Shear thickening and shear-induced agglomeration of chemical mechanical polishing slurries using electrolytes

Chemical mechanical polishing is a fundamental technology used in the semiconductor manufacturing industry to polish and planarize a wide range of materials for the fabrication of microelectronic devices. During the high-shear (∼1,000,000 s^-1) polishing process, it is hypothesized that individual slurry particles are driven together to form large agglomerates (≥0.5 μm). These agglomerates are believed to trigger a shear-induced thickening effect and cause defects during polishing. We examined how the addition of various monovalent salts (CsCl, KCl, LiCl, and NaCl) and electrostatic stabilizing bases (KOH, NaOH, or CsOH) influenced the slurry's thickening behavior. Overall, as the added salt concentration was increased from 0.02 to 0.15 M, the shear rate at which the slurry thickened (i.e., the critical shear rate) decreased. Slurries with added CsCl, NaCl, and LiCl thickened at comparable shear rates (∼20,000-70,000 s^-1) and, in general, followed ion hydration theory (poorly hydrated ions caused the slurry to thicken at lower shear rates). However, slurries with added KCl portrayed thickening behavior at higher critical shear rates (∼35,000-100,000 s^-1) than other chloride salts. Also, slurries stabilized with CsOH thickened at higher shear rates (∼90,000-140,000 s^-1), regardless of the added salt cation or concentration, than the slurries with KOH or NaOH. The NaOH-stabilized slurries displayed thickening at the lowest shear rates (∼20,000 s^-1). The thickening dependence on slurry base cation indicates the existence of additional close-range structure forces that are not predicted by the Derjaguin-Landau-Verwey-Overbeek colloidal stability theory.