Silicon nitride substrate and manufacturing method thereof

Abstract

To reduce color unevenness generated on a surface of a silicon nitride substrate.SOLUTION: A silicon nitride substrate formed by nitriding silicon contained in a sheet-shaped formed body includes a first surface and a second surface opposite to the first surface. If ΔE*ab refers to color difference between a center and an edge of at least one of the first and second surfaces, then ΔE*ab≤1.5 is satisfied.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a silicon nitride substrate and a manufacturing technique thereof, and more particularly to a silicon nitride substrate having a thermal conductivity of 110 W / (m·K) or more and a technique that is effective when applied to the manufacturing technique thereof. [Background technology]

[0002] Japanese Patent Application Laid-Open No. 2003-267786 (Patent Document 1) describes a technology for providing a silicon nitride ceramic sintered body that is blacker in color than conventional materials, has less color unevenness, and has sufficient strength.

[0003] Japanese Patent Laid-Open Publication No. 2005-214659 (Patent Document 2) describes a technique relating to a foreign matter inspection device that can identify foreign matter of a color that has little contrast with the color of a wiring board.

[0004] Japanese Patent Application Laid-Open No. 2016-204206 (Patent Document 3), Japanese Patent Application Laid-Open No. 2016-204207 (Patent Document 4), Japanese Patent Application Laid-Open No. 2016-204209 (Patent Document 5), and Japanese Patent Application Laid-Open No. 2016-204210 (Patent Document 6) describe techniques for providing silicon nitride ceramic members that are lightweight, have high hardness, and have excellent resistance to processing such as polishing, and also have excellent appearance quality.

[0005] Japanese Patent Application Laid-Open No. 9-227240 (Patent Document 7) describes a technology that can reduce the thickness of the surface color layer of a silicon nitride ceramic sintered body and further make the fracture strength characteristics of the surface layer and inner layer uniform. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-267786 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-214659 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-204206 [Patent Document 4] Japanese Patent Application Laid-Open No. 2016-204207 [Patent Document 5] Japanese Patent Application Laid-Open No. 2016-204209 [Patent Document 6] Japanese Patent Application Laid-Open No. 2016-204210 [Patent Document 7] Japanese Patent Application Publication No. 9-227240 Summary of the Invention [Problem to be solved by the invention]

[0007] For example, if foreign matter or dirt adheres to the surface of a silicon nitride substrate, it can cause poor contact with the brazing filler metal or poor insulation of the silicon nitride substrate itself, so it is necessary to remove the foreign matter and dirt. Therefore, visual inspection is performed to detect the adhesion of foreign matter and dirt to the silicon nitride substrate. One visual inspection method, for example, is to take an image of the surface of the silicon nitride substrate with an imaging device such as a CCD camera, and then compare the captured image data with pre-registered reference data to detect foreign matter and dirt.

[0008] However, with the above-described appearance inspection method, if there is color unevenness, for example, between the center and edge of the surface of the silicon nitride substrate, there is a risk that the color unevenness may be erroneously detected as the adhesion of foreign matter or dirt.

[0009] In this regard, it is conceivable to relax the criteria for detecting the attachment of foreign matter or dirt so as not to mistakenly detect color unevenness as the attachment of foreign matter or dirt, but this would result in the risk of missing foreign matter or dirt. For this reason, it is conceivable to improve the accuracy of detecting the attachment of foreign matter or dirt by taking color unevenness into consideration and changing the reference data between the center and the edge of the silicon nitride substrate, but this would make setting the inspection conditions cumbersome.

[0010] As described above, it is clear that color unevenness on the surface of a silicon nitride substrate makes it difficult to perform a highly accurate visual inspection. Therefore, in order to perform a highly accurate visual inspection, it is important to reduce the color unevenness that occurs on the surface of the silicon nitride substrate.

[0011] An object of the present invention is to reduce color unevenness that occurs on the surface of a silicon nitride substrate. [Means for solving the problem]

[0012] The silicon nitride substrate in one embodiment is a silicon nitride substrate formed by nitriding silicon contained in a sheet-like molded body, and has a first surface and a second surface opposite to the first surface. Here, the color difference between the center and the edge of at least one of the first surface and the second surface is ΔE * If ab, then ΔE * ab≦1.5.

[0013] In one embodiment, a method for manufacturing a silicon nitride substrate includes the steps of: (a) preparing a slurry containing silicon powder; (b) obtaining a green body from the slurry; and (c) sintering the green body in a furnace. Step (c) includes a nitriding step of heating the green body at a predetermined temperature, where the temperature difference between the heating temperature and the temperature of the green body in the furnace is 20°C or less. [Effects of the Invention]

[0014] According to one embodiment, color unevenness occurring on the surface of a silicon nitride substrate can be reduced. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a diagram showing a motor circuit including an inverter circuit and a three-phase induction motor. [Figure 2] FIG. 10 is a schematic diagram showing an example of a mounting layout for realizing an inverter circuit. [Figure 3] 1 is a flowchart showing a manufacturing process of a silicon nitride substrate in the related art. [Figure 4]3 is a flowchart showing a manufacturing process of a silicon nitride substrate according to an embodiment. [Figure 5] FIG. 2 is a diagram showing a state in which molded bodies are stacked. [Figure 6] 1 is a photograph showing the surface of a silicon nitride substrate on which color unevenness has occurred. [Figure 7] 1 is a graph showing the measured temperature of the compact, the measured temperature of the furnace, and the temperature difference between the compact and the furnace in a predetermined temperature range in Example 1. [Figure 8] 1 is a graph showing the measured temperature of the compact, the measured temperature of the furnace, and the temperature difference between the compact and the furnace in a predetermined temperature range in Example 2. [Figure 9] 1 is a graph showing the measured temperature of a compact, the measured temperature of a furnace, and the temperature difference between the compact and the furnace in a predetermined temperature range in a comparative example. [Figure 10] FIG. 1 is a schematic diagram showing a three-dimensional image of a "color space." [Figure 11] FIG. 1 is a diagram schematically illustrating a silicon nitride substrate. DETAILED DESCRIPTION OF THE INVENTION

[0016] In all the drawings for explaining the embodiments, the same components are generally designated by the same reference numerals, and repeated explanations thereof will be omitted. In addition, hatching may be used even in plan views to make the drawings easier to understand.

[0017] The silicon nitride substrate in this embodiment is an insulating substrate used in a power module, which is an electronic device that constitutes an inverter circuit that controls a motor provided in, for example, an electric vehicle, a hybrid electric vehicle, a railcar, or industrial equipment.

[0018] <Example of a three-phase inverter circuit configuration> The following description will be given taking a three-phase inverter circuit as an example.

[0019] The power module is used, for example, in the drive circuit of a three-phase induction motor used in an air conditioner, etc. Specifically, this drive circuit includes an inverter circuit, which has the function of converting DC power into AC power.

[0020] FIG. 1 is a circuit diagram showing the configuration of a motor circuit including an inverter circuit and a three-phase induction motor. In FIG. 1, the motor circuit includes a three-phase induction motor MT and an inverter circuit INV. The three-phase induction motor MT is configured to be driven by three different-phase voltages. Specifically, the three-phase induction motor MT uses three-phase AC currents called U, V, and W, which are 120 degrees out of phase, to generate a rotating magnetic field around the conductor rotor RT. In this case, the magnetic field rotates around the rotor RT. This means that the magnetic flux crossing the conductor rotor RT changes. As a result, electromagnetic induction occurs in the conductor rotor RT, and an induced current flows through the rotor RT. The flow of induced current in a rotating magnetic field means that a force is applied to the rotor RT according to Fleming's left-hand rule, and this force causes the rotor RT to rotate.

[0021] As can be seen, a three-phase induction motor MT can rotate the rotor RT by using three-phase AC. In other words, a three-phase induction motor MT requires three-phase AC. Therefore, the motor circuit uses an inverter circuit INV that creates AC from DC to supply three-phase AC to the three-phase induction motor.

[0022] An example of the configuration of this inverter circuit INV will be described below.

[0023] As shown in Fig. 1, for example, the inverter circuit INV is provided with a switching element Q1 and a diode FWD corresponding to each of the three phases. That is, in the inverter circuit INV, the components of the inverter circuit INV are realized by a configuration in which the switching element Q1 and the diode FWD are connected in anti-parallel, as shown in Fig. 1. For example, in Fig. 1, the upper and lower arms of the first leg LG1, the upper and lower arms of the second leg LG2, and the upper and lower arms of the third leg LG3 are each composed of components in which the switching element Q1 and the diode FWD are connected in anti-parallel.

[0024] In other words, in the inverter circuit INV, switching elements Q1 and diodes FWD are connected in anti-parallel between the positive potential terminal PT and each phase (U, V, and W) of the three-phase induction motor MT. Furthermore, switching elements Q1 and diodes FWD are also connected in anti-parallel between each phase of the three-phase induction motor MT and the negative potential terminal NT. That is, two switching elements Q1 and two diodes FWD are provided for each phase, for a total of six switching elements Q1 and six diodes FWD for three phases. A gate control circuit GCC is connected to the gate electrode of each switching element Q1, and the switching operation of the switching element Q1 is controlled by this gate control circuit GCC. In the inverter circuit INV configured in this way, the gate control circuit GCC controls the switching operation of the switching element Q1, converting DC power into three-phase AC power, which is then supplied to the three-phase induction motor MT.

[0025] <Types of switching elements> For example, the switching element Q1 used in the inverter circuit INV may be a power MOSFET or an IGBT (Insulated Gate Bipolar Transistor).

[0026] <Inverter circuit mounting layout example> FIG. 2 is a schematic diagram showing an example of a mounting layout for realizing an inverter circuit.

[0027] In FIG. 2, a power supply wiring VL, wiring WL1 to wiring WL3, and ground wiring GL are formed on a silicon nitride substrate 100, which is an insulating substrate. A power supply potential is supplied to the power supply wiring VL, while a ground potential (earth potential) is supplied to the ground wiring GL. Wire WL1 is connected to the U-phase of a three-phase induction motor, wire WL2 is connected to the V-phase of the three-phase induction motor, and wire WL3 is connected to the W-phase of the three-phase induction motor. The silicon nitride substrate 100 and the wiring pattern formed on this silicon nitride substrate 100 are sometimes collectively referred to as a "silicon nitride circuit substrate." Therefore, as shown in FIG. 2, semiconductor devices SA1 to SA6 are mounted on the "silicon nitride circuit substrate."

[0028] 2, a semiconductor device SA1 is connected between the power supply wiring VL and the wiring WL1, while a semiconductor device SA2 is connected between the wiring WL1 and the ground wiring GL. That is, the semiconductor devices SA1 and SA2 are connected in series between the power supply wiring VL and the ground wiring GL, and constitute the first leg LG1 of the inverter circuit INV shown in FIG. 1. That is, the semiconductor device SA1 constitutes the upper arm of the first leg LG1, and the semiconductor device SA2 constitutes the lower arm of the first leg LG1. Each of the semiconductor devices SA1 and SA2 has a semiconductor chip on which a power MOSFET functioning as a switching element Q1 is formed.

[0029] Similarly, a semiconductor device SA3 is connected between the power supply wiring VL and the wiring WL2, while a semiconductor device SA4 is connected between the wiring WL2 and the ground wiring GL. That is, the semiconductor devices SA3 and SA4 are connected in series between the power supply wiring VL and the ground wiring GL, and constitute the second leg LG2 of the inverter circuit INV shown in FIG. 1. That is, the semiconductor device SA3 constitutes the upper arm of the second leg LG2, and the semiconductor device SA4 constitutes the lower arm of the second leg LG2. Each of the semiconductor devices SA3 and SA4 has a semiconductor chip on which a power MOSFET functioning as a switching element Q1 is formed.

[0030] Furthermore, a semiconductor device SA5 is connected between the power supply wiring VL and the wiring WL3, while a semiconductor device SA6 is connected between the wiring WL3 and the ground wiring GL. That is, the semiconductor devices SA5 and SA6 are connected in series between the power supply wiring VL and the ground wiring GL, and constitute the third leg LG3 of the inverter circuit INV shown in FIG. 1. That is, the semiconductor device SA5 constitutes the upper arm of the third leg LG3, and the semiconductor device SA6 constitutes the lower arm of the third leg LG3. Each of the semiconductor devices SA5 and SA6 has a semiconductor chip on which a power MOSFET functioning as a switching element Q1 is formed.

[0031] In this manner, by arranging six semiconductor devices SA1 to SA6 on a silicon nitride substrate 100 on which power supply wiring VL1, wiring WL1 to wiring WL3, and ground wiring GL are formed (see Figure 2), a mounting layout compatible with an inverter circuit can be realized.

[0032] <Performance required for silicon nitride substrates> As described above, the semiconductor devices SA1 to SA6 are mounted on the silicon nitride substrate 100. At this time, heat is generated in the semiconductor devices SA1 to SA6, and it is necessary to prevent malfunctions and failures of the semiconductor devices SA1 to SA6 caused by the heat. Therefore, the silicon nitride substrate 100 on which the semiconductor devices SA1 to SA6 are mounted is required to have high heat dissipation characteristics. In other words, the silicon nitride substrate 100 is required to have high thermal conductivity. The silicon nitride substrate 100 is also required to be resistant to stresses caused by temperature changes.

[0033] In the following, attention will be paid in particular to the improvement of thermal conductivity.

[0034] <Description of Related Art> First, the related art for manufacturing silicon nitride substrates will be described.

[0035] The term "related art" as used herein refers to art that is not publicly known, but has problems that the inventors have discovered, and is a prerequisite for the present invention.

[0036] FIG. 3 is a flowchart showing the steps of manufacturing a silicon nitride substrate in the related art.

[0037] A rare earth element oxide and a magnesium compound are added to silicon nitride powder as sintering aids, and then a dispersion medium (organic solvent) and, if necessary, a dispersant are added, followed by pulverization in a ball mill to produce a slurry, which is a dispersion of the raw material powder (S101).

[0038] Next, an organic binder or the like is added to the prepared slurry, and the slurry is vacuum degassed as necessary, and the viscosity is adjusted to within a predetermined range to prepare a coating slurry. The prepared coating slurry is then formed into a sheet using a sheet forming machine, cut to a predetermined size, and dried to prepare a sheet-shaped molded product (S102).

[0039] The resulting sheet-like compact is then heated to densify and sinter it (S103). In this manner, a silicon nitride substrate can be manufactured.

[0040] The thermal conductivity of silicon nitride substrates manufactured using the above-mentioned related technology is approximately 90 W / (m·K). In response to this, the present inventors have investigated ways to improve the thermal conductivity of silicon nitride substrates and have found that the use of silicon nitride powder in the related technology makes it difficult to further improve the thermal conductivity. Specifically, the present inventors have found that silicon nitride powder contains a large amount of impurities, making it difficult to obtain high-purity silicon nitride powder, making it difficult to further improve the thermal conductivity of silicon nitride substrates manufactured using the related technology. Therefore, the use of silicon powder instead of silicon nitride powder has been considered. This is because silicon powder contains fewer impurities than silicon nitride powder, making it easier to obtain high-purity silicon powder. In other words, because silicon powder is purer than silicon nitride powder, it is believed that the decrease in thermal conductivity due to impurities can be suppressed. For this reason, in this embodiment, a method for manufacturing a silicon nitride substrate using silicon powder instead of silicon nitride powder is adopted.

[0041] <Method for manufacturing silicon nitride substrate according to the embodiment> The method for manufacturing the silicon nitride substrate employed in this embodiment will be described below.

[0042] FIG. 4 is a flowchart showing the steps of manufacturing a silicon nitride substrate according to this embodiment.

[0043] 1. Slurry preparation process In this embodiment, a slurry is prepared using a raw material powder obtained by adding a rare earth element oxide and a magnesium compound as sintering aids to silicon powder (S201).

[0044] <<Silicon>> In this embodiment, silicon powder of industrially available grade can be used. The silicon before pulverization has, for example, a median diameter D50 of 6 μm or more and a BET specific surface area of ​​3 m 2 / g or less, the oxygen content is 1.0 mass% or less, and the impurity carbon content in silicon is 0.15 mass% or less. Furthermore, it is preferable that the powder has a median diameter D50 of 7 μm or more and a BET specific surface area of ​​2.5 m 2 / g or less, the amount of oxygen is 0.5 mass % or less, and the amount of carbon impurity in silicon is 0.10 mass % or less.

[0045] In particular, the purity of the silicon powder is preferably 99% or more, and more preferably 99.5% or more, because the impurity oxygen contained in silicon is one of the factors that inhibit the thermal conductivity of the silicon nitride substrate obtained by reactive sintering, and therefore, from the viewpoint of improving the thermal conductivity of the silicon nitride substrate, it is desirable that the impurity oxygen content be as low as possible.

[0046] Furthermore, it is desirable to adjust the raw material powder so that the total amount of impurity oxygen contained in the silicon powder and oxygen from the magnesium compound is in the range of 0.1% by mass to 1.1% by mass, based on silicon converted to silicon nitride, by limiting the amount of oxygen from the magnesium compound. Also, the impurity carbon contained in silicon inhibits the growth of silicon nitride particles in the silicon nitride substrate obtained by reaction sintering, which tends to result in insufficient densification, and this is one of the factors that leads to a decrease in the thermal conductivity and insulating properties of the silicon nitride substrate, so it is desirable to keep the amount of impurity carbon as low as possible.

[0047] In this specification, the BET specific surface area (m 2 / g) is the value determined by the BET single-point method using a BET specific surface area meter (JIS R 1626:1996 "Method for measuring the specific surface area of ​​fine ceramic powders by the gas adsorption BET method"). The median diameter D50 (μm) is the particle size at which the cumulative frequency is 50% in the particle size distribution determined by laser diffraction / scattering.

[0048] <<Rare earth element oxides>> In this embodiment, the rare earth element oxides used are easily available and stable oxides such as yttrium (Y), ytterbium (Yb), gadolinium (Gd), erbium (Er), and lutetium (Lu).Specific examples of rare earth element oxides include yttrium oxide (YO), ytterbium oxide (YbO), gadolinium oxide (GdO), erbium oxide (ErO), and lutetium oxide (LuO).

[0049] The content of rare earth element oxide is, for example, 0.5 mol% or more but less than 2 mol% based on the total of silicon (calculated as silicon nitride), rare earth element oxide (calculated as trivalent oxide), and magnesium compound (calculated as MgO). If the content of rare earth element oxide is less than 0.5 mol%, the effect as a sintering aid is insufficient, and the density is not sufficiently high. On the other hand, if the content of rare earth element oxide is 2.0 mol% or more, the thermal conductivity of the sintered body decreases due to an increase in the low thermal conductivity grain boundary phase, and the amount of expensive rare earth element oxide used increases. In particular, the content of rare earth element oxide is preferably 0.6 mol% or more but less than 2 mol%, and more preferably 1 mol% or more but less than 1.8 mol%.

[0050] In this specification, the sum of the number of moles of silicon nitride (Si3N4) obtained when all silicon is nitrided, the number of moles of rare earth element oxides converted to trivalent oxides (RE2O3: RE is a rare earth element), and the number of moles of magnesium compounds converted to MgO may be simply referred to as the "total of silicon (converted to silicon nitride), rare earth element oxides (converted to trivalent oxides), and magnesium compounds (converted to MgO)."

[0051] <<Magnesium compounds>> As the magnesium compound, one or more magnesium compounds containing "Si", "N" or "O" can be used. In particular, it is desirable to use magnesium oxide (MgO), magnesium silicon nitride (MgSiN2), magnesium silicide (Mg2Si), magnesium nitride (Mg3N2), etc.

[0052] Here, the magnesium compound is selected so that 87% by mass or more of the total amount of magnesium silicon nitride is magnesium silicon nitride. By using 87% by mass or more of magnesium silicon nitride, the oxygen concentration in the resulting silicon nitride substrate can be reduced. If the magnesium silicon nitride content in the magnesium compound is less than 87% by mass, the amount of oxygen in the silicon nitride particles after sintering will be high, resulting in a low thermal conductivity of the sintered body (silicon nitride substrate). Therefore, from the perspective of improving the thermal conductivity of the silicon nitride substrate, it is desirable that the magnesium silicon nitride content in the magnesium compound is as high as possible, for example, 90% by mass or more.

[0053] The content of magnesium compounds in silicon nitride substrates (MgO equivalent) is, for example, 8 mol% or more and less than 15 mol% based on the total of silicon (converted to silicon nitride), rare earth element oxides (converted to trivalent oxides), and magnesium compounds (converted to MgO). If the content of magnesium compounds is less than 8 mol%, the effect as a sintering aid is insufficient, and the density is not sufficiently high. On the other hand, if the content of magnesium compounds is 15 mol% or more, the thermal conductivity of the sintered body decreases due to an increase in the grain boundary phase with low thermal conductivity. In particular, the content of magnesium compounds is preferably 8 mol% or more and less than 14 mol%, and even more preferably 9 mol% or more and 11 mol% or less.

[0054] A slurry is prepared using the silicon, rare earth element oxide, and magnesium compound described above (S201). Specifically, a rare earth element oxide and a magnesium compound are added as sintering aids to silicon powder in a predetermined ratio, followed by the addition of a dispersion medium (organic solvent) and, if necessary, a dispersant, and pulverization in a ball mill to prepare a slurry, which is a dispersion of the raw material powder. Examples of the dispersion medium that can be used include ethanol, n-butanol, and toluene. Examples of the dispersant that can be used include sorbitan ester-type dispersants and polyoxyalkylene-type dispersants.

[0055] The amount of the dispersion medium used is preferably, for example, 40% by mass or more and 70% by mass or less relative to the total amount of the raw material powder, and the amount of the dispersant used is preferably, for example, 0.3% by mass or more and 2% by mass or less relative to the total amount of the raw material powder. After dispersion, the dispersion medium may be removed or replaced with another dispersion medium, as necessary.

[0056] 2. Compaction manufacturing process To the slurry obtained as described above, for example, a dispersion medium, an organic binder, a dispersant, etc. are added, and the mixture is vacuum degassed as necessary, and then the viscosity is adjusted to a predetermined range, thereby preparing a coating slurry.

[0057] Next, the prepared coating slurry is formed into a sheet using a sheet forming machine, cut into a predetermined size, and then dried to prepare a sheet-shaped product (S202).

[0058] The organic binder used to prepare the coating slurry is not particularly limited, but examples thereof include PVB resin (polyvinyl butyral resin), ethyl cellulose resin, acrylic resin, etc. The amounts of the dispersion medium, organic binder, dispersant, etc. added are desirably adjusted appropriately depending on the coating conditions.

[0059] The method for forming the coating slurry into a sheet is not particularly limited, but sheet forming methods such as the doctor blade method and extrusion molding method can be used.

[0060] The thickness of the sheet-like molded body formed in the molded body preparation step is, for example, 0.15 mm to 0.8 mm. The prepared sheet-like molded body is cut to a predetermined size using, for example, a punching machine, as necessary.

[0061] 3. Sintering process The produced sheet-like compact is heated to nitride the silicon contained in the compact, followed by a sintering step (S203) for densification. This sintering step includes a degreasing step for removing the organic binder contained in the compact, a nitriding step (S204) for reacting the silicon contained in the compact with nitrogen to form silicon nitride, and a densification sintering step performed after the nitriding step. That is, the silicon nitride substrate in this embodiment is a silicon nitride substrate formed by nitriding the silicon contained in the sheet-like compact. The thickness of the silicon nitride substrate is, for example, 0.15 mm or more and 0.8 mm or less. These steps may be performed sequentially in separate furnaces, or may be performed continuously in the same furnace.

[0062] In the sintering process, as shown in FIG. 5, for example, a plurality of the prepared sheet-like compacts 100A are stacked on a boron nitride (BN) setter 200 with a separator (not shown) sandwiched between them, and a weight 300 is placed on the top layer of the stacked compacts 100A, which are then placed in an electric furnace. In this state, an organic binder removal process (degreasing process) is performed, followed by decarbonization at a temperature of 900°C to 1300°C in a nitriding device, and then nitriding by raising the temperature to a predetermined temperature in a nitrogen atmosphere. Thereafter, a densification sintering process is performed in a sintering device. This sintering process is performed, for example, while applying a load of 10 Pa to 1000 Pa to the compacts 100A using the weight 300.

[0063] The separating material may be, for example, a boron nitride (BN) powder layer having a thickness of approximately 3 μm to 20 μm. The boron nitride powder layer functions to facilitate separation of the silicon nitride substrate (sintered body) after sintering. The boron nitride powder layer is formed, for example, by applying boron nitride powder in a slurry state to one side of each sheet-like compact 100A by spraying, brush coating, or screen printing. The boron nitride powder preferably has a purity of 95% or higher and an average particle size (D50) of 1 μm to 20 μm.

[0064] In this manner, a silicon nitride substrate can be manufactured. In particular, according to the method for manufacturing a silicon nitride substrate in this embodiment, since high-purity silicon powder is used, it is possible to manufacture a silicon nitride substrate having a thermal conductivity of 110 W / (m·K) or more. However, the inventors' investigations have revealed that there is room for improvement in the above-described method for manufacturing a silicon nitride substrate, and this point will be described below.

[0065] <Room for improvement> The above-mentioned method for manufacturing a silicon nitride substrate requires a nitriding treatment because silicon powder is used instead of silicon nitride powder. In this regard, the present inventors have newly discovered that color unevenness occurs on the surface of the manufactured silicon nitride substrate depending on the heating conditions in the nitriding treatment.

[0066] Here, when referring to the "surface" of a silicon nitride substrate in this specification, it is intended to include not only the "surface" as the first surface, but also the "rear surface" as the second surface opposite the first surface. For example, when it is said that color unevenness occurs on the "surface" of a silicon nitride substrate, this will be interpreted as color unevenness occurring on the "surface or rear surface" of the silicon nitride substrate, as appropriate. In other words, when referring to the "surface" in this specification, it is intended to mean "at least one surface of the front surface or rear surface" to avoid cumbersome expression.

[0067] Furthermore, "color unevenness" as used herein means, for example, that the color tone of the center of a rectangular silicon nitride substrate is different from the color tone of the edge. Specifically, Figure 6 is a photograph showing the surface of a silicon nitride substrate on which color unevenness has occurred. In Figure 6, the center of the silicon nitride substrate is whitish, while the edge of the silicon nitride substrate is blackish. Comparing the color tone of the center and the edge reveals that color unevenness has occurred in the silicon nitride substrate.

[0068] For example, foreign matter or dirt adhering to the surface of a silicon nitride substrate can cause poor contact with the brazing filler metal or poor insulation within the silicon nitride substrate itself, so it is necessary to remove the foreign matter or dirt. Therefore, visual inspection is performed to detect the adhesion of foreign matter or dirt to the silicon nitride substrate. However, if color unevenness such as that shown in Figure 6 occurs, there is a risk that the color unevenness will be erroneously detected as the adhesion of foreign matter or dirt. Specifically, considering that foreign matter and dirt are often recognized as black areas (black dots), for example, the blackish edge of the silicon nitride substrate shown in Figure 6 may be erroneously detected as an area where foreign matter or dirt is attached. Therefore, in order to prevent false detection in visual inspection, it is clear that a method for suppressing color unevenness on the surface of a silicon nitride substrate is desired.

[0069] <<Mechanism of color unevenness (speculation)>> Therefore, in order to suppress color unevenness occurring on the surface of a silicon nitride substrate, the present inventors first conducted extensive research into the mechanism by which color unevenness occurs on the surface of a silicon nitride substrate. As a result, the present inventors have deduced the mechanism by which color unevenness occurs, which is shown below. This deduced mechanism by which color unevenness occurs will now be described.

[0070] The nitriding process is performed in a nitrogen atmosphere by, for example, placing multiple sheet-like compacts 100A on a setter 200 and placing a weight 300 on the stacked compacts 100A, as shown in FIG. 5 . In this case, for example, focusing on a first compact sandwiched between the upper and lower compacts 100A among the stacked compacts 100A, heat tends to accumulate in the center of the first compact. This results in a temperature distribution in the first compact where the temperature is high in the center and low in the edge. The nitriding reaction proceeds in the high-temperature center, while the nitriding reaction proceeds less smoothly in the edge. Considering that the nitriding reaction is an exothermic reaction, the temperature in the center where the nitriding reaction proceeds rises rapidly due to positive feedback of the heat generated. If this temperature rise is large, the temperature in the center exceeds the melting point of silicon, causing the silicon to melt, resulting in "thermal runaway." Furthermore, the temperature difference between the center and edge increases, resulting in insufficient nitriding at the edge. If sintering is carried out in such a state where nitriding is insufficient, it is presumed that the silicon that has not been completely nitrided will remain at the edge, resulting in color unevenness. In other words, the inventors presume that color unevenness occurs due to the remaining unnitrided silicon.

[0071] Based on this speculated mechanism, it is believed that the color unevenness that occurs on the surface of a silicon nitride substrate is caused by the temperature rise step in the nitriding process. In other words, if the temperature rise step can be carried out while realizing a temperature distribution that reduces the temperature difference between the center and edge, it is believed that color unevenness can be suppressed while suppressing "thermal runaway." Therefore, in this embodiment, a special feature is incorporated into the temperature rise step in the nitriding process. The technical concept of this special feature in this embodiment will be explained below.

[0072] <Basic Concept of the Embodiment> The basic idea of ​​this embodiment is to heat the compact while maintaining a temperature distribution with a small temperature difference between the center and the edge by suppressing a sudden temperature rise during the nitriding treatment. In other words, the basic idea is to suppress a sudden temperature rise to an extent that allows enough time to sufficiently reduce the temperature difference between the center and the edge.

[0073] According to this basic concept, the temperature can be gradually increased while reducing the temperature difference between the center and the edge, thereby preventing "thermal runaway" caused by a rapid nitriding reaction in the center and eliminating insufficient nitriding at the edge.

[0074] In particular, the basic concept is to ensure that the average temperature rise per unit time (hereinafter referred to as the gradient of the heating temperature) is below a predetermined value in the temperature rise step in the nitriding treatment from 1270°C to 1340°C. For example, the average gradient of the heating temperature is preferably 3.1°C / h or less. Furthermore, the maximum heating temperature is preferably 1390°C or higher and 1500°C or lower.

[0075] According to the basic concept, this allows the temperature to be raised slowly enough to ensure the time required to sufficiently reduce the temperature difference between the center and edge. As a result, according to the basic concept, insufficient nitriding at the edge of the molded body is eliminated, and color unevenness occurring on the surface of the final silicon nitride substrate can be reduced. Therefore, according to the basic concept, false detections in visual inspection due to color unevenness can be reduced, resulting in the significant effect of improving the accuracy of detecting foreign matter or dirt in visual inspection.

[0076] <Specific aspects> A specific embodiment that embodies this basic concept will be described below.

[0077] Silicon nitride substrates of Example 1, Example 2, and Comparative Example were manufactured by the method described in the above "<Method for manufacturing silicon nitride substrate in embodiment>". Each silicon nitride substrate had a first surface and a second surface, and its planar shape was rectangular. The length of each side of each silicon nitride substrate was 200 mm for the long side and 140 mm for the short side. The thickness of each silicon nitride substrate was 0.32 mm.

[0078] When each silicon nitride substrate was manufactured, the molar ratio of the rare earth oxide to the total of silicon, rare earth oxide, and magnesium compound was 1.2 mol %, and the molar ratio of the magnesium compound was 9.8 mol %.

[0079] When producing the silicon nitride substrate of Example 1, in the nitriding step S204, the heating temperature was increased stepwise over the heating time until it reached the maximum heating temperature. The maximum heating temperature was 1400°C. The average gradient of the heating temperature in the temperature range from 1270°C to 1340°C was 2.99°C / h. Figure 7 shows the measured temperature of the compact, the measured temperature of the furnace, and the temperature difference between the compact and the furnace within a given temperature range. The horizontal axis in Figure 7 represents the elapsed time from the reference time when the measured furnace temperature, which is the heating temperature, reached approximately 1300°C. When the measured furnace temperature was approximately 1300°C, the temperature difference between the measured furnace temperature and the temperature of the compact in the furnace was 20°C or less. In Example 1, the compact did not experience a rapid temperature rise during the nitriding process, and "thermal runaway" did not occur. The thermal conductivity of the silicon nitride substrate of Example 1 was 129 W / (m·K).

[0080] When producing the silicon nitride substrate of Example 2, in the nitriding step S204, the heating temperature was increased stepwise over the heating time until it reached the maximum heating temperature. The maximum heating temperature was 1400°C. The average gradient of the heating temperature in the temperature range from 1270°C to 1340°C was 3.02°C / h. Figure 8 shows the measured temperature of the compact, the measured temperature of the furnace, and the temperature difference between the compact and the furnace within a given temperature range. The horizontal axis in Figure 8 represents the elapsed time from the reference time when the measured furnace temperature, which is the heating temperature, reached approximately 1300°C. When the measured furnace temperature was approximately 1300°C, the temperature difference between the measured furnace temperature and the temperature of the compact in the furnace was less than 20°C. In Example 2, the compact did not experience a rapid temperature rise during the nitriding process, and "thermal runaway" did not occur. The thermal conductivity of the silicon nitride substrate of Example 2 was 126 W / (m·K).

[0081] When manufacturing the silicon nitride substrate of the comparative example, in the nitriding step S204, the heating temperature was increased stepwise over the heating time until it reached the maximum heating temperature. The maximum heating temperature was 1400°C. The average gradient of the heating temperature in the temperature range from 1270°C to 1340°C was 4.67°C / h. Figure 9 shows the measured temperature of the compact, the measured temperature of the furnace, and the temperature difference between the compact and the furnace within a given temperature range. The horizontal axis in Figure 9 represents the elapsed time from the reference time when the measured furnace temperature, which is the heating temperature, reached approximately 1300°C. When the measured furnace temperature was approximately 1300°C, the temperature difference between the measured furnace temperature and the temperature of the compact in the furnace exceeded 20°C, reaching a maximum of 44.9°C. In other words, in the comparative example, the compact temperature rose rapidly during the nitriding process, resulting in "thermal runaway." The thermal conductivity of the silicon nitride substrate of the comparative example was 120 W / (m·K).

[0082] Below, the verification results that show that color unevenness can be reduced by using a silicon nitride substrate manufactured under the heating conditions shown in Example 1 or Example 2 will be described.

[0083] <Verification results> <<Quantitative evaluation method for color unevenness>> First, a quantitative evaluation method for color unevenness will be described.

[0084] In this embodiment, a "color space" is used to evaluate color unevenness.

[0085] FIG. 10 is a schematic diagram showing a three-dimensional image of a "color space."

[0086] The constituent parameters of the "color space" shown in FIG. 10 are as follows:

[0087] <<<Lightness L * >>> "Lightness L * " is an index that indicates the brightness of a color tone. * " is 0≦L * The value ranges from 100 to 100. * As the "value" increases, the color tone becomes brighter and whiter. * As the value of " decreases, the color tone becomes darker and more blackish.

[0088] <<<Chromaticness index a * >>> "Chromaticness index a * " is an index that indicates the degree of red to green in a color tone. "Chromaticness index a * " is -60≦a * The value ranges from ≦+60. * As the value of " increases in the positive direction, the color tone becomes redder. * As the value of " increases in the negative direction, the color tone becomes green. * The smaller the absolute value of ", the duller the color tone.

[0089] <<< Chromaticness index b * >>> "Chromaticness index b * " is an index that indicates the degree of yellow to blue in a color tone. "Chromaticness index b * " is -60≦b* The value ranges from ≦+60. Chromaticness index b * As the value of " increases in the positive direction, the color tone becomes yellow. * When the value of " increases in the negative direction, the color tone becomes blue. * The smaller the absolute value of ", the duller the color tone.

[0090] The above-mentioned "Lightness L * ", "Chromaticness index a * " and "chromaticity index b * " is measured in accordance with "JIS Z 8722:2000." For example, in this embodiment, the measurement is performed using the SCI (including reflected light) method, which represents the color of a material. At this time, a "Konica Minolta CR-400" is used as the measuring device, and the measurement is performed with the settings of "visual field: CIE 2° visual field color matching function approximation" and "illuminant: C." The silicon nitride substrate is placed on top of 10 sheets of PPC paper, and the surface of the silicon nitride substrate opposite to the side in contact with the PPC paper is measured using the measuring device.

[0091] <<<Saturation C * >>> "Saturation C * " is calculated based on the following formula 1: "chromaticness index a * " and "chromaticity index b * " is calculated based on the

[0092] C * =√{(a * ) 2 +(b * ) 2} Formula 1

[0093] <<<Color difference ΔE * ab>>> "Color difference ΔE * "ab" is an index that shows the difference between the color and the reference color, and "brightness L * ", "Chromaticness index a * " and "chromaticity index b* " is calculated based on the

[0094] 11 is a diagram showing a silicon nitride substrate 100, in which "Position 1" indicates the region corresponding to the center, while "Position 2," "Position 3," "Position 4," and "Position 5" indicate the regions corresponding to the four edges. In this embodiment, the color at "Position 1" is used as the reference color, and the "color difference ΔE" at "Positions 2" to "Position 5" is calculated. * ab" is calculated.

[0095] Specifically, "position 1" to "position 5" are defined as the positions shown below. "Position 1": The position where the diagonal lines intersect on the surface "Position 2": On the surface, a position 15 mm inward from corner CNR1 toward "Position 1" "Position 3": On the surface, a position 15 mm inward from corner CNR2 toward "Position 1" "Position 4": On the surface, a position 15 mm inward from corner CNR3 toward "Position 1". "Position 5": On the surface, a position 15 mm inward from corner CNR4 toward "Position 1"

[0096] The measurement range was a 20 mm x 20 mm area centered on each of "Position 1" to "Position 5."

[0097] <<Test results for color unevenness>> In the following, the color unevenness of samples #1 to #4 was verified using the configuration parameters of the "color space" described above, and the verification results will be described.

[0098] "Sample #1" is a silicon nitride substrate manufactured by nitriding under the heating conditions of Example 1, and "Sample #2" is a silicon nitride substrate manufactured by nitriding under the heating conditions of Example 2. Furthermore, "Sample #3" and "Sample #4" are silicon nitride substrates manufactured by nitriding under the heating conditions of the comparative examples.

[0099] [Table 1]

[0100] Table 1 shows the results of the color unevenness verification for "Sample #1" to "Sample #4." As shown in Table 1, all of "Sample #1" to "Sample #4" achieved a thermal conductivity of 130 W / (m K).

[0101] However, in "Sample #3" and "Sample #4" which are comparative examples, "color difference ΔE * There are positions where "ab" is larger than 1.5. This means that the silicon nitride substrate manufactured under the heating conditions of the nitriding treatment in the comparative example can obtain high thermal conductivity, but color unevenness becomes apparent.

[0102] In contrast, in "Sample #1" representing Example 1, "color difference ΔE * "ab" is 1.5 or less at all positions from "position 2" to "position 5".

[0103] In addition, in "Sample #2" showing Example 2, "color difference ΔE * "ab" is 0.8 or less at all positions from "position 2" to "position 5".

[0104] This means that high thermal conductivity and reduced color unevenness can be obtained with a silicon nitride substrate manufactured under the heating conditions for the nitriding treatment in Example 1 or Example 2. Therefore, according to Examples 1 and 2, not only can a silicon nitride substrate with excellent heat dissipation characteristics be obtained, but false detections in visual inspections due to color unevenness can also be reduced.

[0105] Furthermore, in Examples 1 and 2, the "brightness L" of the center (position 1) and the edge (positions 2 to 5) * " is 70 or more, and "saturation C *" is 10 or more. This provides a clear contrast with foreign matter or dirt, resulting in a significant effect of improving the accuracy of detecting foreign matter or dirt in visual inspection.

[0106] Here, the term "edge" as used in this specification is described as a concept that includes all of the above-mentioned "position 2" to "position 5." In other words, the term "edge" as used in this specification includes all of "position 2" to "position 5." Furthermore, the term "center" is described as a concept that includes "position 1." Therefore, the term "color difference between the center and edge" is defined as ΔE * If ab, then ΔE * The statement "ab≦1.5" means that the color difference between "Position 1" and "Position 2" is 1.5 or less, the color difference between "Position 1" and "Position 3" is 1.5 or less, the color difference between "Position 1" and "Position 4" is 1.5 or less, and the color difference between "Position 1" and "Position 5" is 1.5 or less.

[0107] Similarly, the statement "the brightness of each of the 'center' and 'edge' is 70 or greater" means that the brightness of each of 'positions 1' to 'positions 5' is 70 or greater.

[0108] Furthermore, the statement "the saturation of each of the 'center' and 'edge' is 10 or greater" means that the saturation of each of the 'positions 1' to 'positions 5' is 10 or greater.

[0109] <Summary> In this embodiment, the temperature increasing step in the nitriding treatment is devised so that the average gradient of the heating temperature is 3.1°C / h or less in the range from 1270°C to 1340°C.

[0110] As a result, according to this embodiment, the "color difference ΔE *It is possible to obtain a silicon nitride substrate in which "ab" is 1.5 or less, and even 0.8 or less. Therefore, the technical idea of ​​this embodiment is excellent in that it can reduce color unevenness. In other words, it can be said that the technical idea of ​​this embodiment has great technical significance in that it can improve the accuracy of detecting foreign matter or dirt in visual inspection by reducing color unevenness.

[0111] The technical idea of ​​this embodiment is effective when applied to a large-sized silicon nitride substrate. Specifically, the technical idea of ​​this embodiment is effective when applied to a configuration in which six semiconductor devices constituting a three-phase inverter circuit are mounted on a single silicon nitride substrate, as shown in Fig. 2. This is because when a large-sized silicon nitride substrate is used, the potential for color unevenness to occur is thought to be greater.

[0112] In this regard, application of the technical idea of ​​this embodiment to the manufacture of large silicon nitride substrates, which have a high potential for color unevenness, is considered to be effective in reducing the occurrence of color unevenness. In particular, the technical idea of ​​this embodiment is effective when applied to the manufacture of rectangular silicon nitride substrates with each side having a length of 100 mm or more.

[0113] The invention made by the inventor has been specifically described above based on the embodiments thereof, but it goes without saying that the present invention is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit of the invention. [Explanation of symbols]

[0114] 100 Silicon nitride substrate 100A molding 200 Setta 300 weights INV Inverter circuit FWD Diode GCC Gate control circuit GL ground wiring LG1 1st Leg LG2 2nd Leg LG3 Third Leg MT 3-phase induction motor NT negative potential terminal PT Positive potential terminal Q1 switching element RT Rotor SA Semiconductor Devices SA1 Semiconductor Device SA2 Semiconductor Device SA3 Semiconductor Device SA4 Semiconductor Device SA5 Semiconductor Devices SA6 Semiconductor Devices VL power wiring WL1 wiring WL2 wiring WL3 wiring

Claims

1. A silicon nitride substrate having a first surface and a second surface opposite the first surface, The planar shape is a rectangle with each side being 100 mm or more, A silicon nitride substrate, wherein, on at least one of the first surface and the second surface, when the color difference between a center portion and an edge portion located 15 mm inward from each corner toward the center portion is ΔE*ab, 0<ΔE*ab≦1.5 is satisfied for all of the edge portions.

2. The silicon nitride substrate according to claim 1 , A silicon nitride substrate, wherein ΔE*ab≦0.

8.

3. The silicon nitride substrate according to claim 1 or 2, A silicon nitride substrate, wherein the brightness of each of the central portion and the edge portion is 70 or more.

4. The silicon nitride substrate according to any one of claims 1 to 3, A silicon nitride substrate, wherein the saturation of each of the central portion and the edge portion is 10 or more.

5. The silicon nitride substrate according to any one of claims 1 to 4, The silicon nitride substrate has a thickness of 0.15 mm or more and 0.8 mm or less.

6. (a) preparing a slurry containing silicon powder; (b) obtaining a compact from the slurry; (c) sintering the compact in a furnace; A method for manufacturing a silicon nitride substrate, comprising: The step (c) is a nitriding process in which the compact is heated at a predetermined heating temperature. Including the process, a temperature difference between the heating temperature of the molded body in the furnace and the heating temperature of the molded body in the nitriding step being 20°C or less when the heating temperature is 1300°C;

7. The method for manufacturing a silicon nitride substrate according to claim 6, The nitriding step is a method for producing a silicon nitride substrate, in which a plurality of the compacts are stacked and heated.

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