Temperature Effects on Reaction Kinetics of Hydrogenated Amorphous Carbon Deposition in Inductively Coupled C2H Plasmas

Abstract

Although extensive research has been conducted on the plasma deposition of hydrogenated amorphous carbon (a-C: H) thin films, studies directly linking a-C: H deposition to plasma characteristics and exploring the reaction kinetics remain limited. In this work, the radical and ion densities were correlated with the a-C: H deposition rate, and different deposition kinetics were proposed for different temperature ranges. The a-C: H thin films were deposited in C2H2 plasmas at various plasma source powers and pressures across different locations within a tubular inductively coupled reactor. The ion densities of the plasmas were measured using an ion probe, while the relative densities of CH and C2 radicals were determined via optical actinometry. The a-C: H deposition rate increased with elevated substrate temperature and ion density when the temperature was below 38 °C. This suggests that the deposition process was primarily limited by surface reaction, which was enhanced by both increased temperature and ion bombardment. The activation energy for a-C: H deposition was found to be 1.47 eV in bias-free, inductively coupled C2H2 plasma. The deposition rate was proportional to the densities of CH and C2 radicals at the surface temperature above 38 °C, indicating that the transport of carbon-containing radicals to the film surface became the limiting factor. Based on these observations, an a-C: H deposition model, comprising mass transfer and surface reaction processes, was proposed. The sp2/sp3 ratios of the a-C: H thin films were also studied, characterized by the intensity ratio of the D and G peaks (ID/IG) in the Raman spectra. The ID/IG ratio increased as the CH radical density decreased, indicating a higher sp2/sp3 ratio.