JP4454520B2 - Silicon casting method - Google Patents

Description

  The present invention relates to a silicon casting method for producing a polycrystalline silicon ingot.

  Solar cells are expected to be put to practical use in a wide range of fields, from small households to large-scale power generation systems, as a clean petroleum alternative energy source. These are classified into crystalline, amorphous, and compound types depending on the type of raw materials used, and most of them currently on the market are crystalline silicon solar cells. This crystalline silicon solar cell is further classified into a single crystal type and a polycrystalline type. Single crystal silicon solar cells have the advantage that the conversion efficiency is easy to increase because the quality of the substrate is good, but the disadvantage is that the manufacturing cost of the substrate is high.

  On the other hand, polycrystalline silicon solar cells have been distributed in the market for a long time, but in recent years, the demand is increasing as interest in environmental issues is increasing, and higher conversion efficiency is required at lower cost. . In order to satisfy these requirements, it is necessary to reduce the cost and quality of the polycrystalline silicon substrate, and it is required to produce a high-purity silicon ingot with a high yield.

  A polycrystalline silicon substrate used for a polycrystalline silicon solar cell is generally manufactured by a method called a casting method. In this casting method, a silicon melt that has been melted by heating at a high temperature is poured into a mold made of quartz or the like coated with a release material to solidify in one direction from the bottom of the mold, or a silicon raw material is placed in the mold and melted once. Then, the silicon ingot is formed by unidirectionally solidifying again from the bottom. The ends of the silicon ingot are removed, cut into a desired size, and the cut silicon ingot is sliced to a desired thickness to obtain a polycrystalline silicon substrate for forming a solar cell.

  As a means for producing such a polycrystalline silicon ingot, a pouring method in which a melting furnace is provided to melt a silicon raw material, and the molten silicon melt is poured into a mold and cooled and solidified. In the casting apparatus, it is necessary to prepare the melted part and the solidified part separately, which increases the size of the apparatus. However, in the method by melting in the mold, in which the silicon raw material is melted in the mold and solidified in the mold as it is, there is no need to provide a melting furnace, so the apparatus is not enlarged, and an expensive member such as a high-purity quartz crucible Can be eliminated, and the cost of the polycrystalline silicon ingot can be reduced. Therefore, it is more advantageous to produce a polycrystalline silicon ingot using the method described later.

  Here, FIG. 4 shows a conventional silicon casting apparatus for in-mold melting.

  In FIG. 4, 11 is a mold, 12 is a cooling section, 13 is a heat insulating wall, 14 is a mold heating section, 15 is a silicon raw material, and 18 is a mold release material. As the mold 11, one in which a mold release material 18 is coated on the inner surface of the mold 11 made of quartz or graphite is usually used. In general, as the release material 18, powders such as silicon nitride which is silicon nitride, silicon carbide which is silicon carbide, silicon dioxide which is silicon oxide, and the like are used, and these powders are mixed with an appropriate binder and solvent. It is known as a known technique to mix and agitate in a solution composed of the following to form a slurry and coat the inner wall of the mold by means such as coating or spraying. In the silicon casting apparatus shown in FIG. 4, a silicon raw material 15 is supplied into the mold 11, the silicon raw material 15 is heated and melted to about 1500 ° C. by the mold heating unit 14, and the mold 11 is gradually lowered to form silicon. The melt 15 is gradually separated from the mold heating unit 14 to be cooled, or the position of the mold 11 is left as it is and the temperature is lowered by the cooling unit 12 to cool, and the silicon melt 15 is solidified in the mold 11 to form silicon. An ingot is formed (for example, refer to Patent Document 1 and Non-Patent Document 1). This silicon ingot can be taken out of the mold 11 by breaking the mold 11 or can be taken out by disassembling the mold 11. These are all arranged in a vacuum vessel (not shown).

  The bulk specific gravity of the silicon raw material in the mold used in such a silicon casting method is usually 1.0 to 1.5, which is about half that of the specific gravity 2.33 of silicon. Therefore, in the conventional silicon casting apparatus shown in FIG. 4, the height of the silicon ingot formed by solidifying the silicon melt 15 is only about half that of the mold 11. The mold 11 and the mold release material 18 applied to the inner wall surface of the mold 11 are expensive consumables and have a large proportion of the running cost. Therefore, the conventional technique has a problem that the required mold 11 is large with respect to the size of the ingot, and the mold 11 and the release material 18 per weight of the ingot are expensive.

  FIG. 5 shows a conventional raw material addition type silicon casting apparatus. With respect to this problem, in Patent Document 2, by using a silicon casting apparatus as shown in FIG. 5, an additional silicon raw material 27 can be supplied into the mold, and a silicon casting method that effectively utilizes the height of the mold. be able to.

  Here, 21 is a mold, 22 is a cooling unit, 23a is a heat insulating material, 23b is a movable door, 24 is a heating unit, 25 is a silicon raw material (silicon melt), 26 is a raw material supply unit, 26a is an introduction member, and 26b is An opening / closing port, 26c is a raw material supply tank, 27 is an additional silicon raw material, and 28 is a release material.

A movable door 23b is provided above the heat insulating wall 23a on the mold, and a raw material supply unit 26 is provided on the movable door 23b so that an additional silicon raw material 27 can be supplied from the raw material supply unit 26 into the mold. ing. As the silicon raw material charged into the mold is melted in this way, the silicon melt can be increased by additionally supplying the raw material into the mold after the bulk of the silicon raw material is reduced. The cost of the hit mold and release material can be reduced.
U.S. Pat. No. 3,898,051 Japanese Patent Laid-Open No. 10-139586 5th International PVSEC p303-305

  However, in the conventional method of supplying the additional silicon raw material 27 as shown in Patent Document 2, the silicon melt 25 in the mold may jump off due to the added additional silicon raw material 27. When the silicon melt 25 in the mold splashes, the silicon melt 25 may adhere to the inner surface of the mold, the heating unit 24, or the heat insulating material 23 in severe cases.

  Then, when the additional silicon raw material 27 is charged, the movable silicon door 23b is opened, so that the silicon melt 25 attached is cooled and solidified, and is heated again after the additional silicon 27 is charged. Due to the difference in coefficient of thermal expansion between the adhered and solidified silicon layer and the release material 28, there was a problem that the release material 28 was peeled and damaged and mixed into the silicon melt. Further, in severe cases, the mold release material 28 is peeled off so that the mold material 28 and the silicon melt 25 come into contact with each other, which may lead to a problem that the silicon ingot is cracked and the product yield is lowered.

  The present invention has been invented in view of such conventional problems. By adding an additional silicon raw material, the mold height can be effectively utilized, and the mold release material on the mold inner surface can be used. It is an object of the present invention to provide a casting method that suppresses a decrease in yield due to mold cracks and mixing of a release material without causing damage.

  As a result of extensive studies by the inventor, the silicon raw material 25 charged in advance in the mold is also heated from the upper part of the mold. If an additional silicon raw material 27 is introduced from the raw material supply unit 26 in this state, the liquid surface of the silicon melt is heated to the highest temperature in the mold and becomes a high temperature, and the viscosity is low and the silicon melt 25 jumps to the inner surface of the mold. I found out that

To achieve the above object, silicon casting method according to the present onset Ming, upper surface has an opening, and the silicon raw material heated and melted in a mold that is heated at least from the upper surface side and the wall side, the silicon melt And a second step of adding an additional silicon raw material to the mold, and a third step of heating and melting the silicon raw material to form a silicon melt, followed by cooling and solidification. In the casting method, in the second step, an additional silicon raw material is charged in a state where heating from the upper surface side of the mold is stopped.

In the second step, the mold is heated from at least the wall surface side.

Further, in the second step, an additional silicon raw material is dropped into the mold and charged in the vicinity of the position where the temperature is lowest on the surface of the silicon melt.

Further, the additional silicon raw material introduced in the second step satisfies pmax ≧ 1 cm when the maximum particle diameter is pmax (cm), and the maximum diameter of a circle included in the opening of the mold The range of pmax / d ≦ 0.12 was satisfied with respect to d (cm).

  As described above, the present invention includes the first step in which the silicon raw material is heated and melted in a mold heated at least from the upper surface side and the wall surface side to have a silicon melt, and the upper surface has an opening. A silicon casting method comprising: a second step of adding an additional silicon raw material to a second step; and a third step of heating and melting the silicon raw material to form a silicon melt, followed by cooling and solidification. In the process, an additional silicon raw material was added while heating from the upper surface side of the mold was stopped.

A first step of heating and melting a silicon raw material in a mold to form a silicon melt, a second step of adding additional silicon raw material to the mold, and heating and melting the silicon raw material to obtain a silicon melt, a third step of cooling to solidify, a silicon casting method including the additional silicon raw material to be introduced in the second step, when the maximum particle diameter is p _{max (cm),} p _max The range of p _max /d≦0.12 is satisfied with respect to the maximum diameter d (cm) of the circle that satisfies ≧ 1 cm and that the opening of the mold encloses.

  Therefore, by using the silicon raw material in the mold with a small bulk specific gravity as the melt and adding the additional silicon raw material to the mold, the height of the mold coated with the release material with high running cost is effective. In addition, the silicon melt in the mold is difficult to splash and the silicon melt can be prevented from adhering to the inner surface of the mold.

  As a result, the cost of the mold and the mold release material per weight of the silicon ingot is reduced, and the mold release material on the inner surface of the mold is not peeled and damaged, and the yield of the silicon ingot is reduced by mixing the mold crack and mold release material. A suppressed casting method can be provided.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a view showing an embodiment of a silicon casting apparatus according to the present invention, wherein 1 is a mold, 2 is a cooling unit, 3a is a heat insulating wall, 3b is a movable door, 4 is a heating unit, 4a is an upper heating unit. 4b is a side heating section, 5 is a silicon raw material or a silicon melt formed by melting a silicon raw material, 6 is a raw material supply section, 6a is an introduction member, 6b is an opening / closing port, 6c is a raw material supply tank, 7 is An additional silicon raw material 8 is a release material.

  The mold 1 is made of graphite, silicon dioxide, or the like, and is configured as an integral structure mold. Such a monolithic mold 1 is disposable in order to destroy and take out the mold 1 when the silicon ingot is taken out, and after the silicon raw material in the mold is melted to effectively use the entire mold, an additional It is preferable to supply the silicon raw material 7.

  A mold release material 8 is applied to the inner surface side of the mold 1 shown in FIG. 1, and the ingot can be taken out after such a mold release material 8 solidifies the silicon melt 5 inside the mold 1. At this time, if the mold release material 8 is not present, the silicon ingot and the mold 1 are fused, and cracks and chips are generated when the mold is taken out. Therefore, the mold release material 8 occupies an extremely important position in the present invention.

  As such a release material 8, each powder of silicon nitride, silicon carbide, silicon oxide or the like or mixed powder is mixed and stirred in a solution composed of an appropriate organic binder and solvent to form a slurry, and a spatula. The inner surface of the mold 1 is coated and dried by using a brush, a brush, a spray method or the like. Common binders include PVA (polyvinyl alcohol), PVB (polyvinyl butyral), PEG (polyethylene glycol), MC (methyl cellulose), CMC (carboxymethyl cellulose), EC (ethyl cellulose), HPC (hydroxypropyl cellulose), and wax. Used for. In addition, when coating the release material 8 mixed with an organic binder aqueous solution to form a slurry, in order to prevent the thermally decomposable product of the organic binder from being mixed into the silicon melt 5 by subsequent heating, A degreasing process is performed. Moreover, as a drying method, the mold release material 8 can be bonded to the mold 1 by performing a degreasing treatment by natural drying or drying using a conventionally known method such as a hot plate or an oven.

  And the casting_mold | template 1 is arrange | positioned inside the heat insulation wall 3a which consists of graphite. The silicon raw material 5 is charged into the mold and heated by the heating unit 4 to obtain a silicon melt 5. Next, an additional silicon raw material 7 is supplied into the mold 1 by the raw material supply unit 6 provided on the upper part of the mold 1. The mold 1, the heat insulating wall 3, the heating unit 4, the raw material supply unit 6, and the release material 8 are installed in a vacuum container (not shown).

  The raw material supply unit 6 includes a raw material supply tank 6c for holding an additional silicon raw material 7 made of, for example, stainless steel, and an introduction member for guiding the additional silicon raw material 7 to the mold 1 below the raw material supply tank 6c. 6a is provided. The introduction member 6a has a through hole h, and an opening / closing port 6b is provided between the through hole h and the additional silicon raw material supply tank 6c.

  When the additional silicon raw material 7 is supplied into the mold 1, the additional silicon raw material 7 can be supplied through the introduction member 6a by opening the opening / closing port 6b from the raw material supply tank 6c. The heating unit 4 is a resistance heating type heater, an induction heating type coil, or the like.

  Thus, since the silicon raw material in the casting_mold | template 1 is heated by the heating part 4 and it is made into a melt state and the additional silicon raw material 7 is supplied, the casting_mold | template 1 with which the release material 8 with the high running cost was apply | coated It is possible to dissolve in the mold by effectively using the height of.

In this case, the density of the silicon ingot (solid) is 2.33 g / cm ³ whereas the density of the silicon melt (liquid) is 2.55 g / cm ^3, when moving from a liquid to a solid In consideration of the volume expansion of about 9% occurring in the above, it is desirable to add the additional silicon raw material 7 so that the silicon melt becomes about 90% of the height of the mold 1.

  In order to make the silicon melt 5 thus formed into a silicon ingot, the mold 1 is removed from the bottom. By removing heat from the lower part of the mold 1, unidirectional solidification of the silicon ingot is realized. However, when the heating is stopped by the side heating section 4b, the silicon melt 5 in the mold has a large heat removal from the liquid surface, so that the liquid surface of the silicon melt 5 is solidified first, and silicon melt If the melt 5 is left inside, the silicon melt 5 left behind will solidify and expand, and the surface of the silicon ingot will appear as if it has erupted, causing cracks in the silicon ingot. In order to prevent this problem, it is desirable to heat the liquid surface of the silicon melt 5 by the upper heating part 4a located above the silicon melt 5 so that the liquid surface of the silicon melt 5 in the mold is not solidified.

  In this way, the silicon melt 5 can be solidified in one direction by removing heat from the lower part of the mold 1 by contacting or approaching the cooling unit 2 provided at the lower part of the mold 1. At this time, as the cooling unit 2, for example, a metal plate such as stainless steel (SUS) can be used, and a coolant such as water is circulated in the inside to effectively remove the silicon melt 5 inside the mold 1. It is desirable to configure so that heat can be removed. Note that the silicon ingot obtained by cooling and solidification is cut and washed to a desired size and used as a polycrystalline silicon substrate material for solar cells.

  The silicon casting method of the first aspect according to the present invention is a first method in which a silicon raw material is heated and melted in a mold heated at least from the upper surface side and the wall surface side to have a silicon melt. A silicon casting method comprising: a second step of adding an additional silicon raw material to the mold; and a third step of heating and melting the silicon raw material to form a silicon melt, followed by cooling and solidification. Then, in the second step, an additional silicon raw material is charged while heating from the upper surface side of the mold is stopped. As described above, since the second step includes the characteristic portion of the invention, the following description will be made focusing on the characteristic portion.

  First, after the first step of the present invention, the inside of the mold is heated and melted and the silicon melt 5 is present. Next, in the second step, the additional silicon raw material 7 is introduced and melted while continuing to heat the mold, but at this time, the additional silicon raw material is introduced while the heating from the upper surface side of the mold 1 is stopped. did.

  Here, the temperature distribution of the silicon melt 5 in the mold at this time will be described.

  Fig.2 (a) is a figure which shows the temperature distribution in the casting_mold | template 1 by the heating part which concerns on this invention. Moreover, FIG.2 (b) is a figure which shows the temperature distribution in the casting_mold | template 1 by the conventional heating part. A1 and A2 are the hottest places inside the mold, B1 and B2 are places where the temperature is the next highest after A1 and A2, and C1 and C2 are places where the temperature is lower than B1 and B2.

  There is a change in the liquid level of the silicon melt 5 of the present invention and the temperature distribution in the prior art, and the liquid surface of the silicon melt 5 has a higher viscosity because the liquid surface temperature of the silicon melt 5 is lower than in the prior art. Become. By supplying the additional silicon raw material 7 in this state, the silicon melt 5 in the mold is less likely to splash and the silicon melt 5 can be prevented from adhering to the inner surface of the mold.

  Hereinafter, the invention mechanism of the effect of the present invention will be described in more detail. FIG. 3 shows a comparison of the jumping of the silicon melt 5 when heated by the heating unit according to the present invention with the conventional one. FIG. 3A is a view schematically showing a silicon casting method (second step) having a heating part according to the present invention, and FIG. 3B is a diagram showing a silicon casting having a conventional heating part 24. It is the figure which showed the method typically.

  Here, 4a is an upper heating part, 4b is a side part heating part, 5 is a silicon melt, and 7 is an additional silicon raw material. As can be seen from the temperature distribution shown in FIG. 2A, in the present invention, since the temperature of the liquid surface of the silicon melt 5 in the mold 1 is low, even when the additional silicon raw material 7 falls, The liquid surface of the silicon melt 5 has a high viscosity and is difficult to splash. As a result, it is possible to prevent the silicon melt from splashing and adhering to the inner surface of the mold, the upper heating unit 4a, the side heating unit 4b, and the heat insulating material 3a.

  However, since the conventional silicon casting method is also heated from the upper surface side of the mold 21, the temperature distribution shown in FIG. As a result, the liquid surface of the silicon melt 25 in the mold 21 has a low viscosity and is likely to splash. Then, as shown in FIG. 3 (b), the silicon melt 25 splashed on the inner surface of the mold adheres, so that the release material 28 at the portion of the adhered silicon melt 25 may be peeled off and mixed into the silicon melt 25. There is.

  Thus, by supplying additional silicon 7 after forming the temperature distribution of the silicon melt 5 in the mold 1 according to the present invention, the mold 1 coated with the release material 8 having a high running cost is applied. The height can be used effectively, and the silicon melt 5 in the mold is difficult to splash, and the silicon melt 5 can be prevented from adhering to the inner surface of the mold. As a result, the cost of the mold 1 and the release material 8 per weight of the ingot is reduced, and the release material 8 on the inner surface of the mold is not peeled and damaged, and the yield of the ingot is reduced due to cracks in the mold 1 and mixing of the release material 8. It is possible to provide a casting method that suppresses this.

  As a more desirable mode, in the first mode of the present invention, heating is performed from at least the wall surface side of the mold 1 in the second step. For example, if the substantially donut-shaped side heating means 4b surrounding the mold 1 is used, the mold 1 may be heated uniformly. And since the temperature of the liquid level of the silicon melt 5 inside the casting_mold | template 1 falls by the heating method of this invention and the thermal radiation from the casting_mold | template 1 to the heat insulation 3a exterior when the movable door 3b is opened, it is preferable.

  Furthermore, in the first embodiment according to the present invention, in the second step, by dropping the additional silicon raw material 7 into the mold 1 and putting it in the vicinity of the position where the temperature is lowest on the liquid surface of the silicon melt 5, It is possible to suppress the silicon melt 5 from adhering to the inner surface of the mold and the silicon melt 5 from scattering to the heating means 4.

Next, in the second aspect of the silicon casting method according to the present invention, a silicon raw material is heated and melted in a mold to form a silicon melt 5, and an additional silicon raw material 7 is charged into the mold 1 for melting. A silicon casting method including a second step of cooling and a third step of cooling and solidifying the silicon melt 5, wherein the silicon raw material charged in the second step has a maximum particle size of p _max , P _max ≧ 1 cm, and the maximum diameter of the circle contained in the opening of the mold 1 at the opening of the mold 1 satisfies the range of p _max /d≦0.12 with respect to d (cm). I did it.

Hereinafter, the maximum particle size p _max of the additional silicon raw material used in the text can be defined using a vibration sieve or the like. The shape of the sieve mesh was a square mesh, and the sieve opening and tolerance were in principle according to JISZ8801. Additional silicon material 7 is classified into first 1cm above, followed by changing the sieve shape to fit the size of the opening of the mold 1 d (cm), maximum particle diameter _p max a (cm) _{p max} /D≦0.12.

  By setting the size of the additional silicon raw material 7 within the above range, the additional silicon raw materials 7 are less likely to collide with each other and energy during the collision is reduced. It drops almost vertically from 6c to mold 1.

  Therefore, since the additional silicon raw material 7 falls within a limited range, the jumping of the silicon melt 5 is suppressed, and the additional silicon raw material 7 protrudes from the opening of the mold 1 or collides with the mold 1. It can suppress that the additional silicon raw material 7 hits the mold 1 wall surface side by the collision of the additional silicon raw materials 7, and it can suppress that the silicon melt 5 adheres to the mold 1 inner surface side. As described above, as a result, the release material 8 is peeled and damaged, and a decrease in yield due to cracks in the mold 1 and mixing of the release material 8 can be suppressed.

  Even if the additional silicon raw material 7 hits the wall surface of the mold 1, the impact is small, so that the release material 8 is peeled off from the wall surface of the mold 1 and mixed into the mold to cause cracks in the silicon ingot. It is possible to suppress problems that occur or the release material 8 is peeled off and taken into the silicon melt 5. Further, it is possible to prevent a problem that the mold release material 8 is peeled off due to the collision and the mold material 8 and the silicon melt 5 are brought into contact with each other to cause cracks in the silicon ingot to reduce the product yield.

  Further, if the additional silicon raw material 7 is within the above range, the surface of the silicon melt 5 in the mold is not rapidly cooled, and the problem that the surface of the silicon melt 5 is solidified is also suppressed. The

  Furthermore, even if additional silicon raw material 7 is added to the mold 1 while continuing to heat the upper surface side of the mold 1, the silicon melt 5 can be prevented from jumping to the inner surface of the mold 1 and the melting time can be further shortened. it can. Further, the liquid level of the silicon melt 5 in the mold is not rapidly cooled, and the problem that the liquid level of the silicon melt 5 is solidified can be further suppressed.

However, when the maximum particle diameter of the additional silicon raw material 7 is in the range of p _max /d>0.12, the additional silicon raw materials 7 are likely to collide with each other, and the impact of the additional silicon raw material 7 at the time of the collision is large. Therefore, the additional silicon raw material 7 is likely to protrude from the opening of the mold 1. As a result, the impact when hitting the wall surface of the mold 1 also increases. And the mold release material 8 peels by the collision with the mold 1 wall surface side, and the produced silicon ingot cracks and the problem that a product yield reduces arises.

  Further, if the through hole h of the introduction portion 6a is narrowed in order to suppress the protrusion of the mold 1 from the opening, it takes time to introduce the additional silicon raw material 7 into the mold, and as a result, the silicon melt in the mold is melted. The liquid level of the liquid 5 is rapidly cooled. Further, even when the size of the additional silicon raw material 7 is increased, the liquid level of the silicon melt 5 in the mold is rapidly cooled, and in a severe case, the liquid level of the silicon melt 5 is solidified. . As a result, a rapid temperature change is given to the release material 8 existing between the mold 1 and the silicon melt 5, which leads to peeling damage of the release material 8.

  Further, when the maximum particle size of the additional silicon raw material 7 is less than 1 cm, the additional silicon raw material 7 is too light and flies by the gas in the melting furnace, and directly sticks to the release material 8 coated on the inner surface of the mold 1. There is a possibility that. The additional silicon raw material 7 adhered to the release material 8 causes the same phenomenon as when the silicon melt 5 jumps and adheres. For example, adhering to the heat insulating material 3 a, the upper heating unit 4 a, etc. is not desirable because it may cause parts to be consumed quickly or the adhering and consumed heat insulating wall 3 a may be mixed into the silicon melt 5.

More preferably, if the maximum particle diameter p _max (cm) of the additional silicon raw material 7 is in the range of 2 cm ≦ p _max and p _max /d≦0.09, the release material 8 is peeled and damaged, and the mold It is possible to more effectively suppress a decrease in yield due to the cracking of 1 and the mixing of the release material.

  In the second embodiment of the present invention, it is desirable to stop heating from the upper side surface of the mold 1 when adding the additional silicon raw material 7 in the second step. Even if the heating from the upper surface is stopped, the additional silicon raw material 7 can be melted, so that the height of the mold 1 coated with the release material 8 having a high running cost can be used effectively. In addition, since the liquid surface temperature of the silicon melt 5 in the mold is low, it is difficult to splash, and the silicon melt 7 can be more reliably prevented from adhering to the inner surface of the mold. As a result, the cost of the mold 1 and the release material 8 per weight of the ingot is reduced, and the release material 8 on the inner surface of the mold is not peeled and damaged, and the yield of the ingot is reduced due to cracks in the mold 1 and mixing of the release material 8. Can be suppressed.

  As a more desirable form, the second form of the present invention is also heated from at least the wall surface side of the mold 1 in the second step. For example, if the substantially donut-shaped side heating means 4b surrounding the mold 1 is used, the mold 1 may be heated uniformly. And since the temperature of the liquid level of the silicon melt 5 inside the casting_mold | template 1 falls by the heating method of this invention and the thermal radiation from the casting_mold | template 1 to the heat insulation 3a exterior when the movable door 3b is opened, it is preferable.

  Furthermore, also in the second embodiment according to the present invention, in the second step, by dropping the additional silicon raw material 7 into the mold 1 in the vicinity of the position where the temperature is lowest on the liquid surface of the silicon melt 5, It is possible to suppress the silicon melt 5 from adhering to the inner surface of the mold and the silicon melt 5 from scattering to the heating means 4.

  Here, in the second step according to the first embodiment or the second embodiment of the present invention, it is desirable to define the additional amount per unit time of the additional silicon raw material 7 to be put into the mold. If the additional amount per unit time of the additional silicon raw material 7 is defined, the additional silicon raw material 7 can always be stably supplied into the mold.

  As a result, a rapid temperature change applied to the mold release material 8 existing between the mold 1 and the silicon melt 5 can be reduced, and further, the silicon melt 5 can be prevented from jumping to the inner surface of the mold 1. The mold material 8 is peeled and damaged, and the decrease in yield can be more effectively suppressed due to cracks in the mold 1 and mixing of the mold release material 8.

As one of the methods, it is desirable to arrange an introduction member 6a having a through hole h above the mold 1, and in the second step, an additional silicon raw material 7 passes through the through hole h of the introduction member 6a. Can be introduced into the mold 1. The minimum diameter of the through hole h is 2.5 times or more the maximum particle diameter p _max (cm) of the additional silicon raw material 7 and 0.48 of the maximum diameter d (cm) of the circle inscribed in the opening of the mold. In a range satisfying the following, more preferably, not less than 3 times the maximum particle diameter p _max (cm) of the additional silicon raw material 7 and not more than 0.42 times the maximum diameter d (cm) of the circle inscribed in the opening of the mold It is desirable to make the range satisfy. When the additional silicon raw material 7 that has entered the raw material supply tank 6 c is put into the mold 1, it is possible to further reduce the scattering of the additional silicon raw material 7 outside the mold 1. Further, since the additional silicon raw material 7 is not clogged at the minimum diameter portion of the through hole h, the additional silicon raw material 7 can be supplied smoothly.

However, when the through hole h of the introduction member 6a is less than 2.5 times the maximum particle diameter p _max (cm) of the additional silicon raw material 7, the additional silicon raw material 7 is clogged with this through hole h. End up. In such a case, the inside of the melting furnace is opened for a long time, the temperature in the melting furnace is lowered by the heat radiation from the inside of the melting furnace, the silicon melt 5 in the mold 1 is solidified, and the height of the mold 1 is increased. There is a possibility that a casting method using this property cannot be obtained. In addition, since no additional silicon raw material 7 can be supplied, a casting method using the height of the mold 1 cannot be obtained. Moreover, since it does not have a role which introduce | transduces the additional silicon raw material 7 into the casting_mold | template 1 when it is larger than 0.48 times the maximum diameter d (cm) of the circle | round | yen inscribed in the opening part of the casting_mold | template 1, additional silicon raw material 7 may be scattered outside the mold, or additional silicon raw material 7 may collide with the inner surface of the mold, and the release material 8 may be peeled and damaged by the impact.

  As described above, the silicon ingot formed by being cooled by the cooling unit 2 in the third step according to the first embodiment or the second embodiment of the present invention has reduced cracks in the mold 1 and mixing of the release material 8. Therefore, the silicon ingot can be manufactured with high reliability without reducing the yield of the ingot. Moreover, since the height of the casting_mold | template 1 can be used effectively by throwing in the additional silicon | silicone raw material 7, the silicon | silicone casting method in which the cost of the casting_mold | template 1 per mold weight and a mold release material becomes small can be provided. it can.

  It should be noted that the embodiment of the present invention is not limited to the above-described example, and it is needless to say that various modifications can be made without departing from the gist of the present invention.

  For example, the mold 1 used in the casting apparatus may be a divided structure mold, and even when such a divided structure mold is used, in order to effectively use the release material 8 applied to the inner surface of the mold. After melting the silicon raw material 5 in the mold, it is preferable to supply the additional silicon raw material 7, and the same effect can be obtained by using the silicon casting method of the present invention.

  Further, the through-hole h of the introduction member 6a may be tapered toward the lower direction. This is preferable because the additional silicon raw material 7 can be stably dropped into the mold.

  The upper heating unit 4a is preferably used as a means for heating the mold 1 during solidification of the silicon melt, but may be used during heating in the second step as long as the temperature is lower than that of the side heating unit 4b.

  Further, the effect of the present invention can be obtained even if a heating function is incorporated in the cooling unit 2 and heating and cooling are performed from the bottom of the mold 1.

  The mold 1 is prepared as an integral mold made of quartz having an inner dimension of 350 mm × 350 mm and a depth of 230 mm, and a silicon nitride powder is stirred and mixed with a 11.0% PVA aqueous solution (polyvinyl alcohol) on the mold inner surface to form a slurry. The release material 8 was applied with a spatula and dried with a hot plate to form a release material layer. Then, 35 kg of silicon raw material was charged into the mold.

  A raw material supply unit 6 is provided on the upper part of the mold 1. The raw material supply unit 6 is provided with a raw material supply unit 6 c made of stainless steel, an opening / closing port 6 b, and an introduction member 6 a having a through hole h made of graphite. It has been. At this time, the minimum diameter on the outlet side of the through hole h of the introduction member 6a was 150 mm. And the raw material supply tank 6c hold | maintained 5 kg of additional silicon raw materials 7 per raw material supply tank, and was set as the raw material supply tank provided with the additional silicon raw materials 7 for a total of six 30 kg.

  Further, a water-cooled cooling unit 2 is provided at the lower part of the mold 1, and this cooling unit 2 uses a stainless steel (SUS) metal plate to circulate water kept at a low temperature inside.

  The heating part 4 of a graphite heater is installed in the upper part and side part of the casting_mold | template 1, and the silicon | silicone raw material 5 in the casting_mold | template 1 is fuse | melted by the upper heating part 4a and the side part heating part 4b. After that, the additional silicon raw material in the first raw material supply tank is supplied into the mold, and after all the silicon raw materials in the mold are melted, the additional silicon raw material 7 in the second raw material supply tank is continuously charged. By repeating a series of operations, about 65 kg of the silicon raw material 7 was melted to obtain a silicon melt 5 in the mold. The silicon melt 5 here could fully utilize the height of the mold 1.

  The silicon melt 5 in the mold 1 was gradually solidified in one direction from the bottom of the mold 1 by the cooling section 2 while the mold 1 was heated by the upper heating section 4a. After cooling, the solidified polycrystalline silicon ingot was taken out from the mold 1 and cut into a thickness of about 300 μm with a wire saw. And it wash | cleaned and the silicon substrate was formed.

At this time, a material in which the maximum particle size p _max of the additional silicon raw material was changed from 0.5 cm to 5 cm based on JISZ8801 was prepared. In addition, when the additional silicon raw material 7 was added, the defect rate (%) of the silicon substrate due to foreign matter contamination when the upper surface side of the mold 1 was heated and when not heated was examined.

  From Table 1, the defect rate when the additional silicon raw material 7 is charged without heating the upper surface side of the mold 1 can be reduced as compared with the case where the additional silicon raw material 7 is heated. In the case of 5 cm, the sample No. 13 was 8.6%, while sample No. No. 10 was added in a state where heating from the upper surface side of the mold 1 was stopped. 14 was greatly improved to 5.4%.

When attention is paid to the sample charged by heating the upper surface side of the mold 1, 7. 7 for the sample having the maximum particle diameter of the additional silicon raw material 7 of 0.5 cm or 5 cm (p _max /d=0.143). There were 7%, 8.6%, and a defect rate of 7% or more.

However, in the sample having a maximum particle size of 1 cm to 4.2 cm (p _max /d=0.12) of the additional silicon raw material 7, the defect rate can be reduced to 6% or less, and some silicon is formed on the inner surface of the mold. Although adhesion of the melt 5 was recognized, the effect of the present invention was confirmed without any particular influence.

In particular, when the maximum particle size of the additional silicon raw material 7 is 2 cm to 3.2 cm (p _max /d=0.09), the defect rate is 4% or less, and the silicon melt 5 is hardly adhered to the inner surface of the mold. It was not recognized, and it was possible to effectively suppress the scattering of the additional silicon raw material 7 introduced into the mold 1 and the silicon melt 5 in the mold from splashing, and the effect of the present invention was confirmed.

  As a result, the height of the mold 1 can be used effectively in the present invention, and the yield of the ingot due to the cracks in the mold 1 and mixing of the mold release material 8 without causing the mold release material 8 on the mold inner surface to be peeled and damaged. It was possible to provide a casting method in which the decrease was suppressed.

It is a cross-sectional schematic diagram which shows one Embodiment of the silicon casting apparatus of this invention. (A) is the temperature distribution of the silicon melt in the mold according to the present invention, and (b) is the temperature distribution of the silicon melt in the conventional mold. (A) It is a figure which shows typically the state which added the additional silicon in the silicon casting method which concerns on this invention, (b) showed the state which added the additional silicon in the conventional silicon casting method typically FIG. It is a cross-sectional schematic diagram which shows the conventional silicon | silicone casting apparatus. It is a cross-sectional schematic diagram which shows the conventional silicon | silicone casting apparatus.

Explanation of symbols

1: Mold 2: Cooling unit 3, 3a: Thermal insulation wall 3b: Movable door 4: Mold heating unit 4a: Mold upper heating unit 4b: Mold side heating unit 5: Silicon raw material (silicon melt)
6: Raw material supply unit 6a: Introduction unit 6b: Opening / closing port 6c: Raw material supply tank 7: Additional silicon raw material 8: Release material

Claims (4)

A first step in which the upper surface has an opening and the silicon raw material is heated and melted in a mold heated at least from the upper surface side and the wall surface side to form a silicon melt;
A second step of adding additional silicon raw material to the mold;
A third step of heating and melting the silicon raw material to form a silicon melt, and then cooling and solidifying;
A silicon casting method comprising:
A silicon casting method in which, in the second step, an additional silicon raw material is charged in a state where heating from the upper surface side of the mold is stopped.   The silicon casting method according to claim 1, wherein in the second step, heating is performed from at least a wall surface side of the mold.   3. The silicon according to claim 1, wherein in the second step, an additional silicon raw material is dropped into the mold and placed in the vicinity of a position where the temperature is lowest on the surface of the silicon melt. Casting method. Said additional silicon raw material is charged in the previous SL second step, when the maximum particle diameter and pmax (cm), filled with pmax ≧ 1 cm, and the maximum diameter d of the circle opening of the mold enclosing The silicon casting method according to claim 1 , wherein a range of pmax / d ≦ 0.12 is satisfied with respect to (cm).

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