Silicon nitride substrate and method of manufacturing the same

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 technology thereof, and more particularly to a silicon nitride substrate with a thermal conductivity of 110 W / (m·K) or higher, and a technology that is effective when applied to the manufacturing technology thereof. [Background technology]

[0002] Japanese Patent Publication No. 2003-267786 (Patent Document 1) describes a technique for providing a silicon nitride-based ceramic sintered body that is blacker in color compared to conventional materials, has less color unevenness, and possesses sufficient strength.

[0003] Japanese Patent Publication No. 2005-214659 (Patent Document 2) describes a foreign object inspection device that can distinguish foreign objects of a color that has low contrast with the color of a wiring board.

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

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

[0006] [Patent Document 1] Japanese Patent Publication No. 2003-267786 [Patent Document 2] Japanese Patent Publication 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 Laid-Open No. 9-227240 [Summary of the Invention] [Problems to be Solved by the Invention]

[0007] For example, when foreign matter or dirt adheres to the surface of a silicon nitride substrate, it may cause poor contact with the "brazing material" or poor insulation of the silicon nitride substrate itself. Therefore, it is necessary to remove foreign matter and dirt. Thus, visual inspection is performed to detect the adhesion of foreign matter and dirt to the silicon nitride substrate. As a visual inspection method, for example, after imaging the surface of the silicon nitride substrate with an imaging device typified by a CCD camera, there is a method of detecting foreign matter and dirt by comparing the data of the captured image with pre-registered reference data.

[0008] However, in the above-described visual inspection method, on the surface of the silicon nitride substrate, for example, if there is color unevenness at the center and the edge, there is a risk of false detection of the color unevenness as the adhesion of foreign matter and dirt.

[0009] Regarding this point, in order not to falsely detect color unevenness as the adhesion of foreign matter and dirt, it is conceivable to relax the criteria for detecting the adhesion of foreign matter and dirt. However, in this case, there is a risk of causing detection omission of foreign matter and dirt. For this reason, it is also conceivable to improve the detection accuracy of the adhesion of foreign matter and dirt by changing the reference data at the center and the edge of the silicon nitride substrate in consideration of color unevenness. However, the setting of inspection conditions becomes complicated.

[0010] As described above, color unevenness on the surface of a silicon nitride substrate makes it difficult to perform highly accurate visual inspections. Therefore, reducing color unevenness on the surface of a silicon nitride substrate is important for performing accurate visual inspections.

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

[0012] In one embodiment, the silicon nitride substrate is a silicon nitride substrate obtained by nitriding silicon contained in a sheet-shaped molded body, and has a first surface and a second surface opposite to the first surface. Here, in at least one of the first surface and the second surface, the color difference between the central part and the edge is ΔE * Let ab be the basis for ΔE. * ab ≤ 1.5.

[0013] A method for manufacturing a silicon nitride substrate in one embodiment comprises (a) a step of preparing a slurry containing silicon powder, (b) a step of obtaining a molded body from the slurry, and (c) a step of sintering the molded body in a furnace. Here, step (c) includes a nitriding step in which the molded body is nitrided by heating it at a predetermined heating temperature, wherein the temperature difference between the heating temperature and the temperature of the molded 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 drawing]

[0015] [Figure 1] This diagram shows a motor circuit including an inverter circuit and a three-phase induction motor. [Figure 2] This is a schematic diagram showing an example implementation layout for an inverter circuit. [Figure 3] This is a flowchart showing the manufacturing process of silicon nitride substrates in related technologies. [Figure 4]This is a flowchart showing the manufacturing process of a silicon nitride substrate in an embodiment. [Figure 5] This figure shows the molded parts stacked in layers. [Figure 6] This is a photograph showing the surface of a silicon nitride substrate where color unevenness has occurred. [Figure 7] This graph shows the measured temperature of the molded body, the measured temperature of the furnace, and the temperature difference between the molded body and the furnace in a predetermined temperature range for Example 1. [Figure 8] This graph shows the measured temperature of the molded body, the measured temperature of the furnace, and the temperature difference between the molded body and the furnace in a predetermined temperature range for Example 2. [Figure 9] This graph shows the measured temperature of the molded body, the measured temperature of the furnace, and the temperature difference between the molded body and the furnace in a specified temperature range for the comparative example. [Figure 10] This is a schematic diagram illustrating the three-dimensional image of a "color space." [Figure 11] This is a schematic diagram showing a silicon nitride substrate. [Modes for carrying out the invention]

[0016] In all the drawings illustrating the embodiments, the same reference numeral is used for identical components, and repeated explanations of them are omitted. Hatching may be used even in plan views to improve clarity.

[0017] The silicon nitride substrate in this embodiment is an insulating substrate used in power modules. A power module is an electronic device that constitutes an inverter circuit for controlling motors in electric vehicles, hybrid electric vehicles, railway vehicles, or industrial equipment.

[0018] <Example of a 3-phase inverter circuit configuration> The following explanation will use a three-phase inverter circuit as an example.

[0019] Power modules are used, for example, in the drive circuits of three-phase induction motors used in air conditioners. Specifically, these drive circuits include inverter circuits, which are circuits that have the function of converting DC power into AC power.

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

[0021] Thus, in a three-phase induction motor MT, the rotor RT can be rotated by utilizing three-phase alternating current. In other words, a three-phase induction motor MT requires three-phase alternating current. Therefore, in the motor circuit, an inverter circuit INV that converts DC to AC is used to supply three-phase AC to the three-phase induction motor.

[0022] The following describes an example configuration of this inverter circuit INV.

[0023] As shown in Figure 1, for example, the inverter circuit INV is provided with switching elements Q1 and diodes FWD corresponding to 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 elements Q1 and diodes FWD are connected in antiparallel, as shown in Figure 1. For example, in Figure 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 elements Q1 and diodes FWD are connected in antiparallel.

[0024] In other words, in the inverter circuit INV, switching elements Q1 and diodes FWD are connected in antiparallel between the positive potential terminal PT and each phase (U phase, V phase, W phase) of the three-phase induction motor MT, and also 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 single phase, and six switching elements Q1 and six diodes FWD are provided for all 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 switching operation of the switching element Q1 is controlled by the gate control circuit GCC, thereby converting DC power into three-phase AC power, and supplying this three-phase AC power to the three-phase induction motor MT.

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

[0026] <Example of an inverter circuit implementation layout> Figure 2 is a schematic diagram showing an example of an implementation layout for realizing an inverter circuit.

[0027] In Figure 2, the silicon nitride substrate 100, which is an insulating substrate, has power supply wiring VL, wirings WL1 to WL3, and ground wiring GL formed on it. Power supply potential is supplied to power supply wiring VL, while ground potential (earth potential) is supplied to ground wiring GL. Wiring WL1 is connected to the U phase of the 3-phase induction motor, wiring WL2 is connected to the V phase of the 3-phase induction motor, and wiring WL3 is connected to the W phase of the 3-phase induction motor. The silicon nitride substrate 100 and the wiring pattern formed on this silicon nitride substrate 100 are sometimes collectively called a "silicon nitride circuit board." Therefore, as shown in Figure 2, semiconductor devices SA1 to SA6 are mounted on the "silicon nitride circuit board."

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

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

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

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

[0032] <Performance requirements for silicon nitride substrates> As described above, semiconductor devices SA1 to SA6 are mounted on the silicon nitride substrate 100. Since heat is generated in semiconductor devices SA1 to SA6, it is necessary to prevent malfunctions or failures of these devices due to heat. Therefore, the silicon nitride substrate 100 on which 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. Furthermore, the silicon nitride substrate 100 is also required to have resistance to stress generated due to temperature changes.

[0033] In the following, we will focus particularly on improving thermal conductivity.

[0034] <Explanation of related technologies> First, we will explain the related technologies for manufacturing silicon nitride substrates.

[0035] In this specification, "related technology" refers to technology that is not publicly known but has a problem identified by the present inventor, and is a technology that forms the basis of the present invention.

[0036] Figure 3 is a flowchart showing the manufacturing process of silicon nitride substrates in related technologies.

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

[0038] Next, an organic binder is added to the prepared slurry, and vacuum degassing is performed as needed, and the viscosity is adjusted to a predetermined range to produce a coating slurry. Then, the prepared coating slurry is formed into a sheet using a sheet forming machine, cut to a predetermined size, and dried to produce a sheet-shaped molded body (S102).

[0039] Next, the resulting sheet-like molded body is heated to densify and sinter it (S103). In this way, a silicon nitride substrate can be manufactured.

[0040] The thermal conductivity of silicon nitride substrates manufactured using the aforementioned related technologies is approximately 90 W / (m·K). In this regard, the inventors have investigated ways to improve the thermal conductivity of silicon nitride substrates and have found that it is difficult to further improve the thermal conductivity using the related technologies due to the use of silicon nitride powder. Specifically, the inventors have found that it is difficult to further improve the thermal conductivity of silicon nitride substrates manufactured using the related technologies because silicon nitride powder contains many impurities and it is difficult to obtain high-purity silicon nitride powder. Therefore, the use of silicon powder instead of silicon nitride powder is being 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, since silicon powder is of higher purity than silicon nitride powder, it is thought that the decrease in thermal conductivity caused by impurities can be suppressed. For this reason, in this embodiment, a method for manufacturing silicon nitride substrates using silicon powder instead of silicon nitride powder is employed.

[0041] <Method for manufacturing a silicon nitride substrate in an embodiment> The following describes the method for manufacturing the silicon nitride substrate used in this embodiment.

[0042] Figure 4 is a flowchart showing the manufacturing process of the silicon nitride substrate in this embodiment.

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

[0044] <<Silicon>> In this embodiment, industrially available grade silicon powder can be used. For example, the silicon before grinding has a median diameter D50 of 6 μm or more and a BET specific surface area of ​​3 m². 2 It is desirable that the powder has a concentration of 1.0% by mass or less per gram, an oxygen content of 1.0% by mass or less, and an impurity carbon content of 0.15% by mass or less in silicon. Furthermore, it is desirable that the median diameter D50 is 7 μm or more and the BET specific surface area is 2.5 m². 2 It is more desirable that the powder be less than / g, has an oxygen content of 0.5% by mass or less, and has an impurity carbon content in silicon of 0.10% by mass or less.

[0045] In particular, the purity of the silicon powder should preferably be 99% or higher, and preferably 99.5% or higher. This is because the impurity oxygen contained in silicon is one of the factors that inhibits the heat conduction of the silicon nitride substrate obtained by reaction sintering, and therefore, from the viewpoint of improving the heat conductivity of the silicon nitride substrate, it is desirable to have less impurity oxygen.

[0046] Furthermore, it is desirable to adjust the raw material powder so that the total amount of impurity oxygen in the silicon powder and oxygen from the magnesium compound is in the range of 0.1% to 1.1% by mass relative to the silicon equivalent in silicon nitride, by limiting the amount of oxygen from the magnesium compound. In addition, impurity carbon in silicon inhibits the growth of silicon nitride particles in the silicon nitride substrate obtained by reaction sintering, resulting in insufficient densification. This is one of the factors that leads to a decrease in the thermal conductivity and insulation properties of the silicon nitride substrate, so it is desirable to keep the amount of carbon as low as possible.

[0047] In this specification, the BET specific surface area (m²) 2 The value ( / g) is obtained using a BET specific surface area meter by the BET single-point method (JIS R 1626:1996 "Method for measuring the specific surface area of ​​fine ceramic powders by gas adsorption BET method"). The median diameter D50 (μm) is the particle size at which the cumulative frequency reaches 50% in the particle size distribution determined by laser diffraction and scattering method.

[0048] <<Rare earth element oxides>> In this embodiment, oxides of yttrium (Y), ytterbium (Yb), gadolinium (Gd), erbium (Er), and lutetium (Lu) are used as rare earth element oxides because they are readily available and stable as oxides. Specifically, examples of rare earth element oxides include yttrium oxide (Y2O3), ytterbium oxide (Yb2O3), gadolinium oxide (Gd2O3), erbium oxide (Er2O3), and lutetium oxide (Lu2O3).

[0049] The content of rare earth element oxides is, for example, 0.5 mol% or more and less than 2 mol% relative to 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 rare earth element oxides is less than 0.5 mol%, the effect as a sintering aid will be insufficient, and the density will not be sufficiently high. On the other hand, if the content of rare earth element oxides is 2.0 mol% or more, the thermal conductivity of the sintered body will decrease due to the increase in grain boundary phases with low thermal conductivity, and the amount of expensive rare earth element oxides used will increase. In particular, it is desirable that the content of rare earth element oxides be 0.6 mol% or more and less than 2 mol%, and even more desirable that it be 1 mol% or more and 1.8 mol% or less.

[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 is sometimes simply referred to as "the sum 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), or magnesium nitride (Mg3N2).

[0052] Here, the magnesium compound is selected such that 87% by mass or more of the total magnesium compound is magnesium 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 amount of magnesium silicon nitride 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 lower thermal conductivity of the sintered body (silicon nitride substrate). Therefore, from the viewpoint of improving the thermal conductivity of the silicon nitride substrate, it is desirable for the amount of magnesium silicon nitride in the magnesium compound to be high, for example, 90% by mass or more is desirable.

[0053] The magnesium compound content (in MgO equivalent) in the silicon nitride substrate is, for example, 8 mol% or more and less than 15 mol% relative to the total of silicon (in silicon nitride equivalent), rare earth element oxides (in trivalent oxide equivalent), and magnesium compounds (in MgO equivalent). If the magnesium compound content is less than 8 mol%, its effect as a sintering aid will be insufficient, and the density will not be sufficiently high. On the other hand, if the magnesium compound content is 15 mol% or more, the thermal conductivity of the sintered body will decrease due to an increase in grain boundary phases with low thermal conductivity. In particular, the magnesium compound content 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 oxides, and magnesium compounds described above (S201). Specifically, rare earth element oxides and magnesium compounds are added to silicon powder as sintering aids in predetermined ratios, then a dispersion medium (organic solvent) and, if necessary, a dispersant are added, and the mixture is ground in a ball mill to produce a slurry, which is a dispersion of the raw material powder. Ethanol, n-butanol, toluene, etc., can be used as the dispersion medium. As a dispersant, for example, sorbitan ester type dispersants, polyoxyalkylene type dispersants, etc., can be used.

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

[0056] 2. Molded body manufacturing process To the slurry obtained as described above, for example, a dispersion medium, an organic binder, a dispersant, etc., and after vacuum degassing as necessary, the viscosity is adjusted to a predetermined range to produce a coating slurry.

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

[0058] The organic binder used in the preparation of the coating slurry is not particularly limited, but examples include PVB resins (polyvinyl butyral resins), ethyl cellulose resins, and acrylic resins. The amounts of the dispersion medium, organic binder, dispersant, etc., should be appropriately adjusted according to 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 can be used.

[0060] The thickness of the sheet-like molded body formed in the molded body manufacturing process is, for example, 0.15 mm to 0.8 mm. The manufactured sheet-like molded body is cut to a predetermined size using, for example, a punching machine, as needed.

[0061] 3. Sintering process The fabricated sheet-like molded body is heated to perform a sintering process in which the silicon contained in the molded body is nitrided and then densified (S203). This sintering process includes a degreasing step to remove the organic binder contained in the molded body, a nitriding step to react the silicon contained in the molded body with nitrogen to form silicon nitride (S204), and a densification sintering step performed after the nitriding step. In other words, the silicon nitride substrate in this embodiment is a silicon nitride substrate obtained by nitriding the silicon contained in a sheet-like molded body. 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 carried out sequentially in separate furnaces or continuously in the same furnace.

[0062] In the sintering process, for example, as shown in Figure 5, multiple sheets of the fabricated molded body 100A are stacked on a boron nitride (BN) setter 200 with a separating material (not shown) in between, and a weight 300 is placed on the top layer of the stacked molded bodies 100A before being placed in an electric furnace. In this state, after the organic binder removal process (degreasing process) is carried out, a decarbonization treatment is performed in a nitriding apparatus at a temperature of 900°C to 1300°C, and the temperature is raised to a predetermined temperature in a nitrogen atmosphere for nitriding treatment. After that, a densification sintering process is carried out in a sintering apparatus. Such a sintering process is carried out while applying a load of 10 Pa to 1000 Pa or less to the molded body 100A using the weight 300.

[0063] As the separation material mentioned above, for example, a boron nitride (BN) powder layer with a thickness of approximately 3 μm to 20 μm is used. The boron nitride powder layer has the function of facilitating the 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 slurry form to one side of each sheet-shaped molded body 100A by spraying, brush coating, or screen printing. It is desirable that the boron nitride powder has a purity of 95% or more and an average particle size (D50) of 1 μm to 20 μm.

[0064] As described above, a silicon nitride substrate can be manufactured. In particular, the silicon nitride substrate manufacturing method in this embodiment uses high-purity silicon powder, so for example, a silicon nitride substrate with a thermal conductivity of 110 W / (m·K) or higher can be manufactured. However, the inventors have found that there is room for improvement in the above-described method for manufacturing a silicon nitride substrate, and this point will be explained below.

[0065] <Room for improvement> In the silicon nitride substrate manufacturing method described above, silicon powder is used instead of silicon nitride powder, thus requiring nitriding treatment. In this regard, the inventors have newly discovered that color unevenness occurs on the surface of the manufactured silicon nitride substrate depending on the heating conditions during the nitriding treatment.

[0066] Here, when we refer to the "surface" of a silicon nitride substrate in this specification, we intend to include not only the "front" as the first surface, but also the "back" as the second surface opposite the first surface. For example, when we say that color unevenness occurs on the "surface" of a silicon nitride substrate, we will appropriately rephrase this as color unevenness occurs on the "front or back" of the silicon nitride substrate. In other words, when we refer to the "surface" in this specification, we intend to use it to mean "at least one surface of the front or back" to avoid complexity in expression.

[0067] Furthermore, as used herein, "color unevenness" refers to, for example, a difference in color between the central and edge areas of a rectangular silicon nitride substrate. Specifically, Figure 6 is a photograph showing the surface of a silicon nitride substrate where color unevenness has occurred. In Figure 6, the central part of the silicon nitride substrate is whitish, while the edge area is blackish. Comparing the colors of the central and edge areas reveals that color unevenness has occurred in the silicon nitride substrate.

[0068] For example, if foreign matter or dirt adheres to the surface of a silicon nitride substrate, it can cause poor contact with the brazing material or insulation failure 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. However, if color unevenness occurs as shown in Figure 6, there is a risk that the color unevenness will be mistakenly 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 edge of the silicon nitride substrate shown in Figure 6 has a dark appearance, so there is a risk that this edge area will be mistakenly detected as an area where foreign matter or dirt has adhered. Therefore, in order to suppress false detections in visual inspection, it is desirable to devise ways to suppress the color unevenness that occurs on the surface of the silicon nitride substrate.

[0069] <<Mechanism of color unevenness (speculation)>> Therefore, in order to suppress color unevenness occurring on the surface of silicon nitride substrates, the inventors first diligently investigated the mechanism by which color unevenness occurs on the surface of silicon nitride substrates. As a result, the inventors have estimated the following mechanism for color unevenness, and will now explain this estimated mechanism.

[0070] Nitriding is performed by heating in a nitrogen atmosphere, for example, as shown in Figure 5, by arranging multiple sheet-like molded bodies 100A on a setter 200 and placing a weight 300 on top of the stacked molded bodies 100A. In this case, for example, if we focus on the first molded body sandwiched between the upper and lower molded bodies 100A, heat tends to accumulate in the center of this first molded body. As a result, a temperature distribution occurs in the first molded body where the temperature is high in the center and low at the edges. At this time, the nitriding reaction proceeds in the high-temperature center, while the nitriding reaction does not proceed as well at the edges. Considering that the nitriding reaction is an exothermic reaction, in the center where the nitriding reaction is progressing, positive feedback of heat generation occurs, resulting in a rapid rise in temperature. If this temperature rise is large, the temperature in the center exceeds the melting point of silicon, causing "thermal runaway" where silicon melts, and the temperature difference between the center and the edges becomes large, resulting in insufficient nitriding at the edges. If sintering is performed in a state of insufficient nitriding, it is presumed that the silicon that was not completely nitrided will remain at the edges, resulting in uneven coloring. In other words, the inventors of this invention surmise that uneven coloring occurs due to the remaining unnitrided silicon.

[0071] Based on this inferred mechanism, the color unevenness that occurs on the surface of the silicon nitride substrate is thought to be caused by the heating process during the nitriding treatment. In other words, if the heating process can be carried out while achieving a temperature distribution that minimizes the temperature difference between the center and the edges, it is thought that color unevenness can be suppressed while "thermal runaway" is suppressed. Therefore, in this embodiment, improvements have been made to the heating process during the nitriding treatment. The technical concept of this embodiment, in which improvements have been made, will be explained below.

[0072] <Basic Concept in the Embodiment> The basic idea in this embodiment is to heat the molded body while maintaining a temperature distribution with a small temperature difference between the center and the edges by suppressing a rapid rise in temperature during the nitriding process. In other words, the basic idea is to suppress a rapid rise in temperature to the extent that sufficient time is available to sufficiently mitigate the temperature difference between the center and the edges.

[0073] According to this basic principle, the temperature can be gradually increased while reducing the temperature difference between the center and the edges. This suppresses "thermal runaway" caused by a rapid nitriding reaction in the center, while also eliminating insufficient nitriding at the edges.

[0074] In particular, the basic concept involves ensuring that the average temperature rise per unit time (hereinafter referred to as the temperature gradient) during the heating process in the nitriding treatment, specifically in the range of 1270°C to 1340°C, is below a predetermined value. For example, it is preferable that the average temperature gradient be 3.1°C / h or less. Furthermore, it is preferable that the maximum heating temperature be between 1390°C and 1500°C.

[0075] This allows the temperature to rise slowly enough to allow sufficient time to mitigate the temperature difference between the center and the edges, according to the basic principle. As a result, according to the basic principle, insufficient nitriding at the edges of the molded body is eliminated, and consequently, color unevenness on the surface of the final manufactured silicon nitride substrate can be reduced. Therefore, according to the basic principle, a significant effect can be obtained in that false detections in visual inspection caused by color unevenness can be reduced, thereby improving the accuracy of detecting foreign objects or dirt in visual inspection.

[0076] <Specific details> The following sections will describe specific examples that embody this fundamental idea.

[0077] Silicon nitride substrates of Example 1, Example 2, and Comparative Example were manufactured using the method described in "<Method for Manufacturing Silicon Nitride Substrates in Embodiments>" above. 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 on the long side and 140 mm on the short side. The thickness of each silicon nitride substrate was 0.32 mm.

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

[0079] When manufacturing the silicon nitride substrate of Example 1, in the nitriding process S204, the heating temperature was increased in a stepwise manner as the heating time progressed, up to the maximum heating temperature. The maximum heating temperature was 1400°C. The average slope of the heating temperature in the temperature rise range from 1270°C to 1340°C was 2.99°C / h. Figure 7 shows the measured temperature of the molded body, the measured temperature of the furnace, and the temperature difference between the molded body and the furnace in a predetermined temperature range. The horizontal axis in Figure 7 shows the elapsed time from the reference point when the measured temperature of the furnace, which is the heating temperature, reached around 1300°C. When the measured temperature of the furnace was near 1300°C, the temperature difference between the measured temperature of the furnace and the temperature of the molded body inside the furnace was 20°C or less. In Example 1, there was no rapid increase in the temperature of the molded body 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 manufacturing the silicon nitride substrate of Example 2, in the nitriding process S204, the heating temperature was increased in a stepwise manner as the heating time progressed, up to the maximum heating temperature. The maximum heating temperature was 1400°C. The average slope of the heating temperature in the temperature rise range from 1270°C to 1340°C was 3.02°C / h. Figure 8 shows the measured temperature of the molded body, the measured temperature of the furnace, and the temperature difference between the molded body and the furnace in a predetermined temperature range. The horizontal axis in Figure 8 shows the elapsed time from the reference point when the measured temperature of the furnace, which is the heating temperature, reached around 1300°C. When the measured temperature of the furnace was near 1300°C, the temperature difference between the measured temperature of the furnace and the temperature of the molded body inside the furnace was 20°C or less. In Example 2, there was no rapid temperature rise of the molded body 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 process S204, the heating temperature was increased in a stepwise manner as the heating time progressed, up to the maximum heating temperature. The maximum heating temperature was 1400°C. The average slope of the heating temperature in the temperature rise range from 1270°C to 1340°C was 4.67°C / h. Figure 9 shows the measured temperature of the molded body, the measured temperature of the furnace, and the temperature difference between the molded body and the furnace in a predetermined temperature range. The horizontal axis in Figure 9 shows the elapsed time from the reference point when the measured temperature of the furnace, which is the heating temperature, reached around 1300°C. When the measured temperature of the furnace was near 1300°C, the temperature difference between the measured temperature of the furnace and the temperature of the molded body inside the furnace exceeded 20°C and reached a maximum of 44.9°C. In other words, in the comparative example, a rapid increase in the temperature of the molded body occurred 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] The following describes the verification results showing that color unevenness can be reduced when using silicon nitride substrates manufactured under the heating conditions shown in Example 1 or Example 2.

[0083] <Verification Results> <<Quantitative evaluation method for color unevenness>> First, a method for quantitatively evaluating 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 the "color space".

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

[0087] <<<Lightness L * >>> "Lightness L * " is an index indicating the light and dark of the color tone. "Lightness L * " takes a value in the range of 0 ≦ L * ≦ 100. As the value of "Lightness L * " increases, the color tone becomes brighter and has a white tint. On the other hand, as the value of "Lightness L * " decreases, the color tone becomes darker and has a black tint.

[0088] <<<Chromaticness index a * >>> "Chromaticness index a * " is an index indicating the degree from red to green of the color tone. "Chromaticness index a * " takes a value in the range of -60 ≦ a * ≦ +60. As the value of "Chromaticness index a * " increases in the positive direction, the color tone becomes red. On the other hand, as the value of "Chromaticness index a * " increases in the negative direction, the color tone becomes green. And, as the absolute value of "Chromaticness index a * " decreases, the color tone becomes duller.

[0089] <<<Chromaticness index b * >>> "Chromaticness index b * " is an index indicating the degree from yellow to blue of the color tone. "Chromaticness index b * " is -60 ≦ b* It takes values ​​in the range of ≤ +60. "Chromaticity index b" * As the value of "" increases in the positive direction, the color tone becomes yellow. On the other hand, "Chromaticity Index b" * As the value of "Chromaticity Index b" increases in the negative direction, the color tone becomes blue. * The smaller the absolute value of "", the duller the color tone becomes.

[0090] The above-mentioned "Brightness L * "Chromaticness Index a * " and "Chromaticness Index b * The measurement is performed in accordance with "JIS Z 8722:2000". For example, in this embodiment, the SCI (including reflected light) method is used to measure the color of the material. At this time, a "Konica Minolta CR-400" is used as the measuring instrument, and the measurement is performed with the settings "Field of view: CIE2° field of view color matching function approximation" and "Light source: C". The silicon nitride substrate is placed on top of 10 sheets of PPC paper stacked on top of each other, and the measurement is performed on the surface of the silicon nitride substrate opposite to the side in contact with the PPC paper using the above measuring instrument.

[0091] <<<Saturation C * >>> "Saturation C * " is based on the following formula 1, and is called "chromaticity index a * " and "Chromaticness Index b * It is calculated based on the following.

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

[0093] <<<Color difference ΔE * ab>>> "Color difference ΔE * "ab" is an index that shows the difference in color from a reference color, and is called "lightness L * "Chromaticness Index a * " and "Chromaticness Index b* It is calculated based on the following.

[0094] Figure 11 is a schematic diagram of the silicon nitride substrate 100, where "Position 1" represents the central region, while "Position 2," "Position 3," "Position 4," and "Position 5" represent the four edge regions. In this embodiment, the color of "Position 1" is used as the reference color, and the "color difference ΔE" in "Position 2" to "Position 5" is used. * The code is calculating "ab".

[0095] Specifically, "Position 1" to "Position 5" are defined as the following positions. "Position 1": The position on the surface where the diagonals intersect. "Position 2": On the surface, 15 mm inward from corner CNR1 in the direction of "Position 1". "Position 3": On the surface, 15 mm inward from corner CNR2 in the direction of "Position 1". "Position 4": On the surface, 15 mm inward from corner CNR3 in the direction of "Position 1". "Position 5": On the surface, 15 mm inward from corner CNR4 in the direction of "Position 1".

[0096] The measurement range is a 20mm x 20mm area centered on each of the five positions, "Position 1" through "Position 5".

[0097] <<Results of color unevenness verification>> Below, we will explain the results of our verification of color unevenness in samples #1 to #4 using the "color space" configuration parameters described above.

[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, a thermal conductivity of 130 W / (m·K) was achieved for all of "Sample #1" to "Sample #4".

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

[0102] In contrast, in "Sample #1" showing Example 1, "Color difference ΔE * The value of "ab" is 1.5 or less at all of "position 2" through "position 5".

[0103] Furthermore, in "Sample #2" which shows Example 2, "Color difference ΔE * The value of "ab" is 0.8 or less at all of "position 2" through "position 5".

[0104] This means that silicon nitride substrates manufactured under the nitriding heating conditions of Example 1 or Example 2 can achieve high thermal conductivity and reduced color unevenness. Therefore, according to Examples 1 and 2, it is possible to obtain silicon nitride substrates with excellent heat dissipation characteristics, as well as reduce false detections in visual inspection caused by color unevenness.

[0105] Furthermore, in Examples 1 and 2, the "brightness L" of the central part (position 1) and the edge parts (positions 2 to 5) * " is 70 or higher, and "Saturation C *The value is 10 or higher. This results in a clearer contrast with foreign objects and dirt, which has the significant effect of improving the accuracy of detecting foreign objects or dirt during visual inspection.

[0106] Here, the term "edge" as used in this specification is defined as a concept that includes all of the above-mentioned "positions 2" to "position 5". In other words, the term "edge" as used in this specification includes all of "positions 2" to "position 5". Also, the term "center" is defined as a concept that includes "position 1". Therefore, the color difference between the center and the edge is ΔE * Let ab be the basis for 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 the 'center' and 'edge' is 70 or higher" means that the brightness of each of the 'positions' from 'position 1' to 'position 5' is 70 or higher.

[0108] Furthermore, the statement "the saturation of the 'center' and 'edge' is 10 or higher" means that the saturation of each of the 'position 1' through 'position 5' is 10 or higher.

[0109] <Summary> In this embodiment, during the heating step in the nitriding treatment, measures have been taken to ensure 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] Therefore, according to this embodiment, "color difference ΔE *A silicon nitride substrate can be obtained in which "ab" is 1.5 or less, and more specifically, 0.8 or less. Therefore, the technical concept of this embodiment is excellent in that it can reduce color unevenness. In other words, the technical concept 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] Furthermore, the technical concept in this embodiment is effective when applied to large silicon nitride substrates. Specifically, the technical concept in this embodiment is effective when applied to configurations such as those shown in Figure 2, in which six semiconductor devices constituting a three-phase inverter circuit are mounted on a single silicon nitride substrate. This is because, when using a large silicon nitride substrate, the potential for color unevenness to occur is considered to be greater.

[0112] In this regard, applying the technical concept of this embodiment to the manufacturing of large silicon nitride substrates, which have a high potential for color unevenness, is considered effective in reducing the occurrence of color unevenness. In particular, the technical concept of this embodiment is effective when applied to the manufacturing of silicon nitride substrates with a rectangular shape where the length of each side is 100 mm or more.

[0113] The present invention has been described in detail above based on its embodiments, but it goes without saying that the present invention is not limited to the above embodiments and can be modified in various ways without departing from its essence. [Explanation of symbols]

[0114] 100 Silicon Nitride Substrate 100A Molded body 200 Setta 300 weights INV Inverter Circuit FWD diode GCC gate control circuit GL ground wiring LG1 1st Leg LG2 Leg 2 LG3 3rd Leg MT 3-phase induction motor NT negative potential terminal PT Positive potential terminal Q1 Switching element RT Rotor SA Semiconductor Equipment SA1 Semiconductor Device SA2 semiconductor device SA3 Semiconductor Device SA4 Semiconductor Device SA5 Semiconductor Device SA6 Semiconductor Device 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. 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 molded body is heated at a predetermined heating temperature. Including the process, A method for manufacturing a silicon nitride substrate, wherein in the nitriding step, when the heating temperature is 1300° C., a temperature difference between the heating temperature and the temperature of the molded body in the furnace is 20° C. or less.

7. The method for manufacturing a silicon nitride substrate according to claim 6, The nitriding step comprises stacking a plurality of the molded bodies and heating the stacked bodies.

Images