Finfet with sigma recessed source/drain and un-doped buffer layer epitaxy for uniform junction formation

A method of forming a semiconductor structure comprising:

• forming a gate structure over a semiconductor fin that extends upwards from a semiconductor substrate portion, wherein the gate structure comprises a gate stack straddling a channel portion of the semiconductor fin and a gate spacer present on sidewalls of the gate stack;
• forming a sigma cavity within the semiconductor fin on each side of the gate structure, wherein the sigma cavity comprises a horizontal tip region extending beneath the gate spacer and a bottom region extending towards the semiconductor substrate portion;
• epitaxially growing a semiconductor buffer region from faceted surfaces of the sigma cavity, wherein the semiconductor buffer region completely fills the horizontal tip region and the bottom region of the sigma cavity, wherein an unfilled portion of the sigma cavity has substantially vertical sidewalls;
• and
• epitaxially growing a doped semiconductor region from the semiconductor buffer region, wherein the doped semiconductor region completely fills the unfilled portion of the sigma cavity and has substantially vertical sidewalls.