JPH0848514A - Method for removing phosphorus in silicon - Google Patents

Abstract

(57) [Summary] [Purpose] To reduce the phosphorus concentration in silicon to 1 ppm or less. [Structure] After melting the raw material silicon in the graphite crucible 2 in a vacuum atmosphere having a degree of vacuum of 0.05 Torr or less, the molten silicon L having a melting point of 16
The stirring bar 12 is immersed in the molten silicon L while maintaining the temperature range of 00 ° C. or less, and the molten silicon L is stirred by the stirring bar 12.

Description

Detailed Description of the Invention

[0001]

This invention relates to a method for removing phosphorus in silicon.

In this specification, "ppm" is based on weight.

[0003]

2. Description of the Related Art For example, silicon used for manufacturing a solar cell has a purity of 99.999 to 99.99999 wt.
%, And a phosphorus content of 1 ppm or less is required.

By the way, commercially available industrial crude silicon used for producing such high-purity silicon contains iron, calcium, aluminum, boron and the like in addition to phosphorus. It is known that impurities other than phosphorus and boron can be efficiently removed by the unidirectional solidification method utilizing the well-known principle of segregation solidification or the zone melting method. However, since the equilibrium segregation coefficient of phosphorus in silicon is relatively large at 0.35, it is necessary to repeat the work a number of times in order to remove it in such a method together with boron having an equilibrium segregation coefficient of 0.8. There is a problem that it takes time and the cost becomes high.

In order to solve such a problem, therefore, a method of removing phosphorus in silicon by evaporating and removing phosphorus by melting and holding the raw material silicon in a vacuum atmosphere has been considered. (See "Journal of the Japan Institute of Metals", Vol. 54, No. 2 (1990) 161-167).

[0006]

However, according to the conventional method, the raw material silicon containing 32 ppm of phosphorus is dissolved in a vacuum atmosphere having a degree of vacuum of 6.8 × 10 ^-3 Torr and kept at 1450 ° C. or 1550 ° C. for 1 hour. Even if the concentration of phosphorus is 6-7
It is only reduced to ppm and the present situation is that the phosphorus reduction effect is not further improved even if the holding time is further extended.

An object of the present invention is to solve the above problems and provide a method for removing phosphorus from silicon which can obtain silicon having a phosphorus concentration of 1 ppm or less, which is required for solar cells.

[0008]

The method for removing phosphorus in silicon according to the present invention is to melt raw silicon in a graphite crucible in a vacuum atmosphere having a vacuum degree of 0.05 Torr or less and then melt silicon in the vacuum atmosphere. Is kept above the melting point and not higher than 1600 ° C., the stir bar is immersed in the molten silicon, and the molten silicon is stirred by the stir bar.

The present invention has been made by the inventors of the present invention by discovering suitable conditions for vacuum processing through various experimental studies.

In the above, the reason why the material is kept in a vacuum atmosphere having a vacuum degree of 0.05 Torr or less during the melting of the raw material silicon and the subsequent stirring is that the vaporization of phosphorus is delayed when the vacuum degree exceeds 0.05 Torr. It is not industrially preferable.

The graphite crucible is used for the following reasons. That is, as the crucible material having heat resistance, there are glass, alumina, etc. in addition to graphite.
This is because when the temperature exceeds 500 ° C., SiO reacts with molten silicon to form SiO, which is evaporated, so that a high degree of vacuum cannot be maintained.

The holding temperature of the molten silicon is set to 1600 ° C. or less because if the holding temperature exceeds 1600 ° C., the generation of SiC due to the reaction between the graphite and the molten silicon becomes severe.

In the above method for removing phosphorus in silicon,
A graphite baffle for improving the stirring effect of the stirrer may be placed in the graphite crucible in advance.

The baffle plate is made of graphite for the same reason as that of the graphite crucible. Further, the number of baffle plates may be one or plural.

[0015]

According to the method of the present invention, the degree of vacuum is 0.05 Torr.
After melting the raw material silicon in the graphite crucible in the following vacuum atmosphere, while maintaining the molten silicon in the temperature above the melting point and 1600 ° C. or less in this vacuum atmosphere, the stir bar is immersed in the molten silicon, Since the molten silicon is stirred by this stirrer, phosphorus in the silicon can be quickly and efficiently removed.

By disposing a graphite baffle for improving the stirring effect of the stirrer in the graphite crucible, the removal of phosphorus in silicon can be further promoted.

[0017]

Embodiments of the present invention will be described below.

FIG. 1 shows an apparatus for carrying out the method of the invention.

In FIG. 1, the device for removing phosphorus in silicon comprises a vacuum melting furnace (1) and a graphite crucible (2) arranged in the vacuum melting furnace (1). The vacuum melting furnace (1) consists of a furnace body (3) with an open upper end and an exhaust port on the peripheral wall, and a lid (4) that closes the upper end opening of the furnace body (3) and is connected to the exhaust port. A vacuum exhaust device (not shown) brings the inside into a vacuum state.

The graphite crucible (2) is mounted on a mounting table (5) installed on the bottom wall of the vacuum melting furnace (1). Crucible
A baffle plate (6) extending vertically is fixedly provided on the inner peripheral surface of (2). The upper end of the baffle plate (6) is equal to the upper end of the crucible (2), and the lower end of the baffle plate (6) is located slightly lower than the upper and lower central portions of the crucible (2). The baffle plate (6) may be attached to the lid (4) of the vacuum melting furnace (1) in a hanging manner. In this case, the baffle (6) is a crucible
It should be placed close to the inner surface of (2). Further, the baffle plate (6) is not always necessary. A cylindrical graphite heater (7) is arranged around the crucible (2) so as to surround it. The graphite heater (7) is a support member whose lower end penetrates the bottom wall of the vacuum melting furnace (1) from below.
It is fixed to (8). A cylindrical heat shield member (9) is arranged around the graphite heater (7). The lower end of the heat shield member (9) extends to the lower end of the mounting table (5).

A vertical graphite rotating shaft (11) is arranged in an airtight manner through the central portion of the lid (4) of the vacuum melting furnace (1). The rotating shaft (11) can move up and down. The lower end of the rotating shaft (11) extends slightly below the lower end of the baffle plate (6) in the graphite crucible (2). A stir bar (12) at the lower end of the rotating shaft (11)
Is fixedly provided. The stirrer (12) has a disk shape, and a plurality of blades are provided on the circumferential surface of the stirrer (12) at intervals in the circumferential direction. The rotating shaft (11) can be rotated by a motor (not shown) arranged above the vacuum melting furnace (1). The rotation speed of the rotating shaft (11), that is, the stirrer (12) is 20.
It is preferably in the range of 0 to 800 rpm.

In such a structure, in order to remove phosphorus in silicon, first the lid (4) of the vacuum melting furnace (1) is opened and a predetermined amount of raw material silicon is put into the graphite crucible (2). Close the lid (4) again, and use a vacuum exhaust device (not shown) to create a vacuum melting furnace.
(1) The inside is made a vacuum state having a vacuum degree of 0.05 Torr or less. Then, the raw material silicon is heated and melted by the graphite heater (7). At this time, the upper end of the baffle plate (6) projects above the liquid surface of the molten silicon (L). After that, the stirring bar (12) is immersed in the molten silicon (L) while maintaining the molten silicon (L) in the temperature range not lower than the melting point and not higher than 1600 ° C. ) Is stirred.
Then, phosphorus is quickly evaporated and removed.

Next, more specific experimental examples of the present invention will be described.

5 kg with various phosphorus concentrations shown in Table 1
After melting the raw material silicon in the graphite crucible (2) in a vacuum atmosphere by using the raw material silicon of, the molten silicon (L) is kept in the vacuum atmosphere at 1500 ° C. while being melted in the molten silicon (L). Immerse the stirrer (12), then this stirrer (12)
Silicon (L) by rotating at 500 rpm
Was stirred. Further, without stirring by the stirrer (12),
The same process as above was performed. The degree of vacuum, the presence or absence of the baffle plate (6), and the processing time are as shown in Table 1. Then, the phosphorus concentration after the treatment was measured. Table 1 also shows the results.

[0025]

[Table 1]

[0026]

As described above, according to the method for removing phosphorus in silicon of the present invention, phosphorus in silicon can be removed quickly and efficiently. Therefore, it is possible to achieve the phosphorus concentration of 1 ppm or less required for silicon used for manufacturing a solar cell.

[Brief description of drawings]

1 is a vertical cross-sectional view showing an apparatus for carrying out the method of the present invention.

[Explanation of symbols]

 (2) Graphite crucible (12) Stirrer (L) Molten silicon

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ryoda Otsuka 6-224 Kaiyama-cho, Sakai City, Showa Aluminum Co., Ltd.

Claims (2)

[Claims] 1. After melting raw material silicon in a graphite crucible in a vacuum atmosphere having a degree of vacuum of 0.05 Torr or less, molten silicon having a melting point of 16 or more and a melting point of 16 or more is melted in the vacuum atmosphere.
A method for removing phosphorus from silicon, which comprises immersing a stirrer in molten silicon while maintaining the temperature within a range of 00 ° C. or lower, and stirring the molten silicon with the stirrer. 2. The method for removing phosphorus from silicon according to claim 1, further comprising disposing a baffle plate made of graphite for improving the stirring effect of the stirrer in the graphite crucible.