Silicon carbide powder and method for producing the same

Abstract

To provide a silicon carbide powder that uniformly contains an acceptor element and a donor element and can easily produce a silicon carbide single crystal having low resistivity and sufficient performance, and to provide a method for producing the same.SOLUTION: A method comprises the steps of: mixing a carbonaceous raw material, an inorganic silicic raw material, a boron compound, and a nitrogen compound to prepare a powder raw material containing boron at 5 ppm or more and 500 ppm or less and nitrogen at 10 ppm or more and 2,000 ppm or less; firing the powder raw material at a temperature of 2,000°C or more and 3,000°C or less; and pulverizing the fired product obtained by firing. Thus, a silicon carbide powder serving as a raw material for producing a silicon carbide single crystal having low resistivity and sufficient performance can be produced.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to silicon carbide powder and a method for producing the same.

Background Art

[0002] In SiC power devices, it is important to lower the resistance value of the device by lowering the resistivity of the single crystal substrate. As a method for lowering the resistivity of an n-type SiC single crystal substrate, increasing the nitrogen concentration is known. Further, for the purpose of suppressing basal plane dislocations and the like of the crystal, it is known to dope a silicon carbide single crystal with an acceptor element such as boron together with a donor element such as nitrogen (see Patent Document 1).

[0003] For example, in Patent Document 1, in order to dope an acceptor element and a donor element, a raw material other than silicon carbide is mixed and sublimated during single crystal production. However, when a raw material other than silicon carbide is mixed with the raw material during single crystal production, there is a problem that the sublimation rate varies between the raw materials and the quality is not stable (see Patent Documents 1 and 2).

[0004] On the other hand, a technique for producing silicon carbide powder in which boron is contained in the whole particles as a raw material for single crystal production is known (see Patent Document 3). Patent Document 3 describes a method of firing a raw material obtained by mixing an inorganic siliceous raw material, a carbonaceous raw material, and a boron compound at 2200°C or higher, and boron nitride is mentioned as the boron compound.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

[0006] However, in the technique described in Patent Document 3, although the concentration of boron, which is an acceptor element contained in silicon carbide powder, is controlled, the concentration of donor elements is not controlled.

[0007] The present invention has been made in view of such circumstances, and an object thereof is to provide silicon carbide powder that contains acceptor elements and donor elements uniformly, has a low resistivity, and can easily produce a silicon carbide single crystal having sufficient performance, and a method for producing the same. [Means for Solving the Problems]

[0008] (1) In order to achieve the above object, the present invention takes the following means. That is, the method for producing silicon carbide powder of the present invention includes a step of mixing a carbonaceous raw material, an inorganic siliceous raw material, a boron compound, and a nitrogen compound to prepare a powder raw material containing 5 ppm or more and 500 ppm or less of boron and 10 ppm or more and 2000 ppm or less of nitrogen, a step of firing the powder raw material at a temperature of 2000°C or more and 3000°C or less, and a step of pulverizing the fired product obtained by the firing.

[0009] Thereby, silicon carbide powder, which is a raw material for producing a silicon carbide single crystal having a low resistivity and sufficient performance, can be produced. Further, a raw material capable of easily producing a silicon carbide single crystal in which nitrogen and boron are uniformly contained can be obtained.

[0010] (2) Further, the method for producing silicon carbide powder of the present invention is characterized by further including a step of classifying the pulverized fired product so that 90% by mass or more of the particle size range is 20 μm or more and 1400 μm or less. Thereby, a raw material capable of suppressing the generation of carbon inclusion and silicon droplets when producing a silicon carbide single crystal by the sublimation recrystallization method can be obtained.

[0011] (3) Further, the silicon carbide powder of the present invention is characterized by containing boron in an amount of 10 ppm or more and 350 ppm or less, and nitrogen in an amount of 200 ppm or more and 3000 ppm or less.

[0012] When the concentration of nitrogen in the silicon carbide particles is 200 ppm or more and the concentration of boron is 350 ppm or less in this way, the improvement of the electrical conductivity due to nitrogen addition is not hindered when manufacturing a silicon carbide single crystal using this as a raw material. On the other hand, when the concentration of nitrogen in the silicon carbide particles is 3000 ppm or less and the concentration of boron is 10 ppm or more, the generation of transition defects occurring in the silicon carbide single crystal due to nitrogen can be suppressed. Further, since the silicon carbide particles contain nitrogen and boron, a silicon carbide single crystal in which these are uniformly contained can be manufactured by the sublimation recrystallization method.

[0013] (4) The silicon carbide powder of the present invention is characterized in that the proportion of powder having a particle size of 20 μm or more and 1400 μm or less in terms of the mesh opening size of the sieve is 90% by mass or more. Thereby, when manufacturing a silicon carbide single crystal by the sublimation recrystallization method, it can be used as a raw material capable of suppressing the generation of carbon inclusion and silicon droplets.

Advantages of the Invention

[0014] According to the present invention, a silicon carbide single crystal that uniformly contains acceptor elements and donor elements, has a low resistivity, and has sufficient performance can be easily manufactured.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0016] Hereinafter, embodiments of the present invention will be described in detail.

[0017] [Composition of Silicon Carbide Powder] The silicon carbide powder (SiC) of the present invention contains, in addition to carbon (C) and silicon (Si), boron (B) as an acceptor element and nitrogen (N) as a donor element as impurities. The silicon carbide powder doped with these elements is used as a raw material for manufacturing a silicon carbide single crystal, and is effective for uniformly containing the acceptor element and the donor element in the product.

[0018] The concentrations of nitrogen and boron in the silicon carbide powder are controlled to contain boron of 10 ppm or more and 350 ppm or less, and nitrogen of 200 ppm or more and 3000 ppm or less. By using such silicon carbide powder as a raw material, a silicon carbide single crystal that uniformly contains an acceptor element and a donor element, has a low resistivity, and has sufficient performance can be easily manufactured. Note that the performance of a power device includes on-resistance, breakdown voltage, switching frequency, and the like.

[0019] Specifically, when the concentration of nitrogen in the silicon carbide particles is 200 ppm or more and the concentration of boron is 350 ppm or less, the improvement of the electrical conductivity due to nitrogen addition is not hindered when a silicon carbide single crystal is manufactured using this as a raw material. On the other hand, when the concentration of nitrogen in the silicon carbide particles is 3000 ppm or less and the concentration of boron is 10 ppm or more, the generation of transition defects generated in the silicon carbide single crystal by nitrogen can be suppressed.

[0020] Further, it is preferable that the proportion of the powder having a particle size of 20 μm or more and 1400 μm or less in terms of the mesh opening size of the sieve is 90% by mass or more. Thereby, when manufacturing a silicon carbide single crystal by the sublimation recrystallization method, the generation of carbon inclusion and silicon droplets can be suppressed.

[0021] [Method for Producing Silicon Carbide Powder] The manufacturing method of silicon carbide powder configured as described above will be described. First, a carbonaceous raw material, an inorganic siliceous raw material, a boron compound, and a nitrogen compound are mixed to prepare a powder raw material. At that time, the input amounts of the respective raw materials are calculated and weighed so that the powder raw material contains 5 ppm or more and 500 ppm or less of boron and 10 ppm or more and 2000 ppm or less of nitrogen. Note that these elements do not reduce the purity of the silicon carbide single crystal required as a power device.

[0022] Examples of the carbonaceous raw material include crystalline carbon such as natural graphite and artificial graphite, and amorphous carbon such as carbon black, coke, and activated carbon. These may be used alone or in combination of two or more. The average particle size of the carbonaceous raw material is appropriately selected depending on the environment during firing, the state of the raw material (crystalline, amorphous), and the reactivity with the carbonaceous material.

[0023] Examples of the inorganic siliceous raw material include crystalline silica such as silica, and amorphous silica such as silica fume and silica gel. These may be used alone or in combination of two or more. However, it is preferable to use amorphous silica because of its good reactivity during firing and easy furnace control. Also, the average particle size of the inorganic siliceous raw material is appropriately selected depending on the environment during firing, the state of the raw material (crystalline, amorphous), the reactivity with the carbonaceous material, and the like.

[0024] Note that the mixing molar ratio (C / Si) of the carbonaceous raw material and the inorganic siliceous raw material is preferably selected in consideration of the environment during firing, the particle size of the raw material for silicon carbide powder, the reactivity, and the like. That is, it is selected so as to improve the yield of silicon carbide obtained by firing and to reduce the remaining amount of unreacted inorganic siliceous raw material and carbonaceous raw material.

[0025] From the perspective of the purity of silicon carbide single crystals required for power devices, boron compounds are preferably compounds containing any one or two or more of nitrogen, carbon, silicon, oxygen, hydrogen, Group 1 elements, and Group 2 elements. Specifically, boron carbide, boric acid, boron nitride, sodium borate, etc. can be mentioned. These may be used alone or in combination of two or more.

[0026] Among them, it is more preferable to use boric acid and sodium borate. This is because in addition to boric acid and sodium borate being inexpensive, boron is easily incorporated into silicon carbide. When boric acid is used, the reduction reaction of boric acid occurs near the formation temperature of silicon carbide. As a result, boric acid is changed to boron carbide, and boron is more likely to be uniformly incorporated into silicon carbide. Also, when sodium borate is used, since the decomposition temperature of sodium borate is close to the formation temperature of silicon carbide, boron is more likely to be incorporated into silicon carbide.

[0027] From the perspective of the purity of silicon carbide single crystals required for power devices, nitrogen compounds are preferably compounds containing any one or two or more of boron, carbon, silicon, oxygen, hydrogen, Group 1 elements, and Group 2 elements. Specifically, silicon nitride and boron nitride, etc. can be mentioned as nitrogen compounds. These may be used alone or in combination of two or more.

[0028] Among them, it is more preferable to use silicon nitride. When silicon nitride is used, since the decomposition temperature of silicon nitride in an inert atmosphere is high and the volatilization is small, nitrogen is more efficiently incorporated into silicon carbide.

[0029] The amount of boron compound added is determined so that the boron concentration of the powder raw material is 5 ppm or more and 500 ppm or less. The boron concentration of the powder raw material is more preferably 10 ppm or more and 350 ppm or less. The amount of nitrogen compound added is determined so that the nitrogen concentration contained in the powder raw material is 10 ppm or more and 2000 ppm or less. The nitrogen concentration of the nitrogen compound of the powder raw material is more preferably 30 ppm or more and 1700 ppm or less.

[0030] By setting the boron concentration in the powder raw material to 5 ppm or more and 500 ppm or less, the boron content in the produced silicon carbide powder is 10 ppm or more and 350 ppm or less. This is because the concentration varies due to the volatilization and uneven distribution of boron during sintering. In addition, by setting the nitrogen concentration in the powder raw material to 10 ppm or more and 2000 ppm or less, the nitrogen concentration in the produced silicon carbide powder is 200 ppm or more and 3000 ppm or less. This is because the concentration varies due to the uneven distribution of nitrogen during sintering.

[0031] The higher the boron and nitrogen concentrations in the raw materials, the higher the boron concentration in the produced silicon carbide powder. However, some of the boron volatilizes during the firing process. Therefore, the boron concentration in the raw materials and the boron concentration in the silicon carbide powder do not necessarily match, and the boron concentration in the silicon carbide powder tends to be lower than that in the raw materials.

[0032] The raw materials may be mixed by any method, such as wet mixing or dry mixing, using, for example, a Hobart mixer, a Huddle mixer, or a Henschel mixer.

[0033] Next, the powder raw material is sintered. In the sintering process, the powder raw material is sintered at a temperature of 2000°C or more and 3000°C or less to obtain a sintered product of the powder raw material. The sintered product is a lump of SiC. The sintering temperature is more preferably 2200°C or more, which is the formation temperature of α-SiC, and 2730°C or less, which is the melting point of SiC.

[0034] The firing can be carried out, for example, by the Acheson method using an Acheson furnace. By using an Acheson furnace, the powder raw material can be fired at an appropriate temperature, and silicon carbide powder with a particle size suitable for sublimation recrystallization can be easily obtained. FIG. 1 is a cross-sectional view showing the configuration of an Acheson furnace. As shown in FIG. 1, the Acheson furnace 1 is a box-shaped indirect resistance heating furnace with an open top. In an indirect resistance heating furnace, heating is performed by a heating element that generates heat through an electric current, rather than by directly passing an electric current through the object to be heated.

[0035] The heating element 2 is provided in a rod shape connecting the electrode cores 4 provided at both ends in the current flow direction inside the Acheson furnace main body 1a. The material of the heating element 2 may be any material that has the property of conducting electricity, such as graphite powder or carbon rod. The shape of the heating element 2 is not particularly limited, and examples thereof include powder and lump. The heating element 2 is provided in a rod shape as a whole connecting the electrode cores provided at both ends in the current flow direction of the Acheson furnace. Examples of the rod shape here include a cylindrical shape and a rectangular column shape. The heating element 2 heats the object to be heated by generating heat due to the current flowing from the electrode cores 4 provided on the inner surface of the Acheson furnace 1.

[0036] The heated material is a powdered raw material, which is sintered by being filled inside an Acheson furnace. The heated material filled on the lower side of the furnace, separated by the heating element 2, is called the filling material 3a, and the heated material filled on the upper side is called the covering material 3b. In consideration of the difference in thermal conductivity, the composition ratio of each raw material constituting the powdered raw material may be different between the filling material 3a and the covering material 3b.

[0037] When using an Acheson furnace, first, the Acheson furnace body 1a is filled with furnace loading material 3a, and the heating element 2 is arranged so as to linearly connect the electrode cores. Then, the covering material 3b is filled so as to cover the heating element 2. Then, power is supplied to the electrode cores to reach an appropriate temperature, and the object to be heated is heated, causing the reaction shown in formula (1) below, and a mass of silicon carbide is produced in the furnace. SiO2+3C→SiC+2CO…(1)

[0038] Boron (e.g., boric acid) reacts with a part of SiO2 at around 1500 °C, and since boron is incorporated, it is considered that silicon carbide crystals with uniformly incorporated boron grow (B substitutes for Si). Nitrogen (e.g., silicon nitride) decomposes nitrogen compounds at around 1500 °C when silicon carbide begins to form, and it is considered that silicon carbide crystals with uniformly incorporated nitrogen grow (N substitutes for C). Since silicon carbide particles are often aggregates of crystallites, the coarser the particles, the stronger this tendency becomes.

[0039] After energization, a massive fired product made of silicon carbide is formed in the furnace. Then, it is cooled until the furnace reaches room temperature. Then, the obtained fired product made of silicon carbide is pulverized. Examples of the pulverization method include pulverization using a top grinder, a disk grinder, a jet mill, a ball mill, etc.

[0040] Thereafter, it is preferable to classify the pulverized material so as to fall within a desired particle size range. Classification using a sieve is the simplest and preferable method. However, the classification is not limited to the method using a sieve and may be either dry or wet. Also, as a dry classification method, for example, a centrifugal classification method using an air flow can be used.

[0041] The classification is preferably carried out such that particles having a particle size exceeding 100 μm are included. Further, it is more preferable that 90 mass% or more of the silicon carbide powder is classified so that the particle size range according to the mesh opening size of the sieve is 20 to 1400 μm, and it is most preferable that the above particle size range is 30 to 1000 μm. Fine particles are likely to cause carbon inclusion due to fine carbon powder, and coarse particles tend to have a small specific surface area, resulting in a slow sublimation rate and an increased tendency to generate silicon droplets. Also, the pulverized material may be appropriately washed with hydrochloric acid or the like to remove contamination caused by pulverization.

[0042] The silicon carbide powder of the present invention thus obtained is composed of silicon carbide particles containing boron and nitrogen as impurities, and boron and nitrogen are contained in the whole of these silicon carbide particles. Here, "boron and nitrogen are contained in the whole" means that boron and nitrogen are contained from the surface layer to the central part of the particles.

[0043] In a preferred embodiment of the silicon carbide powder of the present invention, boron and nitrogen are contained almost uniformly from the surface layer to the central part of the silicon carbide particles constituting the silicon carbide powder. Here, "almost uniform" means that the difference between the part with the highest concentration and the part with the lowest concentration is within 70% based on the higher concentration.

[0044] [Examples] Each example and each comparative example using the method for producing silicon carbide powder will be described below. However, the present invention is not limited by these examples.

[0045] First, an inorganic siliceous raw material, a carbonaceous raw material, a boron and a nitrogen compound were mixed to prepare a powder raw material. Specifically, a boron compound and a nitrogen compound were further added to 700 kg of carbon black and amorphous silica adjusted so that the weight ratio of amorphous silica to carbon black was 2, and then they were mixed using a mortar mixer. In this way, the powder raw materials of Examples 1 to 7 and Comparative Examples 1 to 3 were prepared.

[0046] In each example and each comparative example, the added boron compound and nitrogen compound are as shown in Table 1 below. As shown in Table 1, in Comparative Example 1, no boron compound and nitrogen compound were added.

[0047]

Table 1

[0048] The particle sizes of the respective raw materials constituting the powder raw material are as follows. Inorganic silicate raw material: Amorphous silica, average particle size 2 mm or less Carbonaceous raw material: Carbon black, average particle size 2 mm or less Boron compound: Boric acid powder, particle size range 10 μm to 1000 μm Boron carbide powder, particle size range 1 μm to 45 μm Sodium tetraborate powder, particle size range 10 μm to 1000 μm Nitrogen compound: Silicon nitride powder, particle size range 1 μm to 45 μm Boron-nitrogen compound: Boron nitride powder, particle size range 1 μm to 45 μm

[0049] Next, using an Acheson furnace, the powder raw materials were fired at 2600 °C for 6 hours to obtain a fired product. At this time, graphite powder was used as the heating element. Next, the fired product was recovered at two locations, the upper and lower parts of the electrode core. Further, in each fired product, samples at three points, namely the unreacted raw material side (analysis point 6), the position close to the heating element (analysis point 8), and the center part (analysis point 7), were collected by scraping with a stainless steel spatula. That is, six samples were collected in each example and each comparative example. Figure 2 is a cross-sectional view showing the positions of the respective analysis points in the fired product. For each sample, the weight was measured and silicon carbide powder was obtained by pulverizing using a jaw crusher and a ball mill.

[0050] Next, the boron concentration, nitrogen concentration, and oxygen concentration in each part sample were measured. The boron concentration was measured by wet analysis. More specifically, after preparing a measurement solution by alkali fusion method or pressurized acid decomposition, the boron concentration was measured using an ICP emission spectroscopic analyzer "ULTIMA2" (manufactured by Horiba, Ltd.). Also, the nitrogen concentration and oxygen concentration were measured using an oxygen-nitrogen-hydrogen simultaneous analyzer "TCH-600" (manufactured by LECO Corporation).

[0051] Table 2 shows the boron and nitrogen concentrations and evaluation results at each analysis point in each example and each comparative example. Note that the concentrations at each analysis point shown in Table 2 are the average values of the concentrations at the upper and lower parts of the electrode core. In Table 2, "inside" refers to the concentration at analysis point 8, which is close to the heating element, "outside" refers to the concentration at analysis point 6, which is far from the heating element, and "center" refers to analysis point 7, which is at the center of the other analysis points 6 and 8. Also, the weighted average concentration shown in Table 2 is the average value obtained by weighting the measured concentration by the weight of the sample in which the concentration was measured.

[0052]

Table 2

[0053] As shown in Table 2, in Comparative Example 1 where no boron compound and nitrogen compound are added, there are variations in the measured values of boron concentration and nitrogen concentration among the analysis points, and concentration gradients of boron and nitrogen are generated. Also, in Comparative Example 2, there are variations in the measured values of nitrogen concentration among the analysis points, and a concentration gradient of nitrogen is generated. Further, in Comparative Example 3, there are variations in the measured values of boron concentration among the analysis points, and a concentration gradient of boron is generated.

[0054] On the other hand, in Examples 1 to 7 where the boron concentration of the silicon carbide powder is 350 ppm or less and the nitrogen concentration is 3000 ppm or less, the variations in the measured values of boron concentration and nitrogen concentration at each analysis point are small, and boron and nitrogen are uniformly contained.

[0055] From the above, it was confirmed that by adjusting the addition amounts of the respective raw materials so that the boron concentration of the silicon carbide powder is 350 ppm or less and the nitrogen concentration is 3000 ppm or less, silicon carbide powder in which boron and nitrogen are uniformly incorporated can be produced.

Description of Symbols

[0056] 1 Acheson furnace 1a Acheson furnace main body 2 Heating element 3a Furnace filling material 3b Covering material 4 Electrode core 5 Fired product 6 Analysis point in the outer part 7 Analysis point in the central part 8 Analysis point in the inner part

Claims

1. A step of preparing a powder raw material containing 24.45 ppm or more and 252.49 ppm or less of boron and 35.93 ppm or more and 1181.64 ppm or less of nitrogen by mixing a carbonaceous raw material, an inorganic siliceous raw material, a boron compound, and a nitrogen compound; A step of firing the powder raw material at a temperature of 2000 °C or higher and 3000 °C or lower; A step of pulverizing the fired product obtained by the firing, the method for producing silicon carbide powder being characterized by including these steps.

2. The method for producing silicon carbide powder according to claim 1, further including a step of classifying the pulverized fired product so that the particle size range of 90% by mass or more is 20 μm or more and 1400 μm or less.

3. As the silicon carbide powder, 10 ppm or more and 350 ppm or less of boron and 200 ppm or more and 3000 ppm or less of nitrogen, the method for producing silicon carbide powder according to claim 1 or 2, for obtaining the silicon carbide powder.

4. The method for producing silicon carbide powder according to claim 3, wherein the proportion of the powder having a particle size of 20 μm or more and 1400 μm or less in terms of the mesh opening size of the sieve is 90% by mass or more.

Images