The present paper reports on annealing of Ge implanted SiO2 films and emphasize the observation of voids and the mechanism behind their formation which is considered new. Samples were prepared by ion implanting fluences of 3 x 10(16) and 1 x 10(17) cm(-2) respectively of 100 keV Ge into amorphous SiO2 films which were subsequently annealed up to 1000 degreesC in a N-2 atmosphere. The structure of the films was studied by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS). The most striking of the observations is that spherical voids with diameters up to tens of nanometers are observed in the films after annealing at 1000 degreesC for 1 h. The volume fraction of voids increases with the Ge fluence. The mechanism behind the void formation is indicated by the evolution of the sample structure after increasing annealing time or temperature; Ge first segregates into nanocrystals which then increase in size by diffusion and Oswald ripening. Ge is quite mobile in SiO2, and as oxygen or moisture from the annealing atmosphere diffuse in from the surface, the Ge will be bonded in an oxide closer to the surface than the precipitate. There is thus a net flux of Ge out of the nanoprecipitate into an oxide closer to the surface. The volume occupied by the Ge precipitate becomes a void. This model is discussed and it is concluded that it fits the observations. We also report on the filling of the voids by beam induced migration under TEM electron beam exposure. (C) 2003 Elsevier Science B.V. All rights reserved.