JPS6235691A - Manufacture of semiconductor laser - Google Patents

Abstract

PURPOSE:To form different diffraction gratings with the same phase relation on one substrate by a method wherein, after a thin film of compound containing InP is formed in a region of an InP substrate where the first diffraction grating is to be formed, a resist film is formed over the whole surface. CONSTITUTION:After an InGaAsP layer 22 is formed on the surface of an InP substrate 21, the InGaAsP layer 22 is removed by etching except the part on a region where the first diffraction grating is to be formed. Then a resist film 23 is formed over the whole surface and the pattern of the diffraction grating is formed by a double-beam interference method. If etching is carried out with sodium hydroxide after exposure, only the InGaAsP layer is etched. Then, if etching is carried out with a solution of hydrochloric acid, where the InGaAsP layer remains, this layer serves as a mask and prevents the substrate from being etched and, on the other hand, where the InGaAsP layer is not left, only the resist serves as a mask and the substrate is etched. As a result, two different etched shapes are formed on the substrate so that the first diffraction grating 24 and the second diffraction grating 25 can be obtained. Then, the resist film and the InGaAsP layer are removed.

Description

[Detailed Description of the Invention] [Summary] The present invention provides a distributed feedback semiconductor laser having different diffraction gratings on the surface of a single semiconductor substrate, and provides a method for easily forming two types of diffraction gratings with different shapes. In this method, a thin film of the same type as the substrate is formed on a part of the surface of a semi-insulating substrate, and a resist is applied to the surface and exposed and etched to form a 2N type diffraction grating with the same phase on the same substrate. It is formed on the surface.

[Industrial Field of Application] The present invention relates to a method of manufacturing a semiconductor laser, and particularly to a method of forming a diffraction grating of a semiconductor laser having two different types of diffraction gratings.

With the spread of optical communications, semiconductor lasers have become extremely important devices as light emitting elements, and many structures have been proposed for stable operation, higher output, and higher efficiency of semiconductor lasers.

For optical transmission, single longitudinal mode lasers with low loss even over long distances have been put into practical use, and the most practical one is the distributed feedback laser (DFB), which has a diffraction grating formed near the active layer of a semiconductor laser. laser).

DFB lasers are made by forming a periodic structure diffraction grating in a crystal layer near the active layer so that a laser beam of a specific wavelength can be extracted. There is a structure that suppresses the broadening of the laser emission spectrum by arranging two types of DFB lasers in parallel and combining the two types of DFB lasers, but this creates two different types of diffraction gratings within the same semiconductor crystal. However, there is a need for a method for easily forming these different diffraction gratings, since it is difficult to achieve the same phase relationship.

[Prior Art] FIG. 2 is a schematic cross-sectional view of main parts of a typical FB laser.

A first diffraction grating 2 and a second diffraction grating 3 are formed on the surface of an InP substrate 1, and an indium gallium arsenide phosphide (InGaAsP) layer is laminated as a light guide layer 3 on the surface. InG with a thickness of 0°1 am
There is an active layer 4 of aAsP, a cladding layer 5 of InP with a thickness of 2 pm, a contact layer 6 of InGaAsP with a thickness of 0.5 μm, and furthermore electrodes 7.8 are arranged on both sides of the element.

The first diffraction grating and the second diffraction grating have a coupling constant determined by their structure, and need to be formed so that their phase relationships are in phase.

FIGS. 3(a) to 3(d) are schematic cross-sectional views of essential parts showing a conventional method of forming a diffraction grating.

FIG. 3(a) shows that the resist film 12 is thinned to a thickness of 1 in order to form the first diffraction grating in the region A on the surface of the InP substrate 11.
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Figure 3('b) shows that a resist film is exposed using the three-beam interference method, which utilizes the fact that two types of light waves are superimposed to obtain interference fringes, and the interference fringes are sharp and have sharp intensity. For example, the pitch of the resist II odor is 2,000 lines, and the width and height of the stripes are about 1,000 lines.

Figure 3 (C) shows an InP substrate etched using a saturated solution of bromine as the etching solution. Grooves 13 are formed.

In FIG. 3(d), the resist film is removed after etching the InP substrate, and a predetermined first diffraction grating is formed.

After the first diffraction grating is formed in this way, the second diffraction grating is placed at I so that it is in phase with the first diffraction grating.
Although the second diffraction grating is formed in region B on the surface of the nP substrate, the formation of the second diffraction grating is performed according to the manufacturing method shown in FIGS. 3(a) to 3(d) above. A drawback is that it is extremely difficult to form the gratings at positions with the same phase relationship.

[Problems to be Solved by the Invention] A problem with conventional DFB semiconductor lasers is that it is difficult to form different diffraction gratings on the same substrate with the same phase relationship as a manufacturing method.

[Means for Solving the Problems] The present invention proposes a method for manufacturing a semiconductor laser to solve the above-mentioned problems.
After forming a thin film of a compound containing InP in the region where the first diffraction grating is to be formed on the substrate, a resist film is deposited on the entire surface of the substrate, and the resist film is exposed to light to create a bond between the resist film and the thin film. By performing etching on both sides, a mask pattern having a predetermined diffraction grating shape was formed, and then by etching the InP substrate with an aqueous solution of bromine, diffraction gratings with different shapes were formed on the InP substrate. There are things.

[Function] The present invention enables the formation of a first diffraction grating using a laminated layer of a resist film and a thin film of an InP compound as a mask, and the formation of a second diffraction grating using only a resist film, by simultaneous exposure and etching. This process takes advantage of the fact that the etching characteristics of the substrate differ depending on these masks.
By changing the etching shape during transfer, it becomes possible to easily and accurately form different diffraction gratings.

[Example] FIG. 1(al) to FIG. 1(f) are schematic cross-sectional views of main parts for explaining a method of manufacturing a DFB type semiconductor laser according to the present invention.

FIG. 1(a) shows an indium gallium arsenide phosphide (InGaAsP) layer 22 with a thickness of 0.2 mm on the surface of an InP substrate 21.
pm, and was formed by a normal liquid phase growth method.

FIG. 1(b) shows In in the region forming the first diffraction grating.
The InGaAsP layer in the region where the second diffraction grating is to be formed is removed by etching, leaving the GaAsP layer 22.

FIG. 1(C) shows that after a resist film is applied to the entire surface, patterning of a diffraction grating with a spacing of 300 μm and a width of 150 μm is applied to the resist film 23 using conventional photolithography and chemical etching. In this embodiment, a three-beam interferometry method using a HeCd laser with a wavelength of 4416 nm is used to form a diffraction grating having a period of 4000 nm.

FIG. 1 (d+) shows that when etching is performed using 0.5N caustic soda after exposure, only the InGaAsP layer is etched.

Figure 1(e) shows that when a diffraction grating is transferred to an InP substrate using an aqueous solution of hydrochloric acid, the InGaAsP layer serves as a mask in the area where the InGaAsP layer is, and
Because the AsP layer has extremely good adhesion to the substrate, the InGaAsP layer prevents etching of the substrate by etching with hydrochloric acid.On the other hand, the resist serves as a mask in the areas where the InGaAsP layer is not present, so the bottom surface of the resist film is The substrate is etched by the hydrochloric acid.

As a result, the etched shape of the substrate differs depending on the type of mask, and the first diffraction grating 24 and the second diffraction grating 25 are formed on the InP substrate.

FIG.
By removing the GaAsP layer, different diffraction gratings can be formed.

[Effects of the Invention] As described in detail above, the method for manufacturing a semiconductor laser according to the present invention can easily and accurately form a diffraction grating on the same substrate surface, and can provide a high-quality semiconductor laser. There is a great effect.

[Brief explanation of drawings]

1(al) to 1(f) are schematic cross-sectional views of main parts for explaining the manufacturing method of a semiconductor laser according to the present invention, FIG. 2 is a schematic cross-sectional view of main parts of a DFB laser, and FIG. a) to FIG. 3(d) are schematic cross-sectional views of main parts showing a conventional method of forming a diffraction grating. In the figures, 21 is an InP substrate, 22 is an InGaAsP layer, and 2
3 is a resist film, 24 is a first diffraction grating 25, and a second diffraction grating.

Claims (1)

[Claims] On the first diffraction grating formation region of the semiconductor substrate (21),
A thin film (22) is formed, a resist film (23) is deposited on the semiconductor substrate (21) and the thin film (22), and the resist film (23) is patterned into stripes having a constant pitch. Using the resist film on the formation area of the first diffraction grating as a mask, perform etching with the thin film to form a mask pattern of the thin film having the shape of the first diffraction grating, and using the resist film and the thin film as a mask. , the substrate (21
), a first diffraction grating (24) and a second diffraction grating (24) having a different pitch are formed on the substrate.
25) A method for manufacturing a semiconductor laser, characterized by forming.