Semiconductor device and manufacturing method thereof

A manufacturing method of a semiconductor device comprising:

• forming an oxide insulating layer;
• forming a depressed portion including side surfaces substantially perpendicular to a top surface of the oxide insulating layer in the oxide insulating layer by performing first plasma etching treatment;
• forming a trench including curved lower corner portions and curved upper corner portions by performing second plasma etching treatment and third plasma etching treatment on the depressed portion;
• forming an oxide semiconductor film in contact with a bottom portion, the curved lower corner portions, and side portions of the trench in the oxide insulating layer;
• forming a source electrode and a drain electrode electrically connected to the oxide semiconductor film;
• forming a gate insulating layer over the oxide semiconductor film;
• and
• forming a gate electrode over the gate insulating layer,
• wherein the first plasma etching treatment is performed with application of a first bias power and a first power,
• wherein the second plasma etching treatment is performed with application of a second bias power and a second power,
• wherein the third plasma etching treatment is performed in a rare gas atmosphere, and
• wherein the second bias power is lower than the first bias power.