Comparison of diamond-like carbon films synthesized by 2.45 GHz microwave and 13.56 MHz multi-jet radiofrequency plasma sources

Abstract

Diamond-like carbon films (DLC) were grown by two different plasma deposition systems: the RF–MW system employing a radio frequency (RF) powered substrate holder with an additional 2.45 GHz slot antenna (SLAN) microwave (MW) plasma source and the RF–RF system employing a RF powered substrate holder with an additional RF (13.56 MHz) jet matrix plasma source (JeMPS). Helium and methane were used as carrier gas and carbon source, respectively. When operating the RF–MW system, ion densities approached 4×10 10 cm −3. Because of the large working distance used in our experiments the MW plasma did not contribute significantly to the charged particle density. In the RF–RF system substantially higher ion concentrations of up to 1.6×10 11 cm −3 were measured. Optical and structural properties of the coatings deposited were compared using ellipsometry, Raman spectroscopy and Fourier transform infrared (FTIR). When depositing with the RF–MW system a gradual and controllable change from polymer-like to DLC-based films with increasing substrate RF-power was obtained. This is reflected by a refractive index variation from 1.6 to 2.1. Furthermore, the film growth rates decreased with increasing RF-bias. Typical growth rates were 30 nm min −1 at −400 to −500 V bias. A similar change of the films deposited in the RF–RF system was not observed. Instead, DLC films were produced in all cases. The growth rate peaked at 70–80 nm min −1 when using −350 to −450 V bias. The refractive index (at λ=632 nm) and Vickers hardness were approximately 2.3 and 30 GPa, respectively.