We fabricated a barrierless Cu-alloy seed buffer layer for use as a reliable interconnect. In microelectronic devices, Cu diffuses rapidly into the Si layer, deteriorating the device properties and necessitating a diffusion barrier between Cu and Si to prevent device failure. However, the synthesized Cu-alloy films were stable when used as a buffer layer and did not dissolve the solder even when reactions occurred at the interface. Our results are expected to facilitate development of innovative low-electrical-resistance, low-thermal-resistance, and high-thermal-conductivity metallic materials with good adhesive properties. These materials would be useful in barrierless Cu-metallization solder bump flip chip applications. We measured the film resistivity and current–voltage characteristics of the seed layer and assessed the reliability of the seed layer in Cu interconnects by studying the long-term aging characteristics of Cu and Sn. The results reveal that the fabricated Cu-alloy film exhibits potential for high thermal stability.