Influence of sidewall perturbations of CD-SEM line roughness metrology

Microscopically, a rough edge can be considered as a set of exclusions (i.e., bumps) and inclusions (i.e., divots) along a sidewall. These local perturbations along the sidewall can be thought of as the most basic building blocks of the geometry of rough edges. However, these two structural geometries image differently under critical dimension scanning electron microscopes (CD-SEM), and also when scanned from different directions. An understanding of these imaging differences should be important to improving roughness measurement accuracy. In this work, images from using Monte Carlo and analytical simulations of different sizes of exclusions and inclusions on flat edges are used to better understand the effects of the local microgeometry of the edge, and also how various SEM algorithm choices, parameters, beam size/shape, charging, scan direction, and pixel size/scanning scheme influence SEM line edge uncertainties for such features. Furthermore, how these errors interact with roughness power spectral density (PSD) metrics will be explored, imparting knowledge for optimizing roughness PSD measurement with minimized error.