Effects of ion irradiation on lateral diffusion have been studied in device geometry structures prepared by deposition of Cr films on patterned Si substrates with scanning electron microscopy and transmission electron microscopy. Lateral diffusion can be greatly reduced, and even completely suppressed by using arsenic ion implantation, but not by using silicon irradiation. In Cr/Si planar structures the dopant atoms accumulate near the Si‐CrSi2interface or redistribute into the unreacted Cr films. The presence of 1–2 at. % arsenic substantially slows down the CrSi2formation. We attribute the suppression of lateral diffusion in device geometry structures to interactions of impurities with the matrix atoms.