2.1.4 From 1990s to Modern Times

In the late 1980s and 1990s, the introduction of pure copper-based interconnects took place. Copper was proposed because of its lower electrical resistivity and, therefore, much lower sensitivity to electromigration than aluminum [149]. These results, together with the development of novel process technologies, encouraged IBM and Motorola in 1997 to replace aluminum with copper in IC interconnects. In parallel to the development of the interconnect technology, new aspects of electromigration physics were introduced to the modeling. Different models of electromigration failure phenomenon for VLSI interconnections, based on the electromigration stress-induced voiding, were carried out [91,93,133,63]. Furthermore, a new class of models based on the void surface evolution theory was initiated in the mid-1990s [3,71,107,11].

In the last 20 years, the main focus of the electromigration study on copper-based interconnections was to improve the processing techniques in order to support the circuit speed requirement, and meet the demand for miniaturization of electronic devices. Furthermore, the advent of emerging interconnect structures, such as TSVs, poses a new challenge regarding electromigration reliability. For this purpose, mathematical modeling can significantly contribute to the comprehension of the electromigration failure mechanism in these new technologies. Since electromigration is influenced by different physical phenomena, modeling has became an extreme challenge. The complexity of the analytical models therefore requires the adoption of numerical simulations. The development of TCAD tools for simulating electromigration in interconnects permits to efficiently solve numerical calculations. In this way, several experimental observations can be explained, and the design of reliable interconnects can be improved.