Optical waveguide and wiring board

Abstract

In an optical waveguide (1) according to an embodiment, a lower cladding (21), a core (3), and an upper cladding (22) are stacked in this order, and a part of the core (3) on the side where an optical signal is incident and a part of the core (3) on the side where the optical signal is emitted are exposed. The optical waveguide (1) comprises an upper cladding non-formation region (1a) and an upper cladding formation region (1b). The upper cladding non-formation region (1a) has a core-exposed portion (3a) in which an upper surface (31) of the core (3) is exposed at at least one end (11) of the optical waveguide (1). The upper cladding formation region (1b) has a core-unexposed portion (3b) in which the upper surface (31) of the core (3) is not exposed. The width (W1) of the core (3) of the core-exposed portion (3a) and the width (W2) of the core (3) of the core-unexposed portion (3b) satisfy the relationship of W1 > W2.

Description

Optical waveguide and wiring board

[0001] The present invention relates to an optical waveguide and 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 2014-81586 A

[0004] Patent Document 1 discloses a combination in which a silicon chip on which a polymer waveguide and a silicon waveguide are formed is adiabatically coupled with a face-down orientation. In the polymer waveguide, at the optical coupling point with the silicon waveguide, the width of the core of the polymer waveguide is not large enough relative to the width of the core of the silicon waveguide. Therefore, the tolerance for alignment between the polymer waveguide and the silicon waveguide is narrow, and the polymer waveguide and the silicon waveguide may not be optically coupled with sufficient coupling efficiency. In a combination of a polymer waveguide and a silicon waveguide, misalignment between the core of the polymer waveguide and the core of the silicon waveguide due to thermal history during use may result in poor coupling.

[0005] The optical waveguide of the present invention is an optical waveguide in which a lower cladding, a core, and an upper cladding are laminated in this order, and a part of the core on the optical signal input side and a part of the core on the optical signal output side are exposed. The optical waveguide comprises an upper cladding non-forming region and an upper cladding formed region, the upper cladding non-forming region has a core exposed portion where the top surface of the core is exposed at at least one end of the optical waveguide, and the upper cladding formed region has a core unexposed portion where the top surface of the core is not exposed, and the core width (W1) of the core exposed portion and the core width (W2) of the core unexposed portion satisfy the relationship of the following formula 1: W1 > W2 (Formula 1).

[0006] The wiring board of the present invention is a wiring board of an insulating layer and a conductor layer, which includes the optical waveguide and a component mounting pad, and the optical waveguide is disposed on the surface of the wiring board.

[0007] According to the embodiment of the present invention, it is possible to widen the tolerance range for alignment between an optical waveguide and an optical component that transmits and receives optical signals to and from the optical waveguide, and therefore it is believed that the optical waveguide and the optical component can be optically coupled with good coupling efficiency. Furthermore, it is believed that poor coupling due to misalignment between the optical waveguide and the optical component can be suppressed even with thermal history during use.

[0008] 10 is a plan view showing an example of an optical waveguide according to an embodiment of the present invention. FIG. 1 is a cross-sectional view of the optical waveguide according to the embodiment taken along line IIA-IIA. FIG. 1 is a right side view of the optical waveguide according to the embodiment of FIG. 1. FIG. 1 is a cross-sectional view of the optical waveguide according to the embodiment of FIG. 1. FIG. 1 is a cross-sectional view of the optical waveguide according to the embodiment of FIG. 1. FIG. 1 is a cross-sectional view of the optical waveguide according to the embodiment of FIG. 1. FIG. 1 is a cross-sectional view of the optical waveguide according to the embodiment of FIG. 1. FIG. 1 is a cross-sectional view of the optical waveguide according to the embodiment of FIG. 1. FIG. 1 is a cross-sectional view of the optical waveguide according to the embodiment of FIG. 1. FIG. 1 is a cross-sectional view of the optical waveguide according to the embodiment of FIG. 1. FIG. 1 is a cross-sectional view of the optical waveguide according to the embodiment of FIG. 1. Fig. 1 is a cross-sectional view showing an example of a wiring board according to a second embodiment of the present invention; Fig. 2 is a cross-sectional view showing a modified example of the wiring board according to the second embodiment together with an optical fiber; Fig. 3 is a cross-sectional view showing a modified example of the wiring board according to the second embodiment together with an external optical waveguide; Fig. 4 is a front view showing an example of a manufacturing process for an optical waveguide according to an embodiment; Fig. 5 is a plan view showing an example of a manufacturing process for an optical waveguide according to an embodiment; Fig. 6 is a front view showing an example of a manufacturing process for an optical waveguide according to an embodiment;

[0009] The optical waveguide and wiring board according to the embodiments of the present invention will be described with reference to the drawings. In each drawing referred to in the following description, certain parts may be drawn enlarged to facilitate understanding of the disclosed embodiments, and components may not be drawn to exact proportions with respect to size or length.

[0010] <Structure of Optical Waveguide of Embodiment> Fig. 1 shows a plan view of an optical waveguide 1, which is an example of an optical waveguide of an embodiment. Fig. 2A shows a cross section of the optical waveguide 1 of Fig. 1 taken along line IIA-IIA. Fig. 2B shows a right side view of the optical waveguide 1 of Fig. 1. Fig. 2C shows a cross section of the optical waveguide 1 of Fig. 1 taken along line IIC-IIC. Note that the optical waveguide 1 illustrated in Fig. 1 and other figures is merely an example of an optical waveguide of this embodiment. The structure of the optical waveguide of the embodiment is not limited to the structure shown in each drawing, such as Fig. 1.

[0011] 1 and 2A to 2C, the optical waveguide 1 of this embodiment includes a core 3 that transmits an optical signal, and a clad 2 that surrounds the core 3. The clad 2 is composed of a lower clad 21 and an upper clad 22. The core 3 is sandwiched between the lower clad 21 and the upper clad 22. That is, in the optical waveguide 1, the lower and upper clads 21, 22 and the core 3 are layered in the order of the lower clad 21, the core 3, and the upper clad 22.

[0012] The core 3 has an upper surface 31 facing in a direction along the stacking direction of the clad 2 and the core 3, and a lower surface 32 opposite to the upper surface 31. The stacking direction of the clad 2 and the core 3 is also referred to as the "Z direction" (see FIG. 2A) hereinafter. The upper surface 31 may face in either of two directions (+Z and -Z) along the Z direction. In the example of FIG. 2A, the upper surface 31 is the surface of the core 3 facing in the +Z direction, and the lower surface 32 is the surface of the core 3 facing in the -Z direction. Hereinafter, in the optical waveguide 1, the lower clad 21 side will also be referred to as the "lower side" or simply "lower", and the upper clad 22 side will also be referred to as the "upper side" or simply "upper".

[0013] The lower cladding 21 is located on the lower surface 32 side of the core 3 and is in contact with the lower surface 32. The upper cladding 22 is located on the upper surface 31 side of the core 3 and is in contact with the upper surface 31. The upper cladding 22 covers the upper surface 31 of the core 3. The upper cladding 22 also covers the upper surface 211 of the lower cladding 21 (the surface facing the upper cladding 22). As shown in Fig. 2B , in the optical waveguide 1, the upper cladding 22 also covers the side surface 35 of the core 3. The side surface 35 is a surface that connects the upper surface 31 and the lower surface 32 and extends along the direction in which the core 3 extends.

[0014] In the optical waveguide 1 shown in FIGS. 1 and 2A, the core 3 extends along the +X direction and 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 1 has two opposing ends (one end 11 and the other end 12). In the example of FIG. 1, the one end 11 and the other end 12 are opposed to each other in the X direction. In the optical waveguide 1, an optical signal is incident on the one end 11 or the other end 12, and is emitted from the other end 12 or the one end 11. When an optical signal is incident on the one end 11 side, the optical signal is emitted from the other end 12 side. When an optical signal is incident on the other end 12 side, the optical signal is emitted from the one end 11 side.

[0015] In the optical waveguide 1, a portion of the core 3 on the side where an optical signal is incident and a portion of the core 3 on the side where an optical signal is emitted are exposed. In the optical waveguide 1, an optical signal may be incident on one end 11 and may be emitted from the one end 11. Similarly, an optical signal may be incident on the other end 12 and may be emitted from the other end 12. In the optical waveguide 1, a portion of the core 3 is exposed at both the one end 11 and the other end 12. As shown in FIGS. 1 , 2A, and 2C, at the one end 11, the top surface 31 of the core 3, one end face 33 of the core 3 in the X direction, and a side face 35 are exposed from the cladding 2. At the other end 12, as shown in FIGS. 1 , 2A, and 2B, another end face 34 in the X direction opposite to the end face 33 is exposed from the cladding 2.

[0016] 1 and 2A, in the optical waveguide 1, the upper cladding 22 covers the upper surface 31 of the core 3 and the upper surface 211 of the lower cladding 21. The optical waveguide 1 is composed of an upper cladding-free region 1a, which is a region where the upper cladding 22 is not formed in a planar view, and an upper cladding-formed region 1b, which is a region where the upper cladding 22 is formed in a planar view. That is, the optical waveguide 1 is composed of the upper cladding-free region 1a and the upper cladding-formed region 1b. In a planar view, the upper surface 31 of the core 3 and the upper surface 211 of the lower cladding 21 are exposed in the upper cladding-free region 1a. In the example of FIG. 1, the side surface 35 of the core 3 is also exposed in the upper cladding-free region 1a. Note that "planar view" means viewing an object with a line of sight along the Z direction.

[0017] In the optical waveguide 1, the upper cladding non-forming region 1a has an exposed core portion 3a where the top surface 31 of the core 3 is exposed. In the optical waveguide 1, the upper cladding non-forming region 1a is provided on the one end 11 side. Therefore, as shown in FIGS. 1 and 2A, the exposed core portion 3a is at the one end 11 of the optical waveguide 1. The exposed core portion 3a is a part of the core 3 on the one end 11 side of the optical waveguide 1. In the optical waveguide 1 shown in FIG. 1 etc., there is an upper cladding region 1b adjacent to the upper cladding non-forming region 1a, and the upper cladding region 1b is arranged on the other end 12 side of the upper cladding non-forming region 1a. Note that the upper cladding region 1b may be arranged over the entire area on the other end 12 side of the upper cladding non-forming region 1a.

[0018] The exposed core portion 3a refers to a portion in the upper cladding non-forming region 1a where the top surface of the core is exposed and where the core width W1 is greater than the core width W2 in the upper cladding forming region 1b. The exposed core portion 3a is overlapped with the light receiving portion or light emitting portion E1a of the component E1 to couple an optical signal. The end of the exposed core portion 3a refers to the portion of the core 3 where the width is widest from the core 3 in the non-exposed core portion 3b along the exposed core portion 3a. The exposed core portion 3a may be optically coupled to an external optical connecting component (for example, an optical fiber or an optical waveguide other than the optical waveguide 1).

[0019] The upper cladding region 1b has a non-exposed core portion 3b where the top surface 31 of the core 3 is not exposed. The non-exposed core portion 3b is a part of the core 3 on the other end 12 side of the optical waveguide 1. In the optical waveguide of the embodiment, the upper cladding non-formed region 1a may be provided at both ends of the optical waveguide. That is, the upper cladding non-formed region 1b may be disposed between two upper cladding non-formed regions 1a in the X direction. In the optical waveguide of the embodiment, the upper cladding non-formed region 1a is provided at at least one end of the optical waveguide (for example, one end 11 and / or the other end 12 in FIG. 1 ). Therefore, the exposed core portion 3a is also located at at least one end of the optical waveguide.

[0020] The core 3 and cladding 2 constituting the optical waveguide 1 are formed of any light-transmitting material. The optical waveguide 1 can be formed, for example, of an organic material (organic substance), an inorganic material (inorganic substance), or a hybrid material containing an organic material and an inorganic material, such as an inorganic polymer. Examples of inorganic materials include quartz glass and silicon, and examples of organic materials include acrylic resins such as polymethyl methacrylate (PMMA), polyimide resins, polyamide resins, polyether resins, silicone resins, phenolic resins, and epoxy resins. An optical waveguide 1 formed from an organic material tends to be lightweight and highly tough. Furthermore, an optical waveguide 1 formed from an organic material tends to be flexible. Therefore, an optical waveguide 1 formed from an organic material may increase the degree of freedom in the relative positioning in the Z direction between an optical component optically coupled at one end 11 of the optical waveguide 1 and an optical component optically coupled at the other end 12. Note that "optically coupled" is also referred to as "optically coupled" hereinafter.

[0021] The core 3 and the clad 2 may be made of different materials or may be made of the same material. However, 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. 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 1 may be formed using a forming method called a photobleaching method, for example. The optical waveguide 1 may be formed on a support that supports the optical waveguide 1 during the manufacturing process and then used by separating it from the support, or it may be used together with the support.

[0022] When the optical waveguide 1 is in use, the core 3 is optically coupled at one end 11 and the other end 12 to optical components such as a photoelectric conversion component, such as a semiconductor device including a photoelectric conversion element, and / or a connector member, such as an optical fiber or an optical connector, that connects the waveguide to the outside. That is, 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. In Figures 1 and 2A, a component E1 including a photoelectric conversion element (not shown) is shown by a two-dot chain line as an example of an optical component that is optically coupled to the core 3 at one end 11.

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

[0024] 1 and 2A, the core 3 is positioned so that the upper surface 31 faces the light receiving portion or light emitting portion E1a of the component E1 at one end 11 of the optical waveguide 1, thereby achieving optical coupling (adiabatic coupling). That is, a portion of the optical signal propagating through the core 3 toward the one end 11 leaks out of the core 3 from the upper surface 31 as evanescent light and enters the light receiving portion or light emitting portion E1a of the component E1. Because the upper surface 31 faces the light receiving portion or light emitting portion E1a of the component E1 without the cladding 2 interposed therebetween, it is believed that highly efficient optical coupling is achieved.

[0025] The optical waveguide 1 illustrated in FIG. 1 and other figures has four parallel cores 3. The optical waveguide 1 of the embodiment is not limited to four cores 3 and may have any number of cores 3, one or more. For example, the number of cores 3 is in the range of 2 to 64. When multiple cores 3 are provided, the arrangement pitch P1 of the cores 3 is, for example, 10 μm to 300 μm, and preferably 20 μm to 250 μm. Note that the arrangement pitch P1 of the multiple cores 3 is not limited to these numerical examples. In the example of FIG. 1 , the arrangement pitch P1 of the multiple cores 3 is constant between one end 11 and the other end 12. The arrangement pitch P1 of the cores 3 may vary between the one end 11 and the other end 12. The arrangement pitch P1 of the cores 3 is the distance between the centers of the cores 3.

[0026] The core 3 has a width W1 at the core exposed portion 3a. The core 3 has a width W2 at the core unexposed portion 3b. The core exposed portion 3a is a portion of the core 3 that is located within the upper cladding non-forming region 1a and has the width W1. The core unexposed portion 3b is a portion of the core 3 that is located within the upper cladding forming region 1b and has the width W2.

[0027] The "width" of the core 3, such as width W1 and width W2, is the length of the core 3 in a direction perpendicular to the propagation direction of the optical signal propagating within the core 3 (the X direction in FIG. 1 ) and the lamination direction of the optical waveguide 1 (the Z direction). The direction along the width of the core 3 is also referred to as the +Y direction or −Y direction (see FIG. 1 ) below. The +Y direction and −Y direction are also collectively referred to simply as the "Y direction." If the core 3 is bent along its length, the propagation direction of the optical signal within the core 3 is not constant but changes depending on the portion (section) of the core 3. Therefore, the direction along the width of the core 3 may change from one end of the core 3 to the other.

[0028] In the optical waveguide of the embodiment, the width W1 of the core exposed portion 3a and the width W2 of the core unexposed portion 3b satisfy the relationship of the following formula 1: W1 > W2 (Formula 1) In other words, the width W1 of the core exposed portion 3a and the width W2 of the core unexposed portion 3b are different. The width W1 of the core exposed portion 3a is larger than the width W2 of the core unexposed portion 3b. In the example of Figure 1, the width W1 of the core 3 at one end 11 of the optical waveguide 1 is larger than the width W2 of the core 3 at the other end 12. Therefore, the difference between the width W1 of the core exposed portion 3a and the width We of the light receiving portion or light emitting portion E1a of the component E1 is larger than the difference between the width W2 of the core unexposed portion 3b and the width We of the light receiving portion or light emitting portion E1a.

[0029] Therefore, in this embodiment, compared to when the core 3 has the same width in the core exposed portion 3a as the width W2 of the core non-exposed portion 3b, the tolerance for misalignment in the direction along the width of the core 3 when aligning the optical signal input / output portion of an optical component such as the light receiving unit or light emitting unit E1a with the core 3 is larger. That is, when the core 3 and the light receiving unit or light emitting unit E1a are overlapped in the core exposed portion 3a as shown in FIG. 1, the width Wg of the marginal portion of the core 3 that does not overlap with the light receiving unit or light emitting unit E1a in a planar view is larger than the width of the marginal portion of the core 3 when the core 3 and the light receiving unit or light emitting unit E1a are overlapped in the core non-exposed portion 3b. Therefore, even if the alignment between the optical component such as component E1 and the core 3 is slightly misaligned in the direction along the width of the core 3 (the +Y direction or the −Y direction in FIG. 1), it is considered that an optical signal emitted from one of the optical component and the core 3 is likely to be incident on the other. That is, even if such a positional deviation occurs, it is believed that the substantial optical coupling efficiency between the optical component and the core 3 is likely to be maintained.

[0030] Therefore, in this embodiment, it is considered that alignment for properly optically coupling an optical component such as component E1 with the core 3 is easy. In particular, it is considered that alignment in the direction along the width of the core 3 is easy. Therefore, it is considered that the optical waveguide of the embodiment easily achieves high coupling efficiency in coupling with an optical component. Furthermore, it is considered that poor coupling due to misalignment between the core 3 and an optical component such as component E1 is suppressed even with thermal history during use.

[0031] Additionally, in this embodiment, since the non-exposed core portion 3b has a width W2 smaller than the width W1, it is believed that a desired transmission mode can be easily achieved in the non-exposed core portion 3b. For example, in a light-guiding member such as the optical waveguide 1, the cross-sectional size of an optical path such as the core 3 is selected according to the refractive index difference between the constituent material of the core and the constituent material of the cladding 2 surrounding the core, which is the optical path. In other words, by realizing a core 3 having a cross-sectional size appropriate to the refractive index of each material, an optical signal can be propagated in a desired transmission mode. In this regard, in this embodiment, the core 3 is not formed over its entire length with the width W1 of the exposed core portion 3a, which has a large width for high coupling efficiency with optical components, but the non-exposed core portion 3b has a width W2 smaller than the width W1. Therefore, it is believed that optical transmission in a mode requiring a small core cross-section can be easily achieved. For example, single-mode optical transmission requiring a core diameter of 1 to 7 μm may be possible. Note that the core diameter in this case may be either the core width or the core thickness. In plan view, the non-exposed core portion 3b may have a constant width. The shape of the core unexposed portion 3b may be tapered, or may be a combination of a tapered portion and a portion of constant width.

[0032] As described above, according to this embodiment, the exposed core portion 3a can be optically coupled (especially adiabatic coupled) with an external optical component such as component E1 with excellent coupling efficiency. Furthermore, optical signals can be transmitted in a desired transmission mode in portions other than the optically coupled portion with the optical component. For example, optical signals may be propagated in a high-speed, low-loss transmission mode such as single mode. In other words, the optical waveguide of this embodiment is believed to be able to provide high-speed optical transmission with high efficiency and low loss between two or more optical components optically coupled to the optical waveguide.

[0033] 1 and the like illustrates an example of the dimensions of each portion of the optical waveguide of this embodiment. The width W1 of the core exposed portion 3a and the width W2 of the core unexposed portion 3b are not particularly limited as long as the width W1 is wider than the width W2, but it is desirable that the width W1 of the core 3 in the core exposed portion 3a and the width W2 of the core in the core unexposed portion 3b satisfy the relationship of the following formula 2: 1.0<(W1 / W2)≦3.0 (Formula 2) In other words, the core exposed portion 3a preferably has a width W1 that is larger than the width W2 of the core unexposed portion 3b and is no more than three times the width W2.

[0034] The width W1 of the core 3 in the core exposed portion 3a is preferably 3 μm or more and 30 μm or less, and more preferably 5 μm or more and 20 μm or less. Having a width of this range for the core 3 in the core exposed portion 3a facilitates alignment with optical components such as component E1, and is thought to avoid the need for an excessively large area for optical coupling with the optical component. The width W2 of the core unexposed portion 3b is preferably, for example, 1 μm or more and 10 μm or less. The width W1 of the core exposed portion 3a is the width of the widest portion of the core 3 in the core exposed portion 3a, and the width W2 of the core unexposed portion 3b is the width of the narrowest portion of the core 3 in the core unexposed portion 3b.

[0035] The width of the core 3 is determined by the average value of the length measurements at three points in the X direction. The core width of the core exposed portion 3 a may be constant or may vary. For example, the core width of the core exposed portion 3 a may have a width that gradually increases or decreases from one end to the other end. When the width of the core exposed portion 3 a varies, the core width W1 of the core exposed portion 3 a is determined by the average value of the length measurements at three points within the core exposed portion 3 a in the X direction. The core width of the core unexposed portion 3 b may be constant or may vary. For example, the core width of the core unexposed portion 3 b may have a width that gradually increases or decreases from one end to the other end. When the width of the core unexposed portion 3 b varies, the core width W2 of the core unexposed portion 3 b is determined by the average value of the length measurements at three points within the core unexposed portion 3 b in the X direction.

[0036] The thickness of the core 3 is determined by the average value of measurements taken at three points in the Z direction. The thickness of the core 3 is not particularly limited, but is 1 μm or more and 20 μm or less, and preferably 3 μm or more and 10 μm or less.

[0037] The ratio of the width to the thickness of the core 3 in the core exposed portion 3a is not particularly limited, but is preferably core 3 width: core 3 thickness = 1.0:1.0 to 4.0:1.0. The ratio of the width to the thickness of the core 3 in the core unexposed portion 3b is not particularly limited, but is preferably core 3 width: core 3 thickness = 0.9:1.0 to 1.4:1.0. The cross-sectional shape of the core 3 in the core unexposed portion 3b is square or rectangular.

[0038] The length L1 of the core 3 at the core exposed portion 3 a is preferably 100 μm or more and 3000 μm or less, and more preferably 100 μm or more and 2500 μm or less. When the core exposed portion 3 a has a length in this range, it is thought that there is a high degree of freedom in selecting an optical component to be optically coupled at the core exposed portion 3 a, and that a necessary and sufficient tolerance can be obtained in aligning the optical component in a direction perpendicular to the direction along the width of the core 3.

[0039] <Lower Cladding> The thickness of the lower cladding 21 is determined by the average value of measurements taken at three points in the Z direction. The thickness of the lower cladding 21 is not particularly limited, but is in the range of 5 μm to 100 μm, and preferably in the range of 10 μm to 50 μm.

[0040] <Upper Cladding> The thickness of the upper cladding 22 is determined by the average value of measurements taken at three points in the Z direction. The thickness of the upper cladding 22 is not particularly limited, but is in the range of 5 μm to 100 μm, and preferably in the range of 10 μm to 50 μm.

[0041] An example of the combination of dimensions of each part of the optical waveguide 1 of the embodiment is as follows: The core width in the core exposed portion 3a is 10 μm, and the core thickness is 7 μm. The core width in the core unexposed portion 3b is 7 μm, and the core thickness is 7 μm. The thickness of the lower cladding is 20 μm, and the thickness of the upper cladding is 25 μm.

[0042] Note that the width of the core 3, the thickness of the core 3, the length of the core exposed portion 3a, the thickness of the lower clad 21, and the thickness of the upper clad 22 exemplified above are merely examples, and the dimensions of each portion of the core 3 and the thickness of each clad are not limited to the numerical values ​​exemplified above.

[0043] <Connection Portion> In the optical waveguide 1 shown in Fig. 1 , the core 3 has a connection portion 3c between the exposed core portion 3a and the unexposed core portion 3b, connecting the exposed core portion 3a and the unexposed core portion 3b. As shown in Fig. 1 , the connection portion 3c refers to the core 3 formed in the upper cladding non-forming region 1a between the exposed core portion 3a and the unexposed core portion 3b in a planar view. Here, the shape of the core 3 in the connection portion 3c may have a constant width, may have a tapered shape tapering toward the unexposed core portion 3b, or may be a combination of a constant width shape and a tapered shape. That is, the connection portion 3c may have a taper in a planar view, may be arranged as a straight line with a constant width, or may be arranged as a combination of a straight line with a constant width and a tapered shape. The tapered shape of the core 3 at the connection portion 3c may be a shape in which the width gradually increases linearly (or smoothly) on both sides from the core non-exposed portion 3b to the core exposed portion 3a, a shape in which the width gradually increases linearly on only one side, or a shape in which the width gradually increases in a stepped manner on both sides or only one side.

[0044] The core width W3 of the connection portion 3c is smaller than the core width W1 of the core exposed portion 3a and is equal to or larger than the core width W2 of the core unexposed portion 3b in the upper cladding forming region 1b. In other words, the core widths W1, W2, and W3 satisfy the relationship of the following formula 3: Core width W1 > Core width W3 ≥ Core width W2 (Formula 3)

[0045] It is believed that the core width W3 of the connecting portion 3c suppresses unintended leakage of an optical signal from the exposed core portion 3a toward the unexposed core portion 3b. That is, in the optical waveguide of the embodiment, the width W1 of the exposed core portion 3a is larger than the width W2 of the unexposed core portion 3b, and therefore the connecting portion 3c connecting the exposed core portion 3a and the unexposed core portion 3b has a tapered shape, which is believed to suppress unintended leakage of an optical signal from the exposed core portion 3a toward the unexposed core portion 3b. The core width W3 of the connecting portion 3c is not particularly limited, but is preferably 1 μm to 30 μm.

[0046] The taper ratio ((W1-W2) / L2) of the tapered shape of the core 3 is, for example, 1 or more and 2 or less. L2 is the length of the tapered portion of the connection portion 3c in the X direction. If the connection portion 3c has a taper ratio in this range, leakage of optical signals from the core 3 can be efficiently suppressed within a limited length. In this case, the tapered shape formation ratio R of the core 3 is preferably 0.01<R<0.2, and more preferably 0.03<R<0.15. The tapered shape formation ratio R is the ratio of the length of the tapered portion to the overall length of the core 3.

[0047] <Planar Shape of Core 3> The planar shape of the core 3 will be described with reference to Figures 2D and 2E as well. Figures 2D and 2E show first and second modified examples of the connection portion 3c of the core 3 in the optical waveguide of the embodiment. Note that although two cores 3 are depicted in each of Figures 2D and 2E, any number of cores 3 (e.g., 2 or more and 64 or less) may be arranged in both the first and second modified examples. In Figures 2D and 2E, components similar to those shown in Figure 1 are denoted by the same reference numerals as those in Figure 1 or are omitted as appropriate, and repeated description thereof will be omitted.

[0048] The planar shape of the core 3, when viewed in combination with the upper cladding 22, is a combination of the core exposed portion 3a, the connecting portion 3c, and the core unexposed portion 3b, but when viewed by itself, the core 3 has a portion corresponding to the core exposed portion 3a having a constant width in Fig. 1 and a portion corresponding to the core unexposed portion 3b having a constant width. Between the core exposed portion 3a and the core unexposed portion 3b, there is disposed a connecting portion 3c having a shape with a constant width, a tapered shape, or a shape combining a portion with a constant width and a tapered shape.

[0049] That is, the shape of the connecting portion 3c may include a portion with a constant width and a tapered portion as in the example of Fig. 1, or may have a shape with a constant width as in the first modified example shown in Fig. 2D. Furthermore, the shape of the connecting portion 3c may be tapered over the entire area between the core exposed portion 3a and the core unexposed portion 3b as in the second modified example shown in Fig. 2E.

[0050] Furthermore, when viewed as a single core 3, the shape of the core 3 may include a portion corresponding to the core exposed portion 3a having a constant width and a portion corresponding to the core unexposed portion 3b having a shape that combines a tapered shape with a shape having a constant width (see FIGS. 4 and 5). A connecting portion 3c having a shape that combines a portion with a constant width, a tapered shape, or a portion with a tapered shape may be disposed between the core exposed portion 3a and the core unexposed portion 3b. When both the connecting portion 3c and the core unexposed portion 3b have tapered shapes, the shape of the core 3 in a plan view may be a tapered shape whose width changes continuously from the connecting portion 3c to the core unexposed portion 3b (see FIG. 5). Furthermore, when viewed in a plane, the tapered shape of core 3 may be a shape in which the width changes linearly on both sides (i.e., on both the +Y side and the -Y side of a line passing through the midpoint of the width of core 3), a shape in which the width changes linearly on one side (the +Y side or the -Y side), or a shape in which the width changes stepwise on both sides or one side.

[0051] <Core Surrounded by Lower Cladding> Figure 3 shows another example of the cladding 2 in the optical waveguide 1 of the embodiment. Figure 3 shows a cross section of the example of Figure 3 taken along a cutting line corresponding to line IIC-IIC shown in Figure 1. In the example shown in Figure 3, the core 3 is surrounded by the lower cladding 21. Therefore, the side surface 35 of the core 3 is covered by the lower cladding 21. Therefore, the side surface 35 of the core 3 is covered by the cladding 2 not only in the upper cladding-formed region 1b (see Figure 1) but also in the upper cladding-free region 1a (see Figure 1). Specifically, it is covered by the lower cladding 21. Therefore, it is considered that the transmission mode of the optical signal is less likely to be disturbed in the upper cladding-free region 1a.

[0052] On the other hand, the upper surface 31 of the core 3 is substantially flush with the upper surface 211 of the lower cladding 21. This can prevent unintended contact between the core 3 and an optical component optically coupled to the core 3. Also, the core 3 can be easily formed by the photobleaching method.

[0053] 4 shows an optical waveguide 10a, which is a modified example of the optical waveguide of the embodiment. In the optical waveguide 10a, an exposed core portion 3a is arranged in the upper cladding non-forming region 1a, and the unexposed core portion 3b in the upper cladding forming region 1b has a combination of a tapered shape and a shape having a constant width. When the tapered shape of the unexposed core portion 3b is covered with the upper cladding 22 in this way, it is thought that there is little disturbance in the transmission mode of the optical signal in the core 3.

[0054] FIG. 5 shows an optical waveguide 10b, which is a first modified example of the optical waveguide of the embodiment. As shown in FIG. 5, the optical waveguide 10b has upper cladding-free regions at both one end 11 and the other end 12 of the optical waveguide 10b. A first upper cladding-free region 1aa is located at one end 11 of the optical waveguide 10b, and a second upper cladding-free region 1ab is located at the other end 12 of the optical waveguide 10b. Therefore, in the optical waveguide 10b, the first upper cladding-free region 1aa, the upper cladding region 1b, and the second upper cladding-free region 1ab are arranged in this order. The first upper cladding-free region 1aa includes a first core exposed portion 3aa and a tapered connecting portion 3c, and the second upper cladding-free region 1ab includes a second core exposed portion 3ab and a tapered connecting portion 3c. In the upper cladding forming region 1b, the non-exposed core portion 3b is a combination of two tapered portions and a portion with a constant width.

[0055] When an imaginary line X1 is provided at the middle of the upper cladding formation region 1b in the X direction, the region of the upper cladding formation region 1b closer to the first upper cladding non-formation region 1aa than the imaginary line X1 becomes the upper cladding formation region 1ba, and the region of the upper cladding formation region 1b closer to the second upper cladding non-formation region 1ab than the imaginary line X1 becomes the upper cladding formation region 1bb. Note that the imaginary line X1 is not formed in the optical waveguide 10b, but is provided for convenience of explaining the embodiment.

[0056] The core 3 has exposed core portions at both one end and the other end of the core 3. The core 3 has a first exposed core portion 3aa in the first upper cladding non-forming region 1aa and a second exposed core portion 3ab in the second upper cladding non-forming region 1ab. The width of the first exposed core portion 3aa is W11, and the width of the second exposed core portion 3ab is W12. The upper cladding forming region 1ba has an unexposed core portion 3b1, and the upper cladding forming region 1bb has an unexposed core portion 3b2. The width of the unexposed core portion 3b1 is W21, and the width of the unexposed core portion 3b2 is W22. The widths W11, W12, W21, and W22 of the core 3 satisfy the relationships of the following equations 4 and 5. W11>W21 (equation 4) W12>W22 (equation 5)

[0057] That is, the width W11 of the first core exposed portion 3aa is different from the width W21 of the core unexposed portion 3b1. The width W11 of the first core exposed portion 3aa is larger than the width W21 of the core unexposed portion 3b1. The width W12 of the second core exposed portion 3ab is different from the width W22 of the core unexposed portion 3b2. The width W12 of the second core exposed portion 3ab is larger than the width W22 of the core unexposed portion 3b2.

[0058] The optical waveguide 10b in FIG. 5 is optically coupled to components E1 and E2. Like component E1, component E2 may be a semiconductor device including a photoelectric conversion element. The light-receiving or light-emitting portion E2a of component E2 is coupled to the second exposed core portion 3ab. The optical waveguide 10b has an exposed core portion (first exposed core portion 3aa) at one end 11 of the optical waveguide 10b and an exposed core portion (second exposed core portion 3ab) at the other end 12 of the optical waveguide 10b. Therefore, at the two ends, optical components such as components E1 and E2 can be efficiently coupled to the core 3 by a coupling method using evanescent light, such as adiabatic coupling. In addition to being optically coupled to components E1 and E2, the optical waveguide 10b in FIG. 5 may also be optically coupled to an external optical connecting component, such as an exposed core of an optical fiber or another optical waveguide, instead of component E2.

[0059] Furthermore, because the width W12 of the second core exposed portion 3ab is larger than the width W22 of the non-exposed core portion 3b2, it is believed that alignment for properly optically coupling an optical component such as component E2 to the core 3 is also easy in the second core exposed portion 3ab. Therefore, it is believed that high coupling efficiency is easily achieved in the coupling of optical component E1 and optical component E2. Meanwhile, optical signals may be transmitted in a desired transmission mode, such as single mode, in portions other than the optically coupled portions with the optical components. In other words, the optical waveguide 10b illustrated in FIG. 5 is believed to provide high-efficiency, low-loss, high-speed optical transmission between two optical components, such as semiconductor devices including photoelectric conversion elements, that are optically coupled to the optical waveguide 10b.

[0060] 5 shows an example of the dimensions of each portion of the optical waveguide 10b. The width W11 of the first core exposed portion 3aa and the width W21 of the non-exposed core portion 3b1 are not particularly limited as long as the width W11 is wider than the width W21, but it is desirable that the width W11 of the core 3 in the first core exposed portion 3aa and the width W21 of the core in the non-exposed core portion 3b1 satisfy the relationship of the following formula 6: 1.0<(W11 / W21)≦3.0 (Formula 6) In other words, the first core exposed portion 3aa preferably has a width W11 that is greater than the width W21 of the non-exposed core portion 3b1 and is no more than three times the width W21.

[0061] The width W12 of the second core exposed portion 3ab and the width W22 of the core non-exposed portion 3b2 are not particularly limited as long as the width W12 is wider than the width W22, but it is desirable that the width W12 of the core 3 in the second core exposed portion 3ab and the width W22 of the core in the core non-exposed portion 3b2 satisfy the relationship of the following equation 7: 1.0<(W12 / W22)≦3.0 (Equation 7) In other words, the second core exposed portion 3ab preferably has a width W12 that is larger than the width W22 of the core non-exposed portion 3b2 and is no more than three times the width W22.

[0062] The width W11 of the core 3 at the first core exposed portion 3aa and the width W12 of the core 3 at the second core exposed portion 3ab are preferably 3 μm or more and 30 μm or less, more preferably 5 μm or more and 20 μm or less. Having the widths of the core 3 at the first core exposed portion 3aa and the second core exposed portion 3ab within this range facilitates alignment with optical components such as the components E1 and E2, and also avoids the need for an excessively large area for optical coupling with the optical components. The width W21 of the core non-exposed portion 3b1 and the width W22 of the core non-exposed portion 3b2 are preferably, for example, 1 μm or more and 10 μm or less. The width W11 of the first core exposed portion 3aa is the width of the widest portion of the core 3 at the first core exposed portion 3aa, and the width W12 of the second core exposed portion 3ab is the width of the widest portion of the core 3 at the second core exposed portion 3ab. The width W21 of the core unexposed portion 3b1 is the width of the narrowest part of the core 3 in the core unexposed portion 3b1, and the width W22 of the core unexposed portion 3b2 is the width of the narrowest part of the core 3 in the core unexposed portion 3b2.

[0063] 5, the core of the connection portion 3c has a tapered shape that narrows toward the non-exposed core portion 3b1 or the non-exposed core portion 3b2. The tapered shape of the core of the connection portion 3c may be such that the width gradually increases linearly on both sides from the non-exposed core portion 3b1 toward the first exposed core portion 3aa and from the non-exposed core portion 3b2 toward the second exposed core portion 3ab, or may be such that the width gradually increases linearly on only one side, or may be such that the width gradually increases in a stepwise manner on both sides or only one side.

[0064] The core width W31 of the connection portion 3c on the first upper cladding non-forming region 1aa side is smaller than the core width W11 of the first core exposed portion 3aa and is equal to or larger than the core width W21 of the core non-exposed portion 3b1. In other words, the relationship: core width W11 > core width W31 ≥ core width W21 holds. It is believed that the core width W31 of the connection portion 3c suppresses unintended leakage of an optical signal from the first core exposed portion 3aa toward the core non-exposed portion 3b1. The core width W32 of the connection portion 3c on the second upper cladding non-forming region 1ab side is smaller than the core width W12 of the second core exposed portion 3ab and is equal to or larger than the core width W22 of the core non-exposed portion 3b2. In other words, the relationship: core width W12 > core width W32 ≥ core width W22 holds. It is believed that the core width W32 of the connection portion 3c suppresses unintended leakage of an optical signal from the second core exposed portion 3ab to the core non-exposed portion 3b2. The core widths W31 and W32 of the connection portion 3c are not particularly limited, but are preferably 1 μm to 30 μm.

[0065] 5 , the core 3 has a shape including a portion corresponding to the first core exposed portion 3aa having a constant width, a portion corresponding to the second core exposed portion 3ab having a constant width, and portions corresponding to the non-core exposed portions 3b1 and 3b2 having a shape that combines a tapered shape with a shape having a constant width. A tapered connecting portion 3c is disposed between the first core exposed portion 3aa and the non-core exposed portion 3b1, and a tapered connecting portion 3c is also disposed between the second core exposed portion 3ab and the non-core exposed portion 3b2. In plan view, the core 3 has a tapered shape in which the width continuously changes from the connecting portion 3c to the non-core exposed portion 3b1 in the first upper cladding non-forming region 1aa, and a tapered shape in which the width continuously changes from the connecting portion 3c to the non-core exposed portion 3b2 in the second upper cladding non-forming region 1ab.

[0066] 6A and 6B show modified examples of the optical waveguide of the embodiment. Fig. 6A shows an optical waveguide 10c of a second modified example, and Fig. 6B shows an optical waveguide 10d of a third modified example. Like the optical waveguide 10b of Fig. 5, the optical waveguides 10c and 10d have a first core exposed portion 3aa at one end of the core 3 and a second core exposed portion 3ab at the other end of the core 3. The width of the first core exposed portion 3aa is larger than the width of the core unexposed portion 3b1. The width of the second core exposed portion 3ab is larger than the width of the core unexposed portion 3b2. Therefore, in both the optical waveguide 10c and the optical waveguide 10d, it is easy to align the component E1 (see Figure 5) with the first core exposed portion 3aa, and it is easy to align the component E2 (see Figure 5) with the second core exposed portion 3ab, and it is thought that optical transmission in a desired transmission mode, such as single mode, can be easily achieved in parts other than the optical coupling part with the component E1 or the component E2.

[0067] In the optical waveguide 10c shown in FIG. 6A, the cores 3 have an L-shape in plan view. That is, each core 3 has one bend 3d, and the bend 3d is bent at an angle of approximately 90°. The angle of the bend 3d can be in the range of 45° to 135°. This allows for the flexible arrangement of optical components optically coupled to the optical waveguide 10c, allowing the optical waveguide to be designed accordingly. The bend 3d may be either straight or curved, but is preferably a curved portion, as a curved bend 3d facilitates the transmission of optical signals near the bend 3d.

[0068] The optical waveguide 10c is formed in a rectangular shape in plan view, having a side S1 of the optical waveguide 10c and a side S2 substantially perpendicular to the side S1. The lower cladding 21 is formed in a shape having sides S1 and S2. The upper cladding non-forming region 1a is formed in a shape that follows the sides S1 and S2. When an imaginary line X2 is provided along the bent portion 3d of the core 3 in the upper cladding forming region 1b, the upper cladding forming region 1b is divided into an A1 region and an A2 region by the imaginary line X2. The A1 region is a region where the core 3 of the core non-exposed portion 3b is formed along the side S2 (i.e., substantially parallel to the side S2), and the A2 region is a region where the core 3 of the core non-exposed portion 3b is formed along the side S1 (i.e., substantially parallel to the side S1). Note that the imaginary line X2 is not formed on the optical waveguide 10c, but is provided for convenience of explaining the embodiment.

[0069] In FIG. 6A , the first core exposed portion 3aa at one end of the core 3 is located outside the A1 region. The second core exposed portion 3ab at the other end of the core 3 is located outside the A2 region. Four first core exposed portions 3aa at one end of the core 3 are lined up along side S1. Four second core exposed portions 3ab at the other end of the core 3 are lined up along side S2. Because the core 3 has an L-shape as shown in FIG. 6A , the component E1 (see FIG. 5 ) can be placed in the region between the A1 region and side S1, and the component E2 (see FIG. 5 ) can be placed in the region between the A2 region and side S2. It is considered that alignment of the component E1 with the first core exposed portion 3aa is easy. It is considered that alignment of the component E2 with the second core exposed portion 3ab is easy. In addition, even if the components E1 and E2 are exposed to heat during use after being mounted, poor coupling due to misalignment is suppressed, and therefore optical coupling with good coupling efficiency is achieved. It is also believed that it is easier to achieve a desired transmission mode.

[0070] The core 3 has a first core exposed portion 3aa outside the A1 region and a second core exposed portion 3ab outside the A2 region. The width of the first core exposed portion 3aa is W11, and the width of the second core exposed portion 3ab is W12. A core unexposed portion 3b1 is located in the A1 region of the upper cladding forming region 1b, and a core unexposed portion 3b2 is located in the A2 region of the upper cladding forming region 1b. The width of the core unexposed portion 3b1 is W21, and the width of the core unexposed portion 3b2 is W22. The widths W11, W12, W21, and W22 of the core 3 satisfy the relationships of the following equations 8 and 9. W11>W21 (Equation 8) W12>W22 (Equation 9)

[0071] That is, the width W11 of the first core exposed portion 3aa is different from the width W21 of the core unexposed portion 3b1. The width W11 of the first core exposed portion 3aa is larger than the width W21 of the core unexposed portion 3b1. The width W12 of the second core exposed portion 3ab is different from the width W22 of the core unexposed portion 3b2. The width W12 of the second core exposed portion 3ab is larger than the width W22 of the core unexposed portion 3b2.

[0072] In the optical waveguide 10d shown in Figure 6B, the cores 3 have an overall U-shape in plan view. That is, each core 3 has a turn-back portion 3dd, which has two bends 3d, each bent at an angle of approximately 90°. The turn-back portion 3dd may be formed by a curve, a combination of straight and curved lines, or a combination of straight and curved lines. The turn-back portion 3dd is preferably curved, and when the turn-back portion 3dd is curved, optical signals are more easily transmitted near the turn-back portion 3dd. The optical waveguide 10d has an upper cladding-free region 1a along a side S1 of the optical waveguide 10d, which is rectangular in plan view. That is, in the optical waveguide 10d, the core 3 has a first core exposed portion 3aa at one end and a second core exposed portion 3ab at the other end, but the upper cladding non-forming region 1a is provided only in the region near one end 11 of the optical waveguide 10d (the region along side S1 in Figure 6B).

[0073] The optical waveguide 10d is formed in a rectangular shape in plan view, with a side S1 of the optical waveguide 10d and a side S2 substantially perpendicular to the side S1. The lower cladding 21 is formed in a shape having a side S1 and a side S2. The upper cladding forming region 1b is divided into a region A1 and a region A2 by a virtual line X3 passing through the middle of the folded portion 3dd of each core 3. The region A1 is a region where the cores 3 of the core non-exposed portion 3b are formed in the upper half (the half on the first core exposed portion 3aa side) of the upper cladding forming region 1b, and the region A2 is a region where the cores 3 of the core non-exposed portion 3b are formed in the lower half (the half on the second core exposed portion 3ab side) of the upper cladding forming region 1b. Note that the virtual line X3 is not formed on the optical waveguide 10d, but is provided for convenience in explaining the embodiment.

[0074] In FIG. 6B , the first core exposed portion 3aa at one end of the core 3 is located outside the A1 region. The second core exposed portion 3ab at the other end of the core 3 is located outside the A2 region. Outside the A1 region, the four first core exposed portions 3aa at one end of the core 3 are lined up along the side S1. Outside the A2 region, the four second core exposed portions 3ab at the other end of the core 3 are lined up along the side S1. Because the core 3 has a U-shape as shown in FIG. 6B , the component E1 (see FIG. 5 ) can be placed outside the A1 region, and the component E2 (see FIG. 5 ) can be placed outside the A2 region. It is considered that alignment of the component E1 with the first core exposed portion 3aa is easy. It is considered that alignment of the component E2 with the second core exposed portion 3ab is easy. In addition, even if the components E1 and E2 are exposed to heat history during use after being mounted, poor coupling due to misalignment is suppressed, and therefore optical coupling with good coupling efficiency is achieved. It is also believed that it is easier to achieve a desired transmission mode.

[0075] The core 3 has a first core exposed portion 3aa outside the A1 region (towards side S1), and a second core exposed portion 3ab outside the A2 region (towards side S1). The width of the first core exposed portion 3aa is W11, and the width of the second core exposed portion 3ab is W12. A core unexposed portion 3b1 is located in the A1 region of the upper cladding forming region 1b, and a core unexposed portion 3b2 is located in the A2 region of the upper cladding forming region 1b. The width of the core unexposed portion 3b1 is W21, and the width of the core unexposed portion 3b2 is W22. The widths W11, W12, W21, and W22 of the core 3 satisfy the relationships of the following equations 10 and 11. W11>W21 (Equation 10) W12>W22 (Equation 11)

[0076] That is, the width W11 of the first core exposed portion 3aa is different from the width W21 of the core unexposed portion 3b1. The width W11 of the first core exposed portion 3aa is larger than the width W21 of the core unexposed portion 3b1. The width W12 of the second core exposed portion 3ab is different from the width W22 of the core unexposed portion 3b2. The width W12 of the second core exposed portion 3ab is larger than the width W22 of the core unexposed portion 3b2.

[0077] In the optical waveguides 10c and 10d, each core 3 has one or two bends 3d that bend at approximately 90 degrees. However, in the optical waveguides of the embodiment, the core 3 may have any number of bends that bend at any angle. The bends of the core 3 may also be rounded. That is, in the optical waveguides of the embodiment, each core 3 is not limited to a linear shape (I-shape) as shown in FIG. 1 or an L- or U-shape as shown in FIGS. 6A and 6B , and may have any shape in a plan view.

[0078] <Structure of Wiring Board of Embodiment> Next, a wiring board of a first embodiment will be described with reference to the drawings. FIG. 7 shows a cross-sectional view of a wiring board 100, which is an example of a wiring board of the first embodiment. Note that the wiring board 100 shown in FIG. 7 is merely an example of a wiring board of the present embodiment. The layered structure of the wiring board of the embodiment, and the number of conductor layers and insulating layers, are not limited to the layered structure of the wiring board 100 shown in FIG. 7, and the number of conductor layers and insulating layers included in the wiring board 100. Furthermore, the material of the wiring board is not limited.

[0079] 7, wiring board 100 is a wiring board composed of conductor layers and insulating layers, including conductor layers 41 to 43 as conductor layers and insulating layers 51 and 52 as insulating layers. Wiring board 100 includes optical waveguide 101 and component mounting pad 4. Wiring board 100 has first surface 100a, which is the mounting surface for component E1, and second surface 100b, which is the surface opposite first surface 100a. Optical waveguide 101 is disposed on the surface (first surface 100a) of wiring board 100.

[0080] The conductor layers 41-43 and the insulating layers 51 and 52 are stacked in the following order from the second surface 100b of the wiring board 100 toward the first surface 100a: conductor layer 43, insulating layer 52, conductor layer 42, insulating layer 51, and 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 wiring board 100 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 wiring board 100 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 external components located on the second surface 100b. Incidentally, wiring board 100 may be used as a motherboard on which bumps 8 are not arranged.

[0081] 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, and modified polyimide (MPI) resin. The resins listed as insulating layer materials are merely examples of the materials that can form each insulating layer. Each insulating layer may be formed using any material that provides insulation between conductor layers within the wiring board 100. Although not shown, each insulating layer may include a core (reinforcing material) made of glass fiber, aramid fiber, or the like, or may include an inorganic filler consisting of fine particles such as silica (SiO), alumina, or mullite.

[0082] The solder resists 61 and 62 are formed of, for example, a photosensitive epoxy resin or polyimide resin.

[0083] The conductor layers 41-43 and the via conductor 7 may be formed using any metal having suitable conductivity, such as copper or nickel. Although each of these conductors is depicted as a single layer in FIG. 7 for simplicity, they may have a multi-layer structure including two or more films. For example, the conductor layers 41-43 and the via conductor 7 may have a two-layer structure including an electroless plated film and an electrolytic plated film.

[0084] The component mounting pads 4 are conductor pads formed on the conductor layer 41. An opening 61a is provided in the solder resist 61, and the component mounting pads 4 are exposed in the opening 61a. An optical waveguide 101 is disposed on the solder resist 61. Although not shown, the optical waveguide 101 is fixed to the surface of the wiring board 100 with any fixing member such as an adhesive.

[0085] The optical waveguide 101 is the optical waveguide of the embodiment described above. The optical waveguide 101 may be the optical waveguide 1 shown in FIGS. 1 and 3, or may be the optical waveguides 10a, 10b, 10c, and 10d shown in FIGS. 4, 5, 6A, and 6B, respectively. FIG. 7 shows the optical waveguide 1 of FIG. 1 as an example. Thus, the optical waveguide 101 of FIG. 7 includes a laminated lower cladding 21, a core 3, and an upper cladding 22, and has an upper cladding non-forming region 1a and an upper cladding formed region 1b. The core 3 has, at one end, an exposed core portion 3a exposed in the upper cladding non-forming region 1a. The width of the exposed core portion 3a is greater than the width of the unexposed core portion 3b.

[0086] A component E1 is mounted on the wiring board 100. The component E1 is an optical component such as a semiconductor device including a photoelectric conversion element, as described in the description of the optical waveguide 1 in FIG. 1 and other figures. The component E1 includes a light-receiving portion or a light-emitting portion 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 light-receiving portion or the light-emitting portion E1a, which functions as a light-emitting portion, toward the core 3. When the component E1 is a light-receiving element, an electrical signal based on an optical signal incident from the light-receiving portion or the light-emitting portion E1a, which functions as a light-receiving portion, is generated and output from the electrode E1b.

[0087] The component E1 is mounted on the wiring board 100 by connecting the electrode E1b to the component mounting pad 4 using, for example, solder. In FIG. 7 , the component E1 is flip-chip mounted. The light-receiving or light-emitting portion E1a is positioned facing the exposed core portion 3a of the core 3 of the optical waveguide 101 for optical coupling. Because the optical waveguide 101 is an optical waveguide of an embodiment such as the optical waveguide 1 illustrated in FIG. 1 , it is easy to align the exposed core portion 3a with the light-receiving or light-emitting portion E1a of the component E1, which is considered to enable optical coupling (especially adiabatic coupling) with excellent coupling efficiency. Additionally, optical signals from or to the component E1 may be transmitted through the optical waveguide 101 in a desired transmission mode, such as single mode. That is, the wiring board 100 of this embodiment can provide high-efficiency, low-loss, high-speed optical transmission for optical components optically coupled to the optical waveguide 101.

[0088] Fig. 8 shows wiring board 100α, which is an application example of wiring board 100 shown in Fig. 7. Wiring board 100α includes optical waveguide 101, similar to wiring board 100 of Fig. 7. Optical waveguide 101 shown in Fig. 8 may be optical waveguide 1 shown in Fig. 1 or the like, or may be optical waveguide 10a shown in Fig. 4. Optical waveguide 101 is disposed on first surface 100a of wiring board 100α. Component E3 is mounted on first surface 100a of wiring board 100 when wiring board 100α is in use.

[0089] Component E3, like component E1 shown in Fig. 7, is an optical component such as a semiconductor device including a photoelectric conversion element. Component E3 includes a light receiving or emitting portion E3a that functions similarly to light receiving or emitting portion E1a of component E1, and a ball-shaped electrode E3b that functions similarly to electrode E1b of component E1 shown in Fig. 7. Light receiving or emitting portion E3a of component E3 is optically coupled to core 3 at one end 11 of optical waveguide 101. Electrode E3b is connected to component mounting pad 4 of wiring board 100α.

[0090] Wiring board 100α has a structure similar to that of wiring board 100, except that the number of component mounting pads 4 connected to electrodes E3b of component E3 is different from the number of component mounting pads 4 of wiring board 100 in Fig. 7. In wiring board 100α, components similar to those of wiring board 100 in Fig. 7 are assigned the same reference numerals in Fig. 8 as those assigned in Fig. 7 or are omitted as appropriate, and repeated descriptions of similar components will be omitted.

[0091] In the wiring board 100α of FIG. 8 , when the wiring board 100α is in use, the core 3 in the upper cladding forming region 1b is optically coupled to an optical fiber FB, an external optical connecting component, at the other end 12 of the optical waveguide 101. In the example of FIG. 8 , the optical fiber FB and the optical waveguide 101 are optically coupled via an optical connector CN. An optical signal propagating through the core FB3 of the optical fiber FB can enter the core 3 of the optical waveguide 101 from the other end 12 via the optical connector CN. Furthermore, an optical signal propagating within the core 3 from one end 11 to the other end 12 can enter the core FB3 of the optical fiber FB via the optical connector CN. Note that the wiring board 100 and the wiring board 100α can be applied to any wiring board, regardless of material or structure. The waveguide of the wiring board 100α of FIG. 8 has an exposed core portion at one end and no exposed core portion at the other end.

[0092] <Variation 1 of the Wiring Board of the First Embodiment> FIG. 9 illustrates Variation 1 of the wiring board 100α of the first embodiment illustrated in FIG. 8 . The wiring board 100α illustrated in FIG. 9 includes an optical waveguide 101, similar to the wiring board 100α of FIG. 8 . FIG. 9A illustrates a cross section of a coupling portion between the optical fiber FB and the optical waveguide 101 in FIG. 9 . The optical waveguide 101 illustrated in FIG. 9 may be the optical waveguide 1 illustrated in FIG. 1 or the like, or the optical waveguide 10a illustrated in FIG. 4 . The structure of the wiring board 100α of the example of FIG. 9 is the same as that of the wiring board 100α of FIG. 8 , except for the orientation of the optical waveguide 101. In the wiring board 100α of FIG. 9 , components similar to those of the wiring board 100α of FIG. 8 are denoted by the same reference numerals in FIG. 8 or omitted as appropriate, and repeated descriptions of similar components will be omitted.

[0093] In the example of Figure 9, when wiring board 100α is in use, component E4 is placed on wiring board 100α. Component E4 is an optical component such as a semiconductor device including a photoelectric conversion element. Component E4 includes a light-receiving or light-emitting portion E4a, which is the portion where an optical signal enters component E4 or the portion where an optical signal exits component E1, and also includes an electrode E4b, through which an electrical signal is input or output. Light-receiving or light-emitting portion E4a is exposed on the side surface of component E4. Electrode E4b is connected to component mounting pad 4 of wiring board 100α.

[0094] The optical waveguide 101 is disposed so that the core 3 in the upper cladding forming region 1b of the optical waveguide 101 is optically coupled by end-to-end connection to the light receiving or emitting portion E4a of the component E4 at the other end 12 of the optical waveguide 101. That is, the optical waveguide 101 in Fig. 9 is disposed on the wiring board 100α so that the end face of the core 3 exposed at the end face of the optical waveguide 101 on the other end 12 side faces the light receiving or emitting portion E4a exposed at the side face of the component E4.

[0095] On the other hand, in the example of Fig. 9, the core 3 exposed in the upper cladding-free region 1a including one end 11 of the optical waveguide 101 is optically coupled to the core of an external optical connecting component. In the example of Fig. 9, the core 3 in the upper cladding-free region 1a of the optical waveguide 101 is optically coupled to the core FB3 of the optical fiber FB. That is, in the example of Fig. 9, one end 11 of the optical waveguide 101 is positioned near the edge of the wiring board 100α that is optically coupled to the external optical connecting component. On the other hand, the other end 12 of the optical waveguide 101 is positioned so as to face the component mounting region Ea of the wiring board 100α where the component E4 is disposed. By arranging the optical waveguide 101 in this manner, an optical signal can be propagated between the optical fiber FB and the component E4 via the optical waveguide 101.

[0096] In the portion of the optical fiber FB in Fig. 9 that is optically coupled to the optical waveguide 101, the core FB3 is exposed over a predetermined length, as shown in Fig. 9 and Fig. 9A . Note that Fig. 9A shows an enlarged cross section of the coupling portion between the optical fiber FB and the optical waveguide 101, and the cross section shown in Fig. 9A is a cross section perpendicular to the direction in which the core 3 extends. As shown in Fig. 9A , the core FB3 and cladding FB2 in half of the circular cross section of the optical fiber FB are removed over a predetermined length along the direction in which the core FB3 extends. As a result, the core FB3 is exposed over a predetermined length with a width substantially equal to the diameter of the core FB3, and the exposed core FB3 faces the core 3 exposed in the upper cladding-free region 1a of the optical waveguide 101. This allows the optical fiber FB to be optically coupled (adiabatic coupled) with high efficiency to the optical waveguide 101.

[0097] 9A , optical resin RG is filled between optical fiber FB and optical waveguide 101. The optical resin RG may, for example, adjust the refractive index between core 3 and core FB3, or suppress the intrusion of foreign matter into the joint between core 3 and core FB3. Furthermore, optical resin RG may also bond optical fiber FG and optical waveguide 101. The waveguide of wiring board 100α in FIG. 9 has an exposed core portion on one end side and no exposed core portion on the other end side.

[0098] <Variation 2 of the Wiring Board of the First Embodiment> Figure 10 shows Variation 2 of the wiring board 100α of the first embodiment shown in Figure 8. The wiring board 100α shown in Figure 10 includes an optical waveguide 101, similar to the wiring board 100α of Figure 8. The optical waveguide 101 shown in Figure 10 may be the optical waveguide 1 shown in Figure 1 or the like, or the optical waveguide 10a shown in Figure 4. The structure of the wiring board 100α in the example of Figure 10 is the same as that of the wiring board 100α of Figure 8, except for the orientation of the optical waveguide 101. In the wiring board 100α of Figure 10, components similar to those of the wiring board 100α of Figure 8 are assigned the same reference numerals in Figure 10 as those assigned in Figure 8, or are omitted as appropriate, and repeated descriptions of similar components will be omitted.

[0099] In the example of Figure 10, when wiring board 100α is in use, component E4 is placed on wiring board 100α. Component E4 is an optical component such as a semiconductor device including a photoelectric conversion element. Component E4 includes a light-receiving or light-emitting portion E4a, which is the portion where an optical signal enters component E4 or the portion where an optical signal exits component E1, and also includes an electrode E4b, through which an electrical signal is input or output. Light-receiving or light-emitting portion E4a is exposed on the side surface of component E4. Electrode E4b is connected to component mounting pad 4 of wiring board 100α.

[0100] The optical waveguide 101 is disposed so that the core 3 in the upper cladding forming region 1b of the optical waveguide 101 is optically coupled by end-to-end connection to the light receiving or emitting portion E4a of the component E4 at the other end 12 of the optical waveguide 101. That is, the optical waveguide 101 in Fig. 10 is disposed on the wiring board 100α so that the end face of the core 3 exposed at the end face of the optical waveguide 101 on the other end 12 side faces the light receiving or emitting portion E4a exposed at the side face of the component E4.

[0101] On the other hand, in the example of Fig. 10 , the core 3 exposed in the upper cladding-free region 1a including one end 11 of the optical waveguide 101 is optically coupled to the core of an external optical connecting component. In Fig. 10 , the core 3 in the upper cladding-free region 1a of the optical waveguide 101 is optically coupled to the core OG3 of the external optical waveguide OG. That is, in the example of Fig. 10 , one end 11 of the optical waveguide 101 is positioned near the edge of the wiring board 100α that is optically coupled to the external optical connecting component. On the other hand, the other end 12 of the optical waveguide 101 is positioned so as to face the component mounting region Ea of the wiring board 100α where the component E4 is disposed. By positioning the optical waveguide 101 in this manner, an optical signal can be propagated via the optical waveguide 101 between the external optical waveguide OG and the component E4.

[0102] 10 , the external optical waveguide OG optically coupled to the optical waveguide 101 is composed of a lower cladding OG21, a core OG3, and an upper cladding OG22. The external optical waveguide OG includes an upper cladding no-region OGaa and an upper cladding no-region OGab, which are regions where the upper cladding OG22 is not formed. The core OG3 exposed in the upper cladding no-region OGaa is optically coupled (adiabatic coupled) to the core 3 exposed in the upper cladding no-region 1a of the optical waveguide 101. The waveguide of the wiring board 100α in FIG. 10 has an exposed core portion on one end and no exposed core portion on the other end.

[0103] <Structure of a Wiring Board of the Second Embodiment> Figure 11 shows a wiring board 100β, which is an example of a wiring board of the second embodiment. The wiring board 100β includes an optical waveguide 102 disposed on a first surface 100a of the wiring board 100β. The optical waveguide 102 is the optical waveguide of the previously described embodiment and may be the optical waveguide 10b shown in Figure 5. That is, the optical waveguide 102 has a first upper cladding non-region 1aa on one end 11 and a second upper cladding non-region 1ab on the other end 12. Like the wiring board 100β, the wiring board of the second embodiment includes an optical waveguide having upper cladding non-regions on both one end 11 and the other end 12.

[0104] Wiring board 100β in FIG. 11 has a layer configuration similar to that of wiring board 100 and wiring board 100α shown in FIGS. 7 to 10. That is, in wiring board 100β, conductor layers and insulating layers are formed in the following order from second surface 100b of wiring board 100β toward first surface 100a: conductor layer 43, insulating layer 52, conductor layer 42, insulating layer 51, and conductor layer 41. Furthermore, solder resist 62 is formed to cover conductor layer 43 and insulating layer 52, and solder resist 61 is formed to cover conductor layer 41 and insulating layer 51. In wiring board 100β in FIG. 11, components similar to those of wiring board 100 or wiring board 100α in FIGS. 7 to 10 are assigned the same reference numerals in FIG. 11 as those assigned in FIGS. 7 to 10, or are omitted as appropriate, and repeated description will be omitted.

[0105] On the other hand, the wiring board 100β of FIG. 11 differs from the wiring boards 100 and 100α shown in FIGS. 7 to 10 in that it has component mounting pads 4 on both the one end 11 and the other end 12 of the optical waveguide 102. That is, when the wiring board 100β is used, a component E1 is disposed on the one end 11 of the optical waveguide 101, and a component E2 is disposed on the other end 12. The component E1 shown in FIG. 11 may be the component E1 shown in FIG. 1 or the like, and the component E2 shown in FIG. 11 may be the component E2 shown in FIG. 5. That is, the components E1 and E2 in FIG. 11 may be photoelectric conversion components such as semiconductor devices including photoelectric conversion elements, or the components E1 and E2 may be the same type of photoelectric conversion components. The component E1 has a light-receiving or light-emitting portion E1a and an electrode E1b, and the component E2 has a light-receiving or light-emitting portion E2a and an electrode E2b. An electrode E1b of the component E1 is connected to a component mounting pad 4 on one end 11 side of the optical waveguide 102, and an electrode E2b of the component E2 is connected to a component mounting pad 4 on the other end 12 side of the optical waveguide 102.

[0106] The component E1 is disposed on the wiring board 100β so that the light receiving portion or light emitting portion E1a faces the core 3 exposed in the first upper cladding non-forming region 1aa of the optical waveguide 102. The component E2 is disposed on the wiring board 100β so that the light receiving portion or light emitting portion E2a faces the core 3 exposed in the second upper cladding non-forming region 1ab of the optical waveguide 102. As a result, the core 3 exposed in the first upper cladding non-forming region 1aa of the optical waveguide 102 is optically coupled (adiabatic coupled) to the light receiving portion or light emitting portion E1a of the component E1. Also, the core 3 exposed in the second upper cladding non-forming region 1ab is optically coupled (adiabatic coupled) to the light receiving portion or light emitting portion E2a of the component E2. By arranging the optical waveguide 102, the component E1, and the component E2 in this manner, it is believed that the influence of electromagnetic noise can be suppressed and transmission can be performed at high speed by converting an electrical signal into an optical signal between one end 11 and the other end 12 of the optical waveguide 102. The waveguide of the wiring board 100β in Fig. 11 has an exposed core portion at one end and an exposed core portion at the other end. In other words, the waveguide of the wiring board 100β in Fig. 11 has two exposed core portions, and may have the configurations shown in Figs. 6A and 6B as well as Fig. 5.

[0107] <Variation 1 of the Wiring Board of the Second Embodiment> Figure 12 shows a wiring board 100γ, which is variation 1 of the wiring board of the second embodiment. The wiring board 100γ has the same structure as the wiring board 100α shown in Figures 8 to 10 in terms of the structures of the conductor layers 41-43, the insulating layers 51-52, and the solder resists 61-62. On the other hand, the wiring board 100γ includes an optical waveguide 102 that is different from the optical waveguide 101 included in the wiring board 100α. The optical waveguide 102 included in the wiring board 100γ may be the optical waveguide 10b shown in Figure 5, similar to the optical waveguide 102 included in the wiring board 100β of Figure 11.

[0108] In the wiring board 100γ of Fig. 12, when the wiring board 100γ is in use, a component E1 is disposed on one end 11 of the optical waveguide 102, similar to the wiring board 100β of Fig. 11. In the example of Fig. 12, the component E1 may be the same as the component E1 shown in Fig. 11 and includes a light-receiving or light-emitting portion E1a and an electrode E1b. The optical waveguide 102 and the component E1 are disposed so that the core 3 exposed in the first upper cladding-free region 1aa of the optical waveguide 102 faces the light-receiving or light-emitting portion E1a of the component E1 and is optically coupled (adiabatic coupled). The electrode E1b of the component E1 is connected to a component mounting pad 4 provided on the wiring board 100γ.

[0109] 12 , the core 3 exposed in the second upper cladding non-forming region 1ab on the other end 12 side of the optical waveguide 102 is optically coupled to a core of an external optical connecting component when the wiring board 100γ is in use. That is, in the example of Fig. 12 , the core 3 exposed in the second upper cladding non-forming region 1ab of the optical waveguide 102 is optically coupled to a core FB3 of the optical fiber FB. In the optical fiber FB shown in Fig. 12 , similar to the optical fiber FB shown in Figs. 9 and 9A , the core FB3 and the cladding FB2 are partially removed by a predetermined length in the extension direction of the core FB3 at the portion optically coupled to the optical waveguide 102. As a result, the core FB3 is exposed over a predetermined length in the extension direction of the core FB3, and the exposed core FB3 and the core 3 exposed in the second upper cladding non-forming region 1ab are optically coupled (adiabatic coupled) facing each other. The waveguide of wiring board 100γ in Fig. 12 has an exposed core portion at one end and an exposed core portion at the other end. In other words, the waveguide of wiring board 100γ in Fig. 12 has two exposed core portions, and may have the configurations shown in Fig. 6A and Fig. 6B as well as Fig. 5.

[0110] <Modification 2 of the Wiring Board of the Second Embodiment> Figure 13 shows a wiring board 100γ, which is modification 2 of the wiring board of the second embodiment. The wiring board 100γ has the same structure as the wiring board 100α shown in Figures 8 to 10 with respect to the structures of the conductor layers 41-43, the insulating layers 51-52, and the solder resists 61-62. On the other hand, the wiring board 100γ includes an optical waveguide 102 that is different from the optical waveguide 101 included in the wiring board 100α. The optical waveguide 102 included in the wiring board 100γ may be the optical waveguide 10b shown in Figure 5, similar to the optical waveguide 102 included in the wiring board 100β of Figure 11.

[0111] In the wiring board 100γ of Fig. 13, when the wiring board 100γ is in use, a component E1 is disposed on one end 11 of the optical waveguide 102, similar to the wiring board 100β of Fig. 11. In the example of Fig. 13, the component E1 may be the same as the component E1 shown in Fig. 11 and includes a light-receiving or light-emitting portion E1a and an electrode E1b. The optical waveguide 102 and the component E1 are disposed so that the core 3 exposed in the first upper cladding-free region 1aa of the optical waveguide 102 faces the light-receiving or light-emitting portion E1a of the component E1 and is optically coupled (adiabatic coupled). The electrode E1b of the component E1 is connected to a component mounting pad 4 provided on the wiring board 100γ.

[0112] 13, the core 3 exposed in the second upper cladding non-forming region 1ab on the other end 12 side of the optical waveguide 102 is optically coupled to a core of an external optical connecting component when the wiring board 100γ is in use. That is, in the example of Fig. 13, the core 3 exposed in the second upper cladding non-forming region 1ab of the optical waveguide 102 is optically coupled to a core OG3 of the external optical waveguide OG. The external optical waveguide OG shown in Fig. 13 has the same structure as the external optical waveguide OG shown in Fig. 10. That is, the external optical waveguide OG shown in Fig. 13 is composed of a lower cladding OG21, a core OG3, and an upper cladding OG22, and includes an upper cladding non-forming region OGaa and an upper cladding non-forming region OGab. The core OG3 exposed in the upper cladding free region OGaa is optically coupled by adiabatic coupling to the core 3 exposed in the second upper cladding free region 1ab of the optical waveguide 102. The waveguide of the wiring board 100γ in Fig. 13 has an exposed core portion at one end and an exposed core portion at the other end. In other words, the waveguide of the wiring board 100γ in Fig. 13 has two exposed core portions, and may have the configurations shown in Figs. 6A and 6B as well as Fig. 5.

[0113] Note that the connection between the optical waveguide of the embodiment or the optical waveguide included in the wiring board of the embodiment and an external optical component is not limited to the examples shown in Figures 8 to 13. The optical waveguide of the embodiment and the optical waveguide included in the wiring board of the embodiment may be optically coupled to an external optical component in any manner.

[0114] <Method for Manufacturing an Optical Waveguide of an Embodiment> A method for manufacturing an optical waveguide of an embodiment will be described with reference to FIGS. 14A to 14D. Hereinafter, as an example, a method for directly forming an optical waveguide on a wiring board (e.g., wiring board 100 of FIG. 7) will be described. As shown in FIG. 14A, wiring board 100 is prepared, and underclad 21 is formed on the surface of a solder resist (not shown) of wiring board 100. Underclad 21 is formed, for example, by thermocompression bonding a film-shaped constituent material of underclad 21 to wiring board 100 or by applying a resin composition by spin coating to form a film. For example, a constituent material of underclad 21, such as PMMA, is formed into a film and thermocompression bonded to wiring board 100. Note that the optical waveguide may be formed on a surface of wiring board 100 on which solder resist is not formed.

[0115] As shown in FIG. 14B , a core 3 having a desired shape in plan view is formed. The core 3 is formed by any method. As an example, the core 3 is formed using photolithography. That is, a photosensitive layer made of a material for the core 3, such as PMMA, is formed on the entire surface of the lower clad 21. As an example, the material for the core 3 is applied by spin coating to form a film made of the material for the core 3. Alternatively, the material for the core 3 may be formed into a film and thermocompressed onto the surface of the lower clad 21. Alternatively, a material for the core 3 having a higher refractive index than the lower clad 21 may be applied, or a film-like material having a higher refractive index than the lower clad 21 may be thermocompressed. Then, the layer made of the material for the core 3 on the lower clad 21 is patterned by exposure through a mask corresponding to the shape of the core 3 to be formed and development, thereby forming a desired number of cores 3 having a desired shape.

[0116] Each core 3 is formed with a portion corresponding to the non-exposed core portion 3b having a width W2, a portion corresponding to the exposed core portion 3a having a width W1 larger than the width W2, and a portion corresponding to the connecting portion 3c connecting the non-exposed core portion 3b and the exposed core portion 3a. In the example of FIG. 14B , the portion corresponding to the connecting portion 3c is formed by combining a tapered shape with a shape of a constant width. The exposed core portion 3a is formed to be located in the upper cladding non-forming region 1a (see FIG. 14D ) to be provided in a subsequent process, and the non-exposed core portion 3b is formed to be located in the upper cladding non-forming region 1b (see FIG. 14D ) to be provided in a subsequent process. The connecting portion 3c is formed to connect the exposed core portion 3a and the non-exposed core portion 3b in the upper cladding non-forming region 1a. Note that the connecting portion 3c may have only a tapered shape or only a shape of a constant width, rather than a combination of a tapered shape and a constant width.

[0117] In addition to photolithography, examples of methods for forming the core 3 in a desired shape include a photobleaching method and a core cutting method. In the photobleaching method, a layer having a higher refractive index than the lower cladding 21 and formed on the entire surface of the lower cladding 21 is irradiated with ultraviolet light or the like in the unmasked region using a mask that shields the region where the core 3 is to be formed. The core 3 is formed by reducing the refractive index of the irradiated region. In the core cutting method, a layer having a higher refractive index than the lower cladding 21 and formed on the entire surface of the lower cladding 21 is cut into the desired shape using laser light or the like, thereby forming the core 3 in a desired shape. The core 3 may be formed by such a photobleaching method or core cutting method.

[0118] As shown in Figure 14C, the upper clad 22 is formed on the core 3 and the lower clad 21. For example, similar to the formation of the lower clad 21, a material for the upper clad 22, such as PMMA, is spin-coated onto the lower clad 21 and the core 3 to form a film made of the material for the upper clad 22. Alternatively, the material for the upper clad 22, such as PMMA, may be molded into a film and then thermocompressed. The upper clad 22 is integrated with or at least closely adhered to the lower clad 21, thereby forming the clad 2 surrounding the core 3. At the stage shown in Figure 14C, the upper clad 22 is in contact with the top surface 31 and the side surface of the core 3, and the upper clad 22 is formed to surround the core 3, and the lower clad 21 is formed on the bottom surface 32 of the core 3. As a result, the clad 2 is formed to surround the core 3. Alternatively, the upper clad 22 may be formed on the upper surface 31 of the core 3 , and the lower clad 21 may be in contact with the side surface and lower surface 32 of the core 3 , forming the clad 2 so as to surround the core 3 .

[0119] As shown in Fig. 14D, the portion of the upper cladding 22 covering the exposed core portion 3a and the connection portion 3c of the core 3 is removed. For example, the portion of the upper cladding 22 on the exposed core portion 3a side is removed over the entire length in the direction of the width W1 (see Fig. 14B) of the exposed core portion 3a. As a result, the exposed core portion 3a of the core 3 is exposed from the cladding 2. That is, by removing a portion of the upper cladding 22, an upper cladding non-formed region 1a and an upper cladding formed region 1b are provided. The non-exposed core portion 3b remains covered by the upper cladding 22 in the upper cladding formed region 1b.

[0120] The portion of the upper cladding 22 to be removed is, for example, removed using photolithography. The portion of the upper cladding 22 to be removed may also be removed by laser processing, but the removal method is not limited to these methods. In addition, a film that is a constituent material of the upper cladding 22 may be thermocompressed so as not to cover the exposed core portion 3 a.

[0121] A support plate (not shown) may be prepared instead of the wiring board 100, and the optical waveguide may be formed on the support plate. The support plate supports the optical waveguide during manufacture and provides appropriate rigidity to the optical waveguide during manufacture. A support plate having approximately the same size and shape as the optical waveguide to be manufactured in a plan view may be prepared, or a support plate slightly larger than the optical waveguide to be manufactured in a plan view may be prepared. The support plate may have an adhesive layer (not shown) on the surface on which the lower clad 21 is formed. In this case, after the optical waveguide is completed, the support plate may be peeled off from the completed optical waveguide at the adhesive layer. The manufactured optical waveguide may be used with the support plate attached after completion.

[0122] The support plate is preferably formed of a material having higher rigidity than the optical waveguide to be manufactured. The support plate may be formed of a material having a thermal expansion coefficient lower than that of the optical waveguide to be manufactured. Displacement of the optical waveguide due to temperature changes may be suppressed. 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.

[0123] The support plate may have a suitable flexibility, and may be made of, for example, a resin film. When the optical waveguide is used with the support plate attached, the degree of freedom in the shape of the optical waveguide during use may be increased.

[0124] When the manufactured optical waveguide is to be used without a support plate, the support plate is removed after defining the upper cladding-free region 1a as shown in FIG. 14D . Removal of the support plate completes the formation of a standalone optical waveguide. The support plate can be removed by any method. For example, if the support plate and the lower cladding 21 are bonded together with an adhesive layer (not shown) made of a thermoplastic adhesive, the optical waveguide with the support plate is heated to reduce the adhesive strength of the adhesive, and then the support plate is peeled off from the lower cladding 21. Alternatively, if the lower cladding 21 is bonded to a translucent support plate with an adhesive layer (not shown) made of a photoplastic adhesive, light may be irradiated from the support plate side toward the lower cladding 21 to reduce the adhesive strength of the adhesive, and then the support plate may be peeled off from the lower cladding 21. The optical waveguide after the support plate removal is then disposed on, for example, the wiring board 100 described with reference to FIG. 7 .

[0125] 7 may be manufactured by sequentially forming conductor layer 43, insulating layer 52, conductor layer 42, insulating layer 51, and conductor layer 41, and then forming optical waveguide 101 on this laminate by the manufacturing method described with reference to FIGS. 14A to 14D. Optical waveguide 101 may be separately manufactured on a support plate (not shown) that supports optical waveguide 101 during manufacture, and then disposed on wiring board 100 before optical waveguide placement, either after removing the support plate or with the support plate still attached.

[0126] 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 such as openings 61 a that expose parts of the conductor layer 41 and the conductor layer 43 .

[0127] Then, the optical waveguide 101 is manufactured on the surface of the conductor layer 41 side of the laminate of the conductor layers 41-42 and the insulating layers 51, 52 by the method described with reference to FIGS. 14A to 14D . Alternatively, the optical waveguide 101 may be separately manufactured on a support plate (not shown) and then disposed thereon. For example, a thermosetting, room temperature curing, or photocurable adhesive (not shown) is applied to the surface of the solder resist 61, and the optical waveguide 101 is mounted thereon. If necessary, the adhesive is cured by heating or the like, and the optical waveguide 101 is fixed onto the laminate of the conductor layers and the insulating layers. After the optical waveguide 101 is manufactured or disposed, the bumps 8 are formed by mounting conductive balls using solder or the like and performing a reflow process. The wiring board 100 of the example shown in FIG. 7 is completed through the above steps.

[0128] 7 , the optical waveguide 101 may be manufactured not on the solder resist 61 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, or may be disposed in an area on the surface of the insulating layer 51 where the solder resist 61 and the conductor layer 41 are not provided. Furthermore, the optical waveguide 101 may be disposed on the surface of a laminate of conductor layers and insulating layers, with a support plate (not shown) used in manufacturing the optical waveguide 101 still attached.

[0129] 8 to 13 can be manufactured using a method similar to that of the above-described wiring board 100. That is, wiring boards 100α to 100γ can be manufactured by sequentially forming each conductor layer, each insulating layer, and each solder resist, and then forming optical waveguide 101 or optical waveguide 102 on the surface of solder resist 61 or insulating layer 51 using the method described with reference to FIGS. 14A to 14D. Alternatively, optical waveguide 101 or optical waveguide 102 may be formed on a support plate and then disposed on the surface of separately formed solder resist layer 61 or insulating layer 51.

[0130] The optical waveguides 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 may have any laminated structure and may include any number of conductor layers and insulating layers. For example, the wiring boards of the embodiments may be build-up wiring boards including a core substrate, multilayer wiring boards without build-up layers, or double-sided or single-sided wiring boards. Bumps 8 may not be formed, and via conductors 7 are not necessarily provided. Furthermore, the thickness of the core of the optical waveguide of the embodiments may vary between the core exposed portion and the core unexposed portion.

[0131] 1, 10a to 10d, 101, 102 Optical waveguide 100, 100α to 100γ Wiring board 11 One end of optical waveguide 12 Other end of optical waveguide 1a Upper clad non-forming region 1b Upper clad forming region 2 Clad 21 Lower clad 22 Upper clad 3 Core 3a Core exposed portion 3b Core unexposed portion 3c Connection portion 31 Upper surface of core 4 Component mounting pad 41 to 43 Conductive layer 51 to 52 Insulating layer L1 Core length at core exposed portion W1, W11, W12 Width of core exposed portion W2, W21, W22 Width of core unexposed portion W3 Width of core connection portion

Claims

1. An optical waveguide in which a lower cladding, a core, and an upper cladding are laminated in this order, with a portion of the core on the side where an optical signal is incident and a portion of the core on the side where an optical signal is output being exposed, said optical waveguide comprising an upper cladding non-forming region and an upper cladding formed region, said upper cladding non-forming region having a core exposed portion where the top surface of the core is exposed at at least one end of said optical waveguide, said upper cladding formed region having a core unexposed portion where the top surface of the core is not exposed, and a core width (W1) of said core exposed portion and a core width (W2) of said core unexposed portion satisfy the relationship of the following formula 1. W1>W2 ... (Formula 1) 2. An optical waveguide according to claim 1, wherein the core width (W1) and the core width (W2) satisfy the relationship of the following formula 2: 1.0<(W1 / W2)≦3.0... (Formula 2) 3. An optical waveguide as claimed in claim 1, wherein the width (W1) of the core is 3 μm or more and 30 μm or less.

4. An optical waveguide as claimed in claim 1, wherein the length of said core at said core exposed portion is 100 μm or more and 3000 μm or less.

5. An optical waveguide according to claim 1, wherein the core has a connection portion connecting the exposed core portion and the unexposed core portion.

6. An optical waveguide according to claim 1, wherein the core has a connection portion connecting the exposed core portion and the unexposed core portion, the connection portion having a taper in a plan view.

7. An optical waveguide as claimed in claim 5, wherein the connection portions are arranged in a straight line of a constant width in a plan view.

8. An optical waveguide as claimed in claim 5, wherein the connection portions are arranged in a combination of straight lines of a constant width and tapers in a plan view.

9. An optical waveguide according to claim 5, wherein the width (W3) of the core at the connection portion, the width (W1) of the core and the width (W2) of the core satisfy the relationship of the following formula 3: W1>W3≧W2 ... (Formula 3) 10. An optical waveguide as claimed in claim 1, wherein the upper cladding non-forming region has the core exposed portion at one end of the core and the core exposed portion at the other end of the core, the core in the core exposed portion located at the one end has a width (W11), the core in the core exposed portion located at the other end has a width (W12), the core in the core unexposed portion at the one end has a width (W21), and the core in the core unexposed portion at the other end has a width (W22), and the core width (W11), core width (W12), core width (W21), and core width (W22) satisfy the relationships of the following formulas 4 and 5: W11>W21 ... (Formula 4) W12>W22 ... (Formula 5) 11. An optical waveguide according to claim 10, wherein the core width (W11) and the core width (W21) satisfy the relationship of the following formula 6: 1.0<W11 / W21≦3.0 ... (Formula 6) The core width (W12) and the core width (W22) satisfy the relationship of the following formula 7: 1.0<W12 / W22≦3.0 ... (Formula 7) 12. An optical waveguide as set forth in claim 10, wherein the core width (W11) and the core width (W12) are 3 μm or more and 30 μm or less, respectively.

13. A wiring board of insulating layers and conductor layers, comprising the optical waveguide of claim 1 and a component mounting pad, wherein the optical waveguide is disposed on the surface of the wiring board.

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