JPS62262004A - Production of diffraction grating - Google Patents

Abstract

PURPOSE:To produce a diffraction grating having a good shape without irregularity with satisfactory yield by using a photoresist having good resolution. CONSTITUTION:An AZ1350B.SF film 2 which is a novolak type photoresist is selectively formed on an InP semiconductor substrate 1 and a silicon nitride film 3 and an RV-1100N film 4 which is a novolak type negative photoresist are successively formed over the entire surface; thereafter, the photoresists are subjected to interference exposing by using two beams of He-Cd laser light. The RV-1100N film 4 is then developed and with the periodically formed RV-1100N film 4 as a mask, the silicon nitride film 3 is etched and further with the silicon nitride film 3 and the AZ1350B.SF film 2 as a mask, the InP semiconductor substrate 1 is etched to obtain the diffraction grating 5. After the RV-1100N film 4 is removed, the silicon nitride film 3 is removed and an NNR film 6 which is the negative type photoresist is formed only on the diffraction grating 5. The AZ1350B.SF film 2 is developed and with the NNR film 6 and the AZ1350B.SF film 2 as a mask, the InP semiconductor substrate 1 is etched to obtain the diffraction grating 7.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a diffraction grating, and particularly to a method for manufacturing a diffraction grating on a semiconductor substrate.

(Prior Art) In recent years, research and development of single-axis mode semiconductor lasers have been actively conducted as light sources for long-distance, high-capacity optical transmission systems. Among these, distributed feedback (DFB) semiconductor lasers are popular due to their stability, such as controllability of single-axis mode and wide temperature range, and because they are easier to manufacture compared to other single-axis mode semiconductor lasers. I'm collecting fresh eyes.

A DFB laser has a structure having a uniform diffraction grating within the element, and oscillates at a single wavelength near the Bragg wavelength determined by the period of the diffraction grating. However, in a conventional DFB laser with a uniform diffraction grating, there are two closely oscillatable modes that are symmetrical about the Bragg wavelength. This means biaxial mode operation, which is not preferred as a single axis mode laser. Normally, a DFB laser has a structure in which one end uses a cleavage surface to extract light, and the other end is coated with an anti-reflection coating to suppress the 7-ab1 Le Perot mode. A DFB with a structure in which the end face reflectance is asymmetric like this
The reflector loss characteristics of a laser are asymmetric with respect to the Bragg wavelength, and one longitudinal mode tends to be selected. However, the reflector loss characteristics vary significantly depending on the phase of the diffraction grating at the position of the end reflector. Therefore, in conventional DFB lasers, the difference in mirror loss between the main mode and the next mode is often small, and there are many elements that oscillate in two modes. Therefore, in order to eliminate such instability, a DFB laser was prototyped with a structure in which the period of the diffraction grating inside the DFB laser was shifted by λg/4 (λg is the wavelength of light propagating within the element). An example of literature related to this is Utaka et al., November 2, 1984.
Electronics Letters magazine (Electr) published on the 2nd
onics Letters) Volume 20 No. 4 1008~
I can list a paper with 1010 pages.

A DFB laser having such a structure is called a V4 shift type DFB laser.

, V4 shift type DFB laser is characterized by oscillation in one axial mode that completely matches the Bragg wavelength. For this reason, the number of devices that oscillate in the biaxial mode, which is seen in conventional devices, is extremely rare, and this is very promising in terms of yield improvement. Incidentally, the period Δ of the diffraction grating of the DFB laser has the following relationship with the oscillation wavelength λ.

Here, ne is the isolateral refractive index within the element, m is the order of the diffraction grating, and λg is the wavelength of light propagating within the element.

Therefore, the period of the first-order diffraction grating (m=1) is given by λg A=-. From this, V using a first-order diffraction grating
It can be seen that in the case of a four-shift type diffraction grating, the unevenness of the left and right diffraction gratings can be reversed with the shift region as a boundary. Typical methods for manufacturing a V44 shift type folded lattice using a first-order V44 shift type folded lattice include the following four methods.

The first method is to apply interference exposure by overcoating a novolac-based positive photoresist and a cyclized rubber-based negative photoresist, and utilize the reverse photosensitivity of the positive photoresist and negative photoresist. There is. Documents on this method include Electronics Letters, published November 22, 1984, by Utaka et al.
ics Letters) Volume 20 No. 4 1008-10
I can list a 10-page paper.

The second method is to perform interference exposure by bringing a quartz plate with a difference corresponding to a phase shift into close contact with a substrate coated with photoresist. Technical report 0QE85-60,
The article listed on pages 57-64 can be cited.

A third method is to incorporate a quartz plate with a difference corresponding to the phase shift into the optical system and perform interference exposure.
Proceedings of the 46th Japan Society of Applied Physics Academic Conference, Autumn 202
According to the 2a-N-10r phase plane projection type interference exposure method described on page 2. The manufacturing process of the shifted diffraction grating can be mentioned.

A fourth method is to perform interference exposure by sandwiching an intermediate layer between a novolac negative photoresist and a novolac positive photoresist. 2a-N-9r novolac negative/positive resist) J4 shift diffraction grating described in the proceedings of the 46th Japan Society of Applied Physics Academic Conference, page 202]
can be given.

(Problems to be Solved by the Invention) The conventional method for manufacturing a diffraction grating described above has the following drawbacks.

The first method has disadvantages in that the resolution of the cyclized rubber-based negative photoresist is poor and the reproducibility is poor due to the large number of process steps. In the second method, the transition region is ~
It has the disadvantage of being large, at 10 pm. In the third method,
Due to aberrations, the area where the diffraction grating is formed becomes very small (~
7mm2). In the fourth method, since a novolac-based negative photoresist is used, the first
Unlike the method described above, this method has good resolution, but it has the disadvantage that it is very difficult to control the developing conditions and the yield is low.

An object of the present invention is to provide a method for producing a diffraction grating with no irregularities and a good shape with a high yield, as a method for producing a diffraction grating for use in a J4 shift type DFB laser by improving the fourth method. There is a particular thing.

(Means for Solving the Problems) A method for manufacturing a diffraction grating according to the present invention includes a first step of forming a first photoresist in a first region on a substrate, and a second step after the first step. a second step of forming a second film to cover the first photoresist on the first region of the substrate and the exposed surface of a second region of the substrate other than the first region; After the second step, a third photoresist having a photoresist opposite to that of the first photoresist is formed to cover the second film; After exposing the first and third photoresists in a striped manner, the third photoresist is developed, and the second photoresist is exposed using the third photoresist as a mask.
a fourth step of etching the film; and after the fourth step, the second region of the substrate is etched using the second film as a mask, and then the third photoresist is removed; a fifth step of removing the second film; and after the fifth step, applying a fourth photoresist so as to cover only the second region of the substrate but not the first region of the substrate; a sixth step of selectively forming a photoresist, and after developing the first photoresist after the sixth step, etching the substrate using the first photoresist and the fourth photoresist as a mask. 7th to do
It consists of the following steps.

(Function) The present invention will be explained in detail. Since the method of the present invention uses a photoresist with good resolution, it is possible to obtain a diffraction grating with a good shape, unlike the first method. In addition, in the second method, the step of the phase shift plate is ~2 pm.
Therefore, the transition region is ~10 pm due to Fresnel diffraction.
However, in the method of the present invention, the difference in level during overcoating of photoresist is about 500, and the transition region is ~1N1 m.

In the third method, since the phase shift plate is incorporated into the optical system, the area where the diffraction grating is formed is 7 mm due to the influence of aberrations.
However, in the method of the present invention,
Since the optical system consists only of mirrors, lenses, beam splitters, and beam expanders, the formation area of the diffraction grating is not limited by the influence of aberrations. Below, the principle of the present invention will be described by comparing the method of the present invention and the fourth method using figures. Figure 2(a)
-Cg) are diagrams illustrating the conventional steps for manufacturing an M4 shift diffraction grating using a novolak-based negative/positive photoresist in the order of the steps. The steps will be briefly explained below in order. In the step shown in FIG. 2(a), a positive photoresist film 2 is selectively formed on the InP substrate 1. As shown in FIG. In the step shown in FIG. 2(b), a silicon nitride film 3 is formed to cover the exposed surface of the InP substrate 1 and the positive photoresist film 2. As shown in FIG. In the step shown in FIG. 2(e), a negative photoresist film 4 is formed to cover the silicon nitride film 3, and then interference exposure is performed using two He--Cd laser beams with a wavelength of 3250 nm. In the step shown in FIG. 2(d), only the negative photoresist film 4 above the area where the positive photoresist film 2 does not exist under the silicon nitride film 3 is patterned periodically, and the remaining area is patterned. The negative photoresist film 4 is developed to be completely removed. Figure 2 (
In the step shown in e), the silicon nitride film 3 is formed in a periodic pattern in the area where the negative photoresist film 4 remains, and the silicon nitride film 3 is formed in the area where the negative photoresist film 4 does not remain. Etching is performed using the negative photoresist film 4 as an etching mask so that the etching is completely removed. In the step shown in FIG. 2(g), the InP substrate 1 is etched with a hydrogen bromide etchant using the periodically patterned silicon nitride film 3 and the positive photoresist film 2 as an etching mask.

The above is the conventional method. Below, we will discuss the reasons why the yield was poor in the conventional method.

In the conventional method, as shown in the step of FIG. 2(g), In
By etching the P substrate 1 only once, InP
In order to be able to form the diffraction grating 8 on the entire surface of the substrate 1, a developing method as shown in FIG. 2(d) was used. That is, under the silicon nitride film 3, there are a region where there is no positive photoresist film 2 (this will be named the second region) and a region where the positive photoresist film 2 is present (this will be named the first region). However, development conditions are set such that only the negative photoresist film 4 on the second region is periodically patterned, and the negative photoresist film 4 on the first region is completely removed. had to be set. However, this development condition is very strict, and when the development is actually carried out, both the negative photoresist film 4 on the first and second regions will be patterned and remain, or the negative photoresist film 4 on the first and second regions will remain patterned. In many cases, the square photoresist film 4 was also removed.

If the negative photoresist films 4 on the first and second regions are both patterned and remain, it becomes impossible to form a diffraction grating on the InP substrate 1 under the first region. Furthermore, if the negative photoresist above the first and second regions are both removed, it becomes impossible to form a diffraction grating on the InP substrate 1 below the second region. For this reason, the conventional method had a poor yield.

In the method used in the present invention, 1
Negative photoresist film 4 on the first and second regions
Developed so that both are patterned (this development condition is looser than in conventional methods). Then, after etching the silicon nitride film 3 into a periodic pattern using the negative photoresist film 4 as an etching mask, this pattern is transferred to the substrate 1 under the second region. At this time, since the positive photoresist film 2 has not yet been developed, the substrate portion in the first region is not etched. After that, the negative photoresist film 4 is completely removed, and while the silicon nitride film 3 is removed, only the substrate portion where the diffraction grating 5 is formed, that is, only the second region, is covered with the film 6. This film 6 is for protecting the already formed diffraction grating 5 from being etched when etching the substrate 1 after developing the positive photoresist film 2, and is therefore named the protective film 6. Then, a positive photoresist film 2
After developing, the substrate 1 is etched using the positive photoresist film 2 and film 6 as an etching mask.

The method of the present invention is effective in that a diffraction grating with no irregularities and a good shape can be manufactured with a high yield.

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

FIGS. 1(a) to 1(i) are diagrams illustrating a method for manufacturing a diffraction grating according to an embodiment of the present invention in the order of steps.

In the step shown in FIG. 1(a), A21350, which is a novolak-based positive photoresist, is applied on the InP semiconductor substrate 1.
B-8F membrane 2 (manufactured by Hoechst) with a thickness of 500 people and a cycle of 3
00pm selectively formed. In the step shown in FIG. 1(b), a silicon nitride film 3 is formed to a thickness of 80 nm so as to cover the InP substrate 1 and the A21350B-8F film 2. In the step shown in FIG. 1(c), the silicon nitride film 3 is covered with A21350B.

8F film 2 is a reverse-sensitivity novolak-based negative photoresist RV-1100N film 4 (manufactured by Hitachi Chemical Co., Ltd.), which is formed to a film thickness of 1000 nm, and then a wavelength of 3250 nm.
Interference exposure is performed using two human He-Cd laser beams at an incident angle of 43.2° with respect to a horizontal plane. Figure 1(d)
In the process shown in , the RV-1100N film 4 is
Develop with a deherotspa (manufactured by Hitachi Chemical Co., Ltd.) exclusively for N film. 1st
In the step shown in Figure (e), the RV-1
Using the 100N film 4 as an etching mask, the silicon nitride film 3 is etched with buffered hydrofluoric acid (B, H, F, etc.). In the step shown in FIG.
The diffraction grating 5 can be obtained by etching with a mixed solution of O2:H2O (10:0.1:100). Figure 1 (g
) Steps shown in step 1: convert RV-1100 membrane 4 to RV-11
After removing it with a developer dedicated to 00N film, remove the silicon nitride film 3 with B, HoF, and then apply NNR film 6 (manufactured by Nagaya Co., Ltd.), which is a cyclized rubber-based negative photoresist, to a film thickness of 5000 nm. It is formed so as to cover only the top of the diffraction grating 5. In the process shown in Figure 1 (h), AZ135
0 B・SF film 2 using AZ developer (manufactured by Hoechst)
Develop it with In the step shown in FIG. 1(i), the NNR film 6
Using the periodic AZ1350 B-8F film 2 as an etching mask, the InP semiconductor substrate was etched with HBr.
The diffraction grating 7 can be obtained by etching with a mixed solution of :H2O2:H2O (10:0.1:100).

(Effects of the Invention) The method for manufacturing a diffraction grating of the present invention has the effect that a V44-shifted diffraction grating with deep grooves can be obtained with a high yield. The yield is the same (80% or more) as when manufacturing a normal diffraction grating without V4 shift.

According to the present invention, the period 2000-2500 people, depth ~10
A diffraction grating with a good shape (sawtooth shape) could be formed on the entire surface of the semiconductor substrate. In addition, the transition region is 1μ
m or less. When a semiconductor laser was manufactured using this diffraction grating, the threshold current was several times lower than that of a semiconductor laser using a uniform diffraction grating without J4 shift, and the single mode oscillation yield was 98%. It was found that the direction of the present invention is effective.

[Brief explanation of drawings]

FIGS. 1(a) to 1(i) are process diagrams of an embodiment of the present invention. In FIG. 1, 1 is an InP substrate, 2 is a novolac positive photoresist, 3 is silicon nitride, 4 is a novolac negative photoresist, and 5 is a negative photoresist 4 and silicon nitride 3 as a mask. The formed diffraction gratings include a cyclized rubber-based negative photoresist 6 and a novolak-based positive photoresist 2 as a mask. FIGS. 2(a) to 2(g) are process diagrams of the conventional method. In Figure 2, 1 is an InP substrate, 2 is a novolac positive photoresist, 3 is an intermediate layer, 4 is a novolac negative photoresist, and 8 is an intermediate layer 3 and a novolac positive photoresist 2 as a mask. second area formed
First area <e> Figure 2 Second area First area (a) (d) (e) (f) (g)

Claims (1)

[Claims] a first step of forming a first photoresist on a first region of the substrate; and after the first step, applying a second film to the first photoresist on the first region of the substrate; and a second step of forming a second photoresist to cover the exposed surface of a second region other than the first region of the substrate; a third photoresist formed to cover the second film;
After the third step, the first and third photoresists are exposed in stripes, the third photoresist is developed, and the second photoresist is exposed using the third photoresist as a mask. a fourth step of etching the film;
After the fourth step, a fifth step of etching the second region of the substrate using the second film as a mask, removing the third photoresist, and then removing the second film. and a sixth step of selectively forming a fourth film after the fifth step so as to cover only the second region of the substrate and not cover the first region of the substrate;
A seventh step of developing the first photoresist after the sixth step and etching the substrate using the first photoresist and the fourth film as a mask. A method of manufacturing a diffraction grating.