Method and structure for gap filling improvement

Semiconductor devices having void-free dielectric structures and methods of fabricating same are disclosed herein. An exemplary semiconductor device includes a plurality of fin structures disposed over a substrate having isolation features disposed therein and a plurality of gate structures disposed over the plurality of fin structures. The plurality of gate structures traverse the plurality of fin structures. The semiconductor device further includes a dielectric structure defined between the plurality of fin structures and the plurality of gate structures. The dielectric structure has an aspect ratio of about 5 to about 16. The dielectric structure includes a first dielectric layer disposed over the substrate and a second dielectric layer disposed on the first dielectric layer. The first dielectric layer and the second dielectric layer are disposed on sidewalls of the plurality of fin structures and sidewalls of the plurality of gate structures.