Method for welding silicon members, and welded silicon body

Abstract

Provided is a method for welding silicon members that enables efficient, low-cost welding of an abutting section of two silicon members in a short time with no possibility of carbon impurity contamination during welding. A method for welding silicon members according to the present invention is characterized in that heat conduction welding is performed on an abutting section (10) of two silicon core wires (8, 9), and the silicon is melted until bottom surfaces (8d, 9d) in a meltable region that includes the abutting section (10) are reached.

Description

Welding method for silicon members and silicon welded body

[0001] The present invention relates to a method for welding silicon core wires used as silicon core wire structures in Siemens process reactors and plate-shaped bodies used as bedding materials for silicon curing, and in particular to a method for welding the butt joints of two silicon members (silicon core wires or silicon plate-shaped bodies) to produce silicon core wires of a desired length or silicon plate-shaped bodies of a desired size, and to a silicon welded body welded by this method.

[0002] A commonly known method for producing polycrystalline silicon, a raw material for wafers and semiconductors used in photovoltaic power generation, is the Siemens process, in which a source gas is reacted in a heated reactor inside a reactor to deposit silicon on the reactor surface. The Siemens process will now be described in detail with reference to FIG. 6 . FIG. 6 is a schematic external view of a reactor used to produce polycrystalline silicon using the Siemens process. As shown in FIG. 6 , the reactor 1 includes a bottom plate 3 on which a silicon core structure 2 made of gate-shaped silicon core wires is mounted, and a bell-jar-shaped cover 4 detachably attached to the bottom plate 3. The bottom plate 3 is provided with a gas inlet 3a for supplying a source gas consisting of a mixture of hydrogen and a gas of a silane compound, such as trichlorosilane or monosilane, into the reactor 1, and a gas outlet 3b for discharging waste gas generated inside the reactor 1 to the outside. An end of the silicon core wire is connected to an electrode 5 mounted on the bottom plate 3, and a power supply 6 is connected to the electrode 5 via a cable 5a. In the reactor 1 having such a structure, silicon is deposited on the surface of the silicon core structure 2 to produce a polycrystalline silicon rod 7. The silicon core constituting the silicon core structure 2 generally has a square pillar shape measuring 6 to 8 mm on each side.

[0003] A silicon core structure 2 to be installed in a large reactor 1 requires a long silicon core, and such a silicon core is manufactured by welding and connecting two shorter silicon cores. That is, to manufacture a silicon core of a desired length, it is necessary to weld the butt joints of two silicon cores arranged in a straight line. However, with conventional methods, it is not easy to weld the butt joints of two silicon cores arranged in an L-shape, and it has been difficult to manufacture a silicon core structure 2 consisting of gate-shaped silicon cores of a desired size.

[0004] Furthermore, silicon plates are used for silicon containers such as protective slabs and storage boxes for high-purity silicon products, chutes used when feeding crushed silicon material from a hopper into such containers, and guides for conveying equipment (belt conveyers) for crushed silicon material, but when slicing polycrystalline silicon, the size (width) of the resulting plates is limited. Therefore, there has been a demand for a technology for reliably welding the butt joints of two silicon plates.

[0005] To address such issues, for example, Patent Document 1 discloses an invention entitled "Silicon core wire welding apparatus, silicon core wire manufacturing method, and polycrystalline silicon manufacturing method," which relates to a welding apparatus that can not only weld two silicon core wires arranged in a straight line to manufacture a single straight silicon core wire, but also weld two silicon core wires arranged in an L shape to manufacture a single L-shaped silicon core wire. The silicon core wire welding device disclosed in Patent Document 1 includes a work coil connected to a high-frequency generator, outputting an alternating magnetic field, and including a curved portion from one terminal to the other that forms a space capable of accommodating a welded portion of a silicon core wire, and a carbon portion that is installed so as to be able to be inserted and removed from the space, covers the silicon core wire when inserted into the space, and is heated by the alternating magnetic field to preheat the silicon core wire. The space is formed by forming two imaginary planes, and the work coil is processed so that these imaginary planes face each other at a distance, thereby opening the space so that the silicon core wire can be inserted and removed in a direction substantially parallel to the two imaginary planes, and the silicon core wire is heated in the space by the alternating magnetic field. As such, in the silicon core wire welding device, the space that accommodates the welded portion of the silicon core wire is open in one direction, so that a welded portion can be placed in the space not only when the two silicon core wires are arranged in a straight line in the axial direction but also when they are arranged in an L-shape. Therefore, with the silicon core welding device, it is possible to weld two silicon cores together to produce a single silicon core, not only when the silicon cores are arranged in a straight line, but also when the silicon cores are arranged in an L-shape.

[0006] Although not related to a method for welding the butt joints of two silicon core wires arranged at right angles, Patent Document 2 discloses an invention entitled "Method for Welding Semiconductor Materials" relating to a method for welding semiconductor materials such as silicon. Patent Document 2 describes electron beam welding or laser welding of stacked single-crystal silicon components from a horizontal direction, targeting the stacked portion, and rotating the workpiece to enable welding of the entire circumference of the stacked portion. It also describes preheating the semiconductor material before welding. The welding method disclosed in Patent Document 2 allows the entire circumference of the workpiece to be welded even when the power density of the laser beam is low by rotating the silicon workpiece. Furthermore, since the heat generated during welding is small, thermal degradation and brittleness of the silicon are unlikely to occur. Therefore, the method disclosed in Patent Document 2 makes it possible to perform laser welding on silicon, which has previously been considered difficult to laser weld because the heat generated during welding reduces toughness and makes it more susceptible to breakage.

[0007] Patent No. 6809923 Publication JP-A-11-179565

[0008] The silicon core wire welding method disclosed in Patent Document 1 requires a preheating process before the high-frequency heating process, which increases the time required for welding and complicates the structure of the silicon core wire welding device. Furthermore, there is a risk that carbon used in the carbon part that preheats the silicon core wire may be mixed into the silicon as an impurity during the preheating process. The welding method disclosed in Patent Document 2 allows silicon to be welded using a laser beam, but requires the workpiece to be rotated, which complicates the structure of the welding device, resulting in a problem of increased manufacturing costs for the entire welding device. Furthermore, the welding method disclosed in Patent Document 2 is characterized by preheating silicon so that silicon can be welded even when the power density of the laser beam is low. However, in this case, the preheating process must be performed before laser welding, which increases the time required for welding and complicates the structure of the welding device. Furthermore, if the preheater is made of carbon, there is a risk that carbon may be mixed into the silicon to be welded as an impurity. In addition, the inventions disclosed in Patent Documents 1 and 2 were not designed for welding silicon plate-shaped bodies, and therefore had the problem that they could not be used to weld the butt joints of two silicon plate-shaped bodies.

[0009] The present invention has been made in response to the above-mentioned conventional circumstances, and has as its object to provide a method for welding silicon members that is free from the risk of carbon being mixed in as an impurity into the silicon being welded, and that is capable of welding the butted portions of two silicon members efficiently, in a short time, and at low cost.

[0010] To achieve the above object, the first invention is a method for welding the butt joints of two silicon members using a laser beam, characterized in that the laser beam is irradiated onto the butt joints to melt the silicon until it reaches the bottom of the silicon members using heat conduction welding. The laser beam used in the first invention is the same as that used for heat conduction welding. Unlike the laser beam used in keyhole welding, the power density of the laser beam is low, and the thermal energy imparted to the silicon from the laser beam is conducted slowly through the silicon. This reduces the risk of the temperature of the portion adjacent to the molten member rapidly increasing due to the confinement of the reflected laser beam in the keyhole, which occurs in keyhole welding. This rapid increase in the laser absorption rate leads to thermal degradation and makes the portion brittle, resulting in easy breakage. Furthermore, since a carbon heater is not used, carbon is not introduced as an impurity into the silicon to be welded. Furthermore, the absence of a preheating step reduces the welding time for silicon members.

[0011] The second invention is characterized in that, in the first invention, the silicon member is a silicon core wire, and the laser light is irradiated in a long and narrow irradiation range that straddles the butt joint of the silicon core wire and is along the axial direction of the silicon core wire, with the end face located at the butt joint. In addition to the effect of the first invention, the second invention has the effect of being able to melt silicon in the region that requires melting, including the butt joint, in a short time.

[0012] The third invention is characterized in that, in the first invention, the silicon members are silicon plates, and the laser light is irradiated in a long and narrow irradiation range that straddles the butt joints of the silicon plates and is parallel to the end faces located at the butt joints. In the third invention, the same effect as that of the second invention is exerted when welding the silicon plates.

[0013] A fourth invention is a silicon component with a welded butt joint, characterized in that, where a is the longest distance from the point that formed the end face before melting to the boundary of the fused part measured parallel to the direction perpendicular to the end face, and b is the maximum penetration depth of the fused part, a ≧ b is satisfied. In the fourth invention, the fused part is sufficiently secured, and the part adjacent to it has not become brittle due to thermal degradation, so there is no risk of it easily breaking at that part.

[0014] According to the first invention, the butt joints of two silicon members can be welded without reducing the strength of the silicon due to the heat generated during welding using a laser beam, making it possible to manufacture gate-shaped silicon core structures of desired sizes for use in Siemens process reactors, silicon curing mats, etc. Furthermore, since there is no separate preheating process compared to conventional high-frequency welding, the silicon members can be welded efficiently in a short time, and since a preheating device is not required, the manufacturing costs of the overall equipment used for welding can be kept low. Furthermore, since silicon has an extremely low linear expansion coefficient, there is also the advantage that even if the butt joint is melted over a wide area, the distortion remaining in the silicon members after welding can be kept as small as possible.

[0015] The second invention has the effect that a silicon core structure of a desired size used in the Siemens process can be manufactured inexpensively, and the butted portions of two silicon cores can be welded efficiently in a short time.

[0016] The third invention has the advantage that silicon plates of a desired size, which can be used as a bedding material for silicon curing, can be produced inexpensively, and the butt joints of two silicon plates can be welded efficiently in a short time.

[0017] According to the fourth invention, there is no risk that the silicon core wire and silicon plate-shaped body will easily break at the part adjacent to the molten part, so they can be safely used as silicon core wire structures to be used in Siemens process reactors or as paving materials for silicon curing.

[0018] 1A and 1B are schematic diagrams illustrating how butt portions of two silicon core wires arranged in a straight line are welded by a method for welding silicon members according to a first embodiment of the present invention and by keyhole-type laser welding, respectively. 1A to 1C are diagrams for explaining a method for welding silicon members according to a second embodiment of the present invention. 1A to 1C are diagrams for explaining a method for welding silicon members according to a third embodiment of the present invention. 1A is a schematic perspective view illustrating how butt portions of two silicon plate-shaped bodies arranged in a plane are welded by a method for welding silicon members according to a fourth embodiment of the present invention, and 1B and 1C are diagrams for explaining the welding method for silicon members. 1A is a schematic perspective view illustrating how butt portions of two silicon plate-shaped bodies arranged in an L-shape in side view are welded by a method for welding silicon members according to a fifth embodiment of the present invention, and 1B is a diagram for explaining how laser light is irradiated onto the silicon member. 1 is an external view showing an outline of a reactor used when producing polycrystalline silicon by the Siemens process.

[0019] The welding method for silicon members of the present invention, the structure of a silicon welded body welded by the method, and the functions and effects of the invention will be specifically described with reference to Figures 1 to 5. In the following description, the silicon core wire is an elongated body consisting of a rectangular prism, but this is not limited to a rectangular prism, and it may be another type of rectangular prism or a cylinder. The functions and effects of the present invention described below are similarly exhibited even when the silicon core wire has a shape other than a rectangular prism. However, since a "bottom surface" does not exist when the silicon core wire is a cylinder, the term "bottom" is used, which is a broader concept of "bottom surface." Furthermore, in the present invention, the laser light used for heat conduction welding is a low-power-density laser light that does not form a keyhole during welding, unlike the laser light used for keyhole-type welding.

[0020] In the following description using a rectangular prism-shaped silicon core wire and a silicon plate as examples, the surface of the silicon core wire and the silicon plate on which the laser light is irradiated is referred to as the "first surface," a pair of surfaces (side surfaces) perpendicular to the "first surface" are referred to as the "second surface" and the "third surface," respectively, and a surface parallel to the "first surface" is referred to as the "bottom surface." Furthermore, after welding, the positions of the end faces at the butted portions of the two silicon core wires and the butted portions of the two silicon plates are melted and the boundaries cannot be identified except for the bottom surfaces. However, for convenience, in Figures 1 to 3, the locations that formed the end faces before melting are shown as end faces 8e and 9e. Furthermore, Figures 1(a), 1(b), 2(c), 3(c), and 3(d) only describe the state of the second surfaces 8b and 9b of the two silicon core wires 8 and 9, but the state of the third surfaces 8c and 9c is substantially the same as the state of the second surfaces 8b and 9b. In addition, in FIG. 4(b), only the state of the second surfaces 14b and 15b of the two silicon plates 14 and 15 is described, but the state of the third surfaces 14c and 15c is also almost the same as the state of the second surfaces 14b and 15b.

[0021] The arrows in Figures 1(a), 1(b), 4(a), 4(b), and 5(a) indicate the direction of laser light irradiation. The dashed lines in Figures 1(a), 1(b), 2(a), 2(b), 2(c), 3(a), 3(b), and 3(d) represent butt joint 10, and the dashed lines in Figure 4(b) represent butt joint 16. The hatched lines in Figures 1(a), 1(b), 2(b), 2(c), 3(b), 3(c), and 3(d) represent fusion zone 12, and the hatched lines in Figures 4(a), 4(b), and 5(a) represent fusion zone 18.

[0022] 1( a) and 1(b) are schematic diagrams illustrating welding of a butt portion 10 of two silicon core wires 8, 9 arranged in a straight line by a silicon core wire welding method according to a first embodiment of the present invention and keyhole-type laser welding, respectively. As shown in FIG. 1(a), the silicon core wire welding method according to the first embodiment of the present invention is characterized in that the butt portion 10 of the two silicon core wires 8, 9 arranged in a straight line is welded by heat conduction laser welding, thereby melting silicon in a region including the butt portion 10 until it reaches bottom surfaces 8d, 9d.

[0023] The laser beam used in thermal conduction welding is absorbed by the surface of the material and converted into heat, and the thermal energy is transferred to the interior, melting the material. In thermal conduction laser welding, the power density of the laser beam is low, so the speed at which heat is transferred in the direction perpendicular to the first surfaces 8a, 9a is approximately equal to the speed at which heat is transferred in the longitudinal direction of the first surfaces 8a, 9a (the axial direction of the silicon cores 8, 9 or the direction perpendicular to the end faces 8e, 9e). Therefore, the longest distance from the location where the end faces 8e, 9e were formed before melting to the boundaries 8f, 9f of the molten portion 12 measured parallel to the axial direction is approximately equal to the maximum depth (maximum penetration depth) of the molten portion 12 measured in the direction perpendicular to the first surfaces 8a, 9a. Here, the maximum depth refers to the depth at which the molten portion 12 reaches the bottom of the silicon cores 8, 9. However, it is preferable that the state at which the molten portion 12 reaches the bottom be limited to a level where the melted range at the bottom remains unmelted and the silicon melt does not fall. Furthermore, the state of having reached the bottom may not only mean the state in which the silicon core wires are completely melted to the bottom as described above, but may also include a state in which the silicon core wires are melted to a thickness of 10% or less, preferably 5% or less, of the distance b between the first surfaces 8a, 9a and the bottom surfaces 8d, 9d of the silicon core wires 8, 9, leaving the bottom portions.

[0024] When the molten zone 12 reaches the bottom surfaces 8d, 9d of the silicon core wires 8, 9, the maximum penetration depth described above becomes equal to the distance b (see FIG. 1(a)) between the first surfaces 8a, 9a and the bottom surfaces 8d, 9d of the silicon core wires 8, 9, and does not change any further. On the other hand, the melting of silicon in the axial direction does not necessarily stop immediately when the molten zone 12 reaches the bottom surfaces 8d, 9d of the silicon core wires 8, 9. Therefore, the maximum distance is often longer than the maximum penetration depth. Therefore, the maximum distances from the points that formed the end surfaces 8e, 9e before melting to the boundaries 8f, 9f of the molten zone 12 measured parallel to the axial direction are respectively a １ and a ２ (See FIG. 1(a)). Then, the maximum penetration depth in the fusion zone 12 is equal to the distance b. １ , a ２ The relationship between a and b is expressed by the following equations (1) and (2). １ , a ２ is preferably equal to or smaller than twice the value of b.

[0025]

[0026]

[0027] On the other hand, if the power density of the laser beam is high, part of the molten material evaporates, and the resulting pressure creates a depression 13 on the surface of the material as shown in FIG. 1( b). If this depression 13 becomes deep, it becomes a keyhole (cavity), through which the laser beam can reach the interior of the material. This type of welding method is generally called keyhole welding. However, when keyhole welding is performed on the butt joint 10 of two silicon core wires 8 and 9, the power density of the laser beam is high and the aforementioned thermal energy is transmitted quickly into the interior of the silicon, so the temperature of the portion adjacent to the molten portion 12 rises rapidly. As a result, this portion undergoes thermal degradation, becoming very brittle and prone to breaking.

[0028] In contrast, when heat conduction-type laser welding is performed on the butt joint 10 of the silicon core wires 8, 9, as in the silicon core wire welding method according to the first embodiment of the present invention, the power density of the laser beam is low and the rate at which the thermal energy is transmitted into the silicon is slow, so the temperature of the portion adjacent to the molten zone 12 does not rise rapidly. Therefore, the phenomenon of the portion becoming brittle and prone to breakage due to thermal degradation, which is seen in keyhole welding, is unlikely to occur. Therefore, according to the silicon core wire welding method of the present invention, the butt joint 10 of the two silicon core wires 8, 9 can be welded without reducing the strength of the silicon due to the heat generated during welding. This makes it possible to manufacture a single straight silicon core wire of a desired length. Furthermore, unlike the inventions disclosed in Patent Documents 1 and 2, the silicon core wire welding method of the present invention does not require a separate preheating step, so the silicon core wires 8, 9 can be welded efficiently in a short time. Furthermore, since a preheating device is not required, the manufacturing costs of the overall equipment used for welding can be kept low.

[0029] 2( a) to 2(c) are diagrams for explaining a welding method for silicon core wires according to a second embodiment of the present invention, in which Fig. 2(a) shows an irradiation range 11 of a laser beam on first surfaces 8a, 9a of two silicon core wires 8, 9 arranged in a straight line. Also, Figs. 2(b) and 2(c) schematically show the state of a molten zone 12 on the first surfaces 8a, 9a and second surfaces 8b, 9b of the two silicon core wires 8, 9, respectively. In the welding method for silicon core wires according to the second embodiment of the present invention, as shown in Fig. 2(a), when performing heat conduction laser welding on a butt portion 10 of two silicon core wires 8, 9 arranged in a straight line, it is preferable that the laser beam is irradiated on an irradiation range 11 such that the laser beam crosses the butt portion 10 and forms the molten zone 12 shown in Figs. 2(b) and 2(c). Specifically, a laser beam having an irradiation area equal to the irradiation range 11 may be fixed and irradiated. Alternatively, for example, when the irradiation area of ​​the laser beam is small, the laser beam may be scanned and irradiated onto the first surfaces 8a, 9a so as to describe a rectangle having long and short sides parallel to the axial direction (the longitudinal direction of the first surfaces 8a, 9a or the direction perpendicular to the end surfaces 8e, 9e) and the width direction (the lateral direction of the first surfaces 8a, 9a) of the silicon core wires 8, 9. In the above-described laser beam scanning mode, if the irradiation width of the laser beam is sufficiently large compared to the width of the silicon core wires 8, 9 (the length in the lateral direction of the first surfaces 8a, 9a), the laser beam may not be scanned in the width direction, but may be scanned only in the axial direction (the longitudinal direction of the first surfaces 8a, 9a or the direction perpendicular to the end surfaces 8e, 9e). When scanning the laser beam, it is desirable to scan the irradiation range so that at least a portion of the butted portion 10 overlaps. It is also possible to use a plurality of laser oscillators to irradiate laser light onto a long and narrow range (irradiation range 11) along the axial direction of the silicon core wires 8, 9 (the longitudinal direction of the first faces 8 a, 9 a or the direction perpendicular to the end faces 8 e, 9 e) and across the butted portion 10. Furthermore, it is preferable from the standpoint of safety that the irradiation width of the laser light be less than the width of the silicon core wire.

[0030] As already mentioned, in thermal conduction laser welding, the power density of the laser beam is low and the rate at which the thermal energy is transmitted into the silicon is slow. However, as shown in Fig. 2(a), when the laser beam is irradiated to a long, narrow range along the axial direction of the silicon core wires 8, 9 so as to straddle the portions of the two silicon core wires 8, 9 that formed the end faces 8e, 9e before melting, the silicon melts in a short time over the region that needs to be melted, including the butt joint 10. Therefore, according to this method, the butt joint 10 of the two silicon core wires 8, 9 arranged in a straight line can be welded in a short time to efficiently manufacture a single straight silicon core wire of a desired length. Note that the longest distances from the portions that formed the end faces 8e, 9e before melting to the boundaries 8f, 9f of the fusion zone 12 measured parallel to the axial direction of the silicon core wires 8, 9 are respectively a １ and a ２ (See FIG. 2(b)), the maximum penetration depth in the fusion zone 12 is equal to the distance b (See FIG. 2(c)) between the first surfaces 8a, 9a and the bottom surfaces 8d, 9d. １ , a ２ The relationship between a and b is also expressed by the above-mentioned formulas (1) and (2).

[0031] In a straight silicon core wire formed by welding the butt joints 10 of two silicon core wires 8, 9 together by the welding method described with reference to Figures 1(a) and 2(a), the state of the molten zone 12 is expressed by Figures 1(a), 2(b), and 2(c) and the above-mentioned formula (1), but this silicon core wire does not easily break at the portion adjacent to the molten zone 12 because the portion adjacent to the molten zone 12 is not thermally deteriorated and becomes brittle. As such, the above-mentioned silicon core wire does not easily break at the portion adjacent to the molten zone 12 and can be used safely in the straight portion of the silicon core wire structure 2 of a Siemens process reactor.

[0032] 3( a) to 3(d) are diagrams for explaining a welding method for silicon core wires according to a third embodiment of the present invention, in which Fig. 3(a) shows a laser beam irradiation range 11 on first surfaces 8a, 9a of two silicon core wires 8, 9 arranged to form an L-shape. Fig. 3(b) schematically shows the state of molten portions 12 on the first surfaces 8a, 9a of the two silicon core wires 8, 9, and Figs. 3(c) and 3(d) schematically show the state of molten portions 12 on second surfaces 8b, 9b, respectively. Figs. 3(c) and 3(d) correspond to views viewed in the direction of arrow A and the direction of arrow B, respectively, in Fig. 3(b).

[0033] 3( a), the welding method for silicon core wires according to the third embodiment of the present invention is characterized in that, when performing heat conduction laser welding on a butt joint 10 (a portion where an end face 9 e abuts on a third surface 8 c) of two silicon core wires 8, 9 arranged to form an L shape, the laser beam is irradiated onto an irradiation range 11 across the butt joint 10 and forming a molten zone 12 shown in Fig. 3( b) to 3(d). Specifically, a laser beam having an irradiation area equal to the irradiation range 11 may be fixed and irradiated. Alternatively, when the irradiation area of ​​the laser beam is small, for example, the laser beam may be irradiated while scanning the first surfaces 8 a, 9 a so as to describe a rectangle having long and short sides parallel to the axial direction of the silicon core wires 9 (a direction perpendicular to the longitudinal direction of the first surface 9 a or the end face 9 e) and the width direction (a direction perpendicular to the first surface 9 a), respectively. In addition, when the irradiation width of the laser light is sufficiently large compared to the width of the silicon core wire 9 (the length in the short direction of the first surface 9a), the laser light may not be scanned in the width direction, but may be scanned only in the axial direction (the longitudinal direction of the first surface 9a or the direction perpendicular to the end surface 9e). Furthermore, as described above, when scanning with the laser light, it is preferable to scan so that the irradiation ranges overlap at least a part of the butt joint 10. It is also possible to use multiple laser oscillators to irradiate the laser light onto a long and narrow range (irradiation range 11) spanning the butt joint 10. Furthermore, as described above, it is preferable from the standpoint of safety that the irradiation width of the laser light be less than the width of the silicon core wire.

[0034] As described above, in thermal conduction laser welding, the power density of the laser beam is low and the rate at which the thermal energy is transmitted into the silicon is slow. However, when the laser beam is irradiated onto a long and narrow range along the axial direction of the silicon core wires 9 so as to straddle the portion where the end face 9e abuts against the third surface 8c in two silicon core wires 8, 9 arranged as shown in Fig. 3(a), the silicon melts in a short time over the region that needs to be melted, including the butt portion 10. Therefore, according to this method, the butt portion 10 of the two silicon core wires 8, 9 arranged to form an L shape can be welded in a short time, and a single silicon core wire having an L shape of a desired length can be efficiently manufactured. Note that the longest distance from the portion that formed the end face 9e before melting to the boundary 9f of the fusion zone 12 measured parallel to the axial direction of the silicon core wire 9 is defined as a ２ (See FIG. 3(b)), the maximum penetration depth in the fusion zone 12 is equal to the distance b between the first surfaces 8a, 9a and the bottom surfaces 8d, 9d (See FIGS. 3(c) and 3(d)). ２ The relationship between a and b is also expressed by the above-mentioned formula (2).

[0035] In the L-shaped silicon core wire in which two silicon core wires 8, 9 are welded together by the welding method described with reference to Fig. 3(a), the state of the molten portion 12 is expressed by Fig. 3(b) to Fig. 3(d) and the above-mentioned formula (2), but this silicon core wire does not easily break at the portion adjacent to the molten portion 12 because the portion adjacent to the molten portion 12 is not thermally deteriorated and becomes brittle. As such, the above-mentioned silicon core wire does not easily break at the portion adjacent to the molten portion 12, and therefore can be safely used in the L-shaped portion of the silicon core wire structure 2 to be installed in a reactor for the Siemens process.

[0036] Next, the results of an experiment in which butt portions of two silicon cores were welded by the silicon core welding method of the present invention will be described. Sample 1 is a sample in which the butt portions of two silicon cores arranged in a straight line were welded by the silicon core welding method according to the second embodiment described with reference to Fig. 2( a). Sample 2 is a sample in which the butt portions of two silicon cores arranged in an L-shape were welded by the silicon core welding method according to the third embodiment described with reference to Fig. 3( a). Samples 3 and 4 are sample in which the butt portions of two silicon cores arranged in a straight line and two silicon cores arranged in an L-shape were welded by the keyhole laser welding described with reference to Fig. 1( b).

[0037] When both ends of the four types of samples were held by hand and a bending stress was slowly applied, Samples 3 and 4 easily broke at the portions adjacent to the molten parts. On the other hand, Samples 1 and 2 did not break even when subjected to a force of the same magnitude as that applied to Samples 3 and 4. These results show that, unlike keyhole-type laser welding, the silicon core wire welding method of the present invention can firmly weld the butt joints of two silicon core wires without causing thermal degradation, even when the silicon core wires are arranged in either a straight line or an L-shape.

[0038] FIG. 4( a) schematically illustrates welding of the butt joints 16 of two silicon plates 14, 15 arranged in a plane by a welding method for silicon members according to a fourth embodiment of the present invention, and FIG. 4( b) illustrates the state of the end faces 14e, 15e of the second surfaces 14b, 15b of the silicon plates 14, 15. FIG. 4( c) illustrates the irradiation range of the laser beam on the first surfaces 14a, 15a of the silicon plates 14, 15. Note that hatching is omitted in FIG. 4( a) for the molten portions 18 on the first surfaces 14a, 15a. Also, hatching is omitted in FIG. 4( b) to indicate cross sections of the silicon plates 14, 15. As shown in FIGS. 4( a) and 4(b), the welding method for silicon members according to the fourth embodiment of the present invention is characterized in that heat conduction laser welding is performed on the butt joints 16 of the silicon plates 14, 15, thereby melting the silicon in the region including the butt joints 16 until it reaches the bottom surfaces 14d, 15d.

[0039] 4(b), when the butt joints 16 of the silicon plates 14, 15 are welded by the above-described method, the longest distance from the locations where the end faces 14e, 15e were formed before melting to the boundaries 14f, 15f of the molten zone 18 measured perpendicular to the end faces 14e, 15e is approximately equal to the maximum depth (maximum penetration depth) of the molten zone 18 measured in a direction perpendicular to the first surfaces 14a, 15a. Note that the maximum depth refers to the depth at which the molten zone 18 reaches the bottom of the silicon plates 14, 15. However, it is preferable that the state at the bottom is stopped when the melted range of the bottom is such that an unmelted portion remains to prevent the silicon melt from falling. Furthermore, the state in which the molten portion 18 has reached the bottom may not only refer to the state in which the molten portion 18 is completely melted to the bottom as described above, but also to a state in which the molten portion 18 is melted to a thickness of 10% or less, preferably 5% or less, of the distance b between the first surfaces 14a, 15a and the bottom surfaces 14d, 15d of the silicon plate-like bodies 14, 15, leaving the bottom portion.

[0040] When the molten zone 18 reaches the bottom surfaces 14d, 15d of the silicon plates 14, 15, the maximum penetration depth described above becomes equal to the distance b (see FIG. 4B) between the first surfaces 14a, 15a and the bottom surfaces 14d, 15d of the silicon plates 14, 15, and does not change any further. On the other hand, melting of silicon in a direction perpendicular to the end surfaces 14e, 15e does not necessarily stop immediately when the molten zone 18 reaches the bottom surfaces 14d, 15d of the silicon plates 14, 15. Therefore, the maximum distance is often longer than the maximum penetration depth. Therefore, the maximum distance from the points that formed the end surfaces 14e, 15e before melting to the boundaries 14f, 15f of the molten zone 18 is defined as a １ and a ２ (See FIG. 4B) Then, the maximum penetration depth in the fusion zone 18 is equal to the distance b, so a １ , a ２ The relationship between a and b is expressed by the above-mentioned formula (1) and formula (2). １ , a ２ is preferably equal to or smaller than twice the value of b.

[0041] This welding method allows the butt joints 16 of the two silicon plates 14, 15 to be welded together without reducing the strength of the silicon due to heat generated during welding. This makes it possible to manufacture a single flat silicon plate of a desired size. Furthermore, unlike the inventions disclosed in Patent Documents 1 and 2, this welding method does not require a separate preheating step. This allows the silicon plates 14, 15 to be welded efficiently in a short time, and since no preheating device is required, the manufacturing costs of the overall welding equipment can be reduced.

[0042] The butt joint 16 of the two silicon plates 14, 15 is welded by spot welding several locations on the butt joint 16 or by scanning a laser beam parallel to the end faces 14e, 15e as shown by arrow C in FIG. 4( a). In either case, as shown in FIG. 4( c), it is desirable to irradiate a narrow irradiation area 17 with the laser beam in the linear direction of the butt joint 16 of the silicon plates 14, 15 (parallel to the end faces 14e, 15e). This ensures a preheating zone for the silicon, which is expected to result in shorter welding times. It is preferable to experimentally determine the irradiation time of the laser beam on the irradiation area 17 in advance, based on the time it takes for the melted silicon to reach the bottom.

[0043] 5( a) is a schematic diagram illustrating welding of the butt joints 16 of two silicon plates 14, 15 arranged to form an L-shape in side view by a welding method for silicon members according to a fifth embodiment of the present invention. Note that hatching is omitted for the molten zones 18 on the end faces 14e and the first surfaces 15a in FIG. 5( a). As shown in FIG. 5( a), the welding method for silicon members according to the fifth embodiment of the present invention is characterized in that, when performing thermal conduction laser welding on the butt joints 16 of the silicon plates 14, 15 (the portions where the end faces 15e abut against the bottom faces 14d), the laser beam crosses the butt joints 16 and irradiates an elongated irradiation range 17 in the linear direction of the butt joints 16 of the silicon plates 14, 15 (a direction parallel to the bottom faces 14d and the end faces 15e). In this way, when two silicon plates 14, 15 arranged to form an L-shape in side view are irradiated with laser light in a narrow irradiation range 17 along the linear direction of the butt joint 16 of the silicon plates 14, 15 so as to straddle the point where the end face 15e abuts against the bottom face 14d, a preheating zone for the silicon can be secured, thereby completing welding in a shorter time. In this case, instead of scanning the laser light parallel to the bottom face 14d and the end face 15e as shown by arrow D in FIG. 5( a), several locations on the butt joint 16 may be spot-welded. This method allows the butt joint 16 of two silicon plates 14, 15 arranged to form an L-shape to be welded in a short time, thereby efficiently producing a single silicon welded body of a desired size that forms an L-shape in side view.

[0044] In the welding method for silicon members according to the present invention, the laser beam is irradiated in an inert gas atmosphere such as argon gas. Fig. 5(b) is a schematic diagram illustrating the irradiation of a silicon member 19, such as silicon core wires 8 and 9 or silicon plate-like bodies 14 and 15, with a laser beam. As shown in Fig. 5(b), an irradiation unit cover 21 is installed between the silicon member 19 and the laser irradiation device 20 so as to cover an upper portion of the silicon member 19 where the laser beam is irradiated from the laser irradiation device 20. The irradiation unit cover 21 includes a cylindrical cover body 22 arranged adjacent to the silicon member 19 without contacting the upper surface 19a, a transparent glass plate 23 installed on the cover body 22 so as to close the upper end opening 22a, and a gas supply pipe 24 for supplying argon gas into the cover body 22.

[0045] The irradiation unit cover 21 moves integrally with the laser irradiation device 20, which moves horizontally to scan the laser beam as shown by arrow E in FIG. 5( b) while argon gas is supplied to the interior thereof via a gas supply pipe 24. As a result, the laser beam irradiation portion is always surrounded by argon gas while the silicon member 19 is being welded. Note that another irradiation unit cover 21 may be disposed on the lower surface 19b of the silicon member 19 to prevent gases other than argon gas present on the lower surface 19b of the silicon member 19 from flowing into the upper surface 19a of the silicon member 19 where the laser beam irradiation portion is located. Furthermore, when the silicon member is a silicon core wire 9, it is preferable to provide the irradiation unit cover 21 so as to surround the butted portion 10.

[0046] The present invention can be used to manufacture gate-shaped silicon core structures to be installed in reactors in the Siemens process, and plate-shaped bodies to be used as flooring materials for curing silicon.

[0047] DESCRIPTION OF SYMBOLS 1...Reactor 2...Silicon core structure 3...Bottom plate 3a...Gas supply port 3b...Gas exhaust port 4...Cover 5...Electrode 5a...Cable 6...Power supply unit 7...Polycrystalline silicon rod 8...Silicon core 8a...First surface 8b...Second surface 8c...Third surface 8d...Bottom surface 8e...End surface 8f...Boundary 9...Silicon core 9a...First surface 9b...Second surface 9c...Third surface 9d...Bottom surface 9e...End surface 9f...Boundary 10...Butt joint 11...Irradiation range 12...Melted portion 13...Depression 14...Silicon plate-like body 14a...First surface 14b...Second surface 14c...Third surface 14d...Bottom surface 14e...End surface 14f...Boundary 15...Silicon plate-like body 15a...first surface 15b...second surface 15c...third surface 15d...bottom surface 15e...end surface 15f...boundary 16...butt joint 17...irradiation range 18...molten part 19...silicon member 19a...upper surface 19b...lower surface 20...laser irradiation device 21...irradiation unit cover 22...cover body 22a...upper end opening 23...glass plate 24...gas supply pipe a １ ...Longest distance a ２ …Longest distance b…Distance

Claims

1. A method for welding the butt joints of two silicon members using a laser beam, characterized in that the laser beam is irradiated onto the butt joints by thermal conduction welding, melting the silicon until it reaches the bottom of the silicon members.

2. The method for welding silicon members according to claim 1, characterized in that the silicon member is a silicon core wire, and the laser light is irradiated in a long, narrow irradiation range along the axial direction of the silicon core wire that straddles the butt joint portion of the silicon core wire and has an end face located at the butt joint portion.

3. The method for welding silicon members according to claim 1, wherein the silicon members are silicon plates, and the laser light is irradiated in a long, narrow irradiation range that straddles the butted portion of the silicon plates and is parallel to the end faces located at the butted portion.

4. A silicon welded body having a butt welded portion, characterized in that, when a is the longest distance from the point that formed the end face before melting to the boundary of the molten portion measured parallel to the direction perpendicular to the end face, and b is the maximum penetration depth in the molten portion, a ≧ b.

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