Preparation, microstructure and properties of LaCePr polishing slurry for optical glass CMP

Objectives: In recent years, cerium-based rare earth polishing liquids have been widely used in the field of chemical mechanical polishing (CMP) of glass and other materials due to their excellent selectivity and good polishing efficiency, but their polishing performance needs to be further improved. Using Baotou mixed rare earth ore as the raw material and other additives as auxiliary materials, the LaCePr rare earth CMP polishing solution with good comprehensive performance was prepared through a complex process. The microstructure, the element distribution, and phases were studied, and specific polishing experiments were conducted to evaluate its polishing performance. Methods: The LaCePr chlorination solution was prepared using the product of Baotou mixed rare earth ore, which underwent concentrated sulfuric acid enhanced roasting, water leaching, neutralization and impurity removal, P₅₀₇ extraction transformation and grading. The mixture of ammonium bicarbonate and ammonia water was used as a precipitant, hydrofluoric acid was used as a fluorinating agent, and sodium polyacrylate, sodium hexametaphosphate, sodium hydroxide and other additives were added. The LaCePr rare earth CMP polishing solution samples were prepared through processes such as co-flow precipitation, fluorination, high-temperature roasting, introduction of additives, slurry mixing, and wet ball milling. Subsequently, the macroscopic structure of the prepared polishing solution was observed, and the elemental distribution in the sample was measured using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The solid polishing powder sample of the LaCePr rare earth polishing solution after air drying was analyzed using X-ray diffraction (XRD). The CMP polishing performances of the prepared LaCePr rare earth polishing solution were tested, and the polishing effect of the workpiece was evaluated using the planar precision polishing machine and the optical 3D surface profilometer. Results: (1) The morphology of the solid polishing powder after air-drying of the polishing liquid consists of irregular polygonal spherical grains tightly aggregated, which mainly shows that the fine particles are agglomerated together. In addition, there are no coarse particles in the whole cluster, indicating that the introduction of F and doping of Pr have a positive effect on the lattice distortion of CeO₂. (2) The microstructure and the element distribution diagram of the solid polishing powder show that the solid solution second-phase particles appear around the grain boundaries of individual particles, indicating that the doped La and Pr elements mainly enter the CeO₂ lattice in a solid-solution manner. The La, Ce, Pr and O elements are uniformly distributed, confirming that the introduction of the F element can play a role in grain refinement. (3) In addition to the characteristic peaks of the CeO₂ phase, the diffraction peaks of LaOF, Pr₆O₁₁, LaF₃, PrOF and other phases are also observed in the rare earth polishing solution sample, indicating that the doped La, Pr and F elements enter the CeO₂ lattice in the form of a solid solution of solute atoms. The face-centered cubic Pr₆O₁₁ structure is the same as that of CeO₂, so it can exert a synergistic polishing effect together with CeO₂. (4) The initial polishing ability of the LaCe rare earth polishing solution is 182.6 nm/min. After polishing for 40, 60, 80 and 120 minutes, the polishing abilities of the LaCe polishing solution are 199.3, 199.9, 193.8 and 158.2 nm/min, respectively. The initial polishing ability of the newly prepared LaCePr rare earth polishing solution is 203.4 nm/min. After polishing for 40, 60, 80, and 120 minutes, the polishing abilities of the LaCePr polishing solution are 219.7, 214.7, 206.3 and 189.8 nm/min, respectively. (5) The surface roughness S_a of the glass after five CMP cycles with the LaCe polishing solution is 0.659 nm, while the surface roughness S_a of the glass after five CMP cycles with the LaCePr polishing solution is 0.668 nm. Conclusions: By using LaCePr rare earth chloride solution, the LaCePr rare earth polishing solution can be successfully prepared through processes such as precipitation, fluorination, high-temperature calcination, additive blending, and wet ball milling. The entire process of the preparing polishing solution has achieved zero wastewater discharge, which is a green polishing solution preparation process. After air drying, the LaCePr rare earth polishing solution, the medium particles in the sample are well-formed and evenly distributed. The doped La, Pr and F elements all enter the CeO₂ lattice through solute atom solid solution. The cubic fluorite structure of CeO₂, the tetragonal structure of LaOF, and the face-centered cubic structure of Pr₆O₁₁ in the LaCePr rare earth polishing solution exhibit a synergistic polishing effect. After 120 minutes of cumulative polishing of H-K9L optical glass, the maximum polishing rate of the LaCe polishing solution is 199.9 nm/min, while the maximum polishing rate of the LaCePr polishing solution can reach 219.7 nm/min. The polishing quality of H-K9L glass is basically the same after polishing with the two kinds of polishing solutions, and the surface roughness S_a decreases from the initial 1.123 nm to 0.659 nm and 0.668 nm, respectively. Therefore, the comprehensive properties of the prepared LaCePr CMP polishing liquid are better.