JP5303103B2 - Silicon carbide sintered body and method for producing the same - Google Patents

Description

  The present invention relates to a silicon carbide sintered body and a method for producing the same. More specifically, the present invention relates to a silicon carbide sintered body having a stable conductivity with little variation in volume resistance and a method for manufacturing the same.

  As a method of obtaining a silicon carbide sintered body having good conductivity, a method of introducing a nitrogen source into the silicon carbide sintered body is known. For example, Patent Document 1 discloses that a silicon carbide powder is heated in a nitrogen atmosphere to incorporate nitrogen atoms into the silicon carbide powder to produce a silicon carbide powder that is a raw material of a silicon carbide sintered body, and the nitrogen atoms are Disclosed is a method for producing a silicon carbide sintered body having electrical conductivity comprising a step of sintering a mixture of incorporated silicon carbide powder and a nonmetallic sintering aid in a hot press sintering furnace.

  Patent Document 1 discloses a silicon carbide sintering method in which a mixed powder composed of a silicon carbide powder and a nonmetallic sintering aid is sintered in a hot press sintering furnace while nitrogen gas is introduced. A method for producing a knot is disclosed.

However, in the method of introducing nitrogen gas, it is costly to repair the damaged portion of the heat insulating material disposed around the hot press sintering furnace, so that high cost has been a problem. In addition, since it is difficult to control the amount of nitrogen source introduced, there is a large variation in the conductivity of the obtained silicon carbide sintered body, that is, a variation in the distribution of volume resistance values. For this reason, when the silicon carbide sintered body obtained by such a method is used as a heater member, it is difficult to apply it to a semiconductor manufacturing apparatus because temperature unevenness occurs.
Japanese Patent Laid-Open No. 11-79840

  There has been a demand for a silicon carbide sintered body having little variation in volume resistance and having stable conductivity and a method for producing the same.

That is, the present invention relates to the following items:
(1) In a method for producing a silicon carbide sintered body in which a mixed powder containing silicon carbide powder and a nonmetallic sintering aid is sintered under high temperature and pressure, the silicon carbide powder generates carbon by heating a silicon source. A mixture containing 1.5 to 6 parts by weight of the nitrogen source-containing compound with respect to 100 parts by weight of the carbon source to obtain a solid matter. Obtained by the following steps: obtaining a pre-fired powder by heating and carbonizing a solid in a non-oxidizing atmosphere; and obtaining a silicon carbide powder by further firing the pre-fired powder in a non-oxidizing atmosphere. A method for producing a sintered silicon carbide body.

(2) The method for producing a silicon carbide sintered body according to (1), wherein the boiling point of the carbon source is 150 ° C. or higher.

(3) The method for producing a silicon carbide sintered body according to (1) or (2), wherein the nitrogen source-containing compound is a compound selected from the group consisting of N-methyldiethanolamine, phthalimide, and ammonium benzoate.

(4) A silicon carbide sintered body having a volume resistance of 10 ⁻³ Ωcm or less and a variation in volume resistance at room temperature satisfying the following formula: minimum volume resistance value / maximum volume resistance value = 80% to 100%.

  ADVANTAGE OF THE INVENTION According to this invention, there is little variation in volume resistance and the silicon carbide sintered compact which has the stable electroconductivity, and its manufacturing method are obtained.

  Hereinafter, the present invention will be described with reference to embodiments, but it goes without saying that the present invention is not limited to the following embodiments.

(Method for producing silicon carbide powder)
A method for producing silicon carbide powder as a raw material for a silicon carbide sintered body includes (a) a silicon source, a carbon source that generates carbon by heating, a nitrogen source-containing compound, a polymerization or crosslinking catalyst, and 100 parts by weight of the carbon source. On the other hand, a step of curing and drying a mixture containing 1.5 to 6 parts by weight of a nitrogen source-containing compound to obtain a solid, and (b) heating and carbonizing the solid in a non-oxidizing atmosphere to obtain a calcined powder. And (c) a step of further firing the pre-fired powder in a non-oxidizing atmosphere to obtain a silicon carbide powder. Hereinafter, each process will be described in detail.

(I) Step of obtaining solid material As the silicon source, one or more selected from high-purity tetraalkoxysilane, a polymer thereof, and silicon oxide can be used. Here, “silicon oxide” refers to those including silicon dioxide and silicon monoxide. Specific examples of silicon sources include alkoxysilanes typified by tetraethoxysilane, low molecular weight polymers (oligomers) thereof, silicate polymers having a higher degree of polymerization, and oxidation of silica sol, fine powder silica, etc. A silicon compound is mentioned. Examples of the alkoxy silane include methoxy silane, ethoxy silane, propoxy silane, butoxy silane and the like. Among these, ethoxy silane is preferably used from the viewpoint of handling properties. Here, the “oligomer” refers to a polymer having a degree of polymerization of about 2 to 15. Among these silicon sources, from the viewpoint of good homogeneity and handling properties, an oligomer of tetraethoxysilane and a mixture of an oligomer of tetraethoxysilane and fine powder silica are preferable. These silicon sources are high-purity substances, and the initial impurity content is preferably 20 ppm or less, more preferably 5 ppm or less.

  The substance used as the carbon source can be a high-purity organic compound that contains oxygen in the molecule and remains carbon by heating. Specific examples include various sugars such as phenol resin, furan resin, epoxy resin, phenoxy resin, monosaccharides such as glucose, oligosaccharides such as sucrose, polysaccharides such as cellulose and starch. For the purpose of homogeneously mixing with the silicon source, these are mainly used in liquid form at room temperature, those that dissolve in a solvent, those that soften or become liquid when heated, such as thermoplastic or heat-meltable. Of these, resol type phenol resins and novolac type phenol resins are preferred. The carbon source is preferably one having a boiling point of 150 ° C. or higher.

  The ratio of carbon to silicon (hereinafter abbreviated as C / Si ratio) is defined by elemental analysis of a carbide intermediate obtained by carbonizing a mixed cured solid. Stoichiometrically, when the C / Si ratio is 3.0, the free carbon in the generated silicon carbide should be 0%. However, in practice, the low C / Si ratio is caused by volatilization of the SiO gas generated at the same time. Free carbon is generated in It is important to determine the formulation in advance so that the amount of free carbon in the resulting silicon carbide powder does not become an amount that is not suitable for single crystal production applications. Usually, in firing at 1600 ° C. or more near 1 atm, free carbon can be suppressed when the C / Si ratio is set to 2.0 to 2.5, and this range can be suitably used. When the C / Si ratio is 2.5 or more, free carbon significantly increases. However, since this free carbon has an effect of suppressing grain growth, it may be appropriately selected according to the purpose of grain formation. However, when the atmosphere is fired at a low pressure or a high pressure, the C / Si ratio for obtaining pure silicon carbide varies, and in this case, the range is not necessarily limited to the above C / Si ratio.

  The purity of the silicon source and the carbon source is preferably 1 ppm or less for each impurity element, but is not necessarily limited to this as long as it is within the allowable range of purification in the firing step. Here, the impurity element belongs to the group 1 to group 16 element in the periodic table of the 1989 IUPAC inorganic chemical nomenclature revised edition, and has an atomic number of 3 or more, and the atomic numbers 6 to 8 and 14 to 14 An element excluding 16 elements.

  The nitrogen source-containing compound is preferably a compound selected from the group consisting of N-methyldiethanolamine (MDA), phthalimide, and ammonium benzoate. Of these, MDA is preferred from the viewpoint of being a resin generally used as a carbon source and being a high-boiling compound.

  The mixing ratio of the nitrogen source-containing compound is preferably 1.5 to 6 parts by weight of the nitrogen source-containing compound with respect to 100 parts by weight of the carbon source. If the amount is less than the above lower limit value, the effect due to the addition is hardly exhibited, and if it is more than the above upper limit value, cracks tend to occur after sintering.

  The polymerization or crosslinking catalyst is not particularly limited, but maleic acid or the like can be used.

  In the present invention, when obtaining a homogeneous mixture of a silicon source and a carbon source, the mixture is cured to form a solid as necessary. For example, as described above, when a liquid carbon source is used, a homogeneous mixture of a silicon source and a carbon source is cured, and a subsequent silicon carbide generating step is performed. Examples of the curing method include a method of crosslinking by heating, a method of curing with a curing catalyst, and a method of electron beam or radiation. As the curing catalyst, when the carbon source is a phenol resin or a furan resin, toluenesulfonic acid, toluenecarboxylic acid, acetic acid, oxalic acid, hydrochloric acid, sulfuric acid and other acids, and amines such as hexamine can be used. When various saccharides are used as the carbon source, it is preferable to proceed to a carbonization step in a non-oxidizing atmosphere after insolubilization in an oxygen, ozone or air atmosphere during heating.

  Furthermore, a halogen compound can be added to the raw material mixture in an amount of 0.5 to 5% by weight as a means for improving purity and increasing uniformity. By adding a halogen compound to the raw material mixture, the impurities are halogenated in the silicon carbide synthesis process, and the impurities are removed by vaporizing and scattering in the firing process. This method is particularly effective for removing impurities mixed in the raw material. If impurities are up to 5 ppm based on silicon carbide, the impurities after treatment can be suppressed to 0.5 ppm or less. The timing of adding the halogen compound is most suitable when the raw materials are mixed. As the halogen compound to be added, when the raw materials are mixed in the form of liquid or aqueous solution, addition in a liquid form containing no impurities such as ammonium chloride and aqueous hydrochloric acid is desirable. When a thermoplastic phenol resin or furan resin is used as the carbon source and a solid silicon source is used, it is preferable to add a halogen-containing polymer such as polyvinyl chloride, chlorinated polyethylene, or polychloroprene.

  Impurity removal in the case of adding a halogen compound is a carbonization step of a raw material solidified solid, or in the case of direct firing, a firing step. The halogen compound decomposes and reacts with the impurity element at each temperature rise and is discharged. However, it is more preferable from the viewpoint of the effect of removing impurities that the reaction is carried out in the vicinity of the decomposition temperature of the halide for 10 to 30 minutes and then the temperature is raised to the temperature of the subsequent firing step. In addition, if a method of adding 1 to 5% by volume of halogen or hydrogen halide in the non-oxidizing atmosphere of heating and firing in the silicon carbide production step, the content of each impurity element is reduced to 0.3 ppm or less. Is also possible. This method is effective against impurity contamination in all steps before the firing step, and exhibits a sufficient effect against contamination of 20 ppm or less.

  A raw material mixed solid is obtained by curing and drying a mixture comprising the above components at 120 ° C. to 200 ° C. for 1 to 10 hours.

  A baking process (preheating carbonization baking process) which heats a solid substance to 500-1000 degreeC beforehand as needed can further be added. When the halogen compound is added as described above, the raw material mixture is heated at a temperature of 500 to 600 ° C. for 10 to 30 minutes in a non-oxidizing atmosphere, and then at a temperature of 800 to 1000 ° C. in a non-oxidizing atmosphere. It is preferable to perform two-stage carbonization (preheating carbonization baking process) which is heated for 30 minutes to 2 hours. The heating at 800 to 1000 ° C. is insufficient as a pretreatment when it is 30 minutes or less, and even if the heating is continued for more than 2 hours, the effect is not improved. As the non-oxidizing atmosphere, nitrogen or argon is used, but nitrogen is preferable for economic reasons.

(B) Step of obtaining a calcined powder A solid is heated and carbonized at 800 ° C. to 1000 ° C. in a non-oxidizing atmosphere to obtain a calcined powder. As this non-oxidizing atmosphere, argon is desirable because it is non-reactive even at high temperatures. In the firing process, first, in order to produce silicon carbide, it is necessary to heat the raw material solids to a temperature higher than that necessary for production in a heating furnace. Generally, the temperature is 1350 to 1800 ° C., but 1600 to 1800 ° C. is desirable for more efficient production. Since this silicon carbide formation reaction is an endothermic reaction, care must be taken in measuring the temperature of the product. In addition, since the gas containing SiO and CO generated during firing is accompanied by a large amount of impurity elements, these generated gases are introduced into the reaction vessel by appropriately introducing a certain amount of non-oxidizing atmosphere gas into the heating furnace. It is desirable to continuously discharge out of the system and remove it.

(C) Step of obtaining silicon carbide powder A silicon carbide powder is produced by heating and baking a pre-carbonized solid or heating and baking an uncarburized material. As this non-oxidizing atmosphere, argon is desirable because it is non-reactive even at high temperatures. In the firing process, first, in order to produce silicon carbide, it is necessary to heat the raw material solids to a temperature higher than that necessary for production in a heating furnace. Generally, the temperature is 1350 to 1800 ° C., but 1600 to 1800 ° C. is desirable for more efficient production. Since this silicon carbide formation reaction is an endothermic reaction, care must be taken in measuring the temperature of the product. In addition, since the gas containing SiO and CO generated during firing is accompanied by a large amount of impurity elements, these generated gases are introduced into the reaction vessel by appropriately introducing a certain amount of non-oxidizing atmosphere gas into the heating furnace. It is desirable to continuously discharge out of the system and remove it.

  In order to grow the generated silicon carbide particles, it is preferable to maintain the temperature by heating to a certain high temperature thereafter in a post-treatment step. Specifically, it is preferable to hold at 1700 ° C. or higher and lower than 2000 ° C. It is clear from the results of impurity analysis that during this heating, impurities move to the vicinity of the particle surface as the crystallites of the silicon carbide particles increase and the particles grow.

  While maintaining the silicon carbide powder obtained by the silicon carbide production step at a temperature of 1700 ° C. or higher and lower than 2000 ° C., the heating is further performed at a temperature of 2000 ° C. to 2100 ° C. for 5 to 20 minutes. By performing the process once, impurities near the surface are removed to the outside along with further grain growth and partial sublimation decomposition, and a silicon carbide powder that achieves the target purity can be obtained.

  The heat treatment for 5 to 20 minutes at a temperature of 2000 ° C. to 2100 ° C. performed in the post-treatment step after the production of silicon carbide particles is not only from the viewpoint of removing impurities as described above, but also from the viewpoint of controlling the crystal grain size. preferable. That is, further grain growth occurs by heating, and a powder having a desired particle diameter can be obtained. At this time, if the heating temperature is less than 2000 ° C., removal of impurities is insufficient, and if it exceeds 2100 ° C., decomposition of silicon carbide proceeds excessively and a uniform product may not be obtained.

  The heat treatment at 2000 ° C. to 2100 ° C. in this post-treatment step may be performed only once, for example, for 5 to 20 minutes during the temperature holding of 1700 ° C. or more and less than 2000 ° C. after the silicon carbide particles are generated. The temperature can be lowered to the ambient temperature (that is, the temperature of 1700 ° C. or more and less than 2000 ° C.) after the temperature is raised and held for 5 to 20 minutes, and then the same operation can be repeated any number of times at intervals. At this time, the heating and temperature holding time in one step is preferably 5 to 20 minutes, and if it is less than 5 minutes, the effect is insufficient, and if it exceeds 20 minutes, decomposition of silicon carbide proceeds excessively. There is a fear. In addition, when this heat treatment is repeated several times, it is heated from the atmospheric temperature to a temperature of 2000 ° C. to 2100 ° C. and held at that temperature for 5 to 20 minutes, and then the temperature is lowered to the atmospheric temperature and the temperature is maintained. Then, it is preferable to repeat the above temperature rise, temperature hold, and temperature drop at equal intervals during the temperature hold of 1700 ° C. or more and less than 2000 ° C. In addition, when the temperature is lowered to the atmospheric temperature after being heated to a temperature of 2000 ° C. to 2100 ° C. and held at that temperature for 5 to 20 minutes, the atmosphere is heated for at least twice as long as the temperature is raised and held. It is preferable to repeat the above temperature rise, temperature hold and temperature drop after holding at the temperature. When this heat treatment is repeated several times, the total time for maintaining the temperature at 2000 ° C. to 2100 ° C. is preferably 60 minutes or less. The heating time in this heat treatment is preferably performed under the above conditions. However, when the temperature is heated near 2000 ° C., the heating time is long, and when the temperature is heated near 2100 ° C., It is preferable that the heating time be short.

(Method for producing silicon carbide sintered body)
A method for producing a silicon carbide sintered body according to an embodiment of the present invention is a method for producing a silicon carbide sintered body in which a mixed powder containing a silicon carbide powder and a nonmetallic sintering aid is sintered under high temperature and pressure. The silicon carbide powder contains a silicon source, a carbon source that generates carbon by heating, a nitrogen source-containing compound, a polymerization or crosslinking catalyst, and the nitrogen source-containing compound is 1.5 to 6 weights per 100 parts by weight of the carbon source. A step of curing and drying the mixture containing parts to obtain a solid, a step of heating and carbonizing the solid in a non-oxidizing atmosphere to obtain a pre-fired powder, and further firing the pre-fired powder in a non-oxidizing atmosphere And a step of obtaining silicon carbide powder. That is, it is preferable to use a silicon carbide powder produced by the method for producing silicon carbide powder.

According to the method for manufacturing a silicon carbide sintered body according to the embodiment, the volume resistance is 10 ⁻³ Ωcm or less, and the variation in volume resistance at room temperature is represented by the following formula:
Minimum volume resistance value / maximum volume resistance value = 80% to 100%, preferably 90% to 100%,
A silicon carbide sintered body satisfying the above is obtained.

(Modification of the embodiment)
The present embodiment is not limited to the above contents, and various improvements can be added based on, for example, the following related manufacturing methods. The related manufacturing methods are listed below as a reference for the manufacturing method of the silicon carbide sintered body.
(Hot press method)
As the silicon carbide powder, α-type, β-type, amorphous, or a mixture thereof can be widely used, and commercially available products may be used. Of these, β-type silicon carbide powder is preferably used. In order to increase the density of the silicon carbide sintered body, the silicon carbide powder used should have a smaller particle size. Preferably it is about 0.01-10 micrometers, More preferably, it is 0.05-2 micrometers. When the particle size is less than 0.01 μm, handling in processing steps such as weighing and mixing becomes difficult. On the other hand, when the particle size exceeds 10 μm, the specific surface area of the powder, that is, the contact area with the adjacent powder becomes small. This is not preferable because high density becomes difficult. The use of high-purity silicon carbide powder is preferable because the resulting silicon carbide sintered body also has high purity. The high-purity silicon carbide powder is obtained, for example, by mixing a silicon compound (hereinafter sometimes referred to as “silicon source”), an organic material that generates carbon by heating, and a polymerization catalyst or a crosslinking catalyst. It can be produced by firing the product in a non-oxidizing atmosphere. As the silicon source, liquid and solid compounds can be widely used, but at least one liquid compound is used. Examples of the liquid silicon source include polymers of alkoxysilanes (mono-, di-, tri-, tetra-). Among the alkoxysilane polymers, tetraalkoxysilane polymers are preferably used. Specific examples include methoxysilane, ethoxysilane, propoxysilane, butoxysilane and the like, and ethoxysilane is preferable from the viewpoint of handling. When the degree of polymerization of the tetraalkoxysilane polymer is about 2 to 15, a liquid low molecular weight polymer (oligomer) is obtained. In addition, there is a liquid silicate polymer having a high degree of polymerization. Examples of the solid silicon source that can be used in combination with the liquid silicon source include silicon carbide. In this case, silicon carbide includes silicon monoxide (SiO), silicon dioxide (SiO ₂ ), and silica sol (a colloidal ultrafine silica-containing liquid containing colloidal molecules containing OH groups or alkoxy groups. ), Fine silica, quartz powder and the like. Among these silicon sources, an oligomer of tetraalkoxysilane or a mixture of an oligomer of tetraalkoxysilane and fine powder silica, which has good homogeneity and handling properties, are preferable. Further, these silicon sources are preferably highly pure. Specifically, the initial impurity content is preferably 20 ppm or less, and more preferably 5 ppm or less.

  As an organic material that generates carbon by heating, in addition to a liquid material, a liquid material and a solid material can be used in combination. An organic material that has a high residual carbon ratio and is polymerized or crosslinked by a catalyst or heating is preferable. Specifically, monomers such as phenol resin, furan resin, polyimide, polyurethane, polyvinyl alcohol, and prepolymer are preferable. In addition, liquid materials such as cellulose, sucrose, pitch, and tar are also used. Among them, a resol type phenol resin is preferable in terms of thermal decomposability and purity. The purity of the organic material may be appropriately controlled according to the purpose. In particular, when high-purity silicon carbide powder is required, it is preferable to use an organic material having an impurity element content of less than 5 ppm each.

  The blending ratio of the silicon source and the organic material can be determined in advance by using a molar ratio of carbon to silicon (hereinafter abbreviated as “C / Si”) as a guide. Here, C / Si is C / Si obtained from elemental analysis of a silicon carbide intermediate obtained by carbonizing a mixture of a silicon source and an organic material at 1000 ° C., and analyzing the result. As represented by the following reaction formula, carbon reacts with silicon oxide and changes to silicon carbide.

Formula (I) SiO ₂ + 3C → SiC + 2CO Therefore, in terms of stoichiometry, when C / Si is 3.0, the free carbon in the silicon carbide intermediate is 0%. Volatilizes, and free carbon is generated even if C / Si is a lower value. Since free carbon has an effect of suppressing grain growth, C / Si may be determined according to the particle size of the target powder particles, and the silicon source and the organic material may be blended so as to obtain the ratio. For example, when firing a mixture of a silicon source and an organic material at about 1 atm and 1600 ° C. or higher, the generation of free carbon is suppressed by blending so that C / Si is in the range of 2.0 to 2.5. can do. When C / Si is blended so as to exceed 2.5 under the same conditions, the generation of free carbon becomes remarkable and a silicon carbide powder with small particles can be obtained. Thus, the blending ratio can be appropriately determined according to the purpose. In addition, since the action and effect of free carbon resulting from the silicon carbide powder are very weak compared with the action and effect of free carbon generated from the sintering aid, the free carbon resulting from the silicon carbide powder is the present embodiment. The effect of is essentially unaffected.

  A mixture of a silicon source and an organic material can be cured to form a solid. Examples of the curing method include a method using a crosslinking reaction by heating, a method using a curing catalyst, a method using an electron beam and radiation. The curing catalyst to be used can be appropriately selected according to the organic material to be used. However, when phenol resin or furan resin is used as the organic material, acids such as toluenesulfonic acid, toluenecarboxylic acid, acetic acid, oxalic acid, hydrochloric acid, sulfuric acid, hexamine, etc. And the like. The solid containing the silicon source and the organic material is heated and carbonized as necessary. Carbonization is performed by heating at 800 ° C. to 1000 ° C. for 30 to 120 minutes in a non-oxidizing atmosphere such as nitrogen or argon. Furthermore, silicon carbide is produced when heated at 1350 ° C. to 2000 ° C. in a non-oxidizing atmosphere. The firing temperature and firing time affect the particle size and the like of the resulting silicon carbide powder, and may be appropriately determined. However, firing at 1600 to 1900 ° C. is efficient and preferable. The method for obtaining the high-purity silicon carbide powder described above is described in detail in JP-A-9-48605.

  Next, the nonmetallic sintering aid will be described. The silicon carbide sintered body used in the present embodiment has a free carbon content of 2 to 10% by weight. This free carbon is attributed to the organic material used for the nonmetallic sintering aid, and the amount of free carbon is adjusted to the above-mentioned range by adjusting the addition conditions such as the addition amount of the nonmetallic sintering aid. Can be.

  As the non-metallic sintering aid, those containing an organic material that can be a free carbon source as described above, that is, carbon that is generated by heating (hereinafter sometimes referred to as “carbon source”) are used. The above-mentioned organic material alone or a material obtained by coating the above-mentioned organic material on the surface of silicon carbide powder (particle size: about 0.01 to 1 micron) may be used as a sintering aid. Therefore, it is preferable to use an organic material alone. Specific examples of organic materials that generate carbon by heating include coal tar pitch, pitch tar, phenol resin, furan resin, epoxy resin, phenoxy resin, and various sugars such as glucose, which have a high residual carbonization rate. Examples thereof include small sugars such as monosaccharides and sucrose, and polysaccharides such as cellulose and starch. In order to uniformly mix the organic material with the silicon carbide powder, the organic material is preferably liquid at room temperature, dissolved in a solvent, or softened by heating such as thermoplasticity and heat melting. Among these, the use of a phenol resin is preferable because the strength of the silicon carbide sintered body is improved, and a resol type phenol resin is more preferable. Although the action mechanism of these organic materials is not clear, when the organic materials are heated, inorganic carbon compounds such as carbon black and graphite are produced in the system. It is considered that this inorganic carbon-based compound acts effectively as a sintering aid. However, the same effect cannot be obtained even if carbon black or the like is used as a sintering aid.

  The nonmetallic sintering aid may be dissolved in an organic solvent if desired, and the solution and silicon carbide powder may be mixed. The organic solvent to be used varies depending on the nonmetallic sintering aid. For example, when a phenol resin is used as the sintering aid, lower alcohols such as ethyl alcohol, ethyl ether, acetone, or the like can be selected. When producing a high-purity silicon carbide sintered body, it is preferable to use not only a high-purity silicon carbide powder but also a sintering aid and an organic solvent having a low impurity content.

The addition amount of the nonmetallic sintering aid to the silicon carbide powder is determined so that the free carbon of the silicon carbide sintered body is 2 to 10% by weight. If the free carbon is outside this range, the chemical change to SiC that progresses during the bonding process, and the bonding between the silicon carbide sintered bodies becomes insufficient. Here, the content (% by weight) of free carbon is determined by heating the silicon carbide sintered body at 800 ° C. for 8 minutes in an oxygen atmosphere, and measuring the amount of generated CO ₂ and CO with a carbon analyzer. It can be calculated from the measured value. The amount of sintering aid added varies depending on the type of sintering aid used and the amount of surface silica (silicon oxide) in the silicon carbide powder. As a guideline for determining the addition amount, the surface silica (silicon oxide) content of the silicon carbide powder is quantified in advance using hydrogen fluoride water, and the stoichiometry sufficient to reduce the silicon oxide (formula (I)) The stoichiometry calculated in (1) is calculated. In consideration of this and the ratio of the nonmetallic sintering aid to generate carbon by heating, the amount added can be determined so that the free carbon falls within the above-mentioned suitable range. The description of the non-metallic sintering aid for the silicon carbide sintered body described above is described in more detail in the specification of Japanese Patent Application No. 9-041048.

  Next, a method for sintering a mixture of silicon carbide powder and a nonmetallic sintering aid will be described. The silicon carbide powder and the nonmetallic sintering aid are mixed homogeneously. In order to obtain a homogeneous mixture, a solution obtained by dissolving a sintering aid in an organic solvent as described above may be used. Examples of the mixing method include known methods such as a method using a mixer, a planetary ball mill, and the like. The instrument used for mixing is preferably made of a synthetic resin material in order to prevent mixing of metal element impurities. Mixing is preferably performed for about 10 to 30 hours, particularly about 16 to 24 hours, and mixed sufficiently. After thorough mixing, the solvent is removed and the mixture is evaporated to dryness. Thereafter, the mixture is sieved to obtain a raw material powder of the mixture. For drying, a granulating apparatus such as a spray dryer may be used.

  The raw material powder thus obtained is placed in a molding die. It is preferable that the molding die to be used is made of graphite because metal impurities are not mixed in the silicon carbide sintered body. In order to prevent direct contact between the raw material powder and the metal part of the mold, even if it is a metal mold, the contact part is made of graphite, or a polytetrafluoroethylene sheet (Teflon (registered (Trademark) sheet) can be used preferably. In particular, when it is desired to produce a high-purity silicon carbide sintered body, it is preferable to use a high-purity graphite material for the mold and the heat insulating material in the furnace. Specifically, a graphite material or the like that is sufficiently baked in advance at a temperature of 2500 ° C. or higher and does not generate impurities even when used at a high temperature.

The raw material powder placed in the molding die is subjected to hot pressing. There is no restriction | limiting in particular about the pressure of a hot press, It can carry out by the pressure of the wide range of 300-700 kgf / cm < ² >. However, when pressurizing at 400 kgf / cm ² or more, it is necessary to use hot-press parts, for example, dies, punches, etc. that have excellent pressure resistance.
The hot pressing is performed at 2000 ° C. to 2400 ° C., and it is preferable to raise the temperature up to the hot pressing temperature gently and stepwise. When the temperature is increased in this way, chemical changes, state changes, and the like that occur at each temperature can be sufficiently advanced. As a result, it is possible to prevent impurities from being mixed, cracks and vacancies. An example of a preferred temperature raising step is shown below. First, a molding die containing raw material powder is placed in a furnace, and the inside of the furnace is brought to a vacuum state of 10 ⁻⁴ torr. Gently raise the temperature from room temperature to 200 ° C. and keep it at 200 ° C. for about 30 minutes. Thereafter, the temperature is raised to 700 ° C. in 6 to 10 hours and maintained at 700 ° C. for 2 to 5 hours. In the temperature raising process from room temperature to 700 ° C., desorption of adsorbed moisture and organic solvent occurs, and carbonization of the nonmetallic sintering aid proceeds. The holding time at the constant temperature varies depending on the size of the silicon carbide sintered body, and may be set to a suitable time as appropriate. In addition, the determination of whether the holding time is sufficient can be based on the time point when the decrease in the degree of vacuum is reduced to some extent. Next, the temperature is raised from 700 ° C. to 1500 ° C. in 6 to 9 hours, and held at 1500 ° C. for about 1 to 5 hours. While the temperature is maintained at 1500 ° C., a reaction in which silicon oxide is reduced and converted into silicon carbide proceeds (formula (I)). Insufficient holding time is not preferable because silicon dioxide remains and adheres to the surface of the silicon carbide powder, preventing densification of the particles and causing large grains to grow. The determination as to whether or not the holding time is sufficient is based on whether or not the generation of carbon monoxide as a by-product is stopped, that is, the reduction of the vacuum degree is stopped, and the reduction reaction start temperature is 1300 ° C. It can be used as a guide whether the vacuum has been recovered.
The hot pressing is preferably performed after the inside of the furnace is heated to about 1500 ° C. at which the sintering starts, and then filled with an inert gas in order to make the inside of the furnace a non-oxidizing atmosphere. Nitrogen gas or argon gas is used as the inert gas, but it is preferable to use argon gas that is non-reactive even at high temperatures. When it is desired to produce a high purity silicon carbide sintered body, an inert gas having a high purity is used. After making the inside of the furnace a non-oxidizing atmosphere, the inside of the furnace is heated and pressurized so that the temperature becomes 2000 ° C. to 2400 ° C. and the pressure becomes 300 to 700 kgf / cm ² . If the maximum temperature is less than 2000 ° C., densification is insufficient. On the other hand, if the maximum temperature exceeds 2400 ° C., there is a possibility that the powder or the molded body raw material may be sublimated (decomposed). It is preferable to raise the temperature from about 1500 ° C. to the maximum temperature over 2 to 4 hours and hold at the maximum temperature for 1 to 8 hours. Sintering proceeds rapidly at 1850-1900 ° C. and is completed during the maximum temperature holding time. The pressurization condition, insufficient is the density is less than 300 kgf / cm ^2, sometimes graphite of the mold exceeds 700 kgf / cm ² is damaged, the manufacturing efficiency is not preferable.

The silicon carbide sintered body to be used has a high density, preferably a density of 2.9 g / cm ³ or more and a porosity of 1% or less, a density of 3.0 g / cm ³ or more, and a porosity of 0. .8% or less is particularly preferable. Use of a densified silicon carbide sintered body improves mechanical properties such as bending strength and fracture strength, and electrical physical properties of the resulting silicon carbide bonded body. In addition, when a silicon carbide sintered body having a high density is used, the constituent particles are reduced in size, which is preferable in terms of contamination. On the other hand, when a low-density, for example, porous silicon carbide sintered body is used, the heat resistance, oxidation resistance, chemical resistance, and mechanical strength of the silicon carbide bonded body are inferior and the bonding strength is insufficient. In some cases.

As a method for increasing the density of a silicon carbide sintered body, there is a method in which a forming step is performed in advance prior to the sintering step. This molding process is performed at a lower temperature and lower pressure than the sintering process. By carrying out this sintering step, the bulky powder can be made compact (small volume) in advance, and by repeating this step many times, it becomes easy to produce a large compact. An example of various conditions of the molding process performed in advance prior to the sintering process is shown below. The raw material powder obtained by homogeneously mixing the silicon carbide powder and the nonmetallic sintering aid is placed in a molding die, and the temperature is 80 ° C. to 300 ° C., preferably 120 ° C. to 140 ° C., pressure 50kgf / cm ² ~100kgf / cm ² at 5 to 60 minutes, preferably to obtain a molded body and pressed 20-40 minutes. The heating temperature may be appropriately determined according to the characteristics of the nonmetallic sintering aid. The density of the obtained molded body is 1.8 g / cm ² or more when a powder having an average particle size of about 1 μm is used, and 1. 1 when a powder having an average particle size of 0.5 μm is used. It is preferable to press so that it may become 5 g / cm < ² >. It is preferable for the density of the molded body to be used to be within this range because the silicon carbide sintered body can be easily densified. The molded body may be cut so that the obtained molded body is compatible with a molding die used in the sintering process.

  Total content of impurity elements (elements with atomic number of 3 or more excluding C, N, O, Si in the periodic table of the 1989 IUPAC inorganic chemical nomenclature revised edition) in the sintered silicon carbide used in this embodiment The amount of 5 ppm or less is preferable because it can be used for a process requiring high cleanliness, for example, a semiconductor manufacturing process. More preferably, it is 3 ppm or less, Especially preferably, it is 1 ppm or less. However, the impurity content by chemical analysis has only a meaning as a reference value in actual use. For example, even if the impurity content is the same, the evaluation of the contamination with respect to the silicon carbide bonded body may differ depending on whether the impurities are uniformly distributed or locally distributed. If the materials specifically exemplified above and the exemplified sintering method are used, a silicon carbide sintered body having an impurity content of 1 ppm or less can be obtained. Moreover, in order to reduce the impurity element content of the silicon carbide sintered body, the content of impurity elements contained in the raw materials used (for example, silicon carbide powder and nonmetallic sintering aid) and the inert gas is 1 ppm or less. Or a method of removing impurities by adjusting various sintering conditions such as sintering time and temperature. The impurity element here is the same as described above, and has an atomic number of 3 or more (except for C, N, O, Si) in the periodic table of the 1989 IUPAC inorganic chemical nomenclature revised edition. This element.

Other physical property values of the silicon carbide sintered body used in the present embodiment are: bending strength at room temperature of 200 to 800 kgf / mm ² , Young's modulus of 250 GPa to 500 GPa, Vickers hardness of 2000 to 3500 kgf / mm ² , Poisson's ratio of 0.14 to When the thermal expansion coefficient is 0.21, the thermal expansion coefficient is 3.8 × 10 ^{−6 to} 4.5 × 10 ⁻⁶ 1 / ° C., and the volume resistivity is 0.1 Ωcm or less, various characteristics of the obtained silicon carbide joined body are improved. Therefore, it is preferable. As the silicon carbide sintered body of the present embodiment, a silicon carbide sintered body described in Japanese Patent Application No. 9-041048 can be suitably used.

Examples of the present invention will be shown below, but the present invention is not limited to these examples.
Example 1
Preparation of silicon carbide powder: Ethyl silicate as a silicon source, phenol resin as a carbon source, N-methyldiethanolamine (MDA) as a nitrogen source, and maleic acid as a polymerization catalyst were mixed at a weight ratio shown in Table 1 to obtain a mixture. This mixture was cured and dried to obtain a solid. The obtained solid was heated and carbonized at 900 ° C. in an argon gas atmosphere to obtain a pre-fired powder. The obtained calcined powder was further calcined at 1900 ° C. in an argon gas atmosphere to obtain silicon carbide powder.
Preparation of sintered silicon carbide: The obtained silicon carbide powder is mixed with a phenol resin as a non-metallic sintering aid, and the obtained mixed powder is sintered at 2300 ° C. and 400 kgf / cm ^2. A silicon carbide sintered body was obtained. The obtained silicon carbide sintered body was sliced to obtain a wafer-like silicon carbide sintered body having a diameter of 350 mm and a thickness of 100 mm.
Evaluation: An evaluation experiment was performed based on the following criteria. The obtained results are shown in Table 1.
The nitrogen content in the silicon carbide powder was measured using a nitrogen analyzer manufactured by LECO.
The volume resistivity (× 10 ⁻³ Ωcm) of the surface of the wafer-like silicon carbide sintered body was measured at an interval of 1 cm using a Loresta apparatus manufactured by Mitsubishi Chemical Corporation. The preparation conditions and the results obtained are shown in Table 1.

FIG. 1 shows the results of measuring the ρv distribution (ρv / 10 ⁻³ Ωcm) at room temperature on the surface of the wafer-like silicon carbide sintered body at intervals of 1 cm.

(Examples 2-3)
An experiment was conducted in the same manner as in Example 1 except that the amount of MDA added was changed to the amount shown in Table 1. The preparation conditions and the results obtained are shown in Table 1.

(Comparative Examples 1 and 2)
The experiment was performed in the same manner as in Example 1 except that MDA was not used (Comparative Example 1) and 8 parts by weight of MDA was used (Comparative Example 2). The preparation conditions and the results obtained are shown in Table 1 and FIG.

(Examples 4 and 5)
Experiments were performed in the same manner as in Example 3 except that phthalimide was used instead of MDA (Example 4), and ammonium benzoate was used instead of MDA (Example 5).

  It was confirmed by the examples that a silicon carbide sintered body having a small conductivity and a stable conductivity can be obtained.

It is a figure which shows the result of having measured the volume resistivity value of the surface of the silicon carbide sintered compact at room temperature at a 1-cm space | interval.

Claims (2)

In a method for producing a silicon carbide sintered body, in which a mixed powder containing silicon carbide powder and a nonmetallic sintering aid is sintered under high temperature and pressure,
The silicon carbide powder is
A mixture comprising a silicon source, a carbon source that generates carbon by heating, a nitrogen source-containing compound, a polymerization or crosslinking catalyst, and a mixture containing 1.5 to 6 parts by weight of the nitrogen source-containing compound with respect to 100 parts by weight of the carbon source Curing and drying to obtain a solid,
A step of heating and carbonizing the solid in a non-oxidizing atmosphere to obtain a pre-fired powder;
A step of further firing the pre-fired powder in a non-oxidizing atmosphere to obtain a silicon carbide powder;
Obtained by
The method for producing a silicon carbide sintered body, wherein the nitrogen source-containing compound is a compound selected from the group consisting of N-methyldiethanolamine, phthalimide, and ammonium benzoate.   The method for producing a silicon carbide sintered body according to claim 1, wherein the carbon source has a boiling point of 150 ° C. or higher.

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