Silicon nitride sintered body, substrate for semiconductor device, and member for molten metal

Abstract

Provided is a silicon nitride sintered body which can be used for both a substrate for a semiconductor device and a member for molten metal. A silicon nitride sintered body according to an embodiment contains a plurality of silicon nitride crystal particles and a grain boundary phase. The plurality of silicon nitride crystal particles include solid solution crystal particles obtained by dissolving into solid at least one first element selected from the group consisting of tungsten and molybdenum. In the solid solution crystal particles, the ratio of the number of atoms of the first element to the number of silicon atoms is 0.0001 to 0.01. In the solid solution crystal particles, the ratio of the number of oxygen atoms to the number of silicon atoms is 0.04 or less. The silicon nitride sintered body has a thermal conductivity of 50 W / (m·K) or more.

Description

Silicon nitride sintered body, substrate for semiconductor device, and component for molten metal.

[0001] The embodiments described later generally relate to silicon nitride sintered bodies, substrates for semiconductor devices, and components for molten metal.

[0002] Silicon nitride sintered bodies are used as substrates and wear-resistant components for semiconductor devices. In recent years, silicon nitride sintered bodies with high thermal conductivity of 50 W / (m·K) or higher have been developed.

[0003] For example, Japanese Patent Publication No. 6293772 (Patent Document 1) discloses a silicon nitride substrate with good insulating properties achieved by controlling the grain boundary phase distribution in the thickness direction.

[0004] High thermal conductivity silicon nitride sintered bodies are also used in components for molten metal. For example, Japanese Patent Publication No. 2005-59084 (Patent Document 2) discloses a mold using a silicon nitride sintered body. Also, Japanese Patent Publication No. 2018-98164 (Patent Document 3) discloses a heater protection tube using a silicon nitride sintered body. Since components for molten metal are used in contact with molten metal such as aluminum, they require not only strength but also heat dissipation.

[0005] Japanese Patent Publication No. 6293772, Japanese Unexamined Patent Publication No. 2005-59084, Japanese Unexamined Patent Publication No. 2018-98164

[0006] For example, molten aluminum is at a high temperature of approximately 700°C. The silicon nitride sintered body described in Patent Document 1 did not necessarily have sufficient durability when exposed to high-temperature molten metal. Furthermore, the silicon nitride sintered bodies described in Patent Documents 2 and 3 did not necessarily have sufficient insulating properties as substrates for semiconductor devices. For this reason, conventional silicon nitride sintered bodies could not be used for both semiconductor device substrates and molten metal components. In other words, when the applications are different, individual material designs for silicon nitride sintered bodies are required according to the application. As a result, there was a problem of reduced productivity.

[0007] Embodiments of the present invention are intended to address these problems and aim to provide a silicon nitride sintered body that can be used as both a substrate for semiconductor devices and a component for molten metal.

[0008] The silicon nitride sintered body according to the embodiment includes a plurality of silicon nitride crystal particles and a grain boundary phase. The plurality of silicon nitride crystal particles include solid solution crystal particles in which at least one first element selected from the group consisting of tungsten and molybdenum is solid-dissolved. In the solid solution crystal particles, the ratio of the number of atoms of the first element to the number of atoms of silicon is 0.0001 or more and 0.01 or less. In the solid solution crystal particles, the ratio of the number of atoms of oxygen to the number of atoms of silicon is 0.04 or less. The thermal conductivity of the silicon nitride sintered body is 50 W / (m·K) or more.

[0009] A figure showing an example of the cross-sectional structure of a silicon nitride sintered body according to the embodiment. A perspective view showing an example of a substrate for a semiconductor device. A schematic diagram showing a metal molten metal member according to the embodiment.

[0010] The silicon nitride sintered body according to the embodiment includes a plurality of silicon nitride crystal particles and a grain boundary phase. The plurality of silicon nitride crystal particles include solid solution crystal particles in which at least one first element selected from the group consisting of tungsten and molybdenum is solid-dissolved. In the solid solution crystal particles, the ratio of the number of atoms of the first element to the number of atoms of silicon is 0.0001 or more and 0.01 or less. In the solid solution crystal particles, the ratio of the number of atoms of oxygen to the number of atoms of silicon is 0.04 or less. The thermal conductivity of the silicon nitride sintered body is 50 W / (m·K) or more.

[0011] Figure 1 shows an example of the cross-sectional structure of a silicon nitride sintered body according to an embodiment. In Figure 1, reference numeral 1 denotes the cross-section, reference numeral 2 denotes silicon nitride crystal particles, reference numeral 3 denotes the grain boundary phase, and reference numeral 4 denotes metal compound particles. The cross-sectional structure of a silicon nitride sintered body is sometimes simply called the cross-section. Cross-section 1 contains the grain boundary phase 3 in the gaps between the silicon nitride crystal particles 2. The grain boundary phase 3 may contain metal compound particles 4.

[0012] Multiple silicon nitride crystal particles 2 include solid-solution crystal particles 2a in which a first element is solid-solutioned. The first element is at least one selected from the group consisting of tungsten and molybdenum. Transmission electron microscopy-energy-dispersive X-ray spectroscopy (TEM-EDS) is used to measure the solid-solution element. The TEM-EDS conditions are set to an acceleration voltage of 200 kV, an irradiation current of 1.00 nA, a spot diameter of 1 nm during analysis, an analysis time of 30 seconds, and a sample angle of X = 10°, Y = 0°.

[0013] A portion of the silicon nitride crystal particles 2 is measured in any cross-section 1 of the silicon nitride sintered body. An analysis spot S is set near the center of the silicon nitride crystal particles 2. This analysis determines whether one or two types of tungsten or molybdenum are dissolved in the silicon nitride. The sample used for TEM analysis is thin, about 10 to 100 nm. Also, the cross-section 1 shows the cut surface of the silicon nitride crystal particles 2. Therefore, if tungsten or molybdenum is detected at the analysis spot near the center of the silicon nitride crystal particles 2, it can be determined that the element is dissolved in the silicon nitride crystal. If neither W nor Mo is detected in the measurement of one silicon nitride crystal particle, the measurement position may be changed within the same silicon nitride crystal particle and the measurement may be repeated. If neither W nor Mo is detected in one silicon nitride crystal particle even after changing the measurement position three times, it is determined that the first element (W or Mo) is not dissolved in the silicon nitride. For measuring solid solution elements, a field emission scanning electron microscope (FESEM)-EDS may be used instead of TEM-EDS.

[0014] If a first element (W or Mo) is detected at the analysis spot S, the number of atoms of the first element and the number of atoms of silicon are obtained from the measurement results. At this time, the number of atoms of other elements, such as oxygen and aluminum, which will be described later, may also be obtained. If the ratio of the number of atoms of the first element to the number of atoms of silicon is 0.0001 or more and 0.01 or less, the silicon nitride crystal particle 2 in which the analysis spot S is set is determined to be a solid solution crystal particle 2a.

[0015] In silicon nitride crystal particles 2, the heat resistance can be improved by setting the ratio of the number of atoms of the first element to the number of atoms of Si within the range of 0.0001 to 0.01. This is thought to be because the difference in thermal expansion characteristics between silicon nitride crystal particles 2 and grain boundary phase 3 becomes smaller. As a general value for materials, the linear expansion coefficient of silicon nitride is approximately 2.8 × 10⁻⁶. －６ It is / K. The coefficient of linear expansion of yttrium oxide is approximately 7.4 × 10⁻⁶. －６ It is / K. The coefficient of thermal expansion of tungsten is approximately 4.5 × 10⁻⁶. －６ It is / K. The coefficient of linear expansion of molybdenum is approximately 4.8 × 10⁻⁶. －６ It is / K. Yttrium oxide is a component used as a sintering aid. Sintering aids are components that make up the grain boundary phase.

[0016] Here, the ratio of the number of atoms of the first element to the number of atoms of silicon in the silicon nitride crystal particles 2 is also called the "atomic ratio (1)". If the atomic ratio (1) is less than 0.0001, the effect of improving heat resistance is insufficient. If the atomic ratio (1) exceeds 0.01, the amount of solid solution of the first element is too large, which reduces the insulating properties of the silicon nitride crystal particles 2. For this reason, the atomic ratio (1) is preferably in the range of 0.0001 or more and 0.01 or less, and more preferably in the range of 0.0005 or more and 0.002 or less.

[0017] Furthermore, in the solid solution crystal particles 2a, the ratio of the number of oxygen (O) atoms to the number of Si atoms is 0.04 or less. Here, the ratio of the number of O atoms to the number of Si atoms in the solid solution crystal particles 2a is also called the "atomic ratio (2)". By controlling the amount of oxygen in the solid solution crystal particles 2a, the thermal conductivity of the silicon nitride crystal particles 2 can be improved. As a result, the thermal conductivity of the silicon nitride sintered body can be improved.

[0018] The atomic ratio (2) is preferably 0.04 or less, and more preferably 0.02 or less. The lower limit of the atomic ratio (2) is not particularly limited, but is preferably 0.001 or more. The atomic ratio (2) may be less than 0.001, but this may lead to increased costs.

[0019] The combination of atomic ratios (1) and (2) makes it possible to improve heat resistance through solid solution of W or Mo while also improving thermal conductivity by reducing the amount of oxygen. This expands the applications of silicon nitride sintered bodies, for example, to molten metals.

[0020] Furthermore, even in silicon nitride crystal particles other than solid solution crystal particles, the ratio of the number of O atoms to the number of Si atoms is preferably 0.04 or less.

[0021] The thermal conductivity of the silicon nitride sintered body according to this embodiment is 50 W / (m·K) or higher. By controlling the atomic ratios (1) and (2), the thermal conductivity can be made 50 W / (m·K) or higher, and it is also possible to make it 80 W / (m·K) or higher. The thermal conductivity is measured by the flash method.

[0022] In solid solution crystalline particles, the ratio of the number of aluminum atoms to the number of Si atoms is preferably less than 0.001. This ratio of the number of aluminum atoms to the number of Si atoms in solid solution crystalline particles is also called the "atomic ratio (3)".

[0023] Aluminum is an element that readily combines with oxygen. Reducing the amount of Al in solid solution crystal particles can improve thermal conductivity. Further improvements can be achieved by setting the atomic ratio (3) to less than 0.001. The atomic ratio (3) may also be 0 (including values ​​below the detection limit).

[0024] The number of oxygen atoms and algae atoms is measured by setting an analysis spot S near the center of the silicon nitride crystal grain 2 and analyzing it with TEM-EDS. The number of W, Mo, O, Al, and Si atoms may be measured simultaneously at a single analysis spot S.

[0025] In any cross-section of the silicon nitride sintered body, the ratio of the number of solid solution crystal particles 2a to the number of silicon nitride crystal particles 2 in a measurement area of ​​20 μm × 20 μm is preferably within the range of 30% to 100%.

[0026] To determine the number of silicon nitride crystal particles, count the silicon nitride crystal particles with a length of 0.5 μm or more observed in a cross-sectional photograph of a measurement area of ​​20 μm × 20 μm. Select any 10 or more silicon nitride crystal particles 2 with a length of 0.5 μm or more. Hereafter, the silicon nitride crystal particles selected by this process will be referred to as "selected silicon nitride crystal particles". Measure the amount of W or Mo dissolved in each silicon nitride crystal particle 2 and determine if the atomic ratio (1) is between 0.0001 and 0.01. Count the number of silicon nitride crystal particles 2 determined to be solid-solution crystal particles 2a. The ratio is calculated as [number of solid-solution crystal particles / number of selected silicon nitride crystal particles] × 100 (%). If 10 silicon nitride crystal particles with a length of 0.5 μm or more are not observed in the measurement area of ​​20 μm × 20 μm, measure another 20 μm × 20 μm area.

[0027] The major axis of the solid solution crystal particles 2a is preferably 0.5 μm or larger. By solid-dissolving W or Mo into silicon nitride crystal particles 2 with a major axis of 0.5 μm or larger, the effect of reducing the difference in thermal expansion with the grain boundary phase 3 can be obtained. However, solid solution crystal particles with a major axis of less than 0.5 μm may also be present.

[0028] Here, the ratio of the number of solid solution crystal particles 2a to the number of silicon nitride crystal particles 2 is also simply called the "number ratio." Increasing the number ratio makes it easier to reduce the difference in thermal expansion between the silicon nitride crystal particles 2 and the grain boundary phase 3. For this reason, the number ratio is preferably in the range of 30% to 100%, and more preferably in the range of 60% to 100%. The number ratio is most preferably 100%.

[0029] The average length of the major axis of the silicon nitride crystal particles 2 is preferably 0.5 μm or more and 10 μm or less. The average aspect ratio of the silicon nitride crystal particles 2 is preferably within the range of 2 or more and 10 or less.

[0030] Cross-sectional SEM images are used to measure the average length of the major axis and the average aspect ratio of silicon nitride crystal particles 2. An SEM image with a measurement area of ​​100 μm × 100 μm is used. First, the major axis and minor axis of each silicon nitride crystal particle 2 in the SEM image are measured. The major axis is represented by the length of the line segment connecting the two furthest points on the outer edge of a single silicon nitride crystal particle 2. The minor axis is the length of the silicon nitride crystal particle 2 in a direction that passes through the midpoint of that line segment and is perpendicular to the line segment. The average value of the major axis of each silicon nitride crystal particle 2 is defined as the "average length of the major axis". In addition, the ratio of the major axis to the minor axis of each silicon nitride crystal particle 2 is calculated as the "aspect ratio". The average value of the aspect ratio of each silicon nitride crystal particle 2 is defined as the "average aspect ratio". The average length of the major axis and the average aspect ratio are calculated based on all silicon nitride crystal particles 2 that are not cut off within the measurement area of ​​100 μm × 100 μm, regardless of whether they are solid solution crystal particles or not.

[0031] By controlling the average length of the major axis and the average aspect ratio of the silicon nitride crystal particles 2, the effects of the solid solution crystal particles 2a can be more easily obtained. When the average length of the major axis and the average aspect ratio are within the above range, a structure in which the silicon nitride crystal particles 2 are intricately intertwined is formed, which can improve the strength of the silicon nitride sintered body. For example, if the silicon nitride crystal particles 2 are intricately intertwined, the size of the grain boundary phase 3 formed in the gaps between the silicon nitride crystal particles 2 tends to vary. By controlling the average length of the major axis and the average aspect ratio of the solid solution crystal particles 2a, the variation in the size of the grain boundary phase 3 can be reduced. Furthermore, even if there is variation, the presence of solid solution crystal particles 2a makes it easier to mitigate the difference in thermal expansion. In other words, the synergistic effect of reducing the variation in the size of the grain boundary phase 3 and mitigating the difference in thermal expansion by the solid solution crystal particles 2a can further improve the strength of the silicon nitride sintered body.

[0032] It is preferable that metal compound particles 4 containing a first element are present in the grain boundary phase 3. The grain boundary phase 3 exists in the gaps between the silicon nitride crystal particles 2. As will be described later, the grain boundary phase 3 is formed through a sintering process by a sintering aid. Since the sintering aid is mainly an oxide, the main component of the grain boundary phase 3 is also an oxide. The thermal conductivity of the grain boundary phase 3 mainly composed of an oxide is lower than that of the silicon nitride crystal particles 2. The presence of the metal compound particles 4 containing the first element in the grain boundary phase 3 can strengthen the grain boundary phase 3 and obtain the effect of improving the strength of the silicon nitride sintered body.

[0033] The metal compound particles 4 are preferably crystalline metal compounds. When the metal compound particles 4 are crystalline, it becomes easier to obtain the effect of strengthening the grain boundary phase 3. The strengthening of the grain boundary phase 3 also leads to an improvement in thermal conductivity and heat resistance. Whether the metal compound particles 4 are crystalline can be determined by examining the XRD peaks. XRD peaks may be used for the qualitative analysis of crystalline metal compounds. It is preferable to use the Powder Diffraction File (PDF) for the qualitative analysis. The XRD peaks include peaks based on the silicon nitride crystal particles, peaks based on the first element dissolved in the silicon nitride crystal particles, and peaks of crystal particles contained in the grain boundary phase. In the XRD analysis results, by using the peaks other than the peaks derived from the silicon nitride crystal particles, it can be determined whether the crystalline metal compound 4 is present in the grain boundary phase 3.

[0034] For example, when a crystalline tungsten compound is present in the grain boundary phase 3, an XRD peak corresponding to the tungsten compound is detected. At this time, by referring to the PDF, it can be confirmed that the XRD peak is derived from the tungsten compound. Also, when the metal compound particles 4 are amorphous compounds, no peaks corresponding to crystalline metal compounds are detected in the XRD measurement results. EDS analysis may be used for the qualitative analysis of amorphous compounds.

[0035] Incidentally, the first element dissolved in the silicon nitride crystal particles 2 can also form minute crystals with Si or N. However, in the silicon nitride sintered body according to the embodiment, the atomic ratio (1) is 0.0001 or more and 0.01 or less, and the number of the first element is very small compared with Si or N. In the XRD analysis, the peak based on the first element in the silicon nitride crystal particles 2 is very small or does not appear (including below the detection limit). Therefore, when a peak based on the compound of the first element is observed in the XRD analysis, the compound can be regarded as existing in the grain boundary phase 3.

[0036] The XRD analysis method is as follows. The measurement surface is an arbitrary cross section of the silicon nitride sintered body. A polished surface with a surface roughness Ra of 1 μm or less is used for the measurement surface. The XRD analysis is performed using a Cu target (Cu-Kα), a tube voltage of 40 kV, a tube current of 40 mA, a scan speed of 2.0° / min, and a slit (RS) of 0.15 mm.

[0037] The presence of the first element in the grain boundary phase 3 may be confirmed by EDS analysis. In the EDS analysis, the same conditions as those of the aforementioned TEM-EDS are used. When both the EDS analysis and the XRD analysis are performed, either one may be performed first.

[0038] The main component of the metal compound particles 4 is preferably tungsten or molybdenum. The main component refers to the component contained most in terms of mass ratio. The metal compound particles 4 are preferably one or more selected from oxides, carbides, nitrides, silicides, or composite compounds thereof of the first element. The composite compounds are oxynitrides, oxycarbides, etc. The content of the metal compound particles 4 in the silicon nitride sintered body is preferably in the range of 0.5 mass% or more and 5 mass% or less.

[0039] The proportion of grain boundary phase 3 in the silicon nitride sintered body is preferably within the range of 1% by mass to 20% by mass. The mass of grain boundary phase 3 includes the mass of metal compound particles 4. If the proportion of grain boundary phase 3 is less than 1% by mass, the strength of the silicon nitride sintered body may decrease. If the proportion of grain boundary phase 3 exceeds 20% by mass, the thermal conductivity of the silicon nitride sintered body may decrease. For this reason, the proportion of grain boundary phase 3 is preferably within the range of 1% by mass to 20% by mass, and more preferably within the range of 3% by mass to 15% by mass. When the relative density of the silicon nitride sintered body is 97% or more, the mass of the grain boundary phase may be considered to be the value obtained by subtracting the mass of silicon nitride from 100%. The relative density is calculated as [density measured by Archimedes method / theoretical density] × 100%.

[0040] With the silicon nitride sintered body described above, it is possible to achieve a thermal conductivity of 50 W / (m·K) or higher, and even 80 W / (m·K) or higher. Furthermore, it is possible to achieve a three-point bending strength of 600 MPa or higher, and even 700 MPa or higher.

[0041] The silicon nitride sintered body according to the embodiment can be used as a substrate for a semiconductor device. Figure 2 is a perspective view showing an example of a semiconductor device substrate. In Figure 2, reference numeral 5 denotes the semiconductor device substrate. The semiconductor device substrate 5 is made of a silicon nitride sintered body. In the example shown in Figure 2, the top surface of the semiconductor device substrate 5 is rectangular. The top surface of the semiconductor device substrate 5 may be of various shapes, such as circular (including elliptical), L-shaped, U-shaped, or polygonal.

[0042] The thickness of the semiconductor device substrate 5 is preferably within the range of 0.2 mm to 3 mm. If the thickness is less than 0.2 mm, the insulating properties of the semiconductor device substrate 5 may decrease. The thicker the semiconductor device substrate 5, the easier it is to improve heat dissipation, but beyond a thickness of 3 mm, it becomes difficult to further improve heat dissipation. For this reason, the thickness of the semiconductor device substrate 5 is preferably within the range of 0.2 mm to 3 mm, and more preferably within the range of 0.2 mm to 1 mm.

[0043] A silicon nitride circuit board can be obtained by providing a metal portion on the semiconductor device substrate 5. The metal portion may be a metal plate, a metallized layer, or a thin film layer.

[0044] The metal plate is, for example, a copper plate (including a copper alloy plate) or an aluminum plate (including an aluminum alloy plate). The thickness of the metal plate is preferably 0.2 mm or more, and more preferably 0.8 mm or more. By increasing the thickness of the metal plate, heat dissipation and electrical conductivity can be improved.

[0045] The metal plate may be bonded to the semiconductor device substrate 5 via an active metal bonding layer. When the metal plate is a copper plate, the active metal bonding layer contains titanium (Ti). In addition to Ti, the active metal bonding layer contains one or more selected from, for example, silver (Ag), copper (Cu), tin (Sn), and indium (In). When the metal plate is an aluminum plate, the active metal bonding layer contains one or more selected from, for example, aluminum (Al), silicon (Si), and magnesium (Mg).

[0046] The silicon nitride sintered body according to the embodiment can be used as a component for molten metal. A component for molten metal is a component that comes into contact with molten metal. Examples of components for molten metal include molds, nozzles, pipes, containers, heater tubes, thermocouple protection tubes, degassing equipment components, and molten metal processing chambers.

[0047] Figure 3 is a schematic diagram showing a metal molten metal component according to an embodiment. In Figure 3, reference numeral 6 denotes a mold, reference numeral 7 denotes a heater tube, reference numeral 8 denotes a thermocouple protection tube, reference numeral 9 denotes a stalk, reference numeral 10 denotes a component for a degassing device, reference numeral 11 denotes molten metal, reference numeral 12 denotes a metal molten metal processing chamber, and reference numeral 13 denotes a metal molten metal processing device.

[0048] The molten metal processing apparatus 13 includes a molten metal processing chamber 12. The raw material for the molten metal is introduced into the molten metal processing chamber 12. Molten metal 11 is obtained by heating and melting the raw material. The molten metal processing chamber 12 is equipped with a heater tube 7, a thermocouple protection tube 8, a stalk 9, a degassing device component 10, and the like.

[0049] The heater tube 7 is a protective tube for the heater housed inside. The heater tube 7 is sometimes called a heater protection tube. When the heater generates heat, the raw materials for the molten metal melt, becoming molten metal 11. The raw materials for the molten metal are, for example, solid metal ingots. When casting, the molten metal may be heated with a heater in order to uniformly heat and melt the pre-melted molten metal. In Figure 3, one heater tube 7 is provided, but two or more heater tubes 7 may be provided as needed.

[0050] A thermocouple is housed inside the thermocouple protection tube 8, which protects the thermocouple. The thermocouple can measure the temperature of the molten metal 11. The heating temperature of the heater is controlled according to the temperature of the molten metal 11 measured by the thermocouple protection tube 8.

[0051] The stalk 9 is a pipe that transports the molten metal 11. In Figure 3, the molten metal 11 is supplied to the mold 6 via the stalk 9. The molten metal 11 is poured into the mold 6, and a casting is obtained when the molten metal 11 solidifies.

[0052] The degassing device component 10 is capable of recovering gas generated from the molten metal 11. The degassing device component 10 has the role of exhausting gas from within the molten metal 11. By rotating the degassing device component 10, the gas is exhausted while stirring the molten metal 11. In the example shown in Figure 3, a propeller is attached to the tip of the degassing device component 10.

[0053] The molten metal processing chamber 12 is a container for producing molten metal 11. The size of the molten metal processing chamber 12 can be designed as appropriate.

[0054] The mold 6, heater tube 7, thermocouple protection tube 8, stalk 9, degassing device component 10, and molten metal treatment chamber 12 are all components that come into contact with the molten metal 11. The silicon nitride sintered body according to this embodiment can also be used for these molten metal components.

[0055] The metal molten metal component is suitable for metal molten metals at temperatures below 1000°C. As described later, the sintering temperature of the silicon nitride sintered body is approximately 1800°C. If the metal molten metal temperature is too high, the durability of the silicon nitride sintered body may decrease.

[0056] The temperature of molten metal is near or above the melting point of the base metal. For example, molten aluminum is approximately 700°C, molten zinc is approximately 420°C, molten magnesium alloy is approximately 650°C, and molten iron alloy is approximately 1500°C.

[0057] In environments where molten metal components are used for long periods or repeatedly, it is preferable that the temperature of the molten metal be 1000°C or lower. According to the embodiment, the silicon nitride sintered body has good heat dissipation due to improved thermal conductivity, and as a result, the thermal stress on the molten metal component can be relieved. Furthermore, since the first element is solid-solved in at least some of the silicon nitride crystal particles, not only is the heat dissipation good, but the durability at high temperatures is also good. In addition, by including metal compound particles 4 containing the first element in the grain boundary phase 3, the durability at high temperatures can be further improved.

[0058] The operating temperature of SiC semiconductor devices is expected to rise to around 200°C. The metal molten metal components are used in high-temperature environments of 400°C or higher. The silicon nitride sintered body according to this embodiment has excellent high-temperature durability and is therefore suitable for these applications.

[0059] The thickness of the molten metal component is preferably 2 mm or more, and more preferably 5 mm or more. Increasing the thickness of the molten metal component can improve its durability. The thickness of the molten metal component is the thickness at the thinnest point. For example, in the case of a cylindrical shape, the thickness is half the difference between the outer diameter and the inner diameter of the cylinder. There is no particular upper limit to the thickness of the molten metal component, but it is preferably 50 mm or less. If the thickness exceeds 50 mm, the manufacturability of the molten metal component may decrease. Since the molten metal component is used in a higher temperature environment than the substrate for semiconductor devices, it is preferable that the thickness be 2 mm or more.

[0060] The silicon nitride sintered body may be polished as needed. For example, when the silicon nitride sintered body is used as a substrate for semiconductor devices, the surface roughness Ra of the silicon nitride sintered body is preferably 1 μm or less. When the silicon nitride sintered body is used as a component for molten metal, the surface roughness Ra is preferably 10 μm or less. For components for molten metal, if the surface of the part that comes into contact with the molten metal is rough, dross and other materials tend to accumulate on the surface. For example, if dross accumulates on the surface of components such as heater tubes 7 and thermocouple protection tubes 8, it can lead to a decrease in heating effect or temperature measurement performance. If dross accumulates on the inner surface of components such as stalks 9 and degassing device components 10, it can lead to a decrease in the supply capacity of molten metal or degassing capacity. If the inner surface of the mold is rough, the finished casting may be difficult to separate from the mold due to the anchoring effect. If the casting is difficult to separate from the mold, the mold or the casting may be damaged. Furthermore, if the silicon nitride sintered body has the aforementioned surface roughness after sintering or honing, polishing may not be necessary.

[0061] Next, a method for manufacturing a silicon nitride sintered body according to the embodiment will be described. The manufacturing method of the silicon nitride sintered body according to the embodiment is not particularly limited, as long as it has the above-described structure. Here, an example of a method for obtaining a silicon nitride sintered body with good yield will be described.

[0062] First, the raw material powders are prepared. The raw material powders are silicon nitride powder and sintering aid powder. The average particle size of the silicon nitride powder is preferably 3 μm or less. The oxygen content of the silicon nitride powder is preferably 3% by mass or less, and the alpha-gelatinization rate is preferably 90% or more. Furthermore, in order to reduce the amount of aluminum solid-dissolved in the silicon nitride crystal particles, it is preferable not to use SiAlON powder.

[0063] The sintering aid powder contains rare earth element component powder. The sintering aid powder further contains one or two selected from tungsten component powder and molybdenum component powder. The average particle size of the sintering aid powder is preferably 4 μm or less.

[0064] The rare earth element component powder is, for example, at least one of powders of rare earth element oxides, nitrides, and oxynitrides. The rare earth element is one or more selected from yttrium and lanthanoid elements.

[0065] The tungsten component powder and the molybdenum component powder are, for example, powders of oxides, nitrides, carbides, sulfides, oxynitrides, oxycarbides, or carbonitrides. Among these compounds, oxides or carbides are preferred. For example, the oxide is tungsten oxide (WO ３ ), or molybdenum oxide (MoO ３ ). The carbide is tungsten carbide (WC, W ２ C), or molybdenum carbide (Mo ２ C).

[0066] In particular, it is preferable to use an oxide. The melting point of WO ３ is 1473 °C, the melting point of MoO ３ is 795 °C, the melting point of WC is 2870 °C, and the melting point of Mo ２ C is 2687 °C. The oxide has a melting point lower than the sintering temperature described later. By using an oxide having a melting point lower than the sintering temperature, W or Mo can be dissolved in the silicon nitride crystal particles.

[0067] To dissolve W or Mo in the silicon nitride crystal particles, it is also effective to use silicon nitride powder pre-coated with W or Mo. To produce silicon nitride powder coated with W or Mo, the mechanical alloying method is effective. In the mechanical alloying method, since the metal material is coated while being accompanied by refinement of the material, the reactivity between the silicon nitride and the coating material can be increased. Therefore, an effect of promoting the solid solution of the coating element can be expected along with the grain growth of the silicon nitride.

[0068] In the step of mixing the raw material powders described later, a method of mixing powders with different crushing conditions is also effective. When the total of the silicon nitride powder and one or two selected from the tungsten component powder and the molybdenum component powder is 100 parts by mass, it is preferable to subject 30 to 95 parts by mass of the powder to preliminary crushing and mixing treatment. The preliminary crushing and mixing treatment is performed on the average particle size D of the mixed powder before the preliminary crushing and mixing treatment ５０It is preferable to perform a process that reduces the amount to 2 / 3 or less. It is preferable to perform a step of mixing pre-crushed powder and unpre-crushed powder with the raw material powder. In the sintering process, the powder with small particle size acts as a nucleus, which can promote the solid solution of W or Mo.

[0069] If necessary, sintering aids other than those listed above may be added. These other sintering aids include, for example, one or more selected from titanium component powder, hafnium component powder, zirconium component powder, cobalt component powder, iron component powder, and magnesium component powder. Furthermore, in order to reduce the amount of Al solid solution, it is preferable not to use Al component powder as a sintering aid.

[0070] When the total of "silicon nitride powder" + "rare earth element component powder" + "one or two component powders selected from tungsten and molybdenum" + "other component powders" is taken as 100% by mass, it is preferable that "rare earth element component powder" is 1% to 13% by mass, "one or two component powders selected from tungsten and molybdenum" is 0.5% to 5% by mass, "other component powders" is 0% to 8% by mass, and the remainder is "silicon nitride powder". It is preferable that the total amount of sintering aid powders other than silicon nitride is 20% by mass or less. Adjusting the total amount of sintering aids is effective in controlling the amount of grain boundary phase in the silicon nitride sintered body. Binders and solvents are not counted in the total of silicon nitride powder and sintering aid powders.

[0071] Next, the raw material powder is mixed. The mixing process is carried out using a crushing and mixing machine such as a ball mill. The ball mill can crush the raw material powder by optimizing the crushing media and solvent. By crushing the raw material powder, the remaining aggregates can be suppressed. The ball mill may be either wet or dry. In addition, the mixing process may be carried out by adding an organic binder or solvent as needed.

[0072] Next, the molding process is carried out using the raw materials that have undergone the mixing process. The raw materials may be granulated before molding. The molding process can be carried out by methods such as die molding, rolling granulation, cold isostatic pressing (CIP), doctor blade method, and injection molding. When manufacturing substrates for semiconductor devices, the doctor blade method is effective. When manufacturing components for molten metal, a molding method appropriate to the shape is selected. Multiple molding methods may be combined. For example, a combination of multiple molding methods is to perform CIP after die molding.

[0073] A molded body can be obtained through the molding process. The molded body is subjected to a drying process as needed. The drying process has the effect of removing the solvent that was present when the materials were wet-mixed in the mixing process. The drying process may be natural drying, heat drying, etc. In heat drying, it is preferable to set the temperature within the range of 80°C to 200°C. Below 80°C, the drying efficiency may decrease. Above 200°C, uneven drying may occur. For this reason, the drying temperature is preferably within the range of 80°C to 200°C, and more preferably within the range of 100°C to 160°C.

[0074] Next, the molded body is subjected to a degreasing process as necessary. By performing the degreasing process, the organic binder can be removed. The degreasing temperature is preferably in the range of 400°C to 700°C. The degreasing process may be performed in the atmosphere, a nitrogen atmosphere, etc. A degreased body can be obtained by the degreasing process.

[0075] Next, a sintering process is performed in which the degreased body is heat-treated at a higher temperature than the degreasing process. The sintering process plays a role in densifying the degreased body and controlling its relative density and microstructure. In the sintering process, the sintering temperature is preferably in the range of 1600°C to 2000°C. Normal pressure sintering, pressure sintering, hot isostatic pressing (HIP), etc., can be used for the sintering process. The sintering process may be carried out in air, a non-oxidizing atmosphere, a reducing atmosphere, or a vacuum. Normal pressure sintering is sintering carried out under controlled conditions of 1 atmosphere (0.9 to 1.1 atm = approximately 0.09 to 0.11 MPa). Pressure sintering is sintering carried out by applying a pressure higher than atmospheric pressure. For example, uniaxial pressurization may be performed as pressure sintering. Uniaxial pressurization is sometimes called hot pressing. HIP is a sintering method that applies isotropic pressure using gas or the like. HIP can reduce internal defects such as voids and cracks in the sintered body. In HIP, the pressure is preferably in the range of 10 MPa to 200 MPa. A combination of atmospheric pressure sintering, pressure sintering, and HIP may also be used.

[0076] Regarding the sintering time, it is preferable that the holding time at the sintering temperature be 1 hour or more. In other words, "sintering temperature" refers to a temperature within the range of 1600°C to 2000°C at which the temperature is held for 1 hour or more. The holding time is preferably determined according to the thickness and volume of the silicon nitride sintered body. For example, if the thickness is 1 mm or less, such as a substrate for semiconductor equipment, it is preferable that the holding time at the sintering temperature be within the range of 1 hour to 7 hours. If the thickness is 2 mm or more, such as a component for molten metal, it is preferable that the holding time at the sintering temperature be within the range of 8 hours to 20 hours.

[0077] In the sintering process, if the sintering temperature is higher than the melting point of the tungsten or molybdenum powder, it is preferable to perform an intermediate holding step. In the intermediate holding step, the tungsten or molybdenum powder added as a sintering aid is held at or near its melting point for 3 hours or more. "Near the melting point" refers to a range of ±50°C from the melting point. For example, WO ３ Since the melting point of the powder is 1473°C, the temperature held during the intermediate holding process is within the range of 1473 ± 50°C.３ Since the melting point of is 795°C, the temperature held during the intermediate holding process is 795°C ± 50°C. By holding the mixture near the melting point of the tungsten or molybdenum powder added as a sintering aid, the solid solution of W or Mo into the silicon nitride crystal particles can be promoted. If both tungsten and molybdenum powders are added, it is preferable to perform the intermediate holding process near the melting point of each respective powder.

[0078] When an intermediate holding process is performed, the heating rate from the intermediate holding temperature to the sintering temperature is preferably 150°C / hr (hours) or less. When the thickness of the silicon nitride sintered body is 1 mm or less, such as in the case of a substrate for a semiconductor device, the heating rate is preferably in the range of 40°C / hr to 150°C / hr, and more preferably in the range of 50°C / hr to 100°C / hr. Furthermore, when the thickness of the silicon nitride sintered body is 2 mm or more, such as in the case of a component for molten metal, the heating rate is preferably in the range of 150°C / hr or less, and more preferably in the range of 50°C / hr or less. When the silicon nitride sintered body becomes thicker, slowing down the heating rate makes it possible to make the heat transferred to the degreased body more uniform. This makes it possible to make the solid solution amount of W or Mo to the silicon nitride crystal particles more uniform. When the thickness of the silicon nitride sintered body is 2 mm or more, there is no particular lower limit to the heating rate, but it is preferably 10°C / hr or more. A heating rate of 10°C / hr ensures that the heat transferred to the degreased material is sufficiently uniform. Further reductions in the heating rate lead to decreased productivity. The heating rate may be constant within the aforementioned range, or it may vary within that range.

[0079] A silicon nitride sintered body can be obtained through a sintering process. When the obtained silicon nitride sintered body is used as is as a substrate for a semiconductor device, the obtained silicon nitride sintered body corresponds to a semiconductor device substrate. When the obtained silicon nitride sintered body is used as is as a component for molten metal, the obtained silicon nitride sintered body corresponds to a component for molten metal. If necessary, the obtained silicon nitride sintered body may be subjected to honing or polishing.

[0080] (Examples 1-5, Comparative Examples 1-2) Silicon nitride powder and sintering aid powder were prepared as raw material powders. These powders were mixed under the conditions shown in Table 1. The mixing ratio was calculated assuming the total of silicon nitride powder and sintering aid powder was 100% by mass. In Example 2, Si processed by mechanical alloying was used. ３ N ４ Powder and WO ３ Powder is used. In Example 3, Si treated by mechanical alloying is used. ３ N ４ Powder and MoO ３ Powder was used. The raw material powder was mixed using a ball mill. In the ball mill mixing process, an organic binder and a solvent were added to the raw material powder.

[0081]

[0082] Next, the molding process was carried out. In the example of manufacturing silicon nitride sintered bodies for use in semiconductor device substrates, the doctor blade method was used. In the example of manufacturing silicon nitride sintered bodies for use in molten metal components, mold molding was used. In mold molding, the mixed raw materials were molded to take the shape of a mold for the molten metal component.

[0083] In each of Examples 1 to 5 and Comparative Examples 1 to 2, molding was performed using the doctor blade method and mold molding. Here, the examples in which molding was performed using the doctor blade method are referred to as Examples 1A to 5A and Comparative Examples 1A to 2A. The examples in which mold molding was performed are referred to as Examples 1B to 5B and Comparative Examples 1B to 2B.

[0084] In Examples 1A to 5A and Comparative Examples 1A to 2A, the silicon nitride sintered body (substrate for semiconductor device) was molded to a thickness of 0.32 mm after sintering. In Examples 1B to 5B and Comparative Examples 1B to 2B, the silicon nitride sintered body (member for molten metal) was molded to a thickness of 5 mm after sintering. For example, the thickness of the molded body is adjusted so that the sintered body reaches a predetermined thickness, taking into account the shrinkage of the degreased body during the sintering process.

[0085] Next, the molded body was subjected to a degreasing process. The degreasing process was carried out within the range of 400 to 650°C. The resulting degreased body was then subjected to a sintering process. In the sintering process of this example, an intermediate holding temperature was set. Here, the sintering process for producing a substrate for semiconductor devices is called sintering process A. The sintering process for producing a component for molten metal is called sintering process B. The sintering conditions are as shown in Table 2.

[0086]

[0087] In each example, the mixture was held at an intermediate holding temperature for a period of 3 to 6 hours. Examples 1A to 5A and Comparative Examples 1A to 2A performed sintering process A for manufacturing semiconductor device substrates. Examples 1B to 5B and Comparative Examples 1B to 2B performed sintering process B for manufacturing molten metal components.

[0088] The presence or absence of solid solution elements was investigated in the obtained silicon nitride sintered body. The measurement conditions were as described above. From the silicon nitride crystal particles observed in a measurement area of ​​20 μm × 20 μm, more than 10 silicon nitride crystal particles with a length of 0.5 μm or more were selected. The number of atoms of Si, W, Mo, O, and Al in each selected silicon nitride crystal particle was measured. Based on the measurement results, the atomic ratios (1) to (3) were calculated for each silicon nitride crystal particle. In addition, the ratio of silicon nitride crystal particles with an atomic ratio (1) of 0.0001 or more and 0.01 or less to the number of selected silicon nitride crystal particles was calculated. The results are shown in Table 3. In Table 3, the minimum and maximum values ​​for each of the atomic ratios (1) to (3) are shown.

[0089]

[0090] As can be seen from Table 3, in the examples, the atomic ratio (1), i.e., (W + Mo) / Si, was in the range of 0.0001 to 0.01. In the examples, the atomic ratio (2), i.e., O / Si, was 0.04 or less. In the examples, the atomic ratio (3), i.e., Al / Si, was below the detection limit of less than 0.001, and was practically zero.

[0091] In contrast, in Comparative Example 1, neither W nor Mo was added, so the atomic ratio (1) was 0. In Comparative Example 2, W was added, but the atomic ratio (1) was still 0. The results for Comparative Example 2 show that the intermediate holding step is effective for solid solution of W or Mo into silicon nitride crystal particles. Also, in Comparative Examples 1 and 2, because aluminum components were added, the atomic ratio (3) exceeded 0.001.

[0092] Next, the thermal conductivity and three-point bending strength of the silicon nitride sintered bodies for each example and comparative example were measured. Thermal conductivity was measured according to the flash method of JIS-R-1611 (2010). Three-point bending strength was measured according to JIS-R-1601 (2008).

[0093] Next, arbitrary cross-sections of the silicon nitride sintered body were observed using a scanning electron microscope (SEM). From the SEM observation results, the presence or absence of metal compound particles containing W or Mo, the average length of the major axis of the silicon nitride crystal particles, and the average aspect ratio were determined. XRD analysis was used to determine the presence or absence of metal compound particles containing W or Mo. In addition, the relative density of the silicon nitride sintered body was measured. The measurement methods for each were as described above. These measurement results are shown in Table 4.

[0094]

[0095] As can be seen from Table 4, in each embodiment, the thermal conductivity was 50 W / (m·K) or higher, and the three-point bending strength was 600 MPa or higher. The silicon nitride sintered bodies in each embodiment are suitable for semiconductor device substrates because they have high thermal conductivity. Although the silicon nitride sintered bodies in the comparative examples had high strength, their thermal conductivity was low. Due to their low thermal conductivity, the silicon nitride sintered bodies in the comparative examples are not suitable for semiconductor device substrates.

[0096] Next, high-temperature durability tests were conducted using the continuous casting molds produced in Examples 1B to 5B and Comparative Examples 1B to 2B. The surface roughness Ra of the inner surface of the molds was 10 μm or less. In the high-temperature durability tests, molten aluminum (approximately 700°C) was repeatedly cast using the continuous casting molds. After simultaneously starting the high-temperature durability tests for each continuous casting mold, cracks occurred in the molds of the comparative examples due to repeated use. At this time, no cracks occurred in the molds of the examples. Therefore, it can be seen that the molds of the examples have improved durability compared to the molds of the comparative examples. It was found that an improvement in thermal conductivity leads to an improvement in durability.

[0097] From the above results, it was found that the silicon nitride sintered body according to the example can be applied to both semiconductor device substrates and molten metal components.

[0098] In this specification, "or" indicates that "at least one" of the items listed in the text may be adopted.

[0099] Although several embodiments of the present invention have been illustrated above, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. Modifications of these embodiments are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents. Furthermore, the embodiments described above can be implemented in combination with each other.

[0100] 1…Cross-section of silicon nitride sintered body 2…Silicon nitride crystal particles 2a…Solid solution crystal particles 3…Grain boundary phase 4…Metal compound particles 5…Substrate for semiconductor equipment 6…Mold 7…Heater tube 8…Thermocouple protection tube 9…Stork 10…Parts for degassing equipment 11…Molten metal 12…Molten metal processing chamber 13…Molten metal processing equipment S…Analysis spot

Claims

1. A silicon nitride sintered body comprising a plurality of silicon nitride crystal particles and a grain boundary phase, wherein the plurality of silicon nitride crystal particles include solid solution crystal particles in which at least one first element selected from the group consisting of tungsten and molybdenum is solid-dissolved, and in the solid solution crystal particles, the ratio of the number of atoms of the first element to the number of atoms of silicon is 0.0001 or more and 0.01 or less, the ratio of the number of atoms of oxygen to the number of atoms of silicon is 0.04 or less, and the thermal conductivity is 50 W / (m·K) or more.

2. The silicon nitride sintered body according to claim 1, wherein in the solid solution crystal particles, the ratio of the number of aluminum atoms to the number of silicon atoms is less than 0.

001.

3. The silicon nitride sintered body according to claim 1 or 2, wherein, in a measurement area of ​​any cross-section of 20 μm × 20 μm, the ratio of the number of solid solution crystal particles to the number of silicon nitride crystal particles of 0.5 μm or larger is within the range of 30% to 100%.

4. The silicon nitride sintered body according to any one of claims 1 to 3, characterized in that the average length of the major axis of the plurality of silicon nitride crystal particles is in the range of 0.5 μm or more and 10 μm or less, and the average aspect ratio is in the range of 2 or more and 10 or less.

5. A silicon nitride sintered body according to any one of claims 1 to 4, comprising metal compound particles containing the first element in the grain boundary phase.

6. A silicon nitride sintered body according to any one of claims 1 to 5, wherein the three-point bending strength is 600 MPa or more.

7. A substrate for a semiconductor device using a silicon nitride sintered body according to any one of claims 1 to 6.

8. A metal molten metal component using a silicon nitride sintered body according to any one of claims 1 to 6.

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