JPH07128510A - Formation of diffraction grating - Google Patents

Abstract

PURPOSE:To provide the process for formation of diffraction gratings capable of forming >=2 kinds of the fine diffraction gratings varying in periods on a substrate by one time of interference exposing. CONSTITUTION:At least two kinds of the diffraction gratings; the diffraction grating 5 of the first period and the diffraction grating 6 of the second period are formed on the substrate 1. First, the diffraction grating 2 of the first period is formed. Next, the diffraction grating 2 is coated with a material 3 different from the grating forming material. Patterns 4 are formed on this different material 3. The diffraction grating 6 of the second period different from the first period is formed by etching including simultaneous etching of the different material 3 and the grating forming material 1 of the diffraction grating.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a diffraction grating, and more particularly to a method for producing a diffraction grating in which diffraction gratings having different periods are formed on a substrate.

[0002]

Diffraction gratings are used in the field of optoelectronics such as filters, optical couplers, and distributed feedback type (DF).
B) Used in various optical circuit devices such as lasers and distributed Bragg reflection (DBR) lasers. Especially DFB, D
A diffraction grating formed in a wavelength tunable semiconductor laser typified by a BR laser is used as a resonator of a laser,
The period, shape, and depth of the diffraction grating are laser characteristics (oscillation threshold,
It is an important factor that determines the coupling coefficient, etc.), and it is an important issue to make a highly accurate diffraction grating in such a laser with good controllability.

Conventionally, the diffraction grating is formed by a two-step method of forming a grating photoresist mask and etching. Here, since the period of the diffraction grating in the field of optoelectronics is as small as about 0.1 to 1.0 μm, the conventional photolithography technique cannot be applied to the production of the photoresist mask. Therefore, the holographic exposure method has been generally used. This is an exposure method using the interference of laser light, and its process will be described with reference to FIG.

In this method, first, as shown in FIG. 2 (a), a photoresist 22 is applied to a substrate 21, and two laser beams 23 and 24, which are sufficiently parallel rays, are applied in two directions (in the illustrated example, perpendicular lines). On the other hand, the photoresist 22 is periodically exposed by irradiating it from two directions forming an angle of θ) to form an interference fringe, which is developed and baked. As a result, a lattice-shaped photoresist mask 25 as shown in FIG. 2B is created. Next, the photoresist mask 25 is used as an etching mask for etching (wet or dry) as shown in FIG. 2C, and then the etching mask is peeled off to form a periodic structure (diffraction grating) on the substrate 21 as shown in FIG. 2D. ) 26
Is transcribed.

In the holographic interference exposure method, if the incident angle of the two laser beams is θ and the wavelength of the laser beams is λ, the lattice spacing Λ (FIG. 2) that can be created.
(See (b)) can be expressed as Λ = λ / 2sin θ. As an exposure laser, an Ar laser (λ = 351 nm) or H
An e-Cd laser (λ = 325 nm) is suitable. Λ ≦
A He—Cd laser is used to make a mask of about 0.25 μm.

When two types of diffraction gratings having different periods are formed on a substrate, holographic interference exposure is performed twice by changing the incident angle θ of the two laser beams.

[0007]

However, DFB,
The following problems have been encountered in the two types of diffraction grating fabrication methods for DBR semiconductor lasers having different periods. That is, in the conventional holographic interference exposure method, only one kind of diffraction grating with a period can be obtained by one exposure, and when two kinds of diffraction gratings with different periods are formed on a substrate, the conventional method is used. It is necessary to perform two times of interference exposure by changing the angle of the holographic interference exposure method. Therefore, there is a problem that it is very difficult to match the phase between the diffraction gratings.

Therefore, the present invention has been made in view of the above problems.
Regarding a method for producing a diffraction grating used for a resonator such as an FB or DBR laser, a method for producing a diffraction grating capable of forming two kinds of fine diffraction gratings having different periods on a substrate by one-time interference exposure is provided. The purpose is to do.

[0009]

The present invention provides a first substrate on a substrate.
A method of forming at least two types of diffraction gratings, that is, a diffraction grating having a period of 2 and a diffraction grating having a second period, the step of forming the diffraction grating of the first period, A second step different from the first period by a step of coating with a material, a step of forming a pattern on the foreign material, and an etching including etching the exposed foreign material and a grating forming material of the diffraction grating at the same time;
And a step of forming a diffraction grating having a period of.

More specifically, the second period is half of the first period, and the step of forming the diffraction grating of the first period comprises the steps of applying a photoresist on a substrate and The step of exposing the photoresist by interference and the step of performing the first etching by using the photoresist subjected to the interference exposure as a mask to obtain the diffraction grating of the first period, the different material is photoresist, or the different material. Simultaneously etching the grating forming material of the diffraction grating may be dry etching of the entire surface as the second etching.

[0011]

In the method of forming the diffraction grating of the present invention, the interference exposure is performed only when the diffraction grating of the first period is obtained.
If the process can be short-circuited and dry etching is performed,
No irregularity of the diffraction grating shape occurs. Further, if the different material and the grating forming material of the diffraction grating are simultaneously etched by dry etching, the plane orientation is not limited and the irregular shape is eliminated. Therefore, the stability of the pattern accuracy of the diffraction grating is improved, and the yield is also improved.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a process chart showing the steps of one embodiment of a method for producing a diffraction grating according to the present invention.

First, the first diffraction grating 2 having a desired period is formed on the semiconductor substrate 1 by the conventional holographic interference exposure method described with reference to FIG. 2 (FIG. 1A). The details are as follows.

The semiconductor substrate 1 is cleaned with a surfactant, and then ultrasonic cleaning with an organic solvent is repeated 2-3 times, and then N
₂ blow dry and heat treatment at 200 ℃ for 30 minutes
To do. Subsequently, a photoresist layer for two-beam interference exposure (trade name AZ-1350J: Hoechst, diluted to a desired degree) is applied and formed on the entire surface of the substrate 1 and dried at 80 ° C. Thickness of photoresist layer is 600Å-8
A value of about 00Å is appropriate. Next, an incident angle θ corresponding to a desired period Λ is applied to the photoresist layer by a two-beam interference exposure method using a He-Cd laser (wavelength λ = 325 nm).
To expose. After that, the photoresist layer is developed with an AZ developing solution to form a resist pattern (diffraction grating) corresponding to the degree of photosensitivity according to the intensity distribution of interference fringes generated by interference exposure. Then, using the resist pattern as an etching mask, the semiconductor substrate 1 is wet-etched by using, for example, an H ₃ PO ₄ -based or NH ₄ OH-based etching solution, and dry-etched by using a C ₁₂ -based gas. By etching and removing the etching mask layer, the first diffraction grating 2 is obtained on the substrate 1 (FIG. 1A).

Next, a process of forming the second diffraction grating 6 having a half period of the first diffraction grating 2 will be described. at first,
The semiconductor substrate 1 having the first diffraction grating 2 is subjected to ultrasonic cleaning with an organic solvent 2 to 3 times, dried by N ₂ blow, and heat-treated at 90 ° C. for 30 minutes. Then, as an etching mask layer 3, a photoresist (product name AZ-
1350J: manufactured by Hoechst Co., Ltd.) is diluted to a desired value and applied and formed on the entire surface of the semiconductor substrate 1.
Drying is performed at 20 ° C. for 20 minutes (FIG. 1B).

Then, for example, photoresist (trade name OM
R-85: manufactured by Tokyo Ohka Co., Ltd.) is applied and formed on the entire surface of the etching mask layer 3 and dried at 80 ° C. for 20 minutes. At this time, the appropriate thickness of the photoresist is 1.0 μm.

Next, mask exposure is performed by using a normal photolithography technique. Further, development and rinsing are sequentially performed with a dedicated developing solution and a dedicated rinsing solution for the photoresist layer. As a result, a photoresist stripe pattern 4 is formed on the semiconductor substrate 1 (FIG. 1C).

Next, the exposed etching mask layer 3 is etched for a proper time using reactive ion beam etching (RIBE) with C1 ₂ gas using the stripe pattern 4 of photoresist as an etching mask. The condition at this time is that the ultimate vacuum is 2 × 10 ⁻⁶ T
orr, C1 ₂ flow 15SCCM or less, etching pressure ^{7.5 × 10 -4 Torr~1.5 × 10 -3} Torr, an ion extraction voltage 400V, the etch rate of the semiconductor substrate 2 in the etching conditions 1600 Å / min The selection ratio with respect to the resist mask (etching mask) 3 was about 4: 1.

In the above etching, the etching mask layer 3 applied and formed on the entire surface is first etched and reduced in the etching process (see FIG. 1D), and the lower semiconductor substrate 1 is exposed. Due to the difference in the selection ratio between the etching mask layer 3 and the semiconductor substrate 1, the rate of the etching rate is limited at the boundary between the etching mask layer 3 and the semiconductor substrate 1, and the unique etching proceeds (FIGS. 1E and 1F). )reference). In this way, the second diffraction grating 6 having a period half that of the first diffraction grating 2 initially formed is formed.

Finally, the remaining etching mask layer 3 and the photoresist stripe pattern 4 are removed, and the semiconductor substrate 1 is diffracted into two types: a diffraction grating 5 of a first period and a diffraction grating 6 of a second period. A lattice is obtained (FIG. 1 (g)).

As described above, in this embodiment, the diffraction grating obtained by the holographic interferometry of the prior art is used as the diffraction grating of the first period, the first diffraction grating 2 is coated with the photoresist over the entire surface, and the second diffraction grating is formed. A simple method of forming a mask with a region for forming a periodic diffraction grating as a window, and then performing dry etching as it is without exposing or developing the mask, (1/2 cycle)
The diffraction grating can be obtained accurately and stably.

Thus, two types of diffraction gratings having different periods can be formed on the substrate with high accuracy and stability. Particularly, the diffraction grating of the second period has a half period of the period of the diffraction grating of the first period. Therefore, by setting the period of the first period diffraction grating to the short wavelength (λ = 0.8 μm) DFB and the secondary diffraction grating for the DBR semiconductor laser (period = 2500 Å), the normal holographic interference It is possible to create a first-order diffraction grating (period = 1300Å) in the short wavelength (λ = 0.8 μm) band, which could not be created in the air by the exposure method.

By the way, the present invention is not limited to the embodiment described above. For example, the sectional shape of the diffraction grating 2 having the first period is triangular, but it may be rectangular, trapezoidal, sine wave or the like. It may be determined according to the finally required specifications, and in that case, the forming method is essentially the same. Further, in the above-mentioned embodiment, photoresist is used as the etching mask layer 3 of the diffraction grating 6 of the second period.
An oxide film or a nitride film of an iO ₂ or Si ₃ N ₄ film, or a metal material may be used. In short, the material of the etching mask layer 3 may be determined according to the etching conditions during dry etching. The substrate is not limited to the semiconductor substrate 1, but glass,
Optical glass may be used.

Further, although reactive ion beam etching (RIBE) was used in the etching method of the diffraction grating 6 of the second period, sputter etching, ion milling, reactive ion etching (RIE), etc. may be used, and etching gas may be used. it may be selected in accordance with the is not limited to C1 ₂ material.

If the fabrication method of the above embodiment is applied to the same substrate a plurality of times, the second period can be set to 1/4, 1/8, etc. of the first period, and three or more types of diffraction can be performed on the same substrate. A grid can be formed.

[0026]

As described above, according to the present invention, after the first period formed by the conventional method is formed, a different material such as photoresist is applied over the entire surface, and the second period is applied. After forming a mask having a window in a region where a diffraction grating is desired to be formed, the entire surface is directly etched by dry etching or the like to form a diffraction grating having a second cycle with a fine cycle. Therefore, two or more types of diffraction gratings having different periods can be easily formed on the substrate, and the process can be shortened. At the same time, it is not necessary to align the phases of the diffraction gratings having different periods, the stability of the pattern accuracy can be improved, the yield can be improved, and a fine diffraction grating can be produced. Furthermore, it is possible to improve the characteristics of a laser, a filter, etc. that generally use first-order diffraction in the short wavelength (λ = 0.8 μm) band.

[Brief description of drawings]

FIG. 1 is a cross-sectional view illustrating a process of an example of the present invention.

2A to 2C are cross-sectional views showing steps of a conventional method of forming a diffraction grating by holographic interferometry.

[Explanation of symbols]

 1 Semiconductor Substrate 2 First Period Diffraction Grating 3 Etching Mask Layer 4 Stripe Pattern 5 First Period Diffraction Grating 6 Second Period Diffraction Grating 21 Substrate 22 Photoresist 23, 24 Laser Beam 25 Photoresist Mask 26 Periodic Structure

Claims (2)

[Claims] 1. A diffraction grating having a first period and a second grating are formed on a substrate.
A method for producing at least two types of diffraction gratings having a period of, a step of forming a diffraction grating of a first period, a step of coating the diffraction grating with a material different from the grating forming material, Forming a pattern on the substrate, and forming a diffraction grating having a second period different from the first period by etching including simultaneously etching the different material and the grating forming material of the diffraction grating. A method for producing a diffraction grating characterized by. 2. The method for forming a diffraction grating according to claim 1, wherein the second period is half of the first period.