U.S. patent application number 12/881399 was filed with the patent office on 2010-12-30 for process for producing surface emitting laser, process for producing surface emitting laser array, and optical apparatus including surface emitting laser array produced by the process.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to Mitsuhiro Ikuta, Tetsuya Takeuchi, Tatsuro Uchida.
Application Number | 20100329745 12/881399 |
Document ID | / |
Family ID | 41129105 |
Filed Date | 2010-12-30 |
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United States Patent
Application |
20100329745 |
Kind Code |
A1 |
Uchida; Tatsuro ; et
al. |
December 30, 2010 |
PROCESS FOR PRODUCING SURFACE EMITTING LASER, PROCESS FOR PRODUCING
SURFACE EMITTING LASER ARRAY, AND OPTICAL APPARATUS INCLUDING
SURFACE EMITTING LASER ARRAY PRODUCED BY THE PROCESS
Abstract
Provided is a process for producing a surface emitting laser
including a surface relief structure provided on laminated
semiconductor layers, including the steps of transferring, to a
first dielectric film, a first pattern for defining a mesa
structure and a second pattern for defining the surface relief
structure in the same process; and forming a second dielectric film
on the first dielectric film and a surface of the laminated
semiconductor layers to which the first pattern and the second
pattern have been transferred. Accordingly, a center position of
the surface relief structure can be aligned with a center position
of a current confinement structure at high precision.
Inventors: |
Uchida; Tatsuro;
(Machida-shi, JP) ; Ikuta; Mitsuhiro;
(Kawasaki-shi, JP) ; Takeuchi; Tetsuya;
(Yokohama-shi, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
1290 Avenue of the Americas
NEW YORK
NY
10104-3800
US
|
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
41129105 |
Appl. No.: |
12/881399 |
Filed: |
September 14, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12509551 |
Jul 27, 2009 |
7807485 |
|
|
12881399 |
|
|
|
|
Current U.S.
Class: |
399/220 |
Current CPC
Class: |
H01S 5/18311 20130101;
H01S 5/423 20130101; H01S 5/34326 20130101; H01S 5/209 20130101;
H01S 5/18369 20130101; H01S 2301/166 20130101; H01S 5/18377
20130101; H01S 5/18391 20130101 |
Class at
Publication: |
399/220 |
International
Class: |
G03G 15/04 20060101
G03G015/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 31, 2008 |
JP |
2008-198936 |
Claims
1-13. (canceled)
14. An image forming apparatus, comprising: a surface emitting
laser array formed of a plurality of surface emitting lasers; a
photoreceptor for forming an electrostatic latent image with a beam
from the surface emitting laser array; a charging unit; and a
developing unit, wherein each surface emitting laser includes a
surface relief structure provided on a laminated semiconductor
layer, and wherein each surface emitting laser of the plurality of
surface emitting lasers is formed by a process that includes:
forming a first dielectric film on the laminated semiconductor
layer, forming, on the first dielectric film, a first pattern for
defining a mesa structure and a second pattern for defining the
surface relief structure in a same process, forming the first
pattern and the second pattern on a surface of the laminated
semiconductor layer by using the first dielectric film on which the
first pattern and the second pattern have been formed, forming a
second dielectric film on the first dielectric film and the
laminated semiconductor layer on which the first pattern and the
second pattern have been formed, removing at least a portion of the
second dielectric film formed on the laminated semiconductor layer
on which the first pattern has been formed, and forming the mesa
structure at the portion where the second dielectric film has been
removed.
15. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a process for producing a
surface emitting laser, a process for producing a surface emitting
laser array, and an optical apparatus including the surface
emitting laser array produced by the process.
[0003] 2. Description of the Related Art
[0004] A vertical cavity surface emitting laser (hereinafter
referred to as VCSEL) has been known as one of surface emitting
lasers.
[0005] In the surface emitting laser, an active region is
sandwiched on both sides thereof by two reflectors to form a
resonator in a direction perpendicular to a substrate, and light is
emitted in the direction perpendicular to the substrate.
[0006] It is important for the surface emitting laser to control
transverse mode oscillation. When the surface emitting laser is to
be applied to communications, a transverse mode output is required
to be a single-mode output.
[0007] Therefore, according to the surface emitting laser, a
current confinement structure is provided in an inner portion
thereof by selective oxidation to limit a light emitting region of
an active layer, thereby realizing a single transverse mode.
[0008] However, when the single transverse mode is to be realized
by only the current confinement structure, it is necessary to
reduce the confinement diameter. When the confinement diameter
reduces, the light emitting region becomes smaller, and hence it is
difficult to obtain larger laser power.
[0009] Thus, up to now, there have been studied methods of
introducing an intentional loss difference between a fundamental
transverse mode and a high-order transverse mode to enable
single-transverse mode oscillation while maintaining a light
emitting region somewhat wider than in the case where the single
transverse mode is realized by only the current confinement
structure.
[0010] Of the methods, so-called surface relief methods are
disclosed in Japanese Patent Application Laid-Open No. 2001-284722
and H. J. Unold et al., Electronics Letters, Vol. 35, No. 16
(1999).
[0011] The surface relief methods are methods of performing level
difference processing for reflectance control on a surface of a
surface emitting laser device, to make a high-order transverse mode
loss larger than a fundamental transverse mode loss.
[0012] Herein, a level difference structure provided for
reflectance control in a light output region of a light emission
surface of a reflector as described above is hereinafter referred
to as a surface relief structure.
[0013] Next, the surface relief structures in the conventional
examples described above are described with reference to FIGS. 2A
and 2B.
[0014] In FIGS. 2A and 2B, reference numeral 200 denotes
low-refractive index layers; 202, high-refractive index layers;
204, high-reflectance regions; 206, low-reflectance regions; 208,
fundamental transverse mode light distributions; and 210,
high-order transverse mode light distributions.
[0015] A mirror used for the VCSEL is normally a multilayer
reflector in which a low-refractive index layer and a
high-refractive index layer are alternately laminated each in an
optical thickness equal to 1/4 of a laser oscillation wavelength
.lamda. so as to form multiple pairs.
[0016] In general, the multilayer reflector is terminated at the
high-refractive index layer, and hence a high reflectance equal to
or larger than 99% is obtained by the use of reflection on a final
boundary with air (refractive index=1).
[0017] A convex surface relief structure illustrated in FIG. 2A is
described. The convex surface relief structure is disclosed in H.
J. Unold et al., Electronics Letters, Vol. 35, No. 16 (1999).
[0018] As illustrated in FIG. 2A, a part of the high-refractive
index layer 202 which is a final layer in the low-reflectance
region 206 is removed by a thickness equal to .lamda./4, and hence
the multilayer reflector is terminated at the low-refractive index
layer 200. Therefore, the convex surface relief structure is
obtained.
[0019] According to the convex surface relief structure, a phase of
a beam reflected at a boundary between the low-refractive index
layer 200 and air which is bottom in refractive index than the
low-refractive index layer 200 is shifted by `.pi.` from phases of
all reflected beams of the multilayer reflector which are arranged
under the low-refractive index layer 200.
[0020] As a result, the reflectance in the low-reflectance region
206 is reduced to a value equal to or smaller than 99%, and hence
the reflection loss may be made higher than in the high-reflectance
region 204.
[0021] In order to introduce the loss difference between the
fundamental transverse mode and the high-order transverse mode
based on this principle, the low-reflectance region 206 is formed
in the vicinity of the light output region so that the
low-reflectance region 206 largely overlaps with the high-order
transverse mode light distribution 210.
[0022] In contrast, the fundamental transverse mode light
distribution 208 is set so as to largely overlap with the
high-reflectance region 204 in which the high-refractive index
layer 202 is left as the final layer.
[0023] When the surface relief structure is formed as described
above, the reflection loss in the high-order transverse mode may be
increased, and hence the high-order transverse mode oscillation may
be suppressed. As a result, the single-mode oscillation of only the
fundamental transverse mode may be obtained.
[0024] As illustrated in FIG. 2B, when the low-refractive index
layer 200 having a thickness equal to .lamda./4 is additionally
provided on the high-refractive index layer 202 which is the final
layer, the low-reflectance region 206 may be formed to obtain a
concave surface relief structure. The concave surface relief
structure is disclosed in Japanese Patent Application Laid-Open No.
2001-284722.
[0025] As described above, even in the case of the concave surface
relief structure, the reflectance may be reduced based on the same
principle as in the convex surface relief structure, and hence the
single-mode oscillation of only the fundamental transverse mode may
be obtained.
[0026] When the surface relief structure is to be formed, alignment
between the surface relief structure and the current confinement
structure in an in-plane direction is important.
[0027] That is, in order to effectively obtain the single-mode
oscillation of the fundamental transverse mode, it is desirable to
accurately align the surface relief structure with the current
confinement structure which determines a light intensity
distribution.
[0028] For example, when a central axis of the surface relief
structure is shifted from a central axis of the current confinement
structure, an unintended loss is introduced to a desired
oscillation mode (for example, fundamental transverse mode).
[0029] In order to accurately perform the alignment, a method
called a self-alignment process is disclosed in H. J. Unold et al.,
Electronics Letters, Vol. 35, No. 16 (1999).
[0030] This method is used to form the surface relief structure and
a mesa structure with high precision through alignment patterning
using the same mask.
[0031] Hereinafter, the self-alignment process disclosed in H. J.
Unold et al., Electronics Letters, Vol. 35, No. 16 (1999) is
described in detail with reference to FIG. 3.
[0032] As illustrated in FIG. 3, a resist is formed on a
semiconductor layer 304 and patterned using photolithography to
obtain a first resist pattern 300.
[0033] An outer region of the first resist pattern 300 is used as a
mask for forming the mesa structure, and an inner annular pattern
of the first resist pattern 300 is used as a mask for forming the
surface relief structure. The shape of the first resist pattern 300
is defined by photolithography, and hence a surface relief
structure 302 can be formed with high precision by the inner
annular pattern.
[0034] When the mesa structure is to be formed by wet etching using
the outer region of the first resist pattern 300, the mesa
structure with high size precision may be obtained. Specifically,
the surface relief structure 302 is formed and then a second resist
pattern 306 is formed thereon as a protective layer.
[0035] After that, the mesa structure is formed using the outer
region of the first resist pattern 300.
[0036] The formed mesa structure is oxidized from side surfaces
thereof to form the current confinement structure.
[0037] As described above, the surface relief structure and the
mesa structure can be formed with high precision through alignment
patterning using the same mask. As a result, the surface relief
structure and the current confinement structure which is defined by
the shape of the mesa structure can be also formed with high
precision.
[0038] According to the conventional production method disclosed in
H. J. Unold et al., Electronics Letters, Vol. 35, No. 16 (1999),
the central axis of the convex surface relief structure can be
aligned with the central axis of a non-oxidized region of the
current confinement structure, and hence a device capable of
single-transverse mode oscillation can be manufactured.
[0039] In the production method disclosed in H. J. Unold et al.,
Electronics Letters, Vol. 35, No. 16 (1999), the mesa structure
(trench structure) is formed by wet etching.
[0040] However, in the case where dry etching is required to form
the mesa structure, because the resist has a low resistance to dry
etching, a problem occurs in processing precision when the mesa
structure having a certain level of height is to be formed.
[0041] In particular, in the case of a short-wavelength VCSEL (for
example, 680 nm band), the number of pairs in the multilayer
reflector serving as a top reflector is large. Therefore, the
height of the mesa structure to be formed becomes high, and hence
dry etching is used instead of wet etching. Thus, the method
disclosed in H. J. Unold et al., Electronics Letters, Vol. 35, No.
16 (1999) has a problem in terms of processing precision.
SUMMARY OF THE INVENTION
[0042] The present invention has been made in view of the
above-mentioned problem. An object of the present invention is to
provide a surface emitting laser production process capable of
aligning a surface relief structure with a current confinement
structure at high precision. Another object of the present
invention is to provide an optical apparatus including a surface
emitting laser array produced by a surface emitting laser array
production process using the surface emitting laser production
process.
[0043] In an aspect of the present invention, there is provided a
process for producing a surface emitting laser including a surface
relief structure provided on laminated semiconductor layer, the
process comprising the steps of forming a first dielectric film on
the laminated semiconductor layer; transferring, to the first
dielectric film, a first pattern for defining a mesa structure and
a second pattern for defining the surface relief structure in the
same process; transferring the first pattern and the second pattern
to a surface of the laminated semiconductor layers by using the
first dielectric film to which the first pattern and the second
pattern have been transferred; forming a second dielectric film on
the first dielectric film and the semiconductor layers to which the
first pattern and the second pattern have been transferred;
removing the second dielectric film which has been formed on the
semiconductor layers to which the first pattern has been
transferred; and forming the mesa structure at a portion where the
second dielectric film has been removed.
[0044] According to the present invention, a center position of the
surface relief structure may be aligned with a center position of
the current confinement structure at high precision. In addition,
according to the present invention, the optical apparatus including
the surface emitting laser array having the surface emitting laser
produced by the surface emitting laser production process can be
realized.
[0045] As described above, when a mesa structure is to be formed
not by wet etching but by dry etching, the mask for forming the
mesa structure is provided as the resist in H. J. Unold et al.,
Electronics Letters, Vol. 35, No. 16 (1999). Therefore, the etching
resistance is low, and hence the mesa structure cannot be formed
with high shape precision.
[0046] Thus, the inventors of the present invention studied a
structure in which, in order to improve the dry etching resistance,
a first mask for defining the mesa structure and the surface relief
structure is comprised of a dielectric film and a second mask for
protecting the surface relief structure is comprised of a
resist.
[0047] When the dielectric film (made of, for example, silicon
oxide) serving as the first mask and the resist serving as the
second mask are used to form the mesa structure by chlorine-based
dry etching, the inventors of the present invention had the
following problem.
[0048] That is, the resist serving as the second mask is altered by
dry etching using a chlorine-based gas. In order to remove the
altered resist, oxygen plasma ashing is necessary. However, the
surface relief structure formed using the first mask is also etched
by the oxygen plasma ashing.
[0049] Therefore, a reduction in film thickness from a design value
or roughness of the surface structure occurs, and hence a
sufficient loss difference cannot be provided, thereby affecting a
single mode oscillation characteristic.
[0050] Thus, the inventors of the present invention found that,
when the surface relief structure is protected by a second
dielectric film 426 (see FIG. 5A), the damage to the surface of the
surface relief structure is reduced even in a case where the oxygen
plasma asking for removing the resist is performed.
[0051] Further features of the present invention will become
apparent from the following description of exemplary embodiments
with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] FIGS. 1A and 1B are schematic explanatory views illustrating
a structure of a vertical cavity surface emitting laser according
to Embodiment 1 of the present invention.
[0053] FIGS. 2A and 2B are schematic explanatory views illustrating
surface relief structures in conventional examples disclosed in H.
J. Unold et al., Electronics Letters, Vol. 35, No. 16 (1999) and
Japanese Patent Application Laid-Open No. 2001-284722,
respectively.
[0054] FIG. 3 is an explanatory view illustrating a self-alignment
process in the conventional example disclosed in H. J. Unold et
al., Electronics Letters, Vol. 35, No. 16 (1999).
[0055] FIGS. 4A, 4B, 4C, 4D, 4E and 4F are explanatory views
illustrating a process for producing the vertical cavity surface
emitting laser according to Embodiment 1 of the present
invention.
[0056] FIGS. 5A, 5B, 5C, 5D, 5E and 5F are explanatory views
illustrating the process for producing the vertical cavity surface
emitting laser according to Embodiment 1 of the present
invention.
[0057] FIGS. 6A, 6B, 6C and 6D are explanatory views illustrating
the process for producing the vertical cavity surface emitting
laser according to Embodiment 1 of the present invention.
[0058] FIGS. 7A, 7B and 7C are schematic views illustrating a first
resist pattern formed during the process for producing the vertical
cavity surface emitting laser according to Embodiment 1 of the
present invention.
[0059] FIGS. 8A and 8B are schematic explanatory views illustrating
a structure of a vertical cavity surface emitting laser according
to Embodiment 2 of the present invention.
[0060] FIGS. 9A, 9B and 9C are schematic views illustrating a first
resist pattern formed during a process for producing the vertical
cavity surface emitting laser according to Embodiment 2 of the
present invention.
[0061] FIGS. 10A and 10B are schematic explanatory views
illustrating an electrophotographic recording type image forming
apparatus including a laser array using the vertical cavity surface
emitting laser, according to Embodiment 3 of the present
invention.
DESCRIPTION OF THE EMBODIMENTS
[0062] Preferred embodiments of the present invention will now be
described in detail in accordance with the accompanying
drawings.
EMBODIMENTS
Embodiment 1
[0063] In Embodiment 1, a process for producing a vertical cavity
surface emitting laser having a convex surface relief structure is
described.
[0064] FIGS. 1A and 1B are schematic explanatory views illustrating
a structure of the vertical cavity surface emitting laser in this
embodiment.
[0065] FIG. 1A is a schematic cross sectional view illustrating the
surface emitting laser in this embodiment. The surface emitting
laser includes an n-side electrode 100, a substrate 102, a bottom
distributed Bragg reflector (DBR) 104, an active layer 106, a
current confinement portion (oxidized region) 108, and a
non-oxidized region 110.
[0066] The surface emitting laser further includes atop DBR 112, an
insulating film 114, a p-side electrode (pad electrode) 116, a
surface relief structure 118, and a light output region 120.
[0067] FIG. 1B is an enlarged view illustrating the light output
region 120 and the surroundings thereof. The light output region
120 includes a high-reflectance region 122 and a low-reflectance
region 124.
[0068] According to the vertical cavity surface emitting laser in
this embodiment, the surface relief structure and the non-oxidized
region (light emitting region) of the current confinement structure
(current confinement portion) are provided such that the central
axes thereof coincide with a design axis. For example, the central
axes thereof are aligned with each other.
[0069] For example, when the diameter of the non-oxidized region
110 is equal to or smaller than 7 .mu.m, the high-reflectance
region 122 in the surface relief structure 118 is formed such that
(diameter of the high-reflectance region 122)=(half of diameter of
non-oxidized region).+-.1 [.mu.m], desirably (half of diameter of
non-oxidized region).+-.0.5 [.mu.m]. The low-reflectance region in
the surface relief structure is formed such that the width of the
low-reflectance region is 3.5 .mu.m. Therefore, high-power
single-transverse mode oscillation can be realized.
[0070] That is, when the central axis of the surface relief
structure is aligned with the central axis of the non-oxidized
region of the current confinement structure and the relationship
between the diameter of the non-oxidized region and the diameter of
the high-reflectance region 122 in the surface relief structure is
suitably set, high-power single-transverse mode oscillation of the
fundamental mode can be realized.
[0071] Next, the process for producing the surface emitting laser
according to this embodiment is described.
[0072] FIGS. 4A to 4F, 5A to 5F, and 6A to 6D are explanatory views
illustrating the process for producing the vertical cavity surface
emitting laser according to this embodiment.
[0073] As illustrated in FIG. 4A, multiple semiconductor layers
including a bottom reflector, an active layer, a selective
oxidation layer (current confinement layer), and a top reflector
are sequentially laminated on the substrate.
[0074] Specifically, a metal organic chemical vapor deposition
(MOCVD) crystal growth technique is used. A bottom n-type DBR layer
402 of Al.sub.0.9Ga.sub.0.1As/Al.sub.0.5Ga.sub.0.5As is grown on an
n-type GaAs substrate 400 through a buffer layer (not shown).
[0075] An n-type spacer layer 404 of AlGaInP and a multi-quantum
well (MQW) active layer 406 of GaInP/AlGaInP are sequentially grown
on the bottom n-type DBR layer 402.
[0076] Then, a p-type spacer layer 408 of AlGaInP and a p-type
selective oxidation layer 410 of Al.sub.0.98Ga.sub.0.02As are grown
on the active layer 406.
[0077] A top p-type DBR layer 412 of
Al.sub.0.9Ga.sub.0.1As/Al.sub.0.5Ga.sub.0.5As, a p-type etching
stop layer 414 of AlInGaP, and a p-type GaAs contact layer/p-type
AlGaAs layer 416 are sequentially grown on the selective oxidation
layer 410.
[0078] A first dielectric film 418 is formed above the laminated
semiconductor layers. The first dielectric film 418 serves as a
mask for forming a mesa structure through etching, as described
below. Thus, when designing the top reflector so as to have a large
thickness, it is necessary to form the first dielectric film 418 so
as to have a large thickness as well. For example, the first
dielectric film 418 is formed to have a thickness of 1 .mu.m.
[0079] Silicon oxide, silicon nitride, silicon oxynitride or the
like may be used as a material of the first dielectric film
418.
[0080] Subsequently, a first resist pattern 420 is formed on the
first dielectric film 418 by a lithography technique.
[0081] FIGS. 7A to 7C are schematic views illustrating the first
resist pattern 420 formed as described above. FIG. 7A is a
perspective view, FIG. 7B is a plan view, and FIG. 7C is a cross
sectional view along the line 7C-7C of FIG. 7A. As illustrated in
FIGS. 7A to 7C, the first resist pattern 420 is formed on the first
dielectric film 418 to provide annular opening patterns (first
pattern 422 and second pattern 424) which have substantially the
same central axis and are different in size from each other.
[0082] The small circular annular opening pattern (second pattern
424) is a pattern for defining the surface relief structure which
is a level difference structure. The large circular annular opening
pattern (first pattern 422) is a pattern for defining a diameter of
the mesa structure.
[0083] As described above, the first and second patterns are formed
on the first dielectric film 418 at the same time by the
lithography technique, and hence the surface relief structure and
the mesa structure can be manufactured with high position
precision. As a result, a positional relationship between the
oxidized confinement structure and the surface relief structure can
be set with high precision.
[0084] As illustrated in FIGS. 7A to 7C, the first pattern 422 and
the second pattern 424 are provided as the two large and small
concentric circular annular patterns having different diameters.
However, the present invention is not limited to such patterns.
[0085] For example, two large and small concentric square annular
patterns having different side lengths may be provided instead of
the two large and small concentric circular annular patterns.
[0086] Next, as illustrated in FIG. 4B, the first resist pattern
420 is transferred to the first dielectric film 418 by wet etching
using buffered hydrogen fluoride (BHF).
[0087] This transfer may be performed by dry etching instead of wet
etching.
[0088] In this case, the transfer may be performed such that the
first pattern and the second pattern which are the two large and
small circular annular opening patterns having the same central
axis are formed on the first dielectric film 418.
[0089] After the wet etching using buffered hydrogen fluoride, the
first resist pattern 420 is removed.
[0090] After the removal of the first resist pattern 420, the first
dielectric film 418 including the first pattern 422 and the second
pattern 424 is used as a mask to transfer a pattern to the p-type
GaAs contact layer/p-type AlGaAs layer 416 by wet etching.
[0091] Specifically, the etching stop layer 414 (for example,
AlGaInP layer) having a thickness of 10 nm is introduced in advance
at a distance of .lamda./4n from an uppermost surface.
[0092] The p-type GaAs contact layer/p-type AlGaAs layer 416 having
a thickness of .lamda./4n is selectively removed from the uppermost
surface by wet etching using a phosphoric acid based etchant. In
this case, the etching depth is set to, for example, .lamda./4n.
Note that .lamda. denotes an oscillation wavelength and n denotes a
refractive index of a semiconductor layer to be etched.
[0093] Therefore, the surface relief structure is formed.
[0094] The etching stop layer 414 can selectively stop etching, and
hence the etching depth can be accurately controlled. The etching
depth may be adjusted by time control without using the etching
stop layer.
[0095] Wet etching is desirable in view of the influence of damage
on the surface. However, dry etching may be performed.
[0096] Next, as illustrated in FIG. 4C, a second dielectric film
426 is formed by a CVD film deposition technique on the laminated
semiconductor layers including the first dielectric film 418 in
which the first pattern 422 and the second pattern 424 are
provided. The second dielectric film 426 is a layer for protecting
the surface relief structure and is formed, for example, to 230 nm.
Examples of the second dielectric film 426 include a silicon oxide
film, a silicon nitride film, and a silicon oxynitride film.
[0097] As described above, in this embodiment, the surface relief
structure is protected by the second dielectric film 426.
Therefore, even when oxygen plasma asking for removing a resist is
performed in a subsequent step (FIG. 5A), the damage to the surface
of the surface relief structure may be prevented. In this step, the
second dielectric film 426 is formed so as to cover the first
pattern 422 provided in the first dielectric film 418, and hence
the pattern shape determined by photolithography is changed.
[0098] However, the second dielectric film 426 is formed using, for
example, a plasma CVD method, and hence the second dielectric film
426 is deposited in a uniform thickness on side walls of the first
pattern 422 provided in the first dielectric film 418.
[0099] Therefore, the width of the first pattern 422 for defining
the mesa structure narrows. However, the second dielectric film 426
having the uniform thickness is isotropically formed. Thus, it is
unlikely to cause trouble by dry etching during the formation of
the mesa structure in a later step (FIG. 4F).
[0100] Next, as illustrated in FIG. 4D, a second resist pattern 428
is formed by a lithography technique.
[0101] In this case, the second resist pattern 428 is formed so as
to cover the second pattern 424 provided with the second dielectric
film 426.
[0102] Next, as illustrated in FIG. 4E, the second resist pattern
428 is used as a mask to remove a portion of the second dielectric
film 426 by wet etching using buffered hydrogen fluoride. The
second dielectric film 426 is a layer for protecting the surface
relief structure and is desirably so formed as to have a thickness
that is smaller than that of the first dielectric film 418. If it
is formed to have a large thickness, a long etching time is
required in removing part of the second dielectric film 426,
thereby increasing the amount of side etching for the first
dielectric film 418. As a result, the initial design values may not
be achieved.
[0103] Next, as illustrated in FIG. 4F, a trench 430 is formed by
dry etching to expose the bottom n-type DBR layer 402, thereby
manufacturing the post of the mesa structure. The second resist
pattern 428 formed on the second pattern 424 that defines the
surface relief structure is desirably so formed as to remain after
the dry etching. This is because if the resist does not remain, the
second dielectric film 426 is removed during the dry etching, which
may damage the previously formed surface relief structure.
[0104] In FIG. 4F, dry etching is performed until the bottom n-type
DBR layer 402 is exposed. However, it is sufficient that etching is
performed until the selective oxidation layer 410 for forming the
current confinement structure is exposed. Therefore, dry etching is
not necessarily performed until the bottom n-type DBR layer 402 is
exposed.
[0105] Next, as illustrated in FIG. 5A, the second resist pattern
428 is removed by an oxygen plasma asking technique.
[0106] In this case, the surface relief structure is protected by
the second dielectric film 426, and hence the damage to the surface
of the surface relief structure can be prevented.
[0107] Next, as illustrated in FIG. 5B, the p-type selective
oxidation layer 410 of Al.sub.0.98Ga.sub.0.02As is selectively
oxidized, for example, in a water-vapor atmosphere at a substrate
temperature of 450.degree. C., to form the current confinement
structure (oxidized region 432 and non-oxidized region 434).
[0108] In this case, the surfaces of the laminated semiconductor
layers other than the trench 430 are covered with one of the first
dielectric film 418 and the second dielectric film 426, and hence
the surfaces of the laminated semiconductor layers can be protected
against oxidization.
[0109] Therefore, electrodes may be formed with an excellent
contact resistance state in a subsequent step (FIG. 6C).
[0110] Next, as illustrated in FIG. 5C, the second dielectric film
426 and the first dielectric film 418 are removed by wet etching
using buffered hydrogen fluoride.
[0111] In this embodiment, the second dielectric film 426 and the
first dielectric film 418 are completely removed. However, the
second dielectric film 426 and the first dielectric film 418 are
not necessarily removed and may be both left.
[0112] Alternatively, the second dielectric film 426 may be left
only on the surface relief structure.
[0113] For example, when the following relationship is satisfied
(where film thickness of the second dielectric film 426 is
expressed by d),
d=(N.lamda.)/2n.sub.d
reflectance is not reduced even in the case where the second
dielectric film 426 is left on the surface relief structure. Note
that .lamda. denotes oscillation wavelength, n.sub.d denotes
refractive index of the second dielectric film, and N denotes a
natural number equal to or larger than 1.
[0114] Next, as illustrated in FIG. 5D, an insulating film 436 made
of silicon oxide is formed by a CVD film formation technique so as
to cover the entire surface of the device.
[0115] Next, as illustrated in FIG. 5E, a third resist pattern 438
is formed by a lithography technique.
[0116] Next, as illustrated in FIG. 5F, the insulating film 436 is
removed by wet etching using buffered hydrogen fluoride to expose
the p-type GaAs contact layer/p-type AlGaAs layer 416 to which the
first pattern is transferred.
[0117] In this embodiment, the insulating film 436 is completely
removed. However, it is not always necessary to remove the
insulating film 436. The insulating film 436 may be left in a light
output region (including the surface relief structure) for surface
protection. For example, when the following relationship is
satisfied (where film thickness of the insulating film 436 is
expressed by a),
a=(N.lamda.)/2n.sub.a
reflectance is not reduced even in the case where the insulating
film 436 is left on the surface relief structure. Note that .lamda.
denotes oscillation wavelength, n.sub.a denotes refractive index of
the insulating film, and N denotes a natural number equal to or
larger than 1.
[0118] After that, the third resist pattern 438 is removed.
[0119] Next, as illustrated in FIG. 6A, a fourth resist pattern 440
is formed by a lithography technique to cover the light output
region.
[0120] Next, as illustrated in FIG. 6B, a metal film 442 made of
Ti/Au is deposited on a surface including the fourth resist pattern
440 by a metal deposition technique.
[0121] Next, as illustrated in FIG. 6C, pad electrodes 444 are
formed by a liftoff technique to expose the light output
region.
[0122] Next, as illustrated in FIG. 6D, an n-side electrode
(AuGe/Ni/Au) 446 is formed on a back surface of the n-type GaAs
substrate 400 by a metal deposition technique.
[0123] According to the process described above in this embodiment,
the photolithography technique with high position precision is used
to form the two large and small circular annular opening patterns
whose central axes are aligned with each other.
[0124] The surface relief structure having a controlled reflectance
is defined by the small circular annular opening pattern. The
diameter of the mesa structure is defined by the large circular
annular opening pattern to define the non-oxidized region of the
current confinement structure.
[0125] Therefore, the central axis of the surface relief structure
and the central axis of the non-oxidized region of the current
confinement structure can be controlled with high position
precision.
[0126] When the large circular annular opening pattern is used to
form the mesa structure by dry etching, the surface relief
structure provided in the small circular annular opening pattern is
protected by the dielectric film and the resist, and hence the
surface relief structure is not exposed to the outside.
[0127] Even in the case of the removal of the resist after the dry
etching, the surface relief structure is protected by the
dielectric film, and hence the surface relief structure is not
exposed.
[0128] In this embodiment, the 680 nm band surface emitting laser
is described. However, the present invention is not limited to this
and may be applied to, for example, an 850 nm band (GaAs/AlGaAs
active layer system) surface emitting laser.
[0129] Processes (apparatuses) used for growth, lithography,
etching, asking, and vapor deposition in this embodiment are not
limited to the described processes (apparatuses). When the same
effects are obtained, any process (apparatus) may be employed.
[0130] In this embodiment, the process for producing the surface
emitting laser of a single device is described. When multiple
surface emitting lasers, each of which is the surface emitting
laser of a single device, are arranged in array, the producing
process described above may be applied.
Embodiment 2
[0131] In Embodiment 2, a process for producing a vertical cavity
surface emitting laser having a concave surface relief structure is
described.
[0132] FIGS. 8A and 8B are schematic explanatory views illustrating
a structure of the vertical cavity surface emitting laser in this
embodiment.
[0133] FIG. 8A is a schematic explanatory cross sectional view
illustrating a light output region 920. FIG. 8B is an enlarged view
illustrating the light output region 920.
[0134] In FIGS. 8A and 8B, the same constituent elements as in
Embodiment 1 illustrated in FIGS. 1A and 1B are expressed by the
same reference numerals, and hence the duplicated descriptions are
omitted.
[0135] In Embodiment 1, a high-reflectance region 922 is convex. In
contrast, a low-reflectance region 924 is convex in this
embodiment, and hence a concave surface relief structure is
formed.
[0136] Next, the process for producing the surface emitting laser
according to this embodiment is described.
[0137] A difference from the producing process according to
Embodiment 1 is the second pattern of the first resist pattern.
[0138] The other steps are the same as in Embodiment 1.
[0139] FIGS. 9A to 9C are schematic explanatory views illustrating
a first resist pattern formed during the process for producing the
surface emitting laser according to Embodiment 2 of the present
invention, which is different from the first resist pattern (FIGS.
7A to 7C) in Embodiment 1.
[0140] FIG. 9A is a perspective view, FIG. 9B is a plan view, and
FIG. 9C is a cross sectional view along the line 9C-9C of FIG. 9A.
As illustrated in FIGS. 9A to 9C, a first resist pattern 1020 is
formed on a first dielectric film 1018 and has a first pattern 1022
and a second pattern 1024.
[0141] As illustrated in FIGS. 9A to 9C, the first resist pattern
1020 is formed on the first dielectric film 1018 to provide large
and small opening patterns (first pattern 1022 and second pattern
1024) which have the same central axis and are different in shape
from each other.
[0142] The first pattern 1022 is a pattern for forming the mesa
structure. The second pattern 1024 is a pattern for forming the
surface relief structure.
[0143] As illustrated in FIGS. 9A to 9C, the patterns which have
the same central axis and are different in shape from each other
are provided as a circular annular opening pattern having a large
diameter and a circular opening pattern having a small diameter.
However, the present invention is not limited to such patterns.
[0144] For example, the circular annular opening pattern having the
large diameter may be a square annular opening pattern.
Embodiment 3
[0145] In Embodiment 3, a structural example of an optical
apparatus using the vertical cavity surface emitting laser produced
by the producing process according to any one of the embodiments
described above is described.
[0146] A structural example of an image forming apparatus including
a laser array using the surface emitting lasers is described as the
optical apparatus.
[0147] FIGS. 10A and 10B are schematic explanatory views
illustrating an electrophotographic recording type image forming
apparatus in which the laser array using the vertical cavity
surface emitting lasers is mounted, according to this
embodiment.
[0148] FIG. 10A is a plan view illustrating the image forming
apparatus and FIG. 10B is a side view illustrating the image
forming apparatus. In FIGS. 10A and 10B, the image forming
apparatus includes a photosensitive drum (photoreceptor) 1100, a
charging unit 1102, a developing unit 1104, a transfer charging
unit 1106, a fixing unit 1108, a rotatable polygon mirror 1110, and
a motor 1112. The image forming apparatus further includes a
surface emitting laser array 1114, a reflector 1116, a collimator
lens 1118, and an f-.theta. lens 1120.
[0149] In this embodiment, the rotatable polygon mirror 1110 is
driven to rotate by the motor 1112 illustrated in FIG. 10B.
[0150] The surface emitting laser array 1114 serves as a light
source for recording and is turned on or off by a laser driver (not
shown) based on an image signal.
[0151] An optically modulated laser beam is emitted from the
surface emitting laser array 1114 to the rotatable polygon mirror
1110 through the collimator lens 1118.
[0152] The rotatable polygon mirror 1110 is rotating in a direction
indicated by the arrow. The laser beam output from the surface
emitting laser array 1114 is reflected on a reflecting surface of
the rotatable polygon mirror 1110 as a deflection beam whose
emission angle is continuously changed according to the rotation of
the rotatable polygon mirror 1110.
[0153] The reflected laser beam is subjected to distortion
correction or the like by the f-.theta. lens 1120. Then, the
photosensitive drum 1100 is irradiated with the laser beam through
the reflector 1116 and scanned therewith in the main scanning
direction. In this case, a multiple-line image corresponding to the
surface emitting laser array 1114 is formed in the main scanning
direction of the photosensitive drum 1100 by a laser beam reflected
on a surface of the rotatable polygon mirror 1110.
[0154] In this embodiment, the 4.times.8 surface emitting laser
array 1114 is used, and hence a 32-line image is formed.
[0155] The photosensitive drum 1100 is charged in advance by the
charging unit 1102 and continuously exposed by the scanning of the
laser beam to form an electrostatic latent image.
[0156] The photosensitive drum 1100 is rotating in a direction
indicated by the arrow. The formed electrostatic latent image is
developed by the developing unit 1104. A visible image obtained by
development is transferred to a transfer paper by the transfer
charging unit 1106.
[0157] The transfer paper to which the visible image is transferred
is transported to the fixing unit 1108 and fixed thereby, and then
delivered to the outside of the apparatus.
[0158] The structural example of the image forming apparatus is
described as the optical apparatus. However, the present invention
is not limited to the structural example.
[0159] For example, an optical apparatus such as a projection
display may be provided, in which a light source including the
vertical cavity surface emitting laser according to the present
invention is used and a beam from the light source is made incident
on an image display member to display an image.
[0160] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all such modifications and
equivalent structures and functions.
[0161] This application claims the benefit of Japanese Patent
Application No. 2008-198936, filed Jul. 31, 2008, which is hereby
incorporated by reference herein in its entirety.
* * * * *