Amorphous hydrogenated carbon films deposited by PECVD in CH4 atmosphere were exposed to nitrogen plasma. The r.f.-plasma treatments were performed with a chamber pressure of 3 Pa and the self-bias voltage, Vb, in the range between - 50 and - 500 V. The time of the plasma treatment was 1200 s. The surface layer was profiled by MEIS. The profiles of carbon and nitrogen, as well as of the oxygen contamination, are quite similar and independent on the self-bias voltage. The chemical bonds were probed by XPS and two phases can be identified: nitrogen inside the aromatic structures and bonded to sp1 carbon atoms. The modifications of the surface roughness were followed by AFM and it shows a sharp peak at around Vb = - 100 V. These results are independent on the film thickness and will be discussed in terms of the energy of the incident ions and the displacement energy of the carbon atoms. © 2006 Elsevier B.V. All rights reserved.