Electromigration (EM) failure in metallization is one of the important failure mechanisms in ULSI today. When copper was suggested to replace aluminum to reduce the RC delay, its electromigration performance is thought to be better than aluminum because copper has a higher melting point, and hence much slower diffusion coefficient. This is from the consideration of the intrinsic electromigration performance of the material itself In actual fabrication, metal is surrounded by barrier metal and dielectric. The confinement of metal indeed improves the electromigration performance of the metal interconnect in most cases. In this work, the mechanism of the improvement will be explored from the microscopic consideration. Comparison of the intrinsic electromigration performance of metal with and without the surrounding materials, including the barrier metals, will be used to explain the mechanisms. The understanding of this mechanism will enable us to design a metallization system that can further improve the electromigration performance of metal interconnection. (C) 2004 Elsevier B.V. All rights reserved.