Polycrystalline silicon rod and method for manufacturing polycrystalline silicon rod

Abstract

To provide a polycrystalline silicon rod with reduced risk of a polycrystalline silicon collapse caused due to crack propagation during machining by reducing residual stress of the polycrystalline silicon rod without lowering production efficiency as much as possible.SOLUTION: A polycrystalline silicon rod is manufactured by the Siemens method and has a length of 1 m or more in a longitudinal direction, wherein an absolute value of a difference between a compressive stress and a tensile stress of a residual stress in the longitudinal direction of a rod side surface is 22 Mpa or less.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present invention relates to polycrystalline silicon produced by the Siemens process and to a method for producing polycrystalline silicon rods. [Background technology]

[0002] Polycrystalline silicon is a raw material for single-crystal silicon used in semiconductor manufacturing and silicon used in solar cell manufacturing. The Siemens process is a well-known method for manufacturing polycrystalline silicon. In this process, a silane-based raw material gas is generally brought into contact with a heated silicon core wire, and polycrystalline silicon is deposited on the surface of the silicon core wire by a chemical vapor deposition (CVD) method.

[0003] In the Siemens method, silicon core wires are assembled into a torii gate shape (inverted U-shape), with two vertical and one horizontal, and both ends of the wires are connected to a core wire holder, which is then fixed to a pair of metal electrodes placed on a base plate.Generally, multiple sets of inverted U-shaped silicon core wires are arranged inside the reactor.

[0004] The surface of the inverted U-shaped silicon core wire is heated to 900°C to 1200°C by passing electricity through it, and a raw material gas, such as a mixture of trichlorosilane and hydrogen, is brought into contact with the silicon core wire.This causes polycrystalline silicon to grow in the vapor phase on the silicon core wire, and a polycrystalline silicon rod of the desired diameter is formed in an inverted U shape.

[0005] As peripheral technologies improve, the diameter required for polycrystalline silicon rods is gradually increasing, and there is a trend toward larger diameters and longer lengths. The larger the diameter and longer the length, the more power is required during production.

[0006] Polycrystalline silicon has the property that its electrical resistivity decreases as the temperature increases, so it tends to be hotter near the center and cooler near the surface, and as the diameter increases, the temperature difference also increases.

[0007] Therefore, as the diameter of the polycrystalline silicon rod increases, the stress contained in the polycrystalline silicon rod increases during the reaction, and particularly when cooling after the end of deposition, making it more likely to suffer damage such as cracks and peeling, or even collapse. As a countermeasure, Patent Document 1 and others have proposed a method of reducing the force applied to the polycrystalline silicon rod by making the electrodes movable, thereby reducing damage.

[0008] However, polycrystalline silicon rods with little damage produced by these methods tend to inevitably have large residual stresses because they are cooled to room temperature without releasing the stress. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 2805457 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-48098 [Patent Document 3] WO97 / 44277 Summary of the Invention [Problem to be solved by the invention]

[0010] Therefore, when the rod is cut to the required length in the next process, cracks are likely to occur due to the imbalance of residual stress caused by the stress released in part by cutting the rod. Also, once a crack occurs, it propagates throughout the entire rod, making it unusable for products that need to be shipped in rod form.

[0011] Furthermore, when cracks spread throughout the entire body of a polycrystalline silicon rod, the impact caused fragments to fly, especially in the longitudinal direction of the rod, posing a danger to workers, and improvement was an urgent issue.

[0012] New methods for reducing the residual stress in polycrystalline silicon rods have been proposed.

[0013] For example, Patent Document 2 proposes a method in which the polycrystalline silicon rod deposition stage during deposition is divided into a region near the silicon core, an R / 2 region, and an outermost surface region, and the surface temperature of the polycrystalline silicon rod is lowered toward the later stage of the reaction. This method has the drawback of slowing down the reaction rate and reducing the production rate because the surface temperature of the polycrystalline silicon rod must be lowered as needed.

[0014] Patent Document 3 proposes a method for reducing distortion by performing a heat treatment after the reaction is complete. This method requires a heat treatment step in addition to the normal precipitation step, which also has the disadvantage of reducing productivity.

[0015] As described above, conventional techniques for reducing residual stress (strain) in large-diameter, long polycrystalline silicon rods have resulted in a significant drop in production efficiency. Therefore, an object of the present invention is to provide a polycrystalline silicon rod that reduces the residual stress in the polycrystalline silicon rod without reducing production efficiency as much as possible, thereby reducing the risk of collapse due to crack propagation during processing. [Means for solving the problem]

[0016] In order to solve the above problems, the polycrystalline silicon rod of 1 m or more produced by the Siemens process of the present invention is characterized in that the absolute value of the difference between the compressive stress and the tensile stress of the residual stress in the longitudinal direction of the side of the rod is 22 MPa or less.

[0017] The present invention also provides a method for producing a polycrystalline silicon rod by depositing polycrystalline silicon on a silicon core wire using the Siemens process, characterized in that the polycrystalline silicon rod is produced in such a way that the temperature difference ΔT in the cross section of the silicon polycrystalline rod is 200°C or less for at least one hour before the end of the reaction.

[0018] The above manufacturing method includes: (1) For one hour or more before the end of the reaction, the supply current for producing polycrystalline silicon rods is continuously or intermittently reduced, and the amount of raw material gas is continuously or intermittently reduced. (2) A mode in which high frequency waves are applied to the surface of the polycrystalline silicon rod by a high frequency power supply device for at least one hour before the end of the reaction; (3) A mode in which a dopant is included in the supply gas for at least one hour before the end of the reaction to grow polycrystalline silicon rods; and / or (4) When the polycrystalline silicon is deposited, dummy polycrystalline silicon rods are grown outside the periphery of the polycrystalline silicon. may be adopted. [Effects of the Invention]

[0019] The present invention makes it possible to reduce damage such as cracks that occur when processing polycrystalline silicon after growth is completed. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a schematic diagram illustrating a reactor according to an embodiment of the present invention. [Figure 2] FIG. 2 is a top plan view of the inside of a reaction vessel showing an aspect in which a high frequency power supply device is used in an embodiment of the present invention. [Figure 3] FIG. 2 is a top plan view of the inside of a reaction vessel showing a mode in which dummy polycrystalline silicon rods are grown outside the periphery in an embodiment of the present invention. [Figure 4] FIG. 10 is a top plan view showing a polycrystalline silicon rod to which a strain gauge is attached. [Figure 5] FIG. 1 is a perspective view showing a polycrystalline silicon rod to which a strain gauge is attached. [Figure 6] FIG. 10 is a perspective view showing a manner in which a polycrystalline silicon rod with a strain gauge attached thereto is cut. [Figure 7] 1 is a graph showing residual stresses measured by a strain relief method in conventional polycrystalline silicon rods and polycrystalline silicon rods according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] 1, a manufacturing apparatus (reactor) used to manufacture polycrystalline silicon rods includes a reaction vessel 10, one or more pairs of electrodes 220 provided in the reaction vessel 10, U-shaped electrode wires 210 attached to the pair of electrodes 220, a supply unit 260 for supplying a feed gas containing a raw material gas to the reaction vessel 10, and an exhaust unit 270 for exhausting exhaust gas from the reaction vessel 10. In the reaction vessel, polycrystalline silicon rods are produced using, for example, the Siemens method, which grows polysilicon by a CVD reaction.

[0022] The polycrystalline silicon rod manufacturing apparatus may include an input unit 370 including a personal computer, a smartphone, a tablet terminal, or the like through which an operator inputs, a control unit 350 that performs various controls in the polycrystalline silicon rod manufacturing apparatus, and a storage unit 360 that stores various information. A series of procedures such as a recipe may be stored in the storage unit 360, and the polycrystalline silicon rod may be manufactured according to a predetermined procedure by the control unit 350 reading out the recipe.

[0023] It was previously believed that the surface temperature, which is the reaction temperature, would affect the final residual stress, but the inventors have confirmed that a sufficiently low residual stress can be obtained by reducing the temperature difference in the cross section of the polycrystalline silicon rod (a transverse section perpendicular to the longitudinal direction of the polycrystalline silicon rod) at least one hour before precipitation is stopped. In this embodiment, the absolute value of the difference between the compressive stress and the tensile stress of the residual stress in the longitudinal direction of the side surface of the rod is 22 MPa or less, but is preferably 20 MPa or less, more preferably 16 MPa or less, and even more preferably 10 MPa or less.

[0024] Any method can be used to keep the temperature difference ΔT at any cross section of a polycrystalline silicon rod below 200°C. For example, the following method can be considered: In a polycrystalline silicon rod grown in a U-shape, the current flows more strongly in the straight body section toward the center of the rod, resulting in a higher temperature. However, in the curved section between the straight body and the bridge, the current flows more strongly toward the inner surface of the curved section, resulting in a higher heat generation per unit volume than at the center of the rod in the straight body section. Meanwhile, due to heat removal by thermal radiation and convection, the temperature inside the curved section of the rod and the center of the straight body section are close. These temperatures can be analyzed, for example, using a finite element method (FEM) numerical analysis that couples thermal fluid and electromagnetic fields. Although the temperature difference varies depending on the conditions related to various reactions, it is generally proportional to the diameter of the polycrystalline silicon rod. Therefore, the temperature difference ΔT at the cross section of a polycrystalline silicon rod can be calculated by measuring the temperature inside the straight body section with a radiation thermometer to estimate the temperature inside the straight body and then calculating the temperature difference ΔT in the straight body of the polycrystalline silicon rod. Alternatively, the rod temperature can be calculated by simply calculating ΔT using FEM analysis with various reaction conditions. The lower limit of the time for which the temperature difference ΔT becomes 200°C or less is preferably 1 hour. From the viewpoint of reducing the difference between compressive stress and tensile stress, the lower limit of the time for which the temperature difference ΔT becomes 200°C or less is more preferably 1.5 hours, and even more preferably 2 hours. On the other hand, from the viewpoint of not significantly reducing production efficiency, the upper limit of the time for which the temperature difference ΔT becomes 200°C or less is preferably 5 hours, more preferably 3.5 hours, and even more preferably 2 hours.

[0025] (Method 1) It is conceivable that the temperature difference can be reduced by reducing the current before the end of the reaction to lower the temperature inside the polycrystalline silicon and by reducing the reaction gas, thereby preventing a drop in the surface temperature (see Example 1 described below). In this case, the current may be continuously or intermittently reduced to 70% of the maximum current over 3 to 5 hours to lower the temperature inside the polycrystalline silicon. Furthermore, to prevent a drop in the surface temperature, the amount of source gas (kg / h) may be continuously reduced to 30% of the maximum over 3 to 5 hours. This control may be performed by the control unit 350 reading out a recipe stored in the memory unit 360, or may be performed by an operator's instructions via the input unit 370.

[0026] (Method 2) Furthermore, in order to make the surface temperature of the polycrystalline silicon rod uniform, it is also possible to concentrate Joule heat on the rod surface using the high frequency power supply 400, thereby reducing the temperature difference for at least one hour before the end of the reaction (see FIG. 2). As shown in FIG. 2, an antenna 410 of the high frequency power supply 400 may be provided on the inner surface of the reaction vessel 10, and may receive output from the high frequency power supply main body 400 to provide high frequency waves into the reaction vessel 10.

[0027] To raise the surface temperature of the polycrystalline silicon rod to a temperature suitable for the deposition reaction, power is supplied to the electrode 220 to increase the surface temperature. However, as the diameter increases, the surface area of ​​deposition increases, and therefore the power supplied to the electrode 220 also increases. Because the electrical resistivity of silicon decreases with increasing temperature, a synergistic effect results between the temperature increase at the rod center and the current increase. As a result, the temperatures of the interior of the deposited polycrystalline silicon and the electrode wire 210, such as the silicon core, increase, increasing the temperature difference between the surface of the polycrystalline silicon rod. To eliminate this temperature difference, a high-frequency power supply device 400, as shown in FIG. 2, can be used to generate high-frequency waves on the surface of the silicon rod and reduce the temperature difference between the interior and surface of the silicon rod. The high-frequency power supply device 400 may be operated by the control unit 350 reading a recipe stored in the memory unit 360, or by an operator's instructions via the input unit 370.

[0028] (Method 3) In the latter half of the reaction (at least one hour before the end of the reaction), dopants such as B, P, As, and Al may be added to the source gas containing trichlorosilane and hydrogen to reduce the electrical resistivity of the polycrystalline silicon rod at the outer periphery of the rod, concentrating Joule heat and reducing the temperature difference. This configuration reduces the heat lost during the formation of polycrystalline silicon, thereby relatively reducing the difference between the internal and surface temperatures of the polycrystalline silicon rod. In other words, intentionally increasing the dopant content can reduce the electrical resistivity of the outer periphery of the rod compared to the center, thereby increasing the current flowing to the outer periphery. Note that if the source gas temperature is too high, polycrystalline silicon will not only be generated properly on the surface but may also be generated within the reaction vessel 10, potentially resulting in particles within the reaction vessel 10. Therefore, careful attention must be paid to temperature control. A raw material gas containing trichlorosilane and hydrogen is supplied from a raw material gas supply unit 110, and a dopant is supplied from a dopant supply unit 120. A dopant supply pipe 121 that supplies the dopant supply unit 120 and a raw material gas supply pipe 111 that supplies the raw material gas are connected via a valve 123, and opening and closing of the valve 123 may be performed under the control of a control unit 350 (see FIG. 1). When the valve 123 is in an open state, the dopant is mixed into the raw material gas, and when the valve 123 is in a closed state, mixing of the dopant into the raw material gas is stopped.

[0029] (Method 4) Alternatively, polycrystalline silicon rods (dummy polycrystalline silicon rods) intended to be crushed may be arranged so as to surround the outer periphery of the polycrystalline silicon rod (see FIG. 3). By increasing the power supplied to the electrode 220d to raise the temperature of the electrode wire 210d, the temperature difference between the surface and the interior of the polycrystalline silicon rod (a polycrystalline silicon rod different from the dummy polycrystalline silicon rod) to be manufactured may be reduced by the radiant heat from the dummy polycrystalline silicon rods, which deposit and grow silicon around the electrode wire 210d (see Example 2 described later). A plurality of dummy polycrystalline silicon rods may be provided, and the number of dummy polycrystalline silicon rods may be 1.5 to 3 times the number of polycrystalline silicon rods to be manufactured. For example, 4 to 8 target polycrystalline silicon rods (2 to 4 pairs) may be grown, and 6 to 24 dummy polycrystalline silicon rods (3 to 12 pairs) may be grown around them. In FIG. 3, the electrode used to produce the dummy polycrystalline silicon rods is shown as 220d, and the U-shaped electrode wire used to produce the dummy polycrystalline silicon rods is shown as 210d.

[0030] (Method 5) In addition, by lowering the pressure inside the furnace, the amount of heat removed from the surface of the polycrystalline silicon rod by the supplied gas can be reduced, and the temperature difference ΔT can be reduced.

[0031] Furthermore, two or more of the above-mentioned methods 1 to 5 may be appropriately combined to reduce the temperature difference ΔT.

[0032] To measure the internal temperature of the polycrystalline silicon rod, a sample polycrystalline silicon rod may be prepared, and thermometers may be installed at appropriate radial positions during the growth process of the polycrystalline silicon rod to measure how the temperature changes inside the polycrystalline silicon rod. In this case, it may be assumed that the internal temperatures of polycrystalline silicon rods grown under the same conditions will exhibit the same behavior. In this case, multiple polycrystalline silicon rods may be prepared as samples, and the internal temperature of the polycrystalline silicon rod may be measured from the average value of these samples. Heat removal factors on the surface of the polycrystalline silicon rod may be reduced as much as possible, and the current may be controlled in accordance with the rod surface temperature. Note that heat removal factors include convection due to a supply gas, such as a raw material gas, and the heat removal factors may be reduced by, for example, reducing the supply amount of the supply gas. [Example]

[0033] [Comparative Example] After the polycrystalline silicon rod was grown to a diameter of 125 mm (straight body diameter 125 mm) at a surface temperature of approximately 1050 to 950°C, the current and supply gas were stopped, and after it had cooled sufficiently, the reactor was purged with nitrogen, opened, and removed.

[0034] [Example 1] Polycrystalline silicon rods were grown to a diameter of 121 mm at a surface temperature of 1050-950°C according to the standard manufacturing method. Subsequently, to lower the internal temperature, which was higher than the surface temperature, the current was reduced to lower the temperature inside the bent U-rod, which was assumed to be an internal temperature of -30°C. Specifically, the current was continuously reduced from 2012 A to 1490 A over two hours. Furthermore, to maintain the surface temperature during the reaction at 1050-950°C, the gas supply was gradually reduced to 60% of the maximum supply gas volume over two hours. In this way, the difference between the internal and external temperatures was maintained below 200°C for at least two hours. After the polycrystalline silicon rods had grown to a diameter of 125 mm, the current supply was stopped, the gas supply was stopped, and after sufficient cooling, the reactor was purged with nitrogen, opened, and removed.

[0035] [Example 2] Twelve polycrystalline silicon rods were placed around six polycrystalline silicon rods and grown to a diameter of 121 mm at a surface temperature of 1050 to 950°C. As in Example 1, the current was reduced while gradually reducing the supply gas so as to maintain the rod surface temperature at 1050 to 950°C. Specifically, the current was continuously reduced under the same conditions as in Example 1, but the supply gas rate was continuously reduced from 302 kg / h to 181 kg / h over two hours. In this way, the time during which the difference between the internal and external temperatures remained below 200°C was maintained for at least two hours. Since the radiant heat of the 12 polycrystalline silicon rods surrounding the six polycrystalline silicon rods was utilized, there was no need to reduce the supply gas rate compared to Example 1, improving production efficiency. The current supply to the six polycrystalline silicon rods was stopped, and one hour later, the current supply to the surrounding 12 polycrystalline silicon rods was stopped. After the rods had sufficiently cooled, the reactor was purged with nitrogen, opened, and removed. These six rods were used as the rods of Example 2.

[0036] The polycrystalline silicon rod of the comparative example, the polycrystalline silicon rod of Example 1, and one of the six polycrystalline silicon rods of Example 2 were each sliced ​​at the center to form a cylindrical shape with a length of 1 m. Strain gauges 510 were attached at three locations on the side of the rod in the longitudinal direction so as to be equally spaced circumferentially (at an angle of 120 degrees) (see FIG. 4). 50 mm from the tip of the rod, including the strain gauges 510, was cut at a constant speed while rotating the rod so that the incision was constant over 360 degrees from the rod surface (see FIG. 5). During this process, the polycrystalline silicon rod was held by a chuck 530, and the polycrystalline silicon rod was cut by a peripheral cutting edge 540. The difference between the compressive stress and the tensile stress of the residual stress in the longitudinal direction of the side of the rod during the cutting process was recorded by a data logger 520 (see FIG. 6).

[0037] The Young's modulus of polycrystalline silicon was set to 165 GPa, and the average values ​​of the obtained strains as residual stresses are shown in Figure 7. The release stress on the vertical axis represents the difference between the compressive stress and tensile stress of the residual stress in the longitudinal direction of the side of the rod.

[0038] In the polycrystalline silicon rod produced in the comparative example, the block containing the strain gauge 510 broke when the cut was made past 60 mm in the radial direction. The stress just before the break was about 23 MPa, so it is clear that the rod had residual stress of even greater magnitude.

[0039] It was confirmed that the rod produced in Example 1 had a pressure drop of 16 MPa, and the rod produced in Example 2 had a pressure drop of less than half of the approximately 23 MPa observed in the comparative example.

[0040] The above description of the embodiment and the disclosure of the drawings are merely examples for explaining the invention described in the claims, and the invention described in the claims is not limited by the description of the embodiment or the disclosure of the drawings. Furthermore, it should be noted that the description of the claims at the time of filing is merely the claimed scope of rights at the time of filing, and can be changed as appropriate. [Explanation of symbols]

[0041] 10 Reaction vessel 110 Raw material gas supply unit 120 Dopant supply unit 210 Electrode wire 220 electrode 350 control section 400 High frequency power supply

Claims

1. A method for producing a polycrystalline silicon rod by depositing polycrystalline silicon on a silicon core wire by the Siemens process, comprising: The polycrystalline silicon rod has a longitudinal length of 1 m or more, and the absolute value of the difference between the compressive stress and the tensile stress of the residual stress in the longitudinal direction of the side surface of the rod is 22 MPa or less, by controlling the temperature difference ΔT at the cross section of the polycrystalline silicon rod to be 200°C or less for at least 1 hour before the end of the reaction.

2. 2. The method for producing a polycrystalline silicon rod according to claim 1, wherein the supply current for producing the polycrystalline silicon rod is continuously or intermittently reduced and the amount of the raw material gas is continuously or intermittently reduced for at least one hour before the end of the reaction.

3. 3. The method for producing a polycrystalline silicon rod according to claim 1, wherein a high frequency power supply is used to apply high frequency waves to the surface of the polycrystalline silicon rod for at least one hour before the end of the reaction.

4. 3. The method for producing a polycrystalline silicon rod according to claim 1, wherein a dopant is contained in the supply gas for at least one hour before the end of the reaction to grow the polycrystalline silicon rod.

5. 3. The method for producing a polycrystalline silicon rod according to claim 1, wherein, when the polycrystalline silicon is deposited, a dummy polycrystalline silicon rod is grown outside the periphery of the polycrystalline silicon.

Images