Data retention is one of the major device reliabilities of NAND Flash memory. We found that the lower Refractive Index (RI) of the Passivation Silicon Oxynitride (SiON) layer deposited by PECVD, the better data retention behavior was achieved. The hydrogen content and the stress analysis of the films are analyzed to find out which is more important in this case. Generally, when the RI of SiON decreases, both parameters also decrease, so it is impossible to find out which parameter is major factor of data retention. To analyze the effects of two parameters separately, we applied two conditions which had the same H contents but quite different stress values. The final data retention levels are same in both conditions. In addition, even if the layer has the same H content, the retention characteristic is changed by how hydrogen is bonded in the film. In conclusion, the data retention characteristic can be explained by mobile ions generated by the hydrogen weakly bonded in PECVD SiON films in our experiment.