Comparison of the properties of a-C:H films deposited from methane and heptane precursors: study of the mechanical, chemical and structural properties

Abstract

In this work, thin films of hydrogenated amorphous carbon (a-C:H) were deposited on silicon substrates from heptane and methane precursors by the radio frequency plasma-enhanced chemical vapor deposition (PECVD) technique, varying the self-bias voltage from −100 V to −900 V. As a liquid, heptane is safer, easier to handle, to store and stock, and presented advantages over methane as a precursor in the deposition of a-C:H films by PECVD as it allows deposition rates more than 4 times higher than of methane. Diamond-like carbon films deposited from heptane were also harder (22.5 GPa) and presented smaller intrinsic stress (3.5 GPa) than those obtained from methane (18.8 GPa and 4.1 GPa, respectively). In addition, these films have a smaller hydrogen content and a reduced sp2 character. The observed behavior is consistent with the subplantation model, caused by the ion bombardment energy during deposition. The differences observed in the deposition from both precursors are related to the different number of carbon atoms of the parent molecules and lead to a change of structure and hardness that could be closely related to the structural changes observed by Raman spectroscopy. Films deposited from heptane may be promising for applications due to a higher hardness to elastic modulus ratio H/E of approximately 0.13 when compared to films deposited from methane.