Optical waveguide and wiring board

Abstract

An optical waveguide (1) according to an embodiment includes a lower cladding (21), a core (3), and an upper cladding (22). The optical waveguide (1) is composed of a core exposed section (1a) and a core non-exposed section (1b). In the core exposed section (1a), an upper surface (31at) of the core (3) and an upper surface of the lower cladding (21) are exposed. In the core non-exposed section(1b), the upper cladding (22) is formed over the core (3) and the lower cladding (21). The lower cladding (21) in the core exposed section (1a) is provided with an incline (21as) that extends toward an end portion of the optical waveguide (1). A wiring board according to an embodiment includes a substrate (10) including an insulating layer and a conductor layer formed on the insulating layer, wherein the optical waveguide (1) according to the embodiment is disposed on the substrate (10).

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 structure in which a silicon waveguide and a polymer waveguide are optically coupled. By exposing a part of the upper surface of the core formed on the lower cladding, an adiabatic coupling for optically coupling the core of the polymer waveguide and the core of the silicon waveguide is realized. It is also disclosed that the gap between the waveguide arrays of the polymer waveguide and the silicon waveguide by adiabatic coupling is filled with a filling resin which is an optical epoxy or a UV adhesive.

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

[0004] In the optical waveguide disclosed in Patent Document 1, the gap between the waveguide arrays is filled with a filling resin which is an optical epoxy or a UV adhesive. The filling resin in the gap between the waveguide arrays may prevent the intrusion of foreign matters. In Patent Document 1, the distance of the gap between the waveguide arrays is arranged to be about 3 μm.

[0005] However, it is considered that it is difficult for the filling resin to enter the gap between the waveguide arrays, and problems such as voids (bubbles) due to insufficient resin filling may occur. If problems such as voids (bubbles) occur in the filling resin at the optical coupling portion between the waveguide arrays, it is considered that the optical coupling at the optical coupling portion is not stable due to poor propagation of the optical signal due to reflection and refraction in the filling resin. Furthermore, it is also considered that the reliability is not stable due to the misalignment of the waveguide due to the thermal stress caused by the thermal history from a semiconductor element or the like.

[0006] The optical waveguide of the present invention is composed of a lower cladding, a core, and an upper cladding. The optical waveguide is composed of a core exposed portion and a core non-exposed portion. In the core exposed portion, the upper surface of the core and the upper surface of the lower cladding are exposed, and in the core non-exposed portion, an upper cladding is formed on the core and the lower cladding. An inclination is provided in the lower cladding within the core exposed portion toward the end of the optical waveguide.

[0007] The wiring board of the present invention includes a substrate having an insulating layer and a conductive layer formed on the insulating layer, and an optical waveguide disposed on the substrate.

[0008] According to an embodiment of the present invention, the lower cladding within the core exposure portion is provided with a slope toward the end of the optical waveguide. By providing a slope in the lower cladding, the thickness of the lower cladding in the non-sloped region is stabilized. Furthermore, when using a wiring board in which an optical waveguide with a slope in the lower cladding is arranged, if the core exposure portion of the optical waveguide and the optical terminal of the optical element are arranged to face each other, a gap is formed between the optical waveguide and the optical element. Even if a filler resin such as optical resin is filled into the gap, problems such as incomplete resin filling are suppressed. Therefore, even if the distance of the gap between the optical terminal and the core is small, it is considered that air gaps are less likely to occur within the gap. As a result, when using a wiring board in which an optical waveguide is arranged, it is considered that problems caused by misalignment of the waveguide due to poor propagation of optical signals between the optical waveguide and the optical element, or thermal stress due to thermal history, are less likely to occur, and reliability can be stabilized.

[0009] A plan view showing an example of an optical waveguide according to an embodiment of the present invention. A cross-sectional view of the optical waveguide in the example of Figure 1A along line I-I. A cross-sectional view of the optical waveguide in the example of Figure 1A along line II-II. A cross-sectional view illustrating the mounting configuration of optical elements in the optical waveguide according to an embodiment of the present invention. A cross-sectional view illustrating the mounting configuration of optical elements in the optical waveguide according to an embodiment of the present invention. A cross-sectional view showing a first modified example of the optical waveguide according to an embodiment of the present invention. A cross-sectional view showing a second modified example of the optical waveguide according to an embodiment of the present invention. A cross-sectional view showing a third modified example of the optical waveguide according to an embodiment of the present invention. A cross-sectional view showing an example of a wiring board according to an embodiment of the present invention.

[0010] An optical waveguide of the embodiment will be described with reference to the drawings. Figure 1A shows a plan view of optical waveguide 1, which is an example of an optical waveguide of the embodiment. Figure 1B shows a cross-sectional view taken along line I-I in Figure 1A, and Figure 1C shows a cross-sectional view taken along line II-II in Figure 1A. Note that optical waveguide 1 is merely one example of an optical waveguide of the embodiment. As will be described in detail later with reference to Figure 7, optical waveguide 1 is formed or arranged on a substrate 10 and optically coupled to optical elements such as light-emitting elements and light-receiving elements mounted on the substrate 10. The referenced drawings are drawn to facilitate understanding of the features of the present invention, and the size and proportions of each component may not be accurate.

[0011] As shown in Figures 1A to 1C, the optical waveguide 1 of this embodiment includes a core 3 that transmits optical signals, and a lower cladding 21 and an upper cladding 22 that surround the core 3. The core 3 and the upper cladding 22 are formed on one side of the lower cladding 21.

[0012] In the following description, among the surfaces constituting the lower cladding, the side on which the core and upper cladding are formed will be referred to as "upper," "upper side," "upward," or "upward direction," and the surface facing upward will be referred to as the "upper surface." The side facing the opposite direction from the "upper surface" will be referred to as "lower," "lower side," "downward," or "downward direction," and the surface facing downward will be referred to as the "lower surface."

[0013] Furthermore, the thickness direction of the optical waveguide 1 is also referred to as the "Z direction," with the upward direction being called the +Z direction and the downward direction being called the -Z direction. In addition, the core 3 in the optical waveguide 1 is formed along the "X direction," and the optical signal propagating through the core 3 propagates in the +X direction or the -X direction. The direction perpendicular to the X and Z directions is referred to as the "Y direction."

[0014] As shown in Figure 1A, the optical waveguide 1 is formed from a core exposed portion 1a and a core non-exposed portion 1b. For the sake of explanation, the core 3 consists of core 31a and core 31b, with core 3 in the core exposed portion 1a being core 31a and core 3 in the core non-exposed portion 1b being core 31b. The lower cladding 21 consists of lower cladding 21a and lower cladding 21b, with lower cladding 21 in the core exposed portion 1a being lower cladding 21a and lower cladding 21 in the core non-exposed portion 1b being lower cladding 21b. Therefore, in the core non-exposed portion 1b, core 31b and lower cladding 21b are not exposed in a plan view. In the core exposed portion 1a, core 31a, which is part of core 3, and lower cladding 21a, which is part of lower cladding 21, are exposed. Specifically, core 3 is formed as one or more cores on the upper surface of lower cladding 21. The upper cladding 22 is formed to cover the core 3 and the lower cladding 21 in the core-non-exposed portion 1b, but not in the core-exposed portion 1a. Therefore, in the core-non-exposed portion 1b, the core 3 is not exposed in a plan view. In the core-exposed portion 1a, the upper surface 31at of core 31a, which is part of core 3, and the upper surface 21at of lower cladding 21a, which is part of the upper surface of lower cladding 21, are exposed. The optical waveguide 1 is formed in the area where the lower cladding 21 is formed, and the area where core 3 is formed is formed in the area where core 3 is formed. Hereinafter, the lower cladding 21 and the upper cladding 22 are collectively referred to as cladding 2.

[0015] The optical waveguide 1 has two opposing ends in the X direction, one end 1d and the other end 1c. In the optical waveguide 1, an optical signal is incident on either the one end 1d or the other end 1c, and an optical signal is emitted from either the other end 1c or the one end 1d. In Figure 1A, the optical waveguide 1 is configured to have a core exposure portion 1a only on one end 1d, but core exposure portions 1a may be provided on both the one end 1d and the other end 1c.

[0016] The optical waveguide 1 in this embodiment has three cores 3, but the number of cores 3 is not limited to three and may 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 formed in parallel, the arrangement pitch of the cores 3 is not particularly limited, but is preferably 10 to 300 μm, and preferably 20 to 250 μm. Also, as shown in Figure 1A, the multiple cores 3 do not necessarily have to be arranged in parallel or in a straight line. These shapes may be changed according to the shape and structure of optical components connected to the other end 1c of the optical waveguide 1, for example, as the cores 3 are formed radially in Figure 1A.

[0017] The thickness of the core 3 is not particularly limited, but is between 1 μm and 20 μm, preferably between 3 μm and 10 μm. The thickness of the lower cladding 21 is not particularly limited, but is between 5 μm and 50 μm, preferably between 20 μm and 40 μm. The thickness of the upper cladding 22 is not particularly limited, but is between 5 μm and 40 μm, preferably between 10 μm and 30 μm. The thicknesses of the core 3, lower cladding 21, and upper cladding 22 are determined by the average value of the thickness measured at three points in the SEM image.

[0018] The core 3 and cladding 2 forming the optical waveguide 1 are made of any translucent material. The optical waveguide 1 can be made of, for example, a resin material or a hybrid material containing a resin material and an inorganic material, such as an inorganic polymer. As the resin material, thermosetting resins or photocurable resins are preferred, and specifically, examples include single or multiple resins such as acrylic resins such as polymethyl methacrylate (PMMA), polyimide resins, polyamide resins, polyether resins, phenolic resins, fluororesins, silicone resins, and epoxy resins. By constructing the optical waveguide 1 from a resin material, it can be made lightweight, have high toughness, and be flexible. Inorganic materials such as glass or silicon may also be used.

[0019] The core 3 and cladding 2 may be composed of different materials or of the same type of material. In either case, the core 3 is made of a material with a higher refractive index than the material used for cladding 2. Alternatively, the core 3 and cladding 2 may be formed from materials with the same refractive index and then have their refractive indices differed using a method called photobleaching.

[0020] The optical waveguide 1 can be fabricated, for example, by thermocompression bonding of the core 3 and cladding 2 components, which are formed into a film, onto a substrate, or by forming a film by spin-coating a resin composition. Alternatively, the optical waveguide 1 may be formed on a separately prepared support, and then the support may be removed to form a standalone optical waveguide 1, which can then be placed on the substrate.

[0021] When the optical waveguide 1 is in use, the core 3 is optically coupled at one end 1d and the other end 1c to optical components such as photoelectric conversion elements like light-emitting diodes (LEDs) and photodiodes (PDs), and / or connector members that connect to optical fibers. In other words, the positional relationship between each optical element and the core 3 is adjusted so that optical signals can be transmitted and received between the core 3 and these optical elements.

[0022] As shown by the dashed lines in Figures 1A to 1C, an optical element E1 is arranged in the core exposed portion 1a, which is optically coupled with the core 3 at one end 1d of the optical waveguide 1. The optical element E1 has, for example, a photoelectric conversion function and includes an optical terminal E1a, which is a portion into which an optical signal is incident or an outgoing portion. Examples of optical elements E1 include light-emitting elements such as LEDs, organic light-emitting diodes (OLEDs), laser diodes (LDs), and vertical-resonant surface-emitting lasers (VCSELs), as well as photodetectors such as photodiodes (PDs).

[0023] In the exposed core portion 1a, the optical terminal E1a of the optical element E1 is superimposed with the core 3 to transmit optical signals. In the example shown in Figures 1A to 1C, the optical terminal E1a of the optical element E1 and the upper surface 31at of the core 31a are positioned opposite each other to perform adibatic coupling. As a result, for example, an optical signal propagating through the core 3 toward one end 1d enters the optical terminal E1a of the optical element E1 from the upper surface 31at of the core 31a as evanescent light. Optical coupling is achieved because the optical terminal E1a and the upper surface 31at of the core 31a are opposite each other without passing through the upper cladding 22. It is preferable that the space between the optical element E1 and the optical waveguide 1 in the region where the optical element E1 is located and its surrounding area is filled with an arbitrary filler resin TR that is optically transparent and has an appropriate refractive index. This stabilizes the optical coupling efficiency.

[0024] In the optical waveguide 1 of this embodiment, as shown in Figures 1A and 1B, when viewing the cross-section indicated by the line I-I of the optical waveguide 1, the cross-sectional shape of the core 3 in the core exposed portion 1a is rectangular. The cross-sectional shape of the core 3 only needs to have a region where the upper surface 31at of the core 31a in the core exposed portion 1a is flat or substantially flat.

[0025] As shown in Figures 1A and 1B, the cross-sectional shape of the lower cladding 21 in the core exposed portion 1a has a slope 21as formed toward one end 1d of the optical waveguide 1. The slope 21as is formed by a sloped surface in which the thickness of the lower cladding 21 decreases toward one end 1d.

[0026] As shown in Figure 1B, the line representing the inclined surface of inclined 21as is denoted as imaginary line A, and the line parallel to the lower surface 21ab of the lower cladding 21 and intersecting the end of inclined 21as is denoted as imaginary line B. The interior angle where imaginary line A and imaginary line B intersect is called the inclination angle θ. The inclination angle θ of inclined 21as is not particularly limited, but it is desirable to set it to 45 degrees or less. It is even more desirable that the inclination angle θ of inclined 21as satisfies Equation 1. 0 degrees < θ ≤ 40 degrees ... Equation 1 Furthermore, it is even more desirable that the inclination angle θ of inclined 21as satisfies Equation 2. 1 degree ≤ θ ≤ 10 degrees ... Equation 2 As an example, the inclination angle θ of inclined 21as may be 2 degrees.

[0027] As shown in Figure 1A, the inclination 21as is formed at a distance D1 in plan view. The inclination 21as is outside the area where the core 3 is formed. The distance D1 is not particularly limited, but is preferably 1 mm or more, and more preferably 1 to 3 mm. The distance D2 is the distance between the upper end 12ast of the inclination 21as on the upper surface 21at of the lower cladding 21a and the end 31ae of the core 3. The distance D2 between the inclination 21as and the end 31ae of the core 3 is not particularly limited, but is preferably satisfied by Equation 3. 0 μm ≤ D2 ≤ 1000 μm ... Equation 3

[0028] In this embodiment, the optical waveguide 1 has a slope 21as which stabilizes the thickness of the lower cladding 21 in the non-slope region. When the lower cladding 21 is formed by coating or the like, providing a slope at the end suppresses the reduction of the resin film in the non-slope region, and the thickness of the film in the non-slope region can be stabilized. It is believed that the formation of the optical waveguide is also stabilized as a result of the film stabilization. Furthermore, the effect of the slope 21as of the lower cladding 21 can also be obtained when using a wiring board on which the optical waveguide 1 is arranged. This effect will be explained using Figure 2. The mounting configuration in which the optical waveguide 1 and the optical element E1 are arranged on the substrate 10 will be described. Figure 2 shows a cross-sectional view of an embodiment in which the optical element E1 is mounted on the substrate 10 on which the optical waveguide 1 is arranged. Figure 2 shows only a part of the outermost layer of the substrate 10. The structure of the substrate 10 is not particularly limited, but as an example, it is explained using Figure 7. Specifically, the substrate 10 has a conductive layer 41 formed on the outermost insulating layer 51, and a conductive layer 42 formed below the insulating layer 51. The conductive layer 41 and the conductive layer 42 are connected by via conductors 71 in vias that penetrate the insulating layer 51. A solder resist layer 61 is formed on the insulating layer 51 and the conductive layer 41. By providing an opening 6c in the solder resist layer 61, a conductive pad 4P is formed in which a part of the conductive layer 41 is exposed. The optical waveguide 1 and the optical element E1 are arranged on the substrate 10. The optical element E1 includes an optical terminal E1a and a ball-shaped electrode E1b. The optical element E1 is mounted on the substrate 10 by connecting the electrode E1b to the conductive pad 4P at one end 1d of the optical waveguide 1, for example, using solder. In Figure 2, the optical element E1 is arranged face-down (flip-chip mounting). The optical terminal E1a is positioned so that the upper surface 31at of the core 31a of the core exposed portion 1a, which is provided at one end 1d of the optical waveguide 1, faces each other.

[0029] As shown in Figure 3, a core exposure portion 1a may also be provided at the other end 1c of the optical waveguide 1, and adiabatic coupling may be used at both ends of the optical waveguide 1. That is, in the X direction, the regions may be arranged in the order of core exposure portion 1a, core non-exposure portion 1b, and core exposure portion 1a, and adiabatic coupling may be used at one end 1d and the other end 1c. Furthermore, a slope 21as may also be formed on the lower cladding 21 on the other end 1c side.

[0030] It is preferable that the gap between the core 3 of the optical waveguide 1 and the optical terminal E1a of the optical element E1 be filled with a filler resin TR. It is preferable to use an optically transparent optical resin for the filler resin TR. Furthermore, it is even more preferable to use an optical resin having an appropriately adjusted refractive index. This makes it possible to stabilize the optical coupling efficiency.

[0031] The filler resin TR is filled into the gap in the optical coupling portion of the substrate 10 after the optical waveguide 1 and optical element E1 are mounted on the substrate 10 at the position where the core 3 and optical terminal E1a are optically coupled. Filling with the filler resin TR after mounting the optical element E1 stabilizes optical transmission and prevents the intrusion of foreign matter such as dust.

[0032] The filler resin TR, which is filled into the gap between the optical element E1 and the lower cladding 21 of the optical waveguide 1, penetrates from the periphery to the center of the optical element E1. It is desirable that the gap between the optical terminal E1a of the optical element E1 and the core 3 of the optical waveguide 1 be filled with the filler resin TR. However, filling with the filler resin TR becomes difficult if the gap between the lower cladding 21 and the optical element E1 is narrowed. Even if filling is possible, it is thought that defects such as the formation of voids may occur.

[0033] In contrast, in the optical waveguide 1 of this embodiment, a slope 21as is formed on the lower cladding 21 exposed to the core exposed portion 1a. When the filling resin TR is filled into the optical coupling portion between the core 3 and the optical terminal E1a from the core exposed portion 1a side, the slope 21as on the lower cladding 21 makes it easier for the filling resin TR to flow into the gap between the core 3 and the optical terminal E1a. In other words, the filling resin TR flows through the region of the lower cladding 21 where the slope 21as is not formed. When the filling resin TR reaches the region of the lower cladding 21 where the slope 21as is formed, the filling resin TR flows as if spilling over the slope 21as. Therefore, it is thought that the filling resin TR does not stagnate in the optical coupling portion and is filled evenly. As a result, it is thought that no voids are formed.

[0034] Furthermore, when using a wiring board on which the optical waveguide 1 is arranged, even if the gap between the optical terminal E1a and the core 3 is small, the inclined surface 21as is provided on the lower cladding 21 of the optical waveguide 1. Therefore, it is considered that no voids will be formed in the gap between the optical waveguide 1 and the optical element E1 due to insufficient filling of the filling resin TR. In addition, it is considered that malfunctions such as poor propagation of optical signals and stress due to thermal history such as heat generated from the optical element E1 will be less likely to occur, and the reliability of the coupling of the optical waveguide 1 will be stabilized.

[0035] In Figure 1B, the inclination 21as is formed only on a portion of the thickness direction of the lower cladding 21, but it is not limited to this, and the inclination 21as may be formed over the entire thickness direction of the lower cladding 21.

[0036] The inclined surface 21as can be formed by any possible method, such as creating a bevel at the corner between the upper surface and the end surface of the lower cladding 21, for example, by chamfering. Other methods for forming the inclined surface 21as include physical methods such as mechanical polishing or chemical methods such as dissolving it with a chemical solution.

[0037] Figure 4 is a cross-sectional view showing a first modified example of the optical waveguide 1 in the embodiment, and shows a cross-sectional view corresponding to the cross-section along line I-I in Figure 1. As shown in Figure 4, in the optical waveguide 1, the lower cladding 21 may be formed of two layers: a lower first lower cladding 211 and an upper second lower cladding 212. The second lower cladding 212 is formed on the first lower cladding 211. In this case, the inclination 21as of the lower cladding 21 may be formed on all or part of the inclined surface of the second lower cladding 212. By making the lower cladding 21 multiple layers, the formation of the film of the lower cladding 21 is stabilized. Note that the first lower cladding 211 and the second lower cladding 212 may each be a single layer or a multilayer of two or more layers.

[0038] Figure 5 is a cross-sectional view showing a second modified example of the optical waveguide 1 in the embodiment, and shows a cross-sectional view corresponding to the cross-section along line I-I in Figure 1. As shown in Figure 5, in the optical waveguide 1, the lower cladding 21 may be formed of two layers: a lower first lower cladding 213 and an upper second lower cladding 214. The second lower cladding 214 may be formed to cover the end face of the first lower cladding 213. In the example shown in Figure 5, the inclination 21as of the lower cladding 21 is formed on a part of the inclined surface in the thickness direction of the second lower cladding 214, but is not limited to this, and the inclination 21as of the lower cladding 21 may be formed on the entire inclined surface in the thickness direction of the second lower cladding 214. By making the lower cladding multiple layers, the formation of the film of the lower cladding is stabilized. Furthermore, the formation of the film in the non-inclined region of the lower cladding 21 is stabilized.

[0039] Figure 6 is a cross-sectional view showing a third modification example of the optical waveguide 1 in the embodiment, and shows a cross-sectional view corresponding to the cross-section along line I-I in Figure 1. As shown in Figure 6, in the optical waveguide 1, the lower cladding 21 may be formed of two layers: a lower first lower cladding 215 and an upper second lower cladding 216. In the example shown in Figure 6, the inclination 21as of the lower cladding 21 may be formed as an inclined surface that extends over the entire thickness direction of the lower cladding 21 formed by the first lower cladding 215 and the second lower cladding 216. By making the lower cladding 21 multiple layers, the formation of the film of the lower cladding 21 is stabilized. Note that the first lower cladding 215 and the second lower cladding 216 may each be a single layer or may be composed of two or more layers. The ratio of the thickness of the first lower cladding 215 (T1) to the thickness of the second lower cladding 216 (T2) is not particularly limited, but it is preferably 0.5:1.0 to 1.0:0.5. As an example, the thickness of the first lower cladding 215 (T1) may be 15 μm, and the thickness of the second lower cladding 216 (T2) may be 15 μm.

[0040] In each modified example of the embodiment, the lower cladding 21 within the exposed core portion 1a is provided with a slope 21as toward the end of the optical waveguide 1. The provision of a slope stabilizes the formation of the lower cladding 21 in the non-sloped region of the lower cladding 21. When the filling resin TR is filled into the optical coupling portion between the core 3 and the optical terminal E1a from the exposed core portion 1a side, it is thought that the slope 21as of the lower cladding 21 makes it easier for the filling resin TR to flow into the gap between the core 3 and the optical terminal E1a. As a result, it is thought that no voids will be formed. In each modified example of the embodiment, when the lower cladding 21 is composed of two or more layers, each lower cladding may be made of any translucent material. Each lower cladding may be made of different materials or of the same type of material.

[0041] As shown in Figure 1A, the inclination 21as of the lower cladding 21 does not need to be formed over the entire end face 1d of one end of the optical waveguide 1 in the Y direction, but may be formed only on a portion of the end face in the Y direction.

[0042] Next, the wiring board of the embodiment will be described in detail with reference to the drawings. FIG. 7 shows a cross-sectional view of a wiring board 100 of an embodiment including an optical waveguide 1 disposed on a substrate 10. Note that the optical waveguide 1 and the wiring board 100 shown in FIG. 7 are merely examples of the optical waveguide and the wiring board of the embodiment. The laminated structure and the number of layers of the substrate on which the optical waveguide is disposed are not limited to the laminated structure and the number of layers of the substrate 10 in FIG. 7.

[0043] As shown in FIG. 7, the wiring board 100 includes a substrate 10 and an optical waveguide 1 disposed on the substrate 10. The substrate 10 includes insulating layers and conductor layers laminated alternately. The substrate 10 includes conductor layers 41 to 43 as conductor layers and insulating layers 51 and 52 as insulating layers. The substrate 10 includes a conductor pad 4P included in the conductor layer 41 on a mounting surface of a semiconductor such as an element or an IC. The optical waveguide 1 is formed on the mounting surface of the substrate 10.

[0044] The conductor layers 41 to 43 and the insulating layers 51 and 52 are laminated in the order of the conductor layer 43, the insulating layer 52, the conductor layer 42, the insulating layer 51, and the conductor layer 41 from the lower side to the mounting surface side of the substrate 10. The conductor layer 41 and the conductor layer 42 are connected by a via conductor 71 penetrating the insulating layer 51. The conductor layer 42 and the conductor layer 43 are connected by a via conductor 72 penetrating the insulating layer 52. The substrate 10 includes a solder resist layer 61 covering the conductor layer 41 and the insulating layer 51, and a solder resist layer 62 covering the conductor layer 43 and the insulating layer 52. The substrate 10 includes bumps 8 connected to the respective conductor pads of the conductor layer 43 and protruding from the solder resist layer 62. The bumps 8 are made of a conductor such as solder and are used for electrical and mechanical connection between the substrate 10 and external components (for example, a motherboard of an arbitrary electric device). Note that the substrate 10 may be used as a motherboard on which the bumps 8 are not disposed. Note that wiring is formed by the conductor layers.

[0045] The insulating layers 51 and 52 are formed using insulating resins such as epoxy resin, polyimide resin, BT resin (bismaleimide-triazine resin), polyphenylene ether resin, and phenolic resin. The insulating layers 51 and 52 may also contain any of the following: fluororesin, liquid crystal polymer (LCP), fluoroethylene resin (PTFE), polyester resin (PE), and modified polyimide resin (MPI). The insulating layers 51 and 52 may also contain inorganic fillers such as fine particles made of silica (SiO2), alumina, or mullite. The insulating layers 51 and 52 may also contain a core material (reinforcement material) made of glass fiber or aramid fiber. Note that these resins are merely examples of materials capable of forming insulating layers. Each insulating layer can be formed from any material capable of providing insulation between the conductive layers in the substrate 10.

[0046] The conductor layers 41-43 and via conductors 71 and 72 are formed using any material having appropriate conductivity. The conductor layers 41-43 and via conductors 71 and 72 may have a single-layer structure or a multilayer structure including two or more films. For example, the conductor layers 41-43 and via conductors 71 and 72 have a two-layer structure consisting of a metal film layer and a plating layer formed on the metal film layer. The metal film layer is preferably a sputtered film layer. The plating layer is preferably an electroplated layer with the metal film layer as the power supply layer. The metal film layer is preferably formed using a conductive material including titanium, titanium alloy, nickel, nickel alloy, copper, or copper alloy. The plating layer is preferably formed using a conductive material including copper or copper alloy.

[0047] The solder resist layers 61 and 62 are formed using, for example, a photosensitive polyimide resin or epoxy resin, and form an insulating layer on the surface of the substrate 10.

[0048] As shown in FIG. 7, the optical waveguide 1 is formed on the solder resist layer 61 that constitutes the surface layer of the substrate 10. The optical waveguide 1 is an optical waveguide in an embodiment such as the optical waveguide 1 shown in FIGS. 1A to 6. That is, the optical waveguide 1 in FIG. 7 includes a stacked lower cladding 21, a core 3, and an upper cladding 22, has a core exposed portion 1a and a core non-exposed portion 1b, and an inclination 21as is provided in the lower cladding 21 of the core exposed portion 1a toward the end of the optical waveguide 1.

[0049] An optical element E1 is disposed on the substrate 10. The optical element E1 is an optical component having a photoelectric conversion function and the like, which is described in FIG. 1A and the like. The optical element E1 includes an optical terminal E1a and a ball-shaped electrode E1b. Examples of the optical element E1 include light-emitting elements such as LEDs, OLEDs, LDs, and VCSELs, and light-receiving elements such as PDs.

[0050] An opening 6c is formed in the solder resist layer 61, and the upper surface of a conductor pad 4P, which is a part of the conductor layer 41, is exposed on the bottom surface of the opening 6c. The optical element E1 is disposed on the solder resist layer 61, and the optical element E1 is mounted on the substrate 10 by being electrically connected to the conductor pad 4P via the electrode E1b. In FIG. 7, the optical element E1 is flip-chip mounted. The optical terminal E1a is positioned so as to face the upper surface 31at of the core 3 in the core exposed portion 1a of the optical waveguide 1, and an adiabatic coupling is realized. When the optical element E1 is a light-emitting element, the optical element E1 generates an optical signal based on an electrical signal input to the electrode E1b, and emits the optical signal from the optical terminal E1a functioning as a light-emitting portion toward the core 3. When the optical element E1 is a light-receiving element, an optical signal is incident on the optical element E1 from the optical terminal E1a functioning as a light-receiving portion, and the optical element E1 generates an electrical signal based on the optical signal and outputs it from the electrode E1b.

[0051] The gap between the core 3 of the optical waveguide 1 and the optical terminal E1a of the optical element E1 is filled with a filler resin TR. The filler resin TR supplied to the periphery of the optical element E1 penetrates the gap between the optical element E1 and the lower cladding 21 towards the center of the optical element E1. In the optical waveguide 1 of this embodiment, a slope 21as is formed on the lower cladding 21 that is exposed to the core exposed portion 1a. The slope 21as on the lower cladding 21 stabilizes the formation of the lower cladding 21 in the non-sloped region. Furthermore, when the filler resin TR is filled into the optical coupling portion between the core 3 and the optical terminal E1a from the core exposed portion 1a side, the slope 21as on the lower cladding 21 makes it easier for the filler resin TR to flow into the gap between the core 3 and the optical terminal E1a. As a result, it is thought that no voids will be formed.

[0052] Furthermore, even if the gap between the optical terminal E1a and the core 3 is small, since the lower cladding 21as of the optical waveguide 1 is provided, it is considered that no void will be formed in the gap between the optical waveguide 1 and the optical element E1 due to insufficient filling of the filling resin TR. In addition, it is considered that malfunctions such as poor propagation of optical signals and stress due to thermal history such as heat generated from the optical element E1 will be less likely to occur, and the reliability of the coupling of the optical waveguide 1 will be ensured.

[0053] As described above, in the optical waveguide 1 of this embodiment, a slope 21as is formed on the core exposed portion 1a related to adiabatic coupling. As a result, the filling resin TR in the optical coupling portion between the core 3 and the optical terminal E1a is filled without the formation of voids (air bubbles), and the reliability of the coupling of the optical waveguide 1 is less likely to decrease due to poor propagation of optical signals in the gap between the optical waveguide 1 and the optical element E1 due to insufficient filling of the filling resin TR, or due to stress caused by thermal history. Thus, reliability regarding optical coupling is ensured.

[0054] 1 Optical waveguide 1a Core exposed portion 1b Core not exposed portion 2 Cladding 21, 21a, 21b Lower cladding 21at Upper surface of lower cladding 21ab Lower surface of lower cladding 21as Inclined 22 Upper cladding 3, 31a, 31b Core 31at Upper surface of core 31ae End of core 10 Substrate 41, 42, 43 Conductor layer 51, 52 Insulating layer 61, 62 Solder resist layer 71, 72 Via conductor 100 Wiring board E1 Optical element TR Filling resin

Claims

1. An optical waveguide comprising a lower cladding, a core, and an upper cladding, wherein the optical waveguide consists of a core-exposed portion and a core-non-exposed portion, the upper surface of the core and the upper surface of the lower cladding are exposed in the core-exposed portion, the upper cladding is formed on the core and the lower cladding in the core-non-exposed portion, and the lower cladding within the core-exposed portion is provided with a slope toward the end of the optical waveguide.

2. The optical waveguide according to claim 1, wherein the inclination is formed by an inclined surface in which the thickness of the lower cladding becomes thinner towards the end of the core exposed portion.

3. The optical waveguide according to claim 1, wherein the inclination is provided between the end of the core exposed in the core exposed portion and the end of the optical waveguide.

4. The optical waveguide according to claim 1, wherein the inclination angle θ of the inclination with respect to the lower surface of the lower cladding satisfies the relationship in Equation 1. 0 degrees < θ ≤ 40 degrees ... Equation 1 5. The optical waveguide according to claim 1, wherein the distance D2 between the inclination and the end of the core satisfies the relationship in Equation 3: 0 μm ≤ D2 ≤ 1000 μm ... Equation 3 6. The optical waveguide according to claim 1, wherein the inclination constitutes the entire end face of the lower cladding.

7. The optical waveguide according to claim 1, wherein the lower cladding consists of multiple layers.

8. The optical waveguide according to claim 7, wherein the lower cladding comprises an upper layer in contact with the core and a lower layer covered by the upper layer, and the inclination is formed only in the upper layer.

9. The optical waveguide according to claim 7, wherein the gradient is formed across the multiple layers of the lower cladding.

10. A wiring board comprising: a substrate including an insulating layer and a conductive layer formed on the insulating layer; and an optical waveguide according to claim 1 disposed on the substrate.

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