Method for etching insulating film and method for manufacturing semiconductor optical device

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

• growing a semiconductor layer;
• forming an insulating film on the semiconductor layer;
• forming a first resin layer composed of a non-silicon-containing resin on the insulating film;
• forming a pattern including projections and recesses in the first resin layer;
• forming a second resin layer composed of a silicon-containing resin to cover the projections and the recesses of the pattern in the first resin layer;
• etching the second resin layer by reactive ion etching with etching gas containing CF4 gas and oxygen gas until the projections of the first resin layer are exposed, a Si component of the second resin layer being oxidized in etching the second resin layer;
• selectively etching the first resin layer until the insulating film is exposed using as a mask the second resin layer buried in the recesses of the first resin layer to form a resin layer mask;
• etching the insulating film using the resin layer mask;
• and
• etching the semiconductor layer using the insulating film as a mask to form the diffraction grating,
• wherein in the step of etching the second resin layer, the reactive ion etching is performed under applying a self-bias voltage so that a Si product is removed, the Si product being provided from the Si component remaining in the second resin layer without being oxidized.