Effect of pH on ceria-;silica interactions during chemical mechanical polishing
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Rajiv K. Singh Department of Materials Science and Engineering, and Particle Engineering Research Center, Gainesville, Florida 32611; and Microelectronics Research Center, University of Texas, Austin, Texas 78758 (Received 27 May 2004; accepted 10 December 2004)
To understand the ceria–silica chemical mechanical polishing (CMP) mechanisms, we studied the effect of ceria slurry pH on silica removal and surface morphology. Also, in situ friction force measurements were conducted. After polishing; atomic force microscopy, x-ray photoelectron spectroscopy, and scanning electron microscopy were used to quantify the extent of the particle–substrate interaction during CMP. Our results indicate the silica removal by ceria slurries is strongly pH dependent, with the maximum occurring near the isoelectric point of the ceria slurry. I. INTRODUCTION
Within a decade, chemical mechanical polishing (CMP) has grown from a niche application to an enabling technology. The CMP process has been implemented in multilevel metallization,1 shallow trench isolation (STI),2 and microelectromechanical system (MEMS)3 applications and has been successfully applied to metallic, dielectric, and polymeric materials. The rapid growth of CMP is due to its ability to achieve global planarity in one step. Traditionally, silica or alumina slurries have been used to polish the interlayer dielectric or interconnect metal. However, novel processes such as STI and the constant scaling of device dimensions have generated increased interest in designer slurries, which require the use of abrasives other than silica or alumina.4 By controlling chemical and surface chemical effects, the CMP process can be optimized for specific applications. Some chemical effects that may be controlled during dielectric CMP include the abrasive type (i.e., silica versus ceria), slurry pH (i.e., acidic versus basic), or the amount of slurry additives such as dissolution agents. Chemical effects in dielectric CMP are dominated by the dissolution of the substrate by water.5 Surface chemical effects may be controlled by manipulation of the abrasive size or surface area, the slurry pH, and the use of surfactants. Adsorption phenomena dominate surface chemical effects.6 In our research, we investigated basic CMP mechanisms to achieve desired CMP performance, i.e., material removal rate, surface finish, and selectivity,
DOI: 10.1557/JMR.2005.0176 J. Mater. Res., Vol. 20, No. 5, May 2005
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all with minimal defectiveness. In this work, we studied the effects of ceria slurry pH and particle agglomeration on the silica removal rate. Recently, considerable interest has been shown in ceria-based CMP slurries due to their high selectivity in STI structures. Ceria abrasives have been used for decades in glass polishing as an alternative to extremely hard abrasives like diamond or acid-based solutions. Ceria slurries remove the silica surface at a higher rate, with improved surface finish, and contain one-fifth the abrasive particles (by
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