Electromigration reliability issues in dual-damascene Cu interconnections

Electromigration studies on Cu interconnects are reviewed. Some history and more recent results are discussed along with a description of the present interpretations of the active mass transport mechanisms involved in Cu electromigration. The issue of the dual-damascene process and its potential effect on EM reliability is described with special focus on the peculiarities of the dual-damascene interconnect architecture compared to more conventional subtractively etched Al-based interconnects. Experiments performed on dual-damascene interconnects that highlight electromigration reliability issues such as early failure, a tentative explanation for via electromigration failure, and the Blech effect, are summarized. Emphasis is placed on an experimental methodology that uses large interconnect ensembles in a multi-link configuration. Such a large scale study of nearly 10000 interconnects has shown statistical evidence of bimodal failure behavior consistent with the presence of a weak and strong failure mode, which have been identified as voiding, respectively, within the via and the trench at the cathode end of an interconnect. A multi-link approach has also demonstrated a length-dependent distribution of failures that yields a (j·L)_c product value of about 9000 A/cm in dual-damascene Cu/oxide interconnections and is consistent with mass transport that is controlled by the presence of extended defects within Cu such as grain boundaries, interfaces, and/or surfaces. The study of dual-damascene Cu has demonstrated the importance of statistics in analyzing EM reliability.