Hydrogenated amorphous carbon nitride (a-CNx:H) films were deposited on silicon substrates by plasma enhanced chemical vapor deposition using methane and nitrogen mixture as precursors. The structure and mechanical properties of the as-deposited films were investigated by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and nanoindentation. Based on XPS and FTIR measurements, the nitrogen content in the films is revealed to about 28 at.% with a N/C ratio of 0.39 and N atoms are bonded to C atoms via forming C-N, C=N and C N bonds. In addition, the -CH, and -NH, groups in the films are detected by FTIR spectra. It is found that film hardness depends significantly on the ratio of the C-N to C N bonds and gradually increases with increasing N, fraction in the precursors. (C) 2008 Elsevier B.V All rights reserved.