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Analysis of photomask distortion caused by blank materials and open ratios

As optical lithography error budgets on pattern displacement become more and more stringent for features as small as <180 nm, overlay control will be one of the top challenges facing lithography in the future. However, mask induced error budgets are less considered than uniformity of a mask. In this paper, we demonstrate a pattern displacement caused by stress induced distortion, in-plane distortion(IPD) values of chrome-on-glass (COG) and phase-shift-mask (PSM) blanks, and overlay errors. The magnification after AR/Cr layer removal for a COG is 0.37 ppm, which corresponds to an IPD of 33 nm across 100 x 100 mm2 area. The IPD for PSM corresponds to 43 nm across 100 x 100 mm2 area with 0.48 ppm. The IPD for PSM increases dramatically with increasing open ratios, while that for COG only slightly increases. It is found that mix-match between steppers and scanners should be avoided as long as COG mask and PSM are mixed because errors such as skew and scan direction magnification are uncorrectable with steppers.