Green body and method for producing ceramic sintered body

Abstract

The present invention pertains to a method for producing a ceramic sintered body, the method comprising: an application step for applying boron nitride powder to at least one surface of a green body containing ceramic powder to obtain a green body having a boron nitride layer; a lamination step for laminating a plurality of green bodies having a boron nitride layer to form a laminate; and a firing step for firing the laminate to produce a laminate of a ceramic sintered body, wherein, in a scanning electron microscope observation image of at least 90 μm×120 μm obtained when the green body having a boron nitride layer is observed from the perpendicular direction after blowing air at the boron nitride layer, the ratio of the total area of boron nitride particles having an aspect ratio of greater than 2.5, as calculated by the major diameter / minor diameter of primary particles, to the area of the surface of the green body on which the boron nitride layer is provided is at most 10%.　According to the present invention, a plurality of green bodies can be laminated and fired, and the maximum cross-sectional height Wt of the waviness curve of the obtained sintered body can be reduced.

Description

Method for manufacturing green bodies and sintered ceramic bodies

[0001] The present invention relates to a method for producing a green body and a sintered ceramic body.

[0002] In recent years, ceramic sintered bodies have been widely used as substrates for semiconductor modules and structural components. For example, substrates for power element modules that mount semiconductor elements with high power and large heat generation values ​​require high mechanical strength, high thermal conductivity, and high electrical insulation. Ceramic sintered bodies excel in these properties, and in recent years, ceramic sintered bodies have been widely used as substrates for power element modules.

[0003] Ceramic sintered bodies are produced by firing green bodies whose main component is ceramic powder. The green bodies are fired using an electric furnace or the like, but to reduce production costs, multiple green bodies are stacked and fired simultaneously. When multiple green bodies are stacked and fired, it is common to coat the surfaces of the green bodies with a separating material to facilitate separation of the sintered ceramic bodies after firing. Boron nitride is a commonly used separating material.

[0004] In the manufacturing method of a ceramic sintered body using such a separating material, the state of the applied separating material affects the surface state of the ceramic sintered body, which in turn affects the bonding state between the ceramic substrate and a metal plate when a metallized substrate is formed by bonding the ceramic substrate to a metal plate.

[0005] For example, Patent Document 1 describes that excessive waviness on the surface of a ceramic substrate leads to the generation of voids at the bonding interface between the ceramic substrate and the metal substrate, a decrease in the strength of the bonding interface, and other degradations in bonding quality, thereby degrading the mechanical, thermal, and electrical properties of the circuit board (metallized substrate). It also describes that by applying a boron nitride paste containing hexagonal boron nitride powder, an organic binder, and an organic solvent to the surface of a green body by screen printing, the flat surfaces of the hexagonal boron nitride particles are uniformly arranged so that they are approximately parallel to the surface of the green sheet (green body), i.e., so that the c-axes of the hexagonal boron nitride particles are approximately perpendicular to the surface of the green sheet, thereby reducing the surface waviness (arithmetic mean waviness Wa) of the substrate.

[0006] WO2013 / 054852

[0007] As described in Patent Document 1, reducing the arithmetic mean waviness (Wa) can improve the physical properties of a metallized substrate. However, according to the inventors' investigations, even if the arithmetic mean waviness (Wa) of the surface of a ceramic substrate is small, if there are localized large wavinesses, it is difficult to highly improve the bonding condition between the ceramic substrate and the metal plate. Since the arithmetic mean waviness (Wa) is the average value of the waviness height within the measurement range, even if there are localized large wavinesses, they are smoothed out and cannot be evaluated. Therefore, it was considered necessary to reduce the maximum cross-sectional height (Wt) of the waviness curve in order to highly improve the bonding condition between the ceramic substrate and the metal plate. Therefore, an object of the present invention is to reduce the maximum cross-sectional height (Wt) of the waviness curve of the resulting sintered body while enabling multiple green bodies to be stacked and sintered.

[0008] In order to solve the above problems, the present inventors conducted extensive research. As a result, they found that the state of the boron nitride particles in contact with the green body is important for reducing the maximum cross-sectional height Wt of the waviness curve. Specifically, when boron nitride particles are normally applied to a green body, a boron nitride layer consisting of multiple stacked boron nitride particles is formed on the surface of the green body. Of these, they found that controlling the state of the boron nitride particles near the bottom layer of the boron nitride layer is important for reducing the maximum cross-sectional height Wt of the waviness curve.

[0009] In contrast, when a boron nitride paste containing boron nitride powder, an organic binder, and an organic solvent is applied to the surface of a green body, as in Patent Document 1, it is thought that the boron nitride particles will form aggregates and settle in a disordered manner, similar to the gravitational settling behavior of particles in a general slurry. As a result, at the top of the paste, i.e., at the outermost surface of the boron nitride layer, the flat surfaces of the boron nitride particles are uniformly arranged so that they are approximately parallel to the surface of the green body, but at the bottom of the paste, i.e., the lowermost surface of the boron nitride layer that is in contact with the green body, it is difficult to make the orientation of the boron nitride particles uniform, and particles that are approximately parallel to the surface of the green body and particles that are approximately perpendicular to the surface of the green body are thought to be mixed together in a disordered manner.

[0010] As a result of further investigation, the inventors discovered that it was possible to control the state of the particles near the bottom surface of the boron nitride layer, and that the state of the particles could be confirmed in a scanning electron microscope image taken from a vertical direction after blowing air against the surface of the green body to which the boron nitride particles had adhered, thereby completing the present invention.

[0011] That is, the present invention is a method for producing a ceramic sintered body, comprising: a coating step of applying boron nitride powder to at least one surface of a green body containing ceramic powder to obtain a green body having a boron nitride layer; a lamination step of stacking a plurality of green bodies having the boron nitride layer to form a laminate; and a firing step of firing the laminate to produce a laminate of ceramic sintered bodies, wherein in a scanning electron microscope image of at least 90 μm × 120 μm taken from a vertical direction after blowing air onto the boron nitride layer, the proportion of the total area of ​​boron nitride particles having an aspect ratio, calculated as the major axis / minor axis of the primary particles, of more than 2.5 to the area of ​​the surface of the green body having the boron nitride layer is 10% or less. In a scanning electron microscope image of at least 90 μm × 120 μm observed from a vertical direction after blowing air onto the boron nitride layer, the green body preferably has a total area of ​​boron nitride particles having an aspect ratio, calculated as the long axis / short axis of the primary particles, of 2.5 or less, of 2.0% or more relative to the area of ​​the surface of the green body having the boron nitride layer. The average particle size of the boron nitride powder is preferably 3 μm or more and 25 μm or less. In a scanning electron microscope image of at least 90 μm × 120 μm observed from a vertical direction without blowing air onto the boron nitride layer, the green body preferably has a total area of ​​boron nitride particles having an aspect ratio, calculated as the long axis / short axis of the primary particles, of 2.5 or less, of 10% or more relative to the area of ​​the surface of the green body having the boron nitride layer.

[0012] The present invention also relates to a green body containing ceramic powder, having a boron nitride layer on at least one surface, wherein, in a scanning electron microscope image of at least 90 μm × 120 μm observed from a perpendicular direction after blowing air onto the boron nitride layer, the proportion of the total area of ​​boron nitride particles having an aspect ratio, calculated as the major axis / minor axis of the primary particles, of more than 2.5 to the area of ​​the surface of the green body having the boron nitride layer is 10% or less. Preferably, in a scanning electron microscope image of at least 90 μm × 120 μm observed from a perpendicular direction after blowing air onto the boron nitride layer, the proportion of the total area of ​​boron nitride particles having an aspect ratio, calculated as the major axis / minor axis of the primary particles, of 2.5 or more to the area of ​​the surface of the green body having the boron nitride layer is 2.0% or more. In the green body having the boron nitride layer, in a scanning electron microscope image of at least 90 μm × 120 μm observed from a vertical direction without blowing air onto the boron nitride layer, the ratio of the total area of ​​boron nitride particles to the area of ​​the surface of the green body having the boron nitride layer is preferably 10% or more. Furthermore, the present invention is a silicon nitride substrate having a maximum cross-sectional height Wt of 4.0 μm or less.

[0013] According to the present invention, a ceramic sintered body having a reduced maximum cross-sectional height Wt of the waviness curve can be efficiently obtained.

[0014] 1 is a schematic diagram of a scanning electron microscope image of a green body of the present invention, observed from a vertical direction after blowing air onto the boron nitride layer, and a schematic diagram showing a method for determining the ratio of the total area of ​​boron nitride particles having a specific aspect ratio to the area of ​​the surface of the green body coated with boron nitride, using the obtained scanning electron microscope image of the green body of the present invention.

[0015] The method for producing a ceramic sintered body of the present invention comprises a coating step of applying boron nitride powder to at least one surface of a green body containing ceramic powder to obtain a green body having a boron nitride layer; a lamination step of stacking a plurality of green bodies having the boron nitride layer to form a laminate; and a firing step of firing the laminate to produce a laminate of ceramic sintered bodies, wherein the green body having the boron nitride layer is one in which, in a scanning electron microscope image of at least 90 μm × 120 μm taken from a vertical direction after blowing air onto the boron nitride layer, the proportion of the total area of ​​boron nitride particles having an aspect ratio, calculated as the major axis / minor axis of the primary particles, of more than 2.5 to the area of ​​the surface of the green body to which the boron nitride has been applied is 10% or less.

[0016] The boron nitride layer formed on the surface of the green body is provided to prevent the ceramic sintered bodies from adhering to each other when they are stacked and fired. The boron nitride particles forming the boron nitride layer are preferably hexagonal boron nitride particles. The boron nitride layer may be formed of a single boron nitride particle layer, but is usually formed by stacking multiple boron nitride particle layers.

[0017] In a scanning electron microscope image of at least 90 μm × 120 μm observed perpendicularly after blowing air against the boron nitride layer, the ratio of the total area of ​​boron nitride particles with an aspect ratio (calculated as the long axis / short axis of the primary particles) exceeding 2.5 to the area of ​​the surface of the green body having the boron nitride layer is 10% or less. When blowing air against the boron nitride layer, boron nitride particles in contact with the green body remain on the green body due to their interaction with the green body, while particles not in contact with the green body have a relatively weak interaction and can be largely removed. When observed under a scanning electron microscope in this state, the state of the boron nitride particles near the bottom surface of the boron nitride layer can be confirmed. Specifically, a method of blowing air against the boron nitride layer includes blowing compressed air at 0.7 MPa perpendicularly against the green body at 20 L / min from a vinyl tube with an inner diameter of 6 mm through a flow meter with a valve for 30 seconds.

[0018] After blowing air onto the boron nitride layer, the boron nitride layer (relative to the surface of the green body) is observed perpendicularly using a scanning electron microscope to obtain a scanning electron microscope image of an area of ​​at least 90 μm × 120 μm. The conditions for observation using a scanning electron microscope are not particularly limited, but a measurement magnification of 500 times or more and 1500 times or less is preferred. Furthermore, observation using a backscattered electron image or a secondary electron image is preferred, as this allows the boron nitride particles to be clearly seen.

[0019] Using the scanning electron microscope image thus obtained, the ratio of the total area of ​​boron nitride particles having an aspect ratio, calculated as the major axis / minor axis of the primary particle, of more than 2.5 to the area of ​​the surface of the green body to which the boron nitride has been applied is determined.

[0020] Specific methods for determining the proportion of boron nitride particles with an aspect ratio exceeding 2.5 and the proportion of boron nitride particles with an aspect ratio of 2.5 or less (described later) are described below with reference to Figures 1 and 2. First, a 90 µm x 120 µm area is randomly designated as the measurement range (the area surrounded by the dashed line in Figure 2) from the obtained scanning electron microscope image. Then, the aspect ratio is measured for all boron nitride particles whose entire particles fall within the measurement range. The aspect ratio is calculated as the major axis / minor axis, where the major axis is the length of the longest-spaced straight line connecting the edges of the boron nitride particles and the minor axis is the length of the longest-spaced straight line connecting the edges of the boron nitride particles perpendicular to the major axis. Using the aspect ratio thus determined as a standard, all boron nitride particles within the measurement range are classified into boron nitride particles (3a) having an aspect ratio of 2.5 or less and boron nitride particles (3b) having an aspect ratio of more than 2.5. Here, boron nitride particles partly existing outside the measurement range, i.e., boron nitride particles (3c) lying on the boundary line of the measurement range, are not included in the classification.

[0021] Next, in the scanning electron microscope image, the total area (S1) of all boron nitride particles (3a) present within the measurement range and having an aspect ratio of 2.5 or less, the total area (S2) of all boron nitride particles (3b) present within the measurement range and having an aspect ratio of more than 2.5, and the total area (S3) of the portions of the boron nitride particles (3c) that lie on the boundary lines of all measurement ranges within the measurement range are calculated. Then, excluding the area of ​​the boron nitride particles (3c) that lie on the boundary lines, the proportion (R1) of the area of ​​boron nitride particles in the measurement range that have an aspect ratio of 2.5 or less and the proportion (R2) of the area of ​​boron nitride particles in the measurement range that have an aspect ratio of more than 2.5 are calculated. That is, the total area (10,800 μm) of the measurement range is calculated. ２ ) is S0, R1 (%) can be calculated as follows: R1 (%) = (S1) / (S0-S3) x 100, R2 (%) = (S2) / (S0-S3) x 100. R1 is the ratio of the total area of ​​boron nitride particles having an aspect ratio, calculated as the major axis / minor axis of the primary particles, of 2.5 or less to the area of ​​the surface of the green body having the boron nitride layer in the scanning electron microscope image. R2 is the ratio of the total area of ​​boron nitride particles having an aspect ratio, calculated as the major axis / minor axis of the primary particles, of more than 2.5 to the area of ​​the surface of the green body having the boron nitride layer in the scanning electron microscope image.

[0022] By using such green bodies, it is possible to obtain ceramic sintered bodies with reduced maximum cross-sectional heights Wt of the waviness curves. While the reason for this is unclear, the inventors believe it to be as follows: Boron nitride particles are typically flat, and boron nitride particles with a large aspect ratio in the observed image are arranged on the green body with their flat surfaces nearly perpendicular to the surface of the green body. If such boron nitride particles with a large aspect ratio are present in the boron nitride powder in direct contact with the green body, localized forces will be applied to the surface of the green body when the green bodies are stacked and a load is applied, which can cause localized increases in waviness. In contrast, we believe that by reducing the proportion of boron nitride particles with a large aspect ratio (aspect ratio exceeding 2.5), it is possible to prevent localized increases in waviness and reduce the maximum cross-sectional height Wt.

[0023] In a scanning electron microscope image of at least 90 μm × 120 μm observed from a vertical direction after blowing air onto the boron nitride layer, the ratio of the total area of ​​boron nitride particles having an aspect ratio calculated by the major axis / minor axis of the primary particles exceeding 2.5 to the area of ​​the surface of the green body having the boron nitride layer is more preferably 5.0% or less, and even more preferably 2.0% or less. The smaller the ratio of the total area of ​​boron nitride particles having an aspect ratio calculated by the major axis / minor axis of the primary particles exceeding 2.5 to the area of ​​the surface of the green body having the boron nitride layer, the better, and it may even be 0%.

[0024] In a scanning electron microscope image of at least 90 μm × 120 μm observed from a vertical direction after blowing air onto the boron nitride layer, the proportion of the total area of ​​boron nitride particles with an aspect ratio of 2.5 or less, calculated as the long axis / short axis of the primary particle, is preferably 2.0% or more, more preferably 3.0% or more, and even more preferably 7.0% or more. It can be said that boron nitride particles with a small aspect ratio in the image are arranged on the green body with their flat surfaces approximately parallel to the surface of the green body. When the boron nitride powder in direct contact with the green body is in this state, it becomes easy to prevent localized forces from being applied to the surface of the green body when the green bodies are stacked and a load is applied. In a scanning electron microscope image of at least 90 μm × 120 μm obtained by blowing air onto the boron nitride layer and then observing it from a vertical direction, the upper limit of the proportion of the total area of ​​boron nitride particles having an aspect ratio, calculated as the long axis / short axis of the primary particles, of 2.5 or less is not particularly limited and may be 100%, but from the viewpoint of ease of production and cost, it may be, for example, 50% or less, further 30% or less, and particularly 20% or less.

[0025] In a scanning electron microscope image of at least 90 μm × 120 μm observed from a vertical direction before blowing air onto the boron nitride layer, the green body preferably has an area ratio of boron nitride particles to the area of ​​the surface of the green body having the boron nitride layer of 10% or more, more preferably 20% or more. This makes it easy to separate the sintered body after sintering. In a scanning electron microscope image of at least 90 μm × 120 μm observed from a vertical direction before blowing air onto the boron nitride layer, the upper limit of the area ratio of boron nitride particles to the area of ​​the surface of the green body having the boron nitride layer is not particularly limited and may be 100%, but from the viewpoint of ease of production and cost, it may be, for example, 70% or less, further 50% or less, particularly 40% or less.

[0026] The method for applying the boron nitride powder to the green body is not particularly limited, but examples include application using a sieve, and application by a method in which the powder is adhered to the surface of a rotating roller and then peeled off from the roller and scattered.

[0027] Generally, boron nitride powder contains aggregates of primary particles of boron nitride. If these aggregates are applied to a green body as they are, the boron nitride particles will be arranged in a disordered manner. However, the above method makes it easy to apply the boron nitride particles as single particles and align them.

[0028] When applying the boron nitride powder through a sieve, agglomerates of the boron nitride particles can be removed and the particles can be applied as single particles. The mesh size of the sieve used can be determined appropriately depending on the primary particle size of the boron nitride particles to be applied. The distance between the sieve and the green body is not particularly limited, but is preferably 1 mm to 500 mm, more preferably 10 mm to 300 mm, in order to facilitate uniform and efficient application of the boron nitride powder.

[0029] Electrostatic screen printing, in which a voltage is applied between a sieve and the green body, is also a preferred method. In electrostatic screen printing, the application of a voltage generates an electric field between the electrostatic screen plate (sieve) and the green body, and the boron nitride particles reach the green body along the electric field, making it easy to achieve a uniform coating. Coating by this electrostatic screen printing method can be performed using a commercially available device such as an electrostatic screen printer. Examples of such devices include an electrostatic screen printing device (Berg Industrial Co., Ltd.). The direction of the voltage between the sieve and the green body is not particularly limited, and the green body side may be positive or negative. The voltage between the sieve and the green body is not particularly limited, but from the viewpoint of facilitating the generation of a stable electric field, it is preferably 0.1 kV to 20 kV, and more preferably 1 kV to 10 kV.

[0030] When applying through a sieve or using electrostatic screen printing, the boron nitride powder on the sieve may be electrically charged. By charging the boron nitride powder, the agglomeration of boron nitride particles is suppressed due to electrical repulsion, making it easier to apply the powder to the green body as single particles. The boron nitride powder may be either positively or negatively charged.

[0031] The method for charging the boron nitride powder is not particularly limited, and can be carried out by a known method. A simple method includes, for example, charging by friction between powders or between a powder and an apparatus. The boron nitride powder can be charged by placing it on a sieve and then rubbing it with another object, or by rubbing the boron nitride powder against a stainless steel plate, aluminum plate, or the like after incorporating it into something that can hold the powder, such as a sponge. The powder may also be rubbed against the mesh portion of a sieve or the side of the container over the sieve. The rubbing time is not particularly limited, and may be, for example, from 3 seconds to 100 seconds.

[0032] The boron nitride powder may be positively or negatively charged. The absolute value of the charged potential of the boron nitride immediately before application is 0.30 kV or more, preferably 0.50 kV or more. There are no particular limitations on the upper limit of the absolute value of the charged potential, but if it is too high, handling becomes difficult, so it is preferable to set it to 5.0 kV or less, particularly 3.0 kV or less.

[0033] The amount of charged potential can be measured using a surface electrometer. The charged boron nitride powder on the sieve can be directly measured. When charging the boron nitride powder by rubbing it against a sponge or the like, the amount of charge on the sieve and the boron nitride powder in the sponge form is approximately the same immediately after rubbing, so it is acceptable to measure the boron nitride powder attached to the sponge or the like.

[0034] A method for depositing powder on the surface of a rotating roller and then peeling and scattering the powder from the roller can be achieved, for example, by using an apparatus equipped with a roller at the bottom opening of a powder box containing the powder. In this apparatus, boron nitride powder is loaded into the powder box, and the roller is rotated to deposit the boron nitride powder on the roller surface. The boron nitride powder is then peeled off from the roller surface facing the green body placed below the roller, thereby applying the boron nitride powder to the green body. The method for peeling the boron nitride powder from the roller surface is not particularly limited, and known methods can be used, such as physical action or corona discharge. The method of peeling off the boron nitride powder by corona discharge (corona spraying method) is particularly preferred. This method involves peeling the boron nitride powder from the roller surface and applying it, which reduces the generation of agglomerates during application and allows it to be applied in a state close to single particles. This facilitates controlling the boron nitride particles on the green body and thus the surface roughness. Examples of devices for applying boron nitride powder in this manner include the Nikka K-VII Spray (Nikka Co., Ltd.), which uses a brush roller to remove the powder as a physical action, and the Nikka K-III Spray (Nikka Co., Ltd.), which uses corona discharge to remove the powder.

[0035] The boron nitride used in the present invention is preferably hexagonal boron nitride because it allows for easier separation of the sintered body. The physical properties of the boron nitride powder used in the coating step are not particularly limited, and known boron nitride powders can be used. The average particle diameter D50 of the boron nitride powder is preferably 0.5 to 50 μm, more preferably 3 to 25 μm, and particularly preferably 4 to 20 μm. By setting the particle diameter within the above range, handleability is improved and the occurrence of surface waviness is more easily prevented. The average particle diameter D50 of the boron nitride powder can be measured by a laser diffraction / scattering method.

[0036] In the manufacturing method of the present invention, the amount of boron nitride powder to be applied to the green body is not particularly limited, but the amount of boron nitride in the boron nitride layer is preferably 0.01 mg / cm ２ ~0.60mg / cm ２It is preferable to set the density to 0.05 mg / cm ２ ~0.50mg / cm ２ More preferably, it is 0.10 to 0.40 mg / cm ２ It is more preferable that the boron nitride layer has a too small amount of boron nitride, which limits its effectiveness as a separator. If the boron nitride layer has a too large amount of boron nitride, a large amount of boron nitride powder will remain on the sintered body obtained by firing the green body. If a large amount of boron nitride powder remains, this may adversely affect the bondability with copper and the heat cycle characteristics when the sintered body is used as a heat dissipation substrate or the like.

[0037] In the manufacturing method of the present invention, the boron nitride powder may be applied to only one side of the green body or to both sides of the green body. When applying boron nitride powder to both sides of the green body, only one side may satisfy the above conditions and application method, but it is preferable that both sides satisfy the above conditions and application method.

[0038] The ceramic powder used in the green body of the present invention is not particularly limited as long as it is a ceramic powder used as a raw material for a ceramic sintered body, and examples thereof include silicon nitride, aluminum nitride, titanium nitride, aluminum oxide, titanium oxide, and zirconium oxide. These ceramic powders can be used alone or in combination of two or more. From the viewpoint of obtaining a ceramic sintered body having excellent insulating properties and high thermal conductivity and being used in applications where suppression of surface waviness is particularly important, silicon nitride powder, aluminum nitride powder, aluminum oxide powder, and a mixed powder of aluminum oxide and zirconium oxide are preferred, and silicon nitride powder is more preferred.

[0039] From the viewpoint of sinterability, the specific surface area of ​​the ceramic powder has a lower limit of preferably 1 m ２ / g or more, especially 2m ２ / g or more, especially 5m ２ / g or more, especially 7m ２ / g or more, and the upper limit is preferably 20m ２ / g or less, especially 15m２ The specific surface area of ​​the ceramic powder means the BET specific surface area measured by the BET single-point method using nitrogen gas adsorption.

[0040] The green body may contain, in addition to the ceramic powder, a sintering aid, a binder resin, and the like.

[0041] As the sintering aid, any sintering aid generally used for sintering ceramic powders can be used without any particular limitation, and examples thereof include yttria, magnesia, ceria, silica, calcia, etc. In particular, when the ceramic powder contains silicon nitride powder, Y ２ Si ４ N ６ C, Yb ２ Si ４ N ６ C, Ce ２ Si ４ N ６ C, MgSi ４ N ６ Carbonitride compounds such as C, MgSiN ２ The amount of sintering aid in the green body is preferably 1 to 20 parts by mass, and more preferably 3 to 10 parts by mass, per 100 parts by mass of ceramic powder.

[0042] The binder resin is not particularly limited, but examples thereof include polyvinyl alcohol, polyvinyl butyral, methyl cellulose, alginic acid, polyethylene glycol, carboxymethyl cellulose, ethyl cellulose, acrylic resin, etc. The content of the binder resin in the green body is preferably 1 to 40 parts by mass, more preferably 5 to 30 parts by mass, per 100 parts by mass of the ceramic powder.

[0043] The method for producing the green body is not particularly limited, and the green body may be produced by press molding using a powder obtained by dry mixing the constituent components of the green body, or by wet mixing the constituent components of the green body to produce a slurry for granule production, which is then dried using a spray dryer or the like to produce granules.

[0044] Alternatively, the green body may be produced by wet-mixing the constituent components to prepare a green body molding slurry, and then molding the green body molding slurry into a sheet. The green body molding slurry may be prepared, for example, by weighing out the components in predetermined amounts and stirring and mixing them so that the ceramic powder is dispersed in the dispersion medium. Examples of dispersion devices used for stirring and mixing include ultrasonic dispersion devices, bead mills, ball mills, roll mills, homomixers, ultramixers, dispersing mixers, homomixers, penetrating-type high-pressure dispersion devices, collision-type high-pressure dispersion devices, porous high-pressure dispersion devices, clump-removing-type high-pressure dispersion devices, (collision + penetrating)-type high-pressure dispersion devices, and ultra-high-pressure homogenizers. After stirring and mixing, the green body slurry may be filtered, if necessary, to remove clumps.

[0045] The method for producing a green body from the green body molding slurry is not particularly limited, and known molding methods can be used, but molding by the doctor blade method is preferred from the viewpoint of good sheet thickness uniformity, etc. The obtained green body can be processed into an appropriate size and any shape. For example, it is generally processed into a roughly rectangular parallelepiped shape with a side length of 100 mm to 2000 mm and a thickness of 0.3 mm to 1.2 mm. Note that after molding into a size larger than that used for firing, cutting can be performed to obtain the desired shape. Cutting can be performed before or after drying, which will be described later.

[0046] When obtaining a green body from the green body molding slurry, it is preferable to carry out a drying process as necessary. The drying process is a process for further removing the dispersion medium from the green body, which makes it easier to subsequently sinter the green body. For example, when the dispersion medium is water, the drying process can be carried out by leaving the green body at a temperature of about 30°C to 150°C, and it is preferable to dry the green body so that the water content is 10% or less.

[0047] In the lamination step, a plurality of green bodies are laminated to form a laminate having a boron nitride layer between adjacent green bodies. The number of laminated green bodies may be any number, ranging from several to several tens of green bodies, for example, from 5 to 40, and particularly from 10 to 30.

[0048] When the green bodies contain organic components such as binders, it is preferable to degrease the green body stack prior to firing. The degreasing temperature for degreasing the green body stack is preferably 300°C to 1200°C, more preferably 400°C to 1000°C. Degreasing of the green body stack is typically performed in an atmosphere of an oxidizing gas such as oxygen or air, a reducing gas such as hydrogen, an inert gas such as argon or nitrogen, carbon dioxide, or a mixture of these, or in a humidified gas atmosphere containing a mixture of these gases and water vapor. The degreasing time at the above degreasing temperature can be appropriately selected depending on the type and amount of organic components, such as binders, used in the green bodies, and the degreasing atmosphere, but is typically 30 minutes to 12 hours, preferably 2 hours to 10 hours.

[0049] In the firing step, the laminate is fired to produce a laminate of sintered ceramic bodies. The firing conditions are not particularly limited, and the firing may be carried out under known firing conditions for obtaining sintered ceramic bodies.

[0050] For example, when the ceramic of the ceramic powder is silicon nitride or aluminum nitride, firing is preferably carried out under an inert gas atmosphere. An inert gas atmosphere refers to, for example, a nitrogen atmosphere or an argon atmosphere. The firing pressure is not particularly limited, but because high pressure increases the cost of equipment, etc., firing may be carried out at a pressure of, for example, 10 MPa·G or less, more preferably 3 MPa·G or less, and even more preferably 1 MPa·G or less. Furthermore, when the ceramic of the ceramic powder is silicon nitride, silicon nitride may decompose during firing at low pressures, so a firing temperature of 0 MPa·G or more is preferred. The firing temperature is not particularly limited as long as the desired sintering reaction proceeds. For example, when the ceramic of the ceramic powder is silicon nitride, it can be, for example, 1200°C or higher and 2000°C or lower, and more preferably 1500°C or higher and 1900°C or lower. The firing time is not particularly limited as long as the desired sintering reaction proceeds, but for example, when the ceramic of the ceramic powder is silicon nitride, it is generally about 3 to 20 hours.

[0051] After the firing step, the ceramic sintered body is separated from the stack of ceramic sintered bodies to obtain a ceramic sintered body. In the stack of ceramic sintered bodies, a boron nitride layer is provided between adjacent ceramic sintered bodies, so that the ceramic sintered body can be easily separated from the stack of ceramic sintered bodies.

[0052] Boron nitride powder may remain on the surface of the ceramic sintered body separated from the laminate. Therefore, after separating the ceramic sintered body from the laminate, it is preferable to remove the boron nitride powder remaining on the surface of the ceramic sintered body from the ceramic sintered body. For example, it is preferable to subject the ceramic sintered body separated from the laminate to honing using free abrasive grains. In this case, it is preferable to use alumina abrasive grains with an average particle size of 10 to 100 μm as the free abrasive grains. Furthermore, after honing, the ceramic sintered body may be ultrasonically cleaned or washed with a spray of water to remove the alumina abrasive grains and boron nitride powder. It is preferable that no boron nitride particles are observed on the surface of the ceramic sintered body when observed with a scanning electron microscope. Polishing may or may not be performed after honing.

[0053] The ceramic sintered body separated from the laminate is cut mechanically or by laser, as required, to obtain a ceramic sintered body of a desired size and shape.

[0054] The manufacturing method of the present invention makes it possible to obtain a ceramic sintered body with highly suppressed surface waviness. Specifically, it is possible to obtain a silicon nitride sintered body having a maximum cross-sectional height Wt of the waviness curve of 4.0 μm or less. Furthermore, it is possible to obtain a silicon nitride substrate formed from the silicon nitride sintered body having a maximum cross-sectional height Wt of the waviness curve of 4.0 μm or less. The maximum cross-sectional height Wt is preferably 3 μm or less, and more preferably 2 μm or less. The lower limit of the maximum cross-sectional height Wt is ideally 0 μm, but the maximum cross-sectional height Wt may be 0.1 μm or more.

[0055] The ceramic sintered body obtained by the production method of the present invention can be used for any purpose, but is particularly suitable for substrate applications requiring high suppression of surface waviness, such as various heat dissipation substrates, power module substrates (for automotive applications, electric railway applications, and high-power semiconductor applications), high-frequency circuit boards, LED packages, and optical pickup submounts (for DVDs and CDs).

[0056] The present invention will be described in more detail below using examples and comparative examples. The present invention is not limited to the following examples. The measurements of the various items in the examples and comparative examples were performed by the following methods.

[0057] (1) Evaluation of the boron nitride layer of the green body: After the coating process, compressed air at 0.7 MPa was blown from a vinyl tube with an inner diameter of 6 mm through a valved flowmeter at 20 L / min for 30 seconds in a direction perpendicular to the boron nitride layer. Next, using a scanning electron microscope (JEOL Ltd.: JCM-7000) in backscattered electron imaging mode at 1000x magnification, the boron nitride layer of the green body after the blowing process was observed perpendicular to the plane of the green body, and a scanning electron microscope image was obtained. A 90 μm × 120 μm area was designated from the obtained image, and the aspect ratio calculated from the major axis / minor axis of each primary particle within that area was calculated. The ratio of the total area of ​​boron nitride particles with an aspect ratio of 2.5 or less (R1) and the ratio of the total area of ​​boron nitride particles with an aspect ratio of more than 2.5 (R2) were calculated relative to the area of ​​the surface of the green body bearing the boron nitride layer. Similarly, a portion of the green body was observed with a scanning electron microscope before blowing air onto it, and from the obtained observation image, the area ratio (R3) of boron nitride particles to the area of ​​the surface of the green body having the boron nitride layer was calculated.

[0058] (2) Evaluation of waviness of silicon nitride substrate As the surface waviness of the ceramic sintered body, the filtered center line waviness was measured using a surface roughness meter, and the arithmetic mean waviness Wa and the maximum cross-sectional height Wt of the waviness curve were measured. The measurement conditions were an evaluation length of 4 mm, a measurement speed of 0.3 mm / s, and a cutoff value of 0.8 mm.

[0059] (3) Average particle size of boron nitride powder Boron nitride powder was dispersed in pure water and dispersed for 30 seconds at 300 W using an ultrasonic homogenizer (manufactured by Nippon Seiki Seisakusho: US-600T). The suspension was subjected to wet measurement using a particle size distribution measuring device (manufactured by Microtrac: MT3300EXII) equipped with a standard sample circulator (manufactured by Microtrac: SDC).

[0060] The silicon nitride substrate was manufactured using raw materials containing the following silicon nitride powder and sintering aid. <Silicon nitride powder> - β-phase ratio: 99% - Average particle size D50: 0.9 μm <Sintering aid> - Yttria (manufactured by Shin-Etsu Chemical Co., Ltd.) - Magnesia (manufactured by Ube Materials Co., Ltd.) <Binder and dispersant> - Binder resin: acrylic resin (manufactured by Fujikura Kasei Co., Ltd.) - Dispersant: Cerna D735 (manufactured by Chukyo Yushi Co., Ltd.)

[0061] Example 1 (Preparation of Green Body) 100 parts by weight of silicon nitride powder, 5 parts by weight of yttria, 3 parts by weight of magnesia, 0.5 parts by weight of dispersant, and 22 parts by weight of binder were weighed and mixed in a ball mill using water as a solvent and a resin pot and silicon nitride balls for 48 hours. Next, degassing and viscosity adjustment were performed using a vacuum degasser (manufactured by Sayama Riken Co., Ltd.) to prepare a raw material slurry. The raw material slurry was then molded into a sheet with a width of 750 mm and a thickness of 420 μm using a doctor blade method. The obtained molded body was dried in air at 40° C. to evaporate the solvent, and then cut to a size of 200 mm x 268 mm on the main surface to obtain a green body (green sheet). (Coating Step) A sponge was impregnated with commercially available hexagonal boron nitride powder (average particle size 1 μm: manufactured by Sanwa Materials), and the sponge was rubbed against a SUS plate for at least 10 seconds to obtain hexagonal boron nitride powder charged to 0.85 kV by electrostatic friction. The charged hexagonal boron nitride powder was then placed on a sieve with a mesh size of 200 μm, and the sieve was vibrated to coat (sieve-coated) the charged hexagonal boron nitride powder (average particle size 1 μm) on the green body placed on a table, obtaining a green body with a boron nitride layer. The amount of hexagonal boron nitride powder coated was 0.2 mg / cm. ２In the same manner, hexagonal boron nitride powder was applied to one side of a total of 11 green bodies. (Evaluation of the boron nitride layer of the green body) One green body was extracted from the 11 green bodies after the coating process, and the condition of the bottom layer of the boron nitride layer was confirmed based on the evaluation of the boron nitride layer of the green body described above. Since the 11 green bodies were produced under the same conditions, if the ratio of the total area of ​​boron nitride particles with an aspect ratio of greater than 2.5, calculated by the major axis / minor axis of the primary particles, to the area of ​​the green body surface having the boron nitride layer in the extracted one green body is 10% or less, then the remaining 10 green bodies can be determined to similarly satisfy this requirement. Similarly, if the ratio of the total area of ​​boron nitride particles with an aspect ratio of 2.5 or less, calculated by the major axis / minor axis of the primary particles, to the area of ​​the green body surface having the boron nitride layer in the extracted one green body is 1.0% or more, then the remaining 10 green bodies can be determined to similarly satisfy this requirement. (Lamination Process) Of the green bodies having the boron nitride layer, 10 green bodies, excluding one used for evaluating the boron nitride layer of the green body, were stacked so that the side of one green body having the boron nitride layer was in contact with the side of another green body not having the boron nitride layer, to produce a stack with a separator layer between adjacent green bodies. (Firing Process) The stack was degreased by holding it in an air atmosphere at 400°C for 5 hours. It was then fired in a nitrogen atmosphere at 1830°C for 9 hours under a pressure of 0.8 MPa·G. After firing, the stack was removed, and 10 silicon nitride sintered bodies were separated from the stack. The silicon nitride sintered bodies were easily separated. (Evaluation) All 10 silicon nitride sintered bodies obtained were evaluated for waviness. The evaluation results of the ceramic sintered body with the largest Wt value among the 10 are shown in Table 1. The separability in Table 1 indicates whether or not the 10 silicon nitride sintered bodies could be easily separated from the sintered body after the firing step.

[0062] Examples 2 to 5 Silicon nitride sintered bodies were obtained and evaluated in the same manner as in Example 1, except that the average particle size and coating amount of the boron nitride powder were changed as shown in Table 1.

[0063] Example 6 A silicon nitride sintered body was obtained and evaluated in the same manner as in Example 1, except that the coating step was carried out using the corona spray method described below. (Coating Step) Hexagonal boron nitride powder was charged into the powder filling section of a Nikka K-VII sprayer (manufactured by Nikka Co., Ltd.), and after a trial run until the powder coating was stabilized, the green body was conveyed by a belt conveyor and passed through the spray section, thereby obtaining a green body having a boron nitride layer. The amount of hexagonal boron nitride powder coated was 0.2 mg / cm. ２ The boron nitride discharge speed and the belt conveyor speed were adjusted so that the following was true: In the same manner, the charged hexagonal boron nitride powder was applied to one side of a total of 11 green bodies.

[0064] Example 7 A silicon nitride sintered body was obtained and evaluated in the same manner as in Example 1, except that the coating step was performed using the electrostatic screen printing method described below. (Coating Step) Hexagonal boron nitride powder (average particle size 9 μm) was impregnated into a sponge, and the sponge was rubbed against an SUS plate for 10 seconds or more to obtain hexagonal boron nitride powder charged by electrostatic friction. The charged boron nitride powder was filled onto the electrostatic screen plate of a T-1 type tabletop electrostatic screen printing machine (manufactured by Berg Kogyo Co., Ltd.), a voltage of 3 kV was applied between the electrostatic screen plate and the green body to generate an electrostatic field, and the boron nitride powder was released into the electrostatic field and coated onto the green body, obtaining a green body with a boron nitride layer. The coating amount of hexagonal boron nitride powder was 0.2 mg / cm ２ In the same manner, a total of 11 green bodies were coated on one side with charged hexagonal boron nitride powder.

[0065] Comparative Example 1: A silicon nitride sintered body was obtained and evaluated in the same manner as in Example 1, except that the coating process was performed using the wet coating method described below. (Coating Process) 100 g of boron nitride powder (average particle size D50: 9 μm), 100 g of binder (acrylamide polymer solution), 10 g of dispersant (glycerol monostearate), and 2000 g of water were mixed and stirred for 48 hours to obtain a boron nitride powder slurry. The boron nitride powder slurry was applied to one side of a green body using a spray coater. The green body was then dried at 120°C for 180 minutes to remove the dispersant from the applied boron nitride powder slurry, yielding a green body having a boron nitride layer. Similarly, a total of 11 green bodies were coated on one side with charged hexagonal boron nitride powder.

[0066]

[0067] The evaluation results showed that in Examples 1 to 7, in which green bodies were sintered in which the ratio of the total area of ​​boron nitride particles with an aspect ratio (calculated by the major axis / minor axis of the primary particles) exceeding 2.5 to the area of ​​the surface of the green body having the boron nitride layer was 10% or less, all of the resulting sintered bodies had a maximum cross-sectional height Wt of 4.0 μm or less, and localized large waviness was suppressed. Examples 2 to 4, in which the average particle size of the boron nitride powder was 3 μm or more and 25 μm or less, were particularly able to reduce the maximum cross-sectional height Wt. On the other hand, in Comparative Example 1, in which a green body was sintered in which the ratio of the total area of ​​boron nitride particles with an aspect ratio exceeding 2.5 exceeded 10%, the maximum cross-sectional height Wt exceeded 4.0 μm, and localized waviness was greater than that of the sintered bodies of the Examples.

[0068] In all examples and comparative examples, in scanning electron microscope images of at least 90 μm × 120 μm observed from a vertical direction before blowing air onto the boron nitride layer, the area ratio of boron nitride particles to the area of ​​the surface of the green body having the boron nitride layer was 20% or more, and separation was easy.

[0069] 1 Scanning electron microscope image 1a Measurement range 2 Surface of green body 3 Boron nitride particle 3a Boron nitride particle whose entire particle is within the measurement range and has an aspect ratio of 2.5 or less 3b Boron nitride particle whose entire particle is within the measurement range and has an aspect ratio of more than 2.5 3c Boron nitride particle that falls on the boundary line of the measurement range 3d Boron nitride particle that is outside the measurement range

Claims

1. A method for producing a ceramic sintered body, comprising: a coating step of applying boron nitride powder to at least one surface of a green body containing ceramic powder to obtain a green body having a boron nitride layer; a lamination step of stacking a plurality of green bodies having said boron nitride layers to form a laminate; and a firing step of firing said laminate to produce a laminate of ceramic sintered bodies, wherein in a scanning electron microscope image of at least 90 μm x 120 μm taken from a vertical direction after blowing air onto the boron nitride layer, the green body having said boron nitride layer has a ratio of the total area of ​​boron nitride particles having an aspect ratio, calculated as the major axis / minor axis of the primary particles, of more than 2.5 to the area of ​​the surface of the green body having the boron nitride layer of 10% or less.

2. A method for producing a ceramic sintered body as described in claim 1, wherein, in a scanning electron microscope image of at least 90 μm x 120 μm observed from a vertical direction after blowing air onto the boron nitride layer, the ratio of the total area of ​​boron nitride particles having an aspect ratio, calculated as the long axis / short axis of the primary particles, of 2.5 or less to the area of ​​the surface of the green body having the boron nitride layer is 2.0% or more.

3. The method for producing a ceramic sintered body according to claim 1, wherein the average particle size of said boron nitride powder is 3 μm or more and 25 μm or less.

4. A method for producing a ceramic sintered body according to any one of claims 1 to 3, wherein in a scanning electron microscope image of at least 90 μm x 120 μm observed from a perpendicular direction without blowing air onto the boron nitride layer, the ratio of the total area of ​​boron nitride particles to the area of ​​the surface of the green body having the boron nitride layer is 10% or more.

5. A green body containing ceramic powder, having a boron nitride layer on at least one surface, in which, in a scanning electron microscope image of at least 90 μm x 120 μm observed from a vertical direction after blowing air onto the boron nitride layer, the proportion of the total area of ​​boron nitride particles with an aspect ratio, calculated as the long axis / short axis of the primary particles, exceeding 2.5 to the area of ​​the surface of the green body having the boron nitride layer is 10% or less.

6. A green body according to claim 5, wherein in a scanning electron microscope image of at least 90 μm x 120 μm observed from a vertical direction after blowing air onto the boron nitride layer, the ratio of the total area of ​​boron nitride particles having an aspect ratio, calculated as the long axis / short axis of the primary particles, of 2.5 or less to the area of ​​the surface of the green body having the boron nitride layer is 2.0% or more.

7. A green body according to claim 5 or 6, wherein in a scanning electron microscope image of at least 90 μm × 120 μm observed from a vertical direction without blowing air onto the boron nitride layer, the ratio of the total area of ​​boron nitride particles to the area of ​​the surface of the green body having the boron nitride layer is 10% or more.

8. A silicon nitride substrate having a maximum cross-sectional height Wt of 4.0 μm or less.

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