Shear thickening and defect formation in chemical mechanical polishing slurries

Abstract

Chemical mechanical polishing (CMP) is used by the semiconductor manufacturing industry to polish and planarize materials for the fabrication of microelectronic devices (e.g., computer chips). During the CMP process, the polishing slurry is exposed to shear rates [greater than or equal to]1,000,000 s-1. It is believed that under this extreme shear, individual slurry particles are driven together to form agglomerates which cause the slurry to shear thicken. These shear-induced agglomerates create defects such as scratches or gouges, costing the semiconductor industry billions of dollars annually. To study the shear thickening behavior of fumed silica CMP slurries (20-34 wt%) under high shear, we employed a conventional rotating rheometer with a parallel-plate geometry at small gaps ([less than]100 [mu]m). The fumed silica slurries displayed discontinuous and irreversible thickening at shear rates [greater than]10,000 s-1. The thickening response of the slurries can be tuned by adjusting silica concentration, adding monovalent chloride salts, and/or changing the stabilizing hydroxide base. Overall, the thickening shear rate decreases as silica concentration is increased and as the added salt concentration is increased from 0.02 to 0.15 M. Small-angle light scattering in conjunction with rheology at high shear rates was utilized to probe for shear-induced structure formation during shear thickening. The investigated 25 wt% fumed silica slurry with 0.17 M KCl displayed almost zero scattering in the absence of thickening. However, once discontinuous thickening is observed at [approximate]20,000 s-1, a distinct scattering pattern develops; indicating the formation of 2-3 [mu]m sized structures. The scattering pattern persists as the shear rate is reduced and as the slurry's viscosity increases, suggesting the micron sized agglomerates endure throughout the shear rate reduction step. Using in situ polishing during rheological characterization, we examined the shear thickening behavior of a 25 wt% fumed silica slurry with 0.15 M KCl. Optical microscopy and non-contact profilometry linked slurry thickening behavior to scratching of a "polished" wafer. The thickened slurry generated up to 7x more scratches than a non-thickened slurry. Both slurry thickening and surface scratching were associated with a population of "large'' particles ([greater than or equal to]300 nm), which were undetected in the non-thickened slurries.