Amorphous carbon nitride, a-CNx, films were deposited by reactive radio frequency magnetron sputtering of a graphite target in nitrogen gas. This kind of films could be used as novel electric and optical devices. The authors investigated effects of the substrate temperature up to 873 K on the films in this study. The films were characterized with x-ray photoelectron spectroscopy (XPS), ellipsometry, atomic force microscopy, and nanoindentation tests. XPS studies show that the decreasing tendency in the composition ratio of carbon to nitrogen in a-CNx films with the substrate temperature is observed; however, the bonding fraction of sp(3)C-N increases depending on the substrate temperature. The nanoindentation tests reveal that the film hardness increases from 2 to 12 GPa as the substrate temperature increases from room temperature to 823 K. These results suggest that the film hardness is closely related to the bonding states between carbon and nitrogen. (C) 2008 American Institute of Physics.