The heart of three-dimensional (3D) Si integration is the copper filled Through Silicon Via (TSV), which allows the shortest chip-to-chip interconnections. The copper filling of the TSV is usually achieved with an electro-plating method. Even though copper electro-plating for interconnects is a well-known technology due to the wide application of the copper damascene process, it proves to be quite different from the copper filling of the TSV, where the via diameter changes from nanometers to tens of microns and via depth changes from sub microns to hundreds of microns. What we have learned from the copper damascene process and what works there could not be applied directly to TSV copper filling. Although some void-free TSVs have been achieved, there are still many technical issu...[ Read more ]

The heart of three-dimensional (3D) Si integration is the copper filled Through Silicon Via (TSV), which allows the shortest chip-to-chip interconnections. The copper filling of the TSV is usually achieved with an electro-plating method. Even though copper electro-plating for interconnects is a well-known technology due to the wide application of the copper damascene process, it proves to be quite different from the copper filling of the TSV, where the via diameter changes from nanometers to tens of microns and via depth changes from sub microns to hundreds of microns. What we have learned from the copper damascene process and what works there could not be applied directly to TSV copper filling. Although some void-free TSVs have been achieved, there are still many technical issues to be solved for the copper filling process, including low throughput, high cost ( accounts for nearly 40% of the overall TSV cost), heavy overburden and inconsistent void-free filling. Copper electro-plating is traditionally conducted utilizing copper sulfate based electrolyte containing three different functional additives. Recently a new acidic copper methane sulfonate based electrolyte has been promoted as one candidate copper plating bath. However, literature survey shows that less research work has been performed on the copper electrodeposition in the TSV filling process. It is important and necessary to understand the electrochemical performance of the new sulfonate based electrolyte, TSV filling capability and compatibility with the existing additives.

This thesis is devoted to implementing a comprehensive study of the performance characteristics of the new copper methane sulfonate based electrolyte and its compatibility with the existing additives. The ultimate goal is to screen and develop an additive system that is compatible with sulfonate based electrolytes and which can successfully fill most of the vias without any voids. A number of experimental methods such as wetting-ability tests, Hull-cell studies, current efficiency analysis, polarization behavior studies, electrochemical evaluation and TSV copper filling were carried out to investigate the performance characteristics of the new electrolyte.

Performances of three industrial additives were firstly investigated based on the sulfonate electrolyte. The sulfonate bath is compatible with the industrial additives and void-free copper filling is achieved for 55/220 μm (diameter/depth) vias, utilizing the sulfonate bath containing industrial additives within 6 hours. However, void-free copper filling cannot be realized for vias of 25 μm in diameter and 200 μm in depth due to poor wetting-ability. The polyethylene glycol (PEG)/ Bis-(3-Sulfopropyl)-disulfide sodium salt (SPS) system was then screened and involved in the sulfonate electrolyte. The wetting-ability of the sulfonate bath may be improved by the addition of a SPS accelerator, while the incorporation of PEG suppressor has a minor influence on the wetting performance. In addition, void-free copper filling is accomplished for the 55/220 μm vias using the electrolyte containing PEG/SPS within 4 hours. However, the system cannot successfully fill the vias of 25 μm in diameter and 200 μm in depth.

Finally, a single additive system was developed and successfully applied in the copper filling of three different vias with the diameters of 55, 25 and 10μm. In addition, this single additive copper filling process may simplify the additive replenishment and monitoring procedure during the TSV filling process. The correlation of the copper filling of TSV and cyclic voltammetry striping (CVS) monitoring of the plating bath was built up to predict the formation of voids inside the vias. The proposed void-free copper filling criteria may be further applied in other plating electrolytes. Finally, wafer-level TSV copper filling was successfully achieved utilizing the single additive system.