Development and perfection of ellipsometer systems as well as progress in data interpretation increased their acceptance as a thin-film metrology tool of choice for semiconductor and other industrial applications. Absolute accuracy and repeatability of the measured parameters are commonly used to characterize the performance of metrology systems. Accurate estimation of system repeatability is emerging as a critically important tool in designing new systems, optimizing measurement recipes, and selection (qualification) of equipment for process monitoring and characterization. (C) 2003 Elsevier B.V. All rights reserved.