Sensitivity-limiting factors of at-wavelength EUVL mask blank inspection

Extreme ultraviolet lithography (EUVL) is one of the promising candidates among several lithography options for hp45nm node and beyond. However, fabrication and qualification of defect-free multilayer mask blanks are critical challenges for the implementation of EUVL. MTRAI developed an at-wavelength dark-field inspection tool for mask blanks using 20× Schwarzschild imaging optics and a backside-illuminated CCD, and demonstrated the detection of multilayer defects accurate to 70nm in width and 2nm in height (Tezuka et al., 2004). We are continuously characterizing the tool aiming at the design of a prototype that can inspect the whole area of a mask blank in two hours.