Imaging evaluation of low reflectivity low-n EUV mask for LS at 0.33NA

Mask stacks comprising of alternative absorber materials with various optical properties (n and k values) may allow further improvements in EUV imaging. In a strive towards dose reduction and advancement of resolution limits in EUV lithography, such masks are brought up for consideration. In this work, we evaluate a novel low-n absorber mask with a low EUV absorber reflectivity for dark field Line/Space (LS) printing and compare it to a traditional Ta-based absorber mask. For the novel low-n mask, we experimentally confirm the reflectivity vs. the Ta-based reference mask. Through simulations and experiments at 0.33 numerical aperture (NA), we evaluate the LS imaging performance in terms of best focus through pitch. At the anchor LS pitch 28nm, we report the exposure latitude and the Mask Error Enhancement Factor (MEEF) and compare these metrics to the imaging performance of a Ta-based mask. This work adds understanding to the patterning benefits and limitations of alternative absorber mask stacks in the case of Metal direct print applications.