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X‐ray lithography exposures of PMMA on silicon have been simulated with a Monte–Carlo method including the energy backscattering from the substrate. The line‐edge acuity of the resist image has been modeled with experimentally determined x‐ray spectra of various sources (aluminum, tungsten, and palladium) and various source diameters (0.1, 1.5, and 3 mm). The intrinsic line‐edge degradation due to secondary electron exposure is derived from the point source images while the finite‐source effect (penumbra) is extracted from the additional degradation at finite source diameters. The penumbra effect is found to be rather small in PMMA. Depth profiles of the energy absorbed in PMMA demonstrate the significance of the energy backscattering from the substrate for energetic x rays. This effect can be significantly reduced by an organic spacer layer. The extent of the geometric projection effect has been modeled with the aluminum source. The results show that a finite angle of incidence of the x rays causes a siz...