# Hybrid CMP Slurry Supply System Using Ionization and Atomization
## 4 Discussion  
We evaluated a hybrid system that combines the ionization and atomization systems. The ionization of the slurry affects the MRR and the atomization system affects the uniformity, MRR, and pad temperature. Consequently, the MRR improved and the NU reduced by more than 23% and 25%, respectively, in comparison with the conventional system in global planarization. The ions generated during the electrolysis in the ionization slurry  
Figure 11: Pad temperature comparison between the conventional and hybrid systems based on the process time. Figure 10: Cu dishing dependency on the over-polishing. The conventional and hybrid systems are compared considering different pitches. supply system reduce the repulsive force between the surface and abrasives. This increases the number of particles involved in the polishing process and improves the chemical reactivity at the Cu surface. Hence, the ionized slurries increase the MRR. Additionally, the hybrid system reduced the defects, such as dishing, by 5%-28% in the PW CMP process. Typically, a slurry comprises compounds in which various atoms and elements are chemically bonded. In the ionization system, the chemical components in the slurry exchange electrons directly, and the compounds break the bonds to decompose and generate ions. The ionization of the slurry increases the ion motion and transportation, resulting in adequate charge conduction. The isoelectric points of the Cu oxide surface exhibit a negative potential under basic conditions [18]. In the case of conventional slurry, this generates a repulsive force between the reacted layer and the abrasives (Figure 12a). However, the ionized slurry reduces the electrostatic repulsion (Figure 12b). The ions in the ionized slurry are adsorbed on the abrasive or the surface of the Cu oxide, which reduces the repulsive force between the Cu oxide surface and the silica abrasives, owing to the screening of the surface charges [19]. And this phenomenon can also be explained by the zeta potential theory. The ionized slurry has a double-layer with non-uniform ion distribution due to the Cu oxide surface charge, which is composed of a stern layer and a diffused layer. The potential at the boundary between the double-layer and the area where the potential is balanced is called the zeta potential. The potential of the stern layer decreases linearly from the Cu oxide surface potential, and the potential of the diffused layer decreases exponentially with the ion concentration opposite to the surface charge contained in the ionized slurry. Therefore, the zeta potential decreases due to the increase of cations contained in the ionized slurry, resulting in an attractive force. And the electrostatic force between the reacted layer of Cu and the abrasives determines the actual contact pressure and the number of usable active abrasives on the surface [20]. As the ionized slurry contains a high amount of usable active abrasives and applies a high contact pressure on the surface, the MRR increases. Figure 12: _Cont._  
In addition, the ionized slurry accelerates oxidation and decreases the hardness of the Cu surface as the ions react with Cu atoms to oxidize and remove the atoms simultaneously [21]. Although the reaction layer is removed mechanically by the abrasives, it regenerates rapidly owing to the chemical reaction. Thus, this repeated cycle removes the material [22; 23]. Figure 13 illustrates the measured results of the hardness of the reaction layer on the Cu surface obtained using a nano-indenter (HM2000, Fischerscope, Germany). Typically, nanoindentation measures the mechanical properties of thin films with a thickness of less than several nanometers to determine the wear resistance [24]. In this study, the results indicate that the surfaces etched into the slurries with higher ionization are more deeply indented with the same force. Moreover, the hardness of the surface is low at deeper indentations.