Semiconductor device and method for manufacturing the same

A semiconductor device comprising:

• a gate electrode;
• a gate insulating film overlapping with the gate electrode;
• a stacked-layer oxide film including a channel formation region and overlapping with the gate electrode with the gate insulating film provided therebetween;
• and
• a source electrode and a drain electrode in contact with the stacked-layer oxide film,
• wherein the stacked-layer oxide film includes a first oxide layer and a second oxide layer over the first oxide layer,
• wherein the second oxide layer comprises:
• a depression portion between the source electrode and the drain electrode;
• and
• low-resistance regions in portions in contact with the source electrode and the drain electrode and part of side surfaces of the depression portion, and
• wherein a depth of the depression portion is greater than a depth of the low-resistance regions in a film thickness direction.

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

• forming a gate insulating film over a gate electrode;
• forming an oxide semiconductor layer over the gate insulating film;
• forming an oxide layer over the oxide semiconductor layer to form an stacked-layer oxide film containing the oxide semiconductor layer and the oxide layer;
• processing the stacked-layer oxide film into a predetermined shape;
• forming a conductive film containing Ti as a main component over and in contact with the stacked-layer oxide film;
• etching the conductive film to form a source electrode and a drain electrode, wherein a depression portion is formed in the oxide layer between the source electrode and the drain electrode;
• and
• performing a heat treatment, wherein low-resistance regions are formed in portions of the oxide layer which are in contact with the source electrode and the drain electrode, wherein a depth of the depression portion is greater than a depth of the low-resistance regions in a film thickness direction.