JPH02213182A - Manufacture of diffraction grating - Google Patents

Abstract

PURPOSE:To enable formation of a phase shift diffraction grating by a simple resist process, by applying a resist on a substrate provided with steps by etching in a part wherein a phase shift is desired to be caused and by forming a diffraction grating pattern on the resist by a two-beam interference exposure method. CONSTITUTION:Only an area wherin a phase shift is desired to be cuased is etched to provide steps on a substrate by an ordinary technique of photolithography, and a photoresist 2 is applied thereon. The semiconductor substrate 1 is inclined by an angle psi10 from the direction vertical to the central axis 5 and two parallel laser beams of an incident angle theta are applied from above to execute interference exposure. When it is assumed that 0-points 6 in the figure are points to which the optical paths of the laser lights from two directions are equal and at which the lights are intensified, the 0-points on a broken line are points whereat the lights intensity each other in the case when there are no step. At the bottom of the stepped part, the points 7 whereat the lights intensify each other are shifted by a dimension 8 in the direction of the central axis under the condition that the optical paths are not varied. By varying the depth d9 of a groove and the angle a10 of the substrate, accordingly, the amount 8 of the phase shift can be controlled precisely.

Description

[Detailed description of the invention] Industrial applications The present invention relates to a method for manufacturing a diffraction grating.

A conventional distributed feedback semiconductor laser (hereinafter referred to as a DFB laser) oscillates in a single longitudinal mode, that is, at a single wavelength, around the Bragg wavelength determined by the period of a diffraction grating built into the device. Therefore, the practical use of optical fibers that use optical fibers with wavelength dispersion as light sources that can be used over long distances and with large capacity is progressing.
Ru. However, DI with a uniform diffraction grating within the element
In the B laser, the wavelength that provides the minimum oscillation threshold gain is in principle the Bragg wavelength (does not coincide with it, and there are two adjacent wavelengths on both sides).
This is because it has a longitudinal mode that can oscillate.

Due to instability caused by competition between these two longitudinal modes, there was a point where stable single longitudinal mode oscillation could not be obtained with good yield.

Therefore, in order to make all the elements oscillate in a single longitudinal mode at the Bragg wavelength by matching the Bragg wavelength with the wavelength that gives the minimum threshold gain, the phase of the left and right diffraction gratings is changed to λn/4 ( A phase-shifted DFB laser having a structure shifted by λn: wavelength of light propagating within the device has been proposed.

By the way, in the case of a first-order diffraction grating, in order to create a λ/4 shift diffraction grating, it is necessary to reverse the unevenness of the left and right diffraction gratings. Such a λ/4-shift type diffraction grating requires a more complicated fabrication process compared to one fabrication process for a uniform diffraction grating.

Pages 1008-1o1o (published November 22, 1984)
An interference exposure method that utilizes the inverse photosensitivity of the positive and negative type 7 photoresists described above is known. This is achieved by forming positive and negative type photoresists adjacent to each other on a semiconductor substrate and exposing both types of photoresists simultaneously using the conventional two-beam interference exposure method. The diffraction grating has concavities and convexities that are inverses of each other. As a method for producing a phase shift diffraction grating other than the above, there is a method described in Japanese Patent Application Laid-Open No. 61-289688. The following is a simple explanation of the method: Apply photol/cyst onto a semiconductor substrate, form a dielectric film with a thickness of t only in the areas where the phase is to be reversed, and hold the semiconductor substrate at an angle of ψ. 2 of the angle of incidence θ from above to the horizontal plane
Interference exposure is performed by irradiating the book with a laser beam. As a result, the phase of the diffraction grating pattern exposed on the photoresist through the dielectric film is out of phase with the phase of the diffraction grating pattern on the photoresist in the portion where the dielectric film is not present.

The phase difference is controlled by the thickness t of the dielectric film, the refractive index n, and the stiffness angle ψ.

Problems to be Solved by the Invention However, the above-mentioned conventional technology has disadvantages in that it requires two types of resist coating processes, requires the formation of a dielectric film whose thickness is accurately controlled, and the process becomes complicated. There is.

The present invention provides a method of obtaining an effect equivalent to that of a phase shift diffraction grating using simple resist etching.

The only way to solve the problem is to apply a resist onto a semiconductor substrate on which a phase shift is to be made, on which a diffraction grating is to be formed, on which a step is formed by etching.1. form a diffraction grating pattern.

In order to fabricate a phase shift diffraction grating with uniform action and good reproducibility, it is better to use a type of Jv photoresist. The present invention uses a single 7-photoresist to shift the phase of a diffraction grating between concave and convex portions on a substrate. The specific effect will be explained using FIG. 2. Etch only the area where you want to shift the phase by one level using normal photolithography technology.

After providing a step on the substrate as shown in FIG. 2, a photoresist 2 is applied. The semiconductor substrate 1 is tilted by an angle ψ10 from a direction perpendicular to the central axis 6, and two parallel laser beams having an incident angle θ are irradiated from above to perform interference exposure. Assume that the optical path lengths of the laser beams from the two directions are equal at point 0 6 in the figure, and this is the point where the light is intensified. The 0 point with break a is the point where the light intensifies each other when there is no step. At the bottom of the step, the point 7 where the light intensifies is shifted in the direction of the central axis due to the condition that the optical path length does not change. When the depth 9 of the step is d, at the bottom of the step, the point where the light intensifies is δ=d t&nψ in the direction along the substrate surface (1)
The phase of the diffraction grating is shifted by the equation.

Therefore, by changing the depth d9 of the groove and the angle α1° of the substrate, it is possible to precisely control one phase shift 8.

EXAMPLES Below, examples of the present invention will be explained in detail with reference to the drawings.

Fig. 1 shows the manufacturing method of the diffraction grating of the present invention? It is a diagram explaining the process in order. (IL) is a negative photoresist (for step formation) with a film thickness of 5000 on the XnP semiconductor substrate 1.
2 is applied, and the resist in the portions corresponding to the recesses is removed by mask exposure using an ordinary mercury lamp. In (b), the remaining resist is used as a mask.

H2SO at 25°C: H,02: H20=4:
Etch with a 1:1 mixed solution, and the depth of the step is d.
3,600 people. In (0), after coating a negative photoresist (for forming a diffraction grating) 3 with a film thickness of 700 mm on a substrate with steps, the substrate is 20 mm thick from the direction perpendicular to the central axis 5. 'Only in a state of collapse.

Using a Ha-Cd laser beam with a wavelength of 3260.

Interference exposure is performed so that the angle of incidence of the two light beams is 43.7° with respect to the central axis. The resist pattern (e) is obtained by development after exposure. Using this resist pattern as a mask, saturated bromine water: H, PO4:H20=2:1
:15 as etching solution.

Etch for 20fi at 26°C. After etching, remove the mask resist using a resist remover, (f)
Obtain a phase-shift grating pattern on the substrate as shown in D.

In this embodiment, the period A of the first diffraction grating is 2,500, which is the period trap of the first-order diffraction grating necessary for the DFB laser in the 1.55 μyrL band.

Further, the shift amount of the diffraction gratings at the top and bottom portions with a one-step difference is 1000 from equation (1). Therefore, the unevenness is reversed at the top and bottom in the direction along the surface of the substrate (10 in FIG. 1), and a diffraction grating for a λ/4 shift DFB laser is obtained.

Note that by changing the depth d of one step and the inclination angle ψ of the substrate, one diffraction grating can be freely opened and opened by 77 degrees.

DFB using this phase shift diffraction grating substrate with steps.
When performing laser crystal growth, this step is flattened during the growth of the guide layer below the active layer, so it does not affect the characteristics of the laser device.

Effects of the Invention As described below, by providing steps on a semiconductor substrate, a phase-shifted diffraction grating can be formed in nine steps, similar to the conventional three-beam interference exposure method using a single photoresist. can. Because it is relatively easy to open and open the depth when etching to create a step.

It was found that the reproduction and control of the amount of phase shift are excellent.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining the manufacturing method of a diffraction grating, which is an actual example of the present invention, in step S, and Figure 2 is a diagram explaining the principle of the manufacturing method of a phase shift diffraction grating on a semi-stepped substrate. be. 1... Semiconductor substrate, 2... Photoresist (
(for step formation), 3... photoresist (for diffraction grating formation), 4... diffraction grating, 5... center,
Sleeves, 6... The point where there is no step, the light intensifies each other, 7... The point where the light intensifies each other at the bottom of the step, 8...
・Phase shift amount δ9...Depth of step d, 1o...
...The rolling angle ψ of the board. Name of agent: Patent attorney Takashi Awano and 1 other person 1st
Figure (υ I-・Lord S Hayakure Himetsu 3-・Photo Shizuto

Claims (2)

[Claims] (1) A step of providing a step on a semiconductor substrate, a step of applying a photoresist on the substrate, and an interference exposure of the photoresist by irradiating the surface of the substrate with two laser beams of the same wavelength with different incident angles. a step of developing the photoresist to form a periodic resist pattern; and a step of etching the semiconductor substrate using the periodic resist as an etching mask. . (2) A distributed feedback semiconductor laser incorporating a diffraction grating manufactured by the manufacturing method according to claim 1.