Significant Enhancement Of Inorganic Electrolytes In Ceria Slurry To Material Removal Rate In Polishing Of ZF7 Glass

The synthesis of abrasive particles with controlled microstructure and the modulation of polishing slurry are key topics in the field of chemical mechanical polishing (CMP). Any breakthrough in CMP technology could significantly promote the development of super-large-scale integration circuits and ultra-high precision optical devices. In this paper, some inorganic electrolytes were employed as dispersants to modulate polishing slurry of ceria. Their material removal rate (MRR) for polishing ZF7 glass and the surface quality (Ra) were evaluated. The promotion mechanism of dispersant for the polishing was explained by examinating the relationship of MRR with Zeta potential and size distribution of abrasive particles as well as the dispersion stability of slurries. Some effective mothods to enhance MRR value then proposed.It is found that the addition of NaCl into ceria slurry resulted in MRR decreasing, whereas the introduction of sodium hexametaphosphate showed an evident increase of MRR. When sodium chloride and sodium hexametaphosphate were co-added into ceria slurry, a synergistic enhancement for the MRR value has been observed. The MRR values for pure ceria slurry and ceria slurry containing 0.5mol/L sodium chloride and 1.05wt% sodium hexametaphosphate were 199.36nm/min and 351.26nm/min, showing a 76.2% increase of MRR. The corresponding surface roughness R_a values of as-polished ZF7 optical glasses were 0.754nm and 0.799nm, which only increased by 5.6%, and can meet the need of optical application. The linear relationship between MRR value and the zeta potential of ceria particles in slurries shows that the increase of MRR value is attributed to the increase of minus surface zeta potential. This result indicates that the polishing ability of ceria can be greatly enhanced by adjusting surface zeta potential of abrasive particles using sodium chloride and sodium hexametaphosphate. Furthermore, some other 1:1 type electrolytes such as NH₄Cl,NaNO₃ and NH₄NO₃ also showed similar characteristics and rules.With the addition of cerium ion into ceria slurry, the MRR value for the polishing of ZF7 glass increased by times and greatly depended on the pH of slurry. No significant improvement was observed for the neutral or weak basic slurries. It is found that both MRR value and surface roughness R_a values are increased greatly with slurry pH decreasing from 5 to 1. The MRR value also varied with the concentration of cerium ion in ceria slurry and showed a maxinium value at 0.1M of Ce³⁺. Therefore, the pH and cerium ion concentration in ceria slurry for the polishing of ZF7 glass with a high MRR value and an acceptable R_a value were determined to be between 4 and 4.5 and 0.1M respectively. At the same time, the increase of MRR value was related with the improvement of slurry dispersion, which was attributed from the change of electric charge on particle surface.Furthermore, the addition of Zn(NO)₂ and Al(NO₃)₃ into ceria slurry can also accelerate the polishing process with similar rules.