Effect of radio-frequency and low-frequency bias voltage on the formation of amorphous carbon films deposited by plasma enhanced chemical vapor deposition

The effect of radio-frequency (RF) or low-frequency (LF) bias voltage on the formation of amorphous hydrogenated carbon (a-C:H) films was studied on silicon substrates with a low methane (CH₄) concentration (2-10 vol.%) in CH₄+Ar mixtures. The bias substrate was applied either by RF (13.56 MHz) or by LF (150 kHz) power supply. The highest hardness values (∼18-22 GPa) with lower hydrogen content in the films (∼20 at.%) deposited at 10 vol.% CH₄, was achieved by using the RF bias. However, the films deposited using the LF bias, under similar RF plasma generation power and CH₄concentration (50 W and 10 vol.%, respectively), displayed lower hardness (∼6-12 GPa) with high hydrogen content (∼40 at.%). The structures analyzed by Fourier Transform Infrared (FTIR) and Raman scattering measurements provide an indication of trans-polyacetylene structure formation. However, its excessive formation in the films deposited by the LF bias method is consistent with its higher bonded hydrogen concentration and low level of hardness, as compared to the film prepared by the RF bias method. It was found that the effect of RF bias on the film structure and properties is stronger than the effect of the low-frequency (LF) bias under identical radio-frequency (RF) powered electrode and identical PECVD (plasma enhanced chemical vapor deposition) system configuration.