JPH0590129A - Method of forming diffraction grating - Google Patents

Abstract

PURPOSE:To improve uniformity in the surface of a diffraction grating, by a method wherein the film thickness distribution of resist is made approximate to the light intensity distribution of interference fringes of two luminous fluxes with which resist is irradiated, and the resist film thickness in a region where the light intensity is lowered from the substrate center toward the periphery is made small. CONSTITUTION:A substrate 1 is coated with resist 2 so as to have the film thickness distribution wherein the film thickness is reduced in accordance with the distance further from the center. Single wavelength laser light is divided into two luminous fluxes after being expanded, and the resist 2 is exposed to interference light 4 of two luminous fluxes wherein interference fringes are formed. At this time, the light intensity 3 of interference fringes of the interference light 4 of two luminous fluxes is reduced as Gaussian distribution in accordance with distance further from the center of time substrate 1 toward the end portions. On account of film thickness distribution resist like light, the thick film part is irradiated with the interference light of large intensity, and the thin film part is irradiated with the interference light of small intensity. Thereby the degree of exposure of the resist is averaged all over the resist, and a diffraction grating excellent in surface uniformity can be formed.

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 diffraction grating, and more particularly to a method for obtaining a diffraction grating having a shape with excellent in-plane uniformity.

[0002]

2. Description of the Related Art A distributed feedback laser is a semiconductor laser incorporating a diffraction grating. This is a semiconductor laser designed to achieve single longitudinal mode oscillation, and the laser characteristics of this semiconductor laser greatly change depending on the shape of the diffraction grating. Therefore, it is desired to make the shape of the diffraction grating as uniform as possible.

2A to 2C are process sectional views schematically showing a conventional method of forming a diffraction grating, in which 1 is a substrate and 2 is a substrate.
Is a resist, 2a is an unexposed portion of the resist 2, 2b is an exposed portion of the resist 2, 3 is a light intensity distribution of interference fringes, and 4 is two-beam interference light.

The forming process will be described below. First, substrate 1
The resist 2 is applied on top of it to a uniform film thickness. And FIG.
As shown in (a), an Ar ion laser having a single wavelength is used as a light source for exposure, the laser light is expanded by a beam expander, and then split into two light fluxes.
The resist 2 is exposed by irradiating the two-beam interference light 4 on which interference fringes are formed. As a result, an exposed portion 2a and an unexposed portion 2b corresponding to the interference fringes are formed on the resist 2.
The diameter of the two-beam interference light 4 used here is about 60 m.
The light intensity is about 5 m at a distance of 15 mm from the center.
%, And the light intensity decreases according to a Gaussian distribution as the distance from the center decreases. Subsequently, when the resist 2 exposed by the two-beam interference light is developed, the resist exposed portion 2a is dissolved, the resist unexposed portion 2a is not dissolved, and the substrate is exposed as shown in FIG. 2 (b). A resist pattern is formed on the resist pattern 1 by the resist unexposed portion 2a. Then
When the substrate 1 is etched using the resist pattern as a mask as shown in FIG. 2C, a diffraction grating as shown in FIG. 2D is formed.

[0005]

In the conventional diffraction grating forming process, the laser beam is expanded by the beam expander so that the entire surface of the resist on the substrate 1 is exposed to the two-beam interference as described above. Forming light 4. However, since the two-beam interference light 4 formed by expanding the light beam by the beam expander has the light intensity distribution 3 of the interference fringes, the exposure intensity on the resist 2 also has a distribution.
The resist pattern obtained by development has many resist unexposed portions 2a from the center to the end of the substrate 1, and as a result, the groove on the substrate 1 obtained by the etching process using such a resist pattern as a mask is There is a problem that the shapes are not uniform and the formed diffraction grating has poor in-plane uniformity.

The present invention has been made to solve the above problems, and an object thereof is to obtain a method for forming a diffraction grating having a shape excellent in in-plane uniformity.

[0007]

In the method of forming a diffraction grating according to the present invention, the film thickness distribution of the resist film thickness is brought close to the light intensity distribution of the interference fringes of two-beam interference for irradiating the resist,
The thickness of the resist is thinned in the region where the light intensity from the center of the substrate to the periphery of the substrate becomes weak.

[0008]

According to the present invention, the resist is thicker in the portion irradiated with the interference light having a high light intensity and thinner in the portion irradiated with the interference light having a weak light intensity. The degree of masking by the resist pattern becomes substantially uniform over the entire substrate, and the shape of the groove on the substrate obtained by the etching process using the resist pattern as a mask can be made substantially uniform.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a cross-sectional view schematically showing steps of a method of forming a diffraction grating according to an embodiment of the present invention. The same reference numerals as those in FIG. Has a film thickness distribution that becomes thinner from the center of the substrate toward the edge of the substrate.

Next, the process of forming the diffraction grating will be described. First, as shown in FIG. 1, a resist 2 is applied on a substrate 1 so as to have a film thickness distribution in which the film thickness decreases as the distance from the center of the substrate 1 increases. This laser light is expanded by a beam expander using an ion laser, then divided into two light beams, and the resist 2 is exposed by the two light beam interference light 4 in which interference fringes are formed. At this time, the light intensity 3 of the interference fringes of the two-beam interference light 4
Shows that the light intensity decreases from the center of the substrate 1 toward the edges according to the Gaussian distribution, but the resist 2 formed with the above-described thickness distribution has a thinner thickness from the center of the substrate toward the edges. Therefore, the exposure energy required to expose the resist on the surface portion of the substrate 1 is small, and the two-beam interference light 4 can be exposed to the resist on the surface portion of the substrate 1 at any portion. .. Next, when development is performed, as shown in FIG. 1 (b), the resist exposed portion 2b is dissolved and removed, the resist unexposed portion 2a remains on the substrate 1, and the resist 2a is used as a mask. When the substrate 1 is etched, grooves having a uniform shape can be formed in any region of the substrate 1 as shown in FIG. 1 (c). Finally, when the resist 2a on the substrate 1 is removed, the diffraction grating shown in FIG. 1 (d) is formed.

In the step of forming the diffraction grating according to the present embodiment, since the form of the film thickness distribution of the resist 2 and the form of the light intensity distribution 3 of the interference fringes of the two-beam interference light 4 match.
Interference light having a high light intensity is irradiated to a portion having a large film thickness, and interference light having a low light intensity is irradiated to a portion having a small film thickness, whereby the degree of exposure to the resist 2 is averaged over the entire resist 2, It is possible to form a mask pattern having the same degree of masking on any portion of the substrate 1, the grooves on the substrate 1 obtained by etching are aligned, and the diffraction having a shape with excellent in-plane uniformity is obtained. A lattice is obtained.

In the above embodiments, the case where the positive resist is used has been described, but the same effect can be obtained when the negative resist is used.

[0013]

As described above, according to the method for forming a diffraction grating of the present invention, the resist film thickness is made to have a distribution in accordance with the light intensity distribution of the laser beam for irradiating the resist,
Since the film thickness of the resist corresponding to the region where the light intensity toward the periphery of the substrate becomes weak is formed thin, grooves of almost the same shape can be formed on any part of the substrate, and excellent in-plane uniformity is achieved. There is an effect that a diffraction grating having a curved shape can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view schematically showing a step of a method of forming a diffraction grating according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view schematically showing a conventional diffraction grating forming process.

[Explanation of symbols]

 1 substrate 2 resist 2a resist unexposed part 2b resist exposed part 3 interference fringe light intensity distribution 4 two-beam interference light

Claims (2)

[Claims] 1. A step of exposing a resist on a substrate to two-beam interference light having interference fringes, a step of developing the exposed resist to form a resist pattern, and etching the substrate using the resist pattern as a mask. A method of forming a diffraction grating including a step of processing, wherein the film thickness distribution of the resist is similar to the light intensity distribution of the interference fringes. 2. The method for forming a diffraction grating according to claim 1, wherein the resist has a film thickness that is reduced from a center of the substrate toward a peripheral portion thereof.