A semiconductor structure includes a semiconductor substrate, a gate structure, an etch stop layer, a dielectric structure, and a conductive material. The gate structure is on the semiconductor substrate. The etch stop layer is over the gate structure. The dielectric structure is over the etch stop layer, in which the dielectric structure has a ratio of silicon to nitrogen varying from a middle layer of the dielectric structure to a bottom layer of the dielectric structure. The conductive material extends through the dielectric structure.