A comparative analysis of a-C:H films deposited from five hydrocarbons by thermal desorption spectroscopy

Thermal induced gas evolution studies were performed on hydrogenated amorphous carbon (a-C:H) films deposited by plasma enhanced chemical vapour deposition (PECVD). The hydrocarbon–argon discharges were powered by asymmetric bipolar voltage pulses. Five different hydrocarbons (acetylene C2H2, isobutene C4H8, cyclopentene C5H8, toluene C7H8 and cycloheptatriene C7H8) were compared as film growth precursors.The films were heated in vacuum upto 1000 °C and at the same time the gaseous effusion products were measured with a quadrupole mass spectrometer (QMS). The release of hydrogen, water, argon and different hydrocarbons from CH4 upto C7H8 was proved. Large hydrocarbons were detected only when heating up soft films. When annealing hard films, they almost only lost hydrogen – mainly as H2 but also in remarkable portion as H2O. Possibly, the water in the a-C:H films contains the main fraction of hydrogen unbounded to carbon.The characteristic temperatures for the onset of gas evolution of the molecular gases and of argon were found to be strictly correlated with the film hardness – independent of the film growth precursor. The threshold temperature for the release of the dominating effusion product, molecular hydrogen, is significantly higher than that of argon only for films with hardness below about 13 GPa.