Void Evolution due to Electromigration

In modern integrated circuits interconnects are arranged in several levels of metalization, possibly containing several millions of interlevel connections. The continuous scaling of the interconnect lines results in increased operating current densities and temperatures. Under these conditions, electromigration has become a major reliability concern and posed several challenges for interconnect design and fabrication. For the state of the art copper dual-damascene technology several physical mechanisms act cooperatively during void evolution under electromigration stress. Surface energy, electromigration itself, and elastic strain energy are driving forces for material transport along the void surface. Furthermore, the interaction of the void with local geometric features, such as material interfaces and microstructure, has a considerable impact on the void migration, growth, and shape stability. In particular, the high diffusivity at the copper/capping layer interface is one major issue which controls void migration and growth along the interconnect line. However, as new materials and processes have been introduced, the adhesion between the copper and the surrounding layers is improved, so that grain boundary diffusion starts to become more and more significant. As a consequence, a multitude of void evolution mechanisms have been observed. Although models of the void evolution problem have already been well investigated, most of the published three- dimensional implementations neglect relevant physical phenomena, hindering an accurate simulation of void development. Moreover, the existing numerical schemes are mostly suitable for simulation of two-dimensional or simple three-dimensional interconnects only. Therefore, they offer only a very limited predictive capability of the failure development due to electromigration in modern dual-damascene interconnects. In the scope of this project a general electromigration void evolution model will be theoretically and numerically investigated. The influence of the various driving forces for atomic transport along the void surface in connection with the local geometric properties of the interconnect will be analyzed. The model will be implemented in a simulation tool following a careful study of the corresponding numerical algorithms. Special attention will be paid to evaluation and implementation of suitable numerical methods for fully three-dimensional simulations. The model implementation will be validated by comparison of simulation results with the available literature data and relevant experiments.

A general electromigration void evolution model has been theoretically developed, implemented in a simulation tool, and numerically investigated. The continuous scaling of interconnects in integrated circuits causes increased operating current densities and temperatures. This development increasingly puts pressure on the interconnect design and fabrication, as reliability issues, such as electromigration, are more and more a concern for modern integrated circuits, impeding continuous advancement in the area of microelectronics. In state-of-the-art copper dual-damascene technology different physical mechanisms contribute to void evolution under electromigration-induced stress. In particular, surface energy, electromigration, and elastic strain energy are the primary forces at play, triggering material transport along the void surface. Additionally, the interaction of the void with local geometric features (e.g. material interfaces and microstructures) further impacts void migration, growth, and shape stability. In particular, the high diffusivity at the copper/capping layer interface is one major issue which controls void migration and growth along an interconnect structure. The shortcomings of early approaches based on surface-tracking methods have been tackled with a so-called diffusive interface model. An in-house, three-dimensional simulation tool (FEDOS) has been augmented with this model, enabling simulations of electromigration-induced void evolutions based on our general electromigration void evolution model. The results obtained from our simulation tool, utilizing our developed models, have demonstrated that the ability of predicting the void evolution has been significantly improved as compared to previous approaches. The state-of-the-art has thereby been significantly advanced.