LOCOS

LOCOS (local oxidation of silicon), the isolation technique that preceded shallow trench isolation, grows a thick field oxide directly on the silicon surface between active device regions by selectively oxidizing the silicon itself rather than etching and filling a trench. LOCOS's main limitation, the characteristic bird's-beak lateral encroachment where the oxidation process unavoidably consumes some silicon beneath the edge of the masking layer protecting adjacent active areas, consumed active device area in a way STI's more compact trench structure largely eliminated, which is precisely why STI displaced LOCOS once transistor density scaling made that lost active area unacceptable.