Real-time optical CD metrology(1), employing fast numerical solutions to the diffraction of light from microelectronic features, is beginning to gain wide usage for sub-130nm IC processing. Applications of this method heretofore have concentrated on two steps in the patterning process: the 'develop inspect' (DI) step, at which point the pattern features are formed in the photoresist but prior to etch, and the 'final inspect' (FI) step, after the etch process has transferred the feature into the underlying IC layer(s). In this article, we examine another application: use of optical CD metrology for rapid characterization of stepper/scanner performance and optimization. In order to be useful to characterize a litho process, we found it necessary to expand the set of fitting parameters to include the pitch of the measured line/space array. This is due to two practical matters: first, the pitch imprinted on a wafer is the result of the pitch on reticle and second, the pitch on the wafer is a result of imaging magnification. As a result, this parameter can vary by several percent typically, for a pitch target of 240 to 350nm. Without taking this parameter into account, high-quality fitting of results over a full-field focus-exposure matrix (FEM) wafer, for example, is problematic. With the pitch included as a fitting parameter, it is possible to obtain excellent data-model fits across an entire FEM with a single metrology recipe.