Wiring board

Abstract

A wiring board (1) according to an embodiment comprises an optical waveguide (102) formed by laminating, on a substrate (10), a lower cladding (21), a core (3), and an upper cladding (22) in this order, and an intermediate layer (M) is disposed between the substrate (10) and the lower cladding (21). An end surface of the intermediate layer (M) is located outside of an end surface of the lower cladding (21).

Description

wiring board

[0001] The present invention relates to a wiring board.

[0002] Patent Document 1 discloses a polymer waveguide array formed on a polymer film and a silicon waveguide array formed on a silicon chip, in which the cores of the polymer waveguide and the silicon waveguide are arranged to overlap over a predetermined distance in the optical axis direction and are optically coupled by adiabatic coupling.

[0003] JP 2014-81587 A

[0004] Patent Document 1 discloses that the core width of the polymer waveguide is approximately several micrometers. It is believed that the core thickness must also be extremely thin to accommodate the core width of the polymer waveguide. One example of a manufacturing method for forming the core of a polymer waveguide involves applying a core-forming material to a lower cladding formed on a polymer substrate, exposing the material using a photomask corresponding to the waveguide pattern, and then removing the resin material other than the waveguide pattern through a photolithography process. However, the core thickness may be thinner than the specified thickness. If the core thickness is not formed as designed, it is believed that optical transmission will not be successful. Therefore, it is believed that optical transmission will not be possible even when optically coupling between the core of a polymer waveguide and the core of a silicon waveguide. It is also necessary to prevent problems such as waveguide peeling due to thermal history, such as heat cycles, during use of the wiring board.

[0005] The wiring board of the present invention has an optical waveguide formed by laminating a lower clad, a core, and an upper clad on a substrate in this order. An intermediate layer is disposed between the substrate and the lower clad. The end face of the intermediate layer is located outside the end face of the lower clad.

[0006] According to an embodiment of the present invention, by positioning the end face of the intermediate layer outside the end face of the lower cladding and making it protrude, it is believed that uniformity in the core thickness is ensured and the core is formed according to the design value.

[0007] 1 is a plan view showing an example of an optical waveguide component for a wiring board according to an embodiment of the present invention. FIG. 1 is a cross-sectional view taken along line II of the wiring board according to the example of FIG. 1. FIG. 1 is a cross-sectional view taken along line II-II of the wiring board according to the example of FIG. 1. FIG. 1 is a cross-sectional view taken along line III-III of the wiring board according to the example of FIG. 1. FIG. 2 is a cross-sectional view showing a modified example of the optical waveguide component for a wiring board according to an embodiment of the present invention. FIG. 3 is a cross-sectional view showing an example of a modified example of the wiring board according to an embodiment of the present invention. FIG. 4 is a cross-sectional view showing an example of a modified example of the wiring board according to an embodiment of the present invention. FIG. 5 is a cross-sectional view showing an example of a manufacturing process for an optical waveguide component for a wiring board according to an embodiment. FIG. 6 is a cross-sectional view showing an example of a manufacturing process for an optical waveguide component for a wiring board according to an embodiment. FIG. 7 is a cross-sectional view showing an example of a manufacturing process for an optical waveguide component for a wiring board according to an embodiment. FIG. 8 is a cross-sectional view showing an example of a manufacturing process for an optical waveguide component for a wiring board according to an embodiment.

[0008] A wiring board according to an embodiment of the present invention will be described with reference to the drawings.

[0009] <Structure of Wiring Board of Embodiment> Fig. 1 shows a plan view of a wiring board 1 including an optical waveguide component 101, which is an example of an optical waveguide component of a wiring board of an embodiment. Fig. 2A shows a cross-sectional view taken along line I-I in Fig. 1, Fig. 2B shows a cross-sectional view taken along line II-II in Fig. 1, and Fig. 2C shows a cross-sectional view taken along line III-III in Fig. 1. Note that the wiring board 1 illustrated in Fig. 1 and other figures is merely an example of a wiring board of an embodiment. The structure of the wiring board of the embodiment is not limited to the structure shown in each drawing, such as Fig. 1.

[0010] As shown in FIGS. 1 to 2C , the wiring board 1 of the embodiment includes an optical waveguide 102 composed of a core 3 that transmits an optical signal and a clad 2 that surrounds the core 3. The core 3 of the optical waveguide 102 includes two cores, a first core 31 and a second core 32. The clad 2 is composed of a lower clad 21 and an upper clad 22. In the embodiment, a solder resist 61 is formed on the substrate 10. An intermediate layer M is formed on the solder resist 61 of the substrate 10. The optical waveguide 102 is formed on the intermediate layer M. The lower clad 21 is formed on the intermediate layer M. The core 3 is formed on the lower clad 21. The upper clad 22 is formed on the lower clad 21 and the core 3. The optical waveguide 102 is composed of the lower clad 21, the core 3, and the upper clad 22. That is, the wiring board 1 is arranged in the order of substrate 10, solder resist 61, intermediate layer M, and optical waveguide 102. Furthermore, the optical waveguide 102 is arranged in the order of lower clad 21, core 3, and upper clad 22. An optical waveguide component 101 having the function of an optical waveguide is arranged on the wiring board 1. The optical waveguide component 101 having the function of an optical waveguide is composed of the optical waveguide 102 and intermediate layer M. Furthermore, the substrate 10 does not necessarily need to have a solder resist formed thereon.

[0011] In the description of the embodiments, the side of the wiring board 1 farther from the substrate 10 in the thickness direction is also referred to as the "upper side" or "upper", or simply "upper", and the side closer to the substrate 10 is also referred to as the "lower side" or "lower", or simply "lower". Furthermore, in the clad and core, the surface facing away from the substrate 10 is also referred to as the "upper surface", and the surface facing the substrate 10 is also referred to as the "lower surface".

[0012] 1 to 2C, the thickness direction of the wiring board 1 is also referred to as the Z direction, the upward direction as the +Z direction, and the downward direction as the -Z direction. Furthermore, in the wiring board 1, the core 3 extends along the X direction. An optical signal propagating through the core 3 propagates in the +X direction or the -X direction. Hereinafter, the propagation direction of the optical signal, collectively referred to as the +X direction and the -X direction, will also be simply referred to as the "X direction." The optical waveguide 102 has two opposing ends, one end 11 and the other end 12. The one end 11 and the other end 12 face each other in the X direction. In the optical waveguide 102, an optical signal is incident on the one end 11 or the other end 12, and the optical signal is emitted from the other end 12 or the one end 11. When an optical signal is incident on the one end 11, the optical signal is emitted from the other end 12. When an optical signal is incident on the other end 12, the optical signal is emitted from the one end 11. Of the surfaces that constitute the core and cladding extending in the X direction, the surfaces facing the +Y direction or the −Y direction are referred to as “side surfaces.” Hereinafter, the +Y direction and the −Y direction will also be collectively referred to simply as the “Y direction.”

[0013] In the wiring board 1, the intermediate layer M is formed on the upper surface 10a of the solder resist 61 of the substrate 10. The lower clad 21 is formed on the upper surface Ma of the intermediate layer M. The core 3 is formed on the upper surface 21a of the lower clad 21. The upper surface 21a of the lower clad 21 is composed of a core 3 formation region 21aa and a core 3 non-formation region 21ab, and two cores 3 are formed in the core 3 formation region 21aa. No core is formed in the core 3 non-formation region 21ab. The upper clad 22 is disposed in the upper clad formation region 1b. Thus, the upper clad 22 is formed so as to cover the upper surface 3a and part of the side surface 33 of the core 3, and the region 21ab of the upper surface 21a of the lower clad 21, which is the region where the core 3 is not formed, of the upper surface 21a of the lower clad 21. The side surface 33 of the core 3 is a surface of the surface of the core 3 extending in the X direction and facing the +Y direction or the −Y direction.

[0014] In the optical waveguide 102, a portion of the core 3 is exposed on both the one end 11 side and the other end 12 side. As shown in Figures 1 to 2C, the core 3 on the one end 11 side has a core upper surface 3ab which is a portion of the upper surface 3a of the core 3, one end surface 3b of the core 3 in the X direction, and a portion of the side surface 33 exposed from the upper clad 22 of the clad 2. As shown in Figures 1 to 2C, the core 3 on the other end 12 side has an end surface 3c of the core 3 opposite to the end surface 3b of the core 3 which is another end surface of the core 3 in the X direction exposed.

[0015] As shown in FIGS. 1 to 2C , the wiring substrate 1 has an upper cladding-free region 1a, which is a region where the upper cladding 22 is not formed in a plan view, and an upper cladding-formed region 1b, which is a region where the upper cladding 22 is formed. That is, the optical waveguide 102 is composed of the upper cladding-free region 1a and the upper cladding-formed region 1b. The optical waveguide 102 has the upper cladding-formed region 1b adjacent to the upper cladding-free region 1a. In the upper cladding-free region 1a in a plan view, the optical waveguide 102 has a core exposed portion 30a, where the core upper surface 3ab, which is a part of the upper surface 3a of the core 3, is exposed, and a lower cladding exposed portion, where the region 21ab of the upper surface 21a of the lower cladding 21 is exposed. In the example of FIG. 1 , a part of the side surface 33 of the core 3 is also exposed in the upper cladding-free region 1a. Note that "plan view" means viewing an object with a line of sight along the Z direction.

[0016] In the wiring substrate 1, the upper cladding non-forming region 1a has a core exposed portion 30a where a core upper surface 3ab, which is a portion of the upper surface 3a of the core 3, is exposed. The upper cladding non-forming region 1a is provided on one end 11 side of the optical waveguide 102. Therefore, as shown in FIGS. 1 to 2C, the core exposed portion 30a is located on one end 11 side of the optical waveguide 102. The core exposed portion 30a is a portion of the core 3 on the one end 11 side of the optical waveguide 102. The upper cladding forming region 1b has a core unexposed portion 30b where the upper surface 3a of the core 3 is not exposed. The core unexposed portion 30b is a portion of the core 3 on the other end 12 side of the optical waveguide 102.

[0017] In the wiring substrate 1 of the embodiment, the upper cladding non-forming region 1 a may be provided on both ends of the optical waveguide 102. That is, the upper cladding non-forming region 1 a, the upper cladding region 1 b, and the upper cladding non-forming region 1 a may be arranged in this order in the X direction.

[0018] The core 3 and cladding 2 that form the optical waveguide 102 are made of any light-transmitting material. The optical waveguide 102 can be made of, for example, an organic material or a hybrid material containing an organic material and an inorganic material, such as an inorganic polymer. Examples of organic materials include acrylic resins such as polymethyl methacrylate (PMMA), polyimide resins, polyamide resins, polyether resins, phenolic resins, fluorine resins, silicone resins, and epoxy resins. The optical waveguide 102 made of an organic material can be lightweight, highly tough, and flexible.

[0019] The core 3 and the clad 2 may be made of different materials or may be made of the same type of material. In either case, the core 3 is made of a material having a higher refractive index than the material used for the clad 2 so that an optical signal can be totally reflected at the interface between the core 3 and the clad 2. Alternatively, the core 3 and the clad 2 may be formed of materials having the same refractive index and then subjected to an appropriate process to make their refractive indices different. That is, the optical waveguide 102 may be formed using a forming method called photolithography or photobleaching, for example.

[0020] When the wiring board 1 is in use, the core 3 is optically coupled at one end 11 and the other end 12 of the optical waveguide 102 to optical components such as photoelectric conversion components including a photoelectric conversion element and / or optical components such as connector members for connecting to the waveguide, such as optical fibers or optical connectors. In other words, the core 3 is positioned with respect to each optical component so as to have a positional relationship that allows transmission and reception of optical signals between the core 3 and these optical components. Note that "optically coupled" is also referred to as "optically coupled" hereinafter.

[0021] 1 to 2C, a component E1 including a photoelectric conversion element (not shown) is indicated by a two-dot chain line as an example of an optical component optically coupled to the core 3 at one end 11 of the optical waveguide 102. When the wiring board 1 is in use, the region between the component E1 and the portion of the optical waveguide 102 that overlaps with the component E1 in a planar view is preferably filled with any optically transparent transmissive resin TR. By filling the space between the component E1 and the optical waveguide 102 with the transmissive resin TR, the refractive index of the space between the component E1 and the optical waveguide 102 is adjusted to a more appropriate refractive index than that of air. Furthermore, the transmissive resin TR can prevent foreign matter such as dust from entering the space between the component E1 and the optical waveguide 102.

[0022] The length of the core 3 in the core exposed portion 30a is not particularly limited, but is preferably 100 to 3000 μm, and more preferably 100 to 2500 μm. If the core exposed portion 30a has a length in this range, it is believed that there is a high degree of freedom in selecting optical components, such as the component E1, that are optically coupled in the core exposed portion 30a. It is also believed that a necessary and sufficient tolerance can be obtained for the alignment of the optical components and the optical waveguide 102 in the X direction.

[0023] The component E1 includes an optical terminal E1a, which is a portion where an optical signal enters the component E1 or a portion where an optical signal exits the component E1. The component E1 is optically coupled to the core 3 at the optical terminal E1a. An optical signal propagating through the core 3 from the other end 12 enters the component E1 via the optical terminal E1a at one end 11. Meanwhile, an optical signal exiting from the optical terminal E1a of the component E1 enters the core 3 at the one end 11, propagates within the core 3, and exits from the other end 12.

[0024] The exposed core portion 30a of the optical waveguide 102 is a portion that overlaps with the optical terminal E1a of the component E1 and transmits and receives optical signals. In the example of FIGS. 1 to 2C, the core 3 is positioned at one end 11 of the optical waveguide 102 so that the core top surface 3ab of the exposed core portion 30a faces the optical terminal E1a of the component E1 and is adiabatic coupled. That is, a portion of the optical signal that has propagated through the core 3 toward the one end 11 leaks out of the core 3 from the core top surface 3ab as evanescent light and enters the optical terminal E1a of the component E1. Because the core top surface 3ab of the exposed core portion 30a faces the optical terminal E1a of the component E1 without the cladding 2 interposed therebetween, it is believed that highly efficient optical coupling is achieved.

[0025] The wiring board 1 of the embodiment has two parallel cores 3 in the optical waveguide 102. The optical waveguide 102 is not limited to two and can have any number of cores 3 equal to or greater than one. For example, the number of cores 3 is in the range of 2 to 64. When multiple cores 3 are provided, the arrangement pitch of the cores 3 is not particularly limited, but is, for example, 10 to 300 μm, and preferably 20 to 250 μm. Note that the arrangement pitch of the multiple cores 3 is not limited to these numerical examples. The arrangement pitch of the cores 3 is the distance between the center of one core 3 and the center of the other core 3 of two adjacent cores 3.

[0026] In the wiring board 1 of the embodiment, as shown in Fig. 2B, the cross-sectional shape of the core 3 in the core exposed portion 30a is rectangular. The cross-sectional shape of the core 3 may be any shape as long as the core upper surface 3ab of the core exposed portion 30a has a flat or substantially flat region. Similarly, as shown in Fig. 2C, the cross-sectional shape of the core 3 in the core non-exposed portion 30b is also rectangular, but is not limited to this.

[0027] <Components of Wiring Board of the Embodiment> The shapes and materials of the intermediate layer M, clad 2, and core 3 of the wiring board of the embodiment will be described with reference to FIGS. 1 to 2C. As shown in FIGS. 1 to 2C, in the embodiment, the intermediate layer M is disposed between the substrate 10 and the lower clad 21. As shown in FIGS. 1 and 2A, when viewing the cross section of the wiring board 1 taken along line II, the end face Mb of the intermediate layer M is disposed outward of the end face 21b of the lower clad 21. A distance DL1 is formed between the end face Mb of the intermediate layer M and the end face 21b of the lower clad 21. The distance DL1 is not particularly limited, but is preferably 1 mm or more, and more preferably 1 to 3 mm.

[0028] As shown in FIGS. 1 and 2B or 2C , when viewing the cross section of the wiring substrate 1 taken along line II-II or line III-III, the side surface Md of the intermediate layer M is disposed outside the side surface 21d of the lower clad 21. A distance DS11 is formed between the side surface Md of the intermediate layer M and the side surface 21d of the lower clad 21. The intermediate layer M is also disposed such that the side surface Me of the intermediate layer M is disposed outside the side surface 21e of the lower clad 21. A distance DS12 is formed between the side surface Me of the intermediate layer M and the side surface 21e of the lower clad 21. While the distance DS11 or the distance DS12 is not particularly limited, it is preferably 1 mm or more, and more preferably 1 to 3 mm. The distances DS11 and DS12 may be the same or different.

[0029] 1 and 2A, when viewing the cross section of the wiring board 1 taken along line II, the end face 3b of the core 3 is disposed so as to be located more inward than the end face 21b of the lower clad 21. A distance DL2 is formed between the end face 3b of the core 3 and the end face 21b of the lower clad 21. The distance DL2 is not particularly limited, but is preferably 1 mm or more, and more preferably 1 to 3 mm. In other words, the end face 21b of the lower clad 21 is disposed so as to be more outward than the end face 3b of the core 3.

[0030] As shown in FIGS. 1 and 2B , when viewing the cross section of the wiring board 1 taken along line II-II, two cores 3 are arranged. Of the side surfaces 33 of these two cores 3, the side surface 3d of one core 3 located at the outermost edge of the optical waveguide 102 is arranged so as to be more inward than one side surface 21d of the lower clad 21. Similarly, of the side surfaces 33 of these two cores 3, the side surface 3e of the other core 3 located at the outermost edge of the optical waveguide 102 is arranged so as to be more inward than the other side surface 21e of the lower clad 21. The distance DS21 between the one side surface 3d of the core 3 and the one side surface 21d of the lower clad 21 is not particularly limited, but is preferably 1 mm or more, and more preferably 1 to 3 mm. Similarly, the distance DS22 between the other side surface 3e of the core 3 and the other side surface 21e of the lower clad 21 is not particularly limited, but is preferably 1 mm or more, and more preferably 1 to 3 mm. In other words, the side surfaces 21 d and 21 e of the lower cladding 21 are disposed outside the side surfaces 3 d and 3 e of the core 3 .

[0031] 2A and 2B, the thickness of the intermediate layer M is T1, the thickness of the lower clad 21 is T2, the thickness of the core 3 is T3, and the thickness of the upper clad 22 is T4. The thickness T2 of the lower clad 21 is not particularly limited, but is preferably 5 to 100 μm, and more preferably 10 to 50 μm.

[0032] The thickness T3 of the core 3 is not particularly limited, but is preferably 1 to 20 μm, and more preferably 3 to 10 μm. The thickness T2 of the lower cladding 21 is thicker than the thickness T3 of the core 3. In other words, the ratio (T2 / T3) of the thickness T2 of the lower cladding 21 to the thickness T3 of the core 3 satisfies Equation 3: 1 < T2 / T3 (Equation 3) The ratio (T2 / T3) of the thickness T2 of the lower cladding 21 to the thickness T3 of the core 3 preferably satisfies Equation 4: 1.5 < T2 / T3 (Equation 4) The ratio (T2 / T3) of the thickness T2 of the lower cladding 21 to the thickness T3 of the core 3 preferably satisfies Equation 5: 2.0≦T2 / T3≦4.0 (Equation 5) For example, when the thickness T3 of the core 3 is 5 μm, the thickness T2 of the lower cladding 21 is preferably 10 to 20 μm. The thickness T2 of the lower cladding 21 and the thickness T3 of the core 3 are determined as the average values ​​of measurements taken at three points in the Z direction on the SEM photograph.

[0033] The thickness T4 of the upper cladding 22 is not particularly limited, but is preferably 5 to 100 μm, and more preferably 10 to 50 μm, and is determined by averaging values ​​measured at three points in the Z direction on an SEM photograph.

[0034] As shown in FIGS. 1 to 2C , in the wiring substrate 1, an intermediate layer M is disposed between the substrate 10 and the lower clad 21. This mitigates the effects of surface irregularities on the substrate 10 during the process of forming the lower clad 21, suppresses variations in the thickness of the lower clad 21, and achieves thickness uniformity. This is particularly effective in mitigating the effects of surface irregularities on the lower clad and achieving thickness uniformity when the lower clad is formed by coating. Furthermore, it is believed that variations in the thickness of the core 3 formed on the lower clad 21 and the thickness of the upper clad 22 are suppressed, and uniformity of these two thicknesses is also achieved. It is also believed that the intermediate layer M also suppresses problems such as peeling of the lower clad due to thermal history, such as heat cycles, during use of the wiring substrate 1.

[0035] As shown in FIGS. 1 to 2C , the intermediate layer M is disposed such that its end face Mb is located outside the end face 21b of the lower clad 21. Furthermore, the side faces Md and Me of the intermediate layer M are located outside the side faces 21d and 21e of the lower clad 21, respectively. In other words, the intermediate layer M protrudes beyond the lower clad. In a conventional core formation process in which a core material is applied, a core is formed on the lower clad. At this time, the core material flows down from the end of the lower clad. If the flow rate of this core material increases, the amount of core material near the end of the lower clad 21 decreases more than expected. As a result, the thickness of the core 3 becomes smaller than the predetermined thickness. In the wiring board of FIGS. 1 to 2C , by disposing the intermediate layer M on the substrate 10, the end face Mb of the intermediate layer M is located outside the end face 21b of the lower clad 21.

[0036] By forming the intermediate layer M and the lower clad 21, the core material flows down from the end of the lower clad during the core formation process by coating. However, the flowing-down core material temporarily remains on the intermediate layer M, which is positioned outside the end face 21b of the lower clad 21 and protrudes beyond the end face 21b of the lower clad 21. This reduces the outflow rate of the core material flowing down from the end face of the lower clad 21, thereby suppressing the loss of the core material. As a result, the uniformity of the thickness of the core 3 near the end face 21b, side faces 21d, and 21e of the lower clad 21 is ensured, and it is believed that the thickness of the core 3 can be formed according to the design value. In particular, when forming a predetermined core with a thin thickness, the formation of the intermediate layer M is effective in achieving uniformity of the core thickness. Forming a thin core means forming the core with a thickness of, for example, 1 to 20 μm.

[0037] 1 to 2C, the core 3 is arranged such that the end face 3b of the core 3 is located more inward than the end face 21b of the lower cladding 21. The side faces 3d and 3e of the core 3 are located more inward than the side faces 21d and 21e of the lower cladding 21, respectively. The end face 3b of the core 3 is located more inward than the end face Mb of the intermediate layer, and is also located more inward than the end face 21b of the lower cladding 21. In the cross-sectional shapes of FIGS. 2A and 2B, the end face 21b of the lower cladding 21 is located more outward than the end face 3b of the core 3, and the end face Mb of the intermediate layer M is located more outward than the end face 21b of the lower cladding 21. In other words, the intermediate layer M, the lower cladding 21, and the core 3 form a stepped structure.

[0038] In a conventional core formation process in which a core material is applied, a core is formed on a lower clad. During this process, the core material flows down from the end of the lower clad 21. As the core material flows down from the end of the lower clad 21, the amount of core material near the end of the lower clad 21 is reduced. As a result, the thickness of the core 3 becomes smaller than the predetermined thickness. When the core 3 is formed, the core material in the region near the end of the lower clad 21 is removed by patterning, and the end face 3b of the core 3 is positioned so that it is located inside the end face 21b of the lower clad 21. Furthermore, the side faces 3d and 3e of the core 3 are positioned inside the side faces 21d and 21e of the lower clad 21, respectively. The lower clad 21 and intermediate layer M are positioned outside the end of the core 3. It is believed that the uniformity of the thickness of the core 3 is ensured, and the core 3 can be formed to the predetermined thickness as designed. Note that making the core thickness a predetermined thickness means making the core thickness the designed thickness, and as an example, making the core thickness the optimal value for the core thickness used in the waveguide within the range of 1 to 20 μm.

[0039] As shown in FIG. 2A , the line connecting the upper surface of the end of the intermediate layer M and the upper surface of the end of the lower cladding is designated as imaginary line A, and the line representing the surface of the substrate 10 is designated as imaginary line B. The interior angle at which imaginary line A and imaginary line B intersect is referred to as the forming crossing angle θ. It is desirable for this forming crossing angle θ to be 45 degrees or less, and it is even more desirable for the forming crossing angle θ to be 1 to 40 degrees. By setting the forming crossing angle θ to 45 degrees or less, the amount of core material flowing down near the end of the lower cladding 21 during core formation by coating can be reduced. As a result, the thickness of the core 3 is prevented from becoming smaller than the predetermined thickness. Furthermore, it is believed that setting the forming crossing angle θ to be 1 to 40 degrees not only reduces the amount of core material flowing down near the end of the lower cladding 21, but also ensures uniformity in the core thickness.

[0040] The intermediate layer M will be described in detail. The relationship between the intermediate layer M and the lower clad 21 will also be described. Here, there are two correlations between the thermal expansion coefficient λ1 of the intermediate layer M and the thermal expansion coefficient λ2 of the lower clad 21. One is when the thermal expansion coefficient λ1 of the intermediate layer M is the same as or approximately the same as the thermal expansion coefficient λ2 of the lower clad 21, and the other is when the thermal expansion coefficient λ1 of the intermediate layer M is smaller than the thermal expansion coefficient λ2 of the lower clad 21. To explain these, the former is described below as a first configuration example of the wiring board 1 of the embodiment, and the latter is described below as a second configuration example of the wiring board 1 of the embodiment.

[0041] In a first configuration example of the wiring board 1 of the embodiment, the thermal expansion coefficient λ1 of the intermediate layer M is the same as or approximately the same as the thermal expansion coefficient λ2 of the lower clad 21. The material constituting the intermediate layer M is not particularly limited, but is preferably a thermosetting resin or a photocurable resin. Furthermore, the material constituting the intermediate layer M is more preferably an optical material. An optical material refers to a material that can adjust the refractive index of light and control the refraction and reflection of light, or a material that has little scattering or absorption of light. The material constituting the intermediate layer M may be a material that does not contain particles. The intermediate layer M may be composed of the material constituting the lower clad 21 or a similar material. For example, the intermediate layer M may be formed of an organic material or a hybrid material containing an organic material and an inorganic material, such as an inorganic polymer. Examples of organic materials include acrylic resins, polyimide resins, polyamide resins, polyether resins, phenolic resins, fluorine-based resins, silicone resins, and epoxy resins. In the first configuration example of the wiring board 1 of the embodiment, by forming the intermediate layer M using the same material as the lower clad 21, it is possible to suppress the generation of stress due to external heat and reduce phenomena such as warping of the wiring board 1. The intermediate layer M may be used as a lower layer of the lower clad 21. In this case, it is preferable that the material of the intermediate layer M is the same as the material of the lower clad 21.

[0042] In the first configuration example of the wiring board 1 of the embodiment, it is preferable that the thermal expansion coefficient λ1 of the intermediate layer M and the thermal expansion coefficient λ2 of the lower clad 21 satisfy the relationship of Equation 6: λ2 = λ1 (Equation 6) Here, "the thermal expansion coefficient λ1 of the intermediate layer M and the thermal expansion coefficient λ2 of the lower clad 21 being the same" means that the difference between the thermal expansion coefficient λ1 of the intermediate layer M and the thermal expansion coefficient λ2 of the lower clad 21 is within 10%. Furthermore, "the thermal expansion coefficient λ1 of the intermediate layer M and the thermal expansion coefficient λ2 of the lower clad 21 being approximately the same" means that the difference between the thermal expansion coefficient λ1 of the intermediate layer M and the thermal expansion coefficient λ2 of the lower clad 21 is within ±1%. Note that if the material of the intermediate layer M and the material of the lower clad 21 have the same composition, it can be said that Equation 6 is satisfied.

[0043] The thickness T1 of the intermediate layer M is not particularly limited, but is preferably 5 to 50 μm, and more preferably 10 to 30 μm. The intermediate layer M may be a single layer or a multi-layer structure of two or more layers.

[0044] In a second configuration example of the wiring board 1 of the embodiment, the thermal expansion coefficient λ1 of the intermediate layer M is smaller than the thermal expansion coefficient λ2 of the lower cladding 21. The material constituting the intermediate layer M is not particularly limited, but is preferably a thermosetting resin or a photocurable resin. The material constituting the intermediate layer M may be an optical material or a non-optical material. Here, a non-optical material refers to a material whose refractive index is not adjusted and which cannot control the refraction or reflection of light, or a material that significantly scatters or absorbs light. The material constituting the intermediate layer M may contain particles. The particles are preferably 1 μmφ or less, and examples include inorganic fillers or resin fillers made of fine particles such as silica (SiO), alumina, or mullite. The intermediate layer M may be composed of the same material as the substrate 10 or a similar material. If the substrate 10 has an insulating layer on its surface, the intermediate layer M may be composed of the same or a similar material as the insulating layer. For example, the intermediate layer M may be formed using a thermosetting resin such as an epoxy resin, a bismaleimide triazine resin, or a phenolic resin, or a thermoplastic resin such as a fluororesin, a liquid crystal polymer, a fluorinated ethylene resin, a polyester resin, or a modified polyimide resin. In the second configuration example of the wiring board 1 of the embodiment, by forming the intermediate layer M using a material similar to the substrate 10 or the insulating layer that the substrate 10 has on its surface, the generation of stress due to external heat, etc., may be suppressed, and phenomena such as warping of the wiring board 1 may be reduced.

[0045] In the second configuration example of the wiring board 1 of the embodiment, it is preferable that the thermal expansion coefficient λ1 of the intermediate layer M and the thermal expansion coefficient λ2 of the lower clad 21 satisfy the relationship of Equation 7: λ2>λ1 (Equation 7) Here, the thermal expansion coefficient λ1 of the intermediate layer M being smaller than the thermal expansion coefficient λ2 of the lower clad 21 means that the difference between the thermal expansion coefficient λ1 of the intermediate layer M and the thermal expansion coefficient λ2 of the lower clad 21 exceeds 10%.

[0046] The thickness T1 of the intermediate layer M is not particularly limited, but is preferably 5 to 50 μm, and more preferably 10 to 30 μm. The intermediate layer M may be a single layer or a multi-layer structure of two or more layers.

[0047] In the first configuration example of the wiring board 1 of the embodiment, it is preferable that the thermal expansion coefficient λ2 of the lower clad 21, the thermal expansion coefficient λ1 of the intermediate layer M, and the thermal expansion coefficient λ0 of the insulating layer on the surface of the substrate 10 satisfy the relationship of Equation 2: λ2 = λ1 > λ0 (Equation 2) When the thermal expansion coefficient λ2 of the lower clad 21, the thermal expansion coefficient λ1 of the intermediate layer M, and the thermal expansion coefficient λ0 of the insulating layer on the surface of the substrate 10 satisfy the relationship of Equation 2, in the first configuration example of the wiring board 1 of the embodiment, the generation of stress due to external heat is suppressed, and phenomena such as warping of the wiring board 1 may be reduced.

[0048] In the second configuration example of the wiring board 1 of the embodiment, it is preferable that the thermal expansion coefficient λ2 of the lower clad 21, the thermal expansion coefficient λ1 of the intermediate layer M, and the thermal expansion coefficient λ0 of the insulating layer on the surface of the substrate 10 satisfy the relationship of Formula 1: λ2 > λ1 > λ0 (Formula 1) When the thermal expansion coefficient λ2 of the lower clad 21, the thermal expansion coefficient λ1 of the intermediate layer M, and the thermal expansion coefficient λ0 of the insulating layer on the surface of the substrate 10 satisfy the relationship of Formula 1, in the second configuration example of the wiring board 1 of the embodiment, the generation of stress due to external heat is suppressed, and phenomena such as warping of the wiring board 1 may be reduced.

[0049] <Structure of a Wiring Board According to a Modification of the Embodiment> Fig. 3 shows a cross-sectional view of a wiring board 1 including an optical waveguide component 101, which is a modification of the optical waveguide component of the wiring board according to the embodiment. As shown in Fig. 3, the intermediate layer M may be composed of a first intermediate layer M1 and a second intermediate layer M2. The second intermediate layer M2 is formed on the solder resist 61 of the substrate 10. Furthermore, the first intermediate layer M1 is formed on the second intermediate layer M2. As a result, an intermediate layer M consisting of two layers, the first intermediate layer M1 and the second intermediate layer M2, is formed on the substrate 10. In other words, multiple intermediate layers M are arranged on the substrate 10.

[0050] The optical waveguide 102 is formed on the intermediate layer M. The lower clad 21 is formed on the intermediate layer M. The core 3 is formed on the lower clad 21. The upper clad 22 is formed on the lower clad 21 and the core 3. The optical waveguide 102 is composed of the lower clad 21, the core 3, and the upper clad 22. That is, the wiring board 1 is arranged in the order of the substrate 10, the solder resist 61, the intermediate layer M, and the optical waveguide 102. The optical waveguide 102 is arranged in the order of the lower clad 21, the core 3, and the upper clad 22. The wiring board 1 is provided with an optical waveguide component 101 having the function of an optical waveguide. The optical waveguide component 101 having the function of an optical waveguide is composed of the optical waveguide 102 and the intermediate layer M. The substrate 10 does not need to have a solder resist formed thereon. By increasing the number of intermediate layers M, the influence of unevenness on the surface of the substrate 10 is more efficiently alleviated, and uniformity in the thickness of the lower cladding 21, the core 3 and the upper cladding 22 is ensured.

[0051] As shown in Figure 3, the intermediate layer M may be formed in multiple layers, with the end face of each intermediate layer protruding from the end face 21b of the lower clad 21 or the intermediate layer M above it, forming a stepped structure. By forming the stepped structure, the core material that attempts to flow down from the end of the lower clad during the core formation process by coating can be temporarily retained by the multiple intermediate layers, i.e., the first intermediate layer M1 and the second intermediate layer M2. This reduces the outflow rate of the core material flowing down from the end of the lower clad 21, thereby suppressing the loss of core material. As a result, the uniformity of the film thickness of the core 3 is stabilized, and it is believed that the core 3 thickness is formed as designed.

[0052] Here, the thermal expansion coefficients of the multiple intermediate layers and the thermal expansion coefficient λ2 of the lower clad 21 have the following relationship. The thermal expansion coefficient of the first intermediate layer M1 is referred to as λ11, and the thermal expansion coefficient of the second intermediate layer M2 is referred to as λ12. There are four combinations of multiple intermediate layers and the lower clad 21. The first is a combination in which the thermal expansion coefficients of each intermediate layer and the thermal expansion coefficient λ2 of the lower clad 21 are all the same. The second is a combination in which the thermal expansion coefficients of each intermediate layer are different, but the thermal expansion coefficient λ11 of the first intermediate layer M1 and the thermal expansion coefficient λ2 of the lower clad 21 are the same. The third is a combination in which the thermal expansion coefficients of each intermediate layer are the same, but the thermal expansion coefficient λ2 of the lower clad 21 is different. Finally, the last is a combination in which the thermal expansion coefficients of each intermediate layer and the thermal expansion coefficient λ2 of the lower clad 21 are different.

[0053] In a wiring substrate 1 according to a modified example of the embodiment in which the thermal expansion coefficients of the intermediate layers and the thermal expansion coefficient λ2 of the lower clad 21 are all the same, the thermal expansion coefficient λ11 of the first intermediate layer M1, the thermal expansion coefficient λ12 of the second intermediate layer M2, and the thermal expansion coefficient λ2 of the lower clad 21 preferably satisfy the relationship of Equation 8: λ2 = λ11 = λ12 (Equation 8) Here, the thermal expansion coefficients λ11 of the first intermediate layer M1, λ12 of the second intermediate layer M2, and the thermal expansion coefficient λ2 of the lower clad 21 being the same means that the difference in the thermal expansion coefficients of all layers is within 10%. Note that if the thermal expansion coefficient of the intermediate layer is the same as the thermal expansion coefficient λ2 of the lower clad 21, the intermediate layer may be used as a lower layer of the lower clad 21.

[0054] In a wiring substrate 1 according to a modified example of the embodiment in which the thermal expansion coefficients of the intermediate layers are different but the thermal expansion coefficient λ11 of the first intermediate layer M1 and the thermal expansion coefficient λ2 of the lower clad 21 are the same, the thermal expansion coefficient λ11 of the first intermediate layer M1, the thermal expansion coefficient λ12 of the second intermediate layer M2, and the thermal expansion coefficient λ2 of the lower clad 21 preferably satisfy the relationship of Equation 9: λ2 = λ11 > λ12 (Equation 9) Here, the thermal expansion coefficient λ11 of the first intermediate layer M1 and the thermal expansion coefficient λ2 of the lower clad 21 being the same means that the difference in the thermal expansion coefficients of the layers is within 10%. Note that if the thermal expansion coefficient of the intermediate layer is the same as the thermal expansion coefficient λ2 of the lower clad 21, the intermediate layer may be used as a lower layer of the lower clad 21.

[0055] In a wiring board 1 according to a modified embodiment in which the intermediate layers of each layer have the same thermal expansion coefficient but the thermal expansion coefficient λ2 of the lower clad 21 is different, it is preferable that the thermal expansion coefficient λ11 of the first intermediate layer M1, the thermal expansion coefficient λ12 of the second intermediate layer M2, and the thermal expansion coefficient λ2 of the lower clad 21 satisfy the relationship of Equation 10: λ2 > λ11 = λ12 (Equation 10) Here, the thermal expansion coefficient λ11 of the first intermediate layer M1 and the thermal expansion coefficient λ12 of the second intermediate layer M2 being the same means that the difference in the thermal expansion coefficients of the layers is within 10%.

[0056] In a wiring board 1 according to a modified embodiment in which the thermal expansion coefficients of the intermediate layers and the thermal expansion coefficient λ2 of the lower clad 21 are different from each other, it is preferable that the thermal expansion coefficient λ11 of the first intermediate layer M1, the thermal expansion coefficient λ12 of the second intermediate layer M2, and the thermal expansion coefficient λ2 of the lower clad 21 satisfy the relationship of Equation 11: λ2 > λ11 > λ12 (Equation 11) Here, the different thermal expansion coefficients of the layers means that the difference in the thermal expansion coefficients of the layers exceeds 10%.

[0057] 3, the thermal expansion coefficients of the multiple intermediate layers are adjusted so that they increase in the order of the first intermediate layer M1 and the second intermediate layer M2 from the same thermal expansion coefficient λ2 as the lower cladding 21, thereby efficiently suppressing the influence of heat from outside the wiring board 1. In FIG. 3, the wiring board 1 of the embodiment has two intermediate layers M, but the number is not limited to two and may have any number of intermediate layers equal to or greater than one.

[0058] The combination of materials for the first intermediate layer M1, the second intermediate layer M2, and the lower cladding 21, which are multiple intermediate layers, will be described based on the thermal expansion coefficient of each intermediate layer and the thermal expansion coefficient of the lower cladding 21. When the thermal expansion coefficient λ2 of each intermediate layer and the thermal expansion coefficient λ2 of the lower cladding 21 are all the same, the first intermediate layer M1, the second intermediate layer M2, and the lower cladding 21 may all be optical materials. Alternatively, the first intermediate layer M1, or the first intermediate layer M1 and the second intermediate layer M2 may be used as a lower layer for the lower cladding 21.

[0059] Specifically, the first intermediate layer M1, the second intermediate layer M2, and the lower cladding 21 are all made of optical material. In this case, the optical material of the lower cladding 21 and the optical materials of the first intermediate layer M1 and the second intermediate layer M2 may all be the same material. If the lower cladding 21, the first intermediate layer M1, and the second intermediate layer M2 are all made of the same material, there are three patterns for the configuration of the portion of the optical waveguide 102 that functions as a lower cladding. First, only the lower cladding 21 functions as a portion that functions as a lower cladding. Second, the lower cladding 21 and the first intermediate layer M1 functions as a portion that functions as a lower cladding. Third, the lower cladding 21, the first intermediate layer M1, and the second intermediate layer M2 functions as a portion that functions as a lower cladding. Note that it is preferable that the portion that functions as a lower cladding is the lower cladding 21 and the first intermediate layer M1, or the lower cladding 21, the first intermediate layer M1, and the second intermediate layer M2.

[0060] Alternatively, the lower cladding 21 and the first intermediate layer M1 may be made of an optical material, and the second intermediate layer M2 may be made of a non-optical material. Alternatively, the lower cladding 21 may be made of an optical material, and the first intermediate layer M1 and the second intermediate layer M2 may be made of a non-optical material. Even with these material combinations, it is preferable that the thermal expansion coefficient λ11 of the first intermediate layer M1, the thermal expansion coefficient λ12 of the second intermediate layer M2, and the thermal expansion coefficient λ2 of the lower cladding 21 satisfy the relationship of Equation 8.

[0061] In the case where the thermal expansion coefficients of the respective intermediate layers are different but the thermal expansion coefficient λ11 of the first intermediate layer M1 is the same as the thermal expansion coefficient λ2 of the lower cladding 21, the first intermediate layer M1, the second intermediate layer M2, and the lower cladding 21 may all be made of optical materials. Alternatively, the first intermediate layer M1, or the first intermediate layer M1 and the second intermediate layer M2, may be used as a lower layer of the lower cladding 21.

[0062] Specifically, the first intermediate layer M1, the second intermediate layer M2, and the lower cladding 21 are all made of an optical material. In this case, the optical material of the lower cladding 21 and the optical material of the first intermediate layer M1 may be the same material. If the lower cladding 21 and the first intermediate layer M1 are made of the same material, the configuration of the portion of the optical waveguide 102 having the lower cladding function may be such that only the lower cladding 21 has the lower cladding function, or the lower cladding 21 and the first intermediate layer M1 have the lower cladding function. It is preferable that the portion having the lower cladding function be the lower cladding 21 and the first intermediate layer M1.

[0063] Alternatively, the lower cladding 21 and the first intermediate layer M1 may be made of an optical material, and the second intermediate layer M2 may be made of a non-optical material. In this case, the optical material of the lower cladding 21 and the optical material of the first intermediate layer M1 may be the same material. If the lower cladding 21 and the first intermediate layer M1 are made of the same material, only the lower cladding 21 may function as a lower cladding, or the lower cladding 21 and the first intermediate layer M1 may function as lower cladding. It is preferable that the lower cladding function be the lower cladding 21 and the first intermediate layer M1. Even with these material combinations, it is preferable that the thermal expansion coefficient λ11 of the first intermediate layer M1, the thermal expansion coefficient λ12 of the second intermediate layer M2, and the thermal expansion coefficient λ2 of the lower cladding 21 satisfy the relationship of Equation 9.

[0064] In a combination in which the thermal expansion coefficients of the intermediate layers are the same but the thermal expansion coefficient λ2 of the lower cladding 21 is different, the first intermediate layer M1, the second intermediate layer M2, and the lower cladding 21 may all be made of optical materials. Alternatively, the first intermediate layer M1, or the first intermediate layer M1 and the second intermediate layer M2, may be used as a lower layer of the lower cladding 21.

[0065] Specifically, the first intermediate layer M1, the second intermediate layer M2, and the lower cladding 21 are all made of optical materials. In this case, the optical material of the first intermediate layer M1 and the optical material of the second intermediate layer M2 may be the same material. If the first intermediate layer M1 and the second intermediate layer M2 are made of the same material, only the lower cladding 21 may function as a lower cladding, or the lower cladding 21 and the first intermediate layer M1 may function as lower cladding. It is preferable that the lower cladding 21 and the first intermediate layer M1 function as the lower cladding.

[0066] Alternatively, the lower cladding 21 and the first intermediate layer M1 may be made of an optical material, and the second intermediate layer M2 may be made of a non-optical material. If the lower cladding 21 and the first intermediate layer M1 are made of an optical material, only the lower cladding 21 may function as a lower cladding, or the lower cladding 21 and the first intermediate layer M1 may function as a lower cladding. It is preferable that the lower cladding 21 and the first intermediate layer M1 function as the lower cladding. Furthermore, the lower cladding 21 may be made of an optical material, and the first intermediate layer M1 and the second intermediate layer M2 may be made of a non-optical material. Even with these material combinations, it is preferable that the thermal expansion coefficient λ11 of the first intermediate layer M1, the thermal expansion coefficient λ12 of the second intermediate layer M2, and the thermal expansion coefficient λ2 of the lower cladding 21 satisfy the relationship of Equation 10.

[0067] When the thermal expansion coefficient λ2 of each intermediate layer and the thermal expansion coefficient λ2 of the lower cladding 21 are different, the first intermediate layer M1, the second intermediate layer M2, and the lower cladding 21 may all be made of optical materials. Alternatively, the first intermediate layer M1, or the first intermediate layer M1 and the second intermediate layer M2 may be used as layers below the lower cladding 21.

[0068] Specifically, the first intermediate layer M1, the second intermediate layer M2, and the lower cladding 21 are all made of optical materials. In this case, the optical material of the first intermediate layer M1 and the optical material of the second intermediate layer M2 may be the same material. If the first intermediate layer M1, the second intermediate layer M2, and the lower cladding 21 are all made of optical materials, only the lower cladding 21 may function as a lower cladding, or the lower cladding 21 and the first intermediate layer M1 may function as lower cladding. It is preferable that the lower cladding 21 and the first intermediate layer M1 are the parts that function as a lower cladding.

[0069] Alternatively, the lower cladding 21 and the first intermediate layer M1 may be made of an optical material, and the second intermediate layer M2 may be made of a non-optical material. If the lower cladding 21 and the first intermediate layer M1 are made of an optical material, only the lower cladding 21 may function as a lower cladding, or the lower cladding 21 and the first intermediate layer M1 may function as a lower cladding. It is preferable that the lower cladding 21 and the first intermediate layer M1 function as the lower cladding. Furthermore, the lower cladding 21 may be made of an optical material, and the first intermediate layer M1 and the second intermediate layer M2 may be made of a non-optical material. Even with these material combinations, it is preferable that the thermal expansion coefficient λ11 of the first intermediate layer M1, the thermal expansion coefficient λ12 of the second intermediate layer M2, and the thermal expansion coefficient λ2 of the lower cladding 21 satisfy the relationship of Equation 11.

[0070] As shown in FIG. 3 , the line connecting the upper surface of the end of the lowest layer (second intermediate layer M2) of the intermediate layer M and the upper surface of the end of the lower cladding is designated as imaginary line A', and the line representing the surface of the substrate 10 is designated as imaginary line B'. The interior angle at the intersection of imaginary lines A' and B' is referred to as the formed crossing angle θ'. This formed crossing angle θ' is preferably 45 degrees or less, and more preferably 1 to 40 degrees. By setting the formed crossing angle θ' to 45 degrees or less, the loss of core material near the end of the lower cladding 21 is suppressed. As a result, the thickness of the core 3 can be prevented from becoming smaller than a predetermined thickness. Furthermore, setting the formed crossing angle θ' to 1 to 40 degrees is believed to not only suppress the loss of core material near the end of the lower cladding 21 but also stabilize the core thickness.

[0071] <Detailed Structure of Wiring Board of the Embodiment> Next, the wiring board of the embodiment will be described in detail with reference to the drawings. FIG. 4 shows a cross-sectional view of a wiring board 100, which is an example of a wiring board of the embodiment. Note that the wiring board 100 shown in FIG. 4 and the wiring board 100α shown in FIGS. 5 to 6, which will be referred to later, are merely examples of wiring boards of the embodiment. The layered structure of the wiring board of the embodiment is not limited to the layered structure of the wiring board 100 or the wiring board 100α shown in FIGS. 4 to 6. Furthermore, the number of conductor layers and the number of insulating layers included in the wiring board of the embodiment are not limited to the number of conductor layers and the number of insulating layers included in the wiring board 100 or the wiring board 100α.

[0072] The wiring board 100 of the embodiment shown in FIG. 4 includes a substrate 110 instead of the substrate 10 of the wiring board 1 shown in FIGS. 1 to 3. That is, the wiring board 100 includes the substrate 110 and an optical waveguide component 101 disposed on the substrate 110. The substrate 110 includes an insulating layer and a conductor layer laminated on the insulating layer. The substrate 110 includes conductor layers 41 to 43 as conductor layers and insulating layers 51 and 52 as insulating layers. The wiring board 100 has a first surface 100a, which is a mounting surface for the component E1, and a second surface 100b, which is the surface opposite the first surface 100a. The optical waveguide component 101 is disposed on the first surface 100a. The wiring board 100 includes a component mounting pad 4, which is a conductor pad included in the conductor layer 41, on the first surface 100a.

[0073] The conductor layers 41 to 43 and the insulating layers 51 and 52 are stacked in the following order from the second surface 100b side of the wiring board 100 toward the first surface 100a side: conductor layer 43, insulating layer 52, conductor layer 42, insulating layer 51, conductor layer 41. The conductor layers 41 and 42 are connected by via conductors 7 that penetrate the insulating layer 51. The conductor layers 42 and 43 are connected by via conductors 7 that penetrate the insulating layer 52. The substrate 110 includes a solder resist 62 that covers the conductor layer 43 and the insulating layer 52, and a solder resist 61 that covers the conductor layer 41 and the insulating layer 51. The substrate 110 also includes bumps 8 that are connected to the conductor pads of the conductor layer 43 and protrude from the solder resist 62. The bumps 8 are made of a conductor such as solder and are used for electrical and mechanical connection between the wiring board 100 and an external component (e.g., a motherboard of an electrical device) located on the second surface 100b side. The wiring substrate 100 may be used as a motherboard on which the bumps 8 are not arranged.

[0074] The insulating layers 51 and 52 may be formed using a thermosetting insulating resin such as epoxy resin, bismaleimide triazine resin (BT resin), or phenolic resin. The insulating layers 51 and 52 may also be formed using a thermoplastic insulating resin such as fluororesin, liquid crystal polymer (LCP), fluoroethylene (PTFE) resin, polyester (PE) resin, or modified polyimide (MPI) resin. The resins listed as insulating layer materials are merely examples of materials that can be used to form the insulating layers. The insulating layers may be formed using any material that can provide insulation between conductor layers within the substrate 110. Although not shown, each insulating layer may include a core made of a reinforcing material such as glass fiber or aramid fiber, or may include an inorganic filler made of fine particles of silica (SiO), alumina, or mullite.

[0075] The solder resists 61 and 62 are formed using, for example, a photosensitive material such as epoxy resin or polyimide resin, and form an insulating layer on the surface of the substrate 110 .

[0076] Examples of conductors constituting the conductor layers 41-43 and via conductor 7 include copper, nickel, and silver, and it is preferable to use copper or an alloy primarily made of copper. Although each of these conductors is depicted as a single layer in Fig. 4 for simplicity, they may have a multilayer structure including two or more films. For example, the conductor layers 41-43 and via conductor 7 may have a two-layer structure including an electroless plated film and an electrolytic plated film.

[0077] An opening 61a is provided in the solder resist 61, and the component mounting pad 4 is exposed in the opening 61a. The optical waveguide component 101 is disposed on the solder resist 61. Although not shown, the optical waveguide component 101 may be fixed to the surface of the substrate 110 by any fixing member such as an adhesive.

[0078] The optical waveguide component 101 is an optical waveguide component in the wiring board of the embodiment described in Figures 1 to 3. The optical waveguide component 101 includes an intermediate layer M and an optical waveguide 102, and may be an optical waveguide component similar to the optical waveguide component 101 shown in Figures 1 to 3. Figure 4 shows, as an example, an optical waveguide component 101 that is identical to the optical waveguide component of Figure 1. Thus, the optical waveguide component 101 of Figure 4 includes a laminated intermediate layer M, a lower clad 21, a core 3, and an upper clad 22, and has an upper clad non-forming region 1a and an upper clad formed region 1b. The optical waveguide component 101 has a core exposed portion 30a in the upper clad non-forming region 1a and a core unexposed portion 30b in the upper clad formed region 1b. In the core exposed portion 30a, the end face of the intermediate layer M of the optical waveguide component 101 is disposed outward in the -X direction from the end face of the lower clad 21. In other words, the end face of the intermediate layer M protrudes from the end face of the lower clad 21. Furthermore, in the -X direction, the end face of the core 3 is disposed so as to be inward in the -X direction from the end face of the lower clad 21. Although not shown, in the core exposed portion 30a, the side face of the intermediate layer M of the optical waveguide component 101 is disposed outward in the Y direction from the side face of the lower clad 21. In other words, the side face of the intermediate layer M protrudes from the side face of the lower clad 21. Furthermore, in the Y direction, the side face of the core 3 is disposed so as to be inward in the side face of the lower clad 21.

[0079] A component E1 is mounted on the wiring board 100. The component E1 is an optical component including a photoelectric transducer, as described in the description of the wiring board 1 in FIG. 1 and other drawings. The component E1 includes an optical terminal E1a and a ball-shaped electrode E1b. Examples of the component E1 include a light-receiving element such as a photodiode, and a light-emitting element such as a light-emitting diode (LED), an organic light-emitting diode (OLED), a laser diode (LD), and a vertical-cavity surface-emitting laser (VCSEL). When the component E1 is a light-emitting element, the component E1 generates an optical signal based on an electrical signal input to the electrode E1b, and emits the optical signal from the optical terminal E1a, which functions as a light-emitting unit, toward the core 3. When the component E1 is a light-receiving element, the component E1 receives an optical signal from the optical terminal E1a, which functions as a light-receiving unit, generates an electrical signal based on the optical signal, and outputs it from the electrode E1b.

[0080] The component E1 is mounted on the wiring substrate 100 by connecting the electrode E1b to the component mounting pad 4 using, for example, solder. In Fig. 4, the component E1 is flip-chip mounted. The optical terminal E1a and the core exposed portion 30a of the core 3 of the optical waveguide component 101 are positioned to face each other, thereby achieving optical coupling.

[0081] As shown in FIG. 4 , the wiring board 100 is configured by mounting an optical waveguide component 101 on a solder resist 61, which is one of the insulating layers that constitute the surface layer of the substrate 110. In the wiring board 100, the thermal expansion coefficient λ2 of the lower clad 21, the thermal expansion coefficient λ1 of the intermediate layer M, and the thermal expansion coefficient λ0 of the insulating layer that constitutes the surface layer of the substrate 110 are adjusted to satisfy the relationship of Equation 1 or Equation 2: λ2 = λ1 > λ0 (Equation 2) λ2 > λ1 > λ0 (Equation 1) When the thermal expansion coefficient λ2 of the lower clad 21, the thermal expansion coefficient λ1 of the intermediate layer M, and the thermal expansion coefficient λ0 of the insulating layer that constitutes the surface layer of the substrate 110 satisfy the relationship of Equation 1 or Equation 2, problems such as peeling of the lower clad 21 due to thermal history such as heat cycles during use of the wiring board 100 are suppressed. Furthermore, the influence of external heat is effectively suppressed, which may prevent warping or deformation of the wiring board 100.

[0082] 5 and 6 show a wiring board 100α, which is an example of a wiring board according to a modified embodiment. FIG. 5 shows a cross-sectional view of the wiring board 100α, and FIG. 6 shows a plan view of the wiring board 100α. The wiring board 100α includes a substrate 110α and an optical waveguide component 101 disposed on the substrate 110α. The optical waveguide component 101 is an optical waveguide component according to an embodiment, such as the optical waveguide component 101 shown in FIGS. 1 to 4, and is composed of an intermediate layer M and an optical waveguide 102.

[0083] 5 and 6 , the optical waveguide component 101 has two exposed core portions, a first exposed core portion 301a and a second exposed core portion 302a, at both ends, and a non-exposed core portion 30b in the center. Therefore, at the two exposed core portions, the end faces of the intermediate layer M of the optical waveguide component 101 are located outward from the end face of the lower clad 21. In other words, the intermediate layer M protrudes from the end face of the lower clad 21. That is, at both ends of the optical waveguide component 101, the end faces of the intermediate layer M are located outward from the end face of the lower clad 21. The intermediate layer M protrudes from the end face of the lower clad 21. Furthermore, at the first exposed core portion 301a and the second exposed core portion 302a, the end faces of the core 3 of the optical waveguide component 101 are located inward from the end face of the lower clad 21. That is, at both ends of the optical waveguide component 101, the end faces of the core 3 are located inward from the end face of the lower clad 21. Each component of wiring board 100α and board 110α in Figures 5 and 6 is given the same symbol as the symbol given to each component of wiring board 100 and board 110 having similar functions in Figure 4, or is omitted as appropriate, and repeated explanations of each component are omitted.

[0084] The substrate 110α has a similar structure to the substrate 110 in the embodiment shown in Fig. 4, except for the arrangement of the conductor patterns and via conductors 7 included in each of the conductor layers 41 to 43. The conductor patterns and via conductors 7 included in each of the conductor layers 41 to 43 are provided on both ends of the substrate 110α. Although Fig. 5 shows the conductor patterns and via conductors 7 included in each of the conductor layers 41 to 43 as having a bilaterally symmetrical configuration, the arrangement of the conductor patterns and via conductors 7 is not limited to bilateral symmetry.

[0085] When the wiring board 100α is in use, the components E11 and E12 are mounted on the component mounting surface of the wiring board 100α. In the wiring board 100α, the components E11 and E12 are mounted on the first surface 100a of the board 110α. The component E11 is optically coupled to the first core exposed portion 301a of the core 3 of the optical waveguide component 101, and the component E12 is optically coupled to the second core exposed portion 302a of the core 3 of the optical waveguide component 101. Like the component E1 shown in FIG. 4 , the components E11 and E12 are optical components including a photoelectric conversion element and the like, and each has an optical terminal E1a and an electrode E1b.

[0086] When the component E11 is a light-emitting element, the component E11 generates an optical signal based on an electrical signal input to the electrode E1b, and emits the optical signal from the optical terminal E1a, which functions as a light-emitting unit, toward the first core exposed portion 301a of the core 3. The optical signal propagates through the core 3 from the first core exposed portion 301a to the second core exposed portion 302a. When the component E12 is a light-receiving element, the optical signal is output from the second core exposed portion 302a, and the component E12 generates an electrical signal based on the optical signal incident on the optical terminal E1a, which functions as a light-receiving unit, and outputs the electrical signal from the electrode E1b. The electrical signal input to the component E11 is converted into an optical signal, propagated, and then reconverted back into an electrical signal and extracted from the component E12. This configuration is thought to enable signal propagation with low noise and low loss, even in an environment where the components E11 and E12 are far apart or where significant electrical noise occurs between the components E11 and E12.

[0087] 5 and 6, similar to FIG. 4, the wiring board 100α shown in FIGS. 5 and 6 is configured by mounting an optical waveguide component 101 on a solder resist 61, which is one of the insulating layers that constitute the surface layer of the board 110α. In the wiring board 100α, the thermal expansion coefficient λ2 of the lower cladding 21, the thermal expansion coefficient λ1 of the intermediate layer M, and the thermal expansion coefficient λ0 of the insulating layer that constitutes the surface layer of the board 110α are adjusted to satisfy the relationship of the above formula 1 or 2. When the thermal expansion coefficient λ2 of the lower cladding 21, the thermal expansion coefficient λ1 of the intermediate layer M, and the thermal expansion coefficient λ0 of the insulating layer that constitutes the surface layer of the board 110α satisfy the relationship of the above formula 1 or 2, the influence of external heat is effectively suppressed, and warping and deformation of the wiring board 100α may be prevented.

[0088] <Method of Manufacturing Optical Waveguide Component of Embodiment> A method of manufacturing an optical waveguide component for a wiring board according to an embodiment will be described with reference to FIGS. 7A to 7F. Hereinafter, as an example, a method will be described in which the optical waveguide component 101 shown in FIGS. 1 to 3 is directly formed on the substrate 110 of the wiring board 100 shown in FIG. 4. FIG. 7A shows a plan view of the optical waveguide component 101 formed on the substrate 110. As shown in FIG. 7B, the substrate 110 is prepared, and an intermediate layer M is formed on the surface of a solder resist (not shown) formed on the surface of the substrate 110. The intermediate layer M is formed, for example, by thermocompression bonding a film-shaped material for the intermediate layer M to the substrate 110 or by applying a resin composition by spin coating to form a film. For example, the material for the intermediate layer M, such as PMMA, is applied to the substrate 110, followed by heat treatment or the like, to form the intermediate layer M.

[0089] Next, as shown in FIG. 7C , the lower clad 21 is formed on the surface of the intermediate layer M. The lower clad 21 is formed, similarly to the intermediate layer M, by, for example, thermocompression bonding a film-shaped constituent material of the lower clad 21 to the intermediate layer M or by spin-coating a resin composition to form a film. The constituent material of the lower clad 21, such as PMMA, is applied to the intermediate layer M on the substrate 110 and then subjected to heat treatment or the like to form the lower clad 21. By forming the lower clad 21 after forming the intermediate layer M on the substrate 110, the influence of the unevenness of the surface of the substrate 110 is mitigated, compared to when the lower clad 21 is formed directly on the substrate 110, and the thickness of the lower clad 21 may be more uniform. This may also ensure the thickness uniformity of the core 3 and the upper clad 22 formed on the lower clad 21.

[0090] 7A, the optical waveguide 102 has two opposing ends, one end 11 and the other end 12. On the side of the one end 11, the end face 21b of the lower clad 21 is formed so as to be located more inward than the end face Mb of the intermediate layer M. That is, the lower clad 21 is formed so that the end face Mb of the intermediate layer M protrudes beyond the end face 21b of the lower clad 21. The distance DL1 between the end face Mb of the intermediate layer M and the end face 21b of the lower clad 21 is, for example, 1 mm or more, and preferably 1 to 3 mm. The thickness T2 of the lower clad 21 is preferably 15 to 20 μm.

[0091] Next, as shown in FIG. 7D , cores 3 having a desired shape in plan view are formed. The cores 3 are formed by any method. As an example, the cores 3 are formed using photolithography. That is, a layer made of a photosensitive material for the core 3, such as PMMA, is formed on the entire surface of the lower cladding 21. The material for the core 3 is applied by spin coating to form a film made of the material for the core 3. The material for the core 3 has a higher refractive index than the lower cladding 21. The layer made of the material for the core 3 on the lower cladding 21 is then exposed to light through a mask corresponding to the shape of the core 3 to be formed, and developed, thereby patterning the layer made of the material for the core 3 on the lower cladding 21, and a desired number of cores 3 having a desired shape are formed. The end faces 3 b of the cores 3 are formed so as to be located inside the end face 21 b of the lower cladding 21. The distance DL2 between the end face 3 b of the core 3 and the end face 21 b of the lower cladding 21 is, for example, 1 mm or more, preferably 1 to 3 mm. The thickness T3 of the core 3 is smaller than the thickness T2 of the lower cladding 21. For example, the lower cladding 21 and the core 3 are formed so that the thickness T2 of the lower cladding 21 is 1.5 times or more, and preferably 2.0 to 4.0 times, the thickness T3 of the core 3. More specifically, when the thickness T3 of the core 3 is 5 μm, the thickness T2 of the lower cladding 21 is preferably 15 to 20 μm.

[0092] In the core formation process shown in FIG. 7D , the material for the core 3 has fluidity. Therefore, when applied to the lower clad 21, the material for the core 3 tends to flow down from the edge of the lower clad 21, such as the end face 21b of the lower clad 21, toward the substrate 110. This presumably reduces the thickness of the core 3 near the end of the lower clad 21, resulting in unevenness and failure to achieve the designed thickness. As shown in FIG. 7D , an intermediate layer M is disposed between the substrate 110 and the lower clad 21, and the end face Mb of the intermediate layer M is positioned outside the end face 21b of the lower clad 21. In the subsequent core formation process, the material for the core 3 temporarily stagnates on the edge of the intermediate layer M as it attempts to flow down from the end of the lower clad 21. This suppresses the outflow rate of the material for the core 3. Therefore, the thickness of the material for the core 3 applied to the lower clad 21 near the end of the lower clad 21 is uniform, ensuring that the thickness T3 of the core 3 after pattern formation is the desired thickness. Furthermore, the vicinity of the end of the lower cladding is patterned, and unnecessary cores are removed, and the core 3 is formed so that the end face 3b of the core 3 is positioned inside the end face 21b of the lower cladding 21. This is thought to stabilize the uniformity of the thickness of the core 3, and to form the core according to the design value.

[0093] In addition to photolithography, examples of methods for forming the core 3 include a photobleaching method and a core cutting method. In the photobleaching method, a mask is used to shield the region where the core 3 is to be formed, and the unmasked region is irradiated with ultraviolet light or the like, so that a portion of a resin layer formed on the entire surface of the lower clad 21 that has a higher refractive index than the lower clad 21 is used as the core layer. The refractive index of the irradiated region is reduced, thereby forming the core 3 in a desired shape. In the core cutting method, a layer formed on the entire surface of the lower clad 21 and having a higher refractive index than the lower clad 21 is cut with a laser beam or the like, thereby forming the core 3 in a desired shape.

[0094] The core 3 may be formed by such a photobleaching method or a core cutting method. In either formation method, the intermediate layer M is disposed between the substrate 110 and the lower clad 21, and the end face Mb of the intermediate layer M protrudes beyond the end face 21b of the lower clad 21. This allows the constituent material of the core 3 to temporarily remain on the intermediate layer M during the core formation process, thereby suppressing the outflow rate of the constituent material of the core 3 applied to the lower clad 21. This may improve the accuracy and uniformity of the thickness T3 of the formed core 3. Furthermore, by removing the constituent material of the core 3 near the end of the lower clad and positioning the end face 3b of the formed core 3 inside the end face 21b of the lower clad 21, the uniformity of the thickness of the core 3 is further improved, and the core 3 is believed to be formed according to the design value.

[0095] 7E, the upper cladding 22 is formed on the core 3 and the lower cladding 21. For example, similar to the formation of the lower cladding 21, a material for the upper cladding 22, such as PMMA, is applied to the lower cladding 21 and the core 3 by spin coating to form a film made of the material for the upper cladding 22. Alternatively, the material for the upper cladding 22, such as PMMA, may be formed into a film and then thermocompressed. The upper cladding 22 is integrated with or at least closely adhered to the lower cladding 21 to form the cladding 2 surrounding the core 3. Here, the upper cladding 22 is formed by application.

[0096] 7F , a portion of the upper cladding 22 covering the portion of the core 3 corresponding to the exposed core portion 30 a is removed. For example, the portion of the upper cladding 22 on the side of the exposed core portion 30 a is removed over the entire width of the core 3. As a result, part of the core 3 is exposed from the cladding 2, thereby providing the exposed core portion 30 a. Furthermore, by removing part of the upper cladding 22, an upper cladding non-formed region 1 a and an upper cladding formed region 1 b are provided.

[0097] In the upper cladding non-forming region 1 a, the upper cladding 22 in the portion to be removed is removed by, for example, photolithography, or may be removed by laser processing, but the removal method is not limited to these methods.

[0098] 7E, a film that is a constituent material of the upper clad 22 may be formed by thermocompression bonding or coating so as not to cover the core exposed portion 30a. Through the above steps, the optical waveguide component 101 having the core exposed portion 30a is completed on the substrate 110, as shown in FIG.

[0099] Alternatively, a support plate (not shown) may be prepared instead of the substrate 110, and the optical waveguide component 101 may be formed on the support plate. The support plate is preferably made of a material having higher rigidity than the optical waveguide component 101 to be manufactured. The support plate may be made of a material having a lower thermal expansion coefficient than the optical waveguide component 101 to be manufactured. This may suppress displacement of the optical waveguide component 101 due to temperature changes. Examples of materials for the support plate include glasses such as soda-lime glass, borosilicate glass, and quartz glass; various metals such as tungsten, titanium, and molybdenum; and various ceramics such as alumina, silicon nitride, and silicon oxide. The support plate provides appropriate rigidity to the optical waveguide component 101 during manufacturing.

[0100] When the optical waveguide component 101 is formed on a support plate, the support plate is removed after defining the upper cladding-free region 1a as shown in FIG. 7F . Removal of the support plate completes the production of a single optical waveguide component 101. The support plate can be removed by any method. For example, when the support plate and the intermediate layer M are bonded together with an adhesive layer (not shown) made of a thermoplastic adhesive, the optical waveguide component 101 with the support plate is heated to reduce the adhesive strength of the adhesive, and then the support plate is peeled off from the intermediate layer M. Alternatively, a peel layer may be provided between the support plate and the intermediate layer M, allowing the support plate to be separated after the waveguide is formed.

[0101] <Method for Manufacturing a Wiring Board According to an Embodiment> An example of a method for manufacturing a wiring board according to an embodiment will be described using the wiring board 100α shown in Figure 5 as an example. The wiring board 100α is manufactured by forming a substrate 110α by sequentially forming a conductor layer 43, an insulating layer 52, a conductor layer 42, an insulating layer 51, and a conductor layer 41, and then forming an optical waveguide component 101 on the substrate 110α. The optical waveguide component 101 may be separately formed on a support plate (not shown) and then disposed on the substrate 110α after removing the support plate.

[0102] The conductor layer 43 is formed on a support substrate (not shown), such as a double-sided copper-clad laminate, by a semi-additive method or the like. After the conductor layer 43 is formed, an insulating layer 52 covering the conductor layer 43 is formed, for example, by laminating an insulating resin, such as a film-like epoxy resin, and thermocompression bonding. Through holes are formed in the insulating layer 52, for example, by irradiating it with a carbon dioxide laser beam. After the through holes are formed, the conductor layer 42 is formed on the insulating layer 52 by a semi-additive method or the like, and via conductors 7 are formed in the through holes in the insulating layer 52. Furthermore, an insulating layer 51 is formed on the conductor layer 42 and the insulating layer 52 in the same manner as the insulating layer 52. The conductor layer 41 is formed on the insulating layer 51 in the same manner as the conductor layer 42, and the via conductors 7 are formed in the insulating layer 51 in the same manner as the via conductors 7 in the insulating layer 52. After the support substrate is removed by peeling or the like, solder resists 61 and 62 are formed by applying or spraying a photosensitive epoxy resin. The solder resists 61 and 62 are exposed and developed to form openings that expose parts of the conductor layer 41 and the conductor layer 43. As a result, the substrate 110α is completed.

[0103] The optical waveguide component 101 is then formed on the surface of the substrate 110α using the method described with reference to FIGS. 7B to 7F. Alternatively, the optical waveguide component 101 may be formed separately on a support plate (not shown). The formed optical waveguide component 101 may then be placed on the substrate 110α as shown in FIG. 8. For example, a thermosetting, room temperature curing, or photocurable adhesive AH is applied to the surface of the solder resist 61 on the surface of the substrate 110α, and the optical waveguide component 101 is then mounted thereon. If necessary, the adhesive AH is cured by heating or other means, and the optical waveguide component 101 is fixed to the substrate 110α. After the optical waveguide component 101 is formed, the bumps 8 are formed by mounting conductive balls using solder or other means and performing a reflow process. The wiring substrate 100α of the example shown in FIG. 5 is completed through the above steps.

[0104] In addition, the optical waveguide component 101 may be formed not on the solder resist 61 on the surface of the substrate 110α, but on an area on the surface of the insulating layer 51 where the solder resist 61 and the conductor layer 41 are not provided, and a separately formed optical waveguide component 101 may be placed in such an area on the surface of the insulating layer 51.

[0105] The optical waveguide components and wiring boards of the embodiments are not limited to those having the structures illustrated in the drawings and the structures, shapes, and materials illustrated in this specification. The wiring boards of the embodiments can have any laminated structure and can include any number of conductor layers and insulating layers. For example, the substrate constituting the wiring board of the embodiments may be a build-up wiring board including a core substrate, a multilayer wiring board without a build-up layer, or a double-sided or single-sided wiring board. Bumps and / or via conductors are not necessarily provided.

[0106] 1, 100, 100α Wiring board 101 Optical waveguide component 102 Optical waveguide 10, 110, 110α Substrate 10a Upper surface of substrate 100a First surface of wiring board 1a Upper clad non-forming region 1b Upper clad forming region 2 Cladding 21 Lower cladding 21a Upper surface of lower cladding 21b End surface of lower cladding 21d, 21e Side surface of lower cladding 22 Upper cladding 3 Core 3a, 3ab Upper surface of core (core upper surface) 3b, 3c End surface of core 33, 3d, 3e Side surface of core 30a Exposed core portion 30b Unexposed core portion 41 to 43 Conductive layers 51 to 52 Insulating layers 61, 62 Solder resist M Intermediate layer Ma Upper surface of intermediate layer Mb End face of the intermediate layer Md, Me Side face of the intermediate layer DL1 Distance between the end face of the intermediate layer and the end face of the lower cladding DS11, DS12 Distance between the side face of the intermediate layer and the side face of the lower cladding DL2 Distance between the end face of the lower cladding and the end face of the core DS21, DS22 Distance between the side face of the lower cladding and the side face of the core T1 Thickness of the intermediate layer T2 Thickness of the lower cladding T3 Thickness of the core

Claims

1. A wiring board having an optical waveguide formed by laminating a lower clad, a core, and an upper clad on a substrate in this order, wherein an intermediate layer is disposed between the substrate and the lower clad, and the end face of the intermediate layer is outside the end face of the lower clad.

2. A wiring board according to claim 1, wherein the end of the core is located inside the end of the lower clad.

3. A wiring board according to claim 1, wherein the thickness of the core is smaller than the thickness of the lower clad.

4. A wiring board according to claim 1, wherein the thermal expansion coefficient λ1 of said intermediate layer is the same as or approximately the same as the thermal expansion coefficient λ2 of said lower clad.

5. A wiring board according to claim 4, wherein the intermediate layer is formed using an optical material.

6. A wiring board according to claim 4, wherein the intermediate layer is formed using a material that does not contain particles.

7. A wiring board according to claim 1, wherein the thermal expansion coefficient λ1 of said intermediate layer is smaller than the thermal expansion coefficient λ2 of said lower clad.

8. A wiring board according to claim 7, wherein the intermediate layer is formed using a non-optical material.

9. A wiring board according to claim 7, wherein the intermediate layer is formed using a material containing particles.

10. A wiring board according to claim 1, wherein the optical waveguide comprises an upper clad formed region and an upper clad non-formed region, and the core is exposed in the upper clad non-formed region.

11. A wiring board according to claim 1, wherein the intermediate layer is made up of multiple layers.

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