Optical waveguide and manufacturing thereof

[0002] The present application claims priority to Japanese Application No. P10 204945 filed Jul. 21, 1998, which application is incorporated herein by reference to the extent permitted by law.

[0003] The present invention relates to an optical waveguide and a method of manufacturing the optical waveguide, and particularly to an optical waveguide including a lower clad layer, core portions formed on the lower clad layer, and an upper clad layer formed to cover the lower clad layer and the core portions, and a method of manufacturing the optical waveguide.

[0004] In the field of semiconductor devices, it has been attempted to make the operational speed higher and make the scale of integration larger. For example, significant development has been made to make the performance of microprocessors higher and make the capacity of memory chips larger. To expedite the above development even further, it is required to make the operational speed higher along signal interconnections, make the arrangement density of signal interconnections larger, and improve a delay in signal along electric wiring, and also it is essential to take a suitable measure against EMI (Electro Magnetic Interference) caused by making the operational speed higher and making the arrangement density of signal interconnections larger. As a means for solving the above-described problems, an optical interconnection has been examined. The optical interconnection may be considered to be used in various conditions, typically, between apparatuses, between boards in an apparatus, or between chips in a board. In particular, it may be considered that an optical transmission/communication means using an optical waveguide as a transmission path be suitable for relatively short distance transmission of signals, for example, between chips. In order to get the optical transmission/communication means using the optical waveguide into widespread use, it is important to establish a process of manufacturing the optical waveguide.

[0005] The optical waveguide is required to have a small propagation loss of light, and to be manufactured through simple manufacturing steps. With respect to the propagation loss of light, it may be considered that the optical waveguide be manufactured using a material small in propagation loss of light such as quartz. As verified in the case being applied to an optical fiber, quartz is very good in light transmittance, and in actual fact, the optical waveguide manufactured using quartz exhibits a low propagation loss in a range of 0.1 dB/cm or less. The optical waveguide manufactured using quartz, however, problems in requiring a large number of manufacturing steps, in particular, a heat-treatment step at a high temperature of 800° C. or more, and being difficult to ensure a large area of the optical waveguide. For this reason, the optical waveguide has been manufactured using a high polymer material capable of being treated at a low temperature, for example, polymethyl methacrylate or polyimide. Hereinafter, a related art optical waveguide in which core portions made from a high polymer are formed on a substrate will be described with reference to FIGS. 3A to 3D which are schematic diagrams illustrating steps of manufacturing the optical waveguide.

[0006] First, as shown in FIG. 3A, a lower clad layer 2 is formed on a substrate 1 made from silicon or glass by spin-coating and necessary heat-treatment.

[0007] A core layer 3 having a refractive index larger than that of the lower clad layer 2 is, as shown in FIG. 3B, formed on the lower clad layer 2.

[0008] The core layer 3 is, as shown in FIG. 3C, subjected to patterning by photolithography and etching such as RIE (Reactive Ion Etching), to form rectangular core portions 3ato be taken as an optical waveguide pattern.

[0009] Finally, as shown in FIG. 3D, an upper clad layer 4 is formed to cover the core portions 3aand the lower clad layer 2 by spin-coating and necessary heat-treatment, to obtain an embedded channel-type optical waveguide.

[0010] If the core portions 3aare formed by dry etching such as RIE at the step shown in FIG. 3C, the dry etching is generally performed in an oxygen atmosphere. In such a dry etching, if the RF power becomes large, the surface roughness of each core portion 3abecomes large, and if the gas pressure becomes smaller, the surface roughness of the core portion 3abecomes smaller but the side wall of the core portion 3atends to be etched. Accordingly, to form the core portions 3ahaving a small surface roughness and also having highly accurate rectangular shapes, it is required to perform the dry etching at a small RF power and an optimum gas pressure.

[0011] However, if the RF power is set at a small value in the above dry etching for forming the core portions 3ato be taken as an optical waveguide, there arise problems that the number of manufacturing steps becomes large for a multi-mode optical waveguide in which the thickness of the core portions 3ais in a range of several tens μm to several hundreds μm, although it does not become large for a single mode optical waveguide in which the thickness of the core portions 3ais in a range of several μm to 10 μm, and that the surface roughnesses of both a side wall of each core portion 3aand the lower clad layer 2 become larger to thereby increase the propagation loss of light passing through the core portions 3aconstituting the optical waveguide.

[0012] An object of the present invention is to provide a method of manufacturing an optical waveguide including core portions having a small surface roughness and also having highly accurate rectangular shapes without requiring a large number of the manufacturing steps, and to provide an optical waveguide with a small propagation loss of light.

[0013] To achieve the above object, according to a first aspect of the present invention, there is provided an optical waveguide including: a substrate; a lower clad layer formed on the substrate; core portions formed on the lower clad layer; and an upper clad layer formed to cover the lower clad layer and the core portions; wherein the substrate is made from a material which is transparent to light having a specific wavelength, and the core portions are made from a material which is hardened by irradiation of the light having the specific wavelength. If the above light having the specific wavelength is ultraviolet light, the substrate may be made from a material transparent to ultraviolet light, for example, either quartz or glass, and the core portions may be made from a material hardened by irradiation of ultraviolet light, for example, an ultraviolet hardening-type resin.

[0014] According to a second aspect of the present invention, there is provided a flexible optical waveguide including: a lower clad layer; core portions formed on the lower clad layer; and an upper clad layer formed to cover the lower clad layer and the core portions; wherein the substrate is made from a material which is transparent to light having a specific wavelength, and the core portions are made from a material which is hardened by irradiation of the light having the specific wavelength. If the above light having the specific wavelength is ultraviolet light, the substrate may be made from a material transparent to ultraviolet light, for example, either quartz or glass, and the core portions may be made from a material hardened by irradiation of ultraviolet light, for example, an ultraviolet hardening-type resin.

[0015] According to a third aspect of the present invention, there is provided a method of manufacturing an optical waveguide, including the steps of: forming a lower clad layer on a substrate made from a material transparent to light having a specific wavelength; forming a light cutoff film, which cuts off the light having the specific wavelength, on the lower clad layer; patterning the light cutoff film into a pattern having openings; forming a core layer, which is made from a material hardened by irradiation of the light having the specific wavelength, on the light cutoff film; irradiating the core layer with the light having the specific wavelength from the substrate side, to harden portions, positioned at the openings of the light cutoff film, of the core layer; removing non-hardened portions of the core layer, to render the hardened portions left as core portions; removing the light cutoff film; and forming an upper clad layer to cover the lower clad layer and the core portions. If the above light having the specific wavelength is ultraviolet light, the substrate may be made from a material transparent to ultraviolet light, for example, either quartz or glass; the light cutoff film may be made from a film of a metal such as chromium; and the core portions may be made from a material hardened by irradiation of ultraviolet light, for example, an ultraviolet hardening-type resin. In addition, to form core portions having a small surface roughness and also having highly accurate rectangular shapes, the light having the specific wavelength, entered from the substrate side, is desirable to be adjusted as a collimated light beam having a uniform energy distribution in the irradiation direction.

[0016] According to a fourth aspect of the present invention, there is provided a method of manufacturing a flexible optical waveguide, including the steps of: forming a separation film on a substrate transparent to light having a specific wavelength; forming a lower clad layer on the separation film; forming a light cutoff film, which cuts off the light having the specific wavelength, on the lower clad layer; patterning the light cutoff film into a pattern having openings; forming a core layer, which is made from a material hardened by irradiation of the light having the specific wavelength, on the light cutoff film; irradiating the core layer with the light having the specific wavelength from the substrate side, to harden portions, positioned at the openings of the light cutoff film, of the core layer; removing non-hardened portions of the core layer, to render the hardened portions left as core portions; removing the light cutoff film; forming an upper clad layer to cover the lower clad layer and the core portions; and removing the separation film, to collectively separate, from the substrate, the lower clad layer, the pattern of the core portions, and the upper clad layer which are formed on the separation film. If the above light having the specific wavelength is ultraviolet light, the substrate may be made from a material transparent to ultraviolet light, for example, either quartz or glass; the separation film may be made from a material soluble typically in a weak hydrofluoric acid, such as silicon oxide; the light cutoff film may be made from a film of a metal such as chromium; and the core portions may be made from a material hardened by irradiation of ultraviolet light, for example, an ultraviolet hardening-type resin. In addition, to form core portions having a small surface roughness and also having highly accurate rectangular shapes, the light having the specific wavelength, entered from the substrate side, is desirable to be adjusted as a collimated light beam having a uniform energy distribution in the irradiation direction.

[0017] According to the present invention having the above configuration, it is possible to form the core portions having a small surface roughness and also having highly accurate rectangular shapes following those of the openings of the light cutoff film, and hence to provide an optical waveguide readily with a small propagation loss in light without requiring a large number of the manufacturing steps.

[0018] FIGS. 1A to 1G are schematic diagrams illustrating steps of manufacturing an optical waveguide according to a first embodiment of the present invention;

[0019] FIGS. 2A to 2H are schematic diagrams illustrating steps of manufacturing a flexible optical waveguide according to a second embodiment of the present invention; and

[0020] FIGS. 3A to 3D are schematic diagrams illustrating steps of manufacturing a prior art optical waveguide.

[0021] The present invention is applicable to an optical waveguide including a lower clad layer, core portions formed on the lower clad layer, and an upper clad layer formed to cover the lower clad layer and the core portions, and to a method of manufacturing the optical waveguide. Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1A to 1G and FIGS. 2A to 2H. In these figures, parts corresponding to those described in the related art are designated by the same reference numerals.

[0022] <First Embodiment>

[0023] In this embodiment, the present invention is applied to an optical waveguide including a substrate, a lower clad layer formed on the substrate, core portions formed on the lower clad layer, and an upper clad layer formed to cover the lower clad layer and the core portions. Hereinafter, a method of manufacturing such an optical waveguide will be described with reference to FIGS. 1A to 1G which are diagrams schematically illustrating steps of manufacturing the optical waveguide.

[0024] First, as shown in FIG. 1A, a lower clad layer 2 is formed on a substrate 1.

[0025] The lower clad layer 2 can be formed by a usual method capable of forming a film having a uniform thickness, for example, a spin-coating method, a spraying method, or a method of laminating a previously formed film. At this step, if the substrate 1 is coated with a material, which is capable of being hardened by irradiation of light, by spin-coating or spraying to form the lower clad layer 2, the material thus applied is hardened by irradiation of light. Meanwhile, if the substrate 1 is coated with a material, which is capable of being hardened by heating, by spin-coating or spraying to form the lower clad layer 2, the material thus applied is hardened by heating at a high temperature.

[0026] As shown in FIG. 1B, a light cutoff film 5 is formed on the lower clad layer 2 and is then patterned such that portions, arranged at positions where core portions 3aare to be formed later, of the light cutoff film 5 are opened. That is to say, the pattern of the light cutoff film 5, which has openings at the positions where the core portions 3aare to be formed, is a negative pattern to the core portions 3a.The light cutoff film 5 is patterned by a usual patterning method, for example, a method of forming a resist pattern on the light cutoff film 5 using photolithography and etching the light cutoff film 5 using the resist pattern as a mask, or a method of previously forming a resist pattern on the lower clad layer 2, forming the light cutoff film 5 thereon, and patterning the light cutoff film 5 by lift-off. The light cutoff film 5 is made from a material which cuts off light 6 having a specific wavelength used for hardening the core portions 3ato be formed at the subsequent step and which has good adhesiveness to the lower clad layer 2. Specific examples of the materials may include a metal such as chromium or tantalum, and an organic material. If the light cutoff film 5 is made from chromium, it is desirable to be formed to a thickness of 10⁻⁸m or more capable of sufficiently cutting off the light 6 having the specific wavelength used for hardening the core portions 3a,and further it is more desirable to be formed to a thickness of about 10⁻⁷m in consideration of the subsequent step of removing the light cutoff film 5.

[0027] Then, as shown in FIG. 1C, a core layer 3 is formed to cover the light cutoff film 5 and the lower clad layer 2. The core layer 3 can be formed by a usual method of capable of forming a film having a uniform thickness, for example, the spin-coating or spraying method. In addition, the core layer 3 is made from a material capable of being hardened by irradiation of the light 6 having the specific wavelength, for example, made from an ultraviolet hardening-type resin capable of being hardened by irradiation of ultraviolet rays.

[0028] Next, as shown in FIG. 1D, the core layer 3 is irradiated with the light 6 having the specific wavelength from the substrate 1 side to harden the portions positioned at the openings of the pattern of the light cutoff film 5. To be more specific, the light 6 having the specific wavelength, entered from the substrate 1 side, passes through both the substrate 1 and the lower clad layer 2. The light 6 is then cutoff by the solid portions of the light cutoff film 5, but passes through the openings of the light cutoff film 5. That is to say, the light 6 is made incident on the portions to be taken as the core portions 3a,formed at the openings of the light cutoff film 5, of the core layer 3, to harden the portions to be taken as the core portions 3a,of the core layer 3, thereby forming the core portions 3ahaving shapes following those of the openings of the light cutoff film 5. In this case, to form the core portion 3ahaving a side wall being small in surface roughness and also having a highly accurate rectangular shape following that of the opening of the pattern of the light cutoff film 5, the light 6 having the specific wavelength is desirable to be adjusted as a collimated light beam having a uniform energy distribution in the irradiation direction.

[0029] As shown in FIG. 1E, the portions, not exposed to the light 6 having the specific wavelength, of the core layer 3 are removed, to form the highly accurate rectangular core portions 3ahaving the shapes following those of the openings of the pattern of the light cutoff film 5. The portions, not exposed to the light 6 having the specific wavelength, of the core layer 3 can be easily removed by an organic solvent. If the core layer 3 is made from an epoxy based ultraviolet hardening-type resin, ethanol may be used as the organic solvent. The core portion 3athus formed by irradiation of the light 6 having the specific wavelength has a side wall being small in surface roughness and also has a highly accurate rectangular shape having a vertical plane with respect to the principal plane of the substrate 1.

[0030] As shown in FIG. 1F, the light cutoff film 5 remaining on the lower clad layer 2 is removed. If the light cutoff film 5 is made from a chromium film, it can be easily removed by hydrochloric acid or the like. In this case, it is important to remove the light cutoff film 5 without exerting adverse effect on both the core portions 3aand the lower clad layer 2.

[0031] Finally, as shown in FIG. 1G, an upper clad layer 4 is formed to cover the core portions 3aand the lower clad layer 2, to thus obtain an optical waveguide including the core portions 3awhich are highly accurately patterned and are small in propagation loss of light. In addition, the upper clad layer 4 can be formed by a usual method capable of forming a film having a uniform thickness, for example, the spin-coating or spraying method, or the method of laminating a previously film. At this step, if the core portions 3aand the lower clad layer 2 are coated with a material, which is capable of being hardened by irradiation of light, by spin-coating or spraying to form the upper clad layer 4, the material thus applied is hardened by irradiation of light; and if the core portions 3aand the lower clad layer 2 are coated with a material, which is capable of being hardened by heating, by spin-coating or spraying to form the upper clad layer 4, the material thus applied is hardened by heating at a high temperature.

[0032] <Second Embodiment>

[0033] In this embodiment, the present invention is applied to a flexible optical waveguide including a lower clad layer formed on a substrate, core portions formed on the lower clad layer, and an upper clad layer formed to cover the lower clad layer and the core portions. Hereinafter, a method of manufacturing such a flexible optical waveguide will be described with reference to FIGS. 2A to 2H which are diagrams schematically illustrating steps of manufacturing the flexible optical waveguide.

[0034] First, as shown in FIG. 2A, a separation film 7 is formed on a substrate 1, and a lower clad layer 2 is formed on the separation film 7. Like the first embodiment, the substrate 1 is made from a material transparent to the light 6 having the specific wavelength, for example, quartz or glass such as BK-7. The separation film 7 is also made from a material transparent to the light 6 having the specific wavelength, for example, a dielectric film such as silicon oxide or a film made from an organic material. Further, the lower clad layer 2 is formed by a usual method capable of forming a film having a uniform thickness, for example, the spin-coating or spraying method, or the method of laminating a previously formed film. At this step, if the separation film 7 is coated with a material, which is capable of being hardened by irradiation of light, by spin-coating or spraying to form the lower clad layer 2, the material thus applied is hardened by irradiation of light. Meanwhile, if the separation film 7 is coated with a material, which is capable of being hardened by heating, by spin-coating or spraying to form the lower clad layer 2, the material thus applied is hardened by heating at a high temperature.

[0035] The subsequent steps shown in FIGS. 2B to 2F are the same as those shown in FIGS. 1B to 1F, and therefore, the overlapped description thereof is omitted.

[0036] Then, as shown in FIG. 2G, an upper clad layer 4 is formed to cover core portions 3aand the lower clad layer 2. In addition, the upper clad layer 4 can be formed by a usual method capable of forming a film having a uniform thickness, for example, the spin-coating or spraying method, or the method of laminating a previously film. At this step, if the core portions 3aand the lower clad layer 2 are coated with a material, which is capable of being hardened by irradiation of light, by spin-coating or spraying to form the upper clad layer 4, the material thus applied is hardened by irradiation of light; and if the core portions 3aand the lower clad layer 2 are coated with a material, which is capable of being hardened by heating, by spin-coating or spraying to form the upper clad layer 4, the material thus applied is hardened by heating at a high temperature.

[0037] Finally, as shown in FIG. 2H, the lower clad layer 2, the core portions 3a,and the upper clad layer 4 formed on the separation film 7 are collectively separated from the substrate 1 by removing the separation film 7, to thus obtain a flexible optical waveguide including the core portions 3awhich are highly accurately patterned and are small in propagation loss of light. In addition, it is important to remove the separation film 7 without exerting adverse effect on the core portions 3a,the lower clad layer 2 and the upper clad layer 4. If the separation film 7 is made from silicon oxide, it can be removed by a separation liquid such as a weak hydrofluoric acid.

[0038] While the preferred embodiments of the present invention have been described using the specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the sprit or scope of the following claims.