JP2002128515A - Method of manufacturing high-purity inorganic material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a high-purity inorganic material at a lower cost than conventional method. SOLUTION: The method utilizes the fact that when two materials left still having different solubilities (sum of chemical potentials) of impurity in a state wherein the impurity can be mobile by diffusion such as contact state, the diffusion of the impurity between the two materials is progressed until the chemical potentials become same. A material (capturing material) having a lower sum of chemical potentials (having a higher solubility) than the inorganic material to be purified for the constant concentration of the impurity and the inorganic material are contacted, and after the impurity contained in the inorganic material was transferred by diffusion to the capturing, the material having the dissolved impurity is removed and a high-purity inorganic material is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a high-purity ceramic (silicon carbide, silicon nitride, aluminum nitride, etc.) suitable for a substrate of a thin film solar cell, a member for semiconductor processing, a buffer material for preventing fusion during wafer heat treatment, and the like. And high-purity inorganic substances such as metals (silicon and the like).

[0002]

2. Description of the Related Art It is necessary to prevent inorganic substances used in semiconductor devices and processes from contaminating a target semiconductor. In order to achieve this object, a method of purifying the inorganic substance itself and a method of providing a diffusion prevention film so that impurities do not diffuse from the inorganic substance are conventionally known.

As a conventional technique for increasing the purity of an inorganic substance such as ceramics, there is known a molded article made of silicon carbide and carbon disclosed in Japanese Patent Application Laid-Open No. Sho 64-72964.
A method of removing impurities by heating to 000 to 1200 ° C. and treating with chlorine gas; washing a silicon carbide powder disclosed in JP-A-5-32458 with a mixed acid of hydrofluoric acid and nitric acid and pure water; Japanese Patent Application Laid-Open No. 8-
No. 12437 discloses a method for producing silicon carbide in which a compact comprising silicon carbide and carbon is impregnated with metallic silicon at a temperature equal to or higher than the melting point of silicon in a non-oxidizing atmosphere. Method of performing a step of evaporating impurities by heat treatment
There is a method in which a silicon carbide powder disclosed in JP-A-11923 is mixed with a mineral acid such as hydrogen fluoride and heat-treated under pressure.

As a technique for preventing diffusion of impurities from an inorganic substance, a heat treatment material disclosed in Japanese Patent No. 2844939 is disclosed in Japanese Patent Application Laid-Open No. 2844939. And a film of a substance having a small diffusion constant of the impurity used in an impurity analysis method for collecting and recovering the impurity released from the heat-treated material in the substance.

[0005]

However, JP-A-64-72964, JP-A-5-32458, JP-A-8-12437, and JP-A-11-119.
The method disclosed in Japanese Patent Publication No. 23 cannot sufficiently say that impurities are removed, and it is necessary to repeatedly use a large amount of corrosive substances such as chlorine and hydrofluoric acid. In order to ensure sufficient performance, a large cost was required. In the method of evaporating impurities, 16
Since processing at a high temperature of 00 ° C. or more is required, a great deal of cost is required including peripheral devices.

In the method of providing an impurity diffusion preventing film on an inorganic substance, the prevention film is formed by a vapor phase method such as a CVD method or a vapor deposition method, a sol-gel method, etc., all of which are expensive.

Therefore, an object of the present invention is to provide a production method capable of producing a high-purity inorganic substance at a lower cost than the conventional method.

[0008]

Means for Solving the Problems In order to solve the above problems, the present inventors have conducted intensive studies and as a result, have made the following inventions. That is, the method for producing a high-purity inorganic substance of the present invention includes a pretreatment step of coating or contacting a predetermined inorganic substance with a collector that is a material having a higher impurity dissolving ability than the inorganic substance; Has a heating step of heating the inorganic substance coated or in contact therewith to transfer impurities of the inorganic substance to the trapping material, and a removing step of removing the trapping material.

That is, if the impurities are allowed to move by diffusion, such as a state in which two substances having different impurity dissolving abilities (sum of chemical potentials) are in contact with each other, the impurities have the same chemical potential between the two substances. Based on the fact that diffusion progresses to the point that a substance (collection material) with a lower total chemical potential (higher dissolving capacity) than an inorganic substance to be made highly pure for a certain concentration of impurities, After the impurities contained in the inorganic substance are diffused and moved to the collecting material by contacting with the inorganic substance, a high-purity inorganic substance is obtained by removing the collecting material in which the impurities are dissolved. Can be. Examples of the trapping material include substances in which the concentration of impurities is equal to or lower than a predetermined concentration corresponding to the substance.

[0010] The method of bringing the collecting material into contact with the inorganic substance can be achieved by coating the inorganic substance with the collecting material in addition to the method of simply contacting the collecting substance. In addition, since the diffusion of the impurity proceeds faster at a higher temperature, the impurity is heated to a necessary concentration within a required time.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments.

The method for producing a high-purity inorganic substance according to this embodiment includes a pretreatment step, a heating step, and a removal step. The pretreatment step is a step of contacting or coating a substance that transfers impurities with the inorganic substance, the heating step is a step of actually diffusing and transferring the impurities from the inorganic substance, and the removing step is a step of removing the substance that has diffused and transferred the impurities. This is a step of removing impurities.

The pretreatment step is a step of coating or contacting a predetermined inorganic substance with a collecting material which is a material having a higher impurity dissolving ability than the inorganic substance. The specified inorganic substance is
It is an inorganic substance to be highly purified. For example, high-purity ceramics (such as carbides, nitrides and oxides such as silicon carbide, silicon nitride, aluminum nitride, and alumina) suitable for substrates of thin-film solar cells, members for semiconductor processes, buffers for preventing fusion during wafer heat treatment, and the like. Object, etc.) and metal (silicon, etc.).

It is necessary that these inorganic substances have a purity necessary to maintain the function of a semiconductor or the like in contact therewith. Therefore, impurities to be reduced by the present manufacturing method vary greatly depending on the purpose of use of the inorganic substance. Further, the kind of the contained inorganic substance differs depending on the production method of the inorganic substance itself. Examples of the impurities in the inorganic substance include, for example, Fe contained in the raw material or mixed in the manufacturing process for silicon carbide, Fe, Al, Ca for silicon nitride, Ca, Fe, Ni for aluminum nitride, and Fe for silicon. Ti and the like can be exemplified as main impurities depending on the purpose of use.

The shape of the inorganic substance may be the same as that of the inorganic substance, and examples thereof include a powder, an ingot, and a compact obtained by sintering a powder. Since this manufacturing method is a method of removing impurities by diffusion, when it is necessary to purify the entire inorganic substance, it is better to use a form having a large specific surface area such as a powder form in a shorter time to increase the inorganic substance. Can be purified. Therefore, when only the surface of the inorganic substance needs to be highly purified, the specific surface area does not need to be particularly considered. In addition, a longer time is required as compared with the method of increasing the specific surface area.However, the method is applied again after dissolving the inorganic substance to make the distribution of impurities uniform after purifying by the present manufacturing method. By repeating the above, high purity of the entire inorganic substance can be achieved.

The trapping material is a material having a higher impurity dissolving ability than the inorganic substance to be subjected to the production method, and the state of the substance such as a solid, a liquid, and a gas is not limited. Here, the high solubility of impurities means that the sum of chemical potentials for impurities is lower than that of inorganic substances. For example, since the amount of dissolved impurities is low (low concentration), chemical potential is low. Are low, and the chemical potential between the trapping material and the impurity is low because the trapping material itself has a high affinity for the impurity. A preferable trapping material has a small amount of impurities from the beginning and has a higher affinity for impurities than an inorganic substance. However, in general, it is easier to select a substance having a high affinity for impurities as a trapping material, and a substance having a lower total chemical potential than an inorganic substance can be obtained.

The trapping material may be selected independently of the inorganic substance, or may be a reactant generated by reacting the inorganic substance with any reactant or a transition substance transferred by heat or the like. It can also be collected. Examples of the reactant include an oxidant such as oxygen.

The method of coating or contacting the inorganic substance with the trapping material is not particularly limited, but is preferably a method capable of reliably coating or contacting all surfaces of the inorganic substance to be purified. For example, a method of simply contacting a trapping material in a form according to the form of the inorganic substance (powder if the inorganic substance is a powder, or a form formed according to the form if the inorganic substance is in an ingot). Prepared in advance, such as a method of immersing the inorganic substance in the liquefied trapping material, a method of pressing the liquefied trapping material into the inorganic substance if it is in the form of a powder or a sintered body, etc. There is a method of contacting or coating the trapping material with an inorganic substance.

In addition, when a reactant or a transition product generated by chemically changing an inorganic substance with a certain reactant or transferring by heating satisfies the properties as a trapping material, one of the inorganic substances is used. The inorganic substance can be coated by using a reactant or a transition product generated by reacting or transferring the part with a reactant as a trapping material. As described above, in the method of inducing the trapping material from the inorganic substance itself with the reactant or the like, the surface of the inorganic substance can be reliably covered with the trapping material by appropriately selecting the induction conditions and the like. Then, depending on the method, the pretreatment step of coating the trapping material and the heating step can be performed simultaneously. Also,
Since the adhesion between the inorganic substance and the trapping material increases, impurities can be trapped reliably.

For example, with respect to the powder shown in FIG. 1, the sintered body shown in FIG. 2, and the ingot shown in FIG. 3, the surface portion of the inorganic substance is reacted with a reactant to form a collecting material. When the state of coating the surface of A is illustrated, a reactant is made to act on each inorganic substance A shown in FIGS. 1 to 3A, and the trapping material B is applied to the surface as shown in FIGS. Form a coating. After the impurities in the inorganic substance A are diffused and moved to the film of the trapping material B in a heating step described later, the trapping material is removed in a removing step described later.

When silicon carbide is used as a substrate for a thin-film solar cell, it is preferable that silicon carbide has conductivity. Then, the silicon of the solar cell main body can be directly provided without providing an electrode on the silicon carbide. Thus, to impart conductivity to silicon carbide,
It is a common practice to diffuse Al, B, N, P, and the like as specific impurities into silicon carbide. Therefore, by causing the specific impurities in the trapping material to be contained at an appropriate concentration, in the heating step described below, the impurities in the inorganic substance are transferred to the trapping material, and at the same time, the specific impurities in the trapping material are converted to the inorganic substance. It is diffused and moved inside. Therefore, a step of diffusing the specific impurity is not required separately, and the total number of steps can be reduced. Therefore, it is preferable that the trapping material contains the specific impurity.

The heating step is a step of heating the inorganic substance covered or in contact with the trapping material to transfer impurities of the inorganic substance to the trapping material. In other words, the diffusion rate of the impurities from the inorganic substance to the trapping material has a positive correlation with the temperature, so that the temperature at which the diffusion of the impurities into the trapping material reaches the required concentration within the required time is reached. Heat so that

In the case where the dissolving ability of impurities differs depending on the state (solid, liquid, etc.) of the trapping material, it is necessary to set the necessary temperature (for example, the trapping material to a liquid state) in order to improve the dissolving ability. In some cases, it is heated up to the melting point or higher. The heating method is not particularly limited, and a method such as an electric furnace or an infrared lamp can be used. The heating atmosphere can be freely selected as needed, such as a non-oxidizing (inactive) atmosphere or an oxidizing atmosphere. The heating step performed for these purposes is a step that can be performed simultaneously with the pretreatment step described above or the removal step described below.

When the trapping material contains specific impurities, the heating step preferably includes a step of diffusing and transferring the specific impurities from the trapping material to the inorganic substance. The step of transferring the specific impurities is not particularly different from the heating step, except that it requires a time for the specific impurities to sufficiently diffuse and transfer into the inorganic substance.

The removing step is a step of removing the trapping material. The method of removing the trapping material varies greatly depending on the type of trapping material and the relationship between the trapping material and the inorganic substance. For example, when the trapping material is merely brought into contact with the inorganic substance, the inorganic substance and the trapping material may be physically separated.

When there is a difference in reactivity with respect to a chemical reaction or the like between the inorganic substance and the trapping material, the inorganic substance and the trapping material can be separated by utilizing the difference in reactivity. . For example, in the case where the trapping material has high reactivity with a certain acid, the trapping material is dissolved and removed in the acid. Also,
As described above, when a collector is formed as a reaction product of the inorganic substance around the inorganic substance by causing a reactant to act on the inorganic substance, since the trapping material exists around the inorganic substance,
By treating the inorganic substance with the trapping material with an acid or the like, it is possible to preferentially react and remove from the surrounding trapping material. That is, only the trapping material can be selectively removed by removing the trapping material from the surroundings with an acid or the like and terminating the reaction when the reaction of the acid or the like proceeds to the inorganic substance portion.

Hereinafter, the production method of the present embodiment will be described again based on silicon carbide as an inorganic substance.

Silicon carbide (SiC) is mainly produced from SiO ₂ and carbon by the Acheson method. The produced silicon carbide contains many impurities, and the main impurity is Fe. Even if the amount of Fe in the silicon carbide is small, it is several tens ppm, usually 300 to 500 ppm, and is uniformly contained throughout. First, in order to remove this Fe,
As a pretreatment step, the silicon carbide powder is heated at a temperature of 1100 to 1600 ° C. for a predetermined time in an air atmosphere containing oxygen as a reactant, so that the surface of the silicon carbide has SiO _2.
An oxide layer mainly composed of It is considered that the generated SiO ₂ exists in a glassy state. The chemical potential of Fe in SiO ₂ is lower than the chemical potential in SiC. Specifically, the chemical potential of Fe in SiO ₂ is 1/10 or less of that in SiC.

This heating also serves as the above-mentioned heating step. That is, S as a trapping material as a pretreatment step
While covering the periphery of the inorganic substance with the iO ₂ layer, at the same time, Fe, which is an impurity, is diffused and moved in the coated trapping material by utilizing the difference in chemical potential.

Here, heating at 1100 ° C. or more
If the treatment is performed at a temperature lower than this, it takes a long time to form the oxide layer, and also the diffusion rate of Fe into the formed SiO ₂ layer becomes slow. And, as the heating temperature, 1
The reason why the temperature is set to 600 ° C. or less is that when the treatment is performed at a higher temperature,
This is because the surface of the silicon carbide is rapidly and unevenly oxidized, and the surface of the finally formed silicon carbide becomes rough.
The predetermined time for heating at 1100 to 1600 ° C. is as follows:
The value largely varies depending on the purity of silicon carbide finally required, but roughly ranges from several hours to several hundred hours.

Thereafter, the removal of the SiO ₂ layer as a removal step can be achieved by dissolving with hydrofluoric acid or by heating under a reducing atmosphere. Since SiO ₂ reacts with carbon or carbon monoxide to form SiO and O ₂ and evaporates and is scattered, the silicon carbide coated with the SiO ₂ layer on the surface is heated at 1000 ° C. in a carbon container or carbon powder.
By heating under reduced pressure as described above, the SiO ₂ layer on the surface can be removed. With the removal of the SiO ₂ layer, impurities can also be removed.

The SiO ₂ layer is hardly permeable to oxygen once it is formed. Therefore, when the amount of impurities is large or when high-purity silicon carbide is required, this operation is repeated to further improve the carbonization. Silicon can be brought to the required purity.

In addition to forming an SiO ₂ layer on the surface of silicon carbide, high purity silicon carbide can be achieved by contact with molten silicon in a non-oxidizing atmosphere. The heating temperature in this case is the melting point of silicon (1420).
C) or more. Since silicon does not hinder the reaction as in the case of the SiO ₂ layer, there is an advantage that the number of repetitions for high-purification processing can be reduced.

As described above, silicon nitride, aluminum nitride, alumina, and silicon can be highly purified. For example, in order to highly purify silicides such as silicon and silicon nitride, it is preferable to use silicon dioxide as a trapping material. In order to purify a nitride such as silicon nitride with high purity, it is preferable to use silicon as a trapping material. For example, silicon is used in such a manner that it is brought into contact in a molten state. By using such molten silicon, the amount as a trapping material can be secured,
This is preferable in that a large amount of impurities can be dissolved.

[0035]

The present invention will be described in more detail with reference to the following examples.

Example 1 Silicon carbide powder produced by the Acheson process (average particle size 50 μm, Fe content 350 p)
pm) was oxidized at 1400 ° C. for 8 hours in an air atmosphere. The surface of the silicon carbide powder was oxidized to form a SiO ₂ layer. This SiO ₂ layer was dissolved and removed with hydrofluoric acid. The Fe content of the remaining silicon carbide powder was 10 ppm. In addition,
The Fe content in the dissolved SiO ₂ layer was 4700 ppm.

Example 2 α-silicon carbide (average particle size 2
0 μm, Fe content 250 ppm) and carbon black (Fe content 7 ppm) were mixed at a weight ratio of 88:12 to form a molded body of 50 × 70 × 3 mm. This compact and a crude silicon powder having an Fe content of 5500 ppm were placed in a carbon container, and heated and reacted at 1450 ° C. for 20 minutes in a vacuum. The obtained reaction sintered body is a composite composed of α-silicon carbide used as a raw material, β-silicon carbide generated by reacting carbon and silicon, and silicon remaining between silicon carbide grains. there were. The ratio of each component was approximately 7: 3: 2. Fe in this composite
Was 2000 ppm.

The composite was immersed in a mixed acid of hydrofluoric acid and nitric acid to obtain a silicon carbide porous body in which silicon remaining between silicon carbide grains was dissolved and removed. The Fe content of this silicon carbide porous body was 470 ppm. When this porous silicon carbide body was oxidized at 1400 ° C. for 24 hours in an air atmosphere, a 1.5 μm deep SiO ₂ layer was formed on the surface. This SiO ₂ layer was dissolved and removed with hydrofluoric acid. The average Fe content of the obtained porous body was 250 ppm, and the Fe content on the surface was 5 ppm or less.

(Example 3) Diameter 25 mm, thickness 2.0 m
A commercially available sintered silicon carbide having an Fe content of 40 ppm and a Fe content of 40 ppm was subjected to a heat treatment at 1400 ° C. for 48 hours in an air atmosphere containing humidity to oxidize the surface to form a SiO ₂ layer. Thereafter, the SiO ₂ layer on the surface was dissolved and removed with hydrofluoric acid. F near the surface
As a result of analyzing the e content by SIMS, the Fe content within a surface depth of 1 μm was 1 ppm or less. This is because, by performing the heat treatment in an atmosphere containing humidity, the progress of oxidation of the surface of the sintered body is accelerated, and the treatment is completed in a short time.

Example 4 Carbon black (Fe content: 7 ppm) and coarse silicon powder (Fe content: 5500 pp)
and m) were mixed at a weight ratio of 3: 7, formed into an appropriate size, and reacted at 1500 ° C. for 1 hour in an Ar atmosphere. As a result of X-ray diffraction analysis, the produced powder was β-silicon carbide. The Fe content of this β-silicon carbide powder was 49
It was 00 ppm. In order to promote sintering, carbon and B ₄ C were added to this powder in a mass conversion of 1.5% and 0.5%, respectively.
%, Mixed and dried to obtain a powder for sintering. After the sintering powder is press-molded, it is heated to 2100 ° C. in an Ar atmosphere.
For 2 hours.

The sintered body had an Fe content of 4700 ppm. This sintered body was immersed in high-purity silicon contained in a quartz glass container and heat-treated at 1450 ° C. for 4 hours in an Ar atmosphere. Silicon adhering to the surface is vacuum, 14
Heat to 50 ° C. and evaporate. The content of Fe on the surface of this sintered body was 5 ppm or less as a result of SIMS analysis.

The silicon carbide of each of the embodiments described above can achieve a sufficiently high degree of purity, for example, a substrate for a thin film solar cell, a member for a semiconductor process, a buffer material for preventing fusion during wafer heat treatment, and the like. The purity was sufficient for use.

Therefore, according to the manufacturing method of this embodiment, impurities in ceramics (silicon carbide) as an inorganic substance can be reduced as a whole, and only the surface portion can be highly purified. . In particular, when there is a large difference in the dissolving ability of impurities between the ceramics and the trapping material, a concentration distribution of the impurities is generated inside the ceramics, so that the purity of only the surface is high and the purity of the entire ceramics is not improved. Practical ceramics can be obtained.

By using such a ceramic for a substrate for a polycrystalline thin-film solar cell, the above-mentioned impurity diffusion preventing film is not required, so that the structure of the polycrystalline thin-film solar cell can be simplified and manufactured at low cost. it can.

[0045]

Accordingly, the production method of the present invention has an effect that a production method capable of producing a high-purity inorganic substance at lower cost than the conventional method can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view illustrating a state of an inorganic substance to which a manufacturing method according to an embodiment is applied.

FIG. 2 is a schematic cross-sectional view showing a state of an inorganic substance to which the manufacturing method of the present embodiment is applied.

FIG. 3 is a schematic cross-sectional view showing a state of an inorganic substance to which the manufacturing method of the embodiment is applied.

[Explanation of symbols]

 A: Inorganic substance (object to be purified) B: Collection material

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Hitoshi Uchida 1141-1 Okawagahara, Iino, Fujioka-cho, Nishikamo-gun, Aichi Prefecture Inside Aisin Kako Co., Ltd. (72) Inventor Yu Shirouchi, Iino, Fujioka-cho, Nishikamo-gun, Aichi 1141 Okawagahara 1 Aisin Chemical Industry Co., Ltd. (72) Inventor Shigetaka Wada 41-Cho, Yojimichi, Nagakute-cho, Aichi-gun, Aichi F-1 term in Toyota Central R & D Labs., Inc. 4G046 MA14 MA15 MB02 MB10 MC01 MC04 4G072 AA01 HH01 HH14 JJ18 MM08 NN05 UU01 5F051 BA11 GA03 GA20

Claims (5)

[Claims] A pretreatment step of coating or contacting a predetermined inorganic substance with a collecting material that is a material having a higher impurity dissolving ability than the inorganic substance; A method for producing a high-purity inorganic substance, comprising: a heating step of heating a substance to transfer impurities of the inorganic substance to the trapping material; and a removing step of removing the trapping material. 2. The pre-treatment step includes: contacting a reactant that reacts with the inorganic substance to change the inorganic substance into a material having a higher ability to dissolve impurities than the inorganic substance; 2. The step of changing a surface layer of a substance to a material having a higher impurity dissolving ability than the inorganic substance.
3. The method for producing a high-purity inorganic substance according to 1. 3. The inorganic substance is one or more substances selected from silicon carbide, silicon and nitride, and the material having a higher impurity dissolving ability than the inorganic substance is silicon dioxide. Or the method for producing a high-purity inorganic substance according to 2. 4. The method according to claim 1, wherein the inorganic substance is at least one substance selected from silicon carbide and nitride, and the material having a higher impurity dissolving ability than the inorganic substance is silicon. The method for producing a high-purity inorganic substance according to the above. 5. The trapping material contains a specific impurity, and the heating step has a step of diffusing and transferring the specific impurity from the trapping material to the inorganic substance. The method for producing a high-purity inorganic substance according to any one of the first to third aspects.