JPS6325659B2 - - Google Patents

Description

[Detailed Description of the Invention] [Summary] Protrusions and depressions are provided to change the thickness of an exposure mask, and when exposing a diffraction grating for a DFB laser, scattering light can be prevented at the stepped portion between the projections and depressions. Therefore, a light blocking layer is provided on the stepped portion to prevent light from passing 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 structure in which a diffraction grating 8 is formed on a semiconductor chip 7, and an active layer 9 and an electrode 10 are formed above it.
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, if a diffraction grating is simply formed, the phase relationship in the diffraction grating will shift, resulting in the inconvenience that two longitudinal modes of the DFB laser will occur. 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 manufacturing a diffraction grating having such a phase difference, the applicant of the present invention previously proposed a method for manufacturing a diffraction grating having a phase difference.
No. 57455, we proposed an exposure method as shown in Figure 5. In FIG. 5, A is a sectional view showing the basic configuration, and B is an enlarged view of the main parts thereof. 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 covers the oscillation center C
At the top, there is a step 11 between the convex portion 1 and the concave portion 2, 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.

2 on the surface of this medium 4 through the mask 3.
Two light beams 5 and 6 are emitted. At that time, luminous fluxes 5 and 6
is incident at an angle of 2θ, and the two light beams interfere on the medium 4. Moreover, the central axis A formed by the two light beams 5 and 6 is inclined by an angle φ with respect to the normal line V, and the light beams are irradiated in an asymmetrical state.

As shown in FIG. 5B, 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 t1 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 and 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. That is, in order to manufacture a large number of DFB lasers at the same time, a large number of stepped portions 11 are formed on the mask 3 at the same pitch as the laser dimension l. two luminous fluxes 5, 6
When irradiating, the optical path lengths on both sides are different with each stepped portion 11 as a boundary, and the incident directions of the light beams 5 and 6 are tilted by an angle φ with respect to the normal V, and the respective light beams are irradiated asymmetrically. A diffraction grating whose phase is shifted with the stepped portion 11 as a boundary is formed on the medium 4. After exposing to light to form a diffraction grating, the stepped portion 1
The medium 4 is separated at the positions indicated by the chain lines 12 so that the medium 4 is centered.

[Conventional technology and its problems]

By the way, in order to form the uneven surface consisting of the convex parts 1 and the concave parts 2, 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 beams 5 and 6 are 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 14 is formed in advance at the position where the recess 2 is to be formed, and then a film 15 made of the same material as the mask 3 is formed over the mask 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). The bottom surface of the recess 2 after removing the mask 14 becomes a surface with high surface accuracy. However, although the lift-off method is effective when the thickness of the film 15 is about 0.2 μm, it is effective when the thickness of the film 15 is about 0.2 μm.
When the thickness is about 3 μm, 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. Transparent exposure mask 3
has a stepped portion 11 between the convex portion 1 and the recessed portion 2, and a light blocking layer 23 for blocking exposure light is provided in a region near the stepped portion 11 including this stepped portion 11. It is being

[Effect]

In this way, since the light blocking layer 23 is provided in the area near the stepped part 11 including the stepped part 11, when the medium 4 is exposed by irradiating the light beams 5, 6, etc., only the area of the light blocking layer 23 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 area corresponding to the stepped portion 11,
Since the period of the diffraction grating is different between the left and right sides of the center line C, there is no problem with the oscillation of the laser.

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

〔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.

(a) 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 sides of which are optically polished.

(b) A first mask 19 is formed over each light blocking layer 23 at a position corresponding to the recess 2 . For example, a striped Al vapor deposition film 19 of 700 Å is deposited using photoresist lift-off. This stripe is 600μm with a width of 300μm and a spacing of 300μm.
Period.

(c) Sputter a SiO ₂ film 18 on top of it.
Layer 2.14μm. This may be done by a technique such as vapor deposition.

(d) Next, a photoresist is applied on the film 18 to form a resist pattern 21, leaving the photoresist only in the area above the Al film 19.

(e) An Al vapor deposited film of 1200 Å is deposited on this resist pattern 21, and the first
The second layer Al film 22 is left only in the area where the Al film 19 is not present.

(f) Reactive ion etching “RIE” (O ₂ 5%
Using CF ₄ ), the portion of the SiO ₂ film where the second layer Al film 22 is not present is etched to the surface of the first layer Al film 19 . At this time, the Al films 22 and 19 serve as masks, and the SiO ₂ film 18 and quartz substrate 17 below them are not etched.

(g) Finally, by chemical etching, the Al film 19,
22 is removed.

In this way, the light blocking layer 23 is formed in advance in the area corresponding to the stepped portion 11, and then the film 18 for the convex portion 1 is formed on it, so that when the mask is completed as an exposure mask, the light blocking layer 23 is formed in advance. 23 is embedded.

The light blocking layer 23 may be made of nichrome or the like.
A material that can withstand chemical etching of the first mask (Al film) 19 is used. As explained in (b), in a mask for producing phase difference interference fringes with a pitch of 300 μm and a thickness of about 2 μm, the light blocking layer 23 is
It is effective to apply a pattern with a width of ~3 μm.

Moreover, the concave part 2 and the convex part 1 produced by the method of the example
In both cases, during etching, the Al films 19, 20
Since the Al films 19 and 20 are protected by chemical etching, the optical surfaces become highly accurate.

The quartz mask manufactured using this method was
By using the transparent mask 3 as shown in the figure and performing exposure as shown in Figure 1, scattered light from the stepped portion 11 is prevented, and interference fringes are generated only in the regions of the convex portions 1 and concave portions 2 with high surface precision. 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, A light blocking film may also be formed in the portion 11.

〔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 beams 5 and 6 are irradiated from the back side of the uneven surface to create interference fringes, the light beams 5 and 6 are blocked by the light blocking layer 24 and cannot reach the step portion 11, so that the light beams 5 and 6 cannot reach the step portion 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 sectional view explaining the basic principle of the exposure mask according to the present invention, FIG.
A cross-sectional view of a DFB laser, Figure 4 is a cross-sectional view showing a phase difference diffraction grating of a DFB laser, Figure 5 is a cross-sectional view showing a method of forming a phase difference diffraction grating, and Figure 6 is a cross-sectional view showing a method for forming a phase difference diffraction grating. FIG. 7 is a cross-sectional view showing a typical method for forming a recess, and FIG. 8 is a cross-sectional view showing a method for forming a recess by thick film lift-off. In the figure, 1 is a convex part, 2 is a concave part, 8 is a diffraction grating, 17 is a substrate, 18 is a film, 19 is a first mask, 22 is a second mask, 23 is a light blocking layer, 24
indicate areas where no interference fringes are formed.

Claims (1)

[Scope of Claims] 1. An exposure mask having a stepped portion 11 between a convex portion 1 and a concave portion 2, in which a light shielding light for exposure is provided near the stepped portion 11 including the stepped portion 11. An exposure mask characterized by being provided with a blocking layer 23. 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 all or part of the light blocking layer 23 is embedded inside the exposure mask.