The chemical binding states of C and N atoms, and optical properties of carbon nitride (CNx) thin films deposited by unbalanced magnetron sputtering, have been investigated as a function of the negative substrate bias (V-s). The film deposition rate increased slightly with increasing V-s, having a weak maximum at floating potential (similar to 50 V), and decreased sharply to zero for V-s > 150 V-s while N/C ratios did not exhibit any significant variation. Raman spectroscopy was used to reveal that the structure of the film is predominantly amorphous. Fourier transform infra-red spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) analyses showed that N atoms in the films were bound to C atoms through sp(2) and sp(3) configurations. Triple C-N bonds were also detected by FTIR. The ratio of sp(3) to sp(2) bonds increased with increasing V-s. The maximum sp(3) concentration in CNx films was estimated to be similar to 20%. The optical band gap of CNx films was also found to increase with an increase in V-s.