BEoL Damage Evaluation Utilizing Sub-Critical Cu-Pillar Shear Tests, Acoustic Emission, nXCT, and SEM/FIB Analysis

In previous works, the resulting damages in the back end of line (BEoL) stack triggered by Copper pillar (Cu-pillar) shear-off events were evaluated and classified [1]. It was determined, especially by utilizing acoustic emission (AE) measurements, that damage events consist of multiple extremely fast sub-processes. The objective of this work is the development of an approach to enable the identification of the areas of damage initiation and comprehend the damage propagation in a BEoL stack under mechanical load by triggering only the initial sub-processes. Mechanical stress was induced into the BEoL stack utilizing a displacement-controlled sub-critical Cu-pillar loading approach with the approximate parametrization determined in previous experiments [1]. During mechanical loading, AE signals were constantly measured. As soon as significant acoustic events were detected, the experiment was aborted. The occurring damages were analyzed utilizing a customized nano X-ray computed tomography (nXCT) setup and focused ion beam (FIB) milling as well as scanning electron microscopy (SEM) imaging. In this work, a methodology could be developed to enable the evaluation of BEoL damages in an early, sub-critical stage. These results provide a better understanding of the damage formation and propagation in the BEoL stack and enable a design optimization procedure for the most damage prone areas.