GB2234364A

GB2234364A - Method of producing lambda /4-shifted diffraction grating

Info

Publication number: GB2234364A
Application number: GB9006051A
Authority: GB
Inventors: Masatoshi Fujiwara
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1989-07-28
Filing date: 1990-03-16
Publication date: 1991-01-30
Anticipated expiration: 2010-03-16
Also published as: GB9006051D0; DE4020319C2; DE4020319A1; GB2234364B; JPH0361901A

Abstract

A method of manufacturing a lambda /4-shifted diffraction grating includes the steps of applying an image reversible resist (2) on a substrate (1) on which a diffraction grating is to be formed, making interference exposure (3), applying an intermediate layer (4) and a photoresist layer (5), exposing and developing to remove layers (4) and (5) in one area thus exposing layer (2) in this area, developing the exposed image reversible resist, to form a first pattern of the interference exposure (Fig 1 (c)), developing the image reversible resist on the remaining region of the substrate after a reversal process of the image reversible resist, to form a second pattern which is a reversed pattern of the first pattern of the interference exposure (Fig 1 (d)) and etching to transfer the first and second resist patterns into the substrate (Fig 1(e)). Alternatively, the first pattern may be formed by reversal processing. <IMAGE>

Description

3.4 Z-3 6-1 -1

Method of Producing A/4-Shifted Diffraction Grating FIELD OF THE INVENTION

The present invention relates to a method of producing A/4-shifted diffraction grating, and more particularly to a method of producing A/4shifted diffraction grating by which a /4-shifted diffraction grating can be formed In simplified steps. BACKGROUND OF THE INVENTION

A method of providing a A/4-shifted region in a central portion of a diffraction grating has been generally known in order to make a distributed feedback laser possible to oscillate at a longitudinal fundamental mode.

Figures 2(a) to 2(e) show the steps of a prior art method of producing a phase shift type diffraction grating. In these figures, reference numeral 1 designates a semiconductor substrate. An negative resist 12 is deposited on the substrate 1. An intermediate layer 13 is put between the negative resist 12 and a first positive resist 14 for preventing mixing thereof. OBC (OHKA BACK COATING) which is of the same material as the negative resist and contains no photosensitive radicals is herein used as the intermediate layer 13. A first positive resist 14 is deposited on the intermediate layer 13. Reference numeral 15 designates a second positive resist. An oxygen isolating film 16 is deposited on the second positive resist 15 for preventing il the resist from chemically reacting with oxygen at the time of exposure. PVA (polyvinyl alcohol) which Is of the same material as the positive resist and contains no photosensitive radicals is herein used as the oxygen Isolating film 16.

The production process will be described.

A negative resist 12, an intermediate layer 13, and a first positive resist 14 are deposited on a semiconductor substrate 1, as shown in figure 2(a). A portion of the first positive resist 14 is then removed using ordinary photolithographic techniques, as shown in figure 2(b). Utilizing a pattern of this first positive resist 14 as a mask, a portion of the intermediate layer 13 and a portion of the negative resist 12 are etched away by using a solution of nitric acid as an etchant, as shown in figure 2(c). The first positive resist 14 is removed and then, a second positive resist 15 and an oxygen isolating film 16 are formed, as shown in figure 2(d). In this state, interference exposure is made as shown in figure 2(e), followed by development and rinsing, to form a pattern shown in figure 2(f) comprising the negative resist 12 and the second positive resist 15. The substrate 1 is etched utilizing this pattern as a mask and then, the negative resist 12 and the second positive resist 15 are removed. Consequently, a phase shift type diffraction grating shown 1 k in figure 2(g) is obtained.

The prior art method of producing a A/4-shifted diffraction grating, which comprises the foregoing steps, has the disadvantage in that the work becomes complicated in the developing step or the like because different types of resists must be applied. in addition, It has the disadvantage in that the area of a transition region is increased because the boundary between the different resists occurs. SUMMARY OF THE INVENTION

It Is an object of the present invention to provide a method of producing a A/4-shifted diffraction grating capable of obtaining a simplified process steps for producing a A/4-shifted diffraction grating with high precision.

Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the spirit and the scope of the invention will become apparent to those skilled in the art from this detailed description.

According to an aspect of the present invention, in the method of producing a A/4-shifted diffraction grating, an k 1 image reversible resist is applied on a substrate to make interference exposure, thereafter, the image reversible resist on a first region of the substrate is developed to form a first pattern which is a pattern of the interference exposure and then, the image reversible resist 6n a second region other than the first region of the substrate is developed after a reversal process of the image reversible resist to form a second pattern which is a reversed pattern of the pattern of the interference exposure, thereafter, the substrate is etched utilizing, as masks, the first and second patterns. Therefore, a reversed pattern can be formed by a single resist, so that the steps can be simplified. In addition, the precision of the diffraction grating can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

Figures l(a) to l(e) are cross-sectional views showing the steps of a method of manufacturing a ?V4-phase shifted diffraction grating according to an embodiment of the present invention; and Figures 2(a) to 2(e) are cross-sectional views showing the steps of a prior art method of manufacturing a A/4-phase shifted diffraction grating. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A method of producing a A/4-phase shifted diffraction grating according to an embodiment of the present invention c 1 1 1 - 5 will be described with reference to the drawings.

Figures l(a) to l(e) are cross-sectional views showing the steps of a method of manufacturing a A/4-phase shifted diffraction grating according to an embodiment of the present invention. In these figures, referencenumeral 1 designates a semiconductor substrate. An imagereversible resist 2 is deposited on the substrate 1. Reference numeral 3 designates luminous flux for exposure which forms interference fringes. A resist mixing preventing film 4 Is deposited on the reversible resist 2. As this resist mixing preventing film 4, OBC can be used similarly as in the prior art. A cover resist 5 is deposited on the film 4. As the cover resist 5, either a positive resist or a negative resist may be used.

The image reversible resist 2, which has been generally commercially available, has the following property. More specifically, the image reversible resist 2 exhibits the same property as that of the positive resist when it is developed without any modification after exposure, so that a portion exposed to light is removed by the development. On the other hand, the image reversible resist 2 exhibits the property reverse to the above described property when it is developed after reversal baking, so that the portion exposed to light is left by the development. A specific commodity of the image reversible resist includes AZ5206E.

6 The production process will be described.

An image reversible resist 2 is applied on a semiconductor substrate 1 to a thickness of several hundred angstroms to 1000 angstroms, to make interference exposure by a two-luminous flux interference method, as shown in figure l(a). After the interference exposure, a resist mixing preventing film 4 and a cover resist 5 are deposited, successively, as shown in figure l(b), with the image reversible resist 2 not being developed. As shown in figure 1(c), a portion of the cover resist 5 is then removed through exposure and development, and an exposed portion of the resist mixing preventing film 4 is removed utilizing, as a mask, the remaining portion of the cover resist 5, followed by developing the image reversible resist 2 in a portion where the cover resist 5 was removed. Thereafter, the property of the image reversible resist is reversed through reversal baking. The remaining cover resist 5 and resist mixing preventing film 4 are removed, followed by developing the remaining image reversible resist 2, as shown in figure l(d). In this developing process of figure 1(d), in order to raise the etching rate for a developer of the image reversible resist 2 to be removed in this portion, only a portion, in which no pattern is formed, of the image reversible resist 2 is subjected to exposure (flood exposure) after removing the cover resist 5 and the resist Q, A.

k_ mixing preventing film 4, followed by developing the resist. Thereafter, the substrate I is etched utilizing, as a mask, a pattern formed using the resist 2. The pattern is transferred onto the semiconductor substrate 1, to remove the resist as shown in figure l(e). Consequently, the steps are completed.

A A/4-shifted region is formed in a central portion of the diffraction grating obtained in the above described steps. If a distributed feedback semiconductor laser is fabricated using the semiconductor substrate having this diffraction grating formed thereon, it oscillates In a single mode at a Bragg wavelength.

As is evident from the foregoing description according to the present embodiment, the Image reversible resist is used, to be subjected to interference exposure and then, the image reversible resist on the first region of the substrate is developed without the reversal process while the image reversible resist on the remaining region thereof is developed after the reversal process to form a pattern, and the substrate is etched utilizing this pattern as a mask. Therefore, a reversed pattern can be formed by a single resist, allowing the steps to be simplified. Furthermore, the increase in the area of a transition region, which is caused when a pattern is formed using two types of resists, is prevented, allowing the precision of the diffraction

1 1 grating to be improved.

While in the above described embodiment, the pattern shown in figure l(d) is obtained by removing the cover resist 5 and the resist mixing preventing film 4 following baking after the step shown in figure 1(c) and then, conducting flood exposure only onto a portion, in which no pattern is formed, of the image reversible resist 2, followed by developing the resist, the pattern shown in figure l(d) may be obtained by conducting flood exposure onto the entire image reversible resist 2 after the step shown in figure 1(c), followed by baking and then, removing the cover resist 5 and the resist mixing preventing film 4, and conducting flood exposure onto the entire image reversible resist 2 again, followed by development. In such a case, the pattern formed in the step shown in figure l(c) is.baked, so that the pattern can be prevented from losing its shape in the step shown in figure 1(d).

Furthermore, while in the above described embodiment, a ?V4-shifted diffraction grating is directly formed on a semiconductor substrate, a diffraction grating may be formed on other films such as an insulating film formed on a semiconductor substrate. Alternatively, a diffraction grating may be formed on one or multiple semiconductor layers, having composition different from that of the semiconductor substrate, formed on the semiconductor 1 1 1 substrate.

As is evident from the foregoing description, according to the present invention, an image reversible resist is applied on a substrate to make interference exposure, the image reversible resist on a region of the substrate is developed to form a first pattern which is a pattern of the interference exposure and then, the image reversible resist on the remaining region of the substrate is developed after a reversal process of the image reversible resist to form a second pattern which is a reversed pattern of the pattern of the interference exposure, and the substrate is etched utilizing, as masks, the first and second patterns. Therefore, a A/4-shifted diffraction grating formed by simple steps and without increasing the area of a transition region can be obtained.

j 1

Claims

WHAT IS CLAIMED IS

1. A method of manufacturing a A/4-shifted diffraction grating, comprising the steps of: applying an image reversible resist on a substrate on which a diffraction gratiiig is to be formed, to make interference exposure; developing the image reversible resist on a region of said substrate, to form a first pattern of said Interference exposure; developing the image reversible resist on the remaining region of said substrate after a reversal process of said Image reversible resist, to form a second pattern which is a reversed pattern of the first pattern of said Interference exposure; and transferring'said first and second resist patterns onto said substrate.

2. A method of producing a A/4-shifted diffraction grating as defined in claim 1, which further includes a process of conducting flood exposure onto the image reversible resist on said remaining region of said substrate before developing for forming said second pattern.

3. A method of producing a A/4-shifted diffraction grating as defined in claim 1, wherein said reversal process 1 11 - is a reversal baking.

4. A method of producing a A/4-shifted diffraction grating as defined in claim 3, which includes a process of conducting flood exposure before said baking onto the entire image reversible resist including said first patterned portion, and a process of conducting flood exposure after said baking onto the reversible resist on said remaining region of said substrate.

5. A method of producing a A/4-shifted diffraction grating as defined in claim 1, wherein said substrate is a semiconductor substrate used on which a distributed feedback laser structure is produced.

6. A method of producing a 'A/4-shifted diffraction grating when performed substantially as described hereinbefore with reference to Figure 1 of the drawings.

Published 1991 at 7be Patent Wee. State House, 66/71 High Holborn, London WC I R4TP. Further copies maybe obtained from Sales Branch, Unit 6. Nine Mile Point, Curmfelinfach, Cross Keys, Newport, NPI 7HZ. Printed by Multiplex techniques lid. St Mary Cray, Kent.