Study of mask error enhancement factor improvement with low-n absorber extreme ultraviolet lithography mask

In the case of low-k1 imaging, critical dimension (CD) variation on extreme ultraviolet (EUV) mask enhances on wafer, which is called mask error enhancement factor (MEEF). CD variation for tighter line and space (L/S) features on the photomask has taken up a large fraction of wafer CD variation. Understanding the mask contribution to EUV lithography (EUVL) is one of the important initiatives to further optimize 0.33-NA EUVL employability. The optical properties of the EUV mask absorber have the potential to improve Normalized Image Log-Slope (NILS). Low-n attenuated phase-shift mask (attPSM) is expected to mitigate mask three-dimensional effects and to enhance the image contrast compared with a traditional Ta-based mask. We experimentally evaluated the MEEF on the drawn feature size, illumination, and mask absorber properties to characterize the impact of optical factors on MEEF using the NXE:3400B EUV scanner. This paper covers two types of mask absorber materials which are Ta-based and low-n attPSM. We experimentally demonstrated that a low-n mask is expected to improve the balance of MEEF at vertical L/S through pitch due to suppression of best focus variation. We also tried to identify the suitable absorber optical properties in terms of MEEF and NILS on both 0.33/0.55-NA EUVL through lithographic simulations.