Fin field effect transistor (FinFET) device structure and method for forming the same

A method for forming a fin field effect transistor (FinFET) device structure, comprising:

• forming a first plurality of fin structures and a second plurality of fin structures in a first region and a second region of a substrate, respectively, wherein a first pitch between two adjacent fin structures of the first plurality of fin structures is different from a second pitch between two adjacent fin structures of the second plurality of fin structures;
• depositing a hard mask layer is over the first plurality of fin structures and the second plurality of fin structures;
• depositing a dielectric layer adjacent to the hard mask layer, wherein a top surface of the dielectric layer is substantially level with a top surface of the hard mask layer;
• removing the hard mask layer to expose top surfaces of the first plurality of fin structures and top surfaces of the second plurality of fin structures;
• and
• removing a portion of the dielectric layer to form an isolation structure on the substrate.

A method for forming a multiple fin structures, comprising:

• forming first plurality of fin structures in a first region and second plurality of fin structures in a second region of a substrate, wherein a density of the first plurality of fin structures is greater than a density of the second plurality of fin structures;
• forming a plurality of protruding structures between two adjacent second plurality of fin structures in the second region;
• forming a sacrificial layer on the first plurality of fin structures, the second plurality of fin structures and the plurality of protruding structures;
• and
• removing an upper portion of the sacrificial layer to form an isolation structure on the substrate, wherein the plurality of protruding structures are covered by the isolation structure, and the first plurality of fin structures and the second plurality of fin structures extend above the isolation structure.