Impact of mask absorber geometry on imaging stability in extreme-ultraviolet lithography with high numerical aperture

Extreme-ultraviolet lithography (EUVL) mask absorber patterns inherently form trapezoidal cross-sectional profiles, characterized by a sidewall angle (SWA), because of process limitations. However, this feature has often been considered a fabrication imperfection rather than an optical design parameter. To assess process tolerance, we systematically investigate how the geometric profile and optical constants of mask absorbers influence imaging performance in next-generation high-numerical aperture (NA) EUVL. A tapered sidewall alters the scattering condition at the absorber edge and thereby redistributes the amplitude and phase of the diffracted orders. These changes affect the key imaging metrics, including the normalized image log slope, best focus shift, and horizontal–vertical bias. The sensitivity to SWA variation depends strongly on the absorber material’s optical constants. In particular, a high extinction coefficient and a refractive index approaching unity substantially reduce the impact of SWA variation, thereby improving process tolerance. These findings show that absorber geometry and material properties jointly govern imaging behavior in high-NA EUV systems and provide guidelines for designing next-generation absorber materials with reduced sensitivity to SWA variation.