Patent Description:
Power modules that control a large current are used in fields related to automobiles, electric railways, industrial equipment, power generation, and the like. Ceramic substrates are used as insulating substrates mounted on power modules. In such applications, ceramic substrates are required to have favorable heat dissipation characteristics in addition to insulation property. For example, in Patent Literature <NUM>, ceramic substrates made of a material containing aluminum nitride, alumina, silicon nitride, or silicon carbide as a main component have been proposed.

In addition, in Patent Literature <NUM>, technology in which rare earth elements and Mg are added as components of a sintering aid to perform heat treatment under predetermined conditions and a silicon nitride sintered body having a thermal conductivity of greater than or equal to <NUM> W/(m·K) at normal temperature is produced has been proposed.

Since power modules are important parts that control various devices, it is required for the power modules to stably function. In order to improve the stability, not only is it necessary for each member constituting a power module to be reliable, but it is also necessary to reduce the amount of heat generation due to large currents. In order to satisfy such requirements, it is thought that it would be useful to use ceramic substrates which allow excellent heat dissipation and do not easily break even with a large number of heat cycles. However, in ceramic substrates in the related art, both excellent heat dissipation and high reliability are unlikely to be achieved.

In the present disclosure, a silicon nitride sintered body having both excellent heat dissipation and high reliability, and a method for producing the same are provided. In addition, in the present disclosure, a multilayer body including such a silicon nitride sintered body is provided. In addition, in the present disclosure, a power module including the above-described multilayer body is provided.

A method for producing a silicon nitride sintered body according to an aspect of the present disclosure includes a step of molding a raw material powder containing silicon nitride and an oxide-based sintering aid to obtain a molded body, wherein a content of the oxide-based sintering aid in the raw material powder is <NUM> mass % to <NUM> mass%. An α-conversion rate of the silicon nitride contained in the raw material powder is <NUM> mass% to <NUM> mass%, and the silicon nitride sintered body has a fracture toughness KIC of greater than <NUM> MPa·m<NUM>/<NUM>. The method further include the step of firing the molded body at a firing temperature of <NUM>,<NUM> to <NUM>,<NUM> for <NUM> to <NUM> hours, the temperature increase rate to the firing temperature being <NUM>/hour to <NUM>/hour If the α-conversion rate of the silicon nitride of the raw material powder is high, the grain growth rate in the firing step tends to be high. In this case, it is thought that, although sintering is promoted, the number of defects remaining in an obtained sintered body increases.

Whereas, if the α-conversion rate of the silicon nitride of the raw material powder is low, a β-conversion rate of the raw material powder tends to be high and the grain growth rate in the firing step tends to be low. In this case, it is thought that, although it takes time for sintering, the number of defects included in an obtained sintered body decreases, whereby thermal conductivity and fracture toughness improve. Regarding the silicon nitride sintered body obtained through the above-described production method, the silicon nitride sintered body having both excellent heat dissipation and high reliability due to such an action can be produced.

The silicon nitride sintered body obtained through the above-described production method has a thermal conductivity (at <NUM>) of greater than <NUM> W/m·K and a fracture toughness (KIC) of greater than or equal to <NUM> MPa·m<NUM>/<NUM>. In addition, a transverse strength of the silicon nitride sintered body obtained through the above-described production method may exceed <NUM> MPa. Due to such characteristics, the heat dissipation and the reliability of the silicon nitride sintered body are further improved, and the silicon nitride sintered body can be more suitably used, for example, for a substrate of a power module.

A silicon nitride sintered body obtained according to the present method has a thermal conductivity (at <NUM>) exceeding <NUM> W/m·K and a fracture toughness (KIC) of greater than or equal to <NUM> MPa·m<NUM>/<NUM>. Since this silicon nitride sintered body has excellent thermal conductivity and fracture toughness, it allows excellent heat dissipation and has high reliability. For this reason, the silicon nitride sintered body can be suitably used, for example, for a substrate of a power module.

In the above-described silicon nitride sintered body, a transverse strength may exceed <NUM> MPa. Due to such characteristics, the reliability of the silicon nitride sintered body is further improved, and the silicon nitride sintered body can be more suitably used, for example, for a substrate of a power module.

The thermal conductivity of the above-described silicon nitride sintered body at <NUM> to <NUM> may exceed <NUM> W/m·K. Accordingly, the silicon nitride sintered body can be more suitably used for a substrate of a power module to be used under particularly severe conditions.

A multilayer body may include: a metal layer made of a first metal; a heat dissipation portion made of a second metal having a thermal conductivity higher than that of the first metal; and a substrate which is provided between the metal layer and the heat dissipation portion and made of any of the above-described silicon nitride sintered bodies. The substrate of this multilayer body is made of a silicon nitride sintered body having both excellent heat dissipation and high reliability. This substrate is provided between the metal layer and the heat dissipation portion. For this reason, heat generated on the metal layer side can be efficiently radiated from the heat dissipation portion side. Accordingly, the substrate can be suitably used, for example, in a multilayer body for a power module.

A power module may include: the above-described multilayer body; and a semiconductor element electrically connected to the metal layer. Since such a power module includes the above-described substrate, it has excellent heat dissipation and reliability.

According to the present disclosure, a method for producing a silicon nitride sintered body having both excellent heat dissipation and high reliability can be provided. In addition, a multilayer body including such a silicon nitride sintered body can be obtained. In addition, a power module including the above-described multilayer body can be obtained.

Hereinafter, one embodiment of the present invention will be described with reference to the drawings according to circumstances. However, the following embodiments are merely examples for describing the present invention and are not intended to limit the present invention to the following contents. In the description, the same reference numerals are given to the same elements or elements having the same function, and description thereof will not be repeated according to circumstances. The dimensional ratios of elements are not limited to the ratios shown in the drawings.

A silicon nitride sintered body has a thermal conductivity exceeding <NUM> W/m·K at <NUM>. The thermal conductivity (at <NUM>) of the present disclosure can be measured according to JIS R <NUM>:<NUM>. This thermal conductivity (at <NUM>) is calculated by a calculation equation of A × B × C from values of a thermal diffusivity A [m<NUM>/second], a density B [kg/m<NUM>], and a specific heat C [J/(kg·K)]. The thermal diffusivity A is obtained through a laser flash method using a sample having a size of length × width × thickness = <NUM> × <NUM> × <NUM>. Specifically, the thermal diffusivity is obtained by an equation of A = <NUM> × (thickness [mm])<NUM>/t<NUM>/<NUM>. t<NUM>/<NUM> is the time [seconds] required for increasing the temperature to half of ΔT when the total temperature rise width is set to ΔT. The density B is obtained through an Archimedes method. The specific heat C is obtained through differential thermal analysis.

The thermal conductivity (at <NUM>) of the silicon nitride sintered body may be, for example, greater than <NUM> W/m·K, greater than <NUM> W/m·K, or greater than <NUM> W/m·K from the viewpoint of further improving the heat dissipation. The upper limit of the thermal conductivity (at <NUM>) may be, for example, <NUM> W/m·K from the viewpoint of easiness of the production.

For example, the thermal conductivity of the silicon nitride sintered body at <NUM> to <NUM> may be greater than <NUM> W/m·K or greater than <NUM> W/m·K from the viewpoint of sufficiently increasing the heat dissipation when the silicon nitride sintered body is used in a power module or the like. The upper limit of the thermal conductivity at <NUM> to <NUM> may be, for example, <NUM> W/m·K from the viewpoint of easiness of the production. The thermal conductivity in such a temperature range can also be obtained by the calculation equation of A × B × C as described above. At this time, the thermal diffusivity A may be a measurement value obtained by performing the above-described measurement at the temperature, and the specific heat C may be a literature value. A value at <NUM> can be used as the density B as it is.

The silicon nitride sintered body has a fracture toughness (KIC) of greater than <NUM> MPa·m<NUM>/<NUM>. The fracture toughness (KIC) is a value measured through an SEPB method and is measured according to JIS R <NUM>:<NUM>. The fracture toughness (KIC) of the silicon nitride sintered body is greater than <NUM> MPa·m<NUM>/<NUM> from the viewpoint of further improving reliability. The upper limit of the fracture toughness (KIC) may be, for example, <NUM> MPa·m<NUM>/<NUM> from the viewpoint of easiness of the production.

The silicon nitride sintered body may have a transverse strength of greater than <NUM> MPa from the viewpoint of further improving reliability. The transverse strength is a three-point bending transverse strength and can be measured with a commercially available transverse strength meter according to JIS R <NUM>:<NUM>. The transverse strength of the silicon nitride sintered body may exceed <NUM> MPa or exceed <NUM> MPa from the viewpoint of further improving reliability. The upper limit of the transverse strength may be, for example, <NUM> MPa from the viewpoint of easiness of the production.

The silicon nitride sintered body may substantially consist of only silicon nitride, or may contain components derived from a sintering aid and unavoidable components derived from raw materials, production processes, and the like. The content of silicon nitride in the silicon nitride sintered body may be, for example, greater than or equal to <NUM> mol%, greater than or equal to <NUM> mol%, or greater than or equal to <NUM> mol% from the viewpoint of achieving both high thermal conductivity and excellent insulation properties at a high level.

Since the amount of sintering aid to be used can be reduced to produce the silicon nitride sintered body, the total content of rare earth elements in the silicon nitride sintered body can be sufficiently reduced. The total content of rare earth elements in the silicon nitride sintered body may be less than or equal to <NUM> mass% or less than or equal to <NUM> mass%. <NUM> elements in total including the <NUM> elements of scandium (Sc) and yttrium (Y) and the <NUM> lanthanoid elements from lanthanum (La) to lutetium (Lu) correspond to the rare earth elements.

The silicon nitride sintered body may have a dielectric fracture strength of greater than or equal to <NUM> [kV/<NUM>]. The dielectric fracture strength can be measured according to JIS C-<NUM>:<NUM>. The dielectric fracture strength may be, for example, greater than or equal to <NUM> [kV/<NUM>]. The upper limit of the dielectric fracture strength may be, for example, <NUM> [kV/<NUM>] from the viewpoint of easiness of the production.

<FIG> is a schematic cross-sectional view of a power module. A power module <NUM> includes a metal layer <NUM>, a substrate <NUM> (a silicon nitride sintered body <NUM>), a metal layer <NUM>, a solder layer <NUM>, a base plate <NUM>, and a cooling fin <NUM> in this order. The metal layer <NUM> constitutes a metal circuit through, for example, etching. A semiconductor element <NUM> is attached to the metal layer <NUM> via a solder layer <NUM>. The semiconductor element <NUM> is connected to a predetermined portion of the metal layer <NUM> through a metal wire <NUM> such as an aluminum wire.

The substrate <NUM> is made of the silicon nitride sintered body. Accordingly, the metal layer <NUM> and the metal layer <NUM> are electrically insulated from each other. The metal layer <NUM> may or may not form an electrical circuit. The materials of the metal layer <NUM> and the metal layer <NUM> may be the same as or different from each other. The metal layer <NUM> and the metal layer <NUM> may be made of copper corresponding to a first metal. However, the materials thereof are not limited to copper.

The metal layer <NUM> is joined to the base plate <NUM> via the solder layer <NUM>. The shape of the base plate <NUM> may be, for example, a substantially rectangular plate shape with length × width × thickness = <NUM> to <NUM> × <NUM> to <NUM> × <NUM> to <NUM>. A screw <NUM> for fixing the cooling fin <NUM> forming a heat dissipation member to the base plated <NUM> is provided at an end portion of the base plate. The base plate <NUM> may be made of aluminum corresponding to a second metal. The base plate <NUM> is joined to the cooling fin <NUM> via grease <NUM> on a side opposite to the metal layer <NUM> side.

The base plate <NUM> and the cooling fin <NUM> function as heat dissipation portions since these are made of the second metal having a thermal conductivity higher than that of the metal layer <NUM>. Since the substrate <NUM> has a high thermal conductivity, the semiconductor element <NUM>, the metal layer <NUM>, and the metal layer <NUM> are efficiently cooled by the heat dissipation portions.

A resin case <NUM> is attached to one surface side (a side on which the semiconductor element <NUM> is installed) of the base plate <NUM> so as to accommodate the above-described members (the semiconductor element <NUM>, the metal layers <NUM> and <NUM>, and the substrate <NUM>). The above-described members are accommodated in the space formed by the one surface of the base plate <NUM> and the case <NUM>, and the space is filled with a filler <NUM> such as silicone gel so as to fill the gap. In order to electrically connect the exterior of the case <NUM> and the metal layer <NUM>, a predetermined portion of the metal layer <NUM> is connected to an electrode <NUM> provided through the case <NUM> via a solder layer <NUM>.

The power module <NUM> includes a multilayer body <NUM> consisting of the metal layer <NUM>, the substrate <NUM>, the metal layer <NUM>, the base plate <NUM>, and the cooling fin <NUM>. The substrate <NUM> is made of the silicon nitride sintered body having both the high thermal conductivity and a high fracture toughness. Since the substrate <NUM> has a high thermal conductivity, heat can be smoothly radiated from the semiconductor element <NUM> and the metal layer <NUM> to the substrate <NUM> and the cooling fin <NUM>. In addition, the silicon nitride sintered body is not easily broken even if it receives an impact. For this reason, the power module <NUM> can stably exhibit its performance and has excellent reliability. In this manner, the silicon nitride sintered body is suitably used for the substrate of the power module <NUM>. However, use of the silicon nitride sintered body <NUM> is not limited to a power module.

The material of the base plate <NUM> is not limited to aluminum. For example, the base plate <NUM> may be made of the first metal (for example, copper), and only the cooling fin may be made of the second metal (such as aluminum). In this case, only the cooling fin <NUM> functions as a heat dissipation portion. In addition, there may be no cooling fin <NUM> and only the base plate <NUM> may function as a heat dissipation portion. Furthermore, there may be no metal layer <NUM> and the base plate <NUM> and the substrate <NUM> may be joined to each other.

The method for producing a silicon nitride sintered body will be described below. The method for producing the silicon nitride sintered body according to one embodiment includes a step of molding and firing a raw material powder containing silicon nitride. An α-conversion rate of silicon nitride contained in a raw material powder to be used is <NUM> mass% to <NUM> mass%. Accordingly, the grain growth rate of silicon nitride in the firing step can be slowed down. Accordingly, although it takes time for sintering, the number of defects remaining in the silicon nitride sintered body to be obtained can be decreased.

The α-conversion rate of silicon nitride contained in the raw material powder is less than or equal to <NUM> mass% or may be less than or equal to <NUM> mass% from the viewpoint of further increasing the thermal conductivity. The α-conversion rate of silicon nitride contained in the raw material powder is greater than or equal to <NUM> mass% from the viewpoint of increasing the transverse strength of the silicon nitride sintered body.

The raw material powder contains an oxide-based sintering aid in addition to silicon nitride. Examples of the oxide-based sintering aid include Y<NUM>O<NUM>, MgO, and Al<NUM>O<NUM>. The content of the oxide-based sintering aid in the raw material powder is <NUM> to <NUM> mass% or may be <NUM> to <NUM> mass% from the viewpoint of obtaining the silicon nitride sintered body capable of achieving both the high thermal conductivity and excellent insulation properties at a high level.

The above-described raw material powder is pressurized at, for example, a molding pressure of <NUM> to <NUM> MPa to obtain a molded body. The molded body may be produced through uniaxial pressurization or may be produced through CIP. In addition, the firing may be performed while performing molding through hot pressing. The firing of a molded body may be performed in an inert gas atmosphere of nitrogen gas, argon gas, or the like. The pressure during firing may be <NUM> to <NUM> MPa. The firing temperature is <NUM>,<NUM> to <NUM>,<NUM> or may be <NUM>,<NUM> to <NUM>,<NUM>. The firing time at the firing temperature is <NUM> to <NUM> hours or may be <NUM> to <NUM> hours. The temperature increase rate to the firing temperature is <NUM>/hour to <NUM>/hour.

<FIG> is a view schematically illustrating a state of grain growth in the production method of the present embodiment when sintering silicon nitride particles. A melting point of β-SiN is higher than that of α-SiN and grain growth of β-SiN is slower than that of α-SiN. For this reason, the grain growth in a case where the α-conversion rate of silicon nitride contained in the raw material powder is low more slowly proceeds than a case where the α-conversion rate is high. For this reason, the number of defects remaining in grains is controlled in the process of grain growth of silicon nitride, and as a result, the silicon nitride sintered body having few defects is obtained as shown in <FIG>. Such a silicon nitride sintered body has a high thermal conductivity and a high fracture toughness.

<FIG> is a view schematically illustrating a state of grain growth in a production method in the related art when sintering silicon nitride particles. In <FIG>, the α-conversion rate of silicon nitride in a raw material powder is higher than that of <FIG>. For this reason, α-SiN proceeds so as to complement grain growth of β-SiN in the process of grain growth of silicon nitride, and therefore, the grain growth rate of silicon nitride increases. As a result, the number of defects remaining in grains in the finally obtained the silicon nitride sintered body is larger than that of the case of <FIG>. Such a silicon nitride sintered body has a lower thermal conductivity and fracture toughness than those of the silicon nitride sintered body obtained in <FIG>.

Pores, lattice defects such as dislocations, and the like can be considered as the defects contained in the silicon nitride sintered body. The porosity of the silicon nitride sintered body may be, for example, less than or equal to <NUM> volume% or less than or equal to <NUM> volume%.

<FIG> is a graph illustrating an example of change in relative density of the silicon nitride sintered body depending on the firing temperature. In <FIG>, a curve <NUM> shows change in relative density when a raw material powder containing silicon nitride having an α-conversion rate of <NUM> mass% is used. On the other hand, a curve <NUM> shows change in relative density when a raw material powder containing silicon nitride having an α-conversion rate of <NUM> mass% is used. As shown in <FIG>, in the case where a raw material powder having a low α-conversion rate is used, the relative density increases slowly. This shows that the grain growth rate of silicon nitride is low. The silicon nitride sintered body obtained in this manner has a sufficiently reduced number of defects. In addition, the silicon nitride sintered body consists of large columnar crystal grains due to the crystal shape of β-SiN. It is thought that the silicon nitride sintered body has a high thermal conductivity and a high fracture toughness due to these factors.

Since the silicon nitride sintered body has a high thermal conductivity, it has an excellent heat dissipation. In addition, since the silicon nitride sintered body has a high fracture toughness, it can be stably used for a long period of time without a fracture even in a use environment subjected to a so-called heat cycle in which a high temperature and a low temperature are repeated. For this reason, the silicon nitride sintered body has excellent reliability. In this manner, since the silicon nitride sintered body has both excellent heat dissipation and excellent reliability, it can be suitably used as a substrate of a power module.

Some embodiments have been described above, but the present disclosure is not limited to any of the above-described embodiments.

The contents of the present disclosure will be described in more detail with reference to examples and comparative examples, but the present disclosure is not limited to the following examples.

A silicon nitride powder (manufactured by Starck) having an α-conversion rate of <NUM> mass% was prepared. This silicon nitride powder and MgO and Y<NUM>O<NUM> which were sintering aids were formulated at a ratio of Si<NUM>N<NUM>:MgO:Y<NUM>O<NUM>=<NUM>:<NUM>:<NUM> (mass ratio) to obtain a raw material powder. This raw material powder was uniaxially pressed and molded at a pressure of <NUM> MPa to produce a columnar molded body.

This molded body was placed in an electric furnace equipped with a carbon heater, and the temperature was raised to <NUM>,<NUM> at a rate of temperature increase of <NUM>/hour in a nitrogen gas atmosphere (pressure: <NUM> MPa). After firing was performed at a firing temperature of <NUM>,<NUM> for <NUM> hours, the molded body was cooled at a temperature lowering rate of about <NUM>/hour to obtain a silicon nitride sintered body. The content of silicon nitride in the silicon nitride sintered body was <NUM> mol%.

The thermal conductivity (at <NUM>) of the silicon nitride sintered body was measured according to JIS R <NUM>:<NUM>. A thermal diffusivity A [m<NUM>/second] was obtained through a laser flash method using a sample having a size of length × width × thickness = <NUM> × <NUM> × <NUM>. A measurement device (device name: LFA447) manufactured by Netzch was used. A density B was measured through an Archimedes method, and a specific heat C was obtained through differential thermal analysis. The thermal conductivity was calculated by a calculation equation of A × B × C. The results are as shown in Table <NUM>.

The thermal conductivity at <NUM> to <NUM> was measured in the same manner as the thermal conductivity at <NUM> except that the thermal diffusivity A was measured at each temperature and a literature value was used as the specific heat C. These results are plotted in <FIG>. The thermal conductivity at <NUM> is also concurrently plotted in <FIG>. As shown in <FIG>, 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 <NUM> was higher than that of Comparative Example <NUM> at any temperature.

A fracture toughness (KIC) was a value measured through an SEPB method and was measured with a commercially available measurement device (manufactured by Instron, device name: Universal Testing Systems Type <NUM>) according to JIS R <NUM>:<NUM>. The results are as shown in Table <NUM>.

A transverse strength was a three-point bending transverse strength and was measured with a commercially available transverse strength meter (manufactured by Shimadzu Corporation, device name: AG-<NUM>) according to JIS R <NUM>:<NUM>. The results are as shown in Table <NUM>.

A dielectric fracture strength was measured using a commercially available measurement device (manufactured by Keisoku Giken Co. , device name: <NUM> type) according to JIS C-<NUM>:<NUM>. The results are as shown in Table <NUM>.

A fracture surface of the silicon nitride sintered body was observed with a scanning electron microscope (SEM). <FIG> is a photograph (magnification: <NUM>,<NUM> times) of the observation image with the SEM. The proportion of columnar crystals was high.

A silicon nitride sintered body was obtained in the same manner as in Example <NUM> except that a silicon nitride powder having an α-conversion rate of <NUM> mass% was used and fired at a firing temperature of <NUM>,<NUM> for <NUM> hours. The obtained silicon nitride sintered body was evaluated in the same manner as in Example <NUM>. The evaluation results are as shown in Table <NUM> and <FIG>.

A fracture surface of the silicon nitride sintered body was observed with a scanning electron microscope (SEM). <FIG> is a photograph (magnification: <NUM>,<NUM> times) of the observation image with the SEM. The proportion of columnar crystals was smaller than that in <FIG>, and the size of crystal grains was also smaller than that in <FIG>.

Silicon nitride sintered bodies were obtained as shown in Table <NUM> in the same manner as in Example <NUM> except that silicon nitride powders having an α-conversion rate of <NUM> to <NUM> mass% were used. The obtained silicon nitride sintered bodies were evaluated in the same manner as in Example <NUM>. The evaluation results are as shown in Table <NUM>.

Silicon nitride sintered bodies were obtained as shown in Table <NUM> in the same manner as in Example <NUM> except that silicon nitride powders having an α-conversion rate of <NUM> to <NUM> mass% were used. The obtained silicon nitride sintered bodies were evaluated in the same manners as in the examples. The evaluation results are as shown in Table <NUM>.

As shown in Tables <NUM> to <NUM>, it was confirmed that the thermal conductivity can be increased by lowering the α-conversion rate of a silicon nitride powder used in a raw material powder.

According to the present disclosure, a silicon nitride sintered body having both excellent heat dissipation and high reliability, and a method for producing the same can be provided. In addition, according to the present disclosure, a multilayer body including such a silicon nitride sintered body can be provided. In addition, according to the present disclosure, a power module including the above-described multilayer body can be provided.

Claim 1:
A method for producing a silicon nitride sintered body, the method comprising:
a step of molding a raw material powder containing silicon nitride and an oxide-based sintering aid to obtain a molded body, and
a step of firing the molded body at a firing temperature of <NUM>,<NUM> to <NUM>,<NUM> for <NUM> to <NUM> hours, the temperature increase rate to the firing temperature being <NUM>/hour to <NUM>/hour,
wherein an α-conversion rate of the silicon nitride contained in the raw material powder is <NUM> mass% to <NUM> mass%,
wherein a content of the oxide-based sintering aid in the raw material powder is <NUM> mass % to <NUM> mass%,
wherein the silicon nitride sintered body has a fracture toughness KIC of greater than <NUM> MPa·m<NUM>/<NUM>, and
wherein the fracture toughness KIC is measured through an SEPB method according to JIS R <NUM>:<NUM>.