After a certain stress is reached in the structure a crack or a void forms. In the case that a crack forms it can lead to an open circuit failure causing a malfunction of the whole integrated circuit. For the scenario of a phase one failure the estimation of the TTF ends with the analysis shown in the section before.
In the second case the formation of a void is the result of the first phase. Thereby, the structure does not fail immediately, but a void will get formed in the structure. This void starts to move in the structure due to EM at its surface as well as it might grow. For the open TSV segment (cf. Figure 5.16) this simulation was performed using the phase field method described in Section 3.6. Thereby, a void is placed in the aluminium at the region where the highest stress was found in the simulation before. As the highest stress was located at the interface between the aluminium and the tungsten close to the corner in the aluminium, the void was placed under the interconnect joining the TSV to the planar metallization structure. Due to the symmetry of the problem with respect to the blue line indicated in Figure 5.16, only one half of the structure was simulated with appropriate boundary conditions at the boundary formed by cutting through the blue line. These boundary conditions are vanishing fluxes for electric currents, vacancies, and the phase field function as well as free to slip at the cutting surface for the mechanical calculations. The results of this simulations are shown in Figure 5.22.
First the void starts to move in the structure. The movement is in the same direction as the current density and therefore in the direction towards the tungsten. This phase is depicted in Figure 522a. It shows the aluminium layer from the direction where the tungsten is placed. The tungsten layer was left out to reveal the void. On the right side the cut due to the mirror symmetry is located. In Figure 5.22b the void has reached the end of the aluminium structure, where the aluminium touches the silicon oxide. Due to the tendency of the void to minimize the surface energy the before circular void reforms into a semicircle minimizing the surface area. From now on the void stops moving and only continuous to grow as depicted in Figure 5.22c. This evolution of the void influences the cross section of the conducting metal and therefore, affects the resistance of the metallization of ICs.« PreviousUpNext »Contents