Abstract
We discuss the major challenges facing interferometric metrology and review several optical architectures that have evolved to meet the demands of the photolithography industry. Reliable image formation at extreme values of k1 requires the precise characterization of advanced photomasks, which may themselves contain near- and sub-illumination-wavelength feature sizes. The limitations of available photomask phase metrology tools have driven the development of a new actinic phase metrology architecture that facilitates optical path difference measurements of isolated or dense features on a sub-200-nm spatial scale. We describe the optical considerations that affect optical photomask metrology, and illustrate the new coherent-probe technique with preliminary results obtained using a leading-edge chromeless-phase lithography (CPL) reticle and a high-resolution actinic 193-nm microscope.
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