A method includes depositing a flowable film on a substrate by providing a first input flow, the first input flow including plasma effluents of a first precursor, removing a portion of the flowable film from a sidewall of a feature defined within the substrate to obtain a remaining portion of the flowable film by providing a second input flow, the second input flow including plasma effluents of a second precursor, reducing hydrogen content of the remaining portion of the flowable film to obtain a densified film by providing a third input flow, the third input flow including plasma effluents of a third precursor, and treating the densified film in accordance with a film treatment process.

A semiconductor device structure is provided. The semiconductor device structure includes forming semiconductor device structure includes a gate stack wrapping around a plurality of nanowire structures. The gate stack includes a first portion above the plurality of nanowire structures and second portions between the nanowire structures. The semiconductor device structure further includes a gate spacer layer along a sidewall of the first portion of the gate stack, and a plurality of inner spacer layers along sidewalls of the second portions of the gate stack. The gate spacer layer has a first carbon concentration, the inner spacer layers have a second carbon concentration, and the second carbon concentration is lower than the first carbon concentration.

A semiconductor device structure is provided. The semiconductor device structure includes a plurality of nanowire structures over a fin structure, and a gate stack wrapping around the plurality of nanowire structures. The gate stack includes a first portion above the plurality of nanowire structures and second portions between the nanowire structures. The semiconductor device structure further includes a gate spacer layer along a sidewall of the first portion of the gate stack, and a plurality of inner spacer layers along sidewalls of the second portions of the gate stack. The gate spacer layer has a first carbon concentration, the inner spacer layers have a second carbon concentration, and the second carbon concentration is lower than the first carbon concentration.

A method for manufacturing a semiconductor includes: providing a substrate; forming a polysilicon layer on the substrate, a surface, away from the substrate, of the polysilicon layer having a native oxide; and performing a nitriding treatment to the native oxide, to nitrogenize the native oxide into a silicon oxynitride layer. The native oxide is nitrogenized into the silicon oxynitride layer.

A semiconductor device includes a semiconductor substrate with an insulator layer surrounding a semiconductor fin, and a semiconductor material stack structure on the semiconductor fin. Agate spacer covers the sacrificial gate structure and the semiconductor material stack structure. The semiconductor device further includes gate spacer extensions on the insulator layer and against sidewalls of the semiconductor material stack structure.