Method for manufacturing a semiconductor device

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

• forming a source electrode layer and a drain electrode layer over an insulating surface;
• forming an oxide semiconductor layer in a gap between the source electrode layer and the drain electrode layer by a coating method, and then performing heat treatment on the oxide semiconductor layer;
• forming a gate insulating layer in contact with a top surface of the source electrode layer, the drain electrode layer, and the oxide semiconductor layer;
• forming a gate electrode layer in a region overlapping with the oxide semiconductor layer with the gate insulating layer interposed therebetween;
• introducing an impurity element into the oxide semiconductor layer with the gate electrode layer used as a mask, thereby forming in the oxide semiconductor layer a pair of impurity regions and a channel region sandwiched between the pair of impurity regions;
• forming a conductive film covering the gate electrode layer;
• forming an insulating layer over the conductive film;
• processing the insulating layer, thereby forming a first sidewall insulating layer and a second sidewall insulating layer provided on a side surface of the gate electrode layer in a channel length direction with the conductive film interposed therebetween;
• and
• etching the conductive film with the first sidewall insulating layer and the second sidewall insulating layer used as a mask, thereby forming a first conductive layer and a second conductive layer in contact with the side surface of the gate electrode layer in the channel length direction.