Optical waveguide and wiring board

Abstract

An optical waveguide (1) according to an embodiment of the present invention comprises an upper cladding (22), a core (3), and a lower cladding (21), and is made of resin. The resin that forms the optical waveguide (1) includes a thermosetting resin or a photocurable resin. A first compatible layer (C1) is formed at the interface (I1) between the upper cladding (22) and the core (3), and a second compatible layer (C2) is formed at the interface (I2) between the core (3) and the lower cladding (21), along the direction in which an optical signal is transmitted in the core (3).

Description

Optical waveguide and wiring board

[0001] The present invention relates to an optical waveguide and a wiring board.

[0002] Patent Document 1 discloses a polymer waveguide array formed on a polymer film and a silicon waveguide array formed on a silicon chip. It is also disclosed that the core of the polymer waveguide and the core of the silicon waveguide are arranged to overlap along the direction in which an optical signal is transmitted and are optically coupled by adiabatic coupling.

[0003] Japanese Patent Application Laid-Open No. 2014-81587

[0004] In Patent Document 1, a polymer waveguide is formed of a material constituting the core and a material constituting the cladding. However, it is assumed that problems may occur due to the high performance of the semiconductor in response to the demands for higher signal speed and larger capacity. It is also considered that higher density and heat resistance of the polymer waveguide are required for the high performance of the semiconductor. In increasing the density of the polymer waveguide, if the density of the core is increased, such as by reducing the width of the core, it is assumed that the adhesion cannot be ensured because the contact area between the core and the upper cladding or the lower cladding becomes small. Therefore, it is considered that problems such as peeling of the core from the upper cladding and / or the lower cladding may occur. Furthermore, it is assumed that it is necessary to ensure the heat resistance of the polymer waveguide because the heat generated from the chip (element) becomes larger or the time during which heat is generated becomes longer due to the high performance of the semiconductor. When the wiring board using the polymer waveguide is in use, the heat generated by the chip (element) is also transmitted to the polymer waveguide. If the heat resistance of the waveguide is not ensured against the transmitted heat, it is considered that problems such as failure to ensure the adhesion between the core and the upper cladding / lower cladding and displacement of the core due to thermal stress may be caused.

[0005] The optical waveguide of the present invention comprises an upper cladding, a core, and a lower cladding, and is made of resin. The resin includes a thermosetting resin or a photocurable resin, and a first compatible layer is formed at the interface between the upper cladding and the core along the direction in which the optical signal is transmitted through the core, and a second compatible layer is formed at the interface between the core and the lower cladding.

[0006] The wiring substrate of the present invention includes a substrate comprising wiring consisting of an insulating layer and a conductive layer formed on the insulating layer, and the optical waveguide disposed on the substrate.

[0007] The upper cladding, core, and lower cladding constituting the optical waveguide of the present invention are formed from a thermosetting resin or a resin containing a photocurable resin. A compatible layer is formed at the interface between the upper cladding and the core along the direction in which the optical signal is transmitted through the core. The formation of the compatible layer ensures adhesion between the upper cladding and the core. Furthermore, adhesion between the core and the upper cladding is ensured even with increased core density. Therefore, it is believed that defects such as core delamination in the waveguide are suppressed. In addition, heat resistance between the core and the upper cladding is ensured against thermal stress generated during use of the optical waveguide. Therefore, it is believed that defects such as core displacement in the waveguide are suppressed.

[0008] A compatible layer is formed at the interface between the lower cladding and the core, along the direction in which the optical signal of the core is transmitted. The formation of this compatible layer ensures adhesion between the lower cladding and the core. Therefore, it is believed that problems such as core delamination in the waveguide are suppressed. Adhesion between the core and the lower cladding is also ensured even with high core density. Furthermore, heat resistance between the core and the lower cladding is ensured against thermal stress generated during the use of the optical waveguide. Therefore, it is believed that problems such as core displacement in the waveguide are suppressed.

[0009] A plan view showing an example of an optical waveguide according to the first embodiment of the present invention. A cross-sectional view of the optical waveguide in the example of Figure 1 along line II-II. A cross-sectional view of the optical waveguide in the example of Figure 1 along line III-III. A cross-sectional view of the optical waveguide in the example of Figure 1 along line IV-IV. A cross-sectional view showing a first modification example of the thickness of each compatible layer in the optical waveguide of the embodiment. A cross-sectional view showing a second modification example of the thickness of each compatible layer in the optical waveguide of the embodiment. A schematic diagram showing an example of stepwise compatibility in the first compatible layer of the optical waveguide of the embodiment. A schematic diagram showing an example of stepwise compatibility in the second compatible layer of the optical waveguide of the embodiment. A plan view showing an example of an optical waveguide according to the second embodiment of the present invention. A cross-sectional view of the optical waveguide in the example of Figure 8A along line XIIIB-XIIIB. A cross-sectional view showing an example of a wiring board according to embodiment 1 of the present invention. A cross-sectional view showing a modification example of a wiring board according to embodiment 2 of the present invention. A cross-sectional view showing an example of a wiring board according to embodiment 3 of the present invention. A cross-sectional view showing a modification example of a wiring board according to embodiment 4 of the present invention. A cross-sectional view showing an example of the manufacturing process of an optical waveguide according to the first embodiment. A plan view showing an example of the manufacturing process for an optical waveguide according to the first embodiment. A cross-sectional view showing an example of the manufacturing process for an optical waveguide according to the first embodiment. A cross-sectional view showing an example of the manufacturing process for an optical waveguide according to the first embodiment. A cross-sectional view showing an example of the manufacturing process for an optical waveguide according to the first embodiment. A cross-sectional view showing an example of the manufacturing process for an optical waveguide according to the second embodiment. A plan view showing an example of the manufacturing process for an optical waveguide according to the second embodiment. A cross-sectional view showing an example of the manufacturing process for an optical waveguide according to the second embodiment.

[0010] An embodiment of the present invention, including an optical waveguide and a wiring board, will be described with reference to the drawings. In the drawings referenced in the following description, certain parts may be enlarged to facilitate understanding of the disclosed embodiment. Therefore, the size and length of each component may not be depicted in exact proportions.

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

[0012] The optical waveguide 1 in this embodiment includes a core 3 that transmits optical signals, and a cladding 2 surrounding the core 3. The cladding 2 is composed of a lower cladding 21 and an upper cladding 22. That is, the optical waveguide 1 in Figures 1 to 4 consists of the upper cladding 22, the core 3, and the lower cladding 21. The core 3 is formed on top of the lower cladding 21. The upper cladding 22 is formed on top of the lower cladding 21 and the core 3.

[0013] Core 3 has an upper surface 31 that faces the same direction as the lower cladding 21, core 3, and upper cladding 22 are aligned in order, and a lower surface 32 that is the opposite surface of the upper surface 31. The direction in which the lower cladding 21, core 3, and upper cladding 22 are aligned in order is also called the "stacking direction" or "Z direction". As shown in Figure 2, the upper surface 31 of core 3 is a surface that faces the +Z direction, and the lower surface 32 is a surface that faces the -Z direction. Hereafter, in the optical waveguide 1, the side with the lower cladding 21 will be referred to as the "lower side" or simply "down", and the side with the upper cladding 22 will be referred to as the "upper side" or simply "up".

[0014] 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 Figure 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.

[0015] In the optical waveguide 1 shown in Figures 1 and 2, the core 3 is formed along the +X and -X directions. Optical signals propagating through the core 3 propagate in either the +X or -X direction. The propagation direction of optical signals is 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 face each other in the X direction. When an optical signal is input to the one end 11 side, an optical signal is output from the other end 12 side. When an optical signal is input to the other end 12 side, an optical signal is output from the one end 11 side.

[0016] As shown in Figures 2 and 3, at one end 11, the upper surface 31, the end face 33, and the side surface 35 of the core 3 are exposed. As shown in Figures 1 and 2, at the other end 12, the end face 34 of the core 3 is exposed. Note that at the other end 12, the upper surface 31 and the side surface 35 of the core 3 may also be exposed, similar to the end 11.

[0017] As shown in Figures 1 and 2, the optical waveguide 1 has an upper cladding-free region 1a, which is a region where the upper cladding 22 is not formed in a plan view, and an upper cladding-formed region 1b, which is a region where the upper cladding 22 is formed. The upper cladding-free region 1a is located on one end 11 side of the optical waveguide 1. The upper cladding-formed region 1b is located adjacent to the upper cladding-free region 1a. Note that "plan view" means viewing the object with a line of sight along the Z direction.

[0018] In the optical waveguide 1, the upper cladding-free region 1a is a core-exposed portion where the upper surface 31 of the core 3 is exposed, and the upper cladding-formed region 1b is a core-non-exposed portion where the upper surface 31 of the core 3 is not exposed. The core-exposed portion of the optical waveguide 1 is located on one end 11 side. The core-non-exposed portion of the optical waveguide 1 is located on the other end 12 side. The core 3 consists of core 3a and core 3b, with core 3a located in the core-exposed portion and core 3b located in the core-non-exposed portion. The lower cladding 21 consists of lower cladding 21a and lower cladding 21b, with lower cladding 21a located in the core-exposed portion and lower cladding 21b located in the core-non-exposed portion. Therefore, as shown in Figures 1 and 2, core 3a in the core-exposed portion is located on one end 11 side of the optical waveguide 1. Core 3b in the core-non-exposed portion is located on the other end 12 side of the optical waveguide 1.

[0019] The thickness of core 3 is not particularly limited, but may be between 1 μm and 50 μm, preferably between 1 μm and 20 μm, and more preferably between 3 μm and 10 μm. Furthermore, it is desirable to optimize the thickness of core 3 according to the shape and size of the optical waveguide. The thickness of core 3 is determined by the average value of the thickness of core 3 measured along the Z direction at three points in the X direction in the SEM image.

[0020] The thickness of the lower cladding 21 is not particularly limited, but is preferably 5 μm to 30 μm, and more preferably 15 μm to 30 μm. The thickness of the upper cladding 22 is not particularly limited, but is preferably 5 μm to 30 μm, and more preferably 15 μm to 30 μm. The thickness of the lower cladding 21 and the upper cladding 22 are determined by the average value of the thickness measured along the Z direction at three points in the X direction in the SEM image.

[0021] When the optical waveguide 1 is in use, the core 3 is optically coupled at one end 11 and the other end 12 with optical components such as a photoelectric conversion element and / or a connector member that connects to the outside of the waveguide, such as an optical fiber or an optical connector. In Figures 1 to 3, a component E1 equipped with a photoelectric conversion element is shown by a dashed line as an example of an optical component that is optically coupled with the core 3 at one end 11. When the optical waveguide 1 is in use, it is preferable that the region between component E1 and the portion of the optical waveguide 1 that overlaps with component E1 in a plan view is filled with an optically transparent transparent resin TR.

[0022] Component E1 includes an optical terminal E1a, which is a portion into which an optical signal is input or output from component E1. Component E1 is optically coupled with core 3 at optical terminal E1a. The optical signal propagating through core 3 from the other end 12 enters component E1 via optical terminal E1a at one end 11. On the other hand, the optical signal from optical terminal E1a of component E1 enters core 3 at one end 11, propagates within core 3, and is output from the other end 12. In the examples of Figures 1 to 3, the upper surface 31 of the exposed core and the optical terminal E1a of component E1 are facing each other and are adibatially coupled. A portion of the optical signal propagating through core 3 towards one end 11 is emitted from the upper surface 31 as evanescent light and incident on the optical terminal E1a of component E1.

[0023] The optical waveguide 1 shown in Figures 1 to 4 has four parallel cores 3. The number of cores 3 is not limited to four, but can be any number of one or more. For example, the number of cores 3 is in the range of 2 to 128. When multiple cores 3 are provided, the arrangement pitch P1 of the cores 3 is not particularly limited, but is preferably, for example, 10 μm or more and 300 μm or less, and 20 μm or more and 250 μm or less. In the example of Figure 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 change between one end 11 and the other end 12. That is, each core 3 may be curved between one end 11 and the other end 12.

[0024] The core 3a in the exposed core portion of the optical waveguide 1 has a width W1. The width W1 of the core 3a in the exposed core portion is not particularly limited, but is preferably 1 μm to 15 μm, and more preferably 3 to 10 μm. The core 3b in the non-exposed core portion of the optical waveguide 1 has a width W2. In the optical waveguide 1, the core 3 has a substantially constant width from one end 11 to the other end 12, and the widths W1 and W2 are substantially equal. In the optical waveguide of this embodiment, the width of the core in the exposed core portion and the width of the core in the non-exposed core portion may be different. The width of the core 3 is determined by the average value of the widths measured along the Y direction at three points in the X direction in the SEM image. The "Y direction" is the direction perpendicular to the propagation direction of the optical signal propagating within the core 3 and the stacking direction of the optical waveguide 1.

[0025] As an example of the dimensions of each part of the wiring board 1 shown in Figures 1 to 4, the width W1 of core 3a and the width W2 of core 3b are 5 μm, the thickness of core 3 is 5 μm, the thickness of the lower cladding 21 is 30 μm, the arrangement pitch P1 of core 3 is 50 μm, and the thickness of the upper cladding 22 is 20 μm. Note that the width of core 3, the thickness of core 3, the arrangement pitch of core 3, the thickness of the lower cladding 21, and the thickness of the upper cladding 22 shown here are merely examples, and the dimensions of each part of core 3 and the thickness of each cladding are not limited to the values ​​shown here.

[0026] <Compatibility between the core-forming material and the cladding material> In the optical waveguide 1 of this embodiment, as shown in Figures 2 and 4, a first compatible layer C1 is formed at the interface I1 between the upper cladding 22 and the core 3 along the X direction in which the optical signal of the core 3 is transmitted, and a second compatible layer C2 is formed at the interface I2 between the core 3 and the lower cladding 21. The first compatible layer C1 is formed by the compatibility of the resin forming the upper cladding 22 and the resin forming the core 3. The first compatible layer C1 is formed along the X direction in which the optical signal of the core 3 is transmitted. It is considered that the formation of the first compatible layer C1 ensures good adhesion between the resin constituting the core 3 and the upper cladding 22.

[0027] Here, the core resin material and the upper cladding resin material are two resins with different compositions and properties, but it is thought that chemical adhesion can be obtained by making these two different resins compatible at or near the interface. It is thought that the adhesion between the upper cladding 22 and the core 3 will be stable due to this chemical adhesion. In this way, in the optical waveguide of this embodiment, adhesion is ensured between the upper cladding 22 and the core 3. Therefore, even if thermal shrinkage due to heat affects the optical waveguide when semiconductors such as optical elements are mounted and the waveguide is used as a wiring board, it is thought that defects such as delamination at the interface I1 between the core 3 and the upper cladding 22 will be suppressed. Furthermore, even if the width of the core 3 is reduced, it is thought that defects such as delamination of the core 3 from the upper cladding 22 will be suppressed.

[0028] The second compatible layer C2 is formed by the compatibility of the resin forming the lower cladding 21 with the resin forming the core 3. The second compatible layer C2 is formed along the X direction through which the optical signal of the core 3 is transmitted. It is believed that the formation of the second compatible layer C2 ensures good adhesion between the resin constituting the core 3 and the lower cladding.

[0029] Here, the core resin material and the lower cladding resin material are two resins with different compositions and properties, but it is thought that chemical adhesion can be obtained by making these two different resins compatible at or near the interface. It is thought that the adhesion between the lower cladding 21 and the core 3 will be stable due to this chemical adhesion. In this way, in the optical waveguide of this embodiment, adhesion is ensured between the lower cladding 21 and the core 3. Therefore, even if thermal shrinkage due to heat affects the optical waveguide when semiconductors such as optical elements are mounted and the waveguide is used as a wiring board, it is thought that problems such as delamination at the interface I2 between the core 3 and the lower cladding 21 will be suppressed. Furthermore, it is thought that problems such as delamination of the core 3 from the lower cladding 21 will be suppressed even if the width of the core 3 is reduced. In addition, adhesion with the core 3 can also be obtained by forming a second compatible layer C2 on the core exposed portion.

[0030] The formation of the first and second compatible layers C1 and C2 along the X-direction through which the optical signal is transmitted in the core 3 of the optical waveguide 1 ensures adhesion to the core 3, thereby suppressing problems such as delamination of the optical waveguide. Furthermore, when an optical signal is transmitted to the core 3 of the optical waveguide 1 having the first and second compatible layers C1 and C2, each compatible layer acts as a shield for the optical signal, preventing leakage of the optical signal from the core 3 and allowing the optical signal to be properly reflected within the core 3. Therefore, it is thought that it can handle higher speeds and larger capacities of optical signals. In addition, the formation of the first and second compatible layers C1 and C2 in the optical waveguide 1 suppresses the intrusion of moisture into the interface between the core 3 and cladding 2 of the optical waveguide 1, even when reliability tests such as high-temperature and high-humidity storage are performed, making delamination of the core 3 and cladding 2 less likely.

[0031] <Relationship between the first and second miscible layers> In the optical waveguide of the embodiment, the thickness of the first miscible layer C1 and the thickness of the second miscible layer C2 are not particularly limited, but may be approximately the same or different. In the optical waveguide of the embodiment, it is preferable that the thickness of the first miscible layer C1 and the thickness of the second miscible layer C2 are different from each other. Figures 5 and 6 show modified examples in the optical waveguide 1 of the embodiment in which the thickness T1 of the first miscible layer C1 and the thickness T2 of the second miscible layer C2 are different. In the first modified example shown in Figure 5, the thickness T1 of the first miscible layer C1 is greater than the thickness T2 of the second miscible layer C2. In the second modified example shown in Figure 6, the thickness T1 of the first miscible layer C1 is smaller than the thickness T2 of the second miscible layer C2. Alternatively, the thickness T1 of the first miscible layer C1 is equal to the thickness T2 of the second miscible layer C2.

[0032] If the thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 are different, it is thought that thermal stress will be buffered when the optical waveguide 1 is in use. That is, when the optical waveguide 1 is in use, the photoelectric conversion element such as component E1 (see Figure 1) may generate heat. Therefore, the temperature around the optical waveguide 1 may fluctuate depending on the operating state of the photoelectric conversion element, and the core 3 and cladding 2 may expand and contract due to heat. If the thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 are different, the amount of expansion and contraction due to heat will be different in the first compatible layer C1 on the upper side of the core 3 and the second compatible layer C2 on the lower side of the core 3, so it is thought that the generated thermal stress can be buffered. As a result, it is thought that delamination between the core 3 and cladding 2 will be suppressed.

[0033] In the optical waveguide 1 of the first modified example shown in Figure 5, the thickness T1 of the first compatibilization layer C1 is greater than the thickness T2 of the second compatibilization layer C2. That is, the thickness T1 of the first compatibilization layer C1 and the thickness T2 of the second compatibilization layer C2 satisfy the following relationship 1: T1 > T2 ... (Equation 1) When the thickness T1 of the first compatibilization layer C1 and the thickness T2 of the second compatibilization layer C2 satisfy the relationship 1, it is thought that the effects of thermal stress can be buffered and delamination between the core 3 and the cladding 2 is suppressed.

[0034] In the optical waveguide 1 of the second modified example shown in Figure 6, the thickness T2 of the second compatibilization layer C2 is greater than the thickness T1 of the first compatibilization layer C1. That is, the thickness T1 of the first compatibilization layer C1 and the thickness T2 of the second compatibilization layer C2 satisfy the following relationship Equation 2: T1 ≤ T2 ... (Equation 2) When the thickness T1 of the first compatibilization layer C1 and the thickness T2 of the second compatibilization layer C2 satisfy the relationship Equation 2, it is thought that the effect of thermal stress can be buffered and delamination between the core 3 and the cladding 2 is suppressed. Note that if the difference between the thickness T1 of the first compatibilization layer C1 and the thickness T2 of the second compatibilization layer C2 is within 15%, they are considered equivalent. In other words, if the correlation (T1 / T2) between the thickness T1 of the first miscible layer C1 and the thickness T2 of the second miscible layer C2 satisfies 0.85 ≤ (T1 / T2) ≤ 1.15, then the thickness T1 of the first miscible layer C1 and the thickness T2 of the second miscible layer C2 can be considered to be equivalent.

[0035] The thickness T1 of the first compatible layer C1 is not particularly limited, but is, for example, 50 nm to 2000 nm. Preferably, the thickness T1 of the first compatible layer C1 is 100 nm to 1000 nm. The thickness T2 of the second compatible layer C2 is not particularly limited, but is, for example, 50 nm to 2000 nm. Preferably, the thickness T2 of the second compatible layer C2 is 100 nm to 1000 nm. The thickness T1 of the first compatible layer C1 is determined by the average value of the thickness of the first compatible layer C1 measured along the Z direction at three points in the X direction in the SEM image. The thickness T2 of the second compatible layer C2 is determined by the average value of the thickness of the second compatible layer C2 measured along the Z direction at three points in the X direction in the SEM image. Furthermore, since the optical waveguide 1 in the first and second modified examples has a first compatible layer C1 and a second compatible layer C2 formed on it, even when reliability tests such as high-temperature and high-humidity storage are performed, it is possible to suppress the intrusion of moisture into the interface between the core 3 and cladding 2 of the optical waveguide 1, and it is thought that delamination of the core 3 and cladding 2 will be less likely to occur.

[0036] <Materials Constituting the Optical Waveguide> The core 3 and cladding 2 that form the optical waveguide 1 are made of translucent material. Resin material is used for the core 3 and cladding 2. In other words, the optical waveguide 1 is made of resin. Since the optical waveguide 1 is made of resin material, it can be lightweight, have high toughness, and be flexible.

[0037] The optical waveguide in this embodiment is formed from a resin that is either any thermosetting resin, any photocurable resin, or a resin containing both. Therefore, in examples such as Figure 1, the core 3, the lower cladding 21, and the upper cladding 22 are made from a resin that is either a thermosetting resin, a photocurable resin, or a resin containing both.

[0038] The resin constituting the core 3 and cladding 2 is composed of, for example, one type of polymer or two or more types of polymers. The polymers of the resin constituting the core 3 and cladding 2 may be copolymers, and these copolymers may be copolymers of any bonding configuration, such as block copolymers or alternating copolymers.

[0039] The resins constituting the core 3 and cladding 2 are preferably resins in which two or more resins are mixed or compatible. For example, the resins constituting the core 3 and cladding 2 are preferably resins in which two or more polymers or copolymers are mixed or compatible. Furthermore, the resins constituting the core 3 and cladding 2 are preferably resins composed of two or more polymers that are mixed or compatible, or resins composed of two or more copolymers that are mixed or compatible. Note that the various copolymers of "two or more copolymers" include those with different numbers of monomers (degrees of polymerization) and bonding configurations for each copolymer.

[0040] The core 3 and cladding 2 may be composed of a resin that is either a photocurable resin or a thermosetting resin, or a resin containing both, which are formed by mixing two or more polymers or two or more copolymers.

[0041] Core 3 and cladding 2 may be composed of different resin materials, or they may be composed of resin materials of the same type. For example, it is preferable that the main resins of the core resin material and cladding resin material are of the same type. That is, it is more preferable that a resin in which two types of polymer resins are mixed or compatible, or a resin in which two types of copolymer resins are mixed or compatible, is used, and that the main resins in these resins are of the same type. Here, "main resin" means the resin with the highest content among the resins contained as components in the core resin material or cladding resin material. It is thought that using resin materials with the same type of main resin for the core resin material and cladding resin material makes it easier to make the core resin material compatible with the cladding resin material, or vice versa.

[0042] It is more preferable that the resins constituting the core 3 and the cladding 2 are resins in which two or more resins are mixed or compatible and the refractive index is adjusted. The refractive index of the resins constituting the core 3 and the cladding 2 is preferably adjusted to 1.4 to 1.6. By setting the refractive index of the resin of the optical waveguide 1 to 1.4 or more and 1.6 or less, optical transmission becomes easier. Also, it is more preferable that the refractive index of the resins constituting the core 3 and the cladding 2 is less than 1.5. Further specifically, it is desirable that the refractive index of the resins constituting the core 3 and the cladding 2 is 1.4 or more and less than 1.5. In the optical waveguide 1 having the core 3 and the cladding 2 composed of resins with appropriately adjusted refractive indices, it is considered that leakage of the optical signal propagating in the core 3 is small and efficient optical signal transmission can be obtained.

[0043] Also, the refractive index of the resin constituting the core 3 is made higher than the refractive index of the resin constituting the cladding 2 so that total reflection of the optical signal at the interface between the core 3 and the cladding 2 is possible. Thereby, the optical signal is normally transmitted in the core 3. The refractive index difference is preferably 0.03 or more, and more preferably 0.05 or more. 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 considered that transmission of the optical signal propagating in the core 3 to the cladding 2 is small and good transmission performance can be obtained. Note that the core 3 and the cladding 2 may be made of a resin material having the same refractive index and then subjected to appropriate treatment to make their refractive indices different from each other.

[0044] Examples of the resin materials constituting the core 3 and the cladding 2 include acrylic resins, polyimide resins, polyamide resins, polyether resins, phenolic resins, fluorine resins, silicone resins, and epoxy resins. Among these, it is desirable to use acrylic resins, phenolic resins, fluorine resins, silicone resins, and epoxy resins, for which adjustment of the refractive index is easy.

[0045] In the optical waveguide 1 of this embodiment, the glass transition temperature of the resin material constituting the upper cladding 22 is defined as Tg1, and the glass transition temperature of the resin material constituting the core 3 is defined as Tg2. In this case, it is preferable that the glass transition temperature Tg1 of the resin material constituting the upper cladding 22 and the glass transition temperature Tg2 of the resin material constituting the core 3 are different. Having different glass transition temperatures Tg1 and Tg2 makes it easier to make the resin constituting the core 3 and the resin constituting the upper cladding 22 compatible at the interface I1 between the core 3 and the upper cladding 22. By making the resin constituting the core 3 and the resin constituting the upper cladding 22 compatible, adhesion at the interface I1 can be obtained.

[0046] 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 are not particularly limited, but are preferably between 80 and 270°C. It is preferable that the glass transition temperature Tg1 of the upper cladding 22 and the glass transition temperature Tg2 of the core 3 are different. Furthermore, the glass transition temperature Tg1 of the upper cladding 22 may be higher than the glass transition temperature Tg2 of the core 3, or it may be lower than the glass transition temperature Tg2 of the core 3. It is desirable that the absolute value of the difference between the glass transition temperature Tg1 of the upper cladding 22 and the glass transition temperature Tg2 of the core 3 satisfies the following relationship 3-1: 0°C < |Tg1 - Tg2| ≤ 15°C ... (Equation 3-1)

[0047] By setting the absolute value |Tg1 - Tg2| within the range of Equation 3-1, the core 3 and the upper cladding 22 can be made compatible at interface I1, and a first compatible layer can be formed. By making the core 3 and the upper cladding 22 compatible, adhesion at interface I1 between the core 3 and the upper cladding 22 is ensured, and it is believed that defects such as delamination at interface I1 between the core 3 and the upper cladding 22 are suppressed. It is also believed that defects such as delamination of the core 3 from the upper cladding 22 are suppressed even if the width of the core 3 is reduced.

[0048] Furthermore, it is desirable that the absolute value of the difference between the glass transition temperature Tg1 of the upper cladding 22 and the glass transition temperature Tg2 of the core 3 satisfies the relationship of the following formula 3-2. 0°C < |Tg1 - Tg2| ≤ 10°C ··· (Formula 3-2)

[0049] By setting the absolute value |Tg1 - Tg2| within the range of the formula 3-2, the core 3 and the upper cladding 22 can be made compatible at the interface I1, and a first compatible layer can be formed. By making the core 3 and the upper cladding 22 compatible, better adhesion at the interface I1 between the core 3 and the upper cladding 22 is ensured, and it is considered that problems such as peeling at the interface I1 between the core 3 and the upper cladding 22 are more suppressed. Even if the width of the core 3 is reduced, it is considered that problems such as peeling of the core 3 from the upper cladding 22 are suppressed. Also, by setting the absolute value |Tg1 - Tg2| within the range of the formula 3-2, the characteristics of the two resins of the core 3 and the upper cladding 22 become the same or nearly the same, and the stability of the formation of the optical waveguide 1 is also obtained.

[0050] In the optical waveguide 1 of the embodiment, let the glass transition temperature of the resin in the resin material constituting the lower cladding 21 be Tg3, and the glass transition temperature of the resin in the resin material constituting the core 3 be Tg2. At this time, it is preferable that the glass transition temperature Tg3 of the resin in the resin material constituting the lower cladding 21 is different from the glass transition temperature Tg2 of the resin in the resin material constituting the core 3. Since the glass transition temperature Tg3 and the glass transition temperature Tg2 are different, it becomes easier to make the resin constituting the core 3 and the resin constituting the lower cladding 21 compatible at the interface I2 between the core 3 and the lower cladding 21. By making the resin constituting the core 3 and the resin constituting the lower cladding 21 compatible, adhesion at the interface I2 is considered to be obtained.

[0051] The glass transition temperature Tg3 of the resin constituting the lower cladding 21 is not particularly limited, but is preferably between 80 and 270°C. It is desirable that the glass transition temperature Tg3 of the lower cladding 21 is different from the glass transition temperature Tg2 of the core 3. Furthermore, the glass transition temperature Tg3 of the lower cladding 21 may be higher than the glass transition temperature Tg2 of the core 3, or lower than the glass transition temperature Tg2 of the core 3. Additionally, the glass transition temperature Tg3 of the lower cladding 21 may be approximately the same as or different from the glass transition temperature Tg1 of the upper cladding 22. It is desirable that the absolute value of the difference between the glass transition temperature Tg2 of the core 3 and the glass transition temperature Tg3 of the lower cladding 21 satisfies the following relationship in equation 4-1: 0°C < |Tg3 - Tg2| ≤ 15°C ... (Equation 4-1)

[0052] By setting the absolute value |Tg3 - Tg2| within the range of Equation 4-1, the core 3 and the lower cladding 21 can be made compatible at interface I2, and a second compatible layer can be formed. By making the core 3 and the lower cladding 21 compatible, adhesion at interface I2 between the core 3 and the lower cladding 21 is ensured, and it is believed that defects such as delamination at interface I2 between the core 3 and the lower cladding 21 are suppressed. It is also believed that defects such as delamination of the core 3 from the lower cladding 21 are suppressed even if the width of the core 3 is reduced.

[0053] Furthermore, it is desirable that the absolute value of the difference between the glass transition temperature Tg2 of the core 3 and the glass transition temperature Tg3 of the lower cladding 21 satisfies the following relationship, Equation 4-2: 0°C < |Tg3 - Tg2| ≤ 10°C ... (Equation 4-2)

[0054] By setting the absolute value |Tg3 - Tg2| within the range of Equation 4-2, the core 3 and the lower cladding 21 can be made compatible at interface I2, and a second compatible layer can be formed. By making the core 3 and the lower cladding 21 compatible, the adhesion at interface I2 between the core 3 and the lower cladding 21 is better ensured, and it is thought that defects such as delamination at interface I2 between the core 3 and the lower cladding 21 are further suppressed. Even if the width of the core 3 is reduced, defects such as delamination of the core 3 from the lower cladding 21 are thought to be suppressed. In addition, by setting the absolute value |Tg3 - Tg2| within the range of Equation 4-2, the properties of the two resins of the core 3 and the lower cladding 21 become identical or nearly identical, and the stability of the formation of the optical waveguide 1 is also obtained.

[0055] The resins constituting core 3 and cladding 2 are mixtures or miscibles of two or more polymers or copolymers. It can also be said that the resins constituting core 3 and cladding 2 are combinations of two or more resins. The combinations of two or more resins include combinations of thermosetting resins, combinations of photocurable resins, and combinations of both thermosetting and photocurable resins; any combination is acceptable. Furthermore, the resins constituting core 3 and cladding 2 are also resins in which two or more polymers or copolymers are mixed or miscible and whose refractive index is adjusted. Examples of these resins are listed below. Note that the resins listed below are just examples, and other examples are also included.

[0056] When the resin component, which is a polymer or copolymer as a repeating structure, is the same component, an example 1 is a resin in which two types of polymers or copolymers are mixed or compatible, and the refractive index is adjusted, even though they are the same resin component but differ in one or more bonding groups. An example of two types of polymers or copolymers as in Example 1 is combining two resins AA, which are composition AA, where the repeating structure is A. Here, resins AA are the same resin component AA, but are combined with resins that differ in their terminal bonding groups. A more specific example is a combination of resin AAX, which is composition AAX, where the repeating structure is A, and the terminal bonding group of composition AA is X, and resin AAY, which is composition AAY, where the repeating structure is A, and the terminal bonding group of composition AA is Y (where Y ≠ X), and resin AAY. Furthermore, the refractive index of the resin combination in Example 1 is adjusted to a predetermined value.

[0057] Furthermore, when the polymer or copolymer resin components as repeating structures are different, an example 2 is given of a resin in which resin AA, which is a polymer or copolymer composition AA with resin component AA, and resin BB, which is a polymer or copolymer composition BB with repeating structure B, are mixed or miscible and the refractive index is adjusted. In an example of two polymers or copolymers like in example 2, resin BB, which is resin component BB, is a resin component in which the repeating structure is structure B, which is different from the repeating structure A of resin component AA. In addition, the refractive index of the resin combination in example 2 is adjusted to a predetermined value.

[0058] When there are three types of resin components in a polymer or copolymer as a repeating structure, a resin in which the three types are combined, mixed or miscible, and whose refractive index is adjusted is given as example 3. Several examples of cases in which three or more polymers or copolymers are mixed or miscible include a combination of resins in which all resin components are the same, but the bonding groups at the ends or middle of the resin components are different to some extent, a combination of two or more polymers or copolymers in which the resin components are the same, and a combination of two or more polymers or copolymers in which the resin components are different, all of which are given as example 3. In an example of three polymers or copolymers like example 3, it is preferable that the three or more polymers or copolymers constituting these resins are mixed or miscible.

[0059] An example of a combination of resins in Example 3 where all resin components are the same, but the bonding groups at the ends or middle of the resin components differ to some extent, is to combine three resins AA, which are composition AA, which are resin component

[0060] An example of a combination of two 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 to combine two resins, resin AA, which is composition AA, where resin component AA is resin component AA and the repeating structure is A, and three resins, resin BB, which is composition BB, where resin component BB is resin component BB and the repeating structure is B. Here, resin AA is a combination of resins in which resin component AA is the same resin component AA and the terminal bonding group is X, resin AAX is composition AAX, where resin AAX is composition A, where resin component AA is resin component AA and the terminal bonding group is X, resin AAY is composition AAY, where resin AAY is composition A, where resin component AA is resin component AA and the terminal bonding group is Y (where Y ≠ X), and resin BB is composition BB, where resin BB is composition BB, where resin component BB is resin component B.

[0061] When the resin components of a polymer or copolymer as a repeating structure differ, this involves combining three resins, each with entirely different resin components. The combination is as follows: resin AA, which is composition AA, a polymer or copolymer with resin component AA; resin BB, which is composition BB, a polymer or copolymer with resin component BB, where the repeating structure is B; and resin CC, which is composition CC, a polymer or copolymer with resin component CC, where the repeating structure is C. Furthermore, the refractive index of these resin combinations in example 3 is adjusted to a predetermined value.

[0062] However, the resins constituting the core 3 and cladding 2 in each of the above examples are either thermosetting resins, photocurable resins, or mixed resins of thermosetting resins and photocurable resins. For example, if thermoplastic resins are used as the resins constituting the core 3 and cladding 2, it is considered that the compatible layer of this embodiment cannot be formed.

[0063] By using a resin containing a mixture or compatibility of two or more polymers or copolymers for the core 3 and cladding 2, an optical waveguide with desired properties can be formed. A resin containing a mixture or compatibility of two or more polymers or copolymers is a resin in which the resin component is a combination of two identical components. An example of a combination of two identical components is when resin AAX and resin AAY are used as the two or more polymers or copolymers. For example, resin AAX has excellent light transmission, film-forming properties, and heat resistance, while resin AAY has excellent light transmission, resolution, and chemical resistance. These two resins, each with different properties, are combined. By appropriately adjusting the mixing ratio of resin AAX and resin AAY, the amount of curing agents and other additives added to each resin, and the mixing conditions, a mixed or compatible resin can be obtained. The resulting resin will have predetermined properties regarding light transmission, refractive index, glass transition temperature, etc. By making the resulting resin a material with predetermined properties, a material suitable for optical waveguides can be obtained.

[0064] Furthermore, in resins where two components are combined, the refractive index of the core 3 and the cladding 2 is adjusted to a predetermined value.

[0065] A resin in which two or more polymers or copolymers are mixed or compatible is a resin in which the resin component is 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 the resins of two or more polymers or copolymers. For example, resin AA may have excellent light transmission, film formation properties, and heat resistance, while resin BB may have excellent light transmission, resolution, and chemical resistance, thus combining resins with different properties. By appropriately adjusting the mixing ratio of resin AA and resin BB, the amount of curing agents and other additives added to each resin, and the mixing conditions, a mixed or compatible resin can be obtained. The resulting resin will be a material with predetermined properties in terms of light transmission, refractive index, glass transition temperature, etc. By making the resulting resin a material with predetermined properties, a material suitable for optical waveguides can be obtained.

[0066] Furthermore, the resin, which is a combination of two different components, has its refractive index adjusted to a predetermined value. In addition, the resin constituting the core 3 and the resin constituting the cladding 2 are both combinations of two different components, and the refractive index of the core 3 and cladding 2 is adjusted to a predetermined value.

[0067] A resin in which two or more polymers or copolymers are mixed or miscible is a resin whose resin components are a combination of three. Furthermore, we will explain the resin combinations as a resin of three polymers or copolymers. For convenience, we will refer to the three resins as resin α, dendritic β, and resin δ. There are three resin combinations as a resin of three polymers or copolymers. The first is when resin α, dendritic β, and resin δ are all the same resin component; the second is when two of the resins α, dendritic β, and resin δ are the same resin component, and the remaining resin is a different resin component from these two; and the third is when resin α, dendritic β, and resin δ are all different resin components.

[0068] Even with these combinations, resins α, dendritic β, and resin δ are combined, each possessing different properties such as light transmission, film formation ability, heat resistance, resolution, and chemical resistance, with each resin excelling in these properties as needed. By appropriately adjusting the mixing ratio of resins α, dendritic β, and resin δ, the amount 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 becomes a material with predetermined properties in terms of light transmission, refractive index, glass transition temperature, etc. By making this resulting resin a material with predetermined properties, a material suitable for optical waveguides can be obtained.

[0069] Furthermore, the resin, which is a combination of three components, has its refractive index adjusted to a predetermined value. In addition, the resin constituting the core 3 and the resin constituting the cladding 2, which are a combination of three components, have their refractive indices adjusted to predetermined values.

[0070] In addition to the above, even when there are four or more combinations of resins, by utilizing the properties of each resin and combining them, a material with predetermined properties can be obtained that is suitable for optical waveguides. The resins AA, BB, and CC shown in the examples are formed from any thermosetting resin or any photocurable resin. The combinations shown in Examples 1 to 3 are either any thermosetting resin or any photocurable resin, or resins containing both. Furthermore, resins with four or more combinations have their refractive index adjusted to a predetermined value. Moreover, in resins where the resins constituting the core 3 and the resins constituting the cladding 2 are combinations of four or more, the refractive index of the core 3 and cladding 2 are each adjusted to a predetermined value.

[0071] In optical waveguides, it is necessary to create 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 a resin in which two or more polymers or copolymers are mixed or miscible at an appropriate mixing ratio and mixing conditions as the core and cladding materials. Furthermore, by mixing or miscibleing two or more polymers or copolymers, the deterioration of the material from the time of fabrication to use of the optical waveguide material due to mixing or miscibleing can be suppressed. Not limited to two polymers or copolymers, materials with properties suitable for optical waveguides, such as light transmittance and refractive index within a desired range, can also be obtained by mixing or miscibleing three or more polymers or copolymers.

[0072] The resin constituting the core 3 and cladding 2 is preferably a mixture or miscible of two or more polymers or copolymers, and its refractive index is adjusted. In the embodiment, it is desirable that the refractive index of the resin constituting the core 3 and cladding 2 of the optical waveguide 1 is adjusted to 1.4 to 1.6. Setting the refractive index of the resin of the optical waveguide 1 to 1.4 or higher and 1.6 or lower makes optical transmission easier. Furthermore, it is even more desirable that the refractive index of the resin constituting the core 3 and cladding 2 is 1.4 or higher and less than 1.5. In an optical waveguide 1 having a core 3 and cladding 2 made of resin with an appropriately adjusted refractive index, it is considered that there is less leakage of optical signals propagating within the core 3, and efficient transmission of optical signals can be obtained.

[0073] Furthermore, in order to ensure the normal transmission of optical signals within the core 3, the refractive index of the resin constituting the core 3 is made higher than that of the resin constituting the cladding 2. The refractive index difference is preferably 0.03 or greater, and more preferably 0.05 or greater. That is, in the optical waveguide 1 of the embodiment, the refractive index of the core 3 is preferably 0.03 or greater than the refractive index of the lower cladding 21 and the upper cladding 22, and more preferably 0.05 or greater. When there is a refractive index difference of 0.03 or greater between the core 3 and the lower cladding 21 and the upper cladding 22, it is believed that less light signals propagating within the core 3 are transmitted to the cladding 2, resulting in good transmission performance.

[0074] Furthermore, the resins constituting the core 3 and cladding 2 have their refractive indices adjusted to a predetermined value. Specifically, the predetermined refractive index is within the range of 1.4 to 1.6, and the refractive index difference between the core 3 and cladding 2 is set to 0.03 or greater. For example, the predetermined refractive index is thought to be determined by the size of the core 3 of the optical waveguide 1, the form of optical transmission within the core 3, and the design of external connections such as optical elements or optical fibers that optically couple with the optical waveguide 1. By adjusting the refractive index to a predetermined value, when an optical signal is transmitted within the optical waveguide 1, there is no leakage from the optical waveguide 1, and the optical signal can be transmitted normally.

[0075] <Compatibility State in the First and Second Compatibility Layers> Figure 7A shows an example of the compatibility state of the first compatibility layer C1 in the optical waveguide of the embodiment. Figure 7A schematically shows an example of stepwise compatibility in the first compatibility layer C1. The portion of the upper cladding 22 extending in the +Z direction from interface I1 is called the vicinity of the interface N1, and the end face of the vicinity of the interface N1 is shown by the dashed line NL1. The portion of the core 3 extending in the -Z direction from interface I1 is called the vicinity of the interface N2, and the end face of the vicinity of the interface N2 is shown by the dashed line NL2. The length of the vicinity of the interface N1 from interface I1 and the length of the vicinity of the interface N2 from interface I1 are the same.

[0076] In the example shown in Figure 7A, the cladding resin material and the core resin material are compatible from interface I1 to the vicinity N2 of the interface. For ease of explanation, the vicinity N2 of the interface is layered in three stages, and the layers are labeled S-1, S-2, and S-3 in order from the layer closest to interface I1. 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. Note that the number of stages of compatibility from interface I1 to the vicinity N2 of the interface may be three, or it may be more or fewer layers.

[0077] Another example in Figure 7A is compatibility example 1, in which a portion of the core resin material and cladding resin material is formed in a compatible area extending from interface I1 to the vicinity N1 of the interface on the upper cladding 22 side. It is presumed that the degree of compatibility in compatibility example 1 decreases in stages as you move away from interface I1.

[0078] Another example of compatibility is Example 2, in which a portion of the core resin material and cladding resin material is formed in a region extending from interface I1 to the interface vicinity N1 on the upper cladding 22 side and from interface I1 to the interface vicinity N2 on the core 3 side. In Compatibility Example 2 as well, it is presumed that the degree of compatibility decreases in stages as the distance from interface I1 increases. In this case, the length of the interface vicinity N1 from interface I1 and the length of the interface vicinity N2 from interface I1 may be the same or different. In any case, the formation of a portion of the core resin material and cladding resin material that are compatible near interface I1 ensures close contact between the core 3 and the upper cladding 22. The first compatible layer C1 may also be formed by extending it to the side surface of the core 3.

[0079] Figure 7B shows an example of the compatibility state of the second compatible layer C2 in the optical waveguide of the embodiment. Figure 7B schematically shows an example of stepwise compatibility in the second compatible layer C2. The portion extending from interface I2 towards the core 3 in the +Z direction is called the vicinity of the interface N3, and the end face of the vicinity of the interface N3 is shown by the dashed line NL3. The portion extending from interface I2 towards the lower cladding 21 in the -Z direction is called the vicinity of the interface N4, and the end face of the vicinity of the interface N4 is shown by the dashed line NL4. The length of the vicinity of the interface N3 from interface I2 is the same as the length of the vicinity of the interface N4 from interface I2.

[0080] In the example shown in Figure 7B, the cladding resin material and the core resin material are compatible from interface I2 to the vicinity N3 of the interface. For ease of explanation, the vicinity N3 of the interface is layered in three stages, and the layers are labeled S-4, S-5, and S-6 in order from the layer closest to interface I2. In the compatible portion, S-4 has a high degree of compatibility, S-5 has a lower degree of compatibility than S-4, and S-6 has the lowest degree of compatibility. This shows an example in which the degree of compatibility decreases in stages in the compatible portion. Note that the number of stages of compatibility from interface I2 to the vicinity N3 of the interface may be three, or it may be more or fewer layers.

[0081] Another example shown in Figure 7B is compatibility example 3, in which a portion of the core resin material and cladding resin material is found to be compatible from interface I2 to the vicinity N4 of the interface on the lower cladding 21 side. It is presumed that the degree of compatibility in compatibility example 3 decreases in stages as you move away from interface I2.

[0082] Another example of compatibility is Example 4, in which a portion of the core resin material and cladding resin material is formed in a region extending from interface I2 to the interface vicinity N3 on the core 3 side and from interface I2 to the interface vicinity N4 on the lower cladding 21 side. In Compatibility Example 4 as well, it is presumed that the degree of compatibility decreases in stages as the distance from interface I2 increases. In this case, the length of the interface vicinity N3 from interface I2 and the length of the interface vicinity N4 from interface I2 may be the same or different. In any case, the formation of a portion of the core resin material and cladding resin material that are compatible near interface I2 ensures close contact between the core 3 and the lower cladding 21. The second compatible layer C2 may also be formed by extending it to the side surface of the core 3.

[0083] In the example in Figure 7A, a rough surface is formed at the end face of the interface near N2, indicated by the dashed line NL2, that is, at the interface between the mismatched region NC1 of the core 3 and the first mismatched layer C1. Therefore, it is considered that the adhesion between region NC1 and the first mismatched layer C1 is improved. Consequently, it is considered that delamination between region NC1 and the first mismatched layer C1 is suppressed. Similarly, in the example in Figure 7B, a rough surface is formed at the end face of the interface near N3, indicated by the dashed line NL3, that is, at the interface between the mismatched region NC2 of the core 3 and the second mismatched layer C2. Therefore, it is considered that the adhesion between region NC2 and the second mismatched layer C2 is improved. Consequently, it is considered that delamination between region NC2 and the second mismatched layer C2 is suppressed. Here, if we denote the roughness of the interface between region NC1 in Figure 7A and the first compatible layer C1 as roughness R1, and the roughness of the interface between region NC2 in Figure 7B and the second compatible layer C2 as roughness R2, then roughness R1 may be greater than roughness R2. Also, roughness R2 may be greater than roughness R1. Alternatively, roughness R1 and roughness R2 may be approximately the same.

[0084] If the resin material forming the core 3 and cladding 2 is a resin in which two or more polymers or copolymers are mixed or compatible, adhesion between the core 3 and cladding 2 is ensured. Ensuring adhesion makes it less likely for problems such as peeling of the core 3 to occur when the optical waveguide 1 is in use.

[0085] Furthermore, by using a resin in which two or more types of resins are mixed or compatible for the formation of the core 3 and the upper cladding 22, it is considered that the cladding resin material and the core resin material are compatible at or near the interface I1. The formation of compatible portions at or near the interface I1 ensures adhesion between the upper cladding 22 and the core 3. Therefore, defects such as delamination of the core 3 are less likely to occur. Moreover, even if thermal stress due to the temperature difference expected when using the optical waveguide 1 is transmitted to the interface I between the upper cladding 22 and the core 3, it is considered that the stress will be relieved near interface I. In addition, because the cladding resin material and the core resin material are compatible, even when reliability tests such as high-temperature and high-humidity storage are performed, the intrusion of moisture into the interface between the core 3 and cladding 2 of the optical waveguide 1 can be suppressed, and delamination between the core 3 and cladding 2 is less likely to occur. Therefore, reliability against delamination between the core 3 and cladding 2 can also be ensured. In this way, two performance characteristics, adhesion and reliability, can be obtained.

[0086] Furthermore, if the resin material forming the core 3 and the upper cladding 22 is a resin in which two or more polymers or copolymers are mixed or compatible, and the main resin for the core 3 and the upper cladding 22 is the same, the core 3 and the upper cladding 22 will be more compatible, the adhesion will be more stable, and problems such as peeling of the core 3 will be less likely to occur in the optical waveguide.

[0087] Furthermore, by using a resin material in which two or more resin components are mixed or compatible, and in which the main resin is the same, for the formation of the core 3 and the upper cladding 22, it is considered that the upper cladding 22 and the core 3 become more compatible at the interface I1 between the upper cladding 22 and the core 3, or at or near the interface I1. The formation of compatible areas ensures good adhesion between the core 3 and the upper cladding 22.

[0088] In other words, by using the same main resin for both the upper cladding 22 and the core 3, the upper cladding 22 and the core 3 can be made compatible at the interface I1. This compatibility allows for chemical adhesion. The chemical adhesion ensures good adhesion between the core 3 and the upper cladding 22. Furthermore, even if thermal stress due to temperature differences expected during the use of the optical waveguide is transmitted to the interface I1 between the upper cladding 22 and the core 3, it is thought that the stress will be relieved near the interface I1. In addition, because the resin material for the cladding and the resin material for the core are compatible, even when reliability tests such as high-temperature and high-humidity storage are performed, the intrusion of moisture into the interface between the core 3 and the cladding 2 of the optical waveguide 1 can be suppressed, making delamination between the core 3 and the cladding 2 less likely. Therefore, reliability against delamination between the core 3 and the upper cladding 22 can also be ensured. As a result, two performance characteristics, adhesion and reliability, can be obtained.

[0089] Similarly, by using a resin in which two or more resins are mixed or compatible for the formation of the core 3 and the lower cladding 21, it is considered that the cladding resin material and the core resin material are compatible at or near the interface I2. The formation of compatible portions at or near the interface I2 ensures adhesion between the lower cladding 21 and the core 3. Therefore, defects such as delamination of the core 3 become less likely to occur. Furthermore, even if thermal stress due to the temperature difference expected when using the optical waveguide 1 is transmitted to the interface I2 between the lower cladding 21 and the core 3, it is considered that the stress is relieved near the interface I2. In addition, because the cladding resin material and the core resin material are compatible, even when reliability tests such as high-temperature and high-humidity storage are performed, the intrusion of moisture into the interface between the core 3 and the cladding 2 of the optical waveguide 1 can be suppressed, and delamination of the core 3 and the cladding 2 becomes less likely. Therefore, reliability against delamination of the core 3 and the cladding 2 can also be ensured. In this way, two performance characteristics, adhesion and reliability, can be obtained.

[0090] Furthermore, if the resin material forming the core 3 and the lower cladding 21 is a resin in which two or more polymers or copolymers are mixed or compatible, and the main resin for the core 3 and the lower cladding 21 is the same, the core 3 and the lower cladding 21 will be more compatible, the adhesion will be more stable, and problems such as peeling of the core 3 will be less likely to occur in the optical waveguide.

[0091] Furthermore, by using a resin material in which two or more resin components are mixed or compatible, and in which the main resin is the same, for the formation of the core 3 and the lower cladding 21, it is considered that the lower cladding 21 and the core 3 become more compatible at or near the interface I2 between the lower cladding 21 and the core 3. The formation of compatible areas ensures good adhesion between the core 3 and the lower cladding 21.

[0092] In other words, by using the same main resin for both the lower cladding 21 and the core 3, the lower cladding 21 and the core 3 can be made compatible at the interface I2. This compatibility allows for chemical adhesion. The chemical adhesion ensures good adhesion between the core 3 and the lower cladding 21. Furthermore, even if thermal stress due to temperature differences expected during the use of the optical waveguide is transmitted to the interface I2 between the lower cladding 21 and the core 3, the stress is expected to be relieved near the interface I2. In addition, because the resin material for the cladding and the resin material for the core are compatible, even when reliability tests such as high-temperature and high-humidity storage are performed, the intrusion of moisture into the interface between the core 3 and the cladding 2 of the optical waveguide 1 can be suppressed, making delamination between the core 3 and the cladding 2 less likely. Therefore, reliability against delamination between the core 3 and the lower cladding 21 can also be ensured. As a result, two performance characteristics, adhesion and reliability, can be obtained.

[0093] As shown in the examples in Figures 1 to 4, there are cases such as those described in (1) to (3) below: (1) The core 3 is exposed in the upper cladding-free region 1a. (2) The core 3 is a fine linewidth core with a width of 15 μm or less. (3) In the upper cladding-free region 1a, the width of the core 3 is 15 μm or less, and the core 3 is exposed. In the optical waveguide of the embodiment, even if the optical waveguide is constructed in these cases, it is considered that three effects can be obtained: ensuring adhesion between the core 3 and the cladding 2, ensuring reliability, and ensuring the transmission of optical signals.

[0094] As shown in the examples in Figures 1 to 4, even when the core 3 is exposed in the upper cladding-free region 1a, and the core 3 is a fine-linewidth core with a width of 15 μm or less, and the optical signal is transmitted in 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 transmission of the optical signal. Furthermore, it is believed that the transmission of the optical signal in single mode can also be stabilized. In addition, in the cases shown in (1) to (3) above, it is believed that the above effects can be obtained even if the core 3 is a fine-linewidth core with a width of 10 μm or less.

[0095] <Specific Examples of Resin Materials for Core and Cladding> When the core 3 and cladding 2 are formed using a resin composed of one or more polymers, or a resin composed of one or more copolymers, the following are examples of resins that can be used as materials.

[0096] Examples of one polymer or copolymer that constitutes the resin forming the core 3 or cladding 2 include epoxy resin, acrylic resin, phenolic resin, silicone resin, and fluororesin. Examples of two or more polymers or copolymers that constitute the resin forming the core 3 or cladding 2 include: a combination of epoxy resin as the main resin and a resin selected from epoxy resin, acrylic resin, silicone resin, phenolic resin, and fluororesin as the other resins; a combination of acrylic resin as the main resin and a resin selected from epoxy resin, acrylic resin, silicone resin, phenolic resin, and fluororesin as the other resins; a combination of phenolic resin as the main resin and a resin selected from epoxy resin, acrylic resin, silicone resin, phenolic resin, and fluororesin as the other resins; a combination of silicone resin as the main resin and a resin selected from epoxy resin, acrylic resin, silicone resin, phenolic resin, and fluororesin as the other resins; and a combination of fluororesin as the main resin and a resin selected from epoxy resin, acrylic resin, silicone resin, phenolic resin, and fluororesin as the other resins.

[0097] In this case, it is desirable that the main resin and the other resins belong to the same resin family, such as a combination of epoxy resin as the main resin and epoxy resin as the other resin. The interaction parameters of these combinations of resins from the same family are small, and therefore they are favored because they have good compatibility. Note that these resin combinations are just examples, and other combinations may also be used.

[0098] Furthermore, examples of combinations of polymers constituting the resin that forms the core 3 and the polymers constituting the resin that forms the cladding 2 are provided. As the polymer that forms the core 3, the main resin can be any of epoxy resin, acrylic resin, phenolic resin, silicone resin, or fluororesin. As the polymer that forms the cladding 2 that is combined with it, the main resin can be any of epoxy resin, acrylic resin, phenolic resin, silicone resin, or fluororesin.

[0099] Furthermore, examples of combinations of copolymers constituting the resin that forms the core 3 and the copolymers constituting the resin that forms the cladding 2 include: a combination of epoxy resin as the resin constituting the core 3 and a resin selected from epoxy resin, acrylic resin, phenolic resin, silicone resin, and fluororesin as the resin constituting the cladding 2; a combination of acrylic resin as the resin constituting the core 3 and a resin selected from epoxy resin, acrylic resin, phenolic resin, silicone resin, and fluororesin as the resin constituting the cladding 2; a combination of phenolic resin as the resin constituting the core 3 and a resin selected from epoxy resin, acrylic resin, phenolic resin, silicone resin, and fluororesin as the resin constituting the cladding 2; and a combination of fluororesin as the resin constituting the core 3 and a resin selected from epoxy resin, acrylic resin, phenolic resin, silicone resin, and fluororesin as the resin constituting the cladding 2.

[0100] Of these, it is desirable to combine resins of the same type, such as using epoxy resin as the resin constituting the core 3 and epoxy resin as the resin constituting the cladding 2. Combining resins of the same type is preferable because the compatibility between the resin materials forming the core 3 or cladding 2 is high, which ensures high adhesion between the core 3 and cladding 2. Note that these resin combinations are just examples, and other combinations are also acceptable.

[0101] For example, in the optical waveguide of the embodiment, the core 3 is made of acrylic resin, and the cladding 2 is made of acrylic resin. Otherwise, the resin materials constituting the core 3 and cladding 2 may be epoxy resin, phenolic resin, silicone resin, and fluororesin. Also, in a modified example of the optical waveguide of the embodiment, the core 3 may be made of epoxy resin, and the cladding 2 may be made of epoxy resin. Otherwise, the resin materials constituting the core 3 and cladding 2 may be acrylic resin, phenolic resin, silicone resin, and fluororesin. With a core 3 and cladding 2 made of these combinations of materials, the adhesion between the core 3 and cladding 2 is stable. Problems such as peeling of the core 3 are suppressed. In addition, reliability against core peeling is ensured. Furthermore, it is thought that good transmission of optical signals can be obtained. Note that silicone resin is also called silicon resin and refers to materials having silane compounds, siloxane bonds, etc.

[0102] <Structure of the Optical Waveguide in the Second Embodiment> Figure 8A shows a plan view of optical waveguide 1α, which is an example of an optical waveguide in the second embodiment. Figure 8B shows a cross-section of optical waveguide 1α in Figure 8A along the line VIIIB-VIIIB. Optical waveguide 1α, like optical waveguide 1 in Figure 1, is made of resin and consists of an upper cladding 22, a core 3, and a lower cladding 21. The resin constituting optical waveguide 1α includes a thermosetting resin, a photocurable resin, or a resin containing both. Preferably, the resin constituting optical waveguide 1α is a resin in which two or more resins are mixed or compatible and the refractive index is adjusted.

[0103] In the optical waveguide 1α, a first compatible layer C1 is formed at the interface I1 between the upper cladding 22 and the core 3, along the direction in which the optical signal from the core 3 is transmitted, and a second compatible layer C2 is formed at the interface I2 between the core 3 and the lower cladding 21. Therefore, in the optical waveguide 1α, adhesion is ensured between the upper cladding 22 and the core 3, and also between the core 3 and the lower cladding 21. As a result, it is considered that defects such as delamination of the core 3 are unlikely to occur. Furthermore, because the first compatible layer C1 and the second compatible layer C2 are formed, even when reliability tests such as high-temperature and high-humidity storage are performed, the intrusion of moisture into the interface between the core 3 and the cladding 2 of the optical waveguide 1 can be suppressed, and it is considered that delamination of the core 3 and the cladding 2 is unlikely to occur. The thickness of the core 3 is not particularly limited, but it may be 1 μm to 50 μm, preferably 1 μm to 20 μm, and more preferably 3 μm to 10 μm. The width of core 3 is not particularly limited, but it may be 1 μm to 50 μm, preferably 1 μm to 20 μm, and more preferably 3 μm to 10 μm. Furthermore, it is desirable to appropriately adjust the thickness and width of core 3 according to the shape and size of the optical waveguide.

[0104] In the optical waveguide 1α, the thickness of the first compatible layer C1 and the thickness of the second compatible layer C2 may be the same or different. It is preferable that the thickness of the first compatible layer C1 and the thickness of the second compatible layer C2 are different. It is also preferable that the glass transition temperature Tg1 of the upper cladding 22 and the glass transition temperature Tg2 of the core 3 are different. Furthermore, it is preferable that the glass transition temperature Tg3 of the lower cladding 21 and the glass transition temperature Tg2 of the core 3 are different. If the glass transition temperature Tg1 of the upper cladding 22 and the glass transition temperature Tg2 of the core 3 are different, the upper cladding 22 and the core 3 can be easily made compatible. If the glass transition temperature Tg3 of the lower cladding 21 and the glass transition temperature Tg2 of the core 3 are different, the lower cladding 21 and the core 3 can be easily made compatible.

[0105] On the other hand, the difference between optical waveguide 1α and optical waveguide 1 in Figure 1 is that optical waveguide 1α does not have a core exposed area like the upper cladding-free region 1a of optical waveguide 1. That is, an upper cladding-forming region 1b is formed over the entire surface of optical waveguide 1α, where the upper cladding 22 is formed.

[0106] Therefore, in the optical waveguide 1α, in a plan view, there is no core-exposed portion where the upper surface 31 of the core 3 is exposed, only the upper cladding region 1b, and there is a core-non-exposed portion where the upper surface 31 of the core 3 is not exposed. The optical waveguide 1α is composed entirely of the upper cladding region 1b. Thus, at one end 11 of the optical waveguide 1α, only the end face 33 of the core 3 is exposed, and at the other end 12, only the end face 34 of the core 3 is exposed.

[0107] The optical waveguide 1α is optically coupled to an optical component, including a photoelectric conversion element, at one end 11, for example, when in use. In Figures 8A and 8B, component E2 is shown by a dashed line as an example of an optical component that is optically coupled to the core 3 of the optical waveguide 1α at one end 11. Component E2 is equipped with an optical terminal E2a, which is a portion to which an optical signal is input or output. The optical terminal E2a is exposed on the side surface E22 of component E2, and an optical signal is input or output at the portion of the optical terminal E2a exposed to the side surface E22. In the optical waveguide 1α, only the end face 33 of the core 3 is exposed from the cladding 2 at one end 11, so the optical waveguide 1α and component E2 are arranged so that the core 3 and the optical terminal E2a are end-face connected or butt-coupled. With such arrangement, the core 3 of the optical waveguide 1α and the optical terminal E2a of component E2 are appropriately optically coupled. The optical signal propagating from the other end 12 through the core 3 enters component E2 at one end 11 via the optical terminal E2a. On the other hand, the optical signal from the optical terminal E2a of component E2 enters the core 3 at one end 11, propagates within the core 3, and is output from the other end 12. Although not shown, the region between component E2 and the optical waveguide 1α is preferably filled with any optically transparent resin.

[0108] In the optical waveguide 1α, a first compatible layer C1 is formed at the interface between the upper cladding 22 and the core 3, and a second compatible layer C2 is formed between the core 3 and the lower cladding 21. Therefore, since adhesion and reliability of optical coupling are obtained between the core 3 and the cladding 2, the optical waveguide of the embodiment is considered useful even in bonding configurations where the optical waveguide and optical components are joined at the end face, as in the second embodiment.

[0109] In Figures 8A and 8B, components of optical waveguide 1α that have the same functions as components of optical waveguide 1 in Figure 1 are either denoted by the same reference numerals as in Figure 1 or omitted as appropriate, and repetitive explanations regarding these similar components are omitted. The overall configuration of the optical waveguide, as well as the materials, functions, operations of each component, and their variations, as explained for optical waveguide 1 as an example in Figure 1, are also applicable to optical waveguide 1α as an example in Figures 8A and 8B, except for those relating to the core exposed portion, which is the upper cladding-free region 1a. That is, in optical waveguide 1α as well, when an optical signal is transmitted to the core 3 of optical waveguide 1α having the first compatible layer C1 and the second compatible layer C2, each compatible layer acts as a shield for the optical signal, preventing leakage of the optical signal from the core and allowing the optical signal to be properly reflected within the core 3. Therefore, it is considered that it can handle higher speeds and larger capacities of optical signals.

[0110] <Structure of the Wiring Board in the Embodiment> The wiring boards in the embodiments of the present invention are divided into optical waveguides of the first embodiment and optical waveguides of the second embodiment. The wiring board in the optical waveguide of the first embodiment of the present invention is shown in Figures 9A and 9B. Figure 9A shows a cross-sectional view of the wiring board 100a of Embodiment 1, on which an example of the optical waveguide of the first embodiment is arranged, and Figure 9B shows a cross-sectional view of the wiring board 100b of Embodiment 2, on which a modified example of the optical waveguide of the first embodiment is arranged. Furthermore, the wiring board in the optical waveguide of the second embodiment of the present invention is shown in Figures 9C and 9D. Figure 9C shows a cross-sectional view of the wiring board 100c of Embodiment 3, on which an example of the optical waveguide of the second embodiment is arranged, and Figure 9D shows a cross-sectional view of the wiring board 100d of Embodiment 4, on which a modified example of the optical waveguide of the second embodiment is arranged.

[0111] Note that the wiring boards 100a to 100d shown in Figures 9A to 9D are merely examples of wiring boards in the embodiment. The laminated structure of the wiring board in the embodiment is not limited to any of the laminated structures of the wiring boards 100a to 100d shown in Figures 9A to 9D. Furthermore, the number of conductor layers and insulating layers in the wiring board in the embodiment is not limited to the number of conductor layers and insulating layers included in any of the wiring boards 100a to 100d.

[0112] As shown in Figures 9A to 9D, the wiring boards 100a to 100d all include a wiring board 110. The wiring board 110 is a board that includes wiring consisting of an insulating layer and a conductor layer formed on the insulating layer. Specifically, the wiring board 110 includes conductor layers 41 to 43 as a conductor layer and insulating layers 51 and 52 as an insulating layer. The wiring board 100a in Figure 9A includes an optical waveguide 101a arranged on the wiring board 110, and the wiring board 100b in Figure 9B includes an optical waveguide 101b arranged on the wiring board 110. Furthermore, the wiring board 100c in Figure 9C includes an optical waveguide 101c arranged on the wiring board 110, and the wiring board 100d in Figure 9D includes an optical waveguide 101d arranged on the wiring board 110.

[0113] Optical waveguides 101a to 101d are all optical waveguides of the embodiments described above. Therefore, each optical waveguide is made of resin and consists of an upper cladding 22, a core 3, and a lower cladding 21. The resin constituting optical waveguides 101a to 101d includes thermosetting resins or photocurable resins. Preferably, the resin constituting each optical waveguide is a resin in which two or more resins are mixed or compatible and whose refractive index is adjusted. In optical waveguides 101a to 101d as well, a first compatible layer C1 is formed at the interface I1 between the upper cladding 22 and the core 3 along the direction in which the optical signal of the core 3 is transmitted, and a second compatible layer C2 is formed at the interface I2 between the core 3 and the lower cladding 21.

[0114] The wiring board 110 has a first surface 110F which is the mounting surface for component E1 or component E2, and a second surface 110S which is the opposite surface to the first surface 110F. The wiring board 110 has component mounting pads 4, which are conductor pads included in the conductor layer 41, on the first surface 110F. In the wiring boards 100a to 100d, each optical waveguide is arranged on the first surface 110F.

[0115] The conductor layers 41-43 and insulating layers 51 and 52 are stacked in the order of conductor layer 43, insulating layer 52, conductor layer 42, insulating layer 51, and conductor layer 41, from the second surface 110S side to the first surface 110F side of the wiring board 110. Conductor layer 41 and conductor layer 42 are connected by via conductors 7 that penetrate the insulating layer 51. Conductor layer 42 and conductor layer 43 are connected by via conductors 7 that penetrate the insulating layer 52.

[0116] The wiring board 110 includes a solder resist 62 covering the conductor layer 43 and the insulating layer 52, and a solder resist 61 covering the conductor layer 41 and the insulating layer 51. The solder resists 61 and 62 are formed from, for example, a photosensitive epoxy resin or polyimide resin. 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 connections between the wiring board 110 and external components located on the second surface 110S side (for example, a motherboard of any electrical equipment). Note that the wiring boards 100a to 100d may be used as motherboards without bumps 8.

[0117] The insulating layers 51 and 52 can be formed using thermosetting insulating resins such as epoxy resin, bismaleimide triazine resin (BT resin), or phenolic resin. The insulating layers 51 and 52 may also be formed using thermoplastic insulating resins such as fluororesin, liquid crystal polymer (LCP), fluoroethylene (PTFE) resin, polyester (PE) resin, and modified polyimide (MPI) resin. Note that the resins listed as materials for these insulating layers are merely examples of materials that can form each insulating layer. Each insulating layer can be formed from any material capable of providing insulation between the conductor layers in the wiring board 110. Although not shown, each insulating layer may contain a core material made of reinforcing material made of glass fibers or aramid fibers, and may also contain inorganic fillers made of fine particles such as silica (SiO2), alumina, or mullite.

[0118] Examples of conductors constituting the conductor layers 41-43 and the via conductor 7 include copper, nickel, and silver, with copper or an alloy mainly composed of copper being preferred. Each of these conductors may have a multilayer structure including two or more films. For example, the conductor layers 41-43 and the via conductor 7 may have a two-layer structure including an electroless plating film and an electrolytic plating film.

[0119] The solder resist 61 has an opening 61a, and the component mounting pads 4 are exposed within the opening 61a. Each optical waveguide is arranged on top of the solder resist 61. Although not shown, each optical waveguide is fixed to the first surface 110F of the wiring board 110 with any fixing material such as adhesive.

[0120] <Wiring board of Embodiment 1> Component E1 is mounted on the wiring board 100a in Figure 9A. Component E1 is an optical component including a photoelectric conversion element, as described in the description of the optical waveguide 1 in Figure 1, etc. Component E1 comprises an optical terminal E1a and a ball-shaped electrode E1b. Examples of component E1 include light-receiving elements such as photodiodes, and light-emitting elements such as light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs), laser diodes (LDs), and vertical-resonant surface-emitting lasers (VCSELs). When component E1 is a light-emitting element, component E1 generates an optical signal based on an electrical signal input to electrode E1b and outputs that optical signal from the optical terminal E1a, which functions as a light-emitting part, toward the core 3. When component E1 is a light-receiving element, an electrical signal is generated based on an optical signal input from the optical terminal E1a, which functions as a light-receiving part, and output from electrode E1b.

[0121] Component E1 is mounted on the wiring board 110 by connecting its electrode E1b to the component mounting pad 4 using, for example, solder. The optical terminal E1a and the exposed portion of the core 3 are positioned to face each other and are optically coupled in the form of adibatic coupling. The gap between the optical terminal E1a and the core 3 facing the optical terminal E1a is preferably filled with an optically transparent transparent resin TR having an appropriate refractive index. The thickness of the core of the optical waveguide 1 used in the wiring board of Embodiment 1 is not particularly limited, but may be 1 μm to 50 μm, preferably 1 μm to 20 μm, and more preferably 3 μm to 10 μm.

[0122] The optical waveguide 101a of the wiring board 100a shown in Figure 9A has the same structure as the optical waveguide 1 shown in Figures 1 to 4. That is, a first compatible layer C1 is formed at the interface I1 between the upper cladding 22 and the core 3, and a second compatible layer C2 is formed at the interface I2 between the core 3 and the lower cladding 21. In the optical waveguide 101a, the thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 are equal or different. It is desirable that the thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 are different.

[0123] In the optical waveguide 101a shown in Figure 9A, the thickness T1 of the first miscible layer C1 and the thickness T2 of the second miscible layer C2 satisfy the following relationship: T1 > T2 ... (Equation 1)

[0124] Thus, in the optical waveguide 101a, a first compatible layer C1 is formed at interface I1, and a second compatible layer C2 is formed at interface I2. The thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 are different, and the thicknesses T1 and T2 satisfy the relationship in equation 1 above. Therefore, adhesion is ensured between the upper cladding 22 and the core 3, as well as between the core 3 and the lower cladding 21, and it is thought that the stress on the entire core 3 due to thermal history and other factors is reduced. As a result, it is thought that defects such as delamination of the core 3 are less likely to occur.

[0125] The thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 are not particularly limited, but are preferably 50 to 2000 nm. It is considered that the thickness of the core 3 contributes to the formation of each compatible layer. Therefore, it is also considered desirable that the thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 be in the range of 0.5% to 10% of the thickness of the core 3.

[0126] <Wiring board of Embodiment 2> Component E1 is mounted on the wiring board 100b in Figure 9B. Component E1 is an optical component, including a photoelectric conversion element, which has an optical terminal E1a and a ball-shaped electrode E1b, as described above with respect to the wiring board of Embodiment 1. Examples of component E1 include light-receiving elements such as photodiodes, and light-emitting elements such as LEDs, OLEDs, LDs, and VCSELs. Component E1 may generate an optical signal based on an electrical signal input to electrode E1b and output it from optical terminal E1a, or it may generate an electrical signal based on an optical signal input from optical terminal E1a and output it from electrode E1b.

[0127] Component E1 is mounted on the wiring board 110 by connecting its electrode E1b to the component mounting pad 4 using, for example, solder. The optical terminal E1a and the exposed portion of the core 3 are positioned to face each other and are optically coupled in the form of adibatic coupling. The gap between the optical terminal E1a and the core 3 facing the optical terminal E1a is preferably filled with an optically transparent transparent resin TR having an appropriate refractive index. The thickness of the core 3 of the optical waveguide of Embodiment 1 used in the wiring board of Embodiment 2 is not particularly limited, but may be 1 μm to 50 μm, preferably 1 μm to 20 μm, and more preferably 3 μm to 10 μm.

[0128] The optical waveguide 101b of the wiring board 100b shown in Figure 9B has the same structure as the optical waveguide 1 shown in Figures 1 to 4. That is, a first compatible layer C1 is formed at the interface I1 between the upper cladding 22 and the core 3, and a second compatible layer C2 is formed at the interface I2 between the core 3 and the lower cladding 21. In the optical waveguide 101b, it is preferable that the thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 are equal or different.

[0129] In the optical waveguide 101b shown in Figure 9B, the thickness T1 of the first miscible layer C1 and the thickness T2 of the second miscible layer C2 satisfy the following relationship: T1 ≤ T2 ... (Equation 2)

[0130] Thus, in the optical waveguide 101b, a first compatible layer C1 is formed at interface I1, and a second compatible layer C2 is formed at interface I2. The thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 are different, and the thicknesses T1 and T2 satisfy the relationship in equation 2 above. Therefore, adhesion is ensured between the upper cladding 22 and the core 3, as well as between the core 3 and the lower cladding 21, and it is thought that the stress on the entire core 3 due to thermal history and other factors is reduced. As a result, it is thought that defects such as delamination of the core 3 are less likely to occur. Note that if the difference between the thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 is within 15%, it is considered to be equivalent. In other words, if the correlation (T1 / T2) between the thickness T1 of the first miscible layer C1 and the thickness T2 of the second miscible layer C2 satisfies 0.85 ≤ (T1 / T2) ≤ 1.15, then the thickness T1 of the first miscible layer C1 and the thickness T2 of the second miscible layer C2 can be considered to be equivalent.

[0131] The thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 are not particularly limited, but are preferably 50 to 2000 nm. It is considered that the thickness of the core 3 contributes to the formation of each compatible layer. Therefore, it is also considered desirable that the thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 be in the range of 0.5% to 10% of the thickness of the core 3.

[0132] <Wiring board of Embodiment 3> The optical waveguide 101c of the wiring board 100c shown in Figure 9C has the same structure as the optical waveguide 1α shown in Figures 8A and 8B. That is, in the optical waveguide 101c, the upper cladding 22 is provided throughout, as explained with reference to Figures 8A and 8B. In other words, the optical waveguide 101c is formed only in the upper cladding formation region 1b, and the upper surface of the core 3 is not exposed. The thickness of the core 3 of the optical waveguide 2 used in the wiring board of Embodiment 3 is not particularly limited, but may be 1 to 50 μm, preferably 1 μm to 20 μm, and more preferably 3 μm to 10 μm.

[0133] Component E2 is mounted on the wiring board 100c in Figure 9C. Component E2 is an optical component, including a photoelectric conversion element, as described in the explanation of the optical waveguide 1α in Figure 8A, etc. Component E2 has an optical terminal E2a and an electrode E2b exposed on its side surface. Examples of component E2 include light-receiving elements such as photodiodes, and light-emitting elements such as LEDs, OLEDs, LDs, and VCSELs. Component E2 may generate an optical signal based on an electrical signal input to electrode E2b and output it from optical terminal E2a, or it may generate an electrical signal based on an optical signal input from optical terminal E2a and output it from electrode E2b. Component E2 is mounted on the wiring board 110 by connecting electrode E2b to component mounting pad 4, for example, using solder.

[0134] On the wiring board 100c, component E2 is positioned such that the core 3 and the optical terminal E2a of component E2 are optically coupled by end face connection or butt coupling. That is, the side surface of component E2 where the optical terminal E2a is exposed faces one end 11 of the optical waveguide 100c, and as a result, component E2 is mounted on the wiring board 110 such that the exposed portion of the optical terminal E2a faces the end face 33 of the core 3. Although not shown, it is preferable that the gaps between component E2 and each optical waveguide be filled with any optically transparent resin having an appropriate refractive index.

[0135] In the optical waveguide 101c shown in Figure 9C, a first compatible layer C1 is formed at the interface I1 between the upper cladding 22 and the core 3, and a second compatible layer C2 is formed at the interface I2 between the core 3 and the lower cladding 21. In the optical waveguide 101c, the thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 are equal or different. It is preferable that the thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 are different.

[0136] In the optical waveguide 101c shown in Figure 9C, the thickness T1 of the first miscible layer C1 and the thickness T2 of the second miscible layer C2 satisfy the following relationship: T1 > T2 ... (Equation 1)

[0137] Thus, in the optical waveguide 101c, a first compatible layer C1 is formed at interface I1, and a second compatible layer C2 is formed at interface I2. The thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 are different, and the thicknesses T1 and T2 satisfy the relationship in equation 1 above. Therefore, adhesion between the upper cladding 22 and the core 3 is ensured, as is adhesion between the core 3 and the lower cladding 21, and furthermore, the stress on the entire core 3 due to thermal history and other factors is reduced. As a result, it is considered that defects such as delamination of the core 3 are less likely to occur.

[0138] The thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 are not particularly limited, but are preferably 50 to 2000 nm. It is considered that the thickness of the core 3 contributes to the formation of each compatible layer. Therefore, it is also considered desirable that the thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 be in the range of 0.5% to 10% of the thickness of the core 3.

[0139] <Wiring board of Embodiment 4> The optical waveguide 101d of the wiring board 100d shown in Figure 9D has the same structure as the optical waveguide 1α shown in Figures 8A and 8B. That is, in the optical waveguide 101d, the upper cladding 22 is provided throughout, as explained with reference to Figures 8A and 8B. In other words, the optical waveguide 101d is formed only in the upper cladding formation region 1b, and the upper surface of the core 3 is not exposed. The thickness of the core 3 of the optical waveguide 2 used in the wiring board of Embodiment 4 is not particularly limited, but may be 1 to 50 μm, preferably 1 μm to 20 μm, and more preferably 3 μm to 10 μm.

[0140] Component E2 is mounted on the wiring board 100d in Figure 9D. Component E2 is an optical component, including a photoelectric conversion element, which has an optical terminal E2a and an electrode E2b exposed on the side surface of component E2, as described above with respect to the wiring board of Embodiment 3. Examples of component E2 include light-receiving elements such as photodiodes, and light-emitting elements such as LEDs, OLEDs, LDs, and VCSELs. Component E2 may generate an optical signal based on an electrical signal input to electrode E2b and output it from optical terminal E2a, or it may generate an electrical signal based on an optical signal input from optical terminal E2a and output it from electrode E2b. Component E2 is mounted on the wiring board 110 by connecting electrode E2b to component mounting pad 4, for example, using solder.

[0141] On the wiring board 100d, component E2 is positioned such that the core 3 and the optical terminal E2a of component E2 are optically coupled by end face connection or butt coupling. That is, the side surface of component E2 where the optical terminal E2a is exposed faces one end 11 of the optical waveguide 100d, and as a result, component E2 is mounted on the wiring board 110 such that the exposed portion of the optical terminal E2a faces the end face 33 of the core 3. Although not shown, it is preferable that the gaps between component E2 and each optical waveguide be filled with any optically transparent resin having an appropriate refractive index.

[0142] In the optical waveguide 101d shown in Figure 9D, a first compatible layer C1 is formed at the interface I1 between the upper cladding 22 and the core 3, and a second compatible layer C2 is formed at the interface I2 between the core 3 and the lower cladding 21. Furthermore, in the optical waveguide 101d, it is preferable that the thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 are equal or different.

[0143] In the optical waveguide 101d shown in Figure 9D, the thickness T1 of the first miscible layer C1 and the thickness T2 of the second miscible layer C2 satisfy the following relationship: T1 ≤ T2 ... (Equation 2)

[0144] Thus, in the optical waveguide 101d, a first compatible layer C1 is formed at interface I1, and a second compatible layer C2 is formed at interface I2. The thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 are different, and the thicknesses T1 and T2 satisfy the relationship in equation 2 above. Therefore, adhesion between the upper cladding 22 and the core 3 is ensured, as is adhesion between the core 3 and the lower cladding 21, and furthermore, the stress on the entire core 3 due to thermal history and other factors is reduced. As a result, it is considered that defects such as delamination of the core 3 are less likely to occur. Note that if the difference between the thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 is within 15%, they are considered equivalent. In other words, if the correlation (T1 / T2) between the thickness T1 of the first miscible layer C1 and the thickness T2 of the second miscible layer C2 satisfies 0.85 ≤ (T1 / T2) ≤ 1.15, then the thickness T1 of the first miscible layer C1 and the thickness T2 of the second miscible layer C2 can be considered to be equivalent.

[0145] The thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 are not particularly limited, but are preferably 50 to 2000 nm. It is considered that the thickness of the core 3 contributes to the formation of each compatible layer. Therefore, it is also considered desirable that the thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 be in the range of 0.5% to 10% of the thickness of the core 3.

[0146] Furthermore, in the optical waveguides 101a to 101d shown in Figures 9A to 9D, it is preferable that the core 3, upper cladding 22, and lower cladding 21 are formed from the same resin system in which two or more resins are mixed or compatible. When the core 3, upper cladding 22, and lower cladding 21 are formed from the same resin system in which two or more resins are mixed or compatible, a portion in which the core 3 and cladding 2 are compatible is easily formed at or near the interface between the core 3 and cladding 2, and the adhesion between the core 3 and cladding 2 is stabilized. In addition, it is thought that thermal stress is relieved near the interface between the core 3 and cladding 2, and reliability against delamination between the core 3 and cladding 2 can be ensured. In other words, in the wiring boards 100a to 100d, two performance characteristics can be obtained: adhesion between the core 3 and cladding 2, and reliability against delamination between the core 3 and cladding 2.

[0147] <Method for Manufacturing an Optical Waveguide in the First Embodiment> A method for manufacturing an optical waveguide in the first embodiment will be described with reference to Figures 10A to 10E. Hereinafter, as an example, a method in which an optical waveguide 101a is directly formed on a wiring substrate 110 as shown in Figure 9A will be described. As shown in Figure 10A, a wiring substrate 110 is prepared, and a lower cladding 21 is formed on the surface of the solder resist 61 of the wiring substrate 110. The lower cladding 21 is formed by forming a resin film of the lower cladding 21. For example, the resin material of the lower cladding 21 is applied onto the wiring substrate 110 by spin coating. Alternatively, the resin material of the lower cladding 21, which is formed into a film, may be thermosetting resin, photocurable resin, or a resin containing both. As the resin material of the lower cladding 21, a resin in which two or more polymers or copolymers are mixed or compatible may be used. The lower cladding 21 is obtained by thermosetting or photocuring.

[0148] After the lower cladding 21 is formed, a resin film of the core 3 is formed over the entire surface of the lower cladding 21. For example, the resin film of the core 3 is formed by applying the resin material of the core 3 by spin coating and pre-curing it by heating. Alternatively, the resin film of the core 3 may be formed by laminating the resin material of the core 3, which has been molded into a film, onto the surface of the lower cladding 21 and pre-curing it. The resin material of the core 3 may be a thermosetting resin, a photocurable resin, or a resin containing both. Furthermore, a resin having a higher refractive index than that of the lower cladding 21 may be used as the resin material of the core 3.

[0149] As shown in Figure 10B, the resin film of the core 3 is patterned, resulting in the formation of a predetermined number of cores 3 having a desired shape. The patterning of the resin film of the core 3 can be carried out by any method. For example, a predetermined number of cores 3 having a desired shape can be formed by a photolithography technique, which involves exposure and development through a mask (not shown) corresponding to the shape of the cores to be formed. The formation of cores 3 with a desired shape is not limited to photolithography, but can be carried out by any method such as photobleaching.

[0150] After patterning the resin film of core 3, the resin material of core 3 is cured by heating. As shown in Figure 10C, in the manufacturing of the optical waveguide of this embodiment, the resin material of core 3 is cured by heating so that the resin material of core 3 and the resin material of lower cladding 21 become compatible at the interface I2 between core 3 and lower cladding 21. The compatibility of the resin material of core 3 and the resin material of lower cladding 21 forms a second compatible layer C2 at interface I2. The second compatible layer C2 is formed along the X direction in which the optical signal is transmitted when the optical waveguide is in use.

[0151] The resin material of the core 3 and the resin material of the lower cladding 21 can be made compatible at the interface I2 by appropriately adjusting the curing conditions, such as temperature and pressure, when the resin material of the core 3 is cured on the lower cladding 21. By making the resin material of the core 3 and the resin material of the lower cladding 21 compatible, the resin of the core 3 and the resin of the lower cladding 21 can be brought into close contact. In other words, although the resin for the core and the resin for the lower cladding are resins with different compositions and properties, by making the core resin and the lower cladding resin compatible at the interface, it is sometimes possible to chemically bond the core 3 and the lower cladding 21 together.

[0152] Furthermore, if a resin in which two or more polymers are mixed or compatible, or a resin in which two or more copolymers are mixed or compatible, is used as the resin material for the core 3 and the lower cladding 21, it becomes easier to make the resin material of the core 3 compatible with the resin material of the lower cladding 21, and / or the resin material of the lower cladding 21 compatible with the resin material of the core 3. Moreover, if a resin in which two or more polymers are mixed or compatible, or a resin in which two or more copolymers are mixed or compatible, is used as the resin material for the core 3 and the lower cladding 21, and resins of the same type are used for the resin material of the core 3 and the lower cladding 21, it becomes easier to make the resin material of the core 3 compatible with the resin material of the lower cladding 21, and / or the resin material of the lower cladding 21 compatible with the resin material of the core 3.

[0153] Furthermore, it is desirable that the glass transition temperature Tg2 of the resin material of the core 3 is different from the glass transition temperature Tg3 of the resin material of the lower cladding 21. Alternatively, the glass transition temperature Tg3 of the lower cladding 21 may be higher than the glass transition temperature Tg2 of the core 3, or it may be lower than the glass transition temperature Tg2 of the core 3. When the glass transition temperature Tg3 of the resin material of the lower cladding 21 and the glass transition temperature Tg2 of the resin material of the core 3 are different, it becomes easier to make the resin material of the lower cladding 21 and the resin material of the core 3 compatible, and it becomes easier to form the second compatible layer C2.

[0154] Furthermore, it is more desirable that the absolute value of the temperature difference between the glass transition temperature Tg3 of the lower cladding 21 and the glass transition temperature Tg2 of the core 3 be 15°C or less. When the absolute value of the temperature difference between the glass transition temperature Tg3 of the lower cladding 21 and the glass transition temperature Tg2 of the core 3 is 15°C or less, the resin material of the core 3 and the resin material of the lower cladding 21 become more compatible, and the second compatible layer C2 is more easily formed. It is also thought that the second compatible layer C2 is formed under the curing conditions for forming the optical waveguide.

[0155] Furthermore, it is more desirable that the absolute value of the temperature difference between the glass transition temperature Tg3 of the lower cladding 21 and the glass transition temperature Tg2 of the core 3 be 10°C or less. When the absolute value of the temperature difference between the glass transition temperature Tg3 of the lower cladding 21 and the glass transition temperature Tg2 of the core 3 is 10°C or less, the resin material of the core 3 and the resin material of the lower cladding 21 become more compatible, and the second compatible layer C2 is more easily formed. In addition, it is thought that the curing conditions for forming the optical waveguide make it easier to form the second compatible layer C2.

[0156] As shown in Figure 10D, the upper cladding 22 is formed on the core 3 and the lower cladding 21. For example, similar to the formation of the lower cladding 21, the resin material for the upper cladding 22 is applied to the lower cladding 21 and the core 3 by spin coating and pre-cured by heating to form the resin film of the upper cladding 22. Alternatively, the resin material for the upper cladding 22 may be formed into a film, laminated on the lower cladding 21 and the core 3, and pre-cured to form the resin film of the upper cladding 22. The upper cladding 22 and the lower cladding 21 form the cladding 2 surrounding the core 3. The resin material for the upper cladding 22 is a thermosetting resin, a photocurable resin, or a resin containing both. A resin having a lower refractive index than that of the core 3 may also be used as the resin material for the upper cladding 22.

[0157] As shown in Figure 10E, the portion of the resin film of the upper cladding 22 that covers the portion of the core 3 that corresponds to the core exposed portion is removed. For example, the portion of the upper cladding 22 on the core exposed portion side is removed along the entire width of the core 3. As a result, a core exposed portion is provided as a part of the core 3 is exposed from the cladding 2. In addition, by removing a portion of the resin film of the upper cladding 22, the upper cladding non-formed region 1a and the upper cladding formed region 1b are defined. The portion of the resin film of the upper cladding 22 that is removed is not particularly limited, but as an example, it is removed using photolithography.

[0158] After removing a portion of the resin film of the upper cladding 22, the upper cladding 22 is fully cured by heating. In the manufacturing of the optical waveguide according to this embodiment, the resin film of the upper cladding 22 is fully cured so that the resin material of the upper cladding 22 and the resin material of the core 3 become compatible at the interface I1 between the upper cladding 22 and the core 3. As the resin material of the upper cladding 22 and the resin material of the core 3 become compatible, a first compatible layer C1 is formed at the interface I1. The first compatible layer C1 is formed along the X direction in which the optical signal is transmitted when the optical waveguide is in use.

[0159] For example, the resin material of the upper cladding 22 and the resin material of the core 3 can be made compatible at the interface I1 by appropriately adjusting the curing conditions such as temperature and pressure when the resin material of the upper cladding 22 is cured on the core 3 and lower cladding 21. By making the resin material of the upper cladding 22 and the resin material of the core 3 compatible, the resin of the upper cladding 22 and the resin of the core 3 can be brought into close contact. In other words, although the resin for the core and the resin for the upper cladding are resins with different compositions and properties, by making the core resin and the upper cladding resin compatible at the interface, it is sometimes possible to chemically bond the core 3 and the upper cladding 22 together. The end face of the upper cladding formation region 1b of the optical waveguide may be smoothed by dicing or laser.

[0160] Furthermore, if a resin in which two or more polymers are mixed or compatible, or a resin in which two or more copolymers are mixed or compatible, is used as the resin material for the upper cladding 22 and the core 3, it is easier to make the resin material of the upper cladding 22 compatible with the resin material of the core 3, and / or the resin material of the core 3 compatible with the resin material of the upper cladding 22. Moreover, if a resin in which two or more polymers are mixed or compatible, or a resin in which two or more copolymers are mixed or compatible, is used as the resin material for the upper cladding 22 and the core 3, and resins of the same type are used for the resin material of the upper cladding 22 and the core 3, it is easier to make the resin material of the upper cladding 22 compatible with the resin material of the core 3, and / or the resin material of the core 3 compatible with the resin material of the upper cladding 22.

[0161] Furthermore, it is desirable 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. Alternatively, the glass transition temperature Tg1 of the upper cladding 22 may be higher than the glass transition temperature Tg2 of the core 3, or it may be lower than the glass transition temperature Tg2 of the core 3. When the glass transition temperature Tg1 of the upper cladding 22 and the glass transition temperature Tg2 of the core 3 are different, the resin material of the upper cladding 22 and the resin material of the core 3 are more easily miscible, and the first miscible layer C1 is more easily formed.

[0162] Furthermore, it is more desirable that the absolute value of the temperature difference between the glass transition temperature Tg1 of the upper cladding 22 and the glass transition temperature Tg2 of the core 3 be 15°C or less. When the absolute value of the temperature difference between the glass transition temperature Tg1 of the upper cladding 22 and the glass transition temperature Tg2 of the core 3 is 15°C or less, the resin material of the core 3 and the resin material of the upper cladding 22 become more compatible, and the first compatible layer C1 is more easily formed. It is also thought that the first compatible layer C1 is formed under the curing conditions for forming the optical waveguide.

[0163] Furthermore, it is more desirable that the absolute value of the temperature difference between the glass transition temperature Tg1 of the upper cladding 22 and the glass transition temperature Tg2 of the core 3 be 10°C or less. When the absolute value of the temperature difference between the glass transition temperature Tg1 of the upper cladding 22 and the glass transition temperature Tg2 of the core 3 is 10°C or less, the resin material of the core 3 and the resin material of the upper cladding 22 become more compatible, and the first compatible layer C1 is more easily formed. It is also thought that the curing conditions for forming the optical waveguide make it easier to form the first compatible layer C1. Note that 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.

[0164] Through the above steps, the optical waveguide 101a is completed on the wiring board 110. In the step shown in Figure 10D, a film made of the resin material of the upper cladding 22 may be laminated so that the exposed core portion is not covered. Even in that case, the first compatible layer C1 is formed at the interface I1 between the core 3 and the upper cladding 22 by curing the resin material of the upper cladding 22.

[0165] The thickness T1 of the first compatible layer C1 can be adjusted by appropriately adjusting the curing conditions, such as temperature and pressure, when the upper cladding 22 is cured on the core 3 and lower cladding 21. Alternatively, the thickness T1 of the first compatible layer C1 may be adjusted by appropriately selecting the resin material of the upper cladding 22 and the resin material of the core 3 in terms of their glass transition temperatures. For example, the greater the difference between the glass transition temperature Tg1 of the resin material of the upper cladding 22 and the glass transition temperature Tg2 of the resin material of the core 3, the thicker the first compatible layer C1 may be formed. The thickness T1 of the first compatible layer C1 is not particularly limited, but is typically between 50 and 2000 nm.

[0166] Furthermore, the thickness T2 of the second compatibilization layer C2 can be adjusted by appropriately adjusting the curing conditions, such as temperature and pressure, when the resin material of the core 3 is cured on the lower cladding 21. Alternatively, the thickness T2 of the second compatibilization layer C2 may be adjusted by appropriately selecting the resin material of the core 3 and the resin material of the lower cladding 21 in terms of their glass transition temperatures. For example, the greater the difference between the glass transition temperature Tg2 of the resin material of the core 3 and the glass transition temperature Tg3 of the resin material of the lower cladding 21, the more likely it is to form a thicker second compatibilization layer C2. The thickness T2 of the second compatibilization layer C2 is not particularly limited, but is typically between 50 and 2000 nm.

[0167] In other words, by appropriately selecting the resin material of the core 3, the resin material of the lower cladding 21, and the resin material of the upper cladding 22, for example with respect to the glass transition temperature, and by appropriately adjusting the curing conditions such as the temperature and pressure during the main curing of the core 3 and the temperature and pressure during the main curing of the upper cladding 22, the thickness of the first compatibilization layer C1 can be made thicker than the thickness of the thicker second compatibilization layer C2, the thickness of the second compatibilization layer C2 can be made thicker than the thickness of the thicker first compatibilization layer C1, and the thickness of the first compatibilization layer C1 and the thickness of the second compatibilization layer C2 can be made to be the same. Accordingly, the optical waveguide 101a shown in Figure 9A and the optical waveguide 101b shown in Figure 9B can be manufactured as needed.

[0168] Furthermore, when an optical signal is transmitted to the core 3 of optical waveguides 101a and 101b having a first miscible layer C1 and a second miscible layer C2, each miscible layer acts as a shield against the optical signal, preventing leakage of the optical signal from the core 3 and allowing the optical signal to be properly reflected within the core 3. Therefore, it is considered that it can handle higher speeds and larger capacities of optical signals. In addition, because the first miscible layer C1 and the second miscible layer C2 are formed, even when reliability tests such as high-temperature and high-humidity storage are performed, the intrusion of moisture into the interface between the core 3 and the cladding 2 of optical waveguides 101a and 101b can be suppressed, and delamination of the core 3 and cladding 2 is considered unlikely to occur. Note that the thickness T1 of the first miscible layer C1 and the thickness T2 of the second miscible layer C2 are in the range of 0.5% to 10% of the thickness of the core 3, so it is considered that they do not hinder the transmission of optical signals.

[0169] <Method for Manufacturing an Optical Waveguide in the Second Embodiment> The method for manufacturing an optical waveguide in the second embodiment will be described with reference to Figures 11A to 11C. Hereinafter, as an example, a method in which an optical waveguide 101c is directly formed on the wiring board 110 of Figure 9C will be described. Note that the same steps as those in the method for manufacturing an optical waveguide in the first embodiment described with reference to Figures 10A to 10E will be omitted as appropriate.

[0170] First, a lower cladding 21 is formed on the wiring substrate 110 by a process similar to the manufacturing process of the optical waveguide of the first embodiment described with reference to Figures 10A and 10B, and then a predetermined number of cores 3 having a desired shape are formed. Then, the resin material of the cores 3 is cured by heating or the like.

[0171] Upon full curing of the resin material of core 3, the resin material of core 3 and the resin material of the lower cladding 21 become compatible, and a second compatible layer C2 is formed at the interface I2 between the resin material of core 3 and the lower cladding 21, as shown in Figure 11A. The resin material of core 3 and the resin material of lower cladding 21 can be made compatible at the interface I2 by appropriately adjusting the curing conditions such as temperature and pressure when the resin material of core 3 is fully cured on the lower cladding 21. The second compatible layer C2 is formed along the X direction in which the optical signal is transmitted when the optical waveguide is in use. By making the core resin and the lower cladding resin compatible at the interface, it is sometimes possible to chemically bond core 3 and lower cladding 21 together.

[0172] As described in the first embodiment of the method for manufacturing an optical waveguide, if a resin in which two or more polymers are mixed or compatible, or a resin in which two or more copolymers are mixed or compatible, is used as the resin material for the core 3 and the resin material for the lower cladding 21, it is easier to make the resin material for the core 3 and the resin material for the lower cladding 21 compatible. Furthermore, if a resin in which two or more polymers are mixed or compatible, or a resin in which two or more copolymers are mixed or compatible, is used as the resin material for the core 3 and the resin material for the lower cladding 21, and resins of the same type are used for the resin material for the core 3 and the resin material for the lower cladding 21, it is easier to make the resin material for the core 3 and the resin material for the lower cladding 21 compatible.

[0173] Furthermore, it is desirable that the glass transition temperature Tg2 of the resin material of core 3 be different from the glass transition temperature Tg3 of the resin material of lower cladding 21. Also, it is desirable that the glass transition temperature Tg2 of the resin material of core 3 be different from the glass transition temperature Tg3 of the resin material of lower cladding 21. Furthermore, the glass transition temperature Tg3 of lower cladding 21 may be higher than the glass transition temperature Tg2 of core 3, or it may be lower than the glass transition temperature Tg2 of core 3. When the glass transition temperature Tg3 of the resin material of lower cladding 21 and the glass transition temperature Tg2 of the resin material of core 3 are different, it becomes easier to make the resin material of lower cladding 21 and the resin material of core 3 compatible, and easier to form the second compatible layer C2.

[0174] Furthermore, it is more desirable that the absolute value of the temperature difference between the glass transition temperature Tg3 of the lower cladding 21 and the glass transition temperature Tg2 of the core 3 be 15°C or less. When the absolute value of the temperature difference between the glass transition temperature Tg3 of the lower cladding 21 and the glass transition temperature Tg2 of the core 3 is 15°C or less, the resin material of the core 3 and the resin material of the lower cladding 21 become more compatible, and the second compatible layer C2 is more easily formed. It is also thought that the second compatible layer C2 is formed under the curing conditions for forming the optical waveguide.

[0175] Furthermore, it is more desirable that the absolute value of the temperature difference between the glass transition temperature Tg3 of the lower cladding 21 and the glass transition temperature Tg2 of the core 3 be 10°C or less. When the absolute value of the temperature difference between the glass transition temperature Tg3 of the lower cladding 21 and the glass transition temperature Tg2 of the core 3 is 10°C or less, the resin material of the core 3 and the resin material of the lower cladding 21 become more compatible, and the second compatible layer C2 is more easily formed. It is also thought that the curing conditions for forming the optical waveguide make it easier to form the second compatible layer C2. The thickness T2 of the second compatible layer C2 is not particularly limited, but it is preferably 50 to 2000 nm.

[0176] As shown in Figure 11B, the upper cladding 22 is formed on the core 3 and the lower cladding 21. For example, the resin film of the upper cladding 22 is formed on the core 3 and the lower cladding 21 in a manner similar to that described in the method for manufacturing an optical waveguide in the first embodiment. The upper cladding 22 and the lower cladding 21 form the cladding 2 surrounding the core 3. The resin material of the upper cladding 22 is a thermosetting resin, a photocurable resin, or a resin containing both. Alternatively, a resin having a lower refractive index than that of the core 3 may be used as the resin material for the upper cladding 22. The resin material of the upper cladding 22 is cured by heating or other means.

[0177] As shown in Figure 11C, the resin material of the upper cladding 22 and the resin material of the core 3 become compatible at the interface I1 between the upper cladding 22 and the core 3 due to the final curing of the upper cladding 22 by heating, and a first compatible layer C1 is formed at the interface I1. The resin material of the upper cladding 22 and the resin material of the core 3 can be made compatible at the interface I1 by appropriately adjusting the curing conditions such as temperature and pressure when the resin material of the upper cladding 22 is finalized on the core 3 and the lower cladding 21. The first compatible layer C1 is formed along the X direction in which the optical signal is transmitted when the optical waveguide is in use. By making the resin material of the upper cladding 22 and the resin material of the core 3 compatible, it is possible to chemically bond the core 3 and the upper cladding 22. The end faces of the optical waveguide may be smoothed by dicing or laser.

[0178] As described in the first embodiment of the method for manufacturing an optical waveguide, if a resin in which two or more polymers are mixed or compatible, or a resin in which two or more copolymers are mixed or compatible, is used as the resin material for the upper cladding 22 and the resin material for the core 3, it is easier to make the resin material of the upper cladding 22 and the resin material of the core 3 compatible. Furthermore, if a resin in which two or more polymers are mixed or compatible, or a resin in which two or more copolymers are mixed or compatible, is used as the resin material for the upper cladding 22 and the resin material for the core 3, and resins of the same type are used for the resin material of the upper cladding 22 and the resin material of the core 3, it is easier to make the resin material of the upper cladding 22 and the resin material of the core 3 compatible.

[0179] It is desirable 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. Alternatively, the glass transition temperature Tg1 of the upper cladding 22 may be higher than the glass transition temperature Tg2 of the core 3, or it may be lower than the glass transition temperature Tg2 of the core 3. When the glass transition temperatures Tg1 of the upper cladding 22 and Tg2 of the core 3 are different, the resin material of the upper cladding 22 and the resin material of the core 3 are more easily miscible, and the first miscible layer C1 is more easily formed.

[0180] Furthermore, it is more desirable that the absolute value of the temperature difference between the glass transition temperature Tg1 of the upper cladding 22 and the glass transition temperature Tg2 of the core 3 be 15°C or less. When the absolute value of the temperature difference between the glass transition temperature Tg1 of the upper cladding 22 and the glass transition temperature Tg2 of the core 3 is 15°C or less, the resin material of the core 3 and the resin material of the upper cladding 22 become more compatible, and the first compatible layer C1 is more easily formed. It is also thought that the first compatible layer C1 is formed under the curing conditions for forming the optical waveguide.

[0181] Furthermore, it is more desirable that the absolute value of the temperature difference between the glass transition temperature Tg1 of the upper cladding 22 and the glass transition temperature Tg2 of the core 3 be 10°C or less. When the absolute value of the temperature difference between the glass transition temperature Tg1 of the upper cladding 22 and the glass transition temperature Tg2 of the core 3 is 10°C or less, the resin material of the core 3 and the resin material of the upper cladding 22 are more easily miscible, and the first miscible layer C1 is more easily formed. It is also thought that the curing conditions for forming the optical waveguide make it easier to form the first miscible layer C1. Note that 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. The thickness T1 of the first miscible layer C1 is not particularly limited, but it is preferably 50 to 2000 nm.

[0182] By going through the above steps, the optical waveguide 101c is completed on the wiring board 110. As described in the manufacturing method of the optical waveguide of the first embodiment, by appropriately selecting the resin material of the core 3, the resin material of the lower cladding 21, and the resin material of the upper cladding 22, for example with respect to the glass transition temperature, and by appropriately adjusting the curing conditions such as the temperature and pressure during the main curing of the core 3 and the upper cladding 22, the thickness of the first compatibilization layer C1 can be made thicker than the thickness of the thicker second compatibilization layer C2, the thickness of the second compatibilization layer C2 can be made thicker than the thickness of the thicker first compatibilization layer C1, and the thickness of the first compatibilization layer C1 and the thickness of the second compatibilization layer C2 can be made to be the same.Therefore, the optical waveguide 101c shown in Figure 9C and the optical waveguide 101d shown in Figure 9D can be manufactured as needed. Furthermore, since the thickness T1 of the first compatible layer C1 and the thickness T2 of the second compatible layer C2 are in the range of 0.5% to 10% of the thickness of the core 3, it is considered that they will not hinder the transmission of optical signals.

[0183] Furthermore, when an optical signal is transmitted to the core 3 of optical waveguides 101c and 101d, which have a first compatible layer C1 and a second compatible layer C2, each compatible layer acts as a shield for the optical signal, preventing leakage of the optical signal from the core 3 and allowing the optical signal to be properly reflected within the core 3. Therefore, it is considered that this can accommodate higher speeds and larger capacities of optical signals.

[0184] In addition, the optical waveguides of the first and second embodiments may be formed on a support plate (not shown) instead of the wiring board 110. Preferably, the support plate is made of a material having higher rigidity and a lower coefficient of thermal expansion than the optical waveguide to be manufactured. Examples of materials for the support plate include soda-lime glass, borosilicate glass, and quartz glass, various metals, and various ceramics such as alumina. When the optical waveguide is used without a support plate, the support plate is removed after the formation of the upper cladding 22. The removal of the support plate completes the optical waveguide as a standalone unit. The support plate can be removed by any method. In addition, the manufactured optical waveguide may be used with the support plate attached after completion.

[0185] <Method for Manufacturing a Wiring Board of an Embodiment> A wiring board of an embodiment as illustrated in Figures 9A to 9D can be manufactured by forming a wiring board 110 and forming optical waveguides of the embodiment, such as optical waveguides 101a to 101d, on the wiring board 110. The optical waveguides for the wiring board of the embodiment may be manufactured separately on a support plate (not shown), and then placed on the wiring board 110 using any adhesive after the support plate is removed, or while the support plate is still in place.

[0186] The wiring board 110 illustrated in Figures 9A to 9D can be manufactured, for example, by a general build-up wiring board manufacturing method. Specifically, the conductor layer 43 is formed on a support substrate (not shown), such as a double-sided copper-clad laminate, by, for example, a semi-additive method. After the formation of the conductor layer 43, an insulating layer 52 covering the conductor layer 43 is formed, for example, by lamination and thermocompression bonding of an insulating resin, such as a film-like epoxy resin. Furthermore, the conductor layer 42 is formed on the insulating layer 52, and via conductors 7 are formed on the insulating layer 52, for example, by a semi-additive method. 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, and then another conductor layer 41 is formed on top of it in the same manner as the conductor layer 42. After the removal of the support substrate by peeling or the like, solder resists 61 and 62 are formed by coating or spraying a photosensitive epoxy resin. Openings are formed in the solder resists 61 and 62 by exposure and development, exposing a part of the conductor layer 41 or a part of the conductor layer 43. As a result, the wiring board 110 is completed.

[0187] Then, an optical waveguide such as the optical waveguide 101a is formed on the surface of the wiring board 110, for example, in the manner described with reference to Figures 10A to 10E. Alternatively, the optical waveguide may be separately manufactured on any support plate (not shown) and mounted on the surface of the wiring board 110 using any adhesive. For example, the wiring board of this embodiment can be manufactured by following the above steps.

[0188] The optical waveguides and wiring boards of the embodiments are not limited to the structures illustrated in each drawing, nor to the structures, shapes, and materials illustrated herein. The wiring boards of the embodiments may have any laminated structure and may include any number of conductor and insulating layers. For example, the wiring board constituting the wiring board of the embodiments may be a build-up wiring board including a core board, 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. Also, the thickness of the core of the optical waveguide of the embodiments may vary in the core exposed and core non-exposed areas, respectively. In the optical waveguide of the embodiments, upper cladding non-formed regions may be provided at both ends of the optical waveguide, and therefore an upper cladding region may be located between two upper cladding non-formed regions.

[0189] 1 Optical waveguide (first embodiment) 1α Optical waveguide (second embodiment) 101a-101d Optical waveguide 100a-100d Wiring board 110 Wiring board 2 Cladding 21 Lower cladding 22 Upper cladding 3 Core 41-43 Conductor layer 51-52 Insulating layer C1 First compatible layer C2 Second compatible layer I1 Interface between upper cladding and core I2 Interface between lower cladding and core T1 Thickness of first compatible layer T2 Thickness of second compatible layer X Direction in which optical signal is transmitted (direction of optical signal propagation)

Claims

1. An optical waveguide comprising an upper cladding, a core, and a lower cladding, made of resin, wherein the resin includes a thermosetting resin or a photocurable resin, and a first compatible layer is formed at the interface between the upper cladding and the core along the direction in which the optical signal of the core is transmitted, and a second compatible layer is formed at the interface between the core and the lower cladding.

2. The optical waveguide according to claim 1, wherein the thickness of the first compatible layer (T1) and the thickness of the second compatible layer (T2) are different.

3. The optical waveguide according to claim 2, wherein the thickness of the first compatible layer (T1) and the thickness of the second compatible layer (T2) satisfy the relationship in Equation 1: T1 > T2 ... (Equation 1) 4. The optical waveguide according to claim 2, wherein the thickness of the first compatible layer (T1) and the thickness of the second compatible layer (T2) satisfy the relationship in Equation 2: T1 ≤ T2 ... (Equation 2) 5. The optical waveguide according to claim 1, wherein the resin is a resin in which two or more resins are mixed or compatible and whose refractive index is adjusted.

6. The optical waveguide according to claim 1, wherein the glass transition temperature Tg1 of the upper cladding and the glass transition temperature Tg2 of the core are different.

7. The optical waveguide according to claim 6, wherein the glass transition temperature Tg1 of the upper cladding and the glass transition temperature Tg2 of the core satisfy the relationship in equation 3-1: 0°C < |Tg1 - Tg2| ≤ 15°C ... (Equation 3-1) 8. The optical waveguide according to claim 6, wherein the glass transition temperature Tg2 of the core and the glass transition temperature Tg3 of the lower cladding satisfy the relationship in equation 4-1: 0°C < |Tg3 - Tg2| ≤ 15°C ... (Equation 4-1) 9. The optical waveguide according to claim 1, wherein the refractive index of the core is less than 1.

5.

10. The optical waveguide according to claim 9, wherein the refractive index of the core is 0.03 or greater than the refractive index of the upper cladding.

11. A wiring substrate comprising a substrate including a wiring consisting of an insulating layer and a conductive layer formed on the insulating layer, and a wiring substrate comprising an optical waveguide according to claim 1 disposed on the substrate.

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