The effect of variations in the thickness of the multilayer of a mask blank on pattern printability was examined. The multilayer was assumed to consists of 40 Si/Mo bilayers. For a given total thickness, variations in the thicknesses of the individual SI and Mo monolayers produce such a small loss in reflectance that printability remains good for both binary and attenuated phase-shifting masks, even when such variations exist. On the other hand, variations in the total thickness shift the peak of the reflectance spectrum, which degrades the reflectance on a wafer. In this case, printability for a binary mask is determined simply by the reflectance loss on a wafer, while printability for an attenuated PSM is strongly influenced by variations in total thickness because they change the phase and attenuated reflectance.