Acidic Dispersion of Fused SiO 2 Particles
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The acidic dispersion behavior of fused SiO2 particles was investigated and compared to two fumed SiO2 particles. In contrast, the fused SiO2 particles have a larger particle size, broader size distribution, and lower surface area. Fluoride adsorption was used to study surface activity, and acid–base titration was used to study surface charge in 10−1 to 10−3 M NaCl solutions over the pH range of 2–7.5. Each of the three SiO2 particles exhibited similar titration behavior, with the fused SiO2 particles displaying a higher intrinsic pKa2 value of 7.0 as compared to 6.8 and 6.1 for the two fumed SiO2 particles. Rheological experiments designed to test for dispersion and agglomeration/ gellation at 3 and 6 wt% solids loading in 10−3 M NaCl solutions adjusted to pH 2, 4, and 6 showed the fused SiO2 particles to be more stable in suspension, exhibiting lower viscosity results for all test conditions. Results show that the fused SiO2 particles tested in this report display superior dispersion properties as compared to conventional fumed SiO2 particles for slurry applications under acidic conditions.
I. INTRODUCTION
II. MATERIALS AND METHODS
Dispersion of particles in acidic environments is of specific interest to the semiconductor industry for chemical– mechanical planarization, where slurry formulations are typically adjusted to pH 2–7 for the planarization of metal interconnecting layers.1,2 Specific to SiO2 particles in this pH range is the need to produce stable suspensions that exhibit low viscosity without agglomeration or gellation. Flame hydrolysis can produce both fumed and fused particles, with the main difference being the temperature of the environment in which the particles are hydrolyzed and the time that particles spend at elevated temperature before being collected. Fumed SiO2 particles are collected from cool environments and have high surface areas, whereas fused SiO2 particles are collected in hot environments (>1600 °C) and have relatively low surface areas. In previous reports, SiO2 surfaces have been characterized by acid–base titration on dried SiO2 gels3 and on fused SiO2 particles,4 with both materials exhibiting similar surface chemistries. This paper reports on surface chemistries and rheological behavior of aqueous slurries for two commercially available fumed silica particles and compares these results to those of fused silica aqueous slurries.
The three silica particles used in this study were Corning Incorporated’s fused SiO2 (particle “A”), Degussa OX50 fumed SiO2 (particle “B”), and Cabot 10M fumed SiO2 (particle “C”). Morphological analysis was performed for powders dried from aqueous solution using transmission electron microscopy (TEM), and surface area analysis was performed for as-received dry powders using multipoint N2-BET. Total number of active surface OH sites was measured using the fluoride adsorption method of Sigg and Stumm.5 Solutions were prepared in 0.12 M NaF, adjusted with dilute HCl to pH 5.5 ± 0.1, stirred at room temperature for 1 h, and placed into an oven at 50–5
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