Ceramic substrate and method for manufacturing same

Abstract

This ceramic substrate is provided with a base material that contains a sintered body of ceramic particles and a coating that contains a sintered body of alpha-alumina particles. The coating covers at least a part of the surface of the base material. The average particle diameter of the alpha-alumina particles is smaller than the average particle diameter of the ceramic particles.

Description

Ceramic substrate and manufacturing method thereof

[0001] The present disclosure relates to a ceramic substrate and a manufacturing method thereof, and more particularly to a ceramic substrate including a base material containing a sintered body of ceramic particles and a manufacturing method thereof.

[0002] Patent Document 1 discloses a technique in which a small amount of silica glass is contained in an alumina substrate itself, a glass coating is formed on the entire surface of the alumina substrate, and an upper electrode, resistor, etc. are formed on the glass coating.

[0003] JP 2017-168749 A

[0004] A ceramic substrate according to one aspect of the present disclosure includes a base material containing a sintered body of ceramic particles (A) and a coating material containing a sintered body of α-alumina particles (B). The coating material covers at least a portion of the surface of the base material. The average particle size of the α-alumina particles (B) is smaller than the average particle size of the ceramic particles (A).

[0005] A method for manufacturing a ceramic substrate according to one aspect of the present disclosure includes disposing the α-alumina particles (B) on the surface of the base material, and firing the α-alumina particles (B).

[0006] According to the present disclosure, it is possible to provide a ceramic substrate that can have improved thermal conductivity and surface smoothness.

[0007] Fig. 1 is a diagram showing a micrograph of a cross section of a ceramic substrate according to an embodiment of the present disclosure. Fig. 2 is a diagram showing a micrograph of a cross section of a ceramic substrate according to an embodiment of the present disclosure. Fig. 3 is a diagram showing a micrograph of a surface of a ceramic substrate according to an embodiment of the present disclosure. Fig. 4 is a schematic cross-sectional view of an electronic device according to an embodiment of the present disclosure. Fig. 5 is a schematic cross-sectional view of a ceramic substrate according to an embodiment of the present disclosure. Fig. 6 is a diagram showing evaluation results of a ceramic substrate according to an embodiment of the present disclosure.

[0008] [Embodiments] Embodiments of the present disclosure will be described with reference to the drawings. Note that the following embodiments are merely a portion of various embodiments of the present disclosure. Furthermore, the following embodiments can be modified in various ways depending on the design, etc., as long as the object of the present disclosure can be achieved. The drawings referred to below are schematic diagrams, and the dimensional ratios of the components in the drawings do not necessarily reflect the actual dimensional ratios. The mechanism of action described below is a conjecture, and the present disclosure is not bound by the explanation of the mechanism of action below.

[0009] <Outline> The ceramic substrate 1 is made of a base material 2 containing a sintered body of ceramic particles (A) and α-alumina (Al ２ O ３ and a coating 3 containing a sintered body of α-alumina particles (B). The coating 3 covers at least a portion of the surface of the base material 2. The average particle size of the α-alumina particles (B) is smaller than the average particle size of the ceramic particles (A). This makes it possible to provide a ceramic substrate 1 that can have improved thermal conductivity and surface smoothness.

[0010] The reason why the ceramic substrate 1 can exhibit the above-mentioned properties has not been clearly clarified, but is thought to be due to the following reasons.

[0011] A substrate 2 used in the manufacture of an electronic device 10, such as that shown in FIG. 4 , which includes a resistor 14, an electrode 12, etc., may not have sufficient smoothness when observed microscopically. As a result, electrostatic discharge (ESD) may occur in the electronic device 10, etc., which includes the substrate 2. In order to improve the smoothness of the surface of the substrate 2, a film with enhanced smoothness may be formed on the surface of the substrate 2. However, depending on the type and physical properties of the material used to form the film, the thermal conductivity of the substrate 2 itself may be impaired, or it may be difficult to form a film with enhanced smoothness.

[0012] In relation to the above, in the present disclosure, at least a portion of the surface of the base material 2 is coated with a coating 3 containing a sintered body of α-alumina particles (B) that have a specific crystal structure among alumina particles that can impart thermal conductivity. Furthermore, the average particle size of the α-alumina particles (B) is adjusted to be smaller than the average particle size of the ceramic particles (A) in the sintered body contained in the base material 2. As a result, the thermal conductivity of the ceramic substrate 1 can be increased and the surface smoothness can be improved.

[0013] The sintered body of the α-alumina particles (B) can be produced by sintering the α-alumina particles (B). When the α-alumina particles (B) are sintered, volume shrinkage is unlikely to occur, and therefore, unevenness is unlikely to occur in the coating 3 containing the sintered body of the α-alumina particles (B), and therefore the smoothness of the ceramic substrate 1 can be further improved.

[0014] As described above, the ceramic substrate 1 has improved thermal conductivity and surface smoothness. Therefore, when the ceramic substrate 1 is used to manufacture an electronic device 10 or the like, the ceramic substrate 1 can improve the thermal conductivity and ESD resistance of the electronic device 10 or the like. In other words, the ceramic substrate 1 can have the function of imparting thermal conductivity and ESD resistance to the electronic device 10 or the like.

[0015] More specifically, the electronic device 10 includes a ceramic substrate 1 with enhanced thermal conductivity. Therefore, even if heat is generated in the resistor 14, electrodes 12, or the like included in the electronic device, the heat can be smoothly transferred to the ceramic substrate 1. Furthermore, the electronic device 10 includes a ceramic substrate 1 with enhanced surface smoothness. This facilitates uniform shapes of the resistor 14 and electrodes 12 formed on the ceramic substrate 1, thereby enabling the resistor 14 and electrodes 12 to be formed without localized discontinuities and reducing unevenness in the film thickness of the resistor 14 and electrodes 12. This reduces the likelihood of breakdown (disconnection) in the resistor 14 and electrodes 12, thereby suppressing electrostatic discharge (ESD) in the electronic device 10. In other words, the electronic device 10 according to this embodiment can have high ESD resistance.

[0016] In the following description, the average particle size of particles is the D50 value determined from the particle size distribution of the particles. Furthermore, the maximum particle size of particles is the D90 value determined from the particle size distribution of the particles. The particle size distribution of particles can be measured by various methods, but one example is a volume-based distribution measured by a laser diffraction / scattering method.

[0017] <Ceramic Substrate> The ceramic substrate 1 will now be described. Fig. 1 is a micrograph obtained when a part of the ceramic substrate 1 is observed under a microscope. Fig. 5 is a schematic cross-sectional view of the ceramic substrate 1 based on the micrograph of the ceramic substrate 1 in Fig. 1.

[0018] (Base Material) The ceramic substrate 1 includes a base material 2. The shape of the base material 2 in a plan view (the shape when viewed from the thickness direction of the base material 2) is, for example, rectangular.

[0019] The substrate 2 contains a sintered body of ceramic particles (A). The average particle size of the ceramic particles (A) is not particularly limited, but is, for example, 0.5 μm or more and 8 μm or less. The average particle size of the ceramic particles (A) is preferably about 3 μm. In this case, the thermal conductivity of the ceramic substrate 1 can be further improved, and the surface smoothness can be further improved. The ceramic particles (A) may contain particles of about 10 μm.

[0020] In this embodiment, the ceramic particles (A) contain alumina particles (A1). That is, the base material 2 contains a sintered body of the alumina particles (A1). In other words, the base material 2 is an alumina sintered body substrate. In this case, the base material 2 can have high thermal conductivity. This can further increase the thermal conductivity of the ceramic substrate 1.

[0021] The content of the alumina particles (A1) in the entire ceramic particles (A) is preferably 96 mass% or more. In other words, the content of the sintered alumina particles (A1) in the entire base material 2 is preferably 96 mass% or more. In this case, the thermal conductivity of the ceramic substrate 1 can be more sufficiently improved. The upper limit of the content of the alumina particles (A1) in the entire ceramic particles (A) is, for example, 100 mass%.

[0022] Furthermore, the crystal structure of the alumina particles (A1) contained in the ceramic particles (A) is not particularly limited, but is preferably α-type. That is, the ceramic particles (A) preferably contain α-alumina particles (A11). In this case, the thermal conductivity of the ceramic substrate 1 can be further improved.

[0023] The surface roughness of the substrate 2 is, for example, several nm to several thousand nm. The surface roughness can be expressed as the maximum height roughness (Rz) measured using an atomic force microscope (AFM). Specifically, the surface roughness (maximum height roughness) of the substrate 2 is approximately 2500 nm.

[0024] As described above, the substrate 2 used to prepare the ceramic substrate 1 may not be sufficiently smooth when observed microscopically. More specifically, when preparing the substrate 2, the shape of the ceramic particles (A) used as the raw material for the substrate 2 may appear on the surface. The ceramic particles (A) may move or deform due to grain growth associated with sintering. Furthermore, the binder used to prepare the substrate 2 may disappear during sintering, resulting in the formation of recesses 21 in the substrate 2. This may impair the surface smoothness of the substrate 2. The shape of the recesses 21 is not particularly specified, but examples include a V-shape (right side of Figure 5). Furthermore, the recesses 21 may have a complex shape at the back (left side of Figure 5).

[0025] (Coating) The ceramic substrate 1 is provided with a coating 3 .

[0026] As described above, recesses 21 may occur in the base material 2, which may impair the surface smoothness of the base material 2. In contrast, the coating 3 can cover the recesses 21 that occur in the base material 2. This can further improve the surface smoothness of the ceramic substrate 1. As a result, the function of imparting ESD resistance to the ceramic substrate 1 can also be further improved. The maximum height roughness (Rz) of the portion of the ceramic substrate 1 where the coating 3 is disposed is, for example, 30 nm or more and 1000 nm or less.

[0027] Furthermore, a plurality of recesses 21 may occur on the surface of the base material 2, and in this case, it is preferable that at least one of the plurality of recesses 21 occurring on the surface of the base material 2 is covered with the coating 3. This can improve the surface smoothness of the ceramic substrate 1. As a result, the function of imparting ESD resistance to the ceramic substrate 1 can be improved. In addition, it is preferable that more of the plurality of recesses 21 occurring on the surface of the base material 2 are covered with the coating 3, and as the number of recesses 21 covered with the coating 3 increases, the smoothness of the surface of the ceramic substrate 1 is further improved. As a result, the function of imparting ESD resistance to the ceramic substrate 1 can also be further improved.

[0028] Furthermore, as described above, recesses 21 having a complex shape at the back side may occur on the surface of the substrate 2 (left side of Figure 5). When such recesses 21 having a complex shape at the back side occur on the substrate 2, even if the substrate 2 is coated with a film for the purpose of smoothing the surface of the substrate 2, the film may not cover the recesses 21 to the back side. As a result, the film may not sufficiently fill the recesses 21, resulting in voids. When the substrate 2 having a film-coated surface has voids as described above, the function of imparting ESD resistance may not be sufficiently ensured. In contrast, in the present disclosure, the average particle diameter of the α-alumina particles (B) is adjusted to be smaller than the average particle diameter of the ceramic particles (A). Therefore, when forming the coating 3 on the recesses 21, the coating 3 can be formed so as to sufficiently fill the recesses 21, even in recesses 21 having a complex shape at the back side. This makes it less likely that voids will occur between the coating 3 and the substrate 2. As a result, the function of imparting ESD resistance to the ceramic substrate 1 can be sufficiently enhanced.

[0029] Furthermore, it is preferable that the maximum thickness of the coating 3 is smaller than the surface roughness of the substrate 2. In other words, it is preferable that the coating 3 does not cover the entire surface of the substrate 2, and that areas of the substrate 2 where there are no recesses 21 are exposed on the surface of the ceramic substrate 1. This allows the high thermal conductivity of the substrate 2 itself to be exhibited, thereby enhancing the thermal conductivity of the ceramic substrate 1. Furthermore, it is preferable that the coating 3 sufficiently fills the recesses 21 formed on the substrate 2, thereby sufficiently enhancing the smoothness of the surface of the ceramic substrate 1 where the coating 3 is disposed. In other words, it is preferable that the coating 3 does not cover the entire surface of the substrate 2, and that areas of the substrate 2 where there are no recesses 21 are exposed on the surface of the ceramic substrate 1, while the smoothness of the surface of the ceramic substrate 1 where the coating 3 is disposed is sufficiently enhanced. In this case, the thermal conductivity of the ceramic substrate 1 can be particularly enhanced, and the function of imparting ESD resistance can be particularly enhanced. It is preferable that the maximum thickness of the coating 3 is 2500 nm or less.

[0030] The particle shape of the α-alumina particles (B) is preferably spherical. In this case, the thermal conductivity of the ceramic substrate 1 can be increased while the surface smoothness can be improved. In the present disclosure, the term "spherical" includes not only a true sphere but also an approximately sphere. The particle shape of the α-alumina particles (B) may be, for example, a polygonal, plate-like, scale-like, or needle-like shape.

[0031] As described above, the coating 3 contains a sintered body of α-alumina particles (B). The content of the sintered body of α-alumina particles (B) in the entire coating 3 is preferably 50% by mass or more and 100% by mass or less. In this case, the thermal conductivity of the ceramic substrate 1 can be further improved, and the surface smoothness can be further improved. The content of the sintered body of α-alumina particles (B) is more preferably 80% by mass or more, and even more preferably 95% by mass or more. The content of the sintered body of α-alumina particles (B) in the entire coating 3 may be 100% by mass.

[0032] As described above, the coating 3 contains a sintered body of α-alumina particles (B). In other words, the raw materials for producing the coating 3 contain α-alumina particles (B). The proportion of α-alumina particles (B) in the entire raw materials for producing the coating 3 is preferably 50 mass% or more. This proportion of α-alumina particles (B) is more preferably 80 mass% or more, and even more preferably 95 mass% or more. The raw materials for producing the coating 3 may also contain β-alumina particles or γ-alumina particles. The proportion of β-alumina particles and γ-alumina particles in the entire raw materials for producing the coating 3 is preferably 50 mass% or less. In this case, the smoothness of the ceramic substrate 1 can be further improved. This is presumably because β-alumina particles or γ-alumina particles may undergo a phase transition during firing, which may cause deformation of the coating 3. However, if the proportion of β-alumina particles and γ-alumina particles is 50 mass% or less, deformation of the coating 3 is suppressed.

[0033] The average particle diameter of the α-alumina particles (B) is preferably 30 nm or more and 1000 nm or less. In this case, the thermal conductivity of the ceramic substrate 1 can be further improved, and the surface smoothness can be further improved. This average particle diameter is more preferably 50 nm or more, and even more preferably 100 nm or more. This average particle diameter is more preferably 900 nm or less, and even more preferably 500 nm or less. In addition, the ratio of the average particle diameter of the α-alumina particles (B) to the average particle diameter of the ceramic particles (A) is preferably 1 / 3 or less. In this case, the thermal conductivity of the ceramic substrate 1 can be further improved, and the surface smoothness can be further improved. This ratio is more preferably 1 / 6 or less, and even more preferably 1 / 20 or less.

[0034] The maximum particle size of the α-alumina particles (B) is preferably 1500 nm or less. In this case, the thermal conductivity and surface smoothness of the ceramic substrate 1 can be further improved. The maximum particle size is more preferably 1000 nm or less, and even more preferably 500 nm or less.

[0035] Commercially available α-alumina particles (B) can be used. Examples of commercially available α-alumina particles (B) include AKP-50 (manufactured by Sumitomo Chemical Co., Ltd.), MXA-150B (manufactured by Sumitomo Chemical Co., Ltd.), TM-DA (manufactured by Taimei Chemical Industry Co., Ltd.), TM-DAR (manufactured by Taimei Chemical Industry Co., Ltd.), and 642991 (manufactured by Sigma-Aldrich). These commercially available products may be used alone or in combination of two or more.

[0036] The coating 3 may contain pores 22 (see FIG. 2 ). In other words, the sintered body of α-alumina particles (B) contained in the coating 3 may contain pores 22. By appropriately adjusting the proportion of pores 22 contained in the coating 3, the density of the coating 3 can be appropriately adjusted. This can ensure sufficient thermal conductivity in the ceramic substrate 1 and enhance the function of the coating 3 in alleviating stress generated in the ceramic substrate 1. Note that the density of the coating 3 in the present disclosure refers to the apparent density. In addition, the density of the coating 3 is preferably lower than the density of the substrate 2. In this case, the proportion of pores 22 in the coating 3 is appropriately adjusted so that the density of the coating 3 is lower than the density of the substrate 2. Therefore, the coating 3 can have appropriate flexibility. As a result, even if stress is generated in the ceramic substrate 1, the coating 3 can particularly alleviate that stress. The density of the coating 3 is preferably 0.9 times or less, more preferably 0.8 times or less, and even more preferably 0.7 times or less, the density of the substrate 2 .

[0037] Furthermore, the coating 3 may contain, in addition to the sintered body of the α-alumina particles (B), a fired body of an inorganic material other than the α-alumina particles (B) (hereinafter also referred to as inorganic material (C)). This can promote or inhibit the generation of pores 22 in the coating 3. As a result, the density of the coating 3 can be adjusted. Examples of the inorganic material (C) include boehmite (AlOOH), zirconia (ZrO ２ ), silica (SiO ２ The inorganic material (C) preferably contains at least one selected from the group consisting of α-alumina particles (B), magnesia (MgO), calcia (CaO), and aluminum nitride (AlN). Among these, the inorganic material (C) preferably contains boehmite. In this case, the boehmite can enhance the sinterability of the α-alumina particles (B). This can prevent the proportion of pores 22 in the coating 3 from increasing excessively, thereby allowing the density of the coating 3 to be appropriately reduced. This allows the ceramic substrate 1 to maintain its enhanced thermal conductivity.

[0038] The boehmite may contain pseudo-boehmite. When pseudo-boehmite is used, it may be applied as a colloid (alumina sol) in which pseudo-boehmite nanoparticles are dispersed in a solvent.

[0039] The shape of the boehmite is not particularly limited, but is preferably granular. In this case, the boehmite can be easily interposed between the α-alumina particles (B). This can further enhance the sinterability of the α-alumina particles (B). This can more appropriately increase the proportion of pores 22 in the sintered body contained in the coating 3. As a result, the enhanced thermal conductivity of the ceramic substrate 1 can be further maintained.

[0040] Commercially available boehmite products can be used, such as 10A (manufactured by Kawaken Fine Chemicals Co., Ltd.), AS-520A (manufactured by Nissan Chemical Industries, Ltd.), AS-2 (manufactured by Kawaken Fine Chemicals Co., Ltd.), and AS-200 (manufactured by Nissan Chemical Industries, Ltd.).

[0041] Cracks 31 may occur in the coating 3 (see FIG. 2). The number of cracks 31 that the coating 3 has is not particularly limited, but for example, it is preferable that the number is small. In this case, the surface smoothness of the ceramic substrate 1 can be sufficiently ensured. When measuring the maximum height roughness (Rz) of the portion of the ceramic substrate 1 where the coating 3 is disposed, a portion of the coating 3 where no cracks 31 occur is selected for measurement.

[0042] For example, the cracks 31 may occur extending from the outer surface of the coating 3 (the surface of the coating 3 opposite to the surface facing the substrate 2) in the thickness direction of the substrate 2. The depth of the cracks 31 in the thickness direction of the substrate 2 is preferably shorter than the maximum length of the α-alumina particles (B). In this case, the surface smoothness of the ceramic substrate 1 can be sufficiently ensured. As a result, the function of imparting ESD resistance to the ceramic substrate 1 can also be sufficiently ensured. Specifically, the depth of the cracks 31 is preferably 100 nm or less, more preferably 50 nm or less, and even more preferably 30 nm or less. The maximum length of the α-alumina particles (B) can be measured by D90 in particle size distribution measurement if the particles are substantially spherical. However, if the particle shape has different major and minor axes, it can be confirmed by a method of visual measurement on an image. Note that the coating 3 preferably does not have cracks 31. In this case, the surface smoothness of the ceramic substrate 1 can be more sufficiently ensured. Therefore, the function of imparting ESD resistance to the ceramic substrate 1 can be more sufficiently ensured.

[0043] As described above, the coating 3 containing the sintered body of α-alumina particles (B) covers at least a portion of the surface of the base material 2 containing the sintered body of ceramic particles (A). This improves the surface smoothness of the ceramic substrate 1. The structure of the coating 3 is not particularly limited as long as it can improve the surface smoothness of the ceramic substrate 1. For example, the coating 3 may be in the form of a continuous surface. Alternatively, the coating 3 may be in the form of a discontinuous surface. Note that the coating 3 is preferably in the form of a continuous surface, in which case the surface smoothness of the ceramic substrate 1 tends to be more easily improved. This can improve the function of imparting ESD resistance to the ceramic substrate 1.

[0044] It is preferable that the surface of the substrate 2 has a portion that is not covered with the coating 3 (hereinafter, also referred to as an uncoated portion 23) (see FIG. 3). In other words, by covering the recesses 21 with the coating 3 and forming the uncoated portion 23 in the portion of the ceramic substrate 1 where the recesses 21 do not exist, it is preferable that the maximum height roughness (Rz) of the portion of the ceramic substrate 1 where the coating 3 is disposed is adjusted to be smaller than the maximum height roughness (Rz) of the substrate 2. In this case, the high thermal conductivity of the substrate 2 itself tends to be easily exhibited. This can further increase the thermal conductivity of the ceramic substrate 1. It is preferable that the uncoated portion 23 refers to the portion of the surface of the substrate 2 on which the coating 3 is provided that is not covered with the coating 3.

[0045] The ratio of the area of ​​the uncoated portion 23 to the entire surface of the base material 2 on which the coating 3 is provided is preferably 5% or more and 70% or less. When this area ratio is 5% or more, the thermal conductivity of the ceramic substrate 1 can be further improved. When this area ratio is 70% or less, the surface smoothness of the ceramic substrate 1 can be sufficiently ensured.

[0046] <Method for Manufacturing Ceramic Substrate> A method for manufacturing the ceramic substrate 1 will be described.

[0047] Ceramic substrate 1 is produced by disposing α-alumina particles (B) on the surface of base material 2 and firing the disposed α-alumina particles (B). In this manner, coating 3 contains α-alumina particles (B) fired on the surface of base material 2. In this embodiment, a method for producing ceramic substrate 1 will be described, in which a coating liquid containing α-alumina particles (B) (hereinafter also referred to as coating liquid (X)) is applied onto base material 2, and then the applied coating liquid (X) is dried and then fired.

[0048] (Coating Fluid) The coating fluid (X) will be described. As described above, the coating fluid (X) contains α-alumina particles (B). Furthermore, the coating 3 may contain a calcined body of the inorganic material (C). That is, the coating fluid (X) may contain the inorganic material (C) in addition to the α-alumina particles (B).

[0049] As described above, the inorganic material (C) may contain boehmite. That is, the coating liquid (X) may contain boehmite in addition to the α-alumina particles (B). The mass content ratio of the α-alumina particles (B) to the boehmite in the coating liquid (X) is preferably 100:0 to 80:20. In this case, it is possible to appropriately increase the sinterability of the α-alumina particles (B) while suppressing excessive shrinkage of the coating 3 caused by sintering of the boehmite. This makes it possible to prevent the occurrence of breakage or cracks 31 in the coating 3. It is more preferable that this mass content ratio be 95:5 to 67:33.

[0050] The coating liquid (X) may contain a dispersant. This can improve the dispersibility of the α-alumina particles (B) contained in the coating liquid (X). This allows the α-alumina particles (B) to be distributed evenly across the surface of the substrate 2 when the coating liquid (X) is applied to the substrate 2. As a result, the surface smoothness of the substrate 2 can be further improved. The dispersant preferably contains at least one selected from the group consisting of an acrylic acid-based polymer and a carboxylic acid-based polymer. The dispersant preferably contains an acrylic acid-based polymer, in which case the dispersibility of the α-alumina particles (B) can be further improved. Furthermore, commercially available dispersants can be used. Examples of commercially available dispersants include SD-10 (manufactured by Toagosei Co., Ltd.), A-30SL (manufactured by Toagosei Co., Ltd.), and A6114 (manufactured by Toagosei Co., Ltd.). The method of improving the dispersibility of the α-alumina particles (B) in the coating liquid (X) is not limited to using a dispersant. For example, the dispersibility of the α-alumina particles (B) in the coating liquid (X) can be improved by adjusting the pH of the coating liquid (X). The method for adjusting the pH of the coating liquid (X) is not particularly limited, but examples include methods using chemicals containing nitric acid or ammonia. Furthermore, it is desirable that the chemicals do not contain or contain reduced amounts of alkali metals such as Na or halogen elements such as Cl, so as not to adversely affect the actual electronic device 10. Furthermore, the pH range of the coating liquid (X) is not particularly limited, but is preferably adjusted to, for example, 4 or more and 7 or less.

[0051] The coating liquid (X) may contain a solvent. In this case, the ease of application of the coating liquid (X) can be improved. The ratio of the solvent to the entire coating liquid (X) is preferably 20% by mass or more and 80% by mass or less. In this case, the α-alumina particles (B) can be evenly distributed over the surface of the substrate 2. Therefore, the coating 3 covering the surface of the substrate 2 can be prevented from locally forming low-density or high-density areas in the sintered body of the α-alumina particles (B). This improves the thermal conductivity and surface smoothness of the ceramic substrate 1, while also enhancing the function of the coating 3 in alleviating stress generated in the ceramic substrate 1. Examples of the solvent include water and organic solvents. As the organic solvent, for example, a low-volatility solvent is preferably used, and specifically, terpineol is preferably used.

[0052] As described above, recesses 21 may be formed on the surface of the substrate 2 (see FIG. 5 ). By applying a coating liquid (X) containing α-alumina particles (B) to the substrate 2, the recesses 21 are filled with the coating liquid (X). The coating liquid (X) is then fired to form a coating 3 in the recesses 21. Because the coating 3 can be formed in the recesses 21 so as to fill the recesses 21, the surface smoothness of the ceramic substrate 1 obtained by coating the surface of the substrate 2 with the coating 3 can be improved. The average particle diameter of the α-alumina particles (B) is adjusted to be smaller than the average particle diameter of the ceramic particles (A). Therefore, even if the recesses 21 have a complex shape at their innermost portions, the α-alumina particles (B) can be sufficiently distributed throughout the recesses 21. This reduces the likelihood of voids being formed between the substrate 2 and the coating 3 when the coating liquid (X) is fired to form a coating 3 containing a sintered body of α-alumina particles (B) in the recesses 21. As a result, the function of imparting ESD resistance to the ceramic substrate 1 can be improved.

[0053] Furthermore, by appropriately adjusting the particle size distribution of the α-alumina particles (B), it is possible to facilitate filling of the coating liquid (X) into the recesses 21 formed in the substrate 2. The particle size distribution of the α-alumina particles (B) can be adjusted, for example, by appropriately combining and using a plurality of commercially available products having different particle size distributions.

[0054] (Method of Preparing Coating Liquid) The method of preparing the coating liquid (X) is not particularly limited, but can be prepared by the following method. First, materials such as the α-alumina particles (B) are kneaded using a kneading device to obtain a mixture (M). Next, the mixture (M) is dispersed in the mixture by using a stirring device or the like to disperse the α-alumina particles (B) and the like in the mixture. Then, the dispersed mixture (M) is kneaded again using a kneading device, and the obtained product is used as the coating liquid (X). When kneading using a kneading device, it is preferable to perform kneading while degassing. In this case, it is possible to reduce bubbles generated in the coating liquid (X). This allows the coating liquid (X) to be applied evenly to the substrate 2.

[0055] The kneading device is not particularly limited, but examples thereof include a planetary mixer, a pot mill, etc. The stirring device is not particularly limited, but examples thereof include an ultrasonic homogenizer, a bead mill, a jet homogenizer, etc.

[0056] (Method for manufacturing ceramic substrate using coating liquid) The method for manufacturing ceramic substrate 1 using coating liquid (X) includes applying coating liquid (X) containing α-alumina particles (B) to the surface of base material 2, and firing coating liquid (X). Furthermore, the method for manufacturing ceramic substrate 1 using coating liquid (X) may include removing the solvent, if necessary. The method for manufacturing ceramic substrate 1 will be described in detail below.

[0057] First, a coating liquid (X) containing α-alumina particles (B) is applied to the surface of the substrate 2. Examples of the application method include a coating method in which the substrate 2 is immersed in the coating liquid (X), a coating method using a squeegee, a coating method using a bar coater, a coating method using a spin coater, a coating method by printing, and a coating method by spraying. The coating method by printing includes, for example, at least one method selected from the group consisting of gravure printing, flexographic printing, offset printing, screen printing, and inkjet printing.

[0058] Among the above-mentioned application methods, the application method using a squeegee is preferably used. The method of applying the coating liquid (X) using a squeegee was specifically performed as follows. First, the coating liquid (X) was placed on the substrate 2. Next, the coating liquid (X) was applied to the surface of the substrate 2 by scraping off excess coating liquid (X) placed on the substrate 2 while applying downward force with the squeegee. Note that applying the coating liquid (X) using a squeegee allows the coating liquid (X) to be applied to the surface of the substrate 2 while appropriately suppressing unevenness. This can improve the surface smoothness of the ceramic substrate 1. As a result, the function of imparting ESD resistance to the ceramic substrate 1 can be improved. Furthermore, applying the coating liquid (X) using a squeegee can efficiently fill the recesses 21 present on the surface of the substrate 2 with the coating liquid (X). Therefore, it is possible to reduce the occurrence of voids between the formed coating 3 and the substrate 2. This can increase the thermal conductivity of the ceramic substrate 1. Furthermore, by applying the coating liquid (X) to the substrate 2 using a squeegee, it is possible to easily create areas on the surface of the substrate 2 where the coating liquid (X) does not adhere. Therefore, it is possible to create areas (uncoated areas 23) on the surface of the substrate 2 that are not covered with the coating 3. This allows the high thermal conductivity of the substrate 2 itself to be exhibited. As a result, the thermal conductivity of the ceramic substrate 1 can be increased.

[0059] Next, the solvent is removed from the applied coating liquid (X). Specifically, the substrate 2 coated with the coating liquid (X) is placed in a drying furnace and heated to remove the solvent. The heating conditions are preferably conditions that allow efficient removal of the solvent and prevent the solvent from bumping. Specifically, the heating temperature is, for example, 80°C or higher and 150°C or lower. The heating time is, for example, 0.5 hours or higher and 2 hours or lower. The heating may be performed in an atmospheric atmosphere.

[0060] Next, the coating liquid (X) from which the solvent has been removed is fired. The firing is performed by placing the substrate 2 coated with the coating liquid (X) in an electric furnace. The firing temperature is, for example, 1100°C or higher and 1600°C or lower. By setting the firing temperature to 1100°C or higher, the sintering of the α-alumina particles (B) can be promoted slowly. The densification of the sintered body of the α-alumina particles (B) contained in the coating 3 can be adjusted. This maintains the shape of the coating 3, thereby improving the surface smoothness of the ceramic substrate 1. Furthermore, by setting the firing temperature to 1300°C or higher, the sintering of the α-alumina particles (B) can be promoted appropriately. This allows the proportion of pores 22 contained in the sintered body of the α-alumina particles (B) contained in the coating 3 to be appropriately reduced. As a result, the thermal conductivity of the ceramic substrate 1 can be improved. Furthermore, by setting the firing temperature to 1600°C or lower, excessive sintering of the α-alumina particles (B) can be suppressed. This makes it difficult for unevenness to occur on the coating 3, thereby improving the smoothness of the surface of the ceramic substrate 1. As a result, the function of imparting ESD resistance to the ceramic substrate 1 can be improved.

[0061] The firing time is not particularly limited, but is, for example, from 1 hour to 12 hours. In this case, the thermal conductivity of the ceramic substrate 1 can be sufficiently increased, and the surface smoothness can also be sufficiently increased.

[0062] Depending on the components contained in the coating liquid (X), preliminary baking may be carried out before baking (main baking).

[0063] Furthermore, during firing, a temperature profile may be appropriately set in which the temperature is changed stepwise while repeating steps including temperature increase and firing at a constant temperature.

[0064] Although the method for manufacturing the ceramic substrate 1 has been described above, the method for manufacturing the ceramic substrate 1 is not limited to the above method, and any other suitable method may be employed.

[0065] <Application Example> As described above, the ceramic substrate 1 is applied to the electronic device 10 (see FIG. 4). The ceramic substrate 1 has enhanced thermal conductivity. As a result, the electronic device 10 including the ceramic substrate 1 can have high thermal conductivity.

[0066] Furthermore, the ceramic substrate 1 has an improved surface smoothness, which allows the ceramic substrate 1 to have a function of imparting high ESD resistance, and as a result, the electronic device 10 including the ceramic substrate 1 can have high ESD resistance.

[0067] Specific examples of the electronic device 10 include a chip resistor, a thermistor, and a resistance temperature detector.

[0068] (Configuration of Electronic Device) A specific configuration of the electronic device 10 will be described. The electronic device 10 includes a resistor 14 and a pair of electrodes 12 (upper electrodes). The pair of electrodes 12 are disposed at both ends of the surface of the ceramic substrate 1 on which the coating 3 is disposed. Therefore, the electrodes 12 may be in direct contact with the surface of the base material 2 or in contact with the coating 3. The electrodes 12 contain, for example, copper. A method for disposing the electrodes on the surface of the ceramic substrate 1 includes, for example, sputtering.

[0069] The resistor 14 is disposed on the surface (surface) of the ceramic substrate 1 on which the coating 3 is disposed. Therefore, the electrode 12 may be in direct contact with the surface of the base material 2 or in contact with the coating 3. The resistor 14 is disposed between the pair of electrodes 12 and connected to the pair of electrodes 12. The resistor 14 may be coated with a protective layer. The protective layer may contain, for example, an epoxy resin. The resistor 14 may contain, for example, a metal alloy. The metal may contain at least one metal selected from the group consisting of nickel (Ni), chromium (Cr), aluminum (Al), platinum (Pt), silicon (Si), etc. The method for disposing the resistor 14 on the surface of the ceramic substrate 1 is not particularly limited, but examples include a method of patterning a thin film formed from a metal alloy on the surface of the ceramic substrate 1. Specifically, first, a thin film containing a metal alloy is formed on the surface of the ceramic substrate 1. The method for forming the thin film is not particularly limited, but examples include a sputtering method. Next, the thin film is patterned by photolithography to form resistor 14 from the thin film. Photolithography includes operations such as applying a resist, drying the applied resist, exposing the resist, removing unnecessary portions of the thin film by etching, and peeling off the resist. By this method, resistor 14 can be arranged on the surface of ceramic substrate 1.

[0070] The electronic device 10 may further include a pair of electrodes 13 (lower electrodes). For example, the pair of electrodes 13 are arranged at both end portions of the surface (rear surface) of the ceramic substrate 1 opposite the side on which the coating 3 is arranged. The electronic device 10 may also include a pair of end surface electrodes 15. For example, the pair of end surface electrodes 15 are arranged on both end surfaces of the ceramic substrate 1 so as to be electrically connected to the pair of electrodes 12.

[0071] (Characteristics of Electronic Device) As described above, the electronic device 10 according to this embodiment can have high ESD resistance. Specifically, the values ​​of the ESD breakdown voltage and ESD breakdown current density of the resistor 14 included in the electronic device 10 can be increased. The values ​​of the ESD breakdown voltage and ESD breakdown current density of the electronic device 10 can be measured in accordance with the electrostatic discharge resistance test (AEC-Q200).

[0072] <Examples> The present disclosure will be described in more detail below with reference to examples. The present disclosure is not limited to the following examples, and can be implemented with appropriate modifications, all of which are within the technical scope of the present disclosure.

[0073] In this example, the preparation of the coating liquid (X) for producing the coating 3, the preparation of the ceramic substrate 1, the preparation of the electronic device 10 including the ceramic substrate 1, and the evaluation results of the produced electronic device 10 will be described.

[0074] <<Preparation of Coating Liquid>> First, for each Example, a coating liquid (X) for producing the coating 3 was prepared. Specifically, the coating liquid was prepared as follows. First, materials were placed in a kneading container with the composition (parts by mass) shown in FIG. 6 . Next, the kneading container was set in a planetary mixer (manufactured by Thinky Corporation, product name RE-310), and the materials were kneaded while being degassed to prepare a mixture (M). Next, an ultrasonic homogenizer (manufactured by Mitsui Electric Seiki Co., Ltd., product name UX-600) was used to disperse materials such as α-alumina particles in the mixture (M). Then, the mixture (M) was degassed using the planetary mixer to prepare a coating liquid (X). An aqueous solution whose pH had been adjusted with nitric acid was added to the coating solution (X) so that the solid content (α-alumina and inorganic material) concentration and the pH of the coating solution (X) would be the values ​​shown in FIG. 6 .

[0075] The details of the materials used to prepare the coating liquid (X) are as follows.

[0076] - α alumina particles #1: TM-DAR (manufactured by Daimei Chemical Industry Co., Ltd.). Approximately spherical (particle shape). Average particle size 0.16 μm. Maximum particle size 0.35 μm.

[0077] Alpha alumina particles #2: NXA-150 (manufactured by Sumitomo Chemical Co., Ltd.). Spherical (particle shape). Average particle diameter: 0.18 μm. Maximum particle diameter: 1 μm.

[0078] Inorganic material #1: 10A (Kawaken Fine Chemicals Co., Ltd.). Alumina sol. Granular. Average particle size: 10-15 nm. Al ２ O ３ Converted to 10% by mass.

[0079] Inorganic material #2: AS-520A (Nissan Chemical Co., Ltd.). Alumina sol. Granular. Average particle size: 15 nm. Al ２ O ３ Converted to 20.5% by mass.

[0080] Inorganic material #3: AS-2 (manufactured by JGC Catalysts and Chemicals Co., Ltd.). Plate-like (particle shape). Secondary dispersion diameter: 170 to 200 nm. Al ２ O ３ Converted to 10.5% by mass.

[0081] Inorganic material #4: AS-200 (Nissan Chemical Co., Ltd.). Alumina sol. Feather particle shape. Average particle diameter: 10 nm to 100 nm. Al ２ O ３ Converted to 10.5% by mass.

[0082] Dispersant #1: SD-10 (manufactured by Toagosei Co., Ltd.) A dispersant containing an acrylic acid-based polymer.

[0083] Each of the inorganic materials #1 to #4 is made of alumina sol. ２ O ３ The converted values ​​were converted into solid contents, and the values ​​are shown in FIG.

[0084] <<Preparation of Ceramic Substrate>> The coating liquid (X) prepared by the method described above was used to prepare a ceramic substrate 1. Specifically, the ceramic substrate 1 was prepared by the following method.

[0085] First, the coating liquid (X) was applied to the substrate 2 (alumina sintered body substrate (alumina content: 96% by mass) made of alumina particles having an average particle size of 3 μm) by the method shown in FIG. 6 .

[0086] Next, the substrate coated with the coating liquid (X) was placed in a dryer to dry the solvent. The drying conditions were 80°C in an air atmosphere for 3 hours. Thereafter, the substrate 2 coated with the coating liquid (X) was fired in an electric furnace. The firing conditions were 1280°C in an air atmosphere for 3 hours. In this way, a substrate 2 and a ceramic substrate 1 in which the surface of the substrate 2 was coated with a coating 3 were produced.

[0087] Regarding the application method using a squeegee, the screen printing equipment was modified so that the squeegee came into direct contact with the substrate 2 without using a screen, and a predetermined amount of material was supplied by dropping. Note that, regarding the application method using a squeegee, it was confirmed that an uncoated portion 23 was formed on the ceramic substrate 1.

[0088] The method using a spin coater was carried out using a spin coater (manufactured by Mikasa Co., Ltd.) at a rotation speed of 1500 rpm for 30 seconds.

[0089] In the method using a bar coater, a bar coater (manufactured by OSG System Products Co., Ltd., product name A-BAR) was used, and the distance between the substrate 2 and the bar coater was adjusted so that the wet film thickness was 2 μm.

[0090] The maximum height roughness (Rz) of the surface of the substrate 2 used was 2473 nm. The maximum height roughness (Rz) was measured using an atomic force microscope (manufactured by Hitachi High-Tech Science Corporation).

[0091] The micrograph of Fig. 1 was obtained by observing a cross section of the ceramic substrate 1 obtained in Example 6 with a scanning electron microscope (SEM). The micrograph of Fig. 2 was obtained by observing a cross section of the ceramic substrate 1 obtained in Example 10 with a scanning electron microscope (SEM). The micrograph of Fig. 3 was obtained by observing a surface of the ceramic substrate 1 obtained in Example 6 with a scanning electron microscope (SEM).

[0092] <<Method of Fabricating Electronic Device>> A resistor 14 was fabricated on the surface of the ceramic substrate 1 on which the coating 3 was disposed. Specifically, a thin film made of a NiCrAlSi alloy was fabricated by sputtering, and the thin film was then patterned into a line shape with a line width of 10 to 840 μm by photolithography (resist application, drying, exposure, development, etching, and resist peeling) to form the resistor 14. Then, electrodes 12 made of Cu were formed on both ends of the linear resistor 14, thereby obtaining an electronic device 10 for evaluation. The distance between the electrodes was 1.1 mm.

[0093] The electronic device 10 was fabricated using the substrate 2 (alumina sintered body substrate with an alumina content of 96% by mass) used in Examples 1 to 12, but without disposing a coating 3 on the substrate 2, and was shown as Comparative Example 1.

[0094] <Evaluation> ESD Resistance Evaluation Using the obtained electronic device 10 for evaluation, the values ​​of the ESD breakdown voltage and the ESD breakdown current density were confirmed according to the following method.

[0095] First, in accordance with the electrostatic discharge resistance test (AEC-Q200), a voltage was applied in increments of 0.5 kV between the electrodes 12 of the fabricated electronic device for evaluation, and the voltage at which the resistor 14 broke down (ESD breakdown voltage) was measured. The results are shown in FIG.

[0096] The value of the current density (ESD breakdown current density) measured when the resistor 14 was broken was also measured. The results were compared with the measurement results of Comparative Example 1 and shown in FIG. 6 according to the following criteria.

[0097] A: The ESD current density was significantly higher than that of Comparative Example 1.

[0098] B: A higher ESD current density than Comparative Example 1 was exhibited.

[0099] C: The ESD current density was approximately the same as that of Comparative Example 1.

[0100] Comparing the Example and Comparative Example 1, it was confirmed that the ESD resistance of the electronic device 10 can be improved by coating the alumina sintered body substrate with the coating 3. It was also confirmed that the ESD resistance of the electronic device 10 can be further improved by applying the coating liquid (X) using a squeegee application method or a spin coater application method.

[0101] [Modifications] A description will be given of modifications of the ceramic substrate 1. The modifications are examples of variations in which the configuration of the embodiment is partially changed, added, deleted, or the like.

[0102] The use of the ceramic substrate 1 is not limited to fabricating the electronic device 10. The ceramic substrate 1 can be used to fabricate, for example, at least one selected from the group consisting of heat dissipation members, industrial heat-resistant and chemical-resistant members, wear-resistant parts, and electrical insulating parts.

[0103] The substrate 2 is not limited to an alumina sintered body substrate. That is, the ceramic particles (A) may include aluminum nitride (AlN), zirconia (ZrO ２ ) and silicon nitride (Si ３ Ni ４ In other words, the base material 2 may contain, in addition to the sintered body of alumina particles (A1), a sintered body of at least one type of inorganic particles selected from the group consisting of aluminum nitride, aluminum nitride, silicon nitride, etc.

[0104] The coating solution (X) may contain an additive in addition to the α-alumina particles (B), dispersant, and solvent described in the above embodiment. The additive may contain at least one additive selected from the group consisting of, for example, a binder resin, a sintering aid, and an antifoaming agent.

[0105] As described above, the coating liquid (X) may contain a binder resin. When the coating liquid (X) contains a binder resin, the viscosity of the coating liquid (X) can be adjusted. This makes it easier to apply the coating liquid (X) to the substrate 2. The binder resin contains at least one resin selected from the group consisting of, for example, acrylic resins, cellulose resins, and the like.

[0106] The coating fluid (X) may contain a sintering aid. When the coating fluid (X) contains a sintering aid, the sinterability of the α-alumina particles (B) can be improved. Examples of the sintering aid include calcium oxide (CaO), magnesium oxide (MgO), and titanium oxide (TiO ２ ), manganese oxide (MnO), strontium oxide (SrO), barium oxide (BaO), yttrium oxide (Yb ２ O ３ ) and the like.

[0107] The method for producing the coating 3 on the surface of the substrate 2 is not limited to the method using the coating liquid (X). As a method for disposing the α-alumina particles (B) on the surface of the substrate 2, for example, a method using cold spray or an aerosol deposition method can be applied.

[0108] [Aspects] As is clear from the above-described embodiments, the present disclosure includes the following aspects. In the following, reference numerals are given in parentheses only to clarify the correspondence with the embodiments.

[0109] A ceramic substrate (1) according to a first aspect includes a base material (2) containing a sintered body of ceramic particles (A) and a coating (3) containing a sintered body of α-alumina particles (B). The coating (3) covers at least a portion of the surface of the base material (2). The average particle size of the α-alumina particles (B) is smaller than the average particle size of the ceramic particles (A).

[0110] According to this embodiment, it is possible to provide a ceramic substrate (1) that can have improved thermal conductivity and surface smoothness.

[0111] The ceramic substrate (1) according to the second aspect is the ceramic substrate of the first aspect, wherein the ceramic particles (A) contain alumina particles (A1).

[0112] According to this embodiment, the base material (2) can have high thermal conductivity, which can further increase the thermal conductivity of the ceramic substrate (1).

[0113] The ceramic substrate (1) according to the third aspect is the first or second aspect, in which the coating (3) covers at least one of a plurality of recesses (21) formed on the surface of the base material (2).

[0114] According to this embodiment, the smoothness of the surface of the ceramic substrate (1) is improved.

[0115] The ceramic substrate (1) according to the fourth aspect is any one of the first to third aspects, in which there are areas on the surface of the base material (2) that are not covered with the coating (3).

[0116] According to this embodiment, the high thermal conductivity of the base material (2) itself tends to be exhibited, which can further increase the thermal conductivity of the ceramic substrate (1).

[0117] The ceramic substrate (1) according to a fifth aspect is any one of the first to fourth aspects, wherein the coating (3) has cracks (31). The depth of the cracks (31) in the thickness direction of the base material (2) is shorter than the maximum length of the α-alumina particles (B).

[0118] According to this embodiment, the surface smoothness of the ceramic substrate (1) can be sufficiently ensured, and therefore the function of imparting ESD resistance to the ceramic substrate (1) can also be sufficiently ensured.

[0119] The ceramic substrate (1) according to a sixth aspect is any one of the first to fifth aspects, wherein the density of the coating (3) is lower than the density of the base material (2).

[0120] According to this embodiment, the density of the coating (3) is appropriately adjusted so that the density of the coating (3) is lower than the density of the substrate (2). Therefore, the coating (3) can have an appropriate flexibility. As a result, even if stress occurs in the ceramic substrate (1), the stress can be easily alleviated by the coating (3).

[0121] The ceramic substrate (1) according to a seventh aspect is the ceramic substrate (1) of any one of the first to sixth aspects, wherein the coating (3) is produced by firing α-alumina particles (B) on the surface of the base material (2). That is, the coating (3) includes α-alumina particles (B) fired on the surface of the base material (2).

[0122] According to this embodiment, the thermal conductivity of the ceramic substrate (1) can be sufficiently increased, and the surface smoothness can also be sufficiently increased.

[0123] A method for manufacturing a ceramic substrate (1) according to an eighth aspect is a method for manufacturing a ceramic substrate (1) according to any one of the first to seventh aspects, in which α-alumina particles (B) are arranged on a surface of a base material (2) and the α-alumina particles (B) are fired.

[0124] According to this embodiment, a ceramic substrate (1) can be provided that can have improved thermal conductivity and surface smoothness.

[0125] A method for producing a ceramic substrate (1) according to a ninth aspect is the eighth aspect, in which a coating liquid (X) containing α-alumina particles (B) is applied to the surface of the substrate (2) with a squeegee, thereby disposing the α-alumina particles (B) on the surface of the substrate (2).

[0126] According to this embodiment, the function of imparting ESD resistance to the ceramic substrate (1) can be improved.

[0127] REFERENCE SIGNS LIST 1 ceramic substrate 2 base material 3 coating 21 recess 22 pore 23 uncoated portion 31 crack

Claims

1. A ceramic substrate comprising a base material containing a sintered body of ceramic particles and a coating containing a sintered body of alpha-alumina particles, the coating covering at least a portion of a surface of the base material, and the average particle size of the alpha-alumina particles being smaller than the average particle size of the ceramic particles.

2. The ceramic substrate according to claim 1, wherein the ceramic particles contain alumina particles.

3. The ceramic substrate according to claim 1, wherein the coating covers at least one of a plurality of recesses formed on the surface of the base material.

4. The ceramic substrate according to claim 1, wherein the surface of the base material has areas that are not covered with the coating.

5. The ceramic substrate according to claim 1, wherein the coating has cracks, and the depth of the cracks in the thickness direction of the base material is shorter than the maximum length of the α-alumina grains.

6. The ceramic substrate according to claim 1, wherein the density of the coating is lower than the density of the base material.

7. The ceramic substrate according to claim 1, wherein the coating comprises the alpha alumina particles sintered on the surface of the base material.

8. A method for manufacturing a ceramic substrate according to any one of claims 1 to 7, comprising: arranging the alpha-alumina particles on the surface of the base material; and firing the arranged alpha-alumina particles.

9. The method for manufacturing a ceramic substrate according to claim 8, wherein arranging the alpha-alumina particles on the surface of the substrate includes applying a coating liquid containing the alpha-alumina particles to the surface of the substrate with a squeegee, thereby arranging the alpha-alumina particles on the surface of the substrate.

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