Optical waveguides – Integrated optical circuit
Reexamination Certificate
2000-06-13
2002-11-05
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Integrated optical circuit
C385S088000
Reexamination Certificate
active
06477284
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a photo-electric combined substrate having a photo-electric transducing function used in, e.g., optical communication, in which an optical waveguide is combined with an electric interconnection, and a manufacturing process therefor.
This invention also relates to an optical waveguide, in particular an optical waveguide with a higher heat resistance and a higher economic efficiency which can be manufactured by a relatively convenient process, and to a manufacturing process therefor.
2. Description of the Prior Art
Apparatuses such as an optical exchanger and a photo-interconnection device have been intensely investigated and developed for achieving large-capacity and high-speed communication. These apparatuses comprise an electric signal processing portion, an optical signal processing portion, and a transducing portion of an electric signal to an optical signal or vice versa. The transducing portion comprises a photoelectric transducing device (optical device) such as a laser diode (LD) and a photodiode (PD), and an electric element for operating the optical device or amplifying the signal.
In a conventional photo-interconnection, a silicon substrate is, in the light of its properties, used as a substrate on which an optical waveguide is formed and an optical device is mounted, while a ceramic substrate or printed board is frequently used as a substrate on which an electric interconnection is formed and an electric device is mounted. These substrates are mutually connected via a bonding wire in a manner that the substrate for the optical device is placed over the electric substrate.
In the conventional technique where the substrate on which an optical waveguide is formed and an optical device is mounted, is connected, via a bonding wire, with the substrate on which an electric interconnection is formed and an electric device is mounted, however, the wire is relatively longer. Therefore, when increasing an operating frequency for increasing a transmission capacity, a noise is overlaid on a signal. Thus, a higher frequency cannot be achieved.
In an attempt for solving the problem, several techniques have been proposed, in which both optical and electric devices are mounted on a single substrate; for example, JP-A 9-236731 has disclosed a ceramic substrate on which both optical and electric devices are mounted.
When both electric and optical devices are mounted on a ceramic substrate and these devices are closely mounted for high-speed operation of the optical device, the ceramic is responsible for heat insulation. However, a ceramic does not have a sufficiently low heat conductivity to prevent thermal interference between the electric and the optical devices.
On the other hand, when an optical device is mounted on a resin heat-insulating material on a ceramic substrate, the resin is so soft that an optical axis tends to be not in the right position.
For high-speed operation of an optical device, it is necessary to reduce the length of the electric interconnection between the electric and the optical devices. When the electric device and the optical device are mounted by separate methods, there is a restriction in reducing the length of the electric interconnection between the electric and the optical devices. There are also limitations in densification of, e.g., the electric and the optical devices.
A combined substrate described above has a configuration where an optical waveguide consisting of a siloxane polymer is formed on a ceramic multilayer interconnection substrate. Thus, when the interconnection substrate and the optical waveguide are made of different materials, it is necessary to form the electric insulating layer of the interconnection substrate and the optical waveguide with different materials by separate processes. It has been, therefore, difficult in the combined substrate to realize a complete three-dimensional combination or an adequately reduced cost for the optical waveguide and the electric interconnection. Furthermore, for a siloxane polymer used as a resin for an optical waveguide in the combined substrate, it is difficult to form a fine interconnection or a via-hole by photolithography process. The polymer cannot be, therefore, as a material for an electric insulating film.
In JP-A 3-245586, a semiconductor laser device is mounted on a resin such as a fluororesin (Teflon; trade mark) as an insulating material for preventing heat from being transferred from an electric device to the semiconductor laser device as an optical device.
Among others, an optical waveguide made of a resin has been intensely investigated and developed because it can be formed by a low-temperature and low-cost process into various types of substrates, leading to reduction in an overall cost for an optical module.
For example, F. Shimokawa et al., In Pr43rd ECTC (1993), p.705-710 and T. Matsuura et al., MES'97 (the Seventh Microelectronics Symposium), p.77-80 have disclosed an example where a fluorinated polyimide is used as a material for forming an optical waveguide.
According to these publications, the fluorinated polyimide is applied on a substrate and then heated to 300 to 400° C. to form a film. Then, the optical waveguide core is processed into a desired shape by reactive ion etching. A copolymerization ratio between two polyimides can be varied to adjust a refractive index. The glass-transition temperature of the fluorinated polyimide is about 300° C.
Furthermore, K. Enbutsu et al., MOC/GRIN '97 Tech Digest, P3, p.394 and 1998 Electronic Information Communication Association Electronics Society Meeting Proceeding C-3-69 have disclosed an example where an ultraviolet (UV) curable resin (photosensitive epoxy resin) is used as a material for forming an optical waveguide.
As indicated in these publications, an ultraviolet curable resin has an advantage that only the core of the optical waveguide can be irradiated with UV to be cured into a desired shape. A main component in the UV curable resin can be varied to adjust a refractive index. The glass-transition temperature of the UV curable resin is about 250° C.
In JP-A 10-170738, an optical waveguide is made of an asymmetric-spiro-ring containing epoxy acrylate resin.
However, when a fluorinated polyimide is used as a material for an optical waveguide, reactive ion etching is employed in forming the optical waveguide into a desired shape, leading to a longer etching duration, and thus it cannot be conveniently formed. In addition, available substrate materials are limited due to process factors such as a higher deposition temperature. Furthermore, a fluorinated polyimide has a disadvantage of a higher material cost.
On the other hand, an UV curable resin can be used to conveniently form a core shape because only UV irradiation is required. It is, however, used for a multi-mode optical waveguide whose core cross section has a width and a height of several ten micrometers because of an insufficient resolution for forming a fine pattern. Thus, it is not be applied to a single-mode optical waveguide whose core cross section has a width and a height of about several micrometers.
Furthermore, the UV curable resins as described in the prior art publications have a glass-transition temperature of about 250° C., so that it cannot endure an optical device mounting step (about 300° C.) using a gold/tin solder (melting point: 280° C.) having a self-alignment effect, i.e., an effect that an optical device is drawn to a desired position by surface tension of a solder ball.
SUMMARY OF THE INVENTION
An object of this invention is to provide a photo-electric combined substrate in which an optical waveguide and an electric interconnection can be three-dimensionally combined with a reduced cost, and a manufacturing process therefor.
Another object of this invention is to provide a ceramic substrate comprising an optical device and an electric device which can operate with a high speed.
Another object of this invention is to provide a material for an optical wa
Fujiwara Masahiko
Itoh Masataka
Kaneyama Yoshinobu
Kitajo Sakae
Oda Mikio
Foley & Lardner
Knauss Scott
NEC Corporation
Sanghavi Hemang
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