FinFET and fabrication method thereof

A method for fabricating a fin field effect transistor (FinFET), comprising:

• providing a substrate including an NMOS (N-type metal-oxide-semiconductor) region;
• forming a plurality of fins on the substrate;
• forming an isolation layer between adjacent fins of the plurality of fins and on the substrate;
• forming a gate structure across a length portion of each of the plurality of fins and covering a portion of each of a top surface and sidewalls of each of the plurality of fins;
• forming a first mask layer on the top surface and the sidewalls of each of the plurality of fins in the NMOS region;
• etching the first mask layer to expose the top surface of each of the plurality of fins on both sides of the gate structure along a length direction of each of the plurality of fins in the NMOS region;
• removing a first thickness portion of each of the plurality of fins on the both sides of the gate structure along the length direction of each of the plurality of fins, wherein a remaining portion of each of the plurality of fins in the NMOS region and a remaining portion of the first mask layer form a first trench;
• performing an oxidation treatment on a surface of the remaining portion of each of the plurality of fins exposed by the first trench to form an oxide layer;
• after performing the oxidation treatment, performing a thinning treatment of the remaining portion of the first mask layer on a sidewall of the first trench in the NMOS region to increase a width of the first trench;
• and
• forming an in-situ doped epitaxial layer to fill up the first trench, wherein doping ions in the in-situ doped epitaxial layer are N-type ions.