+ BT Instability in P+poly gate MOS structure

Long term reliability of p⁺poly gate MOS structure with thin gate oxide was investigated and compared with n⁺poly gate. Instability in positive low bias temperature (-BT) stress was found out in p⁺poly gate. Increase of interface states and negative flat band voltage shift were observed after +BT stress test. Increase of interface states does not depend on boron dose into the poly Si gate and phosphorus gettring process, and no interface state generation appears in n⁺poly gate containing implanted borons. So, the mobile ions and borons are not origin of the interface state generation. Therefore, the only one mechanism, which is not denied, is that holes are injected from p⁺poly gate into the oxide and generate interface states.