JP7729058B2 - Method for producing silicon nitride sintered body - Google Patents

Description

This application relates to a silicon nitride sintered body, a circuit board, and a method for manufacturing a silicon nitride sintered body.

In recent years, high-power electronic circuits have come to be used in a variety of applications, including power modules for efficiently driving motors used in electric and hybrid cars, and LED drive circuits. Because these circuits use electronic components that generate heat due to the high power, insulating substrates with high heat dissipation properties are used. Ceramic substrates are commonly used for these insulating substrates because they allow large substrates to be manufactured relatively inexpensively. Sintered silicon nitride substrates, in particular, have excellent mechanical strength. For example, Patent Document 1 discloses a sintered silicon nitride substrate with excellent thermal conductivity.

Japanese Patent Application Laid-Open No. 2007-197226

There is a demand for silicon nitride substrates with higher thermal conductivity. The present invention provides a silicon nitride sintered body with high thermal conductivity, a circuit board, and a method for manufacturing the silicon nitride sintered body.

A silicon nitride sintered body according to an embodiment of the present disclosure includes a plurality of silicon nitride particles and a grain boundary triple junction located between three or more of the plurality of silicon nitride particles, wherein the grain boundary triple junction contains Mg and at least one element RE selected from rare earth elements, the ratio of Mg to RE at the grain boundary triple junction (Mg/RE) satisfies Mg/RE≦1, and the O content in the silicon nitride particles is 0.05 mass% or less.

The silicon nitride sintered body may further include a two-particle boundary located between two of the plurality of silicon nitride particles, the two-particle boundary containing Mg and at least one element RE selected from rare earth elements, and the ratio Mg to RE at the two-particle boundary, Mg/RE, may satisfy Mg/RE≦0.7.

The plurality of silicon nitride particles may contain Mg and O, and each of the plurality of silicon nitride particles may include an inner region and an outer region located outside the inner region, in which the concentration of at least one of Mg and O is higher than that of the inner region, and the thickness of the outer region may be 2.5 nm or less.

At least one of the plurality of silicon nitride particles may contain Mg, O, and RE, and may include fine particles having a particle size of 100 nm or less.

The microparticles may be amorphous.

The fine particles may be present at a density of 5 particles/μm ² or more in an image directly observed at a magnification of 10,000 times or more using a transmission electron microscope.

Thermal conductivity may be 130 W/m·K or more.

The strength of the silicon nitride sintered body in a three-point bending test may be 500 MPa or more.

A circuit board according to an embodiment of the present disclosure comprises a silicon nitride substrate made of the silicon nitride sintered body described above and a conductive pattern disposed on at least one surface of the silicon nitride substrate.

A method for producing a silicon nitride sintered body according to an embodiment of the present disclosure includes: a step (a) of mixing Si powder, an RE raw material powder containing at least one element RE selected from rare earth elements, and an Mg raw material powder containing _MgSiN2 in a ratio of y to ₈ mol% to 15 mol%, where x is the number of moles of _Si3N4 obtained when the Si powder is nitrided, y is the number of moles of the RE raw material powder converted into _trivalent oxide _RE2O3 , and z is the number of moles of the Mg raw material powder converted into MgO, where x + y + z is 100 mol%, to obtain a mixed powder; a step (b) of forming a slurry from the mixed powder to obtain a sheet-like compact; and a step (c) of heating and sintering the compact in a nitrogen atmosphere to obtain a silicon nitride sintered body. The Mg raw material powder is a silicon nitride sintered body obtained by nitriding the Si powder and the MgSiN2. ₂ in an amount of 87 mass % or more.

Step (c) may include step (c1) of holding the compact at a temperature T1 of 1300°C or higher and 1800°C or lower for 0.5 hours or more in a nitrogen atmosphere of 1 atmosphere or higher and 20 atmospheres or lower during the period from the start of heating until the maximum temperature is reached, and step (c2) of holding the compact at a temperature T2 of 1300°C or higher and 1800°C or lower, which is higher than temperature T1, for 0.5 hours or more after step (c1).

Step (c) may further include, prior to step (c1), step (c3) of holding the compact at a temperature of 900°C or higher but lower than 1300°C for 0.5 hours or longer in an atmosphere of less than 1 atmosphere.

The heating temperature may be increased at a rate of 1°C/min or less from the end of step (c3) to the start of step (c1).

Embodiments of the present disclosure provide a silicon nitride sintered body that combines high thermal conductivity and high strength, a circuit board, and a method for manufacturing the silicon nitride sintered body.

FIG. 1 is a transmission electron microscope image showing an example of the silicon nitride sintered body of this embodiment. FIG. 2 is a schematic diagram showing the structure of the silicon nitride sintered body shown in FIG. FIG. 3 is a schematic diagram showing the concentration profile of the constituent elements along the line L in FIG. FIG. 4 is a flowchart showing the method for producing a silicon nitride sintered body according to this embodiment. FIG. 5 is a schematic diagram showing the temperature profile of the heat treatment carried out during sintering in the method for producing a silicon nitride sintered body according to this embodiment. FIG. 6 shows an example of the results of composition analysis of the grain boundary phase of Samples 8, 12, and 13 of the example. FIG. 7(a) is an HAADF image of a silicon nitride particle of sample 8, and (b) shows the concentration profile of the constituent elements by EDS at the arrow in the image shown in (a). FIG. 8(a) is an HAADF image of a silicon nitride particle of Sample 12, and (b) shows the concentration profile of the constituent elements by EDS at the arrow in the image shown in (a). FIG. 9(a) is an HAADF image of a silicon nitride particle of Sample 13, and (b) shows the concentration profile of the constituent elements by EDS at the arrow in the image shown in (a).

As shown in Patent Document 1, for example, the theoretical thermal conductivity of silicon nitride single crystal is believed to be 200 W/m·K or higher (thermal conductivity is measured in units of W for the numerator and m·K for the denominator). However, achieving a comparable thermal conductivity with sintered silicon nitride is difficult. To obtain sintered silicon nitride, sintering aids must be added, and the elements used as sintering aids affect the improvement of thermal conductivity. The present inventors conducted detailed research into the raw materials, sintering aids, and sintering conditions, as well as the structure and element distribution within sintered silicon nitride sintered bodies with high thermal conductivity. As a result, they have devised a silicon nitride sintered body with a thermal conductivity of 130 W/m·K or higher and sufficient mechanical strength, and a method for manufacturing the same. Below, embodiments of the silicon nitride sintered body, circuit board, and method for manufacturing the silicon nitride sintered body disclosed herein are described with reference to the drawings.

(Silicon nitride sintered body)
Fig. 1 is a transmission electron microscope (TEM) image showing an example of a silicon nitride sintered body of this embodiment, and Fig. 2 is a schematic diagram showing the structure of the silicon nitride sintered body 10 shown in Fig. 1. The silicon nitride sintered body 10 is composed of a plurality of silicon nitride particles 11 and a grain boundary phase 12.

Silicon nitride particles 11 are the main phase in silicon nitride sintered body 10. Silicon nitride particles 11 are single crystals as a whole, with nitrogen and silicon atoms regularly arranged. When analyzed, for example, by energy dispersive X-ray spectroscopy (EDS) using a transmission electron microscope, silicon nitride particles 11 contain elements other than nitrogen and silicon. Specifically, silicon nitride particles 11 contain O and Mg. Silicon nitride particles 11 may further contain RE. Here, RE is at least one element selected from rare earth elements. More specifically, RE is at least one element selected from Y, La, Ce, Nd, Pm, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, and Hf. Preferably, RE contains at least Y. Mg and RE are derived from a sintering aid. O originates from sintering aids, is inevitably contained in the raw material Si powder, or is inevitably incorporated during the production of silicon nitride sintered body 10.

Silicon nitride particle 11 includes inner region 11A and outer region 11B. Outer region 11B is located outside inner region 11A so as to cover inner region 11A, and is located at the outer edge of silicon nitride particle 11. As described below, outer region 11B is in contact with grain boundary phase 12.

The outer region 11B contains at least one of Mg and O at a higher concentration than the inner region 11A. The outer region 11B may also contain RE at a higher concentration than the inner region 11A. The concentrations of Mg and O can be determined by scanning the inner region 11A and the outer region 11B perpendicularly to the boundary between the inner region 11A and the outer region 11B using EDS. The boundary between the outer region 11B and the inner region 11A can be determined by the position where the X-ray intensity of Mg and O begins to increase. Figure 3 is a schematic diagram showing the X-ray intensity distribution from the start point a to b of the line segment L in Figure 2, when the section of the line segment L is analyzed using EDS. As shown in Figure 3, the intensities of Mg and O are nearly constant in the inner region 11A, but as the concentration of at least one of Mg and O increases at a certain point near the grain boundary phase 12, the X-ray intensity also increases. The boundary between the outer region 11B and the inner region 11A is determined at the position where the X-ray intensity curve rises. The position where the curve rises may be determined, for example, using analysis software provided with a transmission electron microscope capable of analysis by energy dispersive X-ray spectroscopy (EDS).

The thickness of the outer region 11B is 2.5 nm or less. The thickness of the outer region 11B is preferably 2.2 nm or less, and more preferably 1.6 nm or less. The smaller the thickness of the outer region 11B, the better. The lower limit of the thickness of the outer region 11B is, for example, approximately 0.5 nm.

By containing at least one of Mg and O, the outer region 11B has a lower thermal conductivity than the inner region 11A. For this reason, it is preferable that the thickness of the outer region 11B is small. The Mg and O in the outer region 11B are related to the concentrations of Mg and O in the grain boundary phase 12, as described below.

The O content in the inner region 11A is 0.05% by mass or less. Furthermore, component analysis using EDS revealed that both Mg and RE were below the detection limit, indicating that the inner region 11A is highly pure and in a state close to single crystal. As described below, the content of a specific element in the inner region 11A is expressed as a mass ratio relative to the total detected elements determined by EDS using a transmission electron microscope. For example, multiple silicon nitride particles 11 are arbitrarily selected, and for each, measurements are taken at any two locations within the inner region 11A to determine the average content (the sum of the component amounts at the two locations divided by 2). The average value obtained is then further averaged across multiple silicon nitride particles 11.

The inner region 11A exhibits high thermal conductivity because it contains almost no Mg or RE derived from sintering aids. However, the inner region 11A may contain trace amounts of O, which can reduce the ideal thermal conductivity of silicon nitride, so a low O content is preferable.

The silicon nitride particle 11 preferably contains one or more fine particles 11C within the inner region 11A. The fine particles 11C contain Mg, O, and RE. They also contain Si and N. The fine particles 11C are amorphous and have a particle size of 100 nm or less. Here, the particle size of the fine particles 11C is the maximum length of the particle, determined from an image or observation of a transmission electron microscope image.

The fine particles 11C are an amorphous phase composed of Si, N, O, Mg, and RE. Preferably, the silicon nitride particles 11 contain 5 or more fine particles 11C per ^μm2 in a direct magnification image or observation image of a transmission electron microscope image at 10,000 times or more.

In the inner region 11A of the silicon nitride particle 11, the microparticles 11C contain at least one of the elements O, Mg, and RE in greater amounts than the inner region 11A located outside the microparticles 11C. In other words, by concentrating elements other than Si and N to form the microparticles 11C, the purity of the silicon nitride in the parts of the inner region 11A other than the microparticles 11C can be increased, thereby improving thermal conductivity.

The grain boundary phase 12 is an amorphous phase formed between multiple silicon nitride particles 11. The grain boundary phase 12 includes two-particle grain boundaries 12A and grain boundary triple junctions 12B. The two-particle grain boundaries 12A are located in the region sandwiched between two adjacent silicon nitride particles 11 and are in contact with the two silicon nitride particles 11. The grain boundary triple junctions 12B are located between and are in contact with three or more silicon nitride particles 11 among the multiple silicon nitride particles. The grain boundary triple junctions 12B are connected to the two-particle grain boundaries 12A.

The grain boundary phase 12 contains Mg and RE at higher concentrations than the silicon nitride particles 11. The ratio Mg/RE of Mg to RE at the two-particle grain boundary 12A of the grain boundary phase 12 satisfies the following formula (1): For example, quantitative analysis of the element Mg and the element RE in mass units is performed by EDS analysis, and Mg/RE is calculated in mass%.
Mg/RE≦0.7 (1)

More preferably, the ratio Mg/RE of Mg to RE at the two-particle boundary 12A satisfies the following formula (1').
Mg/RE≦0.5 (1')

Furthermore, the ratio Mg/RE of Mg to RE at the grain boundary triple junction 12B satisfies the following formula (2).
Mg/RE≦1 (2)

More preferably, the ratio Mg/RE of Mg to RE at the grain boundary triple junction 12B satisfies the following formula (2').
Mg/RE≦0.5 (2')

Mg/RE is related to thermal history. During the production of silicon nitride sintered body 10, Si, N, O, Mg, and RE constitute a liquid phase. As silicon nitride particles 11 grow from the liquid phase, the concentrations of Mg and RE in the remaining liquid phase increase. The remaining liquid phase ultimately forms grain boundary phase 12. As a result, Mg and RE exist in high concentrations in grain boundary phase 12. Furthermore, while grain boundary phase 12 has a uniform composition overall in the liquid state, during the cooling process, two-particle grain boundaries 12A are formed first, followed by the grain boundary triple junction 12B. At this time, Mg and RE migrate from two-particle grain boundaries 12A to grain boundary triple junction 12B.

Mg and RE are included as sintering aids. Because the melting point of Mg is around 650°C, Mg forms a liquid phase at a relatively low temperature. Because rare earth metals have relatively high melting points, RE forms a liquid phase at a relatively high temperature. Also, Mg vaporizes at around 1400°C, while RE vaporizes at even higher temperatures. For example, Y vaporizes at around 1800°C.

When producing silicon nitride sintered bodies, Mg is generally added in greater amounts than RE in oxide equivalent, but for the reasons mentioned above, Mg evaporates in greater amounts than RE. According to the results of the inventors' research, silicon nitride sintered body 10 exhibits high thermal conductivity when formula (1) is satisfied for the two-particle grain boundary 12A and formula (2) is satisfied for the grain boundary triple point 12B.

The proportion of Mg in the entire silicon nitride sintered body 10 is preferably 0.2 mass% or less. As mentioned above, the amount of Mg added decreases as Mg evaporates during the production of the sintered body. However, if the amount of Mg evaporated is small, the above-mentioned ratio Mg/RE in the grain boundary phase will not be sufficiently small. Furthermore, because rare earth metals have an affinity for oxygen, the presence of high concentrations of rare earth metals in the grain boundary phase facilitates the migration of oxygen from the silicon nitride particles to the grain boundaries. However, if the total amount of Mg is high, the Mg inhibits this oxygen migration, and it is thought that the reduction in the O content in the silicon nitride particles will not proceed sufficiently.

The two-particle boundary 12A has a large area of contact with the silicon nitride particle 11, and it is believed that the Mg concentration at the two-particle boundary 12A affects the thickness of the outer region 11B of the silicon nitride particle 11. From this perspective, it is preferable that the Mg content at the two-particle boundary 12A is low. Specifically, the Mg content at the two-particle boundary 12A is preferably 8 mass% or less. The thickness of the two-particle boundary 12A is, for example, 0.6 nm or more and 1.6 nm or less.

Silicon nitride sintered body 10 has a thermal conductivity of 130 W/m·K or greater. Silicon nitride sintered body 10 preferably has a thermal conductivity of 140 W/m·K or greater, and more preferably has a thermal conductivity of 150 W/m·K or greater. Silicon nitride sintered body 10 also has a strength of 500 MPa or greater. Here, the strength is a value measured in a three-point bending test in accordance with JIS C6481. Silicon nitride sintered body 10 also has a relative density of 98% or greater.

According to the silicon nitride sintered body 10 of this embodiment, the O content in the silicon nitride particles is 0.05 mass% or less, thereby increasing the purity of the silicon nitride particles 11 and increasing the thermal conductivity of the silicon nitride particles 11. Furthermore, the ratio of Mg to RE at the grain boundary triple junction (Mg/RE) satisfies formula (2), and the ratio of Mg to RE at the two-particle grain boundary (Mg/RE) satisfies formula (1), thereby reducing the Mg concentration in the grain boundary phase. This allows the thickness of the outer region 11B of the silicon nitride particles 11 to be reduced, thereby increasing the thermal conductivity of the silicon nitride sintered body 10. Specifically, it is possible to achieve a silicon nitride sintered body with a thermal conductivity of 130 W/m·K or more. If the thickness of the outer region 11B of the silicon nitride particles 11 is 1.8 nm or less, thermal conductivity between the silicon nitride particles 11 can be increased. Furthermore, by including fine particles with high concentrations of Mg and RE in the silicon nitride particles 11, the purity of the inner region 11A of the silicon nitride particles 11 can be increased, providing higher thermal conductivity. Specifically, it is possible to achieve a silicon nitride sintered body with a thermal conductivity of 150 W/m·K or higher.

(circuit board)
The circuit board of this embodiment includes a silicon nitride substrate made of the silicon nitride sintered body of this embodiment and a conductive pattern disposed on at least one of two opposing main surfaces of the silicon nitride substrate. The circuit board has, for example, a square or rectangular shape with a side length of 3 cm to 10 cm. The thickness of the circuit board is, for example, 0.3 mm to 3.0 mm.

The circuit board of this embodiment includes a silicon nitride substrate made of the above-mentioned silicon nitride sintered body, and therefore has excellent thermal conductivity and high bending strength. For example, it has a thermal resistance of 0.2°C/W or less. It also has a resistance to more than 2,000 cycles in a thermal cycle test.

(Method for producing silicon nitride sintered body)
A method for producing a silicon nitride sintered body according to this embodiment will now be described. Fig. 4 is a flowchart showing the method for producing a silicon nitride sintered body according to this embodiment.

(1) Mixing step (a) (S1)
First, raw material powders are prepared. Si powder is prepared as the silicon nitride raw material. Furthermore, Mg raw material powder and RE raw material powder are prepared as sintering aids. Let x be the number of moles of _Si3N4 obtained when all of the Si powder is _nitrided , y be the number of moles of the RE raw material powder converted to trivalent oxide _RE2O3 , and _z be the number of moles of the Mg raw material powder converted to MgO. When x + y + z is 100 mol%, the RE raw material powder is prepared in a ratio of 0.5 mol% to less than 2 mol%, and the Mg raw material powder is prepared in a ratio of 8 mol% to less than 15 mol%, and then mixed. The remaining proportion of Si powder is 83 mol% to less than 91.5 mol%.

In this embodiment, Si powder is prepared as the raw material for silicon nitride. By using Si powder and nitriding silicon using _N2 gas, it is possible to relatively reduce the amount of oxygen derived from the raw material. If the proportion of Si powder is less than 83 mol%, the bending strength and thermal conductivity of the resulting silicon nitride sintered body are too low. On the other hand, if the proportion of Si powder is 91.5 mol% or more, the sintering aid is insufficient, making it difficult to obtain a dense silicon nitride sintered body.

The Mg raw material powder contains _MgSiN2 as a main component. The Mg raw material powder preferably contains 87 mass% or more of _MgSiN2 , and more preferably contains 90 mass% or more of _MgSiN2 . By using MgSiN2 instead of _MgO , the amount of oxygen derived from the raw materials can be reduced, and the amount of O in the silicon nitride sintered body can be reduced.

If the proportion of Mg raw material powder is less than 8 mol%, it becomes difficult to obtain a dense silicon nitride sintered body. Also, if the proportion of Mg raw material powder is 15 mol% or more, the grain boundary phase increases, and the Mg/RE ratio in the grain boundary phase increases, which tends to reduce the thermal conductivity of the silicon nitride sintered body. Also, if the proportion of _MgSiN2 in the Mg raw material powder is less than 87 mass%, as mentioned above, the amount of O in the silicon nitride sintered body increases, making it difficult to achieve high thermal conductivity.

As described above, the RE raw material powder is a raw material powder containing at least one element selected from rare earth elements. More specifically, the RE is at least one element selected from Y, La, Ce, Nd, Pm, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, and Hf. From the viewpoint of increasing the density of the silicon nitride sintered body, it is preferable that the RE contains Y. The RE may be in the form of an oxide or a compound other than oxygen. For example, the RE raw material powder may be a powder of _Y2O3 , YN, or the _like .

If the proportion of RE raw material powder is less than 0.5 mol%, it becomes difficult to obtain a dense silicon nitride sintered body. Furthermore, if the proportion of RE raw material powder is 2 mol% or more, the grain boundary phase increases, and the thermal conductivity of the silicon nitride sintered body tends to decrease.

Si powder, Mg raw material powder, and RE raw material powder are weighed out to obtain the above raw material powder ratios, and mixed with a plasticizer (e.g., a phthalate-based plasticizer), an organic binder (e.g., polyvinyl butyral, polyisobutyl acrylate), and an organic solvent (e.g., ethyl alcohol, butyl alcohol) in a ball mill or similar to produce a slurry containing the raw materials. The solids concentration of the slurry is preferably 30% by mass or more and 70% by mass or less.

(2) Molding process (b) (S2)
After degassing and thickening the slurry, a sheet-like compact (green sheet) is formed, for example, by the doctor blade method. The thickness of the compact is appropriately set taking into consideration the thickness of the silicon nitride sintered body to be formed and the sintering shrinkage rate. The compact formed by the doctor blade method is usually a long strip, so it is punched or cut to the desired shape and size.

After the molding process, the molded body may be degreased by heating it to 400-800°C to remove the organic binder and plasticizer.

(3) Sintering step (c) (S3)
The compact is introduced into a heating furnace and heated in a nitrogen atmosphere to sinter the raw materials, yielding a silicon nitride sintered body. Figure 5 shows a schematic example of the temperature profile T of the heating furnace during the sintering process and the pressure profile P of the nitrogen gas introduced into the heating furnace. As explained below, the preferred holding time for each step has a range, so the time axes in Figure 5 are not necessarily evenly spaced. Therefore, the slopes of the straight lines in the temperature profile T (solid line) and the pressure profile P (dashed line) do not necessarily accurately reflect the temperature rise/decrease rates and pressure rise/decrease rates. Furthermore, two straight lines with the same slope in the temperature profile T and the pressure profile P do not necessarily represent the same temperature rise rate or pressure rise rate.

The sintering process (c) includes a temperature rise period D1, a sintering period D2, and a cooling period D3. The temperature rise period D1 is the period from the start of heating to the maximum temperature in the temperature profile T.

The temperature rising period D1 includes a step of nitriding the raw material Si powder with N ₂ gas to change it into Si ₃ N ₄ particles according to the reaction formula (3).
3Si+2N ₂ → Si ₃ N ₄ (3)

The sintering period D2 is the period during which the maximum temperature is maintained. The cooling period D3 is the period after the sintering period D2 ends during which the temperature of the heating furnace is lowered to room temperature. Here, the maximum temperature refers to a temperature of 1800°C or higher.

The temperature inside the heating furnace during the sintering process can be measured, for example, through a viewing window in the heating furnace using a radiation thermometer to measure a target such as a molded body inside the furnace. Specifically, the heating furnace can be equipped with a sintering container in which the molded body is placed, a carbon cylindrical wall with the sintering container positioned on its inner periphery, and a target located near the outer periphery of the cylindrical wall.

(3-1) Temperature rise period D1
The temperature rise period D1 includes a decarbonization period d1, a nitriding process d2, a first temperature holding period d3, and a second temperature holding period d4. The first temperature holding period d3 and the second temperature holding period d4 are nitriding periods for nitriding Si.

(i) Decarbonization period d1
First, the ambient temperature in the heating furnace is raised from room temperature to the temperature range of the decarbonization period d1. The heating rate is, for example, 60°C/hr. Once the ambient temperature in the heating furnace reaches a temperature between 900°C and 1300°C, it is maintained at this temperature range for 0.5 hours to 2 hours. The atmosphere in the heating furnace is preferably under reduced pressure, i.e., lower than 1 atmosphere. For example, a pressure of 80 Pa or less is preferred. If carbon remains during sintering, voids are likely to form in the silicon nitride sintered body. For this reason, the carbon in the compact is removed by maintaining the compact under reduced pressure.

If the ambient temperature is lower than 900°C, carbon may not be sufficiently removed. Furthermore, if it is higher than 1300°C, the sintering aid may also be removed. It is more preferable that the ambient temperature in the heating furnace be between 1000°C and 1250°C.

(ii) Nitriding process d2
After the decarbonization period is completed, the molded body is slowly heated from the temperature at the end of the decarbonization period to the temperature at the start of the first temperature holding period d3, which is the nitriding period. Specifically, the heating temperature is increased at a rate of 1°C/min or less to increase the temperature of the molded body.

The temperature rise rate during the nitriding step d2 is preferably 0.5°C/min or less. Furthermore, in order to produce silicon nitride sintered bodies under practical conditions, the temperature rise rate is preferably 0.1°C/min or more.

During the nitriding step d2 and subsequent periods, it is preferable to fill the heating furnace with a nitrogen atmosphere. Specifically, nitrogen or a mixed gas containing nitrogen as the main component and an inert gas such as argon, or a mixed gas containing nitrogen gas with approximately 3% or less of hydrogen, can be used. The pressure inside the heating furnace is preferably between 1 atmosphere and 20 atmospheres. While Figure 5 shows the pressure of the nitrogen atmosphere as constant during the nitriding step d2 and subsequent periods, it may vary within the pressure range described above.

(iii) First temperature holding period d3 and second temperature holding period d4
During the first temperature holding period d3, the temperature of the atmosphere in the heating furnace is held at a temperature T1 of 1300° C. to 1800° C. The holding time is preferably 0.5 hours to 10 hours.

Then, during the second temperature holding period d4, the temperature of the atmosphere in the heating furnace is held at a temperature T2 higher than temperature T1, at 1300°C or higher and 1800°C or lower. The holding time is preferably 0.5 hours or higher and 10 hours or lower.

Although not shown, a third temperature holding period d5 may then be performed, in which the temperature of the atmosphere in the heating furnace is held at a temperature T3 higher than temperature T2, in the range of 1300°C to 1800°C. The holding time is preferably 0.5 hours to 10 hours.

For example, temperature T1 is between 1300°C and 1500°C, and temperature T2 is between 1500°C and 1700°C. As an example, temperature T1 is 1400°C and temperature T2 is 1600°C. Temperature T3 is 1800°C.

These processes nitride the Si powder, producing silicon nitride. By nitriding the Si powder at two temperatures, deformations such as waviness in the sheet-shaped compact can be suppressed, even when the compact size is large.

(3-2) Sintering period D2
After the nitriding period is over, the ambient temperature in the heating furnace is set to a temperature higher than 1800°C, and the compact is sintered. The ambient temperature is preferably 1800°C or higher and 2000°C or lower, more preferably 1800°C or higher and 1950°C or lower, and even more preferably 1800°C or higher and 1900°C or lower. If the ambient temperature exceeds 2000°C, the vaporization of the sintering aid and the decomposition of silicon nitride become intense, making it difficult to obtain a dense silicon nitride sintered body. As long as the ambient temperature is 1800°C or higher and 2000°C or lower, the ambient temperature does not need to be constant and may be changed (for example, gradually increased).

The holding time is between 1 hour and 30 hours. A holding time of between 2 hours and 20 hours is preferred, and between 5 hours and 15 hours is even more preferred.

By maintaining the compact at the above-mentioned temperatures, grain growth of the silicon nitride crystals is promoted, and Mg, RE, and O are excluded from the silicon nitride crystals and migrate to the grain boundary phase, thereby increasing the purity of the silicon nitride crystals. Furthermore, some of the Mg, RE, and O within the silicon nitride crystals form amorphous fine particles within them, thereby increasing the purity of the silicon nitride crystals.

(3-3) Cooling period D3
After the sintering period D2, the ambient temperature in the heating furnace is gradually lowered to approximately room temperature. During the cooling period D3, the liquid phase between the resulting silicon nitride particles is cooled and solidified as a grain boundary phase, thereby fixing the position of the main phase of the resulting silicon nitride particles. To rapidly solidify the liquid phase and maintain uniformity in the grain boundary phase distribution, the cooling rate is preferably 100°C/hr or higher, more preferably 300°C/hr or higher, and most preferably 500°C/hr or higher. For practical purposes, a cooling rate of 500°C/hr or higher and 600°C/hr or lower is preferred. Cooling at such a cooling rate suppresses crystallization of the solidifying sintering aid, forms a grain boundary phase mainly composed of a glass phase, and increases the bending strength of the silicon nitride sintered body. Lowering the ambient temperature to 1200°C fixes the grain boundary phase, and the entire silicon nitride sintered body becomes solid. Therefore, the cooling rate at temperatures lower than that is not particularly limited.

According to the method for producing sintered silicon nitride of this embodiment, by using Si powder as the Si source and a Mg raw material powder containing 87 mass% or more of _MgSiN2 , it is possible to reduce the amount of O contained during production and further reduce the amount of O in the sintered silicon nitride. Furthermore, it is believed that maintaining the compact at two temperature stages during the nitriding of the Si powder contributes to the precipitation of amorphous fine particles with concentrated impurities within the silicon nitride particles.

(Example)
The following describes the results of producing silicon nitride sintered bodies under various conditions and examining their properties.

1. Production of silicon nitride sintered body [Samples 1 to 15]
Si powder, Y ₂ O ₃ powder (Y ₂ O ₃ in Table 1) which is an RE raw material powder, and Mg raw material powder containing MgSiN ₂ and MgO (Mg compound in Table 1) were prepared. The number of moles of Si ₃ N ₄ obtained when all the Si powder was nitrided is defined as x, the number of moles of Y ₂ O ₃ powder as y, and the number of moles of the Mg raw material powder converted to MgO as z. The Y ₂ O ₃ powder and Mg raw material powder were weighed out in the proportions shown in Table 1, and Si powder was weighed out in a proportion corresponding to the number of moles converted to Si ₃ N ₄ , calculated by x = 100 - y - z. The proportion of MgSiN ₂ in the Mg raw material powder is as shown in Table 1.

These powders were mixed to form a slurry, and a sheet-shaped compact (green sheet) was created using the doctor blade method.

The compacts were placed in a heating furnace and sintered under the conditions shown in Table 2 to obtain ceramic sintered bodies (samples 1 to 15).

2. Measurement of Properties (1) The relative density, thermal conductivity, and three-point bending strength were measured for the prepared samples 1 to 15. The apparatus and conditions used for the measurements are as follows:
1) Relative density: A value obtained by dividing the density of the substrate material obtained by Archimedes' method by 3.24 g/ ^cm3 and expressing it as a percentage.
Board size: 30mm x 30mm
2) Thermal conductivity: The thermal conductivity was measured by the laser flash method on a substrate processed to a size of 5 mm x 5 mm.
Device: Netzsch Thermal diffusivity measuring device: LFA 467 Hyper Flash
Thermal conductivity is calculated as specific heat x density x thermal diffusivity.
The specific heat was kept constant at 0.68 kJ/(kg·K) and the density and thermal diffusivity obtained by measurement were inserted into the above equation for calculation.
3) Three-point bending strength JIS Distance between supports: 20 mm Substrate size: 30 mm x 25 mm x 0.32 mmt Crosshead speed: 0.5 mm/min
Apparatus: Shimadzu Autograph AGS-100NX

(2) Samples 1 to 15 were observed using a TEM to check for the presence of fine particles within the silicon nitride particles. The oxygen content at the center of the silicon nitride particles was measured using EDS. The ratio of Mg to Y at the grain boundaries between two particles and at the grain boundary triple junction was also examined.
Furthermore, concentration profiles of Mg and O in the silicon nitride particle and in the grain boundary between two adjacent particles were obtained by EDS line analysis, and the depth of Mg and O, which is the thickness of the outer region (i.e., the length from the interface between the silicon nitride particle and the grain boundary phase to the inside of the silicon nitride particle), was measured. The apparatus and conditions used for the measurement were as follows:
1) Sample preparation: Thinning device using ion milling method: PIPS (precision ion milling device): Gatan Model 691
2) Observation equipment: Transmission electron microscope (TEM): JEOL Ltd. JEM-ARM200F
Observation conditions: Acceleration voltage: 200 kV, Electron diffraction: Camera length: 80 cm
STEM (HAADF) mode: 5C, quantitative analysis: 30 Lsec
TEM-EDX conditions: Line analysis: 256Pix, 1.0msec/Pix
STEM: scanning transmission electron microscope
HAADF image: A HAADF-STEM (High-Angle Annular Dark Field Scanning TEM) image is obtained by scanning a sample with a finely focused electron beam and detecting the high-angle scattered electrons among the transmitted electrons with an annular detector.
Heavy elements appear dark in STEM images and bright in HAADF-STEM images. Because contrast proportional to the atomic weight (Z) is obtained, they are also called Z-contrast images.
3) Elemental Analysis The oxygen content of silicon nitride particles was measured by crushing a sintered body in the following manner, removing the grain boundary phase of the sintered body, and then measuring the oxygen content of silicon nitride particles, which are the main phase.
(a) Pulverization of Sintered Body The sintered body was pulverized into powder of 75 μm or less in size using a vibration mill.
(b) Removal of the grain boundary phase by acid treatment Hydrofluoric acid was added to remove the grain boundary phase, and then the sample was washed with distilled water. After washing, the supernatant was removed, and the sample was repeatedly washed by shaking with distilled water and centrifuged. After the washing was completed, the degree of neutrality was confirmed using pH test paper. Ethanol was then added, and the sample after washing was washed by shaking with ethanol (99.5%) and centrifuged, after which the silicon nitride particles were extracted by vacuum heating and drying.
(c) Measurement Oxygen analysis and quantification by peak separation were performed using a LECO TCH-600 with a temperature rise of 8°C/s and a sample amount of approximately 20 mg. Oxygen peaks appear on both the low-temperature and high-temperature sides, but the low-temperature side represents oxygen on the particle surface, while the high-temperature side represents oxygen inside the particle. Therefore, only the peak on the high-temperature side was approximated to a normal distribution, and the oxygen content was calculated from its area.

Sintered bodies manufactured under similar conditions were prepared, and the amounts of the RE components Y and Mg in the entire sintered body were determined by pulverizing the sintered body as described above and quantitatively analyzing the amounts of Y and Mg using ICP analysis.

3. Results and Discussion (1) Sintering Aid Proportion As shown in Sample 5 in Table 1, when the proportion of Y was less than 0.5 mol%, the relative density of the resulting sintered body was less than 98%. This is thought to be because the insufficient sintering aid prevented uniform growth of silicon nitride particles, resulting in a dense sintered body. Furthermore, as shown in Sample 6, when the proportion of Y was 2 mol% or more, the thermal conductivity was 130 W/m·K or less. This is thought to be because the excess sintering aid, which has a high boiling point, caused a large amount of Y to remain in the grain boundary phase of the resulting sintered body, thickening the grain boundary phase and increasing the concentration of Y in the grain boundary phase, thereby reducing the thermal conductivity of the grain boundary phase.

Similarly, as shown in sample 7, when the Mg content is less than 8 mol%, the relative density of the resulting sintered body is less than 98%. This is thought to be because a lack of sintering aid prevents a dense sintered body from being obtained. Furthermore, as shown in sample 8, when the Mg content is 15 mol% or more, the thermal conductivity is 130 W/m·K or less. This is thought to be because a large amount of Mg remains in the grain boundary phase of the resulting sintered body, and the high Mg concentration in the grain boundary phase reduces the thermal conductivity of the grain boundary phase. Furthermore, as the Mg content increases, the amount of MgO, a raw material powder other than _MgSiN2 , contained in the Mg raw material powder also increases. This is thought to result in an increase in the amount of O remaining in the sintered body, reducing the thermal conductivity.

As shown in Sample 9, even if the proportion of Mg raw material powder is less than 15 mol%, when the proportion of _MgSiN2 is less than 87 mass%, the thermal conductivity is 130 W/m K or less. It is thought that the amount of MgO, which is a raw material powder other than _MgSiN2 contained in the Mg raw material powder, also increases, and the amount of O remaining in the sintered body increases, resulting in a decrease in thermal conductivity.

(2) Oxygen Content in Silicon Nitride Particles As shown in Samples 1-4 and 10-15, when the oxygen content in the silicon nitride particles is 0.05% by mass or less, a thermal conductivity of 130 W/m·K or more is obtained. In particular, Samples 13-15, with a low oxygen content of 0.02% by mass, achieve a thermal conductivity of 150 W/m·K or more. In contrast, as shown in Samples 5, 8, and 9, when the oxygen content in the silicon nitride particles is greater than 0.05% by mass, the thermal conductivity falls below 130 W/m·K. Thus, the oxygen content in silicon nitride particles is related to thermal conductivity, with a tendency for lower oxygen content to increase conductivity. Note that in Samples 6 and 7, the oxygen content in the silicon nitride particles is 0.05% by mass or less, but the thermal conductivity is lower than 130 W/m·K. This is thought to be due to the aforementioned decrease in thermal conductivity caused by the Y content being outside the preferred range.

(3) Amount of Mg in the sintered body As shown in samples 1 to 4 and 10 to 15, when the amount of Mg in the entire sintered body is 0.20 mass% or less, a thermal conductivity of 130 W/m K or more is obtained. In contrast, as shown in samples 5 to 9, when the amount of Mg in the entire sintered body exceeds 0.20 mass%, the thermal conductivity is below 130 W/m K. This is thought to be because the amount of Mg in the sintered body is not low enough, and Mg inhibits the function of Y in attracting oxygen in the grain boundary phase.

(4) Mg/Y ratio of the grain boundary phase As shown in sample 8, when the Mg/Y ratio of the grain boundary triple junction is greater than 1, the thermal conductivity is less than 100 W/m K. This is thought to be because the presence of an excess of Mg in the raw material powder results in a high proportion of Mg in the grain boundary phase, which reduces the thermal conductivity of the grain boundary phase and prevents heat conduction between adjacent silicon nitride particles.

Figure 6 shows the composition ratios of constituent elements at the grain boundary triple junction and two-particle grain boundary for samples 8, 12, and 13. When a liquid phase containing large amounts of Mg and Y is cooled during the production of silicon nitride sintered bodies, it is believed that the two-particle grain boundary is formed first, followed by the grain boundary triple junction. At this time, Mg, Y, and O are thought to migrate from the two-particle grain boundary to the grain boundary triple junction, so the amounts of Mg, Y, and O at the grain boundary triple junction are greater than at the two-particle grain boundary. Furthermore, sample 8 contains a large amount of Mg at the grain boundary triple junction due to the large amount of Mg raw material powder added.

(5) Depth of Mg and O As shown in sample 8, when the Mg depth exceeds 2.5 nm, that is, when the thickness of the outer region of the silicon nitride particle exceeds 2.5 nm, the thermal conductivity is less than 100 W/m K. This is thought to be because the inclusion of Mg thickens the outer region, which has low thermal conductivity, making it difficult for the outer region to conduct heat from adjacent silicon nitride particles to the inner region.

Figure 7(a) is a high-angle annular dark-field (HAADF) image of a silicon nitride particle in sample 8, and Figure 7(b) shows the concentration profile of the constituent elements measured by EDS at the arrow in the image shown in (a). Figures 8(a), (b) and 9(a), (b) show HAADF images and concentration profiles of the constituent elements of samples 12 and 13, respectively.

From these figures, it can be seen that the concentrations of Mg and O are nearly constant within the inner region, but the curves of these concentration profiles rise at the boundary with the outer region, where the concentrations of Mg and O become higher. Furthermore, if the concentrations of Mg and O at the grain boundary between two particles increase, the concentrations of Mg and O in the outer region of the silicon nitride particle also increase, which is thought to shift the position of the boundary between the outer and inner regions and increase the thickness of the outer region.

(6) Fine particles in silicon nitride particles No fine particles were observed in the silicon nitride particles in samples 1 to 12, but five or more fine particles (five or more fine particles) were observed in samples 13 to 15. Since the thermal conductivity of samples 13 to 15 was 150 W/m K or higher, it is thought that the generation of fine particles containing Mg, Y, and O caused these elements in the silicon nitride particles to gather inside the fine particles, increasing the purity of silicon nitride in areas other than the fine particles and increasing the thermal conductivity.

From the above embodiments, it can be seen that a silicon _{nitride sintered} body having a thermal conductivity of 130 W/m·K or more can be produced by using raw materials prepared so that the RE raw material powder is 0.5 mol% or more but less than 2 mol%, the Mg raw material powder is 8 mol% or more but less than 15 mol%, and the Si powder is 83 mol% or more but less than 91.5 mol% in terms of Si3N4, totaling 100 mol%. It can also be seen that the thermal conductivity of the silicon nitride sintered body can be 130 W/m·K or more when the oxygen content in the silicon nitride particles is 0.05 mass% or less, the ratio of Mg to RE at the grain boundary triple junction (Mg/RE) is 1 or less, the Mg/RE at the two-particle grain boundary is 0.7 or less, and the thickness of the outer region of the silicon nitride particle is 2.5 nm or less. Furthermore, it can be seen that a thermal conductivity of 150 W/m·K or more can be achieved by having the amount of oxygen in the silicon nitride particles be 0.02 mass% or less, the thickness of the outer region of the silicon nitride particles (e.g., corresponding to the "depth of Mg and O" in Table 1) be 1.6 nm or less, and at least one of the silicon nitride particles contain fine particles containing Mg, O, and RE and having a particle size of 100 nm or less.

(7) Temperature Profile of the Sintering Process The temperature rise rate during the heat-reducing period for Samples 1 and 10-12 was less than 1°C/min, but this resulted in a thermal conductivity of 132 W/m·K or higher. This is thought to be because the slow temperature rise rate allowed the silicon nitriding reaction to proceed more slowly during the heat-reducing period, resulting in silicon nitride being obtained before the sintering period. Furthermore, Samples 13-15 achieved a thermal conductivity of 150 W/m·K or higher. This is thought to be because sintering at a higher sintering temperature allowed for a lower oxygen content within the silicon nitride particles and a lower Mg content in the entire sintered body, thereby reducing the oxygen content within the silicon nitride particles and the Mg content at the grain boundaries.

The silicon nitride sintered body, circuit board, and method for manufacturing the silicon nitride sintered body disclosed herein can be used for a variety of applications, and are particularly suitable for use in circuit boards with high thermal conductivity and high strength for a variety of applications.

10 silicon nitride sintered body 11 silicon nitride particle 11A inner region 11B outer region 11C fine particle 12 grain boundary phase 12A two-particle grain boundary 12B grain boundary triple point

Claims (3)

a step (a) of mixing Si powder, RE raw material powder containing at least one element RE selected from rare earth elements, and Mg raw material powder containing MgSiN2 in a ratio of _0.5 mol% or _more and less than ₂ mol% and ₈ mol% or more and less than 15 mol% when x + y + _z is 100 mol%, where x is the number of moles of Si3N4 obtained when the Si powder is nitrided, y is the number of moles of the RE raw material powder converted into trivalent oxide RE2O3, and z is the number of moles of the Mg raw material powder converted into MgO, to obtain a mixed powder;
(b) forming a slurry from the mixed powder to obtain a sheet-shaped compact;
(c) sintering the molded body by heating in a nitrogen atmosphere to obtain a silicon nitride sintered body;
Including,
The Mg raw material powder contains 87 mass% or more of the _MgSiN2 ,
The step (c) includes, during the period from the start of heating to the time when the maximum temperature is reached,
In a nitrogen atmosphere of 1 atmosphere or more and 20 atmospheres or less,
A step (c1) of holding the molded body at a temperature T1 of 1300 ° C. or higher and 1800 ° C. or lower for 0.5 hours or longer;
After the step (c1), a step (c2) of holding the compact at a temperature T2 that is 1300° C. or higher and 1800° C. or lower and is higher than the temperature T1 for 0.5 hours or more;
A method for producing a silicon nitride sintered body, comprising: The step (c) includes, before the step (c1),
a step (c3) of holding the compact at a temperature of 900°C or higher and lower than 1300°C for 0.5 hours or longer in an atmosphere lower than 1 atmosphere;
The method for producing a silicon nitride sintered body according to claim 1 , further comprising: 3. The method for producing a silicon nitride sintered body according to claim 2 , wherein the heating temperature is increased at a rate of 1° C./min or less from the end of step (c3) to the start of step (c1).