A three-step technique devoted to the synthesis of pure carbon-onion thin films deposited onto silica substrates has been developed. Carbon onions have been synthesized into silver thin films deposited on silica substrates by using 120 keV carbon ion implantations at 500 degrees C. After silver was removed by thermal annealing, pure carbon-onion thin films can be deposited onto silica substrates. The carbon onions thus obtained show an almost perfect spherical shape, their mean size varying with the implanted fluence. The possibility of recovering a carbon onion powder using this new procedure is discussed, thus opening the way to study some carbon-onion properties which are still unknown.