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Lithographic process monitoring using diffraction measurements

Optical diffraction is an attractive process with which to non-invasively examine materials and structures. Waves diffracted by a periodic structure carry information about geometrical characteristics of the diffracting structure as well as intrinsic properties of the material comprising the structure. These features make optical diffraction from surface relief gratings appealing as a monitor for lithographic processes. Considering that lithographic processes are usually quite stringently controlled, a search for 'departures' from a set of expected design values can linearize the highly non-linear relationship between parameter values and diffraction measurements, and hence permits analyses using classical linear methods. We therefore propose a linear inversion technique to determine key parameters of a periodic structure from an analysis of diffraction data that seeks to determine the departures from a set of expected design values. The approach is validated for the retrieval of three geometrical parameters (groove depth, line width, and sidewall angle) based on simulations performed using the rigorous coupled-wave theory (RCWT). The results obtained are very encouraging and highlight the potential of a linear inversion technique in scatterometry.