The effects of ion bombardment conditions on the crystallographic and electrical properties of tantalum thin films grown on SiO2 and Si have been systematically investigated in Ta thin film formation process employing low-energy ( < 100 eV) inert -gas ion bombardment on a growing film surface. It is demonstrated that the properties of Ta films are strongly dependent upon ion energy and ion flux as well as substrate materials. The bcc-Ta can be formed on SiO2 by controlling impinging ion energy and normalized ion flux defined as the ratio of ion flux to Ta flux ranging lower than 20 eV and higher than 13, respectively, for Ar plasma. Based on these results, low-resistivity bcc-Ta thin films (14.8 mu Omega cm at 300 K) have been successfully formed. It is also experimentally shown that the irradiation by ions with different mass numbers has different effects on growing film properties. even if the energy or momentum of the ions is the same. When normalized ion flux is 26, bcc-Ta films can be grown on Si at ion energy lower than 30 eV for Ar ions and at ion energy ranging from 30 to 90 eV for Xe ions. The results also suggest that there are two cases where the phase transition from bcc to beta occurs. One is caused by defects induced by the recoil implantation of Ta atoms due to the excessive higher-energy ion bombardment and the other is induced by defects due to insufficient total energy input to a growing film surface by ion irradiation and resulting poor film quality.