We have investigated the surface stress evolution under nitridation process of silicon oxide by plasma exited nitrogen. The compressive surface stress, which had been formed by thin oxide on Si(1 0 0) were relaxed by the nitridation. During the nitridation, more than 40% of the initial compressive stress in 3 nm oxide was relaxed. We measured the stress evolution for plasma oxide films and thermal oxide films with thickness of 2-5 nm and found the same amount of the relaxation of oxide stress. Less than 3% of nitrogen was incorporated in SiO2, and not located at the SiO2/Si interface but uniformly distributed in the film with forming a N-Si2O bonding. (C) 2003 Elsevier Science B.V. All rights reserved.