Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2002-12-31
2004-09-28
Wong, Don (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S045013, C372S046012, C372S049010
Reexamination Certificate
active
06798807
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a technology for manufacturing a semiconductor laser and a technology for manufacturing a semiconductor laser module with the semiconductor laser built therein, and to, for example, a technology effective for application to a technology for manufacturing a semiconductor laser for optical communications.
In a wavelength division multiplexing (WDM) type optical transmission system, a demand for more speeding-up has been made even to transmission equipment with an increase in the amount (traffic) of data to be transmitted. A DFB (Distributed FeedBack) laser with a modulator for 10 Gbps DWDM (Dense Wavelength Division Multiplexing) built therein has been described in, for example, the November 2001 issue “Electronic Materials” by Institute for Industrial Research, P31-P33. A direct modulation DFB-LD (Laser Diode) or the like has been also described in the present reference.
Unexamined Patent Publication No. Hei 7(1995)-135369 has disclosed, as a semiconductor laser used as a light source for optical communications, a semiconductor laser wherein first and second electrodes are disposed on the same surface so as to allow flip-chip packaging, and a stray capacitance of a device is reduced to thereby enable handling of a high-speed operation. The same reference describes that a cutoff frequency f of a semiconductor laser (semiconductor laser device) becomes f=½&pgr;RC and a reduction in the capacitance C of the device allows high-speed modulation from this equation. The reference also describes that the capacity of a normal semiconductor laser in which two electrodes are opposed with a substrate and respective layers interposed therebetween, is one by adding a junction capacitance Cj formed between an active layer and a clad layer and a stray capacitance Cd developed between the electrodes.
The present reference has described that the semiconductor laser disclosed therein is formed on a semi-insulating substrate, and a minus electrode
12
and a plus electrode
20
are disposed so as to be electrically isolated from each other on the same surface, whereby the stray capacitance developed between the electrodes is greatly reduced.
SUMMARY OF THE INVENTION
In the case of a semiconductor laser having an embedded layer on the main surface side of a semiconductor substrate, the end-point management of embedded growth is difficult and the surface of the semiconductor substrate and the surface of the embedded layer are hard to be identical to each other. A buildup of a boundary face also occurs on the embedded-layer side. As a result, it is difficult to connect both a first electrode provided on the surface of the semiconductor substrate and a second electrode provided on the surface of the embedded layer to their corresponding electrode portions of a printed circuit board in a satisfactory state through the use of bonding materials under a so-called junction down state in which the side of a pn junction is set as a mounting surface. Yield degradation and deterioration in packaging reliability are produced.
Therefore, the present inventors have discussed that in a ridge structure in which the surface of the semiconductor laser is made flat, the first and second electrodes are disposed on the surface of the semiconductor laser to carry out flip-chip packaging. Namely, while trenches or grooves exist on both sides of a ridge (stripe) to form the ridge in the ridge structure, the outer surface of each trench is identical in height to the surface of the ridge.
FIGS. 31 through 33
are respectively diagrams related to a semiconductor laser (semiconductor laser device) for high-speed optical communications, which has been discussed prior to the present invention, wherein
FIG. 31
is a typical perspective view of the semiconductor laser,
FIG. 32
is an enlarged view of a ridge waveguide portion, and
FIG. 33
is an equivalent circuit diagram of the semiconductor laser, respectively.
The semiconductor laser
60
has a structure wherein it has a multilayered semiconductor layer including an active layer on a main surface of a semiconductor substrate
61
, and an anode electrode
75
and a cathode electrode
76
are respectively provided on the surface thereof and the back surface thereof.
The semiconductor substrate
61
serves as an n-InP substrate
61
, for example. A lower SCH (Separate Confinement Heterostructure) layer
62
formed of an n-InGaAlAs layer, an active layer
63
made up of an InGaAlAs layer, an upper SCH layer
64
made up of a p-InGaAlAs layer, a p-InP layer
65
and a p-InGaAs layer
66
are sequentially laminated and formed on the n-InP substrate
61
.
The multilayered crystal layer is provided with two grooves or trenches
67
in parallel on its surface. A ridge (stripe)
68
is formed at a portion interposed between the two trenches
67
. The trenches
67
are defined by removing, by etching, the p-InGaAs layer
66
corresponding to the top layer of the multilayered crystal layer and the p-InP layer
65
provided therebelow. The upper SCH layer
64
is exposed at the bottom of each trench
67
. The ridge
68
has a width of 2 &mgr;m and a length of 200 &mgr;m.
An insulating film
69
formed of an SiO
2
film is provided so as to extend beyond the trenches
67
from both sides of the ridge
68
respectively and extend over the p-InGaAs layers
66
located outside the trenches
67
. Namely, only the upper surface of the ridge
68
is exposed without being covered with the insulating film
69
. An anode electrode (p electrode)
75
, which electrically contacts the p-InGaAs layer
66
for forming the upper surface of the ridge
68
, is provided up to an area extending from the ridge
68
to outer edge portions of the trenches
67
on both sides of the ridge
68
. The anode electrode
75
is formed with wire bonding portions
75
a
which extend out so as to become wide on the center side of the ridge
68
and are connectable with wires (see FIG.
31
). A cathode electrode (n electrode)
76
is provided on the back surface of the n-In substrate
61
.
Incidentally, the active layer
63
takes a multiple quantum well (MQW) structure. Diffraction gratings are provided on the surface of the n-InP substrate
61
along the longitudinal direction of the ridge
68
to thereby configure a distributed feedback semiconductor layer (DFB-LD).
When a predetermined voltage is applied between the anode electrode
75
and the cathode electrode
76
in such a semiconductor laser
60
, laser light is emitted from the end of the active layer
63
corresponding to the ridge
68
.
A reduction in capacitance is essential for a high-speed modulation laser diode (semiconductor laser). As means for achieving the capacitance reduction, there is (1) a method of reducing electrode areas or (2) a method of making thick a dielectric material between electrodes under a structure wherein the electrodes are opposed to each other with an active layer interposed therebetween. When the anode electrode
75
is further reduced to make each electrode smaller in the semiconductor laser
60
shown in
FIG. 31
, the wire bonding portions
75
a
must be further reduced, so that a problem arises in that the wire connecting portions partly extend out from the wire bonding portions
75
a
, for example, and the reliability (bonding property) of wire connectivity is degraded. This is not preferable. FIG.
17
(
c
) shows a dimensional example of the wire bonding portion
75
a
employed in the semiconductor laser
60
. Since a light-emitting property is impaired, each wire is not bonded onto a ridge portion, i.e., an optical waveguide (resonator) but bonds a position distant from it, i.e., the wire bonding portion
75
a
. Even where the area of the wire bonding portion
75
a
is reduced for the purpose of the capacity reduction, the wire bonding portion
75
a
needs a quadrangular area whose one side is about 80 &mgr;m in the case of a wire having a diameter of about 25 &mgr;m.
Even in the junction down state in which the semiconductor laser
60
is mounted
Ikemoto Kiyomi
Nakamura Atsushi
Takahashi Shoichi
Hitachi , Ltd.
Miles & Stockbridge P.C.
Nguyen Dung T
Wong Don
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