A semiconductor device and a method for forming the same are provided. The method includes forming a gate structure over a fin structure. The method further includes forming first gate spacers on opposite sidewalls of the gate structure. The method further includes forming source/drain features in the fin structure and adjacent to the first gate spacers. The method further includes performing a surface treatment process on top surfaces of the source/drain features and outer sidewalls of the first gate spacers. The method further includes depositing a contact etch stop layer (CESL) over the source/drain features and the first gate spacers. A first portion of the CESL is deposited over the top surfaces of the source/drain features at a first deposition rate. A second portion of the CESL is deposited over the outer sidewalls of the first gate spacers at a second deposition rate.