JP2007108228A - Photoelectric consolidated substrate and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photoelectric consolidated substrate capable of sufficiently shortening a distance between a core layer of an optical waveguide and a metallic wiring pattern surface and also sufficiently improving the propagation efficiency of an optical signal, and to provide its manufacturing method. <P>SOLUTION: The method of manufacturing the photoelectric consolidated substrate comprises: a process for forming a 1st cladding layer 2a on a metallic thin film surface of a metallic thin film transfer sheet which is a release base material formed with a metallic thin film thereon; a process for releasing the release base material; a process for forming a predetermined wiring pattern 11 by etching the metallic thin film; a process for forming the core layer 2b on the surface of the 1st cladding layer 2a; a process for forming a 2nd cladding layer 2c to cover the surface of this core layer 2b; and a process for forming a predetermined position of this core layer 2b into an optical path conversion mirror 21. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an opto-electric hybrid board in which an optical waveguide (optical wiring) and a metal wiring pattern (electric wiring) are formed on one substrate, and a method for manufacturing the same.

  Recently, information communication using light as a medium has become widespread. Therefore, an opto-electric hybrid board (for example, see Patent Document 1) in which an optical waveguide (optical wiring) and a metal wiring pattern (electric wiring) are formed is employed as a board used for information communication electronic devices and the like. Yes.

  As shown in FIG. 9, this opto-electric hybrid board is formed by bonding a substrate 50 on which a metal wiring pattern 11 is formed and an optical waveguide 2 via an adhesive layer 60. This opto-electric hybrid board is usually connected on the metal wiring pattern 11 with a light emitting element A for converting an electric signal into an optical signal L and a light receiving element B for converting the optical signal L into an electric signal. Yes. A through hole 51 is formed in the substrate 50 below the light emitting element A and below the light receiving element B so that the optical signal L from the light emitting element A can be transmitted to the light receiving element B through the optical waveguide. The optical path conversion mirror 21 is formed on the core layer 2b. The optical path conversion mirror 21 is an inclined surface formed in the core layer 2b and inclined by 45 ° with respect to the optical axis.

Then, the optical signal L is transmitted by reflecting the optical signal L emitted downward from the light emitting element A by one (left side in FIG. 9) optical path conversion mirror (inclined surface) 21 (the optical path is converted by 90 °). Then, the light is guided into the core layer 2b of the optical waveguide 2 and reflected again by the other (right side in FIG. 9) optical path conversion mirror (inclined surface) 21 (the optical path is converted by 90 °) to receive the upper light receiving element. This is done by transmitting to B. In FIG. 9, 2a is a first cladding layer, and 2c is a second cladding layer.
JP 2004-20767 A

  However, with the increase in the density and speed of signals, the propagation efficiency of the optical signal L has become insufficient in the conventional opto-electric hybrid board.

  Therefore, as a result of repeated research on this cause, the present inventor has found that the cause is that the optical signal L is attenuated particularly between the core layer 2b of the optical waveguide 2 and the light receiving element B. . It was also found that the cause of the attenuation is that the distance between the core layer 2b of the optical waveguide 2 and the light receiving element B is long.

  Based on the cause investigation result, the inventor shortens the distance between the core layer 2b of the optical waveguide 2 and the light receiving element B (the distance between the core layer 2b and the surface of the metal wiring pattern 11). Accordingly, a method of manufacturing an opto-electric hybrid board that does not use the board 50 on which the metal wiring pattern 11 is formed has been considered. This manufacturing method is a method of forming the optical waveguide 2 on one side of a metal sheet such as a metal foil and then forming the predetermined metal wiring pattern 11 by etching the metal sheet.

  However, in this manufacturing method, unless the thickness of the metal sheet is increased to some extent, the handleability of the metal sheet in the manufacturing process is poor, and the workability when forming the optical waveguide 2 and the metal wiring pattern 11 is poor. Therefore, if the metal sheet is thickened to improve the handleability of the metal sheet, the metal wiring pattern 11 is also thickened, so that the distance between the core layer 2b of the optical waveguide 2 and the light receiving element B is sufficiently shortened. As a result, there is a problem that the propagation efficiency of the optical signal L cannot be sufficiently improved.

  The present invention has been made in view of such circumstances, and can sufficiently shorten the distance between the core layer of the optical waveguide and the surface of the metal wiring pattern and sufficiently improve the propagation efficiency of the optical signal. An object is to provide an opto-electric hybrid board and a manufacturing method thereof.

  In order to achieve the above object, the present invention provides an opto-electric hybrid board having optical signal transmission means and electrical signal transmission means, wherein the optical signal transmission means includes a first cladding layer and a second cladding layer. The core layer is sandwiched and encapsulated, and a predetermined position of the core layer is formed on an optical path conversion mirror, and the electric signal transmission means includes an insulating layer on the surface of the first cladding layer. An opto-electric hybrid board made of a metal wiring pattern directly formed without any intervention is taken as a first gist.

  The present invention is also a method for producing the opto-electric hybrid board, comprising preparing a metal thin film transfer sheet in which a metal thin film is formed on the surface of a release substrate, and providing a first cladding layer on the surface of the metal thin film. A step of forming, a step of peeling the release substrate from the metal thin film, a step of forming the metal thin film into a predetermined wiring pattern by etching, a step of forming a core layer on the surface of the first cladding layer, A second summary of a method for manufacturing an opto-electric hybrid board, comprising: a step of forming a second cladding layer so as to cover the surface of the core layer; and a step of forming a predetermined position of the core layer on an optical path conversion mirror. And

  The inventor conducted further research based on the result of the above-described method for manufacturing an opto-electric hybrid board. As a result, in the method for manufacturing an opto-electric hybrid board, a metal thin film transfer sheet having a metal thin film formed on the surface of the peeling base material is used, and after forming the first cladding layer on the surface of the metal thin film, the peeling base material If the metal thin film is peeled off from the metal thin film and the metal thin film is formed into a predetermined wiring pattern by etching, an opto-electric hybrid board with good workability and stable quality can be manufactured. The present inventors have found that the distance between the two can be sufficiently shortened, and have reached the present invention.

  That is, when the metal thin film transfer sheet having the metal thin film formed on the surface of the peeling substrate is used, the metal thin film has a certain degree of rigidity even if the metal thin film is not thickened. The transfer sheet is easy to handle. Thereby, the workability | operativity at the time of forming a 1st clad layer on the surface of the metal thin film of the metal thin film transfer sheet is improving. And even if it peels a peeling base material, the laminated body of the said metal thin film and 1st cladding layer can acquire a certain amount of rigidity by formation of a 1st cladding layer, and the handleability is improving. For this reason, in subsequent processes (such as a process of forming a metal thin film into a predetermined wiring pattern by etching, a process of forming a core layer on the surface of the first cladding layer, etc.) An electric mixed substrate can be obtained. The obtained opto-electric hybrid board does not have a substrate above the core layer (a board on which a metal wiring pattern is formed) that constitutes a conventional opto-electric hybrid board, and the wiring pattern (metal thin film) Since the thickness can be reduced, the distance between the light receiving element connected on the wiring pattern and the core layer of the optical waveguide can be sufficiently shortened, and the propagation efficiency of the optical signal can be sufficiently improved.

  In the opto-electric hybrid board according to the present invention, the metal wiring pattern is directly formed on the surface of the first cladding layer of the optical waveguide without an insulating layer, so that the core layer of the optical waveguide and the surface of the metal wiring pattern The distance between them is sufficiently shortened. As a result, the distance between the light receiving element connected on the wiring pattern and the core layer of the optical waveguide can be sufficiently shortened, and the propagation efficiency of the optical signal can be sufficiently improved.

  In particular, when the thickness of the metal wiring pattern is in the range of 1.0 to 5.0 μm, the thickness of the metal wiring pattern is sufficiently thin, so that the propagation efficiency of the optical signal is further improved. be able to.

  In addition, the method for manufacturing an opto-electric hybrid board according to the present invention uses a metal thin film transfer sheet in which a metal thin film to be formed later on a predetermined wiring pattern is formed on the surface of a release substrate. , It can be handled with a certain degree of rigidity. For this reason, the metal thin film (wiring pattern) of the metal thin film transfer sheet can be made thin. In addition, since the first clad layer is formed on the surface of the metal thin film, even if the peeling substrate of the metal thin film transfer sheet is peeled off, the laminate of the metal thin film and the first clad layer is formed by the first clad layer. A certain degree of rigidity can be obtained, and the handleability is improved. For this reason, in a subsequent process, workability | operativity is good, As a result, the opto-electric hybrid board | substrate with stable quality can be obtained. In the obtained opto-electric hybrid board, the wiring pattern is formed directly on the surface of the first cladding layer of the optical waveguide without an insulating layer, and therefore, between the core layer of the optical waveguide and the surface of the wiring pattern. Can be sufficiently shortened. As a result, the distance between the light receiving element connected on the wiring pattern and the core layer of the optical waveguide can be sufficiently shortened, and the propagation efficiency of the optical signal can be sufficiently improved.

  In particular, when the thickness of the metal thin film is in the range of 1.0 to 5.0 μm, the thickness of the wiring pattern to be formed later can be made sufficiently thin, and the peeling substrate is peeled off. The subsequent laminate of the metal thin film and the first cladding layer can be handled with a certain degree of rigidity. For this reason, the opto-electric hybrid board that further improves the propagation efficiency of the optical signal can be manufactured with stable quality.

  Next, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to this.

  1 to 6 show an embodiment of a method for manufacturing an opto-electric hybrid board according to the present invention. This opto-electric hybrid board manufacturing method is a method of manufacturing the opto-electric hybrid board shown in FIG. As shown in FIG. 6, this opto-electric hybrid board is obtained by forming a metal wiring pattern 11 directly on the surface of the optical waveguide 2 without using an insulating layer.

  More specifically, the optical waveguide 2 sandwiches and encloses the core layer 2b between the first clad layer 2a and the second clad layer 2c, and in a predetermined position of the core layer 2b (in FIG. ) Is formed on the optical path conversion mirror 21. The optical path conversion mirror 21 is a portion in which the core layer 2b is formed on an inclined surface inclined with respect to the optical axis, and the inclination angle is determined by the angle for converting the optical path, and is usually 45 ° with respect to the optical axis. It is set so that the optical path is converted by 90 °. The metal wiring pattern 11 is formed on the surface of the first cladding layer 2 a of the optical waveguide 2.

  Such an opto-electric hybrid board can be manufactured as follows.

  First, a metal thin film transfer sheet 1 in which a metal thin film 1a shown in FIG. 1 is formed on the surface (lower surface in FIG. 1) of a peeling substrate 1b is prepared, and the first cladding layer 2a and A material for forming the second cladding layer 2c and a material for forming the core layer 2b are prepared.

  That is, although the said metal thin film transfer sheet 1 uses a commercial item normally, you may produce (for example, form the metal thin film 1a on the surface of the peeling base material 1b by sputtering method etc.). And the said metal thin film 1a is later formed in the wiring pattern 11 (refer FIG. 4), Although the material is not specifically limited, Usually, copper is used. Although the thickness of the metal thin film 1a is not particularly limited, it is within the range of 1.0 to 5.0 μm from the viewpoint of shortening the distance between the core layer 2b to be formed later and the surface (upper surface) of the metal thin film 1a. It is preferable. If this distance is less than 1.0 μm, the wiring pattern 11 (see FIG. 4) to be formed later is likely to break, and if the distance exceeds 5.0 μm, the propagation efficiency of the optical signal L becomes insufficient. It is. Further, the peeling substrate 1b is subjected to a peeling treatment on the contact surface with the metal thin film 1a, so that the peeling from the metal thin film 1a is easy. Although it does not specifically limit as the peeling base material 1b, Usually, a resin sheet or a metal sheet is used. The thickness of the release substrate 1b is not particularly limited, but is preferably in the range of 10 to 50 μm from the viewpoint of obtaining rigidity to the extent that the handleability of the metal thin film transfer sheet 1 is improved in the thinnest possible range.

  As a material for forming the first cladding layer 2a and the second cladding layer 2c, for example, an epoxy resin is used. Other forming materials include polyimide resin precursors and the like. The forming material of the first cladding layer 2a and the forming material of the second cladding layer 2c may be the same or different.

  Examples of the material for forming the core layer 2b include a photosensitive epoxy resin. Other forming materials include photosensitive polyimide precursors and the like.

  In addition, from the relationship which comprises the optical waveguide 2, the core layer 2b needs to have a higher refractive index than the 1st cladding layer 2a and the 2nd cladding layer 2c, but adjustment of this refractive index is, for example, the kind of epoxy resin And the composition ratio can be adjusted. Specifically, the relationship between the refractive index of the core layer 2b and the refractive indexes of the first cladding layer 2a and the second cladding layer 2c, that is, the relative refractive index difference Δ [specific refractive index difference Δ = (n core− n clad) / n core: n = refractive index] is usually in the range of 0.1 to 5.0%.

  Then, using the metal thin film transfer sheet 1 prepared in this way and various forming materials, an opto-electric hybrid board is manufactured as follows.

  That is, as shown in FIG. 2, an epoxy resin varnish or the like, which is a material for forming the first cladding layer 2a, is formed on the surface (the lower surface in FIG. 2) of the metal thin film 1a of the metal thin film transfer sheet 1 after drying. Is preferably 1 to 30 μm, particularly preferably 5 to 20 μm, and dried to form a resin layer made of an epoxy resin. As the coating method, a general film forming method such as a spin coating method or a casting method can be used. Next, the first clad layer 2a made of epoxy resin is formed on the surface of the metal thin film 1a of the metal thin film transfer sheet 1 by completing the removal of the residual solvent in the resin layer and the curing reaction by UV curing and heating. Form. In general, the first cladding layer 2a is formed with the one shown in FIG. 2 turned upside down.

  Next, as shown in FIG. 3, the peeling substrate 1b of the metal thin film transfer sheet 1 is peeled from the metal thin film 1a, and the surface of the metal thin film 1a exposed by the peeling (on the side opposite to the first cladding layer 2a). Etching resist 3 having the same pattern as the metal wiring pattern is formed on the surface) using a photoresist.

  Next, as shown in FIG. 4, the metal thin film 1 a (exposed from the etching resist 3) other than the etching resist 3 is dissolved and removed using an etching solution. Thereafter, the etching resist 3 is removed. Thereby, the metal wiring pattern 11 of a predetermined pattern is formed.

Next, as shown in FIG. 5, a photosensitive epoxy resin varnish, which is a material for forming the core layer 2b, is preferably formed on the surface of the first cladding layer 2a (the lower surface in FIG. 5) after drying. The photosensitive epoxy resin layer to be the core layer 2b is formed by initial coating so as to be 2 to 60 μm, particularly preferably 6 to 50 μm. Next, a photomask is placed on the photosensitive polyimide resin precursor layer so as to obtain a desired pattern, and ultraviolet rays are irradiated from above. The exposure amount in this ultraviolet irradiation is 1000 to 10000 mJ / cm ² . Thereafter, in order to complete the photoreaction, post-exposure heat treatment called Post Exposure Bake (PEB) is performed, and development is performed using a developer (wet process method). And in order to imidize the desired pattern obtained by image development, it heat-processes normally. The heating temperature at this time is generally 100 to 150 ° C. By curing in this way, the core layer 2b to be a polyimide resin pattern is formed.

  Thereafter, the second cladding layer 2c is formed in the same manner as the formation of the first cladding layer 2a so as to cover the surface (lower surface and side surfaces) of the core layer 2b. Normally, the core layer 2b and the second cladding layer 2c are formed with the one shown in FIG. 5 turned upside down.

  Then, as shown in FIG. 6, the predetermined position of the core layer 2b (the left and right side portions in FIG. 6) is moved from the second clad layer 2c side to the optical axis by laser processing, dicing using a diamond blade, or the like. Are formed on an optical path conversion mirror (inclined surface) 21 that is inclined with respect to. In this embodiment, the laminated portion composed of the core layer 2b and the second cladding layer 2c is cut into a triangular cross section, and a part of the slope of the triangular cross section is used as the optical path conversion mirror 21. The left and right optical path conversion mirrors 21 are formed at positions below light emitting elements A and light receiving elements B (see FIG. 7), which will be described later. In this way, the opto-electric hybrid board is manufactured. Normally, the optical path conversion mirror 21 is formed with the one shown in FIG. 6 turned upside down.

  As shown in FIG. 7, the light-emitting element A and the light-receiving element B are connected to the metal wiring pattern 11 formed above the optical path conversion mirror 21 in the opto-electric hybrid board manufactured as described above. Is done.

  The optical path conversion by the opto-electric hybrid board is performed as follows. That is, the optical signal L emitted downward from the light emitting element A is incident from the surface of the first cladding layer 2a of the opto-electric hybrid board, passes through the first cladding layer 2a, and then passes through one of the core layers 2b ( Reflected by the optical path conversion mirror 21 on the left side in FIG. 7 (the optical path is converted by 90 °), guided into the core layer 2b, and reflected again by the optical path conversion mirror 21 on the other side (right side in FIG. 7) (the optical path is changed). 90 °) and transmitted to the upper light receiving element B.

  In this optical path conversion, the distance between the core layer 2b and the light receiving element B is short because there is no substrate 50 for forming the metal wiring pattern 11 compared with the optical path conversion by the conventional opto-electric hybrid board shown in FIG. The propagation efficiency of the optical signal L is improved. Furthermore, in this embodiment, the distance between the light emitting element A and the core layer 2b is also shortened, and the propagation efficiency of the optical signal L is further improved.

  In the above embodiment, the light emitting element A is also connected to the metal wiring pattern 11 formed on the surface of the optical waveguide 2, but the light emitting element A is connected to the optical axis of the core layer 2b as shown in FIG. The optical path conversion mirror 21 on one side (left side in FIG. 7) of the core layer 2b may not be formed by disposing it near the outside of one end surface (left end surface in FIG. 8).

  Moreover, in the said embodiment, after forming the 1st clad layer 2a in the surface of the metal thin film 1a of the metal thin film transfer sheet 1, the peeling base material 1b of the metal thin film transfer sheet 1 is peeled from the metal thin film 1a, and it is for etching. Although the metal wiring pattern 11 is formed through the formation of the resist 3, the order in which the peeling base material 1b is peeled off is not limited to this, and may be after the core layer 2b is formed, or the second cladding. It may be after the layer 2c is formed or after the optical path conversion mirror 21 is formed. In any case, after the peeling substrate 1b is peeled off, the metal wiring pattern 11 is formed through the formation of the etching resist 3 in the same manner as in the above embodiment.

  Next, examples will be described together with comparative examples and conventional examples.

[Metal thin film transfer sheet]
A commercially available metal thin film transfer sheet [MicoThin MTSD-H (3 μm), manufactured by Mitsui Kinzoku Co., Ltd.] was prepared. In this metal thin film transfer sheet, a copper thin film having a thickness of 3 μm is formed on the surface of a peeling substrate (copper foil) having a thickness of 25 μm.

[Formation materials of the first cladding layer and the second cladding layer]
20 g of BPEFG (sold by Nagase Sangyo Co., Ltd., ONCOAT EX-1020), 4.1 g of Celoxide 2021P (manufactured by Daicel Chemical Industries), and 0.24 g of SP-170 (manufactured by Asahi Denka) are dissolved in 8.47 g of cyclohexane. As a result, a resin solution as a material for forming the first cladding layer and the second cladding layer was obtained.

[Material for forming the core layer]
Dissolve 20 g of BPEFG (sold by Nagase Sangyo Co., Ltd., on coat EX-1020), 10 g of BPFG (sold by Nagase Sangyo Co., Ltd., on coat EX-1010), 0.3 g of SP-170 (manufactured by Asahi Denka) in 20 g of cyclohexane. As a result, a resin solution as a material for forming the core layer was obtained.

[Manufacture of opto-electric hybrid board]
An opto-electric hybrid board was manufactured in the same manner as in the above embodiment (see FIGS. 1 to 6). At this time, the thickness of each of the first cladding layer and the second cladding layer was 20 μm, the core layer was formed in a linear pattern, and the dimensions were 50 μm thickness, 50 μm width, and 5 cm length. The optical path conversion mirror was formed on an inclined surface inclined by 45 ° with respect to the optical axis by laser processing. Incidentally, as the etching solution used in forming the metal wiring pattern, with 5% NaHCO ₃ aq.

  In Example 1 above, a metal thin film transfer sheet having a copper thin film thickness of 5 μm [MicoThin MTSD-H (3 μm), manufactured by Mitsui Kinzoku Co., Ltd.] was used. Otherwise, the same procedure as in Example 1 was performed.

[Comparative Example 1 of Manufacturing Method]
In Example 1 described above, a rolled copper foil (thickness: 18 μm) was used instead of the metal thin film transfer sheet. Otherwise, the same procedure as in Example 1 was performed. That is, after forming the first cladding layer on one side of the rolled copper foil, the rolled copper foil was etched to form a predetermined metal wiring pattern. Then, after forming the core layer and the second cladding layer on the surface of the first cladding layer, an optical path conversion mirror was formed.

[Conventional example 1]
A conventional opto-electric hybrid board shown in FIG. 9 was prepared. In this opto-electric hybrid board, the thickness of the board on which the metal wiring pattern was formed was 143 μm, and the thickness of the metal wiring pattern was 18 μm. The thicknesses of the first cladding layer, the second cladding layer, and the core layer constituting the optical waveguide were the same as those in Example 1. The thickness of the adhesive layer between the substrate and the optical waveguide was 10 μm.

(Measurement of optical propagation loss)
A light emitting element and a light receiving element are mounted on the opto-electric hybrid boards of Examples 1 and 2 and Comparative Example 1 and Conventional Example 1 obtained in this way, and an optical signal is emitted from the light emitting element, through the core layer, It was transmitted to the light receiving element. And the optical propagation loss at that time was measured. As a result, the optical propagation loss of Example 1 was 3.0 dB, that of Example 2 was 3.4 dB, that of Comparative Example 1 was 8.2 dB, and that of Conventional Example 1 was 9.5 dB.

  Thus, it can be seen that the opto-electric hybrid boards of Examples 1 and 2 and Comparative Example 1 have higher optical signal propagation efficiency because the optical propagation loss is smaller than that of the opto-electric hybrid board of Conventional Example 1. In particular, it can be seen that the opto-electric hybrid boards of Examples 1 and 2 have higher optical signal propagation efficiency than the opto-electric hybrid board of Comparative Example 1.

It is explanatory drawing which shows typically one Embodiment of the manufacturing method of the opto-electric hybrid board | substrate of this invention. It is explanatory drawing which shows typically the manufacturing method of the said opto-electric hybrid board | substrate. It is explanatory drawing which shows typically the manufacturing method of the said opto-electric hybrid board | substrate. It is explanatory drawing which shows typically the manufacturing method of the said opto-electric hybrid board | substrate. It is explanatory drawing which shows typically the manufacturing method of the said opto-electric hybrid board | substrate. It is explanatory drawing which shows typically the manufacturing method of the said opto-electric hybrid board | substrate. It is explanatory drawing which shows typically the state which mounted the light emitting element and the light receiving element on the opto-electric hybrid board of this invention. It is explanatory drawing which shows typically the state which mounted the light emitting element and the light receiving element in other embodiment of the opto-electric hybrid board of this invention. It is explanatory drawing which shows the conventional opto-electric hybrid board | substrate typically.

Explanation of symbols

2a First cladding layer 2b Core layer 2c Second cladding layer 11 Wiring pattern 21 Optical path conversion mirror

Claims (4)

  An opto-electric hybrid board having an optical signal transmission means and an electric signal transmission means, wherein the optical signal transmission means sandwiches and encloses the core layer between the first cladding layer and the second cladding layer, The core layer is made of an optical waveguide formed on a light path conversion mirror at a predetermined position, and the electric signal transmission means is made of a metal wiring pattern formed directly on the surface of the first cladding layer without an insulating layer. An opto-electric hybrid board.   2. The opto-electric hybrid board according to claim 1, wherein the metal wiring pattern has a thickness in the range of 1.0 to 5.0 [mu] m.   A method for producing an opto-electric hybrid board according to claim 1, wherein a metal thin film transfer sheet having a metal thin film formed on the surface of a peeling substrate is prepared, and a first cladding layer is formed on the surface of the metal thin film. A step of peeling the release substrate from the metal thin film, a step of forming the metal thin film into a predetermined wiring pattern by etching, a step of forming a core layer on the surface of the first cladding layer, A method for manufacturing an opto-electric hybrid board, comprising: a step of forming a second cladding layer so as to cover a surface of the core layer; and a step of forming a predetermined position of the core layer on an optical path conversion mirror.   The method for manufacturing an opto-electric hybrid board according to claim 2, wherein the thickness of the metal thin film is in the range of 1.0 to 5.0 μm.

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