Patent Application
20250066258
Embodiments described below relate generally to a silicon nitride sintered body, a wear-resistant member, a substrate for semiconductor devices, and a method for producing a silicon nitride sintered body.
Silicon nitride sintered bodies are used in various fields to take advantage of their high strength. For example, in International Publication No. 2020/121752, a silicon nitride sintered body is used in wear-resistant members such as bearing balls, etc. In International Publication No. 2020/044974, a silicon nitride sintered body is used in a substrate for semiconductor devices. According to International Publication No. 2020/121752, a silicon nitride sintered body having a three-point bending strength of not less than 900 MPa was obtained. According to International Publication No. 2020/044974, a silicon nitride sintered body having a three-point bending strength of not less than 700 MPa was obtained.
According to the production methods described in International Publication No. 2020/121752 and International Publication No. 2020/044974, silicon nitride sintered bodies are made by firing at high temperatures of about 1,600 to 2,000° C. Generally, the furnace used to fire the silicon nitride sintered body is an electric furnace. In recent years, the demand for decarbonization is increasing. To reduce the amount of carbon dioxide discharged in the production of a silicon nitride sintered body, it is considered effective to reduce the firing temperature.
For example, Japanese Patent No. 5811391 discusses a method of using a packing powder in a firing chamber. In Japanese Patent No. 5811391, the packing powder is used to suppress the effects of SiO gas and Mg gas discharged from the degreased body. However, the firing temperature is 1,550 to 1,900° C. The firing temperature of the production method described in Japanese Patent No. 5811391 is about equal to the firing temperatures described in International Publication No. 2020/121752 and International Publication No. 2020/044974.
For example, Japanese Patent No. 5062402 discusses the use of reaction sintering to fire at 1,300 to 1,375° C. According to the production method described in Japanese Patent No. 5062402, the firing is performed at a low temperature, but the strength of the silicon nitride sintered body obtained is about 140 MPa. With such low strength, it has been difficult to use the silicon nitride sintered body as a wear-resistant member or a substrate for semiconductor devices.
A silicon nitride sintered body according to an embodiment includes not less than 0.1 mass % and not more than 10 mass % of zirconium when converted to oxide. In XRD analysis (2θ) of any cross section of the silicon nitride sintered body, 0.01≤I35.3/I27.0≤0.5 and 0≤I33.9/I27.0≤1.0 are satisfied; I35.3 is a maximum peak intensity detected at 35.3°±0.2° based on α-silicon nitride crystal grains; I27.0 is a most intense peak detected at 27.0°±0.2° based on β-silicon nitride crystal grains; and I33.9 is a most intense peak detected at 33.9°±0.2° based on zirconium nitride.
A silicon nitride sintered body according to an embodiment includes not less than 0.1 mass % and not more than 10 mass % of zirconium when converted to oxide. The zirconium is converted into an oxide as ZrO2. At least a portion of the zirconium (Zr) component forms a liquid phase at about 1,300° C. Therefore, zirconium has an effect of lowering the firing temperature. Therefore, to fire at a low temperature, it is effective to include not less than 0.1 mass % and not more than 10 mass % of zirconium when converted to oxide. The effect of the addition is small when the zirconium content is less than 0.1 mass % when converted to oxide. When the zirconium content is greater than 10 mass % when converted to oxide, much zirconium nitride is formed. Also, there is a possibility that voids may become large. Therefore, the strength and/or insulative properties degrade. Accordingly, it is favorable for the zirconium content to be within the range of not less than 0.1 mass % and not more than 10 mass % when converted to oxide, and more favorably within the range of not less than 0.5 mass % and not more than 6 mass % when converted to oxide.
Also, when XRD analysis (2θ) of any cross section of the silicon nitride sintered body according to the embodiment is performed, in the analysis results, 0.01≤I35.3/I27.0≤0.5 and 0≤I33.9/I27.0≤1.0 are satisfied; I35.3 is a maximum peak intensity detected at 35.3°±0.2° based on α-silicon nitride crystal grains; I27.0 is a maximum peak intensity detected at 27.0°±0.2° based on β-silicon nitride crystal grains; and I33.9 is a maximum peak intensity detected at 33.9°±0.2° based on zirconium nitride.
First, a method of the XRD analysis (2θ) will be described. In the XRD analysis, any cross section of the silicon nitride sintered body is used as the measurement surface. A cross section that has a surface roughness Ra of not more than 1 μm is used as the measurement surface. A Cu target (Cu—Kα) is used in the XRD analysis. The XRD analysis is performed by setting the tube voltage to 40 kV, the tube current to 40 mA, the scan speed to 2.0°/min, the slit (RS) to 0.15 mm, and the scanning range (2θ) to 10° to 50°. Qualitative analysis or quantitative analysis that corresponds to the crystal compounds can be performed by the XRD analysis. D8 ADVANCE made by BRUKER or a device having equivalent performance is used for the measurement.
In the obtained analysis results, I35.3 is the maximum peak intensity detected at 35.3°±0.2° based on α-silicon nitride crystal grains; I27.0 is the maximum peak intensity detected at 27.0°±0.2° based on β-silicon nitride crystal grains; and I33.9 is the maximum peak intensity detected at 33.9°±0.2° based on zirconium nitride.
The most intense peak refers to the peak having the largest intensity among the peaks detected within each prescribed range. The intensity of the peak top is used as the “intensity”. Also, the skirt of the peak may be outside the prescribed range as long as the peak top is within the prescribed range. The peak based on α-silicon nitride crystal grains also includes α-sialon. The peak based on β-silicon nitride crystal grains also includes β-sialon. The presence of α-silicon nitride crystal grains and β-silicon nitride crystal grains in the silicon nitride sintered body can be confirmed using other XRD peaks. For example, when α-silicon nitride crystal grains are present, peaks are detected at 35.3°±0.2° and 31.0°±0.2°. Also, when β-silicon nitride crystal grains are present, peaks are detected at 27.0°±0.2° and 33.7°±0.2°. In other words, it is determined that α-silicon nitride crystal grains are present in the silicon nitride sintered body when peaks are detected at 35.3°±0.2° and 31.0°±0.2°. Similarly, it is determined that β-silicon nitride crystal grains are present in the silicon nitride sintered body when peaks are detected at 27.0°±0.2° and 33.7°±0.2°.
If necessary, qualitative analysis is used to check the presence of zirconium nitride. When zirconium nitride is not detected by qualitative analysis or quantitative analysis, it is determined that the peak at I33.9 is not present, that is, I33.9=0. For simplicity, for a silicon nitride sintered body confirmed to include zirconium by qualitative analysis or quantitative analysis, the peak that is detected at 33.9°±0.2° may be determined to be a peak (I33.9) based on zirconium nitride.
0.01≤I35.3/I27.0≤0.5 being satisfied means that both α-silicon nitride crystal grains and β-silicon nitride crystal grains are present. α-Silicon nitride crystal grains are round particles that tend to have aspect ratios of less than 2. β-Silicon nitride crystal grains tend to be slender particles having aspect ratios of not less than 2. By α-silicon nitride crystal grains penetrating gaps between the slender β-silicon nitride crystal grains, the silicon nitride sintered body can be densified; and the strength of the silicon nitride sintered body can be increased. As a result, as described below, a high-strength silicon nitride sintered body can be obtained even in low-temperature firing having a firing temperature of less than 1,650° C.
Silicon nitride is a major component of the silicon nitride sintered body. I35.3/I27.0 being not more than 0.5 means that β-silicon nitride crystal grains account for the greatest mass in the silicon nitride sintered body. In other words, the mass ratio of the β-silicon nitride crystal grains is greater than the mass ratio of the α-silicon nitride crystal grains. Also, the peaks that appear in the XRD analysis are affected by the composition, the crystallinity, the amount present, the orientation, etc. Among the peaks related to the β-silicon nitride crystal grains, the likelihood of the intensity of the peak detected at 27.0°±0.2° being a maximum is high. Also, among the peaks related to the α-silicon nitride crystal grains, the likelihood of the peak detected at 35.3°±0.2° being a maximum is high. The composition can be optimized by controlling the ratio of the maximum peak intensity I35.3 based on the α-silicon nitride crystal grains to the maximum peak intensity I27.0 based on the β-silicon nitride crystal grains, which is the major component, and the ratio of the maximum peak intensity based on the other components to the maximum peak intensity I27.0.
When I35.3/I27.0 is within the range of not less than 0.01 and not more than 0.5, α-silicon nitride crystal grains are distributed in the gaps between the β-silicon nitride crystal grains. The slender β-silicon nitride crystal grains are complexly entangled, and the α-silicon nitride crystal grains fill the gaps between the slender β-silicon nitride crystal grains, which leads to increased strength. When I35.3/I27.0 is less than 0.01, there is a possibility that the amount of the α-silicon nitride crystal grains present may be insufficient. The gaps between the β-silicon nitride crystal grains become large, which may cause the strength of the silicon nitride sintered body to decrease. Also, voids are easily formed, and there is a possibility that the insulative properties of the silicon nitride sintered body may degrade. When I35.3/I27.0 is large and is greater than 0.5, the β-silicon nitride crystal grains become insufficient, and a configuration can no longer be made in which the slender β-silicon nitride crystal grains are complexly entangled. As a result, there is a possibility that the strength of the silicon nitride sintered body may be reduced. Accordingly, it is favorable for I35.3/I27.0 to be within the range of not less than 0.01 and not more than 0.5, and more favorably within the range of not less than 0.05 and not more than 0.4.
I33.9 is a peak based on zirconium nitride (ZrN). When XRD analysis of the silicon nitride sintered body according to the embodiment is performed, 0≤I33.9/I27.0≤1.0 is satisfied. I33.9/I27.0 being not more than 1.0 means that the amount of zirconium nitride present with respect to the β-silicon nitride crystal grains is suppressed. When I33.9/I27.0 is greater than 1.0, there is much zirconium nitride, and so the strength of the silicon nitride sintered body is reduced. Also, the insulative properties of the silicon nitride sintered body degrade because zirconium nitride is an electrically-conductive material. It is therefore favorable for I33.9/I27.0 to be within the range of not less than 0 and not more than 1.0, and more favorably to be within the range of not less than 0 and not more than 0.6. I33.9/I27.0=0 means that ZrN is not present, or I33.9 is not detected (is below the detection limit).
In the XRD analysis, it is favorable for 0.01≤I28.2/I27.0≤0.30 to be satisfied, wherein I28.2 is the maximum peak intensity detected at 28.2°±0.2° based on zirconium oxide. Furthermore, it is favorable for 0.02≤I30.1/I27.0≤0.30 to be satisfied, wherein I30.1 is the maximum peak intensity detected at 30.1°±0.2° based on zirconium oxide.
The presence of a peak based on zirconium oxide means that at least a portion of the zirconium in the silicon nitride sintered body is present as zirconium oxide. Examples of zirconium oxide include ZrO2, hybrid Zr oxides, etc. For example, when aluminum oxide is used as a sintering aid, there are also cases where a Zr—Al—O compound is formed as a hybrid Zr oxide. Similarly, when magnesium oxide is used as a sintering aid, there are also cases where a Zr—Mg—O compound is formed as a hybrid Zr oxide. Hybrid Zr oxides also include partially stabilized zirconia, fully stabilized zirconia, etc. Examples of partially stabilized zirconia or fully stabilized zirconia include oxides of zirconium oxide with yttrium oxide, cerium oxide, magnesium oxide, etc., in solid solution.
Zirconium oxide has one of a monoclinic, tetragonal, or cubic crystal structure. The detection of I28.2 or I30.1 indicates that the zirconium oxide is monoclinic or tetragonal. Monoclinic is a crystal structure that is stable at room temperature. Tetragonal is a crystal structure that is stable at about 1,170° C. Cubic is a crystal structure that is stable at about 2,370° C. In the XRD analysis, the detection of a zirconium oxide peak indicates the presence of crystalline zirconium oxide. In the silicon nitride sintered body, zirconium oxide is present in the grain boundary phase. The presence of a crystal compound of zirconium oxide in the grain boundary phase can reinforce the grain boundary phase. As a result, the strength of the silicon nitride sintered body can be further improved.
0.01≤I28.2/I27.0≤0.30 or 0.02≤I30.1/I27.0≤0.30 being satisfied means that the zirconium oxide is monoclinic or tetragonal. Also, 0.01≤I28.2/I27.0≤0.30 and 0.02≤I30.1/I27.0≤0.30 both may be satisfied, or only one may be satisfied. Also, when only one is satisfied, it is favorable for one of the I28.2 or I30.1 peak not to be detected (to be below the detection limit). Also, the detection of I28.2 indicates that the zirconium oxide is monoclinic. The detection of I30.1 indicates that the zirconium oxide is tetragonal.
Monoclinic or tetragonal which is stable from room temperature to about 1,000° C. is used as a major part. On the other hand, cubic is stable at about 2,370° C. Monoclinic or tetragonal being used as the major part means that a composition that is fired at a low temperature of less than 1,650° C. is stable. Much cubic present means that the heat conducted to the sintered body in the firing process is unstable. There is a possibility that this may make it impossible to control the size of the zirconium oxide and/or voids. In other words, little or no cubic zirconium oxide is favorable. Cubic zirconium oxide can be observed using a TEM (transmission electron microscope), etc.
Also, it is favorable for tetragonal zirconium oxide to be present. As described below, when zirconium oxide reacts with the sintering aid of magnesium oxide, etc., in the sintering process, zirconium oxide becomes tetragonal and is stable. In other words, the presence of tetragonal zirconium oxide indicates that the zirconium oxide and the other sintering aids have reacted and are stable. Examples of other sintering aids include components that form partially stabilized zirconia.
It is therefore favorable for I28.2/I27.0 to be within the range of not less than 0.01 and not more than 0.30, and more favorably within the range of not less than 0.05 and not more than 0.20. Also, it is favorable for I30.1/I27.0 to be within the range of not less than 0.02 and not more than 0.30, and more favorably within the range of not less than 0.05 and not more than 0.20.
Also, in the XRD analysis, it is favorable for 0≤I36.7/I27.0≤0.50 or 0≤I41.9/I27.0≤0.50 to be satisfied, wherein I36.7 is the maximum peak intensity detected at 36.7°±0.2°, and I41.9 is the maximum peak intensity detected at 41.9°±0.2°. Both 0≤I36.7/I27.0≤0.50 and 0≤I41.9/I27.0≤0.50 may be satisfied, or only one may be satisfied.
The peak that is detected at 36.7°±0.2° and the peak that is detected at 41.9°±0.2° are based on crystal compounds in the grain boundary phase. The peaks that are observed in these two ranges are based on crystal compounds other than zirconium oxide, and indicate the presence of crystal compounds other than zirconium oxide in the grain boundary phase. Examples of such crystal compounds include compounds including at least one selected from a rare-earth element, aluminum, and magnesium. Examples of the rare-earth element include yttrium and lanthanoid elements. Examples of the crystal compounds include an oxide, a nitride, or an oxynitride including an element selected from a rare-earth element, aluminum, and magnesium. Each may be a hybrid oxide, a hybrid nitride, or a hybrid oxynitride.
As described below, examples of the sintering aid include rare-earth element oxides, magnesium oxide, and aluminum oxide. Rare-earth elements and magnesium dissolve in zirconium oxide to form hybrid Zr oxides. Hybrid Zr oxides form stable phases. There are also cases where aluminum and oxygen dissolve in silicon nitride to form sialon.
0≤I36.7/I27.0≤0.50 or 0≤I41.9/I27.0≤0.50 being satisfied means that the sintering aid contributes to the formation of the stable phase. That is, the amount of the crystal compounds present in the grain boundary phase is suppressed. As a result, the grain boundary phase can be reinforced by the pinning effect of the zirconium oxide particles. In other words, it is favorable for I36.7/I27.0 or I41.9/I27.0 not to be greater than 0.50. When each is greater than 0.50, much of crystal compounds other than zirconium oxide that have low pinning effects are present. It is therefore favorable for I36.7/I27.0 to be within the range of not less than 0 and not more than 0.50, and more favorably within the range of not less than 0 and not more than 0.4. It is favorable for 0≤I41.9/I27.0≤0.50 to be within the range of not less than 0 and not more than 0.5, and more favorably within the range of not less than 0 and not more than 0.4.
Also, the strength and hardness of the silicon nitride sintered body can be improved by the presence of crystal compounds in the grain boundary phase. This is because the crystal compound phase is harder than the amorphous phase. The improvement of the strength and hardness leads to the improvement of the wear resistance. It is therefore favorable for one or more of 0≤I36.7/I27.0≤0.50, 0≤I41.9/I27.0≤0.50, 0<I36.7/I27.0≤0.50, and 0<I41.9/I27.0≤0.50 to be satisfied.
In the analysis results shown in
Also, it is favorable for the silicon nitride sintered body to include zirconium oxide particles. For example, the zirconium oxide particles are dispersed in the grain boundary phase inside the silicon nitride sintered body. It is favorable for the average particle size of the zirconium oxide particles in a 100 μm×100 μm measurement area in any cross section to be not less than 0.05 μm and not more than 2 μm.
As described above, it is favorable for the zirconium oxide to be a crystal compound having a prescribed XRD peak. The particle sizes of the zirconium oxide particles are the sizes of particles of zirconium oxide. The zirconium oxide particles may be either single-crystal bodies or polycrystals. It is favorable for the average particle size of the zirconium oxide particles to be within the range of not less than 0.05 μm and not more than 2 μm. When the average particle size is within the range of not less than 0.05 μm and not more than 2 μm, the function of reinforcing the grain boundary phase is increased. When the average particle size is less than 0.05 μm, there is a possibility that the function of reinforcing the grain boundary phase may be insufficient. Also, when the average particle size is greater than 2 μm, there is a possibility that the gaps between the silicon nitride crystal grains and the zirconium oxide particles may be large. Large gaps cause the occurrence of voids. It is therefore favorable for the average particle size of the zirconium oxide particles to be within the range of not less than 0.05 μm and not more than 2 μm, and more favorably within the range of not less than 0.1 μm and not more than 1 μm.
Also, it is favorable for the total area of the zirconium oxide particles in a 100 μm×100 μm measurement area to be within the range of not less than 0.01% and not more than 15%. By setting the area ratio in the 100 μm×100 μm measurement area to be not less than 0.01% and not more than 15%, the zirconium oxide particles can be distributed more uniformly. The effects can be further increased by controlling the average particle size and area ratio of the zirconium oxide particles. More favorably, the total area of the zirconium oxide particles is within the range of not less than 0.01% and not more than 15% when any measurement area in any cross section is measured.
A method for measuring the average particle size and area ratio of the zirconium oxide particles will now be described. First, any cross section is obtained from the silicon nitride sintered body. The pretreatment of the cross section is not limited as long as the cross section can be observed by a scanning electron microscope (SEM). For example, it is favorable to observe an ion-milled cross section with SEM. The cross section on which the XRD analysis was performed may be used in the observation by SEM. In such a case, first, an enlarged photograph (a SEM photograph) of not less than 1,000 times is imaged by SEM. A 100 μm×100 μm measurement area is randomly extracted from the enlarged photograph. The particle sizes of the zirconium oxide particles visible in the measurement area are measured. The distance between the two most distant points on the outer edge of a particle is measured as the particle size. The average value of the particle sizes measured in the 100 μm×100 μm measurement area is used as the average particle size. When it is difficult to discriminate zirconium oxide particles in the SEM photograph, elemental mapping of zirconium and oxygen may be performed using energy dispersive X-ray analysis (EDX). When a hybrid Zr oxide is included, the zirconium oxide particles may be discriminated by also using mapping of the hybrid elements. For example, the regions where zirconium and oxygen overlap in the EDX elemental mapping can be determined to be zirconium oxide particles.
It is favorable for the maximum diameter of the voids in the silicon nitride sintered body to be not more than 3 μm. A SEM photograph of any cross section of the silicon nitride sintered body is used to measure the maximum diameter of the voids. The SEM photograph that is used to measure the particle sizes of the zirconium oxide particles may be used to measure the maximum diameter of the voids. The major diameters of the voids visible in the SEM photograph are measured. The distance between the two most distant points on the outer edge of a void is measured as the major diameter of the void. The largest major diameter among the voids visible in the 100 μm×100 μm measurement area is used as the maximum diameter of the voids. It is more favorable for the maximum diameter of the voids to be not more than 1 μm.
To observe the voids, the SEM photograph is analyzed using image analysis software. ImageJ or equivalent or better software is used as the image analysis software. Contrast occurs between the voids and other portions in the SEM photograph of the cross section. For example, the color of voids appear blacker and darker than the colors of portions other than the voids. The contrast difference can be utilized to discriminate between the voids and other portions by binarizing the SEM photograph.
A value that is obtained using a “mode technique” or a “discriminant analysis binarization technique” is used as the threshold for binarization. When the image analysis software includes the function of a discriminant analysis binarization technique, the discriminant analysis binarization technique of the image analysis software is used. In the discriminant analysis binarization technique, the threshold is determined uniquely by the analysis software. It is therefore easy to discriminate the voids. For example, in the SEM photograph, the region that appears white around the void is the boundary between the void and the silicon nitride sintered body. Accordingly, the region that appears white around the void should not be counted as the void. Also, the silicon nitride sintered body is gray in the SEM photograph. By using the threshold obtained by the “mode technique” or the “discriminant analysis binarization technique”, the void can be shown as black; and the silicon nitride sintered body and the white region around the void can be shown as white. By using a binarized image, the voids that are visible in the SEM photograph and the other regions can be discriminated.
For example, in a 100 μm×100 μm measurement area, it is favorable for the average particle size of the zirconium oxide particles to be not less than 0.05 μm and not more than 2 μm, the total area of the zirconium oxide particles to be not less than 0.01% and not more than 15%, and the maximum diameter of the voids to be not more than 3 μm. More favorably, in the 100 μm×100 μm measurement area, the average particle size of the zirconium oxide particles is not less than 0.1 μm and not more than 1 μm; the total area of the zirconium oxide particles is not less than 0.01% and not more than 15%; and the maximum diameter of the voids is not more than 1 μm.
For a silicon nitride sintered body having the configuration described above, even for a firing temperature of less than 1,650° C., a three-point bending strength of not less than 600 MPa can be obtained. Also, a dielectric strength of not less than 13 kV/mm can be obtained. The three-point bending strength is measured in accordance with JIS-R-1601 (2008). The dielectric strength is measured in accordance with the dielectric breakdown strength of JIS-C-2141 (1992). JIS-R-1601 corresponds to ISO 14704. JIS-C-2141 corresponds to IEC 672-2.
Such a silicon nitride sintered body can be used in various fields. The silicon nitride sintered body according to the embodiment can be used in a wear-resistant member.
A bearing has a structure in which the bearing balls 1 are placed between an inner ring and an outer ring. Generally, the inner ring and the outer ring are made of bearing steel. There are cases where electrolytic corrosion occurs due to high-speed rotation of the bearing. When the bearing ball 1 according to the embodiment is used, the insulative properties of the silicon nitride sintered body are high, and so the occurrence of electrolytic corrosion can be suppressed.
Also, friction is generated at the surface of the bearing ball 1. The silicon nitride sintered body according to the embodiment has excellent wear resistance, and therefore has excellent durability. Other than bearing balls, wear-resistant members include various roll materials such as those of rolling, engine parts such as cam rollers, bearing members, compressor vanes, gas turbine blades, friction stir welding tool members, etc.
Also, the silicon nitride sintered body according to the embodiment can be used in a substrate for semiconductor devices.
A circuit part, a semiconductor element, etc., are provided on the substrate for semiconductor devices 2. The circuit part is a metal plate, a metalized layer, etc. The circuit part is bonded to the substrate via a bonding layer as necessary. Also, a semiconductor device is obtained by mounting a semiconductor element, etc., to the circuit part. By using the silicon nitride sintered body according to the embodiment, the substrate for semiconductor devices 2 that has excellent strength and insulative properties can be provided.
A method for producing the silicon nitride sintered body according to the embodiment will now be described. The method for producing the silicon nitride sintered body according to the embodiment is not particularly limited as long as the silicon nitride sintered body has the configuration described above. Here, a method for obtaining the silicon nitride sintered body with a high yield will be described.
The method for producing the silicon nitride sintered body according to the embodiment includes a process of making a compact, a process of making a degreased body, a placement process of the degreased body, a placement process of a packing powder, and a firing process. The compact is made by mixing a silicon nitride powder and at least a zirconium compound powder, and by molding the mixed powder. The degreased body is made by degreasing the compact. The degreased body and the packing powder are placed inside a firing chamber. In the firing process, the degreased body is fired at less than 1,650° C. inside the firing chamber in which the packing powder is placed.
First, a process of making the compact is performed. At least a zirconium compound powder is mixed with a silicon nitride powder. It is favorable for the silicon nitride included in the silicon nitride powder to be the α-type. It is favorable for the 10 alpha percentage of the silicon nitride to be not less than 90%. It is favorable for the oxygen amount included in the silicon nitride to be not more than 2 mass %, and for the average particle size of the silicon nitride to be not more than 3 μm. Grain growth of the α-silicon nitride powder into the β-type occurs in the firing process. Due to the grain growth, both α-silicon nitride crystal grains and β-silicon nitride crystal grains can be present. A β-silicon nitride powder may be added as necessary.
Examples of the zirconium compound powder include a zirconium oxide powder or a hybrid Zr oxide. It is favorable for the ratio of zirconium oxide in the zirconium compound powder to be within the range of not less than 90 mass % and not more than 100 mass %. At least one of a monoclinic zirconium oxide powder, a tetragonal zirconium oxide powder, or a cubic zirconium oxide powder can be used. It is favorable to use a monoclinic zirconium oxide powder as the raw material powder. Monoclinic is stable at room temperature. Also, examples of the hybrid Zr oxide include partially stabilized zirconia or fully stabilized zirconia. It is favorable for partially stabilized zirconia or fully stabilized zirconia to be tetragonal. Tetragonal partially stabilized zirconia or fully stabilized zirconia is stable at room temperature. Therefore, crystal change fluctuation due to the effects of the heat of the firing process can be reduced. It is favorable for the average particle size of the zirconium compound powder to be not more than 3 μm. Zirconium oxide is used as a sintering aid to perform low-temperature firing at less than 1,650° C. Also, it is favorable for the added amount of the zirconium compound powder to be within the range of not less than 0.1 mass % and not more than 10 mass % of zirconium when converted to oxide. The XRD peaks described above are easily controlled within this range.
A sintering aid other than zirconium oxide may be added to the raw material powder. For example, one or more elements are selected from a rare-earth element, aluminum, and magnesium; and at least one selected from an oxide, a nitride, and an oxynitride of the elements is used as the sintering aid. The oxide, the nitride, and the oxynitride each may be a hybrid oxide, a hybrid nitride, or a hybrid oxynitride. Examples of the rare-earth element include yttrium and lanthanoid elements.
In particular, it is favorable to add an oxide as the sintering aid. Aluminum oxide or magnesium oxide reacts with zirconium oxide in the firing process to form a hybrid Zr oxide. Fully stabilized zirconia or partially stabilized zirconia is formed according to the solid solution amount of magnesium. Fully stabilized zirconia or partially stabilized zirconia tends to be tetragonal.
Also, a Zr—Al—O crystal compound is formed when zirconium oxide and aluminum oxide react. Also, there are cases where when aluminum oxide reacts with silicon nitride to form α-sialon or β-sialon. Zirconium oxide that is added in the monoclinic state changes to tetragonal and is stabilized by reacting with another sintering aid. The monoclinic zirconium oxide may remain as-is without being completely reacted with the other sintering aids.
The rare-earth element performs the role of forming a stabilizing phase by dissolving in zirconium oxide. Also, the rare-earth element is a sintering aid effective for improving the sinterability. The rare-earth element can form a crystal compound by reacting with silicon nitride, aluminum, and magnesium. Examples include rare-earth element-Al—O, rare-earth element-Mg—O, rare-earth element-Al—Mg—O, rare-earth element-Al—O—N, rare-earth element-Mg—O—N, rare-earth element Al—Mg—O—N, rare-earth element-Si—Al—O, rare-earth element-Si—Mg—O, rare-earth element-Si—Al—Mg—O, rare-earth element-Si—Al—O—N, rare-earth element-Si—Mg—O—N, rare-earth element-Si—Al—Mg—O—N, and other types of crystal compounds. These crystal compounds are effective for controlling I36.7 or I41.9.
It is favorable for the added amount of the compound of the element selected from a rare-earth element, aluminum, and magnesium to be within the range of not less than 1 mass % and not more than 15 mass % in total. The compound is at least one selected from an oxide, a nitride, and an oxynitride. Also, other than Al2O3 and MgO, spinel (MgAl2O4) may be used as the oxide of aluminum or the oxide of magnesium.
Also, examples of sintering aids other than those described above include silicon oxide. Silicon oxide has the effect of lowering the eutectic temperature with zirconium oxide. Therefore, an effect of lowering the firing temperature is obtained. It is favorable for the added amount of silicon oxide to be within the range of not less than 0.1 mass % and not more than 5 mass %. Impurity oxygen included in the silicon nitride powder may be added as the silicon oxide.
The silicon nitride powder, the zirconium oxide powder (including the hybrid Zr oxide), and other sintering aid powders are mixed. Also, a raw material powder slurry is prepared by mixing a binder and a solvent. Then, a compact is made using the raw material powder slurry. Various methods such as die pressing, cold isostatic pressing (CIP), doctor blade, etc., can be used to make the compact.
Then, a process of making a degreased body by degreasing the compact is performed. The degreasing process is a process of removing or reducing the binder and the solvent in the compact. It is favorable to perform the degreasing process within a temperature range of not less than 400° C. and not more than 600° C. The degreasing process can be performed in nitrogen, in ambient air, or in a vacuum. The degreased body is obtained by performing the degreasing process.
Then, a process of placing the degreased body that is made in a sintering chamber is performed. In the example shown in
Then, a process of placing a packing powder inside the sintering chamber is performed. The firing chamber 3 is a room inside the furnace, and is a room for firing the degreased body 7. Also, the exterior wall 4 and the interior wall 5 are inside the firing chamber 3. The packing powder 6 is placed in the gap between the exterior wall 4 and the interior wall 5. The space in which the degreased body 7 is placed communicates with the space between the exterior wall 4 and the interior wall 5. The process of placing the packing powder 6 may be after placing the degreased body 7 in the firing chamber 3, or may be before placing the degreased body 7 in the firing chamber 3.
As shown in
In the example shown in
It is favorable for the packing powder 6 to be at least one selected from silicon nitride, silicon oxide, magnesium oxide, and talc. The silicon nitride powder may be either the α-type and the β-type. Also, talc is a hybrid oxide represented by MgO·SiO2. Such a packing powder 6 is effective for controlling the partial pressure in the firing process.
Then, a firing process of firing the degreased body 7 at less than 1,650° C. is performed. It is favorable for the firing temperature to be less than 1,550° C. Here, “low-temperature firing” refers to firing the degreased body at less than 1,650° C., or even less than 1,550° C. The pressure when firing may be one of atmospheric pressure, pressurized or decompressed. Also, the firing atmosphere may be one of ambient air, a nonoxidizing atmosphere, or in a vacuum. Also, it is favorable for the pressure change to be not more than 0.1 MPa in the firing process. By using the packing powder 6, the pressure change of the firing process can be suppressed to be not more than 0.1 MPa.
As described above, the zirconium oxide powder is a sintering aid effective for firing at less than 1,650° C. On the other hand, zirconium oxide easily forms zirconium nitride by reacting with silicon nitride. In particular, zirconium nitride is easily formed by zirconium oxide and silicon nitride reacting at about 1,600° C.
Also, the magnesium oxide (MgO) of the sintering aid can form a liquid phase at a low temperature. In the firing process, MgO volatilizes as Mg. The volatilized Mg has strong reducibility. ZrO2 has a strong reduction effect when the Mg included in the degreased body 7 volatilizes. As a result, ZrO2 is reduced, and Zr is formed. The Zr reacts with nitrogen to form ZrN (zirconium nitride). Although zirconium oxide or magnesium oxide is a sintering aid effective for low-temperature firing, conventionally, much ZrN, which degrades the characteristics, had been formed. As a result, there were cases where I33.9/I27.0 was greater than 1.0.
ZrN is mainly formed in the surface layer portion of the silicon nitride sintered body. It is possible to remove the ZrN of the surface layer portion by scraping off about several 10 μm of the surface layer portion. On the other hand, the process of deleting the surface layer portion may cause an increase of the production cost. It is therefore favorable to suppress the formation of ZrN in the sintering process. Also, ZrN is electrically-conductive. When a silicon nitride sintered body that includes ZrN is used in a bearing ball, there is a possibility that electrolytic corrosion may be caused. A silicon nitride sintered body that includes ZrN may cause insulation defects when used as a substrate for semiconductor devices. The vapor pressures of rare-earth oxides and aluminum oxide are less than the vapor pressures of ZrO2 and MgO. Therefore, rare-earth oxides and aluminum oxide are sintering aids having low reducibility.
By using the packing powder 6, the change of ZrO2 into ZrN can be suppressed. That is, the reduction of ZrO2 can be suppressed. Also, the reduction of MgO can be suppressed.
The use of the packing powder 6 increases the hermeticity inside the firing chamber 3. Improving the hermeticity leads to suppression of the pressure change inside the firing chamber 3. Also, by using at least one selected from silicon nitride, silicon oxide, magnesium oxide, and talc as the packing powder 6, the pressure change of the firing process can be set to be not more than 0.1 MPa. The vapor pressures of silicon nitride, silicon oxide, magnesium oxide, and talc are equal to or greater than the vapor pressure of zirconium oxide. Because the packing powder 6 is reduced earlier than zirconium oxide, the zirconium oxide inside the degreased body 7 is not easily nitrided.
For example, when the packing powder 6 includes a silicon nitride powder or a silicon oxide powder, SiO (silicon monoxide) gas is discharged from the packing powder 6. The space inside the firing chamber 3 is filled with the SiO gas discharged from the packing powder 6, thereby making it difficult to nitride the zirconium oxide inside the degreased body 7. When the packing powder 6 includes magnesium oxide or talc, magnesium is discharged from the packing powder 6 when firing. The pressure of magnesium in the space inside the firing chamber 3 is increased. As a result, volatilization of magnesium from the degreased body 7 is suppressed, and the reduction of ZrO2 by the volatilization of Mg from the degreased body 7 can be suppressed. As a result, the XRD peaks of zirconium oxide and/or zirconium nitride can be controlled. Also, the reduction of the MgO added as a sintering aid can be suppressed. According to experiments performed by the inventor of the application, it was found that the Mg present in the space inside the firing chamber 3 does not easily have a reduction effect on the ZrO2 included in the degreased body 7. In other words, it was found that even when the partial pressure of Mg in the space inside the firing chamber 3 is high, the reduction reaction of ZrO2 can be effectively suppressed by suppressing the volatilization of Mg from the degreased body 7.
Also, when a large quantity of the degreased body 7 is fired simultaneously to improve the suitability for mass production, gas is produced from the large quantity of the degreased body 7 at the same timing. As a result, the pressure change of the firing process is large. By discharging the gas from the packing powder 6 and by suppressing the production amount of the gas from the degreased body 7, the pressure change of the firing process can be controlled to be not more than 0.1 MPa.
It is favorable for the firing temperature to be less than 1,650° C., and more favorably less than 1,550° C. As described above, an α-silicon nitride powder is used as the raw material powder. The α-silicon nitride powder undergoes grain growth into β-silicon nitride crystal grains in the firing process. By setting the firing temperature to be low, i.e., less than 1,650° C., the grain growth into the β-silicon nitride crystal grains can be suppressed. As a result, the ratio of the α-silicon nitride crystal grains and the β-silicon nitride crystal grains can be controlled. As a result, I35.3/I27.0 can be controlled. Also, the control of the grain growth of the β-silicon nitride crystal grains also leads to control of the distribution of zirconium oxide particles. Also, the maximum diameter of the voids of the silicon nitride sintered body can be set to be not more than 1 μm.
For example, when the same furnace (batch electric furnace) is used to perform the firing process at 1,650° C., a power reduction of about 15% is possible compared to when the firing process is performed at 1,800° C. Also, low-temperature firing makes it possible to fire at atmospheric pressure. Cost reduction is possible because pressure control is unnecessary. Also, the life of setters, insulation, etc., inside the furnace also can be extended. A silicon nitride sintered body that can be fired at a low temperature is effective for power reduction and cost reduction.
According to the production method described above, a silicon nitride sintered body that has a relative density of not less than 98% can be obtained. The relative density can be determined by Archimedes' method. Even for low-temperature firing at less than 1,650° C., the relative density can be not less than 98%. Also, a silicon nitride sintered body that has a three-point bending strength of not less than 600 MPa can be obtained. A silicon nitride sintered body that has a dielectric strength of not less than 13 kV/mm can be obtained. Although the lower limit of the firing temperature is not particularly limited, it is favorable for the lower limit to be not less than 1,200° C. When less than 1,200° C., there is a possibility that the densification of the silicon nitride sintered body may be insufficient, and the maximum diameter of the voids cannot be 1 μm or less. It is therefore favorable for the firing temperature to be within the range of not less than 1,200°° C. but less than 1,650° C., and more favorably not less than 1,300° C. but less than 1,550° C.
An additional heat treatment may be performed on the silicon nitride sintered body thus obtained as necessary. Examples of the additional heat treatment include hot isostatic pressing (HIP), a warp correction process, etc. A silicon nitride sintered body that has a relative density of not less than 99% can be obtained by HIP processing. Also, the three-point bending strength of the silicon nitride sintered body can be increased to be not less than 900 MPa by HIP processing. The heat treatment temperature of the additional heat treatment process is arbitrary. Although it is favorable for the additional heat treatment process to be performed at less than 1,650° C., the additional heat treatment process may be performed at not less than 1,650° C.
An α-silicon nitride powder was prepared as a raw material powder. The average particle size of the α-silicon nitride powder was not more than 3 μm; the impurity oxygen amount was 1.3 mass %; and the alpha percentage was not less than 90%. A zirconium oxide powder (including hybrid Zr oxides), an yttrium oxide powder, an aluminum oxide powder, a magnesium oxide powder, a spinel powder, and a silicon dioxide powder were prepared as sintering aid powders. The average particle size of the sintering aid powders was not more than 3 μm. The silicon nitride powders and the sintering aid powders were mixed according to the mixing ratios of Table 1. The total of the silicon nitride powder and the sintering aid powders was used as 100 mass % for the mixing ratios. In the crystal structure of the zirconium oxide powder (the hybrid Zr oxides) recited in Table 1, “monoclinic” indicates that a monoclinic zirconium oxide powder was used. “Tetragonal” indicates that a tetragonal yttrium-stabilized zirconia powder was used. The spinel powder of the sintering aid was MgAl2O4.
In the examples, a prescribed amount of at least one of the silicon nitride powder, the zirconium oxide powder, the yttrium oxide powder, the aluminum oxide powder, the magnesium oxide powder, the spinel powder, or the silicon dioxide powder was admixed. In the comparative example 1, zirconium oxide was not added. In the comparative example 2, a large amount of zirconium oxide powder was added. In the comparative example 3, firing was performed without using a packing powder as described below. In the comparative example 4, firing was performed at 1,800° C. as described below.
The raw material powders were mixed; a binder and a solvent were added; and a raw material powder slurry was made using a ball mill. Only the example 9 was made using a planetary mill at 400 rpm for 1 hour. The slurries were dried and powderized, and die pressing was used to make compacts. A degreasing process of the compacts was performed in a range of not less than 400° C. and not more than 600° C.
Then, the degreased bodies were placed in a firing chamber. A packing powder was placed between the exterior wall and the interior wall inside the firing chamber for the examples 1 to 14, the comparative examples 1 to 2, and the comparative example 4. Also, the packing powder was placed on the lid part of the firing chamber. On the other hand, the packing powder was not provided for the comparative example 3. The firing process was performed using the conditions shown in Table 2.
The silicon nitride sintered bodies were made by the processes described above. The zirconium content was within the range of not less than 0.1 mass % and not more than 10 mass % when converted to oxide for the examples 1 to 14 and the comparative examples 3 to 4. The silicon nitride sintered body did not include zirconium for the comparative example 1. The zirconium content in the silicon nitride sintered body was greater than 10 mass % when converted to oxide for the comparative example 2. Because the packing powder was not used for the comparative example 3, the pressure change in the firing process was greater than 0.1 MPa. For the comparative example 4, the firing temperature was high, and so the pressure change in the firing process was greater than 0.1 MPa even when a packing powder was used.
Then, XRD analysis of an arbitrary cross section was performed for each silicon nitride sintered body. The conditions and method of the XRD analysis were as described above. The relationships of the peak intensities observed in the XRD analysis are shown in Table 3.
It can be seen from Table 3 that I35.3/I27.0 and I33.9/I27.0 were within the prescribed ranges for the silicon nitride sintered bodies according to the examples. Also, zirconium was present as zirconium oxide in the silicon nitride sintered bodies according to the examples. The peak ratio being 0 means that the peak intensity of the molecule was below the detection limit.
In contrast, I33.9/I27.0 was greater than 1.0 for the comparative example 2. In the comparative example 2, the added amount of the zirconium oxide powder was high, and so a lot of ZrN was formed. In the comparative example 3, a packing powder was not used, and so a lot of ZrN was formed. In the comparative example 4, the firing temperature was high, and so silicon nitride and zirconium oxide were formed in the sintered body interior; and much zirconium nitride was formed at the surface of the sintered body.
Then, arbitrary cross sections of the silicon nitride sintered body were imaged by SEM. The average particle size and total area of the zirconium oxide particles were measured using the SEM photograph thus obtained. Also, the maximum diameter of the voids was measured. The detailed conditions were as described above. It was checked whether or not the total area of the zirconium oxide particles was within the range of not less than 0.01% and not more than 15% for an arbitrary 100 μm×100 μm measurement area. Also, the relative density of the silicon nitride sintered body was measured. The relative density was measured using Archimedes' method. The results are shown in Table 4.
The average particle size and total area of the zirconium oxide particles were within the prescribed ranges for the silicon nitride sintered bodies according to the examples. Also, the maximum diameter of the voids was not more than 3 μm. Also, the relative density was not less than 96%; and the sintered body was sufficiently densified.
On the other hand, in the comparative example 1, zirconium oxide was not added, and so the voids were large, and the relative density was low. In the comparative example 2, the added amount of zirconium oxide was high, and so the voids were large, and the relative density was low. In the comparative example 3, a packing powder was not used, and so a lot of zirconium nitride was formed, and the insulative properties were low as described below. In the comparative example 4, the firing temperature was high, and so a lot of zirconium nitride was formed. In other words, it was found that the compositions of the examples were unsuited to high-temperature firing.
Then, the three-point bending strength and the dielectric strength were measured for each silicon nitride sintered body. The measurement conditions were as described above. It was measured whether or not the dielectric strength was not less than 14 kV/mm. The results are shown in Table 5.
It can be seen from Table 5 that the silicon nitride sintered bodies according to the examples had excellent three-point bending strength and dielectric strength. Excellent characteristics were obtained even for low-temperature firing at less than 1,650° C. In the comparative examples, the silicon nitride sintered body was not densified, and so the strength and insulative properties were low.
Then, HIP processing was performed on the silicon nitride sintered bodies according to the examples 1 to 3. After HIP processing, the relative density and the three-point bending strength were measured. The results are shown in Table 6.
It can be seen from Table 6 that by performing HIP processing, densification proceeded, and the strength of the silicon nitride sintered body was increased.
Also, according to the examples, the consumed energy can be reduced because the firing process can be performed at a low temperature of less than 1,650° C.
Embodiments of the invention include the following features.
A silicon nitride sintered body, comprising:
being satisfied,
The silicon nitride sintered body according to Feature 1, wherein
is satisfied, and
The silicon nitride sintered body according to Feature 2, wherein
is satisfied, and
The silicon nitride sintered body according to any one of Features 1 to 3, wherein
is satisfied,
The silicon nitride sintered body according to any one of Features 1 to 4, comprising:
The silicon nitride sintered body according to any one of Features 1 to 5, wherein
A wear-resistant member using the silicon nitride sintered body according to any one of Features 1 to 6.
A substrate for semiconductor devices using the silicon nitride sintered body according to any one of Features 1 to 6.
A method for producing a silicon nitride sintered body, comprising:
The method for producing the silicon nitride sintered body according to Feature 9, wherein
The method for producing the silicon nitride sintered body according to Feature 10, wherein
The method for producing the silicon nitride sintered body according to Features 9 to 11, wherein
While certain embodiments of the inventions have been illustrated, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. These novel embodiments may be embodied in a variety of other forms; and various omissions, substitutions, modifications, etc., can be made without departing from the spirit of the inventions. These embodiments and their modifications are within the scope and spirit of the inventions, and are within the scope of the inventions described in the claims and their equivalents. Also, the embodiments above can be implemented in combination with each other.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-153196 | Sep 2022 | JP | national |
This is a continuation application of International Patent Application PCT/JP2023/031775, filed on Aug. 31, 2023. This application also claims priority to Japanese Patent Application No. 2022/153196, filed on Sep. 27, 2022. The entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2023/031775 | Aug 2023 | WO |
| Child | 18944087 | US |
