Diamond films were grown by modified hot filament (HF) CVD technique, employing the double bias configuration. Prior to the diamond growth, a voltage source was connected between the substrate and the filaments placed above the substrate to enhance the diamond nucleation. The influence of DC and AC bias voltage has been systematically investigated by scanning electron microscopy, Raman spectroscopy and X-ray diffraction measurements. In the case of DC biasing, a negative voltage varied from 100 to 180 V has been applied to the Si substrate. It was found that total current depends on process duration and substrate bias. A substrate pretreatment at a bias voltage of 120 V results in optimum process conditions with tendency to (100) preferential diamond growth. In the case of AC biasing, a 50 Hz voltage source was used to achieve nucleation either on conductive or non-conductive substrate. An AC bias voltage of 160 V and higher results in a homogeneous film growth. For both bias types, a continuous diamond layer is formed at relatively low nucleation biasing (lower than 180 V) due to using the dual plasma arrangement of HF CVD reactor. Finally, the AC biasing is shown as a promising way to enhance the diamond nucleation on non-conductive substrates. Influences of the DC and/or AC substrate pretreatment and nucleation duration are discussed. (C) 2003 Elsevier Science B.V. All rights reserved.