WO2018110565A1

WO2018110565A1 - Method for producing high-purity silicon nitride powder

Info

Publication number: WO2018110565A1
Application number: PCT/JP2017/044614
Authority: WO
Inventors: 卓司王丸; 耕司柴田; 猛山尾; 山田　哲夫
Original assignee: 宇部興産株式会社
Priority date: 2016-12-12
Filing date: 2017-12-12
Publication date: 2018-06-21
Also published as: JP2020023406A; TW201829299A

Abstract

Provided is a technique for producing silicon nitride powder having excellent crystallinity and low contained amounts of impurities such as iron and aluminum efficiently and inexpensively by a combustion synthesis method. A method for producing silicon nitride powder, the method comprising mixing silicon powder and diluent silicon nitride, packing the mixed powder into a crucible, producing an agglomerate configured from silicon nitride by a combustion synthesis method, and pulverizing the agglomerate, wherein the contained amounts of iron and aluminum in the silicon powder and diluent are each 100 ppm or less, the total contained amount of metal impurities other than iron and aluminum is 100 ppm or less, the mixture ratio of the silicon powder and the diluent is 9:1 to 5:5 by mass ratio, the bulk density of a powder layer of the mixed powder is 0.3-0.65 g/cm³, and the method for producing high-purity silicon nitride powder comprises pulverizing the agglomerate using a pulverizer loaded or equipped with a grinding medium that includes silicon nitride.

Description

Method for producing high purity silicon nitride powder

The present invention is a raw material for various jigs for semiconductor manufacturing that requires particularly high purity by converting metal silicon powder into silicon nitride powder by a combustion synthesis method using a combustion reaction that self-propagates in an atmosphere containing nitrogen. The present invention relates to a method for efficiently and inexpensively producing a silicon nitride powder that can be used as a raw material for a high thermal conductivity silicon nitride substrate, a mold release agent used in the production of a silicon ingot for solar cells, and the like.

As one method for producing silicon nitride powder, a direct nitridation method is known in which metal silicon powder is fired in a non-oxidizing atmosphere containing nitrogen gas or ammonia gas and then nitrided. This method is simple in process, but because it is a reaction between gas (nitrogen gas or ammonia gas) and solid (metal silicon), it is necessary to finely pulverize the metal silicon used as a raw material. There is a problem that impurities such as aluminum are easily mixed. Further, in order to promote the nitriding reaction, it is necessary to add a catalyst such as calcium fluoride or an iron-based compound, so that these catalysts remain and the purity is lowered. It is said that it is not suitable for manufacturing.

If the raw material does not contain a nitriding promotion catalyst, the nitriding reaction will not proceed unless the heating temperature is raised higher than usual, but the nitridation of metallic silicon is an exothermic reaction. It may happen that the raw material silicon powder is melted. Once partially melted, the nitriding reaction of that part does not proceed, so a large amount of unreacted silicon remains. In order to avoid this, a technology for reducing the amount of unreacted silicon by slowly controlling the raw material particle size and nitriding reaction conditions and proceeding the nitriding reaction slowly has been disclosed. Therefore, it takes a long time to grind the product, and there is a problem that a large amount of impurities are mixed from the grinding medium.

In order to significantly improve the productivity of the nitriding reaction, a method for producing silicon nitride powder by a combustion synthesis method using a combustion reaction that self-propagates in an atmosphere containing nitrogen has been developed.

The combustion synthesis method is a synthesis method that actively uses an exothermic reaction in which a chemical reaction between elements constituting a compound is strong. For example, a high heat of formation is released when an intermetallic compound having a high melting point is synthesized. This is a method for synthesizing a target compound by causing an exothermic reaction to proceed from a raw material powder in a chain reaction in a short time in seconds. Accordingly, the energy supply from the outside is unnecessary, and the cost of the production apparatus can be reduced, so that the target compound can be produced at low cost.
In the combustion synthesis method, a chemical reaction is usually excited at a specific part (for example, one end or the center) of the raw material powder layer, and this chemical reaction is propagated as a combustion wave in the raw material powder layer, thereby proceeding with the synthesis reaction. Let In the case of synthesizing silicon nitride, the following reaction occurs.

3Si + 2N ₂ → Si ₃ N ₄ −ΔH (1)
In the reaction formula, [−ΔH] = 735.7 KJ / mol (T = 1500 K), and this heat generation serves as a driving force for the combustion reaction to propagate the combustion wave.

When this method is applied to the synthesis of silicon nitride powder, silicon powder having a particle size of about several μm can be used as a raw material powder, and conditions can be set so that the nitriding reaction can be completed in a short time. It attracts attention as a method for producing silicon nitride powder.

However, since normally available silicon powder contains about several hundred ppm of metal impurities such as iron, chromium and aluminum, there is no example of producing high-purity silicon nitride powder. Further, in the production of silicon nitride powder by the combustion synthesis method, it is common to add a diluent for reaction control. The silicon nitride powder used as the diluent also contains metal impurities such as iron, chromium and aluminum of about several hundred ppm, and no attempt has been made to produce high-purity silicon nitride powder. .

Regarding combustion synthesis, the following prior art documents are disclosed.

Japanese National Patent Publication No. 03-500640 JP 2000-264608 A JP 2005-194154 A JP 2013-63894 A Japanese Patent Laying-Open No. 2015-081205

Patent Document 1 and Patent Document 2 relate to the production of silicon nitride powder by combustion synthesis. However, impurities contained in the raw material silicon and impurities accompanying the pulverization of the generated silicon nitride lump and deterioration of crystallinity (crystal No consideration is given to the refinement of the diameter and the increase in crystal strain. Therefore, there is no description of what the properties of the obtained silicon nitride powder are.

Patent documents 3 and 4 relate to sialon powder synthesis. Sialon is, for example, a substance represented by the general formula Si _6-z Al _z O _z N _8-z and is premised on the presence of aluminum, and has a high aluminum content of 200 ppm or less in the present invention. The synthesis of pure silicon nitride powder has a different purpose.

Similarly, Non-Patent Document 1 and Non-Patent Document 2 also relate to sialon powder synthesis, and have different purposes from the synthesis of high-purity silicon nitride powder having an aluminum content of 200 ppm or less in the present invention. Is.

Patent Document 5 aims to provide a silicon nitride filler which is a condensed particle having a particle diameter of 5 μm or more and 200 μm or less, and which contains 50% by volume or more of condensed particles including columnar silicon nitride particles. is there. The approach to solving the problem is different from the generation of strong agglomerated particles disclosed in Patent Document 5 and the obtaining of a soft silicon nitride agglomerate that is easily pulverized after nitriding in the present invention.

In Patent Document 5, it is particularly preferable that the ratio of particles having a particle diameter of less than 5 μm is reduced, and the ratio of condensed particles having a particle diameter of 5 μm or more and 200 μm or less is 80% by volume or more, more preferably 90% by volume or more. As an embodiment, coarse aggregated particles having an average particle diameter (D ₅₀ ) of 26 to 77 μm are obtained. Patent Document 5 proposes to reduce the presence of defects inside the crystal. However, Patent Document 5 aims to obtain coarse agglomerated particles in which silicon nitride particles are strongly aggregated. The generation of defects is not considered at all.

Furthermore, the silicon nitride filler describes that it can contain unavoidable impurities in addition to silicon nitride, additives added as necessary, auxiliaries, etc., and unavoidable such as iron and aluminum. The present invention of reducing the mixing of impurities is different from the means for solving the problems. For example, in Patent Document 5, alumina balls are used as a grinding medium, and an alumina mortar and pestle are used as grinding tools.

The present invention has been made in view of the problems of the conventional methods as described above, and the object of the present invention is high purity with good crystallinity and low content of impurities such as iron, chromium and aluminum. It is an object to provide a technique for efficiently and inexpensively synthesizing silicon nitride fine powder by combustion synthesis.

In view of the above, the object of the present invention can be used as a raw material for various jigs for semiconductor manufacturing, a raw material for a high thermal conductivity silicon nitride substrate, a mold release agent used when manufacturing a silicon ingot for solar cells, etc. To provide a method for producing high-purity silicon nitride powder at low cost by a combustion synthesis method using a combustion reaction that self-propagates in an atmosphere containing nitrogen. Specifically, providing a method for producing a high-purity silicon nitride powder suitable as a raw material for sintered silicon nitride having both high thermal conductivity and mechanical strength at low cost by a combustion synthesis method, and adhesion to a mold Provides a method for producing high-purity silicon nitride powder suitable as a mold release agent for polycrystalline silicon ingots at low cost by combustion synthesis, which can form a mold release layer that has good properties and mold release properties and is stable up to high temperatures It is to be.

The inventors of the present invention have made extensive studies to solve the above-mentioned problems, and in a method for producing high-purity silicon nitride powder at low cost by a combustion synthesis method using a combustion reaction that self-propagates in an atmosphere containing nitrogen. By setting the production conditions, a silicon nitride powder having a specific surface area, a specific β-type silicon nitride ratio and a specific metal impurity content less than a specific ratio and a crystallite diameter larger than a specific value can be obtained. I found out that When the mold release layer of the casting mold for polycrystalline silicon ingot is formed using the obtained silicon nitride powder, the mold release property and release characteristics of the polycrystalline silicon ingot can be increased even if the melting temperature of silicon during unidirectional solidification is increased. It has been found that the adhesion of the mold layer to the mold is good up to a high temperature, and the present invention has been completed.

Further, the present inventors set the specific manufacturing conditions in the inexpensive manufacturing method of high-purity silicon nitride powder by the combustion synthesis method using the combustion reaction that self-propagates in an atmosphere containing nitrogen, It has been found that silicon nitride powder having excellent characteristics can be produced, and that when silicon nitride powder is used as a raw material for producing a sintered body, a silicon nitride sintered body having both high thermal conductivity and mechanical strength can be produced. The present invention has been completed.

That is, the present invention relates to the following matters.

In the present invention, silicon powder and silicon nitride powder as a diluent are mixed, and the obtained mixed powder is filled in a crucible, and the silicon powder is mixed by a combustion synthesis method using self-heating and propagation phenomena associated with a combustion reaction. A method for producing a high-purity silicon nitride powder comprising producing a coagulated mass composed of silicon nitride by burning, and crushing the coagulated mass,
The silicon powder has an iron and aluminum content of 100 ppm or less, and a total content of metal impurities other than iron and aluminum is 100 ppm or less,
The diluent is a silicon nitride powder having an iron and aluminum content of 100 ppm or less and a total content of metal impurities other than iron and aluminum of 100 ppm or less,
The compounding ratio of the silicon powder and the diluent in the mixed powder is 9: 1 to 5: 5 by mass ratio,
The bulk density of the powder layer composed of the mixed powder filled in the crucible is 0.3 to 0.65 g / cm ³ ,
A method for producing a high-purity silicon nitride powder, characterized in that the agglomerated mass is pulverized using a pulverizer loaded or loaded with a pulverization medium containing at least silicon nitride.

In the present invention, the silicon powder preferably has a bulk density of 0.2 to 0.7 g / cm ³ .

In the present invention, the bulk density of the silicon nitride powder as the diluent is preferably 0.2 to 0.7 g / cm ³ .

In the present invention, the bulk density of the packed bed composed of the mixture is preferably 0.36 to 0.48 g / cm ³ .

In the present invention, the 50% by volume particle diameter D _{50 of the} silicon powder is preferably 1.0 to 15 μm, and the 90% by volume particle diameter D ₉₀ is preferably 10 to 44 μm.

In the present invention, it is preferable that the thickness of the powder layer is 20 to 70 mm, and the silicon powder is burned by being ignited from the top of the powder layer.

In the present invention, the silicon powder has an iron, chromium and aluminum content of 50 ppm or less, and the total content of metal impurities other than iron, chromium and aluminum is 50 ppm or less. It is preferable that the silicon nitride powder has a chromium and aluminum content of 50 ppm or less and a total content of metal impurities other than iron, chromium and aluminum is 50 ppm or less.

In the present invention, it is preferable that the agglomerated lump is subjected to a first crushing (coarse crushing) using a roll crusher equipped with a roll made of a silicon nitride sintered body.

In the present invention, the silicon nitride powder obtained by the coarse pulverization is accommodated in a container loaded with a pulverization medium composed of a silicon nitride sintered body, and the second pulverization (fine pulverization) is performed by a vibration mill or a bead mill. ) Is preferably used.

In the present invention, the grinding medium preferably has a porosity of 2% or less and a Vickers hardness of 14 GPa or more.

In the present invention, the high-purity silicon nitride powder has a β phase ratio of 70% by mass or more, a BET specific surface area of 3.0 to 13.0 m ² / g, and a content of iron, chromium and aluminum. Are each 200 ppm or less, and the total content of metal impurities other than iron, chromium and aluminum is preferably 200 ppm or less.

In the present invention, the high-purity silicon nitride powder, crystallite diameter _{D C} that is calculated from the powder X-ray diffraction pattern is 0.15 ~ 1 [mu] m, the crystal effective strain be 1.5 × ^{10 -4} or less The ratio D _BET / D _C between the sphere equivalent diameter D _BET and the crystallite diameter D _C calculated from the BET specific surface area is preferably 1 to 3.

In the present invention, a high-purity silicon powder is obtained by pulverizing a high-purity silicon material to have an iron and aluminum content of 100 ppm or less and a total content of metal impurities other than iron and aluminum of 100 ppm or less. The chromium content is also preferably 50 ppm or less. The 10% by volume particle diameter (D ₁₀ ) of the high-purity silicon powder after pulverization is preferably 0.2 to 1.0 μm, and the 50% by volume particle diameter (D ₅₀ ) is preferably 0.5 to 15 μm and 90 volume. The% particle diameter (D ₉₀ ) is preferably 3 to 44 μm.

D ₁₀ , D _50, and D ₉₀ are particle sizes that serve as indices of the particle size distribution (volume distribution) of the powder, and the smaller side and the larger side are equal on the basis of D 50 (so-called median diameter). , D ₉₀ is the boundary, and the cumulative distribution on the smaller particle size side is 90%, and the cumulative distribution on the larger particle side is 10%. Further, the cumulative distribution of the particle reduced diameter again side boundary D ₁₀ of 10%, the larger side cumulative distribution is 90%.

This high-purity silicon powder is mixed with silicon nitride powder having a content of iron and aluminum of 100 ppm or less as a diluent and a total content of metal impurities other than iron and aluminum of 100 ppm or less, and contains nitrogen. The high-purity silicon powder is converted into silicon nitride powder by a combustion synthesis method using a combustion reaction that self-propagates in the atmosphere. The chromium content in the silicon nitride powder is also preferably 50 ppm or less. Silicon nitride powder having such a purity is commercially available, for example, as silicon nitride powder produced by an imide decomposition method.

Also, by selecting the amount of silicon nitride powder added as a diluent, the temperature of the reaction field in the combustion reaction can be controlled to a desired temperature of 1900 ° C. or lower.

In the present invention, a high-purity silicon powder having an appropriate bulk density and a diluent having an appropriate bulk density are mixed at a mixing ratio of 9: 1 to 5: 5 between the high-purity silicon powder and the diluent. The bulk density of the packed bed (powder layer) made of a mixture of high-purity silicon powder and silicon nitride powder as a diluent is controlled to be 0.3 to 0.65 g / cm ³ .

The bulk density of the packed bed made of a mixture of high-purity silicon powder and silicon nitride powder as a diluent varies depending on the packing characteristics of the high-purity silicon powder used. In the present invention, a high-purity silicon powder having a bulk density of preferably 0.2 to 0.7 g / cm ³ is used, and this high-purity silicon powder and preferably a bulk density of 0.2 to 0.7 g / cm ³ are used. By mixing the silicon nitride powder of cm ³ , the bulk density of the packed bed made of the mixture of the high purity silicon powder and the silicon nitride powder as the diluent is 0.3 to 0.65 g / cm ^3. Control.

In this way, the mixing ratio of the high-purity silicon powder and the silicon nitride powder as the diluent and the bulk density of the packed bed made of the mixture of the high-purity silicon powder and the silicon nitride powder as the diluent are controlled to a predetermined value. By doing so, high-purity silicon nitride powder with a low content of unreacted free silicon can be produced at low cost.

The rate of progress of the combustion synthesis reaction also depends on the thickness of the powder layer made of a mixture of high-purity silicon powder and diluent. For this reason, in the present invention, a mixture of high-purity silicon powder and a diluent is formed into a powder layer having a thickness of 20 to 70 mm, and is ignited from a specific portion of the powder layer to advance the combustion synthesis reaction. The specific part means, for example, one end or the center of the raw material powder layer, and can be appropriately selected according to the shape and size of the container filled with the raw material powder.

The higher the nitrogen gas pressure, the faster the combustion synthesis reaction proceeds. In the present invention, however, the nitriding reaction proceeds under a nitrogen gas pressure of 0.3 to 1.5 MPa.

In the present invention, a high-purity silicon nitride powder is produced by pulverizing a silicon nitride agglomerate produced by a combustion synthesis method using a pulverizer loaded and loaded with a pulverization medium containing at least silicon nitride.

The silicon nitride agglomerates are coarsely pulverized using a roll crusher equipped with a silicon nitride roll to prevent metal impurities from being mixed to obtain a coarsely pulverized silicon nitride product. The coarsely pulverized product is pulverized using a dry jet mill, a vibration mill or a bead mill. In vibration mill pulverization, a powder contact portion such as a mill container is covered with a resin such as polyurethane, and a pulverizing medium made of a silicon nitride sintered body having a porosity of 2% or less and a Vickers hardness of 14 GPa or more is used. The fine pulverization is performed by controlling so that the mixing of metal impurities by the pulverization becomes a minimum value.

The high-purity silicon nitride powder obtained by the production method as described above has a β-phase ratio of 70% or more, a specific surface area of 3.0 to 13.0 m ² / g, and an iron and aluminum content, respectively. 200 ppm or less, and the total content of metal impurities other than iron and aluminum is 200 ppm or less. The chromium content is also preferably 100 ppm or less.

The resulting high-purity silicon nitride powder has a free silicon content of 0.5% by weight or less. The crystallite diameter obtained by powder X-ray diffraction is 0.15 to 2.0 μm, and the effective crystal strain is 1.5 × 10 ⁻⁴ or less.

In one preferred embodiment, the high-purity silicon nitride powder obtained by the production method as described above is useful as a release agent for polycrystalline silicon ingots,
When the volume-based 50% particle diameter measured by the laser diffraction scattering method is D ₅₀ and the 90% particle diameter is D ₉₀ , D ₅₀ is 1.7 μm or more and 20 μm or less, and D ₉₀ is 10 μm or more and 40 μm. And
Fe content is 100 ppm or less,
Cr content is 100 ppm or less,
Al content is 100 ppm or less,
The total content of metal impurities other than Fe, Cr and Al is 100 ppm or less,
The crystallite size of β-type silicon nitride which is calculated using the Williamson-Hall type from powder X-ray diffraction pattern of β-type silicon nitride is taken as _{D C,} it is _{D C} is 200nm or more.

In one preferred embodiment, the high-purity silicon nitride powder obtained by the production method as described above is useful as a powder for producing a silicon nitride sintered body,
The specific surface area measured by the BET method is 5 m ² / g or more and 20 m ² / g or less,
When the volume-based 50% particle diameter measured by the laser diffraction scattering method is D ₅₀ and the 90% particle diameter is D ₉₀ , D ₅₀ is 0.5 μm or more and 3 μm or less, and D ₉₀ is 3 μm or more and 7 μm. And
The crystallite size of β-type silicon nitride which is calculated using the Williamson-Hall type from powder X-ray diffraction pattern of β-type silicon nitride is taken as _{D C,} and a _{D C} is 120nm or more,
When the specific surface area equivalent diameter calculated from the specific surface area is D _BET , D _BET / D _C (nm / nm) is 3 or less,
It is preferable that the crystal strain of β-type silicon nitride calculated by using the Williamson-Hall formula from the powder X-ray diffraction pattern of β-type silicon nitride is 1.5 × 10 ⁻⁴ or less.

According to the present invention, high-purity silicon nitride that can be used as a raw material for various jigs for semiconductor manufacturing, a raw material for a high thermal conductivity silicon nitride substrate, a mold release agent used when manufacturing a silicon ingot for solar cells, etc. The powder can be produced efficiently and inexpensively by a combustion synthesis method using a combustion reaction that self-propagates in an atmosphere containing nitrogen.

It is a schematic diagram for demonstrating the structural example of the apparatus for performing a combustion synthesis reaction. It is a flowchart which shows the procedure of the synthesis | combining method of the high purity silicon nitride powder which concerns on this invention.

Hereinafter, the present invention will be described in more detail.

As the silicon source used in the present invention, various silicon scraps and silicon debris derived from semiconductor production lines and the like, polycrystalline workpieces, and other high-purity silicon materials are used.

Various silicon scraps, broken silicon, polycrystalline materials, and other high-purity silicon materials are crushed to have an iron and aluminum content of 100 ppm or less and a total content of metal impurities other than iron and aluminum of 100 ppm. The following high purity silicon powder is obtained. The chromium content is also preferably 50 ppm or less. The content of iron, chromium, and aluminum, or the total content of metal impurities other than iron, chromium, and aluminum can be 50 ppm or less, further 20 ppm or less, and 10 ppm or less, respectively.

In order to pulverize the material serving as the silicon source, a vibration ball mill, a jet mill, a bead mill or the like can be used. In order to pulverize with a vibration ball mill, an object to be pulverized is placed in a resin pot, and an appropriate amount of pulverizing silicon nitride balls is added to vibrate and rotate. Since the balls for grinding are worn and mixed into the raw material, the material must be selected in consideration of the amount of mixing. In the present invention, a silicon source material is pulverized using a pulverizing medium made of a silicon nitride sintered body having a porosity of 2% or less, preferably 1% or less and a Vickers hardness of 14 GPa or more. Similarly for grinding using a bead mill, a grinding medium made of a silicon nitride sintered body having a porosity of 2% or less, preferably 1% or less and a Vickers hardness of 14 GPa or more is used.

The dry jet mill is a pulverization method in which particles collide with each other at a high speed. Generally, particles are accelerated by air pressure.

In order to pulverize with a dry jet mill, it is necessary to use a jet mill equipped with a silicon nitride liner at the contact portion. By attaching a silicon nitride liner to the powder contact portion, it is possible to prevent the purchase of impurities due to wear on the inner wall of the apparatus. The part where it is difficult to wear the silicon nitride liner is covered with a resin such as polyurethane to prevent contamination by impurities. Since a pulverizing medium such as a ball is not used, it is suitable for obtaining a high-purity fine powder. In addition, when a dry jet mill is used, the secondary particle diameter can be easily reduced in a shorter time than a vibrating ball mill.

The particle size distribution of the pulverized silicon powder was measured using a laser diffraction / scattering particle size distribution measuring device, the 10 volume% particle diameter (D ₁₀ ) was 0.2 to 1.0 μm, and the 50 volume% particle diameter (D ₅₀ ). Is 1.0 to 15 μm, and 90% by volume particle diameter (D ₉₀ ) is 10 to 44 μm. D ₅₀ is preferably 3.0 to 10 μm and D ₉₀ is preferably 10 to 20 μm. When the particle size is out of the above particle size range, the nitriding reaction does not proceed sufficiently under the same diluent addition conditions, and a large amount of unreacted silicon remains, or the obtained silicon nitride powder has a particle size of It may become large and difficult to grind.

D ₁₀ , D _50, and D ₉₀ are particle diameters that serve as indices of the particle size distribution (volume distribution) of the powder, and the smaller and larger sides of D ₅₀ (so-called median diameter) are equal amounts. Thus, with D ₉₀ as a boundary, the cumulative distribution on the smaller particle size side is 90%, and the cumulative distribution on the larger particle side is 10%. Further, the cumulative distribution of the particle reduced diameter again side boundary D ₁₀ of 10%, the larger side cumulative distribution is 90%.

The content of iron and aluminum contained in the high-purity silicon powder after pulverization is 100 ppm or less, and the total content of metal impurities other than iron and aluminum is 100 ppm or less. The chromium content is preferably 50 ppm or less. In particular, the content of iron, chromium and aluminum is preferably 20 ppm or less, and the total content of metal impurities other than iron, chromium and aluminum is preferably 20 ppm or less. Furthermore, it is particularly preferable that the content of iron, chromium and aluminum is 10 ppm or less, and the total content of metal impurities other than iron, chromium and aluminum is 10 ppm or less. If the silicon powder contains a large amount of impurities, it cannot be used for semiconductor applications that require high purity.

The obtained silicon powder is nitrided by a combustion synthesis reaction described later. The reaction heat generated during the nitridation reaction of silicon is very large, and the temperature of the reaction system rises to about 1900 ° C., which is higher than the melting point of silicon, 1410 ° C. For this reason, in the combustion synthesis reaction, the nitriding reaction is controlled by using a mixed powder of high-purity silicon powder and silicon nitride powder as a diluent as a raw material.

The compounding ratio of the high purity silicon powder and the silicon nitride powder as a diluent is 9: 1 to 5: 5. If the blending ratio of the diluent in the mixed raw material is less than 10% by mass, the combustion synthesis reaction cannot be controlled, the temperature of the reaction system becomes too high, and the silicon powder in the raw material may be fused. The compounding ratio of the high-purity silicon powder and the silicon nitride powder as the diluent may be 8: 2 or less, 7: 3 or less, 6: 4 or less, or 6: 4 or more, 7: 3 or more, 8: Two or more may be sufficient.

The silicon powder used as a raw material has a particle size of about several μm so that the nitriding proceeds completely and rapidly. If silicon powder with such a particle size is fused, not only the surface area per unit mass (specific surface area) is reduced, but also the gaps between the powders that serve as nitrogen gas introduction holes into the raw material are fused. It will be blocked by the silicon. In such a situation, a situation occurs in which part of the charged raw material remains as unreacted silicon at the end of the synthesis reaction. Further, there arises a problem that the obtained silicon nitride powder has a large particle size and is difficult to grind.

On the other hand, when the blending ratio of the silicon nitride powder as the diluent exceeds 50% by mass, the ratio of the reaction product obtained by nitriding silicon decreases. The silicon nitride powder as a diluent does not directly participate in the combustion synthesis reaction. Therefore, for example, when the blending ratio of the high-purity silicon powder and the diluent is 5: 5, even if the silicon raw material is completely converted into silicon nitride by the combustion synthesis reaction, it is newly obtained by the synthesis reaction. Silicon nitride is only 50% of the total charged amount (starting material). In other words, when silicon nitride is added as a diluent and charged, the production efficiency of silicon nitride must be reduced by the amount of addition of the diluent. In addition, if the content of the diluent is 50%, it can be said that half of the energy consumed in the combustion synthesis reaction is consumed without contributing to the production of new silicon nitride. For this reason, from the viewpoint of energy efficiency and from the viewpoint of an inexpensive manufacturing method, the ratio of the silicon nitride powder in the mixed raw material needs to be 50% by mass or less.

Thus, in the combustion synthesis reaction, by using a mixed powder of high-purity silicon powder and silicon nitride powder as a raw material, the nitriding reaction can be easily controlled, and silicon nitride powder having desired characteristics can be easily obtained.

Further, sodium chloride (NaCl), ammonium chloride (NH ₄ Cl), or the like may be added to adjust the ratio of β-type silicon nitride in the combustion product. These additives have the effect of lowering the temperature of the reaction field due to latent heat or endotherm accompanying decomposition or sublimation. Additives such as sodium chloride (NaCl) and ammonium chloride (NH ₄ Cl) for adjusting the proportion of β-type silicon nitride are high-purity silicon powder, silicon nitride powder as a diluent, and the additives May be 40% by mass or less and 20% by mass or less, and preferably 1% by mass or more and 5% by mass or more.

The progress rate of the combustion synthesis reaction also varies depending on the bulk density of the silicon nitride powder as the raw material silicon and the diluent. When the bulk density of silicon nitride powder as raw material silicon or diluent is smaller than 0.2 g / cm ³ , the packing density of the packed bed made of a mixture of high-purity silicon powder and diluent is lowered, and the reaction vessel is filled with Since the raw material mixture that can be filled into the reactor becomes smaller, the weight of the produced silicon nitride powder per reaction batch decreases, and the production efficiency decreases. On the other hand, when the bulk density of the silicon nitride powder as the raw material silicon or the diluent is larger than 0.7 g / cm ³ , the packing density of the packed layer made of the mixture of the high-purity silicon powder and the diluent is increased. Therefore, the silicon nitride powder produced by the combustion reaction is agglomerated tightly and becomes difficult to pulverize, and the amount of metal impurities mixed in the pulverizing process increases, which is not preferable. In addition, the crystallinity of the pulverized silicon nitride powder is deteriorated, the crystallite diameter is small, and the lattice strain is large. The bulk density of silicon nitride powder as raw material silicon or diluent may be 0.3 g / cm ³ or more, 0.4 g / cm ³ or more, 0.5 g / cm ³ or more, 0.6 g / cm ³ or more. Further, it may be 0.6 g / cm ³ or less, 0.5 g / cm ³ or less, 0.4 g / cm ³ or less, or 0.3 g / cm ³ or less.

In the present invention, the bulk density means the initial bulk density.

By appropriately selecting the bulk density of the silicon nitride powder as the raw material silicon and the diluent, the bulk density of the packed bed made of a mixture of the high-purity silicon powder and the diluent is preferably 0.3 to 0.65 g / cm ³ , preferably is 0.34 ~ 0.55g / cm ^3, more preferably controlled to be 0.36 ~ 0.48g / cm ^3. What is important in the combustion synthesis method of the present invention is the bulk density of a packed bed (powder layer) made of a mixture of high-purity silicon powder and a diluent. The bulk density of the packed bed comprising a mixture of high-purity silicon powder and diluent, 0.4 g / cm ³ or more, 0.45 g / cm ³ or more, 0.5 g / cm ³ or more, 0.55 g / cm ³ or more it may also be, also 0.55 g / cm ³ or less, 0.5 g / cm ³ or less, 0.45 g / cm ³ or less, may be 0.4 g / cm ³ or less.

If the bulk density of the packed bed is controlled to be 0.65 g / cm ³ or less, the crushing strength of the massive silicon nitride obtained by the combustion synthesis reaction can be reduced to 6.5 MPa or less. Is controlled to be 0.55 g / cm ³ or less, the crushing strength of the massive silicon nitride obtained by the combustion synthesis reaction can be reduced to 5.5 MPa or less, and the packing density of the packed bed is 0.48 g. If controlled to be not more than / cm ^3, the crushing strength of the massive silicon nitride obtained by the combustion synthesis reaction can be reduced to 3.5 MPa or less. By reducing the crushing strength of the obtained bulk silicon nitride to 6.5 MPa or less, in the pulverization process described later, the pulverization energy increases so that the amount of mixed metal impurities increases and the crystallinity of the silicon nitride deteriorates. Even without large pulverization, it becomes easy to obtain a silicon nitride powder having a specific surface area or crystallinity (crystallite diameter and crystal effective strain) specified in the present invention. If the crushing strength of the obtained bulk silicon nitride is 5.5 MPa or less, pulverization is further facilitated, and if the crushing strength of the obtained bulk silicon nitride is 3.5 MPa or less, crushing is particularly facilitated.

The rate of progress of the combustion synthesis reaction also depends on the thickness of the powder layer (filled layer) made of a mixture of high-purity silicon powder and diluent. In the present invention, a mixture of high-purity silicon powder and a diluent is formed into a powder layer having a thickness of 20 to 70 mm, and is ignited from a specific portion of the powder layer to advance a combustion synthesis reaction. When the thickness of the powder layer is less than 20 mm, the calorific value due to the combustion reaction is reduced, and the heat synthesis to the firing container etc. is hindered and the self-propagation of the combustion heat is hindered, and the combustion synthesis reaction stops halfway. A large amount of unreacted silicon remains. When the thickness of the powder layer exceeds 70 mm, the combustion reaction becomes too intense due to heat accumulation in the powder layer, and silicon powder in the raw material is fused with each other. Such a problem arises that a part of the charged raw material remains as unreacted silicon, and the obtained silicon nitride powder has a large particle size and is difficult to grind. The thickness of the mixture of the high purity silicon powder and the diluent may be 30 mm or more, 40 mm or more, 50 mm or more, 60 mm or more, or 60 mm or less, 50 mm or less, 40 mm or less, 30 mm or less.

The nitrogen gas pressure in the combustion synthesis reaction of the invention is 0.3 to 1.5 MPa. When the nitrogen gas pressure is less than 0.3 MPa, the speed of the nitriding reaction is slowed, the self-propagation of combustion heat is hindered, the combustion synthesis reaction stops halfway, and a large amount of unreacted silicon remains. The higher the nitrogen gas pressure, the faster the combustion synthesis reaction proceeds. However, when the nitrogen gas pressure exceeds 1.5 MPa, the combustion reaction becomes too intense and the silicon powder in the raw material is fused to the inside of the raw material. The gaps between the powders acting as nitrogen gas introduction holes are blocked by the fused silicon, and a part of the charged raw material remains as unreacted silicon, and the resulting silicon nitride powder has a larger particle size. Problems such as difficulty in crushing. The nitrogen gas pressure may be 0.5 MPa or more, 0.7 MPa or more, 1.0 MPa or more, or 1.2 MPa or less, 1.0 MPa or less, or 0.8 MPa or less.

Crush silicon nitride particles obtained by combustion synthesis reaction. When pulverizing silicon nitride particles obtained by the combustion synthesis reaction, the combustion products obtained by the combustion synthesis reaction are agglomerated, so first, coarsely pulverize using a roll crusher equipped with a silicon nitride roll. Is efficient. Since the roll made of silicon nitride is excellent in wear resistance and contains almost no metal impurities, it is possible to obtain a high-purity coarsely pulverized product of silicon nitride that does not contain metal impurities such as iron, chromium and aluminum. The desired coarsely pulverized silicon nitride can be obtained by sieving the obtained coarsely pulverized product to remove particularly coarse particles.

The silicon nitride particles are preferably finally pulverized. Two-stage pulverization is not essential, but it is preferable to first coarsely pulverize as described above, and then further finely pulverize the coarsely pulverized product. As a means for pulverization, there is no particular restriction other than pulverization using a pulverization apparatus loaded with or equipped with a pulverization medium containing at least silicon nitride.For fine pulverization of silicon nitride, a vibration ball mill, a bead mill, An attritor, a jet mill, or the like can be used.

To pulverize with a vibrating ball mill, put the material to be crushed into a resin pot in which the contact part of a mill container or the like is covered with a resin such as polyurethane, add an appropriate amount of silicon nitride balls for pulverization, and vibrate and rotate. Since the balls for grinding are worn and mixed into the raw material, the material must be selected in consideration of the amount of mixing. In the present invention, the coarsely pulverized silicon nitride is pulverized using a pulverizing medium made of a silicon nitride sintered body having a porosity of 2% or less, preferably 1% or less and a Vickers hardness of 14 GPa or more. A silicon nitride powder having a desired specific surface area or particle size distribution can be obtained by appropriately adjusting the conditions (amplitude, frequency, grinding time, etc.) of the vibrating ball mill. Similarly for grinding using a bead mill, coarsely grinding silicon nitride using a grinding medium made of a silicon nitride sintered body having a porosity of 2% or less, preferably 1% or less and a Vickers hardness of 14 GPa or more. Grind things.

In order to pulverize with a jet mill, it is necessary to use a jet mill with a silicon nitride liner attached to the contact part. The part where it is difficult to attach the silicon nitride liner is covered with a resin such as polyurethane. By attaching a silicon nitride liner to the powder contact portion, it is possible to prevent the purchase of impurities due to wear on the inner wall of the apparatus. Since a grinding medium such as a ball is not used, it is suitable for obtaining a high-purity fine powder.

The most important feature of the method for producing silicon nitride powder of the present invention is that the crushing strength of the bulk silicon nitride which is a combustion product obtained by the combustion synthesis reaction is below a predetermined value. Due to the low crushing strength, the pulverization process in the subsequent pulverization process is significantly facilitated. Thus, the silicon nitride powder obtained by the production method of the present invention has a β-phase ratio of 70% or more, a specific surface area of 3.0 to 13.0 m ² / g, and an iron and aluminum content, respectively. 200 ppm or less, and the total content of metal impurities other than iron and aluminum is 200 ppm or less. The chromium content is also 100 ppm or less. The free silicon content is 1.0% by mass or less, preferably 0.5% by mass or less. The crystallite diameter obtained by powder X-ray diffraction is 0.15 to 2.0 μm, and the effective crystal strain is 1.5 × 10 ⁻⁴ or less.

The ratio of β phase is 70% or more. There are two types of polymorphs of silicon nitride powder, α phase and β phase, where α phase is said to be a low temperature phase and β phase is said to be a high temperature phase. In recent years, attempts have been made to improve the quality by increasing the purity of polycrystalline silicon by increasing the casting temperature of silicon ingots and increasing the holding time in the molten state. Tends to prefer β-phase silicon nitride powder which is stable at high temperatures. For this reason, silicon nitride powder having a β phase ratio of 70% or more is suitable for applications such as a release agent. If the ratio of β phase is less than 70%, decomposition of the release material layer is likely to proceed during the casting of the silicon ingot, causing the molten silicon to stick to the crucible wall and causing cracks in the silicon ingot. Further, in addition to the ratio of the β phase, the search for the characteristics of the silicon nitride powder that has a good effect on the releasability of the polycrystalline silicon ingot is in progress.

In addition, the silicon nitride powder produced by the combustion synthesis reaction is basically a β-phase powder, and if the conditions are set such that the β-phase ratio is less than 70%, the combustion reaction itself becomes unstable. This causes problems such as residual reactive silicon.

The specific surface area is 3.0-13.0 m ² / g. When the specific surface area is less than 3.0 m ² / g, since the particle size is too coarse, the sinterability is lowered, and it cannot be used as a raw material for various jigs for semiconductor manufacturing, a raw material for a high thermal conductivity silicon nitride substrate, or the like. . Moreover, the adhesive force to a crucible wall may fall and it may become difficult to use for the mold release agent etc. which are used at the time of manufacture of the silicon ingot for solar cells. When the specific surface area exceeds 13.0 m ² / g, the amount of metal impurities inevitably mixed by pulverization increases, the crystallinity of silicon nitride particles decreases, and the crystallite diameter is less than 0.15 μm. Or the effective crystal strain exceeds 1.5 × 10 ⁻⁴ . Thus, since the quality of the silicon nitride powder obtained deteriorates, it is not preferable.

The iron and aluminum contents are each 200 ppm or less, preferably 100 ppm or less, more preferably 50 ppm or less, and particularly preferably 10 ppm or less. The chromium content is also preferably 100 ppm or less, preferably 50 ppm or less, more preferably 10 ppm or less, and particularly preferably 5 ppm or less. At the same time, the total content of metal impurities other than iron, chromium and aluminum is also 200 ppm or less, preferably 100 ppm or less, more preferably 50 ppm or less, and particularly preferably 10 ppm or less.

If the contents of iron, chromium and aluminum each exceed 200 ppm, they cannot be used as raw materials for various jigs for manufacturing semiconductors, high-heat-conducting silicon nitride substrates, etc. that require high purity. In particular, for applications such as mold release agents used in the production of silicon ingots for solar cells, the contents of iron, chromium and aluminum need to be 50 ppm or less, particularly preferably 10 ppm or less, respectively. If the total content of metal impurities other than iron, chromium and aluminum exceeds 200 ppm, it cannot be used as a raw material for various jigs for semiconductor manufacturing, a raw material for a high thermal conductivity silicon nitride substrate, etc. that require high purity. . In particular, for applications such as mold release agents used in the production of silicon ingots for solar cells, the total content of metal impurities other than iron, chromium and aluminum needs to be 50 ppm or less, particularly preferably 10 ppm or less.

The free silicon content is preferably 1.0% by mass or less, more preferably 0.5% by mass or less. When the free silicon content exceeds 1.0% by mass, the properties of the obtained sintered body deteriorate, so that it may be difficult to use as a raw material for various jigs for semiconductor manufacturing and a raw material for a high thermal conductive silicon nitride substrate. is there. In addition, the use of a release agent or the like used in the production of a silicon ingot for solar cells is not preferable because molten silicon is likely to enter the release material layer and stick to the crucible wall.

In the present invention, the crystallite size and crystal effective strain of the silicon nitride powder were determined as follows. That is, in the powder X-ray diffraction method, the width of the diffraction line increases as the particle diameter of the sample decreases. Between the diffraction peak width β (radian) and the particle diameter D, the relationship of equation (2) is obtained. Here, λ is the wavelength of the X-ray source.

β = Kλ / (Dcos θ) (2)
In equation (2), if β is defined as the half width, K = 0.9, and if defined as the integral width, K = 1. If there is crystal distortion, the distance between the planes is not uniform in the particles, so that the diffraction lines spread. Here, when the maximum strain is ε, the inter-plane distance d takes a value between d (1 + ε) and d (1−ε), and the spread of the diffraction line is expressed by the following equation (3) or (4): It becomes. Here, η is an effective strain, and β is an integration width. Δθ is a difference representing the spread of the diffraction peak, Δd is a difference representing the expansion / contraction width of the interplanar spacing, and δθ / δd is a partial differential coefficient in Bragg's conditional expression (2d sin θ = nλ).

Δθ = Δd (δθ / δd) = 2εtanθ (3)
β = 2η tan θ (4)
When there is a broadening of the integral width due to both the crystallite size and strain, it can be expressed as the following equation (5) as the sum of equations (2) and (4). In the present invention, the Williamson − expressed by the equation (5) with respect to the broadening of the integral width of the diffraction line with respect to the (101), (110), (200), (201) and (210) planes of β-type silicon nitride. The Hall plot was applied, the intercept and slope of the straight line were determined by the least square method, and the crystal effective strain was calculated.

β = Kλ / (Dcos θ) + 2η tan θ (5)
The silicon nitride powder of the present invention has a crystallite diameter obtained by powder X-ray diffraction of 0.15 to 2.0 μm and a crystal effective strain of 1.5 × 10 ⁻⁴ or less. When the crystallite diameter is less than 0.15 μm, a large number of crystallites exist in one primary particle, and the high temperature stability of the silicon nitride particles deteriorates due to a decrease in crystallinity. For this reason, when it is used as a mold release agent for casting a silicon ingot, the decomposition of the mold release layer at the time of holding at a high temperature is accelerated, and there is a possibility that adhesion between the molten silicon and the crucible wall may occur. . In addition, when used as a raw material for various jigs for semiconductor production, a raw material for a high thermal conductivity silicon nitride substrate, dissolution of silicon nitride particles in the molten phase generated by reaction with an auxiliary agent added in the sintering process, When the β-type silicon nitride particles grow by precipitation, non-uniform grain growth occurs from the crystal growth nuclei of the β particles whose crystallinity is lowered, which is not preferable because the characteristics of the obtained sintered body may be deteriorated.

On the other hand, even when the effective crystal strain exceeds 1.5 × 10 ⁻⁴ , the high temperature stability of the silicon nitride particles deteriorates due to the decrease in crystallinity. For this reason, when it is used as a mold release agent for casting a silicon ingot, the decomposition of the mold release layer at the time of holding at a high temperature is accelerated, and there is a possibility that adhesion between the molten silicon and the crucible wall may occur. . In addition, when used as a raw material for various jigs for semiconductor production, a raw material for a high thermal conductivity silicon nitride substrate, dissolution of silicon nitride particles in the molten phase generated by reaction with an auxiliary agent added in the sintering process, When the β-type silicon nitride particles grow by precipitation, uneven grain growth occurs from the crystal growth nuclei of the β particles whose crystallinity is lowered, which is not preferable because the characteristics of the obtained sintered body may be deteriorated.

Incidentally, the crystallite diameter D _C becomes smaller as comminuting silicon nitride particles. In the silicon nitride powder of the present invention, the ratio D _BET / D _{C of the} sphere equivalent diameter D _BET and the crystallite diameter D _C calculated from the BET specific surface area is 1.0 to 3.0. If the ratio _D BET / _{D C} of the sphere equivalent diameter _{D BET} calculated from the BET specific surface area and the crystallite diameter _{D C} is more than 3.0, the semiconductor manufacturing for various jigs raw material, raw material for silicon high thermal conductivity nitride substrate When β-type silicon nitride particles grow due to dissolution and precipitation of silicon nitride particles in the molten phase produced by reaction with the auxiliary added during the sintering process, the crystallinity decreases. This is not preferable because non-uniform grain growth occurs from the crystal growth nuclei of the β grains, and the characteristics of the obtained sintered body may deteriorate.

The high-purity silicon nitride powder produced according to the present invention is useful as a release agent for polycrystalline silicon ingots in one preferred embodiment, and has a volume-based 50% particle size measured by laser diffraction scattering method of D _50. D ₅₀ is preferably 1.7 μm or more and 20 μm or less. If D ₅₀ is within this range, the adhesion between the silicon nitride particles are also adhesion between the silicon nitride particles and the mold is good tends, also because easily form a dense release layer, a polycrystalline silicon ingot away A release layer having good moldability and good adhesion to the mold can be formed. D ₅₀ is preferably 2 μm or more. D ₅₀ may be 5 μm or less. Further, the 90% particle diameter is taken as _{D _90,} _{D 90} is preferably at 10μm or 40μm or less. If _D90 is within this range, the surface of the release layer is likely to be smooth, and a release layer having good release properties of the polycrystalline silicon ingot can be formed. D ₉₀ is more preferably 30 μm or less. D ₉₀ may also be 15μm or more, and may be 20μm or less. Adjustment of the particle size distribution of the silicon nitride powder can be performed by a grinding process.

In another preferred embodiment, the high-purity silicon nitride powder produced according to the present invention is useful as a raw material for a silicon nitride sintered body, and has a volume-based 50% particle diameter measured by a laser diffraction scattering method of D _50. D ₅₀ is preferably 0.5 μm or more and 3 μm or less. If D ₅₀ is within this range, since sufficient compact density is obtained, a dense sintered body tissue is obtained, it is possible to obtain a silicon nitride sintered body having both high thermal conductivity and high mechanical strength . In this respect, D ₅₀ is more preferably 2 μm or less. Further, the 90% particle diameter is taken as _{D _90,} _{D 90} is 3μm or more 7μm or less. If _D90 is within this range, a homogeneous sintered body structure can be obtained, and a silicon nitride sintered body having both high thermal conductivity and high mechanical strength can be obtained. In this respect, D ₉₀ is more preferably 6 μm or less. Adjustment of the particle size distribution of the silicon nitride powder can be performed by a grinding process.

FIG. 1 is a schematic diagram for explaining a configuration example of an apparatus for performing a combustion synthesis reaction, which is used when carrying out the present invention.

Hereinafter, the method for synthesizing the silicon nitride powder of the present invention will be described with reference to the drawings. In the stainless steel reactor 10, a flow passage through which cooling water flows is formed between the outer wall 30 and the side wall of the inner vessel 20. One end face of the reactor 10 is provided with a lid that can be opened and closed, and the inner container 20 is sealed by closing the lid.

A graphite crucible 23 is provided on the bottom surface of the inner container 20. The graphite crucible 23 is a rectangle having an outer diameter of 770 × 320 mm and a height of 90 mm.

The reactor 10 is configured to be evacuated, and the inside of the inner container 20 can be made high vacuum by operating the vacuum pump with the on-off valve 17 provided in the middle of the exhaust pipe opened. .

Further, the reactor 10 is connected to a nitrogen gas cylinder by a nitrogen gas introduction pipe 15, and a nitrogen gas opening / closing valve 16 is provided in the middle of the nitrogen gas introduction pipe 15. By flowing nitrogen gas from the nitrogen gas cylinder with the nitrogen gas on-off valve 16 opened, the inside of the inner container 20 becomes a nitrogen gas atmosphere. A pressure sensor 14 connected to the nitrogen gas on / off valve 16 is installed on the side wall inside the inner container 20, and the pressure regulating valve connected to the nitrogen gas on / off valve 16 keeps the pressure inside the inner container 20 constant. The nitrogen gas on / off valve 16 is controlled so as to be maintained.

Two rod-shaped electrodes 11 are provided on the inner upper surface of the inner container 20 so as to extend in the vertical direction. The upper ends of these two electrodes 11 are connected by a carbon heater 12 arranged above the graphite crucible 23. A voltage is applied to the lower end portions of these two electrodes 11 by an external power source provided outside the reactor 10, whereby the carbon heater 12 generates heat.

A mixture of silicon powder and diluent (silicon nitride powder) in a weight ratio of 9: 1 to 5: 5 is used as starting material 25 (feeding material). After such a starting material 25 is charged into a material charging part (not shown), the ignition material 13 is added to the surface of a specific part of the starting material. When a voltage is applied from the external power source to the lower ends of the two electrodes 11 while the ignition material 13 is in contact with the carbon heater 12, the ignition material 13 is ignited by being induced by the heated carbon heater 12, and the starting material 25. Can generate heat.

FIG. 2 is a flowchart showing a procedure for combustion synthesis of high-purity silicon nitride powder according to the present invention. First, silicon, silicon nitride, and if necessary, sodium chloride, which is the starting material 25, are charged into a planetary ball mill or the like containing a silicon nitride ball, and mixed by pulverization for a few dozen minutes (step S1). The mixed starting material 25 is charged into a raw material charging unit (not shown), and an ignition material (for example, an aluminum molded product) 13 is added to the upper surface of the specific part (step S2). At this time, the ignition material 13 is disposed on the upper surface of the specific portion of the starting material 25 so as to come into contact with the carbon heater 12.

After starting material 25 is charged into the raw material charging section, the lid (not shown) is closed and the inner container 20 is sealed (step S3). After sealing, the air on-off valve 17 is opened, and the vacuum pump is operated to make the inner container 20 high vacuum (step S4).

After reaching the desired degree of vacuum, the nitrogen open / close valve 16 is opened to allow nitrogen to flow into the inner vessel 20 from the nitrogen cylinder outside the reactor 10, and the inside of the inner vessel 20 is brought to a nitrogen atmosphere (step S5). In addition, the pressure in the inner container 20 is maintained at about 1 MPa.

After the inside of the inner container 20 is in a pressurized nitrogen gas atmosphere, the carbon heater 12 is heated by applying a voltage from an external power source, the ignition material 13 is ignited, and the starting material 25 is combusted (step S6). The energization time at the time of ignition is about 10 seconds. The combustion of the starting material 25 causes the nitriding reaction of the above reaction formula (1).

Reaction heat generated by the nitriding reaction of reaction formula (1) causes a combustion synthesis reaction shown in reaction formula (1) to synthesize silicon nitride. The reaction heat generated during the nitridation reaction of silicon is very large, and the temperature of the reaction system rises to about 1900 ° C., which is higher than the melting point of silicon, 1410 ° C. For this reason, in the combustion synthesis reaction, the nitriding reaction is controlled by using a mixed powder of high-purity silicon powder and silicon nitride powder as a diluent as a raw material.

Also, sodium chloride or ammonium chloride can be added as a reaction aid in order to control the β ratio of silicon nitride as a combustion product. Sodium chloride and ammonium chloride added as reaction aids sublimate due to reaction heat, but do not participate in the combustion synthesis reaction shown in the reaction formula (1).

Although the temperature of the reaction system rises due to the combustion synthesis reaction, since the sublimation reaction of sodium chloride and ammonium chloride is an endothermic reaction, the temperature of the combustion synthesis reaction system decreases due to the endothermic reaction. As a result, the melting of silicon is delayed and the fusion of silicon particles is suppressed. It should be noted that by adjusting the addition amount of sodium chloride and ammonium chloride, the temperature of the reaction system, which decreases with the sublimation of sodium chloride, may hinder the progress of the combustion synthesis reaction shown in the reaction formula (1). It can be set to the temperature without.

Combustion reaction time depends on the amount of starting material, but is generally several minutes to several tens of minutes. When the combustion reaction is completed, the running water on-off valve 24 is opened, cooling water flows into the flow passage portion, and the inner container 20 is cooled through the side wall (step S7).

After the inner container 20 is cooled, the synthesized silicon nitride is taken out from the raw material charging part (step S8). Since the silicon nitride after the combustion reaction is agglomerated, it is pulverized by a roll crusher, a ball mill, a vibration mill or the like as necessary (step S9).

Hereinafter, the method for synthesizing silicon nitride of the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples, and may have various aspects.

High-purity silicon material mainly composed of polycrystalline silicon milling material containing iron, chromium, and aluminum using a grinding device (vibrating ball mill, jet mill, etc.) loaded and loaded with grinding media containing silicon nitride A high-purity silicon powder having an amount of 50 ppm or less and a total content of metal impurities other than iron, chromium and aluminum is 50 ppm or less is obtained. The high-purity silicon powder after pulverization has a 10% by volume particle size (D ₁₀ ) of 0.2 to 1.0 μm, a 50% by volume particle size (D ₅₀ ) of 0.5 to 15 μm, and a 90% by volume particle size (D _90). ) Is 3 to 44 μm.

[Example 1]
The average particle diameter of 1.9 .mu.m, a bulk density of 0.48 g / cm ³ average particle size 0.5μm to silicon powder, bulk density by adding silicon nitride powder of 0.30 g / cm ^3, silicon with a diluent ( A synthetic raw material was prepared so that the mixing ratio with silicon nitride was 8.0: 2.0 in terms of silicon nitride. In addition, the value converted into silicon nitride is, for example, that when 3 mol (84.3 g) of silicon and 1 mol (140.3 g) of silicon nitride are included, all the silicon is converted into silicon nitride. , Meaning that the weight ratio is 1: 1. The average particle size means a particle size at an integrated value of 50% of the particle size distribution measured by a laser diffraction / scattering method. The bulk density of the synthetic raw material was controlled by combining those in which the bulk density of the silicon powder of the blending source and the bulk density of the diluent (silicon nitride) each have a predetermined value.

Next, using the apparatus shown in FIG. 1, a lump of silicon nitride powder was synthesized from the synthesis raw material by a self-combustion reaction according to the following procedure.

The above charged raw materials (total weight: 4.0 kg) were filled into a graphite crucible having a bottom surface of 770 mm × 320 mm. The height of the powder layer was 36 mm.

Place a small amount of aluminum molding as an igniting agent in a predetermined part at the upper end of the filled powder layer, store it inside the pressure-resistant inner container 20, and need it via a carbon heater in a nitrogen atmosphere A simple voltage-current was applied for 10 seconds to burn, and the heat generated by the burning of aluminum was used to cause a self-burning reaction of silicon powder. Combustion proceeded in a stable state with the mixed powder of silicon and the diluent (silicon nitride) as the raw material. The nitrogen gas pressure inside the pressure-resistant inner container 20 was 0.9 MPa, and the reaction time was about 25 minutes. After cooling, the lump of silicon nitride powder produced from the pressure-resistant inner container was taken out. The peripheral 10cm about ³ parts arranged an ignition agent (aluminum moldings traces) were removed from the product. Diffusion of the igniting agent to other parts was not detected.

After crushing the lump of silicon nitride powder with a plastic hammer, it is passed through a roll crusher equipped with a roll made of silicon nitride, coarsely pulverized, and sieved using a sieve having an opening of 80 μm. A pulverized product was obtained. According to the analysis method described later, the specific surface area of the coarsely pulverized product was 0.45 m ² / g, the crystallite diameter was 1.9 μm, and the effective crystal strain was 0.18 × 10 ⁻⁴ .

Furthermore, using a super jet mill SJ-1500 with a silicon nitride liner attached to the powder contact part, the coarsely pulverized product was removed under the conditions of air usage of 2.5 m ³ / min and throughput of 40 g / min. Jet mill pulverized.

The obtained finely pulverized silicon nitride was analyzed as follows.

(Identification and quantification of crystal phase by powder X-ray diffraction measurement)
Powder X-ray diffraction measurement using CuKα rays was performed, and the crystalline phase of the generated silicon nitride powder was calculated (calculation of the β phase ratio) by the method of Gazzara & Messier described in Non-Patent Document 3.

The amount of free silicon was also measured by powder X-ray diffraction. The calibration curve was prepared using a standard sample of silicon and a standard sample of silicon nitride, and obtained from a peak intensity ratio in a powder X-ray diffraction pattern of a mixed powder having a known silicon amount.

(Crystallite size of β-type silicon nitride D _C and measuring methods of crystal strains)
In addition, by applying the Williamson-Hall plot, the crystallite diameter and crystal effective strain of the generated silicon nitride powder were obtained.

(Measurement method of specific surface area and calculation method of equivalent spherical diameter D _BET )
The specific surface area of the high-purity silicon nitride powder of the present invention was measured by a BET one-point method by nitrogen gas adsorption using a BET specific surface area measuring device (Macsorb) manufactured by Mountaintech.

Further, the equivalent spherical diameter D _BET was obtained from the following formula (6) on the assumption that all particles constituting the powder are spheres having the same diameter.

D _BET = 6 / (ρ _SN × S) (6)
Here, ρ _SN is the true density of silicon nitride (true density of α-Si ₃ N ₄ is 3.186 g / cm ³ , true density of β-Si ₃ N ₄ is 3.192 g / cm ³ , α phase and β phase The average true density was calculated based on the ratio to the true density). S is a specific surface area (m ² / g).

(Measuring method of particle size distribution)
The particle size distribution of the silicon nitride powder of the present invention and the silicon powder used as a raw material was measured as follows. 60 mg of a measurement sample was put into 200 ml of pure water mixed with 2 ml of a 20% aqueous solution of sodium hexametaphosphate, and the powder was used for 6 minutes at an output of 300 W using an ultrasonic homogenizer equipped with a stainless steel center cone having a diameter of 26 mm. A dilute solution was prepared by dispersion treatment and used as a measurement sample. The particle size distribution of the measurement sample was measured using a laser diffraction / scattering particle size distribution measuring apparatus (Microtrack MT3000 manufactured by Nikkiso Co., Ltd.) to obtain volume-based particle size distribution data. From the obtained particle size distribution curve, 10 volume% particle diameter (D ₁₀ ), 50 volume% particle diameter (D ₅₀ ) and 90 volume% particle diameter (D ₉₀ ) were determined.

(Method of measuring the content of iron, chromium and aluminum and the content of metal impurities other than iron, chromium and aluminum)
The silicon nitride powder of the present invention, the silicon powder used as a raw material, and the content of iron, chromium and aluminum in the raw material mixed powder, and the content of metal impurities other than iron, chromium and aluminum are measured as follows. did.

The sample was weighed in a resin pressure decomposition vessel, mixed acid (nitric acid and hydrofluoric acid solution) was added, microwave heating was performed, pressure acid decomposition was performed, and the sample was completely dissolved. The content of iron, chromium, aluminum and other metal impurities was determined by measuring the volume of the decomposition solution with ultrapure water and using an ICP-AES (SPS5100 type) analyzer manufactured by SII Nanotechnology. Quantify the content of iron, chromium and aluminum in the test solution and the content of metal impurities other than iron, chromium and aluminum from the detected wavelength and the emission intensity, and the content of iron, chromium and aluminum in the sample. The amount and content of metal impurities other than iron, chromium and aluminum were calculated.

In the present invention, the bulk density means the initial bulk density.
(Measurement method of bulk density)
The initial bulk density of high-purity silicon powder and silicon nitride powder as a diluent was determined by a method in accordance with JIS R1628 “Method for measuring bulk density of fine ceramic powder”. Further, the bulk density of the packed bed made of a mixture of high-purity silicon powder and a diluent was determined by the same method.

(Measurement method of crushing strength of combustion products)
The crushing strength of the combustion product obtained in the present invention was measured as follows. From the combustion product, 5 cubes each having a side of 10 mm were cut out and used as measurement samples. The crushing strength of the measurement sample was measured using a manual crushing strength measuring device (model 1 model manufactured by Aiko Engineering Co., Ltd.). A compression test was performed by applying a load to the measurement sample placed on the pedestal, and the crushing strength was calculated from the measured maximum load. The crushing strength of the combustion product obtained in the present invention was an average value of the crushing strength of the five measurement samples.

Tables 1 and 2 show characteristic values such as bulk density and metal impurity content of the silicon powder, silicon nitride powder and mixed raw material powder used in the combustion synthesis. Tables 3 and 4 show the crushing strength of the combustion products obtained by the combustion synthesis reaction and the physical properties of the high-purity silicon nitride powder obtained by pulverizing the combustion products.

[Example 2]
In the same manner as in Example 1, except that silicon powder having a 50% volume particle diameter (D ₅₀ ) of 4.0 μm and a 90% volume particle diameter (D ₉₀ ) of 12 μm was used as the silicon powder used as a raw material, A mixed powder of silicon, which is a synthetic raw material, and a diluent (silicon nitride) was charged into a conductive container, and a lump of silicon nitride powder was obtained from the synthetic raw material by a self-combustion reaction. In the same manner as in Example 1, coarse pulverization was performed using a roll crusher equipped with a silicon nitride roll.

Furthermore, a silicon nitride coarsely pulverized product is put in a resin pot in which a powder contact portion of a mill container or the like is covered with a resin such as polyurethane, and an appropriate amount of silicon nitride balls for pulverization is added to obtain a predetermined frequency of 1780 cpm and amplitude of 5 mm The vibration mill was pulverized by vibrating and rotating only for the time. Since the grinding balls were worn out and mixed into the raw material, the coarsely pulverized silicon nitride was pulverized using a pulverizing medium made of a silicon nitride sintered body having a porosity of 1% or less and a Vickers hardness of 18 GPa. Since the pulverized product of silicon nitride may adhere to the wall surface of the mill container and the pulverization efficiency may decrease, the crushed object adhered to the inner wall of the container was scraped off every hour.

The obtained finely pulverized silicon nitride was analyzed. The results are shown in Tables 1 to 4.

[Example 3 and Example 4]
The silicon powder having particle size distribution, bulk density and metal impurity content described in Table 1, and silicon nitride powder having the average particle diameter, bulk density and metal impurity content described in the table are listed in the table In the same manner as in Example 1 except that the raw material powder mixed at a ratio was used, a pressure-resistant container was charged with a mixed powder of silicon powder as a synthetic raw material and a diluent (silicon nitride powder). Under the described conditions, a lump of silicon nitride powder was synthesized from the synthetic raw material by a self-combustion reaction. The pulverization process was performed in the same manner as in Example 2, and the obtained finely pulverized silicon nitride was analyzed. The results are shown in Tables 1 to 4.

[Example 5]
A raw material powder composed of a silicon powder having a particle size distribution, a bulk density and a metal impurity content described in Table 1, and a silicon nitride powder having an average particle diameter, a bulk density and a metal impurity content described in the table, Sodium chloride was added as a reaction aid. That is, a synthetic raw material was prepared so that the blending ratio was silicon: silicon nitride: sodium chloride = 63: 27: 10 with a value obtained by converting silicon into silicon nitride. In addition, the value in which silicon is converted into silicon nitride is, for example, that when 3 mol (84.3 g) of silicon and 1 mol (140.3 g) of silicon nitride are included, the weight ratio is 1: 1. I mean.

In the same manner as in Example 1, a lump of silicon nitride powder was synthesized from the above synthetic raw material by a self-combustion reaction under the conditions described in Tables 1 and 2. In the same manner as in Example 2, coarse pulverization using a silicon nitride roll crusher and vibration mill pulverization using silicon nitride balls as a pulverization medium were performed, and the resulting finely pulverized silicon nitride was analyzed. The results are shown in Tables 1 to 4.
[Example 6]
The silicon powder having particle size distribution, bulk density and metal impurity content described in Table 1 and silicon nitride powder having the average particle size, bulk density and metal impurity content described in the table are listed in the same table. A lump of silicon nitride powder was synthesized from the synthetic raw material by a self-combustion reaction in the same manner as in Example 1 except that the raw material powder mixed at the mixing ratio was used under the conditions described in Tables 1 and 2. The pulverization process was performed in the same manner as in Example 1, and the obtained finely pulverized silicon nitride was analyzed. The results are shown in Tables 1 to 4.

[Examples 7 to 10]
The silicon powder having particle size distribution, bulk density and metal impurity content described in Table 1 and silicon nitride powder having the average particle size, bulk density and metal impurity content described in the table are listed in the same table. A lump of silicon nitride powder was synthesized from the synthetic raw material by a self-combustion reaction in the same manner as in Example 1 except that the raw material powder mixed at the mixing ratio was used under the conditions described in Tables 1 and 2. In the same manner as in Example 2, coarse pulverization using a silicon nitride roll crusher and vibration mill pulverization using silicon nitride balls as a pulverization medium were performed, and the resulting finely pulverized silicon nitride was analyzed. The results are shown in Tables 1 to 4.

[Example 11]
A raw material powder composed of a silicon powder having a particle size distribution, a bulk density and a metal impurity content described in Table 1, and a silicon nitride powder having an average particle diameter, a bulk density and a metal impurity content described in the table, Sodium chloride was added as a reaction aid. In the same manner as in Example 5, a lump of silicon nitride powder was synthesized from the above synthetic raw material by a self-combustion reaction under the conditions described in Tables 1 and 2. In the same manner as in Example 2, coarse pulverization using a silicon nitride roll crusher and vibration mill pulverization using silicon nitride balls as a pulverization medium were performed, and the resulting finely pulverized silicon nitride was analyzed. The results are shown in Tables 1 to 4.

[Examples 12 to 16]
The silicon powder having particle size distribution, bulk density and metal impurity content described in Table 1 and silicon nitride powder having the average particle size, bulk density and metal impurity content described in the table are listed in the same table. A lump of silicon nitride powder was synthesized from the synthetic raw material by a self-combustion reaction in the same manner as in Example 1 except that the raw material powder mixed at the mixing ratio was used under the conditions described in Tables 1 and 2. In the same manner as in Example 2, coarse pulverization using a silicon nitride roll crusher and vibration mill pulverization using silicon nitride balls as a pulverization medium were performed, and the resulting finely pulverized silicon nitride was analyzed. The results are shown in Tables 1 to 4.

[Comparative Example 1]
As shown in Table 1, the content of iron, chromium and aluminum as the silicon powder is 200 ppm, 140 ppm and 200 ppm, respectively, the total content of other metal impurities is 180 ppm, and the average particle size is 5. A silicon nitride powder having a content of 20 ppm, 8 ppm, and 30 ppm of iron, chromium, and aluminum, and a total content of other metal impurities of 20 ppm, and having an average particle size of 2.0 μm, in a silicon powder of 2 μm. A synthetic raw material was prepared so that the mixing ratio of silicon and diluent (silicon nitride) was 8.5: 1.5 in terms of silicon nitride. The bulk density of the synthetic raw material is 0.49 g / cm ³ by combining the bulk density of the silicon powder of the compounding source and the bulk density of the diluent (silicon nitride), each having a predetermined value. did.
When the charged raw materials (total weight: 5.4 kg) were filled into a graphite crucible of 770 mm × 320 mm, the height of the powder layer was 45 mm.
Under the conditions described in Table 1 and Table 2, a lump of silicon nitride powder was synthesized from the synthetic raw material by a self-combustion reaction. The pulverization process was performed in the same manner as in Example 1, and the obtained finely pulverized silicon nitride was analyzed. The results are shown in Tables 1 to 4. Comparative Example 1 is an example in which silicon powder containing many Fe, Al, Cr, and other metal impurities is used as a raw material.

[Comparative Example 2 and Comparative Example 3]
The silicon powder having particle size distribution, bulk density and metal impurity content described in Table 1 and silicon nitride powder having the average particle size, bulk density and metal impurity content described in the table are listed in the same table. The raw material powder mixed in the blending ratio is charged into a pressure-resistant container, and a lump of silicon nitride powder is synthesized from the synthetic raw material by a self-combustion reaction in the same manner as in Example 2 under the conditions described in Tables 1 and 2. did. In the same manner as in Example 2, coarse pulverization using a silicon nitride roll crusher and vibration mill pulverization using silicon nitride balls as a pulverization medium were performed, and the resulting finely pulverized silicon nitride was analyzed. The results are shown in Tables 1 to 4. Comparative Example 2 is an example in which both the silicon powder as a raw material and the silicon nitride powder as a diluent contain a large amount of Fe, Al, Cr, and other metal impurities. Comparative Example 3 is an example in which the silicon nitride powder that is a diluent contains a large amount of Fe, Al, Cr, and other metal impurities.

[Comparative Example 4]
A raw material powder composed of a silicon powder having a particle size distribution, a bulk density and a metal impurity content described in Table 1, and a silicon nitride powder having an average particle diameter, a bulk density and a metal impurity content described in the table, Sodium chloride was added as a reaction aid. In the same manner as in Example 5, a lump of silicon nitride powder was synthesized from the above synthetic raw material by a self-combustion reaction under the conditions described in Tables 1 and 2.

The obtained mass of silicon nitride powder was coarsely pulverized using a roll crusher equipped with an alumina roll.

Further, a coarsely pulverized product of silicon nitride was put in an alumina pot, an appropriate amount of pulverized alumina balls were added, and the mixture was vibrated and rotated for a predetermined time at a frequency of 1780 cpm and an amplitude of 5 mm, thereby performing vibration mill pulverization. Since the pulverized product of silicon nitride may adhere to the wall surface of the mill container and the pulverization efficiency may decrease, the crushed object adhered to the inner wall of the container was scraped off every hour.

Comparative Example 4 is an example in which alumina is used in a coarse pulverization and fine pulverization apparatus. The results of analyzing the finely pulverized silicon nitride obtained are shown in Tables 1 to 4.

[Comparative Examples 5 to 8]
The silicon powder having particle size distribution, bulk density and metal impurity content described in Table 1 and silicon nitride powder having the average particle size, bulk density and metal impurity content described in the table are listed in the same table. A lump of silicon nitride powder was synthesized from the synthetic raw material by a self-combustion reaction in the same manner as in Example 2 except that the raw material powder mixed at the mixing ratio was used under the conditions described in Tables 1 and 2. In the same manner as in Example 2, coarse pulverization using a silicon nitride roll crusher and vibration mill pulverization using silicon nitride balls as a pulverization medium were performed, and the resulting finely pulverized silicon nitride was analyzed. The results are shown in Tables 1 to 4. Comparative Example 5 is an example in which the bulk density of the mixed powder of silicon powder and silicon nitride powder as a diluent is large, and the crushing strength of the combustion product is large. Comparative Example 6 is an example where the bulk density of the mixed powder is small. Comparative Example 6 is an example with a small proportion of diluent, and Comparative Example 7 is an example with a large proportion of diluent.

In Comparative Example 8, although the evaluation result was not particularly low in terms of characteristics, the mixing ratio of silicon and diluent (silicon nitride) was 4.0: 6.0 in terms of silicon nitride. Therefore, for the reasons described above, it is contrary to the gist of the present invention that silicon nitride powder is produced at low cost.

[Comparative Example 9]
The silicon powder having particle size distribution, bulk density and metal impurity content described in Table 1 and silicon nitride powder having the average particle size, bulk density and metal impurity content described in the table are listed in the same table. A lump of silicon nitride powder was synthesized from the synthetic raw material by a self-combustion reaction in the same manner as in Example 2 except that the raw material powder mixed at the mixing ratio was used under the conditions described in Tables 1 and 2.

The obtained mass of silicon nitride powder was coarsely pulverized using a roll crusher equipped with an alumina roll.
Furthermore, the coarsely pulverized product of silicon nitride was put in an alumina pot, an appropriate amount of alumina balls for pulverization was added, the pulverization treatment was performed in the same manner as in Comparative Example 4, and the finely pulverized product of silicon nitride was analyzed. . Comparative Example 9 is an example in which alumina is used in a coarse pulverization and fine pulverization apparatus. The results are shown in Tables 1 to 4.

(Evaluation of compatibility as a mold release agent for silicon ingot casting)
The silicon nitride powders obtained in Examples 1 to 7 and Comparative Examples 1 to 4 were evaluated for suitability as a release agent for use in producing a polycrystalline silicon ingot.

Evaluation of the silicon nitride powders of Examples and Comparative Examples as mold release agents for casting molds of polycrystalline silicon ingots was performed as follows.

(Weight reduction rate of silicon nitride powder and free silicon production)
As a simulation evaluation for confirming the stability as a release agent for casting a polycrystalline silicon ingot, the weight reduction rate at 1570 ° C. was measured under an argon atmosphere.

That is, the weight reduction rate of the silicon nitride of the present invention was measured by the following method. First, 5.5 g of silicon nitride powder was weighed, filled in an alumina crucible having a square bottom of 200 mm, a depth of 200 mm, and a thickness of 10 mm, and housed in a batch-type firing furnace, and the inside of the furnace was filled with an argon atmosphere. Then, the temperature was raised to 1570 ° C. under an argon atmosphere and held for 5 hours. After cooling to room temperature, the weight of the argon heat treated powder is measured. The weight reduction rate of the silicon nitride powder was calculated from the following formula (7).

Weight reduction rate of silicon nitride powder (mass%) = (weight of silicon nitride powder (g) −weight of argon heat-treated powder (g)) / (weight of silicon nitride powder (g)) × 100. 7)

Further, the amount of free silicon produced in the silicon nitride powder after heat treatment of holding at 1570 ° C. for 5 hours under an argon atmosphere was measured. Using an X-ray diffractometer (RINT-TTRIII, manufactured by Rigaku Corporation), powder X-ray diffraction measurement (XRD) of the silicon nitride powder after heat treatment was performed. The existing crystal phase was β-type silicon nitride, α-type nitride It was a three phase silicon and metallic silicon. The obtained powder X-ray diffraction pattern was subjected to Rietveld analysis using an analysis program JADE manufactured by Rigaku Corporation to determine the amount of free silicon produced.

Table 5 shows the results of evaluation of compatibility as a mold release agent for silicon ingot casting. The weight reduction rate of the silicon nitride powders of Examples 1 to 7 after firing in an argon atmosphere was 0.20 to 0.80% by weight and the amount of free silicon produced was 0.10 to 0.28% by weight. The weight reduction rates of the silicon nitride powders of Comparative Examples 1 to 4 were 2.11 to 5.4% by weight, and the amount of free silicon produced was 0.49 to 1.03% by weight. There is a problem that a mold release agent having a large weight reduction rate and a large amount of free silicon is peeled off from the crucible wall for casting at the time of silicon ingot casting. For this reason, there is a demand for a silicon nitride powder for a release agent that can exhibit stable characteristics up to a high temperature as a release agent. From this evaluation, it was found that the silicon nitride powders of Examples 1 to 7 were superior in high-temperature stability in a polycrystalline silicon ingot casting atmosphere and could exhibit stable characteristics as a release agent. That is, the high-purity silicon nitride powder of the present invention is suitable as a release agent used for casting polycrystalline silicon.

(Evaluation of compatibility as a raw material for manufacturing sintered bodies such as high thermal conductivity silicon nitride substrates)
The silicon nitride powders obtained in Examples 8 to 16 and Comparative Examples 5 to 9 were evaluated for suitability as raw materials for producing a sintered body such as a high thermal conductivity silicon nitride substrate.

The evaluation of the sintering characteristics of the silicon nitride powders of Examples and Comparative Examples was performed as follows.

(Production and characteristic evaluation of sintered silicon nitride)
A blended powder obtained by adding 3.5 parts by weight of yttrium oxide as a sintering aid and 2 parts by weight of magnesium oxide to 94.5 parts by weight of silicon nitride powder was wet mixed in a ball mill for 48 hours using ethanol as a medium. Thereafter, the slurry was dried under reduced pressure. The obtained mixture was molded into a shape of 62 mm × 62 mm × thickness 7.3 mm and a shape of 12.3 mmφ × thickness 3.2 mm at a molding pressure of 30 MPa, and then CIP (Cold Isostatic) at a molding pressure of 150 MPa. Pressing). The obtained molded body was placed in a boron nitride crucible, heated to 1850 ° C. under a nitrogen atmosphere of 0.8 MPa, and held at 1850 ° C. for 22 hours for sintering. The obtained silicon nitride sintered body was cut and ground, and a 3 mm × 4 mm × 40 mm bending strength test piece according to JIS R1601 and a 10 mmφ × 2 mm test piece for measuring thermal conductivity according to JIS R1611 were obtained. Produced. The relative density of the sintered body was measured by Archimedes method. The room temperature 4-point bending strength at room temperature was measured by a method based on JIS R1601 using an Instron universal material testing machine, and the thermal conductivity at room temperature was measured by a flash method based on JIS R1611.

Table 6 shows the evaluation results of compatibility as a raw material for producing a sintered body such as a high thermal conductivity silicon nitride substrate.
The relative reach density of the sintered bodies was 95.6 to 97.7% for the silicon nitride powders of Examples 8 to 16, and 97.3 to 99.3% for the silicon nitride powders of Comparative Examples 5 to 9. there were. The four-point bending strength of the sintered body at room temperature was 756 to 812 MPa for the silicon nitride powders of Examples 8 to 16, and 717 to 768 MPa for the silicon nitride powders of Comparative Examples 5 to 9.
In contrast, the thermal conductivity of the sintered body at room temperature was 89 to 101 W / mK for the silicon nitride powders of Examples 8 to 16, and 53 to 75 W / m for the silicon nitride powders of Comparative Examples 5 to 9. It was found that the silicon nitride powders of Examples 8 to 16 had a higher thermal conductivity and a high-quality silicon nitride sintered body was obtained. That is, the high-purity silicon nitride powder of the present invention is suitable as a raw material for manufacturing sintered bodies such as various jigs for manufacturing semiconductors and high-heat-conducting silicon nitride substrates that require high thermal conductivity.

[Example 17]
Using the silicon nitride powder obtained in Example 2, the properties as a release agent were evaluated by the following method.
That is, a unidirectional solidification experiment of a polycrystalline silicon ingot was performed using a mold prepared by applying the obtained silicon nitride powder as a release agent, and the polycrystalline silicon ingot was released from the mold. An ingot was produced by melting at 1500 ° C. and 1550 ° C., but no release agent adhered to the ingot.
Next, Fe, Cr, Al, and metal impurities other than these (Fe, Cr, and Al) contained in the polycrystalline silicon ingot obtained by the unidirectional solidification experiment at 1550 ° C. were measured as follows. did.
That is, the obtained polycrystalline silicon ingot was divided into two so that the cut surface was parallel to the solidification direction, and the flight time was measured with the position 1 cm above the bottom on the central axis of the cut surface. Surface analysis was performed using a type secondary ion mass spectrometry method (manufactured by ULVAC-PHI (TRIFT V nano TOF type)). As a result, the normalized secondary ion intensity of secondary mass spectra of Fe, Cr, Al, and metal impurities other than these (Fe, Cr, and Al) was less than 1 × 10 ⁻⁴ , so that metal impurities were detected Judged not to have been. Here, the normalized secondary ion intensity is obtained by dividing the secondary ion intensity of each spectrum by the secondary ion intensity of all detected spectra.

[Example 18]
The D _{50 of the} silicon powder is 4.0 μm, the bulk density is 0.4 g / cm ³ , the bulk density of the mixed powder is 0.42 g / cm ³ , and a bead mill (grinding medium and inner wall liner is used as a grinding method) In the same manner as in Example 8 except that a silicon nitride sintered body) was used, a combustion synthesis reaction and pulverization (coarse pulverization and fine pulverization) of a mixed raw material powder of silicon powder and silicon nitride powder were performed, and nitridation was performed. Silicon powder was produced.
The obtained silicon nitride powder has a specific surface area of 8.0 m ² / g, a β-type silicon nitride ratio of 100% by mass, D ₁₀ of 0.85 μm, D ₅₀ of 2.4 μm, D ₉₀ of 5.1 μm, Fe Β-type silicon nitride calculated by applying a Williamson-Hall plot with a content ratio of 7 ppm, a Cr content ratio of 3 ppm, an Al content ratio of 20 ppm, a content ratio of metal impurities other than Fe, Cr and Al The crystallite diameter D _{c of the} powder was 180 nm, the crystal strain was 0.98 × 10 ⁻⁴ , and D _BET / D _c was 1.3.
In the same manner as in Example 8, a silicon nitride sintered body was produced using the obtained silicon nitride powder, and the obtained silicon nitride sintered body was evaluated for properties. The bulk density was 97.7%, the bending strength was 813 MPa, The thermal conductivity was 103 W / mK.

[Example 19]
The silicon powder having particle size distribution, bulk density and metal impurity content described in Table 1 and silicon nitride powder having the average particle size, bulk density and metal impurity content described in the table are listed in the same table. A lump of silicon nitride powder was synthesized from the synthetic raw material by a self-combustion reaction under the conditions described in Tables 1 and 2 in the same manner as in Example 18 except that the raw material powder mixed at the mixing ratio was used. In the same manner as in Example 18, coarse pulverization using a silicon nitride roll crusher and bead mill pulverization using silicon nitride balls as a pulverization medium were performed, and the resulting finely pulverized silicon nitride was analyzed. The results are shown in Tables 1 to 4. In the same manner as in Example 18, a silicon nitride sintered body was produced using the obtained silicon nitride powder, and the characteristics of the obtained silicon nitride sintered body were evaluated. The results are shown in Table 6.

[Example 20]
A raw material powder composed of a silicon powder having a particle size distribution, a bulk density and a metal impurity content described in Table 1, and a silicon nitride powder having an average particle diameter, a bulk density and a metal impurity content described in the table, Sodium chloride was added as a reaction aid. In the same manner as in Example 18, a lump of silicon nitride powder was synthesized from the above synthetic raw material by a self-combustion reaction under the conditions described in Tables 1 and 2. In the same manner as in Example 18, coarse pulverization using a silicon nitride roll crusher and bead mill pulverization using silicon nitride balls as a pulverization medium were performed, and the resulting finely pulverized silicon nitride was analyzed. The results are shown in Tables 1 to 4. In the same manner as in Example 18, a silicon nitride sintered body was produced using the obtained silicon nitride powder, and the characteristics of the obtained silicon nitride sintered body were evaluated. The results are shown in Table 6.

The method for producing high-purity silicon nitride powder according to the present invention is suitable as a raw material for silicon nitride sintered bodies having both high thermal conductivity and mechanical strength, such as raw materials for various jigs for manufacturing semiconductors and raw materials for high thermal conductive silicon nitride substrates. It is useful as a method for producing inexpensive high-purity silicon nitride powder at low cost by a combustion synthesis method. In addition, the method for producing high-purity silicon nitride powder according to the present invention can be used as a mold release agent for polycrystalline silicon ingots, particularly as a method for producing high-purity silicon nitride powder suitable for use at high temperatures at low cost by a combustion synthesis method. Useful.

DESCRIPTION OF SYMBOLS 10 Reactor 11 Electrode 12 Carbon heater 13 Ignition material 14 Pressure sensor 15 Nitrogen gas introduction pipe 16 Nitrogen gas pressure control valve 17 Air on-off valve 20 Inner container 21 Flowing water pipe 22 Support stand equipped with a temperature sensor 23 Graphite crucible 24 Flowing water on-off valve 25 Starting material 30 Outer wall

Claims

By mixing silicon powder and silicon nitride powder as a diluent, filling the obtained mixed powder into a crucible, and burning the silicon powder by a combustion synthesis method utilizing self-heating and propagation phenomena associated with combustion reaction A method for producing a high-purity silicon nitride powder comprising producing a coagulated mass composed of silicon nitride and pulverizing the coagulated mass,
The silicon powder has an iron and aluminum content of 100 ppm or less, and a total content of metal impurities other than iron and aluminum is 100 ppm or less,
The diluent is a silicon nitride powder having an iron and aluminum content of 100 ppm or less and a total content of metal impurities other than iron and aluminum of 100 ppm or less,
The compounding ratio of the silicon powder and the diluent in the mixed powder is 9: 1 to 5: 5 by mass ratio,
The bulk density of the powder layer composed of the mixed powder filled in the crucible is 0.3 to 0.65 g / cm 3 ,
A method for producing a high-purity silicon nitride powder, characterized in that the agglomerated mass is pulverized using a pulverization apparatus loaded or loaded with a pulverization medium containing at least silicon nitride.
The method for producing high-purity silicon nitride powder according to claim 1, wherein the bulk density of the silicon powder is 0.2 to 0.7 g / cm 3 .
3. The method for producing high-purity silicon nitride powder according to claim 1, wherein a bulk density of the silicon nitride powder as the diluent is 0.2 to 0.7 g / cm 3 .
The high-purity silicon nitride powder according to any one of claims 1 to 3, wherein the powder layer composed of the mixed powder has a bulk density of 0.36 to 0.48 g / cm 3. Manufacturing method.
The 50% by volume particle size D 50 of the silicon powder is 1.0 to 15 μm, and the 90% by volume particle size D 90 is 10 to 44 μm. Of high purity silicon nitride powder.
The high powder according to any one of claims 1 to 5, wherein the powder layer has a thickness of 20 to 70 mm and is ignited from an uppermost portion of the powder layer to burn the silicon powder. A method for producing a pure silicon nitride powder.
The silicon powder has an iron, chromium and aluminum content of 50 ppm or less, and the total content of metal impurities other than iron, chromium and aluminum is 50 ppm or less. The diluent is composed of iron, chromium and aluminum. The silicon nitride powder according to any one of claims 1 to 6, wherein the silicon nitride powder has a content of 50 ppm or less and a total content of metal impurities other than iron, chromium, and aluminum is 50 ppm or less. A method for producing high-purity silicon nitride powder.
The high-purity silicon nitride according to any one of claims 1 to 7, wherein the aggregate is subjected to a first pulverization using a roll crusher equipped with a roll made of a silicon nitride sintered body. Powder manufacturing method.
The silicon nitride powder obtained by the first pulverization is accommodated in a container loaded with a pulverization medium composed of a silicon nitride sintered body, and is further subjected to second pulverization by a vibration mill or a bead mill. The manufacturing method of the high purity silicon nitride powder of Claim 8.
The method for producing high-purity silicon nitride powder according to claim 9, wherein the grinding medium has a porosity of 2% or less and a Vickers hardness of 14 GPa or more.
The high-purity silicon nitride powder has a β-phase ratio of 70% by mass or more, a BET specific surface area of 3.0 to 13.0 m 2 / g, and an iron, chromium, and aluminum content of 200 ppm or less, respectively. The method for producing high-purity silicon nitride powder according to any one of claims 1 to 10, wherein the total content of metal impurities other than iron, chromium and aluminum is 200 ppm or less.
The high-purity silicon nitride powder, crystallite diameter D C that is calculated from the powder X-ray diffraction pattern is 0.15 ~ 1 [mu] m, the crystal effective strain is at 1.5 × 10 -4 or less, the BET specific surface area the method according to claim 11 high purity silicon nitride powder, wherein the ratio D BET / D C of the sphere equivalent diameter D BET calculated and crystallite diameter D C is 1-3.
The high purity silicon nitride powder is
When the volume-based 50% particle diameter measured by the laser diffraction scattering method is D 50 and the 90% particle diameter is D 90 , D 50 is 1.7 μm or more and 20 μm or less, and D 90 is 10 μm or more and 40 μm. And
Fe content is 100 ppm or less,
Cr content is 100 ppm or less,
Al content is 100 ppm or less,
The total content of metal impurities other than Fe, Cr and Al is 100 ppm or less,
The crystallite size of β-type silicon nitride which is calculated using the Williamson-Hall type from powder X-ray diffraction pattern of β-type silicon nitride is taken as D C, wherein, wherein D C is 200nm or more Item 12. A method for producing a high purity silicon nitride powder according to Item 11.
The high purity silicon nitride powder is
The specific surface area measured by the BET method is 5 m 2 / g or more and 20 m 2 / g or less,
When the volume-based 50% particle diameter measured by the laser diffraction scattering method is D 50 and the 90% particle diameter is D 90 , D 50 is 0.5 μm or more and 3 μm or less, and D 90 is 3 μm or more and 7 μm. And
The crystallite size of β-type silicon nitride which is calculated using the Williamson-Hall type from powder X-ray diffraction pattern of β-type silicon nitride is taken as D C, and a D C is 120nm or more,
When the specific surface area equivalent diameter calculated from the specific surface area is D BET , D BET / D C (nm / nm) is 3 or less,
The high-purity nitriding according to claim 11, wherein the crystal strain of β-type silicon nitride calculated from the powder X-ray diffraction pattern of β-type silicon nitride using the Williamson-Hall equation is 1.5 × 10 -4 or less. A method for producing silicon powder.