The post-polysilicon gate-process-induced degradation on the underlying gate oxide is studied. The degradation includes an increase in the electron trapping rate and a decrease in the charge-to-breakdown, Qbd, of the gate oxide. It is found that N2O nitrided gate oxide is more robust than O2 gate oxide in resisting the degradation. Also, to grow a thin polyoxide on the polysilicon-gate in N2O rather than in O2 lessens the degradation on the underlying gate oxide. It is nitrogen, which diffuses through the polysilicon gate and piles up at both polysilicon/oxide and oxide/silicon-substrate interfaces, that improves the oxide quality for the N2O process.