A semiconductor structure includes source/drain (S/D) features disposed over a semiconductor substrate, a metal gate stack disposed between the S/D features, where the metal gate stack traverses a channel region between the S/D features, gate spacers disposed on sidewalls of the metal gate stack, and an etch-stop layer (ESL) disposed over the gate spacers and the S/D features. The semiconductor structure further includes an oxide liner disposed on the ESL, where the oxide liner includes silicon oxide and silicon dioxide, and an interlayer dielectric (ILD) layer disposed on the oxide liner, where composition of the ILD layer is different from composition of the oxide liner.