Silicon nitride sintered body

Abstract

The present invention provides a silicon nitride sintered body having a low dissolved oxygen content, high thermal conductivity, and high strength, and suitable as a base substrate of a bonded substrate. This silicon nitride sintered body has a grain boundary phase containing one or more rare earth elements selected from the group consisting of Y, Sc, La, Ce, Sm, Gd, Dy, Ho, Er, Lu, and Yb, and each element of Mg, Si, O, N, and Sr. The Sr content in the sintered body is 5.0×10-3 mol% to 1.0×10-1 mol%. The ratio of the area of β-type silicon nitride particles having a major axis diameter of 10 μm or more to the total area occupied by all the β-type silicon nitride particles at any cross-sectional position of the sintered body is 2% to 50%.

Description

Silicon nitride sintered body

[0001] The present invention relates to a silicon nitride sintered body.

[0002] Heat dissipation substrates used in power modules and the like are required to have high insulation, high mechanical strength, and high thermal conductivity. Furthermore, with the recent demand for higher integration and higher power consumption, the required values ​​for these physical properties are becoming even higher. Silicon nitride (SiN), which is expected to have a single crystal thermal conductivity of 200 to 320 W / (m K), is being considered as a material for such heat dissipation substrates. ３ N ４ ) has been attracting attention. However, sintered silicon nitride is generally used as a heat dissipation substrate. In such sintered silicon nitride, phonon scattering occurs due to the presence of a few percent of oxygen dissolved in the crystal grains. As a result, the thermal conductivity of sintered silicon nitride remains at around 20 to 80 W / (m·K), which is lower than the predicted value for single crystals.

[0003] On the other hand, silicon nitride is a difficult-to-sinter material, so liquid-phase sintering is performed using a sintering aid to achieve densification. When a compound containing a rare earth element such as Y is used as a sintering aid, the oxygen affinity of the rare earth element is utilized to trap oxygen, reducing the amount of dissolved oxygen in silicon nitride. However, because the melting point of compounds containing rare earth elements is high, Mg compounds and other compounds are also added. Technology that achieves high strength and high thermal conductivity by combining such aids is already known (see, for example, Patent Document 1).

[0004] However, in silicon nitride sintered substrates manufactured using conventional techniques, there is a trade-off between strength and thermal conductivity. While improving thermal conductivity by reducing solute oxygen can be achieved by promoting grain growth of silicon nitride particles, the coarse columnar grains that result from grain growth are a cause of strength degradation. To achieve both high thermal conductivity and high strength, it is necessary to reduce solute oxygen while suppressing grain growth, which is a cause of strength degradation.

[0005] Furthermore, in order for a bonded substrate formed by bonding a metal plate to a silicon nitride sintered body as a base substrate (heat dissipation substrate) to have high quality stability, it must at least have excellent durability against thermal cycles.

[0006] Patent No. 4997431

[0007] The present invention has been made in view of the above problems, and aims to provide a silicon nitride sintered body which has a low dissolved oxygen content, high thermal conductivity, and high strength, and is suitable as a base substrate for bonding substrates.

[0008] In order to solve the above problems, a first aspect of the present invention is a sintered body of silicon nitride, which comprises a grain boundary phase containing one or more rare earth elements selected from the group consisting of Y, Sc, La, Ce, Sm, Gd, Dy, Ho, Er, Lu, and Yb, and the elements Mg, Si, O, N, and Sr, and the content of Sr in the sintered body is 5.0 × 10 －３ mol% or more 1.0×10 －１ mol% or less, and the ratio of the area occupied by β-type silicon nitride particles having a major axis diameter of 10 μm or more to the area occupied by all β-type silicon nitride particles at any one cross-sectional position of the sintered body is 2% or more and 50% or less.

[0009] A second aspect of the present invention is the silicon nitride sintered body according to the first aspect, characterized in that the thermal expansion coefficient is 2.9 ppm / K or more and 3.3 ppm / K or less.

[0010] A third aspect of the present invention is the silicon nitride sintered body according to the first or second aspect, characterized in that at least a part of the grain boundary phase is an amorphous phase.

[0011] A fourth aspect of the present invention is the silicon nitride sintered body according to the first or second aspect, characterized in that at least a part of the grain boundary phase is a crystalline phase of melilite structure.

[0012] A fifth aspect of the present invention is the silicon nitride sintered body according to the first or second aspect, characterized in that at least a part of the grain boundary phase is a crystalline phase with a cuspidine structure.

[0013] A sixth aspect of the present invention is the silicon nitride sintered body according to the first or second aspect, characterized in that the grain boundary phase is a crystalline phase having a melilite structure and / or a cuspidin structure and an amorphous phase.

[0014] A seventh aspect of the present invention is the silicon nitride sintered body according to any one of the first to sixth aspects, wherein the content of Al element in the sintered body is 1.5×10 －２ It is characterized in that the content is less than mol %.

[0015] According to the first to seventh aspects of the present invention, a silicon nitride sintered body having both excellent thermal conductivity and strength is realized. Furthermore, a bonded substrate formed by bonding a metal plate to such a silicon nitride sintered body as a base substrate is realized having excellent durability against thermal cycles.

[0016] Figure 1 is a flowchart showing a method for producing a silicon nitride sintered body. Figure 2 is a graph plotting the strength of the sintered bodies of Example A, Comparative Example A, Example D, and Example E against thermal conductivity. Figure 3 is a graph plotting the strength of the sintered bodies of Example B and Comparative Example B against thermal conductivity. Figure 4 is a graph plotting the strength of the sintered bodies of Example C and Comparative Example C against thermal conductivity. Figure 5 is a graph plotting the strength of the sintered body of Example F against thermal conductivity. Figure 6 is a graph plotting the strength of the sintered body of Example G against thermal conductivity.

[0017] <Method for Producing Sintered Silicon Nitride> Figure 1 is a flowchart showing a method for producing a sintered silicon nitride according to an embodiment of the present invention. In this embodiment, the sintered silicon nitride is produced by a reaction sintering technique using silicon (Si) as the main raw materials and, as sintering aids, a magnesium compound and a rare earth element compound.

[0018] Examples of magnesium compounds include magnesium oxide (MgO) and magnesium carbonate (MgCO ３ ), magnesium silicide (Mg ２ Si), magnesium silicon nitride (MgSiN ２However, in view of cost, it is preferable to minimize the use of magnesium silicon nitride.

[0019] The rare earth element of the rare earth element compound is one or more elements selected from the group consisting of Y, Sc, La, Ce, Sm, Gd, Dy, Ho, Er, Lu, and Yb. The rare earth element compound is one or more compounds selected from the group consisting of oxides, silicides, and carbonitrides of these rare earth elements. However, from the viewpoint of cost, it is preferable to minimize the use of carbonitrides.

[0020] In addition to the above-mentioned substances, in this embodiment, powder of elemental strontium (Sr) and / or a strontium compound is added to the raw material. The strontium compound is SrCO ３ , Sr(OH) ２ , Sr(NO ３ ) ２ , SrSi ２ , Sr ３ N ２ , SrCl ２ and the like are exemplified, and one or more compounds selected from the group consisting of these compounds are used.

[0021] Furthermore, silicon carbide (SiC), magnesium silicide (Mg ２ Si), carbonitrides containing rare earth elements or magnesium elements (e.g., Y ２ Si ４ N ６ C, Yb ２ Si ４ N ６ C, MgSi ４ N ６ C) may be added.

[0022] In producing a silicon nitride sintered body, the powders of these raw materials are first weighed (Step S1). At this time, the composition ratio of silicon, the magnesium compound, and the rare earth element compound, which are the main raw materials, is converted into the composition ratio of silicon nitride, and when the total molar ratio of silicon nitride, the magnesium compound, and the rare earth element compound is taken as 100 mol%, the composition ratio of the magnesium compound is 1 mol% to 15 mol% in terms of oxide, the composition ratio of the rare earth element compound is 0.5 mol% to 7 mol% in terms of oxide, and the remainder is silicon nitride.

[0023] For example, yttrium oxide (Y ２ O ３ When magnesium oxide (MgO) is used as the magnesium compound, silicon, MgO and Y are used as the main raw materials. ２ O ３ When the total weight ratio of the powders is 100 wt%, the weight ratio of MgO is 0.4 wt% to 7.7 wt%, and Y ２ O ３ The weight ratio of the above is 1.3 wt % to 18.1 wt %, with the remainder being silicon.

[0024] Furthermore, the weight ratio of the powder of strontium and / or a strontium compound is 100 wt %, which is the total weight ratio of the powders of silicon, magnesium compound and rare earth element compound. ３ Weight ratio when converted (SrCO ３ The external additive amount (equivalent weight ratio) is 5.6 x 10 －２ The content should be between 1.1 wt% and 1.1 wt%.

[0025] The weighed raw materials are mixed with an organic solvent such as IPA to form silicon nitride and zirconia (ZrO ２ The materials are mixed in a ball mill (e.g., a pot mill) using balls made of cellulose nitrate (S1) (Step S2). If necessary, an organic binder, dispersant, or the like may be added. The mixing time is set in the range of 1 hour to 50 hours.

[0026] Next, the mixed raw materials are dried using a nitrogen dryer or the like. This removes the solvent, and a mixed powder of the raw materials is obtained. The obtained powder is molded into a predetermined shape (step S3). Examples of molding methods include metal molding, sheet molding, and isostatic pressing (CIP) molding.

[0027] In this molding, the molded body may be in the form of a plate so that the sintered body obtained after the final firing process can be used as a substrate as is, or the molded body may be in the form of a bulk, on the assumption that the sintered body will be processed by cutting, machining, grinding, polishing, etc. to obtain a substrate.

[0028] If an organic binder, dispersant, or the like has been used during mixing, the compact is subjected to a degreasing treatment in which it is calcined at a temperature of 800° C. or less in order to remove them (step S4).

[0029] Next, a nitriding treatment is performed (step S5), which is a heat treatment for nitriding the silicon of the compact. Specifically, the compact is kept in a nitrogen atmosphere at a temperature of 1200°C to 1500°C for 1 hour to 15 hours. The nitrogen partial pressure in the nitrogen atmosphere is preferably 0.05 MPa to 0.7 MPa.

[0030] Following this nitriding treatment, a firing (sintering) treatment is carried out to obtain a sintered body (step S6). Specifically, the nitriding-treated compact is fired in a nitrogen atmosphere at a temperature of 1700°C to 1950°C for 1 hour to 50 hours. The nitrogen partial pressure in the nitrogen atmosphere is preferably 0.1 MPa to 1.2 MPa. This results in a silicon nitride sintered body.

[0031] The higher the firing temperature and the longer the firing time, the more the crystal grains of β-type silicon nitride tend to grow and become columnar.

[0032] Furthermore, as this columnarization progresses, the amount of dissolved oxygen in the silicon nitride sintered body decreases as the proportion of particles with a major axis diameter of 10 μm or more among the β-type silicon nitride particles that make up the silicon nitride sintered body increases, but on the other hand, the strength decreases. In consideration of this point, in this embodiment, as described above, the firing temperature during the firing treatment is set to 1950° C. or less, and the firing time is set to 50 hours or less.

[0033] If the obtained sintered body is in a plate shape, it can be used as a silicon nitride substrate as is. On the other hand, if the obtained sintered body is in a bulk shape, the silicon nitride substrate can be obtained by appropriately processing the sintered body by cutting, milling, grinding, polishing, or the like.

[0034] <Characteristics of Silicon Nitride Sintered Body> The characteristics of the silicon nitride sintered body according to this embodiment obtained by the above-mentioned procedure will be described below. Hereinafter, the silicon nitride sintered body according to this embodiment will also be referred to as an Sr-doped sintered body, and for comparison, a silicon nitride sintered body produced without intentionally adding strontium and / or a strontium compound to the raw materials will be referred to as an additive-free sintered body.

[0035] The Sr-added sintered body mainly comprises crystal grains of silicon nitride, which is the main component, and a grain boundary phase existing at the grain boundaries of the crystal grains. The grain boundary phase mainly comprises magnesium compounds, rare earth element compounds, and silicon oxide (SiO ２ ) and comes from.

[0036] In addition, the Sr-added sintered body contains Sr element at 5.0 × 10 －３ mol% or more 1.0×10 －１ The Sr content is 50% by mol or less. The Sr element is present in the crystalline phase and / or amorphous phase at the grain boundaries. Note that the Sr-added sintered body may also contain Sr element derived from impurities in the raw materials.

[0037] That is, the grain boundary phase contains one or more rare earth elements selected from the group consisting of Y, Sc, La, Ce, Sm, Gd, Dy, Ho, Er, Lu, and Yb, as well as Mg, Si, O, N, and Sr.

[0038] The contents of the elements contained in the silicon nitride sintered body are determined, for example, by polishing both surfaces of the sintered body to a thickness of 10 μm or more and 50 μm or less, crushing a portion, and performing quantitative analysis (chemical analysis) in accordance with the method described in JIS R 1603. However, in this embodiment, the content of Sr element is defined as the value when the total content of Si, N, O, Mg, rare earth elements, Sr, and Al among the constituent elements of the Sr-added sintered body is taken as 100 mol %.

[0039] The proportion of the grain boundary phase is preferably less than 10 mol %. If the mixing ratio of the magnesium compound and rare earth element compound contained as sintering aids in the raw material is high, an excessive amount of grain boundary phase is generated, which leads to a decrease in thermal conductivity, and is therefore undesirable.

[0040] The proportion of the grain boundary phase in the silicon nitride sintered body can be determined using the quantitative results of the above-mentioned quantitative analysis for Si, N, O, Mg, rare earth elements, Sr, and Al.

[0041] Briefly, as described above, when the total content of Si, N, O, Mg, rare earth elements, Sr, and Al is taken as 100 mol%, the quantitative values ​​(contents) of the rare earth elements, Mg, Sr, and Al are converted into the content of each oxide. Then, the amount of oxygen in each oxide is subtracted from the total amount of oxygen in the sintered body, and the surplus is considered to be the amount of oxygen contained in the oxide of Si, and this value is calculated as SiO ２ The oxides of rare earth elements, Mg, Sr, and Al obtained above and SiO ２ The abundance ratio of the grain boundary phase can be obtained by summing up the contents of

[0042] The grain boundary phase is composed of at least one of a crystalline phase having a melilite structure and a crystalline phase having a cuspidin structure, and / or an amorphous phase, which contain the above-mentioned elements. The presence of these crystalline phases and / or amorphous phases can be confirmed by, for example, X-ray diffraction (XRD).

[0043] On the other hand, like the Sr-added sintered body, the non-added sintered body also comprises crystal grains of silicon nitride, which is the main component, and at least one of a crystalline phase with a melilite structure and a crystalline phase with a cuspidin structure, and / or a grain boundary phase consisting of an amorphous phase. The types of elements contained in the grain boundary phase are also the same as those of the Sr-added sintered body, except for the Sr element. The non-added sintered body may also contain Sr element derived from impurities in the raw materials, but the amount is 5.0 × 10 －３ It is sufficiently small compared to mol %.

[0044] That is, the Sr-added sintered body differs from the non-added sintered body in that the grain boundary phase contains a large amount of Sr element.

[0045] Furthermore, with regard to the microstructure, when the firing temperature and firing time are the same, the higher the Sr element content of the silicon nitride sintered body, including when no Sr is added, the more suppressed the grain growth tends to be, and the smaller the proportion of particles with a major axis diameter of 10 μm or more in the total β-type silicon nitride particles.

[0046] The microstructure of the silicon nitride sintered body is confirmed by SEM-EBSD (scanning electron microscope-electron backscatter diffraction) measurement of the cross section of the sintered body. Furthermore, using the results of SEM-EBSD measurement, the ratio of the area of ​​β-silicon nitride particles with a major axis diameter of 10 μm or more to the area occupied by all β-silicon nitride particles at any cross section position of the sintered body (major axis particle ratio) can be determined.

[0047] In this embodiment, as described above, the firing temperature in the firing treatment is set to 1700°C to 1950°C, the firing time is set to 1 hour to 50 hours, and the content of Sr element is set to 5.0 × 10 －３ mol% or more 1.0×10 －１ By adding strontium and / or a strontium compound to the raw material so that the content is 10 mol % or less, a silicon nitride sintered body having both excellent thermal conductivity and strength can be obtained. In this embodiment, a silicon nitride sintered body having a thermal conductivity of 108 W / (m K) or more and a strength of 550 MPa or more is considered to be a silicon nitride sintered body having both excellent thermal conductivity and strength.

[0048] First, the Sr-added sintered body according to this embodiment has a feature of having a smaller amount of dissolved oxygen than a non-added sintered body with a similar long axis grain ratio. However, in this embodiment, the amount of dissolved oxygen in the silicon nitride sintered body is evaluated according to the method reported by Kitayama et al. (J. Am. Ceram. Soc., 82, 3263-3265 (1999)., J. Am. Ceram. Soc., 83, 1985-1992 (2000).) by crushing the sintered body, treating it with acid, and then measuring it with an oxygen analyzer.

[0049] The decrease in the amount of dissolved oxygen is thought to be due to the fact that the ratio of α-silicon nitride particles to β-silicon nitride particles in the compact after nitriding when Sr is added is different from that when Sr is not added. In the compact with Sr added, the ratio of α-silicon nitride particles after nitriding is higher than in the compact without Sr, and this is presumed to have promoted the decrease in the amount of dissolved oxygen during the sintering process.

[0050] Furthermore, the Sr-added sintered body according to this embodiment has a higher thermal conductivity than a non-Sr-added sintered body with the same long axis grain ratio. This is thought to be due to the effect of the low amount of dissolved oxygen, as with conventional silicon nitride sintered bodies.

[0051] The thermal conductivity of the silicon nitride sintered body is calculated by preparing an evaluation sample by processing the sintered body into a shape of Φ10 mm × 1 mm, a plate shape of Φ10 mm × 0.2 to 0.4 mm, or a plate shape of 10 mm × 10 mm × 0.2 to 0.4 mm, and using the thermal diffusivity of the evaluation sample measured by the flash method, the specific heat capacity measured by a differential scanning calorimeter (DSC), and the density measured by the Archimedes method.

[0052] In addition, the Sr-added sintered body according to this embodiment has a higher strength than a non-added sintered body having a similar long axis grain ratio. However, in this embodiment, the strength of the silicon nitride sintered body is measured by preparing an evaluation sample by processing the sintered body into a shape of 1.5 mm × 2 mm × 20 mm, a plate shape of 0.2 to 0.4 mm × 2 mm × 20 mm, or a plate shape of 0.2 to 0.4 mm × 24 mm × 40 mm, and performing a three-point bending test or a four-point bending test on the evaluation sample.

[0053] According to conventional technical knowledge, there is a trade-off between thermal conductivity and strength in silicon nitride sintered bodies, and therefore a silicon nitride sintered body with high thermal conductivity should have low strength. However, in the case of the present embodiment, the Sr element is added at 5.0 × 10 －３ mol% or more 1.0×10 －１ In a sintered body containing Sr at a content of 50% or less by mol and having a long axis grain ratio of 2% or more and 50% or less at any one cross-sectional position, such a trade-off relationship does not apply, and the body has excellent thermal conductivity and strength.

[0054] This is thought to be because the inclusion of Sr in the grain boundary phase increases the thermal expansion coefficient of the grain boundary phase, which reduces the areas where high stress occurs locally when a load is applied, thereby achieving high strength.

[0055] To confirm, the distribution of β-silicon nitride particles, grain boundary crystal grains, and amorphous phases in the Sr-added sintered body according to this embodiment is random. Therefore, the distribution of β-silicon nitride particles with a major axis diameter of 10 μm or more is also random, so the value of the major axis grain ratio in a single Sr-added sintered body will be roughly the same regardless of the position at which the SEM-EBSD measurement used to calculate it is performed. Therefore, the value of the major axis grain ratio at any one cross-sectional position of a certain Sr-added sintered body can be treated as representing the major axis grain ratio in that Sr-added sintered body.

[0056] Furthermore, in such cases, the greater the proportion of β-type silicon nitride particles having a major axis diameter of 10 μm or more in the Sr-added sintered body, the greater the value of the major axis particle ratio at any one cross-sectional position. Therefore, the major axis particle ratio at any one cross-sectional position of the Sr-added sintered body can be used as an index showing the amount of β-type silicon nitride particles.

[0057] Preferably, the thermal expansion coefficient of the Sr-added sintered body is 2.9 ppm / K or more and 3.3 ppm / K or less. Sr-added sintered bodies whose thermal expansion coefficients fall within this range tend to have better thermal conductivity and / or strength than Sr-added sintered bodies whose thermal expansion coefficients do not fall within this range. Sr-added sintered bodies whose thermal expansion coefficients fall within this range can be realized by suitably setting at least one of the main raw material composition and sintering conditions.

[0058] For example, magnesium oxide (MgO) and magnesium silicon nitride (MgSiN) are used as the main raw material magnesium compounds. ２ ) is used, an Sr-added sintered body having a thermal expansion coefficient that satisfies the above range and that is superior in at least one of thermal conductivity and strength compared to an Sr-added sintered body using only magnesium oxide (MgO) as the magnesium compound can be obtained.

[0059] The thermal expansion coefficient of the silicon nitride sintered body is determined by preparing an evaluation sample by processing the sintered body into a plate having a length of 20 mm, a width of 4 mm, and a thickness of 3 mm, or a length of 20 mm, a width of 5 mm, and a thickness of 0.2 mm to 0.4 mm, measuring the thermal expansion of the evaluation sample, and calculating the average thermal expansion coefficient from 40°C to 800°C of the obtained thermal expansion curve.

[0060] In addition, the amount of strontium and / or strontium compound added to the raw material powder was excessive, and therefore the Sr element was 1.0 × 10 －１In silicon nitride sintered bodies containing an excessive amount of Sr exceeding 1.0×10 mol%, the long axis grain ratio at any one cross-sectional position is less than 2%. In such silicon nitride sintered bodies, the amount of dissolved oxygen increases and the thermal conductivity decreases compared to Sr-added sintered bodies in which the long axis grain ratio at any one cross-sectional position is 2% or more and 50% or less as in this embodiment. Note that in such cases where the Sr element content is 1.0×10 －１ A silicon nitride sintered body having more than mol % of silicon nitride added thereto is called an over-added sintered body.

[0061] Such an excessively doped sintered body is obtained when the weight ratio of the main raw materials is the same as that of the Sr-doped sintered body, but the externally added amount of strontium and / or strontium compound powder exceeds 1.1 wt%.

[0062] Furthermore, in silicon nitride sintered bodies produced by firing at temperatures higher than 1950°C or for firing times longer than 50 hours, the long axis grain ratio at any one cross-sectional position exceeds 50%, regardless of whether Sr is added or not. Such silicon nitride sintered bodies produced at firing temperatures higher than 1950°C or for firing times longer than 50 hours are referred to as over-fired sintered bodies.

[0063] Although the over-fired sintered body has a lower amount of dissolved oxygen and a higher thermal conductivity than the Sr-added sintered body according to the present embodiment, it does not have sufficient strength. This is thought to be due to the large defect size.

[0064] From the viewpoint of ensuring strength, it is preferable that the relative density of the silicon nitride sintered body is 95% or more and the porosity is 5% or less.

[0065] Considering the properties of the excessively doped sintered body and the excessively fired sintered body as described above, the amount of dissolved oxygen, thermal conductivity, and strength of the silicon nitride sintered body do not change monotonically depending on the weight ratio of strontium and / or strontium compound in the raw material and the Sr element content in the obtained silicon nitride sintered body, but rather change in proportion to the weight ratio of strontium and / or strontium compound in the raw material and the Sr element content in the obtained silicon nitride sintered body. －３ mol% or more 1.0×10 －１The optimum condition is when the Sr element is present in the grain boundary phase in a content range of 100 mol% or less, and when the firing conditions are set to be suitable so that the long axis grain ratio at any one cross-sectional position is 2% to 50%.

[0066] In other words, in order to obtain a silicon nitride sintered body having both excellent thermal conductivity and strength, there are optimum ranges for the amount of strontium and / or strontium compound added to the raw material powder and the abundance ratio of β-type silicon nitride particles having a major axis diameter of 10 μm or more in the obtained silicon nitride sintered body, and as a result of being produced so as to satisfy this optimum range, a silicon nitride sintered body having a predetermined content range of Sr element in the grain boundary phase and containing β-type silicon nitride particles having a major axis diameter of 10 μm or more in a predetermined abundance ratio range will have both excellent thermal conductivity and strength, unlike silicon nitride sintered bodies produced without satisfying these ranges.

[0067] In addition, such silicon nitride sintered bodies having a thermal expansion coefficient of 2.9 ppm / K or more and 3.3 ppm / K or less have superior thermal conductivity and / or strength compared to silicon nitride sintered bodies not satisfying this range.

[0068] In addition, the content of Sr element according to this embodiment is 5.0 × 10 －３ mol% or more 1.0×10 －１ A silicon nitride sintered body having a long axis grain ratio of 2% to 50% by mol% or less is excellent in both thermal conductivity and strength as described above, and is also suitable for producing a bonding substrate with a metal plate (e.g., a copper plate).

[0069] For example, a bonded substrate fabricated by using the silicon nitride sintered body according to the present embodiment as a base substrate (heat dissipation substrate) and bonding the base substrate to a copper plate by AMB bonding not only has excellent thermal conductivity and strength, but also has excellent durability against thermal cycles. An example of thermal cycles is exposing the bonded substrate to low temperatures (e.g., −55°C) and high temperatures (e.g., 175°C) alternately (e.g., 3000 cycles).

[0070] The reason why the above-mentioned bonded substrate exhibits excellent thermal cycle durability is not entirely clear, but it is presumed that one reason is that the difference in thermal expansion coefficient between the base substrate and the copper plate is smaller than when no additive is added. The thermal cycle durability of the bonded substrate is thought to be influenced by the difference in thermal expansion coefficient between the base substrate and the metal plate. Also, the silicon nitride sintered body with a higher Sr content tends to have a higher thermal expansion coefficient. When the Sr content is 5.0 × 10 －３ It is believed that by using a silicon nitride sintered body having a silicon nitride content of 100 mol % or more as the base substrate, the difference in thermal expansion coefficient between the base substrate and the copper plate is reduced to an extent that excellent durability against thermal cycles is exhibited.

[0071] However, the content of Sr element is 1.0 × 10 －１ It is not preferable to use a sintered body containing an excess of more than mol % as a base substrate, because this will result in significant problems such as the copper plate peeling off from the base substrate and cracks occurring in the base substrate.

[0072] This is presumably because a reaction between the brazing filler metal and excess Sr element is likely to occur when joining to a copper plate by the AMB method, and the presence (interposition) of a product of this reaction makes it impossible to obtain sufficient thermal cycle durability in the joined substrate.

[0073] The silicon nitride sintered body according to the present embodiment may contain aluminum (Al) as an impurity element derived from the raw material. It is known that the Al element dissolves in the silicon nitride particles and reduces the thermal conductivity. However, as described above, the Sr element is added at 5.0 × 10 －３ mol% or more 1.0×10 －１ In the case of a silicon nitride sintered body containing Al in an amount of 1.5×10 mol% or less, －２ If the content is less than mol %, such a decrease in thermal conductivity hardly occurs, and a silicon nitride sintered body having both excellent thermal conductivity and strength can be obtained.

[0074] This is because the content of Al element in the raw material of silicon nitride sintered body is 1.5 × 10 －２The same applies to the case where a substance containing Al element, such as aluminum nitride (AlN), is intentionally mixed in a range of less than mol %.

[0075] As explained above, according to the present embodiment, when producing a silicon nitride sintered body using silicon powder, strontium and / or a strontium compound is added to the raw material powder in a predetermined weight ratio, thereby causing the Sr element to be contained in the grain boundary phase within a predetermined content range, and by controlling the abundance ratio of particles among β-type silicon nitride particles having a major axis diameter of 10 μm or more within a predetermined range, it is possible to obtain a silicon nitride sintered body having both excellent thermal conductivity and strength.

[0076] Furthermore, when such a silicon nitride sintered body is used as a base substrate for a substrate to be bonded to a metal plate, a bonded substrate having excellent durability against thermal cycles is realized.

[0077] <Modifications> In the above-described embodiment, by adding strontium and / or a strontium compound to the raw material powder, a predetermined content range of Sr element is contained in the grain boundary phase of the silicon nitride sintered body, thereby realizing a silicon nitride sintered body having both excellent thermal conductivity and strength.

[0078] Instead of such strontium and / or strontium compounds, or together with strontium and / or strontium compounds, barium (Ba) element and / or compounds may be added to the raw material powder. In such a case, in the silicon nitride sintered body finally obtained, Ba element is also contained in the grain boundary phase, as well as Sr element. And, even in the silicon nitride sintered body containing Ba element in the grain boundary phase in this way, it is possible to obtain a silicon nitride sintered body having both excellent thermal conductivity and strength, as in the above-mentioned embodiment.

[0079] In the above-described embodiment, a portion of the silicon used as the main raw material may be replaced with silicon nitride. In such a case, when weighing the raw material powder, the molar ratio of silicon is converted to the molar ratio of silicon nitride, and the sum of the converted value and the molar ratio of silicon nitride is treated as the composition ratio of silicon nitride in the above-described embodiment.

[0080] Alternatively, the silicon nitride sintered body may be produced by using silicon nitride instead of silicon as the main raw material and by gas pressure sintering.

[0081] <Example A Group and Comparative Example A Group> As Examples A1 to A9 and Comparative Examples A1 to A4, silicon nitride sintered bodies were produced by the manufacturing method according to the above-described embodiment, and the obtained sintered bodies were evaluated. Magnesium oxide (MgO) was used as the magnesium compound, and yttrium oxide (Y ２ O ３ Hereinafter, Examples A1 to A9 will be collectively referred to as Example Group A, and Comparative Examples A1 to A4 will be collectively referred to as Comparative Examples Group A.

[0082] (Example A1) First, as raw materials, 90.5 wt % silicon, 3.6 wt % MgO, and 5.9 wt % Y were used. ２ O ３ and SrCO were weighed out, and then added to the total of these 100 wt % so that the amount of external additive was 0.06 wt %. ３ was weighed.

[0083] The weighed raw material powder was placed in a pot mill, and pot mill mixing was carried out for 45 hours using IPA as a solvent and silicon nitride balls with a diameter of 5 mm.

[0084] Subsequently, the IPA was removed using a nitrogen dryer, and the resulting powder was molded using a mold to have a shape of Φ50×5 mm, and further subjected to CIP molding at a pressure of 2 tons to obtain a molded body.

[0085] Next, the obtained compact was placed in a sheath made of boron nitride (BN) and subjected to a nitriding treatment in which heat treatment was carried out for 5 hours in a nitrogen atmosphere at a nitrogen partial pressure of 0.15 MPa and a temperature of 1400°C, thereby nitriding the silicon to form silicon nitride.

[0086] Subsequently, the nitriding-treated compact was fired for 8 hours in a nitrogen atmosphere at a nitrogen partial pressure of 0.9 MPa and a temperature of 1900° C. to obtain a sintered body.

[0087] Both surfaces of the obtained sintered body were polished to a thickness of 10 μm or more and 50 μm or less, and then a portion was crushed and the contents of Si, N, O, Mg, rare earth elements, Sr, and Al (unit: wt%) were determined according to the method described in JIS R1603, and the amount of dissolved oxygen (unit: wt%) was determined according to the method reported by Kitayama et al. Based on the obtained quantitative values, the Sr content (unit: mol%) was calculated when the total of Si, N, O, Mg, rare earth elements, Sr, and Al was taken as 100 mol%.

[0088] Another portion of the sintered body was processed into a Φ10 mm x 0.32 mm plate, and the thermal diffusivity was measured by the flash method, the specific heat capacity by a differential scanning calorimeter (DSC), and the density was measured by the Archimedes method.The thermal conductivity was calculated using the obtained thermal diffusivity, specific heat capacity, and density measurements.Furthermore, another portion of the sintered body was processed into a 0.32 mm x 2 mm x 20 mm plate, and four-point bending strength measurements were performed.

[0089] In addition, a sample for evaluating the thermal expansion coefficient was prepared by processing another portion of the sintered body into a plate measuring 20 mm in length, 5 mm in width, and 0.2 mm to 0.4 mm in thickness. The thermal expansion of the evaluation sample was measured using a thermal expansion measuring device (DIL402 manufactured by Netsch) to calculate the thermal expansion coefficient. Specifically, the sample was heated and cooled once between 40°C and 800°C, and then heated again from 40°C to 800°C. The average thermal expansion coefficient from 40°C to 800°C on the thermal expansion curve obtained at that time was calculated and used as the thermal expansion coefficient of the sintered body. The thermal expansion curve during the reheating was used in the calculation to reduce the influence of setting errors of the sample to the device.

[0090] Furthermore, an arbitrary cross-sectional position of the sintered body was subjected to SEM-EBSD measurement, and the long axis grain ratio was calculated based on the results.

[0091] Specifically, first, as a pretreatment, the sintered body was cut into predetermined dimensions at an arbitrary position, and then one exposed cross section was subjected to cross-section processing using Ar ion milling.

[0092] The obtained sample was subjected to SEM-EBSD measurement under the following conditions.

[0093] Measurement area: 300 μm×300 μm; acceleration voltage: 15 kV; probe current: 15 nA; sample tilt: 70°; step size: 0.5 μm.

[0094] The major axis diameter distribution (area distribution) of the β-silicon nitride particles was then determined from the obtained measurement data, and the major axis particle ratio was determined by calculating the area ratio of β-silicon nitride particles having a major axis diameter of 10 μm or more when the total area of ​​all β-silicon nitride particles was taken as 100%.

[0095] (Example A2) SrCO ３ The sintered body was produced and evaluated in the same manner as in Example A1, except that the weighed value of was 0.18 wt %.

[0096] (Example A3) SrCO ３ The sintered body was produced and evaluated in the same manner as in Example A1, except that the weighed value of was 0.26 wt %.

[0097] (Example A4) SrCO ３ The sintered body was produced and evaluated in the same manner as in Example A1, except that the weighed value of was 0.34 wt %.

[0098] (Example A5) SrCO ３ The sintered body was produced and evaluated in the same manner as in Example A1, except that the weighed value of was 0.58 wt %.

[0099] (Example A6) SrCO ３ The sintered body was produced and evaluated in the same manner as in Example A1, except that the weighed value of was 0.82 wt %.

[0100] (Example A7) SrCO ３ The sintered body was produced and evaluated in the same manner as in Example A1, except that the weighed value of was 1.1 wt %.

[0101] Example A8 A sintered body was produced and evaluated in the same manner as in Example A7, except that the firing time was set to 16 hours.

[0102] Example A9 A sintered body was produced and evaluated in the same manner as in Example A7, except that the firing temperature was set to 1950° C. and the firing time was set to 6 hours.

[0103] (Comparative Example A1) SrCO ３ The sintered body was produced and evaluated in the same manner as in Example A1, except that no additive was mixed and the firing temperature was set to 1960°C.

[0104] (Comparative Example A2) SrCO ３ The sintered body was produced and evaluated in the same manner as in Example A1, except that no mixing was performed.

[0105] Comparative Example A3 A sintered body was produced and evaluated in the same manner as in Example A7, except that the firing temperature was set to 1960°C.

[0106] (Comparative Example A4) SrCO ３ The sintered body was produced and evaluated in the same manner as in Example A1, except that the weighed value of was 1.2 wt %.

[0107] (Comparison between Example A and Comparative Example A) The weight ratios of raw materials (main raw material weight ratio and SrCO ３ The amounts of the external additives, the firing temperature, and the firing time are listed in Table 1. The total weight ratio of the main raw materials is 100 wt %. ３ The amount of externally added ingredient is a weight ratio when the total weight ratio of the main raw materials is 100 wt %.

[0108] Furthermore, the Sr element content, long axis particle ratio, dissolved oxygen content, thermal expansion coefficient, thermal conductivity, and strength of the sintered bodies of Example Group A and Comparative Example Group A are listed in Table 2.

[0109]

[0110]

[0111] As shown in Table 2, the Sr content in each sintered body is roughly the same as that of SrCO shown in Table 1. ３ Similarly to the increasing order of the external additive amount, the amount monotonically increased in the order of (Comparative Example A1 and Comparative Example A2) → Example A1 → Example A2 → Example A3 → Example A4 → Example A5 → Example A6 → (Example A7 to Example A9, and Comparative Example A3) → Comparative Example A4, and for Example A group, it was 5.0 × 10 －３ mol% or more 1.0×10 －１The value was in the range of 0.1 mol % or less.

[0112] In addition, the thermal expansion coefficient of the sintered body, which has a common combination of firing temperature and firing time of 1900°C and 8 hours, is SrCO ３ The increase was in line with the increase in the amount of external additives.

[0113] Specifically, the thermal expansion coefficient of Example A was within the range of 2.9 ppm / K or more and 3.3 ppm / K or less, and Comparative Example A3 also fell within this range, whereas the thermal expansion coefficients of Comparative Examples A1 and A2 remained slightly smaller than this range, and the thermal expansion coefficient of Comparative Example A4 was slightly larger than this range.

[0114] However, the long axis grain ratio, dissolved oxygen content, thermal conductivity, and strength did not change monotonically in this order.

[0115] Specifically, the long axis particle ratio was in the range of 2% or more and 50% or less in Example A group and Comparative Example A2, but exceeded 50% in Comparative Examples A1 and A3, and remained at 1% in Comparative Example A4.

[0116] In contrast, the amount of dissolved oxygen was 0.05 wt% to 0.09 wt% in Example A group, 0.05 wt% in Comparative Example A1, and even less at 0.03 wt% in Comparative Example A3, while in Comparative Examples A2 and A4 it was 0.11 wt%, which was larger than that of Example A group.

[0117] In addition, the thermal conductivity was 108 W / (m·K) to 115 W / (m·K) in Example A group, and was higher at 116 W / (m·K) and 121 W / (m·K) in Comparative Examples A1 and A3, whereas it was 101 W / (m·K) and 100 W / (m·K) in Comparative Examples A2 and A4, respectively, which were lower than Example A group.

[0118] In other words, the sintered body with a small amount of dissolved oxygen has a high thermal conductivity.

[0119] On the other hand, the strength of the sintered body was 614 MPa to 771 MPa in Example A group, and 620 MPa and 678 MPa in Comparative Examples A2 and A4, respectively, which were comparable to those of Example A group. However, in Comparative Examples A1 and A3, the strength was 523 MPa and 512 MPa, respectively, which were smaller than those of Example A group.

[0120] 2 is a graph plotting the strength of the sintered bodies of Example A and Comparative Example A against thermal conductivity. From Fig. 2, it can be seen that the data points for Comparative Example A are located in the region on the lower right where thermal conductivity is high and strength is low, and in the region on the left where thermal conductivity is low and strength is high, whereas the data points for Example A are located in the range REa shown by the dashed line, which has a better balance between thermal conductivity and strength than Comparative Example A.

[0121] This indicates that in Example Group A, a silicon nitride sintered body having excellent both thermal conductivity and strength was obtained.

[0122] It should be noted that the range REa is merely shown for the sake of convenience. Depending on the weight ratio of the main raw materials and the manufacturing conditions, the range REa may be exceeded, but the range REa may be exceeded. －３ mol% or more 1.0×10 －１ It is also possible to realize a silicon nitride sintered body in which the Sr element is present in the grain boundary phase in a content range of 100 mol% or less, and the long axis grain ratio at any one cross-sectional position is 2% to 50%, and which has both thermal conductivity and strength that are equal to or better than those of Example A group.

[0123] (Summary of Results for Example Group A and Comparative Example Group A) The results for Example Group A and Comparative Example Group A are summarized as follows: －３ mol% or more 1.0×10 －１ This shows that when the range of mol% or less is satisfied and the range of the long axis particle ratio is satisfied, the amount of dissolved oxygen is reduced compared to when these ranges are not satisfied, and both the thermal conductivity and strength are increased.

[0124] Focusing on Example A, Example A8, which had a longer firing time and a larger long axis grain ratio of 44% than the other Examples, had slightly lower strength than the other Examples. Considering the fact that Comparative Examples A1 and A3, which also had larger long axis grain ratios, had lower strength, it can be said that reducing the long axis grain ratio is effective in improving strength. This suggests that applying manufacturing conditions that do not result in an excessively large long axis grain ratio is effective.

[0125] <Example Group B and Comparative Example Group B> In Examples B1 to B9 and Comparative Examples B1 to B4, magnesium oxide (MgO) and magnesium silicon nitride (MgSiN) were used as the main raw material magnesium compound. ２ The sintered bodies were produced and evaluated under the same conditions as in Examples A1 to A9 and Comparative Examples A1 to A4, except that the above-mentioned sintered bodies were used. Hereinafter, Examples B1 to B9 will be collectively referred to as Example Group B, and Comparative Examples B1 to B4 will be collectively referred to as Comparative Example Group B.

[0126] The weight ratio of the main raw materials was the same for Example B group and Comparative Example B group: silicon: 88.0 wt%, MgO: 1.2 wt%, MgSiN ２ : 4.8 wt%, Y ２ O ３ % by weight. As a result, the weight ratio of the magnesium compound in the main raw material is larger in Example B group and Comparative Example B group than in Example A group and Comparative Example A group.

[0127] The weight ratio of raw materials in Example B group and Comparative Example B group (weight ratio of main raw materials and SrCO ３ The total weight ratio of the main raw materials is 100 wt %, and the amount of SrCO ３ The amount of externally added ingredient is a weight ratio when the total weight ratio of the main raw materials is 100 wt %.

[0128] Furthermore, the Sr element content, long axis particle ratio, dissolved oxygen content, thermal expansion coefficient, thermal conductivity, and strength of the sintered bodies of Example B group and Comparative Example B group are listed in Table 4.

[0129]

[0130]

[0131] As shown in Table 4, the Sr content in each sintered body was roughly the same as in Example A group and Comparative Example A group, as shown in Table 3. ３ The amount of the external additives increased monotonically in the same manner as the amount of the external additives increased. －３ mol% or more 1.0×10 －１ The value was in the range of 0.1 mol % or less.

[0132] As for the thermal expansion coefficient, similarly to the case of Example A group and Comparative Example A group, for the sintered bodies having the same combination of firing temperature and firing time of 1900°C and 8 hours, the SrCO ３ The increase was in line with the increase in the amount of external additives.

[0133] Specifically, the thermal expansion coefficient of Example B was within the range of 2.9 ppm / K or more and 3.3 ppm / K or less, and Comparative Example B3 also fell within this range, whereas the thermal expansion coefficients of Comparative Examples B1 and B2 remained slightly smaller than this range, and the thermal expansion coefficient of Comparative Example B4 was slightly larger than this range.

[0134] Furthermore, the long axis grain ratios of the sintered bodies of Example B group and Comparative Example B2 were within the range of 2% to 50%, but those of Comparative Examples B1, B3, and B4 were outside this range. This was the same tendency as the sintered bodies of Example A group and Comparative Example A group, which were produced under the same conditions except for the main raw materials.

[0135] Furthermore, the dissolved oxygen content and thermal conductivity values ​​in Example B group and Comparative Example B group are roughly the same as those in Example A group and Comparative Example A group. ３ Furthermore, in Example B and Comparative Example B, as in Example A and Comparative Example A, the sintered bodies with a small amount of dissolved oxygen had high thermal conductivity.

[0136] On the other hand, the strength of the sintered bodies of Example Group B was higher than that of the sintered bodies of Example Group A, which were prepared under the same conditions except for the main raw materials. The same was true for Comparative Examples B2 and B4, but the strength of Comparative Examples B1 and B3 was lower.

[0137] 3 is a graph plotting the strength of the sintered bodies of Example Group B and Comparative Example Group B against the thermal conductivity. In Fig. 3, the range REb in which the data points of Example Group B exist is shown by a solid line, and the range REa in which the data points of Example Group A exist, which was shown in Fig. 2, is also shown by a dashed line.

[0138] 3, it can be seen that the range REb is shifted to the higher strength side than the range REa, which indicates that the silicon nitride sintered body obtained in Example B had even greater strength than the Example A sintered body.

[0139] Note that, like the range REa, the range REb is shown for the sake of convenience of explanation. Although it may fall outside the range REb depending on the weight ratio of the main raw materials and the manufacturing conditions, the combination of the main raw materials is the same as that of Example B, and the range REb is 5.0 × 10 －３ mol% or more 1.0×10 －１ It is also possible to realize a silicon nitride sintered body in which the Sr element is present in the grain boundary phase in a content range of 100 mol% or less, and the long axis grain ratio at any one cross-sectional position is 2% to 50%, and which has both thermal conductivity and strength that are equal to or better than those of Example B group.

[0140] <Example C Group and Comparative Example C Group> For Examples C1 to C9 and Comparative Examples C1 to C4, sintered bodies were produced and evaluated under the same conditions as Examples B1 to B9 and Comparative Examples B1 to B4, respectively, except that the weight ratios of the main raw materials were different. Hereinafter, Examples C1 to C9 will be collectively referred to as Example C Group, and Comparative Examples C1 to C4 will be collectively referred to as Comparative Example C Group.

[0141] The weight ratio of the main raw materials was the same for Example C group and Comparative Example C group: silicon: 89.5 wt%, MgO: 2.4 wt%, MgSiN ２ : 2.4 wt%, Y ２ O ３%. That is, in Example C group and Comparative Example C group, magnesium oxide (MgO) and magnesium silicon nitride (MgSiN) were used as the main raw material magnesium compound. ２ ) in a weight ratio different from that of Example B and Comparative Example B. Specifically, in Example C and Comparative Example C, the weight ratio of the magnesium compound in the main raw material is set to an intermediate value between that of Example A and Comparative Example A and that of Example B and Comparative Example B, and magnesium oxide (MgO) and magnesium silicon nitride (MgSiN ２ ) are equal in weight ratio.

[0142] The weight ratios of raw materials in Example C group and Comparative Example C group (weight ratio of main raw materials and SrCO ３ The total weight ratio of the main raw materials is 100 wt %, and the SrCO ３ The amount of externally added ingredient is a weight ratio when the total weight ratio of the main raw materials is 100 wt %.

[0143] Furthermore, the Sr element content, long axis particle ratio, dissolved oxygen content, thermal expansion coefficient, thermal conductivity, and strength of the sintered bodies of Example C group and Comparative Example C group are listed in Table 6.

[0144]

[0145]

[0146] As shown in Table 6, the Sr content in each sintered body was roughly the same as that in Example A group and Comparative Example A group, as shown in Table 5. ３ The amount of the external additives increased monotonically in the same manner as the amount of the external additives increased. －３ mol% or more 1.0×10 －１ The value was in the range of 0.1 mol % or less.

[0147] As for the thermal expansion coefficient, similarly to the case of Example A group and Comparative Example A group, for the sintered bodies having the same combination of firing temperature and firing time of 1900°C and 8 hours, the SrCO ３ The increase was in line with the increase in the amount of external additives.

[0148] Specifically, the thermal expansion coefficient of Example C was within the range of 2.9 ppm / K or more and 3.3 ppm / K or less, and Comparative Example C3 also fell within this range, whereas the thermal expansion coefficients of Comparative Examples C1 and C2 remained slightly smaller than this range, and the thermal expansion coefficient of Comparative Example C4 was slightly larger than this range.

[0149] Furthermore, the long axis grain ratios of the sintered bodies of Example C and Comparative Example C2 were within the range of 2% to 50%, but those of Comparative Examples C1, C3, and C4 were outside this range. This was the same tendency as that of the sintered bodies of Example A and Comparative Example A, which were produced under the same conditions except for the main raw materials.

[0150] On the other hand, the dissolved oxygen content in Example C and Comparative Example C was slightly lower than that in Example A and Comparative Example A, and the thermal conductivity was slightly higher than that in Example A and Comparative Example A. However, SrCO ３ The fact that the order of increase in the amount of externally added oxygen and the magnitude relationship did not match was the same as in Example A and Comparative Example A. Furthermore, in Example C and Comparative Example C, as in Example A and Comparative Example A, the sintered bodies with a small amount of dissolved oxygen showed high thermal conductivity.

[0151] In contrast, the strength of the sintered bodies of Example C group was comparable to that of the sintered bodies of Example A group, which were prepared under the same conditions except for the main raw materials. The same was true for Comparative Example C group.

[0152] 4 is a graph plotting the strength of the sintered bodies of Example C and Comparative Example C against the thermal conductivity. In Fig. 4, the range REc in which the data points of Example C exist is shown by a solid line, and the range REa in which the data points of Example A exist, which was shown in Fig. 2, is also shown by a dashed line.

[0153] 4, it can be seen that the range REc is shifted to the high thermal conductivity side compared to the range REa, which indicates that the silicon nitride sintered body obtained in Example C had an even higher thermal conductivity than that of Example A.

[0154] Note that, like the range REa, the range REc is shown for the sake of convenience in explanation. Although it may fall outside the range REc depending on the weight ratio of the main raw materials and the manufacturing conditions, the combination of the main raw materials is the same as that of Example C, and the range REc is 5.0 × 10 －３ mol% or more 1.0×10 －１ It is also possible to realize a silicon nitride sintered body in which the Sr element is present in the grain boundary phase in a content range of 100 mol% or less, and the long axis grain ratio at any one cross-sectional position is 2% to 50%, and which has both thermal conductivity and strength that are equal to or better than those of Example C group.

[0155] <Examples D and E> In Example D, the weight ratio of magnesium oxide (MgO) in the main raw material was changed to magnesium silicon nitride (MgSiN ２ ) as a rare earth element compound. ２ O ３ A sintered body was produced under the same conditions as in Example A4, except that the weight ratio of ) was reduced, and the obtained sintered body was evaluated.

[0156] The weight ratio of the main raw materials is silicon: 93.6 wt%, MgO: 4.2 wt%, MgSiN ２ : 1.4 wt%, Y ２ O ３ : 0.8 wt%.

[0157] In Example E, the magnesium compound in the main raw material was replaced with magnesium silicon nitride (MgSiN ２ ) alone was prepared, and the obtained sintered body was evaluated.

[0158] The weight ratio of the main raw materials is silicon: 92.0 wt%, MgSiN ２ : 4.4 wt%, Y ２ O ３ : 3.6 wt%, SrCO ３ The amount of external addition was 1.0 wt %. The firing temperature was 1900° C. and the firing time was 6 hours.

[0159] The weight ratios of raw materials in Examples D and E (weight ratio of main raw materials and SrCO ３The total weight ratio of the main raw materials is 100 wt %, and the amount of SrCO ３ The amount of externally added ingredient is a weight ratio when the total weight ratio of the main raw materials is 100 wt %.

[0160] Furthermore, the Sr element content, long axis grain ratio, dissolved oxygen content, thermal expansion coefficient, thermal conductivity, and strength of the sintered bodies of Examples D and E are listed in Table 8.

[0161]

[0162]

[0163] In Examples D and E, as in Examples A, B, and C, the content of the Sr element in the sintered body was 5.0 × 10 －３ mol% or more 1.0×10 －１ mol % or less, and the long axis particle ratio was in the range of 2% to 50%.

[0164] On the other hand, the thermal expansion coefficient was different from Example Group A, Example Group B, and Example Group C, and was a value outside the range of 2.9 ppm / K or more and 3.3 ppm / K or less.

[0165] Data points showing the sets of thermal conductivity and strength values ​​for the sintered bodies of Examples D and E are also shown in FIG. 2, which shows the data points for Example A group and Comparative Example A group.

[0166] 2, similar to Example A group, which is included in the range REa, Examples D and E also have a well-balanced thermal conductivity and strength value compared to Comparative Example A group. However, compared to Example A group, the thermal conductivity and strength values ​​are slightly lower.

[0167] Considering the results of Example Group B and Example Group C shown in FIGS. 3 and 4 (comparison of Range REb and Range REc with Range REa), it is clear that the Sr content is 5.0×10 －３ mol% or more 1.0×10 －１A silicon nitride sintered body having a long axis grain ratio of 2% or more and 50% or less by mol% and a thermal expansion coefficient of 2.9 ppm / K or more and 3.3 ppm / K or less can be said to be superior in both thermal conductivity and strength to a silicon nitride sintered body having a thermal expansion coefficient that does not satisfy the above ranges.

[0168] <Example Group F> As Examples F1 to F5, silicon nitride sintered bodies having a different combination of rare earth element compounds in the main raw materials than Example Group A were produced by the manufacturing method according to the above-described embodiment, and the resulting sintered bodies were evaluated. Only magnesium oxide (MgO) was used as the magnesium compound. Hereinafter, Examples F1 to F5 will be collectively referred to as Example Group F.

[0169] The weight ratios of the main raw materials were all the same: silicon: 87.3 wt %, MgO: 3.5 wt %, rare earth compound: 9.2 wt %, but the rare earth compounds used were different.

[0170] Specifically, in Examples F1 and F2, Yb ２ O ３ In Example F3, Y ２ O ３ :4.0wt%, Yb ２ O ３ In Example F4, Er was used. ２ O ３ In Example F5, Yb ２ O ３ :4.6wt%, Er ２ O ３ : 4.6 wt%.

[0171] Also, SrCO ３ The amount of externally added was 1.1 wt % in Example F2, the same as in Examples A7 to A9, and 0.34 wt % in the others, the same as in Example A4.

[0172] The firing conditions were the same as those of Example A1 and the like except for Example F2, except that the firing time of Example F2 was changed to 16 hours, the same as that of Example A8 and the like.

[0173] The weight ratio of raw materials in Example F group (weight ratio of main raw materials and SrCO ３The total weight ratio of the main raw materials is 100 wt %, and the amount of SrCO ３ The amount of externally added ingredient is a weight ratio when the total weight ratio of the main raw materials is 100 wt %.

[0174] Furthermore, Table 10 shows the Sr element content, long axis grain ratio, dissolved oxygen content, thermal expansion coefficient, thermal conductivity, and strength of the sintered body of Example Group F.

[0175]

[0176]

[0177] As shown in Table 10, the Sr contents in the sintered bodies of Examples F1, F3, F4, and F5 were approximately 3.0 × 10 －２ The Sr content in the sintered body of Example F2 was approximately 8.9 × 10 mol%, which is close to the Sr contents in the sintered bodies of Examples A7 to A9, which have similar amounts of externally added Sr. －２ mol %.

[0178] The long axis grain ratio was in the range of 2% to 50%, and the thermal expansion coefficient was in the range of 2.9 ppm / K to 3.3 ppm / K.

[0179] Furthermore, the amount of dissolved oxygen was about the same as or slightly lower than that of Example C, and the thermal conductivity was about the same as or slightly higher than that of Example C. On the other hand, the strength was generally about the same as that of Example A and Example C.

[0180] Fig. 5 is a graph plotting the strength of the sintered body of Example Group F against the thermal conductivity. In Fig. 5, the range REF where the data points of Example Group F exist is shown by a solid line, and the range REa where the data points of Example Group A exist, which was shown in Fig. 2, is also shown by a dashed line.

[0181] It can be seen from FIG. 5 that the range REF is shifted to the higher thermal conductivity side than the range REa.

[0182] This means that even if the type of rare earth element compound in the main raw material is different from that in Example Group A, as in Example Group F, the Sr content is 5.0 × 10 －３ mol% or more 1.0×10 －１ mol% or less, the long axis grain ratio is in the range of 2% or more and 50% or less, and the thermal expansion coefficient is in the range of 2.9 ppm / K or more and 3.3 ppm / K or less, a silicon nitride sintered body having both excellent thermal conductivity and strength, similar to Example A group, can be obtained.

[0183] Note that, like the range REa, the range REF is shown for the sake of convenience in explanation. Although it may fall outside the range REF depending on the weight ratio of the main raw materials and the manufacturing conditions, the combination of the main raw materials is the same as that of Example F, and the range REF is 5.0 × 10 －３ mol% or more 1.0×10 －１ It is also possible to realize a silicon nitride sintered body in which the Sr element is present in the grain boundary phase in a content range of 100 mol% or less, and the long axis grain ratio at any one cross-sectional position is 2% to 50%, and which has both thermal conductivity and strength that are equal to or better than those of Example F group.

[0184] <Example G Group> As examples G1 to G4, similar to example B group and example C group, magnesium oxide (MgO) and magnesium silicon nitride (MgSiN) were used as the magnesium compound in the main raw material. ２ ) but with a different combination of rare earth element compounds from those of Example Group B and Example Group C, silicon nitride sintered bodies were produced by the manufacturing method according to the above-described embodiment, and the resulting sintered bodies were evaluated. Note that, hereinafter, Examples G1 to G4 will be collectively referred to as Example Group G.

[0185] The weight ratio of the main raw materials is common: silicon: 85.3 wt%, MgO: 1.0 wt%, MgSiN ２ % and rare earth compound: 9.2 wt %, and the rare earth compounds used were different.

[0186] Specifically, in Example G1, Yb ２ O ３ In Example G2, Y ２ O ３:2.8wt%, Yb ２ O ３ In Example G3, Er was ２ O ３ In Example G4, Yb ２ O ３ :4.6wt%, Er ２ O ３ : 4.6 wt%.

[0187] Also, SrCO ３ The amount of externally added was 0.57 wt %, the same as in Example B5. The firing conditions were the same as in Example A1.

[0188] Weight ratio of raw materials in Example G group (weight ratio of main raw materials and SrCO ３ The amounts of the external additives, the firing temperature, and the firing time are listed in Table 11. The total weight ratio of the main raw materials is 100 wt %. ３ The amount of externally added ingredient is a weight ratio when the total weight ratio of the main raw materials is 100 wt %.

[0189] Furthermore, Table 12 shows the Sr element content, long axis grain ratio, dissolved oxygen content, thermal expansion coefficient, thermal conductivity, and strength of the sintered bodies of Example Group G.

[0190]

[0191]

[0192] As shown in Table 12, the Sr content in each sintered body was approximately 5.0 × 10, which was the content in Example B5. －２ Although it is slightly larger than mol%, 5.0 × 10 －３ mol% or more 1.0×10 －１ The value was in the range of 0.1 mol % or less.

[0193] The long axis grain ratio was in the range of 2% to 50%, and the thermal expansion coefficient was in the range of 2.9 ppm / K to 3.3 ppm / K.

[0194] The dissolved oxygen content and thermal conductivity were similar to those of Example F. That is, the thermal conductivity was slightly larger than those of Examples other than Example F.

[0195] On the other hand, the strength was comparable to that of Example B. That is, the strength was slightly greater than that of Examples other than Example B.

[0196] Fig. 6 is a graph plotting the strength of the sintered body of Example Group G against the thermal conductivity. In Fig. 6, the range REg in which the data points of Example Group G exist is shown by a solid line, and the range REa in which the data points of Example Group A exist is shown by a dashed line.

[0197] It can be seen from Fig. 6 that range REg is shifted toward higher thermal conductivity and higher strength than range REa. Moreover, range REg is located on the higher thermal conductivity side than range REb of Example B group shown in Fig. 3, and is also located on the higher strength side than range REc of Example C group shown in Fig. 4 and range REF of Example F group shown in Fig. 5.

[0198] This means that even if the types of magnesium compound and rare earth element compound in the main raw material are different from those in Example Group G, the weight ratio of each compound can be suitably adjusted to a value where the Sr content is 5.0 × 10 －３ mol% or more 1.0×10 －１ This shows that when the range of 1.0 mol% or less is satisfied, the long axis grain ratio is in the range of 2% or more and 50% or less, and the thermal expansion coefficient is in the range of 2.9 ppm / K or more and 3.3 ppm / K or less, a silicon nitride sintered body can be obtained which has a good balance between thermal conductivity and strength and is even better in both respects than Example A group.

[0199] Note that, like the range REa, the range REg is shown for the sake of convenience in explanation. Although it may fall outside the range REg depending on the weight ratio of the main raw materials and the manufacturing conditions, the combination of the main raw materials is the same as that of Example G, and the range REa is 5.0 × 10 －３ mol% or more 1.0×10 －１ It is also possible to realize a silicon nitride sintered body in which the Sr element is present in the grain boundary phase in a content range of 100 mol% or less, and the long axis grain ratio at any one cross-sectional position is 2% to 50%, and which has both thermal conductivity and strength that are comparable to or even superior to those of Example G group.

[0200] <Production and evaluation of silicon nitride sintered body by another production method> In all of Examples A to G and Comparative Examples A to C, the molded body to be fired in the firing treatment was produced by mold forming and subsequent CIP molding. In the following, a slurry was prepared by weighing and mixing raw material powders to have the same composition as some of the above-mentioned Examples and Comparative Examples, and a silicon nitride sintered body was produced using a sheet molding method as the molded body production method, and the results of its evaluation are shown.

[0201] Specifically, 15 types of slurries having the same composition as those of Comparative Example A2, Example A1, Example A4, Example A7, Comparative Example A4, Comparative Example B2, Example B1, Example B3, Example B7, Comparative Example B4, Comparative Example C2, Example C1, Example C2, Example C7, and Comparative Example C4 were prepared, and molded bodies were produced.

[0202] Hereinafter, all 15 sintered bodies produced using these slurries and employing the sheet molding method as a method for producing the compacts will be referred to with the suffix "-2," such as Comparative Example A2-2, Example A1-2, Example A4-2, ..., Comparative Example C-2. For the sake of distinction, the compacts produced using the sheet molding method will be referred to as sheet compacts, and the sintered bodies produced using such sheet compacts will also be referred to as sheet-molded sintered bodies. Furthermore, the sintered bodies obtained in the above-mentioned Examples A to G and Comparative Examples A to C, in which mold molding and CIP molding were employed for producing the compacts, may be referred to as bulk sintered bodies.

[0203] To prepare the sheet compact, a slurry was first prepared. Specifically, the raw material powders were weighed in predetermined proportions, and these and a dispersant (organic solvent) were placed in a pot mill. Pot mill mixing was performed for 30 hours using silicon nitride balls with a diameter of 5 mm. A resin binder was then added, and the mixture was mixed for another 15 hours. The viscosity was then adjusted using a vacuum defoamer to obtain a slurry.

[0204] The obtained slurry was processed into a tape by a doctor blade method and cut to an appropriate size to obtain a sheet-like molded body having a rectangular shape in plan view (i.e., a sheet molded body). The thickness of the sheet molded body was adjusted so that the thickness of the silicon nitride sintered body (sheet molded sintered body) finally obtained would be 0.32 mm.

[0205] The obtained sheet molded body was subjected to a degreasing treatment for removing organic components (dispersant and binder) by heating at 800° C. in a vacuum for 4 hours.

[0206] Next, the degreased sheet molded body was placed in a sheath made of boron nitride (BN), and a plate also made of BN was placed on top of it. As a nitriding treatment, heat treatment was carried out for 5 hours in a nitrogen atmosphere at a nitrogen partial pressure of 0.15 MPa and a temperature of 1400°C, thereby nitriding the silicon to form silicon nitride.

[0207] Subsequently, the sheet molded body after the nitriding treatment was fired for 8 hours in a nitrogen atmosphere at a nitrogen partial pressure of 0.9 MPa and a temperature of 1900°C, thereby obtaining a rectangular plate-shaped silicon nitride sintered body (sheet molded sintered body) having a thickness of 0.32 mm.

[0208] For each of the obtained sheet-molded sintered bodies, the Sr content (unit: mol%) was calculated when the total of Si, N, O, Mg, rare earth elements, Sr, and Al was taken as 100 mol%, and the thermal expansion coefficient, thermal conductivity, and four-point bending strength were evaluated. SEM-EBSD measurements were also performed, and the long axis grain ratio was calculated based on the results. The results of these evaluations are listed in Table 13.

[0209]

[0210] The evaluation results for each of the sheet-molded sintered bodies shown in Table 13 were generally equivalent to the evaluation results for the bulk sintered bodies shown in Tables 2, 4, and 6, which had the same blended composition of raw material powder.

[0211] The results show that even if the manufacturing method of the compact is different, the －３ mol% or more 1.0×10 －１This shows that it is possible to obtain a silicon nitride sintered body in which the Sr element is present in the grain boundary phase in a content range of 100 mol% or less, and the long axis grain ratio at any one cross-sectional position is 2% to 50%, and which has both excellent thermal conductivity and strength.

[0212] <Fabrication of Bonded Substrates and Evaluation of Thermal Cycle Durability> Bonded substrates were fabricated using silicon nitride sintered bodies as base substrates, and the durability of the resulting bonded substrates to thermal cycles was evaluated. The base substrates used were 15 types of silicon nitride sintered bodies (sheet-formed sintered bodies), the characteristics of which are shown in Table 13. The bonded substrates were fabricated by bonding a 0.8 mm thick copper plate to each 0.32 mm thick silicon nitride sintered body using the AMB (active metal brazing) method.

[0213] For the AMB bonding, first, brazing filler metal layers containing 40% by weight to 80% by weight of silver and titanium hydride as active metal brazing filler metal were formed on both sides of the base substrate. Next, a copper plate was placed on each brazing filler metal layer and hot-pressed in a vacuum to obtain a copper / silicon nitride / copper bonded structure. The hot-pressing process involved applying pressure according to a pressure profile with a maximum pressure of 20 MPa and heating according to a temperature profile with a maximum temperature of 815°C to 845°C. Subsequently, etching was performed by spraying an etching solution at a temperature of approximately 45°C onto the bonded structure to obtain a bonded substrate.

[0214] Each of the bonded substrates thus obtained was subjected to a thermal cycle durability evaluation test, in which the bonded substrate was subjected to 3000 thermal cycles of low temperature (-55°C) and high temperature (175°C).

[0215] Then, after exposure to the thermal cycle, each bonded substrate was subjected to an ultrasonic flaw detection test to evaluate whether or not the copper plate had peeled off from the base substrate and whether or not cracks had occurred in the base substrate.

[0216] The results of the thermal cycle durability test for each bonded substrate are shown in the rightmost column of Table 13, along with the results of the characteristic evaluation of the silicon nitride sintered body used as the base substrate in the preparation of the bonded substrate.

[0217] Specifically, for bonded substrates in which neither peeling of the copper plate from the base substrate nor the occurrence of cracks in the base substrate was observed in the ultrasonic flaw detection test after the thermal cycle durability test, the bonded substrates were judged to have excellent thermal cycle durability, and a circle (◯) was marked in the corresponding column in Table 13.

[0218] On the other hand, for bonded substrates in which at least one of peeling of the copper plate from the base substrate and the occurrence of cracks in the base substrate was observed, the bonded substrates were judged to have poor thermal cycle durability, and an "x" (cross mark) was entered in the corresponding column in Table 13.

[0219] As can be seen from Table 13, the content of Sr element is 5.0 × 10 －３ mol% or more 1.0×10 －１ The bonded substrates using as base substrates silicon nitride sintered bodies with a long axis grain ratio of 2% to 50% by mol% or less were judged to have excellent thermal cycle durability. On the other hand, the bonded substrates using as base substrates silicon nitride sintered bodies with no added Sr element and silicon nitride sintered bodies with excessive added Sr element were judged to have poor thermal cycle durability.

[0220] Thus, the content of Sr element is 5.0 × 10 －３ mol% or more 1.0×10 －１ When a silicon nitride sintered body having a long axis grain ratio of 2% to 50% by mol% or less is used as a base substrate for a bonded substrate to be bonded to a copper plate, a bonded substrate having high thermal cycle durability and therefore high quality stability can be obtained. This shows that a silicon nitride sintered body satisfying both the above-mentioned range of the long axis grain ratio and the range of the Sr element content not only has excellent thermal conductivity and strength, but is also suitable as a base substrate for such a bonded substrate.

Claims

1. A silicon nitride sintered body, comprising a grain boundary phase containing one or more rare earth elements selected from the group consisting of Y, Sc, La, Ce, Sm, Gd, Dy, Ho, Er, Lu, and Yb, and Mg, Si, O, N, and Sr, wherein the content of Sr in the sintered body is 5.0 × 10 －３ mol% or more 1.0×10 －１ mol% or less, and the ratio of the area occupied by β-type silicon nitride particles having a major axis diameter of 10 μm or more to the area occupied by all β-type silicon nitride particles at any one cross-sectional position of the sintered body is 2% or more and 50% or less.

2. The silicon nitride sintered body according to claim 1, characterized in that the thermal expansion coefficient is 2.9 ppm / K or more and 3.3 ppm / K or less.

3. A silicon nitride sintered body according to claim 1 or 2, characterized in that at least a portion of the grain boundary phase is an amorphous phase.

4. A silicon nitride sintered body according to claim 1 or 2, characterized in that at least a portion of the grain boundary phase is a crystalline phase of melilite structure.

5. A silicon nitride sintered body according to claim 1 or 2, characterized in that at least a portion of the grain boundary phase is a crystalline phase with a cuspidin structure.

6. A silicon nitride sintered body according to claim 1 or 2, characterized in that the grain boundary phase is a crystalline phase of melilite structure and / or cuspidin structure and an amorphous phase.

7. A silicon nitride sintered body according to any one of claims 1 to 6, wherein the content of Al element in the sintered body is 1.5 x 10 －２ % or less by mol.

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