Shear Thickening of Chemical Mechanical Polishing Slurries

Numerous YouTube videos show people walking on "water". This "water" is a mixture of cornstarch and water that exhibits a phenomenon known as shear thickening. The viscosity of shear thickening fluids dramatically increases upon reaching a critical stress (e.g., when a human foot contacts the fluid), but behaves like a liquid below the thickening threshold. Chemical mechanical polishing (CMP) slurries also exhibit shear thickening. CMP 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 (i.e., computer chips). During the high shear polishing process, it is hypothesized that individual slurry particles (~100 nm) collide with one another to form large agglomerates (>500 nm) that cause the slurry to shear thicken. These agglomerates tend to dig into the material surface triggering defects such as scratches or gouges during polishing (costing the semiconductor industry billions of dollars annually). The project's goal is to understand the high shear rheological behavior of CMP slurries and to link changes in particle structure, both temporary and permanent, to the observed shear thickening response through the use of high shear (>10,000 s-1) rheology. Small-angle light scattering was employed in situ with rheological characterization (rheo-SALS) to identify the formation, shape, and size of agglomerates generated under shear. Current work is transitioning from studying fractal shaped fumed silica (currently used by industry) to spherical colloidal silica nanoparticles.