Fine pitch probing and 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 0.25 μm. The front-end change, in turn, pushes wirebond pad pitch from above 100 μm to 80 μm-66 μm 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 re-metallization 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 Aluminum thickness that can withstand the mechanical stress during probing and wire bonding. Probe mark depth and the impact of probe marks to the underlying barrier and Copper pad were examined. Ball shear, wire rip and corresponding failure modes, intermetallic coverage and cratering analysis were evaluated at various readpoints of thermal aging study to evaluate the integrity of the re-metallization structure as well as the quality of ball bonds onto the new structure. Contact resistance measurement and reliability assessment were also performed. One re-metallization structure was recommended for Copper High Performance wire bonded devices