MULTI-GATE DEVICES WITH MULTI-LAYER INNER SPACERS AND FABRICATION METHODS THEREOF

A semiconductor device includes semiconductor channel members disposed over a substrate, a gate dielectric layer disposed on and wrapping around the semiconductor channel members, a gate electrode layer disposed on the gate dielectric layer and wrapping around the semiconductor channel members, a source/drain (S/D) epitaxial layer in physical contact with the semiconductor channel members, and a dielectric spacer interposing the S/D epitaxial layer and the gate dielectric layer. The dielectric spacer includes a first dielectric layer in physical contact with the gate dielectric layer and a second dielectric layer in physical contact with the first dielectric layer. The first dielectric layer has a dielectric constant higher than that of the second dielectric layer. The second dielectric layer separates the first dielectric layer from physically contacting the S/D epitaxial layer.