Effect of Ionization Fraction on the Structure and Deposition Mechanisms of Diamond-like Carbon Films: A Molecular Dynamics Study

The mechanical properties of DLC films are strongly process dependent, yet the ionization fraction of the carbon plasma, a key parameter, remains difficult to control independently in experiments. Here, classical molecular dynamics simulations with a dual-energy incident flux of 70 eV ions and 5 eV neutrals were used to investigate how the ionization fraction governs the growth of hydrogen-free DLC films over the full 0 to 100% range. Because the energy of each species is fixed and only their ratio is varied, the ionization fraction is isolated as a single variable, which a mono-energetic beam cannot provide. At 0% ionization, adsorption-dominated growth yields a sp2-rich structure with a tensile stress of +6.08 GPa. From 0% to 50%, the dominant growth mode shifts from adsorption to subplantation, and the density and the compressive stress rise concurrently while the sp3-C fraction remains essentially constant, the stress peaking at −18 GPa at 50%. Beyond 50%, further ionization produces only a marginal increase in the density and the sp3-C fraction, together with a slight relaxation of the compressive stress that is attributed to the thermal relaxation of the network under continuous ion bombardment. At full ionization, subplantation dominates and the film reaches a density of 3.09 g/cm3 and an sp3-C fraction of 33.6%, with a residual compressive stress of −16.5 GPa. The saturation observed above 50% indicates that a further increase in the ionization fraction brings little structural gain, and the results provide a quantitative relationship from which the deposition conditions can be selected according to the desired sp3-C content and the tolerable stress level.