Silicon nitride substrate

Abstract

To obtain a silicon nitride substrate that can be used for metallization applications without requiring a user to distinguish between its front and back surfaces.SOLUTION: A silicon nitride substrate has Ra1 / Ra2 ratio of 0.90 to 1.10, which is the ratio of a surface roughness Ra1 of a first surface to a surface roughness Ra2 of a second surface, and Ra1 is 0.30 μm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to silicon nitride substrates. [Background technology]

[0002] Silicon nitride sintered bodies have excellent mechanical strength, thermal conductivity, and electrical insulation properties, and are therefore used as substrates, particularly in applications requiring high voltages and currents, such as power modules. When forming a metal layer on the surface of such silicon nitride substrates using various metallization methods, the surface roughness of the substrate is important for ensuring adhesion between the substrate and the metal layer. For example, Patent Document 1 discloses that the use of a metal nitride sintered substrate with a surface roughness (Ra) of 0.6 μm or less helps to suppress the formation of voids at the interface between the substrate and the metal layer. In the examples, a green body obtained by the doctor blade method is sintered, and then honed to remove any foreign matter adhering to the surface, resulting in a silicon nitride substrate with a surface roughness of 0.4 μm. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] WO2022 / 034810 Summary of the Invention [Problem to be solved by the invention]

[0004] Silicon nitride substrates can be produced by firing a plate-shaped green body. A typical method for producing a plate-shaped green body is the doctor blade method. The doctor blade method involves forming a layer of slurry containing ceramic powder on a film, followed by removing the solvent to obtain a plate-shaped green body. Green bodies produced by the doctor blade method can achieve relatively low surface roughness even without polishing, as described in Patent Document 1. However, because the green body is formed on a film as described above, the green body has a side that was in contact with the film and a side that was not, which typically results in a difference in surface roughness between the two sides. As a result, the surface roughness of both sides of the fired silicon nitride substrate is generally different. Therefore, when the surface roughness of a silicon nitride substrate is important, users must distinguish between the front and back sides of the substrate, which is time-consuming.

[0005] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a silicon nitride substrate that can be used for metallization without the user having to distinguish between the front and back sides. [Means for solving the problem]

[0006] In order to solve the above problems, the present inventors have conducted extensive research and have succeeded in obtaining a silicon nitride substrate having low and uniform surface roughness on both the front and back surfaces.

[0007] The present invention is a silicon nitride substrate in which the ratio Ra1 / Ra2 of the surface roughness Ra1 of the first surface to the surface roughness Ra2 of the second surface is 0.90 to 1.10, and said Ra1 is 0.30 μm or less. DETAILED DESCRIPTION OF THE INVENTION

[0008] In the silicon nitride substrate of the present invention, the ratio Ra1 / Ra2 of the surface roughness Ra1 of the first surface to the surface roughness Ra2 of the second surface is 0.90 to 1.10, and Ra1 is 0.30 μm or less. The surface roughnesses Ra1 and Ra2 in the present invention are the arithmetic mean roughness Ra defined in JIS B0601:2013. The Ra1 / Ra2 ratio of 0.90 to 1.10 indicates that the difference in surface roughness between the first surface and the second surface is small. This characteristic allows users to use the substrate without distinguishing between the front and back surfaces. Specifically, since the surface roughness of the first and second surfaces is equivalent, the amount of brazing material used during metallization can be made uniform, and the adhesion between the silicon nitride substrate and the metal layer after metallization is equivalent. This facilitates stabilization of thermal conduction and thermal cycle resistance, even when users use the substrate without distinguishing between the front and back surfaces.

[0009] The Ra1 / Ra2 is preferably in the range of 0.93 to 1.07, and more preferably in the range of 0.95 to 1.05.

[0010] The Ra1 is 0.30 μm or less. This improves adhesion when a metal layer is formed on the first surface, making it easy to enhance thermal cycle resistance. If Ra1 is 0.30 μm or less, Ra2 also has an equivalent value (0.27 μm or more and 0.33 μm or less), making it easy to similarly enhance thermal cycle resistance. This makes it easy to obtain high thermal cycle resistance even if a user forms a metal layer without distinguishing between the front and back of the substrate. The Ra1 is preferably 0.25 μm or less, and more preferably 0.20 μm or less. The lower limit of the Ra1 is not particularly limited, but may be, for example, 0.10 μm or more. Similarly, Ra2 is preferably 0.30 μm or less, more preferably 0.25 μm or less, even more preferably 0.20 μm or less, and may be 0.10 μm or more.

[0011] The silicon nitride substrate of the present invention is preferably plate-shaped. When the silicon nitride substrate is plate-shaped, the first and second surfaces are the surface with the largest area (main surface) and the surface opposite to it. When the silicon nitride substrate is rectangular, the areas of the first and second surfaces are the same, but the areas of the main surface and the surface opposite to it may be different, and in that case, it does not matter whether the larger surface is the first surface or the second surface. The shapes of the first and second surfaces are not particularly limited, and may be, for example, rectangular or circular. The areas of the first and second surfaces are not particularly limited, and may be, for example, 100 mm 2 More than 40,000 mm 2 The thickness of the silicon nitride substrate is not particularly limited, but is generally 0.1 mm to 2.0 mm.

[0012] The silicon nitride substrate of the present invention preferably has a three-point bending strength of 600 MPa or more and a fracture toughness of 6.0 MPa m 1 / 2 It is preferable that the thermal conductivity is 80 W / (m·K) or more.

[0013] The silicon nitride substrate of the present invention can be obtained, for example, by forming a raw material slurry containing silicon nitride powder, a sintering aid, and water by a doctor blade method, drying the resulting green body to remove water under specific conditions, firing the green body, and then performing a honing process after firing.

[0014] Although the reason why the silicon nitride substrate of the present invention can be obtained by the above-mentioned manufacturing method is unclear, the inventors believe it to be as follows. In a typical silicon nitride substrate manufacturing process, the surface of silicon nitride powder gradually oxidizes during the processes of raw material mixing, molding, firing, etc. Oxidation of silicon nitride powder also occurs during the molding process using the doctor blade method, and oxidation is usually suppressed on the surface in contact with the film compared to the surface not in contact with the film. As oxidation progresses during green body molding, an oxide film forms on the surface of the green body. However, since the degree of oxidation differs between the surface in contact with the film and the surface not in contact with the film as described above, the oxide film on the surface not in contact with the film in the obtained green body is thicker. As a result, the thickness of the oxide film layer on both sides of the substrate immediately after firing is generally different. Generally, foreign matter adheres to the surface immediately after firing, and is removed by honing or other processes. However, the oxide film is also removed during this process, which affects the surface roughness, resulting in a difference in surface roughness between the surface of the silicon nitride substrate that originates from the surface of the green body that was in contact with the film (film surface) and the surface of the silicon nitride substrate that originates from the surface of the green body that was not in contact with the film (air surface).

[0015] Here, when a raw material slurry containing silicon nitride powder, a sintering aid, and water is molded by the doctor blade method as described above, oxidation occurs more easily than when a solvent other than water is used, and the oxide film on the surface in contact with the film also becomes thicker. On the other hand, oxidation by air progresses on the surface not in contact with the film, so the degree of oxidation does not change significantly regardless of whether the solvent is water or not. Therefore, it is thought that using water as a solvent reduces the difference in the thickness of the oxide film layer on both sides and the difference in surface roughness on both sides.

[0016] In addition, water has a low vapor pressure (vapor pressure 2.3 kPa / 20°C), and it is thought that the solvent evaporates more slowly from the entire sheet when drying after sheet molding than toluene (vapor pressure 2.9 kPa / 20°C) or ethanol (vapor pressure 6.4 kPa / 20°C), which are commonly used in producing green bodies, making it possible to make the surface of the green sheet more uniform.

[0017] During drying, in order to gently evaporate water from the entire sheet, it is believed that the effect on surface roughness can be suppressed by not reducing the pressure excessively and controlling the temperature increase rate slowly, specifically by keeping the pressure at atmospheric pressure and increasing the temperature from room temperature to 100°C at a rate of 2°C / min or less during drying. Organic solvents other than water may be used as solvents during production of the green body as long as the effects of the present invention are not impaired, but the proportion of water to the total solvent is preferably 90% by mass or more, and more preferably 99% by mass or more.

[0018] The above manufacturing method makes it easy to reduce the surface roughness of the silicon nitride substrate of the present invention in an unpolished state after firing and to keep the Ra1 / Ra2 within the above range, making it easier to use. Note that the unpolished state refers to a state in which the substrate has been honed after firing to remove foreign matter, but has not been subjected to polishing, grinding, or other treatments.

[0019] The silicon nitride powder may be composed of α-type silicon nitride, β-type silicon nitride, or a mixture thereof. However, β-type silicon nitride is preferred because β-type powder is less susceptible to grain growth during sintering than α-type powder and is easier to control uniformity. The β-phase ratio of the silicon nitride powder is preferably 80% or more, more preferably 90% or more, and even more preferably 99% or more. The β-phase ratio of the silicon nitride powder refers to the peak intensity ratio of the β-phase to the total of the α-phase and β-phase in the silicon nitride powder [100 × (peak intensity of β-phase) / (peak intensity of α-phase + peak intensity of β-phase)], and is determined by powder X-ray diffraction (XRD) measurement using CuKα radiation. More specifically, it can be determined by calculating the mass ratio of the α-phase and β-phase in the silicon nitride powder using the method described in C.P. Gazzara and D.R. Messier: Ceram. Bull., 56 (1977), 777-780.

[0020] The particle size of the silicon nitride powder is not particularly limited, but the average particle size D50 is preferably 0.4 to 1.2 μm, and more preferably 0.6 to 1.0 μm. The average particle size D50 can be measured using a laser diffraction / scattering particle size distribution analyzer. The specific surface area of ​​the silicon nitride powder is preferably 12 to 30 m. 2 / g, and 18 to 24m 2 The specific surface area means the BET specific surface area measured by the BET single-point method using nitrogen gas adsorption.

[0021] The sintering aid may be any known sintering aid without any particular limitation, and examples thereof include oxides such as yttria, magnesia, ceria, and calcia, as well as oxygen-free compounds such as carbonitride compounds and nitride compounds. Examples of carbonitride compounds include Y2Si4N6C, Yb2Si4N6C, Ce2Si4N6C, and MgSi4N6C. Examples of nitride compounds include MgSiN2. These sintering aids may be used alone or in combination of two or more. The amount of the sintering aid is not particularly limited, but is preferably 5 to 20 parts by mass, and more preferably 7 to 10 parts by mass, per 100 parts by mass of the silicon nitride powder.

[0022] The amount of water is not particularly limited, but from the viewpoint of sufficient mixing and forming a sufficient oxide film layer, it is preferable to use an amount that results in a solid concentration in the slurry (slurry concentration) of 50 to 70 mass %.

[0023] The raw material slurry may contain other components in addition to the silicon nitride powder, sintering aid, and water, such as a binder, a dispersant, a plasticizer, and an antifoaming agent.

[0024] For example, the binder is used to improve the shape retention of the green body, and known binders can be used without particular limitation, such as polyvinyl alcohol, polyvinyl acetal, polyvinyl pyrrolidone, acrylic resins, polyacrylamide, urethane resins, polyester, polyether, melamine, epoxy resins, cellulose resins, and starch. These binders may be used alone or in combination of two or more. The content of the binder is preferably 1 to 30 parts by mass per 100 parts by mass of silicon nitride powder.

[0025] For example, a dispersant is used to improve the dispersibility of the silicon nitride powder and sintering aid powder in the slurry, and generally, a surfactant can be suitably used. Known surfactants can be used without any restrictions. Specific examples of surfactants that can be suitably used in the present invention include carboxylated trioxyethylene tridecyl ether, diglycerol monooleate, diglycerol monostearate, carboxylated heptaoxyethylene tridecyl ether, tetraglycerol monooleate, hexaglycerol monooleate, sorbitan laurate, sorbitan oleate, sorbitan trioleate, polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monooleate, and polyoxyethylene sorbitan trioleate. These surfactants may be used alone or in combination of two or more. The amount of the dispersant can be appropriately selected, and can typically be selected from the range of 0.1 to 5 parts by mass per 100 parts by mass of the silicon nitride powder and the sintering aid powder combined. Within this range, the upper limit of the amount of dispersant is preferably 3 parts by mass or less, more preferably 2 parts by mass or less, and even more preferably 1 part by mass or less.

[0026] To obtain the green body, a raw material slurry containing silicon nitride powder, a sintering aid, and water is molded by a doctor blade method. The raw material slurry can be obtained by mixing silicon nitride powder, a sintering aid, water, and, if necessary, other components. The conditions for the doctor blade method are not particularly limited, and any known method may be used.

[0027] The green body obtained by the doctor blade method is dried to remove water before the firing step. For drying, the green body is placed in a dryer and heated. As mentioned above, it is important to heat the green body from room temperature to 100°C at a rate of 2°C / min or less under atmospheric pressure. The final drying temperature may be, for example, about 100°C to 150°C. The drying time (the time the green body is held at the drying temperature after heating) is not particularly limited, but may be, for example, 1 to 5 hours. The atmosphere during drying is not particularly limited, and may be air or an inert gas atmosphere such as nitrogen. For uniform heating during drying, heaters are preferably placed above and below the green body, and heating is performed from both the air side and the film side.

[0028] After the drying, if the molded body contains organic components such as a binder, dispersant, plasticizer, antifoaming agent, etc., the organic components may be removed by degreasing to facilitate firing. The degreasing conditions are not particularly limited, but may be, for example, by heating the molded body to 450 to 650°C in air or an inert atmosphere such as nitrogen or argon.

[0029] The green body thus obtained can be fired to obtain a silicon nitride substrate. The firing conditions are not particularly limited, but the firing temperature is preferably 1700°C to 2000°C, more preferably 1800°C to 1950°C, and the firing time is preferably 1 to 30 hours, more preferably 3 to 20 hours. The firing may be performed at normal pressure or under pressure.

[0030] The silicon nitride substrate of the present invention may be used in any application, but is preferably used in applications in which a metal layer is provided on at least one of the first surface and the second surface. Such applications make it possible to particularly utilize the feature of the silicon nitride substrate of the present invention, namely, its ease of use regardless of whether it is on the front or back. Metal layers may be formed on both surfaces, in which case there is little difference between the two surfaces in properties such as thermal cycle resistance and thermal conductivity, thereby improving product stability. The method for forming the metal layer is not particularly limited, and may be, for example, a brazing method using a brazing material, or a direct bonding method or diffusion bonding method that does not use a brazing material. Metals used in the metal layer include copper, copper alloys, aluminum, and aluminum alloys. [Example]

[0031] Examples will be described below to specifically explain the present invention, but the present invention is not limited to these examples. The measurements of the various items in the examples and comparative examples were performed by the following methods.

[0032] (1) Measurement of surface roughness Ra1 and Ra2 of silicon nitride substrate Measurements were carried out using a stylus surface roughness measuring device (FTA-S4S3000DF: manufactured by Mitutoyo Corporation) in accordance with JIS B0633: 2001. Measurements were taken five times near the center and five near the edge of the substrate surface, and the surface roughness Ra was calculated as an arithmetic average. In the evaluation, the surface roughness of the air surface was defined as the surface roughness of the first surface Ra1, and the surface roughness of the film surface was defined as the surface roughness of the second surface Ra2.

[0033] (2) Evaluation of thermal cycle resistance The silicon nitride substrate was cut to obtain ten 40 mm × 10 mm silicon nitride samples for evaluation. Copper plates were bonded to both sides of the evaluation samples using the AMB (active metal brazing) method to produce metallized substrates. In the AMB method, a brazing filler metal (Ag-Cu-Ti 75:21:4 (mass ratio)) containing an active metal was screen-printed on both sides of the silicon nitride sample, followed by a 0.5 mm thick copper plate and heating in a vacuum at 850°C for 20 minutes. The sample was then immersed in a ferric chloride aqueous solution to form a pattern. To remove the remaining composite material, the copper plate was eroded in an acidic ammonium fluoride aqueous solution and then washed with water to produce metallized samples. The metallized samples were then subjected to a heat cycle test (3000 cycles from -40 to 200°C) and visually evaluated according to the following criteria. A: No peeling was observed in any of the metallized samples. B: Peeling was observed in some (less than half) of the metallized samples. C: Peeling was observed in some (more than half) of the metallized samples. In this evaluation, if the evaluation results for the air side and the film side are equivalent, it can be said that the substrate can be used without distinguishing between the front and back sides.

[0034] The silicon nitride substrate was manufactured using the following raw materials containing silicon nitride powder and a sintering aid.

[0035] <Silicon nitride powder> ·Betaization rate: 99% ·Average particle size D50: 0.9μm <Sintering aid> Yttria (manufactured by Shin-Etsu Chemical Co., Ltd.) Magnesia (manufactured by Ube Materials Co., Ltd.) <Binders and dispersants> Binder resin: Acrylic resin (manufactured by Fujikura Kasei Co., Ltd.) Dispersant: Cerna D735 (manufactured by Chukyo Yushi Co., Ltd.)

[0036] Example 1 100 parts by mass of silicon nitride powder, 5 parts by mass of yttria, 3 parts by mass of magnesia, 0.5 parts by mass of dispersant, and 22 parts by mass of binder were weighed and mixed in a ball mill with water as the solvent for 48 hours using a resin pot and silicon nitride balls. Next, a vacuum defoamer (manufactured by Sayama Riken Co., Ltd.) was used to defoam and adjust the viscosity to produce a raw material slurry. The raw material slurry was then formed into a sheet using a doctor blade method. The resulting molded body was heated in air from room temperature to 100°C at a rate of 0.5°C / min using top and bottom heaters, and then dried by holding at 100°C for 4 hours to evaporate the solvent. The drying pressure was atmospheric pressure. Next, a degreasing treatment was performed in dry air at 550°C to obtain a green body (green sheet). The resulting green sheet was placed in a firing container and fired at 1880°C for 9 hours in a nitrogen atmosphere under a pressure of 0.9 MPaG to obtain a silicon nitride substrate. The evaluation results are shown in Table 1.

[0037] <Example 2> 100 parts by mass of silicon nitride powder, 5 parts by mass of yttria, 3 parts by mass of magnesia, 0.5 parts by mass of dispersant, and 22 parts by mass of binder were weighed and mixed in a ball mill with water as the solvent for 48 hours using a resin pot and silicon nitride balls. Next, a vacuum defoamer (manufactured by Sayama Riken Co., Ltd.) was used to defoam and adjust the viscosity to produce a raw material slurry. The raw material slurry was then formed into a sheet using a doctor blade method. The resulting molded body was heated in air from room temperature to 100°C at a rate of 1°C / min using a bottom heater, and then dried by holding at 100°C for 6 hours to evaporate the solvent. The drying pressure was atmospheric pressure. Next, a degreasing treatment was performed in dry air at 550°C to obtain a green body (green sheet). The resulting green sheet was placed in a firing container and fired at 1880°C for 9 hours in a nitrogen atmosphere under a pressure of 0.9 MPaG to obtain a silicon nitride substrate. The evaluation results are shown in Table 1.

[0038] <Comparative Example 1> A silicon nitride substrate was obtained in the same manner as in Example 1, except that a mixed solution of toluene and ethanol (1:1 by mass ratio) was used as the solvent when producing the raw material slurry. The evaluation results are shown in Table 1.

[0039] <Comparative Example 2> A silicon nitride substrate was obtained in the same manner as in Example 1, except that a mixed solution of water and ethanol (1:1 by mass ratio) was used as the solvent when producing the raw material slurry. The evaluation results are shown in Table 1.

[0040] <Comparative Example 3> A silicon nitride substrate was obtained in the same manner as in Example 1, except that the obtained molded body was heated in air from room temperature to 100°C at a heating rate of 10°C / min using a bottom heater, and then held at 100°C for 4 hours to dry and evaporate the solvent. The evaluation results are shown in Table 1.

[0041] [Table 1]

Claims

1. A silicon nitride substrate, wherein the ratio Ra1 / Ra2 of the surface roughness Ra1 of the first surface to the surface roughness Ra2 of the second surface is 0.90 to 1.10, and said Ra1 is 0.30 μm or less.

2. The silicon nitride substrate of claim 1 , wherein the first side is an air side and the second side is a film side.

3. A metallized substrate comprising the silicon nitride substrate according to claim 1 or 2, and a metal layer provided on at least one of the first surface and the second surface.

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