Semiconductor device and manufacturing method thereof

A semiconductor device comprising:

• a driver circuit portion comprising a first thin film transistor and a pixel portion comprising a second thin film transistor over the same substrate, the first thin film transistor and the second thin film transistor each comprising:
• a gate electrode layer over a substrate;
• a gate insulating layer over the gate electrode layer;
• an oxide semiconductor layer over the gate insulating layer;
• a source electrode layer over the oxide semiconductor layer, the source electrode layer having a shape whose end portion is located on an inner side than an end portion of the oxide semiconductor layer;
• a drain electrode layer over the oxide semiconductor layer, the drain electrode layer having a shape whose end portion is located on an inner side than an end portion of the oxide semiconductor layer;
• a first oxide conductive layer in contact with the source electrode layer and the oxide semiconductor layer;
• a second oxide conductive layer in contact with the drain electrode layer and the oxide semiconductor layer;
• an oxide insulating layer over the oxide semiconductor layer, the source electrode layer and the drain electrode layer;
• a pixel electrode layer over the oxide insulating layer;
• and
• a liquid crystal layer over the pixel electrode layer,
• wherein the first thin film transistor further comprises a conductive layer which overlaps with the oxide semiconductor layer over the oxide insulating layer, and
• wherein the second thin film transistor is electrically connected to the pixel electrode layer.

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

• forming a first gate electrode layer and a second gate electrode layer over a substrate;
• forming a gate insulating layer over the first gate electrode layer and the second gate electrode layer;
• forming an oxide semiconductor layer over the gate insulating layer;
• forming a first conductive layer over the oxide semiconductor layer;
• forming a first resist mask comprising a first region and a second region with a thickness smaller than a thickness of the first region over the first conductive layer;
• etching the first conductive layer and the oxide semiconductor layer to form a first island-shape oxide semiconductor layer overlapped with the first gate electrode layer, a second island-shape oxide semiconductor layer overlapped with the second gate electrode layer, a first island-shape conductive layer over the first island-shape oxide semiconductor layer and a second island-shape conductive layer over the second island-shape oxide semiconductor layer by using the first resist mask;
• ashing the first resist mask to form a second resist mask by removing the second region;
• etching the first island-shape conductive layer and the second island-shape conductive layer to form a first source electrode layer electrically connected to the first island-shape oxide semiconductor layer, a first drain electrode layer electrically connected to the first island-shape oxide semiconductor layer, a second source electrode layer electrically connected to the second island-shape oxide semiconductor layer and a second drain electrode layer electrically connected to the second island-shape oxide semiconductor layer by using the second resist mask;
• forming an oxide insulating layer over the first island-shape oxide semiconductor layer, the second island-shape oxide semiconductor layer, the first source electrode layer, the first drain electrode layer, the second source electrode layer and the second drain electrode layer;
• forming a transparent conductive film over the oxide insulating layer;
• etching the transparent conductive film to form a pixel electrode layer electrically connected to the second drain electrode layer and a second conductive layer overlapped with the first gate electrode layer;
• and
• forming a liquid crystal layer over the pixel electrode layer.