Method for manufacturing silicon nitride substrate

Abstract

The present invention improves the productivity of a silicon nitride substrate in cases where the silicon nitride substrate is produced by subjecting a multilayer body, which is obtained by stacking a plurality of sheet-like molded bodies that contain silicon, to a nitriding treatment. As a means therefor, the present invention employs a method for producing a silicon nitride substrate, wherein a multilayer body, which is obtained by stacking 5 to 20 sheet-like molded bodies that contain silicon and have a sheet form, is nitrided by being conveyed from a carry-in port to a carry-out port of a continuous heating furnace that is provided with the carry-in port and the carry-out port, a heating mechanism that heats the space between the carry-in port and the carry-out port, and a nitrogen supply mechanism that supplies nitrogen to the space between the carry-in port and the carry-out port.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a silicon nitride substrate, and more particularly to a method for producing a silicon nitride substrate in which a plurality of sheet-like molded bodies made mainly of silicon powder are stacked and then nitrided and sintered. [Background technology]

[0002] BACKGROUND ART Ceramic circuit boards in which a silicon nitride substrate is bonded to a metal circuit and a metal heat sink have been used in semiconductor modules, power modules, and the like.

[0003] A known method for manufacturing silicon nitride substrates involves mixing raw materials such as silicon (Si) powder, sintering aids, binders, and solvents to produce a slurry, forming the slurry into a sheet to produce a sheet-like compact (green sheet), and then degreasing, nitriding, and sintering the sheet-like compact to obtain a silicon nitride substrate. In the degreasing, nitriding, and sintering steps, multiple sheets of the sheet-like compact are stacked and then fed into a batch furnace to improve productivity (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-199657 Summary of the Invention [Problem to be solved by the invention]

[0005] However, if the number of stacked sheet-like bodies in the nitriding process is too large, the reaction heat may concentrate, causing a chain reaction and melting of the silicon. This causes problems with the appearance of the silicon nitride substrate produced and problems with insufficient nitriding reaction in the substrate. In other words, there is a limit to how much productivity can be improved by increasing the number of stacked sheet-like bodies during nitriding.

[0006] SUMMARY OF THE INVENTION Accordingly, the present invention has been made with the object of improving the productivity of silicon nitride substrates. [Means for solving the problem]

[0007] In one embodiment, a method for manufacturing a silicon nitride substrate involves nitriding a stack of 5 to 20 sheet-shaped molded bodies containing silicon by transporting them from the inlet to the outlet in a continuous heating furnace equipped with an inlet and an outlet, a heating mechanism for heating the area between the inlet and the outlet, and a nitrogen supply mechanism for supplying nitrogen between the inlet and the outlet. [Effects of the Invention]

[0008] According to the method for manufacturing a silicon nitride substrate of one embodiment, productivity of silicon nitride substrates is improved. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a perspective view showing a schematic configuration of a nitriding furnace according to an embodiment. [Figure 2] 1 is a plan view showing a stack of a plurality of sheet-shaped bodies and a case according to an embodiment. FIG. [Figure 3] FIG. 3 is a cross-sectional view taken along line AA in FIG. 2. [Figure 4] FIG. 2 is a plan view showing an example of a transport mode of a laminate in the embodiment. [Figure 5] FIG. 2 is a plan view showing an example of a transport mode of a laminate in the embodiment. [Figure 6] FIG. 2 is a cross-sectional view showing a laminate made up of a plurality of sheet-shaped molded bodies and a case in a comparative example and examples 1 and 2. [Figure 7] 10 is a plan view showing the conveyance mode of the laminate in the comparative example and examples 1 and 2. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0010] In all the drawings for explaining the embodiments, the same components are generally designated by the same reference numerals, and repeated explanations thereof will be omitted. In order to make the drawings easier to understand, hatching may be used even in plan views and side views.

[0011] <Silicon nitride substrate> The silicon nitride substrate in this embodiment is a silicon nitride substrate having two surfaces as main surfaces, for example, a silicon nitride substrate having a first main surface that is the front surface and a second main surface that is the opposite surface, the back surface. Note that the terms front surface and back surface are used for convenience to distinguish between the surfaces.

[0012] The silicon nitride substrate of this embodiment can be used, for example, as an insulating substrate for use in a power module, which is an electronic device that constitutes an inverter circuit that controls a motor provided in, for example, an electric vehicle, a hybrid electric vehicle, a railroad car, or industrial equipment.

[0013] <Method for manufacturing silicon nitride substrate> Next, a method for manufacturing the silicon nitride substrate of this embodiment will be described.

[0014] (1-1) Preparation of slurry (slurry preparation process) First, rare earth oxide and magnesium compound are added as sintering aids to silicon powder, which is the raw material for the substrate, to prepare raw material powder, which is then pulverized by a method such as media dispersion to prepare a slurry. The raw materials used are described in detail below.

[0015] (a) Silicon The silicon used here can be industrially available grade silicon powder. The purity of the silicon powder is preferably 99% or higher, more preferably 99.5% or higher. The impurity oxygen contained in silicon is one of the factors that inhibit the thermal conduction of silicon nitride substrates obtained by reaction sintering, so it is preferable to have as little of it as possible. Furthermore, the impurity carbon contained in silicon may inhibit the growth of silicon nitride particles in silicon nitride substrates obtained by reaction sintering. As a result, insufficient densification occurs, which is one of the factors that reduces thermal conduction and insulation.

[0016] (b) Rare earth oxides (sintering aids) The rare earth element oxide used here is preferably an oxide of Y, Yb, Gd, Er, Lu, etc., which is easily available and stable as an oxide. Specific examples of rare earth element oxides include Y2O3, Yb2O3, Gd2O3, Er2O3, Lu2O3, etc.

[0017] (c) Magnesium compounds (sintering aids) As the magnesium compound, one or more magnesium compounds containing silicon (Si), nitrogen (N) or oxygen (O) can be used. In particular, it is preferable to use magnesium oxide (MgO), magnesium silicon nitride (MgSiN), magnesium silicide (MgSi), magnesium nitride (MgN), etc.

[0018] (d) Crushing A rare earth element oxide and a magnesium compound are added to silicon powder as sintering aids in a predetermined ratio, followed by the addition of a dispersion medium (organic solvent) and, if necessary, a dispersant, and the mixture is pulverized in a ball mill to produce a slurry (a dispersion of the raw material powder). The types of dispersion medium and dispersant are not particularly limited and can be selected as desired depending on the sheet molding method, etc.

[0019] As the dispersion medium, ethanol, n-butanol, toluene, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), etc. can be used, and as the dispersant, for example, sorbitan ester type dispersant, polyoxyalkylene type dispersant, etc. can be used.

[0020] The pulverization time is not particularly limited, as it varies depending on the milling device used, the amount and properties of the starting materials, etc., but it is preferable to select a time that allows the raw material powder to be sufficiently pulverized and mixed. The pulverization time is preferably, for example, from 6 to 48 hours, and more preferably from 12 to 24 hours. If the pulverization time is too short, sufficient pulverization may not be achieved, and a silicon nitride substrate with the properties required in this embodiment may not be obtained. If the pulverization time is too long, the amount of impurity oxygen may gradually increase, which may reduce the thermal conductivity of the silicon nitride substrate.

[0021] (1-2) Preparation of sheet-shaped molded body (sheet molding process) To the obtained slurry, a dispersion medium, an organic binder, a dispersant, etc. are added as needed, and the mixture is vacuum degassed as needed to adjust the viscosity to within a predetermined range, thereby preparing a slurry for coating.

[0022] The prepared coating slurry is formed into a sheet using a sheet forming machine, cut to a predetermined size, and then dried to obtain a sheet-shaped molded product. The organic binder used to prepare the coating slurry is not particularly limited, but examples include PVB resin (polyvinyl butyral resin), ethyl cellulose resin, and acrylic resin. The amounts of the dispersion medium, organic binder, dispersant, etc. added are preferably adjusted appropriately depending on the coating conditions. The method for forming the coating slurry into a sheet is not particularly limited, but sheet forming methods such as the doctor blade method can be used.

[0023] The sheet-like slurry is then transported to a drying chamber set at a predetermined temperature and humidity, where the solvent is evaporated to form a dried sheet. The coated sheet is then passed through a drying zone where the temperature is gradually raised and the sheet is dried.

[0024] The thickness of the sheet-like molded body formed in the molding process can be adjusted so that the thickness of the silicon nitride substrate finally obtained is the desired thickness, for example, 0.15 mm or more and 0.8 mm or less, and the sheet-like molded body can be cut to a predetermined size using a punching machine or the like, as necessary.

[0025] (1-3) Heating of the molded body (heating process) The resulting sheet-like compact is heated to nitride the silicon contained in the compact and then densify it. This heating process includes a degreasing process to remove the organic binder from the compact, a nitriding process to heat the sheet-like compact in a nitrogen atmosphere to react the silicon (Si) and nitrogen (N) contained in the compact and nitride it, and a sintering process to densify it after nitriding. These processes may be performed sequentially in separate furnaces or continuously in the same furnace.

[0026] In this embodiment, in this heating step, powdered boron nitride (BN) is applied to the first main surface of the sheet-shaped compact to form a boron nitride (BN powder) layer. When multiple sheet-shaped compacts are stacked, this boron nitride (BN) also functions as a separating material that facilitates separation after sintering. Therefore, when multiple sheet-shaped compacts are stacked, boron nitride (BN) is present between the sheet-shaped compacts, and each sintered compact can be easily separated from the stack of sintered compacts obtained after sintering.

[0027] The sheet-shaped compact coated with boron nitride powder (BN powder) is placed in an electric furnace and degreased (to remove organic binders, etc.), then decarbonized in a nitriding furnace at 900°C to 1300°C, heated to a predetermined temperature in a nitrogen atmosphere to nitride, and then sintered in a sintering device. At this time, it is preferable to heat the compact while applying a load of 10 Pa to 1000 Pa. Degreasing is preferably carried out at a temperature of 800°C or less.

[0028] It is preferable to use a BN powder layer with a thickness of about 3 μm to 20 μm as the separating material. The BN powder layer can be formed by spraying, brushing, or screen printing a BN powder slurry onto one surface of each sheet-like compact. The BN powder preferably has a purity of 95% or higher and an average particle size (D50) of 1 μm to 20 μm.

[0029] Here, a stack of sheet-shaped compacts is prepared by stacking a plurality of sheet-shaped compacts so that they are in contact with each other. It is considered that the BN powder layer is present between the sheet-shaped compacts that are stacked one above the other to form the stack. However, in the present application, the stack will be described as being composed of a plurality of sheet-shaped compacts that are stacked in contact with each other.

[0030] The configuration of the nitriding furnace and the configuration of the sheet-like molded body and its laminate that are transported in the nitriding furnace will be described below with reference to FIGS.

[0031] Fig. 1 is a perspective view of a nitriding furnace (continuous heating furnace) 1, with part of the nitriding furnace 1 cut away to show the interior. As shown in Fig. 1, the nitriding furnace 1 has, for example, a cylindrical structure with a rectangular cross section, and a conveyor 4 is disposed inside for transporting a stack 5 of sheet-like compacts in the longitudinal direction of the nitriding furnace 1. The conveyor 4 is, for example, a cylindrical rod supported so as to be rotatable about its axis, which is arranged horizontally, and a plurality of such rods are arranged in the radial direction (the transport direction of the stack 5).

[0032] The nitriding furnace 1 has an inlet 2 through which the laminate 5 is carried in and an outlet 3 through which the laminate 5 is carried out. The nitriding furnace 1 also has a heating mechanism that heats the area between the inlet 2 and the outlet 3, and a nitrogen supply mechanism that supplies nitrogen between the inlet 2 and the outlet 3. In the nitriding process, the laminate 5 is carried into the nitriding furnace 1 through the inlet 2, transported by the conveyor 4, and carried out of the nitriding furnace 1 through the outlet 3.

[0033] While being transported in this manner, each of the multiple sheet-like formed bodies constituting the laminate (degreased body) 5 is nitrided by being heated in a nitrogen atmosphere. That is, the laminate 5 is nitrided by being transported from the inlet 2 to the outlet 3. The inlet 2 and the outlet 3 may each be provided with a shutter that closes except when the laminate 5 is passing through, in order to prevent a drop in the temperature inside the nitriding furnace 1. In FIG. 1, the transport direction of the laminate 5 is indicated by a white arrow.

[0034] In the nitriding step, the nitrogen partial pressure during nitriding is preferably 0.05 MPa or more and 0.7 MPa or less, more preferably 0.07 MPa or more and 0.2 MPa or less. The nitriding temperature is preferably 1350°C or more and 1500°C or less, more preferably 1400°C or more and 1450°C or less. The holding time after heating to the nitriding temperature is preferably 3 hours or more and 12 hours or less, more preferably 5 hours or more and 10 hours or less.

[0035] As shown in Fig. 1, a plurality of laminates 5 are arranged in a first direction (row direction) that intersects with the conveying direction in a plan view. Here, for example, four laminates 5 are arranged in a direction perpendicular to the conveying direction in a plan view. By arranging a plurality of laminates 5 in the row direction and conveying them in this way, it is possible to perform the nitriding treatment on a plurality of laminates 5 at once. Furthermore, by arranging a plurality of rows of a plurality of laminates 5 arranged in the row direction in this way and conveying them in the conveying direction, it is possible to continuously perform the nitriding treatment on a large number of laminates 5.

[0036] 2 and 3 show the specific configuration of the laminate 5 lined up on the conveyor 4 in Fig. 1. Fig. 3 is a cross-sectional view taken along line AA in Fig. 2. As shown in Figs. 2 and 3, the laminate 5 is placed in a case 6 made up of a plate-shaped BN (boron nitride) setter 6a and a BN frame 6b that surrounds the side surface of the laminate 5 on the BN setter 6a. In other words, the laminate 5 is transported on the conveyor 4 while placed in the case 6.

[0037] The planar shape of the sheet-like formed body 5a constituting the laminate 5 is a rectangle having a long side (first long side) and a short side (first short side), that is, a rectangle. The long side of the sheet-like formed body 5a is 100 mm or more, and the short side is 100 mm or more. Here, the long side of the sheet-like formed body 5a is, for example, 257 mm, and the short side is, for example, 190 mm. The shape of the BN setter 6a in plan view is a rectangle having a long side and a short side, that is, a rectangle. The long side of the BN setter 6a is, for example, 280 mm, and the short side is, for example, 210 mm.

[0038] As shown in Fig. 3, four BN setters 6a are stacked at intervals. Here, one laminate 5 is placed on the surface of the lowest BN setter 6a, and another laminate 5 is placed on the surface of the third BN setter 6a from the bottom. In other words, the laminates 5 are stacked in two layers in a plan view. The stacked laminates 5 do not contact each other in the height direction (vertical direction) and are spaced apart at a predetermined interval.

[0039] One of the main features of this embodiment is that the number of sheet-like formed bodies 5a constituting one laminate 5 is within the range of 5 to 20. Specifically, one laminate 5 here is composed of 15 sheet-like formed bodies 5a stacked in contact with each other in the height direction.

[0040] The thickness of one BN setter 6a is, for example, 1 mm or more and 5 mm or less. The thickness of one laminate 5 is, for example, 7 mm. The distance between the laminate 5 and the BN setter 6a above it is, for example, 3 mm or more and 23 mm or less. The shortest distance between overlapping BN setters 6a in the height direction, i.e., the height of the BN frame 6b, is, for example, 10 mm or more and 30 mm or less.

[0041] Here, no laminate 5 is disposed between the second-lowest BN setter 6a and the third-lowest BN setter 6a. One of the main features of this embodiment is that the shortest distance between two stacked laminates 5 in the height direction is 9.0 mm or more and 35.0 mm or less. Specifically, this distance is 27 mm here.

[0042] In this way, here, the laminate 5, which is made up of a plurality of stacked sheet-like bodies 5a, is nitrided at one time. Furthermore, the laminates 5 are spaced apart from one another and then nitrided at one time. This increases the number of sheet-like bodies 5a that are nitrided per unit time, thereby reducing the manufacturing cost of silicon nitride substrates. Here, the number of stacked laminates 5 is two, but depending on the space within the nitriding furnace 1, three or more may be stacked. In this case, too, as described above, it is necessary to ensure that the gap between the overlapping laminates 5 is at least a predetermined distance (9.0 mm or more).

[0043] As described above, by setting the interval between the overlapping laminates 5 in a plan view to 9.0 mm or more, it is possible to prevent the reaction heat from concentrating on the laminates 5 during the heat treatment in the nitriding step, which would cause the sheet-like formed bodies 5a to melt. Furthermore, by setting the interval between the overlapping laminates 5 in a plan view to 35.0 mm or less, it is possible to increase the number of sheet-like formed bodies to be nitrided, even in a nitriding furnace with a relatively low internal space height.

[0044] Furthermore, as shown in Fig. 1, by arranging a plurality of laminates 5 in a direction perpendicular to the conveying direction in a plan view, the manufacturing cost of the silicon nitride substrate can be reduced. Here, as shown in Fig. 4, each laminate 5 is arranged on the conveyor 4 so that its long side direction is along the conveying direction (the direction of the white arrow in Fig. 4). In other words, each of the plurality of laminates 5 arranged in a row is conveyed in the direction along its long side. This makes it possible to maximize the number of laminates 5 (four in this case) arranged in a row on the conveyor 4. Therefore, the number of sheet-like molded bodies 5a that can be nitrided per unit time can be increased, thereby reducing the manufacturing cost of the silicon nitride substrate.

[0045] As shown in FIG. 5, it is also possible to arrange each laminate 5 on the conveyor 4 so that its short side direction is along the transport direction (the direction of the white arrow in FIG. 5). In other words, each of the multiple laminates 5 lined up in a row is transported in a direction along its short side. By arranging the laminates 5 in this manner, it is possible to maximize the number of laminates 5 (four in this example) lined up in a row on the conveyor 4 in the transport direction, i.e., the longitudinal direction of the nitriding furnace 1. Therefore, it is possible to increase the number of sheet-like molded bodies 5a that are nitrided per unit time, thereby reducing the manufacturing cost of silicon nitride substrates.

[0046] Regardless of whether the laminate 5 is arranged in the orientation shown in FIG. 4 or FIG. 5 , the distance L1 between adjacent laminates 5 in the column direction is 10 mm or more. Furthermore, the distance L2 between adjacent laminates 5 in the conveying direction of the laminate 5 is also 10 mm or more. In this way, by making the distances L1 and L2 between adjacent laminates 5 10 mm or more during conveyance in a plan view, it is possible to prevent the reaction heat from concentrating on the laminate 5 during the heat treatment in the nitriding treatment step, which would otherwise cause the sheet-like formed body 5a to melt. It is more preferable that the distances L1 and L2 be 100 mm or more. By making the distances L1 and L2 100 mm or more, it is possible to further prevent the reaction heat from concentrating on the laminate 5 during the heat treatment in the nitriding treatment step, which would otherwise cause the sheet-like formed body 5a to melt. It is even more preferable that the distances L1 and L2 be 200 mm or more. By making the distances L1 and L2 200 mm or more, the reaction heat is concentrated on the laminate 5 during the heating treatment in the nitriding process, which makes it possible to better prevent the sheet-shaped molding 5a from melting and also to prevent deformation due to swelling of the sheet-shaped molding 5a.

[0047] The silicon nitride substrate of this embodiment is obtained by the method described above. The nitriding rate of the silicon nitride substrate is 90% or more. More specifically, the nitriding rate of the silicon nitride substrate is 90% or more in both the central portion SC and the edge portions SE in a plan view of the silicon nitride substrate. The positions of the central portion SC and edge portions SE of the silicon nitride substrate here correspond to the positions of the central portion SC and edge portions SE of the sheet-like molded body 5a shown in FIG. 2. The edge portion SE is, for example, one of the four corners of a rectangular silicon nitride substrate, i.e., a corner portion.

[0048] The nitriding rate of the silicon nitride substrate referred to in this application is calculated based on the following formula (1) from the silicon content of test pieces measuring 10 mm square in plan view cut out from the central region SC and the edge region SE of the silicon nitride substrate, which are measured by X-ray analysis.

[0049] Nitriding rate (%) = 100 - silicon (wt%) (1) For example, when the measured silicon content is 10 mass %, the nitriding rate is 90%, and when the measured silicon content is 0 mass %, the nitriding rate is 100%.

[0050] The silicon nitride substrate thus obtained has, for example, two main surfaces and four side surfaces, and the surface roughness Ra1 of one of the main surfaces, a first main surface, is 0.50 μm or less.

[0051] This silicon nitride substrate is preferably rectangular, with each side measuring 100 mm or more. The thickness of the silicon nitride substrate is preferably 0.15 mm or more and 0.8 mm or less. Silicon nitride substrates have good thermal conductivity and are suitable for the above-mentioned applications, such as power modules. The thermal conductivity is preferably 100 W / m·K or more, more preferably 110 W / m·K or more, and even more preferably 130 W / m·K or more. The method for producing a silicon nitride substrate including the nitriding step described in detail above can improve the purity of silicon nitride, and therefore, it is preferable that a substrate with a thermal conductivity of 110 W / m·K or more can be easily produced.

[0052] By the subsequent sintering step, the laminate 5 becomes a laminate of sintered bodies obtained by sintering the sheet-like compacts 5a. That is, a silicon nitride substrate made of a plurality of sintered bodies is obtained from this laminate.

[0053] The sintered silicon nitride substrate contains β-phase silicon nitride as a main component, a rare earth element, and magnesium. The rare earth element may be in a simple state or may form a compound with other substances. The magnesium contained in the silicon nitride substrate may be in a simple state or may form a compound with other substances.

[0054] (1-4) Other The silicon nitride substrate manufactured as described above preferably has a dense structure with a relative density of 98% or more. If the relative density of the silicon nitride substrate is less than 98%, high thermal conductivity cannot be obtained. Such a dense silicon nitride substrate is less susceptible to voids that hinder thermal conduction, and the silicon nitride substrate of this embodiment in particular preferably has a thermal conductivity of 110 W / m·K or more in the thickness direction.

[0055] The thickness of the silicon nitride substrate is not particularly limited and can be any thickness. For example, when used as an insulating heat dissipation substrate for semiconductor elements or electronic devices, the thickness is preferably 0.05 mm to 2.5 mm, more preferably 0.1 mm to 1 mm, and particularly when used as a silicon nitride circuit substrate for power modules, even more preferably 0.2 mm to 0.7 mm. The thickness of the silicon nitride substrate after sintering can be adjusted to the desired thickness by adjusting the thickness of the sheet molded body in the sheet molding process, taking into account the effect on the thickness during sintering.

[0056] <Effects of this embodiment> In the nitriding step in the nitriding furnace described with reference to Fig. 1, it is conceivable that, for example, 45 to 90 sheet-like molded bodies are stacked in the height direction so that they are in contact with each other and carried into the nitriding furnace, and then heat treatment is performed. The purpose of increasing the number of stacked sheet-like molded bodies in this way is to increase the number of sheet-like molded bodies that are nitrided per unit time and reduce the manufacturing cost of silicon nitride substrates.

[0057] However, as explained in the section on the problem to be solved by the invention, if the number of laminated sheet-like bodies is too large in the nitriding step, the reaction heat may be concentrated, causing a chain reaction and melting of the silicon, which may cause problems with the appearance of the silicon nitride substrate produced and insufficient nitriding reaction in the substrate.

[0058] Another option is to reduce the number of sheets in one stack of sheet-like molded bodies, stack the stacks in two layers with a gap in the height direction, and then carry the stacked stacks into a nitriding furnace for nitriding. However, if the distance between the two stacked stacks is too close, heat may build up during the nitriding process, causing the silicon to melt.

[0059] Therefore, in the method for manufacturing a silicon nitride substrate according to this embodiment, the number of sheet-like bodies 5a constituting one laminate 5 is set to a range of 5 to 20. Furthermore, the distance between overlapping laminates 5 in plan view is set to 9.0 mm or more. This increases the number of sheet-like bodies nitrided per unit time, reducing the manufacturing cost of silicon nitride substrates, while preventing reaction heat from concentrating on the laminate 5 during the heat treatment in the nitriding step and melting the sheet-like bodies 5a. Therefore, productivity of silicon nitride substrates can be improved.

[0060] Furthermore, the nitriding treatment is carried out while transporting a plurality of laminates 5 arranged in a direction intersecting the transport direction, with the spacing between adjacent laminates 5 being 10 mm or more in plan view. This increases the number of sheet-like molded bodies nitrided per unit time, reducing the manufacturing cost of silicon nitride substrates, while preventing the reaction heat from concentrating on the laminates 5 during the heat treatment in the nitriding step and melting the sheet-like molded bodies 5a. This therefore improves the productivity of silicon nitride substrates. (Example)

[0061] The present embodiment will be described in more detail with reference to examples, but the present invention is not limited to these.

[0062] (Slurry preparation process) To silicon powder with a BET specific surface area of ​​2.1 m / g, a median diameter D50 of 8.2 μm, and an oxygen content of 0.3 mass%, 1.2 mol% Y2O3 powder and 9.8 mol% MgSiN2 powder were added as sintering aids relative to the total silicon (silicon nitride equivalent), rare earth oxides (trivalent oxide equivalent), and magnesium compound (MgO equivalent) to obtain a raw powder. A dispersion medium and dispersant were added to this raw powder to make a slurry with a concentration of 42 mass%, and it was milled in a ball mill using silicon nitride balls as media for 24 hours.

[0063] The amount of magnesium compound added is shown in mol% when all magnesium compounds are converted to MgO. The BET specific surface area, median diameter D50, and oxygen content of the silicon powder before milling were measured using a BET specific surface area analyzer using the BET single-point method, a particle size distribution analyzer using the laser diffraction / scattering method, and an oxygen analyzer using the inert gas fusion / non-dispersive infrared absorption method, respectively.

[0064] (Sheet forming process) The resulting slurry was adjusted in concentration by adding a dispersion medium and an organic binder (acrylic resin), and then degassed to form a coating solution in slurry form. This coating slurry was applied to a carrier film by the doctor blade method, and formed into a sheet of 0.38 mm in thickness at a forming speed of 600 mm / min or less, and then cut into a sheet of 257 mm x 191 mm.

[0065] (nitriding process) A boron nitride (BN powder) layer was formed on the main surface of the obtained sheet-like molded body. A stack 5 was produced by stacking 15 sheet-like molded bodies with a boron nitride powder (BN powder) layer (4.5 μm thick) sandwiched between them. In the comparative example and Examples 1 and 2, as shown in FIG. 6, the case 6A is configured by stacking six BN setters 6a at intervals. Here, the first stack 5 is placed on the surface of the lowest BN setter 6a, the second stack 5 is placed on the surface of the third BN setter 6a from the bottom, and the third stack 5 is placed on the surface of the fifth BN setter 6a from the bottom. In other words, the stacks 5 are stacked in three layers in a plan view. The stacked stacks 5 are not in contact with each other in the height direction (vertical direction) and are spaced apart from each other by a gap of 28 mm.

[0066] Using the nitriding furnace shown in FIG. 1, the laminates 5 of the Comparative Example and Examples 1 and 2 were arranged as shown in FIG. 7 and degreased and nitrided as follows. The laminates were designated laminate (1), laminate (2), laminate (3), and laminate (4) from the entrance of the nitriding furnace. The distance L2 between adjacent laminates 5 in the conveying direction of the laminates 5 was 10 mm in the Comparative Example, 100 mm in Example 1, and 200 mm in Example 2. The nitriding furnace used had multiple internal zones. The nitriding furnace measured 330 mm wide, 7200 mm deep, and 110 mm high, and was divided into 16 zones, each 450 mm long. Zones 1 to 13 from the entrance (zones 1 to 13) were the heating area, and zones 14 to 16 (zones 14 to 16) were the cooling area. Here, nitrogen gas supply pipes were provided in each of zones 1 to 16, and internal atmosphere discharge pipes were provided in each of zones 1 to 7. The supply pipes and discharge pipes were arranged below the device and above the device so that they would flow countercurrently to the conveying direction of the sheet-like formed body.

[0067] The nitriding furnace had SiC heaters in zones 1 to 13 as its heating mechanism, and a roller conveyor as its transport mechanism.

[0068] Using this nitriding treatment device, nitriding treatment was carried out under the conditions shown in Table 1. At this time, the holding time indicates the holding time at the nitriding temperature, and the pressure inside the furnace was set to 15 to 20 Pa. In addition, the oxygen concentration inside the nitriding treatment furnace was constantly controlled to 1 ppm or less.

[0069] [Table 1]

[0070] The evaluation results of the sheet-like molded body (nitride body) obtained after the nitriding step are summarized in Table 2. The appearance of the nitride body was observed to confirm whether melting of silicon had occurred. That is, white areas were determined to be silicon nitride where nitriding of silicon had been completed, and black areas were determined to be silicon where nitriding had not been completed. In Table 2, ◯ indicates that no black areas due to melting of silicon were observed in any of the 15 nitride bodies stacked in each layer of the laminate, △ indicates that the proportion of black areas in the nitride body surface area was 1% or less, and × indicates that the proportion of black areas in the nitride body surface area was more than 1%. In addition, the appearance of the nitride body was observed to confirm whether blisters had occurred. That is, protrusions with a height of 100 μm or more relative to the nitride body surface were determined to be blisters. In Table 2, ◯ indicates that no protrusions due to bulging were found in any of the 15 nitride bodies stacked in each layer of each laminate, △ indicates that the proportion of protrusions in the area of ​​the nitride body surface was 5% or less, and × indicates that the proportion of protrusions in the area of ​​the nitride body surface was more than 5%.

[0071] [Table 2]

[0072] In the comparative example, melting of silicon and deformation due to swelling of the substrate were confirmed in the laminates (1) to (4). In Example 1, melting of silicon was not confirmed in the laminates (1) to (4), and swelling of the nitride was confirmed in the first layer of the laminates (1) and (2). In Example 2, melting of silicon was not confirmed in the laminates (1) to (4), and slight swelling of the substrate was confirmed in the first layer of the laminates (1) and (4).

[0073] Therefore, from the viewpoint of suppressing melting of silicon, the interval between adjacent laminated bodies 5 is set to 100 mm or more. Furthermore, from the viewpoint of suppressing melting of silicon and suppressing deformation due to swelling of the nitride, it is more preferable that the interval between adjacent laminated bodies 5 is set to 200 mm or more.

[0074] The invention made by the inventor has been specifically described above based on the embodiments thereof, but it goes without saying that the present invention is not limited to the above-described embodiments and can be modified in various ways without departing from the spirit of the invention.

[0075] The present invention can be widely used in methods for manufacturing silicon nitride substrates. [Explanation of symbols]

[0076] 1. Nitriding furnace (continuous heating furnace) 2 Loading entrance 3 Exit 4 Conveyor 5. Laminate 5a Sheet-shaped molded body 6 cases 6a BN Setter 6b BN frame

Claims

1. a continuous heating furnace including an inlet and an outlet, a heating mechanism for heating the area between the inlet and the outlet, and a nitrogen supply mechanism for supplying nitrogen between the inlet and the outlet, wherein a stack of 5 to 20 sheet-like molded bodies each containing silicon is transported from the inlet to the outlet, thereby nitriding the stack; In a plan view, the stack is transported in a state where a plurality of stacks are arranged in a first direction intersecting a transport direction, In a plan view, the interval between adjacent laminates during transport is 100 mm or more, The laminate is stacked in a plurality of layers in the height direction, The interval between the stacked laminates in the height direction is 9.0 mm or more. A method for manufacturing a silicon nitride substrate.

2. 2. The method for manufacturing a silicon nitride substrate according to claim 1, the planar shape of the laminate is a rectangle having a first long side and a first short side, Each of the stacks arranged in the first direction is transported in a direction along the first long side. A method for manufacturing a silicon nitride substrate.

3. 2. The method for manufacturing a silicon nitride substrate according to claim 1, the planar shape of the laminate is a rectangle having a first long side and a first short side, Each of the plurality of stacks arranged in the first direction is transported in a direction along the first short side. A method for manufacturing a silicon nitride substrate.

4. 2. The method for manufacturing a silicon nitride substrate according to claim 1, the planar shape of the laminate is a rectangle having a first long side and a first short side, The length of the first short side is 100 mm or more. A method for manufacturing a silicon nitride substrate.

5. 2. The method for manufacturing a silicon nitride substrate according to claim 1, The thickness of the sheet-like molded body is 0.05 mm or more and 2.5 mm or less. A method for manufacturing a silicon nitride substrate.

6. 2. The method for manufacturing a silicon nitride substrate according to claim 1, sintering the nitrided laminate to obtain a plurality of laminated sintered bodies; A method for manufacturing a silicon nitride substrate.

7. 2. The method for manufacturing a silicon nitride substrate according to claim 1, The nitriding rate of the nitrided sheet-like molded body is 90% or more. A method for manufacturing a silicon nitride substrate.

8. 7. The method for manufacturing a silicon nitride substrate according to claim 6, the planar shape of the sintered body is a rectangle having a second long side and a second short side, The length of the second short side is 100 mm or more. A method for manufacturing a silicon nitride substrate.

9. 7. The method for manufacturing a silicon nitride substrate according to claim 6, The thickness of the sintered body is 0.15 mm or more and 0.8 mm or less. A method for manufacturing a silicon nitride substrate.

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