JP7278326B2 - Manufacturing method of silicon nitride sintered body - Google Patents

Description

TECHNICAL FIELD The present invention relates to a silicon nitride sintered body used as a circuit board, a heat radiation member, and the like.

Aluminum nitride (AlN) and silicon nitride (Si ₃ N ₄ ) are examples of materials for insulating ceramics (sintered bodies) used as circuit boards, heat radiating members, and the like.
Aluminum nitride has a high thermal conductivity of 150 W/m·K or more, but has a low mechanical strength, so it is prone to cracks and is difficult to use.
Silicon nitride has a thermal conductivity of 50 W/m·K or more, although it is not as good as that of aluminum nitride, and has high mechanical strength. Therefore, in recent years, the development and adoption of silicon nitride sintered bodies are progressing.

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 (with silicon nitride as 1), and a manufacturing method in which green sheets of the material are sintered at 1600-1900° C. in a nitrogen atmosphere, followed by removal of the residual glass phase at 1100-1700° C.

In Patent Document 2, a silicon nitride substrate with improved bonding properties of circuit members and the like and a dielectric breakdown voltage of 36 to 47 kV / mm (Table 6 of the same document) measured with a substrate having a thickness of 3 mm, and a material thereof is placed in a firing furnace in which co-materials of magnesium oxide and erbium oxide are arranged to suppress volatilization of the components, and sintered at 1750° C. for 3 to 5 hours.

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

In Patent Document 4, after sintering a silicon nitride substrate having a warp of 2.0 μm / mm or less and a green sheet of the material at 1800 to 2000 ° C. for 8 to 18 hours in a nitrogen pressurized atmosphere, A manufacturing method is described in which warpage is suppressed by heat treatment at 1550 to 1700° C. while applying a load.

In Patent Document 5, a silicon nitride substrate having small warpage and high strength and a green sheet of the material are placed in a firing container in which filling powder such as magnesium oxide is arranged to suppress volatilization of silicon nitride and magnesia. and sintering at 1860° C. for 5 hours.

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

JP-A-5-279124 WO2011-087055 JP 2017-178776 A JP 2009-218322 A Japanese Patent Application Laid-Open No. 2020-93978 JP 2014-73937 A

However, none of Patent Documents 1 to 6 describes that the dielectric breakdown voltage was measured with a silicon nitride substrate having a thickness of 100 μm and that the dielectric breakdown voltage was 5 kV or more. Patent Document 3 states that "it is possible to reduce the thickness of the substrate to 0.1 mm". Only up to 23 kV/mm is described. Although Patent Document 6 states that "even a thin silicon nitride substrate with a thickness of about 0.1 to 0.4 mm has a dielectric breakdown strength of 31 KV/mm or more, further 35 KV/mm or more". , as an example, only up to 32 kV/mm in dielectric breakdown strength measured on a substrate having a thickness of 0.15 mm as described above.

Silicon nitride sintered bodies are used as circuit boards, heat dissipation members, etc. in various products such as electronic equipment, semiconductor devices, and automotive parts. has become necessary. Therefore, it is required that the dielectric breakdown voltage actually measured on a silicon nitride substrate having a thickness of 100 μm is high.
As described above, the dielectric breakdown voltage is conventionally often measured with a sintered body having a thickness greater than 100 μm, but the numerical value obtained by converting the measurement result per 100 μm is only a theoretical numerical value. Therefore, it is not possible to guarantee the dielectric breakdown voltage when the film is actually formed to a thickness of about 100 μm.

[1] Add 2 to 3% by mass of MgO as a sintering aid to silicon nitride powder, and add 2.7 to 4% by mass of a rare earth oxide having an oxidation number of 3 (however, the amount should be larger than that of MgO). sintering with the added material;
Dielectric breakdown voltage when an AC voltage is applied to a plate-like silicon nitride sintered body having a thickness of 100 μm, which is a silicon nitride sintered body composed of silicon nitride and a grain boundary phase formed by a sintering aid. is 5 kV or more.

[Action]
Since the dielectric breakdown voltage is 5 kV or more when an AC voltage is applied to a plate-shaped silicon nitride sintered body with a thickness of 100 μm, a high dielectric breakdown voltage is required when it is actually formed to a thickness of about 100 μm. It can correspond to the use of the silicon nitride sintered body.
As described above, conventionally, the numerical value obtained by converting the measurement result of the dielectric breakdown voltage measured for a sintered body having a thickness greater than 100 µm per 100 µm is the dielectric breakdown voltage when actually formed to a thickness of about 100 µm. cannot be guaranteed, but the present invention makes it possible.
Note that the "thickness of 100 µm" is only defined as a condition for measuring the dielectric breakdown voltage, and does not define the thickness of the silicon nitride sintered product. In other words, the silicon nitride sintered body product may have any thickness as long as it is processed to a thickness of 100 μm and has a measured dielectric breakdown voltage of 5 kV or more.

According to the present invention, the dielectric breakdown voltage when an AC voltage is applied to a silicon nitride sintered body having a thickness of 100 μm is 5 kV or more. It can be used for silicon nitride sintered bodies that sometimes require a high dielectric breakdown voltage.

FIG. 1 shows the X-ray diffraction pattern diagram of the silicon nitride sintered body, (a) is the same figure of Example 1, and (b) is the same figure of Comparative Example 1. As shown in 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. FIG. 3 is a diagram for explaining the unevenness of voids. FIG. 4 is a diagram for explaining a method of measuring warpage of a silicon nitride sintered body. FIG. 5 is a diagram showing an application example of the silicon nitride sintered body.

The silicon nitride sintered body of the present invention is characterized by having a dielectric breakdown voltage of 5 kV or more when an AC voltage is applied to the plate-like silicon nitride sintered body having a thickness of 100 μm. In addition to the preferred modes exemplified in the above means, the following preferred modes are exemplified.

1. Manufacturing method In the manufacturing method of the silicon nitride sintered body, the sintering step of sintering the mixture of the silicon nitride powder and the sintering aid is 1930 ≤ sintering temperature (°C) + sintering time (hr) x 50 ≤ 2200. It is preferable that the grain boundary phase formed by the sintering aid has an amorphous structure.
It is preferable that no peaks derived from the grain boundary phase are detected in the X-ray diffraction pattern obtained using an X-ray diffraction apparatus equipped with a semiconductor detector.
In the sintering step, it is preferable to place a previously sintered plate-shaped silicon nitride sintered body separate from the silicon nitride sintered body to be manufactured in the closed casing for firing.
Preferably, the sintering aid contains at least MgO or MgSiN ₂ and does not contain SrO.

By setting 1930 ≤ sintering temperature (°C) + sintering time (hr) x 50 ≤ 2200, sintering is realized and volatilization of SiO ₂ during sintering is suppressed to prevent crystallization of the grain boundary phase. Suppressed. By making the grain boundary phase formed by the sintering aid into an amorphous structure, the volume shrinkage is reduced during cooling after sintering, and the sintering aid exists as a liquid phase even at a lower temperature to form silicon nitride crystals. Since it spreads to narrow portions between grains, the voids in the silicon nitride sintered body are reduced, the internal stress of the silicon nitride sintered body is reduced, and the warp is reduced. Moreover, the unevenness of the void shape is reduced.
In the sintering process, a pre-sintered plate-shaped silicon nitride sintered body separate from the silicon nitride sintered body to be manufactured (hereinafter referred to as "dummy silicon nitride sintered body ), the volatilization of the SiO ₂ of the dummy silicon nitride sintered body during firing suppresses the volatilization of the SiO ₂ of the silicon nitride sintered body to be produced, which also suppresses crystallization. Also, a decrease in sintered density is prevented. The dummy silicon nitride sintered body does not need to have the same composition as the silicon nitride sintered body to be produced, but preferably has the same auxiliary agent system.
Also, 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 more difficult to volatilize than MgO or _MgSiN2 , so it remains after firing and becomes a factor that hinders heat conduction, so at least MgO or _MgSiN2 is included. However, by not containing SrO, a silicon nitride sintered body with high thermal conductivity can be obtained.
Further, by densifying the silicon nitride sintered body to a relative density of 98% or more, the bending strength is increased and the dielectric breakdown voltage is also increased.

2. Grain Boundary Phase The silicon nitride sintered body is composed of silicon nitride and a grain boundary phase formed by a sintering aid, and the grain boundary phase preferably has an amorphous structure.
The grain boundary phase preferably contains at least MgO or MgSiN ₂ and does not contain SrO.
The grain boundary phase preferably has an amorphous structure containing at least Mg, a rare earth element (RE), and Si.
The largest integral of the peaks of the crystalline compound in the grain boundary phase existing in the diffraction angle 2θ in the range of 28° to 32° in the X-ray diffraction pattern obtained using an X-ray diffraction device equipped with a semiconductor detector. A structure having an intensity of 5% or less relative to the integrated intensity of the silicon nitride (101) plane is defined as an amorphous structure.
In at least one arbitrary area of 64 μm×48 μm on the polished surface obtained by polishing the surface of the silicon nitride sintered body by 50 μm or more, the planar projected area ratio of voids is preferably 1.0% or less.
The silicon nitride sintered body preferably has a thermal conductivity of 80 W/m·K or more.

Since the grain boundary phase has an amorphous structure, volume shrinkage is reduced during cooling after sintering, and the sintering aid exists as a liquid phase even at lower temperatures and spreads to narrow areas between silicon nitride crystal grains. , the voids in the silicon nitride sintered body are reduced, the internal stress of the silicon nitride sintered body is reduced, and the warpage is reduced. Moreover, the unevenness of the void shape is reduced.

3. Void Calculated by dividing the area within the contour line of the void by the area within the envelope of the void in at least one arbitrary 64 μm × 48 μm area of the polished surface obtained by polishing the surface of the silicon nitride sintered body to 50 μm or more. It is preferable that the number of voids having an unevenness of 0.9 or more accounts for 10% or more of the total number of voids.
Roughness calculated by dividing the area within the outline of the void by the area within the envelope of the void in at least one arbitrary area of 64 μm×48 μm on the polished surface obtained by polishing the surface of the silicon nitride sintered body by 50 μm or more. It is preferable that the number of voids with a degree of 0.8 or more accounts for 30% or more of the total number of voids.
In the area, it is preferable that the plan-projected area ratio of voids is 1.0% or less.

Voids with a degree of unevenness of 0.9 or more account for 10% or more, or voids with a degree of unevenness of 0.8 or more account for 30% or more, so that void-shaped uneven portions are formed when a voltage is applied. Since the unevenness is small, the partial discharge that occurs in is less likely to occur, and the dielectric breakdown voltage increases.
In addition, when the planar projected area ratio of the voids is 1.0% or less, the warpage of the silicon nitride sintered body is reduced.

4. Warp The highest point of the upper surface of the silicon nitride sintered body measured before 1 minute after holding the plate-shaped silicon nitride sintered body at 120 ° C. for 1 hour or more and placing it on a flat sample stand at 25 ° C. The warpage calculated as the ratio of the difference between the height from the sample stage and the height from the lowest point from the sample stage to the maximum transverse length of the silicon nitride sintered body is preferably 0.2% or less.
Here, the maximum crossing 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 the edge to another point, For example, if the plate surface is rectangular, it is the diagonal length, and if the plate surface is circular, it is the diameter length.

Since the warpage measured as described above is 0.2% or less, even if the product in which the silicon nitride sintered body is used as a circuit board, heat dissipation member, etc. is exposed to a high temperature environment exceeding 100 ° C., the nitriding Since the warpage of the silicon sintered body is small, a sufficient heat dissipation effect is obtained and breakage is unlikely to occur.

5. Applications Applications of the silicon nitride sintered body are not particularly limited, but the following applications can be exemplified.
Circuit boards used in semiconductor modules, LED packages, Peltier modules, printers, multi-function machines, semiconductor lasers, optical communications, high frequencies, etc., as shown in FIG. 5(a).
A general-purpose heat dissipation member as shown in FIG. 5(b).
A heat dissipation member (heat sink) for a power semiconductor module as shown in FIG. 5(c).
An insulating plate as shown in FIG. 5(d).
An insulating plate for bonded wafers as shown in FIG. 5(e).
A heat dissipating member embedded in a flexible resin or the like as shown in FIG. 5(f).
Although not shown, it is a high frequency window used in gyrotrons, klystrons, and the like.

Next, examples embodying the present invention will be described with reference to the drawings while comparing them with comparative examples. 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 of Examples 1 to 21 shown in Tables 1 and 2 and silicon nitride sintered bodies of Comparative Examples 1 to 8 shown in Table 3 were produced. Hereinafter, "each example" refers to each of Examples 1 to 21 and Comparative Examples 1 to 8. In addition, Example 21 is a reference example.

[1] Materials As silicon nitride (Si ₃ N ₄ ), which is the main raw material, silicon nitride powder having an average particle size (D50) of about 1.0 μm manufactured by an imide pyrolysis method or a direct nitriding method is used. Used for example.

As the sintering aid, as shown in Tables 1 to 3, powders of MgO, MgSiN ₂ , Y ₂ O ₃ , La ₂ O ₃ , Nd ₂ O ₃ , Sm ₂ O ₃ and Dy ₂ O ₃ were selected. Two species were used in each example. Examples 1-21 use at least MgO or MgSiN ₂ and do not use SrO.

[2] Manufacturing method (i) Material mixing step For each example, the silicon nitride powder was blended with the sintering aid powder in the mass% shown in Tables 1 to 3 (silicon nitride powder and sintering aid powder is 100% by mass). To 100 parts by weight of this blended powder, 0.3 parts by weight of a surfactant dispersant and about 50 parts by weight of a mixed solvent of toluene and ethanol were added, and the mixture was obtained by a ball mill using a resin container and silicon nitride pebbles. Grinding mixing was performed.

To this pulverized mixture, a dissolved binder solution containing 10 parts by weight of polyvinyl butyral as a binder, 4 parts by weight of dioctyl adipate as a plasticizer, and about 20 parts by weight of a mixed solvent of toluene and ethanol was added, and the dissolved binder was dissolved. After stirring and mixing with a ball mill until the solution and the pulverized mixture were completely mixed, a slurry was prepared. Then, the slurry was heated and left in a vacuum to remove bubbles and volatilize the solvent, thereby adjusting the viscosity at 25° C. to 15000 cps.

(ii) Step of producing green sheet Next, a plate-shaped green sheet was obtained from the produced slurry of each example by a doctor blade method. The final drying temperature in the doctor blade forming apparatus was 90°C. The resulting green sheet was die-cut into a rectangle of 180 mm×250 mm by die pressing.

Boron nitride (BN) powder slurry as a release material is sprayed onto the surface of the punched green sheet by spraying, and a green sheet laminate obtained by stacking a plurality of the green sheets is placed in a BN housing and dried in air. A degreasing step was performed by heating to 500° C. for about 4 hours in a flow to remove organic components such as binders.

(iii) Green sheet sintering process For Examples 1 to 21, a green sheet laminate is placed on a BN bottom plate, a BN setter is placed thereon, and a tungsten block is placed on the setter as a load. was placed, and the above-mentioned plate-like dummy silicon nitride sintered body was placed on the load.
Next, a side plate and a top plate made of BN were installed on the bottom plate to assemble a closed case. The housing containing the green sheets and the like was put into a firing furnace, and the inside of the firing furnace was made into a nitrogen atmosphere of 0.9 MPa. Since the housing is not completely airtight, but closed to the extent that nitrogen can flow in, the inside of the housing also has a nitrogen atmosphere of 0.9 MPa.

In this state, each example is heated at the firing temperature shown in Tables 1 and 2 for the firing time to sinter the green sheet laminate, and the laminate after sintering is made into silicon nitride sintered bodies one by one. separated. The BN release material was removed from the separated silicon nitride sintered body by honing. After honing, the four sides of the silicon nitride sintered body were subjected to break treatment with a diamond scriber, and the shape and dimensions of the finally obtained silicon nitride sintered body were rectangular plates of 139.6 mm × 190.5 mm × 0. 0.32 mm.
In Examples 1 to 21, the sintering temperature was in the range of 1830 to 1920° C., and the sintering was performed for a relatively short time so as to satisfy the following formula 1.
1930 ≤ firing temperature (°C) + firing time (hr) x 50 ≤ 2200 (Formula 1)

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

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

(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 of submerging the silicon nitride sintered body in pure water. The theoretical densities of the raw material powder are Si ₃ N ₄ =3.18 g/cm ³ , MgO=3.60 g/cm ³ , MgSiN ₂ =3.07 g/cm ³ , Y ₂ O ₃ =5.01 g/cm 3 . cm ³ , La ₂ O ₃ =6.51 g/cm ³ , Nd ₂ O ₃ =7.24 g/cm ³ , Sm ₂ O ₃ =7.60 g/cm ³ , Dy ₂ O ₃ =7.81 g/cm ³ It was calculated from the mixing ratio of the raw material powders using values such as
Three-point bending strength is measured by processing a silicon nitride sintered body into a test piece of size 40 mm × 20 mm × 0.32 mm, and using a universal testing machine manufactured by Shimadzu Corporation: model "AG-IS", crosshead It was measured at room temperature (23±2° C.) at a speed of 0.5 mm/min and a distance between fulcrums of 30 mm.

Examples 1 to 21 and Comparative Examples 1 to 4, 6, and 7 had a relative density of 98% or more, and were sufficiently densified, so that the three-point bending strength was 600 MPa or more.
Comparative Examples 5 and 8 had relative densities of less than 98% and were not sufficiently densified, so the three-point bending strength was less than 600 MPa, and high-strength silicon nitride sintered bodies could not be obtained.

(ii) Thermal conductivity Thermal conductivity is measured by processing the silicon nitride sintered body into a test piece with a size of 10 mm × 10 mm × 0.32 mm, and after surface treatment (Ag film deposition + carbon blackening treatment), was measured using a thermal conductivity instrument: model "LFA 467 HyperFlash".

(iii) Identification of grain boundary phase by X-ray diffraction method A silicon nitride sintered body was processed into a test piece with a size of 10 mm × 10 mm × 0.32 mm, and an X-ray diffraction device manufactured by Rigaku Co., Ltd.: model "Ultima IV" ( The X-ray diffraction pattern of the test piece plane was obtained by the powder X-ray diffraction method using the Cu-Kα ray, using a Cu-Ni filter as the target of the sealed tube and a one-dimensional semiconductor type detector). rice field.
In the obtained X-ray diffraction pattern, the integrated intensity of the (101) plane of α-Si ₃ N ₄ (hereinafter referred to as “I silicon nitride”) and the grain boundary where the diffraction angle 2θ is in the range of 28° to 32° The integrated intensity of the maximum peak among the Si—Y—N—O compound peaks of the phase (hereinafter referred to as “I grain boundary phase”) is calculated by the following procedure, and the integrated intensity ratio (I grain boundary phase/ I silicon nitride) was obtained.
(1) Perform preprocessing such as background removal, Kα2 removal and smoothing, and perform peak search.
(2) Calculate the background profile by subtracting the peak profile from the measured data, and fit the calculated data with a B-spline function.
(3) The peak shape is represented by a split pseudovoigt function, and the integrated intensity is calculated.

The X-ray diffraction pattern of Example 1 is shown in FIG. 1(a). No peak derived from the grain boundary phase formed by the sintering aid was detected, and as shown in Table 1, the integrated intensity ratio was 0. This indicates that no grain boundary crystal phase exists and the grain boundary phase has a substantially amorphous structure.
Examples 2-19 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 as small as 2.4%, which also had an amorphous structure. There is .

The X-ray diffraction pattern of Comparative Example 1 is shown in FIG. 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 is there a grain boundary crystalline phase, but the grain boundary phase consists essentially of the crystalline phase.
Comparative Examples 2 to 7 were basically the same as 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 as shown in Table 1, the integrated intensity ratio was 0. This indicates that no grain boundary crystal phase exists and the grain boundary phase has a substantially amorphous structure. However, in Comparative Example 8, as will be described later, the relative density is low and there are few voids having an unevenness of 0.8 or more.

(iv) Void The silicon nitride sintered body was surface-treated as follows.
The silicon nitride sintered body was processed into a test piece of 8 mm × 8 mm × 0.32 mm, and fixed to a φ40 aluminum sample stand using Arco wax "5402SL" manufactured by Nikka Seiko Co., Ltd.
Set the sample stand on the sample rotating machine manufactured by IMT Co., Ltd.: model "SP-L1", and use the company's desktop polishing machine: model "IM-P2" to grind # 80, # 600, and # 1200. The surface of the silicon nitride sintered body was then polished using a diamond polishing pad (manufactured by the same company) (polishing load: 15 N, number of rotations of polishing disk: 150 rpm, number of rotations of sample: 150 rpm) to adjust the flatness. The final polishing amount of the diamond polishing pad was adjusted to about 50 μm. After that, using diamond slurries with particle sizes of 15 μm, 6 μm, and 1 μm (manufactured by the same company), the surface was polished for 5 minutes with each diamond slurry (polishing load: 15 N, polishing disk rotation speed: 150 rpm, sample rotation speed: 150 rpm). gone.
Further, polishing was performed for 20 minutes using an alumina slurry (manufactured by Buehler) having a particle size of 0.05 μm as a polishing agent for finishing, thereby obtaining a mirror finish.
After the mirror finishing, plasma etching was performed in CF ₄ gas for 4 minutes using a plasma etching apparatus: model "SEDE-PHL" manufactured by Meiwa Forsys Co., Ltd. to adjust the microstructure observation surface.
After that, an Au film was formed using an ion sputter (type E-1010) manufactured by Hitachi High-Tech Co., Ltd. for the purpose of conducting a conductive treatment on the observation sample surface. The sputtering time is 120 seconds, and according to the operation manual, the thickness of the Au film formed is about 15-20 nm.

The silicon nitride sintered body after the surface treatment was observed using a scanning electron microscope (SEM) model "S-3400N" manufactured by Hitachi High-Tech Co., Ltd. at an acceleration voltage of 10 kV, and an SEM photograph was taken. The SEM photograph of Example 1 is shown in FIG. 2(a), and the SEM photograph of Comparative Example 1 is shown in FIG. 2(b).

The SEM photograph taken is image-analyzed using the software “Azo-kun Ver. In addition to measuring the degree, the unevenness is classified into six (0.9 or more, 0.8 or more and less than 0.9, 0.7 or more and less than 0.8, 0.6 or more and less than 0.7, 0.5 or more less than 0.6, less than 0.5), and the number of voids in each section and the ratio of the number of voids in each section to the total number of voids were calculated.
Here, the degree of unevenness is calculated by the following equation 2 based on the outline and envelope of the void as shown in FIG. The closer the unevenness degree is to 1, the less unevenness, and the less than 1, the more unevenness.
Irregularity=Area within the outline of the void/Area within the envelope of the void (Formula 2)

In Examples 1 to 21, voids with an unevenness of 0.9 or more accounted for 10% or more, and voids with an unevenness of 0.8 or more accounted for 30% or more.
In Comparative Examples 1 to 8, voids with an unevenness of 0.9 or more were less than 10%, and voids with an unevenness of 0.8 or more were less than 30%.

Next, a 64 μm×48 μm area of the SEM photograph of the silicon nitride sintered body was image-analyzed using the above software, and the planar projected area ratio (%) of voids was calculated by the following equation 3.
Planar projected area ratio = (Total planographic projected 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, 4 to 8, the planar projected area ratio exceeded 1.0%.

(v) Warp As shown in FIG. 4, three silicon nitride sintered bodies (139.6 mm × 190.5 mm × 0.32 mm, diagonal length 236 mm) were prepared at 120 ° C. and a relative humidity of 1%rh for each example. After holding it at the same temperature for 1 hour, remove it from the heating furnace and then optical three-dimensional measuring instrument manufactured by GFMesstechnik: A flat natural stone sample stand (25 ° C. ) before 1 minute has elapsed after placing the silicon nitride sintered body on the same measuring instrument, measure 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 from the sample stage. The average value of the three sheets was calculated, and 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, warpage (average value) exceeded 0.2%.

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

Further, as shown in the following Table 4, for Example 14 (composition and firing conditions are average among all Examples), the second silicon nitride sintered body whose warpage was measured was A smaller size (69.8 mm x 95.3 mm x 0.32 mm, diagonal length of 118 mm) and a smaller size (69.8 mm x 47.6 mm x 0.8 mm) were further divided into two. 32 mm, diagonal length 85 mm) and a smaller size by further dividing it into two (34.9 mm × 47.6 mm × 0.32 mm, diagonal length 59 mm) were also held at 120 ° C. in the same manner as above. Later warpage was measured.

The warpage after division (diagonal length 118 mm, 85 mm, 59 mm) was 0.12%, 0.13%, and 0.15%, respectively, compared to the warp 0.14% before division (diagonal length 236 mm). rice field. From this, even if the size is divided into small pieces, the warpage is almost the same as before the division.

(vi) Dielectric Breakdown Voltage A silicon nitride sintered body was cut into individual pieces of 20 mm×20 mm and polished on both sides to a thickness of 100 μm to obtain measurement samples. In addition, a laser microscope manufactured by Keyence Corporation: Model "VKX-150" was used to measure the surface roughness (Sa) in a range of 200 μm × 200 μm on the surface of the polished sample (magnification of the objective lens was 50 times). =0.48 to 0.52 μm. As a measuring electrode, φ10.4 mm conductive copper foil adhesive tape is attached to both sides of the sample, and a withstand voltage tester of Kikusui Electronics Co., Ltd.: Model "TOS5101" is used. (manufactured by Fluorinert FC-43), an alternating voltage (sine wave) was applied. An AC voltage step-up rate was set at 500 V/s, and an average dielectric breakdown voltage was measured in measurements of three samples.

Examples 1 to 21 had a dielectric breakdown voltage of 5 kV or higher.
Comparative Examples 1 to 7 had a dielectric breakdown voltage of less than 5 kV.

The dielectric breakdown voltage is often measured with a sintered body having a thickness greater than 100 μm (for example, 300 μm), but the numerical value obtained by converting the measurement result of such a sintered body into per 100 μm is purely theoretical. It is a numerical value. Therefore, it cannot be guaranteed that the dielectric breakdown voltage of the sintered body actually formed to a thickness of about 100 μm will also be the converted value. The present invention makes that assurance possible.

It should be noted that the present invention is not limited to the above-described embodiments, and can be embodied with appropriate modifications within the scope of the invention.

Claims (1)

A material obtained by adding 2 to 3% by mass of MgO as a sintering aid to silicon nitride powder and adding 2.7 to 4% by mass of a rare earth oxide having an oxidation number of 3 (however, the amount is larger than that of MgO). sintering with
Dielectric breakdown voltage when an AC voltage is applied to a plate-like silicon nitride sintered body having a thickness of 100 μm, which is a silicon nitride sintered body composed of silicon nitride and a grain boundary phase formed by a sintering aid. is 5 kV or more.

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