The study of interface electromigration in copper interconnects

With the use of Cu as an interconnect material for Integrated Circuits (IC), several new technical and theoretical challenges are immerging. One of these is the electromigration (EM) reliability. In particular, the effect of interface on EM reliability is under great debate and has been a subject of recent research. In this study, direct characterization of interface EM using cross-strip pattern method was performed. In order to investigate the EM characteristics by interface EM, cross-strip patterned Cu films were prepared using sputter and electroplating deposition method, and passivation layers were deposited to create an artificial interface. In this configuration, the rate of interface EM is designed to be measured in relative to surface EM. Tantalum (Ta), titanium nitride (TiN), and silicon nitride (Si₃N ₄) layers were used as the passivation layer respectively. Cu lines with cross-striped areas of different passivation layers were EM tested and characterized using scanning electron microscope (SEM). Unlike the conventional belief, the results obtained from cross-strip testing show that passivation layer has a tendency to create an interface with faster EM rate than surface. Furthermore, it is found that Cu/Si₃N₄ interface has lower interface EM rate than Cu/Ta and Cu/TiN interfaces, though many researchers have suggested that the Si₃N₄/Cu interface provides a faster diffusion path than the Ta/Cu, TiN/Cu interfaces.