JP5443247B2 - Method for producing polycrystalline silicon - Google Patents

Description

  The present invention relates to a method for producing polycrystalline silicon by refining metallic silicon, and in particular, melting metallic silicon containing nickel, purifying the obtained molten silicon by a unidirectional solidification method, and obtaining high-purity polycrystalline silicon. It relates to a method of manufacturing.

  In recent years, crystalline solar cells have attracted attention as fossil fuel alternative energy because the conversion efficiency is as high as 30% for the single crystal type and 20% for the polycrystalline type. The semiconductor substrate wafer necessary for manufacturing these crystalline solar cells is manufactured from a single crystal silicon ingot or a polycrystalline silicon ingot, and these single crystal silicon ingot or polycrystalline silicon ingot is manufactured from polycrystalline silicon. However, for polycrystalline silicon used in solar cells, the smaller the amount of impurity metal such as heavy metal contained, the better.

  Therefore, conventionally, as a method of removing impurity metals such as heavy metals in the metal silicon used as a raw material, the metal silicon is once melted in the crucible, and the obtained molten silicon is cooled from the crucible bottom side and moved upward. By solidifying in one direction, the impurity metal is refined while removing it into the liquid phase using the difference in the solid / liquid partition coefficient, thereby removing the impurity metal as much as possible. A unidirectional solidification method has been carried out to create a columnar crystal structure.

  For example, Patent Document 1 describes that unidirectional solidification is performed while maintaining the temperature of liquid-phase unsolidified molten silicon at 1425 ° C. or higher. Patent Document 2 discloses a container having a purity of 3N (99.9% by mass) or more and 6N (99.9999% by mass) or less and containing fine silica in the inner peripheral layer in order to obtain metallic silicon having an average crystal grain size of 1 mm. Describes that the molten crude metal silicon is unidirectionally solidified at a speed of 1 mm / min or less. Furthermore, Non-Patent Document 1 also reports that the unidirectional solidification method is useful as a method for purifying metal silicon.

Here, when refining metallic silicon containing an impurity metal by a unidirectional solidification method, it is known that the concentration distribution of each impurity metal in the silicon block obtained by the unidirectional solidification follows the following formula (1). .
C = k · C ₀ (1−f) ^k−1 (1)
C: Impurity concentration in solidified phase C ₀ : Initial impurity concentration of molten metal before solidification k: Partition coefficient f: Ratio of solidified phase

  In this equation (1), the distribution coefficient k is determined by the type of impurity metal (element), the apparatus for casting molten silicon, and the casting conditions, and the average concentration of the impurity element in the obtained silicon block is as follows. Since the concentration distribution can be found from the equation (1), if it is determined how much the portion (impurity enriched layer) where the impurity is concentrated in the silicon block is to be removed, the polycrystalline silicon manufactured by calculating by weighted average Can be determined.

  However, in the above-described prior art or the like, when producing silicon by refining metal silicon by the unidirectional solidification method, a silicon block having a concentration distribution according to the formula (1) cannot be obtained only for nickel. When the amount of impurity nickel in the metal silicon is large, there is a problem that it is difficult to efficiently produce high-purity polycrystalline silicon of 99.9999% by mass (6N) or more as a result.

Japanese Unexamined Patent Publication No. 5-147918 JP 2008-81394

9th roceedings of the International Conference (1989) 462-465.

  Therefore, the present inventors examined in detail the cause of this problem [that is, the nickel concentration in the silicon block purified by the unidirectional solidification method does not show a concentration distribution according to the formula (1)]. In the following description, the “solidification rate” means the solidification direction height of the silicon block when all molten silicon solidifies when the molten metal silicon is solidified in one direction, and the height It is a value indicating whether or not it has solidified, and is a ratio of the solidified height.

That is, first, the initial concentration of nickel in the metal silicon used as a raw material is set to 5 mass ppm (hereinafter referred to as “ppmw”), and it is refined by a unidirectional solidification method to form a crystal structure. When the distribution coefficient of the distribution coefficient was set to 7.4 × 10 ⁻⁴ and the nickel concentration distribution curve was verified, a solidification rate−nickel concentration relationship as shown in FIG. 1 was obtained. According to the solidification rate-nickel concentration distribution curve shown in FIG. 1, the nickel concentration at the solidification rate of 86% is about 0.03 ppmw, and the nickel concentration of the entire silicon block is about 0.01 ppmw from the weighted average. Therefore, a silicon block having about 1/500 of the initial nickel concentration is obtained. As a result, even when metal silicon having an initial nickel concentration of 5 ppmw is used as a raw material, the yield is 86% (solidification rate is 86%). It can be expected that high purity polycrystalline silicon of 6N or more can be produced.

  However, the actually obtained polycrystalline silicon has a nickel concentration of 0.34 ppmw at a height of 86%, and the nickel concentration is abrupt even if the resulting silicon block has a columnar crystal structure. It was confirmed that the purification effect was significantly impaired.

In the verification experiment of the above solidification rate-nickel concentration, 5 ppmw was assumed as the initial nickel concentration because the maximum value of nickel concentration in the metal silicon used as the raw material is considered to be about 5 ppmw. The effective distribution coefficient was set to 7.4 × 10 ⁻⁴ of the experimentally obtained value, and the nickel concentration at the solidification rate of 86% was examined for the obtained silicon block. This is because it is expensive and the market price is likely to fluctuate. Therefore, it is required to increase the yield as much as possible, and to effectively use at least the position where the solidification rate is 86%.

  Therefore, the present inventors next (1) Nickel has a larger diffusion coefficient than that of other elements for solid silicon, particularly solid silicon at a temperature of 1000 ° C. or more with a boundary of 1000 ° C. [Landolt-Bornstein Zahlenwerte und Funktionen aus Naturwissenschaften und Technik, eds. O. Madelung, M. Schulz and H. Weiss (SpringerVerlag, Berlin, 1984) p.494 It is expected that the degree of nickel contamination due to thermal diffusion is expected to be small even if the silicon is held for a long time, and (3) since the freezing point of silicon is 1414 ° C. Focusing on the fact that the freezing point drop occurs due to the concentration of impurity metal in the impurity concentration layer, and the surface temperature of the generated silicon block at the completion of solidification is about 1400 ° C. Verification to investigate the high temperature exposure time dependence of nickel thermal diffusion from the silicon block surface by changing the time until the surface temperature of the silicon block reaches 1400 ° C to 1000 ° C when lowering the temperature of the silicon block after completion The experiment was conducted.

  As a result of this nickel thermal diffusion verification experiment, the results shown in FIG. 2 were obtained. This experimental result shows that the nickel concentration at the solidification rate of 86% increases as the time until the surface temperature reaches from 1400 ° C. to 1000 ° C. after completion of the solidification of the silicon block increases. Et al., The cause of the above problem, that is, the cause that the nickel concentration in the silicon block purified by the unidirectional solidification method does not show the concentration distribution according to the formula (1) is that the solidification rate on the silicon block surface is 100%. The nickel in the nearby impurity enriched layer diffuses again into the silicon block due to thermal diffusion that occurs in the process of cooling from 1400 ° C. to 1000 ° C. after the completion of the solidification of the silicon block, thereby expanding the region of the impurity enriched layer. I found out that it was caused by that.

  In addition, the inventors of the present invention have made columnar crystal structure, equiaxed crystal structure, dendritic crystal structure, and equiaxed / dendritic crystal structure as the crystal structure of the impurity-concentrated layer immediately below the outermost surface in the unidirectional solidification method. As a result of examining the nickel concentration at the position where the solidification rate is 86%, as shown in FIG. 3 and Table 1 described later, it was found that the degree of thermal diffusion varies depending on the form of the crystal structure immediately below the impurity concentrated layer.

The present invention has been made based on the knowledge obtained from these experiments by the present inventors.
Accordingly, the present invention relates to a method for purifying metallic silicon, and in particular, an object of the present invention is to provide a method for producing high purity polycrystalline silicon of 6N or more by purifying molten silicon containing nickel by unidirectional solidification. It is.

  The present invention solves the above-described problem by the following configuration, and melts metallic silicon containing nickel, purifies the molten metallic silicon by a unidirectional solidification method, and manufactures polycrystalline silicon having a purity of 6N or more. Provide a method.

  [1] A method for producing polycrystalline silicon having a purity of 6N or more by melting metallic silicon containing nickel and purifying the molten metallic silicon by a unidirectional solidification method, which is obtained by the unidirectional solidification method. A method for producing polycrystalline silicon, wherein the surface temperature of the silicon block reaches 1000 ° C. or less within 400 minutes after the solidification rate of the silicon block reaches 100%.

[2] The polycrystalline silicon according to [1], wherein the metal silicon contains nickel having an initial concentration C ₀ , and the nickel concentration of the metal silicon is reduced to 1/500 or less of the nickel initial concentration C _0. Manufacturing method.

  [3] The method for producing polycrystalline silicon according to [1], wherein the silicon block has a silicon crystal structure other than a columnar crystal structure in the impurity concentrated layer.

  [4] A method for producing polycrystalline silicon having a purity of 6N or more by melting metallic silicon containing nickel and purifying the molten metallic silicon by a unidirectional solidification method, which is obtained by the unidirectional solidification method. A method for producing polycrystalline silicon, wherein a silicon crystal structure in an impurity-concentrated layer of a silicon block is formed into a crystal structure other than a columnar crystal structure.

  [5] The method for producing polycrystalline silicon according to [3] or [4], wherein the crystal structure other than the columnar crystal structure is one or both of a dendritic crystal structure and an equiaxed crystal structure.

  According to the present invention, in the purification by the unidirectional solidification method of metallic silicon, the temperature drop time from the completion of solidification is controlled, or the crystal structure immediately below the outermost surface of the silicon block is converted into an equiaxed crystal structure other than columnar crystals and By incorporating into the dendritic crystal structure, contamination due to thermal diffusion of nickel can be reduced, and high purity polycrystalline silicon of 6N or more can be efficiently produced using this nickel concentration as an index.

FIG. 1 is a graph showing the theoretical purification curve in the silicon block and the nickel concentration of the actually obtained production sample when the initial nickel concentration is 5 ppmw, and a solid circle plot indicates the solidification rate of the actually obtained silicon block. The nickel concentration at 86%, and the ◆ plot is the theoretical purification curve when the initial concentration is 5 ppmw.

FIG. 2 is a graph showing the high-temperature exposure time dependence of nickel thermal diffusion at a position where the solidification rate of the silicon block is 86%.

FIG. 3 is an explanatory diagram schematically showing the crystal structure of the silicon block (the boundary of the pattern is set to a solidification rate of 98%), and (a) shows the case where all solidified with a columnar crystal structure, (b) Is the case where the solidification rate is 98% or later solidified with an equiaxed crystal structure, (c) is the case where the solidification rate 98% or later is solidified with a dendritic crystal structure, (d) is the case where the solidification rate 98% or later is equiaxed Each of the cases solidified in a dendritic crystal mixed structure is shown.

FIG. 4 is an explanatory view showing the main part of a resistance heating type silicon block casting furnace used in the method for producing polycrystalline silicon according to the present invention.

  The present invention is a method for producing polycrystalline silicon having a nickel concentration reduced by refining metallic silicon containing nickel by a unidirectional solidification method. From the above findings of the present inventors, the solidification rate of the silicon block is 100%. This is a method for producing polycrystalline silicon characterized in that the surface temperature of the silicon block of about 1400 ° C. is usually reached to 1000 ° C. or less within 400 minutes, preferably within 300 minutes.

  After the solidification to the outermost surface of the silicon block, the longer it is held at high temperature, the more the nickel concentrated in the impurity-concentrated layer just below the outermost surface is thermally diffused in the silicon block obtained by refining by the unidirectional solidification method. Will return to the ingot again, causing nickel contamination. The shorter the time required for lowering the surface temperature of the silicon block at about 1400 ° C. to 1000 ° C. (hereinafter sometimes referred to as “temperature drop time”), the degree of nickel contamination can be reduced. If the amount is too much, the resulting polycrystalline silicon is likely to crack, so the lower limit of the temperature drop time is preferably about 100 minutes.

  In order to make this temperature drop time within 400 minutes, in the present invention, it is necessary to cool the outermost surface of the silicon block and immediately below the outermost surface. As a means for cooling the outermost surface, a method of actively lowering the temperature at a high speed, such as a method of turning off the heater simultaneously with completion of solidification of the silicon block, a flow of inert gas in the furnace A method of cooling the outermost surface, a method of cooling with a water-cooled or gas-cooled plate from the bottom side of the silicon block, and a method of inserting a water-cooled or gas-cooled carbon block into the furnace simultaneously with completion of solidification These methods can be used as appropriate in accordance with the constraints that come from the apparatus, equipment, etc., and some methods can be used in combination if necessary.

  This is because the above-described temperature drop time of the present inventors (time required for the silicon block outermost surface temperature to reach from 1400 ° C. to 1000 ° C .: minutes) and nickel concentration (nickel concentration at the position where the silicon block solidification is 86%: This is based on an experiment that verified the dependence of nickel thermal diffusion on high-temperature exposure time.

  The results obtained in the above-mentioned nickel thermal diffusion verification experiment are as shown in FIG. 2. From FIG. 2, it is necessary to purify metal silicon having an initial nickel concentration of 5 ppmw to a nickel concentration of 1/500 of the initial concentration. The upper limit of the nickel concentration at a height of 86% is required to be 0.15 ppmw or less. Further, as can be seen from FIG. 2, when the temperature drop time exceeds 400 minutes, the nickel concentration exceeds 0.15 ppmw, and the nickel concentration increases in proportion to the time. Therefore, in the present invention, contamination due to nickel thermal diffusion is prevented by causing the silicon block surface temperature to reach 1000 ° C. or less within 400 minutes after the solidification rate of the silicon block reaches 100%. When the temperature drop time exceeds 400 minutes, the nickel concentration at the solidification rate of 86% exceeds 0.15 ppmw, and the nickel concentration increases in proportion to the length of the temperature drop time. It becomes difficult to purify to a nickel concentration of 1/500.

  Here, in the nickel thermal diffusion verification experiment, metallic silicon having an initial nickel concentration of 5 ppmw is used as a raw material. However, as described above, this is because the upper limit of the initial nickel concentration of metallic silicon used as the raw material is limited. This is because it is approximately 5 ppmw, and when using a metal silicon having an initial nickel concentration lower than 5 ppmw as a raw material, the nickel concentration at a solidification rate of 86% is naturally predicted to be lower than 0.15 ppmw. In addition, the silicon block can achieve 6N even if its solidification rate is higher than 86% and can be used effectively, and the yield is further improved. The lower limit of the initial nickel concentration in the metal silicon used as the raw material is not particularly limited, but is about 0.1 ppmw for the purpose of the present invention.

  Further, the present invention is a method for producing polycrystalline silicon having a purity of 6N or more by melting metallic silicon containing nickel and purifying the molten metallic silicon by a unidirectional solidification method, the unidirectional solidification method. The method for producing polycrystalline silicon is characterized in that the silicon crystal structure in the impurity-concentrated layer of the silicon block obtained by the above is formed into a crystal structure other than the columnar crystal structure. This manufacturing method controls the morphology of the crystal structure in the impurity-concentrated layer immediately below the outermost surface of the silicon block, and prevents thermal diffusion of impurity metals in silicon that occurs through the grain boundaries of the crystal structure, especially nickel contamination due to back-diffusion of nickel. It is to prevent as much as possible.

  Since the columnar crystal structure is linear with few crystal grain boundaries, nickel is likely to be thermally diffused deeply into the silicon block from the outermost surface of the silicon block as a path. However, equiaxed crystal structures and dendritic crystal structures intersect with many grain boundaries, and because they are short, nickel is dispersed and only heat is diffused to the shallow part of the silicon block, resulting in nickel contamination as a block. Stops on the surface and is mild. Therefore, contamination caused by thermal diffusion of nickel can be prevented by forming the crystal structure immediately below the outermost surface of the silicon block into an equiaxed crystal structure or a dendritic crystal structure instead of forming a columnar crystal.

  This is because the above-described experiments of the present inventors, that is, the crystal structure of the impurity concentrated layer immediately below the outermost surface in the unidirectional solidification method are columnar crystal structures, equiaxed crystal structures, dendritic crystal structures, and equiaxed / dendritic structures. This is based on the results of a verification experiment on the dependence of nickel thermal diffusion on the silicon crystal structure to investigate the nickel concentration at a position where the solidification rate is 86%. The solidification rate was measured by immersing a quartz rod in the silicon melt.

  In the experiment to verify the dependence of nickel thermal diffusion on the silicon crystal structure, as a method of creating a crystal structure directly below the outermost surface of the silicon block, for example, the solidification rate is increased by adjusting the temperature drop rate of the heater. There is a method of causing compositional supercooling. In this case, when the impurity metal concentration in the silicon is less than 100 ppmw, the equiaxed crystal structure is obtained. When the impurity metal is 100 ppmw or more and less than 500 ppmw, the equiaxed crystal structure is obtained. A dendritic crystal structure can be incorporated into the crystal structure, and if the impurity metal is 500 ppmw or more, it can be incorporated into the dendritic crystal structure. Here, the solidification rate is 0.5 mm / min or more, preferably 0.5 mm / min or more and 1.5 mm / min or less. Another method is to forcibly cool the surface of the silicon melt by blowing an inert gas (about 100 L / min), thereby eliminating the temperature gradient in the vertical direction and hindering the growth of the columnar crystal structure. There is a way. At this time, when the impurity metal in silicon is less than 100 ppmw, it can be formed into an equiaxed crystal structure, and when the impurity metal is 100 ppmw or more, it can be formed into a dendritic crystal structure.

Table 1 shows the results of a verification experiment examining the dependence of nickel thermal diffusion on the silicon crystal structure.

  As is apparent from the results shown in Table 1 above, as a result of examining the relationship between the range of the crystal structure of the silicon block and the nickel contamination, the range of the equiaxed crystal structure, the range of the dendritic crystal structure, or the equiaxed crystal structure When the area where the dendritic crystal structure is mixed is less than 2% of the total height of the silicon block, it is susceptible to contamination by thermal diffusion of nickel. It can be effectively stopped.

As described above, according to the manufacturing method of the present invention, the temperature drop time from 1400 ° C. to 1000 ° C. at the time of completion of solidification is shortened, or the crystal structure immediately below the ingot block outermost surface is columnar. By forming into an equiaxed crystal structure or dendritic crystal structure without using a crystal, or by using these methods in combination, a metal silicon containing nickel having an initial concentration C ₀ is used as a raw material, and from a molten metal silicon Refined by unidirectional solidification, the nickel concentration can be efficiently reduced to 1/500 or less of the initial concentration C ₀ , which makes it possible to easily produce high purity polycrystalline silicon of 6N or more efficiently at low cost. can do.

Below, the manufacturing method of the polycrystalline silicon of this invention is demonstrated based on the Example and comparative example of this invention.
Here, in the following examples and comparative examples, purification by the unidirectional solidification method of metallic silicon containing nickel uses a square mold 1 having an inner size of 40 cm × 40 cm × 40 cm as shown in FIG. This square mold 1 is placed in a resistance heating type silicon block casting furnace (not shown), held at 1500 ° C. in an Ar atmosphere under atmospheric pressure, and 100 kg of molten silicon held at 1500 ° C. is injected therein. This was done by solidifying in one direction. In FIG. 4, a resistance heater 2 is disposed above the rectangular mold 1, a cooling member 3 is disposed on the outer surface of the bottom, and a predetermined interval is disposed outside the rectangular mold 1 and the resistance heater 2. A heat insulating graphite insulating material 4 for heat insulation is disposed, and the molten silicon 5 in the rectangular mold 1 is solidified in one direction in the rectangular mold 1 from the bottom side upward in the direction of the arrow. It is supposed to be.

[Example 1]
In Example 1, in order to examine the time dependence of thermal diffusion, the temperature drop time until the surface temperature of the silicon block at the completion of solidification reaches 1400 ° C. to 1000 ° C. is changed to change the temperature from the surface of the silicon block. An experiment was conducted to investigate the degree of thermal diffusion of nickel.

  In the method of changing the time, that is, the cooling method in the casting furnace, the temperature drop time from 1400 ° C. to 1000 ° C. was adjusted by changing the flow rate of Ar gas. In order to prevent the influence of impurities other than nickel, 0.5 g of nickel was added to semiconductor virgin polysilicon so that the initial concentration of nickel before purification was 5 ppmw.

  The result is that the vertical axis is the nickel concentration (ppmw) at the position where the solidification rate is 86%, and the horizontal axis is the temperature drop time [to reduce the outermost surface temperature (1400 ° C) at the completion of solidification of the silicon block to 1000 ° C. As can be seen from the graph of FIG. 2, the longer the temperature drop time, the greater the degree of nickel thermal diffusion. Here, in order to purify to polycrystalline silicon having a nickel concentration of 1/500 of the initial nickel concentration, it is necessary to make the nickel concentration 0.15 ppmw or less at a position where the solidification rate is 86%. It was found that the temperature drop time should be within 400 minutes. Polycrystalline silicon obtained in Example 1 had a columnar crystal structure as shown in FIG. 3 (a), a purity of 6N or more, and a yield of 94%.

[Example 2]
Production of polycrystalline silicon by unidirectional solidification as in Example 1 except that 0.1 g of nickel was added to virgin polysilicon for semiconductor and the initial concentration of nickel before purification was 1 ppmw. Went.
Polycrystalline silicon obtained in Example 2 had a columnar crystal structure as shown in FIG. 3 (a) as a whole, a purity of 6N or higher, and a yield of 94%.

[Example 3]
In Example 3, when the solidification rate of the silicon block was 98%, the heater power supply was turned off, and Ar gas was sprayed onto the molten silicon surface at a rate of 1000 L / min. Solidified into an equiaxed crystal region.

After confirming that solidification has been completed up to the outermost surface of the generated silicon block, in order to compare and examine the amount of contamination by thermal diffusion of nickel, and to make the same conditions as in Example 1, once in the casting furnace The temperature was raised to 1400 ° C., the solidification completion temperature, and then lowered to 1000 ° C. over 900 minutes, and the degree of thermal diffusion of nickel from the silicon block surface was examined. Here, in order to prevent the influence of impurities other than nickel in the raw material, 0.5 g of nickel was added to virgin polysilicon for semiconductor in the same manner as in Example 1 above, and the initial nickel concentration C ₀ before purification was added. Was 5 ppmw.

  FIG. 3B shows the crystal structure in the impurity-concentrated layer immediately below the outermost surface, and Table 1 shows the nickel concentration (ppmw) at a solidification rate of 86%. According to the data of FIG. 2 of Example 1, when the temperature was lowered from 1400 ° C. to 1000 ° C. over 900 minutes, the nickel concentration at the same solidification rate of 86% was about 0.34 ppmw. In the case of Example 3, it was significantly reduced to 0.04 ppmw, and the effect of setting the crystal structure in the impurity concentrated layer immediately below the outermost surface of the silicon block as an equiaxed crystal region was confirmed. From this, in Example 3, a silicon block corresponding to the theoretical curve of nickel concentration in purified silicon shown in FIG. 1 was obtained up to 86% solidification rate when the initial nickel concentration was 5 ppmw. The polycrystalline silicon obtained in Example 3 has an equiaxed crystal structure as shown in FIG. 3B in which 98 to 100% of the crystal structure is an impurity-concentrated layer. Was 6N or more and the yield was 90%.

[Comparative Example 1]
In Comparative Example 1, polycrystalline silicon was produced in the same manner as in Example 3 except that the heater power supply was turned off when the solidification rate of the silicon block was 99%.

  According to the data of FIG. 2 of Example 1, when the temperature was lowered from 1400 ° C. to 1000 ° C. over 900 minutes, the nickel concentration at the same solidification rate of 86% was about 0.34 ppmw. In the case of this comparative example 1, it was 0.20 ppmw and did not reach the target nickel concentration of 0.15 ppmw or less. In Comparative Example 1, although nickel contamination of the obtained silicon block was reduced, the nickel concentration at the solidification rate of 86% deviated from the theoretical curve of nickel concentration in the purified silicon shown in FIG. In the obtained polycrystalline silicon, the region of 99-100% of the crystal structure is an impurity-enriched layer and has an equiaxed crystal structure as shown in FIG. 3 (b), and its yield is 6N or higher in yield. Was 75%.

[Example 4]
In Example 4, when the solidification rate of the silicon block was 98%, the heater power supply was turned off, and Ar gas was sprayed onto the molten silicon surface at 1000 L / min to rapidly solidify the solidification rate range of 98 to 100%. A dendrite region is added, and 10 g of iron (Fe) is added so that the Fe concentration in the molten silicon at a position where the solidification rate is 98% is 4000 ppmw so that compositional supercooling occurs during rapid cooling. A polycrystalline silicon was produced by purifying by a unidirectional solidification method in the same manner as in Example 3 except that a dendritic crystal structure was easily formed.

  In Example 4, the nickel concentration at a solidification rate of 86% is 0.06 ppmw, and the obtained polycrystalline silicon has a region of 98 to 100% of its crystal structure as an impurity concentrated layer. It was a dendritic crystal structure as shown in 3 (c), and its purity was 6N or more and the yield was 92%.

[Comparative Example 2]
In Comparative Example 2, the heater power supply was turned off when the solidification rate of the silicon block was 99%, and Ar gas was sprayed onto the surface of the molten silicon at 1000 L / min to rapidly solidify the range of the solidification rate of 99 to 100%. Polycrystalline silicon was produced by purification by the unidirectional solidification method in the same manner as in Example 4 except that the dendritic crystal region was used.

  In Comparative Example 2, the nickel concentration at the solidification rate of 86% is 0.22 ppmw, and the obtained polycrystalline silicon has an impurity concentration layer in the region of 99 to 100% of the crystal structure. It was a dendritic crystal structure as shown in 3 (c), and its purity was 6N or more and the yield was 75%.

[Example 5]
In Example 5, when the solidification rate of the silicon block is 98%, the heater power supply is turned off, and Ar gas is blown onto the surface of the molten silicon at 1000 L / min, so that the solidification rate of 98-100% is rapidly solidified. By adding 0.3 g of iron (Fe), the Fe concentration in the molten silicon at a position where the solidification rate is 98% is set to about 200 ppmw. In the same manner as in Example 3 except that the equiaxed / dendritic mixed crystal structure was made easy to occur, polycrystalline silicon was produced.

  In Example 5, the nickel concentration at a solidification rate of 86% is 0.04 ppmw, and the obtained polycrystalline silicon has a region of 98 to 100% of its crystal structure as an impurity concentrated layer. The crystal structure was equiaxed and dendritic mixed as shown in 3 (d). The purity was 6N or more and the yield was 93%.

[Comparative Example 3]
In Comparative Example 3, the heater power is turned off when the solidification rate of the silicon block is 99%, and Ar gas is blown onto the surface of the molten silicon at 1000 L / min. By adding 0.3 g of iron (Fe), the Fe concentration in the molten silicon is about 200 ppmw at a position where the solidification rate is 98%, and this causes compositional supercooling during rapid cooling. Polycrystalline silicon was produced by purifying by the unidirectional solidification method in the same manner as in Example 3 except that it was easy to produce an equiaxed / dendritic mixed crystal structure.

  In Comparative Example 3, the nickel concentration at a solidification rate of 86% is 0.21 ppmw, and the obtained polycrystalline silicon has an impurity concentration layer in a region of 99 to 100% of the crystal structure. It was an equiaxed and dendritic mixed crystal structure as shown in 3 (d), and its purity was 6N or more and the yield was 70%.

  DESCRIPTION OF SYMBOLS 1 ... Square mold, 2 ... Resistance heater, 3 ... Water cooling type cooling member, 4 ... Graphite heat insulating material, 5 ... Molten silicon.

Claims (5)

  A method for producing polycrystalline silicon having a purity of 6N or more by melting metallic silicon containing nickel and purifying the molten metallic silicon by a unidirectional solidification method, wherein the silicon block obtained by the unidirectional solidification method A method for producing polycrystalline silicon, wherein the surface temperature of the silicon block reaches 1000 ° C. or less within 400 minutes after the solidification rate of 100% reaches 100%. 2. The method for producing polycrystalline silicon according to claim 1, wherein the metal silicon contains nickel having an initial concentration C _{0, and} the nickel concentration of the metal silicon is reduced to 1/500 or less of the nickel initial concentration C ₀ .   2. The method for producing polycrystalline silicon according to claim 1, wherein the silicon block has a silicon crystal structure other than the columnar crystal structure in the impurity-concentrated layer.   A method for producing polycrystalline silicon having a purity of 6N or more by melting metallic silicon containing nickel and purifying the molten metallic silicon by a unidirectional solidification method, wherein the silicon block obtained by the unidirectional solidification method A method for producing polycrystalline silicon, wherein the silicon crystal structure in the impurity-concentrated layer is formed into a crystal structure other than the columnar crystal structure.   The method for producing polycrystalline silicon according to claim 3 or 4, wherein the crystal structure other than the columnar crystal structure is one or both of a dendritic crystal structure and an equiaxed crystal structure.

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