Optical waveguide and waveguide mounting substrate

Abstract

An optical waveguide (1) according to an embodiment comprises a lower cladding (21), a core (3), and an upper cladding (22), and a part of the core (3) on the optical signal incident side or emission side is exposed. The optical waveguide (1) is formed from a resin comprising a thermosetting resin or a photocurable resin, and in the interface (I) between the upper cladding (22) and the core (3), the resin forming the upper cladding (22) and the resin forming the core (3) are compatible with each other. A waveguide mounting substrate according to an embodiment includes a wiring substrate including an insulating layer and a conductor layer, and the optical waveguide according to the embodiment.

Description

Optical waveguide and waveguide mounting substrate

[0001] The present invention relates to an optical waveguide and a waveguide-mounted substrate.

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

[0003] JP 2014-81587 A

[0004] In forming the polymer waveguide array disclosed in Patent Document 1, simply forming the core material and the cladding material may result in problems. For example, if the core width is small, problems such as peeling of the cladding may occur if the adhesion between the core and the upper cladding is not ensured. Furthermore, if the adhesion between the core and the upper cladding is not ensured against thermal stress caused by heat generated when an optical element is mounted and used, or when the polymer waveguide array is used, problems such as peeling at the interface between the core and the cladding or core misalignment may occur.

[0005] The optical waveguide of the present invention comprises a lower cladding, a core, and an upper cladding, and a part of the core is exposed on the input or output side of an optical signal. The optical waveguide is formed from a resin made of a thermosetting resin or a photocurable resin, and the resin forming the upper cladding and the resin forming the core are compatible with each other at the interface between the upper cladding and the core.

[0006] The waveguide-mounted substrate of the present invention includes a wiring board including an insulating layer and a conductor layer formed on the insulating layer, and the optical waveguide described above disposed on the wiring board.

[0007] The optical waveguide of the present invention is made of a thermosetting resin or a photocurable resin, and the resin forming the upper cladding and the resin forming the core are compatible with each other. This ensures adhesion between the upper cladding and the core, which is believed to prevent problems such as core peeling.

[0008] Furthermore, even if the core width is reduced, the adhesion between the core and the upper cladding is ensured, which is thought to prevent problems such as core peeling.

[0009] 8 is a plan view showing an example of an optical waveguide according to a first embodiment of the present invention. FIG. 1 is a cross-sectional view taken along line II-II of the optical waveguide according to the example of FIG. 1. FIG. 1 is a cross-sectional view taken along line III-III of the optical waveguide according to the example of FIG. 1. FIG. 2 is a cross-sectional view taken along line IV-IV of the optical waveguide according to the example of FIG. 1. FIG. 3 is a cross-sectional view showing an example of compatibility between a core and an upper clad in an optical waveguide according to an embodiment. FIG. 4 is a schematic view showing an example of gradual compatibility at the interface between a core and an upper clad in an optical waveguide according to an embodiment. FIG. 5 is a plan view showing a modified example of the optical waveguide according to the first embodiment. FIG. 6 is a plan view showing an example of an optical waveguide according to a second embodiment of the present invention. FIG. 7 is a cross-sectional view showing an example of the optical waveguide according to the example of FIG. 8. FIG. 8 is a cross-sectional view showing an example of a waveguide-mounted substrate according to the first embodiment of the present invention. FIG. 9 is a cross-sectional view showing an example of a waveguide-mounted substrate according to the second embodiment of the present invention. FIG. 10 is a cross-sectional view showing a modified example of the waveguide-mounted substrate according to the second embodiment of the present invention. FIG. 11 is a cross-sectional view showing an example of a waveguide-mounted substrate according to a third embodiment of the present invention. FIG. 12 is a front view showing an example of a manufacturing process of the optical waveguide according to an embodiment. 1A to 1C are plan views showing an example of a manufacturing process for an optical waveguide according to an embodiment, front views showing an example of a manufacturing process for an optical waveguide according to an embodiment, cross-sectional views showing an example of a manufacturing process for a waveguide-mounted substrate according to an embodiment, and

[0010] The optical waveguide and the waveguide mounting substrate according to the embodiments of the present invention will be described with reference to the drawings. In each of the drawings referred to in the following description, certain parts may be drawn enlarged to facilitate understanding of the disclosed embodiments. Therefore, the size and length of each component may not be drawn to exact proportions.

[0011] <Structure of Optical Waveguide of First Embodiment> Fig. 1 shows a plan view of an optical waveguide 1, which is an example of an optical waveguide of the first embodiment. Fig. 2 shows a cross section of the optical waveguide 1 of Fig. 1 taken along line II-II. Fig. 3 shows a cross section of the optical waveguide 1 of Fig. 1 taken along line III-III, and Fig. 4 shows a cross section of the optical waveguide 1 of Fig. 1 taken along line IV-IV. Note that the optical waveguide 1 illustrated in Fig. 1 and other figures is merely an example of an optical waveguide of the embodiment. The structure of the optical waveguide of the embodiment is not limited to the structure shown in each drawing, such as Fig. 1, referred to herein.

[0012] 1 to 4, an optical waveguide 1 according to an 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 formed on the lower clad 21. The upper clad 22 is formed on the lower clad 21 and the core 3. That is, in the optical waveguide 1, the lower clad 21, the upper clad 22, and the core 3 are formed in this order: lower clad 21, core 3, upper clad 22. As shown in FIG. 2, the core 3 and the upper clad 22 are in contact with each other at an interface I.

[0013] The core 3 has an upper surface 31 facing in the same direction as the arrangement of the lower cladding 21, the core 3, and the upper cladding 22, and a lower surface 32 opposite the upper surface 31. The direction in which the lower cladding 21, the core 3, and the upper cladding 22 are arranged in order is also referred to as the "stacking direction" or "Z direction."

[0014] 2, the upper surface 31 of the core 3 is a surface facing the +Z direction, and the lower surface 32 is a surface facing the −Z direction. Hereinafter, in the optical waveguide 1, the lower clad 21 side is also referred to as the “lower side” or simply “bottom”, and the upper clad 22 side is also referred to as the “upper side” or simply “top”.

[0015] 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. As shown in FIG. 4 , the upper cladding 22 also covers the upper surface 211 of the lower cladding 21 and the side surface 35 of the core 3.

[0016] In the optical waveguide 1 in FIGS. 1 and 2 , the core 3 is formed 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. The propagation direction of an optical signal, the +X direction and the −X direction, are also collectively referred to as the “X direction.” The optical waveguide 1 has two opposing ends, one end 11 and the other end 12. 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 the optical signal is emitted from the other end 12 or the one end 11. When an optical signal is incident on the one end 11 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.

[0017] Therefore, one end 11 and the other end 12 of the optical waveguide 1 serve as the input and output sides for an optical signal. In the optical waveguide 1, a part of the core 3 on the one end 11 side is exposed. Furthermore, a part of the core 3 on the other end 12 side is exposed.

[0018] As shown in FIGS. 2 and 3, at one end 11, a top surface 31, an end surface 33, and a side surface 35 of the core 3 are exposed.

[0019] 1 and 2 , an end face 34 of the core 3 is exposed at the end face 12. Note that, like the one end 11 side, the top face 31, the end face 34, and the side face 35 of the core 3 may also be exposed at the other end 12 side.

[0020] 1 and 2, the optical waveguide 1 is formed with an upper cladding non-forming region 1a, which is a region where the upper cladding 22 is not formed in a plan view, and an upper cladding forming region 1b, which is a region where the upper cladding 22 is formed, in a plan view. The upper cladding non-forming region 1a is located at one end of the optical waveguide 1. The upper cladding forming region 1b is located adjacent to the upper cladding non-forming region 1a. Note that "plan view" means viewing an object with a line of sight along the Z direction.

[0021] In the upper cladding non-forming region 1a, the upper surface 31 of the core 3 and the upper surface 211 of the lower cladding 21 are exposed. Therefore, the upper cladding non-forming region 1a has a core-exposed portion 3a where the upper surface 31 of the core 3 is exposed.

[0022] In the upper cladding forming region 1b, the upper surface 31 of the core 3 is not exposed. Therefore, the upper cladding forming region 1b has a non-core exposed portion 3b where the upper surface 31 of the core 3 is not exposed.

[0023] In the optical waveguide 1, the exposed core portion 3a is located on the one end 11 side. The unexposed core portion 3b is located adjacent to the exposed core portion 3a. The exposed core portion 3a may also be located on the other end 12 side. In the example of Fig. 1, the side surface 35 of the core 3 is also exposed in the upper cladding non-forming region 1a.

[0024] 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. The thickness of the core 3 is determined as the average value of length measurements taken at three points in the Z direction on an SEM photograph.

[0025] The thickness of the lower cladding 21 is not particularly limited, but is 5 μm or more and 50 μm or less, and preferably 20 μm to 40 μm. The thickness of the upper cladding 22 is not particularly limited, but is 5 μm or more and 40 μm or less, and preferably 10 μm to 30 μm. The thicknesses of the lower cladding 21 and the upper cladding 22 are determined as the average values ​​of measurements taken at three points in the Z direction on an SEM photograph.

[0026] 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. In other words, the core 3 is positioned with respect to each optical component so as to have a positional relationship that allows transmission and reception of optical signals between the core 3 and these optical components. Note that "optically coupled" is also referred to as "optically coupled" hereinafter.

[0027] 1 to 3, 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 optically coupled to the core 3 at one end 11. When the optical waveguide 1 is in use, the region between the component E1 and the portion of the optical waveguide 1 that overlaps with the component E1 in a plan view is preferably filled with any optically transparent transmissive resin TR. The transmissive resin TR adjusts the refractive index of the space between the component E1 and the optical waveguide 1 to be more appropriate than that of air.

[0028] The length L1 of the core 3 in the core exposed portion 3a is not particularly limited, but 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 length L1 of the core 3 in the core exposed portion 3a is within the above range, it is considered that there is a high degree of freedom in selecting optical components such as the component E1 optically coupled to the core exposed portion 3a. It is also considered that a necessary and sufficient tolerance can be obtained for the alignment of the optical component and the optical waveguide 1 in the X direction.

[0029] The component E1 includes an optical terminal E1a, which is a portion where an optical signal enters the component E1 or a portion where an optical signal exits the component E1. The component E1 is optically coupled to the core 3 at the optical terminal E1a. An optical signal propagating through the core 3 from the other end 12 enters the component E1 via the optical terminal E1a at one end 11. Meanwhile, an optical signal exiting from the optical terminal E1a of the component E1 enters the core 3 at the one end 11, propagates within the core 3, and exits from the other end 12.

[0030] The exposed core portion 3a of the optical waveguide 1 is a portion that overlaps with the optical terminal E1a of the component E1 and transmits and receives optical signals. In the example of FIGS. 1 to 3, the core 3 is positioned at one end 11 of the optical waveguide 1 so that the upper surface 31 of the exposed core portion 3a faces the optical terminal E1a of the component E1 and is adiabatic coupled. That is, a portion of the optical signal that has propagated through the core 3 toward the one end 11 leaks out of the core 3 from the upper surface 31 as evanescent light and enters the optical terminal E1a of the component E1. Because the upper surface 31 faces the optical terminal E1a of the component E1 without the cladding 2 passing through, it is believed that highly efficient optical coupling is achieved.

[0031] The optical waveguide 1 shown in FIGS. 1 to 4 has four parallel cores 3. The optical waveguide of the embodiment is not limited to four cores 3 and can 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 not particularly limited, but 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 center of one core 3 and the center of the other core 3 of two adjacent cores 3.

[0032] The core 3 of the optical waveguide 1 has a width W1 at the core exposed portion 3a. The width W1 of the core 3 at the core exposed portion 3a is not particularly limited, but is, for example, 2 μm or more and 15 μm or less. The width W1 of the core 3 at the core exposed portion 3a is desirably 3 to 10 μm. The core 3 has a width W2 at the core non-exposed portion 3b. In the optical waveguide 1 shown in FIG. 1 and other figures, the width W1 and the width W2 are approximately equal. That is, the core 3 of the optical waveguide 1 has an approximately constant width from one end 11 to the other end 12 of the optical waveguide 1. The width W1 and the width W2 may have a relationship of W1 = W2. However, in the optical waveguide of the embodiment, the width of the core at the core exposed portion 3a may be different from the width of the core at the core non-exposed portion 3b, as in the optical waveguide 1α of FIG. 7, which will be referred to later. The width of the core 3 is determined by the average value of values ​​measured at three points in the X direction.

[0033] The "width" of the core 3, such as width W1 and width W2, refers to the length of the core 3 in a direction perpendicular to the propagation direction of the optical signal propagating within the core 3, which is the X direction in FIG. 2, and the Z direction, which is the lamination direction of the optical waveguide 1. 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 position within 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.

[0034] 1 to 4, the widths W1 and W2 of the core 3 are 5 μm, the thickness of the core 3 is 5 μm, the thickness of the lower clad 21 is 30 μm, the arrangement pitch P1 of the core 3 is 50 μm, the thickness of the upper clad 22 is 20 μm, and the length L1 of the core 3 at the core exposed portion 3 a is 2000 μm. Note that the widths of the core 3, the thickness of the core 3, the arrangement pitch of the core 3, the length of the core 3 at the core exposed portion 3 a, the thickness of the lower clad 21, and the thickness of the upper clad 22 shown here are merely examples, and the dimensions of the parts of the core 3 and the thickness of each clad are not limited to the numerical values ​​shown here.

[0035] <Materials for forming the optical waveguide> The core 3 and clad 2 that form the optical waveguide 1 are made of a light-transmitting material. A resin material can be used for the core 3 and the clad 2. The optical waveguide 1 made of a resin material is lightweight, has high toughness, and can be flexible.

[0036] The optical waveguide of the embodiment is formed of any thermosetting resin, any photocurable resin, or a resin containing both. That is, in the examples of Figures 1 to 4, the core 3, the lower clad 21, and the upper clad 22 are formed of any thermosetting resin, photocurable resin, or a resin containing both.

[0037] The resin constituting the core 3 and the clad 2 is composed of, for example, one type of polymer or two or more types of polymers. The polymer of the resin constituting the core 3 and the clad 2 may be a copolymer, and this copolymer may be a copolymer with any bonding mode, such as a block copolymer or an alternating copolymer.

[0038] The resin constituting the core 3 and the clad 2 is preferably a mixture or miscible mixture of two or more polymers or copolymers. The resin constituting the core 3 and the clad 2 is preferably a resin formed by mixing or miscible mixture of two or more polymers, or a resin formed by mixing or miscible mixture of two or more copolymers. The various copolymers referred to as "two or more copolymers" include copolymers differing in the number (degree of polymerization) or bonding mode of the monomers constituting each copolymer.

[0039] The core 3 and the clad 2 may be made of either a photocurable resin or a thermosetting resin that is made by mixing two or more types of polymers or two or more types of copolymers.

[0040] The refractive index of the resin that constitutes the core 3 and the clad 2 is preferably adjusted. The refractive index of the resin that constitutes the core 3 and the clad 2 is preferably adjusted to 1.4 to 1.6. By making the refractive index of the resin of the optical waveguide 1 equal to or greater than 1.4 and equal to or less than 1.6, optical transmission becomes easier. Furthermore, the refractive index of the resin that constitutes the core 3 and the clad 2 is more preferably less than 1.5. More specifically, the refractive index of the resin that constitutes the core 3 and the clad 2 is preferably equal to or greater than 1.4 and less than 1.5. It is believed that an optical waveguide 1 having a core 3 and a clad 2 that are made of resin with an appropriately adjusted refractive index will have less leakage of optical signals propagating within the core 3, resulting in efficient optical signal transmission.

[0041] The resin constituting the core 3 and the cladding 2 has a refractive index adjusted to a predetermined value. Specifically, the predetermined value of the refractive index is a value within the range of 1.4 to 1.6, and is a value that allows for a refractive index difference between the core 3 and the cladding 2. As an example, the predetermined value of the refractive index is considered to be determined by the size of the core 3 of the optical waveguide 1, the form of light transmission within the core 3, and the design of external connections such as optical elements or optical fibers that are optically coupled to the optical waveguide 1. By adjusting the refractive index to a predetermined value, when an optical signal is transmitted within the optical waveguide 1, the optical signal can be transmitted normally without leaking from the optical waveguide 1.

[0042] The refractive index of the resin constituting the core 3 is set higher than the refractive index of the resin constituting the cladding 2. This allows optical signals to be transmitted normally within the core 3. The difference in refractive index is preferably 0.03 or more, and more preferably 0.05 or more.

[0043] That is, in the optical waveguide 1 of the embodiment, the refractive index of the core 3 is preferably 0.03 or more higher, and more preferably 0.05 or more higher, than the refractive indexes of the lower cladding 21 and the upper cladding 22. When there is a refractive index difference of 0.03 or more between the core 3 and the lower cladding 21 and the upper cladding 22, it is thought that transmission of an optical signal propagating in the core 3 to the cladding 2 is reduced, and good transmission properties are obtained.

[0044] In the optical waveguide 1 of the embodiment, the glass transition point of the resin in the resin material constituting the upper clad 22 is Tg1, and the glass transition point of the resin in the resin material constituting the core 3 is Tg2. In this case, it is preferable that the glass transition point Tg1 of the resin in the resin material constituting the upper clad 22 is different from the glass transition point Tg2 of the resin in the resin material constituting the core 3. The difference between the glass transition points Tg1 and Tg2 makes it easier for the resin in the core 3 and the resin in the upper clad 22 to be compatible at the interface I between the core 3 and the upper clad 22. By making the resin in the core 3 and the resin in the upper clad 22 compatible, adhesion at the interface I is obtained. Note that the glass transition point of the lower clad 21 is Tg3, and the glass transition point Tg3 of the lower clad 21 may be the same as the glass transition point Tg1 of the upper clad 22, or may have a glass transition point different from that of the upper clad 22.

[0045] The glass transition point Tg1 of the resin constituting the cladding 2 and the glass transition point Tg2 of the resin constituting the core 3 are not particularly limited, but are preferably 80 to 270°C. Furthermore, the absolute value of the temperature difference |Tg1-Tg2| between the glass transition point Tg1 of the upper cladding 22 and the glass transition point Tg2 of the core 3 preferably satisfies formula 1: 0°C<|Tg1-Tg2|≦15°C (Formula 1)

[0046] By setting the absolute value |Tg1-Tg2| within the range of formula 1, the core 3 and the upper clad 22 can be made to be compatible with each other at the interface I. By making the core 3 and the upper clad 22 compatible with each other, adhesion at the interface I between the core 3 and the upper clad 22 is ensured, and it is thought that the shape of the core 3 is not affected.

[0047] The glass transition temperature Tg1 of the upper cladding 22 is preferably higher than the glass transition temperature Tg2 of the core 3. Furthermore, the temperature difference between the glass transition temperature Tg1 of the upper cladding 2 and the glass transition temperature Tg2 of the core 3, i.e., Tg1-Tg2, preferably satisfies formula 2: 0°C<Tg1-Tg2≦15°C (formula 2).

[0048] By setting the temperature difference: Tg1-Tg2 within the range of formula 2, the upper cladding 22 and the core 3 can be made to be compatible with each other at the interface I. By making the upper cladding 22 and the core 3 compatible with each other, adhesion at the interface I between the upper cladding 22 and the core 3 is ensured. In addition, it is thought that there will be no effect on the shape of the core 3.

[0049] The resin constituting the core 3 and the resin constituting the clad 2 are a mixture or miscibility of two or more types of polymers or copolymers. The resin constituting the core 3 and the resin constituting the clad 2 may also be a combination of two or more resins. Examples of combinations of two or more resins include a combination of thermosetting resins, a combination of photocurable resins, and a combination of both thermosetting and photocurable resins, but any combination is acceptable. Furthermore, the resin constituting the core 3 and the resin constituting the clad 2 may also be a mixture or miscibility of two or more types of polymers or copolymers, with the refractive index adjusted. Examples of these resins include the resins exemplified below. The resins described below are merely examples, and other examples are also included.

[0050] Examples of resins in which two or more polymers or copolymers are mixed or miscible to adjust the refractive index include the resins exemplified below. Note that the resins described below are only examples, and other examples of resins may also be included in the resins in which two or more polymers or copolymers are mixed or miscible to adjust the refractive index.

[0051] When the polymer or copolymer resin component as the repeating structure is the same component, two types of polymers or copolymers that are the same as the resin component but have one or more different bonding groups are mixed or mixed together to adjust the refractive index. Example 1 exemplified a resin in which the refractive index is adjusted by adjusting the resin. An example of two types of polymers or copolymers as in Example 1 is a combination of two resins AA, each of which is a composition AA, where the repeating structure is A and the resin component AA has a different bonding group at the end. Here, the resin AA is a combination of resins in which the resin component AA has the same bonding group at the end, but has a different bonding group at the end. A specific example is a combination of a composition AA, where the repeating structure is A, and a composition AAX, where the polymer or copolymer composition AAX has a bonding group at the end of the composition AA: X, and a composition AA, where the repeating structure is A, and a polymer or copolymer composition AAY has a bonding group at the end of the composition AA: Y (where Y ≠ X). The resin combination of Example 1 also has a refractive index adjusted to a predetermined value.

[0052] Furthermore, when the resin components in the polymer or copolymer as the repeating structure are different, an example of a resin in which a resin AA is a composition AA that is a polymer or copolymer that is resin component AA, and a resin BB is a composition BB that is a polymer or copolymer that is resin component BB that is a resin component BB whose repeating structure is B, are mixed or mixed together is exemplified as Example 2. Resin BB, which is an example of resin component BB of two types of polymers or copolymers as in Example 2, is a resin component in which the repeating structure is structure B, which is different from the repeating structure A of resin component AA. Furthermore, the refractive index of the resin combination of Example 2 is adjusted to a predetermined value.

[0053] When there are three types of polymer or copolymer resin components as a repeating structure, a resin in which the three types are combined, mixed or miscible, and the refractive index is adjusted is exemplified as Example 3. Some examples of a mixture or miscibility of three or more polymers or copolymers include a combination of resins in which all the resin components are the same but the bonding groups at the ends or middle of the resin components are somewhat different, a combination of two or more polymers or copolymers in which the resin components are the same resin but the resin components of the two or more polymers or copolymers are different, and a combination of all the polymers or copolymers in which the resin components are different are exemplified as Example 3. An example of three types of polymers or copolymers such as Example 3 is preferably a mixture or miscibility of the three or more polymers or copolymers constituting these resins.

[0054] An example of a combination of resins in which all the resin components in Example 3 are the same, but the bonding groups at the ends or middle of the resin components are somewhat different is a combination of three resins AA, each of which is a composition AA, and which is a resin component AA whose repeating structure is A. Here, the resin AA is a combination of resins that are the same but have different terminal bonding groups. A combination of resin AAX, which is a resin component AA (composition AA) whose repeating structure is A and a polymer or copolymer whose terminal bonding group is X, resin AAY, which is a resin component AA (composition AA) whose repeating structure is A and a polymer or copolymer whose terminal bonding group is Y (note that Y ≠ X), and resin AAZ, which is a resin component AA (composition AA) whose repeating structure is A and a polymer or copolymer whose terminal bonding group is Z (note that Z ≠ Y).

[0055] An example of a combination of two or more resins in which the resin components of two or more polymers or copolymers are the same resin and two or more resins in which the resin components of two or more polymers or copolymers are different is a combination of two resins: resin AA, which is composition AA, which is resin component AA, in which the repeating structure is A; and resin BB, which is composition BB, which is resin component BB, in which the repeating structure is B. Here, resin AA is a combination of resins in which the resin components AA are the same but have different terminal bonding groups. The combination is: resin AAX, which is composition AA, which is resin component AA, in which the repeating structure is A, and composition AAX, which is a polymer or copolymer in which the terminal bonding group is X; resin AAY, which is composition AA, which is resin component AA, in which the repeating structure is A, and composition AAY, which is a polymer or copolymer in which the terminal bonding group is Y (note that Y ≠ X); and resin BB, which is composition BB, which is a polymer or copolymer in which the repeating structure is B.

[0056] When the resin components of the polymer or copolymer as the repeating structure are different, three resins with different resin components are combined. The combination is: Resin AA, which is composition AA, which is a polymer or copolymer with resin component AA; Resin BB, which is composition BB, which is a polymer or copolymer with resin component BB, which has repeating structure B; and Resin CC, which is composition CC, which is a polymer or copolymer with resin component CC, which has repeating structure C. The refractive indexes of these three exemplary resin combinations are adjusted to predetermined values.

[0057] However, the resin constituting the core 3 and the clad 2 in each of the above examples is either a thermosetting resin, a photocurable resin, or a mixed resin of a thermosetting resin and a photocurable resin. For example, if a thermoplastic resin is used as the resin constituting the core 3 and the clad 2, a gap may be generated between the resin constituting the core 3 and the resin constituting the clad 2 at the interface between the core 3 and the clad 2, which may cause problems such as peeling of the core 3.

[0058] By using a resin in which two or more polymers or copolymers are mixed or compatible for the core 3 and cladding 2, an optical waveguide with desired characteristics can be formed. A resin in which two or more polymers or copolymers are mixed or compatible is a resin in which the resin components are a combination of two identical components. For example, when resin AAX and resin AAY are used as two or more polymer or copolymer resins, resin AAX has excellent light transmittance, film-forming properties, heat resistance, etc., while resin AAY has excellent light transmittance, resolution, chemical resistance, etc., resulting in a combination of resins with different properties. By appropriately adjusting the blending ratio of resin AAX and resin AAY, the amount of curing agent and other additives added to each resin, and the mixing conditions, a mixed or compatible resin can be obtained. This resin becomes a material with predetermined properties such as light transmittance, refractive index, and glass transition point. By converting this resin into a material with predetermined properties, a material suitable for an optical waveguide can be obtained.

[0059] In addition, the refractive index of the resin that is a combination of two of the same components is adjusted to a predetermined value. Furthermore, in the case where the resin that constitutes the core 3 and the resin that constitutes the cladding 2 are a combination of two of the same components, the refractive index of each of the core 3 and the cladding 2 is adjusted to a predetermined value.

[0060] A resin in which two or more polymers or copolymers are mixed or compatible is a resin in which the resin components are a combination of two different components. An example of a combination of two different components is when resin AA and resin BB are used as two or more polymer or copolymer resins. For example, resin AA is excellent in translucency, film-forming properties, heat resistance, etc., while resin BB is excellent in translucency, resolution, chemical resistance, etc. These resins have different properties. By appropriately adjusting the blending ratio of resin AA and resin BB, the amount of curing agent and other additives added to each resin, and the mixing conditions, a mixed or compatible resin can be obtained. This resin becomes a material with predetermined properties such as translucency, refractive index, and glass transition point. By converting this resin into a material with predetermined properties, a material suitable for optical waveguides can be obtained.

[0061] A resin in which two or more types of polymers or copolymers are mixed or miscible is a resin in which the resin components are a combination of three. Furthermore, a resin combination of three types of polymers or copolymers will be described. For convenience, the three resins will be described as resin α, dendrite β, and resin δ. There are three resin combinations of three types of polymers or copolymers. The first is a case in which resin α, dendrite β, and resin δ are all the same resin component. The second is a case in which two of resins α, dendrite β, and resin δ are the same resin component, and the remaining resin is a different resin component from these two. The third is a case in which resin α, dendrite β, and resin δ are all different resin components.

[0062] Even in these combinations, resins α, β, and δ are combined, each with its own unique characteristics, such as excellent light transmittance, film-forming properties, heat resistance, resolution, and chemical resistance. By appropriately adjusting the blending ratios of resins α, β, and δ, the amounts of curing agents and other additives added to each resin, and the mixing conditions, a mixed or compatible resin can be obtained. This resulting resin is a material with predetermined properties, such as light transmittance, refractive index, and glass transition point. By converting this resulting resin into a material with predetermined properties, a material suitable for optical waveguides can be obtained.

[0063] In addition, even when the resin combination is four or more types, it is possible to obtain a material suitable for an optical waveguide by utilizing the properties of each resin and combining them to create a material with predetermined properties. The resins AA, BB, and CC shown as examples are formed from any thermosetting resin or any photocurable resin. The combinations shown in examples 1 to 3 are any thermosetting resin, any photocurable resin, or resins containing both. Furthermore, resins with a combination of four or more types have their refractive indexes adjusted to predetermined values. Furthermore, when the resins constituting the core 3 and the resins constituting the clad 2 are combined in a combination of four or more types, the refractive indexes of the core 3 and the clad 2 are adjusted to predetermined values, respectively.

[0064] In optical waveguides, it is necessary to provide a difference in refractive index between the core and the cladding. This difference in refractive index can sometimes be achieved by appropriately selecting and using resins in which two or more polymers or copolymers are mixed or miscible at appropriate blending ratios and mixing conditions as the core and cladding materials. Furthermore, mixing or miscible two or more polymers or copolymers can also prevent deterioration of the optical waveguide material due to mixing or miscibility from its preparation to use. A material with properties suitable for optical waveguides, such as desired light transmittance and refractive index, can be obtained by mixing or miscible three or more polymers or copolymers, rather than only two polymers or copolymers.

[0065] The resin constituting the core 3 and the clad 2 is preferably a mixture or miscible mixture of two or more polymers or copolymers, and has an adjusted refractive index. The refractive index of the resin constituting the core 3 and the clad 2 of the optical waveguide 1 of the embodiment is preferably adjusted to 1.4 to 1.6. By setting the refractive index of the resin of the optical waveguide 1 to be 1.4 or more and 1.6 or less, optical transmission becomes easier. Furthermore, the refractive index of the resin constituting the core 3 and the clad 2 is more preferably 1.4 or more and less than 1.5. It is believed that an optical waveguide 1 having a core 3 and a clad 2 made of a resin with an appropriately adjusted refractive index will have less leakage of optical signals propagating within the core 3, resulting in efficient optical signal transmission.

[0066] Furthermore, in order to ensure normal transmission of an optical signal within the core 3, the refractive index of the resin constituting the core 3 is made higher than the refractive index of the resin constituting the cladding 2. The difference in refractive index is preferably 0.03 or more, and more preferably 0.05 or more. That is, in the optical waveguide 1 of the embodiment, the refractive index of the core 3 is preferably 0.03 or more higher than the refractive indexes of the lower cladding 21 and the upper cladding 22, and more preferably 0.05 or more higher. It is believed that when there is a refractive index difference of 0.03 or more between the core 3 and the lower cladding 21 and the upper cladding 22, transmission of an optical signal propagating within the core 3 to the cladding 2 is reduced, resulting in good transmission properties.

[0067] In the optical waveguide 1, it is sometimes required that the glass transition temperature Tg1 of the resin constituting the cladding 2 is different from the glass transition temperature Tg2 of the resin constituting the core 3. The temperature difference in the glass transition temperatures can sometimes be achieved by appropriately selecting and using, as the materials for the core 3 and the cladding 2, resins in which two or more polymers or copolymers are mixed or miscible at appropriate blending ratios and mixing conditions. The difference in the glass transition temperature Tg1 and the glass transition temperature Tg2 allows the upper cladding 22 and the core 3 to be miscible at the interface I. By making the upper cladding 22 and the core 3 miscible, adhesion at the interface I is ensured.

[0068] Furthermore, it is preferable that the absolute value of the temperature difference |Tg1-Tg2| between the glass transition point Tg1 of the upper cladding 22 and the glass transition point Tg2 of the core 3 satisfies the following formula: 0°C<|Tg1-Tg2|≦15°C (Formula 1)

[0069] By setting the absolute value |Tg1-Tg2| within the range of formula 1, the upper cladding 22 and the core 3 can be made to be compatible with each other at the interface I. By making the upper cladding 22 and the core 3 compatible with each other at the interface I, it is believed that adhesion between the upper cladding 22 and the core 3 at the interface I is ensured and the shape of the core 3 is not affected.

[0070] The glass transition temperature Tg1 of the upper cladding 22 is preferably higher than the glass transition temperature Tg2 of the core 3. Furthermore, the temperature difference (Tg1-Tg2) between the glass transition temperature Tg1 of the upper cladding 22 and the glass transition temperature Tg2 of the core 3 preferably satisfies the following formula 2: 0°C<Tg1-Tg2≦15°C (Formula 2)

[0071] By setting the temperature difference: Tg1-Tg2 within the range of formula 2, the upper cladding 22 and the core 3 can be made to dissolve at the interface I. By making the upper cladding 22 and the core 3 dissolve at the interface I, it is believed that adhesion at the interface I between the upper cladding 22 and the core 3 is ensured and the shape of the core 3 is not affected.

[0072] Examples of resin materials for forming the core 3 and the clad 2 include acrylic resins, polyimide resins, polyamide resins, polyether resins, phenolic resins, fluorine-based resins, silicone resins, and epoxy resins. Among these, it is desirable to use acrylic resins, phenolic resins, fluorine-based resins, silicone resins, and epoxy resins, which have refractive indexes that can be easily adjusted.

[0073] 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 photolithography or photobleaching, for example.

[0074] <Compatibility Between the Material Forming the Upper Cladding and the Material Forming the Core> Figure 5 shows an enlarged cross section of one core 3 and the cladding 2 near the core 3 in the optical waveguide of the embodiment. In the optical waveguide of the embodiment, the resin forming the upper cladding 22 and the resin forming the core 3 are compatible with each other at the interface I between the upper cladding 22 and the core 3. Figure 5 shows a portion in region S of the core 3 where the resin forming the upper cladding 22 and the resin forming the core 3 are compatible with each other. In the optical waveguide of the embodiment, as shown in region S in Figure 5, the resin forming the upper cladding 22 and the resin forming the core 3 may be compatible with each other not only at the interface I between the upper cladding 22 and the core 3 but also in the vicinity of the interface I. Furthermore, the compatible portion may be formed not only on the top surface 31 of the core 3 but also on the side surface 35 of the core 3.

[0075] The compatibility of the clad resin material forming the upper clad 22 with the core resin material forming the core 3 allows the upper clad 22 and the core 3 to be tightly adhered to each other. That is, although the core resin material and the clad resin material are two separate resins, it is believed that chemical adhesion can be achieved by making them compatible at or near their interface. It is believed that chemical adhesion stabilizes the adhesion between the upper clad 22 and the core 3. In this way, in the optical waveguide of the embodiment, closer adhesion is ensured between the upper clad and the core than in the past. Therefore, it is believed that defects such as core peeling during use of the optical waveguide are suppressed.

[0076] Furthermore, even if the width of the core 3 is reduced, the adhesion between the core 3 and the upper cladding 22 is ensured. Therefore, since the adhesion is obtained, it is thought that problems such as peeling of the core 3 are suppressed. Furthermore, it is thought that loss of the optical signal is not caused in the transmission of the optical signal in the core 3.

[0077] The portion where the upper cladding 22 and the core 3 are compatible with each other is shown in FIG. 5 in the region S where the material forming the upper cladding 22 and the material forming the core 3 are compatible. The portion along the upper surface 31 of the core 3 is referred to as the upper compatible portion, with a thickness Tu of the upper compatible portion, and the portion along the side surface 35 is referred to as the lateral compatible portion, with a thickness Ts of the lateral compatible portion. Here, the thickness Tu of the upper compatible portion and the thickness Ts of the lateral compatible portion may be the same or different. It is preferable that the thickness Tu of the upper compatible portion be thicker than the thickness Ts of the lateral compatible portion. It is believed that a thicker compatible portion near the upper surface 31 ensures better adhesion. It is believed that the adhesion between the upper cladding 22 and the core 3 at the upper surface 31 is more stable.

[0078] The thickness Tu of the upper surface compatible portion is the thickness of the compatible portion of region S in the Y direction of the core 3. The thickness Ts of the side compatible portion is the thickness in the Z direction of the compatible portion in region S along the side surface 35 of the core 3. This thickness is the thickness of the thickest part of the compatible portion.

[0079] The thickness Tu of the top compatible portion and the thickness Ts of the side compatible portion will now be described. The thickness Tu of the top compatible portion and the thickness Ts of the side compatible portion are the same. In other words, it is preferable that the thickness Tu of the top compatible portion and the thickness Ts of the side compatible portion satisfy Equation 3: Tu = Ts ... (Equation 3) By forming compatibility on the top surface 31 and the side surface 35 of the core 3, adhesion between the core 3 and the upper clad 22 is ensured. Therefore, since adhesion is obtained, it is thought that problems such as peeling of the core 3 are suppressed. Furthermore, it is thought that optical signal loss is not caused in the transmission of optical signals through the core 3.

[0080] The thickness Tu of the top compatibility portion and the thickness Ts of the lateral compatibility portion are different. That is, it is preferable that the thickness Tu of the top compatibility portion and the thickness Ts of the lateral compatibility portion satisfy Equation 4: Tu > Ts (Equation 4). In this case, the thickness Ts of the lateral compatibility portion may be constant or may vary downward in the core 3. Formation of the compatibility portion on the top surface 31 and the side surface 35 of the core 3 ensures adhesion between the core 3 and the upper clad 22. Therefore, since adhesion is achieved, problems such as peeling of the core 3 are thought to be suppressed. Furthermore, it is thought that optical signal loss is not caused in the transmission of optical signals through the core 3. Note that, when the thickness Ts of the lateral compatibility portion varies, the thickness Ts of the lateral compatibility portion is the average value of the results of measurements taken at three points by SEM observation.

[0081] The thickness Tu of the upper surface compatible portion along the upper surface 31 of the core 3 in the region S where the material forming the upper cladding 22 and the material forming the core 3 are compatible is not particularly limited, but is, for example, 5 nm to 1000 nm. The thickness Tu of the upper surface compatible portion is preferably 300 nm or more. The ratio of the thickness Tu of the upper surface compatible portion to the thickness Ts of the side surface compatible portion of the core 3 is, for example, 5% to 50%, and preferably 10% to 30%. It is believed that this stabilizes the adhesion between the upper cladding 22 and the core 3 and also stabilizes the transmission of optical signals within the core 3.

[0082] The core resin material constituting the core 3 and the clad resin material constituting the clad 2 are preferably a resin in which two types of polymer resins are mixed or miscible, or a resin in which two types of copolymer resins are mixed or miscible. Furthermore, the core resin material and the clad resin material preferably have the same main resin type. That is, it is more preferable that a resin in which two types of polymer resins are mixed or miscible, or a resin in which two types of copolymer resins are mixed or miscible, is used, and that the main resins in these resins are of the same type. Here, "main resin" refers to the resin with the highest content among the resins contained as components in the core resin material or the clad resin material. It is believed that using resins in which the main resin type is the same for the core resin material and the clad resin material makes it easier to make the core resin material compatible with the clad resin material, or easier to make the clad resin material compatible with the core resin material.

[0083] In order to make the upper cladding 22 and the core 3 compatible at or near the interface I between the upper cladding 22 and the core 3, it is desirable that the glass transition point Tg1 of the resin constituting the upper cladding 22 is different from the glass transition point Tg2 of the resin constituting the core 3. The difference between the glass transition points Tg1 and Tg2 allows the upper cladding 22 and the core 3 to be compatible at the interface I. By making the upper cladding 22 and the core 3 compatible, adhesion at the interface I between the upper cladding 22 and the core 3 can be obtained.

[0084] In order to make the upper cladding 22 and the core 3 compatible at or near the interface I between the upper cladding 22 and the core 3, it is preferable that the absolute value of the temperature difference |Tg1-Tg2| between the glass transition point Tg1 of the resin constituting the upper cladding 22 and the glass transition point Tg2 of the resin constituting the core 3 satisfies formula 1: 0°C<|Tg1-Tg2|≦15°C (formula 1).

[0085] By setting the absolute value |Tg1-Tg2| within the range of formula 1, the upper cladding 22 and the core 3 can be made to be compatible with each other at the interface I. By making the upper cladding 22 and the core 3 compatible with each other at the interface I, it is thought that adhesion between the upper cladding 22 and the core 3 at the interface I can be obtained and the shape of the core 3 is not affected.

[0086] Furthermore, in order to make the upper cladding 22 and the core 3 compatible at or near the interface I between the upper cladding 22 and the core 3, it is desirable that the glass transition temperature Tg1 of the resin constituting the upper cladding 22 is higher than the glass transition temperature Tg2 of the resin constituting the core 3. In order to make the upper cladding 22 and the core 3 compatible at or near the interface I, it is desirable that the temperature difference Tg1 - Tg2 between the glass transition temperature Tg1 of the resin constituting the upper cladding 22 and the glass transition temperature Tg2 of the resin constituting the core 3 satisfies formula 2: 0°C < Tg1 - Tg2 ≦ 15°C (formula 2)

[0087] By setting the temperature difference: Tg1-Tg2 within the range of formula 2, the upper cladding 22 and the core 3 can be made to dissolve together at the interface I. By making the upper cladding 22 and the core 3 dissolve together, it is believed that adhesion at the interface between the upper cladding 22 and the core 3 is ensured and the shape of the core 3 is not affected.

[0088] Figure 6 shows an example of the interface state between the core and the cladding in the optical waveguide of the embodiment. Figure 6 schematically shows an example of gradual compatibility at the interface I between the core 3 and the upper cladding 22 in a portion corresponding to part VI in Figure 5. The portion from the interface I toward the -Z direction of the core 3 is called the interface vicinity N2, and the end face of the interface vicinity N2 is indicated by the dashed line NL2. The portion from the interface I toward the +Z direction of the upper cladding 22 is called the interface vicinity N3, and the end face of the interface vicinity N3 is indicated by the dashed line NL3. Note that the length from the interface I to the interface vicinity N2 is the same as the length from the interface I to the interface vicinity N3.

[0089] The example shown in Figure 6 shows a portion from interface I to near-interface N2 where the clad resin material and the core resin material are compatible. For ease of explanation, the portion from interface I to near-interface N2 is divided into three layers. They are labeled S-1, S-2, and S-3 from the position closest to interface I. In the compatible portion, S-1 has a high degree of compatibility, S-2 has a lower degree of compatibility than S-1, and S-3 has the lowest degree of compatibility. This shows an example in which the degree of compatibility decreases in stages in the compatible portion. The portion from interface I to near-interface N2 may be three layers, or may have a greater or fewer number of layers. The portion in which the core resin material and the clad resin material are compatible is referred to as a compatible layer.

[0090] In this case, the thickness of the compatible portion is not particularly limited, but is preferably 5 to 1000 nm. It is believed that by having the thickness of the compatible portion in the range of 5 to 1000 nm, adhesion between the upper clad 22 and the core 3 can be obtained.

[0091] 6, there is a miscibility example 1 in which a portion where the core resin material and the clad resin material are miscible with each other is formed from the interface I to the interface vicinity N3 on the upper clad 22 side. It is presumed that the degree of miscibility in miscibility example 1 decreases stepwise with increasing distance from the interface I.

[0092] Another example is Compatibility Example 2, in which a portion where the core resin material and the clad resin material are compatible with each other is formed from the interface I to the interface vicinity N3 on the upper clad 22 side and from the interface I to the interface vicinity N2 on the core 3 side. In Compatibility Example 2, too, it is presumed that the degree of compatibility gradually decreases with increasing distance from the interface I. In this case, the length of the interface vicinity N3 from the interface I and the length of the interface vicinity N2 from the interface I may be equal to or different from each other. In either case, the formation of a portion where the core resin material and the clad resin material are compatible with each other provides close contact between the core 3 and the clad 2, particularly between the core 3 and the upper clad 22.

[0093] When the resin material forming the core 3 and the clad 2 is a resin in which two or more types of polymers or copolymers are mixed or mixed together, adhesion between the core 3 and the clad 2 is ensured. By ensuring adhesion, problems such as peeling of the core 3 are unlikely to occur when the optical waveguide 1 is used.

[0094] Furthermore, by using a resin in which two or more types of resins are mixed or miscible to form the core 3 and the cladding 2, it is believed that the cladding resin material and the core resin material are miscible at the interface I between the upper cladding 22 and the core 3 or near the interface I. The formation of a miscible portion (miscible layer) at the interface I or near the interface I ensures adhesion between the upper cladding 22 and the core 3. This makes it less likely that problems such as peeling of the core 3 will occur. Furthermore, even if thermal stress due to a temperature difference expected during use of the optical waveguide 1 is transmitted to the interface I between the upper cladding 22 and the core 3, it is believed that the stress will be alleviated near the interface I. This also ensures reliability against peeling between the core 3 and the cladding 2. In this way, two performances, adhesion and reliability, can be obtained.

[0095] Furthermore, if the resin material forming the core 3 and the clad 2 is a resin in which two or more types of polymers or copolymers are mixed or mixed together, and the main resin in the core 3 and the upper clad 22 is the same resin, the core 3 and the upper clad 22 become more easily mixed together, the adhesion becomes stable, and problems such as peeling of the core 3 in the optical waveguide 1 are less likely to occur.

[0096] Furthermore, by using a resin material in which two or more resin components are mixed or miscible with each other and in which the same resin is the main component to form the core 3 and the clad 2, it is believed that the upper clad 22 and the core 3 become more miscible with each other at the interface I between the upper clad 22 and the core 3 or from the interface I to the vicinity of the interface. The formation of a miscible portion ensures adhesion between the core 3 and the clad 2.

[0097] That is, by using the same resin as the main resin material for the upper clad 22 and the core 3, the upper clad 22 and the core 3 can be made to dissolve in each other at the interface I, forming a compatible layer. This allows chemical adhesion to be achieved by making them dissolve in each other. The adhesion between the core 3 and the clad 2 is ensured by the action of chemical adhesion.

[0098] Furthermore, even if thermal stress due to a temperature difference expected during use of the optical waveguide 1 is transmitted to the interface I between the upper cladding 22 and the core 3, it is believed that the stress is alleviated near the interface. It is also believed that peeling between the core 3 and the cladding 2 is suppressed and the reliability of the optical waveguide 1 is ensured even in reliability tests under high temperature and high humidity. Therefore, reliability against peeling between the core 3 and the cladding 2 can also be ensured. As a result, two performances, namely adhesion and reliability, can be obtained.

[0099] As shown in the examples of Figures 1 to 4, there are the following cases (1) to (3): (1) The core 3 is exposed in the upper cladding non-forming region 1a. (2) The core 3 has a fine line width of 10 μm or less. (3) The core 3 has a width of 10 μm or less and is exposed in the upper cladding non-forming region 1a. In the optical waveguide of the embodiment, even if the optical waveguide is configured in these cases, it is believed that three effects can be obtained: ensuring adhesion between the core 3 and the cladding 2, ensuring reliability, and ensuring the transmittance of optical signals.

[0100] 1 to 4, even when the core 3 is exposed in the upper cladding non-forming region 1a, the core 3 has a fine line width of 10 μm or less, and an optical signal is transmitted in a single-mode transmission mode through the core 3, three effects can be obtained: ensuring adhesion between the core 3 and the cladding 2, ensuring reliability, and ensuring the transmittance of the optical signal. Furthermore, it is believed that the transmission of the optical signal in the single mode can also be stabilized.

[0101] <Specific examples of resin materials for core and clad> When the core 3 and clad 2 are formed from a resin composed of one or more types of polymers or a resin composed of one or more types of copolymers, examples of resins that can be used as materials are given below.

[0102] Examples of one type of polymer or copolymer constituting the resin forming the core 3 or the clad 2 include epoxy resin, acrylic resin, phenol resin, silicone resin, fluororesin, etc. Examples of two or more types of polymers or copolymers constituting the resin forming the core 3 or the clad 2 include: a combination of an epoxy resin as the main resin and a resin selected from epoxy resin, acrylic resin, silicone resin, phenol resin, and fluororesin as the other resin; a combination of an acrylic resin as the main resin and a resin selected from epoxy resin, acrylic resin, silicone resin, phenol resin, and fluororesin as the other resin; a combination of a phenol resin as the main resin and a resin selected from epoxy resin, acrylic resin, silicone resin, phenol resin, and fluororesin as the other resin; a combination of a silicone resin as the main resin and a resin selected from epoxy resin, acrylic resin, silicone resin, phenol resin, and fluororesin as the other resin; and a combination of a fluororesin as the main resin and a resin selected from epoxy resin, acrylic resin, silicone resin, phenol resin, and fluororesin as the other resin.

[0103] Among these, it is desirable that the main resin and the other resins are resins of the same family, such as a combination of an epoxy resin as the main resin and an epoxy resin as the other resin. The interaction parameter of these combinations of resins of the same family is small, which may be preferable because they are highly compatible. Note that these resin combinations are merely examples, and other combinations may also be used. Here, "resins of the same family" also means that the resin components are the same.

[0104] The following also illustrates an example of a combination of a polymer constituting the resin forming the core 3 and a polymer constituting the resin forming the clad 2. The polymer forming the core 3 may be a resin whose main resin is any of epoxy resin, acrylic resin, phenol resin, silicone resin, and fluororesin. The polymer forming the clad 2 to be combined with the core 3 may be a resin whose main resin is any of epoxy resin, acrylic resin, phenol resin, silicone resin, and fluororesin.

[0105] Examples of combinations of the copolymer constituting the resin forming the core 3 and the copolymer constituting the resin forming the clad 2 include: a combination of an epoxy resin as the resin constituting the core 3 and a resin selected from epoxy resin, acrylic resin, phenol resin, silicone resin, and fluororesin as the resin constituting the clad 2; a combination of an acrylic resin as the resin constituting the core 3 and a resin selected from epoxy resin, acrylic resin, phenol resin, silicone resin, and fluororesin as the resin constituting the clad 2; a combination of a phenol resin as the resin constituting the core 3 and a resin selected from epoxy resin, acrylic resin, phenol resin, silicone resin, and fluororesin as the resin constituting the clad 2; a combination of a silicone resin as the resin constituting the core 3 and a resin selected from epoxy resin, acrylic resin, phenol resin, silicone resin, and fluororesin as the resin constituting the clad 2; and a combination of a fluororesin as the resin constituting the core 3 and a resin selected from epoxy resin, acrylic resin, phenol resin, silicone resin, and fluororesin as the resin constituting the clad 2.

[0106] Among these, it is desirable to combine resins of the same family, such as a combination of an epoxy resin as the resin constituting the core 3 and an epoxy resin as the resin constituting the clad 2. Combining resins of the same family may be preferable from the viewpoint of ensuring high adhesion between the core 3 and the clad 2, since the compatibility between the resin materials constituting the core 3 and the clad 2 is high. Note that these resin combinations are merely examples, and other combinations may also be used.

[0107] For example, in the optical waveguide of the embodiment, the core 3 is made of an acrylic resin, and the clad 2 is made of an acrylic resin. Alternatively, the resin materials constituting the core 3 and the clad 2 may be epoxy resin, phenol resin, silicone resin, or fluororesin. Furthermore, in a modified example of the optical waveguide of the embodiment, the core 3 may be made of an epoxy resin, and the clad 2 may be made of an epoxy resin. Alternatively, the resin materials constituting the core 3 and the clad 2 may be acrylic resin, phenol resin, silicone resin, or fluororesin. The core 3 and the clad 2 made of a combination of these materials ensure stable adhesion between the core 3 and the clad 2. Defects such as core peeling are suppressed. Furthermore, reliability against core peeling is ensured. Furthermore, it is believed that good optical signal transmission properties can be obtained. Silicone resin is also called silicon resin, and refers to a silane compound, a compound having a siloxane bond, or the like.

[0108] <Modification of the Optical Waveguide of the First Embodiment> A modification of the optical waveguide of the first embodiment will be described with reference to Fig. 7 . Fig. 7 shows a plan view of an optical waveguide 1α, which is a modification of the optical waveguide of the first embodiment. As shown in Fig. 7 , the optical waveguide 1α, like the optical waveguide 1 of Fig. 1 , is composed of an upper cladding non-forming region 1a and an upper cladding forming region 1b. The upper cladding non-forming region 1a has a core-exposed portion 3a where the upper surface of the core 3 is exposed. The upper cladding forming region 1b has a core-unexposed portion 3b where the core 3 is not exposed. In the optical waveguide 1α of Fig. 7 , components having the same functions as those of the optical waveguide 1 of Fig. 1 are denoted by the same reference numerals as those in Fig. 1 or are omitted as appropriate, and repeated descriptions of these similar components will be omitted.

[0109] In the optical waveguide 1α, the width W1 of the core 3 at the core exposed portion 3a and the width W2 of the core 3 at the core non-exposed portion 3b are different from each other. In the optical waveguide 1α, the width W1 of the core 3 at one end 11 is greater 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 optical terminal E1a of the component E1 is greater than the difference between the width W2 of the core non-exposed portion 3b and the width We of the optical terminal E1a. Therefore, in the optical waveguide 1α, the tolerance for misalignment along the width of the core 3 when aligning the optical signal input / output portion of an optical component, such as the optical terminal E1a, with the core 3 is greater than when the core 3 has the same width at the core exposed portion 3a as the width W2 of the core non-exposed portion 3b. Therefore, in the optical waveguide 1α, it is considered that alignment for appropriate optical coupling between the optical component and the core 3 is easy. Therefore, it is considered that high coupling efficiency can be easily achieved when coupling with an optical component. It is also believed that poor coupling due to misalignment between the core 3 and optical components such as the component E1 is suppressed even with the thermal history during use.

[0110] Additionally, in the optical waveguide 1α, the unexposed core portion 3b has a width W2 smaller than the width W1, which may facilitate realizing a desired transmission mode in the unexposed core portion 3b. For example, in a light-guiding member such as the optical waveguide 1α, the cross-sectional size of an optical path, such as the core 3, is selected based on the refractive index difference between the material constituting the optical path and the material constituting the covering body, such as the cladding 2, surrounding the optical path. That is, by realizing a core 3 having a cross-sectional size appropriate for the refractive index of each material, an optical signal can be propagated in a desired transmission mode. In this regard, in the optical waveguide 1α, 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. Instead, the unexposed 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 is easily realized. For example, single-mode optical transmission, which requires a core diameter of 3 to 10 μm, may be possible.

[0111] The width W1 of the core 3 in the core exposed portion 3a may be constant or may vary depending on the position in the core exposed portion 3a in the X direction. In the optical waveguide 1α in Fig. 7, the maximum width (W1m) of the width W1 of the core 3 in the core exposed portion 3a is wider than the width W2 of the core 3 in the core unexposed portion 3b. That is, in the optical waveguide 1α, the maximum width W1m of the core exposed portion 3a and the width W2 of the core unexposed portion 3b satisfy the relationship of the following formula A: W1m>W2 (Formula A)

[0112] An example of the dimensions of each portion of the optical waveguide 1α shown in Figure 7 is shown below. The width W1 of the core exposed portion 3a and the width W2 of the core unexposed portion 3b are not particularly limited, 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 3 in the core unexposed portion 3b satisfy the relationship of the following formula B: 1.0 < (W1 / W2) ≤ 3.0 (formula B)

[0113] The width W1 of the core 3 in the core exposed portion 3a is not particularly limited, but is preferably 3 μm or more and 30 μm or less, and more preferably 5 μm or more and 20 μm or less. If the core 3 in the core exposed portion 3a has a width in this range, it is thought that the effect of facilitating alignment with an optical component such as component E1 can be obtained, and an excessively large area for optical coupling with the optical component is not required. The width W2 of the core unexposed portion 3b may be, for example, 1 μm or more and 10 μm or less.

[0114] The core width of the core exposed portion 3a may gradually increase or decrease from one end to the other end. When the width of the core exposed portion 3a varies, the core width W1 of the core exposed portion 3a is determined as the average value of length measurements at three points within the core exposed portion 3a in the X direction. The core width of the core unexposed portion 3b may be constant or may vary. As an example, the core width of the core unexposed portion 3b may gradually increase or decrease from one end to the other end. When the width of the core unexposed portion 3b varies, the core width W2 of the core unexposed portion 3b is determined as the average value of length measurements at three points within the core unexposed portion 3b in the X direction.

[0115] 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.

[0116] An example of a combination of dimensions of each part of the optical waveguide 1α is as follows: The width of the core 3 in the core exposed portion 3a is 10 μm, and the thickness of the core 3 is 7 μm. The width of the core 3 in the core non-exposed portion 3b is 7 μm, and the thickness of the core 3 is 7 μm. The thickness of the lower clad 21 may be 20 μm, and the thickness of the upper clad 22 may be 25 μm. Note that the width and thickness of the core 3, as well as the thickness of the lower clad 21 and the thickness of the upper clad 22 exemplified here are merely examples, and the dimensions of each part of the core 3 and the thickness of each clad are not limited to the numerical values ​​exemplified here.

[0117] In the optical waveguide 1α shown in Fig. 7, the core 3 has a connecting portion 3c between the exposed core portion 3a and the non-exposed core portion 3b, connecting the exposed core portion 3a and the non-exposed core portion 3b. As shown in Fig. 7, the connecting portion 3c refers to the core 3 formed in the upper cladding non-forming region 1a between the exposed core portion 3a and the non-exposed core portion 3b. Here, the connecting portion 3c may have a taper in plan view, may be arranged as a straight line of a constant width, or may be arranged as a combination of a straight line of a constant width and a taper. The tapered shape of the core 3 at the connecting portion 3c may be such that the width gradually increases linearly or smoothly on both sides from the non-exposed core portion 3b to the exposed core portion 3a, or may be such that the width gradually increases linearly on only one side, or may be such that the width gradually increases stepwise on both sides or only one side.

[0118] The core width W3 of the connecting 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 other words, the core widths W1, W2, and W3 satisfy the relationship of the following formula C: Core width W1 > Core width W3 ≥ Core width W2 (Formula C)

[0119] 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. In other words, in the optical waveguide 1α, 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.

[0120] 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 RT of the core 3 is preferably 0.01<RT<0.2, and more preferably 0.03<RT<0.15. The tapered shape formation ratio RT is the ratio of the length of the tapered portion to the overall length of the core 3.

[0121] <Structure of Optical Waveguide of Second Embodiment> Figure 8 shows a plan view of an optical waveguide 1β, which is an example of an optical waveguide of the second embodiment. Figure 9 shows a cross section of the optical waveguide 1β taken along line IX-IX of Figure 8. Like the optical waveguide 1 of Figure 1, the optical waveguide 1β includes a core 3 that transmits an optical signal and a clad 2 surrounding the core 3, with the clad 2 being composed of a lower clad 21 and an upper clad 22. The core 3 is formed on the lower clad 21, and the upper clad 22 is formed on the lower clad 21 and the core 3. In the optical waveguide 1β, both the one end 11 and the other end 12 are the input and output sides for the optical signal. Therefore, a portion of the core 3 on the one end 11, which is the input or output side for the optical signal, is exposed, and a portion of the core 3 on the other end 12, which is the input or output side for the optical signal, is exposed. The optical waveguide 1β is formed from a resin containing a thermosetting resin, a photocurable resin, or both, and the resin forming the upper clad 22 and the resin forming the core 3 are compatible with each other at the interface I between the upper clad 22 and the core 3. Therefore, in the optical waveguide 1β as well, adhesion between the core 3 and the clad 2 is ensured.

[0122] Furthermore, in the resin comprising the core 3 and clad 2 of the optical waveguide 1β, which contains a photocurable or thermosetting resin or both, preferably two or more types of polymers or copolymers are mixed or miscible, and the refractive index is adjusted. As in the optical waveguide 1, in the optical waveguide 1β, if the resin material forming the core 3 and clad 2 is a resin in which two or more types of polymers or copolymers are mixed or miscible, adhesion between the core 3 and the clad 2 is ensured. If adhesion is ensured, problems such as peeling of the core 3 are thought to be less likely to occur. The miscible portion may be formed not only on the top surface of the core 3 but also on the side surface of the core 3.

[0123] Furthermore, when the core 3 and the clad 2 are formed from a resin in which two or more types of resin are mixed or miscible with each other, a miscible portion between the core 3 and the clad 2 is likely to be formed at the interface between the core 3 and the clad 2 or in the vicinity of the interface, stabilizing the adhesion between the core 3 and the clad 2. Furthermore, it is believed that thermal stress is alleviated near the interface between the core 3 and the clad 2, ensuring reliability against peeling between the core 3 and the clad 2. In other words, it is believed that the optical waveguide 1β also achieves the two performance characteristics of adhesion and reliability.

[0124] In the optical waveguide 1β, the glass transition temperature Tg1 of the upper clad resin material and the glass transition temperature Tg2 of the core resin material may be different. If the glass transition temperatures of the upper clad resin material and the core resin material are different, the upper clad 22 and the core 3 are more likely to be compatible with each other at the interface I.

[0125] 1 in that it does not have a region like the upper cladding non-forming region 1a of the optical waveguide 1. In other words, the optical waveguide 1β has an upper cladding forming region 1b in which the upper cladding 22 is formed over the entire surface of the optical waveguide 1β.

[0126] Therefore, in a plan view, the optical waveguide 1β does not have a core-exposed portion where the upper surface 31 of the core 3 is exposed, but has only the upper cladding forming region 1b, and has a core-unexposed portion 3b where the upper surface 31 of the core 3 is not exposed. The optical waveguide 1β is entirely composed of the upper cladding forming region 1b. Therefore, the core 3 is exposed from the cladding 2 at the end face of one end 11 and the end face of the other end 12 of the optical waveguide 1β. That is, in the optical waveguide 1β, the end face 33 of the core 3 is exposed at the one end 11, and the end face 34 of the core 3 is exposed at the other end 12.

[0127] When the optical waveguide 1β is in use, it is optically coupled at one end 11 to a photoelectric conversion component such as a semiconductor device including a photoelectric conversion element. In Figures 8 and 9, component E2 is shown by a two-dot chain line as an example of an optical component optically coupled to the core 3 of the optical waveguide 1β at one end 11. Component E2 includes an optical terminal E2a, which is a portion where an optical signal enters or exits. The optical terminal E2a is exposed on a side surface E22 of the component E2, and the optical signal enters or exits at the exposed portion of the optical terminal E2a on the side surface E22. In the optical waveguide 1β, only the end face 33 of the core 3 is exposed from the cladding 2 at one end 11, so the optical waveguide 1β and component E2 are arranged so that the core 3 and the optical terminal E2a are end-to-end connected or back-coupled.

[0128] That is, the optical waveguide 1β and the component E2 are arranged so that the side surface E22 of the component E2 faces one end 11 of the optical waveguide 1β, thereby facing the end face 33 of the core 3 and the exposed portion of the optical terminal E2a at the side surface E22 of the component E2. This arrangement ensures proper optical coupling between the core 3 of the optical waveguide 1β and the optical terminal E2a of the component E2. An optical signal propagating through the core 3 from the other end 12 enters the component E2 via the optical terminal E2a at the one end 11. Meanwhile, an optical signal output from the optical terminal E2a of the component E2 enters the core 3 at the one end 11, propagates through the core 3, and outputs from the other end 12. Although not shown, the region between the component E2 and the optical waveguide 1β is preferably filled with any optically transparent resin.

[0129] In the optical waveguide 1β, the upper clad 22 and the core 3 are also compatible with each other, providing adhesion and reliability. Therefore, the optical waveguide of the embodiment is considered to be useful even in a bonding configuration in which an optical waveguide and an optical component are bonded end-to-end, such as between the optical waveguide 1β and component E2.

[0130] 8 and 9, components having the same functions as those of the optical waveguide 1 shown in Fig. 1 etc. are denoted by the same reference numerals as those used in Fig. 1 or omitted as appropriate, and repeated explanations of those similar components will be omitted. However, the overall configuration of the optical waveguide, as well as the materials, functions, actions, and variations thereof of each component, which have been described for the optical waveguide 1 shown in Fig. 1 etc., can also be applied to the optical waveguide 1β shown in Fig. 8 and 9, except for those related to the upper cladding non-forming region 1a and the core exposed portion 3a.

[0131] <Structure of Waveguide Mounted Substrate According to Embodiments> Next, a waveguide mounted substrate according to an embodiment of the present invention will be described with reference to the drawings. FIG. 10 shows a cross-sectional view of a waveguide mounted substrate 100, which is an example of a waveguide mounted substrate according to the first embodiment. Note that the waveguide mounted substrate 100 shown in FIG. 10 and the waveguide mounted substrates 100α, 100β, and 100γ shown in FIGS. 11 to 14, which will be referred to later, are merely examples of waveguide mounted substrates according to the embodiments. The layered structure of the waveguide mounted substrate according to the embodiments is not limited to the layered structure of the waveguide mounted substrate 100, the waveguide mounted substrate 100α, the waveguide mounted substrate 100β, or the waveguide mounted substrate 100γ shown in FIGS. 11 to 14. Furthermore, the number of conductor layers and the number of insulating layers of the waveguide mounting substrate of the embodiment are not limited to the number of conductor layers and the number of insulating layers included in the waveguide mounting substrate 100, the waveguide mounting substrate 100α, the waveguide mounting substrate 100β, or the waveguide mounting substrate 100γ.

[0132] As shown in FIG. 10 , the waveguide mount board 100 includes a wiring board 110 and an optical waveguide 101 disposed on the wiring board 110. The wiring board 110 includes an insulating layer and a conductor layer formed on the insulating layer. The wiring board 110 includes conductor layers 41 to 43 as conductor layers and insulating layers 51 and 52 as insulating layers. The waveguide mount board 100 has a first surface 100a, which is a mounting surface for a component E1, and a second surface 100b, which is the surface opposite the first surface 100a. The optical waveguide 101 is disposed on the first surface 100a. The waveguide mount board 100 includes a component mounting pad 4, which is a conductor pad included in the conductor layer 41, on the first surface 100a.

[0133] The conductor layers 41 to 43 and the insulating layers 51 and 52 are stacked in the following order from the second surface 100b side toward the first surface 100a side of the waveguide mounting substrate 100: conductor layer 43, insulating layer 52, conductor layer 42, insulating layer 51, conductor layer 41. The conductor layers 41 and 42 are connected by via conductors 7 that penetrate the insulating layer 51. The conductor layers 42 and 43 are connected by via conductors 7 that penetrate the insulating layer 52. The wiring board 110 includes a solder resist 62 that covers the conductor layer 43 and the insulating layer 52, and a solder resist 61 that covers the conductor layer 41 and the insulating layer 51. The wiring board 110 also includes bumps 8 that are connected to each conductor pad 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 waveguide-mounted substrate 100 and an external component (e.g., a motherboard of an electrical device) located on the second surface 100b side. The waveguide-mounted substrate 100 may also be used as a motherboard without the bumps 8.

[0134] 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 materials that can be used to form the insulating layers. Each insulating layer may be formed using any material that can provide insulation between conductor layers within the wiring substrate 110. Although not shown, each insulating layer may include a core made of a reinforcing material such as glass fiber or aramid fiber, or may include an inorganic filler made of fine particles of silica (SiO), alumina, or mullite.

[0135] The solder resists 61 and 62 are formed of, for example, a photosensitive epoxy resin or polyimide resin.

[0136] Examples of conductors constituting the conductor layers 41-43 and via conductor 7 include copper, nickel, and silver, and it is preferable to use copper or an alloy primarily made of copper. Although each of these conductors is depicted as a single layer in Fig. 10 for simplicity, they may have a multilayer structure including two or more films. For example, the conductor layers 41-43 and via conductor 7 may have a two-layer structure including an electroless plated film and an electrolytic plated film.

[0137] An opening 61a is provided in the solder resist 61, and the component mounting pad 4 is 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 110 by any fixing member such as an adhesive.

[0138] 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 to 4 or the optical waveguide 1α shown in FIG. 7. FIG. 10 shows, as an example, an optical waveguide 101 identical to the optical waveguide 1 of FIG. 1. Thus, the optical waveguide 101 of FIG. 10 includes a laminated lower clad 21, a core 3, and an upper clad 22, with a portion of the core 3 exposed on the input or output side of the optical signal. The optical waveguide 101 is formed from a resin containing a thermosetting resin, a photocurable resin, or both, and the resin forming the upper clad 22 and the resin forming the core 3 are compatible with each other at the interface I between the upper clad 22 and the core 3. Since the optical waveguide 101 in FIG. 10 is identical to the optical waveguide 1 in FIG. 1, repeated explanations of the effects obtained by the optical waveguide 101 and the materials and functions of each component of the optical waveguide 101 will be omitted.

[0139] A component E1 is mounted on the waveguide mounting substrate 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 an optical terminal E1a and a ball-shaped electrode E1b. Examples of the component E1 include a light-receiving element such as a photodiode, and a light-emitting element such as a light-emitting diode (LED), an organic light-emitting diode (OLED), a laser diode (LD), and a vertical-cavity surface-emitting laser (VCSEL). When the component E1 is a light-emitting element, the component E1 generates an optical signal based on an electrical signal input to the electrode E1b and emits the optical signal from the optical terminal E1a, which functions as a light-emitting unit, toward the core 3. When the component E1 is a light-receiving element, an electrical signal based on an optical signal input from the optical terminal E1a, which functions as a light-receiving unit, is generated and output from the electrode E1b.

[0140] The component E1 is mounted on the waveguide mounting substrate 100 by connecting the electrode E1b to the component mounting pad 4 using, for example, solder. In Fig. 10, the component E1 is flip-chip mounted. The optical terminal E1a and the exposed portion of the core 3 of the optical waveguide 101 are positioned to face each other and are optically coupled. The gap between the optical terminal E1a and the core 3 facing the optical terminal E1a is filled with an optically transparent transmissive resin TR having an appropriate refractive index.

[0141] In the optical waveguide 101 formed from a resin containing a thermosetting resin or a photocurable resin, or both, in which the resin forming the upper clad 22 and the resin forming the core 3 are compatible with each other, adhesion between the core 3 and the clad 2 is ensured, similar to the optical waveguide 1 in Fig. 1. Furthermore, in the waveguide-mounted substrate 100, if the resin material forming the core 3 and the clad 2 of the optical waveguide 101 is a resin in which two or more types of polymers or copolymers are mixed or compatible with each other, the core 3 and the clad 2 are closely attached to each other, and it is thought that problems such as peeling of the core 3 are unlikely to occur.

[0142] Furthermore, if the core 3 and the upper clad 22 are formed from the same resin in which two or more types of resins are mixed or miscible, a miscible portion between the core 3 and the clad 2 is likely to be formed at the interface between the core 3 and the clad 2 or in the vicinity of the interface, stabilizing the adhesion between the core 3 and the clad 2. Furthermore, it is thought that thermal stress is alleviated near the interface between the core 3 and the clad 2, ensuring reliability against peeling between the core 3 and the clad 2. In other words, the waveguide-mounted substrate 100 may have two performance characteristics: adhesion between the core 3 and the clad 2 and reliability against peeling between the core 3 and the clad 2.

[0143] Waveguide-Mounted Substrate of Second Embodiment FIGS. 11 and 12 show a waveguide-mounted substrate 100α, which is an example of a waveguide-mounted substrate of the second embodiment. FIG. 11 shows a cross-sectional view of the waveguide-mounted substrate 100α, and FIG. 12 shows a plan view of the waveguide-mounted substrate 100α. The waveguide-mounted substrate 100α includes a wiring substrate 110α and optical waveguides 101a and 101b disposed on the wiring substrate 110α. That is, the waveguide-mounted substrate 100α includes two optical waveguides. The optical waveguides 101a and 101b are optical waveguides of the previously described embodiments, such as the optical waveguide 1 shown in FIGS. 1 to 4 or the optical waveguide 1α shown in FIG. 7.

[0144] Therefore, the optical waveguides 101a and 101b include a laminated lower clad 21, a core 3, and an upper clad 22, with a portion of the core 3 exposed on the optical signal input side or output side. The core 3 has an exposed core portion 3a and a non-exposed core portion 3b. The optical waveguides 101a and 101b are formed from a resin containing a thermosetting resin, a photocurable resin, or both, and the resin forming the upper clad 22 and the resin forming the core 3 are compatible with each other at the interface between the upper clad 22 and the core 3. The optical waveguides 101a and 101b are optical waveguides of an embodiment similar to the optical waveguide 1 of FIG. 1 , and therefore repeated explanations of the effects obtained by the optical waveguides 101a and 101b and the materials and functions of the components of the optical waveguides 101a and 101b will be omitted.

[0145] The wiring board 110α has a similar structure to the wiring board 110 in the first embodiment shown in Fig. 10, except that the arrangement of the conductor patterns and via conductors 7 included in each of the conductor layers 41 to 43 is different from that of the wiring board 110 in the first embodiment shown in Fig. 10. In the wiring board 110α, components having the same functions as those of the wiring board 110 in the first embodiment shown in Fig. 10 are assigned the same reference numerals as those assigned in Fig. 10 or are omitted as appropriate, and repeated explanations thereof will be omitted.

[0146] When the waveguide mounting substrate 100α is in use, the components E11 and E12 are mounted on the component mounting surface of the waveguide mounting substrate 100α. In the waveguide mounting substrate 100α, the components E11 and E12 are mounted on the first surface 100a of the wiring substrate 110α. The component E11 is optically coupled to the core 3 of the optical waveguide 101a, and the component E12 is optically coupled to the core 3 of the optical waveguide 101b. Like the component E1 shown in FIG. 10 , the components E11 and E12 are optical components such as semiconductor devices including a photoelectric conversion element, and are provided with optical terminals E1a and electrodes E1b.

[0147] 11 and 12, the optical waveguide 101a and the optical waveguide 101b are arranged so that the exposed core portion 3a of the optical waveguide 101a and the exposed core portion 3a of the optical waveguide 101b are close to each other. Furthermore, as shown in Fig. 12, the optical waveguides 101a and 101b are arranged so that their ends 11 on the exposed core portion 3a side face each other. That is, the optical waveguides 101a and 101b are arranged on the wiring substrate 110α such that the distance G1 between the exposed core portion 3a of the optical waveguide 101a and the exposed core portion 3a of the optical waveguide 101b is closer than the distance G2 between the non-exposed core portion 3b of the optical waveguide 101a and the non-exposed core portion 3b of the optical waveguide 101b. Since the exposed core portion 3a of the optical waveguide 101a and the exposed core portion 3a of the optical waveguide 101b are brought close to each other, the components E11 and E12 that are optically coupled to the cores 3 of the respective optical waveguides can be arranged so that their electrodes E1b are close to each other. Therefore, it is believed that electrical signals can be transmitted well between the components E11 and E12 while avoiding noise interference and the like.

[0148] In the waveguide-mounted substrate 100α, the optical waveguides 101a and 101b are arranged along the same predetermined direction. That is, the cores 3 of the optical waveguides 101a and 101b are arranged along the same predetermined direction. Therefore, the propagation direction of the optical signal inside the optical waveguide 101a and the propagation direction of the optical signal inside the optical waveguide 101b are aligned on a straight line. The exposed core portions 3a of the optical waveguides 101a and 101b face each other, so that the exposed core portion 3a of the optical waveguide 101a faces one of the two opposing ends of the wiring substrate 110α, and the exposed core portion 3a of the optical waveguide 101b faces the other of the two opposing ends.

[0149] That is, the optical waveguide 101a and the optical waveguide 101b are arranged such that, in a plan view, the exposed core portion 3a of the optical waveguide 101a and the exposed core portion 3a of the optical waveguide 101b face each other at an angle φ of 180°. The phrase "the two exposed core portions 3a face each other at an angle φ" means that the direction in which the core 3 of one exposed core portion 3a extends forms an angle φ with the direction in which the core 3 of the other exposed core portion 3a extends. Therefore, it is considered possible to propagate a desired optical signal via the shortest path by applying appropriate electrical processing from one end of the waveguide-mounted substrate 100α to the other end. That is, when an optical signal is input from one end of the waveguide-mounted substrate 100α to, for example, the optical waveguide 101a, the optical signal is output as an electrical signal to the wiring substrate 110α via the component E11. The electrical signal is electrically processed as necessary on the wiring board 110α, and then converted into an optical signal by the component E12, and the optical signal is output from the optical waveguide 101b at the other end of the waveguide mounting substrate 100α.

[0150] The waveguide-mounted substrate of the embodiment may include not only one or two optical waveguides like the waveguide-mounted substrate 100 of FIG. 10 and the waveguide-mounted substrate 100α of FIG. 11 , but also any number of optical waveguides, three or more. FIG. 13 shows a waveguide-mounted substrate 100β, which is a modified example of the waveguide-mounted substrate of the second embodiment including multiple optical waveguides. The waveguide-mounted substrate 100β of the example of FIG. 13 includes a wiring substrate 110β having a stacked structure similar to the wiring substrate 110 of the embodiment of FIG. 10 and the wiring substrate 110α of the embodiment of FIG. 11 , and optical waveguides 101a, 101b, 101c, and 101d disposed on the wiring substrate 110β. The optical waveguides 101a to 101d are optical waveguides of the previously described embodiments, such as the optical waveguide 1 shown in FIGS. 1 to 4 or the optical waveguide 1α shown in FIG. 7 . Although not shown, when the waveguide mounting substrate 100β is in use, an optical component similar to the component E1 shown in FIG. 10 is mounted on the waveguide mounting substrate 100β and optically coupled to the cores 3 of the optical waveguides 101a to 101d.

[0151] In the waveguide-mounted substrate 100β, the optical waveguides 101a to 101d are arranged on the surface of the wiring substrate 110β with the exposed core portions 3a of the optical waveguides close to and facing each other. The exposed core portions 3a of the optical waveguides 101a and 101c face each other on a straight line, and the exposed core portions 3a of the optical waveguides 101b and 101d face each other on a straight line. Meanwhile, the optical waveguides 101a and 101b are arranged such that the exposed core portions 3a of the optical waveguides 101a and 101b face each other at an angle φa of 90° in a plan view. Similarly, the optical waveguides 101b and 101c are arranged such that the exposed core portion 3a of the optical waveguide 101b and the exposed core portion 3a of the optical waveguide 101c face each other at an angle φb of 90° in a plan view. Similarly, the optical waveguides 101c and 101d are arranged such that the exposed core portion 3a of the optical waveguide 101c and the exposed core portion 3a of the optical waveguide 101d face each other at an angle φc of 90° in a plan view. Furthermore, the optical waveguides 101d and 101a are arranged such that the exposed core portion 3a of the optical waveguide 101d and the exposed core portion 3a of the optical waveguide 101a face each other at an angle φd of 90° in a plan view.

[0152] In the waveguide-mounted substrate of the embodiment, as in the waveguide-mounted substrate 100β of FIG. 13 , multiple optical waveguides may be arranged so that the cores 3 of each optical waveguide are aligned along different, unique directions. Furthermore, multiple optical waveguides may be arranged so that the exposed core portions 3 a of each optical waveguide face each other at any angle. For example, multiple optical waveguides may be arranged so that the exposed core portions 3 a of each optical waveguide face each other at any angle between 45° and 180°. It may be possible to output an optical signal input to the waveguide-mounted substrate from any direction as an optical signal from a desired end in a desired direction via a short path and after appropriate electrical processing.

[0153] <Waveguide Mounted Substrate of Third Embodiment> Fig. 14 shows a waveguide mounted substrate 100γ, which is an example of a waveguide mounted substrate of the third embodiment. The waveguide mounted substrate 100γ includes a wiring substrate 110 that is the same as the wiring substrate included in the waveguide mounted substrate 100 of the example of Fig. 10, and an optical waveguide 1β disposed on the wiring substrate 110. That is, the waveguide mounted substrate 100β includes the wiring substrate 110 illustrated in Fig. 10 and the optical waveguide 1β illustrated in Figs. 8 and 9.

[0154] As described with reference to FIG. 8 and other figures, the optical waveguide 1β includes a lower clad 21, a core 3, and an upper clad 22, with a portion of the core 3 exposed on either the input or output side of the optical signal. The optical waveguide 1β is entirely provided with the upper clad 22, and is formed only by the non-exposed core portion. Therefore, since no exposed core portion is formed, the upper surface 31 of the core 3 is not exposed. The optical waveguide 1β is formed from a resin containing a thermosetting resin, a photocurable resin, or both, and the resin forming the upper clad 22 and the resin forming the core 3 are compatible with each other at the interface I between the upper clad 22 and the core 3. In FIG. 14, the components of the optical waveguide 1β are assigned the same symbols as those assigned to the components of the optical waveguide 1β in FIG. 8, or are omitted as appropriate, and repeated explanations of the effects of the optical waveguide 1β and the functions of each component are omitted. Furthermore, each component of the wiring board 110 in FIG. 14 is assigned the same symbol as that assigned to each component of the wiring board 110 in FIG. 10, or is omitted as appropriate, and repeated explanations of them are omitted.

[0155] When the waveguide-mounted substrate 100γ is used, an optical component such as component E2 is mounted thereon. Component E2 is an optical component such as a semiconductor device including a photoelectric conversion element, as described in the description of the optical waveguide 1β in FIG. 8 and other documents. Component E2 includes an optical terminal E2a and an electrode E2b exposed on a side surface of component E2. Examples of component E2 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). Component E2 is mounted on the waveguide-mounted substrate 100γ by connecting electrode E2b to a component mounting pad 4 of the wiring substrate 110 using, for example, solder.

[0156] The waveguide-mounted substrate 100γ includes an optical waveguide 1β in which the core 3 is exposed at both end faces of the waveguide, with the top surface 31 of the core 3 not exposed. In the waveguide-mounted substrate 100γ, the component E2 is positioned so that the core 3 and the optical terminal E2a of the component E2 are optically coupled by end-to-end connection or back-to-back coupling. That is, the side of the component E2 on which the optical terminal E2a is exposed faces one end 11 of the optical waveguide 1β, and thus the exposed portion of the optical terminal E2a faces the end face 33 of the core 3. Although not shown, the gap between the component E2 and the optical waveguide 1β is preferably filled with any optically transparent resin having an appropriate refractive index. If the component E2 is a light-emitting element, the component E2 generates an optical signal based on an electrical signal input to the electrode E2b and emits the optical signal toward the core 3 from the optical terminal E2a, which functions as a light-emitting element. When the component E2 is a light receiving element, an electrical signal is generated based on an optical signal incident from an optical terminal E2a functioning as a light receiving portion, and is output to the wiring board 110 from an electrode E2b.

[0157] <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. 15A to 15D. Hereinafter, as an example, a method for directly forming the optical waveguide 1 shown in FIGS. 1 to 4 on the wiring substrate 110 of the waveguide-mounted substrate 100 of FIG. 10 will be described. As shown in FIG. 15A, the wiring substrate 110 is prepared, and the lower clad 21 is formed on the surface of the solder resist 61 of the wiring substrate 110. The lower clad 21 is formed by forming a resin film for the lower clad 21. For example, a resin material for the lower clad 21 formed in a film form is thermocompression-bonded to the wiring substrate 110. The resin material for the lower clad 21 may be applied to the wiring substrate 110 by spin coating. The resin material for the lower clad 21 is a resin containing a thermosetting resin, a photocurable resin, or both. The resin material for the lower clad 21 may be a resin in which two or more polymers or copolymers are mixed or miscible.

[0158] As shown in FIG. 15B , the core 3 is formed in a desired shape in a planar view. The core 3 can be formed by any method. For example, the core 3 is formed using photolithography. That is, a resin film for the core 3 is formed on the entire surface of the lower clad 21. For example, the resin material for the core 3 is applied by spin coating to form the resin film for the core 3. Alternatively, the resin material for the core 3 may be formed into a film and then thermocompressed onto the surface of the lower clad 21. The resin material for the core 3 is a resin containing a thermosetting resin, a photocurable resin, or both. It is desirable to use a resin in which two or more types of polymers or copolymers are mixed or miscible with each other as the resin material for the core 3. Alternatively, a resin having a higher refractive index than the lower clad 21 may be used as the resin material for the core 3. Furthermore, the glass transition temperature Tg2 of the resin material for the core 3 may be different from the glass transition temperature Tg3 of the resin material for the lower clad 21.

[0159] Then, by exposing the resin material of the cores 3 to light through a mask (not shown) corresponding to the shape of the cores 3 to be formed and developing the mask, the layer made of the resin material of the cores 3 is patterned on the lower clad 21. As a result, a predetermined number of cores 3 having a desired shape are formed. By making the width of the opening in the exposure mask different between the portion corresponding to the exposed core portion 3a (see FIG. 1) and the portion corresponding to the non-exposed core portion 3b (see FIG. 1), it is possible to provide exposed core portions 3a and connecting portions 3c having widths larger than the non-exposed core portions 3b, as in the optical waveguide 1α of FIG. 7. In other words, by appropriately selecting the exposure mask when patterning the cores 3, it is possible to form cores 3 of a desired shape, size, and number.

[0160] In addition to photolithography, a photobleaching method is also an example of a method for forming the core 3 of a desired shape and size. In the photobleaching method, a film made of a resin material for the core 3, which is formed on the entire surface of the lower cladding 21 and has a refractive index higher than that 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 refractive index of the irradiated region is reduced to form the core 3. The core 3 may be formed by such a photobleaching method.

[0161] 15C , 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, the resin material of the upper clad 22 is applied to the lower clad 21 and the core 3 by spin coating, to form a resin film of the upper clad 22. Alternatively, the resin material of the upper clad 22 may be formed into a film and then thermocompressed. The upper clad 22 is integrated with the lower clad 21 or at least closely adheres to it, thereby forming the clad 2 surrounding the core 3.

[0162] The resin material of the upper cladding 22 is a thermosetting resin, a photocurable resin, or a resin containing both. It is preferable to use a resin in which two or more types of polymers or copolymers are mixed or miscible with each other as the resin material of the upper cladding 22. Alternatively, a resin having a lower refractive index than that of the core 3 may be used as the resin material of the upper cladding 22. Furthermore, it is preferable that the glass transition temperature Tg1 of the resin material of the upper cladding 22 is different from the glass transition temperature Tg2 of the resin material of the core 3. It is more preferable that the glass transition temperature Tg1 is higher than the glass transition temperature Tg2. In this case, it is even more preferable that the difference between the glass transition temperatures Tg1 and Tg2 is 15°C or less. The glass transition temperature Tg1 of the resin material of the upper cladding 22 may be the same as or different from the glass transition temperature Tg3 of the resin material of the lower cladding 21.

[0163] In manufacturing the optical waveguide of the embodiment, the upper clad 22 is formed so that the resin material of the upper clad 22 and the resin material of the core 3 are compatible at the interface I between the upper clad 22 and the core 3. As an example, by appropriately adjusting the temperature and pressure when thermocompression bonding the resin material of the upper clad 22 onto the core 3 and the lower clad 21, the resin material of the upper clad 22 and the resin material of the core 3 can be compatible at the interface I.

[0164] Furthermore, when a resin in which two or more types of polymers are mixed or dissolved together, or a resin in which two or more types of copolymers are mixed or dissolved together, is used as the resin material for the upper clad 22 and the resin material for the core 3, it is easy to make the resin material for the upper clad 22 compatible with the resin material for the core 3 and / or the resin material for the core 3 compatible with the resin material for the upper clad 22. Furthermore, when a resin in which two or more types of polymers are mixed or dissolved together, or a resin in which two or more types of copolymers are mixed or dissolved together, is used as the resin material for the upper clad 22 and the resin material for the core 3, and when the resin material for the upper clad 22 and the resin material for the core 3 are of the same type, it is easy to make the resin material for the upper clad 22 compatible with the resin material for the core 3 and / or the resin material for the core 3 compatible with the resin material for the upper clad 22.

[0165] By making the resin material of the upper clad 22 and the resin material of the core 3 compatible with each other, the resin of the upper clad 22 and the resin of the core 3 can be closely adhered to each other. In other words, although the core resin and the upper clad resin are two separate resins, by making the core resin and the upper clad resin compatible with each other at the interface, the core 3 and the upper clad 22 can be chemically adhered to each other. The compatible portion may be formed not only on the top surface of the core 3 but also on the side surface of the core 3.

[0166] As shown in FIG. 15D , a portion of the upper cladding 22 covering the exposed core portion 3 a of the core 3 is removed. For example, the portion of the upper cladding 22 on the exposed core portion 3 a side is removed over the entire width of the core 3. As a result, a portion of the core 3 is exposed from the cladding 2, thereby providing the exposed core portion 3 a. Furthermore, by removing a portion of the upper cladding 22, an upper cladding non-formed region 1 a and an upper cladding formed region 1 b are defined. The portion of the upper cladding 22 to be removed is removed using, for example, 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. Note that in the step shown in FIG. 15C , a film made of a resin material for the upper cladding 22 may be thermocompression bonded so as not to cover the exposed core portion 3 a. Through the above steps, the optical waveguide 1 is completed on the wiring substrate 110.

[0167] 8 and 9 is manufactured, the step of removing a portion of the upper clad 22, which was described with reference to Fig. 15D, is omitted. Instead, from the state shown in Fig. 15C, the upper clad 22 and the lower clad 21, including the core 3 if necessary, are cut along the direction of cutting line C shown by a two-dot chain line so that the end face of the core 3 is exposed. Preferably, the end face of the core 3 exposed by cutting is polished by any appropriate method.

[0168] A support plate (not shown) may be prepared instead of the wiring substrate 110, and the optical waveguide of the embodiment may be formed on the support plate. The support plate is preferably formed of a material having higher rigidity and a lower thermal expansion coefficient than the optical waveguide to be manufactured. Examples of materials for the support plate include glasses such as soda-lime glass, borosilicate glass, and quartz glass, various metals, and various ceramics such as alumina. When the 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. 15D. Removal of the support plate completes the formation of a single optical waveguide. The support plate may be removed by any method. The manufactured optical waveguide may be used with the support plate attached after completion.

[0169] <Method for Manufacturing a Waveguide-Mounted Substrate of an Embodiment> An example of a method for manufacturing a waveguide-mounted substrate of an embodiment will be described using the waveguide-mounted substrate 100 shown in Fig. 10 as an example. The waveguide-mounted substrate 100 is manufactured by forming a wiring substrate 110 by sequentially forming a conductor layer 43, an insulating layer 52, a conductor layer 42, an insulating layer 51, and a conductor layer 41, and then forming an optical waveguide 101 on the wiring substrate 110. The optical waveguide 101 may be separately manufactured on a support plate (not shown) and then disposed on the wiring substrate 110 after removing the support plate or with the support plate still attached.

[0170] The conductor layer 43 is formed on a support substrate (not shown), such as a double-sided copper-clad laminate, by a semi-additive method or the like. After the conductor layer 43 is formed, an insulating layer 52 covering the conductor layer 43 is formed, for example, by laminating an insulating resin, such as a film-like epoxy resin, and thermocompression bonding. Through holes are formed in the insulating layer 52, for example, by irradiating it with a carbon dioxide laser beam. After the through holes are formed, the conductor layer 42 is formed on the insulating layer 52 by a semi-additive method or the like, and via conductors 7 are formed in the through holes in the insulating layer 52. Furthermore, an insulating layer 51 is formed on the conductor layer 42 and the insulating layer 52 in the same manner as the insulating layer 52. The conductor layer 41 is formed on the insulating layer 51 in the same manner as the conductor layer 42, and the via conductors 7 are formed in the insulating layer 51 in the same manner as the via conductors 7 in the insulating layer 52. After the support substrate is removed by peeling or the like, solder resists 61 and 62 are formed by applying or spraying a photosensitive epoxy resin. The solder resists 61 and 62 are exposed and developed to form openings that expose parts of the conductor layer 41 and the conductor layer 43. As a result, the wiring board 110 is completed.

[0171] Then, the optical waveguide 101 is formed on the surface of the wiring board 110 by the method described with reference to FIGS. 15A to 15D. Alternatively, the optical waveguide 101 may be separately manufactured on a support plate (not shown) and then disposed on the wiring board 110 as shown in FIG. 16. For example, a thermosetting, room temperature curing, or photocurable adhesive GL is applied to the surface of the solder resist 61, and the optical waveguide 101 is mounted thereon. If necessary, the adhesive GL is cured by heating or the like, and the optical waveguide 101 is fixed to the wiring board 110. 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. Through the above steps, the waveguide-mounted substrate 100 of the example of FIG. 10 is completed.

[0172] 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 a separately manufactured optical waveguide 101 may be disposed in such an area on the surface of the insulating layer 51. Furthermore, the optical waveguide 101 may be disposed on the wiring board 110 in a state where a support plate (not shown) used in manufacturing the optical waveguide 101 is still provided.

[0173] The optical waveguide and waveguide-mounted substrate 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 waveguide-mounted substrate of the embodiments may have any laminated structure and may include any number of conductor layers and insulating layers. For example, the wiring substrate constituting the waveguide-mounted substrate of the embodiments may be a build-up wiring board including a core substrate, a multilayer wiring board without a build-up layer, or a double-sided or single-sided wiring board. Bumps and / or via conductors are not necessarily provided. Furthermore, the core thickness of the optical waveguide of the embodiments may vary between the core-exposed portion and the core-unexposed portion. In the optical waveguide of the embodiments, upper cladding-free regions may be provided at both ends of the optical waveguide, and therefore, the upper cladding-formed region may be disposed between two upper cladding-free regions.

[0174] 1, 1α, 1β, 101, 101a to 101d Optical waveguide 100, 100α to 100γ Waveguide mounting substrate 110, 110α, 110β Wiring substrate 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 41 to 43 Conductor layer 51 to 52 Insulating layer I Interface between core and upper clad W1 Width of core at core exposed portion

Claims

1. An optical waveguide consisting of a lower clad, a core, and an upper clad, with a portion of the core exposed on the input or output side of an optical signal, wherein the optical waveguide is formed from a resin made of a thermosetting resin or a photocurable resin, and the resin forming the upper clad and the resin forming the core are compatible at the interface between the upper clad and the core.

2. An optical waveguide according to claim 1, wherein the resin is a mixture or miscible mixture of two or more polymers or copolymers, and the refractive index is adjusted.

3. An optical waveguide according to claim 1, wherein the resin forming the upper clad and the resin forming the core are compatible with each other on the upper surface of the core.

4. An optical waveguide according to claim 1, wherein the resin forming the upper clad and the resin forming the core are compatible with each other at the side surfaces of the core.

5. An optical waveguide according to claim 1, wherein the upper clad has a glass transition temperature Tg1, the core has a glass transition temperature Tg2, and the glass transition temperature Tg1 and the glass transition temperature Tg2 are different.

6. An optical waveguide according to claim 5, wherein the glass transition point Tg1 is higher than the glass transition point Tg2.

7. An optical waveguide according to claim 6, wherein the glass transition point Tg1 and the glass transition point Tg2 satisfy the relationship of the following formula 1: 0°C<|Tg1-Tg2|≦15°C (Formula 1) 8. An optical waveguide according to claim 1, wherein the refractive index of said resin is less than 1.

5.

9. An optical waveguide according to claim 8, wherein the refractive index of said core is greater than the refractive index of said lower cladding and the refractive index of said upper cladding by 0.03 or more.

10. An optical waveguide according to claim 1, wherein the optical waveguide has an upper cladding region and an upper cladding non-forming region, and the upper cladding non-forming region has a core-exposed portion where the surface of the core is exposed.

11. An optical waveguide according to claim 10, wherein the width of the core at the core exposed portion is 2 μm or more and 15 μm or less.

12. A waveguide-mounted substrate comprising: a wiring board including an insulating layer and a conductor layer formed on said insulating layer; and the optical waveguide according to claim 1 disposed on said wiring board.

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