Silicon nitride sintered compact

Abstract

To provide a silicon nitride sintered compact having high thermal conductivity.SOLUTION: A silicon nitride sintered compact comprises a plurality of silicon nitride particles, wherein the plurality of silicon nitride particles contain Mg and O. Each of the plurality of silicon nitride particles includes: an inner region; and an outer region located outside the inner region, wherein a concentration of at least one of Mg and O is higher than that in the inner region. A thickness of the outer region is 2.5 nm or less.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] This application relates to a silicon nitride sintered body.

Background Art

[0002] In recent years, high-power electronic circuits such as power modules for driving motors used in electric vehicles, hybrid cars, etc. with high efficiency, and driving circuits for LEDs have been increasingly used in various applications. In such circuits, since electronic components that generate heat due to high power are used, an insulating substrate with high heat dissipation is used. Since it is possible to manufacture a large substrate relatively inexpensively, generally, a ceramic substrate is used for such an insulating substrate. In particular, a silicon nitride sintered substrate has excellent mechanical strength. For example, Patent Document 1 discloses a silicon nitride sintered substrate having excellent thermal conductivity.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] There is a demand for a silicon nitride substrate with higher thermal conductivity. The present invention provides a silicon nitride sintered body having high thermal conductivity.

Means for Solving the Problems

[0005] The silicon nitride sintered body according to an embodiment of the present disclosure is a silicon nitride sintered body including a plurality of silicon nitride particles, wherein the plurality of silicon nitride particles contain Mg and O, and each of the plurality of silicon nitride particles includes an inner region and an outer region located outside the inner region, and at least one of the concentrations of Mg and O is higher than that in the inner region, and the thickness of the outer region is 2.5 nm or less.

Advantages of the Invention

[0006] According to the embodiments of the present disclosure, there are provided a silicon nitride sintered body having both high thermal conductivity and high strength, a circuit board, and a method for manufacturing a silicon nitride sintered body.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Modes for Carrying Out the Invention

[0008] The theoretical thermal conductivity of silicon nitride single crystal is considered to be 200 W / m·K or more, as shown in, for example, Patent Document 1 (the unit of thermal conductivity has W in the numerator and m·K in the denominator). However, it is difficult to achieve the same thermal conductivity in a silicon nitride sintered body. This is because a sintering aid needs to be added to obtain a silicon nitride sintered body, and the elements serving as the sintering aid affect the improvement of the thermal conductivity. The inventors of the present application have carefully studied the raw materials, sintering aids, and sintering conditions, as well as the structure of the silicon nitride sintered body having high thermal conductivity and the element distribution in the sintered body. As a result, they have conceived a silicon nitride sintered body having a thermal conductivity of 130 W / m·K or more and sufficient mechanical strength, and a method for manufacturing the same. Hereinafter, embodiments of the silicon nitride sintered body, circuit board, and method for manufacturing the silicon nitride sintered body of the present disclosure will be described with reference to the drawings.

[0009] (Silicon nitride sintered body) FIG. 1 is a transmission electron microscope (TEM) image showing an example of the silicon nitride sintered body of the present 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.

[0010] The silicon nitride particles 11 are the main phase in the silicon nitride sintered body 10. The silicon nitride particles 11 are single crystals as a whole, and nitrogen atoms and silicon atoms are regularly arranged. The silicon nitride particles 11 contain elements other than nitrogen and silicon, for example, when analyzed by energy-dispersive X-ray spectroscopy (EDS) of a transmission electron microscope. Specifically, the silicon nitride particles 11 contain O and Mg. The 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 or more selected from Y, La, Ce, Nd, Pm, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Hf. It is preferable that RE contains at least Y. Mg and RE are derived from the sintering aid. O is derived from the sintering aid, inevitably contained in the raw material Si powder, or inevitably incorporated during the manufacture of the silicon nitride sintered body 10.

[0011] The silicon nitride particles 11 include an inner region 11A and an outer region 11B. The outer region 11B is located outside the inner region 11A so as to cover the inner region 11A, and is located at the outer edge of the silicon nitride particles 11. As will be described later, the outer region 11B is in contact with the grain boundary phase 12.

[0012] 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 further contain RE at a higher concentration than the inner region 11A. Here, the concentrations of Mg and O can be determined as the boundary between the outer region 11B and the inner region 11A at the position where the X-ray intensities of Mg and O start to increase when scanning the inner region 11A and the outer region 11B perpendicularly to the boundary between the inner region 11A and the outer region 11B by EDS. FIG. 3 is a schematic diagram showing the X-ray intensity distribution from the starting point a to b of the line segment L when the section of the line segment L shown by L in FIG. 2 is analyzed by EDS. As shown in FIG. 3, the intensities of Mg and O are almost constant in the inner region 11A, but as approaching the grain boundary phase 12, the concentration of at least one of Mg and O increases at a certain position, resulting in an increase in the X-ray intensity. The boundary between the outer region 11B and the inner region 11A is determined at the position where the curve of the X-ray intensity rises. The rising position may be determined, for example, by the analysis software provided in a transmission electron microscope capable of analysis by energy dispersive X-ray spectroscopy (EDS).

[0013] 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 more preferable. The lower limit value of the thickness of the outer region 11B is, for example, about 0.5 nm.

[0014] The outer region 11B contains at least one of Mg and O, resulting in a lower thermal conductivity than the inner region 11A. Therefore, it is preferable that the thickness of the outer region 11B is small. Mg and O in the outer region 11B are related to the concentrations of Mg and O in the grain boundary phase 12, as will be described later.

[0015] In the inner region 11A, the content of O is 0.05 mass% or less. Also, in the component analysis by EDS, both Mg and RE are below the detection limit, and the inside of the inner region 11A is in a state close to high purity and single crystal. As will be described later, the content of specific elements in the inner region 11A is shown by the mass ratio to the total detected elements determined by EDS of a transmission electron microscope. The measurement is, for example, to arbitrarily select a plurality of silicon nitride particles 11, and for each of them, obtain the average value of the contents measured at any two locations inside the inner region 11A (the value obtained by dividing the sum of the component amounts at the two locations by 2). The calculated average value is further averaged for a plurality of silicon nitride particles 11.

[0016] Since the inner region 11A contains almost no Mg and RE derived from the sintering aid, it exhibits high thermal conductivity. However, since the inner region 11A may contain a small amount of O and thus may decrease from the ideal thermal conductivity of silicon nitride, it is preferable that the content of O is small.

[0017] The silicon nitride particle 11 preferably contains one or more fine particles 11C inside the inner region 11A. The fine particle 11C contains Mg, O, and RE. Furthermore, it contains Si and N. The fine particle 11C is amorphous and has a particle size of 100 nm or less. Here, the particle size of the fine particle 11C is the maximum length of the particle obtained from the transmission electron microscope image acquisition and the image or observation.

[0018] The fine particle 11C is an amorphous phase composed of Si, N, O, Mg, and RE. Preferably, the silicon nitride particle 11 contains 5 or more fine particles 11C per μm2 in an image or observation image with a direct magnification of 10,000 times or more of the transmission electron microscope image.

[0019] In the inner region 11A of the silicon nitride particles 11, the fine particles 11C contain at least one element among O, Mg, and RE in a larger amount than the inner region 11A located outside the fine particles 11C. That is, by concentrating elements other than Si and N to form the fine particles 11C, the purity of silicon nitride in the portion other than the fine particles 11C in the inner region 11A can be increased, and the thermal conductivity can be increased.

[0020] The grain boundary phase 12 is an amorphous phase formed between a plurality of silicon nitride particles 11. The grain boundary phase 12 includes a two-particle grain boundary 12A and a grain boundary triple point 12B. The two-particle grain boundary 12A is located in a region sandwiched between two adjacent silicon nitride particles 11 and is in contact with the two silicon nitride particles 11. The grain boundary triple point 12B is located between three or more silicon nitride particles among the plurality of silicon nitride particles 11 and is in contact with them. The grain boundary triple point 12B is connected to the two-particle grain boundary 12A.

[0021] The grain boundary phase 12 contains Mg and RE at a higher concentration than the silicon nitride particles 11. The ratio Mg / RE of Mg to RE in the two-particle grain boundary 12A of the grain boundary phase 12 satisfies the following formula (1). For example, quantitative analysis of elements Mg and RE is performed in mass units by EDS analysis, and Mg / RE is calculated in mass%. Mg / RE ≦ 0.7 (1)

[0022] More preferably, the ratio Mg / RE of Mg to RE in the two-particle grain boundary 12A satisfies the following formula (1'). Mg / RE ≦ 0.5 (1')

[0023] Also, the ratio Mg / RE of Mg to RE in the grain boundary triple point 12B satisfies the following formula (2). Mg / RE ≦ 1 (2)

[0024] More preferably, the ratio Mg / RE of Mg to RE in the grain boundary triple point 12B satisfies the following formula (2'). Mg / RE ≦ 0.5 (2')

[0025] Mg / RE is related to the thermal history. During the production of the silicon nitride sintered body 10, Si, N, O, Mg, and RE form 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. Eventually, the remaining liquid phase forms the grain boundary phase 12. Therefore, in the grain boundary phase 12, Mg and RE exist at high concentrations. Also, although the grain boundary phase 12 has a uniform composition as a whole in the liquid state, two-particle grain boundaries 12A are formed first during the cooling process, and then grain boundary triple points 12B are formed. At this time, Mg and RE move from the two-particle grain boundaries 12A to the grain boundary triple points 12B.

[0026] Mg and RE are included in the sintering aid. Since the melting point of Mg is about 650 °C, Mg generates a liquid phase at a relatively low temperature. Since rare earth metals have relatively high melting points, RE generates a liquid phase at a relatively high temperature. Also, Mg vaporizes at about 1400 °C, and RE vaporizes at a higher temperature than that. For example, Y vaporizes at about 1800 °C.

[0027] When manufacturing a silicon nitride sintered body, generally, in terms of oxide conversion, more Mg is added than RE. However, for the reasons described above, Mg vaporizes more than RE. According to the study results of the inventors of the present application, when the two-particle grain boundaries 12A satisfy formula (1) and the grain boundary triple points 12B satisfy formula (2), the silicon nitride sintered body 10 has high thermal conductivity.

[0028] Also, the proportion of Mg in the entire silicon nitride sintered body 10 is preferably 0.2 mass% or less. As described above, during the production of the sintered body, the amount of added Mg decreases due to the vaporization of Mg. However, if the amount of vaporized Mg is small, the above-mentioned ratio Mg / RE in the grain boundary phase will not be small enough. Also, since rare earth metals have an affinity for oxygen, when rare earth metals exist at a high concentration in the grain boundary phase, oxygen easily moves from the silicon nitride particles to the grain boundaries. However, if the total amount of Mg is large, it is considered that Mg inhibits this movement of oxygen, and the reduction of the amount of O in the silicon nitride particles does not proceed sufficiently.

[0029] The two-particle grain boundary 12A has a large area in contact with the silicon nitride particles 11, and it is considered that the Mg concentration in the two-particle grain boundary 12A affects the thickness of the outer region 11B of the silicon nitride particles 11. From this perspective, it is preferable that the Mg content in the two-particle grain boundary 12A is small. Specifically, the Mg content in the two-particle grain boundary 12A is preferably 8 mass% or less. The thickness of the two-particle grain boundary 12A is, for example, 0.6 nm or more and 1.6 nm or less.

[0030] The silicon nitride sintered body 10 has a thermal conductivity of 130 W / m·K or more. The silicon nitride sintered body 10 preferably has a thermal conductivity of 140 W / m·K or more, and more preferably 150 W / m·K or more. Also, the silicon nitride sintered body 10 has a strength of 500 MPa or more. Here, the strength is the value obtained by a three-point bending test according to JIS C6481. Further, the silicon nitride sintered body 10 has a relative density of 98% or more.

[0031] According to the silicon nitride sintered body 10 of the present embodiment, since the O content in the silicon nitride particles is 0.05 mass% or less, the purity of the silicon nitride particles 11 is increased, and the thermal conductivity of the silicon nitride particles 11 is increased. Also, by satisfying the ratio Mg / RE of Mg to RE at the grain boundary triple point with the formula (2) and satisfying the ratio Mg / RE of Mg to RE at the two-particle grain boundary with the formula (1), the Mg concentration in the grain boundary phase can be reduced. Therefore, the thickness of the outer region 11B of the silicon nitride particles 11 can be made small. Thereby, the thermal conductivity of the silicon nitride sintered body 10 can be increased. Specifically, it is possible to realize a silicon nitride sintered body having 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, the thermal conductivity between the silicon nitride particles 11 can be increased. Also, by including fine particles in which the silicon nitride particles 11 contain Mg and RE at a high concentration, the purity of the inner region 11A of the silicon nitride particles 11 can be increased, and higher thermal conductivity can be provided. Specifically, it is possible to realize a silicon nitride sintered body having a thermal conductivity of 150 W / m·K or more.

[0032] (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 main surfaces located on opposite sides of the silicon nitride substrate. The circuit board has, for example, a square or rectangular shape with a side length of 3 cm or more and 10 cm or less. Also, the thickness of the circuit board is, for example, 0.3 mm or more and 3.0 mm or less.

[0033] Since the circuit board of this embodiment includes a silicon nitride substrate made of the silicon nitride sintered body, it has excellent thermal conductivity and high flexural strength. For example, it has a thermal resistance of 0.2 °C / W or less. Also, it has a resistance of 2000 cycles or more in a thermal cycle test.

[0034] (Method for manufacturing silicon nitride sintered body) The method for manufacturing the silicon nitride sintered body of this embodiment will be described. FIG. 4 is a flowchart showing the method for manufacturing the silicon nitride sintered body of this embodiment.

[0035] (1) Mixing step (a) (S1) First, raw material powders are prepared. As a raw material for silicon nitride, Si powder is prepared. Also, as a sintering aid, Mg raw material powder and RE raw material powder are prepared. Let the number of moles of Si3N4 obtained when all the Si powder is nitrided be x, the number of moles of the RE raw material powder when converted to trivalent oxide RE2O3 be y, and the number of moles of the Mg raw material powder when converted to MgO be z. When x + y + z is 100 mol%, the RE raw material powder is prepared and mixed at a ratio of 0.5 mol% or more and less than 2 mol%, and the Mg raw material powder is 8 mol% or more and less than 15 mol%. The ratio of the remaining Si powder is 83 mol% or more and less than 91.5 mol%.

[0036] In this embodiment, Si powder is prepared as a raw material for silicon nitride. By using Si powder and nitriding silicon with N2 gas, it is possible to relatively reduce the amount of oxygen derived from the raw materials. If the proportion of Si powder is less than 83 mol%, the flexural strength and thermal conductivity of the obtained silicon nitride sintered body are too low. On the other hand, when the proportion of Si powder becomes 91.5 mol% or more, the sintering aid is insufficient, and it becomes difficult to obtain a dense silicon nitride sintered body.

[0037] 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.

[0038] When the proportion of the Mg raw material powder is less than 8 mol%, it becomes difficult to obtain a dense silicon nitride sintered body. Further, when the proportion of the Mg raw material powder is 15 mol% or more, the grain boundary phase increases, and the ratio of Mg / RE in the grain boundary phase becomes large, and the thermal conductivity of the silicon nitride sintered body tends to be low. Also, when the proportion of MgSiN2 in the Mg raw material powder is less than 87 mass%, as described above, the amount of O in the silicon nitride sintered body increases, and it becomes difficult to achieve high thermal conductivity.

[0039] The RE raw material powder is a raw material powder containing at least one element selected from rare earth elements as described above. More specifically, RE is at least one selected from Y, La, Ce, Nd, Pm, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Hf. From the viewpoint of densification of the silicon nitride sintered body, it is preferable that RE contains Y. RE may be in the form of an oxide or in the form of a compound other than oxygen. For example, the RE raw material powder may be a powder such as Y2O3, YN, etc.

[0040] When the proportion of the RE raw material powder is less than 0.5 mol%, it becomes difficult to obtain a dense silicon nitride sintered body. Also, when the proportion of the 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 be low.

[0041] Weigh the Si powder, Mg raw material powder, and RE raw material powder so as to obtain the above-mentioned proportion of the raw material powder, and mix a plasticizer (for example, a phthalic acid-based plasticizer), an organic binder (for example, polyvinyl butyral, isobutyl polyacrylate), and an organic solvent (for example, ethyl alcohol, butyl alcohol) with a ball mill or the like to prepare a slurry containing the raw materials. The solid content concentration of the slurry is preferably 30% by mass or more and 70% by mass or less.

[0042] (2) Forming step (b) (S2) After defoaming and thickening the slurry, a sheet-shaped formed body (green sheet) is formed by, for example, the doctor blade method. The thickness of the formed body is appropriately set in consideration of the thickness of the silicon nitride sintered body to be formed and the sintering shrinkage rate. Since the formed body formed by the doctor blade method is usually in the form of a long strip, it is punched or cut into a predetermined shape and size.

[0043] After the forming step, in order to remove the organic binder and the plasticizer, the formed body may be heated to 400 to 800 °C for debinding.

[0044] (3) Sintering step (c) (S3) The green body is introduced into a heating furnace and heated in a nitrogen atmosphere to sinter the raw material, thereby obtaining a silicon nitride sintered body. Fig. 5 shows a schematic example of the temperature profile T of the heating furnace and the pressure profile P of the nitrogen gas introduced into the heating furnace in the sintering process. As will be described below, since there is a range (width) in the preferable holding time for each process, the time axis in Fig. 5 is not necessarily equally spaced. For this reason, the slopes of the straight lines in the temperature profile T (solid line) and the pressure profile P (dashed line) do not necessarily correctly reflect the temperature rise / fall rate and the pressure rise / fall rate. Also, two straight lines with the same slope in the temperature profile T and the pressure profile P are not necessarily at the same temperature rise rate or the same pressure rise rate.

[0045] The sintering process (c) includes a heating period D1, a sintering period D2, and a cooling period D3. The heating period D1 is the period from the start of heating until the maximum temperature in the temperature profile T is reached.

[0046] In the process of this heating period D1, a step of nitriding the raw material Si powder with N2 gas to change it into Si3N4 particles by the reaction formula shown in formula (3) is included. 3Si + 2N2 → Si3N4 (3)

[0047] Also, the sintering period D2 is the period of maintaining the maximum temperature. The cooling period D3 is the period of lowering the temperature of the heating furnace to room temperature after the end of the sintering period D2. Here, the maximum temperature means a temperature of 1800 °C or higher.

[0048] The temperature inside the heating furnace in the sintering process can be, for example, the temperature measured by a radiation thermometer for a target such as the green body inside the furnace through a viewing window provided in the heating furnace. Specifically, inside the heating furnace, a sintering container in which the green body is placed, a carbon cylindrical wall that arranges the sintering container on the inner peripheral side, and a target located near the outer peripheral side of the cylindrical wall can be provided.

[0049] (3-1) Heating period D1 The temperature increase period D1 includes a decarburization 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.

[0050] (i) Decarburization period d1 First, the ambient temperature in the heating furnace is raised from room temperature to the temperature range of the decarburization period d1. The heating rate is, for example, 60 °C / hr. When the ambient temperature in the heating furnace reaches a temperature of 900 °C or higher and 1300 °C or lower, it is held at a temperature within this range for 0.5 hours or more and 2 hours or less. The atmosphere in the heating furnace is preferably under reduced pressure, that is, lower than 1 atmosphere. For example, a pressure of 80 Pa or lower is preferable. If carbon remains during sintering, voids are likely to form in the silicon nitride sintered body. Therefore, by holding the compact under reduced pressure, the carbon in the compact is removed.

[0051] If the ambient temperature is lower than 900 °C, carbon may not be sufficiently removed. Also, if it is higher than 1300 °C, the sintering aid may also be removed. The ambient temperature in the heating furnace is more preferably a temperature of 1000 °C or higher and 1250 °C or lower.

[0052] (ii) Nitriding process d2 After the end of the decarburization period, the compact is slowly heated from the temperature at the end of the decarburization period to the temperature at the start of the first temperature holding period d3, which is the nitriding period. Specifically, the heating temperature is raised at a rate of 1 °C / min or a rate of 1 °C / min or less to raise the temperature of the compact.

[0053] The temperature increase rate of the nitriding process d2 is more preferably 0.5 °C / min or less. Also, in order to manufacture the silicon nitride sintered body under practical conditions, the temperature increase rate is preferably 0.1 °C / min or more.

[0054] In the nitriding process d2 and subsequent periods, it is preferable to fill the heating furnace with a nitrogen atmosphere. Specifically, nitrogen or a mixed gas mainly composed of nitrogen and containing an inert gas such as argon, or a mixed gas containing hydrogen at about 3% or less in nitrogen gas can be used. The pressure in the heating furnace is preferably about 1 atm or more and 20 atm or less. In FIG. 5, the pressure of the nitrogen atmosphere in the nitriding process d2 and subsequent periods is shown as constant, but it may vary within the above-described pressure range.

[0055] (iii) The first temperature holding period d3 and the second temperature holding period d4 As the first temperature holding period d3, the temperature of the atmosphere in the heating furnace is held at a temperature T1 of 1300°C or more and 1800°C or less. The holding time is preferably 0.5 hours or more and 10 hours or less.

[0056] Thereafter, as the second temperature holding period d4, the temperature of the atmosphere in the heating furnace is held at a temperature T2 higher than the temperature T1 and 1300°C or more and 1800°C or less. The holding time is preferably 0.5 hours or more and 10 hours or less.

[0057] Although not shown, thereafter, as the third temperature holding period d5, the temperature of the atmosphere in the heating furnace may be held at a temperature T3 higher than the temperature T2 and 1300°C or more and 1800°C or less. The holding time is preferably 0.5 hours or more and 10 hours or less.

[0058] For example, the temperature T1 is 1300°C or more and 1500°C or less, and the temperature T2 is 1500°C or more and 1700°C or less. As an example, the temperature T1 is 1400°C, and the temperature T2 is 1600°C. The temperature T3 is 1800°C.

[0059] By these steps, the Si powder is nitrided to produce silicon nitride. By performing the nitriding of the Si powder at two temperatures, even when the size of the molded body is large, deformation such as undulation of the sheet-like molded body can be suppressed.

[0060] (3-2) Sintering period D2 After the nitriding period ends, 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. When 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. If the temperature is 1800 °C or higher and 2000 °C or lower, the ambient temperature does not have to be constant and may change (for example, gradually increase in temperature).

[0061] The holding time is 1 hour or more and 30 hours or less. The holding time is preferably 2 hours or more and 20 hours or less, and more preferably 5 hours or more and 15 hours or less.

[0062] By holding the compact at the above-described temperature, the grain growth of silicon nitride crystals is promoted, and Mg, RE, and O are excluded from the silicon nitride crystals and move to the grain boundary phase, thereby increasing the purity of the silicon nitride crystals. In addition, a part of Mg, RE, and O in the silicon nitride crystals generates amorphous fine particles inside, thereby increasing the purity of the silicon nitride crystals.

[0063] (3-3) Cooling period D3 After the sintering period D2, the ambient temperature in the heating furnace is gradually decreased until it reaches about room temperature. In the cooling period D3, the liquid phase between the generated silicon nitride particles is cooled and solidified as a grain boundary phase, fixing the position of the main phase of the obtained silicon nitride particles. In order to rapidly solidify the liquid phase and maintain the uniformity of the grain boundary phase distribution, the cooling rate is preferably 100 °C / hr or more, more preferably 300 °C / hr or more, and most preferably 500 °C / hr or more. Practically, the cooling rate is preferably 500 °C / hr or more and 600 °C / hr or less. By cooling at such a cooling rate, crystallization of the sintering aid to be solidified is suppressed, a grain boundary phase mainly composed of a glass phase is formed, and the bending strength of the silicon nitride sintered body can be increased. When the ambient temperature is decreased to 1200 °C, the grain boundary phase is determined and the entire silicon nitride sintered body becomes solid. Therefore, the cooling rate at a temperature lower than that is not particularly limited.

[0064] According to the method for manufacturing a silicon nitride sintered body of the present embodiment, by using Si powder as the Si source and Mg raw material powder containing 87 mass% or more of MgSiN2, the amount of O contained during manufacturing can be reduced, and the amount of O in the silicon nitride sintered body can be further reduced. Further, it is considered to contribute to the precipitation of amorphous fine particles in which impurities are concentrated in the silicon nitride particles by holding the compact at two-stage temperatures during nitridation of the Si powder.

[0065] (Example) Hereinafter, the results of manufacturing silicon nitride sintered bodies under various conditions and examining their characteristics will be described.

[0066] 1. Manufacture of Silicon Nitride Sintered Body [Samples 1 to 15] Si powder, Y2O3 powder (Y2O3 in Table 1) as the RE raw material powder, and Mg raw material powder containing MgSiN2 and MgO (Mg compound in Table 1) were prepared. Let x be the number of moles of Si3N4 obtained when all the Si powder is nitrided, y be the number of moles of Y2O3 powder, and z be the number of moles when the Mg raw material powder is converted to MgO. The Y2O3 powder and the Mg raw material powder were weighed at the ratios shown in Table 1, and Si powder in a proportion corresponding to the number of moles in terms of Si3N4 obtained from x = 100 - y - z was weighed. The proportion of MgSiN2 in the Mg raw material powder is as shown in Table 1.

[0067] These powders were mixed to form a slurry, and a sheet-shaped compact (green sheet) was created by the doctor blade method.

[0068] The compact was placed in a heating furnace, and ceramic sintered compacts of Samples 1 to 15 were obtained by sintering the compact under the conditions shown in Table 2.

[0069]

Table 1

[0070]

Table 2

[0071] 2. Measurement of properties (1) The relative density, thermal conductivity, and three-point bending strength of the prepared Samples 1 to 15 were measured. The apparatuses and measurement conditions used for the measurement are as follows. 1) Relative density The value expressed as a percentage by dividing the density of the substrate material obtained by the Archimedes method by 3.24 g / cm3. The substrate size was: 30 mm × 30 mm 2) Thermal conductivity For substrates processed to 5 mm × 5 mm, measurement was performed by the laser flash method. Apparatus: Thermal diffusivity measurement apparatus LFA 467 Hyper Flash manufactured by Netzsch The thermal conductivity was calculated as specific heat × density × thermal diffusivity. The specific heat was assumed to be 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 Support distance: 20 mm Substrate size: 30 mm × 25 mm × 0.32 mm t Crosshead speed: 0.5 mm / min Apparatus: Autograph AGS-100NX manufactured by Shimadzu Corporation

[0072] (2) Samples 1 to 15 prepared were observed using TEM to examine the presence or absence of fine particles inside the silicon nitride particles. The amount of oxygen at the center inside the silicon nitride particles was measured by EDS. Also, the ratio of Mg to Y at the grain boundaries and triple grain boundaries of two particles was examined. Further, by line analysis of EDS, the concentration profiles of Mg and O at the silicon nitride particles and the adjacent two-particle grain boundaries were obtained, and the depths of Mg and O, which are the thicknesses of the outer region (i.e., the length from the interface between the silicon nitride particles and the grain boundary phase into the silicon nitride particles), were measured. The apparatus and measurement conditions used for the measurement are as follows. 1) Sample preparation Thinned by ion milling method Apparatus: PIPS (Precision Ion Milling System): Model 691 manufactured by Gatan 2) Observation Apparatus: Transmission electron microscope (TEM): JEM-ARM200F manufactured by JEOL Ltd. 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: 256 Pix, 1.0 msec / Pix STEM: scanning transmission electron microscope HAADF image: The HAADF-STEM (High-Angle Annular Dark Field Scanning TEM) image is obtained by scanning a finely focused electron beam over the sample and detecting, with an annular detector, the electrons that are scattered at high angles among the transmitted electrons. Heavy elements appear dark in the STEM image and bright in the HAADF-STEM image. Since a contrast proportional to the atomic weight (Z) is obtained, it is also called a Z-contrast image. 3) Elemental analysis The oxygen content of the silicon nitride particles was measured by pulverizing the sintered body according to the following procedure to remove the grain boundary phase of the sintered body and then measuring the oxygen content of the silicon nitride particles, which are the main phase. (a) Pulverization of the sintered body The sintered body was pulverized into a powder of 75 μm or less 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 it was washed with distilled water. For the water washing, the supernatant was removed, and the shaking wash with distilled water and centrifugation were repeated. The completion of the water washing was confirmed by a pH test paper. Further, ethanol was added, and after performing the shaking wash and centrifugation of the sample after the water washing with ethanol (99.5%), the silicon nitride particles were taken out by vacuum heating and drying. (c) Measurement For the quantification by oxygen analysis and peak separation, oxygen measurement was carried out using a TCH-600 manufactured by LECO under the temperature rising condition of 8 °C / s and a sample amount of about 20 mg. The oxygen peaks appear on the low-temperature side and the high-temperature side. Since the low-temperature side is the oxygen on the particle surface and the high-temperature side indicates the oxygen inside the grains, only the peak on the high-temperature side was approximated to a normal distribution, and the oxygen content was determined from its area.

[0073] Sintered bodies prepared under the same conditions were prepared, and the component amounts of Y and Mg, which are RE in the whole sintered body, were determined by pulverizing the sintered body as described above and quantitatively analyzing the amounts of Y and Mg by ICP analysis.

[0074] 3. Results and discussion (1) Ratio of the sintering aid As shown in Sample 5 of Table 1, when the proportion of Y is less than 0.5 mol%, the relative density of the obtained sintered body is less than 98%. This is presumably because the sintering aid is insufficient, and uniform growth of silicon nitride particles is not sufficiently carried out, resulting in an inability to obtain a dense sintered body. Also, as shown in Sample 6, when the proportion of Y is 2 mol% or more, the thermal conductivity is 130 W / m·K or less. This is presumably because the sintering aid with a high boiling point is excessive, so a large amount of Y remains in the grain boundary phase in the obtained sintered body, the grain boundary phase becomes thick, or the concentration of Y in the grain boundary phase increases, thereby reducing the thermal conductivity of the grain boundary phase.

[0075] Similarly, as shown in Sample 7, when the proportion of Mg is less than 8 mol%, the relative density of the obtained sintered body is less than 98%. It is presumably because a dense sintered body could not be obtained due to insufficient sintering aid. Also, as shown in Sample 8, when the proportion of Mg is 15 mol% or more, the thermal conductivity is 130 W / m·K or less. This is presumably because a large amount of Mg remains in the grain boundary phase in the obtained sintered body, and the concentration of Mg in the grain boundary phase increases, thereby reducing the thermal conductivity of the grain boundary phase. Also, as the proportion of Mg increases, the amount of MgO, which is a raw material powder other than MgSiN2 contained in the Mg raw material powder, also increases. For this reason, the amount of O remaining in the sintered body also increases, presumably reducing the thermal conductivity.

[0076] As shown in Sample 9, even when the proportion of the Mg raw material powder is less than 15 mol%, if the proportion of MgSiN2 is less than 87% by mass, the thermal conductivity is 130 W / m·K or less. The amount of MgO, which is a raw material powder other than MgSiN2 contained in the Mg raw material powder, also increases, the amount of O remaining in the sintered body increases, presumably reducing the thermal conductivity.

[0077] (2) Oxygen content in silicon nitride particles As shown in Samples 1 to 4 and 10 to 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, in Samples 13 to 15, since the oxygen content in the silicon nitride particles is as low as 0.02% by mass, a thermal conductivity of 150 W / m·K or more is obtained. On the other hand, 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 is below 130 W / m·K. Thus, the oxygen content of the silicon nitride particles is related to the thermal conductivity, and the tendency is that the lower the oxygen content, the higher the conductivity. In Samples 6 and 7, although the oxygen content in the silicon nitride particles is 0.05% by mass or less, the thermal conductivity is less than 130 W / m·K. This is presumably because, as described above, the ratio of Y is outside the preferred range, and the thermal conductivity decreases for the reasons described above.

[0078] (3) Mg content in the sintered body As shown in Samples 1 to 4 and 10 to 15, when the Mg content in the entire sintered body is 0.20% by mass or less, a thermal conductivity of 130 W / m·K or more is obtained. On the other hand, as shown in Samples 5 to 9, when the Mg content in the entire sintered body exceeds 0.20% by mass, the thermal conductivity is below 130 W / m·K. This is presumably because the component amount of Mg in the sintered body is not low enough, and Mg inhibits the function of Y attracting oxygen in the grain boundary phase.

[0079] (4) Mg / Y ratio of the grain boundary phase As shown in Sample 8, when the Mg / Y ratio at the grain boundary triple point is greater than 1, the thermal conductivity is less than 100 W / m·K. This is presumably because the excessive presence 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 the grain boundary phase hinders the heat conduction between adjacent silicon nitride particles.

[0080] Figure 6 shows the composition ratios of the constituent elements at the grain boundary triple points and two-particle grain boundaries of samples 8, 12, and 13. When the liquid phase containing a large amount of Mg and Y is cooled during the production of the silicon nitride sintered body, it is considered that two-particle grain boundaries are first formed, and then grain boundary triple points are formed. At this time, since Mg, Y, and O are considered to move from the two-particle grain boundaries to the grain boundary triple points, the amounts of Mg, Y, and O at the grain boundary triple points are larger than those at the two-particle grain boundaries. Also, in sample 8, since the amount of Mg raw material powder added is large, a large amount of Mg is contained in the grain boundary triple points.

[0081] (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 particles exceeds 2.5 nm, the thermal conductivity is less than 100 W / m·K. This is considered to be because the inclusion of Mg thickens the outer region with low thermal conductivity, making it difficult for the heat conducted from adjacent silicon nitride particles to be conducted to the inner region by the outer region.

[0082] Figure 7(a) is a high-angle annular dark-field scanning (HAADF) image of the silicon nitride particles of sample 8, and Figure 7(b) shows the concentration profiles of the constituent elements by EDS at the arrow in the image shown in (a). Figures 8(a), (b) and Figures 9(a), (b) show the HAADF images and the concentration profiles of the constituent elements of sample 12 and sample 13, respectively.

[0083] From these figures, it can be seen that within the inner region, the concentrations of Mg and O are almost constant, but at the boundary with the outer region, the curves of these concentration profiles rise, indicating that the concentrations of Mg and O are high. Also, when the concentrations of Mg and O at the two-particle grain boundaries increase, the concentrations of Mg and O in the outer region of the silicon nitride particles also increase. Therefore, it is considered that the position of the boundary between the outer region and the inner region shifts and the thickness of the outer region increases.

[0084] (6) Fine particles in silicon nitride particles In Samples 1 to 12, no fine particles were observed in the silicon nitride particles, but in Samples 13 to 15, 5 or more fine particles (5 or more fine particles) were observed. In Samples 13 to 15, since the thermal conductivity is 150 W / m·K or more, it is considered that due to the formation of fine particles containing Mg, Y, and O, these elements in the silicon nitride particles gather in the fine particles, increasing the purity of silicon nitride in the region other than the fine particles and also increasing the thermal conductivity.

[0085] From the above embodiments, by using the raw materials prepared so as to contain the RE raw material powder at 0.5 mol% or more and less than 2 mol%, and the Mg raw material powder at 8 mol% or more and less than 15 mol%, and the Si powder at 83 mol% or more and less than 91.5 mol% in terms of Si3N4 conversion, and the total being 100 mol%, it can be seen that it is possible to produce a silicon nitride sintered body having a thermal conductivity of 130 W / m·K or more. Also, when the oxygen content in the silicon nitride particles is 0.05 mass% or less, the ratio of Mg to RE, Mg / RE, at the grain boundary triple point 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 particles is 2.5 nm or less, it can be seen that the thermal conductivity of the silicon nitride sintered body becomes 130 W / m·K or more. Furthermore, when the oxygen content in the silicon nitride particles is 0.02 mass% or less, the thickness of the outer region of the silicon nitride particles (corresponding to "depth of Mg and O" in Table 1, for example) is 1.6 nm or less, and at least one of the silicon nitride particles contains fine particles having a particle size of 100 nm or less containing Mg, O, and RE, it can be seen that a thermal conductivity of 150 W / m·K or more can be achieved.

[0086] (7) Temperature profile of the sintering process For Samples 1, 10 to 12, the rate of temperature rise during the slow heating period is less than 1 °C / min, but due to this, a thermal conductivity of 132 W / m·K or more is obtained. This is presumably because, due to the small rate of temperature rise, the nitridation reaction of silicon proceeds more slowly during the slow heating period, and silicon nitride is obtained before the sintering period. Also, for Samples 13 to 15, a thermal conductivity of 150 W / m·K or more is obtained. This is presumably because by sintering at a higher sintering temperature, it becomes possible to lower the amount of oxygen in the silicon nitride particles or reduce the amount of Mg in the entire sintered body, and it becomes possible to reduce the amount of oxygen in the silicon nitride particles or reduce the amount of Mg at the grain boundaries.

Industrial Applicability

[0087] The silicon nitride sintered body, circuit board, and method for manufacturing a silicon nitride sintered body of the present disclosure are potentially applicable to various uses, and in particular, are suitably used for circuit boards for various uses having high thermal conductivity and high strength.

Explanation of Signs

[0088] 10 Silicon nitride sintered body 11 Silicon nitride particles 11A Inner region 11B Outer region 11C Fine particles 12 Grain boundary phase 12A Two-particle grain boundary 12B Grain boundary triple point

Claims

1. A silicon nitride sintered body comprising a plurality of silicon nitride particles, the plurality of silicon nitride particles comprises Mg and O; Each of the plurality of silicon nitride particles includes an inner region and an outer region located outside the inner region and having a higher concentration of at least one of Mg and O than the inner region; The thickness of the outer region is 2.5 nm or less. Sintered silicon nitride.

2. In the inner region, the content of O is 0.05 mass% or less. The silicon nitride sintered body according to claim 1.

3. In the inner region, the content of Mg is below the detection limit of component analysis by EDS.

3. The silicon nitride sintered body according to claim 1 or 2.

4. At least one of the plurality of silicon nitride particles includes fine particles containing Mg, O and RE and having a particle size of 100 nm or less; The silicon nitride sintered body according to any one of claims 1 to 3.

5. The microparticles are amorphous. The silicon nitride sintered body according to claim 4.

6. The fine particles are present at a concentration of 5 particles / μm2 or more in an image directly observed at a magnification of 10,000 times or more using a transmission electron microscope.

6. The silicon nitride sintered body according to claim 4 or 5.

Images