A method for manufacturing a semiconductor device, includes: forming a shallow trench isolation structure surrounding a first semiconductor fin and a second semiconductor fin; forming a dummy gate structure across the first and second semiconductor fins; forming a first flowable dielectric layer over the first and second semiconductor fins; curing the first flowable dielectric layer at a first temperature; removing a first portion of the cured first flowable dielectric layer from above the second semiconductor fin; after removing the first portion of the cured first flowable dielectric layer, forming a second flowable dielectric layer over the second semiconductor fin; curing the second flowable dielectric layer at a second temperature different from the first temperature; and replacing the dummy gate structure with a metal gate structure.