Amorphous carbon (a-C) films have been produced by unbalanced magnetron sputtering (UBMS) on silicon (Si), aluminium (Al), and chromium (Cr) substrates as a function of substrate bias voltage. The chemical bonding configurations were investigated by Raman and near-edge X-ray absorption fine structure (NEXAFS) spectroscopies. It was found that the structural changes induced by bias voltage are substrate-dependent, revealing the key role of the substrate type on film microstructures. The Raman G peak position and I-D/I-G ratio were strongly dependent on substrate material and negative bias voltage. Negative bias voltage had a small effect on the structure of carbon films on metal substrates (Al and Cr). However, the films on silicon showed significant change in the structure with bias voltage. The intensity and area of pi* peak at the C K (carbon) edge increased with the increase of substrate bias voltage. The NEXAFS analysis was in agreement with Raman observations, which clearly indicated an increase Of sp(2) content with increasing bias voltage. The films deposited in the low-bias voltage regime (50-100 V) showed reduced sp(2) configurations. The possible changes of structure with substrate bias are thoroughly discussed. (c) 2004 Elsevier B.V. All rights reserved.