JPH04333001A - Formation of lambda/4 phase shift diffraction grating - Google Patents

Abstract

PURPOSE:To obtain the formation of the lambda/4 phase shift diffraction grating with good controllability in its width. CONSTITUTION:Photoresist 11 on a substrate 1 is exposed to two pieces of interfering luminous flux of Ar laser light 6 to form an interference pattern and then irradiated with the light from a CO2 laser 12 through a mask 13 for far infrared rays to form an image reverse area 14. Then the substrate is developed with a positive liquid developer to form the diffraction grating 9 with a lambda/4 phase shift area 8.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a λ/4 phase shift diffraction grating used in a DFB laser or the like.

0002

[Prior art] Figs. 2(a) to 3(c) and 3(a) to 3(a)
(c) is a process cross-sectional view showing a method of forming a λ/4 phase shift diffraction grating using a conventional negative resist and a positive resist (K.UTAKA et al.: ELECTRONIC
S LETTERS 22nd NOVEMBER 1
984 Vol. 20, No. 4 “λ/4-SHIF
TED InGaAsP/InP DFB LASER
S BY SIMULTANEOUS HOLOGRA
PHIC EXPOSURE OF POSITIVE
AND NEGATIVE PHOTORE SIST
(See “S”). In these figures, 1 is an InP-based substrate (hereinafter simply referred to as a substrate), 2 is a first negative resist,
3 is a first positive resist; 4 is an intermediate layer that suppresses mixing of the first negative resist 2 and the first positive resist 3;
6 is a second positive resist, 6 is an Ar laser beam, 7 is a second negative resist, 8 is a λ/4 phase shift region, and 9 is a diffraction grating. In addition, the subscript a of 2, 3, and 4 is the exposed area by normal exposure, the subscript b is also the non-exposed area, the subscript c of 2, 4, and 5 is the area exposed by interference exposure, and the subscript d is also the non-exposed area. Each region's resist is shown. however,
The photosensitive area 3a of the first positive resist 3 has already been removed and is therefore not shown.

[0003] Next, the forming method will be explained. first,
As shown in FIG. 2(a), a 700 Å thick first layer is placed on the substrate 1.
After coating a negative resist 2,600 Å thick and an intermediate layer 4,700 Å thick first positive resist 3, the positive resist 3a is partially removed by a normal exposure method. Next, Figure 2(b
), using the positive resist 3b as a mask, H2
The intermediate layer 4a and the negative resist 2a are sequentially removed using SO4:H2O2 liquid and H2SO4 liquid. next,
After removing the positive resist 3b with a positive resist developer,
As shown in FIG. 2(c), a second positive resist 5 having a thickness of 700 Å is applied. Next, as shown in Figure 3(a), 3
The second positive resist 5 is exposed and developed by interference exposure with a 51 nm Ar laser beam 6, and using this as a mask, the substrate 1 is etched, and a diffraction grating is formed in the area A as shown in FIG. 3(b). form 9. Next, after removing the positive resist 5d, intermediate layers 4c, 4d, and negative resist 2d using a negative developer, a second negative resist 7 is applied to the area A, and the diffraction grating 9 portion is removed using a normal exposure method. After covering, etching is performed using this as a mask to form a diffraction grating 9 in region B, as shown in FIG. 3(c).

0004

The conventional method for forming a λ/4 phase shift diffraction grating is carried out as described above, and has the following problems. That is, (1) It is not easy to remove the first positive resist 3b in the non-photosensitive area without damaging the intermediate layer 4b and the first negative resist 2b. ■ Partially intermediate layer 4b, first negative resist 2
Since the second positive resist 5 is formed on the substrate 1 leaving the portion b, it is difficult to apply the second positive resist 5 uniformly, and it is not easy to control the width of the diffraction grating 9. (2) Since the negative resist 2c exposed by interference exposure is exposed to a negative developer for a long time, the width of the resist pattern tends to vary, making it difficult to control the width of the diffraction grating 9.

The present invention has been made to solve the above-mentioned problems.
A method for forming a four-phase shift diffraction grating is obtained.

[0006]

[Means for Solving the Problems] A method for forming a λ/4 phase shift diffraction grating according to the present invention is to partially apply heat to a positive resist exposed by interference exposure to form an image reverse region. /4 phase shift diffraction grating.

[0007]

[Operation] In the method for forming a λ/4 phase shift diffraction grating in the present invention, after a positive resist with a uniform thickness is exposed to interference light,
Since image reversal regions are formed by applying heat locally, development and etching are performed, making it easy to control the width of the diffraction grating.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. 1(a) to (d) show an embodiment of the present invention.
FIG. 3 is a process cross-sectional view showing a method for forming a four-phase shift diffraction grating. In this figure, the same symbols as in FIG. 2 indicate the same things, 11 is a 700 Å thick positive resist, 12 is far-infrared light such as CO2 laser light, 13 is a mask that blocks this far-infrared light 12, and 14 is a mask that blocks this far-infrared light 12. This is an image reversal area. In addition,
The subscript c of the positive resist 11 is an area exposed by interference exposure, and the subscript d is also an unexposed area. Further, the subscript g is the area heated by the far infrared light 12, and the subscript h
also corresponds to the area that is not heated.

Next, the steps of the method for forming a λ/4 phase shift diffraction grating according to the present invention will be explained. First, Figure 1 (
As shown in a), after coating a positive resist 11 with a thickness of 700 Å on a substrate 1, an interference pattern is formed on the positive resist 11 by two-cross interference exposure using a 351 nm Ar laser beam 6, etc.
form inside. Next, as shown in FIG. 1(b), when manufacturing an LD with a resonator length L=300 μm, for example, 3
Far-infrared light 12 such as CO2 laser light is irradiated through a metallic far-infrared light mask 13 with lines and spaces of 00 μm to heat the non-masked region 11g. By this heating, an image reverse region 14 is formed in which the non-exposed and heated region 11gd in the non-mask region 11g melts in the positive developer, but the exposed and heated region 11gc hardly melts. Next, as shown in FIG. 1C, by performing development with a positive developer, the unheated mask area 11h is exposed by interference exposure, and the unheated area 11hc is a heated area 1 which is a non-mask area.
1g, the non-photosensitive and heated region 11gd melts, and the phase is reversed at the boundary of the mask 13. Next, as shown in FIG. 1(d), the substrate 1 is etched using an HBr-HNO3-H2O-based etching solution, and the resist pattern is transferred to the substrate 1.

[0010] In the above embodiment, the substrate 1 is InP.
However, an InGaAsP multilayer crystal wafer may also be used. Further, although CO2 laser light is used as the far-infrared light 12, YAG laser light or the like may also be used. In addition, the light source used for the two-cross interference exposure is an Ar laser beam 6
The light source is not limited to , but He-Cd laser light or the like may also be used. Further, in the present invention, the image reverse area 14 is formed using the mask 13, but the image reverse area 14 may be formed by scanning the far infrared light 12 under computer control without using the mask 13.

[0011]

[Effects of the Invention] As explained above, according to the present invention,
After exposing the positive resist to an interference pattern using a two-cross interference exposure method, in a predetermined area, the exposed area is difficult to dissolve in the positive resist developer, and the non-exposed area is easily soluble, by forming an image reverse area. A diffraction grating in which the λ/4 phase is inverted at the boundary between the image reversal region and the non-image reversal region can be easily formed.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing the steps of a method for forming a λ/4 phase shift diffraction grating according to the present invention.

FIG. 2 is a cross-sectional view showing the steps of a conventional method for forming a λ/4 phase shift diffraction grating.

FIG. 3 is a sectional view showing a series of steps subsequent to FIG. 2;

[Explanation of symbols]

1 Substrate 6 Ar laser 8 λ/4 phase shift region 9 Diffraction grating 11 Positive resist 11c Region 11d exposed by two-cross interference exposure
Area 11g CO that is not exposed in two-cross interference exposure
2 Area heated by laser 11h Area not heated by CO2 laser 12 Far-infrared light 13 Mask 14 Image reverse area

Claims (1)

[Claims] 1. A step of forming interference fringes on a positive resist coated on a substrate by two-cross interference exposure; a step of heating a predetermined region with far-infrared light to form an image reverse region; and a step of forming an image reverse region. A method for forming a λ/4 phase shift diffraction grating, the method comprising the step of developing a positive resist on which interference fringes including a λ/4 phase shift grating are formed with a developer.