In one exemplary aspect, a method for semiconductor manufacturing comprises forming first and second silicon nitride features on sidewall surfaces of a contact hole, where the contact hole is disposed in a dielectric layer and above a source/drain (S/D) feature. The method further comprises forming a contact plug in the contact hole, the contact plug being electrically coupled to the S/D feature, removing a top portion of the contact plug to create a recess in the contact hole, forming a hard mask layer in the recess, and removing the first and second silicon nitride features via selective etching to form first and second air gaps, respectively.