Mechanical BEoL Stability Investigation at Cu-Pillars under Cyclic Load

In this work, a method is introduced which enables the evaluation of the influence of cyclical mechanical stress on the back end of line (BEoL) stack by applying shear load to single adjacent Copper (Cu-)pillars. This method is a subsequent development of previous work [1]. The objective of the approach presented in this work is to realize micromechanical loading scenarios which emulate application conditions more precisely compared to e.g. pure shear testing [1],[2],[3] or bump assisted BEoL stability indentation (BABSI) testing [4]. These methods are designed to evaluate the mechanical robustness of the BEoL stack and analyze the occurring damage processes, but they do not mimic the actual load conditions of a flip chip (FC) soldered die in which the Cupillars are immobilized by solder joints and, to a lesser extent, the underfill. In this work, a micromechanical loading approach is introduced which creates a loading situation which is closer to the actual application scenario. A customized indenter tip was utilized to provide the immobilization of single Cu-pillars and load them micromechanically. In a first approach, Cu-pillar shear-off experiments were performed in order to identify the maximum shear force before delamination and the related damage mode. In a second step, sub-critical cycling loading experiments were performed on single Cu-pillars to emulate the change of shear force direction as it can also occur in the field and evaluate the effect on the BEoL stack. Acoustic Emission (AE) measurements were utilized as a damage indicator during mechanical loading. The damage analysis was conducted utilizing nano X-ray computed tomography (nXCT) and scanning electron microscopy (SEM) including the imaging of focused ion beam (FIB) milled cross sections. The identified damages are mainly a deformation of the Aluminum contact pad and the polyimide (PI) surface passivation. However, also cracking could be identified which progressed through the Aluminum pad into the BEoL stack. The crack path is mostly defined by the layout of the Copper grid in the uppermost layers of the BEoL stack. The presence of Copper can deflect cracks and prevent the penetration of the BEoL stack. For this reason, the most severe damages, i.e. the deepest penetration of the BEoL stack, were detected in areas where gaps in the grid of the two uppermost metallization layers are located directly above each other.