JP7188633B1 - Silicon nitride substrate and manufacturing method thereof - Google Patents

Abstract

An object of the present invention is to reduce color unevenness that occurs on the surface of a silicon nitride substrate. A silicon nitride substrate obtained by nitriding silicon contained in a sheet-like compact has a first surface and a second surface opposite to the first surface. Let ΔE*ab be the color difference between the central portion and the edge portion on at least one of the first surface and the second surface, and ΔE*ab≦1.5. [Selection figure] None

Description

TECHNICAL FIELD The present invention relates to a silicon nitride substrate and its manufacturing technology, and for example, to a technology effective when applied to a silicon nitride substrate having a thermal conductivity of 110 W/(m·K) or more and its manufacturing technology.

In Japanese Patent Application Laid-Open No. 2003-267786 (Patent Document 1), the color of the sintered body is blackened compared to conventional materials, and the color unevenness is reduced, and a silicon nitride ceramic sintered body having sufficient strength Techniques are described for providing the

Japanese Patent Application Laid-Open No. 2005-214659 (Patent Document 2) describes a technology related to a foreign matter inspection apparatus capable of discriminating foreign matter of a color having a low contrast with respect to the color of the wiring board.

JP 2016-204206 (Patent Document 3), JP 2016-204207 (Patent Document 4), JP 2016-204209 (Patent Document 5) and JP 2016-204210 (Patent Document 6 ) describes a technique for providing a silicon nitride-based ceramic member that is lightweight, has high hardness, is excellent in resistance to processing such as polishing, and has excellent appearance quality.

Japanese Patent Application Laid-Open No. 9-227240 (Patent Document 7) discloses that the thickness of the surface tone layer portion in the silicon nitride ceramic sintered body is reduced, and the breaking strength characteristics of the surface layer and the inner layer are made uniform. It describes a technique that can

JP-A-2003-267786 JP 2005-214659 A JP 2016-204206 A JP 2016-204207 A JP 2016-204209 A JP 2016-204210 A JP-A-9-227240

For example, if foreign matter or dirt adheres to the surface of the silicon nitride substrate, it becomes a cause of poor contact with the "brazing material" or poor insulation of the silicon nitride substrate itself, so it is necessary to remove the foreign matter or dirt. Therefore, a visual inspection is performed to detect adhesion of foreign matter or dirt to the silicon nitride substrate. As an appearance inspection method, for example, after imaging the surface of a silicon nitride substrate with an imaging device typified by a CCD camera, foreign matter and dirt are detected by comparing data of the captured image with pre-registered reference data. There is a method to detect

However, in the above-described appearance inspection method, if there is color unevenness on the surface of the silicon nitride substrate, for example, between the central portion and the edge portion, there is a risk that the color unevenness will be erroneously detected as adhesion of foreign matter or dirt.

Regarding this point, it is conceivable to relax the criteria for detecting adhesion of foreign matter and dirt in order not to mistakenly detect color unevenness as adhesion of foreign matter and dirt. be. For this reason, it is conceivable to improve the detection accuracy of 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 the color unevenness, but setting the inspection conditions is complicated. becomes.

As described above, it is difficult to perform a highly accurate appearance inspection when the surface of the silicon nitride substrate has color unevenness. Therefore, in order to perform the appearance inspection with high accuracy, it is important to reduce the color unevenness that occurs on the surface of the silicon nitride substrate.

An object of the present invention is to reduce color unevenness that occurs on the surface of a silicon nitride substrate.

A silicon nitride substrate in one embodiment is a silicon nitride substrate obtained by nitriding silicon contained in a sheet-like compact, and has a first surface and a second surface opposite to the first surface. . Let ΔE ^* ab be the color difference between the central portion and the edge portion on at least one of the first surface and the second surface, and ΔE ^* ab≦1.5.

A method for manufacturing a silicon nitride substrate according to one embodiment includes the steps of (a) preparing a slurry containing silicon powder, (b) obtaining a compact from the slurry, and (c) sintering the compact in a furnace. a process. Here, the step (c) includes a nitriding step of nitriding the compact by heating it at a predetermined heating temperature, and the temperature difference between the heating temperature and the temperature of the compact in the furnace is 20° C. or less.

According to one embodiment, color unevenness occurring on the surface of the silicon nitride substrate can be reduced.

1 illustrates a motor circuit including an inverter circuit and a three-phase induction motor; FIG. FIG. 4 is a schematic diagram showing an example of a mounting layout for realizing an inverter circuit; It is a flowchart which shows the manufacturing process of the silicon nitride substrate in related technology. 4 is a flow chart showing a manufacturing process of a silicon nitride substrate according to an embodiment; FIG. 4 is a diagram showing a state in which molded bodies are arranged in layers; 4 is a photograph showing the surface of a silicon nitride substrate with color unevenness. 4 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. FIG. 5 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. FIG. 4 is a graph showing 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 of the comparative example. It is a schematic diagram which shows the stereoscopic image of a "color space." It is a figure which shows a silicon nitride substrate typically.

In principle, the same members are denoted by the same reference numerals throughout the drawings for describing the embodiments, and repeated description thereof will be omitted. In order to make the drawing easier to understand, even a plan view may be hatched.

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

<Configuration example of three-phase inverter circuit>
A three-phase inverter circuit will be described below as an example.

Power modules are used, for example, in drive circuits for three-phase induction motors used in air conditioners and the like. Specifically, this drive circuit includes an inverter circuit, and this inverter circuit is a circuit having a function of converting DC power into AC power.

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 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-phase voltages having different phases. Specifically, in the three-phase induction motor MT, a rotating magnetic field is generated around the rotor RT, which is a conductor, using three-phase alternating current called U-phase, V-phase, and W-phase, which are 120 degrees out of phase. In this case, the magnetic field rotates around the rotor RT. This means that the magnetic flux across the rotor RT, which is a conductor, changes. As a result, electromagnetic induction occurs in the rotor RT, which is a conductor, and an induced current flows in the rotor RT. The fact that an induced current flows in the 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.

Thus, it can be seen that the three-phase induction motor MT can rotate the rotor RT by using the three-phase alternating current. That is, the three-phase induction motor MT requires a three-phase alternating current. Therefore, in the motor circuit, an inverter circuit INV that generates alternating current from direct current is used to supply three-phase alternating current to the three-phase induction motor.

A configuration example of the inverter circuit INV will be described below.

As shown in FIG. 1, for example, the inverter circuit INV is provided with switching elements Q1 and diodes FWD corresponding to three phases. That is, in the inverter circuit INV, for example, 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. For example, in FIG. 1, each of 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 reverses the switching element Q1 and the diode FWD. It consists of components connected in parallel.

In other words, in the inverter circuit INV, the switching element Q1 and the diode FWD are connected in anti-parallel between the positive potential terminal PT and each phase (U-phase, V-phase, W-phase) of the three-phase induction motor MT. A switching element Q1 and a diode 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 single phase, and six switching elements Q1 and six diodes FWD are provided 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 as described above, the switching operation of the switching element Q1 is controlled by the gate control circuit GCC to convert the DC power into the three-phase AC power, and the three-phase AC power is converted to the three-phase induction motor. It is designed to be supplied to MT.

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

<Mounting layout example of inverter circuit>
FIG. 2 is a schematic diagram showing a mounting layout example for realizing an inverter circuit.

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

As shown in FIG. 2, the semiconductor device SA1 is connected between the power wiring VL and the wiring WL1, while the 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. That is, the semiconductor device SA1 constitutes an upper arm of the first leg LG1, and the semiconductor device SA2 constitutes a lower arm of the first leg LG1. Each of the semiconductor device SA1 and the semiconductor device SA2 has a semiconductor chip formed with a power MOSFET functioning as the switching element Q1.

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

Further, the semiconductor device SA5 is connected between the power wiring VL and the wiring WL3, while the 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. That is, the semiconductor device SA5 constitutes an upper arm of the third leg LG3, and the semiconductor device SA6 constitutes a lower arm of the third leg LG3. Each of the semiconductor devices SA5 and SA6 has a semiconductor chip formed with a power MOSFET functioning as a switching element Q1.

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

<Performance Required for Silicon Nitride Substrate>
As described above, the semiconductor devices SA1 to SA6 are mounted on the silicon nitride substrate 100. FIG. At this time, since heat is generated in the semiconductor devices SA1 to SA6, it is necessary to prevent malfunctions and failures of the semiconductor devices SA1 to SA6 caused by 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, silicon nitride substrate 100 is required to have high thermal conductivity. In addition, the silicon nitride substrate 100 is required to have resistance to stress generated due to temperature change.

In the following, particular attention is paid to the improvement of thermal conductivity.

<Description of related technology>
First, a related technique for manufacturing a silicon nitride substrate will be described.

The term "related art" as used in this specification means a technology that is not a publicly known technology, but has a problem found by the inventor of the present application, and is a technology that is a premise of the present invention.

FIG. 3 is a flow chart showing a manufacturing process of a silicon nitride substrate in related art.

After adding a rare earth element oxide and a magnesium compound as sintering aids to the silicon nitride powder, a dispersion medium (organic solvent) and, if necessary, a dispersant are added, and the raw material powder is pulverized by a ball mill. A slurry, which is a dispersion, is prepared (S101).

Next, after adding an organic binder or the like to the prepared slurry, vacuum defoaming is performed as necessary, and the viscosity is adjusted within a predetermined range to prepare a coating slurry. After that, the prepared coating slurry is formed into a sheet by a sheet forming machine, cut into a predetermined size, and dried to form a sheet-shaped formed body (S102).

Subsequently, by heating the obtained sheet-like molded body, the molded body is densified and sintered (S103). A silicon nitride substrate can be manufactured in the manner described above.

The thermal conductivity of the silicon nitride substrate manufactured by the above related technology is about 90 W/(m·K). Regarding this point, the present inventors have studied how to improve the thermal conductivity of a silicon nitride substrate, and as a result, in the related art, it is possible to further improve the thermal conductivity due to the use of silicon nitride powder. We have acquired the knowledge that it is difficult to That is, since silicon nitride powder contains many impurities and it is difficult to obtain high-purity silicon nitride powder, it is difficult to further improve the thermal conductivity of silicon nitride substrates manufactured by related techniques. The inventor of the present invention has acquired the knowledge that Therefore, the use of silicon powder instead of silicon nitride powder has been investigated. This is because silicon powder contains fewer impurities than silicon nitride powder, and it is easy to obtain high-purity silicon powder. In other words, since silicon powder has a higher purity than silicon nitride powder, it is thought that a decrease in thermal conductivity due to impurities can be suppressed. For this reason, the present embodiment adopts a method for manufacturing a silicon nitride substrate using silicon powder instead of silicon nitride powder.

<Method for Manufacturing Silicon Nitride Substrate in Embodiment>
A method for manufacturing a silicon nitride substrate employed in this embodiment will be described below.

FIG. 4 is a flow chart showing steps for manufacturing a silicon nitride substrate according to the present embodiment.

1. Slurry Preparation Step In the present embodiment, a slurry is prepared using raw material powder obtained by adding a rare earth element oxide and a magnesium compound as sintering aids to silicon powder (S201).

<<Silicon>>
In this embodiment, industrial grade silicon powder can be used. Silicon before pulverization has, for example, a median diameter D50 of 6 μm or more, a BET specific surface area of 3 m ² /g or less, an oxygen content of 1.0% by mass or less, and an impurity carbon content in silicon of 0.15% by mass or less. A powder is preferred. Furthermore, the powder has a median diameter D50 of 7 μm or more, a BET specific surface area of 2.5 m ² /g or less, an oxygen content of 0.5% by mass or less, and an impurity carbon content in silicon of 0.10% by mass or less. is more desirable.

In particular, the purity of the silicon powder is desirably 99% or higher, preferably 99.5% or higher. This is because the impurity oxygen contained in silicon is one of the factors that hinder the thermal conductivity of the silicon nitride substrate obtained by reaction sintering. This is because the smaller the number, the better.

Furthermore, by limiting the amount of oxygen from the magnesium compound, the total amount of impurity oxygen contained in the silicon powder and oxygen from the magnesium compound is 0.1% by mass or more and 1.1% with respect to silicon converted to silicon nitride. It is desirable to adjust the raw material powder so that the content is in the range of mass % or less. In addition, the impurity carbon contained in silicon inhibits the growth of silicon nitride particles in the silicon nitride substrate obtained by reaction sintering, and as a result, the densification tends to be insufficient, thereby reducing the thermal conductivity of the silicon nitride substrate. Since it is one of the factors that cause deterioration of insulation properties, it is desirable that the amount is as small as possible.

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

<<rare earth element oxide>>
In the present embodiment, yttrium (Y), ytterbium (Yb), gadolinium (Gd), erbium (Er), lutetium (Lu), which are readily available and stable as oxides, are used as rare earth element oxides. ) are used. Specifically, rare earth element oxides include yttrium oxide (Y ₂ O ₃ ), ytterbium oxide (Yb ₂ O ₃ ), gadolinium oxide (Gd ₂ O ₃ ), erbium oxide (Er ₂ O ₃ ), lutetium oxide ( Lu ₂ O ₃ ) and the like.

The content of the rare earth element oxide is, for example, 0.5 mol % or more and 2 mol with respect to the total of silicon (converted to silicon nitride), rare earth element oxide (converted to trivalent oxide) and magnesium compound (converted to MgO). %. If the content of the rare earth element oxide 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, when the content of the rare earth element oxide is 2.0 mol % or more, the grain boundary phase with low thermal conductivity increases, and the thermal conductivity of the sintered body decreases, and the expensive rare earth element oxide is used. increase in quantity. In particular, the content of the rare earth element oxide is desirably 0.6 mol % or more and less than 2 mol %, and more desirably 1 mol % or more and 1.8 mol % or less.

In this specification, the number of moles of silicon nitride (Si ₃ N ₄ ) obtained when all silicon is nitrided and the oxide of a rare earth element are trivalent oxides (RE ₂ O ₃ : RE is a rare earth element). The sum of the number of moles when converted to , and the number of moles when the magnesium compound is converted to MgO is simply "silicon (converted to silicon nitride), rare earth element oxide (converted to trivalent oxide) and magnesium compound (in terms of MgO)".

<<Magnesium compound>>
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 (MgSiN ₂ ), magnesium silicide (Mg ₂ Si), magnesium nitride (Mg ₃ N ₂ ), and the like.

Here, it is selected so that magnesium silicon nitride accounts for 87% by mass or more of the total magnesium compound. 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 content 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 increases, resulting in a low thermal conductivity of the sintered body (silicon nitride substrate). Therefore, from the viewpoint of improving the thermal conductivity of the silicon nitride substrate, the amount of magnesium silicon nitride in the magnesium compound is preferably as large as possible, for example, 90% by mass or more.

The magnesium compound content (in terms of MgO) in the silicon nitride substrate is, for example, the total of silicon (in terms of silicon nitride), rare earth element oxides (in terms of trivalent oxides), and magnesium compounds (in terms of MgO). , 8 mol % or more and less than 15 mol %. If the content of the magnesium compound is less than 8 mol %, the effect as a sintering aid will be insufficient, and the density will not be sufficiently high. On the other hand, when the content of the magnesium compound is 15 mol % or more, the grain boundary phase with low thermal conductivity increases, thereby decreasing the thermal conductivity of the sintered body. In particular, the magnesium compound content is desirably 8 mol % or more and less than 14 mol %, and more desirably 9 mol % or more and 11 mol % or less.

A slurry is prepared using the silicon, rare earth element oxide and magnesium compound described above (S201). Specifically, after adding a rare earth element oxide and a magnesium compound as sintering aids to silicon powder so as to have a predetermined ratio, a dispersion medium (organic solvent) and, if necessary, a dispersant are added, and ball milling is carried out. A slurry, which is a dispersion of the raw material powder, is prepared by pulverizing with. Ethanol, n-butanol, toluene, etc. can be used as the dispersion medium. Moreover, as a dispersant, for example, a sorbitan ester-type dispersant, a polyoxyalkylene-type dispersant, or the like can be used.

The amount of the dispersion medium used is, for example, preferably 40% by mass or more and 70% by mass or less with respect to the total amount of the raw material powder, and the amount of the dispersant used is, for example, 0.3% with respect to the total amount of the raw material powder. It is desirable that the amount is not less than 2% by mass and not more than 2% by mass. After dispersion, the dispersion medium may be removed or replaced with another dispersion medium, if necessary.

2. Molded body production process To the slurry obtained as described above, for example, a dispersion medium, an organic binder, a dispersant, etc. are added, and if necessary, vacuum defoaming is performed, and then the viscosity is adjusted to a predetermined range. By doing so, a coating slurry is prepared.

Next, the prepared coating slurry is formed into a sheet by a sheet forming machine, cut into a predetermined size, and dried to form a sheet-shaped formed body (S202).

The organic binder used to prepare the coating slurry is not particularly limited, but may include PVB-based resin (polyvinyl butyral resin), ethyl cellulose-based resin, acrylic-based resin, and the like. It is desirable to appropriately adjust the amount of the dispersion medium, organic binder, dispersant, etc. added according to the coating conditions.

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

The thickness of the sheet-like molded body formed in the molded body manufacturing process is, for example, 0.15 mm or more and 0.8 mm or less. The produced sheet-shaped molding is cut into a predetermined size using, for example, a punching machine, if necessary.

3. Sintering Step A sintering step is performed in which silicon contained in the formed body is nitrided and then densified by heating the produced sheet-shaped formed body (S203). The sintering step includes a degreasing step of removing the organic binder contained in the molded body, a nitriding step (S204) of reacting silicon and nitrogen contained in the molded body to form silicon nitride, and a nitriding step. and a subsequent densification and sintering step. In other words, the silicon nitride substrate in the present embodiment is a silicon nitride substrate obtained by nitriding silicon contained in a 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 continuously in the same furnace.

In the sintering step, for example, as shown in FIG. 5, a plurality of sheets of the produced sheet-shaped compact 100A are laminated on a setter 200 made of boron nitride (BN) with a separation material (not shown) interposed therebetween. A weight 300 is placed on the uppermost layer of a plurality of compacts 100A, and placed in an electric furnace. In this state, after the organic binder removal step (degreasing step) is performed, decarbonization treatment is performed at a temperature of 900° C. to 1300° C. in a nitriding apparatus, and the temperature is raised to a predetermined temperature in a nitrogen atmosphere for nitriding treatment. is done. After that, a densification sintering step is carried out in a sintering device. Such a sintering process is carried out while applying a load of 10 Pa or more and 1000 Pa or less to the molded body 100A with a weight 300, for example.

As the separation material described above, for example, a boron nitride (BN) powder layer having a thickness of about 3 μm or more and 20 μm or less is used. The boron nitride powder layer has a function of facilitating 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, for example, a purity of 95% or more and an average particle size (D50) of 1 μm or more and 20 μm or less.

A silicon nitride substrate can be manufactured in the manner described above. In particular, according to the method for manufacturing a silicon nitride substrate in the present embodiment, silicon powder with high purity is used, so that for example, a silicon nitride substrate having a thermal conductivity of 110 W/(m·K) or more can be manufactured. can do. However, as a result of examination by the present inventors, it has become clear that there is room for improvement in the above-described method for manufacturing a silicon nitride substrate. Therefore, this point will be described below.

<Room for improvement>
In the method for manufacturing a silicon nitride substrate described above, since silicon powder is used instead of silicon nitride powder, nitriding treatment is required. In this regard, the present inventors have newly discovered that the surface of the manufactured silicon nitride substrate may have color unevenness depending on the heating conditions in the nitriding treatment.

Here, in this specification, the term "front surface" of the silicon nitride substrate includes not only the "front surface" as the first surface, but also the "back surface" as the second surface opposite to the first surface. intended. For example, when it is said that color unevenness occurs on the "front surface" of the silicon nitride substrate, it is appropriately read that color unevenness occurs on the "front surface or rear surface" of the silicon nitride substrate. That is, the term "front surface" in this specification is used with the intention of "at least one of the front surface and the back surface" to avoid complication of expression.

Further, the term "unevenness of color" as used in this specification means, for example, that the hue of the central portion of the rectangular silicon nitride substrate is different from that of the edge portion. Specifically, FIG. 6 is a photograph showing the surface of a silicon nitride substrate on which color unevenness has occurred. In FIG. 6, the central portion of the silicon nitride substrate is tinged with white, whereas the edge portion of the silicon nitride substrate is tinged with black. , it can be seen that color unevenness occurs on the silicon nitride substrate.

For example, if foreign matter or dirt adheres to the surface of the silicon nitride substrate, it becomes a cause of poor contact with the "brazing material" or poor insulation of the silicon nitride substrate itself, so it is necessary to remove the foreign matter or dirt. Therefore, a visual inspection is performed to detect adhesion of foreign matter or dirt to the silicon nitride substrate. However, when color unevenness as shown in FIG. 6 occurs, there is a risk that the color unevenness is erroneously detected as adhesion of foreign matter or dirt. Specifically, considering that foreign substances and stains are often recognized as black areas (black dots), for example, the edges of the silicon nitride substrate shown in FIG. There is a possibility that the area of the part may be erroneously detected as an area where foreign matter or dirt is attached. Therefore, in order to suppress erroneous detection in the appearance inspection, it can be seen that there is a demand for a device for suppressing color unevenness that occurs on the surface of the silicon nitride substrate.

<<Occurrence mechanism of color unevenness (speculation)>>
Therefore, in order to suppress the color unevenness that occurs on the surface of the silicon nitride substrate, the present inventors first made extensive studies on the mechanism by which the color unevenness occurs on the surface of the silicon nitride substrate. As a result, the present inventor has deduced the generation mechanism of color unevenness as described below.

Nitriding is performed in a nitrogen atmosphere, for example, as shown in FIG. by heat treatment at In this case, for example, among the stacked molded bodies 100A, focusing on the first molded body sandwiched between the upper and lower molded bodies 100A, heat tends to accumulate in the central portion of the first molded body. Therefore, in the first molded body, a temperature distribution is generated in which the temperature is high in the central portion and low in the edge portion. At this time, while the nitriding reaction progresses in the central portion where the temperature is high, the nitriding reaction does not progress easily in the edge portion. Considering that the nitriding reaction is an exothermic reaction, the temperature rises sharply in the central portion where the nitriding reaction progresses, as a result of the positive feedback of the calorific value. If this temperature rise is large, the temperature at the central portion exceeds the melting point of silicon, causing "thermal runaway" in which silicon melts, and the temperature difference between the central portion and the edge portion increases. As a result, it is believed that insufficient nitridation occurs at the edges. If the sintering process is carried out in such a state that nitridation is insufficient, silicon that has not been completely nitrided will eventually remain at the edges, and as a result, color unevenness will occur. In other words, the inventors speculate that the color unevenness occurs due to the remaining unnitrided silicon.

Based on this presumed mechanism, it is considered that the color unevenness occurring on the surface of the silicon nitride substrate is caused by the temperature rising process in the nitriding treatment. That is, if the temperature rise process can be performed while realizing a temperature distribution in which the temperature difference between the temperature in the central portion and the temperature in the edge portion is small, it is possible to suppress color unevenness while suppressing "thermal runaway". it is conceivable that. Therefore, in the present embodiment, the temperature rising process in the nitriding treatment is devised. In the following, the technical idea of this embodiment with this ingenuity will be described.

<Basic idea in the embodiment>
The basic idea of the present embodiment is to heat the compact while maintaining a temperature distribution with a small temperature difference between the central portion and the edge portion by suppressing a rapid temperature rise in the nitriding treatment. In other words, the basic idea is to suppress the rapid temperature rise to the extent that the time required to sufficiently reduce the temperature difference between the temperature at the center and the temperature at the edge can be secured.

According to this basic concept, the temperature can be gradually raised while reducing the temperature difference between the temperature in the central portion and the temperature in the edge portion. Insufficient nitridation at the edges can be overcome while being suppressed.

In particular, the basic idea is that the average amount of temperature rise per unit time (hereinafter referred to as the slope of the heating temperature) is less than or equal to a predetermined value in the range from 1270° C. to 1340° C. in the temperature raising process in the nitriding process. It is devised so that For example, the average gradient of the heating temperature is preferably 3.1° C./h or less. Also, the maximum heating temperature is preferably 1390° C. or higher and 1500° C. or lower.

Thereby, according to the basic idea, it is possible to raise the temperature slowly enough to ensure the time necessary for the temperature difference between the temperature of the central part and the temperature of the edge part to be sufficiently relaxed. From this, according to the basic idea, as a result of eliminating insufficient nitridation at the edge of the compact, it is possible to reduce color unevenness that occurs on the surface of the finally manufactured silicon nitride substrate. Therefore, according to the basic idea, it is possible to reduce erroneous detection in visual inspection due to color unevenness, and as a result, it is possible to obtain a remarkable effect of improving the detection accuracy of foreign matter or dirt in visual inspection.

<Specific embodiment>
A specific embodiment of this basic idea will be described below.

Silicon nitride substrates of Example 1, Example 2, and Comparative Example were manufactured by the method described in "<Method for Manufacturing Silicon Nitride Substrate in Embodiment>". Each silicon nitride substrate had a first surface and a second surface, and had a rectangular planar shape. Each side of each silicon nitride substrate had a long side of 200 mm and a short side of 140 mm. Each silicon nitride substrate had a thickness of 0.32 mm.

When manufacturing each silicon nitride substrate, the molar ratio of the rare earth oxide was 1.2 mol % and the molar ratio of the magnesium compound was 9.8 mol % with respect to the total of silicon, rare earth oxide and magnesium compound.

When manufacturing the silicon nitride substrate of Example 1, in the nitriding step S204, the heating temperature was raised to the maximum heating temperature in such a manner that the heating temperature was raised stepwise with the lapse of the heating time. The maximum heating temperature was 1400°C. The average heating temperature gradient in the heating range from 1270°C to 1340°C was 2.99°C/h. FIG. 7 shows 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. The horizontal axis in FIG. 7 indicates the elapsed time from the reference time when the measured temperature of the furnace, which is the heating temperature, reaches around 1300.degree. When the measured temperature of the furnace was around 1300° C., the temperature difference between the measured temperature of the furnace and the temperature of the compact in the furnace was 20° C. or less. In Example 1, a rapid temperature rise of the compact did not occur in the nitriding treatment, and "thermal runaway" did not occur. The thermal conductivity of the silicon nitride substrate of Example 1 was 129 W/(m·K).

When manufacturing the silicon nitride substrate of Example 2, in the nitriding step S204, the heating temperature was raised to the maximum heating temperature in such a manner that the heating temperature was raised stepwise with the lapse of the heating time. The maximum heating temperature was 1400°C. The average heating temperature gradient in the temperature rising range from 1270°C to 1340°C was 3.02°C/h. FIG. 8 shows 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. The horizontal axis in FIG. 8 indicates the elapsed time from the reference time when the measured temperature of the furnace, which is the heating temperature, reaches around 1300.degree. When the measured temperature of the furnace was around 1300° C., the temperature difference between the measured temperature of the furnace and the temperature of the compact in the furnace was 20° C. or less. In Example 2, a rapid temperature rise of the compact did not occur during the nitriding treatment, and "thermal runaway" did not occur. The thermal conductivity of the silicon nitride substrate of Example 2 was 126 W/(m·K).

When manufacturing the silicon nitride substrate of the comparative example, in the nitriding step S204, the heating temperature was raised to the maximum heating temperature in such a manner that the heating temperature was raised step by step with the lapse of the heating time. The maximum heating temperature was 1400°C. The average heating temperature gradient in the heating range from 1270°C to 1340°C was 4.67°C/h. FIG. 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 FIG. 9 indicates the elapsed time from the reference time when the measured temperature of the furnace, which is the heating temperature, reaches around 1300.degree. When the measured temperature of the furnace was around 1300°C, the temperature difference between the measured temperature of the furnace and the temperature of the compact in the furnace exceeded 20°C and reached a maximum of 44.9°C. In other words, in the comparative example, the temperature of the molded body abruptly increased during the nitriding treatment, and "thermal runaway" occurred. The thermal conductivity of the silicon nitride substrate of the comparative example was 120 W/(m·K).

In the following, the verification result that the silicon nitride substrate manufactured under the heating conditions shown in Example 1 or Example 2 can reduce color unevenness will be described.

<Verification result>
<< Quantitative evaluation method of color unevenness >>
First, a quantitative evaluation method for color unevenness will be described.

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

FIG. 10 is a schematic diagram showing a stereoscopic image of the "color space".

The configuration parameters of the "color space" shown in FIG. 10 include the following parameters.

<<< Brightness L ^* >>>
“Brightness L ^* ” is an index indicating the brightness of color tone. “Lightness L ^* ” takes a value in the range of 0≦L ^* ≦100, and as the value of “lightness L ^* ” increases, the color tone becomes brighter and whitish. On the other hand, when the value of "lightness L ^* " becomes small, the color tone becomes darker and takes on a black tint.

<<<Chromaticness index a ^* >>>
"Chromaticness index a ^* " is an index that indicates the degree of color tone from red to green. "Chromaticness index a ^* " takes a value in the range of -60≤a ^* ≤+60. As the value of the "chromaticness index a ^* " increases in the positive direction, the color tone becomes red. On the other hand, when the value of the "chromaticness index a ^* " increases in the negative direction, the color tone becomes green. The smaller the absolute value of the "chromaticness index a ^* ", the duller the color tone.

<<<Chromaticness index b ^* >>>
"Chromaticness index b ^* " is an index that indicates the degree of color tone from yellow to blue. "Chromaticness index b ^* " takes a value in the range of -60≤b ^* ≤+60. As the value of the "chromaticness index b ^* " increases in the positive direction, the color tone becomes yellow. On the other hand, when the value of the "chromaticness index b ^* " increases in the negative direction, the color tone becomes blue. The smaller the absolute value of the "chromaticness index b ^* ", the duller the color tone.

The above-mentioned "lightness L ^* ", "chromaticness index a ^* " and "chromaticness index b ^* " are measured according to "JIS Z 8722:2000". For example, in the present embodiment, the SCI (including reflected light) method representing the color of the material is used for measurement. At this time, "Konica Minolta CR-400" is used as a measuring instrument, and the measurement is performed with the settings of "field of view: CIE 2° 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, and the surface of the silicon nitride substrate opposite to the side in contact with the PPC paper is measured by the above measuring equipment.

<<<Saturation C ^* >>>
The "chroma C ^* " is calculated based on the "chromaticness index a ^* " and the "chromaticness index b ^* " based on Equation 1 shown below.

C ^* =√{(a ^* ) ² +(b ^* ) ² } Equation 1

<<<color difference ΔE ^* ab>>>
"Color difference ΔE ^* ab" is an index indicating a color difference from a reference color, and is calculated based on "lightness L ^* ", "chromaticness index a ^* ", and "chromaticness index b ^* ".

FIG. 11 is a diagram schematically showing the silicon nitride substrate 100. "Position 1" indicates a region corresponding to the central portion, while "Position 2", "Position 3", "Position 4" and " Position 5" indicates an area corresponding to four edges. In the present embodiment, the "color difference ΔE ^* ab" at "position 2" to "position 5" is calculated using the color at "position 1" as a reference color.

Specifically, “Position 1” to “Position 5” are defined as the positions shown below.
"Position 1": On the surface, the position where the diagonal lines intersect "Position 2": On the surface, the position 15 mm inside from the corner CNR1 in the direction of "Position 1""Position3": On the surface, from the corner CNR2 "Position "Position 4": Position 15 mm inside in the direction of "Position 1" from the corner CNR3 on the surface "Position 5": On the surface, the direction of "Position 1" from the corner CNR4 15mm inside position

The measurement range is an area of 20 mm×20 mm centered on each position of "Position 1" to "Position 5".

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

"Sample #1" is a silicon nitride substrate manufactured by performing nitriding treatment under the heating conditions of Example 1, and "Sample #2" is a silicon nitride substrate manufactured by performing nitriding treatment under the heating conditions of Example 2. A silicon nitride substrate. "Sample #3" and "Sample #4" are silicon nitride substrates manufactured by nitriding under the heating conditions of the comparative example.

Table 1 is a table showing the verification results of color unevenness for "Sample #1" to "Sample #4". As shown in Table 1, a thermal conductivity of 130 W/(m·K) was achieved in any of "Sample #1" to "Sample #4".

However, in "Sample #3" and "Sample #4" showing comparative examples, there are positions where the "color difference ΔE ^* ab" is greater 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 a high thermal conductivity, but the color unevenness becomes conspicuous.

On the other hand, in "Sample #1" representing Example 1, "Color difference ΔE ^* ab" is 1.5 or less at all of "Position 2" to "Position 5".

In addition, in "Sample #2" indicating Example 2, "Color difference ΔE ^* ab" is 0.8 or less at all of "Position 2" to "Position 5".

This means that the silicon nitride substrate manufactured under the heating conditions of the nitriding treatment in Example 1 or Example 2 can obtain high thermal conductivity and can reduce color unevenness. Therefore, according to Examples 1 and 2, it is possible not only to obtain a silicon nitride substrate having excellent heat dissipation characteristics, but also to reduce erroneous detection in visual inspection due to color unevenness.

Furthermore, in Examples 1 and 2, the central portion (position 1) and the edge portions (positions 2 to 5) each had a “lightness L ^* ” of 70 or more and a “chroma C ^* ” of 10. That's it. As a result, the contrast between the foreign matter and dirt becomes clear, and as a result, it is possible to obtain a remarkable effect of improving the foreign matter or dirt detection accuracy in the appearance inspection.

Here, the term “edge portion” as used in this specification is described as a concept including all of the above-described “position 2” to “position 5”. That is, the "edge" as used herein includes all of "Position 2" to "Position 5". Also, the "central part" is described as a concept including "position 1". Therefore, the statement "If ΔE ^* ab is the color difference between the central portion and the edge, ΔE ^* 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. is 1.5 or less.

Similarly, the description that "the brightness of each of the 'central portion' and the 'edge portion' is 70 or more" means that the brightness of each of 'position 1' to 'position 5' is 70 or more. there is

In addition, the description that "the saturation of each of the 'central portion' and the 'edge portion' is 10 or more" means that the saturation of each of 'position 1' to 'position 5' is 10 or more. ing.

<Summary>
In the present embodiment, in the heating process in the nitriding process, the average heating temperature gradient is controlled to 3.1° C./h or less in the range from 1270° C. to 1340° C. As shown in FIG.

Thus, according to the present embodiment, it is possible to obtain a silicon nitride substrate having a "color difference ΔE ^* ab" of 1.5 or less, more specifically 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 the present embodiment has great technical significance in terms of improving the detection accuracy of foreign matter or stains in visual inspection by reducing color unevenness.

The technical idea of the present embodiment is effective when applied to a silicon nitride substrate having a large size. Specifically, the technical concept of the present embodiment is effective when applied to, for example, a configuration in which six semiconductor devices constituting a three-phase inverter circuit are mounted on one silicon nitride substrate, as shown in FIG. is. This is because the use of a silicon nitride substrate having a large size increases the potential for the occurrence of color unevenness.

In this regard, it is considered effective from the viewpoint of reducing the occurrence of color unevenness if the technical idea of the present embodiment is applied to the manufacture of a large-sized silicon nitride substrate having a high potential for occurrence of color unevenness. In particular, the technical idea of the present embodiment is effective when applied to manufacture of a silicon nitride substrate having a rectangular shape with each side having a length of 100 mm or more.

Although the invention made by the present inventor has been specifically described based on the embodiment, the invention is not limited to the above embodiment, and can be variously modified without departing from the gist of the invention. Needless to say.

100 Silicon Nitride Substrate 100A Molded Body 200 Setter 300 Weight INV Inverter Circuit FWD Diode GCC Gate Control Circuit GL Ground Wiring LG1 First Leg LG2 Second Leg LG3 Third Leg MT Three-Phase Induction Motor NT Negative Potential Terminal PT Positive Potential Terminal Q1 Switching Element RT Rotor SA Semiconductor device 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 (5)

A silicon nitride substrate obtained by nitriding a sheet-shaped compact containing silicon,
having a first surface and a second surface opposite to the first surface;
The planar shape is a rectangular shape with each side of 100 mm or more,
On at least one of the first surface and the second surface, if the color difference between the central portion and the edge portion is ΔE ^* ab, the edge portion located 15 mm inward from each corner toward the central portion. 0<ΔE ^* ab≦1.5 for all of
Silicon nitride substrate. In the silicon nitride substrate according to claim 1,
A silicon nitride substrate, wherein 0<ΔE ^* ab≦0.8. In the silicon nitride substrate according to claim 1 or 2,
The silicon nitride substrate, wherein the brightness of each of the central portion and the edge portion is 70 or more. In the silicon nitride substrate according to any one of claims 1 to 3,
The silicon nitride substrate, wherein the saturation of each of the central portion and the edge portion is 10 or more. In the silicon nitride substrate according to any one of claims 1 to 4,
A silicon nitride substrate having a thickness of 0.15 mm or more and 0.8 mm or less.

Images