Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Including integrally formed optical element
Reexamination Certificate
1999-09-10
2001-11-13
Chaudhuri, Olik (Department: 2814)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Including integrally formed optical element
C385S014000, C385S049000
Reexamination Certificate
active
06316281
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical integrated circuit; and, more particularly, to a method for preparing an improved hybrid optical integrated circuit which is capable of accommodating optical waveguides, optical devices, such as light emitting devices or light receiving devices, and optical fibers in an effective manner.
2. Description of the Prior Art
As the amount of information has been dramatically increased in a modern society, in order to transmit such great amount of the information, an optical integrated circuit has been suggested as a core technique in view of the performance and price for an optical transmission system. Such an optical integrated circuit includes two types: optoelectronic integrated circuit(OEIC); and a hybrid integrated circuit(HIC). In the OEIC, an optical waveguide, a light emitting device and a light receiving device are prepared by using the same material and optically coupled to each other on a monolithic wafer which is made of a compound semiconductor, for example, InP. While, in the HIC, a light emitting device and a light receiving device are combined on a wafer by using a surface mounting technique. In view of the conventional techniques, the HIC is more realistic to be implemented than the OEIC.
Various efforts have been addressed to prepare a hybrid optical integrated circuit in which an active optical devices, such as a light emitting device and a light receiving device, are mounted on a wafer together with an optical waveguide. Referring to
FIG. 1
, there is shown a hybrid optical integrated circuit described in U.S. Pat. No. 4,735,677, issued to Masao Kawachi et al, entitled “Method for fabricating hybrid optical integrated circuit.” As shown in
FIG. 1
, the hybrid optical integrated circuit includes a high-silica glass optical waveguide
141
formed on a silicon substrate
140
, a semiconductor laser
147
as a light emitting device, a semiconductor photodetector
148
as a light receiving device, guides
142
for aligning the semiconductor laser
147
, guides
143
for aligning optical fibers
149
. The high-silica optical waveguide
141
has a Y-branch shape which defines three end faces
144
to
146
. The semiconductor laser
147
is positioned between the guides
142
on the silicon substrate
140
and coupled to a first end face
144
. The photodetector
148
is mounted on the silicon substrate
140
and coupled to a second end face
145
. The optical fiber
149
is located between the guides
143
and coupled to a third end face
146
.
The optical waveguide
141
is generally integrated on the silicon substrate
140
and the semiconductor laser
147
and the optical fiber
149
are aligned to the first and third end faces
144
and
146
of the optical waveguide
141
by using the semiconductor laser alignment guides
142
and the optical fiber alignment guides
143
, respectively. Usually, high silica optical waveguide
141
should have a thickness of several tens of microns to meet a single mode condition. In this case, the etching process for fully defining or forming the optical waveguide
141
significantly attacks the etching mask layer also so that the dimension of the optical waveguide
141
and alignment guides
142
and
143
may be altered in micron range. Furthermore, the semiconductor laser
147
should have to machined with submicron accuracy in order to be precisely aligned by the semiconductor laser alignment guides
142
, while it is so difficult to treat a compound semiconductor material, for example, InP, GaAs or the like, used in fabricating the semiconductor laser
147
in a submicron-level precision due to the brittleness thereof.
The waveguide device used for transmitting light within optical devices and optical integrated circuits must have a low light transmission loss and a comparable mode size with optical devices to be interconnected with thereof. In some cases, it should have electro-optic or thermo-electric effects, in order to implement passive and active waveguide devices.
Since single crystal silicon widely used in semiconductor integrated circuit have a higher transmittance in a wavelength range of 1.2 to 1.6 micron, it is possible to use the silicon layers as an optical waveguide in the above wavelength range.
FIG. 2
shows a cross-sectional view illustrating a conventional rib-type SOI (Silicon On Insulator) optical waveguide which is typically formed on a SOI wafer. The SOI wafer has been fabricated by the silicon direct bonding or the separation by implantation of oxygen (SIMOS) methods. The SOI waveguide structure includes a buffer layer
151
formed on a silicon substrate
150
, a core layer
152
and a cladding layer
153
. Typically, the buffer layer
151
formed at a thickness of 1 to 2 &mgr;m has been made of a silicon oxide layer. The core layer
152
has been made of a single crystal silicon layer having a thickness of 2 to 10 &mgr;m. And a silicon oxide layer formed by oxidizing the surface of the core layer
152
is used as the cladding layer
153
. In order that light is guided within the core layer
152
of the SOI waveguide and satisfies a single mode condition in the vertical and horizontal directions, the width (W)
155
of a rib, the height (H) of the core layer
157
and the height (rH)
154
of a slab are given by:
r≦
0.5
H
W
≤
0.3
+
r
1
-
r
2
Accordingly, to obtain the maximum optical coupling efficiency when the SOI waveguide is coupled to the laser and the optical fiber, the thickness
157
of the core layer
152
, the width
155
of the rib and the height
156
of the rib should be controlled under the condition of satisfying the above equation.
As an example of conventional hybrid optical integrated circuits using the SOI waveguides,
FIG. 3
shows an alignment between a waveguide and a laser(“ASOC™-A silicon-based integrated optical manufacturing technology,” Tim Bestwick et al, Proceedings of the 48th ECTC, 1998, pp 566-571). As shown in
FIG. 3
, a rib-type SOI waveguide
162
is formed by etching an undesired portion of a single crystal silicon layer
164
which is isolated from a silicon substrate by a buried oxide layer
163
and a laser
161
coupled to the SOI waveguide
162
is mounted on a recess
168
formed when the single crystal silicon layer
164
is etched to form the rib. Accordingly, a guide for aligning the laser
161
to the SOI waveguide
162
is made of a single crystal silicon thin film and a horizontal alignment between the laser
161
and the SOI waveguide
162
is achieved by attaching the laser
161
to both sidewalls
166
and
169
of the recess
168
.
As the prior art employing a silica waveguide, the above-mentioned optical integrated circuit using a waveguide thin film as mechanical stops of optical devices, still has had a problem itself in that semiconductor laser is to be mechanically processed. Since SOI waveguide thin film, the thickness of which is only about 10 &mgr;m, cannot be used as an alignment guide of an optical fiber, the thickness of which is about 125 &mgr;m. As the same as the prior art using a silica waveguide, any optical fiber aligner except for the SOI waveguide thin film such as anisotropically etched silicon V-groove is necessary. However, in the case of making a structure for optical fiber alignment by other means but SOI waveguide thin film, a lot of optical coupling loss may be caused between the optical fiber and the waveguide due to the misalignment therebetween.
A shortcoming of a SOI waveguide is that a lot of Fresnel loss may occur in the front and back facet of the SOI waveguide when light is incident from the atmosphere to the SOI waveguide or radiated from SOI waveguide to the atmosphere because a refractive index of the silicon layer (about 3.5) used as a core material is much larger than that of the atmosphere (1). It is necessary to form anti-reflection film in the front and back facet of the waveguide in order to lessen Fresnel loss. It is desirable that the most appropriate anti-reflection film has a refractive index of 1.8
Hwang Nam
Joo Gwan Chong
Lee Sang Hwan
Song Min Kyu
Chaudhuri Olik
Duy Mai Anh
Electronics and Telecommunications Research Institute
Jacobson & Holman PLLC
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