Impact of microstructure engineering on electromigration resistance of copper redistribution lines

In this study, the electromigration (EM) reliability and failure mechanisms of Cu redistribution lines (RDLs) with four distinct microstructures (regular Cu, nanotwinned Cu (NT-Cu), nanocrystalline Cu (NC–Cu), and annealed NT-Cu (A-NT-Cu)) were systematically investigated. EM testing was performed in ambient atmosphere at a high temperature (220 °C) under a high current density (1.0 × 10 6 A/cm 2 ). It was found that A-NT-Cu lines exhibited the longest EM lifetime, with a mean time to failure (T 50 ) of 2751 h, followed by NT-Cu (2112 h), NC-Cu (1813 h), and regular Cu (1566 h). The enhanced reliability of A-NT-Cu was attributed to its strong (111) texture and high fraction of coherent twin boundaries (CTBs), which effectively suppresses oxide formation and void generation along grain boundaries (GBs). In contrast, NC-Cu accelerated the degradation due to excessive GB diffusion, resulting in the highest hazard rate ( h(t) ) at prolonged testing durations. These findings demonstrate that microstructure engineering through NT design and texture control is a promising strategy for improving the EM reliability of Cu-RDLs in advanced microelectronic packaging.