JPH0558522B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a diffraction grating, and particularly to a method for manufacturing a diffraction grating on a semiconductor substrate.

(Prior Art) Elements in which unevenness is formed on a semiconductor substrate are used in various fields, and semiconductor lasers are particularly important elements among them. Hereinafter, a semiconductor laser will be explained as a specific example.

In recent years, research and development of single-axis mode semiconductor lasers have been actively conducted as light sources for long-distance, high-capacity optical transmission systems. Among them, distributed feedback (DFB)
Semiconductor lasers are rapidly becoming commercialized due to their stability, including controllability of single-axis mode and wide operating temperature range, and because they are easier to manufacture than other single-axis mode semiconductor lasers. Development is underway.

A DFB laser has a structure with a uniform diffraction grating within the element, and oscillates at a single wavelength near the black wavelength determined by the period of this diffraction grating. However, in a DFB laser having a uniform diffraction grating with small end face reflectances on both end faces, there are two oscillating modes that are likely to oscillate with a Bragg wavelength in between. This means that it is likely to operate in two-axis mode, which is not preferable as a single-axis mode laser. DFB lasers typically have a structure in which one end uses a cleavage surface to extract light, and the other end is coated with a non-reflective coating to suppress end-face reflection in order to suppress Fabry-Perot mode. In this way, the structure with asymmetric end face reflectance
The reflector loss characteristics of a DFB laser are asymmetric with respect to the Bragg wavelength, and one longitudinal mode tends to be selected. However, the reflector loss characteristics vary significantly depending on the phase at which the diffraction grating ends at the position of the end face reflector. Therefore,
In many cases, the difference in mirror loss between the main mode and the next mode was small, and there were many elements that oscillated in two modes. Therefore, in order to eliminate such instability, a prototype DFB laser was fabricated with a structure in which the period of the diffraction grating inside the DFB laser was shifted by λg/4 (λg is the wavelength of light propagating within the element). An example of literature related to this is the article written by Utaka et al., published on November 22, 1984, in Electronics Letters, Vol. 20, No. 4, pp. 1008-1010. A DFB laser with such a structure is called a λ/4 shift type DFB laser.

A λ/4 shift DFB laser is characterized by oscillation in one axial mode that perfectly matches the blogging wavelength. For this reason, the number of devices that oscillate in the biaxial mode, which is seen in conventional devices, is extremely rare, and this is very promising in terms of yield improvement.
By the way, the period Λ of the diffraction grating of the DFB laser is
It has the following relationship with the oscillation wavelength λ.

Λ=mλ/2n _e =mλg/2 Here, n _e is the equivalent refractive index within the element, m is the order of the diffraction grating, and λg is the wavelength of light propagating within the element. Therefore, the period of the first-order diffraction grating (m=1) is given by Λ=λg/2. From this, it can be seen that for a λ/4 shift type diffraction grating using a first-order diffraction grating, the unevenness of the left and right diffraction gratings can be reversed with the shift region as the boundary.

The following four methods can be cited as typical methods for manufacturing a λ/4 shift type diffraction grating.

The first method is to apply interference exposure by overcoating a novolak-based positive photoresist and a cyclized rubber-based negative photoresist, making use of the reverse photosensitivity of the positive and negative photoresists. be. Documents on this method include the November 22, 1984 book entitled "Electronics of Luminescence" by Utaka et al.
Letters magazine (Electronics Letters) Volume 20 No. 4
I can cite the papers listed on pages 1008-1010 of the issue.

The second method is to perform interference exposure by sandwiching an intermediate layer between a novolak negative photoresist and a novolak positive photoresist. Written on page 202 of the Autumn Academic Conference Proceedings of the Japan Society of Applied Physics
2a-N-9 "λ/4 shift diffraction grating made of novolac negative/positive resist" can be mentioned.

A third method is to perform interference exposure by closely adhering a quartz plate with a step corresponding to a phase shift to a substrate coated with photoresist.References on this method include Shirasaki et al., published by the Institute of Electronics and Communication Engineers. I can cite the paper described in Technical Report OQE Vol. 85-60, pages 57-64.

A fourth method is to incorporate a quartz plate with a step corresponding to a phase shift into the optical system and perform interference exposure. 2a-N-10 "Preparation of π-shifted diffraction grating by phase plane projection interference exposure method" in the proceedings of the 46th Japan Society of Applied Physics Academic Conference, page 202
can be given.

(Problems to be Solved by the Invention) The method for manufacturing a diffraction grating described above has the following drawbacks.

In the first method, it is difficult to obtain a diffraction grating with a good shape because the resolution of the cyclized rubber-based negative photoresist is poor.

In the second method, unlike the first method, it is possible to obtain a diffraction grating with a good shape, but it is difficult to control the development conditions, and the process is complicated due to the large number of steps, resulting in a poor yield.

The third method has the disadvantage that interference fringes are formed when a quartz plate with a step corresponding to a phase shift and a substrate with photoresist are brought into close contact with each other, making it impossible to form a diffraction grating over the entire surface of the substrate. .

In the fourth method, aberrations increase due to the influence of diffracted light from the step portion of the phase shift plate, but this aberration is corrected by lens difference. However, the effect of aberrations could not be sufficiently canceled, and the area where the diffraction grating was formed was ~7 mm ² (circle with a radius of 1.5 mm).
The disadvantage was that it was extremely small.

An object of the present invention is to provide a method for manufacturing a first-order λ/4-shift type diffraction grating with good shape with good reproducibility, as a method for manufacturing a diffraction grating for use in a λ/4-shift DFB laser. .

(Means for Solving the Problems) A method for manufacturing a diffraction grating according to the present invention irradiates a mirror having a function of producing a desired phase shift with parallel light, and directs the first reflected light from the mirror to a phase conjugate mirror. The second reflected wavefront reflected from the phase conjugate mirror and the reference light beam are caused to interfere with each other.

(Operation) The principle of the present invention will be explained. In order to form a diffraction grating with a good shape, a photoresist with good resolution may be used. Since the method of the present invention uses only one type of photoresist with good resolution, it is not only possible to manufacture a diffraction grating with a good shape, but also requires only one development and one etching process, reducing the number of process steps. Yield is good because it is small. Therefore, the method of the present invention overcomes the drawbacks of the first and second methods.

Furthermore, in the method of the present invention, there is no need to bring the substrate and quartz plate into close contact when performing two-beam interference exposure.
This method does not have the disadvantage that the area for forming the diffraction grating becomes smaller due to moiré fringes as in the third method.

In the fourth method, the shape area of the diffraction grating becomes extremely small due to the influence of aberrations, but as in the method of the present invention, it is possible to use Once the reflected light is made incident on a phase conjugate mirror, and the reflected light beam from the phase conjugate mirror is caused to interfere with the reference light beam, aberrations can be corrected and a diffraction grating can be formed on the entire surface of the substrate.

A phase conjugate mirror is a medium that generates phase conjugate waves. Figure 3 shows a phase conjugate mirror 30 and a normal mirror 33.
FIG. Figure 3 shows phase conjugate mirror 3.
FIG. 3B is a diagram showing the relationship between the incident wave 34 and the reflected wave 35 at a normal mirror 33. FIG. While a normal mirror 33 would cause the reflected wave 35 to diverge, the phase conjugate mirror 30 can return the reflected wave 32 along the optical path along which the incident wave 31 entered without causing the reflected wave 32 to diverge. That is, when a phase conjugate mirror is used, if phase fluctuations and aberrations change slowly over time during the optical path compared to the round trip of the light, the phase aberration will be canceled out just during the round trip. Therefore, it is possible to obtain reflected light that is not affected by disturbances in the optical path.

(Example) Below, one example of the present invention will be described in detail using the drawings. FIG. 1 is a diagram showing an optical system for manufacturing a λ/4 shift type diffraction grating. FIG. 2 is a diagram showing a beam of light incident on a phase shift mirror having a step for producing a phase shift and a beam of reflected light.

In FIG. 1, the light beam 10 and the reference light beam 17 are 1
The laser beam emitted from the He-Cd laser on the stand is split into two beams by a beam splitter, and each beam is passed through a beam expander. Note that the wavelength of the He-Cd laser beam used here was 3250 Å. A phase shift mirror 1 having the function of producing a desired phase shift is irradiated with obliquely incident parallel light 10, and reflected light 11 from the phase shift mirror 1 is made to enter a semi-transparent mirror 2. Next, the transmitted wave 12 from the semi-transparent mirror 2 is focused onto the phase conjugate mirror 4 through the lens 3. This is because the size of the CS ₂ mixed crystal used as the phase conjugate mirror 4 is small. In this embodiment, a CS ₂ mixed crystal excited using a method called four-wave mixing is used as the phase conjugate mirror 4. The reflected light 14 from the phase conjugate mirror 4 is made to enter the lens 3 again, and the transmitted wave 15 from the lens 3 is made to enter the semi-transparent mirror 2. A part of the transmitted light 15 becomes a parallel light beam 16 by the semi-transparent mirror 2. The parallel light beam 16 and the reference light beam 17 are irradiated onto the semiconductor substrate 5 at an incident angle of 42.6 degrees with respect to the horizontal plane. The semiconductor substrate is coated with AZ1350 (manufactured by Hoechst Co., Ltd.), which is a positive type photoresist, to a thickness of 500 Å.
After exposure, it was developed using AZ developer (manufactured by Hoechst) and transferred to a semiconductor substrate using a hydrogen bromide etchant, resulting in a period of 2400 without irregularities or crosslinks.
We were able to form a λ/4-shifted diffraction grating with a depth of 1000 Å.

(Effects of the Invention) The method for manufacturing a diffraction grating of the present invention is characterized in that aberrations of the optical system are canceled by using a phase conjugate mirror. By the method of the present invention, a λ/4-shifted diffraction grating with a period of 2400 Å and a depth of 1000 Å without irregularities or crosslinks could be formed over the entire surface of a semiconductor substrate with good reproducibility. When a semiconductor laser was manufactured using this diffraction grating, the threshold current was lowered by several mA compared to a diffraction grating formed using the conventional method, and the yield was improved to 98%, making the method of the present invention effective. It turns out that it is.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an optical system according to an embodiment of the present invention. 1 is a phase shift plate, 2 is a semi-transparent mirror, 3 is a lens, 4 is a phase conjugate mirror, 5 is a semiconductor substrate, 10 is an incident light beam, 11 is reflected light from the phase shift mirror 1,
12 is the transmitted light from the semi-transparent mirror 2, 13 is the lens 3
14 is the reflected light from the phase conjugate mirror 4, 15 is the transmitted light from the lens 3, 16 is the reflected light from the semi-transparent mirror 2, and 17 is a reference light flux. FIG. 2 is a sectional view showing a phase shift mirror, an incident light beam, and a reflected light beam. 20 is a stepped portion of the phase shift mirror. FIG. 3 is a diagram illustrating the difference between a phase conjugate mirror and a normal mirror, where 30 is a phase conjugate mirror, 33 is a normal mirror, 31 and 34 are incident lights, and 32 and 35 are reflected lights.

Claims (1)

[Claims] 1 In a method for manufacturing a diffraction grating in which a patterned photoresist is formed on a substrate through coating, exposure, and development steps, and the pattern is transferred onto the substrate, parallel light is passed through a mirror that has the function of producing a desired phase shift. irradiating the phase conjugate mirror with the first reflected light from the mirror, and causing a reference light beam to interfere with a second reflected wavefront reflected from the phase conjugate mirror for exposure. A method for manufacturing a diffraction grating.