Method for manufacturing semiconductor optical device

A method for manufacturing a semiconductor optical device, comprising the steps of:

• preparing a mold having an imprint pattern including periodic projections and recesses;
• forming a substrate product including a semiconductor layer that extends along a reference plane;
• forming a first resin layer on the semiconductor layer;
• forming a diffraction grating pattern having periodic projections and recesses in the first resin layer by transferring the imprint pattern using the mold by nanoimprint lithography, the projection of the diffraction grating pattern having a top portion and a base portion;
• changing a duty ratio of the diffraction grating pattern in the first resin layer by dry-etching the first resin layer;
• forming a second resin layer on the first resin layer so as to cover the projection and the recess of the diffraction grating pattern in the first resin layer;
• removing the top portion of the projection of the diffraction grating pattern by etching back the first resin layer and the second resin layer while leaving the base portion of the projection, the second resin layer being left in the recess of the first resin layer;
• selectively etching the first resin layer through the second resin layer left in the recess of the first resin layer, the first resin layer having a reverse pattern to the diffraction grating pattern;
• and
• etching the semiconductor layer through the first resin layer having the reverse pattern to the diffraction grating pattern to form a diffraction grating.