Patent ID: 6979578

Claim:
A method of determining a parameter of interest during processing of a patterned substrate, comprising: obtaining a measured net reflectance spectrum resulting from illuminating at least a portion of the patterned substrate with a light beam having a broadband spectrum; calculating a modeled net reflectance spectrum as a weighted incoherent sum of reflectances from different regions constituting the portion of the patterned substrate, for wavelengths below a selected transition wavelength in the broadband spectrum, using a first optical model to calculate the reflectance from each region as a weighted coherent sum of reflected fields from thin film stacks corresponding to laterally distinct areas constituting the region; for wavelengths above the selected transition wavelength in the broadband spectrum, using a second optical model to calculate the reflectance from each region as a reflected field from a thin film stack obtained by replacing layers in the region with effective homogeneous mediums; and determining a set of parameters that provides a close match between the measured net reflectance spectrum and the modeled net reflectance spectrum.