| abstract |
An object is to provide a semiconductor device using an oxide semiconductor having stable electrical characteristics. Nitrogen or a rare gas (such as argon or helium) for an oxide semiconductor layer Under an inert gas atmosphere or under reduced pressure, heat treatment for dehydration or dehydrogenation treatment is performed, and oxygen, oxygen and nitrogen, or air (preferably dew point of −40 ° C. or lower, more preferably −5). A highly purified and i-type oxide semiconductor layer is formed by performing a cooling step for oxidation treatment under an atmosphere of 0 ° C. or lower. A semiconductor device including a thin film transistor including the oxide semiconductor layer is manufactured. [Selection] Figure 1 |