PRINTED CIRCUIT BOARD AND METHOD OF MANUFACTURING THE SAME

Abstract

Disclosed herein is a printed circuit board, including: a metal substrate; an anodic oxide layer formed by anodizing the metal substrate; circuit layers formed on the anodic oxide layer; and a first sol-gel layer formed by applying a photocatalytic material between circuit wirings of the circuit layers and then curing the applied photocatalytic material. The printed circuit board is advantageous in that it can be realized into a high-voltage package printed circuit board because a sol-gel layer is formed between circuit wirings of circuit layers.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No. 10-2010-0048232, filed May 24, 2010, entitled “Printed circuit board and the method of manufacturing thereof”, which is hereby incorporated by reference in its entirety into this application.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a printed circuit board and a method of manufacturing the same.

2. Description of the Related Art

Recently, alongside the rapid advancement of semiconductor technology necessary for signal processing, the development of semiconductor devices has been remarkable. Simultaneously, semiconductor packages, such as SIPs (system in packages), CSPs (chip sized packages), FCPs (flip chip packages) and the like, which are formed by mounting an electronic device, such as a semiconductor device, on a printed circuit board, have been under active development. Recently, with the advance of semiconductor technology, the size of the die has been decreased, so that the size of a package substrate for mounting a semiconductor device has also been decreased, with the result that the area in which a bond pad formed on a substrate to be connected with an electronic device can be realized has also been decreased.

Power devices, for example, silicon-controlled rectifiers (SCRs), power transistors, insulated gate bipolar transistors (IGBTs), metal-oxide semiconductor field-effect transistors (MOSFETs), power rectifiers, power regulators, inverters, converters, and high-power semiconductor chips formed of combinations thereof, are designed such that they are operated at a voltage of 30˜1000 V or at a voltage of more than 1000 V. Since high-power semiconductor chips, unlike low-power semiconductor chips such as logic devices and memory devices, operate at high voltage, they are required to have a high heat dissipation capacity and excellent insulating properties at high pressure.

FIG. 1 is a sectional view showing a conventional high-power semiconductor package 100. The conventional high-power semiconductor package 100 includes: a substrate 140 including a base layer 110, an insulation layer 120 and a circuit layer 130; a high-power semiconductor chip 150a and a low-power semiconductor chip 150b mounted on the circuit layer 130 of the substrate 140; and bonding pads respectively formed in the high-power semiconductor chip 150a and the low-power semiconductor chip 150b and connected with the circuit layer 130 by wires 170. Here, the circuit layer 130 is connected to leads serving as external terminals after the wire bonding process, and then an epoxy molding process is performed, completing the high-power semiconductor package 100. Generally, since a high-power semiconductor package emits a large amount of heat when it operates, a radiation plate 180 is provided on the base layer 110 of the high-power semiconductor package. The radiation plate 180 is generally made of a metal having high thermal conductivity. The radiation plate 180 may be adhered on the base layer 110 by an adhesive layer 185. Therefore, the conventional high-power semiconductor package 100 provided with the radiation plate 180 is problematic in that the base layer 110 is additionally required in order to provide the radiation plate 180, its thickness cannot be easily adjusted because the radiation plate 180 is additionally provided, and its size cannot be easily decreased. Further, the conventional high-power semiconductor package 100 is problematic in that the rapidity and reliability of the processes are deteriorated because the process of attaching the base layer 110 must be performed in addition to the process of mounting a semiconductor chip using a lead frame and a wire bonding process. Further, the conventional high-power semiconductor package 100 is problematic in that the total manufacturing cost thereof is increased because the base layer 180 is provided and the adhesive layer 185 is used. Furthermore, the conventional high-power semiconductor package 100 is problematic in that the desired heat dissipation effect cannot be sufficiently realized because the rate of heat radiation possible using the radiation plate 180 is limited.

Meanwhile, in the case of a printed circuit board for a semiconductor package, conventionally, an anodic oxide layer formed on an aluminum substrate has been used as an insulation layer. In this case, there is a problem in that an electric short occurs between the circuit wirings of a circuit layer. Moreover, there is a problem in that an electric short occurs due to the instability of the insulation layer during a process of precipitating residues of a metal layer for forming a circuit layer or additives (Mg, Si, Cu and the like) in anodic oxidation.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been devised to solve the above-mentioned problems, and the present invention provides a printed circuit board, which can improve heat radiation performance and can prevent an electric short from occurring between the circuit wirings of a circuit layer because a sol-gel layer is formed between the circuit wirings of the circuit layer and which can increase the heat radiation effect because an anodic oxide layer is removed from one side of a metal substrate, that is, the one side thereof not provided with a circuit layer, and provides a method of manufacturing the same.

An aspect of the present invention provides a printed circuit board, including: a metal substrate; an anodic oxide layer formed by anodizing the metal substrate; circuit layers formed on the anodic oxide layer; and a first sol-gel layer formed by applying a photocatalytic material between circuit wirings of the circuit layers and then curing the applied photocatalytic material.

Here, the photocatalytic material may be alumina or titanium dioxide.

Further, the anodic oxide layer may be formed only on one side and both lateral sides of the metal substrate.

Another aspect of the present invention provides a method of manufacturing a printed circuit board, including: providing a metal substrate; anodizing the metal substrate to form an anodic oxide layer; forming circuit layers on one side of the anodic oxide layer; and applying a photocatalytic material between circuit wirings of the circuit layers and then curing the applied photocatalytic material to form a first sol-gel layer.

Here, in the forming of the anodic oxide layer, the anodic oxide layer may be formed by anodizing only one side and both lateral sides of the metal substrate.

Further, the forming of the circuit layers may include: forming a seed layer on the anodic oxide layer; forming a circuit plating layer on the seed layer by electrolytic plating; applying an etching resist for forming circuit patterns onto the circuit plating layer and then etching the seed layer and the circuit plating layer; and removing the etching resist.

Further, the forming of the circuit layers may include: forming a seed layer on the anodic oxide layer; applying a plating resist for forming circuit patterns onto the seed layer; forming a circuit plating layer on the seed layer; and removing the plating resist to expose the seed layer, and then etching the exposed seed layer.

Further, the forming of the first sol-gel layer may include: applying a photocatalytic material onto the anodic oxide layer formed on the other side of the metal substrate and then curing the applied photocatalytic material to form a second sol-gel layer; removing the second sol-gel layer; and removing the anodic oxide layer formed on the other side of the metal substrate.

Further, the photocatalytic material may be alumina or titanium dioxide.

Further, the photocatalytic material may be applied by spraying, dipping or aerosol deposition.

Further, the applied photocatalytic material may be cured at a temperature of 100˜200° C.

Various objects, advantages and features of the invention will become apparent from the following description of embodiments with reference to the accompanying drawings.

The terms and words used in the present specification and claims should not be interpreted as being limited to typical meanings or dictionary definitions, but should be interpreted as having meanings and concepts relevant to the technical scope of the present invention based on the rule according to which an inventor can appropriately define the concept of the term to describe the best method he or she knows for carrying out the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a sectional view showing a conventional high-power semiconductor package;

FIG. 2 is a sectional view showing a printed circuit board according to a first embodiment of the present invention;

FIG. 3 is a sectional view showing a printed circuit board according to a second embodiment of the present invention;

FIGS. 4 to 11 are sectional views showing a method of manufacturing a printed circuit board according to a third embodiment of the present invention; and

FIGS. 12 to 19 are sectional views showing a method of manufacturing a printed circuit board according to a fourth embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the present invention will be more clearly understood from the following detailed description of preferred embodiments taken in conjunction with the accompanying drawings. Throughout the accompanying drawings, the same reference numerals are used to designate the same or similar components, and redundant descriptions thereof are omitted. Further, in the following description, the terms “first”, “second”, “one side”, “the other side” and the like are used to differentiate a certain component from other components, but the configuration of such components should not be construed to be limited by the terms. Further, in the description of the present invention, when it is determined that the detailed description of the related art would obscure the gist of the present invention, the description thereof will be omitted.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings.

FIG. 2 is a sectional view showing a printed circuit board according to a first embodiment of the present invention. As shown in FIG. 2, the printed circuit board according to a first embodiment of the present invention includes: a metal substrate 10; an anodic oxide layer 20 formed by anodizing the metal substrate 10; circuit layers 30 and 31 formed on the anodic oxide layer 20; and a first sol-gel layer 50 formed by applying a photocatalytic material between circuit wirings of the circuit layers 30 and 31 and then curing the applied photocatalytic material.

The metal substrate 10 is made of a material which can be formed into the anodic oxide layer 10 by anodizing, and which exhibits a heat radiation effect. The metal substrate 10 may be made of aluminum, magnesium, titanium or the like. The raw material of the metal substrate 10 is not particularly limited as long as it can be formed into the anodic oxide layer by anodizing and has heat radiation characteristics.

The anodic oxide layer 20 is formed by anodizing the metal substrate 10. That is, the anodic oxide layer 20 is formed in a uniform thickness by accelerating the oxidation of the metal substrate 10 using the metal substrate 10 as an anode in a specific solution such as a sulfuric acid solution or the like. Here, the thickness of the anodic oxide layer 20 is determined depending on the degree and period of anodizing, and the anodizing of the metal substrate 10 is performed within the range necessary for forming the anodic oxide layer 20 having insulation characteristics.

The circuit layers 30 and 31 are formed on the anodic oxide layer 20. The circuit layers 30 and 31 may be formed in a subtractive manner or an additive manner, and may be formed in various manners in addition to the subtractive or additive manner.

The first sol-gel layer 50 is formed by applying a photocatalytic material between circuit wirings of the circuit layers 30 and 31 and then curing the applied photocatalytic material. Here, the first sol-gel layer 50 is formed by a sol-gel process. The sol-gel process is a process of preparing “sol-gel derived ceramics”. Particularly, in the sol-gel process, the reaction rate of reactants and the structure of a final product are changed depending on various factors, such as the ratio of water and alkoxide, the pH of a solution, the kind and amount of a solvent and the like. In the sol-gel process, reactions which are sensitive to such a degree that experimental results can be changed with respect to each experimenter arise. In the formation of the first sol-gel layer 50, it is important to control a heat treatment temperature and a cooling rate. Meanwhile, in the sol-gel process, an alcohol solvent dispersed with nanosized ceramic powder is coated and then heat-treated, and, in this case, only the ceramic powder excluding organic components is coated, thus forming the first sol-gel layer 50. The first sol-gel layer 50 formed in this way has predetermined adhesivity and serves to prevent the metal substrate 10 from being damaged. The first sol-gel layer 50 may be formed by spraying, dipping or aerosol deposition. The first sol-gel layer 50 may be formed using a photocatalytic material. That is, the first sol-gel layer 50 may be formed by applying the photocatalytic material and then curing the applied photocatalytic material. The photocatalytic material may be made of titanium dioxide (TiO₂) or alumina (Al₂O₃).

The curing of the photocatalytic material is performed to remove organic components, and may be performed at a temperature of 100˜200° C.

FIG. 3 is a sectional view showing a printed circuit board according to a second embodiment of the present invention. As shown in FIG. 3, the printed circuit board according to a second embodiment of the present invention includes: a metal substrate 10; an anodic oxide layer 20 formed by anodizing the metal substrate 10; circuit layers 30 and 31 formed on the anodic oxide layer 20; and a first sol-gel layer 50 formed by applying a photocatalytic material between circuit wirings of the circuit layers 30 and 31 and then curing the applied photocatalytic material. Here, the anodic oxide layer 20 is formed only on one side and both lateral sides of the metal substrate 10. Therefore, the printed circuit board according to a second embodiment of the present invention is advantageous in that the anodic oxide layer 20 is not formed on the other side of the metal substrate 10, so that the other side thereof is exposed, thereby improving the heat radiation effect thereof.

In the second embodiment of the present invention, in order to further improve the radiation performance of the printed circuit board, the other side of the metal substrate 10 may be exposed by removing the anodic oxide layer 20 formed thereon. Although the anodic oxide layer generally has higher radiation performance than an insulation layer, the radiation performance of the printed circuit board can be further improved by directly exposing the other side of the metal substrate 10. Detailed description of constituents other than the anodic oxide layer 20 will be omitted because it overlaps with their descriptions in the first embodiment of the present invention.

FIGS. 4 to 11 are sectional views showing a method of manufacturing a printed circuit board according to a third embodiment of the present invention. Particularly, in the method, circuit layers 30 and 31 are formed in a subtractive manner. Hereinafter, the method of manufacturing a printed circuit board according to a third embodiment of the present invention will be described with reference to FIGS. 4 to 11. The description of the constitution and function of the printed circuit board overlapping with those of the first and second embodiments of the present invention will be omitted.

The method of manufacturing a printed circuit board according to a third embodiment of the present invention includes the processes of: (A) providing a metal substrate 10; (B) anodizing the metal substrate 10 to form an anodic oxide layer 20; (C) forming circuit layers 30 and 31 on one side of the anodic oxide layer; and (D) applying a photocatalytic material between circuit wirings of the circuit layers 30 and 31 and then curing the applied photocatalytic material to form a first sol-gel layer 50.

Hereinafter, the method of manufacturing a printed circuit board according to a third embodiment of the present invention will be described in more detail with reference to FIGS. 4 to 11.

FIG. 4 shows the processes of: (A) providing a metal substrate 10; and (B) forming an anodic oxide layer 20 by anodizing the metal substrate 10. Here, the metal substrate 10 is anodized to form the anodic oxide layer 20 over the entire surface thereof, and may be made of a material having heat radiation properties. For example, the metal substrate may be made of aluminum, magnesium, titanium or the like. The anodic oxide layer 20 has a uniform thickness because the oxidation of the metal substrate 10 was accelerated using the metal substrate 10 as an anode in a specific solution such as a sulfuric acid solution or the like. Here, the thickness of the anodic oxide layer 20 is determined depending on the period and degree of anodizing, and the anodizing of the metal substrate 10 is performed within the range necessary for forming the anodic oxide layer 20 having insulation characteristics.

FIGS. 5 to 8 show the process of (C) forming circuit layers 30 and 31 on the anodic oxide layer 20. Particularly, in this embodiment, since the circuit layers 30 and 31 are formed in a subtractive manner, a method of manufacturing a printed circuit board in a subtractive manner will be described. The process of forming the circuit layers 30 and 31 includes the steps of: (C-1) forming a seed layer 31 on the anodic oxide layer 20; (C-2) forming a circuit plating layer 30 on the seed layer 31 by electrolytic plating; (C-3) applying an etching resist 40 for forming circuit patterns onto the circuit plating layer 30 and then etching the seed layer 31 and the circuit plating layer 30; and (C-4) removing the etching resist 40.

FIG. 5 shows the steps of (C-1) forming a seed layer 31 on the anodic oxide layer 20 and (C-2) forming a circuit plating layer 30 on the seed layer 31 by electrolytic plating. The seed layer 31 serves as an incoming line for electrolytic plating, and may be formed by wet plating (electroless plating) or dry plating (sputtering) in order to form the circuit plating layer 30 thereon later. The seed layer 31 is formed, and then the circuit plating layer 30 is formed on the seed layer 31 by electrolytic plating, thus forming the circuit layers 30 and 31 having the desired thickness.

FIGS. 6 and 7 show the step of (C-3) applying an etching resist 40 for forming circuit patterns onto the circuit plating layer 30 and then etching the seed layer 31 and the circuit plating layer 30. As shown in FIG. 6, the etching resist 40 is applied onto the circuit plating layer 30 in order to form the circuit layers 30 and 31. As shown in FIG. 7, the seed layer 31 and the circuit plating layer 30 are etched to form a circuit pattern. In this case, the circuit pattern may be formed by wet etching or dry etching. However, the etching of the seed layer 31 and the circuit plating layer 30 is not limited thereto, and the circuit pattern may be formed by other etching methods.

FIG. 8 shows the step of (C-4) removing the etching resist 40. As shown in FIG. 8, the etching resist 40 is removed, thus forming the desired circuit layers 30 and 31 on the anodic oxide layer 20.

FIGS. 9 to 11 shows the process of (D) applying a photocatalytic material between circuit wirings of the circuit layers 30 and 31 and then curing the applied photocatalytic material to form a first sol-gel layer 50. Here, the process of forming the first sol-gel layer 50 may include the steps of: (D-1) applying a photocatalytic material onto the anodic oxide layer 20 formed on the other side of the metal substrate and then curing the applied photocatalytic material to form a second sol-gel layer 51; (D-2) removing the second sol-gel layer; and (D-3) removing the anodic oxide layer 20 formed on the other side of the metal substrate.

FIG. 9 shows the steps of: (D) applying a photocatalytic material between circuit wirings of the circuit layers 30 and 31 and then curing the applied photocatalytic material to form a first sol-gel layer 50; and (D-1) applying a photocatalytic material onto the anodic oxide layer 20 formed on the other side of the metal substrate and then curing the applied photocatalytic material to form a second sol-gel layer 51. In the step of (D), a printed circuit board may be finally formed, or may be formed by simultaneously or sequentially attaching the first sol-gel layer 50 and the second sol-gel layer 51 to both sides of the metal substrate 10. Descriptions of the formation of the first sol-gel layer 50 and the second sol-gel layer 51 and the application and curing of the photocatalytic material will be omitted because they are the same as their descriptions in the first and second embodiments of the present invention.

FIG. 10 shows the step of (D-2) removing the second sol-gel layer 51. Here, the removing of the second sol-gel layer 51 is conducted in order to improve the radiation performance of the printed circuit board. FIG. 11 shows the step of (D-3) removing the anodic oxide layer 20 formed on the other side of the metal substrate. Here, the radiation performance of the printed circuit board can be more improved because the metal substrate 10 is directly exposed by removing the anodic oxide layer 20.

FIGS. 12 to 19 are sectional views showing a method of manufacturing a printed circuit board according to a fourth embodiment of the present invention. Particularly, in the method, circuit layers 30 and 31 are formed in an additive manner. Hereinafter, the method of manufacturing a printed circuit board according to a fourth embodiment of the present invention will be described with reference to FIGS. 12 to 19. The description of the manufacturing processes overlapping with those of the third embodiment of the present invention will be omitted.

The method of manufacturing a printed circuit board according to a fourth embodiment of the present invention includes the processes of: (A) providing a metal substrate 10; (B) anodizing the metal substrate 10 to form an anodic oxide layer 20; (C) forming circuit layers 30 and 31 on one side of the anodic oxide layer; and (D) applying a photocatalytic material between circuit wirings of the circuit layers 30 and 31 and then curing the applied photocatalytic material to form a first sol-gel layer 50.

Hereinafter, the method of manufacturing a printed circuit board according to a fourth embodiment of the present invention will be described in more detail with reference to FIGS. 12 to 19.

FIG. 12 shows the processes of: (A) providing a metal substrate 10; and (B) forming an anodic oxide layer 20 by anodizing the metal substrate 10. Detailed description of these processes will be omitted because they are the same as those of the third embodiment of the present invention.

FIGS. 13 to 16 show the process of (C) forming circuit layers 30 and 31 on the anodic oxide layer 20. Particularly, in this embodiment, since the circuit layers 30 and 31 are formed in an additive manner, a method of manufacturing a printed circuit board in a subtractive manner will be described. The process of forming the circuit layers 30 and 31 includes the steps of: (C-1) forming a seed layer 31 on the anodic oxide layer 20; (C-2) applying a plating resist 41 for forming circuit patterns onto the seed layer 31; (C-3) forming a circuit plating layer 30 on the seed layer 31; and (C-4) removing the plating resist 41 to expose the seed layer 31 and then etching the exposed seed layer 31.

FIG. 13 shows the steps of (C-1) forming a seed layer 31 on the anodic oxide layer 20 and (C-2) applying a plating resist 41 for forming circuit patterns onto the seed layer 31. The seed layer 31 serves as an incoming line for electrolytic plating, and may be formed by wet plating (electroless plating) or dry plating (sputtering) in order to form the circuit plating layer 30 thereon later. The seed layer 31 is formed, and then the circuit plating layer 30 is formed on the seed layer 31 by electrolytic plating. The plating resist 41 is applied onto the seed layer 31 in order to form the desired circuit patterns.

FIG. 14 shows the step of (C-3) forming a circuit plating layer 30 on the seed layer 31. Here, the circuit plating layer 30 is formed on the portion of the seed layer 31 which was not coated with the plating resist 41.

FIG. 15 shows the step of removing the plating resist 41 to expose the seed layer 31, and FIG. 16 shows the step of etching the exposed seed layer 31. Here, the plating resist 41 is removed, and then the seed layer 31 exposed between circuit patterns is selectively etched, thereby finally forming circuit layers 30 and 31.

FIG. 17 shows the steps of: (D) applying a photocatalytic material between circuit wirings of the circuit layers 30 and 31 and then curing the applied photocatalytic material to form a first sol-gel layer 50; and (D-1) forming a second sol-gel layer 51 on the other side of the metal substrate 10, on which the circuit layers 30 and 31 are not formed. Subsequently, as shown in FIG. 18, the second sol-gel layer 51 formed on the other side of the metal substrate 10 is removed, thus completing a printed circuit board (step D-2). Moreover, as shown in FIG. 19, the anodic oxide layer 20 formed on the other side of the metal substrate 10 is removed to expose the other side thereof, thus improving the radiation performance of the printed circuit board. Detailed description of other processes will be omitted because they are the same as those shown in FIGS. 9 to 11 of the third embodiment of the present invention.

As described above, according to the present invention, an anodic oxide layer formed on a metal substrate is used as an insulation layer, thus improving the radiation performance of the printed circuit board.

Further, according to the present invention, a sol-gel layer is formed between circuit wirings of circuit layers, thus realizing a high-voltage package printed circuit board.

Further, according to the present invention, a sol-gel layer is formed between circuit wirings of circuit layers, thus preventing the instability of the insulation layer during a process of precipitating residues of a metal layer for forming a circuit layer or additives (Mg, Si, Cu and the like) in anodic oxidation.

Furthermore, according to the present invention, the insulation voltage of the printed circuit board can be improved depending on the raw material and thickness of a sol coating layer without changing the process conditions of anodization which forms the anodic oxide layer.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. Simple modifications, additions and substitutions of the present invention belong to the scope of the present invention, and the specific scope of the present invention will be clearly defined by the appended claims.

Claims

1. A printed circuit board, comprising:

a metal substrate;

an anodic oxide layer formed by anodizing the metal substrate;

circuit layers formed on the anodic oxide layer; and

a first sol-gel layer formed by applying a photocatalytic material between circuit wirings of the circuit layers and then curing the applied photocatalytic material.

2. The printed circuit board according to claim 1, wherein the photocatalytic material is alumina or titanium dioxide.

3. The printed circuit board according to claim 1, wherein the anodic oxide layer is formed only on one side and both lateral sides of the metal substrate.

4. A method of manufacturing a printed circuit board, comprising:

providing a metal substrate;

anodizing the metal substrate to form an anodic oxide layer;

forming circuit layers on one side of the anodic oxide layer; and

applying a photocatalytic material between circuit wirings of the circuit layers and then curing the applied photocatalytic material to form a first sol-gel layer.

5. The method according to claim 4, wherein, in the forming of the anodic oxide layer, the anodic oxide layer is formed by anodizing only one side and both lateral sides of the metal substrate.

6. The method according to claim 4, wherein the forming of the circuit layers comprises:

forming a seed layer on the anodic oxide layer;

forming a circuit plating layer on the seed layer by electrolytic plating;

applying an etching resist for forming circuit patterns onto the circuit plating layer and then etching the seed layer and the circuit plating layer; and

removing the etching resist.

7. The method according to claim 4, wherein the forming of the circuit layers comprises:

forming a seed layer on the anodic oxide layer;

applying a plating resist for forming circuit patterns onto the seed layer;

forming a circuit plating layer on the seed layer; and

removing the plating resist to expose the seed layer, and then etching the exposed seed layer.

8. The method according to claim 4, wherein the forming of the first sol-gel layer comprises:

applying a photocatalytic material onto the anodic oxide layer formed on the other side of the metal substrate and then curing the applied photocatalytic material to form a second sol-gel layer;

removing the second sol-gel layer; and

removing the anodic oxide layer formed on the other side of the metal substrate.

9. The method according to claim 4, wherein the photocatalytic material is alumina or titanium dioxide.

10. The method according to claim 4, wherein the photocatalytic material is applied by spraying, dipping or aerosol deposition.

11. The method according to claim 4, wherein the applied photocatalytic material is cured at a temperature of 100˜200° C..