Patent ID: 7756677

Claim:
A method of characterizing a diffraction structure having one or more layers sandwiched between an incident medium and a substrate, wherein L l is the lth of said layers and wherein said index l increases from incident medium to substrate, the method comprising: a) illuminating said diffraction structure with incident optical radiation, wherein said diffraction structure has one or more structural parameters; b) measuring an optical response of said diffraction structure as a function of an incident radiation parameter p; c) calculating two or more modeled optical responses of said diffraction structure corresponding to two or more sets S k of values of said structural parameters at a predetermined set of incident radiation parameter values p i ; d) providing said two or more modeled optical responses as inputs to a procedure for providing estimates of one or more of said structural parameters; e) providing said estimates of said one or more structural parameters as an output; f) wherein said calculating is performed by nesting an inner loop running over all sets S k within an outer loop running over all values p i , whereby all modeled responses for each incident radiation parameter value are calculated before changing to the next incident radiation parameter value.