Silicon nitride sintered body

Abstract

This silicon nitride sintered body is characterized in that the lattice strain of silicon nitride crystal grains constituting the silicon nitride sintered body is less than 0.003. According to the present invention, it is possible to provide a silicon nitride sintered body which is capable of forming a highly reliable silicon nitride circuit board having excellent temperature cycle characteristics.

Description

Technical Field

[0001] This invention relates to a silicon nitride sintered body. Background Technology

[0002] Silicon nitride sintered bodies possess excellent properties such as high thermal conductivity, high insulation, and high strength, making them highly sought-after ceramic materials in various industrial fields. For example, they are used as circuit boards (silicon nitride circuit boards) for power modules in automotive or semiconductor applications.

[0003] Silicon nitride circuit boards are manufactured, for example, by bonding a metal plate (such as a copper plate) onto a silicon nitride substrate and then etching the metal plate to form a silicon nitride circuit board. Semiconductor components are then mounted on the silicon nitride circuit board for use.

[0004] For example, Patent Document 1 describes an invention concerning a silicon nitride circuit board, characterized in that, as a circuit board with excellent temperature cycling characteristics, a metal cladding material with a thickness of 1 mm or more is bonded to at least one surface of the silicon nitride circuit board, and a plurality of semiconductor elements are mounted on the silicon nitride circuit board.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2003-168770 Summary of the Invention

[0008] The problem that the invention aims to solve

[0009] In recent years, with the increasing density and power of electronic devices and semiconductor devices, the demand for silicon nitride circuit boards with excellent temperature cycling characteristics and high reliability has been growing. Temperature cycling characteristics refer to the properties of a sample as an indicator of its durability when subjected to repeated temperature changes from low temperature to high temperature and back to low temperature.

[0010] On the other hand, conventional silicon nitride circuit boards sometimes suffer damage to a portion of their surface when subjected to repeated temperature changes, leaving room for improvement from a reliability standpoint.

[0011] Therefore, in this invention, a silicon nitride sintered body is provided, which can form a silicon nitride circuit board with better temperature cycling characteristics than before.

[0012] Methods for solving problems

[0013] To achieve the above objectives, the inventors conducted repeated and in-depth research. They discovered that by focusing on the lattice strain of silicon nitride sintered bodies, which had not been previously studied, the aforementioned problems could be solved using silicon nitride sintered bodies with a lattice strain less than 0.003, thus completing this invention.

[0014] The main points of this invention are as follows [1] to [4].

[0015] [1] A silicon nitride sintered body, characterized in that the lattice strain (lattice distortion) of the silicon nitride grains constituting the silicon nitride sintered body is less than 0.003.

[0016] [2] The silicon nitride sintered body as described in [1] above, wherein the dissolved oxygen concentration of the silicon nitride grains is less than 500 ppm.

[0017] [3] The silicon nitride sintered body as described in [1] or [2] above, wherein the average grain size of the silicon nitride grains is 1 to 15 μm.

[0018] [4] The silicon nitride sintered body as described in any one of [1] to [3] above is a heat dissipation substrate.

[0019] Invention Effects

[0020] According to the present invention, a silicon nitride sintered body can be provided, which can form a silicon nitride circuit board with excellent temperature cycling characteristics and high reliability. Attached Figure Description

[0021] Figure 1 This is a diagram used to illustrate the vacancies in the silicon nitride lattice.

[0022] Figure 2 This is a diagram used to illustrate vacancy cluster defects in the silicon nitride lattice.

[0023] Figure 3 This is an example of a TEM (transmission electron microscopy) dark-field image containing silicon nitride particles with helical dislocations. Detailed Implementation

[0024] [Silicon nitride sintered body]

[0025] The silicon nitride sintered body of the present invention will be described in detail below.

[0026] <Lattice strain>

[0027] In the silicon nitride sintered body of the present invention, the lattice strain of the silicon nitride grains constituting the silicon nitride sintered body is less than 0.003. When the lattice strain is less than 0.003, the temperature cycling characteristics of the silicon nitride circuit board manufactured using the silicon nitride sintered body are improved. From the viewpoint of further improving temperature cycling characteristics, the lattice strain of the silicon nitride grains is preferably 0.002 or less, more preferably 0.001 or less. The lower limit value of the lattice strain of the silicon nitride grains is not particularly limited, but ideally it is 0. The lattice strain of the silicon nitride grains can be measured by X-ray diffraction. Details of the measurement method are described in the examples.

[0028] The reason for improving the temperature cycling characteristics of silicon nitride sintered bodies by reducing the lattice strain of silicon nitride grains is presumed as follows.

[0029] The lattice strain of silicon nitride sintered bodies is related to the vacancies formed by dissolved oxygen.

[0030] In silicon nitride crystals (Si3N4) that constitute silicon nitride sintered bodies, dissolved oxygen is known to be present due to the type of raw materials or manufacturing methods. When oxygen atoms (O) replace nitrogen atoms (N) that constitute silicon nitride crystals to form a substitutional solid solution, vacancies of silicon atoms (Si) are created to satisfy electroneutrality.

[0031] For example, Figure 1 The diagram shows a silicon nitride lattice where four oxygen atoms replace four nitrogen atoms, resulting in the loss of one silicon atom and the creation of a vacancy.

[0032] In silicon nitride sintered bodies, it is believed that the number of vacancies generated is also the same when the dissolved oxygen content is the same. However, it is believed that the durability of the substrate differs between the case where vacancies exist individually and the case where several vacancies aggregate to form vacancy cluster defects.

[0033] For example, in Figure 2 The left and right images schematically represent silicon nitride lattices with the same number of vacancies (5). The left image shows 5 vacancies dispersed. On the other hand, the right image, as shown in A, shows several vacancies clustered together to form vacancy cluster defects. These vacancy cluster defects are believed to be the cause of pits and voids on the silicon nitride substrate surface caused by etching during circuit pattern formation. If temperature cycling tests are performed with such pits or voids present, localized damage to the silicon nitride substrate will occur, starting from these defects.

[0034] It is believed that by reducing vacancy cluster defects, the damage to the silicon nitride substrate caused by these defects can be suppressed, thereby improving temperature cycling characteristics.

[0035] Since lattice strain occurs near vacancy cluster defects, the lattice strain decreases as the number of vacancy cluster defects decreases. Therefore, it is believed that a smaller lattice strain improves the temperature cycling characteristics of the silicon nitride sintered body of the present invention during the formation of silicon nitride circuit substrates.

[0036] The presence of vacancy cluster defects in silicon nitride sintered bodies can be confirmed by transmission electron microscopy (TEM). That is, vacancy cluster defects can be observed by TEM, and their manifestations include, for example, dislocation loops or spiral dislocations. Figure 3 An example of a dark-field TEM image of silicon nitride particles with helical dislocations is shown. It can be observed that... Figure 3 Inside the silicon nitride particles in the central part, there are spiral dislocations that appear as spiral-shaped white bright lines.

[0037] There are no particular limitations on the method for adjusting the lattice strain value. For example, it can be adjusted according to the conditions during the manufacture of silicon nitride sintered bodies, especially the cooling conditions after sintering.

[0038] <Dissolved oxygen>

[0039] The dissolved oxygen concentration of the silicon nitride grains constituting the silicon nitride sintered body of the present invention is not particularly limited, but is, for example, 2500 ppm or less. When the dissolved oxygen concentration is 2500 ppm or less, the thermal conductivity of the silicon nitride sintered body can be improved. On the other hand, from the viewpoint of improving thermal conductivity, the lower the dissolved oxygen concentration of the silicon nitride grains constituting the silicon nitride sintered body, the better, but in practice it is 100 ppm or more.

[0040] When the dissolved oxygen concentration is low, there are fewer vacancies in Si atoms, resulting in higher thermal conductivity. Once vacancies are generated, they become scattering sources of lattice vibrations (phonons), leading to a decrease in thermal conductivity. Therefore, generally speaking, the less dissolved oxygen in a silicon nitride sintered body, the fewer phonon scattering sources there are and the higher the thermal conductivity tends to be.

[0041] From this perspective, the dissolved oxygen concentration of the silicon nitride grains constituting the silicon nitride sintered body of the present invention is preferably less than 500 ppm, more preferably less than 450 ppm, and even more preferably less than 400 ppm.

[0042] The silicon nitride sintered body of the present invention, by reducing lattice strain as described above and reducing dissolved oxygen concentration as described above, can improve temperature cycling characteristics and heat dissipation, and can provide a silicon nitride sintered body with extremely excellent physical properties.

[0043] In this invention, the dissolved oxygen concentration refers to the concentration of oxygen dissolved inside the silicon nitride sintered body (internal oxygen), excluding adsorbed oxygen (external oxygen) that inevitably exists on the surface of the sintered body.

[0044] The dissolved oxygen concentration can be adjusted according to the various raw materials used to manufacture silicon nitride powder, the type and amount of sintering aids used in manufacturing sintered bodies, and the firing conditions.

[0045] It should be noted that the dissolved oxygen concentration can be determined by the method described in the examples.

[0046] <Average Particle Size>

[0047] The average grain size of the silicon nitride grains constituting the silicon nitride sintered body of the present invention is not particularly limited, but is preferably 1 to 15 μm, more preferably 3 to 12 μm, and even more preferably 5 to 10 μm.

[0048] When the average grain size of silicon nitride is above these lower limits, the thermal conductivity of the sintered body tends to increase. When the average grain size of silicon nitride is below these upper limits, the mechanical strength of the sintered body tends to increase.

[0049] The average grain size of silicon nitride grains in the silicon nitride sintered body is a weighted average grain size calculated based on a volume-based distribution. The measurement method is as follows: The cross-section of the silicon nitride sintered body is observed using a scanning electron microscope (SEM). Image analysis is performed on more than 300 particle images to determine the equivalent diameter of each particle's circular area (diameter of equal circular area). Then, based on the volume-based distribution of the aforementioned equivalent diameter of circular area, the weighted average grain size is calculated.

[0050] Thermal conductivity

[0051] From the viewpoint of improving heat dissipation when used as a heat dissipation substrate, the thermal conductivity of the silicon nitride sintered body of the present invention is preferably 85 W / (m·K) or higher, more preferably 90 W / (m·K) or higher. Higher thermal conductivity is better, but for example, it is 150 W / (m·K) or lower, and practically 100 W / (m·K) or lower. It should be noted that the thermal conductivity can be measured by laser flash method.

[0052] [Manufacturing method of silicon nitride sintered body]

[0053] The silicon nitride sintered body of the present invention can be obtained by sintering silicon nitride powder, preferably by sintering a mixture of raw materials containing silicon nitride powder and sintering aid.

[0054] (Silicon nitride powder)

[0055] The average particle size of the silicon nitride powder contained in the mixed raw materials is not particularly limited, but from the viewpoint of making the average particle size of silicon nitride grains in the silicon nitride sintered body within the above-mentioned desired range, it is preferably 0.5 to 3 μm, more preferably 0.5 to 2 μm.

[0056] The average particle size of silicon nitride powder can be determined by a laser diffraction / scattering particle size distribution measuring device. It is defined as: after obtaining the volume frequency distribution curve with particle size (μm) as the horizontal axis and volume frequency as the vertical axis, the particle size (D50) corresponding to the cumulative value of 50% on the measured volume reference particle size distribution cumulative curve.

[0057] Silicon nitride powder can be manufactured using known methods. Examples of methods for manufacturing silicon nitride powder include: reduction nitriding, which uses silicon dioxide powder as a raw material and generates silicon nitride by circulating nitrogen gas in the presence of carbon powder; direct nitriding, which involves reacting silicon powder with nitrogen gas at high temperatures; and imide decomposition, which involves reacting silicon halides with ammonia gas. Additionally, silicon nitride powder can also be manufactured using a direct nitriding method based on spontaneous combustion, i.e., combustion synthesis. Combustion synthesis allows for the stable production of silicon nitride powder without consuming energy.

[0058] Combustion synthesis is a method that uses silicon powder as a raw material, forcibly ignites a portion of the powder under a nitrogen atmosphere, and synthesizes silicon nitride through the self-heating of the raw material compound. Combustion synthesis is a well-known method, as can be seen in, for example, Japanese Patent Application Publication No. 2000-264608 and International Patent Application Publication No. 2019 / 167879.

[0059] (Sintering aid)

[0060] Metal oxides are preferred as sintering aids. Using metal oxides facilitates the sintering of silicon nitride powder, resulting in denser and stronger sintered bodies. Examples of metal oxides include yttrium oxide (Y₂O₃), cerium dioxide (CeO), and magnesium oxide (MgO). Yttrium oxide is preferred. One type of metal oxide can be used alone, or two or more can be used in combination.

[0061] As a sintering aid, it is preferable to use an oxygen-free compound in conjunction with the aforementioned metal oxides. By using an oxygen-free compound as a sintering aid, the dissolved oxygen from the sintering aid can be reduced. As a result, a silicon nitride sintered body with high thermal conductivity can be obtained.

[0062] Examples of oxygen-free compounds include carbonitride compounds containing rare earth elements and carbonitride compounds containing magnesium. Among oxygen-free compounds, carbonitride compounds containing magnesium are preferred, and both rare earth carbonitride compounds and magnesium carbonitride compounds are more preferably used.

[0063] Examples of carbonitride compounds containing rare earth elements include Y2Si4N6C, Yb2Si4N6C, and Ce2Si4N6C. From the viewpoint of easily obtaining silicon nitride sintered bodies with high thermal conductivity, Y2Si4N6C and Yb2Si4N6C are preferred, and Y2Si4N6C is more preferred.

[0064] Examples of magnesium-containing carbonitride compounds include MgSi4N6C.

[0065] When using an oxygen-free compound as a sintering aid, the content of the oxygen-free compound in the mixed raw materials is preferably 1 to 15 parts by mass relative to 100 parts by mass of silicon nitride powder, more preferably 3 to 12 parts by mass, and even more preferably 6 to 10 parts by mass.

[0066] By adjusting the content of oxygen-free compounds used as sintering aids to the above range, it is easier to reduce the dissolved oxygen concentration while promoting sintering.

[0067] The content of sintering aids in the mixed raw materials (the total content of all sintering aids) relative to 100 parts by weight of silicon nitride powder is preferably 5 to 20 parts by weight, more preferably 6 to 18 parts by weight, and even more preferably 8 to 15 parts by weight.

[0068] By adjusting the content of sintering aids to the above range, it is easier to reduce the dissolved oxygen concentration while promoting sintering.

[0069] (Firing conditions)

[0070] The silicon nitride sintered body of the present invention can be obtained by sintering the above-mentioned mixed powder containing silicon nitride powder and sintering aid. The mixed powder can be pressed into a molded body and then sintered in a sintering furnace or the like.

[0071] The above-mentioned compression molding is represented by uniaxial compression molding, but it is preferable to use a method of continuing CIP (Cold Isostatic Pressing) molding after uniaxial compression molding.

[0072] The firing temperature can be, for example, between 1500 and 1900°C. The maximum firing temperature is preferably between 1700 and 1800°C, more preferably between 1750 and 1800°C. By setting the firing temperature to such a high level, grain growth is promoted, and a silicon nitride sintered body with high toughness is easily obtained. It is best to set the holding time at the maximum temperature to, for example, about 3 to 20 hours.

[0073] As described above, the material is held at the highest temperature for a certain period of time and then cooled. Here, the cooling rate from the highest temperature to 1000°C is preferably 0.1 to 5°C / min, more preferably 0.1 to 3°C / min. By setting the cooling rate to be slower, it is easier to reduce the lattice strain of the silicon nitride sintered body and promote grain growth, thus easily obtaining a silicon nitride sintered body with high toughness.

[0074] It should be noted that the conditions when cooling from 1000°C to near room temperature (e.g., 25°C) have little effect on the magnitude of the lattice strain, so they can be set appropriately.

[0075] In addition, cooling is preferably carried out under pressure (in-furnace pressure). Specifically, the pressure during cooling is preferably 120 to 1000 kPa, more preferably 150 to 800 kPa, and even more preferably 180 to 600 kPa.

[0076] By adjusting the pressure during cooling to the above range, combined with the effect of adiabatic compression, it is easy to reduce the lattice strain of silicon nitride sintered bodies.

[0077] From the viewpoint of reducing the lattice strain of silicon nitride sintered bodies, it is particularly preferable that the cooling rate from the highest temperature to 1000°C and the pressure during cooling are both adjusted to the above range.

[0078] The firing process is preferably carried out in a non-reactive gas atmosphere. Examples of non-reactive gas atmospheres include nitrogen and argon, with nitrogen being the preferred atmosphere.

[0079] The silicon nitride sintered body of the present invention is suitable for use as a variety of heat dissipation substrates. Furthermore, because the silicon nitride sintered body of the present invention has low lattice strain, it is possible to form silicon nitride circuit substrates with excellent temperature cycling characteristics.

[0080] Example

[0081] The following embodiments are shown to illustrate the invention in more detail, but the invention is not limited to these embodiments.

[0082] [Determination Method]

[0083] The various physical properties in the examples and comparative examples were determined by the following methods.

[0084] <Lattice strain>

[0085] (i) Preparation of the test sample

[0086] A plate-shaped silicon nitride sintered body (approximately 0.3 mm thick) was mechanically polished on one side to a thickness of approximately 0.2 mm for X-ray diffraction analysis. However, during mechanical polishing, both diamond polishing slurry and CMP (chemical mechanical polishing) slurry were used to suppress and remove the polishing damage layer. The type of CMP slurry was not particularly limited; in this embodiment, a basic colloidal silica slurry (50 nm particle size) was used.

[0087] (ii) Measurement

[0088] For the X-ray diffraction sample prepared as described above, the following steps were performed using CuKα X-ray diffraction (XRD). The integral widths of the (101), (110), (200), (201), and (210) crystal planes of the β phase were calculated from the X-ray diffraction pattern obtained by scanning the X-ray detector in a range of 15–80° with a step size of 0.02°, and these integral widths were substituted into the Williamson-Hall equation in Equation 2 below. It should be noted that the integral width is the value obtained by subtracting the background area of ​​the peak from the total area of ​​the peak curve obtained by X-ray diffraction, and then dividing the resulting area by the peak height.

[0089] Plot the graph with “2sinθ / λ” as the X-axis and “βcosθ / λ” as the Y-axis in Equation 2 below, and calculate the lattice strain (η) using the slope of the straight line obtained by the least squares method.

[0090] βcosθ / λ=η×(2sinθ / λ)+(1 / Dc) (2)

[0091] (β: integral width (rad), θ: Bragg angle (rad), η: lattice strain, λ: X-ray wavelength, Dc: crystallite size (nm))

[0092] <Presence or absence of vacancy cluster defects>

[0093] (i) Sample preparation for transmission electron microscopy (TEM) observation

[0094] For the plate-shaped silicon nitride sintered bodies (approximately 0.3 mm thick) manufactured in the various embodiments and comparative examples, the two surfaces were mechanically polished to a thickness of approximately 0.1 mm, and then irradiated with 6 keV argon ions until perforation was achieved using an ion slicer "EM-09100IS" manufactured by NJEOL Ltd., thereby producing TEM observation samples.

[0095] (ii) TEM observation

[0096] Using a transmission electron microscope "JEM-2100" manufactured by Nippon Electron Ltd., 10 fields of view were observed with an accelerating voltage of 200kV and an observation magnification of 2000x.

[0097] In the 10 observed fields of view, the case in which a dislocation loop or screw dislocation is observed in at least one field of view is evaluated as "the presence of particles containing dislocation loops or screw dislocations". In the 10 observed fields of view, the case in which no dislocation loops or screw dislocations are observed in any of the fields of view is evaluated as "the absence of particles containing dislocation loops or screw dislocations".

[0098] <Dissolved oxygen concentration>

[0099] The dissolved oxygen concentration of the silicon nitride grains constituting the silicon nitride sintered body was determined by secondary ion mass spectrometry (SIMS). The ULVAC-PHI ADEPT-1010 secondary ion mass spectrometry instrument was used.

[0100] As a primary ion, Cs + , will Cs + The laser beam is focused to approximately 50 nm and irradiates the sample surface, generating secondary ions. The concentration of dissolved oxygen in the solid solution is determined by measuring these secondary ions using a mass spectrometer. It should be noted that the quantitative determination of dissolved oxygen concentration is achieved by calculating the concentration of dissolved oxygen in the solid solution. 30 Ion implantation in Si 16 The relative sensitivity coefficient (RSF) of the standard sample of O was used.

[0101] To clarify, since adsorbed oxygen exists on the sample surface, when quantifying the dissolved oxygen concentration using SIMS, the concentration in the sample after removing adsorbed oxygen is determined. 16 O / 30 Si intensity ratio. For calculating the dissolved oxygen concentration in a sample to eliminate the influence of adsorbed oxygen, the method described in Japanese Patent Application Publication No. 2018-24548 can be referred to.

[0102] <Average grain size of silicon nitride>

[0103] (i) Sample preparation for scanning electron microscopy (SEM) observation

[0104] The cross-sections of the plate-shaped silicon nitride sintered bodies manufactured in each embodiment and comparative example were mechanically polished to a mirror finish to prepare SEM observation samples.

[0105] (ii) SEM observation

[0106] Using a scanning electron microscope "JCM-7000" manufactured by Nippon Electron Ltd., with an accelerating voltage of 15kV and an observation magnification of 5000x, five fields of view were observed for each sample, resulting in particle images of more than 300 silicon nitride grains.

[0107] Image analysis was performed on over 300 particle images using the image analysis device attached to the SEM apparatus to determine the spherical equivalent diameter of each particle. Then, based on the volumetric distribution of the aforementioned spherical equivalent diameters, the weighted average particle size was calculated.

[0108] It should be noted that the weighted average particle size based on the volumetric standard distribution can be calculated using the following formula.

[0109] Weighted average particle size based on volumetric distribution

[0110] =Σ(Equivalent diameter of circle area^4) / Σ(Equivalent diameter of circle area^3)

[0111] Evaluation of Temperature Cycling Characteristics

[0112] The following evaluation was conducted using the plate-shaped silicon nitride sintered bodies (approximately 0.3 mm thick) prepared in each embodiment and comparative example as silicon nitride substrates.

[0113] For each silicon nitride substrate, copper plates are bonded using an active metal solder. Two copper plates, each 20 mm long × 15 mm wide × 1.0 mm thick, are placed on the surface side of each silicon nitride substrate. Additionally, one copper plate, 45 mm long × 35 mm wide × 1.0 mm thick, is placed on the back side of each silicon nitride sintered body. The active metal solder used is composed of Ag (57 wt%), Cu (30 wt%), Sn (10 wt%), and Ti (3 wt%). A 40 μm thick layer of active metal solder paste is applied to the silicon nitride substrate, and copper plates are then placed on top of the solder paste. Next, the substrate is subjected to a vacuum at 820°C (10... -2 A heat bonding process is performed (below Pa). The copper plate is bonded to both sides of the silicon nitride substrate. Additionally, the sides of the copper plate are etched to create an 80° angle. Furthermore, an active metal solder layer protrudes 40 μm from the sides of the copper plate.

[0114] The temperature cycling characteristics of the obtained silicon nitride circuit board were evaluated. Specifically, one cycle consisted of -40℃×30 min → room temperature (25℃)×10 min → 200℃×30 min → room temperature (25℃)×10 min. After 4000 cycles, the presence of cracks on the silicon nitride substrate was investigated. The presence or absence of cracks was determined by fluorescent flaw detection.

[0115] The following raw materials are used as raw materials for manufacturing silicon nitride sintered bodies.

[0116] <Silicon nitride powder>

[0117] Silicon nitride powder: Silicon nitride powder with an average particle size of 0.95 μm manufactured by combustion synthesis.

[0118] <Sintering aids>

[0119] Metal oxide: Yttrium oxide (Y₂O₃)

[0120] • Carbonitride compounds containing rare earth elements: Y2Si4N6C

[0121] It is prepared by heating and synthesizing using the following reaction formula.

[0122]

[0123] • Carbonitride compounds containing magnesium: MgSi4N6C

[0124] It is prepared by heating and synthesizing using the following reaction formula.

[0125]

[0126] <Example 1>

[0127] To 100 parts by mass of silicon nitride powder, 3 parts by mass of Y₂O₃, 2 parts by mass of Y₂Si₄N₆C, and 6 parts by mass of MgSi₄N₆C, as sintering aids, were added and mixed using a planetary ball mill to obtain a mixed raw material. Next, using this mixed raw material, molded bodies produced by uniaxial pressing and CIP molding were introduced into a firing furnace and fired under a nitrogen atmosphere at atmospheric pressure. Firing was carried out at a maximum temperature of 1760°C and a holding time at the maximum temperature of 10 hours. Then, the maximum temperature was cooled to 1000°C at a cooling rate of 0.5°C / min. Cooling was carried out at a furnace pressure of 200 kPa. Finally, the silicon nitride sintered body was naturally cooled from 1000°C to room temperature to obtain the sintered body.

[0128] The evaluation results of the prepared silicon nitride sintered bodies are shown in Table 1.

[0129] <Examples 2-5, Comparative Examples 1-2>

[0130] Except for the changes in firing conditions shown in Table 1, silicon nitride sintered bodies were fabricated in the same manner as in Example 1. Various evaluation results of the fabricated silicon nitride sintered bodies are shown in Table 1.

[0131] [Table 1]

[0132]

[0133] As shown in Table 1, the silicon nitride sintered bodies of each embodiment with a lattice strain of less than 0.003 did not develop cracks on the silicon nitride substrate after temperature cycling tests.

[0134] On the other hand, the silicon nitride sintered bodies of the comparative examples with lattice strains of 0.003 or higher developed cracks on the silicon nitride substrates after temperature cycling tests.

[0135] The silicon nitride sintered body of the present invention can form a silicon nitride circuit board with excellent temperature cycling characteristics and high reliability. Claims (as amended under Article 19 of the Treaty) 1. A silicon nitride sintered body, characterized in that the lattice strain of the silicon nitride grains constituting the silicon nitride sintered body is less than 0.003, and the dissolved oxygen concentration of the silicon nitride grains is less than 500 ppm. 2. The silicon nitride sintered body according to claim 1, wherein the average grain size of the silicon nitride grains is 1 to 15 μm. 3. The silicon nitride sintered body according to claim 1 or 2 is a heat dissipation substrate.

Claims

1. A silicon nitride sintered body, characterized in that, The lattice strain of the silicon nitride grains constituting the silicon nitride sintered body is less than 0.

003.

2. The silicon nitride sintered body according to claim 1, wherein, The dissolved oxygen concentration in the silicon nitride grains is less than 500 ppm.

3. The silicon nitride sintered body according to claim 1, wherein, The average grain size of the silicon nitride grains is 1–15 μm.

4. The silicon nitride sintered body according to any one of claims 1 to 3, which is a heat dissipation substrate.

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