CNx films have been deposited by reactive DC sputtering using an opposed-target Penning type magnetron geometry. Measurements over content of the films. Significant amounts of nitrogen are bound in IR- invisible structures, suggested to be N-N type bonds. The hardness concentrated at the film-substrate interface whereas the bulk of the film is stress-free. The C=N phase is eliminated by annealing at 600 degrees C in vacuum. This treatment also reduces the concentration of the other carbon-nitrogen bonds and also weakens the structure; this result is confirmed by post-annealing hardness measurement.