Adhesion experiments on Cu-Damascene processed interconnect structures for mode III loading

This study presents a novel approach of micromechanical interfacial testing in loading mode III. The interfaces beneath single copper interconnect structures are investigated. The method is based on lateral nanoindentation experiments, utilized to probe customized copper torsion structures manufactured by the Dual Damascene process. Thus, the test structures resemble product-like length-scales and properties. For the investigation of interface properties, the experimental load-displacement data were reviewed. Two different interfaces occurring in Back End of Line (BEoL) structures are probed: Copper to Ta/Ta-N barrier as well as Copper to Si-N. SEM images and FIB cross-sections are performed, validating the delamination of the targeted interface. While the first interface shows a very high strength and cannot be delaminated by the here presented experiments, the latter one was brought to delamination in mode III several times. The tests yielded reproducible load-displacement data, the load at delamination for the interface Copper to Si-N was measured to be 130 ± 40 μN. A simple analytical approximation for the maximum shear stresses applied in mode III at the interface of interest is provided. The proposed testing approach is able to reproducibly manufacture and test large quantities of the reality like test structures, while avoiding time-consuming FIB based preparation routines. This can help to evaluate the interface properties in modern BEoL stacks and could lead to simulation based design improvements.