Proposal of cap layer design combined with absorber for EUV mask

Abstract

EUV blank is a complicated system including substrate, multilayer, cap layer, absorber, and so on. All those structures contribute to the lithographic performances. In this paper, lithographic impacts of cap layer were simulated. By evaluating lithographic performances for both a binary type mask and a phase shift type mask, impact of cap layer thickness was clarified especially for best focus range of PSM. Second, impacts of cap layer recess induced by mask and lithographic processes were evaluated. Bayesian optimization method was applied to optimize lithographic performances efficiently. The results show thinner cap thickness enables to improve dose aware resolution (NILS x Threshold 0.5 ) and its stability. On the other hand, thinner cap thickness may cause worse durability for the processes such as etching and repairing. So balance between lithographic performances and film durability should be considered.