Electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) process of hydrogenated amorphous carbon (a-C:H) thin films on Si(100) substrate was investigated with varying CH4 flow rate, radio frequency (RF) self-bias voltage and deposition pressure. A continuous a-C:H film was formed on a Si substrate only when emission intensity ratio of CH to H-beta in the plasma was larger than 0.66. Raman characteristic peaks of the a-C:H films were changed from two distinct bands into single broad band when deposition pressure increased from 3 to 36 mTorr. The G band width and the Fourier transform-infrared (FT-IR) spectra indicated that the change in Raman characteristic peaks was associated with the change of bonding nature in a-C:H film from low hydrogen containing film to high hydrogen containing film. The change of bonding nature with increasing deposition pressure was due to the lower impact energy of bombarding ions and the saturation of fragmented CH ions by hydrogen atoms in plasma at high deposition pressure in ECR-PECVD. (C) 2000 Elsevier Science S.A. All rights reserved.