Abstract
The correlation between the coma sensitivity of the Alternating Phase-Shifting Mask (Alt-PSM) and the Alt-PSM's structure is studied. It is found that an optimized Alt-PSM whose phase width is two thirds of its pitch has a higher sensitivity to coma than Alt-PSMs with the same pitch and the different phase widths, when the pitch of the Alt-PSM makes only +/-1 and +/-3 orders diffraction light enter the lens pupil. The optimized Alt-PSM is used as a measurement mark to retrieve coma aberration from the projection optics in lithographic tools. In comparison with an ordinary Alt-PSM mark whose phase width is a half of its pitch, the measurement accuracies of Z7 and Z14 have increased by 15% and 31% respectively.
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