Gate dielectrics formed by N2O oxynitridation using a rapid thermal annealer (RTA) and a furnace were developed and evaluated. The oxynitrides formed by RTA show very large time-to-breakdown (T-bd) values and small stress-induced leakage current, as compared with pure oxides and oxynitrides formed by furnace annealing. The depth profile of the N-1 delta peak For oxynitrides was analyzed by X-ray photoelectron spectroscopy. Nitrogen atoms in the oxynitrides formed by RTA are localized at the Si/SiO2 interface, and those in the oxynitrides formed during furnace annealing exist uniformly throughout the film. The differences in nitrogen bond state in the oxynitride between the RTA and the furnace are due to the decomposition of N2O gas. A nitrogen atom at the Si/SiO2 interface bonds with one oxygen and two silicon atoms (Si-2=N-O), or bonds with three silicon atoms (Si-3=N). A nitrogen atom located In the bull of the film, however, bonds with two oxygen atoms and one silicon atom (Si-N=O-2). We clarified that the electrical and chemical characteristics of the oxynitride depend on the location of N2O gas decomposition. Thus, precise control of the nitrogen profile and the nitrogen bond states is very important for the formation of highly reliable N2O oxynitride.