The impact of oxygen containing plasma treatment on the electrical properties of gate stack is evaluated by measuring the interfacial layer thickness as a function of plasma treatment condition and by characterizing electrical parameters, such as threshold voltage (Vt), mobility, and interfacial trap density. X-ray photoelectron spectroscopy (XPS) measurements show that exposure to an oxygen containing plasma can result in interfacial layer growth when not protected by work function metal. This direct exposure may also result in the incorporation of positive charges into the dielectric layer. Incorporating positive charges into the dielectric layer results in a positive shift of Vt. This is further verified by the fact that the hole mobility is degraded after plasma processing. This impact to hole mobility is negated when the work function metal is in place to act as a diffusion barrier between the oxygen containing plasma and the dielectric layers.