Electron-beam lithography allows three-dimensional shaping of polymethylmethacrylate (PMMA) structures with minimum feature size of 200 nn for diffractive optical element fabrication. The key step in fabrication of continuous PMMA profile is the resist process : contrast curve measurement at 40 and 50 kV as well as substrate thickness influence studies (bulk substrates and thin membrane) on resist slope profile were also carried out to determine the best process conditions. Examples of fabricated three-dimensional resist structures include : eight-level blazed PMMA profiles for x-ray focusing elements and a beam shaper for applications at a wavelength of lambda = 0.633 mu m. The intrinsic flexibility and accuracy of the developed process, by which the wide range of diffractive optical elements were fabricated, is a direct consequence of the specifically developed and implemented three-dimensional proximity correction algorithm. A detailed description of such an algorithm will also be given.