Low frequency noise (LFN) characteristics of HfSiO and HfSiON nMOS with TiN metal gate were compared. Two different methods to introduce nitrogen in HfSiO, plasma and thermal nitridation, were discussed from LFN point of view. Using Multi-stack Unified Noise (MSUN) model, number and mobility fluctuation components were separated to find out the effects of processing on LFN characteristics. Different processing techniques have negligible effects on number fluctuation components. However, mobility fluctuation components were significantly affected. The reason might be due to the presence and distribution of various Coulomb scattering sites at the silicon-high-k interface and in the bulk of high-k oxide due to different nitridation processes.