Ion beam assisted deposition (IBAD) was used to grow thin Al films (similar to 100 nm thick) onto Si substrates at room temperature. Al was deposited by electron gun evaporation, while an Ar+ ion beam bombarded the film during the growth. Ion beam energies of 300, 500, and 650 eV were used, with current densities ranging between 4 and 20 mu A/cm(2). Polycrystalline Al films have been obtained, whose structural properties were strictly related to the IBAD parameters (ion current and energy). Samples were analyzed by transmission electron microscopy and Rutherford backscattering spectrometry, pointing out the following effects : (1) IBAD produces smaller polycrystalline Al grains with respect to a film evaporated in standard high vacuum conditions; (2) the Al grain size decreases by increasing the ion current or decreasing the ion energy; (3) sputtering yields during IBAD are higher with respect to a postdeposition bombardment process, and show an inverse trend with the ion beam energy. Results have been explained in terms of modification of kinetics of growth induced by IBAD. A simple model is also proposed to take into account the unexpected result about the sputtering yield. Finally, the IBAD capability to modify the microstructure of the Al films was used to improve the coverage of artificially stepped substrates.