JP7605751B2 - Polycrystalline silicon manufacturing method - Google Patents

Description

The present invention relates to an apparatus for producing polycrystalline silicon and a method for producing polycrystalline silicon.

Valves are key components for controlling the movement of fluids such as gases flowing through facilities such as chemical plants. When the gas is at high temperature and pressure, valves are required to have heat resistance and strength. For example, Non-Patent Document 1 discloses a valve having a valve body overlaid with a metal that has excellent heat resistance and wear resistance.

Heat-resistant hardfacing welding for valves (Journal of the Japan Welding Society, Vol. 40, No. 10, 1971, p. 988-994)

During the production of polycrystalline silicon, gases containing corrosive components such as hydrogen chloride and silanes are generated from the polycrystalline silicon reactor. These gases emitted from the reactor can corrode the valves that control the exhaust of gases from the reactor.

The inventors have found that the corrosion of the valve can cause contamination of the polycrystalline silicon deposited in the reactor. Therefore, it is preferable that the valve has excellent corrosion resistance. Non-Patent Document 1 discloses the heat resistance and abrasion resistance of the valve, but does not disclose the corrosion resistance of the valve to chemical substances. In particular, it is unclear which valves are effective in preventing corrosion caused by hydrogen chloride, silanes, etc.

One aspect of the present invention aims to reduce contamination of polycrystalline silicon in a reactor by using an exhaust valve that is less susceptible to corrosion by hydrogen chloride and silanes.

In order to solve the above problems, a polycrystalline silicon manufacturing apparatus according to one embodiment of the present invention includes a reactor for depositing polycrystalline silicon, and an exhaust valve for controlling the opening and closing of an exhaust path for exhaust gas containing at least one of hydrogen chloride and silanes from the reactor, the exhaust valve having a valve body and a receiving sheet capable of airtight contact with the valve body, and at least one of the valve body and the receiving sheet has a contact surface with the exhaust gas formed from a corrosion-resistant material.

According to one aspect of the present invention, contamination of polycrystalline silicon in a reactor can be reduced by using an exhaust valve that is less susceptible to corrosion by hydrogen chloride and silanes.

1 is a schematic diagram showing a part of an apparatus for producing polycrystalline silicon according to an embodiment of the present invention; 2 is a cross-sectional view showing a schematic diagram of a discharge valve provided in the manufacturing apparatus shown in FIG. 1 . FIG. 13 is a diagram showing the state of the exhaust valves provided in the manufacturing apparatuses according to the examples and comparative examples after a long period of operation in which the valve bodies have been boiled and then the moisture has been wiped off.

[Polycrystalline silicon manufacturing equipment]
(Configuration of manufacturing equipment)
An embodiment of the present invention will be described below with reference to Figures 1 to 3. As shown in Figure 1, a polycrystalline silicon manufacturing apparatus 1 includes a reactor 10, a supply path 11, a discharge path 12, a supply valve 20, and a discharge valve 30.

The reactor 10 is a component in which polycrystalline silicon is precipitated. The reactor 10 is a bell jar type reactor, but the shape of the reactor 10 is not limited to this. The reactor 10 may be any reaction vessel for precipitating polycrystalline silicon therein. A supply path 11 and a discharge path 12 are connected to the reactor 10.

The supply path 11 is a pipe that supplies a reaction gas containing hydrogen and silanes to the reactor 10. As the silanes, specifically, silane compounds such as monosilane, trichlorosilane, tetrachlorosilane, monochlorosilane, and dichlorosilane are used, and trichlorosilane gas is generally used. The exhaust path 12 is a pipe through which the exhaust gas discharged from the reactor 10 flows. The arrows in FIG. 1 indicate the flow directions of the reaction gas and the exhaust gas.

The exhaust gas contains gas components generated during the reaction in the reactor 10 in addition to the gas components contained in the reaction gas. Specifically, the exhaust gas contains at least one of hydrogen chloride and silanes such as trichlorosilane and tetrachlorosilane. The exhaust gas may further contain hydrogen and other reaction by-products.

The supply valve 20 is disposed on the supply path 11. The supply valve 20 controls the supply of reaction gas from the supply path 11 to the reactor 10 by opening and closing. When the supply valve 20 is in the closed state, the supply valve 20 airtightly blocks the supply path 11.

The exhaust valve 30 is disposed on the exhaust path 12. The exhaust valve 30 opens and closes to control the exhaust of exhaust gas from the reactor 10 to the exhaust path 12. When the exhaust valve 30 is closed, the exhaust valve 30 airtightly blocks the exhaust path 12. When both the supply valve 20 and the exhaust valve 30 are closed, the reactor 10 is in an airtight state, and the reaction gas or exhaust gas is sealed in the reactor 10.

(Method of manufacturing polycrystalline silicon)
Examples of the method for producing polycrystalline silicon using the production apparatus 1 include the Siemens method and the melt deposition method (Vapor to Liquid Deposition method, VLD method), etc. Here, the method for producing polycrystalline silicon will be described taking the Siemens method as an example.

In the Siemens process, a silicon core 13 is erected inside the reactor 10. In this state, the exhaust valve 30 is closed and the supply valve 20 is opened to supply the reactor 10 with an inert gas and/or at least one gas constituting the reaction gas as standby gas from the supply path 11. If the standby gas is an inert gas, nitrogen gas is preferable, and if it is a gas constituting the reaction gas, hydrogen is preferable. After supplying a predetermined amount of standby gas to the reactor 10 in this manner, the supply valve 20 is closed and the reaction gas is sealed in the reactor 10, which is placed in a standby state until the deposition of polycrystalline silicon begins.

Then, when starting the deposition of polycrystalline silicon, the exhaust valve 30 is opened again (discharge process) and the supply valve 20 is opened to supply reactive gas from the supply path 11 to the reactor 10, and the silicon core wire 13 is heated to a temperature equal to or higher than the silicon deposition temperature. Such a temperature is, for example, 600°C or higher and 1250°C or lower, and more preferably 900°C or higher and 1200°C or lower. As a result, silicon is deposited around the silicon core wire 13 in the reactor 10, and polycrystalline silicon is obtained (deposition process).

The operation of heating the silicon core wire 13 to a temperature equal to or higher than the silicon precipitation temperature may be performed in a standby state in which standby gas is sealed in the reactor 10. That is, in the standby state, the silicon core wire 13 may be heated to a temperature equal to or higher than the silicon precipitation temperature, and then the exhaust valve 30 and the supply valve 20 may be opened to supply reactive gas into the reactor 10, thereby progressing the precipitation of silicon onto the silicon core wire 13.

Here, in the above-mentioned precipitation process, in the standby state, the reactor 10 is in an airtight state, and standby gas is sealed in the reactor 10. However, if the exhaust valve 30 is operated for a long period of time, corrosion occurs in the exhaust valve 30. Due to this corrosion, even if the exhaust valve 30 is closed, the exhaust path 12 cannot be completely blocked, and gas leakage may occur in the exhaust valve 30.

When the exhaust valve 30 is closed and the inside of the reactor 10 is in an airtight state, standby gas flows back from the exhaust path 12 to the reactor 10. In addition, the backflow of the standby gas to the reactor 10 becomes more severe when the exhaust valve 30 corrodes, causing a gas leak and reducing its airtightness. Here, the backflowing standby gas may contain metal contaminants resulting from the corrosion of the exhaust valve 30. Therefore, as a result of the inventors' intensive research, it was discovered as a new finding that when polycrystalline silicon is produced in a state in which the exhaust valve 30 is corroded, the resulting polycrystalline silicon is contaminated by metal contaminants contained in the backflowing standby gas in the early stages of precipitation.

In particular, the operation of sealing the standby gas in the reactor 10 is often performed by repeatedly carrying out a gas replacement operation that involves increasing and decreasing the pressure of the gas in the reactor 10. In this case, if a backflow of the standby gas occurs from the exhaust path 12 to the reactor 10, the amount of metal contaminants contained in the backflowing standby gas increases further. Therefore, contamination of the resulting polycrystalline silicon by metal contaminants becomes even more pronounced.

In order to prevent such contamination of the polycrystalline silicon caused by corrosion of the exhaust valve 30, the exhaust valve 30 provided in the manufacturing apparatus 1 of this embodiment has the configuration shown below.

[Configuration of the exhaust valve]
(Basic structure)
The structure of the exhaust valve 30 will be described in detail with reference to Fig. 2. Fig. 2 shows an open state 201 and a closed state 202 of the exhaust valve 30. As shown in Fig. 2, the exhaust valve 30 is disposed on the exhaust path 12. The exhaust valve 30 includes a valve body 31, a valve seat (receiving seat) 32, a stem 33, and a main body 34. The structure and function of each part of the exhaust valve 30 will be described below together with the operation when the exhaust valve 30 is closed.

An opening 31a is formed in the valve body 31. When the exhaust valve 30 is in the open state 201, the opening 31a and the exhaust path 12 are spatially connected. Therefore, the exhaust gas flowing inside the exhaust path 12 passes through the space surrounded by the opening 31a.

A stem 33 is attached to the valve body 31. The stem 33 is a member that rotates the direction of the valve body 31. When the stem 33 rotates, the valve body 31 also rotates, and the direction of the valve body 31 changes. Accordingly, the position of the opening 31a relative to the discharge path 12 changes. This changes the spatial connection state between the opening 31a and the discharge path 12, and the discharge valve 30 is switched between open and closed. When the discharge valve 30 is in the closed state 202, the valve body 31 faces in a direction that blocks the spatial connection between the opening 31a and the discharge path 12.

A valve seat 32 is disposed between the valve body 31 and the main body 34. The valve seat 32 is in airtight contact with the valve body 31. Therefore, the only space within the exhaust valve 30 through which the exhaust gas can pass is the space surrounded by the opening 31a. Therefore, unless the opening 31a and the exhaust path 12 are spatially connected, the exhaust gas cannot pass through the exhaust valve 30.

In this embodiment, the valve seat 32 is disposed only on one exhaust path 12 side (the right side as one faces the paper in FIG. 2). However, the valve seat 32 may be disposed in any manner so long as the valve body 31 and the main body 34 are in airtight contact with each other. As an example of such a disposition, the valve seat 32 may be disposed on the other exhaust path 12 side of the exhaust valve 30 (the left side as one faces the paper in FIG. 2), or on both sides of the exhaust valve 30.

However, the exhaust valve 30 is not limited to this configuration. The exhaust valve 30 may be, for example, a non-sliding valve in which the valve body 31 and the valve seat 32 do not come into contact when the valve body 31 is rotating. In other words, it is sufficient that the valve body 31 and the valve seat 32 are arranged so that they can come into airtight contact with each other. With this configuration, wear of the valve body 31 and the valve seat 32 due to friction caused by the rotation of the valve body 31 can be greatly reduced. As a result, the service life of the exhaust valve 30 can be extended.

(Composition of valve body and valve seat)
As described above, the exhaust valve 30 is disposed on the exhaust path 12. Exhaust gas containing hydrogen chloride, silanes, and the like flows through the exhaust path 12. Therefore, the exhaust valve 30 is easily corroded because it is exposed to corrosive components such as hydrogen chloride, which is a major cause of metal corrosion, and silanes, which cause metal silicidation.

In particular, the exhaust gas from the reactor 10 contains hydrogen chloride at 0 mol% to 5 mol%, and in many cases, hydrogen chloride at 0.5 mol% to 3 mol%. The exhaust gas from the reactor 10 also contains silanes at 0 mol% to 20 mol%, and in many cases, silanes at 5 mol% to 15 mol%. Exhaust gas containing such concentrations of hydrogen chloride and/or silanes is prone to corrode metals.

Furthermore, the exhaust gas from the reactor 10 is heated to a high temperature of, for example, approximately 500°C due to the heating of the silicon core wire 13. The higher the temperature, the more likely it is that metal corrosion caused by hydrogen chloride and silanes will progress. Therefore, it can be said that the exhaust valve 30 is used in an environment where corrosion is particularly likely to occur. To reduce this corrosion, the valve body 31 has a surface that comes into contact with the exhaust gas made of a corrosion-resistant material.

Specifically, the entire surface of the valve body 31 is covered with Stellite (registered trademark), a corrosion-resistant material, and the main body portion inside is made of stainless steel. The main body portion is not limited to stainless steel and may be made of a corrosion-resistant material, or may be made of a material such as a metal that does not have corrosion resistance.

The corrosion-resistant material forming the covering portion of the valve body 31, like the above-mentioned Stellite, preferably has a cobalt content of 25% by mass or more, more preferably 30% by mass or more, more preferably 35% by mass or more, more preferably 40% by mass or more, more preferably 45% by mass or more, and more preferably 50% by mass or more as an alloy. Such a corrosion-resistant material has particularly excellent corrosion resistance against hydrogen chloride and silanes, etc., and can effectively prevent corrosion of the exhaust valve 30.

Table 1 shows the compositions of various Stellites (Nos. 1, 6, 12, and 21) as examples of alloys containing 25% or more by mass of cobalt. However, alloys containing 25% or more by mass of cobalt are not limited to the metals shown in Table 1.

The corrosion-resistant material forming the coating portion of the valve body 31 is not limited to alloys containing 25% or more by mass of cobalt. Table 2 shows the compositions of Ni-Cr-B-Si alloy (metal A1), chrome steel (metal A2), and precipitation hardened stainless steel (metal A3) as examples of corrosion-resistant materials other than alloys containing 25% or more by mass of cobalt. However, the corrosion-resistant material forming the coating portion of the valve body 31 is not limited to these.

Such a corrosion-resistant material formed on the contact surface of the valve body 31 with the exhaust gas preferably has a thickness of 1.5 mm or more. There is no particular limit to the thickness of the corrosion-resistant material, and for example, the entire valve body 31 may be formed from the corrosion-resistant material. However, from the viewpoint of ease of manufacturing the valve body 31 and manufacturing costs, it is preferable that the thickness of the corrosion-resistant material is 10 mm or less. With such a thickness, the corrosion of the valve body 31 can be effectively reduced by the corrosion-resistant material.

The valve seat 32 may be made of a special graphite material such as Gratite (registered trademark), carbon, other resin products, metal, etc. From the viewpoint of easily ensuring airtightness between the valve body 31 and the valve seat 32, it is preferable that the material of the valve seat 32 is a material that is soft enough to be easily processed, such as the special graphite material. With such a material, the contact portion of the valve seat 32 with the valve body 31 can be easily processed into a shape that matches the shape of the valve body 31. Therefore, airtightness between the valve body 31 and the valve seat 32 can be easily and accurately ensured.

In addition, the contact surface of the valve seat 32 with the exhaust gas may be formed of a corrosion-resistant material, instead of the valve body 31. In this case, the composition and thickness of the corrosion-resistant material formed on the contact surface of the valve seat 32 with the exhaust gas may be the same as the corrosion-resistant material formed on the contact surface of the valve body 31 with the exhaust gas. In addition, the contact surfaces of the valve body 31 and the valve seat 32 with the exhaust gas may both be formed of a corrosion-resistant material. In other words, it is sufficient that at least one of the valve body 31 and the valve seat 32 has a contact surface with the exhaust gas formed of a corrosion-resistant material. The contact surface of the valve body 31 with the exhaust gas and the contact surface of the valve seat 32 with the exhaust gas may be formed of the same corrosion-resistant material, or may be formed of different corrosion-resistant materials.

The portion of the valve seat 32 other than the surface that comes into contact with the exhaust gas (main body portion) may be, for example, a special graphite material such as gratite, carbon, other resin products, metal, etc. If the surface of the valve seat 32 that comes into contact with the exhaust gas is formed of a corrosion-resistant material, the composition and thickness, etc. of the corrosion-resistant material may be the same as the corrosion-resistant material used for the valve body 31 described above.

The supply valve 20 may be configured in the same manner as the exhaust valve 30, or a conventional valve may be used. Compared to the exhaust gas, the supply gas contains almost no hydrogen chloride, and in most cases the temperature is below 200°C. Therefore, the supply valve 20 is less susceptible to corrosion than the exhaust valve 30.

(Effect of corrosion-resistant materials)
In this manner, according to the configuration in which the contact surface of the valve body 31 and/or the valve seat 32 with the exhaust gas is formed of a corrosion-resistant material, even if the valve body 31 and/or the valve seat 32 are exposed to the exhaust gas, corrosion is extremely unlikely to occur. Therefore, gas leakage from the exhaust valve 30 due to corrosion of the valve body 31 and/or the valve seat 32 is unlikely to occur. In addition, if the valve body 31 and/or the valve seat 32 are not likely to corrode in this manner, even if the production of polycrystalline silicon is continued, in the reaction furnace 10 in the standby state, it is possible to suppress the inclusion of metal contaminants in the standby gas flowing back from the exhaust path 12 to the reaction furnace 10. In addition, since the airtightness of the exhaust valve 30 is not impaired by corrosion, the amount of exhaust gas flowing back from the exhaust path 12 to the reaction furnace 10 is suppressed to be low in the first place. As a result, the risk of the polycrystalline silicon precipitated in the reaction furnace 10 being contaminated by metal contaminants resulting from the corrosion of the exhaust valve 30 can be greatly reduced.

Before polycrystalline silicon is precipitated in the reactor 10, the backflow of exhaust gas from the exhaust path 12 to the reactor 10 can be prevented by continuing to supply hydrogen gas from the supply path 11. However, in this case, a large amount of hydrogen gas is required, and the production cost of polycrystalline silicon increases. On the other hand, if the exhaust valve 30 is configured to be less corroded, the backflow of exhaust gas from the exhaust path 12 to the reactor 10 and the contamination of polycrystalline silicon by metal contaminants resulting from corrosion of the exhaust valve 30 can be reduced without using a large amount of hydrogen gas that increases the production cost.

In the manufacturing device 1, the deposition of polycrystalline silicon generates a large amount of hydrogen chloride and silanes, which cause metal corrosion, and the exhaust valve 30 is particularly exposed to an environment that is prone to corrosion. On the other hand, polycrystalline silicon, which is the raw material for semiconductors and solar cells, etc., requires extremely high purity. This is particularly noticeable when polycrystalline silicon is used as a raw material for semiconductors.

Corrosion of the exhaust valve 30 not only causes gas leakage of exhaust gas, but also generates metal contaminants due to backflow of standby gas from the exhaust path 12 when the reactor 10 is in standby, which can cause contamination of polycrystalline silicon. This fact is a new finding by the inventor who has been manufacturing polycrystalline silicon for a long time. In the past, it was generally not known that exhaust gas would backflow from the exhaust path 12 to the reactor 10, and it was not at all expected that corrosion of the exhaust valve 30 would cause contamination of polycrystalline silicon. Therefore, by configuring the exhaust valve 30 of the manufacturing apparatus 1 in the present embodiment in such a way that corrosion is unlikely to occur, the problem of high-level contamination prevention specific to the production of polycrystalline silicon is effectively solved based on the above-mentioned new finding.

In addition, since the exhaust valve 30 is configured to be resistant to corrosion, the frequency of maintenance of the exhaust valve 30 can be significantly reduced. The manufacturing apparatus 1 cannot be operated during maintenance of the exhaust valve 30. Maintenance of the exhaust valve 30, such as cleaning, repairing, and replacing the valve body 31 and/or valve seat 32, takes time. Therefore, when maintenance of the exhaust valve 30 is performed frequently, loss of opportunities to manufacture polycrystalline silicon becomes a major problem. Reducing the frequency of maintenance of the exhaust valve 30 not only reduces the maintenance and management costs of the exhaust valve 30, but also reduces opportunities to manufacture polycrystalline silicon.

In addition, the valve body 31 and the valve seat 32 are susceptible to wear due to friction caused by the rotation of the valve body 31. The surfaces of the valve body 31 or the valve seat 32 made of a corrosion-resistant material can be easily repaired by plating the worn areas with a corrosion-resistant material. Therefore, maintenance of the exhaust valve 30 can be easily completed at low cost and in a short time.

〔summary〕
A polycrystalline silicon manufacturing apparatus according to one embodiment of the present invention includes a reactor for depositing polycrystalline silicon, and an exhaust valve for controlling the opening and closing of an exhaust path for exhaust gas containing at least one of hydrogen chloride and silanes from the reactor, the exhaust valve having a valve body and a receiving sheet capable of airtight contact with the valve body, and the contact surface with the exhaust gas of at least one of the valve body and the receiving sheet is formed from a corrosion-resistant material.

In the production of polycrystalline silicon, after the deposition of polycrystalline silicon in a reactor, gases containing hydrogen chloride and silanes are discharged from the reactor. Hydrogen chloride and silanes are major causes of corrosion of metal materials, so the exhaust valve installed in the exhaust path of the gas is easily corroded. In a reactor that is airtight before the deposition of polycrystalline silicon, if the valve body or receiving sheet of the exhaust valve is corroded, gas leakage may occur, and gas may flow back from the exhaust valve into the reactor. The backflowing gas may contain metal contaminants derived from the corrosion of the exhaust valve. Therefore, the inventors' intensive research has revealed new knowledge that polycrystalline silicon may be contaminated by the metal contaminants.

However, with the above-mentioned configuration, at least one of the valve body and receiving sheet of the exhaust valve has a contact surface with hydrogen chloride formed from a corrosion-resistant material. This effectively prevents corrosion of the exhaust valve. This makes it difficult for metal contaminants resulting from corrosion of the exhaust valve to be generated. In addition, backflow of gas from the exhaust valve to the reactor is unlikely to occur, greatly reducing the risk of the polycrystalline silicon precipitated in the reactor being contaminated by metal contaminants resulting from corrosion of the exhaust valve.

In one embodiment of the polycrystalline silicon manufacturing apparatus of the present invention, the corrosion-resistant material may be an alloy containing 25% by mass or more of cobalt. With the above configuration, the corrosion resistance of the corrosion-resistant material is particularly excellent. Therefore, corrosion of the exhaust valve can be more effectively prevented.

A method for producing polycrystalline silicon according to one embodiment of the present invention uses any of the polycrystalline silicon production apparatuses described above and includes a deposition process in which a reaction gas containing hydrogen and silanes is supplied to the reactor to deposit the polycrystalline silicon, and a discharge process in which the exhaust gas is discharged from the reactor.

In one embodiment of the method for producing polycrystalline silicon according to the present invention, in the deposition process, an inert gas and/or at least one gas constituting the reactive gas is sealed in the reactor with the exhaust valve closed, and then, when starting the deposition of polycrystalline silicon, the exhaust valve is opened and the supply of the reactive gas to the reactor is started.

According to the above configuration, in the polycrystalline silicon deposition process, the exhaust valve is closed in the pre-process of supplying the reaction gas to the reactor and starting the deposition of polycrystalline silicon. At this time, the reactor is filled with at least one gas constituting the inert gas and/or reaction gas in an airtight state. Here, if the valve body or receiving sheet of the exhaust valve is corroded, it is difficult to maintain an airtight state. In addition, there is a risk that the exhaust gas will flow back from the exhaust valve to the reactor. Since the backflowing exhaust gas may contain metal contaminants resulting from the corrosion of the exhaust valve, when polycrystalline silicon is deposited later, the resulting polycrystalline silicon will be contaminated by the metal contaminants, especially in the early stages of the deposition.

However, according to the above manufacturing method, at least one of the valve body and receiving sheet of the exhaust valve has a surface that comes into contact with the gas made of a corrosion-resistant material, so the exhaust valve is less likely to corrode. Therefore, gas is less likely to backflow from the exhaust valve to the reactor, which greatly reduces the risk of the polycrystalline silicon deposited in the reactor being contaminated by metal contaminants resulting from the corrosion of the exhaust valve.

[Additional Notes]
The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

(Valve body composition)
A manufacturing apparatus according to an example of the present invention and a manufacturing apparatus according to a comparative example were each operated for a certain period of time, and the valve bodies were collected from the exhaust valves of the respective manufacturing apparatuses, and the degree of corrosion was observed.

Table 3 shows the composition of the valve bodies according to the examples and comparative examples. As shown in Table 3, the valve body according to the examples consists of a main body portion and a covering portion, and the entire surface of the main body portion is covered by the covering portion. The covering portion is formed from a corrosion-resistant material. Moreover, the valve body according to the comparative example consists only of the main body portion shown in Table 3, and does not have a covering portion.

(Corrosion evaluation of valve body)
The polycrystalline silicon manufacturing apparatus 1 used had the structure shown in Fig. 1. In the polycrystalline silicon deposition process, standby gas was once sealed in the reactor 10 with the exhaust valve 30 closed, and the reactor 10 was held for 50 hours. Thereafter, the exhaust valve 30 was opened, and the reaction gas was supplied to the reactor 10 to start deposition of polycrystalline silicon, thereby producing polycrystalline silicon. Such polycrystalline silicon was produced for three years (batch number: 105 times) in the example, and for nine months (batch number: 33 times) in the comparative example.

The standby gas was hydrogen gas, and the hydrogen gas was introduced into the reactor 10 by repeatedly performing gas replacement operations involving increasing and decreasing the pressure of the gas in the reactor 10. The deposition temperature of the polycrystalline silicon ranged between 900°C and 1100°C.

After the polycrystalline silicon was produced, the valve bodies according to the embodiment and the valve bodies according to the comparative example were collected and boiled, and the moisture was wiped off with a cloth, after which the appearance of each valve body was observed. As a result, as shown in Figure 3, the valve body according to the embodiment that had been in operation for three years maintained its surface smoothness, but the valve body according to the comparative example that had been in operation for nine months showed severe surface roughening due to corrosion, and some peeling was observed.

(Contamination evaluation of polycrystalline silicon)
Next, polycrystalline silicon was deposited using the production apparatus according to the example and the production apparatus according to the comparative example, each of which had been operated for one year.

After polycrystalline silicon was deposited using each of the manufacturing devices shown in Table 4, a location including the boundary between the silicon core wire and the deposition layer was sampled and the amount of metal contaminants was analyzed. Specifically, for each of the obtained rod-shaped polycrystalline silicon pieces with a diameter of 150 mm, a cylinder with a diameter of 10 mm and a length of 150 mm was hollowed out in the horizontal direction perpendicular to the longitudinal direction near the midpoint of the longitudinal direction of the rod. The cylinder extended in the horizontal direction.

The cylinder was cut vertically at a position 2 mm radially forward and backward from the boundary between the silicon core wire and the precipitated layer to obtain measurement samples. The metal content of each measurement sample was measured by dissolving the sample in a mixed solution of fluoro-nitric acid and analyzing the amount of each metal element in the resulting solution using inductively coupled plasma mass spectrometry (ICP-MS). The results are shown in Table 4.

The amount of contamination by Cr, Fe, and Ni was greater in the polycrystalline silicon obtained by the manufacturing apparatus according to the comparative example than in the polycrystalline silicon obtained by the manufacturing apparatus according to the embodiment. This is thought to be because, in the manufacturing apparatus according to the comparative example, when the reactor 10 was in standby mode, backflow of standby gas occurred from the exhaust path, and metal contaminants resulting from corrosion of the exhaust valve 30 contaminated the polycrystalline silicon.

REFERENCE SIGNS LIST 1 Manufacturing device 10 Reactor 12 Discharge path 30 Discharge valve 31 Valve body 32 Valve seat (receiving seat)

Claims (1)

a reactor for depositing polycrystalline silicon;
and an exhaust valve for controlling opening and closing of an exhaust path for an exhaust gas containing at least one of hydrogen chloride and silanes from the reactor,
The exhaust valve has a valve body and a receiving sheet capable of airtight contact with the valve body,
At least one of the valve body and the receiving seat has a surface that comes into contact with the exhaust gas and is made of a corrosion-resistant material,
The corrosion-resistant material is an alloy containing 25 mass % or more of cobalt ,
a deposition step of supplying a reaction gas containing hydrogen and silanes to the reaction furnace to deposit the polycrystalline silicon;
and a discharge step of discharging the exhaust gas from the reactor ,
The method for producing polycrystalline silicon, characterized in that in the deposition process, the exhaust valve is closed, and a standby gas which is an inert gas and/or at least one of the reactive gases is sealed in the reactor, and then, when starting deposition of the polycrystalline silicon, the exhaust valve is opened, and the supply of the reactive gas to the reactor with the standby gas sealed therein is started .

Images