Passivation layer and barrier layer in gate structures

A method comprising:

• providing a structure comprising:
• a stack of bottom channel layers,
• an isolation feature disposed over the stack of bottom channel layers,
• a stack of top channel layers disposed over the isolation feature, and
• a first work function metal (WFM) layer wrapping around the stack of top channel layers and the stack of bottom channel layers;
• performing a gas treatment to the structure, thereby forming a barrier layer over the first WFM layer;
• removing the first WFM layer from the stack of top channel layers;
• and
• forming a second WFM layer wrapping around the stack of top channel layers and over the first WFM layer and the barrier layer, such that the barrier layer is disposed between the first WFM layer and the second WFM layer.

A semiconductor structure, comprising:

• a substrate;
• a bottom channel layer disposed over the substrate;
• a dielectric isolation feature disposed over the bottom channel layer;
• a top channel layer disposed over the dielectric isolation feature;
• a first work function metal (WFM) layer wrapping around the bottom channel layer;
• a second WFM layer wrapping around the top channel layer;
• and
• a barrier layer disposed on a sidewall of the second WFM layer,
• wherein the barrier layer comprises silicon oxide, metal carbide, or a combination thereof.