Method for assessing the delamination risk in BEoL stacks around copper TSV applying nanoindentation and finite element simulation

Abstract

The mechanical characterization of copper through silicon vias (TSV) is not feasible by means of classical material characterization methods like tensile testing due to the small size of the vias. Instead, instrumented nanoindentation is an appropriate method. It has been utilized for the work reported. In addition, finite element (FE) simulations have been employed to assess the delamination risk in the BEoL stack around the copper TSV. Force/displacement curves were obtained by instrumented nanoindentation using sharp indenter tips with various tip angles (Berkovich, Cube Corner). The modelling of the resulting force/displacement curves is based on an adjusted response surface method [e.g. 3-5] that takes into account the various tip shapes. The respective response surfaces are generated by FE simulation assuming homogenous material with isotropic hardening (Ramberg-Osgood). Considering the residuum surfaces obtained for all the different tip shapes, a single point can be identified as the global minimum. This point specifies the parameters of the Ramberg-Osgood relation, hardening exponent and yield strength. Hence, the presented method enables a fast prediction of the plastic material behaviour in a very good first order approach. The accuracy of the method is evaluated by a virtual reverse check and compared to other methods found in the literature. The results of experimental investigations go into FE simulations of the copper TSV. They assess the interaction with the surrounding BEoL stack taking into account damage mechanical approaches. Since possible delamination between copper TSV and barrier contributes to increasing TSV pumping and protrusion, the authors considered silicon bi-material interface cracking as well. To this end, the cohesive surface contact method implemented in ABAQUS™ [15] is used in combination with a traction-separation law comprising a damage evolution law. Finally, suggestions for via design were provided reducing the risk of damages of BEoL-layers forced by copper vias.