JPS61217003A - Manufacture of diffraction grating - Google Patents

Abstract

PURPOSE:To manufacture a uniform diffraction grating which has superior reproducibility and controllability by laminating an insulating film and a photosensitive medium layer on a substrate successively and transferring an interference pattern by a two-luminous-flux interference exposing method. CONSTITUTION:An SiO2 film 20 is vapor-deposited as the insulating film on the GaAs substrate 11 and its film thickness is set almost to the maximum value of the level distribution of a standing wave; and a photoresist film 12 is laminated thereupon and exposed to laser light beams 14 and 14' to form a resist diffraction grating mask pattern 13 on the film 20. Then, the film 20 and substrate 11 are etched to obtain a diffraction grating 15 which is sectioned in a saw-tooth shape and the pattern 13 and film 20 are removed. The film 20 uses a material which is nearly equal in refractive index to the film 12 of a photosensitive medium in an exposed wavelength range and the film thickness of the film 12 is determined satisfying the optimum condition determined by the period of an interference pattern during interference exposure. Thus, the diffraction grating having a fine period is manufactured easily and uniformly with good reproducibility.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for forming a diffraction grating on a substrate surface.

BACKGROUND OF THE INVENTION In recent years, functional devices using compound semiconductors have been actively developed. Among these, optical integrated circuits, which monolithically integrate light-emitting elements, light-receiving elements, and their driving/amplifying circuits using guided waveguide technology, are attracting attention as key devices that can form nodes in optical loop network communication systems. ing. Diffraction gratings applied to optical integrated circuits are composed of periodic irregularities formed on an optical guide, and the period is usually as small as a few thousand amps, requiring processing technology that costs $1. Ru.

FIG. 5 shows a process for creating a conventionally used diffraction grating. In FIG. 5, 1 is a substrate on which a diffraction grating is to be created, 2 is a photoresist film that is a photosensitive medium, and 4 is a photoresist film that is a photosensitive medium.
, 4' are laser beams.

First, a photoresist film 2 is spin-coated onto a substrate 1 using a spinner. Next, the photoresist film 2 is exposed by an interference exposure method using laser beams 4 and 4' as shown in FIG. removed and shown in Figure 5 (
As shown in b), a resist mask pattern 3 is formed. After performing a suitable briefing, this substrate is chemically etched using a suitable etching solution, as shown in Fig. 5 (
As shown in C), the diffraction grating 5 is transferred onto the substrate 1. After that, the photoresist pattern 3 on the substrate 1 is removed, and the fifth
Complete the creation of the diffraction grating as shown in figure (d).

Problems to be Solved by the Invention However, the above method has the following drawbacks.

First, the optical system for interference exposure is composed of a laser light source with good coherence and an optical system with a high angle of 11 degrees. It is difficult to achieve uniform exposure with good reproducibility.

Second, in the interference exposure process, reflection of light at the interface between the substrate and the resist film has a large influence. When exposure is performed using, for example, a GaAS substrate as the substrate 1 and, for example, AZ 1350 as the photoresist film 2, the substrate 1
The reflectance at the interface between the photoresist 132 and the photoresist 132 has a large value of about 20 to 30% depending on the difference in refractive index. Therefore, a standing wave is generated due to interference between the incident wave and the reflected wave, and the actual exposure process is performed within this standing wave. The intensity distribution of the standing wave in the substrate 1 and the photoresist 1i12 changes periodically depending on the distance from the substrate, and takes a minimum value at the interface between the substrate 1 and the photoresist 1112.

In order to make the resist mask pattern 3 for forming the diffraction grating after development optimal as a subsequent chemical etching mask, the groove portions of the resist mask pattern 3 must be sufficiently removed. There was a problem in that the exposure conditions were quite long.

For the reasons mentioned above, in the conventional method, variations occur in the resist mask patterns created, making it difficult to obtain uniformity in the mask effect during etching.

Since the depth of the diffraction grating, that is, the etching depth, affects the characteristics such as the diffraction efficiency of the diffraction grating, it is desired that it be etched uniformly with good reproducibility, but this has been difficult with conventional methods.

The present invention solves these problems, and aims to provide a method for manufacturing a diffraction grating that is uniform and has excellent reproducibility and controllability.

Means for Solving the Problems In order to solve the above problems, the present invention provides a first layer made of an insulating film and a second layer made of a photosensitive medium on a substrate on which a diffraction grating is to be fabricated. are sequentially laminated, and the interference pattern is transferred onto the substrate surface using a two-beam interference exposure method. At this point, for example, the first layer made of an insulating film is made of a material having a refractive index almost equal to that of the second layer made of a photosensitive medium in the exposure wavelength range, and the film thickness of the second layer is set to be the same as that at the time of interference exposure. The interference pattern having the desired period can be formed by holographic exposure to suit the optimum conditions determined by the period of the interference pattern.
After the layer is applied to the photosensitive medium, the pattern can be transferred to the second layer using chemical etching or dry etching techniques to obtain a grating mask for fabricating the grating on the substrate. Thereafter, the substrate is chemically etched using an etching solution whose etching rate is surface orientation dependent, thereby obtaining a diffraction grating carved on the substrate surface.

Function: According to the method of the present invention, a diffraction grating with a fine period used in the light beam 11i circuit can be manufactured easily and uniformly with good reproducibility.

EXAMPLE Hereinafter, an example of the present invention will be described in detail with reference to FIGS. 1 to 4. FIG. 1 shows an example in which GaAs is used for the substrate crystal. In FIG. 1, 1 is a Ga As J! for which a diffraction grating is to be created. The plate 20 is Si 021 as an insulating film! f1112 is a photoresist film, 14.
14' is a laser beam.

First, a SiO2 layer 20 is deposited on the substrate 11 using a sputtering method or a thermal CVD method, as shown in FIG. 1(a).

The insulating film does not need to be limited to Si 02 g120, as long as conditions such as adhesion to the substrate and the phosphor cyst film, controllability of film thickness, and refractive index close to that of the resist film are satisfied. Other materials may also be used, such as 5iNX material. S
The thickness of the iO2 film 20 is set so that the interface with the photoresist film exists at a position where the intensity of the standing wave generated on the substrate during three-beam interference exposure is maximum. For example, under the conditions of the exposure system shown in FIG. 3, when the X axis is taken in the direction perpendicular to the substrate 11, the position X wax where the intensity of the standing wave is maximum can be expressed by the following equation.

Xraax s 700 (2m+ +1 >1-0.1
．． 2...<1) (However, the unit is ^) This X5ax position is 70OA from the board 11, 21
00A -... roar.

This relationship is expressed according to the exposure wavelength and the period of the interference pattern. Therefore, the thickness of the SIO2 film 20 may be set to be controllably close to the optimum value, that is, close to the maximum value of the intensity distribution of the standing wave, depending on the conditions of the exposure optical system.

Next, as shown in FIG. 1(b), 20
The upper resist film 12 is spin-coated to an appropriate thickness, and after an appropriate prebaking process, the photoresist film 1 is formed by interference exposure using laser beams 14 and 14'.
Expose 2. When the development process is performed, a diffraction grating mask pattern 13 can be formed on the SiO2 film 20 as shown in FIG. 1(C). After a suitable boss bake, this mask 13 is removed as shown in Fig. 1(d).
Through this process, 5iOzRQ20 is etched by chemical etching or dry etching. A mixed solution of HF-NH3F may be used as the chemical etch, and a CFJ sputtering method may be used as the dry etch, but the dry etch method is more advantageous in terms of mask undercut during etching.

Next, using the pattern-transferred 5iO2 1120 as a mask, the substrate 11 is etched using an appropriate etching solution as shown in FIG. 1(e). At this time, if the diffraction grating pattern is selected to be parallel to the <011> direction of the substrate 11, and if an etching solution that is bidirectionally dependent is used, a diffraction grating 15 having a sawtooth cross-sectional shape can be obtained. Next, by sequentially removing the resist grating mask pattern 13 and S i 02 FJ20, the pattern shown in FIG.
), a diffraction grating 15 formed on a substrate 11 can be obtained.

According to the above-described method, the following effects occur.

In other words, since the resist interface exists in a position where it is easily exposed, the drawbacks of three-beam interference exposure method, which are inherently unstable, can be covered to some extent, and a resist grating mask pattern that is uniform and highly reproducible can be easily obtained. .

In addition, 5iO was used as a mask for etching the substrate.
Since 21Q is used, it has excellent adhesion to the substrate and acid resistance, and deep etching of the substrate can be performed very stably.

Although the present embodiment has been explained using a GaAs M plate, it goes without saying that the present invention can also be applied to other single crystal substrates, such as InP, Si, etc.

In a three-beam interference exposure system, light incident on the substrate is strongly reflected depending on the angle of incidence due to the difference in refractive index between the substrate and the photoresist film.

Figure 2 (a) shows the reflectance R from the substrate depending on the incident angle θ.
shows how it changes. In this case, as shown in FIG. 2(b), a Ga AS I plate is used as the substrate 11, A Z-1350 is used as the photoresist film 12, and a laser beam (S polarized light) with an exposure wavelength of 3250 A is used as the laser beam 14. . When attempting to fabricate a diffraction grating with a fine period, for example, when the period is 2600A, the incident angle is 39° and the reflectance reaches nearly 40%.

Due to the interference between the incident wave and the reflected wave from the substrate, a strong standing wave is generated on the substrate surface. The actual exposure process will be performed using this standing wave.

The intensity distribution of the electric field of this standing wave changes periodically in a direction perpendicular to the substrate surface, that is, within the photoresist film.

FIG. 3 shows how the intensity distribution l of the standing wave changes depending on the distance from the substrate. The substrate 11 is a GaAs substrate and a photoresist! ! J12 is AZ-1350,
As the laser beam 14, the exposure wavelength is 3250A and the period is 2.
A condition of 600^ is used. As is clear from FIG. 3, the substrate 11 and the photoresist l! At the interface with photoresist pattern 12, the light intensity is minimum, and the photoresist pattern 12
It changes periodically inside. In other words, the region near the substrate interface, which will be a problem in the subsequent chemical etching process, is the region most difficult to be exposed to light.

Let us now consider the configuration of an embodiment as shown in FIG.

As the first layer 20, an insulating film having approximately the same refractive index as the photoresist film of the second layer 12 is used. Therefore, the first
The reflection of light at the interface between the layer 20 and the second layer 12 can be ignored, and the intensity distribution of the standing wave can be considered to be the same as in the case of a single resist layer.

The thickness of the second layer 12 is set so that the interface between the first layer 20 and the second layer 12 exists near the region where the intensity distribution of the standing waves is maximum. In other words, the interface between the photoresist film as the second layer 12 and the insulating film as the first layer 20 is close to the region where the intensity of light is maximum. This facilitates exposure at the resist interface even in unstable holographic exposure, making it possible to obtain a resist grating mask with good controllability.

Effects of the Invention As described above, the present invention enables the fabrication of a diffraction grating with a fine period for use in optical integrated circuits, which can be produced uniformly, with good reproducibility, and easily. is extremely large.

[Brief explanation of drawings]

FIGS. 1(a) to (f) are cross-sectional views for explaining a method for producing a diffraction grating according to an embodiment of the present invention, and FIGS. 2(a) and (b)
Figure 3 is a relationship diagram showing the incident angle of laser light on the substrate and its dependence on reflectance, Figure 3 is a relationship diagram showing the intensity distribution of the standing wave depending on the distance from the substrate, and Figure 4 is the relationship diagram showing the intensity distribution of the standing wave depending on the distance from the substrate. A cross-sectional view showing the state of the insulating film and photoresist film, FIG.
) to (d) are cross-sectional views for explaining a conventional method for producing a diffraction grating. DESCRIPTION OF SYMBOLS 11... Substrate, 12... Photoresist Im (second layer), 13... Resist grating mask pattern, 1
4.14'... Laser light, 15... Diffraction grating, 20
... Insulating film (first layer) Agent Yoshihiro Morimoto Figure 1 Figure 2 OJo 60 Incident light 0 (it) Figure 8 Ojoo 1000 1 JOO order 4 (A
) Figure 4 Figure 5 Figure 4'

Claims (1)

[Claims] 1. When forming a diffraction grating having a desired period on a substrate surface, a first layer made of an insulating film and a second layer made of a photosensitive medium are sequentially laminated on the substrate surface. A method for manufacturing a diffraction grating, characterized in that an interference pattern is transferred onto the substrate surface using a two-beam interference exposure method. 2. As the insulating film of the first layer, a material having a refractive index almost equal to that of the photosensitive medium of the second layer is used, and the film has an optimal film thickness determined by the period of the interference pattern during interference exposure. A method for manufacturing a diffraction grating according to claim 1, characterized in that: