A method for manufacturing a semiconductor device is provided. A substrate with an insulation formed thereon is provided, wherein the insulation has plural trenches, and the adjacent trenches are spaced apart from each other. A barrier layer is formed on an upper surface of the insulation and in sidewalls of the trenches, and the barrier layer comprises overhung portions corresponding to the trenches. A seed layer is formed on the barrier layer. Then, an upper portion of the seed layer formed on an upper surface of the barrier layer is removed. An upper portion of the barrier layer is removed for exposing the upper surface of the insulation. Afterwards, the conductors are deposited along the seed layer for filling up the trenches, wherein the top surfaces of the conductors are substantially aligned with the upper surface of the insulation.