A semiconductor structure includes a fin structure formed over a substrate. The structure also includes a gate structure formed across the fin structure. The structure also includes source/drain epitaxial structures formed on opposite sides of the gate structure. The structure also includes an inter-layer dielectric (ILD) structure formed over the gate structure. The structure also includes a contact blocking structure formed through the ILD structure over the source/drain epitaxial structure. A lower portion of the contact blocking structure is surrounded by an air gap, and the air gap is covered by a portion of the ILD structure.