Semiconductor device and manufacturing method thereof

A method comprising:

• forming a fin structure comprising a first semiconductor layer and a second semiconductor layer over a substrate;
• forming a dummy gate structure across the fin structure;
• forming gate spacers respectively on opposite sides of the dummy gate structure;
• removing the dummy gate structure to form a gate trench between the gate spacers;
• removing a native oxide layer formed on a surface of the first semiconductor layer and in the gate trench by using a hydrogen-containing gas mixture;
• after removing the native oxide layer, selectively removing the first semiconductor layer from the gate trench while leaving the second semiconductor layer suspended in the gate trench;
• removing oxygen atoms within the first semiconductor layer after forming the gate trench and prior to selectively removing the first semiconductor layer;
• and
• after selectively removing the first semiconductor layer from the gate trench, forming a metal gate structure in the gate trench.

A method comprising:

• forming first semiconductor layers and second semiconductor layers alternately arranged over a substrate;
• forming a dummy gate structure covering the first semiconductor layers and the second semiconductor layers;
• removing oxygen atoms within the second semiconductor layers;
• after removing the oxygen atoms within the second semiconductor layers, recessing the second semiconductor layers, wherein after recessing the second semiconductor layers, a sidewall of at least one of the first semiconductor layers comprises Si-H bonds;
• after recessing the second semiconductor layers, forming a source/drain structure connected to the sidewall of the first semiconductor layers;
• and
• replacing the dummy gate structure and the second semiconductor layers with a metal gate structure.