WO2025028388A1 - Sintered silicon nitride object - Google Patents

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

The present invention relates to a silicon nitride sintered body.

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 properties, and high strength. For example, they are used as heat dissipation substrates for power modules for automobiles and semiconductors.
When used as a heat dissipation substrate, the substrate needs to have high thermal conductivity to prevent malfunctions and component cracking caused by heat generation, so there is a demand for sintered silicon nitride with high thermal conductivity.

For example, Patent Document 1 discloses a method for producing a silicon nitride sintered body, which comprises preparing silicon nitride powder with an oxygen content of 1.5 mass% or less, filling a sintering mold with the silicon nitride powder, and continuously applying a pulsed direct current with 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, thereby sintering 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 is possible to efficiently release (dissipate) heat generated from the power module.

JP 2019-6673 A

However, in recent years, the heat generation density of power modules has increased along with the increasing density and power output of electronic devices and semiconductor devices. Temperature rise in power modules can lead to cracking of the substrate and malfunction of elements. For this reason, heat dissipation substrates are required to have higher heat dissipation properties than before, and there is a need to improve the thermal conductivity of the silicon nitride sintered bodies used in heat dissipation substrates.

The object of the present invention is to provide a silicon nitride sintered body with high thermal conductivity.

The inventors conducted extensive research to achieve the above-mentioned objective. As a result, they focused on the crystal lattice distortion of silicon nitride sintered bodies, which had not previously been studied, and discovered that the above-mentioned problems could be solved by using silicon nitride sintered bodies with a crystal lattice distortion of 0.003 or more, thus completing the present invention.

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

The present invention provides a silicon nitride sintered body with high thermal conductivity.

FIG. 2 is a diagram for explaining vacancies in a crystal lattice of silicon nitride. FIG. 2 is a diagram for explaining vacancy cluster defects in a silicon nitride crystal lattice. 1 is an example of a TEM dark field image of a silicon nitride grain containing a helical dislocation.

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

<Crystal lattice distortion>
The silicon nitride sintered body of the present invention has a crystal lattice distortion of 0.003 or more in the silicon nitride crystal grains constituting the silicon nitride sintered body. When the crystal lattice distortion is 0.003 or more, the thermal conductivity of the silicon nitride sintered body is high when the solid solution oxygen concentration described later is the same. From the viewpoint of further increasing the thermal conductivity, the crystal lattice distortion of the silicon nitride crystal grains 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.
The upper limit of the crystal lattice distortion of the silicon nitride crystal grains constituting the silicon nitride sintered body is not particularly limited, but the crystal lattice distortion is preferably 0.01 or less. The crystal lattice distortion of the silicon nitride crystal grains can be measured by X-ray diffraction. Details of the measurement method will be described in the examples.

The reason why the thermal conductivity of the silicon nitride sintered body is improved by setting 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 the silicon nitride sintered body is related to vacancies formed by dissolved oxygen.
It is known that silicon nitride crystals (Si ₃ N ₄ ) constituting silicon nitride sintered bodies contain dissolved oxygen due to the type of raw material, manufacturing method, etc. When oxygen atoms (O) dissolve in the silicon nitride crystals by substitution with nitrogen atoms (N), silicon atom (Si) vacancies are generated to satisfy electrical neutrality.
For example, FIG. 1 shows a diagram of a silicon nitride crystal lattice in which four oxygen atoms are substituted for four nitrogen atoms, and one silicon atom is removed to create a vacancy.
When vacancies are generated, they become a scattering source of lattice vibrations (phonons), which causes a decrease in thermal conductivity. Therefore, generally, the more dissolved oxygen there is in a silicon nitride sintered body, the more phonon scattering sources there are, and the lower the thermal conductivity tends to be.

On the other hand, when the dissolved oxygen concentration in silicon nitride sintered bodies is about the same, it is thought that the thermal conductivity will also change depending on the distribution of vacancies within the sintered body. In other words, when the amount of dissolved oxygen is about the same, it is thought that the number of vacancies generated will also be about the same, but when the vacancies are individually dispersed and exist, compared to when several vacancies gather together to form vacancy cluster defects, the latter is thought to have fewer phonon scattering sources and therefore a higher thermal conductivity.

For example, the left and right diagrams in Figure 2 each show a schematic representation of a silicon nitride crystal lattice with the same number of vacancies (five), but when five vacancies are dispersed as in the left diagram, each of the five vacancies is thought to be a phonon scattering source. On the other hand, when several vacancies are aggregated to form a vacancy cluster defect as shown by A in the right diagram, the vacancy cluster defect becomes a single phonon scattering source. Therefore, it is thought that the case on the right diagram, where vacancy cluster defects are formed, has fewer phonon scattering sources and higher thermal conductivity.
Since the crystal lattice is distorted in the vicinity of the vacancy cluster defects, the greater the number of vacancy cluster defects, the greater the crystal lattice distortion. Therefore, it is believed that the silicon nitride sintered body of the present invention, which has a certain level of crystal lattice distortion or more, has high thermal conductivity.

The presence or absence of vacancy cluster defects in sintered silicon nitride 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. Inside the silicon nitride particle in the center of Figure 3, a helical dislocation is observed as a helical white line.

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 when manufacturing the silicon nitride sintered body, particularly the cooling conditions after sintering.

<Soluted oxygen>
The dissolved oxygen concentration of the silicon nitride crystal grains constituting the silicon nitride sintered body of the present invention is not particularly limited, but is, for example, 2500 ppm or less. If the dissolved oxygen concentration is 2500 ppm or less, the thermal conductivity of the silicon nitride sintered body can be increased. On the other hand, the dissolved oxygen concentration of the silicon nitride crystal grains constituting the silicon nitride sintered body is preferably as low as possible from the viewpoint of improving thermal conductivity, but is practically 100 ppm or more.
As described above, when the solute oxygen concentration is high, generally, many vacancies of Si atoms are generated, and the thermal conductivity is likely to decrease. On the other hand, when the solute oxygen concentration is high, if many vacancy aggregate 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, the decrease in thermal conductivity can be effectively suppressed. From such a viewpoint, 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 dissolved oxygen concentration is relatively high, which eliminates the need to use high-purity raw materials to adjust the dissolved oxygen concentration to a low level, and also eliminates the need to apply special manufacturing conditions to reduce the dissolved oxygen concentration, thereby contributing to improved productivity.

In the present invention, it is preferable that the crystal lattice distortion and the dissolved oxygen concentration (ppm) satisfy the following: crystal lattice distortion/dissolved oxygen concentration>2.4× ^10-6 . If the crystal lattice distortion/dissolved oxygen concentration exceeds 2.4× ^10-6 , the crystal lattice distortion is large when the dissolved oxygen concentration is the same, and the thermal conductivity of the sintered body is likely to be improved. From this viewpoint, the crystal lattice distortion/dissolved oxygen concentration is more preferably 2.5× ^10-6 or more, and even more preferably 3.0× ^10-6 or more.

The dissolved oxygen concentration in the present invention means the concentration of oxygen dissolved inside the silicon nitride sintered body (internal oxygen), and does not include adsorbed oxygen (external oxygen) that is inevitably present on the surface of the sintered body.
The dissolved 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, and the like.
The dissolved oxygen concentration can be measured by the method described in the Examples.

<Average particle size>
The average particle size of the silicon nitride crystal grains 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 grains is equal to or larger 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 grains is equal to or smaller than these upper limits, the mechanical strength of the sintered body tends to be high.

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

<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 practically 100 W/(m·K) or less. The thermal conductivity can be measured by the method described in the examples.

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

(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 the 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 by a laser diffraction/scattering particle size distribution measuring device, 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 becomes 50%.

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

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

(Sintering aid)
As the sintering aid, it is preferable to use a metal oxide. By using a metal oxide as a sintering aid, the sintering of the silicon nitride powder is facilitated, and a denser and stronger sintered body is easily obtained. Examples of the metal oxide include yttria (Y ₂ O ₃ ), ceria (CeO), and magnesia (MgO). Among these, yttria is preferable. The metal oxide may be used alone or in combination of two or more kinds.

The content of the sintering aid in the mixed raw material (the 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, relative to 100 parts by mass of the silicon nitride powder.
By adjusting the content of the sintering aid to the above range, it becomes easy to adjust the dissolved oxygen concentration to an appropriate range while proceeding with sintering. Also, as described above, the silicon nitride sintered body of the present invention can maintain high thermal conductivity even if the dissolved oxygen concentration is high, so that reducing the amount of the sintering aid is also effective from the viewpoint of production costs.

(Firing conditions)
The silicon nitride sintered body of the present invention can be obtained by sintering a mixed powder containing the above-mentioned silicon nitride powder and a sintering aid. The mixed powder is preferably press-molded into a molded body, and then sintered in a sintering furnace or the like.
The above 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 employed.

The firing temperature can be, for example, 1500 to 1900°C. The maximum firing temperature is preferably 1700 to 1800°C, and more preferably 1750 to 1800°C. By setting the firing temperature at such a high level, grain growth is promoted, making it easier to obtain a silicon nitride sintered body with high toughness. The holding time at the maximum temperature should 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. By setting the cooling rate at a high rate in this manner, it becomes easier to increase the crystal lattice distortion of the silicon nitride sintered body.
The conditions for cooling from 1000° C. to near room temperature (for example, 25° C.) do not significantly affect the magnitude of crystal lattice distortion, so may be set appropriately.

Moreover, it is preferable to perform the 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 to within the above range, the effect of adiabatic expansion is also achieved, making it easier to increase the crystal lattice distortion of the silicon nitride sintered body.
From the viewpoint of increasing the crystal lattice distortion 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-mentioned ranges.

The firing is preferably carried out in an inert gas atmosphere. Examples of the inert gas atmosphere include a nitrogen atmosphere and an argon atmosphere, with a nitrogen atmosphere being preferred.

The silicon nitride sintered body of the present invention has large crystal lattice distortion and excellent thermal conductivity. Therefore, the silicon nitride sintered body of the present invention can be suitably used as various heat dissipation substrates.

The following examples are provided to further explain the present invention, but the present invention is not limited to these examples.

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

<Crystal lattice distortion>
(i) Preparation of measurement sample One side of a plate-shaped silicon nitride sintered body (thickness about 0.3 mm) was mechanically polished until the thickness became about 0.2 mm, and used as a sample for X-ray diffraction measurement. However, during mechanical polishing, a CMP (chemical mechanical polishing) slurry was used together with the diamond slurry to suppress and remove the generation of a polishing damage layer. The type of CMP slurry is not particularly limited, but in this embodiment, an alkali-based colloidal silica (particle size 50 nm) slurry was used.
(ii) Measurement The sample for X-ray diffraction measurement prepared as described above was calculated by the following procedure using X-ray diffraction (XRD) using CuKα radiation. From the X-ray diffraction pattern obtained by scanning the range of 2θ from 15 to 80° with an X-ray detector in steps of 0.02°, the integral widths of each 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 formula 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 "2 sin θ/λ" in the following formula 2 was plotted 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))

<Presence or absence of vacancy cluster defects>
(i) Preparation of Transmission Electron Microscope (TEM) Observation Samples For the plate-shaped silicon nitride sintered bodies (thickness about 0.3 mm) produced in each Example and Comparative Example, both surfaces were mechanically polished until the thickness was about 0.1 mm, and then 6 keV argon ions were irradiated until perforation was formed using an ion slicer "EM-09100IS" manufactured by JEOL Ltd. to prepare TEM observation samples. (ii) TEM Observation Using a transmission electron microscope "JEM-2100" manufactured by JEOL Ltd., 10 fields of view were observed at an accelerating voltage of 200 kV and an observation magnification of 2000 times.
When dislocation loops or helical dislocations were observed in at least one of the ten visual fields, it was evaluated as "there are particles containing dislocation loops or helical dislocations." When dislocation loops or helical dislocations were not observed in any of the ten visual fields, it was evaluated as "there are no particles containing dislocation loops or helical dislocations."

<Solution oxygen concentration>
The concentration of dissolved oxygen in the 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.
Cs ⁺ was used as the primary ion, and the Cs ⁺ 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 through a mass spectrometer to measure the dissolved oxygen concentration. The dissolved oxygen concentration was quantified by determining the relative sensitivity factor (RSF) of a standard sample in which ^16O was ion-implanted into ^30Si .

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

<Average particle size of silicon nitride crystal particles>
(i) Preparation of Scanning Electron Microscope (SEM) Observation Samples The cross sections of the plate-like silicon nitride sintered bodies produced in each of the Examples and Comparative Examples were mechanically polished until they had a mirror finish to prepare SEM observation samples.
(ii) SEM Observation Using a scanning electron microscope "JCM-7000" manufactured by JEOL Ltd., 5 visual fields were observed per sample at an acceleration voltage of 15 kV and an observation magnification of 5000 times, and particle images of 300 or more silicon nitride crystal particles were obtained.
The images of 300 or more particles were analyzed by an image analyzer attached to the SEM device to determine the area circle equivalent diameter of each particle. Then, the weighted average particle diameter based on the volume-based distribution of the area circle equivalent diameters was calculated.
The weighted average particle size based on the volume-based distribution can be calculated based on the following formula.
Weighted average particle size based on volume distribution = Σ (area equivalent circle diameter ^ 4) / Σ (area equivalent circle diameter ^ 3)

<Thermal Conductivity>
The thermal conductivity was measured by the laser flash method using a thermal conductivity measuring device "LFA467" manufactured by Netzsch.

The following materials were used to manufacture the silicon nitride sintered body.
<Silicon nitride powder>
Silicon nitride powder: Silicon nitride powder with an average particle size of 0.95 μm produced by combustion synthesis.

<Sintering aid>
Metal oxides: yttria (Y ₂ O ₃ ), alumina (Al ₂ O ₃ ), magnesia (MgO)

Example 1
To 100 parts by mass of silicon nitride powder, 3 parts by _mass of _Y2O3 , 1 part by _mass of _Al2O3 , and 2 parts by mass of MgO were added as sintering aids, and mixed in a planetary ball mill to obtain a mixed raw material. Next, the mixed raw material was used to perform uniaxial press molding and CIP molding to produce a molded body, which was introduced into a sintering furnace and sintered under a nitrogen atmosphere at atmospheric pressure. The sintering was performed under conditions of a maximum temperature of 1760°C and a holding time at the maximum temperature of 10 hours. Then, the material was cooled from the maximum temperature to 1000°C at a cooling rate of 20°C/min. The cooling was performed with the furnace pressure at 1 kPa. Next, the material was naturally cooled from 1000°C to room temperature to obtain a silicon nitride sintered body.
The results of various evaluations of the produced silicon nitride sintered bodies are shown in Table 1.

<Examples 2 to 5, Comparative Examples 1 and 2>
A silicon nitride sintered body was produced in the same manner as in Example 1, except that the firing conditions were changed as shown in Table 1. The results of various evaluations of the produced silicon nitride sintered body are shown in Table 1.

As shown in Table 1, the silicon nitride sintered bodies of the Examples, in which the crystal lattice distortion is 0.003 or more, have higher thermal conductivity than the silicon nitride sintered bodies of the Comparative Examples, in which the crystal lattice distortion is less than 0.003.
Generally, the higher the dissolved oxygen concentration, the lower the thermal conductivity, but the above-mentioned examples have higher thermal conductivity than the comparative examples, despite the higher dissolved oxygen concentration. From this, it can be said that it is effective to increase the crystal lattice distortion, especially in the region with a high dissolved oxygen concentration.

Claims (5)

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. 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 silicon nitride sintered body according to claim 2, wherein the crystal lattice distortion and the dissolved oxygen concentration (ppm) satisfy the following: crystal lattice distortion/dissolved oxygen concentration>2.4×10 ⁻⁶ . The silicon nitride sintered body according to claim 1, wherein the average particle size of the silicon nitride crystal particles is 1 to 5 μm. The silicon nitride sintered body according to any one of claims 1 to 4, which is a heat dissipation substrate.

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