Silicon seed rod assembly of polycrystalline silicon, method of forming the same, polycrystalline silicon producing apparatus, and method of producing polycrystalline silicon

1. Field of the Invention

The present invention relates to a silicon seed rod assembly used for producing polycrystalline silicon in a polycrystalline silicon reaction furnace, and a method of forming the same. Further, the present invention relates to a polycrystalline silicon producing apparatus employing the silicon seed rod assembly and a method of producing polycrystalline silicon.

Priority is claimed on Japanese Patent Application No. 2007-338409, filed Dec. 28, 2007, the content of which is incorporated herein by reference.

2. Description of the Related Art

In general, the Siemens method is known as a method of producing high-purity polycrystalline silicon used as a semiconductor material. The Siemens method is a method of making raw gas composed of mixed gas of chlorosilane and hydrogen to contact with a heated silicon seed rod assembly and of making polycrystalline silicon deposited on a surface of the silicon seed rod assembly by means of a reaction of the raw gas. In an apparatus for adopting the Siemens method, a polycrystalline silicon reaction furnace is used in which a plurality of silicon seed rod assemblies is uprightly provided in a hermetic reaction furnace (see Japanese Patent No. 3819252 and Japanese Patent No. 3881647).

Each silicon seed rod assembly includes two rod-shaped silicon seed rods and a connection member connecting the upper end portions of the silicon seed rods to each other, thereby forming the silicon seed rod assembly in a Π-shape. Then, the lower end portion of the silicon seed rod is fixed to an electrode provided in a bottom of the polycrystalline silicon reaction furnace. The silicon seed rod assembly is heated by supplying current from the electrode at the lower end portion thereof. Then, the raw gas is thermally decomposed or is reduced with hydrogen by joule heat produced at this time, thereby making polycrystalline silicon deposited on the surface of the silicon seed rod assembly.

A section of the rod-shaped silicon seed rod is formed in a square shape, a circular shape, or a shape in which angular portions of a square shape are R-chamfered. In general, since it is necessary to make the upper end portion to be inserted through a through-hole having an annular-hole shape and to support the connection member, the upper end portion connected to the connection member is formed in a cylindrical shape so as to have a diameter slightly smaller than that of the through-hole.

That is, a step portion is formed between an upper end portion and a main body portion of the silicon seed rod since a width of the upper end portion is smaller than that of the main body portion. The step portion is provided with a flat support surface disposed in a direction perpendicular to a longitudinal direction of the silicon seed rod. Then, the upper end portions of the two silicon seed rods are inserted through the through-holes formed in both end portions of the connection member, and the support surface supports an opening-end peripheral edge of the through-hole on the side of the lower surface of the connection member, thereby forming the silicon seed rod assembly.

Incidentally, upon forming the through-hole in the connection member, the through-hole is formed by perforating the connection member by means of a drill from the one side surface toward the other side surface thereof. However, a petal-shape defect portion may be formed in the opening-end peripheral edge on the side of the other side surface in a perforation direction due to brittleness of the connection member. In a case where the silicon seed rod assembly is formed so that the other side surface having the defect portion formed thereon comes into contact with the support surface, a contact area between the connection member and the silicon seed rod decreases.

In a case where a contact state between the silicon seed rod and the connection member is poor, a problem arises in that a meltdown of the contact portion may occur due to local overheating during an ohmic heating. In order to obtain a satisfactory contact state between the silicon seed rod and the connection member, a study has been carried out which improves perpendicularity upon connecting the silicon seed rod and the connection member to each other in order to obtain a satisfactory contact between the silicon seed rod and the connection member. However, a countermeasure for coping with a contact between the support surface of the silicon seed rod and the opening-end peripheral edge of the through-hole of the connection member has not been taken. For this reason, in order to prevent the meltdown caused by overheating, it is required to satisfactorily ensure the contact area therebetween.

The present invention is contrived in consideration of the above-described circumstances, and an object of the invention is to provide a silicon seed rod assembly capable of satisfactorily ensuring a contact area between a silicon seed rod and a connection member and of producing polycrystalline silicon through a reliable process, and a method of forming the silicon seed rod assembly. Further, an object of the invention is to provide a polycrystalline silicon producing apparatus using the silicon seed rod assembly and a method of producing polycrystalline silicon.

In order to achieve the above-described object, the present invention adopts the following configuration. That is, according to an aspect of the invention, there is disclosed a silicon seed rod assembly used for producing polycrystalline silicon by means of a vapor deposition method, the silicon seed rod assembly including: two rod-shaped silicon seed rods; and a silicon connection member bridging the silicon seed rods, wherein a width of an upper end portion of each silicon seed rod is smaller than that of a main body portion thereof, wherein a step portion formed between the upper end portion and the main body portion is provided with a support surface disposed in a direction perpendicular to a longitudinal direction of the upper end portion, wherein the connection member is provided with a through-hole into which the upper end portion is inserted, wherein an opening-end peripheral edge of the through-hole on one side surface of the connection member is sharper than that on the other side surface thereof, and an opening-end peripheral surface on the one side surface thereof is formed into a flat contact surface disposed in a direction perpendicular to a perforation direction of the through-hole, and wherein the upper end portion is inserted into the through-hole so that the contact surface comes into contact with the support surface of the silicon seed rod.

According to another aspect of the invention, there is disclosed a method of forming a silicon seed rod assembly which is used for producing polycrystalline silicon by means of a vapor deposition method and includes two rod-shaped silicon seed rods and a silicon connection member bridging the silicon seed rods, the method including the steps of: forming an upper end portion, of which a width is smaller than that of a main body portion, at an upper end of each silicon seed rod; forming a support surface at a step portion between the upper end portion and the main body portion so as to be disposed in a direction perpendicular to a longitudinal direction of the upper end portion; forming a through-hole by perforating the connection member from one side surface thereof by means of a drill in a direction perpendicular to the one side surface; and inserting the upper end portion into the through-hole so that the one side surface comes into contact with the support surface of the silicon seed rod.

According to the silicon seed rod assembly and the method of forming the same according to the invention, the opening-end peripheral edge of the through-hole on one side surface of the connection member is sharper than that on the other side surface thereof, the opening-end peripheral surface on the one side surface thereof is formed into the flat contact surface, the support surface of the silicon seed rod comes into contact with the contact surface, and then the connection member is supported by the two silicon seed rods, thereby forming the silicon seed rod assembly. Accordingly, since it is possible to satisfactorily ensure the contact area between the silicon seed rod and the connection member, it is possible to restrict an increase in the contact resistance therebetween and to prevent the meltdown of the contact portion caused by overheating during an ohmic heating.

Further, in the silicon seed rod assembly according to the invention, the connection member may include an inclined surface of which a width gradually decreases from the downside to the upside. Accordingly, the connection member is formed in a shape in which silica or foreign material flowing from the above of the connection member is hardly collected or attached thereto upon producing the polycrystalline silicon, thereby producing highly pure polycrystalline silicon.

According to still another aspect of the invention, there is disclosed a polycrystalline silicon producing apparatus including: the silicon seed rod assembly according to the above-described aspect provided in a reaction furnace, wherein at least a pair of electrodes is provided in a bottom portion in the reaction furnace, and end portions, disposed on the opposite side of the connection member, of both silicon seed rods of the silicon seed rod assembly are supported by the electrodes, respectively.

Since local overheating of the silicon seed rod assembly is prevented, it is possible to reliably produce polycrystalline silicon.

According to still another aspect of the invention, there is disclosed a method of producing polycrystalline silicon including the steps of: forming a silicon seed rod assembly by uprightly supporting silicon seed rods by a pair of electrodes, respectively, disposed in a bottom portion in a reaction furnace and by providing a silicon connection member between upper end portions of the silicon seed rods; heating the silicon seed rods by supplying current to the electrodes; reacting the raw gas supplied to the reaction furnace; and depositing polycrystalline silicon on surfaces of the silicon seed rods, wherein the step of forming the silicon seed rod assembly includes the steps of: forming an upper end portion, of which a width is smaller than that of a main body portion, at an upper end of each silicon seed rod and forming a support surface at a step portion between the upper end portion and the main body portion so as to be disposed in a direction perpendicular to a longitudinal direction of the upper end portion; forming a through-hole by perforating the connection member from one side surface thereof by means of a drill in a direction perpendicular to the one side surface; and inserting the upper end portion into the through-hole so that the one side surface comes into contact with the support surface of the silicon seed rod in a state where the silicon seed rods are supported by the electrodes provided in the reaction furnace, respectively.

According to the invention, since it is possible to satisfactorily ensure the contact area between the silicon seed rod and the connection member, it is possible to restrict an increase in the contact resistance therebetween and to reliably produce the polycrystalline silicon.