Semiconductor structure and method for forming the same

A method for forming a semiconductor structure, comprising:

• forming an epitaxial feature over a source/drain region of an active region, wherein the active region comprises a fin element and a plurality of channel layers over the fin element, the epitaxial feature comprises first layers interfacing the channel layers and a second layer spaced apart from the channel layers by the first layer, and a concentration of the dopant in the first layer varies from a concentration of the dopant in the second layer;
• forming a first interlayer dielectric layer over a frontside surface of the epitaxial feature;
• forming a first contact structure through the first interlayer dielectric layer and on the frontside surface of the epitaxial feature, wherein a bottom surface of the first contact structure is lower than a bottom surface of a topmost one of the channel layers;
• forming a second interlayer dielectric layer over a backside surface of the epitaxial feature;
• forming an opening through the fin element and exposing the backside surface of the source/drain feature;
• forming a contact liner along sidewalls of the opening;
• etching the epitaxial feature to extend the opening;
• and
• forming a second contact structure in the opening.

A semiconductor structure, comprising:

• first nanostructures vertically stacked over each other;
• a first source/drain feature adjoining the first nanostructures, wherein a thickness of the first source/drain feature is different from a width of the first source/drain feature in a cross-sectional view;
• a first contact etching stop layer covering the first source/drain feature;
• a first interlayer dielectric layer on the first contact etching stop layer;
• and
• a first contact structure through the first interlayer dielectric layer, the first contact etching stop layer and the first source/drain feature, wherein:
• the first contact structure includes a first portion through the first dielectric layer and a second portion through the first source/drain feature,
• a first sidewall of the first portion is connected to a second sidewall of the second portion through a connecting wall, and
• the second sidewall is offset from the first sidewall by a distance.