Thermal diffusion behavior of implanted germanium atoms in silicon dioxide film measured by high-resolution RBS

The depth profile is a very important information for nanoparticle formation in very thin SiO2 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 SiO2 thin films by high-resolution Rutherford backscattering spectrometry (HR-RBS). Samples were prepared by Ge negative ion implantation into 25-nm-thick SiO2 films on Si substrates at 10 keV with a fluence of 1 × 1015 ions/cm2, 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 SiO2, with strong asymmetric shape at the SiO2/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 SiO2 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 SiO2 films on Si.