Method for manufacturing semiconductor device

Provided is a method for manufacturing a semiconductor device with favorable electrical characteristics. The following steps are performed in the following order: forming an oxide semiconductor film over a substrate having a substantially planar surface; selectively etching the oxide semiconductor film to form an oxide semiconductor layer; implanting an oxygen ion on a top surface of the oxide semiconductor layer and a side surface of the oxide semiconductor layer in a cross-section perpendicular to the substantially planar surface in a channel width direction of the oxide semiconductor layer from an angle 0°