Silicon nitride substrate and method for manufacturing the same

Abstract

To provide a silicon nitride substrate that facilitates the manufacture of laminated substrates with high thermal cycling resistance. [Solution] A silicon nitride substrate wherein the porosity in the surface region from the main surface of the silicon nitride substrate to a depth of 50 μm is 2 to 5%, and the porosity in the internal region deeper than 50 μm from the surface of the silicon nitride substrate is 1% or less.

Description

[Technical Field]

[0001] This invention relates to a novel silicon nitride substrate. [Background technology]

[0002] Silicon nitride has excellent mechanical strength, thermal conductivity, and electrical insulation properties, and is used in semiconductor module substrates and other applications.

[0003] Silicon nitride substrates are often used with metal circuit boards attached to their surface. In such cases, it is important that fracture does not occur even after repeated thermal cycling and that delamination does not occur between the silicon nitride substrate and the metal circuit board. Fracture and delamination due to thermal cycling are caused by the large difference in thermal expansion coefficients between the silicon nitride substrate and the metal circuit board, resulting in deformation and warping due to the difference in thermal expansion (see, for example, Patent Document 1). Methods to suppress this include controlling the surface properties of the silicon nitride substrate (for example, Patent Document 2) and controlling the bonding conditions between the silicon nitride substrate and the metal circuit board (for example, Patent Document 1).

[0004] Silicon nitride substrates typically have voids, and since these voids affect physical properties such as thermal conductivity and insulation, studies are being conducted to control them. For example, Patent Document 3 discloses a silicon nitride substrate in which the void ratio (porosity) in the center of the substrate is 1.80% or less and the void ratio (porosity) in the edges of the substrate is 1.00% or less. Patent Document 4 also discloses a silicon nitride substrate in which the area ratio of voids (air pockets) is 2.0% or less. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-145335 [Patent Document 2] Japanese Patent Publication No. 2002-201076 [Patent Document 3] Japanese Patent Publication No. 2019-059639 [Patent Document 4] WO2013 / 146789 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] In laminated substrates in which metal circuit boards are laminated onto a silicon nitride substrate, thermal cycling resistance is an important characteristic. Therefore, the object of the present invention is to provide a silicon nitride substrate that facilitates the manufacture of laminated substrates with high thermal cycling resistance. [Means for solving the problem]

[0007] To solve the aforementioned problems, the inventors conducted intensive research. As a result, they obtained new insights showing that the thermal cycling resistance of a laminated substrate can be improved by controlling the porosity of the silicon nitride substrate. Specifically, by creating a silicon nitride substrate with many voids near the surface and few voids in the interior, they were able to improve the thermal cycling resistance of the laminated substrate.

[0008] In other words, the present invention relates to a silicon nitride substrate, wherein the porosity in the surface region from the main surface to a depth of 50 μm is 2 to 5%, and the porosity in the internal region deeper than 50 μm from the surface of the silicon nitride substrate is 1% or less. The silicon nitride substrate preferably has a surface roughness Ra of 0.3 μm or more and a thickness of 0.2 mm or more. The silicon nitride substrate preferably has a porosity of 2 to 5% in the second surface region from the back surface to a depth of 50 μm. Furthermore, a laminated substrate in which a metal circuit board is laminated on the main surface of the silicon nitride substrate can be cited as one embodiment of the present invention. Moreover, the present invention relates to a method for manufacturing a silicon nitride substrate, comprising at least a firing step of firing a green body containing silicon nitride powder and a magnesium-based sintering aid in an amount of 6 to 15 parts by mass per 100 parts by mass of silicon nitride powder at a firing pressure of 0 to 0.5 MPa·G, a maximum firing temperature of 1800°C or higher, and a holding time of the maximum firing temperature of 6 hours or less. [Effects of the Invention]

[0009] The silicon nitride substrate of the present invention makes it easy to obtain a laminated substrate with high thermal resistance. [Modes for carrying out the invention]

[0010] The silicon nitride substrate of the present invention has a porosity of 2-5% in the surface region from the main surface of the silicon nitride substrate to a depth of 50 μm, and a porosity of 1% or less in the internal region deeper than 50 μm from the surface of the silicon nitride substrate. The reason why laminated substrates using this silicon nitride substrate can have high thermal cycling resistance is not clear, but the inventors speculate that by making the porosity relatively high on the surface of the silicon nitride substrate, the silicon nitride substrate can more easily follow the deformation and warping of the metal circuit board during thermal cycling, while by making the porosity low inside the silicon nitride substrate, fracture toughness is improved and fracture of the silicon nitride substrate itself can be suppressed.

[0011] Fracture and delamination due to thermal cycling are caused by the significant difference in thermal expansion coefficients between the silicon nitride substrate and the metal circuit board. Generally, the metal circuit board has a higher thermal expansion coefficient than the silicon nitride substrate, so during thermal cycling, the metal circuit board deforms and warps significantly, and it is thought that fracture and delamination occur because the silicon nitride substrate cannot keep up with this. Here, by setting the porosity of the surface region from the main surface of the substrate to a depth of 50 μm (hereinafter sometimes referred to as "surface porosity") to 2-5%, the flexibility of the silicon nitride substrate surface is improved, making it easier to follow the deformation and warping of the metal circuit board, and thus effectively suppressing fracture and delamination. However, if the surface porosity is too high, the strength of the silicon nitride substrate surface decreases, and the risk of the silicon nitride substrate fracturing due to stress caused by the deformation and warping of the metal circuit board during thermal cycling increases. From the viewpoint of improving the flexibility of the silicon nitride substrate surface, the surface porosity is preferably 2.1% or higher, and more preferably 2.2% or higher. Furthermore, the surface porosity is preferably 4.9% or less, and more preferably 4.8% or less.

[0012] Furthermore, by making the porosity of the internal region deeper than 50 μm from the surface of the substrate (hereinafter sometimes referred to as "internal porosity") 1% or less, it becomes easier to prevent fracture of the silicon nitride substrate during thermal cycling. Since the silicon nitride substrate of the present invention has a surface porosity of 2 to 5%, as described above, the surface of the silicon nitride substrate can follow the metal circuit board during thermal cycling. Furthermore, by making the internal porosity 1% or less, the fracture toughness inside the silicon nitride substrate is improved, and even if minute cracks occur on the substrate surface during thermal cycling, it is presumed that the propagation of cracks inside the silicon nitride substrate can be prevented, thereby suppressing fracture of the silicon nitride substrate itself. The internal porosity is preferably 0.9% or less. There is no particular lower limit for the internal porosity, and the fewer the voids, the better, but for example, it can be 0.1% or more.

[0013] The surface porosity and internal porosity can be measured by cutting the silicon nitride substrate perpendicular to the main surface and observing the cut surface with a scanning electron microscope. Specifically, the surface porosity (%) can be determined by taking a scanning electron microscope image (magnification 1000x) of the cut surface of the silicon nitride substrate and dividing the area of ​​voids into the total area of ​​the region from the main surface to a depth of 50 μm, and the internal porosity (%) can be determined by dividing the area of ​​voids into the total area of ​​the region deeper than 50 μm from the surface.

[0014] The silicon nitride substrate of the present invention preferably has a porosity in the second surface region from the back surface to a depth of 50 μm (hereinafter sometimes referred to as "second surface porosity") of 2 to 5%, more preferably 2.1 to 4.9%, and even more preferably 2.2 to 4.8%. By setting the second surface porosity within the above range, it becomes easy to obtain high thermal cycling resistance in a laminated substrate in which metal circuit boards are laminated on both the main surface and the back surface. Furthermore, when manufacturing a laminated substrate in which metal circuit boards are laminated on only one side, the front and back surfaces of the silicon nitride substrate can be not particularly distinguished, making it easy to improve manufacturing efficiency. The second surface porosity can be determined as the ratio of the area of ​​voids to the total area of ​​the region from the back surface to a depth of 50 μm, in the same manner as the measurement of the surface porosity described above.

[0015] The silicon nitride substrate of the present invention preferably has a main surface roughness Ra of 0.3 μm or more, and more preferably 0.4 μm or more. By increasing the surface roughness of the main surface, the adhesion between the silicon nitride substrate and the metal circuit board is improved, and the quality of the laminated substrate is easily enhanced. The upper limit of the surface roughness Ra of the main surface is preferably 1.0 μm or less, and more preferably 0.8 μm or less. By setting the surface roughness of the main surface to or below the upper limit, it is easy to suppress the generation of voids between the silicon nitride substrate and the metal circuit board. From the viewpoint of suppressing void formation in a configuration in which metal circuit boards are laminated on both sides, and enabling the manufacture of a laminated substrate without particularly distinguishing between the front and back sides of the silicon nitride substrate, it is more preferable that the surface roughness Ra of the back surface of the silicon nitride substrate of the present invention is in the same range as the surface roughness of the main surface.

[0016] The silicon nitride substrate of the present invention is preferably plate-shaped. In the silicon nitride substrate of the present invention, the main surface is a surface on which a metal circuit board can be laminated, and when there are a plurality of such surfaces, any of them may be the main surface. For example, when the silicon nitride substrate is plate-shaped, usually either the surface with the largest area or the surface on the opposite side thereof becomes the main surface. In the silicon nitride substrate of the present invention, the back surface is the surface on the opposite side of the main surface. The sizes of the main surface and the back surface are not particularly limited. For example, the area of the main surface can be 100 mm 2 or more and 80,000 mm 2 or less, particularly preferably 900 mm 2 or more and 40,000 mm 2 or less. The shape of the main surface is not particularly limited and can be circular or rectangular. The ratio of the length of the long side to the short side of the main surface in the case of a rectangle is not particularly limited.

[0017] The thickness of the silicon nitride substrate of the present invention is not particularly limited, but is preferably 0.2 mm or more, and generally 2.0 mm or less.

[0018] The silicon nitride substrate of the present invention preferably has a three-point bending strength of 600 MPa or more. Also, the thermal conductivity is preferably 80 W / (m·K) or more.

[0019] The silicon nitride substrate of the present invention can be easily obtained by a manufacturing method of a silicon nitride substrate including at least a firing step of firing a green body containing silicon nitride powder and a magnesium-based sintering aid of 6 to 15 parts by mass with respect to 100 parts by mass of the silicon nitride powder at a firing pressure of 0 to 0.5 MPa·G for a short time at a high temperature. Here, the short time at a high temperature specifically means that the maximum firing temperature is 1800°C or more and the holding time at the maximum firing temperature is within 6 hours.

[0020] Although the reason why the silicon nitride substrate of the present invention is obtained by the manufacturing method is not clear, the present inventors presume that voids can be effectively generated near the surface because sintering ends in the middle stage. That is, by using a highly volatile magnesium-based auxiliary agent and performing firing at a high temperature, a part of the magnesium-based auxiliary agent volatilizes during sintering, and pores are generated on the surface of the sintered body. Although these pores disappear as sintering progresses, by completing firing at the middle stage of sintering before the pores disappear with short-time firing, an appropriate amount of voids remain in the surface region of the silicon nitride substrate after sintering, and it is presumed that the silicon nitride substrate of the present invention can be obtained.

[0021] The silicon nitride powder constituting the green body is not particularly limited, and it may be composed of α-type silicon nitride, β-type silicon nitride, or a mixture thereof. The particle size of the silicon nitride powder is not particularly limited, but the average particle size D50 is preferably 0.5 to 3.0 μm, and more preferably 0.7 to 2.0 μm. The average particle size D50 can be measured by a laser diffraction / scattering method particle size distribution measuring device. Further, the specific surface area of the silicon nitride powder is preferably 5 to 30 m 2 / g, and more preferably 8 to 24 m 2 / g. In the present invention, the specific surface area means the BET specific surface area measured using the BET one-point method by nitrogen gas adsorption.

[0022] The magnesium-based sintering aid can be used without particular limitations as long as it contains magnesium; for example, magnesia, MgSi4N6C, and MgSiN2 can be used. One type of magnesium-based sintering aid may be used alone, or two or more types may be used in combination. The amount of magnesium-based sintering aid to be blended is 6 parts by mass or more and 15 parts by mass or less, preferably 7 parts by mass or more and 10 parts by mass or less, per 100 parts by mass of silicon nitride powder. Blending 6 parts by mass or more of magnesium-based sintering aid makes it easy to effectively form voids on the surface of the silicon nitride substrate. By blending 15 parts by mass or less of magnesium-based sintering aid, it is possible to prevent the surface porosity and internal porosity of the silicon nitride substrate from becoming too high.

[0023] The green material may contain other sintering aids in addition to the silicon nitride powder and magnesium-based sintering aid. Examples of other sintering aids include oxides such as yttria, ceria, and calcia, and carbonitride compounds such as Y2Si4N6C, Yb2Si4N6C, and Ce2Si4N6C. These sintering aids may be used individually or in combination of two or more. The amount of the sintering aids is not particularly limited, but it is preferable that the total amount of other sintering aids and magnesium-based sintering aids be 8 to 20 parts by mass, and more preferably 11 to 18 parts by mass, per 100 parts by mass of silicon nitride powder.

[0024] The green body can be obtained by molding a raw material mixture, which is obtained by mixing the silicon nitride powder, a magnesium-based sintering aid, and other components such as other sintering aids as needed, into a plate shape.

[0025] The mixing method is not particularly limited, and known methods can be used. For example, each component can be measured out in a predetermined amount and mixed dry, or mixed wet using a dispersion medium. Examples of mixing equipment for dry mixing include dry bead mills and attritors. Examples of mixing equipment for wet mixing include ultrasonic dispersers, bead mills, ball mills, roll mills, homomixers, ultramixers, disperser mixers, through-type high-pressure dispersers, impact-type high-pressure dispersers, porous-type high-pressure dispersers, clump-catching-type high-pressure dispersers, (impact + through-type) high-pressure dispersers, and ultra-high-pressure homogenizers. Examples of dispersion mediums for wet mixing include water, alcohol, and toluene.

[0026] The raw material mixture may contain other components in addition to the silicon nitride powder and the sintering aids (magnesium-based sintering aids and other sintering aids). Other components may include, in addition to the dispersion medium, binders, dispersants, plasticizers, and defoamers.

[0027] Examples of binders include polyvinyl alcohol, polyvinyl acetal, polyvinylpyrrolidone, acrylic resins, polyacrylamide, urethane resins, polyesters, polyethers, melamine, epoxy resins, cellulose resins, and starch. These binders can be used individually or in combination of two or more. The amount of the binder can be selected as appropriate, but for example, it can usually be selected from 1 to 30 parts by mass, and especially from 10 to 25 parts by mass, per 100 parts by mass of the total amount of silicon nitride powder and the sintering aid.

[0028] The dispersant is used to improve the dispersibility of silicon nitride powder and sintering aids, and generally, surfactants can be suitably used. Known surfactants can be used without any limitations. Specific examples of surfactants that can be suitably used in the present invention include carboxylated trioxyethylene tridecyl ether, diglycerin monooleate, diglycerin monostearate, carboxylated heptaoxyethylene tridecyl ether, tetraglycerin monooleate, hexaglycerin monooleate, sorbitan laurate, sorbitan oleate, sorbitan trioleate, polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monooleate, and polyoxyethylene sorbitan trioleate. These surfactants may be used individually or in combination of two or more. The amount of the dispersant can be appropriately selected, but for example, it can usually be selected from a range of 0.1 to 5 parts by mass per 100 parts by mass of the total amount of silicon nitride powder and sintering aid. Within that 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.

[0029] The method for forming the raw material mixture into a sheet is not particularly limited. It may be formed by press molding from the raw material mixture, press molding may be performed using granules obtained by drying a slurry of the raw material mixture containing a dispersion medium with a spray dryer or the like, or sheet molding may be performed on a slurry of the raw material mixture containing a dispersion medium using a doctor blade method or the like. Among these, sheet molding by the doctor blade method is preferred because it can be manufactured simply.

[0030] If the raw material mixture contains organic components such as a dispersion medium, binder, dispersant, plasticizer, or defoamer, the green body may be obtained by drying to remove the dispersion medium or degreasing to remove the binder, etc., in order to facilitate firing. The drying conditions are not particularly limited, but for example, if the dispersion medium is water, it can be done by heating the molded body to about 30°C to 150°C. The degreasing conditions are not particularly limited, but for example, it can be done by heating the molded body to 450°C to 650°C in air or in an inert atmosphere such as nitrogen or argon.

[0031] The silicon nitride substrate of the present invention can be obtained by firing the aforementioned green body at a firing pressure of 0 to 0.5 MPa·G at a high temperature for a short time. High temperature for a short time means that the maximum firing temperature is 1800°C or higher, and the holding time at the maximum firing temperature is within 6 hours. The upper limit of the maximum firing temperature is not particularly limited, but for example it can be 2400°C or lower. The lower limit of the firing time is not particularly limited and can be adjusted as appropriate according to the physical properties of the obtained silicon nitride substrate, but for example it can be 30 minutes or more. By setting the firing pressure within the above range, the volatilization of the magnesium-based sintering aid is appropriately generated, and the surface porosity, second surface porosity, and internal porosity of the silicon nitride substrate can be controlled to desired values.

[0032] The silicon nitride substrate of the present invention is not particularly limited in its applications, but it is preferable to use it as a laminated substrate with metal circuit boards because it can improve thermal cycling resistance.

[0033] The method for obtaining a laminated substrate by laminating a metal circuit board onto the main surface or back surface of a silicon nitride substrate according to the present invention is not particularly limited. For example, it may be a brazing method using a brazing material, or a direct bonding method or diffusion bonding method that does not use a brazing material. Examples of metals used for the metal circuit board include copper, copper alloys, aluminum, and aluminum alloys. The metal circuit board may be laminated only on the main surface of the silicon nitride substrate, or it may be laminated on both the main surface and the back surface. [Examples]

[0034] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. Each test method is as follows. Each test method is as follows.

[0035] <Measurement of surface porosity, secondary surface porosity, and internal porosity> A silicon nitride substrate was cut perpendicular to the main surface, and the cut surface of one of the pieces was observed using a scanning electron microscope (Hitachi High-Technologies Corporation "TM3030") (10 fields of view at 1000x magnification). From the obtained scanning electron microscope images, the surface porosity (%) was determined as the ratio of the area of ​​voids to the total area of ​​the region from the surface to a depth of 50 μm on the main surface, the second surface porosity (%) was determined as the ratio of the area of ​​voids to the total area of ​​the region from the surface to a depth of 50 μm on the back surface, and the internal porosity (%) was determined as the ratio of the area of ​​voids to the total area of ​​the region deeper than 50 μm from the surface.

[0036] <Measurement of surface roughness> The surface roughness Ra was measured using a surface roughness measuring instrument (Surfcom 480A, manufactured by Tokyo Seimitsu Co., Ltd.) by scanning the needle with an evaluation length of 2.5 mm and a measurement degree of 0.3 mm / s.

[0037] <Evaluation of thermal cycling resistance> In the measurement of surface porosity, secondary surface porosity, and internal porosity, a portion of the other piece of the silicon nitride substrate that was not used for scanning electron microscopy observation was used to laminate a copper plate with dimensions of 45 mm (length) x 35 mm (width) x 1.0 mm (thickness) onto the main surface using brazing material. The brazing material consisted of Ag (57 wt%), Cu (30 wt%), Sn (10 wt%), and Ti (3 wt%). A paste-like brazing material was applied to the silicon nitride substrate to a thickness of 40 μm, and after placing the copper plate on the brazing material, it was subjected to a vacuum at 820°C (10°C). -2The heat bonding process was carried out at a pressure of less than 5 Pa. For the obtained laminated substrate, one cycle consisted of -40°C × 30 minutes → room temperature (25°C) × 10 minutes → 150°C × 30 minutes → room temperature (25°C) × 10 minutes, and a thermal cycle test of 5000 cycles was performed. For the laminated substrate after the test, the presence or absence of breakage and delamination was evaluated according to the following criteria. A: No breakage or delamination was confirmed. B: Breakage or delamination was confirmed.

[0038] The raw materials and abbreviations used in the test are as follows.

[0039] <Silicon nitride powder> ·C1: β-type silicon nitride powder (average particle size D50: 0.95 μm) <Sintering aid> (Magnesium-based sintering aid) ·S1: MgSi4N6C (specific surface area 12 m 2 / g) (Other sintering aids) ·S2: Yttria (specific surface area 20 m 2 / g) <Binder> ·B1: Acrylic resin emulsion (manufactured by Fujikura Kasei Co., Ltd.: Acrybase EMK-02)

[0040] <Example 1> 100 parts by mass of silicon nitride powder C1, 8 parts by mass of sintering aid S1, 3 parts by mass of sintering aid S2, 20 parts by mass of binder B1, and 50 parts by mass of water were stirred and mixed for 48 hours to obtain a raw material mixture. Then, the mixture was formed into a sheet by the doctor blade method to form a sheet with a width of 75 cm and a thickness of 0.4 mm. Then, it was placed on a plate-shaped heater set at 50°C and left standing for 3 hours to dry, and further degreased at 500°C for 60 hours to obtain a green body. The obtained green body was fired at a firing pressure of 0.1 MPa·G, a maximum temperature of 1800°C, and a maximum temperature holding time of 3 hours to obtain a silicon nitride substrate with a thickness of 0.32 mm. The raw material composition and firing conditions are shown in Table 1, and the evaluation results of the obtained silicon nitride substrate are shown in Table 2.

[0041] <Example 2, Comparative Examples 1-3> Except for changing the raw material composition and firing conditions as shown in Table 1, silicon nitride substrates were obtained and evaluated in the same manner as in Example 1. The evaluation results of the obtained silicon nitride substrates are shown in Table 2.

[0042] [Table 1]

[0043] [Table 2]

Claims

1. A silicon nitride substrate, wherein the porosity in the surface region from the main surface of the silicon nitride substrate to a depth of 50 μm is 2 to 5%, and the porosity in the internal region deeper than 50 μm from the surface of the silicon nitride substrate is 1% or less.

2. A silicon nitride substrate according to claim 1, wherein the surface roughness Ra is 0.3 μm or more.

3. A silicon nitride substrate according to claim 1, wherein the thickness is 0.2 mm or more.

4. The silicon nitride substrate according to claim 1, wherein the porosity in the second surface region from the back surface to a depth of 50 μm is 2 to 5%.

5. A laminated substrate comprising a silicon nitride substrate according to any one of claims 1 to 4, wherein a metal circuit board is laminated on the main surface of the silicon nitride substrate.

6. A method for manufacturing a silicon nitride substrate, comprising at least a firing step of firing a green body containing silicon nitride powder and a magnesium-based sintering aid in an amount of 6 to 15 parts by mass per 100 parts by mass of silicon nitride powder at a firing pressure of 0 to 0.5 MPa·G, a maximum firing temperature of 1800°C or higher, and a holding time of the maximum firing temperature of 6 hours or less.

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