Because of the presence of the bird's beak, local oxidation of silicon technology is no longer suitable for submicron circuits. Sealed interface local oxidation by rapid thermal nitridation is a new isolation technique which uses a rapid thermal nitride and a trilayer of nitride/oxide/nitride to limit the lateral diffusion of oxidant species under the nitride mask. Different rapid thermal nitridation conditions (surface preparation, time, temperature) are explored in order to obtain an efficient sealing nitride film. Masking‐layer thicknesses are optimized in order to obtain a defect‐free isolation structure with a bird's beak length shorter than 0.15 μm for a 0.7‐μm‐thick, as‐grown field oxide. The process is then characterized on submicron complimentary metal oxide semiconductor devices.