Impact of Atomic Defects on Ceria Surfaces on Chemical Mechanical Polishing of Silica Glass Surfaces

This study investigates the impact of atomic defects, such as oxygen vacancies and Ce3+ ions, on cerium oxide (ceria) surfaces during chemical mechanical polishing (CMP) for silica glass finishing. Using density functional theory (DFT) and reactive molecular dynamics simulations, the interaction of orthosilicic molecules and silica glass with dry and wet ceria surfaces is explored. Defects alter the surface reactivity, leading to the dissociation of orthosilicic acid on oxygen vacancies, forming a strong Si-O-Ce bond. Hydroxylated surfaces exhibit easier oxygen vacancy formation and thermodynamically favored substitution of hydroxyl groups with orthosilicic acid. A new ReaxFF library for silica/ceria interfaces with defects is validated using DFT outcomes. Reactive MD simulations demonstrate that ceria surfaces with 30% Ce3+ ions on (111) planes exhibit higher polishing efficiency, attributed to increased Si-O-Ce bond formation. The simultaneous presence of oxygen vacancies and various acidic and basic sites on ceria surfaces enhances the polishing efficiency, involving acid-base reactions with silica. Defective surfaces show superior efficiency by removing silicate chains, contrasting with nondefective surfaces removing isolated orthosilicate units. This study provides insights into optimizing CMP processes for high-precision glass industry surface finishing.

New Atomistic Insights on the Chemical Mechanical Polishing of Silica Glass with Ceria Nanoparticles

Reactive molecular dynamics simulations have been used to simulate the chemical mechanical polishing (CMP) process of silica glass surfaces with the ceria (111) and (100) surfaces, which are predominantly found in ceria nanoparticles. Since it is known that an alteration layer is formed at the glass surface as a consequence of the chemical interactions with the slurry solutions used for polishing, we have created several glass surface models with different degrees of hydroxylation and porosity for investigating their morphology and chemistry after the interaction with acidic, neutral, and basic water solutions and the ceria surfaces. Both the chemical and mechanical effects under different pressure and temperature conditions have been studied and clarified. According to the simulation results, we have found that the silica slab with a higher degree of hydroxylation (thicker alteration layer) is more reactive, suggesting that proper chemical treatment is fundamental to augment the polishing efficiency. The reactivity between the silica and ceria (111) surfaces is higher at neutral pH since more OH groups present at the two surfaces increased the Si-O-Ce bonds formed at the interface. Usually, an outermost tetrahedral silicate unit connected to the rest of the silicate network through a single bond was removed during the polishing simulations. We observed that higher pressure and temperature accelerated the removal of more SiO₄ units. However, excessively high pressure was found to be detrimental since the heterogeneous detachment of SiO₄ units led to rougher surfaces and breakage of the Si-O-Si bond, even in the bulk of the glass. Despite the lower concentration of Ce ions at the surface resulting in the lower amount of Si-O-Ce formed, the (100) ceria surface was intrinsically more reactive than (111). The different atomic-scale mechanisms of silica removal at the two ceria surfaces were described and discussed.

Synthesis of Hierarchical Layered Quasi-Triangular Ce(OH)CO3 and Its Thermal Conversion to Ceria with High Polishing Performance

Layered quasi-triangular Ce(OH)CO₃ assembled from primary nanoparticles was synthesized via a solvothermal method and converted into CeO₂ abrasive particles by calcination at 800-1000 °C. With the increase of calcination temperature, the primary particle size increased and the microstructure, mechanical hardness, and chemical activity of the CeO₂ particles changed, thus affecting the polishing performance. The calcined products obtained at 800, 850, and 900 °C maintained the layered edge structure of the Ce(OH)CO₃ precursor and had a relatively high specific surface area and surface Ce³⁺ concentration. The samples calcined at 950 and 1000 °C lost the layered structure due to the large-scale melting of the primary particles, and their surface chemical activity decreased. The polishing experiments on K9 glass showed that, with the calcination temperature rising from 800 to 1000 °C, the material removal rate (MRR) first increased and then decreased sharply. The initial increase of MRR was attributed to the increase of mechanical hardness of the layered quasi-triangular CeO₂, and the subsequent decrease of MRR was related to the decrease in surface chemical activity and disappearance of the layered edge structure. The product calcined at 900 °C had the highest MRR and best surface quality after polishing due to the layered edge structure and optimal match of chemical activity and mechanical hardness.

Polishing performance of a magnetic nanoparticle-based nanoabrasive for superfinish optical surfaces

Superfine optical components are necessary for advanced engineering applications such as x-ray optics, high-power lasers, lithography, synchrotron optics, laser-based sensors, etc. Fabrication of such superfine surfaces is one of the major challenges for optical and semiconductor industries. This research focuses on the development of a magnetic nanoparticle-based nanoabrasive for superfine optical polishing. The superparamagnetic iron oxide nanoparticle (SPION)-based nanoabrasive is synthesized via a hydrothermal route by employing cost-effective precursors. Detailed characterizations of the prepared nanoabrasive are presented. Transmission electron microscopy results confirm the irregular cubic and spherical shaped morphology of the SPION nanoabrasive along with particle size distribution varying from 10-60 nm, enabling the homogenous cutting effect of the aqueous slurry for polishing. Furthermore, the high surface area and pore size are determined by Brunauer-Emmet-Teller analysis and found to be 30.98m²/g and 6.13 nm, respectively, providing homogenous distribution of the nanoabrasive on the surface of a BK7 substrate for material removal. Application of the developed SPION abrasive is demonstrated for superfinish optical polishing on a BK7 optical disc. The experimental polishing results show superfine surface finishing with an average roughness value of 3.5 Å. The superparamagnetic property of the developed nanoabrasive is confirmed by alternative gradient magnetometry, and it helps in recovering the used nanoabrasive after polishing. Moreover, the polishing performance of the SPION nanoabrasives is compared with a cerium nanoabrasive, which is also synthesized in this study.

Changing the calcination temperature to tune the microstructure and polishing properties of ceria octahedrons

Ceria octahedrons with different microstructure and surface characteristics were prepared by calcining an octahedral CeO2 precursor self-assembled from spherical primary nanocrystals of about 5 nm at 500-900 °C. Structural characterization revealed that with the calcination temperature increasing from 500 to 700 °C, the products maintained a hierarchical structure and primary nanocrystals changed from spherical to octahedral particles. Significant fusion occurred between the primary nanocrystals and the surface of the octahedrons became smooth at the calcination temperature of 800 °C. Single crystal CeO2 octahedrons were formed when the calcination temperature reached 900 °C. The change in microstructure induced by elevated calcination temperature led to increased mechanical hardness and decreased surface chemical activity (specific surface area and surface Ce3+ concentration) of the octahedrons, which had an impact on their polishing performance. The polishing experiments on K9 glass showed that the polishing rate first increased and then decreased with the increment of calcination temperature, indicating that in addition to the mechanical hardness, the surface chemical activity of the octahedrons is also important for material removal. Owing to the best matching of chemical activity and mechanical hardness, CeO2 octahedrons calcinated at 700 °C exhibited the highest polishing rate and the best surface quality for K9 glass.

The role of interactions between abrasive particles and the substrate surface in chemical-mechanical planarization of Si-face 6H-SiC

The interactions between abrasive particles and the wafer surface play a significant role in the chemical-mechanical planarization (CMP) process.