JPH075288B2 - Method for purifying divided silicon under plasma - Google Patents

Description

Description: TECHNICAL FIELD The present invention provides a method of dividing silicon under plasma to enable obtaining bulk solid silicon with the purity required for photovoltaic or electronics applications. It relates to a method for purification.

[Prior art]

In order for silicon to be usable in constructing photovoltaic cells, the content of various elements in it must be below a very low threshold value according to each criterion. In particular, the content of elements exhibiting the properties of electronic killer (eg V, Cr, Ti, Zr, Na) is
Must be 50 ppb or less. In addition, neutral elements (especially F
The content of e, Ni, Mg, C, Mn) should not exceed about 1 ppm. In addition, the content of doping elements (among which boron, aluminum, phosphorus and arsenic are the main ones) must be controlled to very low values so that the silicon obtained is p- or n-type. Further, in order for silicon to be usable in the field of electronics, the content of doping impurities must be 10 ppb or less.

According to French patent 2438499, it is known to purify ingot-like solid silicon bodies by a zone melting method under plasma obtained by radiofrequency excitation of a mixture of argon and hydrogen. There is.

Also, according to European Patent Application No. 45689, there is known a method for purifying silicon ingots by a zone melting method under a plasma consisting of a mixture of argon, oxygen and optionally hydrogen.

Thus, up to now, only the purification method of silicon ingot has been proposed.

[Object of the Invention]

The problem to be solved by the present invention is to obtain a method for purifying silicon in a divided state under plasma. However, if one attempts to apply the methods described in the above patents or patent applications to split silicon, it will be appreciated that it is not possible to achieve the desired purification.

Here, the inventor has found a method that makes it possible to purify silicon in divided form under plasma for use in photovoltaic or electronics applications.

In addition, the present invention allows the production of very pure bulky silicon bodies from segmented silicon, which adds value to, or is generally worth, cheap, impure segmented silicon. It is particularly useful for adding value to pristine silicon, i.e., non-standard powdered products, sawdust from sawing rods, and the like.

Another advantage of the method of the present invention is that the method of the present invention, such as a graphite or protective slag placed in contact with the wall, is conventionally used to make a crucible wall or to separate silicon powder from the crucible. It is possible to remove various materials.

[Specific Description of the Invention]

In practice, the present invention, in refining the divided silicon by melting it under the thermal plasma obtained by high-frequency excitation of the plasma generating gas, in the first step, the plasma generating gas 1% to 100% hydrogen and 99%
Melting of divided silicon composed of a mixture with ˜0% argon is carried out, and in the second step, the molten silicon obtained from the first step is treated with argon, hydrogen and oxygen as plasma generating gas. With a proportion of oxygen of 0.005% to 0.05% and a proportion of hydrogen of 1% to 99%.
A method for purifying split silicon under plasma, characterized in that it is treated with a plasma composed of such that it is 995%.

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

When the silicon is used in powder form, its particle size is generally such that the average diameter of the particles is between 40 μm and 1 mm. Furthermore, the European patent 0100 by the applicant
The use of powders consistent with those described in No. 268 can be particularly considered by the method of the invention. Although the method of the present invention is applicable to metallurgical quality silicon, the method is refined to allow silicon to be directly utilized when it has a certain degree of purity and for photovoltaic or electronics applications. More beneficial when you have to.

According to the invention, the melting of the divided silicon is carried out by means of a thermal plasma obtained by the high-frequency excitation of the plasma-generating gas, which plasma itself avoids the risk of contamination associated with the use of arc plasma. It is generated well by induction.

According to the first step of the method of the present invention, the melting of the divided silicon is carried out by using the gas for plasma generation with 1% to 100% hydrogen and 99%.
It is carried out under a plasma consisting of a mixture of 0% to 0% argon.

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, its strength is 5kW / Kg of silicon processed.
It may be between h and 25kWh.

According to a particularly preferred embodiment of the invention, the melting of the divided silicon is carried out locally, with a certain thickness of unmelted silicon being in contact with the crucible. In this divided thin layer of silicon, heat is generated poorly. Therefore, this layer reduces heat loss and prevents impurities originating from the crucible wall from diffusing into the molten silicon while avoiding reaction with the crucible surface. The thickness of this unmelted, split silicon thin layer is generally between 1 mm and 20 mm.

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 in 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 0.005%.
˜0.05% and a hydrogen content of 1% to 99.995%.

The above oxygen content is critical in the present invention. Below this minimum content, the purification of the element desired to be extracted is insufficient. Further, if the content is higher than the maximum content, the desired purification cannot be further obtained. It should be noted that the complementary removal of boron compounds, which can be carried out subsequently by the action of an acid by a technique well known to those skilled in the art, is not economical because the amount of acid to be used in this case is too large. Is proven.

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

The method according to the invention can be carried out on devices known in the art, in particular the devices described in European patent application 0045689 and French patent application 24384999 to the applicant. The outline of this apparatus will be described below.

The device comprises a horizontally arranged cylinder closed by an impermeable end with an opening for a rod supporting a crucible designed to receive the divided silicon to be purified. The boat can be moved parallel to the axis of the cylinder by the mechanism composed of the screw, nut joint and motor. This cylinder has on its upper part a joint in which a tube made of an insulating material, for example a quartz, adapted to create a plasma is connected in an impermeable manner. This tube receives in its upper part a mixture of plasma generating gases into which the enzyme is introduced. The inflow device includes a mixing chamber, which is equipped with a control valve for introducing a plasma generating gas (purified argon and / or hydrogen) and a flow rate of oxygen to a very low level and a predetermined level. An oxygen inlet tube with a controlled leak valve for regulation is passed through. The flow rate of the plasma-generating gas is adjusted according to the technical characteristics of the device, as is known to those skilled in the art.

The high frequency electromagnetic field for creating the plasma is obtained by windings wound around the tube and is supplied by a sufficiently strong high frequency generator.

Further, the cylinder may have a fitting for connecting to a vacuum pump to purge the cylinder or bring the gas circulation to near atmospheric pressure.

Implementation of the method of the present invention in the apparatus shown above follows a typical process. That is, the divided silicon is placed in the crucible. The pumping means is started to evacuate the air contained in the cylinder. The tube is then brought into communication with a plasma generating gas source with a flow rate suitable for obtaining a suitable pressure in the cylinder. The RF source is then operated inside the tube with sufficient strength to create a plasma by RF induction. A plasma jet is formed by the pumping action of the gas and is carried onto the crucible. This crucible is located on the left side of the device at the start of processing. Once the desired zone melting of the divided silicon has been obtained, oxygen is brought into communication with another plasma generating gas while contacting the unmelted thin layer of the divided silicon with the crucible.

Move the crucible to the right. The molten zone, which was initially on the right side of the boat, slowly moves toward the left end while leaving the refined zone behind.

After the treatment is complete, impurities are found at the left end of the resulting ingot, which can be removed by separating this left end as in a typical zone melting process. .

If desired, it is possible to pass the plasma over the melting zone multiple times in each step without departing from the scope of the invention. Furthermore, since the molten silicon occupies a smaller volume than the divided silicon, it is necessary to carry out the operation according to the method of the present invention multiple times by newly filling the crucible with the divided silicon after each operation. Is also possible.

The process of the invention also makes it possible to obtain bulk silicon having 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 In this example, the device described in European Patent Application No. 45689 is used.

The strength of the plasma used is 7kWh. 50 g of silicon powder (its average diameter is 50μ, and the concentration of various impurities is as shown in Table 1 below) crucible (diameter 2.5 cm, height 15 m)
m).

The powder is melted by passing it through a crucible under a plasma torch in which the composition of the gas for plasma generation is 99% argon and 1% hydrogen, and the moving speed of the crucible is 60 cm / h with respect to the torch. Fix it. Argon flow rate in the device
It is fixed at 30 l / min and the hydrogen flow rate is fixed at 0.3 l / min. The melting is carried out in such a way that a thin layer of unmelted silicon of about 2 mm is brought into contact with the crucible.

Once the divided silicon has melted, its composition is changed by introducing 0.03% oxygen into the plasma generating gas. Both steps were performed under atmospheric pressure.

Furthermore, the atmosphere in contact with the silicon was the same as the plasma atmosphere used. Thus, 50 g of purified silicon solid rods were obtained.

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

Example 2 (Comparative Example) Same as Example 1 using the same powder as in Example 1 but directly using a plasma generating gas of the following composition: 99% Argon, 1% Hydrogen and 0.03% Oxygen. Performed on the device. According to this method, a substance was obtained which was still in powder form and could not be purified by any contact time with plasma.

Claims (4)

[Claims] 1. Purification by melting the divided silicon by melting it under thermal plasma obtained by high-frequency excitation of a plasma-generating gas. In the first step, the plasma-generating gas is 1%. ~ 100% hydrogen and 99% ~ 0% argon composed of a mixture of divided silicon is melted, and in the second step, the molten silicon obtained from the first step is plasma-generated. The working gas is a mixture of argon, hydrogen and oxygen, the proportion of oxygen in which is 0.00
Method for purifying split silicon under plasma, characterized by treating with a plasma composed of such that the hydrogen content is between 5% and 0.05% and the hydrogen content is between 1% and 99.995% . 2. Melting of the divided silicon is carried out locally, with a certain thickness of unmelted silicon in contact with the crucible, whereby this thin layer of divided silicon produces less heat. Therefore, it reduces the heat loss and prevents the impurities generated from the crucible wall from diffusing into the molten silicon while avoiding the reaction with the surface of the crucible. The method according to item 1. 3. A method according to claim 2, characterized in that the thickness of the unmelted, divided silicon thin layer is generally between 1 mm and 20 mm. 4. The process according to claim 1, which is carried out under atmospheric pressure or under a pressure slightly above or below atmospheric pressure.