Phase Shift Mask to Compensate for Mask 3D Effect in High-Numerical-Aperture Extreme Ultraviolet Lithography

Abstract

In extreme ultraviolet lithography (EUVL), high numerical aperture (NA) optical system is considered to extend the resolution limit. In order to increase NA higher than 0.45 without interference between incident and reflected light cones at the mask, chief ray angle (CRA) or demagnification factor should be increased. When increasing CRA from 6 degrees to 9 degrees, imbalance of reflected EUV light with different incidence angle as well as mask shadowing effect can deteriorate imaging performance of the mask. In this paper, broad range multilayer mirror and phase-shifting absorber stack are proposed for high-NA EUVL applications. Aerial image simulation was performed at 0.45 NA, 9 degrees CRA and 6x demagnification to compare not only the telecentricity error between conventional and optimized multilayer mirror, but also the imaging properties between TaBN binary intensity mask and new TaBN/Mo phase shift mask. The telecentricity error was reduced with the optimized multilayer, and the mask 3D effect was mitigated by adopting phase shifting absorber stack. As a result, image contrast and normalized image log-slope were improved and horizontal-vertical critical dimension bias was significantly decreased.