Effect of field oxidation ambients on the gate oxide reliability in recessed local oxidation of silicon (LOCOS) with a nitride spacer has been studied. Conventional wet ambient field oxidations produced negative field oxide edge slopes and resulted in gate oxide thinning at the field oxide edges, leading to degraded gate oxide characteristics. We have found that the slope of field oxide edge can be modified from negative to positive by oxidizing in a dry ambient or in a wet and subsequent dry ambient. No gale oxide thinning was observed when the edge slope was controlled to be positive. Considering field-oxide-ungrowth phenomenon, field oxide thinning effect, gate oxide thinning, and gate oxide reliability, a three-step field oxidation method was proposed for the recessed LOGOS.