Magnetron sputter with a rotating ring permanent magnet using Ar and/or N-2 gases were first used to form amorphous carbon (a-C and a-CNx) films on p-Si wafers set on a grounded lower electrode. The a-C film was hard while the a-CNx films were soft. These films include a little O and H atoms unintentionally. Optical band gap, refractive index, Fourier transform infrared spectroscopy absorption spectra, hardness and field emission threshold electric field were significantly different between a-C and a-CNx films. The optical band gap of the a-C film was 0.7 eV while those of a-CNx films were almost constant at about 1.25 eV. The low field emission threshold electric field of 13 V/mu m was obtained in hard a-C film. (C) 2012 Elsevier B. V. All rights reserved.