Patent ID: 7394554

Claim:
A method of selecting a hypothetical profile to model the profile of a structure formed on a semiconductor wafer to use in determining the profile of the structure using optical metrology, the method comprising; obtaining sample diffraction signals from measured diffraction signals of structures formed on the wafer, wherein the sample diffraction signals are a representative sampling of the measured diffraction signals; defining a hypothetical profile to model profiles of the structures formed on the wafer; and evaluating the hypothetical profile using a sample diffraction signal from the obtained sample diffraction signals, wherein obtaining sample diffraction signals comprises: obtaining measured diffraction signals, wherein the measured diffraction signals are obtained from a plurality of locations on the wafer; determining a sample index, wherein the sample index corresponds to a number and a spacing of the sample diffraction signals; determining a cost distribution associated with the determined sample index; and adjusting the sample index when the determined cost distribution does not meet a cost criterion.