In this work, we investigated the effect of water-vapor treatment on the surface morphology of SiO2 and Si3N4 insulators before and after Co-60 gamma-ray irradiation by using the atomic force microscopy (AFM) operated under non-contact mode. Before irradiation, no apparent surface morphology change was found in SiO2 samples even they were water vapor treated. However, bright spots were found on post-irradiated water-vapor-treated SiO2 sample surfaces but not on those without water-vapor treatment. We attributed the bright spots to the negative charge accumulation in the oxide due to charge balancing between hydroxyl (OH-) ions adsorbed on SiO2 surface and electron-hole pairs (ehps) generated during irradiation since they can be annealed out after low temperature annealing process. On the contrary, no bright spots were observed on post-irradiated Si3N4 samples with and without water-vapor treatment. This result confirms that Si3N4 is a better water-resist passivation layer than SiO2 layer. (C) 2008 Elsevier B.V. All rights reserved.