Actinic extreme ultraviolet lithography mask blank defect inspection by photoemission electron microscopy

A new method for the actinic inspection of defects inside and on top of extreme ultraviolet (EUV) lithography multilayer-coated mask blanks is presented. The experimental technique is based on photoemission electron microscopy supported by the generation of a standing wave field inside and above the multilayer mask blank when illuminated near the resonance Bragg wavelength at around $13.5nm$⁠. Experimental results on programed defect samples based on electron beam lithographic structures or silica balls overcoated with an EUV multilayer show that buried defects with a lateral size down to $50nm$ are detectable. Furthermore, phase structures as shallow as $6nm$ in height on a programed phase grating sample have been detected by this technique. The contrast of the phase defect structures has shown to be strongly dependent on and controlled by the phase of the standing wave field at the mask blank surface, and thus can be optimized by tuning the inspection wavelength.