Method for manufacturing semiconductor device

A method for manufacturing a semiconductor device, comprising:

• forming a gate electrode layer over an insulating surface, wherein the gate electrode layer comprises a stack of a first layer comprising Ti and a second layer comprising Cu;
• forming a gate insulating layer over the gate electrode layer, wherein the gate insulating layer comprises a stack of a silicon nitride layer and a first silicon oxide layer, and the silicon nitride layer covers the second layer;
• forming an oxide semiconductor film over the gate electrode layer;
• forming a conductive film over the gate electrode layer;
• forming a first mask layer over the gate insulating layer, the oxide semiconductor film, and the conductive film;
• forming an oxide semiconductor layer and a conductive layer by etching the oxide semiconductor film and the conductive film with the use of the first mask layer in a first etching step;
• forming a second mask layer by ashing the first mask layer;
• and
• forming an oxide semiconductor layer having a depression, a source electrode layer, and a drain electrode layer by etching the oxide semiconductor layer and the conductive layer with the use of the second mask layer in a second etching step,
• wherein the first mask layer is formed using a light-exposure mask,
• wherein wet etching in which an etchant is used is employed in the first etching step,
• wherein dry etching in which an etching gas is used is employed in the second etching step, and
• wherein the oxide semiconductor layer having the depression includes a region with a smaller thickness than a region overlapping with the source electrode layer or the drain electrode layer.

A method for manufacturing a semiconductor device, comprising:

• forming a gate electrode layer over an insulating surface, wherein the gate electrode layer comprises a stack of a first layer comprising Ti and a second layer comprising Cu;
• forming a gate insulating layer over the gate electrode layer, wherein the gate insulating layer comprises a stack of a silicon nitride layer and a first silicon oxide layer, and the silicon nitride layer covers the second layer;
• forming a first oxide semiconductor film and a second oxide semiconductor film over the gate electrode layer;
• forming a conductive film over the gate electrode layer;
• forming a first mask layer over the gate insulating layer, the first oxide semiconductor film, the second oxide semiconductor film, and the conductive film;
• forming a first oxide semiconductor layer, a second oxide semiconductor layer, and a conductive layer by etching the first oxide semiconductor film, the second oxide semiconductor film, and the conductive film with the use of the first mask layer in a first etching step;
• forming a second mask layer by ashing the first mask layer;
• and
• forming an oxide semiconductor layer having a depression, a source region, a drain region, a source electrode layer, and a drain electrode layer by etching the first oxide semiconductor layer, the second oxide semiconductor layer, and the conductive layer with the use of the second mask layer in a second etching step,
• wherein the first mask layer is formed using a light-exposure mask,
• wherein wet etching in which an etchant is used is employed in the first etching step,
• wherein dry etching in which an etching gas is used is employed in the second etching step, and
• wherein the oxide semiconductor layer having the depression includes a region with a smaller thickness than a region overlapping with the source region or the drain region.