Amorphous carbon nitride thin films (a-CNx:H) have been prepared on silicon (100) substrates by remote plasma-enhanced chemical vapor deposition. A N-2 plasma was used to excite a CH4 gas in the vicinity of the substrate. The structural properties and the composition of the a-CNx:H films were investigated using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The structural and compositional modifications induced by the substrate negative d.c. bias ranging from 0 V to -400 V were examined. The deposition rate of the films increases with the bias voltage. Infrared spectra and XPS analysis indicate the formation of carbon-nitrogen bonds, in addition to hydrogenated groups in all the films. The composition ratio of nitrogen to carbon of the films varies from 0.18 to 0.25, and is not clearly dependent on the bias voltage. Raman spectra indicate a progressive graphitization of the films with increasing bias voltage.