Optical semiconductor device, and method for producing the same

A method for producing an optical semiconductor device comprising a first optical waveguide having a first optical coupling portion, and a second optical waveguide having a second optical coupling portion along the first optical coupling portion, the first and second optical coupling portions being spaced from and facing each other, the method comprising:

• (a) preparing an SOI substrate comprising a semiconductor substrate, a first insulator film over the semiconductor substrate, and a semiconductor layer over the first insulator film;
• (b) applying a first photoresist over the semiconductor layer, and subsequently subjecting the SOI substrate with the first photoresist to dry exposure using an ArF excimer laser or to dry exposure using a KrF excimer laser, followed by developing treatment, so as to form a first resist pattern comprising the first photoresist over the semiconductor layer;
• (c) using the first resist pattern as a mask to pattern the semiconductor layer to form a side-surface of the first optical coupling portion that faces the second optical coupling portion, and a side-surface of the second optical coupling portion that faces the first optical coupling portion, and subsequently removing the first resist pattern;
• (d) applying a second photoresist over the semiconductor layer, and subsequently subjecting the SOI substrate with the second photoresist to liquid-immersion exposure using an ArF excimer laser, followed by developing treatment, so as to form a second resist pattern comprising the second photoresist over the semiconductor layer;
• (e) using the second resist pattern as a mask to further pattern the semiconductor layer, so as to form other portions of the first and second optical waveguide, the other portions having opposite side-surfaces and being any portion other than the first and second optical coupling portions, and subsequently removing the second resist pattern;
• and
• (f) forming a second insulator film over the first insulator film to cover the first and second optical waveguides.

An optical semiconductor device comprising:

• a semiconductor substrate;
• a first insulating film formed over the semiconductor substrate;
• a first core comprised of silicon;
• a second core comprised of silicon;
• and
• a second insulating film formed over the first insulating film, the first core, and the second core;
• wherein a first optical waveguide is formed by the first core and surrounding portions of the first and second insulating films,
• a second optical waveguide is formed by the second core and surrounding portions of the first and second insulating films,
• the first and second cores are spaced closer to each other in a coupling region than outside the coupling region,
• a side surface of the first core facing the second core in the coupling region has a line edge roughness larger than that of side surfaces of the first core outside the coupling region,
• a side surface of the second core facing the first core in the coupling region has a line edge roughness larger than that of side surfaces of the second core outside the coupling region,
• another side surface of the first core that is opposite to the side surface facing the second core in the coupling region also has a line edge roughness that is larger than that of the side surfaces of the first core outside the coupling region, and
• another side surface of the second core that is opposite to the side surface facing the first core in the coupling region also has a line edge roughness that is larger than that of the side surfaces of the second core outside the coupling region.