JP2007314403A - Silicon purification method and apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a silicon purification method comprising an efficient boron removal method for preparing solar cell-grade silicon and to provide a silicon purification apparatus. <P>SOLUTION: Provided is a silicon purification method comprising melting crude silicon in a crucible placed in a melting furnace to remove impurities from the crude silicon, wherein in the crucible at least one boride object comprising a boride is in contact with the crude silicon containing an elementary metal identical with the metallic element constituting the boride, whereupon the B concentration in the metal-grade silicon can easily decrease to the B concentration in solar cell-grade silicon. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  More particularly, the present invention relates to a purification method and a purification apparatus for obtaining high-purity silicon used in solar cells and the like.

  In recent years, solar cells have attracted attention as clean energy, and the production volume has increased significantly. Among various types of solar cells, single crystal or polycrystalline silicon solar cells are the mainstream. Such silicon crystal solar cells are manufactured using non-standard polycrystalline silicon for semiconductors purified by the Siemens method or the like, or end materials of single crystal ingots, etc., but the solar cell market is expanding. As a result, the shortage of raw materials has become serious. Silicon for semiconductors purified by the Siemens method or the like has a high purity of 11N, and the purification cost is high. On the other hand, as solar cell grade silicon (hereinafter referred to as “SOG-Si”), a purity of 6N to 7N is sufficient. Therefore, a method for efficiently purifying SOG-Si from metal grade silicon (hereinafter referred to as “MG-Si”) has been developed.

  In order to change MG-Si to SOG-Si, it is necessary to remove metal impurities such as iron (Fe) and impurities such as phosphorus (P) and boron (B) contained in MG-Si. Since these impurities cannot be sufficiently removed by a single purification step due to their nature, they are removed by individual purification steps.

  Metal impurities such as Fe, aluminum (Al), titanium (Ti), and the like are obtained by a so-called unidirectional solidification method utilizing the property that when molten silicon is solidified, it tends to remain in the molten silicon rather than being taken into the solidified silicon. Has been removed. For example, a method is disclosed in which a hollow rotating cooling body is immersed in silicon melted in an inert gas atmosphere, and high purity silicon is crystallized on the outer peripheral surface of the rotating cooling body while feeding a cooling medium into the rotating cooling body. (See Patent Document 1).

  A method of evaporating and removing highly volatile elements such as P, Al, and calcium (Ca) by melting MG-Si in a vacuum is known. For example, a method is disclosed in which silicon is melted in a non-oxidizing atmosphere and kept in a reduced pressure atmosphere of 10 Pa or less in the melted state (see Patent Document 2).

  As a method for removing B, for example, there is disclosed a method for removing B as a vaporizable compound by supplying an oxidizing gas such as water vapor together with slag mainly composed of calcium oxide that absorbs B. (See Patent Document 3).

  However, this method consumes a large amount of argon (Ar) as a carrier gas and hydrogen gas for oxidation prevention together with slag or oxidizing gas such as calcium oxide, and the purification cost increases, and a part of silicon is also consumed. Since it is oxidized and evaporated as SiO, there is a problem that the yield is deteriorated.

  Therefore, as a simple method of removing B without using slag or oxidizing gas, a method of adding at least one of zirconium (Zr), Ti, or vanadium (V) after melting MG-Si is disclosed ( (See Patent Document 4).

  As a series of purification methods, for example, there is a method including a non-volatile impurity (B etc.) reducing process as Step 1, a volatile impurity (P etc.) reducing process as Step 2, and a metal impurity (Fe etc.) reducing process as Step 3. It is disclosed (see Patent Document 5).

JP 63-45112 A Japanese Patent No. 2905353 JP 2003-213345 A JP-A-64-61309 JP 2005-255417 A

However, the above-described conventional purification methods have the following problems. Since the impurity removal methods disclosed in Patent Documents 1 to 3 are individual purification steps, there is a problem that another impurity removal step is required separately. The method for removing B described in Patent Document 4 does not use slag or the like, and thus has an effect of reducing the purification cost. In combination with the technology of Patent Document 1, it is a technology that can simultaneously remove metal impurities and B. However, since there is no disclosure about the removal of P, there is a problem that a separate purification step for removing P is necessary. Moreover, since the B concentration contained in MG-Si is as low as several tens of ppm to several ppm (hereinafter, ppm is based on weight), it reacts with added Zr or Ti to form a boride of ZrB ₂ or TiB _2. The reaction probability is low, and it is difficult to grow to a particle size that agglomerates and precipitates or is centrifuged. Patent Document 5 discloses a series of purification methods for converting MG-Si to SOG-Si, but removes B, P, and metal impurities in separate purification steps. There is a problem that it is difficult to reduce the purification cost.

  Accordingly, an object of the present invention is to provide a silicon purification method and a silicon purification apparatus including an efficient B removal method for obtaining SOG-Si by solving the above-mentioned problems in MG-Si purification.

  The present invention relates to a silicon purification method for melting crude silicon in a crucible disposed in a melting furnace and removing impurities contained in the crude silicon, wherein at least one kind of boride is formed in the crucible. In the silicon purification method, the boride body is in contact with the roughly purified silicon containing the same metal element as the metal element forming the boride.

  Further, the present invention provides a hollow rotating cooling body by immersing a hollow rotating cooling body in silicon melted in the crucible and supplying a cooling medium to the inside of the rotating cooling body while rotating the hollow rotating cooling body. It is characterized by solidifying silicon on the surface.

  In addition, the present invention is characterized in that the melting furnace has a reduced pressure atmosphere after at least roughly refined silicon is melted.

Moreover, the present invention is characterized in that the temperature in the melting furnace is 1450 ° C to 1700 ° C.
Further, the present invention is characterized in that the roughly purified silicon contains the metal element simple substance having a weight of 10 to 1000 times the weight of boron (B) which is an impurity contained in the roughly purified silicon.

  In the present invention, the metal element forming the boride is titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), tantalum (Ta), niobium (Nb), chromium (Cr) or It is characterized by being molybdenum (Mo).

  Further, the present invention is the silicon purification apparatus for purifying silicon by the silicon purification method, comprising a crucible containing silicon, a heating means for heating the silicon and the crucible, and the boride object. .

  Further, the present invention is characterized in that the crucible has a borated object separating means for separating the borated object and the silicon.

  According to the present invention, in a silicon refining method in which roughly purified silicon is melted in a crucible and impurities contained in the roughly purified silicon are removed, at least one type of boride body made of boride forms the boride. It is in contact with roughly purified silicon containing the same metal element as the metal element.

  Thereby, B and the metal element simple substance are adsorbed on the surface of the boride object, and the adsorbed B and metal element simple substance react to generate boride, and boride crystal growth occurs on the surface of the boride object. B contained in the purified silicon is removed. When removing B contained in the roughly purified silicon as a boride, it can be removed more efficiently than by removing by coagulation sedimentation or centrifugation.

  Moreover, according to this invention, a hollow rotary cooling body is immersed in the molten silicon in the said crucible, and silicon is solidified on the surface of the said hollow rotary cooling body.

  As a result, metal impurities can be removed, and the metal impurity removal step can be carried out in the same purification apparatus in parallel with the B removal step, thereby further efficiently purifying silicon.

  According to the present invention, the melting furnace is evacuated at least after the roughly purified silicon is melted.

  As a result, volatile impurities can be removed, and the B removal step, the metal impurity removal step, and the volatile impurity removal step can be performed in parallel with the same purification apparatus, and silicon can be purified more efficiently.

Moreover, according to this invention, the temperature in a melting furnace is 1450 to 1700 degreeC.
As a result, silicon can be stably melted, and the amount of silicon evaporation and power consumption can be suppressed, and B can be more efficiently removed.

According to the invention, the roughly purified silicon contains the metal element simple substance having a weight of 10 to 1000 times the weight of boron (B), which is an impurity contained in the roughly purified silicon.
As a result, the silicon purification time can be shortened and B can be more efficiently removed.

  Further, according to the present invention, the metal element forming the boride is titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), tantalum (Ta), niobium (Nb), chromium (Cr ) Or molybdenum (Mo).

  These metal elements can react with B contained in silicon to produce a stable high melting point boride in molten silicon.

According to the invention, the silicon purification apparatus includes a crucible containing silicon, heating means for heating the silicon and the crucible, and the borated body.
Thereby, impurities contained in silicon can be efficiently removed.

  Further, according to the present invention, the silicon purification apparatus has a borated object separating means for separating the borated object and the silicon.

  This allows the borated body to be immersed in the molten silicon when necessary, and placed outside the crucible when the silicon solidifies, reducing the risk of breakage of the borated body, etc., and enabling repeated use. Cost can be reduced.

FIG. 1 is a schematic sectional view of a silicon purification apparatus for explaining the silicon purification method of the present invention. In a crucible 2 made of graphite or the like placed in the melting furnace 1, MG-Si having a purity of 98.0% to 99.9% (hereinafter,% is based on weight) as crude purified silicon, and a purity of 99. 0 to 99.9% Ti is added to make molten metal 3. Electric power is supplied to the heating mechanism 5 to heat the furnace so that the furnace temperature is 1414 ° C. to 1700 ° C., which is equal to or higher than the melting point of silicon, and MG-Si and Ti are dissolved. The boride body 4 is made of TiB ₂ and arranged along the inner surface of the crucible 2. After melting, a stirring mechanism 6 having a rotation drive unit (not shown) is inserted into the molten metal 3 to stir the molten metal 3. In the molten metal 3, a mixed liquid phase is formed with MG-Si in a state where Ti is also dissolved. The dissolved Ti and B contained in the MG-Si are TiB _x (X = 1−2) on the surface of the boride body 4 arranged in the molten metal 3 or the crucible 2, and B is removed. Is done.

In the present invention, since the boride body 4 made of large TiB ₂ is arranged inside the crucible 2, that is, inside the reaction system of B and Ti, even if B or Ti collide with the surface of the boride body 4 alone, The probability of being adsorbed on the surface becomes very high, and as a result, the reaction probability of B and Ti increases. Moreover, since the solid (crystal body) of the boride body 4 made of TiB ₂ is arranged, B is removed as TiB ₂ grows on the surface thereof. Rather than separating after growing into a particle size that can be separated by coagulation sedimentation or centrifugation, in the molten metal that is in the liquid phase as in the prior art, the individual atoms collide and combine, B can be removed very quickly.

In the silicon purification method disclosed in Patent Document 4, since a boride is not disposed in the crucible and B and Ti only react in the molten metal, B and Ti are TiB _x (X = 1−2). The reaction rate of the reaction that produces) is very low.

  The furnace atmosphere may be an inert gas atmosphere such as Ar or a vacuum atmosphere. Since it does not affect the reaction system of B and Ti, it can be carried out in an atmosphere used in normal silicon purification. Since the crucible 2 does not directly affect the reaction system, a crucible made of graphite, quartz, or the like that is usually used can be used.

The melting temperature is 1414 ° C. or higher at which silicon melts, and the upper limit is preferably high for the reaction in which B and Ti generate TiB ₂ , but is limited by other factors. Other factors include the heat resistance of components such as crucibles and the heat resistance temperature of borides to be arranged, but also depend on the furnace atmosphere. In order to suppress the evaporation amount and power consumption of silicon, 1414 ° C. to 2000 ° C., more preferably 1450 ° C. to 1700 ° C., which melts stably and suppresses excessive power consumption is appropriate. In particular, when the furnace atmosphere is vacuum, a low temperature of 1700 ° C. or lower is preferable in order to suppress the evaporation of silicon and increase the purification yield.

As a heating means for dissolving silicon or the like, a heating means such as a resistance heating method or a high frequency induction heating method used for melting silicon can be used. In addition, MG-Si in the crucible and a metal element simple substance forming a high melting point boride such as Ti can be dissolved, and B and Ti do not affect the reaction of generating TiB ₂ in the molten metal 3, An electron beam melting method or a plasma torch melting method can also be used.

  Regarding the purity of MG-Si used as a raw material, since it does not essentially affect the effects of the present invention, those usually used can be used. When a low purity of 98% or less is used, it takes time for the purification process for raising the entire silicon purity such as removal of not only B but also metals and P, which is undesirable because the purification cost is high. . In addition, high purity MG-Si of 3N or higher is subjected to a purification process such as acid washing at the stage before being supplied as a raw material, so that the raw material cost becomes high, and even if a material with a purity higher than necessary is used, the final purification is performed. Since the cost is not lowered, a material having a purity of about 98.0% to 99.9% is suitable.

  The amount of MG-Si to be loaded is determined by the crucible and melting furnace to be used, and is not particularly limited, but 100 kg to 1000 kg is loaded.

  Similarly, the purity of Ti to be added does not affect the reaction system, but silicon may be contaminated by impurities contained in Ti, which affects the entire purification process. Usually, a material having a purity of about 99.0% to 99.9% used as a Ti material is easy to obtain and is suitable because the degree of back-contamination is small. In addition, regarding the addition amount, at least the number of atoms that is 1/2 of the amount of B contained in MG-Si to be purified is required. Normally, MG-Si contains Ti as an impurity at least as much as B, so in principle it can be purified without addition of Ti, but in order to shorten the purification time, 10% of B contained is included. It is preferable to add about 1 to 1000 times of Ti. Even if the amount of Ti to be added is large, there is no adverse effect on the reaction of B and Ti, but the Ti concentration of the silicon to be refined must also be 0.1 ppm or less, and the purification process for removing Ti takes time and effort. Because of this, too much is inappropriate.

  Further, Ti to be added is used in the form of powder or a lump of several mm to several cm or more. If it becomes too large, the effects of surface oxidation and the like are reduced, but there are few advantages because it takes time for complete dissolution. Conversely, in the case of a fine powder of several μm or less, there are problems such as rising when the furnace air is evacuated, surface oxidation, material handling, etc., and there are few advantages as well. The other metal element materials forming the high melting point boride shown in the following second and third examples are not particularly limited in terms of size and shape, and handling or dissolution time, etc. In view of the above, those having an average particle diameter of several tens of μ to several mm are used.

  Further, the elemental metal element to be added is not limited to Ti, and it is sufficient that the elemental metal material to be added and the metal element forming the boride of the boride object to be arranged are the same. For example, the same effect can be obtained with zirconium (Zr), vanadium (V), hafnium (Hf), tantalum (Ta), niobium (Nb), chromium (Cr) or molybdenum (Mo). These elements form a liquid phase with silicon above the silicon melting temperature, and form a high melting point boride with B in the liquid phase. The number of metal elements to be added is not limited to one, but may be at least one and added so that the total amount added is the required amount described above.

Similarly, the boride forming the boride body to be arranged is not limited to TiB _2, and the boride of the element can be applied. Melting points of these borides are TiB ₂ : 3063 ° C, ZrB ₂ : 3473 ° C, HfB ₂ : 3523 ° C, VB ₂ : 2673 ° C, NbB ₂ : 3273 ° C, TaB ₂ : 3310 ° C, CrB ₂ : 2473 ° C, MoB: It is a high temperature of 2823 ° C. and is stable even at a silicon melting temperature of 1414 ° C. to 2000 ° C.

When the metal element material to be added is Zr, ZrB ₂ body is used as the borated body, and when two types of Zr and Hf are added, the borated body is also ZrB ₂ body and HfB ₂ body 2 Use type. The two types of borides may be an integral mixture or separate. However, in the case of an integral mixture, a lump having a certain surface area (several cm ² or more) is integrated. Further, the composition ratio of the exemplified boride body is shown by the stoichiometric composition ratio of a typical boride, but the actual composition may deviate from the stoichiometric composition ratio. The ratio of B: metal element is not limited to boride having a composition ratio of 2: 1, but other composition ratios such as 1: 1 can be applied as long as the boride is stable in the molten metal 3.

  Further, the borated object 4 may be placed in the crucible 2 in advance before the MG-Si is charged, or may be inserted into the crucible 2 by the borated object moving mechanism 7 after the MG-Si or the like is melted. Equipped with a plurality of borides, some of them may be placed in advance in the crucible, and some of them may be inserted after melting the silicon. However, when the borated object is inserted into the crucible after melting, the heat of the molten metal 3 is suddenly removed, resulting in partial solidification or the like. Further, since it causes cracking or breakage of the borated body 4 due to thermal shock, it is necessary to determine the insertion speed in consideration of the heat capacity and thermal shock property of the borated body 4 to be inserted. In some cases, it is possible to provide a mechanism for preheating the borated object. As shown in FIG. 3, it is necessary to pay attention to such breakage of the borated object even when the stirring mechanism and the borated object are integrated. Further, added Ti may be charged in the crucible in advance, or may be added by a Ti supply mechanism (not shown) after silicon is melted. Further, it may be additionally supplied during the purification. What is necessary is just to add required amount so that the addition amount of Ti may be about 10 to 1000 times the weight of B contained.

Furthermore, the shape of the borated object such as _two TiB bodies to be arranged is not limited to that shown in FIG. For example, the one shown in FIG. 2 can be exemplified. FIG. 2 (a) is the cylindrical type used in FIG. 1, but the columnar and plate-shaped ones shown in FIGS. 2 (b) and 2 (c) or one plate can be used. Alternatively, a plurality of them may be used. Any material may be used as long as boride formation occurs on the surface and can be separated at the time of silicon solidification. Although not shown, even a boride body in the form of particles or small pieces having an average particle diameter of several μm to several mm is effective because boride formation occurs on the surface and the removal rate of B is increased. Even when the particles or small pieces of borides are circulating in the molten metal 3 during the stirring, they are precipitated when the stirring is stopped, or by the centrifugal separation effect during the rotation cooling solidification described later, Since they gather near the inner wall, they can be separated from the solidified silicon. Furthermore, a boride object as shown in FIG. 3 is also applicable. 3 (a) and 3 (b) show a stirring mechanism 6 having a rotating shaft 6a and a stirring unit 6b made of a borated body. The stirring mechanism 6 shown in FIG. 3 (a) is provided with a boric body 6b concentrically around a rotating shaft 6a. The stirring mechanism 6 shown in FIG. 3 (b) is arranged in the axial direction of the rotating shaft 6a. A plurality of plate-like borides 6b are provided in a direction extending in the vertical direction. The boride object can be molded by sintering or shaped by electrical discharge machining.

Stirring has the effect of accelerating the movement of Ti and B in the molten metal 3 to increase the contact with the TiB ₂ body and increase the TiB ₂ formation rate. However, the convection in the molten metal 3 is possible even without the stirring mechanism 6. Therefore, it is possible to supply Ti and B to the surface of the TiB ₂ body.

  As shown in FIG. 5, the rotary cooling mechanism 9 may also serve as a stirring mechanism. The rotary cooling mechanism 9 may be inserted into the molten metal 3 and rotated at several rpm to several hundred rpm to stir the molten metal 3. The rotary cooling mechanism 9 communicates with the hollow rotary shaft 9a, the hollow rotary cooler 9b fixed to the lower end of the hollow rotary shaft 9a, and the internal space of the hollow rotary shaft 9a (not shown) and the internal space of the hollow rotary cooler 9b. And a cooling fluid supply means for supplying a cooling fluid into the hollow rotary cooling body.

A cooling medium such as nitrogen gas (N ₂ gas) is supplied from the cooling fluid supply means to the hollow rotary cooling body 9b through a rotary cooling device communicating with the internal space of the hollow rotary shaft 9a and the internal space of the hollow rotary cooling body 9b. Then, the hollow rotary cooling body 9b immersed in the molten metal 3 is cooled. By cooling in this way, the silicon 3a is solidified on the outer peripheral portion of the hollow rotary cooling body 9b.

The cooling medium may be an inert gas such as helium (He) or argon (Ar) or dry air in addition to N ₂ . Further, instead of gas, a liquid such as water or alcohol or a mixture with gas may be used. However, in the case of liquids such as water, the rotary cooling device or breakage of the hollow rotary cooling body 9b due to rapid cooling, because of the risk of such pressure increase due to vaporization, handling is easy, the gas such as inexpensive N ₂ preferable. The supply amount of the cooling medium is a parameter to be determined from the flow rate necessary for solidifying silicon by heat exchange on the surface of the hollow rotary cooling body 9b and the solidification rate of silicon. When the outer diameter of the hollow rotary cooling body 9b is 150 mm to 200 mm and the solidification rate is 100 g / min to 5 kg / min, an N ₂ flow rate of 100 L / min to 10000 L / min is given as an example. The rotational speed of the rotary cooling mechanism 9 affects the stirring effect of the molten metal 3 around the solidification part, and affects the amount of metal impurities such as Fe contained in the solidified silicon together with the solidification rate due to the cooling medium flow rate. That is, silicon solidification by the rotary cooling mechanism 9 is unidirectional solidification, and impurities such as metals can be removed by solidification segregation at the solid-liquid interface. Since the solidification segregation effect depends on the solidification rate and the impurity concentration in the liquid phase at the solid-liquid interface, increase the rotation speed to replace the liquid phase at the solid-liquid interface with a high impurity concentration with the surrounding low-concentration liquid phase. Alternatively, by mixing and reducing the concentration, the concentration of impurities such as metal contained in the solidified silicon can be lowered even if the solidification rate is increased. In the above solidification speed example, a rotation speed of 30 rpm to 600 rpm can be exemplified. If it is too slow, the metal impurity concentration becomes high. Conversely, even at a high speed of 600 rpm or more, the effect on the impurity concentration is small, and the load on the rotating mechanism is increased, resulting in an increase in apparatus cost or an apparatus failure. After stirring to promote the reaction of Ti and B, the rotary cooling mechanism 9 used as the stirring mechanism is once pulled up from the molten metal 3, and after the stirring flow of the molten metal 3 is suppressed, it may be immersed again and rotationally solidified. It doesn't matter. After 1 kg to 30 kg of silicon is solidified around the hollow rotary cooling body 9b in this way, the hollow rotary cooling body 9b is pulled up from the molten metal 3. The silicon is moved to another location in the melting furnace 1 (not shown) or another apparatus, and the solidified silicon is removed from the surroundings of the hollow rotary cooling body 9b, and rotary cooling solidification is performed again.

Alternatively, the degree of vacuum in the melting furnace 1 may be 10 ⁻² Torr to 10 ⁻⁵ Torr or less, and the melting furnace 1 may be vacuumed. By modifying the vacuum pumping mechanism 8 to a high vacuum vacuum pump, it is possible to remove B, P, and metal impurities in the same melting furnace with a single purification process.

As silicon is solidified sequentially, the silicon in the crucible is reduced, so that new MG-Si is additionally mounted using a material MG-Si supply means (not shown). The method of replenishment is also a method in which 0.1 kg to 10 kg units are supplied continuously or intermittently to supplement the amount of solidification in order, and after purifying 100 kg units of silicon, 100 kg units of MG-Si are supplied into the crucible 2 It does not matter how you do it. A small crucible for supplying molten Si is provided even if the silicon crushed to a few millimeters or less is poured from the top of the Si melt near the crucible wall using a graphite gutter. Then, the crucible 2 may be tilted to add molten Si. Depending on the heat capacity of the furnace, the capacity of the heating mechanism, and the amount of MG-Si to be added, a time for melting the added MG-Si is provided as necessary. In this way, when solidification and replenishment are repeated, the concentration of metal impurities such as Fe in the molten metal 3 increases. Then, the whole if the above concentration limit is determined by the purification process, the boride object moving mechanism 7 connected to the boride body 4 consisting of TiB _2, raising the boride object 4 from the molten metal 3. Thereafter, the remaining molten metal 3 is discharged by a method such as cooling the crucible 2 to solidify the molten metal 3 or tilting the crucible 2.

  Ti may be added every time MG-Si is added, but since it is added in a large amount in the initial dissolution, it may be added as needed.

  It is said that the allowable concentration of metal impurities in purified silicon differs depending on the type of metal, and for example, the Fe concentration needs to be 0.1 ppm or less. Even if the solidification segregation does not become 0.1 ppm or less due to the above-mentioned solidification segregation, the average Fe concentration reaches an impurity concentration that satisfies the SOG-Si specification by solidification segregation at the time of manufacturing the solar cell ingot or another rotation cooling solidification segregation. It is also possible to reduce the concentration.

  Further, even if the silicon solidification method is not the above-described rotary solidification method, after the borated object 4 is pulled up from the molten metal 3, the crucible 2 is gradually moved downward by a crucible 2 moving mechanism (not shown), and the molten metal 3 is moved to the crucible 2. It may be solidified from below. Since the moving speed of the crucible 2 affects the removal of metal impurities such as Fe and Ti, it becomes a process condition set depending on the subsequent purification process. Usually, it moves at 0.01 mm / min to several mm / min.

  The borated object moving mechanism 7 is not necessary as the B removal process, but the borated object moving mechanism 7 pulls up the borated object before the molten metal 3 is solidified in order to use the borated object 4 a plurality of times. Is preferred. It is possible to solidify the crucible 2 while leaving the borated body 4 left, crush the solidified silicon lump and take out the borated body 4, but the risk of breakage is high. The removal in advance before solidification has the advantages of reducing the purification cost and simplifying the time for taking out the solidified silicon.

  Furthermore, the added metal element alone can be removed by the solidification segregation effect at the time of silicon solidification or by a separate metal removal step, so that the performance and purity of the final purified silicon are not adversely affected and the purification process is complicated or complicated. There is almost no inconvenience. In addition, since a B absorber such as slag or an oxidizing gas is not used, a complicated separation process between slag and silicon is not required, and silicon oxide is not generated by the oxidizing gas. Disappear. Moreover, since the boride body can be reused and the amount of the metal element to be added is very small, it is possible to reduce the cost of the secondary member. Furthermore, it is possible to provide a purification method that can be easily combined with other impurity removal steps other than B. By providing a hollow rotating cooling body, the B removal step and the metal removal step can be carried out almost simultaneously with the same purification device, so reduction in purification time and reduction in the number of purification devices are expected. it can. Further, by setting the inside of the melting furnace to a reduced pressure atmosphere, volatile impurities such as P contained in the molten silicon can be removed without providing a separate purification step and a purification device, so that the purification cost can be reduced. it can.

FIG. 4 is a schematic cross-sectional view of the melting furnace used in the first embodiment. The same components as those in FIG. 1 are given the same numbers. Based on this figure, the purification method of MG-Si will be described. A boride body 4 made of TiB ₂ was previously placed in a quartz crucible 2 externally reinforced with a graphite material. Next, 200 kg of MG-Si having a purity of 99.0% and 200 g (0.1%) of a Ti powder having a purity of 99.9% (average particle size: 120 μm) were put in an Ar atmosphere in the melting furnace 1. Replaced with Since MG-Si having a purity of 99.0% contained about 15 ppm of B and 1200 ppm of Ti, the actual weight ratio of B and Ti in the furnace was 15: 1200, and the atomic ratio was about 1:18. After exhausting the furnace atmosphere to 0.1 Torr by the vacuum exhaust mechanism 8, the exhaust system was closed, and Ar gas was introduced from a gas introduction port (not shown). Thereafter, Ar gas was kept flowing until the crucible 2 was cooled to keep the inside of the furnace in a non-oxidizing atmosphere, and Ar gas that became positive pressure was discharged from a gas discharge port (not shown). Next, electric power was supplied to the resistance heating and heating mechanism 5, and MG-Si and the added Ti were dissolved at 1500 ° C. in the crucible 2. After melting, the stirring mechanism 6 was inserted into the molten metal 3 and rotated at 30 rpm, and the molten metal 3 was stirred. After stirring for 2 hours, the stirring mechanism 6 was pulled up from the molten metal 3, and the borated body 4 was pulled up from the molten metal 3 by the borated body moving mechanism 7 connected to the borated body 4 made of TiB ₂ . Thereafter, the crucible 2 was gradually moved downward at 0.1 mm / min by a crucible 2 moving mechanism (not shown), and the molten metal 3 was solidified from below the crucible 2.

  After the solidified silicon was taken out from the crucible, the upper portion with a large amount of metal impurities was cut out and transferred to the next P removal step or metal impurity removal step. The B concentration of the purified silicon was reduced to about 0.2 ppm, which satisfied the SOG-Si specification.

Next, a second embodiment according to the present invention will be described. FIG. 5 is a schematic cross-sectional view of the melting furnace used in the second embodiment. A boride body 4 made of ZrB ₂ was previously placed in the graphite crucible 2. Next, 250 kg of 98.5% purity MG-Si and 250 g (0.1%) of 98.0% purity Zr powder (average particle size: 50 μm) were added, and the melting furnace 1 was filled with an Ar atmosphere. Replaced with MG-Si with a purity of 98.5% contained about 2000 ppm Fe and about 30 ppm B. After exhausting the furnace atmosphere to 0.1 Torr by the vacuum exhaust mechanism 8, the exhaust system was closed, and Ar gas was introduced from a gas introduction port (not shown). Thereafter, Ar gas was kept flowing until the crucible 2 was cooled. Ar gas at a positive pressure was discharged from a gas outlet (not shown). Next, electric power was supplied to the high-frequency induction heating mechanism 5, and MG-Si and the added Zr were melted at 1600 ° C. in the crucible 2. After melting, a rotary cooling mechanism 9 that also serves as a stirring mechanism was inserted into the molten metal 3 and rotated at 40 rpm to stir the molten metal 3. After stirring for 2 hours, a cooling medium of nitrogen gas (N ₂ gas) is supplied at a N ₂ flow rate of 1000 L / min by the cooling fluid supply means, and silicon 3a is provided on the outer peripheral portion of the hollow rotary cooling body 9b having an outer diameter of 200 mm. 10 kg was solidified.

A new MG-Si was added using a material MG-Si supply means (not shown). Using a graphite gutter, silicon crushed to 10 mm or less was added so as to flow from the upper part of the Si melt near the crucible wall. When the concentration reached the limit concentration determined in the overall purification process, the borated object 4 was pulled up from the molten metal 3 by the borated object moving mechanism 7 connected to the borated object 4 made of ZrB ₂ . Thereafter, the remaining molten metal 3 was discharged by a method of inclining the crucible 2. As one cycle, when 1000 kg of silicon was purified and the average impurity concentration was analyzed, the B concentration was 0.3 ppm and Fe was 0.6 ppm. In order to use it as a solar cell, the Fe concentration needs to be 0.1 ppm or less. Therefore, the Fe concentration was reduced to 0.1 ppm or less by solidification segregation at the time of manufacturing the solar cell ingot. As described above, according to the second embodiment of the present invention, B and metal impurities can be simultaneously removed in the same furnace and in almost a single purification step, and the impurity concentration satisfies the specification of SOG-Si. It was.

Next, a third embodiment relating to the present invention will be described with reference to FIG. 5 used in the second embodiment. A boride body 4 made of VB ₂ was previously placed in the graphite crucible 2. Next, 250 kg of MG-Si having a purity of 99.5% and 250 g (0.1%) of a V powder (average particle size: 100 μm) having a purity of 98.0% were put in, and the melting furnace 1 was evacuated. The mechanism 8 was evacuated. 99.5% pure MG-Si contained about 1000 ppm Fe, about 20 ppm B, and about 30 ppm P. After the atmosphere was reduced to 0.1 Torr or less, power supply to the resistance heating and heating mechanism 5 was started. Then, the degree of vacuum in the melting furnace 1 was set to 10 ⁻⁴ Torr or less, and MG-Si and the added Zr were melted at 1600 ° C. in the crucible 2. After melting, a rotary cooling mechanism 9 that also serves as a stirring mechanism was inserted into the molten metal 3 and rotated at 60 rpm to stir the molten metal 3. After about 2 hours of stirring, the silicon was solidified by a rotary cooling mechanism as shown in the second example. B contained in MG-Si was removed by the reaction of B and V in the molten metal 3 and the surface of the borated body 4, and volatile P was also removed at the same time because of the reduced pressure atmosphere. Since stirring was performed, there was an advantage that the removal rate of P was increased. The average impurity concentration obtained by purifying 1000 kg of silicon was 0.2 ppm for B, 0.1 ppm for P, and 0.3 ppm for Fe, and B and P satisfied the SOG-Si specification.

  As described above, according to the third embodiment of the present invention, the melting furnace is made into a vacuum specification, and the vacuum evacuation mechanism is provided with a high vacuum vacuum pump. The process was able to remove B, P and metal impurities.

As a comparative example, the crude silicon was purified by the conventional method disclosed in Patent Document 4. A graphite crucible containing 2 kg of MG-Si with a purity of 99.7% (B concentration: 10 ppm) and 1% of powder Ti with a purity of 99.9% was charged in a graphite crucible and heated to 1500 ° C. in an Ar atmosphere. -Si and Ti were dissolved. After dissolution, the stirring of the molten metal performed 2 hours with a stirring mechanism, and the B and Ti were reacted to generate TiB _2. After stirring, 300 g of silicon was solidified using a rotating cooling body. Thereafter, the temperature of the crucible was lowered, the sample was collected, and the B concentration contained in silicon solidified in the crucible and purified silicon solidified in the cooling rotator was analyzed to be 14 ppm and 11 ppm, respectively.

  The equilibrium segregation coefficient of B is 0.8, and it is 11/14 = 0.79 when it is considered that the change in the B concentration in the experimental result is simply due to segregation. Considering that the solidification rate by the rotating cooling body is high, it is considered that the effect more than the segregation by simple unidirectional solidification is obtained, but in the case of SOG-Si, the residual B concentration is considered to be 0.3 ppm or less. It has been. Therefore, when refining from MG-Si containing B of several ppm to several tens of ppm to a B concentration of 0.3 ppm or less, the purification time becomes too long and cannot be applied to production.

It is a schematic sectional drawing of the silicon purification apparatus explaining the silicon purification method of this invention. It is the schematic which shows the example of a shape of the boride body in connection with this invention. It is the schematic which shows the structural example of the boride body in connection with this invention. It is a schematic sectional drawing of the silicon refinement | purification apparatus explaining the 1st Example of this invention. It is a schematic sectional drawing of the silicon refinement | purification apparatus explaining the 2nd and 3rd Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Melting furnace 2 Crucible 3 Molten metal 4 Boride body 5 Heating mechanism 6 Stirring mechanism 7 Boride body moving mechanism 8 Vacuum exhaust mechanism 9 Rotation cooling mechanism

Claims (8)

  In the silicon purification method for melting impurities in a roughly crucible disposed in a melting furnace and removing impurities contained in the roughly purified silicon, at least one kind of boride body made of boride is contained in the crucible. A method for purifying silicon, characterized in that it is in contact with the roughly purified silicon containing the same metal element as the metal element forming the boride.   A hollow rotating cooling body is immersed in silicon melted in the crucible, a cooling medium is supplied into the rotating cooling body while rotating the hollow rotating cooling body, and silicon is placed on the surface of the hollow rotating cooling body. 2. The silicon purification method according to claim 1, wherein the silicon is solidified.   3. The silicon refining method according to claim 1, wherein the melting furnace is made to be in a reduced pressure atmosphere after at least the roughly purified silicon is melted.   The silicon purifying method according to any one of claims 1 to 3, wherein the temperature in the melting furnace is 1450 ° C to 1700 ° C.   The said rough refined silicon contains the said metal element single-piece | unit of the weight of 10 times-1000 times the weight of the boron (B) which is an impurity contained in the said roughly refined silicon. The silicon purification method according to one.   The metal element forming the boride is titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), tantalum (Ta), niobium (Nb), chromium (Cr) or molybdenum (Mo). The silicon purification method according to claim 1, wherein the silicon purification method is provided.   2. A silicon purification apparatus for purifying silicon by the silicon purification method according to claim 1, comprising a crucible containing silicon, a heating means for heating the silicon and the crucible, and the borated body.   The silicon purification apparatus according to claim 7, further comprising a borated object separating unit that separates the borated object and the silicon from the crucible.

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