Carbon nitride (CNx) thin films about 35 nm thick, prepared by radio-frequency magnetron sputtering at different substrate temperatures, were annealed at 800 degreesC for 40 min in a vacuum. X-ray photoelectron spectroscopy, Raman spectroscopy, and grazing incidence x-ray reflectivity were used to investigate the annealing effects on the composition and structure of the CNx films. An atomic force microscope equipped with a diamond tip was used to measure the hardness of the thin films. Obvious changes in the film thickness and hardness were observed. The thickness of the films prepared at room temperature and 800 degreesC decreased by 1.2 nm and increased by 3.9 nm, respectively, after annealing, but the thickness of the film prepared at 300 degreesC did not change. The film hardness dropped from more than 30 GPa to almost half after the thermal treatment. However, the nitrogen concentration and density of the films only decreased slightly. These results can be explained by the desorption of volatile phases and the graphitization process occurred during annealing. (C) 2004 American Vacuum Society.