For a low dielectric constant intermetal dielectric application, fluorinated silicon oxide (FxSiOy) films were deposited in an electron cyclotron resonance chemical vapor deposition system, with SiH4, O2, and CF4 as the reaction gases. Since the CF4 is an indirect fluorinating precursor, the fluorinating mechanism resembles that of the oxide etching by a fluorocarbon plasma. Thermal stability of the incorporated fluorine (and hence, the dielectric constant) relies heavily on the deposition parameters and technologies. According to experimental results, adding Ar gas during deposition can improve the thermal stability of incorporated fluorine. Such an improvement is due to the fact that Ar sputtering enhances the removal of weakly bonded silicon fluoride on the as-deposited film surface, thereby elevating the mean bonding strength of fluoride remaining in the oxide.