The deposition of hard a-C:H films in a capacitively coupled 13.56 MHz discharge using methane and acetylene as source gases was extensively studied. The plasma CVD processes were analysed with respect to self-bias voltage, optical sheath thickness, neutral gas composition as well as current and energy of the ions hitting the powered electrode. The films were characterised for their deposition rate, surface roughness, hardness, mass density and hydrogen content. From the film deposition characteristics and the ion flux measurements the carbon atom deposition rate, the ionic carbon flux and the ion energy per deposited carbon atom have been calculated. The film with the highest hardness of 25 GPa was formed in the acetylene discharge when the mean energy per deposited carbon atom was about 50 eV.
The hardness obtained with the methane discharge was lower at 17 GPa and less sensitive to changes in the process parameters. It was found that the creation of a-C:H films with a hardness exceeding 15 GPa from both precursors is possible if the mean energy per deposited carbon atom exceeds only bout 15 eV. The estimated contribution of ions to the carbon deposition rate is much larger in methane than in acetylene discharges. This contribution practically does not depend on the RF power and it is drastically reduced at increased chamber pressures. In CH4 discharges at low pressure the ions may be the main film growth particles. Similar power and pressure lead to about 3 to 4 times higher deposition rates with acetylene compared to the methane process. This results from the effective deposition from radicals in the C2H2 discharge. Also other a-C:H film characteristics, like surface roughness and hydrogen content, show the interplay of ion flux and deposition from radicals for forming the a-C:H structure and properties. |