The amorphous hydrogenated carbon (a-C:H) films were deposited on (100) silicon substrates using a divergent Ar + CH4 electron cyclotron resonance plasma. During the deposition sequence, the rf bias was applied to the substrate to increase the ion bombardment energy. The results of the microhardness test and the Raman spectroscopy revealed that these films showed abrupt changes in the mechanical properties and in the bonding characteristics from polymerlike to diamondlike when the rf substrate bias (V-sub) was increased to - 40 V. Such changes were attributed to the structural and compositional modifications of the growing film by the bombarding ions. In addition, the results of the plasma diagnostics and calculation showed that the required ion energy for growing a hard diamondlike carbon film was about 65 eV or higher. The bombardment by high energetic ions caused the a-C:H film to have diamondlike properties with a strong three-dimensional carbon network by dislodging hydrogen atoms from CH3 radicals on the growing surface, as was identified by Fourier transform infrared spectroscopy.