The present disclosure describes a method of forming an intermediate spacer structure between a gate structure and a source/drain (S/D) contact structure and removing a top portion of the intermediate spacer structure to form a recess. The intermediate spacer structure includes a first spacer layer, a second spacer layer, and a sacrificial spacer layer between the first spacer layer and the second spacer layer. The method further includes removing the sacrificial spacer layer to form an air gap between the first spacer layer and the second spacer layer and spinning a dielectric layer on the air gap, the first spacer layer, and the second spacer layer to fill in the recess and seal the air gap. The dielectric layer includes raw materials for a spin-on dielectric material.