The fabrication of the bird beak free device isolated with vertical sidewalls masked with film has been investigated. The oxidation condition and combination of the second and are studied to investigate the defect mechanism for (100) silicon substrate. The characteristics of dislocations are examined by Secco etching, x‐ray topography, transmission electron microscopy, copper decoration, infrared and micrography. Two different mechanisms, responsible for dislocation generation, are identified where dislocations are introduced either by the intrinsic stress of the second or due to the mechanical damage at the corners of the island pedestal. Edge dislocations are found close to the island edges and mainly located outside the active device area. The dislocation lines are long and straight with the simple 60 deg type dislocation in <011̅> direction, on (111) and (111) planes or in <011> direction on (111̅) and (11̅1) planes. The unique characteristics of the generated dislocations are described.