Angle-resolved photoelectron spectroscopy study was performed on the depth profile of nitrogen atoms in silicon oxynitride (SiON) films formed by the plasma nitridation of silicon dioxide using low-electron-temperature microwave plasma. The depth profile of nitrogen near the SiON surface was confirmed to increase and its peak position moves into SiON films with an increase in the nitridation time, which improves boron immunity. A new transport and reaction model of plasma nitridation is proposed to explain the time evolution of nitrogen concentration and its depth profile in the films. Here, the density of radical nitrogen atoms decreases exponentially with an increase in the distance from the surface, and the nitrogen concentration incorporated in the SiON film is approximately proportional to the logarithmic time of plasma nitridation. It was newly found that postnitridation annealing strongly enhances the pile-up of nitrogen atoms at the Si-SiON interface owing to their diffusion from the inward tail f the nitrogen depth profile near the surface. It is deduced that the pile-up of nitrogen atoms induces Si-H bonds at the interface, which become the main trigger for the degradation of the negative bias temperature instability of p-channel metal-oxide-silicon transistors.