Patent ID: 7362435

Claim:
A method of determining an initial thickness of the most influential surface layer in a sample which comprises a substrate and at least one layer on a surface thereof, by the steps of: practicing steps a and b in either order or simultaneously: a) obtaining ellipsometric data over spectroscopic range of wavelengths by causing a spectroscopic polarized beam of electromagnetic radiation to interact with said sample and enter a detector; b) proposing a mathematical model of said sample including a variable thickness parameter for the most influential surface layer, and an assumed reasonable values of optical constants therefore; c) selecting a first wavelength and determining a list of most influential layer thicknesses for which the mathematical model provides a match to the data at that wavelength, in view of said assumed reasonable values of optical constants for said most influential layer; d) sequentially testing each thickness for the most influential layer on the list at least one additional wavelength to determine the thickness on the list for which the mathematical model provides a best-fit match to the ellipsometric data at said at least one additional wavelength.