Fluorine doped oxide (SiO2-XFX) films have been found to exhibit a low dielectric constant. However, the critical issue about SiO2-XFX is its low resistance to moisture which causes the dielectric constant of the film to increase with time. In this work, N2O and NH3 plasma post-treatments are applied to as-deposited SiO2-XFX films. It has been observed that NH3 plasma post-treatments of SiO2-XFX are quite efficient at blocking moisture. The dielectric constant and stress values of the SiO2-XFX films after the N2O and NH3 plasma post-treatments are very stable. The disadvantage of N2O plasma post-treatment is that the dielectric constant of the SiO2-XFX film increases from 3.21 to 3.6 due to fluorine desorption from the SiO2-XFX network. On the other hand, NH3 plasma post-treatment is more efficient at blocking moisture and keeping the low dielectric constant unchanged due to surface nitridation of the SiO2-XFX film.