1. A method for forming a semiconductor device, said method comprising: using an atomic layer deposition (ALD) method to deposit an ALD layer along sidewalls and covering a bottom surface of an opening extending through a dielectric layer; forming an Mn-based barrier layer on said ALD layer; and depositing Cu in said opening such that said Cu contacts said Mn-based barrier layer and substantially fills said openings, thereby forming a conductive structure in said opening.