A nitridation technique involving shallow implantation of nitrogen into the polysilicon gate followed by thermal drive-in process was studied. Successful incorporation of nitrogen into the gate oxide/polysilicon interface has been achieved; and thereby the reliability of gate oxide has been improved due to the reduced boron diffusion into the gate oxide in p(+) polysilicon gate metal oxide semiconductor devices. An amount of nitrogen segregated in the oxide was found to be independent of the nitrogen implantation dose but to depend only on the drive-in thermal budget. Therefore, a high thermal budget in the drive-in process with a low nitrogen implantation dose is a promising direction for this process to have a high nitrogen concentration in the oxide and a low nitrogen concentration in the polysilicon to avoid a serious poly-depletion problem. However, it also has been found that when the amount of nitrogen incorporation in the oxide exceeds a certain limit, gate oxide reliability is degraded due to enhanced electron trapping.