JPH06201909A - Production of diffraction grating - Google Patents

Abstract

PURPOSE:To provide a process for production of a diffraction grating capable of easily producing the diffraction grating having an arbitrary phase shift. CONSTITUTION:The diffraction grating is formed on a substrate 10 by using a two beam interference exposing method and etching. A diffraction grating periodic structure 12 of a mask layer formed on a layer 10 to be etched by the two beam interference exposing method is formed by etching on the layer 10 to be etched, in such a case. The layer to be etched is subjected to etching with high directivity from an etching angle direction having a certain angle thetafrom the normal vector of the layer to be etched within the plane inclusive of the normal vector of the layer to be etched and the diffraction grating vector at this time. The diffraction grating arbitrarily shifted in phase from the diffraction grating periodic structure 12 of the mask layer is, thereby, formed on the layer 10 to be etched. Arbitrary phase shift parts are formed within the diffraction grating of the layer 10 to be etched if the above-mentioned stages are repeated by varying the etching angle.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a diffraction grating, and more particularly, in a method for manufacturing a diffraction grating having a period of about 2000 angstroms on a semiconductor surface, the phase of the diffraction grating can be changed midway. And a method for manufacturing a diffraction grating.

[0002]

2. Description of the Related Art A distributed feedback type (DF) in which a diffraction grating having a constant period is formed on the surface of a semiconductor crystal and is used as a wavelength selective reflector.
B) The laser has advantages that stable single mode operation can be obtained, that the oscillation wavelength can be changed by the current, and that the crystal does not need to be cleaved to form a resonator and is suitable for integration. Have However, in principle, a stable single mode operation cannot be obtained with a DFB laser in which a diffraction grating having only grooves with a constant period is formed on the upper surface of the semiconductor layer in the waveguide. In order to improve this point, it is necessary to shift the Bragg wavelength by λ / 4 in the laser resonator. Therefore, in recent years, a DFB laser has been proposed which oscillates in a stable single mode by shifting the phase of the diffraction grating on the way. Particularly, in the case of the first-order diffraction grating, the unevenness of the diffraction grating is shifted by 1/2 cycle near the center of the resonator,
That is, a DFB laser has been proposed in which the concavities and convexities of the left and right diffraction gratings are inverted with the center of the element as a boundary.

[0003]

In order to shift the phase of the diffraction grating in the middle of the laser resonator as described above, the following method has hitherto been taken.

(1) Electron beam exposure method A method in which a semiconductor substrate is etched with an appropriate etching solution by using a mask obtained by exposing the photoresist corresponding to the grooves of the diffraction grating one by one by an electron beam exposure apparatus and developing it. is there. . However, according to this method, the phase of the diffraction grating can be arbitrarily changed, but it takes a lot of time to manufacture one semiconductor laser, resulting in poor productivity and high cost, which is a practical problem.

(2) Negative / Positive Resist Method A negative resist in which a portion exposed to light is peeled off by development is coated on the right half of the laser resonator, and conversely, a portion not exposed to light is peeled off by development. A method has also been proposed in which a mold photoresist is applied to the left half, and then exposed by a normal two-beam interference exposure device and developed, and used as a mask for etching. The literature on this method is Electronics Letter.
s Vol. 20, No. 4, p. 1008-1010 can be cited. However, this method has the problems that the performance of the negative resist is poor, that the resist is difficult to apply, and that the phase is shifted only by π.

(3) Phase shift plate insertion method This is a method in which a quartz plate having a step corresponding to a phase shift is brought into close contact with a photoresist to perform interference exposure. As a reference for this method, there is a technical report OQE85-6 of the Institute of Electrical Communication of Japan.
0, pages 57-64. But,
This method has a drawback that the phase transition region is as large as ten and several μm.

There is also a method of performing interference exposure by incorporating a quartz plate having a step corresponding to a phase shift into an optical system, and as a reference of this method, there is the autumn 4th 1985.
Proceedings of the 6th JSAP Academic Lecture Proceedings 2a on page 202
-N-10 can be mentioned. However, this method has a drawback that the diffraction grating formation area is extremely small (about 7 mm ² ) due to aberration.

An object of the present invention is to solve the above-mentioned conventional problems, and a method of manufacturing a diffraction grating for easily manufacturing a diffraction grating having an arbitrary phase shift for a DFB laser or the like. To provide.

[0009]

According to the method of manufacturing a diffraction grating of the present invention for achieving the above object, the diffraction grating periodic structure of the first mask layer formed on the layer to be etched is formed when the diffraction grating is formed on the substrate. When etching is formed on the layer to be etched, the direction is from an etching angle direction having an angle from the normal vector of the layer to be etched in a plane including the normal vector of the layer to be etched and the diffraction grating vector. The step of forming a diffraction grating having an arbitrary phase shift with respect to the diffraction grating periodic structure of the mask layer in the layer to be etched by performing highly etching. In the first aspect of the present invention, the final purpose is to form a diffraction grating in the layer to be etched, and in the second aspect of the present invention, the diffraction grating formed in the layer to be etched is further used as a mask layer. The final purpose is to form the diffraction grating in the layer below it.

More specifically, in order to achieve the above object, the present invention provides a method of a semiconductor substrate when a semiconductor layer is etched using a photoresist having a concavo-convex period after two-beam interference exposure as a mask. In the plane including the line vector and the diffraction grating vector, the normal vector is at a certain angle by dry etching having a strong directivity,
The method of the present invention is characterized in that a diffraction grating whose phase is shifted with respect to the diffraction grating on the photoresist is formed on the semiconductor by transferring the periodic structure on the photoresist onto the semiconductor substrate from an oblique direction. By repeating the etching with changing the etching angle, a diffraction grating having an arbitrary phase shift amount is realized. Further, a periodic structure of a resist having a phase shift is formed by using the method for producing a diffraction grating of the present invention, and the semiconductor substrate is etched using this as a mask, so that shapes such as depth are uniform on the left and right of the phase shift. It made it possible to fabricate a diffraction grating.

[0011]

[First Embodiment] A first embodiment of the present invention will be described below. The present embodiment belongs to the first aspect of the present invention, and a method of manufacturing a diffraction grating using the same includes the following steps.

(1) A step of forming a first photoresist on a semiconductor substrate and a step of irradiating the surface of the semiconductor substrate with two identical semiconductor laser beams having different incident angles to expose the first photoresist by interference exposure. And a step of developing the first photoresist to form a periodic pattern, the step of the two-beam interference exposure method. (2) A step of forming a second photoresist on the periodic photoresist, and a step of exposing and removing a part of the second photoresist by a patterning method which is a normal semiconductor process. (3) The first not covered by the second photoresist
In the step of etching a part of the semiconductor substrate by using the photoresist of FIG. 1 as an etching prevention film, etching with strong directivity is performed at an angle θ ₁ (including the case of zero) from the normal line of the substrate. A step of forming a periodic structure of photoresist by φ ₁ phase shift on a semiconductor substrate. (4) The second photoresist on the semiconductor substrate is removed, a portion of the etched semiconductor substrate is covered with a third photoresist, and a part of the third photoresist is a normal semiconductor process. A step of exposing and removing by a patterning method. (5) In the step of etching a part of the semiconductor substrate using the first photoresist that is not covered with the second photoresist as an etching prevention film, etching with strong directivity is performed at an angle θ from the normal line of the substrate. ₂ (including zero, but either θ ₁ or θ ₂ is not zero)
And the step of forming the periodic structure of the first photoresist on the semiconductor substrate by φ ₂ phase shifting, and the step of removing the third photoresist on the semiconductor substrate. (6) A step of removing the first photoresist on the semiconductor substrate. At least the above steps are provided to realize a phase shift of (φ ₁ + φ ₂ ).

First, the steps (3) and (5) which are the features of the first aspect of the present invention will be described in detail.

FIG. 1 shows a semiconductor substrate 10 during dry etching such as reactive ion beam etching having a strong directivity.
Indicates. Conventionally, as shown in FIG. 8, since the semiconductor substrate 100 is etched from the normal direction (θ = 0), the diffraction grating structure of the photoresist 101 as the etching prevention film has a zero phase shift. The grating 102 will be etched on the semiconductor substrate 100.

On the other hand, in the present invention, the semiconductor substrate 1
0 normal direction and θ Etching is applied from the direction
To prevent this, the first photoresist is used as an etching prevention film.
Stroke 12 diffraction grating structure and phase shift φ Diffractive case
The semiconductor will not be etched on the semiconductor substrate 10.
It By repeating this method, any amount of phase shift
Will be formed.

As a method of controlling the relationship between the normal line of the semiconductor substrate 10 and the etching direction, there is a method of changing the position of the etching beam b as shown in FIG.
As shown in FIG. 2, the semiconductor substrate 10 may be tilted and controlled with respect to the etching beam b.

A first embodiment of the method of manufacturing a diffraction grating according to the first aspect of the present invention will be specifically described with reference to FIG.

(1) A first photoresist 12 for two-beam interference exposure is applied on the semiconductor substrate 10 and two-beam interference exposure is performed (FIG. 3 (1)). (2) The exposed first photoresist 12 is developed to form a diffraction grating on the photoresist 12 (FIG. 3 (2)). (3) Second photoresist 1 for normal pattern exposure
3 is applied to the entire surface and then patterned (the phase transition region is allowed to some extent, the boundary of this patterning is located in the middle of the photoresist lacking portion of the diffraction grating of the photoresist 12 as shown in FIG. 3C). The photoresist 13 is removed only in the diffraction grating fabrication region A on the left side (FIG. 3C). (4) The diffraction grating 15 in which the normal line of the semiconductor substrate 10 is inclined by θ _{1 with} respect to the etching beam b and the phase of the diffraction grating on the first photoresist 12 is phase-shifted by φ _{1 is changed} to the semiconductor substrate 10. It is formed on the top (FIG. 3 (4)). (5) Third photoresist 1 for normal pattern exposure
After applying 4 to the entire surface, patterning is carried out (the boundary of this patterning may not exactly come to the central broken line vertically drawn in FIG. 3) and development is carried out, and only the right side diffraction grating manufacturing region B is formed. Remove the photoresist (Figure 3
(5)). (6) The diffraction grating 16 in which the normal line of the semiconductor substrate 10 is tilted by θ _{2 with} respect to the etching beam b and the phase of the diffraction grating on the first photoresist 12 is phase-shifted by φ _{2 is applied} to the semiconductor substrate 10. It is formed on the top (FIG. 3 (6)). (7) The third photoresist 14 is removed (see FIG.
(7)). (8) First photoresist 1 which is a diffraction grating mask
2 is removed (FIG. 3 (8)).

In this way, a phase shift of (φ ₁ + φ ₂ ) can be formed in the phase shift portion C.

Here, the phase shift will be described. In the case of the first-order diffraction grating in the DFB-LD having the oscillation wavelengths of 1.3 μm and 1.55 μm, the period Λ is 200 nm, 24
0 nm, λ for improving single mode stability
_B (Bragg wavelength) / 4 shift means Λ / 2 shift (see FIG. 4 (2)).

On the other hand, in the case of a diffraction grating having a periodic structure of loss and gain, it is said that a zero phase shift is preferable for improving single mode stability, but a periodic structure of pure loss and gain only. Is difficult to realize, and in many cases, a periodic structure of refractive index is also present at the same time. DF like this
For B-LD, λ _B for single mode stability
/ 4 shift is inconvenient, and λ _B / 8 shift, 3λ _B / 8 shift, etc. are required, and Λ / 8 shift, etc. are required.

Further, a DFB-LD having an oscillation wavelength of 0.8 μm
In the case of, in the case of the first-order diffraction grating, its period Λ ₁ is 120
Since it is very small, it is difficult to realize it by the conventional two-beam interference exposure method. Therefore, the secondary diffraction grating is used as a diffraction grating for wavelength selection. In this case, although the period lambda ₂ of second order diffraction grating becomes 240 nm, lambda _B / 4 shift for single-mode stability enhancement of means lambda _2/4 shift (Fig. 4 (3) reference. Here, the zero shift reference is taken at the center of the peak portion of the diffraction grating in FIG. 4 (1). This Λ 2/4 shift was difficult to manufacture by the conventional _two- beam interference exposure method, but it is possible according to the present invention. In the present invention, any amount of phase shift can be achieved.

For example, DFB-with an oscillation wavelength of 1.55 μm
In the case of the first-order diffraction grating in the LD, λ _B / 4 shift is Λ
./2 shift, and the phase of the diffraction grating requires π shift. Since the period Λ of this diffraction grating is 240 nm, a shift of 120 nm is necessary to set φ ₁ + φ ₂ = π. Here, when φ ₁ = φ ₂ , the depth of the diffraction grating is 100 n
Assuming m, it can be seen that it is sufficient to set θ ₁ = θ ₂ = 31.0 deg.

Further, a DFB-L having an oscillation wavelength of 0.83 μm
In the case of D, a second-order diffraction grating is usually used, but this period Λ
₂ 240nm becomes, λ _B / 4 shift means lambda _2/4 shift. Therefore, in order to make φ ₁ + φ ₂ = π / 2, the shift is 60 nm. Here, assuming that φ ₁ = φ ₂ and the depth of the diffraction grating is 100 nm, θ ₁ = θ ₂ = 1
It can be seen that it may be set to 6.7 deg.

Of course, in the present invention, θ ₁ = θ ₂ and φ ₁
It goes without saying that it is not necessary that == φ ₂ .

[0026]

Second Embodiment Next, a second embodiment which is an embodiment of the method of manufacturing a diffraction grating according to the second aspect of the present invention will be described. In this embodiment, the etching target layer having the phase shift acts as a mask layer for forming a diffraction grating on the semiconductor substrate.

5 and 6 show the steps of the method for manufacturing the diffraction grating of this embodiment. (1) A mask material 11 such as SiO ₂ or Si ₃ N _{4 is} formed on the semiconductor substrate 10 by a chemical vapor deposition method or the like, and a first photoresist 12 for two-beam interference exposure is applied thereon ( FIG. 5 (1)). (2) By performing two-beam interference exposure and developing,
A diffraction grating is formed on the entire surface of the photoresist 12 (FIG. 5 (2)). (3) Second photoresist 1 for normal pattern exposure
After applying 3 to the entire surface, patterning is performed (similarly to the patterning in the first embodiment) and development is performed to remove the photoresist 13 only in the diffraction grating fabrication region A on the left side (FIG. 5C). . (4) The diffraction grating 25 having the normal line of the semiconductor substrate 10 tilted by θ _{1 with} respect to the etching beam b and phase-shifted by φ _{1 with} respect to the phase of the diffraction grating on the first photoresist 12 is used as the mask material 11 It is formed on top (FIG. 5 (4)). (5) Third photoresist 1 for normal pattern exposure
After applying 4 to the entire surface, patterning is performed (similar to the patterning in the first embodiment) and development is performed, and the photoresist is removed only in the diffraction grating formation region B on the right side (FIG. 5 (5)). (6) The diffraction grating 26 in which the normal line of the semiconductor substrate 10 is inclined by φ _{2 with} respect to the etching beam b and the phase of the diffraction grating on the first photoresist 12 is shifted by φ ₂ is used as the mask material 11. It is formed on top (FIG. 6 (6)). (7) The third photoresist 14 is removed (see FIG. 6).
(7)). (8) First photoresist 1 which is a diffraction grating mask
2 is removed (FIG. 6 (8)). (9) The normal line of the semiconductor substrate 10 is aligned with the direction of the etching beam b, and the diffraction grating pattern 2 on the mask material 11 is formed.
5, 26 are etched and transferred to the semiconductor substrate 10 (see FIG. 6).
(9)). (10) The mask material 11 is removed (FIG. 6 (10)).

In this way, a phase shift of (φ ₁ + φ ₂ ) can be formed in the phase shift portion C, and diffraction gratings 28 having uniform shapes and depths on the left and right sides A and B are formed.
Since it can be manufactured, a DFB-LD or the like having good characteristics can be manufactured.

[0029]

[Third Embodiment] Next, as a third embodiment which is an embodiment of the method for manufacturing a diffraction grating according to the first aspect of the present invention, when the shape, depth, etc. of the diffraction grating are different on both sides of the phase shift. Will be described.

In recent years, the characteristics required for optical communication LDs have become diversified, and demands for wider wavelength tunable width, narrower spectral line width, lower chirping, higher optical output, light source for high efficiency frequency modulation, etc. There are various. For this purpose, DFB-L
There are various devises to the D diffraction grating, and a DFB-LD having a diffraction grating with different depths along the cavity direction has also been reported (Koji et al., Proceedings of the Autumn Society of Applied Physics 1991, 1
0p-ZM-17). In addition, the shape of the diffraction grating has a complex influence on the coupling coefficient, and thus is closely related to the above characteristics.

FIG. 7 shows a manufacturing method for a case where the diffraction grating has different shapes on the left and right of the phase shift. Basically, it is the same as the manufacturing method of FIG. 3, and thus detailed description of the process is omitted.

In this embodiment, D having an oscillation wavelength of 0.83 μm is used.
In the case of FB-LD, due to λ _B / 4 shift in the secondary diffraction grating, when φ ₁ = π / 2 and φ ₂ = 0, θ ₁
= 31.0 deg and θ ₂ = 0. In the area A, the number of diffracted lights to the upper and lower sides of the diffraction grating 15 increases, and in the area B, the diffraction grating 3
Diffracted light up and down 6 is reduced. As a result, if the structural design is optimized, the distribution of light intensity inside the resonator can be made uniform, and a narrow spectral line width and low chirping DFB-LD can be realized.

By the way, in the case of a DFB-LD having a plurality of phase shifts in the cavity direction, the manufacturing method of the present invention is repeated a plurality of times. In that case, a part of the diffraction grating is used. It is sufficient to repeat the formation, patterning, and peeling of the photoresist for covering.

Although reactive ion beam etching has been described as the directional etching means, focused ion beam etching or other dry etching means may be used.

In the above, an example of engraving a diffraction grating on a semiconductor substrate has been described. In the case of LD, whether the semiconductor substrate has a layer structure after epitaxial growth, and whether or not the semiconductor substrate has a diffraction grating and then epitaxial growth is performed. Whether or not to perform is not related to the essence of the present invention because it depends on the structure of the LD.

The substrate on which the diffraction grating is formed may be a dielectric such as LiNbO ₃ or a glass substrate instead of the semiconductor substrate, and the present invention is effective in that case as well.

Further, in the present invention, the peaks and troughs of the diffraction grating can be formed obliquely as shown in FIG. 1, but this shape is related to the increase and decrease of diffracted light above and below the diffraction grating.
You can control the characteristics. Therefore, the diffraction grating having the peaks and the valleys formed obliquely without having the phase shift may be effective.

[0038]

As is apparent from the above description, according to the method of manufacturing a diffraction grating of the present invention, a distributed feedback semiconductor laser having a diffraction grating whose phase is changed by an arbitrary amount in the middle of a semiconductor laser resonator or the like ( DFB-LD) and the like can be manufactured, and these are used as a light source for optical communication and the like, and stably oscillate in a single mode even when modulated at high speed.

The method for manufacturing a diffraction grating of the present invention is also effective for manufacturing a distributed reflection type (DBR) LD and the like, and these can also be used as a wavelength selection filter.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing how to perform directional etching on a diffraction grating, which is a characteristic of the present invention, and is a diagram when an etching source direction is changed with respect to a normal line of a semiconductor substrate.

FIG. 2 is a conceptual diagram showing another method of performing directional etching on a diffraction grating, which is a feature of the present invention, and is a diagram when a normal line of a semiconductor substrate is shifted from an etching direction.

FIG. 3 is a process chart showing a first embodiment of the present invention,
It is a figure explaining the manufacturing method of the diffraction grating containing a phase shift part.

FIG. 4 is a diagram showing a phase shift in a diffraction grating.

FIG. 5 is a process diagram showing the first half of the second embodiment of the present invention, in which a mask material is formed on a semiconductor substrate, a diffraction grating is once formed on the mask material, and then the diffraction grating is formed on the semiconductor substrate. It is a figure explaining the manufacturing method of the diffraction grating containing a phase shift part which is transferred.

FIG. 6 is a process diagram showing the latter half of the second embodiment of the present invention, in which a mask material is formed on a semiconductor substrate, a diffraction grating is once formed on the mask material, and then the diffraction grating is formed on the semiconductor substrate. It is a figure explaining the manufacturing method of the diffraction grating containing a phase shift part which is transferred.

FIG. 7 is a process chart showing a third embodiment of the present invention,
It is a figure explaining the manufacturing method of the diffraction grating with a different shape on both sides of a phase shift part.

FIG. 8 is a diagram showing a conventional method of etching a diffraction grating.

[Explanation of symbols]

A, B Two areas with different phases of diffraction grating C Phase shift area θ ₁ , θ ₂ Angle formed by substrate normal and etching direction φ ₁ , φ ₂ Angle formed by diffraction grating on photoresist and diffraction grating on substrate b etching beam 10 substrate 11 mask material 12 two-beam interference exposure method photoresist 13 second photoresist 14 third photoresist 15, 16, 28, 36 substrate diffraction grating 25, 26 mask material diffraction grating

Claims (4)

[Claims] 1. When forming a diffraction grating on a substrate using a two-beam interference exposure method and etching, a diffraction grating periodic structure of a first mask layer formed by the two-beam interference exposure method is formed on the layer to be etched. When etching is formed on the etching layer, the directivity is changed from the etching angle direction having a certain angle from the normal vector of the etching target layer in the plane including the normal vector of the etching target layer and the diffraction grating vector. A method of manufacturing a diffraction grating, which comprises a step of forming a diffraction grating, which has an arbitrary phase shift with respect to the diffraction grating periodic structure of the mask layer, in the layer to be etched by performing high etching. 2. When forming a diffraction grating on a substrate, when a diffraction grating periodic structure of a first mask layer formed on an etching target layer is formed by etching on the etching target layer, a normal vector of the etching target layer is formed. And a diffraction grating vector, a highly directional etching is performed from an etching angle direction having a certain angle from the normal vector of the layer to be etched, so that the diffraction grating periodic structure of the mask layer is formed. And a step of forming an arbitrary phase-shifted diffraction grating on the layer to be etched. 3. The etching layer is formed by repeating a step of forming a diffraction grating having an arbitrary phase shift with respect to the diffraction grating of the mask layer on the etching target layer, and changing the etching angle at that time. 3. An arbitrary phase shift is provided in the diffraction grating of 1.
A method for manufacturing the diffraction grating described. 4. The layer to be etched forming the diffraction grating including the phase shift serves as a mask for etching a layer below it as a second mask layer. Manufacturing method of the diffraction grating of.