[en] The fabrication of carbon masks for very small magnetic tunnel junctions (MTJs) was investigated using focused ion beam assisted selective chemical vapor deposition. Gaseous phenanthrene, absorbed on the sample surface, was decomposed into solid carbon by irradiation with a Ga ion beam. The carbon layer deposited showed a lower (higher) etching rate for Ar (O₂) ion etching. The width of the carbon mask patterns varied from about 30 to 500 nm. Arrays of MTJs with size on the 100 nm scale were fabricated successfully using the carbon mask patterns