WO2020203683A1 - 窒化ケイ素焼結体及びその製造方法、並びに積層体及びパワーモジュール - Google Patents

窒化ケイ素焼結体及びその製造方法、並びに積層体及びパワーモジュール Download PDF

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WO2020203683A1
WO2020203683A1 PCT/JP2020/013787 JP2020013787W WO2020203683A1 WO 2020203683 A1 WO2020203683 A1 WO 2020203683A1 JP 2020013787 W JP2020013787 W JP 2020013787W WO 2020203683 A1 WO2020203683 A1 WO 2020203683A1
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Prior art keywords
silicon nitride
sintered body
nitride sintered
thermal conductivity
metal layer
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PCT/JP2020/013787
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English (en)
French (fr)
Inventor
翔二 岩切
武田　真
真一 高田
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デンカ株式会社
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Priority to KR1020217033670A priority Critical patent/KR20210139409A/ko
Priority to EP20782084.6A priority patent/EP3951857A4/en
Priority to JP2021511943A priority patent/JPWO2020203683A1/ja
Priority to CN202080022624.7A priority patent/CN113614910A/zh
Priority to US17/441,772 priority patent/US20220177376A1/en
Publication of WO2020203683A1 publication Critical patent/WO2020203683A1/ja

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Definitions

  • the present disclosure relates to a silicon nitride sintered body, a method for producing the same, a laminate, and a power module.
  • Patent Document 1 proposes a ceramic substrate made of a material containing aluminum nitride, alumina, silicon nitride or silicon carbide as a main component.
  • Patent Document 2 a rare earth element and Mg are added as components of a sintering aid, and heat treatment is performed under predetermined conditions to perform silicon nitride baking having a thermal conductivity of 100 W / (m ⁇ K) or more at room temperature. Techniques for manufacturing the body have been proposed.
  • the power module Since the power module is an important component that controls various devices, it is required to function stably. In order to improve the stability, it is necessary to suppress not only the reliability of each member constituting the power module but also the heat generation due to the large current. In order to satisfy such a requirement, it is considered to be useful to use a ceramic substrate which is excellent in heat dissipation and does not easily break even if the thermal cycle increases. However, with conventional ceramic substrates, it has been difficult to achieve both excellent heat dissipation and high reliability.
  • the present disclosure provides a silicon nitride sintered body having both excellent heat dissipation and high reliability, and a method for producing the same. Further, the present disclosure provides a laminated body including such a silicon nitride sintered body. Further, in the present disclosure, a laminated body including the above-mentioned laminated body is provided.
  • the method for producing a silicon nitride sintered body includes a step of molding and firing a raw material powder containing silicon nitride, and the pregelatinization rate of silicon nitride contained in the raw material powder is 30% by mass or less. Is.
  • a raw material powder having a silicon nitride pregelatinization rate of 30% by mass or less is used.
  • the pregelatinization rate of silicon nitride in the raw material powder is high, the grain growth rate tends to be high in the firing step. In this case, although sintering is promoted, it is considered that the number of defects remaining in the obtained sintered body increases.
  • the pregelatinization rate of silicon nitride in the raw material powder is low, the ⁇ conversion rate in the raw material powder is high, and the grain growth rate in the firing step tends to be slow. In this case, although it takes time for sintering, it is considered that defects contained in the obtained sintered body are reduced and thermal conductivity and fracture toughness are improved.
  • the silicon nitride sintered body obtained by the above manufacturing method can produce a silicon nitride sintered body having both excellent heat dissipation and high reliability by such an action.
  • the thermal conductivity (20 ° C.) of the silicon nitride sintered body obtained by the above-mentioned production method may exceed 100 W / m ⁇ K, and the fracture toughness (K IC ) may be 7.4 MPa ⁇ m 1/2 or more. Further, the bending strength of the silicon nitride sintered body obtained by the above-mentioned production method may exceed 600 MPa. By having such characteristics, heat dissipation and reliability are further improved, and for example, it can be more preferably used for a substrate of a power module.
  • the thermal conductivity (20 ° C.) is greater than 100W / m ⁇ K
  • fracture toughness (K IC) is 7.4 MPa ⁇ m 1/2 or more. Since this silicon nitride sintered body has high thermal conductivity and fracture toughness, it has both excellent heat dissipation and high reliability. Therefore, for example, it can be suitably used for a substrate of a power module.
  • the silicon nitride sintered body may have a bending strength of more than 600 MPa. By having such characteristics, reliability is further improved, and for example, it can be more preferably used for a substrate of a power module.
  • the thermal conductivity of the silicon nitride sintered body at 150 to 200 ° C. may exceed 60 W / m ⁇ K. As a result, it can be more preferably used as a substrate for a power module used under particularly harsh conditions.
  • the laminate according to one aspect of the present disclosure includes a metal layer made of a first metal, a heat radiating portion made of a second metal having a higher thermal conductivity than that of the first metal, and a metal layer.
  • a substrate provided between the heat radiating portions and made of any of the above-mentioned silicon nitride sintered bodies is provided.
  • the substrate of this laminated body is composed of a silicon nitride sintered body having both excellent heat dissipation and high reliability. Then, this substrate is provided between the metal layer and the heat radiating portion. Therefore, the heat generated on the metal layer side can be efficiently dissipated from the heat radiating portion side. Therefore, for example, it can be suitably used as a laminate for a power module.
  • the power module according to one aspect of the present disclosure includes the above-mentioned laminate and a semiconductor element electrically connected to a metal layer. Since such a power module includes the above-mentioned substrate, it is excellent in heat dissipation and reliability.
  • the present disclosure it is possible to provide a silicon nitride sintered body having both excellent heat dissipation and high reliability, and a method for producing the same. Further, according to the present disclosure, it is possible to provide a laminated body including such a silicon nitride sintered body. Further, according to the present disclosure, it is possible to provide a laminated body including the above-mentioned laminated body.
  • FIG. 1 is a schematic cross-sectional view of an embodiment of a power module.
  • FIG. 2 is a diagram schematically showing the state of grain growth when the silicon nitride particles are sintered in one embodiment of the method for producing a silicon nitride sintered body.
  • FIG. 3 is a graph showing the relationship between the firing temperature and the relative density.
  • FIG. 4 is a graph showing the temperature change of the thermal conductivity of the silicon nitride sintered body of Example 1 and Comparative Example 1.
  • FIG. 5 is an electron micrograph of a fracture surface of the silicon nitride sintered body of Example 1.
  • FIG. 6 is a diagram schematically showing the state of grain growth when the silicon nitride particles are sintered in the conventional method of the silicon nitride sintered body.
  • FIG. 7 is an electron micrograph of a fracture surface of the silicon nitride sintered body of Comparative Example 1.
  • the silicon nitride sintered body has a thermal conductivity of more than 100 W / m ⁇ K at 20 ° C.
  • the thermal conductivity (20 ° C.) in the present disclosure can be measured in accordance with JIS R 1611: 2010. This thermal conductivity (20 ° C.) is A ⁇ B ⁇ from the values of thermal diffusivity A [m 2 / sec], density B [kg / m 3 ], and specific heat C [J / (kg ⁇ K)]. It is calculated by the formula of C.
  • A 0.1388 ⁇ (thickness [mm]) 2 / t 1/2 .
  • t 1/2 is the time [seconds] required for the temperature rise to half of ⁇ T, where ⁇ T is the total temperature rise width.
  • Density B is determined by Archimedes' method.
  • the specific heat C is determined by differential thermal analysis.
  • the thermal conductivity (20 ° C.) of the silicon nitride sintered body may exceed 110 W / m ⁇ K, may exceed 120 W / m ⁇ K, or 140 W / m ⁇ K, for example, from the viewpoint of further improving heat dissipation. It may exceed m ⁇ K.
  • the upper limit of the thermal conductivity (20 ° C.) may be, for example, 200 W / m ⁇ K from the viewpoint of ease of manufacture.
  • the thermal conductivity of the silicon nitride sintered body at 150 to 200 ° C. may exceed 60 W / m ⁇ K, and is 65 W / m ⁇ K. It may exceed K.
  • the upper limit of the thermal conductivity at 150 to 200 ° C. may be, for example, 150 W / m ⁇ K from the viewpoint of ease of manufacture.
  • the thermal conductivity in such a temperature range can also be obtained by the formula of A ⁇ B ⁇ C as described above.
  • the thermal diffusivity A is a measured value obtained by performing the above-mentioned measurement at the temperature, and the specific heat C may be a literature value.
  • the density B the value of 20 ° C. can be used as it is.
  • Silicon nitride sintered body has a fracture toughness is 7.4 MPa ⁇ m 1/2 or more (K IC).
  • Fracture toughness (K IC) is a value measured by the SEPB method, JIS R1607: measured according to 2015.
  • Fracture toughness of sintered silicon nitride (K IC) from the viewpoint of further improving the reliability, may be greater than 7.5 MPa ⁇ m 1/2, may be greater than 8 MPa ⁇ m 1/2.
  • the upper limit of the fracture toughness (K IC), from the viewpoint of ease of production for example, be a 15 MPa ⁇ m 1/2.
  • the silicon nitride sintered body may have a bending strength exceeding 600 MPa from the viewpoint of further improving reliability.
  • the bending strength is a three-point bending strength, and can be measured using a commercially available bending strength meter in accordance with JIS R 1601: 2008.
  • the bending strength of the silicon nitride sintered body may exceed 620 MPa or 650 MPa from the viewpoint of further improving reliability.
  • the upper limit of the bending strength may be, for example, 800 MPa from the viewpoint of ease of production.
  • the silicon nitride sintered body may be substantially composed of silicon nitride only, or may contain a component derived from a sintering aid and an unavoidable component derived from a raw material, a manufacturing process, or the like.
  • the content of silicon nitride in the silicon nitride sintered body may be, for example, 90 mol% or more, or 95 mol% or more, from the viewpoint of achieving both high thermal conductivity and excellent insulating properties at a high level. , 98 mol% or more.
  • the silicon nitride sintered body can be produced by reducing the amount of the sintering aid used, the total content ratio of rare earth elements in the silicon nitride sintered body can be sufficiently lowered.
  • the total content ratio of rare earth elements in the silicon nitride sintered body may be 6.0% by mass or less, and may be 3.0% by mass or less.
  • rare earth elements a total of 17 elements including two elements of Sc (scandium) and Y (yttrium) and 15 elements of lanthanoids from La (lanthanum) to Lu (lutetium) correspond.
  • the silicon nitride sintered body may have a dielectric breakdown strength of 10 [kV / 0.32 mm] or more.
  • Dielectric breakdown strength can be measured in accordance with JIS C-2110: 2016.
  • the dielectric breakdown strength may be, for example, 11 [kV / 0.32 mm] or more.
  • the upper limit of the dielectric breakdown strength may be, for example, 15 [kV / 0.32 mm] from the viewpoint of ease of manufacture.
  • FIG. 1 is a schematic cross-sectional view of a power module according to an embodiment.
  • the power module 100 includes a metal layer 11, a substrate 10 (silicon nitride sintered body 10), a metal layer 12, a solder layer 32, a base plate 20, and cooling fins 22 in this order.
  • the metal layer 11 constitutes a metal circuit by, for example, etching or the like.
  • a semiconductor element 60 is attached to the metal layer 11 via a solder layer 31.
  • the semiconductor element 60 is connected to a predetermined portion of the metal layer 11 by a metal wire 34 such as an aluminum wire (aluminum wire).
  • the substrate 10 is made of a silicon nitride sintered body. As a result, the metal layer 11 and the metal layer 12 are electrically insulated. The metal layer 12 may or may not form an electric circuit. The materials of the metal layer 11 and the metal layer 12 may be the same or different. The metal layer 11 and the metal layer 12 may be made of copper corresponding to the first metal. However, the material is not limited to copper.
  • the metal layer 12 is joined to the base plate 20 via a solder layer 32.
  • a screw 23 for fixing the cooling fin 22 forming a heat radiating member to the base plate is provided at the end of the base plate 20, a screw 23 for fixing the cooling fin 22 forming a heat radiating member to the base plate is provided.
  • the base plate 20 may be made of aluminum corresponding to the second metal.
  • the base plate 20 is joined to the cooling fins 22 via grease 24 on the side opposite to the metal layer 12 side.
  • the base plate 20 and the cooling fins 22 function as heat radiating parts by being composed of a second metal having a higher thermal conductivity than the metal layer 11. Since the substrate 10 has a high thermal conductivity, the semiconductor element 60, the metal layer 11, and the metal layer 12 are efficiently cooled by the heat radiating portion.
  • a resin housing 36 is attached to one side of the base plate 20 (the side on which the semiconductor element 60 is installed) so as to accommodate the above-mentioned members (semiconductor element 60, metal layers 11, 12 and substrate 10).
  • Each of the above members is housed in the space formed by one surface of the base plate 20 and the housing 36, and a filler 30 such as silicone gel is filled so as to fill the gap in the space.
  • a filler 30 such as silicone gel is filled so as to fill the gap in the space.
  • a predetermined portion of the metal layer 11 is connected to an electrode 33 provided through the housing 36 via a solder layer 35.
  • the power module 100 includes a laminate 50 composed of a metal layer 11, a substrate 10, a metal layer 12, a base plate 20, and cooling fins 22.
  • the substrate 10 is made of a silicon nitride sintered body having both high thermal conductivity and high fracture toughness. Since the substrate 10 has high thermal conductivity, heat can be smoothly dissipated from the semiconductor element 60 and the metal layer 11 toward the substrate 10 and the cooling fins 22. Further, the silicon nitride sintered body is not easily broken even if it receives an impact. Therefore, the power module 100 can exhibit stable performance and is excellent in reliability. As described above, the silicon nitride sintered body is suitably used for the substrate of the power module 100. However, the use of the silicon nitride sintered body 10 is not limited to the power module.
  • the material of the base plate 20 is not limited to aluminum.
  • the base plate 20 may be made of a first metal (such as copper) and only the cooling fins may be made of a second metal (such as aluminum).
  • only the cooling fins 22 function as heat radiating portions.
  • the cooling fins 22 may not be provided and only the base plate 20 may function as a heat radiating portion.
  • the base plate 20 and the substrate 10 may be joined without the metal layer 12.
  • the manufacturing method of the silicon nitride sintered body will be described below.
  • the method for producing a silicon nitride sintered body according to one embodiment includes a step of molding and firing a raw material powder containing silicon nitride.
  • the pregelatinization rate of silicon nitride contained in the raw material powder used is 30% by mass or less. As a result, the rate of grain growth of silicon nitride in the firing step can be slowed down. Therefore, although it takes time for sintering, defects remaining in the obtained silicon nitride sintered body can be reduced.
  • the pregelatinization rate of silicon nitride contained in the raw material powder may be 20% by mass or less or 15% by mass or less from the viewpoint of further increasing the thermal conductivity.
  • the pregelatinization rate of silicon nitride contained in the raw material powder may be 5% by mass or more from the viewpoint of increasing the bending strength of the silicon nitride sintered body.
  • the raw material powder may contain a sintering aid in addition to silicon nitride.
  • An oxide-based sintering aid can be used as the sintering aid.
  • the oxide-based sintering aid include Y 2 O 3, Mg O and Al 2 O 3 .
  • the content of the oxide-based sintering aid in the raw material powder is, for example, 4.0 to 8.0% by mass from the viewpoint of obtaining a silicon carbide sintered body capable of achieving both high thermal conductivity and excellent insulating properties at a high level. It may be 4.0 to 5.0% by mass.
  • the above-mentioned raw material powder is pressed with a molding pressure of, for example, 3.0 to 10.0 MPa to obtain a molded product.
  • the molded product may be produced by uniaxial pressure or by CIP. Alternatively, it may be fired while being molded by hot pressing.
  • the molded product may be fired in an atmosphere of an inert gas such as nitrogen gas or argon gas.
  • the pressure at the time of firing may be 0.7 to 0.9 MPa.
  • the firing temperature may be 1860 to 2100 ° C. and may be 1880 to 2000 ° C.
  • the firing time at the firing temperature may be 6 to 20 hours and may be 8 to 16 hours.
  • the rate of temperature rise to the firing temperature may be, for example, 1.0 to 10.0 ° C./hour.
  • FIG. 2 is a diagram schematically showing the state of grain growth when the silicon nitride particles are sintered in the production method of the present embodiment.
  • ⁇ -SiN has a higher melting point and slower grain growth than ⁇ -SiN. Therefore, when the pregelatinization rate of silicon nitride contained in the raw material powder is low, the grain growth proceeds more slowly than when the pregelatinization rate is high. Therefore, in the process of grain growth of silicon nitride, it is suppressed that defects remain in the grains, and as shown in FIG. 2, a silicon nitride sintered body having few defects can be obtained as a result. Such a silicon nitride sintered body has high thermal conductivity and high fracture toughness.
  • FIG. 6 is a diagram schematically showing the state of grain growth when the silicon nitride particles are sintered in the conventional manufacturing method.
  • the pregelatinization rate of silicon nitride in the raw material powder is higher than that in FIG. Therefore, in the process of grain growth of silicon nitride, ⁇ -SiN proceeds so as to complement the grain growth of ⁇ -SiN, so that the grain growth rate of silicon nitride increases.
  • the number of defects remaining in the grains in the finally obtained silicon nitride sintered body is larger than that in the case of FIG.
  • Such a silicon nitride sintered body has lower thermal conductivity and fracture toughness than the silicon nitride sintered body obtained in FIG.
  • the porosity of the silicon nitride sintered body of the present embodiment may be, for example, 1.0% by volume or less, or 0.5% by volume or less.
  • FIG. 3 is a graph showing an example of a change in the relative density of the silicon nitride sintered body depending on the firing temperature.
  • curve 1 shows the change in relative density when a raw material powder containing silicon nitride having an pregelatinization rate of 15% by mass is used.
  • curve 2 shows the change in relative density when the raw material powder containing silicon nitride having an pregelatinization rate of 93% by mass is used.
  • the relative density increases slowly. This indicates that the grain growth rate of silicon nitride is slow.
  • the silicon nitride sintered body thus obtained has a sufficiently reduced number of defects. Further, due to the crystal shape of ⁇ -SiN, it is composed of columnar crystal grains having a large size. Due to these factors, it is considered to have high thermal conductivity and high fracture toughness.
  • the silicon nitride sintered body of the present embodiment has excellent heat dissipation because it has high thermal conductivity. Further, since it has high fracture toughness, it can be used stably for a long period of time without breaking even in a usage environment subject to a so-called thermal cycle in which high temperature and low temperature are repeated. Therefore, it is excellent in reliability. As described above, since it has both excellent heat dissipation and excellent reliability, it can be suitably used as a substrate for a power module.
  • This molded product was placed in an electric furnace equipped with a carbon heater, and the temperature was raised to 1900 ° C. at a heating rate of 2.1 ° C./hour under an atmosphere of nitrogen gas (pressure: 0.88 MPa). After firing at a firing temperature of 1900 ° C. for 12 hours, the mixture was cooled at a cooling rate of about 5.0 ° C./hour to obtain a silicon nitride sintered body.
  • the silicon nitride content in the silicon nitride sintered body was 92 mol%.
  • the thermal conductivity at 50 to 200 ° C. was measured in the same manner as the thermal conductivity at 20 ° C. except that the thermal diffusivity A was measured at each temperature and the literature value was used as the specific heat C. These results are plotted in FIG. In FIG. 4, the thermal conductivity at 20 ° C. is also plotted. As shown in FIG. 4, it was confirmed that the thermal conductivity decreased as the temperature increased. However, it was confirmed that the thermal conductivity of the silicon nitride sintered body of Example 1 was higher than that of Comparative Example 1 at any temperature.
  • K IC Fracture toughness
  • the bending strength is the 3-point bending strength, and was measured using a commercially available bending strength meter (manufactured by Shimadzu Corporation, device name: AG-2000) in accordance with JIS R 1601: 2008. The results are shown in Table 1.
  • the dielectric breakdown strength was measured in accordance with JIS C-2110: 2016 using a commercially available measuring device (manufactured by Measurement Technology Laboratory, device name: 7474 type). The results are shown in Table 1.
  • FIG. 5 is a photograph of an observation image by SEM (magnification: 3000 times). The proportion of columnar crystals was high.
  • Example 1 A silicon nitride sintered body was obtained in the same manner as in Example 1 except that silicon nitride powder having an pregelatinization rate of 93% by mass was used and the material was fired at a firing temperature of 1850 ° C. for 4 hours. The obtained silicon nitride sintered body was evaluated in the same manner as in Example 1. The evaluation results are as shown in Table 1 and FIG.
  • FIG. 7 is a photograph of an observation image by SEM (magnification: 3000 times). The proportion of columnar crystals was smaller than in FIG. 5, and the size of the crystal grains was also smaller than in FIG.
  • Example 2 As shown in Table 2, a silicon nitride sintered body was obtained in the same manner as in Example 1 except that silicon nitride powder having an pregelatinization rate of 10 to 25% by mass was used. The obtained silicon nitride sintered body was evaluated in the same manner as in Example 1. The evaluation results are as shown in Table 2.
  • the thermal conductivity can be increased by lowering the pregelatinization rate of the silicon nitride powder used as the raw material powder.
  • the present disclosure it is possible to provide a silicon nitride sintered body having both excellent heat dissipation and high reliability, and a method for producing the same. Further, according to the present disclosure, it is possible to provide a laminated body including such a silicon nitride sintered body. Further, according to the present disclosure, it is possible to provide a laminated body including the above-mentioned laminated body.

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Abstract

窒化ケイ素を含む原料粉末を成形して焼成する工程を有し、原料粉末に含まれる窒化ケイ素のα化率が３０質量％以下である、窒化ケイ素焼結体の製造方法を提供する。窒化ケイ素焼結体の熱伝導率（２０℃）は１００Ｗ／ｍ・Ｋを超え、破壊靭性（ＫＩＣ）は７．４ＭＰａ・ｍ１／２以上である。

Description

窒化ケイ素焼結体及びその製造方法、並びに積層体及びパワーモジュール
　本開示は、窒化ケイ素焼結体及びその製造方法、並びに積層体及びパワーモジュールに関する。
　自動車、電鉄、産業用機器、及び発電関係等の分野には、大電流を制御するパワーモジュールが用いられている。パワーモジュールに搭載される絶縁基板には、セラミックス基板が利用されている。このような用途においては、セラミックス基板は、絶縁性に加えて良好な放熱特性を有することが求められる。例えば、特許文献１では、セラミックス基板として、窒化アルミニウム、アルミナ、窒化ケイ素又は炭化ケイ素を主成分とする材質のものが提案されている。
　また、特許文献２では、希土類元素とＭｇを焼結助剤の成分として添加して所定の条件で熱処理を行い、常温における熱伝導率が１００Ｗ／（ｍ・Ｋ）以上である窒化ケイ素質焼結体を製造する技術が提案されている。
特開２０１７－２１２３３１６号公報 特開２００６－９６６６１号公報
　パワーモジュールは、各種装置を制御する重要な部品であることから、安定的に機能することが求められる。安定性を向上するためには、パワーモジュールを構成する各部材の信頼性のみならず、大電流に伴う発熱を抑制する必要がある。このような要求を満たすためには、放熱性に優れるとともに、熱サイクルが増えても容易に破壊しないセラミックス基板が有用であると考えられる。しかしながら、従来のセラミックス基板では、優れた放熱性と高い信頼性を両立することが難しかった。
　そこで、本開示では、優れた放熱性と高い信頼性を兼ね備える窒化ケイ素焼結体、及びその製造方法を提供する。また、本開示では、このような窒化ケイ素焼結体を備える積層体を提供する。また、本開示では、上記積層体を備える積層体を提供する。
　本開示の一側面に係る窒化ケイ素焼結体の製造方法は、窒化ケイ素を含む原料粉末を成形して焼成する工程を有し、原料粉末に含まれる窒化ケイ素のα化率が３０質量％以下である。この製造方法では、窒化ケイ素のα化率が３０質量％以下である原料粉末を用いる。原料粉末における窒化ケイ素のα化率が高くなると、焼成する工程において粒成長の速度が速くなる傾向にある。この場合、焼結が促進されるものの、得られる焼結体に残存する欠陥が多くなると考えられる。
　一方、原料粉末における窒化ケイ素のα化率低くなると、原料粉末におけるβ化率が高くなり、焼成する工程における粒成長の速度が遅くなる傾向にある。この場合、焼結には時間を要するものの、得られる焼結体に含まれる欠陥が低減され熱伝導率と破壊靭性が向上すると考えられる。上記製造方法で得られる窒化ケイ素焼結体は、このような作用によって、優れた放熱性と高い信頼性を兼ね備える窒化ケイ素焼結体を製造することができる。
　上述の製造方法で得られる窒化ケイ素焼結体の熱伝導率（２０℃）は１００Ｗ／ｍ・Ｋを超え、破壊靭性（ＫＩＣ）は７．４ＭＰａ・ｍ１／２以上であってよい。また、上述の製造方法で得られる窒化ケイ素焼結体の抗折強度は６００ＭＰａを超えてもよい。このような特性を有することによって、放熱性と信頼性がさらに向上し、例えばパワーモジュールの基板用として一層好適に用いることができる。
　本開示の一側面に係る窒化ケイ素焼結体は、熱伝導率（２０℃）が１００Ｗ／ｍ・Ｋを超え、破壊靭性（ＫＩＣ）が７．４ＭＰａ・ｍ１／２以上である。この窒化ケイ素焼結体は高い熱伝導率と破壊靭性を有することから、優れた放熱性と高い信頼性を兼ね備える。このため、例えば、パワーモジュールの基板用として好適に用いることができる。
　上記窒化ケイ素焼結体は、抗折強度が６００ＭＰａを超えてもよい。このような特性を有することによって信頼性が一層向上し、例えばパワーモジュールの基板用として一層好適に用いることができる。
　上記窒化ケイ素焼結体の１５０～２００℃における熱伝導率は、６０Ｗ／ｍ・Ｋを超えてもよい。これによって、特に過酷な条件下で使用されるパワーモジュールの基板用として一層好適に用いることができる。
　本開示の一側面に係る積層体は、第１の金属で構成される金属層と、第１の金属よりも高い熱伝導率を有する第２の金属で構成される放熱部と、金属層と放熱部の間に設けられ、上述のいずれかの窒化ケイ素焼結体で構成される基板と、を備える。この積層体の基板は、優れた放熱性と高い信頼性を兼ね備える窒化ケイ素焼結体で構成される。そして、この基板が金属層と放熱部の間に設けられている。このため、金属層側で生じた熱を放熱部側から効率よく放熱することができる。したがって、例えばパワーモジュール用の積層体として好適に用いることができる。
　本開示の一側面に係るパワーモジュールは、上述の積層体と、金属層と電気的に接続される半導体素子と、を備える。このようなパワーモジュールは、上述の基板を備えることから放熱性と信頼性に優れる。
　本開示によれば、優れた放熱性と高い信頼性を兼ね備える窒化ケイ素焼結体、及びその製造方法を提供することができる。また、本開示によれば、このような窒化ケイ素焼結体を備える積層体を提供することができる。また、本開示によれば、上記積層体を備える積層体を提供することができる。
図１は、パワーモジュールの一実施形態の模式断面図である。 図２は、窒化ケイ素焼結体の製造方法の一実施形態における、窒化ケイ素粒子を焼結する際の粒成長の様子を模式的に示す図である。 図３は、焼成温度と相対密度の関係を示すグラフである。 図４は、実施例１と比較例１の窒化ケイ素焼結体の熱伝導率の温度変化を示すグラフである。 図５は、実施例１の窒化ケイ素焼結体における破断面の電子顕微鏡写真である。 図６は、従来の窒化ケイ素焼結体の方法における、窒化ケイ素粒子を焼結する際の粒成長の様子を模式的に示す図である。 図７は、比較例１の窒化ケイ素焼結体における破断面の電子顕微鏡写真である。
　以下、場合により図面を参照して、本発明の一実施形態について説明する。ただし、以下の実施形態は、本発明を説明するための例示であり、本発明を以下の内容に限定する趣旨ではない。説明において、同一要素又は同一機能を有する要素には同一符号を用い、場合により重複する説明は省略する。各要素の寸法比率は図示の比率に限られるものではない。
　一実施形態に係る窒化ケイ素焼結体は、２０℃で１００Ｗ／ｍ・Ｋを超える熱伝導率を有する。本開示における熱伝導率（２０℃）は、ＪＩＳ　Ｒ　１６１１：２０１０に準拠して測定することができる。この熱伝導率（２０℃）は、熱拡散率Ａ［ｍ２／秒］、密度Ｂ［ｋｇ／ｍ３］、及び比熱Ｃ［Ｊ／（ｋｇ・Ｋ）］の値から、Ａ×Ｂ×Ｃの計算式で算出される。熱拡散率Ａは、縦×横×厚さ＝１０ｍｍ×１０ｍｍ×２ｍｍのサイズの試料を用い、レーザーフラッシュ法によって求められる。具体的には、Ａ＝０．１３８８×（厚み［ｍｍ］）２／ｔ１／２の数式によって求められる。ｔ１／２は、トータルの温度上昇幅をΔＴとしたときに、ΔＴの半分までの温度上昇に所要する時間［秒］である。密度Ｂは、アルキメデス法によって求められる。比熱Ｃは示差熱分析によって求められる。
　窒化ケイ素焼結体の熱伝導率（２０℃）は、例えば、放熱性を一層高くする観点から、１１０Ｗ／ｍ・Ｋを超えてもよく、１２０Ｗ／ｍ・Ｋを超えてもよく、１４０Ｗ／ｍ・Ｋを超えてもよい。熱伝導率（２０℃）の上限は、製造の容易性の観点から、例えば２００Ｗ／ｍ・Ｋであってよい。
　例えばパワーモジュール等に用いられたときの放熱性を十分に高くする観点から、窒化ケイ素焼結体の１５０～２００℃における熱伝導率は、６０Ｗ／ｍ・Ｋを超えてよく、６５Ｗ／ｍ・Ｋを超えてもよい。１５０～２００℃における熱伝導率の上限は、製造の容易性の観点から、例えば１５０Ｗ／ｍ・Ｋであってよい。このような温度範囲における熱伝導率も、上述のとおりＡ×Ｂ×Ｃの計算式で求めることができる。このとき、熱拡散率Ａは上述の測定を当該温度で行って求められる測定値であり、比熱Ｃは文献値であってよい。密度Ｂは２０℃の値をそのまま用いることができる。
　窒化ケイ素焼結体は、７．４ＭＰａ・ｍ１／２以上である破壊靭性（ＫＩＣ）を有する。破壊靭性（ＫＩＣ）は、ＳＥＰＢ法によって測定される値であり、ＪＩＳ　Ｒ１６０７：２０１５に準拠して測定される。窒化ケイ素焼結体の破壊靭性（ＫＩＣ）は、信頼性を一層向上する観点から、７．５ＭＰａ・ｍ１／２を超えてもよく、８ＭＰａ・ｍ１／２を超えてもよい。破壊靭性（ＫＩＣ）の上限は、製造の容易性の観点から、例えば１５ＭＰａ・ｍ１／２であってよい。
　窒化ケイ素焼結体は、信頼性を一層向上する観点から、６００ＭＰａを超える抗折強度を有していてもよい。抗折強度は、３点曲げ抗折強度であり、ＪＩＳ　Ｒ　１６０１：２００８に準拠して市販の抗折強度計を用いて測定することができる。窒化ケイ素焼結体の抗折強度は、信頼性をさらに向上する観点から、６２０ＭＰａを超えてもよく、６５０ＭＰａを超えてもよい。抗折強度の上限は、製造の容易性の観点から、例えば８００ＭＰａであってよい。
　窒化ケイ素焼結体は、実質的に窒化ケイ素のみで構成されていてもよいし、焼結助剤に由来する成分、並びに原料及び製造プロセス等に由来する不可避的成分を含んでいてもよい。窒化ケイ素焼結体における窒化ケイ素の含有量は、高い熱伝導率と優れた絶縁性を高水準で両立する観点から、例えば９０モル％以上であってよく、９５モル％以上であってもよく、９８モル％以上であってもよい。
　窒化ケイ素焼結体は、焼結助剤の使用量を少なくして製造することができるため、窒化ケイ素焼結体における希土類元素の合計含有割合を十分に低くすることができる。窒化ケイ素焼結体における希土類元素の合計含有割合は、６．０質量％以下であってよく、３．０質量％以下であってよい。希土類元素としては、Ｓｃ（スカンジウム）、Ｙ（イットリウム）の２元素と、Ｌａ（ランタン）からＬｕ（ルテチウム）までのランタノイドの１５元素を含む計１７元素が該当する。
　窒化ケイ素焼結体は、１０［ｋＶ／０．３２ｍｍ］以上の絶縁破壊強度を有してよい。絶縁破壊強度はＪＩＳ　Ｃ－２１１０：２０１６に準拠して測定することができる。絶縁破壊強度は、例えば１１［ｋＶ／０．３２ｍｍ］以上であってよい。絶縁破壊強度の上限は、製造の容易性の観点から、例えば１５［ｋＶ／０．３２ｍｍ］であってよい。
　図１は、一実施形態に係るパワーモジュールの模式断面図である。パワーモジュール１００は、金属層１１、基板１０（窒化ケイ素焼結体１０）、金属層１２、ハンダ層３２、ベース板２０、及び冷却フィン２２をこの順に備える。金属層１１は例えばエッチング等によって金属回路を構成している。金属層１１には、ハンダ層３１を介して半導体素子６０が取り付けられている。半導体素子６０は、アルミワイヤ（アルミ線）等の金属ワイヤ３４で金属層１１の所定の部分に接続されている。
　基板１０は、窒化ケイ素焼結体で構成される。これによって、金属層１１と金属層１２は電気的に絶縁される。金属層１２は電気回路を形成していてもよいし、していなくてもよい。金属層１１及び金属層１２の材質は同一であっても異なっていてもよい。金属層１１及び金属層１２は、第１の金属に相当する銅で構成されていてよい。ただし、その材質は銅に限定されるものではない。
　金属層１２はハンダ層３２を介してベース板２０と接合されている。ベース板２０の形状は、例えば、縦×横×厚さ＝９０～１４０ｍｍ×１２０～２００ｍｍ×３～６ｍｍの略長方形板状であってよい。ベース板２０の端部には放熱部材をなす冷却フィン２２をベース板に固定するネジ２３が設けられている。ベース板２０は第２の金属に相当するアルミニウムで構成されていてもよい。ベース板２０は、金属層１２側とは反対側において、グリース２４を介して冷却フィン２２と接合されている。
　ベース板２０及び冷却フィン２２は、金属層１１よりも高い熱伝導率を有する第２の金属で構成されることによって放熱部として機能する。基板１０が高い熱伝導率を有することから、放熱部によって、半導体素子６０、金属層１１及び金属層１２は効率よく冷却される。
　ベース板２０の一方面側（半導体素子６０が設置される側）は、上述の各部材（半導体素子６０、金属層１１，１２及び基板１０）を収容するように樹脂製の筐体３６が取り付けられている。ベース板２０の一方面と筐体３６とで形成される空間内には上記各部材が収容されるとともに、当該空間の隙間を埋めるようにシリコーンゲル等の充填材３０が充填されている。筐体３６の外部と金属層１１とを電気的に接続するため、金属層１１の所定部分は、ハンダ層３５を介して筐体３６を貫通して設けられる電極３３に接続されている。
　パワーモジュール１００は、金属層１１、基板１０、金属層１２、ベース板２０及び冷却フィン２２で構成される積層体５０を備える。基板１０は、高い熱伝導率と高い破壊靭性を兼ね備える窒化ケイ素焼結体で構成される。基板１０は高い熱伝導率を有することから、半導体素子６０、及び金属層１１から基板１０及び冷却フィン２２に向けて円滑に放熱することができる。また、窒化ケイ素焼結体は衝撃を受けても容易に破壊しない。このため、パワーモジュール１００は安定的に性能を発揮することが可能となり、信頼性に優れる。このように窒化ケイ素焼結体はパワーモジュール１００の基板用として好適に用いられる。ただし、窒化ケイ素焼結体１０の用途はパワーモジュールに限定されるものではない。
　なお、ベース板２０の材質はアルミニウムに限定されるものではない。例えば、ベース板２０が第１の金属（例えば銅等）で構成され、冷却フィンのみが第２の金属（アルミニウム等）で構成されていてもよい。この場合、冷却フィン２２のみが放熱部として機能することとなる。また、別の実施形態では、冷却フィン２２を備えず、ベース板２０のみが放熱部として機能してもよい。さらに別の実施形態では、金属層１２を備えず、ベース板２０と基板１０とが接合されていてもよい。
　窒化ケイ素焼結体の製造方法を以下に説明する。一実施形態に係る窒化ケイ素焼結体の製造方法は、窒化ケイ素を含む原料粉末を成形して焼成する工程を有する。用いられる原料粉末に含まれる窒化ケイ素のα化率は３０質量％以下である。これによって、焼成する工程における窒化ケイ素の粒成長の速度を遅くすることができる。したがって、焼結には時間を要するものの、得られる窒化ケイ素焼結体に残存する欠陥を低減することができる。
　原料粉末に含まれる窒化ケイ素のα化率は、熱伝導率を一層高くする観点から、２０質量％以下であってよいし、１５質量％以下であってもよい。原料粉末に含まれる窒化ケイ素のα化率は、窒化ケイ素焼結体の抗折強度を高くする観点から、５質量％以上であってよい。
　原料粉末は、窒化ケイ素の他に焼結助剤を含んでいてもよい。焼結助剤としては酸化物系焼結助剤を用いることができる。酸化物系焼結助剤としてはＹ２Ｏ３、ＭｇＯ及びＡｌ２Ｏ３等が挙げられる。原料粉末における酸化物系焼結助剤の含有量は、高い熱伝導率と優れた絶縁性を高水準で両立できる炭化ケイ素焼結体を得る観点から、例えば４．０～８．０質量％であってよく、４．０～５．０質量％であってもよい。
　上述の原料粉末を例えば３．０～１０．０ＭＰａの成形圧力で加圧して成形体を得る。成形体は一軸加圧して作製してもよいし、ＣＩＰによって作製してもよい。また、ホットプレスによって成形しながら焼成してもよい。成形体の焼成は、窒素ガス又はアルゴンガス等の不活性ガス雰囲気中で行ってよい。焼成時の圧力は、０．７～０．９ＭＰａであってよい。焼成温度は１８６０～２１００℃であってよく、１８８０～２０００℃であってもよい。当該焼成温度における焼成時間は６～２０時間であってよく、８～１６時間であってよい。焼成温度までの昇温速度は、例えば１．０～１０．０℃／時間であってよい。
　図２は、本実施形態の製造方法における、窒化ケイ素粒子を焼結する際の粒成長の様子を模式的に示す図である。β－ＳｉＮは、α－ＳｉＮよりも融点が高く、粒成長が遅い。このため、原料粉末に含まれる窒化ケイ素のα化率が低い場合、当該α化率が高い場合よりも、粒成長が緩やかに進む。このため、窒化ケイ素の粒成長の過程において、粒内に欠陥が残存することが抑制され、図２に示されるように結果的に欠陥が少ない窒化ケイ素焼結体が得られる。このような窒化ケイ素焼結体は、高い熱伝導率及び高い破壊靭性を有する。
　図６は、従来の製造方法における、窒化ケイ素粒子を焼結する際の粒成長の様子を模式的に示す図である。図６では、図２よりも原料粉末における窒化ケイ素のα化率が高くなっている。このため、窒化ケイ素の粒成長の過程では、α－ＳｉＮがβ－ＳｉＮの粒成長を補完するようにして進行するため、窒化ケイ素の粒成長速度が速くなる。その結果、最終的に得られる窒化ケイ素焼結体において粒内に残存する欠陥の数が図２の場合よりも多くなる。このような窒化ケイ素焼結体は、図２で得られる窒化ケイ素焼結体よりも熱伝導率及び破壊靭性が低い。
　窒化ケイ素焼結体に含まれる欠陥としては、転位等の格子欠陥及び気孔等が考えられる。本実施形態の窒化ケイ素焼結体における気孔率は、例えば１．０体積％以下であってよく、０．５体積％以下であってもよい。
　図３は、焼成温度による窒化ケイ素焼結体の相対密度の変化の一例を示すグラフである。図３中、曲線１は、α化率が１５質量％の窒化ケイ素を含む原料粉末を用いたときの相対密度の変化を示している。一方、曲線２は、α化率が９３質量％の窒化ケイ素を含む原料粉末を用いたときの相対密度の変化を示している。図３に示すように、α化率が低い原料粉末を用いた場合は、相対密度の上昇が緩やかになっている。これは、窒化ケイ素の粒成長速度が遅いことを示している。このようにして得られる窒化ケイ素焼結体は、欠陥の数が十分に低減されている。また、β－ＳｉＮの結晶形状に起因して、サイズの大きい柱状の結晶粒で構成されることとなる。これらの要因によって、高い熱伝導率と高い破壊靭性を有すると考えられる。
　本実施形態の窒化ケイ素焼結体は、高い熱伝導率を有することから放熱性に優れる。また、高い破壊靭性を有することから、高温と低温とが繰り返されるいわゆる熱サイクルを受ける使用環境下においても破断することなく長期間に亘って安定的に使用することができる。このため、信頼性に優れる。このように、優れた放熱性と優れた信頼性を兼ね備えることから、パワーモジュールの基板として好適に用いることができる。
　以上、幾つかの実施形態を説明したが、本開示は上述の実施形態に何ら限定されるものではない。
　実施例及び比較例を参照して本開示の内容をより詳細に説明するが、本開示は下記の実施例に限定されるものではない。
（実施例１）
＜窒化ケイ素焼結体の作製＞
　α化率が１５質量％である窒化ケイ素粉末（Ｓｔａｒｃｋ社製）を準備した。この窒化ケイ素粉末と焼結助剤であるＭｇＯ、Ｙ２Ｏ３を、Ｓｉ３Ｎ４：ＭｇＯ：Ｙ２Ｏ３＝９５．２：１．５：３．３（質量比）で配合して原料粉末を得た。この原料粉末を、６．０ＭＰａの圧力で一軸加圧成形し、円柱形状の成形体を作製した。
　この成形体を、カーボンヒータを備える電気炉中に配置し、窒素ガスの雰囲気下（圧力：０．８８ＭＰａ）、昇温速度２．１℃／時間で１９００℃まで昇温した。１９００℃の焼成温度で１２時間焼成を行った後、約５．０℃／時間の降温速度で冷却し、窒化ケイ素焼結体を得た。窒化ケイ素焼結体における窒化ケイ素含有量は９２モル％であった。
＜窒化ケイ素焼結体の評価＞
　窒化ケイ素焼結体の熱伝導率（２０℃）は、ＪＩＳ　Ｒ　１６１１：２０１０に準拠して測定した。熱拡散率Ａ［ｍ２／秒］は、縦×横×厚さ＝１０ｍｍ×１０ｍｍ×２ｍｍのサイズの試料を用い、レーザーフラッシュ法によって求めた。測定装置としては、Ｎｅｔｚｃｈ製（装置名：ＬＦＡ４４７）を用いた。密度Ｂはアルキメデス法によって測定し、比熱Ｃは示差熱分析によって求めた。Ａ×Ｂ×Ｃの計算式で熱伝導率を算出した。その結果は表１に示すとおりであった。
　熱拡散率Ａを各温度において測定したこと、及び、比熱Ｃとして文献値を用いたこと以外は、２０℃における熱伝導率と同様にして５０～２００℃における熱伝導率を測定した。これらの結果を図４にプロットした。図４には、２０℃の熱伝導率も併せてプロットした。図４に示すとおり、温度が高くなるにつれて、熱伝導率は低下することが確認された。しかしながら、実施例１の窒化ケイ素焼結体の熱伝導率は、いずれの温度においても比較例１よりも高いことが確認された。
　破壊靭性（ＫＩＣ）は、ＳＥＰＢ法によって測定される値であり、ＪＩＳ　Ｒ１６０７：２０１５に準拠し、市販の測定装置（インストロン製、装置名：万能試験機５５８２型）を用いて測定した。その結果は表１に示すとおりであった。
　抗折強度は、３点曲げ抗折強度であり、ＪＩＳ　Ｒ　１６０１：２００８に準拠して市販の抗折強度計（島津製作所製、装置名：ＡＧ－２０００）を用いて測定した。その結果は表１に示すとおりであった。
　絶縁破壊強度の測定は、ＪＩＳ　Ｃ－２１１０：２０１６に準拠し、市販の測定装置（計測技術研究所製、装置名：７４７４型）を用いて測定した。その結果は表１に示すとおりであった。
　窒化ケイ素焼結体の破断面を走査型電子顕微鏡（ＳＥＭ）で観察した。図５は、ＳＥＭによる観察画像の写真である（倍率：３０００倍）。柱状の結晶の割合が高かった。
（比較例１）
　α化率が９３質量％である窒化ケイ素粉末を用いたこと、及び、１８５０℃の焼成温度で４時間焼成したこと以外は、実施例１と同様にして窒化ケイ素焼結体を得た。得られた窒化ケイ素焼結体を実施例１と同様にして評価した。評価結果は表１及び図４に示すとおりであった。
　窒化ケイ素焼結体の破断面を走査型電子顕微鏡（ＳＥＭ）で観察した。図７は、ＳＥＭによる観察画像の写真である（倍率：３０００倍）。柱状の結晶の割合が図５よりも少なく、結晶粒のサイズも図５よりも小さかった。
Figure JPOXMLDOC01-appb-T000001
（実施例２～４）
　表２に示すとおり、α化率が１０～２５質量％である窒化ケイ素粉末をそれぞれ用いたこと以外は、実施例１と同様にして窒化ケイ素焼結体を得た。得られた窒化ケイ素焼結体を実施例１と同様にして評価した。評価結果は表２に示すとおりであった。
Figure JPOXMLDOC01-appb-T000002
（比較例２～５）
　表３に示すとおり、α化率が４６～９０質量％である窒化ケイ素粉末をそれぞれ用いたこと以外は、実施例１と同様にして窒化ケイ素焼結体を得た。得られた窒化ケイ素焼結体を実施例と同様にして評価した。評価結果は表３に示すとおりであった。
Figure JPOXMLDOC01-appb-T000003
　表１～表３に示すとおり、原料粉末に用いた窒化ケイ素粉末のα化率を低くすることによって、熱伝導率を高くできることが確認された。
　本開示によれば、優れた放熱性と高い信頼性を兼ね備える窒化ケイ素焼結体、及びその製造方法を提供することができる。また、本開示によれば、このような窒化ケイ素焼結体を備える積層体を提供することができる。また、本開示によれば、上記積層体を備える積層体を提供することができる。
　１０…基板（窒化ケイ素焼結体）、１１，１２…金属層、３０…充填材、３３…電極、２０…ベース板、２２…冷却フィン、２３…ネジ、２４…グリース、３１，３２，３５…ハンダ層、３４…金属ワイヤ、３６…筐体、５０…積層体、６０…半導体素子、１００…パワーモジュール。

Claims (8)

  1. 　窒化ケイ素を含む原料粉末を成形して焼成する工程を有し、
    　前記原料粉末に含まれる前記窒化ケイ素のα化率が３０質量％以下である、窒化ケイ素焼結体の製造方法。
  2. 　前記窒化ケイ素焼結体の熱伝導率（２０℃）は１００Ｗ／ｍ・Ｋを超え、破壊靭性（ＫＩＣ）は７．４ＭＰａ・ｍ１／２以上である、請求項１に記載の製造方法。
  3. 　前記窒化ケイ素焼結体の抗折強度は６００ＭＰａを超える、請求項１又は２に記載の製造方法。
  4. 　熱伝導率（２０℃）が１００Ｗ／ｍ・Ｋを超え、破壊靭性（ＫＩＣ）が７．４ＭＰａ・ｍ１／２以上である、窒化ケイ素焼結体。
  5. 　抗折強度が６００ＭＰａを超える、請求項４に記載の窒化ケイ素焼結体。
  6. 　１５０～２００℃における熱伝導率が６０Ｗ／ｍ・Ｋを超える、請求項４又は５に記載の窒化ケイ素焼結体。
  7. 　第１の金属で構成される金属層と、
    　第１の金属よりも高い熱伝導率を有する第２の金属で構成される放熱部と、
    　前記金属層と前記放熱部の間に設けられ、請求項４～６のいずれか一項に記載の窒化ケイ素焼結体で構成される基板と、を有する、積層体。
  8. 　請求項７に記載の積層体と、
    　前記金属層と電気的に接続される半導体素子と、を備えるパワーモジュール。
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