Hydrogenated amorphous carbon (a-C: H) thin films were deposited on silicon single crystal substrates from toluene vapor using rf plasma at room temperature. After deposition, the a-C: H films were irradiated with a nitrogen ion beam and effects of nitrogen ion beam irradiation on surface morphology and composition were studied. Nitrogen ion irradiation was performed using nitrogen ion beams of 0.2 and 1.5 keV for 10 min under the constant ion current density at room temperature. Surface morphology was observed by atomic force microscopy (AFM). Changes in composition and carbon-nitrogen bonding states were analyzed by x-ray photoelectron spectroscopy (XPS). Carbon structures were examined by Raman spectroscopy. AFM observations revealed that the film surface became smooth after nitrogen ion beam irradiation and a notable difference in surface roughness is hardly observed between 0.2 and 1.5 keV ion irradiation. XPS studies showed that nitrogen was implanted near the surface of the a-C: H films after nitrogen ion irradiation and combined with carbon, resulting in carbon nitride formation. Depth profiles obtained by XPS showed that nitrogen ions were implanted in the a-C:H films more deeply after 1.5 keV ion irradiation than 0.2 keV ion irradiation. The implanted nitrogen ion behavior inside the films has been clarified, which is useful to judge the effectiveness for the formation of carbon nitride layers. Carbon structures did not change remarkably after nitrogen ion irradiation except the surface region, where carbon nitride layers are formed. (c) 2006 American Vacuum Society.