Abstract: A LOCOS process, a process of the type wherein a capped recessed mesa structure (1,3) is defined in single crystal, semiconductor grade, silicon and thereafter, and in the presence of the capping layers (5,7), local oxide (13) is thermally grown to provide an isolation structure. This process is modified (FIG. 3) by introducing a layer (11) of passive oxide deposit to cover the substrate (3), mesa (1) and upper capping layer (7) prior to the thermal growth of further oxide (13). The resulting oxide (13) is then etched back to remove excess material. This modification results in reduced bird's head height non-planarity and also in reduced bird's beak encroachment into the active area of the silicon device (3). Re-entrancy at the base of the bird's head (13) is also eliminated.