Patent ID: 8906756

Claim:
A method for manufacturing a semiconductor device, comprising the sequential steps of: forming a gate electrode layer; forming a gate insulating layer over the gate electrode layer; forming an oxide semiconductor layer over the gate insulating layer so as to overlap with the gate electrode layer; increasing a temperature of the oxide semiconductor layer in an inert gas atmosphere, a dry air atmosphere, or an oxygen atmosphere; performing first heat treatment on the oxide semiconductor layer in an oxygen atmosphere after the step of increasing the temperature of the oxide semiconductor layer; performing second heat treatment on the oxide semiconductor layer under reduced pressure after the step of performing the first heat treatment; performing cooling of the oxide semiconductor layer in an oxygen atmosphere after the step of performing the second heat treatment; forming a source electrode layer and a drain electrode layer which are electrically connected to the oxide semiconductor layer after the step of performing the cooling; and forming an insulating layer over the oxide semiconductor layer, the source electrode layer, and the drain electrode layer.