Alternative absorber materials for mitigation of mask 3D effects in high NA EUV lithography

Mitigation of mask 3D effects is essential for EUV imaging of high resolution features. The 3D EUV masks give rise to phase effects over the diffracted orders and potentially distort the image on the wafer. These phase effects may reduce contrast, result in pattern shifts and result in best focus variations on wafer. Two variations on the current absorber are investigated to their impact on reduction of M3D effects and impact on image quality. Use of high-k absorber materials allows for thinner masks to be used and helps to reduce averse M3D effects. Attenuated phase shift masks work by allowing a higher optical transmission while giving a phase shift to the transmitted light, which further improves image contrast on wafer and also enables thinner absorbers to be used. Attenuated PSM absorbers show a stronger variation in imaging performance through incidence angle onto the reticle. It has been shown that this results in a variation in imaging performance for varying features and pitches. Specifically of interest is how NILS through focus is influenced by the different absorbers. Phase shift masks show better performance for NILS through focus on contact holes, and high-k masks work well for dense lines.