JPS5826019A - Casting method for polycrystalline silicon ingot - Google Patents

Abstract

PURPOSE:To easily separate a polycrystalline silicon ingot from a mold having a higher softening point then the m.p. of silicon and a partially detachable bottom by coating the inside of the mold with silicon carbide and applying a powdered mold releasing agent. CONSTITUTION:Part of the bottom of a carbon mold 1 having a higher softening point than the m.p. of silicon is made detachable with a carbon plate 4, and the whole inside of the mold 1 and the surface of the plate 4 are coated with a silicon carbide film 2. A powdered mold releasing agent 3 is applied to the film 2, and silicon as starting material is put in the mold 1 and melted by heating. After thoroughly melting the silicon in the mold 1, the melt is solidified from the bottom to obtain a polycrystalline silicon ingot. By pushing the plate 4 upward, the ingot can be easily taken out, and since the ingot 1 itself is not damaged, it can be reused.

Description

[Detailed Description of the Invention] This relates to the casting method of the IjIFi polycrystalline silicon ingot of the present invention. Recently, solar cells have been reconsidered as a solar power generation energy source, and development of low-cost solar cells has been active1. In order to obtain this, it is necessary to use K with as few defects as possible and completely crystalline silicon. For this reason, the cost of solar cells has become high, and their use on land has been limited to date. This led to the development of polycrystals as a low-cost solar cell material to replace single-crystal silicon. Polycrystalline silicon is manufactured by a casting method. This casting method has a higher crystal growth rate than the Czochralski method for obtaining single crystals, can obtain ingots of arbitrary shapes, and is easy to operate without requiring any skill. There is great potential for lower prices.

For example, there are reports of using a graphite plotter as a mold to form a polycrystalline ingot and cutting out polycrystalline plates of 1 oclm x 10 tx to obtain a solar cell having a photoelectric conversion efficiency of 10% or more. ([th IEEE.

Photovoltaic 5specialists
Conference p861976χ.

However, since a graphite tube is used as the mold, it is important to take measures to prevent leakage between the silicon melt and the mold.In this regard, it is important to keep the temperature If of the mold at a temperature tVC slightly lower than the melting point of silicon. trying to solve the problem. (Japanese Patent Application Laid-open No. 51-101466) However, the drawback of this method is that the polycrystalline grain size does not increase due to rapid assimilation at low ILR. In general, large polycrystalline grains are used for solar cells. High photoelectric conversion efficiency can be obtained when It is suggested that it be made larger.

However, in the conventional method of forming polycrystalline silicon lumps using a quartz crucible, the quartz crucible and the silicon melt react violently, and when the silicon melt is cooled and solidified, they are strongly stuck together.

For this reason, when the quartz and silicon polycrystalline lumps were cooled, the quartz and silicon polycrystalline lumps were broken into small pieces, making it impossible to obtain one polycrystalline silicon mass.

To solve this problem, Graded Kurjpur (G
A method using a special quartz crucible called ``R Hatake and Crucible'' was developed. Graded Crucible (Grad@d Crucible): A structure in which the inner surface of the crucible is made larger and the outer surface is rougher, so that only the quartz crucible can be finely cracked during cooling. There is. Therefore, cracks do not occur in the silicon polycrystalline flow. With this method, large crystal grain sizes that are almost single-crystal-like can be obtained.

(13th Photovoltaic 5pecia
lists, Conferencep137197
8) The disadvantage of this method is that graded Kurzipul (GrILd)
An expensive special quartz crucible called ``Crucible'' breaks into small pieces after one use.This is a major factor preventing price reduction.

The inventors of the present invention have already proposed a polycrystalline silicon ingot casting method using a powder release agent that solves the above-mentioned drawbacks. This is a method of obtaining polycrystalline silicon ingots by melting silicon raw materials and cooling and solidifying them.◇The presence of a powder release agent alleviates the stress caused by the difference in thermal expansion coefficient between polycrystalline silicon and the mold during cooling. In addition, the purpose is to prevent cracks in the polycrystalline silicon ingot caused by sticking to the mold, and to allow the polycrystalline silicon ingot to be easily separated from the tube casting. Silicon was melted using quartz as the material for the mold that holds the ingot. There was no leakage between the mold and the solution, and the polycrystalline silicon ingot could be easily ejected, but since it was heated above the softening point of quartz, a portion of the quartz hook formed.
There is a disadvantage that it cannot be used for return purposes - 1) fr
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Carbon molds (silicon carbide coated on the inner surface) are a material that solves all of the above disadvantages, but because mold release agent powder gets stuck between the mold and the polycrystalline silicon ingot, it is easy to break the polycrystalline silicon ingot. The current II#
The drawback was that it was difficult to remove from the mold.

An object of the present invention is to provide a method for producing polycrystalline silicon ingots that eliminates such drawbacks. There is a mental image that allows you to use it. To this end, in the present invention, a hole is made in the bottom of the mold with a slope that is large at the top and small at the bottom, and a carbon plate with the same slope as the hole is inserted from the top of the mold to cover the hole. A silicon raw material is placed in this mold, heated and melted, and then cooled and solidified to form a polycrystalline silicon ingot.

Practical examples of the present invention will be explained below using drawings◎ FIG. 1 is a diagram for explaining one embodiment of the present invention.

As shown in the figure, a part of the bottom of the carbon mold (1) is configured to be removable using a carbon plate (4), and a silicon carbide (Sie) II [
Coating (2). Of course, the inner surface of the mold of the carbon plate (4) inserted into the bottom is also coated. Furthermore, a powder mold release agent (silicon nitride) is added on top of the silicon carbide group (3)
The silicon raw material is placed in the casting chimney and heated and melted.

The silicon raw material is not completely melted in the hook, and if it is solidified from the bottom of the casting 110 under these conditions of 0%, it will be completely solidified after 1 hour and a polycrystalline silicon ingot will be obtained.

Drill a round hole with an inclination on the bottom of the box side, and make the carbon plate that goes into that hole with an inclination, so that when the two are put together, the hole is completely filled. When the hole is sealed by the pressure of the silicon melt O, there is no chance of the melt leaking out.

When taking out the solidified polycrystalline silicon ingot from the mold, push up the carbon plate (4) to push up the p-polycrystalline silicon ingot, and remove the powder stuck between the mold and the polycrystalline silicon ingot. The release agent is also pushed up at the same time. In this way, a polycrystalline silicon ingot can be easily carved out of the mold. There is no damage to the mold itself and it can be used again. Due to its low power consumption, it is cheaper than other mold materials, requires less power consumption to heat the casting WLVt, and is a low-cost mold material. Figure 2 shows the deformation of the attachment/detachment structure on the bottom surface, and the same symbols as in Figure 1 indicate the same components.The thus solidified polycrystalline silicon ingot is cooled gradually so as not to generate thermal stress. Reduce temperature to room temperature. With this method, it is easy to obtain polycrystalline silicon 70 particles with a particle size of 3 to 20 cm.
It was done.

The melting point of silicon nitride is as high as 1900°C, and silicon carbide is even higher than this, so there is no reaction between the silicon melt and the mold. Also, some of the silicon nitride components dissolve into the polycrystalline silicon, but this is not possible.
I will never work as a y.

Although we have explained the case where carbon is used as the mold material, the same effect can be obtained by using other materials such as silicon nitride, which has a softening point higher than the melting point of silicon. In forming the polycrystalline silicon ingot, the method of the present invention could be used to form a polycrystalline silicon ingot by directly melting silicon within a layer of material with a softening point above the silicon melting point. . As a result, the silicon ingot can be easily deposited from the inside of the hook and can be used continuously.

The crystallinity of the obtained polycrystalline silicon ingot had no problems, the crystal grain size was large, and a polycrystalline silicon ingot with few defects was easily obtained.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining one embodiment of the present invention, and FIG.
The figure shows a modified tube with a detachable structure on the bottom of a cast MK. In the figure, 1--Carbon mold 2--Silicon coating 3--Silicon nitride (5L3N powder mold release agent) 4--Detachable part on the bottom of the mold '71 times

Claims (1)

[Claims] A mold tube made of a material with a softening point higher than the melting point of silicon and configured so that a part of the bottom part can be detached is used, and after coating the inside of the mold tube with silicon carbide,
A method for casting polycrystalline silicon ingots, characterized in that polycrystalline silicon is cast in a flio coated with a powdered mold release agent.