To inhibit the detrimental diffusion of copper into silicon devices, an effective barrier is strongly demanded. In this study, copper(rhenium) alloy films were successfully electroless-and electroplated on silicon substrates for self-forming an ultrathin rhenium barrier layer. In pure copper films, the interdiffusion of copper and silicon already occurred at 400 • C, forming silicides and increasing the electrical resistivity of the films. In comparison, not any silicide formation was observed in the electroless-and electroplated copper(rhenium) alloy films with minor incorporations of rhenium for 0.10 and 0.39 at% until 600 and 500 • C, respectively, attributed to the prior segregation of rhenium and the self-formation of rhenium barrier layer at the copper/silicon interface at 400 • C. A zero solubility of rhenium in copper and a small solubility in palladium catalysts facilitate the separation of rhenium from copper. The kinetics of rhenium segregation follows uphill diffusion, and the diffusivity of rhenium in copper at 400 • C is estimated to be 1.5 × 10 −17 cm 2 /s.