A method for preparing polycrystalline silicon for etching

Abstract

This invention discloses a method for preparing polycrystalline silicon for etching. The method includes the following steps: S1: First, a low-concentration silicon nitride slurry is sprayed, followed by a high-concentration silicon nitride slurry is brushed on to prepare a composite silicon nitride coating on the crucible; S2: The polycrystalline silicon material is placed in the crucible, and the crucible is placed in a casting furnace; S3: The casting furnace is closed, and polycrystalline silicon is grown inside the furnace. During the evacuation stage, the gas flow rate is 50 L / min; during the heating and melting stage, the gas flow rate is 100 L / min, and the furnace pressure is 50 mbar; during the crystal growth stage, the gas flow rate is 60 L / min, and the furnace pressure is 50 mbar; during the annealing stage, the gas flow rate is 30 L / min, and the furnace pressure is 50 mbar. The polycrystalline silicon prepared by this method has significantly reduced oxygen, carbon, and nitrogen impurity content, with oxygen and carbon content both below 1 ppma and nitrogen content below 0.5 ppma. This significantly improves etching rate stability and surface accuracy, enhancing etching process stability and device quality.

Description

Technical Field

[0001] This invention relates to the field of polycrystalline silicon preparation technology for etching, and particularly to a method for preparing polycrystalline silicon for etching. Background Technology

[0002] Polysilicon is a key material in etching processes (such as etching targets and etching masks), and its impurity content directly affects etching precision, rate stability, and device lifespan. Common harmful impurity elements in polysilicon used for etching include oxygen, carbon, and nitrogen.

[0003] Oxygen in polysilicon readily forms oxygen precipitates or thermal donors, leading to micro-protrusions or uneven hardness on the material surface during etching. This causes fluctuations in etching rate, affecting pattern transfer accuracy, especially in high-precision etching scenarios (such as semiconductor chip manufacturing), potentially resulting in excessive linewidth deviations. Carbon in polysilicon readily forms carbide precipitates, creating "etching residue" during etching, causing burrs or defects at pattern edges. Furthermore, carbon impurities reduce the chemical stability of polysilicon, making it prone to abnormal reactions under the action of etchants (such as fluorine-based gases), shortening the lifespan of etching tools. Nitrogen impurities may form silicon nitride phases with silicon, whose etching rate differs significantly from pure silicon, resulting in "selective etching unevenness" on the etched surface, compromising the structural integrity of the device.

[0004] Therefore, preparing a polycrystalline silicon for etching with low impurity content is of great significance for improving the stability of the etching process and the quality of devices. Summary of the Invention

[0005] This application provides a method for preparing polycrystalline silicon for etching, which solves the problem of high content of harmful impurities (such as oxygen, carbon, and nitrogen) in polycrystalline silicon for etching in the prior art, and achieves the effect of effectively reducing the content of oxygen, carbon, and nitrogen impurities in polycrystalline silicon for etching.

[0006] This application provides a method for preparing polycrystalline silicon for etching, comprising the following steps: S1: The composite silicon nitride coating of the crucible is prepared by the following method: S1.1: Spray the first silicon nitride slurry into the crucible. The mass ratio of the first silicon nitride slurry is: silicon nitride: silica sol: pure water = 1:0.45-0.55:1.45-1.55; S1.2: Apply a second silicon nitride slurry to the inside of the crucible. The mass ratio of the second silicon nitride slurry is: silicon nitride: silica sol: pure water = 1:0.45-0.55:0.45-0.55; S1.3: Bake the silicon nitride coating to complete the preparation of the composite silicon nitride coating; S2: Place the polycrystalline silicon material into the crucible and place the crucible into the casting furnace; S3: Shut down the casting furnace. Polycrystalline silicon is grown inside the furnace. The gas intake, furnace pressure, and temperature are controlled at each growth stage, as detailed below: S3.1: Evacuation stage: Set the argon gas inlet flow rate to 48-52 L / min, open the gas outlet valve and vacuum pump, and fully open the butterfly valve to reduce the furnace pressure to below 0.2 mbar before entering the heating stage; S3.2: Heating stage: Set the argon gas inlet flow rate to 98-102 L / min, maintain the furnace pressure at 48-52 mbar, turn on the heating source, and enter the melting stage after the temperature rises to above 1530℃; S3.3: Melting stage: Maintain argon flow rate of 98-102 L / min and furnace pressure of 48-52 mbar until the polycrystalline silicon material is completely melted. Then turn on the bottom heat sink and enter the crystal growth stage when the temperature drops below 1410℃. S3.4: Crystal growth stage: Set the argon gas inlet flow rate to 58-62 L / min and maintain the furnace pressure to 48-52 mbar, so that the liquid polycrystalline silicon gradually solidifies and grows from the bottom of the crucible upwards until it is completely solidified and then enters the annealing stage. S3.5: Annealing stage: Set the argon gas inlet flow rate to 28-32 L / min, maintain the furnace pressure at 48-52 mbar, close the bottom heat sink, and hold at 1250-1350℃ for at least 3 hours before entering the cooling stage; S3.6: Cooling stage: Set the argon gas inlet flow rate to 28-32 L / min, raise the furnace pressure to above 1000 mbar, turn on the bottom heat sink, turn off the heating source, and open the casting furnace when the temperature drops below 200℃.

[0007] The beneficial effects of the above embodiments are as follows: the polycrystalline silicon prepared by the method of preparing polycrystalline silicon for etching has an oxygen content that is reduced from 5 ppma to below 1 ppma, a carbon content that is reduced from 10 ppma to below 1 ppma, and a nitrogen content that is controlled to below 0.5 ppma. The polycrystalline silicon for etching can significantly improve the stability of etching rate and surface accuracy, improve the stability of etching process and device quality, and extend the service life of etching tools.

[0008] Based on the above embodiments, this application can be further improved as follows: In one embodiment of this application, in step S1, the mass ratio of the first silicon nitride slurry is: silicon nitride: silica sol: pure water = 1:0.5:1.5; the mass ratio of the second silicon nitride slurry is: silicon nitride: silica sol: pure water = 1:0.5:0.5. Technical effect: By optimizing the slurry ratio, combining low-concentration spraying as a base coat with high-concentration brushing enhances the adhesion between the silicon nitride coating and the crucible, preventing the coating from peeling off under low pressure, thereby reducing the nitrogen impurity content in the finished polycrystalline silicon.

[0009] In one embodiment of this application, the spraying method in step S1.1 is as follows: the first silicon nitride slurry is sprayed using a spray gun: the atomization pressure is set to 0.2-0.25 kg, the gun opening pressure to be at least 0.1 kg, the flow rate to be at least 400 g / min, and the spray gun distance to be 30 ± 2 mm. The inner bottom and four sides of the crucible are sprayed 3-4 times until the slurry is used up. Technical effect: The specific spraying parameters ensure that the slurry uniformly covers the inner wall of the crucible, improves the coating density, reduces the penetration of oxygen impurities during subsequent growth, and further reduces the oxygen content of the finished polycrystalline silicon.

[0010] In one embodiment of this application, the brushing method in step S1.2 is as follows: the second silicon nitride slurry is brushed three times onto the inner bottom and four sides of the crucible using a roller brush. Technical effect: Multiple brushings of high-concentration slurry can form a more uniform surface coating, enhance the sealing effect of the crucible, reduce the diffusion of nitrogen and oxygen impurities from the inner wall of the crucible to the silicon material, and improve the purity of polycrystalline silicon.

[0011] In one embodiment of this application, the gas exhaust system of the casting furnace is sequentially equipped with a ball valve, a butterfly valve, a dust collector, and a vacuum pump, with each component connected by a straight pipe with a diameter of at least 150 mm. Technical benefits: The large-diameter pipe design, combined with the efficient gas extraction components, can maintain a high vacuum (low-pressure environment) inside the furnace, accelerate the removal of volatile impurities, and provide a basis for subsequent high-flow-rate gas intake, effectively reducing the carbon and oxygen impurity content in polycrystalline silicon.

[0012] In one embodiment of this application, the gas inlet system of the casting furnace uses a mass flow meter with a range of at least 200 L / min to control the argon gas intake. Technical benefits: High-precision flow control ensures stable argon gas flow at each growth stage, avoids impurity mixing caused by gas disturbance, improves process stability, and further reduces impurity content.

[0013] In one embodiment of this application, in step S3, the argon gas inlet flow rate is set to 50 L / min during the evacuation stage, 100 L / min during the heating stage, and the furnace pressure is maintained at 50 mbar during the melting stage; the argon gas inlet flow rate is maintained at 100 L / min, and the furnace pressure is maintained at 50 mbar during the crystal growth stage; the argon gas inlet flow rate is set to 60 L / min, and the furnace pressure is maintained at 50 mbar during the annealing stage; the argon gas inlet flow rate is set to 30 L / min, and the furnace pressure is maintained at 50 mbar during the cooling stage; and the argon gas inlet flow rate is set to 30 L / min during the cooling stage. Technical effect: Precise control of argon gas flow rate and furnace pressure at each stage accelerates the removal of impurities from the furnace, while reducing carbon impurity adsorption through airflow disturbance, resulting in oxygen and carbon content in the finished polycrystalline silicon being below 1 ppma.

[0014] In one embodiment of this application, in step S3.5, after closing the bottom heat sink, the temperature is maintained at 1300°C for at least 3 hours before entering the cooling stage. Technical effects: High-temperature heat preservation promotes the repair of internal crystal defects, reduces internal stress, improves the crystal quality of polycrystalline silicon, and further reduces the solubility of impurity atoms, ensuring that the nitrogen content is below 0.5 ppma, thus improving the stability of the etching rate.

[0015] One or more technical solutions provided in the embodiments of this application have at least the following technical effects or advantages: 1. The present invention uses a silicon nitride slurry with a specific ratio, and performs a low-solution spraying, high-solution brushing and drying process under specific pressure, which makes the silicon nitride less likely to detach, thereby reducing the nitrogen and oxygen content in the polysilicon used for etching in the finished product; 2. Compared with conventional small-diameter gas extraction, the present invention uses a large-diameter pipe to maintain a high vacuum (low pressure) inside the furnace and doubles the gas flow rate during heating and melting stages, thereby accelerating the removal of impurities inside the furnace and reducing the impurity content in the polysilicon used for etching the finished product.

[0016] 3. Compared with conventional growth processes, the present invention reduces furnace pressure and increases gas flow rate during the polycrystalline silicon growth stage, thereby reducing the carbon and oxygen content in the finished polycrystalline silicon used for etching.

[0017] 4. The polycrystalline silicon for etching prepared using the method of the present invention has an oxygen and carbon content of less than 1 ppma and a nitrogen content of less than 0.5 ppma. This polycrystalline silicon for etching can significantly improve the stability of etching rate and surface accuracy, improve the stability of etching process and device quality, meet the requirements of high-precision etching process, and extend the service life of etching tools. Attached Figure Description

[0018] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0019] Figure 1 This is a flowchart illustrating the steps of a method for preparing polycrystalline silicon for etching according to an embodiment of this application; Figure 2 This is a schematic diagram of the casting furnace in an embodiment of this application. Detailed Implementation

[0020] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. After reading the present invention, any modifications of the present invention in various equivalent forms by those skilled in the art will fall within the scope defined by the appended claims.

[0021] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0022] This application provides a method for preparing polycrystalline silicon for etching, which solves the problem of high content of harmful impurities (such as oxygen, carbon, and nitrogen) in polycrystalline silicon for etching in the prior art, and achieves the effect of effectively reducing the content of oxygen, carbon, and nitrogen impurities in polycrystalline silicon for etching.

[0023] The technical solution in this application is to solve the above problems, and the overall approach is as follows: Example: like Figure 1 As shown in the figure, this application provides a method for preparing polycrystalline silicon for etching, including the following steps: S1: The composite silicon nitride coating for the crucible is prepared using the following method: S1.1: The first silicon nitride slurry is sprayed into the crucible using a spray gun. The mass ratio of the first silicon nitride slurry is: silicon nitride: silica sol: pure water = 1:0.45-0.55:1.45-1.55, preferably 1:0.5:1.5. The specific spraying method is as follows: The first silicon nitride slurry is applied using a spray gun: Set the atomization pressure to 0.2-0.25 kg, the gun opening pressure to at least 0.1 kg, the flow rate to at least 400 g / min, and the spray gun distance to 30 ± 2 mm. Spray 3-4 coats onto the inner bottom and four sides of the crucible until the slurry is used up. The new first silicon nitride slurry formulation, combined with parameters such as spray gun pressure, can effectively enhance the adhesion between the first silicon nitride slurry and the crucible body, preventing the silicon nitride slurry from detaching under low pressure.

[0024] S1.2: Use a roller brush to apply the second silicon nitride slurry to the inside of the crucible. The mass ratio of the second silicon nitride slurry is: silicon nitride: silica sol: pure water = 1:0.45-0.55:0.45-0.55, preferably 1:0.5:0.5. The specific brushing method is as follows: Using a 30mm wide roller brush, apply three coats of the second silicon nitride slurry to the inner bottom and four sides of the crucible. The concentration of the second silicon nitride slurry is higher than that of the first silicon nitride slurry. After spraying the crucible with the low-concentration slurry as a base coat, the high-concentration slurry is then brushed multiple times to allow the high-concentration slurry to bond with the low-concentration slurry, thereby preventing the silicon nitride slurry from falling off under low pressure.

[0025] S1.3: Bake the silicon nitride coating to complete the preparation of the composite silicon nitride coating.

[0026] The silicon nitride coating is baked using a crucible heating rack for more than 1 hour.

[0027] The crucible is a quartz crucible, and the purity of the quartz sand in the inner layer of the crucible is higher than 7N.

[0028] After the first and second silicon nitride slurries are mixed, they must be stirred for at least 0.5 hours.

[0029] S2: Place the polycrystalline silicon material into the crucible and place the crucible into the casting furnace.

[0030] like Figure 2 As shown, the casting furnace uses Ar as a protective gas, with an air inlet system at the top and an air outlet system in the middle of the left side. The casting furnace uses isostatic graphite as a heating source and graphite hard felt as insulation material. The temperature gradient between the top and bottom is adjusted by opening and closing the bottom heat sink to control the melting and growth of polycrystalline silicon.

[0031] The casting furnace's gas inlet system uses a mass flow meter with a range of at least 200 L / min to control the argon gas intake. The casting furnace's gas outlet system is sequentially equipped with a ball valve, a butterfly valve, a dust collector, and a vacuum pump, with each component connected by a straight pipe with a diameter of at least 150 mm. For this purpose, ball valves control on / off switching, butterfly valves regulate flow, the dust collector has a dust handling capacity of at least 330 L / s to protect the vacuum pump, and the vacuum pump's pumping speed is at least 22000 L / min. Conventional silicon nitride coatings are prone to detaching from the crucible at pressures below 400 mbar, contaminating the growth of polycrystalline silicon. Therefore, the outlet pipe diameter of conventional casting furnaces is around 8 mm, controlling the furnace's low pressure at 600 mbar. The improvements to the silicon nitride coating in this invention, combined with improvements to the gas outlet system, provide a foundation for increasing the gas intake and maintaining the ultra-low pressure environment during the subsequent growth process.

[0032] S3: Shut down the casting furnace. Polycrystalline silicon is grown inside the furnace. The gas intake, furnace pressure, and temperature are controlled at each growth stage, as detailed below: S3.1: Evacuation stage: Set the argon gas inlet flow rate to 48-52 L / min, preferably 50 L / min, open the gas outlet valve and vacuum pump, fully open the butterfly valve, and allow the furnace pressure to drop below 0.2 mbar before entering the heating stage; S3.2: Heating stage: Set the argon gas inlet flow rate to 98-102 L / min, preferably 100 L / min, maintain the furnace pressure at 48-52 mbar (adjusted by the butterfly valve), preferably 50 mbar, turn on the heating source, and automatically enter the melting stage after the temperature rises to above 1530℃; S3.3: Melting stage: Maintain argon flow rate of 98-102 L / min, preferably 100 L / min, and furnace pressure of 48-52 mbar, preferably 50 mbar, until the polycrystalline silicon material is completely melted. Then turn on the bottom heat sink and enter the crystal growth stage when the temperature drops below 1410℃. S3.4: Crystal growth stage: Set the argon gas inlet flow rate to 58-62 L / min, preferably 60 L / min, and maintain the furnace pressure at 48-52 mbar, preferably 50 mbar, so that the liquid polycrystalline silicon gradually solidifies and grows from the bottom of the crucible upwards until it is completely solidified and then enters the annealing stage. S3.5: Annealing stage: Set the argon gas inlet flow rate to 28-32 L / min, preferably 30 L / min, maintain the furnace pressure at 48-52 mbar, preferably 50 mbar, close the bottom heat sink, and hold at 1250-1350℃, preferably 1300℃ for at least 3 hours before entering the cooling stage; S3.6: Cooling stage: Set the argon gas inlet flow rate to 28-32 L / min, preferably 30 L / min, raise the furnace pressure to above 1000 mbar, open the bottom heat sink, turn off the heating source, and open the casting furnace when the temperature drops below 200℃.

[0033] The technical solutions described in the embodiments of this application have at least the following technical effects or advantages: 1. The present invention uses a silicon nitride slurry with a specific ratio, and performs a low-solution spraying, high-solution brushing and drying process under specific pressure, which makes the silicon nitride less likely to detach, thereby reducing the nitrogen content and some oxygen content in the polysilicon used for etching in the finished product (preventing oxygen from entering). 2. Compared to conventional small-diameter evacuation pipes, this invention uses a large-diameter pipe to maintain a high vacuum (low pressure) inside the furnace and doubles the airflow during heating and melting stages, thereby accelerating the removal of impurities from the furnace and reducing the impurity content in the polysilicon used for etching. To this end, the exhaust system is improved by incorporating ball valves, butterfly valves, dust collectors, and other equipment.

[0034] 3. Compared with conventional growth processes, the present invention reduces furnace pressure and increases gas flow rate during the polycrystalline silicon growth stage, thereby reducing the carbon and oxygen content in the finished polycrystalline silicon used for etching.

[0035] 4. The polycrystalline silicon for etching prepared using the method of the present invention has an oxygen and carbon content of less than 1 ppma and a nitrogen content of less than 0.5 ppma. This polycrystalline silicon for etching can significantly improve the stability of etching rate and surface accuracy, improve the stability of etching process and device quality, meet the requirements of high-precision etching process, and extend the service life of etching tools.

[0036] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing polycrystalline silicon for etching, characterized in that, Includes the following steps: S1: The composite silicon nitride coating for the crucible is prepared using the following method: S1.1: Spray the first silicon nitride slurry into the crucible. The mass ratio of the first silicon nitride slurry is: silicon nitride: silica sol: pure water = 1:0.45-0.55:1.45-1.55; S1.2: Apply a second silicon nitride slurry to the inside of the crucible. The mass ratio of the second silicon nitride slurry is: silicon nitride: silica sol: pure water = 1:0.45-0.55:0.45-0.55; S1.3: Bake the silicon nitride coating to complete the preparation of the composite silicon nitride coating; S2: Place the polycrystalline silicon material into a crucible, place the crucible in a casting furnace, and the gas outlet system of the casting furnace is equipped with a ball valve, a butterfly valve, a dust collector, and a vacuum pump in sequence. The components are connected by a straight pipe with a diameter of at least 150 mm. The gas inlet system of the casting furnace uses a mass flow meter with a range of at least 200 L / min to control the argon gas inlet volume. S3: Shut down the casting furnace and grow polycrystalline silicon inside the furnace, as detailed below: S3.1: Evacuation stage: Set the argon gas inlet flow rate to 48-52 L / min, and after the furnace pressure drops below 0.2 mbar, proceed to the heating stage; S3.2: Heating stage: Set the argon gas inlet flow rate to 98-102 L / min, maintain the furnace pressure at 48-52 mbar, turn on the heating source, and enter the melting stage after the temperature rises to above 1530℃; S3.3: Melting stage: Maintain argon flow rate of 98-102 L / min and furnace pressure of 48-52 mbar until the polycrystalline silicon material is completely melted. Then turn on the bottom heat sink and enter the crystal growth stage when the temperature drops below 1410℃. S3.4: Crystal growth stage: Set the argon gas inlet flow rate to 58-62 L / min and maintain the furnace pressure to 48-52 mbar, so that the liquid polycrystalline silicon gradually solidifies and grows from the bottom of the crucible upwards until it is completely solidified and then enters the annealing stage. S3.5: Annealing stage: Set the argon gas inlet flow rate to 28-32 L / min, maintain the furnace pressure at 48-52 mbar, close the heat sink, and hold at 1250-1350℃ for at least 3 hours before entering the cooling stage; S3.6: Cooling stage: Set the argon gas inlet flow rate to 28-32 L / min, raise the furnace pressure to above 1000 mbar, turn on the heat sink, turn off the heating source, and open the casting furnace when the temperature drops below 200℃.

2. The preparation method according to claim 1, characterized in that: In step S1, the mass ratio of the first silicon nitride slurry is: silicon nitride: silica sol: pure water = 1:0.5:1.5; the mass ratio of the second silicon nitride slurry is: silicon nitride: silica sol: pure water = 1:0.5:0.

5.

3. The preparation method according to claim 2, characterized in that: The spraying method in step S1.1 is as follows: the first silicon nitride slurry is sprayed with a spray gun: the atomization pressure is set to 0.2-0.25 kg, the gun opening pressure is at least 0.1 kg, the flow rate is at least 400 g / min, and the spray gun distance is 30±2 mm. The inner bottom and four sides of the crucible are sprayed 3-4 times until the slurry is used up.

4. The preparation method according to claim 3, characterized in that: In step S1.2, the brushing method is as follows: the second silicon nitride slurry is brushed onto the inner bottom and four sides of the crucible three times using a roller brush.

5. The preparation method according to claim 1, characterized in that: In step S3, the argon gas inlet flow rate is set to 50 L / min during the evacuation stage, 100 L / min during the heating stage, and the furnace pressure is maintained at 50 mbar during the melting stage, 100 L / min during the melting stage, and 50 mbar during the crystal growth stage, 60 L / min during the crystal growth stage, and 30 L / min during the annealing stage, while maintaining a furnace pressure of 50 mbar. The argon gas inlet flow rate is also set to 30 L / min during the cooling stage.

6. The preparation method according to claim 5, characterized in that: In step S3.5, after the heat sink is turned off, the temperature is kept at 1300℃ for at least 3 hours before entering the cooling stage.

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