The influence of the surface roughness of a glass substrate on the extreme ultraviolet (EUV) reflectivity of Mo/Si multilayer deposited on it by ASET's helicon sputtering system was investigated. Deposition by helicon sputtering was found to produce a smoothing effect on the multilayer. That is, surface roughness with a spatial period below 100 nm is smoothed out. but that with a spatial period longer than 100 nm is not. The result is that the rms roughness of a Mo/Si multilayer is slightly smaller than that of the substrate. The EUV reflectvity of the multilayer improved as the rms roughness decreased down to 0.30 nm. and than leveled off at about 65% below that value, indicating that the EUV reflectivity is limited by some factor(s) other than surface roughness. However, a relatively high EUV reflectivity of 65% was obtained. even for an rms roughness of about 0.30 nm. (C) 2004 American Vacuum Society.