Method for manufacturing semiconductor device

A method of manufacturing a semiconductor device, comprising:

• forming a gate electrode including copper over a substrate;
• forming a first insulating layer over the gate electrode;
• forming an oxide semiconductor layer over the first insulating layer;
• performing a first heat treatment at a temperature equal to or higher than 350° C. and equal to or lower than 700° C.;
• forming a source electrode and a drain electrode after the first heat treatment;
• forming a second insulating layer over and in contact with the source electrode, the drain electrode and a portion of the oxide semiconductor layer;
• and
• performing a second heat treatment at a temperature equal to or higher than 200° C. and equal to or lower than 400° C. after forming the second insulating layer,
• wherein the first insulating layer includes silicon nitride and silicon oxide over the silicon nitride, and
• wherein the second insulating layer includes silicon oxide and silicon nitride over the silicon oxide.