Wirebondability and Solderability of NiAu electroless front side metalized Silicon wafers subjected to high temperature back end assembly process flows

摘要

In this work we analyze the wirebondability and solderability of different electro-less metallization stacks on Aluminum metalized Silicon wafers after exposure to subsequent high temperature back end assembly process steps. In addition, we propose a measure one can take to minimize the adverse impact such high temperature thermal budgets on solderability and wire bondability of various interconnects. In many silicon high power and high speed microelectronic semiconductor applications the need to concurrently and cost effectively implement a solderable, wirebondable and reliable topside metallization is becoming an increasingly desirable feature. This allows for the simultaneous implementation of various front side and back side interconnect solutions in the assembly process which reduce parasitic resistances, inductances and thermal resistances. These features in general enable power devices to achieve higher current ratings, higher switching speeds and improved thermal performance. The challenge associated with the implementation of these features via the use of front side electro-less front side metals is preserving its integrity during exposure to subsequent high temperature back end assembly operations such as thermal annealing and/or high temperature solder die attach. The subsequent high temperature operations have a tendency to degrade the integrity of NiAu based electro-less top metal finishes due to the diffusion of the barrier metal layer (Ni in general) to the top surface. Such mechanisms result in surface oxidization which dramatically reduces the solderability and wirebondability of the front side bond pads. In this work we experiment with different metal stacks of electro-less plated metallization over Aluminum metalized Silicon wafers and study the impact of different thermal budgets and process sequences to the solderability and wirebondability of the final surface.