A new approach for actinic defect inspection of EUVL multilayer mask blanks: Standing wave photoemission electron microscopy

Extreme ultraviolet lithography (EUVL) at 13.5nm is the next generation lithography technique capable of printing sub-50nm structures. With decreasing feature sizes to be printed, the requirements for the lithography mask also become more stringent in terms of defect sizes and densities that are still acceptable and the development of lithography optics has to go along with the development of new mask defect inspection techniques that are fast and offer high resolution (preferable in the range of the minimum feature size) at the same time. We report on the development and experimental results of a new 'at wavelength' full-field imaging technique for defect inspection of multilayer mask blanks for EUV lithography. Our approach uses a photoemission electron microscope (PEEM) in a normal incidence illumination mode at 13nm to image the photoelectron emission induced by the EUV wave field on the multilayer mask blank surface. We show that by these means, buried defects in the multilayer stack can be probed down to a lateral size of 50nm, which was the smallest pre-defined structure size under study so far. The PEEMs spatial resolution has been proven to be as small as 29nm edge slope width in measurements with 250nm radiation. Therefore, the capability of this technique for actinic measurements at 13nm radiation is anticipated to be well below 50nm.