JPH0243792A - Manufacture of diffraction grating - Google Patents

Abstract

PURPOSE:To realize the manufacture of a diffraction grating possessed of an extremely small period necessary for improving an optical semiconductor device such as a shortwave band DFB laser in characteristic by a method wherein a first groove of a specified period is provided to a semiconductor substrate, and a second groove of a specified period is formed on the semiconductor substrate making use of a mask buried in the first groove. CONSTITUTION:A first mask 4 with a pattern of a specified period A is formed on a semiconductor substrate 2, which is subjected to an etching using the first mask 4 to form a first groove 6 of the specified period A on the semiconductor substrate 2. Next, the above first groove 6 is filled with a second mask 8, and the substrate 2 is etched through the second mask 8 after the first mask 4 has been removed to form a second groove 10 of the specified period A. For instance, a positive resist 4 is applied onto the GaAs substrate 2, and the resist 4 is exposed to light using a two light beam interference method and developed to form a pattern of a period A. Then, a groove 6 is formed, then a negative resist 8 is applied, which is exposed to light, developed, and rinsed, and then a groove 10 is formed.

Description

DETAILED DESCRIPTION OF THE INVENTION A fourth step of forming a second groove.

[Industrial Application Field] The present invention relates to a method for manufacturing a diffraction grating, and in particular to a method for manufacturing a diffraction grating, and in particular, a method for manufacturing a diffraction grating, particularly for a short wavelength band DFB laser (Distributed
The present invention relates to a method of manufacturing a first-order diffraction grating that can increase the coupling coefficient, which is an important parameter of a DFB laser, in a distributed feedback laser (Feedback La5er).

[Prior art] For example, diffraction gratings used in optical semiconductor devices such as semiconductor Hello lasers require a short period on the order of the optical wavelength, so it is difficult to obtain sufficient resolution using ordinary photolithography technology. Can not.

For this reason, in the conventional method for manufacturing diffraction gratings, a three-dimensional patterning technique that utilizes interference fringes that occur when two coherent light waves are made to interfere is used as a patterning technique that is effective for obtaining fine periodic exposure patterns on the order of optical wavelengths. Flux interferometry has been used.

In this three-beam interferometry, if the wavelength of the incident laser beam is λ and the angle of incidence of this laser beam on the substrate surface is θ, then the period A of interference fringes formed on the substrate in a direction that includes the incident surface is: Δ=λ/2s i n θ, so by appropriately selecting the wavelength λ and the incident angle θ, an exposure pattern with a desired period is formed, and further, a diffraction grating with a period is formed by development.

At this time, if the incident angle θ is selected appropriately, the minimum circumference 1tfb\1
11n is 8l1lln = λ/2, and H eCd which is usually used as a laser beam
When using 3250 beams of laser, the minimum period A min
An+n=1525 people.

In other words, the y! of a diffraction grating using conventional three-beam interferometry! !
With this method, it is possible to create a diffraction grating with a zero period of approximately 1,600 people as the shortest period.

This forms a diffraction grating built into a long wavelength band laser of, for example, 1.55 μm or 1.31 μm.

[Problems to be Solved by the Invention] However, in such a conventional method of manufacturing a diffraction grating, the period required for a short wavelength band laser is 100
Diffraction gratings with a short period of 0 to 1200 people cannot be manufactured. For example, it is impossible to fabricate a first-order diffraction grating for a short-wavelength DPB laser, so a second-order diffraction grating has been fabricated and used for a short-wavelength DFB laser. Compared to the first-order grating, the coupling coefficient of this second-order grating is about one line smaller when the depth of the grating is the same, and the radiation loss is also large, so it is difficult to set a large coupling coefficient. Can not.

Therefore, there was a problem in that it was difficult to achieve a significant improvement in characteristics.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a method for manufacturing a diffraction grating having an extremely fine period, which is required to improve the characteristics of an optical semiconductor device such as a short wavelength IDFB laser.

[Means for Solving the Problems] The above-mentioned problems include a first step of forming a first mask having a pattern with a predetermined period on a semiconductor substrate, performing etching using the first mask, and performing etching using the first mask. a second step of forming the first grooves with the predetermined period on the semiconductor substrate; a third step of burying the first grooves with a second mask; and after removing the first mask, A fourth step of performing etching using a second mask to form a second pattern having a predetermined period on the semiconductor substrate.

[Function] That is, the present invention forms first grooves with a predetermined period on a semiconductor substrate, and uses a mask embedded in the first groove to form second grooves with a predetermined period on the semiconductor substrate. By forming the grooves, a diffraction grating consisting of the first and second grooves is formed.

As a result, a diffraction grating having a period twice as fine as the predetermined period formed so far is formed.

[Example] The present invention will be specifically described below based on an illustrative example.

FIG. 1 is a process diagram showing a method for manufacturing a diffraction grating according to an embodiment of the present invention.

For example, a (100) GaAs substrate 2 as a semiconductor substrate
A positive resist (for example, Microposit MP1400-31) 4 is applied on the surface in parallel to the [0111 direction.

Then, the positive resist 4 is exposed and developed using the usual three-beam interference method, and a periodic pattern is formed by the positive resist 4.

Now, in this three-beam interferometry, 3250 light of a He-Cd laser is used, and the Ga of this He-Cd laser light is
When the angle of incidence on the two surfaces of the As substrate is θ, the period A of the pattern formed by the positive resist 4 is Δ-3250/2 sin θ(A). Therefore, by appropriately selecting the incident angle θ, in this example, , period Δ=20 due to positive resist 4
Figure 1 (a) to form a pattern of 00 to 2400A
reference).

Next, the positive resist 4 patterned in this way
Using as a mask, the surface of the GaAs substrate 2 is etched,
This forms a curve in the period (see FIG. 1(b)).

Note that in this etching, (
111) If this is done using an anisotropic etchant that exposes the surface, a figure 7-shaped well 6 as shown in Figure 1(b) will be formed.
becomes.

Next, a negative resist (for example, a diluted solution of OMR-85) 8 is applied as a mask material to the entire surface, and a V-shaped resist 6 is embedded in the period. However, the thickness of the negative resist 8 applied at this time is not equal to the positive It is made thinner than the thickness of the resist 4 so that the upper surface of the positive resist 4 is exposed (see FIG. 1(c)).

Subsequently, the entire surface is exposed to light (Fig. 1(d) I
(see). Further, development and rinsing are carried out using a normal photolithography technique, whereby the exposed negative resist 8 remains on the one hand with a period Δ, and the exposed positive resist 4 on the other hand is dissolved and removed.
The surface of the GaAs substrate 2 is exposed (see FIG. 1(e)).
.

Next, using the periodic patterned negative resist 8 as a mask, the exposed surface of the GaAs substrate 2 is etched,
Form a figure 7 cross 10 to the period. Note that this etching is performed with the same etchant and under the same conditions as when forming Eraser 6, so Eraser 6 and Eraser 10 have the same shape (
(See Figure 1(f)).

Subsequently, when the negative resist 8 is removed, the 7-shaped groove 6 in the period and the V-shaped groove 10 in the period A are combined to form a period twice as fine as the period, that is, the period Δ/2=1
A diffraction grating of 000 to 1200 people is formed on a GaAs substrate (see FIG. 1(g)).

Next, a light guide layer 12 made of AlGaAs, for example, is placed on the GaAs substrate 2 having the diffraction grating with a period of A/2.
is epitaxially grown, and further this optical guide layer 12 is grown epitaxially.
An active layer 14 made of, for example, GaAs, for example, A
A cladding layer 16 made of lGaAs and, for example, GaAs.
A contact layer 18 made of As is epitaxially grown (see FIG. 1(h)).

Subsequently, electrodes 20 and 22 are formed on the contact layer 18 and the bottom surface of the GaAs substrate 2, respectively (see FIG.
)reference). Thereafter, a non-reflective film (not shown) is formed on both side walls to manufacture a DFB laser having a built-in diffraction grating of period/2=1000-1200.

As described above, according to this embodiment, by combining the ordinary three-beam interferometry and photolithography technology, it is possible to easily create a diffraction grating with a V&fine period twice that of the period formed using the three-beam interferometry. can be formed into Therefore,
A diffraction grating with a short period of 1000 to 12008, which is necessary for a short wavelength band laser, can be manufactured, for example, a first order diffraction grating in a short wavelength band DFB laser can be manufactured. Because of this, the coupling coefficient can be increased and the radiation loss can also be reduced compared to the conventional case of using a second-order diffraction grating. Therefore, the characteristics of the short wavelength band DFB laser can be improved.

Note that in the above embodiment, GaA is used as the semiconductor substrate.
Although the s-substrate 2 is used, other semiconductors such as GaAlAs of the same GaAs s-based material may be used as long as the material serves as a short wavelength band laser.

In addition, the (100) plane of the GaAs substrate 2 is etched to form the grooves 6 and 10 that expose the (111) plane.
It is not necessary to limit the substrate surface and etching surface. However, these conditions in the above example are as follows:
This is suitable for forming the grooves 6 and 10 into the same figure 7 shape.

In addition, instead of applying the negative resist 8 as a mask material, for example, using the ECR-CVD method, a positive resist 4 can be applied.
It is also possible to fill in the groove 6 by depositing a silicon oxide film or a silicon nitride film at a low temperature that does not cause damage to the silicon oxide film or silicon nitride film used as a mask material instead of the negative resist 8. The thickness of the nitride film must be made thinner than the thickness of the positive resist 4 so that the upper surface of the positive resist 4 is exposed.

When a silicon oxide film or a silicon nitride film is used as a mask material instead of the negative resist 8, the positive resist 4 is not limited to a positive type, and a negative resist may be used.

Furthermore, in the above embodiment, this diffraction grating manufacturing method is applied to a short wavelength band DFB laser, but the method is not limited to this, and can be applied when forming a short-period diffraction grating in an optical integrated circuit. Can be widely applied.

[Effects of the Invention] As described above, according to the present invention, a first groove with a predetermined period is formed on a semiconductor substrate, and a second groove with the same predetermined period is formed using a mask embedded in the first groove. By forming the grooves, it is possible to manufacture a diffraction grating having a period twice as fine as a predetermined period, which is required to improve the characteristics of an optical semiconductor device such as a short wavelength band DFB laser.

4...Active layer, 6... cladding layer, 8...Contact layer, 0.22... Electrode.

[Brief explanation of the drawing]

FIG. 1 is a process diagram showing a method for manufacturing a diffraction grating according to an embodiment of the present invention. In the figure, 2...GaAs substrate, 4...Positive resist, 6.10...Groove, 8...Negative resist, 12... ...light guide layer,

Claims (1)

[Scope of Claims] A first step of forming a first mask having a pattern with a predetermined period on the semiconductor substrate; and performing etching using the first mask to form the pattern with the predetermined period on the semiconductor substrate. a second groove forming a first groove of
a third step of burying the first trench with a second mask; and after removing the first mask, etching is performed using the second mask to form an area on the semiconductor substrate. and a fourth step of forming the second grooves with the predetermined period.