Semiconductor device and manufacturing method thereof

A method, comprising:

• providing a fin on an epitaxial stack formed over on a substrate and comprising at least one sacrificial epitaxial layer and at least one channel epitaxial layer;
• forming a sacrificial gate stack on channel regions of the fin;
• forming gate sidewall spacers on sidewalls of the sacrificial gate stack;
• performing pre-treatment operations to remove impurities from the at least one sacrificial epitaxial layer, wherein performing pre-treatment operations comprise nitridation operations, carbon removal operations, and oxide removal operations;
• recessing the at least one sacrificial epitaxial layer to form a cavity;
• and
• forming inner spacer material in the cavity.

A method, comprising:

• forming, on a substrate, an epitaxial stack comprising a plurality of sacrificial epitaxial layers and a plurality of channel epitaxial layers;
• forming a fin in the epitaxial stack;
• forming a sacrificial gate stack on channel regions of the fin;
• forming gate sidewall spacers on sidewalls of the sacrificial gate stack;
• performing pre-treatment operations to remove impurities from the plurality of sacrificial epitaxial layers, wherein performing pre-treatment operations comprise nitridation operations, carbon removal operations, and oxide removal operations;
• recessing the plurality of sacrificial epitaxial layers to form a plurality of cavities;
• and
• forming inner spacer material in the plurality of cavities.