Abstract
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. In a second step, the validity of the approach is also shown for a second sample manufactured in 22 nm technology and the experimental results are compared. To inflict damage to the BEoL stack of the respective semiconductor device, external forces are applied to the copper pillars (Cu-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 anymore with the piezo sensors of the tribo-indenter. 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.
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