Performance of Polishing Slurries containing Silica Particles grown by Sol-Gel Method
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Performance of Polishing Slurries containing Silica Particles grown by Sol-Gel Method Sun Hyuk Bae, Jae-Hyun So, Seung-Man Yang and Do Hyun Kim Department of Chemical Engineering Korea Advanced Institute of Science and Technology 373-1, Kusong-dong, Yusung-gu,Taejon, 305-701, Korea ABSTRACT Silica slurry used as abrasives in wafer polishing process is made by dispersing silica particles in an alkali solution. Since commercially available colloidal or fumed silica particles need some modifications to be directly used as abrasive slurry due to their small sizes, irregular shapes or broad size distribution, we have prepared silica abrasives by particle growth of fumed silica or colloidal silica as seeds by sol-gel method. Silica slurries prepared by this step-wise growth from commercial seeds were tested using one-armed polisher for the comparison with commercial slurries and showed the performance comparable to commercial slurries. Microstructures of polishing slurries were investigated using transmission electron microscopy and ARES rheometer. From the result, stability of the slurry was found to be more important than the primary particle sizes for the polishing performance. INTRODUCTION Semiconductor industry requires silicon wafers with extremely tight specifications with respect to flatness and surface uniformity. Thickness variations in the sub-µm range as well as RMS (root mean square) roughness values in the sub-Å regime are prerequisite to efficient manufacturing of advanced integrated circuits. Wafer polishing process consists of stock removal polishing and final polishing step. Wafer surface is effectively removed in stock removal polishing step and final polishing determines the flatness and surface smoothness of the silicon wafers in the wafer manufacturing process. In the polishing process, silicon wafer is polished by chemical and mechanical action between silica particles in the slurry and silicon wafer surface. Polishing slurry is prepared by dispersing silica particles of nm size in a alkaline solution like ammonium hydroxide solution. Among several additives, surfactants are used for the stabilizing of colloidal slurry since surfactants with amphiphilic radicals provide steric or electrostatic hindrance between particles in the slurry. Sol-gel method has long been used to prepare silica particles. Stöber et al. synthesized monodispersed particles by hydrolysis and condensation of silicon alkoxide using ammonia as catalyst [1]. However, the slurry containing silica particles per se prepared by Stöber method has too low solid content to be used as a polishing slurry. Bogush et al. used silica particles by Stöber method as seeds to increase the size of particles and developed the relationship between particle size and concentrations of TEOS (tetraethylorthosilicate), ammonia and water [2]. More concentrated particle dispersion with narrow size distribution using Ludox AS-40® (trademark of colloidal silica of Ludox Corp.) as seeds was reported [3]. Commercially available silica particles such as Ludox®
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