Adsorption behaviors of silicate ions at the CeO2/water interface during CMP process: Role of surface chemistry of CeO2 nanoparticles

Abstract

CeO2 nanoparticles (NPs) have been widely investigated as an abrasive for chemical mechanical planarization (CMP) process. CeO2 NPs form a strong Ce−O−Si bonding with silicate ions during the polishing, which directly corresponds with their polishing efficiency. Herein, we have investigated the surface chemistry of CeO2 NPs and the corresponding adsorption properties of silicate ions onto the surfaces. In the first part, the processing-pathway dependent surface functional groups (e.g., −OH, −NO3 groups) on the silicate adsorption have been studied. We demonstrated that calcined and colloidal CeO2 NPs have –OH and −NO3 terminated surface, respectively. The role of –OH and −NO3 group on CeO2 surface for the silicate adsorption was experimentally and theoretically investigated based on the adsorption isotherms and theoretical analyses using density functional theory (DFT) calculation. Experimental results acquired from adsorption isotherms with Freundlich model indicated the −NO3 group shows a much higher affinity with silicate than the −OH groups. These phenomena were demonstrated through the theoretical approaches that exhibit the binding energy of the NO3-CeO2 on the SiO2 surface is much higher than that of the OH-CeO2. In good agreement with the experimental and the theoretical results based on adsorption properties, the CMP results also show that −NO3 groups significantly enhance the removal of SiO2 than −OH groups. In the second part, we have investigated the size-dependent surface chemistry of CeO2 and the corresponding adsorption properties of silicate ions. As the NPs size decreased, the surface density of –OH group increased due to the high Ce3+ concentrations at the smaller size. Results of adsorption isotherm data present that Freundlich model fitted the experimental data better than Langmuir model. The Freundlich constants (KF and 1/n), related to the adsorption affinity of silicate ions, indicated that adsorption affinity of silicate ions increases with decreasing particle size of CeO2 due to the high Ce3+ concentrations and –OH number density. The results investigated in this study will provide researchers, studied CMP slurries using CeO2 NPs, to the guidelines on importance of understanding surface chemistry of CeO2 NPs.