Stacked multi-gate device with low contact via resistance and methods for forming the same

A semiconductor device, comprising:

• a first transistor, comprising:
• a first semiconductor channel layer;
• a first gate structure wrapping around the first semiconductor channel layer;
• and
• first source/drain epitaxy structures on opposite ends of the first semiconductor channel layer;
• and
• a second transistor vertically above the first transistor, comprising:
• a second semiconductor channel layer;
• a second gate structure wrapping around the second semiconductor channel layer;
• and
• second source/drain epitaxy structures on opposite ends of the second semiconductor channel layer;
• and
• a first isolation structure between one of the second source/drain epitaxy structures and one of the first source/drain epitaxy structures, wherein the first isolation structure comprises an oxide-based material;
• a second isolation structure over the one of the second source/drain epitaxy structures, wherein the second isolation structure comprises a nitride-based material;
• and
• a source/drain contact electrically connecting the one of the second source/drain epitaxy structures and the one of the first source/drain epitaxy structures, wherein the source/drain contact comprises a metal plug and a metal liner, the metal liner is spaced apart from the nitride-based material of the second isolation structure.

A semiconductor device, comprising:

• a first transistor, comprising:
• a first semiconductor channel layer;
• a first gate structure wrapping around the first semiconductor channel layer;
• and
• a first source/drain epitaxy structure alongside the first semiconductor channel layer;
• and
• a second transistor vertically above the first transistor, comprising:
• a second semiconductor channel layer;
• a second gate structure wrapping around the second semiconductor channel layer;
• and
• a second source/drain epitaxy structure alongside the second semiconductor channel layer;
• and
• a source/drain contact electrically connecting the second source/drain epitaxy structure and the first source/drain epitaxy structure, wherein the source/drain contact comprises a metal plug and metal liners on opposite sidewalls of the metal plug, and a bottom surface of the metal plug is free of coverage by the metal liners.