This paper presents a comparative study of SiO2 thin films prepared at room temperature by ion beam induced chemical vapor deposition (IBICVD) and plasma enhanced chemical vapor deposition (PECVD) methods. The films are characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), Rutherford backscattering spectroscopy (RBS), electron recoil detection analysis (ERDA), nuclear reaction analysis (NRA), X-ray reflectometry and spectroscopic ellipsometry. While the films prepared by IBICVD are very compact and dense and have a high refractive index (n=1.48 at λ=550 nm), those prepared by PECVD exhibit a lower refractive index value (n=1.45 at λ=550 nm), lower density and have a higher surface roughness. The different microstructure and properties of the two sets of films are discussed in relation to the ballistic effects that occur by the action of the highly energetic ion beams (e.g. 400 eV) impinging on the surface of the films prepared by IBICVD.