The role of carboxylic acids on nanoparticle removal in post CMP cleaning process for cobalt interconnects

As the technology node moves below 10 nm, cobalt with its low resistivity, superior adhesion property, and wettability with copper, promises to change the conductor landscape in many areas, particularly interconnects and logic contacts. The cleaning process of cobalt films after chemical mechanical planarization is vital to ensure devices’ yield and high performance. The objective of this study is to investigate the removal of nanoparticles from cobalt films, employing two types of carboxylic acids and utilizing a wide pH range from 3 to 11. Various analytical techniques were used to understand the cleaning mechanisms, including zeta potential, surface wettability, dissolution rates, and electrochemical analysis. The results show that carboxylic acids with more chains of the carboxyl groups, such as citric acid, could yield excellent particle removal efficiencies in neutral and alkaline regions. Additionally, it can be concluded that the etching effect accelerated by complexation is predominant in acidic regions, whereas the cobalt oxide surface adsorption of carboxyl groups and stronger electrostatic repulsive forces generated by increased absolute zeta potentials play critical roles at high pH values. Based on the results, an efficient approach with minimal passivation for cleaning cobalt interconnects is proposed. A cleaning solution containing 5 mM citric acid in a neutral pH region can achieve an effective particle removal with reduced passivation using the proposed approach.