Structures including a refractory metal collar at a copper wire and dielectric layer liner-less interface, and a related method, are disclosed. In one embodiment, a structure includes a copper wire having a liner-less interface with a dielectric layer thereabove; a via extending upwardly from the copper wire through the dielectric layer; and a refractory metal collar extending from a side of the via and partially along the liner-less interface. Refractory metal collar prevents electromigration induced slit voiding by improving the interface around the via, and prevents void nucleation from occurring near the via. Also, the refractory metal collar provides electrical redundancy in the presence of voids around the via and dielectric layer liner-less interface.