Electromigration mechanisms in conductor lines: Void shape changes and slit-like failure

A detailed analysis of electromigration damage in unpassivated Al-based conductor lines was conducted. Damage observations revealed that slit-like voids in lines having a bamboo structure develop from equi-axed voids through a shape change driven by the “electron wind”. In order to simulate this behavior a numerical model was developed in which electromigration-driven diffusion on the void surfaces is assumed to act as the primary transport mechanism. This theoretical treatment considers the influence of the current density distribution and the temperature field in the vicinity of the void, while at the same time taking account of the finite line width as well as surface tension effects. In addition, the often observed facetting of voids is discussed on the basis of simulations in which anisotropic surface diffusivity was assumed. It will be shown that void shape changes and slit formation have a major influence on the performance of lines with bamboo structure and that the modeling has the potential of predicting their life times.