Visible-near-IR spectrophotometry was combined with single-wavelength beam profile reflectometry to measure all sis layer thicknesses of a silicon on oxide film stack. The spectrometer or beam profile reflectometer alone is insufficient to solve such complicated film stack. Development of a robust measurement recipe required a new global optimization method working in parameter spaces of up to 12 parameters. The recipe obtained was applied to a 50-site mapping of the wafer. We found excellent agreement between our optical measurement results and scanning electron micrograph (SEM) data for all thicknesses.