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

Abstract

Provided is 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 manufacturing process. [Solution] A method for producing metal foil includes the steps of forming 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 metal foil, wherein the arithmetic mean roughness Ra of the metal foil is 50 nm or less.

Description

[Technical Field]

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

[0002] As electronic devices such as power modules, image sensors, and high-performance computing (HPC) become smaller and the amount of information processed increases, the problem of heat generation becomes more pronounced, making it increasingly important to dissipate heat from the heat source. In 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 of LSIs and other devices from rising. The thermally conductive materials can prevent the temperature of the device from rising 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, with the aim of providing a method for producing a high-flatness metal foil material suitable for FMM manufacturing, which method 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 a high-flatness metal foil formed after precision rolling to 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 prevent scratches while suppressing oxidation of the ultra-flat surface 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 an ultra-flat front surface with an arithmetic mean roughness Ra of 30 nm or less, and a recess-dominated back surface with a Pv / Pp ratio of 1.5 or more, where Pv is the maximum peak height Pp of the cross-sectional curve and Pv is the maximum valley depth of the cross-sectional curve (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2023-046321 [Patent Document 2] International Publication No. 2014 / 017135 Summary of the Invention [Problem to be solved by the invention]

[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, which 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 ordinary 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. [Means for solving the problem]

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

[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. [Brief explanation of the drawings]

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

[0011] (Metal foil manufacturing method) [First embodiment] The method for producing a metal foil according to 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 producing a metal foil by removing the resin substrate (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 showing an example of the process of the method for producing a metal foil according to the first embodiment. 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 as long as the arithmetic mean roughness Ra of one surface is 50 nm or less, and can be any suitable one depending on the purpose. It may be a commercially available product or may be formed by a resin substrate forming process.

[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 base material formation process> The resin substrate forming step is a step 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 a thermoplastic resin composition with one surface thereof flattened; a method of curing a curable resin composition with one surface thereof flattened; and a method of flattening the surface of a resin molded body. Examples of methods for flattening one surface of a thermoplastic resin composition or a curable resin composition include a method of flattening the one surface by placing it against a flat mold, a method of flattening each composition by leaving it still, etc. Examples of methods for flattening the surface of a resin molded product include heat treatment, polishing, etc.

[0016] <Metal foil forming process> The metal foil forming step is a step of forming a metal foil on the resin substrate. The method for 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, a combination of sputtering and electroplating, etc. These methods may be performed in a single process, multiple processes, or a combination of multiple processes.

[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 base material removal process> 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. Examples thereof include a method of melting the resin substrate; a method of washing the resin substrate with a solvent; and a combination thereof.

[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] The method for producing a metal foil according to the second embodiment includes the steps 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 removing the resin substrate to produce a metal foil, and may further include 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 showing an example of the process of the metal foil manufacturing method of the second embodiment. 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 continuing from the flat portions 20a, a master 30 having flat portions 30a and concave portions 30b continuing from 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 to prepare the resin substrate 20 (FIG. 6).

[0026] Next, a metal foil 200 is formed on the flat portions 20a and the protruding 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 recessed 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 a flat portion with an arithmetic mean roughness Ra of 50 nm or less and a convex portion connected to the flat portion, 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 process.

[0028] <Resin base material formation process> The resin substrate forming step is a step of forming 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 examples of such a method include a method of heating and plasticizing a thermoplastic resin composition with one surface thereof placed against a mold; a method of curing a curable resin composition with one surface thereof 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 depending on the purpose, and is 50 nm or less, preferably 20 nm or less, and more preferably 10 nm or less.

[0029] <Metal foil forming process> 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 base material removal process> The resin substrate removing step is a step of removing the resin substrate, and the items described in the resin substrate removing step of the first embodiment can be appropriately selected.

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

[0032] Fig. 9 is a schematic cross-sectional view showing an example of the metal foil of 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, seen from the flat portion 201a side. 9 to 11 is a second embodiment of a metal foil 200 having a first surface having flat portions 201a and recessed portions 201b connecting to the flat portions 201a, and a second surface having protruding portions 202a and flat portions 202b connecting to the protruding portions 202a. As shown in Fig. 9, the metal foil 200 has a thickness t, a height t, a diameter d1 of the recessed portions 201b, a diameter d2 of the protruding portions 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 protrusions 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 protrusions 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 it may also be an embodiment in which the arithmetic mean roughness Ra of the xy plane of the multiple convex portions 202a is 50 nm or less, or an embodiment in which the arithmetic mean roughness Ra of both is 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 can 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% or more and 80% or less, and preferably 30% or more and 70% or less, in order to reduce floating, peeling, and void generation from the substrate, and to achieve excellent adhesion to the substrate and excellent reliability. The area ratio of the total area of ​​the xy plane of the convex portion 202a to the area of ​​the metal foil when viewed from above is 20% or more and 80% or less, and preferably 30% or more and 70% or less, in order to reduce floating, peeling, and void generation from the substrate, and to achieve 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, and examples thereof include a pattern in which a plurality of shapes are arranged as the recesses 201b and protrusions 202a. 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, such as a 45° staggered pattern, a 60° staggered pattern, parallel squares, 60° staggered regular hexagons, equilateral triangles, and alternating rectangles.

[0041] (Metal foil for thermally conductive materials) [First embodiment] The metal foil for thermally conductive materials of the first embodiment 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 flat portions and recesses continuous with the flat portions, the arithmetic mean roughness Ra of the flat portions 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 further includes 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 of the constituent members, if necessary.

[0044] The heating element is not particularly limited and can be appropriately selected depending on the purpose. Examples 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 electronic components (heat generating elements), 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 member 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, and examples thereof include copper and aluminum. The heat spreader is usually in a flat plate shape. The heat sink is a member for releasing 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, and examples thereof include copper and aluminum. The heat sink has, for example, a plurality of fins. The heat sink has, for example, a base portion and a plurality of fins extending in a non-parallel direction (for example, a direction perpendicular to) one surface of the base portion. The heat spreader and the heat sink are generally solid structures with no internal voids. 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 heat spreader and a hollow plate-like structure similar to the heat sink. The heat pipe is a hollow structure having a cylindrical, approximately cylindrical, or flattened cylindrical shape, and 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 2a 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 component 3 and side walls 2b erected 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 component 3, such as a semiconductor element. 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 surface of the side wall 2b of the heat spreader 2 is also mounted on the wiring board 6, so that the side wall 2b surrounds the electronic component 3 at a predetermined distance. By providing the metal foil 7 of this embodiment on the main surface 2a of the heat spreader 2, a heat dissipation member that absorbs the heat generated by the electronic components 3 and dissipates the heat from the heat sink 5 is formed. [Example]

[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 <Metal foil manufacturing> According to the manufacturing method shown in FIGS. 1 to 3, the metal foil having a flat surface of the first embodiment was manufactured by the following procedure.

[0051] <<Preparing the 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 composition was left to stand under vacuum for 2 hours to ensure a flat surface. After that, the composition was plasticized by heating at 200°C for 3 hours to form a resin substrate with a flat surface. The surface roughness of the obtained resin substrate was measured using a white light interferometer (device name: NewView7300, 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 × 70 μm.

[0052] <<Formation of metal foil and removal of resin substrate>> Next, the surface of the resin substrate was subjected to electroless plating of copper 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] FIG. 13 shows the measurement results of the surface roughness of the first surface of the metal foil of Example 1, and FIG. 14 shows the measurement results of the surface roughness of the second surface of the metal foil of Example 1. The surface roughness of the resulting metal foil was measured, and the arithmetic mean roughness Ra of the first surface, which was in contact with the resin substrate, was 10 nm, confirming the same level of flatness as the resin substrate. The arithmetic mean roughness Ra of the second surface, which was the opposite side to the first surface of the metal foil, was 14 nm.

[0054] <Manufacturing of the joint body> 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 (SiC substrate with a 0.5 μm Au plating layer on the surface) were used as the substrate and the counter substrate, respectively. The metal foil was placed so that the flat surface of the metal foil was in contact with the copper substrate, and the silicon substrate was then laminated on top of the metal foil to prepare a laminate. The laminate was bonded under pressure and heating for 1 minute using a bonder (model: SB6e, manufactured by SUSS MicroTech) set at a load of 18 MPa and a temperature of 370°C in a N2 atmosphere, to produce the bonded structure of Example 1.

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

[0056] The thermal resistance [℃·cm] of the resulting bonded body was measured using a method in accordance with ASTM-D5470. 2 The thermal resistance of the metal foil was calculated by subtracting the thermal resistance 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 higher. Good: 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 body was evaluated based on the rate of change in thermal conductivity before and after a thermal cycle (-40°C to 150°C, 100 times) in accordance with JIS C 60068-2-14. [Evaluation criteria] ◎: The rate of change is 10% or less. ○: The rate of change is between 10% and 15%. △: The rate of change is 15% or more and 20% or less. ×: The rate of change is 20% or more.

[0058] (Examples 2 and 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 <Metal foil manufacturing> 4 to 9, the metal foil having flat portions and convex portions and the bonded body of the second embodiment shown in Figures 9 to 11 were manufactured and evaluated by the following procedures. The results are shown in Table 1.

[0060] <<Preparing the resin substrate>> The synthesized soluble polyimide resin composition was poured into a 100 mm x 100 mm mold 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. After that, the mold was plasticized by heating at 200°C for 3 hours, forming a resin substrate with flat portion 20a and multiple convex portions 20b consisting of 50 μm diameter, 100 μm high cylinders arranged in a 90° grid pattern at a 200 μm pitch (see Figures 4 to 6). The surface roughness of flat portion 20a of the resin substrate was measured and found to have an arithmetic mean roughness Ra of 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, this conductive substrate was subjected to electroless plating of copper 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 part of the metal foil that was in contact with the flat part 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 part of the resin substrate.

[0063] <Manufacturing of the joint body> 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 (SiC substrate with a 0.5 μm Au plating layer on the surface) were used as the base and counter substrate, respectively. The metal foil was placed so that the flat portion of the metal foil was in contact with the copper substrate, and the silicon substrate was then laminated on top of the metal foil to prepare a laminate. The laminate was bonded under pressure and heating for 1 minute using a bonder (SB6e, manufactured by SUSS MicroTech) set at a load of 18 MPa and a temperature of 370°C in a N2 atmosphere, producing the bonded structure 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 in the following manner. 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 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 flask was stirred at 50°C for 6 hours. Acetic acid (1.4 g) was added to the flask, and the 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 the 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 (phenolic compound, manufactured by Clariant, 0.5 g), and ADK STAB 3010 (phosphorus compound, manufactured by ADEKA Corporation, 0.5 g) were mixed and degassed to obtain a filler.

[0067] <Manufacturing of the joint body> 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 (SiC substrate with a 0.5 μm Au-plated layer on the surface) were used as the substrate and the opposing substrate. 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 then laminated on top of the metal foil to prepare a laminate. A bonder (model: SB6e, manufactured by SUSS MicroTech) was set to a load of 18 MPa and a temperature of 370°C in a N2 atmosphere, and the bonding was performed under pressure and heat for 1 minute. Next, a filler was filled between the multiple protrusions of the metal foil and the silicon substrate, and the laminate was bonded by curing at 100°C for 3 hours, producing the bonded structure of Example 4.

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

[0069] (Comparative Example 2) Evaluation was carried out in the same manner as in Example 1, except that a commercially available copper foil with 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 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] [Table 1]

[0071] [Table 2] [Explanation of symbols]

[0072] 1. Thermal conductive sheet 2 heat spreaders 2a Main surface 3 Heat generating elements (electronic components) 3a Top side 5 Heatsink 6. Wiring board 7 Metal foil 10 Resin substrate 20 Resin substrate 100 Metallic Foil 200 Metal Foil

Claims

1. 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, A method for producing a metal foil, wherein the metal foil has an arithmetic mean roughness Ra of 50 nm or less.

2. 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 removing the resin substrate to produce a metal foil, A method for producing a metal foil, wherein the metal foil has an arithmetic mean roughness Ra of 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. 3. The method for producing a metal foil according to claim 1, wherein the resin substrate is removed by at least one of melting and washing with a solvent.

5. 3. The method for producing a metal foil according to claim 1, wherein the resin substrate is made of a resin selected from the group consisting of polyimide, polycarbonate, and polyurethane.

6. 3. The method for producing a metal foil according to claim 1, 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 claim 1 or 2, wherein the metal foil has an average thickness of 10 nm or more and 1 mm or less.

8. 3. The method for producing a metal foil according to claim 1, wherein the metal foil has an arithmetic mean roughness Ra of 20 nm or less.

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

10. A flat portion and a recessed portion connected to the flat portion, The arithmetic mean roughness Ra of the flat portion is 50 nm or less, A metal foil for use as a thermally conductive material, characterized in that 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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