Impact of EUV mask absorber sidewall angle on patterning robustness

Investigations into extreme ultraviolet lithograph (EEUVL) image formation of horizontal (perpendicular to the illumination angle) features have found asymmetric aerial images. This is an expected outcome of the off-axis chief ray angle (CRA)) illumination in the EUV lithography system. The asymmetry arises from interference effects in the mask multilayer stack as well as interactions with the mask absorber. The aerial image arriving at the wafer displays an asymmetric shape, which contributes too a reduced contrast on the dark-side absorber, while the light-side absorber contrast remains sharp. This is not the preferred situation as the low-contrast dark-side can lead to reduced imaging robustness during manufacturing. Preliminary studies show that mask absorber sidewall angle (SWA) impacts pattern formation partially through aerial image asymmetries. The light and dark-side of the absorbers form a standing wave in the gap between them due to absorber side wall reflection and corner scattering. The absorbers’ standing waves further interact with the standing waves from the mask stack. Optimizing the absorber SWA is hypothesized to improve contrast thereby improving patterning robustness. This study investigated the impact of absorber SWA on aerial image shape using simulation. The study was designed to understand if an optimal SWWA exists that improves patterning robustness in a manufacturing environment. CD, contrast, focus response, and other data were gathered and presented to understand the impact of SWA on patterning. From these simulated data, the possibility of an optimum SWA was explored.