Silicon nitride-based sintered body and heat dissipation substrate

Abstract

This silicon nitride-based sintered body comprises a plurality of silicon nitride-based particles and grain boundary phases formed between the plurality of silicon nitride-based particles. In a cross-section passing through silicon nitride-based particles and grain boundary phases, formula (1) is satisfied when A is taken to be the ratio of the area of a specific region containing at least one of the rare earth elements, alkaline earth elements, and group 4 elements to the area of a first rectangular region with sides of 50 μm contained in a cross-section of the silicon nitride-based sintered body up to 50 μm from the surface of the silicon nitride-based sintered body and B is taken to be the ratio of the area of a specific region containing at least one of the rare earth elements, alkaline earth elements, and group 4 elements to the area of a second rectangular region with sides of 50 μm contained in a cross-section of the silicon nitride-based sintered body 50 μm or more from the surface of the silicon nitride-based sintered body.　1<A / B<4... (1)

Description

Silicon nitride sintered body and heat dissipation substrate

[0001] The present invention relates to a silicon nitride sintered body and a heat dissipation substrate.

[0002] Conventionally, a silicon nitride sintered body containing silicon nitride as a main component is known (for example, Patent Document 1).

[0003] International Publication No. 2020 / 203787

[0004] However, even with prior art techniques such as that disclosed in Patent Document 1, there still remains room for improvement in techniques for suppressing damage caused by temperature changes in silicon nitride sintered bodies.

[0005] An object of the present invention is to provide a technique for suppressing damage caused by temperature changes in a silicon nitride sintered body.

[0006] The present invention has been made to solve at least a part of the above-described problems, and can be implemented as the following aspects.

[0007] (1) According to one aspect of the present invention, there is provided a silicon nitride sintered body. The silicon nitride sintered body includes a plurality of silicon nitride particles and a grain boundary phase formed between the plurality of silicon nitride particles. In a cross-section passing through the silicon nitride particles and the grain boundary phase, a rectangular region each side of which is 50 µm and which is included in the cross-section of the silicon nitride sintered body within 50 µm from the surface of the silicon nitride sintered body is defined as a first region. Let A be the ratio of the area of a specific region containing at least one of rare earth elements, alkaline earth elements, and Group 4 elements included in the first region to the area of the first region. A rectangular region each side of which is 50 µm and which is included in the cross-section of the silicon nitride sintered body at a distance of 50 µm or more from the surface of the silicon nitride sintered body is defined as a second region. Let B be the ratio of the area of a specific region containing at least one of rare earth elements, alkaline earth elements, and Group 4 elements included in the second region to the area of the second region. Then, the following formula (1) is satisfied. 1 < A / B < 4 --- (1)

[0008] According to this configuration, the content of elements including rare earth elements, alkaline earth elements, and at least one of the group 4 elements in the portion of the silicon nitride sintered body including the surface is greater than the content of elements including rare earth elements, alkaline earth elements, and at least one of the group 4 elements in the interior of the silicon nitride sintered body. The fact that the content of elements including rare earth elements, alkaline earth elements, and at least one of the group 4 elements in the silicon nitride sintered body satisfies formula (1) indicates that elements including rare earth elements, alkaline earth elements, and at least one of the group 4 elements are moderately segregated on the surface. As a result, pores that tend to form relatively easily on the surface are filled by these elements, so, for example, the occurrence of cracks due to thermal cycles of repeated heating and cooling can be suppressed. Therefore, damage due to temperature changes can be suppressed.

[0009] (2) In the silicon nitride sintered body of the above form, the rare earth element contained in the silicon nitride sintered body may be at least one of yttrium, ytterbium, and erbium. With this configuration, since the silicon nitride sintered body contains at least one of yttrium, ytterbium, and erbium as a rare earth element, it becomes easier to crystallize. As a result, the thermal conductivity of the entire silicon nitride sintered body increases, and the thermal conductivity of the silicon nitride sintered body can be improved.

[0010] (3) In the silicon nitride sintered body of the above form, the alkaline earth element contained in the silicon nitride sintered body may be at least one of magnesium and calcium. With this configuration, since the silicon nitride sintered body contains at least one of magnesium and calcium as an alkaline earth element, the silicon nitride particles become more densified. As a result, the strength of the silicon nitride particles is improved, and the strength of the silicon nitride sintered body can be improved.

[0011] (4) In the silicon nitride sintered body of the above form, the group IV element contained in the silicon nitride sintered body may be at least one of hafnium and zirconium. With this configuration, since the silicon nitride sintered body contains at least one of hafnium and zirconium as a group IV element, it becomes easier to crystallize. As a result, the thermal conductivity increases, and the thermal conductivity of the silicon nitride sintered body can be improved.

[0012] (5) In the silicon nitride sintered body of the above form, the silicon nitride sintered body contains aluminum, and the concentration of aluminum contained in the silicon nitride sintered body may be 1000 ppm or less. With this configuration, the silicon nitride sintered body does not contain a relatively large amount of aluminum, which is a factor that reduces thermal conductivity by solid dissolution in the silicon nitride particles. This makes it possible to suppress the decrease in thermal conductivity of the silicon nitride sintered body.

[0013] (6) In the silicon nitride sintered body of the above form, each of the rare earth elements, alkaline earth elements, and Group IV elements may be included in the grain boundary phase. With this configuration, many of each of the rare earth elements, alkaline earth elements, and Group IV elements are included in the grain boundary phase. As a result, pores are less likely to form in the grain boundary phase, which is prone to pore formation, and thus damage to the silicon nitride sintered body due to temperature changes can be further suppressed.

[0014] (7) According to another embodiment of the present invention, a heat dissipation substrate is provided. This heat dissipation substrate is formed from a silicon nitride sintered body of the above embodiment. With this configuration, the heat dissipation substrate is formed from a silicon nitride sintered body that is less susceptible to damage due to temperature changes. As a result, for example, when electronic components such as semiconductor elements are mounted on it, damage to the heat dissipation substrate due to temperature changes caused by heat generation in the semiconductor elements can be suppressed.

[0015] (8) In the heat dissipation substrate of the above form, the bending strength of the silicon nitride sintered body may be 600 MPa or more. With this configuration, since the bending strength of the silicon nitride sintered body of the heat dissipation substrate is relatively large, damage to the heat dissipation substrate can be suppressed even when stress due to differences in thermal expansion or the like is applied.

[0016] (9) In the heat dissipation substrate of the above form, the thermal conductivity of the silicon nitride sintered body may be 90 W / (m·K) or more. With this configuration, since the thermal conductivity of the silicon nitride sintered body of the heat dissipation substrate is relatively high, when electronic components such as semiconductor elements are mounted on it, for example, the heat from the electronic components can be efficiently released to the outside through the heat dissipation substrate.

[0017] (10) In the heat dissipation substrate of the above form, the thickness of the silicon nitride sintered body may be 220 μm or more and 690 μm or less. With this configuration, since the thickness of the silicon nitride sintered body of the heat dissipation substrate is relatively thin, heat from the electronic components can be efficiently released to the outside through the silicon nitride sintered body.

[0018] Furthermore, the present invention can be realized in various forms, for example, in the form of a method for manufacturing a silicon nitride sintered body, a substrate for electronic components equipped with a silicon nitride sintered body, an apparatus equipped with a silicon nitride sintered body, a control method for an apparatus equipped with a silicon nitride sintered body, a semiconductor package equipped with a heat dissipation substrate, and a computer program that uses the semiconductor package equipped with a heat dissipation substrate to supply power to electronic devices and perform the mutual conversion of AC and DC.

[0019] This is a cross-sectional view of a heat dissipation substrate equipped with a silicon nitride sintered body according to the first embodiment. This is a diagram showing a cross-sectional SEM image of the silicon nitride sintered body according to the first embodiment. This is a diagram showing a first SEM-EDS image in the cross-section of the silicon nitride sintered body. This is a diagram showing a second SEM-EDS image in the cross-section of the silicon nitride sintered body. This is a diagram illustrating the results of an evaluation test of the silicon nitride sintered body.

[0020] <First Embodiment> Figure 1 is a cross-sectional view of a semiconductor package comprising a silicon nitride sintered body according to the first embodiment. The semiconductor package P1 of this embodiment performs various power conversions by switching operations in the power semiconductor element 5, specifically supplying power to electronic devices and performing mutual conversion between AC and DC. The semiconductor package P1 of this embodiment comprises a power semiconductor element 5, a heat dissipation substrate 10, solder 21, 22, a heat sink 30, grease 40, and a fin portion 50. In the semiconductor package P1 of this embodiment, as shown in Figure 1, the heat dissipation substrate 10, the heat sink 30, and the fin portion 50 are stacked in that order. Note that the thickness relationships of the power semiconductor element 5, the heat dissipation substrate 10, the solder 21, 22, the heat sink 30, the grease 40, and the fin portion 50 in Figure 1 are shown differently from the actual thickness relationships for the sake of explanation.

[0021] The heat dissipation substrate 10 comprises a silicon nitride sintered body 1, a circuit layer 11, and a conductive layer 12. The heat dissipation substrate 10 has the function of efficiently transferring heat generated by the switching operation of the power semiconductor element 5 from the power semiconductor element 5.

[0022] The silicon nitride sintered body 1 comprises a plurality of silicon nitride particles and a rare earth element nitride crystal formed in the grain boundary phase between the plurality of silicon nitride particles. The silicon nitride particles are formed of silicon nitride (Si3N4). The silicon nitride sintered body 1 of this embodiment is a plate-shaped member having a pair of surfaces 1a and 1b. In the silicon nitride sintered body 1 of this embodiment, the thickness in the stacking direction of the heat dissipation substrate 10, the heat dissipation plate 30, and the fin portion 50 is 220 μm or more and 690 μm or less, but the shape and thickness in the stacking direction of the silicon nitride sintered body 1 are not limited to these. Details of the features of the silicon nitride sintered body 1 of this embodiment will be described later. The silicon nitride particles in the silicon nitride sintered body 1 may be formed of SiAlON.

[0023] The circuit layer 11 is arranged on one of the pair of surfaces 1a and 1b of the silicon nitride sintered body 1, surface 1a. The circuit layer 11 and the silicon nitride sintered body 1 are joined together using a bonding material such as brazing material. The circuit layer 11 is preferably made of a metal, and more preferably made of a metal mainly composed of copper.

[0024] The conductive layer 12 is disposed on the other surface 1b of the pair of surfaces 1a and 1b that the silicon nitride sintered body 1 has. The circuit layer 11 and the silicon nitride sintered body 1 are joined together using a bonding material such as brazing material. The conductive layer 12 is preferably made of a metal, and more preferably made of a metal mainly composed of copper.

[0025] The solder 21 is positioned between the circuit layer 11 and the power semiconductor element 5 on the side of the circuit layer 11 opposite to the silicon nitride sintered body 1. The solder 21 electrically connects the circuit layer 11 and the power semiconductor element 5.

[0026] The solder 22 is provided on the side of the conductive layer 12 opposite to the silicon nitride sintered body 1. The solder 22 is connected to the conductive layer 12 and the heat sink 30.

[0027] The heat sink 30 is a plate-shaped member formed from a material with relatively good thermal conductivity. The heat sink 30 is preferably made of metal, and more preferably made of a metal mainly composed of copper.

[0028] The grease 40 thermally connects the heat sink 30 and the fin portion 50, which will be described later. The grease 40 in this embodiment is a thermally conductive grease with a relatively high thermoelectric coefficient.

[0029] The fin portion 50 is a substantially plate-shaped member formed from a material with relatively good thermal conductivity. The fin portion 50 has a plurality of fins 51 and is formed to have a large surface area. The fin portion 50 is preferably made of metal, and more preferably made of a metal mainly composed of copper.

[0030] Next, the features of the silicon nitride sintered body 1 of this embodiment will be described. The silicon nitride sintered body 1 comprises a plurality of silicon nitride particles and a grain boundary phase formed between the plurality of silicon nitride particles. In this embodiment, the silicon nitride sintered body 1 satisfies the following equation (1): 1 < A / B < 4 ... (1)

[0031] Each of the area ratios A and B included in equation (1) is calculated by performing image analysis using the image analysis software WinROOF on an image acquired using an electron probe microanalyzer (EPMA). Each of the area ratios A and B may be the average value of multiple data obtained when multiple first regions and second regions are set. In this embodiment, five first regions and two second regions are set, and equation (1) is satisfied by the average value of the data obtained in each region. In this embodiment, the "surface" of the silicon nitride sintered body 1 refers to the surface of the silicon nitride sintered body 1 obtained after firing a degreased molded product in the silicon nitride sintered body 1 manufacturing method described later, and does not include the cut surface exposed when the silicon nitride sintered body 1 is cut.

[0032] Figure 2 shows a cross-sectional SEM image of the silicon nitride sintered body 1 of this embodiment. Figure 2 shows a cross-sectional SEM image including the surface 1a of the silicon nitride sintered body 1. The image shown in Figure 2 is a secondary electron image taken using a scanning electron microscope (SEM), so the irregularities on the cross-section are clearly shown. In Figure 2, the white area shown near the surface 1a of the silicon nitride sintered body 1 is a part formed of a material other than silicon nitride. It can be seen that most of this white area is located in the range from the surface 1a of the silicon nitride sintered body 1 to 50 μm. In Figure 2, the region corresponding to the first region R1 and the region corresponding to the second region R2 are shown enclosed by dashed lines. For convenience, in Figure 2, the surface 1a of the silicon nitride sintered body 1 is shown with a solid line, and the position 50 μm from the surface 1a of the silicon nitride sintered body 1 is shown with a dashed line BL.

[0033] Figure 3 shows a first SEM-EDS image of a cross-section of the silicon nitride sintered body 1 of this embodiment. The image shown in Figure 3 is an image taken using energy dispersive X-ray spectroscopy (EDS), and shows the locations where rare earth elements and alkaline earth elements exist in the cross-section shown in Figure 2. The rare earth elements contained in the silicon nitride sintered body 1 of this embodiment are at least one of yttrium, ytterbium, and erbium, and the alkaline earth elements contained in the silicon nitride sintered body 1 of this embodiment are at least one of magnesium and calcium. The silicon nitride sintered body 1 of this embodiment contains yttrium as a rare earth element and magnesium and calcium as alkaline earth elements. The white areas shown in Figure 3 indicate the locations where yttrium, magnesium, or calcium is present. Comparing the first region R1 and the second region R2 shown in Figure 3, it can be seen that the first region R1 contains more yttrium, magnesium, or calcium than the second region R2. The rare earth elements and alkaline earth elements contained in the silicon nitride sintered body 1 each play a role in filling the pores formed in the silicon nitride sintered body 1.

[0034] Figure 4 shows a second SEM-EDS image of a cross-section of the silicon nitride sintered body 1 of this embodiment. The image shown in Figure 4 was captured using energy-dispersive X-ray spectroscopy and shows the location of the group IV elements in the cross-section shown in Figure 2. The group IV elements contained in the silicon nitride sintered body 1 of this embodiment are hafnium and at least one of zirconium. The silicon nitride sintered body 1 of this embodiment contains zirconium as a group IV element. The lighter colored areas shown in Figure 4 indicate the locations where zirconium is present. Comparing the first region R1 and the second region R2 shown in Figure 4, it can be seen that there is more zirconium in the first region R1 than in the second region R2. The zirconium contained in the silicon nitride sintered body 1 plays a role in filling the pores formed in the silicon nitride sintered body 1.

[0035] In the silicon nitride sintered body 1 of this embodiment, as shown in Figures 3 and 4, it can be seen that rare earth elements, alkaline earth elements, and Group IV elements are all present in relatively large quantities near the surface 1a of the silicon nitride sintered body 1. In the silicon nitride sintered body 1 of this embodiment, "A / B" in formula (1) is 1.4. In the silicon nitride sintered body 1 of this embodiment, each of the rare earth elements, alkaline earth elements, and Group IV elements is contained in the grain boundary phase.

[0036] The silicon nitride sintered body 1 of this embodiment contains aluminum, and the concentration of aluminum contained in the silicon nitride sintered body 1 is 1000 ppm or less. In this embodiment, the concentration of aluminum in the silicon nitride sintered body 1 is measured by inductively coupled plasma atomic emission spectrometry (ICP-AES) after the silicon nitride sintered body 1 is pulverized into a powder. The silicon nitride sintered body 1 of this embodiment has an aluminum concentration of 500 ppm.

[0037] The silicon nitride sintered body 1 of this embodiment has a thermal conductivity of 90 W / (m·K) or higher. In this embodiment, the thermal conductivity of the silicon nitride sintered body 1 is measured using the xenon flash method. Specifically, the silicon nitride sintered body 1 is processed into a square thermal conductivity measurement sample with a thickness of 0.32 mm and sides of 10 mm, and the thermal diffusivity of the thermal conductivity measurement sample is measured using the xenon flash method. In this embodiment, the specific heat value of the thermal conductivity measurement sample is kept constant at 0.68 kJ / (K·kg). The thermal conductivity of the thermal conductivity measurement sample is calculated using the thermal diffusivity of the thermal conductivity measurement sample measured by the xenon flash method and the density value of the thermal conductivity measurement sample measured by the Archimedes method, as shown in (2) below. The silicon nitride sintered body 1 of this embodiment has a thermal conductivity of 101 W / (m·K). (Thermal conductivity) = (Density) × (Specific heat) × (Thermal diffusivity) ... (2)

[0038] The silicon nitride sintered body 1 of this embodiment has a bending strength of 600 MPa or more. In this embodiment, the bending strength of the silicon nitride sintered body 1 is measured in accordance with ISO 23242 by processing the silicon nitride sintered body 1 to a thickness of 0.32 × 12 × 25 mm and measuring the bending strength by bending at three points with a span of 15 mm. The silicon nitride sintered body 1 of this embodiment has a bending strength of 687 MPa.

[0039] Next, a method for manufacturing the silicon nitride sintered body 1 provided in the heat dissipation substrate 10 will be described. In the method for manufacturing the silicon nitride sintered body 1, first, as a mixing step, silicon nitride raw material powder and a sintering aid are weighed and a mixed powder is prepared. In the method for manufacturing the silicon nitride sintered body 1 of this embodiment, rare earth raw material powder, alkaline earth raw material powder, and group 4 element raw material powder are selected as the sintering aid. As rare earth raw material powder, for example, a raw material powder containing at least one element selected from rare earth elements is used, and examples include oxides and nitrides of rare earth elements. For example, in the case of yttrium raw material powder, examples include Y2O3 and YN. As alkaline earth raw material powder, for example, examples include MgO, MgCO3 and MgSiN2. As group 4 element powder, examples include oxides of group 4 elements. As zirconium raw material powder, examples include ZrO2, but the pebbles for crushing described later may be formed from ZrO2.

[0040] In the method for producing the silicon nitride sintered body 1 of this embodiment, a slurry containing the raw materials is prepared by adding a plasticizer, such as a phthalate-based plasticizer, an organic binder, such as polyvinyl butyral or isobutyl polyacrylate, and an organic solvent, such as ethyl alcohol or butyl alcohol, to the weighed raw material powder and sintering aid, and mixing them in a ball mill. The solid content concentration in the prepared slurry is preferably 30 wt% or more and 70 wt% or less.

[0041] In the method for producing the silicon nitride-based sintered body 1 of the present embodiment, next, as a molding step, after defoaming and thickening the prepared slurry, for example, a sheet-shaped molded body (green sheet) is produced by the doctor blade method. Note that the thickness of the produced molded body is set in consideration of the thickness of the silicon nitride-based sintered body 1 and the sintering shrinkage ratio. The molded body produced by the doctor blade method is processed to have a preset shape and size. After processing the molded body, in order to remove the organic binder and the plasticizer, the molded body is heated at a temperature of 400°C or higher and 800°C or lower to perform degreasing.

[0042] In the method for producing the silicon nitride-based sintered body 1 of the present embodiment, next, as a firing step, the processed molded body is placed into a heating furnace and sintered by heating in a nitrogen atmosphere. The firing conditions in the firing step are, for example, as follows.   Atmosphere temperature: 1800°C or higher and 2000°C or lower (preferably 1800°C or higher and 1950°C or lower; at a temperature of 1800°C or higher, the firing is performed under a nitrogen atmosphere of about 1 MPa)   Holding time: 1 hour or more and 20 hours or less (preferably 2 hours or more and 10 hours or less)

[0043] In the firing step of the method for producing the silicon nitride-based sintered body 1 of the present embodiment, the gap in a saggar is adjusted such that the proportion of the processed molded body in the saggar is 1% or more and 90% or less, and the movement of the sintering aid during firing is controlled, so that each of the rare earth element, the alkaline earth element and the Group 4 element is promoted to move to the surface 1a of the silicon nitride-based sintered body 1. This improves the strength by filling pores that are easily formed near the surface 1a of the silicon nitride-based sintered body 1. In this way, the silicon nitride-based sintered body 1 of the present embodiment is produced. Note that the method for producing the silicon nitride-based sintered body 1 of the present embodiment is not limited to this.

[0044] Next, the results of an evaluation test for a silicon nitride-based sintered body will be described. In this evaluation test, for each of eight types of silicon nitride sintered bodies (hereinafter simply referred to as "samples") produced by different production methods, characteristics related to elements contained in the samples, thermal shock resistance, thermal conductivity and bending strength were measured or calculated and evaluated.

[0045] Figure 5 illustrates the evaluation test results of silicon nitride sintered bodies. Each of the samples 1 to 8 shown in Figure 5 was prepared by a method similar to the manufacturing method of silicon nitride sintered body 1 in this embodiment. Specifically, the proportion of the molded body processed within the casing was adjusted during the firing process. In addition, sample 5 was prepared using a sintering aid containing ytterbium (Yb) in the rare earth raw material powder used as a sintering aid. In sample 6, a sintering aid containing erbium (Er) in the rare earth raw material powder used as a sintering aid, and a sintering aid containing zirconium and hafnium (Hf) in the Group 4 element raw material powder used as a sintering aid were prepared.

[0046] Figure 5 shows the "Types of Rare Earth Elements," "Types of Alkaline Earth Elements," and "Types of Group IV Elements," respectively, which represent the types of rare earth elements, alkaline earth elements, and Group IV elements contained in the sample.

[0047] The "area ratio in the first region (=A)" and the "area ratio in the second region (=B)" shown in Figure 5 represent the area ratio of the total of rare earth elements (Re) and alkaline earth elements (Ae) in the "first region R1" and the "second region R2," respectively, as explained using Figure 2 ("Area ratio of (Re + Ae) (%)") and the area ratio of group 4 elements (G4) ("Area ratio of G4 (%)"). The "area ratio in the first region (=A)" and the "area ratio in the second region (=B)" in Figure 5 were measured by image analysis using the image analysis software WinROOF on images acquired using an electron probe microanalyzer, similar to the silicon nitride sintered body 1 of this embodiment.

[0048] "A / B" shown in FIG. 5 indicates the ratio of A, which is "the area ratio in the first region", to B, which is "the area ratio in the second region". "A / B" in FIG. 5 was calculated using data obtained by image analysis with image analysis software WinROOF on an image acquired using an electron probe microanalyzer, in the same manner as for the silicon nitride sintered body 1 of the present embodiment. In this evaluation test, similarly to the silicon nitride sintered body 1 of the present embodiment, five first regions and five second regions were respectively set, and calculation was performed using the average value of data obtained in each of the regions.

[0049] "Al concentration" shown in FIG. 5 indicates the concentration of aluminum contained in a sample. "Al concentration" in FIG. 5 was measured by inductively coupled plasma optical emission spectrometry in the same manner as for the silicon nitride sintered body 1 of the present embodiment.

[0050] "Number of endurance cycles" shown in FIG. 5 indicates the thermal shock resistance of a sample. "Number of endurance cycles" in FIG. 5 was measured by an endurance test in which heating and cooling were repeated. Specifically, a thermal shock resistance measurement sample having a copper plate joined thereto was set in a thermal cycle tester, a cycle consisting of holding at -40°C for 5 minutes, raising the temperature, holding at 220°C for 5 minutes, and cooling to -40°C was defined as one cycle, and heating and cooling were repeated for a maximum of 2000 cycles. In this endurance test, SAT inspection (ultrasonic flaw detection inspection) was performed every 100 cycles, and it was confirmed whether cracks occurred at the edge of a portion where the sample and the copper plate were joined. "Number of endurance cycles" shown in FIG. 5 indicates the number of cycles at which cracks occurred, and is indicated as "2000" when no cracks occurred even after repeating heating and cooling for 2000 cycles.

[0051] "Thermal conductivity" shown in FIG. 5 indicates the thermal conductivity of a sample. Thermal conductivity was calculated by measuring the thermal diffusivity of the sample using the xenon flash method in the same manner as for the silicon nitride sintered body 1 of the present embodiment, and using formula (2). Note that the specific heat value of the sample in this evaluation test was fixed at 0.68 kJ / (K·kg) in the same manner as for the silicon nitride sintered body 1 of the present embodiment.

[0052] The "bending strength" shown in Figure 5 represents the bending strength of the sample. The "bending strength" was measured by processing the sample using a method compliant with ISO 23242, similar to the silicon nitride sintered body 1 of this embodiment, and then bending it at three points with a span of 15 mm.

[0053] As shown in Figure 5, samples 1 to 6, where "A / B" is greater than 1 and less than 4, had the maximum "durability cycle count" of 2000 cycles, confirming relatively good thermal shock resistance. Furthermore, samples 1 to 6 were confirmed to have a thermal conductivity of 90 W / (m·K) or higher and a bending strength of 600 MPa or higher. Therefore, samples 1 to 6 were confirmed to have excellent thermal shock resistance while simultaneously achieving good thermal conductivity and appropriate bending strength.

[0054] In sample 7, where "A / B" is less than 1 (0.6), the "endurance cycle count" was 300, confirming that it had inferior thermal shock resistance compared to samples 1 to 6. An "A / B" of less than 1 indicates that in the silicon nitride sintered body sample, rare earth elements, alkaline earth elements, and group IV elements remain more in the interior than on the surface. Therefore, it is thought that the pores that tend to form near the surface were not filled, resulting in inferior thermal shock resistance. In sample 8, where "A / B" is greater than 4 (4.5), the thermal shock resistance was similar to that of samples 1 to 6, but the "thermal conductivity" was less than 90 W / (m·K). An "A / B" of greater than 4 indicates that in the silicon nitride sintered body sample, rare earth elements, alkaline earth elements, and group IV elements are extremely segregated on the surface. Therefore, it is thought that the pores formed in the interior, which also act as resistance to heat transfer, were not completely filled.

[0055] As described above, in the silicon nitride sintered body 1 of this embodiment, the content of elements including rare earth elements, alkaline earth elements, and at least one of the group 4 elements in the portion including the surface 1a is greater than the content of elements including rare earth elements, alkaline earth elements, and at least one of the group 4 elements in the interior. The fact that the content of elements including rare earth elements, alkaline earth elements, and at least one of the group 4 elements in the silicon nitride sintered body 1 satisfies formula (1) means that elements including rare earth elements, alkaline earth elements, and at least one of the group 4 elements are appropriately segregated on the surface 1a of the silicon nitride sintered body 1. As a result, pores that tend to form on the surface 1a of the silicon nitride sintered body 1 are filled by these elements, so the occurrence of cracks due to thermal cycles of repeated heating and cooling can be suppressed (see Samples 1 to 6 in Figure 5). Therefore, damage to the silicon nitride sintered body 1 due to temperature changes can be suppressed.

[0056] Furthermore, according to the silicon nitride sintered body 1 of this embodiment, as shown in Figure 5, the thermal conductivity is 90 W / (m·K) or higher, and the bending strength is 600 MPa or higher. This makes it possible to achieve both good thermal conductivity and appropriate bending strength in the silicon nitride sintered body 1.

[0057] Furthermore, the silicon nitride sintered body 1 of this embodiment contains yttrium as a rare earth element, which makes it easier to crystallize. As a result, the overall thermal conductivity of the silicon nitride sintered body 1 increases, thus improving the thermal conductivity of the silicon nitride sintered body 1.

[0058] Furthermore, according to the silicon nitride sintered body 1 of this embodiment, since magnesium and calcium are included as alkaline earth elements in the silicon nitride sintered body 1, the silicon nitride particles become more densified. As a result, the strength of the silicon nitride particles is improved, and thus the strength of the silicon nitride sintered body 1 can be improved.

[0059] Furthermore, according to the silicon nitride sintered body 1 of this embodiment, since zirconium is included as a group 4 element in the silicon nitride sintered body, it becomes easier to crystallize. As a result, the thermal conductivity increases, and thus the thermal conductivity of the silicon nitride sintered body 1 can be improved.

[0060] Furthermore, according to the silicon nitride sintered body 1 of this embodiment, the concentration of aluminum contained in the silicon nitride sintered body 1 is 1000 ppm or less. In other words, the silicon nitride sintered body 1 does not contain a large amount of aluminum, which is a factor that reduces thermal conductivity by solid dissolving in silicon nitride particles. As a result, the decrease in thermal conductivity of the silicon nitride sintered body 1 can be suppressed.

[0061] Furthermore, in the silicon nitride sintered body 1 of this embodiment, many of the rare earth elements, alkaline earth elements, and Group IV elements are contained in the grain boundary phase. This makes it difficult for pores to form in the grain boundary phase, where pores are prone to occur. Therefore, damage to the silicon nitride sintered body 1 due to temperature changes can be further suppressed.

[0062] Furthermore, according to the heat dissipation substrate 10 of this embodiment, the heat dissipation substrate 10 is formed of a silicon nitride sintered body 1 that is less susceptible to damage due to temperature changes. This makes it possible to suppress damage to the heat dissipation substrate 10 due to temperature changes caused by heat generation in the power semiconductor element 5.

[0063] Furthermore, according to the heat dissipation substrate 10 of this embodiment, the bending strength of the silicon nitride sintered body 1 provided in the heat dissipation substrate 10 is relatively high, so it is less likely to break even when stress is applied, for example, due to differences in thermal expansion.

[0064] Furthermore, according to the heat dissipation substrate 10 of this embodiment, since the thermal conductivity of the silicon nitride sintered body 1 provided in the heat dissipation substrate 10 is relatively high, the heat from the power semiconductor element 5 can be dissipated to the outside more efficiently through the heat dissipation substrate 10.

[0065] Furthermore, according to the heat dissipation substrate 10 of this embodiment, since the thickness of the silicon nitride sintered body 1 provided in the heat dissipation substrate 10 is relatively thin, the heat from the power semiconductor element 5 can be dissipated to the outside more efficiently through the silicon nitride sintered body 1.

[0066] <Modifications of this Embodiment> The present invention is not limited to the above-described embodiments, and can be implemented in various forms without departing from the spirit of the invention. For example, the following modifications are also possible.

[0067] [Modification 1] In the above embodiment, the rare earth element contained in the silicon nitride sintered body 1 was yttrium. The rare earth element contained in the silicon nitride sintered body may be ytterbium or erbium, as shown in Figure 5, or at least one of yttrium, ytterbium, and erbium.

[0068] [Modification 2] In the above embodiment, the alkaline earth elements contained in the silicon nitride sintered body 1 were magnesium and calcium. The alkaline earth elements contained in the silicon nitride sintered body are not limited to these. It may be either magnesium or calcium.

[0069] [Modification 3] In the above embodiment, the group IV element contained in the silicon nitride sintered body 1 was zirconium. The group IV element contained in the silicon nitride sintered body 1 may be hafnium, or both hafnium and zirconium. Furthermore, the group IV element contained in the silicon nitride sintered body 1 may be omitted.

[0070] [Modification 4] In the above embodiment, the silicon nitride sintered body 1 contains aluminum, and the concentration of aluminum contained in the silicon nitride sintered body 1 is 1000 ppm or less. The concentration of aluminum may be greater than 1000 ppm, but a lower concentration makes it easier to suppress the decrease in thermal conductivity due to solid solution of aluminum into the silicon nitride particles. Also, it is not necessary to contain aluminum.

[0071] [Modification 5] In the above embodiment, it was assumed that each of the rare earth elements, alkaline earth elements, and group IV elements were included in the grain boundary phase. Although each of the rare earth elements, alkaline earth elements, and group IV elements does not necessarily have to be included in the grain boundary phase, for example, if rare earth elements or group IV elements are included in the grain boundary phase, crystallization is easier, and thus the strength can be improved.

[0072] [Modification 6] In the above-described embodiment, the silicon nitride sintered body was applied to a heat dissipation substrate provided in a semiconductor package. The technical field to which the silicon nitride sintered body of this embodiment is applied is not limited thereto. The silicon nitride sintered body of this embodiment can be applied as a component that requires both relatively high thermal conductivity and relatively high strength.

[0073] The embodiments of this specification have been described above based on the embodiments and modifications described above. The embodiments described above are for the purpose of facilitating understanding of this specification and do not limit it. This specification may be modified and improved without departing from its spirit and the scope of the claims, and equivalents thereof are included in this specification. Furthermore, any technical features that are not described as essential in this specification may be deleted as appropriate.

[0074] <Application Example 1> A silicon nitride sintered body comprising a plurality of silicon nitride particles and a grain boundary phase formed between the plurality of silicon nitride particles, wherein, in a cross-section passing through the silicon nitride particles and the grain boundary phase, a rectangular region with sides of 50 μm included in the cross-section of the silicon nitride sintered body up to 50 μm from the surface of the silicon nitride sintered body is defined as a first region, and the ratio of the area of ​​a specific region included in the first region that contains at least one of rare earth elements, alkaline earth elements, and group 4 elements to the area of ​​the first region is A, and a rectangular region with sides of 50 μm included in the cross-section of the silicon nitride sintered body that is 50 μm or more away from the surface of the silicon nitride sintered body is defined as a second region, and the ratio of the area of ​​a specific region included in the second region that contains at least one of rare earth elements, alkaline earth elements, and group 4 elements to the area of ​​the second region is B, wherein the following formula (1) is satisfied. 1 < A / B < 4 ... (1) <Application Example 2> A silicon nitride sintered body according to Application Example 1, characterized in that the rare earth element contained in the silicon nitride sintered body is at least one of yttrium, ytterbium, and erbium. <Application Example 3> A silicon nitride sintered body according to Application Example 1 or Application Example 2, characterized in that the alkaline earth element contained in the silicon nitride sintered body is at least one of magnesium and calcium. <Application Example 4> A silicon nitride sintered body according to any one example from Application Example 1 to Application Example 3, characterized in that the group 4 element contained in the silicon nitride sintered body is at least one of hafnium and zirconium. <Application Example 5> A silicon nitride sintered body according to any one of Application Examples 1 to 4, wherein the silicon nitride sintered body contains aluminum, and the concentration of aluminum contained in the silicon nitride sintered body is 1000 ppm or less.<Application Example 6> A silicon nitride sintered body according to any one of Application Examples 1 to 5, characterized in that each of the rare earth elements, alkaline earth elements, and group 4 elements is contained in the grain boundary phase. <Application Example 7> A heat dissipation substrate, characterized in that it is formed from a silicon nitride sintered body according to any one of Application Examples 1 to 6. <Application Example 8> A heat dissipation substrate according to Application Example 7, characterized in that the bending strength of the silicon nitride sintered body is 600 MPa or more. <Application Example 9> A heat dissipation substrate according to Application Example 7 or Application Example 8, characterized in that the thermal conductivity of the silicon nitride sintered body is 90 W / (m·K) or more. <Application Example 10> A heat dissipation substrate according to any one of Application Examples 7 to 9, characterized in that the thickness of the silicon nitride sintered body is 220 μm or more and 690 μm or less.

[0075] 1...Silicon nitride sintered body 10...Heat dissipation substrate R1...First region R2...Second region

Claims

1. A silicon nitride sintered body comprising a plurality of silicon nitride particles and a grain boundary phase formed between the plurality of silicon nitride particles, wherein, in a cross-section passing through the silicon nitride particles and the grain boundary phase, a rectangular region with sides of 50 μm included in the cross-section of the silicon nitride sintered body up to 50 μm from the surface of the silicon nitride sintered body is defined as a first region, and the ratio of the area of ​​a specific region included in the first region that contains at least one of rare earth elements, alkaline earth elements, and group 4 elements to the area of ​​the first region is A; and a rectangular region with sides of 50 μm included in the cross-section of the silicon nitride sintered body that is 50 μm or more away from the surface of the silicon nitride sintered body is defined as a second region, and the ratio of the area of ​​a specific region included in the second region that contains at least one of rare earth elements, alkaline earth elements, and group 4 elements to the area of ​​the second region is B, wherein the following formula (1) is satisfied. 1<A / B<4...(1) 2. A silicon nitride sintered body according to claim 1, characterized in that the rare earth element contained in the silicon nitride sintered body is at least one of yttrium, ytterbium, and erbium.

3. A silicon nitride sintered body according to claim 1 or claim 2, characterized in that the alkaline earth element contained in the silicon nitride sintered body is at least one of magnesium and calcium.

4. A silicon nitride sintered body according to claim 1 or claim 2, characterized in that the group IV element contained in the silicon nitride sintered body is at least one of hafnium and zirconium.

5. A silicon nitride sintered body according to claim 1 or claim 2, wherein the silicon nitride sintered body contains aluminum, and the concentration of aluminum contained in the silicon nitride sintered body is 1000 ppm or less.

6. A silicon nitride sintered body according to claim 1 or claim 2, characterized in that each of the rare earth elements, alkaline earth elements, and group 4 elements is contained in the grain boundary phase.

7. A heat dissipation substrate, characterized in that it is formed from a silicon nitride sintered body as described in claim 1 or claim 2.

8. A heat dissipation substrate according to claim 7, characterized in that the bending strength of the silicon nitride sintered body is 600 MPa or more.

9. A heat dissipation substrate according to claim 7, characterized in that the thermal conductivity of the silicon nitride sintered body is 90 W / (m·K) or more.

10. A heat dissipation substrate according to claim 7, characterized in that the thickness of the silicon nitride sintered body is 220 μm or more and 690 μm or less.

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