The effect of oxide charges at LOCOS isolation edges on oxide breakdown

The degradation of time dependent dielectric breakdown (TDDB) characteristics at LOCOS isolation edges has been studied using MOS capacitors with and without field oxide edges under gate electrodes. The wear-out mode was shifted toward shorter breakdown time by field oxide etching right after the conventional LOCOS process. An increase in leakage current was observed at the isolation edge, so that the current enhancement is regarded as one of the main causes of the degradation. The current was reduced by constant current stress, suggesting the neutralization of positive charges due to electron trapping. The authors attribute the current enhancement causing the degradation to the buildup of positive charges at the isolation edges. Carrier injection before TDDB tests was found to improve the degradation of the wear-out mode