The morphological characterization of local oxidation of silicon (LOCOS) isolation has been carried out by using steam oxidation in the temperature range 1050 to 1180 degrees C and as-grown field oxide thickness between 150 and 600 nm. A bird’s beak minimum is observed between 1100 and 1140 degrees C, corresponding to a change in the bird’s beak growth mechanism from a diffusion type to a regime dependent on silicon surface oxidation and nitride oxidation rates. For the first time on LOCOS isolation, the improvement of field oxide thinning by reducing field oxide thickness is reported : that enables a possible scaling for subquarter micrometer design rules.