Method for manufacturing semiconductor device

A method for manufacturing a semiconductor device comprising the steps of:

• forming an insulating layer over a substrate,
• forming a first oxide semiconductor layer over the insulating layer;
• forming a second oxide semiconductor layer over the first oxide semiconductor layer;
• performing a first heat treatment on the first oxide semiconductor layer and the second oxide semiconductor layer;
• after the first heat treatment, cooling the first oxide semiconductor layer and the second oxide semiconductor layer in an atmosphere containing oxygen;
• forming a conductive film over the first oxide semiconductor layer and the second oxide semiconductor layer after the first heat treatment;
• etching the conductive film to form a source electrode layer and a drain electrode layer;
• forming an oxide insulating film in contact with part of the second oxide semiconductor layer;
• and
• performing a second heat treatment after the oxide insulating film is formed,
• wherein the first heat treatment is performed at a first temperature higher or equal to 400° C.

A method for manufacturing a semiconductor device comprising the steps of:

• forming an insulating layer over a substrate,
• forming a first oxide semiconductor layer over the insulating layer;
• forming a second oxide semiconductor layer including crystals over the first oxide semiconductor layer;
• performing a first heat treatment on the first oxide semiconductor layer and the second oxide semiconductor layer;
• after the first heat treatment, cooling the first oxide semiconductor layer and the second oxide semiconductor layer in an atmosphere containing oxygen;
• forming a conductive film over the first oxide semiconductor layer and the second oxide semiconductor layer after the first heat treatment;
• etching the conductive film to form a source electrode layer and a drain electrode layer;
• and
• forming an oxide insulating film in contact with part of the second oxide semiconductor layer.

A method for manufacturing a semiconductor device comprising the steps of:

• forming an insulating layer over a substrate,
• forming a first oxide semiconductor layer over the insulating layer;
• forming a second oxide semiconductor layer over the first oxide semiconductor layer;
• performing a first heat treatment on the first oxide semiconductor layer and the second oxide semiconductor layer;
• after the first heat treatment, supplying oxygen to the first oxide semiconductor layer and the second oxide semiconductor layer in an atmosphere containing oxygen;
• forming a conductive film over the first oxide semiconductor layer and the second oxide semiconductor layer after the first heat treatment;
• etching the conductive film to form a source electrode layer and a drain electrode layer;
• and
• forming an oxide insulating film in contact with part of the second oxide semiconductor layer.