Failure mechanism and improvement on gate oxide failure at the edge of LOCOS

Gate oxide early breakdown was investigated. It was verified that the gate oxide quality is good and failure was due to extrinsic causes. The failure, which was localized at the edge of LOCOS was similar to Kooi effect. However, investigations showed that it was due to nitridation occured during high temperature nitrogen anneal. Investigation methods to find the root cause of failure were explained. Alternative methods to solve the failure were explored; including thickening the sacrificial oxide layer and changing the nitrogen anneal process sequence. Final solution was chosen based on PCM stress test, QBD and TDDB result with minimal process change.