Downscaling transistors in integrated circuits in order to achieve higher performance goes hand in hand with reducing the cross sections of interconnects. At the same time, the interconnects' lengths are increased to accommodate larger chips with increasing functionality. These tendencies make interconnects increasingly sensitive to electromigration and accompanying electro-thermal and thermo-mechanical effects, which have become the main reliability issue in modern integrated circuits. Copper, with its lower resistivity, higher melting point, good mechanical strength, and better electromigration bulk performance, has replaced aluminum as the advanced metallization solution. However, copper-based interconnects have introduced new problems, since copper electromigrates along fast diffusion paths at the interfaces to surrounding layers.

The main challenge in electromigration modeling and simulation is the diversity of the relevant physical phenomena. Electromigration-induced material transport is also accompanied by a material transport which is driven by the gradients of material concentration, mechanical stress, and temperature distribution. A comprehensive, physically based analysis of electromigration for modern copper interconnects lines serves as the basis for deriving sophisticated design rules which will ensure higher steadfastness of interconnects against electromigration. During the operation of the interconnect, a redistribution of vacancies under the influence of the various promoting factors takes place. The increase of the vacancy concentration, for instance, depletion of material at specific places in the interconnect's metal, leads to buildup of tensile stresses. The compressive stress in the area of the copper interconnect carrying the electrical current (Figure 1) comes from the fact that copper has a larger thermal expansion coefficient than the TiN barrier layer.
If a balance between electromigration, vacancy concentration, temperature, and mechanical stress gradients, which characterize the operating condition of the interconnect, is reached, the vacancy concentration will remain at some value independent of the simulation duration: the interconnect structure is virtually immortal.
In our work we study the implications of the comprehensive electromigration reliability simulations on the design of the interconnect layout in order to obtain structures which are resistive to the impact of electromigration and its promoting factors for the predefined operating conditions.