Polysilicon Rods

Abstract

The present invention provides a polycrystalline silicon rod suitable as a raw material for producing single crystal silicon. Wafers (evaluation samples) are collected from polycrystalline silicon rods grown by the Siemens method, and polycrystalline silicon rods in which grains with a grain size of less than 100 nm account for more than 3% by area ratio are screened out as raw materials for producing single crystal silicon. When such polycrystalline silicon rods are used as raw materials and single crystal silicon is grown by the FZ method, the occurrence of dislocations can be significantly suppressed.

Description

Technical Field

[0001] The present invention relates to a polycrystalline silicon rod grown by a Siemens process, and in particular to a polycrystalline silicon rod suitable as a raw material for producing single crystal silicon based on a floating zone melting method (FZ method). Background Art

[0002] Polycrystalline silicon is used as a raw material for single crystal silicon for semiconductors or silicon for solar cells. As a method for producing polycrystalline silicon, the Siemens method is widely known. The Siemens method is generally a method of depositing polycrystalline silicon on the surface of a heated silicon core wire by contacting a silane raw material gas with the heated silicon core wire using a CVD (Chemical Vapor Deposition) method.

[0003] When manufacturing polycrystalline silicon rods using the Siemens method, silicon core wires are assembled into a torii shape (inverted U shape) with two wires in the vertical direction and one wire in the horizontal direction, and then both ends of the torii-shaped silicon core wires are connected to core wire holders and fixed to a pair of metal electrodes arranged on a substrate. Usually, a structure in which multiple groups of torii-shaped silicon core wires are arranged in a reactor is adopted.

[0004] When the torii-shaped silicon core wire is heated to the precipitation temperature by energizing, and a mixed gas of, for example, trichlorosilane and hydrogen is brought into contact with the silicon core wire as a raw material gas, silicon is vapor-phase grown, and polycrystalline silicon of a desired diameter is formed into an inverted U shape.

[0005] In the production of single crystal silicon by the CZ method, a polycrystalline silicon block obtained by crushing the inverted U-shaped polycrystalline silicon rod is filled in a quartz crucible, and a seed crystal is brought into contact with a heated and melted silicon melt to grow a single crystal silicon ingot.

[0006] It is known that when a silicon single crystal is grown by the CZ method, if unmelted polycrystalline silicon remains floating in the silicon melt, this becomes a cause of dislocation.

[0007] In the production of single crystal silicon using the FZ method, both ends of two vertically extending polycrystalline silicon portions of the polycrystalline silicon formed into the inverted U shape are cut to form cylindrical polycrystalline silicon rods, which are used as raw materials.

[0008] Patent document 1 (Japanese Patent Publication No. 2008-285403) reports that: when manufacturing polycrystalline silicon rods, needle-shaped crystals as heterogeneous microstructures are sometimes precipitated. It is reported that when a polycrystalline silicon rod having precipitated such needle-shaped crystals is used as a raw material to grow a single crystal by the FZ method, each microcrystal is not uniformly melted due to the heterogeneous microstructure of the polycrystalline silicon rod, and the unmelted microcrystals pass through the melting zone and reach the solid-liquid interface of the growing single crystal silicon, thereby causing dislocation.

[0009] Once dislocation occurs, single crystal silicon ingots can no longer be obtained, resulting in a decrease in manufacturing yield.

[0010] As can be seen from the above, in the methods disclosed in Patent Documents 1 and 2 (Japanese Patent Application Laid-Open No. 2014-28747), a scheme for suppressing the generation of needle-shaped crystals is proposed.

[0011] In addition, for example, in Patent Document 3 (Japanese Patent Gazette No. 2013-193902), an invention of a polycrystalline silicon rod preferably used as a raw material for manufacturing single crystal silicon using the CZ method or a raw material for manufacturing silicon ingots using a casting method is disclosed. It is reported that by coarsening the grains, the energy required for melting tends to be reduced.

[0012] Furthermore, Patent Document 4 (Japanese Patent Application Publication No. 2014-31297) and Patent Document 5 (Japanese Patent Application Publication No. 2017-057093) disclose inventions for suppressing defect formation during growth of single crystal silicon by setting the range of crystal grain size of polycrystalline silicon rods to an appropriate level.

[0013] Prior art literature

[0014] Patent Literature

[0015] Patent Document 1: Japanese Patent Application Publication No. 2008-285403

[0016] Patent Document 2: Japanese Patent Application Publication No. 2014-28747

[0017] Patent Document 3: Japanese Patent Application Publication No. 2013-193902

[0018] Patent Document 4: Japanese Patent Application Publication No. 2014-31297

[0019] Patent Document 5: Japanese Patent Application Publication No. 2017-057093 Summary of the invention

[0020] Problems to be solved by the invention

[0021] However, studies by the present inventors have revealed that the measures described in these prior art documents are difficult to reliably suppress the occurrence of dislocations during growth of single crystal silicon by the FZ method or the CZ method.

[0022] An object of the present invention is to provide a polycrystalline silicon rod capable of significantly suppressing the occurrence of dislocations during the production of single crystal silicon, particularly during the production of single crystal silicon using the FZ method.

[0023] Methods used to solve problems

[0024] In order to solve the above-mentioned problems, the polycrystalline silicon rod of the present invention is a polycrystalline silicon rod grown by the Siemens method, characterized in that when wafers are collected from areas other than the silicon core wire part and evaluated, grains with a particle size of less than 100 nm account for more than 3% in terms of area ratio.

[0025] The wafer is, for example, a wafer whose main surface is oriented in the axial direction of the silicon core.

[0026] In addition, the wafer is, for example, a wafer whose main surface is oriented in a radial direction perpendicular to the axial direction of the silicon core wire.

[0027] In the polycrystalline silicon rod, the crystal grains having a size of 100 nm or less preferably account for 6% or more in terms of area ratio.

[0028] The polycrystalline silicon rods are, for example, polycrystalline silicon rods grown using trichlorosilane or monosilane as raw materials.

[0029] Effects of the Invention

[0030] When single crystal silicon is grown using the polycrystalline silicon rod of the present invention as a raw material, the occurrence of dislocations can be significantly suppressed, and the polycrystalline silicon rod is particularly suitable as a raw material for single crystal silicon production by the FZ method. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic explanatory diagram showing an example of collecting a sample for observation cut out from a polycrystalline silicon rod.

[0032] Figure 2 This is a photograph of the polycrystalline silicon rod A observed with an optical microscope.

[0033] Figure 3 This is a photograph of the polycrystalline silicon rod of rod E observed with an optical microscope.

[0034] Explanation of symbols

[0035] 1 Silicon core wire

[0036] 10 Polysilicon Rods

[0037] 21, 22 Evaluation sample (wafer) DETAILED DESCRIPTION

[0038] Here, the polycrystalline silicon rod of the present invention is described with reference to the drawings. Here, it is assumed that the polycrystalline silicon rod of the present invention is studied for the production of single crystal silicon by the FZ method and the research results are reported, but the polycrystalline silicon rod of the present invention is also suitable as a raw material for the production of single crystal silicon by the CZ method.

[0039] The present inventors have been studying the subject of what materials are suitable for polycrystalline silicon rods as raw materials for producing single crystal silicon, and initially focused on large grains and needle-shaped crystals of 500 nm or more, which are the main causes of dislocations.

[0040] However, further studies have revealed that even if the content ratio of relatively large grains and needle-shaped crystals is set within a specific range, the probability of dislocation occurrence during growth of single crystal silicon using the FZ method is not necessarily low.

[0041] It is known that, in particular, when growing single crystal silicon using the FZ method, even if there are no crystallites with a large grain size of 500 nm or more in the polycrystalline silicon rod as a raw material, the melting of the polycrystalline silicon does not become uniform, and dislocations frequently occur. Therefore, the inventors of the present invention simply focused on smaller grains and needle-shaped crystals and conducted research.

[0042] It should be noted that, here, regarding the crystal grain size, for example, when an optical microscope is used for observation, it is calculated as the average value of the diameter in the minor axis direction and the diameter in the major axis direction. In addition, for example, when an electron backscatter diffraction determination method (EBSD method) is used for evaluation, as described in Patent Document 4, the area of ​​each crystal grain detected by the analysis of the electron backscatter diffraction image is obtained, and is defined by the diameter of a circle having the area.

[0043] Previous studies have shown that when single crystal silicon is grown using polycrystalline silicon rods as raw material in which the crystal grain size of which is 100 nm or less, as calculated by analysis using the electron backscatter diffraction method (EBSD method) or observation under an optical microscope, accounts for more than 3% of the area, the probability of dislocation is significantly reduced.

[0044] The inventors consider the following mechanism as the reason, namely, when the crystal grains with a crystal grain size of less than 100 nm exist in an area ratio of more than 3%, the smaller-sized grains will be interspersed in the grain boundaries of relatively larger grains, and the adhesion between the grains will increase. As a result, after the polycrystalline silicon rod is melted, the relatively larger grains will float alone in the silicon melt, which can suppress the induced dislocation.

[0045] Figure 1This is a schematic diagram showing an example of collecting a sample for observation cut from a polycrystalline silicon rod 10. When calculating the area ratio of grains with a grain size of 100 nm or less, a wafer 21 with the axial direction (H direction) of the silicon core 1 as the main surface direction can also be used, and a wafer 22 with the radial direction (V direction) perpendicular to the axial direction of the silicon core 1 as the main surface direction can also be used. It should be noted that the method of collecting the wafer is not limited to these methods, and the direction of the main surface of the wafer is an arbitrary direction. However, this wafer is collected from an area other than the silicon core 1 part.

[0046] The raw material used when growing the polycrystalline silicon rod is not particularly limited, but monosilane or trichlorosilane is preferably used.

[0047] When monosilane is used as a raw material gas (silane-based gas) for producing polycrystalline silicon, hydrochloric acid is not produced when silicon is produced, and the CVD temperature is reduced. When monosilane is used, nucleation is easily generated, and polycrystalline silicon with a small crystal grain size can be grown.

[0048] On the other hand, when trichlorosilane is used as the raw material gas for the manufacture of polycrystalline silicon, the generation of Si powder produced by thermal decomposition when monosilane is used as the raw material can be suppressed. By controlling the concentration of the raw material gas and the growth temperature, the crystal grain size can be controlled more easily, and polycrystalline silicon with a small crystal grain size can be cultivated.

[0049] Example

[0050] [Experiment 1]

[0051] First, five types of polycrystalline silicon rods (rods A to E) were prepared using trichlorosilane as a raw material by the Siemens method under different precipitation conditions, and wafers (evaluation samples) with the main surface in the radial direction perpendicular to the axial direction of the silicon core were collected from each of these polycrystalline silicon rods.

[0052] Each of these samples was analyzed by the EBSD method (magnification: 5000 times), and the crystal grain size distribution was obtained from the obtained image. Figure 2 and Figure 3 The following are photos of polycrystalline silicon rods A and E observed with an optical microscope. Table 1 summarizes the crystal grain size distribution (crystal grain size distribution by area ratio) of each sample and the "yield" when the polycrystalline silicon rods are used as raw materials for single crystal silicon production by the FZ method.

[0053] The "yield rate" mentioned here refers to the ratio of the length to the position where the dislocation occurs when the length after the single crystal is formed without dislocation is set as 100% when the single crystal is formed without dislocation. It should be noted that, for rods A to E, 5 polycrystalline silicon rods are prepared for each, and 5 batches of single crystal silicon are grown using the 5 polycrystalline silicon rods, and the "yield rate" is calculated based on the average value.

[0054] Table 1

[0055]

[0056] As shown in Table 1, the yield of rods (A and B) in which the area ratio of grains with a grain size of 100 nm or less was 3% or more exceeded 80%, and the yield of rod A, which was 9%, was 100%.

[0057] [Experiment 2]

[0058] In addition to the above-mentioned rods A and B, three types of polycrystalline silicon rods (rods F to H) were prepared using trichlorosilane as a raw material by the Siemens method under different precipitation conditions. From these five types of polycrystalline silicon rods, wafers (evaluation samples) were collected with the main surface in a direction different from that in "Experiment 1", i.e., the axial direction of the silicon core wire as the main surface.

[0059] Each of these samples was analyzed using the EBSD method (magnification 5000 times), and the crystal grain size distribution was obtained from the obtained image. Table 2 summarizes the crystal grain size distribution (crystal grain size distribution by area ratio) of each of these samples and the "yield rate" when the polycrystalline silicon rod is used as the raw material for single crystal silicon production using the FZ method. It should be noted that the meaning of "yield rate" is as described above.

[0060] Table 2

[0061]

[0062] As shown in Table 2, for rods (A, B, F) in which grains with a diameter of 100 nm or less accounted for 3% or more in terms of area ratio, the yield exceeded 80%, and the yield of rod A, which had a value of 6%, was 100%.

[0063] That is, it was found that when a wafer was collected from a region other than the silicon core portion for evaluation, a polycrystalline silicon rod in which crystal grains having a grain size of 100 nm or less accounted for 3% or more in terms of area ratio showed an extremely high yield.

[0064] In addition, it was found from the above-mentioned rods A and B that: due to the different collection directions of the wafer, there were slight differences in the area ratio of grains with a particle size of less than 100 nm. However, even in this case, the polycrystalline silicon rods in which the area ratio of grains less than 100 nm accounted for more than 6% also showed a yield of 100%.

[0065] As described above, the polycrystalline silicon rod of the present invention is suitable as a raw material for producing single crystal silicon.

[0066] The present invention is also a method for selecting polycrystalline silicon rods suitable as a raw material for producing single crystal silicon.

[0067] That is, a method for screening polycrystalline silicon rods grown using the Siemens method, wherein polycrystalline silicon rods in which grains with a grain size of less than 100 nm account for more than 3% by area when wafers are collected from areas other than the silicon core wire portion for evaluation are screened as raw materials for single crystal silicon manufacturing.

[0068] Industrial Applicability

[0069] According to the present invention, a polycrystalline silicon rod suitable as a raw material for producing single crystal silicon can be provided.

Claims

1. A method for screening polycrystalline silicon rods, which is a method for screening polycrystalline silicon rods grown by Siemens method as raw materials for single crystal silicon production, characterized in that: Polycrystalline silicon rods in which crystal grains having a grain size of 100 nm or less account for 3% or more in terms of area ratio when wafers are collected from regions other than the silicon core portion and evaluated are screened for use as raw materials for single crystal silicon production.

2. The method for screening polycrystalline silicon rods according to claim 1, wherein: The wafer has the axial direction of the silicon core as the main surface direction.

3. The method for screening polycrystalline silicon rods according to claim 1, wherein: The wafer has a main surface oriented in a radial direction perpendicular to the axial direction of the silicon core wire.

4. The method for screening polycrystalline silicon rods according to any one of claims 1 to 3, wherein: When a wafer is collected from a region other than the silicon core portion and evaluated, polycrystalline silicon rods are selected in which crystal grains having a grain size of 100 nm or less account for 6% or more in terms of area ratio.

5. The method for screening polycrystalline silicon rods according to any one of claims 1 to 3, wherein: The polycrystalline silicon rod is a polycrystalline silicon rod grown using trichlorosilane or monosilane as a raw material.

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