Low-temperature reflow anneals of physical-vapor-deposited (PVD) Cu on Ru are achieved at a Cu/low- κ integration-compatible low temperature of 250°C by surface diffusion. Feature-fill capability is also demonstrated in patterned features along with reasonable electrical measurements. As compared to typical impurity levels in the conventional electroplated Cu, the reflowed PVD Cu had higher purity, which then resulted in lower electrical resistance in the structures. Electromigration test results further confirmed the reliability of the reflowed-Cu/Ru interface. © 2011 IEEE.