Role of Surface Chemistry of Ceria Nanoparticles in CMP

Ceria has been widely used as an abrasive for polishing SiO₂ film due to the high removal rate of SiO₂ during Shallow Trench Isolation (STI) CMP.¹ As the scale of transistor has been reduced, the number of maximum acceptable defects was decreased. Recently, numerous researchers have studied prevention of defect formation on the SiO₂ film by using solution-grown ceria with spherical shape. Although solution-grown ceria is spherical shape and smaller particle size, it leads to unexpected results such as removal rate of SiO₂ and higher defects compared to solid-state ceria with irregular shape. Recently, the study of surface chemistry appears to be more important than morphology, as ceria reacts with the SiO₂film to remove it.

Here, the effect of the surface chemistry of solution-grown ceria on the CMP performance was investigated. Nitrate- and hydroxyl-functionalized ceria nanoparticles were prepared by ion-exchange method.

Figure 1(a) shows the adsorption behavior of silicate ions on nitrate- and hydroxyl-ceria particles at pH 7.0. The presence of surface nitrate group induces high affinity between ceria particles and silicate ions at pH 7. Figure 1 (b) shows the effect of nitrate group on the surface on the CMP performance. Nitrate-ceria particles increase the removal rate of SiO₂film, which correlates well with the adsorption behavior of silicate ions on the ceria particles.

To interpret the adsorption behavior of nitrate- and hydroxyl-functionalized ceria particles, the adsorption reaction at the molecular level through quantum mechanical calculation was investigated.² As the second step, the hydroxyl-ceria adsorption on the H-terminated SiO₂ surface is water generating reaction (Figure 2e). The binding energies of the both cases are -2.865 and -3.813 eV, respectively. On the other hand, only one step adsorption was considered for the nitrate-ceria without generating water molecule (Figure 2f). The binding energy of this nitrate ceria on the SiO₂surface is much higher than that of the hydroxyl-ceria as -4.383 eV.

In this study, we revealed that surface functional groups such as hydroxyl and nitrate groups have a significant influence on the characteristics of CMP. The adsorption behavior based on surface chemistry is important for understanding of reaction between ceria and SiO₂.

Acknowledgement This work was supported by the Global Research Laboratory (GRL) Program (K20704000003TA050000310) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (Information and Communication Technologies) and Future Planning, and the International Cooperation program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government of Ministry of Trade, Industry & Energy (2011T100100369)

Reference

[1] Cook, L. M.“Chemical Processes in Glass Polishing.” Journal of Non-Crystalline Solids, 120, (1-3), 152-171, (1990)

[2] Perdew, J. P. et al. “Accurate and Simple Analytic Representation of the Electron-Gas Correlation-Energy” Physical Review B, 45, (23), 13244-13249, (1992)