Semiconductor structure and formation method thereof

A method for forming a semiconductor structure, comprising:

• providing a substrate, wherein the substrate includes a first region and a second region adjacent to the first region;
• forming a plurality of fins over the first region of the substrate;
• forming an isolation layer over a front surface of the substrate, wherein a top of the isolation layer is lower than a top surface of a fin of the plurality of fins;
• forming a power rail opening by etching the isolation layer over the second region and a first portion of the second region of the substrate in a thickness direction;
• forming a through-hole by etching a second portion of the substrate in the thickness direction along the power rail opening;
• forming a first metal layer in the power rail opening and the through-hole;
• thinning a back surface of the substrate until the first metal layer is exposed from the thinned back surface of the substrate;
• and
• back-etching the back surface of the substrate to enable a back surface of the first metal layer to be above the back surface of the substrate.