2.1 The History of Electromigration

Electromigration phenomena have been studied for a long time. The first observation was reported by the french physicist M. Gerardin [79]. In 1861, he discovered electromigration in liquid alloys of lead-tin, potassium-sodium, gold, and bismuth in mercury. He was also the first person who thought in terms of electrostatic interactions between the field and metal ions in liquid metals and molten salts. The work of F. Skaupy [140] in 1914 introduced the idea of an "electron wind", which laid the foundation for the understanding of electromigration. From his studies on mass transport of impurities in molten metals, he underlined the significance of the interaction between atoms and moving charge carriers. In the 1950s, the first systematic studies of electromigration, based on the correlation between the direction of the current flow and the material transport, were presented by W. Seith and H. Wever [136]. By measuring the mass transport of a series of solid alloys and plotting it in a phase diagram, they observed that the direction of transport varies with the sign of the charge carriers, i.e. electrons or holes, in the specific alloy phase. They also introduced the "marker motion" technique, now called vacancy flux method, which permits to measure the electromigration-induced mass transport using the displacement of an indentation on a metal wire [79]. These experiments prompted the concept of an "electron wind" to account for the induced mass transport. The first mathematical formulations of the electromigration driving force have been derived independently by Fiks [55] and Huntington and Grone [82]. These authors treated the electron wind force by developing a "ballistic" approach based on the semiclassical collision process between mobile point defects and charge carriers. The investigations demonstrated the possibility of using electromigration to probe the interaction of moving atoms and charge carriers directly. All these theoretical ideas stimulated considerable technological interest during the 1960s in experimental studies on electromigration in integrated circuits.