1. Field of the Invention
The present invention relates to a method of manufacturing an optical waveguide device for widespread use in optical communications, optical information processing and other general optics.
2. Description of the Related Art
In general, an optical waveguide for an optical waveguide device is constructed in such a manner that cores serving as a passageway for light are formed in a predetermined pattern on a surface of an under cladding layer, and that an over cladding layer is formed so as to cover the cores. Such an optical waveguide is typically formed on a surface of a substrate such as a metal substrate and the like, and is manufactured together with the substrate to provide an optical waveguide device.
A conventional method of manufacturing such an optical waveguide device is as follows. First, as shown in FIG. 6A, an under cladding layer 20 is formed on a surface of a substrate 10. Then, as shown in FIG. 6B, a photosensitive resin for the formation of cores is applied to a surface of the under cladding layer 20 to form a photosensitive resin layer 30A. Next, irradiation light L is directed through a photomask M formed with an opening pattern corresponding to the pattern of cores 30 (with reference to FIG. 6C) toward the photosensitive resin layer 30A. The irradiation light L is caused to reach the photosensitive resin layer 30A through openings of the opening pattern, thereby exposing portions of the photosensitive resin layer 30A thereto. The irradiation light L is directed to the photosensitive resin layer 30A at right angles thereto. A photoreaction proceeds in the portions of the photosensitive resin layer 30A exposed to the irradiation light L so that the exposed portions are hardened. Then, development is performed using a developing solution to dissolve away unexposed portions of the photosensitive resin layer 30A, as shown in FIG. 6C. The remaining exposed portions become the cores 30 formed in a predetermined pattern. The cores 30 are typically rectangular in sectional configuration. Thereafter, as shown in FIG. 6D, a photosensitive resin for the formation of an over cladding layer is applied to the surface of the under cladding layer 20 so as to cover the cores 30, to thereby form a photosensitive resin layer 40A. Next, irradiation light is directed toward the photosensitive resin layer 40A to expose the photosensitive resin layer 40A thereto. This causes the photosensitive resin layer 40A to be formed into an over cladding layer 40. In this manner, an optical waveguide W0 is formed on the surface of the substrate 10, for example, as disclosed in Japanese Published Patent Application No. 2004-341454.
In such a conventional method, however, the cores 30 have side surfaces 30a formed as roughened surfaces in some cases, as shown in FIGS. 7A and 7B. Such cores 30 increase the propagation losses of light propagating inside the cores 30 because of leakage of light from the side surfaces 30a formed as the roughened surfaces or other reasons. There is apprehension that the above-mentioned optical waveguide does not function as optical interconnection. When the surface roughening of the side surfaces 30a of the cores 30 is even slight but is greater than the wavelength of the light propagating inside the cores 30, the surface roughening causes scattering losses. Also in this case, the propagation losses increase, and there is apprehension that the above-mentioned optical waveguide does not function as optical interconnection, as described above. FIG. 7B is a view drawn based on a photograph in perspective of a core 30 enclosed with the circle C of FIG. 7A which is magnified 700 times with an electron microscope. By magnifying the core 30 700 times with the electron microscope in this manner, it can be seen that the side surfaces 30a of the cores 30 are formed as the roughened surfaces.
The present inventors have made studies to diagnose the cause of the formation of the side surfaces 30a of the cores 30 as the roughened surfaces. In the course of the studies, the present inventors have found that the surface roughening of the side surfaces 30a of the cores 30 occurs either in the case (1) where a metal substrate 11 (with reference to FIG. 8) in common use which is made of stainless steel (SUS) foil and the like is used as the substrate 10 (with reference to FIGS. 6A to 6D) or in the case (2) where a substrate (PET substrate) 12 (with reference to FIG. 9) made of a material having polyethylene terephthalate (PET) as a main component is used as the substrate 10.
As a result of further studies on the case (1) where the metal substrate 11 made of stainless steel foil and the like is used, the present inventors have found that the metal substrate 11 made of stainless steel foil and the like includes a roughened surface having an arithmetic mean roughness (Ra) of not less than 0.095 μm. For this reason, in the above-mentioned step of forming the cores 30, the irradiation light L for use in the exposure which passes through the openings of the photomask M is transmitted through the photosensitive resin layer 30A for the core formation and the under cladding layer 20, and thereafter is reflected diffusely from the roughened surface of the metal substrate 11 because of the surface roughening, as indicated by arrows shown in FIG. 8. The diffusely reflected irradiation light L is transmitted through the under cladding layer 20 obliquely upwardly from below. Then, boundary surfaces (surfaces that are to become the side surfaces 30a) for the patterning of the cores 30 are exposed to the diffusely reflected irradiation light L directed obliquely from below, as indicated by the arrows, in future core regions S included in the photosensitive resin layer 30A for the core formation. This exposure to the light directed obliquely from below results from the above-mentioned diffuse reflection, and is uneven. Thus, it has been found that an unwanted photoreaction proceeds unevenly at the surfaces that are to become the side surfaces 30a of the cores 30 because of the exposure to the light directed obliquely from below to result in the increased width of the cores 30 and the formation of the side surfaces 30a of the cores 30 as the roughened surfaces. In other words, the surfaces that are to become the side surfaces 30a of the cores 30 have both portions subjected to a low degree of exposure to light and portions subjected to a high degree of exposure to light because of the diffuse reflection of the irradiation light L. In the subsequent step of development, the portions subjected to a low degree of exposure to light are dissolved away from the surfaces that are to become the side surfaces 30a of the cores 30, and the portions subjected to a high degree of exposure to light remain unremoved. Thus, the side surfaces 30a of the cores 30 are formed as the roughened surfaces.
The present inventors have made further studies on the case (2) where the PET substrate 12 is used. As a result of the further studies, it has been found that the PET substrate 12 contains an additive component such as a lubricant material 12a and the like, as shown in FIG. 9, and that the irradiation light L for use in the exposure in the above-mentioned step of forming the cores 30 impinges upon and is reflected from the additive component such as the lubricant material 12a, to thereby follow irregular paths. It has also been found that most of the irradiation light L reaches the bottom surface (the back surface) of the PET substrate 12. That is, in the above-mentioned step of forming the cores 30, the irradiation light L for use in the exposure which passes through the openings of the photomask M is transmitted through the photosensitive resin layer 30A for the core formation and the under cladding layer 20, and thereafter enters the interior of the PET substrate 12, as shown in FIG. 9. In the PET substrate 12, the irradiation light L impinges upon the lubricant material 12a and the like to follow irregular paths, and then reaches the bottom surface of the PET substrate 12 at an angle. In general, the back surface of the PET substrate 12 is in contact with a mounting surface of a mounting table and the like (not shown) for placing the PET substrate 12 thereon, the mounting surface being impervious to the irradiation light L. For this reason, the irradiation light L reaching the bottom surface of the PET substrate 12 does not exit from the back surface of the PET substrate 12 but is reflected from the bottom surface of the PET substrate 12 obliquely upwardly as indicated by arrows in FIG. 9. Thereafter, the reflected irradiation light L impinges upon the lubricant material 12a and the like in the PET substrate 12 to follow further irregular paths, and then exits from the front surface of the PET substrate 12 obliquely upwardly. The irradiation light L exiting obliquely upwardly is transmitted through the under cladding layer 20 obliquely upwardly from below. Then, the boundary surfaces (the surfaces that are to become the side surfaces 30a) for the patterning of the cores 30 are exposed to the irradiation light L directed obliquely from below in the future core regions S included in the photosensitive resin layer 30A for the core formation. This exposure to the light directed obliquely from below comes from the diffuse reflection resulting from the lubricant material 12a and the like contained in the PET substrate 12 as mentioned earlier, and is uneven. Thus, as in the case (1) where the metal substrate 11 made of stainless steel foil and the like is used, an unwanted photoreaction proceeds unevenly at the surfaces that are to become the side surfaces 30a of the cores 30 because of the exposure to the light directed obliquely from below to result in the formation of the side surfaces 30a of the cores 30 as the roughened surfaces.
The assignee of the present application has proposed a method of manufacturing an optical waveguide device which is capable of suppressing the surface roughening of core side surfaces of an optical waveguide when the optical waveguide is formed on a surface of a substrate, and has already applied for patents (Japanese Patent Applications Nos. 2008-130727, 2008-234735, 2008-239346, and 2008-269688).
Japanese Patent Application No. 2008-130727 discloses a technique for forming an under cladding layer containing an irradiation light absorbing agent on a roughened surface of a metal substrate or forming an irradiation light absorbing layer on the roughened surface of the metal substrate prior to the formation of an under cladding layer. In this technique, irradiation light for the formation of cores is absorbed or attenuated by the irradiation light absorbing agent contained in the under cladding layer or by the irradiation light absorbing layer before or after being reflected diffusely from the surface of the metal substrate. This significantly reduces the amount of irradiation light directed obliquely from below to roughen the surfaces that are to become the side surfaces of the cores by exposing the surfaces thereto. As a result, this technique suppresses the surface roughening of the side surfaces of the cores.
Japanese Patent Application No. 2008-234735 discloses a technique in which a material that absorbs irradiation light for the formation of cores is used for a substrate having a roughened surface. In this technique, the irradiation light for the formation of the cores is absorbed by the substrate when the irradiation light reaches the surface of the substrate. This significantly reduces the amount of irradiation light directed obliquely from below to roughen the surfaces that are to become the side surfaces of the cores by exposing the surfaces thereto. As a result, this technique suppresses the surface roughening of the side surfaces of the cores.
Japanese Patent Application No. 2008-239346 discloses a technique in which either a colored-layer-coated substrate having a back surface on which a colored layer of a color that absorbs irradiation light is formed or a colored substrate entirely colored in a color that absorbs the irradiation light is used as a PET substrate. In this technique, the irradiation light for the formation of cores is absorbed by the colored layer or the colored substrate. This significantly reduces the amount of irradiation light directed obliquely from below to roughen the surfaces that are to become the side surfaces of the cores by exposing the surfaces thereto. As a result, this technique suppresses the surface roughening of the side surfaces of the cores.
Japanese Patent Application No. 2008-269688 discloses a technique in which either only a front surface or both front and back surfaces of a substrate (a silicon wafer, a glass substrate and the like) have an arithmetic mean roughness (Ra) in the range of 1 to 2 nm. In this technique, the surface that reflects irradiation light for the formation of cores is so smooth that the diffuse reflection of the irradiation light from the surface is suppressed. This significantly reduces the amount of irradiation light directed obliquely from below to roughen the surfaces that are to become the side surfaces of the cores by exposing the surfaces thereto. As a result, this technique suppresses the surface roughening of the side surfaces of the cores.
In this manner, the above-mentioned techniques of the Applications already filed by the assignee of the present application can suppress the surface roughening of the side surfaces of the cores to reduce the propagation losses of light. However, even when the surface roughening of the side surfaces of the cores is suppressed, the side surfaces of the cores are formed as roughened surfaces in some cases. The present inventors have made studies to diagnose the cause of such a phenomenon. As a result, the present inventors have found that the cause of the phenomenon lies in the contamination of the photomask for use in the formation of the cores. Specifically, a material for the formation of the cores and the like might be deposited onto portions of the photomask which surround the openings of the photomask because of the use of the photomask. In that case, the deposits on the portions of the photomask which surround the openings cause the irradiation light passing through the openings to follow irregular paths. This makes the irradiation light reaching the side surfaces of the cores uneven to result in the roughened side surfaces of the cores.
In the formation of an optical waveguide on the surface of a substrate, it has hence been required to suppress the surface roughening of the side surfaces of the cores, thereby reducing the propagation losses of light, regardless of the type of substrate.