1.4.5 Impact of Microstructure and Material Interfaces

Testing a group of metal structures at different stress temperatures, for a constant applied current density, permits to extract the activation energy E_a as the angular coefficient of the line-fit to the data of the lnMTTF vs 1/T plot [52]. The activation energy E_a, obtained from the results of the electromigration stress tests, can be taken as a useful parameter to determine the dominant diffusion path for electromigration-induced material transport in the interconnect. One or more diffusion paths for metal atoms dominate the electromigration failure and they can be summarized as bulk, grain boundaries, and material interfaces, as illustrated in 1.12.

Figure 1.12: Main diffusion paths in a passivated interconnect structure: a) bulk, b) grain boundary, and c) material interface.

For copper-based interconnects, the estimated values of the activation energy for the different diffusion paths are listed in table 1.1 [105]. For diffusion paths with low values of activation energy, the diffusion of atoms along the given path is energetically favorable because the energy barrier to promote diffusion at this site is lower compared to paths with high values of activation energy. Low values of activation energy lead to fast diffusion paths. Since the diffusion along these paths is fast, the mechanism of electromigration degradation is accelerated reducing the interconnect lifetime. To sum up briefly, low values of activation energy lead to higher diffusivities resulting in shorter MTTFs, while high values enhance the electromigration lifetime estimation. Since the contribution from the bulk diffusion is very small compared to the contribution from the grain boundary and material interfaces, the role of grain boundaries and material interfaces should be taken into account in the reliability assessment of the interconnect under the influence of electromigration.

Table 1.1: Activation energies for various diffusional paths in copper interconnects [105].

Diffusion path	Activation energy Ea (eV)
Bulk	2.1
Grain boundary	1.2
Material interface	0.8-1.2

Several micro-structural features, such as grain size and texture strongly influence the electromigration lifetime [149]. The ratio (r_g), described as the average grain size divided by line width, is an important parameter to quantify the microstructure properties of an interconnect [49]. If the ratio r_g>1, the grain size is comparable to the line width and the microstructure of the interconnect assumes a bamboo-like pattern. For r_g<1, the interconnect line presents a polycristalline structure in which the network of small grains provides additional paths for material transport due to the formation of grain boundaries running parallel to the current flow and triple points, generated from the intersection of two or more grains, as sites of void nucleation. Since the grain boundary density in bamboo microstructure lines is smaller than in polycristalline lines, the contribution of mass transport by electromigration along grain boundaries is negligible. Grain boundary diffusion is the dominant transport mechanism for polycristalline lines [1]. Texture and grain size distribution are other important factors which influence the electromigration characteristics, especially in polycristalline structures. In particular, as the grain size distribution becomes broader and the grains are randomly oriented, the standard deviation of the lifetime distribution increases leading to a reduction in the MTTF [92].

Since the activation energy value for diffusion at material interfaces in copper-based interconnects is the lowest value measured (table 1.1), it is evident that diffusion of atoms in this metallization lines is interface dominated and has an early effect on electromigration failure. As described in Section 1.2, a typical interconnect consists of a metal line surrounded by a passivation layer. The interfaces between the passivation layer and the metal line are the fast diffusion paths and the adhesion between the two materials influences the quality of the interface [99]. If the adhesion between metal and passivation layer is weak, the interface is considered slightly bonded which allows and enhances the material diffusion along the path. At the site of weak adhesion, void nucleation is much easier because the stress necessary to nucleate a stable void is significantly reduced compared to the threshold stress value for void nucleation reached at the grain boundaries. Furthermore, the void evolution mechanism at this site is a consequence of the material diffusion along the void surface, which is faster in the presence of a poorly adhered interface.