The characteristics of silicon-rich oxide and fluorinated oxide (FxSiOy) films deposited in an electron cyclotron resonance chemical vapor deposition system with SiH4 and O2 as the oxide sources and CF4 as the fluorinating precursor are investigated in this work. According to experimental results, the dangling bonds in Si-rich oxide behave as positively by charged electron traps and degrade the dielectric strength by lowering the barrier height for Fowler-Nordheim tunneling. On the other hand, a small amount of incorporated F atoms will passivate and neutralize these excess Si dangling bonds, thereby elevating the dielectric strength. However, too much incorporated F will degrade the pretunneling leakage performance owing to the porosity and fatigues structure in FxSiOy film. The high leakage and even breakdown at low field strongly limits the incorporated F concentration in FxSiOy film and the lowering of dielectric constant.