Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Including integrally formed optical element
Reexamination Certificate
1999-09-21
2001-09-18
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Including integrally formed optical element
C438S022000
Reexamination Certificate
active
06291256
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a non-regrowth distributed feedback (DFB) semiconductor laser and a method of manufacturing the same and, more particularly, to a method of fabricating diffraction gratings on both sides of a ridge.
2. Description of the Related Art
DFB semiconductor lasers are known as devices which can be used in the fields of optical communication systems such as optical CATVs, pumping light sources for SHG short-wave lasers for high-density information recording or small solid-state lasers, and optical measurement. Conventional distributed feedback semiconductor lasers are formed using two or more steps of epitaxial growth. In a ridge type DFB semiconductor laser formed using two or more steps of epitaxial growth, a grating (diffraction grating) is provided in a laser waveguide layer and thereafter another layer is formed on the waveguide on an epitaxial growth basis.
Recently, in order to avoid the complicated epitaxial growth in two or more steps, the so-called non-regrowth distributed feedback semiconductor lasers have been developed which are fabricated using one step of epitaxial growth, i.e., which does not involve the second epitaxial growth.
For example, in R. D. Martin et al. “CW Performance of an InGaAs-GaAs-AlGaAs Laterally-Coupled Distributed Feedback (LC-DFB) Ridge Laser Diode” IEEE Photonics Technology Letters, Vol. 7, No. 3, pp 244-246, March 1995, an InGaAs-GaAs-AlGaAs distributed feedback semiconductor laser is disclosed in which an active layer and a cladding layer are formed on a substrate by means of epitaxial growth; a ridge stripe is formed on the cladding layer; and a grating is provided on the top portion of the ridge stripe and on flat portions on both sides thereof. In methods of manufacturing such a non-regrowth distributed feedback semiconductor laser, the grating is formed on the entire region of the substrate including the top portion of the ridge type waveguide by means of direct writing with electron beams (EB).
As shown in
FIG. 1
, however, an unnecessary grating
3
exists in the region of a contact layer on the top portion of a ridge
1
into which a current injected immediately after gratings
2
are formed on flat portions on both sides of the ridge
1
. The grating
3
on the top portion of the ridge may obstruct the step of forming an electrode to make connection to the contact layer region. This can result in poor contact and can adversely affect the characteristics of the device.
SUMMARY OF THE INVENTION
The present invention confronts the above-described problem, and it is an object of the present invention to provide a method of manufacturing non-regrowth DFB semiconductor lasers, especially InP type layers, including the step of fabricating simplified gratings and a non-regrowth DFB semiconductor laser free from poor contact.
The object is achieved by a method of manufacturing a DFB semiconductor laser described below, in which a laser substrate having a cladding layer which is a material for a ridge stripe stacked on an active layer is formed. The cladding layer as a material for a ridge stripe is etched to form a ridge stripe having flat portions on both sides thereof and a flat top portion protruding therefrom. A protective film and a resist layer are formed to cover the flat portions on both sides and the flat upper surface. A latent image of a grating having a periodic structure is formed in the direction in which the ridge stripe extends is formed on the resist layer. The resist layer is developed and the flat portions on both sides are etched to form a grating on both lateral portions and to remove the protective film. An electrode is formed on the flat top portion, the interface between the ridge strip and the electrode being a smooth surface.
In a first aspect of the invention, the method of manufacturing a distributed feedback ridge semiconductor laser having a ridge stripe on an active layer and a periodic structure in a direction in which the ridge stripe extends, which comprises the steps of:
forming a laser substrate comprising a cladding layer made of a material for a ridge stripe and a contact layer stacked in order on an active layer;
forming two lateral flat portions from said cladding layer and said contact layer and forming a ridge stripe protruding from said lateral flat portions and having a flat top portion;
forming a mask such as a protective film and a resist layer in order to cover said lateral flat portions and said flat top portion;
forming a latent image of a grating pattern having a periodic structure in the direction in which the ridge stripe extends on said resist layer and forming a grating pattern by developing said resist layer;
etching said grating pattern to form a grating made of said material for the ridge stripe on or in said cladding layer in regions other than said flat top portion;
removing said protective film to form an insulating layer on a surface of said grating layer and said flat top portion; and
performing etching to expose only said flat top portion on said insulating layer and, thereafter, forming an electrode on said flat top portion.
In a second aspect of the invention of the method of manufacturing a semiconductor laser, wherein said step of patterning includes the step of forming said latent image using an electron beam writing process.
In a third aspect of the invention of the method of manufacturing a semiconductor laser, wherein said active layer is a bulk layer, a single quantum well layer, or a multiple quantum well layer mainly composed of In
1−x
Ga
x
As
1−y
P
y
(x and y represent a decimal fraction); said cladding layer is InP; and said contact layer is InGaAsP or InGaAs.
In a fourth aspect of the invention of the method of manufacturing a semiconductor laser, wherein when wet etching is employed for said step of forming the grating, selective wet etching of InP is performed using a hydrochloric acid type etchant such that the direction of said ridge stripe extends substantially coincides with a [
0
{overscore (
1
)}
1
]-direction of said laser substrate.
In a fifth aspect of the invention of the method of manufacturing a semiconductor laser, wherein when dry etching is employed in said step of forming the grating, in said step of forming the mask, said protective film is left on only said flat top portion of the ridge stripe except the two lateral flat portions, and then said resist layer is entirely formed on said protective film and said two lateral flat portions, and then the dry etching is performed in accordance with the difference in thickness between a total both of the resist layer and the protective film, and the only one single of the resist layer.
In sixth aspect of the invention, the distributed feedback ridge type semiconductor laser having a ridge stripe on an active layer and a periodic structure in a direction in which the ridge stripe extends, which comprises:
a laser substrate including a cladding layer made of a ridge stripe material and a necessary contact layer in this order stacked on an active layer;
two lateral flat portions formed from said cladding layer and said contact layer;
a ridge stripe protruding from said lateral flat positions and having a flat top portion;
a protective film and a resist layer formed in order to cover said lateral flat portions and said flat top portion;
a grating layer formed by developing said resist layer on which a latent image of a grating pattern having a periodic structure in the direction in which the ridge stripe extends has been formed;
a grating layer made of said ridge stripe material formed by etching said grating pattern on or in said cladding layer in regions other than said flat top portion;
an insulating layer formed on a surface of said grating layer and said flat top portion such that said flat top portion is exposed; and
an electrode formed on said flat top portion to produce an interface between said ridge stripe and said electrode, said interface being a smooth surface.
According to the present inven
Chen Nong
Chikuma Kiyofumi
Takei Koyoshi
Watanabe Yoshiaki
Fish & Richardson P.C.
Nelms David
Nhu David
Pioneer Corporation
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