Amorphous carbon nitride thin films (a-CNx) have been deposited onto Si(100) substrates by using a rf diode sputtering system. The films were deposited in reactive nitrogen-argon atmospheres. The partial pressure of nitrogen ranged from 0% to 100% at two different deposition pressures (Pd=2 Pa and P-d=8 Pa). The film composition was determined by ion beam techniques : Rutherford backscattering spectrometry and nuclear reaction analysis. The relative amount of carbon and nitrogen in the films is nearly independent of the nitrogen partial pressure in the reactive plasma. The maximum nitrogen content is 48 at. %. The structural characterization was performed by means of Raman and infrared spectroscopies. Raman spectra revealed the presence of the D and G bands, typical of disordered carbon based materials, and a third band, at about 680 cm(-1), also attributed to film disorder. Infrared spectroscopy results showed the D and G Raman bands, IR allowed due to the nitrogen incorporation in the carbon network, the presence of a band at 2220 cm(-1) due to C=N bonds, and a broadband at 3300 cm(-1) that can be attributed to the O-H stretching of water molecules absorbed in the films’ voids. A linear correlation between the density and the internal stress of the films was also determined and the maximum values of the film density and the mechanical internal stress were measured for films deposited at 50% of nitrogen partial pressure (P-d=8 Pa).