1. Introduction

Over the past century, electronic devices have penetrated every aspect of our lives and the use of electronic products has expanded while their size has continuously decreased [122]. The scaling trend leads to an increase of the density of on-chip interconnects from generation to generation of modern integrated circuits (ICs). The combined need for miniaturization of digital functionalities, such as logic and memory storage, and functional diversification requires a decrease in both interconnect width and length, which can be achieved through the introduction of three-dimensional (3D) integration technologies. The interconnect system in a 3D design is arranged in different levels of metallization which provides the vertical connections of devices in a single wafer or between stacked wafers [131]. Due to the continuous shrinking of interconnect sizes in 3D stacking, a wide spectrum of product opportunities became possible, but several reliability issues introduced from the fabrication process as well as from the device operative conditions arose. In particular, when a sufficient high current density passes through a metal interconnect, a failure mechanism is initiated by the resultant material transport in the direction of the current flow. This failure process known as electromigration is one of the main reliability issues in interconnects for 3D integration technologies. It became of technological interest when semiconductor companies observed electromigration damage in ICs. Since then, improvements have been made in developing new interconnect design, integration processes, and novel materials in order to reduce the electromigration effect in interconnects and predict their long term behavior.

In the following, a brief introduction regarding the technological development of integrated circuits and 3D integration approaches is presented. This is followed by the description of the interconnections composing the 3D layout and their main reliability issues. After that, a description of the physical phenomena behind the electromigration is provided.