A major difference between EUV lithography and its conventional optical predecessors is the lack of a usable pellicle.
No material has been found that is both transmissive at the EUV lithography wavelength of 13.5nm and that has
sufficient structural integrity to serve as a pellicle. Therefore, during exposure to EUV light the mask is unprotected and accretes particles. This necessitates repeated cleanings of the mask, which raises concerns about changes in the mask induced by the cleaning process. It is only through metrology that these concerns can be addressed. This paper describes an optical characterization method to determine the effect of cleaning an EUV mask.