Method for improving discharging rate of semi-insulating powder

Abstract

The invention relates to the technical field of semiconductor material preparation processes, in particular to a method for improving the discharging rate of semi-insulating powder, which comprises the following steps of: 1, stirring and mixing two raw materials, namely graphite powder and silicon powder; 2, loading the uniformly mixed raw material mixture into a graphite crucible; 3, a heater in the heating furnace is started to heat the graphite crucible, and the heater controls the raw material area in the graphite crucible and the space above the raw material area to form a forward temperature difference of 200-350 DEG C; 4, introducing inert gas into the gas inlet pipe, wherein the inert gas passes through the space above the raw material area in the graphite crucible and then is discharged through the gas outlet pipe; 5, after the temperature in the furnace body reaches the synthesis temperature, maintaining the specified synthesis time to complete the silicon carbide synthesis reaction; and 6, after the synthesis reaction is completed, cooling the furnace body to room temperature, and taking out the silicon carbide powder. The method can promote the discharge of nitrogen adsorbed in the raw materials, prevents the nitrogen element from being doped in silicon carbide crystal lattices, and effectively improves the preparation qualification rate of the silicon carbide powder.

Description

Technical Field

[0001] This invention relates to the technical field of semiconductor material preparation processes, specifically to a method for improving the yield of semi-insulating powder. Background Technology

[0002] Silicon carbide (SiC), as a typical representative of third-generation semiconductor materials, is considered one of the ideal materials for manufacturing high-temperature, high-frequency, high-power, and high-voltage devices due to its advantages such as wide bandgap, high thermal conductivity, high breakdown electric field, and large electron mobility. Among them, semi-insulating silicon carbide powder can be used as a raw material for conductive silicon carbide ingots as well as as a raw material for AR glasses and moissanite, and has extremely high industrial application potential.

[0003] The preparation of semi-insulating silicon carbide powder requires specialized synthesis equipment. This equipment can create a specific temperature field to heat the mixture of carbon powder and silicon powder in a graphite crucible, promoting a self-propagating reaction between the two at a high temperature to generate silicon carbide powder. However, both the graphite crucible and the carbon powder adsorb nitrogen from the air. This adsorption is primarily based on a physical adsorption mechanism, utilizing van der Waals forces between the adsorbate (nitrogen molecules) and the adsorbent (graphite material or carbon powder). This physical adsorption is characterized by non-selectivity, strong reversibility, and a fast adsorption rate. This leads to nitrogen easily doping into the silicon carbide lattice during the synthesis of silicon carbide powder, ultimately causing a decrease in both the yield and purity of the semi-insulating silicon carbide powder.

[0004] In response, the prior art "A Silicon Carbide Powder Synthesis Apparatus and Method" (Publication No.: CN115896945A) discloses a method for synthesizing silicon carbide powder. Its crucible structure includes multiple overlapping sub-crucibles, and a venting cylinder is provided at the center of each sub-crucible. A central hole is provided at the center of the venting cylinder, and a first venting section is provided on the side wall of the venting cylinder. The first venting section is connected to the central hole, so that nitrogen adsorbed by the raw material located in the sub-crucible and near the venting cylinder can enter the venting cylinder through the first venting section and be discharged outside the crucible through the central hole. Nitrogen near the bottom and side wall of the sub-crucible can permeate to the outside through the bottom and side wall of the crucible, thereby removing the adsorbed nitrogen element from the raw material.

[0005] For example, the prior art "A high-purity silicon carbide powder synthesis device" (publication number: CN215139935U) discloses a device structure, including a crucible, a protective layer, and a gas-venting component. The gas-venting component includes a first porous structure and a second porous structure. The first porous structure extends from the center of the bottom of the device into a holding chamber for storing silicon-carbon mixed powder and is used to introduce hydrogen into the holding chamber. The second porous structure is disposed at the end of the crucible and communicates with the holding chamber. When hydrogen is introduced into the first porous structure, nitrogen in the holding chamber can overflow through the second porous structure, thereby promoting nitrogen discharge and improving the purity of the silicon carbide powder.

[0006] However, the above solutions have the following technical problems: 1. Regarding the existing technology "A silicon carbide powder synthesis apparatus and method" (publication number: CN115896945A), the nitrogen discharge path is as follows: first, the nitrogen enters the central hole of the ventilation cylinder through the first ventilation section in each sub-crucible, and then is discharged sequentially downwards through the central hole to the outside of the entire crucible. The discharge process relies on the spontaneous movement of nitrogen from top to bottom along the central hole to discharge it outside the crucible. The entire gas discharge path is too long, and the nitrogen discharge speed is slow. Furthermore, the nitrogen adsorbed by the raw materials located at the bottom of the crucible and near the inner wall of the crucible is discharged outside the crucible by permeation. This is not only slow, but also because the multiple sub-crucibles are overlapping, the nitrogen in the sub-crucibles closer to the top layer needs to permeate through multiple sub-crucibles below to be discharged, which greatly reduces the denitrification efficiency.

[0007] 2. Regarding the existing technology "A high-purity silicon carbide powder synthesis device" (publication number: CN215139935U), the first porous structure used for introducing hydrogen is cylindrical, and the outer periphery of the first porous structure is in direct contact with the carbon powder and silicon powder in the holding chamber. This will cause the raw materials to have a certain obstruction effect on the introduction of hydrogen, which will significantly reduce the speed at which hydrogen enters the holding chamber from the first porous structure. The efficiency of using hydrogen to squeeze nitrogen out of the holding chamber will decrease. In addition, hydrogen enters the holding chamber laterally from the first porous structure, while nitrogen needs to be discharged from the second porous structure located at the bottom of the device. When hydrogen enters the holding chamber laterally, it is easy to mix with nitrogen, making it difficult to quickly squeeze nitrogen out of the bottom of the device, reducing the nitrogen discharge speed, especially the nitrogen discharge at the top of the holding chamber.

[0008] All of the above-mentioned existing technologies suffer from low nitrogen removal efficiency, which makes it easy for nitrogen elements to be doped during the synthesis of silicon carbide powder, reducing the yield of silicon carbide powder preparation. Therefore, the market urgently needs a silicon carbide synthesis process to solve the above problems. Summary of the Invention

[0009] This invention provides a method for improving the yield of semi-insulating powder, which can solve the problem in the existing silicon carbide powder synthesis process that the nitrogen adsorbed in the raw materials is slowly discharged, which leads to nitrogen being easily doped into the silicon carbide lattice during the silicon carbide powder synthesis process, thereby reducing the yield of silicon carbide powder preparation.

[0010] This application provides the following technical solution: a method for improving the yield of semi-insulating powder, which uses a furnace with an induction heater to synthesize silicon carbide powder, including the following steps: 1. Mix the two raw materials, graphite powder and silicon powder, together. 2. Load the well-mixed raw material mixture into a graphite crucible; 3. Start the heater in the heating furnace to heat the graphite crucible. The heater controls the raw material zone inside the graphite crucible to form a positive temperature difference of 200℃-350℃ with the space above the raw material zone. The heating furnace used in this step is equipped with an inlet pipe and an outlet pipe. The inlet pipe and outlet pipe are horizontally placed above the mouth of the graphite crucible. The graphite crucible wall is equipped with porous graphite splicing parts, which are arranged opposite to the inlet pipe and outlet pipe. 4. Inert gas is introduced into the inlet pipe. After passing through the space above the raw material area in the graphite crucible, the inert gas is discharged through the outlet pipe. 5. Once the temperature inside the furnace reaches the synthesis temperature, maintain the specified synthesis time to complete the silicon carbide synthesis reaction; 6. After the synthesis reaction is complete, wait for the furnace to cool to room temperature and then remove the silicon carbide powder.

[0011] Beneficial effects: 1. This invention utilizes temperature difference to drive the directional convergence of nitrogen gas and uses inert gas to promote nitrogen gas discharge, thereby improving nitrogen gas discharge efficiency. In this scheme, a positive temperature difference of 200℃-350℃ is precisely controlled by a heater between the raw material zone and the space above it. Utilizing the thermal motion characteristics of gases, nitrogen gas adsorbed by graphite powder and silicon powder within the graphite crucible moves from the higher-temperature area inside the crucible to the lower-temperature area above, concentrating in the non-raw material zone above the crucible before being discharged through the outlet pipe. Simultaneously, inert gas introduced through the inlet pipe flows above the graphite crucible, pushing the nitrogen gas gathered above it and promoting its discharge. This invention uses a heater to control a certain temperature difference within the furnace body, causing nitrogen gas to naturally converge upwards due to the temperature difference. Compared to existing technologies that require nitrogen gas to spontaneously flow downwards for discharge or rely on permeation through the crucible wall, this scheme significantly shortens the nitrogen gas discharge path and accelerates the discharge rate through the temperature difference. Existing denitrification methods all attempt to discharge nitrogen downwards from the crucible. This is influenced by two factors: firstly, the conventional equipment structure, where the bottom of the furnace is easier to accommodate inlets and outlets for convenience; and secondly, the conventional approach, which uses lighter hydrogen to expel nitrogen due to its weight, has led to the use of top-down nitrogen discharge for denitrification. Furthermore, existing technologies rely on nitrogen permeation to remove nitrogen from the crucible wall, significantly reducing discharge efficiency and prioritizing convenience over final product quality. In contrast, this invention optimizes the equipment structure, resulting in a shorter nitrogen discharge path and higher efficiency. It solves the problem of nitrogen being dispersed within the raw material layer and difficult to discharge quickly, reducing the probability of nitrogen contacting the raw material and doping into the silicon carbide lattice, thus effectively improving the yield of silicon carbide powder.

[0012] 2. The continuous temperature difference within the furnace promotes the upward convergence of nitrogen, ensuring efficient powder preparation. In this scheme, because the synthesis reaction temperature of silicon carbide is relatively high, requiring temperatures above 1700 degrees Celsius for the self-propagating reaction to gradually occur, and the heating process of the furnace is a slow heating process, the heater controls the temperature difference inside the furnace, causing the nitrogen in the raw materials to continuously move upward and converge above the mouth of the graphite crucible. That is, during the heating process before the synthesis reaction starts, the nitrogen can be enriched and converged as much as possible. Once the temperature reaches the reaction temperature, most of the nitrogen in the raw materials has already detached from the raw materials and is discharged to the outside of the equipment through the introduction of inert gas. The continuous nitrogen discharge during the furnace heating process does not delay the efficiency of the formal synthesis reaction after the temperature reaches the target. It avoids the nitrogen being trapped in the raw materials due to the slow discharge rate, which would cause the nitrogen to be locked in the silicon carbide lattice during the formal synthesis of the powder, thereby effectively improving the powder preparation qualification rate.

[0013] 3. Utilizing airflow to enhance rapid nitrogen removal and avoid nitrogen residue. The structural layout of the inlet pipe, outlet pipe, and porous graphite splice is designed to ensure more efficient nitrogen removal from the equipment. The porous graphite splice, with its multiple pores, allows for gas entry and exit. Placed on the crucible wall, it smoothly introduces the inert gas from the inlet pipe into the graphite crucible, forming an air curtain. This propels the nitrogen gas gathering above the graphite crucible, allowing it to pass through the porous graphite splice on the other side and enter the outlet pipe for discharge, creating a faster gas flow path and ensuring efficient nitrogen removal. Compared to existing technologies where nitrogen relies on spontaneous flow for removal or permeates from the crucible to the outside of the equipment, this solution avoids significant nitrogen retention in the graphite crucible, effectively reducing the nitrogen impurity content in the silicon carbide powder lattice.

[0014] 4. Balancing synthesis efficiency and quality to improve process effectiveness and efficiency. Since the synthesis reaction of silicon carbide is affected by temperature, if the temperature difference within the graphite crucible is set too low, it will reduce the rapid accumulation of nitrogen gas and decrease nitrogen venting efficiency, leading to nitrogen impurities in the silicon carbide powder lattice. Conversely, if the temperature difference is too high, it can easily cause uneven heating of the raw materials in the upper and lower regions of the graphite crucible, reducing the synthesis efficiency of silicon carbide or affecting the final synthesis quality. Controlling the temperature difference within the furnace within the range of 200℃-350℃ can balance nitrogen venting efficiency and raw material synthesis quality, thereby optimizing the synthesis efficiency and quality of silicon carbide to the most balanced state, ensuring high efficiency and high quality of the process.

[0015] Furthermore, in step one, graphite powder and silicon powder are mixed in a molar ratio of 1:1.

[0016] Beneficial effects: This formula strictly matches the chemical composition of silicon carbide crystals, that is, 1 carbon atom corresponds to 1 silicon atom. It can avoid reaction defects caused by excess or deficiency of a certain raw material, prevent excess of any raw material from causing waste, and ensure that the raw materials undergo a full and uniform chemical reaction in the high-temperature self-propagating reaction to generate high-purity silicon carbide powder with a complete crystal lattice.

[0017] Furthermore, in step two, the level of the uniformly mixed raw material mixture after being loaded into the graphite crucible is lower than the lower surface of the porous graphite splice.

[0018] Beneficial effects: The material surface height design ensures that the porous graphite splice is completely in the empty space above the raw material area, completely avoiding the obstruction of the raw material by the inert gas introduced through the air inlet pipe. This allows the inert gas to quickly and evenly form a complete air curtain above the raw material area through the porous graphite splice, accurately pushing the gathered nitrogen gas to flow towards the air outlet pipe and discharge it, greatly improving the nitrogen discharge efficiency, and ultimately improving the output qualification rate and purity of the semi-insulating silicon carbide powder.

[0019] Furthermore, in step three, the heater gradually heats the graphite crucible, and controls the synthesis temperature in the space above the mouth of the graphite crucible to be 2100℃-2200℃, the synthesis temperature in the lower part of the graphite crucible to be 2400℃-2450℃, and the synthesis temperature in the bottom part of the graphite crucible to be 2300℃-2400℃.

[0020] Beneficial effects: Since the lower part of the graphite crucible is the main filling area for raw materials, setting a suitable synthesis temperature for this area helps to maximize the synthesis reaction efficiency and quality. The space above the opening of the graphite crucible is mainly a nitrogen accumulation zone; setting its temperature lower creates a temperature difference, allowing the nitrogen adsorbed by the raw materials inside the graphite crucible to quickly accumulate upwards, ensuring efficient nitrogen removal. Because the heater is located on the outer periphery of the graphite crucible, heat is conducted through the crucible wall to the inside. On the circumferential wall, heat is conducted from the outside to the inside, where it is directly received by the raw material powder. At the bottom of the graphite crucible, heat is conducted radially from the outer periphery towards the center of the bottom, where it is received by the raw material on the upper surface. Therefore, the... Compared to the crucible walls, the heat conduction path at the bottom of the crucible is longer, making it more prone to thermal stress concentration. Over time, this can lead to overheating and damage, reducing the crucible's lifespan. However, existing and known technologies have overlooked this issue because conventional designs often treat the graphite crucible as a uniformly heated container, assuming that external heating will evenly heat the entire crucible. Designers focus more on the temperatures required for the reaction of the raw materials, neglecting the thermal stress problem of the graphite crucible itself. Furthermore, for cost reasons, improving the heating uniformity at the bottom of the crucible may require changing the heater structure or using a more complex temperature control system, increasing costs and design complexity. Therefore, due to these combined factors, conventional designs tend to ignore the thermal stress problem at the bottom of the crucible. The inventors recognize that a long-lasting and stable reaction vessel is a prerequisite for high-quality continuous production. Therefore, this invention sets the temperature at the bottom of the graphite crucible lower than the temperature in the middle and lower parts of the crucible. The aim is to control the heating temperature at the bottom of the crucible, reduce the thermal stress concentration effect at the bottom, and thus extend the overall service life.

[0021] Furthermore, in step four, the inert gas flow rate is 500 sccm.

[0022] Beneficial effects: The inert gas at this pressure can form a stable and moderate airflow, which can accurately push the accumulated nitrogen gas to be discharged quickly from the outlet pipe, without disturbing the raw material layer or disrupting the thermal balance inside the furnace due to excessive pressure.

[0023] Furthermore, in step five, the synthesis time is 10-25 hours.

[0024] Beneficial effects: A synthesis time of 10h-25h allows the raw materials to undergo a full self-propagating reaction, ensuring the full synthesis of silicon carbide powder. This avoids insufficient reaction due to too short a time and increased energy consumption and powder sintering defects due to too long a time, thus balancing powder quality and process economy. Attached Figure Description

[0025] Figure 1 This is a front view of the heating furnace in this invention. Detailed Implementation

[0026] The following detailed description illustrates the specific implementation method: The markings in the attached drawings of the instruction manual include: furnace body 1, air inlet pipe 2, bell mouth 201, heater 3, insulation material 4, graphite crucible 5, porous graphite splice 6, air outlet pipe 7, raw material area 8.

[0027] Example 1 like Figure 1 As shown, a method for improving the yield of semi-insulating powder is to synthesize silicon carbide powder using a furnace with an induction heater 3. The furnace includes a furnace body 1, insulation material 4 disposed within the furnace body 1, a heater 3 located within the insulation material 4, a graphite crucible 5 disposed within the inner ring of the heater 3, and an inlet pipe 2 and an outlet pipe 7 fixed to the furnace body 1. The graphite crucible 5 has a bulk density ≥1.80 g / cm³, a resistivity of 8-13 μΩ·m³, a thermal conductivity ≥80 W / (m·K), and an ash content ≤5 ppm.

[0028] The inlet pipe 2 and outlet pipe 7 are horizontally positioned above the raw material zone 8 near the graphite crucible 5. Both the inlet pipe 2 and outlet pipe 7 extend horizontally from the outside of the furnace body 1 into the furnace body 1. Porous graphite splicing pieces 6 are symmetrically arranged on the crucible wall of the graphite crucible 5. The porosity of the porous graphite is 40 μm, and the density is ≥1.85 g / cm³. The porous graphite splicing pieces 6 are positioned opposite to the inlet pipe 2 and outlet pipe 7. The ends of the inlet pipe 2 and outlet pipe 7 closest to the porous graphite splicing pieces 6 are both flared 201. The heater 3 is an induction heater 3 with a heating coil. The graphite crucible 5 is located at the center of the heating coil, which is a spiral coil and will not obstruct the inert gas introduced from the inlet pipe 2 from passing through the porous graphite splicing pieces 6 into the graphite crucible 5.

[0029] A method for improving the yield of semi-insulating powder includes the following steps: 1. Mix the two raw materials, graphite powder and silicon powder, together. Use a drum mixer to mix the graphite powder and silicon powder at a molar ratio of 1:1.

[0030] 2. The uniformly mixed raw material mixture is loaded into the graphite crucible 5; the material level after the uniformly mixed raw material mixture is loaded into the graphite crucible 5 is lower than the lower surface of the porous graphite splice 6.

[0031] 3. Start the heater 3 in the heating furnace to gradually heat the graphite crucible 5, and finally control the synthesis temperature of the space above the mouth of the graphite crucible 5 to reach 2100℃-2200℃, preferably 2150℃ in this embodiment. The synthesis temperature of the lower part of the graphite crucible 5 (four-fifths of the area from the material surface downwards) is 2400℃-2450℃, preferably 2400℃ in this embodiment. The synthesis temperature of the bottom part of the graphite crucible 5 (one-fifth of the area from the bottom surface upwards in the crucible, accounting for the entire raw material area) is 2300℃-2400℃, preferably 2350℃ in this embodiment. This creates a positive temperature difference of 200℃-350℃ between the raw material area 8 in the graphite crucible 5 and the space above the raw material area 8. The temperature difference effect allows the nitrogen gas adsorbed by the raw material in the graphite crucible 5 to move towards the space above the raw material area 8 and converge.

[0032] Fourth, an inert gas is introduced into the inlet pipe 2. In this embodiment, the inert gas is argon, and the argon flow rate is 500 sccm. The argon enters from the inlet pipe 2, passes through the porous graphite splice 6, and enters the space above the raw material area 8 in the graphite crucible 5. It pushes the nitrogen gas gathered here to the outlet pipe 7 and discharges it from the outlet pipe 7 along with the nitrogen gas, thus achieving the purpose of denitrification.

[0033] 5. Once the temperature inside furnace 1 reaches the synthesis temperature, maintain a uniform temperature field and continue the synthesis for 10-25 hours to complete the silicon carbide synthesis reaction.

[0034] 6. After the synthesis reaction is complete, turn off the heating furnace and wait for furnace body 1 to cool to room temperature. Then, open furnace body 1 to remove the silicon carbide powder.

[0035] To investigate the effect of the temperature difference between the raw material zone and the space above the raw material zone on the nitrogen emission rate, the following comparative experiment was conducted in this embodiment:

[0036] When the temperature difference within the graphite crucible is set within the range of 200-350℃, infrared spectroscopy analysis reveals that the nitrogen residue in the synthesized silicon carbide powder lattice is suppressed to a near-minimum level. However, when the temperature difference is less than 200℃, the nitrogen residue in the silicon carbide powder lattice increases. This is because the small temperature difference results in insufficient driving force for the upward convergence of nitrogen adsorbed in the raw materials during the synthesis reaction, making it difficult to quickly expel the nitrogen. While the nitrogen residue is low when the temperature difference is greater than 350℃, the large temperature difference within the graphite crucible, particularly between the lower and middle regions of the raw material zone and the top region, leads to uneven particle size in the synthesized silicon carbide powder, reducing the yield. Therefore, setting the temperature difference within the graphite crucible within the range of 200-350℃ significantly improves the yield of silicon carbide powder.

[0037] The above are merely embodiments of the present invention, and the invention is not limited to the fields covered by these embodiments. Commonly known structures and characteristics in the solutions are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the structure of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for improving the yield of semi-insulating powder, characterized in that: The synthesis of silicon carbide powder is carried out using a furnace equipped with an induction heater, including the following steps:

1. Mix the two raw materials, graphite powder and silicon powder, together.

2. Load the well-mixed raw material mixture into a graphite crucible; 3. Start the heater in the heating furnace to heat the graphite crucible. The heater controls the raw material zone inside the graphite crucible to form a positive temperature difference of 200℃-350℃ with the space above the raw material zone. The heating furnace used in this step is equipped with an inlet pipe and an outlet pipe. The inlet pipe and outlet pipe are horizontally placed above the mouth of the graphite crucible. The graphite crucible wall is equipped with porous graphite splicing parts, which are arranged opposite to the inlet pipe and outlet pipe.

4. Inert gas is introduced into the inlet pipe. After passing through the space above the raw material area in the graphite crucible, the inert gas is discharged through the outlet pipe.

5. Once the temperature inside the furnace reaches the synthesis temperature, maintain the specified synthesis time to complete the silicon carbide synthesis reaction; 6. After the synthesis reaction is complete, wait for the furnace to cool to room temperature and then remove the silicon carbide powder.

2. The method for improving the yield of semi-insulating powder according to claim 1, characterized in that: In step one, graphite powder and silicon powder are mixed in a molar ratio of 1:

1.

3. The method for improving the yield of semi-insulating powder according to claim 2, characterized in that: In step two, the level of the uniformly mixed raw material mixture after being loaded into the graphite crucible is lower than the lower surface of the porous graphite splice.

4. The method for improving the yield of semi-insulating powder according to claim 3, characterized in that: In step three, the heater gradually heats the graphite crucible and controls the synthesis temperature in the space above the mouth of the graphite crucible to be 2100℃-2200℃, the synthesis temperature in the lower part of the graphite crucible to be 2400℃-2450℃, and the synthesis temperature in the bottom part of the graphite crucible to be 2300℃-2400℃.

5. The method for improving the yield of semi-insulating powder according to claim 4, characterized in that: In step four, the inert gas flow rate is 500 sccm.

6. The method for improving the yield of semi-insulating powder according to claim 5, characterized in that: In step five, the synthesis time is 10-25 hours.

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