A process for fabricating x-ray lithography masks having structures as small as 0.1 μm is described in detail. The patterns are made by electron-beam lithography in PMMA resist laid on 1-μm-thick polyimide membrane. The fact we use a very thin substrate strongly reduces the influence of electron backscattering on the resolution. That way it is possible to transfer 0.1 μm lines and spaces in 0.25-μm-thick PMMA resist. Gold absorber patterns are formed using electroplating, this technique being able to reproduce the smallest details of the lithography. On the other hand it allows high-aspect ratios because electroplated gold may be as high as the resist layer. Examples of 0.25 μm lines with 0.05 μm spaces in 0.25-μm-thick gold and 0.2 μm spatial period gratings in 0.15-μm-thick gold are given.