Stress-induced phenomena in nanosized copper interconnect structures studied by x-ray and electron microscopy

Abstract

We present the first dynamic study of damage mechanisms in nanosized on-chip Cu interconnects caused by stress-induced voiding in advanced integrated circuits. Synchrotron-based transmission x-ray microscopy is applied to visualize the void evolution and conical dark-field analysis in the transmission electron microscopy to characterize the Cu microstructure. Our x-ray microscopy measurements showed, in contradiction to electromigration studies, no void movement over large dimensions during the stress-induced void evolution. We observed in via/line Cu interconnect structures that voids are formed directly beneath the via, i.e., in the Cu wide line at the edge of the via bottom. It is concluded that voids are originally formed at the site where eventually the catastrophic failure occurs. During stress migration tests, Cu atoms migrate from regions of low stress to regions of high tensile stress, and simultaneously, vacancies migrate along the stress gradient (within a limited range of some microns) in the opposite direction to the location where small vias connect wide Cu lines. The stress distribution and the driving forces for atomic transport depend strongly on the particular geometry of the tested structure but also on interface bonding and metal microstructure. Vacancies form agglomerates and subsequently voids that grow further. The void growth rate depends on the Cu thin film material and its microstructure, particularly the grain size and the grain orientations. The Cu microstructure in the surroundings of the formed void shows that Cu grains are predominantly (111) oriented relatively to the wafer surface. Interfaces and grain boundaries, and particularly their orientation, determine the void evolution dynamics

Modeling the copper microstructure and elastic anisotropy and studying its impact on reliability in nanoscale interconnects

“…However, when we consider stress migration which is primarily driven by the hydrostatic stress gradient which typically have local maxima at the grain boundaries the polycrystalline structures might result in easier nucleation of voids along grain boundaries and also growth of voids (Yang et al 2011;Zschech et al 2009;Sukharev et al 2009). Stress migration and electromigration induced mass transport along the copper interconnects is dependent on competing activation energies for atomic migration along Cu/SiCN interface, Cu/TaN interface, and copper grain boundaries (Hau-Riege and Thompson 2001;Ogawa et al 2002;Sukharev and Zschech 2004).…”

Electromigration and the structure of metallic nanocontacts

“…Another class of imaging methods applied successfully for metallic nanocontacts is X-ray investigation. Mostly, intense X-ray radiation from synchrotrons has been used to study electromigration (Zschech et al, 2009;Budiman et al, 2006;Chen et al, 2008a;and Takahashi et al, 2009). Rarely, also X-rays in electron microscopes have obtained sufficient resolution to study electromigration in thick samples (Frigo et al, 2002).…”

Hard x-ray scanning microscopy with coherent radiation: Beyond the resolution of conventional x-ray microscopes

“…Ideally, this is done in the natural environment of the specimen or under working conditions, e. g., the investigation of electromigration in a via of a functioning electronic device 1 or the activity of small catalyst particles inside a chemical reactor. 2 Hard x-ray microscopy is ideal to address these questions, as the large penetration depth of hard x-rays in matter allows one to investigate the inner structures of an object without destructive sample preparation or in-situ inside of special sample environments.…”