In the area of scanning electron microscopy (SEM), there has been a shift from the basic edge operators used in the 1990s to increasingly sophisticated imaging models and the increasing use of tilt SEM to reconstruct three-dimensional (3-D) representations of the surface.3,4 The past two decades have also seen the prodigious rise of scatterometry or optical critical dimension (OCD) metrology for in-line process control.5–7 Despite steadily increasing demands on the accuracy of the required models, both SEM and scatterometry continue to be widely utilized for process metrology.