Bump shape control on high speed copper pillar plating process in lead-free wafer level packaging

Copper pillars have been adopted and implemented in high volume manufacturing environment as early as 2006 as a replacement for high lead bumps. It is not only lead-free, but also offers the added advantage of higher stand-off, finer pitch capability and better electromigration resistance compared to tin-lead solder bumps. Owing to its significant superior thermal and electrical properties, higher stand-off, simpler UBM structure, and lower overall cost, it is not surprising that copper pillar bump has become and will continue to be a key interconnect technology in future semiconductor packages. As the full implementation of RoHS in 2010 approaches, various chemicals have been tested for this application by IDMs and OSATs. In order to simplify the chemical management in plant and shorten learning period, most of the efforts have been made on using RDL copper plating chemistry for Cu pillar applications. During these trials, bump height uniformity and bump shape have been found the main issues when photoresist thickness or bump height approaches 80 ¿m. A flat surface on the top of copper pillar is needed for implementation of the stacking materials. However, a dome surface appears as bump height increases. In extreme cases, the domeness could be over 10% of bump height and force to sacrifice plating speed into cost ineffective process. In this paper, factors to control bump shape in high speed Cu pillar plating process without sacrificing plating speed in single and multi additive systems were studied.