Colloidal Silica based High Selectivity Shallow Trench Isolation (STI) Chemical Mechanical Polishing (CMP) Slurry
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Colloidal Silica based High Selectivity Shallow Trench Isolation (STI) Chemical Mechanical Polishing (CMP) Slurry Kyoung-Ho Bu and Brij M. Moudgil Particle Engineering Research Center, Department of Materials Science and Engineering University of Florida, Gainesville, Florida 32611, USA ABSTRACT Among various properties of chemical mechanical polishing (CMP) slurry, selectivity plays a key role in global planarization of high density and small pattern size shallow trench isolation (STI) process. Lack of adequate selectivity can lead to defects such as dishing and erosion. To improve the selectivity of STI CMP process, CMP characteristics of silica and silicon nitride wafer were investigated using colloidal silica slurry as a function of slurry pH. Sodium dodecyl sulfate (SDS), an anionic surfactant, was added to increase the selectivity of the slurry. As a result, selectivity increased from 3 to 25. It was concluded that selective passivation layer formed on silicon nitride wafer surface at acidic slurry pH range was responsible for the observed selectivity increase. Adsorption characteristics of SDS on silica and silicon nitride were measured as a function of slurry pH and concentration of SDS. As indicated by zeta potential behavior under acidic pH conditions, SDS adsorption on silicon nitride was significantly higher han silica due to the electrostatic forces. Significantly higher SDS coating on silicone nitride seems to have resulted in lubrication layer leading to increased polishing selectivity.
INTRODUCTION There are several defects such as dishing, nitride erosion and failure to clear oxide that hamper global planarization in CMP process. To minimize such defects, current STI CMP process is comprised of multi-step or raw structure modification such as reverse mask, dummy active area and additional active area. For better productivity and process simplicity, a minimum number of processing steps are highly desired and accordingly, approaches for “high selectivity single-step” slurry designs are being widely investigated [1-3]. Usually, selectivity is defined as the ratio of removal rate of silica to silicon nitride. Recently, ceria abrasives have shown selectivity of more than 5, and are being intensely investigated. In ceria CMP, the pH at which maximum polishing rate and maximum selectivity are achieved is about 8, the isoelectric point (IEP) of ceria, this however leads to agglomeration of ceria yielding poor surface morphologies with scratches and higher roughness [2]. Therefore, the research for new slurry with high selectivity and lower defectivity than ceria is highly required. In this respect, silica may be a preferred abrasive for its verified minimum defects. In this study, a strategy to increase selectivity by selectively inhibiting the polishing of silicon nitride was employed. Based on previous work in our group, surfactant mediated lubrication effects were used to achieve the desired selectivity. For example, Basim et al. showed that addition of long chain cationic surfactant (e.g. C10TA
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