Spatial signature in local overlay measurements: what CD-SEM can tell us and optical measurements can not

This work explores the applications of CD-SEM overlay metrology for double patterned one-dimensional (1D) pitch split features as well as double patterned ensembles of two-dimensional (2D) complex shapes. Overlay model analysis of both optical overlay and CD-SEM is compared and found to give nearly equivalent results. Spatial correlation of the overlay vectors is examined over a large range of spatial distances. The smallest spatial distances are shown to have the highest degree of correlation. Correlation studies of local overlay in a globally uniform environment, suggest that the smallest sampling of overlay vectors need to be ~10-15μm, within the spatial sampling of this experiment. The smallest spatial distances are also found to have to tightest mean distributions. The distribution width of the CD-SEM overlay is found to scale linearly with log of the spatial distances over 4-5 orders of magnitude of spatial length. Methodologies are introduced to examine both the overlay of double pattern contacts at the edge of an array and lithographic process-induced overlay shift of contacts. Finally, a hybrid optical- CD-SEM overlay metrology is introduced in order to capture a high order, device weighted overlay response.