Cleaning basket

Abstract

In order to prevent contamination of polycrystalline silicon in a cleaning step, a cleaning basket (1) is a cleaning basket used for cleaning polycrystalline silicon in a state in which the polycrystalline silicon is accommodated in the cleaning basket and includes a plurality of component parts (10) each made of a resin, the plurality of component parts being coupled to each other by screwing screws ( N) each made of a resin. Fig. 1

Description

Cleaning basket

[0001] The present invention relates to a cleaning basket used in cleaning polycrystalline silicon.

[0002] Polycrystalline silicon used as a silicon semiconductor raw material, a solar cell raw material, and the like is required to be highly pure, and therefore the manufacturing process includes a cleaning step for removing contaminants adhering to the polycrystalline silicon. In the cleaning step, the polycrystalline silicon is placed in a dedicated cleaning basket and subjected to etching cleaning and pure water cleaning. For example, Patent Document 1 discloses a cleaning basket used in cleaning this polycrystalline silicon, which is formed into a box shape from a resin plate and which contains the polycrystalline silicon.

[0003] Furthermore, Patent Document 2 discloses that when polycrystalline silicon is placed in a resin washing basket and subjected to etching cleaning, contamination of the surface of the washed polycrystalline silicon occurs with organic components and carbon originating from the washing basket. Moreover, paragraphs 0039 to 0041 of Patent Document 2 disclose that this contamination originating from the resin in the washing basket is more severe when the resin has deteriorated due to repeated etching cleaning than when the washing basket is new.

[0004] Japanese Patent Publication No. 2011-68554 Japanese Patent Publication No. 2020-128332

[0005] In the cleaning basket disclosed in Patent Document 1, when welding is performed on the portion of the cleaning basket that is immersed in hydrofluoric nitric acid as a cleaning liquid during the cleaning process, the welded portion is more likely to deteriorate than other resin portions, and the inventors of the present application have discovered that this is one cause of contamination caused by the cleaning basket. In particular, repeated etching cleaning leads to cracking and peeling due to the acid, and the resulting resin pieces do not dissolve in the cleaning liquid, so the resin pieces adhere to the polycrystalline silicon, further significantly increasing the contamination of the polycrystalline silicon.

[0006] According to one aspect of the present invention, an object is to prevent contamination of polycrystalline silicon during a cleaning process.

[0007] In order to solve the above problems, one aspect of the present invention provides a cleaning basket that is used to clean polycrystalline silicon while containing the polycrystalline silicon, and is characterized in that it includes a plurality of components made of resin, and each of the plurality of components is connected to one another by screwing together screws made of resin.

[0008] According to one aspect of the present invention, contamination of polycrystalline silicon during a cleaning process can be prevented.

[0009] Fig. 1 is a side view of a washing basket according to an embodiment of the present invention. Fig. 2 is a plan view of the washing basket. Fig. 3 is a front view of the washing basket. Fig. 4 is a cross-sectional view of the washing basket along A-A in Fig. 1. Fig. 5 is a plan view of the bottom plate of the washing basket. Fig. 6 is a plan view of the first side plate of the washing basket. Fig. 7 is a plan view of the second side plate of the washing basket. Fig. 8 is a plan view of the third side plate of the washing basket. Fig. 9 is a diagram illustrating locking.

[0010] Hereinafter, one embodiment of the present invention will be described in detail.

[0011] <Method for producing polycrystalline silicon> The method for producing polycrystalline silicon to be cleaned includes, for example, a silicon deposition step in which a chlorosilane compound and hydrogen are reacted in a reactor to deposit polycrystalline silicon. The Siemens process is known as a method for depositing polycrystalline silicon. In the Siemens process, trichlorosilane and hydrogen are reacted in a bell-shaped reactor. Polycrystalline silicon is then deposited on the surface of a polycrystalline silicon deposition core rod installed upright inside the reactor, thereby obtaining a grown polycrystalline silicon rod.

[0012] The method for producing polycrystalline silicon may also include a step of crushing the polycrystalline silicon rods obtained by the silicon deposition step and a step of classifying the crushed polycrystalline silicon. Furthermore, the method for producing polycrystalline silicon may also include a cleaning step of cleaning the crushed polycrystalline silicon or the classified polycrystalline silicon and a drying step of drying the cleaned polycrystalline silicon. This results in high-purity polycrystalline silicon. Since polycrystalline silicon is used in semiconductor components, it must be highly pure.

[0013] <Polycrystalline Silicon Cleaning Method> In the cleaning step, fluoronitric acid is brought into contact with the polycrystalline silicon as a cleaning solution. Fluoronitric acid is an aqueous solution of nitric acid and hydrogen fluoride. An oxide film containing contaminants may be formed on the polycrystalline silicon removed from the reactor by natural oxidation, usually with a thickness of several nanometers. This oxide film is typically made of SiO ２ When polycrystalline silicon is brought into contact with nitric acid and hydrofluoric acid, the following reaction occurs:

[0014] The surface of the polycrystalline silicon is etched, primarily by the action of hydrogen fluoride, and the oxide film present on the surface is removed. Meanwhile, a new oxide film is formed on the surface of the polycrystalline silicon, primarily by the action of nitric acid. The removal of the oxide film and the formation of an oxide film proceed simultaneously, etching the polycrystalline silicon and removing contaminants attached to the surface of the polycrystalline silicon and contaminants incorporated into the polycrystalline silicon. Contaminants include organic matter, metals, resins, etc. Major metals that can become contaminants or impurities include Na, Mg, Al, K, Ca, Cr, Fe, Ni, Co, Cu, Zn, and W.

[0015] A specific method for contacting polycrystalline silicon with fluoronitric acid is to immerse a cleaning basket 1 containing polycrystalline silicon in a first cleaning tank filled with fluoronitric acid. The cleaning basket 1 will be described later. After immersing the cleaning basket 1 in the first cleaning tank, the cleaning basket 1 is lifted out of the first cleaning tank and immersed in a second cleaning tank filled with ultrapure water. This allows the fluoronitric acid adhering to the polycrystalline silicon to be washed away. The cleaning basket 1 according to this embodiment is used at least for cleaning polycrystalline silicon in the first cleaning tank.

[0016] It is also possible to carry out a drying step using the cleaning basket 1. Specifically, after the cleaning basket 1 is immersed in the second cleaning tank, the cleaning basket 1 is removed from the second cleaning tank, and in the drying step, the cleaning basket 1 is placed in a dryer. The polycrystalline silicon placed in the cleaning basket 1 is dried by hot air from the dryer.

[0017] <Configuration of washing basket> Fig. 1 is a side view of a washing basket 1 according to an embodiment of the present invention. Fig. 2 is a plan view of the washing basket 1 shown in Fig. 1. Fig. 3 is a front view of the washing basket 1 shown in Fig. 1. In Fig. 1, the longitudinal direction of the washing basket 1 is the X-axis direction, the height direction of the washing basket 1 is the Z-axis direction, and the depth direction of the washing basket 1 (the short side direction of the washing basket) is the Y-axis direction. Furthermore, the direction from the opening side of the washing basket 1 toward the bottom plate 2 is the downward direction, and the direction from the bottom plate 2 toward the opening side of the washing basket 1 is the upward direction. Fig. 4 is an A-A cross-sectional view of the washing basket 1 shown in Fig. 1.

[0018] 1 to 4, the cleaning basket 1 includes a plurality of components 10 made of resin and a screw N made of resin. The plurality of components 10 are joined to one another by threading the screws N.

[0019] The component 10 and the screw N are each made of fluororesin. Fluororesin has high chemical resistance and can reduce the risk of cracking or peeling due to the acidic cleaning solution used in the cleaning process. Fluororesin also has high heat resistance, which makes it possible to dry the polycrystalline silicon after etching in the cleaning process while it is still contained in the cleaning basket 1. Furthermore, fluororesin has high mechanical strength among resins, which can reduce the amount of wear on the cleaning basket 1 when the polycrystalline silicon comes into contact with the cleaning basket 1, thereby preventing contamination of the polycrystalline silicon by pieces of resin worn off the cleaning basket 1.

[0020] 1 to 4, the cleaning basket 1 has components 10, which are a bottom plate 2, a first side plate 3, a second side plate 4, and a third side plate 5. The thicknesses of the bottom plate 2, the first side plate 3, the second side plate 4, and the third side plate 5 are all the same. Standardizing the thicknesses of these plates can reduce material costs.

[0021] <Bottom Plate> As shown in Fig. 1 , the bottom plate 2 is positioned approximately horizontally relative to the installation surface on which the cleaning basket 1 is placed, and supports the polycrystalline silicon when the polycrystalline silicon is placed in the cleaning basket 1. Fig. 5 is a plan view of the bottom plate 2 of the cleaning basket 1 shown in Fig. 1. As shown in Fig. 5 , the bottom plate 2 is a substantially rectangular plate-shaped member, and is formed with a plurality of elongated holes 21 extending in the longitudinal direction (X-axis direction) of the bottom plate 2. The plurality of elongated holes 21 are arranged in a row along the lateral direction (Y-axis direction) of the bottom plate 2, and a plurality of groups of the plurality of elongated holes 21 arranged in a row along the Y-axis direction are arranged in a row along the X-axis direction. By forming the holes in the bottom plate 2 as elongated holes, the opening ratio, which is the ratio of the total opening area of ​​the plurality of elongated holes 21 to the area of ​​the inner surface 2a of the bottom plate 2, is improved, thereby improving drainage of water and air escape from the cleaning basket 1.

[0022] A plurality of screw holes NH2 are formed along the Y-axis direction of the bottom plate 2 on each of the side surfaces 2d in the shorter direction of the bottom plate 2. The screw holes NH2 are formed so as to be recessed relative to the side surfaces 2d. As shown in FIG. 8 , screw holes NH5-2 are formed on the third side plate 5 at locations corresponding to the screw holes NH2 formed on the bottom plate 2. The third side plate 5 and the bottom plate 2 can be joined by aligning the positions of the screw holes NH2 and NH5-2, passing a screw N through the screw hole NH5-2, and screwing the screw N into the screw hole NH5-2 and the screw hole NH2.

[0023] <First Side Plate> As shown in Fig. 1, the first side plate 3 is connected to the bottom plate 2 along the longitudinal direction of the bottom plate 2, and as shown in Fig. 4, extends in a direction away from the inner surface 2a (surface) of the bottom plate 2. Fig. 6 is a plan view of the first side plate 3 of the cleaning basket 1 shown in Fig. 1. As shown in Fig. 6, the first side plate 3 is a substantially rectangular plate-shaped member, and a plurality of holes 31 are formed in the first side plate 3.

[0024] A plurality of screw holes NH3 are formed along the short side of the first side plate 3 on each of the side surfaces 3d in the short side direction of the first side plate 3. The screw holes NH3 are formed so as to be recessed relative to the side surfaces 3d. As shown in FIG. 8 , screw holes NH5-3 are formed on the third side plate 5 at locations corresponding to the screw holes NH3 formed on the first side plate 3. The third side plate 5 and the first side plate 3 can be joined by aligning the positions of the screw holes NH3 and NH5-3, passing a screw N through the screw hole NH5-3, and screwing the screw hole N into the screw holes NH5-3 and NH3.

[0025] 4 , the first side plate 3 is connected to the inner surface 2a of the bottom plate 2 at an angle upward and outward along the longitudinal direction of the inner surface 2a. The bottom surface 3c of the first side plate 3 abuts against the inner surface 2a of the bottom plate 2 and also abuts along the outer edge 2c in the longitudinal direction of the bottom plate 2. Because the first side plate 3 abuts against the inner surface 2a at an angle relative to the bottom plate 2, the bottom surface 3c of the first side plate 3 is not perpendicular to the inner surface 3a and outer surface 3b of the first side plate 3.

[0026] By providing the first side plate 3 at an angle to the bottom plate 2, when polycrystalline silicon is placed in the cleaning basket 1, the polycrystalline silicon tends to collect on the bottom plate 2. This also improves drainage of the cleaning basket 1. Furthermore, the weight of the polycrystalline silicon placed in the cleaning basket 1 is distributed across the bottom plate 2 and the first side plate 3. This reduces the load applied from the polycrystalline silicon to the bottom plate 2, and reduces deformation of the bottom plate 2 due to the load.

[0027] <Second Side Plate> As shown in Fig. 1, the second side plate 4 is connected to the bottom plate 2 via the first side plate 3 along the longitudinal direction of the bottom plate 2, and extends in a direction away from the inner surface 2a of the bottom plate 2 as shown in Fig. 4. Specifically, the bottom surface 4c of the second side plate 4 abuts against the upper surface 3e of the first side plate 3 so that the second side plate 4 extends in a direction approximately perpendicular to the inner surface 2a of the bottom plate 2. Fig. 7 is a plan view of the second side plate 4 of the cleaning basket 1 shown in Fig. 1. As shown in Fig. 7, the second side plate 4 is a substantially rectangular plate-shaped member, and a plurality of holes 41 are formed in the second side plate 4.

[0028] Furthermore, a plurality of screw holes NH4 are formed along the Z-axis direction on each of the short-side side surfaces 4d of the second side plate 4. The screw holes NH4 are formed so as to be recessed relative to the side surfaces 4d. As shown in FIG. 8 , screw holes NH5-4 are formed on the third side plate 5 at locations corresponding to the screw holes NH4 formed on the second side plate 4. The third side plate 5 and the second side plate 4 can be joined together by aligning the positions of the screw holes NH4 and NH5-4, passing a screw N through the screw hole NH5-4, and screwing the screw N into the screw holes NH5-4 and NH4.

[0029] 1 , the third side plate 5 is connected to the bottom plate 2 along the short side direction of the bottom plate 2 and extends in a direction away from the inner surface 2 a of the bottom plate 2. In this way, the first side plate 3, the second side plate 4, and the third side plate 5 are connected around the periphery of the bottom plate 2.

[0030] 1 and 3, the pair of third side plates 5 are connected to both ends of the bottom plate 2, the first side plate 3, and the second side plate 4, thereby sandwiching the bottom plate 2, the first side plate 3, and the second side plate 4. Also, as shown in Fig. 3, the bottom plate 2, the first side plate 3, and the second side plate 4 are fastened to the third side plates 5 by threading screws N, thereby joining them together. As a result, an accommodation space 20 for accommodating polycrystalline silicon is formed, as shown in Fig. 4.

[0031] Fig. 8 is a plan view of the third side plate 5 of the cleaning basket 1 shown in Fig. 3. The third side plate 5 is a substantially rectangular plate-shaped member, and has a plurality of holes 51 formed in the third side plate 5. The plurality of holes 51 are formed at positions above the connection point between the bottom plate 2 and the third side plate 5 and the connection point between the first side plate 3 and the third side plate 5.

[0032] <Screws> Screws N are flat head screws made of resin. If pan head screws, truss screws, or the like are used to join the bottom plate 2, the first side plate 3, the second side plate 4, and the third side plate 5, the screw heads will protrude from the surface of the third side plate 5. If the polycrystalline silicon is cleaned while the screw heads are protruding from the surface of the third side plate 5, the protruding screw heads will be scraped off, and the polycrystalline silicon may be contaminated with resin fragments produced by the scraping of the screw heads.

[0033] Therefore, in this embodiment, flat head screws are used as the screws N. This allows the top surfaces of the screw heads to be positioned flush with the surface of the third side plate 5, preventing contamination of the polycrystalline silicon caused by the screw heads of the screws N protruding from the surface of the third side plate 5.

[0034] It should be noted that the screws N do not have to be flat head screws, and the screw heads may be milled so that they do not protrude from the surface of the third side plate 5, so that the top surface of the screw head and the surface of the third side plate 5 are positioned on the same plane. Furthermore, the members used to join the components 10 are not limited to screws N. The members used to join the components 10 need only be made of a material that is highly resistant to chemicals and heat, and can join the components 10 without protruding from the surface of the components 10. Rivets made of resin, for example, may also be used as members used to join the components 10.

[0035] <Assembly Method> The assembly method of the cleaning basket 1 will be described below. First, the bottom plate 2 is fixed to the third side plate 5 by screwing in the screws N. After the bottom plate 2 is fixed to the third side plate 5, the first side plate 3 is fixed to the third side plate 5 by screwing in the screws N so that the first side plate 3 abuts against the inner surface 2a of the bottom plate 2 along the longitudinal direction of the bottom plate 2. After the first side plate 3 is fixed to the third side plate 5, the second side plate 4 is fixed to the third side plate 5 by screwing in the screws N so that the bottom surface 4c of the second side plate 4 abuts against the top surface 3e of the first side plate 3 on the side opposite to the bottom plate 2.

[0036] The bottom plate 2, the first side plate 3, and the second side plate 4 are fixed in position relative to each other by being fixed to the third side plate 5. Therefore, for example, since it is not necessary to fix the bottom plate 2 and the first side plate 3 to each other to form the storage space 20, welding between the bottom plate 2 and the first side plate 3 is not required. Similarly, since it is not necessary to fix the first side plate 3 and the second side plate 4 to each other to form the storage space 20, welding between the first side plate 3 and the second side plate 4 is not required.

[0037] In this way, the cleaning basket 1 can be assembled without welding, and therefore the cleaning basket 1 can be constructed without forming any parts that are vulnerable to the cleaning liquid and that may crack or peel during the cleaning process, thereby preventing contamination of the polycrystalline silicon during the cleaning process.

[0038] Furthermore, the positions of the bottom plate 2, first side plate 3, and second side plate 4 are determined by the positions of the multiple screw holes NH5-2 to NH5-4 formed in the third side plate 5. Therefore, by forming the screw holes NH5-2 to NH5-4 in the third side plate 5 in advance, it is possible to reduce variations in the finished product depending on the assembler. Furthermore, it is possible to easily position the first side plate 3, which is disposed at a predetermined angle relative to the bottom plate 2 and second side plate 4.

[0039] FIG. 9 is a diagram illustrating the welded portion W. Based on FIG. 9 , we will explain how to prevent the screw N from loosening when the screw N is screwed. After the screw N is screwed to secure the bottom plate 2, the first side plate 3, and the second side plate 4 shown in FIG. 3 to the third side plate 5, as shown in FIG. 9 , a portion of the boundary between the screw N and the third side plate 5 is melted to form a welded portion W at the boundary between the screw N and the third side plate 5. This prevents the screw N from loosening and coming off from the portion where the screw N is threaded. Furthermore, after the welded portion W is formed, a torque of 45 cN·m or more is applied to the screw N in the reverse rotation direction using a torque driver to confirm that the screw N does not come off from the portion where the screw N is threaded.

[0040] In this way, by not welding between the bottom plate 2, the first side plate 3, the second side plate 4 and the third side plate 5, and by forming the welded joints W and checking that the screws N do not come loose from the points where they are threaded, it is possible to reduce variation in the strength of the cleaning basket 1 caused by the assembler.

[0041] <Effects> As mentioned above, polycrystalline silicon used as a raw material for silicon semiconductors and solar cells is required to be highly pure. Specifically, if even a small amount of heavy metal adheres to the surface of polycrystalline silicon, the polycrystalline silicon becomes defective. In order to maintain high purity, it is necessary to implement meticulous contamination prevention measures to manufacture high-purity polycrystalline silicon.

[0042] In this embodiment, each of the components 10 of the cleaning basket 1 made of resin is joined to one another by threading together screws N made of resin, thereby eliminating the need for welding between the components 10 when assembling the cleaning basket 1. This prevents contamination of the polycrystalline silicon during the cleaning process, realizing a cleaning basket 1 that maintains the high purity described above.

[0043] According to this configuration, it is possible to reduce the amount of polycrystalline silicon that becomes waste due to contamination of the polycrystalline silicon in the polycrystalline silicon manufacturing process, thereby contributing to the achievement of Goal 12 of the Sustainable Development Goals (SDGs), "Responsible Consumption and Production."

[0044] Example 1 Cleaning and Drying of Polycrystalline Silicon The cleaning and drying of polycrystalline silicon in Example 1 will be described. First, a polycrystalline silicon rod manufactured by the Siemens process was crushed into chunks using a hammer. The polycrystalline silicon obtained in this manner was placed in a cleaning basket, and the cleaning basket containing the polycrystalline silicon was immersed in a first cleaning tank filled with fluoronitric acid, thereby cleaning the polycrystalline silicon.

[0045] Next, the cleaning basket was removed from the first cleaning tank and immersed in a second cleaning tank filled with ultrapure water to wash away the fluoronitric acid adhering to the polycrystalline silicon. Thereafter, the cleaning basket was removed from the second cleaning tank and placed in a dryer. Hot air from the dryer was blown onto the polycrystalline silicon placed in the cleaning basket to dry the polycrystalline silicon.

[0046] The above-mentioned washing and drying of the polycrystalline silicon was carried out at least once a day on average for three months using the washing basket of Example 1, and the appearance of the washing basket and the presence or absence of resin pieces in the washing solution were then visually observed. Note that, for Comparative Example 1 described below, washing and drying of polycrystalline silicon was also carried out in the same manner, and the appearance of the washing basket and the presence or absence of resin pieces in the washing solution were observed.

[0047] <Configuration of the cleaning basket> In Example 1, a cleaning basket was used in which the components and screws were each made of PVDF (polyvinylidene fluoride) resin. More specifically, the cleaning basket used in Example 1 had a bottom plate and side plates as the components, and the components were connected to each other by threading the screws. The cleaning basket used in Example 1 did not have any welds to join the components.

[0048] <Results> Table 1 below shows the results of Example 1 and Comparative Example 1. As shown in Table 1, no cracks or peeling due to deterioration were observed in the cleaning basket used in Example 1. Furthermore, no resin pieces were observed in the cleaning solution.

[0049]

[0050] Comparative Example 1 In Comparative Example 1, polycrystalline silicon was washed and dried in the same manner as in Example 1, except that no screws were used in the washing basket and welds were formed to connect the various components of the washing basket.

[0051] As shown in Table 1, slight whitening due to deterioration was observed at the welded portion of the cleaning basket used in Comparative Example 1. In addition, resin pieces generated from the cleaning basket were observed in the cleaning liquid, and resin powder was found in the cleaning liquid filter installed at the outlet of the first cleaning tank.

[0052] [Summary] A cleaning basket according to one aspect of the present invention is a cleaning basket that is used to clean polycrystalline silicon while containing the polycrystalline silicon, and includes a plurality of components made of resin, each of which is joined to another by screwing together screws made of resin.

[0053] According to the above configuration, the resin components of the cleaning basket are joined together by screwing together resin screws, eliminating the need for welding to join the components when assembling the cleaning basket. Therefore, the cleaning basket can be constructed without forming any areas vulnerable to the cleaning solution, such as areas where organic components or carbon are likely to leach out due to deterioration caused by the chemical solution, leading to cracking or peeling. As a result, a cleaning basket that can prevent contamination of polycrystalline silicon during the cleaning process can be realized.

[0054] In the cleaning basket, the components and the screws may each be made of a fluororesin. According to this configuration, the components and the screws are made of a fluororesin with high chemical resistance, making them particularly resistant to degradation by chemicals and reducing the risk of organic components and carbon leaching, cracking, or peeling. Furthermore, using the same material for the components and the screws results in the same thermal expansion coefficient, preventing problems caused by differences in the thermal expansion coefficients between the components and the screws.

[0055] The cleaning basket may also have, as its components, a bottom plate and side plates connected to the periphery of the bottom plate and extending in a direction away from the surface of the bottom plate, and the bottom plate and the side plates may be joined to each other by threading the screws to form a storage space for storing the polycrystalline silicon.

[0056] According to the above configuration, the bottom plate and side plates made of resin are joined together with resin screws to form a storage space for storing polycrystalline silicon. This makes it possible to configure the cleaning cage without forming any areas in the portion where the polycrystalline silicon is stored that are vulnerable to the cleaning solution, such as those that are easily deteriorated by the cleaning solution and that crack or peel off during the cleaning process of cleaning the polycrystalline silicon with the cleaning solution. As a result, it is possible to realize a cleaning cage that can prevent contamination of the polycrystalline silicon during the cleaning process.

[0057] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Configurations obtained by appropriately combining the technical means disclosed in the respective embodiments are also included in the technical scope of the present invention.

[0058] REFERENCE SIGNS LIST 1 Cleaning basket 2 Bottom plate 2a Inner surface (surface) 3 First side plate (side plate) 4 Second side plate (side plate) 5 Third side plate (side plate) 10 Component 20 Storage space N Screw

Claims

1. A cleaning basket used to clean polycrystalline silicon while containing the polycrystalline silicon, the cleaning basket comprising a plurality of components made of resin, each of the plurality of components being joined to one another by screwing together screws made of resin.

2. The cleaning basket according to claim 1, wherein each of said components and said screws is made of fluororesin.

3. A cleaning basket as described in claim 1 or 2, characterized in that the components include a bottom plate and side plates connected to the periphery of the bottom plate and extending in a direction away from the surface of the bottom plate, and the bottom plate and the side plates are joined to each other by threading the screws to form a storage space for storing the polycrystalline silicon.