JP7660738B2 - Silicon Nitride Substrate - Google Patents

Description

The present invention relates to a silicon nitride substrate having excellent thermal conductivity in the thickness direction and a method for producing the same.

In recent years, attempts have been made to use silicon nitride (Si3N4) substrates in semiconductor circuit substrates such as power semiconductors. Alumina (Al2O3) substrates and aluminum nitride (AlN) substrates are used as semiconductor circuit substrates. Alumina substrates have a thermal conductivity of about 30 W/m·K, but they can be manufactured at low cost. Aluminum nitride substrates have a thermal conductivity of 160 W/m·K.
/m·K or more. Meanwhile, silicon nitride substrates with a thermal conductivity of 50 W/m·K or more have been developed.

Silicon nitride substrates have a lower thermal conductivity than aluminum nitride substrates, but have a superior three-point bending strength of 500 MPa or more. The three-point bending strength of aluminum nitride substrates is usually 300 to 400 MPa.
The strength of silicon nitride is about 100 MPa, and the strength tends to decrease as the thermal conductivity increases. By taking advantage of the advantage of high strength, silicon nitride substrates can be made thinner. Thinning the substrate makes it possible to reduce thermal resistance, improving heat dissipation.

Taking advantage of these characteristics, silicon nitride substrates are widely used as circuit boards by providing circuit parts such as metal plates, etc. In addition, there is also a method of using silicon nitride substrates as circuit boards for pressure-welded structures as shown in International Publication No. WO2011/010597 (Patent Document 1).

International Publication No. WO2011/010597 Pamphlet

However, as mentioned above, silicon nitride substrates have a lower thermal conductivity than aluminum nitride and the like, and therefore when used in semiconductor circuit substrates, the heat generated in the semiconductor chip cannot be efficiently dissipated to the heat sink, and the power that can be input to the semiconductor circuit substrate is also limited.
Therefore, silicon nitride substrates are required to have higher thermal conductivity, especially in the thickness direction.

An object of the present invention is to provide a silicon nitride substrate having excellent thermal conductivity in the thickness direction.

In order to solve the above problems, the present invention relates to a silicon nitride substrate, characterized in that when the substrate surface is irradiated with X-rays, it has an X-ray diffraction peak of β-Si ₃ N ₄ , the orientation degree fa indicating the proportion of the long axis (c-axis) of β-Si ₃ N ₄ particles oriented in the thickness direction in the silicon nitride substrate is in the range of 0.0479 to 0.0929 , the average three-point bending strength of ten test pieces is in the range of 675 to 689 MPa when a 4 mm x 35 mm test piece is bent from the midpoint of two supports with a distance between the two supports of 30 mm, and the thermal conductivity in the thickness direction is in the range of 111.2 to 121.6 W/m・K.

fa=(P-P0)/(1-P0) (1).

In formula (1), P is represented by formula (2), and the (10°
≦2θ≦80°) and all X-ray diffraction line intensity ratios related to the c-axis (specifically, (101
) surface, (111) surface, (201) surface, (121) surface, (301) surface, (221) surface, (1
(31), (002), (401), (102), (112), (231),
The X-ray diffraction intensities of the (202) plane, (141) plane, (212) plane, (302) plane, and (501) plane are the targets), and P0 is represented by formula (3), and in the β-type silicon nitride powder, (10°≦2θ≦
80°), all X-ray diffraction line intensity ratios related to the c-axis (specifically, the (101) plane, (
111), (201), (121), (301), (221), (131), (002), (401), (102), (112), (231), (202
) plane, (141), (212), (302), and (501) planes are the targets of X-ray diffraction intensities.

P=(I(101)+I(111)+I(201)+I(121)+I(301)+I
(221)+I(131)+I(002)+I(401)+I(102)+I(112)
+I(231)+I(202)+I(141)+I(212)+I(302)+I(50
1))/(I(100)+I(110)+I(200)+I(101)+I(120)+
I (111) + I (300) + I (201) + I (220) + I (121) + I (130
)+I(301)+I(400)+I(221)+I(131)+I(230)+I(0
02)+I(140)+I(401)+I(102)+I(112)+I(231)+I
(202)+I(500)+I(141)+I(330)+I(212)+I(240)
+I(302)+I(501))...(2).

P0=(I0(101)+I0(111)+I0(201)+I0(121)+I0(
301) + I0 (221) + I0 (131) + I0 (002) + I0 (401) + I0 (
102) + I0 (112) + I0 (231) + I0 (202) + I0 (141) + I0 (
212)+I0(302)+I0(501))/(I0(100)+I0(110)+I
0 (200) + I0 (101) + I0 (120) + I0 (111) + I0 (300) + I
0 (201) + I0 (220) + I0 (121) + I0 (130) + I0 (301) + I
0 (400) + I0 (221) + I0 (131) + I0 (230) + I0 (002) + I
0 (140) + I0 (401) + I0 (102) + I0 (112) + I0 (231) + I
0 (202) + I0 (500) + I0 (141) + I0 (330) + I0 (212) + I
0(240)+I0(302)+I0(501))...(3).

The present invention also relates to a method for producing a silicon nitride substrate, comprising the steps of: mixing silicon powder, a sintering aid and a dispersion medium to prepare a slurry; forming a sheet body from the slurry; heat-treating the sheet body in a nitrogen-containing atmosphere to nitridize the silicon in the sheet body and form silicon nitride; and sintering the sheet body containing the silicon nitride to produce a silicon nitride substrate, wherein at least in the step of forming the silicon nitride, volatilization of the sintering aid is controlled to orient silicon nitride particles in the thickness direction, which is the direction of movement of the sintering aid.

According to the present invention, when silicon is converted into silicon nitride through a nitriding step and then a silicon nitride substrate is obtained through a sintering step, the volatilization of the sintering aid is promoted at least when silicon nitride is obtained.
Therefore, due to the diffusion and movement caused by the volatilization of the sintering aid, the generated silicon nitride β particles become oriented in the thickness direction.

As a result, when the substrate surface is irradiated with X-rays, it has an X-ray diffraction peak of β-Si ₃ N ₄ , the degree of orientation fa, which indicates the ratio of the long axis (c-axis) of the β-Si ₃ N ₄ grains oriented in the thickness direction in the silicon nitride substrate, is in the range of 0.0479 to 0.0929 , the average value of the three-point bending strength of ten test pieces of 4 mm x 35 mm when the test pieces are bent from the midpoint of two supports with a distance between the two supports of 30 mm, is in the range of 675 to 689 MPa, and the thermal conductivity in the thickness direction is in the range of 111.2 to 121.6 W/m·K , so that it has a higher thermal conductivity than conventional silicon nitride substrates. Also, this makes it possible to obtain a silicon nitride substrate with sufficient strength for practical use.

Therefore, when used in a semiconductor circuit board, the heat generated in the semiconductor chip can be efficiently dissipated to the heat sink, and the power that can be input to the semiconductor circuit board can be improved. In other words, combined with the excellent strength of the silicon nitride substrate, it can be applied to various semiconductor circuit boards including power semiconductors.

In the silicon nitride substrate and the manufacturing method thereof of the present invention, the sintering aid is preferably at least one of a rare earth oxide and a magnesium compound, which promotes the movement of the liquid phase generated from the sintering aid in the thickness direction, thereby making it possible to more significantly achieve the above-mentioned effects.

Furthermore, in the silicon nitride substrate of the present invention, the size of the main surface is 400 to 40,000 mm2.
It is preferable that the density is 3.15 to 3.40 g/cm3 and the dielectric strength is 20 kV/mm or more. In this case, a silicon nitride substrate having a dielectric strength sufficient for practical use can be obtained.

As described above, according to the present invention, it is possible to provide a silicon nitride substrate having excellent thermal conductivity in the thickness direction.

1 is a schematic diagram showing the crystal system of β-Si3N4. 1 is a schematic cross-sectional view of a silicon nitride substrate according to an embodiment of the present invention.

FIG. 1 is a schematic diagram showing the crystal system of β-Si3N4, and FIG. 2 is a schematic cross-sectional view of a silicon nitride substrate in an embodiment of the present invention.

The silicon nitride substrate of the present invention preferably has a silicon nitride content of 85 mass% or more,
More preferably, it is 87 mass% or more. As a result, as described below, due to the crystal system (crystal structure) of silicon nitride, the thermal conductivity of the silicon nitride substrate in the thickness direction is improved. If the silicon nitride content is less than 85 mass%, the proportion of silicon nitride is reduced, and the improvement in the thermal conductivity of the silicon nitride substrate in the thickness direction is insufficient.

The silicon nitride content is preferably 95 mass % or less, and more preferably 93 mass % or less.
When the silicon nitride content exceeds 95 mass %, the content of the sintering aid contained in the silicon nitride substrate is reduced, so that the amount of liquid phase is reduced and the movement of the liquid phase in the vertical direction toward the separating agent layer is reduced, so that it becomes difficult for the generated silicon nitride to be oriented in the thickness direction of the silicon nitride substrate, and the thermal conductivity in the thickness direction of the silicon nitride substrate cannot be improved.

In the silicon nitride substrate of the present invention, the content of the sintering aid is preferably 5% by mass or more, more preferably 7% by mass or more. By including the sintering aid in a proportion of 5% by mass or more, as described below, the proportion of the liquid phase when the silicon nitride substrate is sintered to produce it is optimized, the silicon nitride particles are oriented perpendicular to the thickness direction, and the proportion of silicon nitride is optimized.
The thermal conductivity of the silicon nitride substrate in the thickness direction is improved.

On the other hand, in the silicon nitride substrate of the present invention, the content of the sintering aid must be 15 mass % or less. If the content of the sintering aid exceeds 15 mass %, the proportion of silicon nitride decreases, and the thermal conductivity of the silicon nitride substrate in the thickness direction originating from silicon nitride decreases.

In addition to the silicon nitride and sintering aids described above, the silicon nitride substrate of the present invention contains inevitable impurities, such as organic solvents used as dispersion media in the manufacturing process of the silicon nitride substrate, and additives such as binders and plasticizers.

The silicon nitride substrate of the present invention preferably has an X-ray diffraction peak of β-Si3N4 when the substrate surface is irradiated with X-rays, and has an orientation degree fa, which indicates the proportion of the major axes (c-axes) of β-Si3N4 grains oriented in the thickness direction in the silicon nitride substrate, in the range of 0 to 0.3.

As shown in FIG. 1, the crystal system (crystal structure) of β-Si3N4 has a (200) plane and a (1
The silicon nitride substrate has a hexagonal column shape with multiple planes, including the (002) plane, on its side faces and a (002) plane on its end face. Therefore, when the substrate surface is irradiated with X-rays, the β-S oriented in the thickness direction of the silicon nitride substrate is
The degree of orientation fa, which indicates the proportion of i3N4 grains, being in the range of 0 to 0.3 means that the β-Si3N4 grains are preferentially oriented in the thickness direction of the silicon nitride substrate, and most of the columnar β-Si3N4 grains 11 in the silicon nitride substrate 10 are oriented in the thickness direction as shown in Fig. 2. Reference numeral 12 indicates a grain boundary phase resulting from a sintering aid or the like.

Inherently, the thermal conductivity of silicon nitride particles is higher in the length direction of the hexagonal column than in other directions.
The proportion of silicon nitride (Si3N4) oriented in the thickness direction of the silicon nitride substrate increases.
The present invention can exhibit high thermal conductivity in the thickness direction.

In addition, when the degree of orientation fa, which indicates the ratio of β-Si3N4 grains oriented in the thickness direction in the silicon nitride substrate, is less than 0, i.e., in the negative range, the orientation in the plane direction becomes strong, the above-mentioned effects cannot be fully achieved, and high thermal conductivity cannot be obtained in the thickness direction of the silicon nitride substrate. In addition, the upper limit of the degree of orientation fa is currently 0.3, but from the viewpoint of thermal conductivity, the higher the better. However, if this ratio becomes too high, the mechanical strength, such as the fracture strength, in the thickness direction will decrease.

In the silicon nitride substrate of the present invention, the thermal conductivity in the thickness direction is 80 W/m K or more,
Preferably, it is 85 W/m·K or more. As a result, even when the silicon nitride substrate of the present invention is used in a semiconductor circuit substrate, the heat generated in the semiconductor chip can be efficiently dissipated to the heat sink, and the power that can be input to the semiconductor circuit substrate can be improved. That is, in combination with the excellent strength of the silicon nitride substrate, it can be applied to various semiconductor circuit substrates including power semiconductors.

The above thermal conductivity can be obtained when the silicon nitride substrate of the present invention satisfies the above-mentioned contents of silicon nitride and sintering aid, and satisfies the requirements for X-ray diffraction.

In addition, the silicon nitride substrate of the present invention preferably has a flexural strength of 500 MPa or more, as measured by the three-point method, and a thickness of 0.1 to 1.2 mm. This allows for a silicon nitride substrate with sufficient strength for practical use to be obtained. As shown in Table 1 below, the silicon nitride substrate of the present invention has a flexural strength of 650 MPa or more, preferably 700 MPa, as measured by the three-point method.
a or more.

Furthermore, in the silicon nitride substrate of the present invention, the size of the main surface is 400 to 40,000 mm2.
It is preferable that the density is 3.15 to 3.40 g/cm3 and the dielectric strength is 20 kV/mm or more. In this case, a silicon nitride substrate having a dielectric strength sufficient for practical use can be obtained.

Next, we will explain the manufacturing method for the silicon nitride substrate of the present invention.

First, silicon powder and sintering aid powder are prepared as raw materials. The silicon powder preferably has a median diameter D50 of 50 μm or less and an impurity oxygen content of 0.6 mass% or less. The amount of the sintering aid is preferably 15 mass parts per 100 mass parts of the silicon powder.

The sintering aid is preferably a metal compound powder having a median diameter D50 of 10 μm or less. Examples of metal compound powders include oxides of rare earth elements, magnesium, titanium, hafnium, etc., and more preferably rare earth element oxides and magnesium compounds (magnesia, etc.). These sintering aids have excellent fluidity and exhibit the fluid behavior described below, making it easier to orient the silicon nitride (particles) in the thickness direction.

Next, a dispersion medium is added to the silicon powder and the sintering aid, and the mixture is dispersed with media, for example, in a ball mill, and pulverized and mixed to prepare a slurry. As the dispersion medium, an organic solvent such as toluene, ethanol, or butanol can be used.

Next, to the above slurry, a binder, a plasticizer, etc. are added as necessary, and the slurry is degassed in vacuum to adjust the viscosity of the slurry. As the binder, an organic binder such as butyl methacrylate, polyvinyl butyral, polymethyl methacrylate, etc. can be used.

Next, the viscosity-adjusted slurry is formed into a sheet by a sheet forming method such as a doctor blade method or a roll method to form a sheet body having a thickness of, for example, 0.2 to 1.5 mm. The sheet body can be obtained, for example, by applying the slurry onto a film, drying it, and then removing the film.

Next, a slurry consisting of ceramic powder and a dispersion medium is applied to the main surface of the sheet as necessary to form a separating agent layer. As the dispersion medium, an organic solvent such as toluene, ethanol, or butanol can be used as described above. As the application method, a spray method, a bar coating method, a screen printing method, or the like can be used.

Next, the sheet body is degreased as necessary, for example, in a non-oxidizing atmosphere at a temperature of 600° C. or less for several hours. After that, the sheet body is held in a nitrogen-containing atmosphere at a temperature of 1200 to 1500° C. for 2 to 8 hours to nitride the silicon constituting the sheet body, forming silicon nitride. The nitrogen partial pressure in the nitrogen-containing atmosphere is, for example, 0.05 to 0.5 MPa.

Next, the mixture is maintained at a temperature of 1800 to 1950° C. for 6 to 24 hours in the same nitrogen-containing atmosphere to sinter the silicon nitride.

In the present invention, a weight plate is used in the nitriding and sintering process. There are several methods for doing this, including (1) not using a weight plate during the nitriding of silicon, leaving the top surface free and using the weight plate only during sintering, (2) using a porous plate as the weight plate and applying a load to the molded body successively with the nitriding of silicon and the sintering of silicon nitride, and (3) using a dense plate as the weight plate and providing a layer of separating agent between the molded body and the dense plate.

The separating agent layer is made of ceramic powder, which is not particularly limited as long as it is thermally stable during nitriding and sintering and allows the dense plate to be separated after sintering is completed, but boron nitride is preferred.

Furthermore, when boron nitride is used as the ceramic powder, its purity is preferably 95% or more, its average particle size is preferably 5 to 20 μm, and the thickness of the separating agent layer is preferably 10 to 60 μm, or 20 to 60 μm.

A plurality of sheets each having a separating agent layer formed on a main surface thereof can be laminated with the separating agent layer interposed therebetween. In this case, a plurality of silicon nitride substrates can be simultaneously manufactured through the above-mentioned nitriding and sintering steps.

As a result, when the substrate surface is irradiated with X-rays, it has an X-ray diffraction peak of β-Si3N4,
A silicon nitride substrate can be obtained in which the degree of orientation fa, which indicates the proportion of the major axes (c-axes) of β-Si3N4 grains oriented in the thickness direction, is in the range of 0 to 0.3.

(Example)

( Reference Example 1)
Metal Si powder, sintering aid (rare earth oxide and magnesium compound), dispersant (polyoxyalkylene type dispersant), and dispersion medium (ethanol, butanol) were mixed for 35 hours using a ball mill. The metal Si powder and sintering aid were adjusted so that the mass ratio of silicon nitride content to sintering aid content after sintering was 0.89:0.11. Then, a dispersion medium (ethanol, methyl ethyl ketone), an organic binder (acrylic resin), and a plasticizer were added to the mixture and remixed to prepare a slurry. Next, the prepared slurry was removed from the ball mill and transferred to a defoamer, after which the viscosity of the slurry was adjusted by vacuum defoaming, and the mixture was molded into a sheet to prepare a sheet molded body of 100 x 100 x t 0.38 mm. A doctor blade method was adopted as the sheet molding method.

Thereafter, a ceramic slurry made of boron nitride was applied to the sheet compact to form a separating agent layer having a thickness of 10 μm on the surface of the sheet compact, and the sheet compact was then subjected to a degreasing treatment at 550° C. in a non-oxidizing atmosphere.

Next, the sheet molded article having a separating agent layer made of boron nitride formed on its main surface was subjected to a nitriding treatment at 1400° C. for 2 hours in a nitrogen-containing atmosphere with a nitrogen partial pressure of 0.2 MPa.
Further, the resultant was sintered in a nitrogen-containing atmosphere with a nitrogen partial pressure of 0.7 MPa at 1820° C. for 9 hours to prepare the silicon nitride substrate of Example 1.

( Reference Example 2)
The silicon nitride substrate of Reference Example 2 was produced under the same production conditions as Reference Example 1, except that the metal Si powder and the sintering aid were adjusted so that the mass ratio of the silicon nitride content to the sintering aid content after sintering was 0.895:0.105, a separating agent layer having a thickness of 20 μm was formed on the surface of the sheet molded body, and the sheet molded body was subjected to a degreasing treatment in this state.

( Reference Example 3)
The silicon nitride substrate of Reference Example 3 was produced under the same production conditions as Reference Example 1, except that the metal Si powder and the sintering aid were adjusted so that the mass ratio of the silicon nitride content to the sintering aid content after sintering was 0.878:0.122, a separating agent layer of a thickness of 25 μm was formed on the surface of the sheet molded body, and the sheet molded body was subjected to a degreasing treatment in this state.

( Reference Example 4)
The silicon nitride substrate of Reference Example 4 was produced under the same production conditions as Reference Example 1, except that the metal Si powder and the sintering aid were adjusted so that the mass ratio of the silicon nitride content to the sintering aid content after sintering was 0.885:0.115, a separating agent layer of a thickness of 35 μm was formed on the surface of the sheet molded body, and the sheet molded body was subjected to a degreasing treatment in this state.

( Reference Example 5)
The silicon nitride substrate of Reference Example 5 was produced under the same production conditions as Reference Example 1, except that the metal Si powder and the sintering aid were adjusted so that the mass ratio of the silicon nitride content to the sintering aid content after sintering was 0.883:0.117, a separating agent layer of a thickness of 35 μm was formed on the surface of the sheet molded body, and the sheet molded body was subjected to a degreasing treatment in this state.

( Reference Example 6)
The silicon nitride substrate of Reference Example 6 was produced under the same production conditions as Reference Example 1, except that the metal Si powder and the sintering aid were adjusted so that the mass ratio of the silicon nitride content to the sintering aid content after sintering was 0.885:0.115, a separating agent layer of a thickness of 50 μm was formed on the surface of the sheet molded body, and the sheet molded body was subjected to a degreasing treatment in this state.

( Reference Example 7)
The silicon nitride substrate of Reference Example 7 was produced under the same production conditions as Reference Example 1, except that the sheet molding was subjected to a degreasing treatment without a release agent layer on the surface of the sheet molding, and the sheet molding was sintered without a weight plate.

( Reference Example 8)
A porous plate with a porosity of 40% was formed on the surface of the sheet molding in place of the separating agent layer, and the sheet molding was subjected to a degreasing treatment in this state. Except for this, the silicon nitride substrate of Reference Example 8 was produced under the same production conditions as Reference Example 1.

( Reference Example 9)
A silicon nitride substrate of Reference Example 9 was produced under the same production conditions as Reference Example 1, except that a semi-dense plate with a porosity of 15 % was formed on the surface of the sheet molding in place of the separating agent layer, and the sheet molding was subjected to a degreasing treatment in this state.

(Example 10)
The metal Si powder and sintering aid were adjusted so that the mass ratio of the silicon nitride content to the sintering aid content after sintering was 0.921:0.079, a sheet molded body of 240 x 180 x t 0.29 mm was produced, a separating agent layer of 20 μm in thickness was formed on the surface of the sheet molded body, and in that state the sheet molded body was subjected to a degreasing treatment and sintered at 1840° C. for 12 hours in a nitrogen-containing atmosphere with a nitrogen partial pressure of 0.7 MPa. Except for this, the silicon nitride substrate of Example 10 was produced according to the same production conditions as in Reference Example 1.

Example 11
The mass ratio of the metal Si powder and the sintering aid to the silicon nitride content after sintering is 0.
The silicon nitride substrate of Example 11 was produced under the same production conditions as in Example 10, except that the composition ratio was adjusted to 0.92:0.08 and sintered at 1830° C. for 12 hours in a nitrogen-containing atmosphere with a nitrogen partial pressure of 0.7 MPa.

Example 12
The mass ratio of the metal Si powder and the sintering aid to the silicon nitride content after sintering is 0.
The relative density of the silicon nitride substrate in Example 12 was adjusted to 926:0.074, and a sheet molded body of 240 x 180 x t0.38 mm was produced. A separating agent layer of 10 μm in thickness was formed on the surface of the sheet molded body. In this state, the sheet molded body was subjected to a degreasing treatment. Except for this, the silicon nitride substrate in Example 12 was produced under the same production conditions as in Example 11.

Example 13
The mass ratio of the metal Si powder and the sintering aid to the silicon nitride content after sintering is 0.
The silicon nitride substrate of Example 13 was produced under the same production conditions as in Example 12, except that the ratio of the solubility of the silicon nitride film to the thickness of 1.927 was adjusted to 0.073, a releasing agent layer of 20 μm was formed on the surface of the sheet molded body, and the sheet molded body was subjected to a degreasing treatment in this state.

(Comparative Example)

(Comparative Example 1)
The metal Si powder, silicon nitride powder, and sintering aid were adjusted so that the mass ratio of silicon nitride content to sintering aid content after sintering was 0.909:0.091, a sheet molded body of 100 x 100 x t0.38 mm was produced, a release agent layer of 35 μm was formed on the surface of the sheet molded body, and in this state, the sheet molded body was subjected to a degreasing treatment, and sintered at 1820 ° C. for 9 hours in a nitrogen-containing atmosphere with a nitrogen partial pressure of 0.7 MPa to produce a silicon nitride substrate. The silicon nitride substrate of Comparative Example 1 was produced according to the same production conditions as Reference Example 1, except for the above.

(Comparative Example 2)
The silicon nitride substrate of Comparative Example 2 was prepared under the same preparation conditions as in Reference Example 1, except that the metal Si powder, silicon nitride powder, and sintering aid were adjusted so that the mass ratio of silicon nitride content to sintering aid content after sintering was 0.904:0.096.

(Evaluation Method)
The characteristics of the silicon nitride substrates of the examples and comparative examples were evaluated as follows.

(Orientation degree based on X-ray diffraction peak intensity)
X-ray diffraction was performed using Cu-Kα radiation excited at 40 kV and 15 mA, with scanning by the θ-2θ method at a step width of 0.01°.

(Elemental Analysis)
Quantitative analysis of Si, N, Mg and rare earth elements was performed using a ZSX Primu (Rigaku Corporation).
The analysis was performed by fluorescent X-ray analysis using sII.
The measurement was carried out by inert gas fusion-non-dispersive infrared absorption (NDIR) method using an MGA-920. The SiN content was calculated from the amount and ratio of Si and N, and the amount of sintering aid was calculated from the amount and ratio of Mg and O, and the amount and ratio of rare earth elements and O.

(Thermal Conductivity)
The thermal diffusivity was measured by the flash method using a NETZSCH LFA 467 Hyp
The measurement was performed using an IR Flash device. In this device, a xenon flash light with a pulse width of 20 μsec was irradiated, and the AC temperature response was measured with an IR detector, and the thermal diffusivity was calculated from the amplitude of the temperature response and the attenuation rate with respect to the position. The surface of a test piece with a size of 10 mm x 10 mm was blackened and then the measurement was performed.

(Density measurement)
The density was measured by the Archimedes method.

(Three-point bending strength)
The three-point bending strength was measured using a 4 mm x 35 mm test piece according to JIS R1601:2008
According to the above, the three-point bending strength was measured at room temperature (25° C.) when the test piece was bent from the midpoint between the two supports with a distance of 30 mm between the two supports, and the average value of the three-point bending strengths of 10 test pieces was calculated.

Table 1 shows some of the preparation conditions for the silicon nitride substrates of the examples and comparative examples, as well as the evaluation results.

In Examples 1 to 5 and 10 to 13 in which a separating agent layer was formed on one main surface of a sheet body and Examples 6 to 9 in which a porous plate was formed, the β
It was found that the degree of orientation fa, which indicates the proportion of --Si ₃ N ₄ particles, was in the range of 0 to 0.3, and the thermal conductivity in the thickness direction was 80 W/m·K or more.

10 Silicon nitride substrate 11 Silicon nitride particle 12 Grain boundary phase

Claims (3)

When the substrate surface is irradiated with X-rays, it has an X-ray diffraction peak of β-Si ₃ N ₄ ,
A silicon nitride substrate in which the degree of orientation fa, which indicates the proportion of the long axis (c-axis) of β-Si ₃ N ₄ grains oriented in the thickness direction in the silicon nitride substrate, is within the range of 0.0479 to 0.0929 , the average three-point bending strength of ten test pieces of 4 mm x 35 mm, when the test pieces are bent from the midpoint between two supports with a distance of 30 mm between the two supports, is within the range of 675 to 689 MPa, and the thermal conductivity in the thickness direction is within the range of 111.2 to 121.6 W/m·K . 2. The silicon nitride substrate according to claim 1,
Contains rare earth oxides and magnesium compounds as sintering aids
Silicon nitride substrate. The silicon nitride substrate according to claim 1 or 2,
The size of the main surface is 400 to 40,000 mm ² and the density is 3.15 to 3.40 g/cm ³ and the dielectric strength is 20 kV/mm or more.
Silicon nitride substrate.