JPS62109389A - Manufacture of diffraction grating - Google Patents

Abstract

PURPOSE:To manufacture a diffraction grating having preferable shape as the grating used for a lambda/4 shift type DFB laser by separating a photoresist on the upper layer, then baking it before developing the photoresist of lower layer to enhance the bondability between the photoresist and a substrate. CONSTITUTION:A first photoresist 2 is partly formed on a semiconductor substrate 1, a second reverse photosensitive photoresist 3 opposite to the first photoresist is formed to cover the exposed surface of the substrate 1 and the photoresist 2, and the photoresists 2, 3 are exposed by two-optical interference exposing method. After the photoresist 3 is then developed to form a periodic pattern, with the photoresist 3 as a mask the portion not covered with the photoresist 2 of the substraste 1 is etched. The photoresist 3 is then removed, baked, the photoresist 2 is then developed to form a periodic pattern. With the photoresist 2 as a mask the substrate 1 is then etched, and the photoresist 2 is then removed.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a diffraction grating for a λ/4 shift DFB laser on a semiconductor substrate.

(Conventional Techniques) In recent years, research has been actively conducted on single-mode conductor lasers as light sources for long-distance, high-capacity optical transmission systems. Examples of single-mode semiconductor lasers include distributed feedback (DFB) lasers, distributed reflection (DBR) lasers, composite cavity lasers, and short cavity lasers, but their single-mode operation controllability2 and temperature DBR lasers and DFB lasers are being developed intensively due to stability issues such as wide range. Also, the DFB laser is 11111t better than the DBR laser! DF'B lasers are attracting attention because of their simple manufacturing method.

The DFB laser has a structure with three uniform diffraction gratings within the element, and oscillates at a single wavelength near the Bragg wavelength determined by the period of the diffraction grating. However, in a DFB laser having a uniform diffraction grating, there are two closely excavable moats that are symmetrical with respect to the Bragg wavelength, so there were many elements that oscillated in these two modes. Therefore, in order to eliminate instability in a certain sense, the periodic gold λg/4(λy
is the wavelength of light propagating in the element).
A B laser is being prototyped. (Electronics Letters magazine published November 22, 1984)
nics LetterS) $ 20 Volume 4 1008
- Page 1010 A DFB laser with such a structure is λ/4
It is called a shifted DFB laser.

The λ/4 shift DFB laser is said to oscillate in a single axial mode that perfectly matches the Bragg wavelength, and the number of devices that can be found in the 2-axis mode as described above is extremely rare, increasing yield by 1. It is very promising in this respect. By the way, the period A of the diffraction grating of +DFB laser is the oscillation wavelength λ
has the following relationship.

Here, ne is the equivalent refractive index within the element, and m is the order of the diffraction grating. Therefore, the period of the first-order diffraction grating (m=l) is λ = -. From this, we can conclude that the first-order diffraction grating 1-Oe is λ
It can be seen that for the /4 shift type diffraction grating, the unevenness of the left and right diffraction gratings can be reversed with the shift region as a boundary.

The conventional manufacturing method for such a diffraction grating was realized by Jitaka et al. using the reverse photosensitivity of positive and negative type photoresists (Electronics Letters magazine, November 22, 1984).
ters) Vol. 20, No. 4, pp. 1008-1010), the first photoresist is partially formed on the semiconductor substrate, and then the first photoresist is formed so as to cover the semiconductor substrate 3 and the first photoresist. After forming a second photoresist with reverse photoresist and exposing these first and second photoresists, the second photoresist is first developed, etched, and removed, and then the first photoresist is exposed. The resist is developed, etched, and removed.

(Problems to be Solved by the Invention) The conventional method for manufacturing a diffraction grating described above is difficult because the first photoresist has poor adhesion to the substrate and the first photoresist and the ebchant are not compatible with each other. One drawback is that the valleys and peaks of the diffraction grating are not well formed, resulting in the peaks being connected, and a shallow diffraction grating being formed.

An object of the present invention is to provide a method for manufacturing a diffraction grating that is capable of manufacturing a diffraction grating with a good shape as a diffraction grating for use in a λ/4 shift type DFB laser.

(Means for Solving the Problems) The present invention includes a first step of partially forming a first photoresist on a semiconductor substrate, and an exposed surface S of a handle conductor substrate.
and a second step of forming a second photoresist having a photoresist opposite to that of the first photoresist so as to cover the first photoresist; a third step of exposing the second photoresist by a two-light interference exposure method;
After this third step, there is a fourth step of developing the second photoresist to form a periodic pattern, and after this fourth step, a step of developing the semiconductor substrate using the second photoresist as a mask. a fifth step of etching the portion not covered by the first photoresist; a sixth step of removing the second photoresist after the fifth step; and a sixth step of etching the second photoresist after the sixth step. a seventh step of developing a first photoresist to form a periodic pattern; an eighth step of etching the semiconductor substrate using the first photoresist as a mask after the seventh step; 2, the method for manufacturing a diffraction grating comprises the step of removing the first photoresist after the eighth step; The method includes a tenth step of baking.

The method for manufacturing a diffraction grating of the present invention is configured such that in the eighth step, after immersing the semiconductor substrate in alcohol, the semiconductor substrate is etched using an etchant containing at least alcohol and using the first photoresist as a mask. It can also be done.

(Function) Photo-etching is usually performed in the following steps: photoresist coating, pre-baking, exposure, development, rinsing, post-baking, etching, and photoresist stripping. In this step, pre-baking is performed for the purpose of removing the solvent in the resist film, while post-baking is performed for increasing the adhesion between the resist and the substrate during wet etching.

Conventionally, when manufacturing a diffraction grating using two types of photoresists, the second photoresist is peeled off, and then the first one is immediately removed.
was developing photoresist.

However, in this case, the first 7t resist was easily peeled off from the substrate, making it difficult to manufacture a good diffraction grating.

Therefore, after the second photoresist is completely removed, baking is performed to improve the adhesion between the first photoresist and the substrate. This baking increases the adhesion between the first photoresist and the substrate, forming a well-shaped diffraction grating.

Furthermore, when using a synthetic rubber-based photoresist as the first photoresist, if a hydrogen bromide-based etchant is used, the first photoresist will repel water;
As a result, the etchant does not spread over the entire surface of the substrate. As a result, a diffraction grating with a poor shape is formed. Therefore, if alcohol-based methanol is used as the etchant, the etchant will spread over the entire surface of the substrate, forming a well-shaped diffraction grating.However, if methanol is used as the etchant, alcohol immersion is performed before etching. By performing alcohol immersion,
The etchant and the substrate become compatible, and a well-shaped diffraction grating is formed.

If alcohol immersion is not performed, the etched substrate will not fit well and a well-shaped diffraction grating will not be formed.

(Example) Embodiments of the present invention will be described in detail below with reference to the drawings.

FIGS. 1(a) to 1(a) are diagrams illustrating a method for manufacturing a diffraction grating according to a first embodiment of the present invention in the order of steps. First, as shown in FIG.
AZ1350 film 3 (manufactured by Hoechst), which is a novolak resist of λ, is applied.

Then, interference exposure is performed using two He-Cd laser beams 6 with a wavelength of 3zsoX at an incident angle of 53.7 with respect to the horizontal plane. At this time, the optimum exposure time is about 20 seconds for a light intensity of 4 mW/crl of the He-Cd laser.

Next, as shown in FIG. 1(C), insert the Z1350 film 3 into the AZ
Develop with Developer (manufactured by Hoechst). Figure 1 (t) 0
．． Figure 1 shows how the AZ1350 film 3 was removed with methyl ethyl Kent after etching with a mixed solution (1:100).
A diffraction grating 4 is formed. Then, bake at 80℃ for 10 minutes. Next, as shown in Figure 1(e), the OM
Image OMRM2O3 using R developer (manufactured by Tokyo Ohka).

Next, as shown in FIG. 1(f), the InP substrate 1 is etched with a hydrogen bromide etchant using an etching mask for all of the OMR films 2 to obtain a diffraction grating 5. AZ1
Since the 350 film 3 and the OMRM2O3 have opposite photosensitivity, the unevenness of the diffraction grating 4 and the diffraction grating 5 are reversed, resulting in a λ/4 shift type diffraction grating.

In this diffraction grating manufacturing method, baking is performed after removing the AZ 135Q film 3, so the OMR and film 2
The adhesion between the surface and the substrate is improved, and a diffraction grating with a period of ~2000k and a depth of 800λ without pre-fujis (the peaks of the diffraction grating do not connect) can be obtained, and the shape of the diffraction is better than before. A grid 5 can be formed.

In the second embodiment of the present invention, O
When etching the InP substrate 1 as an MRM2O3 etching mask, etching was performed using 0.01% B methanol as an etchant for 40 seconds. For better etching, soak in methanol before etching.

(Effects of the Invention) As explained above, the present invention has a good sawtooth diffraction grating (for example, a period of ~2000K, a depth of 800~1000K).
λ).

When a semiconductor laser was manufactured using this diffraction grating,
Compared to a semiconductor laser using a diffraction grating formed by the conventional method, the method of the present invention was found to be effective as it showed an improvement in the threshold value (1) reduction in flow (2) and yield (5).

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing the process step of a method for manufacturing a diffraction grating, which is one embodiment of the present invention. In color 2, 1 is IIIP substrate, 2 is OM film (negative photoresist), 3 is AZ1350 film (positive photoresist), 4 is positive photoresist) Diffraction grating formed as a k mask , 5 is a diffraction grating formed as a negative photoresist f mask. Agent: Susumu Uchihara, patent attorney

Claims (1)

Claims: 1) a first step of partially forming a first photoresist on a semiconductor substrate; and forming the first photoresist so as to cover the exposed surface of the semiconductor substrate and the first photoresist. What is photoresist? A second step of forming a second photoresist with reverse photosensitivity, a third step of exposing the first and second photoresists by a two-beam interference exposure method, and this third step. After the step, a fourth step of developing the second photoresist to form a periodic pattern, and after this fourth step, developing the first photoresist of the semiconductor substrate using the second photoresist as a mask. a fifth step of etching the portions not covered by the photoresist; a sixth step of removing the second photoresist after this fifth step; and a sixth step of etching the second photoresist after the sixth step; a seventh step of developing a photoresist to form a periodic pattern; and an eighth step of etching the semiconductor substrate using the first photoresist as a mask after the seventh step;
and a ninth step of removing the first photoresist after the eighth step, in which baking is performed after the sixth step and before the seventh step. A method for manufacturing a diffraction grating, comprising a tenth step of performing. 2) Manufacturing the diffraction grating according to claim 1, wherein the eighth step is to immerse the semiconductor substrate in alcohol and then etch the semiconductor substrate using an etchant containing at least alcohol and using the first photoresist as a mask. Method.