Method for forming a metal film

Abstract

A method for forming a metal film using a solid electrolyte membrane, which can form a metal film with a smooth surface and can fully exhibit the function of an additive. A method for forming a metal film successively includes: (a) a step of supplying a solution containing metal ions and an additive to a solution storage space; (b) a step of increasing the pressure of the solution in the solution storage space while the solution storage space is not in communication with the solution tank and the substrate held on the pedestal is in contact with the solid electrolyte membrane; (c) a step of reducing the pressure of the solution in the solution storage space; and (d) a step of forming the metal film on the substrate by applying a voltage between the anode and the substrate while circulating the solution between the solution storage space and the solution tank.

Description

Technical Field

[0001] The present invention relates to a method for forming a metal film. Background Art

[0002] As one of the methods for forming a metal film on the surface of a substrate, there is a plating method. The plating method is widely used in the manufacture of various products such as wiring boards. However, after forming a metal film by the plating method, it is necessary to wash the substrate with water, and there is a problem of generating a large amount of waste liquid. Therefore, in Patent Document 1, a method for forming a metal film is proposed in place of the conventional plating method. In the method described in Patent Document 1, a solid electrolyte membrane is disposed between an anode and a cathode (substrate), a solution containing metal ions is disposed between the anode and the solid electrolyte membrane, the solid electrolyte membrane is brought into contact with the substrate, and a voltage is applied between the anode and the substrate to deposit metal on the surface of the substrate.

[0003] Prior Art Documents

[0004] Patent Documents

[0005] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2014-185371 Summary of the Invention

[0006] In the method described in Patent Document 1, the metal film grows while contacting the solid electrolyte membrane. If the solid electrolyte membrane has wrinkles or there are bubbles between the solid electrolyte membrane and the metal film, a metal film with low surface smoothness may sometimes be formed.

[0007] In the conventional plating method, various additives for controlling the surface shape, appearance, physical properties, etc. of the metal film are generally added to the plating solution. It is desired that additives can also be used to improve the quality of the metal film in the method using a solid electrolyte membrane as described in Patent Document 1.

[0008] Therefore, there is provided a method for forming a metal film using a solid electrolyte membrane, which can form a metal film having a smooth surface and can fully exhibit the function of the additive.

[0009] According to one aspect of the present invention, there is provided a method for forming a metal film on a substrate using a film forming apparatus, the film forming apparatus having:

[0010] An anode;

[0011] A holder for holding the substrate;

[0012] A solid electrolyte membrane disposed between the anode and the holder;

[0013] A housing that demarcates a solution storage space between the anode and the solid electrolyte membrane; and

[0014] A solution tank capable of communicating with the solution storage space,

[0015] The method successively includes:

[0016] (a) A step of supplying a solution containing ions of the metal and an additive to the solution storage space;

[0017] (b) A step of increasing the pressure of the solution in the solution storage space while the solution storage space is not in communication with the solution tank and the substrate and the solid electrolyte membrane held on the pedestal are in contact;

[0018] (c) A step of reducing the pressure of the solution in the solution storage space; and

[0019] (d) A step of forming a film of the metal on the substrate by applying a voltage between the anode and the substrate while circulating the solution between the solution storage space and the solution tank.

[0020] In the method of the present invention, a metal film having a smooth surface can be formed, and the function of the additive can be fully exerted. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic cross-sectional view showing an example of a film forming apparatus used in the method according to the embodiment.

[0022] Figure 2 It is a flowchart showing the method according to the embodiment.

[0023] Figure 3 It is a schematic cross-sectional view showing an example of a film forming apparatus in which a substrate and a solid electrolyte membrane are in contact.

[0024] Figure 4 It is a schematic cross-sectional view showing an example of a film forming apparatus in the step of supplying a solution to a solution storage space.

[0025] Figure 5 It is a schematic cross-sectional view showing an example of a film forming apparatus in the step of increasing the pressure of the solution in the solution storage space.

[0026] Figure 6 It is a schematic cross-sectional view showing an example of a film forming apparatus in the step of forming a metal film.

[0027] Figure 7 It is a photograph of the copper film of Example 1.

[0028] Figure 8 This is a photograph of the copper film of Comparative Example 1.

[0029] Figure 9 This is a photograph of the copper film of Comparative Example 2.

[0030] Explanation of Reference Numerals

[0031] 10: Substrate; 50: Film-forming apparatus; 51: Anode; 52: Solid electrolyte membrane; 53: Housing; 54: Power supply unit; 55: Pressurizing mechanism; 56: Pedestal; 59: Solution storage space; 61: Solution tank; L: Solution. Detailed Description of Embodiments

[0032] Hereinafter, the embodiments will be described with appropriate reference to the drawings. Furthermore, in the drawings referred to in the following description, the same reference numerals are assigned to the same components or components having the same functions, and repeated descriptions may be omitted. In addition, for ease of explanation, the dimensional ratios in the drawings are sometimes different from the actual ratios, or a part of the components is omitted in the drawings. In the present application, a numerical range represented by the symbol "~" includes the numerical values described before and after the symbol "~" as the lower limit value and the upper limit value, respectively. In addition, the present invention is not limited to the following embodiments, and various design changes can be made without departing from the spirit of the present invention described in the claims.

[0033] (1) Film-forming apparatus

[0034] An example of the film-forming apparatus used in the method according to the embodiment will be described. Figure 1 The film-forming apparatus 50 shown in the figure includes an anode 51, a pedestal 56, a solid electrolyte membrane 52, a housing 53, and a solution tank 61.

[0035] The anode 51 has a conductivity capable of functioning as an electrode. The anode 51 is composed of at least one of a metal (such as gold) having a standard redox potential higher than the standard redox potential (standard electrode potential) of the metal to be formed using the film-forming apparatus 50 and the metal to be formed using the film-forming apparatus 50. The shape and size of the anode 51 can be appropriately set. For example, it can have a shape such as a foil shape, a plate shape, or a spherical shape.

[0036] The pedestal 56 holds the substrate 10 such that the surface 10a of the substrate 10 on which the metal film is to be formed faces the anode 51. The pedestal 56 can be, for example, a table capable of placing the substrate 10.

[0037] At least a part of the surface of the substrate 10 has a conductivity that can function as an electrode. The substrate 10 can be, for example, a substrate made of a conductive material or an insulating substrate having a film of a conductive material formed on at least a part of its surface. Examples of the conductive material include Pt, Pd, Rh, Cu, Ag, Au, Ti, Al, Cr, Si, and their alloys, FeSi2, CoSi2, MoSi2, WSi2, VSi2, ReSi 1.75 , silicides such as CrSi2, NbSi2, TaSi2, TiSi2, ZrSi2, especially transition metal silicides, conductive metal oxides such as TiO2, SnO, GeO, ITO (indium tin oxide), and conductive resins. Examples of the insulating substrate include substrates containing resin and glass such as glass epoxy substrates, resin substrates, and glass substrates. Examples of the resin include thermoplastic resins such as PET resin, PI resin, LCP (liquid crystal polymer), epoxy resin, ABS resin, AS resin, AAS resin, PS resin, EVA resin, PMMA resin, PBT resin, PPS resin, PA resin, POM resin, PC resin, PP resin, PE resin, polymer alloy resins containing elastomer and PP, modified PPO resin, PTFE resin, ETFE resin, etc., thermosetting resins such as phenolic resin, melamine resin, amino resin, unsaturated polyester resin, polyurethane, diallyl phthalate, silicone resin, alkyd resin, etc., and resins obtained by adding cyanate resin to epoxy resin.

[0038] The surface 10a of the substrate 10 is electrically connected to the power supply unit 54 described later. In Figure 1 , the surface 10a of the substrate 10 is electrically connected to the power supply unit 54 via the pedestal 56. The film forming apparatus 50 may also include a conductive member (not shown) that covers at least a part of the edge portion of the surface 10a of the substrate 10, and the surface 10a of the substrate 10 may be electrically connected to the power supply unit 54 via the conductive member.

[0039] The solid electrolyte membrane 52 is disposed between the anode 51 and the pedestal 56. As the solid electrolyte membrane 52, for example, a fluorine-based resin such as Nafion (registered trademark) manufactured by DuPont, a hydrocarbon-based resin, a polyamic acid resin, a membrane of a resin having a cation exchange function such as Ceramion (CMV, CMD, CMF series) manufactured by Asahi Glass Co., Ltd. can be used. The solid electrolyte membrane 52 may have a thickness of, for example, about 5 μm to about 200 μm.

[0040] A housing 53 defines a solution storage space 59 between an anode 51 and a solid electrolyte membrane 52. The housing 53 includes a cylindrical or polygonal cylindrical trunk portion 53c having openings at both the upper and lower portions, and a lid portion 53d that closes the opening at the upper portion of the trunk portion 53c. The opening at the lower portion of the trunk portion 53c is covered by the solid electrolyte membrane 52. The anode 51 is disposed between the solid electrolyte membrane 52 and the lid portion 53d, spaced apart from the solid electrolyte membrane 52. A solution storage space 59 is defined between the anode 51 and the solid electrolyte membrane 52. A solution containing metal ions is stored in the solution storage space 59. Further, in Figure 1 the anode 51 is provided in contact with the lid portion 53d, but the anode 51 and the lid portion 53d may be spaced apart. In this case, a solution can also be stored between the anode 51 and the lid portion 53d. A supply port 53a and a discharge port 53b are provided in the housing 53.

[0041] A solution tank 61 is connected to the supply port 53a and the discharge port 53b of the housing 53 via a supply pipe 64a and a discharge pipe 64b, respectively. Valves 63a and 63b are provided at the connection portion between the supply pipe 64a and the supply port 53a and the connection portion between the discharge pipe 64b and the discharge port 53b, respectively. By opening and closing the valves 63a and 63b, the solution storage space 59 can be communicated with the solution tank 61 or disconnected from the solution tank 61. A pump 62 is connected to the supply pipe 64a or the discharge pipe 64b.

[0042] The film forming apparatus 50 may further include a lifting device (not shown) that lifts the pedestal 56 or the housing 53 so that the surface 10a of the substrate 10 held on the pedestal 56 contacts the solid electrolyte membrane 52. The lifting device may include a hydraulic or pneumatic cylinder, an electric actuator, a linear guide, a motor, etc.

[0043] The film forming apparatus 50 may further include a pressurizing mechanism 55 for increasing the pressure of the solution in the solution storage space 59. The pressurizing mechanism 55 may be, for example, a device that pushes the lid portion 53d toward the inside of the housing 53, and may include a hydraulic or pneumatic cylinder, an electric actuator, a linear guide, a motor, etc.

[0044] The film forming apparatus 50 further includes a power supply unit 54. The negative electrode of the power supply unit 54 is electrically connected to the surface 10a of the substrate 10 via the pedestal 56, and the positive electrode of the power supply unit 54 is electrically connected to the anode 51.

[0045] (2) Method for forming a metal film

[0046] The method according to the embodiment is as Figure 2As shown, it successively includes: a step (S1) of supplying a solution to a solution storage space; a step (S2) of increasing the pressure of the solution in the solution storage space; a step (S3) of decreasing the pressure of the solution in the solution storage space; and a step (S4) of forming a metal film while circulating the solution.

[0047] a) Step (S1) of supplying a solution to a solution storage space

[0048] First, a solution containing metal ions and an additive is prepared and put into a solution tank 61.

[0049] Examples of the metal ions include ions of Cu, Ni, Ag, and Au. The solution may further contain at least one of nitrate ions, phosphate ions, succinate ions, sulfate ions, and pyrophosphate ions. The solution may be a solution of a metal salt, such as a nitrate, phosphate, succinate, sulfate, pyrophosphate, or a mixture thereof.

[0050] The additive may be at least one additive selected from polymers, brighteners, and levelers. The additive may be the same as the additives used in ordinary electroplating and may be appropriately selected according to the material of the metal film formed by the method according to the embodiment.

[0051] As the polymer, a nonionic polyether polymer surfactant such as polyethylene glycol, polypropylene glycol, polyethylene oxide, or polyoxyalkylene glycol can be used. The polymer has the effect of improving the thickness uniformity of the formed metal film by forming a monomolecular film on the surface 10a of the substrate 10 and suppressing the precipitation of the metal.

[0052] As the brightener, a sulfur-containing organic compound can be used. More specifically, an organic sulfur compound having a sulfo group such as 3-mercaptopropanesulfonic acid, sodium salt of 3-mercaptopropanesulfonic acid, bis(3-sulfopropyl) disulfide, disodium salt of bis(3-sulfopropyl) disulfide, (3-sulfopropyl) N,N-dimethyldithiocarbamate, or sodium salt of (3-sulfopropyl) N,N-dimethyldithiocarbamate can be used. The brightener has the effect of promoting the precipitation of the metal, refining the precipitated metal particles, and imparting luster to the formed metal film.

[0053] As a leveling agent, nitrogen-containing organic compounds such as Janus Green B (JGB), safranine compounds, phenazine compounds, polyalkyleneimine, thiourea and its derivatives, and polyacrylic acidamide can be used. The leveling agent has the effect of adsorbing more on the convex portions where metal is likely to precipitate, inhibiting the precipitation of metal at the convex portions, and improving the flatness of the formed metal film.

[0054] As Figure 1 shown, the substrate 10 is held by the pedestal 56 of the film forming apparatus 50. Next, as Figure 3 shown, the pedestal 56 is raised and / or the housing 53 is lowered to bring the surface 10a of the substrate 10 into contact with the solid electrolyte film 52.

[0055] Next, as Figure 4 shown, the solution L is supplied from the solution tank 61 to the solution storage space 59. By operating the pump 62, the solution L is sent from the solution tank 61 to the supply pipe 64a and flows into the solution storage space 59 through the supply port 53a. The solution L comes into contact with the solid electrolyte film 52 and penetrates into the solid electrolyte film 52. As a result, the solid electrolyte film 52 contains the solution L inside.

[0056] b) Step (S2) of increasing the pressure of the solution in the solution storage space

[0057] After stopping the operation of the pump 62, the valves 63a and 63b are closed to disconnect the solution storage space 59 from the solution tank 61. That is, the solution storage space 59 is made not to communicate with the solution tank 61. Next, the pressure of the solution L in the solution storage space 59 is increased. For example, by applying an external force to the solution L in the solution storage space 59, the pressure of the solution L in the solution storage space 59 can be increased. Specifically, as Figure 5As shown, by using the pressing mechanism 55 to press the cover portion 53d toward the inside of the housing 53, an external force can be applied to the solution L to increase the pressure of the solution L. When there are wrinkles in the solid electrolyte membrane 52 or there are air bubbles between the solid electrolyte membrane 52 and the substrate 10, by increasing the pressure of the solution L, the wrinkles can be stretched out and the air bubbles can be removed. Since the valves 63a and 63b are closed, the pressure of the solution L in the solution storage space 59 can be increased to a pressure exceeding the allowable pressure of the solution tank 61, the components (such as the supply pipe 64a, the discharge pipe 64b, and the pump 62) between the solution tank 61 and the valves 63a and 63b, and their connection parts. The pressure of the solution L can be appropriately set within the range where the wrinkles and / or air bubbles can be sufficiently removed and the solid electrolyte membrane 52 is not broken, and can be, for example, 0.2 to 1 MPa.

[0058] c) Step (S3) of reducing the pressure of the solution in the solution storage space

[0059] Next, the pressure of the solution L in the solution storage space 59 is reduced. For example, by reducing the external force applied to the solution L in the solution storage space 59 (especially making it zero), the pressure of the solution L in the solution storage space 59 can be reduced. Specifically, by weakening the force with which the pressing mechanism 55 presses the cover portion 53d or stopping the pressing of the cover portion 53d, the pressure of the solution L can be reduced.

[0060] d) Step (S4) of forming a metal film while circulating the solution

[0061] Open the valves 63a and 63b to connect the solution storage space 59 and the solution tank 61. Next, operate the pump 62 so that the solution L circulates between the solution storage space 59 and the solution tank 61 via the supply pipe 64a and the discharge pipe 64b. While continuously circulating the solution L, apply a voltage between the anode 51 and the surface 10a of the substrate 10 using the power supply unit 54. Then, the metal ions contained in the solution L are reduced on the surface 10a of the substrate 10, and the metal is deposited on the surface 10a of the substrate 10. Moreover, metal ions are also reduced on the surface of the deposited metal, and further metal is deposited. Thus, a metal film is formed on the surface 10a of the substrate 10. Furthermore, the voltage applied between the anode 51 and the surface 10a of the substrate 10 can be appropriately set. By applying a higher voltage, the deposition rate of the metal can be increased. In addition, the solution L can also be heated. Thereby, the deposition rate of the metal can be increased.

[0062] Since the wrinkles of the solid electrolyte membrane 52 and / or the bubbles between the solid electrolyte membrane 52 and the surface 10a of the substrate 10 are removed in step S2, it is possible to prevent or reduce the adverse effects of the wrinkles and / or bubbles on the smoothness of the surface of the metal film in step S4. Therefore, a metal film with a smooth surface can be formed. In addition, since the metal film is formed while continuously circulating the solution L, it is possible to prevent the depletion of the additive in the solution storage space 59. Therefore, a sufficient amount of the additive can be supplied to the position where the metal is deposited and its vicinity, and the function of the additive can be fully exerted. For example, when the solution L contains a polymer as an additive, a metal film with a uniform thickness can be formed. When the solution L contains a brightener as an additive, a metal film with a metallic luster can be formed. When the solution L contains a leveling agent as an additive, a metal film with high flatness can be formed.

[0063] After forming a metal film with a desired thickness, the application of the voltage between the anode 51 and the substrate 10 is stopped, and the operation of the pump 62 is stopped to stop the circulation of the solution L. Then, the pedestal 56 is lowered and / or the housing 53 is raised to separate the solid electrolyte membrane 52 and the metal film (not shown). The substrate 10 on which the metal film is formed is removed from the pedestal 56.

[0064] Examples

[0065] The present invention will be specifically described below by way of examples, but the present invention is not limited to these examples.

[0066] Example 1

[0067] A film forming apparatus 50 as shown in Figure 1 was prepared. A net-like titanium container and copper balls accommodated therein were used as the anode 51. As the solid electrolyte membrane 52, Nafion (registered trademark) (thickness: about 8 μm) was used. As the substrate 10, an FR-4 copper clad laminate was prepared and held by the pedestal 56. A solution (“Cu BRITE SED” manufactured by JCU Corporation) containing copper ions and an additive was put into the solution tank 61 and heated to 42°C. Then, as shown in Figure 3 , the substrate 10 and the solid electrolyte membrane 52 were brought into contact with each other.

[0068] The valves 63a and 63b were opened, and the pump 62 was operated. As shown in Figure 4 , the solution L was supplied from the solution tank 61 to the solution storage space 59 at a flow rate of 1 L / min (step S1).

[0069] The operation of the pump 62 was stopped, and the valves 63a and 63b were closed. Then, as shown in Figure 5As shown, the pressure of the solution L in the solution storage space 59 is increased to 0.6 MPa by pushing the lid portion 53d with the pressurizing mechanism 55 (step S2).

[0070] Stop pushing the lid portion 53d with the pressurizing mechanism 55, and reduce the pressure of the solution L in the solution storage space 59 (step S3). The pressure of the solution L in the solution storage space 59 is approximately equal to the atmospheric pressure.

[0071] Open valves 63a and 63b, and operate the pump 62. As Figure 6 shown, the solution L is circulated between the solution tank 61 and the solution storage space 59 at a flow rate of 1 L / minute. A voltage is applied between the anode 51 and the substrate 10, and a current flows at a current density of 6.8 A / dm 2 . As a result, copper is deposited on the surface 10a of the substrate 10, forming a copper film (step S4). A photograph of the formed copper film is shown in Figure 7 . The copper film has a smooth surface and a uniform metallic luster.

[0072] Comparative Example 1

[0073] Step S1 was carried out in the same manner as in Example 1. While circulating the solution L between the solution tank 61 and the solution storage space 59 at a flow rate of 1 L / minute, a voltage was applied between the anode 51 and the substrate 10 in the same manner as in step S4 of Example 1, thereby forming a copper film on the surface 10a of the substrate 10. A photograph of the formed copper film is shown in Figure 8 . Concavities and convexities were observed on a part of the surface of the copper film (refer to the upper left part of Figure 8 ). It is considered that in this comparative example, since the step of increasing the pressure of the solution L in the solution storage space 59 was not carried out before forming the copper film, the wrinkles of the solid electrolyte membrane 52 did not unfold and remained, and these wrinkles were transferred to the copper film.

[0074] Comparative Example 2

[0075] Steps S1 and S2 were carried out in the same manner as in Example 1. While continuously pushing the lid portion 53d with the pressurizing mechanism 55, a voltage was applied between the anode 51 and the substrate 10, thereby forming a copper film on the surface 10a of the substrate 10. A photograph of the formed copper film is shown in Figure 9 . The metallic luster on the surface of the copper film is uneven. In this comparative example, since valves 63a and 63b were closed in order to increase the pressure of the solution L during the formation of the copper film, the solution L was not circulated. It is considered that as a result, the supply of the additive to the position where copper is deposited and its vicinity is insufficient, and the function of the additive cannot be fully exerted, so the metallic luster of the copper film becomes uneven.

Claims

1. A method for forming a metal film on a substrate using a film forming apparatus, wherein the film forming apparatus has: an anode; a pedestal for holding the substrate; a solid electrolyte membrane disposed between the anode and the pedestal; a housing that demarcates a solution storage space between the anode and the solid electrolyte membrane, the housing having: a cylindrical or polygonal cylindrical trunk portion having openings at the upper and lower portions, and a lid portion that closes the opening at the upper portion of the trunk portion, the opening at the lower portion of the trunk portion being covered by the solid electrolyte membrane; and a solution tank that can communicate with the solution storage space, the method successively includes: (a) a step of supplying a solution to the solution storage space, the solution containing ions of the metal and an additive, the additive being at least one selected from a polymer, a brightening agent, and a leveling agent; (b) a step of increasing the pressure of the solution in the solution storage space in a state where the solution storage space is not in communication with the solution tank and the substrate held on the pedestal is in contact with the solid electrolyte membrane; (c) a step of reducing the pressure of the solution in the solution storage space; and (d) a step of forming the metal film on the substrate by applying a voltage between the anode and the substrate while circulating the solution between the solution storage space and the solution tank, in step (b), in order to increase the pressure of the solution in the solution storage space, an external force is applied to the solution by pushing the lid portion toward the inside of the housing using a pressurizing mechanism, in step (c), the application of the external force is stopped.

2. The method for forming a metal film on a substrate using a film forming apparatus according to claim 1, wherein in step (a), the solution is supplied to the solution storage space in a state where the substrate held on the pedestal is in contact with the solid electrolyte membrane.

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