Dopant Condensation beyond Solubility Limit in the Vicinity of Silicon/Silicide Interface Based on First-Principles Calculations

ABSTRACTIn the trend of scaling down metal-oxide-semiconductor field effect transistors (MOSFETs), reduction of contact resistance at the silicide/silicon (Si) interface will be essential for higher performance. Nickel silicide (NiSi) is considered as a substi-tute for a present electrode material in MOSFETs, cobalt silicide (CoSi2), because silicidation temperature can be reduced as compared with the case of the conventional CoSi2. Hence, we have focused on the NiSi/Si Schottky interface. An ordinary method to increase the dopant concentration at the interface is ion implantation before silicidation process. The dopant atoms are consequently condensed around the interface by snowplow effect, leading to the effective lowering of the Schottky bar-rier height (SBH) because of the band bending enhancement of the Si layer. However, this band bending technique does not reduce the SBH in further scaled MOSFETs. In this context, we studied another possibility of SBH modulation technique, based on the first-principles calculations. Throughout our calculations, we found that a large atomic-scale dipole between impurity and silicide atoms is generated across the interface. Impurity atoms are expected to be condensed because of a large energy gain at the interfaces, leading to the dramatic reduction of the SBH. Based on these results, we proposed a novel di-pole comforting Schottky (DCS) junction. We have also found that the thickness of the Si layer interfacing with the NiSi layer can be 1nm or less. In the present work, we applied this idea to the actual process through experimental techniques. The calculated results suggest that B implantation after silicidation leads to larger B concentration at the interface than that before silicidation, and thereby larger SBH modulation due to interface dipoles can be produced. Then, the NiSi/Si Schottky diodes were formed by ion implantation after silicidation process for dopants (As, B). We evaluated the interface dipoles contribution to the measured SBH reduction. As a result, the dopant atoms were found to be condensed beyond solubility limits on the interface Si side and we confirmed the generated interface dipoles actually reduces the SBT. Furthermore, we explored the other possibility of another type of impurity atoms applicable to the DCS junction. Among some other impurity atoms (Al, In, Mg), the calculated SBH modulation due to dipoles generated around these impurity atoms were found to be further enhanced in some cases. Based on these understandings, we propose a principle for choosing dopants towards ulti-mate lowering of the contact resistance in ultimately scaled MOSFETs.