Reliability challenges for advanced Ru-AG BEOL systems

#reliability #BEOL #packaging

Abstract

Ru is a candidate to replace Cu in the lower metal layers of future scaled BEOL interconnects. Because Ru is compatible with the direct metal etch integration approach, putting air (further referred to as air gap or AG) between metal lines could become the preferred approach to isolate adjacent metal lines. Although such advanced Ru-AG nano-interconnect schemes are awesome from a performance point-of-view, their reliability is challenged. In this presentation, the main reliability challenges for advanced Ru-AG BEOL systems will be discussed and future reliability research opportunities will be proposed.

First, metal-reliability challenges will be addressed, where it will be argued that the determination of Jmax-values is difficult because very high currents are needed to make the lines fail and thus Joule heating needs to be properly considered. Also, the fact that Ru is a p-type metal makes data interpretation and failure analysis challenging (but interesting).

Second, TDDB is discussed. Although lifetime predictions using standard TDDB tests look good, the non-existence of a consensus about a) the TDDB lifetime model, b) the conduction mechanism and the related conduction paths and c) the preferred failure locations, are worrisome and will require significant research efforts.

Finally, experimental and modelling work related to potential mechanical issues with the Ru-AG system will be discussed. Where the mechanically weaker places and their mitigation approaches are more-or-less known for Cu-low-k systems, the integration and design knobs to mitigate potential CPI-issues in such systems require future research attention.