EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

To mitigate mechanical failures of semiconductor devices it is necessary to understand the underlying damage mechanisms. Especially important is the identification of the damage initiation location to identify the mechanically most fragile areas. Due to feature sizes of less than 100 µm, a micromechanical approach utilizing a nano indenter system had to be applied in this work. The experiments focused on damages caused by mechanical stress in the so-called Back End of Line (BEoL) of an unpackaged high end microchip bumped with Copper pillars (Cu-pillars). The BEoL is a heterogeneous material stack mainly consisting of Copper and brittle organosilicate glasses as well as other functional sub structures like diffusion barriers, seal layers, passivation etc.
A stress distribution and damage prediction model derived from a combined approach of damage infliction by micromechanical loading and corresponding FEM simulations is presented. A similar approach has been applied previously but only for specific mechanical loading cases [1]. The micromechanical damage infliction process in this work was performed by applying shear force to single Cu-pillars with a customized indenter tool while shear height and velocity were varied to trigger different damage modes [2, 3]. A three-dimensional FEM model has been developed to evaluate the relation between the occurring stresses and the respective damage modes. The focus was put on the stress tensor fields at the Cu pillar base and in the brittle lower layers of the BEoL stack (low k materials). Both have been identified as damage prone in the experiments.
[1] H. Geisler et al., Experimental Analyses of the Mechanical Reliabilityof Advanced BEOL/fBEOL Stacks Regarding CPI Loading, IRPS, 2013
[2] JEDEC Standard JESD22-B117B, May 2014
[3] J. Silomon et al., Crack Identification in BEoL Stacks Using Acoustic Emission Testing and Nano X-ray Computed Tomography, IEEE IPFA, 2020