Electromigration in ULSI interconnects

The first reported work on electromigration (EM) was presented in 1959, and since then extensive studies on the EM are being conducted theoretically, experimentally and numerically. In this work, the history and the evolution on the studies of EM are presented for both Al and Cu interconnection. Electron wind force was proposed to be the driving force for EM. However, as the interconnect line width shrinks to submicrometer level, other driving forces become important and even dominating. As a result, the conventional diffusion path approach for the modeling of EM is inadequate, and the driving force approach is needed. Both approaches are presented in this work. The extensive studies of EM lead to a much better understanding of the physics of EM, and with this understanding, the factors that affect the EM of interconnects, especially at the field operating conditions are identified and presented here. This identification leads to various design and process modifications and inventions in order to face the challenges of high EM reliability for an ever shrinking interconnection. The understanding of EM has also led to a better EM testing methodology in order to accurately assess the EM of an interconnection. Rigorous statistical analysis of EM test data is another key factor for this accurate assessment. In this work, we presented both the wafer level and package EM testing methodologies, and the rigorous data analysis that takes into account of the bimodal distribution of EM test data. (c) 2007 Elsevier B.V. All rights reserved.