Nitrogen ion beam extracted from low-temperature plasmas consists of both atomic and molecular ions, and often the latter is dominant. In preparing nitrides thin films with low-energy nitrogen ions striking the growing surface, the atomic and molecular ions operate at different energy scales. While the atomic ions already provoke the subplantation growth at about 120 eV, the molecular ions that first dissociate at the surface may need roughly a doubled energy. Therefore, the energy dependence of various film characters displays a rather nontrivial behavior. We illustrate this effect with the electron-cyclotron-wave resonance plasma assisted deposition of amorphous CN film, for which the variation of the film composition, microhardness, and surface roughness with the ion energy was investigated. (C) 2004 American Vacuum Society.