Semiconductor device with backside power rail and methods of fabrication thereof

A method for forming a semiconductor device structure, comprising:

• forming a source/drain feature within a region defined by a first side of a substrate, a first fin structure, and a second fin structure;
• forming a contact etch stop layer (CESL) on the source/drain feature;
• forming a first interlayer dielectric (ILD) layer on the CESL;
• removing the first ILD layer and a portion of the CESL to expose the source/drain feature;
• forming a first silicide layer on a first side of the source/drain feature, wherein the first silicide layer has a first thickness;
• forming a first source/drain contact over the first silicide layer;
• forming a back-end-of-line (BEOL) interconnect structure over the first source/drain contact;
• forming a contact opening extending from a second side of the substrate to the first side of the substrate to expose a second side of the source/drain feature;
• and
• after forming the contact opening, forming a second silicide layer on the second side of the source/drain feature, wherein the second silicide layer has a second thickness less than the first thickness;
• and
• forming a second source/drain contact in the contact opening.

A method for forming a semiconductor device structure, comprising:

• forming a fin structure from a substrate, wherein the fin structure comprises a plurality of first semiconductor layers and a plurality of second semiconductor layers alternatingly arranged;
• forming a source/drain feature over a first side of the substrate, wherein the source/drain feature comprises:
• a first epitaxial layer in contact with the first side of the substrate and a sidewall of at least one of the plurality of first semiconductor layer;
• a second epitaxial layer in contact with the first epitaxial layer;
• and
• a third epitaxial layer having sidewalls surrounded by and in contact with the second epitaxial layer;
• removing the plurality of second semiconductor layers;
• forming a gate electrode layer to surround a portion of each of the plurality of first semiconductor layers;
• forming a first silicide layer on a first side of the source/drain feature, wherein the first silicide layer is in contact with the second and third epitaxial layers;
• forming a first source/drain contact over the first silicide layer;
• removing a portion of the substrate to expose a second side of the source/drain feature;
• forming a second silicide layer on the second side of the source/drain feature;
• and
• forming a second source/drain contact over the second silicide layer.