The present application discloses a semiconductor device with a composite barrier structure and a method for fabricating the semiconductor device. The semiconductor device includes a substrate; a first dielectric layer having a feature opening on a substrate; a composite barrier structure in the feature opening, wherein the composite barrier structure includes a barrier layer in the feature opening and an assisting blocking layer on the barrier layer; and a conductive feature on the assisting blocking layer; wherein the barrier layer comprises tantalum, and the assisting blocking layer comprises copper manganese alloy.