In order to fabricate submicrometer‐channel‐length MOSFET’s using x‐ray lithography, a novel mask fabrication technique was developed. The mask enabled PMMA lines with widths of 100, 160, 250, 270, and 5000 nm to be exposed simultaneously on the same substrate. The mask consists of a polyimide membrane with narrow absorber lines on the front side (facing the substrate) and coarse absorber patterns on the back side (facing the x‐ray source). The narrow absorber lines are formed by oblique shadowing of gold onto sidewalls of rectangular mesas molded on the polyimide from wells anisotropically etched in (110)Si. As a result, exposed lines are straight, controlled in width to ∼10%, and have very small edge ripple. Source and drain pattern areas are formed in gold on the back surface of the membrane by conventional photolithography and ion beam etching. PMMA lines exposed with the x‐ray mask were used for channel masking during source–drain ion implantation. Operating MOSFET’s were fabricated with minimum channel length estimated to be 75 nm.