JP4997431B2 - Method for producing high thermal conductivity silicon nitride substrate - Google Patents

Description

  The present invention relates to a high thermal conductivity silicon nitride substrate that has a function of releasing power generated by mounting a power semiconductor and is suitable for a power module substrate that requires high reliability, and a method of manufacturing the same.

  When considering a heat dissipation material as a structural member, the most common metal material cannot be used without cooling or the like under conditions exceeding 500 ° C. Furthermore, these metal materials are inferior in corrosion resistance and oxidation resistance compared to ceramics. Moreover, since it is a conductor, it is difficult to use it as an insulating substrate material that requires high heat dissipation, such as a high-density mounting substrate.

  On the other hand, ceramic materials such as an aluminum nitride sintered body and a silicon carbide sintered body have both high insulating properties and high thermal conductivity, and thus have been partially used as heat dissipation substrate materials. However, these high thermal conductive ceramics have low strength and toughness and lack mechanical reliability, so their uses are very limited.

  A silicon nitride sintered body is known as an excellent structural ceramic material having both high strength and high toughness. Furthermore, the theoretical thermal conductivity of silicon nitride crystals is expected to be 200 W / mK or more due to the similarity in crystal structure with silicon carbide and aluminum nitride. However, in a general silicon nitride sintered body, impurity oxygen is dissolved in silicon nitride particles, so that phonons responsible for heat conduction are scattered, and the thermal conductivity is about several tens of W / mK. .

  In order to develop a high thermal conductivity for the silicon nitride sintered body, it is necessary to reduce the dissolved oxygen in the silicon nitride particles during sintering (see Non-Patent Document 1).

  Since silicon nitride has a strong covalent bond and a low diffusion coefficient, an oxide is generally added as a sintering aid during the sintering. The added sintering aid reacts with impurity oxygen in the silicon nitride raw material to generate an oxynitride melt, and densification and grain growth proceed by the action of the generated liquid phase.

  When a rare earth oxide having a high affinity with oxygen is added as an auxiliary agent, a large amount of oxygen is trapped in the liquid phase, and the amount of dissolved oxygen inside the particles decreases as the silicon nitride particles grow. For this reason, rare earth oxides are important auxiliaries for achieving high thermal conductivity. However, when only the rare earth oxide is added, it is difficult to obtain a dense sintered body excellent in mechanical properties because the melting point of the liquid phase to be generated is high. For this reason, in order to obtain a silicon nitride sintered body that coexists with excellent mechanical properties and high thermal conductivity, various approaches have been taken with respect to process factors such as the type of additive, amount added, sintering temperature, and time. Has been done.

For example, Patent Document 1 discloses that a fine silicon nitride powder having an Al content of 0.1% by mass or less and an average particle size of 1 μm or less is coated with Mg, Ca, Sr, Ba, Y, La, Ce, Pr, Nd, Sm. , Gd, Dy, Ho, Er, Yb, 1 to 15% by mass of an oxide sintering aid of one or more elements selected from the elements, and then forming 0.1-50 MPa nitrogen It is possible to produce a silicon nitride sintered body having a fracture toughness of 7 MPam ^1/2 or more, a strength of 600 MPa or more, and a thermal conductivity of 80 W / mK or more by firing at a temperature of 1700 to 2300 ° C. under gas pressure. It is disclosed.

  Further, in Patent Document 2, after forming raw material powder in which one or more oxides of magnesium and yttrium and / or lanthanoid group elements are added in a total amount of 1.0 mass% or less to silicon nitride powder, the temperature is 1800 to 2000. C., nitrogen pressure 0.5 to 10 MPa, the powder for adjusting the firing atmosphere is fired using a mixed powder composed of silicon nitride, boron nitride and magnesium oxide, the size of the sintered body particles, inside the silicon nitride particles By controlling the amount of oxygen and the amount of the remaining auxiliary component, it is possible to produce a silicon nitride sintered body having a characteristic that the thermal conductivity at normal temperature is 90 W / mK or more and the three-point bending strength is 600 MPa or more. It is disclosed.

  Patent Document 3 discloses that silicon nitride having a β phase fraction of 30 to 100%, an oxygen content of 0.5% by mass or less, an average particle size of 0.2 to 10 μm, and an aspect ratio of 10 or less. 1 to 50 parts by mass of powder, 99 to 50 parts by mass of α-type silicon nitride powder having an average particle size of 0.2 to 4 μm, at least one selected from the group consisting of Mg, Y and rare earth elements (RE) And 1800 at a temperature increase rate of 5.0 ° C./min or less, after being held at a temperature of 1400 to 1600 ° C. for 1 to 10 hours in a nitrogen atmosphere of 0.5 MPa. A high-strength and high-heat-conductivity silicon nitride sintered body produced by a process of sintering at ˜1950 ° C. for 5 to 40 hours and having a thermal conductivity at room temperature of 100 W / mK or more and a three-point bending strength of 600 MPa or more Are listed.

  In preliminary experiments conducted by the present inventors, in the conventional sintering method, as the sintering time increases, the silicon nitride grain growth progresses and the thermal conductivity improves, but excessive grain growth occurs. Therefore, it has been found that the strength and fracture toughness are remarkably lowered as the thermal conductivity is improved.

For example, as shown in a comparative example described later (see Table 1), a silicon nitride molded body obtained by adding ₂ mol% Yb ₂ O ₃ and 5 mol% MgO to a silicon nitride fine powder having an average particle diameter of 0.2 μm is 1850 ° C. The sintered body fired for 12 hours has a thermal conductivity of 95 W / mK and a strength of 850 MPa, but when the same molded body is fired for 48 hours, the thermal conductivity is improved to 105 W / mK. The strength is drastically reduced to 300 MPa.

  As illustrated in the example above, the conventional method uses silicon nitride powder with controlled properties, optimizes process parameters such as the type, amount of additive, and sintering conditions of the sintering aid, and develops a predetermined microstructure. By doing so, a silicon nitride sintered body having a thermal conductivity of about 100 W / mK and a strength of 600 MPa or more can be produced. However, since expensive silicon nitride powder is used in any of the manufacturing methods, the price of the product increases, and when the thermal conductivity exceeds 100 W / mK, the strength and toughness are drastically lowered and the mechanical reliability is reduced. Becomes scarce. Thus, from the two viewpoints of symbiosis between product price, heat conduction, and mechanical properties, the high thermal conductivity silicon nitride obtained by the conventional sintering method has not been satisfactory.

  From the viewpoint of reducing the cost required for the raw material powder, as illustrated below, a so-called reactive sintering method is used in which an inexpensive silicon powder is used as the raw material powder, and the molded body is nitrided in nitrogen and then sintered at high temperature Development of high thermal conductivity silicon nitride materials used is underway. For example, in Patent Document 4, a Group 3a element compound of the Periodic Table is added at a ratio of 2 to 10 mol% in terms of oxides with respect to silicon or a mixed powder of silicon and silicon nitride, and the aluminum content is A mixed powder of 0 to 0.5% by mass in terms of oxide is molded, the molded body is heat treated in a nitrogen content of 800 to 1500 ° C., and the silicon is nitrided, and β-type silicon nitride is 10% or more. After producing the nitride to be contained, the nitride is fired under normal pressure containing nitrogen at 1400 to 1800 ° C., and the sintered body is further fired at 1800 to 1980 ° C. in an atmosphere having a nitrogen pressure of 0.15 MPa or more. And a sintered body structure in which the silicon nitride crystal has an average particle size of 2 μm or more, an average aspect ratio of 15 or less, and 5 or more silicon nitride particles having a length of 20 μm or more in an arbitrary 300 μm × 300 μm region. , There is a high strength, high toughness can be produced silicon nitride having both a high thermal conductivity.

  Patent Document 4 states that in order to improve the thermal conductivity of silicon nitride, it is important that the aluminum content is 0.5% or less. This is because aluminum is a solid solution in the silicon nitride crystal and causes phonons to scatter.

  However, as described in detail in Non-Patent Documents 1 and 2, the largest factor that inhibits the thermal conductivity of the silicon nitride sintered body is impurity oxygen. For this reason, the value 60-78 W / mK of the thermal conductivity achieved in this prior document which does not consider the influence of the amount of impurity oxygen is not sufficient as a heat radiating member.

  In Patent Document 5, the content of oxygen is 1% by mass or less in a process for producing a silicon nitride sintered body by reactive sintering of silicon under the concept that impurity oxygen greatly reduces thermal conductivity. 80 to 99% by mass of silicon powder and 1 to 20% by mass of oxide powder of at least one element of Y, Yb and Sm are mixed, and the compact is heated to a temperature of 1400 ° C. or less in a nitrogen atmosphere. It is disclosed that high thermal conductivity silicon nitride can be obtained by nitriding at 1700-1950 ° C. after nitriding.

  Further, in Patent Document 5, in order to reduce impurity oxygen, a reducing coating agent is added to silicon powder, and heat treatment is performed in a temperature range of 200 to 800 ° C. in a nitrogen-containing atmosphere in a vacuum of 13 kPa or less. It is shown.

Further, in Patent Document 6, silicon nitride, which is one of the phonon scattering factors, is obtained by using a raw material with few dislocations and not applying a pressure of 9.8 MPa or more to the powder and the molded body during the process. It is disclosed that high strength and high thermal conductivity can be achieved by setting the dislocation density in the particles to 10 μm / μm ³ or less.

  Further, in Patent Document 7, the first component other than silicon nitride includes at least one rare earth element, the second component includes at least one alkaline earth element, Li, and Sr, and has a molar ratio converted to an oxide. The first component is 0.95 to 7.7 mol% and the second component is 0.49 to 4.7 mol%, and the concentration of the first component is higher at the surface than at the center, and the concentration of the second component is at the surface It is disclosed that high strength and high thermal conductivity silicon nitride is produced by making the central portion relatively higher.

  As described above, many methods have been developed for the production of a silicon nitride sintered body having a high thermal conductivity exceeding 100 W / mK, also with respect to reactive sintering in which silicon powder can be used. However, in the conventional method, for example, in order to achieve high heat conduction, it is necessary to use silicon powder having an impurity oxygen amount of 1% or less, or to perform a reduction treatment by adding a reducing coating agent ( Use a silicon raw material with a low dislocation density and perform molding at a pressure of 9.8 MPa or less (see Patent Document 6). The composition differs between the outer peripheral portion and the inner peripheral portion of the sintered body. The actual situation is that there are many limitations on usable silicon raw material powders and applicable processes, such as performing control as described above (see Patent Document 7).

JP 09-030866 A JP 2002-293642 A JP 2003-313079 A Japanese Patent Application Laid-Open No. 11-100300 JP 11-314969 A JP 2000-169239 A JP 2000-272968 A Journal of the American Ceramic Society, “Thermal Conductivity of beta-Si3N4 II: Effect of Lattice Oxygen,” 83 [8] 1985-1992 (2000). Journal of the Ceramic Society of Japan, “Effect of oxygen in sintered body on thermal conductivity of silicon nitride”, 109 [12] 1046-1050 (2001)

  As described above, the production method of high thermal conductivity silicon nitride using the reactive sintering method of silicon developed so far greatly limits the available silicon raw material powder and production process, and the advantages of using this method That is, the advantage of obtaining a material having high characteristics by a process similar to the conventional silicon nitride sintering method using an inexpensive silicon powder has not been fully utilized.

  Under such circumstances, the present inventors, in view of the above prior art, use an inexpensive silicon powder and manufacture a material having high characteristics by a process similar to the conventional silicon nitride sintering method. As a result of intensive research aimed at the goal, the present invention has been completed.

  The object of the present invention is to manufacture a silicon nitride sintered body by reactive sintering using silicon nitriding reaction, ranging from low-grade silicon powder containing a large amount of impurity oxygen to high-grade silicon powder containing a small amount of impurity oxygen. Power semiconductors using a method of manufacturing a silicon nitride sintered body that can be used as a starting material, can be applied to conventional molding and firing processes, and has both excellent mechanical properties and high thermal conductivity It is an object of the present invention to provide a highly reliable and highly thermally conductive silicon nitride substrate that has an effect of releasing heat generated from the substrate and is optimal as a power module substrate or the like that requires high reliability.

The present invention relates to silicon powder or a mixed powder of silicon powder and silicon nitride powder, in a ratio when silicon is converted to silicon nitride, 0.5 to 7 mol% rare earth element oxide and 1 to 7 mol% magnesium. A mixture mixed with a mixture,
The magnesium mixture is at least one selected from magnesium oxide (MgO), magnesium silicide (Mg ₂ Si), and magnesium magnesium nitride (MgSiN ₂ );
Adjusting a mixture in which the total amount of impurity oxygen contained in the silicon powder or the mixed powder of silicon powder and silicon nitride powder and oxygen from the magnesium compound is 0.1 to 1.8% by mass,
The mixture is molded and nitrided, and the obtained nitride is heated in nitrogen of 0.1 MPa or more to be densified so that the relative density is 95% or more, and the obtained plate-like silicon nitride sintered body is obtained A metal plate is joined to at least one surface of the metal plate using a brazing material containing at least one metal element of magnesium, titanium, and zirconium.

  In the present invention, a silicon nitride sintered body having both high thermal conductivity, high strength, and high toughness is obtained, and by joining a metal plate to the sintered body, particularly high heat cycle resistance and high reliability. A high strength and high thermal conductivity silicon nitride substrate is obtained. Further, since the silicon nitride sintered body to be used is excellent in mechanical properties, it is possible to join a highly rigid metal plate or a thick metal plate regardless of the metal to be joined. Therefore, when used as a member for a power module, it is possible to adopt a structure in which a silicon nitride plate is directly bonded to the base plate of the power module, or the silicon nitride substrate is directly screwed to the housing without using the base plate. Therefore, there is a possibility that a new structure of the power module can be realized. As a result, the power module can be reduced in size, reduced in cost, and extended in life, so that it can be expected to make a significant industrial contribution, and through these, it can contribute to energy saving and global environmental conservation.

  Further, according to the production method of the present invention, various silicon raw powders can be used as a starting material, from low-grade silicon powder containing a large amount of impurity oxygen to high-grade silicon powder having a small amount of impurity oxygen.

  In the present invention, in order to produce a dense silicon nitride sintered body using the nitridation reaction of silicon powder, a silicon nitride sintering aid is added in advance to the silicon powder, and the mixture of the powder mixture is 1400 ° C. or lower. After heating in nitrogen to convert silicon into silicon nitride, it is necessary to perform densification by heating the resulting nitride in higher temperature nitrogen. For this reason, in the present invention, the design guideline for obtaining a silicon nitride sintered body having high thermal conductivity is basically the same as a general sintering method using silicon nitride as a starting material.

  As already stated, silicon nitride powder inevitably contains about several percent of impurity oxygen. In addition, since silicon nitride does not sinter itself, an oxide is added as a sintering aid when producing a silicon nitride sintered body. The presence of solid solution oxygen in the crystal and the presence of residual low thermal conductivity grain boundary phases are the theoretical characteristics of the sintered silicon nitride crystal, which is predicted to have a thermal conductivity of over 200 W / mK. This is a factor of significantly lowering the thermal conductivity.

  For this reason, in order to improve the thermal conductivity of the silicon nitride sintered body, (1) reducing the amount of dissolved oxygen in the silicon nitride crystal to lower the thermal conductivity of the crystal itself, (2) It is necessary and important to reduce the low thermal conductivity grain boundary phase remaining in the sintered body.

  Therefore, selection of this sintering aid is very important for achieving high thermal conductivity. For the former, rare earth element oxides having high affinity with oxygen and excellent ability to trap oxygen in the grain boundary phase are used as auxiliary agents. In order to satisfy the latter, the melting point of the melt produced during heating is lowered, contributing to densification in the early stage of sintering, and further, magnesium oxide and / or silicon magnesium nitride that evaporates during sintering at a high temperature. Preferably used.

  Like silicon nitride powder, silicon powder inevitably contains impurity oxygen. The amount of oxygen varies greatly depending on the properties of the silicon powder, but is generally in the range of about 0.2 mass% to several mass%. Compared to the conventional sintering method using silicon nitride powder, the method for producing a silicon nitride sintered body using reactive sintering of silicon powder has a great advantage from the viewpoint of reducing the amount of oxygen.

That is, specifically,
(1) With the nitriding reaction of 3Si + 2N ₂ = Si ₃ N ₄ , the sample weight increases by about 70%, so that the proportion of the amount of impurity oxygen relatively decreases.
(2) Since nitriding of Si powder compacts increases in weight without dimensional change, nitrides have a relative density that is more than 10% higher than compacts and can be easily densified in the post-sintering process. is there,
(3) This has advantages such as shortening the firing time and preventing excessive grain growth that adversely affects mechanical properties.

  As described above, the reaction sintering method has an excellent potential from the viewpoint of achieving high thermal conductivity. However, simply reducing the amount of impurity oxygen in the silicon powder raw material will increase the raw material cost as the raw material is highly purified, the usable raw materials are limited, and impurity oxygen is also a sintering aid as silica. Therefore, there is a problem that the sinterability is hindered as the amount of oxygen in the raw material is reduced.

  Therefore, the present inventors, for the purpose of producing by a reactive sintering method of a silicon nitride sintered body excellent in mechanical properties and thermal conductivity, the properties of silicon powder, in particular, the amount of impurity oxygen and the auxiliary composition are The effect of the silicon nitride sintered body obtained by post-sintering the nitride on the heat conduction and mechanical properties was investigated.

  As a result, in order to produce a silicon nitride sintered body in which thermal conductivity, strength, and toughness coexist, the sum of the amount of oxygen contained in the silicon powder and the amount of oxygen from the magnesium compound as a sintering aid The inventors have found a novel finding that the rare earth oxide can be realized for the first time by precisely controlling the addition amount of the rare earth oxide within a specific range, and have reached the present invention.

That is, the present invention is a reaction sintered silicon nitride sintered body synthesized using reaction sintering of silicon powder, which is mainly composed of β-phase silicon nitride, and at least one rare earth element is converted into an oxide. 0.5-7 mol%, Mg abundance is 2 mol% or less in terms of oxide, thermal conductivity is 100 W / mK or more, 3-point bending strength is 600 MPa or more, precracking fracture test A silicon nitride sintered body having a characteristic that the fracture toughness value measured by the method is 7 MPam ^1/2 or more is obtained, and a metal layer is joined to the silicon nitride sintered body to obtain a highly reliable and highly thermally conductive silicon nitride substrate. Obtainable.

Furthermore, by selecting production conditions in the present invention, the thermal conductivity is 120 W / mK or more, the three-point bending strength is 600 MPa or more, and the fracture toughness value measured by the precracking fracture test method is 10 MPam ^1/2 or more. It is possible to produce a silicon nitride sintered body having a silicon nitride substrate with higher reliability than before, and to improve the reliability and miniaturization of a power module using this substrate. Become.

In the present invention, the silicon powder or the mixed powder of silicon powder and silicon nitride powder is mixed with 0.5 to 7 mol% of rare earth element oxide and magnesium oxide as a magnesium compound in the ratio of silicon to silicon nitride. At least one selected from (MgO), silicon magnesium nitride (MgSiN ₂ ), and magnesium silicide (Mg ₂ Si) is used in a mixture of 1 to 7 mol%. Moreover, in the said mixture, the total amount of oxygen from impurity oxygen contained in silicon powder or mixed powder of silicon powder and silicon nitride powder and oxygen from the magnesium compound is 0.1 to 1 in a ratio when silicon is converted into silicon nitride. It is added so that it may become the range of 8 mass%, and it is set as starting raw material powder.

  In the above starting raw material powder, if the amount of rare earth element added is less than 0.5 mol%, oxygen cannot be trapped in the grain boundary phase, and the amount of oxygen dissolved in the silicon nitride particles increases. If the content of the sintered body exceeds 7 mol%, the amount of the grain boundary phase of the thermal conductivity containing yttrium and the like increases and the thermal conductivity of the sintered body decreases. As the rare earth element used in the present invention, Y, Yb, Nd and Sm which are easily available and are stable as an oxide are preferable, and at least one selected from these is added as an oxide.

  When a silicon nitride sintered body is prepared by adding only rare earth oxide as an auxiliary agent, it needs to be fired at an ultra high temperature of 2000 ° C. in a high nitrogen pressure of about 10 MPa in order to perform densification. This requires a large firing furnace, which increases the process cost. In addition, remarkable grain growth occurs by firing at an ultrahigh temperature, leading to a decrease in mechanical properties. For this reason, in order to promote densification at the time of post-sintering and to enable the development of high strength and high toughness, it is indispensable to add a magnesium compound simultaneously with the addition of the rare earth compound. Addition of a magnesium compound becomes a modified ion of oxynitride glass that is formed when Mg ions are heated, lowers the viscosity of the glass, promotes densification, and evaporates during firing, reducing the amount of residual grain boundary phase. There is a function to reduce.

  As described above, the silicon powder contains 0.2 to several mass% of impurity oxygen. Impurity oxygen is a factor that inhibits the thermal conductivity of silicon nitride, but on the other hand, it is an important sintering aid for densifying silicon nitride as silica. In order for the present inventors to coexist with high thermal conductivity, high strength, and high toughness, it is important to simultaneously control the amount of magnesium ions and the total amount of oxygen contained in the impurity oxygen of the silicon powder and the magnesium compound. I found out.

In the conventional method using magnesium oxide as the magnesium source, low-priced silicon powder containing a large amount of impurity oxygen cannot be used as a raw material for the high thermal conductivity material. A major feature of the present invention is that the amount of oxygen is adjusted using magnesium oxide containing oxygen (MgO) and a magnesium compound not containing oxygen. That is, it is to perform using magnesium oxide (MgO), silicon magnesium nitride (MgSiN ₂ ), and magnesium silicide (Mg ₂ Si). That is, at least one selected from magnesium oxide, silicon nitride magnesium, and magnesium silicide is 1 to 7 mol% in a ratio when silicon is converted to silicon nitride, and impurity oxygen and magnesium compound contained in silicon and silicon nitride Is added to the silicon powder or the mixed powder of silicon powder and silicon nitride powder so that the total amount of oxygen is in the range of 0.1 to 1.8% by mass when silicon is converted to silicon nitride.

  In the present invention, a magnesium powder containing no oxygen, such as silicon magnesium nitride or magnesium silicide, is mainly used for silicon powder having a large amount of impurity oxygen, while magnesium oxide is mainly used for silicon powder having a low amount of impurity oxygen. As added. If the added amount of the magnesium source is less than 1 mol%, densification is difficult. On the other hand, if it exceeds 7 mol%, a large amount of magnesium remains even after post-sintering, thereby inhibiting the thermal conductivity of the sintered body.

  Similarly, if the total amount of impurity oxygen contained in silicon and oxygen from the magnesium compound is less than 0.1% by mass in the ratio when silicon is converted to silicon nitride, densification cannot be performed, If it exceeds 1.8% by mass, the amount of oxygen in the grain boundary phase becomes excessive, the amount of oxygen dissolved in the silicon nitride particles increases, and the thermal conductivity of the sintered body is lowered. In order to prevent the silicon powder from fusing during the nitriding reaction, it is also effective to add the silicon nitride powder up to 30% by mass with respect to the composition in which the silicon is completely nitrided. It is necessary to consider the impurity oxygen to be included in the above composition.

As the magnesium compound not containing oxygen in the present invention, magnesium silicides, fluorides, borides, nitrides and these ternary compounds can be used, but they are easy to handle and stable during processing. In view of the fact that no harmful substances are generated, silicon magnesium nitride and magnesium silicide are preferably used. Further, as the silicon magnesium nitride, for example, a predetermined amount of Mg ₂ Si powder, Si powder, silicon nitride powder is preferably mixed so that the molar ratio of Mg to Si is 1: 1, and 1350 in a nitrogen atmosphere. Although the powder obtained by crushing what was synthesize | combined by heating at degreeC (refer Unexamined-Japanese-Patent No. 2003-267709) is used, it is not restrict | limited to these.

  The powder weighed based on the above composition is mixed with a ball mill or a planetary mill by an ordinary method using water or an organic solvent as a solvent, adding an organic binder or a dispersing agent as necessary. Then, after removing the solvent as necessary, it is molded into a predetermined shape by die molding, sheet molding, isostatic pressing (CIP molding), etc., and in some cases, the organic binder used for molding is removed After calcination at a temperature of 800 ° C. or less, nitriding is performed in a temperature range of 1200 to 1400 ° C. Further, the nitride is post-sintered at a temperature of 1700 to 1950 ° C. in nitrogen of 0.1 MPa or more and densified to a relative density of 95% or more.

  In the present invention, a sheet forming method is suitable for producing the silicon nitride sintered body. For example, if an example of the extrusion molding method is shown, each raw material powder is weighed by the above blending, put into a universal mixer, and added with 3 to 10% by weight of an organic binder such as methylcellulose and 10 to 30% by weight of water and mixed. . After kneading with a kneader such as a three-roll mill, the mixture is refrigerated and aged, and extruded with an extruder to form a thickness of about 0.2 to 2 mm. The obtained green sheets are cut to a predetermined size by a punching machine, etc., and then stacked with BN powder and BN plates for release in between, and placed in a high-temperature furnace equipped with a graphite heat insulating material and a heating graphite heater. set. As a high-temperature furnace, when a tight box type electric furnace (for example, PVSG type manufactured by Shimadzu Corporation) that can efficiently discharge the gas generated in the inside to the outside and can perform vacuum and pressurization is used, Since nitriding reaction and post-sintering can be performed in one furnace, productivity is high and convenient.

  The organic binder is decomposed and removed in a vacuum by heating to 800 ° C. or lower, and then, after sufficiently evacuating to about 1 Pa, nitrogen gas having a purity of 99.9% or more is supplied. Thereafter, the temperature is raised to the nitriding start temperature, and the nitrogen gas supply amount and the nitrogen gas pressure are controlled while monitoring the progress of the nitriding reaction. When the nitriding rate calculated from the absorption amount of nitrogen gas reaches 75% or more, the nitrogen gas pressure is increased to 0.9 MPa, the temperature is raised to the post-sintering temperature, the remaining nitriding reaction is advanced, and the nitride is densely formed. Make it.

  When the post-sintering temperature is less than 1700 ° C., sufficient densification cannot be performed. On the other hand, when the post-sintering temperature exceeds 1950 ° C., excessive grain growth occurs and the strength is significantly reduced. It is carried out at 1950 ° C., preferably 1750-1900 ° C. In order to achieve a thermal conductivity of 130 W / mK or higher, the holding time is adjusted under the above-mentioned post-sintering conditions, or heat treatment is performed in a non-oxidizing atmosphere at a temperature of 1700 ° C. or lower after densification, Further, it is necessary and important that the amount of Mg element is volatilized to 0.2% by mass or less in terms of oxide.

  In the present invention, a metal plate is bonded to at least one surface of the obtained silicon nitride sintered body. For the metal plates to be joined, high-purity copper or aluminum plates are used when the thermal conductivity of the substrate is important, and when the mechanical strength is important, alloy plates mainly composed of copper or aluminum, or other metal plates Is preferably used, and can be appropriately selected according to demand. Two different metal plates may be joined to the same silicon nitride sintered body. Moreover, the thickness can be freely selected from about 0.1 mm to 6 mm, and is preferably 0.25 mm or more.

Since the silicon nitride sintered body of the present invention has a very high fracture toughness of 9 MPam ^1/2 or more if the production conditions are selected, even if an alloy plate having a high mechanical strength or a thick metal plate is joined, the outer periphery of the joint It is difficult for micro cracks to occur in the cracks, and even if cracks occur, they are difficult to progress. A normal silicon nitride substrate in which a high-purity copper or aluminum metal plate having a thickness of about 0.1 to 0.5 mm is bonded on both sides can provide a substrate having better heat shock resistance than conventionally known ones. As long as the power module assembled using the module is used in a normal use environment, its lifetime is semi-permanent.

  Since the silicon nitride sintered body used in the present invention has excellent mechanical properties, the area of the silicon nitride sintered body and the metal plate to be joined can be made larger than usual, and usually a plurality of silicon nitride substrates are soldered. It is possible to directly join the silicon nitride plate to the base plate of the power module having a thickness of about 3 to 6 mm.

  As a brazing material used for joining, a brazing material containing at least one metal element of Mg, Ti, and Zr is preferable. Specifically, for aluminum plate bonding, it is preferable to use an Al alloy or a mixture containing one or more selected from the group consisting of Mg, Cu, Si, and Ag. Preferably, when a brazing material having Mg of 0.05 to 3% by mass is used, the bonding reliability is increased. For example, JIS2014, 2017, 2018, 2024, 2030, 2036, 2214, and 2224 are listed as examples of numbers of Japanese Industrial Standards of aluminum alloys that can be used as brazing materials. For joining a copper plate or other metal plate, it is preferable to use an alloy or mixture containing Ag and Cu as main components containing one or more metal elements selected from Ti and Zr.

  The brazing foil is used by rolling a brazing alloy to a thickness of 5 to 40 μm with a rolling mill. For example, the above-mentioned aluminum alloy thin plate is rolled, cut into necessary dimensions according to the bonding area, and inserted between the silicon nitride sintered body and the metal plate. The brazing material mixture is a mixture of metal powders, which is made by dispersing the powder in a solvent such as terpineol to form a paste, and is applied to at least one joint surface of the silicon nitride sintered body or the metal plate by a screen printing method or a brush. It is applied by a coating method or the like so that the paste thickness after drying becomes 10 to 50 μm. If the thickness of the brazing material or paste is below the above lower limit value, only a small amount of magnesium oxide, titanium nitride, or zirconium nitride layer is formed at the joint interface, and if it exceeds the upper limit value, a brittle alloy layer is likely to be formed. In either case, the bonded surface with poor heat cycle resistance is easily peeled off, resulting in a low-reliability substrate.

  The silicon nitride sintered body and the brazing material arranged as described above are put into a heating furnace, and a load is applied as necessary, and in vacuum or nitrogen gas, depending on the type of the metal plate and the brazing material, 520 to Heating to 650 ° C. or 750 to 950 ° C. melts at least a part of the brazing material, and joins the silicon nitride sintered body and the metal plate. When the temperature is out of the above temperature range, bonding becomes insufficient or a brittle bonding layer is formed, and the reliability of the substrate is lowered.

  After bonding, when the cross section is analyzed for the constituent layer of the bonding interface by electron beam microanalysis, etc., in a sufficiently reliable substrate with good heat cycle resistance, magnesium oxide, titanium nitride, or zirconium nitride is nitrided on the interface. It is distributed in layers at a ratio that accounts for 50% or more of the area of the joining plane between the silicon sintered body and the metal plate.

  The present invention will be specifically described based on comparative examples and examples, but the present invention is not limited to the following examples.

[Comparative Examples 1-1 to 1-6]
(Normal sintering method using silicon nitride powder as starting material)
Silicon nitride pot with 2 mol% ytterbium oxide or 2 mol% yttrium oxide and 5 mol% magnesium oxide added to silicon nitride powder with an average particle size of 0.2 μm (impurity oxygen content 1.3 mass%) and methanol as the dispersion medium And a silicon nitride ball were used for 2 hours for planetary mill mixing. Methanol was evaporated using an evaporator, and the resulting powder was molded into a 45 × 50 × 5 mm shape using a mold, and further CIP molded at a pressure of 306 MPa.

  The compact was placed in a boron nitride (BN) crucible and sintered in pressurized nitrogen of 0.9 MPa at 1850 ° C. for 12 hours, 24 hours or 48 hours. The surface of the sintered body was ground, a sample having a shape of 3 × 4 × 40 mm was cut out, and the three-point bending strength measurement of JIS-R1601 and the pre-crack introduction fracture toughness measurement of JIS-R1607 were performed. Further, a disk-shaped test piece having a thickness of about 2 mm was prepared, and the thermal conductivity was measured using a laser flash method.

Table 1 shows values of thermal conductivity, strength, and fracture toughness of the sintered body thus obtained. In a normal sintering method using silicon nitride powder and oxide-based auxiliary, sintering is performed in which the thermal conductivity of 100 W / mK or more, the three-point bending strength of 600 MPa or more, and the fracture toughness of 7 MPam ^1/2 or more coexist. I found out I couldn't get a body. In addition, it was found that the sample with a longer sintering time showed some improvement in thermal conductivity, but it was accompanied by a sharp decrease in strength and fracture toughness.

[Comparative Examples 1-7, 1-8]
(Normal sintering method using silicon nitride powder as starting material)
Magnesium silicide powder having a particle diameter of 150 μm, silicon powder having a particle diameter of 10 μm, purity of 99%, and silicon nitride powder having a particle diameter of 1 μm are 64.6%, 5.9%, and 29.5% by weight, respectively. And weighed using an agate mortar. The mixed powder filled in high-purity boron nitride (BN) crucible is placed in an alumina tube furnace, heated to ˜1350 ° C. in a nitrogen stream and held for 1 hour, then cooled to room temperature in the furnace, and silicon nitride magnesium powder Was synthesized.

  2 mol% of yttrium oxide and 5 mol% of silicon magnesium nitride synthesized by the above method are added to silicon nitride powder having an average particle size of 0.2 μm (impurity oxygen content: 1.3 mass%), and a silicon nitride pot using methanol as a dispersion medium And a silicon nitride ball were used for 2 hours for planetary mill mixing. Methanol was evaporated using an evaporator, and the resulting powder was molded into a 45 × 50 × 5 mm shape using a mold, and further CIP molded at a pressure of 306 MPa.

  The molded body was placed in a boron nitride (BN) crucible and sintered in pressurized nitrogen of 0.9 MPa at 1850 ° C. for 12 hours or 48 hours. The surface of the sintered body was ground, a sample having a shape of 3 × 4 × 40 mm was cut out, and the three-point bending strength measurement of JIS-R1601 and the pre-crack introduction fracture toughness measurement of JIS-R1607 were performed. Further, a disk-shaped test piece having a thickness of about 2 mm was prepared, and the thermal conductivity was measured using a laser flash method.

Table 1 shows values of thermal conductivity, strength, and fracture toughness of the sintered body thus obtained. Although various characteristics such as thermal conductivity are somewhat improved compared to the case of using magnesium oxide, it has a thermal conductivity of 100 W / mK or more, a three-point bending strength of 600 MPa or more, and a fracture toughness of 7 MPam ^1/2 or more. It was found that it was impossible to obtain a sintered body having all of them. Further, it was found that the sample of Comparative Example 1-8 in which the sintering time was extended was accompanied by a rapid decrease in strength and fracture toughness, as in Comparative Examples 1-1 to 1-6.

[Experimental Examples 2-1 to 2-23]
As a silicon powder, a powder having an average particle size of 10 μm, an impurity oxygen content of 0.16% by mass (powder A), an average particle size of 7 μm, a powder having an impurity oxygen content of 0.61% by mass (powder B), an average particle size of 1 μm, an impurity A powder (powder C) having an oxygen content of 1.75% by mass, an average particle size of 0.9 μm, a powder (powder D) having an impurity oxygen content of 2.6% by mass, magnesium oxide having an average particle size of 0.1 μm as a magnesium compound, and Silicon magnesium nitride powder (average particle size 0.5 μm) synthesized by the method described in Comparative Examples 1-7 and 1-8 or magnesium silicide powder having an average particle size of 10 μm was used. Moreover, yttrium oxide having an average particle diameter of 1.5 μm and ytterbium oxide having an average particle diameter of 1.2 μm are used as the rare earth oxide, and a powder having an average particle diameter of 0.2 μm and an impurity oxygen content of 1.3 mass% is used as silicon nitride. used. The impurity oxygen in the silicon powder and silicon nitride powder was measured using a simultaneous nitrogen / oxygen analyzer.

  The above raw material powder was weighed so as to have the composition shown in Table 2, and planetary mill mixing was performed using methanol as a dispersion medium and a silicon nitride pot and silicon nitride balls for 2 hours. Methanol was evaporated using an evaporator, and the resulting powder was molded into a 45 × 50 × 5 mm shape using a mold, and further CIP molded at a pressure of 306 MPa. Next, in order to carry out reaction sintering, the molded body was placed in a boron nitride (BN) crucible and heated at 1400 ° C. in 0.1 MPa of nitrogen for 4 hours for nitriding treatment. In any of the molded bodies, no residual Si was observed by X-ray diffraction.

  Next, as post-sintering, the nitride was sintered in pressurized nitrogen of 0.9 MPa at 1850 ° C. for 6 hours, 12 hours, or 48 hours. The surface of the sintered body was ground, a sample having a shape of 3 × 4 × 40 mm was cut out, and three-point bending strength measurement according to JIS-R1601 and pre-crack introduction fracture toughness measurement according to JIS-R1607 were performed. Further, a disk-shaped test piece having a thickness of about 2 mm was prepared, and the thermal conductivity was measured using a laser flash method. Moreover, about the produced some sintered compact, the quantitative analysis of the magnesium which remains in a sintered compact was performed by ICP analysis. Table 3 summarizes the values of Mg amount, density, thermal conductivity, strength, and fracture toughness of the sintered body thus obtained.

As is apparent from Tables 2 to 3, 0.5 to 7 mol% of the rare earth element oxide, 1 to 7 mol% of the magnesium compound, impurity oxygen contained in silicon and silicon nitride, and oxygen from the magnesium compound When added so that the total amount is in the range of 0.1 to 1.8% by mass in the ratio of silicon to silicon nitride, the thermal conductivity is 100 W / mK or more, the three-point bending strength is 600 MPa or more, It was found that a silicon nitride sintered body having excellent characteristics with a fracture toughness measured by a precracking fracture test method of 7 MPam ^1/2 or more can be obtained.

Furthermore, by increasing the post-sintering time and converting the residual magnesium amount to 0.2% by mass or less in terms of oxide, the thermal conductivity is 130 W / mK or more, and the three-point bending strength is 600 MPa or more. It was found that a silicon nitride sintered body having a characteristic that the fracture toughness value measured by the crack introduction fracture test method is 7 MPam ^1/2 or more can be produced.

[Example 1, Comparative Examples 2-1 to 2-5]
The sintered bodies obtained in Comparative Examples 1-1 to 1-5 were ground using a resin bond grindstone using 200 mesh diamond abrasive grains to produce 0.6 mm × 75 mm × 50 mm white plates. A brazing material paste obtained by kneading 80 parts by mass of Ag powder, 15 parts by mass of Cu powder, 5 parts by mass of Ti powder, and 15 parts by mass of terpineol on both sides of this white plate was used in a 42 mm × 67 mm area using a screen printer. The coating was applied to a thickness of 20 μm. Prepare two copper plates with a size of 1.0 mm x 42 mm x 67 mm and a purity of 99.9%, place them on both sides of a white plate coated with brazing paste, apply a load of 1 kPa to the entire copper plate, I put it in. Under a vacuum atmosphere of 0.01 Pa, heating was performed at 850 ° C. for 10 minutes to join the silicon nitride white plate and the copper plate. About the obtained silicon nitride board | substrate, -70 degreeC and the heat shock test of 350 degreeC were performed 50 times, As a result, peeling of the copper plate edge part was recognized visually.

  A silicon nitride substrate was produced from the sintered body obtained in Experimental Example 2-4 in the same manner as in Comparative Examples 1-1 to 1-5, and the heat shock test was performed 30 times. As a result, no peeling of the copper plate was observed. It was.

[Example 2]
The area to which the brazing paste is applied is 42 mm × 67 mm on one side, and the entire surface is on one side, and a copper plate with a purity of 99.5% and a size of 1.0 mm × 42 mm × 67 mm and a size of 5.0 mm × 70 mm × 95 mm. A silicon nitride substrate was produced in the same manner as in Example 1 except that one copper plate was prepared. As a result of performing the heat shock test at −70 ° C. and 350 ° C. 50 times, peeling of the end portion of the copper plate was not recognized.

  The cross section of the silicon nitride substrate was observed with an electron beam scanning microscope, and the element distribution of Ti and N was examined. When the region where the presence of TiN in which Ti and N are simultaneously distributed is estimated at the bonding interface between the white plate and the copper plate is viewed from the direction perpendicular to the boundary line between the white plate and the copper plate, It was found that the region where Ti and N are simultaneously distributed overlaps the boundary line by 50% or more.

Example 3
After mixing 100 parts by mass of the raw material powder having the same composition as Experimental Example 2-21 and 35 parts by mass of a binder (trade name “Celander” manufactured by YUKEN INDUSTRIES) with a Henschel mixer, the mixture is kneaded with an extrusion molding machine (manufactured by Miyazaki Tekko). Extruded to produce a sheet-like molded body having a thickness of 0.7 mm. This is punched with a mold to produce 10 plate-shaped molded bodies of 90 mm × 60 mm, and boron nitride (BN) powder is applied to the plate-shaped molded body and laminated in a BN container. A 500 g weight was placed and stored. The sample was set in a tight box furnace manufactured by Shimadzu Corporation and heated to 550 ° C. while being evacuated to remove the binder. Thereafter, after confirming that the degree of vacuum reached 2 Pa, nitrogen gas was introduced and the temperature was raised to the nitriding temperature. After nitriding at 1400 ° C. for 8 hours, the nitrogen gas pressure was increased to 0.9 MPa, and post sintering was performed at 1850 ° C. for 12 hours. Thereafter, the plate-like sintered body was taken out from the BN container, and the BN powder or the like adhering to the surface was removed with a sand blasting device to form a white plate, which was used for subsequent evaluation and substrate production.

  When the obtained white plate was cut into a strip shape having a width of 10 mm and a three-point bending strength measurement was performed using a test piece, it was 660 MPa. Moreover, it was 113 W / mK when the thermal conductivity measurement of the white board obtained by the laser flash method was performed.

  Two aluminum plates with a size of 1.0 mm × 42 mm × 67 mm and a purity of 99.95% were prepared, and an aluminum alloy foil of JIS No. 2024 having a thickness of 10 μm and the same size was sandwiched between the aluminum plate and the white plate, It was set in a vacuum furnace. The aluminum plate and the white plate were joined by heating at 610 ° C. for 10 minutes at a vacuum degree of 0.3 Pa to obtain a silicon nitride substrate.

  The heat shock at −70 ° C. and 250 ° C. was repeated 1000 times, but no peeling of the metal plate was visually observed. Therefore, the aluminum plate was removed by etching, and the white plate at the joining end of the metal plate was observed, but no minute cracks were observed.

  The cross section of the silicon nitride substrate was observed with an electron beam scanning microscope, and the element distribution of Mg and O (oxygen) was examined. When the region where the presence of MgO in which Mg and O are simultaneously distributed is estimated at the bonding interface between the white plate and the copper plate is viewed from the direction perpendicular to the boundary line between the white plate and the copper plate, It was found that the region where Mg and O were simultaneously distributed overlaps the boundary line by 50% or more.

Claims (1)

Mixing silicon powder or mixed powder of silicon powder and silicon nitride powder with 0.5 to 7 mol% rare earth oxide and 1 to 7 mol% magnesium mixture in the ratio of silicon to silicon nitride A mixture of
The magnesium mixture is at least one selected from magnesium oxide (MgO), magnesium silicide (Mg ₂ Si), and magnesium magnesium nitride (MgSiN ₂ );
Adjusting a mixture in which the total amount of impurity oxygen contained in the silicon powder or the mixed powder of silicon powder and silicon nitride powder and oxygen from the magnesium compound is 0.1 to 1.8% by mass,
The mixture is molded and nitrided, and the obtained nitride is heated in nitrogen of 0.1 MPa or more to be densified so that the relative density is 95% or more, and the obtained plate-like silicon nitride sintered body is obtained A metal plate is joined to at least one surface of the metal plate using a brazing material containing at least one metal element selected from magnesium, titanium, and zirconium.