A comparative experimental analysis of the a-C:H deposition processes using CH₄ and C₂H₂ as precursors

The plasma enhanced chemical vapor deposition of a-C:H films using methane and acetylene as precursors was studied. Non-invasive in-situ techniques were used to analyze the plasma processes with respect to the self-bias voltage, the displacement currents to the grounded electrode, the neutral gas composition, the optical sheath thickness as well as current and energy of the ions hitting the powered electrode. The a-C:H films were characterized for their deposition rate, surface roughness, hardness, mass density and hydrogen content. Ion mean free paths, suitable for low pressure RF sheaths, have been quantified for both precursors. The film with the highest hardness of 25 GPa was formed in the C2H2 discharge when the mean energy per deposited carbon atom was about 50 eV. The hardness obtained with the CH4 discharge was lower at 17 GPa and less sensitive to changes in the process parameters. It was found that the creation of hard (hardness > 15 GPa) a-C:H films from both precursors is possible if the mean energy per deposited carbon atom exceeds only ~ 15 eV. Further film characteristics like surface roughness and hydrogen content show the interplay of ion flux and deposition from radicals to form the a-C:H structure and properties.