Fine Pitch Probing, Wirebonding and Reliability of Aluminum Capped Copper Bond Pads

The requirement for improved electrical performance and reduced silicon area has driven Copper to replace Aluminum interconnection as silicon technology is scaled beyond 0.25μm. The frontend change, in turn, pushes the wirebond pad pitch from above 100μm to the 80μm-66μm range. This creates challenges for back-end to probe and wire bond at fine pitch geometry onto a readily oxidized Copper surface. After several re-metallization structures and types of metallurgy were evaluated, capping Copper bond pads with Aluminum was selected as the primary approach for probing and wirebonding Copper devices. Aluminum remetallization structure offers many advantages that help leverage existing tooling and knowledge in-fab, probing and wire bonding processes. This paper will describe probe and wirebond experiments used to select the proper adhesion and diffusion barrier between Copper and Aluminum, and the Aluminum thickness that can withstand the mechanical stress experienced during probing and wire bonding. Probe mark depth and the impact of probe marks to the underlying barrier and Copper pad were examined. To evaluate the integrity of the re-metallization structure, as well as the quality of ball bonds onto the new structure, wire bonded units were subjected to a thermal aging study. Ball shear, wire rip and corresponding failure modes, intermetallic coverage and cratering analysis were evaluated at various readpoints of the thermal aging study. Contact resistance measurements and reliability stress tests were also performed. Based on the results, an optimized re-metallization structure was established for Copper High Performance wire bonded devices. Probe and wire bond conditions compatible with this structure were recommended.