Crucible assembly for synthesizing silicon carbide powder and synthesis method thereof

Abstract

The invention belongs to the technical field of semiconductor material preparation, and particularly relates to a crucible assembly for synthesizing silicon carbide powder and a synthesis method of the crucible assembly. The crucible assembly comprises an outer crucible, a charging crucible and a supporting ring. The outer crucible is made of isostatic graphite, the side wall of the inner cavity is provided with a gas guide groove, and the bottom and top crucible covers are provided with vent holes; the charging crucible is made of porous graphite and is accommodated in the outer crucible; the support ring is arranged between the two, and the support surface is provided with a groove. Through the synergistic effect of the bottom vent hole, the supporting ring groove, the outer crucible side wall gas guide groove and the top vent hole, a gas channel of bottom gas inlet, side wall flow guide and top gas exhaust is formed. Impurities such as nitrogen adsorbed by the raw materials can be forcibly discharged, and the nitrogen content of the powder is effectively reduced; and meanwhile, excessive heat on the side wall and the bottom of the crucible is taken away through gas flow, the uniformity of a synthetic thermal field is remarkably improved, local excessive carbonization of the powder is avoided, and therefore the purity and consistency of the silicon carbide powder are synchronously improved.

Description

Technical Field

[0001] This invention relates to the field of semiconductor material preparation technology, and in particular to a crucible assembly for synthesizing silicon carbide powder and its synthesis method. Background Technology

[0002] Silicon carbide (SiC), as a core representative of third-generation wide-bandgap semiconductor materials, possesses excellent properties such as a large bandgap, high thermal conductivity, high critical breakdown field strength, and high-temperature chemical stability, and is widely used in equipment fields such as new energy vehicles, rail transportation, aerospace, and 5G communications. Physical vapor transport (PVT) is the mainstream method for growing SiC single crystals, and high-purity SiC powder is its key raw material.

[0003] Currently, high-temperature self-propagating synthesis is a commonly used method for preparing silicon carbide powder. However, this method has significant problems in practical applications: First, during electromagnetic induction heating, due to uneven magnetic field distribution and the skin effect, the temperature at the bottom and sidewalls of the crucible is much higher than that in the center, resulting in uneven carbonization of the powder. The powder at the edges and bottom is over-carbonized or even decomposes, severely affecting the overall quality of the batch. Second, for synthesizing semi-insulating silicon carbide powder, the raw materials (silicon powder, carbon powder), graphite crucible, and insulation materials adsorb nitrogen from the air, which is difficult to remove completely at high temperatures. Residual nitrogen impurities enter the crystal lattice during single crystal growth, compensating for acceptors and causing the crystal resistivity to fail to meet the semi-insulating requirements (>10). 5 (Ωcm), causing the product performance to fail.

[0004] Several improvements have been made in the prior art. For example, CN117663763A reduces the bottom temperature by introducing gas through a heat-insulating cavity at the bottom of the crucible, but this does not address the high temperature problem on the side walls, and the single gas flow path limits nitrogen removal efficiency. CN115896945A shortens the nitrogen escape path by employing a multi-layer sub-crucible structure and adds an exhaust channel in the center, effectively reducing the nitrogen content in the powder. However, this multi-layer structure is complex to assemble and does not optimize the thermal uniformity of the crucible side walls and bottom, leaving room for improvement in the consistency of powder synthesis.

[0005] Therefore, there is an urgent need in the field for an integrated crucible assembly and method to solve the two major problems of thermal field uniformity and deep denitrification in the prior art. Summary of the Invention

[0006] The purpose of this invention is to provide a crucible assembly for synthesizing silicon carbide powder and its synthesis method, so as to solve the two major problems of thermal field uniformity and deep denitrification in the prior art.

[0007] The technical solution of the present invention is: on one hand, a crucible assembly for synthesizing silicon carbide powder, comprising: An outer crucible has a bottom wall and a side wall extending upward from the bottom wall, and a sealable outer crucible lid is disposed on the top. The outer crucible is made of isostatic graphite, and a plurality of axially extending gas guide grooves are provided on the inner cavity side wall. At least one through vent hole is provided on the bottom wall and the outer crucible lid, respectively. A charging crucible for holding a mixture of carbon powder and silicon powder, and equipped with a charging crucible lid, the charging crucible being made of porous graphite material and housed within the inner cavity of the outer crucible; A support ring is disposed between the bottom wall of the outer crucible and the bottom wall of the loading crucible to support and position the loading crucible.

[0008] Preferably, at least one through-hole is provided on the bottom wall of the outer crucible and the outer crucible cover.

[0009] Preferably, the upper surface of the support ring is a support surface for supporting the charging crucible, and the support surface is provided with a groove for diverting gas.

[0010] Preferably, the number and position of the grooves correspond to the air guide grooves.

[0011] Preferably, the cross-sections of the inner cavity of the outer crucible and the inner cavity of the charging crucible are circular.

[0012] Preferably, the cross-sectional shape of the air guide groove is one of square, semi-circular or triangular.

[0013] Preferably, the outer crucible has a modular structure, with its sidewalls composed of two semicircular rings joined together by an inlay method, or divided into two parts along the vertical direction and assembled by an inlay method.

[0014] On the other hand, the present invention provides a method for synthesizing silicon carbide powder, comprising: (1) The uniformly mixed silicon powder and carbon powder are loaded into the loading crucible; (2) Place the assembled crucible assembly into the furnace chamber of the heating furnace and seal the furnace chamber; (3) Vacuum treatment is performed on the furnace chamber; (4) Inert gas is introduced into the furnace and the gas is continuously introduced through the vent at the bottom of the outer crucible; (5) The crucible assembly is heated by programmed heating to complete the synthesis reaction, crystal transformation and grain growth of silicon carbide powder. During the heating process, the gas flows through the following path: it enters from the bottom vent hole, is diverted through the support ring groove, enters the outer crucible side wall gas guide groove and rises, and at the same time permeates through the porous loading crucible wall, and finally exits from the top vent hole.

[0015] Preferably, the programmed heating specifically includes: (1) Evacuate the furnace to a vacuum level of 4.5~5.5×10⁻⁶. -6 mbar, and heated to 900~1200℃; (2) Pour high-purity argon gas into the furnace to 800 mbar, maintain the pressure for 10-20 minutes, and then evacuate to a vacuum of 4.5-5.5 × 10⁻⁶ mbar. - 6 mbar, and repeat this process multiple times; (3) At a vacuum degree of 4.5~5.5×10 -6 The synthesis reaction was carried out at mbar by heating to 1300~1400℃ and holding for 30~60 min. (4) Continue heating to 2000~2100℃ and hold for 8~10 hours to carry out the transformation of the powder crystal form and the growth of the grains.

[0016] Compared with the prior art, the advantages of the present invention are: (1) A three-dimensional and efficient gas flow path was constructed: Through the coordinated design of bottom vent, grooved support ring, side wall gas guide groove and top vent, an active and forced circulation gas flow path was formed, which can not only completely remove impurities such as nitrogen adsorbed in powder and graphite parts, but also use gas flow to remove excess heat from the crucible side wall and bottom, thus realizing active homogenization of the thermal field.

[0017] (2) Significantly improves powder quality and consistency: This design effectively avoids excessive carbonization or decomposition of powder caused by local overheating in traditional synthesis, while reducing the nitrogen content of powder to an extremely low level (below 5×10). 16 (atoms / cm³), particularly suitable for the growth of high-quality semi-insulating silicon carbide single crystals.

[0018] (3) Ingenious structure and strong practicality: The modular outer crucible design facilitates assembly, disassembly and maintenance, and the functions of each component are coordinated. Attached Figure Description

[0019] The present invention will be further described below with reference to the accompanying drawings and embodiments: Figure 1 This is a horizontal cross-sectional view of the outer crucible described in this invention; Figure 2 This is a vertical cross-sectional view of the outer crucible described in this invention; Figure 3 This is a schematic diagram of the support ring described in this invention; Figure 4 This is a simplified cross-sectional view of the charging crucible described in this invention; Figure 5 This is a cross-sectional view of the crucible assembly of the present invention in the horizontal direction after assembly; Figure 6This is a vertical cross-sectional view of the crucible assembly described in this invention after assembly.

[0020] The components are: 1. outer crucible; 11. gas guide groove; 12. bottom vent hole; 13. outer crucible cover; 14. top vent hole; 2. support ring; 21. groove; 3. loading crucible; 31. loading crucible cover. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to specific embodiments: The working principle of this invention is as follows: During the synthesis process, the inert gas continuously introduced through the bottom vent 12 forms an active circulation system. This gas flow path plays two core roles: 1. Actively homogenizes the thermal field, improving powder consistency: The gas is first evenly distributed through the grooves 21 of the support ring 2 to the gas guide grooves 11 on each side wall. As it rises in the gas guide grooves 11, the gas continuously carries away the heat that is excessively concentrated on the side wall of the crucible due to the electromagnetic skin effect through convective heat transfer, achieving directional cooling of the side wall. At the same time, under the action of pressure difference, a portion of the gas slowly permeates into the interior of the powder bed through the wall of the charging crucible 3 made of porous graphite. This permeating airflow disturbs the static atmosphere between the powder particles, promoting heat transfer and convection within the powder. Thus, from the two dimensions of "external crucible cooling" and "internal powder homogenization," the overall thermal field uniformity of the synthesis reaction is significantly improved, effectively avoiding excessive carbonization or decomposition caused by local overheating.

[0022] 2. Forced removal of impurities for deep purification: The gas that permeates into the powder and flows through the gap between the powder and the crucible wall together form a "purge airflow" that runs through the powder area. This airflow can effectively carry volatile impurities such as nitrogen desorbed from the raw materials and graphite components, and cause them to rise rapidly along the main channel of the side wall gas guide groove 11, and finally be forcibly discharged from the reaction system through the top vent 14. This three-dimensional, active, and forced circulation gas path of "bottom air intake → side wall guide → internal purging → top exhaust" breaks through the limitations of relying solely on vacuum diffusion exhaust and achieves deep removal of nitrogen impurities.

[0023] Example 1

[0024] like Figures 1 to 6 As shown, a crucible assembly for synthesizing silicon carbide powder includes: Structure and Assembly: Outer crucible 1: as shown Figure 1 The outer crucible 1 shown has a cylindrical structure in its horizontal cross-sectional view. It is machined from high-purity isostatic graphite (purity > 99.99%). Figure 2 As shown, this is a vertical cross-sectional view of the outer crucible 1. 36 rectangular gas guide grooves 11 are uniformly machined on the inner wall of the crucible. The grooves are 2 mm deep and 3 mm wide. These gas guide grooves 11 extend axially along the sidewall from near the bottom to near the top. A bottom vent hole 12 with a diameter of 8 mm is opened at the center of the bottom wall of the outer crucible 1, and a top vent hole 14 with a diameter of 8 mm is opened on the outer crucible cover 13.

[0025] Support ring 2: as Figure 3 As shown in the schematic diagram, the support ring 2 is circular in shape and made of isostatically pressed graphite. Its outer diameter is fitted with the inner diameter of the outer crucible 1 with a clearance fit, and its inner diameter is fitted with the outer diameter of the loading crucible 3 with a clearance fit. On its support surface, 36 fan-shaped grooves 21 are radially machined, and their positions and numbers correspond one-to-one with the gas guide grooves 11 to ensure unobstructed gas flow.

[0026] Charging crucible 3: such as Figure 4 The diagram shows a simplified cross-sectional view of the charging crucible 3 in the vertical direction. Its structural schematic is a cylindrical structure with a lid, made of porous graphite with a preferred porosity of 15% to 25%, used to hold a mixture of high-purity silicon powder and high-purity carbon powder. This porous characteristic is key to achieving inert gas permeation into the powder, thereby simultaneously achieving both thermal field homogenization and deep denitrification. If dense isostatic graphite were used, effective internal gas permeation and purging would be impossible.

[0027] During assembly, such as Figure 6 As shown, first place the support ring 2 on the inner bottom wall of the outer crucible 1, ensuring that its groove 21 is precisely aligned with the bottom of the gas guide groove 11 on the inner wall of the outer crucible 1. Then, as... Figure 5 As shown, the filling crucible 3, which is already filled with the mixed raw materials, is carefully placed into the outer crucible 1, so that its bottom rests stably on the support ring 2. Next, the filling crucible lid 31 of the filling crucible 3 is put on, and finally the outer crucible lid 13 is sealed.

[0028] Example 2

[0029] The only difference between this embodiment and Embodiment 1 is that the cross-sectional shape of the gas guide groove 11 on the inner wall of the outer crucible 1 is changed from rectangular to semi-circular. During machining, the cross-sectional area of ​​the semi-circular groove is controlled to be basically the same as that of the rectangular groove in Embodiment 1 for fair comparison. Since the semi-circular cross-section has superior hydrodynamic characteristics, it can effectively reduce the frictional resistance of gas flow. Experiments show that, under the same inlet pressure, the gas flow rate in this embodiment is increased by approximately 5-8% compared to Embodiment 1. This increase brings two direct beneficial effects: To address the issue of uneven thermal field: a slightly higher gas flow rate enhances the convective heat transfer capacity of the sidewall, resulting in a further reduction of the average temperature of the sidewall region by approximately 5-10°C compared to Example 1, thus further improving the uniformity of the thermal field.

[0030] Regarding the issue of residual nitrogen impurities: The slightly higher flow rate enhances the ability to "pump" and "carry" impurity gases inside the powder, which helps to remove impurities such as nitrogen from the central area of ​​the powder more quickly and thoroughly.

[0031] Example 3

[0032] The difference between this embodiment and Embodiment 1 is that the outer crucible 1 adopts a structure that is divided into two halves along the vertical direction, and is connected and sealed by an external flange and high-temperature resistant bolts.

[0033] The corresponding technical problems solved and the beneficial effects: This structure primarily addresses the problems arising from the difficulty of assembling large, heavy crucible components and the high maintenance costs. After loading and synthesis are completed, there is no need to remove the heavy loading crucible 3 from the top; simply loosening the bolts allows the outer crucible 1 to be opened, and the inner loading crucible 3 can be easily removed for material replacement or cleaning. This greatly reduces operational difficulty and labor intensity, while significantly reducing the risk of damage to large, brittle graphite components during frequent assembly and disassembly, demonstrating the practicality and economy of the structural design of this invention.

[0034] Comparative Example 1 A single isostatic graphite crucible is used, with a sealed, insulated cavity at the bottom. Argon gas is introduced into this cavity through an inlet pipe, and the bottom temperature is reduced by gas insulation. The side walls of the crucible are solid, without any gas guide grooves or dedicated vents at the top. Gas exchange mainly relies on natural diffusion within the furnace space and limited bottom airflow.

[0035] Comparative Example 2 A solid isostatic graphite crucible is used, with only one vent hole in the center of the bottom. The side walls are solid and have no venting grooves 11, and there is no vent hole at the top. This is a simple and low-cost conventional solution currently used by many production units to try to solve the problem of high temperature at the bottom.

[0036] Comparative Example 3 The crucible consists of five independent sub-crucibles stacked together. Each sub-crucible has a venting tube with a venting groove at the center of its bottom, which reduces the nitrogen content by shortening the path of nitrogen gas escaping from the center of the powder. This structure does not have a dedicated venting groove 11 for cooling the sidewalls of the crucible, and its thermal management relies on a traditional insulation structure.

[0037] Performance comparison test and results analysis Using the crucibles of Examples 1-3 and Comparative Examples 1-3, silicon carbide powder synthesis experiments were conducted under identical raw materials (fixed ratios of high-purity silicon powder and high-purity carbon powder), identical thermal environment (the same medium-sized medium-frequency induction heating furnace), and identical synthesis process route (using the vacuuming, gas washing, and segmented heating and holding process described in Example 1). The synthesized powder was systematically characterized, and the key results are summarized in Table 1. Table 1 Key performance data of the powder synthesized in each embodiment and comparative example.

[0038] It should be noted that the yield rate in Table 1 refers to the proportion of powder that meets the preset carbon content and crystal form standards. The criteria for judging the uniformity of powder carbonization (yield rate) in Table 1 are as follows: the carbon-silicon molar ratio of silicon carbide powder is between 0.98 and 1.02, and no free carbon or free silicon is detected. At the same time, the powder color is uniform, and it is recorded as good.

[0039] Results Analysis and Conclusions: Regarding nitrogen content: The nitrogen content of the powder in Examples 1 and 2 of this invention is the lowest, significantly lower than that in Comparative Examples 1 and 2 by an order of magnitude. This indicates that the three-dimensional gas passage constructed in this invention has a far superior ability to force the discharge of impurity gases (especially nitrogen) than conventional solutions that only optimize the bottom or use a single vent.

[0040] Although the nitrogen content of Comparative Example 3 (multi-layer sub-crucible) was better than that of Comparative Examples 1 and 2, it was still significantly higher than that of the embodiments of the present invention. This indicates that relying solely on shortening the diffusion path without constructing an active, efficient, and sidewall-penetrating gas circulation network results in poor denitrification performance and fails to achieve the deep denitrification level achievable by the present invention.

[0041] Regarding thermal uniformity and powder quality: Embodiments 1 and 2 of the present invention exhibit the smallest radial temperature gradient and the highest powder yield, which is directly related to the active cooling effect of the sidewall air guide groove 11 and the internal heat convection and homogenization of the powder brought about by gas infiltration.

[0042] Comparative Example 1 can only improve the high temperature at the bottom to a limited extent, but cannot solve the problem of high temperature on the sidewalls, resulting in severe carbonization of the powder at the edges and a low yield.

[0043] Comparative Example 2 had the worst thermal field uniformity and the most unstable powder quality due to the lack of effective active thermal field control methods.

[0044] The structure of Comparative Example 3 improves the axial temperature distribution to some extent through layering, but due to the lack of an active cooling mechanism for the sidewalls, the improvement in radial thermal field uniformity is limited. Therefore, its powder yield and consistency are not as good as those of the present invention.

[0045] Regarding process stability and industrialization potential: The batch stability (measured by the standard deviation of yield) of the synthetic samples in this invention is significantly better than that of all comparative examples. This indicates that the technical solution of this invention is more stable, the process is superior, and it is more suitable for the requirements of large-scale, stable, and reproducible industrial production.

[0046] In summary, through the systematic experimental comparison of the above-mentioned targeted design embodiments and comparative examples, it is fully verified that the crucible assembly provided by the present invention, through its original "three-dimensional gas passage" design, solves the two interrelated technical problems of "severely uneven thermal field" and "deep residual nitrogen impurities" in the high-temperature self-propagating method for synthesizing silicon carbide powder, and has high industrial application value.

[0047] The above embodiments are merely illustrative of the technical concept and features of the present invention, intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly, and should not be construed as limiting the scope of protection of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects. The scope of the present invention is defined by the appended claims rather than the foregoing description, and thus all changes falling within the meaning and scope of the equivalents of the claims are intended to be included within the present invention.

Claims

1. A crucible assembly for synthesizing silicon carbide powder, characterized in that, include: An outer crucible has a bottom wall and a side wall extending upward from the bottom wall, and a sealable outer crucible lid is disposed on the top. The outer crucible is made of isostatic graphite, and a plurality of axially extending gas guide grooves are provided on the inner cavity side wall. At least one through vent hole is provided on the bottom wall and the outer crucible lid, respectively. A charging crucible for holding a mixture of carbon powder and silicon powder, and equipped with a charging crucible lid, the charging crucible being made of porous graphite material and housed within the inner cavity of the outer crucible; A support ring is disposed between the bottom wall of the outer crucible and the bottom wall of the loading crucible to support and position the loading crucible.

2. The crucible assembly according to claim 1, characterized in that, The upper surface of the support ring is a support surface used to support the charging crucible. A groove is provided on the support surface for gas diversion.

3. The crucible assembly according to claim 2, characterized in that, The grooves correspond to the air guide grooves in both number and position.

4. The crucible assembly according to claim 1, characterized in that, The cross-sections of the inner cavity of the outer crucible and the inner cavity of the charging crucible are circular.

5. The crucible assembly according to claim 1, characterized in that, The cross-sectional shape of the air guide groove is one of square, semi-circular or triangular.

6. The crucible assembly according to claim 1, characterized in that, The outer crucible has a modular structure, with its sidewalls composed of two semicircular rings joined together by an inlay method, or divided into two parts along the vertical direction and assembled by an inlay method.

7. The crucible assembly according to claim 1, characterized in that, The support ring is made of isostatically pressed graphite.

8. A method for synthesizing silicon carbide powder, utilizing the crucible assembly according to any one of claims 1 to 7, characterized in that, include: (1) The uniformly mixed silicon powder and carbon powder are loaded into the loading crucible; (2) Place the assembled crucible assembly into the furnace chamber of the heating furnace and seal the furnace chamber; (3) Vacuum treatment is performed on the furnace chamber; (4) Inert gas is introduced into the furnace and continuously introduced through the vent hole at the bottom of the outer crucible; (5) The crucible assembly is heated by programmed heating to complete the synthesis reaction, crystal transformation and grain growth of silicon carbide powder. During the heating process, the gas flows through the following path: it enters from the bottom vent hole, is diverted through the support ring groove, enters the outer crucible side wall gas guide groove and rises, and at the same time permeates through the porous loading crucible wall, and finally exits from the top vent hole.

9. The synthesis method according to claim 8, characterized in that, The programmed heating process specifically includes: (1) Evacuate the furnace to a vacuum level of 4.5~5.5×10⁻⁶. -6 mbar, and heated to 900~1200℃; (2) Pour high-purity argon gas into the furnace to 800 mbar, maintain the pressure for 10-20 minutes, and then evacuate to a vacuum of 4.5-5.5 × 10⁻⁶ mbar. -6 mbar, and repeat this process multiple times; (3) At a vacuum degree of 4.5~5.5×10 -6 The synthesis reaction was carried out at mbar by heating to 1300~1400℃ and holding for 30~60 min. (4) Continue heating to 2000~2100℃ and hold for 8~10 hours to carry out the transformation of the powder crystal form and the growth of the grains.

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