A method for preparing high-purity silicon carbide by chemical vapor deposition

Abstract

The present invention relates to the field of semiconductor technology, and in particular to a method for preparing high-purity silicon carbide by chemical vapor deposition, comprising the following steps: (1) distilling and purifying a raw material monomethyltrichlorosilane to make its purity greater than 99.99% and its impurity content less than 5 ppm; (2) mixing the purified monomethyltrichlorosilane with hydrogen, vaporizing, and reacting; (3) cooling the tail gas after the reaction to room temperature, then pressurizing it to 0.6-1.0 MPa, exchanging heat with a refrigerant, and further cooling it to -40°C to form a condensate, and sending the condensate to a distillation and purification process to separate and obtain high-purity monomethyltrichlorosilane; (4) sending the uncondensed tail gas in step (3) to bubbling elution and distillation separation to separate hydrogen chloride from the adsorption-saturated chlorosilane liquid; and (5) subjecting the tail gas after bubbling elution to activated carbon pressure swing adsorption to separate high-purity hydrogen.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor technology, and in particular to a method for preparing high-purity silicon carbide by chemical vapor deposition. Background Art

[0002] Silicon carbide single crystal substrate (SiC) is a third-generation compound semiconductor material, a compound material composed of two elements, carbon and silicon. It is mainly divided into conductive and semi-insulating types. Among them, conductive silicon carbide substrates (silicon carbide epitaxy) are mainly used to manufacture high-temperature and high-voltage resistant high-power devices. They are currently widely used in the field of electronics and power, such as new energy vehicles, photovoltaics, smart grids, rail transit and other fields, with a large market size; in addition, semi-insulating silicon carbide substrates (gallium nitride epitaxy) are mainly used in filtering radio frequency devices and other fields, such as power amplifiers in 5G communications and radio detectors in national defense. With the accelerated construction of 5G communication networks, market demand has increased significantly.

[0003] The primary method for preparing single-crystal silicon carbide (SiC) materials is PVT (physical vapor transport), which uses high-purity SiC powder as the raw material to grow SiC single crystals, and then obtain SiC single-crystal substrates. Currently, there are four main methods for preparing high-purity SiC powder: chemical vapor deposition (CVD), plasma deposition, sol-gel deposition, and self-propagating high-temperature synthesis. Of these, self-propagating high-temperature synthesis is the primary method for synthesizing SiC powder worldwide and is currently the most mature.

[0004] The most mature method used by global silicon carbide companies is the self-propagating high-temperature synthesis (SHT). This SHT method for producing silicon carbide powder is a solid-phase synthesis process. The improved SHT method uses high-purity silicon powder and high-purity carbon powder as raw materials. The sintering process is carried out in a high-temperature sintering furnace, where the reaction is continuously controlled at temperatures between 1800 and 2000°C, ultimately producing high-purity silicon carbide synthetic powder. However, the shortcomings of this method for producing silicon carbide powder are also significant. First, the raw silicon powder and carbon powder require pretreatment, and impurities in the powder are transferred to the silicon carbide powder. Therefore, the purity of the silicon carbide powder is completely limited by the purity of the silicon and carbon powders, and even new impurities may be introduced during the pretreatment process. Second, the sintering process requires prolonged high temperatures, resulting in low yields. Currently, the output per furnace is only 100-150 kg. Furthermore, the sintering process is prone to incomplete reaction or excess carbon, necessitating further carbon removal. In addition, the sintering reaction requires the use of high-purity graphite crucibles, insulation felt and other auxiliary materials, which not only easily introduce impurities, but also cause large losses in graphite parts, resulting in high costs for the preparation of silicon carbide powder.

[0005] In general, the current self-propagating synthesis of silicon carbide powder has the disadvantages of limited raw material purity, easy introduction of impurities during the process, difficulty in achieving a purity of silicon carbide powder above 6N, insufficient sintering resulting in excessive carbon, large consumption of auxiliary materials such as graphite parts, and high operating energy consumption, which leads to high preparation costs for high-purity silicon carbide powder. Summary of the Invention

[0006] The present invention aims to provide a method for preparing high-purity silicon carbide by chemical vapor deposition, which greatly improves the purity and utilization rate of the raw materials, obtains a product with higher purity, reduces energy consumption in the high-temperature chemical vapor deposition process, and improves reaction efficiency.

[0007] Based on the above objectives, the present invention adopts the following technical solutions:

[0008] A method for preparing high-purity silicon carbide by chemical vapor deposition comprises the following steps:

[0009] (1) distilling and purifying the raw material monomethyltrichlorosilane to a purity greater than 99.99% and an impurity content less than 5 ppm;

[0010] (2) mixing the monomethyltrichlorosilane purified in step (1) with hydrogen in a molar ratio of 1:2 to 20, vaporizing the mixture, and introducing the mixture into a vapor deposition furnace to react;

[0011] (3) The tail gas after the reaction in step (2) mainly comprises monomethyltrichlorosilane, hydrogen, hydrogen chloride, trichlorosilane, silicon tetrachloride, and alkanes. The tail gas after the reaction is cooled to room temperature, then pressurized to 0.6-1.0 MPa, and heat-exchanged with a refrigerant, and further deep-cooled to -38--42°C. Part of the tail gas is liquefied to form a condensate. The main components of the condensate are monomethyltrichlorosilane and part of trichlorosilane and silicon tetrachloride. The condensate is heated to room temperature through heat exchange and then sent to a distillation and purification process to separate and obtain high-purity monomethyltrichlorosilane, which is then used as a raw material for the vapor deposition reaction.

[0012] (4) The uncondensed tail gas in step (3), whose main components are hydrogen, hydrogen chloride, trichlorosilane, silicon tetrachloride, and alkanes, is sent to a bubbling elution process, wherein the bubbling elution liquid is a chlorosilane liquid, and the liquid temperature is -36°C. The trichlorosilane, silicon tetrachloride, and hydrogen chloride gas in the tail gas are bubbled and eluted in the chlorosilane elution liquid and adsorbed. The adsorbed liquid is then distilled and separated, and the hydrogen chloride adsorbed in the chlorosilane is separated. The separated hydrogen chloride can be used for acid production or for hydrogenation of silicon tetrachloride. The separated chlorosilane liquid continues to be used as a bubbling elution liquid;

[0013] (5) The main components of the tail gas after bubbling and elution are hydrogen and part of hydrogen chloride and alkanes. Then it enters the activated carbon pressure swing adsorption, which can adsorb hydrogen chloride and alkanes in the tail gas and separate high-purity hydrogen, which is continued to be used as the carrier gas for chemical vapor deposition reaction. After the activated carbon adsorption is saturated, the pressure can be reduced to desorb hydrogen chloride, alkanes, etc., and then they can be hydrolyzed and neutralized or burned.

[0014] Preferably, in the distillation purification in step (1), the distillation tower adopts a two-tower series method, first removing heavy substances and then removing light substances, the high-boiling substances in the raw material monomethyltrichlorosilane are mainly metal chlorides and phosphides with boiling points between several hundred and more than one thousand degrees Celsius, and the low-boiling substances are mainly boron chloride, trichlorosilane, and silicon tetrachloride. The raw material monomethyltrichlorosilane enters from tower 1, and the high-boiling substances are discharged from the bottom of the tower to reduce metal and phosphorus impurities; monomethyltrichlorosilane and a small amount of trichlorosilane and silicon tetrachloride are extracted from the top of tower 1 and enter tower 2 together, the low-boiling substances are extracted from the top of tower 2, boron chloride, trichlorosilane, and silicon tetrachloride are separated, and high-purity monomethyltrichlorosilane is output from the bottom of the tower.

[0015] Preferably, in step (2), the purity of the hydrogen is greater than 5N, the dew point is less than -60°C, the nitrogen content in the hydrogen is less than 2ppm, and the oxygen content is less than 4ppm.

[0016] Preferably, the vapor deposition reaction temperature in step (2) is 1200° C. to 1600° C., and the deposition process pressure is from atmospheric pressure to 6 bar.

[0017] Preferably, the vapor deposition reaction temperature in step (2) is 1400° C.-1500° C., and the deposition process pressure is 0.5-1.5 bar.

[0018] Preferably, in step (3), the specific process of cooling to room temperature is as follows: the outlet temperature of the tail gas of the vapor deposition reaction is 300-500°C, which is first cooled by a first stage, cooled by heat exchange with normal temperature cooling water, and cooled to 200-350°C, and then cooled by heat exchange with the feed mixed gas (about 70-120°C), and cooled to below 200°C by a second stage of heat exchange in a shell-and-tube heat exchanger, and then the tail gas is cooled by a third stage again, using normal temperature water as the cooling medium, and the tail gas temperature can be reduced to below 40°C to produce condensate; the deep cooling in step (3) is to pressurize the tail gas reduced to less than 40°C to 6-10 bar, and then the tail gas is cooled by a second stage in a shell-and-tube heat exchanger, and the tail gas is deep cooled to -38--42°C, and further produces condensate, the heat exchange medium is Freon, and the temperature is -60°C. At this time, the condensate components are mainly monomethyltrichlorosilane and a small amount of trichlorosilane and silicon tetrachloride.

[0019] Preferably, in step (3), the condensate that has been cryogenically cooled to -38 to -42°C is first subjected to a primary heat exchange with the tail gas in a shell-and-tube heat exchanger to raise the temperature from -40°C to 0 to 5°C, and then subjected to a secondary heat exchange with a normal temperature water medium in a shell-and-tube heat exchanger to raise the temperature to 10 to 20°C. The condensate at this time is then sent to a purification process to remove trichlorosilane and silicon tetrachloride to obtain monomethyltrichlorosilane.

[0020] Preferably, during the bubbling elution process in step (4), the eluent used is a chlorosilane liquid, which can be trichlorosilane or silicon tetrachloride, the tail gas pressure is 5-8 bar, the pressure of the elution tower is controlled at 3-5 bar, and the eluent temperature is -34 to -38°C.

[0021] Preferably, in step (5), the gases not absorbed in the tail gas after bubbling and eluting are mainly hydrogen and alkanes, chlorosilanes, and hydrogen chloride, and an activated carbon adsorption column is used to adsorb the remaining components except hydrogen to obtain high-purity hydrogen.

[0022] Preferably, the adsorption process comprises three activated carbon adsorption columns as a group, and each activated carbon adsorption column works in an alternating cycle controlled by a time program, and performs adsorption, regeneration, and cooling respectively. During the adsorption process, the temperature is low and high pressure, the temperature is 45-50°C, and the pressure is 0.4-0.6MPa. After the activated carbon adsorbs trace amounts of chlorosilane, hydrogen chloride, and alkanes in the hydrogen, the carbon powder is filtered out to obtain high-purity hydrogen.

[0023] The adsorption column used in the present invention is columnar activated carbon, wherein the activated carbon with a particle size of 4-12 mesh accounts for more than 93%, and the bulk specific gravity is 0.48-0.52 g / ml.

[0024] The monomethyltrichlorosilane raw material used in this invention is industrial-grade and has a purity of over 99.9%. It is a key byproduct of the direct synthesis process for preparing organosilicon monomers. Since the raw materials used in this direct synthesis process are methyl chloride and silicon powder, the metals in the silicon powder directly react with the boron and phosphorus impurities, leaving them in the material. According to the current GB / T20434-2006 "Monomethyltrichlorosilane" product standard, its purity requirements do not include any metal elements or boron and phosphorus content. Therefore, this patent uses monomethyltrichlorosilane as the raw material, and the impurities that need to be removed are metal chlorides, boron chloride, phosphorus chloride, and chlorosilanes. Taking advantage of the difference in boiling points between chlorides, chlorosilanes, and monomethyltrichlorosilane, a distillation purification process is used to remove impurities. During purification, the high-boiling substances in the raw material monomethyltrichlorosilane (boiling point 66°C) are primarily metal chlorides and phosphides (boiling points typically range from several hundred to over a thousand degrees Celsius), while the low-boiling substances are primarily boron chloride (boiling point 12.5°C), trichlorosilane (boiling point 32-34°C), and silicon tetrachloride (boiling point 57.6°C). In the distillation purification process, two towers are connected in series, first removing heavy impurities and then removing light impurities. The material enters the middle of tower 1, and the high-boiling substances are discharged from the bottom of the tower, thereby reducing the amount of metal and phosphorus impurities. Monomethyltrichlorosilane is removed from the top of tower 1 and then, along with a small amount of trichlorosilane and silicon tetrachloride (from the tail gas condensate), enters the middle of tower 2. Low-boiling substances are removed from the top of tower 2, effectively separating components such as boron chloride, trichlorosilane, and silicon tetrachloride. The bottom of the tower produces a high-purity monomethyltrichlorosilane product.

[0025] The hydrogen in the present invention comes from two sources. Initially, the hydrogen is produced by electrolysis. After the system is operational, the hydrogen in the tail gas is recovered and recycled as reactor feed, serving as the primary hydrogen source. Electrolysis replaces the small amount of hydrogen lost by the system. Monomethyltrichlorosilane is the reaction gas source, decomposing to produce silicon carbide and hydrogen chloride, accompanied by minor side reactions. The side reaction products are trichlorosilane, silicon tetrachloride, and alkanes such as acetylene. Hydrogen, the reaction gas source, also serves as a carrier gas. Some of the hydrogen participates in the decomposition of monomethylsilane, while some acts as a carrier gas to drive mass transfer across the deposition surface.

[0026] In the present invention, the vapor deposition reaction adopts a bell-shaped reactor, and several pairs of gate-shaped carriers are arranged in the reactor. Several air inlets are arranged on the bottom plate of the reactor, and the air inlets and the carriers are evenly arranged. Several air outlets are arranged in the center of the bottom plate. The bottom plate and bell jar of the bell-shaped reactor are both jacketed structures. Thermal oil or cooling water medium is introduced into the inside of the jacket to cool the bottom plate and the bell jar. The cooling medium absorbs heat and ensures that the bottom plate and the inner wall of the bell jar are always maintained at a low temperature to ensure that no deposition reaction occurs on the inner wall of the reactor. The flow rate of the cooling medium has a significant relationship according to the size of the furnace, the carrier temperature, the carrier size, etc. Taking 4 pairs of carriers as an example, the surface temperature of the carrier is 1200°C, and the cooling water flow rate is required to be greater than 60 cubic meters per hour. During actual operation, the inner wall temperature is generally lower than 300°C, and no deposition reaction occurs on the inner wall surface at this time. During the reaction, as the deposition layer becomes thicker, the surface area of ​​the deposition layer also increases. The monomethyltrichlorosilane and hydrogen in the material can maintain a constant ratio, but the gas volume and the surface area of ​​the deposition layer need to keep growing synchronously. The larger the surface area of ​​the deposition layer, the greater the gas volume.

[0027] In the bubbling elution process of the present invention, the chlorosilane used as the spray liquid is called the lean liquid, and the chlorosilane liquid adsorbed with hydrogen chloride is called the rich liquid. After the rich liquid emerges from the bottom of the elution tower, it first passes through a primary lean-rich liquid heat exchanger and a secondary lean-rich liquid heat exchanger, where its temperature is raised to 25-30°C before entering the middle of the stripping tower. Hydrogen chloride is extracted from the top of the stripping tower and used for hydrolysis and acid production. The lean liquid extracted from the bottom is cooled to -36°C after passing through a secondary lean-rich liquid heat exchanger, a primary lean-rich liquid heat exchanger, and a subcooler, and then circulated to the top of the elution tower as the eluent. The bottom temperature of the stripping tower is 140°C, and the top pressure is 0.8 MPa.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] The preparation method of the present application greatly improves the purity and utilization rate of the raw materials, and the obtained product has higher purity. By recovering and separating the various components in the reaction tail gas, especially the hydrogen and methylchlorosilane raw materials, hydrogen is used as a carrier gas to achieve almost zero consumption, and all methylchlorosilane can be used for reaction conversion.

[0030] The preparation method of the present invention uses methylchlorosilane containing both carbon and silicon as a raw material. After distillation and purification, high-purity methylchlorosilane can be obtained for chemical vapor deposition of silicon carbide. The material used in this method is easy to purify, the purification method is simple, and the impurity content of the purified material is extremely low. The silicon carbide deposited by the present invention has a purity of 6-8N, and the deposited layer is apparently dense, free of defects such as holes and cracks, and has a thickness of 10-100 mm.

[0031] The preparation method of the present invention adopts a jacketed bell-shaped reactor, in which a heat exchange medium is contained in the jacket, which can circulate and absorb the heat dissipated by the carrier, and recycle the heat, thereby reducing the energy consumption in the high-temperature chemical vapor deposition process. The reactor is a bell-shaped reactor, which is composed of a bell-shaped reactor, a base plate, and a carrier. An air inlet and an air outlet are arranged on the base plate at the same time. The material enters from the air inlet and can flow freely in the furnace chamber. The material gas can freely reach the surface of the carrier and undergo a deposition reaction. The reacted material gas flows out from the air outlet. Its carrier is a heat source, and the surface temperature of the carrier needs to be controlled to be maintained above 1200°C, and the temperature of the remaining wall surfaces is significantly lower than the surface temperature of the carrier, ensuring that the deposition reaction only occurs on the surface of the high-temperature carrier. The gas in the reactor can ensure the amount of material and material concentration required for the deposition reaction. The reaction on the surface of the carrier needs to consume fresh material and produce reaction tail gas, while the amount of material and material concentration near the surface of the carrier can be kept constant, so that the deposition reaction can proceed stably. (If the gas flows in one direction, the concentration of the inlet material is high and the concentration of the outlet material is low, resulting in large differences in deposition and poor uniformity. If the reaction chamber space is small, concentration polarization will occur, which will also result in large differences in deposition and poor uniformity.) The present invention has high deposition efficiency and can be regulated by deposition temperature, material amount, and the ratio of raw materials to carriers. During the deposition process, the deposition rate of silicon carbide is always maintained at above 0.5 mm / h, or even higher. Taking 4 pairs of carriers as an example, the carrier height is 2000 mm, and more than 200 kg of high-purity silicon carbide can be prepared in 50 hours.

[0032] The preparation method of the present invention also recovers and separates the components in the reaction tail gas, particularly hydrogen and methylchlorosilane raw materials. Hydrogen is used as a carrier gas, achieving near-zero consumption, and all of the methylchlorosilane can be used for the reaction conversion to produce silicon carbide and the byproducts hydrogen chloride and a small amount of chlorosilane. Furthermore, the transportation and conversion processes are fully enclosed, and the materials are not exposed to the environment throughout the entire process, resulting in high cleanliness and a low risk of contamination from foreign impurities that could affect the purity of the silicon carbide material. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 1 is a process flow chart of the preparation method of the present invention;

[0034] Figure 2 This is the XRD spectrum of the silicon carbide material obtained by vapor deposition in Example 1 of the present invention. DETAILED DESCRIPTION

[0035] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention are described in detail below. However, the following embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other implementation methods obtained by those skilled in the art without making any creative work are within the scope of protection of the present invention.

[0036] Example 1

[0037] A method for preparing high-purity silicon carbide by chemical vapor deposition comprises the following steps:

[0038] (1) distilling and purifying the raw material monomethyltrichlorosilane to a purity greater than 99.99% and an impurity content less than 5 ppm; in the distillation purification in step (1), two distillation towers are connected in series to first remove heavy elements and then remove light elements;

[0039] (2) The monomethyltrichlorosilane purified in step (1) and hydrogen are mixed in a molar ratio of 1:12, vaporized, and introduced into a vapor deposition furnace for reaction; the hydrogen purity is 5N, the dew point is -61°C, the nitrogen content in the hydrogen is less than 2ppm, the oxygen content is less than 4ppm, the vapor deposition reaction temperature is 1500°C, and the deposition process pressure is 1.0 bar; 4 pairs of gate-shaped carriers are installed in the bell-jar reactor, the carrier height is 2000mm, the deposition time is 40 hours, and the deposition rate of silicon carbide is always maintained above 0.5mm / h during the deposition process, and the SiC deposition weight is 200kg;

[0040] (3) The tail gas after the reaction is cooled to room temperature, then pressurized to 0.8 MPa, and heat exchanged with a refrigerant, and further deep-cooled to -40°C. Part of the tail gas is liquefied to form a condensate. The main components of the condensate are monomethyltrichlorosilane and part of trichlorosilane and silicon tetrachloride. The condensate is first subjected to a first-stage heat exchange with the tail gas in a shell-and-tube heat exchanger to increase the temperature from -40°C to 3°C, and then subjected to a second-stage heat exchange with a normal temperature water medium in a shell-and-tube heat exchanger to increase the temperature to 15°C. The condensate at this time is then sent to a purification process to remove trichlorosilane and silicon tetrachloride to obtain monomethyltrichlorosilane, which is continued to be used as a raw material for the vapor deposition reaction;

[0041] The specific process of cooling to room temperature is as follows: the outlet temperature of the tail gas of the vapor deposition reaction is 400°C, which is first cooled by a first stage cooling method, and then cooled by heat exchange with normal temperature cooling water to 280°C, and then cooled by heat exchange with the feed mixed gas (about 100°C), and then cooled by a second stage cooling method in a shell and tube heat exchanger to below 200°C, and then the tail gas is cooled by a third stage cooling method, using normal temperature water as the cooling medium, and the tail gas temperature can be reduced to below 40°C, thereby generating condensate;

[0042] The cryogenic cooling process involves pressurizing the tail gas to 7 bar after it has dropped to a temperature below 40°C. The tail gas is then cooled twice in a shell-and-tube heat exchanger to -40°C, further generating condensate. The heat exchange medium is Freon, and the temperature is -60°C. At this point, the condensate is mainly composed of monomethyltrichlorosilane and a small amount of trichlorosilane and silicon tetrachloride.

[0043] (4) The uncondensed tail gas in step (3), whose main components are hydrogen, hydrogen chloride, trichlorosilane, silicon tetrachloride, and alkanes, is sent to a bubbling elution process, wherein the bubbling elution liquid is a chlorosilane liquid. The trichlorosilane, silicon tetrachloride, and hydrogen chloride gas in the tail gas are bubbled and eluted in the chlorosilane elution liquid and adsorbed. The adsorbed liquid is then distilled and separated, and the hydrogen chloride adsorbed in the chlorosilane is separated. The separated hydrogen chloride can be used for acid production or for hydrogenation of silicon tetrachloride, and the separated chlorosilane liquid continues to be used as a bubbling elution liquid. During the bubbling elution process, the tail gas pressure is 6 bar, the pressure of the elution tower is controlled at 4 bar, and the eluent temperature is -36°C.

[0044] (5) The tail gas after bubbling and eluting is mainly composed of hydrogen and part of hydrogen chloride and alkanes, which then enter the activated carbon pressure swing adsorption, which can adsorb hydrogen chloride and alkanes in the tail gas, separate high-purity hydrogen, and continue to be used as a carrier gas for chemical vapor deposition reaction. After the activated carbon adsorption is saturated, the pressure can be reduced to desorb hydrogen chloride, alkanes, etc., and then they are hydrolyzed, neutralized or burned. In the tail gas after bubbling and eluting, the gases that are not absorbed are mainly hydrogen, a small amount of alkanes and trace amounts of chlorosilane and hydrogen chloride. An activated carbon adsorption column is used to adsorb impurities in hydrogen. In the adsorption process, three activated carbon adsorption columns are grouped together, and each activated carbon adsorption column is controlled by a time program to work in an alternating cycle, and performs adsorption, regeneration and cooling respectively. During the adsorption process, the temperature is low and high pressure, the temperature is 47°C, and the pressure is 0.5MPa. After the activated carbon adsorbs trace amounts of chlorosilane, hydrogen chloride and alkanes in the hydrogen, the carbon powder is filtered out to obtain hydrogen. The adsorption column is a columnar activated carbon with a particle size of 8 mesh, accounting for 94%, and a relative density of 0.48g / ml.

[0045] Example 2

[0046] A method for preparing high-purity silicon carbide by chemical vapor deposition comprises the following steps:

[0047] (1) distilling and purifying the raw material monomethyltrichlorosilane to a purity greater than 99.99% and an impurity content less than 2 ppm; in the distillation purification in step (1), two distillation towers are connected in series to first remove heavy elements and then remove light elements;

[0048] (2) mixing the purified monomethyltrichlorosilane and hydrogen in step (1) in a molar ratio of 1:2, vaporizing the mixture, and introducing the mixture into a vapor deposition furnace for reaction; the hydrogen purity is 5N, the dew point is -63°C, the nitrogen content in the hydrogen is less than 2ppm, the oxygen content is less than 4ppm, the vapor deposition reaction temperature is 1200°C, and the deposition process pressure is less than 6bar, with the optimal pressure being 0.5bar;

[0049] The bell-shaped reactor was equipped with four pairs of gate-shaped carriers with a height of 2000 mm. The deposition time was 50 hours. During the deposition process, the deposition rate of silicon carbide was always maintained above 0.5 mm / h. The SiC deposition weight was 250 kg.

[0050] (3) The tail gas after the reaction is cooled to room temperature, then pressurized to 0.6 MPa, and heat exchanged with a refrigerant, and further deep-cooled to -42°C. Part of the tail gas is liquefied to form a condensate. The main components of the condensate are monomethyltrichlorosilane and part of trichlorosilane and silicon tetrachloride. The condensate is first subjected to a first-stage heat exchange with the tail gas in a shell-and-tube heat exchanger to increase the temperature from -42°C to 0°C, and then subjected to a second-stage heat exchange with a normal temperature water medium in a shell-and-tube heat exchanger to increase the temperature to 10°C. The condensate at this time is then sent to a purification process to remove trichlorosilane and silicon tetrachloride to obtain monomethyltrichlorosilane, which is continued to be used as a raw material for the vapor deposition reaction;

[0051] The specific process of cooling to room temperature is as follows: the outlet temperature of the tail gas of the vapor deposition reaction is 300°C, which is first cooled by a first stage cooling method, and then cooled by heat exchange with normal temperature cooling water to 200°C, and then cooled by heat exchange with the feed mixed gas (about 70°C), and then cooled by a second stage cooling method in a shell and tube heat exchanger to below 200°C, and then the tail gas is cooled by a third stage cooling method, using normal temperature water as the cooling medium, and the tail gas temperature can be reduced to below 40°C, thereby generating condensate;

[0052] The cryogenic cooling process involves pressurizing the tail gas to 6 bar after it has dropped to less than 40°C, and then cooling it twice in a shell-and-tube heat exchanger. The tail gas is cryogenically cooled to -38°C, and condensate is further generated. The heat exchange medium is Freon, and the temperature is -60°C. At this point, the condensate is mainly composed of monomethyltrichlorosilane and a small amount of trichlorosilane and silicon tetrachloride.

[0053] (4) The uncondensed tail gas in step (3), whose main components are hydrogen, hydrogen chloride, trichlorosilane, silicon tetrachloride, and alkanes, is sent to a bubbling elution process, wherein the bubbling elution liquid is a chlorosilane liquid. The trichlorosilane, silicon tetrachloride, and hydrogen chloride gas in the tail gas are bubbled and eluted in the chlorosilane elution liquid and adsorbed. The adsorbed liquid is then distilled and separated, and the hydrogen chloride adsorbed in the chlorosilane is separated. The separated hydrogen chloride can be used for acid production or for hydrogenation of silicon tetrachloride. The separated chlorosilane liquid continues to be used as a bubbling elution liquid. During the bubbling elution process, the tail gas pressure is 5 bar, the pressure of the elution tower is controlled at 3 bar, and the eluent temperature is -34°C.

[0054] (5) The tail gas after bubbling and eluting is mainly composed of hydrogen and part of hydrogen chloride and alkanes, which then enter the activated carbon pressure swing adsorption, which can adsorb hydrogen chloride and alkanes in the tail gas, separate high-purity hydrogen, and continue to be used as a carrier gas for chemical vapor deposition reaction. After the activated carbon adsorption is saturated, the pressure can be reduced to desorb hydrogen chloride, alkanes, etc., and then they are hydrolyzed, neutralized or burned. In the tail gas after bubbling and eluting, the gases that are not absorbed are mainly hydrogen, a small amount of alkanes and trace amounts of chlorosilane and hydrogen chloride. An activated carbon adsorption column is used to adsorb impurities in hydrogen. In the adsorption process, three activated carbon adsorption columns are grouped together, and each activated carbon adsorption column is controlled by a time program to work in an alternating cycle, and performs adsorption, regeneration and cooling respectively. During the adsorption process, the temperature is low and high pressure, the temperature is 45°C, and the pressure is 0.4MPa. After the activated carbon adsorbs trace amounts of chlorosilane, hydrogen chloride and alkanes in the hydrogen, the carbon powder is filtered out to obtain hydrogen. The adsorption column is a columnar activated carbon with a particle size of 12 mesh, accounting for 96%, and a relative density of 0.52g / ml.

[0055] Example 3

[0056] A method for preparing high-purity silicon carbide by chemical vapor deposition comprises the following steps:

[0057] (1) distilling and purifying the raw material monomethyltrichlorosilane to a purity greater than 99.99% and an impurity content less than 3 ppm; in the distillation purification in step (1), two distillation towers are connected in series to first remove heavy elements and then remove light elements;

[0058] (2) mixing the purified monomethyltrichlorosilane and hydrogen in step (1) in a molar ratio of 1:20, vaporizing the mixture, and introducing the mixture into a vapor deposition furnace for reaction; the hydrogen purity is 5N, the dew point is -64°C, the nitrogen content in the hydrogen is less than 2ppm, the oxygen content is less than 4ppm, the vapor deposition reaction temperature is 1600°C, and the deposition process pressure is less than 6bar, and the optimal pressure is 1.5bar;

[0059] The bell-shaped reactor was equipped with four pairs of gate-shaped carriers with a height of 2000 mm. The deposition time was 30 hours. During the deposition process, the deposition rate of silicon carbide was always maintained above 0.7 mm / h. The SiC deposition weight was 200 kg.

[0060] (3) The tail gas after the reaction is cooled to room temperature, then pressurized to 0.1 MPa, and heat exchanged with a refrigerant, and further deep-cooled to -38°C. Part of the tail gas is liquefied to form a condensate. The main components of the condensate are monomethyltrichlorosilane and part of trichlorosilane and silicon tetrachloride. The condensate is first subjected to a first-stage heat exchange with the tail gas in a shell-and-tube heat exchanger to increase the temperature from -38°C to 5°C, and then subjected to a second-stage heat exchange with a normal temperature water medium in a shell-and-tube heat exchanger to increase the temperature to 20°C. The condensate at this time is then sent to a purification process to remove trichlorosilane and silicon tetrachloride to obtain monomethyltrichlorosilane, which is continued to be used as a raw material for the vapor deposition reaction;

[0061] The specific process of cooling to room temperature is as follows: the outlet temperature of the tail gas of the vapor deposition reaction is 500°C, which is first cooled by a first stage cooling method, and then cooled by heat exchange with room temperature cooling water to 350°C, and then cooled by heat exchange with the feed mixed gas (about 120°C), and then cooled by a second stage cooling method in a shell and tube heat exchanger to below 200°C, and then the tail gas is cooled by a third stage cooling method, using room temperature water as the cooling medium, and the tail gas temperature can be reduced to below 40°C, thereby generating condensate;

[0062] The cryogenic cooling process involves pressurizing the tail gas to 10 bar after the tail gas has dropped to less than 40°C. The tail gas is then cooled twice in a shell-and-tube heat exchanger to -42°C, whereupon condensate is generated. The heat exchange medium is Freon, and the temperature is -60°C. The condensate is mainly composed of monomethyltrichlorosilane and a small amount of trichlorosilane and silicon tetrachloride.

[0063] (4) The uncondensed tail gas in step (3), whose main components are hydrogen, hydrogen chloride, trichlorosilane, silicon tetrachloride, and alkanes, is sent to a bubbling elution process, wherein the bubbling elution liquid is a chlorosilane liquid. The trichlorosilane, silicon tetrachloride, and hydrogen chloride gas in the tail gas are bubbled and eluted in the chlorosilane elution liquid and adsorbed. The adsorbed liquid is then distilled and separated, and the hydrogen chloride adsorbed in the chlorosilane is separated. The separated hydrogen chloride can be used for acid production or for hydrogenation of silicon tetrachloride. The separated chlorosilane liquid continues to be used as a bubbling elution liquid. During the bubbling elution process, the tail gas pressure is 5 bar, the pressure of the elution tower is controlled at 5 bar, and the eluent temperature is -38°C.

[0064] (5) The tail gas after bubbling and eluting is mainly composed of hydrogen and part of hydrogen chloride and alkanes, which then enter the activated carbon pressure swing adsorption, which can adsorb hydrogen chloride and alkanes in the tail gas, separate high-purity hydrogen, and continue to be used as a carrier gas for chemical vapor deposition reaction. After the activated carbon adsorption is saturated, the pressure can be reduced to desorb hydrogen chloride, alkanes, etc., and then they are hydrolyzed, neutralized or burned. In the tail gas after bubbling and eluting, the gases that are not absorbed are mainly hydrogen, a small amount of alkanes and trace amounts of chlorosilane and hydrogen chloride. An activated carbon adsorption column is used to adsorb impurities in hydrogen. In the adsorption process, three activated carbon adsorption columns are grouped together, and each activated carbon adsorption column is controlled by a time program to work in an alternating cycle, and performs adsorption, regeneration and cooling respectively. During the adsorption process, the temperature is low and high pressure, the temperature is 50°C, and the pressure is 0.6MPa. After the activated carbon adsorbs trace amounts of chlorosilane, hydrogen chloride and alkanes in the hydrogen, the carbon powder is filtered out to obtain hydrogen. The adsorption column is a columnar activated carbon with a particle size of 4 mesh, accounting for 95%, and a relative density of 0.5g / ml.

[0065] The monomethyltrichlorosilane raw material in Examples 1-3 of the present invention was subjected to rectification and purification. GC analysis, according to the testing method specified in GB / T 20434-2006, indicated that the monomethyltrichlorosilane had a purity greater than 99.99%. ICP-MS (mass spectrometry) analysis indicated that the total impurity content was less than 5 ppm. The results are shown in Table 1.

[0066] Table 1. Mass spectrometry results of Examples 1-3

[0067]

[0068] The silicon carbide material deposited in Example 1 of the present invention was sampled and subjected to XRD detection. The sampling positions were located at the upper, middle and lower parts of the carrier, respectively. The material components therein were analyzed by X-ray powder diffraction. The results showed that there was only 3C crystal SiC, and no other diffraction peaks such as Si and C. It was judged that there was no excessive carbon or silicon. The test results are as follows: Figure 1 shown.

[0069] The silicon carbide material deposited according to the present invention was subjected to GDMS (glow discharge mass spectrometry), which can directly detect the trace element content in the solid sample. The silicon carbide deposited layer of the silicon carbide material obtained in Example 1 was tested. The detected elements and their contents are shown in Table 2, and the total impurity content was 1 ppm.

[0070] Table 2. GDMS test results of the deposited layer of silicon carbide material in Example 1

[0071]

[0072]

[0073] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A method for preparing high-purity silicon carbide by chemical vapor deposition, characterized in that: The following steps are involved: (1) distilling and purifying the raw material monomethyltrichlorosilane to a purity greater than 99.99% and an impurity content less than 5 ppm; (2) mixing the monomethyltrichlorosilane purified in step (1) and hydrogen in a molar ratio of 1:2 to 20, vaporizing the mixture, and introducing the mixture into a vapor deposition furnace for reaction; (3) The tail gas after the reaction in step (2) mainly comprises monomethyltrichlorosilane, hydrogen, hydrogen chloride, trichlorosilane, silicon tetrachloride and alkanes. The tail gas after the reaction is cooled to room temperature, then pressurized to 0.6-1.0 MPa, and heat exchanged with a refrigerant, and further deep-cooled to -38--42°C. Part of the tail gas is liquefied to form a condensate. The main components of the condensate are monomethyltrichlorosilane and part of trichlorosilane and silicon tetrachloride. The condensate is heated to room temperature through heat exchange and then sent to a distillation and purification process to separate and obtain high-purity monomethyltrichlorosilane, which is continued to be used as a raw material for the vapor deposition reaction; (4) the uncondensed tail gas in step (3), whose main components are hydrogen, hydrogen chloride, trichlorosilane, silicon tetrachloride, and alkanes, is sent to a bubbling elution process, wherein the bubbling elution liquid is a chlorosilane liquid, and the trichlorosilane, silicon tetrachloride, and hydrogen chloride gases in the tail gas are bubbled and eluted in the chlorosilane elution liquid and adsorbed, and the adsorbed liquid is then distilled and separated, and the hydrogen chloride adsorbed in the chlorosilane is separated, and the separated hydrogen chloride is used for acid production or for hydrogenation of silicon tetrachloride, and the separated chlorosilane liquid continues to be used as the bubbling elution liquid; (5) The tail gas after bubbling and elution is mainly composed of hydrogen and part of hydrogen chloride and alkanes, which then enter the activated carbon pressure swing adsorption to adsorb hydrogen chloride and alkanes in the tail gas, separate high-purity hydrogen, and continue to be used as a carrier gas for chemical vapor deposition reaction. After the activated carbon is saturated with adsorption, the pressure is reduced to desorb hydrogen chloride and alkanes, which are then hydrolyzed and neutralized or burned; In the distillation purification described in step (1), the distillation tower adopts a two-tower series connection mode, first removes heavy substances and then removes light substances, the high-boiling substances in the raw material monomethyltrichlorosilane are mainly metal chlorides and phosphides with boiling points between several hundred and more than one thousand degrees Celsius, and the low-boiling substances are mainly boron chloride, trichlorosilane, and silicon tetrachloride. The raw material monomethyltrichlorosilane enters from tower 1, and the high-boiling substances are discharged from the bottom of the tower to reduce metal and phosphorus impurities; monomethyltrichlorosilane and a small amount of trichlorosilane and silicon tetrachloride are extracted from the top of tower 1 and enter tower 2 together; the low-boiling substances are extracted from the top of tower 2, and boron chloride, trichlorosilane, and silicon tetrachloride are separated, and high-purity monomethyltrichlorosilane is output from the bottom of the tower; In step (2), the purity of the hydrogen is greater than 5N, the dew point is less than -60°C, the nitrogen content in the hydrogen is less than 2ppm, and the oxygen content is less than 4ppm; In step (2), the vapor deposition reaction temperature is 1200° C. to 1600° C., and the deposition process pressure is from normal pressure to 6 bar; The vapor deposition reaction adopts a bell-shaped reactor, in which a plurality of gate-shaped carriers are arranged, and the deposition thickness is 10-100 mm.

2. The method for preparing high-purity silicon carbide by chemical vapor deposition according to claim 1, characterized in that: In step (3), the specific process of cooling to room temperature is as follows: the exhaust gas outlet temperature of the vapor deposition reaction is 300-500°C, which is first cooled by a first stage cooling, heat exchanged with normal temperature cooling water to 200-350°C, and then heat exchanged with a feed mixed gas of 70-120°C, and cooled to below 200°C by a second stage heat exchange in a shell-and-tube heat exchanger, and then the exhaust gas is cooled by a third stage cooling, using normal temperature water as a cooling medium, and the exhaust gas temperature is reduced to below 40°C to produce condensate; the deep cooling in step (3) is to pressurize the exhaust gas reduced to less than 40°C to 6-10 bar, and then the exhaust gas is cooled by a second stage in a shell-and-tube heat exchanger, and the exhaust gas is deep cooled to -38--42°C to produce condensate, the heat exchange medium is Freon, and the temperature is -60°C. At this time, the condensate components are mainly monomethyltrichlorosilane and a small amount of trichlorosilane and silicon tetrachloride.

3. The method for preparing high-purity silicon carbide by chemical vapor deposition according to claim 1, characterized in that: In step (3), the condensate cooled to -38 to -42°C is first subjected to primary heat exchange with tail gas in a shell-and-tube heat exchanger to raise the temperature from -40°C to 0 to 5°C, and then subjected to secondary heat exchange with normal temperature water medium in a shell-and-tube heat exchanger to raise the temperature to 10 to 20°C. The condensate is then sent to a purification process to remove trichlorosilane and silicon tetrachloride to obtain monomethyltrichlorosilane.

4. The method for preparing high-purity silicon carbide by chemical vapor deposition according to claim 1, characterized in that: During the bubbling elution process in step (4), the eluent used is chlorosilane liquid.

5. The method for preparing high-purity silicon carbide by chemical vapor deposition according to claim 4, characterized in that: The chlorosilane liquid is trichlorosilane or silicon tetrachloride.

6. The method for preparing high-purity silicon carbide by chemical vapor deposition according to claim 1, characterized in that: In step (5), the gases that are not absorbed in the tail gas after bubbling and eluting are mainly hydrogen and alkanes, chlorosilanes, and hydrogen chloride. An activated carbon adsorption column is used to adsorb the remaining components except hydrogen to obtain high-purity hydrogen.

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