Patent ID: 7391524

Claim:
A system for optically determining one or more parameters of a substantially symmetric grating, the system comprising: a) an optical source providing optical radiation at a variable wavelength and illuminating said symmetric grating, wherein said radiation has a nonzero angle of incidence θ on said symmetric grating, and wherein a plane of incidence of said radiation is substantially parallel to lines of said symmetric grating; b) an optical detector receiving radiation from said symmetric grating and providing a measured spectral response; and c) a processor receiving said measured spectral response and providing a modeled spectral response of said symmetric grating having said parameters as variables, wherein said processor determines said one or more parameters by adjusting said variables to fit said modeled spectral response to said measured spectral response; wherein said modeled spectral response is provided by a symmetry-simplified analysis; wherein said symmetry-simplified analysis is a symmetry-simplified rigorous coupled wave (RCWA) analysis; wherein said symmetry-simplified RCWA comprises a symmetric RCWA for symmetric conical diffraction accounting for N positive and N negative diffraction orders using reduced matrices having dimension N+1; wherein R is one of said reduced matrices having a corresponding full matrix F, and wherein R is related to F as follows: R i,j =F i,j +F i,−j and R i,0 =F i,0 , where 0≦i≦N and 1≦j≦N.