Gate oxide thinning at the active device/FOX boundary in submicrometer PBL isolation

The impact of several poly buffer LOCOS processing parameters on the integrity and defect density (yield) of the gate oxide has been investigated by correlating electrical and structural studies. We found that a thin PBL pad-oxide, in general, gives rise to higher defect density. In addition, it results in a sharp active device/field oxide (FOX) boundary which, by the process of screen oxide growth and etch-back and FOX pull-back, creates a step in the Si in the bird's beak region. TEM studies revealed gate oxide thinning by as much as 30% over this step, explaining the early gate oxide breakdown at the active device/FOX region.