Model-based characterisation of complex periodic nanostructures by white-light Mueller-matrix Fourier scatterometry

Optical scatterometry is one of the most important metrology techniques for process monitoring in high-volume semiconductor manufacturing.By comparing measured signatures to modelled ones,scatterometry indirectly retrieves the dimensions of nanostructures and,hence,solves an inverse problem.However,the increasing design complexity of modern semiconductor devices makes modelling of the structures ever more difficult and requires a multitude of parameters.Such large parameter spaces typically cause ambiguities in the reconstruction process,thereby complicating the solution of the inherently ill-posed inverse problem further.An effective means of regularisation consists of systematically maximising the information content provided by the optical sensor.With this in mind,we combined the classical techniques of white-light interferometry,Mueller polarimetry,and Fourier scatterometry into one apparatus,allowing for the acquisition of fully angle-and wavelength-resolved Mueller matrices.The large amount of uncorrelated measurement data improve the robustness of the reconstruction in the case of complex multi-parameter problems by increasing the overall sensitivity and reducing cross-correlations.In this study,we discuss the sensor concept and introduce the measurement strategy,calibration routine,and numerical post-processing steps.We verify the practical feasibility of our method by reconstructing the profile parameters of a sub-wavelength silicon line grating.All necessary simulations are based on the rigorous coupledwave analysis method.Additional measurements performed using a scanning electron microscope and an atomic force microscope confirm the accuracy of the reconstruction results,and hence,the real-world applicability of the proposed sensor concept.