The Role of Impurities on the Reliability of Cu Interconnects-a Challenge for Advanced Packaging Solutions

Abstract

Motivation The need for shrinking dimensions in the area of advanced packaging is creating new challenges for the reliability for future packages. Metal deposits for a variety of different structures as Cu redistribution layers, μ-Vias, Pillars etc. are becoming thinner or smaller thus effecting on the one hand the area of the interconnect surfaces and on the other hand the thermomechanical strength of the structure. As a result material properties are becoming more and more important as they significantly influence the reliability performance. Content One key element to control the material properties is to understand how impurities in the metal layers and the interface affect the reliability and moreover how to control these impurities. This paper describes exemplary how impurities in the low ppm range influence 1. the mechanical properties and thus the reliability of thin Cu redistribution lines and 2. the interface between Cu and Ti or SnAg solder. We identified the critical impurities, measured their concentration level in the metal and the interface by SIMS and linked it to mechanical properties as ductility and tensile strength. We could show that a thermal budget-as it is usually applied in the manufacture of packages -changes indeed mechanical properties depending on the impurity level. The ductility is mostly affected and reduced by certain critical impurities mainly sulfur. This in turn can lead to cracks in Copper RDL especially for sub 5μm lines during thermal treatment. We could show that modified deposits of high purity do not show this thermomechanical change and withstand the thermal budget without degradation of the mechanical properties while fulfilling all other process requirements. Another defect caused by impurities after thermal budget is the appearance of voids at interfaces. We will demonstrate this effect on two different interfaces • between Ti and Cu RDL and • between Cu and SnAg solder material The appearance of voids is again only detected after thermal budget. Void formation is believed to be due to enrichment of the critical impurities at the interface during recrystallization. We applied SIMS on the different interface areas and could show that void appearance correlate with the existence of Sulfur and other critical elements. Again, high purity deposits do not show these voids after thermal budget offering a viable process alternative. A thorough understanding of electrolyte development is necessary to avoid the incorporation of sulfur, chloride and other incorporations as both elements are key constituents of the additive suites. Development of new electrolytes must not only fulfill process needs as shape, via filling etc. but also take purity into consideration. Summary and Outlook Next generations of packages with smaller and thinner Cu structures are depending on high purity deposits to cope with the reliability requirements. We identified the critical impurities and suggest an acceptable level of impurities which may depend on the package generations and types. New bonding technologies as Cu to Cu direct bonding will also be affected by the purity of the deposit and the nature and thickness of Cu oxide formation as well. It is vital to understand the role of both impurities and oxide formation. *Corresponding and presenting Author .