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Optical critical dimension metrology (OCD), an optical-wavelength light-diffraction technique, is currently undergoing an industry-wide evaluation as a fast, accurate, and non-destructive sub-100 nm line-width monitor. The spectroscopic, zeroth-order diffraction signature obtained from a printed diffraction grating allows extraction of structural information, such as linewidth, sidewall angle, and profile characteristics. The OCD spectrometer is compact and designed for integration into etch tools and lithography tracks. Effective process monitoring requires detailed understanding of the correlation between CD-SEM and OCD measurements. Correlation between the OCD technique and CD-SEM is investigated in this paper by measuring photo-resist gratings on a polysilicon gate film stack. The scatter in the correlation plot is shown to reduce significantly when several CD-SEM measurements are averaged from a single grating. A positive offset in the correlation is observed and a mechanism is proposed to account for the discrepancy.