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The sedimentation of slightly Brownian particles toward a flat plate in water has been measured using the optical technique of total internal reflection microscopy (TIRM). The particles used were 15, 20, and 25 μm diameter polystyrene latex spheres and the plate was a polished BK-7 glass slide. The measurements were performed for dimensionless separation distances (gap width/sphere radius) ranging from 0.01 to 0.05, thus the mobility of the particles was greatly reduced by hydrodynamic coupling with the plate. The average sedimentation rate was found to agree closely with predictions made using the drag equations of Brenner [H. Brenner, Chem. Eng. Sci. 16, 242 (1961)] when the measured separation distances were adjusted by a constant offset which is believed to reflect the presence of asperities on the particle surface. In addition, the Brownian movements observed during sedimentation were found to be normally distributed with a mean of zero, also in agreement with theory. Thus by simply measuring the sedimentation rate, the dependence of both the particle’s mobility and diffusion coefficient on separation distance could be obtained. Although surface roughness on the particle limited the distance of closest approach, it did not affect the particle/surface hydrodynamics.

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The detection of surface particles has become important in contamination control over the years. However, the minimum particle size required to be detected has been becoming smaller as IC geometries shrink. Current visible-light detection systems can detect particles down to around 60 nm in polystyrene-latex-equivalent size and so should be adequate for geometries down to around 0.18 µm, but a quick glance at the National Technology Roadmap for Semiconductors shows that geometries are expected to become as small as 0.07 µm in a little over ten years, requiring the ability to detect particles around 20 nm in diameter. This is beyond the capability of current visible-light scanners, so the Semiconductor Research Corporation has recently commissioned our group to conduct research into the limits of optical defect detection and potential of alternative detection technologies. This research centers on short-wavelength optical systems and scanned electron-beam systems as the most likely candidate technologies for high-speed nanoparticle detection. In this paper we develop a model for the analysis of the performance of hypothetical short-wavelength surface inspection systems and examine the manifold difficulties involved with using those wavelengths. The properties of scattering in the transitional region between the UV and X-ray regimes are also examined.