The depth profile is a very important information for nanoparticle formation in very thin SiO 2 films by ion implantation prior to TEM observation, because of the thermal diffusion of implanted atoms. Here, we have investigated depth profiles of Ge atoms in SiO 2 thin films by high-resolution Rutherford backscattering spectrometry (HR-RBS). Samples were prepared by Ge negative ion implantation into 25-nm-thick SiO 2 films on Si substrates at 10 keV with a fluence of 1 × 10 15 ions/cm 2, and subsequent annealing at various temperatures. As implanted, and after annealing at less than 700 °C, the depth profiles were Gaussian and corresponded well to the calculated profiles. Jointed-half-Gaussian curves were fitted to the profiles. The Ge diffusion coefficients were estimated by applying a linear diffusion equation to the fitting curves, and they indicated a rapid diffusion to the surface side rather than the interface side. After annealing at 900 °C, there appeared two clear Ge peaks in the HR-RBS spectra for SiO 2, with strong asymmetric shape at the SiO 2/Si interface. Cross-sectional TEM observations revealed that a Ge concentration of about 1 at.% yields Ge nanoparticles of about 2 nm in diameter. In this low energy implantation, the vacancies due to the implantation were found near the surface, indicating anisotropic diffusion. At high temperature of 900 °C, more normal Ge diffusion behavior should occur, such as strong anisotropic diffusion in the thin SiO 2 film with an interface of crystalline Si. The HR-RBS was useful to reveal nanoscale profiles, and these results will assist in planning Ge nanoparticle fabrication in thin SiO 2 films on Si.