A semiconductor structure includes a semiconductor substrate and a conductive feature formed over the semiconductor substrate, an etch stop layer formed over the conductive feature, a dielectric layer formed over the etch stop layer, a contact trench formed in the dielectric layer, a bottom of the contact trench being disposed over a top surface of the conductive feature, and a self-aligned sealing oxide layer formed on the dielectric layer. The self-aligned sealing oxide layer is conformed to sidewalls of the dielectric layer exposed in the contact trench.