JPS62165392A - Manufacture of diffraction grating - Google Patents

Abstract

PURPOSE:To easily manufacture a lambda/4 shift type diffraction grating whose transition region is very short by utilizing the reversibility of an etching method and a lift-off method. CONSTITUTION:A patterning layer 2 composed of an SiO2 film and novorak system positive type photoresist 3 are formed o an N-type InP semiconductor substrate 1. The photoresist 3 is exposed to form a periodical pattern and the SiO2 film 2 is etched by buffered fluoric acid with the patterned photoresist 3 as a mask to form a periodical pattern and then the photoresist 3 is removed. After the region II is covered with photoresist 4, the n-type InP semiconductor substrate 1 is etched in the region I by HBr+H2O2+H2O with the SiO2 film 2 as a mask to form a diffraction grating 5. After the photoresist 4 is removed, an Al film 6 is formed. If the periodical SiO2 film 2 remaining in the region II is etched and the Al film 6 on the SiO2 film 2 is removed by lift-off, the Al film 6 in the region II is periodically patterned. The InP substrate 1 is etched by HBr+H2O2+H2O2 with the Al film 6 as a mask to form a diffraction grating 5' in the region II.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method for manufacturing a diffraction grating.

[Prior art] Distributed feedback semiconductor laser (hereinafter referred to as DFB laser)
has a diffraction grating within the element and oscillates at a single wavelength near the Bragg wavelength determined by the period of this diffraction grating, so it is expected to be used as a light source for long-distance, high-capacity optical fiber communications. Research and development of DFB lasers has progressed dramatically over the past few years, and the basic characteristics of the device have become equivalent to conventional Fabry-Perot semiconductor lasers.

However, there are many elements in DFB lasers that oscillate in two or more axial modes, and the yield of elements that oscillate in a single wavelength is not necessarily sufficient. The λ/4 Schiff I type DFB was proposed to solve this problem.
It's a laser. A λ/4 shift type DFB laser is characterized in that the periodicity of the left and right diffraction gratings is reversed near the center of the device.

According to this structure, the wavelength of the main mode coincides with the Bragg wavelength, and the oscillation threshold gain difference between the main mode and the sub-modes on both sides becomes large, so that stable single-wavelength oscillation by the main mode can be obtained.

To manufacture a λ, /4 shift 1-type DFB laser,
It is necessary to form a diffraction grating with inverted periodicity on the same substrate (hereinafter such a diffraction grating will be referred to as a λ/4 shift type diffraction grating). The method for manufacturing a λ/4-shift diffraction grating is as follows: 1) Form positive and negative type resists adjacent to each other on a semiconductor substrate, and simultaneously expose both resists using interference exposure method. and a manufacturing method using the reverse photosensitivity of negative photoresist I (1984, January 1984).
Electronics Letters magazine (El
electronics Letters) Vol. 20, No. 4, pp. 1008-1010) and ■When forming a positive photoresist on a semiconductor substrate and performing interference exposure, a phase shift plate made of a quartz plate with an uneven surface is used. A method of exposing the semiconductor substrate to light by bringing it into close contact with the semiconductor substrate (Technical Research Report QQ, Institute of Electronics and Communication Engineers)
E85-60, July 1985).

[Problems to be Solved by the Invention] However, in the former method of manufacturing a λ/4 shift type diffraction grating, when the negative and positive photoresists have different sensitivities, the optimum exposure time for both is naturally different. Both sides I
- It is difficult to perform optimal exposure in both cases. Therefore, it is very difficult to control the exposure time and the reproducibility is poor.

Furthermore, the method for manufacturing the latter λ/4 shift type diffraction grating is as follows:
Due to Fresnel diffraction that occurs at the step part of the phase shift plate,
In the part where the periodicity of the diffraction grating is reversed, the length is 10μ.
This results in a transition region of approximately m in size where no diffraction grating is formed. In the λ/4 shift type DFB laser, the optical phase is changed to π in the phase shift region where the periodicity of the diffraction grating is reversed.
By shifting /2, stable single wavelength oscillation at the Bra-Soge wavelength is obtained, but if there was a long transition region as described above, the amount of optical phase shift would not exceed π/2. This makes it difficult to obtain stable single wavelength oscillation.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for manufacturing a diffraction grating that can easily manufacture a λ/4 shift 1-type diffraction grating having an extremely short transition region, in order to solve the above-mentioned problems.

[Means for solving problems]

The method for manufacturing a diffraction grating according to the present invention includes a step of periodically forming a patterning layer on the surface of a substrate, a step of etching the substrate using the patterning layer as a mask in a first region of the surface of the substrate, and a step of etching the substrate using the patterning layer as a mask. After forming a thin film layer in a second region adjacent to the first region on the surface of the substrate, removing the patterning layer and the thin film layer on the patterning layer by a lift-off method. and a step of etching the substrate using the thin film layer left periodically in the second region as a mask after this step.

[Effect]

Photolithography techniques used for patterning electrodes in semiconductor electronic devices include the so-called etching method, in which the electrodes are etched and patterned using a photoresist as a mask, and the photoresist is patterned on the semiconductor surface in advance, and the electrodes are then patterned on top of the photoresist. There is a so-called lift-off method in which the electrode metal on the photoresist is removed by lift-off after forming the metal. The etching method and the lift-off method reverse the electrode pattern that is finally formed.

In the present invention, a λ/4 shift type diffraction grating is formed by utilizing the reversibility of these etching methods and lift-off methods. For example, after forming a photoresist coated on a semiconductor substrate in a periodic manner using the conventional three-beam interference exposure method, a diffraction grating is formed in region (3) on the semiconductor substrate by etching using the photoresist I as a mask. do. In region (3) on the semiconductor substrate, a thin film of metal or the like is formed on a periodically formed photoresist, and if this thin film is patterned in a periodic manner using a lift-off method, the photoresist is The periodicity with the patterned thin film is reversed. Therefore, the periodicity of the diffraction grating formed in region H by etching the semiconductor substrate using the periodic thin film as a mask and the diffraction grating formed in region I are reversed.

〔Example〕

Next, embodiments of the present invention will be described in detail using the drawings.

FIGS. 1(a) to (g) are diagrams illustrating a method for manufacturing a diffraction grating according to an embodiment of the present invention in order of steps.
), on the n-1nP semiconductor substrate 1, a patterning layer 2 consisting of a 500-thick Si02 film and a 450-thick novola-type positive-type photoresist 1 to 3 are formed. . In the step shown in FIG. 1(b), the photoresist 3 is exposed using the conventional interference exposure method, and the period is 240.
Form into a periodic pattern of 0 people. In FIG. 1(c), using the photoresist 3 as a mask, the SiO□ film 2 is etched using buffered phosphoric acid to form a periodic pattern, and then the photoresist 3 is removed. In Figure 1(d),
A part of the surface of the semiconductor substrate 1 (region ■) is covered with a photoresist 4, and the other part (region ■) is coated with HBr+■20 using the SiO□ film 2 as a mask.
The diffraction grating 5 is formed by etching using □+H20 (10:0.1:100).

In FIG. 1(e), after removing the photoresist 4, the entire surface of the semiconductor substrate 4 has a thickness of 50 mm. The film 6 is formed by a vapor deposition method or the like. In FIG. 1(f), the periodic 5i02 film 2 remaining in the region ■ is etched with buffered hydrofluoric acid while applying ultrasonic waves, and at the same time the Kl film 6 on the SiO□ film 2 is etched using the ref I...off method. Remove by. As a result, the hel film 6 in region (2) becomes periodic. Figure 1 (g>, A
! Using the film 6 as a mask, move the InP substrate 1 to HBr+■202
By etching using +hO (10:0.1:1.00), a diffraction grating 5' is formed in region (2).

Finally, it remained on the surface of the board ^! The film 6 is removed using a KOH solution or the like. The thus obtained diffraction gratings 5 and 5' in regions I and (2) have a λ/4 shift of 1 to 1 with inverted periodicity.
It becomes a type. Further, the transition region between the diffraction gratings 5 and 5' is very short, less than ltzm.

In this embodiment, the AIz film 6 is used as the thin film layer, but the thin film layer may be made of another metal or an organic film such as photoresist. In this example, 5i is used as the patterning layer.
Although the 02 film 2 is used, the patterning layer may be a nitride film such as a SiN film.

Furthermore, the patterning layer may be a photoresist, and in this case, the photoresist serving as the patterning layer may be formed in a periodic manner by direct interference exposure. However, this case involves the following difficulties.

That is, the thickness of the photoresist used for lift-off is generally required to be 500 Å or more, although it depends on the material and thickness of the film to be lifted-off. However, the film thickness of the photoresist used for producing the diffraction grating is approximately 100 times thinner after exposure and development, and it may be difficult to apply this lift-off method. In this regard, Figure 1 (a
In the examples shown in ) to (g), instead of the photoresist, an oxide or nitride film with a thickness of about 500 mm, onto which the pattern of photoresist 1 to has been transferred in advance, is used to facilitate the manufacture of the diffraction grating. There is.

〔Effect of the invention〕

91 According to the method for manufacturing a diffraction grating according to the present invention, since the conventional exposure method of interference exposure of one type of photoresist can be applied, there is an advantage that a λ/4 shift type diffraction grating can be manufactured easily and with good reproducibility. be.

The λ/4 shift type diffraction grating obtained by the present invention has a very short transition region length of 1 μm or less between the diffraction gratings whose periodicities are reversed. The D'riB laser showed stable single wavelength oscillation at the Bragg wavelength with a probability of over 90%.

[Brief explanation of drawings]

FIG. 1 is a process diagram showing a method for manufacturing a diffraction grating according to an embodiment of the present invention in order of steps. 1...InP substrate, 2...5i02 film, 3,4...
・Photoresist, 5,5'... Diffraction grating, 6...
At' film-10=

Claims (1)

[Claims] a step of periodically forming a patterning layer on the surface of the substrate; a step of etching the substrate in a first region of the surface of the substrate using the patterning layer as a mask;
After this step, a thin film layer is formed in a second region adjacent to the first region on the surface of the substrate, and then the patterning layer and the thin film layer on the patterning layer are removed by a lift-off method. and etching the substrate using the thin film layer left periodically in the second region as a mask after this step.