Fluorescence Correlation Spectroscopic Studies of Particle Properties for Colloidal Ceria Abrasives Used in Chemical-Mechanical Planarization
Authors: Schorr Donald K. , Smith Madison A. , Rawat Ashwani K. , Carver Colin T. , Moinpour Mansour , Remsen Edward E. 
Published: February 2020

Fluorescence correlation spectroscopy (FCS) has been demonstrated as a versatile technique for the analysis of the smallest size fraction (diameter < 20 nm) of an abrasive silica particle dispersion and for small molecule adsorption on these particles (1-3). The technique employs the adsorption of a fluorophore at an abrasive particle’s surface, followed by single-particle detection of the dye-bound abrasive particle. Subsequent evaluation of the diffusion coefficients for all fluorescent species in the abrasive dispersion enables the quantitative determination of the fraction of adsorbed and non-adsorbed dye molecules.

In the present study, the FCS method is extended to the analysis of particle size distributions of smaller-sized colloidal ceria CMP abrasives and the quantitative characterization of dye adsorption on ceria particles. This application of FCS reflects the growing commercial importance of ceria-based slurries in CMP processes, such as shallow trench isolation (STI), which rely on both mechanical and chemical mechanisms in the CMP process4. Adsorption of the high quantum yield dye, Rhodamine 110 (R110), on ceria abrasive particles is shown to render the particles fluorescent for abrasives as small as 10 nm in hydrodynamic diameter. The sizing of particles in this diameter range is often problematic for a number of commonly employed techniques, such as static laser light differential centrifugal sedimentation. The FCS approach is demonstrated to have the additional advantage of being a single-particle technique which enables sizing determinations at nanomolar concentrations of fluorescently tagged ceria particle dispersions. Because dye molecules adsorb at specific sites on the particles, the technique provides a window on the surface chemistry of abrasive ceria particles. This characterization is discussed in conjunction with the FCS analysis of competitive adsorption of R110 and a widely employed CMP slurry additive, picolinic acid, which is used in STI processes. Further enhancements of the FCS technique for the characterization abrasive ceria dispersions, such as particle size distribution determination via the method of maximum entropy (MEM-FCS) and the use of the photon counting histogram (PCH) technique for the determination of molecular brightness of dye-bound particles as a metric for quantitative adsorption analysis are discussed.

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