JPS6218559A - Mask for exposure - Google Patents

Abstract

PURPOSE:To prevent previously the adverse influence in a stepped part at exposure by installing a light shielding layer in the area close to the step part. CONSTITUTION:Since the light shielding layer 23 lies in the area close to the stepped part 11 including an oscillation center part C, only the area of the light shielding layer 23 cannot pass through a light beam when fluxes 5 and 6 are irradiated to expose a medium 4. Moreover only the area corresponding to the light shielding layer 23 on the medium 4 cannot form an interference moire. Even if the interference moire is not formed in the area corresponding to the stepped part 11, the diffraction grating at both sides are stepped out, and therefore there is no hindrance to oscillate laser. And the light of the part 11 causing scattered light cannot pass and does not reach the medium surface, whereby the interference moire will not be disturbed by the scattered light caused in the part 11.

Description

[Detailed Description of the Invention] [Summary] Providing a convex portion and a concave portion to change the thickness of an exposure mask,
When exposing a diffraction grating for a DFB laser, in order to prevent light from being scattered at the stepped portion between the convex portion and the recessed portion, a light blocking layer is provided at the stepped portion so that light cannot pass through the stepped portion.

[Industrial application field]

The present invention relates to an exposure mask suitable for manufacturing, for example, a diffraction grating for a DFB laser.

[Applications of exposure mask]

As shown in FIG. 3, the DFB laser has a diffraction grating 8 formed on a semiconductor chip 7, and an active layer 9 and an electrode 10 on top of the diffraction grating 8.
It has a structure in which When electricity is applied between the positive and negative electrodes of this laser, laser oscillation occurs in the diffraction grating 8 and active layer 9, and laser light is emitted.

However, simply forming a diffraction grating causes the phase relationship in the diffraction grating to shift, resulting in the inconvenience that two longitudinal modes of the DFB laser are generated. In order to solve this problem, it is known to shift the pitch of the diffraction grating 8 with the center C of laser oscillation as a boundary, as shown in FIG. 4, thereby shifting the left and right phase relationship in advance.

As a method for producing a diffraction grating with a phase difference like this,
The applicant of the present invention previously proposed an exposure method as shown in FIG. 5 in Japanese Patent Application No. 60-57455.

FIG. 5(A) is a sectional view showing the basic configuration, and FIG. 5(B) is an enlarged view of the main parts. 4 is a medium forming a diffraction grating, and a transparent mask 3 made of glass or the like is placed thereon.

This mask 3 has a step 11 between the convex portion l and the concave portion 2 on the oscillation center C, and the optical path lengths on both sides thereof are different. Alternatively, the refractive index may be different on the left and right sides with the oscillation center C as a boundary.

Two light beams 5 and 6 are irradiated onto the surface of the medium 4 through the mask 3. At this time, the light beams 5 and 6 are incident at an angle of 2.theta., and interference between the two light beams occurs on the medium 4. Further, the central axis A formed by the two light beams 5 and 6 is tilted by an angle φ with respect to the normal line ■, and the light beams are irradiated in an asymmetrical state.

As shown in FIG. 5(B), the thickness of the mask 3 varies across the stepped portion 11, with the thickness t2 on the right side being smaller than the thickness tl on the left side. Therefore, the optical path lengths are different on the left and right sides of the stepped portion 11, and the two light beams 5.6 are irradiated asymmetrically with an angle φ, so that the phase of the interference fringes is shifted with the stepped portion 11 as a boundary. As a result, the phase of the diffraction grating 8, which is formed by exposing the interference fringes of the two light beams, is also shifted across the stepped portion 11.

The mask 3 having this stepped portion 11 is actually manufactured by a method as shown in FIG. i.e. many DFBs at the same time
In order to manufacture the laser, a large number of stepped portions 11 are formed on the mask 3 at the same pitch as the laser dimension l. When two light beams 5.6 are irradiated, the optical path lengths on both sides are different with each step 11 as a boundary, and the incident direction of the light beams 5.60 is asymmetrically tilted by an angle φ with respect to the normal V. By irradiating, a diffraction grating whose phase is shifted with each stepped portion 11 as a boundary is formed on the medium 4.

After exposing to light to form a diffraction grating, the medium 4 is separated at the positions indicated by chain lines 12 . . . so that the stepped portion 11 is centered.

[Conventional technology and its problems]

By the way, in order to form an uneven surface consisting of convex portions 1 and concave portions 2, chemical processing, notching or tri-etching is performed with a mask 13 covering the position where the convex portions 1 are to be formed, as shown in FIG. It is conceivable to remove the mask 13 after forming the recess 2.

However, in this method, the stepped portion 11 becomes a gentle slope, and when the light beam 5.6 is irradiated for exposure, diffuse reflection and scattering occur, making it impossible to obtain the desired interference fringes.

Even when the lift-off method is used as shown in FIG. 8, there is a problem particularly in the finish of the stepped portion 11. In this figure, first, as shown in (A), a mask 1 is placed in advance at the position where the recess 2 is to be formed.
4, and from above, a film 15 made of the same material as the mask 3.
is formed by a method such as vapor deposition.

Thereafter, when the mask 14 is removed with a solvent, the film 15 above it is also removed, resulting in the state shown in (b). mask 14
The bottom surface of the recessed portion 2 after the removal becomes a surface with high surface accuracy. However, in the lift-off method, the thickness of the film 15 is 0.2 μm.
This is effective in cases where the thickness is about 2 to 3 μm, such as the product targeted by the present invention, but even if the mask 14 is removed, it is difficult to remove the film 15 thereon. Therefore, cracks 16 may occur in the stepped portion 11 between the upper removed portion of the mask 14 and the portion remaining as the convex portion 1, making it impossible to obtain a uniform finish. Therefore, even in the exposure mask manufactured by the lift-off method, the step portion 11 has a negative optical effect when the diffraction grating 8 is manufactured.

A technical object of the present invention is to eliminate such problems with conventional exposure masks and to prevent adverse effects at step portions during exposure.

[Means for solving problems]

FIG. 1 is a sectional view showing the basic principle of an exposure mask according to the present invention. The transparent exposure mask 3 has a stepped portion 11 between the convex portion 1 and the four portions 2;
A light blocking layer 23 that blocks exposure light is provided in a region near the stepped portion 11 including the step portion 11 .

[Effect]

As described above, since the light blocking layer 23 is provided in the region near the step portion 11 including the step portion 11, when the medium 4 is exposed by irradiating the light beam 5.6 etc., only the region of the light blocking layer N23 is provided. No interference fringes are formed as shown by 24 only in the area on the medium 4 corresponding to the light blocking layer 23 where no light can pass.

Even if no interference fringes are formed in the region corresponding to the stepped portion 11, the period of the diffraction grating is different between the left and right sides of the center line C, so there is no problem with the oscillation of the laser.

Since the light from the step portion 11 that causes the scattered light cannot pass through and does not reach the medium surface, the interference fringes are not disturbed by the scattered light generated at the step portion 11.

〔Example〕

FIG. 2 shows an example of a method of manufacturing an exposure mask having a light blocking layer in a step portion, and the manufacturing method will be explained in the order of steps shown in the figure.

(al) A light blocking layer 23 is provided in advance at a position corresponding to the stepped portion 11 on a quartz parallel substrate 17 having a thickness of 1 mm and both surfaces of which have been optically polished.

(bl) A first mask 19 is formed over each light blocking layer 23 at a position corresponding to the concave portion 2.For example, 700 strip-shaped AN vapor deposited films 19 are applied using photoresist lift-off. The width of the stripe is 30
The period is 0 μm, the interval is 300 tt m, and the period is 600 pm.

(C) A 5i02 film 18 is deposited on top of it by sputtering.
．． Laminated to a thickness of 14 μm. This may be done by a technique such as vapor deposition.

(d) Next, a photoresist is applied onto the film 18, and a resist pattern 21 is formed, leaving the photoresist only in the area above the Al film 19.

(e) A! on this resist pattern 21! After depositing 1,200 evaporated films, lift-off is performed using the photoresist pattern 21 previously applied, and the second layer of AI is applied only to the area where the first AJ film 9 is not formed. ! ! ! Leave 22.

(f) Reactive ion etching “RIE” (
025% CF4), the 5i02 film in the area where the second layer Al film 2 is not formed is etched to the surface of the first layer Al film 19. At this time, the Al film 2.19 serves as a mask,
The underlying 5i02 film 18 and quartz substrate 17 are not etched.

(gl Finally, by chemical etching, 1.+1 film 19
．． 22 is removed.

In this way, the light blocking layer 2 is placed in advance in the area corresponding to the stepped portion 11.
3 is formed, and then the film 18 for the convex portion 1 is formed thereon, so that the light blocking layer 23 is embedded in the completed exposure mask.

As the light shield N25b, a material that can withstand chemical etching of the first mask (Al film) 19 is used, such as nichrome. Pitch 300μ as explained in (b)
In a mask for producing phase difference interference fringes with a thickness of about 2 μm, it is effective to form the light blocking layer 23 with a pattern having a width of 2 to 3 μm.

In addition, both the four parts 2 and the convex part 1 produced by the method of the example,
During etching, since it is protected by AAA19.20, AβM'ij! 19. After chemical etching in step 20, an optical surface with high surface precision is obtained.

By using the quartz mask manufactured by this method as the transparent mask 3 shown in FIG. 5 and performing exposure as shown in FIG. 1, scattered light from the stepped portion 11 can be prevented and the surface accuracy can be Interference fringes are formed only in the regions of the high convex portions 1 and concave portions 2, and a highly accurate diffraction grating 8 is obtained.

As in the embodiment, if the light blocking layer 23 is provided in advance and then a film is formed on it, the light blocking layer 23 will be embedded in the exposure mask, but after forming the uneven surface, Part 1
A light-blocking film can also be formed on 1.

〔Effect of the invention〕

As described above, according to the present invention, the light blocking layer 23 is provided in the region corresponding to the stepped portion 11 in the exposure mask having the uneven surface. Therefore, when the light beam 5.6 is irradiated from the back side of the uneven surface to create interference fringes, the light beam 5.6 is blocked by the light blocking layer 24 and cannot reach the step part 11, so the light beam 5.6 cannot reach the step part 11 of the exposure mask. Even if the finish of 11 is poor, the problem that disturbance occurs in the phase difference diffraction grating and reduces the efficiency of laser oscillation can be solved.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view explaining the basic principle of the exposure mask according to the present invention, FIG. 2 is a cross-sectional view showing an example of the manufacturing method of the exposure mask in the order of steps, and FIG. 3 is a cross-sectional view of a DFB laser. Fig. 4 is a cross-sectional view showing a phase difference diffraction grating of a DFB laser, Fig. 5 is a cross-sectional view showing a method of forming a phase difference diffraction grating, and Fig. 6 is a cross-sectional view showing a method of forming a large number of phase difference diffraction gratings at the same time. FIG. 7 is a sectional view showing a typical method for forming a recess, and FIG. 8 is a sectional view showing a method for forming a recess by lift-off of a thick film. In the figure, 1 is a convex part, 2 is a concave part, 8 is a diffraction grating, and 17
18 is a substrate, 18 is a film, 19 is a first mask, 22 is a second mask, 23 is a light blocking layer, and 24 is a region where no interference fringes are formed. Patent Applicant: Fujitsu Limited Agent, Patent Attorney, Hoki Aoyanagi Invention Compliance with X-Exposure Net Mask 6ri Figure 1 'DFB Sea S-i Figure 3 Erroneous Figure 4 Figure 5 Figure 6 Figure 7: Lift-off using the force method with a hollow concave shape Figure 8

Claims (3)

[Claims] (1) An exposure mask having a stepped portion (11) between a convex portion 1 and a recessed portion 2, in which light for exposure is emitted near the stepped portion (11) including the stepped portion (11). Light blocking layer (
23) An exposure mask characterized by being provided with. (2) The exposure mask according to claim (1), wherein the light blocking layer (23) is a metal film. (3) The exposure mask according to claim (1), wherein the light blocking layer (23) is entirely or partially embedded inside the exposure mask. .