Chemical mechanical polishing (CMP) is the only method in semiconductor manufacturing that employs the synergistic action of mechanical and chemical forces to remove excess material from device surfaces, reduce surface roughness, and obtain planarized surfaces. It is also a key technology for fabricating multilevel interconnects in semiconductor chips and integrated circuits.
In semiconductor CMP processes, ceria (CeO₂) has long held a central position in dielectric layer planarization. Compared with colloidal silica or alumina abrasives of similar particle size, CeO₂ slurries exhibit a chemical tooth effect toward silicon-based materials such as silicon dioxide (SiO₂) and silicon nitride (Si₃N₄) at equivalent concentrations, resulting in higher polishing quality. This performance gap cannot be explained solely by hardness and particle size parameters-its root lies in the unique variable‑valence chemistry of cerium.
Structural Basis of CeO₂: Oxygen Vacancies and the Origin of Ce³⁺
CeO₂ has a fluorite cubic structure, in which each Ce atom is coordinated by eight oxygen atoms. Under the ideal stoichiometric ratio, cerium exists as Ce⁴⁺. However, the 4f electron orbitals of cerium allow a reversible transition between Ce⁴⁺ and Ce³⁺: when one oxygen ion is removed from the lattice, the surrounding Ce⁴⁺ ions gain electrons and are reduced to Ce³⁺, simultaneously leaving an oxygen vacancy, thereby forming non‑stoichiometric CeO₂₋ₓ.
The Interface Reaction at the Polishing Interface: Formation and Breakage of Ce–O–Si Bonds
The removal of SiO₂ by CeO₂ is a process involving both chemical reaction and mechanical force, which can be summarized in the following steps:
01 Surface hydroxylation
In an aqueous slurry environment, the Si–O–Si bridging oxygen on the SiO₂ surface is hydrolyzed to form silanol groups (Si–OH). At the same time, the Ce³⁺ active sites on the CeO₂ particle surface carry hydroxyl groups (Ce–OH).
02 Formation of Ce–O–Si bonds
Ce–OH and Si–OH undergo condensation dehydration, forming covalent bonds between the particle and the substrate. The Ce–O–Si bond bridges the particle–substrate interface, directly chemically connecting the abrasive particle to the SiO₂ surface. Studies have detected the presence of this bond on polished particles and wafer surfaces using XPS, TEM, and infrared spectroscopy.
Ce–OH + HO–Si → Ce–O–Si + H₂O
03 Mechanical intervention and material removal
Under the action of polishing pad pressure and relative motion, the Ce–O–Si bond pulls Si atoms from the surface of the SiO₂ bulk network, which are released into the slurry as soluble silicates (Si(OH)₄). After detachment, the Ce–O–Si on the particle surface is hydrolyzed to regenerate Ce–OH, completing one removal cycle. This mechanism is commonly referred to as "chemomechanical synergistic removal": the chemical reaction (condensation and bond formation) weakens the binding energy of the SiO₂ surface layer, while the mechanical force accomplishes the final removal-both are indispensable.
Note: Ce⁴⁺ does not directly participate in the interfacial reaction, but it is the prerequisite for the stable operation of the entire system. In oxidizing aqueous solutions (CMP slurries typically operate at pH 4–8), the thermodynamically stable phase of CeO₂ is the Ce⁴⁺‑dominated fluorite structure. The presence of Ce⁴⁺ ensures the structural integrity of the particles under mechanical stress and also serves as an ion reservoir for the regeneration of Ce³⁺.
Current Research Directions for CeO₂ Abrasives
Current research efforts on improving CeO₂ abrasives primarily focus on the following three directions:
01 Morphology optimization
The morphology of CeO₂ (spherical, cubic, rod‑like, polyhedral, etc.) affects its surface energy, the exposure ratio of active crystal facets, and its dispersion stability in the slurry. Different morphologies exhibit differences in surface Ce³⁺ concentration and hydrodynamic behavior. Researchers control hydrothermal synthesis conditions to tune the morphology, aiming to enhance removal rate and slurry stability.
02 Structural modification
Constructing core–shell structures or heterogeneous composites of CeO₂ with SiO₂ or polymeric materials allows simultaneous tuning of particle hardness, surface chemistry, and dispersibility. For example, SiO₂@CeO₂ core–shell structures reduce the risk of mechanical scratching through a soft core while retaining the chemical activity of the CeO₂ surface. Heterogeneous composite structures can improve the selective polishing capability toward specific substrates through synergistic effects between the two materials.
03 Elemental doping to regulate oxygen vacancy concentration
Doping with trivalent elements (La³⁺, Nd³⁺, Yb³⁺, etc.) into the CeO₂ lattice introduces more oxygen vacancies due to charge compensation, simultaneously reducing surrounding Ce⁴⁺ to Ce³⁺ and directly enhancing surface reactivity. Experiments have shown that Nd doping can increase the material removal rate (MRR) to more than three times that of the undoped control.