Simulation studies of roughness-smoothing effect of molybdenum/silicon multilayer coating based on resputtering model

For an extreme ultraviolet lithography (EUVL) mask, one cause of phase defects is bumps and divots on the surface that are a few nanometers high. Though phase defects are believed to originate in the roughness of the underlying substrate, recent reports suggest that a molybdenum/silicon (Mo/Si) multilayer deposited by ion beam sputtering smoothes out the roughness of the underlying substrate and thus relaxes the specifications for the surface roughness of glass substrates for EUVL masks. In this study, we analyzed this smoothing effect mainly through simulation studies. Our simulation is based on the string model and consists of two parts: the deposition of Mo and Si bilayers, and the resputtering of Mo and Si atoms from each layer. The simulation results based only on the deposition part suggest that a Mo/Si multilayer conformally coats the bumps and divots of the underlying substrate. These results are in good agreement with transmission electron microscope observations of Mo/Si multilayers deposited onto bumps and divots by magnetron sputtering. When resputtering is added to the simulation, both bumps and divots on the underlying substrate are smoothed out under the same conditions. These results suggest that resputtering is partially responsible for the roughness-smoothing effect that appears during the deposition of a Mo/Si multilayer.