Silicon nitride powder, and method for producing silicon nitride powder

Abstract

To provide silicon nitride powder with a high α fraction and reduced adverse effects on sintered body properties.SOLUTION: A silicon nitride powder has an α fraction of 90% or more and contains Y2Si3O3 N4 as a minor component.

Description

[Technical Field]

[0001] The present invention relates to a silicon nitride sintered body and a method for producing a silicon nitride sintered body. [Background technology]

[0002] In recent years, with the increasing density and power output of electronic devices and semiconductor devices, the heat density of power modules has also increased. Rising temperatures in power modules can cause malfunctions in components and cracking of the insulating circuit board. Therefore, ceramic substrates such as alumina and aluminum nitride, which have relatively high thermal conductivity, have been used for insulating circuit boards. However, alumina and aluminum nitride have the drawback of low mechanical strength. Consequently, thick copper, which experiences strong thermal stress, cannot be directly bonded to the ceramic substrate, limiting the structure of power modules. Specifically, the need to solder heat sinks made of copper or aluminum to the insulating circuit board leads to larger power modules, which is a problem. Therefore, silicon nitride (Si3N4) is attracting attention as an insulating circuit board material. Because silicon nitride sintered bodies have higher strength and fracture toughness compared to alumina and aluminum nitride sintered bodies, it becomes possible to directly bond thick copper to the insulating circuit board, contributing to the miniaturization of modules.

[0003] Generally, silicon nitride sintered bodies are manufactured by using silicon nitride powder as a starting material, adding a small amount of sintering aid, and firing them at high temperatures. Silicon nitride powder is produced by direct nitriding, silica reduction, and imide pyrolysis. In particular, direct nitriding is a method of producing silicon nitride powder by heat-treating silicon powder in nitrogen, and has the advantage of not being contaminated with impurities such as carbon, so it is a method that is often used to produce high-performance silicon nitride sintered bodies. It is also known that the state of the silicon nitride powder affects the properties of the silicon nitride sintered body, such as thermal conductivity, physical strength, and corrosion resistance.

[0004] Patent Document 1 discloses a method for producing high-alpha-type, high-purity silicon nitride powder. As described in Patent Document 1, there are two types of silicon nitride: alpha-type silicon nitride (α-Si3N4) and beta-type silicon nitride (β-Si3N4), which have different crystalline phases. Alpha-type silicon nitride has the property of irreversibly transforming into beta-type silicon nitride at high temperatures (around the sintering temperature of 1500-1700°C). On the other hand, beta-type silicon nitride is inferior to alpha-type silicon nitride in terms of sinterability when producing silicon nitride sintered bodies. In other words, in silicon nitride powder used as a raw material for silicon nitride sintered bodies, a high proportion of alpha-type silicon nitride (alpha fraction, alpha conversion rate) is desirable in order to sinter without impairing the properties of the sintered body. Therefore, producing high-purity silicon nitride powder with a high proportion of alpha-type silicon nitride has been a conventional challenge. In particular, this direct nitriding method involves the reaction 3Si + 2N2 → Si3N4 at high temperatures, and is known to be accompanied by a large reaction heat of 176 kcal per mole of silicon nitride. Therefore, a problem has been that in silicon nitride powder, the proportion of β-type silicon nitride, which is high temperature stable and can be produced at high temperatures, tends to be dominant. For this reason, various catalysts are added to silicon powder as additives, and the reaction is carried out at a relatively low temperature of 1300-1350°C to increase the proportion of α-type silicon nitride.

[0005] Patent Document 1 describes how, when producing silicon nitride powder by directly reacting silicon (metallic silicon) powder with nitrogen, using silicon powder with a metal impurity content of 2000 ppm or less, an oxygen content of 0.1 to 0.4% by weight, and an average particle size of 5 to 20 μm, and adding 0.2 to 0.7 parts by weight of calcium oxide per 100 parts by weight of silicon powder, it is possible to produce high-alpha silicon nitride powder with an alpha fraction of 90% by weight or more and high purity. In other words, in the manufacturing method of Patent Document 1, by adding a small amount of calcium oxide to the silicon powder, the Ca content detected as an impurity is suppressed to about 0.2% by weight or less, and silicon nitride powder having an alpha fraction of 90% by weight or more is obtained.

[0006] Furthermore, Patent Document 2 discloses a method for producing silicon nitride powder for manufacturing high-strength sintered bodies. The manufacturing method in Patent Document 2 uses a raw material containing 1 to 5 parts by weight of silicon oxide per 100 parts by weight of silicon (metallic silicon) powder, and is characterized by continuously, intermittently, or temporarily supplying at least one type of halide selected from the group consisting of alkali metal halides and alkaline earth metal halides in a gaseous state to perform nitriding. This manufacturing method also yields silicon nitride powder having an α fraction of 90% by weight or more. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 6-219715 [Patent Document 2] Japanese Patent Application Publication No. 2-248309 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, the manufacturing method described in Patent Document 1 involves adding calcium oxide to silicon powder to produce silicon nitride powder with a high alpha fraction, and it is unavoidable that a small amount of Ca compound remains as an impurity in the synthesized silicon nitride powder. It is known that when manufacturing silicon nitride sintered bodies, Ca compounds remain in the sintered body, leading to a decrease in the strength of the sintered body at high temperatures. Furthermore, in the manufacturing method described in Patent Document 2, the addition of halides makes it unavoidable that F and Cl compounds remain as impurities in the synthesized silicon nitride powder. It is known that F and Cl compounds lead to deterioration of moldability and deterioration of the properties of the sintered body when manufacturing silicon nitride sintered bodies. Therefore, the inventors aimed to provide a synthetic silicon nitride powder with a high alpha fraction (90% or more) without containing impurities that adversely affect the properties of the sintered body when manufacturing silicon nitride sintered bodies.

[0009] To solve the above problems, the present invention aims to provide silicon nitride powder having a high alpha fraction and suppressing adverse effects on sintered body properties, and a method for producing said silicon nitride powder. [Means for solving the problem]

[0010] One embodiment of the present invention is characterized by having an α fraction of 90% or more and containing Y2Si3O3N4 as a minor component.

[0011] A further embodiment of the silicon nitride powder of the present invention is more preferably characterized by containing 4.0 to 9.0% by weight of Y and 0.5 to 2.0% by weight of Mg, in terms of metal equivalent.

[0012] A method for producing silicon nitride powder according to one embodiment of the present invention is characterized by comprising the steps of: preparing a mixed powder by mixing 90 to 96% by weight of silicon powder, 2.5 to 6.5% by weight of Y2O3 powder, and 1.5 to 4.5% by weight of MgO powder; and nitriding the mixed powder by a direct nitriding method to obtain the silicon nitride powder.

[0013] A further embodiment of the present invention is a method for producing silicon nitride powder, more preferably characterized in that, in the step of producing the mixed powder, the mixing ratio of Y2O3 powder and MgO powder by weight percentage, Y2O3 / MgO, is 1 or more.

[0014] A further embodiment of the present invention is a method for producing silicon nitride powder, more preferably characterized in that the step of producing the mixed powder further includes adding 0 to 40 parts by weight of silicon nitride powder to 100 parts by weight of the mixed powder.

[0015] A further embodiment of the present invention is a method for producing silicon nitride powder, more preferably characterized in that the step of nitriding the mixed powder includes reducing the nitrogen pressure inside the furnace to a negative pressure relative to atmospheric pressure at a reduced pressure start temperature of 1100 to 1150°C. [Effects of the Invention]

[0016] The silicon nitride powder of the present invention is produced by adding a small amount of yttrium oxide and magnesium oxide as catalysts to silicon powder and nitriding the mixed powder by a direct nitriding method, and has an α fraction of 90% or more. The silicon nitride powder produced in this way contains Y2Si3O3N4 and Mg compounds as by-components as products of nitriding of yttrium oxide and magnesium oxide powders. However, Y2Si3O3N4 and Mg compounds contained as impurities in the silicon nitride powder do not adversely affect the properties of the sintered body when producing a silicon nitride sintered body from the silicon nitride powder. That is, in the manufacturing process of the silicon nitride sintered body, rare earth oxides containing yttrium oxide and magnesium oxide are added as sintering aids for densification of the sintered body. Therefore, the silicon nitride powder of the present invention does not contain impurities that adversely affect the properties of the sintered body during the production of the silicon nitride sintered body, and has a high α fraction (90% or more), enabling the production of a silicon nitride sintered body with more excellent properties.

Brief Description of the Drawings

[0017] [Figure 1] Powder X-ray diffraction pattern of the silicon nitride powder of Example 1. [Figure 2] Powder X-ray diffraction pattern of the silicon nitride powder of Example 8. [Figure 3] Powder X-ray diffraction pattern of the silicon nitride powder of Comparative Example 1.

Modes for Carrying Out the Invention

[0018] The silicon nitride powder of an embodiment of the present invention is mainly used as a raw material powder for manufacturing a silicon nitride sintered body. The silicon nitride powder contains 90% or more of α-type silicon nitride (α-Si3N4) particles among α-type silicon nitride (α-Si3N4) particles and β-type silicon nitride (β-Si3N4) particles. The α fraction (%) indicates the content ratio (mass fraction) of α-type silicon nitride particles and is represented by α / (α + β)×100. Further, the silicon nitride powder contains amounts of Y2Si3O3N4 and Mg compound (Y2O3-SiO2-MgO-based glass) corresponding to the addition amounts as nitrides of yttrium oxide and magnesium oxide added to silicon powder. Preferably, in the silicon nitride powder, the content of Y is 4.0 to 9.0%, and the content of Mg is 0.5 to 2.0%. That is, the silicon nitride powder contains a predetermined amount of Y and Mg as impurities. However, since oxides of Y and Mg are used as sintering aids during the production of the silicon nitride sintered body, the compounds of Y and Mg in the silicon nitride powder do not adversely affect the properties of the sintered body, unlike the compounds of Ca, F, or Cl.

[0019] Subsequently, a method for manufacturing the silicon nitride powder of the present embodiment will be described. The manufacturing method of the present embodiment mainly includes a mixing step of mixing 90 to 96% by weight of silicon powder, 2.5 to 6.5% by weight of yttrium oxide (Y2O3) powder, and 1.5 to 4.5% of magnesium oxide (MgO) powder to produce a mixed powder, and a nitriding step of nitriding the mixed powder by a direct nitriding method, and enables obtaining a silicon nitride powder with a high α fraction (90% or more) without adding impurities that adversely affect the sintered body properties during the production of the silicon nitride sintered body. Hereinafter, each step will be described in more detail.

[0020] In the mixing step, silicon powder, magnesium oxide powder, and yttrium oxide powder are weighed into predetermined amounts, and the powders are uniformly mixed to produce a mixed powder. Preferably, the silicon powder is of high purity and has a low oxygen content. The mixing step may also preferably further include adding 0 to 40 parts by weight of silicon nitride powder per 100 parts by weight of the mixed powder. This silicon nitride powder acts as a diluent that dissipates the heat generated during the nitriding of the silicon powder, and at the same time prevents the silicon nitride powders from fusing together after synthesis. In this invention, high α-ratio silicon nitride powder can be synthesized even without including silicon nitride as a diluent, but the above additional effects can be obtained by including silicon nitride powder. Preferably, the optionally added silicon nitride powder is high α-ratio silicon nitride powder that does not contain impurities that adversely affect the sintered body properties, as obtained by the manufacturing method of this invention.

[0021] Next, the mixed powder is filled into the firing scabbards. The packing density at this stage is 0.4 to 1.0 g / cm³. 3 It is preferable that the powder-filled pods are placed into a carbon heater firing furnace and the nitriding process is performed.

[0022] In the nitriding process, the furnace is evacuated before heating (preferably to 20 Pa or less), and then filled with nitrogen gas to a predetermined nitrogen pressure (preferably atmospheric pressure to about 0.2 MPa). The nitrogen pressure inside the furnace during synthesis can be arbitrarily adjusted by changing the flow rate of nitrogen gas. Then, while flowing nitrogen gas at a predetermined flow rate to maintain the above nitrogen pressure, the furnace is heated at an arbitrary heating rate until it reaches a first temperature. The first temperature is preferably about 1000°C to 1150°C. Next, at the first temperature, the heating rate is adjusted to decrease in order to suppress the occurrence of a rapid thermal reaction. In other words, the heating rate after exceeding the first temperature is slower than the heating rate to reach the first temperature. Preferably, the heating rate after exceeding the first temperature is about 1 to 3°C / min, and more preferably 2°C / min or less. Then, while maintaining the nitrogen pressure, the furnace is slowly heated until it reaches a second temperature. The second temperature is preferably about 1100°C to 1200°C. The first and second temperatures may be the same. Next, at the second temperature, the nitrogen gas flow rate is adjusted so that the nitrogen pressure inside the furnace becomes negative relative to atmospheric pressure. Preferably, the nitrogen pressure inside the furnace is reduced to 20 to 80 kPa. Then, the furnace is slowly heated under reduced pressure from the second temperature to a higher third temperature. The third temperature is preferably about 1200°C to 1300°C. The second and third temperatures represent the start and end temperatures of reduced pressure, respectively, and the pressure is reduced so that the nitrogen pressure becomes negative within this temperature range. Next, at the third temperature, the nitrogen gas flow rate is adjusted so that the nitrogen pressure is equal to or greater than atmospheric pressure, and the furnace is heated to the maximum temperature. This maximum temperature is preferably 1300°C to 1450°C. Then, by maintaining the maximum temperature and carrying out the nitriding reaction for a reaction time of preferably about 1 to 6 hours, silicon nitride powder with a high alpha fraction (90% or more) can be obtained.

[0023] As described above, the silicon nitride powder manufacturing method of this embodiment is a direct nitriding method in which silicon powder is used as a starting material and nitrided by heating in a nitrogen atmosphere. Generally, the synthesis reaction of silicon nitride by the direct nitriding method is accompanied by a large amount of heat, and it is said that the temperature fluctuation caused by the exothermic reaction can reach several hundred degrees. Alpha-type silicon nitride particles are the crystalline form at low temperatures and readily transform into beta-type silicon nitride particles when heated. In other words, because the nitriding reaction of silicon in the direct nitriding method is an exothermic reaction, the powder becomes locally hot due to the heat of this reaction, and in the synthesized powder produced, there are areas where beta-type silicon nitride particles are locally dominant. For this reason, conventionally, synthesis has been carried out by adding catalysts or diluents, which are impurities, to the silicon powder in order to reduce the reaction temperature, but it has been difficult to obtain silicon nitride powder with a high alpha fraction (90% or more) without the additives adversely affecting the sintered body properties.

[0024] The manufacturing method of this embodiment solves the problems of the direct nitriding method described above by simultaneously adding magnesium oxide (MgO) and yttrium oxide (Y2O3) as catalysts. Specifically, the oxide film on the surface of the silicon particles inhibits the reaction between silicon powder and nitrogen, and removing this oxide film allows silicon and nitrogen to react rapidly. Furthermore, yttrium oxide reacts with the oxide film (SiO2) on the silicon surface, effectively removing the oxide film. In addition, magnesium oxide forms a Y2O3-SiO2-MgO system glass, which has the effect of lowering the melting point. As a result, the SiO2 film on the surface of the silicon powder is rapidly removed at low temperatures, and the nitriding initiation temperature is lowered. Moreover, since yttrium oxide and magnesium oxide added as catalysts are generally used as sintering aids in the production of high thermal conductivity silicon nitride sintered bodies, their presence in the silicon nitride powder after synthesis does not cause any problems in terms of properties. Furthermore, by measuring the Mg and Y content in the synthesized powder and adjusting the raw materials, it is possible to produce sintered bodies with any desired sintering aid ratio.

[0025] Furthermore, in the manufacturing method of this embodiment, in order to suppress localized heat generation due to the exothermic reaction of silicon nitriding, the heating rate is reduced at the first temperature in which the nitriding reaction begins, and the nitrogen atmosphere is reduced in pressure at the second temperature in the initial stages of heat generation. This slows down the rate of the nitriding reaction and prevents the powder from becoming locally hot during synthesis. As a result, the formation of β-type silicon nitride particles is suppressed, making it possible to obtain silicon nitride powder with a high α fraction (90% or more). [Examples]

[0026] The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0027] The silicon nitride powders used in Examples 1-10 and Comparative Examples 1-7 were prepared under the following conditions and procedures.

[0028] First, silicon powder, magnesium oxide powder, and yttrium oxide powder (CaF2 powder or MgF2 powder in Comparative Examples 2 and 3, and no additive in Comparative Example 7) were weighed to the specified amounts and pre-mixed in a poly bag. The pre-mixed powders were then fully mixed using a mixer with stirring blades. The uniformity of the mixed powder was confirmed using a microscope. The prepared mixed powder was then filled into firing scabbards. The packing density at this time was 0.6 or 1.0 g / cm³. 3The mixed powder was then packed into a carbon heater furnace. After insertion, the furnace was evacuated to 20 Pa or less, and then filled with nitrogen gas to 0.1 MPa. The furnace was heated to 1100°C at an arbitrary heating rate. In the temperature range above 1100°C, the heating rate was set to 2°C / min or less. Next, in the temperature range from the depressurization start temperature (1100°C to 1200°C) to the depressurization end temperature (1200°C to 1250°C), the nitrogen gas flow rate was adjusted so that the nitrogen pressure inside the furnace was 40 or 70 kPa (no depressurization in Comparative Example 4). In the temperature range above the depressurization end temperature, the nitrogen flow rate was adjusted again so that the nitrogen pressure was above atmospheric pressure, and the temperature was raised to a maximum temperature of 1350°C. By maintaining the maximum temperature of 1350°C for about 3 hours and carrying out the nitriding reaction, silicon nitride powder was obtained.

[0029] Powder X-ray diffraction measurements were performed on each of the prepared samples from Examples 1-10 and Comparative Examples 1-7. By analyzing the X-ray diffraction patterns, the presence or absence of Y2Si3O3N4 precipitation and the α fraction, which indicates the mass fraction of α-type silicon nitride in the synthesized powder, were derived. The nitridation rate and the content of Y and Mg in metal equivalents were also measured for each sample. All measurements and analyses were performed under the following conditions.

[0030] • Powder X-ray diffraction measurement and analysis Using a Rigaku Corporation Ultima IV powder X-ray diffractometer, the X-ray diffraction intensity of each sample was measured by powder X-ray diffraction using Cu-Kα rays. The synthetic powder was coarsely ground in a crusher, and then further ground in a vibratory mill to prepare the measurement sample. The diffraction pattern of the ground synthetic powder was measured by powder X-ray diffraction. The presence or absence of Y2Si3O3N4 precipitation was confirmed by the presence of the Y2Si3O3N4 diffraction peak. Furthermore, the mass fraction of the crystalline phase was measured using the integrated intensities of the diffraction patterns of the silicon phase, α-type silicon nitride phase, and β-type silicon nitride phase, according to the known Jovanovic and Kimura method, expressed by the following three equations.

number

[0031] · Nitridation rate W obtained by the method of Jovanovic and Kimura mentioned above α , W β , W Si Using, (W α + W β ) / (W α + W β + W Si ) × 100 was taken as the nitridation rate (%).

[0032] · Metal content Using a scanning fluorescent X-ray analyzer ZSX PrimusIV manufactured by Rigaku Corporation, the metal-converted content of each contained element was measured. The synthesized powder was pulverized, and coarse particles were removed by passing it through a sieve with a mesh size of 100 μm. The powder after sieving was granulated, and using a mold with a diameter of 20 mm, it was formed into a pellet with a thickness of 5 mm. The content of each contained element was determined by performing an EZ scan measurement on the obtained pellet with a measurement area of φ10 mm.

[0033] The conditions and various measurement results for each of the samples of Examples 1 to 10 and Comparative Examples 1 to 7 are shown in Tables 1 and 2. Also, FIGS. 1 to 3 exemplarily show the X-ray diffraction patterns of the samples of Example 1, 8, and Comparative Example 1. The Miller indices (hkl) are described for each diffraction peak of the α-type silicon nitride phase, β-type silicon nitride phase, and Y2Si3O3N4 phase from the top in the X-ray diffraction pattern.

[0034]

Table 1

[0036] Furthermore, as shown in Table 1, the samples from Examples 1 to 10 all exhibit an α fraction of 95% or higher. It was found that under the blending ratios and manufacturing conditions of at least Examples 1 to 10, silicon nitride powder with an α fraction of 95% or higher can be obtained. In contrast, Comparative Examples 1 to 7 all exhibit an α fraction of less than 90%, indicating that more β-type silicon nitride is produced. In contrast, Comparative Example 1 was a sample in which the amount of Y2O3 added was relatively reduced compared to the amount of MgO added (Y2O3 / MgO<1), but it was found that when the amount of Y2O3 added was relatively small and the amount of MgO added was dominant, the transition to β-type silicon nitride was not suppressed. Comparative Example 2 was a sample in which CaF2 was added instead of MgO, but it was found that the transition to β-type silicon nitride was not suppressed by the addition of CaF2. Comparative Example 3 was a sample in which MgF2 was added instead of MgO, but similar to Comparative Example 2, it was found that the transition to β-type silicon nitride was not suppressed by the addition of MgF2. Comparative Example 4 was a sample in which nitrogen pressure reduction was not introduced in the specified temperature range, but it was found that the transition to β-type silicon nitride could not be suppressed without pressure reduction. Comparative Example 5 was a sample in which the amount of Y2O3 added was reduced to 1.4 wt% (less than 2.5 wt%), and the amount of Y2O3 added was relatively reduced compared to the amount of MgO added (Y2O3 / MgO < 1), but it was found that with this blending ratio, the amount of Y2O3 added was small, so the exothermic reaction could not be sufficiently reduced, and the transition to β-type silicon nitride could not be sufficiently suppressed. Comparative Example 6 was a sample in which the pressure reduction start temperature was set to a relatively high temperature (1200°C), but it was found that when the temperature range for pressure reduction was shifted to the high temperature side, the transition to β-type silicon nitride could not be suppressed, similar to Comparative Example 4. Comparative Example 7 was a sample in which no catalyst was added, but it was found that in the absence of a catalyst, the transition to β-type silicon nitride was dominant.

[0037] The present invention is not limited to the embodiments described above, and can be implemented in various forms as long as they fall within the technical scope of the present invention.

Claims

1. The alpha fraction is 90% or more, and the X-ray diffraction pattern obtained by powder X-ray diffraction measurement shows Y ２ Si ３ O ３ N ４ A silicon nitride powder characterized by the detection of phase diffraction peaks.

2. The silicon nitride powder according to claim 1, characterized in that it contains 4.0 to 9.0% by weight of Y and 0.5 to 2.0% by weight of Mg, in terms of metal equivalent.

3. A method for producing silicon nitride powder according to claim 1 or 2, 90-96% by weight of silicon powder and 2.5-6.5% by weight of Y ２ O ３ A process of preparing a mixed powder by mixing the powder with 1.5 to 4.5% by weight of MgO powder, A method characterized by comprising the step of nitriding the mixed powder by a direct nitriding method to obtain the silicon nitride powder.

4. In the step of producing the mixed powder, the mixing ratio Y ２ O ３ of the powder to the MgO powder by weight percentage Y<… ２ O ３ / MgO is 1 or more, the method according to claim 3. It should be noted that there seems to be some incomplete or incorrect formatting in the original content (the "…" part in the middle). But this is the translation based on the existing text.

5. The method according to 3 or 4, characterized in that the step of preparing the mixed powder further includes adding 0 to 40 parts by weight of silicon nitride powder to 100 parts by weight of the mixed powder.

6. The method according to any one of claims 3 to 5, characterized in that the step of nitriding the mixed powder includes reducing the nitrogen pressure inside the furnace to a negative pressure relative to atmospheric pressure at a depressurization start temperature of 1100 to 1150°C.

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