Transistor device with work function metal layers and method thereof

Abstract

A semiconductor device and a manufacturing method thereof are provided. The method includes the following steps. A fin structure extending along a first direction and having a lower fin structure and an upper fin structure disposed over the lower fin structure is formed, the upper fin structure includes first semiconductor layers and second semiconductor layers alternately stacked. A sacrificial gate structure extending along a second direction perpendicular to the first direction is formed over the upper fin structure. Gate spacers are formed on the sacrificial gate structure. A portion of the sacrificial gate structure is removed to expose the gate spacers. Portions of the exposed gate spacers are removed to form a first gate trench with a first dimension along the first direction. The rest of the sacrificial gate structure is removed to form a second gate trench with a second dimension along the first direction under the first gate trench, wherein the first dimension is greater than the second dimension. A gate dielectric and a first work function metal layer sequentially covering the first and second gate trenches are formed, wherein a first portion of the first work function metal layer merge in the second gate trench, and a second portion of the first work function metal layer is located on sidewalls of the first gate trench. The second portion of the first work function metal layer is removed to expose the gate dielectric located on sidewalls of the first gate trench while remaining the first portion of the first work function metal layer. A second work function metal layer is formed over the first portion of the first work function metal layer to fill the first gate trench.