Thermo-mechanical modeling of stress-induced-voiding in BEOL Cu interconnect structures

The temperature dependent driving force for stress induced voiding of Cu dual damascene interconnects has been studied using finite element modeling. Both 2D axisymmetric and 3D models have been investigated. Interconnect test structures have been simulated at temperatures ranging from 25 to 300degC and a stress free temperature for the structure is demonstrated, consistent with analytical modeling. Typical reliability test structures, symmetrical in design showed a lower in-plane stress gradient, suggesting a reduced driving force for void formation. Hence, they are not ideally suited for reliability assessment. This compares to more realistic narrow-to-wide chain asymmetric structures with a larger stress gradient leads to faster void growth. For the assessment of stress gradients in these asymmetrical structures, the build-up of a 3D-model was required and showed the limitations of 2D axisymmetric models for this application. The effect of low dielectric (k) materials on the stress distribution in Cu interconnects was analysed. When copper is integrated in low-k materials, a higher hydrostatic stress gradient is observed compared to the traditional SiO₂-based dielectrics.