Laser-Induced Plasma Exposure on Extreme Ultraviolet Lithography Masks: Damage Analysis

Extreme ultraviolet lithography (EUVL) is considered as a possible next-generation lithography technology for sub-22-nm feature fabrication. Fabrication of zero printing defect mask blanks is one of the key challenges identified for EUVL under 16 nm. One proposed cleaning mechanism is based on laser-induced plasma (LIP) shock waves in which selective nanoparticle removal is possible. However, due to both shockwave thermomechanical loading and radiation intensity heating from the LIP core during cleaning, there have been concerns over substrate damage. In this paper, computational and experimental damage studies are conducted for assessing damage risk of LIP exposure to EUVL blank samples. Based on a finite element analysis, it is found that the level of radial stress on the surface of nanofilm and nanofilm layers is the critical parameter identified in both excitation mechanisms leading to mechanical material failure. Experimentally, it is determined that above a critical LIP clearance distance, no substrate damage is observed for the EUVL blank during cleaning regardless of the number of laser shots. It is concluded that pressure amplification methods and creation of residual radial tension in nanofilms could be employed to extend EUVL mask damage threshold for LIP exposure during cleaning.