The effect of fluorine incorporation on properties of silicon dioxide thin films has been studied as a function of NF3/O2 gas flow ratio. Fluorine was incorporated into silicon dioxide films during high density plasma chemical vapor deposition with the SiH4/O2/NF3/He gas mixture used to improve the gap-filling ability for shallow trench isolation of devices. Refractive index measured by ellipsometry decreased with increasing NF3/O2 flow ratios for both as-deposited and annealed films. X-ray reflectivity measurements showed that both fluorine incorporation and thermal annealing reduced the film density. The analysis of infrared absorption spectra showed the relaxation of the Si-O-Si bond with increasing NF3/O2 flow ratios and thermal annealing. The secondary ion mass spectroscopy and x-ray photoelectron spectroscopy studies confirmed the behavior of fluorine diffusion and the binding energy for each species in the films, respectively. These results showed that through fluorine incorporation and thermal annealing, the network structures of silicon dioxide could be modified from low order rings to high order rings accompanied by the enlargement of nanovoids.