Method for producing metal foil, and metal foil for thermally conductive material

Abstract

This method for producing metal foil involves a step for forming metal foil on a resin base material having a flat surface with an arithmetic average roughness Ra of 50 nm or less, and a step for removing the resin base material to produce metal foil, wherein the arithmetic average roughness Ra of the metal foil is 50 nm or less.

Description

METHOD FOR PRODUCING METAL FOIL AND METAL FOIL FOR THERMALLY CONDUCTIVE MATERIAL

[0001] The present invention relates to a method for producing a metal foil and a metal foil for use as a thermally conductive material.

[0002] With the miniaturization of electronic devices such as power modules, image sensors, and high-performance computing (HPC), and the increase in information processing volume, the problem of heat generation has become more pronounced, and heat diffusion from the heat source has become increasingly important. In the case of LSIs and other electronic devices, if the LSI itself is exposed to high temperatures for a long period of time due to heat generated by the elements used, this may lead to malfunction or failure. For this reason, thermally conductive materials are widely used to prevent the temperature rise of LSIs and other devices. The thermally conductive materials can prevent the temperature rise of the device by diffusing the heat generated by the elements or by conducting it to a heat dissipation member for release outside the system, such as to the atmosphere.

[0003] Metal foils with flattened surfaces are expected to improve the efficiency, connection reliability, and durability of electronic devices, and are in wide demand in areas such as semiconductor manufacturing, flexible electronic devices, printed circuit boards, and high-precision sensors. To date, a method for producing a high-flatness metal foil material suitable for FMM (fine metal mask) manufacturing has been reported, which includes a metal coarse foil material forming step, a precision rolling step in which the metal coarse foil material is rolled at least once to form a high-flatness metal foil material having a desired precise thickness and a flat surface, and the high-flatness metal foil formed after precision rolling has a thickness of 5 μm to 5 mm, a heat treatment step, a tension leveling step, and a step in which a high-flatness metal foil material is obtained and a roll-shaped metal foil material is formed during continuous molding (see, for example, Patent Document 1).

[0004] Furthermore, for the purpose of providing a metal foil suitable as an electrode substrate for element formation, which can suppress oxidation of an ultra-flat surface while preventing scratches when rolled, a metal foil made of copper or a copper alloy has been reported, which has been polished by chemical mechanical polishing (CMP) to have a front surface that is an ultra-flat surface with an arithmetic mean roughness Ra of 30 nm or less, and a back surface that is a recess-dominated surface in which the Pv / Pp ratio, where Pv is the maximum valley depth of the cross-sectional curve to the maximum peak height Pp of the cross-sectional curve, is 1.5 or more (see, for example, Patent Document 2).

[0005] JP 2023-046321 A International Publication No. 2014 / 017135

[0006] However, the conventional metal foil manufacturing method described in Patent Document 1 requires advanced polishing techniques and chemical treatments to produce metal foil with an ultra-flat surface, and requires extremely precise processing techniques. Furthermore, manufacturing conditions such as temperature, pressure, and chemical concentration must be strictly controlled, which complicates the manufacturing process. These factors increase manufacturing costs, making normal manufacturing difficult.

[0007] The present invention aims to solve the above-mentioned conventional problems and achieve the following object: to provide a method for producing a metal foil that can produce a metal foil that has excellent flatness and is both reliable and highly thermally conductive through a simple production process.

[0008] The means for solving the above problems are as follows. That is, <1> A method for producing a metal foil, comprising: forming a metal foil on a resin substrate having a flat surface with an arithmetic mean roughness Ra of 50 nm or less; and removing the resin substrate to produce a metal foil, wherein the arithmetic mean roughness Ra of the metal foil is 50 nm or less. <2> A method for producing a metal foil, comprising: forming a metal foil on a resin substrate having a flat portion with an arithmetic mean roughness Ra of 50 nm or less and a convex portion connected to the flat portion; and removing the resin substrate to produce a metal foil, wherein the arithmetic mean roughness Ra of the metal foil is 50 nm or less. <3> A method for producing a metal foil according to <1> or <2>, wherein the metal foil is formed by at least one of electroless plating, vapor deposition, sputtering, and a combination of sputtering and electroplating. <4> A method for producing a metal foil according to any one of <1> to <3>, wherein the resin substrate is removed by at least one of melting and solvent washing. <5> The method for producing a metal foil according to any one of <1> to <4>, wherein the resin substrate is made of a resin selected from the group consisting of polyimide, polycarbonate, and polyurethane. <6> The method for producing a metal foil according to any one of <1> to <5>, wherein the metal foil is made of a metal selected from the group consisting of copper and nickel. <7> The method for producing a metal foil according to any one of <1> to <6>, wherein the metal foil has an average thickness of 10 nm or more and 1 mm or less. <8> The method for producing a metal foil according to any one of <1> to <7>, wherein the metal foil has an arithmetic mean roughness Ra of 20 nm or less. <9> A metal foil for a thermally conductive material, having a flat surface, wherein the arithmetic mean roughness Ra of the flat surface is 10 nm or less, and wherein the thermal conductivity is 200 W / (m·K) or more. <10> A metal foil for a thermally conductive material, comprising a flat portion and a recessed portion connected to the flat portion, wherein the flat portion has an arithmetic mean roughness Ra of 50 nm or less, and a thermal conductivity of 100 W / (m K) or more. <11> The metal foil for a thermally conductive material according to <10>, wherein the average thickness of the metal foil is 10 μm or more and 1 mm or less.

[0009] According to the present invention, it is possible to solve the above-mentioned problems in the prior art, achieve the above-mentioned object, and provide a method for manufacturing a metal foil that can produce a metal foil that has excellent flatness and is both reliable and has high thermal conductivity through a simple manufacturing process.

[0010] FIG. 1 is a schematic cross-sectional view (part 1) showing an example of a process for producing a metal foil according to the first embodiment. FIG. 2 is a schematic cross-sectional view (part 2) showing an example of a process for producing a metal foil according to the first embodiment. FIG. 3 is a schematic cross-sectional view (part 3) showing an example of a process for producing a metal foil according to the first embodiment. FIG. 4 is a schematic cross-sectional view (part 1) showing an example of a process for producing a metal foil according to the second embodiment. FIG. 5 is a schematic cross-sectional view (part 2) showing an example of a process for producing a metal foil according to the second embodiment. FIG. 6 is a schematic cross-sectional view (part 3) showing an example of a process for producing a metal foil according to the second embodiment. FIG. 7 is a schematic cross-sectional view (part 4) showing an example of a process for producing a metal foil according to the second embodiment. FIG. 8 is a schematic cross-sectional view (part 5) showing an example of a process for producing a metal foil according to the second embodiment. FIG. 9 is a schematic cross-sectional view (part 6) showing an example of a process for producing a metal foil according to the second embodiment. FIG. 10 is a top view of the metal foil shown in FIG. 9. FIG. 11 is a bottom view of the metal foil shown in FIG. 9. FIG. 12 is a schematic cross-sectional view showing an example of a heat dissipation structure according to this embodiment. Fig. 13 is a diagram showing the measurement results of the surface roughness of the first side of the metal foil of Example 1. Fig. 14 is a diagram showing the measurement results of the surface roughness of the second side of the metal foil of Example 1. Fig. 15 is a diagram showing the measurement results of the surface roughness of the metal foil of Comparative Example 2.

[0011] (Metal Foil Manufacturing Method) [First Embodiment] A metal foil manufacturing method of the first embodiment includes a step of forming a metal foil on a resin substrate having a flat surface with an arithmetic mean roughness Ra of 50 nm or less (metal foil forming step), and a step of removing the resin substrate to manufacture a metal foil (resin substrate removing step), and further includes other steps such as a resin substrate forming step, as necessary. The arithmetic mean roughness Ra of the metal foil is 50 nm or less.

[0012] 1 to 3 are schematic cross-sectional views illustrating an example of the process of the first embodiment of the metal foil manufacturing method. First, a resin substrate 10 having a flat surface 10a with an arithmetic mean roughness Ra of 50 nm or less is prepared (FIG. 1), and a metal foil 100 is formed on the flat surface 10a of the resin substrate 10 (FIG. 2). Next, the resin substrate 10 is removed to produce the metal foil 100 (FIG. 3). This allows the production of a metal foil 100 having a first surface 101 and a second surface 102, both of which are flat surfaces with an arithmetic mean roughness Ra of 50 nm or less. Each step will be described in detail below.

[0013] -Resin substrate- The resin substrate of the first embodiment is not particularly limited and can be any suitable one depending on the purpose as long as the arithmetic mean roughness Ra of one surface is 50 nm or less. It may be a commercially available product or may be formed by a resin substrate forming step.

[0014] The resin substrate is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include polyimide, polycarbonate, polyurethane, etc. These may be used alone or in combination of two or more.

[0015] <Resin substrate forming process> The resin substrate forming process is a process of forming a resin substrate having a flat surface with an arithmetic mean roughness Ra of 50 nm or less, and examples thereof include a method of heating and plasticizing one surface of a thermoplastic resin composition in a flattened state; a method of curing one surface of a curable resin composition in a flattened state; and a method of flattening the surface of a resin molded body. Methods for flattening one surface of a thermoplastic resin composition or a curable resin composition include, for example, a method of flattening the one surface by applying it to a flat mold, and a method of leaving each composition to flatten. Methods for flattening the surface of a resin molded body include, for example, heat treatment, polishing, etc.

[0016] <Metal Foil Forming Step> The metal foil forming step is a step of forming a metal foil on the resin substrate. The method of forming the metal foil is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include electroless plating (chemical plating), vapor deposition, sputtering, and a combination of sputtering and electroplating. These methods may be performed once, multiple times, or a combination of multiple treatments.

[0017] The metal forming the metal foil is not particularly limited and can be appropriately selected depending on the purpose, but preferably contains at least one of copper, nickel, gold, and silver, more preferably at least one of copper, nickel, gold, and silver, and even more preferably selected from the group consisting of copper and nickel.

[0018] <Resin substrate removing step> The resin substrate removing step is a step of removing the resin substrate. The method for removing the resin substrate is not particularly limited and can be appropriately selected depending on the purpose. For example, a method of melting the resin substrate, a method of washing the resin substrate with a solvent, or a combination thereof can be used.

[0019] The solvent used for the solvent washing is not particularly limited as long as it can swell, dissolve, or the like the resin, thereby peeling off the metal foil from the resin substrate, and can be appropriately selected depending on the purpose of the resin to be used, etc. Examples of the solvent include methyl ethyl ketone (MEK), acetone, cyclohexane, ethyl acetate, cellosolve acetate, and trichloroethylene.

[0020] As a result of the above, as shown in FIG. 3, a metal foil 100 can be produced that has a first surface 101 that is a flat surface with an arithmetic mean roughness Ra of 50 nm or less, and a second surface 102 .

[0021] The average thickness of the metal foil is not particularly limited and can be appropriately determined depending on the purpose, but is preferably 10 nm to 1 mm, more preferably 100 nm to 500 μm, and more preferably 10 μm to 1 μm.

[0022] The arithmetic mean roughness Ra of the first surface of the metal foil is not particularly limited and may be appropriately determined depending on the purpose, and is 50 nm or less, preferably 20 nm or less, and more preferably 10 nm or less.

[0023] The arithmetic mean roughness Ra of the second surface of the metal foil can be equal to or slightly greater than that of the first surface depending on the method for forming the metal foil and its thickness, and is preferably 100 nm or less, more preferably 50 nm or less, and even more preferably 20 nm or less.

[0024] [Second embodiment] A method for producing a metal foil according to a second embodiment includes a step of forming a metal foil on a resin substrate having flat portions with an arithmetic mean roughness Ra of 50 nm or less and protrusions connected to the flat portions, and a step of producing a metal foil by removing the resin substrate, and further includes other steps such as a resin substrate forming step, as necessary. The arithmetic mean roughness Ra of the metal foil is 50 nm or less.

[0025] 4 to 9 are schematic cross-sectional views illustrating an example of the process of the second embodiment of the metal foil manufacturing method. First, to prepare a resin substrate 20 (FIG. 6) having flat portions 20a with an arithmetic mean roughness Ra of 50 nm or less and convex portions 20b connected to the flat portions 20a, a master 30 having flat portions 30a and concave portions 30b connected to the flat portions 30a is placed in contact with a curable resin composition 20′ (FIG. 4), and the curable resin composition 20′ is cured to form the resin substrate 20 (FIG. 5). The resin substrate 20 is then removed from the master 30 and prepared (FIG. 6).

[0026] Next, a metal foil 200 is formed on the flat portions 20a and the convex portions 20b of the resin substrate 20 (FIGS. 7 and 8). Here, examples of methods for forming the metal foil 200 include forming a sputtering layer 200' by sputtering (FIG. 7) and then forming the metal foil 200 by electrolytic plating (FIG. 8). Next, the resin substrate 20 is removed to produce the metal foil 200 (FIG. 9). This allows for the production of a metal foil 200 having flat portions 201a and concave portions 201b continuous with the flat portions 201a, with the arithmetic mean roughness Ra of the flat portions 201a being 50 nm or less.

[0027] -Resin substrate- The resin substrate of the second embodiment is not particularly limited as long as it has flat portions with an arithmetic mean roughness Ra of 50 nm or less and convex portions continuous with the flat portions, and can be appropriately selected depending on the purpose. It may be a commercially available product, or may be formed by a resin substrate forming step.

[0028] <Resin substrate forming process> The resin substrate forming process is a process of forming a resin substrate having a flat portion having an arithmetic mean roughness Ra of 50 nm or less and a convex portion connected to the flat portion, and examples thereof include a method of heating and plasticizing one surface of a thermoplastic resin composition while it is placed against a mold; a method of curing one surface of a curable resin composition while it is placed against a mold; and a method of processing the surface of a resin molded body into a desired shape. The contact surface of the mold with the resin substrate has the same pattern shape as the contact surface of the target metal foil with the resin substrate, i.e., the continuous surface having the flat portion and the convex portion. The arithmetic mean roughness Ra of the flat portion of the mold is not particularly limited and can be appropriately set according to the purpose, and is 50 nm or less, preferably 20 nm or less, and more preferably 10 nm or less.

[0029] <Metal foil forming step> The metal foil forming step is a step of forming a metal foil on the resin substrate, and except for using the resin substrate of the second embodiment, the matters described in the metal foil forming step of the first embodiment can be appropriately selected.

[0030] <Resin Substrate Removing Step> The resin substrate removing step is a step of removing the resin substrate, and the matters described in the resin substrate removing step of the first embodiment can be appropriately selected.

[0031] As a result of the above, as shown in Figures 9 to 11, a metal foil 200 can be produced which has a flat portion 201a and a recess 201b continuous with the flat portion 201a, and in which the arithmetic mean roughness Ra of the flat portion 201a is 50 nm or less.

[0032] Fig. 9 is a schematic cross-sectional view showing an example of a metal foil according to a second embodiment. Fig. 10 is a top view of the metal foil shown in Fig. 9. Fig. 11 is a bottom view of the metal foil shown in Fig. 9, viewed from the flat portion 201a side. The metal foil 200 shown in Figs. 9 to 11 is a second embodiment having a first surface having a flat portion 201a and a recessed portion 201b continuous with the flat portion 201a, and a second surface having a protruding portion 202a and a flat portion 202b continuous with the protruding portion 202a. As shown in Fig. 9, the metal foil 200 has a thickness t, a height h, a diameter d1 of the recessed portion 201b, a diameter d2 of the protruding portion 202a, and a pitch p between adjacent recessed portions 201b and protruding portions 202a.

[0033] When the metal foil 200 is used as a thermally conductive material to be bonded between a substrate and an opposing substrate, it is preferable that the xy plane of the multiple convex portions 202a form the same plane so as to correspond to the bonding surface of the substrate or the opposing substrate, and it is also preferable that the flat portion 201a and the xy plane of the multiple convex portions 202a are parallel planes.

[0034] In the metal foil 200 of Figures 9 to 11, the arithmetic mean roughness Ra of the flat portion 201a is 50 nm or less, but the arithmetic mean roughness Ra of the xy plane of the multiple convex portions 202a may be 50 nm or less, or both may have arithmetic mean roughness Ra of 50 nm or less, and either can be selected appropriately depending on the purpose.

[0035] The average thickness t of the metal foil is not particularly limited and can be appropriately determined depending on the purpose, but is preferably 10 nm to 1 mm, more preferably 100 nm to 500 μm, and more preferably 10 μm to 1 μm.

[0036] The average height h of the metal foil is not particularly limited and can be appropriately set depending on the purpose, but is preferably 100 nm to 1 mm, more preferably 200 nm to 500 μm, and more preferably 10 μm to 100 μm.

[0037] The arithmetic mean roughness Ra of the flat portion 201a of the metal foil is not particularly limited and may be appropriately determined depending on the purpose, and is 50 nm or less, preferably 20 nm or less, and more preferably 10 nm or less.

[0038] The arithmetic mean roughness Ra of each convex portion 202a of the metal foil in the xy plane can be set to be equal to or slightly larger than that of the flat portion 201a depending on the surface roughness of the resin substrate, the method for forming the metal foil, etc., and is preferably 100 nm or less, more preferably 50 nm or less, and even more preferably 20 nm or less.

[0039] [Area Ratio] The area ratio of the flat portion 201a to the area of ​​the metal foil when viewed from above is 20% to 80% and preferably 30% to 70% in terms of being able to reduce lifting, peeling, and void generation from the substrate, and providing excellent adhesion to the substrate and excellent reliability. The area ratio of the sum of the xy planes of the convex portions 202a to the area of ​​the metal foil when viewed from above is 20% to 80% and preferably 30% to 70% in terms of being able to reduce lifting, peeling, and void generation from the substrate, and providing excellent adhesion to the substrate and excellent reliability.

[0040] [Pattern] The pattern of the metal foil is not particularly limited and can be appropriately selected depending on the purpose. For example, a pattern in which multiple shapes are arranged as the recesses 201b and protrusions 202a can be mentioned. Examples of the shapes include circles, ellipses, triangles such as equilateral triangles, quadrilaterals such as squares and rectangles, and polygons such as regular polygons. Among these, a pattern in which multiple shapes are regularly arranged is preferred from the viewpoint of uniformity of thermal conductivity. Examples of the pattern include 45° staggered, 60° staggered, parallel squares, 60° staggered regular hexagons, equilateral triangles, and alternating rectangles.

[0041] (Metal foil for thermally conductive material) [First embodiment] The metal foil for thermally conductive material of the first embodiment is a metal foil for thermally conductive material that has a flat surface, the arithmetic mean roughness Ra of the flat surface is 10 nm or less, and the thermal conductivity is 200 W / (m·K) or more, and can be suitably manufactured by the metal foil manufacturing method of the first embodiment.

[0042] [Second embodiment] The metal foil for thermally conductive materials of the second embodiment has a flat portion and a recessed portion continuous with the flat portion, the arithmetic mean roughness Ra of the flat portion is 50 nm or less, and the thermal conductivity is 100 W / (m·K) or more, and can be suitably manufactured by the metal foil manufacturing method of the second embodiment.

[0043] (Heat Dissipation Structure) The heat dissipation structure of this embodiment includes a heat generating element, the metal foil of this embodiment described above, a heat dissipation member, and may further include other members as necessary. The heat dissipation structure includes the metal foil between the heat generating element and the heat dissipation member. The heat dissipation structure may further include an adhesive layer between each component as necessary.

[0044] The heating element is not particularly limited and can be appropriately selected depending on the purpose. Examples of the heating element include electronic components such as a CPU (Central Processing Unit), an MPU (Micro Processing Unit), and a GPU (Graphics Processing Unit).

[0045] The heat dissipation member is not particularly limited as long as it is a structure that dissipates heat generated by an electronic component (heat-generating element) and can be appropriately selected depending on the purpose. Examples include a heat spreader, a heat sink, a vapor chamber, and a heat pipe. The heat spreader is a component for efficiently transferring heat from the electronic component to other components. The material of the heat spreader is not particularly limited and can be appropriately selected depending on the purpose, such as copper or aluminum. The heat spreader is typically flat. The heat sink is a component for dissipating heat from the electronic component into the air. The material of the heat sink is not particularly limited and can be appropriately selected depending on the purpose, such as copper or aluminum. The heat sink, for example, has multiple fins. The heat sink, for example, has a base and multiple fins extending in a non-parallel direction (e.g., a direction perpendicular to) one surface of the base. The heat spreader and the heat sink generally have solid structures without internal spaces. The vapor chamber is a hollow structure. A volatile liquid is sealed in the internal space of the hollow structure. Examples of the vapor chamber include a hollow structure of the heat spreader, and a plate-like hollow structure similar to the heat sink. The heat pipe is a cylindrical, approximately cylindrical, or flattened cylindrical hollow structure. A volatile liquid is sealed in the internal space of the hollow structure.

[0046] 12 is a schematic cross-sectional view showing an example of a semiconductor device as a heat dissipation structure. The metal foil 7 of this embodiment dissipates heat generated by an electronic component 3 such as a semiconductor element, and as shown in FIG. 12 , is fixed to the main surface 2 a of the heat spreader 2 facing the electronic component 3, and is sandwiched between the electronic component 3 and the heat spreader 2. The thermally conductive sheet 1 is sandwiched between the heat spreader 2 and the heat sink 5. The thermally conductive sheet 1 may be the metal foil of this embodiment or another thermally conductive sheet.

[0047] The heat spreader 2 is formed, for example, in the shape of a rectangular plate, and has a main surface 2a facing the electronic components 3 and side walls 2b extending along the outer periphery of the main surface 2a. The heat spreader 2 has a thermally conductive sheet 1 provided on the main surface 2a surrounded by the side walls 2b, and a heat sink 5 provided on the other surface 2c opposite the main surface 2a via the thermally conductive sheet 1. The higher the thermal conductivity of the heat spreader 2, the lower the thermal resistance and the more efficiently it absorbs heat from the electronic components 3, such as semiconductor elements. Therefore, the heat spreader 2 can be formed using, for example, copper or aluminum, which have good thermal conductivity.

[0048] The electronic component 3 is, for example, a semiconductor element such as a BGA, and is mounted on the wiring board 6. The tip surfaces of the side walls 2b of the heat spreader 2 are also mounted on the wiring board 6, so that the side walls 2b surround the electronic component 3 at a predetermined distance. The metal foil 7 of this embodiment is provided on the main surface 2a of the heat spreader 2, thereby forming a heat dissipation member that absorbs heat generated by the electronic component 3 and dissipates the heat through the heat sink 5.

[0049] The present invention will be described in more detail below based on examples, but the present invention is not limited to the following examples.

[0050] Example 1 <Production of Metal Foil> According to the production method shown in FIGS. 1 to 3, a metal foil having a flat surface of the first embodiment was produced by the following procedure.

[0051] <<Preparation of Resin Substrate>> The synthesized soluble polyimide resin composition was poured into a mold measuring 100 mm x 100 mm to a thickness of 0.5 mm or more, and the mixture was left to stand under vacuum for 2 hours to flatten the surface. The mixture was then heated at 200 ° C for 3 hours to plasticize the resin substrate, forming a resin substrate with a flat surface. The surface roughness of the resulting resin substrate was measured using a white light interferometer (device name: NewView 7300, manufactured by Ametec Co., Ltd.). The arithmetic mean roughness Ra was 10 nm, confirming excellent surface flatness. The surface roughness measurement conditions were: measurement magnification: ×100, scan length: 10 μm, scan range: 50 μm x 70 μm.

[0052] <<Formation of Metal Foil and Removal of Resin Substrate>> Next, copper electroless plating was performed on the surface of the resin substrate to form a metal foil having an average thickness of 50 μm. The resin substrate was dissolved in methyl ethyl ketone (MEK) to isolate the metal foil, thereby obtaining the metal foil of Example 1.

[0053] Figure 13 shows the results of measuring the surface roughness of the first side of the metal foil of Example 1, and Figure 14 shows the results of measuring the surface roughness of the second side of the metal foil of Example 1. The surface roughness of the obtained metal foil was measured, and it was confirmed that the arithmetic mean roughness Ra of the first side that was in contact with the resin substrate was 10 nm, which was the same level as the flatness of the resin substrate. Furthermore, the arithmetic mean roughness Ra of the second side opposite the first side of the metal foil was 14 nm.

[0054] <Production of Bonded Assembly> First, the oxide film on the surface of the metal foil was removed with 10% by mass sulfuric acid. A copper substrate (5 mm × 5 mm × 0.3 mm) and a silicon substrate (a SiC substrate having a 0.5 μm Au plating layer on the surface) were used as the substrate and the opposing substrate, respectively. The metal foil was placed on the copper substrate so that the flat surface of the metal foil was in contact with the copper substrate, and the silicon substrate was laminated on top of the metal foil to prepare a laminate. ２ A bonder (device name: SB6e, manufactured by SUSS MicroTech) was set to a load of 18 MPa and a temperature of 370°C under an ambient atmosphere, and the laminates were bonded by applying pressure and heat for a bonding time of 1 minute, thereby producing a bonded body of Example 1.

[0055] <Evaluation> The obtained metal foils and bonded bodies were evaluated for "thermal conductivity" and "reliability" as follows. The results are shown in Table 1.

[0056] The resulting bonded body was measured for thermal resistance [°C cm ２ / W] was measured. The thermal resistance of the metal foil was calculated by subtracting the thermal resistances of the substrate and the opposing substrate from the result, and the thermal conductivity [W / m·K] was calculated from this thermal resistance and the average thickness of the metal foil, and the thermal conductivity was evaluated according to the following criteria. [Evaluation criteria] ⊚: Thermal conductivity is 200 W / m·K or more. ◯: Thermal conductivity is 100 W / m·K or more and 200 W / m·K or less. ×: Thermal conductivity is 100 W / m·K or less.

[0057] <Reliability> The reliability of the resulting bonded bodies was evaluated based on the rate of change in thermal conductivity before and after thermal cycling (-40°C to 150°C, 100 times) using a method in accordance with JIS C 60068-2-14. [Evaluation criteria] ◎: Rate of change is 10% or less. ○: Rate of change is 10% or more and 15% or less. △: Rate of change is 15% or more and 20% or less. ×: Rate of change is 20% or more.

[0058] (Examples 2 to 3) Metal foils of Examples 2 and 3 were produced and evaluated in the same manner as in Example 1, except that the average thickness of the metal foil in Example 1 was changed as shown in Table 1. The results are shown in Table 1.

[0059] (Example 4) <Production of Metal Foil> According to the production method shown in Figures 4 to 9, metal foils having flat portions and convex portions and bonded bodies of the second embodiment shown in Figures 9 to 11 were produced and evaluated by the following procedure. The results are shown in Table 1.

[0060] <<Preparation of Resin Substrate>> The synthesized soluble polyimide resin composition was poured into a mold measuring 100 mm x 100 mm to a thickness of 0.5 mm or more, and the mold was pressed against a master plate and left to stand under vacuum for 2 hours. The mold was then plasticized by heating at 200 °C for 3 hours to form a resin substrate having a flat portion 20a and a plurality of convex portions 20b, each of which consisted of 50 μm diameter, 100 μm high cylinders arranged in a 90° grid pattern at a pitch of 200 μm (see Figures 4 to 6). The surface roughness of the flat portion 20a of the resin substrate was measured to find that the arithmetic mean roughness Ra was 20 nm, confirming excellent surface flatness.

[0061] <<Formation of Metal Foil and Removal of Resin Substrate>> A 100 nm thick sputtering layer made of silver was formed on the surface of the resin substrate by sputtering to form a conductive substrate (see FIG. 7 ). Next, copper was electrolessly plated on this conductive substrate to form a metal foil with an average thickness of 50 μm (see FIG. 8 ). The resin substrate was dissolved in MEK to isolate the metal foil, yielding the metal foil of Example 4 (see FIG. 9 ).

[0062] The surface roughness of the obtained metal foil was measured, and the arithmetic mean roughness Ra of the flat portion of the metal foil that was in contact with the flat portion of the resin substrate was found to be 20 nm, which was confirmed to be at the same level as the flatness of the flat portion of the resin substrate.

[0063] <Production of Bonded Assembly> First, the oxide film on the surface of the metal foil was removed with 10% by mass sulfuric acid. A copper substrate (5 mm × 5 mm × 0.3 mm) and a silicon substrate (a SiC substrate having a 0.5 μm Au plating layer on the surface) were used as the substrate and the opposing substrate, respectively. The metal foil was placed on the copper substrate so that the flat portion of the metal foil was in contact with the copper substrate, and the silicon substrate was laminated on top of the metal foil to prepare a laminate. ２ A bonder (device name: SB6e, manufactured by SUSS MicroTech) was set to a load of 18 MPa and a temperature of 370°C under an ambient atmosphere, and the laminates were bonded by applying pressure and heat for a bonding time of 1 minute, thereby producing a bonded body of Example 4.

[0064] (Example 5) A joined body of Example 5 was produced and evaluated in the same manner as in Example 4, except that the joined body was produced using the metal foil of Example 4 and a filler was prepared and filled according to the following procedure. The results are shown in Table 1.

[0065] <Preparation of Filler> Dimethyldimethoxysilane (500 g) and 3-glycidoxypropylmethyldimethoxysilane (100 g) were placed in a 1000 mL separable flask equipped with a thermometer and a dropping funnel, and the mixture was stirred at 50°C. An aqueous solution of potassium hydroxide (1.3 g) dissolved in water (165 g) was slowly added dropwise to the flask, and after the addition was complete, the mixture was stirred at 50°C for 6 hours. Acetic acid (1.4 g) was added to the flask, and volatile components were removed under reduced pressure. The potassium acetate was filtered to obtain a polymer. The obtained polymer was washed with hexane and water, and volatile components were removed under reduced pressure to obtain Polymer A.

[0066] The polymer A (100 g) obtained above, RIKACID MH-700G (acid anhydride, manufactured by New Japan Chemical Co., Ltd., 25 g), U-CAT SA 102 (curing accelerator, manufactured by San-Apro Co., Ltd., 0.5 g), HOSTANOX O16 (phenol-based compound, manufactured by Clariant, 0.5 g), and ADK STAB 3010 (phosphorus-based compound, manufactured by ADEKA Corporation, 0.5 g) were mixed and degassed to obtain a filler.

[0067] <Production of Bonded Assembly> First, the oxide film on the surface of the metal foil was removed with 10% by mass sulfuric acid. A copper substrate (5 mm × 5 mm × 0.3 mm) and a silicon substrate (a SiC substrate having a 0.5 μm Au plating layer on the surface) were used as the substrate and the opposing substrate, respectively. The metal foil was placed on the copper substrate so that the flat portion of the metal foil was in contact with the copper substrate, and the silicon substrate was laminated on top of the metal foil to prepare a laminate. ２ A bonder (device name: SB6e, manufactured by SUSS MicroTech) was set to a load of 18 MPa and a temperature of 370°C under an ambient atmosphere, and pressure and heat were applied for a bonding time of 1 minute. Next, a filler was filled between the multiple protrusions of the metal foil and the silicon substrate, and cured at 100°C for 3 hours to bond the laminate, thereby producing a bonded body of Example 4.

[0068] Comparative Example 1 Evaluation was performed in the same manner as in Example 1, except that a commercially available copper foil having an average thickness of 50 μm and an arithmetic mean roughness Ra of 100 nm was used instead of the metal foil in Example 1. The results are shown in Table 2.

[0069] Comparative Example 2 Evaluation was performed in the same manner as in Example 1, except that a commercially available copper foil having an average thickness of 50 μm and an arithmetic mean roughness Ra of 200 nm was used instead of the metal foil of Example 1. The results are shown in Table 2. FIG. 15 also shows the measurement results of the surface roughness of the metal foil of Comparative Example 2. As a result of the measurement, the arithmetic mean roughness Ra of the metal foil of Comparative Example 2 was 200 nm.

[0070]

[0071]

[0072] This application claims priority based on Japanese Patent Application No. 2024-141149, filed on August 22, 2024, the entire contents of which are incorporated herein by reference.

[0073] REFERENCE SIGNS LIST 1 Thermally conductive sheet 2 Heat spreader 2a Main surface 3 Heat generating element (electronic component) 3a Upper surface 5 Heat sink 6 Wiring board 7 Metal foil 10 Resin substrate 20 Resin substrate 100 Metal foil 200 Metal foil

Claims

1. A method for producing metal foil, comprising the steps of: forming a metal foil on a resin substrate having a flat surface with an arithmetic mean roughness Ra of 50 nm or less; and removing the resin substrate to produce a metal foil, wherein the arithmetic mean roughness Ra of the metal foil is 50 nm or less.

2. A method for producing metal foil, comprising: a step of forming metal foil on a resin substrate having flat portions with an arithmetic mean roughness Ra of 50 nm or less and protrusions connected to the flat portions; and a step of producing metal foil by removing the resin substrate, wherein the arithmetic mean roughness Ra of the metal foil is 50 nm or less.

3. The method for producing a metal foil according to claim 1 or 2, wherein the metal foil is formed by at least one of electroless plating, vapor deposition, sputtering, and a combination of sputtering and electroplating.

4. The method for producing a metal foil according to any one of claims 1 to 3, wherein the resin substrate is removed by at least one of melting and washing with a solvent.

5. A method for producing a metal foil according to any one of claims 1 to 4, wherein the resin substrate is made of a resin selected from the group consisting of polyimide, polycarbonate, and polyurethane.

6. A method for producing a metal foil according to any one of claims 1 to 5, wherein the metal foil is made of a metal selected from the group consisting of copper and nickel.

7. A method for producing a metal foil according to any one of claims 1 to 6, wherein the average thickness of the metal foil is 10 nm or more and 1 mm or less.

8. A method for producing a metal foil according to any one of claims 1 to 7, wherein the arithmetic mean roughness Ra of the metal foil is 20 nm or less.

9. A metal foil for a thermally conductive material, characterized in that it has a flat surface, the arithmetic mean roughness Ra of the flat surface is 10 nm or less, and the thermal conductivity is 200 W / (m·K) or more.

10. A metal foil for thermally conductive materials, characterized in that it has a flat portion and a recessed portion connected to said flat portion, the arithmetic mean roughness Ra of said flat portion is 50 nm or less, and the thermal conductivity is 100 W / (m·K) or more.

11. The metal foil for a thermally conductive material according to claim 10, wherein the average thickness of the metal foil is 10 μm or more and 1 mm or less.

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