A new technique for fabricating high contrast x-ray masks with precisely controlled linewidths of less than 100 Å is described. The technique is based on the deposition at an oblique angle (shadowing) of x-ray absorber material onto relief structures of triangular or square cross section in a polyimide plastic membrane. Precise linewidth control is possible because shadowing angle, absorber thickness,and relief structure can be precisely determined. The smooth and well-controlled triangular cross-section structures required are produced in silicon by anisotropic chemical etching and then transferred to polyimide by molding. The square structures are made by reactive-ion-etching of SiO2 with CHF3 and are transferred to polyimide by molding. Results of a numerical model of carbon K (45 Å) x-ray exposures in PMMA of shadowed triangular profile masks are presented which indicate that linewidth control of ±50 Å should be possible for submicrometer period grating. Scanning electron micrographs of PMMA gratings of 1 μm, 3200 Å and 1968 Å period with linewidths as small as 400 Å are shown. The successful replication of ∠200 Å linewidth patterns in PMMA using the carbon K x-ray and shadowed square cross-section masks is reported.