Crack Identification in BEoL Stacks Using Acoustic Emission Testing and Nano X-ray Computed Tomography

A novel approach is presented to evaluate the mechanical stability of back end of line (BEoL) stacks. A SRAM test vehicle manufactured in 28 nm technology with copper pillar bumps is used. To inflict damage to the BEoL of the given semiconductor device, external forces are applied to the copper pillars by shear loading using a tribo-indenter system. An acoustic emission (AE) detection sensor is attached to the sample to measure acoustic waves during the shear experiment as an indication for damage. The damage progression in the BEoL stack is extremely fast, hence details can't be resolved with the piezo sensors of the tribo-indenter anymore. However, due to the much higher temporal resolution AE measurements give a more precise insight into the damage process. The resulting damages are imaged using nano X-ray computed tomography (nXCT) as well as scanning electron microscopy (SEM). The results provide a better understanding of the origin and the propagation of damages in the BEoL stack.