BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a crystal growth technique of a polycrystalline silicon rod, and more specifically relates to a technique for producing a polycrystalline silicon rod suitable as a raw material for production of single-crystalline silicon.

Description of the Related Art

Single-crystalline silicon essential for manufacturing a semiconductor device and the like is grown as a crystal according to a CZ method or an FZ method, and a polycrystalline silicon rod or a polycrystalline silicon mass is used as a raw material in such a case. Such a polycrystalline silicon material is often produced according to a Siemens method. The Siemens method is a method including bringing a silane raw material gas of trichlorosilane, monosilane or the like into contact with a silicon core heated, thereby growing polycrystalline silicon on the surface of the silicon core by vapor-phase epitaxy (deposition) according to a CVD (Chemical Vapor Deposition) process.

For example, when single-crystalline silicon is grown as a crystal according to a CZ method, a polycrystalline silicon mass is charged in a quartz crucible, and heated and molten to provide a silicon melt, a seed crystal is dipped in the silicon melt to allow a dislocation line to disappear for no dislocation, and thereafter a crystal is pulled up while the diameter thereof is gradually increased until a predetermined diameter is achieved. If unmolten polycrystalline silicon here remains in the silicon melt, such an unmolten polycrystalline piece drifts in the vicinity of the solid-liquid interface by convection, thereby causing the occurrence of dislocation to be induced, to result in disappearance of a crystal line.

Japanese Patent Laid-Open No. 2008-285403 reports the following: a needle crystal may be deposited in a polycrystalline silicon rod in a step of producing the rod according to a Siemens method, and, if such a polycrystalline silicon rod is used to grow single-crystalline silicon according to a FZ method, an individual crystallite is not uniformly molten depending on the size thereof due to the above-described heterogeneous fine structure, and an unmolten crystallite as a solid particle passes through a molten zone into a single-crystalline rod and is thus incorporated as an unmolten particle into the solidified surface of a single crystal, thereby resulting in the occurrence of defect formation. See also Japanese Patent Laid-Open No. 2012-193902 and Japanese Patent Laid-Open No. 2014-028747, with respect to the needle crystal.

The “needle crystal” referred herein means a needle crystal whose long axis direction corresponds to the deposition direction of the polycrystalline silicon rod (direction vertical to the long axis direction of the polycrystalline silicon rod). The length in the long axis direction of the needle crystal is up to about several nm. Such a needle crystal is considered to be formed by integration due to interconnection of a locally heterogeneous crystal generated in the course of deposition of polycrystalline silicon, with progression of a deposition step. If a polycrystalline silicon rod in which such locally heterogeneous crystal and needle crystal are present is used to perform crystal growth of single-crystalline silicon according to an FZ method, the locally heterogeneous crystal and the needle crystal float in the silicon melt, resulting in failure of crystal growth. Therefore, there is demanded a growth technique of a polycrystalline silicon rod including no locally heterogeneous crystal and no needle crystal. In comparison of Miller indices, a needle crystal crystallographically has a feature where <220> is dominant over <111> (the amount of detection is larger in a <220> in comparison of the amount of detection by X-ray diffraction.), and <111> is dominant in a region where no needle crystal is present.

Japanese Patent Laid-Open No. 2016-150885 reports the following: a locally heterogeneous crystal having a principal plane of <111> is easily generated at the center portion of a polycrystalline silicon rod (portion closer to a core). If the crystal grain size is measured by EBSD, there cannot be obtained any information on heterogeneity of a locally heterogeneous crystal, while even the crystal orientation forming the heterogeneous crystal, besides the outer appearance of the heterogeneous crystal, is determined. The best method for detecting a locally heterogeneous crystal is a method where etching by an aqueous solution of hydrofluoric acid and nitric acid mixed is performed and observation with an optical microscope is made. In observation at a magnification of about 100 times with an optical microscope, a locally heterogeneous crystal can be often confirmed as a crystal portion having a longer diameter of 10 μm or more.

SUMMARY OF THE INVENTION

Of course, single-crystalline silicon grown in either case of a CS method or an FZ method is increased in the diameter thereof, and the diameter thereof is currently predominantly from 6 inches to 8 inches. According to such an increase in the diameter, adverse effects (disappearance of a crystal line, warpage or disorder of a crystal line, and the like) caused by a heterogeneous site present in a polycrystal have been remarkably generated in the step of single-crystallization. Therefore, polycrystalline silicon for use as a raw material for production has been increasingly demanded to have higher crystal homogeneity than ever. Specifically, such polycrystalline silicon is demanded to include neither a needle crystal nor a locally heterogeneous crystal.

The present invention has been made in view of such problems, and an object thereof is to provide polycrystalline silicon suitable as a raw material for production of single-crystalline silicon, resulting in contribution to stable production of single-crystalline silicon.

In order to achieve the object, the polycrystalline silicon rod according to the present invention is a polycrystalline silicon rod grown by deposition according to a chemical vapor deposition process, wherein the polycrystalline silicon rod does not include any needle crystal having a shape where a grain size dvin a direction vertical to a long axis direction of the polycrystalline silicon rod is larger than a grain size dpin a direction in parallel with the long axis direction of the polycrystalline silicon rod (dv>dp).

In addition, the polycrystalline silicon rod according to the present invention is a polycrystalline silicon rod grown by deposition according to a chemical vapor deposition process, wherein, when a surface of a plate-like sample collected so that a direction vertical to a long axis direction of the polycrystalline silicon rod corresponds to a principal plane direction is etched by a mixed liquid of hydrofluoric acid and nitric acid, the etched surface does not include any locally heterogeneous crystal having a grain size of 10 μm or more.

In addition, the polycrystalline silicon rod according to the present invention is a polycrystalline silicon rod grown by deposition according to a chemical vapor deposition process, wherein the polycrystalline silicon rod has a diameter (2R) of 150 mm or more, and, when an X-ray diffraction chart obtained by in-plane rotation with a center of a plate-like sample collected from each of a center region, an R/2 region and an outer region of the polycrystalline silicon rod, as a center of rotation, at an angle φ of 180 degrees is determined, a degree of variation in diffraction intensity from a <220> plane satisfies 0.15 or less in the center region, 0.30 or less in the R/2 region and 0.58 or less in the outer region in evaluation as a 6σn-1/average value, wherein σn-1represents a standard deviation.

Preferably, the degree of variation in diffraction intensity from the <220> plane satisfies 0.12 or less in the center region, 0.30 or less in the R/2 region and 0.54 or less in the outer region.

More preferably, the degree of variation in diffraction intensity from the <220> plane satisfies 0.09 or less in the center region, 0.15 or less in the R/2 region and 0.20 or less in the outer region.

Further preferably, the degree of variation in diffraction intensity from the <220> plane satisfies 0.08 or less in the center region, 0.10 or less in the R/2 region and 0.10 or less in the outer region.

In addition, the method for producing a polycrystalline silicon rod according to the present invention is a method for producing a polycrystalline silicon rod having a diameter of 150 mm or more by deposition according to a chemical vapor deposition process, wherein a D/L value is set within the range of less than 0.40 when multiple pairs of silicon cores are placed in a reaction furnace and it is assumed that an average value of a final diameter of the polycrystalline silicon rod is defined as D (mm) and a mutual interval between the multiple pairs of silicon cores is defined as L (mm).

Preferably, a reaction pressure in a deposition step of polycrystalline silicon is set to 0.2 MPa or more.

The polycrystalline silicon rod according to the present invention can be used to perform crystal growth according to an FZ method and/or crystal growth by use of a mass obtained from a polycrystalline silicon block according to a CZ method, thereby allowing local generation of a partial melt residue to be suppressed, to result in contribution to stable production of single-crystalline silicon.

