In this paper, the established model of NBTI (Negative Bias Temperature Instability) mechanism was reviewed. Based on this mechanism, then, the influence of nitrogen was discussed among other processes. A constant concentration of nitrogen exists inside $SiO_2$ in order to prevent boron from diffusing and to increase dielectric constant. It was shown that NBTI improvement was achieved by controlling nitrogen profile. It was supposed that the existence of low activation energy of Si-N bonds at $Si-SiO_2$ interface attributes the improvement by making hydrogen prevent interface traps. It was also shown that improvement of NBTI can be achieved by more effective control of nitrogen profile. It was supposed that the maximum control of nitrogen profile can be achieved by DPN (Decoupled Plasma Nitridation) process.