The characteristics of silicon-rich oxide and fluorinated oxide (FxSiOy) films deposited in an electron cyclotron resonance chemical vapor deposition system with SiH4 and O-2 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.