JPS627001A - Production of diffraction grating - Google Patents

Abstract

PURPOSE:To obtain a diffraction grating for generating two light flux interference exposure simpler than electron beam exposure and inverting recessed and projected phases periodically by using metals as the 1st and 3rd materials, using only one kind of photoresist for the 2nd material and then lifting off the photoresist. CONSTITUTION:After evaporating a gold thin film 12 to be the 1st material on the surface of a substrate 1, photolisographing and etching are applied to the film 12 so as to leave the film 12 only an area A, only a micropositive photoresist film 13 to be the 2nd material having high resolution is applied to the area A and an area B including no film 12 and uniform 2-light flux interference exposure is applied to the film 13. The exposed film is developed to remove only the exposed part to constitute a diffraction grating by the residual film 13 and the film 12 is etched by using the diffraction grating as a mask. While insulating the surface of the grating from its gaps, a silver thin film 14 to be the 3rd material is evaporated, the area B is covered with a resist film 15 and then etched to leave the grating-like film 12 on the area A, and then the film 15 is removed to leave the grating-like film 14 on the area B.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a method of manufacturing a diffraction grating consisting of periodic irregularities using two-beam interference exposure. The present invention relates to a method for manufacturing a diffraction grating having a structure in which the phase of concave and convex portions is inverted.

(Prior art and its problems) Diffraction gratings consisting of periodic concavities and convexities reflect or pass only light of a desired wavelength, so in the field of optical communication, they are used as filter or distributed feedback semiconductor lasers (hereinafter referred to as It is used inside the rDFB laser (abbreviated as “rDFB laser”).

Among them, the DFB laser, which has a diffraction grating in or near the light emitting region, emits light in a single axis mode.
It has been in the spotlight as a light source for optical communications, and various proposals have been made to date. Particularly recently, inverting the phase of the unevenness near the center of the diffraction grating has attracted attention as a way to achieve more stable single mode operation.

As described above, the oscillation wavelength of the FB laser is determined by the period of the unevenness of the diffraction grating, and stable operation also depends on the manufacturing precision of the diffraction grating. Therefore, the fabrication accuracy of the diffraction grating is DFB
This will affect the characteristics of the laser.

Before describing a conventional method for manufacturing a diffraction grating having a structure in which the phase of the concave and convex portions is inverted, a method for manufacturing a diffraction grating having a structure in which the phase of the concave and convex portions is not inverted will be described first.

FIG. 1 is a diagram showing the principle of manufacturing a uniform diffraction grating by a conventional two-beam interference exposure method. Wavelength λ. For example, He-C
d Laser light 3 is split into two by a half mirror 4, each split light 3 is reflected by a mirror 5, and the combined wave of the split light 3 is placed on a substrate 1 using, for example, a positive type photoresist as shown in the figure. A diffraction grating can be formed by exposing the crystal surface coated with the film 2 to an interference pattern generated when the crystal surface is irradiated, and performing development and etching. Here, the period Δ of the unevenness can be obtained by the following equation, where α is the incident angle of the laser beam 3.

On the other hand, as a method for manufacturing a diffraction grating having a structure in which the phase of the diffraction grating is inverted at the center of the laser, there is electron beam scanning exposure using computer control. This method exposes the portions corresponding to the grooves of the diffraction grating by sequentially scanning and irradiating them with an electron beam, but it can be applied when the period of the diffraction grating is large, but when the period of the unevenness is When the period is small (approximately 2,000 people), such as a first-order diffraction grating, which is half the wavelength λ of light in the crystal, the resolution reaches its limit and manufacturing becomes substantially difficult.

Furthermore, since the electron beam exposure method involves individual sequential scanning, it takes a considerable amount of time to scan the entire surface of the diffraction grating pattern, making it difficult to apply this method to mass production processes.

Next, problems when manufacturing a diffraction grating having a structure in which the phases of concavities and convexities are reversed in adjacent regions using two-beam interference exposure will be described.

(1) Figure 2 is a schematic diagram of the case where a diffraction grating with a phase inversion, that is, a 180° phase shift, is manufactured by the two-beam interference exposure described above, and regions A and B are formed using a metal mask. This is a separate exposure method. The figure shows a case where periodic irregularities are manufactured in region A, and at this time region B is covered with a metal mask 6 having a thickness t (about 50 μm).

Note that a gap d (about several μm) is usually provided on the photoresist film 2. The interference pattern is shown closest to region B, but as is clear from the figure, a region C is created where the laser beam 3 is not irradiated due to the influence of the thickness of the metal mask 6, that is, a region C where no unevenness is formed. Similarly, even if a metal mask 6 is applied to region A and two-beam interference exposure is performed to region B, a region C is formed in which no unevenness is formed, and as a whole, no unevenness is formed over twice as much as area C.

For example, if the period A of the unevenness of the diffraction grating is 2400, H
e - Wavelength λ of the Cd laser. If there are 3,250 people, then the incident angle α is as follows.If the mask thickness t is 50μ- and the gap is d, then the area C where periodic unevenness is not produced is C=(t+d
) tanα−t Htanα= 47 (/Jn).

Therefore, by two times of two-beam interference exposure, the area twice the area C where no unevenness is formed is 94 [μm], and since the total length of the light emitting area is usually about several hundred [μI], D
The operating current of the FB laser increases, and single wavelength operation also becomes unstable. As a solution to this problem, a slight improvement can be made by reducing the thickness t of the metal mask 6 or by providing a slope on the upper surface edge of the inner end of the metal mask 6, but this still results in a region C where no unevenness is formed.

(2) In order to invert the phase of the unevenness by 180 degrees when first exposing area A and then exposing area B, the substrate 1 must be accurately moved by half the period of the unevenness (approximately 1000 people). There must be. However, about 1000 people (0.1 μm)
It is extremely difficult to move the substrate 1 accurately, and it is also extremely difficult in terms of reproducibility.

As described above, it is difficult to manufacture a diffraction grating having a structure in which the phase of periodic asperities is inverted using conventional two-beam interference exposure, and even electron beam exposure has problems in terms of mass production.

(Objective and Features of the Invention) The present invention has been made to solve the above-mentioned conventional drawbacks, and uses two-beam interference exposure, which is simpler and easier to mass-produce than electron beam exposure, to periodically An object of the present invention is to provide a method for manufacturing a diffraction grating that can realize a diffraction grating having a structure in which the phases of the concave and convex portions are inverted.

The feature of the present invention is to use metal as the first and third materials, use only one type of photoresist as the second material, and use lift-off to create a diffraction grating with a highly accurate phase inversion. There is a particular thing.

Another feature of the present invention is that dielectrics are used as the first and third materials, only one type of photoresist is used as the second material, and the diffraction pattern is precisely phase-inverted by using lift-off. The purpose is to create a grid.

(Structure and operation of the invention) The present invention will be described in detail below using the drawings.

(Example 1) FIG. 3 is a first example according to the present invention, which schematically shows the manufacturing process of a diffraction grating having a structure in which the phase of concavities and convexities is inverted.

In this example, a case will be explained in which gold (metal) is used as the first material, a photoresist with excellent resolution is used as the second material, and silver, which is a metal different from the first material, is used as the third material. .

(1) First step (a) Gold film 12, which is the first material, is formed on the entire surface of the substrate 1.
After depositing the thin film 12 by vapor deposition or the like, the gold film 12 is left only in the region A by ordinary photolithography and etching. This is one of the features of the present application, in which a metal that can form regular irregularities even when etched or the like and is easy to process is provided in one region. As the second material, a photoresist film with excellent resolution, for example, a microposit photoresist film 13 (hereinafter referred to as "P film 13") is used.
”) and uniform two-beam interference exposure is performed. That is, in the present invention, the photoresist film 1 with excellent resolution is
Just use the type. Note that the black portions in the figure are exposed.

(C) Next, development is performed to remove the exposed portion to form a diffraction grating of the P film 13.

(2) Second step (dl Using the P film 13 as a mask, the thin film of the gold film 12 in area A is etched with an etching solution of KI:h.

(el) Next, a silver film 14 is deposited as a third material.

At this time, by performing exposure and development so that the shape of the P film 13 becomes a slightly inverted trapezoid, a silver film 14 is formed on the side surface of the P film 13.
The silver film 14 and the silver film 14a can be attached to the substrate l and the P film 13, respectively, as shown in the figure.

(3) Third step (r) A protective photoresist film 15 (hereinafter referred to as "F film 15") is formed in the gi region B by a normal photolithography method, and the P film 13 in the region A and the Membrane 13
The silver film 14a attached thereon is simultaneously removed by lift-off. Note that even if the silver film 14a in region A remains, it will be removed in the next step (shu), so lift-off may not be used here.

(The silver film 14 adhering to the substrate 1 in the phantom area A is etched and removed using dilute nitric acid. At this time, the dilute nitric acid
2, a diffraction grating is formed by the gold film 12 in region A.

(h) The F film 15 and the P film 13 in region B are removed by performing development or the like after exposing the entire surface to acetone or light.

Incidentally, in this step, the silver film 14a attached to the P film 13 is also removed at the same time using a scraper, and as a result, a diffraction grating of the silver film 14 is formed in the region B.

Through the above steps, in region A, the diffraction grating of the gold film 12 becomes
In region B, a diffraction grating of the silver film 14 is formed, and the phases of the periodic irregularities are reversed.

(4) Fourth step (1) Substrate 1 is chemically etched using the metals of region A and region B as masks, and each metal is removed, thereby creating a diffraction grating in which the phase of the unevenness is reversed at the center of substrate 1. can be obtained.

As described above, the present invention uses gold 12 and silver 14 as the first and third materials, and the P film 13 with excellent resolution as the second material, and performs lift-off etc. in a simple and convenient manner. A diffraction grating with phase inversion that is highly mass-producible can be manufactured.

(Example 2) Figure 4 shows a second example of the present invention, and describes a manufacturing process using a dielectric such as a SiN film as an auxiliary material (hereinafter referred to as "auxiliary material") for the second material. This is what is shown. By using this auxiliary material, the process described in Example 1 (
This facilitates the lift-off of h).

In the following explanation, the same work steps as in the above embodiment will be briefly described.

(1) First step (a) After leaving a thin film of the gold film 12 only in area A as in the example process, the entire surface of the substrate I is coated with an auxiliary coating, which is a feature of this example, by plasma CVD or ECR. A SiN film 16 is deposited as a material.

佽) As in Example 1, PM13, which is the second material, is applied to the entire surface of region A and region B, and two-beam interference exposure is performed.

(C) Similarly to Example 1, a diffraction grating of the P film 13 is formed.

(2) Second step (dl) Wet etching with ammonium fluoride or dry etching with Freon
Etch the film 16.

(e) Using the SiN film 16 as a mask, the gold film 12 in region A is etched with an etching solution of Kl:It.

(f) Similar to step (e) in Example 1, the third
A silver film 14 made of the same material is vapor-deposited. However, Example 1
The difference is that in this embodiment, instead of the P film 13, it is deposited on the surface of the SiN film 16, which is an auxiliary material, and the rest is exactly the same.

(3) Third step (gl) As in Example 1, F film 15 is formed in region B. In Example 1, P film 15 is similar in material to F film 15.
Although the film 13 is protected, in this example, the SiN film 16, which is an auxiliary material and is made of a completely different material from the F film 15, is protected, so that the lift-off performed in step [13] described later is facilitated.

(h) The silver films 14 and 14a in region A are etched with dilute nitric acid, and the SiN film 16 is further removed with ammonium fluoride or the like. At this time, since dilute nitric acid and ammonium fluoride do not attack gold, a diffraction grating of gold M12 is formed in region A.

(1) After removing the F film 15 in region B with acetone etc., remove the remaining SiN film 1 with ammonium fluoride etc.
Remove 6. In this step, the upper film 14a on the SiN film 1 is removed by lift-off, and a diffraction grating of the silver film 14 is formed in region B. Therefore, in regions A and B, diffraction gratings of the gold film 12 and the silver film 14 in which the phases of periodic irregularities are reversed are formed in the same manner as in the first embodiment.

(4) Fourth step 0) In the same manner as in Example 1, a diffraction grating in which the phase of the concave and convex portions is reversed is obtained on the substrate 1.

As described above, by using the lift-off method according to this embodiment of the present invention, a structure in which the phase of the concaves and convexes is reversed by conventional uniform two-beam interference exposure and etching can be created by using only one type of photoresist. Although gold and silver are used as the first and third materials, other metals that can be selectively etched may be used. Further, a dielectric material may be used as the first material and the third material.

Furthermore, in the example, gold or silver was directly deposited on the substrate, but in order to improve adhesion, an extremely thin metal film such as chromium was interposed between them, and a step of etching the chromium was added as necessary. Good too.

(Example 3) FIG. 5 is another example according to the present invention, and is a schematic diagram of a manufacturing process using a dielectric as the first material and the third material, and a photoresist as the second material.

(1) First step (a) After depositing a dielectric material such as a Sing film 17 as a first material on the substrate l by ECR method or the like, a second step is performed in the same manner as in step (b) in Example 1. The material P
A film 18 is applied and uniform two-beam interference exposure is performed.

) As in the previous example, P was applied to the entire surface of areas A and B.
Form a diffraction grating for one day.

(C) After dry etching with Freon or the like, the entire SiO□ film 17 is etched, and then the P film 18 is removed. Therefore, the first material S is applied to the entire surface of areas A and B.
A diffraction grating of iO□1)1)7 is formed.

(2) Second step (dl In the Ski area B, the F film 19, which is the second material system, is formed as in the previous example.
(e) Next, P film 18 may be used instead of 9.
A third material, dielectric, for example, a SiN film 20 is deposited over the entire surface of regions A and B. At this time, by using the ECR method with good directivity of raw material plasma, the SiO
□It is possible to minimize the adhesion of the SiN film 20 to the side walls of the entire film 17 1719, and to use Si with a composition similar to 5ixNa, which has a low etching rate with hydrofluoric acid, which will be described later.
An N film 20 can be formed.

(3) Third step (f) 5i of area A is etched using hydrofluoric acid etching solution.
(The diffraction grating of the h film 17 is removed. In this step, the SiN film 20 on the 5i01 film 17 is removed at the same time by lift-off. At this time, the SiO□ film 17 is Since the rate of all 17 lattices is about 10 times higher, all 17 lattices of the SiO□ film 17 are selectively removed.

(Illusion) The F film 19 is lifted off at the same time as the F film 19 in area B by acetone or by developing after exposing the entire surface.
The SiN film 20 above is also removed. Therefore, a diffraction grating made of the third material is formed in region A, and a diffraction grating made of the first material is formed in region B, and the phases of the periodic irregularities are inverted with each other.

In addition, in this embodiment, since lift-off of area B can be performed subsequent to lift-off of area A, work efficiency is extremely improved.

(4) Fourth step (h) SiN film 20 in area A, 5i0 in area B
By chemically etching the two films using each as a mask, it is possible to obtain a diffraction grating on the substrate 1 in which the phases of the concave and convex portions are reversed at the center.

In the above explanation, dielectrics were used as the first material and the third material, but if the first material is a dielectric,
A metal such as gold or silver may be used as the third material.

(Effects of the Invention) As is clear from the above description, since the present invention uses only one two-beam interference exposure, there is no need to move the substrate 1, and a metal mask 6 or the like is used during the two-beam interference exposure. Since there is no portion (region C) in which unevenness is not formed because it is not necessary, a diffraction grating with precise phase inversion can be manufactured. Furthermore, since it is sufficient to use only one type of photoresist with excellent resolution, it is possible to manufacture periodic irregularities with high precision. Therefore, D is stable and has good characteristics.
It can be applied to FB lasers, etc., and its effects are extremely large.

[Brief explanation of drawings]

Figure 1 is a layout diagram for explaining the principle of conventional two-beam interference exposure method.
FIG. 2 is a cross-sectional view illustrating an example of manufacturing a diffraction grating in which the phase of unevenness is partially reversed by conventional two-beam interference exposure;
FIG. 3 is a cross-sectional view for explaining the manufacturing process of a diffraction grating according to the present invention, and FIGS. 4 and 5 are cross-sectional views for explaining the manufacturing process of other embodiments of the present invention. DESCRIPTION OF SYMBOLS 1...Substrate, 2...Positive photoresist film, 3...He-Cd laser beam, 4...Half mirror-15...Mirror, 6...Metal mask, 1
2... Gold film (first material), 13.18... Microposit photoresist (second material), 14, 1
4a... Silver film (third material), 15.19... Protective photoresist, 16... SiN film (second material), 17... 5ift film, 20... SiN film .

Claims (4)

[Claims] (1) A thin film of a first material is formed in a first region on a substrate, and a diffraction grating made of the second material is formed on the entire surface of the substrate including the first region and a second region other than the first region. A first step of forming by a two-beam interference exposure method, and etching a thin film of the first material using the diffraction grating made of the second material as a mask, and then forming a thin film of a third material on at least the exposed substrate. a second step of depositing the protective photoresist and the third material in the second region after etching the third material in the first region by protecting the second region with a photoresist; a third step of removing the first material of the first region on the substrate using the diffraction grating of the first material in the first region and the diffraction grating of the third material in the second region as masks; and a fourth step of forming a diffraction grating in which the phases of the concave and convex portions are inverted with respect to each other in the second region. (2) Manufacturing the diffraction grating according to claim 1, wherein a metal or a dielectric is used as the first material and the third material, and a photoresist is used as the second material. Method. (3) a first step of forming a diffraction grating made of a first material on the entire surface of the substrate by two-beam interference exposure; and covering the diffraction grating only in a second region on the substrate with a second material; After that, a second step of depositing a third material on the entire surface, and removing the diffraction grating of the first material in the first region on the substrate and the second material on the second region. a third step of forming a diffraction grating of the first material in the second region and a diffraction grating of a third material in the first region; a fourth step of forming a diffraction grating on the substrate, using a diffraction grating made of a third material as a mask, in which the phases of projections and recesses are reversed in the first region and the second region; . (4) A dielectric is used as the first material, a photoresist is used as the second material, and a dielectric or a metal is used as the third material. A method for manufacturing a diffraction grating.