JPH02219001A - Production of diffraction grating - Google Patents

Abstract

PURPOSE:To form the diffraction grating of a short period from the diffraction grating having a two-fold period by forming the diffraction grating of the period two-fold the target period by interference exposing and etching and lifting off a 2nd etching mask which is deposited by vapor deposition, etc., using the photoresist layer of this time and then executing the etching again. CONSTITUTION:A photoresist film 2 is provided on the surface of a (100) GaAs substrate 1 and the first diffraction grating 4 is produced by the interference exposing method and etching. As an example, an He-Cd layer is used as the light source in the interference exposing and the incident angle of a luminous flux is set at a prescribed angle. As SiO2 film 3 is then deposited as the 2nd mask film by the vapor deposition, etc., on the surface of the substrate 1 to a prescribed thickness and thereafter, the photoresist film 2 and the SiO2 3 thereon are peeled by using acetone and the etching is executed again. The GaAs diffraction grating 5 of the period half the period of the first diffractiong rating 4 is obtd. when the SiO2 film 3 is finally removed by using hydrofluoric acid. The diffractiong rating 5 of the short period is easily produced in this way from the diffraction grating 4 having the two-fold period.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for forming a diffraction grating on a substrate, and particularly to a method for forming a short-period diffraction grating from a diffraction grating having twice the period. It is something.

[Conventional technology]

Conventionally, a method combining interference exposure and etching has been known as a method for forming a diffraction grating on a substrate. The period A of the diffraction grating that can be formed by this method is given by λ A = 2n sin θ. Here, λ is the wavelength in vacuum of the light to be interfered with, n is the refractive index of the medium on the substrate surface, and θ is the incident angle of the light beam onto the substrate surface.

Therefore, by appropriately selecting the wavelength and incidence angle of the light source, it is possible to fabricate a diffraction grating with an arbitrary period.

[Problem that the invention seeks to solve]

However, in the conventional interference exposure method, the period of the diffraction grating that can be manufactured is theoretically larger than 2 of the wavelength used when exposure is performed in the atmosphere (n=1).

For example, even when a He-Cd laser with a wavelength of 325 nm is used as a light source, the period of the diffraction grating is 0.16.
The diameter is 25 μm or more, and a first-order diffraction grating (A=0.12 to 0.13 μm) for GaAs systems cannot be obtained.

Furthermore, in order to form a short-period diffraction grating, it is necessary to increase the incident angle of the light beam, which causes a practical problem because the light utilization efficiency becomes low. In order to solve these problems, methods using prisms and liquid immersion methods have been attempted, but it is not easy to produce a short-period diffraction grating no matter which method is used.

Therefore, a method of fabricating a diffraction grating with a period of % from a diffraction grating with twice the desired period (A) was proposed and applied to distributed feedback (DFB) lasers (Applied P
hysics Letters, 51(2), 63.1
987).

As shown in FIG. 2, this is done by coating the entire surface of the diffraction grating 10 with a periodicity,
After this phenomenon occurs until it remains only in the recesses of the diffraction grating IO, the substrate is etched using the photoresist 11 as a mask. As a result, a first-order diffraction grating (Δ=0.128 μm) for the GaAs system is obtained.

However, in the above method, the thickness of the remaining photoresist must be controlled depending on the time of the phenomenon, and as a result, there is a problem that the yield is reduced.

[Means for solving problems]

In order to solve the above conventional problems, the present invention first forms a diffraction grating with a period twice the target period by interference exposure and etching, and then uses a photoresist layer as an etching mask. After lifting off the second etching mask deposited by evaporation, sputtering, etc. whose film thickness can be easily controlled, etching is performed again.

The thickness of the second etching mask used in the present invention must be smaller than the depth of the first diffraction grating in order to facilitate lift-off in the next step.

〔Example〕

The present invention will be described in detail below with reference to an example in which a diffraction grating with a period of 0.125 μm is fabricated on a GaAs substrate.

First, as shown in Figure 1 (al), (100) GaAs
A photoresist film 2 as a first mask is formed on the surface of the substrate 1.
The original diffraction grating 4 with a period of 0.25 μm was fabricated using the interference exposure method and etching using an ammonia-based etchant (NLOH + H, O□+11□0) as in the conventional method.
Create. - For example, H as a light source in interference exposure
An e-Cd laser (wavelength λ=325r+m) is used, and the incident angle of the light beam is 40.5'.

Note that, as the above-mentioned etching liquid, other etching liquids such as sulfuric acid type may be used in addition to the ammonia type etching liquid.

Next, as shown in FIG. The Sin on the film and the film 3 are peeled off.

When the first substrate etching is in the forward mesa direction, that is, the pattern is perpendicular to the (07M) plane, the cross-sectional shape is as shown in Figure 1 f.
As shown in a), it becomes a ■groove, and there is an undercut.

Therefore, even if a SiO□ film is deposited on this substrate, if the film thickness is smaller than the depth of the unevenness formed on the substrate l, the photoresist film can be easily peeled off, and the SjO□ remains only in the recesses of the diffraction grating.

When this is etched again, only the exposed convex portions are etched, so that the irregularities of the diffraction grating described above become irregularities with a half period of 0.125 μm (FIG. 1(d)).

Finally, when the SiO□ film 3 is removed using hydronic acid, the first
As shown in the figure (el), a GaAs diffraction grating 5 with a period of 0.125 μm, which corresponds to half of the original diffraction grating 4, is obtained.

Although the present invention has been explained above using a GaAs substrate as an example, the material of the substrate is not limited to 8βGaAs+InP.
It can also be applied to other semiconductor substrates, dielectric substrates such as LiNb0:+ and glass, or metals.

In addition, although SiO□ was used in the embodiment as the second mask film 3, it does not peel off at the recesses of the diffraction grating when the photoresist layer is peeled off, and has sufficient strength as a mask during substrate etching. Any substance that can be removed without damaging the substrate can be used as the mask film.

〔Effect of the invention〕

As described above, according to the present invention, a short-period diffraction grating can be easily produced from a diffraction grating with twice the period.

[Brief explanation of the drawing]

FIGS. 1(a) to 1(a) are sectional views showing step by step an embodiment of the present invention, and FIGS. 2(a) to dl are sectional views showing step by step a conventional method. DESCRIPTION OF SYMBOLS 1...Substrate, 2...Photoresist (first mask) 3...5i02 film (second mask), 4.5...
Diffraction grating. Procedural Amendment Figure March 2'i, 1999!

Claims (1)

[Claims] A step of forming a diffraction grating with a specific period on a substrate by etching using a photoresist as a mask, and forming a metal or dielectric material to a thickness equal to or less than the depth of the diffraction grating while leaving the photoresist. removing the photoresist together with the metal or dielectric material deposited on top of the photoresist; and etching the substrate using the metal or dielectric material remaining only in the recesses of the diffraction grating as a mask. A method for manufacturing a diffraction grating, comprising: forming a diffraction grating having a period that is 1/2 of the specific period.