Semiconductor device and manufacturing method thereof

A method, comprising:

• providing a structure comprising:
• a stack of alternating channel layers and interposer layers,
• an oxide layer over the stack,
• a dummy gate electrode over the oxide layer,
• a source/drain feature connected to a sidewall of the stack,
• a contact etch stop layer (CESL) extending along a top surface of the source/drain feature, and
• an interlayer dielectric (ILD) layer over the CESL, wherein a thickness of the CESL is less than a thickness of the ILD layer;
• removing the dummy gate electrode to form a gate opening;
• forming a mask layer in the gate opening and over the CESL and the ILD layer;
• patterning the mask layer to remove the mask layer from the gate opening;
• performing a first etch process to remove a top portion of the oxide layer using the mask layer as an etch mask;
• performing a second etch process to remove a portion of a topmost channel layer;
• removing a remaining portion of the mask layer;
• removing the interposer layers and a remaining portion of the oxide layer, thereby enlarging the gate opening;
• and
• forming a metal gate structure in the gate opening and wrapping around lower channel layers below the topmost channel layer.

A semiconductor structure, comprising:

• a stack of first channel layers;
• a second channel layer on a side and apart from the stack of first channel layers;
• a first gate structure disposed over and wrapping around the stack of first channel layers, the first gate structure comprising a first top portion above a topmost first channel layer;
• a second gate structure disposed over and wrapping around the second channel layer, the second gate structure comprising a second top portion directly above the second channel layer and extending continuously to a level of a top surface of the first top portion;
• and
• a source/drain feature connected to the stack of first channel layers and the second channel layer,
• wherein a bottom surface of the first top portion is higher than a bottom surface of the second top portion, wherein the source/drain feature is doped with a dopant, wherein the source/drain feature comprises a first layer and a second layer, wherein a concentration of the dopant in the first layer varies from a concentration of the dopant in the second layer.