JP7831336B2 - Method for forming metal films - Google Patents

Description

This invention relates to a method for forming a metal film with a predetermined pattern on the surface of a substrate.

Conventionally, metal films have been formed by depositing metal onto the surface of a substrate using electroplating (see, for example, Patent Document 1). The film-forming apparatus includes a container for holding the plating solution. The container has an opening, which is sealed with an electrolyte membrane. The film-forming apparatus further includes a pressing mechanism that uses the liquid pressure of the plating solution to press the substrate with the electrolyte membrane.

Here, if a metal underlayer with a predetermined pattern is formed on the surface of the substrate, a voltage is applied between the anode and the substrate while pressing the substrate with the liquid pressure of the electrolyte membrane. This allows a metal film with the predetermined pattern to be formed on the underlayer. However, if the substrate does not have an underlayer with the predetermined pattern, it is conceivable to use a masking material, for example, as shown in Patent Document 2.

Japanese Patent Publication No. 2022-001658 Japanese Patent Publication No. 2016-108586

Here, when a screen mask is used as a masking material to deposit a metal film, the screen mask is sandwiched between the substrate and the electrolyte membrane. In this state, to ensure adhesion between the substrate and the screen mask, the screen mask is pressed against the electrolyte membrane by the liquid pressure of the plating solution while the metal film is deposited on the substrate surface. However, during film deposition, if a conductive fluid or similar material is not filled into the perforations of the predetermined pattern formed on the screen mask when current is passed between the anode and the substrate, film deposition defects may occur.

This invention has been made in view of the above points, and aims to provide a method for forming a metal film that can stably form a metal film having a predetermined pattern using a screen mask.

In view of the above problems, the present invention provides a method for forming a metal film, wherein a screen mask is sandwiched between an electrolyte membrane and a substrate, and a metal film having a predetermined pattern is formed on the substrate by electroplating. The method includes: a placement step of placing the screen mask, on which perforations of a predetermined pattern are formed, onto the substrate; a filling step of applying the liquid pressure of the plating solution in a container sealed with the electrolyte membrane to press the substrate with the screen mask through the electrolyte membrane, and filling the perforations with the plating solution that has seeped out from the container through the electrolyte membrane; and a film formation step of applying an electric current between an anode in contact with the plating solution and the substrate while the substrate is being pressed by the screen mask, thereby allowing metal ions contained in the plating solution to pass through the electrolyte membrane, and forming a metal film on the substrate in the predetermined pattern derived from the metal ions. The liquid pressure of the plating solution in the filling step is made greater than the liquid pressure of the plating solution in the film formation step.

In a preferred embodiment, the film formation step involves circulating the plating solution in a circulation path between the tank storing the plating solution and the container by driving a pump, while forming the metal film. In the filling step, after blocking the circulation path, a piston connected to the container increases the liquid pressure of the plating solution within the container to a first liquid pressure. In the film formation step, after opening the circulation path, the pump is driven to adjust the liquid pressure of the plating solution within the container to a second liquid pressure lower than the first liquid pressure.

In a more preferred embodiment, during the filling step, a constant current is passed between the anode and the substrate, and the voltage acting between the anode and the substrate is measured. When the measured voltage falls below a preset value, it is determined that the plating solution has filled the through-hole, and the process proceeds to the film formation step.

According to the present invention, a metal film having a predetermined pattern can be stably formed using a screen mask.

(a) is a schematic cross-sectional view showing an example of a metal film deposition apparatus according to an embodiment of the present invention, (b) is a schematic perspective view of a screen mask, and (c) is a partially enlarged cross-sectional view along the line A-A in (b). This is a flowchart illustrating an example of a method for forming a metal film according to an embodiment of the present invention. (a) is a schematic cross-sectional view illustrating the filling process shown in Figure 2, and (b) is a schematic cross-sectional view illustrating the film formation process shown in Figure 2. A table-like figure shows the results of the examples and comparative examples of the present invention.

First, a film deposition apparatus 1 used in a metal film deposition method according to an embodiment of the present invention will be described. Figure 1 is a schematic cross-sectional view showing an example of a metal film deposition apparatus according to an embodiment of the present invention.

As shown in Figure 1, the film deposition apparatus 1 is a film deposition apparatus that deposits a metal film F of a predetermined pattern P onto a substrate B by electroplating, with a mask structure 60 sandwiched between the electrolyte membrane 13 and the substrate B. Specifically, the film deposition apparatus 1 comprises an anode 11, an electrolyte membrane 13, and a power supply 14 that applies a voltage between the anode 11 and the substrate B.

The film deposition apparatus 1 comprises a housing 15 containing an anode 11 and a plating solution L, a mounting table 40 on which the substrate B is placed, and a mask structure 60 positioned on the surface of the substrate B. The film deposition apparatus 1 further includes a linear actuator 70 for raising and lowering the housing 15. The linear actuator 70 has a rod 72 that moves linearly relative to the main body 71, and the housing 15 is fixed to the tip of the rod 72. In this embodiment, for the sake of explanation, the electrolyte membrane 13 is positioned below the anode 11, and the mask structure 60 and substrate B are positioned further below it. However, the positional relationship between the electrolyte membrane 13 and the substrate B is not particularly limited, as long as a metal film F can be deposited on the surface of the substrate B.

Substrate B functions as a cathode. Substrate B is a plate-shaped substrate. In this embodiment, substrate B is a rectangular substrate. Of the surface of substrate B, the opposing surface Ba facing the electrolyte membrane 13 (screen mask 62) is the film-forming surface that functions as the cathode. The material of substrate B is not particularly limited as long as it functions as a cathode (i.e., a conductive surface). Substrate B may be made of a metallic material such as aluminum or copper. When forming a wiring pattern from a metal film F, substrate B is a substrate with an underlayer of copper or the like formed on the surface of an insulating substrate such as resin. In this case, after forming the metal film F, the underlayer other than the area where the metal film F is formed is removed by etching or the like. This allows a wiring pattern of the metal film F to be formed on the surface of the insulating substrate.

The anode 11 is, for example, a non-porous anode made of the same metal as the metal film. The anode 11 has a block-like or plate-like shape. The anode 11 may also be porous, a mesh, or a cage containing multiple balls. Examples of materials for the anode 11 include copper. The anode 11 dissolves upon application of voltage from the power supply 14. However, when forming a film using only metal ions from the plating solution L, the anode 11 is insoluble in the plating solution L. The anode 11 is electrically connected to the positive electrode of the power supply 14. The negative electrode of the power supply 14 is electrically connected to the substrate B via the mounting base 40. Furthermore, the film deposition apparatus 1 is equipped with a voltmeter 18 for measuring the voltage between the anode 11 and the substrate B during film deposition.

The plating solution L is a liquid containing the metal to be formed into a metal film in an ionic state. Examples of such metals include copper, nickel, gold, or silver. The plating solution L is a solution obtained by dissolving (ionizing) these metals with an acid such as nitric acid, phosphoric acid, succinic acid, sulfuric acid, sulfamic acid, or pyrophosphate. Examples of solvents for this solution include water and alcohol. For example, if the metal is copper, the plating solution L could be an aqueous solution containing copper sulfate, copper pyrophosphate, etc.

The electrolyte membrane 13 is a membrane that, when brought into contact with the plating solution L, can impregnate (contain) metal ions together with the plating solution L. The electrolyte membrane 13 is a flexible membrane. The material of the electrolyte membrane 13 is not particularly limited, as long as it allows metal ions from the plating solution L to move to the substrate B side when a voltage is applied by the power supply 14. Examples of materials for the electrolyte membrane 13 include ion-exchange resins such as fluoropolymer resins like Nafion® manufactured by DuPont. The film thickness of the electrolyte membrane 13 is preferably in the range of 5 μm to 200 μm. More preferably, the film thickness is in the range of 20 μm to 60 μm.

The containment body 15 is made of a material insoluble in the plating solution L. The containment body 15 has a containment space 15a for containing the plating solution L. The anode 11 is positioned in the containment space 15a of the containment body 15. An opening 15d is formed on the substrate B side of the containment space 15a. The opening 15d of the containment body 15 is covered with an electrolyte membrane 13.

As shown in Figures 1(a) and 3(a), the linear actuator 70 raises and lowers the housing 15 by linearly moving the rod 72 so that the electrolyte membrane 13 and the mask structure 60 can move toward and away from each other. In this embodiment, the mounting base 40 is fixed, and the housing 15 is raised and lowered by the linear actuator 70. The linear actuator 70 is an electrically operated actuator that converts the rotational motion of a motor into linear motion using a ball screw or the like (not shown). However, a hydraulic or pneumatic actuator may be used instead of an electrically operated actuator.

The housing 15 has a supply port 15b for supplying the plating solution L to the housing space 15a. The housing 15 also has a discharge port 15c for discharging the plating solution L from the housing space 15a. The supply port 15b and the discharge port 15c are holes communicating with the housing space 15a. The supply port 15b and the discharge port 15c are formed on either side of the housing space 15a. The supply port 15b is connected to the supply pipe 51. The discharge port 15c is fluidly connected to the discharge pipe 52.

The film deposition apparatus 1 further comprises a tank 91, a supply pipe 51, a discharge pipe 52, and a pump 80. As shown in Figure 1(a), the tank 91 contains the plating solution L. The supply pipe 51 connects the tank 91 to the container 15. The pump 80 is provided on the supply pipe 51. The pump 80 supplies the plating solution L from the tank 91 to the container 15. The discharge pipe 52 connects the tank 91 to the container 15. The discharge pipe 52 is provided with a pressure regulating valve 54. The pressure regulating valve 54 adjusts the pressure (liquid pressure) of the plating solution L in the container space 15a to a predetermined pressure. Furthermore, an on-off valve 55A is provided between the pump 80 and the container 15, and an on-off valve 55B is provided between the container 15 and the pressure regulating valve 54.

In this embodiment, the plating solution L is drawn from the tank 91 into the supply pipe 51 by driving the pump 80. The drawn-in plating solution L is then pumped from the supply port 15b into the containment space 15a. The plating solution L in the containment space 15a is returned to the tank 91 via the discharge port 15c. A circulation path 50 for circulating the plating solution L can be formed between the tank 91 and the containment body 15.

Furthermore, by continuously driving the pump 80, the liquid pressure of the plating solution L in the containment space 15a can be maintained at a predetermined pressure by the pressure regulating valve 54. The pump 80 presses the screen mask 62 of the mask structure 60 with the electrolyte membrane 13, which is subjected to the liquid pressure of the plating solution L. Furthermore, the film deposition apparatus 1 has a piston 92 that pressurizes the plating solution L contained in the containment space 15a.

The piston 92 is connected to the pressurized passage 15e of the housing 15. The pressurized passage 15e communicates with the housing space 15a. With the aforementioned on-off valves 55A and 55B closed, advancing the piston 92 pressurizes the plating solution L in the housing space 15a, and retracting the piston 92 depressurizes the plating solution L in the housing space 15a. Although the passages for the piston 92 and the pump 80 leading to the housing space 15a are shown as parallel passages (separate passages), they may also be the same passage.

The mask structure 60 comprises a frame 61 and a screen mask 62. The screen mask 62 has through-portions 68 formed according to a predetermined pattern P of the metal film F. The screen mask 62 includes a mesh portion 64 and a mask portion 65. The screen mask 62 is a mask with a flexibility of approximately 50 μm to 400 μm. The screen mask 62 is supported by the frame 61 on the surface of the frame 61 that faces the substrate B (the opposing surface).

The mesh portion 64 is fixed to the frame 61. The mesh portion 64 is stretched with a predetermined tension so as to cover the opening of the frame 61. The mesh portion 64 has multiple openings 64c formed in a grid pattern. The mesh portion 64 is woven in a mesh-like structure with multiple oriented wires 64a and 64b intersecting each other. The multiple wires 64a are arranged with spacing between them, and the multiple intersecting wires 64b are also arranged with spacing between them. As a result, multiple openings 64c are formed in a grid pattern in the mesh portion 64. The material of the wires 64a and 64b is not particularly limited, as long as it has corrosion resistance to the plating solution L. Examples of materials for the wires 64a and 64b include metal materials such as stainless steel, or resin materials such as polyester.

The mask portion 65 is fixed to the mesh portion 64 on the side of the mesh portion 64 facing the substrate B. The mask portion 65 has through-holes 68 formed according to a predetermined pattern P. The mask portion 65 is the part that adheres to the substrate B during film formation due to pressure from the electrolyte membrane 13. The material of the mask portion 65 is not particularly limited as long as it can adhere to the substrate B. It is preferable that the mask portion 65 undergoes compressive elastic deformation due to pressure from the electrolyte membrane 13. For example, the material of the mask portion 65 can be a resin material such as acrylic resin, vinyl acetate resin, polyvinyl resin, polyimide resin, or polyester resin, or a rubber material such as silicone rubber. A screen mask 62 having a predetermined pattern P can be manufactured using general silkscreen manufacturing techniques using emulsions. Therefore, a detailed explanation of the manufacturing method of the screen mask 62 is omitted.

The frame 61 supports the periphery of the screen mask 62 with its opposing surface facing the base material B (mounting base 40). In this embodiment, the mask structure 60 and the housing 15 are separate components; however, for example, the frame 61 may be fitted into the opening 15d of the housing 15, making the mask structure 60 and the housing 15 a single unit. This not only allows the screen mask 62 to be attached to the housing 15, but also allows the screen mask 62 to support the deflection of the electrolyte membrane 13 caused by the weight of the plating solution L contained in the housing 15.

The screen mask 62 is fixed to the frame 61. In this embodiment, the screen mask 62 has a rectangular outer shape. Therefore, the frame 61 has a rectangular, frame-like shape. The material of the frame 61 is not particularly limited as long as it can maintain the shape of the mask structure 60. For example, the material of the frame 61 can be a metal material such as stainless steel, or a resin material such as thermoplastic resin. The frame 61 is formed, for example, from a metal plate by punching, and has a thickness of about 1 mm to 3 mm.

The following describes each step of the film deposition method shown in Figure 2, referring to Figures 1(a) and 3(a) and 3(b) described above. In this film deposition method, a metal film F with a predetermined pattern P is deposited on the surface of the substrate B via a screen mask 62 on which through-holes 68 of a predetermined pattern P are formed.

First, the placement process S1 is performed. In this process, the base material B is placed on the placement table 40. Specifically, the base material B is placed in the recess 41 of the placement table 40. Next, the arrangement process S2 is performed. Specifically, as shown in Figure 1(a), the mask structure 60 is placed on the base material B. This allows the screen mask 62, with the predetermined pattern P through-portions 68 formed on it, to be placed on the base material B.

Next, the filling process S3 is performed. In this process, as shown in Figure 3(a), the liquid pressure of the plating solution L in the container 15, which is sealed with an electrolyte membrane 13, is applied to press the substrate B with the screen mask 62 through the electrolyte membrane 13. At this time, the plating solution La that seeps out from the container 15 through the electrolyte membrane 13 fills the through-hole 68.

More specifically, in the filling process S3, first, the linear actuator 70 is driven to move the housing 15 towards the mounting base 40, bringing the electrolyte membrane 13 into contact with the mask structure 60. Next, the pump 80 is driven to supply the plating solution L to the housing space 15a of the housing 15. At this time, the on-off valves 55A and 55B are open, and the pressure of the plating solution L in the housing space 15a is set to the pressure set by the pressure regulating valve 54. However, at this point, it is sufficient that the housing space 15a within the housing 15 is filled with the plating solution L; the liquid pressure of the plating solution L is not particularly limited.

Next, the on-off valves 55A and 55B are closed, and the piston 92 is advanced. This, in the filling process S3, after blocking the circulation path 50, increases the liquid pressure of the plating solution L in the container 15 to the first liquid pressure by the piston 92 connected to the container 15. Here, since the electrolyte membrane 13 seals the plating solution L contained in the containment space 15a, the plating solution L contained in the containment space 15a passes through the electrolyte membrane 13 and seeps out toward the screen mask 62. The seeped-out plating solution La fills the through-holes 68 of the screen mask 62. The first liquid pressure of the plating solution L in the filling process S3 is higher than the liquid pressure of the plating solution L set by the pressure regulating valve 54 (the second liquid pressure of the plating solution L in the film formation process S4). This allows the plating solution La to quickly fill the through-holes 68 of the screen mask 62.

Furthermore, if the screen mask 62 is placed on the surface of the substrate B and the surrounding atmosphere is reduced pressure, the plating solution La can easily fill the through-holes 68. Additionally, if a groove or hole for air venting is provided on the surface of the screen mask 62, allowing communication from the through-holes 68 to the outside, the air in the through-holes 68 can be easily replaced with the plating solution La.

Here, in the filling step S3, while a constant current (a current of a certain magnitude) is passed between the anode 11 and the substrate B, the voltage acting between the anode 11 and the substrate B may be measured using a voltmeter 18. When the measured voltage falls below a preset value, it is determined that the plating solution L has filled the through-hole 68, and the process proceeds to the film formation step S4.

On the other hand, if it is determined in the filling step S3 that the plating solution La has not filled the through-hole 68 of the screen mask 62, even if the power supply 14 is controlled to flow a constant current between the anode 11 and the substrate B, the current will not flow easily, and the voltage value between the anode 11 and the substrate B will be higher than the voltage during normal film formation. Therefore, in such cases, the current between the anode 11 and the substrate B is released, the liquid pressure of the plating solution L in the container 15 is maintained at the first liquid pressure, the filling step S3 is continued, and after a predetermined time has elapsed, the voltage acting between the anode 11 and the substrate B is measured again in the same manner to determine whether the plating solution L has filled the through-hole 68. The current supply and voltage measurement are intermittently repeated until it is determined that the plating solution L has filled the through-hole 68.

Next, the film formation process S4 is performed. Here, with the substrate pressed by the screen mask 62, an electric current is passed between the anode 11, which is in contact with the plating solution L, and the substrate B. This allows the metal ions contained in the plating solution L to pass through the electrolyte membrane 13, forming a metal film F derived from the metal ions on the substrate B in a predetermined pattern P.

Specifically, as shown in Figure 3(b), the on-off valves 55A and 55B are opened to release the blockage of the circulation path 50, and then the pump 80 is driven to adjust the liquid pressure of the plating solution L in the container 15 to a second liquid pressure lower than the first liquid pressure. The liquid pressure adjustment to the second liquid pressure can be performed using the pressure regulating valve 54. This prevents excessive liquid pressure from the plating solution L from acting on the electrolyte membrane 13 during film formation, thereby suppressing deterioration of the electrolyte membrane 13. Note that the piston 92 is retracted when the pump 80 is driven.

In this way, by driving the pump 80 in the circulation path 50 between the tank 91 where the plating solution L is stored and the container 15, the plating solution L can be circulated while the metal film F is formed. In particular, since the liquid pressure of the plating solution L in the filling step S3 (first liquid pressure) is greater than the liquid pressure of the plating solution L in the film formation step S4 (second liquid pressure), the plating solution La is rapidly filled into the perforations 68 of the screen mask 62 during the filling step S3. Therefore, the metal film F can be formed stably. During film formation, when a voltage is applied, water molecules contained in the plating solution L pass through the electrolyte membrane 13 along with metal ions passing through the electrolyte membrane 13, so that moisture is stably maintained in the perforations 68 of the screen mask 62. This allows for the stable deposition of metal ions on the surface of the substrate B.

In the film formation process S4, after a predetermined thickness of metal film F is formed by applying current for a predetermined time, the separation process S5 is performed. In this process, the pump 80 is stopped, the plating solution L is discharged from the containment space 15a, and then the linear actuator 70 is driven to move the containment 15 away from the mounting table 40, separating the electrolyte membrane 13 from the mask structure 60. Subsequently, the mask structure 60 is removed from the surface of the substrate B, and the substrate B is removed from the mounting table 40. Through this series of operations, a substrate B with a predetermined pattern P of metal film F can be obtained.

Examples of the present invention are described below.
[Examples]
As a substrate for film formation, a glass epoxy substrate was prepared by impregnating a layer of glass fiber cloth with epoxy resin. Copper foil was formed on the surface of this glass epoxy substrate. Next, a copper film was formed using the film formation apparatus shown in Figure 1. A copper sulfate aqueous solution (Cu-BRITE-SED) manufactured by JCU Corporation was used as the plating solution, and a Cu plate was used as the anode. Nafion®, manufactured by DuPont, was used as the electrolyte membrane. Before film formation, a liquid pressure higher than the liquid pressure at the time of film formation was applied to the electrolyte membrane, and the plating solution that seeped out from the electrolyte membrane was filled into the perforations of the screen mask. As film formation conditions, the plating solution temperature was set to 42°C, the current density to 7 A/ ^dm² (current value 0.03 A), and the energizing time to 388 seconds to form the copper film. Film formation was performed under two conditions: the liquid pressure of the plating solution to 0.6 MPa and 1 MPa. The voltage between these was also measured when a constant current was passed between the substrate and the anode. These results are shown in Figure 4.

[Comparative Example]
A copper film was deposited in the same manner as in the example. The only difference from the example was that the filling process was omitted. The results are shown in Figure 4.

As shown in Figure 4, a good copper film was formed in the example. The state of the electrolyte membrane in the film deposition apparatus of the example and comparative example was checked after film deposition. In the comparative example, at the initial stage of current application (initial voltage application) during film deposition, the voltage between the anode and the substrate was high for a certain period of time, resulting in abnormal deposition of the copper film in some areas, and burn marks were observed on the surface of the copper film. In the comparative example, it is thought that the voltage between the anode and the substrate was high because the plating solution was not sufficiently filled in the perforations of the screen mask. However, it is thought that the voltage stabilized afterward because, during current application, water from the plating solution, along with copper ions, passed through the electrolyte membrane.

1: Film deposition apparatus, 13: Electrolyte membrane, 15: Container, 40: Mounting platform, 50: Circulation path, 62: Screen mask, 68: Through-hole, 90: Tank, B: Substrate, F: Metal film, L: Plating solution, P: Predetermined pattern

Claims (3)

A film formation method in which a metal film having a predetermined pattern is formed on a substrate by electroplating with a screen mask sandwiched between the electrolyte membrane and the substrate,
The aforementioned film formation method is
A placement step of placing the screen mask, which has a predetermined pattern of perforations formed on it, on the substrate,
A filling step in which the plating solution is sealed in a container with an electrolyte membrane, and the liquid pressure of the plating solution inside the container is applied to press the substrate with the screen mask through the electrolyte membrane, and the plating solution that seeps out from the container through the electrolyte membrane is filled into the through-holes,
The process includes a film formation step in which, while the substrate is pressed with the screen mask, an electric current is passed between the anode in contact with the plating solution and the substrate, thereby allowing metal ions contained in the plating solution to pass through the electrolyte membrane, and a metal film derived from the metal ions is formed on the substrate in the predetermined pattern,
A method for forming a metal film, characterized in that the liquid pressure of the plating solution in the filling step is made greater than the liquid pressure of the plating solution in the film formation step. The aforementioned film formation process involves circulating the plating solution in a circulation path between the tank in which the plating solution is stored and the container by driving a pump, while forming the metal film.
In the filling process, after blocking the circulation path, the piston connected to the container increases the liquid pressure of the plating solution inside the container to a first liquid pressure.
The method for forming a metal film according to claim 1, characterized in that, in the film formation step, after opening the blockage of the circulation path, the pump is driven to adjust the liquid pressure of the plating solution in the container to a second liquid pressure lower than the first liquid pressure. In the aforementioned filling process,
With a constant current flowing between the anode and the substrate, the voltage acting between the anode and the substrate is measured.
A method for forming a metal film according to claim 1 or 2, characterized in that when the measured voltage falls below a preset value, it is determined that the plating solution has been filled into the through-hole, and the process proceeds to the film formation step.