Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Including integrally formed optical element
Reexamination Certificate
1999-05-21
2001-05-08
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Including integrally formed optical element
Reexamination Certificate
active
06228671
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing a diffraction grating substrate for a semiconductor laser partially having a diffraction grating, by electron beam exposure or the like, as well as to a process for producing a semiconductor laser partially having a diffraction grating, using the above diffraction grating substrate.
2. Description of the Prior Art
In production of a semiconductor laser having a diffraction grating, it is one of the most important factors in ensuring the satisfactory performance and yield of the semiconductor laser to precisely control the diffraction grating shape of the diffraction grating substrate used in the semiconductor laser. In order to attain the precise control, it is conducted to form, on a semiconductor substrate by electron beam exposure (hereinafter referred to as EB exposure), a fine resist pattern of diffraction grating having precisely controlled aperture intervals. And then the semiconductor substrate is etched to transfer the resist pattern onto the substrate. In this case, formation of a diffraction grating pattern on the whole surface of a wafer by EB exposure takes an enormous exposure time; therefore, it is ordinarily conducted to form a diffraction grating pattern on part of a wafer and only the pattern region is subjected to EB exposure to form an aperture pattern.
FIG. 13
is a schematic drawing of a diffraction grating substrate in which a diffraction grating has been formed by a conventional process. This structure is formed as follows. A resist
305
is coated on a semiconductor substrate
302
; at part of the coated resist, a resist pattern
301
for formation of diffraction grating is formed; then, wet etching is conducted to form a diffraction grating. As shown in
FIG. 13
, when the resist pattern is a repetition of an aperture of given area, the etching rate become larger in the vicinity of the end
303
of resist pattern formation region owing to a pattern effect; as a result, overetched hollowing
304
of diffraction grating takes place in the vicinity of said end, making it impossible to form a diffraction grating of uniform shape.
In a semiconductor laser partially having such a diffraction grating of nonuniform shape, there occur, at the end of diffraction grating formation region, light reflection and scattering caused by the overetched hollowing of diffraction grating, and which brings on laser beam waveguide loss and nonuniform field distribution.
SUMMARY OF THE INVENTION
The present invention aims at providing a process for producing a semiconductor substrate having a diffraction grating of uniform shape by suppressing the overetched hollowing of diffraction grating, and a process for producing a semiconductor laser partially having a diffraction grating by using the above semiconductor substrate.
The present invention relates to a process for producing a diffraction grating substrate for a semiconductor laser partially having a diffraction grating, which process comprises:
a resist patterning step of forming a resist pattern for formation of diffraction grating on a semiconductor substrate so that the aperture area of diffraction grating increases gradually toward the end of diffraction grating formation region, and
an etching step of conducting etching using said resist pattern to form a diffraction grating at the surface of the semiconductor substrate.
The present invention further relates to a process for producing a semiconductor laser partially having a diffraction grating, which comprises a step of forming, on the diffraction grating substrate produced by the above process, a guide layer, an active layer and a clad layer in this order.
The present inventor made a study on the relation between resist pattern and overetched hollowing of diffraction grating and, as a result, found out that particularly in wet etching, an etching solution remains easily at the end of diffraction grating formation region and the overetched hollowing of diffraction grating takes place at said end. Specifically explaining, the consumption rate of etching solution is constant at the region where the resist pattern is formed at constant aperture intervals; however, at the end of diffraction grating formation region, the area to be etched decreases suddenly, the consumption rate of etching solution decreases as well (a pattern effect), as a result, the etching solution remains at said end. Therefore, by gradually increasing the area of each aperture of resist pattern towards the end of diffraction grating formation region, the consumption rate of etching solution can be made substantially the same between the end of diffraction grating formation region and the other inner part of diffraction grating formation region, whereby the remaining of etching solution at said end can be prevented and a diffraction grating of uniform shape can be obtained.
Thus, according to the present invention, by forming a resist pattern for formation of diffraction grating so that the area of each aperture of diffraction grating increases gradually at the end of diffraction grating formation region, it is possible to suppress a rise in etching rate which takes place in a wet etching step owing to a pattern effect at the end of diffraction grating formation region. As a result, there can be provided a process for producing a semiconductor laser partially having a diffraction grating, which semiconductor laser causes neither laser beam waveguide loss nor nonuniform field distribution.
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T. Nishida et al., “Microloading Effect in InP Wet Etching”, pp. 2414-2421, J. Electrochem, Soc., vol. 140, No. 8, Aug. 1993.
Bowers Charles
Christianson Keith
NEC Corporation
Young & Thompson
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