by ARMAND KOOLEN, ASML, Eindhoven University of Technology
In semiconductor device manufacturing, optical wafer metrology is a key technology for advanced control of the lithographic patterning in the multiple layers of an integrated circuit (IC). One such optical wafer metrology method is Fourier scatterometry, where the far-field diffraction pattern of periodic structures on the wafer is measured. In case of grating-on-grating wafer targets the lateral displacement, or overlay, between two IC layers can be measured using a technique called Diffraction-Based Overlay (DBO).
The complexity of signal formation in Fourier scatterometry under realistic conditions, i.e. in the context of optical sensor imperfections, depends a lot on the spatial coherence regime in which the Fourier scatterometer is operated. Only the fully coherent or fully incoherent limit allow for the use of fast 2D convolution models to predict the far-field patterns under such conditions. In the partial coherent regime, where the relative size between illuminated wafer spot and target is variable, 4D overlap integrals arise that do not allow for on-the-fly far-field predictions. In this talk we will present a perturbative technique, together with advanced analytical mathematics, that allows us to reduce the complexity of modelling partial coherent Fourier scatterometry to an ultrafast 2D convolution. The approach is fully vectorized and capable of predicting the impact of partial coherence and optical sensor properties on all Stokes parameters.