Silicon nitride sintered body

Abstract

This silicon nitride sintered body contains at least one element selected from the group consisting of Ti, Ge, Zr, Ag, Ba, and Hf in a solid solution at a concentration of 0.01-0.15 mass% in silicon nitride particles constituting the sintered body. According to the present invention, it is possible to provide a silicon nitride sintered body in which a reduction in volume resistivity at high temperatures is extremely small compared to a conventional silicon nitride sintered body with respect to volume resistivity at room temperature.

Description

Technical Field

[0001] The present invention relates to a new silicon nitride sintered body. Specifically, the present invention provides a silicon nitride sintered body in which the decrease in volume resistivity at high temperature is extremely small relative to the volume resistivity at room temperature. Background Art

[0002] Silicon nitride sintered bodies have excellent properties such as high thermal conductivity, high insulation, and high strength, and are thus attracting attention as various industrial materials. The above-mentioned silicon nitride sintered body exhibits a high volume resistivity at room temperature, but the volume resistivity decreases significantly at high temperature. Therefore, when used as a substrate for power modules or the like with a use temperature of 200°C or higher, it may be difficult to maintain sufficient insulation tolerance.

[0003] Conventionally, a technique has been reported in which, in order to suppress the decrease in volume resistivity at high temperature, the electrical insulation at high temperature is improved by heating a silicon nitride sintered body circuit board having a circuit formed therein in a nitrogen atmosphere (see Patent Document 1). In addition, a silicon nitride sintered body containing a specified amount of Mg or a specified amount of Mg and Y 2 O 3 has been disclosed to have high electrical insulation at high temperature. As its mechanism of action, it is presumed that the above elements are dissolved in the crystal grains of silicon nitride or MgO is dissolved and covers the crystal grains constituting the sintered body (see Patent Document 2).

[0004] However, in any of the above silicon nitride sintered bodies, the suppression of the decrease in volume resistivity at high temperature is not sufficient, and there is still room for further improvement. Prior Art Documents

[0005] Patent Documents Patent Document 1: Japanese Patent Application Laid-Open No. 2001-77245 Patent Document 2: Japanese Patent Application Laid-Open No. 2001-64080 Summary of the Invention Problems to be Solved by the Invention

[0006] Therefore, an object of the present invention is to provide a silicon nitride sintered body in which the decrease in volume resistivity at high temperature is extremely small compared to conventional silicon nitride sintered bodies relative to the volume resistivity at room temperature. Means for Solving the Problems

[0007] In order to solve the above problems, the present inventors repeatedly conducted in-depth studies and found that by dissolving a specific element in a specific amount in the silicon nitride crystal grains constituting the silicon nitride sintered body, it is possible to effectively suppress the decrease in volume resistivity at high temperature, thereby completing the present invention.

[0008] That is, according to the present invention, there is provided a silicon nitride sintered body, characterized in that at least one element selected from Ti, Ge, Zr, Ag, Ba, and Hf is solid-soluted in the silicon nitride particles constituting the sintered body at a concentration of 0.01 to 0.15% by mass.

[0009] In addition, the total concentration of P, Cr, Mn, and Fe elements solid-soluted in the above-mentioned silicon nitride particles of the above-mentioned silicon nitride sintered body is adjusted to 0.05% by mass or less, which is preferable because it can further suppress the decrease in volume resistivity at high temperatures. Advantages of the Invention

[0010] The silicon nitride sintered body of the present invention can maintain a high volume resistivity at room temperature even at a practical use temperature as a substrate for a power module or the like, and can maintain sufficient insulation resistance when used for this purpose. Detailed Embodiments

[0011] In the present invention, the silicon nitride sintered body as the object preferably contains silicon nitride in a proportion of 80% by mass or more, particularly preferably 90% by mass or more. In addition, the total content (mass) of aluminum element is preferably 800 ppm or less. Thereby, a silicon nitride sintered body having high thermal conductivity can be formed. The total content of the above-mentioned aluminum element is preferably 500 ppm or less, more preferably 200 ppm or less.

[0012] The greatest feature of the silicon nitride sintered body of the present invention is that at least one element selected from Ti, Ge, Zr, Ag, Ba, and Hf (hereinafter also referred to as specific first element) is solid-soluted in the silicon nitride particles constituting the sintered body at a concentration of 0.01 to 0.15% by mass, preferably 0.03 to 0.12% by mass. Within the above-mentioned solid-solution concentration range, the solid-solution amount of each specific first element is preferably 0.01% by mass or more, more preferably 0.03% by mass or more. The above-mentioned specific first element may be used alone or in combination of two or more. In the case of using two or more specific first elements in combination, the content of the above-mentioned specific first element refers to the total content of two or more elements.

[0013] Among the above-mentioned specific first elements, at least one element selected from Zr and Hf is particularly preferable.

[0014] In the present invention, the confirmation of the existence of elements solid-soluted in silicon nitride particles and the measurement of their concentrations are specifically described in the following examples, and can be carried out by the following methods.

[0015] (1) Confirmation of the existence of elements solid-soluted in silicon nitride particles "Solid solution" means a state in which, for example, in silicon nitride particles, elements other than nitrogen and silicon do not precipitate as a simple substance or a compound but exist inside the silicon nitride particles. In this state, the elements existing in the silicon nitride particles can be confirmed by observing whether there are precipitates inside the silicon nitride particles using a transmission electron microscope (TEM) and analyzing the elements inside the above-mentioned silicon nitride particles using an energy dispersive X-ray spectroscopy device (EDS) mounted on the TEM.

[0016] (2) Measurement of the concentration of elements dissolved in silicon nitride particles The concentration of elements dissolved in silicon nitride particles can be measured by qualitatively and quantitatively analyzing the elements using EDS.

[0017] The silicon nitride sintered body of the present invention can significantly suppress the decrease in volume resistivity at high temperatures by containing a specific first element in the silicon nitride particles constituting the sintered body at the above-mentioned specific concentration. Even in applications at high temperatures such as the substrate for a power module as described above, a high volume resistivity can be maintained. That is, when the concentration of the specific first element in the silicon nitride particles constituting the sintered body is lower than the above range, the effect of suppressing the decrease in volume resistivity of the silicon nitride sintered body at high temperatures is lacking, and it is difficult to exhibit a high volume resistivity at high temperatures. In addition, when the concentration of the specific first element is higher than the above range, although the effect of suppressing the decrease in volume resistivity at high temperatures is exhibited, there is a tendency to cause problems such as a decrease in the thermal conductivity and insulation of the silicon nitride sintered body.

[0018] In addition, the above-mentioned specific first element is preferably uniformly distributed in each silicon nitride particle constituting the sintered body. Regarding the silicon nitride particles observed in the above-mentioned TEM, it is preferable that the specific first element exists in 80% or more, preferably 90% or more of the particles in terms of the number ratio.

[0019] As long as the silicon nitride sintered body of the present invention satisfies the above configuration, there are no particular limitations on other configurations. Preferred embodiments are as follows.

[0020] In the silicon nitride sintered body of the present invention, it is preferable that there are few solid solution elements other than the above-mentioned specific first element. Among them, the total concentration of elements such as P, Cr, Mn, and Fe dissolved in the above-mentioned silicon nitride particles (hereinafter also referred to as specific second elements) is preferably 0.05% by mass or less, and particularly preferably 0.02% by mass or less.

[0021] The silicon nitride particles constituting the silicon nitride sintered body of the present invention are preferably crystalline particles of silicon nitride. The average particle size (average major axis) is not particularly limited, and is preferably 1 to 5 μm, and particularly preferably 3 to 5 μm, because it can impart high thermal conductivity to the silicon nitride sintered body.

[0022] In addition, the average aspect ratio of the above-mentioned silicon nitride particles is not particularly limited, but in order to impart high strength to the silicon nitride sintered body, it is preferably 1.2 to 2.5, and particularly preferably about 1.7 to 2.5.

[0023] In addition, a compound phase derived from an additive component used in sintering may be present at the grain boundary phase or triple point of the silicon nitride particles constituting the silicon nitride sintered body.

[0024] The shape of the silicon nitride sintered body of the present invention is not particularly limited, but it is plate-shaped for use as a substrate in the above-mentioned power module or the like. The thickness of the plate-like body is generally 0.1 to 1 mm, and particularly about 0.2 to 0.7 mm.

[0025] The manufacturing method of the silicon nitride sintered body of the present invention is not particularly limited. As a representative manufacturing method, the following manufacturing method can be cited, which is characterized by including: a step of preparing silicon nitride powder in which a specific first element is dissolved, and a step of firing the silicon nitride powder to obtain a sintered body.

[0026] The above manufacturing method is characterized in that instead of adding a specific first element during the sintering of silicon nitride powder, the specific first element is previously dissolved in the silicon nitride powder. That is, during sintering, when a specific first element is added to the silicon nitride powder, this element is difficult to dissolve in the silicon nitride particles constituting the sintered body and is likely to exist at the grain boundary. Therefore, in the silicon nitride sintered body obtained by this method, the solid solution concentration of the specific first element in the silicon nitride particles constituting the sintered body is insufficient, and it is difficult to maintain a high volume resistivity at high temperatures.

[0027] The method for obtaining the silicon nitride powder in which the specific first element is dissolved is not particularly limited. For example, it can be cited: when manufacturing silicon nitride powder by the combustion synthesis method, adding a compound of the specific first element to the raw material silicon, specifically adding oxides, nitrides, etc. and reacting; after adding a compound of the specific first element to the silicon nitride powder or bulk material, heating it at 600 to 1600 °C to dissolve it, and pulverizing it as needed.

[0028] From the viewpoint of easily manufacturing silicon nitride powder in which an element is dissolved, the method of the above combustion synthesis method is preferably used. This method is a method of using silicon powder as a raw material, forcibly igniting a part of the raw material powder in a nitrogen atmosphere, and synthesizing silicon nitride by the self-heating of the raw material compound. The combustion synthesis method is a well-known method, and for example, Japanese Patent Laid-Open No. 2000-264608, International Publication No. 2019 / 167879, etc. can be referred to.

[0029] The solid solution concentration of the specific first element in the finally obtained silicon nitride sintered body can be adjusted by the concentration of the specific first element dissolved in the above-mentioned silicon nitride powder.

[0030] On the other hand, as a method for reducing the solid solution concentration of the above-mentioned specific second element in the silicon nitride powder and thus finally in the silicon nitride sintered body, the following can be cited: as a raw material for sintering, a method of manufacturing silicon nitride powder from a high-purity raw material that contains as little as possible elements other than the specific first element; as the material of the grinding medium, or the housing container, the conveying pipeline, etc. used in the manufacture of silicon nitride powder, a method of preventing these from coming into direct contact with the material containing the above-mentioned specific second element.

[0031] In the method for manufacturing the above-mentioned silicon nitride sintered body, the crystal form of the silicon nitride powder used is not particularly limited, and α-type and β-type silicon nitride powders can be used alone or in combination.

[0032] The average particle size of the above-mentioned silicon nitride powder is preferably 0.5 to 3 μm, and particularly preferably 0.7 to 1.7 μm. In addition, the oxygen content of the silicon nitride powder is preferably about 0.5 to 1.5% by mass.

[0033] In the method for manufacturing the above-mentioned silicon nitride sintered body, the method of firing the silicon nitride powder to manufacture the silicon nitride sintered body is not limited, and the conventional methods can be adopted without particular limitation.

[0034] For example, the following can be cited: a method of firing a molded body of a raw material mixture containing silicon nitride powder and a sintering aid at a temperature of 1200 °C or higher and lower than the silicon nitride decomposition temperature under the above pressure for 2 to 30 hours under the supply of an inert gas such as nitrogen or argon at a pressure of 0 to 0.1 MPa·G.

[0035] As the above-mentioned sintering aid, known sintering aids such as oxides of yttrium, magnesium, cerium, and carbonitrides can be used. The amount used is preferably 5 to 20 parts by mass, and particularly preferably 7 to 10 parts by mass, relative to 100 parts by mass of the silicon nitride powder.

[0036] In addition, when manufacturing the molded body, binders such as polyvinyl alcohol, polyvinyl butyral, methyl cellulose, alginic acid, polyethylene glycol, carboxymethyl cellulose, ethyl cellulose, and acrylic resins can be used. The content thereof is preferably 1 to 30 parts by mass relative to 100 parts by mass of the silicon nitride powder, and the ratio can be appropriately determined according to the molding method.

[0037] Furthermore, when molding using a binder, a slurry is prepared using a solvent such as water, alcohol, or hydrocarbon, and is molded into a sheet shape or the like by a molding method such as a doctor blade method. If necessary, it can be dried and degreased for firing.

[0038] When manufacturing a substrate using the method for manufacturing a silicon nitride sintered body of the present invention, after the above-mentioned firing, the surface can be cleaned by sandblasting with abrasive grains such as silicon nitride powder, and the surface can also be polished. Example

[0039] Hereinafter, in order to more specifically illustrate the present invention, examples are shown, but the present invention is not limited thereto.

[0040] In addition, in the examples and comparative examples, various measurement methods are carried out by the following methods.

[0041] (1) Concentration of solid-solution elements in the silicon nitride sintered body The concentration of solid-solution elements in the silicon nitride sintered body is determined by TEM-EDS measurement. This device uses a JEOL TEM JEM-2100 equipped with an EDS JED-2300 made by JEOL. TEM observation is carried out on any 30 silicon nitride particles in the silicon nitride sintered body, and qualitative and quantitative analysis of the elements is carried out by EDS point analysis. The average value of the analysis values of the 30 particles is used as the concentration of solid-solution elements in the silicon nitride sintered body. However, among the 30 silicon nitride particles observed by TEM, even if one silicon nitride particle containing precipitates inside the particle is confirmed, the concentration of solid-solution elements cannot be measured. It should be noted that the acceleration voltage of TEM is 200 kV, and the EDS measurement time for each particle is 1000 seconds.

[0042] (2) Ratio of silicon nitride particles solid-solved with a specific first element in the silicon nitride sintered body Regarding the ratio of silicon nitride particles solid-solved with a specific first element in the silicon nitride sintered body, it is calculated as the ratio of the number of silicon nitride particles in which at least one specific first element is detected in the above EDS analysis to the 30 silicon nitride particles observed by the above TEM. However, for silicon nitride particles in which precipitates are confirmed inside the particles by TEM observation, they are counted as particles that are not solid-solved. It should be noted that the detection limit of the specific first element in the above EDS analysis is 0.005 mass%.

[0043] (3) Total content of aluminum element in the silicon nitride sintered body The total content of aluminum element in the silicon nitride sintered body is measured using an inductively coupled plasma emission spectrometer ("iCAP 6500 DUO" manufactured by Thermo Fisher Scientific).

[0044] (4) Volume resistivity at room temperature and 450 °C The volume resistivity at room temperature (25 °C) and 450 °C is measured using an evaluation device made by combining a microammeter and an electric furnace, and the volume resistivity is obtained according to JIS C 2141. The measurement is carried out in a nitrogen atmosphere under an applied voltage of 500 V. It should be noted that for the measurement at 450 °C, after heating to 450 °C at a heating rate of 20 °C / minute, it is held for 15 minutes and then measured.

[0045] Examples 1 to 22, Comparative Examples 1 and 2 A mixed powder in which silicon powder (semiconductor grade, average particle size 5 μm) is mixed at a ratio of 80% by mass and silicon nitride powder (average particle size 1.5 μm) as a diluent is mixed at a ratio of 20% by mass is blended with oxides of the elements shown in Table 1 at the ratios shown in Table 1 to obtain a raw material powder.

[0046] The above raw material powder is filled into a reaction vessel to form a raw material powder layer. Next, the reaction vessel is placed in a pressure-resistant closed reactor having an ignition device and a gas supply / discharge mechanism. After the inside of the reactor is depressurized and degassed, nitrogen is supplied for nitrogen replacement. Then, nitrogen is slowly supplied to raise the pressure to 0.7 MPa. When the specified pressure is reached (at ignition), the bulk density of the raw material powder is 0.5 g / cm 3 . Then, the end of the raw material powder in the reaction vessel is ignited to carry out a combustion synthesis reaction to obtain a massive product formed of silicon nitride. An appropriate amount of the obtained massive product is put into a vibration mill and subjected to fine pulverization for 6 hours. It should be noted that a conventional device and method are used for the vibration mill and the fine pulverization method. However, as a countermeasure against metal contamination, a polyurethane lining is provided inside the mill, and balls mainly composed of silicon nitride are used as the pulverization medium. In addition, 1% by mass of ethanol is added as a pulverization aid immediately before the start of fine pulverization to carry out fine pulverization, and silicon nitride powder is obtained.

[0047] The obtained silicon nitride powder is composed of β-type silicon nitride, and in the above fine pulverization, the average particle size is about 1.8 μm.

[0048] Weigh 100 parts by mass of the silicon nitride powder obtained by the above method, a compound Y without oxygen bonds 2 Si 4 N 6 2 parts by mass of C, MgSi 4 N 65 parts by mass of C and 3 parts by mass of yttrium oxide were used. Water, a resin pot, and silicon nitride balls as a dispersion medium were used, and they were pulverized and mixed in a ball mill for 24 hours. It should be noted that water was weighed in advance to make the concentration of the slurry 60% by mass, and it was put into the resin pot. After pulverization and mixing, 22 parts by mass of an aqueous resin binder was added, and it was mixed for another 12 hours to obtain a paste-like molding composition. Then, the molding composition was formed into a sheet by a doctor blade method to obtain a sheet-shaped molded body with a width of 75 cm and a thickness of 0.3 mmt. Then, the obtained sheet-shaped molded body was degreased at a temperature of 550 °C in dry air, and the degreased molded body was put into a firing container and fired at 1780 °C for 9 hours under a nitrogen atmosphere and a pressure of 0.02 MPa·G to obtain a silicon nitride sintered body. The total content of aluminum element in the obtained silicon nitride sintered body was 200 ppm. In addition, for the obtained silicon nitride sintered body, the solid solution amount of each element and the volume resistivity at room temperature and 450 °C are shown in Table 2.

[0049] [Table 1]

[0050] [Table 2]

Claims

1. A silicon nitride sintered body, characterized in that, at least one element selected from Ti, Ge, Zr, Ag, Ba, and Hf is solid-soluted in the silicon nitride particles constituting the sintered body at a concentration of 0.01 to 0.15% by mass.

2. The silicon nitride sintered body according to claim 1, wherein, the total concentration of P, Cr, Mn, and Fe elements solid-soluted in the silicon nitride particles is 0.05% by mass or less.

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