Study of MEEF improvement with low-n absorber EUVL mask

Abstract

Extreme ultraviolet lithography (EUVL) system at 0.33 numerical aperture (NA) has been used for high volume manufacturing of advanced technology node devices. As pattern pitch shrinks, low-k1 imaging becomes necessary but it typically suffers from reduced imaging contrast. In the case of low-k1 imaging, critical dimension (CD) variation on 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. Therefore, it is important to balance improvement of Normalized Image Log Slope (NILS) and MEEF. Understanding the mask contribution to EUVL is one of the important initiatives to further optimize 0.33-NA EUVL employability towards pushing down pitch scaling. The optical properties of the EUV mask absorber, which are the refraction index n and extinction coefficient k, have potential to improve NILS and MEEF. Low-n attenuated phase-shift mask (attPSM) is expected to mitigate mask three dimensional (M3D) effects and to enhance the image contrast compared to traditional Ta-based absorber mask. In this study we experimentally evaluated the MEEF on the drawn feature size, illumination, and mask absorber properties to characterize the impact of these optical factors on MEEF for the appropriate condition 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 low-n absorber EUVL 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-NA and 0.55-NA EUVL through rigorous lithographic simulations.