Although extensive research has been conducted on the plasma deposition of hydrogenated amorphous carbon ( a -C: H) 

thin fi lms, 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 diff erent deposition 

kinetics were proposed for diff erent temperature ranges. The a -C: H thin fi lms were deposited in C 2 H 2 plasmas at various 

plasma source powers and pressures across diff erent 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 C 2 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 C 2 H 2 plasma. The deposition rate was proportional to the densities 

of CH and C 2 radicals at the surface temperature above 38 °C, indicating that the transport of carbon-containing radicals 

to the fi lm 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 fi lms were also studied, 

characterized by the intensity ratio of the D and G peaks (I D /I G ) in the Raman spectra. The I D /I G ratio increased as the 

CH radical density decreased, indicating a higher sp2 / sp3 ratio.