JP7684401B2 - Silicon nitride sintered body and method for producing silicon nitride sintered body - Google Patents

Description

The present invention relates to a silicon nitride sintered body and a method for producing a silicon nitride sintered body.

In recent years, the heat generation density of power modules has increased with the increasing density and power output of electronic devices and semiconductor devices. The temperature rise of a power module can cause malfunction of elements and cracks in an insulating circuit board. For this reason, ceramic substrates such as alumina and aluminum nitride, which are materials with relatively high thermal conductivity, have been used for insulating circuit boards. However, alumina and aluminum nitride have the disadvantage of low mechanical strength. Therefore, thick copper, which is subject to strong thermal stress, cannot be directly bonded to a ceramic substrate, which has restricted the structure of the power module. Specifically, a problem is that the power module becomes large because it is necessary to solder a heat sink such as copper or aluminum to the insulating circuit board. Therefore, silicon nitride (Si ₃ N ₄ ) material has been attracting attention as an insulating circuit board. Since silicon nitride sintered bodies have higher strength and fracture toughness than alumina and aluminum nitride sintered bodies, it is possible to directly bond thick copper to an insulating circuit board, which contributes to the miniaturization of modules. For this reason, silicon nitride sintered bodies with improved mechanical strength and thermal conductivity performance are being developed.

For example, Patent Document 1 discloses a method for producing a silicon nitride sintered body substrate with improved mechanical properties and thermal conductivity. In this method, a sintering aid of one or more elements selected from Mg, Ca, Sr, Ba, Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, and Yb is added in a range of 1% to 15% by weight to silicon nitride powder containing 0.1% or less by weight of Al, and the mixture is molded and then fired at a temperature of 1700°C to 2300°C under a nitrogen gas pressure of 1 atm to 500 atm. The silicon nitride sintered body substrate obtained by this method is composed of 85% to 99% by weight of β-type silicon nitride grains and the remainder being a grain boundary phase of oxide or oxynitride. The grain boundary phase contains 0.5% by weight to 10% by weight of one or more metal elements selected from Mg, Ca, Sr, Ba, Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, and Yb. The grain boundary phase contains 1% by weight or less of Al atoms, has a porosity of 5% or less, and the ratio of β-type silicon nitride grains having a minor axis diameter of 5 μm or more in the microstructure of the sintered body is 10% by volume to 60% by volume. In other words, it is known that in order to obtain a silicon nitride sintered body substrate with high thermal conductivity, rare earth compounds and magnesium oxide are added as sintering aids, and the thermal conductivity and mechanical strength can be improved by changing the mixing ratio and the amount of addition.

Patent Document 2 discloses a silicon nitride sintered body substrate and a method for producing the same, which realizes thermal conductivity and mechanical strength by controlling the degree of orientation of β-silicon nitride (β-Si ₃ N ₄ ) particles. In Patent Document 2, silicon nitride powder containing 7% or less of β-silicon nitride is used as a raw material, and a sheet molded body is formed after adjusting the viscosity of the slurry to 13,000 cps or more, so that the intensity ratio I _{101 /I 210} between the intensity I 101 of the X-ray diffraction peak of the (101) plane of the crystal structure of the β-silicon nitride particles of the silicon nitride sintered body substrate produced by the method and the intensity I ₂₁₀ of the X-ray diffraction peak of the ( _{210) plane of the β-silicon nitride is} controlled to be closer to 1. As a result, the relative decrease in the fracture toughness value _KC1 in a first direction parallel to the substrate plane of the silicon nitride sintered substrate was suppressed relative to the fracture toughness value _KC2 in a second direction perpendicular to the first direction, and a silicon nitride sintered substrate having isotropic fracture toughness in both directions was obtained.

Japanese Patent Application Publication No. 9-30866 JP 2019-52072 A

The inventors reviewed the manufacturing method of conventional silicon nitride sintered bodies and focused on further controlling the degree of orientation of β-type silicon nitride (β-Si ₃ N ₄ ) particles, with the aim of further improving the thermal conductivity performance of β-type silicon nitride particles in the substrate thickness direction.

In order to solve the above problems, the object of the present invention is to provide a silicon nitride sintered body in which the orientation of β-type silicon nitride (β-Si ₃ N ₄ ) particles is controlled and the thermal conductivity performance in the thickness direction of the substrate is further improved, and a method for producing the silicon nitride sintered body.

One form of the silicon nitride sintered body of the present invention is characterized in that it forms a substrate containing β-type silicon nitride particles and has a negative Lotgering factor f(hk0), which indicates the degree of orientation of the (hk0) plane of the β-type silicon nitride particles in the substrate plane.

A further embodiment of the silicon nitride sintered body of the present invention is more preferably characterized by a thermal conductivity in the thickness direction of the substrate of 100 W/mK or more.

A silicon nitride sintered body according to a further embodiment of the present invention is more preferably characterized in that it has a fracture toughness value K _C1 perpendicular to the thickness direction and a fracture toughness value K _C2 in the thickness direction, and K _C1 /K _C2 is 0.85 or more.

A silicon nitride sintered body according to a further embodiment of the present invention is more preferably characterized in that, in a cross-sectional photograph taken of a cross-section of a substrate cut perpendicular to the plane, it contains 10 or more β-type silicon nitride particles having a major axis of 50 μm or more in an area of 200,000 ^μm2 .

A silicon nitride sintered body according to a further embodiment of the present invention is more preferably characterized in that, in a cross-sectional photograph taken of a cross-section obtained by cutting a substrate vertically, within an area of 200,000 ^μm2 , there are 8 or more β-type silicon nitride grains whose major axis is 50 μm or more and whose inclination angle with respect to the normal to the substrate surface is 45 degrees or less.

A further form of the silicon nitride sintered body of the present invention is more preferably characterized in that the arithmetic mean height Sa, which indicates the roughness of the substrate surface, is 0.8 μm or more.

One embodiment of the present invention relates to a method for producing a silicon nitride sintered body, _the method comprising the steps of _: mixing silicon powder, rare earth oxide powder, and magnesium silicon nitride powder to produce a mixed powder so that the molar ratio of the silicon nitride sintered body is 85 to 95 mol % of silicon nitride (Si _{3 N 4} ), 1 to 3 mol % of rare earth oxide (RE 2 O 3 ), and 4 to 12 mol % of magnesium silicon nitride (MgSiN 2 ); forming the mixed powder into a sheet to produce a molded body; a nitriding step of heating the molded body from a first temperature to a second temperature in a nitrogen atmosphere; and a densifying step of firing the molded body in a nitrogen atmosphere at a third temperature for a predetermined time to produce a silicon nitride sintered body.

A further embodiment of the method for producing a silicon nitride sintered body according to the present invention is more preferably characterized in that the silicon powder has a specific surface area of 5.0 m ² /g or more and a D _99.9 diameter of 9.5 μm or less.

In a further embodiment of the method for producing a silicon nitride sintered body according to the present invention, the magnesium silicon nitride powder is more preferably characterized in that the specific surface area is 9.0 m ² /g or more.

A further aspect of the method for producing a silicon nitride sintered body according to the present invention is more preferably characterized in that the molded body is produced by a sheet molding method.

A further embodiment of the method for producing a silicon nitride sintered body according to the present invention is more preferably characterized in that the inorganic filling rate of the molded body is 47% or more.

According to the silicon nitride sintered body and its manufacturing method of the present invention, the thermal conductivity in the thickness direction of the substrate is improved by controlling the long axis of the β-type silicon nitride particles to be aligned in the thickness direction of the substrate.

2 is a SEM image of the substrate surface of the silicon nitride sintered body of Example 1 taken at a magnification of 2000 times.

13 is an SEM image of the substrate surface of the silicon nitride sintered body of Example 5 taken at a magnification of 2000 times.

1 is an SEM image of the substrate surface of the silicon nitride sintered body of Comparative Example 1 taken at a magnification of 2000 times.

1 is an X-ray diffraction pattern of the silicon nitride sintered body of Example 1.

X-ray diffraction pattern of the silicon nitride sintered body of Example 5.

X-ray diffraction pattern of the silicon nitride sintered body of Example 12.

X-ray diffraction pattern of the silicon nitride sintered body of Comparative Example 1.

X-ray diffraction pattern of the silicon nitride sintered body of Comparative Example 7.

3 is a photograph of a cross section of a substrate of the silicon nitride sintered body of Example 1.

13 is a photograph of a cross section of a substrate of the silicon nitride sintered body of Example 5.

4 is a photograph of a cross section of a substrate of the silicon nitride sintered body of Comparative Example 1.

FIG. 2A is an exemplary image of a cross section of a substrate of a silicon nitride sintered body after a fracture toughness test, and FIG. 2B is a schematic diagram thereof, for explaining a method for measuring fracture toughness values in a first direction and a second direction in the present invention.

The silicon nitride sintered body of one embodiment of the present invention is formed into a substrate of a predetermined thickness, and can be used as a substrate for mounting electronic components, mainly by brazing (soldering) a metal plate such as a copper plate to the substrate surface. The thickness of the substrate is preferably 0.1 to 1.0 mm.

The silicon nitride sintered body of this embodiment is a sintered body composed of raw material powders of 85 to 95 mol % silicon, calculated as silicon nitride after silicon is completely nitrided, 1 to 3 mol % rare earth oxide, and 4 to 12 mol % magnesium silicon nitride. In this embodiment, the rare earth oxide may be selected from oxides of Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, Yb, or a combination thereof.

The β-type silicon nitride particles contained in the silicon nitride sintered body substrate have an elongated hexagonal columnar crystal structure having a long axis and a short axis. The silicon nitride sintered body of this embodiment has improved thermal conductivity in the thickness direction by controlling the preferential orientation of the β-type silicon nitride in the thickness direction of the substrate. The characteristics of the silicon nitride sintered body of this embodiment are shown below.

The silicon nitride sintered body of this embodiment is configured so that the Lotgering factor f(hk0), which indicates the degree of orientation of the (hk0) plane of β-type silicon nitride in the substrate plane, is a negative value.

The Lotgering factor f(hk0) is an index that represents the degree of orientation of each crystal grain that constitutes a polycrystalline body. The value of the Lotgering factor f(hk0) is calculated by the following formulas 1 to 3 using the integrated intensity of each diffraction peak of the X-ray diffracted from the target crystal plane.
f(hk0)=(ρ−ρ ₀ )/(1−ρ ₀ ) (Formula 1)
ρ ₀ = ΣI ₀ (hk0)/ΣI ₀ (hkl) (Formula 2)
ρ=ΣI(hk0)/ΣI(hkl) (Equation 3)
ρ ₀ is calculated using the diffraction intensity I ₀ in the range of 2θ from 20 degrees to 65 degrees in the X-ray diffraction spectrum of the non-oriented sample, and is obtained by formula 2 as the ratio of the total diffraction intensity ΣI ₀ (hk0) of the diffraction intensity of the (hk0) plane to the sum of all diffraction intensities ΣI ₀ (hkl). Here, the diffraction intensity I ₀ is calculated from the X-ray diffraction spectrum of a reference sample (a non-oriented sample having the same composition as the target sample). In addition, ρ is calculated using the diffraction intensity I in the range of 2θ from 20 degrees to 65 degrees in the X-ray diffraction spectrum of a sample of sintered silicon nitride, and is obtained by formula 3 as the ratio of the total diffraction intensity ΣI (hk0) of the diffraction intensity of the (hk0) plane to the sum of all diffraction intensities ΣI (hkl). Note that h, k, and l are integers and represent plane indices. If the crystal orientation of the obtained sample is random, the difference between ρ and ρ ₀ is small, and according to formula 1, f (hk0) is close to zero. If f(hk0) is a negative value (ρ< _ρ0 ), it means that the obtained sample has a lower degree of orientation of the (hk0) plane than the non-oriented sample. In other words, a negative value of the Lotgering factor f(hk0) of the obtained sample indicates that the (00l) plane (i.e., the long axis) of the β-type silicon nitride grains is strongly oriented in the plate thickness direction.

Another index showing the degree of orientation of β-type silicon nitride particles in the substrate of the silicon nitride sintered body of this embodiment is the intensity ratio I 101 /I ₂₁₀ between the intensity I 101 of the X-ray diffraction peak of the (101) plane of the β-type silicon nitride of the silicon nitride sintered body and the intensity I ₂₁₀ of the X-ray diffraction peak of the (210) plane _of the β-type silicon nitride. In the silicon nitride sintered body of this embodiment, the intensity ratio I ₁₀₁ /I ₂₁₀ is 1.5 or more, _and orientation of the major axis in the plate thickness direction is inferred.

Furthermore, in the silicon nitride sintered body of this embodiment, the arithmetic mean height Sa, which indicates the roughness of the substrate surface, is 0.8 μm or more. Since β-type silicon nitride particles are columnar particles, when the long axis of the β-type silicon nitride particles is preferentially oriented in the plate thickness direction, the particle tip is in a shape protruding from the substrate surface, and the arithmetic mean height Sa is 0.8 μm or more, which is a relatively high value.

In the silicon nitride sintered body of this embodiment, in a cross-sectional photograph taken of a cross section of the substrate cut perpendicular to the plane, it was observed that 10 or more coarse β-type silicon nitride particles having a major axis of 50 μm or more were contained in an area of 200,000 ^μm2 . Among them, it was observed that 8 or more coarse β-type silicon nitride particles having a major axis of 50 μm or more and an inclination angle of 45 degrees or less with respect to the normal line of the substrate surface were contained. In other words, analysis using a microscope showed that the major axes of many coarse β-type silicon nitride particles were oriented in the plate thickness direction.

In other words, the silicon nitride sintered body of this embodiment contains a relatively large number of coarse particles having a long axis of 50 μm or more, and the long axis of the β-type silicon nitride is preferentially oriented in the thickness direction of the substrate.

In the silicon nitride sintered body of this embodiment, the thermal conductivity in the thickness direction of the substrate is 100 W/mK or more. In other words, the β-type silicon nitride particles are rod-shaped, and the thermal conductivity in the long axis direction is approximately twice that in the short axis direction. Therefore, by aligning the β-particles in the thickness direction, the thermal conductivity in the thickness direction is greatly improved.

In addition, the silicon nitride sintered body of this embodiment is densified to a relative density of 98% or more. In the silicon nitride sintered body of this embodiment, the fracture toughness value K _C1 in the first direction parallel to the substrate plane is 5.5 MPa·m ^1/2 or more, and the fracture toughness value K _C2 in the second direction perpendicular to the substrate plane is 5.5 MPa·m ^1/2 or more. Furthermore, the ratio K _C1 /K _C2 between the fracture toughness value K _C1 in the first direction and the fracture toughness value K _C2 in the second direction is 0.85 or more. That is, in the silicon nitride sintered body, the columnar crystals are controlled to be aligned in the vertical direction, so that the mechanical strength (fracture toughness) of the substrate is isotropically exhibited, and high mechanical strength (fracture toughness) is exhibited in both the first direction and the second direction.

Next, a method for producing the silicon nitride sintered body of this embodiment will be described. The method for producing the silicon nitride sintered body is a reaction sintering method in which silicon powder is used as the starting material instead of silicon nitride powder, and the molded silicon powder is heated in a nitrogen atmosphere to simultaneously perform nitridation and densification. In general, the reaction sintering method improves the thermal conductivity of the sintered body because the raw material purity is high, but it is said that the raw material adjustment and sintering conditions for densification are difficult. In addition, in a general reaction sintering method, silicon powder is converted into rod-shaped β-type silicon nitride particles, so it is difficult to control the orientation in the plate thickness direction, and it is known that the orientation is likely to become random.

According to this embodiment, the method for producing a silicon nitride sintered body mainly includes a mixing step of mixing a silicon raw material powder, a rare earth oxide powder, and a magnesium silicon nitride powder to produce a mixed powder so that the silicon nitride sintered body has a molar ratio of 85 to 95 mol % of silicon nitride, 1 to 3 mol % of rare earth oxide, and 4 to 12 mol % of magnesium silicon nitride; a forming step of slurrying the mixed powder and forming it into a sheet to produce a molded body; a nitriding step of heating the molded body from a first temperature to a second temperature in a nitrogen atmosphere; and a densifying step of firing the molded body in a nitrogen atmosphere at a third temperature for a predetermined time to obtain a silicon nitride sintered body. Each step will be described in more detail below.

Silicon powder is prepared as a starting material. The silicon powder, organic solvent, and dispersant are pulverized in a ball mill, and the particle size is adjusted so that the specific surface area of the silicon powder is 5.0 m ² /g or more and the D _99.9 diameter is 9.5 μm or less. Here, in a particle size distribution curve with the horizontal axis being particle size (μm) and the vertical axis being frequency (%), the D ₅₀ diameter (median diameter) is the particle diameter with a frequency of 50%, and the D _99.9 diameter is the particle diameter with a frequency of 99.9% (corresponding to the most frequent value of the distribution). After the particle size adjustment of the silicon powder is completed, a rare earth oxide powder and magnesium silicon nitride powder are mixed as a sintering aid to prepare a mixed powder. The specific surface area of the magnesium silicon nitride powder is preferably 9.0 m ² /g or more. Here, in terms of molar ratio, 85-95 mol % silicon, 1-3 mol % rare earth oxide, and 4-12 mol % magnesium silicon nitride are mixed, calculated as silicon nitride after silicon is completely nitrided. This mixed powder is thoroughly mixed in a ball mill, and then a binder, a plasticizer, and an organic solvent are added to make a slurry.

Next, the slurry is vacuum degassed and the viscosity is adjusted. The proportion of organic solvent in the degassed slurry is adjusted to 35 wt % or less, and the viscosity of the slurry is adjusted to 15,000 to 25,000 cps. A sheet-shaped compact is then produced using a doctor blade or the like.

The molded body is produced so that the inorganic filling rate is 47% or more. In order to make the inorganic filling rate of the molded body 47% or more, it is preferable to adjust the crushing particle size so that the specific surface area of the silicon powder is 9.0 ^{m 2} /g or less (that is, in the range of 5.0 to 9.0 ^{m 2} /g), and to make the ratio of the organic solvent after defoaming 35 wt% or less. If the specific surface area of the silicon powder is larger than 9.0 m 2 /g, the ratio of fine silicon powder increases, which makes it easier to cause aggregation, and therefore the filling property becomes poor. In addition, if the ratio of the organic solvent contained in the slurry after defoaming exceeds 35 wt%, the organic solvent content that volatilizes during sheet molding increases, so drying shrinkage increases and fine bubbles are easily generated in the molded body. The inorganic filling rate is measured as follows. The sheet molded body used for the measurement has a residual organic solvent of 0.1 wt% or less. The volume and weight of the sheet molded body are measured, and the green density ρ _g (g/cm ³ ) of the molded body is measured. Thereafter, the sheet molded body used for the measurement was subjected to a binder removal treatment in the atmosphere at 500° C. for 3 hours. The weight after the binder removal treatment was measured to determine the organic fraction Pi (%), and the inorganic filling rate Fi (%) was calculated by the following formula 4.
Fi=ρ _g × (1-Pi/100)/ρ _th ×100 (Formula 4)
Here, ρ _th is the theoretical density at the time of mill blending, and is a value calculated from the weight ratio of the raw material inorganic components.

Next, the prepared sheet-like molded body was debindered in a dry air atmosphere at about 500 to 800 ° C. After that, it was heated in a vacuum to about 1000 ° C (first temperature) in a furnace, and then the nitrogen pressure atmosphere was changed and the temperature was raised from about 1000 ° C to about 1350 ° C (second temperature). At this time, the molded body can be nitrided by gradually raising the temperature from the first temperature to the second temperature (for example, 1 ° C / min) in the nitrogen pressure atmosphere. Then, the inside of the furnace is changed to a higher pressure nitrogen pressure atmosphere, and the temperature is raised from the second temperature to a third temperature of about 1750 to 2000 ° C (preferably 1900 ° C). After the temperature increase, the third temperature is held for a long time (for example, about 8 hours) to sinter the nitrided molded body and sufficiently densify the molded body, thereby producing a silicon nitride sintered body.

By going through the above-described steps, it is possible to produce a silicon nitride sintered body in which β-type silicon nitride particles are preferentially oriented in the thickness direction of the substrate. That is, in the manufacturing method, by using magnesium silicon nitride and minimizing the amount of added yttrium oxide (i.e., rare earth oxide is set to 1 to 3 mol%) to suppress the amount of oxygen in the molded body, it can be considered that the reduction in the nitriding step and the densification step is increased. When the reduction is increased in this way, the silicon oxide film on the surface of the silicon powder is reduced, and SiO(g) is vaporized in the plate thickness direction. Furthermore, the reduction reaction of SiO(g) + CO(g) → Si(g) + CO ₂ (g) is promoted, and the generated Si(g) undergoes a reaction process of 3Si(g) + 2N ₂ (g) → β-Si ₃ N ₄ in the porous body before densification, and it is considered that β-Si ₃ N ₄ precipitates in the plate thickness direction in the pores. Furthermore, when the heat treatment temperature is increased, a silicon nitride substrate is obtained in which β-type silicon nitride particles are preferentially oriented in the thickness direction, with β-Si ₃ N ₄ precipitated in the pores in the thickness direction as nuclei. It is believed that the extension of the β-type silicon nitride particles in the thickness direction increases the thermal conductivity and improves the heat dissipation properties as an insulating substrate.

The above-described process is merely an example and does not limit the present invention. For example, the method of forming the slurry is not limited to the doctor blade method, and the slurry may be pressure-molded into a sheet body by extrusion molding, casting molding, or the like.

The present invention will now be described in more detail with reference to examples and comparative examples, but the present invention should not be construed as being limited to the following examples.

The silicon nitride sintered bodies of Examples 1 to 20 and Comparative Examples 1 to 8 were prepared under the following conditions and procedures.

A silicon powder having a predetermined powder characteristic and a sintering aid powder were prepared. An appropriate amount of silicon powder was put into a ball mill, and the silicon powder, organic solvent, and dispersant were pulverized with the ball mill, and the particle size was adjusted until the specific surface area and D _99.9 diameter of the silicon powder reached the predetermined values. Here, the compounding composition ratio in each sample, and the values of D _99.9 diameter, D ₅₀ diameter, and specific surface area of the silicon powder are as shown in Table 1. After the particle size adjustment of the silicon powder was completed, a sintering aid was added and mixed with a ball mill for 1 hour. Then, a binder (polyvinyl butyral), a plasticizer (dioctyl adipate), and an organic solvent (mixed solvent of toluene and ethanol) were added to make a slurry. The slurry was vacuum defoamed to adjust the viscosity. The ratio of the organic solvent contained in the slurry after defoaming was set to 35 wt% or less, and the viscosity of the slurry was set to 15,000 to 25,000 cps. The viscosity of the slurry was measured by a TVC-7 viscometer manufactured by Toki Sangyo Co., Ltd. Specifically, a spindle was rotated in the slurry, and the viscosity was calculated from the resistance force. Then, a sheet-shaped molded body was produced by a doctor blade with a molding speed of 200 mm/min or more. Next, the inorganic filling rate Fi (%) of the sheet-shaped molded body of each sample was measured. BN was sprayed onto the surface of the produced sheet-shaped molded body as a release agent to prepare a laminate of 20 sheets per block. The laminate was debindered at 500°C in dry air, then placed in a furnace, heated to about 1000°C in a vacuum, and heated to about 1350°C at 1°C/min in a nitrogen pressure atmosphere of 0.2 MPa. Then, the nitrogen pressure atmosphere was set to 0.9 MPa, and the temperature was raised from about 1350°C to about 1900°C, and fired at about 1900°C for about 8 hours. After firing, the laminated substrate was separated and the sintered surface was honed by blasting alumina abrasive grains (average grain size: 50 μm) at a honing pressure of 0.4 MPa to obtain a silicon nitride sintered body of 190 mm×140 mm with a plate thickness of 0.35 mm.

For each of the samples of Examples 1 to 20 and Comparative Examples 1 to 8, the crystalline phase of each sample was identified by X-ray diffraction measurement, and the X-ray diffraction pattern was analyzed to derive the integrated intensity of each diffraction peak, the Lotgering factor f (hk0), and the intensity ratio I ₁₀₁ /I _210. In addition, for each sample, the β-type silicon nitride particles of the vertical cut section were observed to derive how many coarse particles with a major axis of 50 μm or more exist in an area of 200,000 μm ² , and how many coarse particles with an inclination angle of 45 degrees or less with respect to the normal line of the substrate surface exist. Furthermore, for each sample, the relative density, arithmetic mean height Sa, thermal conductivity (W/mK), and fracture toughness (MPa·m ^1/2 ) were measured. Various measurements were performed under the following conditions.

X-ray diffraction measurement and analysis Using an Ultima IV model manufactured by Rigaku Corporation, the X-ray diffraction intensity of each sample was measured by powder X-ray diffraction method using Cu-Kα radiation. Individual pieces cut to 10 mm x 10 mm were used for the measurement. The substrate surface after honing was used as the measurement surface. The measurement conditions were as follows.
Sampling width: 0.02 degrees Scan speed: 10 degrees/min Divergence slit: 2/3 degrees Divergence vertical slit: 10 mm
Scattering slit: 8 mm
Receiving slit: open Tube voltage/current: 40 kV/40 mA
Detector: Semiconductor detector In the X-ray diffraction pattern of the substrate plane obtained by X-ray incidence on the substrate plane, the integrated intensity of the diffraction peak corresponding to the Miller indices (hkl) of β-type silicon nitride particles was calculated. Based on the calculated peak intensities, the Lotgering factor f(hk0) and the intensity ratio I ₁₀₁ /I ₂₁₀ were derived.

Observation method of β-type silicon nitride particles on vertical cut section Using individual pieces cut to 10 mm x 10 mm, the individual pieces were embedded in epoxy resin, and the vertical section of the substrate was observed. The observation surface was prepared by the following procedure. A flat surface was formed with #800 diamond polishing paper, and polished with diamond slurry in the order of 15 μm, 6 μm, and 1 μm until the polishing scratches generated in each previous process disappeared, and a mirror surface was obtained by finishing polishing with alumina slurry of 50 nm. After mirror processing, _CF4 plasma etching was performed to obtain the observation surface. Then, using a laser microscope VKX-150 manufactured by Keyence Corporation, the above observation surface was observed with an objective lens magnification of 20 times, and a photograph of the cross section was taken. The cross-sectional photographs were subjected to image processing, and in the cross-sectional photographs of an area of 200,000 ^μm2 , the number of coarse particles among the β-type silicon nitride particles whose major axis was 50 μm or more, and further, the number of coarse particles among the coarse particles whose inclination was 45 degrees or less with respect to the normal to the substrate surface were measured.

Relative density: Calculated from the density of the sintered body relative to the theoretical density calculated from the raw material mixing ratio (Equation 5). The density of the sintered body was measured by Archimedes' method using pure water.
Relative density (%) = (sintered body density / theoretical density) × 100 (Equation 5)

Arithmetic mean height Sa
The surface roughness Sa of the substrate surface after honing was measured in an area of 500 μm×500 μm using a laser microscope VKX-150 manufactured by Keyence Corp. The measurement conditions were as follows:
Objective lens magnification: ×20
Image correction: Automatic surface tilt correction Filter type: Gaussian S-filter: 2 μm
F-operation: None L-filter: 0.2 mm
End effect compensation: Yes

Thermal conductivity The flash method was used to measure the thermal conductivity in the thickness direction of the substrate. A thermal conductivity measuring device LFA467 manufactured by NETZSCH Geratebau GmbH was used for the measurement. A piece cut to 10 mm x 10 mm from the substrate was used for the measurement. A gold sputtered film was formed on both sides of the piece in order to suppress the transmission of flash light, and graphene spray was used on both sides of the piece in order to uniformly absorb the pulsed light, and a blackening treatment was performed. When calculating the thermal conductivity, a value of 0.68 J/(g·K) was used as the specific heat of the obtained sintered body.

Fracture toughness The fracture toughness of each sample was measured according to JIS-R1607 using a Vickers hardness tester HV-120 manufactured by Mitutoyo Corporation. That is, a cross section of the substrate cut along the thickness direction of the substrate was mirror-polished, and an indentation having diagonal lengths a1 and a2 was formed on the mirror-finished surface near the center in the thickness direction of the mirror-finished surface as shown in Figures 12(a) and 12(b). The diagonal lengths a1 and a2 of the indentation and the lengths c1 and c2 of the cracks generated from the apex of the indentation were measured, and the fracture toughness value _KC was calculated from the indentation load, the diagonal length of the indentation, the crack length, and the elastic modulus. According to JIS-R1607, the fracture toughness value KC is calculated by the following formula.
K _C =0.026×E ^1/2 ×P ^1/2 ×a/C ^3/2 (Formula 6)
C=((c1/2+c2/2)/2)/2 (Formula 7)
a=((a1/2+a2/2)/2)/2 (Formula 8)
E: modulus of elasticity P: indentation load a: half the average of the diagonal length of the indentation C: half the average of the crack length In contrast, in this embodiment, the fracture toughness value K _C was evaluated separately as the fracture toughness value K _C1 in the first direction (parallel to the substrate plane) and the fracture toughness value K _C2 in the second direction (perpendicular to the substrate plane). Specifically, the crack length generated in the direction parallel to the substrate plane was set to c1, and C in formula 6 was set to c1/2 to calculate K _C1 . In addition, the crack length generated in the direction perpendicular to c1 (thickness direction of the substrate) was set to c2, and C in formula 6 was set to c2/2 to calculate K _C2 . The thickness of the test piece was 0.35 mm, and the indentation load P was 10 kgf.

The conditions and various measurement results for each sample of Examples 1 to 14 and Reference Examples 1 to 8 are shown in Tables 1 and 3. The conditions and various measurement results for each sample of Examples 15 to 20 are shown in Tables 2 and 4. Figures 1 to 3 show exemplary SEM photographs of the substrate surfaces of the samples of Examples 1 and 5 and Comparative Example 1 taken at 2000x magnification. Figures 4 to 8 show exemplary X-ray diffraction patterns for Examples 1, 5, and 12 and Comparative Examples 1 and 7. In the X-ray diffraction patterns, Miller indices (hkl) are listed for each diffraction peak of β-type silicon nitride particles. Figures 9 to 11 show exemplary cross-sectional photographs of Examples 1 and 5 and Comparative Example 1.

In Examples 1 to 20, the raw material powders were composed of 85 to 95 mol % silicon (calculated as silicon nitride), 1 to 3 mol % rare earth oxide, and 4 to 12 mol % magnesium silicon nitride in terms of molar ratio. Here, in Examples 1 to 14, yttrium oxide (Y ₂ O ₃ ) was selected as the rare earth oxide (RE ₂ O ₃ ). In Examples 15 to 20, La ₂ O ₃ , Sm ₂ O ₃ , Gd ₂ O ₃ , Dy ₂ O ₃ , Er ₂ O ₃ , and Yb ₂ O ₃ were selected as the rare earth oxide (RE ₂ O _{3 )} , respectively. The silicon powder as the starting material has a D _99.9 diameter of 8.0 to 9.5 μm (9.5 μm or less) and a specific surface area of 5.0 to 8.0 ^{m 2} /g (5.0 ^{m 2} /g or more and 9.0 ^{m 2} /g or less). In Examples 1 to 20, the inorganic filling rate of the molded body produced in the molding process was 47% or more. On the other hand, Comparative Examples 1 and 2 are samples in which magnesium oxide is used as the raw material powder instead of magnesium silicon nitride. Comparative Example 3 is a sample in which magnesium silicon nitride is 3 mol % (less than 4 mol %). Comparative Example 4 is a sample in which the silicon powder as the starting material has a D _99.9 diameter of 12.3 μm (larger than 9.5 μm) and a specific surface area of 4.3 m ² /g (less than 5.0 m ² /g). Comparative Example 5 is a sample in which the specific surface area of the silicon powder, which is the starting material, is 9.9 ^{m 2} /g (greater than 9.0 m ² /g). Comparative Example 6 is a sample in which yttrium oxide is 0.9 mol % (less than 1 mol %). Comparative Example 7 is a sample in which magnesium silicon nitride is 12.7 mol % (greater than 12 mol %) and the D _99.9 diameter is 10.1 μm (greater than 9.5 μm). Comparative Example 8 is a sample in which magnesium silicon nitride is 0.5 mol % (less than 4 mol %). In Comparative Examples 4, 5, and 7, due to the silicon powder characteristics, the inorganic filling rate of the molded body produced in the molding process was less than 47%. Note that in Comparative Examples 1 to 3, 6, and 8, the measurement of the inorganic filling rate of the molded body was omitted, but since the silicon powder characteristics and manufacturing conditions are similar, it is estimated that the inorganic filling rate will be 47% or more, as in the examples.

In the SEM photographs (magnification 2000x) of the substrate surface of the silicon nitride sintered body shown in Figures 1 to 3, rod-shaped β-type silicon nitride particles can be seen. In particular, in Figures 1 and 2 corresponding to Examples 1 and 5, cross sections in the minor axis direction of β-type silicon nitride particles whose major axes are aligned in the thickness direction of the substrate can be observed. Furthermore, in the X-ray diffraction patterns of Figures 4 to 8, diffraction peaks were confirmed at 2θ corresponding to the (110), (200), (101), (120), (201) and (301) planes of the β-type silicon nitride particles.

Table 1 shows the structural characteristics of each sample of Examples 1 to 14 and Comparative Examples 1 to 8 of the silicon nitride sintered body. According to Table 1, in Examples 1 to 14, the Lotgering factor f(hk0), which indicates the degree of orientation of the (hk0) plane of the β-type silicon nitride particles, is a negative value, and the intensity ratio I ₁₀₁ /I ₂₁₀ is 1.5 or more. In other words, it can be seen that the orientation of the β-type silicon nitride is dominant in the thickness direction of the substrate. In contrast, in Comparative Examples 1 to 8, the Lotgering factor f(hk0) is a positive value, and the intensity ratio I ₁₀₁ /I ₂₁₀ is less than 1.0. Table 1 also shows that in Examples 1 to 14, the arithmetic mean height Sa, which indicates the roughness of the substrate surface, is 0.8 μm or more. On the other hand, in Comparative Examples 1 to 8, except for Comparative Example 6, the arithmetic mean height Sa is less than 0.8 μm. Furthermore, as shown in Table 1 and Figures 9 and 10, the image analysis results of the photographed cross-sectional images of the substrate showed that in Examples 1 to 14, in the cross-sectional photographs of a region of 200,000 ^μm2 , 10 or more coarse β-type silicon nitride particles with a major axis of 50 μm or more were included, and among the coarse particles, 7 or more β-type silicon nitride particles had an inclination angle of 45 degrees or less with respect to the normal line of the substrate surface. On the other hand, in Comparative Examples 1 to 8, as shown in Table 1 and Figure ¹¹ , in the cross-sectional photographs of a region of 200,000 μm2, 8 or less coarse β-type silicon nitride particles with a major axis of 50 μm or more were included, and among the coarse particles, 6 or less β-type silicon nitride particles had an inclination angle of 45 degrees or less with respect to the normal line of the substrate surface.

Table 2 shows the structural characteristics of each sample of Examples 15 to 20 of the silicon nitride sintered body. In Examples 15 to 20, the Lotgering factor f(hk0), which indicates the degree of orientation of the (hk0) plane of the β-type silicon nitride particles, shows a negative value. In Examples 15, 16, and 20, the intensity ratio I ₁₀₁ /I ₂₁₀ shows 1.1 or more, and in Examples 17 to 19, the intensity ratio I ₁₀₁ /I ₂₁₀ shows 1.5 or more. Table 2 also shows that in Examples 15 to 20, the arithmetic mean height Sa, which indicates the roughness of the substrate surface, is 0.8 μm or more. Furthermore, as shown in Table 2, according to the image analysis results of the photographed substrate cross-sectional image, it was shown that in Examples 15 to 20, in the cross-sectional photograph of an area of 200,000 μm ² , 10 or more coarse particles of β-type silicon nitride with a major axis of 50 μm or more are included. Furthermore, Table 2 shows that, except for Example 20 in which the rare earth oxide _{was Yb2O3} , the coarse particles contained eight or more β-type silicon nitride particles having an inclination angle of 45 degrees or less with respect to the normal line to the substrate surface. Seven β-type silicon nitride particles having an inclination angle of 45 degrees or less were identified in Example 20. That is, it was shown that Examples 1 to 20 contained at least seven β-type silicon nitride particles having an inclination angle of 45 degrees or less with respect to the normal line to the substrate surface.

From the above results, it is presumed that in the silicon nitride sintered bodies of Examples 1 to 20, the long axis of β-type silicon nitride is preferentially oriented in the thickness direction of the substrate compared to the sample of the comparative example. It was also found that in the silicon nitride sintered bodies of Examples 1 to 20, a relatively large number of coarse β-type silicon nitride particles with a long axis of 50 μm or more are formed compared to the sample of the comparative example, and that the coarse particles are preferentially aligned in the thickness direction of the substrate (within 45 degrees of the normal). In other words, the silicon nitride sintered body of the present invention is characterized by crystal growth in which the β-type silicon nitride particles coarsen in the plate thickness direction.

Table 3 shows the thermal conductivity and physical strength of each sample of the silicon nitride sintered body of Examples 1 to 14 and Comparative Examples 1 to 8. According to Table 3, in Examples 1 to 14, the thermal conductivity in the thickness direction of the substrate was 100 W/mK or more. On the other hand, in Comparative Examples 1 to 8, the thermal conductivity was less than 100 W/mK. That is, in the crystal structure of the silicon nitride sintered body, it can be seen that the preferential orientation of the major axis of the β-type silicon nitride particles in the thickness direction of the substrate and the coarsening of the β-type silicon nitride particles contribute to the improvement of the thermal conductivity. Also, according to Table 3, in Examples 1 to 14, the fracture toughness value K _C1 in the first direction parallel to the substrate plane was 5.5 MPa·m ^1/2 or more, and the fracture toughness value K _C2 in the second direction perpendicular to the substrate plane was 5.5 MPa·m ^1/2 or more. Furthermore, in Examples 1 to 14, the ratio K _C1 /K _C2 of the fracture toughness value K _C1 in the first direction to the fracture toughness value K _C2 in the second direction was 0.85 to 1.2, indicating that the mechanical strength (fracture toughness) of the substrate was isotropically exerted. In contrast, in Comparative Examples 1 to 8, the ratio K _C1 /K _C2 was about 0.8, indicating that the fracture toughness value K _C2 in the second direction perpendicular to the substrate plane was clearly large.

Table 4 shows the thermal conductivity and physical strength of each sample of the silicon nitride sintered body of Examples 15 to 20. According to Table 4, in Examples 15 to 20, the thermal conductivity in the thickness direction of the substrate was 100 W/mK or more. Also, according to Table 4, in Examples 15 to 20, the fracture toughness value K _C1 in the first direction parallel to the substrate plane was 5.5 MPa·m ^1/2 or ^more , and the fracture toughness value K _C2 in the second direction perpendicular to the substrate plane was almost equal to 5.5 MPa·m ^1/2 or more. Furthermore, in Examples 15 to 20, the ratio K _C1 /K _C2 of the fracture toughness value K _C1 in the first direction to the fracture toughness value K _C2 in the second direction was 0.85 to 1.2, which showed that the mechanical strength (fracture toughness) of the substrate was isotropically exerted.

The present invention is not limited to the above-described embodiment, and can be embodied in various forms within the technical scope of the present invention.

Claims (13)

A silicon nitride sintered body having a substrate containing β-type silicon nitride particles, characterized in that the Lotgering factor f(hk0), which indicates the degree of orientation of the (hk0) plane of the β-type silicon nitride particles in the substrate plane, is a negative value. The silicon nitride sintered body according to claim 1, characterized in that the thermal conductivity in the thickness direction of the substrate is 100 W/mK or more. 2. The silicon nitride sintered body according to claim 1, which has a fracture toughness value K _C1 perpendicular to the thickness direction and a fracture toughness value K _C2 in the thickness direction, and K _C1 /K _C2 is 0.85 or more. The silicon nitride sintered body according to claim ¹ , characterized in that in a cross-sectional photograph taken of a cross section of a substrate cut perpendicular to a plane, there are 10 or more β-type silicon nitride particles having a major axis of 50 μm or more in an area of 200,000 μm2. 2. The silicon nitride sintered body according to claim 1, characterized in that in a cross-sectional photograph taken of a cross section of a substrate cut perpendicular to a plane, an area of 200,000 ^μm2 contains 7 or more β-type silicon nitride particles having a major axis of 50 μm or more and an inclination angle of 45 degrees or less with respect to the normal to the substrate surface. The silicon nitride sintered body according to claim 1, characterized in that the arithmetic mean height Sa, which indicates the roughness of the substrate surface, is 0.8 μm or more. a mixing step of mixing silicon powder, rare earth oxide powder and magnesium silicon nitride powder having a D _99.9 diameter of 9.5 μm or less and a specific surface area of 5.0 ^{m 2} /g or more and 9.0 m 2 /g or less to produce a mixed powder so that the silicon nitride sintered body has a molar ratio of 85 to 95 mol % of silicon nitride, 1 to ³ mol % of rare earth oxide and 4 to 12 mol % of magnesium silicon nitride;
a molding step of forming the mixed powder into a sheet shape to produce a green body;
a nitriding step of heating the compact from a first temperature to a second temperature in a nitrogen atmosphere;
a densification step of sintering the molded body in a nitrogen atmosphere at a third temperature for a predetermined time to produce a silicon nitride sintered body;
A method for producing a silicon nitride sintered body, comprising: 8. The method according to claim 7, wherein the magnesium silicon nitride powder has a specific surface area of 9.0 m ² /g or more. The manufacturing method described in claim 7, characterized in that the molded body is produced by a sheet molding method. The manufacturing method described in claim 7, characterized in that the inorganic filling rate of the molded body is 47% or more. The thermal conductivity in the thickness direction of the substrate is 100 W/mK or more, and
2. The silicon nitride sintered body according to claim 1, which has a fracture toughness value K _C1 perpendicular to the thickness direction and a fracture toughness value K _C2 in the thickness direction, and K _C1 /K _C2 is 0.85 or more. In a cross-sectional photograph taken of a cross section of a substrate cut perpendicular to a plane, 10 or more β-type silicon nitride particles having a major axis of 50 μm or more are contained in an area of 200,000 ^μm2 , and
In a cross-sectional photograph taken of a cross section of the substrate cut perpendicularly to a plane, ⁷ or more β-type silicon nitride particles having a major axis of 50 μm or more and an inclination angle of 45 degrees or less with respect to the normal line of the substrate surface are contained in an area of 200,000 μm2,
12. The silicon nitride sintered body according to claim 1 , wherein the arithmetic mean height Sa, which indicates the roughness of the substrate surface, is 0.8 μm or more. _8. The method _of claim 7 _, wherein _the rare earth oxide is selected from at least one of _the group consisting of _Y2O3 , _La2O3 , _{Sm2O3 , Gd2O3} , _Dy2O3 , _{Er2O3 ,} and _Yb2O3 .