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 10 000 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. Because the future in microelectronics usually means now, electromigration studies on Cu have progressed rapidly since its identification as the future microelectronics metallization of the present. Much of the literature shows a range of observed EM activation energies. The values appear to depend on Cu deposition and/or preparation methods and can indicate the wide variety of pathways available for EM damage formation. Which mechanism is dominant in a given instance can depend on whether faster mechanisms are available for mass transport. The actual fast-path mechanism in confined Cu metallization has not been determined, although recent literature tends to favor, an interface mechanism. Certain other major issues have been identified or addressed such as the Blech effect, low-k integration, and joule heating. The Blech effect in Cu appears to be stronger than in Al and is consistent with the interpretation that extended defects such as interfaces, grain boundaries, and/or surfaces play a major role in mass transport. Low-k integration and its effect on EM through an alteration of mass transport through back-stress effects, interface characteristics, and joule heating are a future reliability challenge. The use of dual-damascene technology in the fabrication of advanced interconnects presents itself as an integration and reliability challenge. The different architecture of the dual-damascene interconnect compared to the more standard W-plug, AI(Cu)-based interconnect means that new mechanisms of failure might exist. Voiding within and around the dual-damascene via as well as intraline leakage breakdown either through corrosion or EM-induced extrusion failure are new and prominent areas of concern. The study of dual-damascene Cu has also demonstrated the importance of statistics in analyzing EM reliability. Multi-link test structures are a potentially important approach toward addressing the real-world problem of extrapolating interconnect lifetime in reliability testing to those in working microelectronics devices. Early failure in dual-damascene interconnects is a major area of interest because bimodal behavior can be intrinsic to EM reliability in Cu. Although dual-damascene interconnects have novel architecture, the more global property of EM threshold behavior is preserved with certain caveats: the issues of back stress development and of early failure occurring below the threshold value. Many questions still remain, however, and more detailed analysis is required to enable a more carefully shaded and colorful picture of EM damage formation, especially because of dimensional scaling pressures. Future work should clarify the reliability picture further as the use of DD Cu technology becomes further entrenched in the technological landscape.