A semiconductor structure, a method for manufacturing a FinFET structure and a method for manufacturing a semiconductor structure are provided. The method for forming a FinFET structure includes: providing a FinFET precursor including a plurality of fins and a plurality of gate trenches between the fins; forming a first portion of the trench dummy of a dummy gate within the plurality of gate trenches; removing at least a part of the first portion of the trench dummy; forming a second portion of the trench dummy over the first portion of the trench dummy; performing a first thermal treatment to the first and second portions of the trench dummy; and forming a blanket dummy of the dummy gate over the second portion of the trench dummy. The present disclosure further provides a FinFET structure with an improved metal gate.