A new spalling mechanism of intermetallics from the adhesion layer in the terminal-stage reaction between Cu and Sn

Three-dimensional integrated circuits provide a promising approach to extend Moore's law by vertically stacking multiple functional chips with microbumps. However, with miniaturization, depletion of the wetting layer of under bump metallurgy becomes a worrisome possibility. Upon depletion of the wetting layer, the adhesion layer will be directly exposed to intermetallics at the terminal stage of solid-state reactions. In this study, the resulting spalling phenomenon is investigated for the first time. The microstructure evolution of ultrathin Cu–Sn microbumps shows that spalling of Cu6Sn5 and Cu3Sn from the Cr adhesion layer occurs when Cu is depleted. The driving force for spalling is the high interfacial energy between intermetallics and the Cr adhesion layer. To prevent such spalling, which deteriorates the integrity of the bumps, the Cu wetting layer must be at least 1.3 times thicker than that of Sn. ? 2021 Elsevier Ltd