In this paper it is shown that the shape of the bird's beak in poly-buffered LOCOS isolation structures, and related to this the local mechanical stress in the substrate near this bird's beak, is highly influenced by nearby structures. It is found that especially a large Si 3 N 4 -field affects the bird's beak geometry at nearby structures. A possible explanation for this effect is the influence of mechanical stress present in the substrate before field oxidation on the oxidation rate.