JPH0374804B2 - - Google Patents

Description

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

(Prior art) Diffraction gratings consisting of periodic unevenness reflect or pass only light of a desired wavelength, so they are used as filters or distributed feedback semiconductor lasers (hereinafter abbreviated as "DFB lasers") in the field of optical communications. ) is used inside etc.

Among these, DFB lasers, which have a diffraction grating in or near the light emitting region, have attracted attention as a light source for optical communications because they emit light in a single-axis mode.
There have been various proposals in the past. 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.

The oscillation wavelength of such a DFB laser is determined by the period Λ of the unevenness of the diffraction grating, and stable operation also depends on the manufacturing precision of the diffraction grating. Therefore, the manufacturing precision of the diffraction grating influences the characteristics of the DFB 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 the structure of a uniform diffraction grating formed by a conventional two-beam interference exposure method. For example, a He-Cd laser beam 3 with a wavelength λ ₀ is split into two by a half mirror 4, and each split beam 3 is reflected by a mirror 5.
A diffraction grating is formed by exposing and etching the synthesized wave of the split light 3 to the interference light generated when the surface of the crystal coated with, for example, a positive type photoresist film 2 on the substrate 1 is irradiated as shown in the figure. be able to. Here, the period Λ of the unevenness is determined by Λ=λ ₀ /2sinα (1), 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. This method can be applied when the period Λ of the diffraction grating is large, but when the period Λ of the asperities is When the period Λ is small (approximately 2000 Å), such as in a first-order diffraction grating, which is half the wavelength λ of light in the crystal, the limit of resolution is reached and the structure becomes practically 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) FIG. 2 is a schematic diagram of a diffraction grating having a phase inversion structure manufactured by the above-mentioned two-beam interference exposure. This 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 (approximately 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, a metal mask 6 is applied to area A, and area B is
Even if two-beam interference exposure is performed, there is a region C in which no unevenness is formed, and as a whole, no unevenness is formed in an area twice as large as the area C.

For example, if the period Λ of the unevenness of the diffraction grating is 2400 Å and the wavelength λ ₀ of the He-Cd laser is 3250 Å, the incident angle α is α = sin ^-1 (λ ₀ /2Λ) = sin ^-1 (3250/480043 [degrees], and the mask thickness t is 50 μm and the gap is d.
Then, the region C where periodic irregularities are not produced
is C=(t+d)tanαt・tanα=47
[μm].

Therefore, by two times of two-beam interference exposure, the area twice the area C where no unevenness is formed is 94 [μm].
The total length of the light-emitting region is usually several hundred [μm].
Because of this, the operating current of the DFB laser becomes large and single wavelength operation becomes unstable. As a solution to this problem, it can be slightly improved 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.
After all, there is a region C where no unevenness is formed.

(2) In order to reverse 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 Λ of the unevenness (approximately 1000 Å). . But about 1000
It is extremely difficult to move the substrate 1 accurately by Å (0.1 μm), and it is also extremely difficult in terms of reproducibility.

As described above, it has been difficult to manufacture a diffraction grating having a structure in which the phase of periodic asperities is inverted using conventional two-beam interference exposure.

(Objectives 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.
It is an object of the present invention to provide a method for manufacturing a diffraction grating that can realize a diffraction grating having a structure in which the phase of periodic irregularities is inverted.

A feature of the present invention is that one of a negative type photoresist film (N film) and a positive type photoresist film (P film) is formed on the substrate in a first region A, and a second region separate from the first region is formed on the substrate. After forming a state in which a positive type photoresist film is formed on the other of the negative type photoresist film and the positive type photoresist film or on the negative type photoresist film in the region B, the first region of the substrate and Two-beam interference exposure is performed on the second area, and by utilizing the fact that the negative type and positive type development characteristics of the photoresist film are reversed, unevenness is created in each area in the first area and the second area. The purpose is to form a diffraction grating in which the phases of the two are reversed.

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

FIG. 3 is an embodiment according to the present invention, which schematically shows the manufacturing process of a diffraction grating having a structure in which the phase of the concave and convex portions is inverted.

(a) N film 7 is formed on substrate 1 by a conventional coating method.

(b) In order to remove the N film 7 in the B region without exposing the N film 7 to light, an auxiliary P film 2a is laminated on the N film 7.

(c) By normal mask exposure, the auxiliary P film 2a in the first region A of the diffraction grating is removed, and the N film 7 in the region A is also removed using the P film 2 in the second region B of the diffraction grating as an etching mask.

(d) The auxiliary P film 2a in region B is removed using an organic solvent or the like.

(e) Next, a P film 2 is formed on the entire surface of region A and region B. This step is the basis for manufacturing a diffraction grating having a structure in which the phase of the concave and convex portions is reversed by one uniform two-beam interference exposure, which will be described below. That is, there is no need to move the substrate as in the conventional case. Further, although a step is formed at the boundary between region A and region B, since the thickness of the N film 7 is very thin (usually about 0.1 μm), region C is almost never formed as in the conventional case.

(f) By performing uniform two-beam interference exposure, concave portions are formed in the exposed portions of the P film 2 where the interference light is reinforced, and convex portions are formed in the unexposed portions of the P film 2, resulting in the overall When viewed from above, periodic unevenness is formed as shown in figure f. However, at this stage, the phase of the unevenness between region A and region B is not reversed.

(g) Using the unevenness of the P film 2 as a mask, unevenness is formed on the substrate 1 in area A by chemical etching.

(h) After normal full-surface exposure is performed on the N film 7 in area B using the unevenness of the P film 2 as a mask, the P film 2 in areas A and B is removed by normal development processing, and the N film is further developed. . Note that the exposed portion of the N film 7 remains as a convex portion, so at this stage, the area A
In region B and region B, irregularities with inverted phases are formed.

(i) An additional P film mask 2b is applied only to region A by mask exposure, and in region B, irregularities are formed on the substrate 1 in region B by chemical etching using the irregularities of the N film 7 as a mask.

(j) Unevenness of N film 7 in region B and P film 2 in region A
By removing , a diffraction grating having a structure in which the phase of the concaves and convexes is reversed near the center of the substrate 1 can be obtained.

FIG. 4 shows another embodiment of the present invention, which schematically shows the manufacturing process of a diffraction grating having a structure in which the phase of the concave and convex portions is inverted.

(a) N film 7 is formed on the entire surface of substrate 1 using a conventional coating method.

(b) P film 2 is laminated on N film 7.

(c) Remove the P film 2 in area A. As a result, different types of photoresist films are laminated on the surface F of the region A and region B, which is a feature of the present application.

(d) Perform one two-beam interference exposure. Since the area A is the N film 7, the exposed areas become convex parts, and the unexposed areas become concave parts. on the other hand,
Since region B is the P film 2, unevenness is formed that is exactly opposite to that of the negative type. Therefore, area A
and region B have a structure in which the phase of the unevenness is reversed. In addition, although a step is created between region A and region B by the thickness of P film 2, since P film 2 is very thin (approximately 0.1 μm), region C where unevenness is not formed as in the conventional method is almost never created. .

(e) In the above (d), the N film 7 located in the concave portion of the P film 2 in region B is removed by normal N film development processing, and then the unevenness of the P film 2 is removed. Therefore, in region A and region B, N
The unevenness of the film 7 is formed as shown in (e). (however,
(f) Using the unevenness of the N film 7 as a mask, form unevenness on the substrate 1 by chemical etching, and then remove the N film 7 to form the desired diffraction grating. It is formed.

As is clear from the above steps, according to the present invention, by laminating different types of photoresist films on the surfaces of area A and area B, highly accurate unevenness can be achieved with one two-beam interference exposure. A diffraction grating having a structure in which the phase of the diffraction grating is inverted is manufactured.

FIG. 5 shows another embodiment of the present invention, which is another process method for laminating different types of photoresist films in areas A and B.

(a) N film 7 is laminated on substrate 1 by a conventional coating method.

(b) An additional P film 2a is laminated in order to remove the N film 7 in the B region without exposing the N film 7 to light.

(c) The additional P film 2a in the first area A of the diffraction grating is removed by normal mask exposure, and the N film 7 in area A is also etched using the additional P film 2a in the second area B of the diffraction grating as an etching mask. remove.

(d) Remove the additional P film 2a in region B.

(e) P film 2 is formed in region A and region B.

(f) By removing the P film 2 in region A by mask exposure, different types of photoresist films are formed in region A and region B.

(g) If uniform two-beam interference exposure is performed, the unevenness of the N film 7 will be formed in area B, and the unevenness of the P film 2 will be formed in area A, and the unevenness in each area will be inverted in phase. .

(h) Next, the substrate 1 is etched using the unevenness of the N film 7 and the P film 2 as masks, and the desired diffraction grating is formed by removing the N film 7 and the P film 2.

As described above, by carrying out the process of laminating different types of photoresist films on a substrate according to this embodiment of the present invention, a structure in which the phase of the concave and convex portions is reversed due to the conventional uniform two-beam interference exposure and etching can be obtained. Diffraction gratings can be manufactured. In FIGS. 3 to 5, when exposing the P film 2 and the auxiliary P film 2a,
Depending on the exposure conditions (light wavelength, intensity, exposure time, etc.) or the light absorption characteristics of the P film, the light may become substantially N.
The case where the film 7 is not reached is explained. Next, referring to FIG. 6, depending on the exposure conditions (light wavelength, intensity, exposure time, etc.) or the light absorption characteristics of the P film, the light exposed to the P film substantially reaches a part of the N film 7. This is an example of a case where the following is the case.

(a) An N film 7 is formed over the entire surface of the substrate 1.

(b) In order to remove the N film 7 in the first area A without exposing the N film 7 in the second area to light, a reflective film made of an ultraviolet absorbing film or a thin gold film, etc. is placed on the N film 7. Layer 8.

(c) Furthermore, an auxiliary P film 2a is laminated.

(d) The auxiliary P film 2a in the first region A of the diffraction grating is removed by normal mask exposure, and the reflective film 8 in the region A is further etched using the auxiliary P film 2a in the second region B of the diffraction grating as an etching mask. The N film 7 is also removed.

(e) Next, the auxiliary P film 2a in region B is removed using an organic solvent or the like, and the reflective film 8 and N film 7 are formed only in region A.

(f) P film 2 is formed on the entire surface of region A and region B.

(g) Periodic irregularities are formed by the P film 2 by performing two-beam interference exposure.

(h) Using the unevenness of the P film 2 as a mask, unevenness is formed on the substrate 1 in area A by chemical etching. Next, in region B, the reflective film 8 in the concave portion of the P film 2
remove.

(i) Further, the N film 7 in the region B is exposed to normal light over the entire surface, and the P film 2 and the reflective film 8 are removed. Also,
The P film 2 in area A is also removed. Since the exposed portion of the N film 7 remains (the opposite is the case for the P film), at this stage unevenness with phase inversion is formed in the region A and region B.

(j) Apply the additional P film 2b only to region A, and in region B, use the irregularities of the N film 7 as a mask to form irregularities on the substrate 1 in region B by chemical etching.

(k) By removing the N film 7 in region B and the additional P film 2b in region A, a diffraction grating having different phases in region A and region B can be obtained.

Still another embodiment of the present invention will be described with reference to FIG. In this example, the laminated film of the P film and the N film is exposed at the same time by effectively utilizing the exposure conditions such that the light exposed to the P film is substantially transmitted through the N film 7.

(a) After uniformly coating the auxiliary P film 2a on the substrate 1, the auxiliary P film 2 in the second region B is formed by mask exposure.
Remove a.

(b) Apply the N film 7 to the entire surface of the substrate 1 from which the auxiliary P film 2a in the second region B has been removed.

(c) Second area B due to lift-off of the auxiliary P film 2a
Only the N film 7 is left.

(d) A P film 2 is applied to the entire surface of the substrate 1 which has undergone the step (c).

(e) Perform two-beam exposure. The black part is the exposed part.

(f) The P film 2 is developed to create irregularities on the P film 2.

(g) Etching the substrate 1 in the first area A.

(h) The P film 2 is removed, the N film 7 is developed, and unevenness of the N film 7 is created in the second region B.

(i) By mask exposure, the additional P film 2b is formed in the first region A, and the substrate 1 in the second region B is etched.

(j) Remove additional P film 2b and N film 7.

(Effect of the invention) As is clear from the above steps, the present invention provides 1
Since only one two-beam interference exposure is used, there is no need to move the substrate 1, and there is no need to use a metal mask 6 during the two-beam interference exposure, so there is no part (region C) where no unevenness is formed. Diffraction gratings with phase inversion can be manufactured. Therefore, it can be applied to DFB lasers, etc., which are stable and have good characteristics, and the effect is extremely large. still,
In the explanation of the embodiments shown in FIGS. 3, 4, and 5, it is assumed that the P film 2 has good infrared absorption. However, if the P film 2 does not have sufficient ultraviolet absorption, as shown in FIG. It is sufficient to laminate a reflective film made of an ultraviolet absorbing film or a thin gold film between the P film 2 and the N film 7 as shown in FIG. The method is valid. Further, although detailed specific explanations regarding mask exposure, post-exposure development steps, photoresist coating, etc. are omitted, ordinary photoresist techniques are used.

[Brief explanation of drawings]

Fig. 1 is a principle diagram of the conventional two-beam interference exposure method, Fig. 2 is a schematic diagram of manufacturing a diffraction grating in which the phase of the concave and convex portions is partially reversed by conventional two-beam interference exposure, and Fig. 3 is a diffraction grating according to the present invention. Manufacturing process diagram of the lattice, Figure 4,
FIG. 5, FIG. 6, and FIG. 7 are manufacturing process diagrams of other embodiments of the present invention. 1...Substrate, 2...Positive type photoresist film, 2a...Auxiliary positive type photoresist film, 2b...Additional positive type photoresist film, 3...He-Cd laser beam, 4...Half mirror , 5...mirror, 6...metal mask, 7...
...Negative photoresist film, 8... Ultraviolet absorbing film or reflecting film (intervening thin film).

Claims (1)

[Scope of Claims] 1. On a substrate, one of a negative type photoresist film and a positive type photoresist film is formed in a first region, and the negative type photoresist film is formed in a second region different from the first region. a first step of forming a positive type photoresist film on the other of the resist film and the positive type photoresist film or on the negative type photoresist film; A second step of performing two-beam interference exposure and utilizing the fact that the negative type and positive type development characteristics of the photoresist film are reversed to form unevenness in each area in the first area and the second area. a third step of forming diffraction gratings whose phases are mutually inverted. 2. In the first step, after sequentially laminating a negative type photoresist film and an auxiliary positive type photoresist film on the entire surface of the substrate, removing the laminated film in the first region by mask exposure and development treatment, Further, by forming a positive type photoresist film on the entire surface after removing the auxiliary positive type photoresist film in the second region, the positive type photoresist film is formed on the negative type photoresist film in the second region. 2. The method of manufacturing a diffraction grating according to claim 1, wherein a film is formed, and the positive type photoresist film is formed in the first region. 3. In the first step, a negative type photoresist film and a positive type photoresist film are sequentially formed on the entire surface of the substrate, and the positive type photoresist film in the first area is removed by mask exposure and development treatment. According to the patent, the negative type photoresist film is formed in the first region, and the positive type photoresist film is formed on the negative type photoresist film in the second region. A method for manufacturing a diffraction grating according to claim 1. 4. In the first step, after laminating a negative type photoresist film and an auxiliary positive type photoresist film on the entire surface of the substrate, the laminated film in the first region is removed by mask exposure and development treatment, and After removing the auxiliary positive type photoresist film in the second region, forming a positive type photoresist film on the entire surface, and further removing the positive type photoresist film in the first region by mask exposure and development treatment. The diffraction grating according to claim 1, wherein the negative type photoresist film is formed in the second region, and the positive type photoresist film is formed in the first region. manufacturing method. 5 In the first step, after sequentially laminating a negative type photoresist film, an intervening thin film that absorbs or reflects ultraviolet rays, and an auxiliary positive type photoresist film on the entire surface of the substrate, the laminated film in the first region is laminated. By removing the auxiliary positive type photoresist film in the second area by mask exposure and development processing and forming a positive type photoresist film on the entire surface, the negative type photoresist film is removed in the second area. Claim 1, wherein the positive type photoresist film is formed on the resist film via the intervening thin film, and the positive type photoresist film is formed in the first region. A method of manufacturing a diffraction grating. 6. In the first step, after the auxiliary positive type photoresist film is formed on the entire surface of the substrate, the auxiliary positive type photoresist film in the second region is removed by mask exposure and development treatment, and then the entire surface is further covered with the auxiliary positive type photoresist film. After forming a negative type photoresist film and removing the auxiliary positive type photoresist film and the negative type photoresist film in the first region by lift-off, further forming a positive type photoresist film on the entire surface, Claim 1, wherein a positive type photoresist film is formed in the first region, and the positive type photoresist film is formed on the negative type photoresist film in the second region. A method of manufacturing the described diffraction grating.