Characterization of Large Particles in Fumed Silica Based CMP Slurry
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1249-E04-08
Characterization of Large Particles in Fumed Silica Based CMP Slurry W. Scott Rader1, Tim Holt1, Kazusei Tamai2, 1 2
Fujimi Corporation, Tualatin, OR USA Fujimi Incorporated, Kakamigahara, Gifu Pref. Japan
ABSTRACT Large particles in fumed silica dispersions were characterized by sedimentation, light scattering techniques, Transmission Electron Microscopy (TEM), and lacunarity. Applying centrifugation to fumed silica dispersions generated differences in sedimentation rates of large particles. The sedimentation rates of the large particles were affected by morphological differences and the particles remaining in the supernatant displayed buoyant behavior. The large particle morphology varied from branch like aggregates containing large primary particles to particles comprised of highly coalesced, tightly packed small primary particles. The results indicate the presence of different types of large particles in fumed silica dispersions to which conventional large particle characterization is unable to distinguish. INTRODUCTION CMP (Chemical Mechanical Planarization) has become a critical process in modern semiconductor device manufacturing. The goal of CMP is to provide a polishing process that abrades residual material by combining mechanical abrasion with a chemical modification providing an ultra flat, defect free surface. The defects generated during CMP can significantly reduce device yields and are therefore important to prevent. CMP induced defects are known to originate from mechanical, chemical and electrochemical effects. However, when studying the cause of defects during the polishing of oxide films, it was determined that the abrasive component plays the majority role. Therefore it is important to understand the nature of the mechanical interactions, and effectively monitor the quality of the abrasive throughout the life cycle of the slurry.
Figure 1: Comparison of fumed silica abrasive particles (A) and colloidal silica abrasive particles (B).
Two common types of abrasives used in CMP of oxide films are precipitated (colloidal) and pyrogenically generated (fumed) silica. Figure 1 shows the morphological difference between fumed and colloidal silica. Fumed silica
particles are aggregate structures 100 – 300 nm, consisting of smaller primary particles approximately 10-30 nm. Much larger agglomerates of fumed silica, ranging in size from 300 nm to a scale of thousands of microns are also present. Fumed silica aggregates and agglomerates are dispersed into an aqueous medium through a shearing and wetting process. These slurries are typically stabilized at high pH. Because not all agglomerates are easily sheared, filtration can be utilized to remove many of the larger particles. However, particles less than 1000 nm can pass though the filtration common to these applications, ultimately reaching the fabrication process and possibly inducing mechanical defects. Further exacerbating the issue is that new generations of IC circuits are requiring smaller structures which are more sensitive to smal
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