Abstract: A method for forming a semiconductor device structure is provided. The method includes forming a transistor layer over a substrate and forming a trench in the transistor layer. A depth to width ratio of the trench is greater than or equal to 3. The method further includes filling the trench with a gap-fill material using a flowable chemical vapor deposition process, wherein a precursor and a reactant are used in the flowable chemical vapor deposition process, and a ratio of the precursor to the reactant is about 1.

Abstract: A semiconductor structure includes a channel structure, a gate structure, two source/drain features, and a plurality of inner spacers. The channel structure includes a plurality of channel features which are spaced apart from each other. The gate structure is disposed to surround the channel features. The source/drain features are disposed at two opposite sides of the channel structure such that each of the channel features interconnects the source/drain features. Each of the inner spacers is disposed to separate the gate structure from a corresponding one of the source/drain features. Each of the inner spacers includes an inner spacer body and a lateral nitrided portion. The lateral nitrided portion is in direct contact with the corresponding one of the source/drain features and has a nitrogen content greater than that of the inner spacer body.