A method for filling a gap includes: providing a semiconductor substrate, at least having an metal interconnect layer and an insulating dielectric layer on top of the underlying metal interconnect layer, the insulating dielectric layer having a gap; forming a diffusion bather layer and a seed layer sequentially in the gap and on a surface of the insulating dielectric layer outside the gap; forming a mask layer on a surface of the seed layer outside of the gap; and depositing a metal layer on the semiconductor substrate with the mask layer, the metal layer filling the gap.