Avoiding the Spherical Particle Assumption: Fractal Particle Density, Size, and Structure Characterization through Combined Sedimentation and Viscometry Measurements.

Physically meaningful characterization of irregularly shaped particles continues to present substantial challenges to the experimentalist. "Equivalent diameters" based on experimental techniques such as static and dynamic light scattering or sedimentation have proliferated to the point that they are often no longer recognized as equivalent. This study demonstrates the use of dual-fluid disk centrifuge photosedimentometry coupled with rheological measurements of viscosity to provide direct insights into both the average mass of a structured particle size distribution and the average hydrodynamic diameter.

Shear-induced structures and thickening in fumed silica slurries.

Chemical mechanical polishing (CMP) is an essential technology used in the semiconductor industry to polish and planarize a variety of materials for the fabrication of microelectronic devices (e.g., computer chips). During the high shear (~1,000,000 s(-1)) CMP process, it is hypothesized that individual slurry particles are driven together to form large agglomerates (≥0.5 µm), triggering a shear thickening effect. These shear-induced agglomerates are believed to cause defects during polishing. In this study, we examined the shear thickening of a 25 wt % fumed silica slurry with 0.17 M added KCl using in situ small-angle light scattering during rheological characterization (rheo-SALS). The salt-adjusted slurry displays a ~3-fold increase in viscosity at a critical shear rate of 20,000 s(-1) during a stepped shear rate ramp from 100 to 25,000 s(-1). As the shear rate is reduced back to 100 s(-1), the slurry displays irreversible thickening behavior with a final viscosity that is 100-times greater than the initial viscosity. Corresponding rheo-SALS images indicate the formation of micrometer scale structures (2-3 µm) that directly correlate with the discontinuous and irreversible shear thickening behavior of the fumed silica slurry; these micrometer scale structures are 10-times the nominal particle diameter (~0.2 µm). The scattering patterns from the 25 wt % slurry were corroborated through rheo-SALS examination of 27 and 29 wt % slurries (C(KCl) = 0.1 M). All slurries, regardless of ionic strength and solids loading, display scattering patterns that are directly associated with the observed thickening behavior. Scattering was only observable during and after thickening (i.e., no scattering was detected in the absence of thickening). This work serves as the first in situ observation of micrometer scale structures within the fumed silica CMP slurry while under shear.

Simultaneous absolute determination of particle size and effective density of submicron colloids by disc centrifuge photosedimentometry.

Disc centrifuge photosedimentometry (DCP) with fluids of different densities is used to simultaneously determine the particle size and effective density of spherical silica particles. Incorporation of a calibrated infrared pyrometer into a DCP instrument is shown to enhance the measurement capability of the DCP technique by correcting for the temperature dependence of the spin fluid's density and viscosity. Advantages of absolute DCP determinations for size and density analysis relative to standardized DCP measurements include the elimination of instrument standardization with a particle of known density and measurements or estimation of the effective particle density. The reliability of diameter determinations provided by absolute DCP was confirmed using silica particles with nominal diameters ranging from 250 to 700 nm by comparison of these analyses with a diameter determination by transmission electron microscopy for silica particle size standards. Effective densities determined by absolute DCP for the silica particles ranged from 2.02 to 2.34 g/cm(3). These findings indicate that the silica particles have little or no porosity. The reported characterization of colloidal silica using absolute DCP suggests applicability of the technique to a variety of particle types including colloidal materials other than silica, core-shell particles, compositionally heterogeneous mixtures of nanoparticles, and irregularly shaped, structured colloids.