Waveguide fabrication by electron beam irradiation of silica

Abstract: A method for manufacturing waveguide-type optical components featuring a lower propagation loss at low cost in which a lower clad section (2) made of silica glass is formed on a substrate (1), and an antistatic layer and a mask layer having a desired pattern are formed on the surface of the lower clad section (2) in the order they are listed. Then, an electron beam is irradiated from above the mask layer to produce a core (6) of the desired pattern in the lower clad section (2), the mask layer and the antistatic layer are removed to expose the surface of the lower clad section (2), and an upper clad section (7) having a lower refractive index than the core (6) is formed on the exposed surface of the lower clad section (2), thus producing an optical component. The surface of the whole optical component is apart from the core; therefore, the scattering loss caused by minute irregularities on the surface of the optical component is reduced.

Description: BACKGROUND OF THE INVENTION The present invention relates to a manufacturing method for waveguide-type optical components, and more particularly, to a method for manufacturing waveguide-type optical components with a lower propagation loss at a low cost. To fabricate optical fiber communication systems for an extensive field of applications, it is necessary to supply, in a high volume at a low cost, optical components such as optical couplers/splitters and optical multiplexers/demultiplexers to be incorporated in the systems. There is especially high demand for high-volume and low-cost supply of the waveguide-type optical components, which use silica glass as the main material, because they ensure good coupling with optical fibers and provide excellent optical propagation. The electron beam irradiation is attracting attention in the industry as a method for manufacturing waveguide-type optical components, which is capable of responding to the demand described above and which requires a relatively low man-hour and a lower cost. Concerning the optical component manufacturing method based on the electron beam irradiation, for instance, a thesis titled "Optical channel waveguide fabrication based on electron beam irradiation of silica" by S. J. Madden et al. appeared in Applied Physics Letters, Vol. 57, No. 27 issued on Dec. 31, 1990, and another thesis titled "Waveguide Fabrication for Integrated Optics by Electron Beam Irradiation of Silica" by D. Barbier et al. appeared in Journal of Lightwave Technology, vol. 9, No. 6 issued in June 1991. According to the conventional typical electron beam irradiation method, an antistatic layer composed of aluminum, for example, and a thin-film layer composed of gold, for example, are formed in turn on a lower clad section which is formed on a Si substrate and which is made of silica glass. Then, only a portion of the thin-film layer, which corresponds to a waveguide pattern, is removed from the thin-film layer to form a mask layer of the desired pattern, and an electron beam is irradiated from above the mask layer. In that portion of the lower clad section which is subjected to the electron beam irradiation, the silica glass contracts and produces a higher refractive index than the other portions, thereby forming a core. Lastly, the mask layer and the antistatic layer are removed in turn. In this manner, waveguide-type optical components, in which cores with the desired patterns in the lower clad assemblies exist as the waveguide routes, can be obtained at a relatively low cost. However, as is generally known about the film forming technology, the surface of the lower clad section does not become completely smooth and flat, but develops minute irregularities. As a result, in an optical component manufactured in such a manner, the top surface of a core formed in the lower clad section is in contact with the foregoing minute irregularities of the lower clad section, and then the top surface of the core is exposed to the atmosphere. Under such a condition, the light, which goes through the core, increases its scattering loss due to the foregoing minute irregularities. Consequently, the propagation loss, which is partly caused by the scattering loss, increases. A method for irradiating an electron beam onto chalcogenide glass to change the refractive index of a required part of the chalcogenide glass, to thereby form an optical waveguide is known from JP-A-61-45202. The chalcogenide glass is a special type of glass which is extremely difficult to handle, as distinct from silica glass. Namely, in order to stabilize characteristics of the optical waveguide, it is inevitably necessary to form a thin metallic film on a surface of the optical waveguide, as mentioned in JP-A-61-45202. Moreover, JP-A-61-45202 teaches a thin film coating layer made of polymethyl methacrylate with a thickness of at most 0.5 µm. This indicates that the polymethyl methacrylate layer of JA-A-45202 is extremely poor in ability of preventing light from being scattered by minor irregularities. Therefore, a waveguide manufactured in accordance with the known method of JP-A-45202 is also subject to scattering loss due to minute irregularities, as set forth in connection with the previous publications. OBJECT AND SUMMARY OF THE INVENTION An object of the present invention is to provide a manufacturing method for waveguide-type optical components with a reduced propagation loss, which manufacturing method has been achieved by solving the above-mentioned problem in the waveguide-type optical components manufactured using the electron beam irradiation and by increasing the distance between the core and the uneven surface of the optical component. The manufacturing method of waveguide-type optical components according to the present invention comprises a process as defined in claim 1. The method in accordance with the present invention is advantageous in that it makes it possible to manufacture optical components in which the cores of the optical components are embedded in the upper clad sections so that they are apart from the surfaces of the whole optical components (the surfaces of the upper clad sections); therefore, the scattering loss due to minute irregularities on the whole surface of an optical component can be reduced, resulting in a reduced propagation loss which is partly caused by scattering loss. Additionally, according to the present invention method, the electron beam irradiation is used to manufacture optical components, making it possible to reduce the required manufacturing man-hour and cost. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic sectional view which shows a lower clad section formed on a substrate in a manufacturing process of a waveguide-type optical component in accordance with the method of the present invention; FIG. 2 is a schematic sectional view which shows an antistatic layer and a metallic thin-film layer formed on the lower clad section in the process; FIG. 3 is a schematic sectional view which shows a mask layer formed in the process; FIG. 4 is a schematic sectional view which shows a core formed by irradiating an electron beam in the process; FIG. 5 is a schematic sectional view which shows the state wherein mask layer and the antistatic layer have been removed in the process; and FIG. 6 is a schematic sectional view which shows an upper clad section formed in the process. DETAILED DESCRIPTION The manufacturing method for waveguide-type optical components according to the present invention will now be described with reference to the accompanying drawings of FIG. 1 through FIG. 6. As shown in FIG. 1, a lower clad section 2 made of silica glass is formed to a thickness of about 10 to 30µm on a silicon substrate 1 by such methods as the flame hydrolysis deposition method, the plasma enhanced CVD method, the sol-gel method, and the electron beam evaporation method. Then, as shown in FIG. 2, an antistatic layer 3, which is made of, for example, metallic aluminum and measures about 300 angstrom thick, and a thin-film layer 4a, which is made of, for example, gold and measures about 6000 angstrom thick are formed on a lower clad section 2 in sequence by, for instance, the sputtering method or the evaporation method. After that, only a portion of the thin-film layer 4a that corresponds to a waveguide route pattern to be formed is removed from the thin-film layer 4a by applying, for example, the photo-lithography and the reactive ion etching (RIE) to the thin-film layer 4a in turn to produce a mask layer 4 of the desired pattern (FIG. 3). Next, as shown in FIG. 4, an electron beam 5 is irradiated from above the mask layer 4. Normally, the accelerating voltage of electron beams ranges from 15 to 25kV with a dose of about 1 to 2C/cm 2 . In that portion of the lower clad section 2 which is subjected to the irradiation of the electron beam, the silica glass contracts and develops a higher refractive index than other portions. In other words, this portion with the higher refractive index provides a core 6, thereby forming the core 6 of the desired pattern in the lower clad section 2. The thickness of the core 6 can be set to an appropriate value by adjusting the accelerating voltage of the electron beam irradiated. Also, a difference in specific refractive index between the core 6 and the lower clad section 2 can be set to any value by adjusting the dose of the electron beam. For instance, when the electron beam is irradiated under the above-mentioned condition, the core 6 will be 2 to 7µm high, the core layer having a difference in specific refractive index of about 0.35%. Then, the mask layer 4 and the antistatic layer 3 are removed in sequence (FIG. 5). As in the case where optical components are manufactured by the conventional method, the optical components manufactured by the method in accordance with the present invention has a large propagation loss in the manufacturing phase shown in FIG. 5 wherein the mask layer 4 and the antistatic layer 3 are removed after the core 6 is formed. This means that the surface of the lower clad section 2 formed on the silicon substrate 1 as shown in FIG. 1 does not become completely smooth and flat, but develops minute irregularities. Therefore, in the manufacturing phase shown in FIG. 5, the top surface of the core 6 formed in the lower clad section 2 is in contact with the foregoing minute irregularities of the lower clad section 2, and then, the top surface of the core 6 is exposed to the atmosphere. Under such a condition, the light, which goes through the core 6, increases its scattering loss due to the foregoing minute irregularities. Consequently, the propagation loss, which is partly caused by scattering loss, increases. To solve the problem, in the method according to the present invention, an upper clad section 7 is provided on the lower clad section 2 to embed the core 6 in the clad sections 2 and 7 as shown in FIG. 6, this process being added to the processes shown in FIG. 1 through FIG. 5. In an optical component as a completed product shown in FIG. 6, the surface of the upper clad section 7 has minute irregularities, but the core 6 is not exposed to the atmosphere. Further, the core 6 is apart from the foregoing minute irregularities by the thickness of the upper clad section 7; therefore, light going through the core 6 incurs a lower scattering loss caused by the minute irregularities. There is no particular limit to the thickness of the upper clad section 7, however, the above-mentioned effect can be maintained more easily in general as the thickness increases. For instance, in the case of a waveguide-type optical component designed to optimize the loss of field mismatch with a regular optical fiber for a wavelength of 1.3µm, even if the upper clad section 7 is about 0.1µm thick, it starts to contribute to reduction in the scattering loss, and when the thickness reaches 20µm or more, the effect will be attained almost completely. The material used for the upper clad section 7 is acceptable as long as it has a lower refractive index than the core. As in the case of the lower clad section 2, the upper clad section 7 can be formed by a method such as the flame hydrolysis deposition method, the plasma enhanced CVD method, the sol-gel method, and the electron beam evaporation method. The flame hydrolysis deposition method is especially suitable. For instance, when the flame hydrolysis deposition method is used to form the upper clad section 7 using the same silica glass as the lower clad section 2, a waveguide-type optical component with an extremely low loss can be obtained. EMBODIMENT The lower clad section 2 consisting of silica glass of about 25µm thick was formed on the Si substrate 1 by the flame hydrolysis deposition method (FIG. 1). Then, the aluminum layer (antistatic layer) 3 of about 300 angstrom thick and the gold thin layer 4a of about 6000 angstrom thick were formed on the lower clad section 2 in turn by the sputtering method (FIG. 2). The gold thin layer 4a was subjected to the photo-lithography and the reactive ion etching to turn the gold thin layer 4a into the mask layer 4 having a pattern wherein the waveguide route width is about 8µm (FIG. 3). Then, the electron beam 5 was irradiated from above the mask layer 4 under an accelerating voltage of 25kV. The dose was about 1C/cm 2 . The core 6, which had a height of about 5µm, a width of about 8µm, and a difference in specific refractive index of about 0.35%, was formed (FIG. 4). Next, the mask layer 4 and the aluminum layer 3 were removed in sequence to expose the lower clad section 2 (FIG. 5). The light propagation loss under this condition was 0.3dB/cm. On the exposed lower clad section 2, the upper clad section 7, which consists of the same silica glass as the lower clad section and which is about 20µm thick, was formed by the flame hydrolysis deposition method, thus embedding the core 6. The propagation loss of the optical component thus obtained was 0.1dB/cm. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims: A manufacturing method of waveguide-type optical components, comprising processes of: (a) forming a lower clad section (2), which is made of silica glass, on a substrate (1); (b) forming an antistatic layer (3) and a mask layer (4), which has a desired pattern, on the surface of said lower clad section (2) in the order they are listed; (c) forming a core (6) of said desired pattern in said lower clad section (2) by irradiating an electron beam (5) from above said mask layer (4); (d) removing said mask layer (4) and said antistatic layer (3) to expose the surface of said lower clad section (2); and (e) forming an upper clad section (7), which has a lower refractive index than said core (6), on the exposed surface of said lower clad (2), wherein said upper clad section (7) is formed to have a thickness which is sufficient to substantially avoid the scattering loss caused by minute irregularities produced on the surface of said upper clad section (7) thereby avoiding propagation loss. The manufacturing method of waveguide-type optical components according to claim 1, wherein said upper clad section (7) is formed by the flame hydrolysis deposition method. The manufacturing method of waveguide-type optical components according to claims 1 or 2, wherein the thickness of said upper clad section (7) reaches at least 20 µm.

Inventors: UEKI KEN, YANAGAWA HISAHARU, UEKI, KEN, YANAGAWA, HISAHARU
Applicants: FURUKAWA ELECTRIC CO LTD, THE FURUKAWA ELECTRIC CO., LTD.
Application year: 1992
Main classification: G02B6
IPC codes: G02B6