Method of producing aluminum nitride green body

Abstract

To provide a production method that can stably be carried out when using machining chips resulting from machining by circulating the same to aluminum nitride slurry upon producing an aluminum nitride green body by molding aluminum nitride slurry and machining the molded article, without adversely affecting an eventual product, i.e., an aluminum nitride sintered compact.SOLUTION: A method of producing an aluminum nitride green body, comprising a slurry preparation step of preparing aluminum nitride slurry including aluminum nitride powder, a sintering aid, an organic binder and an organic solvent, a green body molding step of molding the aluminum nitride slurry and removing the organic solvent therefrom to form a green body, a machining step of machining the green body, a circulation step of returning machining chips resulting from the machining step to the slurry preparation step to reuse the same, is characterized by adjusting the concentration of free ammonia in the aluminum nitride slurry dissolving the machining chips, to 100 ppm or below.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a novel method for producing aluminum nitride green bodies. [Background technology]

[0002] Aluminum nitride has high thermal conductivity and excellent insulating properties, so its sintered bodies are used for a variety of purposes, including substrates for mounting semiconductors, and components for semiconductor manufacturing equipment.

[0003] The above-mentioned sintered body is generally obtained by producing an aluminum nitride green body by molding a molding composition containing aluminum nitride powder, a sintering aid, and a molding binder, and then degreasing and firing the aluminum nitride green body. Specifically, in the case of producing an aluminum nitride sintered substrate, the molding composition is dispersed in a solvent to produce an aluminum nitride slurry, which is molded into a sheet shape by, for example, a doctor blade method, and the solvent is removed to obtain a green body, which is then degreased and fired.

[0004] In the above-mentioned method for producing a green body, the green body obtained by removing the solvent is processed to a desired size and shape prior to degreasing and sintering, but a large amount of processing waste of the green body is generated during the process. Conventionally, since the processing waste has the same composition as the green body, a method has been proposed in which the processing waste is circulated to a slurry preparation process for effective use (see Patent Document 1). However, it has been found that when the above-mentioned processing scraps are circulated through an aluminum nitride slurry to prepare the slurry, the processing scraps may not dissolve sufficiently in the slurry, resulting in a phenomenon that adversely affects the surface properties and strength of the aluminum nitride sintered body finally obtained, and that this phenomenon occurs more noticeably in aluminum nitride powder obtained by a manufacturing method that does not include an oxidation step for removing carbon after the synthesis of aluminum nitride, such as a direct aluminum nitridation method or a combustion synthesis method.

[0005] Although it is conceivable to remove the undissolved processing waste by filtration, in this case, the filtering device will be destroyed in an extremely short time, making it difficult to carry out the method on an industrial scale. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 9-87032 DISCLOSURE OF THEINVENTION [Problem to be solved by the invention]

[0007] Therefore, an object of the present invention is to provide a method for producing an aluminum nitride green body by molding and processing an aluminum nitride slurry, in which scraps generated by the above processing can be circulated and reused in the aluminum nitride slurry in a stable manner without adversely affecting the final aluminum nitride sintered body. [Means for solving the problem]

[0008] The inventors have conducted extensive research into the above-mentioned causes, and have found that in the manufacturing process of the green body, it takes a long time, several tens of hours, to thoroughly mix the raw materials to form an aluminum nitride slurry, and that during this process, the aluminum nitride powder present in the aluminum nitride slurry is decomposed due to the influence of moisture present in the environment, and the ammonia generated during this process inhibits the dissolution of the processing chips into the aluminum nitride slurry. Based on this knowledge, the inventors conducted further research and have found that by adjusting the concentration of free ammonia in the aluminum nitride slurry that dissolves the processing chips to a specific range, the processing chips can be reliably dissolved in the aluminum nitride slurry in a short time, the influence of insufficient dissolution of the processing chips on the resulting sintered body can be completely prevented, and the object of the present invention can be achieved, and the present invention has been completed.

[0009] That is, according to the present invention, there is provided a method for producing an aluminum nitride green body, comprising: a slurry preparation step of preparing an aluminum nitride slurry containing aluminum nitride powder, a sintering aid, an organic binder and an organic solvent; a green body forming step of forming the aluminum nitride slurry and removing the organic solvent; a processing step of processing the green body; and a circulation step of returning scraps generated from the processing step to the slurry preparation step for reuse, wherein in the slurry preparation step, the free ammonia concentration in the aluminum nitride slurry in which the scraps are dissolved is adjusted to 100 ppm or less.

[0010] The present invention also provides a method for producing an aluminum nitride sintered body, which comprises degreasing and sintering the aluminum nitride green sheet obtained by the above-mentioned production method. Effect of the Invention

[0011] According to the method of the present invention, in the production of aluminum nitride green bodies by molding and processing an aluminum nitride slurry to produce aluminum nitride green bodies, when the processing waste generated by the above processing is recycled and used in the aluminum nitride slurry, the processing waste can be reliably dissolved in the aluminum nitride slurry, so that a homogeneous green body can be produced. By degreasing and sintering this green body, an aluminum nitride sintered body having good surface properties and stable strength can be stably produced. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The method for producing an aluminum nitride green body of the present invention includes a slurry preparation step of preparing an aluminum nitride slurry containing aluminum nitride powder, a sintering aid, an organic solvent, and an organic binder, a green body forming step of forming the aluminum nitride slurry and removing the organic solvent, a processing step of processing the green body, and a circulation step of returning processing waste generated from the processing step to the slurry preparation step and reusing it.

[0013] As described below, the slurry preparation step generally includes a dispersing device for mixing raw materials including aluminum nitride powder, a sintering aid, an organic solvent, and an organic binder to prepare an aluminum nitride slurry, and may include a degassing tank for degassing the aluminum nitride slurry, and may further include a dissolving tank for dissolving processing waste into the aluminum nitride slurry, as described below.

[0014] The raw material aluminum nitride powder is not particularly limited and any known aluminum nitride powder can be used, but the manufacturing method of the present invention is particularly effective for aluminum nitride powder having a relatively thin surface oxide film obtained by methods such as direct aluminum nitridation or combustion synthesis, specifically, an oxide film having a thickness of 10 Å or less, particularly 8 Å or less, as measured by X-ray photoelectron spectroscopy (XPS).

[0015] The aluminum nitride powder preferably has an average particle size of about 1 to 3 μm, and is preferably pulverized to the above average particle size as required for use. The above average particle size refers to a value measured by a laser diffraction type particle size distribution measuring device.

[0016] As the sintering aid, compounds generally used as sintering aids for aluminum nitride sintered bodies can be used, for example, rare earth compounds such as yttrium oxide, lanthanum oxide, and cerium oxide, alkaline earth metal compounds such as calcium oxide and magnesium oxide, or mixtures of these compounds. Among these sintering aids, yttrium oxide is preferred in that the resulting sintered body is dense and has high thermal conductivity.

[0017] These sintering aids can be used in an amount usually ranging from 2 parts by mass to 6 parts by mass, and preferably from 3 parts by mass to 5 parts by mass, relative to 100 parts by mass of the aluminum nitride powder.

[0018] The organic binder is not particularly limited, and any known organic binder used in the production of aluminum nitride green bodies may be used without any particular restriction. Taking into consideration flexibility, ease of degreasing, etc., polyvinyl butyral (PVB), polyvinyl alcohol (PVA), acrylic resin, etc. are preferably used.

[0019] These organic binders can be used in an amount of usually 5 parts by mass to 20 parts by mass, preferably 6 parts by mass to 15 parts by mass, based on 100 parts by mass of the aluminum nitride powder.

[0020] Furthermore, the organic solvent is not particularly limited, and a general organic solvent can be used. For example, ketones such as acetone, methyl ethyl ketone, and methyl isopropyl ketone, alcohols such as ethanol, propanol, and butanol, aromatic hydrocarbons such as benzene, toluene, and xylene, halogenated hydrocarbons such as trichloroethylene, tetrachloroethylene, and bromochloromethane, or a mixture of these organic solvents can be used. These organic solvents can be used in an amount of usually 20 parts by mass to 200 parts by mass, preferably 40 parts by mass to 150 parts by mass, based on 100 parts by mass of the aluminum nitride powder.

[0021] In the slurry preparation step of the present invention, additives such as surfactants and plasticizers may be used as other components constituting the aluminum nitride slurry. The above surfactants can be used without particular limitations, but nonionic surfactants are preferred. The surfactants can be used in an amount of usually 0.1 to 10 parts by mass, preferably 0.02 to 3.0 parts by mass, per 100 parts by mass of the aluminum nitride powder. Examples of the plasticizer include polyethylene glycol and its derivatives, phthalates such as dimethyl phthalate, dibutyl phthalate, benzyl butyl phthalate, and dioctyl phthalate, stearates such as butyl stearate, tricresol phosphate, tri-n-butyl phosphate, and glycerin. The plasticizers can be used in an amount of usually 0.1 to 20 parts by mass, preferably 0.4 to 15 parts by mass, per 100 parts by mass of the aluminum nitride powder.

[0022] In the slurry preparation process of the present invention, the dispersing device can be a known one. For example, a ball mill, an ultrasonic dispersing device, a bead mill, a roll mill, an ultra mixer, a dispersing mixer, a homo mixer, a penetrating type high-pressure dispersing device, a collision type high-pressure dispersing device, a multi-hole type high-pressure dispersing device, a lump removal type high-pressure dispersing device, a (collision + penetrating) type high-pressure dispersing device, an ultra-high pressure homogenizer, etc. are mentioned. In addition, the mixing time using the dispersing device varies depending on the device, but for example, when a ball mill is used, 5 hours or more, preferably 10 hours or more are recommended.

[0023] The aluminum nitride slurry obtained by the above mixing may be transferred to a degassing tank via a dissolution tank as required, and may be subjected to operations such as degassing and desolvation. In addition, a filtration operation using a filter may be performed.

[0024] In the slurry preparation step of the present invention, the scraps made of aluminum nitride green bodies, which are circulated in the circulation step described below and dissolved in the aluminum nitride slurry, may be dissolved by supplying them to the aluminum nitride slurry in the dispersing device in the slurry preparation step, or may be dissolved in a separate dissolution tank. The dissolution tank may be, for example, a tank having a known stirring means such as a stirring blade.

[0025] From the slurry preparation step, the aluminum nitride slurry may be continuously or intermittently supplied to the green body forming step. In addition, it is preferable to adjust the mixing ratio and mixing amount of the raw materials so that the composition of the aluminum nitride slurry is constant according to the amount of the circulated processing waste. The amount of the circulated processing waste may be 2 parts by mass or more, particularly 5 parts by mass or more, and may be 40 parts by mass or less, particularly 30 parts by mass or less, per 100 parts by mass of the solid content of the slurry.

[0026] An important requirement in the present invention is that when the scraps are circulated in the slurry preparation step by a circulation step described later, the free ammonia concentration in the aluminum nitride slurry in which the scraps are dissolved is adjusted to 100 ppm or less, particularly 50 ppm or less in the slurry preparation step. That is, when the free ammonia concentration exceeds the above range, the solubility of the circulated scraps in the aluminum nitride slurry is significantly reduced, and even if it is dissolved over a long period of time, undissolved matter remains in a granular or lumpy state on the surface and inside of an aluminum nitride green body obtained by molding the scraps, which leads to poor appearance and reduced strength of an aluminum nitride sintered body obtained by degreasing and firing the scraps.

[0027] The free ammonia concentration in the aluminum nitride slurry can be measured, for example, by a gas chromatograph mass spectrometer (GC / MS).

[0028] In the present invention, the means for adjusting the free ammonia concentration in the aluminum nitride slurry to the above range is not particularly limited, and any means capable of reducing free ammonia is adopted without any particular limitation. A preferred embodiment is an embodiment in which an organic acid such as citric acid is added to the aluminum nitride slurry. The amount of the organic acid added may be 0.1 to 5 parts by mass, particularly 0.2 to 4 parts by mass, relative to 100 parts by mass of the aluminum nitride slurry. The organic acid may be added in powder form, or may be dissolved in a suitable solvent, such as alcohol, before addition.

[0029] In the present invention, the green body forming step is a step of forming the aluminum nitride slurry obtained in the slurry preparation step, removing the organic solvent to obtain a green body, and any known method for forming the aluminum nitride slurry can be adopted without any particular limitation. Specifically, taking a sheet-shaped green body (hereinafter also referred to as a green sheet) as an example, there can be mentioned a method in which the aluminum nitride slurry is formed into a sheet by a doctor blade method, and then at least a part of the organic solvent contained therein is dried and removed.

[0030] In the present invention, the processing step is a step of processing the green body by grinding, cutting, etc. Specifically, in the case of the green sheet, examples include a mode in which both ends of the green sheet are cut to process it into a green sheet of a desired width, and a mode in which the green sheet is punched into a desired shape.

[0031] Furthermore, the circulation step is a step of returning the processing waste generated by the processing in the processing step to the slurry preparation step for reuse. The processing waste may be circulated to the slurry preparation step as it is, but may also be cut into pieces of an appropriate size, for example, a maximum diameter of 50 mm or less, preferably 20 mm or less, and further 10 mm or less, and then circulated to the slurry preparation step.

[0032] The green body obtained by the method of the present invention can be degreased and sintered by a known method to obtain an aluminum nitride sintered body.

[0033] The degreasing is usually carried out 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 gases, or in a humidified gas atmosphere in which these gases are mixed with water vapor. The degreasing temperature can be appropriately selected depending on the type of binder and the degreasing atmosphere, but can usually be selected from a temperature range of 300° C. to 1200° C., preferably 400° C. to 1000° C. The degreasing time can be appropriately selected depending on the type of binder and the degreasing atmosphere, but can usually be selected from a range of 30 minutes to 10 hours, preferably 2 hours to 8 hours.

[0034] The firing is usually carried out in a non-oxidizing gas atmosphere, in vacuum, or under reduced pressure. The non-oxidizing gas may be, for example, a reducing gas such as hydrogen, an inert gas such as argon, helium, or nitrogen, or a mixture of these. The firing temperature may be usually in the range of 1650°C to 1900°C, preferably 1750°C to 1800°C, and more preferably 1760°C to 1790°C.

[0035] When carrying out the above-mentioned sintering, a sintering vessel usually used for sintering aluminum nitride can be used without any problem. For example, a closed vessel made of aluminum nitride or boron nitride may be used.

[0036] The aluminum nitride sintered body obtained by the above-mentioned firing can be used as it is as a structural material for electronic components, etc., as a metal bonded substrate for power modules, etc., and as a circuit board having a metallized surface such as a phi pattern, etc. EXAMPLES

[0037] In order to more specifically explain the present invention, examples are shown below, but the present invention is not limited to these examples. In the examples, various measurements were carried out by the following methods.

[0038] (1) Ammonia concentration in aluminum nitride slurry Measurement was performed using GC / MS (Agilent Technologies, model number: HP5890 / HP5973).

[0039] (2) Surface roughness of sintered aluminum nitride substrate Using a surface roughness meter (Tokyo Seimitsu Co., Ltd., model number: SURFCOM 480A), five samples were cut out from any point on the sintered substrate, each measuring 2 cm x 2 cm. The maximum height R of all the samples was z and the average roughness R of all samples a He asked for:

[0040] (3) Bending strength of sintered aluminum nitride Five samples measuring 4 mm x 50 mm were cut out from random locations of the sintered substrate, and the three-point bending strength was measured at a crosshead speed of 0.5 mm / min and a span of 30 mm in accordance with JIS R1601. The test pieces were prepared by surface grinding to a width of 4 mm. The average value of the five samples was recorded as the measured value.

[0041] Example 1 (Slurry preparation process) Aluminum nitride coarse powder with an average particle size of 10 μm obtained by the combustion synthesis method was pulverized in a ball mill to obtain 100 parts by mass of aluminum nitride powder with an average particle size of 1.5 μm and an oxide film thickness of 6 Å, 5 parts by mass of yttrium oxide, 2 parts by mass of sorbitan trioleate, 38 parts by mass of toluene, 28 parts by mass of ethanol, 4 parts by mass of butanol, and 8 parts by mass of polyvinyl butyral were charged into a ball mill to prepare an aluminum nitride slurry. In addition, 3 parts by mass of citric acid was added to 100 parts by mass of the aluminum nitride slurry of the above composition, and further, 25 parts by mass of aluminum nitride green body processing waste was added to 100 parts by mass of the solid content of the slurry by a circulation process. The aluminum nitride slurry was obtained by dissolving the processing waste by mixing in a ball mill for 10 hours. The free ammonia concentration in the ball mill was maintained at 50 ppm or less.

[0042] The aluminum nitride slurry obtained from the ball mill was fed to a degassing tank, where it was degassed and desolvated under reduced pressure, and the viscosity was adjusted to 20,000 cP to obtain an aluminum nitride slurry.

[0043] (Green body molding process) The slurry obtained above was formed into a sheet by a doctor blade method and then dried to prepare an aluminum nitride green sheet having a width of 20 cm and a thickness of 0.75 mm.

[0044] (Processing process) This aluminum nitride green sheet was punched out using a press machine to obtain an aluminum nitride green body having a width of 68 mm, a length of 68 mm and a thickness of 0.75 mm.

[0045] (circulation process) In the above processing step, the processing waste generated by punching was cut into pieces with a maximum diameter of 5 cm and circulated to the above slurry preparation step.

[0046] (Degreasing and baking process) The green body thus obtained was degreased in dry air at 580°C for 4 hours to obtain an aluminum nitride degreased body. The aluminum nitride degreased body thus obtained was then placed in a sintering vessel and sintered in a nitrogen atmosphere at 1780°C for 5 hours to obtain an aluminum nitride sintered body.

[0047] R of the obtained aluminum nitride sintered body z is 2.388μm, R a The thickness was 0.253 μm and the bending strength was 471 MPa.

[0048] Comparative Example 1 An aluminum nitride sintered body was produced in the same manner as in Example 1, except that citric acid was not added to the ball mill in the slurry preparation step and the free ammonia concentration was not adjusted.

[0049] The ammonia concentration of the aluminum nitride slurry in the ball mill was 234 ppm.

[0050] R of the obtained aluminum nitride sintered body z , 3.26μm, R a The thickness was 0.264 μm and the bending strength was 447 MPa.

Claims

1. A method for producing an aluminum nitride green body, comprising: a slurry preparation step of preparing an aluminum nitride slurry containing aluminum nitride powder, a sintering aid, an organic binder and an organic solvent; a green body forming step of forming the aluminum nitride slurry and removing the organic solvent; a processing step of processing the green body; and a circulation step of returning processing waste generated from the processing step to the slurry preparation step for reuse, wherein in the slurry preparation step, a free ammonia concentration in the aluminum nitride slurry in which the processing waste is dissolved is adjusted to 100 ppm or less.

2. A method for producing an aluminum nitride sintered body, comprising degreasing and sintering the aluminum nitride green sheet produced according to claim 1.