Actinic EUVL mask blank defect inspection by EUV photoelectron microscopy

摘要

A new method for the actinic at-wavelength inspection of defects inside and ontop of Extreme Ultraviolet Lithography (EUVL) multilayer-coated mask blanks is presented. The experimental technique is based on PhotoElectron Emission Microscopy (PEEM) 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 13.5 nm wavelength. Experimental results on programmed defect samples based on e-beam lithographic structures or PSL equivalent silica balls overcoated with an EUV multilayer show that buried defects scaling down to 50 nm in lateral size are detectable with further scalability down to 30 nm and smaller due to the PEEM's instrumental performance. Furthermore, phase structures as shallow as 6 nm in height on a programmed phase grating sample has been detected by this technique. The visibility of the phase defect structures has been 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 illumination wavelength between 12.5 nm and 13.8 nm.