Characterization of line width variation by a three-step methodology is presented. Causes of line width variation with distinct spatial signatures are first isolated by spatial analysis. Sources with similar spatial signatures are then separated by contributor-specific measurements. Unanticipated components are lastly identified by examination of the residual from spatial analysis. Significant sources include photomask error, flare, aberrations, development non-uniformity, and scan direction asymmetry. These components are synthesized to quantify the contributions from the three modules of the patterning process: photomask, exposure system, and post-exposure processing. Although these modules are independent of one another their effects on line width variation may be correlated. Moreover, the relative contributions of the modules are found to vary with exposure tool, development track, and lithography strategy, affirming the usefulness of the methodology in process tracking and optimization.