JP4412919B2 - Casting equipment - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a silicon casting apparatus, for example, a polycrystalline silicon casting apparatus used for manufacturing a semiconductor substrate for a solar cell and the like.
[0002]
[Prior art]
A solar cell converts incident light energy into electrical energy. Major solar cells are classified into crystalline, amorphous, and compound types depending on the type of materials used. Of these, most of the crystalline silicon solar cells currently on the market are in the market. This crystalline silicon solar cell is further classified into a single crystal type and a polycrystalline type. A single-crystal silicon solar cell has the advantage that it is easy to increase the efficiency because the quality of the substrate is good, but has the disadvantage that the manufacturing cost of the substrate is high. On the other hand, the polycrystalline silicon solar cell has the advantage that it can be manufactured at a low cost although it has the disadvantage that it is difficult to increase the efficiency because the substrate quality is poor. In recent years, conversion efficiency of about 18% has been achieved at the research level due to the improvement of the quality of the polycrystalline silicon substrate and the advancement of cell technology.
[0003]
On the other hand, since mass-produced polycrystalline silicon solar cells are low in cost, they have been distributed in the market. However, in recent years, demands are increasing as environmental problems are addressed.
[0004]
A polycrystalline silicon substrate used for a polycrystalline silicon solar cell is generally manufactured by a method called a casting method. This casting method is a method of forming a silicon ingot by cooling and solidifying a silicon melt in a mold made of graphite or the like coated with a release material. An end portion of the silicon ingot is removed or cut to a desired size, and the cut ingot is sliced to a desired thickness to obtain a polycrystalline silicon substrate for forming a solar cell.
[0005]
A general silicon casting apparatus 15 for producing this silicon ingot is shown in FIG.
[0006]
In FIG. 2, 1 is a melting crucible, 2 is a raw material silicon, 3 is a holding crucible, 4 is a tap, 5 is an upper heating device, 6 is a side heating device, 7 is a mold heating device, 8 is a mold, and 9 is a mold. A heat insulating material, 10 is a silicon melt, and 11 is a cooling plate.
[0007]
A melting crucible 1 for melting the raw material silicon 2 is disposed at the upper part of the silicon casting apparatus 15 and held by a holding crucible 3, and the bottom of the melting crucible 1 and the holding 3 crucible 7 is used for discharging a silicon melt. A tap 4 is provided. Further, heating devices 5 and 6 are respectively disposed on the side and the upper portion of the melting crucible 1 and the holding crucible 3, and a mold 8 into which the silicon melt 10 is poured is disposed on the lower portion of the melting crucible 1 and the holding crucible 3, A heat insulating material 9 is provided outside thereof. Further, a cooling plate 11 is provided below the mold 8, and a mold heating device 7 for controlling the solidification of the silicon melt 10 is disposed above the mold 8.
[0008]
For example, high-purity quartz is used for the melting crucible 1 in consideration of heat resistance and the fact that impurities do not diffuse into the silicon melt. The holding crucible 3 is for holding the melting crucible 1 made of quartz or the like because the melting crucible 1 is softened at a high temperature in the vicinity of the melting point of silicon and cannot keep its shape, and the material is made of graphite or the like. . As the heating devices 5, 6, and 7, a resistance heating type heater, an induction heating type coil, or the like is used. The mold 8 is made of graphite, carbon fiber reinforced material, or the like, and is used by applying a release material mainly composed of silicon nitride or the like on the inside thereof. The mold heat insulating material 9 is for suppressing heat removal, and a carbon-based material is used in consideration of heat resistance, heat insulating properties, and the like. The cooling plate 11 is for cooling and solidifying the silicon melt 10 poured into the mold 8. These are all arranged in a vacuum vessel (not shown).
[0009]
When casting a silicon ingot in the casting apparatus 15, if the heat removal from the side of the mold is large, the temperature difference in the in-plane direction when the silicon solidifies increases, which induces the generation of crystal defects due to thermal stress. In addition, in order to prevent the problem of causing cracks in the worst block, the mold heat insulating material 9 is provided so as to cover the mold 8 larger than the mold 8 on the side as shown in FIG.
[0010]
[Patent Document 1]
Japanese Patent Laid-Open No. 09-263489
[Problems to be solved by the invention]
However, in the above-described conventional casting apparatus 15, when the silicon melt dissolved in the melting crucible 1 is poured into the mold 8, the silicon melt 10 jumps from the mold 8 to the mold heat insulating material 9. There was a problem to do.
[0012]
The adhered silicon melt deteriorates the mold heat insulating material 9 and lowers its durability and mechanical strength. Along with this, the mold heat insulating material 9, which contains the silicon melt and becomes brittle, may be mixed into the silicon melt 10 in the mold 8. Carbon constituting the mold heat insulating material 9 has a low solid solubility in silicon, and carbon exceeding the solid solution limit is formed into a silicon ingot when the silicon melt 10 is cooled and solidified as hexagonal α-SiC. In addition to adversely affecting the machinability when slicing the ingot, it becomes a factor of deterioration of characteristics when it is made into a solar cell element.
[0013]
In addition, the thermal conductivity of silicon is about 31.3 W / mK, which is about 80 times larger than the thermal conductivity of carbon felt moldings of about 0.40 W / mK. No. 9 significantly reduces the heat insulation of the mold side surface, resulting in a large temperature gradient in the in-plane direction of the silicon ingot, generating thermal stress that causes various crystal defects that degrade the solar cell characteristics. End up. As described above, the deterioration of the durability of the mold heat insulating material 9 not only induces the mixing of carbon into the silicon melt 10, but also reduces the heat insulating properties of the side surface of the mold and consequently deteriorates the quality of the silicon ingot.
[0014]
In order to avoid this problem, it is necessary to replace the mold heat insulating material 9 with a new one that does not deteriorate each time the silicon ingot is cast. In addition to causing an increase, it is still impossible to avoid a decrease in heat insulation during the solidification of the silicon melt 10.
[0015]
The present invention has been made in view of such problems of the prior art, and the object thereof is a silicon casting having a simple structure and having no foreign matter mixed into the silicon melt 10 and good unidirectional crystal growth. An object of the present invention is to provide a casting apparatus capable of producing a lump at a low cost.
[0016]
[Means for Solving the Problems]
The casting apparatus of the present invention includes a melting part having a melting crucible, a solidified part having a mold, a carbon-based heat insulating material covering a side surface of the mold, and an upper part of the heat insulating material. A protective cover covering the entire surface of the protective cover, wherein the protective cover has a layered structure obtained by rolling graphite.
[0018]
In still another casting apparatus of the present invention, the protective cover has a thickness of 0.2 to 10 mm.
[0019]
[Action]
According to the casting apparatus of the present invention, as described above, in the casting apparatus having the melting part composed of the melting crucible 1 and the heating devices 5 and 6 in the upper part and the solidifying part composed of the mold 8 in the lower part, the mold 8 is covered with a carbon-based heat insulating material 9 and the upper part of the mold 8 is covered with a protective cover, so that when the silicon melt 10 is poured from the hot water outlet 4 of the melting crucible 1, Even if it jumps around, the upper part of the mold heat insulating material 9 is covered with the protective cover 12, so that the problem that the jumped silicon melt adheres to the mold heat insulating material 9 and deteriorates does not occur. Thereby, the problem that the mold heat insulating material 9 including the silicon melt becomes mixed into the silicon melt 10 in the mold 8 can be solved, and the silicon melt adheres to the mold heat insulating material 9. Therefore, the mold heat insulating material 9 can sufficiently secure the heat insulating property, and a high-quality silicon ingot can be cast.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
The casting apparatus according to the present invention will be described in detail below with reference to the drawings.
[0021]
The overall configuration of the casting apparatus according to the present invention is the same as that of the casting apparatus shown in FIG. That is, a melting crucible 1 for melting the raw material silicon 2 is disposed on the upper part and held by a holding crucible 3, and a bottom 4 of the melting crucible 1 and the holding 3 crucible 7 is used to discharge a silicon melt. Is provided. Further, heating devices 5 and 6 are respectively disposed on the side and the upper portion of the melting crucible 1 and the holding crucible 3, and a mold 8 into which the silicon melt 10 is poured is disposed on the lower portion of the melting crucible 1 and the holding crucible 3, A heat insulating material 9 is provided outside thereof. Further, a cooling plate 11 is provided below the mold 8, and a mold heating device 7 for controlling the solidification of the silicon melt 10 is disposed above the mold 8.
[0022]
For example, silicon raw material put in a melting crucible 1 made of high-purity quartz or the like is heated and melted by upper and side heating devices 5 and 6 made of a resistance heating type heater, induction heating type coil, etc. The melt is poured from the bottom outlet 4 into the lower mold 8. The mold 8 containing the silicon melt 10 is made of carbon, carbon fiber reinforced carbon material, or the like, and the inner surface thereof is covered with a release material mainly composed of silicon nitride, for example, so that the silicon melt 10 and the mold 8 are covered. Is prevented, and the cooled and solidified ingot is easily released from the mold 8. Further, the side surface is insulated by the mold heat insulating material 9 and heated from the upper part by the mold heating device 7, and the cooling plate 11 is brought into contact with or brought close to the lower part to remove heat from the lower part to realize unidirectional solidification.
[0023]
FIG. 1 is a view for explaining a casting apparatus according to the present invention, and is a schematic sectional view showing a structure of a solidified portion.
[0024]
In FIG. 1, 8 is a mold, 9 is a mold heat insulating material, 10 is a silicon melt, 11 is a cooling plate, and 12 is a protective cover. Thus, when the silicon melt 10 is poured from the outlet 4 of the melting crucible 1, the upper part of the mold heat insulating material 9 is covered with the protective cover 12 even if the silicon melt jumps around. Therefore, the problem that the jumped silicon melt adheres to the mold heat insulating material 9 and deteriorates does not occur.
[0025]
At this time, the protective cover 12 is preferably a material having a layered structure obtained by rolling graphite. Since graphite is widely used industrially and can be manufactured at low cost, rather than replacing the mold insulation 9 with a new one without deterioration each time a silicon ingot is cast, Replacing the protective cover with a new one has a significant cost advantage. In addition, by rolling and forming into a sheet, generation of dust from the surface can be prevented, and there is an effect of suppressing foreign matter contamination in the mold. Further, since the layered structure has a low fluid permeability, it is possible to more effectively prevent the splashed silicon melt from penetrating into the mold heat insulating material 9 under the protective cover 12.
[0026]
Furthermore, the thickness of the protective cover 12 is desirably in the range of 0.2 to 10 mm. According to the results of experiments conducted repeatedly by the present inventors, if the thickness is 0.2 mm or less, there is a problem that it is not possible to prevent the splashed silicon melt from penetrating into the mold heat insulating material. . If the thickness is 10 mm or more, the temperature distribution between the upper part of the mold 8 heated by the mold heating device 7 and the lower part of the mold 8 cooled by the cooling plate 11 changes, and the formed polycrystalline silicon. This is unsuitable because it adversely affects the quality of the substrate.
[0027]
Thereafter, when pouring of the silicon melt into the mold is completed, the mold is being cooled while the output of the upper and side heating devices 5 and 6 is turned off and the cooling plate 11 is pressed against the bottom of the mold 8. A silicon ingot can be obtained by adjusting the output of the heating device 7 to solidify the silicon melt 10 in one direction from the bottom to the top.
[0028]
【The invention's effect】
As described above, according to the casting apparatus of the present invention, in the casting apparatus having the melting part composed of the melting crucible 1 and the heating devices 5 and 6 in the upper part and the solidified part composed of the mold 8 in the lower part, the mold 8 is The silicon melt 10 is covered with the carbon-based heat insulating material 9 and the upper part of the mold 8 is covered with the protective cover 12 so that the silicon melt 10 is poured into the surrounding area when the silicon melt 10 is poured from the hot water outlet 4 of the melting crucible 1. Even if it jumps, since the upper part of the mold heat insulating material 9 is covered with the protective cover 12, there is no problem that the jumped silicon melt adheres to the mold heat insulating material 9 and deteriorates. As a result, the problem that the mold heat insulating material 9 including the silicon melt becomes mixed into the silicon melt 10 in the mold 8 is solved, and a slice generated due to carbon deposition in the silicon ingot. It is possible to prevent factors that cause a decrease in yield and a decrease in characteristics when a solar cell element is formed.
[0029]
Further, since the silicon melt does not adhere to the mold heat insulating material 9, the mold heat insulating material 9 can sufficiently secure the heat insulating property, and a high-quality silicon ingot can be cast.
[0030]
At this time, by using a material having a layered structure obtained by rolling graphite for the protective cover, generation of dust can be prevented with an inexpensive material, and foreign matter can be prevented from being mixed into the mold. Moreover, since it has a layered structure with low fluid permeability, it is possible to more effectively prevent the splashed silicon melt from penetrating into the mold heat insulating material 9 under the protective cover 12. High silicon ingot can be cast.
[0031]
Moreover, adhesion of the silicon melt to the mold heat insulating material 9 can be effectively prevented by setting the thickness of the protective cover 12 to 0.2 to 10 mm.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining a casting apparatus according to the present invention, and is a schematic sectional view showing a structure of a solidified portion.
FIG. 2 is a schematic sectional view for explaining the structure of a casting apparatus in the prior art.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Melting crucible 2 ... Raw material silicon 3 ... Holding crucible 4 ... Outlet 5 ... Upper heating apparatus 6 ... Side part heating apparatus 7 ... Mold heating apparatus 8 ... Mold 9 ... Mold heat insulating material 10 ... Silicon melt 11 ... Cooling plate 12 ... Protective cover

Claims (2)

A melting part having a melting crucible;
A solidified part having a mold provided at a lower part of the melting part;
A carbon-based insulation covering the side of the mold;
A protective cover covering the entire upper surface of the heat insulating material,
The said protective cover comprises the layered structure which rolled the graphite, The casting apparatus characterized by the above-mentioned.   The casting apparatus according to claim 1, wherein the protective cover has a thickness of 0.2 to 10 mm.

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