JPS62205354A - Lithographic method - Google Patents

Abstract

PURPOSE:To enhance precision and reproducibility of a resist mask by exposing a negative type photoresist formed on a positive type photoresist, and forming a mask made of the CONSTITUTION:The positive type photoresist 2 is applied to a substrate 1, and the negative type photoresist 3 is applied to the photoresist 2. Then, the photoresists 3, 2 are imagewise exposed, and the mask made of the positive type photoresist 2 is obtained by developing the photoresist 2 after development processing of the photoresist 3. At that time, a thickness of the photoresist 2 and the photoresist 3 are controlled to, for example, 300mm and 100mm, respectively, thus permitting a ratio of illumination intensity reaching the photoresist 3 to that reaching the photoresist 2 to be made about 30:1, and a diffraction grating of sufficient depth to be formed by etching on the photoresist 2.

Description

[Detailed Description of the Invention] [Summary] This invention, when it is difficult to obtain high contrast with the exposure method of lithography, coats a negative photoresist in a layered manner on a positive photoresist, exposes both resists, and produces a photoresist made of a positive photoresist. By forming the mask, the effect of improving the contrast of the exposure amount of the positive resist is obtained.

[Industrial application field]

The present invention relates to an improvement in a lithography method, and particularly to a lithography method that applies an exposure method that is difficult to obtain high contrast, such as two-beam interference method.

For example, in distributed feedback (DFB) lasers, feedback is selectively performed using a diffraction grating provided at the interface of the optical waveguide.
The two-beam interferometry method applied to the exposure of this diffraction grating pattern generally has low contrast, making it difficult to obtain a good resist mask, and improvements are desired.

[Conventional technology]

The DFB laser has, for example, a structure as shown in the schematic side sectional view of FIG. In the figure, 21 is an n-type indium phosphide (InP) substrate, 22 is a diffraction grating pattern, and 23 is an n-type indium gallium arsenide phosphide (I) with a thickness of about 0.15 μm.
(nGaAsP) guide layer, 24 is a TnGaAsP active layer with a thickness of approximately 0.15 mm, 25 is a p-type 1nP cladding layer,
26 is a p-type InGaAsP contact layer, 27 is a p-side electrode, and 28 is an n-side electrode.

The pitch A of the diffraction grating pattern 22 at which the oscillation wavelength of this DFB laser is, for example, a 1.3-wavelength band, is about 0.4 μm when using second-order diffracted light, and about 0.2 pm when using first-order diffracted light. As an exposure method for such a periodic pattern where the pitch A is small and it is difficult to form an exposure mask,
A two-beam interferometry as shown in FIG. 2 is generally practiced.

In the figure, 11 is a semiconductor substrate, 12 is a resist film,
13 is, for example, helium-cadmium (with a wavelength of 325 nm).
He-Cd) laser, 14.17.18 is a mirror, 1
5 is a beam expander, 16 is a half mirror,
The laser beam emitted from the laser 13 and made into a parallel beam of the required size by the beam expander 15 is divided into two directions by the half mirror 16, and is irradiated onto the resist film 12 by the mirrors 17 and 18 to form the resist film]2. A striped interference pattern is formed on the top due to the optical path length difference between the two beams.

The resist film 12 is normally a positive type quinone diazide photoresist, such as the He Z1350 series manufactured by Sisoplay Co., Ltd.
Although it is desirable to have a thickness of about 100 nm, it is difficult to increase the contrast of the interference pattern, and the effective thickness at which the resist film 12 after development is separated into stripes is limited, and sometimes remains at about 100 planes. Furthermore, since the intensity changes slowly, there are many changes in the pattern due to changes in the resist film thickness, exposure and development conditions, and there are also problems in the reproducibility of the pattern shape.

[Problem that the invention seeks to solve]

For example, 2 applied to the diffraction grating pattern of a DFB laser.
Conventionally, resist masks obtained by exposure methods such as beam interferometry, which are difficult to obtain high contrast, have many problems as described above, and there is a strong demand for improvement in their accuracy and reproducibility.

[Means for solving problems]

The above problem can be solved by applying a positive photoresist onto a desired substrate, coating a negative photoresist on top of the positive photoresist, exposing the negative photoresist and the positive photoresist, and then exposing the negative photoresist to light. This problem is solved by the lithography method according to the present invention, in which the positive photoresist is developed after development to form a mask made of the positive photoresist.

[For production]

According to the present invention, a cyclized rubber-based negative photoresist, for example, is laminated and coated on a positive photoresist used for mask formation, the negative photoresist and the positive photoresist are simultaneously exposed, and the negative photoresist is developed. After that, the positive photoresist is developed.

For example, cyclized rubber resists, which are most commonly used in negative photoresists, contain bisazide as a photocrosslinking agent.
When irradiated with light, it decomposes, releases nitrogen (N2) gas, and becomes nitrene radicals, which react with the double bonds of the rubber and form a network structure by abstracting hydrogen (H2), becoming insoluble in solvents.

In the state where this network structure is formed, the light absorption rate is reduced to about 2 that of the resist before irradiation with light. Therefore, the absorption of light by the negative photoresist layer rapidly decreases at positions where the intensity of the incident light is high and gradually decreases at positions where the intensity of the incident light is low, and the intensity difference in the exposure amount of the positive photoresist layer is caused by the absorption of light on the top surface of the negative photoresist layer. It is emphasized by the difference in the intensity of the incident light.

〔Example〕

The present invention will be specifically explained below with reference to an example schematically shown in FIG.

In the figure, 1 is a substrate, 2 is a positive photoresist layer,
3 is a negative photoresist layer. In this example, a quinonediazide-based resist, such as the AZ1350 series manufactured by Shisopray Co., Ltd., is used as the positive photoresist, and the thickness thereof is set to 300 nm, and a cyclized rubber-based resist, such as KMER manufactured by Kodasoku Co., Ltd., is used as the negative photoresist. The thickness is, for example, 1100n.

Exposure is performed from the upper surface of the negative photoresist layer 3 by the two-beam interference method, and the interference pattern incident on the upper surface of the negative photoresist layer 3 by the two-beam interference method has an illuminance ratio of about 10:1 at most. On the other hand, the illuminance ratio of the light reaching the positive photoresist layer 2 in this example is 30:1.
It became possible to form a resist mask with a thickness of 3001 m and to form a diffraction grating with sufficient depth by etching.

Note that the positive photoresist is developed with the negative resist pattern remaining on the positive photoresist layer 2, but by sufficiently stirring during this development, the negative resist pattern and the unexposed positive resist can be peeled off. Easy and reliable implementation.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to form a thick and sharp resist mask under exposure conditions in which it is difficult to obtain high contrast, and a great effect can be obtained in the manufacture of various devices such as, for example, DFB lasers.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of an embodiment of the present invention, Figure 2 is a schematic diagram of the two-beam interference method. FIG. 3 is a schematic side sectional view of the DFB laser. In the figure, 1 is the board, 2 is a positive photoresist layer; 3 indicates a negative photoresist layer. Reiiri-cho, an example of a tip Yu 1 garden DFB taupe shellfish I Higo surface■4 3rd

Claims (1)

[Claims] 1) Apply a positive photoresist on a required substrate, apply a layered layer of negative photoresist on the positive photoresist, expose the negative photoresist and the positive photoresist, and apply the negative photoresist to light. 1. A lithography method comprising the step of developing a positive photoresist after developing the mold photoresist to form a mask made of the positive photoresist. 2) The lithography method according to claim 1, wherein the negative photoresist is a cyclized rubber photoresist.