Method for removing carbon impurities from granular polycrystalline silicon

Abstract

The invention relates to a method for removing carbon impurities in granular polycrystalline silicon. The carbon impurities in the granular polycrystalline silicon are removed by adopting the synergistic effect of high-temperature pyrolysis and ozone oxidation. The method comprises the following specific steps: firstly, generating cracks in granular polycrystalline silicon in a vacuum or inert atmosphere through microwave radiation heating, and exposing internal carbon impurities; then, ozone gas reacts with carbon impurities in the cracks, and the carbon impurities are removed. According to the method, through the synergistic effect of microwave heating and ozone oxidation, the content of carbon impurities in the granular polycrystalline silicon is effectively reduced from about 0.4 ppma to 0.1 ppma or below. Compared with the prior art, not only can the purity of the granular polycrystalline silicon be improved, but also the silicon particles can be prevented from being oxidized in the heating process, and the subsequent application performance of the granular polycrystalline silicon is ensured. The method has the advantages of high efficiency, economy and environmental protection, can promote large-scale application of the granular silicon, especially application in monocrystalline silicon production, and has important technical and economic values.

Description

Technical Field

[0001] The present invention relates to the technical field of polysilicon preparation, and in particular to a method for removing carbon impurities from granular polysilicon. Background Art

[0002] At present, the preparation technology of high-purity polysilicon mainly includes the modified Siemens method, fluidized bed method and other methods. The modified Siemens method is a closed-loop silicon production method, which uses the reaction of metallic silicon powder and HCl to generate SiHCl3, which is then distilled and purified, and then H2 reduction deposition is used to obtain high-purity polysilicon. The modified Siemens method currently occupies a major share of the market and is widely used in the production of high-purity polysilicon. However, this method has the problems of low production efficiency, high energy consumption and large fixed investment, which leads to certain economic and technical challenges in large-scale production, especially when high output and low energy consumption are required, it is still difficult to meet the demand.

[0003] The fluidized bed method is a common silicon production method. This method usually uses SiH4 or SiCl4 as the silicon source gas, fills with hydrogen as the carrier gas, undergoes a chemical vapor deposition reaction in the temperature range of 500 to 700°C, and grows on the silicon powder pre-set in the fluidized bed. As the reaction proceeds, the reaction products gradually form granular silicon and are discharged through the bottom of the fluidized bed. The advantages of the fluidized bed method are simple equipment structure, low investment, and high production efficiency, so it is widely used in the production of granular silicon. However, due to the large amount of impurity pollution generated during the production process of the fluidized bed method, especially the impurities are easily mixed on the surface and inside of the granular silicon, resulting in a decrease in the purity of the final product.

[0004] Due to its large specific surface area, granular silicon is prone to adsorbing and mixing carbon-based impurities during its growth process. In particular, various gas substances in the fluidized bed during production cause the silicon particles to "boil" in the fluidized bed. This movement causes the granular silicon to collide and rub against the carbon-based materials on the inner wall of the reactor as it grows, resulting in the attachment of carbon impurities. These carbon impurities will be wrapped inside the particles, significantly increasing the carbon content in the granular silicon (usually around 0.4 ppma). In contrast, the carbon content of rod-shaped silicon prepared by the traditional modified Siemens method is usually lower, about 0.2 ppma.

[0005] The high content of carbon impurities in granular silicon has a significant impact on the quality of monocrystalline silicon, especially in the context of the gradual promotion of N-type monocrystalline silicon, the purity requirements of silicon raw materials are more stringent. The presence of carbon impurities not only affects the physical properties of granular silicon, but also leads to dislocation defects, increased broken edge rate and other problems in the subsequent production process of monocrystalline silicon, further limiting the proportion of granular silicon used in CZO monocrystalline silicon. Therefore, the development of an effective method to remove carbon impurities in granular polycrystalline silicon has become an important technical requirement to improve the quality of granular silicon and expand its application range.

[0006] At present, there is no effective method for removing carbon impurities from granular polysilicon in the industry. Due to the lack of effective technology for removing carbon impurities, CZ-pulling single crystal companies generally use granular silicon as an additive and control its doping ratio below 30%, while the doping ratio of some leading silicon wafer factories can only reach about 40%. Such a doping ratio limits the large-scale application of granular silicon materials and fails to give full play to the advantages of granular silicon. Therefore, the development of an efficient method for removing carbon impurities from granular polysilicon is crucial to improving the purity of granular silicon and promoting its application in monocrystalline silicon production. Summary of the invention

[0007] The technical problem to be solved by the present invention is to provide a method for removing carbon impurities from granular polysilicon, so as to solve the problem in the prior art that the carbon impurity content in granular silicon is too high, affecting the purity of silicon material and the quality of single crystal silicon.

[0008] In order to overcome the above defects of the prior art, the present invention provides a method for removing carbon impurities from granular polysilicon, comprising the following steps: S1: placing granular polycrystalline silicon into a reaction vessel, heating the granular polycrystalline silicon by microwave radiation, heating and keeping the temperature under vacuum or inert atmosphere, until the granular polycrystalline silicon generates cracks due to uneven heating, exposing internal carbon impurities; S2: After the granular polycrystalline silicon is cooled to 100-200°C after microwave irradiation in step S1, ozone gas is filled into the reactor. The ozone gas enters the interior of the granules along the cracks and reacts with carbon impurities to generate carbon dioxide and other gases which are discharged to obtain granular polycrystalline silicon with carbon impurities removed.

[0009] The implementation principle of the method for removing carbon impurities from granular polysilicon in the present application is as follows: Since each substance has different dielectric properties, etc., the microwave power density actually obtained in the same microwave electromagnetic field is inconsistent, and the microwave energy is also inconsistently converted into thermal energy. If there are multiple components in a particle, this will lead to inconsistent temperature rises of the components in the particle, and the differences in the thermal expansion coefficients of the components will also lead to differences in the particles during thermal expansion, resulting in non-uniformly distributed thermal stress concentration inside the particles. Therefore, because the silicon particles contain different substances, they can produce internal cracks under the action of microwave radiation, thereby exposing the carbon impurities inside the silicon particles. Ozone gas has strong oxidizing properties and is easily decomposed into O2 and atomic O at lower temperatures, and atomic O is very active. The carbon impurities in silicon particles mainly come from the continuous impact of the graphite devices in the furnace during the fluidization process of the particles. Graphite is a form of carbon, which is a highly reducing element and is easy to participate in oxidation reactions. In particular, micro-nanographite has more reactive sites. Cataldo studied the reaction of graphite powder ground into nanometer scale with ozone. The results showed that the fine graphite powder can react with ozone gas very quickly at room temperature to generate carbon dioxide gas (J. Nanosci. Nanotechnol. 2007, 7, 1-9). The reason for choosing to fill O3 gas at a lower temperature is mainly to avoid the rapid decomposition of O3 in a high temperature environment and the oxidation of silicon particles.

[0010] Compared with the existing technology, it has the following advantages: Improve removal efficiency: Compared with existing physical or chemical dust removal methods, the present invention effectively improves the efficiency of removing carbon impurities through the synergistic effect of microwave radiation heating and ozone oxidation. Microwave heating can expose the carbon impurities inside the granular polysilicon and completely remove the internal carbon impurities through the chemical reaction of ozone gas, thereby significantly reducing the carbon content of the granular silicon; prevent oxidation: The use of microwave heating under vacuum or inert atmosphere conditions avoids oxidation of the granular silicon surface during the heating process, effectively maintaining the purity of the granular silicon. Compared with traditional high-temperature oxidation treatment, the present invention reduces the negative impact of surface oxidation and ensures the quality of granular silicon; energy-saving and high efficiency: the synergistic effect of microwave heating and ozone oxidation in the present invention can quickly and evenly heat granular polysilicon and complete the removal of carbon impurities with lower energy consumption, significantly improving the processing efficiency and reducing production costs; improved carbon impurity removal effect: compared with traditional methods, the present invention can maintain the good structure and performance of granular polysilicon while removing carbon impurities, and the carbon impurity content can be reduced from about 0.4 ppma to below 0.1 ppma, thereby improving the quality of granular polysilicon and enabling it to meet higher purity requirements, especially in the production of N-type monocrystalline silicon, showing better application prospects. After the above-mentioned microwave radiation pyrolysis-ozone oxidation synergistic effect, the carbon impurities of the original granular silicon can be effectively removed, thereby meeting the subsequent normal use for pulling solar single crystal rods.

[0011] In a possible implementation, in step S1, the reaction vessel is a crucible, and the microwave radiation heating device is a tubular microwave oven.

[0012] Compared with the prior art, the above technical solution can effectively improve the heating efficiency of granular polysilicon and optimize the temperature distribution during the heating process, avoiding the problems of uneven heating or temperature fluctuations in traditional heating methods. Traditional methods (such as resistance heating or furnace heating) often rely on external heat sources, and the temperature conduction is uneven, which may lead to local overheating or insufficient heating during the heating process. By using a combination of a tubular microwave oven and a crucible, it can be ensured that microwaves act directly on the material, and the granular polysilicon can be quickly and evenly heated by microwave radiation, thereby greatly improving the heating efficiency.

[0013] In a possible implementation, in step S1, the power of the microwave radiation heating is 200W-4000W, and the heating rate is 10-200°C / min.

[0014] Compared with the prior art, the above technical solution can significantly improve the removal efficiency of carbon impurities in granular polysilicon, while improving the controllability and uniformity of the heating process. Traditional heating methods usually rely on external heat sources to heat granular silicon, which may cause uneven heat during the heating process, thereby affecting the removal effect of carbon impurities. Through microwave radiation heating, granular polysilicon can absorb energy evenly in a short period of time, thereby accelerating the removal of carbon impurities, significantly improving the removal efficiency of carbon impurities in granular polysilicon, and ensuring the thorough removal of carbon impurities.

[0015] In a possible implementation, in step S1, the insulation temperature is 600-1200° C., and the insulation time is 10-180 min.

[0016] Compared with the prior art, the above technical solution can effectively optimize the removal process of carbon impurities in granular polysilicon; the heating method in the prior art may have uneven temperature control, or the carbon impurities cannot be completely removed due to the temperature being set too low, while in the above possible implementation mode, by setting the insulation temperature to 600-1200°C, it can be ensured that the carbon impurities inside the granular polysilicon are effectively released and removed under high temperature conditions, while controlling a reasonable insulation time (10-180 minutes), so that this process can ensure that the carbon impurities are completely removed while avoiding excessive treatment, thereby improving the removal efficiency, avoiding the problems of uneven heating and excessive treatment, and at the same time ensuring the quality stability of the granular polysilicon and the feasibility of subsequent applications.

[0017] In a possible implementation, in step S1, the inert atmosphere includes one or both of nitrogen and argon, and the gas flow rate is 50-100 mL / min.

[0018] Compared with the prior art, by adopting the above-mentioned technical scheme, by setting an appropriate inert atmosphere and inflation flow rate, the surface of the granular polysilicon is effectively protected, oxidation reaction is avoided, the purity and structural stability of the granular polysilicon are ensured, and the granular polysilicon can be effectively prevented from reacting with oxygen during high-temperature heating. Conventional high-temperature treatment methods usually rely on air or uncontrolled atmospheres, which may lead to oxidation of silicon materials and thus affect the performance and subsequent applications of the particles. The above-mentioned possible implementation methods of the present invention can effectively isolate oxygen and prevent surface oxidation of the granular polysilicon by using an inert atmosphere such as nitrogen and argon, while ensuring stability during the heating process.

[0019] In a possible implementation, in step S2, the flow rate of the ozone gas is 10-100 g / h, and the time for filling the ozone gas is 1 hour.

[0020] Compared with the prior art, the above technical scheme can effectively optimize the reaction process between ozone and carbon impurities in granular polysilicon by reasonably controlling the flow rate and filling time of ozone gas. The higher flow rate ensures that the ozone gas can fully penetrate into the cracks inside the granular polysilicon to react with the carbon impurities. The reasonable reaction time (1 hour) ensures the completeness of the reaction, avoids incomplete reaction or the formation of by-products, and further synergistically removes the carbon impurities in the granular polysilicon, thereby improving the purity and quality of the granular polysilicon.

[0021] In a possible implementation, in step S2, the temperature reduction method is furnace cooling.

[0022] In a possible implementation, in step S2, the carbon impurity content of the granular polysilicon after the carbon impurities are removed is below 0.1 ppma.

[0023] Compared with the prior art, the above technical solution can significantly reduce the carbon impurity content in granular polysilicon and improve its purity and quality; in the prior art, although some carbon impurities can be removed, due to the limited efficiency of the removal method, the carbon impurity content in granular polysilicon is usually high and cannot meet the requirements of high-purity silicon materials. Through the technical solution of the present invention, the carbon impurities can be completely removed in a relatively short time by utilizing the synergistic effect of microwave radiation heating and ozone oxidation, thereby greatly reducing the carbon content in granular polysilicon to below 0.1ppma, significantly improving the purity of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1This is an appearance diagram of granular polysilicon after being processed by the method for removing carbon impurities from granular polysilicon of the present invention. DETAILED DESCRIPTION

[0025] First, those skilled in the art should understand that these implementations are only used to explain the technical principles of the embodiments of the present application, and are not intended to limit the protection scope of the embodiments of the present application. Those skilled in the art can make adjustments to them as needed to adapt to specific application scenarios.

[0026] The present invention provides a method for removing carbon impurities from granular polysilicon, comprising the following steps: S1: placing granular polycrystalline silicon into a reaction vessel, heating the granular polycrystalline silicon by microwave radiation, heating and keeping the temperature under vacuum or inert atmosphere, until the granular polycrystalline silicon generates cracks due to uneven heating, exposing internal carbon impurities; S2: After the granular polycrystalline silicon is cooled to 100-200°C after microwave irradiation in step S1, ozone gas is filled into the reactor. The ozone gas enters the interior of the granules along the cracks and reacts with carbon impurities to generate carbon dioxide and other gases which are discharged to obtain granular polycrystalline silicon with carbon impurities removed.

[0027] As a preferred solution, in step S1, the reaction vessel is a crucible, and the microwave radiation heating device is a tubular microwave oven.

[0028] As a preferred solution, in step S1, the power of the microwave radiation heating is 200W-4000W, and the heating rate is 10-200°C / min.

[0029] As a preferred solution, in step S1, the insulation temperature is 600-1200° C., and the insulation time is 10-180 min.

[0030] As a preferred solution, in step S1, the inert atmosphere includes one or both of nitrogen and argon, and the gas flow rate is 50-100 mL / min.

[0031] As a preferred solution, in step S2, the flow rate of the ozone gas is 10-100 g / h, and the time for filling the ozone gas is 1 hour.

[0032] As a preferred solution, in step S2, the cooling method is furnace cooling.

[0033] As a preferred solution, in step S2, the carbon impurity content of the granular polysilicon after the carbon impurities are removed is below 0.1 ppma.

[0034] The following provides specific data and tests in combination with the above data range to describe the above technical solution of the present invention in detail: Embodiment 1: This embodiment provides a method for removing carbon impurities from granular polysilicon, comprising the following steps: S1: Put the granular polycrystalline silicon into a crucible, put it into a tubular microwave oven to heat the granular polycrystalline silicon by microwave radiation, the power of the microwave radiation heating is 2000W, the heating rate is 100℃ / min, heating and heat preservation are carried out under vacuum or inert atmosphere, the heat preservation temperature is 900℃, the heat preservation time is 55 min, the inert atmosphere includes one or two of nitrogen and argon, and the filling flow rate is 75mL / min; cracks are generated in the granular polycrystalline silicon due to uneven heating, exposing the internal carbon impurities; S2: After the granular polycrystalline silicon after microwave irradiation in step S1 is cooled to 150°C in the furnace, ozone gas is filled into the reactor. The flow rate of the ozone gas is 55g / h, and the time of filling the ozone gas is 1h. The ozone gas enters the interior of the particles along the cracks and reacts with carbon impurities to generate gases such as carbon dioxide and discharge them, thereby obtaining granular polycrystalline silicon with carbon impurities removed.

[0035] In Example 1, the carbon impurity content of the granular polycrystalline silicon after the carbon impurities were removed was 0.4 ppma, and the carbon impurity content of the granular polycrystalline silicon after the carbon impurities were removed was 0.05 ppma.

[0036] Embodiment 2: This embodiment provides a method for removing carbon impurities from granular polysilicon, comprising the following steps: S1: Put the granular polycrystalline silicon into a crucible, put it into a tubular microwave oven to heat the granular polycrystalline silicon by microwave radiation, the power of the microwave radiation heating is 200W, the heating rate is 10℃ / min, heating and heat preservation are carried out under vacuum or inert atmosphere, the heat preservation temperature is 600℃, the heat preservation time is 10min, the inert atmosphere includes one or two of nitrogen and argon, and the filling flow rate is 50mL / min; the granular polycrystalline silicon generates cracks due to uneven heating, exposing the internal carbon impurities; S2: After the granular polycrystalline silicon after microwave irradiation in step S1 is cooled to 100°C with the furnace, ozone gas is filled into the reactor. The flow rate of the ozone gas is 10g / h, and the time of filling the ozone gas is 1h. The ozone gas enters the interior of the particles along the cracks and reacts with carbon impurities to generate gases such as carbon dioxide and discharge them, thereby obtaining granular polycrystalline silicon with carbon impurities removed.

[0037] In Example 2, the carbon impurity content of the granular polycrystalline silicon after the carbon impurities were removed was 0.5 ppma, and the carbon impurity content of the granular polycrystalline silicon after the carbon impurities were removed was 0.09 ppma.

[0038] Embodiment 3: This embodiment provides a method for removing carbon impurities from granular polysilicon, comprising the following steps: S1: Put the granular polycrystalline silicon into a crucible, put it into a tubular microwave oven to heat the granular polycrystalline silicon by microwave radiation, the power of the microwave radiation heating is 4000W, the heating rate is 200℃ / min, heating and heat preservation are carried out under vacuum or inert atmosphere, the heat preservation temperature is 1200℃, the heat preservation time is 100 min, the inert atmosphere includes one or two of nitrogen and argon, and the filling flow rate is 100mL / min; the granular polycrystalline silicon generates cracks due to uneven heating, exposing the internal carbon impurities; S2: After the granular polycrystalline silicon after microwave irradiation in step S1 is cooled to 200°C in the furnace, ozone gas is filled into the reactor. The flow rate of the ozone gas is 100g / h, and the time of filling the ozone gas is 1h. The ozone gas enters the interior of the particles along the cracks and reacts with carbon impurities to generate gases such as carbon dioxide and discharge them, thereby obtaining granular polycrystalline silicon with carbon impurities removed.

[0039] In Example 3, the carbon impurity content of the granular polycrystalline silicon after the carbon impurities were removed was 0.3 ppma, and the carbon impurity content of the granular polycrystalline silicon after the carbon impurities were removed was 0.04 ppma.

[0040] like Figure 1 As shown, Figure 1 The schematic diagram of granular polysilicon after treatment in Example 1 of the present invention is further proved by the above examples that the present invention provides an efficient method for removing carbon impurities from granular polysilicon. The technical principle is to use the synergistic effect of microwave radiation heating and ozone oxidation to cause cracks inside the granular polysilicon by microwave heating, expose carbon impurities, and then react with carbon impurities through ozone gas to generate carbon dioxide and other gases and discharge them, thereby effectively removing carbon impurities in granular polysilicon. The method can significantly reduce the carbon impurity content in granular polysilicon to less than 0.1ppma. The present invention solves the problems of low efficiency and incomplete removal of carbon impurities in granular polysilicon in the prior art, especially in granular polysilicon produced by fluidized bed method, the carbon impurity content is high, which affects the purity and subsequent application of silicon materials. Through the combination of microwave radiation heating and ozone oxidation, the present invention realizes the efficient removal of carbon impurities, ensures the application of granular polysilicon in the production of single crystal silicon, and improves the overall quality of granular silicon. The technical solution of the present invention not only improves the quality of granular polysilicon and reduces the content of carbon impurities, but also avoids the problems of incomplete removal or over-oxidation of carbon impurities in traditional methods. It has higher processing efficiency and lower energy consumption, promotes the large-scale application of granular polysilicon, and meets the market demand for high-purity silicon materials.

[0041] In the description of the present application, the description with reference to the terms "one embodiment", "some embodiments", "in the present embodiment", "specific example", or "some examples" etc. means that the specific features, mechanisms, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, mechanisms, materials or characteristics described may be combined in a suitable manner in any one or more embodiments or examples. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0042] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application shall be based on the protection scope of the claims.

Claims

1. A method for removing carbon impurities from granular polysilicon, characterized in that: The following steps are involved: S1: placing granular polycrystalline silicon into a reaction vessel, heating the granular polycrystalline silicon by microwave radiation, heating and keeping the temperature under vacuum or inert atmosphere, until the granular polycrystalline silicon generates cracks due to uneven heating, exposing internal carbon impurities; S2: After the granular polycrystalline silicon is cooled to 100-200°C after microwave irradiation in step S1, ozone gas is filled into the reactor. The ozone gas enters the interior of the granules along the cracks and reacts with carbon impurities to generate carbon dioxide and other gases which are discharged to obtain granular polycrystalline silicon with carbon impurities removed.

2. The method for removing carbon impurities from granular polysilicon according to claim 1, characterized in that: In the step S1, the reaction vessel is a crucible, and the microwave radiation heating device is a tubular microwave oven.

3. The method for removing carbon impurities from granular polysilicon according to claim 1, characterized in that: In the step S1, the power of the microwave radiation heating is 200-4000W, and the heating rate is 10-200°C / min.

4. The method for removing carbon impurities from granular polysilicon according to claim 1, characterized in that: In the step S1, the insulation temperature is 600-1200° C., and the insulation time is 10-180 min.

5. The method for removing carbon impurities from granular polysilicon according to claim 1, characterized in that: In the step S1, the inert atmosphere includes one or both of nitrogen and argon, and the gas flow rate is 50-100 mL / min.

6. The method for removing carbon impurities from granular polysilicon according to claim 1, characterized in that: In the step S2, the flow rate of the ozone gas is 10-100 g / h, and the time for charging the ozone gas is 1 hour.

7. The method for removing carbon impurities from granular polysilicon according to claim 1, characterized in that: In the step S2, the temperature reduction method is furnace cooling.

8. The method for removing carbon impurities from granular polysilicon according to claim 1, characterized in that: In the step S2, the carbon impurity content of the granular polysilicon after the carbon impurities are removed is below 0.1 ppma.

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