Semiconductor structure having air gap between gate electrode and distal end portion of active area and fabrication method thereof

A method for fabricating a semiconductor structure, comprising:

• providing a semiconductor substrate having a major surface;
• forming a trench isolation region and an active area surrounded by the trench isolation region in the semiconductor substrate, wherein the active area has a longitudinal axis extending along a first direction;
• forming a first gate trench in the active area and a second gate trench in the trench isolation region;
• forming a sacrificial spacer in the second gate trench;
• forming a first gate electrode and a second gate electrode in the first gate trench and the second gate trench, respectively;
• removing the sacrificial spacer from the second gate trench, thereby forming an air gap between the second gate electrode and the semiconductor substrate in the second gate trench, wherein the air gap overlaps with an entire thickness of the second gate electrode along a thickness direction that is perpendicular to a top surface of the semiconductor substrate;
• and
• depositing a cap dielectric layer on the second gate electrode, wherein the cap dielectric layer seals the air gap.