Material Removal Mechanisms of Oxide and Nitride CMP with Ceria and Silica-Based Slurries - Analysis of Slurry Particles
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Material Removal Mechanisms of Oxide and Nitride CMP with Ceria and Silica-Based Slurries - Analysis of Slurry Particles Pre- and Post-Dielectric CMP Naga Chandrasekaran Micron Technology, Inc., 8000 S. Federal Way, P.O. Box 6 Boise, ID 83707, U.S.A. ABSTRACT The effect of CMP process parameters (pressure and pad hardness) on the ceria and silica abrasive particle-size distribution (PSD), morphology, and surface composition when polishing oxide and nitride surfaces was investigated in detail. The PSD was observed to shift post-CMP, with ceria and silica exhibiting a decrease and increase, respectively, in the number of particles towards the tail end of the distribution. The shift in ceria PSD was observed to increase as pad hardness increased. An increase in polish pressure and work surface hardness resulted in an equivalent shift in the PSD when polished on a soft pad. The inclusion of an additive reduced the oxide removal rate, and the abrasive particles exhibited the presence of a thin organic coating on the surface. The difference in material removal mechanisms and selectivity when polishing oxide and nitride with ceria and silica-based slurries was also investigated in detail. INTRODUCTION Shallow trench isolation (STI) is the preferred technology over local oxidation of silicon (LOCOS) for isolating semiconductor devices built on a silicon substrate. STI technology achieves isolation between active areas by patterning trenches into the silicon using nitride as a hard mask. Oxide is deposited to fill the trenches, which is also deposited over the nitride surface and requires to be removed. Chemical mechanical planarization (CMP) is the preferred methodology for removing excess oxide and achieving local and global planarization. The pattern dependence of CMP leads to nonuniform material removal across the entire wafer surface and requires over-polish to completely remove the oxide from the active areas (over nitride surface). This requires STI CMP to exhibit oxide-to-nitride removal selectivity, which is achieved primarily through the slurry-work surface interactions. Silica and ceria abrasives are used widely for STI CMP. In order to formulate optimized STI CMP slurries with high polish rate and selectivity, it is necessary to understand the differences in material removal mechanisms when polishing oxide and nitride surfaces with ceria- and silica-based slurries. Pioneering work has been reported in the literature on the material removal mechanisms of CMP on oxide and nitride surfaces with various abrasives. Park et al. [1] suggested that the selectivity between TEOS and PECVD nitride with ceria and silica slurries is relatively equal when an additive is not present in the ceria slurry. In contrast, it has also been suggested that the ability to remove the hydrated layer from the work surface using a chemical reaction with water depends on the bond strength of the abrasive grain relative to the Si-O bond and the isoelectric point (IEP) of the abrasive surface [2], which is different in ceria and silica
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