Calibration of wafer surface inspection systems using spherical silica nanoparticles

Silica nanospheres with diameters ranging from 60 nm to 269 nm are investigated as an alternative to polystyrene spheres for calibrating laser-scattering-based wafer surface inspection systems, since they are less susceptible to changes upon ultraviolet exposure. Polystyrene and silica spheres were classified by differential mobility analysis before being deposited onto bare silicon wafers, and scattered signals were measured by two commercial tools using 488 nm and 355 nm laser light. The instrument signals were modeled by integrating a theoretically-determined differential cross section over the collection geometry of each tool, and the predicted signals were compared to the measured signals. The resulting calibrations, whether performed using the polystyrene spheres, the silica spheres, or both, were found to be equivalent and to meet industry requirements, provided the index of refraction of the silica spheres was allowed to be a floating parameter. The indices were found to be 1.413 and 1.421 at 488 nm and 355 nm, respectively, consistent with a void fraction of 11.4%.