphite cap in traditional methods can be avoided. Furthermore, it effectively prevents corrosion of the seed crystal's back side, thereby obtaining high-quality silicon carbide single crystals. Specifically, traditional methods directly bond the crystal to the graphite cap, which easily leads to high thermal stress due to the difference in the coefficients of thermal expansion between the seed crystal and graphite. To overcome this problem, existing methods add graphite paper buffer between the seed crystal and the graphite cap, but still rely on bonding to fix the seed crystal, graphite paper, and graphite cap. Bonding the graphite paper buffer cannot completely avoid stress problems, and due to the porosity of the graphite paper material itself and the carbonization failure of the adhesive above 2000℃, weak or uneven bonding often occurs, frequently resulting in seed crystal ablation or reverse corrosion of the back side. This invention addresses the issues of thermal stress and uneven heat dissipation caused by bonding the seed crystal to other graphite components by applying a carbonized coating to the back of the seed crystal and improving the seed crystal support structure. This eliminates the need for bonding the seed crystal to other graphite components, ensuring the final product is a high-quality silicon carbide single crystal. Furthermore, the carbonized coating effectively prevents the volatilization of silicon vapor from the back of the seed crystal, thus preventing back-side corrosion. Simultaneously, the silicon carbide powder layer on the porous graphite plate surface provides sufficient Si, Si₂C, and SiC₂ atmospheres, further preventing back-side corrosion and guaranteeing the final product is a high-quality silicon carbide single crystal. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the crystal growth crucible of the present invention. In the figure, 1 is a graphite cover, 2 is a temperature measuring point of the graphite cover, 3 is a vent hole of the graphite cover, 4 is a silicon carbide powder layer, 5 is a porous graphite plate, 6 is a vent hole of the porous graphite plate, 7 is a seed crystal with coating, 8 is a second graphite support ring, 9 is a first graphite support ring, 10 is a loaded graphite crucible, and 11 is silicon carbide raw material for crystal growth.

[0028] Figure 2 This is a schematic diagram of a seed crystal with a coating. In the diagram, 12 is the seed crystal, 13 is the first phenolic resin adhesive layer, 14 is the graphite emulsion coating, 15 is the second phenolic resin adhesive layer, and 16 is the graphite powder coating.

[0029] Figure 3 The images show the coated seed crystal and silicon carbide crystal prepared in Example 3.

[0030] Figure 4 This is a physical image of the silicon carbide crystal prepared for Comparative Example 1. Detailed Implementation

[0031] This invention provides a method for preparing silicon carbide single crystals, comprising the following steps:

[0032] A coated seed crystal is provided, the coated seed crystal comprising a seed crystal and a carbide coating disposed on the silicon surface of the seed crystal;

[0033] A crystal growth crucible is provided, which, from bottom to top, comprises a loading graphite crucible, a first graphite support ring, a second graphite support ring, a porous graphite plate, and a graphite cap.

[0034] Silicon carbide raw material for crystal growth is placed in the loading graphite crucible. A first graphite support ring is assembled on the loading graphite crucible containing the silicon carbide raw material for crystal growth. The coated seed crystal is placed on the first graphite support ring with the silicon surface of the coated seed crystal facing upward. Then, a second graphite support ring is assembled on the first graphite support ring. The porous graphite plate is placed on the second support ring with a space reserved between the porous graphite plate and the coated seed crystal. A silicon carbide powder layer is laid on the surface of the porous graphite plate. Then, the graphite cover is placed on top, and the plate is heated to perform crystal growth treatment to obtain silicon carbide single crystal.

[0035] This invention provides a coated seed crystal, comprising a seed crystal and a carbonized coating disposed on the silicon surface of the seed crystal; the thickness of the carbonized coating is preferably 150–200 μm. In this invention, one side of the seed crystal is a carbon surface, and the opposite side is a silicon surface. The carbonized coating on the silicon surface prevents corrosion of the carbon surface and protects it. This invention first provides a detailed description of the preparation method of the coated seed crystal.

[0036] In this invention, the method for preparing the coated seed crystal preferably includes the following steps:

[0037] Phenolic resin is mixed with propylene glycol methyl ether acetate to obtain a phenolic resin solution;

[0038] Graphite emulsion, ethanol and phenolic resin are mixed to obtain a diluted graphite emulsion solution;

[0039] The phenolic resin solution is coated onto the silicon surface of the seed crystal, and after drying, a first phenolic resin adhesive layer is formed on the silicon surface.

[0040] The graphite emulsion diluent is coated on the surface of the first phenolic resin adhesive layer, and after drying, a graphite emulsion coating is formed on the surface of the first phenolic resin adhesive layer.

[0041] The phenolic resin solution is coated on the surface of the graphite emulsion coating, and after drying, a second phenolic resin adhesive layer is formed on the surface of the graphite emulsion coating.

[0042] Graphite powder is coated on the surface of the second phenolic resin adhesive layer to form a graphite powder coating, which is then sintered and cured to obtain a seed crystal with a coating.

[0043] This invention mixes phenolic resin with propylene glycol methyl ether acetate to obtain a phenolic resin solution. In this invention, the phenolic resin is preferably a linear phenolic resin, more preferably an electronic-grade high-purity linear phenolic resin (purchased from Jinan Shengquan Group Co., Ltd., product model SH5065 or SH5075); the propylene glycol methyl ether acetate (PGMEA) is preferably an electronic-grade propylene glycol methyl ether acetate. In this invention, the concentration of phenolic resin in the phenolic resin solution is preferably 15-35 wt%, specifically 15 wt%, 20 wt%, 25 wt%, 30 wt%, or 35 wt%. This invention utilizes the phenolic resin solution to prepare a first phenolic resin adhesive layer and a second phenolic resin adhesive layer. The concentration of phenolic resin in the phenolic resin solution used to prepare the first and second phenolic resin adhesive layers can be the same or different, as long as it meets the actual bonding requirements; this invention does not have any special limitations in this regard. In this invention, the phenolic resin is preferably mixed with propylene glycol methyl ether acetate and then heated to ensure that the phenolic resin is fully dissolved; the heating temperature is preferably 75-85°C, more preferably 80°C; the heating time is preferably 2-4 hours, more preferably 3 hours; and the heating method is preferably water bath heating.

[0044] After obtaining the phenolic resin solution, the present invention coats the phenolic resin solution onto the silicon surface of the seed crystal, and after drying, forms a first phenolic resin adhesive layer on the silicon surface. In this invention, the coating method of the phenolic resin solution is preferably spin coating, and the maximum spin coating speed is preferably 2000-3000 rpm, specifically 2000 rpm, 2500 rpm, or 3000 rpm; the spin coating time is preferably 30-80 s, more preferably 50 s. In this invention, the thickness of the first phenolic resin adhesive layer is preferably 2-4 μm, more preferably 2-3 μm. The present invention preferably selects a suitable spin coating process based on the concentration of phenolic resin in the phenolic resin solution and the desired thickness of the first phenolic resin adhesive layer. In an embodiment of the present invention, taking the preparation of a first phenolic resin adhesive layer with a thickness of 2 μm as an example, when the concentration of phenolic resin is 15 wt%, the spin coating process specifically involves sequentially performing spin coating at 500 rpm × 10 s, 1000 rpm × 10 s, 2000 rpm × 20 s, and 500 rpm × 10 s; when the concentration of phenolic resin is 20 wt%, the spin coating process specifically involves sequentially performing spin coating at 500 rpm × 10 s, 1000 rpm × 10 s, 2500 rpm × 20 s, and 500 rpm × 10 s; when the concentration of phenolic resin is 25 wt%, the spin coating process specifically involves sequentially performing spin coating at 500 rpm × 10 s, 1000 rpm × 10 s, 3000 rpm × 20 s, and 500 rpm × 10 s. In this invention, the drying temperature is preferably 100-300°C, and the drying time is preferably 30-40 minutes.

[0045] This invention involves mixing graphite emulsion, ethanol, and phenolic resin to obtain a diluted graphite emulsion solution. In this invention, the D50 of the graphite in the graphite emulsion is preferably <400 nm. In this invention, the type of phenolic resin is preferably the same as that in the phenolic resin solution, and will not be elaborated further. In this invention, the content of graphite emulsion in the diluted graphite emulsion solution is preferably 20–40 wt%, specifically 20 wt%, 25 wt%, 30 wt%, 35 wt%, or 40 wt%; the content of phenolic resin in the diluted graphite emulsion solution is preferably 4–6 wt%, more preferably 5 wt%. This invention preferably involves ultrasonication after mixing the graphite emulsion, ethanol, and phenolic resin to ensure thorough mixing of the components; the ultrasonication time is preferably 0.5–1.5 h, more preferably 1 h.

[0046] After obtaining the graphite emulsion diluent and the first phenolic resin adhesive layer, the present invention coats the surface of the first phenolic resin adhesive layer with the graphite emulsion diluent, and after drying, forms a graphite emulsion coating on the surface of the first phenolic resin adhesive layer. In the present invention, the coating method of the graphite emulsion diluent is preferably spraying, and the spraying distance is preferably 30-40 cm, more preferably 35-40 cm; the pressure of the compressed air is preferably 0.2-0.4 MPa, more preferably 0.25-0.3 MPa. In an embodiment of the present invention, a Japanese Lumina-STS-10SK spray gun is specifically used for spraying. In the present invention, the drying temperature is preferably 80-200°C, and the drying time is preferably 20-30 min. In the present invention, the thickness of the graphite emulsion coating is preferably 60-80 μm, more preferably 60-70 μm.

[0047] After obtaining the graphite emulsion coating, the present invention coats the surface of the graphite emulsion coating with the phenolic resin solution, and after drying, forms a second phenolic resin adhesive layer on the surface of the graphite emulsion coating. In the present invention, the preparation method of the second phenolic resin adhesive layer is preferably the same as the preparation method of the first phenolic resin adhesive layer, and will not be described again here.

[0048] After obtaining the second phenolic resin adhesive layer, the present invention coats the surface of the second phenolic resin adhesive layer with graphite powder to form a graphite powder coating on the surface of the second phenolic resin adhesive layer, and then sintersties and cures it to obtain seed crystals containing the coating. In the present invention, the D50 of the graphite powder is preferably 1-5 μm, more preferably 2-5 μm; the graphite powder is preferably high-purity graphite powder, and the purity is preferably 5N. In the present invention, the coating method of the graphite powder is preferably spraying, the spraying distance is preferably 30-50 cm, more preferably 40-50 cm; the pressure of the compressed air is 0.2-0.4 MPa, more preferably 0.25-0.3 MPa. In the embodiments of the present invention, a Japanese Lumina-STS-10SK spray gun is specifically used for spraying. In the present invention, the thickness of the graphite powder coating is preferably 70-130 μm, more preferably 80-100 μm. In this invention, the sintering and curing temperature is preferably 300–500°C, and the time is preferably 2–3 hours. The sintering and curing is preferably carried out under vacuum conditions, specifically in a vacuum furnace. In this invention, the function of the sintering and curing is to remove the solvent from propylene glycol methyl ether acetate and graphite emulsion, and after the phenolic resin crosslinks and cures, to fix the graphite powder and graphite in the graphite emulsion onto the silicon surface of the seed crystal.

[0049] This invention provides a crystal growth crucible, which, from bottom to top, comprises a loading graphite crucible, a first graphite support ring, a second graphite support ring, a porous graphite plate, and a graphite cap. In this invention, the loading graphite crucible is used to hold silicon carbide raw material for crystal growth, the first graphite support ring is used to support the coated seed crystal, and the second graphite support ring is used to support the porous graphite plate. In this invention, the thickness of the porous graphite plate is preferably 5–10 mm, more preferably 6–8 mm; the porosity is preferably 40–60%, more preferably 50–60%; and the pore size is preferably 8–30 μm. In this invention, the porous graphite plate preferably has vent holes, the diameter of which is preferably 1-2 mm, and the number of vent holes is preferably four. These four vent holes are preferably arranged in a circumferentially evenly spaced array around the center of the porous graphite plate, and the diameter of the inscribed circle of each vent hole is preferably 100 mm. This invention preferably uses vent holes in the porous graphite plate to better maintain pressure and prevent the deposition of silicon carbide on the back of the seed crystal. The porous graphite plate in this invention serves to support the silicon carbide powder layer laid on its surface, ensuring a sufficient atmosphere of Si, Si2C, and SiC2. This invention preferably uses a porous graphite plate with the above-mentioned properties, which can filter out larger clusters of Si, Si2C, and SiC2 gas molecules to prevent deposition on the back of the seed crystal. In this invention, a temperature measuring point is provided at the center of the graphite cover, and vent holes are distributed around the temperature measuring point of the graphite cover; the diameter of the vent holes is preferably 1 to 3 mm, the number of vent holes is preferably 4, and the 4 vent holes are preferably distributed in an array at equal intervals along the circumference; the diameter of the outer tangent circle of the 4 vent holes is preferably 100 to 130 mm.

[0050] The present invention discloses a method for preparing silicon carbide single crystals based on the coated seed crystal and the crystal growth crucible, comprising the following steps: placing silicon carbide raw material for crystal growth in the loading graphite crucible; assembling a first graphite support ring on the loading graphite crucible containing the silicon carbide raw material for crystal growth; placing the coated seed crystal on the first graphite support ring with the silicon face of the coated seed crystal facing upwards; then assembling a second graphite support ring on the first graphite support ring; placing a porous graphite plate on the second support ring with a space reserved between the porous graphite plate and the coated seed crystal; laying a silicon carbide powder layer on the surface of the porous graphite plate; then covering it with the graphite cover; and heating to perform crystal growth treatment to obtain silicon carbide single crystals.

[0051] In this invention, the particle size of the silicon carbide raw material used for crystal growth is preferably 8-40 mesh. The thickness of the silicon carbide powder layer laid on the surface of the porous graphite plate is preferably 20-30 mm, more preferably 23-25 ​​mm; the particle size of the silicon carbide powder used to lay the silicon carbide powder layer is preferably 40-60 mesh. When the porous graphite plate has venting pores, the silicon carbide powder layer laid on the surface of the porous graphite plate is preferably compacted to create a "bridging" effect, preventing leakage from the venting pores. By laying the silicon carbide powder layer on the surface of the porous graphite plate, this invention can provide vapors such as Si, Si2C, and SiC2 during the crystal growth process, preventing reverse corrosion of the seed crystal, maintaining the integrity of the seed crystal, and avoiding seed crystal ablation, thereby ensuring smooth crystal growth. This invention does not have a special limitation on the size of the space reserved between the porous graphite plate and the coated seed crystal, as long as they do not come into contact.

[0052] In this invention, the temperature of the crystal growth treatment is preferably 2100–2200°C, more preferably 2100–2150°C; the time is preferably 80–120 h, more preferably 95–105 h; the crystal growth treatment is preferably carried out under a pressure of 3–5 mbar (i.e., close to vacuum). In this invention, the time to raise the temperature to the required temperature for the crystal growth treatment is preferably 6–12 h, more preferably 10–12 h; the heating process is preferably carried out under a pressure of 600–800 mbar, more preferably 700–800 mbar; the pressure is preferably provided by argon gas, and the flow rate of the argon gas is preferably 100–400 sccm, more preferably 100–200 sccm.

[0053] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0054] The sources of some of the raw materials used in this invention are as follows:

[0055] The phenolic resin is specifically electronic-grade high-purity linear phenolic resin, purchased from Jinan Shengquan Group Co., Ltd., product model SH5075;

[0056] Propylene glycol methyl ether acetate (PGMEA) is specifically electronic grade propylene glycol methyl ether acetate with a purity of 99.9999%.

[0057] The graphite emulsion was purchased from Shanghai Jizhi Biochemical Technology Co., Ltd., under the Acmec brand, with a D50 of <400nm and 99.95% metals basis.

[0058] The graphite powder is high-purity graphite powder (5N) with a particle size of 2-5 μm.

[0059] Example 1

[0060] (1) Preparation of coated seed crystals, including the following steps:

[0061] Phenolic resin and PGMEA were mixed at a mass ratio of 15:85 and heated in a water bath at 80°C for 3 hours to fully dissolve the phenolic resin and obtain a phenolic resin solution. Graphite emulsion, ethanol and phenolic resin were mixed at a mass ratio of 20:75:5, stirred with a glass rod for 5 minutes, and then ultrasonically dispersed for 1 hour to ensure uniform mixing of all components and obtain a diluted graphite emulsion solution.

[0062] The phenolic resin solution was uniformly spin-coated onto the silicon surface of the seed crystal using a spin coater. The spin coating process was as follows: 500 rpm × 10 s, 1000 rpm × 10 s, 2000 rpm × 20 s, and 500 rpm × 10 s were performed sequentially. After spin coating, the silicon surface of the seed crystal was placed upwards and baked at 80°C for 20 min to allow the solvent to fully evaporate, forming a first phenolic resin adhesive layer with a thickness of 2 μm on the surface of the silicon surface.

[0063] The graphite emulsion diluent was sprayed onto the surface of the first phenolic resin adhesive layer using a Japanese Lumina-STS-10SK spray gun at a spray pressure of 0.25 MPa and a spray distance of 35 cm. After spraying, the silicon side of the seed crystal was placed upward and baked at 100°C for 40 minutes to allow the solvent to evaporate fully, thus forming a graphite emulsion coating with a thickness of 60 μm on the surface of the first phenolic resin adhesive layer.

[0064] A second phenolic resin adhesive layer is prepared on the surface of the graphite emulsion coating in the same manner as the first phenolic resin adhesive layer is prepared.

[0065] Graphite powder was sprayed onto the surface of the second phenolic resin adhesive layer using a Lumina-STS-10SK spray gun (Japan). The compressed air pressure was 0.25 MPa, and the spraying distance was 35 cm. After spraying, a 70 μm thick graphite powder coating was formed on the surface of the second phenolic resin adhesive layer. The layer was then placed in a vacuum furnace and sintered at 300°C for 2 hours to form a 150–200 μm thick carbonized coating on the seed silicon surface, resulting in a coated seed crystal (e.g., ...). Figure 2 (As shown).

[0066] (2) Crystal growth loading and seed crystal assembly:

[0067] The seed crystal assembly crucible used, from bottom to top, includes a loading graphite crucible, a first graphite support ring, a second graphite support ring, a porous graphite plate, and a graphite cap; such as Figure 1 As shown, 4 kg ± 50 g of silicon carbide raw material for crystal growth with a particle size of 8-40 mesh is loaded into the charging graphite crucible. Then, the first graphite support ring is assembled, and the coated seed crystal is placed on the first graphite support ring with the silicon face upwards and the carbon face (i.e., the crystal growth face) downwards. Next, a second graphite support ring is assembled on the first graphite support ring, and the porous graphite plate is placed on the second support ring, with a space reserved between the porous graphite plate and the coated seed crystal. The porous graphite plate has a thickness of 6 mm, a porosity of 57%, and a pore size of 8-30 μm. Four first vent holes are evenly spaced around the center of the porous graphite plate, each with a diameter of 1 mm and a preferred inscribed circle diameter of 100 mm. Then, a 25 mm thick layer of silicon carbide powder is laid on the surface of the porous graphite plate (compacted after laying to prevent leakage of silicon carbide powder from the first vent holes). The silicon carbide powder used has a particle size of 40-60 mesh. Finally, a graphite cover is placed on top. A temperature measuring point is located at the center of the graphite cover, and four second vent holes are evenly spaced around the temperature measuring point, each with a diameter of 2 mm and a circumscribed circle diameter of 120 mm.

[0068] (3) Crystal growth:

[0069] First, under a pressure of 800 mbar (provided by argon gas at a flow rate of 150 sccm), the temperature was increased from room temperature (25°C) to 2150°C over a 10-hour period. Then, after another 10-hour period, the pressure was reduced to 3 mbar, and the crystals were grown at 2150°C for 100 hours to obtain SiC single crystals.

[0070] Examples 2-9

[0071] SiC single crystals were prepared according to the method of Example 1, except that the formulations (by mass percentage) of the phenolic resin solution and graphite emulsion diluent used in preparing the seed crystal with coating and the spin coating process used in preparing the first phenolic resin adhesive layer are listed in Table 1.

[0072] Table 1. Raw material formulations and spin-coating processes used in the preparation of coated seed crystals in Examples 1-9.

[0073]

[0074]

[0075] Note: Different spin coating processes are used depending on the viscosity of the phenolic resin solution, where:

[0076] 1) The spin coating process with a maximum speed of 2000 rpm is specifically performed in sequence as follows: 500 rpm × 10 s, 1000 rpm × 10 s, 2000 rpm × 20 s, and 500 rpm × 10 s.

[0077] 2) The spin coating process with a maximum speed of 2500 rpm is specifically performed in sequence as follows: 500 rpm × 10 s, 1000 rpm × 10 s, 2500 rpm × 20 s, and 500 rpm × 10 s.

[0078] 3) The spin coating process with a maximum speed of 3000 rpm is specifically performed in sequence as follows: 500 rpm × 10 s, 1000 rpm × 10 s, 3000 rpm × 20 s, and 500 rpm × 10 s.

[0079] Figure 3 These are physical images of the coated seed crystal and silicon carbide crystal prepared in Example 3. Figure 3 The left side shows a physical image of the seed crystal with coating. The results show that the coating on the back side (Si surface) of the seed crystal is uniform. Figure 3 The middle and right sides are actual images of silicon carbide crystals. The results show that there was no ablation on the back of the crystal after the crystal growth was completed and it was taken out of the furnace, and the seed crystal was intact.

[0080] Three silicon carbide crystal samples were prepared according to the method in Example 3, and their crystal defect indices were tested. The specific test results are shown in Table 2. Table 2 shows that the crystals produced using the method of this invention exhibited good defect indices after furnace exit. The microtube density, detected by the KOH etching method, was <0.5 cells / cm³. 2 Through-type screw dislocations (TSD) < 200 / cm 2 Base plane dislocations (BPD) < 500 / cm 2 Edge dislocations (TED) < 5000 / cm 2 .

[0081] Table 2. Detection results of crystal defect indices for silicon carbide crystals prepared in Example 3.

[0082]

[0083]

[0084] Comparative Example 1

[0085] Silicon carbide crystals were prepared using a traditional bonded seed crystal growth method, specifically by using a phenolic resin solution (25 wt% phenolic resin + 75 wt% PGMEA) to bond graphite paper, which was then bonded to a graphite cap; the crystal growth process was the same as in Example 3.

[0086] Figure 4 This is a physical image of the silicon carbide crystal prepared in Comparative Example 1. Figure 4 The left side shows the assembly diagram of the bonded seed crystal and the graphite cap; the middle side shows the back view of the bonded seed crystal; and the right side shows the front view of the bonded seed crystal. Figure 4 It is known that after the crystal growth is completed and the seed crystal is taken out of the furnace, the seed crystal cracks due to excessive stress. In the case of bonded seed crystals, the graphite paper and graphite cover or the graphite paper and seed crystal separate at the crystal growth temperature, resulting in uneven temperature distribution of the seed crystal and cracking and ablation. Therefore, the crystal growth fails.

[0087] As can be seen from the above embodiments and comparative examples, the present invention, through the optimized design of the seed crystal support structure, the use of a porous graphite plate and the setting of a silicon carbide powder layer on the surface of the porous graphite plate, can avoid the problem of uneven heat dissipation caused by the bonding and fixing of the seed crystal, graphite paper and graphite cover in the traditional method, and can effectively prevent corrosion on the back of the seed crystal, thereby obtaining high-quality silicon carbide single crystals.

[0088] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing silicon carbide single crystals, comprising the following steps: A coated seed crystal is provided, the coated seed crystal comprising a seed crystal and a carbide coating disposed on the silicon surface of the seed crystal; The method for preparing the coated seed crystal includes the following steps: mixing phenolic resin with propylene glycol methyl ether acetate to obtain a phenolic resin solution; mixing graphite emulsion, ethanol, and phenolic resin to obtain a graphite emulsion dilution; coating the silicon surface of the seed crystal with the phenolic resin solution, and after drying, forming a first phenolic resin adhesive layer on the silicon surface; coating the surface of the first phenolic resin adhesive layer with the graphite emulsion dilution, and after drying, forming a graphite emulsion coating on the surface of the first phenolic resin adhesive layer. The phenolic resin solution is coated on the surface of the graphite emulsion coating, and after drying, a second phenolic resin adhesive layer is formed on the surface of the graphite emulsion coating; graphite powder is coated on the surface of the second phenolic resin adhesive layer, and a graphite powder coating is formed on the surface of the second phenolic resin adhesive layer, and then sintered and cured to obtain the coated seed crystal. A crystal growth crucible is provided, which, from bottom to top, comprises a loading graphite crucible, a first graphite support ring, a second graphite support ring, a porous graphite plate, and a graphite cap. Silicon carbide raw material for crystal growth is placed in the loading graphite crucible. A first graphite support ring is assembled on the loading graphite crucible containing the silicon carbide raw material for crystal growth. The coated seed crystal is placed on the first graphite support ring with the silicon face facing upward. Then, a second graphite support ring is assembled on the first graphite support ring. The porous graphite plate is placed on the second graphite support ring with a space reserved between the porous graphite plate and the coated seed crystal. A silicon carbide powder layer is laid on the surface of the porous graphite plate. Then, the graphite cover is placed on top, and the plate is heated to perform crystal growth treatment to obtain silicon carbide single crystal.

2. The preparation method according to claim 1, characterized in that, The thickness of the carbonized coating is 150~200μm.

3. The preparation method according to claim 1, characterized in that, The graphite in the graphite emulsion has a D50 of less than 400 nm; the thickness of the graphite emulsion coating is 60-80 μm.

4. The preparation method according to claim 1, characterized in that, The graphite emulsion diluent is coated by spraying, with a spraying distance of 30-40cm and a compressed air pressure of 0.2-0.4MPa.

5. The preparation method according to claim 1, characterized in that, The graphite powder has a D50 of 1~5μm; the graphite powder coating has a thickness of 70~130μm.

6. The preparation method according to claim 1, characterized in that, The graphite powder is coated by spraying, with a spraying distance of 30-50cm and a compressed air pressure of 0.2-0.4MPa.

7. The preparation method according to claim 1, characterized in that, The phenolic resin content in the phenolic resin solution used to prepare the first phenolic resin adhesive layer and the second phenolic resin adhesive layer is independently 15~35wt%; the thickness of the first phenolic resin adhesive layer and the second phenolic resin adhesive layer is independently 2~4μm.

8. The preparation method according to claim 1, characterized in that, The porous graphite plate has a thickness of 5-10 mm, a porosity of 40-60%, and a pore size of 8-30 μm.

9. The preparation method according to claim 1, characterized in that, The thickness of the silicon carbide powder layer is 20~30mm; the particle size of the silicon carbide powder used to lay the silicon carbide powder layer is 40~60 mesh.

Images