The structural features of a-C films prepared by RF magnetron sputtering, in sequential thin layers, under positive and negative bias voltage onto Si substrate, were examined by transmission electron (TEM) microscopy. The microstructure of the films was investigated by cross section TEM. The amorphous character of the films was obvious from bright field images that also revealed the changes of the surface roughness as a function of the deposition steps and bias conditions. Dark field imaging, with the objective aperture on [111] diamond reflection position, revealed that the films were rich in sp(3) C-C bonds throughout the whole film. Similar images with the objective aperture on [002] graphite reflection showed the distribution of sp(2) C-C bonding in the films.