Atomistic Mechanisms of Chemical Mechanical Polishing of a Cu Surface in Aqueous H₂O₂: Tight-Binding Quantum Chemical Molecular Dynamics Simulations

We applied our original chemical mechanical polishing (CMP) simulator based on the tight-binding quantum chemical molecular dynamics (TB-QCMD) method to clarify the atomistic mechanism of CMP processes on a Cu(111) surface polished with a SiO₂ abrasive grain in aqueous H₂O₂. We reveal that the oxidation of the Cu(111) surface mechanically induced at the friction interface is a key process in CMP. In aqueous H₂O₂, in the first step, OH groups and O atoms adsorbed on a nascent Cu surface are generated by the chemical reactions of H₂O₂ molecules. In the second step, at the friction interface between the Cu surface and the abrasive grain, the surface-adsorbed O atom intrudes into the Cu bulk and dissociates the Cu-Cu bonds. The dissociation of the Cu-Cu back-bonds raises a Cu atom from the surface that is mechanically sheared by the abrasive grain. In the third step, the raised Cu atom bound to the surface-adsorbed OH groups is removed from the surface by the generation and desorption of a Cu(OH)₂ molecule. In contrast, in pure water, there are no geometrical changes in the Cu surface because the H₂O molecules do not react with the Cu surface, and the abrasive grain slides smoothly on the planar Cu surface. The comparison between the CMP simulations in aqueous H₂O₂ and pure water indicates that the intrusion of a surface-adsorbed O atom into the Cu bulk is the most important process for the efficient polishing of the Cu surface because it induces the dissociation of the Cu-Cu bonds and generates raised Cu atoms that are sheared off by the abrasive grain. Furthermore, density functional theory calculations show that the intrusion of the surface-adsorbed O atoms into the Cu bulk has a high activation energy of 28.2 kcal/mol, which is difficult to overcome at 300 K. Thus, we suggest that the intrusion of surface-adsorbed O atoms into the Cu bulk induced by abrasive grains at the friction interface is a rate-determining step in the Cu CMP process.