Sintered silicon nitride object

Abstract

This sintered silicon nitride object is characterized in that crystal grains of silicon nitride which constitute the sintered silicon nitride object have a crystal lattice strain of 0.003 or greater.　The present invention can provide a sintered silicon nitride object having a high thermal conductivity.

Description

Silicon nitride sintered body

[0001] The present invention relates to a silicon nitride sintered body.

[0002] Silicon nitride sintered bodies have attracted attention as ceramic materials in various industrial fields due to their excellent properties, such as high thermal conductivity, high insulation, and high strength, and are used, for example, as heat dissipation substrates for power modules for automotive and semiconductor applications. When used as a heat dissipation substrate, the substrate's thermal conductivity must be high to prevent malfunctions and component cracking due to heat generation. Therefore, silicon nitride sintered bodies with high thermal conductivity are in demand.

[0003] For example, Patent Document 1 discloses a method for producing a silicon nitride sintered body, which comprises preparing silicon nitride powder having an oxygen content of 1.5 mass% or less, filling the silicon nitride powder into a sintering mold, and continuously applying a pulsed direct current having a voltage of 1 V or more but less than 10 V and an output current of 500 A or more and 40,000 A or less to the gaps between the particles of the raw material powder under a pressure of 70 MPa or more to sinter the silicon nitride powder. It also describes that when the silicon nitride sintered body is used as a heat dissipation substrate for a power module, it can efficiently release (radiate) heat generated from the power module.

[0004] Japanese Patent Application Laid-Open No. 2019-6673

[0005] However, in recent years, the heat generation density of power modules has increased due to the increasing density and power output of electronic devices and semiconductor devices. The temperature rise in power modules can lead to cracking of the substrate and malfunction of the elements. Therefore, heat dissipation substrates are required to have higher heat dissipation properties than before, and the thermal conductivity of the silicon nitride sintered bodies used in heat dissipation substrates needs to be improved.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a silicon nitride sintered body having high thermal conductivity.

[0007] The present inventors have conducted extensive research to achieve the above object, and as a result, have focused on the crystal lattice distortion of silicon nitride sintered bodies, which had not been investigated previously, and have found that the above problems can be solved by using a silicon nitride sintered body having a crystal lattice distortion of 0.003 or more, thereby completing the present invention.

[0008] The gist of the present invention is the following [1] to [5]. [1] A silicon nitride sintered body, characterized in that the crystal lattice distortion of the silicon nitride crystal grains constituting the silicon nitride sintered body is 0.003 or more. [2] The silicon nitride sintered body according to [1] above, wherein the silicon nitride crystal grains have a solute oxygen concentration of 500 to 2500 ppm. [3] The crystal lattice distortion and the solute oxygen concentration (ppm) satisfy the following relationship: crystal lattice distortion / solute oxygen concentration > 2.4 × 10 －６ [4] The silicon nitride sintered body according to any one of [1] to [3] above, wherein the silicon nitride crystal grains have an average particle size of 1 to 5 μm. [5] The silicon nitride sintered body according to any one of [1] to [4] above, which is a heat dissipation substrate.

[0009] According to the present invention, it is possible to provide a silicon nitride sintered body having high thermal conductivity.

[0010] 1 is a diagram for explaining vacancies in a silicon nitride crystal lattice, 2 is a diagram for explaining vacancy cluster defects in a silicon nitride crystal lattice, and 3 is an example of a TEM dark-field image of a silicon nitride particle containing a helical dislocation.

[0011] [Silicon nitride sintered body] The silicon nitride sintered body of the present invention will be described in detail below.

[0012] <Crystal Lattice Distortion> In the silicon nitride sintered body of the present invention, the crystal lattice distortion of the silicon nitride crystal particles constituting the silicon nitride sintered body is 0.003 or more. When the crystal lattice distortion is 0.003 or more, the thermal conductivity of the silicon nitride sintered body increases when the dissolved oxygen concentration described below is the same. From the viewpoint of further increasing the thermal conductivity, the crystal lattice distortion of the silicon nitride crystal particles is preferably 0.0035 or more, more preferably 0.004 or more, even more preferably 0.005 or more, and even more preferably 0.006 or more. Although there is no particular upper limit for the crystal lattice distortion of the silicon nitride crystal particles constituting the silicon nitride sintered body, the crystal lattice distortion is preferably 0.01 or less. The crystal lattice distortion of the silicon nitride crystal particles can be measured by X-ray diffraction. Details of the measurement method will be explained in the examples.

[0013] The reason why the thermal conductivity of a silicon nitride sintered body is improved by increasing the crystal lattice distortion of silicon nitride crystal grains to a certain level or more is presumed to be as follows: The crystal lattice distortion of silicon nitride crystal grains constituting a silicon nitride sintered body is related to vacancies formed by dissolved oxygen. ３ N ４ It is known that silicon nitride sintered bodies contain dissolved oxygen due to the type of raw material and manufacturing method. When oxygen atoms (O) displace and dissolve nitrogen atoms (N) that make up the silicon nitride crystal, silicon atom (Si) vacancies are generated to ensure electrical neutrality. For example, Figure 1 shows a diagram of a silicon nitride crystal lattice in which four oxygen atoms replace four nitrogen atoms, leaving one silicon atom missing, creating a vacancy. When vacancies are created, they become a scattering source for lattice vibrations (phonons), causing a decrease in thermal conductivity. Therefore, generally, the greater the amount of dissolved oxygen in a silicon nitride sintered body, the greater the number of phonon scattering sources, and the lower the thermal conductivity tends to be.

[0014] On the other hand, when the solute oxygen concentration in silicon nitride sintered bodies is about the same, it is thought that the thermal conductivity also changes depending on the distribution of vacancies within the sintered body. That is, when the amount of solute oxygen is about the same, it is thought that the number of vacancies generated will also be about the same, but when the vacancies exist individually and dispersedly, compared to when several vacancies aggregate to form vacancy cluster defects, the latter is thought to have fewer phonon scattering sources and therefore higher thermal conductivity.

[0015] For example, the left and right diagrams in Figure 2 schematically show silicon nitride crystal lattices with the same number of vacancies (five). When five vacancies are dispersed as in the left diagram, each of the five vacancies is thought to act as a phonon scattering source. On the other hand, when several vacancies aggregate to form a vacancy cluster defect, as shown in the right diagram, as indicated by A, the vacancy cluster defect acts as a single phonon scattering source. Therefore, the case on the right, where vacancy cluster defects are formed, is thought to have fewer phonon scattering sources and higher thermal conductivity. Because the crystal lattice is distorted near vacancy cluster defects, the greater the number of vacancy cluster defects, the greater the crystal lattice distortion. Therefore, it is thought that the silicon nitride sintered body of the present invention, which has a crystal lattice distortion of a certain level or more, will have high thermal conductivity.

[0016] The presence or absence of vacancy cluster defects in a silicon nitride sintered body can be confirmed by observation with a transmission electron microscope (TEM). That is, vacancy cluster defects are observed by TEM as, for example, dislocation loops or helical dislocations. For example, Figure 3 shows an example of a TEM dark-field image of a silicon nitride particle containing a helical dislocation. A helical dislocation is observed as a helical white line inside the silicon nitride particle in the center of Figure 3.

[0017] The method for adjusting the value of the crystal lattice distortion is not particularly limited, but it can be adjusted, for example, by the conditions under which the silicon nitride sintered body is produced, particularly the cooling conditions after firing.

[0018] <Solute Oxygen> The solute oxygen concentration of the silicon nitride crystal particles constituting the silicon nitride sintered body of the present invention is not particularly limited, but is, for example, 2500 ppm or less. When the solute oxygen concentration is 2500 ppm or less, the thermal conductivity of the silicon nitride sintered body can be increased. On the other hand, from the viewpoint of improving thermal conductivity, the lower the solute oxygen concentration of the silicon nitride crystal particles constituting the silicon nitride sintered body, the better, but in practical terms, it is 100 ppm or more. As described above, when the solute oxygen concentration is high, generally, many Si atom vacancies are generated, which tends to decrease thermal conductivity. On the other hand, when the solute oxygen concentration is high, if many of the above-mentioned vacancy cluster defects are formed, the number of phonon scattering sources decreases, and the decrease in thermal conductivity can be effectively suppressed. That is, in the present invention, when the solute oxygen concentration is relatively high, it is easy to effectively suppress the decrease in thermal conductivity. From this perspective, the solute oxygen concentration of the silicon nitride crystal grains constituting the silicon nitride sintered body of the present invention is preferably 500 to 2500 ppm, more preferably 1000 to 2500 ppm, and even more preferably 1300 to 2500 ppm. Thus, the silicon nitride sintered body of the present invention can maintain high thermal conductivity even when the solute oxygen concentration is relatively high. Therefore, there is no need to use high-purity raw materials to adjust the solute oxygen concentration low, and there is no need to apply special manufacturing conditions to reduce the solute oxygen concentration, which also contributes to improved productivity.

[0019] In the present invention, the crystal lattice strain and the dissolved oxygen concentration (ppm) are such that the crystal lattice strain / dissolved oxygen concentration>2.4×10 －６ It is preferable that the crystal lattice distortion / solid solution oxygen concentration is 2.4×10 －６ If the crystal lattice strain / solid solution oxygen concentration is greater than 2.5×10, the thermal conductivity of the sintered body is likely to be improved because the crystal lattice strain / solid solution oxygen concentration is large when the solid solution oxygen concentration is the same. －６ More preferably, it is 3.0 × 10 or more. －６ More preferably, it is equal to or greater than this.

[0020] In the present invention, the solute oxygen concentration refers to the concentration of oxygen dissolved inside the silicon nitride sintered body (internal oxygen), and does not include adsorbed oxygen (external oxygen) inevitably present on the surface of the sintered body. The solute oxygen concentration can be adjusted by various raw materials used to produce the silicon nitride powder, the type and amount of sintering aid used in producing the sintered body, sintering conditions, etc. The solute oxygen concentration can be measured by the method described in the Examples.

[0021] <Average particle size> The average particle size of the silicon nitride crystal particles constituting the silicon nitride sintered body of the present invention is not particularly limited, but is preferably 1 to 5 μm, more preferably 2 to 5 μm, and even more preferably 3 to 5 μm. If the average particle size of the silicon nitride crystal particles is equal to or greater than these lower limits, the thermal conductivity of the sintered body tends to be high. If the average particle size of the silicon nitride crystal particles is equal to or less than these upper limits, the mechanical strength of the sintered body tends to be high.

[0022] The average particle size of silicon nitride crystal grains in a silicon nitride sintered body is a weighted average particle size based on a volumetric distribution, and is measured as follows: A cross section of the silicon nitride sintered body is observed with a scanning electron microscope (SEM), and images of 300 or more particles are analyzed with an image analyzer to determine the equivalent circle diameter of each particle. The weighted average particle size is then calculated based on the volumetric distribution of the equivalent circle diameters.

[0023] <Thermal Conductivity> The thermal conductivity of the silicon nitride sintered body of the present invention is preferably 85 W / (m·K) or more, more preferably 90 W / (m·K) or more, from the viewpoint of improving heat dissipation when used as a heat dissipation substrate. The higher the thermal conductivity, the better, but it is, for example, 150 W / (m·K) or less, and in practical use, 100 W / (m·K) or less. The thermal conductivity can be measured by the method described in the examples.

[0024] [Method for Producing Sintered Silicon Nitride] The sintered silicon nitride of the present invention can be obtained by firing silicon nitride powder, preferably by firing a mixed raw material containing silicon nitride powder and a sintering aid.

[0025] (Silicon nitride powder) The average particle size of the silicon nitride powder contained in the mixed raw material is not particularly limited, but from the viewpoint of setting the average particle size of silicon nitride crystal particles in the silicon nitride sintered body in the desired range described above, it is preferably 0.5 to 3 μm, more preferably 0.5 to 2 μm. The average particle size of the silicon nitride powder can be measured using a laser diffraction / scattering particle size distribution analyzer, and a volume frequency distribution curve is obtained with the horizontal axis representing particle size (μm) and the vertical axis representing volume frequency. The average particle size of the silicon nitride powder means the particle size (D50) at which the cumulative curve of the measured volume-based particle size distribution reaches 50%.

[0026] Silicon nitride powder can be produced by known methods. Examples of methods for producing silicon nitride powder include a reduction-nitridation method in which silica powder is used as a raw material and nitrogen gas is passed through in the presence of carbon powder to produce silicon nitride, a direct nitridation method in which silicon powder is reacted with nitrogen at high temperatures, and an imide decomposition method in which silicon halide is reacted with ammonia. Silicon nitride powder can also be produced by combustion synthesis, a direct nitridation method that utilizes self-combustion. Combustion synthesis allows for stable production of silicon nitride powder without incurring energy costs.

[0027] The combustion synthesis method uses silicon powder as a raw material, forcibly ignites a portion of the raw material powder under a nitrogen atmosphere, and synthesizes silicon nitride by self-heating of the raw material compound. The combustion synthesis method is a known method, and reference can be made to, for example, JP 2000-264608 A and WO 2019 / 167879 A.

[0028] (Sintering Aid) It is preferable to use a metal oxide as the sintering aid. By using a metal oxide as the sintering aid, the sintering of the silicon nitride powder is facilitated, and a denser and stronger sintered body is more likely to be obtained. As the metal oxide, yttria (Y ２ O ３ ), ceria (CeO), magnesia (MgO), etc. Among these, yttria is preferable. One type of metal oxide may be used alone, or two or more types may be used in combination.

[0029] The content of sintering aids in the mixed raw material (total content of all sintering aids) is preferably 3 to 12 parts by mass, more preferably 4 to 10 parts by mass, and even more preferably 5 to 8 parts by mass, per 100 parts by mass of silicon nitride powder. By adjusting the content of sintering aids within the above range, it becomes easier to adjust the dissolved oxygen concentration within an appropriate range while proceeding with sintering. Furthermore, as described above, the silicon nitride sintered body of the present invention can maintain high thermal conductivity even with a high dissolved oxygen concentration, so reducing the amount of sintering aid is also effective from the perspective of production costs.

[0030] (Firing conditions) The silicon nitride sintered body of the present invention can be obtained by firing a mixed powder containing the silicon nitride powder and sintering aid described above. The mixed powder can be formed into a compact by press molding, and then fired in a firing furnace or the like. The press molding is typically uniaxial press molding, but a method in which uniaxial press molding is followed by CIP (cold isostatic pressing) molding is preferably used.

[0031] The firing temperature (firing temperature) can be, for example, 1500 to 1900°C. The maximum firing temperature is preferably 1700 to 1800°C, more preferably 1750 to 1800°C. Setting the firing temperature at such a high level promotes grain growth, making it easier to obtain a silicon nitride sintered body with high toughness. The holding time at the maximum temperature can be, for example, about 3 to 20 hours. As described above, the material is held at the maximum temperature for a certain period of time and then cooled. The cooling rate from the maximum temperature to 1000°C is preferably 15 to 100°C / min, more preferably 20 to 80°C / min. Setting the cooling rate at a high rate in this way makes it easier to increase the crystal lattice strain in the silicon nitride sintered body. The conditions for cooling from 1000°C to near room temperature (e.g., 25°C) do not significantly affect the magnitude of the crystal lattice strain, so they can be set appropriately.

[0032] Furthermore, it is preferable to perform cooling under reduced pressure (furnace pressure); specifically, the pressure during cooling is preferably 0.005 to 50 kPa, more preferably 0.008 to 10 kPa, and even more preferably 0.01 to 5 kPa. By adjusting the pressure during cooling within the above range, the effect of adiabatic expansion is also added, making it easier to increase the crystal lattice strain of the silicon nitride sintered body. From the perspective of increasing the crystal lattice strain of the silicon nitride sintered body, it is particularly preferable to adjust both the cooling rate from the maximum temperature to 1000°C and the pressure during cooling within the above ranges.

[0033] The firing is preferably carried out in an inert gas atmosphere, such as a nitrogen atmosphere or an argon atmosphere, with a nitrogen atmosphere being preferred.

[0034] The silicon nitride sintered body of the present invention has large crystal lattice distortion and excellent thermal conductivity, making it suitable for use as a variety of heat dissipation substrates.

[0035] EXAMPLES The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0036] [Measurement Methods] Various physical properties in the examples and comparative examples were measured by the following methods.

[0037] <Crystal lattice distortion> (i) Preparation of measurement sample One side of the surface of a plate-shaped silicon nitride sintered body (thickness about 0.3 mm) is mechanically polished until the thickness is about 0.2 mm, and used as a sample for X-ray diffraction measurement.However, during mechanical polishing, in order to suppress and remove the generation of polishing damage layer, CMP (chemical mechanical polishing) slurry is used together with diamond slurry.The type of CMP slurry is not particularly limited, but in this embodiment, alkali-based colloidal silica (particle size 50 nm) slurry is used.(ii) Measurement The sample for X-ray diffraction measurement prepared as above is calculated by X-ray diffraction (XRD) using CuKα radiation according to the following procedure. From the X-ray diffraction pattern obtained by scanning the X-ray detector in 0.02° increments over the 2θ range of 15 to 80°, the integral widths of the (101), (110), (200), (201), and (210) planes of the β phase were calculated, and the integral widths were substituted into the Williamson-Hall equation of the following formula 2. The integral width is the value obtained by dividing the area obtained by subtracting the area of ​​the peak background from the area of ​​the peak waveform obtained by X-ray diffraction by the peak height. The following formula 2 was plotted with "2 sin θ / λ" on the X axis and "β cos θ / λ" on the Y axis, and the crystal lattice distortion (η) was calculated from the slope of the straight line obtained by the least squares method. βcosθ / λ=η×(2sinθ / λ)+(1 / Dc) (2) (β: integral width (rad), θ: Bragg angle (rad), η: crystal lattice strain, λ: X-ray wavelength, Dc: crystallite size (nm))

[0038] <Presence or absence of vacancy aggregate defects> (i) Preparation of transmission electron microscope (TEM) observation sample For the plate-shaped silicon nitride sintered body (thickness approximately 0.3 mm) produced in each example and comparative example, both surfaces were mechanically polished until the thickness was approximately 0.1 mm, and then 6 keV argon ions were irradiated until perforations were formed using an ion slicer "EM-09100IS" manufactured by JEOL Ltd., to prepare a TEM observation sample. (ii) TEM observation Using a transmission electron microscope "JEM-2100" manufactured by JEOL Ltd., 10 visual fields were observed at an acceleration voltage of 200 kV and an observation magnification of 2000 times. If dislocation loops or helical dislocations were observed in at least one of the 10 visual fields, it was evaluated as "there are particles containing dislocation loops and helical dislocations." If dislocation loops or helical dislocations were not observed in any of the 10 visual fields, it was evaluated as "there are no particles containing dislocation loops or helical dislocations."

[0039] <Solute oxygen concentration> The solute oxygen concentration of silicon nitride crystal grains constituting the silicon nitride sintered body was measured by secondary ion mass spectrometry (SIMS). The secondary ion mass spectrometer used was a "PHI ADEPT-1010" manufactured by ULVAC-PHI, Inc. The primary ions were Cs ＋ Use Cs ＋ The beam was focused to about 50 nm and irradiated onto the surface of the sample to generate secondary ions. The generated secondary ions were measured using a mass spectrometer to determine the dissolved oxygen concentration. The dissolved oxygen concentration was determined as follows: ３０ To Si １６ The relative sensitivity factor (RSF) of a standard sample implanted with O ions was determined.

[0040] In addition, since adsorbed oxygen exists on the sample surface, when quantifying the dissolved oxygen concentration by SIMS, the amount of dissolved oxygen in the sample excluding the adsorbed oxygen is １６ O / ３０ The Si intensity ratio is calculated. As a method for calculating the dissolved oxygen concentration in a sample by removing the influence of adsorbed oxygen, the method described in JP 2018-24548 A can be referred to.

[0041] <Average Particle Diameter of Silicon Nitride Crystal Particles> (i) Preparation of Scanning Electron Microscope (SEM) Observation Samples The cross sections of the plate-shaped silicon nitride sintered bodies produced in each Example and Comparative Example were mechanically polished to a mirror finish to prepare SEM observation samples. (ii) SEM Observation Using a JEOL Ltd. scanning electron microscope "JCM-7000," 5 fields of view per sample were observed at an acceleration voltage of 15 kV and an observation magnification of 5000x, and particle images of 300 or more silicon nitride crystal particles were obtained. The 300 or more particle images were analyzed using an image analyzer attached to the SEM device, and the area-equivalent circle diameter of each particle was determined. Then, a weighted average particle diameter based on the volume-based distribution of the area-equivalent circle diameters was calculated. The weighted average particle diameter based on the volume-based distribution can be calculated using the following formula: Weighted average particle diameter based on volume-based distribution = Σ(area-equivalent circle diameter^4) / Σ(area-equivalent circle diameter^3)

[0042] <Thermal Conductivity> The thermal conductivity was measured by a laser flash method using a thermal conductivity measuring instrument "LFA467" manufactured by Netsch.

[0043] The following materials were used as raw materials for producing sintered silicon nitride: <Silicon nitride powder> Silicon nitride powder: silicon nitride powder with an average particle size of 0.95 μm produced by combustion synthesis.

[0044] <Sintering aid> Metal oxide: yttria (Y ２ O ３ ), alumina (Al ２ O ３ ), magnesia (MgO)

[0045] Example 1: 100 parts by mass of silicon nitride powder was mixed with Y as a sintering aid. ２ O ３ 3 parts by mass of Al ２ O ３One part by mass of the above and 2 parts by mass of MgO were added and mixed in a planetary ball mill to obtain a mixed raw material. Next, the mixed raw material was subjected to uniaxial press molding and CIP molding to produce a molded body, which was then introduced into a sintering furnace and sintered under a nitrogen atmosphere at atmospheric pressure. The sintering was carried out under conditions of a maximum temperature of 1760°C and a holding time at the maximum temperature of 10 hours. The material was then cooled from the maximum temperature to 1000°C at a cooling rate of 20°C / min. The furnace pressure was set to 1 kPa. The material was then naturally cooled from 1000°C to room temperature to obtain a silicon nitride sintered body. Table 1 shows the evaluation results of the produced silicon nitride sintered body.

[0046] Examples 2 to 5, Comparative Examples 1 and 2 Silicon nitride sintered bodies were produced in the same manner as in Example 1, except that the firing conditions were changed as shown in Table 1. Table 1 shows the results of various evaluations of the produced silicon nitride sintered bodies.

[0047]

[0048] As shown in Table 1, the silicon nitride sintered bodies of each Example, which have a crystal lattice distortion of 0.003 or more, have higher thermal conductivity than the silicon nitride sintered bodies of each Comparative Example, which have a crystal lattice distortion of less than 0.003. Generally, a high solute oxygen concentration tends to result in lower thermal conductivity, but the above Examples, despite their higher solute oxygen concentrations, have higher thermal conductivity than the Comparative Examples. This suggests that increasing the crystal lattice distortion, especially in regions with high solute oxygen concentrations, is effective.

Claims

1. A silicon nitride sintered body, characterized in that the crystal lattice distortion of silicon nitride crystal grains constituting the silicon nitride sintered body is 0.003 or more.

2. The silicon nitride sintered body according to claim 1, wherein the silicon nitride crystal grains have a dissolved oxygen concentration of 500 to 2500 ppm.

3. The crystal lattice distortion and the dissolved oxygen concentration (ppm) are such that: Crystal lattice distortion / dissolved oxygen concentration>2.4×10 －６ 3. The silicon nitride sintered body according to claim 2, which satisfies the above.

4. The silicon nitride sintered body according to claim 1, wherein the average particle size of said silicon nitride crystal grains is 1 to 5 μm.

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

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