Polishing mechanisms of various surfactants in chemical mechanical polishing relevant to cobalt interconnects

Cobalt (Co), with its low resistivity, superior adhesion property, and void-free seamless fill ability, is being considered by the industry as a promising candidate to replace the conventional tungsten contact plugs for middle-of-the-line and the wiring metal copper interconnects for back-end-of-the-line. Chemical mechanical polishing (CMP), whose demand is greatly increasing as device dimensions shrink to 7-nm node, has been recognized as one of the indispensable manufacturing approaches for the fabrication of multilevel metal interconnection structures. In addition to meeting the appropriate removal rates and removal selection of heterogeneous materials, excellent wafer surface quality and minimized residual particles, also have become the criteria in the developing process of CMP slurries for Co interconnects. This work investigates the optimization process of the final CMP step for Co interconnects by introducing surfactants in hydrogen peroxide based slurries. Various types of surfactants having different functional groups were employed and compared thoroughly by material removal rates (MRRs) and selectivity. The chemical and mechanical mechanisms of various surfactants during CMP process have been clearly revealed by various approaches, including polishing and static etching experiments, electrochemical analysis, tests of absolute zeta potentials, and particle sizes, as well as wafer surface energy measurements. Research results show that Co MRRs are directly correlated with functional groups of various surfactants. Moreover, absolute zeta potentials of abrasive particles can be deeply influenced by different types of surfactants, where the correlation between absolute zeta potentials and Co MRRs presents strong negative. On the other hand, direct positive correlation between oxide MRRs and particle sizes affected by corresponding surfactants can be summarized. Meanwhile, no obvious corresponding relationship between contact angles and MRRs was found, indicating MRRs were barely affected by wettability of wafer surfaces. At last, the selected surfactants were applied to 12-inch wafers for verification, including MRRs, removal selectivity, defects and surface roughness. Each of these surfactants could make Co and TEOS surface Ra maintain at around below 0.7 nm and 0.3 nm, with a number of residual particles lower than 600 particle/wafer.

Automated summary

This study optimized the final chemical mechanical polishing (CMP) step for Cobalt interconnects by introducing surfactants in hydrogen peroxide-based slurries. The research results showed that the functional groups of surfactants directly influenced the removal rates of Cobalt, and different surfactants also had an impact on the absolute zeta potentials of abrasive particles, which showed a strong negative correlation with the Cobalt removal rates. Additionally, there was a positive correlation between oxide removal rates and particle sizes affected by corresponding surfactants.