The present disclosure provides a method of fabricating a semiconductor structure in accordance with some embodiments. The method includes forming a first low-k dielectric layer over a substrate; forming a first and second metal features in the first low-k dielectric layer; forming a first trench in the first low-k dielectric layer, the first trench spanning between the first and second metal features; performing a ultraviolet (UV) treatment to sidewalls of the first low-k dielectric layer in the first trench; forming a first etch stop layer in the first trench; and depositing a second low-k dielectric layer on the first etch stop layer, thereby forming an air gap in the first trench.