Impact of anamorphic scaling on orientational dependence of high-NA EUV lithography

HiNA EUV lithography is employed to expose metal oxide resist on horizontal and vertical line-space features with ~10nm linewidths at 21 nm pitch, using a symmetrical illumination source. Significant differences are observed between horizontal and vertical features across three key metrics: exposure latitude (EL), unbiased line edge roughness (uLER), and mask error enhancement factor (MEEF). Horizontal features demonstrate superior performance, exhibiting lower uLER, higher EL, and reduced MEEF compared to vertical features. These performance differences at identical pitch are typically attributed to the well-known effects of EUV mask shadowing in low numerical aperture (NA) EUV systems. However, this study demonstrates that EUV mask shadowing alone does not fully account for the observed experimental discrepancies, and that other anamorphic effects must also be considered. To differentiate between mask shadowing and potential other anamorphic impacts on the horizontal (8x mag) and vertical (4x mag) line-space patterns, simulations were conducted using both magnifications in each orientation. Lithographic simulations reveal that the maximum normalized image log slope (NILS) of horizontal lines exceeds that of vertical features, aligning with experimental observations. The simulations also corroborate the experimentally observed slight focus shift between horizontal and vertical features. Additionally, simulations indicate that mask sidewall angle and related effects exhibit greater sensitivity in the vertical features compared to the horizontal lines. Further analysis of anamorphic mask effects for both orientations at smaller k₁ factors show that anamorphic imaging effects contribute to the imaging orientational preference between horizontal lines and vertical lines at different k1.