Optical waveguides and wiring boards

Abstract

Suppression of damage at the corners of the core. [Solution] The optical waveguide 1 of this embodiment consists of a lower cladding 21, a core 3, and an upper cladding 22. The optical waveguide 1 consists of an upper cladding non-formed region 1a and an upper cladding formed region 1b, in which the core 3 is exposed in the upper cladding non-formed region 1a, and the corners of the core 3 are rounded in a plan view.

Description

Technical Field

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

Background Art

[0002] Patent Document 1 discloses a polymer waveguide array formed on a polymer film and a silicon waveguide array formed on a silicon chip. The core of the polymer waveguide having a rectangular cross-sectional shape and the core of the silicon waveguide are arranged so as to overlap over a predetermined distance in the optical axis direction and are optically coupled by adiabatic coupling.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In Patent Document 1, the shape of the end portion of the core of the polymer waveguide is rectangular in plan view. For example, when mounting components such as semiconductors and capacitors on a substrate or storing a wiring board, it is conceivable that the core of the polymer waveguide is contacted and the end portion of the core of the polymer waveguide is chipped. Depending on the chipping method of the core of the polymer waveguide, it is assumed that the optical coupling with the optical element is unstable.

Means for Solving the Problems

[0005] The optical waveguide of the present invention includes a lower cladding, a core, and an upper cladding. The optical waveguide includes an upper cladding non-formation region and an upper cladding formation region. In the upper cladding non-formation region, the core is exposed and has rounded corners in plan view.

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

[0007] According to embodiments of the present invention, since the core has rounded corners in a plan view, even if a component comes into contact with the exposed core in the upper cladding-free region of the optical waveguide, for example, it is considered that damage such as chipping of the core is less likely to occur. Therefore, instability in the transmission direction of optical signals due to damage such as chipping of the core is suppressed, and reliable optical coupling between the core of the optical waveguide and the optical element is ensured. [Brief explanation of the drawing]

[0008] [Figure 1] A plan view showing an example of an optical waveguide in an embodiment of the present invention. [Figure 2] Cross-sectional view of the optical waveguide along line II-II in Figure 1. [Figure 3] Cross-sectional view of the optical waveguide at line III-III in Figure 1. [Figure 4] Cross-sectional view of the optical waveguide along the IV-IV line in Figure 1. [Figure 5A] A plan view showing an example of the core in the upper cladding-free region of an optical waveguide according to the embodiment. [Figure 5B] A plan view showing a modified example 1 of the core in the upper cladding-free region of the optical waveguide of the embodiment. [Figure 5C] A plan view showing a modified example 2 of the core in the upper cladding-free region of the optical waveguide of the embodiment. [Figure 6] Enlarged view of the cross-section of the core in the upper cladding-free region of the embodiment. [Figure 7] A diagram illustrating a method for measuring the internal angle of the trapezoidal cross-sectional shape of the core of an optical waveguide according to an embodiment. [Figure 8] A plan view showing a modified example of the core shape of the core exposed portion in the optical waveguide of the embodiment. [Figure 9] A cross-sectional view showing an example of a wiring board according to an embodiment of the present invention. [Modes for carrying out the invention]

[0009] 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.

[0010] <Structure of an optical waveguide in this embodiment> Figure 1 shows a plan view of optical waveguide 1, which is an example of an optical waveguide in one 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-section 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 an embodiment. The structure of the optical waveguide in an embodiment is not limited to the structure shown in Figure 1 and other drawings.

[0011] As shown in Figures 1 to 4, the optical waveguide 1 of the 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. The core 3 is formed on the lower cladding 21. The upper cladding 22 is formed on the lower cladding 21 and the core 3. That is, the optical waveguide 1 is formed in the order of lower cladding 21, core 3, and upper cladding 22.

[0012] Core 3 has an upper surface 31 that faces in the direction along the formation direction of the cladding 2 and core 3, and a lower surface 32 that is the opposite surface of the upper surface 31. The formation direction of the cladding 2 and core 3 will also be referred to as the "Z direction" below. The upper surface 31 may face either of the two directions along the Z direction. In optical waveguide 1, as shown in Figure 2, the upper surface 31 of core 3 is the surface that faces the +Z direction, and the lower surface 32 is the surface that faces the -Z direction. Below, in optical waveguide 1, the lower cladding 21 side will be referred to as the "lower side" or simply "down," and the upper cladding 22 side will be referred to as the "upper side" or simply "up."

[0013] The lower clad 21 is located on the lower surface 32 side of the core 3 and is in contact with the lower surface 32. The upper clad 22 is located on the upper surface 31 side of the core 3 and is in contact with the upper surface 31. The upper clad 22 covers the upper surface 31 of the core 3. The upper clad 22 also covers the upper surface 211 of the lower clad 21. As shown in FIG. 4, the upper clad 22 of the optical waveguide 1 also covers the side surface 35 of the core 3. The side surface 35 of the core 3 is a surface along the direction in which the core 3 extends among the surfaces connecting the upper surface 31 and the lower surface 32.

[0014] The core 3 of the optical waveguide 1 in FIGS. 1 and 2 extends along the +X direction and the -X direction. The optical signal propagating in the core 3 propagates in the +X direction or the -X direction. Hereinafter, the propagation direction of the optical signal is also simply referred to as the "X direction" by collectively referring to the +X direction and the -X direction. The optical waveguide 1 has two opposed ends, one end 11 and the other end 12. The one end 11 and the other end 12 are opposed in the X direction. In the optical waveguide 1, an optical signal is incident on one end 11 or the other end 12, and the optical signal is emitted from the other end 12 or the one end 11. When the optical signal is incident on the one end 11 side, the optical signal is emitted from the other end 12 side. When the optical signal is incident on the other end 12 side, the optical signal is emitted from the one end 11 side. Therefore, in the optical waveguide 1, the one end 11 and the other end 12 serve as the incident side or the emission side of the optical signal. Therefore, a part of the core 3 is exposed on both the one end 11 side and the other end 12 side.

[0015] As shown in FIGS. 2 and 3, at the one end 11, the upper surface 31 of the core 3, the end surface 33 of the core 3, and the side surface 35 of the core 3 are exposed. As shown in FIGS. 1 and 2, at the other end 12, the end surface 34 of the core 3 is exposed. Note that, also on the other end 12 side, the upper surface 31 and the side surface 35 of the core 3 may be exposed in the same manner as on the one end 11 side.

[0016] As shown in FIGS. 1 and 2, the optical waveguide 1 has an upper clad non-formation region 1a and an upper clad formation region 1b. The upper clad non-formation region 1a is a region where the upper clad 22 is not formed in plan view, and the upper clad formation region 1b is a region where the upper clad 22 is formed in plan view. The upper clad non-formation region 1a is disposed on the one end 11 side of the optical waveguide 1. The upper clad formation region 1b is disposed adjacent to the upper clad non-formation region 1a. Note that the upper clad formation region 1b may be disposed on the other end 12 side of the optical waveguide 1, and the region other than the upper clad non-formation region 1a may be the upper clad formation region 1b. In the upper clad non-formation region 1a in plan view, the upper surface 31 of the core 3 and the upper surface 211 of the lower clad 21 are exposed. Note that "plan view" means looking at an object along the line of sight in the Z direction.

[0017] In the optical waveguide 1, the upper clad non-formation region 1a is a core exposure portion where the upper surface 31 of the core 3 is exposed, and the upper clad formation region 1b is a core non-exposure portion where the upper surface 31 of the core 3 is not exposed. The core exposure portion of the optical waveguide 1 is provided on the one end 11 side. Also, the core non-exposure portion of the optical waveguide 1 is provided on the other end 12 side. Note that the core 3 is composed of a core 3a and a core 3b, the core 3a is disposed in the core exposure portion, and the core 3b is disposed in the core non-exposure portion. The lower clad 21 is composed of a lower clad 21a and a lower clad 21b, the lower clad 21a is disposed in the core exposure portion, and the lower clad 21b is disposed in the core non-exposure portion. Therefore, as shown in FIGS. 1 and 2, the core 3a in the core exposure portion is located on the one end 11 side of the optical waveguide 1. The core 3b in the core non-exposure portion is located on the other end 12 side of the optical waveguide 1.

[0018] In the optical waveguide of this embodiment, the upper cladding-free region 1a may be provided at both ends of the optical waveguide. That is, the upper cladding-forming region 1b may be located between the two upper cladding-free regions 1a in the X direction. In other words, the optical waveguide of this embodiment may have core exposure portions at both ends of the optical waveguide. In that case, the core-non-exposed portion will be located between the core exposure portions at both ends. In the optical waveguide of this embodiment, the upper cladding-free region 1a is provided at at least one end of the optical waveguide. For example, the upper cladding-free region 1a is provided at one end 11 and / or the other end 12 in Figure 1. Therefore, the core exposure portion 3a is also located at at least one end of the optical waveguide.

[0019] 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, an organic material, an inorganic material, or a hybrid material containing an organic material and an inorganic material, such as an inorganic polymer. Examples of organic materials include acrylic resins such as polymethyl methacrylate (PMMA), polyimide resins, polyamide resins, polyether resins, and epoxy resins, while examples of inorganic materials include quartz glass and silicon. An optical waveguide 1 made of organic materials can be lightweight, have high toughness, and be flexible.

[0020] Core 3 and cladding 2 may be composed of different materials or of the same type of material. However, core 3 is made of a material with a higher refractive index than the material used for cladding 2, so that total internal reflection of the optical signal at the interface between core 3 and cladding 2 is possible. Core 3 and cladding 2 may be formed from materials with the same refractive index and then subjected to appropriate processing to make their refractive indices different. That is, optical waveguide 1 may be formed using a formation method called photolithography or photobleaching, for example. Optical waveguide 1 may be formed on a support that supports optical waveguide 1 during the manufacturing process, and then used separately from the support, or used together with the support. Optical waveguide 1 may also be placed on a wiring board. In this case, optical waveguide 1 may be formed on the wiring board, or optical waveguide 1 may be formed in advance and then placed on the wiring board.

[0021] The thickness of core 3 is not particularly limited, but is between 1 μm and 20 μm, and preferably between 3 μm and 10 μm. 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 an SEM image.

[0022] 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.

[0023] When the optical waveguide 1 is in use, core 3 is optically coupled at one end 11 and the other end 12 to optical components such as a photoelectric conversion element and / or a connector 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 optically coupled to core 3 at one end 11. When the optical waveguide 1 is in use, the region between component E1 and the portion of the optical waveguide 1 that overlaps with component E1 in a plan view is preferably filled with an optically transparent transparent resin TR.

[0024] The length L1 of the core 3a in the core exposed portion is not particularly limited, but is preferably 100 μm or more and 3000 μm or less, and more preferably 100 μm or more and 2500 μm or less. When the core 3a has a length within the above range, it is considered that there is a high degree of freedom in selecting optical components such as component E1 that are optically coupled to the core 3a. Furthermore, it is considered that a necessary and sufficient tolerance range can be obtained in the alignment of the optical component and the optical waveguide 1 in the X direction.

[0025] Component E1 includes an optical terminal E1a, which is either a portion into which an optical signal is incident or a portion from which an optical signal is emitted. Component E1 is optically coupled with core 3 at optical terminal E1a. An optical signal propagating from core 3 from the other end 12 enters component E1 at one end 11 via optical terminal E1a. On the other hand, an optical signal emitted from optical terminal E1a of component E1 enters core 3 at one end 11, propagates within core 3, and is emitted from the other end 12.

[0026] The exposed core portion of the optical waveguide 1 is superimposed with the optical terminal E1a of component E1 and is the part that transmits optical signals. In the examples shown in Figures 1 to 3, the core 3 is positioned at one end 11 of the optical waveguide 1 such that the upper surface 31 of the core 3a and the optical terminal E1a of component E1 face each other and are adibtably coupled. That is, a portion of the optical signal that has propagated through the core 3 toward the end 11 is emitted as evanescent light from the upper surface 31 to the outside of the core 3 and incident on the optical terminal E1a of component E1.

[0027] The optical waveguide 1 has four parallel cores 3. The optical waveguide 1 of this embodiment is not limited to four, but can have any number of cores 3 from one to four. For example, the number of cores 3 is in the range of 2 to 64. When multiple cores 3 are provided, the arrangement pitch P1 of the cores 3 is not particularly limited, but is 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 vary 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.

[0028] The core 3 of the optical waveguide 1 has a width W1 at core 3a and a width W2 at core 3b. In the optical waveguide 1 shown in Figure 1, widths W1 and W2 are approximately equal. That is, the core 3 of the optical waveguide 1 has a substantially constant width from one end 11 to the other end 12 of the optical waveguide 1. Widths W1 and W2 may have the relationship W1 = W2. However, in the optical waveguide of the embodiment, as in the optical waveguide 1α in Figure 8 which will be referred to later, the width of the core at core 3a and the width of the core at core 3b may be different.

[0029] The "width" of core 3, such as widths W1 and W2, is the length of core 3 at the bottom surface 32 in a direction perpendicular to the propagation direction of the optical signal propagating within core 3 (which is the X direction in Figure 2) and the Z direction, which is the stacking direction of the optical waveguide 1. The direction along the width of core 3 is also referred to as the +Y direction or -Y direction (see Figure 1) below. The +Y direction and -Y direction are collectively referred to simply as the "Y direction". If core 3 is bent along its length, the propagation direction of the optical signal within core 3 is not constant and changes depending on the position within core 3. Therefore, the direction along which the width of core 3 is aligned may change between one end and the other end of core 3.

[0030] As shown in Figure 5A, the optical waveguide 1 of the embodiment has rounded corners 36 of the core 3a in the upper cladding-free region 1a when viewed from above. That is, the optical waveguide 1 of the embodiment has rounded corners 36 of the core 3a in the upper cladding-free region 1a where a pair of edges 37 extending in the longitudinal direction (X direction) of the core 3 intersects with a portion 38 extending in the width direction (Y direction) of the core 3 that connects the pair of edges 37 when viewed from above. Here, the pair of edges 37 are portions that extend in the longitudinal direction at the ends of the core 3 in the width direction (Y direction) of the outer circumference of the core 3 when viewed from above, and are portions that extend left and right at the vertical ends of the core 3 in Figure 5A.

[0031] In the optical waveguide 1 of this embodiment, the rounding of the corners 36 of the core 3a is thought to distribute stress across the rounded portion when components other than the core 3 come into contact with the exposed corners 36 of the core 3a, such as during component mounting. This stress distribution due to the rounding of the corners 36 of the core 3a is thought to suppress damage such as chipping of the core 3a. Therefore, instability in the optical coupling between the core 3a and the optical element (e.g., optical terminal E1a) due to damage to the corners 36 of the core 3a is suppressed, and the transmission of optical signals is ensured.

[0032] The shape of the rounded corners 36 of the core 3a is not limited. In the example shown in Figure 5A, the end of the core 3a, including the corners 36, is formed in a semicircular shape. More specifically, the end of the core 3a is formed in a semicircular shape with a diameter equal to the width W1 of the core 3a (see Figure 1). When the end of the core 3a is formed in a semicircular shape, the entire end of the core 3a is rounded, which is thought to promote stress distribution and suppress damage such as chipping of the core 3a. Therefore, it is thought that optical elements such as optical terminals E1a can be placed close to the outer edge of the core 3a.

[0033] In the modified example shown in Figure 5B, the end of the core 3a, including the corner 36, is formed in a semi-elliptical shape. More specifically, the end of the core 3a is formed in a semi-elliptical shape with a major axis equal to the width W1 of the core 3a. In this case as well, the entire end of the core 3a is rounded, which is thought to promote stress distribution and suppress damage such as chipping of the core 3a. Therefore, it is thought that optical elements such as optical terminals E1a can be placed close to the outer edge of the core 3a.

[0034] In the modified example shown in Figure 5C, the corners 36 of the core 3a are formed in an arc shape, and the two arc-shaped portions are connected by a straight portion. Even in this case, it is thought that damage such as chipping of the core 3a is suppressed by the distribution of stress due to the rounding of the corners 36 of the core 3a. The rounding of the corners 36 of the core 3a may have a shape other than the shape shown. In the embodiment, the ends of the core 3a have a shape that is symmetrical in the width direction of the core 3, but they may have an asymmetrical shape.

[0035] The length of R at the corner 36 of the core 3a is not particularly limited, but it is preferable that it satisfies the relationship shown in equation 1-1 below. 1.0μm≦R≦6.0μm (Formula 1-1)

[0036] If R at the corner 36 of core 3a is within the above range, sufficient length of curvature for stress distribution is ensured. Therefore, it is thought that damage such as chipping of core 3a is further suppressed. Here, R at the corner 36 of core 3a is the radius of curvature of the rounded portion at the end of core 3a in a plan view, as shown in Figure 5A. In the case of Figure 5C, it is the average value of the lengths R1 and R2 of the two rounded portions. Here, the radius of curvature R of the rounded portion of the corner 36 of core 3a in a plan view in Figure 5A can be obtained by the following equation A. Let W (μm) be the length of the imaginary line L1 connecting the start and end points of the rounded portion of core 3a of optical waveguide 1, and let h (μm) be the length of the imaginary line L2 which is perpendicular to the vertex of the rounded portion of core 3a of optical waveguide 1 and the imaginary line L1. R = ((W / 2)) 2 +h2 ) / (2×h) (Formula A) The range of R in the rounded portion of the core 3a of the optical waveguide 1 preferably satisfies the relationship in equation 1-1 above. Within this range, sufficient length of rounding for stress distribution is ensured. Therefore, it is thought that damage such as chipping of the core 3a is further suppressed. Furthermore, it is preferable that the range of the rounded portion R of the core 3a of the optical waveguide 1 satisfies the relationship shown in equation 1-2 below. 1.5μm≦R≦5.0μm (Formula 1-2) Within this range, sufficient length of curvature for stress distribution is ensured. Therefore, it is thought that damage such as chipping of core 3a will be further suppressed. Light transmission is also thought to be more stable. Furthermore, it is preferable that the range of the rounded portion R of the core 3a of the optical waveguide 1 satisfies the relationship shown in equations 1-3 below. 2.0μm≦R≦4.0μm (Formula 1-3) Within this range, sufficient length of curvature for stress distribution is ensured. Therefore, it is thought that damage such as chipping of core 3a will be further suppressed. It is also thought that light transmission will be stable regardless of the width of core 3a.

[0037] In the optical waveguide of this embodiment, as shown in Figure 3, the cross-sectional shape of the core 3a in the core-exposed portion may be trapezoidal. That is, in the cross-sectional shape of the core 3a, both of the two opposing side surfaces 35 may be inclined such that the upper surface 31 side is located more inward of the core 3 than the lower surface 32 side. When the cross-sectional shape of the core 3a in the core-exposed portion is trapezoidal, it is thought that even if stress generated by, for example, heat generation during use or contact with external optical components is transmitted to the core 3a, that stress will be dispersed. As a result, deformation of the core 3a may be suppressed. In addition, stress within the core 3 caused by the placement of the core 3a very close to the optical terminal E1a of component E1 may be relieved. As a result, deformation of the core 3a may be suppressed. Note that, as shown in Figure 4, the cross-sectional shape of the core 3b in the core-non-exposed portion is a rectangle or square with two opposing side surfaces that are substantially parallel. However, in the optical waveguide of this embodiment, the cross-sectional shape of the core 3b in the core-non-exposed portion may be trapezoidal. Furthermore, in the cross-sectional shape of the core 3, one of the two opposing side surfaces 35 may be perpendicular to the bottom surface 32.

[0038] <Cross-sectional shape of the core at the exposed core portion> The trapezoidal cross-sectional shape of the core 3a in the core exposure portion will be explained with reference to Figures 6 and 7. Figure 6 shows an enlarged view of portion VI in Figure 3 as an example of the cross-sectional shape of the core in the core exposure portion of the optical waveguide of the embodiment. Figure 7 is a schematic diagram showing a method for measuring the angle of the interior angle of the trapezoidal cross-sectional shape of the core in the optical waveguide of the embodiment.

[0039] As shown in Figure 6, the cross-sectional shape of the core 3a in the core exposed portion of the optical waveguide 1 of the embodiment is trapezoidal. That is, the cross-sectional shape of the core 3 in the core exposed portion has four sides along the upper surface 31, the lower surface 32, and the two side surfaces 35, and the angle θ of the interior angle IA at both ends of the lower surface 32 is smaller than the angle of the interior angle at both ends of the upper surface 31. Here, the "trapezoidal" shape of the cross-section of the core 3a is a shape enclosed by a pair of substantially parallel opposite sides and another pair of opposite sides that narrow towards one of the first pair of opposite sides, and does not have to have four vertices like a typical quadrilateral. The trapezoidal shape of the cross-section of the core 3a is enclosed by an upper base indicating the upper surface 31 of the core 3a, a lower base indicating the lower surface 32 of the core 3a, and two legs indicating the two opposing side surfaces 35 of the core 3a. Furthermore, the distance between the two opposing sides 35 of the core 3a increases as you approach the bottom surface 32 from the top surface 31 of the core 3a. In other words, the cross-section of the core 3a has a tapered shape in which the width W decreases towards the top surface 31.

[0040] The advantages obtained when the core 3a in the core exposed portion has a trapezoidal cross-sectional shape are explained below. As shown in Figures 1 to 3 and Figure 6, when the optical waveguide 1 is in use, the core 3a is positioned to face the optical terminal E1a of component E1. Furthermore, for efficient optical coupling, the core 3a is positioned in very close proximity to the optical terminal E1a of component E1. For example, the core 3a and the optical terminal E1a of component E1 may be positioned with a gap of only about 3 μm in mind during design. Component E1 may contain elements that generate heat during use. In that case, if the optical terminal E1a of component E1 and the core 3a of the optical waveguide 1 are in close proximity, it is conceivable that stress caused by the expansion or contraction of component E1 due to heat generation during use may be transmitted to the core 3a. In addition, when arranging component E1 and / or the optical waveguide 1, the optical terminal E1a of component E1 and the core 3a of the optical waveguide 1 may come into contact. Therefore, it is conceivable that the optical waveguide 1 may be used while the optical terminal E1a of component E1 and the core 3a of the optical waveguide 1 remain in contact. Such contact between the optical terminal E1a of component E1 and the core 3a of the optical waveguide 1 can generate stress at the contact point, and this stress may be transmitted to the core 3a.

[0041] When stress is transmitted to the core in this manner, if the cross-sectional shape of the core is rectangular, as in the core described in Patent Document 1, deformation of the core may occur. In contrast, in the optical waveguide of this embodiment, the core 3a has a trapezoidal cross-sectional shape, so deformation of the core 3a is suppressed. That is, the cross-sectional shape of the core 3a in the exposed part of the core has a trapezoidal shape in which the width W increases towards the lower surface 32 side of the core 3a. Therefore, even if stress is transmitted to the core 3 due to contact with component E1 or heat generation from component E1, the stress does not concentrate near the upper surface 31 of the core 3a but is easily effectively distributed to the lower surface 32 side as well. For this reason, deformation of the core 3a due to stress caused by contact with component E1 or heat generation from component E1 is suppressed. Since deformation of the core 3a is suppressed, it is considered that, according to the optical waveguide of this embodiment, the core 3a and the optical terminal E1a of component E1 can be optically coupled with good efficiency.

[0042] Furthermore, if the core 3a of the optical waveguide 1 has a trapezoidal cross-sectional shape, the resin-filling properties of the transparent resin TR in the region between the optical waveguide 1 and component E1 may be improved. That is, the resin-filling of the region between the optical waveguide 1 and component E1 with the transparent resin TR is usually performed after the optical waveguide 1 and component E1 are placed in predetermined positions with a small gap between them. The resin-filling is usually performed by allowing the transparent resin TR to penetrate into the region between the optical waveguide 1 and component E1 in the direction indicated by arrow A1 in Figure 6. In the optical waveguide 1, if the cross-sectional shape of the core 3a in the core exposed portion has a trapezoidal shape in which the width W becomes smaller towards the upper surface 31 side of the core 3a, the transparent resin TR supplied near the side surface 35 of the core 3a can easily penetrate into the gap between the core 3a and the optical terminal E1a of component E1, for example, by capillary action. Therefore, it is considered that the region between the optical waveguide 1 and component E1 can be easily filled with the transparent resin TR without containing any unfilled areas such as voids. Consequently, when the core 3a has a trapezoidal cross-sectional shape, the optical waveguide 1 and component E1 can face each other through a region with an appropriate refractive index filled with the transparent resin TR. In other words, it is considered that the core 3a and the optical terminal E1a of component E1 can be optically coupled well.

[0043] In optical waveguide 1, if core 3a has a trapezoidal cross-sectional shape, the two interior angles IA on the lower cladding 21 side of the trapezoidal cross-sectional shape of core 3a have an angle θ smaller than 90°. The angle θ of the interior angle IA in the cross-sectional shape of core 3a satisfies the relationship shown in equation 2 below. 80°<θ<90° (Formula 2)

[0044] If the angle θ of the interior angle IA on the lower cladding 21 side of the trapezoidal cross-sectional shape of core 3a is less than 90°, the above-mentioned stress dispersion effect on the stress transmitted to core 3 is obtained, and it is presumed that the intended suppression effect on the deformation of core 3 is obtained. Also, if the angle θ of the interior angle IA is greater than 80°, it is considered that the area on the upper surface 31 of core 3a that directly faces the optical terminal E1a of component E1 is more easily secured to the size required for good optical coupling. In the cross-sectional shape of core 3a, the angles θ of the two interior angles IA may be equal to or different from each other. Furthermore, in the cross-sectional shape of core 3, one of the interior angles IA at both ends of the lower surface 32 may be a right angle.

[0045] Referring to Figure 7, a method for measuring the angle θ of the internal angle IA of the cross-sectional shape of the core 3a in the core exposed portion will be explained. Figure 7 schematically shows a scanning electron microscope (SEM ×5000) observation image of the internal angle IA in the cross-section of the core 3a in the core exposed portion of the optical waveguide of the embodiment. In measuring the angle θ of the internal angle IA, a virtual straight line IS2 extending over the side surface 35 of the core 3a and a virtual straight line IS1 extending over the bottom surface 32 of the core 3a are set on the observation image of the internal angle IA shown in Figure 7, which is captured by SEM at a magnification of 1000x. The angle between the virtual straight line IS2 and the virtual straight line IS1 is then measured as the angle θ of the internal angle IA by the SEM photograph.

[0046] As an example of an embodiment, the width of the core 3a is 5 μm, the thickness of the core 3a is 5 μm, the angle of one interior angle IA of the core 3a (θ1) is 84.1°, the angle of the other interior angle IA of the core 3a (θ2) is 86.2°, the arrangement pitch of the core 3a is 50 μm, the thickness of the lower cladding 21 is 30 μm, the thickness of the upper cladding 22 is 20 μm, the length L1 of the core 3a in the exposed core portion is 2000 μm, and the length of R at the corner 36 of the core 3a in a plan view is 2.5 μm.

[0047] The cross-sectional shape of core 3a is not limited to a trapezoid. For example, the cross-sectional shape of core 3a may be a rectangle or a square, or any other rectangle other than a trapezoid. In the cross-section of core 3a, the corners where the top surface 31 and the side surface 35 of core 3a intersect may be rounded.

[0048] Referring to Figure 8, a modified example of the core shape in the core-exposed portion of the optical waveguide in the embodiment will be described. Figure 8 shows a plan view of the optical waveguide 1α, including a modified example of the core shape in the core-exposed portion of the optical waveguide in the embodiment. As shown in Figure 8, the optical waveguide 1α, like the optical waveguide 1 in Figure 1, consists of an upper cladding-free region 1a and an upper cladding-formed region 1b. The upper cladding-free region 1a has a core-exposed portion where the upper surface of the core 3a is exposed. The upper cladding-formed region 1b has a core-non-exposed portion where the core 3b is not exposed. In the optical waveguide 1α in Figure 8, components having the same function as the components of the optical waveguide 1 in Figure 1 are either denoted by the same reference numerals as in Figure 1 or are omitted as appropriate, and repetitive explanations regarding these similar components are omitted.

[0049] In the optical waveguide 1α, the width W1 of core 3a in the core-exposed portion and the width W2 of core 3 in the core-non-exposed portion 3b are different. In the optical waveguide 1α, the width W1 of core 3a at one end 11 is greater than the width W2 of core 3b at the other end 12. Therefore, the difference between the width W1 of core 3a and the width We of the optical terminal E1a of component E1 is greater than the difference between the width W2 of the core-non-exposed portion 3b and the width We of the optical terminal E1a. Therefore, in the optical waveguide 1α, the allowable range of misalignment in the direction along the width of core 3a when aligning the input / output portion of an optical component such as the optical terminal E1a with core 3a is larger compared to the case where core 3a has the same width as core 3b in the core-non-exposed portion. Therefore, in the optical waveguide 1α, it is considered that alignment for appropriate optical coupling between the optical component and core 3a is easier. Therefore, it is considered that optical coupling is ensured when coupling with the optical component. Furthermore, it is believed that the thermal history during use will also suppress coupling failures caused by misalignment between core 3a and optical components such as component E1.

[0050] In the optical waveguide 1α of the embodiment, as shown in Figure 8, the width W1 of the core 3a in the core-exposed portion is wider than the width W2 of the core 3b in the core-non-exposed portion, and the corners 36 of the core 3a are rounded in a plan view. In this case, it becomes possible to increase the length of the rounding compared to the case where the width W1 of the core 3a in the core-exposed portion is the same as the width W2 of the core 3b in the core-non-exposed portion (when the width W1 is narrower). By increasing the length of the rounding at the corners 36 of the core 3a in the core-exposed portion, stress is more easily distributed. Therefore, in the optical waveguide 1α shown in Figure 8, in addition to making alignment easier as described above, it is thought that damage such as chipping and cracking of the core 3 is further suppressed.

[0051] In addition, in optical waveguide 1α, the width W2 of core 3b in the core-exposed portion is smaller than the width W1 of core 3a, making it easier to realize a desired transmission mode in core 3b. For example, in optical guide members such as optical waveguide 1α, the size of the cross-section of the optical path, such as core 3, is selected according to the refractive index difference between the constituent material of the optical path and the constituent material of the covering, such as cladding 2, that surrounds the optical path. That is, by realizing core 3 having a cross-section of an appropriate size according to the refractive index of each material, optical signals can be propagated in a desired transmission mode. In this regard, in optical waveguide 1α, core 3 is not formed along its entire length with the width W1 of core 3a in the core-exposed portion, which has a large width for high coupling efficiency with optical components; rather, the width W2 of core 3b in the core-exposed portion is smaller than the width W1 of core 3a. Therefore, it is considered that optical transmission in modes requiring a minute core cross-section is easily realized. For example, single-mode optical transmission requiring a core diameter of 3 to 10 μm may be possible.

[0052] The width W1 of core 3a in the exposed core portion may be constant or may vary depending on the position in the X direction within core 3a. In the optical waveguide 1α in Figure 8, the maximum width (W1m) of core 3a in the exposed core portion is wider than the width W2 of core 3b in the non-exposed core portion. That is, in the optical waveguide 1α, the maximum width W1m of core 3a and the width W2 of core 3b satisfy the relationship shown in equation 3 below. W1m>W2 ···(Equation 3)

[0053] Figure 8 illustrates an example of the dimensions of each part of the optical waveguide 1α, including a modified shape of the core 3a in the core-exposed portion. The width W1 of core 3a and the width W2 of core 3b are not particularly limited, but it is desirable that the width W1 of core 3a in the core-exposed portion and the width W2 of core 3b in the core-non-exposed portion satisfy the relationship in Equation 4 below. 1.0<(W1 / W2)≦3.0 (Formula 4)

[0054] The width W1 of the core 3a in the core exposed portion is not particularly limited, but is preferably 3 μm or more and 30 μm or less, and more preferably 5 μm or more and 20 μm or less. Having a width within this range of the core 3a in the core exposed portion facilitates alignment with optical components such as component E1, and is considered to eliminate the need for an excessively large area for optical coupling with the optical component. The width W2 of the core 3b may be, for example, 1 μm or more and 10 μm or less.

[0055] The width of core 3 is determined by the average value of measurements taken at three points in the X direction. For example, the width of core 3a in the exposed core portion may increase or decrease from one end to the other. If the width of core 3a changes, the width W1 of core 3a in the exposed core portion is determined by the average value of measurements taken at three points within core 3a in the X direction. The width of core 3b in the non-exposed core portion may be constant or vary. For example, the width of core 3b in the non-exposed core portion may increase or decrease from one end to the other. If the width of core 3b changes, the width W2 of core 3b in the non-exposed core portion is determined by the average value of measurements taken at three points within core 3b in the X direction.

[0056] The ratio of the width of core 3a to the thickness of core 3a in the exposed core portion is not particularly limited, but it is preferable that the ratio of the width of core 3a to the thickness of core 3a is 1.0:1.0 to 4.0:1.0. The ratio of the width of core 3b to the thickness of core 3b in the non-exposed core portion is not particularly limited, but it is preferable that the ratio of the width of core 3b to the thickness of core 3b is 0.9:1.0 to 1.4:1.0.

[0057] An example of a combination of dimensions for each part of the optical waveguide 1α is as follows: The width of core 3a in the exposed core portion is 10 μm, and the thickness of core 3a is 7 μm. The width of core 3b in the non-exposed core portion is 7 μm, and the thickness of core 3b is 7 μm. The thickness of the lower cladding 21 may be 20 μm, and the thickness of the upper cladding 22 may be 25 μm. Note that the core width, core thickness, and length of the exposed core portion, as well as the thickness of the lower cladding 21 and upper cladding 22 exemplified here, are merely examples, and the dimensions of each part of the core 3 and the thickness of each cladding are not limited to the values ​​exemplified here.

[0058] In the optical waveguide 1α in Figure 8, the core 3 has a connecting core 3c between the core-exposed core 3a and the core-non-exposed core 3b. As shown in Figure 8, in a plan view, the connecting core 3c refers to the core 3a formed in the upper cladding-free region 1a between the core-exposed core 3a and the core-non-exposed core 3b. Here, in a plan view, the connecting core 3c may have a taper, may be arranged as a straight line of a constant width, or may be arranged as a combination of a straight line of a constant width and a taper. The tapered shape of the connecting core 3c may be a shape that gradually widens linearly or smoothly on both sides from the core-non-exposed core 3b toward the core-exposed core 3a, may be a shape that gradually widens linearly on only one side, or may be a shape that gradually widens in a step-like manner on both sides or only one side.

[0059] The width W3 of the core 3c at the connection point is smaller than the width W1 of the core 3a at the exposed core point, and is greater than or equal to the width W2 of the core 3b at the non-exposed core point. In other words, the core widths W1, W2, and W3 satisfy the following relationship in Equation 5. Core width W1 > Core width W3 ≥ Core width W2 ... (Equation 5)

[0060] It is considered that the width W3 of the core 3c of the connection part suppresses the unintentional leakage of the optical signal from the core 3a of the core exposed part toward the core 3b of the core non-exposed part. That is, in the optical waveguide 1α, since the width W1 of the core 3a of the core exposed part is larger than the width W2 of the core 3b of the core non-exposed part, the core 3c of the connection part connecting the core 3a of the core exposed part and the core 3b of the core non-exposed part is considered to suppress the unintentional leakage of the optical signal from the core 3a of the core exposed part toward the core 3b of the core non-exposed part by having a tapered shape. The width W3 of the core 3c of the connection part is not particularly limited, but is preferably 1 μm to 30 μm.

[0061] The taper ratio ((W1 - W2) / L2) of the tapered shape of the core 3 is, for example, 1 or more and 2 or less. Here, L2 is the length of the tapered shape part of the core 3c of the connection part in the X direction. When the core 3c of the connection part has a taper ratio within this range, it may be possible to efficiently suppress the leakage of the optical signal from the core 3 within a limited length. At this time, the formation ratio RT of the tapered shape in the core 3 is preferably 0.01 < RT < 0.2, and more preferably 0.03 < RT < 0.15. The formation ratio R of the tapered shape is the ratio of the length of the tapered shape part to the total length of the core 3.

[0062] As an example of a modification, the width of the core 3a of the core exposed part: 10 μm, the width of the core 3b of the core non-exposed part: 5 μm, the thickness of the core 3a: 5 μm, the angle θ of one inner angle IA of the core 3a: 81.2°, the angle θ of the other inner angle IA of the core 3a: 82.4°, the arrangement pitch of the core 3: 50 μm, the thickness of the lower cladding 21: 30 μm, the thickness of the upper cladding 22: 20 μm, the length of the core exposed part of the core 3a: 2000 μm, the length of R at the corner 36 of the core 3 in plan view: 2.5 μm.

[0063] <Structure of the wiring board of the embodiment> Next, the wiring board of the embodiment will be described with reference to the drawings. Figure 9 shows a cross-sectional view of a wiring board 100, which is an example of a wiring board of the embodiment. Note that the wiring board 100 shown in Figure 9 is merely one example of a wiring board of the embodiment. The laminated structure of the wiring board of the embodiment is not limited to the laminated structure of the wiring board 100 in Figure 9. Furthermore, the number of conductor layers and insulating layers in the wiring board of the embodiment is not limited to the number of conductor layers and insulating layers included in the wiring board 100.

[0064] As shown in Figure 9, the wiring board 100 includes a wiring board 110 and an optical waveguide 101 disposed on the wiring board 110. The wiring board 110 includes an insulating layer and a conductor layer laminated on the insulating layer. The wiring board 110 includes conductor layers 41-43 as the conductor layer and insulating layers 51 and 52 as the insulating layer. The wiring board 100 has a first surface 100a which is the mounting surface for component E1 and a second surface 100b which is the opposite surface of the first surface 100a. The optical waveguide 101 is disposed on the first surface 100a. The wiring board 100 has component mounting pads 4 which are conductor pads included in the conductor layer 41 on the first surface 100a.

[0065] 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 100b to the first surface 100a of the wiring board 100. 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. The wiring board 110 includes solder resist 62 covering the conductor layer 43 and insulating layer 52, and solder resist 61 covering the conductor layer 41 and insulating layer 51. The wiring board 110 also includes bumps 8 that are connected to each conductor pad of the conductor layer 43 and protrude from the solder resist 62. Bump 8 is made of a conductive material such as solder and is used for electrical and mechanical connections between the wiring board 100 and external components located on the second surface 100b side (for example, the motherboard of any electrical device). Note that the wiring board 100 may also be used as a motherboard without bump 8.

[0066] 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.

[0067] Solder resists 61 and 62 are formed from, for example, a photosensitive epoxy resin or polyimide resin.

[0068] 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. Although each of these conductors is simplified and depicted as a single layer in Figure 9, they 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.

[0069] The solder resist 61 has an opening 61a, and the component mounting pads 4 are exposed within the opening 61a. An optical waveguide 101 is placed on top of the solder resist 61. Although not shown, the optical waveguide 101 is fixed to the surface of the wiring board 110 with any fixing material such as adhesive.

[0070] The optical waveguide 101 is the optical waveguide of the embodiment described above. The optical waveguide 101 can be the optical waveguide 1 shown in Figures 1 to 8. Figure 9 shows, as an example, the same optical waveguide 101 as the optical waveguide 1 in Figure 1. Therefore, the optical waveguide 101 in Figure 9 includes a stacked lower cladding 21, a core 3, and an upper cladding 22, and has an upper cladding non-formation region 1a and an upper cladding formation region 1b. The core 3 has a core-exposed portion 3a in the upper cladding non-formation region 1a and a core-unexposed portion 3b in the upper cladding formation region 1b. In plan view, the core 3a of the core-exposed portion in the upper cladding formation region 1b has rounded corners 36 of the core 3.

[0071] Component E1 is mounted on the wiring board 100. Component E1 is an optical component such as a semiconductor device including a photoelectric conversion element, as described in the description of the optical waveguide 1 in Figure 1. Component E1 has 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-cavity surface-emitting lasers (VCSELs). When component E1 is a light-emitting element, it generates an optical signal based on an electrical signal input to electrode E1b and emits this 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 incident from the optical terminal E1a, which functions as a light-receiving part, and output from electrode E1b.

[0072] Component E1 is mounted on the wiring board 100 by connecting its electrode E1b to the component mounting pad 4 using, for example, solder. In Figure 9, component E1 is flip-chip mounted. The optical terminal E1a and the core 3a of the exposed core portion of the optical waveguide 101 are positioned opposite each other and optically coupled. The core 3a of the exposed core portion of the upper cladding region 1b has rounded corners 36 in a plan view. It is thought that the stress distribution due to the rounding of the corners 36 of the core 3a suppresses damage such as chipping of the core 3a. Therefore, damage to the corners 36 of the core 3a suppresses malfunctions in the optical coupling between the core 3a and the optical element (for example, optical terminal E1a), and ensures the transmission of optical signals.

[0073] 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. Furthermore, the thickness of the core of the optical waveguide of the embodiments may vary in the core-exposed and core-non-exposed portions, respectively. [Explanation of symbols]

[0074] 1 Optical waveguide 1a Upper cladding-free region 1b Upper cladding region 21 Lower cladding 22 Upper cladding 3 cores 36 corners

Claims

1. An optical waveguide consisting of a lower cladding, a core, and an upper cladding, The optical waveguide consists of an upper cladding-free region and an upper cladding-formed region. In the aforementioned upper cladding-free region, the core is exposed. In plan view, the corners of the core are rounded.

2. An optical waveguide according to claim 1, The radius (R) at the corner of the core satisfies the relationship shown in Equation 1 below. 1.0μm≦R≦6.0μm (Formula 1-1)

3. The optical waveguide according to claim 1, wherein the cross-section of the core is trapezoidal.

4. The optical waveguide according to claim 1, wherein the width of the core in the core-exposed portion is wider than the width of the core in the core-non-exposed portion.

5. A substrate including an insulating layer and a conductive layer formed on the insulating layer, The optical waveguide according to claim 1, disposed on the substrate, A wiring board that includes this component.

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