Co-W as an advanced barrier for intermetallics and electromigration in fine-pitch flipchip interconnections

The trend towards thinner packages with embedded active components in case of RF modules, and higher I/O densities in case of multicore processors or 3D ICs, are pushing interconnection technologies to its fundamental limits. The limitations of traditional solder bump technologies in terms of its fatigue resistance, current-handling, electromigration and thermomigration resistance has shifted the interconnection focus to advanced thick copper bump UBMs with solder caps. However, even these interconnections fail to meet thermo-mechanical and electrical reliability requirements for fine-pitch flipchip interconnections at high current densities where electromigration becomes a major concern. This paper explores Co-W as an advanced barrier between copper bump and solder cap for fine-pitch flipchip technology to improve electromigration resistance. By suppressing the intermetallic growth and controlling electromigration, the novel barrier is expected to enhance the current-handling and thermomechanical reliability. In a systematic experimental study, Cu-Sn diffusion and intermetallic growth rate of this new Cu-Co-W-Sn-Ag approach are compared with that of Cu-Sn-Ag with XPS depth-profiling and cross-section analysis using SEM and EDS. Based on the analysis, the benefits of Co-W as a solder barrier for fine-pitch flipchip interconnections at high current densities is presented.