Electromigration: A review

The aggressive scaling planned for integrated circuits is placing more demands on the materials, processes and designs. The circuits must be reliable and electromigration is a key reliability issue. Because many of the factors that contribute to electromigration are not completely understood, manufacturers must proceed cautiously. In this article electromigration is reviewed from the prospective of the reliability engineer, focusing on those areas of greatest applicability to the manufacturing environment. First, the fundamental physics of electromigration are examined to provide a basis for understanding the factors that affect the lifetimes under the various test conditions. Then, empirical data concerning the impact on reliability of metal stripe geometry, structure and composition are reviewed. The care necessary to make fast, wafer-level tests an important process control tool is discussed. It is shown that pulsed-dc and ac waveforms can provide longer lifetimes than predicted previously, providing some relaxation of current density requirements for higher circuit densities. An understanding of these phenomena is necessary for the reliability engineer to assess today's and tomorrow's integrated circuits.