Silicon dioxide films NOD, deposited at room temperature by electron cyclotron resonance (ECR) plasma reactor from a gas phase combination of O-2, SiH4 and He, present excellent structural and electrical properties. However, when fabricating field effect devices it is also crucial to minimize the defect density at the semiconductor/insulator interface. We show that the interface state density, investigated in Al/SiO2/Si MOS capacitors, can be substantially reduced performing post-deposition annealing. In particular we studied the effects of annealing temperature and time in different gas ambient: vacuum, nitrogen and forming gas (5% H-2+N-2). We found that interface state passivation mainly occurs when thermal annealing is performed after Al-contact deposition and that it is quite insensitive to the annealing atmosphere. The present results clearly suggest that the hydrogen passivation mechanism is driven by the H-containing species present in the film and a possible mechanism to explain the results is proposed. (c) 2006 Elsevier B.V. All rights reserved.