Competitive anion/anion interactions on copper surfaces relevant for Damascene electroplating

Abstract

The competitive interaction of chloride and SPS (bis-(sodium-sulfopropyl)-disulfide) at Cu(1 0 0)/electrolyte model interfaces was studied by means of cyclic voltammetry in combination with in situ STM and DFT. This specific anion/anion interaction is of paramount importance for the suppressor ensemble deactivation in the context of the industrial Cu Damascene process used for the state-of-the-art on-chip metallization. It is the interplay between chemisorbed chloride and SPS which regulates the dissociative SPS adsorption on copper as the key step in the course of the surface-confined MPS (mercaptopropane sulfonic acid) production. The latter species is considered as the actual anti-suppressor (depolarizer) in context of the Cu Damascene process. Under competitive conditions the chloride adsorbs and orders much faster on Cu(1 0 0) than the SPS. The resulting c(2 × 2)-Cl adlayer acts as an effective barrier for the dissociative SPS adsorption, at least under non-reactive conditions. Defect sites within the chloride matrix are identified as crucial pre-requisites for the dissociative SPS adsorption. Defects are generated under reactive conditions during copper dissolution or copper deposition due to rapid anion adsorption/desorption dynamics. As consequence of the SPS dissociation a mixed, defect-rich c(2 × 2)-Cl–MPS co-adsorption phase forms on Cu(1 0 0) where every second chloride species of the pristine c(2 × 2)-Cl adlayer is displaced by MPS units. This co-adsorption phase reveals an apparent p(2 × 2) symmetry in the STM experiment since only the sulfonic head groups of the MPS units are imaged while the S and the Cl species chemisorbed on the copper surface remain invisible at the “buried” interface. The relevance of this surface reaction for the Cu Damascene process is discussed in detail.