Process for the synthesis of silicon carbide with reduced nitrogen impurities

Abstract

The application discloses a method for synthesizing silicon carbide with reduced nitrogen impurity content, which comprises the following steps: loading Ti metal into a graphite crucible with a cover, then placing the graphite crucible with the cover into a silicon carbide powder synthesis crucible; uniformly mixing Si and C powder, then placing the mixed Si and C powder into the silicon carbide powder synthesis crucible and sealing the silicon carbide powder synthesis crucible; placing the powder synthesis crucible into a graphite heater, sealing the graphite heater, pumping the graphite heater to a base vacuum, filling argon, maintaining the pressure in the furnace, then performing temperature rising and temperature maintaining according to a preset temperature control program to complete the synthesis of the silicon carbide powder. The Ti metal has the effect of physically and chemically adsorbing nitrogen impurities at high temperature, and generates Ti3N, TiN and other compounds or Ti-N solid solution. Since the physical properties of titanium nitride are stable, the content of nitrogen impurities can be reduced in the process of synthesizing high-purity silicon carbide powder, and the purity of the silicon carbide powder is improved.

Description

Technical Field

[0001] This invention belongs to the technical field of silicon carbide powder synthesis for wide bandgap semiconductor materials. Specifically, it relates to a method for synthesizing silicon carbide with reduced nitrogen impurity content. Background Technology

[0002] Compared to silicon, a traditional semiconductor material, silicon carbide single crystals possess advantages such as high thermal conductivity, large bandgap, high breakdown field strength, and high thermal stability, making them particularly suitable for fabricating high-frequency, high-power power electronic devices and currently a research hotspot both domestically and internationally. Since Lely first used the sublimation method to prepare silicon carbide single crystals in 1955, the most mature and commercially viable method for growing silicon carbide single crystals is the physical vapor transport method (PVT). The PVT method requires placing a seed crystal within the crystal growth space. Silicon carbide powder is heated to 2200-2400℃, and using a suitable temperature gradient, the gaseous substances decomposed from the silicon carbide powder are transported to the silicon carbide seed crystal, forming a supersaturated deposition on the seed crystal for growth.

[0003] In the technology of growing high-purity semi-insulating bulk silicon carbide single crystals, the impurities in the silicon carbide single crystals mainly come from the impurities contained in the silicon carbide polycrystalline powder. During the synthesis of silicon carbide powder, due to the small atomic radius of nitrogen, nitrogen gas is often adsorbed in the graphite crucible and carbon felt insulation layer and cannot be completely removed, resulting in the presence of nitrogen impurities in the silicon carbide powder. This directly leads to a high nitrogen impurity content in the grown high-purity semi-insulating bulk silicon carbide single crystals, affecting the quality of the high-purity semi-insulating crystals.

[0004] Currently, the method for removing residual nitrogen from the thermal field of silicon carbide single crystal growth is to introduce hydrogen gas into a sealed growth chamber to reduce the residual nitrogen content. Although the working principle is not entirely clear, the nitrogen content in silicon carbide powder synthesized by introducing hydrogen gas is significantly reduced. However, since hydrogen is a hazardous gas, its use in the production process increases the complexity and risk of the process, as well as the need for supporting hydrogen gas facilities, thus increasing the cost and complexity of producing high-purity silicon carbide powder. Summary of the Invention

[0005] The purpose of this invention is to provide a method for synthesizing silicon carbide with reduced nitrogen impurity content, in order to solve the problem that residual nitrogen in the hot zone of existing high-temperature vacuum sintering furnaces affects the purity of synthesized high-purity silicon carbide powder.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for synthesizing silicon carbide with reduced nitrogen impurity content, comprising the following steps:

[0008] Step 1: Put Ti metal into a covered graphite crucible, and then put the covered graphite crucible into a silicon carbide powder synthesis crucible.

[0009] Step 2: After mixing Si and C powder evenly, put them into a silicon carbide powder synthesis crucible and seal the silicon carbide powder synthesis crucible.

[0010] Step 3: Place the powder synthesis crucible into the graphite heater, seal the graphite heater and evacuate it to a base vacuum, fill it with argon gas, maintain the pressure inside the furnace, and then perform heating and heat preservation according to the preset temperature control program to complete the synthesis of silicon carbide powder.

[0011] Step four: After the furnace chamber of the graphite heater cools down, open the furnace chamber and take out the synthesized silicon carbide powder.

[0012] Furthermore, in step one, the Ti metal element is selected from high-purity Ti powder.

[0013] Furthermore, in step two, a porous graphite crucible is used.

[0014] Furthermore, in step three, the furnace pressure is maintained at 3 mbar.

[0015] Furthermore, in step three, the preset temperature control program is to raise the temperature to 500℃-750℃ and then hold it, then raise the temperature to 800℃-1000℃ and then hold it; then raise the temperature to 1900℃ and hold it.

[0016] Furthermore, in step three, the temperature is raised to 500℃-750℃ and then held for 18000s; the temperature is raised to 800℃-1000℃ and then held for 7200s.

[0017] Furthermore, in step three, the temperature is raised to 1900℃ and then held for 18000 seconds.

[0018] Furthermore, in step four, the furnace cavity is cooled to 20°C for 10,800 seconds.

[0019] In this invention, the physical and chemical adsorption properties of Ti metal at high temperatures are utilized to generate compounds such as Ti3N and TiN, or Ti-N solid solutions. Titanium nitride exhibits stable physical properties, with a melting point as high as 2950.6~3205.8℃, exhibiting high temperature resistance and stable properties. This invention can reduce the nitrogen impurity content during the synthesis of high-purity silicon carbide powder, which not only improves the purity of the silicon carbide powder but also enhances the purity and quality of high-purity semi-insulating bulk silicon carbide crystals grown from high-purity silicon carbide powder using the vapor phase method.

[0020] The method for removing nitrogen impurities according to the present invention is simple, safe, and easy to implement. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of a high-temperature vacuum sintering furnace for synthesizing silicon carbide powder;

[0022] Figure 2 EDS spectrum of the silicon carbide powder synthesized in Example 1;

[0023] Figure 3 A comparison of the physical samples of silicon carbide powder synthesized in Example 1 and Comparative Example 1.

[0024] Figure 1 In the middle, 1—induction coil, 2—powder synthesis crucible, 3—graphite crucible with lid, 4—quartz holder, 5—upper temperature measuring hole, 6—graphite heater, 7—carbon felt insulation layer, 8—quartz barrel, 9—lower temperature measuring hole. Detailed Implementation

[0025] A typical embodiment of the present invention provides a method for synthesizing silicon carbide with reduced nitrogen impurity content. The synthesis method described in this embodiment, as shown in... Figure 1 The high-temperature vacuum sintering process is carried out in the high-temperature vacuum sintering furnace shown. This furnace includes a powder synthesis crucible 2, a covered graphite crucible 3, a quartz holder 4, an upper temperature measuring hole 5, a graphite heater 6, a carbon felt insulation layer 7, a quartz barrel 8, and a lower temperature measuring hole 9. An induction coil 1 is externally mounted on the graphite heater 6 for heating and maintaining temperature during the synthesis process. The covered graphite crucible 3 is placed inside the powder synthesis crucible 2, which is then placed inside the graphite heater 6. The carbon felt insulation layer 7 is placed on the inner wall of the graphite heater 6. The upper and lower temperature measuring holes 5 and 9 are respectively located at the upper and lower ends of the graphite heater. The bottom of the graphite heater 6 has a quartz holder 4. The entire assembly is placed inside the quartz barrel 8.

[0026] Based on the above-mentioned high-temperature vacuum sintering furnace, this embodiment provides a synthesis method including the following steps.

[0027] Step 1: Put Ti metal into a covered graphite crucible 3, and then put the covered graphite crucible 3 into a silicon carbide powder synthesis crucible 2.

[0028] The Ti metal is selected from high-purity Ti powder to improve the reactivity of metallic Ti.

[0029] Step 2: After mixing Si and C powder evenly, place them into powder synthesis crucible 2 and seal the powder synthesis crucible.

[0030] Preferably, the Si and C powders are high-purity Si powder and C powder with uniform particle size.

[0031] The porous structure of the covered graphite crucible 3 facilitates the entry of residual nitrogen from the thermal field into the covered graphite crucible 3 containing metallic Ti. A powder synthesis crucible 2 with a threaded lid can be used to increase its sealing performance.

[0032] Step 3: Place the powder synthesis crucible into the graphite heater, seal the graphite heater and evacuate it to a base vacuum, fill it with argon gas, maintain the pressure inside the furnace, and then perform heating and heat preservation according to the preset temperature control program to complete the synthesis of silicon carbide powder.

[0033] Preferably, the furnace pressure is maintained at 3 mbar.

[0034] Preferably, the preset temperature control program involves heating to 500℃-750℃ and holding at that temperature, then heating to 800℃-1000℃ and holding at that temperature; then heating to 1900℃ and holding at that temperature. Preferably, the temperature is held at 500℃-800℃ for 18000 seconds; and then held at 800℃-1000℃ for 7200 seconds.

[0035] By increasing the holding time in two temperature ranges, 500℃-750℃ and 800℃-1000℃ respectively, and allowing metallic Ti to fully physical and chemically adsorb nitrogen impurities at temperatures where Si and C do not react (below 1100℃), the nitrogen impurities are allowed to be fully adsorbed by metallic Ti.

[0036] In this invention, the elemental Ti used has the physical properties of general metallic elemental Ti. At room temperature, titanium does not react with nitrogen. At high temperatures, titanium is one of the few metallic elements that can react violently with nitrogen. When the temperature is 500-600℃, titanium can significantly absorb nitrogen and form interstitial solid solutions. When the temperature is greater than 600℃, the nitrogen absorption rate of titanium increases. When the temperature rises to the melting point, the reaction between molten titanium and nitrogen is very violent.

[0037] Preferably, the temperature is raised to 1900℃ and then held for 18000 seconds.

[0038] Step four: After the furnace chamber of the graphite heater cools down, open the furnace chamber and take out the synthesized silicon carbide powder.

[0039] The furnace cavity was cooled to 20°C in 10,800 seconds.

[0040] The technical solutions claimed in this invention will be further described below through some embodiments. However, the embodiments and comparative examples are for explaining the implementation schemes of this invention and do not exceed the scope of the subject matter of this invention. The scope of protection of this invention is not limited by the embodiments. Unless otherwise specified, the materials and reagents used in this invention can be obtained from commercially available products in the art. Example 1

[0041] (1) Weigh 5g of high-purity Ti powder using an electric level and put it into a high-purity graphite crucible 3 with a lid;

[0042] (2) Weigh 70.049g of high-purity silicon powder and 29.95g of high-purity carbon powder, mix them evenly in a mortar, and put them into the powder synthesis crucible 2;

[0043] (3) Place the silicon carbide powder synthesis crucible 2 into the graphite heater 6;

[0044] (4) Place the graphite heater 6 into the hot field with the carbon felt insulation layer 7 and seal the vacuum high-temperature sintering furnace.

[0045] (5) Turn on the mechanical pump of the vacuum high-temperature sintering furnace, set the pressure target value to 0 mbar, and the time length to 600 s;

[0046] (6) When the vacuum degree inside the furnace is below 5 mbar, turn on the molecular pump and use a combination of molecular pump and mechanical pump for pumping;

[0047] (7) When the vacuum level inside the furnace is below 6E-6mbar, turn off the molecular pump and keep the mechanical pump running to continue evacuating the furnace.

[0048] (8) Turn on the intermediate frequency induction power supply RF to energize the induction coil, and set the maximum power to 25%;

[0049] (9) Set the furnace to an automatic operation program, set the target temperature to 550℃, the heating time to 6000s, and hold at 550℃ for 18000s;

[0050] (10) Set the temperature to 1000℃, the heating time to 2700s, and the holding time to 7200s;

[0051] (11) Set the argon gas flow rate to 0.05 L / min and set the pressure control program to maintain the furnace pressure at 3 mbar;

[0052] (12) Set the temperature to 1900℃, the heating time to 5400s, and the holding time to 18000s;

[0053] (13) Set the target temperature to 20℃ and the cooling time to 10800s;

[0054] (14) Set the maximum power to 0 and the running time to 10s. After 10s, turn off the intermediate frequency induction power supply RF, close the argon valve, and turn off the mechanical pump.

[0055] (15) Run the program set in (9), (10), (11), (12), (13), and (14);

[0056] (16) After the furnace body cools to room temperature, fill the furnace cavity with argon gas to one atmosphere, open the furnace cavity, take out the powder synthesis crucible 2, take out the synthesized silicon carbide powder and put it into a plastic bag for storage.

[0057] See Figure 2 The spectrum obtained by EDS testing of the silicon carbide powder synthesized in Example 1. Example 2

[0058] The only difference from Example 1 is in steps (9) and (10).

[0059] (9) Set the furnace to an automatic operation program, set the target temperature to 500℃, the heating time to 6000s, and hold at 500℃ for 18000s;

[0060] (10) Set the temperature to 800℃, the heating time to 2700s, and the holding time to 7200s. Example 3

[0061] The only difference from Example 1 is in steps (9) and (10).

[0062] (9) Set the furnace to an automatic operation program, set the target temperature to 750℃, the heating time to 6000s, and hold at 750℃ for 18000s;

[0063] (10) Set the temperature to 1000℃, the heating time to 2700s, and the holding time to 7200s.

[0064] Comparative Example 1: Silicon carbide powder was synthesized in a covered graphite crucible (3) without Ti metal in a powder synthesis crucible (2). See also Figure 3 The spectrum obtained by EDS testing of the silicon carbide powder synthesized in Comparative Example 1.

[0065] As can be seen from the structures of Example 1 and Comparative Example 1, by Figure 2 It can be seen that the characteristic peak intensity of nitrogen in the silicon carbide powder synthesized in Example 1 is almost zero. Figure 3 It can be seen that the silicon carbide powder synthesized without this method is greenish in color, indicating the presence of nitrogen impurities. In contrast, the silicon carbide powder synthesized using this method is yellow, a typical color of low-temperature phase silicon carbide. Visually, it is clear that the method of this invention effectively reduces nitrogen impurities in the synthesized silicon carbide powder. This invention solves the problem of high nitrogen impurity content in the synthesized high-purity silicon carbide powder.

Claims

1. A method for synthesizing silicon carbide with reduced nitrogen impurities, comprising the steps of: The method comprises the steps of: ​ Step 1: loading Ti metal into a graphite crucible with a cover, and then placing the graphite crucible with the cover into a silicon carbide powder synthesis crucible; Step 2: mixing Si and C powders uniformly, and then placing the mixed powders into the silicon carbide powder synthesis crucible and sealing the silicon carbide powder synthesis crucible; Step 3: placing the powder synthesis crucible into a graphite heater, sealing the graphite heater, pumping to a base vacuum, filling argon, maintaining the pressure in the furnace, and then performing temperature control according to a preset temperature control program to complete the synthesis of the silicon carbide powder; Step 4: after the furnace chamber of the graphite heater is cooled, the synthesized silicon carbide powder is taken out.

2. The method of claim 1, wherein: In step 1, the Ti metal is high-purity Ti powder.

3. The method according to claim 1 or 2, characterized in that: In step 2, the graphite crucible has a porous structure.

4. The method of claim 3, wherein: In step 3, the pressure in the furnace is maintained at 3 mbar.

5. The method according to claim 1 or 4, characterized in that: In step 3, the preset temperature control program is to heat to 500-750 DEG C and then maintain the temperature, continue to heat to 800-1000 DEG C and then maintain the temperature, and then heat to 1900 DEG C and then maintain the temperature.

6. The method of claim 5, wherein: In step 3, the temperature is maintained for 18000 s after heating to 500-750 DEG C, and the temperature is maintained for 7200 s after heating to 800-1000 DEG C.

7. The method of claim 6, wherein: In step 3, the temperature is maintained for 18000 s after heating to 1900 DEG C.

8. The method of claim 1 or 7, wherein: In step 4, the furnace chamber is cooled to 20 DEG C, and the cooling time is 10800 s.

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