Method for Fabricating InP Diffraction Grating and Distributed Feedback Laser

(1) Field of the Invention

The present invention relates to a method for fabricating a distributed feedback laser, and more particularly to a low distortion distributed feedback laser for analog modulation.

(2) Description of the Related Art

Conventionally, a diffraction grating of a distributed feedback laser was formed with an optical interference exposure process. With such a method, however, it was difficult to form a diffraction grating having a non-uniform distribution in pitches and depths thereof or to form a diffraction grating at partial or local regions. It was also difficult to form a diffraction grating in a large area on a semiconductor substrate.

There is a known technique for forming a laser element in which, as a method for forming non-uniform diffraction gratings, an electron beam exposure is used, and which comprises the steps of uniformly coating an electron beam resist 1 on a semiconductor substrate (InP substrate) 2 as illustrated in FIG. 1A, varying electron beam diameters during the application of the electron beam exposure, forming diffraction grating patterns whose widths are narrower at a center portion and wider towards side portions as shown in FIG. 1B, and forming diffraction gratings having different heights by a wet-etching process.

However, in the conventional method explained above in which the electron beam diameters are varied to form non-uniform grating patterns, it was difficult to control the heights of resist by the electron beam exposure, and it was also difficult to control the heights of diffraction grating by dry-etching.

An object of the present invention, therefore, is to overcome the problems existing in the prior art and to provide a method for fabricating a low distortion distributed feedback laser for analog modulation with a high production yield through simple processes.

According to one aspect of the invention, there is provided a method for fabricating an InP diffraction grating for a distributed feedback semiconductor laser, the method comprising the steps of:

applying an electron beam resist on an InP semiconductor substrate;

giving electron beam exposure to the electron beam resist and controlling heights of resist patterns by using fixed electron beam diameters but by varying incident electron doses;

developing the electron beam resist; and

etching the semiconductor substrate using the electron beam resist as a mask,

the electron beam exposure being such that the incident electron doses are made larger at a center portion that at portions towards two sides of a resonator.

The basic principle of the invention resides in the utilization of a proximity effect of electron beams to develop in the electron beam exposure and in controlling resist pattern heights by simple processes.

In the electron beam exposure, the electron beams diffuse within the resist or reflect on a surface of the substrate resulting in a proximity effect by which the exposure patterns affect one another at portions where they are proximate to one another. This proximity effect appears remarkably especially when fine-line diffraction gratings are formed.

FIG. 3A shows a state wherein the exposure amounts of electron beams (incident electron doses) are different and, due to the proximity effect, the portions exposed to the electron beams become larger at portions at which the incident electron doses are larger (to the right in the drawings) and, inversely, the portions exposed become smaller at portions at which the incident electron doses are smaller (to the left in the drawings). Therefore, as shown in FIG. 3B, the resist patterns after the development result in shapes having a lower height and a narrower width at portions at which the incident electron doses are increased and, conversely, in shapes having a higher height and a wider width at portions at which the incident electron doses are decreased.

According to the invention, the doses of electron beams are varied, and, by utilizing the proximity effect, the heights and widths of the resist pattern shapes are controlled.

Thus, the invention provides a method for fabricating a low distortion distributed feedback laser for analog modulation with high production yields through simpler processes than those in the prior art. Furthermore, where the invention is embodied in a coherent type distributed feedback laser, the occurrence of spatial hole burning in the direction of resonator length can be suppressed so that it is possible to obtain a laser with narrow line widths.

The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention explained with reference to the accompanying drawings, in which:

FIGS. 1A and 1B are diagrammatic sectional view of a diffraction grating for a distributed feedback laser for explaining a conventional method for fabricating the same;

FIGS. 2A, 2B and 2C are diagrammatic sectional or a diffraction grating for a distributed feedback laser for explaining a method for fabricating the same according to the present invention; and

FIGS. 3A and 3B are diagrams showing a portion of the diffraction grating for explaining a proximity effect in the electron beam exposure in an embodiment according to the invention.

Now, a preferred embodiment of the invention is explained with reference to the accompanying drawings.

FIGS. 2A, 2B and 2C show in diagrammatic sectional views a diffraction grating of a distributed feedback laser for explaining a fabrication method therefor as an embodiment according to the invention.

First, an electron beam resist PMMA (Poly Methyl Methacrylate) 1 having a thickness of about 1500 Angstroms is uniformly applied on an InP semiconductor substrate 2 as shown in FIG. 2A and, in order to obtain the diffraction grating shapes as designed in advance, the electron beam exposure is applied such that the electron beam doses become larger at a center portion of the resonator than at both end portions thereof. In this case, due to the proximity effect explained above, the resist diffraction patterns formed in the electron beam resist result in a shape having a lower height and a narrower width with an increase in the electron beam doses at center portions of the resonator and is a higher height and a wider width with a decrease in the electron beam doses at both end portions thereof as shown in FIG. 2B. For example, where, by using an electron beam resist in which a PMMA resist of 1500 Angstroms thick is applied, a line-and-space with 0.2 .mu.m pitches which becomes a primary diffraction grating for a 1.3 .mu.m wavelength distributed feedback laser is formed, the electron dose for controlling the proximity effect is 2-4 nC/cm. Also, at the center portion of the diffraction gating, there is formed a phase shift 3 of .lambda./4. This resist pattern is etched by dry-etching using a Cl.sub.2 type gas and is transferred onto the InP substrate 2, whereby a diffraction grating having a non-uniform shape is obtained as shown in FIG. 2C.

On the substrate in which the non-uniform diffraction gratings have been formed, there are sequentially grown by and MOVPE (Metal-Organic Vapor Phase Epitaxy) method an n-InGaAsP waveguide layer, an n-InP spacer layer, an n-In-GaAsP SCH (Separate Confinement Heterostructure) layer, an MQW (Multi-Quantum Well) active layer, a p-InGaAsP SCH layer, and an InP cladding layer, whereby a double hetero-structure wafer having an active optical waveguide is fabricated. Then, the wafer is buried in a DC-PBH (Double-Channel Plainer Buried Heterostructure) by a LPE (Liquid Phase Epitaxy) process, and electrodes are formed at p-side and n-side respectively. Finally, the resulting structure is cleaved to a cavity length 300 .mu.m and both facets are provided with antireflection coatings whereby a low distortion distributed feedback laser for analog modulation is formed.

The element thus formed was evaluated, which showed excellent results with the oscillation threshold value being about 15 mA and the slope effect being 0.4 W/A and, at the modulaation by a 1.5 GHz band 2 tone test, and IMD.sub.3 .ltoreq.-80 dBc was obtained under 20% optical modulation. Also, it showed that the uniformity was good and the production yield was improved to over 50%.

In this embodiment of the invention, the electron beam resist employed the PMMA resist but may employ a P (MMA-co-MAA) resist with the same or similar advantage.

Also, the dry-etching using a Cl.sub.2 is exemplified in the embodiment but a CH.sub.4, C.sub.2 H.sub.6 type gas may well be used.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.