Method for fabricating semiconductor device

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

• forming a semiconductor layer comprising a metal oxide;
• forming, over the semiconductor layer, a first conductive layer and a second conductive layer that are apart from each other over the semiconductor layer;
• performing first plasma treatment using a mixed gas comprising a first oxidizing gas and a reducing gas on a region where the semiconductor layer is exposed;
• forming a first insulating layer over the semiconductor layer, the first conductive layer, and the second conductive layer;
• and
• forming a second insulating layer over the first insulating layer,
• wherein the first insulating layer is formed by a plasma-enhanced chemical vapor deposition method using a mixed gas comprising a first gas comprising silicon, a second oxidizing gas, and a first ammonia gas,
• wherein the first insulating layer is formed successively after the first plasma treatment without exposure to an air,
• wherein the second insulating layer is formed by a plasma-enhanced chemical vapor deposition method using a mixed gas comprising a second gas comprising silicon and a third oxidizing gas, and
• wherein the second insulating layer is formed successively after formation of the first insulating layer without exposure to an air.