Detection of bonding voids in multi-tier stacks with scanning acoustic microscope

Bonding voids are the key failure mechanism for 3D system integrations, especially for advanced, multi-tier stacks. However, to date there is no reliable method to separate the voids from different layers. We introduce bonding voids in a controlled manner with pre-defined lateral sizes, thicknesses, and coordinates in the complex N=4 multi-layer stacks. Hence, the smallest detectable void sizes at different depths of the stacks and axial resolution of scanning acoustic microscope can be precisely determined. This study demonstrates the void detection and void separation capabilities of scanning acoustic microcopy, indicating its advantages in the failure analysis of multi-tier stacks.