Semiconductor device and manufacturing method thereof

A semiconductor device comprising:

• a gate electrode layer;
• a gate insulating layer over the gate electrode layer;
• a high-conductive oxide semiconductor layer over the gate insulating layer;
• an oxide semiconductor layer over the high-conductive oxide semiconductor layer;
• a first buffer layer and a second buffer layer over the oxide semiconductor layer;
• and
• source and drain electrode layers over the first buffer layer and the second buffer layer,
• wherein the first buffer layer and the second buffer layer have higher conductivity than the oxide semiconductor layer and are subjected to reverse sputtering treatment and heat treatment in a nitrogen atmosphere,
• the high-conductive oxide semiconductor layer has higher conductivity than the oxide semiconductor layer and is subjected to reverse sputtering treatment and heat treatment in a nitrogen atmosphere, and
• the oxide semiconductor layer is electrically connected to the source and drain electrode layers with the first buffer layer and the second buffer layer interposed therebetween.

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

• forming a gate electrode layer over a substrate;
• forming a gate insulating layer over the gate electrode layer;
• forming a first oxide semiconductor film over the gate insulating layer, using a sputtering method;
• subjecting the first oxide semiconductor film to heat treatment;
• forming a second oxide semiconductor film over the first oxide semiconductor film using a sputtering method;
• subjecting the second oxide semiconductor film to reverse sputtering treatment;
• subjecting the second oxide semiconductor film to heat treatment in a nitrogen atmosphere;
• etching the first oxide semiconductor film and the second oxide semiconductor film to form an oxide semiconductor layer and a first buffer layer;
• forming a conductive film over the oxide semiconductor layer and the first buffer layer;
• etching the conductive film and the first buffer layer to form source and drain electrode layers, a second buffer layer, and a third buffer layer;
• and
• subjecting the oxide semiconductor layer to heat treatment,
• wherein the second buffer layer and the third buffer layer have higher conductivity than the oxide semiconductor layer.

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

• forming a gate electrode layer over a substrate;
• forming a gate insulating layer over the gate electrode layer;
• forming a first oxide semiconductor film over the gate insulating layer using a sputtering method;
• subjecting the first oxide semiconductor film to heat treatment;
• forming a second oxide semiconductor film over the first oxide semiconductor film using a sputtering method;
• subjecting the second oxide semiconductor film to heat treatment in a nitrogen atmosphere;
• subjecting the second oxide semiconductor film to reverse sputtering treatment;
• etching the first oxide semiconductor film and the second oxide semiconductor film to form an oxide semiconductor layer and a first buffer layer;
• forming a conductive film over the oxide semiconductor layer and the first buffer layer;
• etching the conductive film and the first buffer layer to form source and drain electrode layers, a second buffer layer, and a third buffer layer;
• and
• subjecting the oxide semiconductor layer to heat treatment,
• wherein the second buffer layer and the third buffer layer have higher conductivity than the oxide semiconductor layer.