Fin field-effect transistor and fabrication method thereof

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

• providing a substrate having a first region, a second region, and a plurality of first fins on the first region of the substrate and a plurality of second fins on the second region of the substrate;
• forming a liner oxide layer on side surfaces of the first fins, side surfaces of the second fins and a surface of the substrate;
• forming an insulating barrier layer on a portion of the liner oxide layer on the first region of the substrate;
• forming a precursor material layer on the insulating barrier layer on the first region of the substrate and on the liner oxide layer on the second region of the substrate;
• performing a curing annealing process to convert the precursor material layer into an insulation layer, a first oxide layer being formed on the side surfaces of the first fins, and a second oxide layer being formed on the side surfaces of the second fins, wherein a thickness of the first oxide layer is smaller than a thickness of the second oxide layer after the curing annealing process;
• and
• removing a top portion of the insulation layer to form an isolation layer and removing portions of the liner oxide layer, the insulating barrier layer, the first oxide layer and the second oxide layer higher than a surface of the isolation layer, wherein a half width at half maximum feature size of portions of the first fins higher than the surface of the isolation layer is greater than a half width at half maximum feature size of portions of the second fins higher than the surface of the isolation layer after removing portions of the liner oxide layer, the insulation layer, the first oxide layer and the second oxide layer, that are higher than the surface of the isolation layer.