Method for manufacturing semiconductor laser device

Details Method for manufacturing semiconductor laser device Method for manufacturing semiconductor laser device

2002-07-25

2003-11-04

Wille, Douglas A. (Department: 2823)

Semiconductor device manufacturing: process

Making device or circuit emissive of nonelectrical signal

Including integrally formed optical element

C372S045013

Reexamination Certificate

active

06642075

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor laser device, and more specifically, to a method for manufacturing a distributed feedback semiconductor laser device.
2. Description of the Related Art
In a related-art distributed feedback semiconductor laser (hereinafter referred to as “DFB laser”) comprising a optical-wave guide ridge having a diffraction grating formed therein, when the diffraction grating is formed as a part of the steps in the method for manufacturing a semiconductor laser, etching of a diffraction grating layer having the same width as the ridge width is performed using an SiO
2
film mask pattern having an opening of a diffraction grating shape wider than the ridge width, and a resist pattern formed on the SiO
2
film mask pattern having an opening with the same width as the ridge width extending in the direction of the optical wave guide.
A related-art method for manufacturing a diffraction grating will be described.
FIGS. 33
,
35
, and
39
are plans of a semiconductor laser showing a step in a related-art method for manufacturing a semiconductor laser, for example, disclosed in Japanese Patent Application No. 2000-352450;
FIG. 34
is a sectional view of the semiconductor laser along line
34
—
34
in
FIG. 33
;
FIG. 36
is a sectional view of the semiconductor laser along line
36
—
36
in
FIG. 35
;
FIG. 37
is a sectional view of the semiconductor laser along line
37
—
37
in
FIG. 35
;
FIG. 38
is a sectional view of the semiconductor laser along line
38
—
38
in
FIG. 35
; and
FIG. 40
is a sectional view of the semiconductor laser along line
40
—
40
in FIG.
39
.
First, on an n-type InP substrate (hereinafter “n-type” and “p-type” are described as “n-” and “p-”, respectively), an n-InP clad layer, an n-AlInAs clad layer, an n-AlGaInAs light-confinement layer, an AlGaInAs quantum-well layer, a p-AlGaInAs light-confinement layer, a p-AlInAs clad layer, a p-InP layer, a p-InGaAsP layer, and a p-InP layer are laminated and grown. Then, an SiO
2
insulating film is formed, and a resist film is grown thereon.
Next, EB exposure is performed at a pitch p
10
of about 2000 Å and a width of 10 &mgr;m (½ the pitch p
10
, 1000 Å for the exposed portion) and development is performed to form a resist pattern, the SiO
2
insulating film is etched using the resist pattern to form an SiO
2
insulating film pattern, and the resist pattern is removed. The result of this process is shown in
FIGS. 33 and 34
.
In
FIGS. 33 and 34
, the reference numeral
122
denotes an SiO
2
insulating film pattern, and
122
a
denotes an SiO
2
insulating film opening. The length a
10
of the SiO
2
insulating film opening
122
a
corresponds to the exposure width of EB, 10 &mgr;m, and the width w
10
of the SiO
2
insulating film opening
122
a
corresponds to the ½ the EB exposure pitch p
10
, 1000 Å.
In
FIG. 34
, the reference numeral
101
denotes an n-InP substrate,
102
denotes an n-InP clad layer,
103
denotes an n-AlInAs clad layer,
104
denotes an n-AlGaInAs light-confinement layer,
105
denotes an AlGaInAs quantum-well layer,
106
denotes a p-AlGaInAs light-confinement layer,
107
denotes a p-AlInAs clad layer,
108
denotes a p-InP layer,
110
denotes a p-InGaAsP layer, and
121
denotes a p-InP layer.
Next, referring to
FIGS. 35
,
36
, and
37
, a resist is applied onto the SiO
2
insulating film pattern
122
, and a resist pattern
124
having a resist pattern opening
124
a
is formed using photoengraving along the row of the SiO
2
insulating film openings
122
a
. The result of this step is shown in
FIGS. 35
,
36
,
37
, and
38
.
In
FIG. 35
, the width w
20
of a resist pattern opening
124
a
is 1.8 &mgr;m, which is the same as the width of the optical-wave guide ridge formed later.
FIG. 37
shows a cross section of the portion where the p-InP layer
121
is covered with the SiO
2
insulating film pattern
122
; and
FIG. 38
shows a cross section of the portion where an area of the p-InP layer
121
is not covered with the SiO
2
insulating film pattern
122
, and exposed on the surface at the width w
20
of the resist pattern opening
124
a.
Next, areas of the p-InP layer
121
and the p-InGaAsP layer
110
not covered by the SiO
2
insulating film pattern
122
and the resist pattern
124
, that is the area exposed on the surface in the sectional view of
FIG. 38
are etched by dry etching using the SiO
2
insulating film pattern
122
and the resist pattern
124
as a mask, and using methane gas and hydrogen plasma as etching media, to expose the p-InP layer
108
. Then, the SiO
2
insulating film pattern
122
and the resist pattern
124
are removed. The result is shown in
FIGS. 39 and 40
.
Thereafter, a p-InP layer is grown and filled to form a diffraction grating layer composed of a p-InGaAs/p-InP layer.
In the related-art method for manufacturing a diffraction grating, when dry etching is performed using an SiO
2
insulating film pattern
122
and a resist pattern
124
as a mask and using methane gas and hydrogen plasma as etching media, methane gas and hydrogen plasma, which are etching media, may react with the resist, which is an organic substance, to change the concentration of methane gas and hydrogen plasma, which determine the etching rate; and in some cases, the depth of etching in the direction along the width w
20
of the resist pattern opening
124
a
may lack uniformity.
FIG. 41
is a schematic diagram showing the distribution of depth in the direction along the width w
10
of the SiO
2
insulating film openings
122
a
of a related-art diffraction grating; and
FIG. 42
is a schematic diagram showing the distribution of depth in the direction along the width w
20
of the resist pattern opening
124
a.
As seen from
FIG. 42
, the depth of the grooves closer to the resist becomes shallow due to the lowered etching rate. Also, since the reaction of the resist with methane and hydrogen plasma changes depending on the surface conditions of the resist, dependence of the etching rate on the surface conditions of the resist may occur, and the etching rate may differ between lots. For this reason, there was difficulty in forming a diffraction grating having an even thickness in the width direction of the ridge wave guide, and fluctuation in the laser characteristics of semiconductor lasers, resulting in lowering of the yield of semiconductor lasers.
The known techniques include Japanese Patent Application Laid-Open No. Hei.6-291408 (1994), which discloses the use of a resist, oxide film, or nitride film as a material of a pattern forming film for forming diffraction gratings.
Japanese Patent Application Laid-Open No. Sho.62-165392 (1987) discloses a method for separately etching regions with inverted periodicity, when a &lgr;/4 shifted diffraction grating is formed, using a patterning layer of an SiO
2
oxide film and a patterning layer of an aluminum film.
Furthermore, Japanese Patent Application Laid-Open No. Sho.62-139503 (1987) discloses a method for forming a diffraction grating in a specific region by forming the mask pattern of a first photoresist having a window corresponding to the specific region laminated with the mask pattern of a second photoresist that does not react with the first photoresist, and by using these two types of mask patterns as masks.
SUMMARY OF THE INVENTION
The present invention has been made to overcome the above-described drawbacks and disadvantages of the related art. It is an object of the present invention to provide a method for manufacturing a semiconductor laser that can easily manufacture a semiconductor laser having diffraction gratings of an even thickness, and having uniform laser characteristics.
According to one aspect of the invention, there is provided a method for manufacturing a semiconductor laser device comprising: a first step of sequentially laminating on a semiconductor substrate of a first conductivity type, a first clad layer of a first conductivity type, a

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