WO2024084631A1

WO2024084631A1 - Silicon nitride sintered body

Info

Publication number: WO2024084631A1
Application number: PCT/JP2022/038967
Authority: WO
Inventors: 理松本; 光隆高橋
Original assignee: 株式会社Maruwa
Priority date: 2022-10-19
Filing date: 2022-10-19
Publication date: 2024-04-25

Abstract

[Problem] To reduce volume shrinkage during cooling after sintering so as to reduce voids in a silicon nitride sintered body and reduce warpage of the silicon nitride sintered body. [Solution] Provided is a silicon nitride sintered body in which is used a material obtained by adding, to silicon nitride powder, 2 to 3 mass% of MgO as a sintering aid, and 2.7 to 4 mass% of a rare earth oxide (the amount thereof is greater than the MgO) having an oxidation number of 3, the material being sintered, and the silicon nitride sintered body comprising a silicon nitride and a grain boundary phase formed from the sintering aid, wherein the grain boundary phase has an amorphous structure, and from among the peak of a crystalline compound in the grain boundary phase, in which the diffraction angle 2θ is in a range of 28° to 32° in an X-ray diffraction pattern obtained using an X-ray diffraction device provided with a semiconductor detector, the largest integral intensity is 2.4% or less with respect to the integral intensity of the silicon nitride (101) plane.

Description

Silicon nitride sintered body

The present invention relates to silicon nitride sintered bodies that are used as circuit boards, heat dissipation components, etc.

2. Description of the Related Art Examples of insulating ceramics (sintered bodies) used as circuit boards, heat dissipation members, and the like include aluminum nitride (AlN) and silicon nitride (Si ₃ N ₄ ).
Aluminum nitride has a high thermal conductivity of 150 W/m·K or more, but has low mechanical strength, making it prone to cracks and difficult to use.
Silicon nitride has a thermal conductivity of 50 W/m·K or more, although it is not as high as aluminum nitride, and has high mechanical strength, so it has the advantages of being less susceptible to cracks and being able to be made thinner. For this reason, the development and adoption of silicon nitride sintered bodies has progressed in recent years.

Patent Document 1 describes a silicon nitride sintered body in which the crystal phase present in the grain boundary phase of silicon nitride crystal grains has an X-ray diffraction peak intensity ratio of 0.05 to 0.40 (silicon nitride being 1), and a manufacturing method in which a green sheet of this material is sintered at 1600 to 1900°C in a nitrogen atmosphere, and then the residual glass phase is removed at 1100 to 1700°C.

Patent Document 2 describes a silicon nitride substrate that has improved bonding of circuit components and has a dielectric breakdown voltage of 36 to 47 kV/mm (Table 6 in the same document) measured on a substrate 3 mm thick, and a manufacturing method in which a green sheet of the material is placed in a firing furnace containing magnesium oxide and erbium oxide co-materials to suppress volatilization of the components, and sintered at 1750°C for 3 to 5 hours.

Patent Document 3 describes a silicon nitride substrate with a porosity of 0-1.0%, a maximum pore diameter of 0.2-3 μm, and a dielectric strength of 17-29 kV/mm (Tables 7 and 8 of the same document) measured on a substrate with a thickness of 0.15-0.635 mm, and a manufacturing method in which a green sheet of the material is sintered at 1800-1900°C in a non-oxidizing atmosphere.

Patent Document 4 describes a silicon nitride substrate with a warpage of 2.0 μm/mm or less, and a manufacturing method in which a green sheet of the material is sintered in a pressurized nitrogen atmosphere at 1800-2000°C for 8-18 hours, and then heat-treated at 1550-1700°C while applying a load to suppress the warpage.

Patent Document 5 describes a silicon nitride substrate that has little warping and high strength, and a manufacturing method in which a green sheet of the material is placed in a firing vessel in which packed powder such as magnesium oxide is placed to suppress the volatilization of silicon nitride and magnesia, and sintered at 1,860°C for five hours.

Patent document 6 describes a silicon nitride substrate with a void ratio of 0.1-4% and a dielectric breakdown strength of 32-36 kV/mm (Table 3 in the same document) measured on a substrate with a thickness of 0.15-0.25 mm, and a manufacturing method in which a green sheet of the material is sintered in a nitrogen atmosphere at 1850-1900°C for 3-5 hours.

Japanese Patent Application Laid-Open No. 5-279124 International Publication No. 2011-087055 JP 2017-178776 A JP 2009-218322 A JP 2020-93978 A JP 2014-73937 A

However, none of Patent Documents 1 to 6 describes a silicon nitride sintered body in which the grain boundary phase formed by the sintering aid has an amorphous structure. Rather, Patent Document 1 describes that if the sintering aid remains in the grain boundary phase as a glass phase, high-temperature properties such as high-temperature strength and creep resistance decrease, and therefore, as described above, the remaining glass phase is removed by crystallizing it at 1100 to 1700°C after sintering.

In contrast, the inventors' research has revealed that if the grain boundary phase formed by the sintering aid is a crystalline phase, the volumetric shrinkage during cooling after sintering becomes large, the number of voids in the silicon nitride sintered body increases, and the warping of the silicon nitride sintered body becomes large. It has also been found that if the grain boundary phase formed by the sintering aid has an amorphous structure, the volumetric shrinkage during cooling after sintering becomes small, the number of voids in the silicon nitride sintered body decreases, and the warping of the silicon nitride sintered body becomes small. The present invention was made as a result of further intensive research into this issue.

[1] A material in which 2 to 3 mass% of MgO is added as a sintering aid to silicon nitride powder and 2.7 to 4 mass% of a rare earth oxide having an oxidation number of 3 (however, the amount is greater than the amount of MgO) is added is used and sintered,
A silicon nitride sintered body comprising silicon nitride and a grain boundary phase formed with a sintering aid, wherein the grain boundary phase has an amorphous structure, and the maximum integrated intensity of peaks of crystalline compounds in the grain boundary phase present in a diffraction angle 2θ range of 28° to 32° in an X-ray diffraction pattern obtained using an X-ray diffractometer equipped with a semiconductor detector is 2.4% or less of the integrated intensity of the silicon nitride (101) plane.
[2] The grain boundary phase preferably contains at least MgO or MgSiN ₂ and does not contain SrO.
[3] It is preferable that the thermal conductivity is 72 W/mK or more.
[4] The silicon nitride sintered body is processed into a test piece measuring 40 mm × 20 mm × 0.32 mm, and the three-point bending strength measured at a crosshead speed of 0.5 mm/min, a support distance of 30 mm, and at room temperature (23±2°C) is preferably 625 MPa or more.
[5] It is preferable that the surface of the silicon nitride sintered body is polished by 50 μm or more, and in at least one arbitrary 64 μm × 48 μm area of the polished surface, the plane projection area ratio of voids is 1.0% or less.
[6] It is preferable that the thermal conductivity is 80 W/m·K or more.
[7] A circuit board using the silicon nitride sintered body according to any one of 1 to 6 above.
[8] A heat dissipation member using the silicon nitride sintered body according to any one of 1 to 6 above.
[9] An insulating member using the silicon nitride sintered body according to any one of 1 to 6 above.

[Action]
The grain boundary phase has an amorphous structure, and among the peaks of crystalline compounds in the grain boundary phase that exist in the diffraction angle 2θ range of 28° to 32° in an X-ray diffraction pattern obtained using an X-ray diffractometer equipped with a semiconductor detector, the maximum integrated intensity is 2.4% or less of the integrated intensity of the silicon nitride (101) plane, so that the volume shrinkage is small during cooling after sintering, and the sintering aid exists as a liquid phase until a lower temperature and penetrates to the narrow portions between the silicon nitride crystal grains, so that the number of voids in the silicon nitride sintered body is reduced, and the internal stress of the silicon nitride sintered body is reduced, resulting in less warping. Also, the irregularities in the void shape are reduced.
In addition, the addition of an alkaline earth metal as a sintering aid has the effect of lowering the melting point of the liquid phase. However, even if it is an alkaline earth metal, SrO is less likely to volatilize than MgO or _MgSiN2 , so it remains after sintering and becomes a factor that inhibits thermal conduction, so by containing at least MgO or _MgSiN2 and not containing SrO, a silicon nitride sintered body with high thermal conductivity can be obtained.

According to the present invention, the number of voids in the silicon nitride sintered body is reduced, and the warping of the silicon nitride sintered body is reduced.

FIG. 1 shows X-ray diffraction patterns of sintered silicon nitride bodies, where (a) is the same pattern for Example 1 and (b) is the same pattern for Comparative Example 1. FIG. 2 shows SEM photographs of the silicon nitride sintered body, where (a) is the same photograph of Example 1 and (b) is the same photograph of Comparative Example 1. FIG. 3 is a diagram for explaining the unevenness of voids. FIG. 4 is a diagram for explaining a method for measuring the warpage of a silicon nitride sintered body. FIG. 5 is a diagram showing examples of applications of the silicon nitride sintered body.

The silicon nitride sintered body of the present invention is a silicon nitride sintered body consisting of silicon nitride and a grain boundary phase formed by a sintering aid, characterized in that the grain boundary phase has an amorphous structure. In addition to the preferred embodiments exemplified in the above means, the following preferred forms are exemplified.

1. Manufacturing Method In a method for manufacturing a silicon nitride sintered body, the sintering step of sintering a mixture of silicon nitride powder and a sintering aid is preferably performed such that 1930≦sintering temperature (° C.)+sintering time (hr)×50≦2200, and the grain boundary phase formed by the sintering aid has an amorphous structure.
It is preferable that no peaks derived from grain boundary phases are detected in an X-ray diffraction pattern obtained using an X-ray diffractometer equipped with a semiconductor detector.
In the sintering step, it is preferable to place a pre-sintered plate-shaped silicon nitride sintered body separate from the silicon nitride sintered body to be produced in a closed casing for firing.
It is preferable that the sintering aid contains at least MgO or _MgSiN2 , and does not contain SrO.

By setting the temperature (°C) + firing time (hr) x 50 ≤ 2200, sintering is realized, and the volatilization of _SiO2 during sintering is suppressed, and the crystallization of the grain boundary phase is suppressed. By making the grain boundary phase formed by the sintering aid into an amorphous structure, the volume shrinkage during cooling after sintering is reduced, and the sintering aid exists as a liquid phase at lower temperatures and penetrates to the narrow parts between the silicon nitride crystal grains, so that the number of voids in the silicon nitride sintered body is reduced, and the internal stress of the silicon nitride sintered body is reduced, resulting in less warping. In addition, the unevenness of the void shape is reduced.
In addition, in the sintering step, if a pre-sintered plate-shaped silicon nitride sintered body (hereinafter referred to as a "dummy silicon nitride sintered body") separate from the silicon nitride sintered body to be produced is placed in a closed casing for firing, the SiO ₂ of the dummy silicon nitride sintered body will volatilize during firing, suppressing the volatilization of SiO ₂ of the silicon nitride sintered body to be produced, which also suppresses crystallization and prevents a decrease in sintering density. The dummy silicon nitride sintered body does not need to have the same composition as the silicon nitride sintered body to be produced, but it is preferable that they use the same auxiliary agent system.
In addition, the addition of an alkaline earth metal as a sintering aid has the effect of lowering the melting point of the liquid phase. However, even if it is an alkaline earth metal, SrO is less likely to volatilize than MgO or _MgSiN2 , so it remains after sintering and becomes a factor that inhibits thermal conduction, so by containing at least MgO or _MgSiN2 and not containing SrO, a silicon nitride sintered body with high thermal conductivity can be obtained.
Furthermore, by densifying the silicon nitride sintered body to a relative density of 98% or more, the bending strength and the dielectric breakdown voltage become high.

2. Voids In at least one 64 μm × 48 μm area of a polished surface obtained by polishing the surface of a silicon nitride sintered body by 50 μm or more, the number of voids having a degree of irregularity of 0.9 or more, calculated by dividing the area within the contour line of the void by the area within the envelope line of the void, preferably accounts for 10% or more of the total number of voids.
It is preferable that in at least one 64 μm x 48 μm area of a polished surface obtained by polishing the surface of a silicon nitride sintered body by 50 μm or more, the number of voids having a degree of irregularity of 0.8 or more, calculated by dividing the area within the contour line of the void by the area within the envelope line of the void, accounts for 30% or more of the total number of voids.
In the above area, it is preferable that the plane projected area ratio of voids is 1.0% or less.

When voids with a degree of unevenness of 0.9 or more account for 10% or more, or when voids with a degree of unevenness of 0.8 or more account for 30% or more, partial discharges that occur in the void-shaped uneven parts when voltage is applied are less likely to occur due to the small unevenness, and are reduced, resulting in an increase in the breakdown voltage.
Furthermore, by keeping the plane projected area ratio of the voids at 1.0% or less, warping of the silicon nitride sintered body is reduced.

3. Warpage A plate-shaped silicon nitride sintered body is preferably maintained at 120°C for 1 hour or more, and then placed on a flat sample stage at 25°C. The warpage is measured within 1 minute of placing the plate-shaped silicon nitride sintered body on a flat sample stage at 25°C, and is calculated as a ratio of the difference between the height of the highest point of the upper surface of the silicon nitride sintered body from the sample stage to the height of the lowest point of the upper surface of the silicon nitride sintered body from the sample stage to the maximum transverse length of the silicon nitride sintered body, and is preferably 0.2% or less.
Here, the maximum transverse length of the silicon nitride sintered body refers to the maximum length of a line segment that crosses the plate surface of the silicon nitride sintered body from one point on its edge to another point, and is, for example, the diagonal length when the plate surface is rectangular, and the diameter length when the plate surface is circular.

As described above, the measured warpage is 0.2% or less, so even if products in which the silicon nitride sintered body is used as a circuit board, heat dissipation component, etc. are exposed to high-temperature environments exceeding 100°C, the small warpage of the silicon nitride sintered body ensures sufficient heat dissipation and is less likely to break,

4. Dielectric Breakdown Voltage When an AC voltage is applied to a plate-shaped silicon nitride sintered body having a thickness of 100 μm, the dielectric breakdown voltage is preferably 5 kV or more.

Since the dielectric breakdown voltage when an AC voltage is applied to a 100 μm-thick plate-shaped silicon nitride sintered body is 5 kV or more, the silicon nitride sintered body can be used in applications where a high dielectric breakdown voltage is required when actually formed to a thickness of about 100 μm.
It should be noted that the "thickness of 100 μm" is defined only as a condition for measuring the dielectric breakdown voltage, and does not define the thickness of the silicon nitride sintered body product. In other words, the silicon nitride sintered body product may have any thickness, and it is preferable that the dielectric breakdown voltage measured after processing the silicon nitride sintered body to a thickness of 100 μm is 5 kV or more.

5. Applications Applications of the silicon nitride sintered body are not particularly limited, but the following applications can be exemplified.
As shown in FIG. 5A, a circuit board used in a semiconductor module, an LED package, a Peltier module, a printer, a multifunction machine, a semiconductor laser, optical communication, high frequency, etc.
A general-purpose heat dissipation member as shown in FIG.
A heat dissipation member (heat sink) for a power semiconductor module as shown in FIG.
An insulating plate as shown in FIG.
An insulating plate for bonding wafers as shown in FIG. 5(e).
A heat dissipation member embedded in a flexible resin or the like as shown in FIG.
Although not shown, a high frequency window is used in gyrotrons, klystrons, etc.

Next, examples of the present invention will be described with reference to the drawings, in comparison with comparative examples. Note that the materials, quantities, and conditions of each part in the examples are examples, and can be changed as appropriate without departing from the gist of the invention.

Silicon nitride sintered bodies were produced as Examples 1 to 21 shown in Tables 1 and 2, and as Comparative Examples 1 to 8 shown in Table 3. Hereinafter, the term "each example" refers to each of Examples 1 to 21 and Comparative Examples 1 to 8. Example 21 is a reference example.

[1] Materials Silicon nitride powder having an average particle size (D50) of about 1.0 μm produced by the imide pyrolysis method or the direct nitridation method was used as the main raw material silicon nitride (Si ₃ N ₄ ) in each example.

As the sintering aid, _two _{types selected from the powders of MgO, MgSiN2, Y2O3, La2O3} _, _Nd2O3 _, _Sm2O3 _, _and _Dy2O3 were used in each example, as shown in Tables 1 to 3. In Examples 1 to 21, at least _MgO or _MgSiN2 was used, and _SrO was not used.

[2] Manufacturing method (i) Mixing process of materials For each example, the sintering aid powder was mixed with the silicon nitride powder in the mass% shown in Tables 1 to 3 (the total of the silicon nitride powder and the sintering aid powder was 100 mass%). 0.3 parts by weight of a surface-active dispersant and about 50 parts by weight of a mixed solvent of toluene and ethanol were added to 100 parts by weight of this mixed powder, and the mixture was pulverized and mixed in a ball mill using a resin container and silicon nitride balls.

A dissolved binder solution consisting of 10 parts by weight of polyvinyl butyral as a binder, 4 parts by weight of dioctyl adipate as a plasticizer, and approximately 20 parts by weight of a mixed solvent of toluene and ethanol was added to this pulverized mixture, and the dissolved binder solution and the pulverized mixture were stirred and mixed in a ball mill until they were completely mixed, after which a slurry was produced. The slurry was then heated and left in a vacuum to degas and volatilize the solvent, adjusting the viscosity at 25°C to 15,000 cps.

(ii) Green Sheet Preparation Step Next, a plate-shaped green sheet was obtained from each of the prepared slurries by a doctor blade method. The final drying temperature in the doctor blade molding device was 90° C. The obtained green sheet was punched into a rectangle of 180 mm × 250 mm by die pressing.

A boron nitride (BN) powder slurry was sprayed onto the surface of the die-cut green sheet as a release agent, and the green sheet laminate made by stacking multiple green sheets was placed in a BN case and heated to 500°C in a dry air flow for approximately four hours, performing a degreasing process to remove organic components such as binders.

(iii) Green Sheet Sintering Process For Examples 1 to 21, a green sheet laminate was placed on a bottom plate made of BN, a setter made of BN was placed on top of the laminate, a tungsten block was placed on the setter as a load, and the plate-shaped dummy silicon nitride sintered body described above was placed on the load.
Next, a side plate and a top plate made of BN were attached to the bottom plate to assemble a closed case. The case containing the green sheet and the like was placed in a sintering furnace, and the inside of the sintering furnace was filled with a nitrogen atmosphere of 0.9 MPa. The case was not completely sealed, but was closed to the extent that nitrogen could flow in, so the inside of the case was also filled with a nitrogen atmosphere of 0.9 MPa.

In this state, the green sheet laminate was sintered by heating for the firing time at the firing temperature shown in Tables 1 and 2 for each example, and the sintered laminate was separated into individual silicon nitride sintered bodies. The separated silicon nitride sintered bodies were subjected to honing to remove the BN mold release agent. The silicon nitride sintered bodies after honing were broken on the four outer periphery sides with a diamond scriber, and the shape and dimensions of the silicon nitride sintered bodies finally obtained were rectangular plate-like 139.6 mm x 190.5 mm x 0.32 mm.
In Examples 1 to 21, the firing temperature was in the range of 1830 to 1920° C., and sintering was performed for a relatively short time so as to satisfy the following formula 1.
1930≦firing temperature (° C.)+firing time (hr)×50≦2200 (Equation 1)

In Comparative Examples 1 to 7, the firing temperature was in the range of 1860 to 1880° C., but sintering was carried out for a relatively long time so as to exceed the upper limit of the above formula 1.
In Comparative Example 8, the firing temperature was set to 1800° C., and sintering was performed for a short time so that the temperature was below the lower limit of the formula (1).

[3] Characteristics The characteristics of the silicon nitride sintered body of each example were measured in terms of relative density, three-point bending strength, thermal conductivity, identification of grain boundary phase by X-ray diffraction method, voids, warpage, and dielectric breakdown voltage (shown in Tables 1 to 3).

(i) Relative density and three-point bending strength The relative density of the silicon nitride sintered body is measured density/theoretical density. The measured density was measured by the Archimedes method in which the silicon nitride sintered body was submerged in pure water. The theoretical density was calculated from the mixture ratio of the raw material powder using the following values as the density of the raw material powder: _Si3N4 = _3.18g / ^cm3 , MgO = 3.60g/ ^cm3 , ^MgSiN2 = 3.07g/ ^cm3 , _Y2O3 ₌ 5.01g/ ^cm3 _, _La2O3 ₌ 6.51g/ ^cm3 , Nd2O3 = 7.24g _/ _cm3 _, _Sm2O3 = 7.60g/ ^cm3 , Dy2O3 ₌ 7.81g/ _cm3 ^.
The three-point bending strength was measured by processing the silicon nitride sintered body into a test piece measuring 40 mm × 20 mm × 0.32 mm, and using a universal testing machine, model "AG-IS", manufactured by Shimadzu Corporation, at a crosshead speed of 0.5 mm/min, a support distance of 30 mm, and room temperature (23±2°C).

In Examples 1 to 21 and Comparative Examples 1 to 4, 6, and 7, the relative density was 98% or more, and they were sufficiently densified, so that the three-point bending strength was 600 MPa or more.
In Comparative Examples 5 and 8, the relative density was less than 98%, and therefore sufficient densification was not achieved. Therefore, the three-point bending strength was less than 600 MPa, and a high-strength silicon nitride sintered body could not be obtained.

(ii) Thermal Conductivity The thermal conductivity was measured by processing the silicon nitride sintered body into a test piece having a size of 10 mm × 10 mm × 0.32 mm, subjecting the test piece to a surface treatment (Ag film deposition + carbon blackening treatment), and then using a thermal conductivity measuring device manufactured by NETZSCH, model "LFA 467 HyperFlash."

(iii) Identification of grain boundary phase by X-ray diffraction method The silicon nitride sintered body was processed into a test piece measuring 10 mm × 10 mm × 0.32 mm, and an X-ray diffraction pattern of the test piece plane was obtained by powder X-ray diffraction method using Cu-Kα radiation using an X-ray diffractometer manufactured by Rigaku Corporation: Model "Ultima IV" (Cu target of sealed tube, Ni filter used, one-dimensional semiconductor detector).
In the obtained X-ray diffraction pattern, the integrated intensity of the (101) plane of α-Si ₃ N ₄ (hereinafter referred to as "silicon nitride I") and the integrated intensity of the maximum peak among the peaks of the Si-Y-N-O compound of the grain boundary phase having a diffraction angle 2θ in the range of 28° to 32° (hereinafter referred to as "grain boundary phase I") were calculated by the following procedure to determine the integrated intensity ratio (grain boundary phase I/silicon nitride I).
(1) Preprocessing is performed by removing background, removing Kα2 and smoothing, and then a peak search is performed.
(2) The background profile is calculated by subtracting the peak profile from the measured data, and the calculated data is fitted with a B-spline function.
(3) The peak shape is represented by a split pseudo-Voight function, and the integrated intensity is calculated.

1(a) shows the X-ray diffraction pattern of Example 1. No peak derived from the grain boundary phase formed by the sintering aid was detected, and the integrated intensity ratio was 0 as shown in Table 1. This indicates that no grain boundary crystalline phase exists, and the grain boundary phase has a substantially amorphous structure.
Examples 2 to 19 were similar to Example 1.
In Example 20, a peak derived from the grain boundary phase formed by the sintering aid was detected, but as shown in Table 1, the integrated intensity ratio was only 2.4%, which is also an amorphous structure.

1(b) shows the X-ray diffraction pattern of Comparative Example 1. A peak derived from the grain boundary phase formed by the sintering aid was detected, and as shown in Table 3, the integrated intensity ratio was 24.6%. This indicates that not only was the grain boundary crystalline phase present, but that the grain boundary phase was substantially composed of a crystalline phase.
Comparative Examples 2 to 7 were basically similar to Comparative Example 1 (although the integrated intensity ratios were different).
In Comparative Example 8, no peak derived from the grain boundary phase formed by the sintering aid was detected, and the integrated intensity ratio was 0, as shown in Table 1. This indicates that no grain boundary crystal phase exists, and the grain boundary phase is substantially an amorphous structure. However, as described below, Comparative Example 8 has a low relative density and few voids with an irregularity degree of 0.8 or more.

(iv) Voids The silicon nitride sintered body was subjected to a surface treatment as follows.
The silicon nitride sintered body was processed into a test piece of 8 mm x 8 mm x 0.32 mm, and fixed to a φ40 aluminum sample stage using Alcowax "5402SL" manufactured by Nikka Seiko Co., Ltd.
The sample stage was set on a sample rotator (model "SP-L1") manufactured by IMT Corporation, and the silicon nitride sintered body was surface-polished (polishing load: 15N, polishing plate rotation speed: 150 rpm, sample rotation speed: 150 rpm) using diamond polishing pads (manufactured by the same company) of #80, #600, and #1200 in that order using a tabletop polishing machine (model "IM-P2") manufactured by the same company, and the flatness was adjusted. The final polishing amount with the diamond polishing pad was adjusted to about 50 μm. Then, using diamond slurries (manufactured by the same company) with particle sizes of 15 μm, 6 μm, and 1 μm, surface polishing (polishing load: 15 N, polishing plate rotation speed: 150 rpm, sample rotation speed: 150 rpm) was performed with each diamond slurry for 5 minutes.
Furthermore, the substrate was polished for 20 minutes using an alumina slurry (manufactured by Buehler) with a particle size of 0.05 μm as a finishing abrasive to obtain a mirror finish.
After mirror finishing, the specimen was subjected to plasma etching in _CF4 gas for 4 minutes using a plasma etching device (model: SEDE-PHL) manufactured by Meiwa Force Systems Co., Ltd., to prepare a surface for microstructure observation.
Thereafter, in order to perform a conductive treatment on the surface of the observation sample, an Au film was formed using an ion sputter, model "E-1010" manufactured by Hitachi High-Technologies Corporation. The sputtering time was 120 seconds, and according to the operation manual, the thickness of the formed Au film was about 15 to 20 nm.

The silicon nitride sintered body after the above surface treatment was observed and photographed using a scanning electron microscope (SEM) model "S-3400N" manufactured by Hitachi High-Tech Corporation at an accelerating voltage of 10 kV. Figure 2(a) shows the SEM photograph of Example 1, and Figure 2(b) shows the SEM photograph of Comparative Example 1.

The SEM photographs taken were subjected to image analysis using software "A-zo-kun Ver. 2.58" manufactured by Asahi Kasei Engineering Co., Ltd., and the unevenness of the voids present in any one 64 μm × 48 μm area of the polished surface was measured. The unevenness was divided into six categories (0.9 or more, 0.8 to less than 0.9, 0.7 to less than 0.8, 0.6 to less than 0.7, 0.5 to less than 0.6, and less than 0.5), and the number of voids in each category and the percentage of the number of voids in each category to the total number of voids were calculated.
Here, the unevenness is calculated by the following formula 2 based on the contour line and envelope line of the void as shown in Fig. 3. The closer the unevenness is to 1, the less unevenness there is, and the smaller the unevenness is below 1, the more unevenness there is.
Irregularity=area within the contour line of a void/area within the envelope line of a void (Equation 2)

In Examples 1 to 21, voids with a degree of unevenness of 0.9 or more accounted for 10% or more, and voids with a degree of unevenness of 0.8 or more accounted for 30% or more.
In Comparative Examples 1 to 8, the percentage of voids with a degree of unevenness of 0.9 or more was less than 10%, and the percentage of voids with a degree of unevenness of 0.8 or more was less than 30%.

Next, an area of 64 μm×48 μm in the SEM photograph of the silicon nitride sintered body was subjected to image analysis using the above software, and the plane projection area ratio (%) of voids was calculated according to the following formula 3.
Plane projection area ratio = (total plane projection area of voids / area area) x 100 ... (Equation 3)
In Examples 1 to 21 and Comparative Example 3, the planar projected area ratio was 1.0% or less.
In Comparative Examples 1, 2, and 4 to 8, the planar projected area ratio exceeded 1.0%.

(v) Warpage As shown in FIG. 4, three silicon nitride sintered bodies (139.6 mm×190.5 mm×0.32 mm, diagonal length 236 mm) for each example were placed in a heating furnace adjusted to 120° C. and a relative humidity of 1% rh, and held at the same temperature for one hour. After that, they were removed from the heating furnace and placed on a flat natural stone sample stage (25° C.) equipped with an optical three-dimensional measuring device, model "MikroCAD", manufactured by GFMesstechnik. Before one minute had passed, the difference (μm) between the height of the highest point of the silicon nitride sintered body from the sample stage and the height of the lowest point of the silicon nitride sintered body from the sample stage was measured using the same measuring device, and the average value of the three differences was calculated. The ratio (%) of the average value to the maximum transverse length (diagonal length in this example) of the plate surface of the silicon nitride sintered body was taken as the value of warpage.

In Examples 1 to 21 and Comparative Example 8, the warpage (average value) was 0.2% or less.
In Comparative Examples 1 to 7, the warpage (average value) exceeded 0.2%.

In addition, for Example 1, the holding time at 120°C was increased to 2 hours, 4 hours, and 8 hours, and the warpage was measured in the same manner as above. However, since the measurement result when held for 1 hour was within ±1%, no significant difference due to the holding time was observed.
In addition, the time elapsed from removal from the heating furnace to placing the sample on a flat sample stand at 25°C for measurement was changed to 20 seconds and 40 seconds, and the warpage was measured in the same manner as above, but the results were within ±3% of the measurement results after 1 minute, so there was no significant difference due to the time elapsed from removal from the heating furnace within 1 minute. Note that within ±3% means a variation of 0.194% to 0.206% for a silicon nitride sintered body with a warpage of 0.2%, and it can be said that there is no significant difference.

Furthermore, as shown in Table 4 below, for Example 14 (where the composition and firing conditions are average among all the Examples), the second silicon nitride sintered body for which warpage was measured was divided into four pieces to reduce the size (69.8 mm x 95.3 mm x 0.32 mm, diagonal length 118 mm), further divided into two pieces to reduce the size (69.8 mm x 47.6 mm x 0.32 mm, diagonal length 85 mm), and further divided into two pieces to reduce the size (34.9 mm x 47.6 mm x 0.32 mm, diagonal length 59 mm). The warpage was measured after holding at 120°C in the same manner as above.

The warpage before division (diagonal length 236 mm) was 0.14%, while the warpage after division (diagonal lengths 118 mm, 85 mm, 59 mm) was 0.12%, 0.13%, and 0.15%, respectively. This shows that even when dividing into smaller sizes, the warpage remains almost the same as before division.

(vi) Breakdown voltage The silicon nitride sintered body was cut into individual pieces of 20 mm x 20 mm, and both sides were polished to a thickness of 100 μm to prepare a measurement sample. The surface roughness (Sa) of the polished sample surface in a 200 μm x 200 μm range (magnification of the objective lens: 50 times) was measured using a laser microscope: model "VKX-150" manufactured by Keyence Corporation, and the result was in the range of Sa = 0.48 to 0.52 μm. Conductive copper foil adhesive tape of φ10.4 mm was attached to both sides of the sample as a measurement electrode, and an AC voltage (sine wave) was applied in a fluorine-based inert liquid (Fluorinert FC-43 manufactured by 3M Japan Ltd.) using a withstand voltage tester: model "TOS5101" manufactured by Kikusui Electronics Co., Ltd. The AC voltage was increased at a rate of 500 V/s, and the average breakdown voltage in the measurements of three samples was measured.

In Examples 1 to 21, the dielectric breakdown voltage was 5 kV or more.
In Comparative Examples 1 to 7, the dielectric breakdown voltage was less than 5 kV.

The breakdown voltage is often measured on sintered bodies that are thicker than 100 μm (e.g., 300 μm), but the value obtained by converting the measurement results of such a sintered body to a value per 100 μm is merely a theoretical value. Therefore, it is not possible to guarantee that the breakdown voltage of the sintered body when actually formed to a thickness of about 100 μm will be the converted value. The present invention makes it possible to guarantee this.

The present invention is not limited to the above-described embodiments, and can be modified as appropriate without departing from the spirit of the invention.

Claims

A material in which 2 to 3 mass % of MgO and 2.7 to 4 mass % of a rare earth oxide having an oxidation number of 3 (but in a larger amount than the MgO) are added as sintering aids to silicon nitride powder is used and sintered.
A silicon nitride sintered body comprising silicon nitride and a grain boundary phase formed with a sintering aid, wherein the grain boundary phase has an amorphous structure, and the maximum integrated intensity of peaks of crystalline compounds in the grain boundary phase present in a diffraction angle 2θ range of 28° to 32° in an X-ray diffraction pattern obtained using an X-ray diffractometer equipped with a semiconductor detector is 2.4% or less of the integrated intensity of the silicon nitride (101) plane.
2. The silicon nitride sintered body according to claim 1, wherein the grain boundary phase contains at least MgO or MgSiN2 , and does not contain SrO.
The silicon nitride sintered body according to claim 2 has a thermal conductivity of 72 W/mK or more.
The silicon nitride sintered body according to any one of claims 1 to 3 has a three-point bending strength of 625 MPa or more when processed into a test piece measuring 40 mm x 20 mm x 0.32 mm, and measured at a crosshead speed of 0.5 mm/min, a support distance of 30 mm, and at room temperature (23±2°C).
A silicon nitride sintered body according to any one of claims 1 to 4, in which the planar projection area ratio of voids is 1.0% or less in at least one 64μm x 48μm area of the polished surface of the silicon nitride sintered body that has been polished by 50μm or more.
The silicon nitride sintered body according to any one of claims 1 to 4 has a thermal conductivity of 80 W/m·K or more.
A circuit board using the silicon nitride sintered body according to any one of claims 1 to 6.
A heat dissipation member using the silicon nitride sintered body according to any one of claims 1 to 6.
An insulating member using the silicon nitride sintered body according to any one of claims 1 to 6.