Method for fabricating a semiconductor device comprising an oxide semiconductor

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

• forming a substrate;
• forming a gate electrode layer;
• forming a gate insulating layer over the gate electrode layer;
• forming an oxide semiconductor layer, in which a channel is formed, overlapping with the gate electrode layer with the gate insulating layer provided therebetween;
• forming a first insulating layer over the oxide semiconductor layer;
• forming a second insulating layer over the first insulating layer;
• forming a third insulating layer over the second insulating layer;
• and
• forming a source electrode layer and a drain electrode layer electrically connected to the oxide semiconductor layer,
• wherein the oxide semiconductor layer comprises indium, gallium, and zinc,
• wherein the oxide semiconductor layer is formed by a sputtering method,
• wherein the oxide semiconductor layer comprises a crystal region including a crystal with a particle size greater than or equal to 1 nm and less than or equal to 20 nm in a surface portion,
• wherein the source electrode layer or the drain electrode layer is in contact with a side surface of the oxide semiconductor layer,
• wherein the source electrode layer and the drain electrode layer overlap with the gate electrode layer, and
• wherein the crystal region has c-axis alignment.