Semiconductor device and the manufacturing method thereof are disclosed herein. An exemplary semiconductor device comprises a first semiconductor fin and a second semiconductor fin formed over a substrate, wherein lower portions of the first semiconductor fin and the second semiconductor fin are separated by an isolation structure; a first gate stack formed over the first semiconductor fin and a second gate stack formed over the second semiconductor fin; and a separation feature separating the first gate stack and the second gate stack, wherein the separation feature includes a first dielectric layer and a second dielectric layer with an air gap defined therebetween, and a bottom portion of the separation feature being inserted into the isolation structure.