The effects of nitrogen concentration on the thermal stability and electric properties of the WNx film as the gate electrode are investigated. WNx is deposited by using reactive radio frequency sputtering, and films with composition of WN0.6, WN0.8, and WN1.5 are obtained at 10, 25, and 40% of N-2 partial flow ratio, respectively. The crystal structure of the WN0.6 film indicates that this film is a mixture of W + W2N, while WN0.8 and WN1.5 films both show the W2N phase. After annealing in N-2 + H-2 (N-2/ H-2 = 9: 1) ambient at 500 degrees C, the surface of the WN0.6 film reveals only the W - O bonding but no W - N bonding. In addition, oxygen diffuses from SiO2 into WN0.6 and leads to the formation of a mixing layer. Subsequently, flatband voltage (V-FB) of the metal oxide semiconductor capacitor shifts positively after annealing at 500 degrees C. After annealing at 500 degrees C, WN0.8 and WN1.5 films exhibit better resistance to oxidation than the WN0.6 film, regardless of the surface of the WNx film or the interface between WNx and SiO2. Resistivity of all WNx films increases after annealing and also increases with increasing nitrogen content in the WNx films. However, neither the nitrogen content in the WNx nor the postmetal annealing affect the leakage current of WNx/SiO2/Si capacitors at both positive and negative biases. (C) 2006 The Electrochemical Society.