Method of manufacturing phase-shifted diffraction grating

A photoresist 12 is coated on a substrate 11, and a phase shift medium 13 is formed on the photoresist 12. The phase shift medium 13 is patterned. Thereafter, first and second laser beams 16 and 17 having asymmetric incidence angles with respect to the substrate 11 are radiated on the photoresist 12 to perform an interference exposing operation. The photoresist 12 is developed and post-baked, and the substrate 11 is etched using the photoresist 12 as a mask, thereby forming a diffraction grating having a discontinuous phase portion.

TECHNICAL FIELD 
The present invention relates particularly to a method of manufacturing a 
phase-shifted diffraction grating having a discontinuous phase portion. 
BACKGROUND ART 
A conventional method of manufacturing a diffraction grating using a 
two-beam interference method is mainly employed in a DFB (Distributed 
FeedBack) laser to be described later. 
The DFB laser uses a diffraction grating (periodical projection structure) 
formed in a waveguide as a reflection mechanism for a laser beam. The DFB 
laser oscillates in a single mode at or near the Bragg wavelength defined 
by the period of the diffraction grating. In addition, when the DFB laser 
is to be modulated at a high speed, since it is operated in the single 
mode, it is expected as a light source of a long-distance large-capacity 
optical communication system using optical fibers. 
When a DFB laser having a waveguide on which a uniform diffraction grating 
is formed has both end faces having a small reflectance, since the DFB 
laser easily oscillates at two wavelengths one of which is shorter than 
the Bragg wavelength and the other of which is longer than the Bragg 
wavelength, the DFB laser cannot easily oscillate in a single longitudinal 
mode. Therefore, there is proposed a .lambda./4-shifted DFB laser which 
oscillates at a single mode such that the phase of a diffraction grating 
is shifted by .lambda./4 (a phase amount of .pi./2) in the central portion 
of a laser resonator. In addition, there is proposed a DFB laser which 
oscillates at a wavelength equal to the Bragg wavelength by a two-beam 
interference method using positive and negative photoresists (this DFB 
laser is described in, e.g., Electronics Letters Vol. 20, NO 24, 1984, PP 
1,008-1,010). In specific element performance, it is reported that a high 
production yield can be obtained by using a .lambda./8-shifted diffraction 
grating having a shift amount half that of the .lambda./4-shifted 
diffraction grating (this is described in, e.g., the Institute of 
Electronics and Communication Engineers of Japan, Technical Report 
OQE86-150). 
FIGS. 1A to 1F show a method of manufacturing a diffraction grating using 
positive and negative photoresists. 
As shown in FIG. 1A, a negative photoresist 22 is formed on an InP 
substrate 21. An intermediate layer 23 is formed on the negative 
photoresist 22. A positive photoresist 24 is formed on the intermediate 
layer 23. As shown in FIG. 1B, the positive photoresist 24 is patterned. 
Thereafter, the intermediate layer 23 and the negative photoresist 22 are 
etched using the positive photoresist 24 as a mask. As shown in FIG. 1C, 
the intermediate layer 23 and the positive photoresist 24 are removed. 
Thereafter, a positive photoresist 25 is formed on the entire surface of 
the resultant structure. A two-beam interference exposure operation is 
performed using first and second laser beams 26A and 26B. As shown in FIG. 
1D, the positive photoresist 25 is developed. As shown in FIG. 1E, after 
the InP substrate 21 is etched using a hydrogen bromide-based etchant, the 
positive photoresist 25 is removed. A positive photoresist (protection 
film) 27 is formed on only a portion where the InP substrate 21 is etched. 
Thereafter, the negative photoresist 22 is developed. As shown in FIG. 1F, 
the InP substrate 21 is etched using a hydrogen bromide-based etchant, and 
the negative and positive photoresists 22 and 27 are removed, thereby 
obtaining a phase-shifted diffraction grating. 
In the method using the negative and positive photoresists, the negative 
and positive photoresists are simultaneously used. For this reason, the 
thicknesses of the photoresists and an exposure time of two-beam 
interference cannot easily be adjusted. The shapes of diffraction gratings 
formed in the regions of the negative and positive photoresists may be 
different from each other. In addition, when a shift amount of the phase 
of the diffraction grating is adjusted, the shift amount cannot help being 
set in an amount of .lambda./4 (i.e., a phase amount of .pi./2). 
As a method of obtaining an arbitrary shift amount, there is a 
manufacturing method using a glass mask, e.g., reported in the Technical 
Report OQE85-60, the Institute of Electronics and Communication Engineers 
of Japan. In addition, there is also a manufacturing method using a phase 
shift film (1986, the Japan Society of Applied Physics, lecture No. 
29P-T-8). 
FIGS. 2A to 2D show a manufacturing method using a phase shift film. 
As shown in FIG. 2A, a negative photoresist 22 is formed on an InP 
substrate 21. An intermediate layer 23 is formed on the negative 
photoresist 22. A phase shift negative resist film 28 is formed on the 
intermediate layer 23. Thereafter, the phase shift negative resist film 28 
is patterned. A two-beam interference exposing operation is performed 
using first and second laser beams 26A and 26B. As shown in FIG. 2B, the 
intermediate layer 23 is removed from a portion which is not covered with 
the phase shift negative resist film 28. Thereafter, the negative 
photoresist 22 on this portion is developed. As shown in FIG. 2C, the InP 
substrate 21 is etched using a hydrogen bromide-based etchant. The 
negative photoresist 22 is removed from a portion which is not covered 
with the phase shift negative photoresist 28. In addition, the 
intermediate layer 23 and the phase shift negative resist film 28 are 
removed. Thereafter, a positive photoresist (protection film) 27 is formed 
on only a portion where the InP substrate 21 is etched, and the negative 
photoresist 22 is developed. As shown in FIG. 2D, the InP substrate 21 is 
etched using a hydrogen bromide-based etchant, and the negative and 
positive photoresists 22 and 27 are removed, thereby obtaining a 
phase-shifted diffraction grating. 
In the manufacturing method using the phase shift film, when a photoresist 
used as the phase shift film is used as a phase shift medium, the 
photoresist has poor optical accuracy, since the film thickness of the 
photoresist is decreased during the development. In addition, a 
photoresist used as the phase shift medium naturally absorbs an 
ultraviolet beam. For this reason, optimal exposure times are 
disadvantageously different from each other in the region of the phase 
shift film and other regions. The method using a glass mask has the 
following drawbacks. That is, a process of manufacturing the glass mask is 
complicated, and an undesired multi reflection cannot easily controlled 
such that the glass mask is brought into tight contact with an object to 
be etched during the two-beam interference exposing operation. 
In the manufacturing method using positive and negative photoresists or a 
phase shift film, a phase-shifted diffraction grating is manufactured by 
two etching processes. In the two etching processes, in order to prevent 
loss of a diffraction grating after the first etching operation, the 
diffraction grating is generally protected by coating a photoresist. 
However, a border between a region protected by the photoresist and a 
region etched in the second etching process cannot easily be made clear. 
As described above, according to the conventional manufacturing method, 
when a phase-shifted diffraction grating is to be manufactured, the 
phase-shifted diffraction grating is transferred to a semiconductor 
substrate by two etching processes. For this reason, this manufacturing 
method has a unique drawback, and a phase-shifted portion 
disadvantageously has a step near a border between a region protected by a 
photoresist and a region etched by the second etching process. 
The present invention has been made to solve the above drawbacks, and has 
as its object to provide a method of manufacturing a diffraction grating 
in which a uniform diffraction grating having an arbitrary phase shift 
amount can be formed by one etching process. 
DISCLOSURE OF INVENTION 
In order to achieve the above object, according to the present invention, a 
diffraction grating having a discontinuous phase portion can be obtained 
as follows. A photoresist is coated on a substrate, and a phase shift 
medium is formed on the photoresist. The phase shift medium is patterned. 
Thereafter, first and second laser beams having asymmetric incidence 
angles with respect to the substrate are radiated on the photoresist to 
perform an interference exposure operation. Thereafter, the photoresist is 
developed and post-baked, and the substrate is etched using the 
photoresist as a mask, thereby obtaining a diffraction grating having a 
discontinuous phase portion.

BEST MODE FOR CARRYING OUT THE INVENTION 
An embodiment of the present invention will be described below with 
reference to the accompanying drawings. 
FIGS. 3A to 3F show a method of manufacturing a phase-shifted diffraction 
grating according to the first embodiment of the present invention. 
As shown in FIG. 3A, a first photoresist 12 is coated on an InP substrate 
11. A dielectric film 13 such as a coating oxide film or an SiO.sub.2 film 
is formed on the first photoresist 12 as a phase shift medium. A Cr film 
14 is deposited on the dielectric film 13. A second photoresist 15 is 
coated on the Cr film 14. The Cr film 14 prevents radiation of an 
ultraviolet beam on the first photoresist 12 during exposure of the second 
photoresist 15. Therefore, other metal films each having an effect for 
cutting off an ultraviolet beam may be used in place of the Cr film 14. As 
shown in FIG. 3B, the second photoresist 15 is patterned. Note that end 
portions of the photoresist 15 serve as phase-shifted portions. As shown 
in FIG. 3C, the Cr film 14 is patterned using the second photoresist 15 as 
a mask. After the second photoresist 15 is removed, the dielectric film 13 
serving as a phase shift medium is patterned using the Cr film 14 as a 
mask by, e.g., a hydrogen fluoride-based solution. As shown in FIG. 3D, 
after the Cr film 14 is removed, first and second laser beams 16 and 17 
having asymmetric incidence angles with respect to the InP substrate 11 
are radiated on the first photoresist 12 to perform a two-beam 
interference exposing operation. As shown in FIG. 3E, after the dielectric 
film 13 is removed, the first photoresist 12 is developed and post-baked. 
As shown in FIG. 3F, the InP substrate 11 is etched using the first 
photoresist 12 as a mask by a hydrogen bromide etchant. Thereafter, the 
first photoresist 12 is removed, thereby obtaining a phase-shifted 
diffraction grating. 
FIGS. 4A to 4F show a method of manufacturing a phase-shifted diffraction 
grating according to the second embodiment of the present invention. 
As shown in FIG. 4A, a first photoresist 12 is coated on an InP substrate 
11. A rubber-based resin film 18 is coated on the first photoresist 12 as 
a phase shift medium. A coating oxide film 19 is coated on the 
rubber-based resin film 18. A Cr film 14 and a second photoresist 15 are 
sequentially formed on the coating oxide film 19. As in the first 
embodiment, the Cr film 14 prevent radiation of an ultraviolet beam on the 
first photoresist 12 during exposure of the second photoresist 15. 
Therefore, other metal films each having an effect for cutting off an 
ultraviolet beam may be used in place of the Cr film 14. As shown in FIG. 
4B, the second photoresist 15 is patterned. End portions of the 
photoresist 15 serve as phase-shifted portions. As shown in FIG. 4C, the 
Cr film 14 is patterned using the second photoresist 15 as a mask, and the 
second photoresist 15 is removed. Thereafter, the coating oxide film 19 is 
patterned using the Cr film 14 as a mask by a hydrogen bromide-based 
solution. The rubber-based resin film 18 is patterned using the Cr film 14 
as a mask by a butyl acetate solution. As shown in FIG. 4D, after the Cr 
film 14 and the coating oxide film 19 are removed, a two-beam interference 
exposing operation is performed by first and second laser beams 16 and 17 
having asymmetric incidence angles with respect to the InP substrate 11. 
As shown in FIG. 4E, after the rubber-based resin film 18 is removed, the 
first photoresist 12 is developed and post-baked. As shown in FIG. 4F, the 
InP substrate 11 is etched using a hydrogen bromide-based etchant. 
Thereafter, the first photoresist 12 is removed, thereby obtaining a 
phase-shifted diffraction grating. 
According to the first and second embodiments, a phase-shifted diffraction 
grating is transferred by one etching process. For this reason, when an 
InP substrate is used as a semiconductor substrate, a diffraction grating 
which has a stable shape, a period of 1,900 to 2,600 [.ANG.], without any 
step at a phase-shifted portion can be obtained at a high yield. According 
to the first and second embodiments, when a thickness of a dielectric film 
or a rubber-based resin film is changed, the phase of the diffraction 
grating can be shifted by an arbitrary amount from 0 to .pi.. 
In this case, in the first and second embodiments, when the rotational 
angle .delta. of the InP substrate 11 is set to be 15.times. and the 
period of the diffraction grating is set to be 2,400 [.ANG.], each of the 
thicknesses of the dielectric film and the rubber-based resin film 
required for shifting the phase of the diffraction grating by .pi. is 
about 1 .mu.m (see FIG. 5). 
Upon two-beam interference exposure, when incidence angles .theta. of the 
two beams are fixed, the period and phase shift amount of the diffraction 
grating are defined by the rotational angle .theta. of the InP substrate 
11 and the thickness of a phase shift medium (the dielectric film 13 or 
the rubber-based resin film 18). For example, the rotational angle k of 
the InP substrate 11 is fixed, the period of the diffraction grating is 
constant. In this case, an arbitrary phase shift amount can be obtained by 
changing the thickness of the phase shift medium. In addition, when the 
phase shift medium is removed before the first photoresist 12 is 
developed, a phase-shifted diffraction grating can be transferred to the 
InP substrate by one etching process.