The thermal stability and electrical properties of W2N/Ta2O5/Si metal oxide semiconductor (MOS) capacitors upon post-metal annealing in H-2 or N-2 + H-2 ambient at 400-600degreesC for 30 min are investigated. After annealing at 400-500degreesC, the W2N gate remains intact but, due to loss of nitrogen, partly transforms to WO3 after annealing at 600degreesC. The partial oxidation of W2N is more noticeable when annealing in H-2 ambient than in N-2 + H-2, and also induces greatly increased resistivity. However, the capacitance-voltage (C-V) curves of the W2N/Ta2O5/Si MOS capacitors are similar before and after post-metal annealing. C-V hysteresis measurements showed that charges within the Ta2O5 layer of the as-fabricated MOS structure are positive, and the density is 4.94 x 10(12) cm(-2). After annealing in H-2, the charge density decreased with increasing annealing temperature, and the charge polarity became negative after annealing at 600degreesC. In N-2 + H-2, the charge polarity became negative right after annealing at 400degreesC, and the density was below 10(12) cm(-2). For the I-V curves, the leakage current decreases with increasing annealing temperature at positive bias, and H-2 annealing results in lower leakage currents than N-2 + H-2 annealing. In contrast, the leakage currents are all similar when the samples are stressed at negative bias, regardless of the annealing temperature and ambient. (C) 2004 The Electrochemical Society.