JP5375794B2 - Method for producing silicon single crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a silicon single crystal capable of suppressing generation of dislocation when manufacturing a silicon single crystal, preventing formation of an impurity layer on the inner surface of a quartz crucible, and shortening a useless time in which the silicon single crystal is not manufactured. <P>SOLUTION: This method for manufacturing a silicon single crystal by MCZ method includes: a melting process for melting polycrystal silicon stored in a quartz crucible to obtain silicon melt; and a pulling-up process for bringing a seed crystal into contact with a melt surface of the silicon melt, and pulling up the seed crystal upward, while applying a magnetic field to the silicon melt, to thereby grow a silicon single crystal. In the method for manufacturing the silicon single crystal, at least after the melting process and before the pulling-up process, a process for applying the magnetic field to the silicon melt and leaving it as it is, and thereafter a process for stopping application of the magnetic field and leaving it as it is are performed. <P>COPYRIGHT: (C)2012,JPO&amp;INPIT

Description

  The present invention relates to a method for growing a silicon single crystal by the Czochralski method, and particularly to a method for suppressing the occurrence of dislocations when growing a silicon single crystal by applying a magnetic field.

  Conventionally, a method called Czochralski method has been widely adopted for growing silicon single crystals. Among them, the MCZ method (magnetic field application Czochralski method) in which a magnetic field is applied to the silicon melt in order to suppress convection of the silicon melt in the quartz crucible is known. In the MCZ method, polycrystalline silicon is accommodated in a quartz crucible, a melting process in which polycrystalline silicon is melted by a heater in the quartz crucible, a seed crystal is brought into contact with the melt surface of the silicon melt from above, and silicon is formed by a coil. A silicon single crystal is grown by performing a pulling process in which the seed crystal and the quartz crucible are rotated and moved up and down while pulling up the seed crystal while applying a magnetic field to the melt.

A quartz crucible for containing a silicon melt is made of amorphous SiO ₂ (quartz glass) having an amorphous structure. The quartz crucible reacts with the silicon melt, and a cristobalite crystal layer made of crystalline SiO ₂ is formed on the SiO ₂ / Si interface, that is, the inner surface of the quartz crucible in contact with the silicon melt. During the pulling of the silicon single crystal, the cristobalite crystal layer may peel off, and may be released or dropped from the quartz crucible into the silicon melt to reach the silicon single crystal growth interface being pulled. As a result, it enters the silicon single crystal being pulled and causes dislocation of the silicon single crystal.

  In view of this, various methods have been proposed to prevent cristobalite from peeling off the inner surface of the quartz crucible during the silicon single crystal pulling process and to avoid dislocation of the silicon single crystal. For example, Patent Document 1 discloses a method of growing a silicon single crystal while applying a magnetic field to a silicon melt using a quartz crucible having an aluminum low concentration layer on the inner surface side.

  However, in the above-described prior art, there is a problem that this impurity is contained in the silicon single crystal by forming an aluminum low concentration layer (impurity layer) in the quartz crucible. If an impurity is contained in the silicon single crystal, there is a concern about the influence on the device. Therefore, it is not preferable to form an impurity layer on the inner surface of the quartz crucible. In particular, it is a solution that is not preferable for next-generation devices that require high purity.

  Patent Document 2 describes heating a silicon melt contained in a quartz crucible while intermittently applying a magnetic field to the silicon melt. However, since Patent Document 2 does not describe any specific mode for intermittently applying a magnetic field, means for intermittently applying a magnetic field is unknown. Further, in order to intermittently apply the magnetic field to the silicon melt a plurality of times, it is considered necessary to repeat the excitation and demagnetization operations of the coil, but this is both cumbersome and increases the risk of operation errors. In addition, by repeating excitation and demagnetization of the coil, useless time during which the silicon single crystal is not manufactured becomes longer. For example, Patent Document 2 states that “the total time during which no magnetic field is applied is about 8 to 20 hours if the single crystal is pulled for 100 hours of melting + 30 hours for pulling,” for example. The wasted time that has not been raised is preferably as short as possible. Further, there is also disclosed a method in which the strength of the magnetic field is 1000 gauss to 2000 gauss and the single crystal is pulled after heating for 100 hours or more to melt the raw material of the silicon melt. The time required only for melting the crystalline silicon is approximately 10 to 20 hours depending on the amount of charge. That is, the heating for 100 hours or more described in Patent Document 2 means that unnecessary heating is performed for 80 to 90 hours after the completion of melting of polycrystalline silicon, which is industrially inefficient and unrealistic.

JP 2010-30816 A Japanese Patent Laid-Open No. 2001-240494 JP-A-10-297994

  The present invention has been made in view of the above-described problems, and is a method for producing a silicon single crystal that suppresses the occurrence of dislocations when producing a silicon single crystal, without forming an impurity layer in a quartz crucible. Another object of the present invention is to provide a method for producing a silicon single crystal that can shorten a useless time during which no silicon single crystal is produced.

In order to achieve the above object, in the present invention,
Melting the polycrystalline silicon contained in the quartz crucible into a silicon melt; bringing the seed crystal into contact with the melt surface of the silicon melt and applying a magnetic field to the silicon melt; A method of producing a silicon single crystal by the MCZ method, comprising a pulling step of growing the silicon single crystal by pulling the crystal upward;
At least after the melting step and before the pulling step, a step of applying a magnetic field to the silicon melt and leaving it to stand, and then a step of stopping the application of the magnetic field and leaving it to stand are performed. A manufacturing method is provided.

  Thus, after the melting step, before the pulling step, a step of applying a magnetic field to the silicon melt and leaving it, and then a step of stopping and applying the magnetic field to the silicon single crystal manufacturing method If this is the case, cristobalite is formed on the surface of the quartz crucible by leaving the magnetic field applied to the silicon melt once, and the cristobalite is moderately removed by leaving the application of the magnetic field only once. A silicon single crystal that can be prevented from dislocation because it does not peel off and can be dissolved until the entire surface of the quartz crucible is close to the initial amorphous silica state) The manufacturing method becomes. Furthermore, since the application of the magnetic field and the application of the magnetic field to each other are performed only once, the wasted time when the silicon single crystal is not manufactured can be shortened, and the coil excitation / demagnetization work is not complicated. The single crystal growing apparatus is a silicon single crystal manufacturing method in which a simple one that is usually used can be used.

  In addition, in the step of applying a magnetic field to the silicon melt and leaving it to stand, it is preferable that the applied magnetic field strength is 3000 gauss or more and 5000 gauss or less.

  Thus, in the step of applying a magnetic field to the silicon melt and leaving it to stand, if the applied magnetic field strength is 3000 gauss or more and 5000 gauss or less, the standing time for applying the magnetic field for forming cristobalite can be shortened, It is industrially efficient.

  Further, in the step of leaving the silicon melt by applying a magnetic field, the leaving time is preferably set to 1 hour or more.

  As described above, in the step of leaving the silicon melt by applying a magnetic field, if the standing time is set to 1 hour or more, the time is sufficient to form cristobalite.

  Further, in the step of leaving the application of the magnetic field and leaving it to stand, the leaving time is preferably set to 1 hour or more and less than 8 hours.

  Thus, in the step of leaving the application of the magnetic field and leaving it to stand, if it is a method for producing a silicon single crystal in which the standing time is 1 hour or more and less than 8 hours, the cristobalite is not peeled off during the silicon single crystal pulling step. As described above, the cristobalite is moderately dissolved, and the cristobalite is not dissolved until the entire surface of the quartz crucible becomes close to the initial amorphous silica. This is a method for producing a silicon single crystal that can prevent the formation of the silicon single crystal and thus avoid the dislocation of the silicon single crystal. Further, in this way, in the step of leaving the application of the magnetic field and leaving it to stand, if it is a method for producing a silicon single crystal in which the standing time is 1 hour or more and less than 8 hours, a silicon single crystal is not produced. It becomes the manufacturing method of the silicon single crystal which can shorten time.

  As described above, according to the present invention, after the melting step and before the pulling step, a cristobalite is formed by applying a magnetic field once to the silicon melt and then leaving it, and then applying the magnetic field once. Since the cristobalite can be dissolved to such an extent that the cristobalite does not exfoliate in the process of stopping and allowing to stand, a method for producing a silicon single crystal that can suppress the cristobalite from exfoliating and causing dislocation of the growing silicon single crystal. Can be provided. Furthermore, since the application of the magnetic field and the application of the magnetic field to each other are performed only once, the wasted time when the silicon single crystal is not manufactured can be shortened, and the coil excitation / demagnetization work is not complicated. It is possible to provide a silicon single crystal manufacturing method in which a simple single crystal growing apparatus can be used.

Hereinafter, although an embodiment is described about the present invention, the present invention is not limited to this.
As described above, there has been a demand for the development of a method for manufacturing a silicon single crystal that can reduce a useless time during which no silicon single crystal is manufactured without forming an impurity layer in a quartz crucible.

  In general, in the MCZ method, polycrystalline silicon is accommodated in a quartz crucible made of quartz glass, and the polycrystalline silicon is melted by a heater in the quartz crucible, and a seed crystal is placed on the surface of the silicon melt. The silicon single crystal is grown by performing a pulling process in which the seed crystal and the quartz crucible are rotated and moved up and down while pulling up the seed crystal while applying a magnetic field to the silicon melt with a coil. However, as described above, there is a problem that cristobalite peels into the silicon melt and causes dislocation of the silicon single crystal being grown.

  Therefore, the present inventor has conducted intensive studies and devised a method of heating the silicon melt before pulling up the crystal, thereby shortening the preparation time when the crystal is not pulled up and avoiding dislocation of the silicon single crystal. I found.

  That is, the present inventor pays attention to applying a magnetic field to the silicon melt after the melting step and before the pulling step, and (1) it is not necessary to intermittently apply the magnetic field to the silicon melt. The present invention was completed by finding that the silicon single crystal can be pulled up by the MCZ method after leaving after applying the magnetic field once and after stopping the application of the magnetic field once the silicon is melted. Further, the inventor preferably (2) it is sufficient to leave the magnetic field applied for 3000 hours or more and 5000 gauss or less for 1 hour or more, and (3) leave the magnetic field applied for 1 hour or longer. We have found that less than 8 hours is sufficient. With such a standing time, the preparation time until the pulling process can be remarkably shortened even if 2 hours required for exciting and demagnetizing the coil are added. As a result, the silicon single crystal manufacturing method of the present invention does not require the formation of an impurity layer on the inner surface of the quartz crucible, and there is no need to worry about the influence on the device due to the inclusion of impurities in the silicon single crystal. Good. In addition, since the application of the magnetic field and the application of the magnetic field to each other are performed only once, the wasted time when the silicon single crystal is not manufactured can be shortened, and the coil excitation / demagnetization work is not complicated. A single crystal growing apparatus can be a simple method for producing a silicon single crystal.

  As disclosed in Patent Document 2 and Patent Document 3, cristobalite is formed on the surface of the quartz crucible in an environment in which a magnetic field is applied to the silicon melt, and the cristobalite formed in an environment in which application of the magnetic field is stopped It is gradually dissolved in the liquid. After completion of the melting process of polycrystalline silicon, the present inventor applied a magnetic field of 4000 gauss to the silicon melt and left it for 1 hour, stopped the application of the magnetic field, and then left the silicon single crystal by MCZ method. When cristobalite on the surface of the quartz crucible after the pulling process for pulling up was observed, a cristobalite peeling mark was observed near the center of the so-called brown ring. On the other hand, after completion of the polycrystalline silicon melting step, a magnetic field of 4000 gauss is applied to the silicon melt and left for 1 hour, then the magnetic field is stopped and left for 5 hours, and then the crystal is pulled up by the MCZ method. On the surface of the quartz crucible after the process, the brown ring was dissolved and not observed, and cristobalite was formed in the shape of islands in the amorphous silica sea. Furthermore, dislocations are generated for 10 silicon single crystals grown by the MCZ method without stopping application of the magnetic field and 10 silicon single crystals grown by the MCZ method after stopping application. When the number of occurrences was investigated, dislocations were observed in all 10 silicon single crystals when the application of the magnetic field was not stopped and left untreated, whereas dislocations were observed when the application was stopped and left untreated. There wasn't.

  That is, simply applying a magnetic field of 4000 gauss to the silicon melt and leaving it alone causes the cristobalite formed by the application of the magnetic field to peel off during the pulling process, and the peeled cristobalite is completely dissolved in the silicon melt before it is completely dissolved. It was found that it reached the solid-liquid interface during crystal growth, leading to dislocation of the silicon single crystal. On the other hand, if cristobalite is properly dissolved by stopping application of the magnetic field, cristobalite peeling during silicon single crystal growth (during the pulling process) can be suppressed, and the silicon single crystal being grown should not be dislocated. I understood. In addition, even if cristobalite is peeled from the quartz crucible in a state where cristobalite is dissolved moderately, the cristobalite that has been dissolved moderately is dissolved in the silicon melt because the thickness is thin, so that the solid liquid of silicon single crystal It was found that the interface does not reach the interface and does not cause dislocation of the growing silicon single crystal.

  In addition, after completion of the polycrystalline silicon melting step, a 4000 gauss magnetic field was applied to the silicon melt and left for 1 hour, then the magnetic field was turned off and left for 8 hours, and then the quartz crucible was pulled up by the MCZ method. On the surface, a brown ring was observed again, and a cristobalite flake was observed near the center. This is because the cristobalite formed by leaving the magnetic field applied for 1 hour was considerably dissolved by leaving the magnetic field applied for 8 hours, and the entire surface of the quartz crucible returned to a state close to the initial amorphous silica. This means that cristobalite nucleation / growth and delamination occurred again during the MCZ pulling process. In other words, it is suggested that nucleation of new cristobalite hardly occurs when cristobalite remains on the surface of the quartz crucible, but nucleation can occur when the entire surface is in an amorphous silica state. That is, if the cristobalite is almost dissolved by leaving the application of the magnetic field stopped for a long time (8 hours or more if the magnetic field is applied for 1 hour), the subsequent crystal pulling by the MCZ method is performed. This means that the possibility of dislocations in the silicon single crystal is increased.

  Based on the above findings, the present inventor has determined that the surface state of the silicon single crystal being grown is present if the surface state of the quartz crucible before the crystal pulling step is a state where cristobalite is formed in an island shape in the sea of amorphous silica. This led to the idea that dislocations are suppressed. It is not necessary to intermittently turn on and off the magnetic field with respect to the silicon melt because it only lengthens the useless time during which the silicon single crystal is not manufactured. The cristobalite is formed on the surface of the quartz crucible when left applied, and the cristobalite is moderately removed after leaving the application of the magnetic field only once (the cristobalite does not peel off, and the entire surface of the quartz crucible is (It does not dissolve until it is close to the state of amorphous silica) It can be dissolved, it is possible to avoid dislocation of the silicon single crystal, and it is possible to shorten the useless time when the silicon single crystal is not manufactured It was. This will be described in detail below.

The present invention
Melting the polycrystalline silicon contained in the quartz crucible into a silicon melt; bringing the seed crystal into contact with the melt surface of the silicon melt and applying a magnetic field to the silicon melt; A method of producing a silicon single crystal by the MCZ method, comprising a pulling step of growing the silicon single crystal by pulling the crystal upward;
At least after the melting step and before the pulling step, a step of applying a magnetic field to the silicon melt and leaving it to stand, and then a step of stopping the application of the magnetic field and leaving it to stand are performed. It is a manufacturing method.

[Melting process]
The melting step is a step of melting polycrystalline silicon contained in a quartz crucible to form a silicon melt.

[Step of applying magnetic field to silicon melt and leaving it to stand]
The step of leaving the silicon melt by applying a magnetic field is a step of leaving the silicon melt by applying a magnetic field once after the melting step and before the pulling step. By this step, the quartz crucible reacts with the silicon melt, and cristobalite is formed at the interface between the quartz crucible and the silicon melt. In this step, a magnetic field is applied to the silicon melt so that cristobalite is formed, and the subsequent application of the magnetic field is stopped. It is possible to prevent the crystals from undergoing dislocation.

  The strength of the magnetic field applied to the silicon melt is preferably 3000 gauss or more and 5000 gauss or less. In the step of leaving the silicon melt by applying a magnetic field, when the cristobalite is formed once on the inner surface of the quartz crucible, if the magnetic field strength is 3000 gauss or more, the standing time can be shortened and industrially efficient. . On the other hand, it is sufficient to apply 5000 gauss. In the step of applying a magnetic field to the silicon melt and leaving it to stand, it is preferable that the time of applying and leaving the magnetic field is 1 hour or more. If the time for which the magnetic field is applied and left for one hour or longer, the time is sufficient to form cristobalite on the surface of the quartz crucible, and the time is also sufficiently short from the viewpoint of efficiency. Further, when the time of leaving the magnetic field necessary to make the quartz crucible surface once cristobalite was examined, if a magnetic field of 3000 gauss or more and 5000 gauss or less is applied, 1 hour is sufficient. Further, it is preferable that the maximum time for which the magnetic field is applied is 10 hours. If the standing time in which the magnetic field is applied is 10 hours or less, the formation of cristobalite on the surface of the quartz crucible does not proceed excessively. Therefore, in the subsequent step of stopping the application of the magnetic field and leaving it to stand, the standing time for dissolving cristobalite formed on the surface of the quartz crucible can be shortened, and high-efficiency production of a silicon single crystal is maintained, which is preferable. . In particular, when the standing time in which the magnetic field is applied is 1 hour or more and 10 hours or less, it is preferable that the standing time in the step of stopping the application of the magnetic field is 1 hour or more and less than 8 hours. Therefore, it is possible to shorten a useless time during which no silicon is grown, and to maintain high-efficiency production of silicon single crystals.

[Step of leaving off the application of magnetic field]
The step of stopping and leaving the application of the magnetic field is a step of stopping and leaving the application of the magnetic field once after the step of applying and leaving the magnetic field to the silicon melt. By this step, cristobalite formed on the surface of the quartz crucible is appropriately dissolved. In this step, it is preferable that the application of the magnetic field is stopped so that cristobalite formed on the inner surface of the quartz crucible does not peel during the pulling step, and the cristobalite dissolves to such an extent that it does not peel during the pulling step. This is because if cristobalite peels and reaches the solid-liquid interface during the growth of the silicon single crystal, it may cause dislocation. In this step, it is preferable that the application of the magnetic field is stopped and the cristobalite is not dissolved so that the entire surface of the quartz crucible becomes close to the initial amorphous silica. In this process, if the cristobalite dissolves until the entire surface of the quartz crucible is close to the initial amorphous silica, the cristobalite nucleation is more likely to occur due to the application of a magnetic field during the pulling process, and the cristobalite peels off and dislocations occur. This is because there is a possibility of causing crystallization.

  The time for stopping the application of such a magnetic field is preferably 1 hour or more and less than 8 hours. If the application time of the magnetic field is stopped for 1 hour or longer, the cristobalite can be sufficiently dissolved to such an extent that it does not peel off. This is preferable because it is thin enough to dissolve in the liquid. Further, it is preferable that the time for which the application of the magnetic field is stopped is left for less than 8 hours, because the cristobalite is not excessively dissolved until new cristobalite nucleation occurs. Therefore, if the time to stop applying the magnetic field and leave it for 1 hour or more and less than 8 hours, the cristobalite is appropriately dissolved in order to prevent the cristobalite from peeling during the pulling process of the silicon single crystal, and the entire surface of the quartz crucible is initial. This is preferable because it takes time to dissolve cristobalite until it becomes close to amorphous silica.

  If the standing temperature is increased, it is possible to shorten the standing time of the step of leaving the magnetic field applied and the step of leaving the magnetic field stopped. By raising the standing temperature, the standing time can be set to a shorter time within the range of the present invention. When the quartz crucible is deformed by high temperature and there is a possibility that the production of the silicon single crystal may be hindered, the temperature and time to be left may be appropriately adjusted according to the furnace structure in the puller. These standing temperatures can be adjusted by adjusting the heater power, and since the temperature of the quartz crucible surface is important, it can also be changed by adjusting the crucible rotation speed (CR). When the standing temperature is a seeding temperature that is a temperature at which the seed crystal is brought into contact with the silicon melt in order to pull up the silicon single crystal, the time for which the magnetic field is applied and left is preferably 1 hour or more. It is preferable that the time for which the application is stopped and left to stand is 1 hour or more and less than 8 hours. If the silicon melt is allowed to stand as a seeding temperature in this way, the seed crystal can be brought into contact with the silicon melt and seeded immediately after being left to start growing a silicon single crystal.

[Pulling process]
The pulling step is a step of growing a silicon single crystal by bringing a seed crystal into contact with the melt surface of the silicon melt and pulling the seed crystal upward while applying a magnetic field to the silicon melt. This step may be performed by a silicon single crystal pulling method which is generally performed by setting conditions according to the specifications of the silicon single crystal to be pulled.

  According to the embodiments described above, the present invention is a method of manufacturing a silicon single crystal that can suppress the occurrence of dislocations when manufacturing a silicon single crystal, and shortens a useless time when the silicon single crystal is not manufactured. This is a method for producing a silicon single crystal.

  EXAMPLES Hereinafter, although an Example and a comparative example are shown and this invention is demonstrated more concretely, this invention is not limited to the following Example.

[Method for producing silicon single crystal]
400 kg of raw material polycrystalline silicon was placed in a quartz crucible and melted for 10 hours without applying a magnetic field to obtain a silicon melt (melting step). Thereafter, a magnetic field of 4000 gauss is applied to the silicon melt for the time indicated in the vertical direction in Table 1, and then left for one time, and then the application of the magnetic field is stopped and left for one time for the time indicated in the horizontal direction in Table 1. went. Thereafter, a 4000 gauss magnetic field was again applied to the silicon melt, and the silicon single crystal having a diameter of 300 mm was pulled up (pulling step). The results of Examples and Comparative Examples are shown in Table 1.

  In Table 1, the time when the magnetic field is applied for 0 hours or the time when the application is stopped for 0 hours is a comparison in which the magnetic field is not applied or the magnetic field is not applied. It is an example, and others are an Example (shaded part). Table 1 shows the number of dislocations when 10 silicon single crystals are pulled by the MCZ method. Here, the number of dislocations referred to here is the number of silicon single crystals having dislocations among ten silicon single crystals.

  From the results shown in Table 1, it is clear that dislocation occurs at a high frequency in the case where the application of the magnetic field in the comparative example is stopped for 0 hour, and the case where the application of the magnetic field in the comparative example is applied for 0 hour. In the case of (5), it became clear (10 times) that dislocations occur. On the other hand, in the examples of the present invention, it was shown that the number of dislocations was clearly lower than that in the comparative example. In particular, in the case of the example in which the magnetic field is left for 1 hour or longer and the magnetic field-free standing is 1 hour or longer and less than 8 hours, the number of dislocations is within 2 times. It became clear that this was particularly noticeable.

  Accordingly, after the melting step and before the pulling step, a step of leaving the magnetic field applied to the silicon melt once and a step of leaving the magnetic field once stopped and then leaving the silicon single crystal are provided. It has been clarified that this is a method for producing a silicon single crystal that suppresses the occurrence of dislocations during the production of. In particular, as shown in Table 1, the time for applying the magnetic field once in the embodiment of the present invention and leaving it to stand for 1 hour or more and 10 hours or less, and the time for leaving the application of the magnetic field once for 1 hour or more is 8 hours or more. If the time is less than the time, not only the dislocation of the silicon single crystal is remarkably suppressed, but also the time required for exciting and demagnetizing the coil (2 hours) is not less than 4 hours to 20 hours. It was shown that the useless time when the silicon single crystal was not manufactured can be shortened. The value of 4 hours or more and less than 20 hours indicates that, for example, even when compared with Patent Document 2, the wasted time during which no silicon single crystal is manufactured can be significantly shortened. Further, it has been shown that even when the standing time for applying the magnetic field is 15 hours or longer or the standing time for stopping the application of the magnetic field is 9 hours or more, the dislocation of the silicon single crystal can be suppressed as compared with the comparative example. It was shown that the dislocation of the silicon single crystal can be sufficiently suppressed by appropriately adjusting the standing time in which the application of the magnetic field is stopped according to the length of the standing time in which c is applied (the progress of formation of cristobalite).

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has substantially the same configuration as the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.

Claims (1)

Melting the polycrystalline silicon contained in the quartz crucible into a silicon melt; bringing the seed crystal into contact with the melt surface of the silicon melt and applying a magnetic field to the silicon melt; A method of producing a silicon single crystal by the MCZ method, comprising a pulling step of growing the silicon single crystal by pulling the crystal upward;
At least after the melting step and before the pulling step , the magnetic field strength to be applied to the silicon melt is set to 3000 to 5000 gauss and the magnetic field is applied only once for 1 hour or more, and then, 1 to 8 hours, stopping the application of the magnetic field only once and leaving it alone.