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We demonstrate direct flare measurements of 4-mirror projection optics in the Engineering Test Stand (ETS) using a conventional resist clearing method (the Kirk method). Two extreme ultraviolet (EUV) lithographic projection optics, one with higher flare than the other, have been characterized and the results compared. The measured results have also been compared to analytical calculations based on measured mirror roughness and the extended point spread function (PSF). Full-field flare across the 24 mm field width has been measured, and we have verified that flare is constant across the field for EUV lithography as predicted. Horizontal (H) and vertical (V) flare bias has been observed and the cause of the H-V flare bias has been investigated. The main cause has been identified to be anisotropic mirror polishing. Simulations with the 2 dimensional Power Spectral density (PSD) function have confirmed the experimental results.

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