Development of Low-Temperature, Pressureless Copper-to-Copper Bonding by Microfluidic Electroless Interconnection Process

Abstract

A new bonding technology that will solve all the reliability problems associated with traditional soldering bonding method was developed. This new bonding scheme uses electroless metals as interconnect materials instead of solders, so IMCs will not be a central concern anymore. In addition, void-free, seamless electroless copper pillars interconnection can be achieved at a low bonding temperature without the need to apply any pressure. This study aims to optimize the fluid flow parameters of microfluidic electroless interconnection process to eliminate hydrogen entrapment issue that will lead to the formation of seam or void at the joining interface between copper pillars. Fluids flow was conducted in both continuous flow and batch-flow processing to assess the joining performance of the microfluidic electroless interconnection process. It was found that the batch-flow processing can join copper pillar without creating any voids or seams at the joining interface, while that of the continuous flow is more likely to have a few voids or seams formed at the joining interface. The reason that the batch-flow processing has better joining ability is that the hydrogen gas bubbles, a byproduct of electroless plating, can be effectively ejected from the plating surface to create rooms for the deposition of electroless plating. Furthermore, a high phosphorus level of Ni(P) deposits was found within the gap between copper pillars, especially near the joining interface. Further, microfluidic electroless interconnection process also can successfully join pillar-to-pad or pillars with small-to-large bump configuration.