Semiconductor device and manufacturing method thereof

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

• forming a second insulator over a first insulator;
• forming a first conductor over the second insulator;
• forming a first conductive layer by etching the first conductor;
• forming a third insulator over the second insulator and the first conductive layer;
• forming a fourth insulator over the third insulator;
• forming a semiconductor over the fourth insulator;
• forming a second conductor over the semiconductor;
• forming a second conductive layer by etching the second conductor;
• separating the second conductive layer into a third conductive layer and a fourth conductive layer by etching the second conductive layer;
• forming a semiconductor layer by etching the semiconductor;
• forming a first insulating layer by etching the fourth insulator;
• forming a fifth insulator over the third insulator but not over the semiconductor layer, the third conductive layer, and the fourth conductive layer;
• forming a sixth insulator over the fifth insulator, the semiconductor layer, the third conductive layer, and the fourth conductive layer, and in contact with a side surface of the first insulating layer, a side surface of the semiconductor layer, a side surface of the third conductive layer, and a side surface of the fourth conductive layer;
• forming a third conductor over the sixth insulator;
• and
• forming a fifth conductive layer by etching the third conductor.