JPS63218506A - How to purify split silicon under plasma - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to the production of split silicon under plasma, which makes it possible to obtain bulk solid silicon with the purity required for photovoltaic or electronics applications. Relating to a method of purification.

[Prior art]

In order for silicon to be able to be used to construct photovoltaic cells, the content of various elements in it must be below very low threshold values according to the respective criteria.

In particular, elements that exhibit electron killer properties (e.g. V.

The content of Cr % T i s Z r s N a ) must be below 50 ppb. Also, the content of neutral elements <4 IP (Fe%Ni, Mg, C%Mn) should not exceed about lppm. In addition, the content of doping elements (among which #1 uron, aluminum, phosphorus and arsenic are the main ones) must be controlled to extremely low values so that the resulting silicon is of p- or n-type. Must not be. In addition, in order for silicon to be used in the electronics field, the content of doping impurities must be 10
Must be less than ppb.

According to 7 Lance Patent No. 2,458,499, it is known to purify solid silicon bodies in the form of ingots by a zone melting process under a plasma obtained by radiofrequency excitation of a mixture of argon and hydrogen.

Also known from European Patent Application No. 45 689 is a method for purifying silicon ingots by zone melting under a plasma consisting of a mixture of argon, oxygen and optionally hydrogen.

Thus, until now, only methods for refining silicon ingots have been proposed.

[Purpose of the invention]

The problem to be solved by the present invention is to provide a method for producing silicon in a split state under plasma. However, it is recognized that it is not possible to achieve the desired purification if one attempts to apply the methods described in the aforementioned patents or patent applications to split silicon.

The inventors have now found a method which makes it possible to purify silicon in a split state under plasma in order to make it usable for photovoltaic or electronics applications.

Furthermore, the present invention makes it possible to produce very pure bulk silicon bodies from split silicon, which gives a side shape to cheap impure split silicon or generally side It is particularly useful for giving side molds to silicone which has lost its shape, i.e. off-spec powdered products, scraps from sawing rods, etc.

Another advantage of the method of the present invention is that the method of the present invention is conventionally used for making the walls of a crucible or for separating silicon powder from a crucible, in which graphite or slag is placed in contact with the wall. This means that it is possible to remove the material.

[Detailed description of the invention]

In fact, in the present invention, when refining ζ-split silicon by melting it under a thermal plasma obtained by high-frequency excitation of a plasma-generating gas, in the first step, the plasma-generating gas is % hydrogen and 9
In the second step, the molten silicon obtained from the first step is melted using a mixture of 9% to 0% argon, and the plasma generating gas is argon, hydrogen. and oxygen, in which the proportion of oxygen is (1005%-105-) and the proportion of hydrogen is 1qIb~?-995%. The present invention relates to a method for refining characterized split silicon under plasma.

According to the method of the invention, silicon is used in divided forms such as powder, granules, and shavings.

When silicon is used in powder form, the particle size is generally such that the average diameter of the particles is between 40μ and 1μ. Furthermore, European Patent No. 0 by the applicant
The use of powders consistent with those described in No. 100268
Particularly possible with the method of the invention. Although the method of the present invention can be applied to silicon of metallurgical quality, the method can be used only when the silicon has a certain purity and is purified in order to make the silicon directly available for photovoltaic or electronics applications. It's even more useful when you have to.

According to the invention, the melting of the split silicon is carried out by means of a thermal plasma obtained by high-frequency excitation of a plasma-generating gas, and the plasma itself is induced by induction in order to avoid the risk of contamination associated with the use of arc plasmas. It can be caused to occur conveniently.

According to the first step of the method of the invention, the melting of the fractionated silicon is carried out under a plasma in which the plasma-generating gas consists of a mixture of 1% to 100% hydrogen and 99% to 0% argon. It will be done.

The intensity of the plasma used is appropriately selected depending on the amount of powder to be treated, its purity, and the composition of the plasma generating gas. For example, the intensity may be between 5 kWh and 25 kWh per F of silicon treated.

According to a particularly preferred embodiment of the present invention, the melting of the silicon in one portion is carried out locally while a certain thickness of unmelted silicon is brought into contact with the crucible.

This divided thin layer of silicon generates heat poorly. This layer thus reduces heat loss and prevents impurities originating from the crucible walls from diffusing into the molten silicon while avoiding reactions with the crucible surfaces. The thickness of this thin layer of unmelted divided silicon is generally K 1 m to 20 m.
It is between tg.

The volume of molten silicon is matched to the shape of the crucible used.

According to the second step of the method of the present invention, the molten silicon produced from the first step is such that the plasma generating gas is a mixture of argon, hydrogen and oxygen, in which the proportion of oxygen is 1005.
% to 005% and the proportion of hydrogen is 1 to 99.99
It is treated by a plasma consisting of 5% of the sample.

The above oxygen content is critical for the present invention.

Below this minimum content, the purification of the element desired to be extracted is insufficient. Moreover, if the content is higher than the maximum content, the desired purification cannot be obtained any further.

It should be noted that the supplementary removal of the 19.5-element compound, which can be carried out subsequently by the action of acids by techniques well known to those skilled in the art, is not economical due to the too large amount of acid that has to be used in this case. is proven.

The process of the invention can be carried out at atmospheric pressure or at slightly below or above atmospheric pressure. Furthermore, the atmosphere in contact with the silicon must be the same as the plasma atmosphere.

The method of the invention can be carried out in equipment known in the art, in particular the equipment described in European Patent Application No. 0045689 and French Patent Application No. 24584999 in the name of the applicant. The outline of this device will be explained below.

The apparatus has a horizontally disposed cylinder closed by an impermeable end with a rond inlet supporting a crucible adapted to receive the split silicon to be purified. A mechanism consisting of a screw and nut connection and a motor allows the boat to move parallel to the axis of the cylinder. This cylinder has a joint in its upper part that connects in an opaque manner a tube made of an insulating material, for example quartz, which is adapted to create a plasma. This tube receives in its upper part a mixture of plasma-generating gases into which oxygen is introduced. This inlet device includes a mixing chamber, which includes a plasma generating gas (purified argon and/or hydrogen) inlet tube equipped with a control valve and a flow rate of oxygen at a very low and defined level. An oxygen inlet tube with a controlled leak valve for regulation is threaded. The flow rate of the plasma generating gas is adjusted in a manner known to those skilled in the art depending on the technical characteristics of the device.

The high-frequency electromagnetic field for creating the plasma is obtained by windings around the tube and supplied by a high-frequency generator of sufficient strength.

Furthermore, the cylinder can have a fitting for connection to a vacuum pump to purge the cylinder or bring gas circulation to near atmospheric pressure.

To implement the method of the invention in the apparatus described above, a typical procedure is followed. That is, the divided silicon is placed in a crucible. The pump means is activated to expel the air contained within the cylinder. The tube is then placed in communication with the plasma generating gas supply bowl at a flow rate suitable to obtain the appropriate pressure within the cylinder. The radio frequency source is then activated with sufficient intensity to create a plasma by radio frequency induction within the tube. A plasma jet is formed by the pumping action of the gas and is carried over the crucible. This crucible is.

At the start of processing, it is located on the left side of the device. Once the desired zone melting of the segmented silicon has been achieved, oxygen is communicated with another plasma generating gas while the thin layer of unmelted segmented silicon is brought into contact with the crucible.

Move the crucible to the right side. The melt zone, initially on the right side of the boat, slowly progresses towards the left end, leaving the refined zone behind.

At the end of the process, the impurity is found at the left-hand end of the resulting ingot and can be removed by separating this left-hand end as in a typical zone melting process.

Note that, if not desired, it is also possible to pass the plasma through the melt stopper multiple times in each step without departing from the scope of the present invention. Moreover, since the molten silicon occupies a smaller volume than the divided silicon, the operation according to the method of the invention may be carried out several times, with the crucible being freshly filled with divided silicon after each operation. is also possible.

The method of the invention also makes it possible to obtain bulk silicon of the purity required for photovoltaic or electronics applications.

The following examples illustrate the invention and do not limit its scope in any way.

Example 1 This example uses the apparatus described in European Patent Application No. 45689.

The intensity of the plasma used was 7 kWh. 5゜I
A bed of silicon powder (its average diameter is 50μ, and the concentrations of various impurities are as shown in Table 1 below) is placed in a crucible (diameter 2.scm, height 15B).

The composition of the plasma generating gas is 9996 argon and 1
The powder is melted by passing it through the crucible under a plasma torch, which is hydrogen in es, and the moving speed of the crucible is
Fixed at 60 cwt/h for the torch. The flow rate of argon was fixed at 50 j/min in the device,
The flow rate of hydrogen is fixed at a 517 m1n. Melting is carried out in a force manner such that a thin layer of unmelted silicon of approximately 2 fi is brought into contact with the crucible.

Once the split silicon is melted, its composition is changed by introducing oxygen at α05 into the plasma generating gas. Both steps were carried out under atmospheric pressure.

Furthermore, the atmosphere in contact with silicon was the same as the plasma atmosphere used. A solid rod of 509 purified silicon was thus obtained.

Table 1 below shows the purification results achieved for three different powder samples, the specimens being analyzed by plasma emission spectroscopy.

Table 1 Example 2 (comparative example) The same powder as in Example 1 was used and the following composition was used: 99-
Example 1 is carried out in the same apparatus as in Example 1, using directly a plasma generating gas having a composition of 1% argon, 1% hydrogen and 0.03% oxygen. This method resulted in a material that was still in powder form and could not be purified no matter how long the contact time with the plasma was.

Claims (4)

[Claims] (1) In refining the divided silicon by melting it under thermal plasma obtained by high-frequency excitation of the plasma generating gas, in the first step, the plasma generating gas is % hydrogen and 99% to 0% argon to melt the divided silicon, and in the second step, the molten silicon obtained from the first step is used as a plasma generating gas. is a mixture of argon, hydrogen and oxygen in which the proportion of oxygen is 0.0
05% to 0.05% and the proportion of hydrogen is 1% to 99%
．． A method for purifying split silicon under a plasma, characterized in that it is treated with a plasma composed of 995% of silicon. (2) the melting of the segmented silicon is carried out locally with a certain thickness of unmelted silicon in contact with the crucible, so that this thin layer of segmented silicon does not generate much heat; Claim 1, characterized in that impurities originating from the walls of the crucible are prevented from diffusing into the molten silicon, thus reducing heat losses and avoiding reactions with the surfaces of the crucible. Method described. 3. A method according to claim 2, characterized in that the thickness of the thin layer of unfused, divided silicon is generally between 1 mm and 20 mm. (4) The method according to claim 1, wherein the method is carried out at atmospheric pressure or at a pressure slightly higher or lower than atmospheric pressure.