Assessment of extreme ultraviolet-induced charging of subtractive metal lithography masks

Abstract

An assessment was made of the extent of charging induced in a patterned multilayer-based extreme ultraviolet (EUV) mask and a model mask structure by exposure to EUV radiation. An aluminum subtractive metal lithography mask, consisting of ∼1700 Å of Al spaced from underlying Mo/Si multilayer by ∼500–700 Å of SiO2, was taken as representative of subtractive metal masks in general. Photoelectron spectra measured from patterned and model mask Al samples during EUV exposures were compared with spectra from samples known not to be charging. These studies indicate that EUV-induced charging of the Al absorber layer in the subtractive metal masks is zero to within ∼0.4 V accuracy. We speculate that the mask does not charge up due to an increase in the electrical conduction of the SiO2 spacer layer upon EUV irradiation. Irradiation of the spacer will create large numbers of low-energy secondary electrons in the SiO2 conduction band, providing a conducting path from the Al absorber layer to the electrically grounded Mo/Si multilayer.