Effect of ionic strength on ruthenium CMP in H2O2-based slurries

With the development of ultra-large scale integrated circuits, ruthenium has been selected as one of the most promising barrier metals for copper interconnects to replace traditional Ta/TaN bilayer. This paper mainly investigated the effect of ionic strength on the chemical mechanical polishing performance of ruthenium in H2O2-based slurries. The results show that, the ruthenium removal rate (RR) increases with the increasing concentration of H2O2 due to the formation of ruthenium oxides like Ru(OH)3, RuO2·2H2O and even RuO42−; additionally, the ruthenium RR can be further enhanced with the increase of K+ ionic strength. It is revealed that the added K+ can intensify the electrochemical reactions between H2O2 and the ruthenium surface by increasing the conductivity, meanwhile can also result in the neutralization of the zeta potentials of both silica particles and the ruthenium surface, and thus can lead to the decrease of the electrostatic repulsive force and the increase of the mechanical abrasion intensity between silica particles and the ruthenium surface. Therefore, the ruthenium RR increases with the increase of K+ ionic strength. Furthermore, the effects of K+ ionic strength on the material removal rate (MRR) selectivity of Ru vs. Cu and the galvanic corrosion of Cu/Ru couple are studied. It is found that, in order to achieve higher MRR selectivity than 1.0, KNO3 is preferred for the K+ source; and with H2O2 as the oxidizer, copper galvanic corrosion problem can be effectively suppressed.