Semiconductor device and manufacturing method thereof

A semiconductor device comprising:

• a transistor;
• an oxide insulating layer over the transistor, the oxide insulating layer having a trench;
• an oxide semiconductor film in contact with a bottom portion of the trench, the oxide semiconductor film including indium, gallium, and zinc;
• a gate insulating layer over the oxide semiconductor film;
• and
• a gate electrode over the gate insulating layer,
• wherein a width of the trench in a first position of the trench is greater than or equal to 1 time and less than or equal to 1.5 times a width of the trench in a second position of the trench, and
• wherein the bottom portion of the trench is closer to the second position of the trench than the first position of the trench.

A semiconductor device comprising:

• a transistor;
• an oxide insulating layer over the transistor;
• a trench formed in the oxide insulating layer;
• an oxide semiconductor film in contact with a bottom portion and side portions of the trench, the oxide semiconductor film including indium, gallium, and zinc;
• a source electrode and a drain electrode electrically connected to the oxide semiconductor film;
• a gate insulating layer over the oxide semiconductor film;
• and
• a gate electrode over the gate insulating layer,
• wherein the side portions have side surfaces substantially perpendicular to a top surface of the oxide insulating layer,
• wherein a width between upper ends of the trench is greater than or equal to 1 time and less than or equal to 1.5 times a width between the side surfaces, and
• wherein at least one of the bottom portion and a lower corner portion in which the bottom portion meets one of the side portions is curved.