JPH11199216A - Device for unidirectional solidification of silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the subject device capable of maintaining the solidification rate to be constant as compared with any conventional device for the same purpose. SOLUTION: This device is provided with: a casting mold 3 disposed on a support base that is placed inside a casting chamber 15; a melt supply means 2 for pouring a silicon melt 4 to be cast, into the casting mold 3; a heating means 10 for heating the upper side of the melt 4 in the casting mold 3; a water-cooled copper plate 12 for cooling the bottom of the casting mold 3; and an insulating material 9 placed between the water-cooled copper plate 12 and the bottom of the casting mold 3; wherein the melt supply means 2 and the heating means 10 are placed apart from each other in the upper part of the casting chamber 15, at positions which are the same distance apart from the central axis of the casting chamber 15 in the radial direction. Further, the device is provided with two disks, i.e., an upper disk 5 and a lower disk 6, which overlap each other and are rotatable independently from each other with their center shafts as the rotary shafts, respectively, and also, function as the support base for the casting mold 3 and the support base for the insulating material 9, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a unidirectional solidification apparatus for silicon, and more specifically, to cast a coarsely purified silicon for a solar cell and unidirectionally solidify to remove metal impurity elements. The present invention relates to a device for forming an ingot.

[0002]

2. Description of the Related Art A silicon substrate used for a solar cell must contain many impurity elements in order to exhibit required semiconductor characteristics.
It needs to be reduced to the order of pm. Therefore, the present applicant has researched and developed a technique for removing metallic silicon (purity: 99.3% by weight) as a starting material, and then removing boron and carbon by oxidative refining and removing phosphorus by volatile refining. But,
Even in such a roughly refined silicon, the metal impurity elements titanium, iron, aluminum and calcium are still contained in an amount that does not satisfy the specifications for solar cells. Therefore, attention has been paid to the fact that these elements have a small solid-liquid partition coefficient, and so-called one-way solidification method is used to remove and purify them, and to form a silicon ingot.

By the way, in order to perform unidirectional solidification, the silicon melt poured into the mold is solidified at a constant speed (ideally, it is desirable that the solidification interface is formed as one surface) upward from the bottom. It is necessary to let For this reason, conventionally, the molten metal is heated from above by a graphite heater or the like, and is cooled by applying a water-cooled copper plate to the bottom surface of the mold.

Using this method, at the beginning of solidification,
The heat removal at the bottom is good, and the interface proceeds stably at a relatively high solidification rate from below the silicon melt, but as the solidification progresses, the heat removal from the lower part of the molten silicon deteriorates. However, the solidification rate gradually decreases. When the solidification rate fluctuates in this manner, the movement of the impurity element from the solidified portion to the molten silicon becomes unstable, and it becomes difficult to obtain an ingot having uniform quality in the height direction. Therefore, controlling the solidification rate to be constant has been important for the production of silicon for solar cells.

Conventionally, since the heat removal rate from the bottom of the mold could not be sufficiently controlled, the output of the upper heater was adjusted, that is, the output of the heater was increased in the early stage of solidification.
At the end of solidification, the output of the heater was weakened.
However, the heat removal from the bottom is more dominant to the solidification rate, and it has been difficult to control the heating of the top and the cooling of the bottom sufficiently. Furthermore, since the frequency of controlling the output of the heater at a higher frequency is higher, the unit power consumption tends to be higher.

Therefore, Japanese Patent Application Laid-Open No. 62-260710 proposes a technique in which one kind of heat insulating material is interposed between the bottom surface of a mold and a water-cooled copper plate, and the cooling is performed by changing the thickness of the heat insulating material. It mentions adjusting the speed.

[0007]

However, Japanese Patent Application Laid-Open No. 62-260710 does not describe a specific method of changing the heat insulating material, that is, how to control the heat insulating material. Referring to the figure of the coagulation apparatus described in the publication,
Although the apparatus can easily replace the mold in the casting chamber, it is not possible to easily replace the heat insulating material in a short time. In this case, the heating of the upper part and the cooling of the bottom part cannot be controlled in a well-balanced manner.

[0008] In view of such circumstances, an object of the present invention is to provide a unidirectional solidification apparatus for silicon capable of maintaining a solidification rate more constant than before.

[0009]

Means for Solving the Problems The inventor of the present invention has made intensive studies to achieve the above-mentioned object, and changed the thermal conductivity of a heat insulating material sandwiched between a bottom surface of a mold and a water-cooled copper plate during solidification of silicon. And embodied this idea in the present invention. That is, the present invention provides a mold disposed on a support provided in a casting chamber, hot water supply means for pouring molten silicon cast into the mold, heating means for heating the molten metal in the mold, and a bottom portion of the mold. In a one-way solidification apparatus for silicon, comprising a water-cooled copper plate for cooling the water and a heat insulating material disposed between the water-cooled copper plate and the bottom of the mold, the hot water supply means and the heating means are provided above the casting chamber, Are provided at the same radial position apart from each other, and are provided with two vertically overlapping disks that rotate independently of each other about the center, the upper disk is used as a support for the mold, and the lower disk is thermally insulated. This is a unidirectional solidification apparatus for silicon, which is used as a support for a material.

Further, the present invention is the unidirectional solidification apparatus for silicon, wherein the lower disk is divided into a plurality of regions, and heat insulating plates having different thermal conductivity are arranged in each region. Furthermore, the present invention is characterized in that the heat insulating plate in the plurality of regions is provided with a rotating means of a lower disk which can be freely moved to the bottom of the mold according to temperature information from the water-cooled copper plate during solidification. This is a unidirectional solidification device for silicon.

In addition, the present invention is a unidirectional solidification apparatus for silicon, characterized in that the thickness and physical properties of the heat insulating plate can be freely changed, or that the heating means is a graphite heater. In addition, the present invention provides the hot water supply means,
The present invention is also a unidirectional solidification apparatus for silicon, which is a refractory funnel having a sliding gate.

According to the present invention, a heat insulating material having a low thermal conductivity is arranged in the early stage of solidification because heat is excessively removed from the bottom surface, solidification proceeds, and a space between the silicon melted portion and the mold bottom is formed. When the silicon solidified portion is formed, the solidified portion is subjected to heat transfer rate-determining, and the heat removal from the silicon melted portion is deteriorated. Therefore, the silicon solidified portion can be replaced with a heat insulating material having a high thermal conductivity as the solidification progresses. As a result, the solidification rate can be maintained substantially constant during the period, and a high quality silicon ingot can be obtained. In some cases, depending on the height of the solidified portion, it is better to directly contact the bottom surface of the mold with the water-cooled copper plate. In such a case, it can be dealt with by previously removing the heat insulating plate from the lower disk.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. First, FIG. 1 shows the situation when the molten silicon is received in the unidirectional solidification apparatus according to the present invention.
After replacing the atmosphere in the casting chamber 15 with argon, the sliding gate 1 of the hot water supply means is opened, and the molten silicon 4 is injected into the mold 3 through the funnel 2 made of refractory. When the injection is completed, the sliding gate 1 is closed and the atmosphere in the casting chamber 15 is replaced with argon again.
Subsequently, the upper disk 5 (commonly referred to as a turntable) supporting the mold 3 is rotated by a rotating means 7 described later to move the mold 3 to a solidification work position. When the mold 3 reaches the solidification work position, the heating means 10 and the hood 11 are set above the mold 3 as shown in FIG. After that, the lower disk 6 supporting the heat insulating plate 9 is rotated by the rotating means 8 described below in the same manner as described above, and a predetermined type of heat insulating plate 9 is arranged below the mold 3. Finally, mold 3
The water-cooled copper plate 12 for cooling the bottom of the plate is raised and set below the heat insulating plate 9, and the solidification operation is started.

Here, the upper disk 5 is shown in FIG.
As shown in FIG. 7, a through hole 16 is provided at a position where the mold 3 is placed, so that heat can be easily removed by cooling. As shown in FIG. 3B, the lower disk 6 is divided into a plurality of regions (four in this case), provided with through holes 16 in each region, and fitted with different types of heat insulating plates 9. Have been. Of course, there is a case where the heat insulating plate 9 is not necessary during the solidification.
There may be a through-hole 16 into which is not fitted. Further, the upper and lower disks 5, 6 are separated from each other, and rotating means 7, 8 are provided so that the disks 5 and 6 can be reversibly rotated about the axis of the disk, or can move up and down. As the rotating means 7 and 8, a known means combining a gear, a motor, a cylinder and the like can be used.

Next, the control of the solidification rate of the silicon melt 4 injected into the mold 3 will be described. At the beginning of the solidification, the output of the heating means 10 has a predetermined solidification speed,
The solidification is advanced by the amount of cooling water flowing through the heat insulating plate 9 and the water-cooled copper plate 12. After that, solidification proceeds to some extent, and if an abnormality in the solidification rate is confirmed by information of a thermometer (not shown) provided on the wall of the mold 3, the type of the heat insulating plate 9 is set each time so as to maintain the solidification rate. To change. In other words, the water-cooled copper plate 12 is lowered, the lower disk 6 is rotated, an appropriate heat-insulating plate 9 is placed at the bottom of the mold 3, the water-cooled copper plate 12 is raised again, and solidification is continued. Normally, in the case of a mold 3 having a height of 25 cm, solidification takes about 4 hours, but by repeating this operation several times, the amount of heat removed from the water-cooled copper plate 12 can be appropriately controlled, and a substantially constant solidification speed can be obtained. become.

[0016]

EXAMPLE Using a directional solidification apparatus according to the present invention shown in FIGS. 1 to 3, 50 kg of a silicon melt roughly purified for a solar cell was cast into a graphite mold 3 to produce an ingot. In order to compare the effect of solidification, casting was performed without changing the type of the heat insulating plate 9 during solidification. The size of the mold 3 is 250 mm in height, 300 mm in width, and 300 mm in length. The used heat insulating plate 9 was made of graphite and had the same thermal conductivity and thickness as those shown in Table 1. Here, No. 1 in Table 1 was used. 1, 2, 3 and 4 show the above-mentioned areas of the lower disk 6.

FIG. 4 shows the control state of the solidification rate. FIG.
It is apparent from the above that the solidification rate can be controlled by replacing the heat insulating plate 9. Table 2 shows the impurity concentrations of the used silicon melt 4 and the obtained ingot. From Table 2, it is also clear that the ingot cast by the unidirectional solidification apparatus according to the present invention satisfies the specifications as silicon for solar cells.

[0018]

[Table 1]

[0019]

[Table 2]

In the control of the solidification rate, the output of the graphite heater of the heating means 10 is adjusted so that the solidification rate is constant while the same heat insulating material 9 is used. The device may be narrower than in the case of a device in which the type of heat insulating material cannot be replaced, and the power consumption can be reduced as compared with the conventional device. 1 and 2, the heating means 10 arranged above the mold 3 is a graphite heater. However, when the atmosphere in the casting room is not argon gas but vacuum, an electron gun may be used.

[0021]

As described above, according to the present invention, in the process of solidifying silicon for a solar cell, a uniform solidification rate can be obtained, and the quality and uniformity of the silicon ingot can be improved. . Further, since the amount of heat removal is reduced, the output of the heating means can be reduced, and the power consumption required for coagulation can be reduced as compared with the conventional case.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view showing a unidirectional solidification apparatus for silicon according to the present invention.

FIG. 2 is a diagram showing another situation of FIG. 1;

FIGS. 3A and 3B show a plan view of the apparatus of FIG. 1, wherein FIG. 3A is a view as viewed from an arrow A, and FIG.

FIG. 4 is a diagram showing a control state of a solidification rate.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Sliding gate of silicon hot water supply means 2 Funnel of silicon hot water supply means 3 Mold 4 Melt of silicon (melt) 5 Upper disk 6 Lower disk 7 Upper disk rotating means 8 Lower disk rotating means 9 Heat insulating plate 10 Heating Means (graphite heater) 11 Mold cover 12 Water-cooled copper plate 13 Cooling water supply / drain hose 14 Cooling plate drive device 15 Casting room 16 Through hole

Claims (6)

[Claims] 1. A mold disposed on a support table provided in a casting chamber, a hot water supply means for pouring molten silicon cast into the mold, a heating means for heating the molten metal in the mold, and a bottom of the mold. In a unidirectional solidifying apparatus for silicon comprising a water-cooled copper plate to be cooled and a heat insulating material disposed between the water-cooled copper plate and the bottom of the mold, the hot-water supply unit and the heating unit are provided above the casting chamber, from the center of the casting chamber. Two discs are provided at the same radial position and are separated from each other, and are vertically rotatable about the center independently of each other, and the upper disc is used as a support for a mold, and the lower disc is used as a heat insulating material. A unidirectional solidification apparatus for silicon, wherein the apparatus is used as a support. 2. The unidirectional solidification apparatus for silicon according to claim 1, wherein the lower disk is divided into a plurality of regions, and heat insulating plates having different thermal conductivity are arranged in each region. 3. The apparatus according to claim 1, further comprising: a lower disk rotating means for moving the heat insulating plates in the plurality of regions to the bottom of the mold in accordance with temperature information from the water-cooled copper plate during solidification. Item 3. The unidirectional solidification device for silicon according to Item 1 or 2. 4. The unidirectional solidification apparatus for silicon according to claim 1, wherein the thickness and the physical properties of the heat insulating plate are changeable. 5. The unidirectional solidification apparatus for silicon according to claim 1, wherein said heating means is a graphite heater. 6. The unidirectional solidification apparatus for silicon according to claim 1, wherein said hot water supply means is a refractory funnel having a sliding gate.